Plastics Construction Materials
Heat-Resistant Polyester Laminates Heat resistance of polyester-glass cloth laminates has been studied by measurement of flexural strength at 500" F. after exposure to this temperature for times up to 192 hours. The nature of the monomer was made the only variable in a series of test laminates. Diallyl bicyclo[2,2,l]hept-5-ene-2,3-dicarboxylatewas the only monomer, other than triallyl cyanurate, to survive 192 hours at 500' F. with measurable flexural strength. Mixtures of these two monomers gave laminates that heat aged better than either one alone. This synergism is related to a reduction of laminate crazing and a corresponding decrease in surface exposed to oxidative attack. Acrylonitrile and triallyl aconitate also reduce craze but do not act synergistically with triallyl cyanurate, probably because they represent intrinsically less heat-stable structures than diallyl bicyclo[2,2,l]hept-5-ene-2,3-dicarboxylate.
W. CUMMINGS AND M. BOTWICK Naugatuck Chemical Division, U n i t e d S t a t e s R u b b e r Co., Naugatuck, C o n n .
T
HE projection of polyester-glass
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cloth laminates into ever-expanding fields of use has made it increasingly desirable that the structures possess resistance to high temperature exposure (500' F.). The problem was first considered from a structural point of view by Ebers and coworkers (2) in 1950. These workers investigated the influence of alkyd structure (including glycol and dibasic acid variations), monomer-alkyd ratio, and the cross-linking mechanism on heat resistance. It was concluded that none of these factors was of decisive importance. However, the advantage of a high level of polymerlzable unsaturation in the system was pointed out, and it was established that the decrease in strength during heat exposure was largely the result of oxidative attack since losses in strength could be eliminated by the use of a nitrogen atmosphere. B subsequent study of the influence of monomer structure on heat resistance culminated in the discovery that triallyl cyanurate, in combination with a modified maleic alkyd, gave a polyester which exhibited remarkable repistance to heat: (3). This superiority was ascribed to the thermostability of the s-triazine nucleus and the high degree of cross linking attained with the trifunctional monomer. In the work described here, the influence of a second monomer, used in conjunction with triallyl cyanurate, was studied. PREPARATIOh OF ALKYD-MONOMER BLENDS
A specially modified alkyd ( 4 ) was used in all blends so that the alkyd structure would not be a variable. The blends were prepared by heating the alkyd and monomers with stirring at 60° to 70" C. The compositions all contained 50% by weight of monomers. Sufficient hydroquinone (50 to 100 p.p.m.) was added so that gelation would not occur in less than 24 hours a t room temperature after the addition of 201, benzoyl peroxide. The uncatalyzed blends were Etored in the dark a t room temperature until used in the preparation of laminates. PREPARATION O F LAMINATES
The resin (600 grams, an amount calculated to give a fivefold excess over that appearing in the cured laminate) was added July 1955
to a smooth paste prepared from benzoyl peroxide (2% of resin weight) and an equal weight of styrene. After stirring to completely dissolve the catalyst, any suspended air was removed by evacuation in a vacuum desiccator. The catalyzed resin was poured onto a sheet of cellophane supported in a wooden frame and 14 plies (7 X 13 inches) of glass cloth (either ECC 181-114 or ECC 181-301) were carefully laid in the pool. After soaking overnight (to ensure maximum wetting of the cloth), the cellophane was formed into a bag, and residual entrapped air was removed by careful manipulation with a squeegee. Care was taken to remove only that amount of resin from the cloth which would permit removal of visible air bubbles. A considerably larger amount of resin was squeezed from the lay up in the press in the period (approximately 10 minutes) before gelation occurred. This resin carried with it any minute bubbles of air that might have otherwise been trapped in the laminate. The lay up was placed in an electrically heated press and cured a t 80' to 90' C. a t a specified pressure (15 pounds per square inch for laminates involving 181-114 cloth and 10 pounds per square inch for those using 181-301) during hour. The specified pressures gave a resin content of 37 f 2%. I n most cases the resins had a disk viscosity of 30 to 40 poises a t 25' C. Where the viscosity was outside this range, l/s-inch steel shims were used in the press to ensure that the resin content would be in the desired range. After removal from the press, the laminates were placed for 2 hours in an oven regulated to 120" C. in order to complete the cure. TESTING OF LAMINATES
The cured panels were trimmed and cut into 3 X '/z inch strips using an Alundum cutoff wheel. They were then measured as to width and thickness and exposed to the test temperature in a thermostatically controlled circulating air oven for the specified time. Span-depth ratio was 1 6 : l or greater. The measurement of flexural strength was carried out in an Olsen Lo-Cap universal testing machine equipped with a forced draft electrically heated oven surrounding the jaws so that the test could be carried out a t exposure temperature.
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2,3-dicarboxylate (DET 11) as a monomer and found that the laminate retains a flexural strength of 10,200 pounds per square inch (measured a t 500" F.) after 192 hours' exposure to 500' F. (laminate No. 2). At 400' F. (laminate No. 3) the strength showed no appreciable decrease between 192 and 408 hours' exposure (38,000 to 39,000 pounds per square inch). At 300" F. the resin was evidently still approaching full cure between 192 and 408 hours since the strength increased from 39,000 to 42,000 pounds per square inch. Although DET does not confer heat resistance on the high level experienced with TAC, it nevertheless is significantly better than any other monomer tested which DISCUSSION is not a s-triazine derivative. Heat resistance of unsaturated polyester resins is conveniently Attention was then directed to resins in which the monomer studied by measuring the loss in flexural strength of a glass cloth was a mixture of TAC and D E T (Nos. 4-10). All resins conlaminate during exposure to an elevated temperature. If the taining mixed monomers have the composition: Alkyd, 50 parts; structure 6f the monomer is made the only variable in such a TAC, X parts; comonomer, 50-X parts. Quite unexpectedly test, it is possible to examine the importance of this factor in it was found that any mixture of TAC and D E T gave a resin detail. The other variables are ( a ) character of the alkyd, exhibiting better heat resistance (particularly as measured by ( b ) weave of the glass cloth and finish thereon, (c) resin content 192 hours a t 500" F. exposure) than either monomer used alone. of the laminate, ( d ) cure cycle, ( e ) temperature of exposure, and The optimum ratios (18 to 28% D E T ) give strength values in (f) duration of exposure. I n a study of this type (1) it was the range 28,000 to 30,000 pounds per square inch which represhown that the monomers-styrene, diallyl phthalate, acrylonisents a considerable improvement over the 19,000 pounds per trile, p-chloroallyl cyanurate, 2-vinylpyridine and triallyl carsquare inch attained with TBC alone under comparable condiballylate-all gave laminates that failed to survive 192 hours' tions. exposure to 500' F. with a measurable flexural strength. On The other comonomers failed to exhibit this synergistic effect. the other hand, a laminate (No. 1 in Table I) prepared with Both 12.5 and 25y0 of acrylonitrile (Nos. 11-13) served to detriallyl cyanurate (TAC) as the monomer and the other variables crease the heat resistance of the resin compared to No. l which controlled, had a flexural strength of 19,000 pounds per square contained TAC alone. None of t8he compositions containing inch after such an exposure (measurement a t 500' F.). triallyl aconitate survived 192 hours of 500' F. The 24-hour I n this work and in that discussed below, the other variables strengths (Nos. 14-17) show a regular decrease as the proportion were controlled as indicated in the experimental section. The of triallyl aconitate is increased. glass cloth finish was either 301 (a silane type) or 114 (a methacryThe laminate (No. 1)containing TAC as the sole monomer was lato chromium complex type). As in the previous study ( 1 ) it badly crazed before exposure to 500" F. It was observed that appears that the silane finish gives consistently better heat-re12.5% D E T (No. 9) considerably decreased the amount of this sistant laminates. Such differences are not a factor in the folcraze and that further (smaller) decreases resulted from the use lowing comparisons. of 18.75% D E T (No. 8) and 25% D E T (No. 7). With greater proportions of D E T (Nos. 6 and 5) the amount of craze did not COzCH2CH=CHz change materially. The deterioration of polyester laminates a t CH~-CHCH o ii N bN -OCH~CH=CH~ b - ~ ~ ~ 2 ~/,, ~ , = , ~ ~ 2 -11 I 500' F. is largely an oxidative process since strength losses can N N be eliminated by conducting the exposure in an inert atmosphere \/ \I/\ ( 2 ) . The crazed laminates must present a greater area to hot OCH&H=CHz COJCH~CH=CH~air because the cracks involve the surface. This can be shown by smearing the laminate with ink. After removing the surplus I TAC I1 D E T ink, the craze pattern stands out as black lines. At least part Recently we have examined diallyl bicyclo [2,2,I ]hept-5-eneof the increase in 192-hour strength accompanying increased MATERIALS
Triallyl aconitate was obtained through the courtesy of L. F. Martin of the Southern Regional Research Laboratory. Acrylonitrile and triallyl cyanurate were commercial-grade materials. Diallyl bicyclo[2,2,I] hept-5-ene-2,3-dicarboxylate (DET) was prepared in 87% yield by azeotropic esterification of the corresponding anhydride with excess allyl alcohol using p-toluenesulfonic acid as the catalyst and toluene as the entraining agent-b.p. 163-170' C. at 10-11 mm., mas = 1.4985.
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Table I.
'
1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17 a TAG b DET C After d After
301 301 301
Heat Resistance of Glass Cloth Laminates'as a Function of Monomer Composition
50 0 0
none DET b DET
50 50 50
51,500 60,300 52,900
:
19 600 23 700
...
36,500 30 100 34 :zoo 28 600 37: 100 39,700 44,200 45,000 42 000 44 500 42 ,400 24,600 27 200 34 200 23,600 20,100 17,600 15,400
I 9 000 10 ' 200 39'300 C 38:900d 11,300 19,600 27,400 30 200 29 800 28,800 , 24,200
32,900 61,000 DET 43.75 301 6.25 30,600 73,000 DET 37.5 12.5 301 27 000 80,500 31.25 DET 18.75 301 25 700 67 900 DET 25 25 301 29,400 63 700 18.75 31.25 DET 301 31 200 75 400 12.5 37.5 DET 301 33 200 76 700 6.25 43.75 DFT 301 0 13,700 82,300 A 50 114 0 14 200 11,300 73 200 25 A 114 25 12 100 24,800 5 s : 900 12.5 37.5 A 114 28,600 12.5 37,500 TAAf 114 37.5 0 24,400 35,200 25 TAA 114 25 g 21,200 34,500 37.5 12.5 TAA 114 I 18,700 35,500 50 TAA 114 0 = triallyl cyanurate. fe A TAA = acrylonitrile. = triallyl aconitate. = diallyl bicyclo[2,2,1 hept 5 ene 2 3 dicarboxylate. B No flexural strength measurement possible 408 hours: 38 100 lb.)sq. inch. - ' - ' due t o delamination. 408 hours: 42: 100 lb./sq. inch.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 7
Plastics Construction Materials Dercentage - of DET can therefore be ascribed to the decreased surface resulting from the corresponding loss in craze. Note that the maximum strength coincides with the minimum craze (25% DET, No. 7). Since D E T monomer alone (No. 2) gives a less heat-resistant structure than TAC alone (No. l),it might be expected that with the craze factor stabilized a t DET percentages greater than 2570, the additional D E T would act to decrease the heat resistance. A more heat-resistant monomer (TAC) is being replaced by a lesser one ( D E T ) without any compensating decrease in craze after the percentage of D E T has reached 25%. This is the situation actually observed. The monomers, acrylonitrile and triallyl aconitate, also act to decrease craze. However, they are intrinsically much poorer monomers for heat resistance than D E T (Nos. 2, 11 and 17). It would appear that this factor overshadows the improvement due to craze reduction and for this reason the synergistic effect noted with D E T is absent.
ACKNOWLEDG-VENT
Most of the work reported here was sponsored by the Materials Laboratory, Directorate of Research WADC, USAF, Contract AF 33(600)16825. LITERATURE CITED
(1) Cummings, W., and Botwick, M., Division of Paint, Plastics,
and Printing Ink Chemistry, 124th Meeting, ACS, Chicago, Ill., Sept. 9, 1953. (2) Ebers, E. S., Brucksch, W. F., Elliott, P. M., Holdsworth, R. S., and Robinson, H. W., IND.ENQ.CHEM.,42, 114-19 (1950). (3) Elliott, P. XI., Modern Plastics, 29, No. 11, 113-14, 185-7 (July 1952). (4) Knapp, R. L. (to U. S. Rubber Co.), U. S. Patent 2,671,070 (March 2, 1954). RECEIVED for review September 17, 1954.
ACCEPTSDDecember 15, 1954.
Design of Plastic Structures for Complex Static Stress Systems Design equations for plastics in static stress systems have been developed from the theory of linear viscoelasticity. The time dependent properties of plastics are characterized by two parameters that are easily measured. Development of equations as well as comparison of calculated and measured values are given for deflection of clamped beams and disks, torsion of rods, buckling of columns, and diametral expansion of pipe and tanks under hydrostatic pressure.
A. A. MACLEOD Polychemicals Department, E. I . du Pont de Nemours & Co., Inc., Wilmington, Del.
T
H E engineering design of plastics parts raises some difficult problem because many physical properties of plastics are time dependent. Since these quantities are also sensitive to temperature and frequently to humidity and since in most applications temperature and humidity are ambient, even the measurement of the properties to represent a given application is a task. The properties needed for the design of mechanical parts are the stress to cause fracture and the relation between stress and deformation. Both of these are time dependent for plastics. Conscquently, they are not determined by the usual short-time tests, such as those for elastic modulus and tensile strength. Further, because of the wide variety of plastics and conditions of operation, the data on long-time behavior for the particular design problem are seldom available. I n this article, the relations between stress and deformation are analyzed a t stresses well below the long-time fracture stress. A method of estimating the longtime behavior from quantities determinable in short-time tests is presented. The objective is to describe a way t o obtain design equations for plastics in any static stress system. Both the way to measure the quantities to characterize the time dependence of the plastic and the way to include the quantities in design formulas are described. To be of practical value, these time dependent properties must be determinable easily in a short time. Consequently calculation of long-time behavior involves large extrapolations. The results, then, are order of magnitude values to act as a guide in design. It follows that if the plastic is to be subjected to a range of temperature and humidity, the worst possible combinaJuly 1955
tion of conditions should be chosen for the calculations. dpplication of the method requires measurement of the parameter characterizing time dependence for the worst conditions of operation followed by calculation of the long-time behavior. The assumptions made in developing this method were that the theory of linear viscoelasticity was applicable, that pure volume changes of plastics were independent of time, and that the materials were homogeneous and isotropic and remained so under stress. Applying these assumptions yielded a mathematical model of the time dependent behavior of plastics under stress a t stress levels below the long-time fracture stress. The resulting equations give calculated values in satisfactory agreement with current knowledge. As our knowledge of plastics increases, however, the model will be modified and extended. THEORETICAL BACKGROUND
Development from basic principles of the equations presented here is given by DiDonato ( 3 , 4). The equations are developed from the theory of linear viscoelasticity. The theory proposes that stress be proportional to strain a t all times and that the proportionality coefficient be a function of time. A useful model for the development of the equations is the combination of springs and dashpots. A unit is made up of a spring and a dashpot either in series or in parallel, and the model consists of a number of units in parallel or in series, respectively. For plastics an infinite number of units is desirable. The mathematical equations from this model and other related
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