Polymeric materials

Abstract

A formulation for treatment of a polymeric material, for example recycled polyolefin, to enable relatively high quantities of recycled polyolefin, for example PCR, to be combined with virgin resin to achieve excellent mechanical properties comprises a radical generator and a cross-linking agent. The radical generator may be a peroxide, a hindered amine, a nitroxide or a metal radical source and the cross-linking agent is suitably a bi- or multi-functional chemical compound which is capable of reacting with and cross-linking polyolefin.

Description

[0001]POLYMERIC MATERIALS P45423 This invention relates to polymeric materials and particularly, although not exclusively, relates to polyolefins. Preferred embodiments relate to polymeric materials comprising polyethylene and / or polypropylene; and / or such polymeric materials which include relatively high levels of recycled polyolefin, for example post-consumer resin (PCR) and / or post-industrial resin (PIR). It is becoming increasingly important to maximize the amounts of recycled polyolefin used in producing new molded articles. For example, PCR (which may itself comprise a mixture of polyolefin resins and contamination from other polymers and organic or inorganic species) may be mixed with virgin resin to produce a mass of polymeric material which is formed into pellets or granules by, for example, melt- blending. The pellets or granules may then be extruded or injection molded to produce new articles. However, when quantities of recycled resin (e.g. PCR) increase above a nominal level (e.g. typically 25 wt%), mechanical properties of polymeric materials tend to deteriorate with consequent challenges in processing and articles produced may have relatively poor mechanical properties. For example, inclusion of recycled resin may disadvantageously impact key properties of the polymeric material, such as melt flow, tensile strength, tensile modulus, tensile elongation, flexural strength, flexural modulus, heat deflection temperature (HDT), Vicat softening temperature and / or resistance to environmental stress cracking (ESCR). If PCR used has many heat histories, it could form gels or molecular weight may be lowered during processing due to chain scission. The net impact is that end use applications of polymeric materials incorporating PCR may lead to molded articles having product defects and generate excessive scrap. The use of recycled resin as described may lead to particular challenges in ESCR with a risk that, if too high a level of recycled resin is used, molded articles produced may be susceptible to failure under constant stress below the resin’s yield point. Packaged consumer goods, often produced by extrusion blow molding, are increasingly required to be produced with increasing levels of PCR. However, the greater the level of PCR, the greater the risk of ESCR failure. A number of commercial products are available, directed at solving some of the problems described herein. For example, CaCO3 may be added. However, this provides limited improvement in key mechanical properties and, disadvantageously, very high additional levels are required. Compatibilisers are known but tend to require high additional levels and tend to reduce the flexural strength of the article, in turn limiting its applicability. Special resin grades are commercially available; however, their utility reduces with increasing recycle content. It is an object of the preferred embodiments of the present invention to address the above-described problems. It is an object of preferred embodiments of the present invention to enable higher quantities of recycled polyolefin, for example PCR, to be usable without significantly negatively impacting mechanical properties (eg impact strength or flexural properties) of the polyolefin. It is an object of preferred embodiments of the present invention to enable higher quantities of recycled polyolefin, for example PCR, to be usable without significantly negatively impacting ESCR. According to a first aspect of the invention, there is provided a formulation for treatment of a polymeric material, the formulation comprising: (i) a radical generator; and (ii) a cross-linking agent. Advantageously, a formulation as described herein can be used to treat recycled polyolefin and enable relatively high quantities of recycled polyolefin, for example PCR, to be combined with virgin resin to achieve excellent mechanical properties, including excellent resistance to ESCR. The formulation preferably comprises said radical generator and said cross-linking agent intimately associated with one another, for example by being arranged together, for example dispersed together, in a carrier as hereindescribed; or, alternatively, said formulation may comprise a pack or collocation which comprise separate elements, one comprising said radical generator and another comprising said cross-linking agent. Said radical generator may be organic or an inorganic complex. For example, it may be an inorganic salt or a transition metal complex. Said radical generator may be a thermal radical initiator, a thermal cationic initiator, a photo radical initiator or a photo cationic initiator. Said radical generator is preferably a thermal radical initiator. Said radical generator may be a substituted or unsubstituted phthalimide, peroxyketal, alkyl or aryl peroxide, organic or inorganic radical generator, a combination of one or more radical generators, a polymeric radical initiator, an entity which generates radicals by a macroparticle such as polystyrene or polytetrafluoroethylene. Said radical generator may be a peroxide, a hindered amine, a nitroxide or a metal radical source. Preferably, said radical generator is stable and storable at room temperature (25 °C). Preferably, said radical generator is processible at 220 °C, for example at typical HDPE processing temperature. Preferably, said radical generator has a self-accelerating decomposition temperature (SADT) of more than 80 °C. This may be determined on the radical generator per se as described in Assessment 2. Preferably, said radical generator has a t ½ (half-life period) in the range 100-200°C, more preferably in the range 100-150°C. Preferably, said radical generator has the capability to initiate radical polymerization. Said radical generator, for example peroxide, preferably has a decomposition temperature, suitably measured by differential scanning calorimetry (DSC), of at least 60°C, for example in the range 60°C to 300°C. Said radical generator is preferably a solid peroxide, preferably with a relatively high half life period and SADT as described. Usually peroxides with relatively high molecular weight and with bulky groups are thermally stable and, as such, are preferred. During melt-processing, for example reactive extrusion, preferred peroxides take part in a reaction by the radical generation. Said radical generator is preferably a peroxide. Said radical generator may be a peroxide with alkyl or aryl groups as substituents; or a peroxide with an active functional group selected from: acrylate, ester, ether, diimide and cyclic or acyclic hydrocarbons. Said radical generator may be stabilised with an inorganic species such as silica, kaolin or calcium carbonate. Said radical generator may be selected from 1,1-Di(tert-amylperoxy)cyclohexane, Di(4- tert.butylcyclohexyl)peroxydicarbonate, Dibenzoyl peroxide, 2,2-Di(tert.butylperoxy)butane, Dicetylperoxydicarbonate, 1,1-Di(tert.butylperoxy)cyclohexane, Acetylacetone peroxide, tert.Butylperoxybenzoate, Methyl ethyl ketone peroxide / Cumyl hydroperoxide, Cumyl hydroperoxide, Methyl isobutyl ketone peroxide, Methyl ethyl ketone peroxide, tert.Amylperoxy-2-ethylhexylcarbonate, 2,5-Dimethyl-2,5-di(tert.butylperoxy)hexane, tert.Butylperoxy-2-ethylhexylcarbonate, Di(2,4- dichlorobenzoyl)peroxide, Dicumylperoxide, 2,5-Dimethyl-2,5-di(tert.butylperoxy)hexane, Di(tert.butyl)peroxide, Di(tert.butyl)peroxide, 2,5-Dimethyl-2,5-di(tert.butylperoxy)hexyne-3, Di(2- ethylhexyl)peroxydicarbonate, Di(3,5,5-trimethylhexanoyl)peroxide, Dilauroyl peroxide, Dimyristylperoxydicarbonate, tert.Butylperoxy-2-ethylhexylcarbonate, tert.Butylperoxy-2- ethylhexanoate, tert.Amylperoxy-2-ethylhexanoate, tert.Butylperoxyneodecanoate, tert.Butylperoxy- 3,5,5-trimethylhexanoate, 1,1-Di(tert.butylperoxy)-3,3,5-trimethylcyclohexane, 1,1- Di(tert.butylperoxy)cyclohexane, Di(4-tert.butylcyclohexyl)peroxydicarbonate, Dimyristylperoxydicarbonate, Acetylacetone peroxide, Methyl ethyl ketone peroxide, 1,1- Di(tert.butylperoxy)-3,3,5-trimethylcyclohexane, Acetylacetone peroxide, Methyl ethyl ketone peroxide, Acetylacetone peroxide, Methyl ethyl ketone peroxide / Cumyl hydroperoxide, Methyl ethyl ketone peroxide / Acetylacetone peroxide, tert.Butylperoxybenzoate, Acetylacetone peroxide, Methyl isobutyl ketone peroxide, Methyl ethyl ketone peroxide, Acetylacetone peroxide, tert.Butylperoxybenzoate, Acetylacetone peroxide, Peroxide Dye Concentrates, Disuccinoylperoxide, tert.Amyl hydroperoxide, tert.Amylperoxy-2-ethylhexanoate, Tert. Amylperoxy-2-ethylhexylcarbonate, tert.Amylperoxypivalate, tert.Butyl hydroperoxide, tert.Butylperoxyacetate, tert.Butylperoxy-2-ethylhexanoate, tert.Butylperoxy-2- ethylhexylcarbonate, tert.Butyl-peroxy-isobutyrate, tert.Butylperoxy-3,5,5-trimethylhexanoate, tert.Butylperoxyneodecanoate, tert.Butylperoxypivalate, 1,1-Di(tert.butylperoxy)-3,3,5- trimethylcyclohexane, 1,3-1,4-bis(tert-butylperoxyisopropyl) benzene, Tert-butylcumylperoxide, n-butyl- 4,4-di(tert-butylperoxy)valerate, t-butyl peroctoate, t-butyl peroxypivalate, di-tertiary butyl peroxide, alpha-cumyl peroxyneodecanoate, 3-hydroxy-1,1-dimethyl butyl-peroxyneodecanoate, Di(4- methylbenzoyl) peroxide , 3,3,5,7,7-Pentamethyl-1,2,4-trioxepane and 1,2,4,5,7,8-Hexoxonane, 3,6,9- trimethyl-3,6,9-tris(Et and Pr) derivatives (reaction mass of 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8- hexoxonane and 3,6-diethly-3,6,9-trimethyl-9-n-propyl-1,2,4,5,7,8-hexoxonane and 3-ethyl-3,6,9- trimethyl-6,9-di-n-propyl-1,2,4,5,7,8-hexoxonane and 3,6,9-trimethyl-3,6,9-tri-n-propyl-1,2,4,5,7,8- hexoxonane sold under brand name Trigonox® 501-CS40). Said radical generator may be a di-(optionally-substituted)alkyl peroxide. Where it is optionally- substituted, it may be substituted by a hydrocarbon moiety (eg containing hydrogen and carbon atoms only), for example a phenyl moiety. Said radical generator may be a dialkyl peroxide. Said radical generator may be selected from dicumyl peroxide, 1,1-Di(tert.butylperoxy)cyclohexane, tert.Butylperoxybenzoate, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane and 1-(anthraquinon-2-yl)ethyl imidazolecarboxylate. Said radical generator may include one or more peroxide (O-O) groups in combination only with moieties which include carbon and hydrogen atoms only. Said radical generator may include peroxide groups in combination with moieties of formula -CR50R51R52wherein R50, R51and R52independently are selected from phenyl and methyl groups and the free carbon atom is preferably directly bonded to an oxygen atom of a peroxide group. One preferred group of radical generators have a t ½ (half-life period) in the range 60-200°C. Preferred examples include 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexyne-3 3,6,9-Triethyl-3,6,9-trimethyl-1,4,7- triperoxonane, Di(tert-butyl)peroxide, 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane, Di(tert-butylperoxy- isopropyl)benzene, tert-Butyl cumyl peroxide, Di-(tert-amyl)-peroxide, Dicumyl peroxide,Butyl 4,4- di(tert-butylperoxy)valerate, tert-Butylperoxybenzoate, 2,2-Di(tert-butylperoxy)butane, tert-Amyl peroxy- benzoate, tert-Butylperoxy-acetate , tert-Butylperoxy-(2-ethylhexyl)carbonate, tert-Butylperoxy isopropyl carbonate, tert-Butyl peroxy-3,5,5-trimethyl-hexanoate, 1,1-Di(tert-butylperoxy)cyclohexane, tert-Amyl peroxyacetate,tert-Amylperoxy-(2-ethylhexyl)carbonate,1,1-Di(tert-butylperoxy)-3,5,5- trimethylcyclohexane,1,1-Di(tert-amylperoxy)cyclohexane, tert-Butyl-monoperoxy-maleate,1,1’- Azodi(hexahydrobenzonitrile), Di(4-methylbenzoyl) peroxide, 3,3,5,7,7-Pentamethyl-1,2,4-trioxepane and 1,2,4,5,7,8-Hexoxonane, 3,6,9-trimethyl-3,6,9-tris(Et and Pr) derivatives (reaction mass of 3,6,9- triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexoxonane and 3,6-diethly-3,6,9-trimethyl-9-n-propyl-1,2,4,5,7,8- hexoxonane and 3-ethyl-3,6,9-trimethyl-6,9-di-n-propyl-1,2,4,5,7,8-hexoxonane and 3,6,9-trimethyl- 3,6,9-tri-n-propyl-1,2,4,5,7,8-hexoxonane sold under brand name Trigonox® 501-CS40). Said radical generator may be selected from 2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane and dicumyl peroxide, with the latter being especially preferred.. Said cross-linking agent is subtly arranged to cross-link polyolefin, for example recycled polyolefin when said formulation is contacted and / or processed with polyolefin, for example as described in the fourth aspect. Said cross-linking agent is suitably a bi- or multi-functional chemical compound which is capable of reacting with and cross-linking polyolefin. Preferably, said cross-linking agent includes two or more, preferably at least three, C=C double bonds each of which may, independently, be internal or terminal. Said cross-linking agent may include 2 to 6 C=C double bonds, each of which is preferably terminal. Said cross-linking agent may be a monomer or polymer. Preferably, said cross-linking agent is a monomer. It preferably does not include any repeat unit. Said, cross-linking agent may have a molecular weight of at least 80, preferably at least 140, more prefatory at least 200Da. The molecular weight may be less than 1000Da. Said cross-linking agent preferably does not include any carboxylic acid groups. It preferably does not include any amide groups. It preferably does not include any aldehyde groups. Said cross-linking agent may have a melting point of greater than -100oC; and said melting point may be less than 100oC. Said cross-linking agent may have a boiling point of greater than 80oC; and said boiling point may be less than 700oC. For the avoidance of doubt, the aforementioned boiling points refer to the cross-linking agent in isolation. In some embodiments, a liquid carrier, as hereinafter described, may have the effect of reducing the vapor pressure of the cross-linking agent in the formulation. Said C=C double bonds may be a part of a vinyl moiety, for example as in divinyl benzene; part of an allyl moiety as in triallyl cyanaurate; or part of an acrylate group. Said C=C double bonds may be components of poly vinyl oligomers, for example polybutadiene. In a preferred embodiment, said cross-linking agent includes three or four C=C double bonds, each suitably being a component of a respective moiety pendent from a central fragment. Each of said three or four C=C double bonds is preferably terminal. Preferably, said cross-linking agent includes a fragment (A), wherein at least two, preferably at least three (and preferably no more than six, or no more than five, or no more than four) moieties of structure are bonded to fragment (A) via respective starred (*) carbon atoms. Fragment (A) may include a cyclic, aromatic or heteroaromatic moiety, for example derived from a bisphenol or a cyanuric acid. Fragment (A) may consist of atoms selected only from carbon, hydrogen and oxygen atoms. In some embodiments, it may not include any aromatic or heteroaromatic moiety. In some embodiments, it may not include any alkenyl or alkynyl moiety. In some embodiments, it may not include any cyclic moiety. Fragment (A) preferably includes a moiety Fragment (A) preferably includes at least two, preferably at least three (and preferably not more than six, or not more than five or not more than four) moieties of formula (II). One or each moiety of formula (II) may be bonded to a moiety of formula (I) suitably to define a moiety . Said cross-linking agent preferably includes at least two, preferably at least three (and, preferably, no more than six or no more than five or no more than four) moieties of formula (III). Thus, said cross-linking agent preferably includes at least two, preferably at least three (and, preferably, no more than six or no more than five or no more than four) acrylate moieties. Fragment (A) preferably includes a fragment (B) to which each moiety of formula (II) is bonded. Fragment (B) may consist of atoms selected only from carbon, hydrogen and oxygen atoms. It preferably does not include any aromatic or heteroaromatic moiety. It preferably does not include any alkenyl or alkynyl moiety. It preferably does not include any cyclic moiety. Fragment (B) is preferably saturated. It preferably includes carbon and hydrogen atoms only. Fragment (B) preferably has a molecular weight of less than 150, preferably less than 100, more preferably less than 95. The molecular weight may be at least 12, preferably at least 50. Fragment (B) may include at least 2, suitably at least 3, preferably at least 4, more preferably at least 5 carbon atoms. It may include less than 10, preferably 6 or fewer carbon atoms. Fragment (B) preferably includes a tetra-substituted carbon atom. Preferably, said cross-linking agent of said preferred embodiment includes a plurality, suitably, 2 to 10, preferably 2 to 8, more preferably 3 to 6, especially 3-4 acrylate moieties, wherein, suitably, the alkenyl moieties of the acrylate moieties are each terminal groups of said cross-linking agent. Preferably, said cross-linking agent is a tri– or tetra-acrylate. Preferably, said cross-linking agent of said preferred embodiment is a monomer. It preferably does not include any repeat unit. In said preferred embodiment, said cross-linking agent preferably consists of atoms selected only from carbon, hydrogen and oxygen atoms. It preferably does not include any aromatic or heteroaromatic moiety. It preferably does not include any alkynyl moiety. It preferably does not include any cyclic moiety. Said cross-linking agent preferably does not include any carboxylic acid groups. It preferably does not include any anhydride groups. It preferably does not include any amide groups. It preferably does not include any aldehyde groups. Said cross-linking agent may have a melting point of greater than -100oC; and said melting point may be less than 100oC. Said cross-linking agent may have a boiling point of greater than 80oC; and said boiling point may be less than 700oC. For the avoidance of doubt, the aforementioned boiling points refer to the cross-linking agent in isolation. In some embodiments, a liquid carrier, as hereinafter described, may have the effect of reducing the vapor pressure of the cross-linking agent in the formulation. When said cross-linking agent includes at least two C=C double bonds, it may be selected from Triallyl isocyanurate, Trimethylolpropane triacrylate, Pentaerythritol tetraacrylate, 1,6-Hexanediol diacrylate, Neopentyl glycol diacrylate, Di(trimethylolpropane) tetraacrylate, 1,6-Hexanediol dimethacrylate, Divinyl benzene, Bisphenol A glycerolate diacrylate, Triallyl cyanurate, Triallyl citrate , Triallyl trimellitate, Triallyl orthoformate, Triallyl aconitate, Trimethylolpropane trimethylacrylate, Ethoxylated bisphenol A dimethacrylate, Dipentaerythritol pentaacrylate, propoxylated glyceryl triacrylate, benzyl methacrylate, 2-ethylhexyl methacrylate, Ddipentaerythritol hexaacrylate, ethylene glycol Dimethacrylate, Methacrylic anhydride, Methacrylate anhydride, Neopentyl glycol dimethacrylate and Di(ethylene glycol) divinyl ether. In some embodiments, said cross-linking agent may be an oxaspiro compound, suitably including at least two moieties of formula (I). Such a compound may be a tetraoxaspiro undecane compound, for example incorporating at least two divinyl moieties. Said compound may be 3,9-Divinyl- 2,4,8,10-tetraoxaspiro[5.5]undecane. Said formulation may include 0.0001 to 50.0 wt% of said radical generator. Said formulation may include 1 to 10wt%, preferably 1 to 8wt%, more preferably 2 to 7wt% of said radical generator. The sum of the wt% of all radical generators in the formulation may be in the range 0.0001 to 50.0wt%. Said sum may be in the range 1 to 10wt%, preferably 1 to 8wt%, more preferably 2 to 7wt%. Said formulation may include 0.0001 to 99.0 wt% of said cross-linking agent. Said formulation may include 1 to 50wt%, preferably 5 to 30wt%, more preferably 10 to 25wt% of said cross-linking agent. The sum of the wt% of all cross-linking agents in the formulation may be in the range 0.0001 to 99.0 wt%. Said sum may be in the range 1 to 50wt%, preferably 5 to 30wt%, more preferably 10 to 25wt%. In said formulation, a ratio defined as the wt% of said radical generator divided by the wt% of said cross- linking agent may be in the range 1:1 to 1:100, for example in the range 1:1 to 1:10. In said formulation, a ratio defined as the sum of the wt% of all radical generators divided by the sum of the wt% of all cross-linking agents may be in the range 1:1 to 1:100, for example in the range 1:1 to 1:10. In an embodiment (A1), said formulation may not include a carrier (eg a component which is inert and / or is arranged to dissolve or disperse the radical generator and / or cross-linking agent but otherwise is not intended to react or form reactive species when the formulation is used to treat a polyolefin) separate from said radical generator and said cross-linking agent. In this case, the sum of the wt% of all radical generators and all cross-linking agents in said formulation may be at least 90wt%, for example at least 94wt%, at least 98wt% or at least 99wt%. In embodiment (A1), said cross-linking agent may be selected to act both as a cross-linking agent and a carrier for the radical generator. In this case, said cross-linking agent is preferably a liquid and said radical generator may be a liquid which is miscible with the cross-linking agent or may be a solid which is finely dispersed in the cross-linking agent. In embodiment (A1), said formulation may include 1 to 50 wt% of said radical generator and 50 to 99 wt% of said cross-linking agent. Preferably, said formulation include 10 to 50 wt% of said radical generator and 50 to 90 wt% of said cross-linking agent. In said embodiment (A1), said formulation may include 1 to 50 wt% in total of radical generators and 50 to 99 wt% in total of cross-linking agents. Preferably, said formulation include 10 to 50 wt% in total of radical generators and 50 to 90 wt% in total of cross-linking agents. In said embodiment (A1), a ratio defined as the wt% of said radical generator divided by the wt% of said cross-linking agent may be in the range 1:1 to 1:100, for example in the range 1:1 to 1:10. In said embodiment (A1), a ratio defined as the sum of the wt% of all radical generators divided by the sum of the wt% of all cross-linking agents may be in the range 1:1 to 1:100, for example in the range 1:1 to 1:10. Said formulation of the first aspect may be a liquid formulation or a solid formulation, for example a solid masterbatch. Said formulation may comprise: (i) said radical generator; (ii) said cross-linking agent; and (iii) a carrier. The radical generator and cross-linking agent are preferably dispersed, preferably substantially homogeneously, in the carrier. The radical generator and cross-linking agent are preferably compatible with the carrier. Preferably, unless otherwise stated herein, the reference to “liquid” and “solid” refer to a state at 25°C and standard pressure (101325Pa). When said formulation is a liquid, the carrier is preferably a liquid. Said carrier may have a boiling point at 760mmHg in accordance with ASTM D1078 in the range 150 to 1150°C, preferably in the range 275°C to 1000°C or in the range 275°C to 500°C. Said carrier may have a viscosity measured using a Brookfield viscometer, spindle 2, 20rpm at 20°C in the range 50-3500cP, more preferably in the range 200-1800cP. Said carrier may comprise a hydrocarbon-containing liquid. Said carrier may comprise one or more liquids, for example, one or more hydrocarbon-containing liquids. Said carrier may comprise a low molecular weight wax. Said carrier may comprise a triglyceride. Said carrier may comprise liquid rubber. Said carrier may comprise mineral oil. When said formulation is a liquid, said radical generator is preferably fully dissolved in the carrier at the concentration used or is itself a liquid. When said formulation is a liquid, said cross-linking agent is preferably fully dissolved in the carrier at the concentration used or is a liquid. Examples of liquid rubbers include polyisobutylene of low MW (eg 1000 – 2500) and / or having viscosity in the range 200 – 6000 cSt; polybutadiene (MW 5000 – 10000); and ethylene-alpha-olefin co-polymer (MW 2700 – 16000). In general terms, said carrier may have a viscosity in the range 400- 50000 cps. When said carrier comprises one or more liquids, it may comprise one or more of the carriers referred to. For example, said carrier may comprise a mixture of mineral oil (eg 1 to 30wt%) and polyisobutylene and / or liquid rubber (eg 70 to 99wt%). When said formulation is a solid, said carrier may comprise a polyolefin and / or a thermoplastic elastomer (TPE). Said carrier may comprise a carrier typically used in solid masterbatches. Said carrier may comprise a polyolefin, for example polyethylene (eg HDPE or LDPE) or polypropylene, a styrenic block polymer (eg styrene-ethylene-butylene-styrene (SEBS) or styrene-butadiene copolymer (SBC), ethylene-vinyl acetate (EVA) or ethylene Propylene Rubber (EPR). Said formulation which includes said carrier may include 1 to 10wt%, preferably 1 to 8wt%, more preferably 2 to 7wt% of said radical generator. The sum of the wt% of all radical generators in the formulation may be in the range 1 to 10wt%, preferably 1 to 8wt%, more preferably 2 to 7wt%. Said formulation which includes said carrier may include 5 to 30wt%, preferably 10 to 25wt% of said cross-linking agent. The sum of the wt% of all cross-linking agents in the formulation may be in the range 5 to 30wt%, more preferably 10 to 25wt%. In said formulation which includes said carrier, a ratio defined as the wt% of said radical generator divided by the wt% of said cross-linking agent may be in the range 1:1 to 1:100, for example in the range 1:1 to 1:10. In said formulation which includes said carrier, a ratio defined as the sum of the wt% of all radical generators divided by the sum of the wt% of all cross-linking agents may be in the range 1:1 to 1:100, for example in the range 1:1 to 1:10. Said formulation may include 50 to 94 wt%, preferably 70 to 90wt% of said carrier. The sum of the wt% of all carriers in said formulation may be in the range 50 to 94 wt%, preferably 70 to 90wt%. In said formulation, a ratio defined as the wt% of all carriers divided by the sum of the wt% of all radical generators may be in the range 1 to 95 or 2 to 90. In said formulation, a ratio defined as the wt% of all carriers divided by the sum of the wt% of all cross- linking agents may be in the range 1 to 95 or 2 to 90. Said formulation may include a radical mediator / polymerization inhibitor which is suitably able to accept or donate a radical and prevent pre-reaction of monomers, for example the cross-linking agent. Said radical mediator / polymerization inhibitor may include sterically hindered hydroxy groups. Said radical mediator / polymerization inhibitor may be selected from a hydroquinone, for example monomethyl ether hydroquinone, TEMPO derivatives, lipophilic organic compounds, for example butylated hydroxytoluene (BHT), Copper(II) Dibutyldithiocarbamate, cobalt 2-ethylhexanoate and phenothiazine. Preferably, said radical mediator / polymerization inhibitor is selected from BHT, hydroquinone and TEMPO. Said formulation may include 0 to 5wt%, for example 0.001 to 3 wt%, of said radical mediator / polymerization inhibitor. Said formulation may include a colorant which may be a pigment. Said formulation may include 0 to 50wt%, for example 0 to 10wt%, of colourant, in total. Colorant may be included to counter any discoloration in the polymeric material to which the formulation may be added as described herein. Said formulation may include an antioxidant / heat stabilizer. Said formulation may include an antioxidant which may be a phenolic, phosphite, thio-ether or organic sulfide. Said formulation may include an antioxidant / free radical scavenger / anti-crosslinking agent selected from: glutathione, lipoic acid, vitamins such as ascorbic acid (vitamin C), vitamin B, vitamin D, vitamin-E, tocopherols (synthetic or natural, alpha-, gamma-, delta-), acetate vitamin esters, water soluble tocopherol derivatives, tocotrienols, water soluble tocotrienol derivatives; melatonin, carotenoids including carotenes, lutein, pycnogenol, glycosides, trehalose, polyphenols and flavonoids, quercetin, lycopene, lutein, selenium, nitric oxide, curcuminoids, 2-hydroxytetronic acid; cannabinoids, synthetic antioxidants such as tertiary butyl hydroquinone, 6-amino-3-pyrodinoles, butylated hydroxyanisole, butylated hydroxytoluene, ethoxyquin, tannins, propyl gallate, other gallates, Aquanox™ family; Irganox® and Irganox® B families including Irganox® 1010, Irganox® 1076, Irganox® 1330, Irganox® 1035; Irgafos® family; phenolic compounds with different chain lengths, and different number of OH groups; enzymes with antioxidant properties such as superoxide dismutase, herbal or plant extracts with antioxidant properties such as St. John's Wort, green tea extract, grape seed extract, rosemary, oregano extract, mixtures, derivatives, analogues or conjugated forms of these. Said formulation may include an antioxidant which is a primary antioxidant with reactive OH or NH groups such as hindered phenols or secondary aromatic amines; or secondary antioxidants such as organophosphorus compounds or thiosynergists; or multifunctional antioxidants, hydroxylamines, or carbon centered radical scavengers such as lactones or acrylated bis-phenols. Antioxidants may be selected individually or used in any combination. Antioxidants may be used in conjunction with hydroperoxide decomposers. Said formulation may include 0.0001-50%, preferably less than 5wt%, more preferably 0.0001-2wt%, of antioxidant / heat stabilizer in total. When said formulation is a liquid, said formulation may include a viscosity control agent. A viscosity control agent may be a surfactant and / or fumed silica. A viscosity control agent may be a wax, silicon dioxide, povidone, carbomer, poloxamer, carboxymethyl cellulose, hydroxyethyl cellulose or a polyacrylic acid. Said formulation may include 0 to 50wt%, 0.0001 to 50wt%, 0.0001 to 5wt% or 0.0001 to 3wt% of viscosity control agent. In some cases, said formulation may include one or more fillers (eg inorganic fillers, such as CaCO3). Preferably, said formulation includes less than 1wt%, less than 0.5wt% or less than 0.1wt% of fillers, for example inorganic fillers. According to a second aspect of the invention, there is provides a mixture comprising: (a) a polymeric material; (b) a radical generator or a residue thereof; (c) a cross-linking agent or a residue thereof; and, optionally (but preferably) (d) a carrier. Preferably, the mixture comprises said polymeric material and the formulation of the first aspect. Said mixture may comprise a residue of said radical generator produced by heat processing, for example reactive extrusion of, the mixture. Said mixture may comprise a residue of said cross-linking agent produced by heat processing, for example reactive extrusion of, the mixture. When said carrier is unfunctionalised (eg it comprises an oil such as mineral oil), said carrier may be solubilized in the polymeric material. When said carrier is functionalized, said carrier may undergo cleavage or reaction produced by heat processing, for example reactive extrusion of, the mixture. Said polymeric material preferably comprises one or more polyolefins. It preferably comprises at least two polyolefins from different sources and / or which have different identity and / or properties. Said polymeric material preferably comprises polyethylene. It may include 50 to 100wt% polyethylene. Said polymeric material preferably comprises recycled material, for example PCR or PIR. It may comprise polyethylene PCR, especially HDPE PCR. Said polymeric material preferably comprises at least 25wt% recycled material, for example PCR. Said polymeric material may comprise 25 to 100wt%, 25 to 75wt% or 25 to 5wt0%, recycled material, for example PCR. Said polymeric material may comprise up to 50wt% of polypropylene (PP), for example 10 to 50wt% PP. Compositions of PCR may vary as between one another. For example, PCR may include inorganic contamination and / or may be contaminated with fillers. For example, said PCR may include up to 3wt%, for example 0.5 to 1wt% of inorganic and / or organic contamination. Said polymeric material may comprise virgin polymer, for example virgin polyethylene, especially virgin HDPE. Said polymeric material preferably comprises at least 25wt% virgin polymer. Said polymeric material may comprise 25 to 75wt% or 50 to 75wt% virgin polymer. In some cases, said polymeric material may comprise about 100wt% of virgin polymer, for example PE. Said radical generator, cross-linking agent and carrier may have any feature as described in the first aspect. Said mixture may be in a solid form, for example in the form of pellets or granules. Said mixture may include at least 90wt%, preferably at least 94wt%, more preferably at least 96wt%, of polyolefins in total. It may include less than 99wt% or less than 98wt% of polyolefins in total. Said mixture may include at least 90wt%, preferably at least 94wt%, more preferably at least 96wt%, of polyethylene in total. It may include less than 99wt% or less than 98wt% of polyethylene in total. According to a third aspect, there is provided a product of the treatment (eg melt-processing and / or reaction, for example reactive extrusion) of: (a) a polymeric material; with (b) a radical generator; (c) a cross-linking agent; and, optionally (but preferably) (d) a carrier. The product may be produced by the treatment (eg melt-processing and / or reaction, for example reactive extrusion) of said polymeric material with the formulation of the first aspect. Said product may be in the form of granules or pellets. Alternatively, said product may be in the form of a molded article, for example produced by extrusion, such as by extrusion blow molding, by compression molding or by injection molding. The molded article may be a packaging article such as a receptacle or container or a sheet. Said polymeric material, radical generator, cross-linking agent and carrier may have any feature as described in the first and / or second aspects. According to a fourth aspect of the invention, there is provided a method of improving mechanical properties of a polymeric material which, optionally, includes recycled polymeric material, the method comprising: (i) selecting a first mass of polymeric material, wherein said first mass includes a first polymeric material and, optionally, recycled polymeric material; (ii) contacting said first mass with a radical generator and a cross-linking agent, each being as described in the first aspect and / or contacting said first mass with a formulation of the first aspect and / or producing a mixture of the second aspect; and (iii) melt-processing, for example by reactive extrusion, the combination produced in step (ii). Said first polymeric material may comprise a polyolefin for example polyethylene and / or polypropylene. Said first polymeric material is preferably not recycled polymeric material, for example it is not PCR. Said first polymeric material may be virgin polymeric material, for example virgin polyolefin. It may be virgin polyethylene or virgin polypropylene. It is preferably virgin polyethylene. Said first polymeric material may comprise at least 80 wt%, at least 90 wt% or at least 95 wt% of virgin polyolefin. Said first polymeric material may comprise at least 80 wt%, at least 90 wt% or at least 95 wt% of virgin polyethylene; or said first polymeric material may comprise at least 80 wt%, at least 90 wt% or at least 95 wt% of virgin polypropylene. Said recycled polymeric material may, if included, comprise scrap polymeric material, PCR or PIR. Said recycled polymeric material may comprise recycled polyolefin, for example recycled polyethylene and / or polypropylene. Said recycled polymeric material may comprise PCR. A polymeric material produced in the method may include 50 to 100wt% of said first polymeric material and 0 to 50wt% of recycled polymeric material. Said polymeric material produced in the method may include 50 to 80wt% of said first polymeric material and 20 to 50wt% of recycled polymeric material. The polymeric material produced in the method may be in the form of granules or pellets. The polymeric material may in the form of a molded article, for example produced by extrusion, such as by extrusion blow molding, compression molding or injection molding. The molded article may be a packaging article such as a receptacle or container. The molded article may be an extrusion blow molded article. The molded article may be a packaging article such as a receptacle or container or a sheet. Said method is preferably a method of improving ESCR of said polymeric material. For example, when ESCR is determined as described herein, there may be an improvement of at least 50%, preferably at least 100%, more preferably at least 200%. Such high % improvement may be required to ensure polyolefins, for example polyethylenes can be utilized in higher value applications such as consumer goods. According to a fifth aspect, there is provided a molded article, pellets or granules which comprise a polymeric material, for example as described herein (eg it comprises the first polymeric material and optional recycled polymeric material of the fourth aspect) and a trace amount of one or more of the following: a radical generator or a residue thereof as described herein; a cross-linking agent or a residue thereof as described herein; and / or a carrier or a residue thereof as described herein. The polymeric material is suitably modified by use of the formulation of the first aspect, for example such that the cross-linking agent in the formulation used becomes part of the polymer backbone with an associated advantageous effect on mechanical properties. The reaction of the cross-linking agent with the polymer backbone may be determine via analytical methods designed to analyse polymer fragments such as pyrolysis GC-MS, rheology, NMR, DSC, gel content and density. The presence of remnants / degradation of the radical generators and cross-linking agent may be determined by LC-MS / GC-MS. Preferably, said molded article includes a trace amount of both said radical generator or a residue as described herein and said cross-linking agent or a residue as described herein. Any feature of any aspect of any invention described herein may be combined with any feature of any other invention described herein mutatis mutandis. Specific embodiments of the invention will now be described, by way of example. The following materials are referred to hereinafter: Virgin HDPE - refers to HDPE grade HD5502 S from Ineos; PCR HDPE - refers to commercially-available recycled HDPE; The following assessments were undertaken: Assessment 1 – Environmental Stress-Cracking Test (ESCR) This was assessed by a bent strip test, using ASTM D1693 under Condition B. At condition B, a bent strip of a test resin is placed in a 10% Igepal® solution at 50 °C. The strip is a plaque that is of dimension L x B x Thickness (3.8cm x 1.3cm x 3mm) . A 0.5 mm notch is cut across the strip to create a stress point before immersion into the 10% Igepal® solution at 50 °C. The time to failure is measured. Assessment 2 – Self-accelerating decomposition temperature (SADT) measurement A test peroxide is placed in an oven set at a test temperature. A timer is started when peroxide reaches 2°C below the test temperature. The oven is held at constant temperature for up to one week or, until a runaway event occurs. A peroxide “Passes” if it does not exceed the test (oven) temperature by 6°C within one week. A peroxide “Fails” if it exceeds the test temperature by 6°C within one week. The test is repeated in 5°C increments until a failure is reached. The failure temperature is reported as the SADT for that peroxide. Secondary information about the nature of the decomposition can also be recorded. Example 1 – General procedure for preparation of HDPE granules from recycled HDPE. Virgin HDPE and PCR HDPE in 50:50 wt% ratio were mixed with a formulation comprising a carrier, peroxide and cross-linking agent. After thorough mixing, the samples were processed in a twin-screw extruder at a nominal temperature screw profile of 190-230°C and productivity of 4-5kg / h to produce granules. Example 2 – General procedure for compression or injection molding of granules. Granules as prepared in Example 1 were compression molded or injection molded to prepare test pieces for testing as described. In the case of compresssion molding, test pieces are compression molded plates with dimensions of L x B x Thickness (20 cm x 15 cm x 3mm). Further, the compression molded plates are die punched into rectangular test specimen of dimensions L x B x Thickness (4 cm x 1.5 cm x 3mm). In the case of compresssion molding, injection molding was arranged to produce rectangular test specimens with dimensions L x B x Thickness (4 cm x 1.5 cm x 2mm). Examples 3 to 12 – Specific formulations used in preparation of granules Following the procedure referred to in Example 1, granules were produced using the ingredients in the table below, with the balance being made up of Virgin HDPE and PCR HDPE in a 50:50 wt% ratio. Example Mineral Identity of Amount of Identity of cross- Amount of cross- No. oil peroxide peroxide linking agent linking agent (wt%) (carrier) (wt%) (wt%) 3 0.1 Dicumyl 0.0082 Pentaerythritol 0.01 peroxide tetraacrylate 4 0.1 Dicumyl 0.0096 Pentaerythritol 0.015 peroxide tetraacrylate 5 0.1 Dicumyl 0.015 Trimethylolpropane 0.01 peroxide triacrylate 6 0.1 Dicumyl 0.005 Pentaerythritol 0.01 peroxide tetraacrylate 7 0.1 Dicumyl 0.015 Pentaerythritol 0.02 peroxide tetraacrylate 8 0.1 Dicumyl 0.025 Pentaerythritol 0.01 peroxide tetraacrylate 9 0.1 Dicumyl 0.0106 Pentaerythritol 0.02 peroxide tetraacrylate 10 0.1 Dicumyl 0.0908 Pentaerythritol 0.01 peroxide tetraacrylate 11 0.1 Dicumyl 0.0125 Pentaerythritol 0.0136 peroxide tetraacrylate 12 0.1 Dicumyl 0.0082 1,6-Hexanediol 0.0088 peroxide dimethacrylate 13 0.1 Dicumyl 0.015 Trimethylolpropane 0.03 peroxide trimethacrylate 14 0.1 Dicumyl 0.015 Trimethylolpropane 0.05 peroxide trimethacrylate 15 0.074 Dicumyl 0.0095 Pentaerythritol 0.0148 peroxide tetraacrylate 16 0.1275 Dicumyl 0.0142 Pentaerythritol 0.0222 peroxide tetraacrylate Example 17 - Extrusion and compression molding data for PCR from a general recycle stream Granules comprising a 50:50 mixture of EBM grade virgin HDPE and PCR HDPE from a general recycle stream with a baseline ESCR (F50) 17h were selected and compression molded as described in Example 2 and ESCR and other properties were measured as described in Assessment 1. The aforementioned was compared to granules produced and compression molded from the same HDPE used in Examples 3 to 11. Results are provided below: Impact, Example No Tensile Tensile Tensile ESCR MFI Izod, Tensile of Strength Strength Elongation Increase 190C / 21. Notched (I Modulus, formulation @ Yield @ Break @ Yield, % 6kg SO) ISO (MPa) used (MPa) (MPa) ISO (%) (kJ / m^2) None 0 20.97 21.0 13.4 7.7 57.2 866 3 135% 22.67 20.04 15.0 7.7 59.4 866 4 94% 18.6 20.34 14.6 7.6 49.0 888 The table shows that key mechanical properties of the HDPE resin are largely unchanged by incorporation of the formulations, but there is, advantageously, significant impact on ESCR. Example 18 - Compression molding data for EBM grade PCR The procedure of Example 17 was followed using a 50:50 resin mixture of EBM grade virgin HDPE and PCR from the general recycle stream with a baseline ESCR (F50) of 12 h. Results are provided in the table below: Example No of formulation used ESCR Improvement None 0 3 100% 13 67% 14 67% It should be appreciated from the above results that, by the inclusion of the formulations, the ESCR of the EBM grade resin can be increased significantly. Example 19 - Compression molding data for EBM grade PCR The procedure of Example 17 was followed using a 50:50 resin mixture of EBM grade virgin HDPE and PCR from the general recycle stream with a baseline ESCR (F50) of 4h. Results are provided in the table below: Example No of ESCR Improvement formulation used None 0 5 75% Example 20 - Compression molding data for EBM grade PCR The procedure of Example 17 was followed using a 50:50 resin mixture of EBM grade virgin HDPE and PCR from the general recycle stream with a baseline ESCR (F50) of 27h. Results are provided in the table below: Example No of ESCR Improvement formulation used None 0 12 165% Example 21 – Injection molding data for PCR from a general recycle stream Granules comprising a 50:50 mixture of EBM grade virgin HDPE and PCR from a general recycle stream with a baseline ESCR (F50) 280h. were injection molded as described in Example 2 and ESCR was measured as described in Assessment 1. Results are provided below which show that the presence of formulations described including a cross-linking agent offer significant benefits. Example No of ESCR Improvement formulation used None 0 6 23% 7 71% 8 257% Example 22 – Injection molding data for PCR with lower baseline level of ESCR Following the procedure of Example 21, a 50:50 resin mixture of EBM grade virgin HDPE and PCR from a general recycle stream with a baseline ESCR (F50) of 42 h. was injection molded and results are provided in the table below. Example No of ESCR Improvement formulation used None 0 9 190% 10 53% Example 23 – Injection molding data for PCR with medium baseline level of ESCR Following the procedure of Example 21, a 50:50 resin mixture of EBM grade virgin HDPE and PCR from a general recycle stream with a baseline ESCR (F50) of 147 h. was injection molded and results are provided in the table below. Example No of ESCR Improvement formulation used None 0 3 77% 4 274% 11 512% Example 24 – Injection molding data for flake PCR Flakes comprising a 100% post-consumer recycle with a baseline ESCR (F50) of 17 hr were injection molded as described in example 2 and ESCR was measured as described in Assessment 1. Results are provided below. Example No of ESCR Improvement formulation used 15 124% 16 176% Example 25 – Injection molding data for flake PCR Flakes comprising a 100% post-consumer recycle with a baseline of 130 hours were injection molded as described in example 2 and ESCR was measured as described in Assessment 1. Results are provided below. Example No of ESCR Improvement formulation used 15 137% 16 514% Example 26 – Injection molding data for food grade PCR Granules comprising of 100% food-grade post-consumer recycle with a baseline of 64 hours were injection molded as described in example 2 and ESCR was measured as described in Assessment 1. Results are provided below. Example No of ESCR Improvement formulation used 15 30% 16 105% Example 27-32 – Compression molding data using formulations comprising alternate cross- linking agents. Granules comprising 100% virgin EBM grade virgin HDPE with a ESCR (F50) baseline of 12 h, were selected and compression molded as described in example 2. The formulations in the following table were prepared as described and ESCR was assessed as described in Assessment 1. Example Mineral Identity Amount Identity of branching agent Amount ESCR No. oil of of of improvement (carrier) peroxide peroxide branching (wt%) (wt%) agent (wt%) 27 0.1 Dicumyl 0.015 1,6-Hexanediol 0.016 583% peroxide dimethacrylate 28 0.1 Dicumyl 0.015 1,6-Hexanediol 0.054 583% peroxide dimethacrylate 29 0.1 Dicumyl 0.015 1,6-Hexanediol diacrylate 0.03 233% peroxide 30 0.1 Dicumyl 0.015 1,6-Hexanediol diacrylate 0.05 450% peroxide 31 0.1 Dicumyl 0.015 3,9-Divinyl-2,4,8,10- 0.03 850% peroxide tetraoxaspiro[5.5]undecane 32 0.1 Dicumyl 0.015 3,9-Divinyl-2,4,8,10- 0.05 575% peroxide tetraoxaspiro[5.5]undecane Example 33-36 - Compression molding data using formulations comprising alternate cross- linking agents. Granules comprising a 50:50 mixture of EBM grad virgin HDPE and PCR from a general recycle stream with a ESCR (F50) baseline of 10 h, were selected and compression molded as described in example 2. The formulations in the following table were prepared as described and ESCR was assessed as described in Assessment 1. Example Mineral Identity Amount Identity of branching agent Amount ESCR No. oil of of of improvement (carrier) peroxide peroxide branching (wt%) (wt%) agent (wt%) 33 0.1 Dicumyl 0.015 1,6-Hexanediol diacrylate 0.016 20% peroxide 34 0.1 Dicumyl 0.015 1,6-Hexanediol diacrylate 0.054 110% peroxide 35 0.1 Dicumyl 0.015 3,9-Divinyl-2,4,8,10- 0.03 100% peroxide tetraoxaspiro[5.5]undecane 36 0.1 Dicumyl 0.015 3,9-Divinyl-2,4,8,10- 0.05 130% peroxide tetraoxaspiro[5.5]undecane The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

CLAIMS 1 A formulation for treatment of a polymeric material, the formulation comprising: (i) a radical generator; and (ii) a cross-linking agent. 2 A formulation according to claim 1, wherein said radical generator is a peroxide, a hindered amine, a nitroxide or a metal radical source. 3 A formulation according to claim 1 or clam 2, wherein said radical generator is a peroxide. 4 A formulation according to any preceding claim, wherein said radical generator is selected from dicumyl peroxide, 1,1-Di(tert.butylperoxy)cyclohexane, tert.Butylperoxybenzoate, 2,5-dimethyl-2,5-di- (tert-butylperoxy)hexane, 1-(anthraquinon-2-yl)ethyl imidazolecarboxylate and 1,2,4,5,7,8- Hexoxonane, 3,6,9-trimethyl-3,6,9-tris(Et and Pr) derivatives (reaction mass of 3,6,9-triethyl-3,6,9- trimethyl-1,2,4,5,7,8-hexoxonane and 3,6-diethly-3,6,9-trimethyl-9-n-propyl-1,2,4,5,7,8-hexoxonane and 3-ethyl-3,6,9-trimethyl-6,9-di-n-propyl-1,2,4,5,7,8-hexoxonane and 3,6,9-trimethyl-3,6,9-tri-n- propyl-1,2,4,5,7,8-hexoxonane sold under brand name Trigonox® 501-CS40). 5 A formulation according to any preceding claim, wherein said cross-linking agent includes two or more, preferably at least three, C=C double bonds. 6 A formulation according to any preceding claim, wherein said cross-linking agent includes three or four C=C double bonds, each being a component of a respective moiety pendent from a central fragment, wherein each of said three or four C=C double bonds is terminal. 7 A formulation according to any preceding claim, wherein said cross-linking agent includes a fragment (A), wherein at least two, preferably at least three (and preferably no more than six, or no more than five, or no more than four) moieties of structure are bonded to fragment (A) via respective starred (*) carbon atoms. 8 A formulation according to claim 7, wherein fragment (A) consists of atoms selected only from carbon, hydrogen and oxygen atoms.9 A formulation according to claim 7 or claim 8, wherein fragment (A) includes at least two, preferably at least three (and preferably not more than six, or not more than five or not more than four) moieties of formula. 10 A formulation according to claim 9, wherein one or each moiety of formula (II) is bonded to a moiety of formula (I) to define a moiety. 11 A formulation according to claim 10, wherein said cross-linking agent includes at least two, preferably at least three (and, preferably, no more than six or no more than five or no more than four) moieties of formula (III). 12 A formulation according to any of claims 7 to 11, wherein said fragment (A) includes a fragment (B) to which each moiety of formula (II) is bonded, wherein fragment (B) consists of atoms selected only from carbon, hydrogen and oxygen atoms. 13 A formulation according to claim 12, wherein fragment (B) is saturated, includes carbon and hydrogen atoms only, has a molecular weight of less than 150 and at least 50, and includes at least 2 and less than 10 carbon atoms. 14 A formulation according to any preceding claim, wherein said cross-linking agent includes a plurality, preferably 2 to 8, more preferably 3 to 6, especially 3-4 acrylate moieties. 15 A formulation according to claim 14, wherein the alkenyl moieties of the acrylate moieties are each terminal groups of said cross-linking agent.16 A formulation according to any preceding claim, wherein said cross-linking agent includes at least two. preferably at least three (and, preferably, no more than six or no more than five or no more than four) acrylate moieties. 17 A formulation according to any preceding claim, wherein said cross-linking agent consists of atoms selected only from carbon, hydrogen and oxygen atoms; and / or said cross-linking does not include any aromatic or heteroaromatic moiety, any alkynyl moiety or any cyclic moiety. 18 A formulation according to any preceding claim, wherein said cross-linking agent is selected from Triallyl isocyanurate, Trimethylolpropane triacrylate, Pentaerythritol tetraacrylate, 1,6-Hexanediol diacrylate, Neopentyl glycol diacrylate, Di(trimethylolpropane) tetraacrylate, 1,6-Hexanediol dimethacrylate, Divinyl benzene, Bisphenol A glycerolate diacrylate, Triallyl cyanurate, Triallyl citrate , Triallyl trimellitate, Triallyl orthoformate, Triallyl aconitate, Trimethylolpropane trimethylacrylate, Ethoxylated bisphenol A dimethacrylate, Dipentaerythritol pentaacrylate, propoxylated glyceryl triacrylate, benzyl methacrylate, 2-ethylhexyl methacrylate, Ddipentaerythritol hexaacrylate, ethylene glycol Dimethacrylate, Methacrylic anhydride, Methacrylate anhydride, Neopentyl glycol dimethacrylate and Di(ethylene glycol) divinyl ether. 19 A formulation according to any preceding claim, wherein said cross-linking agent is 3,9-Divinyl- 2,4,8,10-tetraoxaspiro[5.5]undecane. 20 A formulation according to any preceding claim, wherein said formulation includes a carrier. 21 A formulation according to claim 20, wherein said radical generator is fully dissolved or homogenously dispersed in the carrier and / or said cross-linking agent is fully dissolved or homogenously dispersed in the carrier, wherein, optionally, said carrier comprises a hydrocarbon-containing liquid. 22 A formulation according to any of claims 1 to 20, wherein said formulation is a solid which, optionally, is selected from a polyolefin, a Styrenic block polymer, Ethylene-vinyl acetate (EVA) or Ethylene Propylene Rubber (EPR). 23 A formulation according to any preceding claim, wherein said formulation includes: 0.001 to 10wt%, for example 1 to 10 wt%, of said radical generator; 0.005 to 50wt%, for example 1 to 50 wt% of said cross-linking agent;40 to 99.99wt%, for example 49 to 98 wt%, of said carrier. 24 A formulation according to any of claims 20 to 23, wherein: a sum of the wt% of all radical generators in the formulation is in the range 0.001 to 2.0wt%, for example 0.01 to 2.0 wt%; a sum of the wt% of all cross-linking agents in the formulation is in the range 0.001 to 2.0wt%, for example 0.01 to 2.0 wt%; and a sum of the wt% of all carriers in said formulation is in the range 46 to 99.9wt%, for example 50 to 99.9 wt%. 25 A formulation according to any preceding claim, wherein, in said formulation: a ratio defined as the wt% of said radical generator divided by the wt% of said cross-linking agent is in the range 1:1 to 1:100, for example in the range 1:1 to 1:10; and / or a ratio defined as the sum of the wt% of all radical generators divided by the sum of the wt% of all cross- linking agents is in the range 1:1 to 1:100, for example in the range 1:1 to 1:

10. 26 A formulation according to any preceding claim, wherein said formulation includes a radical mediator / polymerization inhibitor able to accept or donate a radical and prevent pre-reaction of monomers. 27 A mixture comprising: (a) a polymeric material; (b) a radical generator or a residue thereof; (c) a cross-linking agent or a residue thereof; and, optionally (but preferably) (d) a carrier. 28 A mixture according to claim 27, wherein the mixture comprises said polymeric material and the formulation of any of claims 1 to 26. 29 A mixture according to claim 27 or claim 28, wherein said polymeric material comprises one or more polyolefins.30 A mixture according to any of claims 27 to 29, wherein said mixture comprises at least two polyolefins from different sources and / or which have different identities and / or properties. 31 A mixture according to any of claims 27 to 30, wherein said polymeric material comprises polyethylene. 32 A mixture according to any of claims 27 to 31, wherein said polymeric material comprises recycled material, for example PCR or PIR. 33 A mixture according to any of claims 27 to 32, wherein said polymeric material comprises at least 25wt% virgin polymer. 34 A mixture according to any of claims 27 to 33, wherein said mixture is in a solid form, for example in the form of pellets or granules. 35 A product of the treatment of: (a) a polymeric material; with (b) a radical generator; (c) a cross-linking agent; and, optionally (but preferably) (d) a carrier. 36 A product according to claim 35, wherein the product is produced by the treatment (eg melt- processing and / or reaction) of said polymeric material with the formulation of any of claims 1 to 26 and said product is in the form of granules or pellets or an article, for example a container or extruded sheet or film. 37 A method of improving mechanical properties of a polymeric material which, optionally, includes recycled polymeric material, the method comprising: (i) selecting a first mass of polymeric material, wherein said first mass includes a first polymeric material and, optionally, recycled polymeric material;(ii) contacting said first mass with a radical generator and a cross-linking agent, each being as described in any of claims 1 to 26 and / or contacting said first mass with a formulation of any of claims 1 to 26 and / or producing a mixture of any of claims 27 to 34; and (iii) melt-processing the combination produced in step (ii). 38 A method according to claim 37, wherein said first polymeric material comprises a polyolefin for example polyethylene and / or polypropylene and comprises at least 80 wt% of virgin polyolefin. 39 A method according to claim 37 or claim 38, wherein a polymeric material produced in the method includes 50 to 100wt% of said first polymeric material and 0 to 50wt% of recycled polymeric material; and, optionally, said polymeric material produced in the method is in the form of granules or pellets. 40 A method according to any of claims 37 to 39, wherein said method is a method of improving environmental stress cracking (ESCR) and / or melt strength of said polymeric material. 41 A molded article, pellets or granules which comprise a polymeric material and a trace amount of one or more of the following: a radical generator or a residue thereof as described in any preceding claim; a cross-linking agent or a residue thereof as described in any preceding claim; and / or a carrier or a residue thereof as described in any preceding claim; and / or wherein said polymeric material includes a part of a cross-linking agent as herein described.

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