G. D. JONES, R. E. FRIEDRICH, T. E. WERKEMA, and R. L. ZIMMERMAN Dow Chemical Co., Midland, Mich.
POLY-ALPHA-METHYLSTY RENE If i t s molecular weight is minimum Poly-a-methylstyrene is moldable and has x
b b b
Outstanding heat resistance Solvent resistance Hardness
ALPHA-methylstyrene has been homopolymerized to high molecular weight by the use of Friedel-Crafts catalysts and low temperatures (2, 4-6, 78). I t is generally known that a-methylstyrene is not homopolymerizable to high molecular weight by free-radical catalysts (75), although it can be copolymerized by the free radical mechanism. In this work, poly-a-methylstyrene of high TOlecular weight has been made by sodiumcatalyzed polymerization a t approximately room temperature and the properties of the polymer have been investigated (8). Poly-a-methylstyrene, as described by Hersberger, Reid, and Heiligmann (4, 6) is more brittle than polystyrene of the same molecular weight. Moreover, it is desirable to keep the molecular weight a t a minimum in order to improve the ease of molding and extrusion. Problems associated with molding and extrusion are the relatively greater stiffness and lower thermal stability in comparison to polystyrene. Therefore, it becomes important to define carefully the lower limit of molecular weight for useful physical properties. Inasmuch as unfractionated samples were used in this work, discrepancies were noted in correlating physical properties and molecular weight measurements. A typical example of molding grade polystyrene, having a solution viscosity of 31 cp. (loyoby weight solution in toluene), was found to have a weight average molecular weight (light scattering) of 306,000. In contrast, a typical poly-a-methylstyrene sample having a solution viscosity of 15.6 cp. was found to have molecular weight 339,000 by light scattering. This difference is caused by a broader molecular weight distribution in the poly-a-methylstyrene. A sample of polystyrene made by sodium-catalyzed polymerization had a broader distribution also.
Poly-a-methylstyrene made by sodium catalysis is clear, colorless, and of excellent electrical properties if pure monomer is used. Minor amounts of impurities such as oxygen, when present during polymerization, cause the for-
200
mation of a yellow colored polymer. Samples 0.1-inch thick have been prepared having a difference in ultraviolet transmission, when measured at 420 and 620 m p of 3% as compared to 2y0 for commercial polystyrene.
c
I60
.
.*
ROCKWELL SUPERFICIP HARDNESS
RECOVER r
X
COMPRESSION MOLD1 NG x-x
140
POLY . a - M E T H Y L S T Y R E N E
I20
100
80
',,. \-
60
'\\
40
\
INDEN TA T/ON
\*
\
\ \
\
20
I 30
I 15
b
45
LOAD, KG.
Figure 1. Hardness measured with a 1 /4-inch Rockwell superficial-hardness ball reading a maximum of 200 Modulus of Unplasticized Plastics Bending Tensile X 1 0 - 6 Poly-a-methylstyrene Polystyrene Poly(methy1 methacrylate)
60,500 48,000 40,200
3.0 2.55 2.55 VOL. 48, NO. 12
Flex. Strength, Lb./Sq. In.
7,500 12,500 1 1,000 DECEMBER 1956
2123
0
POLYMER
‘/a
Figure 2.
The electrical properties of poly-cumethylstyrene have been determined by the M I T Laboratory for Insulation Research.
Electrical Properties Frequency, Cycles 102 10s 108 107
108 1 09
3
x
10’0
109
Dielectric Constant 2.60 2.60 2.59 2.58 2.57 2.57 2.57 2.56
Dissipation Factor,
% 0.0067 0.0067
0.007 0.01
0.021 0.048 0.045 0.03
It appears that in poly-a-methylstyrene as in tert-butylbenzene when compared to polystyrene and cumene, respectively, the extra methyl group fits into holes in the structure necessitated by the imperfect packing of benzene rings. This results in greater density (Figure 2) and greater solvent and heat resistance in the case of the derivative with the extra methyl group. Just as the a-methyl group increases the stiffness of methacrylates as compared to acrylates, so it increases the stiffness of poly-a-methylstyrene relative to that of polystyrene. This difference is reflected in a higher second-order transition point, a higher melt viscosity, more hardness (Figure I), lower elongation (Table 111), and greater solvent resistance. I t is also reflected in greater difficulty in orientation. Polystyrene monofilament is readily oriented to a degree measured by a birefringence of 3 X 10-2, but it is difficult to get quite as high with poly-a-methylstyrene. Poly-a-methylstyrene is soluble in aromatic and halogenated solvents, but
2 1 24
% POLYMER
100
50
Poly-a-methylstyrene in a-methylstyrene
less so than polystyrene. This is shown by the fact that upon addition of methanol to a dilute toluene solution, precipitation occurs at 26% methanol (by volume) in the case of polystyrene and 22y0 in the case of poly-a-methylstyrene. Polystyrene is soluble in methyl ethyl ketone over the whole molecular weight range, although osmotic and viscosity data show that the polymer chains are less open in this solvent than in toluene. Only low molecular weight poly-amethylstyrene is soluble in methyl ethyl ketone. Gasoline affects poly-a-methylstyrene much less than polystyrene. Moreover, solvents which will dissolve poly-a-methylstyrene are much slower in attack upon a molded surface than in the case of polystyrene.
Physical Properties A practical test of solvent resistance is obtained through measuring critical elongation (20)-the elongation where crazing occurs when the specimen is exposed to a vapor. Poly-a-methylstyrene seems to have a higher useful elongation by this test despite its lower absolute elongation. Critical Elongation of CompressionMolded Samples
Solvent Gasoline Heptane Butter
.
Poly-
Poly-a-
styrene,
methylstyrene,
%
%
0.08 0.08
0.29
0.20-1.00 0.24 0.50
degradation than polystyrene, there is greater dependence on the conditions of fabrication. Poly-a-methylstyrene is a noted example of those polymers which have a long “zip” length in degradation-Le., a t elevated temperatures, depolymerization has a relatively long kinetic chain length. Depolymerization occurs at lower temperatures than with polystyrene, as has been shown by Jellinek (7) and Madorsky ( 7 7 ) ; however, these workers have studied depolymerization rate in vacuo, whereas degradation under practical conditions is catalyzed by autoxidation and may be suppressed by antioxidants. A practical test has been developed which involves two steps. First, theground polymer sampleis heated a t 265 O C. in air for 15 minutes, then vacuum is applied a t the same temperature for 15 minutes and the residual weight 4 ,
:h 2 30‘C
I
4 L
\
240’C.
7-
P
HEATING T I M E I N HOUQS
The physical properties of poly-ctmethylstyrene depend on the amount of monomer and polymer of low molecular weight present in the specimen; because of higher sensitivity of the polymer to
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
I
l
l
I
I
I
I
I
I
1
1
1
I
96 L -
.9
$6
.7 .6
60
.5
c 54 -
c,
.3
.2
40
0
4
6
12
16
20
24
28
32
T I M E , HR.
Figure 4. Depolymerization on heating a 4% solution in diphenyl ether. C/Co is relative polymer concentration determined by precipitation in methanol
20
Figure 5. Depolymerization by heating 15 minutes in air and 15 minutes under vacuum at same temperature (right) 2 50
260
I
I
270
280
I
I
290
300
T E M P E R A T U R E , "C.
determined. The results of this method showed that use of an antioxidant reduces the degree of depolymerization to that obtained under vacuum. Sweeping with nitrogen was not equivalent to heating under vacuum. I t does not take much oxygen to promote degradation in the absence of antioxidant.
Weight Loss of Conditioned Samples Sample Conditioned Under: Vacuum
Weight Loss, yo 4.8,5.0,5.7,4.8, 5.3, 5 . 2
Nitrogen Air Air (0.5% 2,6-di-tertbutyl-p-cresol added)
14
14.8, 18.4, 18.7 33.4, 32.5, 38.7 4.0, 2 . 5
I t was expected that stability would be related to residual monomer concentration because the monomer is more readily autoxidized than ;he polymer. Also, poly-a-methylstyrene would be expected to resemble tert-butylbenzene in resistance to autoxidation. I t was found, however, that stability was not improved by reprecipitation to remove monomer. There is apparently end group unsaturation in the polymer itself and correction had to be made for it in developing a bromination analysis for monomer content. Possibly this unsaturation provides a locus for autoxidation. Because use of an antioxidant suppressed only the effect of air and did not improve stability over that determined in vacuo, a search was made for an inhibitor which could be used in conjunction with
the antioxidant. For this search, a test was devised which involved heating polya-methylstyrene in diphenyl ether solution under nitrogen and precipitating samples in order to follow both the viscosity drop and production of monomer. The initial polymer concentration was 4y0 and the stabilizers were tested at 0.4y0 concentration expressed in terms of the sohtion. I t was found that under these conditions, a rapid drop in viscosity occurs (Figure 3). Either zipping (Figure 4) is accompanied by transfer and scission or the zip length is short relative to polymer chain length. It was thought that chain transfer agents could be found that might cause more viscosity drop, and if used alone, not re: duce depolymexization rate but be beneficial if used in conjunction with an antioxidant. Such a synergistic effect has been shown in other systems (70).
I
PE PRECIF
20
16
12
8 4
0
10 80
90
100
110 I20 130 H E A T DISTORTION,
140
150
160
170
'C
Figure 6. Correlation of poly-a-methylstyrene heat distortion with residual monomer content
20 ML
30
40
50
60
70
METHANOL ADDED TO 80 r j l i ( 0 . 0 0 31100 ~ Cone,)
80
TOLUENE
90 SOLN.
I00
Figure 7. Titration curves for samples 3 and 6 having similar viscosities and different molecular weight distribution VOL. 48, NO. 12
DECEMBER 1956
2125
24 ,I
24
PERCENT P R E C l P l TATION
l12 6
I
t IO
I
PERCENT P:R: E TAI T ION
50 60 70 80 90 100 ADDED TO 80 M L . T O L U E N E SOLN. (0.003 g . / l O O C o n c . )
20 30 40 ML. METHANOL
Figure 8. Turbidimetric titration curves for samples having different titration curves and good properties
It was found, however (Table I), that compounds which promoted viscosity drop generally promoted monomer production also, and a combination of one of them, 1-dodecanethiol, with an antioxidant (;V-pheny1-2-naphthylamine) was not beneficial. A conventional inhibitor, 1,4-naphthoquinone, was found effective i n stabilizing against weight loss; when used in the previously described test at a concentration of 0.47, based on the polymer, it improved the stability somewhat (Figure 5). Surprisingly, iodosobenzene also behaved as an inhibitor, although it gave low initial viscosity (that observed when the solution was heated to test temperature) in the solution test. A low value indicates the occurrence of degradation during the period of heating to test temperature. Iodosobenzene was ineffective in the test first described.
IO
I
20
ML.
IC0
Figure 9. Turbidimetric titration curves for samples having poor properties
A prolonged heat-aging experiment, made at 132' C., showed that degradation is responsible for the loss of mechanical properties on continuous exposure to temperatures in this range. The commonly used plastics show as pronounced an effect (73). Compression-molded samples of polya-methylstyrene usually have heat distortion temperatures of 150' to 155' c. Of course, some residual monomer is always present, and Figure 6 gives a correlation of heat distortion with residual monomer content. A comparison of injection- and compressionmolded samples (Table 11) shows that for a given content of residual monomer (1.2 to 1,47c), injection-molded samples have a lower heat distortion. This is caused by strain, for after annealing for 3 days at 240' F., the values agreed with those of compression-molded samples.
Table 1.
30 40 50 60 70 80 90 M E T H A N O L A D D E D TO 80 M L . T O L U E N E S O L N . ( 0 . 0 0 3 g./IOO Cone.)
The lower the molecular weight of the polymer, the lower the melt viscosity and the shorter the cycle time in molding; therefore, it is a perennial problem to determine the minimum molecular weight for satisfactory physical properties. The lower impact strength of the sample (Table 11) having a viscosity of 13.9 cp. indicates that the lower limit for usable strength lies in the range of this sample. In attempting to determine this limit more precisely, it was found that it appeared to shift with different samples. This lack of reproducibility was ascribed to variation in molecular weight distribution. T o investigate the effect of molecular weight distribution, samples were compared by turbidimetric titration (79). Figure 7 shows the titration curves for two samples of nearly the same viscosity and different molecular weight
Effect of Additives on Degradation Rate of Poly-a-methylstyrene in Diphenyl Ether Solution a t 240" C." (Viscosity for 10% sol. in Toluene, 20 cp.) Ajter 10 Hours Polymer ViSC.,
vab,
2 1 26
conen.,
CICOC!%
Blank
19.4 21.5 17.2 19.8
0.75 0.66 0.66 0.66
Air 1,4-Naphthoquinone
20.3 20.3 20'9 9.5 19.2 14.6 20.3 16.4 13.8 17.2 17.7 16.5
0.92 0.91 0.91 0.91 0.60 0.57 0.62 0.59 0.48 0.44 0.13 0.15
Podosobenzene p-Dinitrobenzene Phenyl-2-naphthylamine (PBN) Phenylacetylene 2-Benzimidazolethiol Pyridine, N-oxide Benzoic acid I-Dodecanethiol PBN 1-dodecanethiol a All viscosities for 10% soln. in toluene. Measured when sample reached 240' C. 0 Relative concentration determined by precipitation in methanol. Extrapolated. e After 1 . 5 hours.
+
Figure 10. Photographs of polymer band during sedimentation in Spinco Model E centrifuge
CP.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ViSC.,
CP. 0.41 0.41 0.35 0.35
d'la,
0.36 0.37 0.45 0.71 0.29 0.40 0.39 0.47 0.26 0.20d
0.37e 0.16d
3 00
2.80
.
PRECIPITATION
20
-
16
-
12
-
8 -
4 -
I '
0 10 010
Figure 1 1. same M,
020
040
050 C O N C E N T R A T I O N 0. / I O 0 M L 030
ML.
060
diffusion constant, where M , lies between M , and M,. It appears likely from the boundary gradient photographs taken in the ultracentrifuge that sample 5 had the broader distribution (Figure 10). Samples 1 and 9 (Figure 12) are similar in viscosity but sample 9 has
Physical Properties Property Viscosity, cp. Volatile (in methylene chloride at 213 C.), yo Heat distortion temp., O
CP.
15.6 5.8
c.
Compression molded Injection molded Tensile strength, lb./ sq. inch Elongation, % ' Impact strength, inch-lb. Viscosity after molding,
Table II.
Sample Sample 3 6 17.0 4.6
135 122
....
7780 1.8 0.8
5900 1.3 0.4
14.4
14.7
a
40 50 60 70 BO M E T H A N O L A D D E D TO 80 M L . T O L U E N E (0.003g1100 Cona.)
100
90 SOLN.
a broader distribution curve. The samples were in different physical form; sample 9 is the only one of the series which was in granular form and usually some slight advantage in properties is gained by molding granules rather than ground polymer. Although some difference is shown in Table IV, the com-
Physical Properties as a Function of Polymer Viscosity ~ ~ ~Heat Distortion ~ $ Temp. 1 of~ Molded Samples, C.
Impact Strengthb, V i s c o s i t ? ~ ~ Inch Lb. 0.7 1.2 1.2 1.1 1.1 1.2
13.9 24 28.9 32.4 38.7 48
L 30
Figure 12. Turbidimetric titration curves for samples of high viscosity
Sedimentation constants of two samples of
distribution. These curves differ from true molecular weight distribution curves because the sample is being diluted during the titration; hence, the curve is elongated on the low molecular weight side. The following data were obtained for physical properties of these two samples.
20
Strength, Lb./Sq. Inch
Unannealed injectionc
Annealed injectionC
Compressiom
Elongation,
%
6650 7590 7730 6410 7650 7090
136 139 137 138 130 130
146 144 148 148 143 149
145 150 150 155 140 148
1.3 1.3 1.3 1.3 1.3 1.3
loyo soln. by weight in toluene measured 25' C. Measured on two unnotched bars, 1/8 inch square, placed side by side. Molded at a machine temp. of 520° F. and mold temp. of 230' F.
~
The sample with poorer physical properties is the one for which the turbidimetric titration indicates a broader molecular weight distribution. Although some reduction in viscosity occurred on molding, it affected the turbidimetric titration little, as curve 6M of Figure 7 shows. Figure 8 gives a set of turbidimetric titratims for samples of different viscosity and fairly good properties. I n contrast, the samples represented in Figure 9 had broader titration curves and poor properties. Samples 3 and 6 (Figure 7) were selected for comparing physical properties because the viscosities were similar after molding, but samples 3 and 5 were closely similar before molding and therefore were selected for comparison in the ultracentrifuge. Figure 11, shows that as expected, sample 3 has the higher sedimentation constant which is proportional to the product of M, and the
PEF :ENT PRECIP 'ATION
20
-
-4
16
12
8
4
0 IO
20 30 40 50 60 70 80 90 ML. M E T H A N O ADDED L TO ~ O M L . TOLUENE SOLN. (0.003g.l100 C o n c . ) Figure 13. Turbidimetric curves for samples of low viscosity VOL. 48, NO. 12
DECEMBER 1956
100
2 127
Table 111.
Sample No. 1
9
Temp., 'F. 490 510 510 530 490 510 510
are placed those samples having relatively too much of the low molecular weight polymer which is soluble in methyl ethyl ketone. Below this line, the area to the right of the figure represents samples too high in viscosity for ease of molding. The desirable areas attainable with and without fractionation are shown. The results of precipitation tests on samples dissolved in methyl ethyl ketone are :
Comparison of Samples 1 and 9
Pressure, Lb./Sq. Inch 19,000 19,000 26,600 19,000 19,000 19,000 19,000
viscosity after Molding, Cp. 22.4 22.4 21.7
Tensile Strength, Lb./Sq. Inch
Elongation, %
8320
2.1
8330 8120
2.0 2.0
20.1
Impact Strength, In. Lb. 0.9 1.0 1.0 0.9 0.8 0.9 0.8
Results of Precipitation Tests
parison as shown in Table 111 indicated very similar physical properties. Sample 1 was only slightly altered by molding. The fact that sample 7 is not inferior to sample 1 agrees with the observation of h/lerz, Xielson, and Buchdahl (72) that for any physical property of polystyrene, there is a minimum%, value above which the property is insensitive to further increase. A comparison a t the lower limit of viscosity is given by samples 4 and 8 (Figure 13). The turbidimetric titration shows sample 4 to have narrower distribution and it had superior physical properties (Table IV). The viscosity of sample 4 after molding was slightly higher than that of sample 8. From the osmotic data reported in Table IV, it is believed that the limiting value of M , for good physical properties cited (72) as 20,000 for polystyrene, is too low for poly-a-methylstyrene-at least for unfractionated samples-and that a value of 80,000 to 100,000 is needed. The samples described in Table I V have a rather high content of volatile material; however, the physical properties are essentially unchanged by its progressive removal as shown in Table V. The saniples of Table I V were separated into two fractions according to solubility in methyl ethyl ketone and viscosity. I n interpreting the results of this test, allowance must be made for Table IV. Sample NO.
1
2 3 4 5 6
7 8
Visc.", CP. 24.4 13.7 15.6
voz.,
15.2 17.0 19.4 8.4
4.4 4.6 5.0 3.2
..
%
6.2 4.4 5.8
..
Heat Dist., C. C.M. 129 132 135 135
%
CP.
..
.. .. ..
a
Insoluble jt-om Precipitation Valuea,
83.5 34.4 83.0 18.0 82.9 27.3 76.2 22.2 65.3 40.7 3.1 51.0 19.1 2.9 In area under turbidimetric titration curve, up to 40 ml. methanol added.
the viscosity of the original sample, For a given distribution, higher solubility in methyl ethyl ketone is expected if the original viscosity is low. A correlation of these two factors is given in Figure 14. The solubility in methyl ethyl ketone and solution viscosity were determined for samples of the actual moldings used for impact tests, the values of which are plotted in boxes on the chart. The samples were precipitated in a mixture of equal volumes of toluene and methanol to remove monomer before solution viscosity measurement. The line marked "normal distribution" applies to the samples after devolatilization and molding. I n Figure 13, any point lying to the left of the normal distribution curve is obtained only by fractionation and the area to the left of the line marked "best fractions" is not attainable. Above the borderline region in the area marked "poor"
% 90 78 86 76 77 65
Injection molding of poly-a-methylstyrene has been carried out under conditions described in 'Table VI. Small samples of material listed in Table I V were injection molded in a 10-gram laboratory molding machine a t machine temperatures ranging from 265" to 277" C. (510' to 530' F,), die temperature 100' C., and a t both 17,000 and 26,600 pounds per square inch without alteration of properties. I n a large injection molding machine with many shots of polymer melted and waiting in the machine, it is neither necessary nor desirable to operate a t so high a temperature. I t is necessary, however, to avoid interruptions in order to prevent depolymerization on prolonged heating in the cylinder. Good temperature control is necessary. Full bar ejection is desirable; pin ejection sometimes produces cracks in the specimen. A 2" draft in the die
Properties of Samples Represented in Figures 7, 8, and 9
92 72
3.57 2.95
visc. after Molding, CP. 21.4 12.6 14.4 9.73 12.4 14.7 14.0 7.0
..
3.50
20.1
Melt
vise.*
x
'Mwe
M,d
x10-a
x 10-3
374
143
..
339
126
5.1 2.0
328 212
10-3
8.0 6.0 5.6 3.3 4.8
.. ..
..
.. .. 339 ..
.. .. 99 ..
9 22.6 6.0 Measured from 10% s o h . in toluene. At 500' F., in poises after 10 min. in caplastometer (12.51 X 106 dyne From light scattering by F. L. Saunders, Dow Chemical Co. ( 1 7 ) . From osmotic pressure, by H. P. Frank, Brooklyn Polytechnic Inst. e Wt. loss after 15 min. in air, 15 min. in vacuum at 270' C. 7,Two unnotched bars, 1 / ~in. square side by side. Estimated. 0
.
lMw/Mn 2.62 2.8+ 2.69
..
3.43 3.5+
a
2 128
Soluble, Viscosity, CP.
Viscosity, Sample N o .
.. .. ,.
Insoluble
INDUSTRIAL AND ENGINEERING CHEMISTRY
shear stress).
Degrad. Teste, % Loss 12.5 8.7 11.8
..
8.6 9.6 12.4 8.6 5.4
Tensile Inzpactf Strength Elonga- Strength, Lb./Sp. In. tion,% In. L b . 1.8 0.7 7510 7110 1.8 0.7 1.8 0.8 7780 2.0 0.6 7510 0.5 5540 1.4 0.4 5900 1.3 0.4 5680 1.4 0.2 too 1.4 brittle 2.2 8440 1.0
Table V.
Effect of Removing Volatile on Physical Properties (Samplessof Table IV re-devolatilized)
Heat - Diatortionl c. Cp. Vol.", % C.M. I.M. 1 24.4 6.2 129 118 25.4 1A 210 4.2 139 133 25.3 1B 240 2.1 130 165 24.7 260 2.0 1c 158 137 2 13.7 4.4 131 132 2A 14.0 210 3.4 142 150 2B 13.7 240 1.4 136 157 13.4 145 260 2c 15.6 3 132 5.8 ii5 15.7 3.4 3A 210 130 139 15.1 240 3B 1.7 142 155 1.4 15.5 158 260 3c 139 Measured from a 10% soln. in toluene. Dissolved in CHZCL and devolatilized 1 hour at 213' C. Two unnotched bars, 1/8 in. square, side by side.
Devol. Sample N o . Temp., O C.
vise.,
..
. i
I
a
is needed to facilitate ejection. When molding taxes the heating capacity of the machine, it is helpful to preheat the granules as well as heat the hopper section. Poly-a-methylstyrene has been satisfactorily extruded without degradation on a modified 4l/%-inch National Erie single screw extruder, oil-heated at 430' F. As a result of mechanical working, the polymer temperature was 460' F., the oil actually acting as a coolant. The extruded polymer was then injection molded and the tensile strength was found to be 8570 pounds per square inch elongation, 2.6%, and unnotched impact strength, 0.8 inch pound. Bubble-free extrusions have' also been made on a Welding Engineers' mixing extruder, but with some breakdown. Because some breakdown occurs, it is necessary to have, as this machine does, an open section after the mixing section to let vapors escape. The effect of such extrusion is shown as follows :
Table VI.
Propertv Viscosity, cp. Volatile, % Heat distortion temp., O
c.
Tensile strength, Ib./sq. inch Elongation, yo Impact strength, inch-lb,
After
Ex-
Ex-
128
Mold
8710 8570 2.3 2.6 1.0 0.8
Poly-a-methylstyrene is compression molded at 400' F. and 1000 pounds per square inch. To make large bubblefree moldings of half-inch thickness, it is necessary to use 10 to 15 minutes of preheat with pressure applied gradually and 10 to 15 minutes cooling. For a 0.1-inch thickness, 1 minute is sufficient. Second-order transition-temperature measurements by a dilatometric technique indicated a lower value than was expected. In the following tabulation,
Table VII.
14.4 14.6 14.4 13.6
is forced into the mold is much less than the pressure exerted on the granules and varies depending on the melting conditions in the molding machine. The melt behavior of poly-a-methylstyrene is considerably more non-Newtonian than that of polystyrene. Without applied pressure there is virtually no flow. At 216' C., for example, polystyrene will melt and flow with no applied pressure, whereas poly-a-methvlstyrene particles barely sinter together. Data on commercial molding grades of polystyrene fall between curves D and E of Figure 15. As plotted in this figure, the more nearly horizontal the line the greater the dependence of flow on pres-
WatsonStillman,
4
450 440 410
4 20 5 2 18,000 1'/a
4 02.
02.
16
02.
535 515 460 250 200
535 515 450 265 245
480
8 32 10 10 20,000 2
8 48 40 5 20,000 31/4a
180 total 20,000 10
for 30 min.
Second Order Transition Compared with Heat Distortion Values
Sample No. 3 3A 3B 3c
*
Mold Temp., O F. 490 500 550 540 530 520 550 540 530 520 510 540
Reed Prentice
Fellows, 8 Oz.
200
Cycle, sec. Plunger Closed Open Booster Pressure, lb./sq. in. Molding size, oz. Granules preheated at 280' F.
116
Visc." after Molding, CP. 21.4 22.1 20.8 19.4 12.6 12.7 12.0
Injection Molding Conditions for Poly-a-methylstyrene
Machine Type Capacity Temperature, O F. Front machine Rear machine Nozzle Hopper
trusion tvusion 14.8 12.7 5 7
Impact Strength', Lb. 0.7 0.8 0.9 0.9 0.7 0.9 0.7 0.8 0.8 0.9 0.8 0.8
Elongation, yo 1.8 1.7 1.8 2.1 1.8 2.0 1.9 1.7 1.8 2.0 2.0 1.9
comparing second order transition with heat distortion values, values are shown which do not correlate well with heat distortion or residual monomer content and are therefore open to question. The shrinkage effects of annealing poly-a-methylstyrene specimens are sufficient to make dilatometric measurements less reproducible than with polystyrene. It has been noted that the density of extruded samples actually increases with increasing residual monomer content. This phenomenon is ascribed to facilitation of packing. There is likely to be considerable variation in the density of injection-molded specimens because the effective pressure at which the plastic
Effect of Extrusion Before
Tensile Strength, Lb./Sq. In. 7,510 6,840 7,470 8,080 7,110 7,430 7,610 6,770 7,780 8,070 8,100 8,510
Residual Monomer, % 3.20
Tta, C. 100 87 101 100 5 95 5 106 7 108 1 112 Dilatometric T,measurements by B. B. By bromine titration.
Heat Distortion, C.M., O C. 135 139
Viscositl/,
CP.
158 3.20 138 4.18 135 3.80 142 4.85 148 Hibbard, Dow Chemical Co.
VOL. 48, NO. 12
15.6 15.7 15.1 15.5 15.2 17.0 19.4 24.4
DECEMBER 1956
21 29
o/o
because of degradation and strain, but because of the beneficial effect of lubrication, the lubricated sample may show no reduction in heat distortion. For example, with 37, of the dimer of CYmethylstyrene, the heat distortion of a n injection-molded specimen was the same as with 1y0, or 132' C . Plasticizers of low molecular weight do not generally improve the physical properties of styrene-type plastics; however, an indirect benefit may accrue from reduction in degradation during molding. This beneficial effect is not usually reproducible, because it is observed only when the molding conditions are not right. The addition of as much as 10% cr-methylstyrene dimer makes the plastic brittle.
SOLUBLE IN M F K
too 90 80 70
60 50 40
Experimental
30 20
IO 0 0
5
15
IO
20
25
30
35
40
SOLUTION V I S C O S I T Y (10% IN T O L U E N E )
Figure 14. Correlation of physical properties of molded poly-or-methylstyrene with viscosity and solubility in methyl ethyl ketone
distortion point. Thus, the addition of 3y0 of the dimer of a-methylstyrene reduces the heat distortion temperature (compression molding) only to 146' C., whereas 1yo tributyl aconitate reduces it to 144" C. With 5% of di-tert-octyldiphenyl oxide it was 145' C., with 370 zinc stearate, 141' C.; and with 3yo tris-f-tert-butylphenyl phosphate, 147' C. The heat distortion temperature of injection-molded specimens is lower
sure. While degradation does occur with prolonged heating in the melt viscometer (Figure 16), it appears to be independent of shear. Both chain scission and monomer formation contribute to the decrease in viscosity under these conditions. Internal lubricants can be added to reduce the melt viscosity. As with polystyrene, relatively stiff molecules produce less reduction in the heat
SHEAR STRESS (Dynes / C m p
1
4t 2
0 , O WISES
O
~
/
t
1,000
I
2
3
4
APPARENT FLUIDITY
5 6 7 8 1 0
(-
P 0s ;
Es )
.IO3
Figure 15. Melt viscosity of lubricated poly-or-methylstyrene at 500" F. Unlubricated control, solution viscosity 26 c.p.r. measured at 10% concentration in toluene D. With 10% a-rnethylstyrene dimer; B, C, E. With 2, 5, and 1 O%, respectively, of an incompatible polymer, polypropylene glycol of molecular weight 2000
A.
2 1 30
~
Polymerization. One hundred and eighty grams of a-methylstyrene, freshly redistilled with exclusion of air, was treated with 3.6 grams of sodium shot (1 to 2 mm. in diameter) and sealed in the absence of air in a glass tube. After 115 hours of agitation a t 10" C., the clear yellow solution was filtered to remove suspended sodium and poured with stirring into 1.5 liters of ethyl alcohol. The polymer which precipitated as white fibers was dried at 80' @. (yield, 45 grams); viscosity, 14.39 cp. (loyoby weight in toluene at 25' C . ) ; intrinsic viscosity, 0.67 in toluene. The yellow color observed during polymerization was bleached at once upon opening the polymerization vessel to air; however, when impurities were present in the monomer the color reappeared in molding. Unsaturation Titration. As a chemical method of analysis for residual monomer in polymer samples, a procedure for bromination was developed
INDUSTRIAL AND ENGINEERING CHEMISTRY
10
Figure 16. viscosity
30
50
70 9 110 TIME. M I N U T E S
130
150
Degradation observed by change in melt
Melt viscosity measured at shear stress of 7
X 1 0-5 dynes
(3). I n this procedure a 2-gram sample of polymer dissolved in 25 ml. of methylene chloride was treated with 25 ml. of 0.1 N bromine solution in dry carbon tetrachloride. After a half hour in the dark, the solution was treated with aqueous sodium iodide and the iodine titrated with thiosulfate. A titration with 0.1N caustic was then made to correct for bromine consumed by substitution. T h e accuracy of the method was checked by analysis of reprecipitated polymer. The unsaturation found in these samples was probably largely end group unsaturation rather than residual monomer. Titration of Residual Unsaturation after Precipitation Viscosity, 10% S o h . Toluene, c p . 14.9 19.0
26 10.7
Unsaturation as % Monomer 0.106,O. 133,O.123 0.315,0.322,0.318 0.157, 0.124 0.216, 0.206
Stability Tests. A 2-gram sample of polymer ground to nine mesh was placed in 17/~-inchPetri dishes which were then placed on a rack and lowered into a tube which was immersed in a circulating bath of hot silicone oil. The variation of temperature from the bottom to the top of the rack was from 264' to 261' C. The samples, which sintered but did not flow together, were heated for 15 minutes under air and then a 1mm. vacuum was applied for 15 minutes in the usual test which is referred to as the practical test. T h e degradation of poly-a-methylstyrene in diphenyl ether solution was carried out under prepurified nitrogen in a stirred flask heated by a n oil bath at 250' f 2' C. A 1-liter volume of 4% solution was used and 50-ml. samples withdrawn for precipitation in methanol (400 ml.). The samples were dried to constant weight a t 100' C. for 6 hours in vacuo. They were redissolved in toluene for viscosity measurement a t 10% concentration. The stabilizers were added a t 0.4% concentration based on the solvent. This concentration was selected in order to maintain the same absolute concentration of stabilizer as was used in the molten plastic. Residual Monomer Determination b y Weight Loss. The residual monomer can be determined by loss of weight o n heating in vacuum at a temperature of 213' C. where depolymerization is not rapid enough to prevent the approaching of an end point. I t is advantageous also to dissolve the sample in a low-boiling solvent such as methylene chloride before treatment and then to allow most of the methylene chloride to evaporate before placing the sample in the vacuum oven. The use of methylene chloride seems to promote degradation a t 240' C., but a t 213' C.,
Ultracentrifuge Measurements. Two samples of closely similar M , and different distribution were examined a t the University of Michigan in a Spinco Model E ultracentrifuge a t 52,000 r.p.m. Benzene was used as the solvent. This particular instrument was not provided with temperature control; the temperature ranged from 24' to 29.5' C. and was recorded for each run. Acknowledgment 0
20
40 60 80 100 TIME IN MINUTES
120
I40
Figure 17. Weight loss when heated in vacuum at 240' C. with no solvent added
degradation is slow enough to obtain an approximate value for monomer content (Figure 17). Thus, in 2 hours a t 213' C. with methylene chloride, a value of 3.2% volatile was obtained on the sample for which Figure 17 shows the weight loss a t 240' C. with no solvent added. Turbidimetric Titrations, T o 80 ml. of a dilute solution of poly-amethylstyrene in toluene was added methanol in 2.5-cc. increments a t room temperature. The transmittance of light through the solution was measured with a Lumitron spectrophotometer using a light source filtered by a blue broadband filter. The total addition of 80 ml. of methanol required 35 minutes and the transmittance was measured 30 seconds after each addition. Under these conditions, polystyrene of molecular weight below 10,000 (below 1.5 cp.) is not precipitated; however, in the case of poly-a-methylstyrene anything over a molecular weight of 1000 is precipitated. The change in transmission relative to ultimate loss in transmittance was plotted versus volume of methanol added; therefore, to obtain molecular weight distribution it would be necessary to correct for dilution. Light-Scattering Measorements (77). Light scattering measurements were made in toluene and the dissymmetry correction was based on the assumption of a random coil. The molecular weight results are substantially higher than those of polystyrene of the same viscosity. For example, unfractionated polystyrene having a viscosity of 31 cp. for a 10% solution in toluene has been found to have a molecular weight of 306,000 and a 2, value of 1.18 (76) which is in general agreement with the data summarized by Outer, Carr, and Zimm (74). The statement has been made by Bueche (7) that polystyrene shows a high solution viscosity because of the stiffness of the chain. In view of the greater stiffness of poly-a-methylstyrene, the lower viscosity for the same M , is ascribed to a broader molecular weight distribution.
The molding and physical test measurements were performed under the supervision of Leo Kin, Dow Chemical Co., and the ultracentrifuge data were prepared by H. L. Hobbs, Engineering Research Institute, University of Michigan. References
(1) Bueche, A. M., J . A m . Chem. SOC. 71. 1452 11949). (2) Cooke, M. D., Staudinger, J. J. D. (to Distillers Co., Ltd.), Brit. Patent 611,255, 611,256, (Oct. 27, 1948). (3) Dorn, F. R., Dow Chemical Co., unpublished work. (4)Hersberger, A. B., (to Atlantic Refining Co.), U. S. Patent 2,472,589; (June 7. 1949). Hersberge;, A. B:, Reid, J. C., Heiligmann, R. G., TND. ENG. CHEM. 37, 1073 (1945). Hersbereer. A. B.. Heiliemann, R. G.. h i d : . 2.479.618. "(Aue.. 23. 1949); Heiligmann, R. 'G.," Zbid.; 2,507,338 (May 9, 1950). Jellinek, H. H. G., J . Polymer Sci. 4, 13 (1949). (8) Jones, G. D. (to Dow Chemical Co.), U. S. Patent 2,621,171, (Dec. 9, 1952); Werkema, T. E., Zbid., 2,658,058, (Nov. 3, 1953). (9) Karam, H. J., Dow Chemical Co., unpublished work; Karam, H. J., Cleereman, K. J., Williams, J. L., Modern Plastics 32, No. 7, 129 (1955). (10) Le Bras, J., Hildebrand, R., Comfit. rend. 223, 724 (1946). (11) Madorsky, S. L., J . Polymer Sci. 11, 491 (1953). (12) Merz, E. H., Nielson, L. E., BuchENG.CHEM.43, 1396 dahl, R., IND. (1951). (13) Mesrobian, R. B., Tobolsky, A. V., J . Polvmer Sci.2. 473 (1947). (14) Outer, P., Carr, C. I., Zimm; B. H., J . Chem. Phys. 18, 830 (1950). (15) Rubens, L. C., Boyer, R. F., "Styrene Monograph" (R. H. Boundy, R. F. Boyer, editors) p. 256, Reinhold, New York, 1952. (16) Sanders, J. W., Dow Chemical Co., unpublished work. (17) Saunders, F. L., Dow Chemical Co., unpublished work. (18) Sparks, W. J., Kellog, H. B., Field, D. C. (to Standard Oil Development Co.), U. S. Patent 2,436,614 (Feb. 24, 1948). (19) Young, L. J., Johnson, W. A., Dow Chemical Co., unpublished work. (20) Ziegler, E. E., Brown, W. E., Plasttcs Technology 1, No. 6, 341 (1955); Ibad., No. 7, p. 409. RECEIVED for review December 27, 1955 ACCEPTED June 4, 1956 VOL. 48, NO. 12
DECEMBER 1956
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