Applied Polymer Science - American Chemical Society

14

Flammability of Polymers 1

2

Y. P. KHANNA and E. M. PEARCE 1

Corporate Research & Development, Allied Corporation, Morristown, NJ 07960 Department of Chemistry, Polytechnic Institute of New York, Brooklyn, NY 11201

Downloaded by PRINCETON UNIV on April 13, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch014

2

Polymer Flammability Principles of Flammability Flame Retardation Polymer Structure and Flammability Flame Retardation of Polymers Synergism in Flame Retardation Selection of Fire Retardants Flame Retardation of Polymeric Materials

The use of polymeric materials is growing in a number of areas such as home furnishings, domestic and industrial buildings, appliances, fabrics, and transportation vehicles. An expanded growth of polymers concurrent with the proliferation of safety standards being set by the government and private agencies has stressed that reducing the flammability of polymeric materials is of primary importance. Selection or design of a flame retardant system for a particular application is often difficult. A flame retardant polymer should have high resistance to ignition, low rate of combustion and smoke generation, low toxicity of product gases, retention of low flammability during use, acceptability in appearance and properties, no environmental or health safety impact, and little or no economic penalty. To select or design a polymeric material with desirable flammability properties, a familiarity with the principles of combustion, the relationship of flammability to polymer structure, and the modes of flame inhibition is essential. This chapter reviews the basic principles of polymer flammab i l i t y , the relationship of polymer flammability to polymer structure, and the general approaches to flame retardation. In addition, examples illustrating the concepts of polymer flammability and flame retardation are presented. 0097 6156/ 85/ 0285-O305S06.00/ 0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

306

APPLIED POLYMER SCIENCE

Polymer Flammability


This section deals b r i e f l y with the basics of polymer flammability and the approaches to flame retardation. A f a m i l i a r i t y with both of these s u b j e c t s i s e s s e n t i a l f o r d e s i g n i n g p o l y m e r i c m a t e r i a l s of desired flammability properties. P r i n c i p l e s of Flammability. The presence of f u e l , heat, and oxygen i s necessary to i n i t i a t e and sustain a f i r e . A basic flammability c y c l e i s diagrammed i n F i g u r e 1. When s u f f i c i e n t l y heated by the e x t e r n a l i g n i t i o n source, the p o l y m e r i c m a t e r i a l reaches a c h a r a c t e r i s t i c temperature at which i t begins to degrade. The extent to which the m o l e c u l a r oxygen p l a y s a r o l e i n t h i s s u r f a c e decomposition ( i . e . , thermal or t h e r m o - o x i d a t i v e ) depends on the s p e c i f i c polymer used. Gaseous c o m b u s t i b l e products may then be formed at a r a t e dependent upon f a c t o r s such as the i n t e n s i t y of external heat, temperature, and rate of polymer decomposition. Flammable gases (fuel) thus produced diffuse to the flame front where a s e r i e s of h e a t - g e n e r a t i n g , complex, f r e e - r a d i c a l c h a i n reactions take place i n the presence of surrounding oxygen. Because the flaming of organic polymers simply represents the oxidation of hydrocarbons, i t would be i n s t r u c t i v e to c o n s i d e r the w e l l understood methane-oxygen combustion system. This system, shown i n R e a c t i o n s 1-11, can s e r v e as a model f o r the more complex polymer flames. The mechanism i n R e a c t i o n s 1-11 i n d i c a t e s t h a t methane combustion i s a complex f r e e - r a d i c a l chain reaction c o n s i s t i n g of p r o p a g a t i o n , c h a i n - b r a n c h i n g , and t e r m i n a t i o n s t e p s . Oxygen i s found only i n the major branching step. In the branching step, two chain-carrying r a d i c a l s are produced for each one consumed, and t h i s c o n d i t i o n accounts f o r the e x p l o s i v e nature of b u r n i n g . The main chain-carrying r a d i c a l s i n the propagation s t e p s are the hydrogen atom and hydroxyl r a d i c a l s . This mechanism indicates that hydrogen, not carbon, i s the f u e l s p e c i e s most r e s p o n s i b l e f o r f l a m i n g hydrocarbons. A f t e r i g n i t i o n and removal of the i g n i t i o n source, combustion becomes s e l f - p r o p a g a t i n g i f s u f f i c i e n t heat i s generated and radiated back to the material to continue the decomposition process. The combustion process i s governed by such v a r i a b l e s as the rate of heat generation, rate of heat transfer to the surface, surface area, and rate of decomposition. Flame Retardation. Polymer combustion, a h i g h l y complex process, i s composed of a vapor phase, i n which the r e a c t i o n s r e s p o n s i b l e f o r the formation and propagation of the flame take p l a c e , and a condensed phase, i n which f u e l f o r the gas r e a c t i o n s i s produced. Flame r e t a r d a n c y , t h e r e f o r e , can be improved by a p p r o p r i a t e l y .pa modifying e i t h e r one or both of these phases (2). The approaches aimed at reducing the flammability of polymer systems can be grouped i n t o the f o l l o w i n g three categories: Vapor Phase. In the vapor-phase approach, a f l a m e - r e t a r d a n t or modified polymer unit releases upon heat exposure a chemical agent that i n h i b i t s f r e e - r a d i c a l reactions i n v o l v e d i n the flame formation and propagation. For example, HX (where X i s halogen), produced by


Flammability of Polymers

KHANNA AND PEARCE

HEAT


(Heat of Combustion Produces Fuel Through More Material Decomposition)

FUEL

+

OXYGEN

(External Flame Initiates Polymer Decomposition Which Produces the Fuel Required for Combustion) Figure 1.

(Vapor Phase Oxidation Reactions of the Fuel Generate Heat)

Schematic representation of the f l a m m a b i l i t y c y c l e

PROPAGATION C H + HO •

• CH - +H 0

1

CH + H •

•

CH * + H

2

CH - +0-

•

CH 0 +H •

•

CHO* + C H

4

3

4

3

CH 0 + CH 2

3

2

3

2

3

2

4

4

C H 0 + HO"

— •

CHO- + H 0

5

CH 0 +H •

— •

CHO- + H

6

CH 0 + 0 •

•

CHO* +H0-

7

CHO-

•

CO + H •

8

CO + HO-

•

C 0 + H-

9

• HO- +0-

10

• RH + M*

11

2

2

2

2

2

2

CHAIN BRANCHING H- + 0

2

TERMINATION H- +R- +M

R e a c t i o n s 1-11



308


the p y r o l y s i s of a h a l o g e n - c o n t a i n i n g o r g a n i c m a t e r i a l i n the polymer, a c t s as a r a d i c a l scavenger and r e p l a c e s l e s s r e a c t i v e halogen atoms with a c t i v e chain c a r r i e r s (3, 4): H* + HX

> H + X*

HO' + HX

> H 0 + X-

2

2

Halogenated flame r e t a r d a n t s such as c h l o r i n a t e d p a r a f f i n s , c h l o r o c y c l o a l i p h a t i c s , and c h l o r o - and bromoaromatic a d d i t i v e s , which are commonly employed i n f l a m e - r e t a r d i n g p l a s t i c s , are postulated to function p r i m a r i l y by a vapor-phase f l a m e - i n h i b i t i o n mechanism. Flame retardation could be implemented by incorporating f i r e - r e t a r d a n t a d d i t i v e s , impregnating the material with a flamer e t a r d a n t substance, or u s i n g f l a m e - r e t a r d a n t comonomers i n the polymerization or grafting. Condensed phase. In condensed-phase m o d i f i c a t i o n , the flame r e t a r d a n t a l t e r s the decomposition chemistry so t h a t the t r a n s formation of the polymer to a char residue i s favored. This r e s u l t c o u l d be a c h i e v e d w i t h a d d i t i v e s t h a t c a t a l y z e char r a t h e r than flammable product formation or by designing polymer structures that f a v o r char f o r m a t i o n . C a r b o n i z a t i o n , which occurs at the c o s t of flammable product formation, a l s o s h i e l d s the r e s i d u a l substrate by i n t e r f e r i n g w i t h the access of heat and oxygen. Phosphorus-based a d d i t i v e s are t y p i c a l examples of flame retardants that could act by a condensed phase mechanism. M i s c e l l a n e o u s . These approaches i n c l u d e d i l u t i o n of the polymer w i t h nonflammable m a t e r i a l s ( f o r example, i n o r g a n i c f i l l e r s ) , incorporation of materials that decompose to nonflammable gases such as carbon d i o x i d e , and f o r m u l a t i o n of products t h a t decompose e n d o t h e r m i c a l l y . A t y p i c a l example of such a flame r e t a r d a n t i s aluminum oxide trihydrate (A^Og.SHoO). This type of material acts as a thermal s i n k to i n c r e a s e the neat c a p a c i t y of the combusting system, lower the polymer s u r f a c e temperature v i a endothermic events, and d i l u t e the oxygen supply to the flame, thereby reducing the f u e l concentration needed to sustain the flame. Of the s e v e r a l test methods for e v a l u a t i n g the burning behavior of different polymers, the l i m i t i n g oxygen index (LOI) (5) w i l l be used here to i l l u s t r a t e the r e l a t i v e f l a m m a b i l i t y of m a t e r i a l s . This test measures the minimum concentration of oxygen i n an oxygenn i t r o g e n atmosphere that i s necessary to i n i t i a t e and support a flame. Polymer Structure and Flammability The f l a m m a b i l i t y of a p a r t i c u l a r polymer depends m a i n l y upon i t s structure. The amounts of char and incombustible gases formed upon thermal decomposition determine to a great e x t e n t the flame r e s i s t a n c e of a p o l y m e r i c m a t e r i a l . A m a t e r i a l w i t h LOI 26). The lower flammability i n t h i s case i s a t t r i b u t e d to factors such as r e d u c t i o n i n the amount of c o m b u s t i b l e v o l a t i l e s , h i g h energy requirements for continuous fuel generation, and i n s u l a t i n g effects of the resultant char. C l a s s I I I c o n s i s t s of h a l o g e n - c o n t a i n i n g polymeric materials. Some of them form a s m a l l char r e s i d u e , o t h e r s form none at a l l . These polymers are i n h e r e n t l y flame r e t a r d a n t because halogen r a d i c a l s a c t as r a d i c a l scavengers i n the vapor phase and, t h e r e fore, i n h i b i t combustion, as described e a r l i e r . A l s o the s p l i t t i n g o f f of noncombustible gases such as H C l , Hf, and C 0 F 4 s e a l s the polymer surface from the combustion a i r and i s thereby p a r t l y r e s p o n s i b l e f o r the flame r e t a r d a n c y . A l l these f a c t o r s thus influence the i n t e r a c t i o n between p y r o l y s i s and i g n i t i o n .



1

Out

2

Nylon 66

Polyethylene Terephthalate

Cellulose

Polystyrene

2

2

2

f

n

J

"

C00-CH -CH -

CH,0H

H OH

n

f H N -(CH -HNC0-(CH^C0+ 2)6

H OH

+0C - ^ Q ^ -

-

2

-CH 0H

2

rCH -CH4

2

+CH -CH=CH—CH 4

n

Polybutadiene

2

+CH -CH 4

Polyethylene 2

Structure

Name

POLYMER

n

Degradation Products

2

2

4

2

4

6

6

2

2

Saturated and Unsaturated Hydrocarbons

2

H 0, C0 , Cyclopentanone, Traces of

and More Complex Chain Fragments

2

C H , H 0, CH , C H , Terephthalic Acid

Acetaldehyde Major Product with C0 , CO,

Tar Containing Principally Levoglucosan

H 0 and Small Amounts of C0 , CO and a

Remainder Dimer, Trimer and Tetramer

About 41% Monomer, 2% Toluene,

Saturated and Unsaturated Hydrocarbons

About 2% Monomer In Addition to

Unsaturated Hydrocarbons from C^-Cto

Continuous Spectrum of Saturated and

Table I. Effect of Structure on Polymer Flammability Properties


21.5

20.6

19.9

18.3

18.3

17.4


III

II

Polytetrafluoroethylene

Polyvinylidene Chloride

Polyvinyl Chloride

Polyvinyidene Fluoride

Polybenzimidazole

Kynol®

Nomex®

Polycarbonate

+

0 "

H

N

N

2

9" "

ι

F

I

C—

I

F

Cl

Cl

_ F

" F F

F

-A—i 1 ï H Cl

"H

H

-M-

H

~H ι -C

H

CH, H

n

n

n

n

„ . 0 C - ^ C 0 4

\^CH -|^N-CH^V

^ .

CH,

N

- o/Ôy>—c—/oVo-c-

CHj

11

2

4

2

29.8

29.4

2

Fragments

3

6

>95% Monomer, 2 - 3 % C F , No Larger

High Yields of HCI

and Aromatic Hydrocarbons

Quantitative Yields of HCI, Aliphatic

Involatile at 25^, Some Carbonization

35% HF and High Yields of Products

3

May Include NH ,H etc.

Very High Char Yield, Small Volatiles

(76%). Traces of Phenol, Cresol. Benzene

95.0

60.0

47.0

43.7

41.5

High Char Yield; Volatiles Comprise Xylene 35.5

3-Cyanobenzoic Acid

H 0,1.3-Dicyanobenzene,

High Char Yield ; Volatiles Contain CO,, CO,

Much Char

Minor Products CO, CH , 4-Alkyl Phenols;

Major Products C0 , Bisphenol A;


3· f

2"

η m

m >

D

ζ > >

•ζ

>

2C



312

I

0

I

I

I

1

1

20

40

60

80

100

CHAR RESIDUE, CR(%) Figure 2. C o r r e l a t i o n between oxygen index and char residue. 1. polyformaldehyde; 2. polyethylene, polypropylene; 3. polystyrene, polyisoprene; 4. nylon; 5. c e l l u l o s e ; 6. p o l y ( v i n y l a l c o h o l ) ; 7. PETP; 8. p o l y a c r y l o n i t r i l e ; 9. PPO; 10. polycarbonate; 11. Nomex; 12. p o l y s u l f one; 13. Kynol; 14. polyimide; 15. carbon. (Reproduced with permission from Ref. 6. Copyright 1975 IPC Business Press.


14.

KHANNA AND PEARCE


313

Flame Retardation of Polymers


Much l i t e r a t u r e discusses the flame retardation of various polymeric materials (10-15). The techniques of reducing the flammability of polymers, i n p r i n c i p l e , are based on one or more of the t h r e e fundamental approaches described e a r l i e r . This section deals with the c o n c e p t o f s y n e r g i s m and i t s a p p l i c a t i o n i n r e d u c i n g f l a m m a b i l i t y , s e l e c t i o n of f i r e - r e t a r d a n t a d d i t i v e s , and flame retarding some s p e c i f i c polymer systems. Synergism i n Flame Retardation. The effect of a mixture of two or more flame retardants may be a d d i t i v e , s y n e r g i s t i c , or antagonistic. Synergism i s the case i n which the effect of two or more components taken together i s greater than the sum of t h e i r i n d i v i d u a l effects. The concept of synergism i s very important i n f i r e r e t a r d a t i o n because i t can l e a d to e f f i c i e n t flame r e t a r d a n t s w i t h l e s s e x p e n s i v e polymer systems and minimal e f f e c t s on other d e s i r a b l e properties. One of the c l a s s i c i l l u s t r a t i o n s of synergism observed i n flame retardation i s the addition of Sb203 to halogen-containing polymers. The r e a c t i o n of a c h l o r i n e source w i t h SboOg produces SbOCl as an i n t e r m e d i a t e w i t h lower energy b a r r i e r , wnich then on thermal decomposition e v o l v e s S b C ^ , the a c t u a l flame r e t a r d a n t working by gas phase i n h i b i t i o n (16). The existence of phosphorushalogen and nitrogen-phosphorus synergisms has a l s o been suggested. Compounds based on the combinations of Sb-X (X i s halogen), N-P, and P-X have been used as flame-retardant a d d i t i v e s for thermoplastics. S e l e c t i o n of F i r e R e t a r d a n t s . The c h o i c e of flame r e t a r d a n t s depends on the nature of the polymer, the method of processing, the proposed s e r v i c e conditions, and economic considerations. Although the processing, s e r v i c e , and economic factors are impor.pa tant, the flame-retardancy p o t e n t i a l of an a d d i t i v e i s of primary importance, and t h i s factor can be r e a d i l y evaluated by thermal a n a l y s i s . Einhorn (17) has d e s c r i b e d the use of TGA i n s e l e c t i n g the a p p r o p r i a t e f i r e r e t a r d a n t s . The technique i n v o l v e s matching the degradation of candidate a d d i t i v e s with that of the polymer under consideration. F i g u r e 3 r e p r e s e n t s the TGA thermogram f o r a hypothetical polymer that e x h i b i t s a simple unimolecular degradation p r o c e s s . P o i n t A r e p r e s e n t s the r e g i o n of i n i t i a l d e c o m p o s i t i o n , and p o i n t B r e p r e s e n t s the t e m p e r a t u r e o f maximum r a t e o f degradation. F i r e retardants are screened so that a material having t h e r m a l c h a r a c t e r i s t i c s s i m i l a r to those i n F i g u r e 3 i s s e l e c t e d . The e f f i c i e n c y of matching the degradation curve of the polymer with the v o l a t i l i z a t i o n or d e g r a d a t i o n c h a r a c t e r i s t i c s of the f i r e r e t a r d a n t has been c i t e d as the key to e f f e c t i v e flame retardancy (18). I f the f l a m e - r e t a r d a n t a d d i t i v e possesses low t h e r m a l s t a b i l i t y compared to t h a t of the polymer, i t w i l l be l o s t before i t s f u n c t i o n i s needed; i f the a d d i t i v e has g r e a t e r s t a b i l i t y , i t may be i n t a c t at the time i t s f u n c t i o n i s needed. The use of more than one fire-retardant a d d i t i v e depends on the thermal degradation c h a r a c t e r i s t i c s of the flammable substrate. S e v e r a l manufacturers or s u p p l i e r s of flame r e t a r d a n t s have l i s t e d TGA w e i g h t - l o s s data to f a c i l i t a t e the s e l e c t i o n of appropriate fire-retardant a d d i t i v e s . These data are r e p o r t e d i n Table I I only for those a d d i t i v e s with i d e n t i f i a b l e structures.




314

Figure 3.

TGA thermogram for a hypothetical polymer. (Reproduced with permission from Ref. 17. Copyright 1971 National Academy of Sciences.)


14.

KHANNA AND PEARCE


315

Table II. TGA Weight Loss Data for Various Flame Retardants

TGA weight loss at(°C)


Flame retardant Alumina, hydrated Analine,2,4,6-tribromoBarium metaborate Benzene, hexabromoBenzene, pentabromoethylBiphenyl, hexabromoBiphenyl, octabromoBisphenol-A, tetrabromoBisphenol-A, tetrabromo-, bis(methylether) Bisphenol-A, tetrabromo-, bis(2,3-dibromopropylether)Bisphenol-A, tetrabromo-. bis(2-hydroxyethyl ether)Bisphenol-A, tetrabromo-, bis(allyl ether)Bisphenol-A, tetrabromo-, bis(2,3-dibromopropyl carbonate)Bisphenol-A, tetrabromo-, diacetate Bisphenol-S, tetrabromo-, butenediol, dibromoCarbonate, bis(2,4,6-tribromophenyl)Cyclododecane, hexabromoCyclohexane, pentabromo-. chloroDiphenylamine, decabromoDiphenyloxide, pentabromoDiphenyloxide, octabromoDiphenyloxide, decabromoMethylenedianiline, tetrabromoNeopentyl alcohol, tribromoNeopentyl glycol, dibromoPhosphate, tris(2-chlorothyl)Phosphate, tris(/3-chloropropyl)Phosphate, tris(2,3-dibromopropyl)Phosphate, tris(dichloropropyl)Phosphonate, bis(2-chloroethyl)-vinylPhosphonate, diethyl N,/V-bis-(2-hydroxyethyl) amino methylPhosphonate, dimethyl methylPhosphonium bromide, ethylene bis,-tris(2-cyanoethyl)Phosphonium bromide, tetrakis(2-cyanoethyl)-

1%

5%

— 121 200 232 180

156 350 265 217

— —

— —

245 244 284 284 220

—

247 240 227

284 280 315 322 245

—

278

—

274 317

287 230 200 277 247 325 357

— —

— —

115 __

135 170 155 270 210 135 165 60 — —

— 175 260

—

— 215

— — — 250 253

" Compiled from various product data literature.


10% 290 174 1000 280 232 299 336 298 296 322 337 261 328 283

—

308 255 235 351

—

340 373 268 160 150 190 175 285 225 155 180 75 285 307

316



Flame Retardation of P o l y m e r i c M a t e r i a l s . The s u i t a b i l i t y of a p a r t i c u l a r flame-retarding method i s often determined by the nature of the polymer type (e.g., thermoplastic or thermoset). A b r i e f and general d i s c u s s i o n r e g a r d i n g the flame r e t a r d a t i o n of thermoplastics, thermosets, elastomers, and f i b e r s f o l l o w s . Thermoplastics. Many flame-retardant chemicals have been developed f o r use i n t h e r m o p l a s t i c s . Most of these flame r e t a r d a n t s are of the a d d i t i v e t y p e , u s u a l l y h a l o g e n - and/or phosphorus-based compounds. Some examples of the f i r e - r e t a r d a n t a d d i t i v e s used i n p o l y o l e f i n s (12) are h e x a b r o m o c y c l o d o d e c a n e , o c t a b r o m o d i p h e n y 1, hexabromobipheny1, c h l o r o w a x , c h l o r i n a t e d t r i p h e n y l , c h l o r e n d i c a c i d , tris(tribromophenyl)phosphite, and t r i s ( d i b r o m o p r o p y l ) p h o s phate. T o u v a l (19) has d e s c r i b e d the use of antimony-halogen s y n e r g i s t i c mixtures as flame retardants for polyethylene and p o l y p r o pylene. Red phosphorus used as a flame retardant for high-density polyethylene (HDPE) has been described by Peters (20). He suggested that red phosphorus increased the thermo-oxidative s t a b i l i t y of HDPE by s c a v e n g i n g oxygen at the polymer s u r f a c e , thus r e t a r d i n g the o x i d a t i v e degradation processes. This type of i n t e r a c t i o n between HDPE and phosphorus i n the condensed phase l e d to a r e d u c t i o n i n polymer f l a m m a b i l i t y , as expected. I n c o r p o r a t i n g halogens e i t h e r as a d d i t i v e s or as p a r t of the polymer i s the most common technique of flame retarding polystyrene. P r e f e r a b l y , the halogen s h o u l d be p a r t of the p o l y m e r i c c h a i n because the halogenated a d d i t i v e s tend to be noncompatible and can cause s e v e r a l other side effects. P r i n s et a l . (21) d e s c r i b e d the lower f l a m m a b i l i t y of p o l y bromostyrene r e l a t i v e to t h a t of p o l y s t y r e n e . On the b a s i s of thermal a n a l y s i s experiments, they suggested that bromine i n h i b i t e d most of the o x i d a t i v e chain reactions, and thus the combustion was not supported (vapor-phase mechanism). Khanna and Pearce (16) and Brauman (22) demonstrated that polystyrene could be flame retarded by a p p r o p r i a t e l y modifying i t s s t r u c t u r e w i t h s u b s t i t u e n t s t h a t promote the char y i e l d of the system (condensed-phased mechanism). P o l y ( v i n y l c h l o r i d e ) (PVC) has a high l e v e l of c h l o r i n e and as a r e s u l t i s considered i n h e r e n t l y flame retardant. However, i n many circumstances further improvement i n flammability reduction of PVC i s desired. An e f f i c i e n t way to enhance flame retardation of PVC i s by the addition of Sb20g as a synergist. A d d i t i v e retardants based on halogen and/or phosphorus are a l s o employed. Sobolev and Woycheshin (23) pointed out that the burning rate of PVC c o u l d be lowered by u s i n g Al 0o*3Ho0 as a f l a m e - r e t a r d a n t filler. 9

Z

Thermosets. F i r e retardancy i n thermosetting polymers i s achieved l a r g e l y by the use of r e a c t i v e f i r e r e t a r d a n t s because the common f i r e - r e t a r d i n g a d d i t i v e s l a c k permanence. The f l a m m a b i l i t y of thermosetting m a t e r i a l s can be reduced by the additions of inorganic f i l l e r s and/or r e a c t i v e flame retardant components. Flame-retardant v i n y l monomers or other c r o s s - l i n k i n g agents are a l s o f r e q u e n t l y employed.



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A large number of halogen-containing r e a c t i v e d i o l s , p o l y o l s , a n h y d r i d e s , and other f u n c t i o n a l groups c o n t a i n i n g i n t e r m e d i a t e s have been used to produce f l a m e - r e s i s t a n t unsaturated p o l y e s t e r resins, Flame-retardant p o l y e s t e r r e s i n s have been made by u s i n g bromostyrene as p a r t i a l replacement of styrene for c r o s s - l i n k i n g (21). As i n p o l y e s t e r r e s i n s , r e a c t i v e halogens c o n t a i n i n g f i r e r e t a r d a n t c h e m i c a l s are most often used i n epoxy m a t e r i a l s . Tetrabromobisphenol A i s perhaps the most widely used component for flame-retarding epoxy resins. Nara and Matsuyama (24) and Nara et a l . (25) described the thermal degradation and flame retardance of t e t r a b r o m i n a t e d b i s p h e n o l A d i g l y c i d y l ether compared to the nonbrominated structure. Their r e s u l t s indicate that bromine acts by vapor-phase as w e l l as condensed-phase mechanisms of flame inhibition. F i r e retardancy i n p o l y u r e t h a n e s i s p r i m a r i l y of commercial importance i n foams. Foams are g e n e r a l l y flame r e t a r d e d by the i n c o r p o r a t i o n of h a l o g e n s , phosphorous, and/or n i t r o g e n o u s compounds. Conley and Quinn (10) reviewed the f i r e r e t a r d a t i o n of various polyurethanes. E l a s t o m e r s . Many a p p l i c a t i o n s of rubbers such as t i r e s , g a s k e t s , and washers n o r m a l l y do not r e q u i r e flame r e s i s t a n c e . When improvement i n flammability i s required, i t can be achieved by the a d d i t i o n of h a l o g e n - c o n t a i n i n g m a t e r i a l s , phosphorus compounds, o x i d e s of antimony, and combinations of these m a t e r i a l s . Rubbers c o n t a i n i n g c h l o r i n e and s i l i c o n atoms, f o r example, Neoprene and S i l i c o n e , have s e l f - e x t i n g u i s h i n g properties. Rogers and F r u z z e t t i (10) described the flame retardance of elastomers. Fibers. The flame r e t a r d a t i o n of f i b e r - f o r m i n g polymers i s g e n e r a l l y a c h i e v e d by the i n c o r p o r a t i o n of a d d i t i v e s based on h a l o g e n , phosphorus, and/or n i t r o g e n . Use of antimony oxide as a synergist with halogen i s a l s o common. The most common route f o r making c e l l u l o s i c f i b e r s flame retardant i s the use of c a t a l y s t s such as antimony t r i c h l o r i d e and phosphoric acid that enhance the formation of char. Because of the major r o l e of c e l l u l o s i c f i b e r s i n the t e x t i l e market, flame r e t a r d a t i o n of c e l l u l o s e has been the s u b j e c t of s e v e r a l r e v i e w s ( 1 0 , JJL, 15). Wool has been regarded as a r e l a t i v e l y safe f i b e r from the flammability point of view. However, i t could be flame retarded to a higher degree i f required. Hendrix et a l . (26) suggested a large improvement i n f i r e resistance of wool by treatment with 15% H 3 P O 4 . Beck et a l . (27) showed t h a t weak a c i d i c m a t e r i a l s , such as b o r i c a c i d and dihydrogen phosphate, are e f f e c t i v e a d d i t i v e s f o r flame r e t a r d i n g wool by the condensed-phase mechanism ( i n c r e a s e d char residue). Retardation of the combustion of nylon f i b e r s has been reviewed by Pearce et a l . (10). S e v e r a l bromine and phosphorus compounds have been suggested to be e f f e c t i v e for flame retarding nylon 6, but none of them i s used i n practice on a reasonable s c a l e (12). The f l a m m a b i l i t y of p o l y ( e t h y l e n e t e r e p h t h a l a t e ) can be decreased by u s i n g bromo-compounds i n c o n j u n c t i o n w i t h compounds such as Sb203 s y n e r g i s t , red phosphorus, and t r i p h e n y l p h o s p h i n e



318


oxide. Flame retardation of poly(ethylene terephthalate) has been reviewed by Lawton and Setzer (10). Their review indicates that halogen and/or phosphorus compounds added to the melt prior to fiber spinning are useful flame retardants. In the past decade several high-temperature fibers have been made commercially available. These materials based on aromatic and heterocyclic structures are inherently flame resistant; those based on heterocyclic structures originate from the heavy carbonaceous char that rapidly forms on the surface exposed to a flame. The flammability of even high-temperature fiber-forming polymers can be improved. Durette, made by chlorination or oxychlorination of Nomex, a polymer of jn-phenyleneisophthalamide, exhibits much improvement in flammability properties over Nomex. Khanna and Pearce (28) described the reduction in flammability of aromatic polyamides by the use of halogen-substituted diamines. The mechanism of flame retardation in this case was reported to involve vapor phase as well as condensed phase. Burning resistance of these high temperature fibers can also be achieved by thermal treatments, which has been substantiated by Bingham and H i l l (29) for Kynol (cross-linked phenolics) and Durette fabrics. Hirsch and Holsten (30) reported that the fabric made from poly(m-phenylenebis(mbensamido)terephthalamide) could be stabilized thermally so that the product would not burn even if heated red hot in a gas flame. Such treatments are believed to involve cyclodehydration of the original polymer. Polymer flammability continues to be an important field of research even though it is reasonably well understood at present. Current efforts w i l l contribute to the design of safer polymeric materials of the future. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Fristom, R. M.; Westenberg, A. A. "Flame Structure"; McGrawH i l l : New York, 1965. Frisch, K. C. Int. J. Polym. Mater. 1979, 7, 113. Factor, A. J . Chem. Educ. 1974, 51, 453. Petrella, R. V. "Flame Retardant Polymeric Materials"; Lewin, M.; Atlas, S. M.; Pearce, Ε. M., Eds.; Plenum: New York, 1978; Vol. 2, p. 159. Fenimore, C. P. Martin, F. J. Mod. Plast. 1966, 44(3), 141. van Krevelen, D. W. Polymer 1975, 16, 615. Grassie, N.; Scotney, A. "Polymer Handbook"; Brandrup, J . ; Immergut, Ε. H., Eds.; Wiley: New York, 1975; Vol. II, p. 473. Williams, D. J. "Polymer Science & Engineering"; Prentice Hall: New Jersey, 1971; p. 32. Pearce, Ε. M.; Khanna, Y. P.; Raucher, D. "Thermal Analysis in Polymer Characterization"; Turi, Ε. Α., Ed.; Academic: New York, 1981; Chap. 8, p. 793. "Flame Retardant Polymeric Materials"; Lewin, M.; Atlas, S. M.; Pearce, Ε. M., Eds.; Plenum: New York, 1975; Vol. 1. "Flame Retardant Polymeric Materials"; Lewin, M.; Atlas, S. M.; Pearce, Ε. M., Eds.; Plenum: New York, 1978; Vol. 2. "Flame Retardancy of Polymeric Materials"; Kuryla, W. C.; Papa, A. J., Eds.; Dekker: New York, 1973; Vol. 1&2.


14. KHANNA AND PEARCE 13. 14. 15. 16.


17. 18. 19.

20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.


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"Flame Retardancy of Polymeric Materials"; Kuryla, W. C.; Papa, A. J., Eds.; Dekker: New York, 1975; Vol. 3. "Flame Retardancy of Polymeric Materials"; Kuryla, W. C.; Papa, A. J., Eds.; Dekker: New York, 1978; Vol. 4. "Flame Retardancy of Polymeric Materials"; Kuryla, W. C.; Papa, A. J . , Eds.; Dekker: New York, 1979; Vol. 5. Khanna, Y. P.; Pearce, Ε. M. "Flame Retardant Polymeric Materials; Lewin, M.; Atlas, S. M.; Pearce, Ε. M., Eds.; Plenum: New York, 1978; Vol. 2, Chap. 2. Einhorn, I. N. Reprint from "Fire Research Abstracts and Reviews"; National Academy of Sciences: Washington, D.C., 1971; Vol. 13, p. 3. Pitts, J. J. J. Fire Flammability 1972, 3, 51. Touval, I. "Flame Retardants for Plastics"; Presented at Flame Retardant Polymeric Materials course, Plastics Institute of America, Stevens Institute of Technology, Hoboken, New Jersey, 1975. Peters, Ε. N. J. Appl. Polym. Sci. 1979, 24, 1457. Prins, M.; Marom, G.; Levy, M. J . Appl. Polym. Sci. 1976, 20, 2971. Brauman, S. K. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1129. Sobolev, I.; Woycheshin, E. A. in "Flammability of Solid Plastics"; Hilado, C. J., Ed.; Fire and Flammability Series; Technomic: Westport, Connecticut, 1974; Vol. 7, p. 295. Nara, S.; Matsuyama, K. J. Macromol. Sci., Chem. 1971, 5(7), 1205. Nara, S.; Kimura, T.; Matsuyama, K. Rev. Electr. Commun. Lab. 1972, 20, 159; Chem. Abst. 77, 75562h. Hendrix, J. E.; Anderson, T. K.; Clayton, T. J . ; Olson, E. S.; Barker, R. H. J. Fire Flammability 1970, 1, 107. Beck, P. J . ; Gordon, P. G.; Ingham, P. E. Text. Res. J. 1976, 46, 478. Khanna, Y. P.; Pearce, Ε. M. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 2835. Bingham, Μ. Α.; Hill, B. J. J. Thermal Anal. 1975, 7, 347. Hirsch, S. S.; Holsten, J. R. Polym. Prepr. 1968, 9, 1240.


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