Recycling to promote the harmony between plastic p

2022-06-25
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Recycling promotes the harmony between plastic packaging materials and the environment (VI)

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 has various processes, technologies, characteristics and evaluations. However, it has been proved by practice that the best way to recover polymer is material recovery, whether it is single component or uniform mixing, or polymer materials with known composition or low hybrid procedure. Material recovery method is the most efficient way to recover energy. Assuming that the recycled material can replace the new material in the ratio of 1:1, the "substitution coefficient" is set to 1, which means that the recycled material can meet the performance standard of the new material. In this case, the material recovery method consumes less energy than other recovery methods, and the exhaust emission is also very low. These advantages of the material recovery method will decrease with the decrease of the substitution coefficient

5.4.1 sorting technology of plastic wastes

there are many kinds of plastics. Their production raw materials are different, and the products after waste degradation are also different. The mixing of different impurities has different effects on the performance after recycling. The differences in physicochemical properties and incompatibility of various plastics greatly affect the processing performance of the directly recycled mixture. In order to improve the utilization value of recycled products, the collected waste plastics are generally classified and screened first, and then treated with different recycling technologies according to different materials and different requirements. In the past 20 years, the classification and separation of waste plastics mainly focused on five main plastics, such as PE, PP, PVC and PS, and polyethylene terephthalate (PET). The computer automatic sorting system has been developed abroad to realize the continuous automation of the sorting process. In China, the most primitive manual selection method is still the main method, relying on manual selection, which has low efficiency and high labor intensity. The Swiss bueher company can separate PE, PP, PS, PVC and pet waste plastics with a pc386 computer, and the separation capacity is 1t/h. The density of PE and PP is less than 1.0g/cm3, the density of PVC and pet is more than 1.3g/cm3, and the density of PS is between 1.0~1.1g/cm3. Therefore, flotation can be carried out according to the density of plastics. PE and PP can be separated from other plastics with water as the medium. However, PE and PP are difficult to be separated by gravity separation because of their similar density. The one-time separation rate of the small suspension separation device of Japan Plastics processing promotion association can reach more than 99.9%. Dow Chemical Company of the United States has also developed a similar separation technology to separate mixed waste plastics with liquid hydrocarbons instead of water, and achieved better results. Kellogg and Rensselaer Institute of comprehensive technology jointly developed the solvent selective separation and recovery technology, which can separate the mixed waste plastics without manual sorting. The shredded waste plastics are added to a certain solvent, and the solvent can selectively dissolve different polymers at different temperatures to separate them. Tokyo Metropolitan University has successfully developed a new technology for high-efficiency separation of waste plastic sheets. The waste plastic sheets in the water tank are strongly magnetized. Due to different magnetic fields of different plastic belts and different floating and sinking depths, separation occurs. In addition, some new separation technologies such as electromagnetic rapid heating and reactive blending have also been reported. The electromagnetic rapid heating method can recover and separate metal polymer components, and the reactive blending method can recover and separate the waste bumper with coating layer

5.4.2 additives to improve the quality of recycled materials

when recycled materials are reused, only a small amount of recycled materials such as leftover materials (5%) will be added to the new materials. Product waste is processed under appropriate conditions, and its performance will not be damaged with sufficient stabilizers. However, the addition of high proportion of waste materials or the mixing of waste materials after consumption will have a significant impact on the properties of materials, such as affecting the melt flow index (MFI) and damaging the anti-aging performance. It is usually not cost-effective to add a large amount of waste to a small amount of new material to prevent melt stagnation or adhesion. At present, great efforts are being made to use recycled materials and additive formulations, which can be more advantageous in terms of cost and performance than the mixing of new materials and recycled materials

additives play an important role in the utilization of recycled materials, especially when preparing to replace new materials with recycled materials, additives can improve the quality of recycled materials, open up new application fields and effectively improve the substitution coefficient. In order to prepare recycled materials with different components to a unified technical standard, a series of additives with different specifications have been developed, such as polymer additives, enhancers and functional additives (see Table 3). In addition, compatibilizers and stabilizers for polymer recycling materials have been specially developed. Compatibilizers and stabilizers are also the most widely used additives in practice, so they have received full attention

compatibilizer is used to improve the mechanical properties of the mixture. Different kinds of polymers usually do not form homogeneous mixtures, but only exist in multiphase systems, including continuous and dispersed phases. The low adhesion between the continuous phases will reduce the mechanical properties of this incompatible mixture. Compatibilizer can destroy the interphase interface and make it produce adhesion. Its use concentration is usually greater than 5% to produce satisfactory results

the use of compatibilizer depends on the chemical structure of the application object. We should also consider that some mechanical properties restrict each other. For example, compatibilizers can increase the impact strength, but at the same time reduce the flexural strength and rigidity. In addition, the use of compatibilizers will have a long-term negative impact on some properties, because some groups in the development of rail welding material processing technology have thermal sensitivity and photooxidation effects, which should be taken into account when designing the formula. It is difficult to separate layered materials such as pe/pa or pe/pet by general methods, and compatibilizers are most suitable for the treatment of layered recycled materials, where their active groups can play a good role

simply adding reinforcing fiber and filler into the plastic body can greatly improve its overall performance, especially its mechanical performance. The application of fillers is also to reduce costs. When using fillers, their adsorption and desorption and impurities should be considered, which will have a long-term impact on the properties of plastics, so stabilizers should be used at the same time. In order to improve the quality of recycled materials, long bundles of glass fiber reinforced thermoplastic materials can be added. This can compensate for the degradation of material properties caused by the destruction of original glass fibers in the regeneration process

Table 3 additives to improve the quality of recycled materials

normal> polymer additives

normal> new materials

normal> modifier (impact modifier)

normal> compatibilizer

normal> fortifier

normal> fiber (glass fiber, cellulose fiber)

normal> fillers (talc, wollastonite, calcium carbonate, wood flour)

normal> functional additives

normal> Color material

normal> processing auxiliary material (lubricant)

normal> stabilizer (processing, heat and light stabilizer)

normal> reactive molecules (for increasing molecular weight)

the test shows that organic pigments can be used for coloring and color matching. For example, carbon black can be used to cover uneven colors. Carbon black also has good light stability, but has a significant adverse effect on the long-term heat resistance of the material. Therefore, the carbon black concentration in the formula and the selection of stabilizers should be considered

the rheological properties of the recycled material can be improved by adding additives and lubricants and changing the flow rate. For polyolefin recovery 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

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