Polymer Degradation and Stability 7 (1984) 1-12

The Mechanisms of Action of Diestertin PVC

Stabilisers

B. B. C o o r a y

Akzo Chemic UK Limited, Littleborough OL15 0BA, Great Britain

(Received: 10 March, 1983)

ABSTRACT

Di(carbobutoxy ethyl)tin bis(2-ethyl hexyl thioglycollate) is a typical

estertin stabiliser widely employed in rigid PVC processing and its
mechanisms of action have been investigated. It is shown that the estertin
is more effective than dioctyltin bis(2-ethyi hexyl thioglycollate) in
controlling unsaturation (colour ) and per oxide formation in P VC during
processing in a Brabender and on a two-roll mill. This is attributed to
better compatibility of the estertin, faster hydrogen chloride neutrali-
sation and allylic chlorine replacement, increased thiol addition efficacy
and reduced Lewis acidity of its mono- and dichloride derivatives.
However, the estertin is shown to be a less efficient peroxide decomposer
relative to the alkyltin, whilst being somewhat more susceptible to
tin-carbon bond cleavage upon extensive thermal treatment.

INTRODUCTION

A number of workers have demonstrated that highly efficient PVC

stabilisers participate in a number of chemical reactions within the
polymer and thereby extend the period during which mechanical and
aesthetic properties are retained. 1 - 3 It is also well known that synergism
between chemical processes brought about by such an additive or an
additive mixture causes performance optimisation accompanied by cost
reductions in commercial PVC formulations. 4 0 r g a n o t i n stabilisers
1
Polymer Degradation and Stability 0141-3910/84/$03.00 © Elsevier Applied Science
Publishers Ltd, England, 1984. Printed in Great Britain
2 B.B. Cooray

exhibit autosynergism and this is particularly applicable to thiotin

compounds. 2
In recent years a new class of organotin compounds called estertins 5
has become available and this study deals with the stabilisation
mechanisms of di(carbobutoxy ethyl)tin bis(2-ethyl hexyl thioglycollate).
The mechanistic study has been carried out in rigid PVC under the actual
processing and stability test conditions usually employed in the PVC
industry. The chemical changes have been carefully monitored. The
changes occurring in the polymer include unsaturation formation (colour
accumulation) through dehydrochlorination and peroxide formation
through oxidative processes. 6'7 The concentrations and rates of
formation of these species in PVC in the presence of stabilisers measure
the efficiency of the overall stabilisation process. The chemical changes
undergone by the stabiliser during polymer processing may be a guide to
intervening mechanistic processes.

EXPERIMENTAL
Materials: PVC was Breon M90/50 and Corvic $57/116 as obtained from
the manufacturers.
Di(carbobutoxy ethyl)tin bis(2-ethyl hexyl thioglycollate) was pre-
pared from di(carbobutoxy ethyl)tin dichloride by reaction with 2-ethyl
hexyl thioglycollate in the presence of sodium hydrogen carbonate at
40 °C until neutral pH was reached. After separating sodium chloride, the
estertin stabiliser is isolated in near theoretical yield. (~o Sn: observed,
15.06; theory, 15.16; %S: observed, 8.2; theory, 8.17; %C1: observed,
<0.05; theory, 0.)
Dioctylin bis(2-ethyl hexyl thioglycollate)was prepared similarly from
dioctyltin dichloride and 2-ethyl hexyl thioglycollate. (% Sn: observed,
15.7; theory, 15.81; KS: observed, 8.5; theory, 8.53; %C1: observed,
< 0.05; theory, 0.)
Processing:
(a) 100 parts of M90/50 PVC was formulated with 0.50 parts of
Wax E and 0.65 parts of calcium stearate and processed in a
Brabender mixer at 180 °C with the chamber completely filled.
After processing for the required length of time, the polymer was
removed rapidly from the mixer and chilled to minimise further
degradation. The polymer was stored in a sealed bag until
required.
 Mechanisms of action of diestertin PVC stabilisers 3

(b) A rigid calendering formulation of PVC (Corvic S67/116, 100

parts; glycerol mono-oleate, 0.9 parts; Wax E, 0.3 parts; K120N,
0.5 parts; K175, 0.7 parts; K.A.B. 22, 2.0 parts and Resoform
violet BL 0.001 parts) was subjected to a mill stability test at 185 °C
and samples were withdrawn at 5-min intervals and stored after"
chilling.
Determination of the peroxide concentration was by the method
previously described. 7
Unsaturation was measured by the method described by Scott et al. s
Equimolar amounts of the two stabilisers were used throughout this study
(2.6 × 10 -3 mol/100g PVC).

RESULTS

The formation of unsaturation and peroxides during the processing of

rigid PVC in a Brabender mixer at 180°C is shown in Fig. 1. In the
absence of an organotin compound, unsaturation and peroxide
formation occur rapidly during the first 3 min of processing. Further
processing causes a slower and almost linear accumulation of

3.C 1"2

2 _= -1'0

4"

- ~

0.5 ~ .~. ~

0 5 10 15 20 25 30 35
Processing t i m e (rain)

Fig. 1. Unsaturation and peroxideformation in PVC during Brabenderprocessingat

180°C. A. Estertin. B. Octyltin. C. Control (lubricants only).
4 B.B. Cooray

unsaturation in the polymer. Peroxide concentration is initially reduced

from its maximum level but, after a short equilibration period, further
processing causes rapid and autocatalytic accumulation of the species in
the absence of a stabiliser.
The level ofunsaturation produced in PVC formulated with the estertin
compound is significantly lower than that observed in its absence. During
the first 3 min of processing, a lower unsaturation maximum was observed
with the estertin. This unsaturation is rapidly eliminated during the next
few minutes of processing and a clear induction period is observed during
which unsaturation in the polymer is below the detectable level. This
induction period corresponds to the time taken for initial colour
formation in PVC. The magnitude of the induction period is higher with
the estertin than with the alkyltin compound. After this induction period
further processing leads to an accumulation of unsaturation in the
polymer. The alkyltin was found to cause a higher level ofunsaturation in
the polymer during this period and consequently the colour of PVC was
visually lighter when the estertin compound was used. The level of
unsaturation in the polymer formulated with the estertin became higher
than that produced with the alkyltin when the polymer was processed for
a further 15 rain (total processing time, 50min).
Peroxide formation was also lower in the presence of organotin
stabilisers. The initial maximum was slightly higher in the presence of the
estertin relative to that observed with the alkyltin. Peroxide concentration
was also reduced to an undetectable level in the presence of the alkyltin
but the estertin compound showed a higher equilibrium level of peroxide
after 5 min of processing. This equilibrium level was maintained during
further processing in the presence of the estertin. The induction period
during which peroxide was zero with the alkyltin was soon followed by
relatively rapid and linear peroxide formation. This led to a curve cross-
over soon after 20 min of processing and subsequently the peroxide level
in PVC formulated with alkyltin was significantly higher than in PVC
formulated with the estertin.
Figure 2 shows the formation ofunsaturation in PVC in the presence of
the two tin stabilisers during a mill stability test at 185 °C. The stabilisers
were incorporated into PVC by compounding a rigid calendering
masterbatch in a Papenmier at 110 °C and allowing the polymer to cool to
room temperature, The compounded polymer was then processed on an
oil heated two-roll mill at 185 °C and samples were withdrawn at 5-min
intervals. Unsaturation measurement in the samples reveals that, in the
 Mechanisms of action of diestertin PVC stabilisers 5

presence of the estertin compound, lower levels of unsaturation are

observed. The alkyltin compound appears to be rather less efficient in
controlling early and mid-colour formation in PVC.
Further processing causes an almost linear accumulation of un-
saturation (and colour) in PVC formulated with dioctyltin bis(2-ethyl-
hexyl thioglycollate) while the estertin compound causes faster dehydro-
chlorination. This effect is also observed with some calcium-zinc
stabilisers and is generally overcome by suitable formulation
modifications.
The higher level of unsaturation formed in the polymer during
extensive processing in the presence of the estertin compound is reduced

ooL
~_ ~F ~x/ B

, ,
0 5 10 15 20 25 30 35 40
Milling time at 185oC (rnin)

Fig. 2. Unsaturation formation in PVC under hot milling conditions (185°C).

A. Alkyltin stabiliser. B. Estertin stabiliser.

to a level similar to that observed with the alkyltin by using a wide variety
of additives. These include calcium stearate, calcium alkyl half-maleates,
dibutyltin dilaurate and di(carbobutoxy ethyl)tin bis(monobutylmaleate).
PVC films were cast from dichloromethane solution after adding
equimolar amounts of di(carbobutoxy ethyl)tin bis(2-ethyl hexyl
thioglycoUate) and dioctyltin bis(2-ethyl hexyl thioglycollate), respec-
tively. After vacuum drying at 20 °C to constant weight, the films were
subjected to a temperature of 185 °C in an air circulating Heraeus oven.
During the first 25 rain of heating, the original stabilisers were converted
to their respective half-chlorides at different rates, as shown in Fig. 3.
Cast films were used in this study in order to ensure that effects due to
lubrication and shear are eliminated.
Half-chloride formation in PVC films was monitored by making use of
previously constructed calibration curves using known concentrations of
6 B.B. Cooray

30-

A
t-
O

~20-
oo
t-
O

io10 /j

0 10 20 30
Oven heating tim= (rain)

Fig. 3. Stabiliser conversion to half-chloride, during oven heating at 190 °C. A. Estertin
stabiliser. B. Alkyltin stabiliser.

the stabiliser, its half-chloride and 2-ethyl hexyl thioglycollate in cast

films, as shown in the following equation:
:l
R2Sn(2-EHTG)2 + HC1 , R2Sn + 2EHTG
\2-EHTG
2-EHTG - 2-ethyl hexyi thioglycollate

The ratio of the infra-red absorption peaks at 1665 cm- 1 (co-ordinated

carbonyl) and at 1720cm-1 (free carbonyl) was plotted against the
percentage conversion of the original stabiliser to its corresponding half-
chloride in constructing the calibration curves.
The results show that the estertin compound is converted to its half-
chloride at a faster rate than the alkyltin in PVC under oven heating
conditions. A relatively small amount of unsaturation was also generated
in the polymer during the experiment. These results suggest that the
estertin compound is more reactive towards hydrogen chloride and
towards replaceable chlorine sites along the polymer, e.g. allylic chlorine,
relative to the alkyltin compound.
The interaction between stabilisers and hydrogen chloride could only
be followed during the early stages of heating due to the complex nature
of the infra-red spectra upon further heating.
 Mechanisms of action of diestertin PVC stabilisers 7

In a model study, the reaction between tertiary butyl hydroperoxide

(TBH) and the two organotin stabilisers was monitored by measuring the
hydroperoxide concentration using the calibrated infra-red absorption at
3538cm-i in a Perkin-Elmer 457 spectrophotometer using ordinate
expansion. 9 Using chlorobenzene as the solvent, a first order plot of the
concentration of TBH as a function of reaction time at 60 °C is shown in
Fig. 4. Both dioctyltin bis(2-ethyl hexyl thioglycollate) and di(carbo-
butoxy ethyl)tin bis(2-ethyl hexyl thioglycollate) were found to react with

Reaction t i m e (x lO'2min)
0 1 2 3 4

no stabiiiser (control)

-0.2

~ -0,4

-0.6

Fig. 4. Reaction betweenhydroperoxidesand stabilisers in solution. Decompositionof

0.5 Mtertiary butyl hydroperoxideby 0-025Mtin stabilisers at 60°C in chlorobenzene.
A. Estertin stabiliser. B. Alkyltin stabiliser.

TBH in initially rapid stoichiometric processes followed by slower

catalytic reactions. The rate of the catalytic reaction was found to be
faster with the alkyltin stabiliser but catalysis commenced at an earlier
stage with the estertin compound.
The effect of dioctyltin dichloride formation in PVC under Brabender
processing conditions (180°C) is shown in Fig. 5. It is clear that the two
organotin dichlorides are not affecting unsaturation formation adversely.
The estertin dichloride causes only a slight increase in rate ofunsaturation
8 B.B. Cooray

,¢-

o 1<~

e-
l I I I 1
0 2 4 6 8 10
Processing t i m e (mln)

Fig. 5. Unsaturation formation during Brabender processing at 180 °C in the presence

of dichlorides. A. Di(carbobutoxy)ethyltin dichloride. B. Dioctyltin dichloride. C.
Control without stabiliser.

formation, thereby demonstrating only a slight Lewis acid activity. The

alkyltin dichloride is more catalytic than the estertin.
PVC films obtained by processing the polymer in the Brabender mixer
for 5 min in the presence o f the two organotin dichlorides were subjected
to Heraeus air oven heating at 190 °C. As shown in Fig. 6, the rate o f
unsaturation formation in the polymer was similar to that observed in the
presence o f only lubricants. The alkyltin dichloride gave a slightly faster
rate than the estertin dichloride.

2'5~

~ 2.C

C
.9

~1.5

1.0 I I I i J
0 5 10 15 20 25
Oven heating time (rain)

Fig. 6. Unsaturation formation during Hcraeus oven heating at 190 °C in the presence
of dichlorides. A. Di(carbobutoxy)ethyltin dichloride. B. Dioctyltin dichloride.
C. Control without organotin.
 Mechanisms of action of diestertin PVC stabilisers 9

DISCUSSION

The results demonstrate that estertin stabilisers performed a number of

functions in PVC under technological conditions.
Under processing conditions involving the application of shear and
heat the stabiliser eliminates the initially formed unsaturation by the
addition of 2-ethyl hexyl thioglycollate to developing double bonds.
It has previously been shown that a similar reaction occurs with
dioctyltin bis(iso-octyl thioglycollate). 7
The lower level of unsaturation generated during the early stages of
processing, the longer induction period during which unsaturation is
undetectably low and the lower level of unsaturation subsequently
detected (Figs 1 and 2) demonstrate the higher efficiency of the thiol
addition reaction when the estertin is used. Since the thiol ligand is the
same in the case of the two stabilisers, the higher efficiency of the estertin
may be associated with the greater compatability of this stabiliser with
PVC. This ensures that the stabiliser is available more readily at sites of
chemical change along the polymer chain, thereby favouring reaction
between liberated thiol ligands and developing unsaturation. The thiol
addition reaction has been shown to be peroxide catalysed under similar
processing conditions. *° Thiols are also known to undergo oxidative
processes in the presence of peroxides 1~ and the greater compatibility
of the estertin compound with PVC may contribute to favoured
polymer-thiol reaction at the expense of thiol oxidation processes.
Figure 2 shows that, during extensive processing, the rate of
dehydrochlorination of PVC was faster in the presence of the estertin
compound. Although such prolonged processing is usually not
encountered in PVC fabrication, the reduced efficiency of the estertin
under these conditions could almost certainly be related to tin-carbon
bond cleavage under extensive heating, t 5 This may lead to the formation
of species such as tin tetrachloride which are stronger Lewis acids than
di(carbobutoxy ethyl)tin dichloride, hence promoting dehydro-
chlorination under extreme processing.
Thiol oxidation processes in polymers do contribute to significant
peroxidolytic activity 1* and the peroxide concentrations plotted in Fig. 1
demonstrate that such activity is initially more pronounced with the
alkyltin compound. Figure 4 shows that the thiotin stabilisers are
themselves effective peroxide decomposers and it has been previously
demonstrated that dioctyltin bis(iso-octyl thioglycollate) reacts with
I0 B. B. Cooray

peroxides via a multi-stage mechanism, t 2 The catalytic rate observed with

the alkytin compound was also faster than that caused by the estertin
compound (Fig. 4) and this may be attributed to some important
structural differences between the two classes of organotins:

SCH2COOCsHI 7
C,H9__O /CH2--CH2\ 1 . 0 ~ c / ' O - - - C 4 H 9
c .... sh y I
%O ...... NCH2/CH2
SCH2COOCaHI 7
Scheme 1. Schematicrepresentation of an estertin molecule.

As shown in Scheme 1, estertin compounds show pronounced internal

coordination of the carbonyl group from carboalkoxyethyl fragments to
the tin atom t3 and this leads to greater steric crowding around the tin
atom in di(carbobutoxy ethyl)tin bis(2-ethyl hexyl thioglycollate) relative
to the tin atom in dioctyltin(2-ethyl hexyl thioglycollate). In both
stabilisers co-ordination contributions may also come from the carbonyl
groups in 2-ethyl hexyl thioglycollate fragments.
Such steric hindrance may not affect the relatively rapid reactions at the
tin atom, such as reaction of tin-sulphur bonds with hydrogen chloride
and with hydroperoxides as shown in Figs 3 and 4. The rate of reaction
with hydrogen chloride to yield stabiliser half-chlorides was actually
faster with the estertin and the rate of stoichiometric reaction with
hydroperoxide similar in the two stabilisers examined.
Subsequent peroxide decomposition rates are, however, determined by
the concentrations of catalytic peroxide decomposer species ~2 and it has
previously been shown that the formation of such species is preceded by
rearrangement and further stoichiometric oxidation of the sulphenate
ester formed as a result of initial stoichiometric interaction with peroxides
in the case of dioctyltin bis(iso-octyl thioglycollate).~2
In the case of the estertin stabiliser, both rearrangement and further
oxidation of the sulphenate esters are likely to be affected by steric
hindrance around the central tin atom, thereby causing a relatively slow
separation of the oxidised sulphur moiety from the metal atom. Such a
separation and the release of catalytic species requires either hydrolysis or
alcoholysis of oxidized sulphur groups attached to tin atoms ~2 and these
processes may also be impeded by steric interference.
 Mechanisms of action of diestertin PVC stabilisers 11

The slower peroxidolytic activity of the estertin compound in PVC is

therefore consistent with its structural features and their influence on the
mechanism of peroxide decomposition through the formation of catalytic
species.
However, the estertin compound is more effective in maintaining an
almost equilibrium level of peroxide over a longer processing time than
the alkyl tin compound. This may be related to better control of other
reactions that contribute to peroxide accumulation when the estertin
compound is present. Efficient thiol addition and faster hydrogen
chloride neutralisation by the estertin compound both contribute to
lowering the level of potential oxidation. Thiol addition reduces the
oxidizable allylic hydrogen atom sites 2 and removal of hydrogen chloride
reduces the build up of peroxides through peroxide/hydrogen chloride
interactions by a radical pathway. ~4
The internal carbonyl co-ordination characteristic of estertin com-
pounds also contributes to a reduction in the Lewis acidity of the
corresponding dichlorides relative to dioctyltin dichloride. In the latter
the tin atom is easily accessible for chemical interactions. This is
consistent with the results shown in Figs 5 and 6. None of the organotin
dichlorides was found to be a powerful Lewis acid catalyst in PVC,
however. Their effect on PVC degradation under processing conditions
and in oven heating conditions was only a minor one with the diestertin
dichloride being least harmful.
This is in contrast to conclusions drawn by Hoang et al.~6 regarding
the Lewis acidity of estertin dichlorides. The evidence from the author's
study clearly shows that the alkyltin dichloride causes slightly higher
dehydrochlorination than the estertin dichloride in PVC under actual
processing conditions. Internal carbonyl co-ordination in the estertin
compound contributes to a reduction in the Lewis acidity of the
dichloride, thereby eliminating any measurable pro-degradation.

CONCLUSIONS

Di(carbobutoxy ethyl)tin bis(2-ethyl hexyl thioglycollate) neutralises

hydrogen chloride and the resultant thiol adds to developing un-
saturation. Both these processes appear to be faster than with dioctyl tin
bis(2-ethyl hexyl thioglycollate). The estertin stabiliser also decomposes
peroxides over a long period of PVC processing, thereby controlling
12 B. B. Cooray

oxidative degradation. Di(carbobutoxy ethyl)tin dichloride does not

affect PVC degradation to any significant extent during processing or
oven heating.

ACKNOWLEDGEMENTS

T h e a u t h o r wishes to thank Drs J. W. Burley, P. H o p e and R. E. H u t t o n

for helpful discussions during the course of this study and Akzo Chemie
for granting permission to publish the results.

REFERENCES

1. G. Ayrey and R. C. Poller, Developments in polymer stabilisation--2,

Chapter I (G. Scott (Ed.)), Applied Science Publishers Ltd, London (1980).
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Chapter 2 (G. Scott (Ed.)), Applied Science Publishers Ltd, London (1980).
3, W. Starnes, Stabilisation and degradation of polymers, Adv. in Chem.
Series, 169 (1978).
4. L. I. Nass, Encyclopedia of PVC, Vol. I, Chapter 9, Dekker, New York
(1976).
5. J. W. Burley, R. E. Hutton and B. R. Iles, Ger, Pat. 2,540,210 (25th March,
1976).
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(1978).
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