Historical Aspects of Polymer Fire Retardance - ACS Symposium

Chapter 7

Historical Aspects of Polymer Fire Retardance Raymond R. Hindersinn

Fire and Polymers Downloaded from pubs.acs.org by UNIV LAVAL on 10/03/15. For personal use only.

Hindersinn Associates, 4288 Lower River Road, Youngstown, NY 14174

The h i s t o r y of polymer fire retardance i s reviewed from its i n c e p t i o n with the e a r l y Egyptians to the most recent developments in intumescent fire r e t a r d a n t s and i n h e r e n t l y fire retardant polymers.

EARLY HISTORY The c o n t r o l o f polymer flammability, which has enjoyed c o n s i d e r a b l e success i n the l a s t f o r t y years, had i t s beginnings i n a n t i q u i t y with the f i r s t e a r l y attempts t o reduce the flammability o f n a t u r a l c e l l u l o s i c m a t e r i a l s such as cotton and wood (1-3) . Some o f these e a r l y developments are summarized i n Table I . Perhaps the e a r l i e s t reference t o t h i s development was reported by the Greek h i s t o r i a n Herodotus (484-431 BC) who noted t h a t the Egyptians were imparting a degree o f f i r e retardance t o wood by soaking i t i n alum (potassium aluminum s u l f a t e ) . About two c e n t u r i e s l a t e r , the Romans "improved" the process by adding vinegar t o the mixture. Military a p p l i c a t i o n s f o r f i r e retardant wood were subsequently reported i n the f i r s t century B.C. by V i t r u v i u s where e a r l y siege towers were protected against i n c e n d i a r i e s by a t h i c k c o a t i n g o f c l a y r e i n f o r c e d with h a i r . An "incombustible c l o t h " was subsequently developed i n the 17th century f o r P a r i s i a n t h e a t e r c u r t a i n s by t r e a t i n g canvas with a mixture o f c l a y and gypsum. The f i r s t patent on a f i r e r e t a r d a n t treatment f o r wood and t e x t i l e s was issued t o Wyld i n 1735. 0097-6156/90/0425-0087$06.00/0 © 1990 American Chemical Society


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A b a s i c s c i e n t i f i c i n v e s t i g a t i o n o f f i r e retardancy, however, remained t o be i n i t i a t e d by Gay-Lussac i n France at the request o f King Louis XVIII i n 1821 who was again interested i n reducing the f l a m m a b i l i t y o f t h e a t e r curtains. T h i s researcher noted t h a t the ammonium s a l t s of s u l f u r i c , h y d r o c h l o r i c and phosphoric a c i d s were very e f f e c t i v e f i r e r e t a r d a n t s on hemp and l i n e n and t h a t the e f f e c t c o u l d be improved c o n s i d e r a b l y by u s i n g mixtures o f ammonium c h l o r i d e , ammonium phosphate and borax. This work has withstood the t e s t o f time and remains v a l i d t o t h i s day. Thus the b a s i c elements o f modern fire r e t a r d a n t chemistry had been d e f i n e d e a r l y i n recorded h i s t o r y and remained the s t a t e o f the a r t u n t i l e a r l y i n the twentieth century. The most e f f e c t i v e treatments f o r c e l l u l o s i c m a t e r i a l s being concentrated i n Groups I I I , V and V I I elements. In 1913, the renowned chemist W i l l i a m Henry P e r k i n became i n t e r e s t e d i n the problem o f reducing the high flammability of a then popular fabric known as " f l a n n e l e t t e " and i n the process o f h i s work f i r s t d e f i n e d the most important requirements f o r a f i r e r e t a r d a n t f a b r i c which i n c l u d e d p r o p e r t i e s such as d u r a b i l i t y , f e e l , non-poisonous nature, low c o s t and p r i n t a b i l i t y a f t e r treatment. The Perkins treatment consisted of impregnating the f a b r i c with aqueous s o l u t i o n s o f sodium stannate and ammonium s u l f a t e . A subsequent heat treatment converted the chemicals t o i n s o l u b l e s t a n n i c oxide which was b e l i e v e d t o be the a c t i v e r e t a r d a n t . The Perkins process d i d not win popular favor and f u r t h e r s c i e n t i f i c work on f i r e retardance remained dormant u n t i l World War I I when new developments i n s y n t h e t i c polymers ushered i n a new e r a o f f i r e r e t a r d a n t chemistry. MODERN FIRE RETARDANT DEVELOPMENTS The advent o f s y n t h e t i c polymers was o f s p e c i a l s i g n i f i c a n c e s i n c e the water s o l u b l e i n o r g a n i c s a l t s d e f i n e d up t o t h a t time were o f l i t t l e o r no u t i l i t y i n these l a r g e l y hydrophobic m a t e r i a l s . Modern developments t h e r e f o r e were concentrated on the development o f polymer compatible permanent fire retardants. Although a multitude of individual products have since been developed, Table I I attempts t o l i s t the most s i g n i f i c a n t developments with the l a r g e s t impact on the d i r e c t i o n o f f i r e r e t a r d a n t chemistry. CHLORINATED PARAFFIN AND ANTIMONY OXIDE. The demands o f the armed f o r c e s i n World War I I f o r a f i r e r e t a r d a n t , waterproof treatment f o r canvas t e n t i n g l e d t o the development o f a combination treatment containing a chlorinated p a r a f f i n (CP), antimony oxide and a b i n d e r (4., 5) . T h i s was the f i r s t d e f i n i t i o n o f the halogenantimony s y n e r g i s t i c combination which has s i n c e been shown t o be so e f f e c t i v e i n many f i r e r e t a r d a n t polymer

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Historical Aspects of Polymer Fire Retardance

TABLE I

Early Historical Fire Retardant Developments


Development

Date

• Alum used to reduce theflammabilityof wood by Egyptians.

About 450 B.C.

• Romans used a mixture of alum and vinegar on wood.

About 200 B.C.

• Mixture of clay and gypsum used to reduceflammabilityof theater curtains.

1638

• Mixture of alum, ferrous sulfate and borax used on wood and textiles by Wyld in Great Britain.

1735

• Alum used to reduceflammabilityof balloons.

1783

• Gay-Lussac reported a mixture of (NH ) P0 , NH C1 and borax to be effective on linen and hemp.

1821

• Perkin described a FR treatment for cotton using a mixture of sodium stannate and ammonium sulfate.

1912

4 3

TABLE

4

4

II

Most Important Modern Developments in Polymer Fire Retardance 1) Chlorinated paraffin, antimony oxide and a binder as a treatment on canvas. 2) Chlorine containing unsaturated polyesters. 3) Filler-like retardants. 4) Oxygen index method of evaluating relative polymer flammability. 5) Intumescent fire retardant systems. 6) Inherently fire retardant polymers.

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products and the f i r s t i n t r o d u c t i o n of organic halogen compounds i n p l a c e of the i n o r g a n i c s a l t s p r e v i o u s l y i n vogue. REACTIVE FIRE RETARDANTS. T h i s new treatment was then immediately a p p l i e d t o p o l y v i n y l c h l o r i d e (PVC) and unsaturated p o l y e s t e r s ; both products which entered the development stage during World War I I . The new products enjoyed a degree of success i n PVC, because o f the need f o r a p l a s t i c i z e r t o allow processing of t h a t polymer. The p l a s t i c i z i n g nature of the CP, however, not only reduced the d e s i r a b l e p h y s i c a l p r o p e r t i e s of the p o l y e s t e r laminates but tended t o "wash out" i n many environments i n which these products were being used thus reducing o r e l i m i n a t i n g the d e s i r a b l e f i r e retardant p r o p e r t i e s f o r which they had been added i n the f i r s t p l a c e . These major d e f i c i e n c i e s o f CP r a p i d l y l e d t o the c o n c l u s i o n t h a t a r e a c t i v e f i r e retardant system would be p r e f e r r e d which would be c h e m i c a l l y reacted i n t o the p o l y e s t e r a t some stage of the p o l y e s t e r s y n t h e s i s and/or f a b r i c a t i o n o f the f i n a l product and thus confer permanent f i r e r e t a r d a n t p r o p e r t i e s t o the f i n a l product. The first f i r e retardant p o l y e s t e r c o n t a i n i n g a r e a c t i v e f i r e retardant monomer was introduced by the Hooker E l e c t r o c h e m i c a l Corporation i n the e a r l y 1950*s c o n t a i n i n g c h l o r e n d i c a c i d as the r e a c t i v e monomer (6) . This pioneering development rapidly l e d t o the i n t r o d u c t i o n of v a r i e t y of r e a c t i v e halogen and phosphorus c o n t a i n i n g monomers, such as tetrabromophthalic anhydride, chlorostyrene and tetrabromobisphenol A, which found a p p l i c a t i o n i n a wide v a r i e t y of condensation polymer systems. FIRE RETARDANT FILLERS. The next major f i r e r e t a r d a n t development r e s u l t e d from the need f o r an acceptable f i r e retardant system f o r such new thermoplastics as polyethylene, polypropylene and nylon. The p l a s t i c i z e r approach of CP o r the use of a r e a c t i v e monomer were not a p p l i c a b l e t o these polymers because the c r y s t a l l i n i t y upon which t h e i r d e s i r a b l e p r o p e r t i e s were dependent were reduced or destroyed i n the process of adding the f i r e r e t a r d a n t . A d d i t i o n a l l y , most halogen a d d i t i v e s , such as CP, were thermally unstable a t the high molding temperatures r e q u i r e d . The i n t r o d u c t i o n of i n e r t f i r e r e t a r d a n t f i l l e r s i n 1965 defined two novel approaches t o f i r e retardant polymers. One of these products was a thermally stable i n s o l u b l e chlorocarbon prepared from c y c l o o c t a d i e n e and hexachlorocyclopentadiene. The diadduct, dodecachlordodecahydrodi-methanodibenzo [a, e] cyclooctene, has the s t r u c t u r e given i n I and i s commonly r e f e r r e d t o as a c y c l o a l i p h a t i c c h l o r i n e compound ( 7 ) .

7.

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ci

91

•ci

cr


CI

I

CI

The extreme i n s o l u b i l i t y of t h i s high melting thermally s t a b l e hydrocarbon allowed i t t o be compounded into most thermoplastics without decomposition or discoloration. I t s high c h l o r i n e content (65.1% by weight) and f i l l e r - l i k e p r o p e r t i e s not only increased the heat d i s t o r t i o n and f l e x u r a l modulus of the o r i g i n a l polymer, without the degradation of such important p r o p e r t i e s as e l e c t r i c a l and water r e s i s t a n c e , but was e s s e n t i a l l y non-migrating at elevated temperatures and i n aqueous environments. Although s i n c e d i s p l a c e d by more e f f e c t i v e aromatic bromocarbons i n p o l y o l e f i n s such as polystyrene and ABS, the product s t i l l f i n d s c o n s i d e r a b l e u t i l i t y i n f i r e r e t a r d a n t nylon compositions. The other f i r e retardant f i l l e r , hydrated aluminum oxide (or alumina) (8), exerts i t s f i r e r e t a r d a n t e f f e c t in polymer compositions by dehydrating under flame c o n d i t i o n s and preventing burning by i n j e c t i n g l a r g e amounts of non-flammable water vapor i n t o the atmosphere adjacent to the heated polymer surface. Self extinguishment i s conferred by c o o l i n g the s u r f a c e and displacing the oxygen necessary for continued flammability. Because of i t s r e l a t i v e l y low decomposition temperature (245-320°C) hydrated alumina f i n d s i t s g r e a t e s t u t i l i t y i n polymer compositions r e q u i r i n g low p r o c e s s i n g temperatures, such as p o l y e s t e r s , where i t s low c o s t , hydrophobicity and r e i n f o r c i n g p r o p e r t i e s can be used t o great advantage. I t s e f f e c t i v e n e s s i s a l s o l i m i t e d when a p p l i e d t o p o l y o l e f i n s because the l a r g e q u a n t i t i e s r e q u i r e d f o r e f f e c t i v e f i r e retardance make p r o c e s s i n g d i f f i c u l t or impossible. The major advantages of t h i s f i r e retardant system are low smoke production and no hydrogen h a l i d e off-gases produced during p y r o l y s i s on f i r e exposure. OXYGEN INDEX TEST METHOD. P r i o r to the i n t r o d u c t i o n of the Oxygen Index Test (01) i n 1966 by Fenimore and Martin (9), f i r e retardant polymer research was dependent upon a plethora of t e s t methods with v a r i a b l e degrees of s t r i n g e n c y f o r f i r e retardant e v a l u a t i o n . Not only were test r e s u l t s r e l a t e d only to a burning or selfextinguishing set of observations, but compositions passing one set of t e s t c o n d i t i o n s could not e a s i l y be r e l a t e d t o behavior i n a more demanding set of c o n d i t i o n s . A d d i t i o n a l l y , the r e q u i r e d t e s t specimens v a r i e d from f i l m s , coatings, foams and r i g i d p l a s t i c s , a l l of which a f f e c t e d the degree of i g n i t a b i l i t y of the specimen.


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The 01 t e s t i s c a r r i e d out on a f i l m specimen i n a v a r i a b l e mixture of oxygen and n i t r o g e n with t h a t gas mixture being determined i n which a small change i n gas composition a l t e r s the burning c h a r a c t e r i s t i c s of the material from self extinguishing to a burning classification. The oxygen c o n c e n t r a t i o n a t t h i s p o i n t was reported as a r a t i o or index of oxygen c o n c e n t r a t i o n t o the t o t a l c o n c e n t r a t i o n of oxygen and n i t r o g e n i n the mixture m u l t i p l i e d by a hundred. The 01 f o r a i r under t h i s system i s reported as 21. As i n d i c a t e d i n Table I I I , a l l m a t e r i a l s could now be assigned an 01 value t h a t would r e l a t e i t s r e l a t i v e flammability t o any other m a t e r i a l provided t h a t the m a t e r i a l was capable of burning i n pure oxygen. Such an assignment was extremely u s e f u l i n f i r e retardant research activities since the simple determination of a composition's oxygen index allowed an estimate of i t s r e l a t i v e flammability. Of p a r t i c u l a r i n t e r e s t i s the r e l a t i v e p o s i t i o n of carbon on t h i s s c a l e s i n c e i t s high 01 value of 65 has o f t e n been used t o decrease the flammability of more flammable polymers. Subsequent research on the t e s t has a l s o shown t h a t the temperature of the t e s t c o n d i t i o n s could be increased t o allow the assignment of 01 values t o f i r e r e s i s t a n t m a t e r i a l s not e a s i l y flammable i n pure oxygen at room temperature (10. 11). Fenimore and co-workers (12) a l s o used the oxygen index t e s t c o n d i t i o n s t o study the mechanism of f i r e retardant systems by comparing the 01 i n oxygen/nitrogen and n i t r o u s oxide/nitrogen oxidizing atmospheres. Despite i t s usefulness as a research t o o l , however, the t e s t c o n d i t i o n s cannot be r e l a t e d t o r e a l fire s i t u a t i o n s and l a r g e s c a l e t e s t i n g i s s t i l l r e q u i r e d to determine the f i r e r e s i s t a n c e of m a t e r i a l s i n a c t u a l f i e l d conditions. INTUMESCENT FIRE RETARDANT SYSTEMS. As previously mentioned, the r e l a t i v e l y high 01 value f o r elemental carbon i n Table I I I has l e d t o the recent development of FR a d d i t i v e systems f o r many h i g h l y flammable polymers which o b t a i n t h e i r f i r e retardant e f f e c t by c a t a l y z i n g the p y r o l y s i s of the polymer backbone i n t o carbonaceous char or by supplying the carbonaceous i n g r e d i e n t s i n the a d d i t i v e mixture. Thus the polymer i s then converted from a composition with an 01 of 20 i n t o an 01 approaching t h a t of carbon, w e l l above the value of 25-30 r e q u i r e d f o r most f i r e r e t a r d a n t systems. T h i s approach t o polymer f i r e retardance not only r e q u i r e s a lower l e v e l of a d d i t i v e s t o o b t a i n a r e q u i r e d degree of f i r e retardance but o f t e n reduces the l a r g e volumes of heavy smoke evolved during exposure of the halogen/antimony oxide compositions t o flame c o n d i t i o n s . The l a r g e q u a n t i t i e s of u n d e s i r a b l e halogen h a l i d e t h a t accompanies the p y r o l y s i s of these l a t t e r compositions i s a l s o e l i m i n a t e d .


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93

Although the use of intumescent combinations of a p o l y h y d r i c organic compound, an a c i d forming c a t a l y s t and a gas forming component have long been known t o form excellent fire protective coatings for flammable s u b s t r a t e s (13) , the i n c o r p o r a t i o n of one or more of these components i n t o the b a s i c polymer composition t o c o n f e r an intumescent c h a r a c t e r when exposed t o flame temperature i s only a recent development. The chemical composition of the polymer s u b s t r a t e seems t o be an important v a r i a b l e i f not the most important v a r i a b l e i n determining the e f f e c t i v e n e s s of t h i s approach t o polymer f i r e retardance and d i r e c t l y determines the l o a d i n g l e v e l and number of ingredients required for e f f e c t i v e results. Some of the intumescent systems and their effectiveness i n the recommended polymer s u b s t r a t e s are summarized i n Table IV together with r e p r e s e n t a t i v e r e s u l t s of a commercial halogen/antimony oxide composition f o r comparison. The first system, designated "Melabis" f o r convenience, contains a l l three components of the intumescent c o a t i n g compositions i n c o r p o r a t e d i n t o a s i n g l e water i n s o l u b l e thermally stable additive that can be c o n v e n i e n t l y compounded i n t o polypropylene and i n j e c t i o n molded without premature decomposition. As can be seen, a 20% f i r e retardant l e v e l by weight i s s u f f i c i e n t t o c o n f e r a VO r a t i n g by the UL 94 f l a m m a b i l i t y t e s t (14) while a 48% l o a d i n g i s r e q u i r e d f o r the same degree of f i r e retardance with the conventional c y c l o a l i p h a t i c chlorine/antimony oxide system. By comparison, only one weight percent or l e s s of an aromatic metal s u l f o n a t e (an a c i d forming component) i s r e q u i r e d t o a t t a i n a f i r e r e t a r d a n t r a t i n g by the ASTM D635 t e s t c o n d i t i o n s (15, 16) t o the h i g h l y aromatic polycarbonate s u b s t r a t e . The t h i r d composition i n Table IV seems t o be r e l a t e d to the aromatic sulfonate/polycarbonate technology j u s t d i s c u s s e d with some m o d i f i c a t i o n s being necessary i n order to compensate for the aliphatic nature of the polypropylene (17, 18) s u b s t r a t e . In t h i s case the aromatic s u l f o n a t e i s r e p l a c e d with a metal salt ( p r e f e r a b l y magnesium s t e a r a t e ) . A s i l i c o n e o i l and or gum has been added t o enhance the intumescent c h a r a c t e r and a small amount of i n e r t f i l l e r and decabromodiphenyl oxide i s i n c l u d e d probably t o improve the molding c h a r a c t e r i s t i c s of the t o t a l composition. F i r e r e t a r d a n t compositions with a good s u r f a c e char can be obtained at t o t a l loadings only about h a l f that r e q u i r e d f o r the halogen/antimony oxide composition. THERMALLY STABLE POLYMERS No d i s c u s s i o n of polymer f i r e retardance would be complete without a t l e a s t a b r i e f mention of the h i g h l y aromatic polymers, a l l of which are very d i f f i c u l t l y flammable i f they burn at a l l (19). Although the low f l a m m a b i l i t y of p h e n o l i c and furane r e s i n s are w e l l known, these t h e r m a l l y

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TABLE III

Limiting Oxygen Indexes of Various Materials


01 Material Poly oxy methylene Candle Polymethylmethacrylate Polypropylene Polystyrene Chlorinated Polyether Polycarbonate Polyphenylene Oxide Polyvinyl Chloride (No Plasticizer) Polyvinylidene Chloride Carbon Polytetrafluoroethylene

[%i 15 16 17 17 18 23 27 29 45 60 65 95

SOURCE: Data from ref. 9.

TABLE IV

Some Representative Intumescent FR Systems

FR System

Recommended Application

Approx. Loading Level Required for V-0 Designation Oxygen ViaUL-94 Index

> = r ° ^ o l o ^ ( / > o o-J o© ^—o Polypropylene

20

Polycarbonates

1*

-

Magnesium Stearate/ Suicone/Talc

Polypropylene

21.8

30

Cycloaliphatic Chlorine Antimony Oxide

Polypropylene

48

26

NH® 3

N

P)

N

H N^ ^NH2 2

N

MELABIS Aromatic Sulfonates

* This composition has only a SE rating by ASTM D 635


7. HINDERSINN

95


stable linear polymers are unique because of their controlled chemical structure yields exceptional properties without the need for fillers and/or reinforcements or a long curing cycle for their high temperature p r o p e r t i e s and thermal r e s i s t a n c e . The h i g h cost o f the f i n i s h e d polymers and t h e i r often s p e c i a l i z e d f a b r i c a t i o n techniques, however, w i l l l i m i t t h e i r u t i l i t y for the foreseeable future to specialized applications where economics a r e o f secondary i m p o r t a n c e . The thermal and flammability characteristics of only a few r e p r e s e n t a t i v e commercial polymers a r e summarized i n Table V. The oxygen indexes o f t h e s e u n m o d i f i e d polymers a r e w e l l above t h a t necessary f o r f i r e r e t a r d a n t r a t i n g s and m o s t h a v e VO c l a s s i f i c a t i o n s u n d e r U L 9 4 t e s t c o n d i t i o n s . Some w i l l also withstand direct flame c o n d i t i o n s f o r appreciable lengths o f time without l o s i n g t h e i r coherence and without emitting large quantities of smoke or unusually noxious gases.

TABLE V: Some Thermally Stable Polymers Chemical Structure

-o-(c\l*

'

Mp.

c

Tg

c

Oxygen

UL-94

index

Rating

427

Jn

—

190

30

285

88-93

46-53

VO

421

369

42

VO

_n

n

- - ® - c - o - @ o

/—\ o

-o-l-fQ-'c_n SOURCE: Data from ref. 17.

FIRE AND POLYMERS

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Literature Cited 1)

2)


3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) Chem., 15) 16) 17) 18) 19)

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RECEIVED November1,1989

Historical Aspects of Polymer Fire Retardance - ACS Symposium

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