Coating rubber composition for conveyor belts and its use therefor

Abstract

Coating rubber composition for conveyor belts, comprising 10 to 65 parts by mass of carbon black, which has a specific surface area according to nitrogen adsorption of 50m² 2 / g or less, 20 to 100 parts by mass calcium carbonate, 5 to 20 parts by mass silicon dioxide and 40 parts by mass or less oil per 100 parts by mass of mixed rubber comprising natural rubber and at least one of butadiene rubber and SBR.

Description

TECHNICAL AREA

[0001] The present invention relates to a coating rubber composition for conveyor belts. The present invention relates in particular to a coating rubber composition for conveyor belts, which can suitably be used as coating rubber-forming materials and the like, which are used for adhesion between a rubber covering and a conveyor belt canvas and / or between canvases. State of the art

[0002] If an endless conveyor belt is repeatedly bent in the head and tail drum regions, the rubber covering of the belt at the end can curl, leading to delamination at the end joint if adhesion is insufficient. The endless conveyor belt is particularly susceptible to such delamination if the top surface rubber covering is too thick. Once the delamination of the end rubber covering reaches the core canvas, the conveyor belt can break due to abrasion and deterioration.

[0003] Accordingly, methods for joining a layered belt body comprising rubber covers and a core canvas arranged between them are used at the end regions of the same, forming a conveyor belt in an endless configuration, wherein the upper surface rubber cover of each end region of the belt body is cut from the core canvas towards the outside in a step-like pattern to form a joining surface, and the joining is carried out while each end part of the belt body is pressed against every other (Patent Documents 1 and 2).

[0004] Currently, to achieve adhesion of a sailcloth to a rubber cover enclosing a conveyor belt, a coating rubber-forming material is used between the rubber cover and the sailcloth, or between sailcloths. Alternatively, the coating rubber-forming material is used both between the rubber cover and the sailcloth, and also between sailcloths. While good adhesion between the coating rubber and the sailcloths results in excellent conveyor belt durability, there is a tendency for the removal process, intended to expose the core sailcloth during the continuous processing of the conveyor belt, to become difficult. This also applies between sailcloths. To facilitate continuous processing, an approach is used that involves increasing the amount of calcium carbonate contained in the coating rubber-forming material.However, this approach can unfortunately result in reduced durability.

[0005] The applicant of the present invention previously proposed a coating rubber composition for conveyor belts as a coating rubber-forming material used for adhesion between a rubber cover and a canvas and / or between canvases when an endless belt is produced by joining the end parts of a conveyor belt constructed such that a canvas adheres to a rubber cover, wherein the coating rubber composition has a suitably weakened adhesive strength to facilitate endless processing, while the adhesion durability between the rubber cover and the canvases or between canvases is improved compared to the case where the release force is reduced as a result of an increased amount of calcium carbonate in the coating rubber-forming material.That is, a coating rubber composition for conveyor belts is proposed in which carbon black and calcium carbonate are compounded as fillers in diene-based rubber, comprising mixed rubber consisting of 82.5 to 52.5 wt% diene-based rubber and 17.5 to 47.5 wt% recycled rubber component, and 10 to 85 wt% calcium carbonate per 100 wt% of the mixed rubber (Patent Document 3).

[0006] In the case where the above coating rubber composition for conveyor belts is used as a coating rubber-forming material intended for application between a rubber cover and a conveyor belt canvas, between canvases, or between both to form a reinforced layer, the adhesive strength is suitably weakened to facilitate continuous processing involving peeling, while maintaining sufficient permanent adhesion between them. Therefore, the above coating rubber composition can adequately achieve the desired goal of ensuring the conveyor belt can operate stably over an extended period.

[0007] For durability purposes, the coating rubber used between the rubber cover and the sailcloth, or between sailcloths, is required to have good adhesion in addition to high peel strength. When high peel strength is required, carbon black with a small particle diameter is generally compounded with the coating rubber to achieve this. However, in this case, the rubber's adhesion (observed visually by observing that the rubber adheres to the surface of a sailcloth after a peel test, taking cases where separation occurs across the entire surface of the coating rubber layer as 100%) will decrease on the peeled surface.A process involving the compounding of carbon black with a large particle diameter and a large amount of calcium carbonate can now be used to improve rubber adhesion at the peeled surface. However, in this case, the peel strength will decrease. As described above, peel strength and rubber adhesion at the adhesion surface are inversely related and therefore cannot be satisfied simultaneously.

[0008] Furthermore, the applicant of the present application has proposed a coating rubber composition for conveyor belts as a coating rubber-forming material used for adhesion between a rubber cover and a canvas and / or between canvases, wherein the coating rubber composition is capable of providing a vulcanized material that simultaneously exhibits both improved peel strength and rubber adhesion between the rubber cover and the canvas or between canvases, and which comprises carbon black and calcium carbonate as fillers in diene-based rubber, wherein the coating rubber composition consists of mixed rubber comprising 83.0 to 97.5 wt% diene-based rubber and 2.5 to 17.0 wt% recycled rubber component, and 45 to 75 wt% carbon black, which has a specific surface area according to nitrogen adsorption of 30 to 50 m². 2 / g and has a DBP oil absorption of 95 to 125 ml / 100 g, and comprises 5 to 50 parts by mass of calcium carbonate per 100 parts by mass of the mixed rubber (Patent document 4).

[0009] While adhesion to the canvas was the main problem for conventional coating rubbers, there is also a need to reduce energy loss at the core to give a conveyor belt energy-saving capability. There is a need for a coating rubber capable of reducing the energy loss that occurs when a moving conveyor belt flexes as a result of the objects being loaded on it.

[0010] Furthermore, if it is used as an endless belt in which the ends of the belt body are connected, the belt is exposed to a state of overvulcanization as a result of prolonged vulcanization, e.g., revulcanization in the endless section. Therefore, sufficient adhesion performance must be achieved even in the case of overvulcanization.

[0011] However, the rubber adhesion to the canvas is low when overvulcanized, and achieving good adhesion is difficult. To improve rubber adhesion in cases of overvulcanization, an adhesive-based compounding agent can be added. However, this cannot prevent the reduction in pull-off strength when vulcanized normally.

[0012] JP 2009-292960 A describes a coated rubber compound for conveyor belts. More precisely, the present invention relates to a coating rubber compound for a conveyor belt, which is suitable as a coating rubber molding material used for adhesion between a cover rubber and a canvas of an endless conveyor belt and / or between canvases.

[0013] WO 2000 / 026279 A1 describes a process for the formation of a free-flowing polymer / filler masterbatch powder, wherein the process comprises the intimate mixing of a polymer and a filler for a time and under shear conditions sufficient to transform the components into a free-flowing, associated composition of polymer and filler. DOCUMENT OF THE STATE OF THE TECHNOLOGY PATENT DOCUMENT Patent document 1: Japanese utility model JP 2562069 Y2 Patent document 2: Published Japanese patent application JP 2000-344318 A Patent document 3: Published Japanese patent application JP 2009-040815 A Patent document 4: Published Japanese patent application JP 2009-292960 A OUTLINE OF THE INVENTION PROBLEM THAT IS TO BE SOLVED BY THE INVENTION

[0014] The primary object of the present invention is to provide a coating rubber composition for conveyor belts which not only has improved pull-off force as a criterion for adhesion to canvases, but also has improved energy-saving capability while maintaining the durability of the conveyor belt.

[0015] A further object of the present invention is to provide a coating rubber composition for conveyor belts which is able to ensure sufficient rubber adhesion to canvases when overvulcanized, and which has good adhesion while maintaining pull-off strength and low energy loss when subjected to normal vulcanization. MEANS TO SOLVENT THE PROBLEM

[0016] The primary object of the present invention can be achieved by providing a coating rubber composition for conveyor belts comprising 10 to 65 parts by mass of carbon black, which has a specific surface area according to nitrogen adsorption of 50 m². 2 / g or less, containing 20 to 100 parts by mass of calcium carbonate, 5 to 20 parts by mass of silicon dioxide and 40 parts by mass or less of oil per 100 parts by mass of mixed rubber comprising natural rubber and at least one of butadiene rubber and SBR. EFFECT OF INVENTION

[0017] The coating rubber composition for conveyor belts according to the present invention can not only improve the adhesion of the conveyor belt to the canvas, but also its energy-saving capacity, while maintaining its durability.

[0018] Furthermore, compounding 5 to 20 parts by mass of silicon dioxide ensures sufficient rubber adhesion to the sailcloth, resulting in good bond strength even in the case of overvulcanization, while maintaining pull-off strength and energy efficiency during normal vulcanization. This can provide an endless tape in which the ends of the tape body are well vulcanized and adhere to each other. EXECUTIONAL FORMS FOR IMPLEMENTING THE INVENTION

[0019] The present invention uses a mixed rubber comprising natural rubber and at least one of butadiene rubber and SBR. The mixed rubber comprises natural rubber in a proportion of 15 to 65 wt%, preferably 25 to 55 wt%, and butadiene rubber and / or SBR in a proportion of 85 to 35 wt%, preferably 75 to 45 wt%. If natural rubber is used outside this range, the rubber's adhesive properties deteriorate. It is emphasized that any emulsion-polymerized SBR, solution-polymerized SBR, or oil-extended products thereof can be used as the SBR.

[0020] Soot, which has a specific surface area according to nitrogen adsorption of 50 m² 2Carbon black (GPF), FEF, SRF, and similar materials with a specific surface area of ​​20 to 65 parts by mass per 100 parts by mass of the blended rubber is used. If the amount of carbon black is less than this, the rubber adhesion deteriorates. Conversely, if the amount of carbon black is higher, energy loss increases. Furthermore, energy loss increases when carbon black with a larger specific surface area (according to nitrogen adsorption) than the above is used, for example, HAF carbon black.

[0021] Calcium carbonate is preferably used in a proportion of 25 to 100 parts by mass, preferably 30 to 70 parts by mass, per 100 parts by mass of the mixed rubber. If the amount of calcium carbonate is lower than this, the rubber's adhesive properties deteriorate. Conversely, if the amount of calcium carbonate is higher than this, the pull-off strength decreases and the hardness increases.

[0022] Aroma oil, paraffin oil, and the like are used as the oil, and an oil component contained in oil-enhanced SBR is also counted as oil. Oil is used in a proportion of 40 parts by mass or less, preferably 10 to 35 parts by mass, per 100 parts by mass of the blended rubber. If the amount of oil is higher than this, the energy loss increases. It is emphasized that in the examples in patent documents 3 and 4, oil is used in a proportion higher than this.

[0023] In the coating rubber composition for conveyor belts according to the present invention, a rubber composition in which the amounts of carbon black and calcium carbonate are changed to 10 to 65 parts by mass and 20 to 100 parts by mass respectively, and in addition 5 to 20 parts by mass of silicon dioxide are compounded, is used to improve the rubber's adhesion to sailcloth when it is overvulcanized.

[0024] Additional compounding of silicon dioxide increases the values ​​for hardness and tan δ, and thus the amounts of carbon black and calcium carbonate can be easily changed to achieve such values.

[0025] For the silicon dioxide used, a type with a specific surface area according to BET (as per ASTM D1993-03) of 70 to 200 m² is used with regard to its reinforcing properties and dispersibility. 2 / g, preferably from 70 to 190 m 2 / g has. These silicon dioxides are dry-process silicon dioxide, produced by pyrolysis of silicon halides or organosilicon compounds or the like, or wet-process silicon dioxide, produced by acid decomposition of sodium silicate and the like. Wet-process silicon dioxide is preferred in terms of cost and performance. In fact, commercially available products currently on the market for use in the rubber industry can be used as is.

[0026] The compounding ratio of silicon dioxide is 5 to 20 parts by mass, preferably 10 to 15 parts by mass, per 100 parts by mass of the mixed rubber. Compounding with silicon dioxide can improve the rubber's adhesion to the sailcloth when overvulcanized. If the compounding ratio of silicon dioxide is less than this, the rubber's adhesion to sailcloth cannot be improved to a desirable degree. Conversely, if the compounding ratio of silicon dioxide is greater than this, energy loss increases.

[0027] To improve the properties required for silicon dioxide, as described above, and to enhance its dispersibility in diene-based rubber (silicon dioxide has a poor affinity for rubber polymer and tends to form hydrogen bonds through silanol groups, thereby reducing its dispersibility in rubber), a silane adhesion promoter is preferably compounded. A polysulfide-based silane adhesion promoter is preferably used, which has an alkoxysilyl group reactive with a silanol group on the surface of silicon dioxide and a sulfur chain reactive with the polymer, e.g.: Bis(3-triethoxysilylpropyl)tetrasulfide, Bis(2-triethoxysilylethyl)tetrasulfide, Bis(3-trimethoxysilylpropyl)tetrasulfide, Bis(3-triethoxysilylpropyl) disulfide.

[0028] The vulcanization of the coating rubber composition for conveyor belts is carried out with at least one of the following vulcanization accelerators, e.g., sulfur- and thiazole-based accelerators (MBT, MBTS, ZnMBT and the like), sulfenamide-based accelerators (CBS, DCBS, BBS and the like), guanidine-based accelerators (DPG, DOTG, OTBG and the like), thiuram-based accelerators (TMTD, TMTM, TBzTD, TETD, TBTD and the like), dithiocarbamate-based accelerators (ZTC, NaBDC and the like), and xanthate-based accelerators (ZnBX and the like). Sulfur or various vulcanization accelerators are preferably used in a proportion of about 1 to 5 parts by mass, preferably about 2 to 4 parts by mass, per 100 parts by mass of the mixed rubber.Over-vulcanization can occur if a large amount of sulfur or various vulcanization accelerators are used, however, the vulcanization time has far more significant effects.

[0029] The following can suitably be compounded into the coating rubber composition for conveyor belts according to the present invention in addition to the components above: vulcanization aids, vulcanization retarders, and various compounding agents, e.g., various reinforcing agents other than carbon black, calcium carbonate, and silicon dioxide (fillers), anti-aging agents, antioxidants, pigments (colorants), plasticizers, flame retardants, solvents, surfactants (including leveling agents), dispersants, dehydrating agents, anti-corrosion agents, adhesion promoters, antistatic agents, fillers (mass agents), and processing aids, in a range that does not interfere with the objectives of the present invention.

[0030] After kneading with rollers, kneaders, Banbury mixers, and the like, the resulting conveyor belt coating rubber composition can be used as a coating rubber-forming material for adhesion between a rubber cover and a conveyor belt canvas, or between canvases (or both). Specifically, the conveyor belt coating rubber composition is pressure-bonded to both surfaces of the canvas, using calender rollers and the like to create a film-like reinforcing layer. Then, at least one layer of the aforementioned film-like reinforcing layer is laminated. Top and bottom surface rubber covers are then laminated onto the top and bottom surfaces of the reinforcing layer. Subsequently, the rubber covers are bonded to the canvas, e.g., by adhesive.The conveyor belt is produced by pressing for approximately 10 to 60 minutes at a temperature of approximately 140 to 170°C. The canvas is manufactured by weaving organic fibers, such as nylon, polyester, vinylon, and aramid, into a sailcloth-like fabric, which is then used as a core reinforcement layer.

[0031] If the tape is used here as an endless tape, with the ends of the tape body joined together, it will be subject to overvulcanization as a result of extended vulcanization, e.g., revulcanization in the endless section. Nevertheless, even if it is overvulcanized, sufficient adhesion must be achieved. Therefore, the total vulcanization time is extended to approximately 90 to 150 minutes. EXAMPLES

[0032] Next, the present invention will be described by means of examples. Example 1 Natural rubber (RSS #3) 50 mass-produced parts Butadiene rubber 50 mass-produced parts GPF soot 37 mass parts (has a specific surface area according to nitrogen adsorption of 29 m² 2 / G) Calcium carbonate 30 mass-produced parts Aroma oil 22.50 bulk parts (A-OMIX, product of Sankyo Yuka Gogyo KK) sulfur 2.5 mass parts Vulcanization accelerator CZ (product of Ouchi Shinko Chemical Industrial Co., Ltd.) 1.0 mass part

[0033] The above components were kneaded together using a Banbury mixer and the like to produce a coating rubber composition for conveyor belts. Example 2

[0034] A coating rubber composition for conveyor belts was manufactured as in Example 1, except that 68.74 parts by mass of oil-extended SBR (Nipol 1723, product of Zeon Corporation) (50 parts by mass SBR and the oil component is 18.74 parts by mass) was used instead of butadiene rubber, and that the amount of flavored oil was changed to 3.80 parts by mass (the total amount of oil component was 22.54 parts by mass). Example 3

[0035] A coating rubber composition for conveyor belts was produced as in Example 1, except that the amount of butadiene rubber was changed to 24.54 parts by mass, and that 35.00 parts by mass of oil-extended SBR (Nipol 1723) (25.46 parts by mass SBR, oil component was 9.54 parts by mass) was also used, and that the amount of flavor oil was also changed to 13.00 parts by mass (the total amount of oil component was 22.54 parts by mass). Example 4

[0036] A coating rubber composition for conveyor belts was used as in Example 3, except that instead of GPF carbon black, the same amount of FEF carbon black (with a specific surface area according to nitrogen adsorption of 42 m²) was used. 2 / g) was used, manufactured. Example 5

[0037] A coating rubber composition for conveyor belts was produced as in Example 3, except that instead of GPF carbon black, the same amount of SRF carbon black (with a specific surface area according to nitrogen adsorption of 24 m²) was used. 2 / g) was used. Example 6

[0038] A coating rubber composition for conveyor belts was produced as in Example 4, except that the amounts of natural rubber, butadiene rubber, oil-enhanced SBR and flavored oil were changed to 19.88 parts by mass, 46.96 parts by mass, 45.60 parts by mass (33.16 parts by mass SBR, where the oil component was 12.44 parts by mass) and 20.00 parts by mass respectively (the total amount of oil component was 32.44 parts by mass) and that 50.0 parts by mass FEF carbon black were used.

[0039] The following properties were measured for the conveyor belt coating compositions obtained in each of the above examples:

[0040] Normal physical properties: 100% modulus according to JIS K6251-2004 and hardness according to JIS K6253 at room temperature were measured for a vulcanized material obtained by press vulcanizing the target composition at 150°C for 30 minutes. The advantageous hardness here is 45 to 60.

[0041] Adhesion to sailcloth: The peel force for a polyester sailcloth (according to JIS K6256-1:2006 “Peel force Adhesion to textile fabric”) and the rubber adhesion (rubber bonded to the surface of a sailcloth is visually observed after peel tests; if tearing as a result of peeling is observed across the entire surface of the coating rubber layer, the rubber adhesion is 100%, indicating the best adhesion) were measured for a vulcanized material that had undergone normal vulcanization (at 150°C for 30 minutes).

[0042] Here, coating rubber compositions with a pull-off force of 6.0 N / mm or more and a rubber adhesion of 60% or more are considered to exhibit excellent durability as coating rubber for conveyor belts.

[0043] Calender processability (Vm): Mooney-Scorch was measured according to JIS K6300-1:2013 “unvulcanized rubber” to determine the minimum viscosity.

[0044] The advantageous minimum viscosity here is 20 to 40.

[0045] tan δ (20°C): Using a viscometer (viscoelastic spectrometer) (product of Toyo Seiki Seisaku-Sho Ltd.), tan δ at 20°C was measured for a vulcanized material that had undergone normal vulcanization (at 150°C for 30 minutes).

[0046] It is emphasized that measurements of tan δ were carried out while a test piece was stretched by 10% and vibrations with an amplitude of ±2% at a vibration frequency of 20Hz were applied.

[0047] Here, a tan δ value of 0.15 or less can reduce the energy loss resulting from bending of a moving conveyor belt and can be considered to have excellent energy-saving capabilities.

[0048] Measurement results are shown in Table 1 below. Table 1 measured sizes Example Example Example Example Example Example 1 2 3 4 5 6 Normal physical properties 100% module (MPa) 1,8 1,9 1,8 2,1 1,6 2,1 hardness 52 51 51 54 50 54 Adhesive strength on sailcloth Deduction force (N / mm) 7,8 7,5 8,2 7,9 7,3 8,9 Rubber adhesive properties (% ) 80 65 65 70 60 90 Vm (Point) 25,3 31,4 28,9 30,9 27,7 28,3 tan δ (20°C) 0,090 0,117 0,099 0,121 0,088 0,148 Comparative example 1

[0049] A coating rubber composition for conveyor belts was produced as in Example 3, except that the amount of GPF carbon black was changed to 10.00 parts by mass. Comparative example 2

[0050] A coating rubber composition for conveyor belts was produced as in Example 3, except that the amount of GPF carbon black was changed to 80.00 parts by mass. Comparative example 3

[0051] A coating rubber composition for conveyor belts was produced as in Example 3, except that the amount of calcium carbonate was changed to 20.00 parts by mass. Comparative example 4

[0052] A coating rubber composition for conveyor belts was produced as in Example 3, except that the amount of calcium carbonate was changed to 105.00 parts by mass. Comparative example 5

[0053] A coating rubber composition for conveyor belts was manufactured as in Example 3, except that the amounts of natural rubber, butadiene rubber, oil-enhanced SBR and flavored oil were changed to 39.77 parts by mass, 27.07 parts by mass, 45.60 parts by mass (33.16 parts by mass as SBR and 12.44 parts by mass as oil component) and 30.00 parts by mass (the total amount of oil component was 42.44 parts by mass), respectively, and that 63.0 parts by mass of FEF carbon black were used instead of GPF carbon black. Comparative example 6

[0054] A coating rubber composition for conveyor belts was prepared as in Example 3, except that the amount of oil-extended SBR was changed to 103.75 parts by mass (75.46 parts by mass as SBR and 28.29 parts by mass as oil component) and that natural rubber and aroma oil were not used. Comparative example 7

[0055] A coating rubber composition for conveyor belts was produced as in Example 6, except that the amount of natural rubber was changed to 66.84 parts by mass and the amount of oil-enhanced SBR was changed to 45.60 parts by mass (33.16 parts by mass as SBR and 12.44 parts by mass as oil component).<die Gesamtmenge an Ölkomponente war 32,44 Massenteile> ). Comparative example 8

[0056] A coating rubber composition for conveyor belts was manufactured as in Example 2, in which butadiene rubber was not used, except that the amounts of natural rubber, oil-extended SBR, GPF carbon black, calcium carbonate, and flavored oil were changed to 25.00 parts by mass, 64.93 parts by mass (47.22 parts by mass as SBR and 17.71 parts by mass as the oil component), 10.00 parts by mass, 80.00 parts by mass, and 3.00 parts by mass, respectively, and that, in addition, 55.56 parts by mass of tire-reclaimed rubber, grade A (Tire Rec Purple line, product of Muraoka Rubber Reclaiming Co., Ltd.) (27.78 parts by mass as the rubber component, 16.67 parts by mass as the carbon component) were added.<die Gesamtmenge an Kohlenstoffkomponente war 26,67 Massenteile> and 11.11 parts by mass as oil component<die Gesamtmenge an Ölkomponente war 31,82 Massenteile> ) were used. Comparative example 9

[0057] A coating rubber composition for conveyor belts was produced as in Example 2, in which no butadiene rubber was used, except that the amounts of natural rubber, oil-extended SBR, GPF carbon black, calcium carbonate, and flavored oil were changed to 30.00 parts by mass, 74.20 parts by mass (53.97 parts by mass as SBR and 20.23 parts by mass as the oil component), 60.00 parts by mass, 61.00 parts by mass, and 28.00 parts by mass, respectively, and that, in addition, 32.00 parts by mass of tire-regenerated rubber, grade A (Tire Rec Purple Line) (16.00 parts by mass as the rubber component, 9.60 parts by mass as the carbon component) were added.<die Gesamtmenge an Kohlenstoffkomponente war 69,60 Massenteile> and 6.40 parts by mass as oil component<die Gesamtmenge an Ölkomponente war 54,63 Massenteile> ) were used.

[0058] The results obtained from the comparison examples described above are shown in Table 2 below. Table 2 measured sizes Comparative examples 1 2 3 4 5 6 7 8 9 Normal physical properties 100% module (MPa) 1,4 2,8 1, 9 2,8 2,1 2,2 2,2 1, 4 1, 7 Hardness, adhesion to sailcloth 46 63 51 65 55 55 54 52 53 Deduction force (N / mm) 7,9 8,9 7,7 2,7 9,5 6,5 9,8 5,8 7,3 rubber is able (%) 40 100 50 100 100 50 50 60 100 Vm (Point) 25,8 36,6 27,5 25,9 26,8 31,9 28, 5 30, 9 24,2 tan δ (20°C) 0,069 0,174 0,096 0,126 0,207 0,105 0,169 0,153 0,207

[0059] The above results suggest the following conclusions: (1) In comparative example 1, where the amount of carbon black is too low, the rubber adhesion is poor. (2) In comparative example 2, where the amount of soot is too large, the energy loss is large. (3) In comparative example 3, where the amount of calcium carbonate is too low, the rubber adhesion is poor. (4) In comparative example 4, where the amount of calcium carbonate is too large, the pulling force is low and the hardness is high. (5) In comparative example 5, where the amount of oil is too large, the energy loss is large. (6) In comparative example 6, where no natural rubber is used, the rubber adhesion is poor. (7) In comparative example 7, where the proportion of natural rubber in the mixture is too high, the rubber adhesion is poor and the energy loss is large. (8) In comparative example 8, where a universal coating is used, continuous processing is improved, but the peeling force is low. (9) In comparative example 9, where a universal coating is used, the energy loss is large. Example 7

[0060] For the coating rubber composition for conveyor belts produced in Example 3, various properties were measured for a normally vulcanized material thereof (vulcanized at 150°C for 30 minutes), and the adhesion to a sailcloth (the peeling force and the rubber adhesion to a polyester sailcloth) was measured for an overvulcanized material thereof (vulcanized at 150°C for 140 minutes). Example 8

[0061] A coating rubber composition for conveyor belts was prepared as in Example 7, except that the amounts of GPF carbon black and calcium carbonate were changed to 32 parts by mass and 25 parts by mass, respectively, and that 10 parts by mass of silicon dioxide (Nipsil AQ, a product of Tosoh Silica Corporation) were also compounded. Measurements were then carried out on normally vulcanized and overvulcanized samples of this composition in a manner similar to that described above. Example 9

[0062] A coating rubber composition for conveyor belts was prepared as in Example 8, except that the amounts of GPF carbon black, calcium carbonate, and silicon dioxide were changed to 29 parts by mass, 20 parts by mass, and 15 parts by mass, respectively. Measurements were then carried out on normally vulcanized and overvulcanized materials of this composition in a manner similar to that described above. Comparative example 10

[0063] A coating rubber composition for conveyor belts was prepared as in Example 8, except that the amounts of GPF carbon black and silicon dioxide were changed to 17 parts by mass and 30 parts by mass, respectively. Measurements were then carried out on normally vulcanized and overvulcanized samples of this composition in a manner similar to that described above.

[0064] Comparative example 11

[0065] A coating rubber composition for conveyor belts was produced as in Example 3, except that in addition Adhesive-based compounding agent I (Penacolite Resin B-18-S, product of INDSPEC Chemical Co.) 2 mass parts Adhesive-based compounding agent II (CYREZ964, product of Allnex) 2 mass parts The materials were compounded. Then measurements were carried out on normally vulcanized and overvulcanized materials in a similar manner to that described above.

[0066] Measurement results from examples 7 to 9 and comparison examples 10 to 11, as described above, are shown in Table 3 below. Table 3 Measured sizes Example 7 Example 8 Example 9 cf. example 10 Cf. - e.g. 11 [normal vulcanized material] Normal physical Characteristics 100% module (MPa) 1,8 1,8 1,7 1,6 1,9 hardness 51 52 53 54 51 Adhesive strength on sailcloth Deduction force (N / mm) 9,0 8,9 8,9 9,2 4,2 Rubber adhesive properties (% ) 85 95 100 100 100 Vm (Point) 26,8 35,2 39,4 53,4 29,8 tan δ (20°C) 0, 109 0,124 0,132 0,157 0,118 [overvulcanized material] Adhesive strength on sailcloth Deduction force (N / mm) 7,56 7,54 7,52 7,50 7,08 Rubber adhesive properties (% ) 60 90 100 100 90

[0067] The above results suggest the following conclusions: (1) In Example 7, the rubber adhesion of the overvulcanized material is only 60%. (2) In Example 8, where 10 parts by mass of silicon dioxide are compounded, the rubber adhesion of the overvulcanized material has improved significantly. (3) In Example 9, where 20 parts by mass of calcium carbonate and 15 parts by mass of silicon dioxide are compounded, the rubber adhesion of the overvulcanized material has improved significantly. (4) In comparative example 10, in which 30 parts by mass of silicon dioxide are compounded, the minimum viscosity is high, resulting in impaired calender processability. Furthermore, the tan δ value (20°C) is high, resulting in increased energy loss. (5) The adhesion of the overvulcanized material to sailcloth is improved by compounding an adhesive-based compounding agent, however, the adhesion to the sailcloth (pull-off force) is reduced for the normally vulcanized material.

Claims

[1] A coating rubber composition for conveyor belts comprising 10 to 65 parts by mass of carbon black having a specific surface area according to nitrogen adsorption of 50m 2 / g or less, 20 to 100 parts by mass of calcium carbonate, 5 to 20 parts by mass of silicon dioxide and 40 parts by mass or less of oil per 100 parts by mass of mixed rubber comprising natural rubber and at least one of butadiene rubber and SBR. [2] The coating rubber composition for conveyor belts according to claim 1, wherein the mixed rubber comprises 15 to 65 mass% of natural rubber and 85 to 35 mass% of butadiene rubber or SBR. [3] The coating rubber composition for conveyor belts according to claim 1, wherein the amount of oil is 10 to 35 parts by mass. [4] Use of the coating rubber composition for conveyor belts according to claim 1 as a coating rubber-forming material between a rubber cover and a canvas of a conveyor belt and / or between canvases. [5] Use of a coating rubber composition according to any one of claims 1 to 4 for producing a conveyor belt comprising an upper surface rubber covering layer, a sheet-like reinforcing layer and a lower surface rubber covering layer, wherein the coating rubber composition for conveyor belts according to claim 4 is used as a coating rubber-forming material for the reinforcing layer when the coating rubber composition is press-bonded to both surfaces of a canvas to form a sheet-like reinforcing layer. [6] Use of a coating rubber composition according to any one of claims 1 to 4 for producing the conveyor belt according to claim 5 as an endless belt in which the ends of a belt body are connected to one another.