3 PVC Stabilizers

The Economic and Technical Importance of PVC Stabilizers 427

3 PVC Stabilizers

Dr. R. Bacalogulu, Dr. M. H. Fisch, Polymer Additives, Crompton Corp., Tarrytown, NY,USA, Dipl. Chem. J. Kaufhold, Dipl. Chem. H. J. Sander*, Polymer Additives, Witco VinylAdditives GmbH, Lampertheim, Germany

3.1 The Economic and Technical Importance of PVC Stabilizers

Polyvinyl chloride (PVC) was one of the first thermoplastics developed. It has becomeworldwide a very important bulk plastic over its almost 70 year history. PVC consumptionin different geographic areas and expected demand through 2000 are shown in Fig. 3.1.

Fig. 3.1 PVC consumption from 1980 to 2000

PVC – including the various copolymers of vinyl chloride and chlorinated PVC – is expectedto remain important among thermoplastics because of its compatibility with a large numberof other products (e. g., plasticizers, impact modifiers), in contrast to other plastics. BecausePVC’s mechanical properties can be adjusted over a wide range, yielding everything fromrigid to flexible end products, there are many different processing methods and applicationsfor PVC. The toxicological problems which at one time were major obstacles in themanufacture and processing of PVC were solved satisfactorily many years ago [1, 2].

* Recent address: Baerlocher GmbH, Unterschleissheim, Germany

0

5

10

15

20

25

30

1980 1985 1990 1995 2000

Year

PV

C c

onsum

ption (

Mio

. t)

World

Asia

North America

Western Europe

428 PVC Stabilizers

When PVC was first developed, flexible PVC was dominant, but rigid PVC production hasincreased continually and is now approximately two-thirds of total consumption in manycountries.

The low thermal stability of PVC is well known. Despite this fact, processing at elevatedtemperatures is possible by adding specific heat stabilizers that stop the damage. This is oneof the main reasons PVC has become a major bulk plastic. The development and productionof suitable heat stabilizers followed the production of PVC from the beginning, and remainsa precondition for processing and application in the future. Consumption of heat stabilizersin Western Europe was approximately 150,000 tons in 1995 and is estimated to be 170,000tons by the year 2000 [3]. The consumption of thermal stabilizers for PVC worldwide isestimated to be 450,000 tons [4].

3.2 Thermal Degradation and Stabilization of PVC

3.2.1 Mechanism of PVC Degradation

When PVC is processed at high temperatures, it is degraded by dehydrochlorination, chainscission, and crosslinking of macromolecules. Free hydrogen chloride (HCl) evolves and dis-coloration of the resin occurs along with important changes in physical and chemical proper-ties. The evolution of HCl takes place by elimination from the polymer backbone; discolora-tion results from the formation of conjugated polyene sequences of 5 to 30 double bonds(primary reactions). Subsequent reactions of highly reactive conjugated polyenes crosslinkor cleave the polymer chain, and form benzene and condensed and/or alkylated benzenes intrace amounts depending on temperature and available oxygen (secondary reactions).

3.2.1.1 Dehydrochlorination of PVC in the Absence of Air (Primary Degradation)

Any mechanism of degradation has to explain a series of experimental facts. Structuralirregularities, such as tertiary or allylic chlorine atoms, increase the degradation ratesmeasurably at the beginning of the process by a rapid dehydrochlorination that starts thedegradation process (Scheme 3.1). Initial rates of degradation are proportional to thecontent of these irregularities. However, PVC degrades even if these irregularities areeliminated by special polymerization conditions or treatments because of the dehydrochlori-nation of normal monomer units (random elimination) (Scheme 3.1).

It is estimated that after allowing for the differences in concentrations and reaction rates,the rate of random degradation in commercial PVC because of normal chain secondarychlorine atoms has the same order of magnitude as does degradation that results fromstructural irregularities [5, 6, 7]. Cis-ketoallylic structures, although very reactive indehydrochlorination (Scheme 3.1), are not present in commercial PVC but can be generatedby thermal oxidative processes [7, 8]. After the reactive irregularities initially present areexhausted, degradation continues because of the elimination initiated from normalmonomer units [6, 7, 9]. These findings indicate that thermal degradation in PVC is anintrinsic property of this polymer and that changes in synthesis conditions or specialtreatments that eliminate structural irregularities improve the stability of PVC, but can notcompletely eliminate its degradation. Stabilizers must be used.

Thermal Degradation and Stabilization of PVC 429

Scheme 3.1

Not all allylic chlorine atoms preexisting and/or formed in the degradation process accel-erate degradation. Single double bonds can be identified in degraded PVC by NMR spec-troscopy. Double bond sequences, once formed, do not increase by continuation of degrada-tion [6]. There are allylic chlorides with some forms of alkenic double bonds that are stableunder degradation conditions [6].

The conjugated polyene sequences are generated in apparently parallel processes from thefirst moment of degradation. For relatively low conversions, their concentrations increaselinearly with time. Zero order rate constants calculated as slopes of these lines decreaseexponentially with the increase of the number of double bonds in the sequence [6, 10].

In the thermal degradation of solid PVC, an induction period is observed, and then forhigher conversions, the degradation rate increases with time, indicating an autocatalyticprocess. Hydrogen chloride formed in the degradation increases both the degradation rateand the mean number of double bonds in the polyene sequence, and consequently plays anessential catalytic role in PVC degradation [11, 12, 13].

Some local configurations and conformations of the polymer chain of PVC, such as theconformation GTTG (G for Gauche T for Trans) at the end of certain isotactic sequences,favor degradation. These conformations exhibit a high local mobility relative to theremaining structures in PVC and possess some chlorine atoms with very high degrees of

Cl

Cl Cl

Cl

Cl

Dehydrochlorination of structural

Allylic chlorides

Tertiary chlorides

O Cl O

Cis keto allylic chlorides

Dehydrochlorination of normal monomer

Cl Cl

Cl

Cl

Dehvdrochlorination of normal monomer units

Dehvdrochlorination of structural irregularities

430 PVC Stabilizers

freedom. Both features make possible the adoption of the conformation enabling theelimination reaction [14, 15]. It follows that dehydrochlorination is possible only forspecific local conformations. Along the same line, PVC molecules at the surface of primaryparticles in the solid state have a much higher conformational mobility than molecules inthe interior. PVC degradation consequently is expected to take place predominantly at thesurface of primary particles.

It is well known that dehydrochlorination of PVC proceeds violently in the presence ofLewis acids such as FeCl3 [111], ZnCl2, [112],AlCl3, [113] SiCl4, GeCl4, SnCl4, BCl3, andGaCl3 [16, 17]. This process is responsible for the very fast discoloration of PVC in thepresence of Zn or Sn carboxylates that act as stabilizers till the corresponding halides areformed and fast dehydrochlorination starts.

The reaction mechanism of a complex chemical process such as PVC degradation definesthe sequence of elementary reactions leading from reactants to products and describes eachof these reactions. The mechanism of PVC degradation should explain the abovefundamental observations and should also agree with the observations related to PVCstabilization that are discussed later in this chapter.

The dehydrochlorination of PVC is a very specific chemical process because of the existenceof a long series of alternating CHCl and CH2 groups in the polymer backbone that makespossible a chain of multiple consecutive eliminations. However, the parallel formation ofconjugated polyene sequences containing 1 to 30 double bonds cannot be explained by asimple consecutive elimination. The chain reaction model from Scheme 3.2 can explain thisapparent contradiction [6].

Scheme 3.2

Cl Cl

[]n

I1

I

Cl

[ ][ ]2

2

n-1

I

Cl

[]

[]

m-1 m n-m+1

Initiation

Propagation

Propagation

Propagation

Termination

Termination

Termination

-HCl

-HCl

-HCl

-HCl........

Cl

[ ]n

I- active intermediates

HCl catalyzed

HCl catalyzed

HCl catalyzed

HCl catalyzed

PVC

-HCl

Termination

-HClCl

[

3

]

]

]

][n-2

kik

k

k

k

-HCl

-HCl

k'

k'

k'


The first elimination from a monomer residue from the chain (-CH2-CHCl-) or a structuralirregularity such as a tertiary chlorine atom (-CH2-CCl<) forms an active intermediate (I1)or a stable monoalkene. This active intermediate partitions between a stable sequence oftwo double bonds and a new intermediate (I2). The fate of the second intermediate isanalogous to that of the first one and the process continues in this way, generating all thedouble bond sequences. The concentration of each intermediate is lower than the concen-tration of the previous one. A simple steady state approximation shows that all polyenesequences in the distribution are formed simultaneously and the apparent rate constantsdecrease exponentially in agreement with experiment [6].

There is a general consensus that the intermediates in the degradation process are allylicsequences with progressively increased numbers of conjugated double bonds [7, 18]However, the mechanism of initiation, propagation, and termination steps is controversial.An early mechanism hypothesized that the intermediates were allylic radicals [19, 20](Scheme 3.3).

S

c

h

e

m

CH

CH2

CH

CH2

CH

CH2

CH

Cl Cl Cl Cl

CH

CH

CH

CH2

CH

CH2

CH

Cl Cl Cl Cl

CH

CH

CH

Cl ClCl

CH

CH2

CH

CH2

CH

CH

CH

CH

CH

CH2

CH

Cl Cl Cl

.

.

.

CH

CH

CH

CH

CH

Cl Cl

CH

CH2

R.

Cl.

Cl

RH

.

. . .

HCl

Scheme 3.3

The major problem with this mechanism is that the chlorine atom is known to be so reactiveas to be non-selective. Data on model compounds showed that the allylic hydrogen atom,(>C=CH-CH2-CCl<), has only slightly higher reactivity toward abstraction by a chlorineatom that is free to diffuse throughout the polymer matrix than does a hydrogen atom from asecondary carbon (-CH2-CCl<). The above mechanism consequently generates primarilyisolated double bonds and not the observed sequences of conjugated polyenes, owing to themuch higher concentration of hydrogen on secondary carbon atoms [7]. This radicalmechanism also fails to explain the very important catalytic role of HCl. In addition, thereare no reliable proofs that radicals are intermediates in PVC degradation in the absence ofinitiators and/or oxygen [18].

An ion pair mechanism (Scheme 3.4) was considered for the initiation step by ionization ofchlorine followed by rapid elimination of a proton. A much faster ionization of the acti-vated allylic chlorine formed was considered responsible for the chain reactions [21, 22].

432 PVC Stabilizers

Scheme 3.4

However, this mechanism does not explain the previously presented experimental facts.Moreover, it does not postulate any interruption reactions of the degradation chain. Theonly product of degradation should be a polyene resulting from elimination of all chlorineatoms from the PVC molecule. Consequently, the ion pair mechanism cannot explain thereal distribution of polyene sequences as a function of the number of double bonds. Thismechanism also does not explain the catalytic role of HCl. Formation enthalpy of Cl2H-

complexes of 3 to 4 kcal/mol as an intermediate for the catalyzed process in this mechanismdoes not compensate for the high activation enthalpy of C-Cl ionization (140–180 kcal/mol).

A concerted elimination mechanism postulated by A. R. Amer and J. S. Shapiro [23],modified by M. Fisch and R. Bacaloglu [18], and based on experimental data and molecularorbital calculations [6, 18, 24, 25, 26, 27] and additional experimental data from W. H.Starnes and coworkers [28] best explains the experimental facts (Scheme 3.5).

The first step is slow formation of a double bond randomly along the polymer chain via a1,2-unimolecular elimination of HCl through a four-center transition state (Scheme 3.5) or a

Cl Cl Cl ClCl

Cl ClCl Cl

+

Cl_

Cl ClCl Cl

ClClClCl

_+

ClClCl

HCl

HCl_

_


six-center transition state in the catalytic presence of HCl or metal chloride dike ZnCl2(Scheme 3.6). Structural irregularities such as allylic or tertiary chlorine atoms eliminatemuch faster than do normal secondary chlorines in the chain.

Scheme 3.6

The second and third steps of the processes constitute the chain elimination, regardless ofthe initiation site. In the second step, an HCl molecule is eliminated from a cis �-alkyl-allylchlorine through a six-center transition state, generating a conjugated diene or polyene.

Cl ClH Cl Cl

ClCl

Cl H ClClCl

Trans stable

Cis reactive

Cis reactive

PVCInitiation

Propagation

Propagation

ClH Cl

Cl Cis reactive

Propagation

Trans stable

Cl Cl ClClCl

HCl

Cl Cl Cl

Cl Cl Cl ClCl

1,3-Rearrangement

ClH

H Cl Cis-trans isomerization(HCl catalysis)

Cl ClCl ClH

Trans stable

1,3-Rearrangement

1,3-Rearrangement

Elimination(HCl catalysis)

(HCl catalysis)

ClCl ClH

Trans stable

H Cl

etc.

(HCl catalysis)

(HCl catalysis)

(HCl catalysis)

Scheme 3.5

H

Cl

Cl

H H

Cl

H Cl

Cl

Cl Cl H

H Cl

H H

H HCl

Cl

H

Cl

Cl

H H

Cl

Cl

Cl Cl H

Zn Cl

H H

Cl

Zn Cl

Cl

434 PVC Stabilizers

Next is an HCl-catalyzed, 1,3 chlorine rearrangement, generating a new cis �-alkyl-allylchlorine from the conjugated polyene. The second and third processes may continue as longas HCl is still present in the system. Elimination of HCl stops the 1,3-rearrangement ofchlorine and consequently, the reaction chain. This explains the very important role of HClin PVC degradation. All the metallic chlorides that are Lewis acids and can form complexeswith chloroalkanes may have a similar role. (Scheme 3.7).

Scheme 3.7

The analogy with chloroalkanes and chloroalkenes provides important support for theabove mechanism [25], as it was shown that activation parameters for the initiation of PVCdegradation correlate with the activation parameters of gas phase elimination from second-ary chloroalkanes and fall on the same straight line. This suggests that both processes mayhave the same mechanism: a 1,2-elimination of hydrogen chloride from a synperiplanarconformation involving backbone chlorine through a transition state of four centers. In thesame way, it was shown that the chain reactions may have the same mechanism as the elim-ination from cis �-alkyl-allylic chlorides (chloroalkenes): a 1,4-elimination of hydrogenchloride through a transition state of six atoms from a cis allylic structure [25].

Only the cis-allylic system with one double bond that has a relatively low activationenthalpy of dehydrochlorination is reactive in chain propagation in the degradation process.Trans conjugated polyenes are much more stable in the absence of HCl. To dehydrochlo-rinate, they require a 1,2-elimination at the one end that has a much higher activationenthalpy. This process is similar to a random initiation and is much more likely statisticallyto occur at a different place in the polymer molecule or on a different polymer moleculeentirely. In this way, a chain reaction, once interrupted, does not continue, and thesequences of conjugated polyenes remain as such in the system. Trans conjugated polyenicsystems are known to be very stable relative to their cis isomers because of their favorableconformation that allows polymer packing to occur [29].

Cl

Cl

HCl

Cl

H

Cl

Cl

Cl

Zn

Cl

Cl

Cl

Zn

Cl


3.2.1.2 Thermal Oxidative Degradation of PVC

During processing, in addition to thermal dehydrochlorination, the polymer is exposed tothermo-oxidative degradation resulting from oxygen; in addition, mechanical stress maycause chain scission. The main feature in thermo-oxidative degradation is dehydrochlori-nation as in thermal degradation. The presence of oxygen causes the dehydrochlorinationprocess to accelerate, but the discoloration is not as severe as during thermal degradation [5,30]. The polyene sequences are shorter as a result of the reaction between them and oxygen.The overall activation energy of dehydrochlorination is practically the same for thermal andthermo-oxidative processes [30]. The initial dehydrochlorination proceeds by the samemechanism. The most significant damage during the commercial processing of PVC occurs

Scheme 3.8

CH2

CH2

CH2

Cl Cl Cl Cl

CH2

CH2

Cl Cl

CH2

Cl Cl

.

.

CH2

CH2

CH

Cl Cl Cl

+

Thermal dehydrochlorination

Sher

CH2

CH2

CH

Cl Cl Cl

O

O.

CH2

CH CH

Cl Cl Cl

.

O2

CH2

CH CH

Cl Cl Cl

O

OH

.

CH2

CH2

CH

Cl Cl Cl

O.

OH.

CH2

CH2

CH

Cl O Cl

Cl.

Cl

CH2CH

3CH

2CH

Cl O

.

Thermo dehydrochlorination

Shear

436 PVC Stabilizers

as a result of mechano-chemical reactions in the presence of entrapped oxygen. The shearforces cause chain scission, generating radicals. Thermally-initiated HCl loss is followed byradical oxidation of polyenes to form peroxy radicals and hydroperoxides. Hydroperoxidesdecompose to generate alkoxy and hydroxy radicals that accelerate the oxidation processand form ketones and acid chlorides [31]. Ketoallylic chlorides initiate the thermaldehydrochlorination process, as described earlier.

Although the radical process is much more complex than shown in Scheme 3.8, it is clearthat thermo-oxidative degradation does not differ in any essential way from thermaldegradation. Dehydrochlorination, the most important process, has the same mechanism inboth types of degradations.

3.2.1.3 Secondary Processes in PVC Degradation

In PVC degradation at low dehydrochlorination levels, polyene concentrations increaselinearly and in parallel with HCl evolution. At higher dehydrochlorination levels, theincrease in polyene concentration levels off. The plateau value is lower when degradationtemperatures and oxygen pressures are higher. When the plateau is reached, dehydrochlori-nation level for all double bond sequences show that no consecutive reactions to longerpolyenes take place [32].

In the absence of oxygen during the thermal degradation of solid samples, a measurableincrease in molecular weight occurs and the molecular weight distribution becomes widerand shifts toward higher values. At some point during degradation, the melt viscosityincreases considerably, as can be observed by increasing torque in a Brabender Plasticorderexperiments [33]. The crosslinking process is catalyzed by HCl [34]. The most importantcrosslinking reaction is the Diels-Alder condensation of cisoid trans-trans dienes with otherpolyenes [35] (Scheme 3.9).

Cl Cl

Cl

Cl

Cl

Cl

Cl

Cl

Diels Alder condesation

Cl

Cl

Cl

Cl

Cl

Cl

Benzene formation

Scheme 3.9


Benzene is formed in very small amounts even at temperatures as low as 160 to 170 °C byan intramolecular process [36, 37] (Scheme 3.9). At higher temperatures, substitutedbenzenes and condensed aromatic hydrocarbons are formed by radical scission of Diels-Alder condensation products and radical cyclization of polyenes [38]. In the presence ofoxygen, the same reactions take place, but they are more complex because of the processesdescribed earlier. Oxidative scission of the chain predominates and, in general, themolecular weight of the polymer decreases [33].

3.2.2 Heat Stabilization of PVC

The degradation of PVC at elevated temperatures required in thermoplastic processing is anintrinsic characteristic of the polymer and consists of dehydrochlorination, auto-oxidation,mechano-chemical chain scission, crosslinking, and condensation reactions. Thisdegradation must be controlled by the addition of stabilizers. The heat stabilizer mustprevent the dehydrochlorination reaction that is the primary process in degradation. Thereare two ways the stabilizer can act:

• By reacting with allylic chlorides, the intermediates in the zipper degradation chain. This

process should be faster than the chain propagation itself, requiring a very active nucleo-

phile. However. the reactivity of the nucleophile should not be so high as to react with the

secondary chlorine of the PVC chain, a process that rapidly exhausts the stabilizer. To be

effective, the stabilizer must be associated by complex formation with polymer chlorine

atoms, which means it should have a Lewis acid character. This association should take

place in regions where the polymer molecules have maximum mobility; in other words,

where the conformation of the polymer can favor the degradation processes.

• Once the degradation starts, it is very fast and can be stopped only if the stabilizer is

already associated with the chlorine atom that becomes allylic. These regions are the

surfaces of the primary particles of PVC, where the stabilizer molecules are associated

with the chlorine atoms. The exceptional effectiveness of such stabilizers at very low

concentrations is explained by their entropically favorable position for stopping

degradation. In general, these stabilizers, because of their effectiveness, prevent the

formation of polyenes longer than four to five double bonds and maintain very good early

color in the polymer. These stabilizers are called primary stabilizers.

• Scavenging the hydrogen chloride generated by degradation is another way to stop the

process as the HCl is a catalyst for the chain propagation reaction and the initiation step.

However, the diffusion of HCl is quite slow because HCl is associated with the double

bond where it was generated. When HCl diffuses away from the reaction center, the

zipper degradation reaction stops. The stabilizer should scavenge HCl with high

effectiveness to avoid its catalytic effect in chain initiation that starts another zipper

dehydrochlorination chain. Because this type of stabilizer cannot prevent the dehydro-

chlorination in its early stages, polyenes longer than four to five double bonds are formed.

PVC discolors and the initial color is not maintained. However, by scavenging HCl, this

type of stabilizer avoids the autocatalytic degradation and consequently, overall

degradation is much slower. These stabilizers provide very good long term stability and

are usually referred to secondary stabilizers.

438 PVC Stabilizers

To have good stabilization of PVC with good early color and long term stability, the twotypes of stabilizer should be combined appropriately for each particular PVC formulation.

Stabilization is complicated by the fact that primary stabilizers become strong Lewis acidsby reacting with the HCl that catalyzes the initiation and propagation of PVC degradation.To avoid this, secondary stabilizers should react efficiently with HCl to protect the primarystabilizers. Another possibility is to include compounds called costabilizers in the system.Costabilizers form relatively stable complexes with the chloro derivatives of primarystabilizers (the Lewis acids) and suppress their degradative effect.

The most important classes of stabilizers and how they act in PVC stabilization is brieflydiscussed below.

3.2.2.1 Alkyltin Stabilizers

The most commercially important alkyltin derivatives are the mono and dimethyl-, butyl-and octyltin alkyl thioglycolates, mercaptopropionates and alkyl maleates. All thesecompounds react with HCl to form the corresponding tin chlorides (Scheme 3.10).However, their stabilization effect does not correlate with the amount of HCl reacted norwith the rate of this reaction [39]. It has been established that in the stabilization of PVCwith alkyltin alkyl thioglycolates, alkyl thioglycolates are released by reaction with HCl[40]. These alkyltin compounds consequently function as secondary stabilizers, but this isnot the main mechanism of their action.

Scheme 3.10

During PVC stabilization with alkyltin alkyl thioglycolates, thioglycolate groups areincorporated into the polymer chain as was determined by 113Sn and 14C tagging [41, 42].Alkyltin thioglycolates exchange thioglycolate groups with chlorine atoms in reactions withmodel allylic chlorides and the reactivity in this process parallels the PVC stabilizationeffect [42]. The main stabilization mechanism of these compounds is consequently substi-tution of allylic chlorine and they are primary stabilizers (Scheme 3.11).

In alkyltin thioglycolates, one thioglycolate group bonds to tin to form a complex and is notactive in stabilization. Alkyltin mercaptopropionate groups do not form such complex struc-tures; all mercaptopropionate groups are active in stabilization and their activity is highercompared to the corresponding thioglycolates on a molar basis. In general, monoalkyltins aremore reactive than dialkyltin derivatives; however, as a result of the very fast exchange of

Sn

OC

CH2

S

S

H3C

H3C

CH2

C

O

O

O

RR

Sn

OC

CH2

S

Cl

H3C

H3C

O

R

H Cl

H

S

CH2

C

O

O

R


thioglycolate groups, compositions comprised of at least 40 to 50% mono content exhibitactivity equal to that of the monoalkyltin derivatives themselves. Consequently, pure mon-alkyltin derivatives are not required to obtain maximum stabilization. [43, 44]. Based on theirhigh compatibility with PVC and difficulty of extracting them from PVC blends, it has beenpostulated that tin stabilizers associate with chlorine atoms at the surface of PVC primaryparticles which explains their high efficiency in PVC stabilization [43, 44] (Scheme 3.12).

Scheme 3.12

Mercapto compounds generated by the reaction of alkyltin mercapto derivative with HCladd to double bond sequences and by this process, retard PVC’s discoloration [45].Dialkyltin di(alkyl maleates) are able to add in a Diels Alder reaction to polyene sequencesand reduce the discoloration of degraded PVC [46]. In both cases, the average polyenesequence length is shortened, thereby shifting the absorption maximum toward theultraviolet and away from the visible wavelengths.

3.2.2.2 Mixed Metal Stabilizers

Metal carboxylates stabilize PVC by either mechanism, depending on the metal. Stronglybasic carboxylates derived from metals such as K, Ca, or Ba, which have little or no Lewisacidity are mostly HCl scavengers. Metals such as Zn and Cd, which are stronger Lewisacids and form covalent carboxylates, not only scavenge HCl, but also substitutecarboxylate for the allylic chlorine atoms [7] (Scheme 3.13).

It has been shown that when the concentration of the metal carboxylates is decreased, theester group introduced into the backbone by direct substitution can be eliminated by

Cl Cl Cl Cl Cl

RO

O

CCH2

S

RO

OC

CH2

SnH3CH3C S

PVC i ti l

PVC primary particles

Scheme 3.11

Sn

OC

CH2

S

Cl

H3C

H3C

O

RS

CH2

CO

OR

Cl

Cl

H3C

H3C

RO

S

CH2 C

O

Sn

SCH

2

C

O

O

RCl

440 PVC Stabilizers

reaction with HCl or by thermal degradation at higher temperatures (reversible blockingmechanism) [7, 48] (Scheme 3.13). IR spectroscopy has shown that Zn carboxylatesassociate with PVC molecules at the surface of primary particles [49] and are,consequently, very effective in the substitution of allylic chlorine. The synergism betweenZn or Cd carboxylates and Ba or Ca carboxylates is attributed to fast exchange reactionsbetween zinc or cadmium chlorides and barium or calcium carboxylates. These reactionsregenerate the active zinc or cadmium carboxylates and also avoid the catalytic effect ofzinc or cadmium chlorides in PVC degradation (Scheme 3.14). However, it has been shownthat the synergistic effect is increased by preheating zinc and calcium stearates together[50]. In this way, a complex zinc stearate is formed that is more active in allylic chlorinesubstitution (Scheme 3.14).

Scheme 3.14

O

R

O O

R

O

Ca

+ HCl2 CaCl2 +

O

R

OH

2+2

_ _

C

OZn

OC

Cl

R O

R

OCl

C

OZn

R O

Cl

OC

R

OCl

+

ClCl

R C

O

OH+

ClH

ClO

R

CO

C

O

R

O O C

R

O

Ca

+ CaCl2

++2

_ _

C

O

R

O O C

R

O

Ca+2

_ _

ZnCl2 Zn OO

CO

C O

R

R

Zn OO

CO

C O

R

R

+Zn OO

CO

C O

R

R

OC

O

R

_

O C

R

OCa+2

_

Scheme 3.13


The damaging effect of Zn or Cd chlorides in PVC degradation can be considerablyreduced by using costabilizers that form metal complexes with them. The most commoncostabilizers used with solid Cd and Zn carboxylates are polyols [51].

3.2.2.3 Alkyl Phosphites Stabilizers

Dialkyl phosphites have no effect on PVC degradation. Trialkyl phosphites scavenge HClby an Arbuzov reaction and form dialkyl phosphites. They react also with allylic chlorides,but this process plays a secondary role [27, 52, 114] (Scheme 3.15). When used alone,phosphites are secondary stabilizers, giving good long term stability but poor early color.However, in the presence of zinc di(dialkyl phosphites) (formed from zinc salts and trialkylphosphites as stabilization proceeds), allylic substitution is considerably increased andbecomes the dominant process in PVC stabilization. The early color is very considerablyimproved [27].

Scheme 3.15

3.2.2.4 β- Diketones Stabilizers

β-Diketones and similar compounds with active methylenes react in the presence of Zncarboxylates as catalysts with allylic chlorides generated by PVC degradation by a C-alkylation process [54, 114]. The stabilization effect increases with the CH acidity of thesecompounds [55].

3.2.2.5 Epoxidized Fatty Acid Esters Stabilizers

Epoxides are HCl scavengers and are also reported to be effective in allylic chlorinereplacement in the catalytic presence of Zn and Cd salts (Scheme 3.16) [56, 57].

P

RO OROR

H Cl

P

RO OROR

H

+

Cl_

P

O OROR

HRCl

Cl

P

RO OROR

P

RO OROR

Cl_

P

O OROR

RCl

+

442 PVC Stabilizers

Scheme 3.16

3.2.2.6 Hydrotalcites Stabilizers

Hydrotalcite, a natural mineral, is the hydroxycarbonate of Mg and Al with the exactformula: Mg6Al2 (OH)16CO3.4H2O. It is constituted from infinite sheets of octahedra ofMg2+ six-fold coordinated to OH-, sharing edges (brucite-like sheets), where Al3+ substitutesfor some of the Mg2+ ions. A positive charge is generated in the hydroxyl sheet that iscompensated for by CO3

2- anions, which lie in the interlayer regions between two sheets. Inthe free space of these interlayers, there is water of crystallization, associated by hydrogenbonds with both OH- and CO3

2- anions. Hydrotalcite-like clays with anions of weak acidsreact with strong acids such as HCl and exchange the anions with Cl-. This reaction allowshydrotalcite-like clays to be used as HCl scavengers in PVC stabilization [58, 59].

3.3 Product Groups and Their Specific Chemical and Application Characteristics

Despite the great variety of thermostabilizers known, only a few have gained industrialimportance. According to their chemical composition, they are usually divided into fourgroups: tin stabilizers, mixed metal carboxylates, lead stabilizers, and metal free stabilizers.Besides the thermostabilizers, there is the important group of costabilizers, which areproducts with no significant efficiency alone, but which are used together with stabilizers toprovide strongly enhanced effects.

3.3.1 Tin Stabilizers

As early as 1936, Yngve recommended not only tetraalkyltin but also alkyltin carboxylatesas PVC stabilizers [60]. In 1950, Firestone filed patent applications for organotin mercap-tides, which became extremely important in further developments in PVC technology [61].

Organotin compounds with at least one tin-sulfur bond are generally called organotin mer-captides, sulfur-containing tin stabilizers, or thiotins. Organotin salts of carboxylic acids –

O

HCl

HO Cl

O

Zn Cl2

OZnClCl

Cl OCl

ZnCl2

Product Groups and Their Specific Chemical and Application Characteristics 443

mainly maleic acid or half esters of maleic acid – are usually known as organotin carboxy-lates, and the corresponding stabilizers are sometimes called sulfur-free tin stabilizers.

3.3.1.1 Organotin Mercaptides and Organotin Sulfides

Sulfur-containing organotin compounds are among the most efficient and most widely usedheat stabilizers. They can be described by the following structures (Scheme 3.17):

Scheme 3.17

Sn- R1

S- R2

CH3-/ Methyltin -S-CH2-CO-O-alkyl thioglycolates (alkyl is mostly ethylhexyl or iso-octyl)

n-C4H9- Butyltin -S-CH2~CH2-CO-O-alkyl mercaptopropionates

n-C8H17- Octyltin -S-CH2-CH2-O- CO alkyl mercaptoethanol esters (so-called reverse esters)

-S-alkyl alkylmercaptides

-S- sulfides

Among these, the liquid thioglycolates are the predominant group on the market.

Mono- and diorganotin mercaptides are often used in combination, because these mixturesimprove initial color as well as the long-term heat stability of PVC (synergistically) [62–65]. This is shown in Fig. 3.2.

Fig 3.2 Synergism of mono- and dioctyltin isooctyl thioglycolates exemplified by yellowness Index

(YI) as a function of milling time (t) in the dynamic heat stability test on a two-roll mill at 200 °C

a: dioctyltin-bis(isooctylthioglycolate),

b: monooctyltin-tris(isooctylthioglycolate),

c: 80% dioctyltin-bis(isooctylthioglycolate) and 20% monooctyltin-tris(isooctylthioglycolate)

Sn

S

S

R1

R1

R2

R2

S R2

S R2

Sn

S R2

R1

444 PVC Stabilizers

The term “dialkyltin” is also used for mixtures of dialkyltin with smaller amounts ofmonoalkyltin compounds in order to exploit the synergistic effect of the combination ofmono- and dialkyltin.

Very efficient, solid stabilizers of a type not mentioned above are derived from ß-mercapto-propionic acid (Scheme 3.18):

Scheme 3.18

Sulfides of mono- and diorganotin are used in liquid mixtures with certain tin stabilizers,mainly together with thioglycolates and reverse esters.

The heat stabilizing effect of these organotin stabilizers depends on the type of mercaptogroup they contain. These groups are directly involved in the stabilizing reaction; they caneither replace the labile chlorine directly according to Scheme 3.11, or add onto polyenesequences after intermediate formation of the mercaptide HSR [45, 66].

Organotin mercaptides are able to react with HCl, to annihilate initiating sites by substi-tution and also help impede auto-oxidation. The combination of these functions gives theorganotin mercaptides exceptional thermostabilizing properties not found in any other classof stabilizer.

Details about the mechanism of organotin stabilization can be found in Section 3.2 and inreferences 36–42.

The organotin-sulfur stabilizers, especially as mixtures of mono- and dialkyl-tin i-octylthioglycolates, can be used in all PVC applications where high thermostability is required.They can stabilize all homopolymers, emulsion, suspension, and bulk PVC (E-, S-, M-PVC), as well as copolymers of vinyl chloride, graft polymers, polyblends, and postchlo-rinated PVC (CPVC).

One of the most appreciated properties of the whole organotin stabilizer group is theabsolute crystal clarity of finished articles, an advantage in the manufacture of rigid PVCpackaging and transparent film, bottles, and containers. Organotin thioglycolates are usedalso in the manufacture of plasticized PVC hoses, profiles, sheet, and transparent top coatsor layers. Sulfur-containing organotin stabilizers are not, in general, self-lubricating.Therefore, the high processing temperatures necessary for optimum transparency may causethe hot melt to adhere to the metal surfaces of processing equipment, unless suitablelubricants are added.

Adding high-polymeric processing aids based on PMMA to organotin-stabilized PVCimparts better flow properties and improves the surface quality of finished articles, e.g., incalendering films, extruded profiles and sheet, blown bottles, and injection molded fittings.

Sn CH2

CH2

R

R

C

O

O

Sn

O

CH2

CH2

SR

R

n

n

C

O

Product Groups and Their Specific Chemical and Application Characteristics 445

Organotin mercaptides should not be used with cadmium- or lead-containing stabilizers orpigments because the resulting formation of cadmium or lead sulfide can discolor the PVC(“sulfur staining”).

Organotin stabilizers migrate from rigid PVC only very slightly [67–69]. This fact, togetherwith favorable toxicological properties, is the basis for the worldwide approval of certaintypes of methyl- and octyltin isooctylthioglycolates for use in food packaging and potablewater pipe [70, 71].

As described in the previous Section (3.2.2.), organotin stabilizers are transformed duringprocessing into the corresponding organotin chlorides. Methyltin chlorides have consid-erably higher vapor pressure than the analogous butyl- or octyltin compounds. Because oftheir volatility during processing, the maximum allowed concentration (MAC) for tin (0.1mg/m3) must be monitored and enforced strictly, especially in open systems such ascalendering.

Butyltin mercaptides are widely used as stabilizers in the production of films, sheet,injection moldings, floor tiles, and wall coverings. In the US, they are also used for pipeextrusion, and siding with high titanium dioxide content is manufactured almost exclusivelywith organotin mercaptides. Transparent and translucent articles for outdoor use can bestabilized with sulfur-containing organotin stabilizers only if suitable UV absorbers are alsopresent.

A special application for sulfur-containing organotin stabilizers is the production offoamed, rigid PVC profiles and sheet.

Reverse esters are mercaptides of mono- or di-methyl or butyltin, based on mercaptoethanolesters of long chain fatty acids. They are especially effective in the manufacture of PVCpipe and siding. Approvals for water pipe exist for certain organotin reverse esters.

The liquid estertin iso-octyl thioglycolates (alkyl-O-CO-CH2CH2)2Sn(SCH2COO-i-octyl)2are also efficient non- toxic stabilizers, but they have not developed significant marketshare.

3.3.1.2 Organotin Carboxylates

Only carboxylates carboxylate with the following structures are of practical interest(Scheme 3.19):

Scheme 3.19

R1 = Butyl or Octyl, R2 = Alkyl or -CH=CH-CO-O-Alkyl

Sn

O

O

R1

R1

CO R2

CO R2

446 PVC Stabilizers

As already mentioned, organotin derivatives of maleic acid may have an additionalstabilizer function, i.e., the Diels-Alder reaction [46]. Their performance is good in all typesof suspension, emulsion, and bulk PVC. Optimum results are obtained when they arecombined with small amounts of phenolic antioxidants, particularly in plasticized PVC,impact-modified PVC, and PVC copolymers.

Because stabilizers containing maleic acid occasionally lead to eye and mucous membraneirritations, there have been many attempts to replace them with other systems. For manyyears, organotin stabilizers free of maleic acid have been on the market. These consist of acombination of organotin carboxylates, e.g., laurates, and a small amount of an organotinmercaptide [72]. Just as with sulfur-free organotin stabilizers, when used in a suitableformulations, this combination gives rigid PVC high transparency and excellent weatheringstability. In the melt, PVC stabilized with alkyltin maleates tends to stick to hot contactareas of the processing equipment. However, this problem can be prevented by suitablelubricants.

Organotin carboxylates work especially well in the manufacture of rigid or plasticized PVCarticles for outdoor use, as for example, transparent and translucent double-walled panelsfor greenhouses, siding, and window profiles [74, 75], particularly when pigmented [73].

3.3.1.3 Recommended Formulations

Organotin MercaptidesGlass-clear Rigid Film for Food Packaging

S/M-PVC (K-value: 57 to 60) 100 partsMBS Modifier 40. – 12 partsProcessing Aid 0.5 – 2.0 partsDi n-octyltin Mercaptide 1.0 – 1.5 partsInternal Lubricant 0.2 – 1.0 partsExternal Lubricant 0.3 – 0.6 parts

Potable Water Pipes (Pressure Pipes)S-PVC (K-value: 68) 100 partsCa Carbonate 00. – 2.0 partsDimethyl or Dioctyltin Mercaptide 0.3 – 0.5 partsCa Stearate 0.4 – 1.0 partsParaffin Wax 0.6 – 1.0 partsPE Wax 0.0 – 0.6 partsOxydized PE Wax 0.1 – 0.2 parts

Siding Top and BaseCapstock Substrate

S-PVC (K-value: 64 to 67) 100 parts 100 partsImpact Modifier 4.0 – 6.0 parts 4.0 – 6.0 partsCa Stearate 1.3 – 1.75 parts 1.0 – 1.4 partsCa Carbonate 0.0 – 5.0 parts 8.0 – 12.0 partsTiO2 6.0 – 10.0 parts 1.0 – 2.0 parts

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3 PVC Stabilizers