AU2020223672B1 - Method for coagulating natural rubber latex using high-pressure carbon dioxide - Google Patents

Abstract

The present invention provides a method for coagulating a natural rubber latex using high-pressure carbon dioxide (C 2 ), and belongs to the technical field of natural rubber latex coagulation. The method provided by the present invention includes the following steps: under sealed conditions, introducing CO2 until a pressure of 10-30 MPa, and coagulating a natural rubber latex to obtain a rubber lump. Because the present invention coagulates the natural rubber latex with high-pressure C02, the rubber latex coagulates rapidly; the whole coagulation process is conducted in a sealed reactor, largely preserving non-water-soluble non-rubber components (e.g., protein, phosphates, etc.) in the rubber latex and high product performance; moreover, because CO2 is a chemically inert gas, the natural rubber latex is coagulated in the presence of C0 2 , and components in the natural rubber latex will not be damaged, which stay the same; finally prepared natural rubber latex product has optimal quality, providing a new idea for natural rubber latex coagulation.

Description

METHOD FOR COAGULATING NATURAL RUBBER LATEX USING HIGH-PRESSURE CARBON DIOXIDE TECHNICAL FIELD

The present invention relates to the technical field of natural rubber latex coagulation, and in particular to a method for coagulating a natural rubber latex using high-pressure carbon dioxide (C0 2). BACKGROUND

Natural rubber latex bled from a rubber tree has to coagulate to enter processes of thinning, creping and dehydration, hammer milling and granulation, and drying and finally prepare a solid natural rubber latex product (raw rubber). Main components of the natural rubber latex include rubber hydrocarbon, water, and 5% non-rubber components; under the protection of an electric double layer formed by these components, fresh rubber latex maintains a stable emulsion state. Natural rubber latex coagulation is caused by breaking a charge balance of a latex system through chemical, biological, or physical method; so far, commonly used chemical and biological coagulation methods include acid coagulation, inorganic salt coagulation, microorganism coagulation, natural coagulation, enzymatic coagulation, etc. In addition, there are some physical coagulation methods, for example, heating, coagulation and maturation of the rubber latex by using a small thermal gelatinization machine. The following patents have been reported: (1) a method for solidifying nature rubber latex with microwave radiation (CN101519461B), by which nature rubber latex is radiated by microwave under the flowing or static state to obtain flocculent gel, and the flocculent gel is solidified and cured to obtain curds; (2) a method for promoting natural rubber latex to coagulate in vacuum (CN102477109A), by which, with a small amount of acetic acid as a primer, particles in the latex are instantly broken by using a vacuum technology, and the stability of the latex is lost so that the latex is coagulated; and (3) a natural latex jet-flow solidification method (CN103275248B), including the following steps of: pressurizing the natural rubber latex by using a pressure pump, jetting the natural latex into a gasifying chamber through a jet-flow port under the action of pressure, so as to form natural latex solidified grains. Moreover, physical methods for natural rubber latex coagulation further include freezing, stirring, etc., but in general, physical coagulation methods are seldom investigated. Early in 1951, Fraser D had used high-pressure C02 in research on sterilization (Fraser D. Bursting bacteria by release of gas pressure [J]. Nature, 1951,167(4236):33-34.). Use of high-pressure CO2 and supercritical CO2 in rubber industry mainly focuses on ionomer grafting or composite preparation, desulfurization and regeneration of rubber powder and waste tire rubber, preparation of rubber or polymer foam materials, and diffusion, degradation, peeling of rubber in supercritical CO2 fluid. So far, coagulation of natural rubber latex with high-pressure CO2 has not been reported yet. SUMMARY

An objective of the present invention is to provide a method for coagulating a natural rubber latex using high-pressure C02 ; the method has rapid coagulation speed, and the natural rubber product prepared thereby has optimal quality. To achieve the above objective, the present invention provides the following technical solutions. The present invention provides a method for coagulating a natural rubber latex using high-pressure C02, including the following steps: under sealed conditions, introducing CO2 until a pressure of 10-30 MPa, and coagulating a natural rubber latex to obtain a rubber lump. Preferably, in percent by mass, the natural rubber latex has an ammonia content of 0-0.45% and a dry rubber content of>25%. Preferably, the coagulation is conducted at a constant temperature of 25-40°C. Preferably, the constant temperature is achieved by water bath. Preferably, the coagulation is achieved by standing, and the standing lasts for 1-5 h. Preferably, the sealed conditions are provided by a reactor. Preferably, the pressure is held for 5-10 min after the desired pressure is reached. Preferably, after the rubber lump is obtained, the rubber lump is thinned, creped, dehydrated, granulated, and dried successively to obtain a natural rubber latex product. Preferably, the drying is conducted at 60-100°C. The present invention provides a method for coagulating a natural rubber latex using high-pressure C0 2 , including the following steps: under sealed conditions, introducing C02 until a pressure of 10-30 MPa, and coagulating a natural rubber latex to obtain a rubber lump. Because the present invention coagulates the natural rubber latex with high-pressure C0 2 , the rubber latex coagulates rapidly; the whole coagulation process is conducted in a sealed reactor, largely preserving non-water-soluble non-rubber components (e.g., protein, phosphates, etc.) in the rubber latex and high product performance; moreover, because CO2 is a chemically inert gas, the natural rubber latex is coagulated in the presence of C02, and components in the natural rubber latex will not be damaged, which stay the same; a natural rubber latex product prepared thereby has optimal quality, providing a new idea for natural rubber latex coagulation. Results of examples indicate excellent performance of natural rubber products prepared from the rubber lumps coagulated by the method of the present invention. In the method of the present invention, no coagulant is needed to add during coagulation; waste water produced can be treated directly without neutralization; compared with the existing acid coagulation method, the method of the present invention saves acids and bases, CO 2 is recyclable and reusable, the environment will not be damaged, and the process is green and eco-friendly. DETAILED DESCRIPTION

The present invention provides a method for coagulating a natural rubber latex using high-pressure C02, including the following steps: under sealed conditions, introducing CO2 until a pressure of 10-30 MPa, and coagulating a natural rubber latex to obtain a rubber lump. In the present invention, unless otherwise specified, all desired raw materials are commercially available products well known to those skilled in the art. In the present invention, the natural rubber latex is fresh natural rubber latex harvested on the day of harvest; in order to ensure no deterioration before processing, preservation of the fresh natural rubber latex with ammonia water makes the rubber latex stay at an alkaline pH. In the present invention, in percent by mass, the natural rubber latex has an ammonia content of 0-0.45%, and more preferably 0-0.1%, and a dry rubber content of >25%. The process of adding ammonia water is not particularly limited in the present invention, as long as a natural rubber latex with the above ammonia content can be obtained. In the present invention, controlling the ammonia content and the dry rubber content within the above ranges can ensure the quality of the fresh natural rubber latex. In the present invention, the ammonia content and the dry rubber content of the natural rubber latex are for raw materials of the natural rubber latex; when using the natural rubber latex falling within the above content ranges as a raw material, the present invention preferably coagulates the natural rubber latex directly or dilutes and then coagulates the raw materials of the natural rubber latex. Neither process of dilution nor dilution factor is particularly limited in the present invention, but may be adjusted according to the actual requirements. In the present invention, the sealed conditions are provided by a reactor, i.e., the present invention preferably puts the natural rubber latex in the reactor and introduces C02 thereinto after the reactor is sealed. The reactor is not particularly limited in the present invention, as long as the reactor is well known in the art; in examples of the present invention, the reactor is specifically made of stainless steel and has a specific volume of 2.5 L, 10 L, or 30 L. Flow of the CO 2 introduced is not particularly limited in the present invention, as long as desired pressure is reached in the reactor; after introducing the C02, the pressure is preferably -25 MPa; after reaching the desired pressure, the pressure is preferably held for 5-10 min, and more preferably 6-8 min. The present invention preferably uses a pump to achieve the process of introducing C02 ; the pump is not particularly limited in the present invention, as long as pumps can achieve the desired pressure for introducing CO 2 . The present invention controls the pressure to shorten the coagulation time of the natural rubber latex. Because the natural rubber latex has approximately 70% of water, the present invention pressurizes CO2 to increase the solubility of CO 2 in the natural rubber latex; further, carbonic acid produced neutralizes ammonia (for preservation) in the natural rubber latex and breaks the charge balance of the natural rubber latex system, leading to natural rubber latex coagulation; meanwhile, CO2 pressurization can extrude rubber particles in the natural rubber latex and thus force these particles to clump together to accelerate rubber latex coagulation, finally achieving natural rubber latex coagulation. In the present invention, the coagulation is preferably conducted at a constant temperature, and the constant temperature is preferably 25-40°C, and more preferably 28-31°C; the constant temperature is preferably achieved by water bath, i.e., the reactor is placed in a water bath for coagulation. In the present invention, controlling the temperature within the above range can prevent denaturation of the natural rubber latex due to overheating. In the present invention, the coagulation is preferably achieved by standing, and the standing preferably lasts for 1-5 h, and more preferably 2-3 h. In the standing process, the natural rubber latex coagulates slowly and fully. After the coagulation is completed, CO2 is recovered by depressurization, and a rubber lump is obtained. In the present invention, after the rubber lump is obtained, in order to prepare natural rubber with the rubber lump, the rubber lump is preferably thinned, creped, dehydrated, granulated, and dried successively to obtain a natural rubber latex product. The process of thinning, creping, dehydration, and granulation is not particularly limited in the present invention, as long as the natural rubber latex product is obtained through the process well-known in the art. In the present invention, the drying is preferably conducted at 60-100°C, and more preferably 70-80°C; the drying time is not particularly limited in the present invention, as long as the natural rubber latex product is obtained through the process well-known in the art. The technical solutions of the present invention will be described below clearly and completely with reference to the examples of the present invention. It is clear that the described examples are only a part of examples of the present invention, not all examples of the present invention. All other examples obtained by persons of ordinary skill in the art based on the example of the present invention without creative efforts shall fall within the scope of the present invention. Example 1 In percent by mass, 2 kg of fresh natural rubber latex (ammonia content: 0.05%; dry rubber content: 31.20%) harvested on the day of harvest was put in a 2.5 L stainless steel reactor; the

A reactor was capped; a thermostatic water bath was adjusted and held at 30°C; after the temperature was constant, CO2 was introduced into the reactor to pressurize to 10 MPa and hold for 5 min; after closing a valve and letting stand for 5 h under pressure, CO 2 was recovered by depressurization, and a rubber lump was obtained. Example 2 In percent by mass, 5 kg of fresh natural rubber latex (ammonia content: 0.1%; dry rubber content: 32.78%) harvested on the day of harvest was diluted with 3.2 kg of water until the dry rubber content thereof was diluted to 20%, followed by putting in a 10 L stainless steel reactor; the reactor was capped; a thermostatic water bath was adjusted and held at 30°C; after the temperature was constant, CO2 was introduced into the reactor to pressurize to 20 MPa and hold for 10 min; after closing a valve and letting stand for 4 h under pressure, C02 was recovered by depressurization, and a rubber lump was obtained. Example 3 In percent by mass, 20 kg of fresh natural rubber latex (ammonia content: 0.08%; dry rubber content: 27.94%) harvested on the day of harvest was diluted with 2.35 kg of water until the dry rubber content thereof was diluted to 25%, followed by putting in a 30 L stainless steel reactor; the reactor was capped; a thermostatic water bath was adjusted and held at 30°C; after the temperature was constant, CO2 was introduced into the reactor to pressurize to 30 MPa and hold for 10 min; after closing a valve and letting stand for 3 h under pressure, CO2 was recovered by depressurization, and a rubber lump was obtained. Comparative Example 1 Natural rubber latex was coagulated by the acid coagulation method: in percent by mass, 5 kg of fresh natural rubber latex (ammonia content: 0.05%; dry rubber content: 31.20%) harvested on the day of harvest was mixed with 2070 mL of water and 730 mL of 2 wt% formic acid solution until a rubber latex coagulation concentration was 20%; the natural rubber latex was allowed to stand in a plastic tray and coagulated for 16 h to obtain a rubber lump. Performance tests The rubber lump prepared in Example 1 was thinned, creped, dehydrated, granulated, and dried in a vacuum drying oven at 70°C successively, and a resulting natural rubber product was labeled as NR-hl. The rubber lump prepared in Example 2 was thinned, creped, dehydrated, granulated, and dried in a vacuum drying oven at 80°C successively, and a resulting natural rubber product was labeled as NR-h2. The rubber lump prepared in Example 3 was thinned, creped, dehydrated, granulated, and dried in a vacuum drying oven at 90°C successively, and a resulting natural rubber product was labeled as NR-h3. The rubber lump prepared in Comparative Example 1 was thinned, creped, dehydrated, granulated, and dried in a vacuum drying oven at 80°C successively, and a resulting natural rubber product was labeled as NR-a. Natural rubber products prepared from the rubber lumps of Examples 1 to 3 and Comparative Example 1 were subjected to performance tests using conventional methods. Tests for rubber vulcanizing properties were conducted in accordance with ACS I Formula of Pure Rubber (natural rubber 100.00 parts, zinc oxide 6.00 parts, sulfur 3.50 parts, stearic acid 0.50 parts, and MBT 0.50 parts) specified in NY/T 1403-2007. Results are shown in Table 1. Table 1 Performance data for natural rubber products prepared in Examples 1 to 3 and Comparative Example 1

Initial Plasticity retention Mooney viscosity Tensile Tear Example plasticity (PO) index (PRI) (ML, 1+4) strengthl/MPa strength/kN-m 1

Example 1 56 96 85 25.16 32.39

Example 2 46 94 80 23.82 29.10

Example 3 51 93 83 24.67 30.89

Comparative Example 1 36 79 75 18.66 26.44

As is apparent from the data in Table 1, the PRI and the tensile strength of the natural rubber products prepared from the rubber lumps of Examples 1 to 3 are obviously superior to those of an acid-coagulated sample of Comparative Example 1; the method for coagulating a natural rubber latex using high-pressure C02 provided by the present invention has no effect on the performance of the natural rubber products. The foregoing descriptions is merely preferred examples of the present invention; it should be noted that various modifications and variations can be made by those skilled in the art without departing from the principles of the present invention and are within the scope of the invention.

Claims (6)

1. What is claimed is: 1. A method for coagulating a natural rubber latex using high-pressure carbon dioxide (C0 2 ), comprising the following steps: under sealed conditions, introducing CO2 until a pressure of 10-30 MPa, and coagulating a natural rubber latex at a constant temperature of 28-31 °C, to obtain a rubber lump, wherein the natural rubber latex has an ammonia content of 0-0.45% and a dry rubber content of>25%, and the coagulation is achieved by standing, and the standing lasts for 1-5 h.
2. 2. The method according to claim 1, wherein the constant temperature is achieved by water bath.
3. 3. The method according to claim 1, wherein the sealed conditions are provided by a reactor.
4. 4. The method according to claim 1, wherein the pressure is held for 5-10 min after the pressure is reached.
5. 5. The method according to any one of claims 1 to 4, wherein, after the rubber lump is obtained, the rubber lump is thinned, creped, dehydrated, granulated, and dried successively to obtain a natural rubber latex product.
6. 6. The method according to claim 5, wherein the drying is conducted at 60-100°C.