Pure polyvinyl chloride (PVC) resin belongs to a class of strong polar polymers, and its intermolecular force is large, which leads to high softening temperature and melting temperature of PVC. Generally, it takes 160~210℃ to process. In addition, substituted chlorine group contained in PVC molecule easily leads to dehydrochlorination reaction of PVC resin, thus causing degradation reaction of PVC, so PVC is extremely unstable to heat, and temperature rise will greatly promote dehydrochlorination of PVC. De-HCl reaction begins, which leads to degradation of PVC.

 

In view of above two deficiencies, PVC needs to be added with additives during processing, so that various soft, hard, transparent, good electrical insulation, foaming and other products that meet people's needs can be produced. When choosing variety and dosage of additives, we must comprehensively consider various factors, such as physical-chemical properties, flow properties, molding properties, and finally establish an ideal formula.
In addition, according to different uses and processing methods, we also need to choose type of resin. Combination of different types of PVC resins and various additives is what we often call PVC formula design. So how to design specific formula? Following will explain selection of each raw and auxiliary material, hoping to be beneficial to everyone.

Viscosity or K value commonly used in industry represents average molecular weight (or average degree of polymerization). Molecular weight of resin is related to physical and mechanical properties of product. The higher molecular weight, the higher tensile strength, impact strength and elastic modulus of product, but fluidity and plasticity of resin melt decrease.
At the same time, different synthesis processes lead to differences in shape of resin. The most common one is loose resin produced by suspension method, commonly known as SG resin. Therefore, SG type resin absorbs plasticizers quickly, and plasticization speed is fast. Main uses of suspension resin are shown in table below. Emulsion resin should be used as PVC paste to produce artificial leather.
Suspension PVC resin type and main use
Model Grade Main Use
SG1 Class A Advanced Electrical Insulating Material
SG2 Class A Electrical insulating material, film
Class 1 B, Class 2 General soft products
SG3 Class A electrical insulating material, agricultural film, artificial leather surface film
Class B, Class II all-plastic sandals
SG4 Grade A Industrial and Civilian Films
Class 1 B, Class 2 hose, artificial leather, high-strength tubing
SG5 Grade A Transparent Products
Class I B, Class II Rigid pipe, rigid sheet, monofilament, conduit, profile
SG6 Class A record, transparencies
Class 1 B, Class 2 Rigid plate, electrode, fiber
SGG7 Grade A bottle, transparencies
Class 1 B, Class 2 Rigid injection pipe fittings, perchlorinated vinyl resin

Addition of plasticizers can reduce force between PVC molecular chains, so that glass transition temperature, flow temperature and melting point of crystallites contained in PVC plastics are reduced. Good low temperature performance. Plasticizer can improve plasticity of resin, making product soft and good low temperature resistance.
When plasticizer is less than 10 parts, effect on mechanical strength is not obvious. When adding about 5 parts of plasticizer, mechanical strength is the highest, which is so-called reverse plasticization phenomenon. It is generally believed that phenomenon of anti-plasticization is that after adding a small amount of plasticizer, activity of macromolecular chain increases, so that molecules are ordered to produce crystallites. Impact strength of hard products with a small amount of plasticizer added is smaller than that without adding time, but after a certain dosage is increased, impact strength increases with increase of dosage, which satisfies universal law.
In addition, when plasticizer is added, heat resistance and corrosion resistance of product decrease. For each additional plasticizer, Martin heat resistance decreases by 2~3. Therefore, generally hard products do not add plasticizers or add less plasticizers. Sometimes only a few parts of plasticizer are added to improve processing fluidity. Soft products need to add a large amount of plasticizer. The larger amount of plasticizer, the softer product.
Types of plasticizers include phthalates, straight-chain esters, epoxy, phosphates, etc. In terms of their comprehensive properties, DOP is a better variety and can be used in various PVC product formulations. Linear esters such as DOS are cold-resistant plasticizers and are often used in agricultural films. They have poor compatibility with PVC. Generally, no more than 8 parts are suitable. In addition to good cold resistance, epoxy plasticizers also have heat resistance and light resistance, especially when used in combination with metal soap stabilizers. General dosage of epoxy plasticizers is 3 to 5 parts.
Wire and cable products need to be flame-retardant, plasticizers with relatively good electrical properties should be selected. PVC itself is flame-retardant, but most of plasticized soft products are flammable. In order to make soft PVC products flame-retardant, flame-retardant plasticizers such as phosphate ester and chlorinated paraffin should be added. Electrical properties of these two types of plasticizers are also better than other plasticizers, but with increase of amount of plasticizers, electrical properties generally show a downward trend. For PVC products used for non-toxic purposes, non-toxic plasticizers such as epoxidized soybean oil should be used.
As for the total amount of plasticizer, it should be different according to requirements for softness of product and use, process and use environment. Generally, calendering process produces PVC film, and the total amount of plasticizer is about 50 parts. Blown film is slightly lower, generally 45 to 50 parts.