DESCRIPTION
COAGULATION METHOD OF NATURAL RUBBER LATEX
Field of the Invention
The present invention relates to the coagulation method of natural rubber latex.
Background of the Invention
Heretofore, coagulation of natural rubber latex has been carried out by a batch coagulation process- Fresh natural rubber latex is placed in a coagulation bath and diluted with equal amounts of water. Dilute formic acid is added into the diluted natural rubber latex to adjust the pH to 4.5-5.5. After 9 hr on standing the crumbs are separated on the surface of the serum. By application of stirring the separation of crumbs occurs after 3-4 hr. In both cases, the separated crumbs mat together to form a thick spongelike sheet. The sheet is pulled up from the bath and subjected to dehydrate, cut into small pieces and washing with water. Although the coagulation process is a batch type, coagulation bathes are placed in parallel to make the subsequent processes like a continuous production of solid natural rubber. However, the ordinary coagulation method involves following problems to be solved: (1) The rate of coagulation is slow. (2) Batch coagulation process is inevitable due to slow coagulation. (3) The coagulated crumbs form a thick sheet, which costs a great deal of electric power and labor to dehydrate, cut into small pieces and washing with water. (4) Removal of the impurities by washing is difficult due to a thick mat-like form of the coagulated crumbs , whichmakes solidnatural rubber containing a large quantity of impurities, (5) Cutting into small pieces and drying of a thick mat-like form of the coagulated crumbs require a large amount of energy and spent a lot of time.
(6) The cost of equipments is expensive, (7) The waste water including formic acid and residual rubber particles cause water pollution.
Disclosure of the Invention
An object of the present invention is to provide a new method to solve the problems of coagulation of natural rubber latex, which makes it possible to coagulate natural rubber latex quickly and continuously with low cost of operation and equipment.
Another object of the present invention is to provide a novel continuous coagulation method of natural rubber latex and process thereof to solve these problems.
Other objects and advantages of the present invention will be apparent from the following description.
The present invention attains the objects and shows advantages mentioned above by adding (1) an acid and (2) (i) at least one salt selected from the group consisting of calcium salts or ammonium salts of nitric acid, sulfuricacid, carbonic acid, phosphoric acid, hydrochloric acid or formic acid, and/or (ii) a polymer flocculant to a natural rubber latex with stirring to form porous slurry crumbs.
Brief description of the drawings Figure 1
Figure 1 is a graph showing the coagulation preferable for low solid rubber content in a coagulation tank.
Figure 2
Figure 2 is a graph showing the coagulation preferable for high solid rubber content in a coagulation tank.
Best mode for carrying out the Invention
The coagulation of SBR {styrene-butadiene rubber) by
emulsion polymerization is generally carried out by using a continuous and instantaneous coagulation method by the addition of sodium chloride and sulfuric acid into the SBR latex. The resulting coagulum is the form of porous crumbs slurry, which facilitate the subsequent washing and drying processes. On the other hand, natural rubber latex is difficult to apply simply this procedure, because natural rubber latex is composed of high molecular weight natural rubber, the colloid membrane, which is different from synthetic rubber latex, and contains sticky non-^rubber components . As a result of extensive studies, the present inventors have found a new coagulation method and process of natural rubber latex, in which natural rubber latex is coagulated quickly and preferably facilitates to employ a continuous coagulation operation. Consequently, the new coagulation system solves the problems of the ordinary coagulation process and greater increase in the productivity. Namely, the coagulation system of the present invention provides a quick coagulation process for natural rubber latex and forms the coagulum of porous crumbs slurry, which facilitates drying of the crumbs and separation of the clear serum fraction.
Fresh natural rubber latex or concentrated ammoniated latex is utilizable as natural rubber latex.
The coagulation system in the present invention is composed of (1) acid and (2) specified salt and/or polymer flocculant.
Any of organic acids and inorganic acids are utilized as examples of the acids. For example, formic acid and acetic acid are favorable as to the organic acids and sulfuric acid, hydrochloric acid and carbonic acid are favorable as to the inorganic acids. One or a combination of two or more acids is able to use as these acids.
Examples of the salts are one of the salts selected from calcium or ammonium salt of nitric acid, sulfuric aci d,
carbonic acid, phosphoric acid, hydrochloric acid and formic acid. One or a combination of two or more salts is able to use as these salts. Sodium chloride, which is generally used for the coagulation of SBR, is not preferable because of low coagulation rate.
Examples of the polymer f locculant are any of the anionic flocculants, nonionic flocculants , and cationic f locculants . Examples of the anionic flocculants, are sodium alginate, sodium carboxymethyl cellulose, sodium polyacrylate, acrylamide-sodium aerylate copolymer, and partially hydrolyzed polyacrylamide.
Examples of the cationic polymer flocculant are water-soluble aniline-formaldehyde resin, polyethylene imine, polydiallyldimethylammonium chloride, chitosan, hexamethylenediamine-epichlorohydrin condensation product, polyvinylimidazoline, polydialkylaminomethaacrylate, and Mannich reaction product of polyacrylamide. Examples of the nonionic polymer flocculant are starch, guar gum, gelatin, polyacrylamide, and polyethylene oxide.
In these coagulation systems , the pH value of coagulation is adjusted to 4.0-6.5, preferably 4.5-5.5, by controlling the amount of acid to add. The concentration of the salt and polymer flocculant is adjusted to 0.5-3.0% (w/v), more preferably 1,0-2.0% (w/v), against the total mixture. The concentration of polymer flocculant is adjusted to 0.001-1.0 weight parts, more preferably 0.01-0.75 weight parts, against the 100 parts of rubber weight in the natural rubber latex, respectively.
The coagulation is carried out by mixing a specified salt and/or polymer flocculant with natural rubber latex beforehand, followed by the addition of an acid, or by adding the specified salt and/or polymer flocculant into natural rubber latex that is mixed with the acid beforehand . Of course , it is possible to add both of the acid and specified salt
and/or polymer flocculant into natural rubber latex at the same time In these coagulation processes natural rubber latex is coagulated quickly and the resulting coagulum is porous crumbs slurry, which is suitable for washing and drying.
In general, a system involving phase transition process such as coagulation is liable to face many problems for the application of the laboratory scale result to the development of a continuous operation method suitable for industrial production. A typical example of the continuous coagulation can be seen in the production of emulsion polymerized SBR. Direct application of the coagulation system of SBR to the coagulation of natural rubber latex encounters with serious difficulty due to the sticky character of natural rubber crumbs to facilitate the formation of large crumbs in the coagulation tank, which is different from the fine dispersed crumbs slurry in the case of emulsion polymerized SBR.
As a result of extensive studies, the present inventors have found a new continuous coagulation process suitable for the coagulation of natural rubber latex based on entirely new idea. The new process facilitates to provide porous crumbs slurry, which has high surface area and can be easily dehydrated, washed and dried, and send to the subsequent processes. Accordingly, it is possible to do continuous operation for the coagulation and finishing processes of natural rubber latex.
Fundamentally, the continuous coagulation process is composed of the first equipment to mix uniformly the chemicals for coagulation and the second equipment to accomplish the coagulation. The first equipment aims to mix the chemicals with natural rubber latex uniformly followed by creaming and coagulation of the latex. Although an agitator is in popular use for mixing, it is also available to apply a line mixer or static mixer. The second equipment is a vessel of enough volume to hold a period of residence time and is equipped
with an Agitator or a device for forcing liquid flow to agitate liquid in the vessel for the accomplishment of coagulation. However, it is not always necessary to use two steps. It is possible to carry out the creaming, flocculation and coagulation in an equipment such as coagulation tank. One of the important points of continuous coagulation process is the coagulation operation not to form crumbs in a form of large coagulum and discharge of the resulting crumbs of desirable form from the coagulation tank. The present invention solved these problems by establishing suitable conditions for coagulation and the structure of the coagulation tank.
Figure 1 shows an example of coagulation operation for low solid rubber concentration in a coagulation tank (crumb slurry concentration) such as lower than 20% (w/v). Natural rubber latex, acid and polymer f locculant are fed to a creaming vessel with an agitator to get a cream fraction. The cream fraction is subjected to the coagulation tank to form porous crumbs slurry according to the progress of coagulation. The resulting crumbs are overflowed from the coagulation tank and supplied to the dehydrating equipment. The resulting coagulum is porous , which facilitates dehydration and washing . In Figure 1, an example is shown to mix the latex with acid in a creaming tank f ollowedby themixing of polymer f locculant. This process, however, is an example of the procedure, which does not specify the order of adding the acid and polymer flocculant.
Figure 2 shows an example of coagulation operation for high solid rubber concentration in a coagulation tank (crumb slurry concentration) such as higher than 10% (w/v) . A mixer is applied in this case. Natural rubber latex, acid and polymer f locculant are mixed uniformly in the mixer and the coagulation proceeds at the same time. The resulting coagulum with a form of fine crumbs or porous blocks of appropriate size is supplied
to the dehydration equipment. The form of the coagulum can be adjusted according to the structure of the mixer and operation conditions• In Figure 2, an examples is shown to mix the latex with acid followed by the mixing of polymer flocculant. This process, however, is an example of the procedure, which does not specify the order of adding the acid and polymer flocculant.
In these continuous coagulation systems, it is possible to reuse the water from the serum fraction after separation of the crumbs and that used for washing of crumbs for control of the latex concentration and to make aqueous solution of the acid and polymer flocculant.
Although there is no special limit for the dry rubber content (DRC) of natural rubber latex, DRC can be adjusted preferably to 1-60 %, morepref erably to 5-40 %. The temperature for the coagulation is preferable to be at room temperature to 80°C in general, although there is no special limitation.
Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention shall not be limited by these Examples in any way.
The present invention provides a coagulation method, which facilitates to coagulate natural rubber latex quickly, form the coagulum of simple processing, and reduce the content of impurities in the resulting natural rubber.
Examples
The natural rubber latex and chemicals used for the experiments are as follows: Natural rubber latex
The natural rubber latex was prepared by the addition of aqueous ammonia in fresh natural rubber latex (FL-latex) (about 30 % DRC) to adjust pH range to 10-11.

Aqueous solution (0.025% (w/v)) of a cationic polymer flocculant composed of dimethylalminoethylacrylate-acrylamide copolymer.

Aqueous solution (0.025% (w/v)) of an anionic polymer flocculant composed of sodium acrylate-acrylamide copolymer.

Aqueous solution of formic acid (5% w/v).
The phr value means parts per 100 parts of rubber.
Examples 1-6 and Comparative Examples 1-3
Examples of acid/salt coagulation are shown. A salt given in Table 1 was added into FL-latex (DRC 10% (w/v)) to make 1 % (w/v) aqueous solution and mixed well. An aqueous acid solution 2 mL, adjusted to 5% (w/v) , was added dropwise into 50 mL of the FL-latex containing the salt with stirring. The coagulation time was remarkably shortened and the resulting coagulum was porous crumbs slurry with high surface area, which facilitated washing with water, as shown in Table 1. As comparative examples, the coagulation system of formic acid/sodium chloride and sulfuric acid/sodium chloride was shown. Both systems coagulation was very slow.

Examples 7-10
Examples of acid/polymer flocculant coagulation are shown.
Into FL-latex (DRC 10% (w/v) ) was added 0.01 phr polymer flocculant given in Table 2 and mixed well. Into 20 mL of this FL-latex was added 0.2 mL of an aqueous acid solution, adjusted to 1% (w/v) for sulfuric acid and 5% (w/v) for formic acid, dropwise with stirring. Polymer flocculants were found also to be effective for rapid coagulation and the resulting coagulum was porous crumbs with high surface area, which facilitated washing with water, as shown in Table 2.

Examples 11-14
Examples of acid/salt and acid/polymer flocculant coagulation are shown.
Into 50 mL of FL-latex (DRC 10% (w/v)) an acid given in Table 3 was added and adjusted the pH to 4.5-5.5. Into this FL-latex was added 10 mL of aqueous solution of polymer flocculant or salt (5% (w/v)) given in Table 3 successively with stirring. The necessary time for coagulation and the state of the serum phase is shown in Table 3. The resulting coagulum was porous crumbs with high surface area, which facilitated washing with water. No difference was observed for the order of addition for salt and polymer flocculant on the coagulation process.

Examples 15-23 and Examples 4-9
Examples are given to confirm the effect of inorganic salt.
The necessary time for coagulation was determined by adding dropwise with stirring the following (A) into (B). The degree of transparency was observed for the serum fraction after finishing the coagulation.
(A) Latex: 5 mL (Add 5 g inorganic salt given in Table 4 into
100 mL of FL-latex (30% (w/v) DRC) and adjusting to the
prescribed DRC)
(B) Formic acid: 20 mL (5% (w/v))
The resulting coagulum was porous crumbs with high surface area as shown in Table 4.

Examples 24-29
Examples are given to confirm the effect of polymer flocculant.
The necessary time for coagulation was determined by adding dropwise the following (A) into (B) with stirring. The degree of transparency was observed for the serum fraction
after finishing the coagulation.
(A) Latex: 5 mL (Adding 0.025phr polymer flocculant given
in Table 5 into FL-latex (30% (w/v) DRC) and adjusting to
the prescribed DRC)
(B) Formic acid: 20 mL (5% (w/v))
The resulting coagulum shown in Examples 24 to 29 was porous crumbs with high surface area as shown in Table 5.

Examples 30-35
Examples are given to confirm the effect of stirring. Into 100 phr of FL-latex (30% (w/v) DRC) was added 50 phr of 0.025% (w/v) aqueous solution of Floerger and adjusted the prescribed DRC given in Table 6. The latex was coagulated by the addition of formic acid to give the pH value of serum phase to 4.5-5.5 with or without stirring. The effect of stirring was confirmed in these experiments. The result was shown in Table 6.

Examples 36-41
Examples are given to confirm the effect of stirring. Into 100 phr of FL-latex (DRC of 30% (w/v)) was added 5% (w/v) formic acid. The amount of formic acid was adjusted to give the pH value of serum phase to 4.5-5.5. Water was added into the latex to get prescribed DRC. Into the latex was added 50 phr of 0.025% (w/v) aqueous solution of Floerger with or without stirring. The effect of stirring was confirmed also in these experiments. The result was shown in Table 7.

Example 42-47
Examples are given to confirm the effect of stirring.
An aqueous solution including Floerger and formic acid was prepared beforehand. The aqueous solution was mixed with FL-latex (30% (W/v) DRC) with or without stirring in a ratio to make prescribed concentration of the resulting crumbs. Here, the concentration of aqueous solution of Floerger and formic acid was dependent on the concentration of crumbs. As to Floerger, the amounts of aqueous solution of Floerger and FL-latex were adjusted to keep the ratio of FL-latex (30% (w/v) DRC) and the amount of Floerger (0,025% (w/v) aqueous solution) to 1:0.5. As to formic acid, the amounts of formic acid and FL-latex were adjusted to give the pH value of 4.5-5.5 for the resulting serum phase. Effectiveness of stirring was also confirmed in these experiments. Here, the concentration of crumbs indicates the dry rubber content in a mixture of the aqueous solution and FL-latex (30% (w/v) DRC) . The result was shown in Table 8.

CLAIMS
1 A coagulation method of natural rubber latex, which comprises adding (1) an acid and (2) (i) at least one salt selected from the group consisting of calcium salts or ammonium salts of nitric acid, sulfuric acid, carbonic acid, phosphoric acid, hydrochloric acid or formic acid, and/or (ii) a polymer flocculant, to a natural rubber latex with stirring to form porous slurry crumbs.
2. The coagulation method of claim 1, in which (1) an acid
and (2) (i) the salt mentioned above and/or (ii) a polymer
f locculant are added into the continuous flow of natural rubber
latex with stirring to form continuously porous slurry crumbs
in the state of floating.
3. The coagulation method of claim 1 or 2, in which the
stirring is carried out by a stirrer or compulsory fluid of
continuous flow.