dibasic lead phosphite as a stabiliser against the natural weathering of plasticised pvc

Br. Polym. J . 1974, 6, 13-23

Dibasic Lead Phosphite As A Stabiliser Against The Natural Weathering Of Plasticised PVC Derek Pearsona and John Morley Associated Lead Manufacturers Limited, Hayhole Lead Works Willington Quay, Wallsend, Northumberland, England and Associated Lead Manufacturers Limited, Research Laboratories 7 Wadsworth Road, Perivale, Greenford, Middlesex, England

(Paper received 27 April 1973, amendedpaper accepted 13 December 1973)

Dibasic lead phosphite is known to be a stabiliser for plasticised PVC against weathering. The stabilisation mechanism, however, is not known and could be due to absorption of U.V. radiation by the dibasic lead phosphite, or to an antioxidant action.

It is suggested that the mechanism, in fact, depends upon an antioxidant action against the harmful effect of atmospheric oxygen absorbed during the test, and also that a similar effect occurs during thermal degradation of the PVC in free air. This beneficial effect is due to the presence of the phosphite grouping, but it is also necessary to have sufficient basic lead present to remove chlorine ions or radicals in the form of lead chloride. It has been found that equivalent molecular amounts of dibasic lead phosphite and normal lead phosphite are equally efficient in the presence of basic lead sulphate compounds.

An interesting discovery is that the weather conditions during the early stages of an exposure test may be more important than the total radiationreceived by the test piece at a later date. The conditions in the early stages of a test influence the manner in which the basic lead stabiliser removes the harmful chlorine ions or radicals liberated from the polymer. Mild conditions allow sufficient time for lead to diffuse through the plastic to the surface region where it reacts to form a precipitate of lead chloride in the form of needle like crystals. Harsher conditions result in the basic lead stabiliser particles themselves absorbing the chlorine ions or radicals to form lead chlorine in situ.

Tetrabasic lead sulphate improves the stability of the PVC by absorbing some of the harmful U.V. radiation with the result that the diffusion mechanism is favoured even under severe conditions.

1. Introduction

Dibasic lead phosphite is well known as a stabiliser for plasticised PVCl against weathering. Chevassus and De Brutelles2 have suggested that the stabilisation mechanism is the absorption of harmful U.V. radiation by the dibasic lead phosphite particles. However, it has been shown3 that tribasic lead sulphate which, although it has greater U.V. absorption characteristics, nevertheless gives less stability to weathering PVC than

aPreviously working at the Research Laboratories, Perivale, Middlesex.

13

14 D. Pearson and J. Morley

dibasic lead phosphite. It, therefore, seems more likely that the phosphite ion acts as an anti-oxidant, in similar manner to organic phosphite compound^.^

Lehmann and Scholy5 have shown that normal lead phosphite is a better stabiliser than dibasic lead phosphite for plasticised PVC in artificial weathering conditions. The present work compares the stabilising effect of mixtures containing normal lead phosphite or dibasic lead phosphite together with tri or tetra basic lead sulphate; in plasticised PVC during outdoor exposure at two sites. The thermal stability of PVC containing these mixtures has also been determined in free and restricted air. It is hoped by this means to elucidate the action of dibasic lead phosphite in the stabilisation of the PVC.

2. Experimental

In order to obtain results in a reasonably short time from the exposure tests, the PVC compounds were made using smaller concentrations of lead phosphite compound than would normally be used in commercial practice. The amounts of lead phosphite compound used are such that pairs of panels stabilised with normal and dibasic lead phosphite contain the same amount of phosphite ion.

Tribasic lead sulphate and tetrabasic lead sulphate have been used as thermal stabilisers, the latter having some stabilising property for PVC against outdoor exposure due to its high U.V. absorption characteristic^.^

2.1. Preparation of samples

The PVC mix was compounded with the following components:- 100 parts 50 parts 4 part x parts Y Parts

Thermal stabilisqr Tribasic lead sulphate Tribasic lead sulphate Tetrabasic lead sulpbate Tetrabasic lead sulphate Phosphite stabiliser Normal lead phosphite Normal lead phosphite Normal lead phosphite Normal lead phosphite Normal lead phosphite Dibasic lead phosphite Dibasic lead phosphite Dibasic lead phosphite Dibasic. lead phosphite

Cor& D65/8 polymer dioctyl phthalate normal lead stearate thermal stabiliser normal or dibasic lead phosphite

29 parts 5 parts 24 parts 5 parts

Y 0 parts 0.1 parts 0.19 parts 0.39 parts 0.58 parts 0.25 parts 0.50 parts 1.0 parts 1.5 parts

X

Natural weathering of plasticised PVC 15

The mixes were milled on a two roll mill for 10 min at 160"C, then removed from the rollers to give a sheet of two mm thickness.

2.2. Determination of the weathering properties of the panels

Two identical sets of panels were prepared; one set was placed in position at 45" to the southern horizon at Perivale, Middlesex, England and the other at 45" to the northern horizon at Jacobs, Natal, South Africa. The panels in England were erected on 1st June 1970 and the panels in South Africa were erected on 22nd July 1970. The panels were examined at four months and eight months exposure time both visu- ally and by the microscopical technique described in a previous paper.6

The most common change in the appearance of a plasticised PVC panel during outdoor exposure is the appearance of brownish or orange coloured scales. These scales appear at scattered sites, then spread to cover the surface of the panel. The amount of scaling or crazing is determined by measuring the proportion of surface covered with scales to the total area.

The scales appear on microscopic examination to consist of degraded polymer and are harder than the undegraded polymer. Low powered microscopical examination of the surface reveals that the scales are formed by a pattern of cracks and craters similar to a dried up mud patch. There has clearly been a loss of plasticiser from the surface during scaling, but no attempt has been made to quantify this loss.

2.3. Determination of the thermal stability of the samples

Congo red tests were carried out on the samples. These tests comprised of placing small quantities of the PVC sheet in test tubes which were placed in an aluminium metal block at 180 & 0.2"C, and measuring the time taken before hydrogen chloride vapours were evolved from the PVC. The vapours were detected using Congo red paper placed in the neck of the test tube. The tests were carried out both in free air, with the test tube unstoppered, and in restricted air, with the test tube stoppered with a rubber Bunsen valve.

3. Results

3.1. Exposure tests TABLE 1 . Meteorological information

Site Perivale, England Jacobs, South Africa

Period of exposure Hours of sunshine Rainfall

humidity Min. (%)

4 months 8 months 829.5 h 1081.5 h

208 mm 486 mm 21.3 15.8 1 1 . 1 7.8 16.2 11.8 73 79 56 65

4 months 762.7 h 312 mm

23.2 13.4 18.1 76 62

8 months 1412.6 h 743 mm

25.0 16.6 20.5 75 66

16 D. Pears00 and J. Morley

TABLE 2. Visual examination of panels containing tribasic lead sulphate after exposure at Perivale (crazed area %l

Amount of lead phosphite compound

2.5 p.h.r. 5 p.h.r. Tribasic lead sulphate Tribasic lead sulphate

after after after after 4 months 8 months 4 months 8 months

0 0.25 p.h.r. DLP 0.5 p.h.r. DLP 1.0 p.h.r. DLP 1.5 p.h.r. DLP 0.1 p.h.r. NLP 0.19 p.h.r. NLP 0.39 p.h.r. NLP 0.58 p.h.r. NLP

100 95 70 30 10 100 85 40 20

100 100 95 70 30 100 95 80 40

100 90 80 25 10 80 60 30 25

100 100 90 60 25 100 90 60 50

TABLE 3. Visual examination of panels containing tetrabasic lead sulphate after exposure at Perivale (crazed area %)



2.5 p.h.r. 5 p.h.r. Tetrabasic Iead sulphate Tetrabasic lead sulphate


90 100 75 80 30 75 20 lost 10 30 10 30 5 15 3 10

trace 15 trace 5 25 90 20 25 10 30 5 I5 5 15 trace 5

trace 15 trace trace

TABLE 4. Visual examination of panels containing tribasic lead sulphate after exposure at Jacobs (crazed area %)


2.5 p.h.r. Tribasic lead sulphate

after after 4 months 8 months

0 100 100 0.25 p.h.r. DLP 10 100 0.5 p.h.r. DLP 2 40 1.0 p.h.r. DLP trace 5 1.5 p.h.r. DLP no change no change 0.1 p.h.r. NLP 15 100 0.19 p.h.r. NLP 5 30 0.39 p.h.r. NLP trace 5 0.58 p.h.r. NLP no change trace

5 p.h.r. Tribasic lead sulphate

after after 4 months 8 months

10 1 L

trace no change no change

10 2

trace no change

100 70 30 5

no change 90 20

trace no change


TABLE 5. (crazed area %).

Visual examination of panels containing tetrabasic lead sulphate after exposure at Jacobs


2.5 p.h.r. 5 p.h.r.


Tetrabasic lead sulphate Tetrabasic lead sulphate


~

2 trace

no change no change no change

2 no change no change no change

~

100 100

5 2

trace 5 5

trace trace

no change no change no change no change no change no change no change no change no change

10 5

trace trace

no change 5

trace trace

no change

3.2. Microscopic examination of the exposed panels Microscopical examination of sections cut perpendicular to the exposed surface shows that reactions are occurring to considerable depths (up to 500 pm). These reactions can occur in one or several forms when the reaction products appear as layers or strata beneath the surface of the panel. From past experience it is possible to identify the reactions from the appearance of the reaction products.6 The various reactions that have been distinguished are as follows. -

Reaction of the stabiliser particles in situ with hydrogen chloride or chlorine radicals evolved from the PVC. The reaction products have the same physical dimensions as the original stabiliser particles, but show greatly reduced exhibition between crossed polars and an increased optical density. Reaction between the stabiliser particles and stearate radicals or stearic acid. The stabiliser paIticles dissolve into the PVC matrix leaving an area without stabiliser particles. Precipitation of lead chloride. The dissolved lead stearate compound reacts with hydrogen chloride or chlorine radicals from the PVC to form lead chloride in the polymer matrix. The lead chloride nucleates and grows into needles with characteristic form and exhibition between crossed polars.

Reactions (b) and (c) are those which occur in plasticised PVC undergoing slow degradation with good stabilisation conditions, and reaction (a) occurs when the for- mation of chloride ions or radicals takes place at a faster rate than the dissolution of the stabiliser particles.

Examination of panels containing 2.5 p.h.r. of tribasic lead sulphate and exposed at Perivale for eight months, shows, that with equivalent pairs of panels with the same phosphite concentration, the dibasic lead phosphite appears to be more efficient as a stabiliser. The depth and area of the reactions is greater with normal lead phosphite and with increase in phosphite content the area and depth of reactions are more rapidly reduced in the case of dibasic lead phosphite.

At 5 p.h.r. of tribasic lead sulphate comparison of panels containing the same amount of phosphite ion show that reactions occur to similar depths below degraded L


parts of the surface. At both 2.5 and 5 p.h.r. tribasic lead sulphate, the reaction products are in the form described as (a) in panels containing dibasic lead phosphite. In panels containing normal lead phosphite the reaction products are in the form of (a) beneath the surface and (b) at greater depths.

Similar examination of panels exposed in South Africa for eight months shows that there is little difference between equivalent panels containing normal lead phosphite and dibasic lead phosphite at both 2.5 and 5 p.h.r. of tribasic lead sulphate. The amount of degradation and the depth of reactions are both reduced with increasing the tribasic lead sulphate content. The reaction products are in the form (a) beneath the exposed surface, then a layer of form (c) and at greatest depth a layer of form (b).

Examination of panels containing tetrabasic lead sulphate shows that there are no significant differences between panels containing 2.5 p.h.r. and 5 p.h.r. or between panels containing equivalent amounts of phosphite ion. The reaction products are in form (a) beneath the surface and form (b) at greater depths in the panel.

3.3. Thermal stability tests TABLE 6. Congo Red times in unrestricted air of PVC stabilised with 2.5 p.h.r. of basic lead sulphate and additions of lead phosphite compounds

Basic lead sulphate Tribasic lead sulphate Tetrabasic lead sulphate

(a) Additions of dibasic lead

0 0.25 0.50 1 .o 1.5

lead phosphite (p.h.r.) 0 0.1 0.19 0.39 0.58

phosphite (p.h.r )

(b) Additions of normal

3 h 15 min 3 h 4 6 m i n 4 h 03 min 4 h 4 5 m i n 5 h 05 min

3 h 15 min 3 h 24 min 3 h 2 4 m i n 3 h 2 6 m i n 3 h 5 1 m i n

3 h 3 0 m i n 3 h 24 rnin 3 h 52 rnin 4 h 30 rnin 5 h 0 0 m i n

3 h 30 rnin 2 h 5 4 m i n 3 h 0 8 m i n 3 h 31 rnin 3 h 39 rnin

TABLE 7. Congo red times in unrestricted air of PVC stabilised with 5 p.h.r. of basic lead sulphate and additions of lead phosphite compounds.


(a) Additions of dibasic lead phosphite (p.h.r.)

0 0.25 0.50 1 .oo 1.50

(b) Additions of normal lead phosphite (p.h.r.)

0 0.1 0.19 0.39 0.58

8 h 50 min 9 h 0 5 m i n 9 h 20 min 9 h 4 7 m i n

10 h 10 rnin

8 h 50 min 8 h 47 min 9 h 20 min 9 h 4 4 m i n 9 h 5 0 m i n

6 h 27 rnin 7 h 0 0 m i n 7 h 45 rnin 8 h 0 5 m i n 8 h 40 rnin

6 h 27 rnin 7 h 28 rnin 7 h 57 min 8 h 0 5 m i n 8 h 40 rnin


TABLE 8. Congo red times in restricted air of PVC stabilised with 2.5 p.h.r. of basic lead sulphate and additions of lead phosphite compounds


,(a) Additions of dibasic lead phosphite (p.h.r.)

0 0.25 0.50 1 .o 1.5

(b) Additions of normal lead phosphite (p.h.r.)

0 0.1 0.19 0.39 0.58

3 h 50 min 4 h 14 min 4 h 28 min 4 h 48 min 5 h 05 min


3 h 59 min 4 h 1 4 m i n 4 h 39 min 4 h 35 min 5 h 02 min


'TABLE 9. and additions of lead phosphite compounds

Congo red times in restricted air of PVC stabilised with 5 p.h.r. of basic lead sulphate


(a) Additions of dibasic lead phosphite (p.h.r.)

0 10 h 30 min 9 h 50 min 0.25 10 h 25 rnin 10 h 55 min 0.50 10 h 45 min 10 h 15 min I .o 12 h 50 min 10 h 45 min 1.5 15 h 39 rnin 13 h 10 min

(b)Additions of normal lead phosphite (p.h.r.)

0 10 h 30 min 0.1 10 h 25 min 0.19 10 h 15 min 0.39 10 h 19 min 0.58 10 h 25 min

9 h 5 0 m i n 10 h 25 min 10 h 35 min 10 h 30 min 10 h 55 min

4. Discussion 4.1. Exposure tests

From the results of the visual examinations (Tables 2 to 5) it is evident that the concentration of dibasic lead phosphite and normal lead phosphite are the prime factors in the weather stability of the panels exposed. On comparing the visual examinations of the panels containing dibasic lead phosphite with the panels containing normal lead phosphite there is a remarkable similarity of results between the sets of panels containing the same phosphite ion concentration and thermal stabiliser. This shows clearly that it is the phosphite ion that prevents degradation on exposure and not the


lead phosphite compound. The microscopical examination of exposed panels apart from those exposed at Perivale containing 2.5 p.h.r. of tribasic lead sulphate agree with the above results from the visual examinations. At 2.5 p.h.r. of tribasic lead sulphate exposed at Perivale for four and eight months (Table 2), the area and depth of reactions are less with dibasic lead phosphite than with normal lead phosphite and microscopical examination reveals that with increase in phosphite content the area and depth of reactions are more rapidly reduced in the case of dibasic lead phosphite. This suggests that there must be a minimum amount of basic lead in the PVC compound for the phosphite ion to act as a stabiliser. Tetrabasic lead sulphate has good U.V. absorption characteristics3 and mixtures of tetrabasic lead sulphate and phosphite give better stability to the panels than similar mixtures of tribasic lead sulphate and phosphite (Tables 2 to 5). Some of the harmful radiation will be absorbed by the tetrabasic lead sulphate particles so that less degradation is taking place at 2.5 p.h.r. in the panel exposed at Perivale. The phosphite ion in this case controls the degradation without regard to the composition of the lead phosphite compound. This suggests that the amount of basic lead necessary to allow the phosphite ion to act as a stabiliser will depend on the amount of radiation being absorbed by the polymer.

The panels were erected during summer in Perivale and winter in South Africa. During the first four months the panels at Perivale received more exposure to sunshine at similar temperatures to the panels in South Africa but over the full eight months period the panels in South Africa received by far the greater amount of exposure to sunshine at a higher average temperature (Table 1). It can be seen that degradation is more advanced in the panels exposed at Perivale both at four months and eight months exposure. This suggests that the conditions of exposure during the first four months of a test are more important than the total period.

Microscopic examination of the panels exposed in South Africa for eight months containing 2.5 p.h.r. of tribasic lead sulphate shows little difference in area and depth of reactions in panels containing equivalent amounts of normal and dibasic lead phosphite. Under these conditions, the amount of basic lead is sufficient for the phosphite ion to act as a stabiliser against exposure. The products from the stabilisation reactions under the mild early exposure conditions in South Africa are in a different form from the reaction products under the more stringent early exposure conditions in the U.K. The reaction products from the mild conditions show that the basic lead sulphate particles have dissolved and migrated to the exposed surface of the panel where lead chloride has nucleated and grown into needle like crystals. In the more stringent early exposure conditions, the basic lead sulphate particles in the surface region have been converted to lead chloride by the absorption of chloride ions or radicals. The presence of tetrabasic lead sulphate or normal lead phosphite tends to cause the migration effect rather than the absorption effect even in the panels exposed at Perivale. The migration of the lead compounds from the body of the panel to the surface may explain why the panels in South Africa are less degraded after eight months compared with the panels in the U.K. The conditions in which the panels were initially exposed appear, therefore, to control the stabilising reaction. Low amounts of radiation result in the migration or diffusion mechanism and high amounts of radiation result in the absorption mechanism.


4.2. Thermal stability of the PVC panels Antioxidant reactions are usually revealed by a synergistic lengthening of the Congo red time for a PVC compound with addition of the antioxidant compound. This effect can be seen with the addition of phenolic compounds to PVC stabilised with basic lead compounds.'

Normal lead phosphite contains no basic lead which might contribute to the Congo red time, and it can be seen that additions of normal lead phosphite have little effect in conditions of restricted air (Tables 8 and 9), and in free air when only a small amount of basic lead is available (Table 6). A steady increase in Congo red time is obtained with increments of dibasic lead phosphite, under these conditions, which can be attributed to increase in the basic lead content of the polymer.

Under the conditions of free air and a substantial amount of basic lead a dramatic reversal is noted in Table 7. The Congo red test time of PVC containing increments of normal lead phosphite increases to match almost exactly the times of PVC containing equivalent molecular amounts of dibasic lead phosphite. It is clear that under these conditions normal lead phosphite is acting synergistically, but is the increase in the panels containing dibasic lead phosphite due to increase in basic lead or to the phosphite ion?

The common factor between the pairs of panels containing normal and dibasic lead phosphite is the identical amounts of phosphite ion. No increase in Congo red time can be correlated with the lead ion in the normal lead phosphite even in the high Congo red times obtained in Table 9. It is difficult to see therefore why identical results should be obtained if normal lead phosphite and dibasic lead phosphite were reacting by different mechanisms. It would appear that the phosphite ion in the dibasic lead phosphite is reacting synergistically with the basic lead present in the sulphate and the basic lead in the dibasic lead phosphite is unimportant. No explanation can be given for this effect, it can be pointed out that dibasic lead phosphite is an inferior thermal stabiliser compared with tribasic lead sulphate even though the compounds have a similar particle size. This can be seen by comparing the increments in Congo red test time in Table 6 with increase in dibasic lead phosphite content compared with the increase of 5 hours 35 min for an increase of 2-5 p.h.r. of tribasic lead sulphate from Table 6 to Table 7 for tribasic lead sulphate alone.

Careful comparison of the results in Tables 6 and 8 show that identical panels containing dibasic lead phosphite gave identical results at the high range of dibasic lead phosphite contents i.e. the degradation rate of the polymer is reduced in free air as compared with the results obtained in restricted air. This also indicates an antioxidant reaction in the presence of dibasic lead phosphite which is not occurring in the presence of normal lead phosphite. It is believed that the explanation for this difference lies in the fact that stabilisation/degradation reactions can occur in two ways. If the diffusion of lead ions takes place at such a rate that the supply of lead is in excess of that required to react with the chlorine atoms being formed by the degrading polymer, then a stable situation is established. Hydrogen chloride is not evolved from PVC nor does the colour of the PVC change until over 90% of the basic lead stabiliser has been utilised to form lead chloride and normal lead salt, (sulphate or phosphite). If, however, the diffusion rate of lead is less than that required to react


with the chiorine radicals, then degradation will be autocatalytic and hydrogen chloride evolution and colour changes can take place from localised positions, whilst the bulk of the stabiliser is still unchanged.

It would appear that there must be sufficient stability in the specimen for the phosphite ions to act as antioxidants in thermal excitation and this must be achieved by increasing the thermal stabiliser concentration (basic lead) to a sufficient level. Further work at different temperatures may yield interesting results. It can be said that the results shown in Tables 6 to 9 indicate that it is possible for lead phosphite compounds to act as an antioxidant during thermal excitation of PVC in certain circumstances.

5. Conclusions

Plasticised PVC panels stabilised with mixtures of basic lead sulphate and dibasic lead phosphite or normal lead phosphite have been exposed to outdoor weathering in South Africa and the U.K. Visual examination of the panels after four months and eight months exposure has shown that dibasic lead phosphite and normal lead phosphite improve the stability of the panel according to the amount of phosphite ion that is present in the panel, always providing that there is sufficient basic lead to remove liberated chloride ions or radicals in the form of lead chloride.

The unexposed panels have also been subjected to thermal degradation at 180°C in free and restricted amounts of air. It has been found that, in free air a synergistic improvement in stability is obtained when normal lead phosphite is present together with basic lead sulphate but no improvement is obtained from the phosphite ion in restricted amounts of air nor when the basic lead content is below 2.5 parts per hundred of resin.

It is concluded from the comparison of these results that, under certain conditions, the phosphite ion can act as an anti-oxidant against the absorption of atmospheric oxygen during both thermal and photo excitation.

It has also been found that radiation intensity in the early stages of an exposure test may be more important than the total radiation received by the test panel. Panels exposed in the summer months in the U.K. degraded faster than panels exposed in the winter months in South Africa, and moreover continued to degrade at a faster rate at eight months exposure even though the South African panels had received many more hours of sunshine.

Microscopic examination of the exposed panels suggests that the initial exposure conditions may set up one of two stabilisation mechanisms. The milder conditions allow the basic lead particles to dissolve into the polymer and diffuse to concentrate in the exposed surface there to react and eventually crystallise as lead chloride needles.

In more stringent early exposure conditions, the basic lead particles in the surface region are converted to lead chloride in situ by absorption of chloride ions or radicals from the degrading polymer.

Mixtures of tetrabasic lead sulphate and dibasic or normal lead phosphite give better stability to PVC on outdoor exposure than equivalent panels containing tribasic lead sulphate mixtures. This is due to the high U.V. absorption characteristics of the tetra-


basic lead sulphate. The more effective diffusion stabilisation mechanism was found in panels containing tetrabasic lead sulphate even when they had been exposed to high light intensities in the early stages of weathering.

References 1. Thorensen, F.; Kroken, P. Berger Met. 1968, 28 (i), 8. 2. Chevassus, F.; De Brutelles, R. The Stubilisution of PVC. Edward Arnold. London 1963, 81. 3. Pearson, D. Microscope 1968, 16 (3), 243. 4. Ya, Gordon, G. Stabilisarion of Synthetic Polymers 1963.72 and 73. (Israel Progrom for Transla-

tion Ltd.). 5 . Lehmann and Scholy. E. Ger. Pat, 61 095. 1968. 6. Pearson, D. Polymer Age 1971, 2 (11), 429. 7. Minsker, K. S, Soviet Plust. 1969, 1, 56.

dibasic lead phosphite as a stabiliser against the natural weathering of plasticised pvc

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