Recycling: Promoting the Harmonious Development of Plastic Packaging Materials and the Environment (7)

5.4 New progress in recycling technology Due to ecological and economic reasons, the recycling of plastic waste (also known as recycling) has become the focus of global attention. Plastic recycling process is diverse, technology varies, features vary, and evaluation varies. However, it has been proved that the best method for recovering the polymer, whether it is single component or homogeneous mixing, or known or low-mixed polymer materials, is the material method recycling. The material recycling method is the highest energy recovery method. Assuming that the recycled material can replace the new material by a 1:1 ratio, the “substitution coefficient” is set to 1, which means that the recycled material can meet the performance standards of the new material. Under such circumstances, the material recycling method consumes less energy than other recycling methods, and the exhaust emissions are also low. These advantages of the material recycling method will decrease as the substitution coefficient decreases.

5.4.1 sorting technology of plastic waste There are many kinds of plastics, and their production raw materials are different, and the products after the waste degradation are also different. The impact of different impurities on the performance after recycling is not the same, all kinds of plastic The differences in physical and chemical properties and incompatibility have greatly affected the processing performance of the directly recycled mixture. In order to increase the use value of recycled products, the collected waste plastics are generally categorized and screened first, and then treated according to different materials and different requirements using different recycling technologies. In the past 20 years, the separation and separation of waste plastics has mainly focused on five main plastics such as PE, PP, PVC and PS, and polyethylene terephthalate (PET). A computer automatic sorting system has been developed abroad to achieve continuous automation of the sorting process. Our country still adopts the most primitive manual selection method, relying on manual sorting, which is inefficient and labor-intensive. The Bueher company in Switzerland uses four kinds of filters to identify the halogen lamp as a strong light source. The PC386 computer can separate PE, PP, PS, PVC and PET waste plastics with a separation capacity of 1t/h. The density of PE and PP is less than 1.0g/cm3, the density of PVC and PET is greater than 1.3g/cm3, and the density of PS is between 1.0~1.1g/cm3. Therefore, flotation can be performed according to different plastic densities. PE and PP are separated from other plastics, but because PE and PP are close in density, they are difficult to separate by re-selection. The separation rate of the small suspension separation unit of Japan Plastics Processing Promotion Association can reach 99.9%. The Dow Chemical Company of the United States has also developed a similar separation technology to replace mixed waste plastics with liquid hydrocarbons instead of water, and achieved better results. Kellogg and the Rensselaer Polytechnic Institute have jointly developed a solvent selective separation and recovery technology to separate mixed waste plastics without manual sorting, adding chopped waste plastics to certain solvents. At different temperatures, solvents can selectively dissolve different polymers and separate them. Tokyo Metropolitan University has successfully developed a new technology for the efficient sorting of waste plastic sheets, which applies strong magnets to the waste plastic sheets in the water tank. Separation occurs due to different magnetic tapes and different depths of floatation. In addition, some new separation techniques such as electromagnetic heating and reactive blending have also been reported. Electromagnetic rapid heating method can recover and separate metal-polymer components, and reactive blending method can realize the recovery and separation of waste bumpers with paint layer.

5.4.2 Additives to improve the quality of recycled materials When recycled materials are reused, only a small amount of recycled material (<5%), such as scrap, is generally added to the new material. Product wastes are processed under moderate conditions and, together with sufficient stabilizers, their properties are generally not damaged. The addition of a high percentage of waste or the incorporation of post-consumer waste materials can have a significant effect on the properties of the material, such as affecting the melt flow index (MFI) and destroying anti-aging properties. Adding a large amount of waste to a small amount of new material is usually not cost-effective. At present, the use of recycled materials and additive formulations is vigorously pursued, which can be more advantageous in terms of cost and performance than new materials and recycled materials.

Additives play an important role in the use of recycled materials. In particular, when preparing fresh materials instead of recycled materials, additives can improve the quality of recycled materials, open up new fields of application, and effectively increase the substitution coefficient. In order to prepare different recycled materials with uniform technical standards, a series of additives of different specifications, such as polymer additives, reinforcing agents and functional additives, have been developed (see Table 3). In addition, compatibilizers and stabilizers for polymer recycling materials have also been developed in particular. Compatibilizers and stabilizers are also the most widely used additives and are therefore given full attention.

The role of the compatibilizer is to improve the mechanical properties of the mix. Different kinds of polymers usually do not form a homogeneous mixture, but only exist in heterogeneous systems, including continuous and dispersed phases. The low adhesion between the continuous phases will reduce the mechanical properties of this non-compatible mixture. Compatibilizers can break the interface between phases and make them adhesive. Its use concentration is usually greater than 5% to produce satisfactory results.

The use of compatibilizers depends on the chemical structure of the application subject. Also consider that some mechanical properties are mutually exclusive. For example, compatibilizers increase the impact strength but at the same time they reduce the flexural strength and rigidity. In addition, the use of compatibilizers can have long-term negative effects on some properties because some of these groups have thermal and photo-oxidative effects that should be taken into account when designing formulations. It is difficult to separate the layered materials such as PE/PA or PE/PET by general methods, and the compatibilizer is most suitable for the treatment of the layered recyclate, and the active group can play a good role here.

Simply adding reinforcing fibers and fillers to the plastic body can greatly improve its overall performance, especially the mechanical properties. The application of fillers is also to reduce costs. The use of fillers should take into account their adsorption and desorption and impurities, which can have a long-term effect on the properties of the plastic. Therefore, stabilizers must be used at the same time. In order to improve the quality of the recycled material, it is possible to add a long bundle of glass fiber-reinforced thermoplastic material. This can compensate for the deterioration of the material properties caused by the destruction of the original glass fiber during the regeneration process.

Table 3 Additives to Improve the Quality of Recycled Materials

Normal > Polymer additives

Normal > New material

Normal > Modifier (impact modifier)

Normal > compatibilizer

Normal > fortifier

Normal > fiber (glass fiber, cellulose fiber)

Normal > filler (talc, wollastonite, calcium carbonate, wood flour)

Normal > Functional additives

Normal > color

Normal > Processing Accessories (grease)

Normal > Stabilizer (process, heat, light stabilizer)

Normal > reactive molecules (used to increase molecular weight)

Experiments show that organic colorants can be used for coloring and toning. For example, carbon black can be used to cover uneven colors. Carbon black also has good light stability, but it has a significant adverse effect on the long-term heat resistance of the material, so the carbon black concentration in the formulation and the selection of the stabilizer must be taken into consideration.

By adding auxiliaries and lubricants and changing the flow rate, the rheological properties of the recycled material can be improved. For polyolefin recycled materials, low molecular weight polyolefins can be added and the melt viscosity can be reduced. However, increasing the amount of additives will reduce the mechanical properties. Larger amounts will also cause incompatibility, so the effect of the adjustment is limited.