Introduction to PVC and various additives

The PVC plastic formula is mainly composed of PVC resin and additives, which are divided into heat stabilizers, lubricants, processing modifiers, impact modifiers, fillers, aging resistant agents, colorants, etc. according to their functions. Before designing PVC formulas, one should first understand the properties of PVC resin and various additives.

Raw materials and additives

PVC resin

The resin used to produce PVC plastic profiles is polyvinyl chloride resin (PVC), which is a polymer formed by polymerization of vinyl chloride monomers, with a yield second only to PE.

PVC resin can be divided into two types due to the different dispersants used in polymerization: loose type (XS) and tight type (XI). The loose type has a particle size of 0.1-0.2mm, with an irregular and porous surface in the form of cotton balls, which are easy to absorb plasticizers. The tight type has a particle size of less than 0.1mm, and has a regular surface, solid surface, which is in the form of ping-pong balls, which are not easy to absorb plasticizers. Currently, loose type is commonly used.

PVC can also be divided into ordinary grade (toxic PVC) and sanitary grade (non-toxic PVC). Hygienic grade requires that the content of vinyl chloride (VC) be lower than lOXl0-6, which can be used in food and medicine. With different synthesis processes, PVC can be divided into suspension PVC and lotion PVC. According to the national standard GB/T5761-93 "Inspection Standard for General Purpose PVC Resins by Suspension Method", suspension method PVC is divided into PVC-SGl to PVC-SG8Jk resins. The smaller the number, the higher the degree of polymerization, molecular weight, and strength, but the more difficult the melting flow and processing are. When making soft products, PVC-SGl, PVC-SG2, and PVC-SG3 types are generally used, and a large amount of plasticizer needs to be added. For example, the PVC film uses SG-2 resin with 50-80 parts of plasticizer added. When processing hard products, plasticizers are generally not added or a small amount is added, so PVC-SG4, VC-SG5, PVC-SG6, PVC-SG7, and PVC-SG8 types are used. For example, PVC hard pipes use SG-4 resin, plastic door and window profiles use SG-5 resin, hard transparent sheets use SG-6 resin, and hard foam profiles use SG-7 and SG-8 resins. The lotion PVC paste is mainly used for artificial leather, wallpaper, floor leather and dipped plastic products. Some PVC resin manufacturers classify their PVC resins according to their degree of polymerization (polymerization degree is the number of unit chain links, and polymerization degree multiplied by chain link molecular weight equals polymer molecular weight). For example, the PVC resin produced by Shandong Qilu Petrochemical Factory is SK-700; SK-800; SK-1000; SK-1100; SK-1200, etc. The polymerization degree corresponding to its SG-5 resin is 1000-1100. The physicochemical properties of PVC resin can be seen in Chapter 4.

PVC powder is a white powder with a density between 1.35-1.45g/cm3 and an apparent density between 0.4-0.5g/cm3. Depending on the content of the plasticizer, it can be classified as soft or hard products. Generally, 0-5 parts of the plasticizer content are considered hard products, 5-25 parts are considered semi hard products, and more than 25 parts are considered soft products.

PVC is a non crystalline, polar polymer with high softening and melting temperatures. Pure PVC generally needs to be plasticized at temperatures between 160-210 ° C. Due to the polar bonds between macromolecules, PVC exhibits hard and brittle properties. Moreover, PVC molecules contain chlorine groups, and when the temperature reaches 120-C, pure PVC begins to undergo a dehydrochlorination reaction, which can lead to thermal degradation of PVC. Therefore, various additives must be added during processing to modify and impact PVC, so that it can be processed into useful products.

PVC resin is mainly used for the production of various types of films (such as daily printing film, industrial packaging film, agricultural greenhouse film, and heat shrink film), various types of boards and sheets (whose sheets can be used for plastic suction products), various types of pipes (such as non-toxic water pipes, building threading pipes, transparent hoses, etc.), various types of special profiles (such as doors, windows, decorative panels), hollow blow bottles (used for cosmetics and beverages), cables, various types of injection molded products and artificial leather, floor leather, plastic toys, etc.

stabilizer

Pure PVC resin is extremely sensitive to heat. When the heating temperature reaches 90Y or above, a slight thermal decomposition reaction occurs. When the temperature rises to 120C, the decomposition reaction intensifies. At 150C and 10 minutes, the PVC resin gradually changes from white to yellow red brown black. The decomposition process of PVC resin is a series of chain reactions caused by the HCL removal reaction, which then leads to the breakage of macromolecular chains. The thermal stability mechanism to prevent PVC thermal decomposition is achieved through the following aspects.

By capturing HCl generated during the thermal decomposition of PVC, the catalytic degradation of HCl is prevented. Qihong lead salt stabilizers QY-298 mainly act according to this mechanism, as well as metal soaps, organic tin, phosphite lipids, and epoxides Replace active allyl chloride atoms. Metal soaps, phosphite lipids, and organic tin can act according to this mechanism. React with free radicals and terminate the reaction of free radicals. Qihong organic tin stabilizer YJ-906 and phosphite act according to this mechanism. The addition of conjugated double bonds leads to the growth of conjugated chains. Organic tin and epoxy react according to this mechanism Decompose peroxides to reduce the number of free radicals. Organic tin YJ-906 and phosphite act according to this mechanism. Passivation of metal ions that catalyze the removal of HCl. Organic tin stabilizer YJ-906: suitable for PVC transparent products, with extremely high thermal stability and excellent initial coloring properties, pure color, minimal gas, and no precipitation.

lubricant

The function of lubricants is to reduce the friction between materials and the surface of materials and processing equipment, thereby reducing the flow resistance of the melt, reducing the viscosity of the melt, improving the fluidity of the melt, avoiding adhesion between the melt and equipment, and improving the surface smoothness of the product.

According to different forming methods, the lubrication effect is emphasized differently:

Rolling forming to prevent molten material from sticking to the rollers;

Injection molding, improving flow and demolding performance;

Extrusion molding, improving flow and mold separation;

Pressing and laminating are beneficial for separating the pressing plate from the product.

Classification of lubricants:

According to the composition of lubricants, there are mainly saturated hydrocarbons and halogenated hydrocarbons, fatty acids, fatty acid esters, aliphatic ester amines, metal soaps, fatty alcohols, and polyols.

According to the function of lubricants, they are classified into internal and external lubricants.

Its main differentiation is based on its compatibility with the resin. The internal lubricant has a high affinity with the resin, and its function is to reduce intermolecular forces; The affinity between external lubricants and resins is low, and their function is to reduce friction between resins and metal surfaces.

The distinction between internal and external lubricants is only relative and does not have strict standards for classification. In resins with different polarities, the role of internal and external lubricants may change. For example, stearic acid alcohol, stearic acid amide, butyl stearate, and stearic acid monoglyceride have an internal lubrication effect on polar resins such as PVC and PA; But for non polar resins (such as PE, PP), external lubrication is shown. On the contrary, polymer paraffin and other polar resins have poor compatibility, such as being used as an external lubricant in polar PVC, while being an internal lubricant in non polar resins such as PE and PP.

At different processing temperatures, the effects of internal and external lubricants also change. For example, stearic acid and stearic alcohol used in the early stage of PVC rolling forming mainly play an external lubrication role due to low processing temperature and poor compatibility with PVC; When the temperature increases, the compatibility with PVC increases, and it becomes an internal lubricant.

According to the composition of lubricants, they can be divided into: saturated hydrocarbons, metal soaps, aliphatic amides, fatty acids, fatty acid esters, and fatty alcohols.

1. Saturated hydrocarbons

Saturated hydrocarbons can be divided into non-polar hydrocarbons (such as polyethylene wax and polypropylene wax) and polar hydrocarbons (such as chlorinated paraffin, oxidized polyethylene, etc.) according to their polarity. Saturated hydrocarbons can be classified according to their molecular weight; Liquid paraffin (C16-C21), solid paraffin (C26-C32), microcrystalline paraffin (C32-C70), and low molecular weight polyethylene (molecular weight 1000-10000) are mainly used as non-toxic external lubricants for PVC.

(1) Liquid paraffin: commonly known as white oil, it is a colorless and transparent liquid that can be used as a transparent external lubricant for PVC, with a dosage of about 0.5 parts. The dosage can seriously affect the strength of welding corners.

(2) Solid paraffin, also known as natural paraffin, is a white solid that can be used as an external lubricant for PVC, with a dosage of 0.1-1.0 parts. Excessive dosage can affect transparency.

(3) Microcrystalline paraffin, also known as high melting point paraffin, has a white or light yellow solid appearance and is called microcrystalline paraffin due to its fine crystals. The lubrication effect and thermal stability are better than other paraffin waxes. The amount used in PVC is relatively small, usually ranging from 0.1 to 0.3 parts.

(4) Low molecular weight polyethylene, also known as polyethylene wax, has a white or light yellow solid powder appearance and poor transparency. It can be used as an external lubricant for PVC extrusion and rolling processing, with a dosage of generally less than 0.5 parts.

(5) Oxidized polyethylene wax is a partially oxidized product of polyethylene wax, with a white powder appearance. It has excellent internal and external lubrication, good transparency, low price, and a dosage of 0.2-1.0 parts.

(6) Chlorinated paraffin has good compatibility with PVC, poor transparency, and good effect when used in combination with other lubricants. The dosage should be less than 0.5 parts.

2. Metal Soaps

It is not only an excellent heat stabilizer but also a lubricant, with both internal and external lubrication effects. Different varieties have slightly different emphasis, and the lubricity is better with calcium stearate and lead stearate.

3. Aliphatic acid amine

It includes two major categories: monofatty acid amides and bifatty amides. Monofatty acid amides mainly exhibit internal lubrication, and the main varieties include ethyl bis stearamide, N, N ethylidene bis ricinolamide, etc.

4. Fatty acids

Qihong stearic acid G01 is a widely used lubricant, second only to metal soaps, and can be used in PVC. When used in small amounts, it has an internal lubrication effect; When used in large quantities, it acts as an external lubricant. The amount of stearic acid added is less than 0.5 parts.

5. Fatty acid esters

(1) Butyl stearate, appearing as a colorless or light yellow oily liquid, mainly provides internal lubrication and external lubrication in PVC, with a dosage of 0.5-1.5 parts.

(2) Monostearic acid glyceride, codenamed GMS, has a white waxy solid appearance and is an excellent internal lubricant for PVC. It has little effect on transparency and can be used in combination with stearic acid at a dosage of less than 1.5 parts.

(3) Ester wax and saponified wax mainly refer to the subsequent products made from lignite wax as the main raw material and through bleaching and other processes. Bleaching wax includes S wax and L wax, saponification wax includes 0 wax and OP wax. Mainly used for HPVC, dosage 0.1-0.3 parts.

6. Fatty alcohols

Stearic alcohol, with a white bead like appearance, has an internal lubricating effect and good transparency. It is used in PVC at a dosage of 0.2-0.5 parts. It can also be used in PS. Ruqihong High Temperature Lubricant GW-916916 is a polymer oxide with excellent performance as a functional processing aid, suitable for plasticizing and post release of PVC foam board products, increasing extrusion torque. As a high-temperature lubricant for PVC, the dosage is 0.2-0.5 parts.

Processing modification additives

1. The working principle of processing aids

Due to poor ductility of PVC melt, it is easy to cause melt breakage; The slow relaxation of PVC melt can easily lead to rough, matte, and shark skin on the surface of the product. Therefore, PVC processing often requires the addition of processing aids to improve the above-mentioned defects in its melt.

Processing aids are additives that can improve the processing performance of resins, with three main ways of action: promoting resin melting, improving melt rheological properties, and endowing lubrication functions.

Promoting resin melting: When PVC resin melts under a certain shear force under heating, the processing modifier first melts and adheres to the surface of PVC resin particles. Its compatibility with the resin and its high molecular weight increase PVC viscosity and friction, effectively transferring shear stress and heat to the entire PVC resin, accelerating PVC melting.

Improving the rheological properties of PVC melt: PVC melt has disadvantages such as poor strength, poor ductility, and melt fracture, while processing modifiers can improve the aforementioned rheological properties of the melt. The mechanism of its action is to increase the viscoelasticity of PVC melt, thereby improving off mold expansion and increasing melt strength.

Provide lubricity: The compatible part of the processing modifier and PVC first melts, promoting melting; The parts that are incompatible with PVC migrate outside the molten resin system, thereby improving demolding performance.

PVC processing modifier K-311 is an acrylic processing aid prepared by lotion polymerization process,

It can promote the melting of PVC at low temperatures, enhance the cohesion and homogeneity of PVC melting, and reduce processing temperature.

Gives products excellent transparency, especially suitable for high transparency products and high-quality profiles.

Improve the rheological properties of PVC, promote the plasticization and melting of PVC resin, and improve the appearance quality of products.

Especially suitable for the processing and molding of PVC high transparency products.

2. Commonly used processing modifier - ACR

ACR is a copolymer of methyl methacrylate and monomers such as acrylate and styrene. In addition to being used as a processing aid, it can also be used as an impact modifier. The ACR in China can be divided into several types: QH201, QH301, QH401, and QH402. Foreign brands include K120N, K125, K175, P530, P501, P551, P700, PAl00, etc..

The important role of ACR processing modifiers is to promote the plasticization of PVC, shorten the plasticization time, improve the uniformity of melt plasticization, and reduce the plasticization temperature. Table 4 shows the effect of ACR on plasticization time, temperature, etc. measured using the BLANBENDE plastometer.

ACR201 or ACR401 is generally used in PVC plastic door and window profiles, with a dosage of 1.5-3 parts.

Impact modifier

An important aspect of polymer material modification is to improve its impact resistance. PVC resin is a polar amorphous polymer with strong intermolecular forces, making it a hard and brittle material; Low impact strength. After adding an impact modifier, the elastic particles of the impact modifier can reduce the total craze induced stress and utilize the changes in the particles themselves