WO2022159042A1

WO2022159042A1 - Method for preparation of natural rubber with constant viscosity using aldehyde compounds

Info

Publication number: WO2022159042A1
Application number: PCT/TH2021/000072
Authority: WO
Inventors: Suriyakamon Montha; Piyada SUWANDITTAKUL; Chaveewan Kongkaew; Surapich Loykulnant; Pipatcha RAKDEE
Original assignee: National Science And Technology Development Agency
Priority date: 2021-01-22
Filing date: 2021-12-14
Publication date: 2022-07-28

Abstract

The present invention relates to a method for preparation of natural rubber with constant viscosity, a method for preparing block rubber with constant viscosity, and a method for preparing sheet rubber with constant viscosity. A mixture of aldehyde compound solution and acid solution is used as natural rubber latex coagulant. Aldehyde compound is selected from a group consisting of formaldehyde (FA), glutar aldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), and combinations thereof. Acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, and combinations thereof. It is a simple method using chemicals of low toxicity. Shorter storage time of rubber cup lump is required before block rubber is produced.

Description

METHOD FOR PREPARATION OF NATURAL RUBBER WITH CONSTANT VISCOSITY USING ALDEHYDE COMPOUNDS

FIELD OF THE INVENTION

The present invention relates to the field of chemistry and relates, in particular, to a method for preparation of natural rubber with constant viscosity using aldehyde compounds.

BACKGROUND OF THE INVENTION

Natural rubber is an important raw material in rubber industries (such as tires, belts, shoe soles or hoses) due to its high strength and elasticity. Since natural rubber has some properties that change over time, especially an increase in hardness of the rubber during storage, this makes it difficult to control the production process of rubber products and requires more energy to be used in the rubber compounding process. It is assumed that a mechanism of storage hardening in natural rubber is caused by branched and network bonding between rubber molecules and non-rubber molecules such as proteins and phospholipids. This reaction creates natural rubber with higher gel content and Mooney viscosity.

There are two types of raw materials used in a manufacture of constant viscosity natural rubber: rubber latex and rubber cup lump. At present, a production of constant viscosity natural rubber is done by adding monofunctionaldehydic condensing reagents to natural rubber. These chemicals react with aldehyde radicals and prevent the reaction between aldehyde groups and amino groups in rubber molecules. The most popular viscosity modifier is hydroxylamine neutral sulfate, HNS. However, the use of HNS is highly toxic and it has limitations in controlling the viscosity of natural rubber.

In addition, other chemicals were also used to prevent the increase in viscosity of the rubber. US patent no. 5693695A disclosed natural rubber containing a viscosity stabilizer and a method for manufacturing the same, which impart a good processability and excellent physical properties, an additive for natural rubber having the effect of stabilizing viscosity, a rubber composition containing the additive, and a method for suppressing an increase in the viscosity of natural rubber with the additive. A viscosity stabilizer included a hydrazide compound of the following general formula, R-CONHNH, wherein R is an alkyl group having 1 to 5 carbon atoms or a cycloalkyl group having 3 to 5 carbon atoms. As a result, the gelation of rubber (storage hardening) was suppressed and a decrease in molecular weight was prevented.

US patent no. 5710200A disclosed natural rubber containing viscosity stabilizers treated under drying conditions resulting in 10% or less of the difference in gel content and 85% or more of molecular weight retention between before and after drying, and methods for manufacturing natural rubber comprising treating under drying conditions to result in 10% or less of the difference in gel content and 85% or more of molecular weight retention between before and after drying. Natural rubber containing viscosity stabilizers was treated with a strainer, and methods for manufacturing natural rubber comprising treating with a strainer after compounding hydrazide stearate into the natural rubber. Gelation (storage hardening) was suppressed and a decrease in molecular weight was prevented.

China patent no. 103627044A disclosed a method for preparing constant viscosity natural rubber by adding more than or equal to 2.5% of a hydroxylamine solution and more than or equal to 8.0% of a hydrazine hydrate solution in natural latex. The preparation method of the constant viscosity natural rubber comprised processes of purification, constant viscosity treatment, solidification and curing, dehydration and impurity removal, and drying. The constant viscosity natural rubber was stable in Mooney viscosity, high in consistency of sizing materials, excellent in performance, simple in production process, strong in interchangeability, and environmentally friendly, and had a relatively high popularization value.

China patent no. 108440800B disclosed a method for preparing a low-Mooney constant viscosity natural rubber. This invention adopted a dry mixing (stirring) method physical reaction preparation process, took low molecular weight rubber material with low Mooney viscosity as a plasticizer, lubricated the chain link between natural rubber molecular chains by an internal mixing method, inhibited the reaction of functional groups such as phospholipid group, aldehyde group and the like, delayed the crosslinking between rubber hydrocarbon macromolecules, prevented hardening, ensured that the prepared natural rubber met the standard of low Mooney viscosity, had small storage and hardening change value, simple preparation process, no pollution and high production efficiency, and had characteristics of low Mooney viscosity, small change value, long constant time, stable performance, good consistency and the like.

Patent documents US5693695A, US5710200A and CN103627044A disclosed a method for using hydrazide compounds to control the viscosity of natural rubber, and patent document CN108440800B disclosed a technique for using plasticizers in combination with the mechanical dry mixing process.

From above patent documents, it was found that highly toxic chemicals are still used to control the viscosity of natural rubber and several steps are required in a production of constant viscosity natural rubber. The present invention aims to solve such problems by using aldehyde compounds such as glutaraldehyde to bind the protein molecules in natural rubber latex to a higher molecular weight. This method keeps the rubber viscosity constant during storage.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a method for preparing rubber cup lump for use in preparation of block rubber with constant viscosity comprising the steps of: a) preparing a mixture of aldehyde compound solution and acid solution in a weight ratio of 3: 100 to 25: 1; b) adding a mixture to natural latex and then leaving it for 2 to 4 hours to obtain a rubber coagulum; c) storing the rubber coagulum until the moisture content is in a range of 20 to 30% by weight. wherein the aldehyde compound is selected from a group consisting of formaldehyde (FA), glutaraldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), or a mixture thereof; and acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, or a mixture thereof.

In another embodiment, a method for preparing block rubber with constant viscosity comprising the steps of: a) preparing a mixture of aldehyde compound solution and acid solution in a weight ratio of 3: 100 to 25: 1; b) adding a mixture to natural latex and then leaving it for 2 to 4 hours to obtain a rubber coagulum; c) washing the rubber coagulum with water until the pH of washing water is neutral, and then reducing the size of the rubber. d) drying and compressing the rubber. wherein the aldehyde compound is selected from a group consisting of formaldehyde (FA), glutaraldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), or a mixture thereof; and acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, or a mixture thereof. In another embodiment of the invention , a method for preparing sheet rubber with constant viscosity comprising the steps of: a) preparing a mixture of aldehyde compound solution and acid solution in a weight ratio of 3: 100 to 25: 1; b) adding a mixture to natural latex and then leaving it for 2 to 4 hours to obtain a rubber coagulum; c) sheeting the rubber coagulum to get the thickness in a range of 1 to 4 mm; d) drying the sheet rubber at 50 to 80 °C for 3 to 5 days. wherein the aldehyde compound is selected from a group consisting of formaldehyde (FA), glutaraldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), or a mixture; and acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, or a mixture thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the appearance of pores inside the cup lump after keeping the cup lump for 30 days; (a) GA 0% (b) GA 0.01% (c) GA 0.02% (d) GA 0.03% (e) GA 0.05% (f) GA 0.10% (g) GA 0.2% (h) GA 0.3% (i) GA 0.4% (j) GA 0.5% (k) GA 1.0% and (1) GA 2.0% by weight of natural latex.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for preparation of natural rubber with constant viscosity, a method for preparing block rubber with constant viscosity, and a method for preparing sheet rubber with constant viscosity using aldehyde compounds such as glutaraldehyde. Aldehyde compounds act as protein binders and acid solutions act as latex coagulants. It is a simple method using chemicals of low toxicity. A shorter storage time of rubber cup lump is required before block rubber is produced.

Any aspects described herein are meant to include other aspects of the present invention, unless stated otherwise.

Definition

Technical terms or scientific terms used here have definitions as understood by persons skilled in the art unless stated otherwise.

Any tools, equipment, methods, or chemicals named here mean tools, equipment, methods, or chemicals being used commonly by a person skilled in the art unless stated otherwise that they are tools, equipment, methods, or chemicals specific only to the present invention.

Use of singular nouns or pronouns when used with "comprising" in claims and/or specification means "one" and also includes "one or more", "at least one", and "one or more than one".

All compositions and/or methods disclosed and claimed are aimed to include aspects of the invention from actions, operation, modifications, or changing of any parameters without performing significantly different experiments from the present invention, and obtaining similar objects with the same utilities and results of the present invention according to persons skilled in the art although without mention of the claims specifically. Therefore, substitution or similar objects to the present invention including minor modifications or changes which can be clearly seen by persons skilled in the art should be considered within the scope, spirit, and concept of the invention as appended claims.

Throughout this application, the term “about” means any number that appears or shows here that could be varied or deviated from any error of equipment, method, or person using said equipment or method.

Hereafter, invention embodiments are shown without any purpose to limit any scope of the invention.

In an embodiment of the invention, a method for preparing rubber cup lump for use in preparation of block rubber with constant viscosity comprising the steps of: a) preparing a mixture of aldehyde compound solution and acid solution in a weight ratio of 3: 100 to 25: 1; b) adding a mixture to natural latex and then leaving it for 2 to 4 hours to obtain a rubber coagulum; c) storing the rubber coagulum until the moisture content is in a range of 20 to 30% by weight. wherein the aldehyde compound is selected from a group consisting of formaldehyde (FA), glutaraldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), or a mixture thereof; and acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, or a mixture thereof.

The aldehyde compound content may be in a range of 0.005 to 5% by weight of natural latex. The aldehyde compound content may be in a range of 0.02 to 1% by weight of natural latex.

The acid solution concentration may be in a range of 1.0 to 10.0% by weight.

The aldehyde compound is preferably glutaraldehyde.

The acid solution is preferably formic acid.

A pH of the mixture in step b may be in a range of 4 to 5.

A storage time of rubber coagulum in step c may be in a range of 12 to 20 days.

In another embodiment of the invention, the present invention relates to rubber cup lumps obtained by the method of this invention.

In another embodiment of the invention, a method for preparing block rubber with constant viscosity comprising the steps of: a) preparing a mixture of aldehyde compound solution and acid solution in a weight ratio of 3: 100 to 25: 1; b) adding a mixture to natural latex and then leaving it for 2 to 4 hours to obtain a rubber coagulum; c) washing the rubber coagulum with water until the pH of washing water is neutral, and then reducing the size of the rubber. d) drying and compressing the rubber. wherein the aldehyde compound is selected from a group consisting of formaldehyde (FA), glutaraldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), or a mixture thereof; and acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, or a mixture thereof.

The aldehyde compound content may be in a range of 0.005 to 5% by weight of natural latex.

The aldehyde compound content may be in a range of 0.02 to 1% by weight of natural latex.

The acid solution concentration may be in a range of 1.0 to 10.0% by weight.

Preferably, the aldehyde compound is glutaraldehyde.

Preferably, the acid solution is formic acid.

A pH of the mixture in step b may be in a range of 4 to 5.

In another embodiment of the invention, the present invention relates to block rubber obtained by the method of this invention. In another embodiment of the invention, a method for preparing sheet rubber with constant viscosity comprising the steps of: a) preparing a mixture of aldehyde compound solution and acid solution in a weight ratio of 3: 100 to 25: 1; b) adding a mixture to natural latex and then leaving it for 2 to 4 hours to obtain a rubber coagulum; c) sheeting the rubber coagulum to get the thickness in a range of 1 to 4 mm; d) drying the sheet rubber at 50 to 80 °C for 3 to 5 days. wherein the aldehyde compound is selected from a group consisting of formaldehyde (FA), glutaraldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), or a mixture ; and acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, or a mixture thereof.

The acid solution concentration may be in a range of 1.0 to 10.0% by weight.

Preferably, the aldehyde compound is glutaraldehyde.

Preferably, the acid solution is formic acid.

A pH of the mixture in step b may be in a range of 4 to 5.

In another embodiment of the invention, the present invention relates to sheet rubber obtained by the method of this invention.

Any invention characteristics shown here have included their application to other aspects of this invention unless otherwise specified.

The following are non-limiting examples, which disclose the preparation of representative methods of this present invention.

EXAMPLE

Example 1

Various amounts of glutaraldehyde, GA (0, 0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 2.0, 5.0, 10.0 and 15.0% by weight of natural latex) were mixed with 5 wt% formic acid solution (as shown in Table 1). This mixture was gradually added to natural latex in a proportion of 5% by weight of natural latex, and the pH was adjusted to 4-5. Then, the latex was left for 3 hours to obtain a rubber coagulum. The rubber coagulum was then squeezed to record the appearance of the rubber coagulum before being creped and dried.

Comparative example 1

Various amounts of glutaraldehyde, GA (0, 0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 2.0, 5.0, 10.0 and 15.0% by weight of latex) were mixed with natural latex (as shown in Table 1). A solution of 5 wt% formic acid was then gradually added to the latex in a proportion of 5% by weight of natural latex, and the pH was adjusted to 4-5. Then, the latex was left for 3 hours to obtain a rubber coagulum. The rubber coagulum was then squeezed to record the appearance of the rubber coagulum before being creped and dried.

The integrity of latex coagulation of example 1 and comparative example 1 was calculated using the following equation; weight of dried rubber in sample

Integrity of latex coagulation = - - - — - - — — - - - — x 100% weight of dried rubber in control sample

The results in Table 1 show that the sequence of mixing glutaraldehyde solution, formic acid solution, and natural latex had an effect on the appearance of the rubber coagulum. Mixing glutaraldehyde solution with formic acid solution before adding it to the latex (according to this invention) provided hard, soft, and firm rubber coagulum in a range of 0.01- 5% concentrations of glutaraldehyde solutions. The rubber coagulum was soft and disintegrated when using gluratraldehyde concentrations more than or equal to 5%. On the other hand, mixing glutaraldehyde solution with natural latex before adding with formic acid solution (according to comparative examples) provided hard, soft, and firm rubber coagulum in a range of 0.01-0.4% concentrations of glutaraldehyde solutions. The rubber coagulum was soft and disintegrated when using gluratraldehyde concentrations of 5%. The rubber coagulum did not occur when using gluratraldehyde concentrations more than or equal to 1%. The appearance of the rubber coagulum was related to the integrity of the latex coagulation.

Table 1: Appearance of rubber coagulum and integrity of latex coagulation

Example 2

Various amounts of glutaraldehyde, GA (0, 0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 2.0, 5.0, 10.0 and 15.0% by weight of natural latex) were mixed with 5 wt% formic acid solution (as shown in Table 2). This mixture was then gradually added to natural latex in a proportion of 5% by weight of latex and the pH was adjusted to 4-5. Then, the latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from a cup and left for 14 days.

Comparative example 2

Various amounts of glutaraldehyde, GA (0, 0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 2.0, 5.0, 10.0 and 15.0% by weight of natural latex) were mixed with natural latex (as shown in Table 2). A solution of 5 wt% formic acid was then gradually added to the latex in a proportion of 5% by weight of natural latex and the pH was adjusted to 4-5. Then, the latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from a cup and left for 14 days.

The moisture content of example 2 and comparative example 2 was calculated using the following equation; weight of wet rubber — weight of dried rubber

Moisture content = - - - - - — - x 100% weight of wet rubber

The moisture content in the rubber coagulum from example 2 and comparative example 2 was reduced with increasing amounts of glutaraldehyde. The moisture content in the rubber coagulum from example 2 was lower than that from the comparative example 2. This result indicated that the rubber coagulum prepared according to this invention required storage time to achieve the moisture standard (approximately 25% moisture) shorter than the conventional method, which took approximately 30 days.

Table 2: Moisture contents in the rubber coagulum from example 2 and comparative example 2 after storage the rubber coagulum for 14 days

Example 3

Various amounts of glutaraldehyde, GA (0, 0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0 and 2.0% by weight of natural latex) were mixed with 5 wt% formic acid solution (as shown in Table 3). This mixture was then gradually added to natural latex in a proportion of 5% by weight of natural latex and the pH was adjusted to 4-5. Then, the latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from the cup and left for 30 days. The rubber coagulum was cut into 2 parts. Rotten smell level and pores in the rubber coagulum were recorded. Determination of the rotten smell level used the following criteria;

Level 0 (no smell): No smell

Level 1 (very mild smell): General people do not smell but people who are responsible for smell testing and people who are particularly sensitive to smells will smell it.

Level 2 (mild smell): Smell is very mild and faint which must be intentionally inhaled to feel.

Level 3 (moderate smell): Smell disturbs the surrounding community.

Level 4 (strong smell): Smell was annoying.

Level 5 (very strong smell): Smell is not suitable for breathing.

It was found from Table 3 that the rotten smell level and pores in the rubber decreased with increasing glutaraldehyde content.

Table 3: Rotten smell level and pores in the rubber coagulum

Example 4

Various amounts of glutaraldehyde, GA (0, 0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0 and 2.0% by weight of natural latex) were mixed with 5 wt% formic acid solution (Table 4). This mixture was then gradually added to natural latex in a proportion of 5% by weight of natural latex and the pH was adjusted to 4-5. The latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from the cup, left for 30 days, creped, washed and dried at 80 °C for 24 h. Physical properties of dried rubber such as initial Wallace plasticity (Po), plasticity retention index (PRI) and Mooney viscosity (MV) were determined. It was found that PRI of the rubber from this invention increased with increasing glutaraldehyde content, and MV of the rubber from this invention decreased with increasing glutaraldehyde content.

Table 4: Physical properties of the rubber from this invention

Example 5

Various amounts of glutaraldehyde, GA (0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0 and 2.0% by weight of natural latex) were mixed with 5 wt% formic acid solution (as shown in Table 5). This mixture was then gradually added to natural latex in a proportion of 5% by weight of natural latex and the pH was adjusted to 4-5. The latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from the cup, left for 30 days, creped, washed and dried at 80 °C for 24 h.

Comparative example 5

Various amounts of glutaraldehyde, GA (0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0 and 2.0% by weight of natural latex) were mixed natural latex (as shown in Table 5). A solution of 5 wt% formic acid was then gradually added to the natural latex in a proportion of 5% by weight of natural latex and the pH was adjusted to 4-5. The latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from the cup, left for 30 days, creped, washed and dried at 80 °C for 24 h.

Mooney viscosity (MV) of the rubber from example 5 and comparative example 5 before and after aging at 60 °C for 3 days was measured. Delta MV was calculated from the difference between MV before aging and MV after aging. It was found that the delta MV of the rubber from example 5 was lower than that from comparative example 5. This indicated that this invention provided rubber with more constant MV.

Table 5: Delta MV of the rubber

Example 6

Various amounts of glutaraldehyde, GA (0, 0.1, 0.2, 0.3, 0.4 and 0.5% by weight of natural latex) were mixed with 5 wt% formic acid solution (Table 6). This mixture was used as latex coagulant for the sheet rubber preparation.

Natural latex (2.5 kg) was put into a rectangular tray. Dilution of natural latex was done by adding 2.5 kg of water. The coagulant was then gradually added to natural latex in a proportion of 5% by weight of natural latex which made the natural latex have pH in a range of 4-5. The latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from the tray and sheeted by using a two-roll mill machine to get the sheet rubber with 2-3 mm thickness. Washing and smoke drying of the sheet rubber were done at 60 °C for 4 days. Physical properties of dried sheet rubber such as initial Wallace plasticity

(P0), plasticity retention index (PRI) and Mooney viscosity (MV) were determined. Delta MV was calculated from the difference between MV before and after aging at 60 °C for 3 days. Comparative example 6

Natural latex (2.5 kg) was put into a rectangular tray. Dilution of natural latex was done by adding 2.5 kg of water. Various amounts of glutaraldehyde, GA (0.1, 0.2, 0.3, 0.4, 0.5, 2.0% by weight of natural latex) were added into the natural latex (Table 7). Then, 5% formic acid was gradually added to natural latex in a proportion of 5% by weight of natural latex which made the natural latex have pH in a range of 4-5. The latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from the tray and sheeted by using a two-roll mill machine to get the sheet rubber with 2-3 mm thickness. Washing and smoke drying the sheet rubber were done at 60 °C for 4 days. Mooney viscosity (MV) before and after aging at 60 °C for 3 days were determined. Delta MV was calculated from the difference between MV before aging and MV after aging.

From the results in Table 6, the smoked sheet rubber from this invention exhibited an increase of PRI property with increasing glutaraldehyde content and decrease of Mooney viscosity with increasing glutaraldehyde content. From the results in Table 7, the smoked sheet rubber from this invention showed constant viscosity properties better than that from the comparative smoked sheet rubber.

Table 6: Physical properties of the smoked sheet rubber from this invention

Table 7: Delta MV of the smoked sheet rubber

Example 7

Formaldehyde (FA), glutaraldehyde (GA) or diazolidinyl urea (DU) was mixed with 5 wt% formic acid solution (Table 8). This mixture was used as latex coagulant for block rubber preparation.

Natural latex (2.5 kg) was put into a rectangular tray. Dilution of natural latex was done by adding 2.5 kg of water. The coagulant was then gradually added to natural latex in a proportion of 5% by weight of natural latex which made the natural latex have pH in a range of 4-5. The latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from the tray, creped, washed, crushed, dried at 80 °C and compressed into block rubber. Physical properties of the block rubber such as initial Wallace plasticity (P0), plasticity retention index (PRI) and Mooney viscosity (MV) were determined. Delta MV was calculated from the difference between MV before and after aging at 60 °C for 3 days. Comparative example 7

Natural latex (2.5 kg) was put into a rectangular tray. Dilution of natural latex was done by adding 2.5 kg of water. Various amounts of glutaraldehyde, GA (0.1, 0.2, 0.3, 0.4, 0.5, 2.0% by weight of natural latex) were added into the natural latex (Table 9)._Then, 5% formic acid was gradually added to natural latex in a proportion of 5% by weight of natural latex which made the natural latex have pH in a range of 4-5. The latex was left for 3 hours to obtain a rubber coagulum. Then, the rubber coagulum was removed from the tray, creped, washed, crushed, dried at 80 °C and compressed into block rubber. Mooney viscosity (MV) before and after aging at 60 °C for 3 days were determined. Delta MV was calculated from the difference between MV before aging and MV after aging.

From the results in Table 8, the block rubber from this invention showed an increase of PRI property with increasing glutaraldehyde content and decrease of Mooney viscosity with increasing glutaraldehyde content. From the results in Table 9, the block rubber from this invention exhibited constant viscosity properties better than the comparative block rubber.

Table 8: Physical properties of the block rubber

Table 9: Delta MV of block rubber

The principle of the device and the methodology described herein is intended to disclose the qualification of the present invention that is not acted, performed, modified, or changed any factor significantly different from the present invention. The resultant qualification and benefits of use are equivalent or similar to the qualification of the present invention, in accordance with those who are skilled in the art. Though not specifically stated in the claims, any substitution or similar qualification to those described herein, which include any modifications and minimal changes that are apparent to those who are skilled in the art, may be considered being in the scope of this invention as described in the attached claims.

BEST MODE OF THE INVENTION Best mode or preferred embodiment of the invention is as provided in the description of the invention.

Claims

Claims:

1. A method for preparing rubber cup lump for use in preparation of block rubber with constant viscosity comprising the steps of: a) preparing a mixture of aldehyde compound solution and acid solution in a weight ratio of 3: 100 to 25: 1; b) adding a mixture to natural latex and then leaving it for 2 to 4 hours to obtain a rubber coagulum; c) storing the rubber coagulum until the moisture content is in a range of 20 to 30% by weight. wherein the aldehyde compound is selected from a group consisting of formaldehyde (FA), glutaraldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), or a mixture thereof; and acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, or a mixture thereof.

2. The method for preparing rubber cup lump for use in preparation of block rubber with constant viscosity of claim 1, wherein aldehyde compound content is in a range of 0.005 to 5% by weight of natural latex.

3. The method for preparing rubber cup lump for use in preparation of block rubber with constant viscosity of claim 2, wherein said aldehyde compound content is in a range of 0.01 to 2% by weight of natural latex.

4. The method for preparing rubber cup lump for use in preparation of block rubber with constant viscosity of claim 1, wherein acid solution concentration is in a range of 1.0 to 10.0% by weight.

5. The method for preparing rubber cup lump for use in preparation of block rubber with constant viscosity of claim 1, wherein said aldehyde compound is glutaraldehyde.

6. The method for preparing rubber cup lump for use in preparation of block rubber with constant viscosity of claim 1, wherein said acid solution is formic acid.

7. The method for preparing rubber cup lump for use in preparation of block rubber with constant viscosity of claim 1, wherein pH of the mixture in step b is in a range of 4 to 5.

8. The for preparing rubber cup lump for use in preparation of block rubber with constant viscosity method of claim 1, wherein storage time of rubber coagulum in step c is in a range of 12 to 20 days.

9. The rubber cup lumps obtained from the method according to any one of claims 1 to 8.

10. A method for preparing block rubber with constant viscosity comprising the steps of: a) preparing a mixture of aldehyde compound solution and acid solution in a weight ratio of 3: 100 to 25: 1; b) adding a mixture to natural latex and then leaving it for 2 to 4 hours to obtain a rubber coagulum; c) washing the rubber coagulum with water until the pH of washing water is neutral, and then reducing the size of the rubber. d) drying and compressing the rubber respectively. wherein the aldehyde compound is selected from a group consisting of formaldehyde (FA), glutaraldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), or a mixture thereof ; and acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, or a mixture thereof.

11. The method for preparing block rubber with constant viscosity of claim 10, wherein aldehyde compound content is in a range of 0.005 to 5% by weight of natural latex.

12. The method for preparing block rubber with constant viscosity of claim 11, wherein said aldehyde compound content is in a range of 0.01 to 2% by weight of natural latex.

13. The method for preparing block rubber with constant viscosity of claim 10, wherein acid solution concentration is in a range of 1.0 to 10.0% by weight.

14. The method for preparing block rubber with constant viscosity of claim 10, wherein said aldehyde compound is glutaraldehyde.

15. The method for preparing block rubber with constant viscosity of claim 10, wherein said acid solution is formic acid.

16. The method for preparing block rubber with constant viscosity of claim 10, wherein pH of the mixture in step b is in a range of 4 to 5.

17. The block rubber obtained from the method according to any one of claims 10 to 16.

18. The method for preparing sheet rubber with constant viscosity comprising the steps of: a) preparing a mixture of aldehyde compound solution and acid solution in a weight ratio of 3: 100 to 25: 1; b) adding a mixture to natural latex and then leave it for 2 to 4 hours to obtain a rubber coagulum; c) sheeting the rubber coagulum to get the thickness in a range of 1 to 4 mm; d) drying the sheet rubber at 50 to 80 °C for 3 to 5 days. wherein the aldehyde compound is selected from a group consisting of formaldehyde (FA), glutaraldehyde (GA), butylaldehyde (BA), crotoaldehyde (CA), diazolidinyl urea (DU), or a mixture thereof; and acid solution is selected from a group consisting of sulfuric acid, acetic acid, formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, or a mixture thereof.

19. The method for preparing sheet rubber with constant viscosity of claim 18, wherein aldehyde compound content is in a range of 0.005 to 5% by weight of natural latex.

20. The method for preparing sheet rubber with constant viscosity of claim 19, wherein said aldehyde compound content is in a range of 0.01 to 2% by weight of natural latex.

21. The method for preparing sheet rubber with constant viscosity of claim 18, wherein acid solution concentration is in a range of 1.0 to 10.0% by weight.

22. The method for preparing sheet rubber with constant viscosity of claim 18, wherein said aldehyde compound is glutaraldehyde.

23. The method for preparing sheet rubber with constant viscosity of claim 18, wherein said acid solution is formic acid.

24. The method for preparing sheet rubber with constant viscosity of claim 18, wherein pH of the mixture in step b is in a range of 4 to 5.

25. The sheet rubber obtained from the method according to any one of claims 18 to 24.