The plastics industry has extremely stringent requirements for additives: they not only need to efficiently solve processing problems but also must possess characteristics such as high temperature resistance, chemical inertness, and non-damage to the mechanical properties of plastics. Molecular sieves' extremely high thermal stability, deep drying capacity, selective adsorption, chemical inertness, and non-toxic safety can effectively meet these requirements.
Working Principle of Zeolite Molecular Sieve
Molecular sieves mainly rely on their polar adsorption and sieving effects. Water molecules or other polar molecules are adsorbed by cations within the pores. Due to the huge specific surface area inside molecular sieves, their adsorption capacity far exceeds that of ordinary desiccants. Furthermore, the pore size distribution of molecular sieves is extremely uniform (0.3nm-1.0nm). Only molecules with a kinetic diameter smaller than the pore size of the molecular sieve can enter the pores and be adsorbed. For example, 3A molecular sieves only adsorb water molecules, while blocking larger propylene or ethylene monomers.
Selection of Molecular Sieves
In the plastics industry, the selection of molecular sieves is mainly based on two dimensions: the molecular size of the target adsorbate and the processing temperature and matrix type of the plastic.
For water removal only: zeolite 3A molecular sieve is the first choice. Using zeolite 4A molecular sieve or zeolite 13X molecular sieve may adsorb plasticizers, antioxidants, or monomers from the resin, leading to formulation imbalance.
For deodorization/monomer removal: Choose 13X or NaY zeolite, utilizing their large pores to adsorb volatile organic compounds.
Specific Application Scenarios
Monomer and Solvent Purification in the Polymerization Stage
In the Ziegler-Natta catalytic polymerization process of polyolefins (PE/PP), the catalyst is extremely sensitive to poisons. 3A molecular sieves are widely used to dry monomers such as ethylene, propylene, and butene, as well as comonomers, ensuring that the moisture content of the raw materials is below 5 ppm.
Water Hydrolysis Protection for Engineering Plastics
Pyropolymers such as polyester (PET), polyamide (nylon), and polycarbonate (PC) are highly susceptible to hydrolysis when exposed to water in the molten state, resulting in a sharp drop in molecular weight and loss of mechanical properties.
Plastic Recycling and Regeneration
In the post-consumer plastics (PCR) recycling process, waste plastics typically contain residual oil, low-molecular-weight degradation products, and malodorous substances. By using 13X or NaY molecular sieves as highly efficient adsorbents and combining them with the vacuum exhaust system of a twin-screw extruder during the regeneration granulation process, the purity and color of the recycled material can be significantly improved, achieving high-value recycling.
