Additive masterbatch

Abstract

Described herein are compositions comprising a polymer / plastic carrier and potassium metabisulphite for imparting antimicrobial activity to a plastic material. Methods for making the same, and uses of the same for microbial control, are also disclosed.

Description

[0001] ADDITIVE MASTERBATCH

[0002] The present invention relates to the addition of an antimicrobial agent into or onto plastics or polymer material, and the provision of an additive masterbatch that imparts antimicrobial properties to a plastic or polymer material when added to such a material.

[0003] A masterbatch is a concentrate that is used to impart properties, such as colour (known as a colour masterbatch), or antimicrobials and UV, humidity or temperature resistance (known as additive masterbatches), to polymers during the manufacturing process. The composition of a masterbatch is called its recipe.

[0004] A masterbatch, also termed a polymeric concentrate, is a concentrated mixture of one or more additives, or active components, blended and extruded together in a carrier matrix that is used to add the additive(s) or active component(s) to a final polymer or plastic product. After extrusion, the extrudate is cooled, cut and formed into granules, mini beads, pellets or powders depending on its destined application. The carrier matrix may be resin, wax or a polymer such as polyethylene (PE) or polypropylene (PP), ethylene vinyl acetate (EVA), etc. that is compatible with the polymer or plastics material into which the masterbatch is added and diluted. The additive in a masterbatch is encapsulated in the carrier material. This encapsulation reduces or, in many cases, eliminates any dangerous properties of an additive, as well as protecting temperature-sensitive additives during the final product manufacturing process. Encapsulation also protects an additive from exposure to humidity, temperature and UV during storage.

[0005] Masterbatches are added or “let down” into a raw polymer or plastics material and are passed through a blending process which feeds into a moulding or extruding machine where the final plastic product is made.

[0006] An advantage of imparting particular properties to a polymer by way of a masterbatch, compared to adding one or more compounds perse, is that masterbatches are highly concentrated. This not only saves on raw component bulk and expense but also allows a higher accuracy of dosing of expensive components and, thus, minimal chance of variance during the manufacturing process of the final product. A masterbatch also provides a form that results in consistent dispersal of active in a finished polymer product, as well as having a long shelf life due to being in solid form and being solvent-free. A masterbatch typically contains 40% to 65% additives, but the range may be from 15% to 80%. Furthermore, melting processes are improved because the carrier in a masterbatch is matched with the polymer(s) into which the masterbatch is added and diluted.

[0007] Antimicrobial masterbatches are well known but most are metal based systems using the likes of silver, zinc and copper salts which exhibit high thermal stability but limited antimicrobial activity. For example, US 2016 / 0135470 describes the masterbatches, concentrates and / or intermediates containing copper iodide as having broad-spectrum, fast acting antimicrobial activity. US 2008 / 0306183 discloses polyolefin or polyester fibres that include silver as an antimicrobial agent. However, the additional safety and environmental concerns of metal particles have caused manufacturers and consumers to turn from these technologies and seek alternative / naturally inspired alternatives. US 2017 / 0240720 is directed to an antimicrobial masterbatch polymer composition in which the antimicrobial property is provided by the inclusion of chlorinated phenols (Triclosan derivatives) or quaternary ammonium compounds. The use of triclosan in food storage containers was banned in the European Union in 2010. In an alternative, US 2003 / 0235605 describes a polymeric article in which the antimicrobial property is effected by the release of an antimicrobial gas, such as chlorine dioxide, chlorine, hydrogen peroxide, and sulphur dioxide, on exposure to light and / or humidity.

[0008] Another drawback for these technologies is the growing rise in antimicrobial resistance due to overuse of a single active for a long time.

[0009] WO 2016 / 207183 describes the synergistic effect of zinc pyrithione with food-approved antimicrobials, including sodium sulphite, sodium bisulphite, potassium metabisulphite, potassium sulphite, calcium hydrogen sulphite, potassium hydrogen sulphite sodium nitrate, potassium nitrate and lysozyme. The combination has particular use when incorporated in or coated onto materials used for food packaging. The antimicrobial component is exemplified only as being carried in water.

[0010] WO 2021 / 138144 demonstrates that a single antimicrobial component may be suitably effective when formulated in a plastic. In particular, described is the inclusion of some organic acids in plastic substrates. However, the Applicant finds that not all organic acids are effective because not all are sufficiently stable at the temperatures required for plastic processing, namely temperatures of about 200°C and above.

[0011] US 2003 / 089891 describes an antimicrobial cleaning composition for use in cleaning semiconductor substrates. The composition comprises a solvent, a cleaning agent and at least one antimicrobial agent, in which potassium metabisulphite is provided as an example of a suitable antimicrobial agent. The composition is formulated and used as a liquid.

[0012] It is against this background that the present invention has been devised. In particular, the present invention encompasses a composition for imparting antimicrobial activity to a plastic material in which the composition comprises a single component that imparts antimicrobial properties when added and diluted to a polymer or plastic material. The resulting polymer or plastic material shows antimicrobial efficacy against both gram-negative and gram-positive bacteria, is cost-effective, and is safe including food.

[0013] Specifically, the present invention resides in a composition for combination with or application to a plastic material comprising: i) a thermoplastic or thermoplastic elastomer carrier; and ii) an antimicrobial agent. The antimicrobial agent consists essentially of potassium metabisulphite in a solid or dry form and the composition has a substantially neutral pH i.e. a pH in a range of from about 6 to 8, such as from about 6.5 to about 7.5 or around 7. The term “consisting essentially of” is used in the context to denote a composition in which the only active component in the composition is potassium metabisulphite. It will be understood that the composition may include additional components present in amounts such that the antimicrobial property of potassium metabisulphite is not affected. In the alternative, the composition may not include additional components, that is to say, the composition for combination with or application to a plastic material may consist essentially of: i) a thermoplastic or thermoplastic elastomer carrier; and ii) an antimicrobial agent. The antimicrobial agent may consist essentially of potassium metabisulphite as described above.

[0014] Potassium and sodium metabisulphite are well known as antimicrobial and antioxidant agents. However, their use in masterbatches has not been well explored because both are known to decompose above 150°C. Some have investigated sodium, rather than potassium, metabisulphite. For example, WO 2009 / 051594 describes a multi-layered coextrusion film in which sodium metabisulphite is dispersed in a middle layer of the film. Sodium metabisulphite is present as a solid in which different sized particles give off gas at different rates when activated by moisture. It is the release of gas that imparts an antimicrobial property to the film.

[0015] Others have used potassium metabisulphite in combination with other antimicrobial antioxidant agents. For example, Foralosso, F.B. et a / ((2014) Food Bioprocess Technol. 7, 1483-1495) report on the effect of PVC impregnated with a 1 :1 mixture of pure and encapsulated potassium metabisulphite on the shelf-life of apples stored. Encapsulated potassium metabisulphite was prepared using a sol-gel method in which encapsulated potassium metabisulphite was synthesised in pure “bare” silica and the use of an organosilane aminopropyl linker. The antimicrobial effects of 0.1%, 1%, 2% w / w additive in PVC were tested on Salmonella sp. No details are provided on how the additive was incorporated into PVC. Control and 0.1% additive showed no difference between the two data points, but films with 1% and 2% additive showed one log cycle reduction in antimicrobial population. The authors hypothesise that the pure potassium metabisulphite fraction immediately undergoes dissociation, generating an antimicrobial effect in the first days of storage, whereas the encapsulated fraction - because of the silica matrix - requires longer periods to generate sulphur dioxide (SO2) antimicrobials, thereby extending the antimicrobial effect.

[0016] The use of sodium or potassium metabisulphite as a sole antimicrobial agent in a plastic is counter intuitive. This is because their antimicrobial activity is most efficacious at low (acidic) pHs and in aqueous conditions. Furthermore, both sodium and potassium metabisulphite decompose above about 150°C. In complete contrast, the composition of the present invention is in a solid or dry state and has a substantially neutral pH.

[0017] While the existing science suggests that, because sodium metabisulphite is an effective antimicrobial agent, potassium metabisulphite should also be similarly effective. However, the inventors of the present invention have, very surprisingly, found that sodium metabisulphite, when incorporated as the sole agent in a thermoplastic or thermoplastic elastomer carrier, actually shows very little antimicrobial activity. In contrast, potassium metabisulphite shows very good antimicrobial efficacy against both gram-negative and grampositive bacteria, even more surprisingly in both low and high melting point thermoplastics and thermoplastic elastomers.

[0018] It will be appreciated that the most effective incorporation or application of the active component is achieved where the carrier is selected to be compatible with the plastic material into which the composition may be incorporated as a masterbatch or applied to as a coating. The carrier protects the active ingredient to minimise the loss of activity where the active ingredient is thermally sensitive. Examples of plastic materials encompassed by the present invention include those listed as a thermoplastic or thermoplastic elastomer carrier.

[0019] Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. Most modern plastics are derived from fossil fuel-based chemicals such as natural gas or petroleum, but more recent industrial methods use variants made from renewable materials, including corn and cellulose derivatives.

[0020] A thermoplastic, or thermosoftening plastic, is a plastic polymer material that softens and becomes pliable or mouldable when heated and solidifies on cooling. Once softened, a thermoplastic may be processed using methods such as extrusion, injection moulding, thermoforming and blow moulding. These types of plastics can be reshaped or remoulded numerous times and, once cooled, show no changes in chemical property after being heated and cooled multiple times, making them easily recyclable and reusable.

[0021] Examples of thermoplastics include: polyolefins and copolymers thereof; polypropylene (PP); polyethylene including ultra-high molecular weight polyethylene (UHMWPE), high density polyethylene (HDPE), medium density polyethylene (MDPE), linear low density polyethylene (LLDPE), and low density polyethylene (LDPE); polyesters including lactone-based polymers such as poly(lactide) (PLA), polycaprolactone (PCL), valeralactone, glycolide, and pentadecalactone; polyoxymethylene (POM); polyvinyl chloride (PVC); acrylics including ester derivatives (PAc) thereof; poly(methyl methacrylate) (PMMA) or acrylic; ethylene-vinyl acetate (EVA); polyvinyl alcohol (PVA); acrylonitrile-butadiene-styrene (ABS); acrylonitrile styrene acrylate (ASA); styrene acrylonitrile resin (SAN); and combinations thereof.

[0022] Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers (TPR), are a class of copolymers or a physical mix of polymers (usually a plastic and a rubber) that consist of materials with both thermoplastic and elastomeric properties. Thermoplastic elastomers show advantages typical of both rubbery materials and plastic materials. To qualify as a thermoplastic elastomer, a material must have three essential characteristics: i) the ability to be stretched to moderate elongations and, upon the removal of stress, return to something close to its original shape; ii) be processable as a melt at an elevated temperature; and show no significant creep (slow deformation under persistent mechanical stresses).

[0023] Examples of thermoplastic elastomers include thermoplastic polyurethanes (TPU); styrenic block copolymers (TPS); thermoplastic polyolefin elastomers (TPO); thermoplastic copolyester (TPC); unclassified thermoplastic elastomers (TPZ); and combinations thereof.

[0024] The polymer / plastic may be present in the composition in a powdered form. In an embodiment, the polymer / plastic is not present in the composition as a resin. The polymer / plastic is suitably low-density polyethylene. It is suitably present in the composition in a powdered form. In an embodiment, low-density polyethylene is not present in the composition as a resin. In another example, the polymer / plastic is polypropylene or ethylene-vinyl acetate.

[0025] The polymer / plastic carrier makes up the bulk of the composition and may be present in an amount of between about 50 wt% and about 99.5 wt%, between about 80 wt% and about 99.5 wt%, between about 90 wt% and 99.5 wt%, or between about 95 wt% and 99.5 wt%.

[0026] In an example, the composition may have an active concentration (i.e. a concentration of potassium metabisulphite) of 0.5 wt% to 60 wt%, 5 wt% to 60 wt%, 0.5 wt% to 30 wt%, less than 20 wt%, less than 10 wt%, 5 wt% or less, between about 0.5 wt% to about 5 wt% of the composition. In some embodiments, a composition as described herein comprises between about 1 wt% and about 90 wt% of potassium metabisulphite, e.g., about 1 , 2, 3, 5, 7, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 8%, or 90 wt% of potassium metabisulphite, including all values and ranges therebetween. In some embodiments, the masterbatch comprises between about 1 and about 2 wt%), between about 2 and about 5 wt%, between about 5 and about 10 wt%, between about 10 and about 15 wt%, between about 15 and about 20 wt%, between about 20 and about 25 wt%, between about 25 and about 30 wt%, between about 30 and about 40 wt%, between about 40 and about 50 wt%, between about 50 and about 60 wt%, between about 60 and about 70 wt%, between about 70 and about 80 wt%, between about 80 and about 90 wt%, between about 1 and about 5 wt%, between about 2 and about 10 wt%, between about 5 and about 15 wt%, between about 10 and about 20 wt%, between about 15 and about 25 wt%, between about 20 and about 30 wt%, between about 25 and about 40 wt%, between about 30 and about 50 wt%, between about 40 and about 60 wt%, between about 50 and about 70 wt%, between about 60 and about 80 wt%, between about 70 and about 90 wt%, between about 1 and about 10 wt%, between about 2 and about 15 wt%, between about 5 and about 20 wt%, between about 10 and about 25 wt%, between about 15 and about 30 wt%, between about 20 and about 40 wt%, between about 25 and about 50 wt%, between about 30 and about 60 wt%, between about 40 and about 70 wt%, between about 50 and about 80 wt%, between about 60 and about 90 wt%, between about 1 and about 15 wt%, between about 2 and about 20 wt%, between about 5 and about 25 wt%, between about 10 and about 30 wt%, between about 15 and about 40 wt%, between about 20 and about 50 wt%, between about 25 and about 60 wt%, between about 30 and about 70 wt%, between about 40 and about 80 wt%, between about 50 and about 90 wt%, between about 1 and about 20 wt%, between about 2 and about 25 wt%, between about 5 and about 30 wt%, between about 10 and about 40 wt%, between about 15 and about 50 wt%, between about 20 and about 60 wt%, between about 25 and about 70 wt%, between about 30 and about 80 wt%, between about 40 and about 90 wt%, between about 1 and about 25 wt%, between about 2 and about 30 wt%, between about 5 and about 40 wt%, between about 10 and about 50 wt%, between about 15 and about 60 wt%, between about 20 and about 70 wt%, between about 25 and about 80 wt%, between about 30 and about 90 wt%, between about 1 and about 30 wt%, between about 2 and about 40 wt%, between about 5 and about 50 wt%, between about 10 and about 60 wt%, between about 15 and about 70 wt%, between about 20 and about 80 wt%, between about 25 and about 90 wt%, between about 1 and about 40 wt%, between about 2 and about 50 wt%, between about 5 and about 60 wt%, between about 10 and about 70 wt%, between about 15 and about 80 wt%, between about 20 and about 90 wt%, between about 1 and about 50 wt%, between about 2 and about 60 wt%, between about 5 and about 70 wt%, between about 10 and about 80 wt%, between about 15 and about 90 wt%, between about 1 and about 60 wt%, between about 2 and about 70 wt%, between about 5 and about 80 wt%, between about 10 and about 90 wt%, between about 1 and about 70 wt%, between about 2 and about 80 wt%, between about 5 and about 90 wt%, between about 1 and about 80 wt%, or between about 2 and about 90 wt%, potassium metabisulphite, including all ranges and subranges therebetween. In some embodiments, the composition comprises between about 15 and about 30 wt% potassium metabisulphite. In some embodiments, the composition comprises between about 20 and about 80 wt% potassium metabisulphite. In some embodiments, the composition comprises between about 20 and about 40 wt% potassium metabisulphite. In some embodiments, the composition comprises between about 30 and about 50 wt% potassium metabisulphite. In some embodiments, the composition comprises between about 40 and about 60 wt% potassium metabisulphite. In some embodiments, the composition comprises about 20 wt% of potassium metabisulphite.

[0027] The composition may have an active concentration (i.e. a concentration of potassium metabisulphite) of about 10-90 wt%, such as 15-85 wt% or 20-80 wt% potassium metabisulphite. The active concentration may be about 10-25%, such as about 15-25%. In one embodiment, the active concentration is about 20 wt%.

[0028] In an embodiment, the composition comprises only potassium metabisulphite and a polymer / plastic carrier as described herein. That is to say, the composition may not comprise any additional components. In one embodiment, the composition does not comprise stearic acid, dimethyl silicone oil and / or other fillers.

[0029] Suitably the composition may comprise or essentially consist of a polymer / plastic carrier as described herein and about 20-80 wt% potassium metabisulphite. For example, the composition may consist of around 80 wt% of a polymer / plastic carrier as described herein and around 20 wt% potassium metabisulphite.

[0030] In some embodiments, the potassium metabisulphite is in a dry form. The potassium metabisulphite may be in the form of a mini bead, pellet or granules. In some embodiments, the potassium metabisulphite granules have an average size of between about 0.1 pm and about 500 pm, e.g., about 0.1 pm, 0.2 pm, 0.5 pm, 1 pm, 2 pm, 5 pm, 10 pm, 20 pm, 50 pm, 100 pm, 200 pm, or 500 pm, including all values and ranges therebetween. In some embodiments, the granules have an irregular shape and the size is determined by the longest length of the granule (for example, the longest length of a cube is its body diagonal). In another example, the potassium metabisulphite is present in the composition in a powdered form. A powdered form has a larger surface area which may improve antimicrobial activity. In addition, powders finer than 50 pm are below the best eye resolution and so cannot be seen in the final product.

[0031] In some embodiments, all or substantially all of potassium metabisulphite is in direct contact with the polymer / plastic carrier. In embodiments, more than about 60%, more than about 70%, more than about 80%, more than about 85%, more than about 90%, more than about 95%, more than about 97%, more than about 98%, more than about 99%, or 100%, of the potassium metabisulphite, including all values and ranges therebetween, is in direct contact with the polymer / plastic carrier.

[0032] In some embodiments, no potassium metabisulphite in the polymer / plastic carrier is encapsulated in a non-plastic material (which in turn may be embedded in the polymer / plastic carrier). In some embodiments, no potassium metabisulphite in the polymer / plastic carrier is encapsulated in a non-polymer material (which in turn may be embedded in the polymer / plastic carrier). In some embodiments, no potassium metabisulphite in the polymer / plastic carrier is encapsulated in a metal oxide, silicon dioxide, or other non-plastic materials, for example via a sol-gel process. In some embodiments, less than about 40%, less than about 30%, less than about 20%, less than about 15%, less than about 10%, less than about 5%, less than about 3%, less than about 2%, or less than about 1%, of potassium metabisulphite, including all values and ranges therebetween, in the polymer / plastic carrier is encapsulated in a non-plastic material (e.g., metal oxide or silicon dioxide). In a further example, the composition may be in a solid form, such as granules, mini beads, pellets or a powder. In this way, the composition is in a form particularly suitable for use as a polymeric concentrate or masterbatch, or a polymeric binder for coatings.

[0033] Where the composition is for use as a polymeric binder for coatings, the composition may further include additional components that enable the specified use. Examples include binders, cross-linkers, stabilisers and / or compatibilisers which may be added in any combination with any suitable homopolymer, copolymer or elastomer.

[0034] As described above, active ingredient(s) are typically incorporated into plastics through masterbatching, in which a high weight percent of active ingredient(s) is formed into granules, mini beads, pellets or a powder which is then diluted by the manufacturer during injection moulding, extrusion, blow moulding and other common plastic forming techniques. Thus, in a particular example, the polymeric concentrate or masterbatch may have an active concentration (i.e. a concentration of potassium metabisulphite) of 1 % to 60%, or 5% to 40% by weight of the composition. Alternatively, the polymeric binder may have an active concentration of about 0.1 % to about 10% based on solution % solids. The polymer binder may be non-aqueous or contain a trace amount of co-solvent.

[0035] By way of illustration, a mixture containing 30 wt% of dry powderised potassium metabisulphite and 70 wt% of dry powderised polypropylene can be placed into a TMS oven at 80 °C for at least an hour. Extrusion can be performed in a Process 11 Parallel Twin Screw Extruder (Thermo Scientific), using the heating profile from doser as 150, 170, 185, 190, 195, 195, 185 and 170 at a dosing speed of 30% with the extruder set to 185 RPM. The extruded composition can be cooled and pelletized using a Varicut Pelletiser (Thermo Scientific) at 3 max speed at setting L3, to form a granular composition. The composition pellets can be stored in zip lock bags and stored in the fridge for freshness.

[0036] Also encompassed by the present invention is a masterbatch comprising, or consisting essentially of, a composition as described and defined herein. Expressed in another way, the present invention also encompasses the use of a composition as described and defined herein either as a masterbatch for incorporation into a plastic material, or as a coating for application to a surface of a plastic material.

[0037] A method for microbial control on a plastic material surface is also provided, the method comprising either i) incorporating into the plastic material, or ii) applying onto a surface of the plastic material, a composition as herein described comprising: i) a thermoplastic or thermoplastic elastomer carrier; and ii) an antimicrobial and / or antioxidant agent, wherein the antibacterial and / or antioxidant agent consists essentially of potassium metabisulphite in a solid or dry form and the composition has a substantially neutral pH.

[0038] In a particular example, the carrier and potassium metabisulphite may be combined as powders before being heated and extruded into granules, mini beads, pellets or a powder.

[0039] In an embodiment, the carrier is not a resin. For example, the carrier is suitably low-density polyethylene. It is suitably combined with potassium metabisulphite in the composition as a powdered. In an embodiment, low-density polyethylene is not combined with potassium metabisulphite in the composition as a resin.

[0040] It will be appreciated that the compositions, methods, use, masterbatch and coatings described and defined herein have many applications. Examples of products that benefit from having an antimicrobial profile include: conventional and technical textiles and fabrics, facemasks, high performance non-woven materials such as filters, ventilation systems, surgical gowns, wet wipes, nappies, hygienic pads, wound dressings, food packaging such as plastic bags, vacuum packing, cling and stretch films, rigid and soft containers, bottles and lids for foods and beverages, inserts for packaging and articles including food packaging, household products such as storage boxes, containers, domestic liquids bottles, waste bins and dustbins, gloves and thin film coatings for various applications including but not limited to hospitals, industrial kitchens, restaurants, mobile telephones or similar devices, touchscreens, keypads, keyboards, buttons and knobs, handles, screen protectors, furniture such as a seat, chair, toilet seat, cup or mug, worktop, banknotes, upholstery. High contact materials and appliances include kitchen appliances, doors, taps or faucets, steering wheels, hospital appliances, medical devices, medical components.

[0041] The invention will now be described further with reference to the following, non-limiting experiments and figures, in which:

[0042] Figure 1 : Antibacterial activity against Staphylococcus aureus (Figure 1a) and Escherichia coli (Figure 1b) of 0.5 wt%, 2.5 wt% and 5 wt% potassium metabisulphite in low-density polyethylene (LDPE) compared to an untreated control sample. Asterisks indicate full inhibition of bacterial growth.

[0043] Figure 2: Antibacterial activity against Staphylococcus aureus (Figure 2a) and Escherichia coli (Figure 2b) of 0.5 wt%, 1%, 2 wt% and 5 wt% potassium metabisulphite in polypropylene (PP) compared to an untreated control sample and a commercially available silver formulation with a 20 wt% loading. Figure 3: Antibacterial activity against Staphylococcus aureus (Figure 3a) and Escherichia coli (Figure 3b) of potassium and sodium metabisulphite carried in an ethylene-vinyl acetate (EVA) masterbatch at 20 wt% which was diluted into PP to 2 wt% and 5 wt% at final concentration. Asterisks indicate full inhibition of bacterial growth.

[0044] Figure 4: Antifungal activity of potassium metabisulphite carried in PP against the yeast Candida albicans and the fungus Aspergillus brasiliensis within 24 hours of contact time.

[0045] METHODS

[0046] Powderised potassium metabisulphite (Sigma Aldrich, BioUltra, 97%) was combined with powderised polymer, extruded into pellets at temperatures up to 190°C, and heat pressed at a temperature of up to 170°C into a film for testing.

[0047] All samples were tested for a bacterial contact time of 24 h following a modified ISO 22196 standard.

[0048] RESULTS

[0049] Experiment 1

[0050] The aim of this experiment was to assess the antimicrobial efficacy of potassium metabisulphite in low-density polyethylene (LDPE, ExxonMobil 605BA). LDPE was selected as the thermoplastic because potassium metabisulphite is known to have low thermal stability. It was hypothesised that use of the compound would be better suited to polymers processed at low temperatures, i.e. temperatures below 150°C.

[0051] As shown in Figure 1 , concentrations of 0.5 wt%, 2.5 wt% and 5 wt% potassium metabisulphite all showed an average of 6.5 log (>99.9999%) reduction in Staphylococcus aureus (S. aureus) (Figure 1a) and just under 6 log (99.9999%) reduction in Escherichia coli (E. coli) (Figure 1b), completely inhibiting bacterial growth for both bacteria.

[0052] Experiment 2

[0053] The aim of this experiment was to assess the antimicrobial activity of potassium metabisulphite at a higher temperature. To do this, powderised potassium metabisulphite was incorporated into polypropylene (Impact Polypropylene, Sabie PHC31) at concentrations of 0.5 wt%, 1 wt%, 2 wt% and 5 wt%. The results were compared to PP alone (control) and a commercially available masterbatch loaded with 20% silver. As can be seen in Figure 2, whilst a slightly lower antibacterial effect is observed in E. coli (Figure 2b), the antibacterial activity remains far higher than the silver antimicrobial composition, which contains a much higher concentration of antimicrobial agent than the test samples of the present invention. These results are surprising as they suggest that potassium metabisulphite is able to retain its antimicrobial activity up to 190°C. Additionally, as shown in Figure 2a, the lowest concentration of potassium metabisulphite (0.5%) exhibited almost 6 logs reduction for S. aureus.

[0054] Experiment 3

[0055] The aim of this experiment was to compare the antimicrobial effects of potassium metabisulphite with its closely related chemical, sodium metabisulphite (Sigma Aldrich, 97%). Ethylene-vinyl acetate (EVA, ExxonMobil Escorene 00728CC) was used as the carrier for the antimicrobial at 20 wt% which was mixed with PP, diluting the active to final concentrations of 2 wt% and 5 wt%.

[0056] As shown in Figure 3, sodium metabisulphite, very unexpectedly, yielded no appreciable antibacterial effect against either S. aureus (Figure 3a) or E. coli (Figure 3b).

[0057] Experiment 4

[0058] The aim of this experiment was to determine the antifungal efficacy of potassium metabisulphite incorporated into PP at 1 wt %. The antifungal efficacy was measured at 24 hours of contact with the yeast Candida albicans and the fungus Aspergillus brasiliensis.

[0059] As shown in Figure 4, Candida albicans showed an almost 4 log (99.98%) reduction, while Aspergillus brasiliensis exhibited just over 3 log (99.92%) reduction upon 24 hours of contact with a test masterbatch.

[0060] The aim of the present invention is to enable the creation of an antimicrobial surface using safe, food-based additives / preservatives. From the results provided herein, it is clear that potassium metabisulphite is suitable for incorporation into, or application onto, a solid surface for various applications.

Claims

CLAIMS1. A composition for imparting antimicrobial activity to a plastic material comprising: i) a thermoplastic or thermoplastic elastomer polymer / plastic; and ii) an antimicrobial agent, wherein the antimicrobial agent consists essentially of potassium metabisulphite in a solid or dry form and the composition has a substantially neutral pH.

2. The composition of claim 1 , wherein the plastic material comprises the same material as the thermoplastic or thermoplastic elastomer polymer / plastic.

3. The composition of claim 1 or claim 2, the thermoplastic or thermoplastic elastomer is selected from one or more of polyolefins and copolymers thereof; polypropylene (PP); polyethylene including ultra-high molecular weight polyethylene (UHMWPE), high density polyethylene (HDPE), medium density polyethylene (MDPE), linear low density polyethylene (LLDPE), and low density polyethylene (LDPE); polyesters like poly(lactic acid) (PLA) and polycaprolactone (PCL); polyoxymethylene (POM); polyvinyl chloride (PVC); acrylics including ester derivatives (PAc) thereof; poly(methyl methacrylate) (PMMA) or acrylic; ethylene-vinyl acetate (EVA); polyvinyl alcohol (PVA); acrylonitrile-butadiene-styrene (ABS); acrylonitrile styrene acrylate (ASA); styrene acrylonitrile resin (SAN); thermoplastic polyurethanes (TPU); styrenic block copolymers (TPS); thermoplastic polyolefinelastomers (TPO); thermoplastic vulcanisates (TPV); thermoplastic copolyester (TPC); thermoplastic polyamides (TPA); unclassified thermoplastic elastomers (TPZ); and combinations thereof.

4. The composition according to any one of claims 1 to 3, wherein the polymer / plastic is present in the composition in an amount of between about 50 wt% and about 99.5 wt%, between about 80 wt% and about 99.5 wt%, between about 90 wt% and 99.5 wt%, or between about 95 wt% and 99.5 wt%.

5. The composition according to any one of claims 1 to 4, wherein composition has an active concentration of 0.5 wt% to 60 wt%, 5 wt% to 60 wt%, 0.5 wt% to 30 wt%, less than 20 wt%, less than 10 wt%, 5 wt% or less, between about 0.5 wt% to about 5 wt% of the composition.

6. The composition according to any one of claims 1 to 5, wherein potassium metabisulphite is present in the composition in a powdered form.

7. The composition according to any one of claims 1 to 6, wherein the polymer / plastic is present in the composition in a powdered form.

8. The composition according to any one of claims 1 to 7, wherein the composition is formulated as a masterbatch for incorporation to the plastic material, or a polymer binder for application to a surface of the plastic material9. The composition according to claim 8, wherein the polymeric concentrate or masterbatch has an active concentration of 1 % to 60%, or 5% to 40% by weight of the composition, or the polymer binder has an active concentration of about 0.1% to about 10% based on solution % solids.

10. The composition according to any one of claims 1-9, wherein the composition consists essentially of a dry powdered thermoplastic or thermoplastic elastomer polymer / plastic and dry powdered potassium metabisulphite.11 . The composition according to claim 10, wherein the polymer / plastic is present in an amount of around 70 wt% and the potassium metabisulphite is present in an amount of around 30 wt%.

12. Use of a composition as claimed in any one of claims 1 to 11 either as a masterbatch for incorporation into a plastic material, or as a coating for application to a surface of a plastic material.

13. A method for microbial control on a plastic material surface, the method comprising either i) incorporating into the plastic material, or ii) applying onto a surface of the plastic material, a composition as claimed in any one of claims 1 to 11 comprising: i) a thermoplastic or thermoplastic elastomer carrier; and ii) an antimicrobial and / or antioxidant agent, wherein the antibacterial and / or antioxidant agent consists essentially of potassium metabisulphite in a solid or dry form and the composition has a substantially neutral pH.

14. The method according to claim 13, wherein the carrier and potassium metabisulphite are combined as powders before being heated and extruded into granules, mini beads, pellets or a powder.

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