Photocurable composition and use in additive manufacturing process

Abstract

A photocurable composition comprising 20-80 wt% urethane (meth)acrylate oligomer, 15-70 wt% acrylamide monomer, such as N-acryloyl morpholine or N,N-diethyl acrylamide, and 5-60 wt% (meth)acrylate mon

Description

Field of the Invention The present invention relates to a photocurable composition that is particularly suitable for use in an additive manufacturing resin. Background Additive manufacturing involves the construction of a three-dimensional object, usually in which material is deposited, joined or solidified under computer control, with the object being formed typically layer by layer. Additive manufacturing encompasses three-dimensional (3D) printing. In resin-based 3D printing, a liquid photopolymer resin may be stored in a vat (known as vat polymerisation). Resin-based 3D printing processes are centred on the polymerisation (curing) of resins, such as (meth)acrylic-based resins. The curing process can be triggered by means of an initiator, such as a photoinitiator that is sensitive to actinic radiation, e.g. ultraviolet (UV) radiation. Selective exposure of photopolymer resin within the vat to UV light can build the desired 3D printed product layer by layer. This process of raising or lowering the build platform and exposure to UV light is repeated until the final 3D object is formed. Such resin-based 3D printing technologies include stereolithography (SLA), digital light processing (DLP) and liquid crystal display (LCD), which mainly differ by the light source and how it is used to cure the resin. Another example is hot lithography, which utilises a coating and heating unit, in combination with a movable material vat to facilitate 3D printing at elevated temperatures and with chemistries unsuitable with the aforementioned technologies. Another example is volumetric printing, which is similar to SLA printing but, rather than building layer by layer, radiation is projected into an entire vat of transparent resin simultaneously, curing the entire product at once three-dimensionally. A further example process is multiphoton (including two-photon) lithography. These resin-based (vat polymerisation) 3D printing processes are distinguished from processes in which a material is extruded or deposited out of a nozzle moving over a print bed and then solidified or cured. Such processes include fused filament fabrication (in which a thermoplastic filament is heated while fed through a nozzle in an extrusion head), material jetting (in which tiny droplets of material are deposited and then solidified or cured on a build plate), binder jetting (in which a recoating blade spreads a thin layer of powder before a printhead with inkjet nozzles selectively deposits droplets of a binding agent to bind the powder particles together), and directed energy deposition (in which metal material is fed and fused by powerful energy at the same time as it is deposited). Furthermore, they are also distinguished from processes where materials are selectively fused together in a powder / granular bed. Such processes include selective laser sintering (in which a laser is used to sinter powdered material such as thermoplastic and glass into a desired 3D shape), selective laser melting (in which a laser is used to melt metal powder to create full dense metal materials) and electron beam melting (which involves layer-by-layer melting of metal powder with an electron beam under vacuum to yield void-free materials). Traditional (meth)acrylic-based resins are known to have poor elongation properties after curing, resulting in fragile 3D printed products. In WO 2020 / 073149 A1,3D printing resins involving a composition of a (meth)acrylate oligomer, an acrylamide compound and a (meth)acrylate monomer, together with a photoinitiator, are disclosed, which appear to exhibit improved elongation properties, but still have limited toughness. The (meth)acrylate monomers used in the exemplified compositions of WO 2020 / 073149 A1 have a glass transition temperature (Tg) of at most 88 °C, while the (meth)acrylate oligomers used are difunctional. EP 2436510 A1 is focused on producing 3D objects with large cross-sections in short time with high accuracy. Compositions are disclosed which comprise a (meth)acrylate oligomer, an acrylamide compound a (meth)acrylate monomer, together with a photoinitiator. The exemplified compositions use a difunctional oligomer, such as CN 981 (Sartomer) which has a Tg of 35 °C, These compositions demonstrate reasonable tensile properties, but limited elongation at break. Meanwhile, US 10239255 B2 discloses polymerisable liquids, including those that comprise a urethane acrylate oligomer and a (meth)acrylate monomer, together with a photoinitiator. A monofunctional to oligomeric double bond ratio of at least 10:1 for resin 3D printing is disclosed. Similarly, good elongation properties but limited toughness is observed, when polymerised. Indeed, it is known in the art to be difficult to obtain resins that are suitable for additive manufacturing applications and that have a good combination of properties relating to toughness and flexibility. Other resins are also known in the prior art, such as those disclosed in WO 2020 / 064523 A1, which involve a combination of a urethane acrylate oligomer with a vinyl monomer, such as vinyl methyl oxazolidinone (VMOX). Consequently, there remains a need within the art for improvements in resins for additive manufacturing applications; in particular, to photocurable resins for use in vat polymerisation processes that yield cured products having an improved balance of properties relating to toughness and flexibility. The present invention has been devised in light of the above considerations. Summary of the Invention A first aspect of the invention is a photocurable composition comprising 20 to 80 wt% of a urethane (meth)acrylate oligomer, 15 to 70 wt% of an acrylamide monomer, and 5 to 60 wt% of a (meth)acrylate monomer, based on the total weight of the photocurable composition; wherein the (meth)acrylate monomer has a functionality of greater than 1.5 and a glass-transition temperature (Tg) of greater than 120 °C. As used herein, the term “(meth)acrylate” refers to both or any one of “acrylate” and “methacrylate”. The weight percent of the urethane (meth)acrylate oligomer, the acrylamide monomer or the (meth)acrylate monomer is defined as the weight of the urethane (meth)acrylate oligomer, the acrylamide monomer or the (meth)acrylate monomer respectively, divided by the sum of the weights of the components of the photocurable composition (i.e. the total weight of the photocurable composition), multiplied by 100. The components of the photocurable composition comprise at least the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer, and may also include, by way of example, a pigment, an optical brightener, a defoamer, a deaerating agent, a free radical inhibitor, a filler, a catalyst, a thermal initiator, and / or a photoinitiator, as described herein. The present invention provides a photocurable composition that comprises a urethane (meth)acrylate oligomer component and an acrylamide monomer component, the addition to which of a (meth)acrylate monomer component having a functionality of greater than 1.5 and particularly a glass-transition temperature (Ta) of greater than 120 °C surprisingly increases the tensile strength and tensile modulus of resins of the composition, while maintaining good elongation properties. Resins of the composition are particularly, but not exclusively, suitable in additive manufacturing applications, such as 3D printing applications. The invention also relates to a resin comprising the photocurable composition. The uncured resin also beneficially has sufficiently low viscosity to allow it to flow out and away from cured structures, prior to the next layer being printed, particularly when used in a vat polymerisation process. Suitably, the photocurable composition of the first aspect can undergo a curing process in which polymerisation takes place. A photoinitiator can initiate the curing process, for example when exposed to UV radiation. In some embodiments, the photocurable composition comprises a photoinitiator-that is to say, the photoinitiator is pre-added to the composition. In other embodiments, the photocurable composition is supplied together with a photoinitiator in a kit - that is to say, the photoinitiator is not preadded to the composition. In addition to a photoinitiator, a thermal initiator can initiate the curing process, for example when exposed to an elevated temperature. In some embodiments, the photocurable composition comprises a thermal initiator - that is to say, the thermal initiator is pre-added to the composition. In other embodiments, the photocurable composition is supplied together with a thermal initiator in a kit - that is to say, the thermal initiator is not pre-added to the composition. Thus, a second aspect of the invention is a kit comprising the photocurable composition of the first aspect and a photoinitiator. In some embodiments, the kit further comprises a thermal initiator. A third aspect of the invention is a cured product of the photocurable composition of the first aspect. The cured product may be or may be comprised in an article or an object, to which the invention also relates. A fourth aspect of the invention provides the use of the photocurable composition of the first aspect in an additive manufacturing process. The fourth aspect also provides the use of the kit of the second aspect in an additive manufacturing process. The fourth aspect also provides the use of the cured product according to the third aspect in an additive manufacturing process. In some embodiments, the additive manufacturing process is a 3D printing process. Suitably, an additive manufactured article is produced by the additive manufacturing process. For example, a 3D printed article is produced by the additive manufacturing process, where the process is a 3D printing process. A fifth aspect of the invention provides an additive manufacturing process comprising the use of the photocurable composition of the first aspect. The fifth aspect also provides an additive manufacturing process comprising the use of the kit of the second aspect. The fifth aspect also provides an additive manufacturing process comprising the use of the cured product according to the third aspect. In some embodiments, the additive manufacturing process is a 3D printing process. Suitably, an additive manufactured article is produced by the process of the fifth aspect. For example, a 3D printed article is produced by the process of the fifth aspect, where the process is a 3D printing process. A sixth aspect of the invention provides an additive manufactured article made from the photocurable composition of the first aspect. The sixth aspect also provides an additive manufactured article made from the kit of the second aspect. The sixth aspect also provides an additive manufactured article comprising the cured product according to the third aspect. The sixth aspect also provides an additive manufactured article made according to the process of the fifth aspect. In some embodiments, the additive manufactured article is a 3D printed article. A seventh aspect of the invention provides a method of making the photocurable composition of the first aspect, comprising combining the acrylamide monomer, the (meth)acrylate monomer, and the urethane (meth)acrylate oligomer or precursor(s) thereof. An eighth aspect of the invention provides use of a cured product of a photocurable composition, such as the cured product according to the third aspect, such that the cured product provides a tensile modulus as measured by ISO 527-1 of at least 800 MPa (such as at least 1000 MPa, or at least 1100 MPa, or at least 1300 MPa, or at least 1500 MPa, or at least 1750 MPa, or at least 2000 MPa, or at least 2250 MPa, or at least 2500 Pa), and / or a tensile strength as measured by ISO 527-1 of at least 15 MPa (such as at least 20 MPa, or at least 25 MPa, or at least 30 MPa, or at least 35 MPa, or at least 40 MPa, or at least 45 MPa, or at least 50 MPa), Preferably, in the eighth aspect, the cured product also provides an elongation at break as measured by ISO 527-1 of at least 2% (such as at least 4%, or at least 5%, or at least 6%, or at least 8%, or at least 10%, or at least 12%, or at least 15%, or at least 20%). Preferably, in the eighth aspect, the cured product also provides a flex strength as measured by ISO 178 of at least 50 MPa (such as at least 60 MPa, or at least 70 MPa, or at least 80 MPa, or at least 85 MPa), and / or a flex modulus as measured by ISO 178 of at least 1500 MPa (such as at least 1750 MPa, or at least 2000 MPa, or at least 2100 MPa, or at least 2200 MPa, or at least 2300 MPa, or at least 2400 MPa), and / or a shore D hardness as measured by ISO 868 of at least 50D (such as at least 60D, or at least 70D, or at least 75D, or at least 80D, or at least 85D). Preferably, in the eight aspect, the cured product also has a heat deflection temperature (HDT), as measured by ISO 75 method A flatwise, of at least 30 °C (such as at least 35 °C, or at least 40 °C, or at least 45 °C, or at least 47 °C, or at least 49 °C, or at least 51 °C). In a further aspect, there is provided a photocurable composition comprising 25 to 40 wt% of a urethane (meth)acrylate oligomer, 30 to 60 wt% of a reactive monofunctional monomer, and 10 to 40 wt% of a (meth)acrylate monomer, based on the total weight of the photocurable composition; wherein the urethane (meth)acrylate oligomer has a functionality of greater than 2.5, such as greater than 3.5, and the (meth)acrylate monomer has a glass-transition temperature (Tg) of greater than 120 °C and a functionality of greater than 1.5, such as about 2. The reactive monofunctional monomer may optionally be selected from an acrylamide monomer (such as N-acrylolyl morpholine or N,N-diethyl acrylamide) and an N-vinyl monomer (such as 1-vinyl-2-pyrrolidinone). Surprisingly, it has been found that the use of a higher functionality oligomer together with a greater amount of the (meth)acrylate monomer than in prior art compositions can lead to improved tensile strength and tensile modulus of resins of the composition, while maintaining good elongation properties. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Preferred embodiments of the invention may also include any one or more of the following preferred features. Preferred features mentioned in relation to the first aspect of the invention may apply equally to the other aspects. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. All compositional ranges expressed herein are limited in total to and do not exceed 100 percent (volume percent or weight percent) in practice. Where multiple components can be present in a composition, the sum of the maximum amounts of each component can exceed 100 percent, with the understanding, and as those skilled in the art readily understand, that the amounts of the components actually used will conform to the maximum of 100 percent. The term “(meth)acrylate” is extensively used within the art to denote “acrylate or methacrylate” and this well-known abbreviation will be used throughout. As used herein, the term “monomer” refers to a small molecule that may chemically react and become chemically bonded with itself or other monomers to form a polymer. Acrylamide monomer The acrylamide monomer component of the photocurable composition refers to a compound having at least one group represented by CH2=C(R)-C(O)-N in each molecule, wherein R is either hydrogen or a methyl group. Suitably, the acrylamide monomer is a reactive monofunctional monomer. By “monofunctional”, it is meant that the monomer has a functionality of about 1, i.e. one bonding site. In some embodiments, the acrylamide monomer is selected from N-acrylolyl morpholine (ACMO), N,N-diethyl acrylamide (DEAA), N,N-dimethyl acrylamide (DMAA) and N-hydroxyethyl acrylamide (HEAA). In some embodiments, the acrylamide monomer is selected from N-acrylolyl morpholine (ACMO) and N,N-diethyl acrylamide (DEAA). In some embodiments, the acrylamide monomer is N-acrylolyl morpholine (ACMO). In some embodiments, the acrylamide monomer is N,N-diethyl acrylamide (DEAA). The acrylamide compound can be used alone or in combination. The photocurable composition comprises the acrylamide monomer in an amount from 15 to 70 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the acrylamide monomer in an amount from 15 to 70 wt%, based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the acrylamide monomer in an amount of at least 20 wt%, or at least 25 wt%, or at least 30 wt%, or at least 34 wt%, or at least 38 wt%, or at least 42 wt%, or at least 46 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the acrylamide monomer in an amount of at most 65 wt%, or at most 60 wt%, or at most 56 wt%, or at most 52 wt%, or at most 48 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the acrylamide monomer in an amount from 15 to 65 wt%, or from 15 to 60 wt%, or from 15 to 56 wt%, or from 15 to 52 wt%, or from 15 to 48 wt%, or from 20 to 70 wt%, or from 20 to 65 wt%, or from 20 to 60 wt%, or from 20 to 56 wt%, or from 20 to 52 wt%, or from 20 to 48 wt%, or from 25 to 70 wt%, or from 25 to 65 wt%, or from 25 to 60 wt%, or from 25 to 56 wt%, or from 25 to 52 wt%, or from 25 to 48 wt%, or from 30 to 70 wt%, or from 30 to 65 wt%, or from 30 to 60 wt%, or from 30 to 56 wt%, or from 30 to 52 wt%, or from 30 to 48 wt%, or from 34 to 70 wt%, or from 34 to 65 wt%, or from 34 to 60 wt%, or from 34 to 56 wt%, or from 34 to 52 wt%, or from 34 to 48 wt%, or from 38 to 70 wt%, or from 38 to 65 wt%, or from 38 to 60 wt%, or from 38 to 56 wt%, or from 38 to 52 wt%, or from 38 to 48 wt%, or from 42 to 70 wt%, or from 42 to 65 wt%, or from 42 to 60 wt%, or from 42 to 56 wt%, or from 42 to 52 wt%, or from 42 to 48 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the acrylamide monomer in an amount from 30 to 60 wt% based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the acrylamide monomer in an amount from 42 to 56 wt% based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. Reactive monofunctional monomer Suitably, the acrylamide monomer component of the photocurable composition is a reactive monofunctional monomer. Alternative photocurable compositions may contain alternative reactive monofunctional monomers. Such reactive monofunctional monomers may be selected from an acrylamide monomer, an N-vinyl monomer (such as 1-vinyl-2-pyrrolidinone [NVP]), a vinyl ether monomer, a vinyl ester monomer, a vinylamide monomer, a styrene monomer, a (meth)acrylate monomer, a cyanoacrylate monomer, a vinyl carbonate monomer, an acryloyl monomer and a vinyl carbamate monomer. (Meth)acrylate monomer The (meth)acrylate monomer component of the photocurable composition may be any acrylate or methacrylate monomer capable of polymerisation, which has a functionality of greater than 1.5 and has a glass-transition temperature (Tg) when homopolymerised of greater than 120 °C. The functionality of the (meth)acrylate monomer denotes the number of bonding sites for polymerisation. The functionality may be expressed as an average, allowing for non-integer values. In some embodiments, the (meth)acrylate monomer has a functionality of at least 2.0, or greater than 2.0, or at least 2.5, or greater than 2.5, or at least 3.0, or greater than 3.0, or at least 3.5, or greater than 3.5, or at least 4.0, or greater than 4.0. In some embodiments, the (meth)acrylate monomer has a functionality of at most 6.0, or at most 5.5, or at most 5.0, or at most 4.5, or at most 4.0, or at most 3.5, or at most 3.0, or at most 2.5. In some embodiments, the (meth)acrylate monomer has a functionality of at least 2.0. In some embodiments, the (meth)acrylate monomer has a functionality of about 2.0. Thus, in some embodiments, the (meth)acrylate monomer is difunctional. By “difunctional”, it is meant that the monomer has a functionality of about 2, i.e. two bonding sites, and hence the monomer’s repeating unit forms two bonds in a polymer with other monomers. The difunctionality results in the formation of a linear polymer chain. Analogously the monomer may be, for example, trifunctional (i.e. may have a functionality of about 3) or tetrafunctional (i.e. may have a functionality of about 4). The term, glass-transition temperature (Tg), is well known in the art, indicating the temperature below which an amorphous material behaves as a glassy solid and above which the same material turns into a viscous or rubber-like state. The (meth)acrylate monomer has at least one group represented by the general formula H2C=C(R)-C(O)-in which R is either hydrogen or a methyl group. Suitably, the (meth)acrylate monomer has two groups represented by the general formula H2C=C(R)-C(O)- in which the two R groups are independently selected from hydrogen and a methyl group. In some embodiments, the two R groups are both hydrogen or both a methyl group. In some embodiments, the two R groups are both hydrogen. In some embodiments, the two R groups are both a methyl group. In some embodiments, the (meth)acrylate monomer is a methacrylate monomer. In some embodiments, the (meth)acrylate monomer is an acrylate monomer. In some embodiments, the (meth)acrylate monomer is selected from 2-ethoxylated bisphenol A dimethacrylate, tricyclodecanedimethanol diacrylate, ethylene glycol dimethacrylate and bisphenol A diacrylate ester. In some embodiments, the (meth)acrylate monomer has a glass-transition temperature (Tg) of at least 125 °C, or at least 130 °C, or at least 135 °C. In some embodiments, the (meth)acrylate monomer has a glass-transition temperature (Tg) of at least 130 °C. In some embodiments, the (meth)acrylate monomer has a glass-transition temperature (Tg) of at most 300 °C, or at most 250 °C, or at most 225 °C, or at most 210 °C, or at most 200 °C, or at most 190 °C, or at most 185 °C. In some embodiments, the (meth)acrylate monomer has a glass-transition temperature (Tg) of from greater than 120 °C to at most 300 °C, or from greater than 120 °C to at most 250 °C, or from greater than 120 °C to at most 225 °C, or from greater than 120 °C to at most 210 °C, or from greater than 120 °C to at most 200 °C, or from greater than 120 °C to at most 190 °C, or from greater than 120 °C to at most 185 °C, or from at least 125 °C to at most 300 °C, or from at least 125 °C to at most 250 °C, or from at least 125 °C to at most 225 °C, or from at least 125 °C to at most 210 °C, or from at least 125 °C to at most 200 °C, or from at least 125 °C to at most 190 °C, or from at least 125 °C to at most 185 °C, or from at least 130 °C to at most 300 °C, or from at least 130 °C to at most 250 °C, or from at least 130 °C to at most 225 °C, or from at least 130 °C to at most 210 °C, or from at least 130 °C to at most 200 °C, or from at least 130 °C to at most 190 °C, or from at least 130 °C to at most 185 °C, or from at least 135 °C to at most 300 °C, or from at least 135 °C to at most 250 °C, or from at least 135 °C to at most 225 °C, or from at least 135 °C to at most 210 °C, or from at least 135 °C to at most 200 °C, or from at least 135 °C to at most 190 °C, or from at least 135 °C to at most 185 °C. The above glass-transition temperature (Tg) values relate to when the (meth)acrylate monomer is homopolymerised. The Tg values disclosed herein are as measured by differential scanning calorimetry. The photocurable composition comprises the (meth)acrylate monomer in an amount from 5 to 60 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the (meth)acrylate monomer in an amount from 5 to 60 wt%, based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the (meth)acrylate monomer in an amount of at least 6 wt%, or at least 10 wt%, or at least 15 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the (meth)acrylate monomer in an amount of at most 50 wt%, or at most 40 wt%, or at most 37 wt%, or at most 32 wt%, or at most 27 wt%, or at most 22 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the (meth)acrylate monomer in an amount from 5 to 50 wt%, or from 5 to 40 wt%, or from 5 to 37 wt%, or from 5 to 32 wt%, or from 5 to 27 wt%, or from 5 to 22 wt%, or from 6 to 60 wt%, or from 6 to 50 wt%, or from 6 to 40 wt%, or from 6 to 37 wt%, or from 6 to 32 wt%, or from 6 to 27 wt%, or from 6 to 22 wt%, or from 10 to 60 wt%, or from 10 to 50 wt%, or from 10 to 40 wt%, or from 10 to 37 wt%, or from 10 to 32 wt%, or from 10 to 27 wt%, or from 10 to 22 wt%, or from 15 to 60 wt%, or from 15 to 50 wt%, or from 15 to 40 wt%, or from 15 to 37 wt%, or from 15 to 32 wt%, or from 15 to 27 wt%, or from 15 to 22 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the (meth)acrylate monomer in an amount from 10 to 60 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the (meth)acrylate monomer in an amount from 5 to 40 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the (meth)acrylate monomer in an amount from 10 to 40 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the (meth)acrylate monomer in an amount from 10 to 22 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. Urethane (meth)acrylate oligomer The urethane (meth)acrylate oligomer component of the photocurable composition may be any of the types familiar to those skilled in the art. In some embodiments, the urethane (meth)acrylate oligomer is a urethane methacrylate oligomer. In some embodiments, the urethane (meth)acrylate oligomer is a urethane acrylate oligomer. Commercially available urethane (meth)acrylate oligomers that are useful in the photocurable compositions of the invention include, for example: BR116, BR144B, XR-145S (all manufactured by Bomar); Genomer4312, Genomer 4316, Genomer 4425, Genomer4690 (all manufactured by Rahn); Photomer 6010, Photomer 6184, Photomer6892 (all manufactured by IGM Resins); Ebecryl4201, Ebecryl 4513m Ebecryl 8209, Ebecryl 8465 (all manufactured by Allnex). These oligomers have a functionality of greater than 2.5. Urethane (meth)acrylate oligomers are usually the reaction products of precursors, which precursors can include an isocyanate component containing at least two isocyanate functional groups (e.g. a diisocyanate), and a (meth)acrylate component containing at least one (meth)acrylate functional group and at least one group reactive with the isocyanate functional group (such as a hydroxyl, thiol or amino group; where the reaction of a hydroxyl group with the isocyanate functional group results in the production of a urethane linkage). Thus, for example, a simple type of urethane methacrylate oligomer could be prepared by reacting two moles of hydroxyethyl methacrylate with one mole of an aliphatic alkylene diisocyanate or with one mole of a cycloaliphatic diisocyanate. A chain-extending reagent may also be used in the production of the urethane (meth)acrylate oligomer. For example, the urethane (meth)acrylate oligomer may be the reaction product of: (i) a di- or higher isocyanate having at least two isocyanate groups; (ii) a chain-extending reagent which has at least two groups selected from hydroxyl, thiol and / or amino groups, each capable of reaction with an isocyanate group of (i); (iii) a hydroxymethacrylate component or a hydroxyacrylate component. The isocyanate component may also be a prepolymer, formed by the reaction of a stoichiometric excess of an isocyanate containing at least two isocyanate functional groups with a chain-extending compound containing two or more functional groups (such as hydroxyl, thiol or amino) reactive with the isocyanate functional groups. One example of a chain-extending group is a polyether diol such as polyethylene glycol. Another example is amine terminated polyethers. Thus, for example, a type of prepolymer isocyanate component could be prepared by reacting two moles of an aliphatic alkylene diisocyanate (or two moles of a cycloaliphatic diisocyanate) with one mole of a polyethylene glycol (these are commercially available in a variety of molecular weights). The resulting prepolymer could then be reacted with two moles of hydroxyethyl methacrylate to give the urethane methacrylate oligomer, which would have a theoretical structure A-X-Y-X-A, in which A is the acrylate component, X is the diisocyanate and Y is the polyethylene glycol chain-extender. Urethane (meth)acrylates useful in the photocurable compositions of the invention may be of the simple (A-X-A) type or the chain-extended (A-X-Y-X-A) type. A greater or lesser degree of chain extension can be achieved by controlling the relative amounts of isocyanate (X) and polyol chain-extender (Y) to provide a theoretical structure A-(X-Y)n-X-A where n is usually in the range of 1 to 5. Preferred chain-extended type urethane (meth)acrylate oligomers have a theoretical structure in which n = 1. As is well understood by those skilled in the art, the preparation of products of this nature inevitably results in a mixture of molecular structures with differing molecular weights, having a variety of values of n. The relative proportion of isocyanate (X) and polyol chainextender (Y) determines the average value of n for what is, in practice, a mixture of similar oligomer structures. Hence, “preferred chain-extended type urethane (meth)acrylate oligomers have a theoretical structure in which n = 1” refers to such a mixed product in which the average value of n is 1. In addition, as is well understood in the field of urethane (meth)acrylates, an oligomer is a molecule having a relatively large molecular weight, obtained by reaction of molecules having lower molecular weights; the oligomer may in some instances contain a small number of repeating sequences of the lower molecular weight molecules. Exemplary urethane (meth)acrylate oligomers typically have a weight average molecular weight (Mw) below about 25,000 and particularly below about 20,000, and a number average molecular weight (Mn) below about 10,000 and especially below about 7,500. In some embodiments, the urethane (meth)acrylate oligomer component is a blend comprising two or more (meth)acrylate oligomers. In some embodiments, the blend comprises two or more urethane acrylate oligomers, or two or more urethane methacrylate oligomers, or one or more urethane acrylate oligomer(s) and one or more urethane methacrylate oligomer(s). In some embodiments, the urethane (meth)acrylate oligomer component is a blend comprising two (meth)acrylate oligomers in a ratio of 10:1 to 1:10, such as 5:1 to 1:5 or 3:1 to 1:3. Similar to the (meth)acrylate monomer as described herein, the urethane (meth)acrylate oligomer component can also be described as having a functionality, which denotes the number of bonding sites for polymerisation. The functionality may be expressed as an average (e.g. in a blend), allowing for noninteger values. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of at least 1.5, or greater than 1.5, or at least 2.0, or greater than 2.0, or at least 2.5, or greater than 2.5, or at least 3.0, or greater than 3.0, or at least 3.5, or greater than 3.5, or at least 4.0, or greater than 4.0. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of at most 6.0, or at most 5.5, or at most 5.0, or at most 4.5, or at most 4.0. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of greater than 2.5. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of greater than 3.5. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of at least 4.0. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of about 4.0 (i.e. the oligomer is tetrafunctional). In some embodiments, the urethane (meth)acrylate oligomer has a glass-transition temperature (Ta) of at most 150 °C, or at most 100 °C, or at most 80 °C, or at most 60 °C, or at most 40 °C, or at most 20 °C, or at most 10 °C, or at most 0 °C. In some embodiments, the urethane (meth)acrylate oligomer has a glass-transition temperature (Tg) of at most 40 °C. In some embodiments, the urethane (meth)acrylate oligomer has a glass-transition temperature (Tg) of at most 20 °C. In some embodiments, the urethane (meth)acrylate oligomer has a glass-transition temperature (Tg) of at most 10 °C. In some embodiments, the urethane (meth)acrylate oligomer has a glass-transition temperature (Tg) of at most 0 °C. In some embodiments, the urethane (meth)acrylate oligomer has a glass-transition temperature (Tg) of at least -50 °C, or at least -30 °C, at least -20 °C, or at least -10 °C. In some embodiments, the urethane (meth)acrylate oligomer has a glass-transition temperature (Tg) from -50 °C to 150 °C, or from -50 °C to 100 °C, or from -50 °C to 80 °C, or from -50 °C to 60 °C, or from -50 °C to 40 °C, or from -50 °C to 20 °C, or from -50 °C to 10 °C, or from -50 °C to 0 °C, or from -30 °C to 150 °C, or from -30 °C to 100 °C, or from -30 °C to 80 °C, or from -30 °C to 60 °C, or from -30 °C to 40 °C, or from -30 °C to 20 °C, or from -30 °C to 10 °C, or from -30 °C to 0 °C, or from -20 °C to 150 °C, or from -20 °C to 100 °C, or from -20 °C to 80 °C, or from -20 °C to 60 °C, or from -20 °C to 40 °C, or from -20 °C to 20 °C, or from -20 °C to 10 °C, or from -20 °C to 0 °C, or from -10 °C to 150 °C, or from -10 °C to 100 °C, or from -10 °C to 80 °C, or from-10 °C to 60 °C, or from -10 °C to 40 °C, or from -10 °C to 20 °C, or from-10 °C to 10 °C, or from-10 °C to 0 °C. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of greater than 2.5 or greater than 3.5, and a glass-transition temperature (Tg) of at most 20 °C. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of greater than 2.5 or greater than 3.5, and a glass-transition temperature (Tg) of at most 10 °C. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of greater than 2.5 or greater than 3.5, and a glass-transition temperature (Tg) of at most 0 °C. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of greater than 3.5, and a glass-transition temperature (Tg) of at most 20 °C. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of greater than 3.5, and a glass-transition temperature (Tg) of at most 10 °C. In some embodiments, the urethane (meth)acrylate oligomer has a functionality of greater than 3.5, and a glass-transition temperature (Tg) of at most 0 °C. The above glass-transition temperature (Tg) values relate to when the urethane (meth)acrylate oligomer is homopolymerised. The photocurable composition comprises the urethane (meth)acrylate oligomer in an amount from 20 to 80 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the urethane (meth)acrylate oligomer in an amount from 20 to 80 wt%, based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the urethane (meth)acrylate oligomer in an amount of at least 25 wt%, or at least 29 wt%, or at least 33 wt%, or at least 37 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the urethane (meth)acrylate oligomer in an amount of at most 70 wt%, or at most 65 wt%, or at most 60 wt%, or at most 55 wt%, or at most 50 wt%, or at most 48 wt%, or at most 44 wt%, or at most 40 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the urethane (meth)acrylate oligomer in an amount from 20 to 70 wt%, or from 20 to 65 wt%, or from 20 to 60 wt%, or from 20 to 55 wt%, or from 20 to 50 wt%, or from 20 to 48 wt%, or from 20 to 44 wt%, or from 20 to 40 wt%, or from 25 to 80 wt%, or from 25 to 70 wt%, or from 25 to 65 wt%, or from 25 to 60 wt%, or from 25 to 55 wt%, or from 25 to 50 wt%, or from 25 to 48 wt%, or from 25 to 44 wt%, or from 25 to 40 wt%, or from 29 to 80 wt%, or from 29 to 70 wt%, or from 29 to 65 wt%, or from 29 to 60 wt%, or from 29 to 55 wt%, or from 29 to 50 wt%, or from 29 to 48 wt%, or from 29 to 44 wt%, or from 29 to 40 wt%, or from 33 to 80 wt%, or from 33 to 70 wt%, or from 33 to 65 wt%, or from 33 to 60 wt%, or from 33 to 55 wt%, or from 33 to 50 wt%, or from 33 to 48 wt%, or from 33 to 44 wt%, or from 33 to 40 wt%, or from 37 to 80 wt%, or from 37 to 70 wt%, or from 37 to 65 wt%, or from 37 to 60 wt%, or from 37 to 55 wt%, or from 37 to 50 wt%, or from 37 to 48 wt%, or from 37 to 44 wt%, or from 37 to 40 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the urethane (meth)acrylate oligomer in an amount from 25 to 55 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the photocurable composition comprises the urethane (meth)acrylate oligomer in an amount from 25 to 40 wt%, based on the total weight of the photocurable composition, or optionally based on the sum of the weights of the urethane (meth)acrylate oligomer, the acrylamide monomer and the (meth)acrylate monomer. Chain-extending reagents Suitable chain-extending polyols are simple diols / polyols (such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, 1,3 or 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, glycerol, trimethylolpropane, penta erythritol and the like), polyether polyols (homopolymers of ethylene oxide or propylene oxide or copolymers of the two, bearing two or more terminal hydroxyl groups, or simple diols / polyols extended by alkoxylation, especially ethoxylation / propoxylation), polyester polyols, polycaprolactone polyols, simple diols / polyols extended by reaction with lactones such as y-butyrolactone, 6-valerolactone or E-caprolactone, polytetramethylene glycols, polytetrahydrofurans, polycarbonate polyols, or sugars (such as isosorbide, sorbitol, sorbitan, xylitol, erythritol and the like). Preferred chain-extending reagents have two or more, preferably three or more, hydroxyl groups available for reaction with an isocyanate group. Particularly preferred chain-extending polyols are polyether polyols, polyester polyols, polycarbonate polyols, and sugars. The particular polyol may be chosen based on its molecular weight, in order to influence the cross-linking density of the oligomer. For polyols of the same functionality, the molecular weight of the polyol is generally inversely proportional to the degree of oligomer cross-linking density obtained. Thus, where a low degree of oligomer cross-linking density is desirable, a polyol having a relatively high molecular weight may be used, such as at least 500 g / mol or at least 750 g / mol. Polyether polyols Especially preferred polyether polyols are alkoxylated (typically ethoxylated and / or propoxylated) derivatives of tri-, tetra- or higher functional polyols. Examples of suitable triols include trimethylolethane, trimethylolpropane and glycerol. Examples of tetrafunctional polyols include pentaerythritol and ditrimethylolpropane. Hexa-functional polyols include dipentaerythritol and sorbitol. Preferred polyether polyols are derived from tri- or tetra-functional polyols. Such chain-extending reagents include ethoxylated and / or propoxylated derivatives of glycerol, ethoxylated and / or propoxylated derivatives of trimethylolpropane and, more preferably ethoxylated and / or propoxylated derivatives of pentaerythritol. Such derivatives may contain from about 3 to 60 units of ethylene oxide (EO), propylene oxide (PO) or combinations thereof, for example. Example polyether polyols suitable for use in the present invention include the alkoxylated range of polyols available from Perstorp Specialty Chemicals, Sweden. This range includes alkoxylated polyols having functionalities of 2, 3, 4 and 6 with OH values of 150 to 1000 mg KOH / g, and molecular weights ranging from about 150 to 1100 g / mol. Such polyols available from Perstorp Specialty Chemicals include Polyol R2490 (hydroxyl functionality of 2); Polyols 3165, 3380, 3610, 3611,3990, R3430, R3530, R3540, R3600 (all hydroxyl functionality of 3); Polyols 4290, 4360, 4525, 4640, R4410, R4630, R4631 (all hydroxyl functionality of 4); and Polyol R6405 (hydroxyl functionality of 6). Additional examples of polyether polyols are available in the Voranol® range from The Dow Chemical Company. This range includes polyols having OH values of about 25 to 820, and molecular weights ranging from about 255 to 6000 g / mol. Further example polyether polyols are also available in the Arcol® range of Covestro. Particularly preferred polyether polyols which are alkoxylated derivatives of tri- or higher functional polyols include aliphatic hydrophilic triols and tetrols in the Simulsol® range from Seppic SA of Paris, France. The range of hydrophilic triols includes ethoxylated derivatives of trimethylolpropane; the range of tetrols includes ethoxylated and propoxylated derivatives of pentaerythritol. A particularly suitable example polyether polyol may have a molecular weight in the range 250 to 1100 g / mol. Furthermore, preferred polyether polyols may have OH values in the range 150 to 1100 mg KOH / g. Some commercially available products may also be a blend of polyether polyols and, in those cases, the number of (EO) / (PO) units might not be an integer. Preferred polyether polyols for use in the present invention contain from 3 to 30 units of ethylene oxide, propylene oxide, or combinations thereof, more preferably from 3 to 20 such units and especially from 3 to 10 units. Especially preferred polyether polyols are tri- and tetrafunctional polyols which contain a theoretical average of up to 6 (EO) / (PO) units per arm of the polyol, and more preferably up to 3, suitably 0.5 to 3 (EO) / (PO) units per arm. Such units need not be distributed equally to all arms of the polyol. Particularly preferred polyether polyols have average molecular weights of less than or equal to 1100 g / mol, especially less than or equal to 1000 g / mol, more preferably less than or equal to 900 g / mol, and particularly preferably from 150 to 1000 g / mol. Some specific examples of polyether polyols suitable for use in the present invention include 3-mole ethoxylate of pentaerythritol, 5-mole ethoxylate of pentaerythritol, 10-mole ethoxylate of pentaerythritol, 20-mole ethoxylate of pentaerythritol, 5-mole propoxylate of pentaerythritol and 3-mole ethoxylate of tri methylolpropane. Polyester polyols Example polyester polyols may be obtainable by condensation polymerisation of a polyol with a polycarboxylic acid. However, preferred polyester polyols are obtainable by reaction of a di- or higher functional polyol with a hydroxy-acid or a lactone. Such preferred polyester polyols are obtainable by polymerisation of a lactone having a 4- to 10-membered ring, more preferably a 5-, 6- or 7-membered ring. Examples include y-butyrolactone, 6-valerolactone and E-caprolactone, with £-caprolactone especially preferred. Generally, lactone polymerisation is performed in the presence of an alcohol to facilitate the polymerisation. This may be a simple monofunctional alcohol, or a diol, or a tri- or higher functional alcohol. Example polyhydric alcohol compounds include sugar alcohols, especially those containing 5 to 6 carbon atoms, such as arabitol, xylitol, ribitol, mannitol, sorbitol, dulatol and idiol. Particularly preferred polyester polyols are derived from £-caprolactone and a tri- or higher functional alcohol. Especially preferred polyester polyols are derived from £-caprolactone and a sugar alcohol. Most preferably a polyester polyol is derived from £-caprolactone and a hexitol, and especially sorbitol. Polycaprolactone polyols suitable for use as chain-extending reagents in the present invention are also available from Ingevity, in the CAPA® range. Diols, triols and tetrols are available. The triols are typically derived from trimethylolpropane and the tetrols from pentaerythritol. Preferred polycaprolactone polyols may have a molecular weight of 2000 g / mol, or less. Particularly preferred polycaprolactone polyols may contain from 3 to 20 caprolactone units in the molecule, preferably from 3 to 10 caprolactone units. Also preferred polycaprolactone polyols may contain a theoretical average of up to 6 caprolactone units per arm of the polyol, more preferably from 1 to 3 caprolactone units per arm. Such units need not be distributed equally to all arms of the polyol. Preferred chain-extender polyols, as described above in the categories of both polyether polyols and polyester polyols, are simple diols / polyols extended by ethoxylation / propoxylation or by reaction with £-caprolactone. Especially preferred as chain-extending polyols are the reaction products of sorbitol with £-caprolactone and polyethoxylated or polypropoxylated derivatives of pentaerythritol. Amine-containing chain-extending reagents Example chain-extending reagents which include two or more terminal amino groups include amine terminated polyethers, based on the polyethers as described above. Example polyether amines are available from Huntsman Advanced Materials (Switzerland), in the Jeffamine® range. This range includes diamines having backbones based on polyethylene glycol, polypropylene glycol, and mixtures thereof. Triamines are also available, prepared by reaction of propylene oxide with a triol, followed by amination of terminal hydroxy groups. Products of various molecular weights are available in each category. Preferred polyetheramines would have a molecular weight of 1000 g / mol, or less. Polycarbonate polyols Example polycarbonate polyols may be obtainable by the catalysed (e.g. tetrabutoxy titanium, dibutyltin oxide or bases) reaction of a diol with either dimethyl carbonate or diphenyl carbonate, before excess carbonate reagent and mono-alcohol (methanol or phenol) are removed under reduced pressure to uncap the terminal hydroxyl functionality. Polycarbonate polyols are typically primary diols that have two hydroxyl groups available for reaction with an isocyanate group. Example polycarbonate polyols suitable for use in the present invention include the commercially available polyols in the Eternacoll® range supplied by UBE, such as Eternacoll UH-50, UH-100, UH-200, UH-300, PH-50, PH-100, PH-200 and PH-300. Isocyanate Suitable di- / poly-isocyanates include, for example, aliphatic alkylene diisocyanates and cycloaliphatic diisocyanates. Examples of aliphatic alkylene diisocyanates include 1,6-hexamethylene diisocyanate (HDI), 1,4-tetramethylene diisocyanate and 1,12-dodecane diisocyanate. Examples of cycloaliphatic diisocyanates include 1,3-and 1,4-cyclohexane diisocyanates and isophorone diisocyanate (IPDI), as well as hydrogenated aromatic diisocyanates such as hydrogenated diphenylmethane diisocyanate (hMDI), hydrogenated toluene diisocyanate (hTDI), hydrogenated tetramethylxylylene diisocyanate (hTMXDI) and hydrogenated polymethylene polyphenyl diisocyanate (hPAPI). Also, modified versions of such diisocyanates may be available, including dimers, trimers (IPDI trimer and HDI trimer are commercially available, for instance), uretidone-modified, urea modified, urethane modified and allophanate-modified diisocyanates. In some embodiments, the isocyanate is a monomeric diisocyanate having a functionality of about 2. In some embodiments, the isocyanate is a diisocyanate in which the two isocyanate functional groups are non-equivalent, i.e. the two isocyanate groups do not have equal reactivity. This may be observed in cases where the molecular structure of the diisocyanate is not symmetrical. In some embodiments, the isocyanate component is a blend comprising two or more di- / poly-isocyanates. (Meth)acrylate end-capping agent An ethylenically unsaturated component having at least one functional group that is reactive with an isocyanate group of the isocyanate component is typically employed in the preparation of the urethane (meth)acrylate oligomer. For example, the functional group may be a hydroxyl, thiol or amino group; where the reaction of a hydroxyl group with the isocyanate functional group results in the production of a urethane linkage. Suitably, the ethylenically unsaturated component is a (meth)acrylate component, having at least one (meth)acrylate functional group and at least one group (e.g. at least one hydroxyl group) which is reactive with an isocyanate group. Thus, the (meth)acrylate component acts as an “end-capping agent”. A (meth)acrylate component may be any of those familiar from the numerous published syntheses of urethane (meth)acrylate oligomers. Particularly suitable (meth)acrylate components have a hydroxyl group and examples include 2-hydroxyethyl acrylate (HEA), hydroxypropyl acrylate (HPA), 2-hydroxyethyl methacrylate (HEMA), hydroxypropyl methacrylate (HPMA); ethoxylated and / or propoxylated derivatives of HEA, HEMA, HPA or HPMA, and reaction products of HEA, HEMA, HPA and HPMA with lactones, for example such as y-butyrolactone or e-caprolactone. Hydroxy-functional species containing more than one (meth)acrylate functional groups, such as glycerol dimethacrylate and pentaerythritol triacrylate, may also be used. Preferred (meth)acrylate components are HEA, HEMA, HPA, HPMA and the polyethoxylated and / or polypropoxylated derivatives thereof. In some embodiments, the (meth)acrylate component is selected from polyethylene glycol monoacrylates, polyethylene glycol monomethacrylates, polypropylene glycol monoacrylates, and polypropylene glycol monomethacrylates, each of which may contain from 3 to 9 or 10 repeating units of ethylene oxide (EO), propylene oxide (PO) or combinations thereof. Preferred components contain 5 to 10, especially 5 or 6 repeating (EO) or (PO) units. Polyethylene glycol monomethacrylates and polypropylene glycol monomethacrylates are particularly preferred. Preferred example polyethylene glycol (meth)acrylates have a molecular weight between 200 and 750, and especially between 200 and 550. Preferred example polypropylene glycol methacrylates have a molecular weight between 240 and 960, and especially between 240 and 700. The particular (meth)acrylate may be chosen based on its molecular weight, in order to influence the cross-linking density of the oligomer. For (meth)acrylates of the same functionality, the molecular weight of the (meth)acrylate is generally inversely proportional to the degree of oligomer cross-linking density obtained. Thus, where a low degree of oligomer cross-linking density is desirable, a (meth)acrylate having a relatively high molecular weight may be used, such as at least 240 g / mol or at least 300 g / mol. Examples of suitable components are available from the Bisomer™ range from GEO Specialty Chemicals, and the Bimax™ range from Gelest. Such polyethylene glycol (meth)acrylates and polypropylene glycol (meth)acrylates may be employed in combination with a chain-extending reagent which is a polyether polyol as described above in relation to embodiments of the photocurable composition. According to some embodiments a preferred (meth)acrylate component is HEA; according to other embodiments a preferred component is a tetrapropoxylated derivative of HPMA. In a first group of embodiments of the invention, a urethane (meth)acrylate oligomer for the photocurable composition may be derived from: (i) an aliphatic alkylene diisocyanate or a cycloaliphatic diisocyanate; (ii) a polyester polyol obtainable by polymerisation of a lactone, especially £-caprolactone; and (iii) hydroxyethyl acrylate. In a particularly preferred first group of embodiments, a urethane (meth)acrylate for the photocurable composition may be derived from: (i) a cycloaliphatic diisocyanate; (ii) a polyester polyol derived from e-caprolactone and a tri- or higher functional alcohol, especially a sugar alcohol; and (iii) hydroxyethyl acrylate. In a second group of embodiments, a urethane (meth)acrylate oligomer may be derived from: (i) an aliphatic alkylene diisocyanate or a cycloaliphatic diisocyanate; (ii) a polyol selected from alkoxylated (such as ethoxylated or propoxylated) derivatives of trimethylol propane and alkoxylated (such as ethoxylated or propoxylated) derivatives of pentaerythritol; and (iii) a polyethylene glycol mono(meth)acrylate or a polypropylene glycol mono(meth)acrylate. In a particularly preferred second group of embodiments, a urethane (meth)acrylate oligomer may be derived from: (i) a cycloaliphatic diisocyanate; (ii) a polyol selected from ethoxylated and propoxylated derivatives of trimethylol propane and ethoxylated and propoxylated derivatives of pentaerythritol, wherein the number of units of ethylene oxide, propylene oxide or combinations thereof is in the range of 3 to 20; and (iii) a polyethylene glycol mono(meth)acrylate or a polypropylene glycol mono(meth)acrylate, each of which contains from 5 to 10 repeating units of ethylene oxide or propylene oxide. In a further group of embodiments, a blend of the urethane (meth)acrylate oligomers of the first and second group of embodiments can be employed. Example blends of urethane (meth)acrylate oligomers for use as a component of the photocurable composition may include oligomers of these embodiments in a ratio of 10:1 to 1:10, especially 5:1 to 1:5, more preferably 3:1 to 1:3. Preferred blends of urethane (meth)acrylate oligomers for use in the photocurable composition may comprise (a) at least one first oligomer derived from a polyester polyol chain-extending reagent and (b) at least one second oligomer derived from a polyether polyol chain-extending reagent, in a ratio (a):(b) of 10:1 to 1:10, especially 5:1 to 1:5, more preferably 3:1 to 1:3. Preferably, such urethane (meth)acrylate oligomers (a) correspond to the first group of embodiments as described above and urethane (meth)acrylate oligomers (b) correspond to the second group of embodiments as described above. In some embodiments, it is preferred that urethane meth(acrylate) oligomers (a) provide at least 50 wt% of the oligomer blend. In these blends, in some embodiments, either oligomer (a) or (b) may be replaced by an oligomer (c) derived from a polycarbonate chain-extending reagent. In some embodiments, the urethane (meth)acrylate oligomer is of a first type, which is a reaction product of sorbitol, £-caprolactone, IPDI and HEA. In some embodiments, the urethane (meth)acrylate oligomer is of a second type, which is a reaction product of a polyethoxylated pentaerythritol, IPDI and the tetrapropoxylate of hydroxypropyl methacrylate. In some embodiments, the urethane (meth)acrylate component is a mixture of said first and second types of urethane (meth)acrylate. Preferably the (i) isocyanate, (ii) chain-extending reagent and (iii) hydroxy (meth)acrylate undergo reaction until the isocyanate (NCO) content of the urethane (meth)acrylate oligomer is 1% or less, preferably 0.8% or less, more preferably 0.6% or less. A urethane (meth)acrylate oligomer which is the reaction product of (i), (ii) and (iii) above typically takes the form of a solution of oligomer in monomer(s) otherwise known as a resin. Some of the monomer content of the resin may be provided by an excess of reagents (ii) and / or (iii). Generally, in both cases the reaction between the isocyanate and the chain-extending reagent, and the reaction between the isocyanate and end-capping hydroxyl(meth)acrylate require a catalyst. A preferred catalyst is a metal salt, especially a metal carboxylate. Tin catalysts are preferred. Particularly preferred catalysts are tin carboxylates, especially organotincarboxylates. Reaction between isocyanate component (i) and chain-extending reagent (ii), and the isocyanate component (i) and hydroxyl(meth)acrylate (iii) will generally be performed at temperatures in the range of 40 to 130°C, and especially, in many cases, at a temperature in the range of 50 to 110°C. The composition and characteristics of the resin may be further modified by the addition of diluent monomer(s). This may be done for the purpose of adjusting the viscosity / solids content of the resin, for example, or for influencing the subsequent curing characteristics of the resin. Suitably, a diluent monomer has one ethylenically unsaturated bond. Example diluent monomers include vinyl aromatics, such as styrene, methylstyrene, ethylstyrene, tert-butylstyrene, halogenated styrene and vinyl toluene. Additional diluent monomers include (meth)acrylate ester monomers, such as esters of Ci-Ce monofunctional alcohols with (meth)acrylic acid [such as methyl acrylate, methyl methacrylate, ethyl acrylate or methacrylate, n-propyl or isopropyl acrylates or methacrylates, butyl (meth)acrylates (all isomers) and hexyl (meth)acrylates], esters of higher molecular weight alcohols having up to about 12 carbon atoms [such as lauryl (meth)acrylate, 2-ethylhexyl (meth)acrylate, hexyl (meth)acrylate, isodecyl (meth)acrylate and the like], esters of cycloaliphatic alcohols [such as cyclohexyl (meth)acrylate], hydroxy-functional (meth)acrylate esters of di / poly-functional alcohols [such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate or pentaerythritol tri(meth)acrylate], di / poly-esters of di / poly-functional alcohols [such as ethylene glycol di(meth)acrylate, 1,3 or 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, or trimethylolpropane tri(meth)acrylate], or (meth)acrylate esters formed by alcohols bearing other functional groups [such as tetra hydrofurfury I (meth)acrylate or benzyl (meth)acrylate]. Such monomers can be added prior to or during formation of the urethane (meth)acrylate oligomer. Additionally or alternatively, diluent monomer may be added to the urethane (meth)acrylate reaction product. Diluent monomer content in such a resin may be up to 80 wt% of the resin, for example. The isocyanate (i) may firstly undergo reaction with end-capping hydroxy (meth)acrylate (iii), followed by reaction with a chain-extending reagent (ii). Alternatively, reaction between isocyanate (i) and chainextending reagent (ii) may take place before reaction with hydroxy (meth)acrylate (iii). Photoinitiators In some embodiments, the photocurable composition comprises a photoinitiator. In some embodiments, the photoinitiator is pre-added to the photocurable composition. In some embodiments, the photoinitiator is not pre-added to the photocurable composition. In such embodiments, the photocurable composition may be supplied without the photoinitiator, or the photocurable composition may be supplied together with the photoinitiator in a kit, for example. Thus, the invention also relates to a kit comprising the photocurable composition and a photoinitiator. In such a kit, the user can add the photoinitiator to the photocurable composition at their convenience. Photoinitiators tend to initiate, enable, facilitate or catalyse chemicals reactions that result in or improve the rate or amount of photopolymerisation in a given composition when it is subjected to appropriate amounts of actinic radiation. A photoinitiator is a compound that undergoes a photoreaction upon absorption of actinic radiation (particularly in the visible and / or UV spectral region), whereupon reactive species are created. These reactive species are capable of catalysing, initialising or carrying out chemical reactions that result in significant changes in the physical properties of suitable formulations. Hence, the photoinitiator is a compound that can transform the physical energy of light into suitable chemical energy in the form of reactive intermediates. Common types of photoinitiators include cationic photoinitiators and free-radical photoinitiators. The photoinitiator may be any common photoinitiator. A plurality of photoinitiators may be used. The photoinitiator(s) may serve to initiate polymerisation of the urethane (meth)acrylate oligomer, acrylamide monomer and / or (meth)acrylate monomer. In some embodiments, the photoinitiator initiates polymerisation in response to UV light and / or visible light. In some embodiments, the photoinitiator initiates polymerisation at a wavelength of from 100 to 700 nm, or from 100 to 430 nm, or from 380 to 700 nm, or from 300 to 470 nm, or from 300 to 395 nm, or from 325 to 365 nm, or from 345 to 430 nm. In preferred embodiments, the photoinitiators comprise a free-radical photoinitiator. In some embodiments, the free-radical photoinitiator is selected from a phosphine oxide (such as a benzoyl phosphine oxide), an aryl ketone, a benzophenone, a hydroxylated ketone, a ketal, a metallocene, or a combination thereof. In some embodiments, the free-radical photoinitiator is selected from 2,4,6-trimethylbenzoyl diphenylphosphine oxide, ethyl(2,4,6-trimethylbenzoyl)-phenyl phosphinate, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide, 2-methyl-1-[4- (methylthio)phenyl]-2-morpholinopropanone-1,2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone, 2-dimethylamino-2-(4-methyl-benzyl)-1-(4-morpholin-4-yl-phenyl)-butan-1-one, 4-benzoyl-4'-methyl diphenyl sulphide, 4,4'-bis(diethylamino) benzophenone, 4,4'-bis(N,N'-dimethylamino) benzophenone [Michler's ketone], benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, dimethoxybenzophenone, acetophenone, chlorinated acetophenone, dialkoxyacetophenones, dialkylhydroxyacetophenones, dialkylhydroxyacetophenone esters, benzoin acetate, benzoin, benzoin alkyl ethers, dimethoxybenzoin, dibenzylketone, 1-hydroxycyclohexyl phenyl ketone, phenyl (l-hydroxyisopropyl)ketone, 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone, 4-isopropylphenyl(1-hydroxyisopropyl)ketone, oligo-[2-hydroxy-2-methyl-1-[4-(1 -methylvinyl)phenyl] propanone], camphorquinone, 4,4'-bis(diethylamino) benzophenone, benzil dimethyl ketal, bis(eta 5-2-4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1 H-pyrrol-1-yl)phenyl]titanium, benzoylcyclohexanol, acyloxime esters, acylphosphine oxides, acylphosphonates, ketosulfides, dibenzoyldisulfides, diphenyl dithiocarbonate, and any combination thereof. Further free-radical photo-initiators include: benzoylphosphine oxides, such as, for example, 2,4,6-trimethylbenzoyl diphenylphosphine oxide (Lucirin TPO from BASF) and 2,4,6-trimethylbenzoyl phenyl, ethoxy phosphine oxide (Lucirin TPO-L from BASF), bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Irgacure 819 or BAPO from Ciba), 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone-1 (Irgacure 907 from Ciba), 2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl) phenyl]-1-butanone (Irgacure 369 from Ciba), 2-dimethylamino-2-(4-methyl-benzyl)-1-(4-morpholin-4-yl-phenyl)-butan-1-one (Irgacure 379 from Ciba), 4-benzoyl-4'-methyl diphenyl sulphide (Chivacure BMS from Chitec), 4,4'-bis(diethylamino) benzophenone (Chivacure EMKfrom Chitec), 4,4'-bis(N,N'-dimethylamino) benzophenone (Michler's ketone), camphorquinone, 4,4'-bis(diethylamino) benzophenone (Chivacure EMK from Chitec), 4,4'-bis(N,N'-dimethylamino) benzophenone (Michler's ketone), bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Irgacure 819 or BAPO from Ciba), and metallocenes such as bis(eta 5-2-4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl] titanium (Irgacure 784 from Ciba), ora mixture thereof. In some embodiments, the photoinitiator is a phosphine oxide. In some embodiments, where a plurality of photoinitiators are used, the photoinitiators comprise a phosphine oxide. In some embodiments, the photocurable composition comprises the photoinitiator in an amount of up to 7 wt%, or up to 6 wt%, or up to 4 wt%, or up to 2 wt%, or up to 1 wt%, or up to 0.5 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the photoinitiator in an amount of up to 7 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the photoinitiator in an amount of at least 0.01 wt%, or at least 0.1 wt%, or at least 0.2 wt%, or at least 0.5 wt%, or at least 1 wt%, or at least 2 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the photoinitiator in an amount from 0.01 to 7 wt%, or from 0.01 to 6 wt%, or from 0.01 to 4 wt%, or from 0.01 to 2 wt%, or from 0.01 to 1 wt%, or from 0.01 to 0.5 wt%, or from 0.1 to 7 wt%, or from 0.1 to 6 wt%, or from 0.1 to 4 wt%, or from 0.1 to 2 wt%, or from 0.1 to 1 wt%, or from 0.1 to 0.5 wt%, or from 0.2 to 7 wt%, or from 0.2 to 6 wt%, or from 0.2 to 4 wt%, or from 0.2 to 2 wt%, or from 0.2 to 1 wt%, or from 0.2 to 0.5 wt%, or from 0.5 to 7 wt%, or from 0.5 to 6 wt%, or from 0.5 to 4 wt%, or from 0.5 to 2 wt%, or from 0.5 to 1 wt%, or from 1 to 7 wt%, or from 1 to 6 wt%, or from 1 to 4 wt%, or from 1 to 2 wt%, or from 2 to 7 wt%, or from 2 to 6 wt%, or from 2 to 4 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the photoinitiator in an amount from 0.5 to 2.0 wt%, based on the total weight of the photocurable composition. Thermal initiators In some embodiments, the photocurable composition comprises a thermal initiator. In some embodiments, the thermal initiator is pre-added to the photocurable composition. In some embodiments, the thermal initiator is not pre-added to the photocurable composition. In such embodiments, the photocurable composition may be supplied without the thermal initiator, or the photocurable composition may be supplied together with the thermal initiator in a kit, for example. Thus, the invention also relates to a kit comprising the photocurable composition and a thermal initiator. In such a kit, the user can add the thermal initiator to the photocurable composition at their convenience. The kit may further include a photoinitiator. Thermal initiators tend to initiate, enable, facilitate or catalyse chemicals reactions that result in or improve the rate or amount of polymerisation in a given composition when it is subjected to appropriate temperatures. A thermal initiator is a compound that undergoes decomposition at specific temperatures, whereupon reactive species are created. These reactive species are capable of catalysing, initialising or carrying out chemical reactions that result in significant changes in the physical properties of suitable formulations. Hence, the thermal initiator is a compound that can yield an appreciable amount of reactive intermediates at elevated temperatures. Common types of thermal initiators include peroxide thermal initiators and azo compound thermal initiators. The thermal initiator may be any common thermal initiator. A plurality of thermal initiators may be used. The thermal initiators) may serve to initiate polymerisation of the oligomer and / or any reactive diluent monomers. In some embodiments, the thermal initiator initiates polymerisation in response to an elevated temperature. In some embodiments, the thermal initiator initiates polymerisation at a temperature of at least 25 °C, or at least 30 °C, or at least 40°C, or at least 50 °C, or at least 60 °C. In some embodiments, the thermal initiator initiates polymerisation at a temperature of from 25 °C to 200 °C, or from 25 °C to 180 °C, or from 60 °C to 200 °C, or from 60 °C to 180 °C. In some embodiments, the thermal initiator is a peroxide thermal initiator. In some embodiments, where a plurality of thermal initiators are used, the thermal initiators comprise a peroxide thermal initiator. In some embodiments, the thermal initiator is selected from tert-amyl peroxybenzoate, 4,4-azobis(4-cyanovaleric acid), 1,T-azobis(cyclohexanecarbonitrile), 2,2'-azobisisobutyronitrile (AIBN), benzoyl peroxide, 2,2-bis(tert-butylperoxy)butane, 1,1-bis(tert-butylperoxy)cyclohexane, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane, 2,5-bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne, bis(1-(tert-butylperoxy)-1-methylethyl)benzene, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butyl hydroperoxide, tert-butyl peracetate, tert-butyl peroxide, tert-butyl peroxybenzoate, tert-butylperoxy isopropyl carbonate, cumene hydroperoxide, cyclohexanone peroxide, dicumyl peroxide, lauroyl peroxide, 2,4-pentanedione peroxide, peracetic acid, potassium persulfate, and any combination thereof. Further thermal initiators include: peroxide compounds, such as, for example, Diisobutyryl peroxide (TRIGONOX 187 from Nouryon), Cumyl peroxyneodecanoate (TRIGNOX 99 from Nouryon), 1,1,3,3-Tetramethylbutyl peroxyneodecanoate (TRIGONOX 423 from Nouryon), tert-Amyl peroxyneodecanoate (TRIGONOX 123 from Nouryon), Di-sec-butyl peroxydicarbonate (TRIGONOX SBP from Nouryon), Di(4-tert-'-butylcyclohexyl) peroxydicarbonate (PERKADOX 16 from Nouryon), Di(2-ethylhexyl) peroxydicarbonate (TRIGONOX EHP from Nouryon), tert-Butyl peroxyneodecanoate (TRIGONOX 23 from Nouryon), Dicetyl peroxydicarbonate (PERKADOX 24 from Nouryon), Dimyristyl peroxydicarbonate (PERKADOX 26 from Nouryon), 1,1,3,3-Tetramethylbutyl peroxypivalate (TRIGONOX 425 from Nouryon), tert-Amyl peroxypivalate (TRIGONOX 125 from Nouryon), tert-Butyl peroxypivalate (TRIGONOX 25 from Nouryon), Di(3,5,5-trimethylhexanoyl) peroxide (TRIGONOX 36 from Nouryon), Dilauroyl peroxide (LAUROX from Nouryon), 2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane (TRIGONOX 141 from Nouryon), Di(4-methylbenzoyl) peroxide (PERKADOX PM-W75 from Nouryon), 1,1,3,3-Tetramethylbutyl peroxy-2-ethylhexanoate (TRIGONOX 421 from Nouryon), tert-Amyl peroxy-2-ethylhexanoate (TRIGONOX 121 from Nouryon), Dibenzoyl peroxide (PERKADOX Lfrom Nouryon), tert-Butyl peroxy-2-ethylhexanoate (TRIGONOX 21S from Nouryon), tert-Butyl peroxyisobutyrate (TRIGONOX 41 from Nouryon), 1,1-Di(tert-amylperoxy)cyclohexane (TRIGONOX 122 from Nouryon), 1,1-Di(tert-butylperoxy)-3,3,5-trimethylcyclohexane (TRIGONOX 29 from Nouryon), 1,1 -Di(tert-butylperoxy)cyclohexane (TRIGONOX 22 from Nouryon), tert-Amylperoxy 2-ethylhexyl carbonate (TRIGONOX 131 from Nouryon), tert-Butyl peroxy-3,5,5-trimethylhexanoate (TRIGONOX 42 from Nouryon), 2,2-Di(tert-butylperoxy)butane (TRIGONOX D-C50* from Nouryon), tert-Butylperoxy isopropyl carbonate (TRIGONOX BPIC-C75* from Nouryon),), tert-Amyl peroxybenzoate (TRIGONOX 127 from Nouryon), tert-Butyl peroxyacetate (TRIGONOX F-C50 from Nouryon), tert-Butyl peroxybenzoate (TRIGONOX C from Nouryon), Dicumyl peroxide (PERKADOX BC-FF from Nouryon), Di(tert-butylperoxyisopropyl)benzene (PERKADOX 14S from Nouryon), 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane (TRIGONOX 101 from Nouryon), Di-tert-butyl peroxide (TRIGONOX B from Nouryon), 3,6,9-Triethyl-3,6,9-trimethyl-1,4,7-triperoxonane (TRIGONOX 301 from Nouryon), 1,2,4,5,7,8-Hexoxonane, 3,6,9-trimethyl-3,6,9-tris (Et and Pr) (TRIGONOX 501 from Nouryon), 1,1,3,3-Tetramethylbutyl hydoperoxide (TRIGONOX TMBH-L from Nouryon), Cumyl hydroperoxide (TRIGONOX K from Nouryon), tert-Butyl hydroperoxide (TRIGONOX A from Nouryon), tert-Amyl hydroperoxide (TRIGONOX TAHP-W85 from Nouryon), and azo compounds, such as, for example, 2,2’-Azodi(isobutyronitrile) (PERKADOX AIBN from Nouryon), 2,2’-Azodi(2-methylbutyronitrile) (PERKADOX AMBN from Nouryon), 1,1’-Azodi(hexahydrobenzonitrile) (PERKADOXACCN from Nouryon), ora mixture thereof. In some embodiments, the photocurable composition comprises the thermal initiator in an amount of up to 10 wt%, or up to 8 wt%, or up to 6 wt%, or upto 4 wt%, or up to 2 wt%, or up to 1 wt%, or up to 0.5 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the thermal initiator in an amount of up to 10 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the thermal initiator in an amount of at least 0.01 wt%, or at least 0.1 wt%, or at least 0.2 wt%, or at least 0.5 wt%, or at least 1 wt%, or at least 2 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the thermal initiator in an amount from 0.01 to 10 wt%, or from 0.01 to 8 wt%, or from 0.01 to 6 wt%, or from 0.01 to 4 wt%, or from 0.01 to 2 wt%, or from 0.01 to 1 wt%, or from 0.01 to 0.5 wt%, or from 0.1 to 10 wt%, or from 0.1 to 8 wt%, or from 0.1 to 6 wt%, or from 0.1 to 4 wt%, or from 0.1 to 2 wt%, or from 0.1 to 1 wt%, or from 0.1 to 0.5 wt%, or from 0.2 to 10 wt%, or from 0.2 to 8 wt%, or from 0.2 to 6 wt%, or from 0.2 to 4 wt%, or from 0.2 to 2 wt%, or from 0.2 to 1 wt%, or from 0.2 to 0.5 wt%, or from 0.5 to 10 wt%, or from 0.5 to 8 wt%, or from 0.5 to 6 wt%, or from 0.5 to 4 wt%, or from 0.5 to 2 wt%, or from 0.5 to 1 wt%, or from 1 to 10 wt%, or from 1 to 8 wt%, or from 1 to 6 wt%, or from 1 to 4 wt%, or from 1 to 2 wt%, or from 2 to 10 wt%, or from 2 to 8 wt%, or from 2 to 6 wt%, or from 2 to 4 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the thermal initiator in an amount from 0.1 to 10 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises a photoinitiator and a thermal initiator. Where both a photoinitiator and a thermal initiator are used, this may lead to enhanced curing outcomes, such as improving the curing of thicker products. Further additives The photocurable composition may optionally further comprise one or more additives. Possible additives include stabilisers, dyes, pigments, optical brighteners, antioxidants, wetting agents, photosensitisers, chain transfer agents, free radical inhibitors, levelling agents, defoamers, deaerating agents, surfactants, fillers, and the like. Further possible additives include antioxidants, acid scavengers, thickeners, flame retardants, silane coupling agents, resin particles, core-shell particle impact modifiers, soluble polymers and block polymers. In some embodiments, the photocurable composition further comprises one or more of a pigment and an optical brightener. In some embodiments, the photocurable composition comprises one or more pigment(s). In some embodiments, the photocurable composition comprises one or more optical brightener(s). In some embodiments, the photocurable composition comprises one or more pigment(s) and one or more optical brightener(s). In some embodiments, the pigment is a dye. In some embodiments, the pigment is an absorber of light, particularly ultraviolet (UV) light. Suitable examples of such absorbers of light include, but are not limited to, titanium dioxide, carbon black, and / or an organic UV light absorber, such as a hydroxybenzophenone, hydroxyphenylbenzotriazole, oxanilide, benzophenone, thioxanthone, hydroxyphenyltriazine, and / or benzotriazole ultraviolet light absorber. In some embodiments, the optical brightener is a fluorescent brightener. Suitable examples of such brighteners include, but are not limited to, benzoxazole derivatives, bis-benzoxazoles, bis-benzoxazolyl-stilbenes, bis-benzoxazolyl-thiophenes, thiophenediyl benzoxazoles, 2,5-thiophenediylbis-(5-tert-butyl-1,3-benzoxazoles). In some embodiments, the photocurable composition comprises the pigment in an amount of upto 3.0 wt%, or up to 2.5 wt%, or up to 2.0 wt%, or up to 1.5 wt%, or up to 1.2 wt%, or up to 1.0 wt%, or up to 0.7 wt%, or up to 0.5 wt%, or up to 0.3 wt%, or up to 0.1 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the pigment in an amount of at least 0.001 wt%, or at least 0.01 wt%, or at least 0.1 wt%, or at least 0.3 wt%, or at least 0.5 wt%, or at least 0.7 wt%, or at least 1.0 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the optical brightener in an amount of up to 3.0 wt%, or up to 2.5 wt%, or up to 2.0 wt%, or up to 1.5 wt%, or up to 1.2 wt%, or up to 1.0 wt%, or up to 0.7 wt%, or up to 0.5 wt%, or up to 0.3 wt%, or up to 0.1 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the optical brightener in an amount of at least 0.001 wt%, or at least 0.01 wt%, or at least 0.1 wt%, or at least 0.3 wt%, or at least 0.5 wt%, or at least 0.7 wt%, or at least 1.0 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the pigment and the optical brightener in an amount of up to 3.0 wt%, or up to 2.5 wt%, or up to 2.0 wt%, or up to 1.5 wt%, or up to 1.2 wt%, or up to 1.0 wt%, or up to 0.7 wt%, or up to 0.5 wt%, or up to 0.3 wt%, or up to 0.1 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the pigment and the optical brightener in an amount of at least 0.001 wt%, or at least 0.01 wt%, or at least 0.1 wt%, or at least 0.3 wt%, or at least 0.5 wt%, or at least 0.7 wt%, or at least 1.0 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the pigment and / or the optical brightener in an amount of up to 2 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the pigment and / or the optical brightener in an amount from 0.001 to 2 wt% or from 0.01 to 2 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition further comprises a defoamer and / or a deaerating agent. The action of defoamers is based essentially on preventing foam formation or destroying foam that has already been formed. Deaerating agents act essentially to promote coalescence of finely distributed gas or air bubbles to larger bubbles in the medium to be deaerated, such as the photocurable composition, and hence accelerate the escape of the gas (or the air). Defoamers can often also be used as deaerating agents and vice versa. In some embodiments, the defoamer or the deaerating agent is a silicon-free polymer or a silicon-containing polymer. Suitable examples of silicon-containing polymers include, but are not limited to, unmodified or modified polydialkylsiloxanes or branched copolymer, comb copolymers or block copolymers composed of polydialkylsiloxane and polyether units (the latter being obtainable from ethylene oxide or propylene oxide, for example). In some embodiments, the defoamer or the deaerating agent is an organic polymer. Suitable examples of organic polymers include, but are not limited to, polyethers and polyacrylates, dialkylpolysiloxanes (particularly dimethylpolysiloxanes), organically modified polysiloxanes (such as arylalkyl-modified polysiloxanes), and fluorosilicones. In some embodiments, the photocurable composition comprises the defoamer and / or the deaerating agent in an amount of upto 2.0 wt%, or upto 1.5 wt%, or up to 1.2 wt%, or up to 1.0 wt%, or up to 0.7 wt%, or up to 0.5 wt%, or up to 0.3 wt%, or up to 0.1 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the defoamer and / or the deaerating agent in an amount of at least 0.001 wt%, or at least 0.01 wt%, or at least 0.1 wt%, or at least 0.3 wt%, or at least 0.5 wt%, or at least 0.7 wt%, or at least 1.0 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the defoamer and / or the deaerating agent in an amount of upto 1 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the defoamer and / or the deaerating agent in an amount from 0.001 to 1 wt% or from 0.01 to 1 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition further comprises one or more fillers. Examples of fillers include both organic and inorganic particulate fillers. The filler may possess a surface functionality or not, the surface functionality comprising a polymerisation group that is capable of (co)polymerisation with one or more of the urethane (meth)acrylate oligomer, acrylamide monomer and (meth)acrylate monomer. The filler may comprise organic or inorganic particles of micron size or less, such as nanoparticles. Examples include core-shell particles, inorganic particles, pigments or plasticisers. In some embodiments, the particulate filler comprises an inorganic filler, such as SiO2, AIO2, TiO2, ZnO2, SnO2, Am-SnO2, ZrO2, Sb-SnO2, AI2O3 or carbon black. In some embodiments, the particulate filler comprises an organic filler, such as polyurethane particles, polystyrene particles, poly(methyl methacrylate) particles or polycarbonate particles. Further examples of fillers include, but are not limited to, glass fibres, glass microspheres, silicas, talc, boron fibres, carbon fibres, clays, waxes, graphite, graphene, carbon nanotubes, and aluminium trihydrate. In some embodiments, the photocurable composition comprises the one or more fillers in an amount of at least 0.1 wt%, or at least 1 wt%, or at least 5 wt%, or at least 10 wt%, or at least 15 wt%, or at least 20 wt%, or at least 30 wt%, or at least 40 wt%, or at least 50 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the one or more fillers in an amount of up to 90 wt%, or up to 80 wt%, or up to 70 wt%, or up to 60 wt%, or up to 50 wt%, or up to 40 wt%, or up to 30 wt%, or up to 20 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the one or more fillers in an amount of up to 50 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition comprises the one or more fillers in an amount from 0.1 to 50 wt% or from 1 to 50 wt%, based on the total weight of the photocurable composition. In some embodiments, the photocurable composition further comprises one or more free radical inhibitor(s). Suitable examples of free radical inhibitors include, but are not limited to, 2,6-di-tert-butyl-a-dimethylamino-p-cresol, butylated hydroxytoluene (BHT), butylated hydroxyanisole, hydroquinone, toluhydroquinone, para-methoxyphenol, mono tert-butyl hydroquinone, tert-butyl catechol, phenothiazine, alpha-tocopherol, pyrogallol, (2,2,6,6-tetramethyl-1-piperidinyl)oxyl, N-nitroso-N-phenylhydroxylamine aluminum salt, tetraethylthiuram disulfide, di-tert-butyI nitroxide, and 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide. Method of making the photocurable composition The invention also relates to methods of making the photocurable composition of the invention. Preferably, the photocurable composition is made in a reactor. In some embodiments, two or more of the components of the photocomposition are combined in the same reactor, such as a single reactor. In some embodiments, all components of the photocomposition are combined in the same reactor, such as a single reactor. Thus, in some embodiments, the method of making the photocurable composition is a “one-pot” process. In some embodiments, the method of making the photocurable composition comprises combining the acrylamide monomer, the (meth)acrylate monomer, and the urethane (meth)acrylate oligomer or precursor(s) thereof. By “combining”, it is meant that the components may either be brought together and reacted in a single step, or across multiple steps (i.e. a stepwise process) in any possible combination thereof. In some embodiments, the method of making the photocurable composition comprises combining in the same reactor (e.g. a single reactor) the acrylamide monomer, the (meth)acrylate monomer, and the urethane (meth)acrylate oligomer or precursors) thereof. In some embodiments, the method of making the photocurable composition comprises combining the acrylamide monomer, the (meth)acrylate monomer, and the urethane (meth)acrylate oligomer, optionally all in the same reactor (e.g. a single reactor). In some embodiments, the method of making the photocurable composition comprises combining the acrylamide monomer, the (meth)acrylate monomer, and precursor(s) of the urethane (meth)acrylate oligomer, optionally all in the same reactor (e.g. a single reactor). Some possible precursors of the urethane (meth)acrylate oligomer are described herein. For example, the precursors of the urethane (meth)acrylate oligomer may include an isocyanate component containing at least two isocyanate functional groups (e.g. a diisocyanate), and a (meth)acrylate component containing at least one (meth)acrylate functional group and at least one group (such as hydroxyl, thiol or amino) reactive with the isocyanate functional group. The precursors may further include a chainextending reagent. Hence, in some embodiments, the method of making the photocurable composition comprises combining the acrylamide monomer, the (meth)acrylate monomer, a di- or higher isocyanate having at least two isocyanate groups, a chain-extending reagent which has at least two groups selected from hydroxyl, thiol and / or amino groups (each capable of reaction with an isocyanate group), and a hydroxymethacrylate or hydroxyacrylate component. Thus, the urethane (meth)acrylate oligomer may be made in situ from precursors, such as before combination with the (meth)acrylate monomer and / or the acrylamide monomer. In some embodiments, the precursors of the urethane (meth)acrylate oligomer are combined before combination with the acrylamide monomer. In some embodiments, the precursors of the urethane (meth)acrylate oligomer are combined before combination with the acrylamide monomer and the (meth)acrylate monomer. In some embodiments, the precursors of the urethane (meth)acrylate oligomer are combined in the presence of the (meth)acrylate monomer. As described above, one or more catalyst(s) may be used in the preparation of the urethane (meth)acrylate oligomer from its precursors, such as in a reaction between an isocyanate and a chainextending reagent, and / or a reaction between an isocyanate and an end-capping hydroxyl(meth)acrylate. Such catalyst(s) may remain present in the photocurable composition once prepared. A preferred catalyst is a metal salt, especially a metal carboxylate. Tin catalysts are preferred, particularly tin carboxylates, especially organotincarboxylates. In some embodiments, the method of making the photocurable composition comprises combining the urethane (meth)acrylate oligomer with the (meth)acrylate monomer, followed by addition of the acrylamide monomer, optionally all in the same reactor (e.g. a single reactor). In some embodiments, the method of making the photocurable composition comprises combining precursor(s) of the urethane (meth)acrylate oligomer with the (meth)acrylate monomer, followed by addition of the acrylamide monomer, optionally all in the same reactor (e.g. a single reactor). Additionally or alternatively, in methods of the invention, the acrylamide monomer may be produced from one or more precursors) and / or the (meth)acrylate monomer may be produced from one or more precursor(s). In some embodiments, the method of making the photocurable composition further involves the addition of a photoinitiator, as disclosed herein. In some embodiments, the method of making the photocurable composition further involves the addition of a thermal initiator, as disclosed herein. In some embodiments, the method of making the photocurable composition further involves the addition of one or more of a pigment and an optical brightener, as disclosed herein. In some embodiments, the method of making the photocurable composition further involves the addition of a defoamer and / or a deaerating agent, as disclosed herein. In some embodiments, the method of making the photocurable composition further involves the addition of one or more fillers, as disclosed herein. In some embodiments, the method of making the photocurable composition further involves the addition of one or more free radical inhibitor(s), as disclosed herein. In some embodiments, the method of making the photocurable composition further involves the addition of one or more additional additive(s). Curing The invention also relates to cured products of the photocurable composition of the invention. The curing of the photocurable composition of the invention may comprise one or more steps (such as two steps, three steps, four steps, or more) of exposing the composition to actinic radiation, suitably actinic radiation having sufficient energy to initiate polymerisation or a cross-linking reaction. The actinic radiation may include, but is not limited to, a-rays, y-rays, ultraviolet (UV) radiation, visible light and electron beams. The actinic radiation may be provided by a source that may be selected from a laser, LED, lamp, or any other conventional means. Preferably, the source is one or more LEDs. The source may be coupled to an image projection system (for instance, in a DLP or LCD 3D printer). The curing may be initiated, enabled, facilitated or catalysed by a photoinitiator. Examples of photoinitiators are described herein. For instance, the photoinitiator may be a phosphine oxide. In addition to a photoinitiator, a thermal initiator may be used. Examples of thermal initiators are described herein. For instance, the thermal initiator may be a peroxide thermal initiator. In some embodiments, the photocurable composition is cured by actinic radiation at a wavelength of from 100 to 700 nm, or from 100 to 430 nm, or from 380 to 700 nm, or from 300 to 470 nm, or from 300 to 395 nm, or from 325 to 365 nm, or from 345 to 430 nm, for example at a wavelength of about 405 nm. Preferably, UV radiation is the source of energy for the curing of the photocurable composition. In some embodiments, the photocurable composition is cured by UV LED. In some embodiments, the curing of the photocurable composition comprises one or more steps of exposing the composition to UV radiation with an energy of up to about 100 mw / cm2 for about 0.5 to about 20 seconds, such as about 1 to about 20 seconds. In some embodiments, the curing of the photocurable composition comprises one or more steps of exposing the composition to UV radiation with an energy of about 0.5 to about 100 mw / cm2 for about 0.5 to about 20 seconds, such as about 1 to about 20 seconds. In some embodiments, the curing of the photocurable composition comprises one or more steps of exposing the composition to UV radiation with an energy of about 2 to about 100 mw / cm2 for about 0.5 to about 20 seconds, such as about 1 to about 20 seconds. In some embodiments, the curing of the photocurable composition comprises one or more steps of exposing the composition to UV radiation with an energy of about 10 to about 100 mw / cm2 for about 0.5 to about 20 seconds, such as about 1 to about 20 seconds. In some embodiments, the curing of the photocurable composition comprises one or more steps of exposing the composition to UV radiation at room temperature or at greater than room temperature, such as at least 25 °C, or at least 30 °C, or at least 40 °C, or at least 50 °C, or at least 60 °C, or at least 70 °C, or at least 80 °C. The foregoing statements (relating to exposing the composition to actinic radiation) may refer to one or more initial curing steps. For instance, initial curing of the photocurable composition may occur as the composition is printed (e.g. in an additive manufacturing process, such as a 3D printing process). For example, in a typical hot lithography process, highly viscous material is heated within the print chamber and applied to the build platform and then cured layer by layer. In a typical stereolithography (SLA) process, the resin is cured by a laser that traces the dimension of the build part for each layer. Meanwhile, in typical LCD and DLP processes, the whole image for each layer is projected and cured simultaneously. In an additive manufacturing process, the photocurable composition preferably comprises a photoinitiator (e.g. a phosphine oxide). Additionally (i.e. subsequent to one or more initial curing steps) or alternatively, the curing of the photocurable composition of the invention may comprise one or more “post-curing” steps. Postcuring generally refers to the exposure of an initially cured object to temperatures at or above the temperature of any initial curing steps for an extended period, which may be in the presence of actinic radiation (such as UV radiation). Therefore, the cured products described herein, in addition to having been (initially) cured, may have been post-cured. In some embodiments, the curing of the photocurable composition comprises post-curing at a temperature greater than the temperature of any initial curing steps. In some embodiments, the curing of the photocurable composition comprises post-curing at a temperature of at least 60 °C, or at least 70 °C, or at least 80 °C, or at least 90 °C, or at least 100 °C, or at least 110 °C, or at least 120 °C, or at least 130 °C, or at least 140 °C, or at least 150 °C, or at least 160 °C, or at least 170 °C, or at least 180 °C. These temperatures may also relate to a step of a multi-step post-curing method. That is to say, the photocurable composition may be post-cured over more than one step (such as two steps, three steps, four steps, five steps, or more), wherein each step is at a temperature greater than the curing temperature. In some embodiments, the photocurable composition is post-cured over more than one step, wherein the steps are of increasing temperature. In some embodiments, the photocurable composition is post-cured for at least 10 minutes, or at least 20 minutes, or at least 30 minutes, or at least 60 minutes, or at least 90 minutes, or at least 120 minutes, or at least 150 minutes, or at least 180 minutes. In some embodiments, the photocurable composition is post-cured for at most 360 minutes, or at most 300 minutes, or at most 240 minutes, or at most 180 minutes, or at most 150 minutes, or at most 120 minutes. In some embodiments, the photocurable composition is post-cured for at least 15 minutes, or at least 30 minutes or at least 60 minutes, at 60 °C or more. These times may refer either to the duration of post-curing as a whole, or to a step of a multi-step postcuring method. In a multi-step post-curing method, one step, more than one step, or all steps may be carried out in the presence of actinic radiation (such as UV radiation). In some embodiments, at least the first step of a multi-step post-curing method is carried out in the presence of actinic radiation (such as UV radiation). The foregoing statements relating to exposing the composition to actinic radiation (such as UV radiation) may apply to any post-curing steps (e.g. type, photoinitiator, source, wavelength, energy, time, temperature). In some embodiments, the curing of the photocurable composition comprises an initial curing step as the composition is printed (e.g. in an additive manufacturing process, such as a 3D printing process), followed by one or more post-curing steps (such as two, three, four, five or more post-curing steps). In some embodiments, the photocurable composition is post-cured at 60 °C or more (such as at 70 °C or more, or at 80 °C or more) in a first post-curing step, and then at 100 °C or more (such as at 120 °C or more, or at 140 °C or more, or at 150 °C or more) in a second post-curing step. In some embodiments, the photocurable composition is post-cured at 140 °C or more (such as at 150 °C or more, or at 160 °C or more, or at 170 °C or more) in a third post-curing step. In some embodiments, the photocurable composition is post-cured at 160 °C or more (such as at 170 °C or more, or at 180 °C or more, or at 190 °C or more) in a fourth post-curing step. In some embodiments, a post-curing step is carried out for at least 10 minutes (such as at least 20 minutes, or at least 30 minutes, or at least 60 minutes, or at least 90 minutes, or at least 120 minutes). One or more initial curing steps may precede the post-curing steps, wherein the initial curing steps are carried out at lower temperature than the post-curing steps. In some embodiments, the curing of the photocurable composition comprises an initial curing step (optionally in the presence of actinic radiation such as UV radiation), a first post-curing step at 60 °C or more (such as at 70 °C or more, or at 80 °C or more) for at least 90 minutes (such as at least 120 minutes) in the presence of actinic radiation (such as UV radiation), and then a second post-curing step at 140 °C or more (such as at 150 °C or more) for at least 30 minutes (such as at least 60 minutes, or at least 90 minutes). In some embodiments, the curing of the photocurable composition comprises an initial curing step in the presence of UV radiation, a first post-curing step for at least 120 minutes at 80 °C or more in the presence of UV radiation, followed by a second post-curing step for at least 60 minutes at 150 °C or more. In some embodiments, the curing of the photocurable composition comprises an initial curing step (optionally in the presence of actinic radiation such as UV radiation), a first post-curing step at 60 °C or more (such as at 70 °C or more, or at 80 °C or more) for at least 90 minutes (such as at least 120 minutes) in the presence of actinic radiation (such as UV radiation), then a second post-curing step at 100 °C or more (such as at 120 °C or more, or at 130 °C or more) for at least 20 minutes (such as at least 30 minutes), then a third post-curing step at 120 °C or more (such as at 150 °C or more, or at 160 °C or more) for at least 20 minutes (such as at least 30 minutes), and then a fourth post-curing step at 140 °C or more (such as at 160 °C or more, or at 180 °C or more) for at least 60 minutes (such as at least 90 minutes, or at least 120 minutes). In some embodiments, the curing of the photocurable composition comprises an initial curing step in the presence of UV radiation, a first post-curing step for at least 120 minutes at 80 °C or more in the presence of UV radiation, followed by a second post-curing step for at least 30 minutes at 130 °C or more, followed by a third post-curing step for at least 30 minutes at 150 °C or more, followed by a fourth post-curing step for at least 120 minutes at 180 °C or more. Heat deflection temperature (HPT) The heat deflection temperature (HDT) can be measured by different techniques. Two of the most commonly used methods are ISO 75 and ASTM D648. In both methods, the bars of the test specimens are placed under a deflection measuring device. A load is placed on each specimen, using edgewise specimen orientation for ASTM D648, and either edgewise or flatwise specimen orientation for ISO 75. The specimens are then lowered into a silicone oil bath where the temperature is raised at 2 °C per minute until they deflect 0.25 mm for ASTM D648, or 0.34 mm for ISO 75 flatwise, or 0.32 mm for ISO 75 edgewise. The specimen sizes differ depending on the method. ASTM D648 uses a standard bar of 5 inches x 0.5 inches x 0.25 inches; ISO 75 flatwise uses a bar of 80 mm x 1 o mm x 4 mm; ISO 75 edgewise uses a bar of 120 mm x 10 mm x 4 mm. ASTM D648 may be carried out using a load of 0.455 MPa or 1.82 MPa. Meanwhile, ISO 75 can be measured with either method A (load of 1.80 Mpa), method B (load of 0.45 MPa), or method C (load of 8.00 MPa). In some embodiments, a cured product of the photocurable composition has an HDT, as measured by ISO 75 method A flatwise, of at least 30 °C, or at least 35 °C, or at least 40 °C, or at least 45 °C, or at least 47 °C, or at least 49 °C, or at least 51 °C. In some embodiments, a cured product of the photocurable composition has an HDT, as measured by ISO 75 method A flatwise, of at most 80 °C, or at most 70 °C, or at most 65 °C, or at most 60 °C, or at most 57 °C, or at most 55 °C, or at most 54 °C. The skilled person will be aware of suitable general curing methods, and how the steps and conditions of these methods may be varied. Examples of such methods and conditions of curing are described herein, particularly in the “Curing” section above. In some embodiments, a cured product of the photocurable composition has an HDT as disclosed herein, when the photocurable composition is cured by a method comprising a first post-curing step for at least 30 minutes (e.g. at least 60 minutes) at 60 °C or more in the presence of UV radiation. The curing method may additionally comprise an initial curing step, which initial curing step may be in the presence of UV radiation. The initial curing step may occur as the composition is printed (e.g. in an additive manufacturing process, such as a 3D printing process, such as those described herein). The UV radiation of the initial curing step and / or the first post-curing step may have a wavelength of about 360 to about 420 nm. The UV radiation of the initial curing step and / or the first post-curing step may have a wavelength of about 385 nm or about 405 nm. The measurement of the HDT by ISO 75 method A flatwise may be in accordance with the methods of the description. Viscosity As described herein, it is desirable that the photocurable composition has sufficiently low viscosity, particularly for suitability in additive manufacturing applications. In particular, it is desirable that the uncured resin has sufficiently low viscosity to allow it to flow out and away from cured structures, prior to the next layer being printed, particularly when used in a vat polymerisation process. Furthermore, it is additionally desirable that said low viscosity does not require heating during the printing process, as many commercially available resin-based 3D printing hardware lack this functionality. Generally, machines capable of printing higher-viscosity resins are particularly expensive. It is also found that high viscosity can lead to poor resolution and poor surface detail in printed articles, as well as incidents of printer failure due to excessive peel forces. In some embodiments, the photocurable composition has a viscosity (ICI viscosity at 25 °C) of at least 2.0 poise, or at least 4.0 poise, or at least 5.0 poise, or at least 6.0 poise, or at least 6.4 poise, or at least 6.7 poise, or at least 7.0 poise. In some embodiments, the photocurable composition has a viscosity (ICI viscosity at 25 °C) of at most 30 poise, or at most 25 poise, or at most 20 poise, or at most 15 poise, or at most 13 poise, or at most 11 poise, or at most 10 poise, or at most 9.5 poise, or at most 9.0 poise. Mechanical properties The following properties relate to cured products of the photocurable composition (or, analogously, to the photocurable composition when cured). For example, the photocurable composition may be cured by the methods described herein. The sample preparation and testing methodology are discussed further below. In some embodiments, a cured product of the photocurable composition has a tensile modulus, as measured by ISO 527-1, of at least 500 MPa, or at least 650 MPa, or at least 800 MPa, or at least 1000 MPa, or at least 1100 MPa, or at least 1300 MPa, or at least 1500 MPa, or at least 1750 MPa, or at least 2000 MPa, or at least 2250 MPa, or at least 2500 MPa. In some embodiments, a cured product of the photocurable composition has a tensile modulus, as measured by ISO 527-1, of at most 6000 MPa, or at most 5000 MPa, or at most 4000 MPa, or at most 3500 MPa, or at most 3000 MPa, or at most 2750 MPa, or at most 2500 MPa. In some embodiments, a cured product of the photocurable composition has a tensile strength, as measured by ISO 527-1, of at least 15 MPa, or at least 20 MPa, or at least 25 MPa, or at least 30 MPa, or at least 35 MPa, or at least 40 MPa, or at least 45 MPa, or at least 50 MPa. In some embodiments, a cured product of the photocurable composition has a tensile strength, as measured by ISO 527-1, of at most 150 MPa, or at most 125 MPa, or at most 100 MPa, or at most 90 MPa, or at most 85 MPa, or at most 80 MPa, or at most 75 MPa. In some embodiments, a cured product of the photocurable composition has an elongation at break, as measured by ISO 527-1, of at least 2%, or at least 4%, or at least 5%, or at least 6%, or at least 8%, or at least 10%, or at least 12%, or at least 15%, or at least 20%. In some embodiments, a cured product of the photocurable composition has an elongation at break, as measured by ISO 527-1, of at most 70%, or at most 60%, or at most 50%, or at most 45%, or at most 40%, or at most 36%, or at most 34%, or at most 30%. In some embodiments, a cured product of the photocurable composition has a flex strength, as measured by ISO 178, of at least 50 MPa, or at least 60 MPa, or at least 70 MPa, or at least 80 MPa, or at least 85 MPa. In some embodiments, a cured product of the photocurable composition has a flex strength, as measured by ISO 178, of at most 200 MPa, or at most 150 MPa, or at most 125 MPa, or at most 110 MPa, or at most 100 MPa, or at most 95 MPa, or at most 90 MPa. In some embodiments, a cured product of the photocurable composition has a flex modulus, as measured by ISO 178, of at least 1500 MPa, or at least 1750 MPa, or at least 2000 MPa, or at least 2100 MPa, or at least 2200 MPa, or at least 2300 MPa, or at least 2400 MPa. In some embodiments, a cured product of the photocurable composition has a flex modulus, as measured by ISO 178, of at most 4000 MPa, or at most 3500 MPa, or at most 3000 MPa, or at most 2800 MPa, or at most 2700 MPa, or at most 2600 MPa, or at most 2500 MPa. In some embodiments, a cured product of the photocurable composition has a shore D hardness, as measured by ISO 868, of at least 50D, or at least 60D, or at least 70D, or at least 75D, or at least 80D, or at least 85D. In some embodiments, a cured product of the photocurable composition has a shore D hardness, as measured by ISO 868, of at most 150D, or at most 125D, or at most 110D, or at most 100D, or at most 95D, or at most 90D. In some embodiments, a cured product of the photocurable composition has: a tensile strength, as measured by ISO 527-1, of at least 35 MPa (such as at least 40 MPa, or at least 45 MPa, or at least 50 MPa); and / or a tensile modulus, as measured by ISO 527-1, of at least 1300 MPa (such as at least 1500 MPa, or at least 1750 MPa, or at least 2000 MPa, or at least 2250 MPa, or at least 2500 Pa); and / or an elongation at break, as measured by ISO 527-1, of at least 2% (such as at least 4%, or at least 5%, or at least 6%, or at least 8%, or at least 10%, or at least 12%, or at least 15%, or at least 20%); and / or an HDT, as measured by ISO 75 method A flatwise, of at least 40 °C (such as at least 45 °C, or at least 47 °C, or at least 49 °C, or at least 51 °C). In some embodiments, a cured product of the photocurable composition has: a tensile strength, as measured by ISO 527-1, of at least 35 MPa (such as at least 40 MPa, or at least 45 MPa, or at least 50 MPa); and a tensile modulus, as measured by ISO 527-1, of at least 1300 MPa (such as at least 1500 MPa, or at least 1750 MPa, or at least 2000 MPa, or at least 2250 MPa, or at least 2500 Pa); and an elongation at break, as measured by ISO 527-1, of at least 2% (such as at least 4%, or at least 5%, or at least 6%, or at least 8%, or at least 10%, or at least 12%, or at least 15%, or at least 20%); and an HDT, as measured by ISO 75 method A flatwise, of at least 40 °C (such as at least 45 °C, or at least 47 °C, or at least 49 °C, or at least 51 °C). In some embodiments, a cured product of the photocurable composition has: a tensile strength, as measured by ISO 527-1, of at least 35 MPa (such as at least 40 MPa); and / or a tensile modulus, as measured by ISO 527-1, of at least 1300 MPa (such as at least 1500 MPa); and / or an elongation at break, as measured by ISO 527-1, of at least 10% (such as at least 12%, or at least 15%); and / or an HDT, as measured by ISO 75 method A flatwise, of at least 40 °C (such as at least 45 °C, or at least 47 °C, or at least 49 °C). In some embodiments, a cured product of the photocurable composition has: a tensile strength, as measured by ISO 527-1, of at least 35 MPa (such as at least 40 MPa); and a tensile modulus, as measured by ISO 527-1, of at least 1300 MPa (such as at least 1500 MPa); and an elongation at break, as measured by ISO 527-1, of at least 10% (such as at least 12%, or at least 15%); and an HDT, as measured by ISO 75 method A flatwise, of at least 40 °C (such as at least 45 °C, or at least 47 °C, or at least 49 °C). Density In some embodiments, the photocurable composition has a density of at least 1.10 g / cm3, or at least 1.15 g / cm3, or at least 1.20 g / cm3, or at least 1.24 g / cm3, at least 1.26 g / cm3, at least 1.28 g / cm3. In some embodiments, the photocurable composition has a density of at most 1.50 g / cm3, or at most 1.40 g / cm3, or at most 1.36 g / cm3, or at most 1.34 g / cm3, at most 1.32 g / cm3, at most 1.30 g / cm3. Additive manufacturing process A photocurable composition of the invention, or a kit of the invention, or a cured product of the invention may be used in an additive manufacturing process as appropriate. Thus, the invention also relates to an additive manufacturing process comprising the use of a photocurable composition of the invention, or the use of a kit of the invention, or the use of a cured product of the invention. The additive manufacturing process may be a 3D printing process, and may comprise the use of any suitable 3D printer. Suitably, the 3D printer can print the photocurable composition of the invention. Suitably, the 3D printing process is a vat polymerisation process. In some embodiments, the 3D printing process comprises the use of a stereolithography (SLA) 3D printer, a digital light processing (DLP) 3D printer, a liquid crystal display (LCD) 3D printer, a volumetric 3D printer, a hot lithography printer, or a multiphoton lithography 3D printer. In some embodiments, the 3D printing process comprises the use of a stereolithography (SLA) 3D printer. In some embodiments, the 3D printing process comprises the use of a digital light processing (DLP) 3D printer. In some embodiments, the 3D printing process comprises the use of a liquid crystal display (LCD) 3D printer. In some embodiments, the 3D printing process comprises the use of a volumetric 3D printer. In some embodiments, the 3D printing process comprises the use of a hot lithography 3D printer. In some embodiments, the 3D printing process comprises the use of a multiphoton lithography 3D printer. The additive manufacturing process may comprise a step of providing a first layer of the photocurable composition, which layer may be provided in an oxygen-rich environment (which is particularly preferable for SLA and DLP processes). The additive manufacturing process may optionally comprise a step of heating at least a portion of the first layer of the photocurable composition. The additive manufacturing process may comprise an additional step of exposing at least a portion of the first layer of the photocurable composition imagewise to actinic radiation to form a cured cross-section (the curing preferences disclosed herein may apply). The additive manufacturing process may comprise a further step of providing an additional layer of the photocurable composition onto at least a portion of the cured cross-section, and then exposing at least a portion of the additional layer of the photocurable composition imagewise to actinic radiation to form an additional cured cross-section (the curing preferences disclosed herein may apply). The steps may be repeated several times sufficient to form the complete shape of the additive manufactured article to be built. The layers of the photocurable composition may be provided in a number of ways, as will be appreciated by those skilled in the art. The layer may be of any suitable thickness and shape and is dependent on the additive manufacturing process utilised. For example, in an SLA process, liquid resin lies in a vat, and the first layer is set and controlled by a vertically movable platform that raises or lowers (dips) the liquid level in the vat, along with the intensity, orientation and focus of a laser that cures to a specified depth below the surface of the liquid resin, initiating curing through selective exposure. Such layers will typically be substantially of a uniform thickness in SLA processes. In similar LCD-based processes, instead of a layer determined by a laser depth, the radiation is imparted image-wise in a mask-based pattern, typically by light collimated by a multitude of LED sources. Alternatively, where DLP technology is employed, a projected light source is used to initiate curing of an entire layer in the print structure by exposing the liquid resin to a complete image of that layer, resulting in the curing of all points in said layer simultaneously. In alternative iteration, layers may be selectively deposited on a substrate or previous cured layer, as is performed in known jetting processes. The substrate may be a planar build plate or platform, for example. Layers may further be provided in a rolling or extrusion process, and may be transported on a movable foil, film, or carrier, as is described in, e.g., U.S. Pat. No. 8,678,805. Thus, in some embodiments, the additive manufacturing process may comprise one or more steps of curing a photocurable composition of the invention. For example, the additive manufacturing process may comprise one or more initial curing steps, which may occur as the composition is printed (e.g. in a 3D printing process). Additionally (i.e. subsequent to one or more initial curing steps) or alternatively, the additive manufacturing process may comprise one or more post-curing steps. For example, the one or more postcuring steps may take place separately from a 3D printer where one or more initial curing steps take place, but the one or more post-curing steps may still be considered to be part of the overall additive manufacturing process. The invention also relates to additive manufactured articles produced by the additive manufacturing process. The additive manufactured article may be a 3D printed article. Thus, the invention also relates to additive manufactured articles made from a photocurable composition of the invention, or a kit of the invention. The invention also relates to additive manufactured articles comprising a cured product of the invention. Vat polymerisation methods of additive manufacturing are particularly suited to applications where highly detailed parts are required with features in the range of tens of microns. Nevertheless, the same methods can also be used to produce large-scale objects of up to and in excess of 1 m3. Examples of articles that require high detail include dental models, microfluidics, war-gaming miniatures, functional prototypes and electronic components. Examples of large-scale objects include architectural features, artistic sculptures, educational aids, and plugs and moulds for composite manufacturing. Further embodiments Preferably, a photocurable composition according to the first aspect may comprise, based on the total weight of the photocurable composition: (A) from 20 to 80 wt% of the urethane (meth)acrylate oligomer, such as from 25 to 55 wt% or from 25 to 40 wt%; (B) from 15 to 70 wt%, such as from 30 to 60 wt%, of the acrylamide monomer, such as N-acrylolyl morpholine or N,N-diethyl acrylamide; (C) from 10 to 60 wt% of the (meth)acrylate monomer, having a functionality of greater than 1.5, such as about 2, and a glass-transition temperature (Tg) of greater than 120 °C, such as greater than 130 °C; and (D) optionally, up to 7 wt% of a photoinitiator, such as from 0.5 to 2.0 wt%. Preferably, a photocurable composition according to the first aspect may comprise, based on the total weight of the photocurable composition: (A) from 20 to 80 wt% of the urethane (meth)acrylate oligomer, such as from 25 to 55 wt% or from 25 to 40 wt%, wherein the urethane (meth)acrylate oligomer has a functionality of greater than 2.5, such as greater than 3.5; (B) from 15 to 70 wt%, such as from 30 to 60 wt%, of the acrylamide monomer, such as N-acrylolyl morpholine or N,N-diethyl acrylamide; (C) from 10 to 40 wt% of the (meth)acrylate monomer, having a functionality of greater than 1.5, such as about 2, and a glass-transition temperature (Tg) of greater than 120 °C, such as greater than 130 °C; and (D) optionally, up to 7 wt% of a photoinitiator, such as from 0.5 to 2.0 wt%. Preferably, a photocurable composition according to the first aspect may comprise, based on the total weight of the photocurable composition: (A) from 25 to 55 wt% of the urethane (meth)acrylate oligomer, such as from 25 to 40 wt%, wherein the urethane (meth)acrylate oligomer has a functionality of greater than 2.5, such as greater than 3.5, and a glass-transition temperature (Tg) of at most 60 °C, such as at most 40 °C or at most 20 °C; (B) from 30 to 60 wt% of the acrylamide monomer, such as N-acrylolyl morpholine and N,N-diethyl acrylamide; (C) from 10 to 40 wt% of the (meth)acrylate monomer, having a functionality of greater than 1.5, such as about 2, and a glass-transition temperature (Tg) of greater than 120 °C, such as greater than 130 °C; and (D) optionally, up to 7 wt% of a photoinitiator, such as from 0.5 to 2.0 wt%. Preferably, a photocurable composition according to the first aspect may comprise, based on the total weight of the photocurable composition: (A) from 20 to 80 wt% of the urethane (meth)acrylate oligomer, such as from 25 to 55 wt% or from 25 to 40 wt%, wherein the urethane (meth)acrylate oligomer has a functionality of greater than 2.5, such as greater than 3.5, and a glass-transition temperature (Ta) of at most 20 °C, such as at most 10 °C or at most 0 °C; (B) from 15 to 70 wt%, such as from 30 to 60 wt%, of the acrylamide monomer, such as N-acrylolyl morpholine or N,N-diethyl acrylamide; (C) from 10 to 40 wt% of the (meth)acrylate monomer, having a functionality of greater than 1.5, such as about 2, and a glass-transition temperature (Tg) of greater than 120 °C, such as greater than 130 °C; and (D) optionally, up to 7 wt% of a photoinitiator, such as from 0.5 to 2.0 wt%. Preferably, a photocurable composition according to the first aspect may comprise, based on the total weight of the photocurable composition: (A) from 20 to 80 wt% of the urethane (meth)acrylate oligomer, such as from 25 to 55 wt% or from 25 to 40 wt%; (B) from 15 to 70 wt%, such as from 30 to 60 wt%, of the acrylamide monomer, such as N-acrylolyl morpholine or N,N-diethyl acrylamide; (C) from 5 to 60 wt%, such as from 10 to 60 wt%, of the (meth)acrylate monomer, having a functionality of greater than 1.5, such as about 2, and a glass-transition temperature (Tg) of greater than 120 °C, such as greater than 130 °C; and (D) optionally, up to 7 wt% of a photoinitiator, such as from 0.5 to 2.0 wt%; such that, when cured, the composition has a tensile strength, as measured by ISO 527-1, of at least 35 MPa (such as at least 40 MPa, or at least 45 MPa, or at least 50 MPa); and / or a tensile modulus, as measured by ISO 527-1, of at least 1300 MPa (such as at least 1500 MPa, or at least 1750 MPa, or at least 2000 MPa, or at least 2250 MPa, or at least 2500 Pa); and / or an elongation at break, as measured by ISO 527-1, of at least 2% (such as at least 4%, or at least 5%, or at least 6%, or at least 8%, or at least 10%, or at least 12%, or at least 15%, or at least 20%); and / or an HDT, as measured by ISO 75 method A flatwise, of at least 40 °C (such as at least 45 °C, or at least 47 °C, or at least 49 °C, or at least 51 °C). Preferably, a photocurable composition according to the first aspect may comprise, based on the total weight of the photocurable composition: (A) from 20 to 80 wt% of the urethane (meth)acrylate oligomer, such as from 25 to 55 wt% or from 25 to 40 wt%; (B) from 15 to 70 wt%, such as from 30 to 60 wt%, of the acrylamide monomer, such as N-acrylolyl morpholine or N,N-diethyl acrylamide; (C) from 5 to 60 wt%, such as from 10 to 60 wt%, of the (meth)acrylate monomer, having a functionality of greater than 1.5, such as about 2, and a glass-transition temperature (Tg) of greater than 120 °C, such as greater than 130 °C; and (D) optionally, up to 7 wt% of a photoinitiator, such as from 0.5 to 2.0 wt%; such that, when cured, the composition has a tensile strength, as measured by ISO 527-1, of at least 35 MPa (such as at least 40 MPa); and / or a tensile modulus, as measured by ISO 527-1, of at least 1300 MPa (such as at least 1500 MPa); and / or an elongation at break, as measured by ISO 527-1, of at least 10% (such as at least 12%, or at least 15%); and / or an HDT, as measured by ISO 75 method A flatwise, of at least 40 °C (such as at least 45 °C, or at least 47 °C, or at least 49 °C). Preferably, a photocurable composition according to the first aspect may comprise, based on the total weight of the photocurable composition: (A) from 20 to 80 wt% of the urethane (meth)acrylate oligomer, such as from 25 to 55 wt% or from 25 to 40 wt%, wherein the urethane (meth)acrylate oligomer has a functionality of greater than 2.5, such as greater than 3.5, and a glass-transition temperature (Tg) of at most 20 °C, such as at most 10 °C or at most 0 °C; (B) from 15 to 70 wt%, such as from 30 to 60 wt%, of the acrylamide monomer, such as N-acrylolyl morpholine or N,N-diethyl acrylamide; (C) from 10 to 40 wt% of the (meth)acrylate monomer, having a functionality of greater than 1.5, such as about 2, and a glass-transition temperature (T0) of greater than 120 °C, such as greater than 130 °C; and (D) optionally, up to 7 wt% of a photoinitiator, such as from 0.5 to 2.0 wt%; such that, when cured, the composition has a tensile strength, as measured by ISO 527-1, of at least 35 MPa (such as at least 40 MPa, or at least 45 MPa, or at least 50 MPa); and / or a tensile modulus, as measured by ISO 527-1, of at least 1300 MPa (such as at least 1500 MPa, or at least 1750 MPa, or at least 2000 MPa, or at least 2250 MPa, or at least 2500 Pa); and / or an elongation at break, as measured by ISO 527-1, of at least 2% (such as at least 4%, or at least 5%, or at least 6%, or at least 8%, or at least 10%, or at least 12%, or at least 15%, or at least 20%); and / or an HDT, as measured by ISO 75 method A flatwise, of at least 40 °C (such as at least 45 °C, or at least 47 °C, or at least 49 °C, or at least 51 °C). Preferably, a photocurable composition according to the first aspect may comprise, based on the total weight of the photocurable composition: (A) from 20 to 80 wt% of the urethane (meth)acrylate oligomer, such as from 25 to 55 wt% or from 25 to 40 wt%, wherein the urethane (meth)acrylate oligomer has a functionality of greater than 2.5, such as greater than 3.5, and a glass-transition temperature (Tg) of at most 20 °C, such as at most 10 °C or at most 0 °C; (B) from 15 to 70 wt%, such as from 30 to 60 wt%, of the acrylamide monomer, such as N-acrylolyl morpholine or N,N-diethyl acrylamide; (C) from 10 to 40 wt% of the (meth)acrylate monomer, having a functionality of greater than 1.5, such as about 2, and a glass-transition temperature (Tg) of greater than 120 °C, such as greater than 130 °C; and (D) optionally, up to 7 wt% of a photoinitiator, such as from 0.5 to 2.0 wt%; such that, when cured, the composition has a tensile strength, as measured by ISO 527-1, of at least 35 MPa (such as at least 40 MPa); and / or a tensile modulus, as measured by ISO 527-1, of at least 1300 MPa (such as at least 1500 MPa); and / or an elongation at break, as measured by ISO 527-1, of at least 10% (such as at least 12%, or at least 15%); and / or an HDT, as measured by ISO 75 method A flatwise, of at least 40 °C (such as at least 45 °C, or at least 47 °C, or at least 49 °C). AAA The features disclosed in the foregoing description, or in the following claims, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organisational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the words “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about”, it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means, for example, +1- 10%. Examples Materials The acrylamide monomer, N-acryloyl morpholine (ACMO), was obtained from Rahn, having a To of 145 °C. The acrylamide monomer, N,N-diethyl acrylamide (DEAA), was obtained from KJ Chemicals, having a Tg of 81 °C. Urethane (meth)acrylate oligomer A is a tetrafunctional urethane acrylate oligomer that is the reaction product of diisocyanate, a polyether polyol and a hydroxyl functional acrylate monomer. Oligomer A has a GPC Mn molecular weight of 2880 g / mol, and a Tg of -6 °C (when homopolymerized and post-cured for 3 hours and measured by DSC [BS EN ISO 11357]). Urethane (meth)acrylate oligomer B is a polyester / polyether based difunctional urethane acrylate oligomer that is commercially available from Sartomer, having the product name CN981. Oligomer B has a GPC Mn molecular weight of 1487 g / mol and a Tg of 35 °C (TDS data). Urethane (meth)acrylate oligomer C is a difunctional urethane acrylate oligomer that is commercially available from Allnex, having the product name Ebecryl 4491. 1-Vinyl-2-pyrrolidinone (NVP) was obtained from BASF. The photoinitiator used was a phosphine oxide obtained from IGM Resins. The difunctional (meth)acrylate monomers used are shown in Table 1 below, in which “MW” is molecular weight and the Tg values are from supplier TDS data. Table 1 - (Meth)acrylate monomers Product Name Chemical Name Supplier MW (g / mol) Tg (°C) SR348C 3-Ethoxylated bisphenol A dimethacrylate Sartomer 496 116 SR348L 2-Ethoxylated bisphenol A dimethacrylate Sartomer 452 162 SR206 Ethylene glycol dimethacrylate Sartomer 198.2 135 SR833S Tricyclodecanedimethanol diacrylate Sartomer 304 185 Ebecryl 130 Tricyclodecanedimethanol diacrylate Allnex 304 160 Ebecryl 600 Bisphenol A diacrylate ester Allnex 1246.5 130 Sample preparation and testing methodology ISO 527-2 Type 5A tensile specimens were produced by printing flat to the build plate on an Asiga MAX UV 3D printer (385 nm wavelength). The layer height was 50 microns. The light intensity was 5.08 mW / cm2 and the exposure time for each layer was 3.429 s. Specimens were then washed in a Form Wash IPA bath for 5 minutes. The IPA was then allowed to evaporate. A Form Cure was used for UV and thermal post-curing of the specimens for 16 minutes at 60 °C with 405 nm light. Halfway through the post-cure, the specimens were turned over to promote homogeneous post-curing. Tensile testing was then undertaken according to ISO 527-1 at 5 mm / min using a video extensometer and a 1 kN load cell on an Instron universal testing machine. The elongation at break is in the increase in gauge length after break divided by the original gauge length. Table 2 - Inventive Example 1 and Reference Examples 1 to 3 Inventive Example 1 Reference Example 1 Reference Example 2 Reference Example 3 Urethane (meth)acrylate oligomer 38.4 wt% Oligomer A 38.4 wt% Oligomer A 47 wt% Oligomer C 38.4 wt% Oligomer A Acrylamide monomer 44.1 wt% ACMO 44.1 wt% ACMO 47 wt% NVP 44.1 wt% NVP Di(meth)acrylate monomer 16.5 wt% SR348L 16.5 wt% SR348C 5 wt% Ebecryl 130 16.5 wt% SR348L Photoinitiator 1.0 wt% 1.0 wt% 1.0 wt% 1.0 wt% Tensile modulus (MPa) 2810 1660 919 1720 Tensile strength (MPa) 63.6 52.3 25.3 51.5 Elongation at break (%) 11.8 20.1 50.5 17.0 In Inventive Example 1 and Reference Examples 1 to 3 (as shown in Table 2 above), the urethane (meth)acrylate oligomer was prepared without added reactive diluent. The material was heated to 60 °C in an oven to reduce its viscosity and allow easier processing. Speedmixer cups and mixer were utilised to prepare the samples. 100 g samples were produced by combining the constituents in a 100 g Speedmixer cup. The mixing was undertaken using a DAC 600 FVZ Speedmixer planetary mixer for 5 minutes at 2000 RPM. Reference Example 1 uses a difunctional (meth)acrylate monomer (SR348C) having a Tg of 116 °C, while Inventive Example 1 uses a difunctional (meth)acrylate monomer (SR348L) having a Tg of 162 °C. The mechanical performance benefit of adding a high Tg (i.e. >120 °C) difunctional (meth)acrylate monomer to a urethane (meth)acrylate oligomer and acrylamide monomer-based 3D printing resin, compared with a <120 °C Tg difunctional (meth)acrylate monomer, is shown by the surprisingly significant improvements observed in tensile modulus and tensile strength, while good flexibility is maintained. It is thought that the fewer ethoxylations (2 versus 3) between the bisphenol A and the methacrylate groups in the SR348C (lower Tg) and SR348L (higher Tg) monomers allow fewer degrees of freedom for the polymer chain to move (i.e. steric hindrance), resulting in a stiffer network that offers more resistance to applied tensile forces. Meanwhile, Reference Example 2 is a reproduction of a prior art composition, which utilises 1-vinyl-2-pyrrolidinone (NVP) as a monomer instead of an acrylamide monomer, as well as difunctional oligomer C instead of tetrafunctional oligomer A. Reference Example 2 demonstrates the use of a Tg >120 °C difunctional (meth)acrylate monomer with a urethane (meth)acrylate oligomer and an N-vinyl monomer. In order to allow a direct comparison with Inventive Example 1 to be made, Reference Example 3 then adjusts the conditions from Reference Example 2, using the same difunctional (meth)acrylate monomer (Tg >120 °C), the same photoinitiator, the same urethane (meth)acrylate oligomer and the same amount of the components as Inventive Example 1, whilst utilising the same monomer (NVP) as Reference Example 2. It can be seen that Inventive Example 1, while retaining good elongation properties, exhibits a greater tensile modulus and tensile strength than both Reference Examples 2 and 3. In particular, Inventive Example 1 differs from Reference Example 3 only by the use of ACMO as the monomer instead of NVP, and the result of this change is improved tensile modulus (2810 MPa versus 1720 MPa) and improved tensile strength (63.6 MPa versus 51.5 MPa) in Inventive Example 1, which is thought to be attributable to the higher Tg of ACMO (145 °C). Moreover, it is believed that good elongation properties are retained through the use of an acrylamide monomer, as the acrylamide group preferentially homopolymerizes (as opposed to copolymerising with the urethane (meth)acrylate oligomer or the difunctional (meth)acrylate monomer), which creates long pol(acrylamide) chains, resulting in good elongation. Table 3 - Inventive Examples 2 to 5 and Reference Example 4 Reference Example 4 Inventive Example 2 Inventive Example 3 Inventive Example 4 Inventive Example 5 Urethane (meth)acrylate oligomer 38.4 wt% Oligomer A 38.4 wt% Oligomer A 38.4 wt% Oligomer A 38.4 wt% Oligomer A 38.4 wt% Oligomer A Acrylamide monomer 44.1 wt% DEAA 44.1 wt% DEAA 44.1 wt% ACMO 44.1 wt% DEAA 44.1 wt% ACMO Di(meth)acrylate monomer 16.5 wt% SR348C 16.5 wt% SR348L 16.5 wt% SR833S 16.5 wt% SR206 16.5 wt% Ebecryl 600 Photoinitiator 1.0 wt% 1.0 wt% 1.0 wt% 1.0 wt% 1.0 wt% Tensile modulus (MPa) 179 831 2130 1340 2382 Tensile strength (MPa) 13.1 25.8 45.7 28.9 52.8 Elongation at break (%) 67.3 34.0 17.8 16.4 35.2 Reference Example 4 and Inventive Examples 2 to 5 (shown in Table 3 above) were prepared in the same manner as the examples in Table 2. The inventive examples in Table 3 show the effects on mechanical performance from varying the difunctional (meth)acrylate monomer (that has a Tg of greater than 120 °C) in the 3D printing resin system. As an alternative to ACMO, DEAA is also used as the acrylamide monomer. It can be seen that the effect of using high Tg (i.e. >120 °C) difunctional (meth)acrylate monomers also occurs with the alternative acrylamide monomer, with the advantageous effects on tensile modulus and tensile strength particularly pronounced for the examples using DEAA. Table 4 - Inventive Examples 6 to 8 Inventive Example 6 Inventive Example 7 Inventive Example 8 Urethane (meth)acrylate oligomer 30.5 wt% Oligomer A 40.5 wt% Oligomer A 50.5 wt% Oligomer A Acrylamide monomer 52.0 wt% ACMO 42.0 wt% ACMO 32.0 wt% ACMO Di(meth)acrylate monomer 16.5 wt% SR348C 16.5 wt% SR348C 16.5 wt% SR348C Photoinitiator 1.0 wt% 1.0 wt% 1.0 wt% Tensile modulus (MPa) 2010 1780 970 Tensile strength (MPa) 57.3 49.5 31.9 Elongation at break (%) 10.7 17.9 25.5 Table 4 above shows Inventive Examples 6 to 8, which were prepared in the same manner as the examples in Table 2. The examples in Table 4 show that formulations can be made and printed with a range of oligomer contents. Increasing the oligomer content results in reductions in tensile modulus and tensile strength, 10 which makes the addition of the high Tg (i.e. >120 °C) difunctional (meth)acrylate monomer more effective. Table 5 - Inventive Examples 9 to 11 Inventive Example 9 Inventive Example 10 Inventive Example 11 Urethane (meth)acrylate oligomer 42.9 wt% Oligomer B 42.9 wt% Oligomer B 42.9 wt% Oligomer B Acrylamide monomer 39.6 wt% ACMO 39.6 wt% ACMO 39.6 wt% ACMO Di(meth)acrylate monomer 16.5 wt% SR348L 16.5 wt% SR833S 16.5 wt% Ebecryl 600 Photoinitiator 1.0 wt% 1.0 wt% 1.0 wt% Tensile modulus (MPa) 3021 2846 2681 Tensile strength (MPa) 74.3 74.9 69.7 Elongation at break (%) 7.2 6.8 10.2 15 Table 5 above shows Inventive Examples 9 to 11, which were prepared in the same manner as the examples in Table 2. Inventive Examples 9 to 11 corroborate the effects observed for Inventive Example 1, but using oligomer B, which has a higher Tg (35 °C) than oligomer A (-6 °C) and a lower functionality (2) than oligomer A (4). As such, the beneficial effects (namely high tensile modulus and tensile strength, whilst retaining good elongation) resulting from adding a high Tg (i.e. >120 °C) difunctional (meth)acrylate monomer are also observed for other oligomers. The effects are not limited to oligomers with low Ta and high functionality. In particular, the examples show effectiveness with oligomers having a Tg of less than or equal to 40 °C and a functionality of greater than 1.5. Nevertheless, by comparison of the mechanical results when using Oligomer B with those using Oligomer A, it is observed that an especially beneficial combination of properties is obtained when using the tetrafunctional, low Tg Oligomer A. For instance, the composition of Inventive Example 5 is very similar to that of Inventive Example 11, except for the difference in oligomer. Inventive Example 5 (using tetrafunctional Oligomer A) has significantly superior elongation at break compared with Inventive Example 11 (using difunctional Oligomer B). This is surprising, because it might be expected that the use of a higher functionality oligomer would lead to increased rigidity and reduced elongation. However, without wishing to be bound by theory, it is believed that the use of a high functionality oligomer that specifically has a low Tg (-6 °C in the case of tetrafunctional Oligomer A), in a composition along with the acrylamide monomer and the high Tg (meth)acrylate monomer as claimed, results in a particular cured structure / network which surprisingly yields an improved balance of properties. Specifically, the balance of properties relating to toughness (e.g. tensile modulus and / or tensile strength) and flexibility (e.g. elongation at break) is particularly beneficial compared with known compositions, which do not comprise this particularly favourable combination of components including an oligomer that has a low Tg (e.g. at most 20 °C, or at most 10 °C, or at most 0 °C) and a high functionality (e.g. greater than 2.5, or greater than 3.5). Indeed, as aforementioned, it is known in the art to be difficult to obtain resins that are suitable for additive manufacturing applications and that have a good balance of these properties. This means that cured products of the composition of the invention are especially useful in applications where both good toughness and good flexibility are desirable. Meanwhile, it has also been found that the viscosity of such compositions of the invention remains acceptably and advantageously low for ease of use in straightforward additive manufacturing processes (e.g. low energy intensive processes). Table 6 - Inventive Examples 12 and 13 Inventive Example 12 Inventive Example 13 Urethane (meth)acrylate oligomer 38.4 wt% Oligomer A 28.5 wt% Oligomer A Acrylamide monomer 54.0 wt% ACMO 34.2 wt% ACMO Di(meth)acrylate monomer 6.6 wt% SR348L 36.3 wt% SR348L Photoinitiator 1.0 wt% 1.0 wt% Tensile modulus (MPa) 1590 2300 Tensile strength (MPa) 47.1 71.7 Elongation at break (%) 33.0 6.0 Table 6 above shows Inventive Examples 12 and 13, which were prepared in the same manner as the examples in Table 2. Inventive Examples 12 and 13 outline the effects on mechanical performance of varying particularly the (meth)acrylate monomer content. The advantageous effects on the invention were observed across the range of weight percentages tested. In particular, in relation to the balance of properties, it can be deduced that elongation can be prioritised when lower concentrations of the (meth)acrylate monomer (SR348L) are used, while tensile strength and tensile modulus can be prioritised with higher concentrations of the (meth)acrylate monomer (SR348L). References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein. WO 2020 / 073149 A1 US 10239255 B2 EP 2436510A1 WO 2020 / 064523 A1

Claims

1. A photocurable composition comprising 20 to 80 wt% of a urethane (meth)acrylate oligomer, 15 to70 wt% of an acrylamide monomer, and 5 to 60 wt% of a (meth)acrylate monomer, based on the total weight of the photocurable composition; wherein the (meth)acrylate monomer has a functionality of greater than 1.5 and a glass-transition temperature (Tg) of greater than 120 °C.

2. The photocurable composition of claim 1, comprising 10 to 60 wt% of the (meth)acrylate monomer.

3. The photocurable composition of either claim 1 or claim 2, comprising 10 to 40 wt% of the (meth)acrylate monomer.

4. The photocurable composition of any one of claims 1 to 3, comprising 30 to 60 wt% of the acrylamide monomer.

5. The photocurable composition of any one of claims 1 to 4, comprising 25 to 55 wt% of the urethane (meth)acrylate oligomer.

6. The photocurable composition of any one of claims 1 to 5, comprising 25 to 40 wt% of the urethane (meth)acrylate oligomer.

7. The photocurable composition of any one of claims 1 to 6, wherein the urethane (meth)acrylate oligomer has a functionality of greater than 2.5 or greater than 3.5.

8. The photocurable composition of any one of claims 1 to 7, wherein the urethane (meth)acrylate oligomer has a glass-transition temperature (Tg) of at most 40 °C, or at most 20 °C, or at most 10 °C, or at most 0 °C.

9. The photocurable composition of any one of claims 1 to 8, wherein the acrylamide monomer is selected from N-acrylolyl morpholine and N,N-diethyl acrylamide.

10. The photocurable composition of claim 9, wherein the acrylamide monomer is N-acryloyl morpholine.

11. The photocurable composition of any one of claims 1 to 10, wherein the (meth)acrylate monomer has a functionality of about 2.

12. The photocurable composition of any one of claims 1 to 11, wherein the (meth)acrylate monomer is selected from 2-ethoxylated bisphenol A dimethacrylate, tricyclodecanedimethanol diacrylate, ethylene glycol dimethacrylate and bisphenol A diacrylate ester.

13. The photocurable composition of any one of claims 1 to 12, wherein the (meth)acrylate monomer has a glass-transition temperature (Tg) of at least 130 °C.

14. The photocurable composition of any one of claims 1 to 13, further comprising one or more of a pigment and an optical brightener.

15. The photocurable composition of claim 14, wherein the pigment and / or the optical brightener is present in up to 2 wt% of the composition.

16. The photocurable composition of any one of claims 1 to 15, further comprising a defoamer and / or a deaerating agent.

17. The photocurable composition of claim 16, wherein the defoamer and / or the deaerating agent is present in upto 1 wt% of the composition.

18. The photocurable composition of any one of claims 1 to 17, further comprising a photoinitiator.

19. The photocurable composition of claim 18, wherein the photocurable composition comprises the photoinitiator in an amount selected from up to 7 wt% and from 0.5 to 2.0 wt%, based on the total weight of the composition.

20. The photocurable composition of either claim 18 or claim 19, wherein the photoinitiator is a phosphine oxide.

21. A kit comprising the photocurable composition of any one of claims 1 to 17 and a photoinitiator, wherein the photoinitiator is optionally a phosphine oxide.

22. Use of the photocurable composition of any one of claims 1 to 20 or the kit of claim 21 in an additive manufacturing process, wherein the additive manufacturing process is optionally a 3D printing process.

23. The use according to claim 22, wherein the additive manufacturing process comprises the use of a stereolithography (SLA) 3D printer, a digital light processing (DLP) 3D printer, a liquid crystal display (LCD) 3D printer, a volumetric 3D printer, or a multiphoton lithography 3D printer.

24. A cured product of the photocurable composition according to any one of claims 1 to 20.

25. An additive manufactured article comprising the cured product according to claim 24.