Effect of Some Tin and Sulfur Additives on the Thermal Degradation of

Chapter 9

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 30, 2015 | http://pubs.acs.org Publication Date: July 21, 1995 | doi: 10.1021/bk-1995-0599.ch009

Effect of Some Tin and Sulfur Additives on the Thermal Degradation of Poly (methyl methacrylate) 1

Jayakody A. Chandrasiri and Charles A. Wilkie

Department of Chemistry, Marquette University, Milwaukee, WI 53233

A detailed understanding of the course of the reaction between an additive and a polymer will lead to useful information that may enable the effective design of a flame retardant for that polymer. In this study we present an interpretation of the reaction between poly(methyl methacrylate) and tetrachlorotin, phenyltin trichloride, diphenyltin dichloride, triphenyltin chloride, tetraphenyltin, and diphenyl disulfide.

The thermal degradation of poly (methyl methacrylate), PMMA, has been studied for many years. It has been well understood that essentially the only product of this degradation is monomer but it has never been clear exactly how this monomer is produced. The problem is that if the initial step is a random scission, then both a primary macroradical and a tertiary macroradical are produced and it is not reasonable to assume that these will both depolymerize at the same rate to give the same products. Recently Kashiwagi (1) and Manring (2) have proposed solutions to this problem. Kashiwagi has suggested that random main chain scission occurs first and that the tertiary macroradical depolymerizes as expected. The primary macroradical, on the other hand, loses the side chain to give a macromolecule with an unsaturated chain end which degrades to monomer. Manring suggests that the initial step is cleavage of the side chain, producing only a tertiary macroradical which then degrades to give monomer. Over the past several years this research group has examined the effects of a wide variety of additives upon the thermal degradation of poly(methyl methacrylate). Additives that have been investigated include red phosphorus (3-4), Wilkinson's catalyst, ClRh(PPh ) (5-6), Nafions (7), copolymers of 2-sulfoethyl methacrylate and methyl methacrylate (8), and a variety of transition metal halides, MnCl , CrCl , FeCl , FeCl , NiCl , CuCl , and CuCl (9-11). The goal of this 3

2

1

3

2

3

3

2

2

Corresponding author 0097-6156/95/0599-0126$12.00/0 © 1995 American Chemical Society In Fire and Polymers II; Nelson, Gordon L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

9.

CHANDRASIRI & WILKIE

Polyfmethyl methacrylate) Thermal Degradation 127

work has been to understand, in detail, how a wide variety of additives effect the thermal degradation of PMMA so that this information may be used to design an additive that will prevent the thermal degradation of the polymer. In this paper we report on some of the more recent investigations using some tin additives (12-13) and diphenyl disulfide (Chandrasiri, J. Α.; and Wilkie, C. Α., Polym. Degrad. Stab., in press).


EXPERIMENTAL Thermal degradation is usually studied in this laboratory in the absence of oxygen by pyrolysis in sealed tubes at the appropriate temperature. Typically the additive alone, or a 1:1 by mass mixture of the additive with polymer, is placed in a sealed tube and thoroughly evacuated. After the tube has been sealed off from the vacuum line, it is placed in the oven at 375°C for some time period. The tube is then carefully opened and the contents are separated into four fractions and analyzed by conventional techniques. The four fractions are non-condensable gases (methane and CO are typically present in small amounts in these studies), condensable gases, chloroform solubles, and chloroform insolubles. The gases are separated by conventional vacuum line techniques (14). The tube is then removed from the vacuum line and treated with chloroform to dissolve materials that are chloroform soluble. The remaining material is denoted as chloroform insoluble but may be further separated by treatment with other solvents. The gases are identified by infrared spectroscopy while the chloroform solubles may be identified by nuclear magnetic resonance (GE 300 Omega instrument), GC-MS (Hewlett Packard 5890 gas chromatograph with a Hewlett-Packard 5970 mass selective detector), infrared spectroscopy (Mattson 4020 Galaxy Series Spectrometer), etc. Chloroform insolubles are identified by similar techniques. Degradation of the Additives Alone. Both tetraphenyltin and tin tetrachloride are thermally stable and undergo no degradation when they are heated to 375 °C in a sealed tube. The major products that are obtained when phenyltin trichloride is pyrolyzed for 25 hours are: 34% SnCl , 38% SnCl , 15% bi- and poly-phenyls, 3% chlorobenzene, 10% benzene, and 1% elemental chlorine. From diphenyltin dichloride one obtains 55% SnCl , 29% bi- and poly-phenyls, and 16% benzene. From triphenyltin chloride the products are 23% SnCl , 17% elemental tin, 40% bi- and ter-phenyls, and 20% benzene (15). The degradation of diphenyl disulfide produces thiophenol, and diphenyl sulfide as major products with smaller amounts of elemental sulfur, diphenyl trisulfide, diphenyl tetrasulfide, and thianthrene (Chandrasiri, J. Α.; and Wilkie, C. Α., Polym. Degrad. Stab., in press). 4

2

2

2

Degradation of PMMA in the Presence of Phenyltin Trichloride. The pyrolysis of a 1:1 by mass mixture of additive and polymer produces 32% condensable gases and 49% chloroform insoluble. The major condensable products are carbon dioxide, water, benzene, methyl chloride, isobutyric acid, and methacrylic acid. Smaller amounts of monomeric methyl methacrylate, an anhydride, and other products were also found. The chloroform insoluble fraction is separated to obtain

In Fire and Polymers II; Nelson, Gordon L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

128

FIRE A N D POLYMERS II

17% tin (II) chloride and 32% completely insoluble, non-volatile residue, identified as char. Degradation of PMMA in the Presence of Diphenyltin Dichloride. The same products were observed as for the trichloride; 47% condensables including C 0 , benzene, methyl chloride, water, methacrylic acid, and isobutyric acid and minor amounts of methyl methacrylate, methyl 2-methylbutyrate, an anhydride, and other products; 12% SnCl , and 32% non-volatile char. 2


2

Degradation of PMMA in the Presence of Triphenyltin Chloride. The pyrolysis of this mixture produced 49% of volatiles containing C 0 , benzene, water, methacrylic acid, and isobutyric acid; the minor products consisted of methyl methacrylate, methyl 2-methylbutyrate, methyl isobutyrate, methanol, methyltin trichloride, and an anhydride; a trace of methyl chloride was also obtained. The chloroform insoluble fraction contains 6% SnCl and 35% non-volatile char. 2

2

Degradation of PMMA in the Presence of Tin Tetrachloride. The condensable products included methyl chloride, carbon dioxide, HCI, water, methacrylic acid, isobutyric acid, methyltin trichloride with minor amounts of methyl methacrylate, methyl isobutyrate and anhydrides. When the amount of SnCl is increased the production of the two methyl esters is reduced. In addition to SnCl , a sixmembered cyclic anhydride, and char were obtained. 4

2

Degradation of PMMA in the Presence of Tetraphenyltin. The condensable products observed in this reaction consist of 17% benzene, 29% methyl methacrylate, 2% methyl isobutyrate, 3% toluene, 3% methyl 2-methylbutyrate, C 0 , and trace amounts of other materials. The chloroform soluble fraction was 5 % of the total and consisted of methyl esters of phenyl substituted propionic acids, methylphenyltin compounds, and other products. The chloroform insoluble fraction contained 11% elemental tin and 20% char. 2

Degradation of PMMA in the Presence of Diphenyl Disulfide. The condensable gases consist of 6% of a mixture of C 0 , CS , and methyl formate, 12% methyl isobutyrate, 13% thiophenol, 4% methyl methacrylate, and 4% thioanisole and traces of methylated benzenes. The chloroform soluble fraction accounted for 47% of the total and contained diphenyl sulfide, methyl phenyl disulfide, and methyl 2methyl-3-(phenylthio)propionate). The insoluble fraction accounted for 8% of the total and contained sulfur. 2

2

Oxygen Index. The oxygen index for these blends of additives and polymers were measured on a home built apparatus using bottom ignition as previously described (6). RESULTS AND DISCUSSION Pyrolysis of Additives Alone. Six additives have been examined in this study, SnCl , PhSnCl , Ph SnCl , Ph SnCl, Ph Sn and Ph S . Both tetraphenyltin and tin 4

3

2

2

3

4

2

2



9.


Poly (methyl methacrylate) Thermal Degradation 129

tetrachloride are quite thermally stable and undergo essentially no degradation when they are heated at 375°C for 2 hours. The other materials are all degraded by this treatment. For the phenyltin chlorides the initial step is the cleavage of a Sn-Ph bond with the formation of a phenyl radical and the corresponding tin-based radical. A detailed description of these degradations has been published (75) but these are the radicals that are initially produced and present in largest concentration and are the species responsible for reactions with the polymer. The degradation of diphenyl disulfide has been studied for several years (7622). The initial step is the cleavage of the S-S bond with the formation of two arenethiyl radicals. Subsequent reactions lead to the formation of thianthrene, diphenyl sulfide, thiophenol, and other products. Again, the arenethiyl radical is initially formed and is present in greatest concentration and is responsible for the reactions with PMMA. Degradation of PMMA in the Presence of Phenyltin Chlorides, Ph SnCl . . The degradation of PMMA is significantly effected by the presence of phenyltin trichloride. As noted above, essentially the only product in the degradation of PMMA is monomeric methyl methacrylate. It is surprising to note that methacrylic and isobutyric acids are the dominant products and that only a small amount of methyl methacrylate is observed when the polymer is degraded in the presence of any of the phenyltin chlorides. It is clear that the degradation scheme has been completely changed by the presence of the additive. The presence of phenyl and tin-based radicals has a significant effect on the degradation. A pathway to account for the products observed in this reaction is presented below as Scheme 1. The phenyl radical may interact with the polymer to form toluene and a carboxyl radical on the polymer (eq. 1). The tin-based radical can interact with an ester group of the polymer to give a methyltin chloride and a carboxyl radical (eq. 2). This carboxyl radical may hydrogen abstract to give a methacrylic acid unit (eq. 3), or it can interact with the tin-based radical to form a tin ester (eq. 4). The tin ester can lose a chlorine atom (eq. 5) and the resulting tin based radical can combine with another carboxyl radical (eq. 6). According to equation 3, methacrylic acid units are produced. The normal degradation pathway for polymethacrylic acid is the loss of water with the formation of anhydrides. Water, anhydrides, and methacrylic acid are identified amongst the products of this reaction but monomeric methacrylic acid is not obtained for the degradation of polymethacrylic acid. When methacrylic acid units are adjacent they may lose water with the formation of anhydrides, however when they are separated it is believed that they will simply depolymerize as is observed for the ester. The C NMR spectrum of the residue shows resonances that may be attributable to carbonyl, aromatic carbons, backbone carbons, and pendant methyl group. The chemical shift of the carbonyl is quite downfield (187.25 ppm) from the normal position and this may be explained as arising from the tin ester. The C NMR spectrum of tin (IV) acetate shows the ester carbonyl at 184.7 ppm. The positions of the backbone and methyl resonances are those expected for a methacrylate. x

13

13


3 x

130

FIRE AND P O L Y M E R S II

CH,

ChU

I

3

-Cwv

(1>

+ PhCH,

C =0

A.

icH, CH„

CH,

^CH —Cwv


I

+ SnR CI

2

2

3

N * A , C H

— +

2

CH SnR CI

(2)

2

3

C =0

I

C =0

OCH,

CH,

I

H-Abstract

3

(3)

-CWV C =0

C =0

CH,

CH,

I «CH

7

I

C~~

4-

SnR CI=i>

5

~~CH~

9

(4)

I

I

C =0

C =0

0·

C)SnRCI 2

CI H ,3

C!H3,

^CH,,

= 0

wvCH

0

C'vw

+

CI-

(5)

I C =0

C =0

CSnR CI

0SnR

2

2

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CH,

I

CH,

I 2

ι

I

I 2

I

C =0

C =0

C =0

isnR

A-

AsnR C-

2

2

I C =0 wsCH^— Cwv

I CH,

Scheme 1


9.

Poly(methyl methacrylate) Thermal Degradation 131


Degradation of PMMA in the Presence of Tin Tetrachloride. The products of the degradation in the presence of tin tetrachloride are quite similar to those that are obtained in the presence of the phenyltin chlorides. There is, however, an important difference. The phenyltin chlorides undergo thermal degradation when they are heated alone but tin(IV) chloride does not degrade when it is heated alone but is completely consumed when heated with PMMA. A description of the interaction between these materials is shown in Scheme 2. The initial step is coordination of SnCl to the carbonyl oxygen of the polymer (eq. 1). Similar interactions occur between thermally stable transition metal halides (9-11, 23-24) and lead to the loss of methyl chloride with the formation of the salt of the transition metal. A similar reaction occurs in this case (eq. 2). The tin ester is not thermally stable and the SnCl radical that is produced (eq. 3) can interact with the polymer to form methyltin trichloride and a carboxyl radical (eq. 4). Subsequent reactions are reminiscent of those with the phenyltin chlorides. Another possible pathway for the interaction of SnCl and PMMA is the reaction of the tin compound with the monomer produced from PMMA. When a mixture of MMA and SnCl (1.0 g each) is thermolyzed under the same conditions used for the reaction of polymer and tin(IV) chloride, all of the same products are obtained except that the anhydride is not produced. The lack of the anhydride makes it likely that the tin compound interacts directly with the polymer but does not preclude reaction with the monomer. The measured oxygen index is near 40; it is very unlikely that any amount of volatile monomer could be liberated and still have an oxygen index of this value.


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Degradation of PMMA in the Presence of Tetraphenyltin. When tetraphenyltin is heated alone, a small amount of degradation occurs with the formation of benzene and elemental tin. It is likely, considering the Sn-Ph bond energy (25), that this bond does undergo some cleavage but the radical must recombine to starting material. In the presence of PMMA the phenyl radical and triphenyltin radical have other opportunities for reaction. This may be described as a mutuallyassisted degradation. The acids that are produced with the phenyltin chlorides and tetrachlorotin are not observed and methyl methacrylate and benzene are the major products of this reaction; in addition methyl 2-methylbutyrate, methyl 2-methyl-3phenylpropionate, and methyl 2-methylene-phenylpropionate are obtained. The details of the interaction are unknown at this time and no scheme can be written to describe this reaction. One would imagine that if phenyl radicals are available, they would interact as seen for the phenyltin chlorides. The absence of methacrylic acid as a product indicates that this reaction does not occur. Some of the products are attributable to reactions of a primary macroradical and this supports the Kashiwagi mechanism for PMMA degradation. Degradation of PMMA in the Presence of Diphenyl Disulfide. The compounds that are of interest are those that contain both phenyl and methacrylate fragments and only these shall be addressed herein. The initial degradation reaction of diphenyl disulfide is the formation of arenethiyl radicals and these account for the reaction with PMMA. Of these, the most interesting is methyl 2-methyl-3(phenylthio)propionate and its oligomers. These products arise from the interaction


132

FIRE AND POLYMERS II

H

H

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2

—

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.

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(β)

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—

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C

—

OH

(9) Scheme 2


9.


Poly(methyl methacrylate) Thermal Degradation 133

between the primary macroradical and either the arenethiyl radical or diphenyl disulfide. The formation of thioanisole may arise from an S 2 reaction of diphenyl disulfide with a methyl radical. The relatively small amounts of thioanisole, CO, C 0 together with the formation of ester containing oligomers indicates that diphenyl disulfide inhibits the cleavage of PMMA. Diphenyl disulfide terminates degradation of macroradicals and initiates the formation of arenethiyl radicals. This leads to the high yield of sulfur-containing oligomers and char formation that are observed in this reaction. H


2

Oxygen Index. The oxygen index values that have been obtained for these blends of polymer and additive are shown in Table I. Except for the case of tetrachlorotin, the measured 01 values indicate little ability for these additives to function as flame retardants. The improvement that is seen for SnCl is remarkable and an additive of this type, but without the disadvantageous properties of SnCl such as water instability, may prove useful as aflameretardant. 4

4

Table I Oxygen Index for 1:1 blends of additive and PMMA Additive

Oxygen Index

SnCl PhSnCl Ph SnCl Ph SnCl Ph Sn Ph S PMMA

40 22 26 24 21 17 17

4

3

2

3

4

2

2

2

Poly(Methyl Methacrylate) Degradation. Two mechanisms have been suggested to account for the thermal degradation of PMMA. Kashiwagi (7) has suggested that the main chain is initially cleaved to produce a primary and a tertiary macroradical. They have proposed that the tertiary macroradical degrades to monomer while the primary macroradical undergoes side chain cleavage which then will degrade to monomer. Manring (2) has proposed that side chain cleavage occurs first and that the macroradical that is formed then degrades to monomer. It is significant that primary macroradical are never obtained in the Manring scheme. At least one product in each of the degradations studies must arise from the combination of Kashiwagi's primary macroradical and a radical produced during the degradation; these products are shown in Table II. The observation of these products seems to confirm that main chain cleavage occurs first and is then followed by side chain cleavage; this supports the Kashiwagi mechanism for the thermal degradation of PMMA.


134

FIRE AND POLYMERS II

Table II Products from Primary Macroradical R-CH -CH(CH )COOCH 2

R


CH

3

3

Additive 3

Ph SnCl . x

4 x

Ph

Ph Sn

PhS

Ph S

4

2

2

It is difficult to imagine routes whereby these products could be formed that do not involve the primary macroradical and this offers significant support for the Kashiwagi main chain scission pathway for PMMA degradation.

CONCLUSION The radical species that are first produced when a thermally unstable additive is used appear to be responsible for all degradation reactions and it is probably unnecessary to know the complete degradation pathway of the additive in order to understand its reaction with the polymer. None of the additives that have been studied appear to be directly useful as flame retardants for PMMA but the knowledge about reaction pathways may prove useful in the design of suitable additives. Specifically good Lewis acids, such as tetrachlorotin, may facilitate the formation of non-volatile, cross-linked ionomers that will provide the necessary stabilization under thermal conditions. The Kashiwagi main chain cleavage mechanism for the degradation is supported by these results. LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8.

Kashiwagi, T; Inabi, A.; Hamins, A.; Polym. Degrad. Stab., 1989 26, 161. Manring, L.E.; Macromol., 1991, 24, 3304. Wilkie, C.A.; Pettegrew, J. W.; Brown, C. E.; J. Polym. Sci., Polym. Lett. Ed., 1991, 19, 409. Brown, C. E.; Wilkie, C. Α.; Smukalla, J.; Cody Jr.,R. B.; Kinsinger, J. Α.; J. Polym. Sci., Polym. Chem. Ed., 1986, 24, 1297. Sirdesai, S. J.; Wilkie, C. Α.; J. Appl. Polym. Sci., 1989, 37, 863. Sirdesai, S. J.; Wilkie, C. Α.; J. Appl. Polym. Sci., 1989, 37, 1595. Wilkie, C. Α.; Thomsen, J. R.; Mittleman, M . L.; J. Appl. Polym. Sci.,1991, 42, 901. Hurley, S. L.; Mittleman, M . L.; Wilkie, C. Α.; Polym. Degrad. Stab., 1993, 39, 345.


9.


9. 10. 11.


12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

Poly (methyl methacrylate) Thermal Degradation 135

Wilkie, C. Α.; Leone, J. T.; Mittleman, M . L.; J. Appl. Polym. Sci., 1991, 42, 1133. Beer, R. S.; Wilkie, C. Α.; Mittleman, M . L.; J. Appl. Polym. Sci., 1992, 46, 1095. Chandrasiri, J. Α.; Roberts, D. E.; Wilkie, C. Α.; Polym. Degrad. Stab., 1994, 45, 97. Chandrasiri, J. Α.; Wilkie, C. Α., Polym. Degrad. Stab., 1994, 45, 83. Chandrasiri, J. A. Wilkie, C. Α., Polym. Degrad. Stab., 1994, 45, 91. Shriver, D. F.; Drezdzon, M . A. The Manipulation of Air-Sensitive Compounds, Wiley-Interscience, New York, New York, 1986, p. 104. Chandrasiri, J. Α.; Wilkie, C. Α.; Appl. Organometal. Chem., 1993, 7, 599. Graebe, C.; J. Liebigs Ann. Chem. 1874, 174, 177. Schonberg, Α.; Mustafa, A. J. Chem. Soc., 1949, 889. Schonberg, Α.; Mustafa, Α.; Askar, W. Science 1949, 109, 522. Mayer, R.; Frey, H. -J. Angew. Chem. Int. Ed. Engl. 1964, 3, 705. Harpp, D.N.; Kader, H.A.; Smith, R.A. Sulfur Letters 1982, 1, 59. Stepanov, B.I.; Rodionov, V. Ya.; Chibisova, T.A. J. Org. Chem. USSR, 1974, 10, 78. Zandstra, P.J.; Michaelsen, J.D. J. Chem. Phys. 1963, 39, 933. McNeill, I.C.; McGuiness, R.C.; Polym. Degrad. Stab., 1984, 9, 167. McNeill, I.C.; McGuiness, R.C.; Polym. Degrad. Stab., 1984, 9, 209. Chambers, D. B.; Glocking, F.; Weston, M . J., J. Chem. Soc., A, 1967, 1759.

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Effect of Some Tin and Sulfur Additives on the Thermal Degradation of

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