EFFECT OF TEMPERATURE ON STRENGTH OF LAMINATES P
PATRICK NORELLI AND W. H. GARD
ROPER use of plastic laminates requires a knowledge of their physical properties over the range of temperatures at which the product might be employed. This paper deals with this problem in the field of commercial phenolic laminates: one melamine laminate is included. The test program has not yet been completed; therefore, this article reports only the tensile, compressive, and shear characteristics of various types of laminates from -55" to 200" C. The minimum temperature in this range is a practical one; laminates have been used a t a much lower temperature. The maximum temperature employed in these tests is higher than that normally recommended for laminates. Phenolic laminates can be used continuously at 100" to 125' C. and intermittently at 150" to 175" C. Five test specimens were employed a t ea& temperature for each test. The materials constitute a representative cross section of those generally used in the mechanical grades of phenolic laminates. The samples were obtained from regular productionline boards or sheets. A commercial type of cresylic acid resin was used in their manufacture. A description of the grades utilized follows:
Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa. This paper deals with the specialized field of phenolic laminates, and reports their tensile, Eompressive, and shear characteristics from -55' to 200' C. The elevated temperatureawere attained by means of a small cylindrical furnace constructed to employ electrical resistance heating. To reach subnormal temperatures, a bath of solid carbon dioxide and alcohol was employed in a cylindrical vessel of annular construction. The yield strength, .ultimate strength, modulus of elasticity intension, ultimate strength in compression, ultimate strength in shear are reported a t -55", -20°, O', 25', 75", EO', and 200" C. We may conclude from the results that the tensile, compressive, and shear strengths of phenolic laminates are inversely proportional to temperature, and that the cellulose-filled materials are more sensitive to temperature change than their mineral-filled counterparts. The rate of loss of strength as a function of temperature increases above room temperature for the cellulose-filled laminates, whereas it decreases for the mineral-filled materials. Evidence of this variable change is supported by thermal expansion data for a typical laminate. The thermal expansion curve is shown to have a transition point a t a temperature well within the range a t which the accelerated change in physical properties seems to take place.
Materia Designation A B
C D
E
F G
NEMA or ASTM Grade
C
X
Ah
XX LE
(Exptl.) AA
Filler Coarse-weave cotton fabria Kraft paper Asbestos cloth Alpha paper Fine-weave cotton fabric Fiberglas fabTic Asbestos fabric
Resin, Phenolio, Phenolic, Phenolic. Phenolic' Phenolic' Phenolic' Melamink,
% 35-40 40-45 40-45 50-65 50-55 35-40 40-45
TEST METHODS
GENERAL. A standard Amsler 3(rton test machine was employed for all tests. This machine has a maximum head speed a t no load of 0.050 inch per minute. The elevated temperatures were attained by a small cylindrical furnace constructed to employ electrical resistance heating
Figure 2.
Figure I .
Apparatus for Tensile Testing 580
Equipment for Shear Test8
I-,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1945
s81
aloohol wns used in B cylindrical vassel of nnnulsr conof dry tee Fxgure 1 shows the setup for the tensile teste, and F~gure 2 shorn the shew test setup. The tat specimen wns mounted ~nthe machine wth the heating or cooling chsmbr m place and the temperature adjusted. The temperature wa9 measured hy s thermocouple fnstened to the test specimen (Figure 3). The time required to sttarn thermal eqmbbnum for the vmous temperatures wa9 as follows. 3-6
-56
-20
3 2
0 25
76
2
...
P
Figure 3. Method of Foxteninp, Therrnoconple tu Terl Specimen
3
150 200
...
-
..
No attempt wm made (0 precondition the test specimens; they were tested ns received. While it is admitted t h a t tho time
TABLEI. A V E R A ~ EVALUESor T E V ~ ~PHOPERT~ES LE (IN P O E N D S PER S O U A R E INCH)
T~~~
-55-
c.
-209
c.
00
c.
25*
c.
750
c. isel c.
UILimds Tenails Strength 9,770 9*z90 8,720 5.310 22.W0 22.880 22,700 11.530 10,530 in nu0 9 380 8.890 14.~30 i3:9zn IZ:ZJO 7.720 13,7no 12 700 12.330 7340 44,330 12:540 37sou d 4 s o i 2 . z ~ ~ 11,460 i n . 3 ~ 9.220
2uv
c.
A.
A
R C
D
E F
G
10.350 21.570 1I,7w 15,320 14.830 n.4m IZ.P~U
A
8.640
c
iu.080
B
.,..
E . 0.2% Yield Stranpth in Tension 6.780 7.290 6.200 3,030 .... 13,000 8,200
i:oia
R
D E
ll.840 13.835 30,880
11,010 10.110 28,8n0
9 30
0
10.540
8.520
8
A
1.13
3,330
~ .. . 1,2"0 , ... 7,320 .... . . ..
7,940
zb.'z80
8.310 1.575
. ~. .
23:iio 6.830
i.300
... .
10
i;jan
C. Modvlua of Eiestioity in Tension ( X IO')
B
c D E
F
G
*
2.37
2.10 2.19
0.99 a.16 2.0
1.5
1.52
1.4
1.3
2.8b
3.1
3.3 3.3
4
3.7
R m m lam emture.
b .t
0.99 1.85 1.84 1.4
- 46- 8.
0.87 2.08 1.85 1.2
1.2 2.9 2.0
0.51
1.39 1.37 0.85 0.83 2.8 1.5
0.21 ,
..
1.1
u,7n
2.6
2.2 1.3
.... .... 1.0
....
....
required to sttsiil thermal equilibrium does &ect the t w t specim e n ~(and the effect is detrimental at the higher ternpentturd, thia prooedure ww followed beesuse it w m practical. TENSION. The stress-strain curves were obtained w i t h a Beidwin-Southwsrk recorder. Figum 3 shows the mothod of clamping the extel~ometerto the test specimen. The end3 of eneh tensile teat specimen were alweys soored and sandpapered to minimize slippage in the grips of the machine. The tensile test spocimenu were shaped according to the A.S.T.M. standards on plwtics and were %pproximsteiy '1. inch thick; they were a little longer (191/* inches) then usual to keep the grips of the ksting machine out oi thc heating or cooling chnmber. COXELE~~NXJ. The compression 1081 samples were sppmnimat& oneinch cubes cut from oneinch-thick laminated boards. The heating snd cooling chambers employed for the tensile tests were nlso used ioi tho compressive tests. SKEAE.Severs1 shesr fixtures were tried before it WBJ decided ta use thc punch and die type which is under consideration for approvd by A.S.T.M. Committee D-20 on plastics. A crosssectional view of tho sheer test fixture is given in Figure 4. A phenolic laminrtc shear fixture was made ior uae st the lower temperatures. Its insulating properties shortened the time required to reach and maintain these lower temperatures. A tool steel fixture WM mnde for ulle at elevated temperatures. The punoh and die type of shear fixture wlyl employed beoauae it seemed to minimize bending of the specimen during the teat. All shear test specimens were '/t inoh thick.
Figun 1. PrppoMd A.S.T.M. S h u r Toot (Punch Type) for 0.010 to 0.500 Inch
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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
Vol. 37, No. 6
The ultimate shear strength was determined by dividing the load (in pounds) by the area of the shear edge which is taken as the product of the thickness of the specimen by the circumference of the punch. TENSILE PROPERTIES
The ultimate tensile strengths are listed in Table 1A; the'yield strength at 0.2% of?set is given in Table IB with the exception of material 13 (kraft paper filler), which does not seem to have such a point except at the most elevated temperatures. The stressstrain curve for material B is a straight line up to its breaking point. Table IC contains values of modulus of elasticity in tenTENSILE S T R A I N = I O 3 - INCH I INCH sion which were calculated from the straightFigure 5. Stress-Strain Curves at Various 'I'emperatures for Type A line portion of the stress-strain curves. Laminate Figures 5 and 6 illustrate the stress-strain diagram for specific materials at four temDEFINITIONS peraturw. Each curve represents an actual test taken from approximately the middle of the range for the samples The yield strength in tension was determined by drawing a tested; that is, the specimens represented by these curves line parallel to the initial straight-line portion of the curve and have approximately the average tensile properties shown at an offset of 0.002 inch per inch. The stress which corresponds in Table I. The yieM to the intersection of this line with the stress-strain curve is points for each arbitrarily defined as the yield strength. 240 material are connected The modulus of elasticity in tension is taken as the ratio of by a dotted line to stress over strain (streas/strain) for any point on the straighbline show i t s v a r i a t i o n portion of the streas-strain curve. with temperature. The ultimate compressive strength is defined as the ultimate I60 C o m p a r i s o n of t h e compressive load divided by the original bearing area of the test slopes of the straightspecimen. line portion of these I 80 -50 -25 0 25 50 75 c u r v e s shows h o w t h e elastic modulus TABLE11. ULTIMATE COMPRESSIVE STRENGTHS (IN POUNDS in tension . decreases PER SQUARE INCH) with increasing temType -55' C. - 2 O O C. Oo C. 26O C. 76* C. 150' C. 200' C. perpture. A. Fiatwise Direotion Figures 7 and 8 42,800 42,120 29,850 19.460 11,600 A 51,000 44.930 44.800 41,800 44.030 33.530 B 48,800 show the effect of 48.700 43 630 48 550 3 8 4 3 0 25;300 30;780 C 54,980 48,250 44:630 39:850 29:150 . . ... D 54,100 temperature on the 4 3 9 0 0 4 3 2 0 0 31 730 45,300 E 54,100 ultimate and yield F 73,200 66,900 47:400 e9:7ao 48:870 3533'0 23;ido Q 51,820 47,530 41,800 46,800 37,660 27,160 38,500 strengths of specific B. Edgewise Direotion materials. The per9.825 8,800 27,050 24,280 15,1180 A 37,350 30,300 c e n t a g e c h a n g e in 80,450 26,750 26,075 16.750 B 33,400 -50 -25 0 25 50 70 19 800 20 575 16225 12,'a'iO 1i:djb C 22.250 22,450 TEMPERATURE OC. strength is greater 29:700 26:700 I5b75 30.800 D 38.300 for cellulose-filled .... . i:di5 33 100 27 900 19 900 E 311,850 33.900 Figure 7. Temperature Effect 7,325 18:830 14:195 13,'700 F 20.400 14,800 than for 22,100 21,000 21,400 11,098 12.070 23.300 0 23,450 on Strength of Type B and Type D Laminates filled plastics. The following table compares the two:
*u
8
...
..
....
% LOWfor
% Loas for
Temperature Rsnge, C.
Laminates
CeUu)ose
Mineral Laminatea
-55 to 25 -55 to 76 -65 to 200
13.5 48.7 88
22 31 45
COMPRESSIVE PROPERTI ES
TENSILE S T R A I N INCH /INCH
Figure 6. Temperature Effect on Stress-Strain Properties of Type'F Laminate
Figure 9 shows how the ultimate compressive strength varies with temperature. The curves were drawn from the data in Table IIA. The rates of change in strength are about the same except for asbestos-filled laminates C and
June, 1945
INDUSTRIAL AND ENGINEERING
CHEMISTRY
583
TEMPERATURE-OC.
Figure 9. Effeot of Temperature on Flatwise Compressive Strength of Laminate TEMPERATURE-OC.
Figure 8. Temperature Effect on the Strength of Type F Laminate
TEMPERATURE- OC.
Figure 10. Effectof Temperature on Edgewise Compressive Strength of Laminates
G, which show a slower rate of decrease in strength with increase in temperature. Material F which contains Fiberglas filler gives about the asme rate of decrease in strength as do the cellulose-Bled materials. The significance of this fact appears to be that the resin is more critical than the filler in the range of temperatures employed in this test. The Fiberglas-
Figure 11. Effect of Temperature on Strength of Type D and Type E Laminates
filled laminate is 40% better in ultimate strength than any of the others (Figure 9). The data for ultimate compreaeive strength in the edgewise direction is recorded in Table ILB and was obtained according to the outlined procedure. Figure 10 shows graphically the deet of temperature on the edgewise compressive strength of plastics
INDUSTRIAL A N D ENGINEERING CHEMISTRY
v0i. 31,
no.8
SHEAR PKOPEKTIES
Material G was not available for this experiment. Five testa on each of the others were made at the six temperaturan. Three testa were mn in a flatwise direction and two in the e d g e d direotion with reference to the Guer in the laminatas. Table I11 contaim the data on ultimate Eatwise and e d g e d shear s b n g t h . Figure 11 shows how theultimateshearstrengthsofmsterials D and F vary with temperature. Testa on materials B and D showed what might be called an "initial ultimate'' and a "final ultimate". At a point which varied between 50 and 90% of full load, there was a momentary falling off of the load. A t temperatthis was aecomFisura 12. Laminatee after Being Stressed to the Initial Ultimata Point at panied by an audible snap and visible a Subnormal Temperature (-20' C.) in am Edgewise Direction cracking of the speoimen; it hsppened The fdnt u&* ~ s n i n ps r d I d to the ladlutlon tho 8nt imdtortinn nf hi1n.r. . .. in only the edgewise direction and WBB not evident above mom temperature. Investigation of the specimen at this point disclosed minute crsoks ninning parallel to the laminations. Figure 12 shows the two types of specimen atfected, with the hairline craeks readily discernible. One might at first believe this initial failure due to bending rather than shear. Actually it is failure in hew, except that in this ease it ie p a l l e l to #e lamioatiom whereas the &SI failure ocours BEthe lsminstions. Material 8, which contains B kraft paper flier, nutlera #e initial ultimate at 50 to 60% of ita 6nal ultimate; material D, which contains an alpha paper filler, suffers ie initial ultimste at 70 to 90% of ita final ultimate. This again ten& to prove that #e initial failure is due to shear pardle1 to the laminatiom because the alpha-paper-filled laminate should and does have better bond strength than the kraft-paperfilled laminate.
T ~ L 111. E AVERAGE ULTIMATE SKEAKTEST VALVBS (E3POUNDS PER 8QlJaKEINCH) TW -M* c. -zo* c. c. zs'c. 75- c. i s n o c. z n v c. 'A.
Flatwine Dim&ition
The euwm of shear strength against temveratnre show that, again, the Fiherglasdlled laminate is superior to the others in the fislwise sense, hut that the diKerence between ita Batwise and edgewise shear strength is muoh greater than for the other matenals. The IORS in strength (for mwt of the maMaI.7 tested)
changes abruptly at room temperature. In severs1 casea the rate strength is greater above room temperature; in a few cases it is leas. This is similar to the phenomenon o b 4 in the tensile testa and to a certain extent in the c o m p h v e tests. The Isminates which have a slower rate of s b n g t h loas sbove room temperature w e of the mineral filled type--Fiherglas fabrio and asbnstos fabrio of loss in
TEMPERATURE-.^.
Figure 13. Thermal Erpndon Teat on M i a r t r
June, 1945
585
INDUSTRIAL AND ENGINEERING CHEMISTRY CONCLUSIONS
The results of theae teats show that the tensile, compressive, and shear strength of phenolic laminatea are inversely proportional to temperature, and that the Oellulose-Wed materials are more sensitive to temperature change than their mineral-filled counterparts. The rate of loss of strength as a function of temperature increases above room temperature for the aelluloee-filled laminates, whereas it decreases for the mineral-filled materials. At the time these teat reaulta were being analyzed, thermal expansion teats on paper-8lled laminates were being conducted. It was noticed that a change in the coefficient of thermal expansion took place at approximately 60' 0. Figure 13 shows the expansion u.9. temperature characteristica of this material; the transid $ion point can be located accurately by a straight edge. The
temperature at which t h i transition occurs lies within the range where critical variation in the physical properties of the laminates was noted. Many more determinations will have to be made before any general conclusion can be drawn with regard to the effect of second-order transition point on physical properties of laminates and of plastics in general. This phenomenon should be of general interest and must be considered in any future investigation of the effect of temperature on the physical properties of plastics and plastics laminates. ACKNOWLEDGMENT
The authors are pleased to acknowledge the assistance and suggeations of R. W. Auxier and P.G. McVetty of the Westinghouse b e a r c h Laboratories.
AZEOTROPIC DEHYDRATION OF PYRIDINE AND ITS HOMOLOGS LLOYD BERG, J. M. HARRISON, AND C. W.MONTGOMERY Gul ReMarch de Development Cornpony,Pittsburgh, Pa. By means of the hydrogen bond c l ~ i f i c a t i o nof liquids, the search for entrainers for effecting a desired azeotropic mparation b considerably simpllfied. Thb rgstem is applied to the eeleation of entrainers suitable for the azeotropic dehydration of pyridine and ita homologs. A number of satisfactory entrainers are listed, and experimental result0 are given for the dehydration of pyridine, 2-picoline, and 2,6lutidine, using toluene, methyl isobutyl ketone, and propyl imbutyrate, respectively.
I THE
N study of azeotropic distillation, very little information has been presented to enable an investigator to select a suitable entrainer for Q desired separation. Recently (8)a system waa proposed for grouping all liquids into five classes, depending upon their hydrogen-bond-forming potentialities. The object of this paper is to make u8e of the generalizations derived from this classification in the selection of azeotropic systems, and specifically to determine suitable processes for the dehydration of pyridine and its homologs. The high water-solubility of pyridine and its homologs makes ita recovery from water-insoluble media easy to accomplish by extraction. The attractiveness of this method is lessened, however, by the difficulty of separating pyridine or its homologs from water. Pyridine, picolines, and lutidines form minimum-boiling azeotropes with water 80 that they cannot be separated by straight rectiiication alone. Furthermore, the composition of these azeotropes is high enough in the heterocyclic compound 80 that they contain too much pyridine oz homolog to be discarded but an insufficient amount to be treated as reasonably pure. Previously reported work on the azeotropic dehydration of these heterocyclic compounds is limited (2,6). HYDROGEN BOND C W S S I F I C A T l O N OF LIQUIDS
Based on the hydrogen bond classification of liquids, generalizations concerning the probable nature of the azeotropes formed between any two classes of liquids can be made. Briefly the classe~ are as follows (3):
CLASS I. Liquids capable of forming three-dimensional networks of stron hydrogen bonds. CLASS11. &her liquids composed of molecules containing
both active hydrogen atoms and donor atoms (oxygen, nitrogen, and fluorine). CLASS 111. Liquids composed of molecules containing donor atoms but no active hydrogen atoms. CLASSIV. Liquids composed of molecules containing active hydrogen atoms but no donor atoms. CLASSV. All other liquids-i.e., liquids having no hydrogenbond-forming capabilities. By this system of classification it is possible to predict the nature of the deviations from Raoult's law, and the hydrogen bonding in a mixture makes it possible to judge approximately the extent of these deviations. When a system which shows positive (+) deviations from Raoult's law forms an azeotrope, it will be a minimum-boiling azeotrope. When the deviations are negative (-), any azeotrope formed will be maximum boiling. On the basis of this liquid classification, a systematic summary of deviations for water (class I) and the pyridine heterocyclia compounds (class 111) is the following: CLASSBS I and V E and EV I and I I and I1 I and I11 I11 and I1 I11 and IV
$ i: :"}
DEVIATIONB
}
1
Always (+) deviations; frequently limited
solubility
Usually (+) deviations; very complicated groups
Always (- ) deviations Quasi-ideal systems; always (+) deviations or ideal ENTRAINER PROPERTIES
With the understanding of azeotrope formation afforded by this uwsification, it is possible to design methode for sepamting
