JP5400108B2 - Process for producing modified natural rubber - Google Patents

Description

The present invention relates to a method for producing a modified natural rubber.

  In recent years, there is an increasing demand for lower fuel consumption of automobiles, and tires with low rolling resistance are required. Therefore, as a rubber composition used for tire treads or the like, a rubber composition having a low tan δ (hereinafter referred to as low loss property) and excellent in low heat generation properties is required. In addition, a rubber composition for a tread is required to have excellent wear resistance and fracture characteristics in addition to low loss. On the other hand, in order to improve the low loss property, wear resistance and fracture characteristics of the rubber composition, it is necessary to improve the affinity between the rubber component and the filler such as carbon black and silica in the rubber composition. It is valid.

  For example, in order to improve the affinity between the filler and the rubber component in the rubber composition and to improve the reinforcing effect by the filler, synthetic rubber or functional group having improved affinity with the filler by terminal modification Synthetic rubbers and the like have been developed in which the monomers are copolymerized to improve the affinity with the filler.

  On the other hand, although natural rubber is used in large quantities by taking advantage of its excellent physical properties, it is a technology that improves the natural rubber itself to improve the affinity with the filler and greatly enhance the reinforcing effect of the filler. There is no.

  For example, a technology for epoxidizing natural rubber has been proposed. However, since this technology cannot sufficiently improve the affinity between the natural rubber and the filler, the reinforcing effect of the filler can be sufficiently improved. Can not. In addition, a technique (see Patent Documents 1 to 6) in which a vinyl monomer is added to natural rubber latex and graft polymerization is known, and the grafted natural rubber obtained by the technique has already been used as MG latex or the like. Has been produced. However, in order to achieve high grafting efficiency by this method, it is necessary to use a high-purity latex, so that it is necessary to separately perform an operation such as centrifugation, and there is a problem that the manufacturing cost increases.

JP-A-5-287121 JP-A-6-329702 Japanese Patent Laid-Open No. 9-25468 JP 2000-319339 A JP 2002-138266 A JP 2002-348559 A

Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art, and can be manufactured at low cost, and by using it as a rubber component of the rubber composition, the low loss property, wear resistance and destruction of the rubber composition It is to provide a method for producing a modified natural rubber capable of improving the characteristics.

As a result of intensive investigations to achieve the above-mentioned object, the present inventor has added a polar group-containing mercapto compound to a solid natural rubber raw material that can be obtained at low cost by applying a mechanical shearing force to the rubber composition. The inventors have found that a modified natural rubber capable of improving low loss, wear resistance, and fracture characteristics can be obtained, and the present invention has been completed.

That is, the method for producing a modified natural rubber according to the present invention imparts a mechanical shearing force to a polar group-containing mercapto compound on at least one natural rubber raw material selected from the group consisting of a natural rubber latex coagulum and a natural rubber cup lamp. a method for producing a modified natural rubber to be added Te, the amount added of the polar group-containing mercapto compound is 0.01 to 5.0 wt% with respect to solid rubber component of the natural rubber raw material, the polar group-containing mercapto compound Polar group is amino group, imino group, nitrile group, ammonium group, imide group, amide group, hydrazo group, azo group, diazo group, hydroxyl group, carboxyl group, carbonyl group, epoxy group, oxycarbonyl group, nitrogen-containing heterocycle It is at least one selected from the group consisting of a group, an oxygen-containing heterocyclic group, and a tin-containing group.

According to the present invention, a low-loss property, wear resistance, and fracture properties of a rubber composition are improved by adding a polar group-containing mercapto compound to a solid natural rubber raw material that can be obtained at low cost by applying a mechanical shearing force. It is possible to provide a method for producing a modified natural rubber.

The present invention is described in detail below. The modified natural rubber produced by the method of the present invention imparts a mechanical shearing force to a polar group-containing mercapto compound on at least one natural rubber raw material selected from the group consisting of natural rubber latex coagulum and natural rubber cup lamp. The addition amount of the polar group-containing mercapto compound is 0.01 to 5.0% by mass with respect to the solid rubber component in the natural rubber raw material, and the polar group of the polar group-containing mercapto compound is an amino group or an imino group. Group, nitrile group, ammonium group, imide group, amide group, hydrazo group, azo group, diazo group, hydroxyl group, carboxyl group, carbonyl group, epoxy group, oxycarbonyl group, nitrogen-containing heterocyclic group, oxygen-containing heterocyclic group And at least one selected from the group consisting of tin-containing groups. Since the polar group of the polar group-containing mercapto compound is excellent in affinity to various fillers such as carbon black and silica, the modified natural rubber has an affinity for various fillers compared to unmodified natural rubber. high. Therefore, the rubber composition using the modified natural rubber as a rubber component has a high dispersibility of the filler with respect to the rubber component, the reinforcing effect of the filler is sufficiently exhibited, and excellent in fracture characteristics and wear resistance. Low exothermicity (low loss) is also greatly improved. Further, by using the rubber composition for a tire, particularly a tread of the tire, it is possible to remarkably improve fracture resistance and wear resistance while greatly reducing rolling resistance.

As raw materials for the modified natural rubber produced by the method of the present invention , various natural rubber latex coagulated products (including unsmoked sheets) or natural rubber cup lamps can be used. Or may be used in combination of two or more. In the method for producing a modified natural rubber according to the present invention, it is not necessary to use a natural rubber latex having a high purity, and therefore a modified natural rubber can be produced at a relatively low cost. Among the natural rubber raw materials, cup lamps and the like can be obtained at a low cost, so that there is a great merit in terms of cost. Note that when cup lamps or the like are used as raw materials, the modification efficiency of natural rubber may be slightly reduced, but the merit is superior in terms of the overall cost and modification efficiency.

In the present invention, the polar group-containing compound by addition reaction to the natural rubber molecule in the natural rubber material, the polar group-containing compound have a mercapto group in the molecule, a polar group-containing mercapto compound.

As a means for giving mechanical shearing force to the mixture of the natural rubber raw material and the polar group-containing mercapto compound, a twin-screw extrusion kneader and a dry prebreaker are preferable. Here, the polar group-containing mercapto compounds case of addition reaction to the natural rubber molecule in the natural rubber raw material, the natural rubber raw material and the polar group-containing mercapto compound were charged into a device given the mechanical shear forces, required to Accordingly, by adding an organic peroxide or the like and applying mechanical shearing force, a polar group-containing mercapto compound can be added to the double bond of the main chain of the natural rubber molecule in the natural rubber raw material. .

The polar group of the polar group-containing mercapto compound to be added to the natural rubber molecule in the natural rubber raw material is an amino group, imino group, nitrile group, ammonium group, imide group, amide group, hydrazo group, azo group, diazo group, It is at least one selected from the group consisting of a hydroxyl group, a carboxyl group, a carbonyl group, an epoxy group, an oxycarbonyl group, a nitrogen-containing heterocyclic group, an oxygen-containing heterocyclic group, and a tin-containing group. These polar group-containing mercapto compounds may be used alone or in combination of two or more.

  Examples of the mercapto compound containing an amino group include mercapto compounds having at least one amino group selected from primary, secondary and tertiary amino groups in one molecule. Among the mercapto compounds having an amino group, a tertiary amino group-containing mercapto compound is particularly preferable. Here, as the primary amino group-containing mercapto compound, 4-mercaptoaniline, 2-mercaptoethylamine, 2-mercaptopropylamine, 3-mercaptopropylamine, 2-mercaptobutylamine, 3-mercaptobutylamine, 4-mercaptobutylamine Etc. The secondary amino group-containing mercapto compounds include N-methylaminoethanethiol, N-ethylaminoethanethiol, N-methylaminopropanethiol, N-ethylaminopropanethiol, N-methylaminobutanethiol, N- And ethylaminobutanethiol. Further, the tertiary amino group-containing mercapto compounds include N, N-dimethylaminoethanethiol, N, N-diethylaminoethanethiol, N, N-dimethylaminopropanethiol, N, N-diethylaminopropanethiol, N, N N, N-disubstituted aminoalkyl mercaptans such as -dimethylaminobutanethiol and N, N-diethylaminobutanethiol. Among these amino group-containing mercapto compounds, 2-mercaptoethylamine, N, N-dimethylaminoethanethiol and the like are preferable. These amino group-containing mercapto compounds may be used alone or in combination of two or more.

  Examples of the mercapto compound having a nitrile group include 2-mercaptopropane nitrile, 3-mercaptopropane nitrile, 2-mercaptobutane nitrile, 3-mercaptobutane nitrile, 4-mercaptobutane nitrile, etc., and these nitrile group-containing mercapto compounds May be used individually by 1 type, and may be used in combination of 2 or more type.

  Examples of the mercapto compound containing a hydroxyl group include mercapto compounds having at least one primary, secondary, or tertiary hydroxyl group in one molecule. Specific examples of the hydroxyl group-containing mercapto compound include 2-mercaptoethanol, 3-mercapto-1-propanol, 3-mercapto-2-propanol, 4-mercapto-1-butanol, 4-mercapto-2-butanol, 3 -Mercapto-1-butanol, 3-mercapto-2-butanol, 3-mercapto-1-hexanol, 3-mercapto-1,2-propanediol, 2-mercaptobenzyl alcohol, 2-mercaptophenol, 4-mercaptophenol, etc. Among these, 2-mercaptoethanol and the like are preferable. These hydroxyl group-containing mercapto compounds may be used alone or in a combination of two or more.

  Examples of the mercapto compound containing a carboxyl group include mercaptoacetic acid, mercaptopropionic acid, thiosalicylic acid, mercaptomalonic acid, mercaptosuccinic acid, mercaptobenzoic acid and the like. Among these, mercaptoacetic acid is preferred. These carboxyl group-containing mercapto compounds may be used individually by 1 type, and may be used in combination of 2 or more type.

  In the mercapto compound containing the nitrogen-containing heterocyclic group, the nitrogen-containing heterocyclic ring includes pyrrole, histidine, imidazole, triazolidine, triazole, triazine, pyridine, pyrimidine, pyrazine, indole, quinoline, purine, phenazine, pteridine, melamine. Etc. The nitrogen-containing heterocycle may contain other heteroatoms in the ring. Here, as a mercapto compound containing a pyridyl group as a nitrogen-containing heterocyclic group, 2-mercaptopyridine, 3-mercaptopyridine, 4-mercaptopyridine, 5-methyl-2-mercaptopyridine, 5-ethyl-2-mercapto Examples of mercapto compounds containing other nitrogen-containing heterocyclic groups include 2-mercaptopyrimidine, 2-mercapto-5-methylbenzimidazole, 2-mercapto-1-methylimidazole, and 2-mercapto. Examples include benzimidazole and 2-mercaptoimidazole. Among these, 2-mercaptopyridine and 4-mercaptopyridine are preferable. These nitrogen-containing heterocyclic group-containing mercapto compounds may be used alone or in combination of two or more.

  Examples of the mercapto compound having a tin-containing group include 2-mercaptoethyltri-n-butyltin, 2-mercaptoethyltrimethyltin, 2-mercaptoethyltriphenyltin, 3-mercaptopropyltri-n-butyltin, and 3-mercaptopropyl. Mention may be made of tin-containing mercapto compounds such as trimethyltin and 3-mercaptopropyltriphenyltin. These tin-containing mercapto compounds may be used individually by 1 type, and may be used in combination of 2 or more type.

The above-described components are charged into a device that can be given mechanical shearing force, and mechanical shearing force is given in the device, whereby a modified natural rubber in which the polar group-containing mercapto compound is added and reacted with natural rubber molecules is obtained. In this case, the modification reaction of the natural rubber molecule may be carried out by heating, preferably 30 to 160 ° C., more preferably 50 to 130 ° C., so that the modified natural rubber has sufficient addition efficiency. Can be obtained.

In the modified natural rubber produced by the method of the present invention, the addition amount of the polar group-containing mercapto compound is in the range of 0.01 to 5.0% by mass with respect to the solid rubber component in the natural rubber raw material, and 0.02 to 3.0% by mass. % Is preferable, and a range of 0.03 to 2.0% by mass is even more preferable. If the addition amount of the polar group-containing mercapto compound is less than 0.01% by mass, the low loss and wear resistance of the rubber composition may not be sufficiently improved. In addition, if the addition amount of the polar group-containing mercapto compound exceeds 5.0% by mass, natural physical properties such as viscoelasticity and SS characteristics (stress-strain curve in a tensile tester) are greatly changed. The inherent physical properties of rubber are impaired, and the processability of the rubber composition may be greatly deteriorated.

  The rubber composition of the present invention is characterized by using the modified natural rubber, and preferably further contains a filler. Here, the blending amount of the filler is not particularly limited, but is preferably in the range of 5 to 100 parts by mass, more preferably in the range of 10 to 70 parts by mass with respect to 100 parts by mass of the modified natural rubber. If the blending amount of the filler is less than 5 parts by mass, sufficient reinforcement may not be obtained, and if it exceeds 100 parts by mass, the workability may be deteriorated.

Examples of the filler used in the rubber composition of the present invention include carbon black and inorganic filler. Here, as the inorganic filler, silica and the following formula (I):
nM · xSiO _y · zH ₂ O (I)
[Wherein, M is a metal selected from the group consisting of aluminum, magnesium, titanium, calcium and zirconium, an oxide or hydroxide of these metals, and a hydrate thereof, or a carbonate of these metals. And n, x, y, and z are each an integer of 1 to 5, an integer of 0 to 10, an integer of 2 to 5, and an integer of 0 to 10]. Is mentioned. These fillers may be used alone or in a combination of two or more.

Examples of the carbon black include GPF, FEF, SRF, HAF, ISAF, and SAF grades, and examples of the silica include wet silica, dry silica, and colloidal silica. Furthermore, the inorganic compound of the above formula (I) includes alumina (Al ₂ O ₃ ) such as γ-alumina and α-alumina; alumina monohydrate such as boehmite and diaspore (Al ₂ O ₃ .H ₂ O). Aluminum hydroxide [Al (OH) ₃ ] such as gibbsite and bayerite; aluminum carbonate [Al ₂ (CO ₃ ) ₃ ], magnesium hydroxide [Mg (OH) ₂ ], magnesium oxide (MgO), magnesium carbonate ( MgCO ₃ ), talc (3MgO · 4SiO ₂ · H ₂ O), attapulgite (5MgO · 8SiO ₂ · 9H ₂ O), titanium white (TiO ₂ ), titanium black (TiO _2n-1 ), calcium oxide (CaO), calcium hydroxide [Ca (OH) _2], magnesium aluminum oxide _{(MgO · Al 2 O 3)} , clay _{(Al 2 O 3 · 2SiO 2} ), kaolin _{(Al 2 O 3 · 2SiO 2} · 2 ₂ O), pyrophyllite _{(Al 2 O 3 · 4SiO 2} · H 2 O), bentonite _{(Al 2 O 3 · 4SiO 2} · 2H 2 O), aluminum silicate _{(Al 2 SiO 5, Al 4} · 3SiO 4・ 5H ₂ O etc.), magnesium silicate (Mg ₂ SiO ₄ , MgSiO ₃ etc.), calcium silicate (Ca ₂ SiO ₄ etc.), aluminum calcium silicate (Al ₂ O ₃ · CaO · 2SiO ₂ etc.), Magnesium calcium oxide (CaMgSiO ₄ ), calcium carbonate (CaCO ₃ ), zirconium oxide (ZrO ₂ ), zirconium hydroxide [ZrO (OH) ₂ .nH ₂ O], zirconium carbonate [Zr (CO ₃ ) ₂ ], various zeolites Examples thereof include crystalline aluminosilicates containing hydrogen, alkali metals, or alkaline earth metals that correct electric charges.

  In addition to the above modified natural rubber and filler, the rubber composition of the present invention includes compounding agents commonly used in the rubber industry, such as anti-aging agents, softeners, silane coupling agents, stearic acid, zinc white. Further, a vulcanization accelerator, a vulcanizing agent, and the like can be appropriately selected and blended within a range that does not impair the object of the present invention. As these compounding agents, commercially available products can be suitably used. The rubber composition of the present invention can be produced by blending various modified additives appropriately selected as necessary with the modified natural rubber, kneading, heating, extruding and the like.

  The tire of the present invention is characterized by using the rubber composition, and the rubber composition is preferably used for a tread. A tire using the rubber composition as a tread is excellent in fuel efficiency, fracture characteristics, and wear resistance. The tire of the present invention is not particularly limited except that the rubber composition described above is used for any of the tire members, and can be produced according to a conventional method. Moreover, as gas with which this tire is filled, inert gas, such as nitrogen, argon, helium other than normal or the air which adjusted oxygen partial pressure, can be used.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

<Production Example 1>
Formic acid was added to the field latex to adjust the pH to 4.7, the latex was coagulated, and the obtained solid was treated 5 times with a creper and crushed through a shredder. Next, the dry rubber content of the obtained coagulum was determined, and 600 g of coagulum, 3.0 g of N, N-diethylaminoethyl methacrylate, tert-butyl hydroperoxide (t-BHPO) 1.2 in terms of dry rubber. were kneaded in a kneader at room temperature at 30 rpm for 2 minutes to be uniformly dispersed. Next, while uniformly adding 1.2 g of tetraethylenepentamine (TEPA) to the obtained mixture, a twin-screw kneading extruder made of Kobe Steel [same-direction rotating screw diameter = 30 mm, L / D = 35, three vent holes The modified modified natural rubber A was obtained by extrusion while applying mechanical shearing force at a barrel temperature of 120 ° C. and a rotation speed of 100 rpm.

  Further, based on the mass of the resulting modified natural rubber A, the conversion rate of N, N-diethylaminoethyl methacrylate added as a monomer was 83%. Further, when the modified natural rubber A was extracted with petroleum ether and further extracted with a 2: 1 mixed solvent of acetone and methanol, homopolymer was separated, and 7% of the charged monomer amount was detected. . Accordingly, the graft amount of N, N-diethylaminoethyl methacrylate in the modified natural rubber A is 0.39% by mass with respect to the solid rubber component in the natural rubber raw material.

<Production Examples 2-7>
Instead of 3.0 g of N, N-diethylaminoethyl methacrylate, 2.1 g of 2-hydroxyethyl methacrylate was added in Production Example 2, 1.7 g of 4-vinylpyridine was added in Production Example 3, and 1.4 g of methacrylic acid was added in Production Example 4. In Production Example 5, 1.7 g of acrylonitrile is added, 2.3 g of glycidyl methacrylate is added in Production Example 6, 2.8 g of methacrylamide is added in Production Example 7, and other than that, the modified natural rubber B˜ G was obtained. Further, in the same manner as in the modified natural rubber A, the graft amount of the polar group-containing compound added as a monomer in the modified natural rubbers B to G was analyzed, and the results shown in Table 1 were obtained.

<Production Example 8>
Other than using 1.7 g of N, N-dimethylaminoethanethiol instead of 3.0 g of N, N-diethylaminoethyl methacrylate without adding tert-butyl hydroperoxide (t-BHPO) and tetraethylenepentamine (TEPA) Obtained modified natural rubber H in the same manner as in Production Example 1 above. Moreover, when the addition amount of dimethylaminomethanethiol in the obtained modified natural rubber H was analyzed using a pyrolysis gas chromatograph-mass spectrometer, it was 0.22% by mass with respect to the solid rubber component in the natural rubber raw material. .

<Production Examples 9-12>
Instead of 1.7 g of N, N-dimethylaminoethanethiol, 1.8 g of 4-mercaptopyridine was added in Production Example 9, 1.3 g of mercaptoethanol was added in Production Example 10, and 1.5 g of mercaptoacetic acid was added in Production Example 11. In Example 12, modified natural rubbers I to L were obtained in the same manner as in Production Example 8 except that 3.3 g of 3-mercaptopropyltrimethoxysilane was added. Further, in the same manner as the modified natural rubber H, the addition amount of the mercapto compound in the modified natural rubbers I to L was analyzed, and the results shown in Table 1 were obtained.

<Production Example 13>
Formic acid was added to the field latex to adjust the pH to 4.7, the latex was coagulated, and the obtained solid was treated 5 times with a creper, passed through a shredder and crushed, and then heated at 110 ° C. with a hot air dryer. And dried for 210 minutes to obtain natural rubber M.

<Production Examples 14 to 26>
In Production Examples 1 to 13, in place of the solidified product obtained by adding formic acid to the field latex, a cup lamp (obtained by natural coagulation of latex obtained from a natural rubber tree) was used as a natural rubber raw material. Modified natural rubbers N to Y and natural rubber Z were obtained in the same manner as in Production Examples 1 to 13 except that they were used. Further, in the same manner as in the modified natural rubber A or the modified natural rubber H, the graft amount or addition amount of the polar group-containing compound in the modified natural rubbers N to Y was analyzed, and the results shown in Table 1 were obtained.

  Next, a rubber composition having the formulation shown in Table 2 was prepared by kneading with a plast mill, and the Mooney viscosity, tensile strength (Tb), tan δ, and abrasion resistance were measured for the rubber composition by the following methods. Measured and evaluated. The results of the rubber composition according to Formula 1 are shown in Table 3, and the results of the rubber composition according to Formula 2 are shown in Table 4.

(1) Mooney viscosity The Mooney viscosity ML _{1 + 4} (130 ° C.) of the rubber composition was measured at 130 ° C. according to JIS K6300-1994.

(2) Tensile strength The vulcanized rubber obtained by vulcanizing the rubber composition at 145 ° C for 33 minutes was subjected to a tensile test according to JIS K6301-1995, and the tensile strength (Tb) was measured. . It shows that fracture resistance is so favorable that tensile strength is large.

(3) tanδ
Using a viscoelasticity measuring device [manufactured by Rheometrics Co., Ltd.] for the vulcanized rubber obtained by vulcanizing the above rubber composition at 145 ° C for 33 minutes, loss tangent (tanδ ) Was measured. It shows that it is excellent in low-loss property, so that tan-delta is small.

(4) Abrasion resistance For the vulcanized rubber obtained by vulcanizing the above rubber composition at 145 ° C for 33 minutes, the amount of wear at a slip rate of 60% at room temperature was measured using a Lambone type abrasion tester. Table 3 reference examples 1-7, and 100 the reciprocal of the worn amount of Comparative example 1 for examples 8-11 and Comparative example 1, reference example 12-18 in Table 3, examples 19-22 and Comparative example 2 The reciprocal of the wear amount of Comparative Example 2 is set to 100, and the reciprocal of the wear amount of Comparative Example 3 is set to 100 for Reference Examples 23 to 29, Examples 30 to 33, Reference Example 34 and Comparative Example 3 in Table 4, For Reference Examples 35 to 41, Examples 42 to 45, Reference Example 46, and Comparative Example 4 in Table 4, the reciprocal of the amount of wear in Comparative Example 4 was set as 100, and each index was displayed. The larger the index value, the smaller the wear amount and the better the wear resistance.

* 1 The types of rubber components used are shown in Table 3 and Table 4.
* 2 “Nipsil AQ” manufactured by Nippon Silica Kogyo.
* 3 Degussa “Si69”, bis (3-triethoxysilylpropyl) tetrasulfide.
* 4 N- (1,3-Dimethylbutyl) -N'-phenyl-p-phenylenediamine.
* 5 N, N'-dicyclohexyl-2-benzothiazolylsulfenamide.
* 6 Diphenylguanidine.
* 7 Dibenzothiazyl disulfide.
* 8 Nt-butyl-2-benzothiazylsulfenamide.

From the comparison between Examples and Comparative Examples in Tables 3 and 4, it was found that by using a modified natural rubber modified with a polar group-containing mercapto compound in place of natural rubber, the fracture characteristics, low loss and resistance of the rubber composition were reduced. It can be seen that the wear resistance can be greatly improved.

Claims (3)

A method for producing a modified natural rubber, wherein a polar group-containing mercapto compound is added with mechanical shearing force to at least one natural rubber raw material selected from the group consisting of natural rubber latex coagulum and natural rubber cup lamp,
The addition amount of the polar group-containing mercapto compound is 0.01 to 5.0 mass% with respect to the solid rubber component in the natural rubber raw material,
The polar group of the polar group-containing mercapto compound is an amino group, imino group, nitrile group, ammonium group, imide group, amide group, hydrazo group, azo group, diazo group, hydroxyl group, carboxyl group, carbonyl group, epoxy group, A method for producing a modified natural rubber, which is at least one selected from the group consisting of an oxycarbonyl group, a nitrogen-containing heterocyclic group, an oxygen-containing heterocyclic group, and a tin-containing group. The method for producing a modified natural rubber according to claim 1 , wherein the means for applying the mechanical shearing force is a biaxial extrusion kneader or a dry prebreaker. The method for producing a modified natural rubber according to claim 1 or 2 , wherein the polar group-containing mercapto compound is added to the natural rubber raw material at 50 to 160 ° C.