Properties of PVC stabilizer

Generally classified according to the chemical composition of heat stabilizers, they can be basic lead salts, metal soaps, organotin compounds, epoxy compounds, hypophosphite esters, polyols, etc. If PVC stabilizers are classified into main stabilizers and auxiliary stabilizers according to their size of action. The auxiliary stabilizer itself has only a small stabilizing effect or no thermal stabilizing effect, but it has a synergistic effect when used together with the main stabilizer; The main stabilizer is generally a heat stabilizer containing metals. Pure organic compounds such as epoxy compounds, phosphites, polyols, etc. are generally used as auxiliary stabilizers.

Because achieving good stability of polyvinyl chloride often requires the simultaneous use of multiple PVC heat stabilizers, some commercial PVC stabilizers are compounded with multiple components to form composite stabilizers, such as barium cadmium stabilizers, barium zinc stabilizers, etc. These composite stabilizers are usually added with lubricants and other additives required for polyvinyl chloride processing to facilitate user use. There are many types of composite stabilizers, including powder, paste, liquid, and three forms. Therefore, there are several classifications of heat stabilizers:

1. Lead salt composite stabilizer

2. OBS organic based stabilizer

3. Organic tin stabilizers

4. Liquid composite stabilizer

5. Calcium zinc composite stabilizer

6. Barium zinc composite stabilizer

7. Potassium zinc composite stabilizer

Principle of action editing (1) Absorption of hydrogen chloride Me (OOCR) 2+2HCl==MeCl2+2HOOCR, where Me is Pb, Ba, Cd, Ca, Zn, Sn, Sb, Mg, Sr, etc. (2) Eliminate unstable chlorine atom substitution or eliminate unstable chlorine atoms. (3) Prevent the automatic oxidation of polyvinyl chloride, which is easily degraded by O2 oxidation under the action of thermal oxygen and shear force. (4) Adding metal salts of maleic acid can inhibit or eliminate double bonds or shorten or reduce them.

Feature Editing

Compatibility: People always hope that PVC stabilizers can be easily dispersed when mixed with species, and can be used for a long time without precipitation after being made into products. However, when the compatibility of the stabilizer is poor, it will move from the inside of the product to the outside surface and remain on the surface of the product after Z. If the sprayed material is a powdery solid substance, it is called frost spray; If liquid is sprayed out, it is called "sweating". Generally speaking, certain metal soaps, lubricants, and maleic acid ester organotin can easily cause frosting; PVC stabilizers such as hypophosphite and phosphate esters can also increase the amount of frost spray. For metal soaps, those with low electronegativity tend to spray less frost; Aromatic acid radicals spray less cream than fatty acid radicals; The longer the carbon chain in fatty acid radicals, the more severe the frost spraying.

Transparency 1. PVC stabilizers and transparent organotin compounds, especially maleic acid ester and thiol organotin compounds, are a type of PVC stabilizer with good transparency, mainly used in hard transparent products; Metal soap has transparency and is often used in semi transparent products; Inorganic lead salts are opaque and can only be used in opaque products.

2. Whitening phenomenon: Transparent plastic products may lose their transparency if soaked in water, exposed to outdoor sunlight, bent, stretched, or subjected to other conditions. Water immersion whitening refers to the phenomenon of transparent plastic boards appearing white, turbid, and opaque after being immersed in water for a certain period of time; Exposure whitening refers to the phenomenon of plastic hard boards being exposed to air due to the effects of moisture, carbon dioxide, sulfur dioxide, and light in the atmosphere; Stress whitening refers to the whitening phenomenon that occurs in the creases of plastic products when subjected to mechanical external forces.

3. Fish eye bubbles refer to transparent or semi transparent plastic sheets that, when viewed in the presence of light, contain small circular particles resembling fish eyes. This is known as "fish eyes". Bubbles "are caused by the imbalance of volatile components or processability in PVC stabilizers

The reason for instability (1) is that PVC is polymerized by vinyl chloride monomer through free radical initiation. In the reaction, during the growth of molecular chains, chain transfer reactions occur to generate tertiary carbon atoms. The chlorine and hydrogen atoms connected to the tertiary carbon atoms become active atoms due to their low electron cloud distribution density and bond energy, making them easy to remove HCl from adjacent H and Cl. The molecular structure of PVC resin is arranged end-to-end as shown in the following formula:

If the original molecular structure of PVC resin is purely linear and composed of secondary carbon atoms and chlorine atoms, then its stability is relatively good. But in fact, even high-purity PVC resin begins to decompose into HCl above 100 ℃, indicating that there are still unstable factors in its molecular structure. The unstable structure of PVC is believed to be caused by the free radical initiated polymerization of vinyl chloride, which may contain the following structures on the molecule

After the decomposition of the molecular structure, one end contains secondary and tertiary chlorine, the other end has a β - unstable group, and there is also the presence of oxygen in the structure. The reason for this may be partly due to the peroxide reduction of initiators, and the oxygen-containing structure may be formed by polymerization reactions in the presence of trace amounts of oxygen or by post oxidation of polymers.

(2) The solution is that due to the poor thermal stability of PVC, corresponding stabilizers must be added to repair the defects in the PVC chain, while timely absorbing the HCL produced by PVC dechlorination. Due to the high viscosity and shear characteristics of PVC during processing, corresponding lubricants such as polyethylene wax must be added along with stabilizers. Polyethylene wax, also known as PE wax, is a low molecular weight polyethylene that is directly polymerized from ethylene. Products synthesized by different synthesis processes and catalytic systems have differences in molecular weight, molecular weight distribution, and molecular chain structure, resulting in significant differences in the performance of the corresponding products.

PE wax is usually a white powder with an average molecular weight of 1500-5000 and a melting point of 100-120 degrees. It has excellent external lubrication effect in PVC processing, which can effectively improve the fluidity, yield, dispersion, surface brightness, and demolding properties of PVC processing. Due to its large molecular weight, high melting point, and excellent high-temperature stability, it exhibits strong external lubrication effects under both high temperature and high shear conditions.

Polyethylene wax products can effectively improve the dynamic thermal stability and flowability of PVC during processing, increase the extrusion efficiency of PVC products, improve the surface glossiness of products, and effectively reduce the precipitation phenomenon during PVC product processing. There are several types of PE wax available in the domestic market:

Ethylene homopolymerization is used to synthesize PE wax. The PE wax produced by this method has good external lubrication performance, high glossiness, narrow molecular weight distribution, and very stable quality. Representative products include Honeywell's A-C series;

Low molecular weight polyethylene cracking product, commonly known as cracking wax, is a simple and easy to operate cracking process. However, the molecular weight distribution of the product fluctuates with the production process, and the quality is relatively good, with a small amount of low melting point components still present;

The byproduct of ethylene polymerization, commonly known as secondary wax, is purified through a refining process to obtain PE wax. This product has low viscosity, high melting point, excellent external lubrication performance, and low price, but its quality fluctuates with changes in raw materials and processes. Due to the characteristics of the refining process, there are inevitably many low melting point components in the product.