FACTORS INFLUENCING PROCESSABILITY J.
S.
RUGG
AND
G.
w.
SCOTT
E. 1. du Pont de Nemours & Co., Wilmington, Del.
P
ROCESSISG studies of hdiprene B urethane rubber have resulted in an understanding of a number of the factors required for processability. This report describes these findings in terms of: 1. Characteristics of the polymer t h a t influence processing 2. Principles of processing 3. Application of these principles to some practical aspects of processing. Control of viscosity a n d temperature i s important in processing
Equipment and Procedures. Plasticity nieasurements n-ere made with both the JTilliams parallel plate plastometer according to ASTiLl Xethod D926-46 T ( 1 ) using onion skin paper instead of holland cloth and the Nooney viscometer according to ASTM Method D927-47 T ( 1 ) uaing the small rotor. Williams plasticity and recovery measurements were made a t 80" C. The h1ooney viscosity values reported mere obtained after 4 minutes' shear unless otherwise noted. Special LIooney viscosity determinations were made using the modified equipment described. RIooney scorch determinations mere made according t o ASTM D1077-49 T. The temperature of the test was 121' C. unless othernise shown. The minimum value and the time required for the viscosity to rise 10 units above the minimum TI ere recorded. A 14 x 30 inch two-roll rubber mill and a 6-inch Royle extruder with an 18-inch tread extruding head ne1e used extensively throughout this work. Extrusions lvere run at a screw speed of 18 r.p.m. with a head temperature of 90" C. and barrel and screw temperature of 20 to 25" C. The natural rubber used for all comparisons v a s smoked sheets. All polymers used in plasticity comparisons n-ere milled 15 minutes on a 6 X 12 inch mill having 15' C. n-ater circulating through the rolls. Viscosity and Plasticity. Mooney viscosity and Williams plasticity tests are widely used to estimate the ability of elastomers to flow and retain shapes. These properties of Adiprene B are compared with natural rubber and GR-S 1500: Adiprene
B
Mooney viscosity, L1S-4/100° C. 65 Williams lasticity and recovery at 800 390-55
8.
Smoked Sheets
GR-S
25
1500 30
160-50
130-87
Examination of these data might lead to the conclusion that Adiprene B is very difficult to process because of its high Mooney viscosity and Williams plasticity. Actually, this is not true since Adiprene B bands readily on a mill and shoii s smoothness comparable with natural rubber. Thermoplasticity. Adiprene B does not break down during milling under conditions usually used to break down other elastomers. It softens but undergoes no permanent change in plasticity or Mooney viscosity. The high viscosity of the polymer causes rapid heat generation during milling n-hich in turn
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produces rapid softening and smooth-out on the mill. The ease of milling suggests that the polymer is thermoplastic. The influence of temperature oil the flow characteristics of compounded Adiprene B is shoivn in Figure 1. Adiprene B is very t,hermoplastic betxveen 80" and 100" C., but a t higher temperat'ures, up to 121' C., it is relatively 11011thermoplastic. Above 121O C. a pronounced change in apparent viscosity and plasticity occurs suggesting a change in flow characteristics. Satural rubher is relatively nonthermoplastio over the temperature range st'udied. Adiprene B and natural rubber are similar in their response to temperature within the temperature range normally encountered during processing (1OOo to 121 C.)) and they behave similarly during milling. Investigation of the change in viscosity above 121' C. revealctl that this correlat,es with observed differences during milliiig. The optimum processing temperature range for compounds of Adiprene B is 100" to 121' C. Higher temperatures produei; a, very marked deterioration in appearance and handling char:ic:terist'ics. During milling a t temperatures n.ithin the optimum range a very smoot'li plastic sheet is obtained, xhereas above 121" C. t,he stock becomes rough, granular, and discontinuoiw. This effect is reversible and appears to he dependent o n temperctture. Return to a lower stock temperature brings back :L smooth plastic sheet. The t,emperature at n-hich this occurs is called the "apparent transition point,.' ' Apparent Transition Point. The influence of temperature o n this transition point was studied by nieasiiring Nooney viscosity as the sample temperature increased continuously from TO" C. to 150" C. I t mas postulated that when the temperature rose to the transition point, the polymer mmld crumble and tear around the rotor which in turn should cause the slope of the temperatiueviscosity curve to decrease markedly. The break point in tlie curve is believed to be indicative of the t r a i d i o n . This test ir run using a small rotor and a starting temperature of TO" C. The test specimen is placed in the machine arid readings are t'aIic.11 until a constant viscosity is obtained. Then, TT-ith the machine running, the platen temperature is increased to 175' C. As thP specimen temperature rises, viscosity readings are recorded at each 5" C. rise in sample temperature, measured by a thermocouple imbedded in the specimen. The data are shown i i i Figure 2 for a gum stock and one plasticized x i t h a liquid mpolymer of butadiene and acrylonitrile. These data confirm the hypothesis by the presence of a defiiiite break in the temperature-viscosity curve n-hich correlates with the temperature a t r h i c h the t'ransition is observed during milling. The presence of the plast'icizer increases the transition temperature from about 120" C. in the gum stock to about 150" C. 9 transition temperature above normal operating temperaturr,s is necessary to ensure smooth trouble-free processing. A more complete study of this phenomenon is continuing using t h r Mooney viscometer. Thixotropy, Processing observnt'ions indicate that Adiprenr: I3 shows a different response to temperature under shear than it does in a static state. I t appears to be less plastic under static
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 5
URETHANE RUBBER conditions than under dynamic conditions-compounds flow better during extrusion than they do in a relatively static state, as molding. I n addition, better molding is often obtained at a low temperature-e.g., 90" t o 121" C.-than at higher temperatures. This suggests t h a t Adiprene B is thixotropic, and its flow characteristics change with rate of shear.
j.400
\
100-
I
-.300
>-I
I
>-
5
8
--.zoo
75-
t MOONEY, VISCOSITY AT
m
0
a
Y
-I e
' 50-
-.IO0
> z w MOONEY
25-
01
- SMOKED SHEETS
I I 100 121 TEMPERATURE
Figure 1.
I 135
=!
P
I 150
-OG,
Temperature vs. plasticity and viscosity
Thixotropy of liquids is easily identified by viscosity tests at increasing and decreasing rates of shear using an instrument such as the Brookfield viscometer. Methods of measuring thixotropy in solid polymers are not common. A method was devised t o measure the Mooney viscosity a t various rates of shear by using a machine having variable rotor speeds similar t o those reported by Decker and Roth ( 2 ) . I n this modified machine, as the rotor speed increases the rate of shear relative to the die surface also increases.
70
100 121 TEMPERATURE - "C.
Figure. 2.
Transition vs. temperature
150
By analogy with the flow curve of a thixotropic fluid, curves of viscosity versus increasing and decreasing shear rate for a thixotropic solid should be displaced and have the appearance of a hysteresis loop with the curve for decreasing speed showing lower viscosity. Experiments were made with Adiprene B and other elastomers using the small rotor at 100" C. starting at 0.1 r.p.m. When a constant viscosity reading was obtained, the speed was increased to the next increment and held until the reading again became constant. This procedure was continued until a maximum speed of 20 r.p.m. was reached after which the speed was reduced in a similar stepwise fashion until the slowest speed was May 1956
ROTOR
Mooney viscosity vs. rotor speed
"i -0
0
I
Figure 3.
- SMALL
I
0 --
80
v,
IOO'G.
reached. The speeds used were 0.1, 0.5, 1.0, 2.0, 4.0, 10, and 20 r.p.m. The results of a comparison of milled smoked sheets and Adiprene B using this procedure are shown in Figure 3. The curves of viscosity for increasing and decreasing rotor speed are of the same general nature but widely separated for Adiprene B and close together for natural rubber. Both elastomers show a characteristic plastic flow curve, but Adiprene B has a higher yield point. These data indicate t h a t Adiprene B is more thixotropic than natural rubber. Practically, this means that processing speed affects both polymers similarly, but Adiprene B is more viscous a t all speeds, and continuous working reduces its apparent viscosity and tends t o improve processing. The viscosity of Adiprene B a t the lowest speed is higher than the viscosity of natural rubber a t the highest speed tested. Nerve. Recovery from deformation, commonly called nerve, is an important indicator for processing smoothness and dimensional stability of processed compounds. Attempts t o judge nerve by Williams recovery led to a conclusion inconsistent with experience. Conventional Williams recovery values, a t 100" t o 121' C., are high, yet in actual processing a t these temperatures, stock shrinkage and smoothness are at least equal t o other elastomers. It is believed t h a t the poor correlation of Williams recovery data with processing experience is due to the stiffness of Adiprene B which resists compression of the pellet during t h e standard Williams test. A specimen compressed only slightly could be expected to recover more fully than one compressed to a greater extent. A test was devised which would avoid this difficulty by compressing specimens t o 50% deflection in ASTM compression set Method B clamps. Standard ASTM compression set pellets. were cut from uncured sheets with a rotating die. The specimens were compressed for 3 minutes a t the test temperature then released and allowed to recover for 30 minutes a t the test temperature. Per cent recovery was calculated by Recovered height X 100 Original height-compressed height
=
70recovery
A comparison of this procedure with the standard Williams recovery using Adiprene B and natural rubber is as follows:
80' c. 100' c. 121" c. 150' C.
Williams Recovery, 0.001 Inch Adiprene Smoked B sheets 55 50 125 12 136 0 114 0
INDUSTRIAL AND ENGINEERING CHEMISTRY
50% Recovery Deflection, After Adiprene Smoked B sheets 60 48 38 49 23 33 6 9
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
t
IO
1u
U
T1f.i:
Figure 4.
I
LU
3c "ADIPRENE" B EPC CARBON PLASTICIZER
I
i
5
2
PARTS
- MINUTES Figure 6.
Influence of temperature on Mooney scorch
140
9
co rn U N P L A S T I C I Z E D
I
BLACK
100 I5 15
I
5
4 "HYLENE" DB!M
Effect of curing agent level on Mooney scorch at 121 C.
-
c1: w
3
4
120-
E a w
5 + e
2W
15
PARTS
-
PLASTICIZER
'O0I-
20
I
0
Figure
5.
I I 20 PARTS EPC B L A C K
Effect of compounding on Mooney viscosity
Adiprene and natuial rubber are practically equivalent in per cent recovery or nerve when tested at equal deflection. This confirms processing experience. The conventional Tilliams recovery value is very high for Adiprene B and shows very little response to temperature, whereas natural rubber reaches the point of no recovery a t 121' C. However, by the modified compression set test both elastomers lose nerve as the temperature increases. Mooney viscosity was considered to be of more value in processing studies since it is a dynamic test rather than a static compression test, and it was used for most testing. Scorch. Scorching in elastomer stocks is a change from a plastic to an elastic state brought about by a reaction of the polymer and a curing agent. Llanifestation of scorch in 9diprene B stocks is different than in other elastomers. Stocks containing Bdiprene B and curing agent increase in viscosity during scorching until they are too stiff to process. They do not become lumpy and unmillable when partially cured or scorched. Instead, they become progressively tougher and more viscous while remaining "processable" even in an advanced state of scorch. Since scorching in stocks of Adiprene B is mainly a change in viscosity and there are no complicating factors like cured lumps to contend with, the Mooney scorch test is an excellent method for estimating the scorching tendency. I n addition to the usual 5or 10-point rise, the low reading is also recorded since it is an indication of viscosity and may be used to determine the state of scorch present in the stock a t the beginning of the test. Temperature and type of curing agent have a major influence on the rate of scorch of Adiprene B. Figure 4 shows the influence
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I
I 40
20
40
I'OONEY
Figure 7.
VISCOSITY
I
i
- M S - 4/121°C.
60
Heat generation in 6-inch extruder vs. Mooney viscosity
of temperature and t n o cuiing agents on the scorch resistance of 3 gum stock. Hylene DM 11 (i,i'-methy lenedi-o-tolylisocyanate) is pi c ferred over HJ lene T U (3,3'-diisocyanato-4,.l-'-dimethj lcaihariilide) for proccssing because it is less scorchy. The slope of the cuive for 121' C. is steeper than the one at 100' C. This confirms actual processing experience that temperatures of 121" C. and highei sharply reduce safe processing time. Plasticizers reduee viscosity to desired level for processing
Processing stocks containing rldiprene B is difficult when they become so viscous that they d l not mill or extrude properly. It follow then that there is a maximum viscosity above which a stock cannot be processed. The viscosity limit depends on the machine to be used-i.ct., extruder, calender, etc.-and varies depending on size, design, and condition of an individual machine, Typical values foiinti for specific equipment are: Maximum processing viscosity Tread extruder, 6 inch Conventional extruder, 2 inch Calender 12 inch Mill, 30 $ch Mill, 12 inch
INDUSTRIAL AND ENGINEERING CHEMISTRY
MS-4/121' C. 60 110 150 150 200
Vol. 48, No. 5
U R E T H A N E RUBBER Extrusion is the most critical operation. Stocks which can be extruded usually can be processed by other methods with relative ease. Stock viscosity is influenced by two factors-compounding and scorching. Compounding with carbon blaclc and plasticizers id a practical means of varying stock viscosity over a wide range. As in other elastomers, the use of carbon black increases the stock viscosity and makes processing more difficult. Therefore, plasticizers are necessary in most stocks to reduce the viscosity to the desired level for processing. A liquid butadiene-acrylonitrile copolymer, available commercially as Hycar 1312, has proved effective for plasticizing Adiprene B. Figure 5 shows the influence of this plasticizer in combination with E P C black on the Mooney viscosity of Adiprene B. These data show the amount of plasticizer required with a range of carbon black loadings t o yield any selected stock viscosity. Fifteen parts of plasticizer is usually ample for most types of processing. The use of larger quantities of plasticizer tends t o affect vulcanizate properties adversely, as is true with other elastomers, so minimum quantities should be used. Scorching increases stock viscosity. Therefore, a slow rate of scorch at processing temperatures is necessary to keep stock viscosity within the processable range. Scorch rate is influenced by the amount of curing agent and the compounding ingredients used. Figure 6 shows the influence of curative level on the RIooney scorch rate at 121" C. of a gum stock and a stock containing carbon black and plasticizer. As would be expected, the blooney scorch rate increases as the quantity of Hylene D M M is increased, and use of the smallest quantity consistent with good vulcanizate properties is desirable. I n most stocks, 2 to 3 parts of Hylene DMM is optimum; however, for some uses up to 5 parts is warranted. The stock containing carbon black and plasticizer is more scorch resistant than the gum stock. This is of considerable importance in processing since by compounding to a lower viscosity, heat generation is reduced which, coupled with a slower scorch rate, results in compounds capable of being processed under commercial conditions. Maximum processing temperature should not exceed 121' C.
Influence of Viscosity on Processing Temperature. The maximum processing temperature of stocks containing Adiprene B should not exceed 121" C. Above that temperature, stocks may be granular and semielastic because of transition. The rate of scorch is also faster than at lower temperatures, and stocks reach a maximum processable viscosity much sooner. Processing temperature is a function of stock viscosity. Control of stock viscosity is essential t o maintain processing temperature below 120' C. Since extrusion is the most exacting of the common processing operations, a study was made of the influence of Mooney viscosity on heat generation in a 6-inch tread extruder, Figure 7 shows the results of this study. It is evident from these data that to attain an extrusion temperature no higher than 121' C., a maximum viscosity of 50 is indicated. However, consistently better processing will be obtained if a viscosity of 30 to 40 MS-4/121° C. is used. Moisture Control. Moisture should be avoided since it increases scorching and tends to reduce the quality of the vulcanisate. Compounded stocks will absorb moisture from the air. Stocks containing curing agents should be relatively dry and provision made to keep them dry until they are cured. Adequate control of moisture is accomplished by: 1. Using dry compounding ingredients 2. Milling the stock immediately prior t o the addition of curing agent for approximately 10 minutes a t 121O C. (A normal stock warm-up prior t o calendering or extrusion usually suffices to dry the stock if it has been stored between mixing and finish processing. )
M a y 1956
3 . Cooling mixed stock in air and avoiding external sources of moisture 4. Slabbing mixed stocks as thick as possible t o reduce surface area
Bin Storage. Stocks containing curing agent increase in viscosity during storage and will become unprocessable if stored too long. Close scheduling of operations is necessary to ensure that the stock viscosity does not exceed the processable range for the operations t o be used. T h e useful life of stocks is increased a t low temperatures and humidity and when stocks are stored in thick slabs. Six days of storage time appears to be a maximum for stocks which afe to be extruded. If other processing operations, less critical than extrusion, are contemplated, the safe storage time is increased accordingly. Compound Development. Development of practical compounds suitable for most types of processing is accomplished through the use of the foregoing data on compounding. For example, assume that an extrudable tire-tread compound is desired. This type of stock usually requires 15 parts of EPC black for the best wear resistance, but such a stock cannot be extruded because of high viscosity. Figure 7 shows t h a t t o attain an extrusion temperature of about 110" C. in a 6-inch tread extruder the &looneyviscosity MS-4/12lo C. should be about 40. Figure 5 indicates t h a t a stock containing 15 parts of EPC black has a viscosity of 75, but the addition of 15 parts of plasticizer reduces the viscosity to 36 and places it in the right range for extrusion. This stock, containing 5 parts of Hylene DRIM, has a hlooney scorch rate a t 121" C. of 23 minutes to a 10-point rise. This is adequate in view of the fact that the stock should process a t about 110" C., and a t that temperature the scorch rate is even slower. This formulation has been extruded repeatedly into a smooth tread cap for experimental tire testing using conventional equipment. On the other hand, if this hypothetical stock is not intended for extrusion but only for calendering or molding, the stock viscosity may be higher. This reduces the amount of plasticizer required since in these operations, processing temperatures of high viscosity stocks are more easily controlled. The limiting factor then becomes the maximum processable viscosity that allows more latitude in compounding. Summary
Adiprene B urethane rubber is a high viscosity thermoplastic elastomer that can be compounded and otherwise processed on conventional rubber manufacturing equipment. There are fundamental differences between Adiprene B and the more common elastomers in processing which must be taken into consideration. 1. Processing temperatures should be less than 121" C. t o ensure smooth plastic flow and to reduce the rate of scorching. 2. Processing temperature is proportional t o Mooney viscosity and both can be reduced by the use of plasticizers. 3. Moisture increases the rate of scorch and i t should be held to a minimum through all phases of processing. 4. Since stocks containing Adiprene B increase in viscosity during storage, close scheduling of operations will ensure the maximum processable life. Literature cited
Testing Materials, Philadelphia, Pa., Standards o n Rubber Products, December 1954. (2) Decker, G. E., Roth, F. L., Rubber World 128, 399 (1953). (1) Am. Soc.
RECEIVED for review May 19, 1955. ACCEPTEDJanuary 28, 1966. Division of Rubber Chemistry, ACS, Detroit, Mioh.. May 1955. Contribution No. 109, Rubber Laboratory, E. I . du P o n t de Nemours 8: Co., Wilmington, Del.
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