Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1997-12-05
2004-06-01
Mulcahy, Peter D. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C523S351000
Reexamination Certificate
active
06743853
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a viscosity stabilizer for natural rubber, a viscosity-stabilized natural rubber composition blended with said viscosity stabilizer and a process for producing the same, and a method of controlling a rise in the viscosity of natural rubber in which said viscosity stabilizer is used for controlling a rise in the viscosity of natural rubber.
2. Description of the Related Art
In general, natural rubber is produced in tropical countries such as Thailand, Malaysia and Indonesia. Natural rubber is used much and widely in the rubber industry and the tire industry because of excellent physical properties thereof.
Natural rubber has as low Mooney viscosity (JIS K6300) as 60 to 70 immediately after production, but the viscosity rises close to 90 to 100 with the passage of time during storage and transportation to Japan.
This phenomenon is called gelation (storage hardening), and it is generally noted that an aldehyde group contained in an isoprene chain of natural rubber reacts with protein and amino acids to cause the gelation. However, the mechanism thereof has not yet distinctly been clarified.
Gelation of natural rubber caused in production sites deteriorates the processability in producing rubber products and therefore has to be prevented.
In order to prevent this gelation, natural rubber latex is processed with 0.08 to 0.30% of hydroxylamine sulfate [(NH
2
OH)
2
·H
2
SO
4
] to prepare a viscosity-stabilized natural rubber composition in Rubber Research Institute of Malaysia (RRIM). Further, a type of composition (SMR-GP) obtained by blending dried natural rubber with a solution of hydroxylamine sulfate described above has been developed as well.
However, the viscosity-stabilized natural rubber composition obtained by using hydroxylamine sulfate has the following problems:
(1) Hydroxylamine sulfate is designated as a poison in Japan and therefore hardly used.
(2) A rise in the viscosity is observed at the beginning of storage.
(3) A viscosity-stabilizing effect is low under a severe condition (in an oven of about 60° C.).
(4) Compatibility with rubber is inferior, and therefore there is a fear of bad dispersibility.
(5) The decomposition temperature is low, and therefore a sufficiently high effect can not be obtained in kneading at high temperatures.
On the other hand, semicarbazide (NH
2
NHCONH
2
) is described in a literature [B. C. Sekhar, J. Polymer Science, Vol. XLVIII, 133 (1960)] in which the results of screening of additives having a viscosity-stabilizing effect on natural rubber are shown. Further, hydroxylamine, semicarbazide and dimedone (1,1-dimethylcyclohexane-3,5-dione) are disclosed as additives for natural rubber which are likely to have a viscosity-stabilizing effect.
However, the respective compounds are expensive, and it has been confirmed that if an amount used is so small though it is scattered, the viscosity-stabilizing effect is basically low from the beginning or the viscosity rises after some period while the viscosity-stabilizing effect is observed at the beginning.
The following two matters can be considered as the cause thereof.
a) Sufficiently high dispersion can not be obtained because of addition in a small amount.
b) An amount of some level is required in order to block an aldehyde group which is considered to cause gelation.
On the other hand, it is well known that carbon black and silica are added as reinforcing fillers for rubber.
However, it is not known at all that carbon black and silica prevent gelation (storage hardening) of natural rubber, that is, have a viscosity-stabilizing effect.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the conventional problems described above and to provide a low-cost viscosity stabilizer for natural rubber having a viscosity-stabilizing effect continuing over an extended period of time, a viscosity-stabilized natural rubber composition blended with said viscosity stabilizer and a production process for the same, and a method of controlling a rise in the viscosity of natural rubber in which said viscosity stabilizer is used for controlling a rise in the viscosity of natural rubber.
Intensive investigations made by the present inventors in order to solve the conventional problems described above have resulted in finding that the specific rubber compounding agents already used as compounding agents for rubber have a viscosity-stabilizing effect. Further researches have resulted in successfully obtaining a viscosity stabilizer for natural rubber, a viscosity-stabilized natural rubber composition and a production process for the same, and a method of controlling a rise in the viscosity of natural rubber, and thus coming to complete the present invention.
That is, the viscosity stabilizer for natural rubber according to the present invention is characterized by comprising carbon black and/or silica. In addition, at least one selected from the group consisting of a plasticizer, a processing aid, a softening agent and a dispersant (hereinafter referred to as rubber chemicals) may be added to them.
The viscosity-stabilized natural rubber composition of the present invention is prepared by blending natural rubber with the viscosity stabilizer described above.
In the process for producing the viscosity-stabilized natural rubber composition of the present invention, the viscosity stabilizer described above is added to natural rubber latex in producing a natural rubber master batch to produce the viscosity-stabilized natural rubber composition.
In the method of controlling a rise in the viscosity of natural rubber according to the present invention, the viscosity stabilizer described above is added to natural rubber latex in producing a natural rubber master batch to control a rise in the viscosity of natural rubber. The viscosity stabilizer is added preferably in an amount of 5 to 80 parts by weight per 100 parts by weight of natural rubber.
The reason why the viscosity stabilizer for natural rubber according to the present invention has a viscosity-stabilizing property is presumed as follows. That is, it is considered that carbon black and silica used for natural rubber reacts with an aldehyde group contained in an isoprene chain to block the aldehyde group, thereby preventing a gelation reaction with protein and the like contained in natural rubber to control a rise in the viscosity due to gelation, and that as a result, natural rubber is provided with a viscosity-stabilizing property.
Since the viscosity-stabilized natural rubber composition of the present invention is blended with carbon black and/or silica which provide natural rubber with a viscosity-stabilizing property, gelation of natural rubber is prevented, and a rise in the viscosity of natural rubber is controlled over a long period of time.
The viscosity-stabilized natural rubber composition of the present invention exhibits an excellent viscosity-stabilizing effect over an extended period of time, and therefore treatment such as mastication is not required in using. This can improve the production efficiency to a large extent at a kneading stage after mastication and can reduce the addition of carbon black and silica as reinforcing fillers. Accordingly, the kneading efficiency is elevated, and the processability can be improved as well to a large extent. Further, bale deformation (cold flow) is not caused. In addition, materials which have so far been used in the rubber industry are blended as a viscosity stabilizer, and therefore no any influences are exerted on the physical properties of rubber after vulcanization.
In the process for producing the viscosity-stabilized natural rubber composition of the present invention, carbon black and/or silica having the viscosity-stabilizing property described above are merely added to natural rubber latex in producing a natural rubber master batch, whereby the viscosity-stabilized natural rubber composition preventing the gelation of natural rubber and controlling a rise in the viscosity
Bridgestone Corporation
Mulcahy Peter D.
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