Flame retardancy and smoke suppression in a tertiary polymer blend

Polymer Degradation and Stability 44 (1994) 93-97 ELSEVIER

Flame retardancy and smoke suppression in a tertiary polymer blend P. Carty Department of Chemical and Life Sciences, University of Northumbria at Newcastle, UK, NE7 7XA

&

S. White Cookson Ceramics and Minerals Ltd, Newcastle upon Tyne, UK, NE28 6UQ (Received 6 September 1993; accepted 8 October 1993)

This paper deals with the effects which basic iron(III) oxide has on the flammability, smoke production, char formation, rate of heat release and thermal stability in a specific blend of three polymers---ABS (acrylonitrilebutadiene-styrene), PVC (polyvinyl chloride) and PP (polypropylene). The iron compound when incorporated in modest amounts strongly modifies the thermal degradation processes which occur. The data are briefly compared to previous results obtained for the two-polymer blend of ABS and PVC. Incorporating polypropylene into the ABS/PVC system has significant effects on flammability and smoke production. Stabilisation of the polymer system by the iron compound is confirmed by experiment and it is one of the very few additives available today which exerts a flame retarding/smoke suppressing effect via a char forming mechanism.

INTRODUCTION

cially those containing PVC. In this paper the authors discuss some important results which have been obtained using a tertiary polymer blend containing ABS (acrylonitrile-butadienestyrene), PVC (polyvinyl chloride) and PP (polypropylene). These three polymers are very cheap, widely available and separately have a myriad of uses. 8 The recent interest and activity in the engineering field of polymer blends have been a result of greater competition for new materials, where applications call for combinations of properties not previously available in a single polymer, and it has been found that it is less expensive to combine existing plastics rather than develop totally new polymers on which to base new products. It is of interest to compare the use of polymer blends with developments in the textile industry where combinations of fibres such as polyester and cotton have been used to produce fabrics which are very much better than either fibre when used separately. Of course ABS is a polymer blend in its own right.

In a series of recent publications 2-6 the authors have outlined the effects which some iron compounds have on flammability, smoke production, char formation and heat stability in semi-rigid PVC and in blends of ABS and PVC. The authors have also very recently reported on a synergistic interaction occurring between antimony(Ill) oxide and basic iron(Ill) oxide in the same blend of ABS and PVC. 7 The plastics industry is constantly looking for new additives for thermoplastics which, in addition to improving their resistance to ignition and burning, enhance polymer properties in terms of both heat and light stability. In recent publications the authors have commented on the effects of iron compounds in improving both heat and light stability in some formulations, espePolymer Degradation and Stability 0141-3910/94/$07.00 1994 Elsevier Science Limited 93

P. Carty, S. White

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Having obtained excellent flame retardancy/ smoke suppressing effects in blends of ABS and PVC, the authors decided to extend the polymer range and look at the effects of iron compounds in a combination of ABS, PVC and PP. After looking at the possibility of alloying these three polymers and using previous experience in producing blends it was finally decided, on the basis of producing a useful plastic using a two-roll mill, that a 50 ABS/30 PVC/20 PP blend would be the most suitable combination to investigate.

Limiting oxygen index (LOI) and smoke density (/)MAX) determination LOI values were determined according to ASTM D-2863-77 (BS 2782 Part 14b). 9 DMAX values were obtained in the flaming mode according to BS 6401. "~ Values in Table 1 of DMAX are all per gram of sample and represent a mean of at least three values.

Thermal stability and char formation The determination of thermal stability is fully described in Ref. 6 and that of carbonaceous char in Ref. 4.

EXPERIMENTAL Blending ABS, PVC and PP

Heat release This was carried out in the usual way by adding the following polymers to the nip of the rollers in a heated two-roll mill: (i) ABS (ii) PP (iii) UPVC

Cyclolac GSM 100 phr Hostalen 100 phr Corvic $67/111 100phr Tribasic lead sulphate 5 phr (stabiliser) Calcium stearate (lubricant) 1 phr

Values of heat release were determined for the authors by Cookson America using an Ohio state University Heat Release R H R Calorimeter (OSU R H R Calorimeter). The furnace heat flux was 25 kW -2

RESULTS

The temperature was set at 180°C and ABS polymer chips melted between the nip of the rolls. Powdered U P V C was carefully added a little at a time and finally PP polymer chips were added carefully to the melt of ABS and PVC. By careful cutting and mixing on the two-roll mill the three polymers seemed readily to form a homogeneous gel. After cooling, the hide produced appeared to have good strength, an excellent surface sheen and lacked the brittleness which so often occurs when dissimilar polymers are mixed.

Flammability (LOI), smoke density and rate of heat release The LOI, smoke density and R H R values are given in Table 1.

Thermal stability The thermal stability results are shown in Table 2. Sample dimensions: (25 x 25 x 3) mm; temperature: 200(+2)°C; duration of experiment: 4h.

Table 1. LOI, smoke density and rate of heat release values

Polymer blend

LOI

Smoke density (OMAx g-')

Maximum heat release rate (kW m -2 g-t)

50 ABS/30 PVC/20 PP 50 ABS/30 PVC/20 PP/5 FeOOH 50 ABS/30 PVC/20 PP/10 FeOOH

21-6 24.8 26-0 (v-o in UL94")

82.0 42-6 32.8

6.06 N/A 2-43

UL94 values were determined according to Ref. 11.

Flame retardancy in a tertiary polymer blend

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Table 2. Thermal stability results Polymer blend: mass loss (g) Time

50 ABS/30 PVC/20 PP

50 ABS/30 PVC/20 PP/I0 feOOH

0.100 0.300 0-103 0.244

0.007 0.019 0-031 0-039

(h) 1 2 3 4

Char values

The char values, determined at 650°C for 10 min, are given in Table 3. Table 3. Char values

Polymer blend 50 ABS/30 PVC/20 PP 50 ABS/30 PVC/20 PP/5 FeOOH 50 ABS/30 PVC/20 PP/10 FeOOH

Per cent char 3.86 18.40 24.95

DISCUSSION

In earlier work the authors found that iron compounds present in quite low concentrations have very significant effects on the LOI and smoke density values in A B S / P V C blends. In 7 0 A B S / 3 0 PVC, for example, adding 5 phr of F e O O H to the blend raised the LOI value from 21.8 for the iron-free formulation to 33.4 for the formulation containing iron, a very significant increase of 15 LOI units. Smoke is also reduced in the presence of iron. Adding 5 phr of F e O O H to 70 ABS/30 PVC reduces smoke production by about 45%. Increasing the concentration of F e O O H to 10 phr reduces smoke production by almost 50%. In the tertiary blend under investigation in this work, similar but subtly different effects are observed. Having polypropylene in the system modifies the changes which the iron c o m p o u n d has on the properties. Polypropylene is a low smoke (DMAxg - 1 = 14"0) but very flammable (LOI = 17.6) polymer. In addition, polypropylene has a very high heat of combustion. '2 These features of polypropylene influence the effects which the iron c o m p o u n d has on the polymer blend. Even though it is present as about one-fifth of the total polymer present polypropylene has significant effects on the properties of the blend as a whole.

Smoke production is reduced but, unlike the 7 0 A B S / 3 0 P V C system, very high LOI values are not obtained. Looking at Table 1 it is very obvious that raising the LOI from 21.6 for the iron-free formulation to 24-8 for a blend containing 5 p h r of F e O O H and to 26.0 for a blend containing 10phr F e O O H is not so dramatic a change when compared with the 70 ABS/30 PVC system. This effect is essentially a result of having a polymer present which is highly flammable. However, the 5 0 A B S / 3 0 P V C / 2 0 P P / 1 0 F e O O H formulation has a v-o rating at 1/16th of an inch in the UL94 test and so could be classed as being sufficiently flame retarded for commercial application. Smoke reduction by basic iron(III) oxide in the tertiary blend is quite dramatic. In an iron-free 50 ABS/30 PVC/20 PP formulation the DMAx gvalue is 82. Adding 5 phr of F e O O H to the blend reduces the DMAx g-I value to 42-6 (a 48% smoke reduction) while having 10phr F e O O H present the smoke is further reduced to 32.8, a very important 60% reduction in smoke. This very low value for DMAX iS unquestionably a consequence of having an appropriate combination of PP and F e O O H in the blend. The authors have fully reported on charring reactions in A B S / P V C blends ~ and it seems that the charring system of F e O O H / P V C is also very active in this tertiary polymer blend. Table 3 shows quite clearly the effect of the iron additive on the formation of char. The iron-free formulation produces only 3.86%, char but when 5 phr F e O O H is added, 18.40% char is produced. At an additive concentration of 10 phr F e O O H about 25% char is produced. These high values are indicative of low smoke formulations. The effects of the F e O O H / P V C system are to retain polymer carbon in the solid phase as carbonaceous char by a mechanism the authors suggested in an earlier publication.' Thermally stable polymers are becoming

96

P. Carty, S. White 250

p50ABS/ x 30~ /

MassLoss (nag) 200150 100-

50-

O~

1

2 3 Time(hr)

4

Fig. 1. Thermal stability of a 50 ABS/30 PVC/20 PP blend at 200°C.

increasingly important. ~3'~4 in many applications and we have shown that iron c o m p o u n d s readily stabilise certain polymer systems by a mechanism as yet unknown to us. It is well known that increasing polypropylene content as well as increasing quaternary carbon tends to decrease the thermal stability of polymers. ~5 Figure 1 shows the results of the authors' thermal stability experiments. The iron-free formulation begins to lose mass after being heated at 200°c for two hours. After four hours at 200°C the sample lost about 9% of its original mass. In comparison the 50 ABS/30 PVC/20 PP/10 F e O O H sample loses only about 1% of its original mass after four hours at 200°C in air. No paper describing the flammability properties of polymers would be complete without some reference to heat release. The authors have determined several flammability parameters using an OSU R H R calorimeter and were especially interested in the rate of heat release data. The results are shown in Table 1. Once again it is difficult to explain why, but the presence of basic iron(Ill) oxide reduces the rate at which heat is released when 5 0 A B S / 3 0 P V C / 2 0 P P is forced to burn. The iron-free formulation produces about 6 k W m - : g -~ but when 1 0 p h r F e O O H is c o m p o u n d e d into the system the rate of heat release falls to 2 - 4 3 k W m - 2 g -j, a fall of about 60%. If we compare the rate of heat release for the 50 ABS/30 PVC/20 PP/10 F e O O H formulation against that which we have obtained for 100% ABS we find that the rate of heat release is reduced by 72%. Unfortunately at this time we cannot explain how this iron c o m p o u n d seems to be so effective in its ability to stabilise this polymer system, effectively reducing its tendency

to be decomposed by a variety of different agencies. We can speculate that the iron compound is very active in removing HCI as it is formed from the decomposing PVC part of the blend. However, as the 7 0 A B S / 3 0 P V C work has shown, confirmed by the data published here, even blends with quite low proportions of PVC present are stabilised by iron compounds. In conclusion we have shown that there are real benefits in replacing some of the ABS in a 7 0 A B S / 3 0 P V C system with polypropylene. In the tertiary system of 5 0 A B S / 3 0 P V C / 2 0 P P having polypropylene present results in a formulation which can be realistically classed as being 'low smoke'. Certainly there are very few commercial polymers currently in use which have values for smoke density (DMAx g-~) of less than 40. In real fires it is very well d o c u m e n t e d that it is the smoke which claims many of the lives which are lost. As the 5 0 A B S / 3 0 P V C / 20 PP/10 F e O O H system meets the flammability requirements for commercial application and the fact that the iron c o m p o u n d present reduces smoke production very significantly and stabilises the system, such a tertiary polymer formulation could have very interesting commercial applications.

ACKNOWLEDGEMENTS The authors would like to thank Jan Eastwood who produced the manuscript and Cookson plc who supported this work.

REFERENCES 1. Carty, P. & White, S., Polymer, 33 (1992) 2704. 2. Carty, P. & White, S., Appl. Organometallic Chem., 5 (1991) 51. 3. Carty, P. & Adger, B. M., Appl. Organometallie Chem., 4 (1990) 127. 4. Carty, P. & White, S., Polymer, 35(2) (1994) 343. 5. Carty, P. & White, S., Fire and Materials, 18 (1994). 6. Carty, P. & White, S., Conference paper. In Organic Flame Retardants--All Change: Royal Society of Chemistry, London, Jan. 1993. 7. Carty, P. & White, S., Polym. Deg. & Stab., in press. 8. Crawford, R. J., Chapter 1 in Plastics Engineering (2nd edn). Pergamon Press, London, 1987. 9. see Troitzsch, J. (ed.), International Plastics Flammability Handbook, Principles, Regulations, Testing and Approval (2nd edn). Hanser, Munich, 1990, p. 217. 10. see Troitzsch, J. (ed.), International Plastics Flammability Handbook, Principles, Regulations, Testing and Approval (2nd edn). Hanser, Munich, 1990, p. 408.

Flame retardancy in a tertiary polymer blend 11. see Troitzsch, J. (ed.), International Plastics Flam-

mability Handbook, Principles, Regulations, Testing and Approval (2nd edn). Hanser, Munich, 1990, p. 346. 12. see Troitzsch, J. (ed.), International Plastics Flammability Handbook, Principles, Regulations, Testing and Approval (2nd edn). Hanser, Munich, 1990, p. 20. 13. Wadehra, I. L., Conference paper. In Flame Retardancy of Polymeric Materials. Business Communications Co.,

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Stanford, CT, May 1990. 14. Garrison, W. E., Conference paper. In High Perfor-

mance F l a m e Retardant Thermoplastics for Electrical/Electronic Components. Autumn Meeting, Fire Retardant Chemicals Association, USA, 1991, p. 55. 15. Shibryaeva, L. S., Kiryushkin, S. G. & Zaikov, G. E., Polym. Deg. & Stab., 36 (1992) 17.