Method of controlling rise in viscosity of natural rubber

Abstract: Provided are a viscosity stabilizer comprising carbon black and/or silica and further blended with at least one selected from the group consisting of a plasticizer, a processing aid, a softening agent and a dispersant, a viscosity-stabilized natural rubber composition blended with the viscosity stabilizer described above in an amount of 5 to 80 parts by weight per 100 parts by weight of natural rubber, a process for producing a viscosity-stabilized natural rubber composition, which comprises adding the viscosity stabilizer described above to natural rubber in producing a natural rubber master batch, and a method of controlling a rise in the viscosity of natural rubber, which comprises adding the viscosity stabilizer described above to natural rubber in producing a natural rubber master batch.

Description: The present invention relates to a method for controlling a rise in the viscosity over time of natural rubber. In general, natural rubber is produced in tropical countries such as Thailand, Malaysia and Indonesia. Natural rubber is used widely in the rubber industry and the tire industry because of the excellent physical properties thereof. Natural rubber has a Mooney viscosity (JIS K6300) as low 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. 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 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 has been 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 the 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) The viscosity-stabilizing effect is low under severe conditions (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 the 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 that 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. 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 for controlling a rise in the viscosity over time 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 inventor 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 for controlling a rise in the viscosity over time of natural rubber, and thus the present invention has been accomplished. That is, a viscosity stabilizer for natural rubber used 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 is prepared by blending natural rubber with the viscosity stabilizer described above. In the process for producing the viscosity-stabilized natural rubber composition, 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 for controlling a rise in the viscosity over time of natural rubber according to the present invention, the viscosity stabilizer described above comprising carbon black and/or silica is added to natural rubber to produce a natural rubber master batch to control a rise in the viscosity of natural rubber, wherein the viscosity stabilizer is added in an amount of 40 to 80 parts by weight per 100 parts by weight of natural rubber. When the viscosity stabilizer is carbon black, it may be added 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 used 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 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 exhibits an excellent viscosity-stabilizing effect over an extended period of time, and therefore treatment such as mastication is not required in use. 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 increased, 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 influences are exerted on the physical properties of rubber after vulcanization. In the process for producing the viscosity-stabilized natural rubber composition, 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 over a long period of time can be produced. Accordingly, no specific modifications are required for conventional natural rubber production steps. Further, in the method of controlling a rise in the viscosity of natural rubber according to the present invention, carbon black and/or silica having the viscosity-stabilizing property described above are merely added to natural rubber in producing a natural rubber master batch, whereby the gelation of natural rubber is prevented, and therefore a rise in the viscosity of natural rubber can be controlled. In addition, the method described above is safe in terms of handling. The embodiment of the present invention shall be explained in detail. The viscosity stabilizer used in the present invention comprises carbon black and/or silica, and at least one selected from the group consisting of a plasticizer, a processing aid, a softening agent and a dispersant may further be added thereto. As explained in the above, these carbon black and silica are known materials and used as reinforcing fillers for rubber, and the production processes for the same are known as well. However, it has not so far been known at all that they exhibit a viscosity-stabilizing effect on natural rubber, and such new properties have been found by the present inventor. They can be handled safely and are available at low cost. As apparent from the examples described later, when they are used for natural rubber, a viscosity-stabilizing effect is displayed from the time of addition and maintained over an extended period of time. A range of applying carbon black and silica as the viscosity stabilizers can distinctly be distinguished from a range of using them as conventional reinforcing fillers. That is, carbon black and silica used as conventional reinforcing fillers are added at a step of preparing compounded rubber before vulcanizing and molding in producing rubber products, while carbon black used as a viscosity stabilizer is added when natural rubber latex is processed to produce a master batch of natural rubber used as a raw material of rubber products. Accordingly, an applying step and an applying timing in both uses can clearly be distinguished, and the particularly marked effect described above is displayed in use as the viscosity-stabilizing agent. The viscosity-stabilized natural rubber composition is produced by adding carbon black and/or silica and the rubber chemicals described previously to natural rubber latex and/or dried natural rubber, for example TSR (natural rubber obtained after drying by hot air) and RSS (natural rubber obtained after drying by smoking) and kneading them by means of a mixer or an extruder in producing a natural rubber master batch, or at a step of manufacturing the natural rubber master batch produced in the process of tapping-natural rubber latex-coagulation-cleaning (washing by water)-dehydration-drying-packing, and can be molded into sheet-shaped and bale-shaped articles. The kneading conditions are varied depending on the kind and grade of natural rubber used (produced), and the kneading is carried out preferably for 45 seconds or more, preferably 60 seconds or more to come to a discharging temperature of 130°C or higher. Further, in the method of controlling a rise in the viscosity of natural rubber according to the present invention, carbon black and/or silica and the rubber chemicals described previously are added to natural rubber in producing the natural rubber master batch described above, whereby a rise in the viscosity of natural rubber is controlled. When the above ingredients are added to natural rubber after drying, they have to be added to natural rubber subjected to drying treatment as early as possible since gelation (storage hardening) continues even when left standing at room temperature. The viscosity-stabilized natural rubber composition controls a rise in the viscosity caused by the gelation of the polymer of natural rubber by compounding carbon black and/or silica, thereby producing a viscosity-stabilizing effect. This viscosity-stabilized natural rubber composition has a higher viscosity than that of natural rubber itself because of compounding of carbon black and/or silica, but a subsequent rise in the viscosity is limited, that is a rise in the viscosity attributable to the gelation of the polymer is not found and thus the viscosity-stabilizing effect is exerted. A viscosity-stabilized natural rubber composition obtained by adding a conventional viscosity stabilizer has large bale deformation (cold flow) because of a low viscosity level and therefore handling is difficult. However, in the viscosity-stabilized natural rubber composition used in the present invention, the viscosity is maintained, as described above, at a high level as compared with those obtained by adding conventional viscosity stabilizers, and therefore bale deformation (cold flow) is not found, which in turn increases the workability. Carbon black usually used in the rubber industry can be used in the present invention. Carbon black of various grades such as SAF and HAF can be used alone or in combination in the present invention. As for silica, anhydrous silicic acid fine powder usually used as a white reinforcing filler for rubber is used. As the rubber chemicals, stearic acid and zinc oxide are used. In the present invention, carbon black or silica can be used alone, and in addition thereto there can be used carbon black in combination with silica, carbon black in combination with the rubber chemicals, silica in combination with the rubber chemicals, and carbon black and silica in combination with the rubber chemicals. The viscosity stabilizer is added in total in an amount of 40 to 80 parts by weight, preferably 40 to 70 parts by weight per 100 parts by weight of natural rubber, although when the viscosity stabilizer is carbon black it can be used in an amount of 5 to 80 parts by weight. An amount of this viscosity stabilizer of less than 5 parts by weight makes it impossible to provide a viscosity-stabilizing effect, and an amount exceeding 80 parts by weight deteriorates the properties of rubber as well as the workability. That is, the sheet becomes ragged and breaks, so that a continuous operation of producing a natural rubber master batch undersirably stops each time. The amounts of carbon black and silica added later as reinforcing fillers for rubber can be reduced or set at zero in case that they are used in the above-mentioned range of 40 to 70 parts by weight. Thus, the workability of an after-kneading step is improved, and the properties of the rubber obtained after the addition are not deteriorated. The addition amount described above is varied to some extent in the range described above depending on the kind and the grade of natural rubber used (produced) and the kind of the viscosity stabilizer used. Next, specific and detailed explanations shall be given with reference to examples and comparative examples, but the present invention shall not be restricted to these examples. The respective natural rubber compositions prepared in the examples and the comparative examples were measured for a change in Mooney viscosity with the passage of time to evaluate a viscosity-stabilizing effect. Further, the properties of the rubbers, the handling property and the productivity were evaluated as well. The Mooney viscosity, the properties of the rubbers, the handling property and the productivity were determined and evaluated by the following methods. (1) Mooney viscosity (JIS K6300) In order to check whether the viscosity is stabilized, the respective natural rubber compositions were left standing at room temperature in a hardening-accelerating condition, whereby a change in the Mooney viscosity with the passage of time was determined. (2) Property of rubber The respective natural rubber compositions were evaluated with the naked eye according to the following evaluation criteria. Evaluation criteria: o ○: good ○: quite good ×: ragged and unsuitable for transportation (3) Handling property The respective natural rubber compositions were evaluated with the naked eye according to the following evaluation criteria. Evaluation criteria: o ○: good handling property and suitable for transportation ○: quite good handling property ×: large deformation and not preferred (4) Productivity The total (from mastication to final kneading stage) productivity (time and cost required for processing) was shown by index, wherein the productivity of Comparative Example 1 was set at 100. The higher the value is, the more the productivity is improved. Examples 1 to 5 and Comparative Examples 1 to 2 Carbon black, which has a viscosity-stabilizing effect, was added in the proportions shown in the following Table 1 based on 100 parts by weight of natural rubber (PSR type), and the mixtures were kneaded by means of a Banbury mixer to prepare the respective natural rubber compositions. In Examples 1 to 3 and Comparative Example 1, the mixtures were put into the mixer at 50 to 60°C and kneaded for 60 seconds to come to a discharge temperature of 130°C, and in Examples 4 and 5 and Comparative Example 2, the mixtures were kneaded for 60 seconds to come to a discharge temperature of 160° C. The blend proportions and the evaluation results are shown in Table 1. Examples 6 to 10 and Comparative Example 3 Silica, which has a viscosity-stabilizing effect, was added in the proportions shown in the following Table 2 based on 100 parts by weight of natural rubber (PSR type), and the mixtures were kneaded by means of the Banbury mixer to prepare the respective natural rubber compositions. In Examples 6 to 8, the mixtures were put into the mixer at 50 to 60°C and kneaded for 60 seconds to come to a discharge temperature of 130°C , and in Examples 9 and 10 and Comparative Example 3, the mixtures were kneaded for 60 seconds to come to a discharge temperature of 160°C. Example 6 is outside the scope of the invention because the amount of the silica added is less than 40 parts by weight. The blend proportions and the evaluation results are shown in Table 2. Examples 11 to 12 The rubber compositions were prepared in the same manner as in Example 3, except that carbon black and silica, each of which has a viscosity-stabilizing effect, were used in combination in the proportions shown in the following Table 2, or carbon black, silica and stearic acid were used in combination. The blend proportions and the evaluation results are shown in Table 2. Examples 1 to 12 (other than Example 6) fall in the scope of the present invention, and it has been confirmed that the respective natural rubber compositions do not rise in Mooney viscosity and have a viscosity-stabilizing effect. Further, it has become clear that the properties and the handling properties of the rubbers are good and mastication can be eliminated and that the productivity can be elevated to a large extent. In contrast with this, it has been confirmed that in Comparative Example 1, the Mooney viscosity rises to a large extent as time goes on and mastication is required and that the deformation is large and therefore the handling property is deteriorated. The blend amount of carbon black is as large as 90 parts by weight in Comparative Example 2, and the blend amount of silica is as large as 90 parts by weight in Comparative Example 3. Accordingly, it has been confirmed that the rubbers show a ragged property and the handling property is deteriorated and that such amount is not preferred.

Claims: A method for controlling a rise in the viscosity over time of natural rubber, which comprises adding a viscosity stabilizer for natural rubber comprising carbon black and/or silica to natural rubber to produce a natural rubber master batch, wherein the viscosity stabilizer is added in an amount of 40 to 80 parts by weight per 100 parts by weight of natural rubber. A method for controlling a rise in the viscosity over time of natural rubber, which comprises adding a viscosity stabilizer for natural rubber comprising carbon black to natural rubber to produce a natural rubber master batch, wherein the viscosity stabilizer comprising carbon black is added in an amount of 5 to 80 parts by weight per 100 parts by weight of natural rubber. A method as claimed in claim 1 or 2, which comprises adding said viscosity stabilizer for natural rubber and at least one selected from the group consisting of a plasticizer, a processing aid, a softening agent and a dispersant to natural rubber to produce a natural rubber master batch. Use of silica and/or carbon black in a method for controlling a rise in the viscosity over time of natural rubber, wherein a viscosity stabilizer for natural rubber comprising carbon black and/or silica is added to natural rubber to produce a natural rubber master batch, wherein the viscosity stabilizer is added in an amount of 40 to 80 parts by weight per 100 parts by weight of natural rubber. Use of carbon black in a method for controlling a rise in the viscosity over time of natural rubber, wherein a viscosity stabilizer for natural rubber comprising carbon black is added to natural rubber to produce a natural rubber master batch, wherein the viscosity stabilizer comprising carbon black is added in an amount of 5 to 80 parts by weight per 100 parts by weight of natural rubber. Use of silica and/or carbon black according to claim 4 or 5, wherein said viscosity stabilizer for natural rubber and at least one selected from the group consisting of a plasticizer, a processing aid, a softening agent and a dispersant are added to natural rubber to produce a natural rubber master batch.

Inventors: MITARAI KUNINORI, MITARAI, KUNINORI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1997
Main classification: C08K3
IPC codes: C08K3, C08L7, C09K3