Detailed Explanation of Raw And Auxiliary Materials for PVC Formulation

Pure polyvinyl chloride (PVC) resin belongs to a class of highly polar polymers, with high intermolecular forces, resulting in high softening and melting temperatures of PVC. Generally, it takes 160-210 ℃ to process. In addition, the substituted chlorine groups contained within PVC molecules can easily lead to the dehydrochlorination (HCl) reaction of PVC resin, thereby causing the degradation reaction of PVC. Therefore, PVC is extremely unstable to heat, and an increase in temperature will greatly promote the HCl removal reaction of PVC. Pure PVC starts the HCl removal reaction at 120 ℃, leading to PVC degradation.

In view of the shortcomings in the above two aspects, PVC needs to add additives in processing in order to produce various products that meet people's needs, such as soft, hard, transparent, good electrical insulation, foaming, etc. When selecting the variety and dosage of additives, it is necessary to comprehensively consider various factors, such as physical and chemical properties, flowability, and formability, in order to ultimately establish an ideal formula.

In addition, according to different uses and processing methods, it is also necessary to choose the type of resin. The combination of different types of PVC resin and various additives is commonly known as PVC formula design. Below is a detailed explanation of each raw material in the PVC formula for reference.

01 Selection of resin

In industry, viscosity or K value is commonly used to represent the average molecular weight (or average degree of polymerization). The molecular weight of the resin is related to the physical and mechanical properties of the product. The higher the molecular weight, the higher the tensile strength, impact strength, and elastic modulus of the product, but the fluidity and plasticity of the resin melt decrease.

At the same time, different synthesis processes result in differences in the morphology of the resin. The common one is the loose resin produced by suspension method, commonly known as SG resin. Its structure is loose, surface shape is irregular, and the cross-section is loose and porous, forming a network. Therefore, SG type resin absorbs plasticizers quickly and has a fast plasticizing speed. Lotion resin should be used as PVC paste to produce artificial leather.

02 Plasticizer

The addition of plasticizers can reduce the interaction force between PVC molecular chains, resulting in a decrease in the glass transition temperature, flow temperature, and melting point of the microcrystals contained in PVC plastic. Plasticizers can improve the plasticity of the resin, making the product soft and resistant to low temperatures.

The effect of plasticizers on mechanical strength is not significant when the dosage is less than 10 parts. When about 5 parts of plasticizers are added, the mechanical strength is actually the highest, which is a so-called reverse plasticizer phenomenon. It is generally believed that the phenomenon of anti plasticization is the effect of adding a small amount of plasticizer, which increases the activity of large molecular chains and causes molecular ordering to produce microcrystals. The impact strength of hard products with a small amount of plasticizer added is actually smaller than that without added time, but when it is increased to a certain dosage, its impact strength increases with the increase of dosage, meeting the universal law.

In addition, adding plasticizers reduces both the heat and corrosion resistance of the product. For every additional part of plasticizer, Martin's heat resistance decreases by 2-3. Therefore, generally hard products do not add plasticizers or add less plasticizers. Sometimes a few plasticizers are added to improve processing fluidity. Soft products, on the other hand, require the addition of a large amount of plasticizer, and the higher the plasticizer dosage, the softer the product will be.

There are various types of plasticizers, including phthalates, straight chain esters, epoxides, phosphate esters, etc. In terms of their comprehensive performance, DOP is a good variety that can be used in various PVC product formulations. Straight chain esters such as DOS belong to cold resistant plasticizers and are commonly used in agricultural films. They have poor compatibility with PVC and are generally not more than 8 parts. In addition to good cold resistance, epoxy plasticizers also have heat resistance and light resistance, especially when used in combination with metal soap stabilizers. The general dosage of epoxy plasticizers is 3-5 parts.

Wire and cable products should have flame retardancy, and plasticizers with relatively good electrical performance should be selected. PVC itself has flame retardancy, but most plasticized soft products are flammable. In order to make soft PVC products flame retardant, flame retardant plasticizers such as phosphate esters and chlorinated paraffin should be added. These two types of plasticizers also have better electrical properties than other plasticizers. However, with the increase of plasticizer dosage, the overall electrical performance shows a downward trend.

For PVC products used for non-toxic purposes, non-toxic plasticizers such as epoxy soybean oil should be used. As for the total amount of plasticizers, it should vary according to the requirements for the softness of the product, as well as different uses, processes, and usage environments. The general rolling process for producing PVC films requires a total amount of plasticizer of about 50 parts. Blow molded film is slightly lower, usually between 45-50 parts.

03 Stabilizer

PVC is processed at high temperatures and easily releases HCl, forming an unstable polyene structure. Meanwhile, HCl has a self catalytic effect, which can further degrade PVC. In addition, if there is oxygen or iron, aluminum, zinc, tin, copper, and cadmium plasma present, they will catalyze the degradation of PVC and accelerate its aging. Therefore, plastic will exhibit various adverse phenomena, such as discoloration, deformation, cracking, decreased mechanical strength, decreased electrical insulation performance, and brittleness. To solve these problems, stabilizers must be added to the formula, especially heat stabilizers are essential. The stabilizers used for PVC include heat stabilizers, antioxidants, UV absorbers, and chelating agents. When designing the formula, different varieties and quantities of stabilizers are selected based on the requirements for product use and processing technology.

（1） Heat stabilizer

The heat stabilizer must be able to capture the self catalytic HCl released by PVC resin or react with the unstable polyene structure produced by PVC resin to prevent or reduce the decomposition of PVC resin.

Lead salt stabilizers are mainly used in hard products. Lead salt stabilizers have the characteristics of good thermal stabilizers, excellent electrical performance, and low cost. However, it has high toxicity and is prone to contaminating products, so it can only produce opaque products. In recent years, a large number of composite stabilizers have emerged, and single component stabilizers have shown a trend of being replaced. The characteristics of composite stabilizers are strong specificity, low pollution, and simple ingredients for processing enterprises. However, due to the lack of unified standards, there are significant differences in the composite stabilizers used by different companies.

Barium cadmium stabilizers are a type of heat stabilizer with good performance and are widely used in PVC agricultural films. Usually, it is a combination of barium, cadmium, zinc, organic phosphites, and antioxidants.

Calcium zinc stabilizers can be used as non-toxic stabilizers in food packaging, medical devices, pharmaceutical packaging, and other fields, but their stability is relatively low. When calcium stabilizers are used in large amounts, their transparency is poor and they are prone to frost spraying. Calcium zinc stabilizers generally use polyols and antioxidants to improve their performance.

Organic tin based heat stabilizers have good performance and are a good variety for PVC hard and transparent products.

Epoxy stabilizers are typically used as auxiliary stabilizers. When used in combination with barium cadmium calcium zinc stabilizers, this type of stabilizer can improve the stability of light and heat, but its disadvantage is that it is prone to exudation. Other auxiliary stabilizers include polyols and organic phosphites.

In recent years, rare earth stabilizers and hydrotalcite stabilizers have also emerged. Rare earth stabilizers are mainly characterized by excellent processing performance, while hydrotalcite is a non-toxic stabilizer.

（2） Antioxidants

During the processing and use of PVC products, oxidation occurs due to the effects of heat and ultraviolet radiation, and its oxidative degradation is related to the generation of free radicals. The main antioxidant is a chain breaking terminator or free radical scavenger. Its main function is to bind with free radicals, form stable compounds, and terminate the chain reaction. The main antioxidant used in PVC is generally bisphenol A. There are also auxiliary antioxidants or hydrogen peroxide decomposition agents, and PVC auxiliary antioxidants are triphenyl phosphite and diisooctyl phosphite. The combination of main and auxiliary antioxidants can exert a synergistic effect.

（3） UV absorber

PVC products used outdoors, due to exposure to sensitive wavelength range ultraviolet radiation, PVC molecules become excited or their chemical bonds are broken, causing free radical chain reactions, promoting PVC degradation and aging. In order to improve the ability to resist ultraviolet rays, ultraviolet absorbers are often added. The commonly used UV absorbers for PVC include triazine-5, UV-9, UV-326, TBS, BAD, and OBS. Triazine 5 has the best effect, but due to its yellow color, the film is slightly yellowish. Adding a small amount of phthalocyanine blue can improve it.

UV-9 is commonly used in PVC agricultural films, with a general dosage of 0.2-0.5 parts. TBS, BAD, and OBS, which belong to the salicylic acid class, have mild effects and can achieve good aging resistance when used in combination with antioxidants. For non transparent products, weather resistance is generally improved by adding opaque rutile titanium dioxide. In this case, adding ultraviolet absorbers requires a large amount and is not very cost-effective.

（4） Chelating agent

In the stable system of PVC plastics, commonly added phosphite esters not only serve as auxiliary antioxidants but also act as chelating agents. It can form metal complexes with harmful metal ions that promote the removal of HCl from PVC. The commonly used phosphite esters include triphenyl phosphite, diisooctyl phosphite, and diphenyl octyl phosphite. In PVC agricultural film, the general dosage is 0.5-1 parts. When used alone, it is easy to color in the early stage and has poor thermal stability. It is generally used in combination with metal soap.

04 Lubricants

The function of lubricants is to reduce the friction between polymers and equipment, as well as the internal friction between polymer molecular chains. The former is called external lubrication, while the latter is called internal lubrication. Products with external lubrication effects such as silicone oil and paraffin, and products with internal lubrication effects such as monoglycerides, stearic alcohols, and esters. As for metal soaps, they have both.

Furthermore, it should be noted that the term "internal and external lubrication" is only a common term without clear boundaries. Some lubricants, such as stearic acid, play different roles under different conditions. They can provide internal lubrication at low temperatures or in small amounts, but their external lubrication gradually becomes more advantageous as the temperature or dosage increases. Another special example is calcium stearate, which is used as an external lubricant when used alone, but becomes an internal lubricant that promotes plasticization when used in combination with hard lead and paraffin.

In hard PVC plastics, excessive lubricant can lead to a decrease in strength and also affect process operations. For injection products, peeling may occur, especially near the gate. For injection products, the total amount of stearic acid and paraffin is generally 0.5-1 parts; Extruded products generally do not exceed one portion.

In the formula of soft products, too much lubricant can cause frost and affect the strength, high-frequency welding, and printability of the product, while too little lubricant will stick to the rollers. For blow molded films, too little lubricant will stick to the mold, making it easy for the plastic to coke inside the mold. Meanwhile, in order to improve the sticking phenomenon of the blown film, it is advisable to add a small amount of internal lubricant monoglyceride. When producing PVC soft products, the amount of lubricant added is generally less than 1 part.

05 Filler

Adding certain inorganic fillers as incremental agents to PVC to reduce costs, while improving certain physical and mechanical properties (such as hardness, thermal deformation temperature, dimensional stability, and reducing shrinkage), and increasing electrical insulation and flame resistance. In recent years, the nanomaterialization of inorganic fillers and their application as modifiers in plastics has been a research hotspot, and some research achievements have been made, such as the toughening and strengthening of PVC with nano calcium carbonate. The important problem to be solved is how to evenly disperse nano products in plastics.

In the process of hard extrusion molding, the general fillers for PVC products are calcium carbonate and barium sulfate. For injection molded products, good fluidity and toughness are required, and titanium dioxide and calcium carbonate are generally recommended. The filler content of hard products within 10 parts has little effect on the performance of the products.

In terms of soft products, adding an appropriate amount of filler can make the film have good elasticity and a smooth and dry surface without appearing bright. It also has advantages such as high heat resistance and low permanent deformation. In soft product formulations, fillers such as talc powder, barium sulfate, calcium carbonate, titanium dioxide, and clay are commonly used. Talcum powder has a relatively small impact on transparency.

When producing thin films, the amount of filler can reach 3 parts, which affects the performance. Adding calcium carbonate mainly to ordinary cables; Adding calcined clay to the insulation layer of cables can improve the heat resistance and electrical insulation of plastics. In addition, antimony trioxide can also be added as a filler to soft products to improve their flame resistance

06 Colorants

The colorants used for PVC plastics are mainly organic pigments and inorganic pigments. PVC plastics have high requirements for pigments, such as resistance to high temperatures during processing, unaffected by HCl, no migration during processing, and light resistance. Commonly used ones include:

（1） Red mainly includes soluble azo pigments, cadmium red inorganic pigments, iron oxide red pigments, phthalocyanine red, etc;

（2） Yellow mainly includes chrome yellow, cadmium yellow, and fluorescent yellow, etc;

（3） Blue mainly includes phthalocyanine blue

（4） The green color is mainly phthalocyanine green;

（5） White mainly uses titanium dioxide powder;

（6） Purple is mainly plastic purple RL;

（7） Black is mainly carbon black.

In addition, fluorescent whitening agents are used for whitening, gold and silver powders are used for color printing, and pearl powder gives plastics a pearl like astigmatism.

07 Foaming agents and flame retardants

The foaming agents used for PVC are mainly ADC foaming agents, azodiisobutyronitrile and inorganic foaming agents. In addition, lead and cadmium salts also contribute to foaming, which can lower the decomposition temperature of AC foaming agent to 150-180 ℃. The amount of foaming agent is determined based on the foaming rate.

Due to its high chlorine content, rigid PVC plastic itself has flame retardancy. Adding flame retardants to PVC cables, decorative walls, and plastic curtains can increase their flame resistance. Commonly used flame retardants include chlorinated paraffin, antimony trioxide (2-5 parts), and phosphate esters. Phosphates and chlorinated plasticizers also have flame retardancy.

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