Rubber composition and method of producing rubber products using the same composition

This invention presents a rubber composition wherein a determined amount of protein is mixed with unvulcanized rubber. Rubber products made of this rubber composition have sufficient strength for practical use and are harmless to human bodies. The rubber composition of the invention not requiring vulcanizing process is advantageous in eliminating the need of large-scale vulcanizing equipment. Combined use of a curing agent and protein will increase modulus higher than using the curing agent alone.

FIELD OF THE INVENTION 
The present invention relates to a rubber composition containing protein 
and a method of producing rubber products using the same composition, or 
more particularly to the rubber composition which, whether unvulcanized or 
vulcanized at low density, is given sufficient strength for practical use 
by containing protein therein and to the method of producing rubber 
products using the same composition. 
BACKGROUND OF THE INVENTION 
Conventionally, natural rubber and synthetic rubbers has been vulcanized. 
Vulcanization means producing bridged structure among rubber molecules by 
using a curing agent mixed with the crude rubber. Vulcanization increases 
elasticity of the rubber, giving sufficient strength for practical use. 
Known curing methods include sulfur cure, peroxide cure and radiation 
cure. 
However, rubber products using the above curing methods have the following 
problems. When rubber products by sulfur cure are used in acid solution, 
zinc ion may be eluted from zinc oxide which was added during production 
process. Furthermore, because of some vulcanization accelerators, the 
rubber product itself may cause skin irritation. 
If rubber products are made of rubber prepared by peroxide cure, the 
products themselves may cause skin irritation because peroxide has a 
property of irritating skin. 
Rubber products made of rubber prepared by radiation cure are quite harmful 
to human bodies because they contain carbon tetrachloride or organic 
multifunctional monomers as a cross linking agent. 
When the rubber products made of the rubbers vulcanized by any of the 
curing methods are used in direct contact with the human bodies, 
particularly when they are used in the human bodies, they have a fatal 
drawback of being harmful. Whereas unvulcanized rubbers do not have the 
above harmfullness because any curing agent is not added to them. However, 
the unvulcanized rubbers cannot achieve sufficient strength, and so a 
single substance of them is rarely used as a structural material. 
Further improvement in the modulus of the vulcanized rubber is sometimes 
required. In this case, the above vulcanizations cannot give sufficient 
modulus to the rubber. 
SUMMARY OF THE INVENTION 
A main object of the present invention is to overcome the technical 
problems mentioned above, and to present a rubber composition enabling 
production of rubber products which is harmless to human bodies and is 
strong enough for practical use, and a method of producing rubber products 
using the same composition. 
Another object of the invention is to present a rubber composition which 
without using a filler, enables production of rubber products having a 
higher modulus than conventionally vulcanized rubbers, and a method of 
producing rubber products using the same composition. 
After continued efforts and study for achieving the above objects, the 
inventors have found an surprising fact that when a certain amount of 
protein is mixed with rubber, the rubber may have a sufficient strength 
for practical use even though it is not vulcanized. Thus the inventors 
have completed the present invention. 
The rubber composition of the invention is characterized by mixing 1 to 20 
parts by weight of protein in solid with 100 parts by weight of rubber. 
The method of producing rubber products according to the invention is 
characterized by that after mixing an aqueous solution of protein to 
rubber latex in a ratio of 1 to 20 parts by weight of protein in solid to 
100 parts by weight of rubber in solid, and the resultant mixture is dried 
and molded. 
In the present invention, the reason why mixing protein with rubber will 
enhance the strength of the unvulcanized rubber is not clearly known. It, 
however, is presumed that protein may act as a kind of reinforcing agent 
and at the same time it may bridge rubber molecules to some extent. Since 
the rubber composition of the invention only needs mixing protein and does 
not employ the conventional curing agents such as sulfur, it will not do 
any harm to human bodies. 
In addition, if protein is mixed with the conventional vulcanized rubber 
composition, the modulus of the vulcanized rubber may be improved even 
further. 
In order to manufacture the vulcanized rubber products according to the 
present invention, a protein and a curing agent are mixed into a rubber 
latex, and subsequently the resultant mixture is molded and vulcanized. In 
this case, the above aqueous protein solution containing 1 to 20 parts by 
weight of protein in solid is mixed with the rubber latex containing 100 
parts by weight of rubber in solid, and then dried, molded and vulcanized. 
It is preferable that the aqueous protein solution is mixed with the above 
latex rubber at a ratio of 1 to 20 parts by weight of protein in solid to 
100 parts by weight of rubber in solid, and the resultant mixture is 
subsequently mixed with a curing agent, dried, molded and vulcanized. 
It has been known that if a small amount of casein, normally by about 0.1 
part by weight, is added to natural rubber latex, the mechanical stability 
of the rubber is enhanced. The mechanical stability means a property of 
not being coagulated by mechanical shock or the like during production 
process. However, a fact that mixing 1 to 20 parts by weight of protein in 
solid to 100 parts by weight of rubber will enhance the strength or 
modulus of 10 the rubber product is not yet known in the related field 
prior to the present invention.

DETAILED DESCRIPTION OF THE INVENTION 
Examples of protein preferably used by the invention include keratin and/or 
casein, and keratin is particularly preferred. The reason may be that 
because keratin contains a lot of SH groups, strength of the rubber can be 
enhanced by cross-linking between the SH groups and rubber molecules. 
The type of rubber used by the invention is not particularly limited. 
Preferable examples of the rubber may include natural rubber, chloroprene 
rubber, isoprene rubber, acrylonitrile-butadiene rubber, styrene-butadiene 
rubber and the like. 
These rubbers may be unvulcanized or may contain a curing agent. As 
described above, however, if the rubber product is used in direct contact 
with the human body, it is preferable to use the unvulcanized rubber or a 
low-density vulcanized rubber mixed with a minimum allowable amount of the 
curing agent that may not affect the human body. On the other hand, if the 
rubber is used as industrial materials which may not come in contact with 
the human body and which require even higher strength, the curing agent 
such as sulfur may be added. 
The unvulcanized rubber products of the invention may include those in 
which protein bridges among rubber molecules to a certain extent. Those 
unvulcanized rubber products significantly differ from the conventional 
rubbers by sulfur cure in the method and extent of bridging. 
A preferable mixing ratio of protein according to the invention is 1 to 20 
parts by weight to 100 parts by weight of rubber. If the mixing ratio of 
the protein is lower than 1 part by weight, there is no effect of adding 
the protein, and therefore the strength of the rubber cannot be improved. 
If the mixing ratio of the same is more than 20 parts by weight, the 
amount of protein solution becomes excessive with less solid material 
therein. This will result in lower workability of the rubber or longer 
hours for drying process, incurring loss increase. 
As described above, to produce the rubber products using the rubber 
composition of the invention, a deter- mined amount of an aqueous protein 
solution is added to rubber latex, and the resultant mixture is dried and 
molded. The rubber latex is normally made anionic so that colloid may be 
stably dispersed. To make the latex of natural rubber anionic, ammonia may 
be added, and to make the latex of synthetic rubber anionic, ammonia or 
potassium hydroxide may be added. 
To produce the vulcanized rubber or low-density vulcanized rubber, an 
aqueous protein solution is added to the rubber latex and then further a 
curing agent is added. The example of sulfur cure agents may include 
sulfur and sulfide such as 4,4'-dithiomorpholine, dipentamethylenethiuram 
tetrasulfide and the like. The example of vulcanization accelerators may 
include amines such as hexamethylenetetramine and n-butylaldehyde aniline; 
guanidines such as diphenylguanidine and di-o-tolylguanidine; and sulfides 
such as N,N'-diphenylthiourea, N,N'-diethylthiourea, dibutylthiourea, 
dilaurylthiourea, mercaptobenzothiazole, sodium salt, zinc salt or 
cyclohexylamine salt of mercaptobenzothiazole, dibenzothiazole disulfide, 
tetramethylthiuram disulfide, tetraethylthiuram disulfide, 
tetrabutylthiuram disulfide, tetramethylthiuram monosulfide, 
dipentamethylenethiuram tetrasulfide, piperidine 
pentamethylenedithiocarbamate, sodium diethyldithiocarbamate, sodium 
dibutyldithiocarbamate, zinc dimethyldithiocarbamate, zinc 
diethyldithiocarbate, zinc dibutyldithiocarbamate, and zinc 
N-ethyl-N-phenyldithiocarbamate. Further, activating agents such as zinc 
oxide, lead oxide and magnesium oxide may be added. 
The example of peroxide curing agents may include p-quinone dioxime, 
p,p'-dibenzoyl quinone dioxime and 4,4'-dithiodimorpholine. 
The mixing ratio of said curing agents used for sulfur cure or peroxide 
cure is not especially specified and mixing it by 0.5 to 2 parts by weight 
to 100 parts by weight of rubber is satisfying. 
The example of radiation curing agents may include bifunctional monomers 
having two double bonds in one molecule such as 
1,3-butyleneglycolacrylate, 1,3-butyleneglycol dimethacrylate, 
1,6-hexaneglycol diacrylate and 1,6-hexaneglycol dimethacrylate, 
neopentylglycol diacrylate, neopentylglycol dimethacrylate; and 
monofunctional monomers having one double bonds in one molecule such as 
ethylacrylate, n-butylacrylate, n-hexylacrylate and 2-ethylhexylacrylate. 
A mixing ratio of the curing agents used for radiation cure is 1 to 20 
parts by weight, preferably 2 to 10 parts by weight to 100 parts by weight 
of rubber. 
According to the rubber composition and the method of producing using the 
same composition of the invention, the unvulcanized rubber is mixed with a 
determined amount of protein to produce the rubber products which are 
strong enough for practical use and harmless to the human body. Therefore, 
the rubber may be preferably applicable to artificial organs, gloves, 
condoms and catheters, which are used in direct contact with the human 
body, requiring high strength. Further according to the invention, the 
rubber products may be readily produced by mixing rubber latex with the 
aqueous protein solution. The present invention is advantageous in 
eliminating the need of a large-scale vulcanizing equipment because it 
does not need the vulcanization process as required by the conventional 
methods. 
Combined use of the curing agent and protein may achieve higher modulus 
than using the curing agent alone. 
EXAMPLES 
Referring to Examples of the invention, the rubber composition of the 
invention will be described in details. 
EXAMPLE 1 
1.0 part by weight of water-soluble keratin in solid was mixed with 100 
parts by weight of natural rubber latex containing 0.7% by weight of 
ammonia. The resultant mixture was poured onto a glass plate to be dried 
and molded at room temperatures, and thus was obtained a 0.3 mm thick 
film. The film is in unvulcanized state not containing the curing agent 
such as sulfur. 
The water-soluble keratin which was used in this example was prepared by a 
method presented at "2 A247 Lecture of the 63th Annual Meeting of the 
Chemistry Society of Japan in Spring." That is, wool was shaken with 8M 
urea, a reducing agent (2-mercaptoethanol) and a surfactant, at 50.degree. 
C. for 12 hours. After filtering, the mixture was dialyzed through 
cellophane tube to obtain an aqueous keratin solution. The ratio of 
keratin contained in this aqueous solution was about 2 to 3%. 
EXAMPLE 2 
A 0.3 mm thick film was obtained by the same manner as in Example 1 except 
that 2.5 parts by weight of water-soluble keratin in solid was mixed with 
100 parts by weight of natural rubber latex. 
EXAMPLE 3 
A 0.3 mm thick film was obtained by the same manner as in Example 1 except 
that 5.0 parts by weight of water-soluble keratin in solid was mixed with 
100 parts by weight of natural rubber latex. 
EXAMPLE 4 
A 0.3 mm thick film was obtained by the same manner as in Example 1 except 
that 10.0 parts by weight of water-soluble keratin in solid was mixed with 
100 parts by weight of natural rubber latex. 
EXAMPLE 5 
A 0.3 mm thick film was obtained by the same manner as in Example 1 except 
that 20.0 parts by weight of water-soluble keratin in solid was mixed with 
100 parts by weight of natural rubber latex. 
EXAMPLE 6 
A 0.3 mm thick film was obtained by the same manner as in Example 1 except 
that 5.0 parts by weight of water-soluble keratin in solid was mixed with 
100 parts by weight of chloroprene rubber latex produced by Denka Co., 
Ltd. 
EXAMPLE 7 
A 0.3 mm thick film was obtained by the same manner as in Example 1 except 
that 5.0 parts by weight of water-soluble keratin in solid was mixed with 
100 parts by weight of isoprene rubber latex produced by Sumitomo Seika 
Chemicals Co., Ltd. 
EXAMPLE 8 
A 0.3 mm thick film was obtained by the same manner as in Example 1 except 
that 5.0 parts by weight of water-soluble keratin in solid was mixed with 
100 parts by weight of acrylonitrile-butadiene rubber latex produced by 
Nippon Zeon Co., Ltd. 
EXAMPLE 9 
A 0.3 mm thick film was obtained by the same manner as in Example 1 except 
that 5.0 parts by weight of water-soluble keratin in solid was mixed with 
100 parts by weight of styrene-butadiene rubber latex produced by Nippon 
Zeon Co., Ltd. 
EXAMPLE 10 
An aqueous casein solution containing 1.0 part by weight of casein in solid 
was mixed with 100 parts by weight of natural rubber latex containing 0.7% 
by weight of ammonia. The resultant mixture was poured onto a glass plate 
to be dried and molded at room temperatures, and thus was obtained a 0.3 
mm thick film. The film is in unvulcanized state not containing the curing 
agent such as sulfur. 
A casein solution was prepared by dissolving solid casein in aqueous 
ammonia. 
EXAMPLE 11 
A 0.3 mm thick film was obtained by the same manner as in Example 10 except 
that a solution containing 2.5 parts by weight of casein in solid was 
mixed with 100 parts by weight of natural rubber latex. 
EXAMPLE 12 
A 0.3 mm thick film was obtained by the same manner as in Example 10 except 
that a solution containing 5.0 parts by weight of casein in solid was 
mixed with 100 parts by weight of natural rubber latex. 
EXAMPLE 13 
A 0.3 mm thick film was obtained by the same manner as in Example 10 except 
that a solution containing 10.0 parts by weight of casein in solid was 
mixed with 100 parts by weight of natural rubber latex. 
EXAMPLE 14 
A 0.3 mm thick film was produced by the same manner as in Example 10 except 
that a solution containing 20.0 parts by weight of casein in solid was 
mixed with 100 parts by weight of natural rubber latex. 
EXAMPLE 15 
A 0.3 mm thick film was obtained by the same manner as in Example 10 except 
that a solution containing 5.0 parts by weight of casein in solid was 
mixed with 100 parts by weight of chloroprene rubber latex produced by 
Denka Co. Ltd. 
EXAMPLE 16 
A 0.3 mm thick film was obtained by the same manner as in Example 10 except 
that a solution containing 5.0 parts by weight of casein in solid was 
mixed with 100 parts by weight of isoprene rubber latex produced by 
Sumitomo Precision Chemical Co., Ltd. 
EXAMPLE 17 
A 0.3 mm thick film was obtained by the same manner as in Example 10 except 
that an aqueous solution containing 5.0 parts by weight of casein in solid 
was mixed with 100 parts by weight of acrylonitrile-butadiene rubber latex 
produced by Nippon Zeon Co., Ltd. 
EXAMPLE 18 
A 0.3 mm thick film was obtained by the same manner as in Example 10 except 
that an aqueous solution contain- ing-5.0 parts by weight of casein in 
solid was mixed with 100 parts by weight of styrene-butadiene rubber latex 
produced by Nippon Zeon Co., Ltd. 
COMATIVE EXAMPLE 1 
Natural rubber latex containing 0.7% by weight of ammonia was poured onto a 
glass plate to be dried and molded at room temperatures, and thus was 
obtained a 0.3 mm thick film. 
COMATIVE EXAMPLE 2 
Chloroprene rubber latex produced by Denka Co., Ltd. was poured onto a 
glass plate to be dried and molded at room temperatures, and thus was 
obtained a 0.3 mm thick film. 
COMATIVE EXAMPLE 3 
Isoprene rubber latex produced by Sumitomo Precision Chemical Co., Ltd. was 
poured onto a glass plate to be dried and molded at room temperatures, and 
thus was obtained a 0.3 mm thick film. 
COMATIVE EXAMPLE 4 
Acrylonitrile-butadiene rubber latex produced by Nippon Zeon Co., Ltd. was 
poured onto a glass plate to be dried and molded at room temperatures, and 
thus was obtained a 0.3 mm thick film. 
COMATIVE EXAMPLE 5 
Styrene-butadiene rubber latex produced by Nippon Zeon Co., Ltd. was poured 
onto a glass plate to be dried and molded at room temperatures, and thus 
was obtained a 0.3 mm thick film. 
EVALUATION TEST 
The films obtained by the Examples and Comparative Examples were cut into 
JIS-4 dumbbell specimens, which were subject to a tensile test in 
accordance with JIS-K 6301 to measure the strength at break of each of the 
Examples and Comparative Examples. In the test, 3 samples for each Example 
or Comparative Example were measured, and the measurements were stated in 
the order of higher values as S.sub.1 24 S.sub.2 .gtoreq.S.sub.3. The 
average value was found by the following expression. 
EQU Strength at break=0.7S.sub.1 +0.2S.sub.2 +0.1S.sub.3 
The measurement values are given in Table 1. 
TABLE 1 
______________________________________ 
Mixing ratio 
of protein Tensile 
Type Amount stength 
of (parts by 
at break 
rubber Type weight) (MPa) 
______________________________________ 
Example No. 
Example 1 
Natural rubber 
Keratin 1.0 7.2 
Example 2 
Natural rubber 
Keratin 2.5 8.9 
Example 3 
Natural rubber 
Keratin 5.0 2.4 
Example 4 
Natural rubber 
Keratin 10.0 16.3 
Example 5 
Natural rubber 
Keratin 20.0 22.2 
Example 6 
Chloroprene Keratin 5.0 9.0 
rubber 
Example 7 
Isoprene rubber 
Keratin 5.0 3.6 
Example 8 
Acrylonitrile- 
Keratin 5.0 8.8 
butadiene rubber 
Example 9 
Styrene-butadiene 
Keratin 5.0 4.3 
rubber 
Example 10 
Natural rubber 
Casein 1.0 8.3 
Example 11 
Natural rubber 
Casein 2.5 8.7 
Example 12 
Natural rubber 
Casein 5.0 9.9 
Example 13 
Natural rubber 
Casein 10.0 10.9 
Example 14 
Natural rubber 
Casein 20.0 12.8 
Example 15 
Chloroprene Casein 5.0 6.2 
rubber 
Example 16 
Isoprene rubber 
Casein 5.0 3.2 
Example 17 
Acrylonitrile- 
Casein 5.0 3.1 
butadiene rubber 
Example 18 
Styrene-butadiene 
Casein 5.0 3.0 
rubber 
Comparative 
Example No. 
Example 1 
Natural rubber 
-- -- 6.4 
2 Chloroprene -- -- 5.9 
rubber 
3 Isoprene rubber 
-- -- 0.4 
4 Acrylonitrile- 
-- -- 2.9 
butadiene rubber 
5 Styrene-butadiene 
-- -- 1.3 
rubber 
______________________________________ 
As shown in Table 1, the films obtained by Examples 1 to 18, containing 
protein therein, have higher strength at break than those of Comparative 
Examples 1 to 5. The higher mixing ratio of protein gives the higher 
strength at break. 
EXAMPLES 19 to 28 (sulfur cure) 
Casein or keratin was mixed with 100 parts by weight of natural rubber 
latex in ratios given in Table 2. Then, 1.0 part by weight of sulfur as 
well as 1.0 part by weight of zinc oxide as curing agents, and 0.6 part by 
weight of dibutylthiocarbamate as a vulcanizing accelerator were added. 
The resultant mixture was left to dry for 24 hours at a room temperature 
of about 30.degree. C., and thus was obtained a 0.3 mm thick dry film. 
Each of the films of Examples 19 to 28 was cut into a JIS-4 dumbbell 
specimen for measuring modulus (M.sub.300 and M.sub.500) at elongations of 
300% and 500%. For comparison, the same test was conducted on the natural 
rubber not containing protein. The results are shown in Table 2. 
TABLE 2 
______________________________________ 
Mixing ratio of protein 
Example Amount (parts 
M.sub.300 
M.sub.500 
No. Type by weight) (kgf/cm.sup.2) 
(kgf/cm.sup.2) 
______________________________________ 
Example 19 
Casein 1.0 11.6 26.2 
Example 20 
Casein 2.5 13.4 33.9 
Example 21 
Casein 5.0 19.5 48.4 
Example 22 
Casein 10.0 33.8 83.2 
Example 23 
Casein 20.0 49.2 126.0 
Example 24 
Keratin 1.0 11.3 25.4 
Example 25 
Keratin 2.5 13.0 29.3 
Example 26 
Keratin 5.0 16.4 48.0 
Example 27 
Keratin 10.0 26.7 78.3 
Example 28 
Keratin 20.0 34.5 18.1 
Control -- -- 10.4 20.8 
______________________________________ 
Table 2 shows that mixing 5 phr of protein will increase the modulus to 
more than twofold of that of Control not containing protein.