Method for preparing light-color natural rubber by complex enzyme hydrolysis method

Abstract

The invention relates to the technical field of rubber and latex processing, in particular to a method for preparing light-color natural rubber by a compound enzyme hydrolysis method. According to the specific technical scheme, the method for preparing the light-color natural rubber by reducing the non-rubber component content of the rubber particles through the complex enzyme hydrolysis method comprises the steps that after natural rubber latex is treated with protease and lipase and / or phospholipase, the light-color natural rubber is obtained through centrifugation, solidification, water washing and drying. The contents of non-rubber components mainly including proteins and lipids in the natural rubber are respectively reduced through enzymatic treatment, and the content of color substances is reduced from the source so as to reduce the occurrence of non-enzymatic browning of the natural rubber.

Description

Technical Field

[0001] The invention relates to the technical field of rubber and latex processing, and in particular to a method for preparing light-colored natural rubber by a composite enzyme hydrolysis method. Background Art

[0002] The natural color produced by natural rubber processing is an important sensory indicator of rubber products, which directly affects people's visual and psychological perception of it, and also affects its application in certain fields. Some types of natural rubber, such as white crepe, are intuitively graded according to the transparency of the dry rubber color (see GB / T 8089), which also determines its economic value. Light-colored standard rubber is the latex-grade standard rubber with the highest technical specifications among process-graded natural rubbers. It is an excellent rubber type for the preparation of medical, health, food and high-end daily rubber products.

[0003] Natural rubber will change color during processing, and its mechanism is mainly due to the fact that phenols and aminophenols in latex combine with oxygen to form quinones under the catalysis of polyphenol oxidase. Quinones react with amino acids and proteins in latex to form melanin-like substances, and melanin continues to oxidize and turn black. However, this is not a key factor affecting the final color of natural rubber. As long as the process regulations are strictly followed and timely and reasonable processing is carried out before the latex becomes corrupt and deteriorates, these will not have a decisive impact on the final color. Because the chemical properties of substances such as polyphenols and polyphenol oxidase are extremely unstable, they are easily destroyed during processing or slightly treated to reduce the occurrence of browning. Many natural color substances, especially non-rubber components, are present in natural rubber particles. These substances are prone to non-enzymatic browning during processing (mainly Maillard reaction involving proteins and lipid oxidation leading to the production of melanin), which is the fundamental reason why the appearance of natural rubber is brown or even black.

[0004] The technical regulations for the production of natural raw rubber light-colored rubber in my country (NY / T 385-2019) recommend that the antioxidants added are sodium pyrophosphite (SMS, ≤0.05%) or tin dioxide (≤0.02%). However, SMS has a strong pungent odor (H 2 S), and will be oxidized into sodium sulfate if left in the air for a long time. In actual production, there are high requirements for storage and use. Hathaichanok Boonyang et al. found that SMS bleaching treatment did not have a significant decolorization effect on NR, while the color index of NaOH-treated rubber or saponified rubber (SP) was the lightest, and the color of urea-treated rubber or deproteinized rubber (DP) was close to that of centrifuged rubber. Studies have shown that high-purity NR prepared by saponification treatment has a light yellow color due to the removal of protein; further use of organic solvent extraction to remove non-rubber components in rubber, especially proteins and lipids, can reduce the yellowness index of NR to a level comparable to that of synthetic polyisoprene.

[0005] Therefore, the current standardized production process of light-colored natural rubber has the disadvantage of relying solely on antioxidant-bleaching agent treatment, which leads to unstable color of rubber products and a quality drop below expectations, and cannot meet the market demand for high-quality light-colored natural rubber raw materials. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a method for preparing light-colored natural rubber by a composite enzyme hydrolysis method, specifically, reducing the content of non-rubber components mainly composed of proteins and lipids in natural rubber through enzymatic treatment, thereby reducing the content of color substances from the source to reduce the occurrence of non-enzymatic browning of natural rubber.

[0007] Protease is a general term for a class of enzymes that hydrolyze protein peptide chains and can break down proteins into smaller polypeptides and amino acids. By adding proteases and surfactants to natural rubber latex, the content of proteins and analogs contained in natural rubber can be effectively reduced, reducing its allergenicity. Usually, proteins and phospholipids are combined with the molecular ends of natural rubber, and the proteins themselves and phospholipids themselves are further combined and associated at their ends to form a high-order branched structure. The lipid substances in natural rubber latex can also be continuously hydrolyzed by ammonia to form long-chain fatty acids, which further react with ammonia water to form fatty acid ammonium, and are adsorbed on the surface of latex particles to stabilize the colloid of latex. Phospholipids, which account for more than 60% of lipid substances, are decomposed at the branching points of this high-order branched structure, so the branching degree will be reduced.

[0008] In the light-colored rubber prepared by enzymatic hydrolysis of natural latex, if the protein and phospholipids forming the above-mentioned side chains are partially decomposed in a limited manner, physical properties (including strength and anti-aging properties) will not be greatly reduced. Therefore, the method of gradually and limitedly removing the protein and lipid substances in the latex by enzymatic hydrolysis can be used to reduce the possibility of browning in the latex processing, thereby producing a light-colored natural rubber product with a low color index. The latex can maintain a stable colloidal state, partly because the rubber particles in the latex are protected by the double electron layer composed of protein. Enzymes can promote the hydrolysis of proteins and lipid substances under certain conditions (temperature, pH value, dosage and substrate concentration), so that the rubber particles lose the protection of the double electron layer and the latex coagulates. Therefore, the correct use of enzymes in the present invention is key, which is to ensure that the protein and lipid substances can be removed to the maximum extent, and to ensure the stability of natural latex, to ensure that it does not coagulate during the enzymatic hydrolysis process, and to meet the technical requirements for further processing of rubber.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0010] The invention discloses a method for preparing light-colored natural rubber by composite enzyme hydrolysis. After natural rubber latex is treated with protease, lipase and / or phospholipase, the light-colored natural rubber is obtained by centrifugation, coagulation, water washing and drying.

[0011] Preferably, the total solid content of the natural rubber latex is 10-30%.

[0012] Preferably, the protease is one or more of alkaline protease, neutral protease, bromelain, papain, trypsin, thermolysin, staphylococcal protease and clostripain;

[0013] The lipase is one or more of phosphatase, sterolase, and hydroxy acid lipase;

[0014] The phospholipase includes one or more of phospholipase A, phospholipase B, phospholipase C and phospholipase D.

[0015] Preferably, based on the total solid content, the amount of the protease added is 0.005 to 1.0 part, and the amount of the lipase and / or phospholipase added is 0.005 to 0.5 part per 100 parts of natural rubber latex.

[0016] Preferably, the enzymatic activity of the protease is ≥100 U / g, and the enzymatic activity of the lipase and / or phospholipase is ≥1000 U / g, respectively.

[0017] Preferably, the natural rubber latex contains a surfactant, and the surfactant is a nonionic surfactant or an anionic surfactant; the amount of the nonionic surfactant and the anionic surfactant is 0.01 to 3% of the mass of the natural rubber latex;

[0018] The nonionic surfactant is any one or more of Tween, Span, alginate, stachyose, glucoside, peregal "O", long-chain fatty alcohol polyoxyethylene ether, alkylphenol polyoxyethylene ether, nonylphenol polyoxyethylene ether, isomeric tridecyl alcohol polyoxyethylene ether and alkyl polyglucoside; the anionic surfactant is a higher fatty acid soap, and the higher fatty acid soap is sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and fatty acid soap.

[0019] Preferably, a pH regulator is added during the process of treating the natural rubber latex with the protease, and the pH regulator includes but is not limited to ammonia, sodium hydroxide, potassium hydroxide, sodium carbonate and sodium bicarbonate.

[0020] Preferably, the temperature of the enzymatic hydrolysis reaction is 40-70°C; the centrifugal speed is 9000-13000 rpm.

[0021] Preferably, the natural rubber latex is treated with protease and lipase and / or phospholipase sequentially or simultaneously, or the natural rubber latex is treated with lipase and / or phospholipase and protease sequentially.

[0022] The present invention includes two enzymatic reaction treatment processes: protein hydrolysis removal and lipid hydrolysis removal. First, the natural rubber latex is treated with protease, and after the latex is treated with hydrolase and purified by a purification process, lipase and / or phospholipase are added to the obtained latex to perform enzymatic reaction, and then the purification treatment is performed again to gradually and orderly reduce the non-rubber substances in the rubber.

[0023] Preferably, after the natural rubber latex is centrifuged, its solid content is adjusted to 25-30%, and its pH is adjusted to 4.0-4.5, and the latex is allowed to stand until the rubber is completely coagulated.

[0024] The present invention has the following beneficial effects:

[0025] 1. The present invention introduces a composite enzyme system comprehensive hydrolysis technology, through which various enzymes are fully contacted with various non-rubber components in natural rubber latex, especially proteins and lipids, and the catalytic properties of different types of enzyme reagents are brought into play to gradually hydrolyze proteins and lipid substances partially, thereby reducing the possibility of non-enzymatic browning of non-rubber components. While maintaining the inherent physical properties of rubber at the critical point of not destroying the stability of natural rubber latex, the non-rubber substances that affect the color index of rubber appearance are removed from the source, thereby solving the problems of unstable chromaticity and quality decline below expectations caused by bleaching with bleaching agents in the current process regulations.

[0026] 2. The present invention utilizes the combined effects of multiple enzymatic reactions to partially decompose or convert proteins and lipids attached to or adsorbed by rubber particles into low molecular weight products, which are then transferred to the aqueous phase. Proteins and phospholipids that form natural rubber side chains are partially decomposed in a limited manner, which does not lead to a significant reduction in physical properties and retains good performance and processing properties.

[0027] 3. The present invention provides a new method for obtaining a light-colored natural rubber standard rubber, namely, fresh natural rubber latex or concentrated natural rubber latex is first treated with alkaline protease, and then treated with at least one of lipase, lysozyme, etc., or treated in the opposite order, or treated simultaneously, so that the non-rubber substance of the natural rubber latex is partially dissolved to be reduced to a certain extent, thereby reducing the possibility of non-enzymatic browning of the non-rubber components, and producing high-quality light-colored natural rubber with excellent performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a photo of the appearance of natural rubber particles after compound enzyme treatment and low-temperature drying. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0031] The present invention discloses a method for preparing light-colored natural rubber by composite enzyme hydrolysis, which comprises the following steps: 1) protein removal treatment: treating the purified natural rubber latex which has been filtered, clarified and impurity-removed with protease;

[0032] 2) Removal of lipid substances: Treat natural rubber latex with lipase and / or phospholipase; 3) Remove non-rubber substances by combining multiple high-speed centrifugation and water washing; 4) Combine hanging dehydration and low-temperature drying to prepare light-colored natural rubber with a low color index. Among them, steps 1) and 2) are not restricted in order, and the two steps can be carried out in any order or simultaneously, specifically: protease and lipase and / or phospholipase can be added to natural rubber latex at the same time for enzymatic treatment. Due to the enzymatic reaction, the lipids and phospholipids attached to or adsorbed by the rubber particles are decomposed or converted into low molecular weight products, which are then transferred to the water phase, and the non-rubber substances are first separated from the latex mixture in the separation step.

[0033] Protease, lipase, phospholipase and other enzyme reagents are highly specific. Lipase and phospholipase are only highly active on ester bonds and have no hydrolysis activity on proteins and sugars. Therefore, the combination of enzymes will not have an adverse effect on their respective activities. Enzyme treatment can be carried out while standing, and appropriate stirring can be performed at intervals to accelerate the enzymatic reaction.

[0034] During the whole enzymolysis reaction, the temperature is controlled at 40-70° C., preferably 50-55° C. Before enzymolysis, the total solid content of the natural rubber latex is diluted to 10-30%, preferably 20-25%. The latex is diluted to a certain concentration in order to increase the probability of sufficient contact and collision between the latex particles and the enzyme reagent, so as to facilitate the subsequent enzymolysis reaction. The concentration of the ammonia water used for dilution can be selected between 0.1% and 2%.

[0035] Furthermore, after the enzymatic hydrolysis is completed, the natural rubber latex is centrifuged at a speed of 9000-13000 rpm, the upper layer is removed, the cream part is separated, and the rubber hydrocarbon is separated from the whey containing more non-rubber substances. Then the rubber hydrocarbon part is dispersed in the same volume of water, a certain amount of surfactant is added, mixed evenly, and centrifuged again. Repeat the above operation two to three times. The surfactant added can be 0.1-1.0 parts, which can ensure that the latex does not coagulate.

[0036] Adjust the solid content of the centrifuged natural rubber latex to 25-30%, then add 5% formic acid or acetic acid to adjust the pH to 4.0-4.5. Let the latex stand for a while to solidify, and completely immerse the solidified latex in hot water at 30-80°C for more than 24 hours; take out and squeeze the rubber sheet with a crepe press or roller and wash it with water to remove various substances, and repeat the above operation three to five times.

[0037] After the cleaned rubber is pressed into a thin crepe, it is either hung up to dry the water or directly dried in an oven at a heating temperature below 70°C to obtain a light-colored solid natural rubber. In order to take into account both efficiency and quality performance, the drying temperature is preferably 45-60°C, and more preferably 45-50°C.

[0038] Furthermore, the natural rubber latex can be fresh (field) natural rubber latex, or it can be concentrated natural rubber latex or commercial centrifugal ammoniation concentrated natural rubber latex. The clarification and impurity removal treatment means that the natural rubber latex must be filtered through a 40-mesh stainless steel filter screen to remove various impurities such as clots, sand and gravel, leaves, etc. that can affect the appearance color, and then stand to clarify and remove finer impurities to obtain purified natural rubber latex.

[0039] Further, the protease is one or more of alkaline protease, neutral protease, bromelain, papain, trypsin, thermolysin, staphylococcal protease and clostripain. For the present invention, the source and type of the enzyme are not critical, as long as it has the corresponding enzymatic hydrolysis activity; the lipase is one or more of phosphatase, sterolase, and hydroxy acid lipase; the phospholipase includes one or more of phospholipase A, phospholipase B, phospholipase C and phospholipase D; the above lipase and / or phospholipase are not limited, and any enzyme from bacteria, fungi and yeast can be used. The enzyme activity of the protease is ≥100U / g, preferably ≥1000U / g, more preferably ≥10000U / g, and more preferably ≥100000U / g. The enzyme activity of the lipase and / or phospholipase is ≥1000U / g, preferably ≥10000U / g, and more preferably ≥100000U / g.

[0040] Further, based on the total solid content, the amount of the protease added is 0.005 to 1.0 parts, preferably 0.1 to 0.5 parts, per 100 parts of natural rubber latex; the natural rubber latex and the protease are stirred evenly and then left to stand. The time for enzymatic treatment of the latex is not subject to specific restrictions and can be determined based on the factors in the treatment: the protein content in the natural rubber latex, the type, activity and amount of the protease. When the amount of protease added is small, the decomposition reaction of the protein is not ideal and a satisfactory effect cannot be achieved. When the amount of protease used is large, excessive protease will increase the production cost, and the activity of the enzyme will also decrease. Almost all of the protein contained in the natural rubber latex is decomposed, the stability of the latex decreases too quickly, and high-speed centrifugation is impossible, which reduces the tensile strength and wear resistance.

[0041] The amount of lipase and / or phospholipase added is 0.005-0.5 parts, preferably 0.01-0.3 parts. After the natural rubber latex and lipase and / or phospholipase are stirred evenly, they are allowed to stand. The time for enzymatic treatment of the latex is not subject to specific restrictions and can be determined according to the factors in the treatment: the content of lipid substances in the natural rubber latex, the type, activity and quantity of lipase and phospholipase, etc. If the addition amount is within the above range, the phospholipids contained in the natural rubber latex will be properly decomposed, and the processing performance and good physical properties will be maintained. If the addition amount (total amount) of lipase and / or phospholipase is less than 0.005 parts by mass, the decomposition reaction of phospholipids is not ideal, and the improvement effect of reducing the color index cannot be fully obtained; if the addition amount (total amount) exceeds 0.5 parts by mass, the fatty acids contained in the natural rubber latex are almost completely decomposed, the stability of the latex decreases too quickly, and high-speed centrifugation is impossible, the crystallinity of the obtained product is reduced, and the tensile strength and wear resistance are reduced.

[0042] Further, the temperature for storing and using natural rubber latex is controlled at -15 to 70°C, and the most suitable temperature is 15 to 30°C. Enzyme reagents such as proteases and lipases are usually active within 40 to 70°C, and the optimal temperature is 50 to 55°C. When the temperature is higher than 65°C, the enzyme reagent itself will rapidly degrade and quickly inactivate, and when the temperature is lower than 40°C, the enzymatic reaction of proteases and lipases is difficult to proceed. The enzyme treatment process of the present invention is carried out at a temperature of 70°C or lower, preferably 60°C or lower. If the enzyme treatment temperature exceeds 70°C, the stability of natural rubber latex will decrease, and it is very easy to coagulate during the enzyme treatment process. Once the latex coagulates, the hydrolysis effect of the enzymatic reaction is greatly reduced, and it is difficult to effectively remove the non-rubber substances of natural rubber. Taking into account factors such as the preservation of natural rubber latex and the optimal use temperature of enzyme reagents, the most suitable temperature for enzymatic hydrolysis is preferably 50 to 55°C.

[0043] Furthermore, in the present invention, natural rubber latex can be treated by the above enzyme treatment method in combination with a surfactant. The surfactant is a nonionic surfactant or an anionic surfactant; the amount of the nonionic surfactant and the anionic surfactant is 0.01-3% of the mass of the natural rubber latex; it can also be a cationic surfactant and an amphoteric surfactant. The role of the surfactant is mainly to use anionic surfactants to enhance the water solubility of protein and phospholipid decomposition products, while increasing the difference in specific gravity between skim and latex, so as to facilitate subsequent centrifugal separation to remove most of the non-rubber components including protein and phospholipids. The present invention does not have any special restrictions on the ratio of different types of surfactants, and they can be added as needed according to the relevant knowledge known in the art to give full play to their beneficial effects.

[0044] The nonionic surfactant is any one or more of Tween, Span, alginate, stachyose, glucoside, peregal "O", long-chain fatty alcohol polyoxyethylene ether, alkylphenol polyoxyethylene ether, nonylphenol polyoxyethylene ether, isomeric tridecyl alcohol polyoxyethylene ether and alkyl polyglucoside; the anionic surfactant is a higher fatty acid soap, and the higher fatty acid soap is sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and fatty acid soap.

[0045] Further, during the protease treatment process, some other additives, such as pH regulators, may be added to enable the enzyme reagent to better exert its effect. Any commonly used pH regulator may be used, including but not limited to ammonia, sodium hydroxide, potassium hydroxide, sodium carbonate and sodium bicarbonate.

[0046] The present invention will be further described below in conjunction with specific embodiments.

[0047] Comparative Example 1

[0048] Take 5kg of concentrated natural rubber latex (total solid content is 62.1%, dry content is 60.3%, the same as in the following examples) and filter it with a 40-mesh stainless steel filter screen to remove various impurities. Without any treatment, add 5% by mass of formic acid or acetic acid to adjust the pH to 4.0-4.5. Let the latex stand until the rubber is completely coagulated, immerse it completely in 70℃ hot water and soak it for 24h (no heating is required in the later stage), then use a roller to squeeze the coagulated rubber component and use water to remove the whey. After that, the rubber sheet is repeatedly washed with water. After being pressed into a thin sheet, hang it up to dry the surface moisture, and dry it in an oven below 70℃ to obtain a solid natural rubber as a comparative sample A.

[0049] Example 1

[0050] 10kg concentrated natural rubber latex is diluted with 0.4% ammonia water to a total solid content of 25%, 1% boric acid is added as a preservative, and various impurities are removed by filtering with a 40-mesh stainless steel filter screen. Then, 0.1 part of protease is added to every 100 parts (in terms of solid content) of natural rubber latex, stirred evenly, and left to stand at room temperature (32°C) for 24 hours, stirring for 10 minutes at intervals of 3 hours. Then, 0.1 part of lipase is added relative to 100 parts by mass of the solid content in the natural rubber latex, and left to stand at room temperature (32°C) for 24 hours, stirring for 10 minutes at intervals of 3 hours. After standing, various impurities are removed by filtering with a 40-mesh stainless steel filter screen, and the latex is centrifuged at 13000rpm for 30 minutes. Then, the latex after centrifugation is dispersed in the same volume of water, an appropriate amount of sodium dodecyl sulfate is added, and the above centrifugation operation is repeated twice. The total solid content of the natural rubber latex is adjusted to 30%, and then 5% by mass of formic acid or acetic acid is added to adjust the pH to 4.0-4.5. The latex is left to stand until the rubber is completely coagulated, and then it is completely immersed in hot water at 70°C for 24 hours (no need to heat it later), and then the coagulated rubber component is squeezed with a roller and the whey is removed with water. After that, the rubber sheet is repeatedly washed with water. After being pressed into a thin sheet, it is hung up to dry the surface moisture, and dried in an oven below 70°C to obtain a light-colored solid natural rubber B.

[0051] Example 2

[0052] 10kg concentrated natural rubber latex is diluted with 0.4% ammonia water to a total solid content of 25%, 1% boric acid is added as a preservative, and various impurities are removed by filtering with a 40-mesh stainless steel filter screen. Then, 0.1 parts of protease and 0.2 parts of lipase are added to every 100 parts (in terms of solid content) of natural latex, and stirred evenly in a water bath (45°C) for 24 hours, with a stirring interval of 3 hours for 10 minutes. After standing, various impurities are removed by filtering with a 40-mesh stainless steel filter screen, and the latex is centrifuged at 13000rpm for 30 minutes. Then, the centrifuged latex is dispersed in the same volume of water, an appropriate amount of sodium dodecyl sulfate is added, and the above centrifugation operation is repeated twice. The total solid content of the natural rubber latex is adjusted to 30%, and then 5% by mass of formic acid or acetic acid is added to adjust the pH to 4.0-4.5. The latex is allowed to stand until the rubber is completely coagulated, and it is completely immersed in 70°C hot water and soaked for 24 hours (no heating is required in the later stage), and then the coagulated rubber component is squeezed with a roller and the whey is removed with water. After that, the rubber sheet was repeatedly washed with water, pressed into a thin sheet, hung up to dry the surface moisture, and dried in an oven below 70°C to obtain a light-colored solid natural rubber C.

[0053] Example 3

[0054] 10kg concentrated natural rubber latex is added with 0.4% ammonia water by weight of latex, and various impurities are removed by filtering with a 40-mesh stainless steel filter screen. Then, 0.1 part of sodium dodecyl sulfate, 0.2 part of protease and 0.1 part of lipase are added to each 100 parts (in terms of solid content) of natural rubber latex, and stirred evenly at room temperature (32°C) for 24 hours, with a stirring interval of 3 hours for 10 minutes. After standing, various clots are removed by filtering with a 40-mesh stainless steel filter screen, and the latex is centrifuged at 13000rpm for 30 minutes to remove the whey containing more non-rubber substances. Then the latex after centrifugation is dispersed in the same volume of water, and an appropriate amount of surfactant sodium dodecyl sulfate is added. The above centrifugation operation is repeated two to three times. The solid content of the modified natural rubber latex is adjusted to 30%, and then 5% formic acid or acetic acid is added to adjust the pH to 4.0-4.5. The latex is left to stand until the rubber is completely coagulated, and then it is completely immersed in hot water at 70°C for 24 hours (no heating is required in the later stage), and then the coagulated rubber component is squeezed with a roller and the whey and non-rubber components are removed with water. After that, the rubber component is repeatedly washed with water. After being pressed into a thin sheet, it is hung up to dry the surface moisture, and dried in an oven below 70°C to obtain a light-colored solid natural rubber D.

[0055] Example 4

[0056] 10kg concentrated natural rubber latex is diluted with 0.4% ammonia water to a total solid content of 25%, 1% boric acid is added as a preservative, and various impurities are removed by filtering with a 40-mesh stainless steel filter screen. Then, 0.2 parts of protease and 0.2 parts of lipase are added to every 100 parts (in terms of solid content) of natural rubber latex, stirred evenly, and left to stand at room temperature (30°C) for 24 hours, with a stirring interval of 3 hours for 10 minutes. After standing, various impurities are removed by filtering with a 40-mesh stainless steel filter screen, and the latex is centrifuged at 13000rpm for 30 minutes. Then, the centrifuged latex is dispersed in the same volume of water, an appropriate amount of sodium dodecyl sulfate is added, and the above centrifugation operation is repeated twice. The total solid content of the natural rubber latex is adjusted to 30%, and then 5% by mass of formic acid or acetic acid is added to adjust the pH to 4.0-4.5. The latex is left to stand until the rubber is completely coagulated, and it is completely immersed in 70°C hot water and soaked for 24 hours (no heating is required in the later stage), and then the coagulated rubber component is squeezed with a roller and the whey is removed with water. After that, the rubber sheet was repeatedly washed with water, pressed into a thin sheet, hung up to dry the surface moisture, and dried in an oven below 70°C to obtain a light-colored solid natural rubber E.

[0057] Example 5

[0058] 10kg concentrated natural rubber latex is diluted with 0.4% ammonia water to a total solid content of 25%, 1% boric acid is added as a preservative, and various impurities are removed by filtering with a 40-mesh stainless steel filter screen. Then, 0.3 parts of protease and 0.3 parts of lipase are added to every 100 parts (in terms of solid content) of natural rubber latex, stirred evenly, and left to stand at room temperature (30°C) for 24 hours, with a stirring interval of 3 hours for 10 minutes. After standing, various impurities are removed by filtering with a 40-mesh stainless steel filter screen, and the latex is centrifuged at 13000rpm for 30 minutes. Then, the centrifuged latex is dispersed in the same volume of water, an appropriate amount of sodium dodecyl sulfate is added, and the above centrifugation operation is repeated twice. The total solid content of natural latex is adjusted to 30%, and then 5% by mass of formic acid or acetic acid is added to adjust the pH to 4.0-4.5. The latex is left to stand until the rubber is completely coagulated, and it is completely immersed in 70°C hot water and soaked for 24 hours (no heating is required in the later stage), and then the coagulated rubber component is squeezed with a roller and the whey is removed with water. After that, the rubber sheet was repeatedly washed with water, pressed into a thin sheet, hung up to dry the surface moisture, and dried in an oven below 70°C to obtain a light-colored solid natural rubber F.

[0059] Example 6

[0060] 10kg concentrated natural rubber latex is diluted with 0.4% ammonia water to a total solid content of 25%, 1% boric acid is added as a preservative, and various impurities are removed by filtering with a 40-mesh stainless steel filter screen. Then, 0.5 parts of protease and 0.5 parts of lipase are added to every 100 parts (in terms of solid content) of natural rubber latex, stirred evenly, and left to stand at room temperature (30°C) for 24 hours, with a stirring interval of 3 hours for 10 minutes. After standing, various impurities are removed by filtering with a 40-mesh stainless steel filter screen, and the latex is centrifuged at 13000rpm for 30 minutes. Then, the centrifuged latex is dispersed in the same volume of water, an appropriate amount of sodium dodecyl sulfate is added, and the above centrifugation operation is repeated twice. The total solid content of natural latex is adjusted to 30%, and then 5% by mass of formic acid or acetic acid is added to adjust the pH to 4.0-4.5. The latex is left to stand until the rubber is completely coagulated, and it is completely immersed in 70°C hot water and soaked for 24 hours (no heating is required in the later stage), and then the coagulated rubber component is squeezed with a roller and the whey is removed with water. After that, the rubber sheet was repeatedly washed with water, pressed into a thin sheet, hung up to dry the surface moisture, and dried in an oven below 70°C to obtain a light-colored solid natural rubber G.

[0061] Example 7

[0062] 10 kg of fresh natural rubber latex (solid content of 31.4%) was added with 1% boric acid by weight of the latex, and various impurities were filtered out with a 40-mesh stainless steel filter screen. Then, 1 part of sodium lauryl sulfate, 0.3 parts of protease and 0.3 parts of chlorinated paraffin were added to every 100 parts (based on solid content) of natural rubber latex.

[0063] Lipase, stir evenly and stand at room temperature (32°C) for 24 hours, stirring for 10 minutes at intervals of 3 hours. After standing, filter with a 40-mesh stainless steel filter to remove various clots, centrifuge the latex at 13,000 rpm for 30 minutes, and remove the whey containing more non-glue substances. Then disperse the centrifuged latex in the same volume of water, add an appropriate amount of surfactant sodium dodecyl sulfate, and repeat the above centrifugation operation two to three times. Adjust the solid content of the modified natural latex to 30%, and then add 5% formic acid or acetic acid to adjust the pH to 4.0-4.5. Let the latex stand until the rubber is completely coagulated, immerse it completely in 70°C hot water and soak it for 24 hours (no heating is required in the later stage), then squeeze the coagulated rubber component with a roller and remove the whey and non-glue components with water. After that, the rubber component is repeatedly washed with water. After being pressed into a thin sheet, hang it up to dry the surface moisture, and dry it in an oven below 70°C to obtain a light-colored solid natural rubber H.

[0064] Performance test: The residual protein in natural rubber is obtained by analyzing its nitrogen content using the Kjeldahl method; the determination of lipid content is based on the characteristic that natural rubber cannot be dissolved in isopropanol but can be dissolved in n-hexane. The natural rubber is dissolved in a composite solvent of n-hexane and isopropanol to extract lipids in NR, and the lipid content is calculated based on the change in the rubber mass before and after the extraction of the two. The color index is determined according to GB / T 14796-2023, and the plasticity value is tested using a rapid plastic meter in accordance with GB / T 3510-2023. Its processing performance is determined by a Mooney viscometer (GOTECH Mooney viscometer MV-3000-A) to obtain its Mooney viscosity, Mooney relaxation time, torque and other parameters. The performance results of light-colored natural rubber obtained by different treatment processes are shown in Table 1.

[0065] Table 1 Properties of light-colored natural rubber obtained by different compound enzyme treatment processes

[0066]

[0067]

[0068] The above results show that with the increase of the amount of protease and lipase added and the subsequent centrifugal purification process, the protein and lipid content on the surface of the rubber particles gradually decreased, and its color index also gradually decreased. The sample showed a gradual change from brown to light yellow and light color, such as Figure 1As shown. Due to the removal of the outer protective layer of latex particles - protein and lipid, the thermal stability of natural rubber decreases, showing that the initial value of plasticity, PRI and Mooney viscosity have decreased to varying degrees, and the corresponding weight-average molecular weight has also decreased to varying degrees. However, from the perspective that the Mooney relaxation time (the time required for the Mooney to drop by 80% from 1 second after the motor stops, reflecting the interaction and viscoelastic behavior between rubber molecular chains) is basically the same, its internal viscoelasticity has not changed significantly, and the processing performance has not changed. Therefore, the appearance chromaticity of the light-colored standard rubber prepared by the present invention becomes significantly lighter, while maintaining the original processing performance.

[0069] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing light-colored natural rubber by a composite enzyme hydrolysis method, characterized in that: After the natural rubber latex is treated with protease, lipase and / or phospholipase, the light-colored natural rubber is obtained through centrifugation, coagulation, water washing and drying.

2. The preparation method according to claim 1, characterized in that: The total solid content of the natural rubber latex is 10-30%.

3. The preparation method according to claim 1, characterized in that: The protease is one or more of alkaline protease, neutral protease, bromelain, papain, trypsin, thermolysin, staphylococcal protease and clostripain; The lipase is one or more of phosphatase, sterolase, and hydroxy acid lipase; The phospholipase includes one or more of phospholipase A, phospholipase B, phospholipase C and phospholipase D.

4. The preparation method according to claim 1, characterized in that: Calculated on the basis of total solid content, the amount of the protease added is 0.005 to 1.0 part, and the amount of the lipase and / or phospholipase added is 0.005 to 0.5 part per 100 parts of natural rubber latex.

5. The preparation method according to claim 1, characterized in that: The enzyme activity of the protease is ≥100 U / g, and the enzyme activity of the lipase and / or phospholipase is respectively ≥1000 U / g.

6. The preparation method according to claim 1, characterized in that: The natural rubber latex contains a surfactant, which is a nonionic surfactant or an anionic surfactant; the amount of the nonionic surfactant and the anionic surfactant is 0.01 to 3% of the mass of the natural rubber latex; The nonionic surfactant is any one or more of Tween, Span, alginate, stachyose, glucoside, peregal "O", long-chain fatty alcohol polyoxyethylene ether, alkylphenol polyoxyethylene ether, nonylphenol polyoxyethylene ether, isomeric tridecyl alcohol polyoxyethylene ether and alkyl polyglucoside; the anionic surfactant is a higher fatty acid soap, and the higher fatty acid soap is sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and fatty acid soap.

7. The preparation method according to claim 1, characterized in that: During the process of treating the natural rubber latex with the protease, a pH regulator is added, and the pH regulator includes but is not limited to ammonia, sodium hydroxide, potassium hydroxide, sodium carbonate and sodium bicarbonate.

8. The preparation method according to claim 1, characterized in that: The temperature of the enzymatic hydrolysis reaction is 40-70°C; the rotation speed of the centrifuge is 9000-13000 rpm.

9. The preparation method according to claim 1, characterized in that: The natural rubber latex is treated with protease and lipase and / or phospholipase sequentially or simultaneously, or the natural rubber latex is treated with lipase and / or phospholipase and protease sequentially.

10. The preparation method according to claim 1, characterized in that: After the natural rubber latex is centrifuged, its solid content is adjusted to 25-30%, and its pH is adjusted to 4.0-4.5 to make it coagulate.

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