Stabilization of Polymers and Stabilizer Processes

22 Fire Retardants for Thermoplastics

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Phosphonium Halides A L L A N E . SHERR, H E L E N C . G I L L H A M , and H A R V E Y G . K L E I N 1

Central Research Division, American Cyanamid Co., Stamford, Conn.

Mono- and diphosphonium halides have been found to be flame retardants for plastic materials. Their effectiveness can be related to the formation of various active phosphorus compounds, as well as to many of the postulated mechanisms forflameretardant action. The compounds are postulated to be effective because they decompose on ignition to thermally stable phosphine oxides or phosphonic acids which, in turn, are decomposed to continuous films of phosphate glass. In addition, the phosphonium halides form alkyl halides which cool the flame and/or form halogen acids which are flame retardants. Τ η the preceding chapter (19) we described the use of phosphine oxides, phosphonic acids, and phosphinic acids asflameretardants for thermoplastic materials. We also have found phosphonium halides to be effective flame retardants for plastics (5, 6). These compounds were either the monophosphonium halides,

A

• χ-

Ρ

or diphosphonium halides (top of p. 319). Present address: Organic Chemicals Division, American Cyanamid Co., Bound Brook, N. J. 1

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

22.

319

Phosphonium Halides

SHERR ET A L .

y Un +/ (CH ) P^


2

m

Surprisingly, these halides are compatible with a wide range of plastic materials while inorganic salts are not, and moisture does not appear to be damaging. The thermoplastic polymers we studied included the polyolefins as polyethylene and polypropylene; the polyacrylates and methacrylates as poly (methyl methacrylate ) ; styrene polymers including both clear and impact types; and acrylonitrile-butadiene-styrene ( A B S ) plastics. Fire retardance was evaluated by the D-635 procedure as described previ ously (19). Early in our studies, we found that ethylene-bis[tris(2-cyanoethyl)phosphonium bromide] (6) is an effective fire retardant for polyethylene. W h e n this compound showed flame retardant activity, a wide variety of other related compounds were synthesized and evaluated. Preparation of Monopbospbondum Halides The monophosphonium halides can be formed readily by the reaction of a phosphine with an alkyl halide. Usually the phosphine and the halide are heated i n a solvent such as butyl alcohol or acetonitrile. The phosphonium halide is isolated by filtration (if insoluble), by precipita tion with ethyl ether, or by evaporation of the solvent (10, 11, 12, 13, 14, 16, 18). Yields are from 50 to 100%. v

R

Ri—Ρ + R X

R

z

^P^

-

3

R

\ + ^

R

v-

Ri

2

3

Carboxyalkyl phosphonium compounds can be prepared by the fol lowing reaction. ^ ^ - P H

2

+ 3CH =CHCOOH

^ ^ ~

2

P

+

(

C

H

2

C

H

2

C

O

CH CH COO2

HC1

2

H 0 2

Or

P

+

(CH CH COOH) · Cl 2

2

3

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

O

H

)

2

320

STABILIZATION OF POLYMERS AND STABILIZER

PROCESSES

Preparation of Diphosphonium Halides The diphosphonium halides are prepared i n high yields (9, 12) i n a manner similar to the monophosphonium halides: R P + X ( C H ) X -> R P ( C H ) P R · 2X" 3

2

W

3

2

n

3


Grayson (12) gives details of the preparation of the ethylenebis[tris ( 2-cyanoethyl ) phosphonium bromide]. Experimental The phosphonium halides were incorporated into the thermoplastic polymers by the milling and molding technique described previously (19). The samples were tested for flame retardance using the modified A S T M D-635-56T test (19). Results Tables I and II list some of the many phosphonium compounds studied and indicate their effectiveness. M a n y of our evaluations were halted by limited supplies of the phosphonium halides. Ethylenebis[tris(2-cyanoethyl)phosphonium bromide] was effective at 1 % i n natural crystal polystyrene. As soon as the burner i n the D635 Flame Test was removed, the sample containing the flame retardant extinguished. Ethylenebis [tris ( 2-cyanoethyl ) phosphoium bromide] at 2 % i n pigmented crystal polystyrene gave a self-extinguishing polymer, and at 4 % i n natural impact polystyrene such samples were selfextinguishing. Effect of Phosphonium H olid e on Properties of Polymer Table III lists some of the physical properties of polymers which contain ethylenebis [tris (2-cyanoethyl) phosphonium bromide]. This addi tive caused an increase i n the dissipation factor and dielectric constant and lowered the dielectric strengths of polyethylene and poly (methyl methacrylate). The effects on mechanical properties were mixed. Obvi ously, lower concentrations of phosphonium halides would have less effect on mechanical and electrical properties. A t levels of 1^3% very little change i n properties would be expected. It was surprising that the phosphonium salts were compatible with such a range of polymers. W e did not observe any tendency for the phosphonium salts to plate out of or exude from the polymer. In a l l cases homogeneous blends were obtained.


22.

SHERR E T A L . Table I.

Monophosphonium Halides as Flame Retardants Melting Point, °C.

Compound

@

P (CH CH COOH) · Cl+

2

2

3


2

2

Empirical Formuh

Polymer

Concentration, %

PE

20

3

240-243 C H N»PI

PMMA

10

240-243 C H N P I 3

PE

10

183-186 C i H P I

PE

10

PE

10

PE

10

163-164 C H N P B r

PE

10

24.5-246 C H 0 P I

PE

10

1 B

( 0 ) » 3· Ι -

2 0

e

1B

1 0

CH.P

9

1 5

1 8

158-165 C H N P C l i e

9

3

2

2

5

1 7

3

C H NPBr

( C H ) P C H C H N ( C , H ) • Br" 4

b

176-178 C H O P C l 10

C H P ( C H C H C N ) · I3

321

Phosphonium Halides

l 8

2

4 1

3

a

CH=CH ( C H ) PCH CH —N 4

9

3

2

/

\

. N

2

1 8

3 e

2

I

ÇH CH» CH,

v

c

CH

S

ι

> r

3

3

° 1 2

CH

2 3

3

3

CH P

(O).-Br-

230-232 C i H P B r

PE

15

C H„P

(Q)»"

203-208 C o H P B r

PE

15

PE

15

PE

15

3

a

Q CO)

Br

C H P ( 0 ) 3 Cl2

s P C H C H O H · Br2

9

2

>280

1 8

20

C H PCl 2 r i

2 2

214-218 C H O P B r 2 0

2 0

2

* The concentrations reported are not necessarily the lowest level at which the specific additive is effective in the polymer shown. ' PE = polyethylene. PMMA = poly (methyl methacrylate).


322


Table II.

Diphosphonium Halides as Flame Retardants Melting Point, °C.

Compound

(NCCH CH ) PCH CH P2

2

3

2

2

>

2

8

Q

Empirical FormuL·

c H N P Br 2 0

2 8

e

2


2

Ο

C H

Concentration, %

i PMMA

10

PE

2

/

3

(©)

Polymer

h

(CH CH CN) · 2Br 2

PROCESSES

* P

>285

C H P C1 4 4

3 8

2

2

p

s

a

15 1

PE

15

PE

15

PE

20

(0)a-2Cl(NCCH CH ) P(CH ) P2

2

8

2

1 0

104-106

C., H N P Br

297-308

C H P Br

8

44

H

2

2

(CH CH CN) · 2Br 2

(O)

2

3

sPCH CH F2

(Q)

2

3 8

3 4

2

2

3'2Br-

" The concentrations reported are not necessarily the lowest level at which the specific compound is effective in the polymer. PMMA = poly (methyl methacrylate ). PE = polyethylene. PS = crystal polystyrene. b

Mechanism of Action Previously we described the high thermal stability of phosphine oxides and phosphonic acids and indicated that this stability may allow a shielding of the polymer from the flame ( 19 ). The thermal disproportionation of secondary phosphine oxides into secondary phosphines and phosphinic acids ( 1 ) was also reviewed.

Η 2R P— Ο

R PH + R P 2

2

2

OH

The thermal decomposition of phosphonium halides is reported to give phosphines and alkyl halides as shown below (7,15): R P X -> R P + RX 4

3


22.

323

Phosphonium Halides

SHERR ET A L .


If R X is methyl iodide, bromide, or chloride, it may cool the flame and, consequently, reduce combustion. Further, the phosphine can be oxidized readily to the phosphine oxide. The latter reaction may lead to self-extinguishing properties either by absorption of energy to complete the reaction and/or more likely by formation of the polyphosphates and the subsequent shielding of the polymer by this thermally stable glass (19). Table III.

Physical Properties of Polymers with and without a Phosphonium Bromide 0

Polypropylene

Polyester*

Polyfmethylmethacrylate)

Polymer-*

Polyethylene*

Cone, of

0%

15%

0%

15% 0%

25%

0%

12.5%

Dielectric strength, volts/mil

960

930

1030

625

356

340

650

600

Dielectric constant, 60 c.p.s.

2.16

2.92

2.17

2.88

3.24

4.11

3.56

4.19

e

9

R P R ' P R · 2Br 3

3

Dissipation factor, 0.0007 0.0138 0.0009 0.035 0.0034 0.104 0.054 0.093 60 c.p.s. Flexural modulus, X 10 p.s.i.g.

0.035

0.052

0.26

0.35

0.67

0.63

0.38

0.38

Flexural strength, X 10 p.s.i.g.

1.4

1.6

—

—

15.1

4.8

13.3

12.5

Tensile modulus, X 10 p.s.i.g.

0.028

0.037

_

_

% Elongation at yield

62.8

20.2

9.8

4.5

—

0.9

Tensile strength at 1300 yield, p.s.i.g.

1200

4800

4000

—

2300 8800 7200

Vicat softening point, °C.

86

83

128

152

—

—

—

—

Density

0.927

0.960

0.912

0.965

—

—

—

—

e

3

_

_

_

_

e

5.9

3.9

( N C C H C H ) P C H C H P ( C H C H C N ) · 2Br. * Union Carbide DYNH-1, sample thickness 0.04 inch. Enjay Escon 125, sample thickness 0.04 inch. American Cyanamid Co., Laminae 4123, sample thickness 0.125 inch, casting. Rohm & Haas Plexiglas VM-1000, sample thicloiess 0.06 inch.

β

2

2

3

2

2

2

2

3

c

d

β


324


PROCESSES

The mechanism of action of the phosphonium halides as flame re tardants also may be related to the formation of phosphine oxides and/or phosphinic acids from the halide, according to the following reactions (8,14,17). R P X " + H O H -> R P O H + H X 4

+

4


OH

-

R P + O H " ^ R P O H -> R P — O 4

+

4

4

H,0

R H + O H " -> R + R P = 0 +

3

In this case, presumably H X also is formed. The H X often is con sidered an effective flame retardant (4). Tetrakishydroxymethyl phosphonium chloride ( T H P C ) is well estab lished as a flame retardant agent with textiles (3). Collins (2) has sug gested that T H P C and urea break down to produce phosphoric acid via a phosphine oxide, phosphinic acid, and phosphonic acid. For cellulose, Collins concludes flameproofing is essentially caused by the dehydrating action of the phosphoric acid formed.

Stabilization of Polymers and Stabilizer Processes

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