1 METHODG) FOR PRODUCING MI CROPLAST IC FRAGN MENI'S Field of invention 5 The invention relates to a method for producing microplastic fragments. Background Plastic is an environmental pollutant as it degrades slowly, and can enter aquatic 10 systems and the food chain. Over time, large fragments (macroplastics) are gradually broken into smaller fragments (microplastics) which are generally less than a millimetre in diameter and may be only a few microns. Another source of microplastics is products such as facial and body cleansers, e.g. exfoliating scrubs, face exfoliators, and body exfoliators. 15 However, in order to test the effects of microplastics on the environment, a reliable source is required for the corresponding research. Unfortunately it is not possible to simply manually pulverize fragments into very small 20 particles due to the elasticity of the plastic which prevents such action. One known method of producing microplastics is to sieve them out from the environment using filtration and/or electrostatic charge as for example described in CN103304077 or US20140138288. However, these microplastics are contaminated due 25 to their exposure to the environment, which could affect the results of any subsequent research using the same. It is also possible to synthesise microplastics, e.g. in the forn of beads, but this is very expensive, and tend to be available on the market for only certain polymers. 30 An aim of the invention therefore is to provide a more cost-effective method for producing microplastics for use in research, where the microplastics are free from environmental contamination.
2 Summary of Invention In an aspect of the invention, there is provided a method for producing microplastic 5 fragments comprising the steps of providing a source of plastic pieces; cooling the plastic pieces until they are friable; grinding the plastic pieces into fragments; sifting the fragments with a coarse sieve to separate small fragments therefrom; 10 mixing the small fragments with a liquid to form a mixture; sifting the mixture through one or more fine sieves to separate microplastics therefrom; and centrifuging at least part of the sieved mixtures to create a pellet and a supernatant, each of which are dried to form microplastic fragments. 15 In one embodiment the plastic pieces may include one or more virgin or recycled polymers including Polyethylene Terephthalate (PET), [High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), Polyvinyl Chloride (PVC), Low-Density Polypropylene (LDPP), High-Density Polypropylene (HDPP), and Polystyrene (PS). 20 Advantageously this method can therefore be used to produce a wide range of microplastic materials and sizes, for a much lower cost than the synthetic beads, and where the microplastics are free from contaminants. 25 In one embodiment the plastic pieces are cooled with a cooling liquid such as liquid nitrogen. This makes the plastic pieces friable i.e. fragile and inelastic, so that they can be ground into fragments. In one embodiment the coarse sieve has pores of around 1000 microns in diameter, but 30 it will be appreciated that any suitably sized sieve may be used to separate out small fragments from large pieces.
3 in one embodiment the liquid is an organic solvent to which the plastic is resistant. Typically the organic solvent is ethanol or methanol. In an alternative embodiment the liquid is distilled water. Advantageously the liquid prevents clumping of the small fragments, improving homogeneity and allowing plastic particles i.e. microplastics to be 5 separated therefrom. It will be appreciated by the person skilled in the art that any liquid could be used, the important criteria for the choice of liquid being: firstly compatibility of the plastic material with the liquid; and secondly the density of the plastic material, which should be higher than the solvent so fragments sink rather than floating on the surface and clumping. 10 In one embodiment the ratio of small fragments to liquid ranges from about 1:5 (w/v) to 1:60 (w/v), preferably from about 1:10 (w/v) to 1:50 (w/v), more preferably from about 1:20 (w/v) to 1:40 (W/v) 15 In one embodiment fine sieves with progressively smaller pores are used to separate different sizes of microplastics. In one embodiment the fragments and/or mixture are sifted through the same sieve several times. Advantageously this improves homogeneity of the fragments retained by 20 each sieve, as it reduces the number of irregular shaped particles that may have been passed through the pores the first time, but been retained by the sieve another time. In one embodiment the plastic pieces are cooled by dipping into the cooling liquid. In an alternative embodiment the cooling liquid is added to the grinder to cool the plastic 25 pieces before being ground. In one embodiment the mixture is stirred to reduce clumping of small fragments. In one embodiment the sieved mixture is allowed to settle to separate into a pellet and 30 supernatant. In one embodiment at least part of the sieved mixture, typically the post-settlement supernatant, is centrifuged at around 200rpm for about 2 minutes.
4 Typically the pellet and/or supernatant are dried using an oven and/or a rotary evaporator. 5 Brief Description of Drawins It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the 10 accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. Figure 1 illustrates a method for producing microplastic fragments according to an embodiment of the invention. 15 Detailed Description With regard to Figure 1, there is illustrated a method for producing microplastic fragments from different polymer types including virgin and recycled Polyethylene 20 Terephthalate (PET), High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), Polyvinyl Chloride (PVC), Low -Density Polypropylene (LDPP), High-Density Polypropylene (FIDPP), and Polystyrene (PS). The method starts with placing plastic pieces 2 (e.g. resin granules) in a sieve made of 25 stainless steel or other cold-resistant material and then dipping 4 the sieve in a cooling liquid such as liquid nitrogen for a few seconds. If the grinder is resistant to cold (e.g. made of stainless steel) then the liquid nitrogen may be placed directly therein with the plastic pieces, instead of dipping the pieces in a 30 sieve. Thereafter, the fragile plastic parts are immediately ground 6 using an electric grinder. A mortar and pestle may alternatively be used to grind the plastic pieces.
5 The resulting ground plastic fragments are sifted 8 through a 1000 pm porous layer (coarse sieve) to separate small fragments (diameter <1 mm) from the bigger fragments (diameter > mm). The previous steps may be repeated 10 several times for the bigger 5 fragments. The small fragments are mixed 12 with a liquid at a proportion of 1:20-40 (w/v) then optionally stirred 14 using a magnetic stirrer for 15 min. This ratio is optimised to avoid wasting large amounts of liquid and having to dry the same, while being sufficient to 10 prevent clumping of the fragments to promote sieving thereof. The liquid used depends on the plastic, but is typically an organic solvent such as methanol or ethanol. Distilled water may be used for some plastics (e.g. for HDPE), unless they float (e.g. LDPE). 15 Depending on target sizes, fragments are filtered 16 through one or more porous layers (fine sieves). Several sieves may be placed on top of each other in a superposed arrangement, the top sieve having the largest pores, the bottom sieve having the smallest pores. The suspension is loaded onto the top sieve (preferably in a dropwise manner 20 using a burette) and the mobile liquid phase carries the fragments onto the bottom porous layers (sieves), facilitating separation. After sorting, fragments are collected from each sieve, then resuspended in the carrier liquid and passed through the fine sieves as described above. Typically, the fine sieving 25 process is repeated 18 three to five times. The repetitions help to homogenise the small fragments in each fine sieve, and the fragments are then collected 20 and dried in an oven to form a product comprising microplastic fragments size A(1-n), where A represents the procedural step from which 30 the fragments are derived, and n is the number of fine sieves. The particle sizes correspond to the sieve pore size, e.g. Al may contain microplastic fragments with a diameter of >60 ptm, A2 a diameter of 46-60 ptm, and A3 a diameter of 32-46 pm.
6 Drying is usually conducted at a temperature of <120 'C, preferably lower such as around 50 'C, to avoid clumping of the fragments. The carrier liquid which holds smaller particles is allowed to settle 22 down for a few 5 minutes until it separates into two phases (pellet and supernatant). The pellet is dried 24 in an oven to form a product comprising microplastic fragments size B (typically having a diameter of 25-32 pm) and the supernatant is centrifuged 26 at around 200 rpm for about 2 min. 10 The resulting pellet and supernatant are separately collected 28, 30 respectively and dried in an oven or rotary evaporator to respectively form products comprising microplastic fragments size C and D. Fragments size C is typically 20-25 [tm, whereas fragments size D is <20 tm, usually averaging around 10pum. 15 It will be appreciated by persons skilled in the art that the fragment sizes A-D indicated above are for example only, as the dimensions could vary according to the type of material and the pore size of the sieves. 20 It will be appreciated by persons skilled in the art that the present invention may also include further additional modifications made to the system which does not affect the overall functioning of the system.