302
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
+
Sel H HSe: HSe: H + Hr $- Se: hv Se: -+ SeQ
+
+
Similarly, hydrocarbon free radicals may be deactivated by reactions such as the following:
-
+ +
RCHZ Sez RCHZSe: RCHzSe: H + RCH, Se: RCHz R CH2.CH2 R RCHzSe;
+
+
-f
(15)
+ Se:
(16) (17)
Because small amounts of hydrogen selenide greatly decrease the flame speeds of propane-air mixtures, it may be concluded that Sea is an efficient chain breaker a t relatively low concentrations and serves t o deactivate hydrogen atoms and other freeradical species such as RCHa-, whose concentration and diffusion rates may partially determine the flame speed. ACKNOWLEDGMENT
This work was done under the sponsorship of the Flight Research Laboratory, Wright Air Development Center, WrightPatterson Air Force Base, Ohio.
Vol. 47, No. 2
The author wishes to acknowledge the contribution of A. A. Putnam, Battelle Memorial Institute, n h o derived the modified equation for Zc,/c: which was used in this work. He also wishes to express thanks to R. S. Litton, who ably and faithfully assisted in carrying out the experimental work, to Fred Benington, consulting chemist in fuels research a t Battelle, to Paul Woodberry, who prepared and purified the h j drogen selenide, and to J. F. Foster, division chief of fuclq rrwarch at Battelle, for his helpful interest and ronsideratiori LITEKA’I’URE CITEI)
(1) tlooth, H . S., ed., “Inorganic S y n t l e a e s . ” Vol. 11,p . 183,RlcGrawHill Book Co., New York, 1946. ( 2 ) Coward, H. F., and Greenwald, I f . l’., 1T.S. Bur. M i n e d , Tech. Paper 427 (1928).
(3) Kurs, P. p., INn. Esr:. C’EEM.. 45, 2072 (lYR.7). (4) I h i d . , p. 2361. ( 5 ) Paynian, Wr..C h e n ~Soc., . 115, 1436 (1919). (6) Stock, A , and Friederici, K., Rer.. 46, 1960 (1913). (7) Stock, A., a n d Kuss, E., lhid., 47, ,3124 (1914). (8) Stjock, A , , a n d Xlassenez, C., Ihid.. 45, 35.79 (1912).
RECEIVED for review March 1.5. 1954.
Xr,or:msn Auguat I U , IY.5.L.
Heat-Resistant Allvl Resins J
HOWARD W. STARICWEATIIER, JR.], ARNOLD ADICOFF, ~ N D FREDERICK R . EIRICH I n s t i t u t e of Polymer Research, Polytechnic- I n s t i t u t e of Brooklyn, Hrookl-srt, Y. Y .
T
H E R E is a need for a plastic casting material which combines excellent transparency with hardness, fabricability, and good heat and impact resistance. Polymethyl methacrylate has been used for a number of years but is thermoplastic above 80’ to 100’ C . and very easily scratched. Partial success has been achieved by the use of allyl resins (5). The two allyl monomers which have received the greatest attention are diethylene glycol bisallyl carbonate (I) and diallyl phthalate (11).
0 /I
CH~-CH~-O-~-O-CH~-CH=CH~
/
e
0
0
\
CHa-CHz-4-
&-O-CHa--CH=CHa I
0
nionoiiier~are shown in Figure 1, in comparison with polymethyl methacrylate. The latter, a typical linear thermoplastic polymer, shows a “glassy state,” or L‘second-order,”transition temperature between 80” and 100” (”. In the aame range, the resin from diethylene glycol bisallyl carbonate changes from a glass to a hard crosslinked rubber which exhibits moderate elastic properties and does not flow a t temperatures as high as 210” C. The polymer of diallyl phthalate softens a t about 140’ C. At higher temperatures, its properties are intermediate between tliose of the other two resins: Tt does not flo~vas readily as polymethyl methacrylate but does undergo strainb which are only partiall) recovered on the release of Ptress. Foi both allyl resins, the location of the transition temperat ui e reflects the melting of secondary bonds between the polymer chains, while the primary covalent crosslinks hold the specimen in shape. Bemuse of the much smaller number of crosslinks per unit volume, the specinienr break above the transition tempcrxturv a t a much reduced strew COPOLYMERS BASED ON 1)IAI.LYL PHTHALATE
I1 The former is commercially known as CR-39, and was developed by the Pittsburgh Plate Glass Co. ( 4 ) . I t s disadvantages lie mainly in a certain brittleness and a considerable, though finite, heat distortion a t approximately 100’ C. Diallyl phthalate is a product of the Shell Chemical Co. and is widely used as a crosslinking agent rather than as a homopolymer. The heat-distortion curves (flexural deformation vs. temperature under constant stress) of resins prepared from these two 1 Present address, Polschemicals Department, E. I. du Pont de Nemours & C o . , Ino., Wilmington, Del.
Maximum strength in a thermoset resin should result from a geometrically regular pattern of flexible crosslinks, but such regularity is found only in three-dimensional crj-ntals, where flexi1)ility is lacking. A strong structure phould also result from a conipletely random distribution of crosslinks. However, in any homopolymerization of divinyl or diallyl compounds, there is a teiidericy to form crosslinks in clusters, with corresponding estermive density fluctuations. The polymeriention products are chnracterized by brittleness and oft,en by wcnk, cheesy stages, pawed just beyond the gel point, which may be defined as the short^ t,ime iuterval during which the polymerizing solution undergoes R very rapid irreversible change in viscosity. This effect is the more pronounced the shorter the primary clinins are. The functionality of diallyl phthalate should lead to a tightly cropslinked structure; however, the fact t,hat it appears to undergo
INDUSTRIAL AND ENGINEERING CHEMISTRY
February1955
1
$ 4 X
I*
3 3
I'
i 8 9 01 g I-
U
Y
I 'ATURE, "C.
Figure 1. A. B. C.
Heat-distortion curves
Polymeth,I mrthucrylate Polymer from dieth) Irne glycol bi,all, I rurbnniiir Polymer fmm diull>I phthalate
permanent deformation a t high temperatures shows that ita crosslinking is largely imperfect. The most likely cause for this lies in the ortho position of the two allyl groups. The latter will Frequently polymerize a t t8he same time and enter t.he same polymer chain, thus forming a ring instead of a crosslink. Simpson and coworkers (6) studied the polymerization of diallyl phthalate in detail. They concluded that each primary chain consists of about 14 diallyl phthahte units, 6 of which are doubly reacted. Several approaches were tried to improve the quality of crosslinking in diallyl phthalate polymers while retaining their att,ractive high temperature properties. A series of copolymers of diallyl phthalate containing 25, 50, and 75% by weight of diethylene glycol bisallyl carbonate was prepared using 3, 5 , and 10% bensoyl peroxide as initiator. [Large amounts of initiators are required for allyl polymerizations, because the growing chains are terminated by degradative chain transfer after only a few inonomer units have been added ( 1 , 3). Whereas fairly large amounts of catalyst fragments are thus introduced into the polymer, higher temperature polymerizations with di-tert-butyl peroxide offered no particular advantage over the use of benzoyl peroxide.] The heat-distortion curves of samples made with 10% initiator are shown in Figure 2. The copolymer containing only 25% tiiallJ-1phthalate had a softeuing point not much higher than that of the homopolymer of diethylene glycol bisallyl carbonate and vvas less stiff above 100" C. The copolymers containing 50 and 75% diallyl phthalate exhibited plastic flow above their glassy state transitions, indicating poor crosslinking. Reasonably successful results, however, were obtained with the following copolymers of diallyl phthalate, which were completely elastic above their glassy state transitions; the heat-distortion curves are shown in Figure 3. However, they were all brittle. Copolymers containing 10 and 25% triethylene glycol bisallyl carbonate were prepmed with 6% benzoyl peroxide. These resins
NEEDED.,
softened at about 12.5' C., but did not flow a t higher temperatures. Apparently, triethylene glycol bisallyl carbonate is a more effective_,crosslinking agent for diallyl phthalate than i h lower homolog because the longer chain, separating the two allyl groups, makes them less likely to enter closely related polymer structures. Copolymers of diallyl phthalate with either 10% allyl methacrylate or 10% triallyl cyanurate also have high glassy state transition temperatures with no high temperature plastic flow. Allyl methacrylate is probably effective because the methacrylate group is more reactive than the allyl group ( 6 ) ,and will tend to react a t an earlier stage of the polymerization. These groups are, therefore, less likely to form rings by entering the same polymer chain. Triallyl cyanurate (111) has three allyl groups symmetrically substituted on a triazine ring. I t s high functionality plus the fact that the meta positions are less likely to form ring structures than o-diallyl phthalate apparently helps to promot? thorough crosslinking. O--CH2--CH=CHp I
I1
CHFCH--CH~-O-C
1
C-O--CHp-CH-('H
POLYMERS BASED ON DIETHYLENE GLYCOL BISALLYI. CARBONATE
The polymer from diethylene glycol bisallyl carbonate hap several attractive features, but it is handicapped by its comparatively low glassy state transition temperature. The latter can
PS
50
75 100 TEMPERATURE,
125
A. R.
0% 25%
E.
.
c.
D. 100%
50% 75%
200" C.
aircraft glazing and other applications
..
copolymers of triallyl cyanurate and diethylene glycol bisallyl carbonate L
150
175
O C
Figure 2. Heat-distortion curves for polymers from diethylene glycol bisallyl carbonate and various amounts of diallyl phthalate
.
PRESCRIPTION.
2
\" TI1
a transparent and strong plastic to withstand temperatures to
FOR..
303
INDUSTRIAL AND ENGINEERING CHEMISTRY
304
be raised to some extcnt by copolymerization with diallyl phthalate but, as shown, this expedicnt leads to incomplete crosslinlcing and excessive brittleness. ,4 more successful approach consists of Copolymerization with triallyl cyanurate.
Vol. 47, No. 2
CASTING TECIlUIQUE
Allyl resins mere cast betn ecn CHI cfully cleaned polished glass plates, separated by a strip gasket of Koroseal of cross section 0.15 X 0.30 inch, bent t o form a square. These gaskets were used only once. The assembly was held in position by springloaded U-clamps. The polymerizing mixture was introduccd through a hypodermic needle placed in a gap in the gasket a t a corner of the plate. The mold n-as completely filled, to cliniinate air pockets. HEAT-DI STORTION TEST
25
50
75 100 TEMPERATURE,
O
125 C.
150
175
Figure 3. Heat-distortion curves for polymer containing diallyl phthalate and other comonomers A.
Homopolymor
D.
10% allyl methacrylate 10% triallyl cyanurate
This test is a modification of ASThl D 648-45T. The diinensions of the specimen Fere 0.5 X 2.5 X '/a inch. The length of the span was 2 inches, and a load of 347 grams was applied a t the center. From this, a maximum fiber stress of about 300 pounds per square inch can be calculated. The sample was immcrwd in a bath of silicone oil which was heated at a n average rate of 6" C. per minute with mechanical stirring. The stress was maintained throughout the heating and the distortion was measured in units of 0.01 mm. with a gage attachcd to the loading column. The heat distortion was defined as the increment over the reading under load a t 25" C.
B . 10% triethylene glycol bisallyl carbonate C. 25% triethylene glyool bisallyl carbonate E.
A series of copolyniers of dietliylenc glycol bisallyl carbonatc with from 7.7 to 41.7% triallyl cyanurate was prepared, using 3% benzoyl peroxide as initiator. The heat-distortion curves for these resins are shown in Figure 4. The magnitude of the deformation xhen passing untler coiixtarr t load through the t>ransition point is seen to dccrcase as the concentration of triallyl cyanurate is increascd. The tlc illation, furthemorc, is very gradual as the temperature is raised above 80' C., whereas the homopolymers deform over n fairiy narrow temperature rangc. The sample containing 41.7y0 triallyl cyanurate underwent essentially no heat distortion as it \vas heated to 210' C. under load, but was very brittle. During polymerization the triallyl cyanurate copolymers do not exhihit the checsv consistericy usually found in allyl polymcrs just bcyonti the gel stage, but are harder niid tougher throughout. The copolymers are also less likely to crack during polymerization, are more easily released from the mold, and are more easily obt'ained with perfect surfaces. The optimum composition v a s found t o contain 10% tiiallyl clmiurate, which could be incorporated without increafiing the hrittleness of the basic clictshylcneglycol bisallyl carbonate rcsin. \Vith larger amounts of comonomer, the resins became progressively more brittle, This is shown by the ball drop heights given in Table I. The copolymerization was best carried out with 3% benzoyl peroxide. With less initiator thcrc was a tendency to craze during polymerization. Samples polymerized with more than 3% initiator were apt, to tlcgrade when heated to high temperatures. The most sat,isfactory polymerization cycle tried consisted of 42 hours a t 60" C., follon-ed by 24 hours a t 90" C. and 1 hour at 110' C.
'fable I. Effect of l'riall>I Cyanurate on Toughness of Copolyniers with Diethylene Glycol Bisallyl Carbonate T i i d l l > l Cganuiatc, 0 0
7 7 10 0 16 0
35 2 41 7
6'
3Iinimum Ball Drop Height t o Break, Fret 5-6 5 5
5 4 5
4
Very low
25
50
75 100 TEMPERATURE,
' C.
125
150
175
Figure 4. Heat-distortion curves for copolymers of diethylene glycol bisallyl carbonate with various poreentage of triallyl cyanurate H A L L DROP TEST
Samples approximately 2 X 2 X '/e inch were placed on a brass ring having an internal diameter of 1.4 inches. A steel ball, 1 inch in diameter and weighing 33 grams, was dropped from a height of 3.5 feet on the part of the sample directly over the center of the ring. The test was repeated, increasing the drop height 6 inches a t a time until the sample broke. The minimum height a t which the sample brolcc as rccorded as the ball drop height of the material. Usually, drop heights measured in this manner were reproducible within = t 6 inches. LITERATURE CITED
(1) B a r t l e t t , P. D., and Altschul. R., J . A m . Chem. Soc., 67,S12, 810 (1945). (2) Gaylord, N. G., and Eirich, F. R., I b i d . , 74, 334,337 (1952). (3) Lynn, Lawrence, M o d e r n Plaslics, 31, No. 2, 139 (1953). (4) Muskat, J. E., a n d S t r a i n , F. (to P i t t s b u r g h Plate Glass Co.), U. S. Patent 2,370,565 (Feb. 27, 1945). (5) R u t o v s k y , B. N.,and Shur. A . >,I., J . A w l . Chem. ( ( * . S . S . E . ) ,
24, 1325 (1951). (6) Simpson, W., Holt, T., and Zctic, 11. J. J . Polymer Sci..10, 459
(1953). RECEIVEDfor review April 8 , 1954. -4CcEPTED October 1 6 1054. Part of a research program carried out with the support of the Rwcau of Aeronautics, Department of the Navy. Bbstracted from the dissartation presented b y Howard W. Starkweather. Jr., to the Graduate F ~ c u l t yof the Polytechnic Institute of Brooklyn in partial fulfillment of the rewircmenta for the degree of doctor of philosophy.
