GB2179355A - A method for the treatment of natural rubber field latex - Google Patents

Abstract

A method for the treatment of fresh natural rubber field latex comprises incubating the field latex with a proteolytic enzyme at a pH suitable for the enzyme. The amount of enzyme present and the incubation conditions are such that the enzyme-treated field latex, when subsequently processed into epoxidised natural rubber latex, has improved coagulation and crepeing properties. A method for the preparation of epoxidised natural rubber from fresh natural rubber field latex is also described. This comprises i) incubating the field latex with a proteolytic enzyme at a pH suitable for the enzyme, ii) epoxidising the enzyme-treated field latex to the desired mole % level of epoxidation, iii) coagulating the epoxidised natural rubber latex, and iv) crepeing, washing, crumbling and drying the epoxidised natural rubber.

Description

SPECIFICATION A method for the treatment of natural rubber field latex This invention relates to the use of natural rubber (NR) field latexforthe production of epoxidised natural rubber (ENR). In particular it relates to a method of treatment of No field latex so that ENR can be prepared from it.

Epoxidised natural rubber is a relatively newform of rubberwhich has some useful properties similarto those possessed by more specialised rubbers. For example, depending on the level of epoxidation, it has low gas permeability, good oil resistance, good wet grip, low rolling resistance and high damping. The epoxidation of natural rubber and other unsaturated polymers is well known. ENR may be prepared from centrifuged latex concentrates which is a few weeks old (hereinafter referred to as "matured latex concentrate") by epoxidation with peracetic or performic acid under controlled conditions. Afterthe epoxidation reaction, the latex is coagulated and the coagulated rubber is converted to crumbs which are then dried with through circulation of hot air.Since the epoxidation is carried out under acidic conditions, the latex is stabilised with a non-ionicsurfactant during the reaction. It iswell known that latexstabilised with a non-ionic surfactant can be coagulated by heating to a temperature close to the cloud point ofthe surfactant.

Large scale production of ENR involves the following steps: (a) epoxidation of the latex, (b) coagulation ofthe latex with steam, (c) crepeing and washing ofthe coagulum and hammermilling the crepe to form crumbs, (d) chemical treatment to improve properties, (e) drying ofthecrumbsand (f) pressing and palleting ofthe dried crumbs.

Forthe production of 50 mole % epoxidised natural rubber(ENR5O) latex from matured latex concentrate it is usual to add 25 parts per hundred parts rubber (phr) of common salt to the latex to lower its colloidal stability before coagulating the latex by passing steam directly into the latex held in containers. This is a batch-wise process. The coagulum is allowed to consolidate or mature for2 hours and then passed through thecrepeing mill or a series ofcrepeing mills. After one pass through the creper, a continuous sheet or crepe isformed.The crepe is usually passed through thecreper many times (about8times) before itis comminuted to crumbs in a creper-hammermill.These operations, i.e. step (c) of the above process, are important because besides dewatering the coagulum they also facilitate the removal from the coagulum of excess water soluble reactants and reaction by-products which, if they wereto remain, could cause adverse properties in the rubber.

The processing ofthe ENR50 coagulum into dry crumbs uses the same conventional machinery and equipment asthat used for producing crumb rubber e.g. Heveacrumb.

However, if the starting material for epoxidation is fresh NR field latex, rather than matured latex concentrate, the ENR latex (ENR50 and ENR25) obtained is very much more difficultto coagulate by heating with direct steam even with the addition of common salt. It requires a longer period of heating forcoagulation to occur and this gives rise to a lot of very fine particles; however, coagulation is even then still incomplete.

After maturation of the coagulum for a few hours or even overnight, the coagulum, on repeated passage through the creper, does not form a crepe but breaks up into small pieces and into fine particles. In factthe coagulum behaves somewhat like a paste. The fine particles can be dispersed in waterto give a milky dispersion resembling a latex. Hence it is difficult to dewaterthe coagulum and to wash excess reactants and reaction by-products off the rubber without the loss of a great deal of the rubber itself. Moreover, this paste-like coagulated rubber is difficultto dry. As a result, ENR latex prepared from fresh field latex cannotbe economically processed into dry rubber using the conventional rubber processing machinery and equipment.

A newer method for coagulating ENR latex, which is infact preferred, is the continuous coagulation method using the apparatus and method described in our UK Patent Application No. 8427736. According to this method, ENR latex is passed down a substantially vertical stainless steel column as a thin film on the inner surfaces thereof until it comes into contact with steam which has been introduced into the interior ofthe column, whereupon the latex is rapidly heated by the steam and coagulates. The resulting coagulum passes through the remainder of the column and is collected atthe exitthereof.

EN R50 latex prepared from matured latex concentrate can be coagulated in the column coagulator butthe matured coagulum breaks up into small pieces and into fine particles on the creper. Ifthe small pieces of coagulum are repeatedly passed through the creper, a crepe is formed after 5 to 10 passes through the creper; the finer particles, however, still do not form a crepe. EN R25 latex prepared from matured latex concentrate may also be coagulated by this method and the coagulum can be converted to crepe and crumbs without much difficulty.

However, in the case of ENR50 or EN R25 latex prepared from fresh field latex, the latex does not coagulate in the column coagulator. Sometimes the latex merely thickens slightly and some flocks are formed which even on maturation behave somewhat like a paste and do not form a crepe even on repeated passage through the creper.

In many rubber-producing countries, itwould be more economical to use fresh field latex instead of matu red latexconcentrate asthe starting material from which ENR is prepared. However, in view of the aforementioned problems this has so far not been possible. These problems are rather unique and it is believed that difficulties of a similar nature have not been encountered before in the processing of natural rubber latex into dray rubber.

It is to be understood that the term "field latex" as used herein includes field latex in which the bottom fraction and sludge have been removed by clarifying with a centrifugal clarifier.

There are a number of differences between fresh field latex and matured latex concentrate, such as a difference in particle size. However, it may be supposed that for present purposes the most important difference isthe presence of a much larger amount of non-rubber substances in field latex. There are a lotof non-rubber substances in natural rubber latex including the following classes of substances: inositols, carbohydrates, proteins, lipids, amino acids, other organic acids, nitrogeneous bases, thiols, nucleic acids and metalliccations and inorganic anions. It may seem obvious two try to solve the problems by removing the non-rubber substances, but it is not at all obvious as to which of these are causing the problems.

British Patent No. 1,366,934 describes a method of removing protein from natural rubberwhich comprises incubating natural rubber latex with a proteolytic enzyme at a pH suitable forthe enzyme in the presence of a soap to prevent premature thickening or coagulation of the latex and subsequently separating proteinaceous material from the rubber. The resulting deproteinisednatural rubber(DPNR) contains not more than 1% of proteinaceous material It has now been found thatthe afore-mentioned problems associated with the use of fresh field latex forthe production ofepoxidised natural rubber are caused by the presence of a large amount of protein in the field latex and, more specifically, it is the molecular size ofthe proteins which causesthe problems.

According to the present invention there is provided a method for the treatment offresh natural rubberfield latex which comprises incubating the field latex with a proteolytic enzyme at a pH suitable for the enzyme, the amount otenzyme present and the incubation conditions being such that the enzyme-treated field latex, when subsequently processed into epoxidised natural rubber latex, has improved coagulation and crepeing properties.

According to a further embodiment ofthe present invention there is provided a method forthe preparation of epoxidised natural rubberfrom fresh natural rubber field latex which comprises the following steps:i) incubating the field latex with a proteolytic enzyme at a pH suitable forthe enzyme, ii) epoxidising the enzyme-treated field latex to the desired mole % level of epoxidation, iii) coagulating the epoxidised natural rubber latex, and iv)crepeing,washing,crumbling and drying the epoxidised natural rubber.

The latex to be treated with enzyme could also be skim latex orfield latex to which some skim latex has been added before epoxidation.

It has been found thata limited enzymetreatment offresh or matured latexconcentrate beforeepoxidation to high epoxidation levels (e.g. ENR 50)-enables the epoxidised latex to be coagulated using either the batch coagulation method orthe continuous column coagulation method without the need to add common saltto the latex and the resulting coagulum has good crepeing properties.

The present invention therefore provides a way of overcoming the previously described problems by providing a method of reducing the size ofthe protein molecules in the field latex by enzymatic hydrolysis using any proteolytic enzyme. The level of enzyme added to the field latex and the incubation time arevery important and are much greater than those required for preparing enzyme deproteinised natural rubber bythe previously known method referred to above. After the enzyme treatment, it is not necessary to remove the degraded protein fragments from the latex. The enzyme-treated field latex, after the appropriate incubation period, is ready for epoxidation to the required mole % epoxidation level. The ENR latexthus prepared can be successfully coagulated by steam according to either (a) the batch coagulation method, or (b) the continuous coagulation method.In both processes of coagulation, no addition of common salt to the latex is required.

In the batch coagulation method, steam is passed directly into the ENR latex held in a series of containers until thetemperaturereaches about98"C. The hot coagulum is leftto maturetypically forfrom about 1/2 hour to3 hours. During this period the smallerpieces of coagulum consolidate and form a big and quitecoherent mass. Coagulation is completed giving a clearserum. During the maturation period, the coagulum istested at intervals for its ability to form a crepe after on-e pass through the creper. As soon as this is possible the coagulum is creped and washed about 8 times and is then comminuted to crumbs using the creper-hammermill. Forthefinal size-reduction other conventional machinery, e.g. a creper-shredder or extruder or pelletiser, may also be used.The crumbs are then dried with through circulation of hotair(about 80'Cto 100'C) in theusual way. Itis notadvisableto leavethe hotcoagulum to mature for longerthan is necessary as excessive heat is known to degrade the rubber molecules. Maturation of the hot coagulum for different periods oftime may be used to prepare ENR of varying molecular weight and hence generally different Mooneyviscosity.

In the continuous coagulation method, the ENR latex is passed down a vertical stainless steel column and is coagulated with steam inside the column as described earlier. The coagulum is collected in a container placed atthe exitofthe column. Itis then leftto maturetypicallyforfrom 1/2 hourto 3 hours and isthen creped and washed and converted to crumbs and dried in a similarway as in the batch coagulation method. Forthis method to work effectively, it is desirable for the ENR latexto have a dry rubber content of about 25% or higher.

In the present process, we have used Savinase 8.OL and Alcalase 2.5L both of which are alkaline proteinases but other proteolytic enzymes may be used. Both these enzyme preparations are commercially available.

These are supplied in liquid form and consist ofthe active enzyme dissolved in a solvent system consisting of 1 ,2-propanediol, stabiliser and water. Savinase 8.OL has an activity of 8i0 Kilo Novo Proteinase Units per gram (KNPU/g) whileAlcalase 2.5L has an activity of 2.SAnson Units per gram (AU/g).

The physical form ofthe enzyme is not important,forexampleAlcalase 2.OTwhich is available as dry granules having an activity of2.OAU/g has also been used successfully. Adisadvantage of using the granular form is that the inert carrier, e.g.titanium dioxide, which is insoluble in water must be removed by sedimentation or centrifugation after the enzyme has been dissolved. This operation results in the loss of some ofthe enzyme. Moreover, if sedimentation is used to remove the inert carrier, a dilute solution ofabout 5% must be prepared to obtain maximum recovery ofthe enzyme-solution. This dilute enzyme solution causes undesirable dilution of the field latex.

The amount of enzyme may be chosen to obtain a desired rate or degree of proteolysis. We have used 0.05 to 1 phrofthe liquid enzymeforfresh field latex. Time andtemperature of incubation may also be chosen to achieve the desired rate and degree of proteolysis, typical figures being from 12 to 96 hours at from 25"C to 60"C. The pH range forthe enzymes is from 7.5 to 1 1 .0. Itwill be appreciated that if the enzymatic hydrolysis is carried out at a high temperature (40-600),the incubation time and/orthe level of enzyme required can be reduced.

The amount of enzyme and thetime of incubation arevery important. Low levels of enzyme and short incubation times which are sufficient for preparing DPNR are inadequate for solving the problems of coagulation and crepeing satisfactorily. These are illustrated in Examples 1 to 3. Forthe preparation of DPNR as described in British Patent No. 1 366,934 the latex after incubation with enzyme is diluted two a solids content of about3% priorto coagulation with acid so asto avoid entrapping proteinaceous material in thecoagulum.

We have used this method to assess approximately the extent of protein breakdown using different levels of enzyme and different incubation times. This is illustrated in Example 1. The nitrogen content ofthe DPNR provides some indication of the extent of protein breakdown.

The ENR produced from enzyme-treated field latex has a low nitrogen content, typically about 0.04% on the weightofthe rubber. This value is even lowerthan the lowestvalue obtained, about 0.06%, for DPNR prepared by enzyme deproteinisation of field latex (Example 1 ). The reason forthis is probably dueto further hydrolysis ofthe enzyme-degraded protein fragments and/or hydrolysis of other nitrogen-containing compounds (e.g.

phospholipids) underthe conditions of the epoxidation reaction, i.e. heat and performic acid. The increased solubility ofthe protein fragments during the heat coagulation ofthe ENR latex could also accountforthis lower value. The ash content of the ENR is typically 0.08% by weight. It is noted that the nitrogen content of ENR prepared from matured latex concentrate is about 0.11% on the weight of the rubber.

If it is desired to improve the properties ofthe ENR e.g. Wallace plasticity and plasticity retention index,this may be achieved using known chemical methods. For example an antioxidant may be added to the latex before coagulation and the ENR crumbs may be treated with an antioxidant before drying.

It is not fully understood why enzymatic hydrolysis ofthe proteins in field latex should solve the problems of difficulty in coagulation of ENR latex and inability ofthecoagulum to form a crepe, but-it seems likely thatthe following factors contribute to the result. Underthe epoxidation conditions which consist of heating the latex with formic acid and hydrogen peroxide in the presence of a non-ionicsurfactant, (a) the protein molecules are chemically converted to some form of steric stabiliser and/or (b) the protein molecules interact chemicallywith the non-ionic surfactantto form biggerstericstabiliser molecules. (Non-ionicsurfactants are stericstabilisers themselves).These protein-derived steric stabiliser molecules havethe effect of inhibiting or reducing the probability ofthe latex particles cohering and coalescing with one anotherto form a quite continuous and coherent mass, when collision between particles occurs at temperatures of less than about 100"C. Hence the ENR latex is difficult to coagulate by heating with steam. In the presence of salt the protein-derived steric stabiliser and the non-ionic surfactant gradually lose some of their stabilisation property towards heat. Hence on heating the ENR latex in the presence of common salt, some coagulation occurs; this is the result of rubber particles coalescing to form loose aggregates.Depending on the size, each loose aggregate contains many rubber particles having some contact with one another but because of the presence ofthe protein derived steric stabiliser molecules on the surface of the particles, the latter are inhibited or hindered from further coalescing with one anotherto form a bigger and quite continuous and coherent mass. The loose aggregates are also similarly inhibited or hindered from coalescing with one anotherto form a bigger mass. In factthe matured coagulum on passing through the creper breaks up into loose aggregates.

When the protein molecules are hydrolysed into small fragments e.g. polypeptides and amino acids and under the epoxidation reaction conditions, the reaction products that may be formed from these small fragments have a lower steric stabilisation property compared with the very much biggerprotein-derived stericstabiliser; the smaller the fragments, the lower is the stabilisation property. This probably explains why a higher level ofenzyme and longer incubation time which result in a greater degree of proteolysis are more effective in solving the problems described.

The following examples are included to illustrate the present invention.

Example 1 Field latex was preserved with 0.25% ammonia on latex weight. Potassium oleate was added to the latex at a level of 1 phrto stabilisethe latex when the proteins were degraded. The enzymes, Savinase 8.OLandAlcalase 2.5L,were aded to two samples ofthe latex at levels of 0.1 to 0.5 phrandthe latex mixtures were incubated at room temperature (about 30"C) for 1 to 6 days.

After various incubation periods, a sample of each latex was diluted to a solids content of about 3% before coagulation with formic acid in the usual way. The coagulated rubberwas creped and dried in warm airinthe usual way. The nitrogen content of the rubber is given in Table 1.

TABLE 1 Nitrogen content ofrubber (weight%) from fieldlatex treated with Savinase andAlcalase Enzyme level, phr Days 0.10 0. 15 0.20 0.30 0.40 0.50 1 0.08 0.09 0.08 0.08 0.06 0.08 (0.25) (0.13) (0.09) (0.10) (0.08) (0.10) 2 0.09 0.08 0.07 0.08 0.06 0.07 (0.09) (0.08) (0.09) (0.09) (0.07) (0.07) 3 0.07 0.06 0.06 0.06 0.06 0.07 (0.08) (0.07) (0.07) (0.06) (0.06) (0.07) 4 0.07 0.07 0.07 0.07 0.06 0.07 (0.07) (0.07) (0.07) (0.06) (0.07) (0.06) 6 0.07 0.07 0.06 0.07 0.07 0.06 (0.06) (0.07) (0.07) (0.08) (0.08) (0.07) Unbracketed values are for Savinase treated latex while bracketed values are for Alcalase treated latex.

Nitrogen content of control rubber (i.e. no enzyme treatment) = 0.35% It is seen that for an incubation time of 1 day and at enzyme levels of less than about 0.2 phr, Savinase8.OLis more effectivethan Alcalase 2.5L but at longer incubation times both enzymes are equally effective in hydrolysing the proteins in the latex.

Example 2 Fresh field latex was preserved with 0.25% by weight of ammonia (System A) or 0.25% by weight of ammonia plus 0.013% oftetramethyl thiuram disulphide (TMTD) and 0.013% of zinc oxide (System B).

Preservative system B is known to keep field latex stable and fluid for a longer period than system A. Non-ionic surfactant (e.g. Teric16A29) used to stabilise the latex for the epoxidation reaction was added at a level of 2 phr. (Teric 16A29 is a condensation product of one molecule of a long chain aliphatic alcohol mainlycetyl alcohol and abouttwenty nine molecules of ethylene oxide.)The liquid enzyme preparation was added to the latex mixture whichwas then allowed to incubate at room temperature for 24to 66 hours. After incubationthe latex was epoxidised to ENRSO by heating with formic acid and hydrogen peroxide for about 24 hours.The reaction was then stopped by neutralising the acid with ammonia.The ENRSO latex was then coagulated with steam by either (a) the batch coagulation method or (b) the continuous coagulation method.

In the batch coagulation method, steam was passed directlyintothe latex in a series of containers until the temperature reached about 95'C. The latex coagulated and the coagulum was left to mature typically for about 1/2 hourto 3 hours until it could form a creped after one pass through the-creper. Itwasthen crepe and washed about 8 times and then size-reduced to crumbs on the creper-hammermill in the usual way. The crumbs were dried with through circulation of hot air in the usual way.

In the continuous coagulation method,the ENRSO latex was passed down a vertical stainless steel column as athinfilm and was coagulated with steam inside the column as described in UK Patent Application No.

8427736. The coagulum was collected in container placed at the exit of the column. Itwas then left to mature typicallyfor 1/2 hourto3 hours and was creped and washed 8times and converted to crumbs and dried in a similar way as for the batch coagulation method. For this method to work effectively it is desirable forthe ENRSO latex to have a dry rubber content of about25% or more.

The conditionsfor enzyme treatment and their effect upon the coagulation of ENR50 latex and the ability of the coagulum to form a crepe are shown in Table 2.

TABLE2 Effect of enzyme treatment conditions on the coagulation and crepeing of ENR50 Experimental Enzyme Incubation field latex level,phr Time,hours Coagulation Crepeing 1. System A Nil -- poor cannotform crepe 2. System A 0.40Savinase 24 good poor 3. System A 0.25Savinase 42 good poor 4. System A 0.25 Savinase 66 good good 5. System A 0.25Alcalase 66 good good 6. System B 0.35 Savinase 42 good good 7. System B 0.35Alcalase 42 good good Good coagulation indicates that complete coagulation occurred, while poor coagulation indicatesthat coagulation waseither incomplete or little coagulation occurred.

Good crepeing indicates that the coagulum formed a crepe after 1 passthroughthe creper,while poor crepeing indicates that many passes were needed before a crepe was formed.

The nitrogen contents ofthe NERSO obtained were 0.03 to 0.04weight % for Experiments (2) to (7).These values were lowerthan those of ENR50 (average = 0.11 weight%) prepared from matured latex concentrate or those of DPNR from enzyme deproteinisation offield latex shown in Table 1. The average ash content ofthe ENR 50 (45 samples) prepared according to Experiments (4) to (7) was 0.08% by weight with a standard deviation of 0.02%.

it is seen that the presence of a small amountofTMTD and zinc oxide in the latex did notaffectthe effectiveness of the enzymes used.

From Table 2, it is noted that the level of enzyme added and the time of incubation are very important in order to solve the problems of coagulation and crepeing satisfactorily. Low levels of enzyme and short incubation times which are sufficientfor preparing DPNR (Table 1 )are inadequateforsolving these problems satisfactorily.

Example3 Fresh field latex was preserved with 0.25% by weight of ammonia and stabilised with 1.6 phrofnon-ionic surfactant (e.g.Teric 16A29) and then treated with Savinase 8.0L. After incubation the latex was epoxidised to ENR25 by heating with formic acid and hydrogen peroxideforabout 24 hours. The reaction was then stopped byneutralising the acidic latex mixture with ammonia. The latex was coagulated with steam by either (a)the batch coagulation method or (b) the continuous coagulation method and the coagulum was creped and converted to crumbs and dried in a similarway as for ENR50 in Example 2. The initial sizes of the rubberflocs appeared to be smallerthan those of ENR50.However, on maturation for 1/2 hourto 2 hours,theseflocs consolidated into a big mass which could be creped and converted to crumbs without problems if enzymatic hydrolysis was sufficient.

The effect of the conditions of enzyme treatment on the coagulation of ENR25 latex and crepeing behaviour ofthe coagulum is shown in Table 3.

TABLE3 Effect of enzyme treatment conditions on the coagulation and crepeing ofENR25 Savinase Incubation Experiment levelphr Time,hours Coagulation Crepeing 1 0.3 42 poor -2 0.4 66 poor 3 0.6 66 good good 4 0.40 96 good good The nitrogen content ofthe ENR25 obtained was 0.04weight%for Experiments (3) and (4) and the ash content was similartothose ofENR50 in Example 3.

Example4 This example demonstrates heat accelerated enzymatic hydrolysis.

The incubation time and/orthe level of enzyme needed can be reduced by accelerating the enzymatic hydrolysis of the proteins present in field latex. This is achieved by carrying outthe hydrolysis at elevated temperature (e.g. 40"C- 60"C). It has been found that it is not necessary to maintain the temperature ofthe enzyme-treated latex at a constant level. Hence on day zero 4000 litres of enzyme-treated field latex (treated in a similarway as in Example 2forENR 50 and Example 3forENR 25) were heated to 55 C,whereuponthe heating was discontinued to save energy (and thus reduce costs). The latex mixture was covered and left undisturbed overnight (about 18 hours) sothatthe hydrolysis could proceed.The next day (dayone)the temperature ofthe latex was found to have dropped to about46'C. Atthe end ofthis 18 hourenzyme treatment,the latexwas readyforepoxidation to ENR 50 in a similarway as in Example 2.

For a 42 hour enzyme treatment, the latex was again heated to 55"C on day one, whereupon the heating was discontinued and the latex left undisturbed for 24 hours. It was then epoxidised to ENR 50 in a similarway asin Example 20r epoxidised to ENR 25 as in Example 3.

Similarly for a 66 hour enzyme treatment, the latex was again heated to 550Con day two, the heating was then discontinued and the latex leftforanother24 hours. Itwasthen epoxidisedto ENR 25 in a similar way as in Example 3.

The epoxidised latex, after neutralisation with ammonia, was coagulated with steam by either (a) the batch coagulation method or (b) the continuous column coagulation method and the coagulum was creped and converted to crumbs and dried in a similarway as in Example 2.

The effects ofthe above conditions of heat and enzyme treatment on the coagulation of epoxidised latex and the crepeing behaviourofthe coagulum are shown in TabIe4.

TABLE4: Effect enzyme treatment conditions (4- 55'C) on the coagulation andcrepeing ofENR 50 and ENR25 Enzyme level, Incubation Experiment phr Time, hours Coagulation Crepeing 1.ENR50 0.25Alcalase 18 Good Poor 2.ENR50 0.40Alcalase 18 Good Good 3.ENR50 0.40Savinase 18 Good Good 4. ENR 50 0.20 Alcalase 42 Good Poor 5.ENR50 0.30Alcalase 42 Good Good 6. ENR 50 0.30 Savinase 42 Good Good 7.ENR25 0.55Alcalase 42 Good Good 8.ENR25 0.35Alcalase 66 Good Good 9. ENR 25 0.35Savinase 66 Good Good Alcalase refers to Alcalase 2.5 Lwhile Savinase refers to Savinase 8.0. L.

For ENR 25, the coagulation ofthe epoxidised latex (Experiments 7 to 9) was much better than that in Experiments 3 & of Example 3 sincethe initial sizes of the coagulated rubber appeared to be biggerand therefore the coagulum could be creped in a shortertime.

The nitrogen and ash contents of the epoxidised rubbers were similarto those in Examples 2 & .

Claims (11)

1. A method forthe treatment of fresh natural rubberfield latex which comprises incubating thefield latex with a proteolytic enzyme at a pH suitableforthe enzyme, the amount of enzyme present and the incubation conditions being such that the enzyme-treated field latex, when subsequently processed into epoxidised natural rubber latex, has improved coagulation and crepeing properties.

2. A method as claimed in claim 1 ,wherein the natural rubberfield latex is incubated with from 0.05 to 1 phr of a proteolytic enzyme having an activity of 8.0 KN PU/g enzyme of 2.5 AU/g enzyme forfrom 12 to 96 hours at from 25into 60"C.

3. A method as claimed in claim 1 or claim 2, wherein Savinase orAlcalase or other alkaline proteinaseis used as the enzyme art a pH of from 7.5 toll.

4. A method as claimed in any one of claims 1 to 3, wherein a non-ionic surfactant is present at a level of from 1 to 5 phrsoto stabilise the latex during the enzyme treatment and prevent prematurecoagulation.

5. Epoxidised natural rubber latex which has been prepared from natural rubberfield latex treated according to the method as claimed in any one of claims 1 to4.

6. Amethodforthe preparation of epoxidised natural rubberfromfresh natural rubberfield latexwhich comprises the following steps: i) incubating the field latex with a proteolytic enzyme at a pH suitable forthe enzyme, ii) epoxidising the enzyme-treated field latex to the desired mole % level of epoxidation, iii) coagulating the epoxidised natural rubberlatex, and iv) crepeing, washing, crumbling and drying the epoxidised natural rubber.

7. A method as claimed in claim 6, wherein the epoxidation of step ii) is performed by heating the enzyme-treated field latex with formic acid and hydrogen peroxide.

8. A method as claimed in claim 6 orclaim 7, wherein the coagulation of step iii) is performed bypassing steam directly into the epoxidised natural rubber latex until the temperature reaches about 98"C.

9. Amethod as claimed in claim 6 orclaim 7, wherein the coagulation of step iii) is performed by passing the epoxidised natural rubber latex down a stainless steel column counter-currentto steam.

10. A method as claimed in any one of claims 6to 9, wherein additional chemicals, such as an antioxidant, are added to the epoxidised natural rubber latex before coagulation and/orto the epoxidised natural rubber crumbs before drying.

11. Epoxidised natural rubber which has been prepared from natural rubberfield latex treated according to the method as claimed in any one of claims 1 to 10 and wherein the nitrogen content is not more than 0.08% by weight.