Some aspects of the effects of fillers on the behaviour of PVC in fires

Some Aspects of the Effects of Fillers on the Behaviour of PVC in Fires George Matthews and George S..Plemper

The effects of particle size and concentration of calcium carbonate fillers on the critical oxygen index and the evolution of hydrogen chloride during pyrolysis of unplasticised PVC has been shown to parallel the effects in plasticised PVC but to differ from them in two respects. Coarser varieties of calcium carbonate (i.e. appreciably greater than 0.1 micron) react with pyrolysis hydrogen chloride more efficiently in UPVC than in plasticised PVC. Plots of critical oxygen index against concentration of calcium carbonate in UPVC show two inflexions, maxima at low concentrations (i.e. < 30 PHR) and minima at high concentration (i.e. > 40 PHR). Similar inflexions are also shown with alumina trihydrate and silica. In both plasticised and unplasticised PVC, calcium oxide produces a steady fall in critical oxygen index with increasing concentration, more marked than with calcium carbonate at equal hydrogen chloride retentions. The complex behaviour is attributed to the number of factors contributing to overall burning behaviour. Of these, char formation, in particular, is not readily assessed numerically, and consequently it has not been possible to quantify the contributions of the individual effects.

1. INTRODUCTION

The evolution of toxic substances and smoke from materials in fires constitutes a serious hazard additional to the obvious one of fire itself, and reduction of this hazard is regarded as being almost as important an objective as decreasing ignitability. One possible way of reducing or eliminating the evolution of toxic substances from polymeric materials is to incorporate fillers or other additives that react with or absorb the objectionable substances, or modify pyrolysis and combustion reactions so that they are not produced. In a previous paper evolution of hydrogen chloride during pyrolysis or combustion of PVC by the incorporation of calcium carbonate, and showed that in PVC plasticised with 60 PHR of DOP this is dependent not only on concentration but also on particle size of the filler, the efficiency of the suppression increasing with reduction in particle size. On the basis of critical oxygen index measurements reduction in evolution of hydrogen chloride by 0.1 micron calcium carbonate is accompanied by an increased tendency to burn unless the concentration of calcium carbonate is well above that required to retain all of the hydrogen cloride. Calcium carbonate of this particle size appeared to behave in unplasticised PVC in a similar fashion to its behaviour in plasticised polymer. Further work has been directed towards comparing effects of particle size of calcium carbonate in unplasticised PVC with those found in plasticised PVC, and in particular to elucidate which of the possible mechanisms are involved, and also to quantify their contributions. It was thought that this might be achieved by comparison of the effects of calcium carbonate with those of other fillers, such as calcium oxide which would react with hydrogen chloride but would not release carbon dioxide, silica, which would be expected to be inert, and alumina hydrate which would demonstrate the effect of released water.

Polytechnic of the South Bank, London (Manuscript received 24 May 1982)

we reported on the suppression of

In an attempt to detect synergism between combinations of fire-retardants Lyons *, calculated the concentrations of each fire-retardant in a combination that would be expected to yield the same level of fire-retardancy as that achieved with a given concentration of one of them alone. If, in combination, lower concentrations than indicated were required synergism was supposed to have been demonstrated. This assumes that the effect of each fire- retardant increases linearly with its concentration. As Weil and indeed is often certainly not operative. Weil postulated that the existence of synergism would be established if a plot of flame-retardancy response (or flame resistance), as exemplified by critical oxygen index, against concentration of one retardant containing a fixed amount of a second showed a steeper slope than the corresponding plot for the first retardant alone at all concentrations. Anita and coworkers used this approach to detect chemical inter- action between aluminium oxides and halogen-containing fire-retardants. However, even this approach will only be true in any meaningful sense if the scale of flame- retardancy is known to be linear, and it is not obvious that this will be so. In the case of the critical oxygen index, for example, linearity would imply that a change of two per cent, say, has the same significance at high, medium, or low critical oxygen indices. At first sight the effectiveness of a flame-retardant might be expected to operate according to a law of diminishing returns, that i$ to vary with its concentration according to an equation of the type R = R, (l-e-kc), where R, is the flame retardancy effectiveness at infinite concentration, and R is the effectiveness at any concentration C. Indeed ‘diminishing returns’ relationships are frequently encountered, at least at relatively low concentrations. However some flame-retardants, and other addivites, would be quite incombustible even in oxygen, so that at very high concentrations the relationships must tend upwards. If plots were made against per cent concentration of additive rather than the conventional PHR, curves for the same additive in a variety of polymers would presumable regress towards a common point at 100 per cent of additive, as

points out, linearity of response cannot be assumed

THE BRITISH POLYMER JOURNAL, VOLUME 15, JUNE 1983 95

illustrated in Fig. 1, where curves I , 11,111 and IV represent imaginary plots of critical oxygen index against per cent concentration of some particular filler in four different polymer systems. The curves should extrapolate to a common point Pa t 100 per cent filler, and the points C1, C,, C3 and Cq, where the curves cross the 100 per cent oxygen line, represent the limiting concentrations of filler above which the different polymers will not burn in pure oxygen.

Critical oxygen Inde, (% 02)

I

/

P I $7 I c 4

YI -

1c 0'

% Filler

,

Fig.1 different polymer systems.

'Idealised' extrapolation of COI vs filler concentration curves for

We now wish to report on our investigations into the effects of varying particle size of calcium carbonates in unplasticised PVC and in PVC plasticised at an intermediate concentration. Also we report on some comparative studies of the type indicated above, aimed at evaluating the contributions made by the various possible mechanisms involved.

2. EXPERIMENTAL

Materials, apparatus an'd procedures were as reported previously but an additional form of calcium carbonate was included, namely a sample of 'Calopake', of nominal mean particle size 0.5 micron, supplied by John & E. Sturge Ltd. Other fillers included in this study were calcium oxide, alumina trihydrate and silica. The effects of calcium carbonate and calcium oxide on critical oxygen index of LD polyethylene were also examined.

3. RESULTS

A comparison of Fig. 2 with Fig. 4 in our previous paper 1

reveals that retention of hydrogen chloride on pyrolysis of unplasticised PVC containing calcium carbonate follows a similar pattern to that observed with PVC containing 60 PHR of DOP, i.e. that retention increases with concentration and decreases with increasing particle size of calcium carbonate. However, although the behaviour of fine particles (i.e. 0.1 micron and less) is similar in unplasticised and plasticised PVC efficiency of retention by the coarser varieties is higher in unplasticised than in plasticised PVC. This is seen clearly in Fig. 3 , which shows the variation in retention of hydrogen chloride by 100 PHR of calcium carbonate of 14 microns nominal mean particle size over a DOP plasticiser concentration of 0 to 60 PHR. The calcium carbonate/hydrogen chloride reaction with this grade of filler appears to be some three times more efficient in unplasticised PVC than in PVC containing 60 PHR of DOP. Figures 4 and 5 show the relationships between hydrogen chloride retention in UPVC and measured surface area, and estimated surface area, respectively. As with plasticised PVC no single simple relationship has been found, efficiency of retention in relation to surface area increasing with increasing particle size, The effects of varying concentration and particle size of calcium carbonate on the critical oxygen index of unplasticised PVC are shown in Fig. 6. The curve for each of the four particle sizes examined appears to exhibit a maximum at relative low concentrations (i.e. ca 5 to 30 PHR) as well

HCl retention (%I

Concentration of calcium carbonate (PHR)

Fig.2 fillers on retention of hydrogen chloride in pyrolysed PVC ( V = 0.1 micron, A = 0.5 micron; B = 1 micron; 0 = 14 microns; @ = 14 microns in 30 DOP; X = 14 microns in 60 DOP).

Effect of concentration and particle size of calcium carbonate

96 THE BRITISH POLYMER JOURNAL, VOLUME 15, JUNE 1983

HC1 retention (%) HC1 retention (%)

Concentration of DOP (PHRI

Fig.3 Variation of retention of hydrogen chloride on pyrolysis of PVC containing 100 PHR of coarse (14 microns) calcium carbonate with concentration of dioctyl phthalate.

HCl retention (%)

Total surface area relative to 120 PHR of 0.1 micron calcium carbonate

Fig.5 UPVC with relative surface of calcium carbonate estimated on the assumption that particles are uniformly sized and spherical. ( 0 = 0.08 micron; B 0.1 micron; A = 0.5 micron; i3 = 1 micron;

Variation of retention of hydrogen chloride on pyrolysis of

0 = 14 microns).

COI (% oxygen)

Surface area of filler (m2/g polymer)

Fig.4 Variation of retention of hydrogen chloride on pyrolysis of UPVC with surface area of calcium carbonate filler. ( 0 = 0.08 mici V = 0.1 micron; = 1 micron; 0 = 14 microns).

as the expected minimum at higher concentrations (i.e. 70 to 120 PHR) exhibited with plasticised PVC. In other respects the effect of particle size parallels that observed with plasticised PVC in that the finer calcium carbonate (0.1 micron) has the most deleterious effect on critical oxygen index, certainly at concentrations above 20 PHR, while at the other extreme the coarser calcium carbonate

30

0 10 20 30 40 50 60 70 80 90 100 110 120

'on; Concentration of calcium carbonate (PHR)

Fig.6 and particle size of calcium carbonate filler. ( 0 = 14 microns; EI = 1 micron; A = 0.5 micron, V = 0.1 micron).

(14 microns) generally produces an increase in critical oxygen index. At constant filler content the effect of DOP plasticiser on critical oxygen index parallels the effect on unfilled PVC. The relationship for 100 PHR of the coarser calcium carbonate is shown in Fig. 7.

Variation of critical oxygen index of UPVC with concentration


CO I (56 oxygen) HCl retention (56)

0 10 20 30 40 50 60

Concentration of DOP ( P H R )

Fig.7 dioctyl phthalate in PVC containing 100 PHR coarse (14 microns) calcium carbonate filler ( @ ) and in unfilled PVC ( ).

Fig. 8 shows the results obtained in pyrolysis experiments with unplasticised PVC and PVC plasticised with 60 PHR of DOP containing various proportions of calcium oxide. Difficulties were experienced in determining the particle size of this calcium oxide, but transmission and scanning electron microscopy on the filler dispersed in toluene and in unplasticised PVC suggested that the mean particle size was of the order of 5 microns and that it tended to agglomerate. As was found with the coarser varieties of calcium carbonate the efficiency of hydrogen chloride retention is seen to be greater in unplasticised than in plasticised PVC, but even in the former case is less than 70 per cent of theoretical, Silica, selected as a model of an ‘inert’ filler, was found to retain at most only very small amounts of hydrogen chloride, so that evolution of the gas was found to be greater than 99 per cent even at 200 PHR of the filler. Alumina hydrate, selected as a filler offering fire retardancy by virtue of combined water, was also found to have very low hydrogen chloride retention, of similar magnitude to that of silica. In combinations with the coarser calcium carbonate, alumina hydrate was found to reduce the hydrogen chloride retention, by as much as 10 to 15 per cent in some cases. Figs. 9 and 10 illustrate the effects of calcium oxide, silica and alumina hydrate on the critical oxygen index of unplasticised and plasticised PVC. In the latter the inert filler, silica, produces a steady increase in critical oxygen index with concentration, while the alumina hydrate gives an appreciably greater increase, presumably mainly arising from its possession of combined water. In unplasticised PVC, however, both of these fillers exhibit maxima at around 55 PHR, similar to the maxima exhibited by calcium carbonates. Although, when compared with fine particle calcium carbonate, calcium oxide is a less effective scavenger for hydrogen chloride, its effect on critical oxygen index is even more marked than that of the

Variation of critical oxygen index with concentration of

Concentration of filler (PHR)

1:ig.B in pyrolysed PVC ( 0 = calcium oxide in UPVC; = calcium oxide in PVC containing 60 PHR dioctyl phthalate; - . - . = 0.1 micron calcium carbonate in UPVC --- = theoretical for calcium oxide; ....... ..... = 0 1 micron calcium carbonate corrected to equivalent molar proportions of calcium oxide in UPVC).

Effect of calcium oxide on the retention of hydrogen chloride

COI (oxygen)


Fig.9 PHR DOP) with concentration of various fillers ( D = alumina trihydrate A = silica; 0 = calcium oxide; carbonate, -.-.- = 14 microns calcium carbonate).

Variation of critical oxygen index of plasticised PVC (60

= 0.1 micron calcium


COi (%oxygen)

50

40

30

0 10 20 30 40 50 60 70 80 90 100

0 100 200


Fig.10 tration of various fillers ( b = alumina trihydrate; A = silica; o = calcium oxide, -- = 0.1 micron calcium carbonate; -.-.- = 14 microns calcium carbonate).

Variation of critical oxygen index of UPVC with concen-

COI (%oxygen)

0 10 20 30 40 50 60 70 80 90 100

Retention of hydrogen chloride (%)

Fig.11 Relationship between critical oxygen index and retention of hydrogen chloride on pyrolysis of PVC containing 60 PHR DOP and various concentrations of calcium oxide.

carbonate, when compared at equal concentrations or, as shown in Figs. 11 and 12, at equal retentions of hydrogen chloride when pyrolysed. Fig. 12 also shows a similar dependence in unplasticised PVC, of the relationship between critical oxygen index and hydrogen chloride retention by calcium carbonates on particle size to that reported previously for plasticised PVC. However, unlike the latter case, increase in hydrogen chloride retention by increase in concentration of even the coarser particle calcium carbonates eventually produces a decrease in the critical oxygen index of unplasticised PVC. As with the plots of the latter against concentration of calcium carbonates (Fig. 6 ) each curve appears to possess both a minimum at high concentrations and a maximum at low.

Retention of hydrogen chloride (%)

Fig.12 retention of hydrogen chloride on pyrolysis by calcium carbonate and calcium oxide fillers ( 0 = 14 microns calcium carbonate;

ate I = 0.1 micron calcium carbonate; 0 = calcium oxide).

Relationships between critical oxygen index of UPVC and

= 1 micron calcium carbonate; A = 0.5 micron calcium carbon-

Fig. 13 shows relationships between the critical oxygen index of unplasticised PVC and various combinations of filler mixtures comprising alumina hydrate and the coarser (14 microns) calcium carbonate. At lower concentrations (i.e. up to about.50 PHR) the carbonate boosts the critical oxygen index in relation to the values obtained with the hydrate alone. In so far as it is possible to put any quantitative significance to comparisons of critical oxygen index the increases obtained by the mixtures of filers are generally greater than the sum of those that would be expected from each filler alone. At 80 PHR of the coarser calcium carbonate the critical oxygen index is slightly lower than that obtained with the alumina hydrate alone, but this is around the minimum, and higher concentrations than this would be expected to boost critical oxygen index above that obtained with the alumina hydrate alone. In a further attempt to assess the significance of the reaction with hydrogen chloride in determining the effect of calcium carbonate on burning behaviour, samples of low density polyethylene containing various concentrations of filer were examined. As seen in Fig. 14, the finer particle size calcium carbonates produce a steady, small, but significant increase in critical oxygen index, but assuming an index of 21 per cent as being the minimum target to aim for to produce a material that will not burn in air quite high concentrations (i.e. greater than 85 PHR) would be required. Calcium oxide and the coarser (14 microns) calcium carbonate appear t o have similar relatively small effects.

4. DISCUSSION

4.1 Hydrogen chloride retention Since the reaction of fine particle (i.e. 0.1 micron and below) calcium carbonates with hydrogen chloride liberated by pyrolysed plasticised PVC is highly efficient it


COI (% oxygen! throughout the polymer matrix, and not merely to effects of relative surface areas, so that liberated hydrogen chloride molecules have a smaller chance of escape from the system before contacting and diffusing into a filler particle. A similar explanation might account for the decrease in efficiency with the introduction of plasticiser (Fig. 3), which effectively decreases the concentrations of both Tdler and hydrogen chloride. In this connection it may be noted that at 60 PHR of DOP in PVC containing 100 PHR of calcium carbonate the plasticiser is present at a volume concentration of around 36 per cent. O’Mara studied the absorption of hydrogen chloride by a range of fillers during pyrolysis and combustion of PVC containing 100 PHR of DOP, and found that micro-pulver- ised calcium carbonate had an efficiency of retention of hydrogen chloride of between 40 per cent at 80 PHR and 53.6 per cent at 20 PHR. Estimating an adjustment for the fact that he used 100 PHR of DOP, the results reported here and previously carbonate had a mean particle size of around 0.6 micron, at which size the filler would not be expected to have a large effect on critical oxygen index (cf Reference 1, Fig. 7). Indeed on the basis of results obtained with other fillers, namely calcium oxide, magnesium oxide, sodium carbonate, aluminium oxide and alumina trihydrate, O’Mara found little relationship between critical oxygen index and hydrogen chloride retention and concluded that hydrogen chloride plays a minor role in flame retardation, He also suggested that the low efficiency of hydrogen chloride retention by calcium carbonate or calcium oxide might be due to the formation of an intermediate calcium oxychlor- ide which is impervious to further attack. This seems unlikely as calcium oxychlorides are generally regarded as being unstable with hydrogen chloride. Moreover if such a mechanism was operating it would be expected to limit reaction of the finer particle calcium carbonates to well below 100 per cent. In our view it is more likely that penetration of filler particles is determined by a balance between particle radius, rate of diffusion into the particles, and rate of diffusion out of the polymer matrix. Once reaction on the surface of a particle has commenced liberated water and carbon dioxide might also constitute a hindrance to further penetration.

suggest that O’Mara’s calcium

0 10 20 30 40 50 60 70 80

Concentration of calcium carbonate (PHR)

Fig.13 (14 microns) calcium carbonate on the critical oxygen index of UPVC ( 0 = No alumina trihydrate; EI = 5 PHR alumina trihydrate; 0 = 10 PHR alumina trihydrate, A = 15 PHR alumina trihydrate).

Effects of combinations of alumina trihydrate and coarse

COI (% oxygen)

0 10 20 30 40 50 60 70 80 90 100


Fig.14 density polyethylene ( V = 0.1 micron calcium carbonate; 0 = 0.08 micron calcium carbonate, 0 = 14 microns calcium carbonate;

Effects of various fillers on critical oxygen index of low

B = calcium oxide).

is not to be expected that these fillers would exhibit appreciably different behaviour in unplasticised polymer. It is possible that the higher reactivity of these forms of calcium carbonate compared with the coarser varieties (Fig. 2 ) is due in part to their more uniform distribution

4.2 Critical oxygen index Although the rankmg of calcium carbonates on the basis of their effects on critical oxygen index of unplasticised PVC (Fig. 6 ) is the same as for plasticised PVC, the relationships between index and concentration differ in two respects. The maxima at low concentrations exhibited at all particle sizes, though to different extents, is paralleled in plots of critical oxygen index against retention of hydrogen chloride. To attempt explanations for the shapes of these curves it is necessary to consider the factors that might conceivably contribute to changes in critical oxygen index when calcium carbonate and other filers are incorporated into PVC. (i) Dilution of organic feedstock would generally be

expected to increase critical oxygen index, perhaps in accordance with a simple exponential relationship of the type mentioned earlier, i.e. COI = COI, (l-e-kc). However, it is possible that burning of unfilled polymer might tend to produce an excess of flammable vapours in relation to the available oxygen supply so that small proportions of filler could result in flammable vapour/oxygen ratios closer to the


optimum, so that simple dilution could produce a reduction in the critical oxygen index. Moreover, as suggested in the introduction, at high concentrations of non-combustible inorganic fillers the critical oxygen index must tend upwards to reach 100 per cent at the concentration above which combustion in oxygen will not occur. Endothermic and exothermix reactions will clearly affect burning behaviour in respect of their effect on the overall demand for heat. The dehydration of alumina trihydrate, which occurs at around 302"C, is highly endothermic, to the extent of about 1170 J/g.7> The decomposition of calcium carbonate to calcium oxide and carbon dioxide, which does not occur rapidly until much higher temperatures in the region of 600"C, requires about 1800J/g. On the other hand the reactions of calcium carbonate and calcium oxide with hydrogen chloride are exother- mic, to the extent of about 400J/g for the former and 3900J/g for the latter.

(iii) Heat capacity of a filler and its solid degradation products is generally regarded as acting in the manner of a simple diluent, taking heat from the flame that would otherwise be effective in degrading the polymer. It may be noted that the specific heat capacities of the fillers under consideration are around 0.8J/gK, UPVC. However it is also possible that once heated to polymer degradation temperatures the filler might act as a heat sink, tending to resist any fall in temper- ature that might result from changing chemical and thermal conditions. Thermal conductivities of fillers exceed that of UPVC by factors of from 6 to over 20 times. Here again fillers could be seen to influence burning behaviour in two opposing ways, in that increased conduction of heat from the flaming region would tend to reduce temperatures there, but, on the other hand to produce higher temperatures in the polymer away from the flame. Chemical interference with polymer degradation processes has been studied but little detailed infor- mation about specific behaviour appears t o be available. In view of the autocatalytic effect of hydrogen chloride in the degradation of PVC, its fixation by calcium carbonate or oxide might be expected to reduce the degradation rate, but calcium carbonate is not generally regarded as a stabiliser for PVC and the results presented here offer no evidence to the contrary. Evolution of gases and vapours is most likely to act by diluting and possibly cooling flammable vapours in the flame. This is clearly the case with water from alumina trihydrate lo or calcium carbonate, and carbon dioxide from the latter. I t may be noted that in the reaction between hydrogen chloride and calcium carbonate the resultant water and carbon dioxide have higher heat capacity than the hydrogen chloride they replace. This supports the suggestion that, contrary to previous reports, hydrogen chloride inhibits combustion chemically, l1 and its futation by calcium carbonate or oxide clearly tends to reduce the critical oxygen index.

(ii)

compared with about 1 J/gK for

(iv)

(v)

(vi)

(vii) Modification of char formation can affect burning behaviour in different ways. Char is made up of

residues from degraded polymer and filler, so that any increase in the contribution from the former is effectively equivalent to a reduction in fuel supply.

While generally being regarded as acting as a barrier between flame and polymer, limiting heat flow from the former to the latter, formation of char carries the solid phase further into the flame and hence increases thermal conduction from flame to polymer. Char might also act as a preheater for volatiles passing from the degrading polymers to the flame. The presence of a filler may prevent disintegration of residues from polymer degradation, thus reducing the liberation of fresh polymer surfaces for further degradation and at the same time reducing removal of heat and spread of fire by burning drops. On the other hand it is not inconceivable that some fillers might interfere with cohesion between different regions of degraded material and thus reduce any tendency for char formation. Thus the nature of the char and its formation can have a pronounced but largely unpredictable effect on burning behaviour, and in particular in tests such as the critical oxygen index.

Silica would be expected to behave as a typical inert filler l 2 and thus its behaviour might yield some quantitative estimate of what might be called the 'inert effect' i.e. the contribution made by dilution of polymer by filler independently of any chemical interactions, involving functions (i), (iii) and (iv) of the above. The overall effect might be expected to vary smoothly in relation to concentration. This seems to be borne out by the results obtained with PVC containing 60 PHR of DOP (Figs. 9 and 1 5), although there is a hint of a plateau or even a slight 'hump' occurring in the concentration range of 50 to 80 PHR ( 2 5 to 33 per cent) of filler. Over the range of concentrations investigated the gradient of the critical oxygen index/ concentration curves increase with concentration, but the curves do not appear to fit any simple exponential equation. The same may be said of the results obtained with alumina hydrate. It is tempting to equate the differences between the curves for these two fillers to the combined effects of the heat required for dehydration of the hydrate and of the evolution of water vapour into the volatile phase, but meaningful quantification would be difficult. Likewise the differences between the curves for silica and calcium oxide must be attributable in the main to reaction of the oxide with hydrogen chloride with consequent reduction in concentration of the latter accompanied by generation of heat of reaction, both of which would tend to reduce the critical oxygen index. To determine the effect of the difference in heat of reaction plus the additional generation of carbon dioxide arising from the use of calcium carbonate, it is necessary to eliminate the inert effect by comparing samples of these fillers that have similar hydrogen chloride versus concentration relationships. None of the varieties of calqium carbonate examined matches calcium oxide in this respect in plasticised PVC. It is estimated that a calcium carbonate of mean particle diameter around 0.3 micron would be required to match the calcium oxide used in this work.

The difficulties of putting any quantitative interpretation on the results obtained with plasticised PVC are magnified when attention is directed to unplasticised PVC because of the much greater inflexions in the relationships between critical oxygen indices and filler concentrations (Fig. 6, 10, 12 and 16). Over a wide range of concentrations the retention of hydrogen chloride by calcium oxide is close to that of calcium carbonate of 0.1 micron mean particle size,


0 % Filler 100

Fig.15 with 60 PHR DOP ( D = alumina trihydrate; A = silica; o = CaO, -.-.- = 14 microns CaC03; -- = 0.1 micron CaCO3).

COl vs Concentration of various fillers in PVC plasticised

COI (%

0 % Filler 100

Fig.16 and unplasticised PVC, and low density polyethylene (A = alumina trihydrate; S = silica; 14 = 14 microns CaC03; 1 = 1 micron CaC03; 0.5 = 0.5 micron CaC03; 0.1 = 0.1 micron CaCO3; C = CaO; U = UPVC; P = PVC with 60 PHR DOP; L = LDPE).

COI vs % concentration of various fillers in plasticised

and it is tempting to conclude that the near parallelism of the critical oxygen index versus hydrogen chloride retention curves for these two materials (Fig. 12) demonstrates an increase of about 8.5 critical oxygen index units as characteristic of the effect of lower heat of reaction plus carbon dioxide evolution of calcium carbonate. Given the presence of maxima, minima are bound to OCCUI With calcium carbonate and oxide, once the concentration of filler is high enough for its maximum retention of pyrolytic hydrogen chloride to be attained, any further increase will tend only to increase the dilution of organic feedstock by noncombustible inorganic material and possibly of flammable volatiles by carbon dioxide. The

deleterious effects of the coarser calcium carbonates above about 30 PHR in unplasticised PVC, compared with their effects in plasticised polymer, might be attributable to their more efficient retention of hydrogen chloride. Sobolev and Woychesim ‘y8 included ‘rigid PVC’ containing 40 per cent by weight (ca 67 PHR) of a 4 microns particle size calcium carbonate in their studies of endothermic flame retardants. They reported a modest increase in critical oxygen index for this composition compared with ‘unfilled PVC’, agreeing qualitatively with the results reported herein but differing in magnitude. Their ‘rigid’ PVC is not characterised, but since its critical oxygen index is quoted as 36.0, compared with our 47.9, it is possible that the PVC examined was either a copolymer or lightly plasticised. A DOP content of only 10 PHR would be expected to produce a value of 36, and at this concentration the PVC would commonly be regarded as ‘rigid’. It is significant that the maxima inflexions in the critical oxygen curves are present with silica and alumina hydrate as well as with calcium carbonates. This suggests that the maxima arise from changes in mode rather than mechanisms of combustion, and the evidence suggests that this is connected with variations in char formation. The relatively rapid increase in critical oxygen index at low concentrations of filler could result from the formation of char that acts mainly as a barrier between flame and polymer surface. Reduction of fuel by dilution in the solid phase and the partial combustion involved in char formation would also contribute to the increase in critical oxygen index. With calcium carbonate, retention of hydrogen chloride eventually exerts such a large effect as to produce the more or less marked fall which continues until dilution of organic material by inorganic filler and its decomposition products becomes the over-riding effect. With silica and alumina trihydrate explanations for the reductions in critical oxygen index in the concentration range of 50 to 80 PHR are less obvious. One possibility is that the char is now in a form that acts as an effective heat exchanger, raising the temper- ature of volatile combustibles on their passage from polymer to flame, and also conducting heat from the flame to the polymer surface, The minima might also be due, at least in part, to an optimisation of concentration of combustible fuel in relation to the external gas supply, such that its oxygen demand would be at a minimum. The increases in critical oxygen index as concentrations are increased beyond those at which minima occur indicating that diluent effects are now over-riding the others.

5. CONCLUSIONS

1. Calcium carbonate and calcium oxide fillers in unplasticised PVC react with and retain hydrogen chloride evolved during pyrolysis and combustion in a similar fashion to their behaviour in plasticised PVC.’ Retention of hydrogen chloride increases with increase in concentration and decrease of the particle size of calcium carbonate. Retention of hydrogen chloride by calcium carbonates of mean particle size appreciably greater than 0.1 micron is reduced by the presence of plasticiser.

2.

3.


4. Retention of hydrogen chloride by reactive fillers in plasticised and unplasticised PVC tends to increase flammability; but the total effect of such fders is dependent on a combination of the level of retention, heat of reaction between filler and hydrogen chloride, the nature of the reaction products, dilution of organic feedstock, formation of char, and the combustion conditions. Because of the difficulties of separating these effects, estimating the’significance of thermal properties quantitatively, and in particular quantifying the complex nature of char formation, it is at present not possible to quantify these separate contributions to burning behaviour. Comparison of results obtained with unfilled PVC and PVC filled with calcium oxide or calcium carbonate suggest that hydrogen chloride exerts a chemical and not merely a physical inhibiting effect on combustion.

5.

References 1 2

3. 4.

Matthews,G. & Plemper,G.S.,Brft. Polym. J., 1981,13 ,17 . Lyons, J.W., ‘The Chemistry and Uses of Fire Retardants’, 1970, pp 20-24, New York; Wiley-Interscience. Lyons, W.J., J. Fire and Flamm., 1980,1,302. Weil, E.D., ‘Flame Retardancy of Polymeric Materials’, eds Kuryla, W.C. & Papa, A.J., Vol. 3,1975, pp 186-198, New York: Marcel Dekker. Antia, F.K., Cullis, C.F. & Hirschler, M.M.,Eur. Polym. J.,

O’Mara,M.M.,J. Polym. Sci., Part A-l,1971,9,1387-1400. Sobolev, I. & Woycheshin, E.A., J. Fire F b m m fFire Retard. Chem., 1974,1 ,13 . Sobolev, I . & Woycheshin, E.A., Coatings and Plastics Reprints, 1976,36,2,497-504. Katz, Harry S. & Milewski, John V., eds, ‘Handbook of Fillers and Reinforcements for Plastics: 1978, Amsterdam: Van Nostrand Reinhold. Connolly, W.J., & Thornton, A.M.,Mod. Plastics, 1965,43, (2), 154. Matthews, G . & Plemper, G.S., unpublished work, paper in preparation.

5

6 7

8

9

1981,17,451-455.

10

11

12 Smith, D.A.,PolymerLetters, 1966,4,215-221.


Some aspects of the effects of fillers on the behaviour of PVC in fires

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