JP7696482B2 - Deproteinized natural rubber crosslinked product - Google Patents

Description

The present invention relates to a cross-linked product of protein-removed natural rubber, and more specifically to a cross-linked product of protein-removed natural rubber that is less susceptible to volume expansion and deterioration in physical properties when immersed in water.

The rubber materials commonly used in wet areas are synthetic rubbers such as EPDM, (hydrogenated) NBR, and fluororubber, but from the perspective of resource sustainability, it is desirable to use plant-derived natural rubber.

However, because natural rubber contains hydrophilic proteins in its structure, there is an issue that its volume increases during use.

Patent Document 1 proposes protein-free natural rubber, in which proteins are separated from natural rubber, but even in this case, such a tendency is still observed.

WO 2011/027739 A1

An object of the present invention is to provide a deproteinized cross-linked natural rubber that is less susceptible to volume expansion and deterioration in physical properties when immersed in water.

The object of the present invention is achieved by a cross-linked product of a rubber composition comprising protein-removed natural rubber and an organic peroxide cross-linking agent, the cross-linked product being used as a seal for aqueous systems , which has a volume change of 4.3% or less after a water resistance test at 80°C for 500 hours in accordance with JIS K-6258.

A cross-linked product of a rubber composition in which protein-removed natural rubber is blended with an organic peroxide cross-linking agent, and which has a volume change of 4.3% or less after a water resistance test at 80°C for 500 hours in accordance with JIS K-6258, effectively prevents volume expansion when immersed in water with almost no deterioration in physical properties.

The cross-linked product of protein-removed natural rubber according to the present invention contains protein-removed natural rubber and an organic peroxide cross-linking agent.

In this invention, deproteinized natural rubber refers to deproteinized natural rubber having a nitrogen content of 0.019% by weight or less, as measured by the Kjeldahl method (based on JIS K6451-2, which corresponds to ISO 1407; a sample is wet-decomposed using a mixed catalyst of potassium sulfate and copper (II) sulfate and sulfuric acid, the nitrogen in the sample is converted to ammonium sulfate, a strong alkali is added to this and the liberated ammonia is subjected to steam distillation, introduced into an aqueous boric acid solution and then titrated with sulfuric acid or hydrochloric acid to determine the amount of ammonia, from which the nitrogen content is calculated).

Such protein-removed natural rubber can be obtained by adding a urea compound, preferably urea represented by the general formula _RNHCONH2 (R: hydrogen atom or an alkyl group having 1 to 5 carbon atoms) or a lower alkyl derivative thereof, an anionic, nonionic or cationic surfactant and a polar organic solvent, preferably a water-miscible polar organic solvent such as an aliphatic alcohol having an alkyl group having 1 to 5 carbon atoms, a ketone having 3 to 4 carbon atoms, an aliphatic carboxylic acid having 1 to 5 carbon atoms, or a carboxylate ester having an alkyl group having 1 to 5 carbon atoms, to natural rubber latex, and then denaturing and removing the proteins in the latex.

In addition, such protein-removed natural rubber has a protein content in the solid rubber of 0.5 μg/g or less as measured by the modified Lowry method.

Kjeldahl method (JIS K6451-2):
0.1g of rubber, 0.65g of catalyst (potassium sulfate: copper (II) sulfate pentahydrate: selenium = weight ratio 15:2:1), and 2.5ml of concentrated sulfuric acid were weighed and placed in a Kjeldahl flask, and heated with a gas burner for about 30 minutes until the solution turned green. A steam distillation apparatus was assembled, and an appropriate amount of distilled water was placed in a 1L flask, which was then heated in an oil bath, and the 300ml two-neck flask and Liebig condenser were washed with steam. The contents of the Kjeldahl flask were transferred into the two-neck flask using 20ml of distilled water, and 10ml of 67w/v% sodium hydroxide aqueous solution and 10ml of distilled water were added to the two-neck flask, and steam was passed through. Ammonia was captured using an Erlenmeyer flask containing 10ml of 2w/v% boric acid aqueous solution as a receiver. The distillation was stopped when the content of the Erlenmeyer flask in the steam distillation apparatus reached about 20ml. The inside of the Liebig condenser was washed with distilled water, and the washings were poured into an Erlenmeyer flask. The nitrogen content in the rubber sample was determined by titration with 0.005 mol/L sulfuric acid using methyl red as an indicator.
The nitrogen content was calculated using the following formula.
Nitrogen content (%) = V/1000 (L) × N (molL ^-1 ) × 14 × 2 (gmol ^-1 ) × 1/w (g ^-1 ) × 100
Here, V is the titration amount of 0.005 mol/L sulfuric acid, w is the mass of rubber, and N is the concentration of 0.005 mol/L sulfuric acid used in the titration multiplied by a factor (1.004).

Examples of the organic peroxide crosslinking agent that can be used include dialkyl peroxides such as dicumyl peroxide, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy) hexyne-3 , 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, and 1,3-bis(tert-butylperoxyisopropyl)benzene; diacyl peroxides such as benzoyl peroxide; and peroxy esters such as 2,5-dimethyl-2,5-bis(benzoylperoxy)hexane and tert-butylperoxyisopropyl carbonate, with dicumyl peroxide being preferred.

These organic peroxide crosslinking agents are used in a ratio of about 0.1 to 10 parts by weight, preferably about 0.2 to 5 parts by weight, per 100 parts by weight of protein-removed natural rubber.

A polyfunctional unsaturated compound such as triallyl isocyanurate, triallyl cyanurate, trimethylolpropane trimethacrylate, etc. may be used in combination with the organic peroxide crosslinking agent in an amount of about 10 parts by weight or less, preferably about 0.2 to 5 parts by weight, per 100 parts by weight of the protein-removed natural rubber.

In addition, crosslinking aids such as zinc oxide, magnesium oxide, litharge, stearic acid, zinc stearate, etc. can be added in a ratio of about 0.1 to 10 parts by weight, preferably about 0.2 to 5 parts by weight, per 100 parts by weight of protein-removed natural rubber.

In addition, p-phenylenediamines such as N-isopropyl-N'-phenyl-p-phenylenediamine, N-octyl-N'-phenyl-p-phenylenediamine, N,N'-diallyl-p-phenylenediamine, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, and N,N'-di(1,4-dimethylpentyl)-p-phenylenediamine, as well as anti-aging agents such as ketone-amine reaction products such as 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline, and other optional ingredients may be appropriately blended and used.

Various reinforcing agents or fillers can be compounded in the rubber composition in an amount of about 90 parts by weight or less, preferably about 35 to 60 parts by weight, per 100 parts by weight of the protein-removed natural rubber. The proportion of the reinforcing agent or filler improves the breaking strength and modulus of the crosslinked product, but compounding in a larger proportion may impair the breaking elongation and compression set properties.

As reinforcing agents or fillers, generally various grades of carbon black, preferably FEF carbon black, and white fillers such as silica produced by the dry or wet process are used. When silica is used, it is preferable to use a silane coupling agent in combination.

The rubber composition is prepared by kneading the protein-removed natural rubber and other compounding ingredients in a sealed kneader or an open roll.

Crosslinking of the rubber composition is carried out by heating in a hot press at about 140 to 200° C. for about 3 to 30 minutes depending on the type of crosslinking agent used.

The resulting deproteinized natural rubber cross-linked product is effectively used as a rubber product which is used in contact with water, particularly as a sealing material for water systems, for example.

Next, the present invention will be explained using examples.

Reference Example 100 parts by weight of natural rubber latex (product of Malaysia Golden Hope Pvt. Ltd., imported by Toyotsu Chemiplas; rubber content 60.2% by weight, ammonia content 0.7% by weight) was added with 1 part by weight of sodium dodecyl sulfate and 0.1 part by weight of urea and stirred at room temperature for 90 minutes.

The latex was then centrifuged at 15°C and 9000 rpm for 30 minutes to separate the upper cream layer. Distilled water was added to the cream layer, followed by the addition of 0.5 parts by weight of sodium dodecyl sulfate and 2.5 parts by weight of acetone, and the mixture was stirred for 1 hour.

This centrifugation and redispersion process (rubber concentration is adjusted to 30% by weight using 1% anionic surfactant-0.025% ethanol aqueous solution) was repeated a total of three times to prepare protein-removed natural rubber latex, which was then dried to obtain protein-removed natural rubber.

The resulting natural rubber (PFNR) had a nitrogen content of 0.001% by weight or less, which is the detection limit, as measured by the Kjeldahl method (JIS K-6451) under the above-mentioned conditions , and a protein content of 0.5 μg/g or less, which is the detection limit, as measured by the modified Lowry method.

Example 1
PFNR obtained in the reference example 100 parts by weight
FEF Carbon Black 50 〃
Dicumyl peroxide 2.5 〃
Crosslinking agent zinc oxide 5 〃
Crosslinking agent stearic acid 1 〃
Anti-aging agent (Kawaguchi Chemical Industry Products Antage RD) 2 Same
Of the above components, PFNR was masticated in a closed kneader, and then the compounding components other than dicumyl peroxide were added and kneaded at 120° C. Furthermore, dicumyl peroxide was added and kneaded using an open roll to obtain a protein-removed natural rubber composition.

The resulting protein-removed natural rubber composition was thermally crosslinked at 170°C for 7 minutes, and crosslinked until the time corresponding to t90 on the vulcanization curve, to obtain a crosslinked sheet having a thickness of 2 mm. The crosslinked sheet was subjected to the following tests.

Normal values (hardness, tensile strength, elongation at break): Compliant with JIS K-6253 and K-6251 Water immersion test : Compliant with JIS K-6258
Measure the change in normal value and the volume change rate after immersion in water at 80℃ for 500 hours
JIS No. 6 test pieces and 2 x 5 cm test pieces for measuring volume change were cut from the rubber sheet.
Each set consisted of three pieces.
Then, 100 ml of ion-exchanged water was poured in and the container was covered with aluminum foil.
After the test, the test tube was placed in a thermostatic chamber at a specified temperature.
The volume was measured by placing the test piece in water.
Calculate the weight of the suspended object using Archimedes' principle.
The percentage change in volume was calculated.

Example 2
In Example 1, the same amount (100 parts by weight) of deproteinized NR (imported by Toyotsu Chemiplas; nitrogen content 0.019% by weight) was used in place of the PFNR obtained in the Reference Example.

Comparative Example In Example 1, the same amount (100 parts by weight) of natural rubber (imported by Toyotsu Chemiplas; nitrogen content 0.480% by weight) was used in place of the PFNR obtained in the Reference Example.

The results obtained in the above examples and comparative examples are shown in the following table.

table
Measurement items Example 1 Example 2 Comparative example
[Normal value]
Hardness (Durometer A) 53 59 61
Tensile strength (MPa) 20.3 23.4 24.5
Elongation at break (%) 340 380 350

[After immersion in water for 500 hours at 80℃]
Hardness change rate (%) -6 -14 -10
Tensile strength change rate (%) -10 -16 -32
Elongation change at break (%) +3 -8 -14
Volume change rate (%) +4.1 +4.3 +9.1

These results show that when PFNR or deproteinized NR is used instead of natural rubber, the volume change rate after immersion in water at 80°C for 500 hours is small, and in particular for PFNR, the rate of change in various physical properties is also kept small.

Claims (6)

A cross-linked product of a rubber composition comprising protein-removed natural rubber and an organic peroxide cross-linking agent, the cross-linked product being used as a seal for aqueous systems and having a volume change of 4.3% or less after a water resistance test at 80°C for 500 hours in accordance with JIS K-6258. The protein-removed cross-linked natural rubber according to claim 1, wherein the nitrogen content of the protein-removed natural rubber is 0.019% by weight or less as measured by the Kjeldahl method specified in JIS K-6451, which corresponds to ISO 1407. The protein-removed cross-linked natural rubber according to claim 1, wherein the rubber composition further comprises a reinforcing agent or a filler. The cross-linked product of protein-removed natural rubber according to claim 3, in which the reinforcing agent or filler is blended in an amount of 90 parts by weight or less per 100 parts by weight of protein-removed natural rubber. The cross-linked product of protein-removed natural rubber according to claim 3, in which the reinforcing agent or filler is blended in a ratio of 35 to 60 parts by weight per 100 parts by weight of protein-removed natural rubber. 2. A seal for an aqueous system which is the cross-linked product of protein-removed natural rubber according to claim 1.