Review of recycling of plastic packaging material waste in various countries (9)

5.4.2 Additives to Improve the Quality of Recycled Materials

When recycled material is reused, only a small amount of scrap (<5%), such as scrap, will generally be 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 in a small amount of new material is usually not cost-effective. At present, the use of recycled materials and additive formulations is vigorously pursued. It is possible to combine R&D with new materials and recycled materials in terms of cost and performance.

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 areas of application, and effectively increase the substitution coefficient. In order to prepare different recycled materials with uniform technical standards, a series of additives with 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 have therefore received sufficient 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 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 its reactive groups 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 requires consideration of 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

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, and large amounts will also lead to incompatibility, so the effect of adjustment is limited. The addition of peroxide to PP can cause degradation, and special attention should be paid to the added stabilizer.

Specific polymers such as PA, PET, PBT/PC blends, etc. can use additives to repair previously damaged parts. Increasing the molecular weight can shorten the polymer chain of the reaction product, so that the original performance can be restored. At the same time, the parameters related to the molecular weight can also be selectively improved.

Stabilizers are very important for improving the quality of recycled materials. Therefore, for the new polymer materials, special attention should be paid to preventing oxidation and photo-oxidation. When using other additives, also consider this issue. Re-stabilization can maintain the stability of the material's existing performance indicators during processing to facilitate long-term and special applications (heat or light). The amount of stabilizer added in the regrind should be adjusted according to the application and quality requirements of similar materials. Even so, the effect may not be ideal. For the wastes of food packaging materials that have been used for a short period of time, there are generally insufficient residual stabilizers and no light stabilizers, and these are necessary for the recycling of long-lasting packaging products.

Oxidation and impurity mixing can cause changes in molecular weight and material denaturation, and reduce the heat resistance and light stability of recycled materials. Therefore, the re-stabilization process should be adjusted accordingly. The post-stabilizer process has the following four advantages, which is also a concern for regrind.

·Maintain material properties during processing and reduce raw material consumption for thin-walled products;

· Under severe processing conditions (such as long production cycle or large throughput), there is also good stability, and reduce decolorization treatment;

· UV resistance, ie optical and mechanical qualities under light and outdoor conditions;

· Products that meet quality standards can obtain high added value at lower prices and high quality applications.

Most of the recycled materials that can be seen currently come from LDPE/LLDPE membranes, shrink films, HDPE containers, bottles, containers, and automotive battery housings, and will continue to grow in the coming years. Basel's Ciba Spezalitatenchemie AG based on phenolic antioxidants, co-stabilizers and hindered amines (HALS) developed the Rycyclostab and Recyclossorb product families for these materials, particularly polyolefins.

Additives can be widely used for the recycling of various plastic wastes, including packaging materials, trays, PA and PP injection molding, PET/HDPE blow molded bottles, and PS.

If the re-stabilization can reach the best condition, the quality of the recycled material can even reach the standard of new material (RAL standard). Trash cans made of recycled materials are a successful example.

In recent years, Ciba has developed a compound additive, trade name “Recycloblend 660”, which contains antioxidants, co-stabilizers, and reaction additives. These additives can directly act on plastic waste, thereby increasing its molecular weight or impurities therein. The reaction or integration, reducing its harmful effects, has been successfully used in the recovery process of filler-containing polyolefin recycled materials.

(to be continued)