WO2005023874A1

WO2005023874A1 - Process for coagulating and concentrating epoxidised natural rubber latex

Info

Publication number: WO2005023874A1
Application number: PCT/SG2004/000001
Authority: WO
Inventors: Balachandra Chakkingal Sekhar
Original assignee: B.C. Sekhar Sdn Bhd; Nair, Sushil, Sukumaran
Priority date: 2003-09-11
Filing date: 2004-01-06
Publication date: 2005-03-17

Abstract

The present invention discloses a process of treating epoxidised latices with ionic salts to reduce the phase separation temperature of the non-ionic detergent used in epoxidation of natural rubber latex and thence effecting coagulation of the latex with acid at a reduced temperature including room temperature. Preferably, the ionic salt is ammonium sulphate at a concentration range of about 0.2 to 1% based on the water phase. The treated latex may then be coagulated by the addition of formic acid at temperatures ranging from 28 to 40 °C. Following treatment with salt, the epoxidised latex may be treated with ammonia and subjected to centrifugation or creaming to obtain epoxidised latex concentrate having dry rubber content (DRC) in excess of 50%.

Description

PROCESS FOR COAGULATING AND CONCENTRATING EPOXIDISED NATURAL RUBBER LATEX

Field of Invention

This invention relates to a treatment of epoxidised natural rubber latex. In particular, it relates to effecting coagulation of the epoxidised natural rubber latex for the purpose of concentrating said latex.

Background of Invention

In the epoxidation process natural rubber latex is first stabilised using non-ionic detergents which are known in the industry as LUBROL™ L , vTJLCASTAB™ LW, ETHOMOMEEN™ etc. These compounds are ethylene oxide condensates of long chain alcohols. The non-ionic detergents enable the addition of high concentrations of acid to the natural rubber latex without flocculation or coagulation of the latex.

It is well known that the epoxidation of natural rubber latex is carried out by reaction of formic acid and hydrogen peroxide in si tu in the natural rubber latex. The method is described in many publications, in particular, the British Patent No. GB-2,113,692 taken out by the Malaysian Rubber Research and Development Board now known as the Malaysian Rubber Board (MRB) . Hydrogen peroxide is converted into performic acid, which then interacts with the rubber in the latex, thus forming epoxidic structure at the double bonds . The extent of epoxidation is determined by the proportions of formic acid and hydrogen peroxide introduced into the non-ionic detergent stabilised natural rubber latex.

In the normal process, natural rubber latex is coagulated using formic acid. Even if preservation is done with ammonia, formic acid can effectively coagulate the preserved rubber latex. However, epoxidised latex cannot be coagulated either by acid or base as it is too stable to be reacted upon by the acid or base.

It is therefore necessary in practice, as described in said British Patent No. GB-2,113,692 to coagulate epoxidised latex using high temperature in the vicinity of 100°C or more. When epoxidised latex is thus coagulated it first forms a powdery material, which finally agglomerates, with assistance, into a coagulum. This is a cumbersome and expensive process for recovering epoxidised rubber.

Epoxidised latex can neither be centrifuged nor creamed because of the densities involved. Ultra-filtration using expensive membranes has to be resorted to, to achieve adequate concentration of epoxidised latex. Even then, because of the stability of epoxidised latex against acids, manufacturing operations to make dipped goods is rendered difficult . Objects and Summary of Invention

Our' present invention overcomes both difficulties described above, i.e. the disadvantage of having to coagulate epoxidised latex at high temperatures, and the problem of concentrating epoxidised latex by creaming or centrifugation. Our invention enables the epoxidised latex to be treated so that it may coagulate at room temperature and may be concentrated by centrifugation or creaming.

In accordance with the objects, the present invention discloses a process of treating epoxidised latices with ionic salts to reduce the phase separation temperature of the non- ionic detergent used in epoxidation of natural rubber latex and thence effecting coagulation of the latex with acid at a reduced temperature including room temperature .

Preferably, the ionic salt is ammonium sulphate at a concentration range of about 0.2 to 1% based on the water phase. The treated latex may then be coagulated by the addition of formic acid at temperatures ranging from 28 to 40°C.

The salts may also be chosen as follows : cations chosen from any one of : - ammonium and any other metallic ions; and anions chosen from any one of : - sulphate, chloride, nitrate, phosphate and the like which are selected in accordance with their respective ionic activity and concentration to achieve the desired phase separation temperature and thence coagulation temperature. In another preferred embodiment, ammonium sulphate used is in concentration ranges from about 0.2 to 1% based on total epoxidised latex.

Following treatment with salt, the epoxidised latex may be treated with ammonia and subjected to centrifugation or creaming to obtain epoxidised latex concentrate having dry rubber content (DRC) in excess of 50%.

In yet another preferred embodiment, the salt used is ammonium sulphate at 0.2 to 2% concentration on the latex phase . Other salts may be used in increased proportions or concentrations .

With the treatment according to the present invention, the coagulation of epoxidised natural rubber latex may be effected at reduced or room temperature and the concentration of epoxidised latex over 50% DRC may be achieved.

Detailed Description of the Invention

The underlying principle of the process of our invention is essentially based on the Gibbs phase rule. According to this principle, the non-ionic detergent can be brought out of phase at different temperatures using suitable salts. While the detergent has a phase separation temperature in excess of 100°C, by suitable addition of selected salts, the phase separation temperature could be lowered to room temperature. This principle can be applied to any non-ionic detergent- stabilised latex. Epoxidised latex, therefore, when admixed with suitable concentration of metallic salts or ammonium salts, the non- ionic detergent in the latex and the surface of the rubber particles comes out of phase and this then renders the latex rubber particles to behave as normal natural rubber latex particles. Accordingly, the treated latex particles may then be subjected to coagulation under acid conditions as in the case of normal natural rubber latex. The following examples illustrate this innovation :-

Example 1

(i) 500 grams of epoxidised latex is mixed with 10ml of a 10% solution of ammonium sulphate. This is then mixed with 2 - 3 ml of 10% formic acid. The mixture is stirred and allowed to stand, (ii) A second lot of 500ml of epoxidised latex is mixed with 2ml of 10% formic acid and stirred and kept standing. (iii) Further 500ml of epoxidised latex at PH7 is admixed with 2ml of 10% formic acid and the mixture is allowed to stand. After 10 minutes the following results are obtained: - (a) The latex is totally coagulated and the aqueous phase is clear. (b) The latex remains fluid with no signs of instability. (c) The latex is flocculated and shows agglomeration. Example 2

The same procedure is repeated as in Example 1 except that, in place of ammonium sulphate, zinc sulphate is used.

The following results are obtained: - Epoxidised latex containing the added zinc sulphate is also observed to coagulate fully on the addition of formic acid.

We have found that other salts, including formates, can replace ammonium sulphate and zinc sulphate. The efficacy of phase separation and thus the coagulation of the epoxidised latex vary according to the cation and anion in the salt. The choice of the cation and anion and the concentration of the salt determine the phase separation temperature and thus the coagulation temperature.

Different detergents have differing phase separation temperatures and this can be easily determined. The concentration of salt required can also be ascertained by mixing the detergent with the salt and establishing the temperature at which the phase separation takes place, depending on the detergent .

In respect of centrifugation or creaming of epoxidised latex there is need for not only effecting phase separation of the detergent used but also to establish a density difference between the epoxidised rubber particles and the aqueous phase. This can again be achieved by the addition at the appropriate concentration of a metal salt followed by a clustering agent e.g. creaming agent. The following example illustrates the effectiveness of the innovation.

Example 3

(i) Epoxidised latex is treated with ammonium sulphate at a concentration of 2% based on the total epoxidised latex at pH 7. (ii) A small amount of ammonia is added at a concentration of 0.3% on total latex, (iii) Then ammonium alginate 0.i2% on total latex is added and left standing for 2 to 24 hours . (iv) One portion of the latex is subjected to centrifugation and another portion is allowed to cream, (v) A control epoxidised latex at pH 7 is also divided into 2 portions; one is subjected to centrifugation and the other to creaming by ammonium alginate.

The results are shown in Table I below:

Table 1

The epoxidised latex used in the examples is commercially available 50% epoxidised latices. We have also established that all levels of epoxidised latex can also be in similar fashion coagulated at room or reduced temperature as well as centrifuged or creamed.

Claims

CLAIMSWhat we claim is:

1. A process of treating epoxidised latices with ionic salts to reduce the phase separation temperature of the non- ionic detergent used in epoxidation of natural rubber latex and thence effecting coagulation of the latex with acid at a reduced temperature including room temperature .

2. A process according to Claim 1 in which the epoxidised latex is treated with ammonium sulphate at a concentration range of about 0.2 to 1% based on the water phase and then coagulated by the addition of formic acid at temperatures ranging from 28 to 40°C.

3. A process according to any one of claims 1 and 2 wherein the salts added comprise: cations chosen from any one of: - ammonium and any other metallic ions; and anions chosen from any one of : - sulphate, chloride, nitrate, formate and phosphate; and which are selected in accordance with their respective ionic activity and concentration to achieve the desired phase separation temperature and thence coagulation temperature.

4. A process according to claim 3 wherein the salt is ammonium sulphate and the concentration ranges from about 0.2 to 1% based on total epoxidised latex.

5. A process for treating epoxidised latex with the salt followed by ammonia and a clustering agent, including ammonium alginate and tamarind seeds, and subjected to centrifugation or creaming to obtain epoxidised latex concentrate having dry rubber content (DRC) in excess of 50%.

6. A process according to claim 5 in which the salt used is ammonium sulphate at 0.2 to 2% concentration on the latex phase.

7. A process according to any one of the preceding claims wherein the coagulation of epoxidised natural rubber latex is effected at reduced or room temperature .

8. A process according to any one of claims 5 and 6 wherein the concentration of epoxidised latex achieved is over 50% DRC.

9. An epoxidised latex concentrate obtained by a process according to any one of claims 1 to 8.

10. A product made from epoxidised latex concentrate according to claim 9.