Stabilization of elastomers with aliphatic-phenyl diamines and aliphatic phosphite compounds

Elastomers such as guayule rubber is stabilized by the addition of an alkyl, cycloalkyl or alkyl substituted cycloalkyl phenyl-phenylenediamine compound with an alkyl, cycloalkyl or alkyl substituted cycloalkyl phosphite compound which, in combination, yield an unexpectant and synergistic result.

TECHNICAL FIELD 
The present invention relates to stabilization of elastomers, especially 
guayule rubber, through the combination of two types of compounds, namely 
an aliphatic phenyl-phenylenediamine along with an aliphatic phosphite 
compound. 
BACKGROUND ART 
Guayule rubber can be recovered from the guayule shrub Parthenium 
Argentatum which grows in semiarid regions. There has been a renewed 
interest in the commercialization of guayule rubber to supplement natural 
rubber obtained from the rubber tree Hevea brasiliensis. Both guayule and 
Hevea are considered "natural rubber." 
Unlike Hevea, guayule rubber does not contain natural antioxidants and must 
be stabilized to prevent rapid degradation upon contact with air (Guayule, 
An Alternate Source of Natural Rubber; National Academy of Science; 
Washington D.C. 1977; page 25). An antioxidant to stabilize guayule rubber 
is usually added during the solution phase purification of the crude 
guayule rubber (ibid, p. 36). Since guayule and Hevea rubbers have 
chemical and physical properties that are virtually identical (ibid, p. 
3), it is reasonable to expect that conventional antioxidants for rubber 
(specifically, cis-1,4-polyisoprene) would be adequate for the 
stabilization of guayule rubber. However, Keller, Winkler and Stevens 
(Paper No. 55, "Degradative Effects of Guayule Resin on Natural Rubber," 
presented at the 117th Meeting of the Rubber Division, ACS; Las Vegas, 
Nev., May 1980, determined that linoleic acid, which is a component of 
guayule resin (which in turn is a potential contaminant of guayule rubber) 
has an adverse effect on the stability of guayule rubber. Therefore, from 
a practical standpoint, a stabilizer additive(s) for guayule rubber 
preferably will function to prevent the deleterious effects of heat and 
oxygen (air) on the rubber itself (cis-1,4-structure) as well as the 
possible adverse effects of certain components of guayule resin. 
There are many chemical compounds which have been shown to be effective 
antioxidants for elastomers. For example, Dunn in his article, "Review of 
Unsolved Problems in the Protection of Rubber Against Oxidative 
Degradation," Rubber Chemistry and Technology, 47 960 (1974); pointed out 
that there were over three thousand patents issued in a ten year period on 
the protection of polymers against aging. Dunn also pointed out that 
because of "antagonism," an antioxidant which proves highly effective in 
one rubber can be totally ineffective in another. Howard [Rubber Chemistry 
and Technology 47 976 (1974)] discussed antioxidant synergism. 
The following references disclose the use of specific compounds as 
stabilizers for guayule rubber. 
Gonzales (Paper No. 2, 116th Meeting of the Rubber Division, ACS, 
Cleveland, Ohio, Oct. 23-26, 1979) reported the results of stabilization 
of guayule rubber with phenolic compounds (thio-, methylene coupled as 
well as alkylated) and N,N'-diphenyl-p-phenylenediamine. The synergistic 
results of our invention is not suggested by Gonzales. 
Butyl Zimate.RTM. which is a product of R. T. Vanderbilt Co., Inc. and 
chemically identified as the zinc salt of di-n-butyldithiocarbamic acid 
has been advertized as an excellent stabilizer for guayule rubber 
(Chemical & Engineering News, May 11, 1981). 
The C & EN May 11, 1981 advertisement also disclosed that Butyl Zimate.RTM. 
alone or with Agerite.RTM. Geltrol.RTM. is the optimum stabilizer for 
Kraton.RTM. 1107 - based hot melts. Agerite.RTM. Geltrol.RTM. is also a 
product of R. T. Vanderbilt and is identified chemically (Rubber World; 
Materials and Compounding Ingredients for Rubber, 1968 Edition, page 97) 
as a modified high molecular weight hindered phenol. Hindered phenols are 
not included in the synergistic combination of the present invention. 
A somewhat more specific identification of Agerite.RTM. and Geltrol.RTM. is 
an alkylated-arylated bisphenolic phosphite (The Vanderbilt Handbook, page 
388). 
Baldemar Motomochi B, Pasquale G. Galioto and Hans R. Strop disclosed in 
their publication entitled, "Mechanical Drying of Guayule and Hevea: A 
Pilot Plant Study) a paper presented at 3rd International Guayule 
Conference, Pasadena, Calif., Apr. 30, 1980). the use of 50 percent Butyl 
Zimate/50 percent AgeRite Superlite.RTM. at a 1 percent loading in guayule 
rubber as a stabilizer system. AgeRite Superlite.RTM. was identified as a 
polybutylated Bisphenol A which is a phenolic compound and no mention of 
synergism was made. 
"BHT" which is butylated hydroxy toluene was disclosed and the antioxidant 
used to stabilize guayule rubber in the Saltillo, Mexico pilot plant 
(Paper No. 19, "Vulcanization of Guayule Rubber;" L. F. Ramos and B. 
Motomochi presented at the 116th meeting of the Rubber Division, ACS, Oct. 
23-36, 1978 in Cleveland, Ohio. 
U.S. Pat. Nos. 1,753,184 and 1,753,185 relate to the use of 
dimethyl-para-phenylenediamine as a stabilizing agent for guayule rubber. 
However, these patents fail to suggest the utilization of a second 
stabilizing compound or any synergistic result obtained thereby. 
No reference relating to the use of a positive synergistic combination of 
para-phenylenediamines with organic phosphites for the stabilization of 
elastomers such as guayule rubber has been found. 
DISCLOSURE OF INVENTION 
It is therefore an aspect of the present invention to provide stabilizing 
compounds for elastomers which yield a synergistic result. 
It is yet another aspect of the present invention to provide synergistic 
stabilizers wherein one of said stabilizers is an 
aliphatic-phenylphenylenediamine and the other stabilizer is an 
aliphatic-phosphite. 
It is yet another aspect of the present invention to provide stabilization 
of guayule rubber to a degree which would allow the guayule rubber to be 
stored for a reasonable length of time. 
These and other aspects of the present invention will become apparent from 
the following specification which describes in detail the present 
invention. 
In general, a stabilized elastomer, comprises: approximately 100 parts by 
weight of the elastomer; and from about 0.05 to about 1.0 parts by weight 
of a compound having the formula: 
##STR1## 
wherein R is an alkyl having from 1 to 12 carbon atoms, a cycloalkyl or an 
alkyl substituted cycloalkyl having from 5 to 12 carbon atoms, and 
combinations thereof; and from about 0.05 to about 1.0 parts by weight of 
a phosphite having the formula 
EQU (R'--O).sub.3 P 
where R' is an alkyl having from 1 to 12 carbon atoms, a cycloalkyl or an 
alkyl substituted cycloalkyl having from 5 to 12 carbon atoms, and 
combinations thereof. 
BEST MODE FOR CARRYING OUT THE INVENTION 
Generally, elastomers can be stabilized utilizing the two types of 
compounds of the present invention. Examples of such elastomers include 
styrene-butadiene rubber, nitrile rubber, butyl rubber, a random copolymer 
of ethylene-propylene, a block copolymer of ethylene-propylene, EPDM, that 
is a terpolymer of ethylene-propylene and a diene, for example norbornene, 
neoprene, polybutadiene, and especially guayule rubber. With regard to the 
synthetic elastomers, they can be obtained from conventional sources as 
well known to the art. 
Considering guayule rubber, it is utilized as attained in any conventional 
manner. Typically, the whole guayule plant can be crushed or ground to a 
small particle size, for example about 1/8 inch length. A hammer mill is 
commonly utilized. The ground shrub can then be slurried in water and 
further ground to release the rubber as described in reference, see page 
1. This process is generally referred to as the "Water Flotation Process." 
Alternatively, the crushed particles can be treated with a nonpolar, 
hydrocarbon solvent to extract the rubber therefrom. Typical solvents 
include the alkanes having from 4 to 9 carbon atoms, for example butanes, 
pentanes, and hexanes, with hexane being preferred. Other suitable 
solvents include the various cycloalkanes having from 5 to 10 carbon atoms 
such as cyclohexane. The aromatics or alkyl substituted aromatics having 
from 6 to 12 carbon atoms, as for example, benzene, toluene, etc., can 
also be utilized. 
Since guayule rubber has poor stability, it must be stabilized with various 
compounds. The two types of compounds of the present invention have been 
found to impart a good degree of stability and is truly a synergistic 
stability in that the total improvement is much greater than the 
additives' stability of each separate compound. 
The first type of compound is a phenylenediamine compound having the 
formula 
##STR2## 
where R is an alkyl having 1 to 12 carbon atoms, with 1 to 8 carbon atoms 
being desired. The preferred alkyl group is hexyl and preferably the 
dimethylbutyl isomer thereof. R can also be a cycloalkyl or an alkyl 
substituted cycloalkyl having from 5 to 12 carbon atoms with from 6 to 10 
carbon atoms being preferred. Representative examples of such compounds 
include N-isopropyl-N'-phenyl-p-phenylenediamine, 
N-cyclohexyl-N-phenyl-p-phenylenediamine, with 
N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine being preferred. 
The second type of compound is a phosphite compound having the formula 
EQU (R'--O).sub.3 P 
where R' is an alkyl having from 1 to 12 carbon atoms, desirably having 
from 4 to 10 carbon atoms, with octyl or specifically isooctyl being 
preferred. R' can also be a cycloalkyl or an alkyl substituted cycloalkyl 
having from 5 to 12 carbon atoms, with from 6 to 8 carbon atoms being 
preferred. 
The amount of the phenylenediamine compound as well as the phosphite 
compound is generally from about 0.05 to about 1.0 parts by weight and 
preferably from about 0.1 to about 0.5 parts by weight based upon 100 
parts by weight of the elastomer. The ratio of the phenylenediamine 
compound to the phosphite compound, based upon weight, is generally from 
about 10 to about 0.1, and desirably from about 5 to about 0.5. 
Preferably, an approximate one-to-one weight ratio is utilized. 
The phenylenediamine type stabilizer and the phosphite type stabilizer can 
be mixed or blended with the elastomer, as for example guayule rubber, 
until generally a random distribution is obtained in accordance with any 
conventional manner. For example, they can be added either sequentially or 
simultaneously to the rubber in any common mixing apparatus such as a 
Banbury, two mill roll, or the like. Often times, a two roll mill is 
utilized at ambient temperature or which is heated to a temperature of 
from about 25.degree. to about 75.degree. C. 
The stabilized guayule rubber of the present invention can generally be 
utilized in any situation wherein guayule rubber is utilized. Accordingly, 
it can be utilized in making a great number of rubber products such as 
conveyor belts, shoe soles, tires, especially aircraft tires, tank pads, 
and the like. Moreover, the elastomer can contain various conventional 
additives such as accelerators, fillers, curing agents, and the like. 
The following examples illustrate the synergistic result obtained with 
regard to stabilization. By stabilization, it is meant that guayule rubber 
functions in an evaluation test similar to commercially acceptable Hevea 
rubber. 
To evaluate the thermooxidative stability of the rubber containing the 
synergistic stabilizer combinations of the invention, the Wallace 
Plastimeter was used. Experimental results obtained with this instrument 
are generally accepted by those skilled in the art as relatively accuarate 
evaluation of the thermooxidative stability of Hevea natural rubber. 
ASTM D3194-73 procedure was followed in our initial experimental work (1980 
Annual ASTM Standards; Rubber, Natural and Synthetic-General; part 37, 
page 712). However, in order to differentiate between very effective 
positive synergistic stabilizer combinations of the present invention, we 
modified the ASTM D3194-73 procedure by aging single samples (rather than 
three samples) at 140.degree. C. for the designated 30 minutes, as well as 
60, 90 and 120 minutes to increase the severity of the test. By the 
modified procedure, it developed trends of oxidative stability under 
rather severe aging conditions. It should be stated that if the 
experimental data summarized in the following tables contain only a Po and 
PRI value for 30 minutes of aging, the ASTM D3194-73 test procedure was 
followed. If Po and PRI values given for 30, 60, 90 and 120 minutes are 
recorded, the modified ASTM procedure was followed. It is emphasized that 
the ASTM procedure was modified in the sense that one, rather than three, 
samples were used and the time of forced-air aging at 140.degree. C. was 
extended from 30 to 120 minutes. Sample preparation and actual testing in 
the Wallace Plastimeter were the same as designated in the ASTM D3194-73 
procedure. 
For the sake of complete disclosure, Po is the plasticity value on the 
unaged rubber sample, PA the plasticity value on the sample aged for the 
designated time and PRI (Plasticity Retention Index) is the ratio of aged 
plasticity/unaged plasticity expressed as a percent; (Po/PA)100 =PRI.

EXAMPLE 1 
Three stocks of guayule rubber were used (designated 1, 2 and 3 in Table 
I). The guayule rubber was recovered by the "water flotation process." As 
will be discussed later, the three stocks of guayule rubber gave 
essentially equivalent test results and were used in both Examples 1 and 
2. 
TABLE I 
______________________________________ 
STABILITY OF REFERENCE ELASTOMERS 
Wallace Plastimeter Data 
Rubber Pa After Minutes 
PRI After Minutes 
Sample Po 30 60 90 120 30 60 90 120 
______________________________________ 
Guayule 
No. 1 42 2 -- -- -- 5 -- -- -- 
No. 2 43 2 -- -- -- 5 -- -- -- 
No. 3 45 3 0 0 0 7 0 0 0 
Hevea 45 19 -- -- -- 42 -- -- -- 
Hevea 48 20 9 7 4 42 19 15 8 
______________________________________ 
EXAMPLE 2 
The same stock material was utilized in Example 2 as for Example 1, with 
the exception that the listed additives were added on a two roll mill, 
with the mill being at ambient temperature and mixed until the additives 
were well dispersed. 
TABLE II 
______________________________________ 
POSITIVE SYNERGISTIC 
ADDITIVE COMBINATION A & B 
Guayule Rubber; 
Wallace Plastimeter Data 
phr Additive 
Pa After Minutes 
PRI After Minutes 
A B Po 30 60 90 120 30 60 90 120 
______________________________________ 
1.00 -- 43 28 22 19 24 65 51 44 56 
0.50 -- 43 25 -- -- -- 58 -- -- -- 
0.50 -- 43 20 18 15 10 47 42 35 23 
0.25 -- 45 25 -- -- -- 55 -- -- -- 
0.25 -- 44 17 14 12 9 39 32 27 20 
-- 0.50 41 11 6 0 0 27 15 0 0 
-- 0.25 41 3 0 0 0 7 0 0 0 
0.25 0.25 41 34 29 17 13 83 71 41 32 
0.25 0.25 43 39 -- -- -- 91 -- -- -- 
______________________________________ 
Data on unstabilized guayule rubber as well as Hevea rubber are summarized 
in Table I. Results on the Hevea sample are included as a reference point. 
That is, the Hevea sample is representative of a commercial product, the 
thermooxidative stability of which is adequate for long term storage of 
the rubber as well as adequate to prevent undue degradation during mixing 
of the rubber with compounding ingredients prior to vulcanization to 
finished rubber products. 
Referring to Table I, results on three samples of unstabilized guayule 
rubber are recorded. The Po values on these samples range from 42 to 45 as 
compared to 45 and 48 Po values obtained on the Hevea rubber sample. These 
results are indicative that the bulk viscosities of guayule and Hevea 
rubbers are similar. 
Referring to the 30 minute PA values, it is readily apparent that the PA 
value of 2 to 3 obtained on the guayule rubber sample are significantly 
lower than the 19 and 20 PA results on Hevea. The results demonstrate that 
the guayule rubber samples are not adequately stabilized. Samples of these 
unstabilized quayule rubber stocks were used in our subsequent 
investigation. That is, selected additives were added to the unstabilized 
guayule rubber samples and the resulting mixture evaluated in the Wallace 
Plastimeter to determine the effects of the additive relative to 
stabilization of the Guayule rubber. 
It should be noted that the PA values on the guayule rubber samples after 
further forced-air oven treatment were essentially zero (too low to 
measure accurately). In contrast, the PA values of the Hevea sample 
gradually decreased from 20 after 30 minutes to 4 after 120 minutes. Since 
Hevea rubber performs very well in industrial applications, Wallace 
Plastimeter data on experimental stabilized guayule rubber samples greater 
than the results obtained on Hevea would indicate that the thermooxidative 
stability of the guayule rubber sample would be adequate for commercial 
use. 
Wallace Plastimeter data on guayule rubber containing the positive 
synergistic additive combinations of our invention are summarized in Table 
II. 
For the sake of brevity in our specification, the commercially available 
N-(1,2-dimethylbutyl)N'-phenyl-p-phenylenediamine is designated additive 
"A" and the tris(isooctyl) phosphite is designated additive "B". Guayule 
rubber containing additive "A" is in accord with expected results; 
additive "A" functions as an effective stabilizer; and, as the 
concentration of additive "A" is decreased, the thermooxidative stability 
of the experimental samples is decreased. 
As indicated by the PA and PRI values on the samples containing additive 
"B" only, additive "B" is not a particularly effective stabilizer for 
guayule rubber. However, the combination of 0.25 phr of additive "A" and 
0.25 phr additive "B" does impart good thermooxidative stability to 
guayule rubber. Actually, the results on the sample containing 0.25 phr 
each of the additives "A" and "B" are considered better than the 30 and 60 
minute results obtained on the sample containing either 1.0 phr additive 
"A" or 0.50 phr additive "B". 
As apparent from the utilization of both additives "A" and "B," the overall 
values are considerably higher than the additive effect of "A" plus "B," 
thus clearly indicating a synergistic result. 
While in accordance with the patent statutes, a preferred embodiment and 
best mode has been described in detail, the scope of the present invention 
is measured by the scope of the attached claims.