1. Thermal degradation mechanism of PVC
PVC is obviously decomposed at 100-150 °C. Ultraviolet light, mechanical force, oxygen, ozone, hydrogen chloride and some active metal salts and metal oxides will greatly accelerate the decomposition of PVC. The thermal aging of PVC is complicated. Some literature reports that the thermal degradation process of PVC is divided into two steps. (1) Dehydrochlorination: the removal of active chlorine atoms from the molecular chain of PVC polymer to produce hydrogen chloride and the formation of conjugated multiolefins; (2) Formation of longer chain polyenes and aromatic rings: as the degradation proceeds further, The chlorine atom on the allyl group is extremely unstable and easily desorbed, resulting in a longer chain conjugated multiolefin, so-called "zipper" dehydrogenation, while a small amount of CC bonds are broken and cyclized, resulting in a small amount of aromatic Class of compounds. Decomposing dehydrochlorination is the main cause of PVC aging. The degradation mechanism of PVC is complicated, and there is no unified conclusion. The researchers mainly propose free radical mechanism, ion mechanism and single molecule mechanism.
2. Thermal stability mechanism of PVC
During the processing, the thermal decomposition of PVC has little change for other properties, mainly affecting the color of the finished product, and adding a heat stabilizer can suppress the initial coloration of the product. When the mass fraction of HCl removed reaches 0.1%, the color of the PVC begins to change. Depending on the number of conjugated double bonds formed, PVC will appear in different colors (yellow, orange, red, brown, black). If oxygen is present during the thermal decomposition of PVC, there will be formation of colloidal carbon, peroxide, carbonyl and ester based compounds. However, the thermal degradation of PVC has a great influence on the performance of the material during the long-term use of the product. Adding a heat stabilizer can delay the degradation time of PVC or reduce the degree of PVC degradation.
The addition of a heat stabilizer during the processing of PVC can inhibit the degradation of PVC. The main functions of the heat stabilizer are: by replacing unstable chlorine atoms, absorbing hydrogen chloride, and adding reaction with unsaturated sites. Degradation of PVC molecules. The ideal heat stabilizer should have multiple functions: (1) substitution of reactive, unstable substituents such as a chlorine atom or allyl chloride attached to a tertiary carbon atom to form a stable structure; (2) absorption and neutralization And HCl released during PVC processing to eliminate the autocatalytic degradation of HCl; (3) neutralize or passivate metal ions and other harmful impurities that catalyze degradation; (4) resist by various forms of chemical reactions The unsaturated unsaturated bond continues to grow, inhibiting degradation and coloration; (5) It is better to have protective shielding effect on ultraviolet light.
3. PVC stabilizer, mechanism of action and use
3.1 lead salt stabilizer
Lead salt stabilizers [7] can be divided into three categories: (1) simple lead salt stabilizers, mostly salt-based salts containing PbO; (2) heat stabilizers with lubrication, mainly neutrality of fatty acids And a salt-based salt; (3) a composite lead salt stabilizer, and a solid and liquid composite stabilizer containing a synergistic mixture of lead salts and other stabilizers and components.
The lead salt stabilizer has strong thermal stability, good dielectric properties, and low price. The reasonable ratio of lubricant to PVC can widen the processing temperature range of PVC resin, and the quality of processed and post-processed products is stable. A commonly used stabilizer. Lead salt stabilizers are mainly used in hard products. Lead salt stabilizers have the characteristics of good heat stabilizer, excellent electrical properties and low cost. However, lead salts are toxic, cannot be used for products that come into contact with food, nor can they be made transparent, and are easily contaminated with sulfides to form black lead sulfide.
The salt-based lead salt is the earliest and most widely used heat stabilizer for polyvinyl chloride. It is alkaline and can react with the produced HCL to stabilize. Lead salts are not ideal in terms of toxicity, stain resistance and product transparency. However, it has a good stabilizing effect and low price, so it is still widely used in inexpensive PVC extrusion and calendering products. Because of its excellent electrical properties and low water absorption, it is widely used as a stabilizer for PVC electrical insulation products, records and foams.
1. Tribasic lead sulfate (also known as tribasic lead sulfate)
White powder, specific gravity 7.10, sweet and toxic, hygroscopic, non-flammable and corrosive. It is insoluble in water, but soluble in hot amine acetate. It will discolor and decompose when exposed to light. Refractive index of 2.1, often used as a stabilizer for electrical insulation products.
2. Dibasic lead phosphite
This is a fine needle-like crystalline powder; the specific gravity is 6.1, sweet and toxic; it becomes grayish black at around 200 °C, and turns yellow at around 450 °C. This product is insoluble in water and organic solvents, soluble in hydrochloric acid. It has a refractive index of 2.25 and is an antioxidant. It is an excellent weathering stabilizer.
3.2 Metal soap stabilizers
Stearic acid soap heat stabilizers are generally prepared by saponification of alkaline earth metals (calcium, cadmium, zinc, antimony, etc.) with stearic acid, lauric acid, and the like. There are many types of products, each with its own characteristics. Generally, lubricious stearic acid is superior to lauric acid, and PVC compatible lauric acid is superior to stearic acid.
Because metal soap can absorb HCl, some varieties can also replace the Cl atom of the active site with the fatty acid root by the catalytic action of its metal ions, so it can exert different degrees of thermal stabilization on PVC. There are very few metal soap compounds in the PVC industry, but usually a composite of several metal soaps. Common are calcium zinc soap stabilizers. According to the Frye-horst mechanism, the stabilization mechanism of the calcium/zinc composite stabilizer can be considered as follows: first, the zinc soap reacts with allyl chloride on the PVC chain, and then the calcium soap and the zinc soap react with the chlorine chloride to form an unstable metal chloride. At this time, the auxiliary stabilizer as an intermediate medium transfers the chlorine atoms to the calcium soap to regenerate the zinc soap, delaying the formation of zinc chloride having a function of promoting dehydrochlorination.
Calcium-zinc stabilizers can be used as non-toxic stabilizers in food packaging, medical devices, and pharmaceutical packaging, but their stability is relatively low. When the amount of calcium stabilizers is large, the transparency is poor and easy to spray. Calcium-zinc stabilizers generally use polyols and antioxidants to improve their performance. There are already transparent calcium-zinc composite stabilizers for hard pipes in China.
Metal soaps are also a class of widely used polyvinyl chloride heat stabilizers. It is used as a simple substance or a mixture of cerium carboxylate, cadmium carboxylate, zinc carboxylate, and calcium carboxylate. Its stabilizing effect is due to its esterification at the point where the decomposition of the PVC molecular chain begins. The strength of the stabilizing effect is related to the metal ratio in the metal soap, the type of the carboxylic acid, and the presence or absence of a synergistic agent such as a phosphite, an epoxidized oil, or an antioxidant in the formulation. Among them, cadmium soap and zinc soap have the greatest stabilizing effect.
1. Lead stearate
This is a fine powder which is insoluble in water, soluble in hot ethanol and ether, dissolved in an organic solvent, and cooled to a gel. In the case of strong acid decomposition into stearic acid and the corresponding lead salt, it is susceptible to moisture. It has good lubricity and low melting point to ensure good dispersion.
2, 2-ethyl acetate lead
It is soluble in solvents and plasticizers. It is usually sold as a solution of 57-60% mineral oil or plasticizer. It is widely used as an activator for the foaming agent azodicarbonamide in foams.
3. Lead salicylate
This is a white crystalline powder with a specific gravity of 2.36 and a refractive index of 1.76. It has the functions of PVC heat stabilizer and light stabilizer.
4. Tribasic lead stearate
This is a white powder with a specific gravity of 2.15 and decomposition at 280-800 ° C. It is easy to agglomerate when exposed to high temperatures above 100 ° C. Soluble in ether, toxic, non-flammable and corrosive. The refractive index is 1.60. This product has good lubrication and good light stability, and is widely used in FVC record formulations.
5. Dibasic lead phthalate
White fine crystalline powder with a specific gravity of 4.5. Do not dissolve in common solvents. This product is weakly acidic, and its base part is easily carbonated. The refractive index is 1.99. When the formulation contains a saponified plasticizer, the stabilizing effect is better than that of the tribasic lead sulfate.
6. Tribasic maleic acid lead (tribasic maleate maleate)
It is a yellowish fine powder with a specific gravity of 6.0 and a refractive index of 2.08. It is toxic, non-flammable and corrosive, has good color stability, and has the function of eliminating unstable diene structure.
7, barium stearate
White fine powder, strontium content 19.5-20.6%, specific gravity 1.145%, melting point 225 ° C or more. Do not dissolve in water, but simmer in hot ethanol. It is heated and dissolved in an organic solvent to form a gel after cooling. In the case of strong acid decomposition into petty acid and the corresponding barium salt, it is easy to get wet. It is a heat stabilizer that must be used when sulfur contamination is avoided. It is also a lubricant used in processing at high temperatures.
8, barium laurate
9, ricinoleic acid
This is a yellowish-white powder with a melting point of l16-124, which is a stabilizer that gives the product good transparency.
10. Cadmium stearate
White fine powder, cadmium content 16.5-17.5%, insoluble in water, soluble in hot ethanol, heated and dissolved in organic solvent, cooled to gel, in the case of strong acid decomposition into stearic acid and corresponding cadmium salt, easy to get wet . It is a heat and light stabilizer for PVC that requires good transparency.
11. Cadmium ricinoleate
This is a white powder with a melting point of 96-104 ° C. It is combined with heat and light stabilizer for PVC.
12. Calcium stearate
White fine powder, insoluble in water, soluble in hot ethanol and ether. In the case of strong acid decomposition into stearic acid and the corresponding calcium salt, it is susceptible to moisture. It is a non-toxic stabilizer and lubricant for PVC. It is generally not used alone, but is often used in combination with zinc soap, magnesium soap or epoxy secondary stabilizers.
13, calcium ricinoleate
This is a white powder with a melting point of 74-82 ° C, a non-toxic stabilizer for PVC.
14, zinc stearate
White fine powder, insoluble in water, soluble in hot alcohol, turpentine, benzene and other organic solvents. After heating and dissolving in an organic solvent, it is cooled to a gel. In the case of strong acid decomposition into stearic acid and the corresponding zinc salt, it is susceptible to moisture. Non-toxic stabilizers and lubricants for PVC.
15. Magnesium stearate
White powder, specific gravity 1.07, pure melting point of 85 ° C industrial product melting point of l08-115 ° C. Slightly soluble in water, soluble in hot ethanol, decomposed into stearic acid and magnesium salt in the presence of strong acid. This is a non-toxic stabilizer and lubricant for PVC and a lubricant for phenolic and urea-formaldehyde resins.
3.3 organotin stabilizer
The alkyl tin in organotin is usually methyl, n-butyl or n-octyl. Most of the products produced in Japan are butyl tins. European octyl tins are more common. This is a standard non-toxic stabilizer approved in Europe. In the United States, methyl tin is used more. There are three main types of commonly used organotin stabilizers: (1) aliphatic acid salts, mainly dibutyltin dilaurate, di-n-octyltin dilaurate, etc.; (2) maleate salts, mainly horses Dibutyltin acid, bis(monobutyl maleate) dibutyltin, di-n-octyl maleate, etc.; (3) thiolates, of which bis(thiocarboxylic acid) ester is the most used.
Organotin-based heat stabilizers have good performance and are good varieties for PVC hard products and transparent products. Especially, octyl tin is an indispensable stabilizer for non-toxic packaging products, but its price is relatively expensive.
The organotin heat stabilizer (tin thioacetate) has a good stabilizing effect on PVC. Especially liquid organotin stabilizers, liquid organotin stabilizers can be better mixed with PVC resins than solid heat stabilizers. The organotin stabilizer (tin thioacetate) can replace the unstable Cl atoms on the polymer, giving the PVC resin long-term stability and initial color retention. The stabilizing mechanism of tin thioglycolate is proposed: (1) S atoms can replace unstable Cl atoms, thus inhibiting the formation of conjugated polyenes. (2) HCl, as a product of thermal degradation of PVC, can accelerate the formation of conjugated polyenes. Tin thioacetate can absorb the produced HCl.
Organotin stabilizers are derivatives of various tin carboxylates and tin mercaptides. The main products are organic compounds of dibutylphosphonium and dioctyltin, of which dioctyltin compounds are used as non-toxic stabilizers in more countries. Organotin is mainly used to stabilize hard PVC products. Especially those products that require excellent transparency and thermal stability can of course stabilize soft products, but they are generally not used because they are expensive.
1. Dibutyltin dilaurate
This is a light yellow clear liquid that is soluble in all industrial plasticizers and solvents. It is toxic. It is the oldest variety of organotin, with excellent lubricity, transparency and weather resistance, sulfur resistance but poor heat resistance. Used as a primary stabilizer for soft, transparent articles, used as a lubricant in rigid transparent articles.
2. Dibutyltin maleate
This is a white amorphous powder, and the melting point and volatility vary depending on the degree of polymerization, and are between about 100 and 140 °C. This product is toxic and tearful. Mainly used for hard transparent products requiring high softening point and high impact strength. Because it has no lubricating effect, it is often used together with dibutyltin dilaurate in an amount of 0.5-2 parts.
3. Bis(monobutyl maleate) dibutyltin
Light yellow transparent liquid, non-toxic (allows less than 3 parts). This product has good weather resistance, transparency, coloring resistance and thermal stability, does not cause sulfur stains, and is often used in PVC transparent hard products, the dosage is 2.0-4.0 parts.
4. Bis(isobutyl thioglycolate) di-n-octyltin
It is a pale yellow liquid that is insoluble in water and readily soluble in esters, ethers, alcohols, aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons and the main types of plasticizers. It is the main variety in tin thiolate, one of the most commonly used non-toxic organotin stabilizers for rigid transparent PVC (2-3 parts) and soft transparent PVC (1 part). The disadvantages are poor weather resistance, odor, sulfur, and no lubrication. In addition, due to the ester group, there is a certain plasticization effect.
5. Di-n-octyltin dilaurate
This is a yellow liquid having a specific gravity of 1.01-1.02 at 25 ° C, a refractive index of 1.46-1.47, and a viscosity at 60 ° C below 60 ° C. This product is non-toxic and has good lubricity. It is mainly used for hard transparent PVC food packaging materials, and the dosage is less than 1.5 parts.
6. Di-n-octyl tin maleate ’
This is a white powder with a melting point of 87-105 ° C, insoluble in water, soluble in benzene, ethanol, acetone. This product is non-toxic, has excellent long-term heat resistance, and is mainly used as a secondary stabilizer for tin thiolate. The dosage is usually 0.3-0.5 parts.
3.4 Rare Earth Stabilizer
China's rare earth resources are abundant, accounting for more than 80% of the world's total. Rare earth heat stabilizers give PVC excellent thermal stability in plastic processing. The mechanism of rare earth stabilizers is: the special electronic structure of rare earth lanthanides (most 2 electrons in the outer layer, 8 electronic structures in the outer layer, and many empty orbits are determined. Many empty orbits can be used as the central ion to accept the ligand, and the ion pairing is formed by electrostatic attraction. Under the action of external thermal oxygen, The outer or sub-outer electrons are excited, and the strong coordination energy between the rare earth atoms and the chlorine atoms on the PVC chain can control the generation of free chlorine atoms, thereby preventing or delaying the auto-oxidation chain reaction of Ha. Play a role in heat stability.
Rare earth heat stabilizers mainly include organic weak acid salts and inorganic salts of light rare earth lanthanum, cerium and lanthanum. The types of organic weak acid salts are rare earth stearate, rare earth fatty acid, rare earth salicylic acid, rare earth citrate, rare earth laurate, rare earth octoate and the like.
The preliminary study on the mechanism of action of rare earth stabilizers is as follows: (1) The special electronic structure of rare earth lanthanides (8 electrons in the outermost layer, 8 electronic structures in the outer layer, and many empty orbits) Very small, under the action of external thermal oxygen or under the action of polar groups
