WO2005023875A1

WO2005023875A1 - Improvements in or relating to the preparation of latex concentrate by centrifugation, creaming or evaporation

Info

Publication number: WO2005023875A1
Application number: PCT/SG2004/000031
Authority: WO
Inventors: Krishnan Namboodiripad; Balachandra Chakkingla Sekhar
Original assignee: B.C. Sekhar Sdn Bhd; Rk Latex (P) Ltd; Nair, Sushil, Sukumaran
Priority date: 2003-09-11
Filing date: 2004-02-03
Publication date: 2005-03-17
Also published as: MY148315A

Abstract

A method of reducing thixotropy of ammoniated latex concentrates, by the addition of proteolytic enzymes under alkaline conditions to the latex concentrate. The method is effective for both high and low ammonia latex concentrates. The reduction of viscosity includes the reduction of the Brookfield viscosity of the latex concentrate. The proteolytic enzyme used is pepsin, including papain. Preferably, the concentration of proteolytic enzyme used is in the range of from about 0.005 to 0.20% based on dry rubber content (DRC). Potassium hydroxide (KOH) may preferably be used to effectively remedy any adverse effects of the enzyme on the latex concentrate's properties, including chemical stability time (CST), mechanical stability time (MST), volatile fatty acid (VFA) number, and filterability.

Description

Improvements in or relating to the preparation of latex concentrate by centrifugation, creaming or evaporation

Technical Field

This invention relates to improvements in or relating to preservation of rubber latex in the form of a concentrate, With stable Brookfield viscosity, a property related to thixotropy. More particularly, it concerns preserving the latex concentrate without the attendant problems of thixotropy, thereby achieving viscosity stability, of the preserved latex concentrate.

Background Art

Fresh latex collected from Hevea trees is neutral but volatile fatty acids (VFA) due to rapid bacterial action causes the lowering of pH and putrefaction. The organic acids formed by the bacterial degradation also cause the latex to autocoagulate. Hence, fresh latex is usually preserved with ammonia at about 0.1 to 0.3% based on DRC. Consumer demands for low ammonia content results in lower levels of ammonia concentration being used in the field with a variety of other preservatives before it is fed into a bulking tank for settling and removal of sludge.

For manufacturing purposes, such as for dipped latex articles, the Hevea latex is usually concentrated by centrifugation, creaming or evaporation. The objective of such a process of concentration is to produce concentrated latices wherein the dry rubber content (DRC) exceeds 50% and more commonly 60%. Normal Hevea field latex has a DRC ranging from about 28% to 35%.

The concentrated latex is required for producing or manufacturing latex dipped rubber articles such as balloons, gloves, catheters, bladders, latex thread, latex foam, etc. The time between the production of latex concentrates and that of the dipped articles may vary from that of a few weeks to several months . The concentrate may be stored in tanks or drums at ambient temperatures ranging from about 5°C to over 45°C.

The stability and quality of the latex concentrate and the processability properties of the latex during manufacture are determined by properties such as volatile fatty acid (VFA) , ammonia content, viscosity (as indicated by Brookfield viscosity) , mechanical stability time (MST) , chemical stability time (CST) , filterability, etc.

The volatile fatty acid (VFA) content of a latex is indicated by the Volatile Fatty Acid number (VFA no.) which should not exceed the range of from about 0.1 to 0.2 and generally should remain below 0.05 if proper hygiene is observed.

The mechanical stability of a latex is indicated by the time taken for visible aggregation of latex particles under highspeed agitation with a specified rotor and container. The time for the first sign of flocculation to appear is noted as the Mechanical Stability Time (MST) . The mechanical stability time (MST) normally rises from 100s in a freshly prepared concentrate to over 700s in 4 - 5 weeks. Usually, the MST increase tapers off to reach a constant value in 3 - 6 weeks' time. The mechanical stability can also be artificially increased by the addition of suitable soaps such as oleates, laureates and stearates .

Chemical stability of a latex refers to the ability of the latex to withstand the destabilizing effects of additives such as vulcanizing ingredients and more particularly zinc oxide. The test to determine the chemical stability is the Zinc Stability Time (ZST) test which involves adding definite amounts of potassium oleate and zinc oxide to the latex and allowing the mixture to stand at a certain temperature and then performing a mechanical stability test on the sample. The mechanical stability time of the matured mixture is then taken as a measure of the chemical stability time (CST) of the latex.

The viscosity of a latex is taken to be indicative of the thixotropy of the latex. Thixotropy refers to the properties of gel-like liquids or colloids such as latex which liquefy when subjected to vibratory forces such as agitation, ultrasonic waves or simple shaking and then solidify again when left standing. The viscosity of the latex is commercially determined by rotational viscometry whereby the test liquid is sheared between two coaxial cylinders as is done using the Brookfield viscometer.

The aforesaid VFA, viscosity, CST, MST and other properties are also dependent on the type of planting material or tree variety from which the latex is tapped, seasonal variations in temperatures in the tree-growing areas, soil conditions, and other unknown factors. Adequate scientific and technological information is now available to ensure very low VFA, adequate MST and CST, and retention of ammonia concentration which acts as both a preservative and bactericide.

Objects of the Invention

It is well-known in all rubber-growing areas that the viscosity of the concentrated latex, as indicated by the Brookfield viscosity, is subject to wide seasonal variations. This may move from levels of below 60 to higher than 110 cps.

No proven method has yet been scientifically established to counter this change of Brookfield viscosity which may be described as indicative of the thixotropy of latex.

Thixotropy of latex may be of detriment to the manufacture of dipped latex articles. Both low levels and high levels are detrimental. VFA and MST of latex can be controlled by collection and production hygiene with adequate ammonia preservation in the field and factory.

There is yet no known method of adjusting Brookfield viscosity levels of latex concentrate. When creaming agents such as ammonium alginate are present in the latex concentrate, the Brookfield viscosity may even exceed 200 cps . Staterαent of Disclosure

We have now discovered that the addition of enzymes that break down protein levels under alkaline conditions could significantly influence the level of Brookfield viscosity.

In the general embodiment of the invention, the method of reducing thixotropy of ammoniated latex concentrate comprises the addition of proteolytic enzymes under alkaline conditions to said latex concentrate, wherein the ammoniated latex concentrate includes both high and low ammonia latex concentrates .

In a specific embodiment, the reduction of the viscosity includes the reduction of the Brookfield viscosity of the latex concentrate .

In a preferred embodiment, the enzyme used is pepsin, preferably vegetable pepsin, including papain, as derived from papaya { Carica papaya L.) and botanically similar plants.

In another preferred embodiment, the concentration of proteolytic enzyme used is in the range of from about 0.005 to 0.20% based on dry rubber content (DRC) and may be in suitable commercially-available grades and forms.

In another aspect of the invention, the method includes improving or adjusting the seasonal variation in Brookfield viscosity by treatment of the latex concentrates with proteolytic enzyme under high pH conditions. Alkali hydroxides, preferably, potassium hydroxide (KOH) is used in small amounts to effectively remedy any adverse effects of the enzyme on the latex concentrate's properties, including chemical stability time (CST) , mechanical stability time (MST) , volatile fatty acid (VFA) number, and filterability.

Detailed Description of Specific Embodiments

According to our present invention therefore is provided a method of altering the Brookfield viscosity levels of latex concentrate by the addition of about 0.002 to 0.200% based on DRC of a proteolytic enzyme, i.e. pepsin, including vegetable pepsin (or papain which is derived from papaya. Papain may be isolated from the latex of the green fruit and leaves of Carica papaya L.

Pepsin is commercially available as Puerzym™ (from Firma) while papain is available under the commercial name of Velardon™ (from Organon) . This enzyme is variously used in the meat industry to breakdown protein and tenderizes the meat. It has also been used for lowering the nitrogen levels of latex rubbers for solid rubber applications. It is not known, however, to be used for lowering the levels of Brookfield viscosity, in other words remedying the problem of thixotropy.

Our invention however is primarily and solely to influence the property of thixotropy and, as the enzymes are used in low concentrations, they do not significantly influence the nitrogen level of the rubbers in latex concentrate. The manner in which the invention may be carried out is illustrated in the following examples in which the level of papain or pepsin added to the latex concentrate is based on the dry rubber content .

Example I

Fresh latex concentrate prepared in the usual manner and ammoniated to 0.7% and having a pH of 10.6 is divided into 3 lots. While the first is used as control, the second and third are treated at 0.007 and 0.013 of papain based on DRC. These 3 samples are then tested for Brookfield viscosity. The Table I shows that Brookfield viscosity of these 3 samples from day 1 to day 33.

Table I

Example 2

In Example 2 the exercise is repeated using pepsin in the similar fashion and the Brookfield viscosity results are shown in Table 2A.

Table A

Any adverse influence on mechanical stability of the concentrate may be adjusted by the addition of soaps or detergents usually employed in the industry. The addition of a small amount of KOH, from about 0.2 to 0.5% based on DRC, would effectively remedy any adverse influence on chemical stability, which is exemplified in Table 2B below.

Table 2B

The addition of KOH is to essentially neutralize the formation of any volatile fatty acids (VFA) through the hydrolysis of protein by the additions of the enzyme. Example 3

One sample comprises centrifuged latex that has been treated with tamarind seed powder (TSP) and papain at 0.2% on DRC. After 2 days, KOH (0.2 - 0.5% based on DRC) was added to cease the activity of papain. Two further samples of one liter each of TSP-treated centrifugal latex was taken and treated with papain at 0.05% and 0.1% on DRC. Control (0.00%) is included as well.

Table 3

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Claims

CLAIMSWhat we claim is:

1. A method of reducing thixotropy of ammoniated latex concentrate, by the addition of proteolytic enzymes under alkaline conditions to said latex concentrate.

2. A method according to Claim 1 wherein the ammoniated latex concentrate includes both high and low ammonia latex concentrates .

3. A method according to Claim 1 wherein the reduction of the viscosity includes the reduction of the Brookfield viscosity of the latex concentrate.

4. A method according to Claim 1 where the enzyme used is pepsin.

5. A method according to Claim 4 wherein the pepsin is vegetable pepsin, including papain, as derived from papaya { Carϊca papaya L.) and botanically similar plants.

6. A method according to any one of Claims 4 and 5 wherein the concentration of proteolytic enzyme used is in the range of from about 0.005 to 0.20% based on dry rubber content (DRC) .

7. A method according to any of the preceding claims wherein the proteolytic enzyme used is in any one or combination of the following: grades : technical, industrial, purified; and/or forms: powder, aqueous solution.

8. . A method of improving or adjusting the seasonal variation in Brookfield viscosity by treatment of the latex concentrates with proteolytic enzyme under high pH conditions .

9. A method according to Claim 8 wherein alkali hydroxides, including potassium hydroxide (KOH) , is used in small amounts to effectively remedy any adverse effects of the enzyme on the latex concentrate's properties, including chemical stability time (CST) , mechanical stability time (MST) , volatile fatty acid (VFA) number, and filterability.

10. Latex concentrate centrifuged, creamed or evaporated that is treated by the methods claimed in any one of the preceding claims .

11. The use of proteolytic enzymes, including any one of pepsin and papain, in reducing thixotropy of latex concentrates, including reducing the Brookfield viscosity of said latex concentrates .

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