Plasticizing GR-S and Natural Rubber ~
ARNOLD R. DAVIS American Cyanamid Company, Stamford, Conn.
years that the nature of the plasticization process became known. The first indication that atmospheric oxygen might play a part in plasticizing rubber is said (16) to be a conclusion to a report of the Research Association of British Rubber Manufacturers in 1921 stating: "The effect produced by milling ,can be imitated by heating rubber in the presence of oxygen or air." The work supporting this conclusion was also published (9) by Fry and Porritt in 1927. A few years later Grenquist (12) showed that a breakdown analogous to that brought about by milling could be obtained by heating rubber in air. In a study of the loss of unsaturation MILS 1 1 in r u b b e r d u r i n g le0 milling, Fisher and Gray (8) suggested 110 that the breakdown 100 or p l a s t i ciza t ion 90 during milling is chemical in nature eo and is caused by f 70 oxygen. Schack€4 lock (15) showed that a new com40 pound was formed from rubber by milling it in air. He assumed this c o m p o u n d was formed by oxida10 PERCENT O,O'-DIEENZAMIDODIPHEMLD~SI~LFIDE tion. Cotton (5), and a little later Figure 2. Hot Banbury Plasticizing Busse (I), showed of GR-S with o,o'-Dibenzamidodiphenyl Disulfide conclusively t h a t oxygen is necessary Banbury jacket, 153' C.; rotors 100' C.; batch, 250 grams; total time in'Banbury, for the plasticiza6 minutes tion of rubber during mastication or milling. Comes (4) showed that rubber could be plasticized by mastication in an internal mixer at high temperatures (up t o 400 F.). Busse and Cunningham (3) showed that oxygen is required for the plasticization of rubber with mastication in an internal mixer at temperatures of 160" to 325" F. After a long study of thermal plasticization of rubber, Farberov and Margolina (6) concluded that, for a given set of thermal conditions, oxygen is the fundamental factor in thermal plasticization. Hagen (IS) stated that oxygen is required for the thermal plasticization of the butadiene-styrene copolymer, Buna S. Since it is now evident that the plasticization of rubber and butadiene-styrene copolymers involves a chemical reaction, one would expect to find a number of catalysts for the reaction. These chemicals would be used in relatively small amounts to decrease the time or to reduce the temperature required to give a, desired plasticity, as well as to give increased plasticity in a given time. The plasticization catalysts might avoid the use of large amounts of commonly used softeners which function largely through dilution, swelling, and lubrication of the elastomer. Ac-
Another class of chemicals, the o,o'-diacylaminodiphenyl disulfides, are shown to promote the plasticization of GR-S with hot mastication. o,o'-Dibenzamidodiphenyl disulfide has been shown to be a catalytic plasticizer for GR-S and natural rubber with hot mastication. The data point out the possibility of decreasing the time and thereby the power required to obtain a desired plasticity, or the possibility of producing softer rubber in a given time. The control of additional gel formation and also actual reduction of gel with hot processing of GR-S in the presence of o,o'-dibenzamidodiphenyl disulfide is shown. This indicates possible improvement in quality, and, with lower viscosities, possible improvement i n processing characteristics. Physical properties obtained with cold masticated GR-S and natural rubber may be obtained with the same elastomers plasticized by use of o,o'-dibenzamidodiphenyl disulfide with hot mastication. The similarity in the chemical nature of the plasticization of GR-S and natural rubber is shown.
k
B
UTADIENE-STYRENE copolymers have been plasticized by mastication and thermal processes similar to those used for natural rubber. It is generally known that the copolymer GR-S does not yield t o plasticization processes as well as or t o the same extent as does rubber. Several explanations for this difference have been advanced (16). The tendency for GR-S (17) to cyclize or form gel in hot processing and the presence of an antioxidant undoubtedly have counter effects on the plasticization. The mastication of rubber and a crude use of the thermal process of softening rubber date back over 125 years to the days of Thomas Hancock (14). However, it was only during the last 25
50 45
0 b 35 30 5 4 2 5 Feo EI 8 15
-
40
BEFORE
MILLING
AFTER BANBURY MILLING
I
-
.
I
O
>
B1L
.
0 0,0'-6d.d
0,O-D.dd.
0,O'-Did.
Figure 1. Cold and Hot Banbury Milling Compared with Hot Banbury Milling with o,o'-Dibenzamidodiphenyl Disulfide in GR-S Total milling time 6 minutes; batch, 230 grams; cold Banbury inidal temperature, 30' C. (cold water on); hot Banbury jacket, 1 5 3 O C.; rotora, 1000 c.
94
95
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1947
MILS
"h 35
-
30% O,O'-Lldd. ON OR-S
I-MIN.REMlMRYal 100.C.
% GEL IN GR-S BEFORE MILLINOsO
90
80 70
-1 60
I
if 40
I
I
I
20
5 10 15 TOTAL T W IN BANBW, MINUTES
Figure 3. Hot Banbury Plasticizing of GR-S with o,o'-Dibenzamidodiphenyl Disulfide
10 0
Banbury jacket, 153' C.; rotors, 100' C.; batch, 250 grams
tually, a number of such catalysts have been found, and several have had considerable practical application during the past ten years. Zimmerman and Cooper (94) stated in 1928 that diphenylguanidine as well as some antioxidants were known to be softeners for rubber. To date many rubber technologists have undoubtedly observed that the commonly used guanidine accelerators are effective plasticizers for rubber, particularly with hot milling. Piperidinium pentamethylene dithiocarbamate has been shown to plasticize lightly vulcanized rubber on milling (90). Mercaptobenzothiazole (3) and some of its derivatives are known to promote the plasticizing of rubber with hot mastication. The hydrazines (2, W1), particularly phenylhydrazine and its salts, thiophenols (aromatic mercaptans) (3, 32), and various nitroso compounds, including nitroso-p-naphthol ( 7 , B ) have been used
TABLE
Figure 4. Plasticities of GR-S Containing o,o'-Dibenzamidodiphenyl Disulfide at Different Banbury Temperatures Batch, 250 grams; total time in Banbury, 6 minutes
to plasticize rubber. Certain aromatic mercaptans (IO), the zinc (18) salts of aromatic mercaptans, and nitroso-p-naphthol have found some applicacion in plasticizing GR-S with hot mastication. The fact that several of these chemicals, although in different ratios, aid in plasticizing .GR-S and natural rubber under like conditions again indicates similarity in the nature of the plasticization process for the two elastomers. Busse and Cunningham (3) called the hydrazine compounds and thiophenols true mastication accelerators ct oxidation catalysts. Since these materials do not affect the aging qualities of the cured rubber stocks, it might be better to call them plasticization catalysts or catalytic plasticizers. As a result of research work on the problems of plasticizing I. HOTBANBURY4 PLASTICIZING O F GR-S K I T H TYPICAL GR-S, a number of o,o'-diacylaminodiphenyl disulfides or bis (oacylaminophenyl) disulfides were found t o be effective plasticizers ~,~'-DIACYLAMINODIPHENYL DISULFIDES for GR-S with hot mastication. Several of this class of chemicals were also found to be very active in plasticizing natural rubber during hot mastication. Table I shows the results obtained with a number of typical o,o'-diacylaminodiphenyl disulfides in GR-S Williams 1-Minute % with hot Banbury milling. Plasticizer 3-Min. Y b Recovery on
GR-S Control GR-S (blend of 4 polymers) o,o'-Diacetamidodiphenyl disulfide o,o'-Dibutyramido diphenyl disulfide o,o'-Di-isobutyramidodipbenyl disulfide o,o'-Dicrotonami,dodiphenyl disulfide GO'-Dipalmitamidodiphenyl disulfide 4-Dlbensamjdodjphenyl djsulfide o,o'-Dibensamidodiphenyl disulfide
0
1.5 1.5
1.6 1.5 2.4 1.5 1.0
at 100' C.. a t 100' C., Mils Mils 103 49 85 33 93 45
86 94 97 81 86
41 47 46 35 38
B a n b u y jacket, 162' ,C., rotors, 100' C.: batch, 200 grams GR-S; total time in Banbury, 6 minutes. Lower Y values and recovery figures indicate softer GR-S.
*
TABLE 11. HOTBAN BURY^ PLASTICIZING %
Plasticizer
Gg-S Control GR-S (blend of 4 polymers) p,p'-Diacetamidodiphenyl disulfide ( a form) p,p'-Diacetamidodiphenyl disulfide ( P form) u,p'-Dibutyramidodiphenyl disulfide p,p'-Dibenzamido+phenyl +sulfide od-Dibenzamido&phenyl disulfide Control GR-S (blend of 3 polymers) o,o'-Dib,ensamid,odiphenyl disulfide m,m'-Dibenzamidodiphenyl &sulfide
0
Williams 3-Min. Y a t 100' C., Mils 103
.
I-Min. Recovery a t looo C.. Mils 49
1.5
101
51
1.5 1.5 2.0 1.5 0 1.5 1.5
100 102 106 81 119 93 144
44 51 53 35 40 24 91
a Banbury jacket at 162: C.; rotors a t 100° C.; batch, 200 grams GR-8; t o t a l time in Banbury, 6 minutes.
EXPERIMENTAL METHOD?
A small laboratory Banbury mixer (approximately 310-320 grams of polymer capacity) was used for most of the work. The jacket of the Banbury could be heated to 162" C. (80 pounds steam pressure) and the rotors to 100" C. The chemical plasticizer was added 0.5 minute after the elastomer was loaded into the Banbury. A 6 X 12 inch laboratory mill a t 50-60" C. was used to crepe out the batches from the Banburv and to facilitate cooling. Some experiments in open mill plasticizing were run with the-6 X 12 inch mill a t 115" to 120" C. I n these experiments the plasticizer was added 1minute after the polymer was put on the mill. Banbury and mill temperatures were checked with a surface pyrometer. The temperature of the rubber on dumping from the Banbury was determined with a needle pyrometer. With Banbury temperatures of 153" C. (jacket) and 100" C. (rotors), GR-S showed a temperature of 149 C. on dumping. When temperatures of 162' C. (Banbury jacket) and 100' C. (Banbury rotors) were used, the GR-S showed a temperature of 157" C. I n the case of natural rubber, temperatures somewhat higher than the Banbury jacket temperature were observed (Figure 9). GR-S from one source was used for some of the work, whereas a blend of GR-S polymers from several sources was used for other parts of the work. In the Banbury, batches of 200 to 310 grams of GR-S were used, whereas on the 6 X 12 inch mill batches of 320 grams were used. Separate batches of 250 grams were milled for the indicated time in determining the change of plasticity with time of milling, instead of individual samples being removed from a single
.
INDUSTRIAL AND ENGINEERING
