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The effect of thermal processing on PVC—VII. Reactive antioxidants as thermal stabilizers
European Polymer Journal Vol. 16, pp. 1145 to 1151
0014-3057/80/1201.1145502.00/0
¢~ Pergamon Press Lid 1980. Printed in Great Britain
THE EFFECT OF THERMAL PROCESSING ON PVC--VII REACTIVE ANTIOXIDANTS .AS THERMAL STABILIZERS
B.
andG.ScoTr
B. Cooxxv Department of Chemistry, University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET, England
(Received 27 March 1980) Abstract--A benzyl thiol (BHBM) containing a hindered phenolic antioxidant group has been compared as a thermal stabilizer for PVC with a typical tin maleate stabilizer (DBTM). It has been shown that BHBM both adds to the developing double bond in PVC during processing and destroys hydroperoxides. It is thus an effective stabilizer but it is portia.By destroyed by side.reactions. In combination with DBTM, BHBM shows powerful synergistic effects which are optima] at a molar ratio of DBTM:BHBM "-4:1. The reasons for the side-reactions and the synergism are rationalized on the basis of the known reactions of BHBM.
INTRODUCTION In previous publications, it has been demonstrated that the early stages of PVC procesaing are characterized by the rapid formation of both unsaturation and hydroperoxides [1-3]. A commercial PVC stabilizer based on the tin male,ate structure appeared not to inhibit the initial formation of unsaturation but no peroxides could he detected with the analytical procedure employed [4]. It was argued that prior formation of both ummturation and of peroxides is responsible for the sensitization of the polymer to photooxidation and that both are a direct result of the processing operation ['5,6]. Other studies have shown ['7, 8] that it is possible to react thiol-containing antioxidants with double bonds in polymers in the presence of free radicals; the purpose of the present study is to examine the possibility of destroying the developing unsaturation in PVC during proeeuing by this technique, using (3,5.di-tert-butyl-4-hydroxy)benzyl mercaptan (BHBM). A more sensitive method for the measurement of hydroperoxides was employed in the present work than that used previously and the effect of dibuty] tin mnieate (DBTM) was compared with that of the thiol. oH tBu
,~tBu
OCO~. / CH
(BubSq It O~O.~"CH CH~:,SH (8HBM)
(DBTM)
EXPERIMENTAL
Materials PVC was Breon M90/50 as obtained from the manufacturers. BHBM was made as degaibed previously [7,8] (m.p. 28-30°). DBTM was Irgastab DBTM from CibaGeigy. Procedure ProcessinO. Suitably formulated PVC was processed as
described previously [2] in the closed mixer of a torque rheometer at 170° with the chamber completely filled. The rotor speed was constant at 60 rpm. After p r ~ for the required period, the polymer was removed rapidly from the mixer and chilled in cold water to minimize further degradation. The polymer was dried and stored in a sealed
bag.
Compression namlding. The processed polymer was pow. dered by abrasion and the power was compremion moulded between' stainlem~eel plates at 170~ for 90see using special grade cellophane to separate the polymer from the metal. The polymer was preheated between the platens of the press for 45 sec prior to application of a pressure of 100kg/em2. Uniform films of thickness 3 x 10-acre were chosen for analysis and thermal exposure studies. DetermMatuTn of peroxide concentration. The following experimental procedure was =_daptedfrom Bocek [9]. 0.5 g of p r ~ powdered polymer was ~voilen in 75 ml of deaerated chloroform for 12 hr in a conical flask fitted with a rubber seal. One ml of deaerated 0.004 M phosphoric acid was added, followed by 2 nd of a deaerated solution of 0.005 M ammonium ferrous sulphate in methanol. Both reagents were added using a syrm~ to prevent contamination by O=. After standing in the dark for 4 hr, 1 ml of a 0,5% solution of 1-10 phenanthroline in de,aerated benzene was added. The reaction mixture was allowed to stand for 15 rain after swirling. Five ml of the reaction mixture free of polymer was carefully withdrawn and its absorption in the visible region (510am) was measured against the blank reference in a Perkin--Elmer model 137 u.v. visible spectrophotometer. Peroxide concentration was then read off using a calibration curve. The polymer samples containing BHBM were exten. Sively extracted with cold methanol (72 hr) in constant circulation before peroxide determination. Total unsaturation was measured as described previously [2]. For samples processed with BHBM, the deterruination was carried out after extensive extraction with cold methanol (72 hr) and it was found that the polymer. bound BHBM did not re,act with iodine to any significant extent, until after 3 hr reaction time and hence did not interfere with the determination of polymer ummturation. Mono-ummturation was determined by the following method adapted from Martin [17]. 0.5g of powdered processed polymer was swollen in" 75 ml of chloroform for 12 hr in a conical flask fitted with a rubber seal. Four g of
1145
//
1146
B.B. Cooa^v and G. SCOTT ,0--
)-¢
I-¢
6 c
zo
4
4=
.o 0 c
/
Q.
0
I
I
/
I
4
8
12
16
BHBM concentration,
.J 20
m o t / l O 0 0 x I0 3
FiB. 1. Relation between hydroxyl index (3620 c m - t ) and BHBM concentration in PVC. acid free mercuric acetate (AR grade, ex. Fisons) and 30 ml of methanol were added. After adding 75 ml of saturated aqueous (AR) sodium chloride and 100ml of distilled water, the liberated acetic acid was titrated with 0.1 N NaOH after 3 hr and the phenolphthaicin end-point determined. Blank runs were similarly examined for each set of experiments. BHBM treated PVC samples were exhaustively cold extracted prior to mono-unsaturation determination. When DBTM was used, the contribution to the total unsaturatinn from the male,ate groups, some of which were unextractable, after processing was determined with the aid of i.r. spectroscopy. A 5% solution of the processed polymer in dichioromethane showed a peak at 1605 eradue to the olefmic absorption of the nucleate, group. A 5?/0 solution of unprocessed PVC in dichloromethane containing a known concentration of DBTM was then placed in the reference beam. When the total concentration of
male,ate groups in the processed polymer was equal to that in the refereno~ sample, then the peak at 1605cm -~ just disappeared. The concentration of D W r M required to can(:elout the ~ unsaturation in the ~ sample was accurately determined for each processing time using a suitable range of known concentration. The volume of sodium thimulphate corres~nding to this concentratinn of DBTM for each pro~ssing time was then subtracted from the titrimetriereading, to determine the actual unsatura~on deveJoped m the polymer dunng p t ~ m ~ n ~ The optimum swellin$ time of 12 hr and the optimum reaction time of I hr were determined in seperttte experiment& Total colour difference was measured as ~ h e d previonsly [2]. i.r. Examinatico of the polymer treated with BHBM
showed a sharp phmmlic OH a b , o ~
at 3620m-t
arm- methanol extraction. Using u n w o o m ~
cast PVC
2.0 --
/
x
Tom
vl.
/
L.b,ice..s~one
E c• o o
1.0
i
OBTM ( 5.781
6
8
I0
Proce~inQ time,
12
_
14
16
18
20
rain
Fig. 2. Effectof B H B M alone and in synergisticcombination with D B T M on the formation of unsaturation in P V C during processing at 170° (lubricantsCaSt, 0.8% + W a x E, 0.65~) present in ¢agh formulation except control (numbers in parentheses are concentrations in mol/100 g x 10s).
°I M
0.8
T
The effect of thermal processing on PVC~VII
I~
0.6:
/
No additive
/~e
¢~
jr / \
" 0.4
e,.~,,l~, D ~
a.
TM (5.78)
DBTM (4.48)+ BHBM ( 0
2
4
6
8
/
e/~-
j
Lubricont$ olone
=
1147
I0
~2
F~roces$ing time,
.
/4
18
16
20
rain
Fig. 3, Effect of BHBM alone and in synergistic combination with DgTM on the formation of peroxides in PVC during processing at 170~ (other data as for Fig. 2). films (thicknms 3 x 10-3¢m) containing known concen. trstiom of BHBM, a linear calibration plot was construtted wing the absorption peak at 3620 cm-: {see Fig. 1) and this was employed in calculating the concentration of retained BHBM in PVC after
fall to zero. The former began to reduce after less than I rain and the latter after about 3 rain. When DBTM and BHBM were used together at the same total concentration (5.78 x 10-3 tool/100 8), synergistic effects were observed. In Figs 2 and 3 the molar ratio of the two components was 3A4:1 (see below) and it is clear that this system gives both a lower level of ummturation and of peroxide formation during the initial stage and a lower rate of formation of both species.at the end of the induction period. The fommtion of colour (Fig. 4) followed the same general pattern as the formafon of unsaturation for all the additives. Although the level of colour was higher with BHBM than with DBTM, solvent extraction of the films reduced it indicating that BHBM underwent some decomposition to coloured products during processing. The synergistic mixture showed much less colour formation than either of the individual components. Exhaustive extraction with methanol of the films containing BHBM processed for shor~ times resulted in the substantial removal of the additive as measured by residual phenolic OH in the polymer (see Fig. 5). The amount remaining after extraction increased sharply with processing time
processing.
ItESULTS
The formation of tmmturation and of peroxide as a fuaation of prorating time is shown in Figs 2 and 3 f o r 5 PVC focmuiations. As found previously [1,2] the control without lubricants or stabilizers gave the highest level of both, ri=ing rapidly during the first 2 rain of proce=fing. Unsaturation continued to rise at a iow~ rate after 2 rain but peroxide concentration fell rapidly, I d t h o u l ~ With the more sensitive method used in this study, not to zero. The two lubricants,
calcium stearat= (0.8%) and Wax E (0.65%) when used together somewhat reduced the formation of both species but did not change the shapes of the curves. The additional prep-nee of DBTM (5.78 x 10- a tool/100 g) rednLJed but did not eliminate the formarion of both species. BHBM by contrast caused the concentration of both unsaturation and peroxides to an e-
70
No:
~., 60
.~ 50 o
4O _
~.,.~f~Lubr
-
~ 2O
0
/
e
/
icont s olone
y
e"'e
BHBM (5 7'8)
/p.'~
2
( ~.?m
4
,
~
8
..p,
I0
PrOcelsing time,
12
14
;
16
;
18
T
20
min
Fig. 4. Elfcct of BHBM alone and in synergistic oombiemfion with DBTM on colour formation in PV¢ during processing at 170' (other data as for Fig. 2).
I 148
B.B. Co(mAYand O. SCOTT " r _-_ . _ _ . . . .
_8.".B_". _(
........................
I~t~ 8HBM(5.78) unext ; 3 L~- ~" . ~" E ],p,/BHgM(S,Te) ext
V
-o
~ ~
-
"
o
"
"--o
OBTM (4.48) + 8HBM (I. 31 unext u a ~ • c
=
:3-4
I
I
{
._u_ f ~I"~=~DBTM (4,48)+ BHBM (I.3)ext
~"
I
0
f
I
I
5
I
1
I
10 15 Proceuing time, rain
I
20
I
25
Fig. 5. R©tention of BHBM in PVC during processing at 170 in the presence and absence of DBTM (other data as for Fig. 2).
until after 3 rain no further change was observed; the amount of phenol remaining substantially constant for 20 min. The unextracted films showed a drop in phenolic OH during the first few minutes of processing to the level of the bound component. The point of disappemm~ce of unsaturation in the polymer (Fig. 2) corresponded to the time to maximum bonding of the antioxidant in the polymer (Fig. 5). Similar behaviour was observed for the synergistic mixture except that in this case the antioxidant appeared to be completely retained by the polymer at the end of 3 min (Fig. 5) and processing for up to 20 min led to no loss of polymer-bound antioxidant. PVC was processed without additives for various periods in separate experiments and 5.78 x l0 -3 mol/t00 g BHBM added after processing. After processing for a further 3 min, the concentration of retained thiol was determined from the calibration curve (see Fig. l) and was found to be remarkably similar to the concentration of mono-unsaturation produced in the unstabilized polymer (see Fig. 6). Unsaturation remaining in the polymer treated with
BHBM was conjugated (difference between total and mono-unsaturation) and similar in concentration to this species in the polymer processed without additives (see Fig. 6). The formation of carbonyl groups in the polymer is shown in Fig. 7 both in the presence and absence of additives. The trend is the same in all the formulations examined, although the extent is different.The extent of initialformation of carbonyl {1725 cm- ~) is much greater in the case of B H B M than in the unstabilizedpolymer whereas initialcarbonyl formation is suppressed in the polymer containing D B T M , after correction for carbonyl in the additive. Extraction of the sample containing B H B M caused a slight reduction in carbonyl (1725cm -=) concentration during the later stages but most of it was part of the polymer chain. Conjugated carbonyl (1690 cm" t) which was also present in the polymer after processing was completely removed by extraction. The synergistic system gave rise to the lowest carbonyl {1725 cm -t) formation and no conjugated carbonyl was observed. Figures 8 and 9 compare the induction periods to
2.0 --
-~ 2.0 x
. I0
~....~
~'
0
(no BHBM )
"7 /
ft
~pJ"~."~
,~,"
~1
t ~/~" 2
I
4
;'"
\I
6
~/.-"
.
--I0
\ u,,atu,onon o,e, ~ odding 8HBM
1 ~ . .
Conjugated unsaturotion ( no BHBM )
I
8
1
I0
Processinq time,
I
12
I
16
I
t2
g
.~ ~
I
18
min
Fig. 6. Effect of adding BHBM to PVC processed for various times at |70. After each processing without additives, 5.78 × 10 -3 mol/100g of BHBM added and processed for further ] rain fol]owed by methanol extraction.
The effect of thermal processing on PVC--VII
OBTM(5.78)
i o
.
-
.
~
I
~
-
l
•
--
_ _ .
E
'<" o -
o
g
t
0
@I
I]49
I
2
I
4
[
I
[
I
I
6 8 I0 12 t4 Processing time, man
16
I
18
20
Fig. 7. Effect of BHBM alone and in synergistic combination with DBTM on carbonyl formation in PVC during processing at 170' (other data as for Fig, 2).
~_0 3 x T I
e..~'e-e NOadditive fe/" /e
~
Lubricants alone
..4
-
--
l, /
HBM (5.78)
DBTM (5.78)
:~
29o ( 2.5g/,OOg )
0
I0
20
30 40 50 60 Oven ageing time, hr
70
80
FiB. & Effect of BHBM alone and in synergistic combination with DBTM on the formation of unsaturation in PVC during oven ageing at 140 (other data as for Fig. 2).
o..O'~'e NO additive
/
kubriconts alone
HBM 15.78)
/ D B T M
IO
f
_ 0
l,
i~.,...,-,~,-.-i 10
2'
~'
,~o
Oven ageing time,
T290(2.Sg/lOOg )
._._.-.---"
.._~l-~-'~.~.'~
~
(5.78)
a'----"
~
DBTM (4.48) *BHBM
•
~o
' 70
' 80
(,3)
hr
Fig. 9. Effect of BHBM alone and in synergistic combination with DBTM on colour formation in PVC during oven ageing at 140' (other data as for Fig. 2).
1150
B.B. Co(m^Yand G. SCOTT 8O
80
20 e-
o_
so g
~ eo
/
,+o.+
o 4o
O.
o
/~Unsaluration
2o
~ -~0.5
~ 1 % 8MBM
Fig. 10. Synergistic optima for DBTM/BHBM mixtures (total concentration 5.8/100g x 10-3) during oven ageing at 140", as measured by induction period to unsaturation formation, unsaturation at 60 hr and colour formation at 60 hr. unsaturation and colour formation respectively for PVC films heated in an air oven at 140°. The total concentrations of all the additives were maintained constant at 5.78 x 10-3 mol/100g but for comparison a commercial tin maleate based stabilizer (T290) was used at the normally recommended concentration of 2.5 g/100 g, The induction periods to the formation OH
tSu~tBu CH~SH
0
tBu~tBu
formation is virtually 100";, and the colour and conjugated carbonyl formation is eliminated. Since a major function of DBTM is to scavenge HC1 1"4], it seems likely that in its absence direct attack of HCI on BHBM leads to a rapid acid cataiysed elimination of H2S with subsequent formation of the characteristic red coloured quinone [10"1 and loss of phenolic OH in
tBu.
"
CH2
+~,,s of and the extent of formation and unsaturation formation at 60 hr were used to measure the combination effects of the two additives at constant total molar concentration. The results, plotted as a function of tool% BHBM in Fig. 10, show that a synergistic optimum is obtained at a DBTM:BHBM molar ratio in the region of 4:1. DISCUSSION
The rapid reduction of chemically measured unsaturation in processed PVC to zero in the presence of BHBM (Fig. 2) together with the binding of the antioxidant to the polymer (Fig. 5) clearly indicates that the thiol group has reacted with the developing unsaturation. The thiol addition reaction appears to be particularly effective in eliminating mono-unsaturation while conjugated unsaturation remains largely unaffected (Fig. 6). Thiol adduct formation is known to be a radical catalysed process [7, 8] and the rapid formation of peroxides before the unsaturation begins to decrease confirms the sequence of events described previously [2, 3]. However, it is clear from both the formation of an orange-red colour from BHBM and of an extractable conjugated carbonyl compound together with the fact that adduct formation does not reach 100% of the BHBM added (Fig. 5) that sidereactions must also be occurring under these conditions. In the presence of DBTM, BHBM adduct
the polymer (Fig 5). The stilbenequinone is, however, readily extracted from the polymer and cannot therefore be the reason for the increase in carbony[ concentration in the polymer during processing since the latter is not extracted and is in any case non-coloured. It seems more likely that this is an example of the well-known pro-oxidant action of thiols, monosulphides and their derived oxidation products during the initial stages of processing as a result of redox reactions with hydroperoxides ['6, 11, 12]. It has been shown 1"13] that both BHBM and its derived monosulphides are converted to very effective catalysts for the ionic decomposition of hydroperoxides and there is little doubt that the disappearance of initially formed hydropcroxide and the long induction period to further oxidation (Fig. 2) are due to this reaction. However. the effective "catalyst is formed after an induction period which involves the redox reaction of derived sulphenic acids etc. with hydroperoxide El 1-13] and even the presence of the phenolic antioxidant group in the molecule appears to be incapable of inhibiting catbonyl formation. The reason for the inhibition of the pro-oxidant reaction in the presence'of DBTM is less obvious. However, there is evidence~that HCI can catalyse the homolysis of hydroperoxides [6, 14, 15] and that its removal by DBTM [15] can reduce its pro-oxidant effect. The role of HCI in the thermal degradation of PVC is discussed in detail elsewhere [6, 15].
1151
The effect of thermal processing on PVC-- VII The reasons for the synergistic interaction between BHBM and DBTM are clear on the basis of the complementary mechanisms proposed. Removal of unsaturation and hydroperoxides by BHBM and scavenging of hydrogen chloride, reaction with allylic chlorine and with poly-conjugation by DBTM are complementary processes which lead to very effective synergism in all the thermally produced changes normally associated with PVC degradation. Furthermore, these effects can be achieved at synergistic concentrations (1.8 g/100 g for the optimum ratio) considerably lower than those observed with commercial synergistic systems (2.5 g/100 g) (Fig. 8). The main side-reaction when BHBM is used alone (stilbenequinone formation) is substantially eliminated at 4:1 ratio of DBTM to BHBM due to the scavenging of HC1 which catalyses the reaction. The behaviour of the synergistic mixture of BHBM and DBTM closely resembles that of the dialkyltin thioglycollates [16] in its ability both to add to mono-unsaturation resulting from HCI elimination and to behave as a peroxide decomposing antioxidant even in the presence of HCI [15]. It is clear however that a 4-fold molar excess of HCI scavenger is necessary to achieve the optimal effectiveness of the thiol antioxidant.
,4cknowledgemems--We are indebted to Victor Wolf Ltd for the provision of a grant to one of us (BBCI. We also thank Dr N. R. Clark and Dr R. Grantham for helpful discussion of the results.
REFERENCES
1. G. Scott. M. Tahan and J. Vyvoda. Ctwml Ind, 903 (1976). 2. G. Scott, M. Tahan and J. Vyvoda, E,r. Poh'm J. 14, 377 (1978). 3. G. Scott, Polym. Plast. Technol. Engng II, I (1978). 4. G. Scott, M. Tahan and J. Vyvoda, Eur. Polvm. J. 14, 913 (1978). 5. G. Scott. M. Tahan and J. Vyvoda. E,r, Polym. J. 14, 1021 (1978). 6. B. B. Cooray and G. Scott, Derelopmems in Polymer Stabilisationm2, (Edited by G. Scott). Applied Science Publishers, p. 53. (1980). 7. M. R. N. Fernando, G. Scott and J. E. Stuckey, J Rubh. Res. Inst. Sri Lanka 54, 520 (19771: G. Scott. Plast. Rubb Process. 41 (1977). 8. K. W. S. Kularatne and G. Scott. Eur. Polym. J. 15, 827 (1979). 9. P. Boeek, Chem. Prumsyl. 17, 439 (1967k 10. L. J. Filar and S. Winstein, Tetrahedron Lett. 25, 9 (1960). 11. G. Scott, Eur. Polym J. Suppl. 189 (1969): G. Scott, Mechanisms of Reactions of Sulphur Compounds (Edited by N. Kharaschk Vol. 4. pp. 99 11969h C. Armstrong and G. Scott. J. chem. Soc. 1747 (19711. 12. B. B. Cooray and G. Scott, Eur. Potym. J. In press. 13. V. M. Farzaliev. W. S. E Fernando and G. Scott. Eur. Polym. J. 14, 785 (1978). 14. B. B. Cooray and G. Scott, Chemv Ind. 741 (1979k 15. B. B. Cooray and G. Scott, Eur. Polym. ,1. 16, 169 (1980). 16. B. B. Cooray and G. Scott. Polym Degrad. Stabil. 2, 35 (1980). 17. R. W. Martin. Analyt. Chem. 21,921 (1979).
0014-3057/80/1201.1145502.00/0
¢~ Pergamon Press Lid 1980. Printed in Great Britain
THE EFFECT OF THERMAL PROCESSING ON PVC--VII REACTIVE ANTIOXIDANTS .AS THERMAL STABILIZERS
B.
andG.ScoTr
B. Cooxxv Department of Chemistry, University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET, England
(Received 27 March 1980) Abstract--A benzyl thiol (BHBM) containing a hindered phenolic antioxidant group has been compared as a thermal stabilizer for PVC with a typical tin maleate stabilizer (DBTM). It has been shown that BHBM both adds to the developing double bond in PVC during processing and destroys hydroperoxides. It is thus an effective stabilizer but it is portia.By destroyed by side.reactions. In combination with DBTM, BHBM shows powerful synergistic effects which are optima] at a molar ratio of DBTM:BHBM "-4:1. The reasons for the side-reactions and the synergism are rationalized on the basis of the known reactions of BHBM.
INTRODUCTION In previous publications, it has been demonstrated that the early stages of PVC procesaing are characterized by the rapid formation of both unsaturation and hydroperoxides [1-3]. A commercial PVC stabilizer based on the tin male,ate structure appeared not to inhibit the initial formation of unsaturation but no peroxides could he detected with the analytical procedure employed [4]. It was argued that prior formation of both ummturation and of peroxides is responsible for the sensitization of the polymer to photooxidation and that both are a direct result of the processing operation ['5,6]. Other studies have shown ['7, 8] that it is possible to react thiol-containing antioxidants with double bonds in polymers in the presence of free radicals; the purpose of the present study is to examine the possibility of destroying the developing unsaturation in PVC during proeeuing by this technique, using (3,5.di-tert-butyl-4-hydroxy)benzyl mercaptan (BHBM). A more sensitive method for the measurement of hydroperoxides was employed in the present work than that used previously and the effect of dibuty] tin mnieate (DBTM) was compared with that of the thiol. oH tBu
,~tBu
OCO~. / CH
(BubSq It O~O.~"CH CH~:,SH (8HBM)
(DBTM)
EXPERIMENTAL
Materials PVC was Breon M90/50 as obtained from the manufacturers. BHBM was made as degaibed previously [7,8] (m.p. 28-30°). DBTM was Irgastab DBTM from CibaGeigy. Procedure ProcessinO. Suitably formulated PVC was processed as
described previously [2] in the closed mixer of a torque rheometer at 170° with the chamber completely filled. The rotor speed was constant at 60 rpm. After p r ~ for the required period, the polymer was removed rapidly from the mixer and chilled in cold water to minimize further degradation. The polymer was dried and stored in a sealed
bag.
Compression namlding. The processed polymer was pow. dered by abrasion and the power was compremion moulded between' stainlem~eel plates at 170~ for 90see using special grade cellophane to separate the polymer from the metal. The polymer was preheated between the platens of the press for 45 sec prior to application of a pressure of 100kg/em2. Uniform films of thickness 3 x 10-acre were chosen for analysis and thermal exposure studies. DetermMatuTn of peroxide concentration. The following experimental procedure was =_daptedfrom Bocek [9]. 0.5 g of p r ~ powdered polymer was ~voilen in 75 ml of deaerated chloroform for 12 hr in a conical flask fitted with a rubber seal. One ml of deaerated 0.004 M phosphoric acid was added, followed by 2 nd of a deaerated solution of 0.005 M ammonium ferrous sulphate in methanol. Both reagents were added using a syrm~ to prevent contamination by O=. After standing in the dark for 4 hr, 1 ml of a 0,5% solution of 1-10 phenanthroline in de,aerated benzene was added. The reaction mixture was allowed to stand for 15 rain after swirling. Five ml of the reaction mixture free of polymer was carefully withdrawn and its absorption in the visible region (510am) was measured against the blank reference in a Perkin--Elmer model 137 u.v. visible spectrophotometer. Peroxide concentration was then read off using a calibration curve. The polymer samples containing BHBM were exten. Sively extracted with cold methanol (72 hr) in constant circulation before peroxide determination. Total unsaturation was measured as described previously [2]. For samples processed with BHBM, the deterruination was carried out after extensive extraction with cold methanol (72 hr) and it was found that the polymer. bound BHBM did not re,act with iodine to any significant extent, until after 3 hr reaction time and hence did not interfere with the determination of polymer ummturation. Mono-ummturation was determined by the following method adapted from Martin [17]. 0.5g of powdered processed polymer was swollen in" 75 ml of chloroform for 12 hr in a conical flask fitted with a rubber seal. Four g of
1145
//
1146
B.B. Cooa^v and G. SCOTT ,0--
)-¢
I-¢
6 c
zo
4
4=
.o 0 c
/
Q.
0
I
I
/
I
4
8
12
16
BHBM concentration,
.J 20
m o t / l O 0 0 x I0 3
FiB. 1. Relation between hydroxyl index (3620 c m - t ) and BHBM concentration in PVC. acid free mercuric acetate (AR grade, ex. Fisons) and 30 ml of methanol were added. After adding 75 ml of saturated aqueous (AR) sodium chloride and 100ml of distilled water, the liberated acetic acid was titrated with 0.1 N NaOH after 3 hr and the phenolphthaicin end-point determined. Blank runs were similarly examined for each set of experiments. BHBM treated PVC samples were exhaustively cold extracted prior to mono-unsaturation determination. When DBTM was used, the contribution to the total unsaturatinn from the male,ate groups, some of which were unextractable, after processing was determined with the aid of i.r. spectroscopy. A 5% solution of the processed polymer in dichioromethane showed a peak at 1605 eradue to the olefmic absorption of the nucleate, group. A 5?/0 solution of unprocessed PVC in dichloromethane containing a known concentration of DBTM was then placed in the reference beam. When the total concentration of
male,ate groups in the processed polymer was equal to that in the refereno~ sample, then the peak at 1605cm -~ just disappeared. The concentration of D W r M required to can(:elout the ~ unsaturation in the ~ sample was accurately determined for each processing time using a suitable range of known concentration. The volume of sodium thimulphate corres~nding to this concentratinn of DBTM for each pro~ssing time was then subtracted from the titrimetriereading, to determine the actual unsatura~on deveJoped m the polymer dunng p t ~ m ~ n ~ The optimum swellin$ time of 12 hr and the optimum reaction time of I hr were determined in seperttte experiment& Total colour difference was measured as ~ h e d previonsly [2]. i.r. Examinatico of the polymer treated with BHBM
showed a sharp phmmlic OH a b , o ~
at 3620m-t
arm- methanol extraction. Using u n w o o m ~
cast PVC
2.0 --
/
x
Tom
vl.
/
L.b,ice..s~one
E c• o o
1.0
i
OBTM ( 5.781
6
8
I0
Proce~inQ time,
12
_
14
16
18
20
rain
Fig. 2. Effectof B H B M alone and in synergisticcombination with D B T M on the formation of unsaturation in P V C during processing at 170° (lubricantsCaSt, 0.8% + W a x E, 0.65~) present in ¢agh formulation except control (numbers in parentheses are concentrations in mol/100 g x 10s).
°I M
0.8
T
The effect of thermal processing on PVC~VII
I~
0.6:
/
No additive
/~e
¢~
jr / \
" 0.4
e,.~,,l~, D ~
a.
TM (5.78)
DBTM (4.48)+ BHBM ( 0
2
4
6
8
/
e/~-
j
Lubricont$ olone
=
1147
I0
~2
F~roces$ing time,
.
/4
18
16
20
rain
Fig. 3, Effect of BHBM alone and in synergistic combination with DgTM on the formation of peroxides in PVC during processing at 170~ (other data as for Fig. 2). films (thicknms 3 x 10-3¢m) containing known concen. trstiom of BHBM, a linear calibration plot was construtted wing the absorption peak at 3620 cm-: {see Fig. 1) and this was employed in calculating the concentration of retained BHBM in PVC after
fall to zero. The former began to reduce after less than I rain and the latter after about 3 rain. When DBTM and BHBM were used together at the same total concentration (5.78 x 10-3 tool/100 8), synergistic effects were observed. In Figs 2 and 3 the molar ratio of the two components was 3A4:1 (see below) and it is clear that this system gives both a lower level of ummturation and of peroxide formation during the initial stage and a lower rate of formation of both species.at the end of the induction period. The fommtion of colour (Fig. 4) followed the same general pattern as the formafon of unsaturation for all the additives. Although the level of colour was higher with BHBM than with DBTM, solvent extraction of the films reduced it indicating that BHBM underwent some decomposition to coloured products during processing. The synergistic mixture showed much less colour formation than either of the individual components. Exhaustive extraction with methanol of the films containing BHBM processed for shor~ times resulted in the substantial removal of the additive as measured by residual phenolic OH in the polymer (see Fig. 5). The amount remaining after extraction increased sharply with processing time
processing.
ItESULTS
The formation of tmmturation and of peroxide as a fuaation of prorating time is shown in Figs 2 and 3 f o r 5 PVC focmuiations. As found previously [1,2] the control without lubricants or stabilizers gave the highest level of both, ri=ing rapidly during the first 2 rain of proce=fing. Unsaturation continued to rise at a iow~ rate after 2 rain but peroxide concentration fell rapidly, I d t h o u l ~ With the more sensitive method used in this study, not to zero. The two lubricants,
calcium stearat= (0.8%) and Wax E (0.65%) when used together somewhat reduced the formation of both species but did not change the shapes of the curves. The additional prep-nee of DBTM (5.78 x 10- a tool/100 g) rednLJed but did not eliminate the formarion of both species. BHBM by contrast caused the concentration of both unsaturation and peroxides to an e-
70
No:
~., 60
.~ 50 o
4O _
~.,.~f~Lubr
-
~ 2O
0
/
e
/
icont s olone
y
e"'e
BHBM (5 7'8)
/p.'~
2
( ~.?m
4
,
~
8
..p,
I0
PrOcelsing time,
12
14
;
16
;
18
T
20
min
Fig. 4. Elfcct of BHBM alone and in synergistic oombiemfion with DBTM on colour formation in PV¢ during processing at 170' (other data as for Fig. 2).
I 148
B.B. Co(mAYand O. SCOTT " r _-_ . _ _ . . . .
_8.".B_". _(
........................
I~t~ 8HBM(5.78) unext ; 3 L~- ~" . ~" E ],p,/BHgM(S,Te) ext
V
-o
~ ~
-
"
o
"
"--o
OBTM (4.48) + 8HBM (I. 31 unext u a ~ • c
=
:3-4
I
I
{
._u_ f ~I"~=~DBTM (4,48)+ BHBM (I.3)ext
~"
I
0
f
I
I
5
I
1
I
10 15 Proceuing time, rain
I
20
I
25
Fig. 5. R©tention of BHBM in PVC during processing at 170 in the presence and absence of DBTM (other data as for Fig. 2).
until after 3 rain no further change was observed; the amount of phenol remaining substantially constant for 20 min. The unextracted films showed a drop in phenolic OH during the first few minutes of processing to the level of the bound component. The point of disappemm~ce of unsaturation in the polymer (Fig. 2) corresponded to the time to maximum bonding of the antioxidant in the polymer (Fig. 5). Similar behaviour was observed for the synergistic mixture except that in this case the antioxidant appeared to be completely retained by the polymer at the end of 3 min (Fig. 5) and processing for up to 20 min led to no loss of polymer-bound antioxidant. PVC was processed without additives for various periods in separate experiments and 5.78 x l0 -3 mol/t00 g BHBM added after processing. After processing for a further 3 min, the concentration of retained thiol was determined from the calibration curve (see Fig. l) and was found to be remarkably similar to the concentration of mono-unsaturation produced in the unstabilized polymer (see Fig. 6). Unsaturation remaining in the polymer treated with
BHBM was conjugated (difference between total and mono-unsaturation) and similar in concentration to this species in the polymer processed without additives (see Fig. 6). The formation of carbonyl groups in the polymer is shown in Fig. 7 both in the presence and absence of additives. The trend is the same in all the formulations examined, although the extent is different.The extent of initialformation of carbonyl {1725 cm- ~) is much greater in the case of B H B M than in the unstabilizedpolymer whereas initialcarbonyl formation is suppressed in the polymer containing D B T M , after correction for carbonyl in the additive. Extraction of the sample containing B H B M caused a slight reduction in carbonyl (1725cm -=) concentration during the later stages but most of it was part of the polymer chain. Conjugated carbonyl (1690 cm" t) which was also present in the polymer after processing was completely removed by extraction. The synergistic system gave rise to the lowest carbonyl {1725 cm -t) formation and no conjugated carbonyl was observed. Figures 8 and 9 compare the induction periods to
2.0 --
-~ 2.0 x
. I0
~....~
~'
0
(no BHBM )
"7 /
ft
~pJ"~."~
,~,"
~1
t ~/~" 2
I
4
;'"
\I
6
~/.-"
.
--I0
\ u,,atu,onon o,e, ~ odding 8HBM
1 ~ . .
Conjugated unsaturotion ( no BHBM )
I
8
1
I0
Processinq time,
I
12
I
16
I
t2
g
.~ ~
I
18
min
Fig. 6. Effect of adding BHBM to PVC processed for various times at |70. After each processing without additives, 5.78 × 10 -3 mol/100g of BHBM added and processed for further ] rain fol]owed by methanol extraction.
The effect of thermal processing on PVC--VII
OBTM(5.78)
i o
.
-
.
~
I
~
-
l
•
--
_ _ .
E
'<" o -
o
g
t
0
@I
I]49
I
2
I
4
[
I
[
I
I
6 8 I0 12 t4 Processing time, man
16
I
18
20
Fig. 7. Effect of BHBM alone and in synergistic combination with DBTM on carbonyl formation in PVC during processing at 170' (other data as for Fig, 2).
~_0 3 x T I
e..~'e-e NOadditive fe/" /e
~
Lubricants alone
..4
-
--
l, /
HBM (5.78)
DBTM (5.78)
:~
29o ( 2.5g/,OOg )
0
I0
20
30 40 50 60 Oven ageing time, hr
70
80
FiB. & Effect of BHBM alone and in synergistic combination with DBTM on the formation of unsaturation in PVC during oven ageing at 140 (other data as for Fig. 2).
o..O'~'e NO additive
/
kubriconts alone
HBM 15.78)
/ D B T M
IO
f
_ 0
l,
i~.,...,-,~,-.-i 10
2'
~'
,~o
Oven ageing time,
T290(2.Sg/lOOg )
._._.-.---"
.._~l-~-'~.~.'~
~
(5.78)
a'----"
~
DBTM (4.48) *BHBM
•
~o
' 70
' 80
(,3)
hr
Fig. 9. Effect of BHBM alone and in synergistic combination with DBTM on colour formation in PVC during oven ageing at 140' (other data as for Fig. 2).
1150
B.B. Co(m^Yand G. SCOTT 8O
80
20 e-
o_
so g
~ eo
/
,+o.+
o 4o
O.
o
/~Unsaluration
2o
~ -~0.5
~ 1 % 8MBM
Fig. 10. Synergistic optima for DBTM/BHBM mixtures (total concentration 5.8/100g x 10-3) during oven ageing at 140", as measured by induction period to unsaturation formation, unsaturation at 60 hr and colour formation at 60 hr. unsaturation and colour formation respectively for PVC films heated in an air oven at 140°. The total concentrations of all the additives were maintained constant at 5.78 x 10-3 mol/100g but for comparison a commercial tin maleate based stabilizer (T290) was used at the normally recommended concentration of 2.5 g/100 g, The induction periods to the formation OH
tSu~tBu CH~SH
0
tBu~tBu
formation is virtually 100";, and the colour and conjugated carbonyl formation is eliminated. Since a major function of DBTM is to scavenge HC1 1"4], it seems likely that in its absence direct attack of HCI on BHBM leads to a rapid acid cataiysed elimination of H2S with subsequent formation of the characteristic red coloured quinone [10"1 and loss of phenolic OH in
tBu.
"
CH2
+~,,s of and the extent of formation and unsaturation formation at 60 hr were used to measure the combination effects of the two additives at constant total molar concentration. The results, plotted as a function of tool% BHBM in Fig. 10, show that a synergistic optimum is obtained at a DBTM:BHBM molar ratio in the region of 4:1. DISCUSSION
The rapid reduction of chemically measured unsaturation in processed PVC to zero in the presence of BHBM (Fig. 2) together with the binding of the antioxidant to the polymer (Fig. 5) clearly indicates that the thiol group has reacted with the developing unsaturation. The thiol addition reaction appears to be particularly effective in eliminating mono-unsaturation while conjugated unsaturation remains largely unaffected (Fig. 6). Thiol adduct formation is known to be a radical catalysed process [7, 8] and the rapid formation of peroxides before the unsaturation begins to decrease confirms the sequence of events described previously [2, 3]. However, it is clear from both the formation of an orange-red colour from BHBM and of an extractable conjugated carbonyl compound together with the fact that adduct formation does not reach 100% of the BHBM added (Fig. 5) that sidereactions must also be occurring under these conditions. In the presence of DBTM, BHBM adduct
the polymer (Fig 5). The stilbenequinone is, however, readily extracted from the polymer and cannot therefore be the reason for the increase in carbony[ concentration in the polymer during processing since the latter is not extracted and is in any case non-coloured. It seems more likely that this is an example of the well-known pro-oxidant action of thiols, monosulphides and their derived oxidation products during the initial stages of processing as a result of redox reactions with hydroperoxides ['6, 11, 12]. It has been shown 1"13] that both BHBM and its derived monosulphides are converted to very effective catalysts for the ionic decomposition of hydroperoxides and there is little doubt that the disappearance of initially formed hydropcroxide and the long induction period to further oxidation (Fig. 2) are due to this reaction. However. the effective "catalyst is formed after an induction period which involves the redox reaction of derived sulphenic acids etc. with hydroperoxide El 1-13] and even the presence of the phenolic antioxidant group in the molecule appears to be incapable of inhibiting catbonyl formation. The reason for the inhibition of the pro-oxidant reaction in the presence'of DBTM is less obvious. However, there is evidence~that HCI can catalyse the homolysis of hydroperoxides [6, 14, 15] and that its removal by DBTM [15] can reduce its pro-oxidant effect. The role of HCI in the thermal degradation of PVC is discussed in detail elsewhere [6, 15].
1151
The effect of thermal processing on PVC-- VII The reasons for the synergistic interaction between BHBM and DBTM are clear on the basis of the complementary mechanisms proposed. Removal of unsaturation and hydroperoxides by BHBM and scavenging of hydrogen chloride, reaction with allylic chlorine and with poly-conjugation by DBTM are complementary processes which lead to very effective synergism in all the thermally produced changes normally associated with PVC degradation. Furthermore, these effects can be achieved at synergistic concentrations (1.8 g/100 g for the optimum ratio) considerably lower than those observed with commercial synergistic systems (2.5 g/100 g) (Fig. 8). The main side-reaction when BHBM is used alone (stilbenequinone formation) is substantially eliminated at 4:1 ratio of DBTM to BHBM due to the scavenging of HC1 which catalyses the reaction. The behaviour of the synergistic mixture of BHBM and DBTM closely resembles that of the dialkyltin thioglycollates [16] in its ability both to add to mono-unsaturation resulting from HCI elimination and to behave as a peroxide decomposing antioxidant even in the presence of HCI [15]. It is clear however that a 4-fold molar excess of HCI scavenger is necessary to achieve the optimal effectiveness of the thiol antioxidant.
,4cknowledgemems--We are indebted to Victor Wolf Ltd for the provision of a grant to one of us (BBCI. We also thank Dr N. R. Clark and Dr R. Grantham for helpful discussion of the results.
REFERENCES
1. G. Scott. M. Tahan and J. Vyvoda. Ctwml Ind, 903 (1976). 2. G. Scott, M. Tahan and J. Vyvoda, E,r. Poh'm J. 14, 377 (1978). 3. G. Scott, Polym. Plast. Technol. Engng II, I (1978). 4. G. Scott, M. Tahan and J. Vyvoda, Eur. Polvm. J. 14, 913 (1978). 5. G. Scott. M. Tahan and J. Vyvoda. E,r, Polym. J. 14, 1021 (1978). 6. B. B. Cooray and G. Scott, Derelopmems in Polymer Stabilisationm2, (Edited by G. Scott). Applied Science Publishers, p. 53. (1980). 7. M. R. N. Fernando, G. Scott and J. E. Stuckey, J Rubh. Res. Inst. Sri Lanka 54, 520 (19771: G. Scott. Plast. Rubb Process. 41 (1977). 8. K. W. S. Kularatne and G. Scott. Eur. Polym. J. 15, 827 (1979). 9. P. Boeek, Chem. Prumsyl. 17, 439 (1967k 10. L. J. Filar and S. Winstein, Tetrahedron Lett. 25, 9 (1960). 11. G. Scott, Eur. Polym J. Suppl. 189 (1969): G. Scott, Mechanisms of Reactions of Sulphur Compounds (Edited by N. Kharaschk Vol. 4. pp. 99 11969h C. Armstrong and G. Scott. J. chem. Soc. 1747 (19711. 12. B. B. Cooray and G. Scott, Eur. Potym. J. In press. 13. V. M. Farzaliev. W. S. E Fernando and G. Scott. Eur. Polym. J. 14, 785 (1978). 14. B. B. Cooray and G. Scott, Chemv Ind. 741 (1979k 15. B. B. Cooray and G. Scott, Eur. Polym. ,1. 16, 169 (1980). 16. B. B. Cooray and G. Scott. Polym Degrad. Stabil. 2, 35 (1980). 17. R. W. Martin. Analyt. Chem. 21,921 (1979).