JP2015117321A - Natural rubber and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide natural rubber having improved Po (Wallace plasticity degree) and plasticity degree residual ratio, PRI than conventional technique for meet needs for maintaining PRI high without depending seasons and to overcome problem that PRI of natural rubber varies depending on seasons which the natural rubber is collected due to characteristics that the a raw material is a natural product, where PRI represents a ratio of plasticity after aging to before aging when the natural rubber is applied to air heating aging, i.e. higher PRI means higher aging resistance, and further to prevent change of viscosity over time.SOLUTION: Above problem is solved by natural rubber obtained by cutting a coagulum of natural rubber latex and contacting the cut product and citric acid and/or a solution thereof. Further viscosity change can be reduced by adding semicarbazide and a salt thereof and/or a solution thereof.

Description

  The present invention relates to natural rubber having improved Po (Wallace plasticity), plasticity retention index (PRI), and reduced viscosity change rate, and a method for producing the same.

  Natural rubber is widely used in large quantities in the rubber industry because of its excellent physical properties and low cost. Natural rubber is made from rubber sap (natural rubber latex) collected by tapping at a rubber plantation. As a production method thereof, a method is generally employed in which an acid such as formic acid is added to a latex to coagulate, and after dehydration, drying and smoking, measurement and molding are performed. Other than this, unsmoked sheets before soot smoke, rubber called cup lamps that have been naturally solidified by collecting the latex that has flowed out in the cup, rubber and wet rubber called tree races that have been solidified in a string in the cut groove of the rubber tree Slabs), semi-finished products such as RSS scraps, etc. are pulverized, washed, dried, weighed, and molded.

  However, the natural rubber produced by these conventional methods has a problem that the PRI varies depending on the season in which it is collected, because the raw material is a natural product. PRI represents the ratio of plasticity after aging to natural aging when natural rubber is aged by air heating. That is, the higher the PRI, the higher the aging resistance. Therefore, it is required to suppress the fluctuation of PRI depending on the season and keep the PRI high.

  In the prior art, the decrease in PRI is considered to be one of the causes that the auto-oxidation reaction of the polymer is catalyzed by the metal ions present in the natural rubber latex. Therefore, chelate such as ethylenediaminetetraacetic acid (EDTA) is used. Means for adding an agent have been proposed (see, for example, Patent Documents 1 to 3, Non-Patent Documents 1 and 2). However, in the conventional technique using a chelating agent such as a general sodium salt of EDTA, metal soap is generated at the time of washing, and bubbles must be generated and eliminated. In addition, there is a problem that treatment is necessary because safety is a concern and there is no biodegradability.

JP 2013-185095 A UK patent application 48702/65 (SEKHAR, B.C .; Improvements in and relating to the preparation of lower grade natural rubbers and skim rubbers) UK patent application 14393/66 (SMITH, J.F. and SEKHAR, B.C., Lower grade and skim rubbers)

SIVABALASUNDERAM, j. AND NADARAJAH, M; Factors affecting the plasticity retention index of natural rubber; Q. Jl. Rubb. Res. Inst. Ceylon., 42, 13 (1966) A.A.WATSON; Improve aging of natural rubber by chemical treatment; J. Rubber.Res. Inst.Malaya., 22 (1), 104-119 (1969)

  An object of the present invention is to provide a natural rubber having improved Po (Wallace plasticity) and plasticity residual ratio (PRI) and reduced viscosity change rate as compared with the prior art, and a method for producing the same.

As a result of intensive research, the inventors cut the natural rubber latex coagulum and brought it into contact with an aqueous citric acid solution, which caused an increase in Po (Wallace plasticity) and PRI (plasticity residual ratio). Further, the present inventors have found that the increase in viscosity is suppressed and that the rate of change in viscosity can be further reduced by adding a semicarbazide compound to solve the above problems, and the present invention has been completed.
That is, the present invention is as follows.
1. A natural rubber obtained by bringing a coagulated natural rubber latex into contact with citric acid and / or a solution thereof.
2. 2. The natural rubber as described in 1 above, obtained by further adding semicarbazide or a salt thereof and / or a solution thereof after contacting the coagulated product with the citric acid and / or a solution thereof.
3. A natural rubber obtained by adding semicarbazide or a salt thereof and / or a solution thereof to the natural rubber according to the above 1 or 2 having a heating loss rate of 0.8% or less.
4). 4. The natural rubber according to any one of 1 to 3, wherein the citric acid concentration in the citric acid solution is 0.1% by mass or more.
5. 5. The natural rubber according to any one of 1 to 4, wherein a contact time between the coagulated product and the aqueous citric acid solution is 20 minutes or more.
6). (1) cutting the natural rubber latex coagulum,
(2) A method for producing natural rubber, comprising at least a step of contacting the obtained cut product with citric acid and / or a solution thereof.
7). 7. The method for producing natural rubber as described in 6 above, further comprising a step of adding semicarbazide or a salt thereof and / or a solution thereof to the cut product after the step (2).
8). 8. The method for producing natural rubber as described in 6 or 7 above, wherein the citric acid concentration in the citric acid solution is 0.1% by mass or more.
9. 9. The method for producing natural rubber according to any one of 6 to 8, wherein a contact time between the cut product and the aqueous citric acid solution is 20 minutes or more.
10. The method for producing natural rubber according to any one of 6 to 9, wherein the step of bringing the obtained cut product into contact with an aqueous citric acid solution is a step using a creper.

According to the present invention, it is possible to provide a natural rubber having an increased Po (Wallace plasticity) and plasticity residual ratio (PRI) and a reduced viscosity change rate as compared with the prior art, and a method for producing the same. Further, in the present invention, it is an essential requirement that the coagulate of natural rubber latex is cut and brought into contact with citric acid and / or its solution that is contained in a large amount in citrus fruits and also present in the living body. There is no problem with safety.
According to a preferred embodiment of the present invention, semicarbazide or a salt thereof and / or a solution thereof is added after drying the cleaved product after contact with the citric acid and / or the solution thereof, or the cleaved product. The odor during the production of natural rubber is reduced, and the effect of achieving a constant viscosity can be achieved.

Hereinafter, the present invention will be described in more detail.
As described above, the natural rubber of the present invention is obtained by cutting and chopping a coagulated product of natural rubber latex and bringing the cut product into contact with an aqueous citric acid solution.

The natural rubber latex coagulated product used in the present invention is not particularly limited. For example, an unsmoked sheet (latex coagulated with acid, dried by sheeting), smoked sheet (unsmoked sheet smoked). ), A cup lamp (a product in which latex is naturally coagulated in a cup), a slab, or a mixture thereof.
These coagulated materials are preferably cut roughly once or twice before being subjected to the following cutting step, and then contacted with an aqueous sodium pyrosulfite solution for the purpose of preserving. By this operation, an increase in Po (Wallace plasticity), PRI and an increase in viscosity are suppressed. In this embodiment, the concentration of sodium pyrosulfite in the aqueous solution is, for example, 0.01 to 30% by mass, and preferably 0.05 to 10% by mass. Moreover, as a contact method of both, there exist the method of spraying the aqueous solution of sodium pyrosulfite, the method of immersing, etc. with respect to the cut coagulum. The immersion time is preferably 10 minutes or longer. In this state, it is generally stored in an outdoor warehouse with a roof called a cup lamp pool for about 3 days to 1 month.

The natural rubber latex coagulated product is subsequently cut, washed and sheeted repeatedly, and finally becomes a small particle size cut product (crumb). The particle size of the cut product is preferably 10 mm or less, and preferably 2 to 5 mm from the viewpoint of easy hot air drying.
According to a preferred embodiment of the present invention, it is preferable to sprinkle an aqueous citric acid solution when forming a sheet by a creper treatment. This process facilitates sheeting and removes metal ions that promote the deterioration of natural rubber, increasing Po (Wallace plasticity) and plasticity residual index (PRI), and suppressing an increase in viscosity. . In addition, there is a method of immersing the cut product in a citric acid aqueous solution in the cutting and washing steps. The particle size of the cut product in contact with the citric acid aqueous solution is, for example, 200 mm or less, and is preferably 100 mm to 1 mm from the viewpoint of improving the effect of the present invention. The particle size of the cut product can be adjusted by combining a plurality of known slab cutters, rotary cutters, sheeting by a creper process, and cutting by a shredder process.

  In view of improving the effect of the present invention, the citric acid solution is preferably an aqueous solution, and the citric acid concentration in the aqueous citric acid solution is, for example, 0.01% by mass or more, and 0.1 to 30% by mass. More preferably, the contact time between the cut product and the citric acid aqueous solution is, for example, 20 minutes or more, and more preferably 20 minutes to 60 minutes.

  The cut product that has been contacted with the aqueous citric acid solution is preferably subjected to a drying step through the steps of sheeting by creper treatment, washing, and cutting by a shredder treatment.

The drying temperature in the drying step is, for example, in the range of 75 ° C to 140 ° C. When the drying temperature is less than 75 ° C., the water used in the water washing hardly evaporates. On the other hand, if it exceeds 140 ° C., the physical properties of the natural rubber may be deteriorated, which is not preferable.
The drying time is, for example, 100 to 400 minutes.

  According to a preferred embodiment of the present invention, semicarbazide or a salt thereof and / or a solution thereof is added to the cleavage product after contact with an aqueous citric acid solution. According to this preferred embodiment of the present invention, the odor during the production of natural rubber is reduced, and the increase in viscosity can be suppressed. Examples of the salt of semicarbazide include bicarbonate and hydrochloride. Examples of the addition method include a method of kneading the dried product of the cut product and semicarbazide or a salt thereof with a kneader such as an extruder. The addition amount of semicarbazide or a salt thereof is, for example, 0.01 parts by mass or more, preferably 0.05 to 3.0 parts by mass with respect to 100 parts by mass of natural rubber. There is also a method of drying after sprinkling on the crumb before drying. The natural rubber preferably has a heat loss rate of 0.8% or less.

The natural rubber of the present invention thus obtained has improved Po (Wallace plasticity) and plasticity residual index (PRI) over the prior art. Moreover, in the form in which semicarbazide or a salt thereof is added, odor reduction and constant viscosity are achieved.
Although natural rubber exhibits a low viscosity immediately after its production, a phenomenon called so-called storage hardening, in which the viscosity increases (gels) with time, occurs. When the elapsed time is about 1 to 2 months due to storage or transportation, the Mooney viscosity increases by 10 points or more, and the workability deteriorates. According to the study of the present inventor, it has been found that the storage and curing of natural rubber is caused by a cross-linking reaction involving an aldehyde group in an isoprene chain generated by an oxidation reaction during the drying process.
When a general nucleophile such as an amine compound is reacted to trap the aldehyde, there is a return in the equilibrium reaction, and the aldehyde cannot be completely captured. For this reason, crosslinking due to aldehyde cannot be suppressed, resulting in an increase in viscosity. On the other hand, when semicarbazide or a salt compound thereof is used, there is no return as described above, and the aldehyde can be completely captured, so that an increase in viscosity due to aldehyde crosslinking can be suppressed.
On the other hand, malodorous components evaporating from natural rubber are aldehydes and fatty acids, and aldehydes and fatty acids are trapped by semicarbazide or a salt compound thereof by the above mechanism, and odor reduction is achieved.
Therefore, according to the present invention, it is possible to solve the problems of storage curing, achieve a constant viscosity over a long period of time, and also provide a rubber composition containing natural rubber having a significantly reduced odor and a constant viscosity of the natural rubber. An odor suppression method can be provided.

  The natural rubber of the present invention includes fillers such as carbon black and silica, vulcanization or crosslinking agents, vulcanization or crosslinking accelerators, various oils, anti-aging agents, plasticizers, vulcanization aids, processing aids, etc. And it can use suitably for the raw material of various rubber products, for example, the rubber composition for tires.

  EXAMPLES Hereinafter, although an Example and a comparative example further demonstrate this invention, this invention is not restrict | limited to the following example.

Examples 1-4 and Comparative Examples 1-6
The natural rubber latex collected by tapping was naturally coagulated in a cup to prepare a cup lamp (CL). Subsequently, the CL was cut, washed with water, subjected to a creper treatment to form a sheet, and further cut by a shredder treatment to obtain a cut product having a particle size of about 2 cm.
Next, this cut product was immersed in various acids described in Table 1 for a predetermined time and formed into a sheet using a creper. The sheet was dried with a dryer at 105 ° C. for 3 hours, and Wallace plasticity (Po) and residual plasticity (PRI) were measured. The results are also shown in Table 1.
The Wallace plasticity (Po) was measured at 100 ° C. using a high-speed plasticity meter manufactured by Wallace. The sample size is a disk shape with a diameter of 10mm and a thickness of 3.2-3.6mm.
Residual plasticity index (PRI) was measured by heating the disk-shaped sample at 140 ° C for 30 minutes, allowing it to cool for 30 minutes, measuring Wallace plasticity (P ₃₀ ), and expressing the percentage of residue from Po as a percentage. expressed. _{(PRI = (P 30) /} (Po) × 100)

  From the results of Table 1, for Comparative Example 1 that was not treated with acid, or for Comparative Example 2 that was treated with water instead of acid treatment, Comparative Examples 3 and 5 that were treated with formic acid, and further treated with oxalic acid In Comparative Examples 4 and 6, a certain degree of PRI improvement effect was confirmed, but each Example treated with citric acid showed a higher PRI improvement effect. For example, when each example acid-treated at a concentration of 0.5 mass% is compared, Example 1 treated with citric acid shows the highest PRI improvement effect. This tendency is the same even when 1.0% by volume of acid is used. In addition, when Examples 2 to 4 were compared, Example 2 with the longest immersion time showed the highest PRI improvement effect. In addition, referring to the result of Po, it can be seen that the citric acid immersion is higher than the other acids.

Examples 5-9 and Comparative Examples 7-11
In the above example, the immersion treatment of various acids is not performed on the cut product having a particle diameter of about 2 cm. Instead, when the cut product is formed into a sheet by the creper process, the dispersion treatment of various acids is performed on the cut product in Table 2. The above example was repeated except that it was performed under the conditions indicated. The results are also shown in Table 2. In addition, the frequency | count in Table 2 is the number of repetitions of a creper process, and the dispersion | spreading process of various acids was performed only in the first time of a creper process. The application amount of various acids is about 0.5 liters in total.

  From the results of Table 2, it was confirmed that Comparative Example 8 and 10 treated with formic acid and Comparative Examples 9 and 11 treated with oxalic acid had some PRI improvement effect compared to Comparative Example 7 not treated with acid. However, each of the examples treated with citric acid showed a higher PRI improvement effect. For example, when each example acid-treated at a concentration of 0.5% by mass is compared, Example 5 treated with citric acid shows the highest PRI improvement effect. This tendency is the same even when 1.0% by mass of acid is used. When Examples 6 to 9 were compared, Example 9 with the highest citric acid concentration showed the highest PRI improvement effect. In addition, referring to the result of Po, it can be seen that the citric acid immersion is higher than the other acids.

Examples 10-17 and Comparative Examples 12-19
In Example 1, the cut material was not immersed in the first stage sodium pyrosulfite for 30 minutes, and the aqueous citric acid solution was sprayed for the first time in the second stage, then formed into a sheet by passing through a creper 10 times, and dried. As a third step, after drying, 0.15 phr of propionic acid hydrazide 50% aqueous solution or semicarbazide hydrochloride (25% aqueous solution 0.3 phr or powder 0.075 phr) was added to the dry rubber (in some cases, phr is not added). It means a part by mass with respect to 100 parts by mass of the cut product). The addition was performed by passing the dried product and semicarbazide hydrochloride through a roll 25 times. Wallace plasticity (Po) and residual plasticity (PRI) were measured in the same manner as in Example 1. As a result, Po was equivalent or improved by sodium pyrosulfite immersion and / or spraying with an aqueous citric acid solution. PRI decreases with the addition of propionic acid hydrazide, but slightly improves with semicarbazide hydrochloride.
Next, 3 g of the natural rubber obtained as described above was sealed in a polyvinyl chloride bag (4 × 6 inches), heated in an oven at 125 ° C. for 1 hour, and then allowed to cool for 30 minutes. The detector tube of the odor measuring device was put into a plastic bag and the odor was measured. The result was the average value of the obtained natural rubber after 0 day and 21 days later. As a result, when the odor of the natural rubber of Comparative Example 12 was set to 100, the odor of the natural rubber obtained in the example was lower when the treatment was performed (lower means lower odor).
In addition, the Mooney viscosity (MV) after 0 days and 21 days of the natural rubber obtained in each example was measured. Mooney viscosity is a measured value of the viscosity of natural rubber at 100 ° C. according to JIS K6300. As a result, the increased value of Mooney viscosity of natural rubber (the value obtained by subtracting the value of Mooney viscosity after 21 days from the Mooney viscosity after 21 days) was lower in the example in which citric acid was washed.
The results are shown in Table 3 below.

Claims (10)

  A natural rubber obtained by bringing a coagulated natural rubber latex into contact with citric acid and / or a solution thereof.   The natural rubber according to claim 1, obtained by further adding semicarbazide or a salt thereof and / or a solution thereof after contact of the coagulated product with the citric acid and / or a solution thereof.   A natural rubber obtained by adding semicarbazide or a salt thereof and / or a solution thereof to the natural rubber according to claim 1 or 2 having a heating loss rate of 0.8% or less.   The natural rubber according to any one of claims 1 to 3, wherein the citric acid concentration in the citric acid aqueous solution is 0.1 mass% or more.   The natural rubber according to any one of claims 1 to 4, wherein a contact time between the coagulated product and the aqueous citric acid solution is 20 minutes or more. (1) cutting the natural rubber latex coagulum,
(2) A method for producing natural rubber, comprising at least a step of contacting the obtained cut product with citric acid and / or a solution thereof.   The method for producing natural rubber according to claim 6, further comprising a step of adding semicarbazide or a salt thereof and / or a solution thereof to the cut product after the step (2).   The method for producing natural rubber according to claim 6 or 7, wherein the citric acid concentration in the citric acid aqueous solution is 0.1 mass% or more.   The method for producing natural rubber according to any one of claims 6 to 8, wherein a contact time between the cut product and the aqueous citric acid solution is 20 minutes or more.   The method for producing natural rubber according to any one of claims 6 to 9, wherein the step of bringing the obtained cut product into contact with an aqueous citric acid solution is a step using a creper.