The Stability Problem Of PVC Production
Heat stabilizer is one of the most important auxiliaries in the process of PVC processing. The number of PVC heat stabilizers used is not much, but its effect is huge. The use of heat stabilizers in PVC processing can ensure that PVC is not easy to degrade and is more stable. The heat stabilizers commonly used in PVC processing are alkaline lead salts stabilizers, metal soap stabilizers, organotin stabilizers, rare earth stabilizers, epoxy compounds and so on. The mechanism of PVC degradation is complex, and the mechanisms of different stabilizers are different, and the stability results are also different.
1.Thermal degradation mechanism of PVC
PVC decomposes at 100~150 degrees. Ultraviolet, mechanical, oxygen, ozone, hydrogen chloride and some active metal salts and metal oxides will greatly accelerate the decomposition of PVC. The thermal oxygen aging of PVC is more complicated. Some literature reports that the thermal degradation process of PVC is divided into two steps. (one) dehydrochlorination: PVC polymer chains from active chlorine atoms from hydrogen chloride, and formation of conjugated polyene; (two) the longer chain olefins and aromatic ring formation: with the further degradation of the allylic chlorine atoms, unstable easy to take off, a longer conjugated chain the number of olefins, the so-called "zipper" at the same time, dehydrogenation, fracture, C-C key small ring, have a small amount of aromatic compounds. The decomposition of dehydrochlorination is the main cause of PVC aging. The degradation mechanism of PVC is complex and there is no unified conclusion. The researchers mainly have the mechanism of  free radical, ion mechanism and single molecule mechanism.
2.Thermal stability mechanism of PVC
During the processing, the thermal decomposition of PVC has little effect on other properties, mainly affecting the color of finished products. Adding heat stabilizer can inhibit the initial coloring of products. When the loss of HCl is 0.1%, the color of PVC begins to change. According to the number of conjugated double bonds formed, PVC presents different kinds of color (yellow, orange, red, brown, black). If there is oxygen in the process of PVC thermal decomposition, there will be the formation of colloidal carbon, peroxide, carbonyl and ester based compounds. However, the thermal degradation of PVC has great influence on the performance of materials during a long period of use. Adding heat stabilizer can delay the degradation time of PVC or reduce the degree of PVC degradation.
The addition of heat stabilizer can inhibit the degradation of PVC during the processing of PVC. The main function of heat stabilizer is to inhibit the degradation of PVC molecule by replacing unstable chlorine atoms, absorbing hydrogen chloride and adding reaction to unsaturated parts. The ideal heat stabilizer should have a variety of functions: (1) the substituent replacement lively, unstable, such as connection in the tertiary carbon atom of chlorine atom or allyl chloride, the formation of stable structure; (2) the absorption and release of PVC and HCl in the process of processing, eliminate the automatic catalytic degradation of HCl (; 3) metal ions on catalytic degradation of neutralization or passivation and other harmful impurities; (4) by chemical reaction of various forms of blocking unsaturated bonds continue to grow, inhibits the degradation of coloring; (5) the best protective effect on UV shielding.
3.PVC stabilizer, mechanism and use
3.1 lead salt stabilizer
Lead salt stabilizer can be divided into 3 categories: (1) the lead salt stabilizer alone, mostly containing PbO base salt; (2) heat stabilizers with lubrication, mainly fatty acids and neutral salt base; (3) the compound lead salt stabilizer, and contain lead salts and other stabilizing agent the group with a synergistic mixture of solid and liquid composite stabilizer.
Lead salt stabilizer has strong thermal stability, good dielectric properties and low price. It can make the temperature range of PVC resin wider and the quality of processed and post-processed products stable, which is the most commonly used stabilizer at present. Lead salt stabilizers are mainly used in hard products. The stabilizers of lead salts have the characteristics of good heat stabilizers, excellent electrical properties and low price. But lead salt is toxic and cannot be used in contact with food products, can not be made transparent products, and easily generate black lead sulfide sulfide pollution.
Salt based lead salts are the earliest and most widely used heat stabilizers used in polyvinyl chloride, which are alkaline, so it can be stable with the HCL reaction. In terms of toxicity, antifouling and product transparency, the lead salt is not ideal. However, it has good stability and low price, so it is still widely used in cheap PVC extrusion and calendering products. Because of its excellent electrical properties and low water absorption, it is widely used as a stabilizer for PVC's electrical insulating products, records and foam plastics.
1，three salt base lead sulfate (also known as three alkaline lead sulphate) White powder, the proportion of 7.10, sweet and toxic, easy to absorb moisture, no combustible and corrosive. It dissolves in water but can dissolve in hot acetic amine, and is discoloration and discoloration of light when it is moist. The refractive index is
2.1, which is often used as a stabilizer for electrical insulation.
2, two lead phosphite
This is a fine needle crystal powder; the proportion is 6.1, sweet and toxic; 200 degrees turn into dark gray, 450 DEG turn yellow. This product is insoluble in water and organic solvents, soluble in hydrochloric acid. The refractive index is 2.25. It has an antioxidant effect and is an excellent stabilizer for climate resistance.
3.2 Metal soap stabilizers
Stearic acid soap heat stabilizer is usually made from alkaline earth metal (calcium, cadmium, zinc, barium, etc.) with stearic acid, lauric acid and other saponification. There are many kinds of products and they have their own characteristics. In general, lube stearic acid is superior to lauric acid, but the compatibility of PVC with lauric acid is better than stearic acid.
Because the metal soap can absorb HCl, some varieties can also replace the Cl atoms of the active sites by fatty acids instead of the Cl atoms by the catalysis of their metal ions, so they can play different degrees of thermal stabilization on the metal. The PVC industry is rarely a single metal soap compound, usually a complex of several metal soaps. Calcium and zinc soap stabilizers are common. According to the mechanism of Frye-horst, the stability mechanism of calcium / zinc composite stabilizer can be considered as follows: first, zinc soap is reacted with allyl chloride on PVC chain, then calcium soap, zinc soap and chlorinated chloride react to form unstable metal chlorides.
At this time, the secondary stabilizer, as an intermediate medium, transfers the chlorine atoms into the calcium soap to regenerate the zinc soap, which delays the generation of zinc chloride with the function of dehydrochlorination.
Calcium and zinc stabilizer can be used as a non-toxic stabilizer, used in food packaging and medical devices, medicine packaging, but its stability is relatively low. When the dosage of calcium stabilizer is large, the transparency is poor, and it is easy to frost. Calcium and zinc stabilizers generally use polyols and antioxidants to improve their performance. There are already transparent calcium and zinc composite stabilizers for hard pipes.
Metal soaps are also a kind of widely used polyvinyl chloride heat stabilizers. Use of barium carboxylate, cadmium carboxylate, zinc carboxylate, calcium carboxylate or mixtures of calcium carboxylates. Its stabilizing effect is because it can be esterified in the place where the polyvinyl chloride molecular chain begins to decompose. The effect of stabilization is related to the metal ratio of the metal soap, the type of carboxylic acid, and the existence of synergistic agents such as phosphite, epoxy oil and antioxidant. The stability of cadmium soap and zinc soap is the greatest.
1, lead stearate
This is a fine powder, insoluble in water, soluble in hot ethanol and ether, heated and dissolved in organic solvents, and then cooled to a colloidal substance. When strong acid is decomposed into stearic acid and corresponding lead salt, it is easy to be dampened. It has good lubricity and low melting point, which ensures good dispersion.
2, 2 - ethyl acetate
It is soluble in solvents and plasticizers. A solution that is usually sold as a solution of 57 - 60% mineral oil or plasticizer. It is widely used as an activator of azo two formamide in foam plastics.
3, lead salicylate
This is a white crystalline powder with a specific gravity of 2.36 and a refractive index of 1.76. It also has the effect of PVC heat stabilizer and light stabilizer.
4, three salt base stearate
This is a white powder which is decomposed at 2.15，280-800 ℃ and is easily caking at high temperatures above 100℃. Dissolved in ether, toxic, flammable and corrosive. The index of refraction is 1.60. This product is well lubricated and has good light stability. It is widely used in the FVC record formula.
5, two lead phthalate lead formate
White fine crystalline powder with a specific gravity of 4.5. Insoluble in ordinary solvents. This product is weak acid, its salt base is easily carbonated. The index of refraction is 1.99. When the formula contains the plasticizer that is easily saponified, the stabilizing effect is better than that of three base lead sulfate.
6, three salt base maleic acid lead (three base maleic acid lead)
It is yellow and fine powder with a specific gravity of 6 and a refractive index of 2.08. It is toxic, flammable and corrosive. It has good color stability and eliminates the structure of unstable diene.
Fine white powder, barium content 19.520.6%, the proportion of 1.145%, the melting point of 225℃. Insoluble in water, but melted in hot ethanol. It is heated and dissolved in an organic solvent to form a gelatinous substance after cooling. When strong acid is decomposed into fatty acid and corresponding barium salt, it is easy to be dampened. It is a heat stabilizer which must be used to avoid sulfur pollution. It is also a lubricant used in the process of high temperature processing.
8, the acid barium
9, Barium ricinus
This is a yellowish white powder, the melting point of L16 - 124, can make the products have good transparency stabilizer.
10, cadmium stearate
White fine powder, cadmium content is 16.5 - 17.5%, insoluble in water, soluble in hot ethanol, dissolved in organic solvents, cooled to form colloid, and decomposed into strong acid and stearic acid and cadmium salt, which is easy to get damp. It is a heat and light stabilizer for PVC with good transparency.
11, cadmium ricinate
This is a white powder with a melting point of 96-104 degrees centigrade, and PVC is used for both heat and light stabilizers.
12, calcium stearate
White fine powder, insoluble in water, soluble in hot ethanol and ether. When strong acid is decomposed into stearic acid and corresponding calcium salt, it is easy to be dampened. It is a non-toxic stabilizer and lubricant for PVC. It is usually not used alone, but often used with zinc soap, magnesium soap or epoxy auxiliary stabilizer.
13, calcium ricinate
This is a white powder with a melting point of 74-82 ℃, a nontoxic stabilizer for PVC.
14, zinc stearate
Fine white powder, insoluble in water, dissolved in hot ethanol, turpentine, benzene and other organic solvents. It is heated and dissolved in organic solvents and retreated and cooled into a colloid. When strong acid is decomposed into stearic acid and corresponding zinc salt, it is easy to be dampened. Non toxic stabilizers and lubricants for PVC.
15, magnesium stearate
White powder, the proportion of 1.07, the melting point of pure product is 85℃, the melting point of industrial product is 108-115 ℃. It dissolves in water, soluble in hot ethanol, and is decomposed into stearic acid and magnesium salt with strong acid. This is a non-toxic stabilizer and lubricant for PVC, and it is also a lubricant for phenolic and urea formaldehyde resins.
3.3 organic tin stabilizer
The alkyl tin in organotin is usually three kinds, such as methyl, positive Ding Ji and n- octyl. Most of Japan produces butyltin, and European octyltin is more common. This is a European standard nontoxic stabilizer. The United States uses more methyl tin. Organic tin stabilizer used in three categories: (1) fatty acid salts, mainly refers to the two Ding Ji tin, two lauric acid two lauric acid two octyl tin; (2) maleic acid salts, mainly refers to the two Ding Ji tin, maleic acid bis (maleic acid monobutyl tin, Ding Ji) two Maleic acid two octyl tin; (3) thiol salts, two (thio carboxylic acid) ester is the largest amount of.
Organotin thermal stabilizers have better properties, and are good varieties for PVC hard products and transparent products. Especially Xin Jixi has become an indispensable stabilizer for nontoxic packaging products, but its price is more expensive.
Organotin heat stabilizer (mercapto acetate) has a good stability effect on PVC. In particular, liquid organotin stabilizers, compared with solid heat stabilizers, liquid organotin stabilizers can be better mixed with PVC resin. Organotin stabilizers (mercapto acetate) can replace the unstable Cl atoms on the polymer and make the PVC resin have long-term stability and initial color retention. The stability mechanism of tin thiol acetate is also proposed: (1) S atoms can replace unstable Cl atoms, thus inhibiting the formation of conjugated polyolefin. (2) as a product of thermal degradation of PVC, HCl can also accelerate the formation of conjugated polyolefin. And tin thiol acetate can absorb the produced HCl.
Organic tin stabilizer is tin mercaptide and carboxylic acid derivatives, the main products are organic compounds two and two - butyl octyl tin, of which two octyl tin compounds are more and more countries as a non-toxic stabilizer. Organotin is mainly used to stabilize hard PVC products, especially those that need excellent transparency and thermal stability, of course, it can also stabilize soft products. However, because of its high price, organic tin is generally not used.
1. two lauric acid two butyl tin
This is a light yellow clear liquid, soluble in all industrial plasticizers and solvents, this product is toxic. It is the oldest species in organotin, with excellent lubricity, transparency and weatherability, sulfur - resistant but poor heat resistance. The main stabilizer used as a soft and transparent product and used as a lubricant in hard transparent products.
2. butyltin maleate and two butyl tin
This is a white amorphous powder. The melting point and volatility vary with the degree of polymerization, about 100 - 140 ℃. This product is poisonous and teary. It is mainly used for hard and transparent products with high softening point and high impact strength. As a result of no lubrication, often with two lauric acid two butyl tin and use 0.5-2 portion.
3. dibutyl tin maleate (monbutyl maleate) two butyl tin
Light yellow transparent liquid, non-toxic (allow for the amount of less than 3 copies). This product has good weather resistance, transparency, prevention of coloring and thermal stability, no sulfur pollution, used in PVC transparent hard products, the amount of 2.0-4.0.
4. Bis(2-ethylheryl thioglycolato)dioctyltin
It is a light yellow liquid, insoluble in water, easily soluble in esters, ethers, alcohols, fatty hydrocarbons and aromatics, chlorinated hydrocarbons, and the main types of plasticizers. It is the main variety of thiol tin, one of the most widely used nontoxic organotin stabilizers, for hard transparent PVC (2-3) and soft transparent PVC (1). Its disadvantages are poor weatherability, odor, sulfur and no lubrication. In addition, it has a certain plasticizing effect because of the ester group.
5. Di-n-octyltin dilaurate
This is a yellow liquid, 25 DEG C when the proportion of 1.01-1.02, the refractive index is 1.46-1.47 at 30 DEG in viscosity below 60 cps. This product is nontoxic and has good lubricity. It is mainly used in the hard transparent PVC food packaging material, with the dosage of less than 1.5 copies.
This is a white powder with a melting point of 87-105 degrees centigrade, insoluble in water and soluble in benzene, ethanol and acetone. This product is nontoxic and has excellent long-term heat resistance. It is mainly used as a secondary stabilizer for thiol tin. The dosage is often used in 0.3-0.5 parts.
3.4 rare earth stabilizer
China's rare earth resources rich reserves, accounting for more than 80% of the world, given the thermal stability of rare earth heat stabilizer PVC excellent in plastics processing, a preliminary study on the mechanism of rare earth stabilizer: special electronic structure of rare earth lanthanides (the outermost layer of 2 electrons, 8 times outer electronic structure, there are many empty orbit) decided, a large number of empty orbit as central ion accept ligands, forming ion coordination bond through electrostatic attraction in external thermal oxygen under the action of outer or outer electrons to be intensified, and the chlorine atom of rare earth atom and PVC chain between the strong with potential, it can control the free chlorine atomic generation, automatic oxidation chain reaction so as to prevent or delay Ha, play a role of thermal stability.
The rare earth heat stabilizers mainly include rich rare earth lanthanum, cerium, neodymium, organic weak acid salts and inorganic salts. The kinds of organic weak acid salts include rare earths of stearate, rare earth of fatty acid, rare earth of salicylate, rare earth citric acid, rare earth of lauric acid, rare earth of octanic acid and so on.
A preliminary study on the mechanism of rare earth stabilizer: (1) the special electronic structure of rare earth lanthanides (the outermost layer of 2 electrons, 8 times outer electronic structure, there are many empty orbits) determined the empty orbital energy difference is very small, in the role of the outside world or in thermal oxygen polar groups under the action of the outer layer or outer electrons can be intensified, and the instability of PVC chain of Cl ligand, hydrogen chloride and PVC processing and decomposition of the ligands at the same time, there is a strong attraction between the rare earth elements and chlorine, chlorine free control can play a role, which can prevent the autoxidation of chain reaction or delay hydrogen chloride, to thermal stabilization. (2) rare-earth multifunctional stabilizers can physically adsorb the oxygen impurities in PVC and the ionic impurities contained in PVC itself, and enter the lattice holes of rare-earth multifunctional stabilizers, avoiding their impact vibration on the parent C Cl bond. Therefore, the activation energy of PVC HCl can be increased by the effect of rare earth multifunction stabilizers, so that the thermal degradation of PVC plastics can be delayed. (3) the proper anionic groups in rare earth compounds can replace allyl chloride atoms on PVC macromolecules, eliminate the weakness of degradation, and achieve stable purposes. There are more research on rare earth stabilizers in China.
Overall, stable effect of rare earth heat stabilizer is superior to metal soap stabilizers, has better thermal stability, and with other kinds of stabilizers have synergistic effect widely, is well tolerated, without sulfur pollution, storage stability, non-toxic environmental protection. In addition, rare earth elements have unique coupling effects with CaCO3, and promote the plasticizing effect of PVC. Therefore, the amount of CaCO3 can be increased, the use of processing aids ACR can be reduced, and the cost can be effectively reduced. The characteristics of the stabilizing effect of rare earth on PVC are its unique synergistic effect. The proper coordination of rare earth with some metals, ligands and stabilizers can greatly improve the stability.
3.5 other stabilizers
3.5.1 epoxy class
Epoxidized soybean oil, linseed oil, epoxy, epoxy tallol, butyl epoxy stearate, epoxy ester and other compounds are commonly used in PVC heat stabilizer side, they use a higher synergistic effect combined with the stabilizer has the advantages of light stability and non-toxic, suitable for soft, especially soft FVC products to exposure to sunlight, not normally used for rigid PVC products, its shortcoming is easy to seep.
Studies have shown that adding epoxy to sunflower oil containing metal salt soap different (Ba/Cd and Ca/Zn) in PVC, by measuring the thermal stability of the materials, found that the sunflower oil and metal soap salt has a good synergistic effect, can enhance the thermal stability of PVC materials, the reason analysis of the synergy: HCl produced by the degradation of absorbed sunflower oil and metal salt soap, HCl concentration decreased while reducing the speed of HCl de PVC (HCl has a catalytic effect on PVC degradation), improved the thermal stability of PVC.
3.5.2 polyhydroxyl group
Polyhydroxy compounds, such as pentaerythritol and xylitol, have a certain thermal stability on PVC, and are commonly used as the auxiliary heat stabilizers of PVC.
The rate of dehydrochlorination and thermal stability test, found extension does not contain heavy metal and zinc heat stabilizer PVC/ polyhydroxy compound heat stable time to 200 DEG C, type and the stability effect of hydroxyl number and polyhydroxy compounds, especially polyhydroxy compounds containing hydroxyl end PVC to promote the long-term thermal stability, cause absorption degradation of HCl.
Phosphite, β-two ketone, two hydropyrimidine, two phenyl thiourea, 2-phenyl indole, β - line butylene ester, three hydroxymethylpropane and thiolaic anhydride can also play synergistic role with main heat stabilizer. It can be used as the auxiliary heat stabilizer of PVC to absorb the generated HCl and delay the discoloration of PVC.
4, PVC light stabilizer
The mechanism of light stabilizers because of its structure and different varieties and different, some can shield ultraviolet or UV absorbing and transforming it into harmless heat; some can quench the excited molecules by ultraviolet or group, to make it return to the ground; others capture due to the photo oxidation of free radicals, inhibit photooxidation of chain reaction, make the polymer from UV damage.
PVC is a kind of polymer which is not sensitive to ultraviolet ray, but the residual photosensitive impurities, catalyst residue or other photosensitive additives in PVC will cause the degradation of PVC. When exposed to sunlight, PVC plastics are damaged by ultraviolet radiation, which is absorbed by ultraviolet radiation at 290 to 400 nm, and cause chain reaction. The performance of PVC plastics decreases, such as reducing impact strength or discoloring products. The addition of ultraviolet absorbents in the formula can effectively inhibit the photodegradation. Therefore, the most commonly used UV absorbent is the light stabilizer used in PVC.
The requirements for UV stability of PVC hard products are mainly in outdoor building materials, such as wall panels, louvers and window profiles. Soft products are mainly used for seat covers, garden gardening hoses and lawn facilities.
There are many kinds and varieties of light stabilizers. The main types of PVC are two benzophenones, benzodiazoleds, three azine and carbon black. Common varieties are: UV-9 (2- hydroxy -4- methoxy benzophenone (two), UV-531 2- -4- is hydroxyl octyloxy - two benzophenone (2'-), UV-326[hydroxy -3'- tert butyl -5'- methyl phenyl) -5- chloro benzene and three with UV-P[2- (2'- -5'-, hydroxyl methyl phenyl) benzene and three triazole], UV-24 (2, 2'- two - hydroxy methoxy benzophenone two), three in -5[2, 4, 6- three (2'- hydroxy -4'- is octyloxy phenyl) -1, 3, 5- in three].
Carbon black can absorb light and convert it into heat to re release it without damaging the polymer. But it can only be used in dark polyvinyl chloride products. Its dosage also depends on the color of the products, so the use of carbon black in PVC is restricted and restricted. The choice of light stabilizers for PVC should consider the interaction between them and heat stabilizers. The application of light stabilizers should not depend on the effect of heat stabilizers. For example, when benzophenone light stabilizer and barium cadmium heat stabilizer are used together, the soft PVC products will be yellowed, reducing the alkalinity of the barium cadmium stabilizer, and the yellowing phenomenon will be weakened. Two Benzo three azole light stabilizers are very effective for improving the photostability of PVC, especially for hard PVC. However, in the hard polyvinyl chloride, some benzo three azole light stabilizers and thiazine heat stabilizers are used to form a pink complex. Therefore, when the heat stabilizer is metal soap, UV-P is often used, and the amount is 0.2%-0.5%. When thiol organotin is used as a heat stabilizer, UV-531 is often used, and the amount is 0.3-0.5%. In the polyvinyl chloride (PVC) agricultural film, three azozine -5 has a prominent anti aging effect, and the amount is 0.2-0.5%.
5，Current status and development trend of PVC heat stabilizers
After entering in twenty-first Century, due to the global requirements of the increasingly strict environmental protection regulations, restrictions on the heavy metal stabilizer is increasing, the production and consumption of heat stabilizer to the further development of non-toxic, low toxicity and high efficiency composite direction, lead-free, cadmium free developed countries has attracted universal attention, and the application of alternative products appear, lead and cadmium (especially CD) stabilizer application has showed a gradual downward trend, there are some non-toxic or low toxic heat stabilizers (such as organic tin compounds, calcium / zinc soap salt and rare earth stabilizer etc.).
Although the production and development of compound, non-toxic and low toxic heat stabilizers have made considerable progress in recent years, there are many deficiencies and gaps compared with the advanced level in the world, such as fewer varieties and smaller production scale. The production and application of new heat stabilizers in China is far from enough to meet the development of domestic PVC industry. The heat stabilizers of some high-grade PVC products also rely mainly on imports. The rapid development of PVC industry in China has provided a good market guarantee and wide development space for the development of heat stabilizer industry.
At the same time, it also put forward higher requirements for heat stabilizer industry. To strengthen China's new research and development of heat stabilizer, should pay attention to the following points: (a) strengthen the original lead and cadmium free calcium zinc stabilizers research and improvement, improve the quality of the original products; (two) according to the sources of raw materials and market distribution, and gradually establish the relative concentration of the large-scale production plant auxiliary agent group; (three) coordination development and the production of other PVC additives, the development of multiple composite products, further reduce resource waste and environmental pollution, promote the sustainable development of the green additives industry.