What is PVC
I. Overview of Polyvinyl Chloride (PVC) Resin
Polyvinyl chloride resin is a thermoplastic polymer formed by the polymerization of vinyl chloride monomer (VCM), with a structural unit of (-CH₂-CHCl-)ₙ (where ₙ represents the degree of polymerization). Its English name is Polyvinyl Chloride Polymer, commonly abbreviated as PVC or PVC resin.
In 1872, Baumann synthesized polyvinyl chloride. In 1912, German chemist Fritz Klatte synthesized PVC and filed a patent in Germany, but failed to develop applicable products before the patent expired. In 1926, W.L. Simon from the United States dissolved unused PVC powder in a high-boiling solvent under heating; upon cooling, he accidentally obtained plasticized PVC that was soft, easy to process, and elastic. This accidental discovery opened the door to the industrialization of PVC, making it the first industrialized product among the five major general-purpose resins. In 1931, Germany's IG Farbenindustrie produced PVC using the emulsion polymerization method in Bitterfeld. In 1941, the United States developed the suspension polymerization technology for PVC production.
In 1958, China's first self-designed PVC plant (with an annual output of 3,000 tons) was completed and put into operation at Jinxi Chemical Plant in Liaoning Province, marking a new era for China's plastics industry. Over decades, PVC has become the world's second-largest general-purpose resin (only after polyethylene resin), accounting for 29% of the global total consumption of synthetic resins. Its prominent advantages include abundant raw materials (petroleum, limestone, coke, salt, and natural gas), mature manufacturing processes, low cost, and wide applications.
PVC resin appears as white powder or granules, with a particle size of 60–250 μm and an apparent density of 0.40–0.60 g/cm³, and it has a glossy appearance. Its transparency is better than that of polyethylene and polypropylene but worse than polystyrene. Depending on the amount of plasticizers added, PVC can be divided into soft and rigid products: soft products are flexible and tough with a sticky feel, while rigid products have higher hardness than low-density polyethylene but lower than polypropylene, and whitening may occur at bending points.
PVC has the advantages of flame retardancy (flame retardant value ≥ 40), high chemical resistance (resistant to concentrated hydrochloric acid, 90% sulfuric acid, 60% nitric acid, and 20% sodium hydroxide), good mechanical strength, and excellent electrical insulation. However, it has poor heat resistance: its softening point is 80°C, and it starts to decompose, discolor, and release hydrogen chloride (HCl) at 130°C. PVC products are classified into soft, semi-rigid, and rigid types based on plasticizer content: generally, 0–5 parts of plasticizer for rigid products, 5–25 parts for semi-rigid products, and >25 parts for soft products. PVC is easy to process and can be manufactured via molding, lamination, injection molding, extrusion, calendering, blow molding (for hollow products), etc. It is mainly used to produce wires and cables, films for various purposes, flooring materials, fabric coatings, artificial leather, hoses, gloves, toys, plastic shoes, special coatings, sealants, as well as rigid products such as plates, doors and windows, pipes and valves, rigid sheets, and hollow plastic containers.
II. Physicochemical Properties and Analytical Indicators
1. Physicochemical Properties
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Item
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Details
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Dangerous Goods No.
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-
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UN No.
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Polyvinyl chloride; PVC
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CAS No.
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9002-86-2
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Physicochemical Properties
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PVC is an amorphous polymer, appearing as white powder. Its glass transition temperature ranges from 105°C to 75°C depending on molecular weight. Compared with other general-purpose plastics, PVC is flame-retardant and self-extinguishing, with excellent chemical resistance. It is insoluble in water, alcohol, and gasoline, but can swell or dissolve in ethers, ketones, and aromatic hydrocarbons. PVC resin is a stable white powder with a particle size of 60–250 μm and an apparent density of 0.4–0.6 g/mL. The dilute solution viscosity (0.5 g resin in 100 mL cyclohexanone) is 80–160 mL/g, and 100 g resin absorbs 20–30 g plasticizer at room temperature.
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Combustion and Explosion Hazards
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Flammability: CombustibleHazardous characteristics: Dust can form explosive mixtures with airCombustion (decomposition) products: Hydrogen chloride, carbon monoxide, benzeneStability: StableFire-fighting methods: Mist water, foam, dry powder, carbon dioxide, sand
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First Aid
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Skin contact: Remove contaminated clothing and rinse with running waterEye contact: Rinse with plenty of running water or normal saline; seek medical attentionInhalation: Move to a well-ventilated area; provide oxygen if breathing is difficult; seek medical attentionIngestion: Administer plenty of warm water to induce vomiting; seek medical attention
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Protective Measures
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Use closed systems; ventilation and lighting equipment must be explosion-proof; avoid tools that easily generate sparks; wear self-priming filter dust masks, anti-static work clothes, and chemical-resistant gloves; no smoking or eating at the work site
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Toxicity and Health Hazards
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Exposure routes: Inhalation, ingestionToxicity: LD₅₀: -Health hazards: Long-term or repeated exposure may cause changes in lung function and symptoms such as dermatitis and skin desquamation
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Spill Handling
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For large spills: Cut off power supply; emergency personnel should wear full protective equipment; isolate the spill area and restrict access; cover with clean plastic sheeting or canvas to avoid dust; carefully sweep up, collect for recycling, or dispose of as waste
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Packaging and Storage
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Storage precautions: Store in a cool, well-ventilated area; keep away from ignition sources and heat; prevent exposure to sunlight, rain, moisture, and high temperatures; equip with appropriate types and quantities of fire-fighting equipment
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2. Introduction to Key Indicators
(1) Viscosity Number (K Value or Average Degree of Polymerization)
The viscosity number is usually used to indicate the molecular weight of the polymer and classify polymer grades; internationally, K value or degree of polymerization is also used for this purpose.
PVC resin is an amorphous linear polymer. A higher relative molecular weight leads to a higher viscosity number, which improves the material's strength, rigidity, toughness, heat resistance, and low-temperature resistance but impairs its processing performance. The reason is that as the relative molecular weight of PVC resin increases, the van der Waals forces or entanglement between PVC macromolecular chains increase (enhancing mechanical properties), and abnormal weak structures and chain end groups in the molecular chains decrease (improving heat resistance and weather resistance).
The following table shows the correspondence between the grades of suspension-polymerized PVC resin and various parameters:
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Grade
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Viscosity Number (mL/g)
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Absolute Viscosity (mPa·s)
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K Value
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Average Degree of Polymerization
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SG-0
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>156
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-
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>77
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>1785
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SG-1
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156–144
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2.1–2.2
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77–75
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1785–1536
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SG-2
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143–136
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2.0–2.1
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74–73
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1535–1371
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SG-3
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135–127
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1.9–2.0
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72–71
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1370–1251
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SG-4
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126–119
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1.8–1.9
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70–69
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1250–1136
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SG-5
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118–107
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1.7–1.8
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68–66
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1135–981
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SG-6
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106–96
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1.6–1.7
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65–63
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980–846
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SG-7
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95–87
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1.5–1.6
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62–60
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845–741
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SG-8
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86–73
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1.4–1.5
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59–55
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740–650
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SG-9
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<73
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-
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<55
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<650
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(2) Apparent Density
Apparent density refers to the mass of monomer powder per unit volume without compression. According to national standards, the apparent density of PVC resin should be determined in accordance with GB/T 20022. It is related to the particle morphology, average particle size, and particle size distribution of the resin. A high apparent density indicates regular resin particles, which is conducive to the generation of shear heat during mixing, the pre-plasticization of mixed raw powder, and uniform mixing. In addition, a high apparent density generally increases the bulk density of dry blends, facilitating high-speed extrusion of rigid products.
Soft PVC products require PVC resin with high molecular weight, porous structure, and low apparent density; rigid products prefer resin with lower molecular weight and higher apparent density (to enable high-speed extrusion and improve production efficiency). Generally, PVC resin with an apparent density of approximately 0.54–0.58 g/mL or higher is selected.
Research by Wu Mingjing from Qingdao Haijing Group [1] shows that a 0.05 g/mL difference in resin apparent density has little impact on mixing time; the apparent density of the resin is closely related to that of the dry blend but does not determine the flowability of the dry blend (which is more affected by other factors). A 0.05 g/mL difference in resin apparent density affects plasticization time: higher apparent density leads to faster plasticization. During extrusion, the torque difference is small, but the product quality is significantly affected. Observation of the cross-section of extruded sheets shows that materials with low apparent density have poorer fluidity in the barrel than those with high apparent density, resulting in severe delamination, bubbles, and surface blistering (attributed to poor plasticization).
The impact of apparent density differences on processing performance can be adjusted through formula and process parameters, without causing substantial defects in the products.
(3) Volatiles (Including Water)
Most volatiles in PVC resin are water, along with residual vinyl chloride and some solvents. Moisture affects the screening rate of the resin and the quality of the final products: excessive volatiles easily cause bubbles during molding (impairing product quality, resulting in rough surfaces, or even product warping). Excessively low volatile content may generate static electricity, affecting resin fluidity and causing difficulties in mixing and kneading. Generally, the volatile content is preferably controlled at 0.1%–0.3%.
The level of volatile content is mainly related to the stripping and drying processes after PVC polymerization, determined by temperature and control standards.
According to national standards, the determination of volatile content should comply with GB/T 2914: the sample is heated at (110±2)°C for 1 hour, and the result is calculated based on the weight loss within 1 hour.
(4) Number of Impurity Particles
There are two main types of impurity particles: ① external impurities introduced during production; ② discolored resin particles (commonly known as "black-yellow spots") generated during PVC resin production, which have significantly lower stability.
Observation and research by Li Jun from Ningxia Western Chlor-Alkali Co., Ltd. [1] found that the main causes of excessive impurity particles in PVC resin are the shedding of scale in the slurry stripping tower, material accumulation in the steam pipeline of the stripping tower, and material accumulation at the bottom of the air dryer. In general, the number of impurity particles in PVC resin is mainly related to the post-polymerization treatment and packaging processes. To reduce impurity particles, it is necessary to strengthen inspection and management of these production links: regularly clean the air filter of the blower to ensure effective filtration (reducing impurity entry from air into the final PVC product); regularly clean the centrifuge to prevent yellow spots on the centrifuge drum from entering the final product; strengthen the cleaning of reactors and tanks to prevent particle adhesion to the reactor/tank walls (which may turn black due to secondary heating). Additionally, using high-quality additives and improving the quality of monomers and deionized water are also important.
A small number of impurity particles in resin is normal, but excessive impurities adversely affect processing, product performance, and appearance. In particular, external impurity particles impair the dielectric properties and mechanical properties of products. For products with strict appearance requirements and electrical products, the number of impurity particles in PVC resin should be strictly controlled.
According to national standards, the determination of the number of impurity particles should comply with GB/T 9348.
(5) Number of "Fish Eyes"
"Fish eyes" refer to transparent, unplasticized particles (gel particles) under normal processing conditions. They affect the strength of films, the insulation performance of cable materials, and the sound quality of records; thus, some products require limiting the number of "fish eyes" in the resin. For most products, excessive "fish eyes" cause rough surfaces, reduce the physical properties of the entire product, and may even become centers of degradation (leading to product discoloration).
The essence of "fish eyes" is a small number of ultra-high molecular weight PVC particles with a three-dimensional network structure, formed due to improper conditions during polymerization. These particles have very low plasticizer absorption capacity and can only swell (but not plasticize) with plasticizers under normal processing conditions. However, most "fish eyes" encountered in processing are not of this three-dimensional structure but are "pseudo-fish eyes" that can be plasticized. These pseudo-fish eyes are linear resin with a relatively high molecular weight, whose physical structure is too dense due to packaging.
"Fish eyes" can form through multiple pathways. For example, a broad molecular weight distribution of PVC causes low-molecular-weight components to plasticize quickly and high-molecular-weight components to plasticize slowly under the same processing conditions. If processing conditions are fixed, the slow-plasticizing high-molecular-weight components will appear as pseudo-fish eyes in the product. The formation of "fish eyes" is related to both the formulation and process of suspension polymerization (for PVC resin production) and the processing formulation and process of PVC resin; thus, careful analysis and differentiation are required.
Measures to reduce or eliminate "fish eyes" in PVC resin production include:
1. Ensuring the content of high-boiling point substances in monomers is <1.0×10⁻³;
2. Strictly controlling the content of anions and cations in deionized water for polymerization and the water content in monomers;
3. Adding chain transfer agents to eliminate free radicals;
4. Selecting a dispersion system with low surface tension, moderate colloid protection ability, and moderate stirring intensity;
5. Reducing reactor fouling (adhered materials);
6. Adding anti-fish eye agents.
For PVC processing enterprises, in addition to selecting PVC resin with a low number of "fish eyes", attention should be paid to the plasticizability and lubrication balance of the processing formula, as well as the matching of processing temperatures. Necessary adjustments should be made according to the structural morphology of the resin to avoid the massive occurrence of pseudo-fish eyes.
According to national standards, the determination of the number of "fish eyes" should comply with GB/T 4611.
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