Polymers are widely used in many products, from nylon stockings to commercial airplanes and artificial heart valves. PVC is one of the polymers most resistant to chemicals, but its strength varies due to the use of additives for processing and improving properties. Thermoplastics are polymers that soften and flow when heated and harden again when cooled, making it possible to reprocess during manufacturing or recycling after use. Of the four main monomers used in thermoplastics (ethylene, propylene, styrene and vinyl chloride), polypropylene has grown most rapidly in recent years.
Thermoplastic polyesters, mainly PET, are growing even faster today. Blending two or more existing polymers is often done to achieve a material with a combination of unique properties, such as chemical resistance and toughness, or to reduce cost. Fusion extrusion processes are often the most convenient, economical and environmentally friendly for the manufacture of films and sheets. Process optimization requires a fundamental understanding of material properties and processing characteristics.
The manufacture of plastics is an important part of the national economy, with important innovations continuing to be introduced in processes and products. To control the chemical resistance of a plastic film during manufacturing, it is important to understand the properties of the polymer being used. Normal impact grades are generally considered to have better chemical resistance than high-impact grades because the chemical resistance of PVC is influenced by the impact modifier. Mixing, compounding and devolatilization are also often involved in processing formulations that include special additives, such as antioxidants, plasticizers, flame retardants, lubricants, pigments, fillers and other polymers. Screw extruders are at the heart of these processes and are designed with the help of extensive computer-aided modeling.
Films are formed by blowing thin-walled tube films or stretching and bending molten films. Mathematical models or simulations can be used to develop sophisticated manufacturing methods; however, much remains to be done. The most essential ingredient for successful control of chemical resistance is a fundamental understanding of material properties and processing characteristics. Extruder screws can be very sophisticated and mass and energy balances can be combined with heat, mass and momentum transfer with different levels of detail. It is also important to consider the compatibility between polymers when blending two or more existing polymers.
Process optimization requires a fundamental understanding of material properties and processing characteristics. The systems approach facilitates the rapid identification of critical material and processing parameters and helps manufacturing procedures. Future work in this area is expected to be extensive and more limited by the nature of the available physical models. A variety of anti-lock types are available to address blocking problems faced by manufacturers of polyolefin films and coatings.
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