Characteristic and Application of Common Plastic – Part 2
PVC
PVC stands for Polyvinyl Chloride, which is obtained from the polymerization process of the vinyl chloride monomer, VCM. These are initiated upon adding initiators like peroxides or azo compounds, while others experience free radical polymerization by light or heat. PVC is a homopolymer and a copolymer of vinyl chloride, and it is known to be non-crystalline. In practical use, PVC usually contains an addition of stabilizers, lubricants, processing aids, colorants, impact modifiers, and other additives. It has flame resistance, high strength, weather resistance, and excellent geometric stability. PVC is highly resistant to oxidizers, reducers, and strong acids. However, it can be corroded by concentrated oxidizing acids such as sulfuric acid and nitric acid, and shall not be suitable for contact with aromatic hydrocarbons or chlorinated hydrocarbons.
The bright colors, resistance to corrosion, and durability mainly originate from pure polyvinyl chloride. However, PVC products are usually not used for storing food and drugs because, in the processing of PVC, many types of toxic plasticizers and aging inhibitors will be added to make it more resistant to heat, improve its toughness, and enhance its flexibility. It is quite popular with synthetic materials of high usage globally, only ranking second to other synthetic materials. In 1995, the quantity of PVC produced in Europe was approximately 5 million tons, while the consumed amount was 5.3 million tons. The average production and consumption of PVC in Germany were approximately 1.4 million tons. The growth rate for PVC is worldwide 4%. In Southeast Asia, its growth rate is huge because of the urgent requirements for infrastructure construction. Amongst the materials that can produce three-dimensional surface films, PVC is the most suitable one. The monomer structure of PVC is CH2=CHCl. In view of good processability, low manufacturing cost, resistance to corrosion, and insulation, PVC enjoys large-scale applications. It is used in PVC card manufacturing, PVC label manufacturing, PVC wire manufacturing, PVC curtain manufacturing, PVC-coated welding mesh manufacturing, PVC foam board manufacturing, PVC ceiling panel manufacturing, PVC pipes, PVC skirting boards, and wire conduits, cable insulation, plastic doors and windows, plastic bags, and more. Thus, it can be said that products made of PVC surround us in every form throughout our lives. They find application in a good number of artificial leather goods: luggage, sporting goods such as balls: Basketballs, footballs, and rugby balls, Uniform belts, but also specialized protective equipment.
Some characteristics of PVC during burning are: difficult to ignite, extinguished if it is taken out from flame, produces a yellow flame with white smoke, and has a smell similar to chlorine, irritating when burning. Different additives can change the physical and mechanical properties of PVC resin, which is a multi-component plastic. Normally, a number of advantages are among the main features of PVC products: chemical stability, flame retardancy, abrasion resistance, the ability to dampen sound and vibration, high strength, good electrical insulation, low cost, wide availability of material, and good airtightness. Bad thermal stability and easy aging under light, heat, and oxygen are disadvantages. PVC resin itself is non-toxic, and if combined with non-toxic plasticizers and stabilizers, the resultant products are harmless to human and animal contact. However, most PVC products in the market use toxic plasticizers and stabilizers, so unless a product is specifically marked as non-toxic, it should not be used to store food.
Physical Properties
PVC resin is an amorphous thermoplastic plastic. Under ultraviolet light, hard PVC radiates a pale blue or violet-white fluorescence; soft PVC emits blue or blue-white fluorescence. At 20°C, its refractive index is 1.544 and the specific gravity is 1.40. Products with plasticizers and fillers normally have a density ranging from 1.15 to 2.00. Soft PVC foam plastic ranges in density from 0.08 to 0.48. Hard foam plastics range from 0.03 to 0.08. The water absorption rate for PVC shall not be more than 0.5%. The physical and mechanical properties of PVC depend on the molecular weight of the resin, plasticizer content, and contents of fillers. Molecular weight: Higher molecular weight improves mechanical properties and increases cold and thermal stability resistance, but on the other hand, it raises processing temperature and makes molding difficult; lower molecular weight has reverse effects. Increasing filler content reduces tensile strength.
Thermal Properties
The softening point of PVC resin is close to its degrading temperature. Decomposition begins around 140°C and increases drastically above 170°C. In light of the fact that PVC resin must be processed with proper molding, there are two critical process indicators: decomposition temperature and thermal stability. Temperature for decomposition refers to a breakdown with a large amount of hydrochloric acid emitted while thermal stability refers to a time when a large quantity of hydrochloric acid is not emitted at a certain temperature usually 190°C. PVC plastic exposed to 100°C for extended periods will decompose unless alkaline stabilizers are added and it will decompose rapidly if temperatures exceed 180°C. Long-term use over 55°C should be avoided with most PVC plastic products; specially formulated PVC can go up to 90°C. Soft PVC products become rigid at low temperatures. Due to the presence of chlorine atoms in PVC molecules, it is generally flame-resistant and self-extinguishing without dripping.
Stability
Being basically an unstable material, PVC resin tends to decompose upon exposure to light and heat, which is accompanied by hydrochloric acid release and structural changes; however, the degree is relatively minor. A combined action of mechanical forces, oxygen, odor, HCl, and some active metal ions can accelerate the decomposition. In the process, after HCl is removed, conjugated double bonds form on the main chain, ultimately changing the color of the resin. Released amounts of hydrochloric acid in increased portions can make PVC resin turn from white to yellow, rose, red, brown, or even black.
Electrical Properties
The electrical properties of PVC are a function of the amount of residual materials in the polymer and the kind and amount of additives in the formulation. Heating also has some effects on the electrical properties of PVC. In the course of the heat decomposition of PVC, because of the existence of chloride ions, its electrical insulation will be reduced. If a large number of chloride ions cannot be consumed by alkaline stabilizers (such as lead salts), it will result in a sharp drop in electrical insulation performance. Unlike non-polar polymers, including polyethylene and polypropylene, the electrical performance of PVC changes with frequency and temperature; for example, the dielectric constant decreases with increasing frequency.
Chemical Properties
PVC is very stable chemically and is highly valued as a corrosion-resistant material. PVC is stable to most inorganic acids and bases and does not dissolve when heated but decomposes to release hydrochloric acid. It reacts with potassium hydroxide to produce the brown insoluble unsaturated product. The solubility of PVC depends on molecular weight and the method of polymerization. Usually, an increase in molecular weight is correlated with reduced solubility, and the resins prepared by the emulsion method are less soluble than those prepared by the suspension method. Some of the various solvents in which PVC can dissolve include ketones (e.g., methyl ketone, cyclohexanone), aromatic solvents such as toluene or xylene, dimethylformamide, and tetrahydrofuran. At room temperature, PVC resin is practically insoluble in plasticizers, while at raised temperatures, it swells to a big extent and even dissolves.
Processing Properties
PVC is an amorphous polymer with no distinct melting point. It becomes plastic at temperatures between 120-150°C. Due to poor thermal stability, it loses small amounts of HCl at this temperature, which favors further decomposition; thus, alkaline stabilizers and HCl scavengers have to be added to suppress catalytic cracking. Pure PVC is rigid in nature, and a sufficient amount of plasticizers must be added to make it flexible. Other additives, which include UV absorbers, fillers, lubricants, pigments, and antifungal agents, are required to attain optimum performance of the PVC products. The quality and processing conditions of the product, like in other plastics, are decided by the properties of the resin. In the case of PVC, some processing-related resin properties include particle size, thermal stability, molecular weight, fisheyes, bulk density, purity, foreign impurities, and porosity. In PVC paste, the viscosity and gelation properties of the paste must also be measured for easy control of the processing conditions and product quality.
Characteristic Molding
It is an amorphous material with low moisture absorption and bad fluidity. That is to say, to improve the fluidity of plastics, they can be pre-dried, and to avoid the formation of air bubbles in the plastic, pre-drying can be applied. The mold gate system is supposed to be short and coarse; there should be large gate cross-sections; and also there must not be any dead corners. Mold must be cooled and chrome-plated. Because it has corrosive characteristics and flow, specialized equipment and molds are rather preferred. For all products, different kinds and amounts of additives should be added according to need. Readily decomposed. Specifically readily decomposed in contact with steel or copper at 200°C with liberation of corrosive and irritating gases. Narrow temperature range for molding. With screw injection machines the nozzle aperture should be large to avoid stagnation of the material. Best without inserts but if they are present, then they must be preheated. PVC stands for Polyvinyl Chloride. It is a flame-retardant material, chemically stable, which generates only low tensile stress. Some of the material properties include high strength, high rigidity, and high hardness. Working temperature range: -15°C to 60°C. The material can be bonded and welded.
Key Properties of PVC:
- Corrosion Resistance: Best suited for corrosion-resistant equipment in the chemical industry.
- Ease of Processing: Easy to cut, weld, and bend.
- High Strength, Rigidity, and Hardness; Good electrical insulation; Fire Retardancy - Chemical Stability; Low Water Absorption makes it easy to bond and paint; Low Cost.
Applications:
- Construction: Plumbing, electrical conduit pipes, floor tiles.
- Artificial Leather: Electronics - cables and wires.
- Industrial: Packaging materials, coating, and film.
- Consumer Goods: Toys, household products.