The Stability Problem Of PVC Production

December 18, 2017

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 [4] 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.


7,Barium stearate


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.


6. Dioctyl(maleate)tin 


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 purpose