Low-protein vulcanized natural latex, preparation method and application thereof, vulcanized rubber film and application thereof

Abstract

The invention provides low-protein vulcanized natural latex, a preparation method and application thereof, a vulcanized rubber film and application thereof, and relates to the technical field of latex. The preparation method comprises the following steps: firstly, mixing natural latex, a surfactant, a stabilizer, protease and a vulcanizing agent, and simultaneously carrying out enzymolysis reaction-vulcanization; or mixing the natural latex, the surfactant, the stabilizer and the protease, carrying out enzymolysis reaction, and adding the vulcanizing agent for vulcanizing; and then carrying out centrifugal treatment to obtain the low-protein vulcanized natural latex. Protein is hydrolyzed by adding protease, and the surfactant is added, so that hydrolysis of protein can be promoted while natural latex is stabilized; by adding the strong alkaline stabilizer, the effect of stabilizing the latex is achieved. According to the preparation method provided by the invention, centrifugal treatment is performed after vulcanization is completed, protein can be removed, the unreacted vulcanizing agent can be removed, the obtained low-protein vulcanized natural latex cannot be continuously vulcanized in the storage process, and the performance stability of the low-protein vulcanized natural latex is good.

Description

Technical Field

[0001] The present invention relates to the technical field of latex, and in particular to a low-protein vulcanized natural rubber latex, a preparation method and application thereof, a vulcanized rubber film and application thereof. Background Art

[0002] Natural rubber (NR), a biosynthesized natural macromolecular compound, contains 6-8% by weight of non-rubber substances, with protein being the primary component. Fresh natural rubber latex has a protein content of approximately 1-2% by weight, while concentrated natural rubber latex prepared through a centrifugal process has a protein content of 1.5-3.5%. With the widespread use of latex products, allergic reactions caused by protein in latex have become a major concern for medical natural rubber latex products. Research has shown that water-soluble protein in natural rubber latex is the primary cause of allergies. It is generally accepted internationally that when the water-soluble protein content of latex is less than 0.0001%, individuals allergic to latex exhibit little to no allergic reaction in skin irritation tests. However, most countries require a water-soluble protein content of less than 0.0005. Consequently, the US Food and Drug Administration (FDA) issued a 1999 advisory to the medical glove industry, stipulating that the soluble protein content in each pair of medical gloves should be less than 1200 μg. How to reduce the content of water-soluble protein in natural rubber latex and natural rubber latex products has become the focus of latex product manufacturers around the world.

[0003] The deproteinization methods for natural rubber latex mainly include protease treatment, urea denaturation, and multiple centrifugation. Protease treatment is the most mature technology for hydrolyzing proteins into polypeptides and amino acids under relatively mild conditions. The raw materials for the protease treatment method can be fresh natural latex and concentrated natural latex, and it is most economical to use fresh latex as the raw material. The deproteinized natural latex obtained after purification by multiple centrifugation has a lower nitrogen content, but its process is more complicated and has a low yield. The vast majority of low-protein concentrated natural latex products on the market are processed by secondary centrifugation of concentrated latex, which is costly and unfavorable for widespread use.

[0004] Vulcanized natural rubber latex is a latex product made by adding vulcanizing agents and other additives to concentrated natural rubber latex, causing it to undergo a partial or complete vulcanization reaction. Compared to unvulcanized concentrated natural rubber latex, vulcanized natural rubber latex offers improved processing performance and product quality stability, playing an important role in the medical field and can be used to manufacture condoms, gloves, catheters, and other products. However, due to the mobility of the cross-linked structure of vulcanized natural rubber latex, residual vulcanizing agents can still trigger the vulcanization reaction, causing the vulcanized latex to become increasingly vulcanized during storage, affecting its shelf life. Summary of the Invention

[0005] In view of this, the present invention aims to provide a low-protein vulcanized natural rubber latex, a preparation method thereof, and applications thereof. The preparation method of the present invention is simple, and the prepared low-protein vulcanized natural rubber latex has a long pot life, good process stability, and excellent overall performance, which can meet the needs of downstream product manufacturers for deproteinized natural rubber latex.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing low-protein vulcanized natural rubber latex, comprising method 1 or method 2;

[0008] The method 1 comprises the following steps: mixing natural rubber latex, a surfactant, a stabilizer, a protease and a vulcanizing agent, performing an enzymatic hydrolysis reaction-vulcanization and then centrifuging to obtain low-protein vulcanized natural rubber latex;

[0009] The method 2 comprises the following steps: mixing natural rubber latex, a surfactant, a stabilizer and a protease, performing an enzymatic hydrolysis reaction, adding a vulcanizing agent for vulcanization and then performing centrifugal treatment to obtain low-protein vulcanized natural rubber latex.

[0010] Preferably, the protease includes one or more of alkaline protease, papain, trypsin and bromelain; the amount of the protease is 20,000 to 200,000 u / kg of dry glue.

[0011] Preferably, the surfactant includes anionic surfactant and / or nonionic surfactant;

[0012] The mass of the surfactant is 0.03-3% of the dry weight of natural rubber latex.

[0013] Preferably, the anionic surfactant includes one or more of a carboxylic acid surfactant, a sulfonic acid surfactant, a sulfate surfactant, and a phosphate surfactant;

[0014] The nonionic surfactant includes one or more of polyoxyalkylene ether surfactants, polyoxyalkylene ester surfactants, polyol fatty acid ester surfactants and sugar fatty acid ester surfactants.

[0015] Preferably, the strong alkaline stabilizer comprises an alkali metal hydroxide;

[0016] The mass of the strong alkaline stabilizer is 0.05-0.3% of the dry weight of the natural rubber latex.

[0017] Preferably, the mass of the vulcanizing agent is 0.5 to 5% of the dry weight of natural rubber latex;

[0018] The vulcanizing agent includes sulfur, zinc oxide and dithiocarbamoyl vulcanization accelerator; the mass of the sulfur is 0.3-2% of the dry weight of natural rubber latex; the mass of the zinc oxide is 0.1-1% of the dry weight of natural rubber latex; the mass of the dithiocarbamoyl vulcanization accelerator is 0.1-2% of the dry weight of natural rubber latex.

[0019] Preferably, the temperatures of the enzymatic hydrolysis reaction-sulfurization, the enzymatic hydrolysis reaction and the sulfurization are independently 20-55°C.

[0020] The present invention also provides a low-protein vulcanized natural rubber latex prepared by the preparation method described in the above technical solution, wherein the nitrogen content of the low-protein vulcanized natural rubber latex is ≤0.1wt% and the water-extracted protein content is ≤35μg / g.

[0021] The present invention also provides a vulcanized rubber film, which is obtained by forming a film from the low-protein vulcanized natural rubber latex described in the above technical solution.

[0022] The present invention also provides the use of the low-protein vulcanized natural rubber latex described in the above technical solution or the vulcanized rubber film described in the above technical solution in gloves, balloons, condoms, urinary catheters, rubber hoses / rubber threads or sports equipment.

[0023] The present invention comprises mixing natural rubber latex, a surfactant, a stabilizer, a protease, and a vulcanizing agent, performing an enzymatic hydrolysis reaction, performing vulcanization, and then performing centrifugation to obtain a low-protein vulcanized natural rubber latex; or mixing natural rubber latex, a surfactant, a stabilizer, and a protease, performing an enzymatic hydrolysis reaction, adding a vulcanizing agent, performing vulcanization, and then performing centrifugation to obtain a low-protein vulcanized natural rubber latex. Protein in the latex can stabilize the latex and prevent the latex from agglomerating and coagulating. After removing the protein, the protective layer of the latex particles is destroyed. The present invention hydrolyzes the protein by adding a protease, and by adding a surfactant, it can stabilize the natural latex while promoting the hydrolysis of the protein. The addition of a strongly alkaline stabilizer stabilizes the latex.

[0024] The preparation method provided by the present invention is subjected to centrifugal treatment after vulcanization is completed, which can not only remove protein but also remove unreacted vulcanizing agent. The obtained low-protein vulcanized natural rubber latex will not continue to vulcanize during storage. The low-protein vulcanized natural rubber latex has good performance stability and excellent comprehensive performance. The nitrogen content of the low-protein vulcanized natural rubber latex prepared by the present invention is below 0.1wt%, which meets the industry standard of NY / T1813-2009, and the water-extracted protein content is ≤35μg / g, which greatly reduces the allergic reaction of the latex product. Moreover, the preparation method provided by the present invention has simple process, simple operation, low energy consumption, low production cost, and is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1This is the tensile stress-strain curve of the vulcanized rubber film prepared in Test Example 1. DETAILED DESCRIPTION

[0026] The invention provides a preparation method of low-protein vulcanized natural rubber latex, which includes method 1 or method 2. Method 1 includes the following steps: mixing natural latex, a surfactant, a stabilizer, a protease and a vulcanizing agent, performing an enzymatic hydrolysis reaction, performing vulcanization, and then performing centrifugal treatment to obtain low-protein vulcanized natural rubber latex; Method 2 includes the following steps: mixing natural latex, a surfactant, a stabilizer and a protease, performing an enzymatic hydrolysis reaction, adding a vulcanizing agent, performing vulcanization, and then performing centrifugal treatment to obtain low-protein vulcanized natural rubber latex.

[0027] Unless otherwise specified, the materials and equipment used in the present invention are all commercially available products in the art.

[0028] In the present invention, the natural latex is preferably fresh natural latex, and the preservation system of the natural latex is preferably an ammonia preservation system.

[0029] In the present invention, the protease preferably comprises one or more of alkaline protease, papain, trypsin and bromelain; the protease is preferably used in the form of a protease aqueous solution, the concentration of which is preferably 5-10wt%, and in specific embodiments can be 5wt%, 6wt%, 7wt%, 8wt%, 9wt% or 10wt%. In the present invention, the mass (dry weight) of the protease is selected from 0.02-2% of the dry weight of the natural rubber latex, more preferably 0.1-0.5%, and in specific embodiments can be 0.02%, 0.05%, 0.1%, 0.2%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8% or 2%. In the present invention, the amount of the protease is preferably 20,000 to 200,000 U / kg of dry gelatin, more preferably 100,000 to 200,000 U / kg of dry gelatin, and in specific embodiments, it can be 20,000 U / kg of dry gelatin, 50,000 U / kg of dry gelatin, 80,000 U / kg of dry gelatin, 100,000 U / kg of dry gelatin, 120,000 U / kg of dry gelatin, 150,000 U / kg of dry gelatin, 180,000 U / kg of dry gelatin, or 200,000 U / kg of dry gelatin. The present invention hydrolyzes protein by adding protease.

[0030] In the present invention, the surfactant preferably includes an anionic surfactant and / or a nonionic surfactant. In the present invention, the anionic surfactant preferably includes one or more of a carboxylic acid surfactant, a sulfonic acid surfactant, a sulfate surfactant and a phosphate surfactant, and may specifically include sodium lauryl sulfate (SDS) and / or sodium dodecylbenzenesulfonate (SDBS). In the present invention, the nonionic surfactant preferably includes one or more of a polyoxyalkylene ether surfactant, a polyoxyalkylene ester surfactant, a polyol fatty acid ester surfactant and a sugar fatty acid ester surfactant, and may specifically include Tween 20 and / or peregal O. In the present invention, the surfactant is preferably used in the form of an aqueous surfactant solution, and the concentration of the aqueous surfactant solution is preferably 5 to 30 wt%, and in specific embodiments, it can be 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt% or 30 wt%. In the present invention, the mass (dry weight) of the surfactant is selected to be 0.03-3% of the dry weight of the natural rubber latex, more preferably 0.1-1%. In specific embodiments, it can be 0.03%, 0.05%, 0.1%, 0.2%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8% or 3%. Since protein can stabilize the latex in latex and prevent the latex from agglomerating and coagulating, and the protective layer of the latex particles is destroyed after the protein is removed, the present invention can promote the hydrolysis of protein while stabilizing the natural rubber latex by adding a surfactant. The present invention plays the role of stabilizing the latex by adding a strong alkaline stabilizer.

[0031] In the present invention, the strong alkaline stabilizer preferably comprises an alkali metal hydroxide, and in specific embodiments, it can be potassium hydroxide and / or sodium hydroxide. The strong alkaline stabilizer is preferably used in the form of an aqueous solution of the strong alkaline stabilizer, and the concentration of the aqueous solution of the strong alkaline stabilizer is preferably 10-30wt%, and in specific embodiments, it can be 10wt%, 15wt%, 20wt%, 25wt% or 30wt%. In the present invention, the mass (dry weight) of the strong alkaline stabilizer is selected to be 0.05-0.3% of the dry weight of the natural rubber latex, more preferably 0.1-0.2%, and in specific embodiments, it can be 0.05%, 0.08%, 0.1%, 0.15%, 0.2%, 0.25% or 0.3%.

[0032] In the present invention, the mass (dry weight) of the vulcanizing agent is selected to be 0.5-5% of the dry weight of the natural rubber latex, and in specific embodiments, it can be 0.5%, 0.8%, 1%, 1.02%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%. In the present invention, the vulcanizing agent preferably includes sulfur, zinc oxide and a dithiocarbamoyl vulcanization accelerator; and the dithiocarbamoyl vulcanization accelerator preferably includes one or more of zinc diisobutyldithiocarbamate (ZDBC), zinc diethyldithiocarbamate (ZDEC), zinc dibenzyldithiocarbamate, zinc dialkyldithiophosphate and diallyl sulfide. In the present invention, the mass (dry weight) of the sulfur is selected to be 0.3-2% of the dry weight of the natural rubber latex, and in specific embodiments, it can be 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, or 2%. In the present invention, the mass (dry weight) of the zinc oxide is selected to be 0.1-1% of the dry weight of the natural rubber latex, and in specific embodiments, it can be 0.1%, 0.17%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%. In the present invention, the mass (dry weight) of the dithiocarbamoyl vulcanization accelerator is selected to be 0.1-2% of the dry weight of the natural rubber latex, and in specific embodiments, it can be 0.1%, 0.2%, 0.3%, 0.35%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, or 2%.

[0033] In the present invention, the vulcanizing agent is preferably used in the form of a vulcanizing agent suspension, and the vulcanizing agent suspension is preferably a commercial vulcanizing agent suspension emulsion or a homemade vulcanizing agent suspension. In the present invention, the concentration of the vulcanizing agent suspension is preferably 10 to 60 wt%, and in specific embodiments can be 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt% or 60 wt%.

[0034] In the present invention, the self-made vulcanizing agent suspension preferably includes a vulcanizing agent, a dispersant, aqueous ammonia and water; the dispersant preferably includes a diffusant NF and / or a casein aqueous solution; the concentration of the casein aqueous solution is preferably 5-25wt%, and in specific embodiments, it can be 5wt%, 8wt%, 10wt%, 11.5wt%, 15wt%, 18wt%, 20wt%, 22wt% or 25wt%; the concentration of the aqueous ammonia is preferably 1.5-25wt%, and in specific embodiments, it can be 1.5wt%, 3wt%, 5wt%, 8wt%, 10wt%, 12wt%, 15wt%, 18wt%, 20wt%, 22wt% or 25wt%. In the present invention, the preparation method of the vulcanizing agent suspension preferably includes the following steps: mixing the vulcanizing agent, the dispersant, aqueous ammonia and water to obtain the vulcanizing agent suspension. In the present invention, the mixing is preferably grinding, and the grinding preferably includes ball milling and / or sand milling; the ball milling time is preferably 48 to 60 hours, and in specific embodiments it can be 48 hours, 50 hours, 52 hours, 55 hours, 58 hours or 60 hours; the sand milling time is preferably 10 to 20 hours, and in specific embodiments it can be 10 hours, 12 hours, 15 hours, 18 hours or 20 hours.

[0035] In the present invention, the method 1 comprises the following steps: mixing natural rubber latex, a surfactant, a stabilizer, a protease and a vulcanizing agent, performing an enzymatic hydrolysis reaction-vulcanization and then centrifuging to obtain low-protein vulcanized natural rubber latex.

[0036] In the present invention, the temperature of the enzymatic reaction-sulfurization is preferably 20 to 55°C, more preferably 40 to 50°C, and can be specifically 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C or 55°C; the time of the enzymatic reaction-sulfurization is preferably 6 to 48h; the time of the enzymatic reaction-sulfurization is preferably adjusted according to the temperature of the enzymatic reaction-sulfurization. Specifically, when the temperature of the enzymatic reaction-sulfurization is room temperature (20 to 30°C), the time of the enzymatic reaction-sulfurization is 24 to 72h (can be specifically 24h, 36h, 48h, 60h). h or 72h); when the temperature of the enzymatic reaction-sulfurization is 30-45°C and not 30°C (specifically 31°C, 35°C, 40°C or 45°C), the time of the enzymatic reaction-sulfurization is 12-22h (specifically 12h, 14h, 16h, 18h, 20h or 22h); when the temperature of the enzymatic reaction-sulfurization is 45-55°C and not 45°C (specifically 46°C, 48°C, 50°C, 52°C or 55°C), the time of the enzymatic reaction-sulfurization is 6-10h (specifically 6h, 7h, 8h, 9h or 10h).

[0037] In the present invention, the vulcanization degree of the vulcanized latex obtained by the enzymatic hydrolysis reaction-vulcanization is preferably 2:3. In the present invention, the vulcanization degree is preferably tested once every 30 minutes using a chloroform gel state method, and vulcanization is stopped when the vulcanization degree is 2:3.

[0038] In the present invention, the centrifugal treatment is preferably performed by centrifuging the sample at room temperature using an industrial disc centrifuge. The present invention removes protein by centrifugation.

[0039] In the present invention, the method 2 comprises the following steps: mixing natural rubber latex, a surfactant, a stabilizer and a protease, performing an enzymatic hydrolysis reaction, adding a vulcanizing agent for vulcanization and then centrifuging to obtain low-protein vulcanized natural rubber latex.

[0040] In the present invention, the temperature of the enzymatic hydrolysis reaction is preferably 20 to 55°C, more preferably 40 to 50°C, and in specific embodiments can be 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C or 55°C; the time of the enzymatic hydrolysis reaction is preferably 6 to 48 hours; the time of the enzymatic hydrolysis reaction is preferably adjusted according to the temperature of the enzymatic hydrolysis reaction. Specifically, when the temperature of the enzymatic hydrolysis reaction is room temperature (20 to 30°C), the time of the enzymatic hydrolysis reaction is 24 to 72 hours (specifically 24 hours, 36 hours, 48 ​​hours, 60 hours, etc.). h or 72h); when the temperature of the enzymatic hydrolysis reaction is 30-45°C and not 30°C (specifically 31°C, 35°C, 40°C or 45°C), the enzymatic hydrolysis reaction time is 12-22h (specifically 12h, 14h, 16h, 18h, 20h or 22h); when the temperature of the enzymatic hydrolysis reaction is 45-55°C and not 45°C (specifically 46°C, 48°C, 50°C, 52°C or 55°C), the enzymatic hydrolysis reaction time is 6-10h (specifically 6h, 7h, 8h, 9h or 10h).

[0041] In the present invention, the vulcanization temperature is preferably 20-55°C, more preferably 40-50°C, and in specific embodiments can be 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, or 55°C. In the present invention, the vulcanization degree of the vulcanized latex obtained by the vulcanization is preferably 2:3. In the present invention, the vulcanization degree is preferably tested every 30 minutes using a chloroform gel state method, and the vulcanization is stopped when the vulcanization degree is 2:3.

[0042] In the present invention, the centrifugal treatment is preferably performed by centrifuging the sample at room temperature using an industrial disc centrifuge. The present invention removes protein by centrifugation.

[0043] The present invention removes protein from natural rubber latex by protease hydrolysis, and simultaneously adds a vulcanization aid to vulcanize the latex. The protein removal and vulcanization are carried out simultaneously. When a certain degree of vulcanization is reached, the vulcanized latex is centrifuged once, which not only removes protein but also removes unreacted vulcanizing agent. The resulting low-protein vulcanized natural rubber latex does not continue to vulcanize during storage, and the low-protein vulcanized natural rubber latex has good performance stability and excellent comprehensive properties. The preparation method provided by the present invention has a simple process, saves energy consumption, has low production cost, and is suitable for industrial production.

[0044] The present invention also provides a low-protein vulcanized natural rubber latex prepared by the preparation method described in the above technical solution, wherein the nitrogen content of the low-protein vulcanized natural rubber latex is ≤0.1wt% and the water-extracted protein content is ≤35μg / g.

[0045] In the present invention, the low-protein vulcanized natural rubber latex is preferably stored in a mixture of aqueous ammonia and lauric acid. In the present invention, the concentration of the aqueous ammonia is preferably 10 to 25 wt%, and in specific embodiments, it can be 10 wt%, 11.5 wt%, 15 wt%, 18 wt%, 20 wt%, 22 wt%, or 25 wt%. The amount of aqueous ammonia added is preferably 0.4 to 0.8% of the mass of the natural rubber latex, and in specific embodiments, it can be 0.7%. The amount of lauric acid added is preferably 0.02 to 0.08% of the mass of the natural rubber latex, and in specific embodiments, it can be 0.05%. Low-protein vulcanized natural rubber latex quickly coagulates after centrifugation. By adding aqueous ammonia and lauric acid, the present invention can prevent coagulation of the low-protein vulcanized natural rubber latex and improve its mechanical stability.

[0046] The present invention also provides a vulcanized rubber film, which is formed by forming a film of the low-protein vulcanized natural rubber latex described in the above technical solution. The present invention has no particular limitation on the film-forming method, and any film-forming method well known to those skilled in the art can be used.

[0047] The present invention also provides the use of the low-protein vulcanized natural rubber latex described in the above technical solution or the vulcanized rubber film described in the above technical solution in gloves, balloons, condoms, urinary catheters, rubber hoses, rubber threads or sports equipment.

[0048] To further illustrate the present invention, the low-protein vulcanized natural rubber latex provided by the present invention, its preparation method and application are described in detail below with reference to the examples, but they should not be construed as limiting the scope of protection of the present invention.

[0049] In the following examples and comparative examples, fresh natural rubber latex preserved in high ammonia was taken, its dry rubber content was detected, and then deionized water was added to dilute it to a dry rubber content of 30% by mass to obtain a fresh natural rubber latex aqueous suspension.

[0050] The chloroform gel state method was used to test the degree of vulcanization according to the literature (Li Zhijun. Analysis and Testing of Natural Rubber: China Agricultural University Press; 2007).

[0051] Preparation method of 50% sulfur suspension: add 100g sulfur powder, 40mL of 10wt% casein aqueous solution, 2.5mL of 11.5wt% ammonia water, and 57.5mL of deionized water, grind and obtain 50% sulfur dispersion.

[0052] Preparation method of 40% ZnO suspension: add 100g ZnO, 6g diffusant NF, 80mL of 10wt% casein aqueous solution, 10mL of 20wt% ammonia water, and 60mL of deionized water, grind and obtain 40% ZnO dispersion.

[0053] Preparation method of 50% ZDBC suspension: add 100g ZDBC, 2.3g diffusing agent NF, 40mL 10wt% casein aqueous solution, 57.7mL deionized water, grind and obtain 50% ZDBC dispersion.

[0054] The dispersion of sulfur, ZnO and ZDBC is ground by ball milling for 48 to 60 hours.

[0055] Example 1

[0056] Table 1 Raw materials for the preparation of low protein vulcanized natural rubber latex

[0057]

[0058] According to the dry weight ratios shown in Table 1, a fresh natural rubber latex aqueous suspension (30 wt% dry rubber content), a KOH aqueous solution (20 wt% concentration), and an SDS aqueous solution (10 wt% concentration) were added and stirred at room temperature (20-25°C) for 30 minutes. An alkaline protease aqueous solution (5 wt% concentration) was then added and the protease hydrolysis reaction was carried out at 40°C for 8 hours. A 50% sulfur suspension, a 50% ZDBC suspension, and a 40% ZnO suspension were then added and the reaction was continued at 40°C for 12 hours. The degree of vulcanization of the latex was tested every 30 minutes using the chloroform gelation method. When the degree of vulcanization reached the end of the second stage and the beginning of the third stage, the latex was centrifuged at room temperature using a butterfly centrifuge to obtain a low-protein vulcanized natural rubber latex (denoted as LDPNR-1). The latex was then preserved by adding 0.7% ammonia water and 0.05% lauric acid relative to the mass of the natural latex.

[0059] Example 2

[0060] According to the dry weight ratios shown in Table 1, a fresh natural rubber latex aqueous suspension (30 wt% dry rubber content), a KOH aqueous solution (20 wt% concentration), an SDS aqueous solution (10 wt% concentration), and an alkaline protease aqueous solution (5 wt% concentration) were added and stirred at room temperature for 30 minutes. Subsequently, a sulfur suspension (50 wt% concentration), a ZDBC suspension (40 wt% concentration), and a ZnO suspension (50 wt% concentration) were added and reacted at room temperature for 48 hours. The degree of vulcanization of the latex was monitored every 30 minutes using the chloroform gelation method. When the degree of vulcanization reached the end of the second stage and the beginning of the third stage, the latex was centrifuged in an industrial disc centrifuge at room temperature to obtain a low-protein vulcanized natural rubber latex (LDPNR-2). This was then preserved by adding 0.7% ammonia and 0.05% lauric acid relative to the mass of the natural latex.

[0061] Comparative Example 1

[0062] The only difference from Example 2 is that no alkaline protease aqueous solution was added for deproteinization. The prepared vulcanized natural rubber latex was recorded as NR-0 or control.

[0063] Test Example 1

[0064] Performance test of the vulcanized natural rubber latex and vulcanized rubber film prepared in Examples 1-2 and Comparative Example 1

[0065] (1) Basic property test of low protein vulcanized natural rubber latex

[0066] The total solids content of the vulcanized natural rubber latex was tested according to GB / T 8298-2017, the alkalinity was tested according to GB / T 8300-2016, the mechanical stability was tested according to GB / T 8301-2008, and the apparent viscosity was tested according to NY / T 1037-2016. The test results for the vulcanized natural rubber latex are shown in Table 2. It can be seen that the characteristic indicators of the vulcanized natural rubber latex prepared in Examples 1-2 and Comparative Example 1 all meet the requirements of GB / T 14797.1.

[0067] Table 2 Basic properties test results of vulcanized natural rubber latex

[0068] sample LDPNR-1 LDPNR-2 NR-0 Total solid content (mass fraction) / % 60.38 60.49 61.24 Alkalinity (based on the ammonia content of latex, mass fraction) / % 0.83 0.83 0.84 Viscosity / (mPa·s) 44.00 41.80 38.70 Mechanical stability (storage for 21 days) / s 600 690 850

[0069] (2) Comparison of deproteinization effects of different methods

[0070] The nitrogen content test method is GB / T 8088-2008, and the protein content is 6.25 times the measured value of the nitrogen content. The test method for water-extractable protein is T / CRIA 19004-2023. Table 3 shows the test results of nitrogen content, protein content and water-extractable protein of vulcanized natural rubber latex. It can be seen that comparative example 1, in which no protease was added, has the highest nitrogen content and water-extractable protein content. Example 1 has the best deproteinization effect at a temperature of 40°C, with a nitrogen content as low as 0.08wt%, and a water-extractable protein content of only 31.77% of that of comparative example 1. In Example 2, since the activity of protease at room temperature is worse than that at 40-50°C, even if the reaction time is extended, the deproteinization effect is worse than that of Example 1. However, compared with comparative example 1, the water-extractable protein content in Example 2 has decreased by nearly 50%.

[0071] Table 3 Nitrogen content, protein content and water-extractable protein test results of vulcanized natural rubber latex

[0072]

[0073] (3) Comparison of tensile properties of vulcanized rubber films

[0074] The vulcanized natural rubber latex prepared in Examples 1 to 2 and Comparative Example 1 was diluted to a total solid content of 40 wt %, respectively, and spread on a clean glass plate. After drying, the film was drained for 24 h and air-dried to obtain a vulcanized rubber film with a thickness of 1 mm, which was labeled as VLDPNR-1 (Example 1), VLDPNR-2 (Example 2) and VNR-0 (Comparative Example 1), respectively.

[0075] The vulcanized film was cut into dumbbell-shaped specimens and the tensile properties of the vulcanized film were measured using a universal tensile testing machine according to GB / T528-2009 (tensile speed was 500 mm / min). Table 4 shows the tensile properties of the vulcanized film. Figure 1 is the tensile stress-strain curve of the vulcanized rubber film.

[0076] Table 4 Tensile properties of vulcanized rubber

[0077]

[0078] From Table 4 and Figure 1 It can be seen that with the decrease of protein content, the tensile strength, 100% modulus of elongation, 300% modulus of elongation and 500% modulus of the vulcanized film show a downward trend.

[0079] In summary, the present invention uses fresh natural latex as raw material and prepares low-protein vulcanized concentrated natural latex through a single centrifugation process. The preparation process is simple and the cost is low. Since the centrifugation process not only removes water-soluble proteins but also removes excess vulcanization auxiliaries, the prepared low-protein concentrated vulcanized natural latex has a long applicability period, good process stability, and excellent comprehensive performance, and can meet the needs of downstream product companies for deproteinized natural latex.

[0080] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing low-protein vulcanized natural rubber latex, comprising method 1 or method 2; The method 1 comprises the following steps: The natural rubber latex, a surfactant, a stabilizer, a protease and a vulcanizing agent are mixed, and enzymatic hydrolysis and vulcanization are performed followed by centrifugation to obtain a low-protein vulcanized natural rubber latex. The method 2 comprises the following steps: mixing natural rubber latex, a surfactant, a stabilizer and a protease, performing an enzymatic hydrolysis reaction, adding a vulcanizing agent for vulcanization and then performing centrifugal treatment to obtain low-protein vulcanized natural rubber latex.

2. The preparation method according to claim 1, characterized in that The protease comprises one or more of alkaline protease, papain, trypsin and bromelain; and the dosage of the protease is 20,000 to 200,000 u / kg of dry glue.

3. The preparation method according to claim 1, characterized in that The surfactant includes an anionic surfactant and / or a nonionic surfactant; The mass of the surfactant is 0.03-3% of the dry weight of natural rubber latex.

4. The preparation method according to claim 3, characterized in that The anionic surfactant includes one or more of a carboxylic acid surfactant, a sulfonic acid surfactant, a sulfate surfactant, and a phosphate surfactant; The nonionic surfactant includes one or more of polyoxyalkylene ether surfactants, polyoxyalkylene ester surfactants, polyol fatty acid ester surfactants and sugar fatty acid ester surfactants.

5. The preparation method according to claim 1, characterized in that The strong alkaline stabilizer includes an alkali metal hydroxide; The mass of the strong alkaline stabilizer is 0.05-0.3% of the dry weight of the natural rubber latex.

6. The preparation method according to claim 1, characterized in that The mass of the vulcanizing agent is 0.5 to 5% of the dry weight of natural rubber latex; The vulcanizing agent includes sulfur, zinc oxide and dithiocarbamoyl vulcanization accelerator; the mass of the sulfur is 0.3-2% of the dry weight of natural rubber latex; the mass of the zinc oxide is 0.1-1% of the dry weight of natural rubber latex; the mass of the dithiocarbamoyl vulcanization accelerator is 0.1-2% of the dry weight of natural rubber latex.

7. The preparation method according to claim 1, characterized in that The temperatures of the enzymatic reaction-sulfurization, the enzymatic reaction and the sulfurization are independently 20-55°C.

8. The low-protein vulcanized natural rubber latex prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The nitrogen content of the low-protein vulcanized natural rubber latex is ≤0.1wt%, and the water-extractable protein content is ≤35μg / g.

9. A vulcanized rubber film, characterized in that: The film is obtained by forming a low-protein vulcanized natural rubber latex according to claim 8.

10. Use of the low-protein vulcanized natural rubber latex according to claim 8 or the vulcanized rubber film according to claim 9 in gloves, balloons, condoms, urinary catheters, rubber hoses / rubber threads or sporting goods.

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