INDUSTRIAL AND ENGINEERING CHEMISTRY
FACTORS INfLUEWNG THE
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INDUSTRIAL AND ENGINEERING
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121 '-132°C.
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and the magnitude of this maximum apparently increases as the temperature of milling increases. After 20-minute milling in this laboratory test, the effect has entirely disappeared and recovery shows a normal value. Recovery has frequently been associated with gel formation, and it is possible that the first effect of increased temperature is to cause rapid gel formation. Further mechanical action finally breaks down the gel to a polymer with low recovery. If this is the true explanation, it may indicate the reason for a lack of correlation between plasticity recovery values and processability; the past history of the polymer in arriving a t the measured value for the plasticity and recovery might well be expected to have a marked effect on the processability of the stock. Since the results obtained in this series of tests were unexpected, the effect of high-temperature breakdown on a few additional samples of GR-S was investigated. The results are given in Table 11. The recovery values for these samples after various milling periods are shown in Figure 2. The data show that the marked increase in recovery on short milling a t high temperatures is characteristic of OR-S. The absolute value of the maximum and its position on the recoverytime of milling curve is, however, dependent upon the particular lot of GR-S tested. With the sample of GR-S of high viscosity (85 Mooney) and the sample from source B, the first effect of milling is a decrease in recovery. Following this decrease, the recovery values increase with increased milling, pass through a maximum, and finally decrease as the time of milling is prolonged.
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EFFECT OF C H E M I C A L PLASTICIZERS
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Sturgis and Vincent ( 1 ) developed chemical agents which markedly improve the milling characteristics of GR-S. The first series of experiments was therefore repeated with 2% of agent RPA No. 5 as peptizer. (See Table I11 a r d Figure 3.) The results obtained in the presence of a chemical plasticizer are entirely different from those in its absence. The speed of breakdown, as measured by plasticity, is markedly increased by increasing temperatures of milling. I n this respect the results are similar to those obtained when natural rubber is broken down in the presence o! chemical plasticizers. I n contrast to the results obtained in the absence of RPA No. 5 , the recovery figures show a constant decrease throughout the milling operation regardless of the milling temperature of GR-S. I n fact, just as with the plasticity determination, recovery appears to fall off more rapidly, the higher the temperature a t which GR-S is broken down. It is also noteworthy that, judged either by plasticity or recovery, the extent of breakdown, even a t the conclusion of the milling operation, is much greater in the presence of the chemical plasticizer than in its absence.
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Table I.
Influence of M i l l i n g Temperature on Mastication of without Chemical Plasticizer
h4illing Time. Rlin.
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FIGURE 3. G R - S CONTAINING 2% RPA N O . 5 A S T IGATE D AT V A R I O U S T E M P F R A T U R E :
L,..
o MILL TEMP 32"-43"C. x MILL TEMP 7Oo-89'C. A MILL TEMP 121 O- 132'C
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GR-S
Williams Plasticity Recovery a t Milling Temp. of: 32-43' C . 70-89' C. 121-132' C. 128-45 128-45 128-46 131-52 142-96 122-47 138-130 130-69 124-37 132-180 131-127 123-26 126-149 123-50 119-42 105-20 117-33 111-13
II. Mill Mastication of Three Lots of GR-S at 111-132" C.
Milling Time, SIin.
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3 5 10 15 20
Plasticity Recovery of GR-S Samples Mooney 85 Source B Source C 185-133 131-61 138-61 154-90 166-88 129-33 124-36 142-77 165-108 137-62 161-138 120-49 128-34 158-166 108-15 127-31 154-128 97-7
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1944
I n view of these surprising differences, it seemed important to determine the minimum concentration of the chemical plasticizer required to produce this effect. I n this work the intermediate temperature of milling was selected, and the test was run at a mill roll temperatureof 700 c. (158" F.) and a maximum temperature in GR-S of 89" c, (1920 F.). The results are plotted in
A 2% RPA N0.5
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EFFECT O F S T O R A G E AFTER MILLING
Since the results obtained in this study had indicated a lack of thermal stability of GR-S in the absence of chemical plasticizer, the effect of varying times of storage after milling was investigated. The samples selected for test were those that had been
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Table
111.
Influence of M i l l i n g Temperature on Williams Plasticity Recovery of GR-S Containing 2% RPA No. 5
Milling Time, Min. 0 3 5 10 15 20
Table
Plasticity Recovery a t Mining Temp. of: 32-43' C. 70-89' C. 121-132°C. 128-45 128-45 84-9 113-25 88-8 105-16 98-97 83-5 89-8 85-5 82-3 85-2
IV. Plasticity Recovery Changes of Masticated GR-SO Stored at 26.7" C.
% of RPA
Plasticity Recovery a t Milling Temp. of: 32-43' C. 70-89' C. 121-132O C. 0 111-13 117-33 105-20 107-7 118-117 115-66 118-18 118-22 122-106 131-21 120-30 124-52 1 112-21 105-26 108-46 114-12 110-23 ii3-40 115-16 111-20 117-42 111-15 114-22 129-96 2 98-3 82-3 85-2 97-5 81-2 85-1 94-4 81-2 91-8 96-7 81-2 94-15 a Plasticity recovery of unmasticated GR-S, 128-45. 6 Original values.
No. 5
Storage Period '24 hr.b 3 days 1 week 2 weeks 24 hr.b 3 days 1 week 2 weeks 24 hr.6 3 days 1 week 2 weeks
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MINUTES MILLING
Figure 4. Under these conditions of milling, between 1 and 2% of RPA No. 5 is required t o remove the maximum in the recovery vs. time of milling curve. When only 1% of the agent is employed, the maximum is still apparent although it is shifted toward the left and is markedly decreased in magnitude. Undoubtedly the amount of RPA No. 5 required to eliminate the maximum will be dependent both on the lot of GR-S used and the milling temperature.
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milled for 20 minutes in the earlier set of experiments. Plasticity pellets were stored at a constant temperature of 26.7" C. (80' F.) in the dark, and plasticity recovery measurements were made after varying storaGe periods. The results are given in Table IV. The results for GR-S which had been broken down at the highest temperature-mill roll 121' C. (250' F.), maximum GR-S 132' C. (270" F.)-are plotted in Figure 5. GR-S, broken down in the absence of a chemical plasticizer, tends to revert to a condition resembling its original unmilled state on storage at room temperature. This effect is the more pronounced, the higher k e temperature at which the G R S is broken down, and is particularly apparent in the increased recovery of samples on storage at room temperature. When masticated at 121' C., GR-S had a recovery 2 days after milling equal to the recovery of the original unmilled polymer; this change was still progressing after 2-week storage at which time the recovery was three times that of the original unmilled polymer. GR-S, broken down in the presence of a chemical plasticizing agent, does not show this effect. When the test was discontinued after 2-week storage at room temperature, the GR-S broken down in the presence of 2y0 RPA No. 5 at low and intermediate temperatures was in essentially the same condition, judged by either plasticity or recovery, as it was immediately after milling; GR-S broken down a t elevated temperatures showed a small decrease in plasticity and a small increase in recovery. LITERATURE CITED
(1) Sturgis and Vincent, fall meeting, Div. of Rubber Chem.,
A.C.S., New York, 1943.
(2) Vila, I b i d .
PREWENTED before the spring meeting of the Division of Rubber Chemistry, AMBRICAN CHIPMICAL SOCIETT, in New York, N. Y.,1944.
