Plastic is a kind of polymer - resin as the basic component, and then add some used to improve its performance of various additives made of polymer materials. At present almost all food, no matter how much, is packed in plastic. As a rising star of packaging materials, plastic packaging material has become the fastest developing packaging material in the world in recent 40 years because of its rich raw materials, low cost, excellent performance, light weight and beautiful characteristics.

The safety of plastic packaging materials is mainly reflected in the migration and dissolution of toxic and harmful substances left inside the material, which leads to food pollution. The main sources are as follows:

1.1 The resin itself has certain toxicity; Unpolymerized free monomers, cracking substances (vinyl chloride, styrene, phenols, nitrile gum, formaldehyde), degradation substances and aging toxic substances in the resin have an impact on food safety. The US Food and Drug Administration has banned PVC products as food packaging materials because of the carcinogenic potential of oral ingestion of vinyl chloride (VCM), not PVC itself, but residual vinyl chloride (VCM) in PVC. Polyvinyl chloride (VCM), a free monomer of polyvinyl chloride (VCM), has an anesthetic effect and can cause pain by constricting blood vessels in human limbs. It also has carcinogenic and teratogenic effects. It forms vinyl oxide in the liver and has a strong alkylation effect, which can combine with DNA to produce tumors [1]. The residual substances in polystyrene, such as styrene, ethylbenzene, toluene and isopropylbenzene, are harmful to food safety. Styrene can inhibit the birth of rats and reduce the weight of liver and kidney. Low molecular weight polyethylene dissolves in oil to produce preserved flavor, which affects product quality. The raw material of polycarbonate resin used for making milk bottles produces phenol, which has certain toxicity and produces peculiar smell. The impact of these harmful substances on food safety depends on the concentration of these substances in the material, the tightness of the combination, the nature of contact with the material, time, temperature and solubility in food, etc. [2].

1.2 Surface pollution of plastic packaging. Because plastic is easy to charge, easy to absorb dust impurities and microorganisms, the formation of food pollution.

1.3 Toxicity of stabilizers, plasticizers, colorants and other additives added to plastic products in the manufacturing process. Food packaging commonly used plastic PE(polyethylene), PP(polypropylene) and PET(polyester), because the processing process of additives use less, the resin itself is relatively stable, their safety is very high.

1.4 Food pollution caused by a large number of toxic additives, heavy metals, pigments and viruses in illegally used recycled plastics. Recycling of plastic materials is a general trend, because of the complexity of recycling channels, recycling containers often residual harmful substances, it is difficult to ensure complete cleaning treatment. Some in order to cover up the quality defects of recycled products, often add a large amount of paint, resulting in the residue of paint pigment, resulting in food pollution. For regulatory reasons, even a large amount of medical waste plastic is recycled, which poses a risk to food safety. The state stipulates that recycled polyethylene shall not be used to make food packaging materials.

1.5 Ink pollution. Ink can be roughly divided into benzene ink, benzene - free ink and alcohol ink, water-based ink and other types. The main material in ink pigments, resins, additives and solvents. Ink manufacturers often consider the impact of resins and additives on safety, while ignoring the indirect harm of pigments and solvents on food safety. Domestic small ink manufacturers even use dyes to replace pigments for ink production, and dye migration will seriously affect the safety of food; In addition, some ink to improve the fastness to adhesion will add some promoters, such as siloxane substances, such substances will be in a certain drying temperature group bond fracture, methanol and other substances, and methanol will harm the human nervous system. Printing ink on plastic food packaging bags, because benzene and other toxic substances are not easy to volatilize, the impact of food safety is greater. In recent years, all over the plastic food packaging bag sampling rate is generally low, only 50% ~ 60%, the main unqualified items are benzene residue exceeded standards, and the main reason for benzene exceeded standards is in the process of plastic packaging printing in order to dilute the use of solvents containing benzene. In July 2005, "Weekly Quality Report" reported that CCTV reporters in Gansu, Qinghai, Zhejiang, Jiangsu, four provinces of more than a dozen different scales of plastic color printing enterprises survey found that because of the low price of toluene, enterprises in order to quickly print thick ink on the plastic film, it is used as the main raw material for mixing solvents. Lanzhou quality inspectors random spot check the seven plastic color printing in the production of composite food packaging film, sent to the product quality inspection center in gansu province and the national packaging products quality inspection center testing, the results showed that 7 samples with 5 benzene residue was detected to exceed bid, dried beef, milk powder, candy, halogen bean curd, potato chips, five kinds of food packaging.