Application of heat stabilizers in PVC plastics
Polyvinyl chloride (PVC) is extremely sensitive to heat, poor thermal stability, processing temperature greater than 90 degrees Celsius that decomposition, so PVC processing must add the appropriate amount of heat stabilizers to inhibit its decomposition.
Heat stabilizers mainly include lead salts, metal soaps, organic tin, compound heat stabilizers, phosphite esters, mono-alcohols and polyols, epoxy and nitrogen compounds, the first four categories account for the vast majority of the amount of heat stabilizers. Heat stabilizers have been the direction of research, development and production of efficient and durable, low-cost, low coloring and electrical insulation. 1980s, due to the increasing awareness of environmental protection worldwide, non-toxic and environmentally friendly heat stabilizers have become a hot spot for development.In the process of thermal degradation of PVC, almost no monomer is produced, but a large amount of HC1 is generated. the mechanism of thermal degradation of PVC is very complex, likewise, the mechanism of action of heat stabilizers is also very complex. Comprehensive research results so far, the role of heat stabilizers can be summarized as preventive (neutralizing HC1, replacing unstable chlorine atoms and preventing auto-oxidation) and remedial (reacting with unsaturated parts in PVC to destroy carbon positive ions). Details are as follows.
(1) Neutralization of HCL removed from PVC to inhibit its autocatalytic effect. For example, lead salts, organic acid metal soaps, organic tin compounds, epoxy compounds, amines, metal alcohol salts and phenol salts and metal thiol salts can react with HC1 and inhibit the deHC1 reaction of PVC.
Me(RCOO)2+2HCI→Me C12 +2RCOOH
(2) Displacement or elimination of destabilizing factors such as allyl chloride atoms or tertiary carbon chloride atoms in PVC molecules to eliminate the initiation point of deHCI. For example, the organotin stabilizer tin atoms and PVC molecules of unstable chlorine atoms ligand bonding, while the sulfur atoms in organotin complexed with the corresponding carbon atoms in PVC, ligands in the sulfur atoms and unstable chlorine atoms replacement, when there is the presence of HC1, ligand bond splitting, sulfhydryl groups and PVC molecules in the carbon atoms firmly bonded, thereby inhibiting further de HCI formation of double bond reaction. Among the metal soaps, zinc and cadmium soaps have the fastest substitution reactions with unstable chlorine atoms, barium soaps are the slowest, calcium soaps are slower, and lead soaps are in the middle. At the same time, the resulting metal chlorides, which have different degrees of catalytic effect on the dechlorination of hydrogen chloride, are in the following order of strength
ZnCl2> CdCl2>> BaCl2 , CaCl2 > R2SnCl2
(3) Adds to double bonds and conjugated double bonds to prevent the development of polyene structures and reduce coloration. The salts or complexes of unsaturated acids have double bonds and undergo double-ene addition reactions with conjugated double bonds in PVC molecules
thereby destroying its conjugated structure and inhibiting discoloration. In addition, the metal soap substitution of allyl chloride is accompanied by double bond transfer, which breaks the polyene structure and thus inhibits discoloration.
(4) Trap free radicals and prevent auto-oxidation. Such as the addition of phenolic heat stabilizers can block the deHCL is due to the phenol given by the hydrogen atom radicals can be coupled with the degradation of PVC macromolecule radicals, the formation of substances that can not react with oxygen, and has a heat stabilizing effect. This heat stabilizer can have one or several roles.
There are synergistic effects between different heat stabilizers. Domestic and foreign medical community is generally considered safe and non-toxic calcium and zinc complex stabilizers are so. The U.S. Health Administration and the Japanese Health Bureau that calcium and zinc composite series stabilizers are a non-toxic additives for plastic products. It is foreseeable that calcium and zinc compound stabilizers will eventually build the future non-toxic PVC stabilizer system in each country
Zinc carboxylate and PVC allyl chloride reaction is the main reaction of calcium-zinc complex stabilizer stabilization effect, the presence of calcium carboxylate zinc soap and PVC unstable chlorine atom reaction generated by the harmful ZnCl2 regeneration into zinc soap, which on the one hand activates the zinc soap, on the other hand, and greatly reduces its catalytic capacity, and itself into a harmless calcium chloride, thereby inhibiting the destructive effect of ZnCl2, the principle as shown in Figure 11-1
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The rapid development of the PVC industry has greatly driven the development and progress of heat stabilizers. The development and development of heat stabilizers show new weather, continue to low toxicity, non-toxic development, lead system, cadmium system of heat stabilizers accounted for the proportion of declining year by year; organic tin development momentum slowed down; zinc system of heat stabilizers to accelerate the development of speed; new heat stabilizers water talc series, rare earth series, organic antimony series, perchlorate series and other development and application of the development of positive development momentum; multiple compound “One package” has become the development trend. Between different heat stabilizers, heat stabilizers and plasticizers, lubricants, antioxidants and other additives, sometimes there is a synergistic effect. In order to achieve the desired stabilization effect, they will be compounded and mixed in the appropriate proportion and method, made of “a package” stabilizer system, not only to improve the stabilization effect, and easy to use, reduce dust pollution.
Formally due to the successful development of effective heat stabilizers, there is today’s PVC industry. The amount of heat stabilizers at home and abroad has increased dramatically with the surge in PVC consumption, only differing in the variety of heat stabilizers. It can be expected that the demand for heat stabilizers will increase with the growth of PVC consumption.
According to the degree of hardness and softness of the product, the degree of transparency, whether the requirement of non-toxic, and other additives synergistic compounding to choose the best heat stabilizer, in order to play its due role. Such as PVC non-toxic transparent hard bottle formulations are shown in Table 11-3.
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Table 11-3 in the addition of organotin heat stabilizer to meet the requirements of non-toxic and non-hazardous, but also to improve the thermal stability of the product. Organotin heat stabilizers are one of the faster developing non-toxic heat stabilizers today.
PVC primary stabilizers are those compounds that have a stabilizing effect when used alone, while secondary stabilizers are those compounds that are ineffective when used alone but have a synergistic effect when combined with the primary stabilizer. Certain primary stabilizers or certain primary and secondary stabilizers have synergistic effects when used in combination with each other.
(A) Salt-based lead salts
Salt-based lead salts are the earliest and most widely used heat stabilizers for PVC, and are alkaline in nature, so they can react with the HCL generated and play a stabilizing role. In terms of toxicity, stain resistance and product transparency, lead salts are not ideal. However, it has good stabilization effect and low price, so it is still used in large quantities in cheap PVC extrusion and calendering products. Because it has excellent electrical properties and low water absorption, it is widely used as stabilizer for PVC electrical insulation products, records and foam.
1、Tri-salt lead sulfate (also called tri-basic lead sulfate)
White powder, specific gravity 7.10, sweet and toxic, easy to absorb moisture, non-combustible and corrosive. It is insoluble in water, but soluble in hot amine acetate, and will change color and decompose when it is wet and exposed to light. Refractive index 2.1, often used as a stabilizer for electrical insulation products.
2、Dibasic lead phosphite
This is a fine needle-like crystalline powder; specific gravity 6.1, sweet taste toxic; about 200 ℃ into gray-black, about 450 ℃ into yellow. This product is insoluble in water and organic solvents, soluble in hydrochloric acid. Refractive index of 2.25, the role of antioxidants, is an excellent climate-resistant stabilizer.
(B) metal soaps
Metal soaps are also a class of widely used polyvinyl chloride heat stabilizers. Barium carboxylate, cadmium carboxylate, zinc carboxylate, calcium carboxylate are used as monomers or mixtures. Its stabilizing effect is due to its ability to act as an ester at the point where the decomposition of the polyvinyl chloride molecular chain begins. The strength of the stabilizing effect is related to the metal ratio in the metal soap, the type of carboxylic acid, and the presence of cofactors such as phosphite, epoxidized oil, and antioxidants in the formulation. Among them, cadmium soap and zinc soap has the greatest stabilizing effect.
1, lead stearate
This is a fine powder, it is insoluble in water, soluble in hot ethanol and ether, heated in organic solvents to dissolve, and then cooled to become gelatinous. It decomposes into stearic acid and the corresponding lead salt when it meets strong acid, and is easily affected by moisture. It has good lubricity and low melting point to ensure its good dispersibility.
2、Lead 2-ethylacetate
It can be soluble in solvents and plasticizers. It is usually sold as 57-60% solution of mineral oil or plasticizer. It is widely used as activator of foaming agent azodicarbonamide in foam.
3、Lead salicylate
It is a white crystalline powder, specific gravity 2.36, refractive index 1.76. It is also a PVC heat stabilizer and light stabilizer.
4、Lead trisodium stearate
It is a white powder, specific gravity 2.15, decompose at 280-800℃, easy to agglomerate at high temperature above 100℃. It is soluble in ether, toxic, non-combustible and corrosive. Refractive index 1.60. This product has good lubricating parts, good light stability, widely used in the formulation of FVC records.
5、Lead dibasic phthalate
White fine crystalline powder, specific gravity 4.5. insoluble in common solvent. The product is weakly acidic, its salt part is easy to carbonate. The refractive index is 1.99. When the formula contains plasticizer which is easy to saponify, the stabilizing effect is better than lead sulfate.
6、Tri-salt lead maleate (tri-salt lead maleate)
Fine yellow powder, specific gravity 6.0, refractive index 2.08, toxic, non-flammable and corrosive, good color stability, and has the effect of eliminating unstable diene structure.
7、Barium stearate
White fine powder, barium content 19.5-20.6%, specific gravity 1.145%, melting point 225℃ or more. Insoluble in water, but smelt in hot ethanol. It dissolves in organic solvent by heating and becomes gelatinous after cooling. It decomposes into fatty acid and corresponding barium salt when it meets strong acid, and it is easily affected by moisture. It is a heat stabilizer for selection when sulfur pollution must be avoided, and it is also a lubricant used in processing under high temperature.
8、Barium tannate
9、Barium ricinoleate
It is a yellowish-white powder with melting point l16-124, which can make the products get good transparency stabilizer.
10、Cadmium stearate
White fine powder, cadmium content 16.5-17.5%, insoluble in water, soluble in hot ethanol, dissolved by heating in organic solvent and become gelatinous after cooling, decomposed into stearic acid and corresponding cadmium salt when meeting strong acid, easy to moisture. It is the heat and light stabilizer of PVC which requires good transparency.
11、Cadmium ricinoleate
This is a white powder, melting point 96-104 ℃, PVC with both heat and light stabilizer role
12、Calcium stearate
White fine powder, insoluble in water, soluble in hot ethanol and ether. Decompose into stearic acid and corresponding calcium salt when meet strong acid, easy to moisture. It is a non-toxic stabilizer and lubricant for PVC. Generally not used alone, but often used with zinc soap, magnesium soap or epoxy type of secondary stabilizer.
13、Calcium ricinoleate
This is a white powder, melting point 74-82 ℃, PVC with non-toxic stabilizer.
14、Zinc stearate
White fine powder, insoluble in water, soluble in hot ethanol, turpentine, benzene and other organic solvents. Dissolved in organic solvents after heating and receding into gel. It decomposes into stearic acid and corresponding zinc salt when it meets strong acid, and is easily affected by moisture. Also PVC non-toxic stabilizer and lubricant.
15、Magnesium stearate
White powder, specific gravity 1.07, pure product melting point of 85 ℃ industrial products melting point of l08-115 ℃. Slightly soluble in water, soluble in hot ethanol, decomposed into stearic acid and magnesium salt when encountering strong acid. This is a non-toxic stabilizer and lubricant of PVC, but also phenolic and urea-formaldehyde resin lubricant.
(C) compound stabilizers
1. Liquid barium cadmium and liquid barium cadmium zinc compound approved agent
This type of compound stabilizer is mainly used in the processing of soft PVC products, the basic components include.
(1) barium salts – can be alkylphenol barium, barium 2-ethylhexanoate, barium laurate, barium benzoate and substituted benzoate, barium neodecanoate, etc.. Barium salt in the complex of 6-7%, that is, the ratio with the cadmium salt is about 2: 1-32 (noted by metal, weight percent, the same below).
(2) Cadmium salt – can be 2-ethylhexanoic acid cadmium, cadmium laurate, cadmium oleate, phenyl acid and substituted cadmium benzoate, cadmium naphthenate, cadmium neodecanoate, etc. Cadmium salts account for about 3-4% in the complex.
(3) Zinc salt – can be zinc 2-ethylhexanoate, zinc laurate, zinc naphthenate, zinc neodecanoate, benzoic acid and substituted benzoic acid, etc. Zinc salts make up 0.5-1% of the complex.
(4) Phosphite – it can be triphenyl phosphite, diphenyl mono-octyl phosphite, diphenyl monodecyl phosphite, tris(nonyl phenyl) phosphite, etc. Phosphite ester in the complex of about 15-20% (by weight), as a chelating agent used.
(5) Other – including a small amount of 2,6-ditertiary butyl p-cresol, bisphenol A, nonylphenol and other phenolic antioxidants and ultraviolet absorbers, as well as liquid paraffin, white oil, diesel oil, ingot oil and other mineral oils as solvents, in addition to a small amount of senior alcohol and other defoamers.
Liquid barium cadmium zinc compound stabilizer due to different composition, the nature also varies. But generally is light yellow to yellow clear liquid, specific gravity 0.95-1.02 at room temperature, viscosity less than 100 centipoise, freezing point in -15 ℃ or so. Liquid barium-cadmium and liquid barium-cadmium-zinc are similar in that they have excellent thermal and optical stability, small initial coloring, good transparency and color stability. Their stabilizing effect is stronger than the solid compound soap, so the dosage can be reduced, generally 2-3 parts, no dust poisoning, and in general plasticizer completely dissolved, have good dispersion, precipitation tendency is small. The initial coloring of liquid barium-cadmium-zinc is smaller than liquid barium-cadmium.
2. Liquid barium-zinc complexes
It is generally light yellow to yellow-brown oily liquid, with specific gravity of 1.0-1.1 at room temperature, good heat resistance, not polluted by sulfide, and can be used with epoxy plasticizer to improve the stabilization effect, and it has activation effect on foaming agent.
3、Liquid calcium-zinc compound
The main components of non-toxic liquid calcium and zinc stabilizers are calcium stearate, calcium ricinoleate, zinc stearate, zinc ricinoleate, and epoxidized soybean oil, ultraviolet absorbers, etc. Liquid calcium and zinc stabilizers are generally used to be more soluble in organic solvents with less carbon number of fatty calcium salts and zinc salts.
It is a non-toxic stabilizer for PVC and is mainly used as a stabilizer for food packaging films, utensils and foam artificial leather.
It is a non-toxic stabilizer of PVC, mainly used as a stabilizer of food packaging film, utensils and foam artificial leather.
(D) Organic tin stabilizer
Organotin stabilizers are various derivatives of tin carboxylate and tin thiol, the main products are organic compounds of dibutyltin 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 requiring excellent transparency and thermal stability, but it can also stabilize soft products, but because of its high price, it is generally not used.
1、Dibutyltin dilaurate
This is a light yellow clear liquid, soluble in all industrial plasticizers and solvents, this product is toxic. It is the oldest variety of organic tin, has excellent lubricity, transparency and weather resistance, sulfur stain resistance but poor heat resistance. It is used as the main stabilizer of soft transparent products and as lubricant in hard transparent products.
2、Dibutyltin maleate
This is a white amorphous powder, melting point and volatility vary with the degree of polymerization, about 100-140 ℃ between. This product is toxic and tear-forming. It is mainly used for hard transparent products requiring high softening point and high impact strength. Because of no lubricating effect, it is often used with dibutyltin laurate, the amount of 0.5-2 copies.
3. Bis(monobutyl maleate)dibutyltin
Light yellow transparent liquid, non-toxic (allowable dosage of 3 parts or less). This product has good weather resistance, transparency, coloring prevention and thermal stability, does not occur sulfur pollution, commonly used in PVC transparent hard products, dosage 2.0-4.0 copies.
4. Bis(isobutyl mercaptoacetate) di-n-octyltin
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