Flame-Resistant Polyesters from Hexachlorocyclopentadiene P. ROBITSCHEK AND C. THO3IAS BEAN Hooker Electrochemical Co., Niuguru Fulls, N . Y .
Experimental results are presented that indicate that the diene reaction adduct of hexachlorocyclopentadiene with maleic acid can be utilized as a major constituent in the synthesis of polyester resins that are unusually flame resistant, particularly in the presence of a small proportion of antimony oxide. A comparison with essentially equivalent resins made from phthalic anhydride and tetrachlorophthalic anhydride shows that the new resins disclosed in this paper are substantially more flame resistant and heat resistant. Since glass reinforced polyester laminates are commonly used in thin sections and such sections may be highly flammable when conventional phthalic based polyesters are used, the flame resistant polyester resins derived from hexachlorocyclopentadiene have a distinct practical utility. In addition to the adduct of hexachlorocyclopentadiene and maleic acid, other adducts suitable as intermediates for flame resistant polyester resins are described.
EXACHLOROCYCLOPENTSDIEKE can be reacted with a number of dienophiles in the diene reaction to form a variety of polybasic acids and polyhydric alcohols. These acids and alcohols in turn can be utilized as major constituents in the synthesis of unsaturated polyester resins. The adduct of hexachlorocyclopentadiene with maleic acid is now commercially available, and this compound in particular JTas found to be interesting in the preparation of resins that show unusually high flame resistance, reeistance to heat and water degradation, and excellent electrical properties. Typical cured polyestel resins may contain up t o 50% of hexachlorocyclopentadieneby weight.
pentadiene (236" C. @ 760 mm.) many reactions may be run at high temperatures a t atmospheric pressure.
Hexachlorocyclopentadiene
bIaleic Anhydride
HEX.ACHLOROCYCLOPENTADIE?JE
Hexachlorocyclopentadicnc is a chlorocarbon that was first identified in 1930 (8). The compound was prepared by reaction of cyclopentadiene F i t h alkaline potassium hypochlorite, a method that can be used on a large scale but is comparatively expensive. Later it was discovered that hexachlorocyclopentadiene can be prepared from chlorinat,ed aliphatic pentanes. The chlorinated pentanes are prepared by chlorination in the presence of light and are converted by a complex thermal dehydrohalogenation reaction into hexachlorocyclopentadiene. Kot only npentane but, also isopentane lends itself for the process, and therefore inexpensive Tax materials may be used (4,6 ) . Hexachlorocyclopentadiene is currently manufactured on a plant scale by a continuous process. It is used on a large scale as a n intermediate ior insecticides such as Chlordane, Heptachlor, -4ldrin, and Dieldrin. Diene Reactions of Hexachlorocyclopentadiene. The conjugated double bonds in hexachlorocyclopentadiene are react,ive toward certain dienophiles such as cyclopentadiene and maleic anhydride. The ease with which the diene reaction proceeds Kith selected dienophiles is surprising, particularly since i t has been held that dienes with multiple chlorine substitution in the double bonds do not react a t all or only nith great difficulty (6). Hexachlorobutadiene for instance is unreactive in the diene reaction (1). The diene reaction is greatly accelerated by high temperatures. Fortunately, because of the high boiling point of hexachlorocyclo-
1628
1:4,5,6)7, 7-Hexachlorobicyclo- [2.2.1.]5-heptene-2,3-dicarboxylic anhydride Difunctional Intermediates. A great many unsaturated dicarboxylic acids, anhydrides, or dihydric alcohols may be reacted with hexachlorocyclopentadiene to form intermediates suitable for polyesterification. Table I lists some of the adducts and their properties. The adduct, of hexachlorocyclopentadiene with maleic anhydride is currently of most intereet as a n intermediate for polgester resins. It is readily prepared by heating the compounds in a solvent a t elevated temperatures. It may be crystallized in a highly pure &ate as a snow white crystalline solid. The anhydride, however, is unstable and is converted to t,he acid when exposed t o humid air. It is the acid which was used for all the experimental work described in the formation of polyester resinfi. [2.2.I .I -5This adduct, namely, 1,4,~,6,7,7-hexachlorobicycloheptene-2,3-dicarboxylic acid, also known as chlorendic acid, will be designated henceforth as HET (Registered trade-mark acid. Hooker Electrochemical Co., Xiagara Falls, S . P.)
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46,No. 8
-Unsaturated TABLE
effect. Local overheating or presence of iron may cause severe discoloration and should be avoided.
I. DIFUNCTIONAL ADDUCTS
Dienophile
M.P., O C.
Lit. Cited
238 208-2104 243.5 240-2476 131.0-134.6 134-136 67
(a)
Form
Hexachlorocyclopentadiene Maleic anhydride Crystalline solid Maleic acid Citraconic anhydride Chloromaleic anhydride ltaconic anhydride Fumaroyl chloride Diethyl maleate s-Allyl glyceryl ether LiquidC 2-Butyne diol Crystalline solid
(7)
.......
49-51
(3)
Difluorotetraohlorocyclopentadiene Maleic anhydride
Crystalline solid
179.0-180.5
(3)
Derived by hydrolysis of anh,ydride reaction product. b Mixture of anhydride and acid. 6 Boiling point 204-214' C. a t 1 mm.
a
I?
I00
K
i
i
I
Polyesters-
I
COMPARISON OF POLYESTER R E S I N S
The effect of HET in comparison with phthalic anhydride and tetrachlorophthalic anhydride was investigated in resin formulations. The formulas used were based on mole equivalency of the dibasic acids or anhydrides and of glycols. Maleic anhydride was employed as the unsaturation active in copolymerization. The mole ratio of the test acids or anhydrides to maleic anhydride was 1.00 to 0.70. The glycols, ethylene glycol and diethylene glycol, were used in equimolar proportion. The experimental reactions were carried out in a 5-liter flask fitted with a stirrer, nitrogen inlet, thermometer, and an outlet. The flask was charged with the ingredients and immersed in a thermostatically controlled heated oil bath. Reaction progress was followed by measuring acid number6 of the resins. Toward the end of the reaction hydroquinone iTas introduced in a proportion of 0.03% by weight based on the weight of the resin. The inhibited resin was cooled and compounded with 30 parts of styrene per 100 parts of resin by weight. The compounded resins were catalyzed with 0.5Yo benzoyl peroxide, cast into glass tubes, cured for 2 days a t 50" C. followed by a 80' C. post cure for 1 day. Table I1 presents preparation and physical properties data. Since the molecular weight ratios of H E T anhydride t o tetrachlorophthalic anhydride and phthalic anhydride are 2.51 to 1.93 to I , respectively, the experimental H E T resin contained the least amount of maleic anhydride on a weight basis.
PYTUALIC LNUYDRIDE.
I .----.
!a1
I
4
1
~
I
d
I
1
0
1
12 IC 20 REACTION TIME IN H O U R S AT 1 9 5 - C .
4
1 24
*
to
1
HET Acid Resin
26
Figure 1. Comparison of Reaction Rates of HET Acid and Phthalic Anhydride
RESIN FORMATION
H E T can be readily esterified with dihydric alcohols to form linear polyester resins. Even a t early stages of the reaction the products are solid at room temperature. The rate of esterification with ethylene glycol resembles closely that of phthalic anhydride, as shown in Figure 1. The experiments were carried out under identical conditions. The resins resulting from the esterification of H E T and ethylene glycol are high melting, thermoplastic, and brittle. Addition of benzoyl peroxide to the resin does not convert it to a thermoset product even after prolonged heating. Addition of styrene and benzoyl peroxide is equally ineffective in this respect. It can therefore be postulated that the double bond in HET is inactive under polymerizing conditions. The chlorine atoms in N E T are apparently firmly held with respect to chain transfer. This is substantially different from hexachlorocyclopentadiene which in itself is an effective chain transfer agent. I n order to form unsaturated polyester resins that are capable of cross-linking, i t is necessary to incorporate a n unsaturation active in copolymerization. Such an unsaturation is provided by an unsaturated dibasic anhydride or acid such as maleic anhydride or fumaric acid. The experimental data given are based on typical HET-containing resins incorporating a mixture of ethylene and diethylene glycols, and maleic anhydride. Since H E T is more sensitive to heat discoloration than phthalic anhydride, the preparation is best carried out between 160' to 180" C. in an atmosphere of inert gas. The usual variables such as rate of stirring and rate of inert gas passage have a conventional August 1954
~~
TABLE 11. COMPARISON OF POLYESTER RESINS Phthalic Anhydride Resin
Tetrachlorophthalic Anhydride Resin
Resin preparation Reaction time, hr. Reaction temperature, ' C. Acid No. of base resin Chlorine content of base 38.0 0 29.9 resin, wt. % Styrene, parts/hundred of 30 30 30 resin, by weight Chlorine content of styren29.2 0 23.0 ated resin, wt. % Viscosity of styrenated resin, 21-82 z1 21-22 Gardner Iloldt Scale Physical properties of cast resins Compression strength, lb./ 20,000 17,000 23,000 sq. in.a 3.2 12.1 19 Hot compression creep, % Compression strength, lb./ 7,000 Below 1 , 0 0 0 Below 1,000 sq. in. at 100' C.a Weight loss after 7 days a t 3.3 7.2 7.3 2000 C., 7%" Weight loss after 30 days a t 14.4 25.6 ... 2000 c., %" Flame resistance, -4STM . , Burns freely ... D 757-49 0.18 ... 0.79 Inches burned/minute 2 . . . 0 Time t o ignite, sec. 150 ... >180 Time flame out, sea. Flame resistance, $STM . Burns freely ... D 635-44 0.20 ... 0.26 Inches burned/minute 2 ... 2 No. of ignitions Yes NO Yes Self-extinguishing O C ... 150 Flame-out time, sec. Cylindrical casting l l / u inches dia. X.1 inch tall. 6 Cylindrical casting 6/a inch dia. X 1 inch tall under load approximately 200 lb. after 15 minutes a t looo C. C Immediately self-extinguishing after removal of flame.
.
. .
HEAT RESISTANCE
The data indicate that the resin prepared from HET differs from the comparable phthalic and tetrachlorophthalic resins by its considerably higher strength at elevated temperatures and lower susceptibility to heat degradation. The resistance to heat degradation on prolonged exposure a t 175" C. of the two resins already described is illustrated in Figure 2. The castings (ll/a
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1629
21 I
,
I
1
20
25
w '5 I5
3
z
Y,
9
=
8
tz
! 5
0
9
f 08
6
VI
3
z
B 8
I 2
3
04
4
b
I2
05
16
Figure 2. Volatile Losses versus Exposure at 1%' C .
inches diameter X 1 inch tall) though severely disrolored showed no loss of their compressive strength. At temperatures above 330" C. pyrolysis of HET derived polyester resins takes place. When heated in air in a glass container the chlorine contained in the compounds is liberated as hydrogen chloride. No phosgene can be detected. Table TI1 summarizes data on pyrolysis of resins derived from HET.
10 TUICKNESS
W E E K S OF EXPOSURE
Figure 3.
5
OF SAMPLE
Burning Itate versus Thickness
Method of hSTR.1 D 757-%9
Inches Time to Burned/ Ignite, Materia1 ('is-inch thickness) Minute Sec. HET, polyester casting 0.18 2 Phthalic polyester casting 0.93 0 0.69 0 Hickory wood Phthalic polyester 50% glass laminates 0,59 0 HET polyester 50% glass laminate 0.05 15 HET polyester 50% glass laminate, 3 0% SbiOs Does not ignite .,
Time of FIaining. See. 150 >180 > 180 >180 51
6.791,-i.e., less than l t ' 8 of the corresponding HET based resin and about 1/'4 of the corresponding phthalic resin. This is a n indication of high thermal stability. F L 4 M E RESISTANCE
Conventional po1yel;ter resins based on phthalic and maleic anhydrides, and styrene as a cross-linking agent may be easily ignited and once ignited continue to burn freely. Introduction of tetrachloropht,halic anhydridc retards burning rate somexvhat, but the effect is not comparable to that, obtained by HET (see Table 11). Addition of antimony oxide t o chlorine containing polycstcr resins improves flame resistance. h t a chlorine content above about 25%, a n addition of approximately 391, antimony oxidc is entirely adequate for cxcellrnt additional flame resistance, In general, the higher the chlorine content the less antimony oxide is required for equal flame resistance. Hen-ever, beloiv certain levels of chlorine content (about 20y0,)even excessive amounts of antimony oxide cannot compensate for deficiency in chlorine Figure 4 . Equipment for ASTi!l D 757-49 content. Addition of lOy0 antimony oxide to phth Flammabilit> T e s t , Hot War at 1000" C. resins does not measurably reduce flammability. The XST11 t,cst D 757-49 TWH used t,o determine co flame resistance of different materials. This test c A polyester resin based on the adduct of difluorotctrachloropressing a lls-inch thick ?trip against a bar heated to 900" to cyclopent,adiene with nialeic anhydride shows further improve1000" C. a t nhieh temperature it g l o w bright cherry red. ment in heat resistance. Exposure to 200" C. for 30 days of a I n addition to the requkite va!ues of inches burned per niinutc, casting l l / s inches diameter by 1 inch tall gave a \Tolatile loss of an additional record n-as marl(, of the time required lo], the flame to burst out' after contact ~ IHET S R a s ~ uPOLYESTER RESIS TABLE 111. P Y R O L YOF with the bar, and of the time of Oxygen burning. The distance burned Remaining Sample. Wt. c/a in .4ir in inches 17-88 divided by 3 to Temperature Carbon HydroIllumi- Passed Over obtain inches burned per of Test, monge? carbon nant as Sample, CI1lorine, ~ t % . c. oxide cliloride Residue dioxide ethylene I'ol. 70 Distillate Resid'.ie minute. Some tests have been 2J i o 2.50
