Fire Retardants for Thermoplastics

21 Fire Retardants for Thermoplastics Phosphine Oxides, Phosphonic Acids, and Downloaded by NANYANG TECHNOLOGICAL UNIV on October 12, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch021

Phosphinic Acids ALLAN E. SHERR,1 HELEN C. GILLHAM, and HARVEY G. KLEIN Central Research Division, American Cyanamid Co., Stamford, Conn. Phosphine oxides, phosphonic acids, and phosphinic acids have been found to be flame retardants for various thermoplastic polymers. While there are many reasons for their effectiveness, we postulate that the acidity of the compounds is directly related to their activity and that the formation of polyphosphates (or phosphate glasses) is vital to the mechanism by which they function.

W / e have had a continuing interest inflameretardants, methods of test and mechanisms of action of such materials. Recently, we reported studies involvingfireretardant additives, syntheses of monomers, and the preparation of copolymers to achieveflameresistance (41). In addition, considerable synthetic work in phosphorus chemistry has been pursued at our Stamford Research Laboratories. Some of this has been reported by Grayson, Rauhut, Buckler, Wystrach, and co-workers. This chapter and the one following result from this background. In this paper we report the use of some phosphine oxides, phosphonic acids, and phosphinic acids to impartfireretardant properties to polymers. In addition, we postulate a mechanism by which these compounds behave asflameretardant agents. Fire Retardant Schemes

There are many techniques for imparting self-extinguishing characteristics to polymers (1, 2, 8,10,11, 32, 34, 36, 37, 38,43). The following is a partial list. 1

Present address: Organic Chemicals Division, American Cyanamid Co., Bound Brook,

N. J.

307

Platzer; Stabilization of Polymers and Stabilizer Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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308

STABILIZATION OF POLYMERS AND STABILIZER

PROCESSES

Inorganic Fillers. Smith (43) listed the physical addition of inor ganic fillers such as ammonium phosphate, ammonium bromide, ammo nium sulfamate, or antimony oxide as one such scheme. Borate salts also are used as inorganic fillers. Organic Additives. Boyer and Vajda have described the physical addition of organic fire retardants (5). Examples are phosphate esters, chlorinated waxes, halogenated phosphate esters, and/or halogen-con taining polymeric materials. Gouinlock et al. (18), among others, have discussed highly halogenated compounds. Polymer Structure Modification. Chemical modification of the poly mer structure includes the use of tetrahalophthalic acids or anhydrides, diallylphenyl phosphonate, brominated allyl phosphates, chlorendic an hydride, and tetrakishydroxymethyl phosphonium chloride. Inherently Stable Structure. Inherent stability of the polymer struc ture includes resins such as unplasticized poly (vinyl chloride) containing about 56% chlorine which is self-extinguishing and poly( vinylidene) chloride. Fire Retardant Tests There is a wide variety of procedures for measuring the selfextinguishing properties of plastic materials. These include A S T M Tests D568, D635, D757, D1692, and D1433; Military Specification L P 406b Test 2023.1; A S T M Test E84 (Underwriters Tunnel Test); etc. Summa ries and discussions of these tests have been presented by Sauber (40), Feuer (14), and Hammerl (21). Briber has questioned the significance of fire retardant tests for plastics (6). Thus, the value of the individual tests is a subject of considerable debate, particularly regarding the rela tionship of the test to the actual occurrence and prevention of fires. Preparation of Phosphine Oxides Generally, the phosphine oxides were prepared by mild oxidation of the corresponding phosphine (26). This was accomplished by refluxing an alcoholic solution of the phosphine in the presence of oxygen and/or hydrogen peroxide. The resultant phosphine oxide was isolated by filtration or solvent evaporation.


21.

SHERR E T A L .

309

Phosphine Compounds

Feshchenko and Kirsanov more recently have reported the reaction of red phosphorus with iodine and an alcohol to prepare tertiary phosphines in 85-90% yield (13). R O H + I + Ρ —» R ,P=0 + RI 2


Preparation

:

of Ρ bos phonic Acids

Phosphonic acids can be prepared in yields of 50-100% by the types of reactions shown in the equations below (22, 27, 28, 29, 30, 31, 41, 44). Ο

II

RPHo + H 0 2

a

+P 0 2

^ RP(OH)

2

* - °,

2 5

2

/T^P(OH)

0O 325 c

autoclave

2

\ — /

Ο /7~\V

CH

£j)

3 H

/7~vV

(1) A1C1,

+PC, _ _

C

H

s

'I

^ ^ - P ( O H )

2

The Arbuzov reaction is frequently used to prepare phosphonic acids as shown below (22). Ο CH C1 + P ( O C H ) 2

2

5

HCl

• ^^-CH P(OC H )

3

2

Ο /—\ II (/ \ ) — C H P ( O H )

W

Preparation

of Phosphinic

2

a

n

2

2

Acids

The reaction of secondary phosphines with hydrogen peroxide in a suitable solvent is a convenient method for preparing the phosphinic acids (27). Yields are usually from 50 to 90% of theoretical.

\

P H + HoOo

,

\"

P—OH


310


PROCESSES


Burning Test The synthesized phosphorus compounds were evaluated as follows. The additive was added gradually to the polymer fused on a two-roll mill at 170°-174°C. After addition, polymer sheets were taken off the mill and put back on the mill endwise. Several such passes were made until the sample was thoroughly mixed. The specimen was removed from the mill i n thin sheets and, while hot, cut into small pieces. The polymer was compression molded at 700 p.s.i.g. and a temperature of ca. 155°C. into a 6 X 6-inch sheet of about 0.045-inch thickness. This sheet was cut into the 5 X 1/2 X 0.045-inch specimens for burning i n the modified D635 test. The sample was initially evaluated with 2 5 % additive. If the compound was effective, lower concentrations were used until the additive would not confer fire retardant activity, or until the supply of additive was exhausted. W i t h poly (methyl methacrylate), P M M A , cast samples also were prepared. A slight modification of A S T M Test D635-56T appeared suitable for use i n our studies to obtain preliminary evaluations of new phosphorus compounds. Our deviations were to use a 5-inch specimen, 0.5-inch wide, and 0.045-inch thick instead of the 0.05-inch thickness prescribed. W e omitted the use of a wire gauze under the specimen. Effective Compounds In our studies we found that phosphonic acids (16), phosphinic acids (25), and phosphine oxides (17) are additives capable of imparting fire retardant properties to thermoplastic polymers. Tables I and II pre sent data for some of these compounds when added to polyethylene or to poly (methyl methacrylate). The concentration reported is not necessarily the lowest effective concentration for the additive i n the polymer. These additives also were effective in other thermoplastic polymers such as polystyrene, impact polystyrene, polypropylene and A B S . The compounds were completely compatible with the polymers. Reported Mechanisms of Action The mechanism of action of flame retardants i n thermoplastic mate rials (polyethylene, polypropylene, polystyrene, cellulosics, P M M A , etc.) is unknown and is certainly quite complex. Broido (7) presented a good example i n the difficulties of explaining how fire retardants work. H e found that materials which were most effective i n preventing flaming combustion of cellulose were also effective i n causing sugar cubes to support flame! Generally, fire retardant activity is attributed to one of the following mechanisms: (a) Modification of the mechanism of decomposition or change i n the rate of decomposition of the polymer. Broido (7) uses this explana-


21.

Phosphine Compounds

SHERR E T A L .

Table I.

Phosphine Oxides as Flame Retardants

Compound

~C^

( C 1

C H

2)3

311

^

Melting Point, °C.

Empirical Formula

Polymer"

ConcentraHon, %

185-188

0 Η 01βΡΟ 1Γ)

ΡΕ

15

C H ,PO

PMMA

5

21

Cl Downloaded by NANYANG TECHNOLOGICAL UNIV on October 12, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch021

156 (\=/) (C=^)

3

F

°

=

2

C H

2

156 C H

2CH NH 2

2

1 8

1 ;

C H PO 1 8

PE

i n

g

C H NPO

166-167

C H PO

103-105

C H N PO

1

5

i n

PMMA

1 8

15 15

O (f %\ 2 Ρ

1 8

2 1

PMMA

15

PMMA

10

O Ρ (CH CH CN) 2

2

2

1 2

9

2

O H N C CH CH 2

2

Ρ (CH.,CH CN)

2

2

Il

11

Ο

O 2

3

(HOOCCH,CH )

2

2

2

2

//

(/

_CH ) 2

%)

3

—CH ) 2

2

F T

1 4

14

2

PMMA

15

PMMA

15

2

·

164 2

O (HOOCCH CH )

C H N.P0

ft

(H N C C H C H ) 2

160-

2

203-206

C H N P0

P=0

155-156

C H ,P0

7

PE

15

Ρ H Il O

127-130

C H P0

5

PE

15

P=0

212-214

C H PO

PE

15

105.5-107.5

C H PO

PE

10

3

P=0

Ρ H

F T

N

E

2 1

1 4

1 8

3

I R

N

2 1

1 R

4

O \_/)

2

P

H

72-75

C H PO 1 2

2 8

PE

O "PE = Polyethylene; PMMA = Poly(methyl methacrylate).


15

312

STABILIZATION OF POLYMERS AND STABILIZER PROCESSES

Table II.

Phosphonic and Phosphinic Acids as Flame Retardants Melting Point, °C.

Compound / ^ _ C H

2

Ρ (OH)


-CH

2

Polymer

ConcentraHon, %

PMMA

2.5

a

2

II Ο

Q

Empirical Formula

166

Ρ (OH)

C H PO

188M89

2

7

9

a

7

9

3

C H C1P0 7

8

3

PE

15

II

ο

Cl ^J)-P

(OH)

2

1

6

C H P0

2

e

7

PMMA

3

2.5

Ο Q-P N0

H

(OH),

i

5

(

m

5

4

C

e

H

e

N

p

0

PMMA

5

15

Ο

2

3

C - ^ - P

(OH),

189

C H P0

g o

_

9 4

CH C1P0

8 0

_

8 2

CH C1 P0

3

144-145

C H P0

2

PE

5

lSPi-192

C

2

PE

15

7

9

3

PMMA

15

PMMA

5

PMMA

15

Ο C1CH Ρ ( O H ) · H 0 2

2

2

6

4

Ο C1 C Ρ ( O H ) 3

2

2

3

Ο ({f})

2 J OH

1 2

2 3

Ο

(TS\

2 Ρ OH

1 2

H P0 n

Ο α

Ρ Ε = Polyethylene; PMMA = Poly (methyl methacrylate).


21.

SHERR E T A L .

Phosphine Compounds

313

tion as have Gruntfest and Young (19) i n discussing flame retardants for P M M A .


( b ) Retardation of combustion i n the vapor phase, including cooling of the flame by radiation from carbon particles. (c) Boyer discussed insulation of the substrate from the flame (4). Amosov (1) and Broido (7) mention isolation of the fuel from oxygen or dilution of oxygen with nonflammable gases. Bubbling, foaming, or evolution of nonflammable gases such as ammonia, carbon dioxide, nitro gen or water isolate the fuel from oxygen. Decomposition products may provide nonflammable coatings which fuse to the surface, usually as glassy or foamy coatings and insulate the substrate from the flame. Formation of stable chemical compounds with the fuel prevent access of oxygen. Amosov (1) reports that phosphonic acid and/or phosphates form stable esters with cellulose. (d) Another possible mechanism is to increase the specific heat of the surface layer of the substrate. (e) Gruntfest (19) and Broido (7) discuss the reduction of the flame temperature as a route to fire retardance. O p p (35) gives as an example a mixture of 10% petroleum ether, 4 0 % carbon disulfide and 50% carbon tetrachloride which w i l l burn when ignited but w i l l not kindle other articles with which it comes i n contact. The combustible liquids burn, but the noncombustible carbon tetrachloride evaporates rapidly and keeps the temperature below the fire point of other substances. (f) Gruse (20) and Amosov (1) describe the breaking of the chain reactions i n the flame; Rosser (39) has postulated the following reactions for combustion and inhibition of a hydrocarbon flame where R H is any hydrogen containing species i n the flame. The reduction of H O - radicals with H B r slows the exothermic propagation reaction. HO-+CO->C0

+H-

Exothermic propagation

(1)

+ Ο

Chain branching

(2)

HO- + HBr -> H O H + Br-

Inhibition

(3)

Br-

Regeneration

(4)

Η-

2

+Oo -»ΗΟ·

+ HR -> HBr + R-

(g) Rosser et al (39) attribute action to thermal decomposition of a free radical initiator, accelerating the breakdown of polymer, and pro moting reaction between polymer fragments and halogen-œnteming materials. This leads to a delay i n loss of halogen from the polymer mass. Fenimore and Jones (12) report the first of their investigations into the modes of inhibiting polymer flammability. They show that chlorine substituted i n polyethylene inhibits by affecting the pyrolysis of the con densed phase, but the pair of reactants, antimony plus a little chlorine, poisons the flame. Bromine is more effective than chlorine because it also poisons the flame. They speculate that halogen seems necessary to vaporize Sb from S b 0 . 2

3


314


PROCESSES

W h i c h of the above is responsible for the effectiveness of a particular agent is a matter of conjecture at present It seems likely that a combina tion of mechanisms is required for fire retardant activity!


Mode of Action of Phosphine Oxides, Phosphonic Acids, and Phosphinic Acids W e believe that the activity of the phosphine oxides, the phosphonic and the phosphinic acids is related to the acidity of the compounds, as well as the thermal stability of the carbon-phosphorus bond i n the com pounds and of the phosphorus-oxygen bond of the derived acids. Collins (9) proposed that phosphine oxides are converted to phos phinic acids and phosphonic acids. These, in turn, produce phosphoric acids. W e suggest that the phosphoric acids then are decomposed into polyphosphates, (P0 )./~ and exist as stable phosphate glasses. The thermal stability of the phosphate glass [as well as the stability of the oxides and phosphonic or phosphinic acids (15)] enables it to shield the polymer from the flame by formation of a continuous glassy coating. Thus, the fuel ( polymer ) is isolated from oxygen. Secondary phosphine oxides thermally disproportionate into second ary phosphines and phosphinic acids ( 3 ) . The phosphine can be reoxidized to the phosphine oxide and continue this cycle. 3

RoPH + R P

2R P = Ο

2

2

I

Η

^ -OH

Dicyclohexylphosphine oxide would thermally disproportionate into dicyclohexylphosphinic acid and dicyclohexylphosphine. W e found that both the phosphine oxide, m.p. 7 2 ° - 7 5 ° C , at 15%, and the phosphinic acid, m.p. 144°-145°C., at 5 % , i n polyethylene are flame retardants. Hudson (23) has suggested that tertiary phosphine oxides form phosphinic acids on heating. Such acids

R

3

P

=

R P ^ °

0

™

2

,

^ O H

R P ^ ° ^(OH)

2

can be further oxidized to the phosphonic acid and then to the phosphoric acid (9). W e believe the phosphoric acid next forms a polyphosphate * H P 0 - + (P0 )/- + x H 0 2

4

3

2

Note also the formation of water, which would aid flame retardance.


21.

SHERR E T A L .

Phosphine Compounds

315


The acidic nature of the phosphorus compounds (24) alters the course of the pyrolysis of certain polymers. Gruntfest and Young (19) previously postulated that with poly (methyl methacrylate) an acid func tions as a chain stopper or causes a primary alteration of the P M M A , such as crosslinking. They noted the formation of high yields of dimethyl CH,

CH,

-TcHo—cJ

-fcH —cl

L

L

\ n J

2

n

COOCH,

2CH,OH

+ CHoOH

|J

COOH

-

CH,OCH,+ H 0 2

CHo

CH,

COOH

0=C

CH,

C=0

CH, I

Ί

--CH —cl L I J c=o I 2

n

ο

I

Γ

c=o I "

~ *CH.>—C — —

L

ι -I ηι

ι

CH,

ether during pyrolysis. The ether may well arise by acid hydrolysis of the methacrylate, liberating methyl alcohol which dehydrates to the ether. The poly(methacrylic acid) then probably dehydrates to poly(methacrylic anhydride) and water, further helping to promote flame retardance.


316

STABILIZATION OF POLYMERS AND STABILIZER PROCESSES


Literature Cited

(1) Amosov, G. Α., Fire Res. Abstr. Rev. 7 (2), 136 (1965). (2) Bailey, W. J., U. S. Patent 3,334,064 (Aug. 1, 1967). (3) Bailey, W. J., Muir, W. M., Marktscheffel, F., J. Org. Chem. 27, 4404 (1962). (4) Boyer, Ν. E., PlasticsTechnol:8(11), 33 (1962). (5) Boyer, Ν. E., Vajda, A. E., SPE (Soc. Plastics Engrs.) Trans. 4, 45 ( 1964 (6) Briber, A. J., SPE Soc. Plastics Engrs. Tech. Papers 13, 1041 (1967). (7) Broido, Α., Science 133, 1701 (1961). (8) Carpenter, C. H., Mack, G. P., British Plastics 34, 541 (1961). (9) Collins, J. D., Chem. Ind. 1965, 1286. (10) Dahms, R. H., Petrol. Refiner 41 (3), 132 (1962). (11) Drake, Jr., G. L., Beninate, T. V., Guthrie, J. D., Am. Dyestuff Reptr. 50 (4), 27 (1961). (12) Fenimore, C. P., Jones, G. W., Combustion Flame 10, 295 (1966). (13) Feshchenko, N. G., Kirsanov, Α. V.,J.Gen. Chem. USSR 1966 (3), 36. (14) Feuer, S. S., Torres, A. F., Chem. Eng. 69 (7), 138 (1962). (15) Freedman, L. D., Doak, G. O., Chem. Rev. 57, 479 (1957). (16) Gillham, H. C., Klein, H. G., U. S. Patent 3,345,432 (Oct. 3, 1967). (17) Gillham, H. C., Sherr, A. E., U. S. Patent 3,341,625 (Sept. 12, 1967). (18) Gouinlock, Jr., Ε. V., Long, F. W., Creighton, S. M., Plastics Technol. 8 (12), 40 (1962). (19) Gruntfest, I. J., Young, Ε. M., Am. Chem. Soc. Div. Organic Coatings Plastics Chem. 21 (1), 113 (1961). (20) Gruse, A. B., Natl. Fire Protection Assoc. Quart. 53, 330 (1960). (21) Hammerl, A. J., Reinforced Plastics 2 (5), 22 (1963). (22) Harvey, R. G., De Sombre, E. R., "Topics in Phosphorus Chemistry," M. Grayson, Ed., pp. 57-112, Vol. 1, Interscience, New York, 1964. (23) Hudson, R. F., private communication. (24) Kabachnik, M. I., Mastrukova, Τ. Α., Shipov, A. E., Melentyeva, Τ. Α., Tetrahedron 9, 10 (1960). (25) Klein, H. G., Gillham, H. C., U. S. Patent 3,322,716 (May 30, 1967). (26) Kosolapoff, G. M., "Organic Phosphorus Compounds," pp. 99-120, Wiley, New York, 1950. (27)Ibid.,p. 137. (28) Ibid., pp. 121-170. (29) Kosolapoff, G. M.,J.Am. Chem. Soc. 74, 4119 (1952). (30) Kosolapoff, G. M., Huber, W. F.,J.Am. Chem. Soc. 69, 2020 (1947). (31) Lecher, Η. Z., Chao, T. H., Whitehouse, K. C., U. S. Patent 2,717,906 (1955). (32) Mack, G. P., "Modern Plastics Encyclopedia," p. 479, McGraw-Hill, New York, 1962. (33) Milks, J. E., Wystrach, V. P., Siegele, F. H., U. S. Patent 3,032,500 (1962). (34) "Modern Plastics Encyclopedia," p. 451, McGraw-Hill New York, 1966. (35) Opp, C. J., Official Digest 346, 840 (1953). (36) Parkyn, B., British Plastics 32, 29 (1959). (37) Roberts, C. W., SPE Trans. 3, 111 (1963). (38) Rockey, K. W., Plastics (London) 26 (283), 103 (1961). (39) Rosser, W. Α., Wise, H., Miller, J., "Seventh Symposium (International) on Combustion," Butterworth & Co., London, 1959. Platzer; Stabilization of Polymers and Stabilizer Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

21.

SHERR ET AL.

Phosphine Compounds

317

(40) Sauber, W. J., Patten, G. Α., Plastics World 18 (12) (1960). (41) Saunders, B. C., Stacey, G. J., Wild, F., Wilding, I. G. E., J. Chem. Soc. 1948, 699. (42) Sherr, A. E., Klein, H. G., J. Appl. Polymer Sci. 11, 1431 (1967). (43) Smith, J., Chem. Ind. 1965, 1289. (44) Van Winkle, J. L., Morris, R. C., U. S. Patent 2,874,184 (1959). 1967.


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Fire Retardants for Thermoplastics

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