Transition Temperature and Cubical Expansion of Plastic Materials
FRED E. WILEY Plax Corporation, Hartford, Conn.
The standard specinleu is a slab, 3 X 1 X '18 inch. Standard specimens are used as a matter of coiivenience but are not necessary. Any shape weighing about 20 grams may be utilized, Prior to testing, the specimens are dried in an oven at 75" C. for 2 hours and cooled in a desiccator over anhydrous calcium chloride. If desired, the specimen may be conditioned as recommended by the American Society for Testing Materialsnamely, 25' C. and 50 per cent relative humidity. However, in this case the specimen \vi11 lose water during the test, and fiomewhat, different results will be obtained. The specimen is suspended from a balance arm by a length of silk thread, and its weight in still air is determined to the nearest 5 mg. The oil bath is brought. t: a low temperature (0-20" C.) which is maintained within 0.5 C. Stirring is necessary to ensure uniformity of temperature. The specimen is hung fully submerged in the bath. Five minutes are alloxved for the specimen t o assume the temperature of the bath. Stirring is then stopped and the neight of the submerged sample determined. The v,-eighing is repeated at time intervals until no further change of weight is noted. Constant weight is obtained some time after temperature equilibrium has been established. The bath temperature is then raised and the measurements are repented at the higher temperature level. This procedure is repeated until a temperature of about 130-150" C. is reached. The same procedure is then followed in steps as the bath is cooled. When measurements are made below room temperature, the beaker of oil is placed in a pail of acetone. Solid carbon dioxide is added to the acetone until the desired temperature is reached.
A test to measure the "transition temperatures" o f plastics and their cubical coefficients o f expansion is described, and data obtained by this test are presented. The apparatus is simple and map be readily constructed from standard laboratory equipment. No inordinate sltill is required on the part of the laboratory technician i n securing precise results by this method. It is shown that changes i n the slope o f the temperature-expansion curve correspond to the softening points found by several commonly used tests. Data on the temperature limitations of plastics have been obtained which are of value in determining the utility of plastics for industrial use.
NE of the more important physical properties of a
0
plastic is its softening point. This is the transition temperature a t which internal strains may be relieved, strength drops rapidly, high elasticity sets in, and excessive cold flow takes place. Ample evidence has been presented by Carswell, Hayes, and Nason to support the assumption that some plastics undergo a fundamental change in structure a t a definite temperature, which in some respects resembles the melting of crystals (4). I n this paper a method of measuring the transition temperature of a plastic is described. This method requires a minimum of apparatus and is therefore available to any laboratory. The test is capable of determining the density and volume expansion of plastics and hence serves a triple purpose. It is apparent that one test which mill determine separately three fundamental properties of a plastic is aell suited to formulation control. This method consists essentially of measuring the volume expansion of a plastic sample over a wide temperature range. The transition temperature is indicated by a change in slope of the expansion-temperature curve.
Calculations An accurate calibration chart must be obtained for the density-temperature relation of the bath medium. The weight of the sample in oil (either measured or found by extrapolation) at 0' C. is used to calculate the specific volume of the sample by Equation 1:
Apparatus and Procedure Figure 1 shows the apparatus, assembled from standard laboratory equipment. I t consists of an electric hot plate, a 2-liter beaker filled to about 1 inch of the top with mineral oil, a 500-watt thermostatically controlled immersion-type heater, a laboratory thermometer, a balance sensitive to 2 mg., and a spool of silk thread. For low temperature work a pail of acetone and solid carbon dioxide is used. The mineral oil used is Thermol Medium, a Stanco product which has been found satisfactory for most materials. At low temperatures an aircraft compass oil is employed to avoid excessive viscous damping. 1052
HIGH-FREQUENCY RADIO P k R T S LIMITINGT E V F E R I T U R E OB 82" C.
POLYSTYRENE
POSSESS
.i
September, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
1053
While pure mineral oil is relatively stable, continued use a t elevated temperaturps will change its appearance from water clear to deep orange. The density-temperature calibration will hold true until the oil becomes cloudy and yellowish in color. If the plastic to be tested is affected by the oil, such action is evidenced by a change in weight of the specimen. By heating a plastic in the bath and then weighing again in air, the effect by oil may be investigated. Ry this method a limiting temperature will usually be found, above which the plastic will rapidly and permanently change its weight. Hence polystyrene will begin to change weight rapidly in pure mineral oil a t about 210" F. (99" C.). If such a change occurs during a test, the curve obtained by increasing temperature will differ from that obtained when the temperature is lowered. Moreover, the weight in air of the specimen a t the end of the test will differ from its initial value. It is interesting to note, however, that if a volume-expansion curve is secured for certain plastic samples and these samples are then heated in oil until a change in weight is observed, the original curve may be obtained by repeating the test using the new weight in air. In other cases, however, oil will plasticize the material and valid results may not be secured. Ethylcellulose shows this action, while polystyrene and Vinylite are characteristic of the former action.
FIGURE1. APPARATUSFOR MEASURING THE CUBICALExPANSION OF PLASTICS
During the cooling of a melt, the tendency of the molecules to cohere will increase as a consequence of the diminished heat motion. This action is indicated by a decrease in volume on cooling. It has been found that amorphous materials pass
1.0400
where Vo = specific volume at 0" C., cc./gram Wa = weight of sample in air, grams WO = weight of sample in bath at 0" C., grams PO = density of bath medium at 0" C., grams/cc.
,
Discussion of Results
,
i'
TEMPERATURE IN OC.
,z;
,
,
,
,
io;,
,
Unit volume is found by Equation 2: 2,
=
(--) w,- w1
w,- wo
po Pt
where v = unit volume, ratio Wt = weight of sample in bath at T oC., grams p , = density of bath medium at T oC., grams/cc.
A curve of unit volume us. temperature is then plotted on coordinate paper as in Figure 2. The slope of such a curve is the coefficient of cubical expansion. A break in the curve indicates a transition temperature. If density is calculated, appropriate correction for the buoyant effect of air should be made. However, because of the nature of Equation 2, such a correction is not necessary when calculating unit volume.
Peculiarities of the Test At temperatures close to the transition point it is necessary to wait for the volume to become constant a t constant temperature. This apparently occurs some time after temperature equilibrium has been established, as indicated by the gradual stabilization of the weight. With some materials this stabilization time may exceed one hour. When no further change of weight is noted, this value is recorded for the particular temperature. Some plastics possess a high degree of relatively low-boiling material which, under conditions of the test, forms bubbles within the specimen. It is obvious that tests are not valid when bubbling occurs.
TEMPERATURE IN OF.
FIGURE 2. VOLUME-TEMPERATVR~ CURW OF POLYSTYRENE EXHIBITINQ A TRANSITXON ZONE WITH A TRANSITION POINT AT 82' C. from the "liquid" into the "solid" state without a discontinuous volume change (16). However, as the temperature falls below the transition point, Tg,the volume shrinkage due to falling temperature proceeds at a slower rate (7). (Tois an abbreviation of the German Temperatur des Glasxustandes, sometimes referred to as the brittle point.) Hence the transition point may be determined by a break in the expansiontemperature curve of an amorphous material. For a pure material this break is very sharp and the transition temperature may be accurately located. Patnode and Scheiber showed that the expansion-temperature curve of a pure polystyrene polymer consists of two intersecting straight lines (14). The point of intersection is
1054
Vol. 34, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY 0
20
TEMPERATURE IN ’C. 40
60
80
100
mercial polystyrene. The above phenomenon is shown in Figure 2. The curve of Figure 3 for a methyl methacrylate copolymer molding compound is characteristic of a pure amorphous material (7). The phenomenon of amorphous materials is termed a ‘[second-order phase transition’’ and is characterized by a change in the degrees of freedom of the chain molecules. Presumably this would mean that, below a second-order transition point, individual segments of the chain molecules do not carry out rotational and vibrational movements as a whole, while they do carry out such internal Brownian movements above this point (IS). The curves for the cellulosic esters shown in Figure 4 are somewhat different in character. The portions of the curve representing the “melted” and “frozen” regions do not intersect even when extended, but are connected by a region of rapid expansion. Such materials possess two transition points with a temperature interval in which softening takes place. This type of curve was found for both strained and annealed cellulose acetate. The character of the curve was
120
w
1.000020
60
100
140
180
220
TEMPERATURE IN OF.
CURVEOF METHYL FIGURE3. VOLUME-TEMPERATURE METHACRYLATE COPOLYMER (CRYSTALLITE A-loo), CHARACTERISTIC OF A PURE AMORPHOUS MATERIAL
TEMPERATURE INOC.
TEMPERATURE INOC. 0
called the “transition temperature”. Jenckel and Ueberreiter (8) attempted to show that the transition temperature of polystyrene is a function of molecular chain length. Their data indicate an upper limit for transition temperature of about 100” C. regardless of molecular weight. 1 It is rather infrequent that the condition observed by Patnode obtains. Commercial polystyrene consists of a mixture of molecules of widely varying size and of impurities such as monomeric styrene, ethylbenzene, paraffin, and other materials. When such a sample is tested, we find that, instead of a sharp break in the volume-temperature curve, the inversion takes place over a temperature range of perhaps 20” C. and thus gives a “transition zone”. If, however, we extend the straight sections of this curve until they intersect, it is interesting to note that the transition point thus indicated is a t 82” C. T h i s value is the temperature found by experience t o be t h e softening point of com-
20
40
60
80
100
120
0
20
40
60
80
1.0500
1.0400
PROPIONATE
W
2 1.0300 3 _1
9
k z
3
1.0200
1.0 I60
1.0000
IO0
60
TEMPERATURE
140
TEMPERATURE
INOF.
180 INOF.
TEMPERATURE INOC.
TEMPERATURE I N OC. I.050C
1.040C
W
z
1.030C
2 -J
9
k
2 I.OZ0C
3
1.0 I oc
l,OOOC
60
100
140
TEMPERATURE
180
220
INOF.
TEMPERATURE INOF:
CURVESO F PLASTICIZED CELLULOSIC ESTERS FIamE 4. VOLUME-TEMPERATURE Transition is indicated by a zone of rapid expansion.
100
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1942
TEMPERATURE IN%.
20
80
100
140
TEMPERATURE IN "C.
220
I80
TEMPERATURE
260
INOF.
FIGURE5. VOLUME-TEMPERATURE CURVES OF THERMOSETTING
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PLASTICS
Transition temperature corresponds t o point at whioh internal strain can be relieved.
'=
TEMPERATURE
FIGURE6.
not changed by holding the specimen a t 310" F. (154" C.) for one hour before testing. This type of change is called a "first-order phase transition'' and is characterized by a discontinuous volume change which takes place upon passing through the transition zone. The first-order transition is akin to crystallization and indicates a fusion of the material (13). If the lower section of such a curve is extended to the upper transition temperature, an estimate of the volume change on fusion can be made. Baker, Fuller, and Pape showed by x-rays that cellulose acetate may exist partially in the crystalline state (2). The volume change on fusion is an indication of this crystalline portion. Baker also showed that by appropriate treatment, cellulose acetate may exist predominately in an amorphous or disordered state (2). I n such a state cellulose acetate should exhibit a second-order transition. There is another observation to be made. With some materials that do not progressively soften with temperature rise, such as cellulose nitrate (Figure 4) and phenol plastics (Figure 5 ) , the coefficient of expansion above the transformation interval is lower than that found below the interval, This feature is not necessarily true for all infusible resins. It is thought that the second-order transition temperature and the lower temperature of a first-order transition correspond to internal softening and are representative of the temperature limitation of a plastic for industrial use. Plastics that do not entirely soften with heat, such as cellulose nitrate and phenol plastics, appear to be exceptions. The upper temperature of cellulose nitrate corresponds to the softening point found from experience, while the upper point for the phenolic plastic corresponds to the annealing temperature a t which internal strains are relieved. The curves of Figures 2 to 7 are all reversible and show no thermal hysteresis. Qach reading of volume was stabilized a t constant temperature. In most cases constant volume wag attained within 20 minutes; a t temperatures close to the transition point a considerably longer time was necessary. Stabilization is an important part of any procedure by which transition points are obtained. Without stabilization the points may be shifted (3). Certain transition temperatures for glass have been found by Lillie (19)t o disappear
INOF.
VOLUME-TEMPERATURE CURVES ACRYLATE~COPOLYMER
OF
METH-
High-temperature break a t 124" C. aorresponds to softening point of Norton material.
TEMPERATURE INOC.
W
30 > t
2 I.OZO0 3
1.0 IO0 9
I.oooozo
t
TEMPERATURE
INOF.
F I G U R7.~ VOLUME-TEMPERATURE CURVEOF POLYVINYL CHLORIDE ACETATE
entirely upon stabilization. Hence the solid state of amorphous matter is not to be considered as different from the liquid state. The transitionpoint must be interpreted as the temperature at which the lag between temperature and the adjustment of the molecular bonding becomes appreciable. The glassy state of a thermoplastic resin must be interpreted as a liquid state in which conditions are far removed from equilibrium (11). Materials that actually crystallize are excluded from these statements, as indicated by the work of Goggin and Lowry (6). According to the work of FUOSS, viscosity seems to be the property that retards rapid attainment of equilibrium (6).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 9
is approximately 4 X 10l2poises. This value will vary, depending upon the type of plastic tested and the rate of temperature rise used. Natl. Bur. of Expansion VolumeA softening point test used a t the National Bureau of Standards Softening Transitbon Standards embodies a fairly large stabilization period and Plastic Material Point, C. Temp.. C. should yield fairly consistent results (IO). This test measures Methacrylate x-6 127 124 the sag of a cantilever beam in 1 hour a t constant temperCellulose nitrat,e F-2 68 72 Cellulose acetate C-23 57 53 ature. The procedure is repeated a t 10" F. (5.6' C.) intervals. 57 Cellulose acetate B-96 Cellulose acptate butyrate AA-7 56 58 The temperature a t which the beam sags a total of 0.1 inch is Cellulose aretate butyrate AA-5 54 56 taken as the softening point. Calculation and experiment Cellulose acetate propionate CP-1 61 55 Vinyl copolynier L-9 54 53 show t.hat this test measures an average visMethacrylate K-24 61 60 cosity of 1.4 X 10l2poises. It is felt that optimum results could be obtained by measuring THE TRANSFORMATION TABLE11. DATAOBTAINED I N PASSING THROUGH the rate of sag of a beam at the end of 2 hours INTERVAL^ a t various temperatures. I n this manner the Transition Cpefficient of Vol. Change in temperature a t which viscosity was 2 x 1012 Temperature, Cubical Expansion, Transformation c. TJ 10 - 5 / " c. Interval poises (or a more suitable value) could be measMaterial TO A B c % vu ured as a softening point which would be 82 ... 21.5 46.3 .. 0 Polystyrene XRIS-10023 relatively free from errors due t o lack of stabil91 ... 19.8 32.8 .. 0 Styramic Methacrylate copolymer A-100 54 . _ . 23.5 47.0 .. 0 izat,ion. Ethylcellulose 1911-MS 43 ... 34.6 50.5 .. Lucite 57 68 27.0 61.8 42:s .. Table I shows the transition temperature of 60 104 25.3 50.0 42.8 Methacrylate K-24 several types of plastics as compared to results Vinyl copolymer L-9 53 85 19.6 55.7 44.0 i:i4 Cellulose nitrate F-2 60 72 40 0 .. 07 50.0 27.1 0.10 of tests a t the National Bureau of Standards 4 Cellulose acetate B-96 53 70 58.8 46.7 0.30 Cellulose acetate C-23 57 71 41.4 51.4 39.3 0.29 on the same materials (9); there is fair general 58 73 41.4 Cellulose acetate butyrate AA-7 66.7 37.8 0.34 agreement between the two sets of data. 46.4 Cellulnse acetate butyrate AA-5 56 73 94.2 54 1 0 30 Cellulose acetate propionate CP-1 55 72 38.6 94.2 34.3 0.48 Table I1 presents the transition temperaWood-flour-filled molded phenolic 43 75 9.8 19.4 2.9 0.30 Cast phenolic R-1001 50 75 30.4 53.6 29.5 0.48 tures, coefficients of expansion, and volume a Nomenclature as follows: change on passing through the transformation F o r A . Vi = V O [ I + At . A applies below Tg. interval for a typical group of commercial For Bl Vr = Vo 11 + A J i + B ( t - T ) ] ; B applies below Tf. Vo [ l + A T g + B (TI - $0). + C ( t - T f ) l : C applies above Tf. For C: Vt plastics. T - lowest temperature a t which a break in volume-temperature curve occurs. next higher temperature a t which a break in volume-temperature curve occurs. Tj? Acknowledgment t = temperature C. V O = volume a t 0 4 C. The writer wishes to acknowledge the assistance of James Bailey and Donald Whinnem of the Plax Corporation and t o thank G. bl.Kline Tamman (16) found by the ordinary methods of determinaof the National Bureau of Standards for obtaining samples and furnishing check results. tion (17) that the viscosity a t the transition temperature of amorphous materials is always about 10l2 to 1013 poises. Literature Cited This fact is the basis of many empirical softening point tests, (1) Am. SOC.for Testing Materials, Method D48-39. However, until stabilized, viscosity will vary with time a t (2) Baker, W. O., Fuller, C. S.,Pape, N. R., J . A m . Chem. Soc., constant temperature in the transformation interval. This 64, 776 (1942). variation will be a function of the stabilization rate for each (3) Bekkedahl and Wood, IND.ENG.CREM.,33, 381 (1941). plastic. Usually the time required for stabilization is too (4) Carswell, T. H., Hayes, It. F., and Nason, H. K., Ibid., 34 454 (1942). great for rapid practical tests. Hence, minor discrepancies (5) Fuoss, R. M., J. Am. Chem. Soc., 63, 374 (1941). must be expected when the softening points of different (6) Goggin and Lowry, IND. ENG.CHEM.,34, 327 (1942). plastics are measured by empirical methods. The greater the (7) Houwink, "Elasticity, Plasticity and Structure of Matter", period of stabilization embodied in the test, the greater will p. 45, London, Cambridge Univ. Press, 1937. (8) Jenckel and Ueberreiter, 2. physik Chem., 182A,361 (1938). be the accuracy obtainable. It is felt that this method of (9) Kline, G. M . , private communication, Feb. 14, 1942. determining transition temperature embodies sufficient Ibid., March 2, 1942. stabilization time. Lillie, H. R., Glass I n d . , 19, 296 (1938). The A. S.T. M. thermal distortion test, which measures Lillie, H. R., J . Am. Ceram. SOC.,16, 619 (1933). ESG. CHEW,34, 449 (1942). Mark, H., IND. the sag of a loaded beam as the temperature is increased Patnode and Scheiber, J . Am. Chem. SOC.,61, 3449 (1939). continuously until a deflection of 0.010 inch is attained, emTamman, G., "Der Glaszustand", 1933. bodies very little stabilization time (1). The writer has Tamman, G., "States of Aggregation", New Yorlr, D. TTan found both by calculation and experiment that this test Kostrand Co., 1925. Trouton, F. T., Proc. R o y . Soc. (London), A77, 426 (1906) measures the temperature a t which the viscosity of a plastic TABLEI.
TRANSITION TEMPERATURES AND SOFTENINQ POINTS
COMPARISON OF
-
O
