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Bottom PVC heat stabilizer
Bottom PVC heat stabilizer
1. Why does PVC necessarily need heat stabilizers?
PVC is one of the largest plastic products in the world. PVC is a white powder with an amorphous structure, a small degree of branching, and poor stability to light and heat. Due to the extremely poor processing stability of PVC, the melting process is as high as 160℃, and PVC is decomposed when the temperature reaches 140℃, so PVC resin should be processed and produced by the method of melting and plasticizing through modification treatment.
2. The role of heat stabilizers
In the process of resin processing, it can prevent or inhibit the removal of HCl and the discoloration caused by the removal of HCl.
During the effective use period of the product, maintain its sufficient thermal stability and slow down the degradation caused by heat, light, and prolong its service life.
3. The mechanism of action of heat stabilizers
1. Absorb HCl generated during the degradation of PVC;
2. Replace the unstable allyl chlorine atom or tertiary carbon chlorine atom in the PVC molecule to inhibit the removal of PVC and HCL;
3. React with free radicals to terminate the transmission of free radicals;
4. Addition reaction with conjugated double bonds to reduce coloration;
5. Inhibit the oxidation of PVC;
6. Passivation of metal ions with catalytic HCl removal.
4. Classification of heat stabilizers
According to their role, they can be divided into three categories:
Main heat stabilizer (lead salts, metal soaps, organotin, etc.)
Auxiliary heat stabilizers (epoxy compounds, phosphites, polyols, etc.)
Composite stabilizer composed of primary and secondary stabilizers
1. Main heat stabilizer
PVC stabilizers mainly include lead salts, fatty acid metal salts, and organotin.
1. Lead salts include all lead salts except lead soap, most of which contain lead salts with a base, namely lead monoxide ( PbO ). Lead salt stabilizers are the most commonly used and most effective stabilizers for PVC, but are not suitable for transparent products; lead salt stabilizers have gradually withdrawn from the market in recent years due to their high toxicity.
name | abbreviation | Performance characteristics |
Tribasic lead sulfate (trisalt) | TLS | It has long-lasting heat resistance, good light and water resistance, and excellent electrical insulation. High toxicity, lack of lubricity, should be used in conjunction with lubricants |
Dibasic lead phosphite (disalt) | DL | weather resistance is very prominent, the toxicity is high, and the lubricity is lacking, so it should be used with lubricants |
Dibasic lead stearate (dishard salt) | DLS | It has the functions of lead salt and metal soap, and has excellent lubricity, but poor weather resistance |
2. Mostly salts of metals (barium, cadmium, lead, zinc, calcium, magnesium) of fatty acids (lauric acid, stearic acid, etc.). Its general formula is (RCOO) nM , in addition to its interaction with HCl, its main function is to replace the active allyl chloride atom. The stabilization effect of metal soaps is general, transparent, toxic or non-toxic. They are rarely used alone, but are generally used in combination, mainly for soft products.
name | abbreviation | Performance characteristics |
Cadmium Stearate | CdSt | As one of the important transparent heat stabilizers, it has small initial coloration and is often used in combination with barium stearate. |
barium stearate | BaSt | It has excellent lubricity, and the initial hue is not good. It is often used in conjunction with PbSt , CdSt , etc. Good anti- sulfur pollution , but easy to precipitate |
Calcium stearate | CaSt | Good processability, non-toxic, generally not used alone, but often used in combination with zinc soap, magnesium soap or epoxy secondary stabilizer |
lead stearate | PbSt | It has good thermal stability, can also be used as a lubricant for PVC, and has a good synergistic effect when used together with cadmium, barium and organic tin. However, it has poor plasticizing performance, easy to precipitate, poor transparency, high toxicity, and serious vulcanization pollution. |
Zinc stearate | ZnSt | Very high activation, a small amount of addition can improve the initial coloration of PVC, and has the advantages of significant anti-sulfur pollution and anti-precipitation, but poor later stability, easy to cause "zinc burning", calcium and barium soap should be used together |
3. The general formula of organotin is RmSnY4-m (R is an alkyl group, and Y is a group connected to Sn through an oxygen atom or a sulfur atom). Depending on Y, there are three main types of organotin stabilizers: fatty acid salt type, maleate type and thiolate type. Organotin has good stabilization effect, is suitable for transparent products, and has non-toxic specifications; but its price is more expensive.
category | Performance characteristics |
fatty acid salt | Good lubricity and processing performance, but poor thermal stability and transparency, and obvious initial coloration when used alone. |
Maleate | Good heat and weather resistance, mainly used as the main stabilizer for PVC rigid transparent products, can prevent initial coloring, but lacks lubricity, easy to bloom |
thiolate | Outstanding heat resistance and good transparency, no initial coloring, especially suitable for hard transparent products, good processing performance, but high price |
2. Auxiliary heat stabilizer
1. Epoxy compound
Epoxy compounds are important auxiliary heat stabilizers for PVC. They can enhance the heat resistance and weather resistance of the main heat stabilizer. They are mainly divided into two categories: plasticizing type and resin type.
The plasticizing types are mainly epoxy compounds such as epoxy soybean oil, epoxy linseed oil, epoxy butyl stearate, and octyl ester.
The main resin type is epichlorohydrin bisphenol A epoxy resin.
2. Phosphite compounds
There are many types of phosphites, including trialkyl phosphites, triaryl esters, mixed alkyl aryl esters , trithioalkyls , bisphosphites , and polymeric phosphites.
3. Organic Phosphite Compounds
Organic phosphites are peroxide decomposers, widely used as auxiliary antioxidants in polyolefins and synthetic rubbers, and as chelating agents in PVC. When used in combination with metal stabilizers, it can combine metal ions to prevent the catalytic degradation of metal ions, thereby improving the heat resistance and weather resistance of PVC.
4. Polyols
Polyols can improve the thermal stability of PVC and improve color. At present, it is believed that the addition of polyols can absorb impurity ions to inhibit its catalytic degradation and electrical conductivity. The main varieties are pentaerythritol, xylitol, mannitol, sorbitol and so on.
3. Compound heat stabilizer
There are many kinds of composite stabilizers in PVC additives, the more common ones are composite calcium zinc stabilizer , lead salt composite stabilizer, barium cadmium zinc composite stabilizer, organic tin composite stabilizer and so on.
The advantages of composite heat stabilizer: good compatibility with resin, good transparency, not easy to precipitate, easy to disperse, easy to measure, and good processing performance.
5. Development Trend of Heat Stabilizers for PVC Materials
(1) Develop low-toxic and non-toxic varieties
(2) Vigorously develop organotin stabilizers
(3) Actively develop organic auxiliary heat stabilizers
In short, with the gradual strengthening of people's awareness of environmental protection and health, heat stabilizers will develop towards low toxicity, no pollution, and compound high efficiency.
