Thermal Stabilization of PVC by “Plasticizer Thiols”

W. H. STARNES, JR., B. DU, andV. G. ZAIKOV

Departments of Chemistry and Applied Science College of William and Maq

P. 0. Box8795 WiUiamsburg, VA 23187-8795

Various new carboxylate esters containing one or more sulfhydryl groups are shown to be remarkably effective as both thermal stabilizers and plasticizers for poly(viny1 chloride) (PVC). These “plasticizer thiols” function well as primary stabi- lizers in the absence of metal-containing additives when they are introduced at ei- ther a typical plasticizer level or a conventional stabilizer level. Their syntheses are straighfforward and. in some cases, require only an acid-catalyzed esterification performed with commercially available starting materials. Unlike typical thiols, the purified plasticizer thiols do not have offensive odors when their molecular weights are relatively high.

INTRODUCTION

lasticizers and heat stabilizers are extremely use- P ful as additives for poly(viny1 chloride) (PVC). The best heat stabilizers incorporate certain heavy metals, which, in recent years, have fallen into disfavor be- cause of their toxicities. Thus the replacement of these stabilizers with effective ones containing no metals has been highly desirable and the focus of much research. We now describe, in a very prelimi- nary way, the first results of a new approach to this problem that involves the use of materials in which a stabilizing sulfhydryl function has been bonded to a plasticizer to form a “plasticizer thiol” (PT). A much more detailed account of our work in this area will be published in due course.

At elevated temperatures in homogeneous solu- tions, simple organic thiols are quite effective as color stabilizers for PVC (1, 2). Their ability to perform as such derives, at least in part, from their deactivation of labile defect structures in a process that may in- volve reductive dechlorination under certain condi- tions (1-5). However, when solvents are absent, the PT additives disclosed here are fa r more effective as stabi- lizers than simple thiols, because of their excellent compatibility with the polymer. Not surprisingly, their stabilization efficacies are enhanced when they are used in the large amounts that plasticization requires. Moreover, owing in part to their high molecular masses, their odors tend to be low and inoffensive when the PTs are pure.

In 1999, the plasticizers produced worldwide were valued at ca. $5 billion (61, while heat stabilizers were marketed for about $1.8 billion in toto (6). From these

figures it is apparent that the potential commercial and technological impacts of the PT additives are enormous.

RESULTS AND DISCUSSION

Preparation of Plasticizer Thiols

Aromatic ester thiols la-c were obtained by the con-

la, 0- b, m-

H c, P-

ventional acid-catalyzed esterification of the corre- sponding acids with 2-ethyl- 1-hemol. The acid prog- enitor of la (thiosalicylic acid) was purchased directly, while the other precursory acids were acquired from a simple reaction sequence involving (a) diazotization of m- or paminobenzoic acid in water, (b) treatment of the resultant diazonium salt solutions with aqueous EtOCS,K, and (c) basic hydrolysis of the aryl xan- thates thus formed.

Application of such a sequence to the preparation of acid 3 gave rather low yields of an impure final prod- uct and presented scale-up problems. Much better re- sults were obtained for 3 with the method in Scheme 1 , which uses a Newman-Kwart rearrangement (7) to generate compound 2. Yields for all of the steps in Scheme 1 , including the esterification to form PT 4,

250 JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, DECEMBER2001, Vol. 7, No. 4

Thermal Stabilization of PVC by "Plasticizer Thiols"

Scheme 1 . Synthesis of plasticizer thwl4.

&OH. H' BC.* 2. 1. DABCOmF, Me2NCSCI

H &C02H A * H

3 4

were nearly quantitative. Compound 4, a straw-col- ored oil, contained a few percent of the corresponding disulfide but was quite pure otherwise.

Several aliphatic plasticizer thiols also were pre- pared. Two examples are compounds 5 and 6, which were made in high yields from commercial precursors by direct esterification. These F'Ts are prototypes for

0

SH

6

those in which one or more SH groups reside on either the alkyl part or the acyl portion of a carboxylate di- ester. Mercapto esters obtained by esterification of pentaerythritol and other polyols were synthesized as well and will be discussed in a later report (8).

Stabilization and Plasticization by Compounds la-c and 4-6

Reductive dechlorination of PVC by thiols in solu- tion greatly enhances the rate of evolution of HCl (1). However, in striking contrast, that effect was not ob- served when PVC was heated alone with la-c or 4-6 at 170°C under argon. In fact, under those conditions at the kinetic steady state, 30 parts by weight (phr) of, for example, F T 4 actually decreased the rate of dehy- drochlorination by about 30%. as compared to that of PVC containing 30 phr of the well-known plasticizer, bis(2-ethylhexyl) phthalate (DOP). Compounds la+, when used at the 30-phr level, did cause steady-state rate enhancements, but these were never greater than about 40430% and thus were insufficient to disquaiify the mercaptobenzoates for use as PVC additives. Im- portantly, when la-c or 4-6 were present, the dehy- drochlorination rates were autodecelerating, an effect that was not observed when the additive was DOP. Moreover, unlike DOP, I T S la-c and 4-6 caused color stabilization of the polymer during kinetic runs.

The effect of FT 4 on color was studied further by comparing the colors produced by heating PVC/addi- tive blends for 1.5 h at 170°C under argon. When the additive was compound 4 (0.044 mol per mol of monomer units). the blend remained perfectly white

JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, DECEMBER2001, Vol. 7, No. 4 251

W. H. Stames, Jr., B. Du, and V. G. Zaikov

throughout the heating period. However, when the ad- ditive was 1-dodecanethiol (used at the same molar level), a dark reddish-brown mixture resulted after 1.5 h. 1-Dodecanethiol is essentially insoluble in PVC. Thus its ineffectiveness as a stabilizer strongly sup- ports the proposition that the color stabilization of PVC by thiols relates directly to the degree of compati- bility of these additives with the polymer.

Table 1 reports the results of a number of ASTM tests that were used to compare the effects of PT 4 with those produced by DOP and two commercial sta- bilizers containing metals. Major conclusions that emerge from the tabulated data are as follows: (a) In all respects. compound 4 compares very favorably with DOP as a primary plasticizer. (b) The heat stabi- lization conferred by 4, when it is used at a typical plasticizer concentration, is equal to or much better than the stabilization achieved with typical loadings of a commercial lead or barium/zinc heat stabilizer. (c) A commercial barium/zinc stabilizer does not enhance the stabilization caused by 4 but decreases it instead. (d) Excellent stabilization can be achieved with 4 without the addition of an HCl scavenger such as epoxidized soybean oil.

In short, the data in Table 1 demonstrate most con- vincingly that PT 4 can function well as a primary sta- bilizer and plasticizer in the absence of other addi- tives.

Test results for FTs 5 and 6 appear in Table 2. A s was the case for the runs with PT 4, data for controls (C-1 and C-2 in Table 21 were obtained simultaneously

with the data for formulations that contained the FTs. It is apparent that PT 5 and PT 6 performed excep- tionally well in these trials.

Stability tests also were carried out with formula- tions containing only 3-5 phr of a number of PTs. Again the results were generally comparable to or bet- ter than those obtained with the barium/zinc or lead additives in Tables 1 and 2. especially when the PTs were accompanied by a small amount (5 phr) of epoxi- dized soybean oil. Details of these observations will be reported later (8).

Mechanism of Stabilization by Plasticizer Thiols

Our preliminary work on this problem suggests that the PT additives stabilize color by destroying labile structural defects (e.g., internal allylic chloride), thereby preventing the initiation and growth of conjugated polyene chromophores. The gradual removal of labile defects could account for the autodeceleration of HC1 loss that all of the PTs cause. In theory, the defects might be excised by reductive dechlorination (Eq 1) (1) or by the nucleophilic displacement of labile chloride (Eq 2). Both of these reactions give HC1, but even so,

RCI + 2 R S H - RH + R'SSR' + HCI (1)

RCl+ R S H __f RSR + HCl (2)

they still could tend to reduce its net yield per unit time by retarding polyene growth. Thus far we have obtained no structural evidence for reductive de- chlorination, but we have found that small amounts of

Table 1. Comparison of PT-4 With Conventional Additives for the Stabilization and Plasticization of PVC.

Formulation, phra

Ingredient A B C D E F

P V C b 100.0 100.0 100.0 100.0 100.0 100.0 DOPC 30.0 30.0 30.0 4 35.0 30.0 30.0 5.0 ESOd 5.0 5.0 5.0 5.0 BdZn liquide 3.0 3.0 3.0 DythalITribase' 4.0 stearic acid 0.1 0.1 0.1 0.1 0.1 0.1

Test9 A B C D E F

hardness, Shore Ch 85/74 86/75 94/85 94/86 95/91 86/75 sp gri 1.27 1.30 1.29 1.29 1.29 1.27 tensile strength,' psi 331 5 3472 3467 342 1 321 8 3322 elongation,) % 338 305 281 328 306 341

initial yellow,I min 15 10 10 10 15 15 dynam heat stab,k min 24 60 60 60 60 3

dec time,m min 45 >60 >60 >60 25 20

aParts by weight per hundred parts of PVC. bOxyVinyls 455F. CBis(2-ethylhexyl) phthalate. dEpoxidized soybean oil. e%eneral Purpose" commercial stabilizer. 'Blend of dibasic lead phthaiate and tribasic lead sulfate. gDeviations are not reported because the tabulated values were obtained from smgle runs. hASTM D 2240-86. 'ASTM D 792-91. 'ASTM D 638-91. "Dynamic heat stability (205"C, 100 rpm, #5 bowl), ASTM D 2538-95. '210°C. ASTM D 2115-92. "'Oven heat stability, 210°C. ASTM D 21 15-92.

252 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, DECEMBER 2001, Vol. 7, No. 4

Thermal Stabilization ofPVC by "Plasticizer Thwls"

Table 2. Comparison of PT 5 and PT 6 With Conventional Additives for the Stabilization and Plasticization of PVC.

Formulation, phP

Ingredient c-1 c-2 T-1 T-2 T-3 T- 4 . .

P V C b 100.0 100.0 100.0 100.0 100.0 100.0 DOPC 30.0 30.0 5 35.0 30.0 6 35.0 30.0 ESOd 5.0 5.0 5.0 BdZn liquide 3.0 DythalTTribase' 4.0 stearic acid 0.1 0.1 0.1 0.1 0.1 0.1

Test9 c-I c-2 T-I T-2 T-3 T-4 -

hardness, Shore Ch 85/75 91/85 70/60 70f60 80172 80/73 sp gri 1.27 1.32 1.30 1.30 1.27 1.27 tensile strength) psi 3482 3725 3759 3630 3588 3504 elongation) % 327 288 262 259 31 1 342 dynam heat stab,k rnin 19 29 57 58 >60 >60 initial yellow,I min 15 10 15 15 15 15 dec time,m rnin 45 60 >60 >60 50 >60

aParts by weight per hundred parts of PVC. bOxyVinyls 455F. CBis(2-ethylhexyl) phthalate. dEpoxidized soybean oil. e"General Purpose" commercial stabilizer. 'Blend of dibasic lead phthalate and tribasic lead sulfate. 'JDeviations are not reported because the tabulated values were obtained from single runs 'ASTM I) 2240-86. 'ASTM D 792-91. IASTM D 638-91. kDynamic heat stability (205"C, 100 rpm, #5 bowl), ASTM D 2538-95. '210"C, ASTM D 21 15-92. '"Oven heat stability, 210"C, ASTM D 21 15-92.

FT 4 become chemically bonded to the polymer upon heating. That result is explicable in terms of the reac- tion shown as Eq 2, but it can also be accounted for by a free-radical addition of compound 4 to conju- gated polyene sequences (a process that would short- en these sequences and thus reduce their color). Fur- ther research on the mechanism of stabilization is planned.

EXPERIMENTAL

Reaction products were identified by using high- field 'H and 13C NMR spectroscopies together with (gas chromatography)/(mass spectrometry). All of the chemicals had the highest purities available commer- cially and were used as received. The formulations listed in Tables 1 and 2 were prepared by using stan- dard blending techniques. Rates of HC1 evolution were determined by adapting a published procedure (9). The methods used to prepare the plasticizer thiols will be described in detail elsewhere (8, 10).

CONCLUDING REMARKS The results presented in this paper show that plas-

ticizer thiols are promising candidates for commercial use in PVC. In this regard it should be noted, how- ever, that the specific thiols we have studied thus far were intended to be prototypes and thus are not nec- essarily the best PTs for particular applications. We

are continuing our studies of these unusual ac litives and are concentrating our efforts pnmanly on the de- velopment of improved methods of synthesis and on the identifhtion of new types of effective PT structures.

ACKNOWLEDGMENTS

We thank Maryellen Cobb of the Teknor Apex Com- pany for providing the information that appears in Tables 1 and 2. The research performed at the College of William and Mary was supported by the Edison Polymer Innovation Corporation.

REFERENCES

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Plitz, C. L. Schosser, F. C. Schilling, and D. J. Freed, PoZym Prepr. (Am Chern. Soc.. Div. Polym. C h e n ~ ) , 21 (21, 138 (1980).

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