The Economic and Technical Importance of PVC Stabilizers
3
427
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 Vinyl Additives GmbH, Lampertheim, Germany
3.1 The Econ Economi omicc and Techni echnical cal Importa Importance nce of of PVC Stab Stabilizers ilizers Polyvi Polyvinyl nyl chlori chloride de (PVC) (PVC) was one one of the first first thermo thermopla plasti stics cs develo develope ped. d. It has has becom becomee worldwide a very important bulk plastic over its almost 70 year history. PVC consumption in different geographic areas and expected demand through 2000 are shown in Fig. 3.1. 30
World 25
) t . o i 20 M ( n o i t p 15 m u s n o c 10 C V P
sia orth America
5
Western Europe 0 1980
1985
1990
1995
2000
Year
Fig. 3.1
PVC consumption cons umption from fro m 1980 to 2000
PVC – including the various copolymers of vinyl chloride and chlorinated PVC – is expected to remain important among thermoplastics because of its compatibility with a large number of other products (e. g., plasticizers, impact modifiers), in contrast to other plastics. Because PVC’s mechanical properties can be adjusted over a wide range, yielding everything from rigid to flexible end products, there are many different processing methods and applications for PVC. PVC. The The toxi toxico colo logi gical cal proble problems ms whic which h at one time time were were major major obst obstac acle less in the the manufacture and processing of PVC were solved satisfactorily many years ago [1, 2]. * Recent address: address: Baerlocher Baerlocher GmbH, Unterschleiss Unterschleissheim, heim, Germany Germany
428
PVC Stabilizers
When PVC was first developed, flexible PVC was dominant, but rigid PVC production has increased continually and is now approximately two-thirds of total consumption in many countries. The low thermal stability of PVC is well known. Despite this fact, processing at elevated temperatures is possible by adding specific heat stabilizers that stop the damage. This is one of the main reasons PVC has become a major bulk plastic. The development and production of suitable heat stabilizers followed the production of PVC from the beginning, and remains a precondition for processing and application in the future. Consumption of heat stabilizers in Western Europe was approximately 150,000 tons in 1995 and is estimated to be 170,000 tons by the year 2000 [3]. The consumptio consumption n of thermal thermal stabiliz stabilizers ers for PVC worldwide worldwide is estimated to be 450,000 tons [4].
3.2 3.2 Therma Thermall Degrada Degradati tion on and and Sta Stabi biliza lizati tion on of of PVC 3.2. 3.2.1 1
Mecha echanis nism m of PVC PVC Deg Degra rada dati tion on
When PVC is processed at high temperatures, it is degraded by dehydrochlorination, chain scission, and crosslinking of macromolecules. Free hydrogen chloride (HCl) evolves and discoloration coloration of the resin occurs along with important important changes in physical and chemical properproperties. The evolution of HCl takes place by elimination from the polymer backbone; discoloradiscoloration results from the formation of conjugated polyene sequences of 5 to 30 double bonds (primary reactions). Subsequent reactions of highly reactive conjugated polyenes crosslink or cleave the polymer chain, and form benzene and condensed and/or alkylated benzenes in trace amounts depending on temperature and available oxygen (secondary reactions). 3.2.1.1 3.2.1.1 Dehydrochlorina Dehydrochlorination tion of PVC in the Absence Absence of Air (Primary Degradatio Degradation) n)
Any mechanism of degradation has to explain a series of experimental facts. Structural irregu irregulari larities ties,, such as tertiar tertiary y or allylic allylic chlorine chlorine atoms, atoms, increase increase the degrada degradatio tion n rates rates measurably at the beginning of the process by a rapid dehydrochlorination that starts the degr degrada adatio tion n proces processs (Sch (Scheme eme 3.1) 3.1).. Initi Initial al rate ratess of degrad degradat ation ion are are prop proport ortio ional nal to the the content content of these these irregula irregularit rities. ies. Howeve However, r, PVC PVC degrade degradess even even if these these irregula irregularit rities ies are eliminated eliminated by special polymerization polymerization conditions or treatments treatments because of the dehydrochloridehydrochlorination 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 secondary chlo chlori rine ne atoms atoms has has the the same same order order of magni magnitud tudee as does does degra degradat datio ion n that that resu results lts from from struct structur ural al irre irregul gular ariti ities es [5, 6, 7]. 7]. Cis-ketoal -ketoallyli lylicc structur structures, es, althoug although h very very reacti reactive ve in dehydrochlorination (Scheme 3.1), are not present in commercial PVC but can be generated by thermal oxidative processes [7, 8]. After the reactive irregularities initially present are exha exhaust usted ed,, degr degrada adatio tion n conti continu nues es becaus becausee of the the elimi eliminat natio ion n initi initiat ated ed from from norma normall monomer units [6, 7, 9]. 9] . These findings indicate that thermal degradation in PVC is an intrin intrinsic sic prope propert rty y of this this polym polymer er and and that that chang changes es in synt synthe hesi siss cond conditio itions ns or spec specia iall treatments that eliminate structural irregularities improve the stability of PVC, but can not completely completely eliminate eliminate its degradation. degradation. Stabilizers must be used.
Thermal Degradation and Stabilization of PVC
429
Dehvdrochlorination structural irregularities Deh drochlorination ofofstructural Cl
Cl
Cl All lic chlorides Cl
Cl
Cl Tertiar chlorides
O
Cl
O Cis keto all lic chlorides
Dehvdrochlorination normalmonomer monomer units Deh drochlorination ofofnormal Cl
Cl
Cl
Scheme 3.1
Not all allylic chlorine atoms preexisting and/or formed in the degradation process accelerate degradation. Single double bonds can be identified in degraded PVC by NMR spectroscopy. Double bond sequences, once formed, do not increase by continuation of degradation [6]. There are allylic chlorides with some forms of alkenic double bonds that are stable under degradation conditions [6]. The conjugated polyene sequences are generated in apparently parallel processes from the first moment of degradation. For relatively low conversions, their concentrations increase linearly with time. Zero order rate constants calculated as slopes of these lines decrease exponentially 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 for higher conversions, the degradation rate increases with time, indicating an autocatalytic process. Hydrogen chloride formed in the degradation increases both the degradation rate and the mean number of double bonds in the polyene sequence, and consequently plays an essential catalytic role in PVC degradation [11, 12, 13]. Some local configurations and conformations of the polymer chain of PVC, such as the conformation 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 the remaining structures in PVC and possess some chlorine atoms with very high degrees of
430
PVC Stabilizers
freedom. Both features make possible the adoption of the conformation enabling the elimination reaction [14, 15]. It follows that dehydrochlorination is possible only for specific local conformations. Along the same line, PVC molecules at the surface of primary particles in the solid state have a much higher conformational mobility than molecules in the interior. PVC degradation consequently is expected to take place predominantly at the surface of primary particles. It is well known that dehydrochlorination of PVC proceeds violently in the presence of Lewis acids such as FeCl 3 [111], ZnCl 2, [112],AlCl 3, [113] SiCl 4, GeCl4, SnCl4, BCl3, and GaCl3 [16, 17]. This process is responsible for the very fast discoloration of PVC in the presence of Zn or Sn carboxylates that act as stabilizers till the corresponding halides are formed and fast dehydrochlorination starts. The reaction mechanism of a complex chemical process such as PVC degradation defines the sequence of elementary reactions leading from reactants to products and describes each of these reactions. The mechanism of PVC degradation should explain the above fundamental observations and should also agree with the observations related to PVC stabilization that are discussed later in this chapter. The dehydrochlorination of PVC is a very specific chemical process because of the existence of a long series of alternating CHCl and CH 2 groups in the polymer backbone that makes possible a chain of multiple consecutive eliminations. However, the parallel formation of conjugated polyene sequences containing 1 to 30 double bonds cannot be explained by a simple consecutive elimination. The chain reaction model from Scheme 3.2 can explain this apparent contradiction [6].
Cl
Cl [
PVC
Cl -HCl
]n
k i Initiation HCl catalyzed
[
k Termination
-HCl
I Propagation k' HCl catalyzed
I
[
]] [ 2
1
k Termination -HCl [
][ 3
k Termination
m-1
Scheme 3.2
]
n-2
Cl [
k Termination
I- active intermediates
]] n-1 Cl
-HCl
Propagation k' -HCl HCl catalyzed ........ -HCl Propagation k' -HCl HCl catalyzed
I
Cl
-HCl
2
]n
[ ] m
]
n-m+1
Thermal Degradation and Stabilization of PVC
431
The first elimination from a monomer residue from the chain (-CH 2-CHCl-) or a structural irregularity such as a tertiary chlorine atom (-CH 2-CCl<) forms an active intermediate (I1) or a stable monoalkene. This active intermediate partitions between a stable sequence of two double bonds and a new intermediate (I2). The fate of the second intermediate is analogous to that of the first one and the process continues in this way, generating all the double bond sequences. The concentration of each intermediate is lower than the concentration of the previous one. A simple steady state approximation shows that all polyene sequences in the distribution are formed simultaneously and the apparent rate constants decrease exponentially in agreement with experiment [6]. There is a general consensus that the intermediates in the degradation process are allylic sequences 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). Cl
Cl
Cl
Cl
Cl
CH
CH
CH
CH
CH
CH2
CH2 R
Cl
Cl
CH
CH
CH
CH
CH2
CH2
CH2
.
.
RH Cl
Cl CH
Cl
.
CH CH
Cl
Cl
Cl
CH
CH
CH
CH2
CH
Cl
Cl CH
CH CH
CH
Cl
CH
CH
CH
.
Cl
CH CH
CH
Cl
CH2
S
.
c
HCl
h e
CH .
CH2
.
.
.
m
Scheme 3.3
The major problem with this mechanism is that the chlorine atom is known to be so reactive as 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 chlorine atom that is free to diffuse throughout the polymer matrix than does a hydrogen atom from a secondary carbon (- CH 2-CCl<). The above mechanism consequently generates primarily isolated double bonds and not the observed sequences of conjugated polyenes, owing to the much higher concentration of hydrogen on secondary carbon atoms [7]. This radical mechanism also fails to explain the very important catalytic role of HCl. In addition, there are no reliable proofs that radicals are intermediates in PVC degradation in the absence of initiators and/or oxygen [18]. An ion pair mechanism (Scheme 3.4) was considered for the initiation step by ionization of chlorine followed by rapid elimination of a proton. A much faster ionization of the activated allylic chlorine formed was considered responsible for the chain reactions [21, 22].
432
PVC Stabilizers
Cl
Cl
Cl
Cl
Cl
Cl _
Cl
Cl
Cl
Cl _
Cl
Cl
Cl
Cl
+ _ Cl
HCl
Cl
Cl
Cl
Cl
+ _ Cl
HCl
Cl
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. The only product of degradation should be a polyene resulting from elimination of all chlorine atoms from the PVC molecule. Consequently, the ion pair mechanism cannot explain the real distribution of polyene sequences as a function of the number of double bonds. This mechanism also does not explain the catalytic role of HCl. Formation enthalpy of Cl 2Hcomplexes of 3 to 4 kcal/mol as an intermediate for the catalyzed process in this mechanism does 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 molecular orbital 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 a 1,2-unimolecular elimination of HCl through a four-center transition state (Scheme 3.5) or a
Thermal Degradation and Stabilization of PVC Trans stable Cl Cl Cl
Elimination Cl Cl (HCl catalysis)
H Cl Cl Cl Cl
H Cl
Cl Cl
PVC
Initiation (HCl catalysis)
H Cl
Cis-trans
isomerization (HCl catalysis)
Cl Cl Cl
Trans stable
Propagation
Cl H
Cl
433
Cis reactive
H
Cl Cl Cl
1,3-Rearrangement (HCl catalysis) Propagation
Trans stable
Cl H Cl Cl Cis reactive
Cl
Cl
Cl
Cl
Cl
H
Cl Cl
1,3-Rearrangement (HCl catalysis) Propagation
Cl
Cl H
Trans stable
Cl Cl
Cl
Cis reactive
H
Cl
1,3-Rearrangement (HCl catalysis
etc.
Scheme 3.5
six-center transition state in the catalytic presence of HCl or metal chloride dike ZnCl 2 (Scheme 3.6). Structural irregularities such as allylic or tertiary chlorine atoms eliminate much faster than do normal secondary chlorines in the chain. H H Cl
Cl
Cl
Cl
Cl
H
Cl
H
Cl H
H
H
H
Cl Cl
Cl Zn Cl
Cl
Cl
H
Zn
Cl H
H
Cl
Cl
Cl
H Cl
H
Cl
H
H
H Cl
Scheme 3.6
The second and third steps of the processes constitute the chain elimination, regardless of the initiation site. In the second step, an HCl molecule is eliminated from a cis -alkyl-allyl chlorine through a six-center transition state, generating a conjugated diene or polyene.
434
PVC Stabilizers
Next is an HCl-catalyzed, 1,3 chlorine rearrangement, generating a new cis -alkyl-allyl chlorine from the conjugated polyene. The second and third processes may continue as long as HCl is still present in the system. Elimination of HCl stops the 1,3-rearrangement of chlorine and consequently, the reaction chain. This explains the very important role of HCl in PVC degradation. All the metallic chlorides that are Lewis acids and can form complexes with chloroalkanes may have a similar role. (Scheme 3.7).
H
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Zn Cl
Zn
Cl
Cl
Scheme 3.7
The analogy with chloroalkanes and chloroalkenes provides important support for the above mechanism [25], as it was shown that activation parameters for the initiation of PVC degradation correlate with the activation parameters of gas phase elimination from secondary chloroalkanes and fall on the same straight line. This suggests that both processes may have the same mechanism: a 1,2-elimination of hydrogen chloride from a synperiplanar conformation involving backbone chlorine through a transition state of four centers. In the same way, it was shown that the chain reactions may have the same mechanism as the elimination from cis -alkyl-allylic chlorides (chloroalkenes): a 1,4-elimination of hydrogen chloride 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 activation enthalpy of dehydrochlorination is reactive in chain propagation in the degradation process. Trans conjugated polyenes are much more stable in the absence of HCl. To dehydrochlorinate, they require a 1,2-elimination at the one end that has a much higher activation enthalpy. This process is similar to a random initiation and is much more likely statistically to occur at a different place in the polymer molecule or on a different polymer molecule entirely. In this way, a chain reaction, once interrupted, does not continue, and the sequences of conjugated polyenes remain as such in the system. Trans conjugated polyenic systems are known to be very stable relative to their cis isomers because of their favorable conformation that allows polymer packing to occur [29].
Thermal Degradation and Stabilization of PVC
435
3.2.1.2 Thermal Oxidative Degradation of PVC
During processing, in addition to thermal dehydrochlorination, the polymer is exposed to thermo-oxidative degradation resulting from oxygen; in addition, mechanical stress may cause chain scission. The main feature in thermo-oxidative degradation is dehydrochlorination as in thermal degradation. The presence of oxygen causes the dehydrochlorination process 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 and thermo-oxidative processes [30]. The initial dehydrochlorination proceeds by the same mechanism. The most significant damage during the commercial processing of PVC occurs Cl
Cl
Cl
Cl
.
CH2
CH2
Cl
Cl
Cl
CH2
.
Shear Sher Cl +
CH2
CH2
CH2
Cl
Cl
Thermal dehydrochlorination
Cl
CH2
Cl
O2
Cl
Cl
Cl
CH2
CH
Cl
CH2
O
.
CH2
CH
.
Cl
CH
Cl
.
O O
CH2
Cl
CH
CH2
CH
CH
CH2
Cl
Cl
Cl
Cl
CH2
CH O
.
OH OH
.
Cl
.
Cl
O
CH2
CH
Cl
Cl
CH2
CH2
Thermo dehydrochlorination Scheme 3.8
Cl
O
CH
CH3
CH2 .
436
PVC Stabilizers
as a result of mechano-chemical reactions in the presence of entrapped oxygen. The shear forces cause chain scission, generating radicals. Thermally-initiated HCl loss is followed by radical oxidation of polyenes to form peroxy radicals and hydroperoxides. Hydroperoxides decompose to generate alkoxy and hydroxy radicals that accelerate the oxidation process and form ketones and acid chlorides [31]. Ketoallylic chlorides initiate the thermal dehydrochlorination process, as described earlier. Although the radical process is much more complex than shown in Scheme 3.8, it is clear that thermo-oxidative degradation does not differ in any essential way from thermal degradation. Dehydrochlorination, the most important process, has the same mechanism in both types of degradations. 3.2.1.3 Secondary Processes in PVC Degradation
In PVC degradation at low dehydrochlorination levels, polyene concentrations increase linearly and in parallel with HCl evolution. At higher dehydrochlorination levels, the increase in polyene concentration levels off. The plateau value is lower when degradation temperatures and oxygen pressures are higher. When the plateau is reached, dehydrochlorination level for all double bond sequences show that no consecutive reactions to longer polyenes take place [32]. In the absence of oxygen during the thermal degradation of solid samples, a measurable increase in molecular weight occurs and the molecular weight distribution becomes wider and shifts toward higher values. At some point during degradation, the melt viscosity increases considerably, as can be observed by increasing torque in a Brabender Plasticorder experiments [33]. The crosslinking process is catalyzed by HCl [34]. The most important crosslinking reaction is the Diels-Alder condensation of cisoid trans-trans dienes with other polyenes [35] (Scheme 3.9). Diels Alder condesation Cl
Cl
Cl
Cl
Cl
Cl Cl
Cl
Benzene formation
Cl
Cl
Cl
Cl
Cl Scheme 3.9
Cl
Thermal Degradation and Stabilization of PVC
437
Benzene is formed in very small amounts even at temperatures as low as 160 to 170 °C by an intramolecular process [36, 37] (Scheme 3.9). At higher temperatures, substituted benzenes and condensed aromatic hydrocarbons are formed by radical scission of Diels Alder condensation products and radical cyclization of polyenes [38]. In the presence of oxygen, the same reactions take place, but they are more complex because of the processes described earlier. Oxidative scission of the chain predominates and, in general, the molecular 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 an intrinsic characteristic of the polymer and consists of dehydrochlorination, auto-oxidation, mechano-chemical chain scission, crosslinking, and condensation reactions. This degradation must be controlled by the addition of stabilizers. The heat stabilizer must prevent the dehydrochlorination reaction that is the primary process in degradation. There are 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 nucleophile. 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 dehydrochlorination 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 stabili zers.
438
PVC Stabilizers
To have good stabilization of PVC with good early color and long term stability, the two types of stabilizer should be combined appropriately for each particular PVC formulation. Stabilization is complicated by the fact that primary stabilizers become strong Lewis acids by 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 primary stabilizers. Another possibility is to include compounds called costabilizers in the system. Costabilizers form relatively stable complexes with the chloro derivatives of primary stabilizers (the Lewis acids) and suppress their degradative effect. The most important classes of stabilizers and how they act in PVC stabilization is briefly discussed below. 3.2.2.1 Alkyltin Stabilizers
The most commercially important alkyltin derivatives are the mono and dimethyl-, butyland octyltin alkyl thioglycolates, mercaptopropionates and alkyl maleates. All these compounds 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 nor with the rate of this reaction [39]. It has been established that in the stabilization of PVC with alkyltin alkyl thioglycolates, alkyl thioglycolates are released by reaction with HCl [40]. These alkyltin compounds consequently function as secondary stabilizers, but this is not the main mechanism of their action. H H3C H3C
R
C
H3C
S Sn
S O
H3C
Cl
CH2
O
CH2
S
Sn
Cl
O
CH2 C O
C O
R O
R S
O
H
C CH2
R O
Scheme 3.10
During PVC stabilization with alkyltin alkyl thioglycolates, thioglycolate groups are incorporated into the polymer chain as was determined by 113Sn and 14C tagging [41, 42]. Alkyltin thioglycolates exchange thioglycolate groups with chlorine atoms in reactions with model allylic chlorides and the reactivity in this process parallels the PVC stabilization effect [42]. The main stabilization mechanism of these compounds is consequently substitution 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 not active in stabilization. Alkyltin mercaptopropionate groups do not form such complex structures; all mercaptopropionate groups are active in stabilization and their activity is higher compared to the corresponding thioglycolates on a molar basis. In general, monoalkyltins are more reactive than dialkyltin derivatives; however, as a result of the very fast exchange of
Thermal Degradation and Stabilization of PVC H3C O CH2
C
H3C
S Sn
Cl
O
O
CH2 C
CH2 O
S H3C
C
R
R O
Sn
S
H3C
Cl
Cl
439
O
O
R
C
S
CH2
O R
Cl
Scheme 3.11
thioglycolate groups, compositions comprised of at least 40 to 50% mono content exhibit activity equal to that of the monoalkyltin derivatives themselves. Consequently, pure monalkyltin derivatives are not required to obtain maximum stabilization. [43, 44]. Based on their high compatibility with PVC and difficulty of extracting them from PVC blends, it has been postulated that tin stabilizers associate with chlorine atoms at the surface of PVC primary particles which explains their high efficiency in PVC stabilization [43, 44] (Scheme 3.12). O CH2 S H3C H3C Cl
Cl
Sn Cl
R
C O CH2 C S Cl
O O
R
Cl
PVC primary particles Scheme 3.12
Mercapto compounds generated by the reaction of alkyltin mercapto derivative with HCl add 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 sequences and reduce the discoloration of degraded PVC [46]. In both cases, the average polyene sequence length is shortened, thereby shifting the absorption maximum toward the ultraviolet and away from the visible wavelengths. 3.2.2.2 Mixed Metal Stabilizers
Metal carboxylates stabilize PVC by either mechanism, depending on the metal. Strongly basic carboxylates derived from metals such as K, Ca, or Ba, which have little or no Lewis acidity are mostly HCl scavengers. Metals such as Zn and Cd, which are stronger Lewis acids and form covalent carboxylates, not only scavenge HCl, but also substitute carboxylate for the allylic chlorine atoms [7] (Scheme 3.13). It has been shown that when the concentration of the metal carboxylates is decreased, the ester group introduced into the backbone by direct substitution can be eliminated by
440
PVC Stabilizers
O
O _ _ O O +2 R Ca R O C
R
O +
+
H O
2 R
R
R
O Zn Cl
CaCl2
2 HCl
C
Cl
O
C Cl
O
O
R
O
O C O
+
Cl Zn
R Cl
C
H
O
Cl
Cl
O
O
Cl R
C
+
O
H
Scheme 3.13
reaction with HCl or by thermal degradation at higher temperatures (reversible blocking mechanism) [7, 48] (Scheme 3.13). IR spectroscopy has shown that Zn carboxylates associate with PVC molecules at the surface of primary particles [49] and are, consequently, very effective in the substitution of allylic chlorine. The synergism between Zn or Cd carboxylates and Ba or Ca carboxylates is attributed to fast exchange reactions between zinc or cadmium chlorides and barium or calcium carboxylates. These reactions regenerate the active zinc or cadmium carboxylates and also avoid the catalytic effect of zinc or cadmium chlorides in PVC degradation (Scheme 3.14). However, it has been shown that 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 chlorine substitution (Scheme 3.14). R O C R
_ O
_ O Ca+2
O
O
C
+
ZnCl2
CaCl2
+
C O
Zn
O
R
C R
R R
R O C R
_ _ O +2 O Ca
O C
O +
R
O
C O
Zn
O C R
O
C O Ca+2 _ O
C O
_ Zn O O
C R
C R
Scheme 3.14
O
O
O
Thermal Degradation and Stabilization of PVC
441
The damaging effect of Zn or Cd chlorides in PVC degradation can be considerably reduced by using costabilizers that form metal complexes with them. The most common costabilizers 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 HCl by 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 trialkyl phosphites as stabilization proceeds), allylic substitution is considerably increased and becomes the dominant process in PVC stabilization. The early color is very considerably improved [27].
H
H
Cl
P
P
OR OR
RO
RO _ Cl
Cl P RO
OR OR
H
+ OR OR
P O
RCl OR OR
_ + P Cl OR RO OR
P
RCl O
OR OR
Scheme 3.15
3.2.2.4 β- Diketones Stabilizers
β-Diketones and similar compounds with active methylenes react in the presence of Zn carboxylates as catalysts with allylic chlorides generated by PVC degradation by a Calkylation process [54, 114]. The stabilization effect increases with the CH acidity of these compounds [55]. 3.2.2.5 Epoxidized Fatty Acid Esters Stabilizers
Epoxides are HCl scavengers and are also reported to be effective in allylic chlorine replacement in the catalytic presence of Zn and Cd salts (Scheme 3.16) [56, 57].
442
PVC Stabilizers HCl O
HO
Zn Cl2
Cl
Cl
Cl
O
O Cl
OZnCl ZnCl2
Scheme 3.16
3.2.2.6 Hydrotalcites Stabilizers
Hydrotalcite, a natural mineral, is the hydroxycarbonate of Mg and Al with the exact formula: Mg 6Al2 (OH)16CO3.4H2O. It is constituted from infinite sheets of octahedra of Mg2+ six-fold coordinated to OH -, sharing edges (brucite-like sheets), where Al3+ substitutes for some of the Mg 2+ ions. A positive charge is generated in the hydroxyl sheet that is compensated for by CO 32- anions, which lie in the interlayer regions between two sheets. In the free space of these interlayers, there is water of crystallization, associated by hydrogen bonds with both OH- and CO32- anions. Hydrotalcite-like clays with anions of weak acids react with strong acids such as HCl and exchange the anions with Cl -. This reaction allows hydrotalcite-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 industrial importance. According to their chemical composition, they are usually divided into four groups: tin stabilizers, mixed metal carboxylates, lead stabilizers, and metal free stabilizers. Besides the thermostabilizers, there is the important group of costabilizers, which are products with no significant efficiency alone, but which are used together with stabilizers to provide strongly enhanced effects. 3.3.1
Tin Stabilizers
As early as 1936, Yngve recommended not only tetraalkyltin but also alkyltin carboxylates as PVC stabilizers [60]. In 1950, Firestone filed patent applications for organotin mercaptides, which became extremely important in further developments in PVC technology [61]. Organotin compounds with at least one tin-sulfur bond are generally called organotin mercaptides, sulfur-containing tin stabilizers, or thiotins. Organotin salts of carboxylic acids –
Product Groups and Their Specific Chemical and Application Characteristics
443
mainly maleic acid or half esters of maleic acid – are usually known as organotin carboxylates, 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 used heat stabilizers. They can be described by the following structures (Scheme 3.17): 1
R
S
R
R
Sn 1
R
S
2
S
2 1
Sn
2
S
2
R
R
S
R 2
R
Scheme 3.17 1
Sn- R
S- R
2
CH3 -/ Methyltin -S-CH2 -CO-O-alkyl n-C 4H9 - Butyltin -S-CH 2~ CH2 -CO-O-alkyl n-C 8H17 - Octyltin -S-CH2 -CH2-O- CO alkyl -S-alkyl -S-
thioglycolates (alkyl is mostly ethylhexyl or iso-octyl) mercaptopropionates mercaptoethanol esters (so-called reverse esters) alkylmercaptides sulfides
Among these, the liquid thioglycolates are the predominant group on the market. Mono- and diorganotin mercaptides are often used in combination, because these mixtures improve 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)
444
PVC Stabilizers
The term “dialkyltin ” is also used for mixtures of dialkyltin with smaller amounts of monoalkyltin compounds in order to exploit the synergistic effect of the combination of mono- and dialkyltin. Very efficient, solid stabilizers of a type not mentioned above are derived from ß-mercaptopropionic acid (Scheme 3.18): R Sn
R
O CH2
CH2
R
O R
n
CH2
Sn
n
C
S
CH2 O
C O
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 mercapto group they contain. These groups are directly involved in the stabilizing reaction; they can either replace the labile chlorine directly according to Scheme 3.11, or add onto polyene sequences after intermediate formation of the mercaptide HSR [45, 66]. Organotin mercaptides are able to react with HCl, to annihilate initiating sites by substitution and also help impede auto-oxidation. The combination of these functions gives the organotin mercaptides exceptional thermostabilizing properties not found in any other class of stabilizer. Details about the mechanism of organotin stabilization can be found in Section 3.2 and in references 36–42. The organotin-sulfur stabilizers, especially as mixtures of mono- and dialkyl-tin i-octyl thioglycolates, can be used in all PVC applications where high thermostability is required. They can stabilize all homopolymers, emulsion, suspension, and bulk PVC (E-, S-, MPVC), as well as copolymers of vinyl chloride, graft polymers, polyblends, and postchlorinated PVC (CPVC). One of the most appreciated properties of the whole organotin stabilizer group is the absolute crystal clarity of finished articles, an advantage in the manufacture of rigid PVC packaging and transparent film, bottles, and containers. Organotin thioglycolates are used also in the manufacture of plasticized PVC hoses, profiles, sheet, and transparent top coats or layers. Sulfur-containing organotin stabilizers are not, in general, self-lubricating. Therefore, the high processing temperatures necessary for optimum transparency may cause the hot melt to adhere to the metal surfaces of processing equipment, unless suitable lubricants are added. Adding high-polymeric processing aids based on PMMA to organotin-stabilized PVC imparts better flow properties and improves the surface quality of finished articles, e.g., in calendering films, extruded profiles and sheet, blown bottles, and injection molded fittings.
Product Groups and Their Specific Chemical and Application Characteristics
445
Organotin mercaptides should not be used with cadmium- or lead-containing stabilizers or pigments 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, together with favorable toxicological properties, is the basis for the worldwide approval of certain types of methyl- and octyltin isooctylthioglycolates for use in food packaging and potable water pipe [70, 71]. As described in the previous Section (3.2.2.), organotin stabilizers are transformed during processing into the corresponding organotin chlorides. Methyltin chlorides have considerably higher vapor pressure than the analogous butyl- or octyltin compounds. Because of their volatility during processing, the maximum allowed concentration (MAC) for tin (0.1 mg/m3) must be monitored and enforced strictly, especially in open systems such as calendering. 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 pipe extrusion, and siding with high titanium dioxide content is manufactured almost exclusively with organotin mercaptides. Transparent and translucent articles for outdoor use can be stabilized with sulfur-containing organotin stabilizers only if suitable UV absorbers are also present. A special application for sulfur-containing organotin stabilizers is the production of foamed, rigid PVC profiles and sheet. Reverse esters are mercaptides of mono- or di-methyl or butyltin, based on mercaptoethanol esters of long chain fatty acids. They are especially effective in the manufacture of PVC pipe and siding. Approvals for water pipe exist for certain organotin reverse esters. The liquid estertin iso-octyl thioglycolates (alkyl-O-CO-CH 2CH2)2Sn(SCH 2COO-i-octyl) 2 are also efficient non- toxic stabilizers, but they have not developed significant market share. 3.3.1.2 Organotin Carboxylates
Only carboxylates carboxylate with the following structures are of practical interest (Scheme 3.19): 1
R
2
O
CO
R
O
CO
R
Sn 1
R
2
Scheme 3.19
R1 = Butyl or Octyl, R 2 = Alkyl or -CH=CH-CO-O-Alkyl
446
PVC Stabilizers
As already mentioned, organotin derivatives of maleic acid may have an additional stabilizer function, i.e., the Diels-Alder reaction [46]. Their performance is good in all types of suspension, emulsion, and bulk PVC. Optimum results are obtained when they are combined 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 membrane irritations, there have been many attempts to replace them with other systems. For many years, organotin stabilizers free of maleic acid have been on the market. These consist of a combination of organotin carboxylates, e.g., laurates, and a small amount of an organotin mercaptide [72]. Just as with sulfur-free organotin stabilizers, when used in a suitable formulations, this combination gives rigid PVC high transparency and excellent weathering stability. In the melt, PVC stabilized with alkyltin maleates tends to stick to hot contact areas of the processing equipment. However, this problem can be prevented by suitable lubricants. Organotin carboxylates work especially well in the manufacture of rigid or plasticized PVC articles for outdoor use, as for example, transparent and translucent double-walled panels for greenhouses, siding, and window profiles [74, 75], particularly when pigmented [73]. 3.3.1.3 Recomm ended Formulations
Organotin Mercaptides Glass-clear Rigid Film for Food Packaging S/M-PVC (K-value: 57 to 60) MBS Modifier Processing Aid Di n-octyltin Mercaptide Internal Lubricant External Lubricant
100 parts 40. – 12 parts 0.5 – 2.0 parts 1.0 – 1.5 parts 0.2 – 1.0 parts 0.3 – 0.6 parts
Potable Water Pipes (Pressure Pipes) S-PVC (K-value: 68) Ca Carbonate Dimethyl or Dioctyltin Mercaptide Ca Stearate Paraffin Wax PE Wax Oxydized PE Wax
100 parts 00. – 2.0 parts 0.3 – 0.5 parts 0.4 – 1.0 parts 0.6 – 1.0 parts 0.0 – 0.6 parts 0.1 – 0.2 parts
Siding Top and Base S-PVC (K-value: 64 to 67) Impact Modifier Ca Stearate Ca Carbonate TiO2
Capstock 100 parts 4.0 – 6.0 parts 1.3 – 1.75 parts 0.0 – 5.0 parts 6.0 – 10.0 parts
Substrate 100 parts 4.0 – 6.0 parts 1.0 – 1.4 parts 8.0 – 12.0 parts 1.0 – 2.0 parts
