AU734999B2 - Discoloration inhibitor of deproteinized natural rubber latex, deproteinized natural rubber latex, and method for preserving the same - Google Patents

Description

-1- P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: DISCOLORATION INHIBITOR OF DEPROTEINIZED NATURAL RUBBER LATEX, DEPROTEINIZED NATURAL RUBBER LATEX, AND METHOD FOR PRESERVING THE SAME The following statement is a full description of this invention, including the best method of performing it known to us: GH REF: P23053-E:SDS:RK I'l lA- TITLE OF THE INVENTION DISCOLORATION INHIBITOR OF DEPROTEINIZED NATURAL RUBBER LATEX, DEPROTEINIZED NATURAL RUBBER LATEX, AND METHOD FOR PRESERVING THE SAME BACKGROUND OF THE INVENTION The present invention relates to a discoloration inhibitor of a deproteinized natural rubber latex, the deproteinized natural rubber latex using the same, and a method for preserving the deproteinized natural rubber latex.

A natural rubber has widely been used in industrial rubber products such as automobile tire, belt, adhesive, etc.

and domestic rubber products such as glove, etc., heretofore.

The natural rubber is obtained as a latex comprising a non-rubber content such as protein, lipid, ash content, 15 etc., in addition to a rubber content. Therefore, the above non-rubber contents are contained as impurities in a natural V rubber product obtained by solidifying the rubber content from the above latex and subjecting the raw rubber (crepe rubber or smoked sheet rubber) thus obtained to the steps of mastication, mixing of additional components, molding and vulcanization.

It has recently been reported that the allergic symptom develops by use of the natural rubber product, and i. assumed that the cause thereof is a protein in the natural rubber. Therefore, a deproteinized natural rubber latex

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i, ii ~I A q,, 2 wherein the protein is removed as possible has been developed (EP-A-584,597).

On the other hand, the non-rubber contents such as protein, liphid, ash content, etc. have the effect of preventing aging of the rubber caused by heat, oxidation, ozone, etc. in the natural rubber latex. Therefore, the above deproteinized natural rubber latex is inferior in aging resistance and it becomes necessary to add an antioxidant.

As the antioxidant for natural rubber latex, various antioxidants such as phenolic antioxidant, amine antioxidant, organic phosphite antioxidant, imidazole antioxidant, etc. are known. The present inventors have found that, among the above antioxidants, the phenolic antioxidant is used most preferably because of excellent emulsion or dispersion stability in the 15 latex.

However, there arises a new problem by blending the phenolic antioxidant. That is, the natural rubber product of the deproteinized natural rubber latex has an advantage of excellent transparency, but has a problem that there arises a phenomenon of changing the whole white latex to pink when preserving for a long period of time after the addition of the phenolic antioxidant (so-called "pinking").

It is considered that this discoloration is different from that caused with the progress of a normal aging because the discoloration does not arise in a latex of an 3 isoprene rubber (IR) having almost the same molecular structure as that of the natural rubber, and the formation of a chelate compound by a magnesium ion (Mg 2 in the natural rubber latex and a phenol in the antioxidant is assumed as one of causes thereof.

The natural rubber product obtained by using a white deproteinized natural rubber is transparent, while a pinkcolored natural rubber product is obtained from a latex wherein pinking has arisen. Accordingly, the transparency as a feature of the deproteinized natural rubber product is deteriorated. Thus, it would be advantageous to prevent the deproteinized natural rubber latex from causing discoloration with preventing from causing aging.

SUMMARY OF THE INVENTION

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The present inventors have studied and found that, when blending at least one selected from the group consisting of a salt of a water-soluble dithiocarbamic acid derivative (hereinafter referred to as "a water-soluble dithiocarbamate"), a salt of a pyrosulfurous acid (hereinafter referred to as "pyrosulfite"), a cycloalkanethiol and a halogenated thiophenol is blended in the deproteinized natural rubber latex, the discoloration and aging of the deproteinized natural rubber latex may be reduced.

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4 In one aspect, the present invention provides a method for preserving a deproteinized natural rubber latex, which comprises preserving the deproteinized natural rubber latex after blending a phenolic antioxidant and at least one discoloration inhibitor selected from the group consisting of a salt of a water-soluble dithiocarbamic acid derivative, a salt of a pyrosulfurous acid, a cycloalkanethiol and a halogenated thiophenol.

According to the above preserving method, not only the aging of the deproteinized natural rubber caused by the action of the phenolic antioxidant may be reduced but also the discoloration caused by the phenolic antioxidant may be reduced by the action of the above discoloration inhibitor.

The discoloration inhibitor reduces the discoloration of the deproteinized natural rubber latex caused by the phenolic antioxidant, by adding to the deproteinized natural rubber latex, or adding to the natural rubber latex before removing the protein therefrom.

9 In another aspect, the present invention provides a deproteinized natural rubber latex containing a phenolic antioxidant and at least one discoloration inhibitor selected from the group consisting of a salt of a water-soluble dithiocarbamic acid derivative, a salt of a pyrosulfurous acid, a cycloalkanethiol and a halogenated thiophenol, and the discoloration inhibitor being contained in an amount of 0.005 to 15 parts by weight based on 100 parts by weight of a rubber content of the latex.

<C I 5 According to the deproteinized natural rubber latex, the discoloration of the latex caused by the blending of the phenolic antioxidant may be reduced, thereby making it possible to provide a natural rubber product which is superior in transparency.

DETAILED DESCRIPTION OF THE INVENTION In the present invention, the water-soluble dithiocarbamate, pyrosulfite, cycloalkanethiol and halogenated thiophenol are used in the form of a powder or the form of an aqueous solution. It is particularly preferred to use the above compound in the form of an aqueous solution containing 25 it in an amount of about 1 to 30% by weight with the view of facilitating dissolution in the natural rubber latex.

The water-soluble dithiocarbamate is a salt of a dithiocarbamic acid derivative and an alkaline metal, and is represented by the general formula:

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R

1

S

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/N-C-SM

R

1 s (wherein R 1 and R 2 are the same or different and indicate a hydrogen atom, an alkyl group or an aryl group; and M indicates an alkaline metal).

Examples of the groups R 1 and R 2 include an alkyl group such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tbutyl, etc. and an aryl group such as phenyl, etc., in addition to a hydrogen atom. Examples of the alkaline metal represented by the group M include sodium, potassium and the like.

15 Specific examples of the salt of the water-soluble dithiocarbamate include sodium dimethyldithiocarbamate, sodium diethyldithiocarbamate, sodium dibutyldithiocarbamate, potassium diethyldithiocarbamate, sodium dibutyldithiocarbamate and potassium diethyldithiocarbamate, sodium dithiocarbamate and the like.

Examples of the pyrosulfite (diphosphonate) include a salt of pyrosulfurous acid and an alkaline metal, such as sodium pyrosulfite, potassium pyrosulfite and the like.

Examples of the cycloalkanethiol include a thiol containing a cycloalkyl group having 5 to 8 carbon atoms, such as cyclopentanethiol, cyclohexanethiol, cycloheptanethiol, S1" a I 7 cyclooctanethiol and the like.

Examples of the halogenated thiophenol include those wherein hydrogen of the benzene ring is substituted with a halogen atom such as chlorine, bromine, iodine, etc., such as 4-bromothiophenol, pentachlorothiophenol, 1,3,5triiodothiophenol and the like.

The above discoloration inhibitor is used by blending in the natural rubber latex before or after the deproteinization treatment.

The deproteinized natural rubber latex of the present invention contains the phenolic antioxidant, and further contains the above discoloration inhibitor in the amount of 0.005 to 15 parts by weight, preferably from 0.01 to 10 parts by weight, more preferably from 0.5 to 10 parts by weight, 15 particularly from 1 to 5 parts by weight, based on 100 parts by weight of the rubber content of the latex.

When the amount of the discoloration inhibitor is smaller than 0.005 parts by weight, the effect of preventing the discoloration of the latex is lowered and, therefore, it is not preferred. On the other hand, even when the above discoloration inhibitor is contained in the amount of more than 15 parts by weight, the effect of preventing the discoloration of the latex does not change. On the contrary, the discoloration inhibitor serves as the excess component after the molding processing and there is a fear of exerting a t, 8 bad influence to the quality of the rubber product.

The natural rubber latex in the present invention means a field latex obtained from a natural rubber plant, and any of a commercially available ammonia-treated latex and a fresh field latex can be used. In the present invention, the deproteinized natural rubber latex refers to those wherein the protein content is smaller than that of a normal natural rubber latex, but refers generally to those wherein the content of a protein in the raw rubber obtained from the deproteinized natural rubber latex is not more than 0.05% by weight in terms of the nitrogen content.

The deproteinized natural rubber latex in the present invention can be obtained by subjecting a natural rubber latex as a raw material to a deproteinization treatment which has hitherto been known, for example, a treatment of adding a protease in the natural rubber latex to decompose a protein, adding a surfactant to wash the latex repeatedly and removing the decomposed protein.

.The above discoloration inhibitor may be blended in the latex during the deproteinization treatment or may previously be blended in the natural rubber latex which has never been subjected to the deproteinization treatment.

:The above protease used in the deproteinization treatment of the natural rubber latex is not specifically limited and there can be used those derived from bacteria, 9 mold, yeast and the like. Among them, a protease derived from bacteria is preferably used.

As the surfactant used in the deproteinization treatment of the natural rubber latex, for example, there can be used the anionic surfactant and/or nonionic surfactant.

Examples of the anionic surfactant include salt of carboxylic acid, salt of sulfonic acid, sulfuric ester, phosphoric ester, etc. are suitably used.

As the salt of carboxylic acid anionic surfactant, for example, there can be used those having 6 to 30 carbon atoms, such as fatty acid salt, polyhydric carboxylate, salt of rhodinic acid, salt of dimer acid, salt of polymer acid, salt of tall oil fatty acid, etc. Among them, a carboxylate having 10 to 20 carbon atoms is preferably used. When the 15 number of carbon atoms is not more than 6, the dispersion and

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emulsion of the non-rubber content such as protein are insufficient. On the other hand, when the number of carbon atoms is not less than 30, it becomes difficult to be dispersed in water. Examples of the salt of sulfonic acid anionic surfactant include alkylbenzenesulfonate, alkylsulfonate, alkylnaphthalenesulfonate, naphthalenesulfonate, diphenyl ether sulfonate and the like.

Examples of the sulfuric ester anionic surfactant include salt •of alkyl sulfuric ester, salt of polyoxyalkylene alkyl sulfuric ester, salt of polyoxyalkylene alkylphenyl ether

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r 10 sulfate, salt of tristyrenated phenol sulfuric ester, salt of polyoxyalkylene distyrated phenol sulfuric ester and the like.

Examples of the phosphoric ester anionic surfactant include salt of alkyl phosphoric ester, salt of polyoxyalkylene phosphoric ester and the like.

Examples of the salt in the above anionic surfactants include a metal salt Na salt, K salt, Ca salt, Mg salt, Zn salt, etc.) and an amine salt (e.g.

triethanolamine salt, etc.).

As the nonionic surfactant, for example, there can preferably used a polyoxyalkylene ether, a polyoxyalkylene ester, a polyhydric alcohol fatty acid ester, a sugar fatty acid ester, an alkyl polyglycide and the like.

Examples of the polyoxyalkylene ether nonionic 15 surfactant include polyoxyalkylene alkyl ether, polyoxyalkylene alkyl phenyl ether, polyoxyalkylene polyol alkyl ether, polyoxyalkylene styrenated phenol ether, polyocyalkylene distyrenated phenol ether, polyoxyalkylene tristyrenated phenol ether and the like. Examples of the polyoxyalkylene ester nonionic surfactant include a polyoxyalkylene fatty acid ester and the like. Examples of the polyhydric alcohol fatty acid ester nonionic surfactant :include an aliphatic acid ester of a polyhydric alcohol having 2 to 12 carbon atoms or a fatty acid ester of a polyoxyalkylene polyhydric alcohol. Examples of the sugar 11 fatty acid ester include fatty acid esters of sucrose, glucose, maltose, fructose, polysaccharides and the like. It is also possible to use polyalkylene oxide adducts thereof.

Examples of the alkyl polyglucoside nonionic surfactant include alkyl glucoside, alkyl polyglucoside, polyoxyalkylene alkyl glucoside, polyoxyalkylene-alkyl polyglucoside and the like. Also, there are fatty acid esters thereof. It is also possible to use polyalkylene oxide adducts thereof.

Examples of the alkyl group in these surfactants include those having 4 to 30 carbon atoms. Examples of the polyoxyalkylene group include those having 2 to 4 carbon atoms, such as those wherein the number of mols of ethylene oxide added is from about 1 to 50 mols. Examples of the fatty acid include straight-chain or branched saturated or 15 unsaturated fatty acid having 4 to 30 carbon atoms.

In order to decompose the protein in the natural rubber latex using the protease, the protease may be added in the amount of about 0.001 to 10% by weight based on the field latex or ammonia-treated latex.

The time of treating with the enzyme is not specifically limited, but the treatment is preferably conducted for about several minutes to one week. The latex o. may be stirred or allowed to stand. If necessary, the 0* temperature may be adjusted. The temperature which is suitable for the treatment with the enzyme is from 5 to 90 0

C,

12 12 preferably from 20 to 60°C. When the temperature of the treatment exceeds 90°C, the enzyme is rapidly deactivated. On the other hand, when it is smaller than 5°C, the reaction of the enzyme hardly proceeds.

As the method of washing the latex with the surfactant, a method of adding the surfactant to the latex which has already been subjected to an enzyme treatment, followed by centrifugation is suitably used. In that case, the surfactant is suitably added in the amount within the range from 0.001 to 10% by weight based on the latex. The centrifugation may be conducted once or several times.

Furthermore, in place of the centrifugation, a washing method of agglomerating latex particles, followed by separating can also be used.

15 In the above description, after enzymolysis, the A surfactant was added and the latex was washed, but the treatment may be conducted after adding the enzyme and surfactant at the same time.

The phenolic antioxidant formulated in the deproteinized natural rubber latex of the present invention is not specifically limited, and examples thereof include various antioxidants which have hitherto been known, such as 2,6-di(tbutyl)-4-methylphenol, 4,4'-thiobis(6-t-butyl-3-phenol), di(t-butyl)hydroquinone, styrene-phenol condensate and the like.

13 The amount of the phenolic antioxidant blended is not specifically limited, and is set within the range from 0.01 to 10 parts by weight, preferably from 0.05 to 5 parts by weight, more preferably from 0.1 to 3 parts by weight, based on 100 parts by weight of the rubber content of the deproteinized natural rubber latex. When the amount of the antioxidant is smaller than the above range, it is likely to become insufficient to prevent the rubber from aging. On the other hand, when the amount of the antioxidant blended exceeds the above range, it is likely to exert a bad influence on the quality of the resulting rubber product. When the amount is too large, pinking is likely to arise even if the above discoloration inhibitor of the present invention is used.

The method of blending the phenolic antioxidant with 15 the latex is not specifically limited, but a method of adding S: after converting into an o/w type emulsion or suspension is suitable. The surfactant used for converting the phenolic antioxidant into the o/w type emulsion or suspension is not specifically limited, but the above anionic surfactant and nonionic surfactant may normally be used.

The phenolic antioxidant may be blended in the deproteinized natural rubber latex, or may also be blended in the natural rubber latex which is not subjected to the .deproteinization treatment, previously.

In the deproteinized natural rubber latex of the 14 present invention, the amount of the rubber content is suitably within the range from 58 to 62% by weight, preferably from 59 to 61% by weight.

The method of preserving the deproteinized natural rubber latex of the present invention is characterized by preserving after blending the above discoloration inhibitor and phenolic antioxidant in the deproteinized natural rubber latex, and the amount of the discoloration inhibitor blended as well as kind and amount of the phenolic antioxidant blended are the same as those described above.

The conditions in case of preserving the deproteinized natural rubber latex are not specifically limited, but it is preferred to preserve at the temperature within the range from 20 to 30 "C after light screening and 15 shielding with the view of certainly preventing the aging and discoloration of the latex.

EXAMPLES

Reference Example (Production of deproteinized natural rubber latex) "Alkalase 2.0 M" manufactured by Novo Nordisk Bioindustry Ltd. was used as a protease, and "Soctex" (rubber content: 60.2%) manufactured by Socfin company (Malaysia) were used as a natural rubber latex.

•The above natural rubber latex (15 ml) was diluted with distilled water (200 ml) and then stabilized with 0.12% 15 sodium napthenate. Then, sodium dihydrogenphosphate was added to this diluted natural rubber latex to adjust the pH to 9.2.

After the above "Alkalase 2.0 M" (0.78 g) was dispersed in distilled water (10 ml), the dispersion was added to the above diluted natural rubber latex. After the pH was adjusted again to 9.2, the-latex was allowed to stand at 37°C for 24 hours.

To the latex which has already been subjected to an enzyme treatment, a nonionic surfactant "Triton X-l" (manufactured by Toho Kagaku Kogyo Co., Ltd) was added in a concentration of followed by centrifugation at 11,000 rpm for 30 minutes. The creamy fraction thus obtained was dispersed again in distilled water (200 ml) containing 1% Triton X-l (aforecited), followed by centrifugation again.

15 After repeating this operation three times, a creamy deproteinized natural rubber was obtained.

The nitrogen content of the deproteinized natural rubber thus obtained was not more than 0.01%. Incidentally, the 0* 6** above nitrogen content was determined by the RRIM test method

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(Rubber Research Institution of Malaysia (1973). "SMR Bulletin No. 0 Example 1 To an aqueous solution containing "Emulvin W" and "Tamol N" (both are surfactants manufactured by BASF Co., 1% by weight each), an antioxidant (2.6-di(t-butyl)-4- 16 methyiphenol, trade name "Nocrac 200"1 of Ohuchi Shinco icagalcu Co., Ltd.) was added so that the concentration thereof becomes by weight to prepare an emulsion.

On the other hand, an aqueous solution wherein the concentration of sodium diethyl.dthocarbamate (trade name of "Nocceler SDC" of Ohuchi Sinko Kagaku Co., Ltd.) to 20% by weight is adjusted (discoloration inhibitor) was prepared.

To the cream of deproteinized natural rubber obtained in the above Reference Example, water was added so that the concentration of the rubber becomes 60% by weight to form a latex. Then, an emulsion of the above antioxidant was added so that the amount of the phenolic antioxidant blended.

becomes 3 parts by weight based on 100 parts by weight of the rubber content of the above latex.

Furthermore, the above discoloration inhibitoi was added so that the amount of sodium diethyldithiocarbauate blended becomeB 0.01 parts by weight based on 100 parts by weight of the rubber content of the above latex to obtain a deproteinized natural rubber latex.

Example 2 According to the same manner as that described In Example 1 except for changing the amount of sodium diethyldithiocarbamate blended to 0.1 parts by weight based on Os .100 parts by weight of the rubber content, a deproateinized 25 natural rubber'latex was obtained.-

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17 Example 3 According to the same manner as that described in Example 1 except for changing the amount of sodium diethyldithiocarbamate blended to 1.0 parts by weight based on 100 parts by weight of the rubber content, a deproteinized natural rubber latex was obtained.

Example 4 According to the same manner as that described in Example 1 except for using an aqueous solution (concentration: 20% by weight) of sodium dibutyldithiocarbamate (trade name "Nocceler TP", manufactured by Ohuchi Shinko Kagaku Co., Ltd.) in place of the aqueous solution of sodium diethyldithiocarbamate as the discoloration inhibitor and adjusting the amount of sodium diethyldithiocarbamate blended 9 15 to 1.0 parts by weight based on 100 parts by weight of the rubber content, a deproteinized natural rubber latex was obtained.

Example According to the same manner as that described in Example 1 except for changing the amount of sodium diethyldithiocarbamate blended to 2.0 parts by weight based on 100 parts by weight of the rubber content, a deproteinized natural rubber latex was obtained.

.9 Example 6 According to the same manner as that described in i.

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I Ir 18 Example 1 except for changing the amount of sodium diethyldithiocarbamate blended to 8.0 parts by weight based on 100 parts by weight of the rubber content, a deproteinized natural rubber latex was obtained.

Example 7 According to the same manner as that described in Example 1 except for changing the amount of sodium diethyldithiocarbamate blended to.10.0 parts by weight based on 100 parts by weight of the rubber content, a deproteinized natural rubber latex was obtained.

Comparative Example To the cream of deproteinized natural rubber obtained in the above Reference Example, water was added so that the concentration of the rubber content becomes 60% by i 15 weight to form a latex. Then, an emulsion of the above antioxidant was added so that the amount of the phenolic antioxidant becomes 3 parts by weight based on 100 parts by weight of the rubber content of the above latex to obtain a deproteinized natural rubber latex.

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(Evaluation of discoloration inhibition effect) The deproteinized natural rubber latexes obtained in the above Examples 1 to 7 were allowed to stand under the atmosphere at 25 0 C, and the color of the latexes was visually observed immediately after the addition of the discoloration inhibitor, after a elapse of 10 days and 50 days after 19 addition, respectively. The results are shown in Table 1.

Incidentally, when the color of the latex is white, the resulting rubber product becomes transparent.

Table 1 Comp. Ex. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex.5 Ex. 6 Ex. 7 added amount (parts by.weight) rubber content' 100 100 100 100 100 100 100 100 phenolic antioxidant 3.0 3.0 3.0 3.0 3.0 3.0 3.0 discoloration inhibitor Nocceler SDC 0.01 0.1 1.0 2.0 8.0 10.0 Nocceler TP 1.0 color of latex.

immediately after addition white white white white white white white white days after addition pink light white white white white white white Spink days after addition pink light light white white whtie white white pink pink ii 15 *o 21 As is apparent from Table 1, regarding the latex of Comparative Example 1 wherein no discoloration inhibitor was added, the color of the latex changed to pink by standing for days.

On the other hand, in Examples 1 to 2, the degree of discoloration after 10 days and 50 days was a little. In Examples 3 to 7, the color of the latexes could be maintained to be white even after 50 days.

Example 8 According to the same manner as that described in Example 1 except for using an aqueous solution (concentration: by weight) of sodium pyrosulfite in place of the aqueous solution of sodium diethyldithiocarbamate as the discoloration inhibitor and adjusting the amount of sodium pyrosulfite blended to 2.0 parts by weight based on 100 parts by weight of the rubber content, a deproteinized natural rubber latex was obtained.

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Example 9 According to the same manner as that described in Example 1 except for using an aqueous solution (concentration: 20% by weight) of cyclohexanethiol in place of the aqueous solution of sodium diethyldithiocarbamate as the discoloration inhibitor and adjusting the amount of hexanethiol blended to parts by weight based on 100 parts by weight of the rubber content, a deproteinized natural rubber latex was obtained.

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22 Example According to the same manner as that described in Example 1 except for using an aqueous solution (concentration: by weight) of 4-bromothiophenol in place of the aqueous solution of sodium diethyldithiocarbamate as the discoloration inhibitor and adjusting the amount of 4-bromothiophenol blended to 2.0 parts by weight based on 100 parts by weight of the rubber content, a deproteinized natural rubber latex was obtained.

Example 11 According to the same manner as that described in Example 1 except for using an aqueous solution (concentration: by weight) of pentachlorothiophenol in place of the aqueous solution of sodium diethyldithiocarbamate as the 15 discoloration inhibitor and adjusting the amount of pentachlorothiophenol blended to 2.0 parts by weight based on 100 parts by weight of the rubber content, a deproteinized natural rubber latex was obtained.

With respect to the deproteinized natural rubber latexes obtained in Examples 8 to 11, the evaluation of the discoloration inhibition effect was conducted according to the same manner as that described above. The results are shown in *.oo Table 2.

0* *x 8 Ex 9 Ex 10 Ex 11 added amount,(parts by weight)' rubber content 10 100 100 100 phenolic antioxidant'' 3.0 3.0 3.0 discoloration inhibitor sodium pyrosulfite 2.0 cyclohexaflethiol 2.0 4-bromothiophenol 2.0 pentachlorothiophenol color of latex immediately after addition white white: white white days after addition white white white white days after addition white white white white 24 As is apparent from Table 2, in Examples 8 to 11, the color of the latexes could be maintained to be white even after 50 days.

In the claims which follow and in the preceding summary of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprising" is used in the sense of "including", i.e. the features specified may be associated with further features in various embodiments of the invention.

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Claims (13)

1. A method for preserving a deproteinized natural rubber latex, which comprises preserving the deproteinized natural rubber latex after blending a phenolic antioxidant and at least one discoloration inhibitor selected from the group consisting of a salt of a water-soluble dithiocarbamic acid derivative, a salt of a pyrosulfurous acid, a cycloalkanethiol and a halogenated thiophenol.

2 The method according to claim 1, wherein the salt of the water-soluble dithiocarbamic acid derivative is a salt of a dithiocarbamic acid derivative and an alkaline metal.

3. The method according to claim 1, wherein the salt of the water-soluble dithiocarbamic acid derivative is sodium diethyldithiocarbamate or sodium dibutyldithiocarbamate.

4. The method according to claim 1, wherein the salt of S: a pyrosulfurous acid is sodium pyrosulfite or potassium i* pyrosulfite.

The method according to claim 1, wherein the cycloalkanethiol is a thiol containing a cycloalkyl group having 5 to 8 carbon atoms.

6. The method according to claim 1, wherein the halogenated thiophenol is 4 -bromothiophenot, pentachlorothiophenol or 1,3,5-triiodethiophenol. 'NC -26

7. A product produced by the method of any one of claims 1 to 6.

8. A deproteinized natural rubber latex containing a phenolic antioxidant and at least one discoloration inhibitor selected from the group consisting of a salt of a water-soluble dithiocarbamic acid derivative, a salt of a pyrosulfurous acid, a cycloalkanethiol and a halogenated thiophenol, and the discoloration inhibitor being contained in an amount of 0.005 to 15 parts by weight based on 100 parts by weight of a rubber content of the latex.

9. The deproteinized natural rubber latex according to claim 8, wherein the discoloration inhibitor is contained in an amount of 1 to 5 parts by weight based on 100 parts by weight of a rubber content of the latex.

10. The deproteinized natural rubber latex according to claim 8, wherein the phenolic antioxidant is contained in an amount of 0.01 to 10 parts by weight based on 100 parts by weight of a rubber content of the latex.

11. The deproteinized natural rubber latex according to claim 8, wherein the content of a protein is not more than 9 0.05% by weight in terms of the nitrogen content. 9 LU o 27

12. A deproteinized natural rubber latex substantially as herein described with reference to any one of Examples 1 to 11.

13. A method of preserving a deproteinized natural rubber latex substantially as herein described with reference to any one of Examples 1 to 11. Dated this 7 1h day of November 2000 SUMITOMO RU13BER INDUSTRIES, LTD. By his Patent Attorney GRIFFITH HACK