What are the properties of PVC plastics and what are the chemical modification methods?


PVC plastic has a variety of properties such as physical properties, thermal properties, stability, electrical properties, and chemical properties; its chemical modification methods include copolymerization modification, grafting reaction, chlorination, etc. The following is a specific introduction to "PVC plastic properties And chemical modification methods".


1. Various properties of PVC plastic

1. Physical properties

Polyvinyl chloride resin is a thermoplastic with an amorphous structure. Under ultraviolet light, hard PVC produces light blue or purple-white fluorescence, while soft PVC produces blue or blue-white fluorescence. The refractive index is 1.544 and the specific gravity is 1.40 when the temperature is 20℃. The density of products with plasticizers and fillers is usually in the range of 1.15~2.00, the density of soft PVC foam is 0.08~0.48, and the density of rigid foam is 0.03. ~0.08. The water absorption rate of PVC is not more than 0.5%.

2. Thermal performance

The softening point of polyvinyl chloride resin is close to the decomposition temperature. It has begun to decompose at 140°C, and it decomposes more rapidly at 170°C. In order to ensure the normal progress of the molding process, two most important process indicators are specified for polyvinyl chloride resin, namely decomposition temperature and thermal stability. The so-called decomposition temperature is the temperature when a large amount of hydrogen chloride is released, and the so-called thermal stability is the time when a large amount of hydrogen chloride is not released under a certain temperature condition (usually 190°C). Polyvinyl chloride plastic will decompose when exposed to 100℃ for a long time unless an alkaline stabilizer is added. If it exceeds 180℃, it will decompose rapidly.

3. Stability

Polyvinyl chloride resin is a relatively unstable polymer, which will also degrade under the action of light and heat. The process is to release hydrogen chloride, causing structural changes, but the degree is relatively light. At the same time, the decomposition will be accelerated in the presence of mechanical force, oxygen, odor, HCl and some active metal ions.

4. Electrical performance

The electrical properties of PVC depend on the amount of residue in the polymer, the type and amount of various additives in the formulation. The electrical properties of PVC are also related to the heating conditions: when the PVC is decomposed by heating, its electrical insulation will be reduced due to the presence of chloride ions. If a large amount of chloride ions are generated, they cannot be neutralized by alkaline stabilizers (such as lead salts). It will lead to a significant decrease in its electrical insulation performance. Unlike non-polar polymers such as polyethylene and polypropylene, PVC's electrical properties change with frequency and temperature, for example, the dielectric constant decreases with increasing frequency.

5. Chemical properties

Polyvinyl chloride has very good chemical stability and is extremely valuable as an anti-corrosion material. PVC is stable to most inorganic acids and alkalis, and will not dissolve when heated, but will be separated into hydrogen chloride. Azeotrope with potassium hydroxide to prepare brown and insoluble unsaturated products. The solubility of PVC is related to molecular weight and polymerization method. Generally speaking, the solubility decreases as the molecular weight of the polymer increases, and the solubility of the emulsion resin is worse than that of the suspension resin. It can be dissolved in ketones (such as methylhexanone, cyclohexanone), aromatic solvents (such as toluene, xylene), dimethylformyl, and tetrahydrofuran. Polyvinyl chloride resin is almost insoluble in plasticizers at room temperature, and swells significantly at high temperatures, and even dissolves.

6. Processing performance

PVC is an amorphous polymer with no obvious melting point and has plasticity when heated to 120-150°C. Because of its poor thermal stability, a small amount of HCl is released at this temperature, which promotes its further decomposition, so alkaline stabilizers and HCl must be added to inhibit its catalytic cracking reaction. Pure PVC is a hard product, and an appropriate amount of plasticizer needs to be added to make it soft. For different products, additives such as ultraviolet absorbers, fillers, lubricants, pigments, anti-fungal agents, etc. are needed to improve the quality of PVC products. Use performance. Like other plastics, the performance of the resin determines the quality and processing conditions of the product.

Second, the chemical modification method of PVC plastic

1. Copolymerization modification: that is, let vinyl chloride monomer and other monomers undergo copolymerization reaction. For example, it can be copolymerized with monomers such as vinyl acetate, vinylidene chloride, acrylonitrile, acrylate, maleic anhydride, etc., to improve the molding processability, or reduce the molding temperature, or open up new uses, or appear as a new material.

2. Grafting reaction: Introduce another monomer group or another polymer on the side chain of PVC to carry out the grafting reaction. For example: ethylene-vinyl acetate is grafted with vinyl chloride to control the number of vinyl chloride grafted parts and the degree of polymerization, so as to improve the impact performance, low temperature brittleness, and aging resistance of this modified material.

3. Chlorination: The PVC is chlorinated by the aqueous phase suspension method (or gas phase method) to increase the chlorine content from 57% to about 65%. The purpose of this modification is to improve the heat resistance and use temperature of PVC. It is 35-40°C higher than the original PVC and is called chlorinated polyvinyl chloride (CPVC). The density of CPVC is higher than that of PVC, 1.7 g/cm3. Its flame retardancy is better than PVC, and its tensile strength is also better than PVC. The disadvantage is low impact strength. CPVC can be used to produce products by extrusion, injection, and calendering methods. CPVC can be used for pipes, plates, profiles, foaming materials, adhesives, coatings, modifiers, etc.