AU2007273485A1 - Rubber composition for conveyor belt and conveyor belt - Google Patents

Description

CERTIFICATE OF TRANSLATION I, Fumio MITSUHASHI, am a patent attorney of ION PATENT of Hayakawa-Tonakai Bldg. 3F, 12-5, Iwamoto-cho 2-chome, Chiyoda-ku, Tokyo, Japan, do solemnly and sincerely declare that I am conversant with the Japanese and English languages and I have executed with the best of my ability this translation into English of PCT Application No. PCT/JP2007/C63894 attached hereto which was filed on July 12, 200/ in the name of THE YOKOHAMA RUBBER CO., LTD. And believe that the translation is true and correct. Tokyo: December 26, 2008 Fumio MITSUHASHI Patent Attorney DESCRIPTION RUBBER COMPOSITION FOR CONVEYOR BELT AND CONVEYOR BELT Technical Field [0001] The present invention relates to a rubber composition for conveyor belts, and to a convEyor belt. Background Art [0002] Conveyor belts are commonly used for transporting materials. However, owing to increases in transport volume, improvements in transport efficiency and other factors, there exists a desire for conveyor belts of larger size and higher strength. Recently, conveyor belts with total lengths of up to several kilometers have even emerged. As a result, equipment costs and power consumption have grown, leading to a desire for low-cost, low-power consumption belt conveyor systems. In particular, ways for lowering the cost and power consumption of be-t conveyors by improving the properties of the rubber making up the belt are being investigated. [0003] For example, Patent Document 1 discloses "a conveyor belt which is wrapped around and travels between a 2 drive pulley and an idler pulley and is furnished to a goods transporting system, the conveyor belt being characterized in that the loss factor (tan 8) and dynamic modulus (E'), which are properties of rubber at a belt inner surface that comes into contact with the pulleys, respectively satisfy the conditions 0.04 5 tan 8 0.12 and E' 20 kgf/cm 2 ," and also discloses "a conveyor belt which is wrapped around and travels between a drive pulley and an idler pulley and is furnished to a goods transporting system, the conveyor belt being characterized in that rubber at an inner surface of the conveyor belt includes, with respect to a polymer composed of 40 to 100 parts by weight of natural rubber and from 60 to 0 parts by weight of polybutadiene rubber, from 20 to 55 parts by weight of carbon black." [0004] Patent Document 2 discloses "a conveyor belt having, on at least the top or bottom side of a core layer, a cover rubber in contact with a belt supporting member, the cover rubber being characterized by including silica and a silane coupling agent." [0005) In addition, the present applicant has disclosed "a rubber composition for conveyor belts, comprising from 30 to 65 parts by weight of carbon black having the colloidal properties shown below per 100 parts by weight of 3 a rubber component: 1) a nitrogen adsorption specific surface area (N 2 SA) of up to 80 m 2 /g, 2) an iodine adsorption (IA) of up to 70 mg/g, and 3) a dibutyl phthalate (DBP) oil absorption of at least 100 cm 3 /100 g and a loss factor tan 8 at a frequency of 10 Hz, 2% dynamic strain and 20 0 C of more than 0.120 and up to 0.200 (see Patent Document 3). [0006] Patent Document 4 discloses, in Claim 1, "a rubber compound which has an improved reversion resistance and includes the following: (a) a rubber selected from the group consisting of natural rubber, rubber derived from a diene monomer, and mixtures thereof; (b) from about 0.1 to about 10 phr of a bismaleimide compound of the following general formula (I) O 0 I it Y-C-C C-C-Y HC-C 0 0 (wherein R is a divalent acyclic aliphatic grcup of from about 2 to about 16 carbons, a divalent cyclic aliphatic group of from about 5 to about 20 carbons, a divalent aromatic group of from about 6 to about 18 carbons or a divalent alkyl aromatic group of from about 7 to about 24 carbons, which divalent groups may include a heteroatom 4 selected from among oxygen, nitrogen and sulfur; X is 0 or an integer from 1 to 3; and Y is hydrogen or -CH 3 ); and (c) from about 0.1 to about 10 phr of a bisbenzottiazolyldithio compound of the following general formula (II) S 2- R

-

S 2 s ONN (wherein R 1 is selected from the group consisting of alkylene groups having from 1 to 20 carbons, cycloalkylene groups having from 6 to 24 carbons, arylene groups having from 6 to 18 carbons, alkarylene groups having from 7 to 25 carbons and divalent groups of the following formula --(- R--O 0-(- R1 -)- ). Patent Document 4 also discloses, in Claim 2, "the rubber compound of claim 1 in the shape of a tire, hose, belt or shoe sole." (0007] Patent Document 1: JP 11-139523 A Patent Document 2: JP 2004-10215 A Patent Document 3: JP 2004-18752 A Patent Document 4: JP 10-77361 A Disclosure of the Invention Problems to be Solved by the Invention [0008] However, in the case of the conveyor belt described in Patent Document 1, the purpose of which is to 5 lower the tan 8 value and reduce power consumption, when the tan 5 value is made too small, the breaking strength (TB) and the elongation at break (EB) decline, sometimes leading to an inferior tear strength and fatigue resistance. As a result, surface damage in cover rubber and the like proceeds during conveyor belt travel, which in turn triggers surface failure of the conveyor belt, resulting in unstable operation. The conveyor belt described in Patent Document 2 has a high energy loss index, making the reduction in power consumption insufficient. Because the conveyor belt used in the rubber composition for conveyor belts described in Patent Document 3 has a high energy loss index, the reduction in power consumption is sometimes insufficient owing to disparities in the line on which the belt operates (e.g., line gradient and curvature). Moreover, it has been found that belts in which the compound described in Patent Document 4 is used have a poor abrasion resistance when the amount of natural rubber (NR) exceeds 80 parts by weight. It has also been found that, at a natural rubber content of less than 25 parts by weight, the 25% modulus (s 25 ) decreases and the energy loss index (AH) increases, as a result of which a reduction in 6 power consumption cannot be achieved. In addition, because the compound described in Patent Document 4 contains silica but contains neither a silane coupling agent ror diethylene glycol, the loss factor tan 8 and the energy loss index (AH) are both large, preventing a reduction in the power consumption from being achieved. [0009] It is therefore an object of the prEsent invention to provide both a rubber composition for conveyor belts and a conveyor belt which enable a sufficient reduction in power consumption to be achieved while retaining such basic properties as a 'high breaking strength and a good abrasion resistance. Means for Solving the Problems [0010] The inventors have conducted extensive investigations in order to resolve the above problems. As a result, they have discovered that conveyor belts in which the back side is formed with a rubber composition containing specific amounts of a rubber component composed of natural rubber (NR) and polybutadiene rubber (BR) in a specific proportion, carbon black, silica, a E.ilane coupling agent and diethylene glycol, or a rubber composition having a specific loss factor tan 8 and a specific energy loss index (AH) are able to achieve a 7 sufficient reduction in power consumption while retaining basic properties such as a high breaking strength and a good abrasion resistance. [0011] Accordingly, the present invention provides the following (1) to (15). (1) A rubber composition for conveyor belts, comprising a rubber component composed of natural rubber (NR) and polybutadiene rubber (BR), carbon black, silica, a silane coupling agent and diethylene glycol, wherein: the natural rubber and the polybutadiene rubber in the rubber component have a weight ratio NR/B*R therebetween of from 80/20 to 25/75, the carbon black content per 100 parts by weight of the rubber component is from 15 to 35 parts by weight, the silica content per 100 parts by weight of the rubber component is from 5 to 25 parts by weight, the silane coupling agent content per 100) parts by weight of the rubber component is from 0.5 to 3 parts by weight, and the diethylene glycol content per 100 parts by weight of the rubber component is from 0.5 to 4.5 parts by weight. (2) The rubber composition for conveyor belts of (1), further comprising 1,3-bis(citraconimidomethyl) benzene and/or hexamethylene-1,6-bis(thiosulfate) disodium salt 8 dihydrate. (3) The rubber composition for conveyor belts of (2), wherein the content of 1,3-bis(citraconimidomethyl) benzene and/or hexamethylene-1,6-bis(thiosulfate) disodium salt dihydrate is from 0.1 to 2 parts by weight per 100 parts by weight of the rubber component. (4) The rubber composition for conveyor belts of any one of (1) to (3), wherein the silica has a nitrogen adsorption specific surface area (N 2 SA) of from 100 to 250 m2 m 2/g. (5) The rubber composition for conveyor belts of any one of (1) to (4), wherein the polybutadiene rubber (BR) is an end-modified polybutadiene rubber. [0012] (6) A conveyor belt comprising a top cover rubber layer, a reinforcing layer and a bottom cover rubber layer, wherein at least a back side of the bottom cover rubber layer is formed of the rubber composition for conveyor belts of any one of (1) to (5). [0013] (7) A rubber composition for conveyor belts which has: a loss factor tan 5, as measured at a temperature of 200c, at 10% extension and under the application of ±2% amplitude strain oscillations at a frequency of 10 Hz, of from 0.04 9 to 0.07; and an energy loss index (AH), as defined by formula (1) below, of up to 0.080 AH = (SpGr x tan 8)/M 2 s (1), where SpGr is the specific gravity (g/cm 3 ) at 20 0 C, tan 8 is the loss factor measured at a temperature of 20 0 C, at 10% extension and under the application of ±2% amplitude strain oscillations at a frequency of 10 Hz, and M 25 is the tensile stress (MPa) at 25% elongation. (8) The rubber composition for conveyor belts of (7), comprising a rubber component composed of natural rubber (NR) and polybutadiene rubber (BR). (9)The rubber composition for conveyor belts of (7), comprising a rubber component composed of natural rubber (NR) and polybutadiene rubber (BR), carbon black, silica, a silane coupling agent and diethylene glycol. (10) The rubber composition for conveyor belts of (7), comprising a rubber component composed of natural rubber (NR) and polybutadiene rubber (BR), carbon black, silica, a silane coupling agent and diethylene glycol, wherein: the natural rubber and the polybutadiene rubber in the rubber component have a weight ratio NR/BR therebetween of from 80/20 to 25/75, the carbon black content per 100 parts by weight of 10 the rubber component is from 15 to 35 parts by weight, the silica content per 100 parts by weight of the rubber component is from 5 to 25 parts by weight, the silane coupling agent content per 100 parts by weight of the rubber component is from 0.5 to 3 parts by weight, and the diethylene glycol content per 100 parts by weight of the rubber component is from 0.5 to 4.5 parts by weight. (11) The rubber composition for conveyor belts of (9) or (10), wherein the silica has a nitrogen adsorption specific surface area (N 2 SA) of from 100 to 25J m 2 /g. (12) The rubber composition for conveyor belts of any one of (8) to (11), wherein the polybutadiene rubber (BR) is an end-modified polybutadiene rubber. [0014] (13) The rubber composition for conveyor belts of any one of (7) to (12), further comprising 1,3 bis(citraconimidomethyl) benzene and/or hexamethylene-1,6 bis(thiosulfate) disodium salt dihydrate. (14) The rubber composition for conveyor belts of (13), wherein the content of 1,3-bis(citraconimidomethyl) benzene and/or hexamethylene-1,6-bis(thiosulfate) disodium salt dihydrate is from 0.1 to 2 parts by weight per 100 parts by weight of the rubber component.

11 [0015] (15) A conveyor belt comprising a top cover rubber layer, a reinforcing layer and a bottom cover rubber layer, wherein at least a back side of the bottom cover rubber layer is formed of the rubber composition for conveyor belts of any one of (7) to (14). Advantages of the Invention [0016] As described below, the present invEntion is useful because it provides a conveyor belt which is capable of achieving a sufficient reduction in power consumption while retaining basic properties such as a high breaking strength and good abrasion resistance. Brief Description of the Drawing [0017] FIG. 1 is a schematic cross-sectional view of a preferred embodiment of a first conveyor belt according to the present invention. Legend [0018] 1: Conveyor belt 2: Top cover rubber layer 3: Reinforcing layer 12 4: Bottom cover rubber layer 5: Article transporting surface 11,16: Outer layers 12, 15: Inner layers Best Mode for Carrying Out the Invention [0019) The invention is described more fully below. The rubber composition for conveyor belts according to a first aspect of the invention (sometimes referred to below simply as "the first rubber composition for conveyor belts of the invention") is a rubber composition which includes a rubber component composed of natural rubber ard polybutadiene rubber, carbon black, silica, a silane coupling agent and diethylene glycol, wherein the natural rubber and the polybutadiene rubber in the rubber component have a weight ratio NR/BR therebetween of from. 80/20 to 25/75, the carbon black content per 100 parts by weight of the rubber component is from 15 to 35 parts by weight, the silica content per 100 parts by weight of the rubber component is from 5 to 25 parts by weight, the silane coupling agent content per 100 parts by weight of the rubber component is from 0.5 to 3 parts by weight, and the diethylene glycol content per 100 parts by weight of the rubber component is from 0.5 to 4.5 parts by weight.

13 The respective ingredients in the first rubber composition for conveyor belts of the invention are described below in detail. [0020] Rubber Component The rubber component is made up of natural rubber and polybutadiene rubber. The weight ratio (NR/BR) of natural rubber to polybutadiene rubber in the rubber component is from 80/20 to 25/75, preferably from 70/30 to 50/50, and more preferably from 70/30 to 60/40. By having the relative proportions of the natural rubber and the polybutadiene rubber fall withIn the foregoing range, the resulting first rubber composition for conveyor belts of the invention has, following vulcanization, both a good breaking strength and a good abrasion resistance, thus making it possible :o retain the basic physical properties of a conveyor belt. This appears to be attributable to the good compatibility of natural rubber and polybutadiene rubber, which further enhances the reinforcing properties. [0021) In the first rubber composition for conveyor belts of the present invention, the polybutadiene rubber has a weight-average molecular weight of preferably at 14 least 500,000, and more preferably at least 550,000. A weight-average molecular weight in this range enhances the breaking strength and tear strength of the first rubber composition for conveyor belts of the invention after vulcanization, and also results in a better abrasion resistance. [0022] In the first rubber composition for conveyor belts of the invention, the polybutadiene rubber is preferably an end-modified polybutadiene rubber. The end-modified polybutadiene rubber is not s.ibject to any particular limitation. The method used for end-modifying polybutadiene rubber may be, for example, one in which an end (an active end) of the polybutadiene rubber is modified using a modifier. Illustrative examples of such modifiers include tin halides such as tin tetrachloride and tin tetrabromide, halogenated organotin compounds such as tributyltin chloride, silicon compounds such as silicon tetrachloride and chlorotriethylsilane, isocyanate group-bearing compounds such as phenyl isocyanate, amide compounds such as N methylpyrrolidone (NMP), lactam compounds, urea compounds, and isocyanurate derivatives. [0023] By using such an end-modified polybutadiene rubber, the subsequently described loss factor tan 8 and 15 energy loss index (AH) of the resulting first rubber composition for conveyor belts following vulcanization are both within good ranges, making it possible to achieve a sufficient reduction in power consumption. This appears to be attributable to the contribution by the modified end moiety to crosslinking, and to an increase in the crosslink density following vulcanization. [0024) Commercial products may be used as the end modified polybutadiene having a weight-average molecular weight of at least 500,000. A specific example is Nipol BR1250H (weight-average molecular weight, 570,000; NMP-modified) produced by Zeon Corporation. [0025] Carbon Black The carbon black is not subject to any particular limitation, although carbon black which includes GPF (General Purpose Furnace) black is preferred. It is also possible to include any of the other carbon bl-acks shown below. [0026] Examples of other carbon blacks include HAF (High Abrasion Furnace), SAF (Super Abrasion Furnace), ISAF (Intermediate Super Abrasion Furnace), FEF (Fast Extruding Furnace), SRF (Semi-Reinforcing Furnace), FT (Fine Thermal) 16 and MT (Medium Thermal). [0027] Commercial products may be used as such carbon blacks. Exemplary GPF blacks include Asahi #55 (produced by Asahi Carbon Co., Ltd.), Seast V (Tokai Carbon Co., Ltd.) and Diablack G (Mitsubishi Chemical Corporation). Exemplary HAF blacks include Seast 3 (Tokai Carbon Co., Ltd.) and Shoblack N339 (Showa Cabot KK). An example of an ISAF black is Shoblack N220 (Showa Cabot KK), an example of a SAF black is Seast 9 (Tokai Carbon Co., Ltd.), an example of a FEF black is HTC #100 (Shin Nikka Carbon), examples of SRF blacks include Asahi #50 (Asahi Carbon Co., Ltd.) and Mitsubishi Diablack R (Mitsubishi Chemical Corporation), and examples of FT blacks include Asahi #15 (Asahi Carbon Co., Ltd.) and HTC #20 (Shin Nikka Carbon). [0028] In the first rubber composition for conveyor belts of the invention, the content of such carbon blacks is from 15 to 35 parts by weight, preferably from 20 to 30 parts by weight, and more preferably from 25 to 30 parts by weight, per 100 parts by weight of the rubber component. At a carbon black content in the above range, the breaking strength and abrasion resistance of t.he resulting first rubber composition for conveyor belts of the invention 17 following vulcanization are both good, thus enabling the conveyor belt to retain its basic properties. Moreover, because the subsequently described loss factor tan 8 and energy loss index (AH) are both in good ranges, a sufficient reduction in the power consumption can be achieved. This appears to be attributable to large molecular interactions between the carbon black and the above-described rubber component, which enhances the reinforcing properties. [0029] In the first rubber composition for conveyor belts of the present invention, by using at lEast GPF as the carbon black, the energy loss index of a first conveyor belt according to the invention formed from tl.e resulting rubber composition is further improved. [0030] Silica The silica is not subject to any particular limitation. Illustrative examples include fumed silica, fired silica, precipitated silica, pulverized silica, fused silica, finely divided anhydrous silica, finely divided hydrated silica, hydrated aluminum silicate and hydrated calcium silicate. Of these, finely divided hydrated silica is preferable because the breaking strength and the abrasion resistance 18 of the resulting first rubber composition for conveyor belts following vulcanization become better. [0031] Commercial products may be used as such silicas. Illustrative examples of finely divided hydrated silicas include Nipsil AQ (Nippon Silica Kogyo KK) an. Tokusil GU (Tokuyama Corporation). [0032] In the first rubber composition for conveyor belts of the invention, the content of such silicas is from 5 to 25 parts by weight, and preferably from 10 to 20 parts by weight, per 100 parts by weight of the rubber component. At a silica content in the above range, the loss factor tan 8 and the energy loss index (AH) of the resulting first rubber composition for conveyor belts of the invention following vulcanization are both in good ranges, enabling a sufficient reduction in power consumption to be achieved. The reason is thought to be that interactions between molecules of the silica and the above-described rubber component can be made smaller than for carbon black. (0033] In the first rubber composition for conveyor belts of the invention, the silica has a nitrogen adsorption specific surface area (N 2 SA) of preferably from 100 to 250 m 2 /g, and more preferably from 125 to 200 m 2 /g. The nitrogen adsorption specific surface area is an alternative characteristic for the surface area available 19 for use by silica in adsorption to rubber molecules, and is measured as the amount of nitrogen adsorption to the silica surface. When this value is in the above range, the subsequently described loss factor tan 8 and energy loss index (AH) of the first rubber composition for conveyor belts of the invention following vulcanization are both in even better ranges, enabling a more sufficient reduction in power consumption to be achieved. [0034] Silane Coupling Agent It is preferable to use, as the silane coupling agent, a polysulfide-type silane coupling agent employed in rubber applications. Illustrative examples of such polysulfide-type silane coupling agents include bis(3 triethoxysilylpropyl)tetrasulfide and bis(3 triethoxysilylpropyl)disulfide. Of these, bis(3-triethoxysilylpropyl)tetrasulfide is preferred because the breaking strength of the first rubber composition for conveyor belts of the invention following vulcanization is even better. [0035] Commercial products may be used as such silane coupling agents. Specific examples include bis(3- 20 triethoxysilylpropyl)tetrasulfide (Si69, produced by Degussa) and bis(3-triethoxysilylpropyl)disulfide (Si75, produced by Degussa). [00361 In the first rubber composition for conveyor belts of the invention, the content of such silane coupling agents is from 0.5 to 3 parts by weight, and preferably from 1 to 2 parts by weight, per 100 parts by weight of the rubber component. At a silane coupling agent content in the above range, the resulting first rubber composition for conveyor belts of the invention has, following vulcanization, a good breaking strength. This is thought to be on account of an increase in chemical bonding between the silane coupling agent and the silica. [0037] Diethylene Glycol The diethylene glycol is a compound having the chemical formula (CH 2 0HCH 2

)

2 0. Commercial products produced by Nippon Shokubai Co., Ltd. may be used as the diethylene glycol. [0038] In the first rubber composition for conveyor belts of the present invention, the content of the diethylene glycol is from 0.5 to 4.5 parts by weight, preferably from 0.5 to 2 parts by weight, and more 21 preferably from 0.6 to 1.8 parts by weight, per 100 parts by weight of the rubber component. At a diethylene glycol content in the above range, the subsequently described loss factor tan 8 and Energy loss index (AH) of the resulting first rubber composition for conveyor belts of the invention following vulcanization are both in good ranges, thus making it possible to achieve a sufficient reduction in power consumption. This is believed to be on account of the ability to rEduce molecular interactions between the silica and the rubber component. [0039] In addition to the above-described ingredients, in one preferred embodiment, the first rubber composition for conveyor belts of the invention also includes 1,3 bis(citraconimidomethyl) benzene and/or hexamethylene-1,6 bis(thiosulfate) disodium salt dihydrate. Here, 1,3-bis(citraconimidomethyl) benzene is the compound having chemical formula (1) below, and hexamethylene-1,6 bis(thiosulfate) disodium dihydrate is the compound having chemical formula (2) below. [0040] 22

H

3 C O O CH 3

N-H

2 C CH2-N (1) o O Na+-0 3

S-S-(CH

2 ) -S-S0 3 Na 4 2H 2 0 (2) [00411 Including these compounds makes the subsequently described energy loss index (AH) of the resulting first rubber composition for conveyor belts of the invention smaller, enabling a sufficient reduction in pcwer consumption to be achieved. This is because the subsequently described tensile stress at 25% elongation (M 2 5 ) of the resulting first rubber composition for conveyor belts of the invention is higher and the loss factor tan 8 is lower. Yet, given that these compounds are reagents which prevent reversion of the rubber composition (anti-reversion agents), such an effect was unanticipated. [0042] In the first rubber composition for conveyor belts of the invention, the content of these compounds is preferably from 0.1 to 2 parts by weight, and more preferably from 0.2 to 1 parts by weight, per 100 parts by weight of the rubber component. In cases where both 1, 3-bis (citraconimidomethyl) benzene and hexamethylene-1,6-bis(thiosulfate) disodium salt 23 dihydrate are included, this content refers tc the combined content thereof. [00431 Commercial products may be used as these compounds in the first rubber composition for conveyor belts of the invention. For example, Perkalink 900 (produced by Flexsys) may be used as the 1,3-bis(citraconimidomethyl) benzene, and Duralink HTS (Flexsys) may be used as the hexamethylene 1,6-bis(thiosulfate) disodium dehydrate. [0044] In addition to the various above-mentioned ingredients, the first rubber composition for conveyor belts of the invention may also include crosslinking agents such as vulcanizing agents, vulcanizing aids and vulcanization accelerators, and vulcanization retarders. In addition, various other compounding ingredients may be included insofar as the objects of the invention are attainable. [0045] Exemplary vulcanizing agents include sulfur-based vulcanizing agents, organic peroxide-type vulcanizing agents, metal oxide-type vulcanizing agents, phenolic resins and quinone dioxime. Illustrative examples of sulfur-based vulcanizing agents include powdered sulfur, precipitated sulfur, highly dispersible sulfur, surface-treated sulfur, insoluble 24 sulfur, dimorpholine disulfide and alkylphenol disulfides. Illustrative examples of organic peroxide-typE vulcanizing agents include benzoyl peroxide, t-butylhydroperoxide, 2,4 dichlorobenzoyl peroxide, 2,5-dimethyl-2,5-di(t butylperoxy)hexane and 2,5-dimethylhexane-2,5-di(peroxyl benzoate). Other examples of suitable vulcanizing agents include magnesium oxide, litharge, p-quinone dioxime, p dibenzoylquinone dioxime, poly-p-dinitrosobenzene and methylene dianiline. [0046] Exemplary vulcanizing accelerators include aldehyde ammonia, guanidine, thiourea, thiazole, sulfenamide, thiuram and dithiocarbamate-type accelerators. An example of an aldehyde ammonia-type accelerator is hexamethylene tetramine (H). An example of a guanidine-type accelerator is diphenylguanidine. An example of a thiourea-type accelerator is ethylenethiourea. Examples of thiazole-type accelerators include dibenzothiazyl disulfide (DM), 2-mercaptobenzot.hiazole and the zinc salt thereof. Examples of sulfenamide-type accelerators include N cyclohexyl-2-benzothiazolylsulfenamide (CZ) anc' N-t-butyl- 25 2-benzothiazolylsulfenamide (NS). Examples of thiuram-type accelerators include tetramethylthiuram disulfide (TMTD) and dipentamethylenethiuram tetrasulfide. Examples of dithiocarbamate-type accelerators include sodium dimethyldithiocarbamate, zinc dimethyldithiocarbamate, tellurium diethyldittiocarbamate, copper dimethyldithiocarbamate, iron dimethyldithiocarbamate and pipecoline pipecolyl dithiocarbamate. [0047] Vulcanizing aids that may also be used include common rubber aids such as zinc oxide, stearic acid, oleic acid and the zinc salts thereof. [0048] When such vulcanizing agents, vulcanization accelerators and vulcanizing aids are included, the combined content thereof is preferably from 0.1 to 10 parts by weight, and more preferably from 0.5 to 5 parts by weight, per 100 parts by weight of the rubber component. At a content in this range, the resulting first rubber composition for conveyor belts of the invention has, after vulcanization, a better breaking strength, and the subsequently described loss factor tan 8 and energy loss index (AH) are also better. [0049] Illustrative examples of vulcanization retarders 26 include organic acids such as phthalic anhydride, benzoic acid, salicylic acid and acetylsalicylic acid; nitroso compounds such as N-nitrosodiphenylamine, N-nitrosophenyl @-naphthylamine and N-nitrosotrimethyldihydroquinoline polymer; halogen compounds such as trichloromelanine; 2 mercaptobenzimidazole; and Santogard PVI. When a vulcanization retarder is included, the content thereof is preferably from 0.1 to 0.3 part by weight, and more preferably from 0.1 to 0.2 part by weight, per 100 parts by weight of the rubber component. At a content in this range, the resulting first rubber composition for conveyor belts of the invention has an improved scorch stability and manufacturability when extruded to form a conveyor belt. [0050) Examples of compounding ingredients include reinforcing agents (fillers) other than the above-described carbon black, antidegradants, antioxidants, pigments (dyes), plasticizers, thixotropic agents, ultraviolet absorbers, flame retardants, solvents, surfactants (including leveling agents), dispersants, dehydrating agents, rust inhibitors, tackifiers, antistatic agents and processing aids. These compounding ingredients may be ingredients that are commonly used in rubber compositions. The amounts in which 27 such compounding ingredients are included may be selected as appropriate without any particular limitation. [0051] Preparation of the first rubber composition for conveyor belts of the invention may be carried out by adding together the above-described rubber component, carbon black, silica, silane coupling agent and diethylene glycol, as well as various compounding ingredients to be included as desired and kneading with a Banbury mixer or the like, then working in the vulcanizing agent, vulcanizing aid and vulcanizing accelerator with a roll mill or the like. Vulcanization may be carried out under commonly used conditions. For example, vulcanization may be carried out by heating at a temperature of about 140 0 C to about 150'C for 0.5 hour. [0052] The conveyor belt according to the f:.rst aspect of the invention (sometimes referred to below simply as "the first conveyor belt of the invention") is a conveyor belt composed of a top cover rubber layer, a reinforcing layer and a bottom cover rubber layer, wherein at least a back side of the bottom cover rubber layer is formed of the above-described first rubber composition for conveyor belts of the invention. The first conveyor belt of the invention is described below 28 in conjunction with FIG. 1. However, the construction of the first conveyor belt of the invention is not subject to any particular limitation, provided the above-described first rubber composition for conveyor belts of the invention is used in the back side of the bottom cover rubber layer. [0053] FIG. 1 is a schematic cross-sectional view of a preferred embodiment of the first conveyor belt according to the invention. In FIG. 1, the symbol 1 denotes a conveyor belt, 2 is a top cover rubber layer, 3 is a reinforcing layer, 4 is a bottom cover rubber layer, 5 is an article transporting surface, 11 and 16 are outer layers, and 12 and 15 are inner layers. As shown in FIG. 1, the conveyor belt 1 has a ::einforcing layer 3 which serves as a center layer and is provided on both sides with a top cover rubber layer 2 and a bottom cover rubber layer 4. The top cover rubber layer 2 is composed of two layers: an outer layer 11 and an inner layer 12. The bottom cover rubber layer likewise is composed of two layers: an outer layer 16 and an inner layer 15. Here, the outer layers and inner layers (outer layer 11 and inner layer 12, outer layer 16 anc. inner layer 15) of the top cover rubber layer 2 and the bottom cover rubber layer 4 may each be formed using different rubber 29 compositions. [0054] In FIG. 1, the top cover rubber layer 2 is composed of two layers: an outer layer 11 and an inner layer 12. However, in the first conveyor belt of the invention, the number of layers making up the top cover rubber layer 2 is not limited to two, and instead may be one or may be three or more. In cases where the number of layers is three or more, these layers may be formed using mutually differing rubber compositions. The same applies to the bottom cover rubber layer 4. Because it is desirable for the outer layer 11 serving as the article transporting surface 5 of the top cover rubber layer 2 to be formed of a rubber composition having excellent properties such as heat resistance, abrasion resistance and oil resistance, the top cover rubber layer 2 is preferably composed of two layers. The outer layer 16 serving as the back side of the bottom cover rubber layer 4 is formed of the first rubber composition for conveyor belts of the invention. Moreover, owing to the importance placed on production costs and adhesion with the reinforcing layer 3, it is desirable that the inner layer 15 of the bottom cover rubber layer 4 be formed of another rubber composition. Hence, the cover rubber layer 4 is preferably composed of two layers.

30 [0055] The core of the reinforcing layer 3 is not subject to any particular limitation. Any material used in conventional conveyor belts may be suitably selected for use. Illustrative examples of such materials include a material made of cotton cloth and man-made or synthetic fibers which has been coated or impregnated with rubber cement or RFL (resorcinol-formaldehyde latex) treated then folded, specially woven nylon sailcloth, and steel cord. Any one of these may be used alone, or two or more may be laminated and used together. The shape of the reinforcing layer 3 is not subject to any particular limitation. For example, it may be in the form of a sheet, as shown in FIG. 1, or may be composed of a parallel array of embedded reinforcement wires. [0056] The rubber composition which forms the inner layer 12 of the top cover rubber layer 2 and the inner layer 15 of the bottom cover rubber 4 is not subject to any particular limitation. Rubber compositions used in conventional conveyor belts may be suitably sElected for use. A single type may be used alone, or a combination of two or more types may be mixed and used together. [0057] The rubber composition which forms the outer layer 11 of the top cover rubber layer 2 is not subject to any particular limitation. Rubber compositions used in 31 conventional conveyor belts may be suitably selected for use, in accordance with the basic properties (e.g., heat resistance, abrasion resistance, oil resistance) required of this outer layer. [0058] Because at least the back side of the bottom cover rubber layer in the first conveyor belt of the invention is formed of the first rubber composition for conveyor belts of the invention, it is possible to achieve a sufficient reduction in power consumption while retaining such basic properties as a high breaking strength and a good abrasion resistance. [0059] In the first conveyor belt of the invention, the bottom cover rubber layer has a thickness of preferably from 5 to 20 mm, and more preferably from 6 to 15 mm. Here, when the bottom cover rubber layer is made of an inner layer and an outer layer, the thickness of the bottom cover rubber layer refers to the combined thickness of these layers. If the bottom cover rubber layer has a thickness in this range, even when used to transport high-temperature articles, cupping of the belt due to rubber deterioration and other causes can be prevented. [0060] No particular limitation is imposed on the method of manufacturing the first conveyor belt of the invention.

32 Any commonly used method may be employed for -:his purpose. An example of a preferred method is one in which, first, the starting materials are worked together using a roll mill, kneader, Banbury mixer or the like, following which the rubber composition for each cover rubber Layer is formed into sheet using a calender or the like. Next, the respective layers thus obtained are laminated in a predetermined order with the reinforcing layer interposed therebetween, following which pressure is applied at a temperature of from 140 to 1700C for a period of from 10 to 60 minutes. [0061] The rubber composition for conveyor belts according to a second aspect of the invention (also referred to below simply as "the second rubber composition for conveyor belts of the invention") is a rubber composition having a loss factor tan 6, as measured at a temperature of 200C, at 10% extension and under the application of ±2% amplitude strain oscillations at a frequency of 10 Hz, of from 0.04 to 0.07, and having an energy loss index (AH), as defined by formula (1) below, of up to 0.080. AH = (SpGr x tan 8)/M 25 (1) Here, SpGr is the specific gravity (g/cm 3 ) at 200C, tan 8 is the loss factor measured at a temperature of 200C, at 33 10% extension and under the application of ±20 amplitude strain oscillations at a frequency of 10 Hz, and M 25 is the tensile stress (MPa) at 25% elongation. (0062] The second aspect of the invention focuses on energy loss in the belt conveyor system, and particularly energy loss which arises when the conveyor belt passes over the rollers during operation. The object is to reduce such energy loss, and thereby reduce the power consumption of the overall belt conveyor system. The properties specified for the second rubber composition for conveyor belts of the invention are described below. [0063] Loss Factor Tan 8 The loss factor tan 6, expressed as the ratio between the storage modulus E' and the loss modulus E" which represent the dynamic character of the rubber compositicn, namely tan 6 = E"/E'. The lower this value, the smaller the amount of energy dissipated as heat (amount of energy loss) during deformation of the rubber composition. Hence, this value can be used as a measure of energy loss. At the same time, although a low tan 6 value appears to indicate that lower power consumption is possible, as already noted above in the section entitled "Problems to be Solved by the Invention," if the tan 8 value i.s too low, 34 the breaking strength (TB) and the elongation at break (EB) decrease, making conveyor belt operation unstable. Accordingly, in the present invention, the tan 5 value is set within a specific range so as to achieve both a lower power consumption and good basic properties such as breaking strength and elongation at break, and also to enable use in conveyor lines that require conveyor belts having elevated properties. [0064] The loss factor tan 8 of the second rubber composition for conveyor belts of the invention is thus from 0.04 to 0.07, preferably from 0.05 to 0.07, more preferably from 0.05 to 0.065, and even more preferably from 0.055 to 0.065. In the present invention, the loss factor tan 3 refers to the loss factor obtained, using a test specimen cut as a strip (20 mm (L) x 5 mm (W) x 2 mm (T)) from a vulcanizate obtained by vulcanizing the second rubber composition for conveyor belts of the invention at 148 0 C for 3D minutes, by measurement at a temperature of 200C, at 10% extension and under the application of ±2% amplitude strain oscillations at a frequency of 10 Hz. [0065] Energy Loss Index (AH) The energy loss index (AH) is represented by above formula 35 (1). Formula (1) is a formula which has hitherto been used in the tire industry as a guide to the efficiency of reductions in friction with road surfaces, but is believed to be effective also for evaluating the reduction in power consumption by rubber compositions for conveyor belts. SpGr in above formula (1) indicates the specific gravity (g/cm 3 ) at 200C. If this value is small, it is possible to reduce the total mass, enabling a low power co:-isumption effect equivalent to a small load to be achieved. Moreover, as mentioned above, tan 5 in above formula (1) exerts an influence on energy loss due to deformation of the rubber composition when the conveyor belt passes over the rollers. When this value is small, a low power consumption effect can be achieved.

M

25 in above formula (1) indicates the tensile stress (MPa) at 25% elongation. Because this represents belt stiffness, which can be regarded as an alternative characteristic for hardness, it has an influence on the magnitude of deflection by the rubber composition. At a high value, the deflection is low, enabling a low power consumption effect to be achieved. In this invention, M 25 is the value obtained, using a test specimen punched in the shape of a Nc. 3 dumbbell from a vulcanizate obtained by vulcanizing 36 the second rubber composition for conveyor belts of the invention at 148*C for 30 minutes, by carrying out a tensile test at a test rate of 500 mm/min in accordance with JIS K6251-2004 and measuring at room temperature the tensile stress (M 25 ) (MPa) at 25% elongation. Hence, the technical significance of above formula (1) is that, by dividing the product of SpGr and tan 8 by M 2 5 , based on these properties, it is possible to comprehensively assess energy loss when the rubber composition passes over the rollers. The resulting value thus serves as an indicator of whether the rubber composition for conveyor belts is suitable for conveyor belts aimed at reducing power consumption. [0066] The energy loss index (AH) shown in above formula (1) for the second rubber composition for conveyor belts of the invention is up to 0.080, preferably up to 0.07, more preferably less than 0.07, and even more preferably from 0.030 to 0.065. [0067) The second rubber composition for conveyor belts of the invention is a rubber composition for which, as noted above, the loss factor tan 8 measured under predetermined conditions is from 0.04 to 0.07 and the energy loss index (6H) indicated by above formula (1) is up to 0.080.

37 The reason for setting the value of the loss factor tan S in a range of 0.04 to 0.07 in spite of the fact that tan 6 is included within above formula (1) is that, even when the energy loss index (AH) is 0.080 or less, basic properties such as breaking strength and elongation at break canno-t be guaranteed if tan 8 is less than 0.04. [0068] The second rubber composition for conveyor belts of the invention preferably includes a rubber component composed of natural rubber and polybutadiene rubber. By including a rubber component composed of natural rubber and polybutadiene rubber, the second rubber composition for conveyor belts of the invention has, following vulcanization, both a good breaking strength and a good abrasion resistance, enabling the basic properties of a conveyor belt to be retained. Here, use may be made of the same polybutadiene rubber described above in connection with the first rubber composition for conveyor belts of the invention. For the same reasons as those given in connection with the first rubber composition for conveyor belts of the invention, the polybutadiene rubber has a weight-average molecular weight of preferably at least 500,000 and is preferably an end modified polybutadiene. [00691 The second rubber composition for conveyor belts 38 of the invention preferably includes a rubber component composed of natural rubber and polybutadiene rubber, carbon black, silica, a silane coupling agent and diethylene glycol. By including not only natural rubber and polybutadiene rubber, but also carbon black, silica, a silane coupling agent and diethylene glycol, the second rubber composition for conveyor belts of the invention has, following vulcanization, both a good breaking strength and a good abrasion resistance. As a result, the Dasic properties as a conveyor belt can be retained and, because the loss factor tan 8 and the energy loss index (AH) are both in good ranges, a more sufficient reduction in power consumption can be achieved. Here, the carbon black, silica, silane coupling agent and diethylene glycol used may be those described in detail in connection with the first rubber composition for conveyor belts of the invention. [0070] In addition, it is preferable for the second rubber composition for conveyor belts of the invention to include a rubber component made of natural rubber and polybutadiene rubber, carbon black, silica, a :3ilane coupling agent and diethylene glycol, for the natural rubber and the polybutadiene rubber in the rubber component to have a weight ratio NR/BR therebetween of f::om 80/20 to 39 25/75, for the carbon black content per 100 parts by weight of the rubber component to be from 15 to 35 parts by weight, for the silica content per 100 parts by weight of the rubber component to be from 5 to 25 parts by weight, for the silane coupling agent content per 100 parts by weight of the rubber component to be from 0.5 to 3 parts by weight, and for the diethylene glycol content per 100 parts by weight of the rubber component to be from 0.5 to 4.5 parts by weight. By including specific amounts of carbon black, silica, silane coupling agent and diethylene glycol, the breaking strength and the abrasion resistance of the second rubber composition for conveyor belts of the invention after vulcanization are both good, thus making it possible to retain the basic properties of a conveyor belt. Moreover, because the loss factor tan 8 and the energy loss index (AH) are within even more preferable ranges, a reduction in power consumption can be more fully achieved. [0071] In the second rubber composition for conveyor belts of the invention, the weight ratio NR/BR of natural rubber to polybutadiene rubber in the rubber component is more preferably from 70/30 to 50/50, and even more preferably from 70/30 to 60/40. The carbon black content per 100 parts by weight of the rubber component is more preferably from 20 to 30 parts by 40 weight, and even more preferably from 25 to 30 parts by weight. The silica content per 100 parts by weight of the rubber component is more preferably from 10 to 20 parts by weight. The silane coupling agent content per 100 parts by weight of the rubber component is more preferably from 1 to 2 parts by weight. The diethylene glycol content per 100 parts by weight of the rubber component is more preferably from 0.5 to 2 parts by weight, and even more preferably from 0.6 to 1.8 parts by weight. [0072] In the second rubber composition for conveyor belts of the invention, as with the first rubber composition for conveyor belts of the invention, it is desirable to use a silica having a nitrogen ac.sorption specific surface area (N 2 SA) of preferably from 100 to 250 m 2 /g, and more preferably from 125 to 200 m 2 /g. [0073] In one preferred embodiment, the second rubber composition for conveyor belts of the invention, as in the case of the first rubber composition for conveyor belts of the invention, includes 1,3-bis (citraconimidomethyl) benzene and/or hexamethylene-1,6-bis(thiosulfate) disodium salt dihydrate. Here, as in the first rubber composition for conveyor belts 41 of the invention, the 1,3-bis(citraconimidomethyl) benzene is a compound of above chemical formula (1) and the hexamethylene-1,6-bis(thiosulfate) disodium salt dihydrate is a compound of above chemical formula (2). [0074) Including these compounds makes the energy loss index (AH) of the resulting second rubber composition for conveyor belts of the invention smaller, enabling a sufficient reduction in power consumption to be achieved. This is because the tensile stress (M 2 5 ) at 25% elongation of the second rubber composition for conveyor oelts of the invention improves and the loss factor tan 5 i:s smaller. Yet, given that these compounds are reagents which prevent reversion of the rubber composition (anti-reversion agents), such an effect was unanticipated. [0075] In the second rubber composition for conveyor belts of the invention, the content of these compounds per 100 parts by weight of the rubber component is preferably from 0.1 to 2 parts by weight, and more preferably from 0.2 to 1 parts by weight. This content, in cases where 1,3-bis(citraconinidomethyl) benzene and hexamethylene-1,6-bis(thiosulfate) disodium salt dihydrate are both included, refers to the combined content thereof. [0076] Commercial products like those described above in 42 connection with the first rubber composition for conveyor belts of the invention may be used as these compounds in the second rubber composition for conveyor bel:s of the invention. [0077) In addition to the various above-menzioned ingredients, the second rubber composition for conveyor belts of the invention may also include crosslinking agents such as vulcanizing agents, vulcanizing aids and vulcanization accelerators, and vulcanization :cetarders. In addition, various other compounding ingredients may be included insofar as the objects of the invention are attainable. The vulcanizing agents, vulcanizing aids, vulcanization accelerators, vulcanization retarders and various compounding ingredients used here may be the same as those described in detail in connection with the fir3t rubber composition for conveyor belts of the invention. [0078] The conveyor belt according to the second aspect of the invention (sometimes referred to below as simply "the second conveyor belt of the invention") is a conveyor belt composed of a top cover rubber layer, a reinforcing layer and a bottom cover rubber layer, wherein at least a back side of the bottom cover rubber layer is formed of the above-described second rubber composition for conveyor 43 belts of the invention. That is, instead of the first rubber composition for conveyor belts of the invention used in the above-described first conveyor belt of the invention, the second conveyor belt of the invention uses the second rubber composition for conveyor belts of the invention. Aside fiom this, other features of the second conveyor belt of the invention are the same as those of the first conveyor belt of the invention. [0079] In the second conveyor belt of the invention, because at least the back side of the bottom cover rubber layer is formed of the second rubber composition for conveyor belts of the invention, a sufficient reduction in power consumption can be achieved while retaining such basic properties as a high breaking strength and a good abrasion resistance. [0080] No particular limitation is imposed on the method for manufacturing the second conveyor belt of -he invention. Any commonly used method may be employed. Production may be carried out by the same method as for the first conveyor belt of the invention. Examples [0081) The rubber compositions for conveyor belts of the 44 invention are described more fully in the following examples, which are illustrative and should not be construed as limiting the invention. Examples 1 to 26, Comparative Examples 1 to 8, Reference Examples 1 and 2 Rubber compositions for conveyor belts were prepared using the ingredients in the amounts (parts by weight) shown in Table 1 below per 100 parts by weight of the rubber component. Various properties following vulcanization were measured and evaluated for each of the resulting rubber compositions. The results are shown in Table 1 below. Reference Examples 1 and 2 are examples in which the same rubber compositions as in Example 5 and Comparative Example 2 of JP 11-139523 A were prepared. Both of the reference examples contain no silica, no diethylene glycol and no silane coupling agent, and are thus comparative examples. Physical properties other than abrasion resistance were measured and evaluated by the methods shown below. (0082] Physical Properties After Vulcanization (1) Breaking strength 45 Vulcanized rubber compositions were prepared by subjecting each of the rubber compositions obtained to 30 minutes of vulcanization at 1480C. Using test specimens punched in the shape of No. 3 dumbbells from each of the vulcanized rubber compositions prepared, a tensile test was carried out at a test rate of 500 mm/min in accordance with JIS K6251-2004 and the breaking strength (TB) (MPa) was measured at room temperature. When the breaking strength was 14 MPa or more, the specimen was rated as having a high breaking strength. (0083] (2) Abrasion resistance DIN abrasion tests were carried out in accordance with JIS K6264-2-2005 using test specimens punched in the shape of disks (16.2 mm diameter x 6 mm thick) from eaci of the vulcanized rubber compositions prepared. The amount of abrasion (mm 3 ) on carrying out a DIN abrasion 1est at room temperature was measured. Specimens having an amount of abrasion of 120 mm 3 or less were rated as having abrasion resistance (Good). [0084] (3) Loss factor tan S Using test specimens punched in the shape of strips (20 mm 46 (L) x 5 mm (W) x 2 mm (T) ) from each of the vulcanized rubber compositions prepared, the loss factor tan 5 was measured with a viscoelasticity spectrometer manufactured by Toyo Seiki Seisaku-Sho Ltd. Measurement was carried out at a temperature of 200C, at 10% extension and under the application of ±2% amplitude strain oscillations at a frequency of 10 Hz. [0085] (4) Energy Loss Tndex (AH) The specific gravities of each of the vulcani2zed rubber compositions prepared were measured. Using test specimens punched in the shape of No. 3 dumbbells from each of the vulcanized rubber compositions prepared, tensile tests at a test rate of 500 mm/min were carried out in accordance with JIS K6251-2004 and the tensile stress (M 25 ) (MPa) at 25% elongation was measured. Next, using the respective values for specific gravity, M 25 and the loss factor tan 5 measured as described above, the energy loss index (AH) for each of the vulcanized rubber compositions prepared was determined from above formula (1). [0086] 47 Table 1 (part 1) Comparative Examples 1 2 3 4 5 6 7 8 NR 65.00 100.00 80.00 70.00 70.00 20.00 50.00 50.00 VCR 35.00 BR1 20.00 30.00 30.00 80.00 50.00 50.00 Carbon black 1 25.00 Carbon black 2 25.00 25.00 25.00 25.00 25.00 40.00 20.00 Silica 1 10.00 10.00 10.00 10.00 10.00 10.00 30.00 Silica 2 Diethylene glycol 1.00 1.00 1.00 Silane coupling agent 1.00 1.00 1.00 1.00 Vulcanizing agent 1 2.5 3.2 3.2 3.2 3.2 3.2 3.2 3.2 Vulcanization accelerator 1 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Properties after vulcanization Breaking strength (MPa) 21.0 23.0 18.2 16.9 17.5 17.6 18.6 19.1 Abrasion resistance Good Poor Fair Good Good Good Good Good Specific gravity (g/cm 3 ) 1.08 1.09 1.09 1.09 1.09 1.09 1.13 1.13 M25 (MPa) 1.10 0.80 0.78 0.80 0.90 0.90 1.12 1.18 tan 8 0.121 0.064 0.065 0.067 0.068 0.068 0.087 0.085 6H 1 0.119 0.087 0.091 0.091 0.082 0.082 0.088 0.081 [00871 Table 1 (part 2) Examples of the Invention 1 2 3 4 5 6 7 NR 80.00 70.00 70.00 70.00 70.00 70.00 70.00 VCR BR1 20.00 30.00 30.00 30.00 30.00 30.00 30.00 Carbon black 1 Carbon black 2 20.00 20.00 25.00 25.00 25.00 25.00 25.00 Silica 1 10.00 10.00 10.00 10.00 10.00 10.00 10.00 Silica 2 Diethylene glycol 1.00 1.00 1.00 2.00 4.00 2.00 2.00 Silane coupling agent 1.00 1.00 1.00 1.00 1.00 2.00 3.00 Vulcanizing agent 1 3.2 3.2 3.2 3.2 3.2 3.2 3.2 Vulcanization accelerator 1 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Properties after vulcanization Breaking strength (MPa) 18.6 18.1 18.7 19.5 19.9 20.2 20.4 Abrasion resistance Good Good Good Good Good Good Fair Specific gravity (g/cm 3 ) 1.08 1.08 1.09 1.09 1.10 1.09 1.09

M

25 (MPa) 1.02 1.05 1.10 1.20 1.24 1.29 1.34 tan 6 0.051 0.054 0.060 0.056 0.059 0.056 0.056 AH 0.054 0.055 0.059 0.051 0.052 0.047 0.046 48 [0088] Table 1 (part 3) Examples of the Invention Ref. Ex. 8 9 10 11 12 1 2 NR 50.00 50.00 50.00 25.00 50.00 VCR BRI 50.00 50.00 50.00 75.00 50.00 Carbon black 1 Carbon black 2 25.00 25.00 30.00 25.00 20.00 Silica 1 10.00 10.00 10.00 20.00 Silica 2 10.00 Diethylene glycol 1.00 1.00 1.00 1.00 1.00 Silane coupling agent 1.00 1.00 1.00 1.00 1.00 Vulcanizing agent 1 3.2 3.2 3.2 3.2 3.2 Vulcanization accelerator 1 1.5 1.5 1.5 1.5 1.5 Properties after vulcanization Breaking strength (MPa) 19.5 19.8 19.9 18.1 18.9 20.7 19.0 Abrasion resistance Good Good Good Good Good -- - Specific gravity (g/cm 3 ) 1.09 1.09 1.10 1.09 1.11 1.06 1.06

M

25 (MPa) 1.03 1.10 1.07 0.90 1.10 0.60 0.53 tan 6 0.061 0.055 0.067 0.066 0.069 0.090 0.089 AH 0.065 0.055 0.069 0.080 0.070 0.160 0.178 [0089) Table 1 (part 4) Examples of the Invention 13 14 15 16 17 NR 70.00 70.00 70.00 70 00 70.00 VCR BR1 30.00 30.00 30.00 30.00 30.00 Carbon black 1 Carbon black 2 30.00 30.00 30.00 30 00 30.00 Silica 1 10.00 10.00 10.00 10 00 5.00 Silica 2 Diethylene glycol 1.00 1.00 1.00 1.00 1.00 Silane coupling agent 1.00 1.00 1.00 1.00 1.00 Vulcanizing agent 1 3.2 3.2 3.2 3.' 3.2 Vulcanization accelerator 1 1.5 1.5 1.5 1.5 1.5 Anti-reversion agent 1 0.25 0.50 0.50 Anti-reversion agent 2 0.25 0.50 Properties after vulcanization Breaking strength (MPa) 20.6 20.1 19.8 18.9 14.3 Abrasion resistance Good Good Good Good Good Specific gravity (g/cm 3 ) 1.10 1.10 1.10 1.: 0 1.09 M25 (MPa) 1.19 1.25 1.19 1.:9 1.14 tan 6 0.059 0.058 0.058 0.057 0.049 AH 0.054 0.051 0.054 0.053 0.046 49 [0090] Table 1 (part 5) Examples of the Invention 18 19 20 21 22 23 NR 70.00 70.00 70.00 70.CO 70.00 70.00 VCR BRI 30.00 30.00 30.00 30.C0 30.00 30.00 Carbon black 1 Carbon black 2 30.00 30.00 30.00 30.CO 30.00 30.00 Silica 1 Silica 2 Silica 3 10.00 5.00 10.00 Silica 4 10.00 5.00 10.00 Diethylene glycol 1.00 1.00 1.00 1.00 1.00 1.00 Silane coupling agent 1.00 1.00 1.00 1.00 1.00 1.00 Vulcanizing agent 1 3.2 3.2 3.2 3.2 3.2 3.2 Vulcanization accelerator 1 1.5 1.5 1.5 1.5 1.5 1.5 Anti-reversion agent 1 0.50 0.50 0.50 0.50 Properties after vulcanization Breaking strength (MPa) 18.9 15.4 19.4 18.7 17.6 18.0 Abrasion resistance Good Good Good Good Good Good Specific gravity (g/cm 3 ) 1.10 1.09 1.10 1.09 1.10 1.10

M

25 (MPa) 1.30 1.08 1.38 1.24 1.15 1.22 tan 8 0.056 0.046 0.051 0.048 0.057 0.051 AH 0.047 0.047 0.040 0.042 0.055 0.046 [0091] Table 1 (part 6) Examples of the Invention 14 18 20 24 25 26 NR 70.00 70.00 70.00 70.0) 70.00 70.00 BR1 30.00 30.00 30.00 BR2 30.00 30.00 30.00 Carbon black 2 30.00 30.00 30.00 30.0) 30.00 30.00 Silica 1 10.00 10.0) Silica 3 10.00 10.00 Silica 4 10.00 10.00 Diethylene glycol 1.00 1.00 1.00 1.00 1.00 1.00 Silane coupling agent 1.00 1.00 1.00 1.00 1.00 1.00 Vulcanizing agent 1 3.2 3.2 3.2 3.2 3.2 3.2 Vulcanization accelerator 1 1.5 1.5 1.5 1.5 1.5 1.5 Anti-reversion agent 1 0.50 0.50 0.50 0.50 0.50 0.50 Properties after vulcanization Breaking strength (MPa) 20.1 18.9 19.4 21.3 20.3 23.5 Abrasion resistance Good Good Good Good Good Good Specific gravity (g/cm 3 ) 1.10 1.10 1.10 1.10 1.10 1.10 Ms (MPa) 1.25 1.30 1.38 1.21 1.36 1.41 tan 5 0.058 0.056 0.051 0.04~ 0.044 0.041 AH 0.051 0.047 0.040 0.04? 0.035 0.032 [0092] The following materials were used as the rubber 50 component and other ingredients making up the compositions shown in Table 1 above. Natural rubber (NR): RSS#3 Polybutadiene rubber (VCR): VCR617 (Ube Industries) BR1: Nipol BR1220 (weight-average molecular weight, 460,000; end-not-modified; Zeon Corporation) BR2: Nipol BR1250H (weight-average molecular weight, 570,000; end-modified with NMP; Zeon Corporation) Carbon black 1: HAF (Shoblack N339; Showa Cabot) Carbon black 2: GPF (Diablack G; Mitsubishi. Chemical Corporation) Silica 1: Finely divided hydrated silica (nitrogen adsorption specific surface area (N 2 SA) 202 m 2 /g; Nipsil AQ; Nippon Silica Kogyo) Silica 2: Finely divided hydrated silica (nitrogen adsorption specific surface area (N 2 SA): 120 m 2 /g; Tokusil GU; Tokuyama Corporation) Silica 3: Finely divided hydrated silica (nitrogen adsorption specific surface area (N 2 SA): 144 m 2 /g; Zeosil 165 GR; Rhodia Silica Korea) Silica 4: Finely divided hydrated silica (nitrogen adsorption specific surface area (N 2 SA) : 159 m 2 /g; Ultrasil 7000GR; United Silica Industrial) Diethylene glycol: Produced by Nippon Shokubai 51 Silane coupling agent: Bis(3 triethoxysilylpropyl)tetrasulfide (Si69; Decussa) Vulcanizing agent 1: Sulfur (oil-treated sulfur; Hosoi Chemical Industry) Vulcanization accelerator 1: N-tert-butyl-2 benzothiazolylsulfenamide (Nocceler NS; Ouchi Shinko Chemical Industrial Co.) Anti-reversion agent 1: 1,3 bis(citraconimidomethyl)benzene (Perkalink < 900; Flexsys) Anti-reversion agent 2: Hexamethylene-1,6 bis(thiosulfate) disodium salt dihydrate (Duralink HTS, Flexsys) [0093] It is apparent from the results shown in Table 1 that the rubber compositions obtained in ExampLes 1 to 26 of the invention, owing to their various physical properties following vulcanization, are rubber compositions suitable for conveyor belts capable of achieving a sufficient reduction in power consumption while retaining basic properties such as a high breaking strength and a good abrasion resistance. By contrast, rubber compositions containing no diethylene glycol or the like (Comparative Examples 1 to 5), a rubber composition containing rubber component having a low 52 proportion of NR (Comparative Example 6), a rubber composition containing a large amount of carbon black (Comparative Example 7), a rubber composition containing a large amount of silica (Comparative Example 8), and the rubber compositions of Reference Examples 1 and 2 were all found, based on the physical properties following vulcanization, to be rubber compositions having a high energy loss index (AH) and an insufficient reduction in power consumption. [00941 The rubber compositions obtained in Examples 13 to 21 which contained an anti-reversion agent had an increased M 2 5 and a low loss factor tan 6, and thus had an even lower energy loss index (AH). Accordingly, they were found to be rubber compositions suitable for conveyor belts capable of more fully achieving a reduction in power consumption. In particular, the rubber compositions obtained in Examples 18 to 21 which contained an anti-reversion agent and in which the silica had a nitrogen adsorption specific surface area in the preferred range (125 to 200 m 2 /g) Lad an improved M 25 and a low loss factor tan 5, and thus had an even lower energy loss index (AH). Accordingly, these were found to be rubber compositions suitable for conveyor belts capable of more fully achieving a reduction in power 53 consumption. [0095] The rubber compositions obtained in Examples 24 to 26 which contained as the polybutadiene an end-modified polybutadiene having a weight-average molecular weight of at least 500,000 had an improved M 2 5 and a low loss factor tan 8, and thus had an even lower energy loss index (AH). Accordingly, these were found to be rubber compositions suitable for conveyor belts capable of more fully achieving a reduction in power consumption.

Claims (15)

1. A rubber composition for conveyor belts, comprising a rubber component composed of natural rubber (NR) and polybutadiene rubber (BR), carbon black, silica, a silane coupling agent and diethylene glycol, wherein: the natural rubber and the polybutadiene rubber in the rubber component have a weight ratio NR/BR therebetween of from 80/20 to 25/75, the carbon black content per 100 parts by weight of the rubber component is from 15 to 35 parts by weight, the silica content per 100 parts by weight of the rubber component is from 5 to 25 parts by weight, the silane coupling agent content per 100 parts by weight of the rubber component is from 0.5 to 3 parts by weight, and the diethylene glycol content per 100 parts by weight of the rubber component is from 0.5 to 4.5 pars by weight.

2. The rubber composition for conveyor belts of claim 1, further comprising 1,3-bis(citraconimidomethyll benzene and/or hexamethylene-1, 6-bis (thiosulfate) disodium salt dihydrate. 55

3. The rubber composition for conveyor belt;3 of claim 2, wherein the content of 1,3-bis(citraconimidomethyl) benzene and/or hexamethylene-1,6-bis(thiosulfate) disodium salt dihydrate is from 0.1 to 2 parts by weight per 100 parts by weight of the rubber component.

4. The rubber composition for conveyor belts of any one of claims 1 to 3, wherein the silica has a nitrogen adsorption specific surface area (N 2 SA) of front 100 to 250 m2 m2/g.

5. The rubber composition for conveyor belts of any one of claims 1 to 4, wherein the polybutadiene rubber (BR) is an end-modified polybutadiene rubber.

6. A conveyor belt comprising a top cover rubber layer, a reinforcing layer and a bottom cover rubber layer, wherein at least a back side of the bottom cover rubber layer is formed of the rubber composition for conveyor belts of any one of claims 1 to 5.

7. A rubber composition for conveyor belts which has: a loss factor tan 6, as measured at a temperature of 20*C, at 10% extension and under the application of ±2% amplitude 56 strain oscillations at a frequency of 10 Hz, of from 0.04 to 0.07; and an energy loss index (AH), as defined by formula (1) below, of up to 0.080 AH = (SpGr x tan 8)/M 25 (1), where SpGr is the specific gravity (g/cm 3 ) at 200C, tan 5 is the loss factor measured at a temperature cf 200C, at 10% extension and under the application of ±2% amplitude strain oscillations at a frequency of 10 Hz, and M 2 5 is the tensile stress (MPa) at 25% elongation.

8. The rubber composition for conveyor belts of claim 7, comprising a rubber component composed of natural rubber (NR) and polybutadiene rubber (BR).

9. The rubber composition for conveyor belts of claim 7, comprising a rubber component composed of natural rubber (NR) and polybutadiene rubber (BR), carbon black, silica, a silane coupling agent and diethylene glycol.

10. The rubber composition for conveyor belts of claim 7, comprising a rubber component composed of natural rubber (NR) and polybutadiene rubber (BR), carbon black, silica, a silane coupling agent and diethylene glycol, wherein: 57 the natural rubber and the polybutadiene rubber in the rubber component have a weight ratio NR/BR therebetween of from 80/20 to 25/75, the carbon black content per 100 parts by weight of the rubber component is from 15 to 35 parts by weight, the silica content per 100 parts by weight of the rubber component is from 5 to 25 parts by weigt, the silane coupling agent content per 100 parts by weight of the rubber component is from 0.5 to 3 parts by weight, and the diethylene glycol content per 100 parts by weight of the rubber component is from 0.5 to 4.5 parts by weight.

11. The rubber composition for conveyor belts of claim 9 or 10, wherein the silica has a nitrogen adsorption specific surface area (N 2 SA) of from 100 to 250 m 2 /g.

12. The rubber composition for conveyor belts of any one of claims 8 to 11, wherein the polybutadiene rubber (BR) is an end-modified polybutadiene rubber.

13. The rubber composition for conveyor belts of any one of claims 7 to 12, further comprising 1,3 bis(citraconimidomethyl) benzene and/or hexamethylene-1,6- 58 bis(thiosulfate) disodium salt dihydrate.

14. The rubber composition for conveyor belts of claim 13, wherein the content of 1,3-bis(citraconimidomethyl) benzene and/or hexamethylene-1,6-bis(thiosulfate) disc-dium salt dihydrate is from 0.1 to 2 parts by weight per 100 parts by weight of the rubber component.

15. A conveyor belt comprising a top cover rubber layer, a reinforcing layer and a bottom cover rubber layer, wherein at least a back side of the bottom cover rubber layer is formed of the rubber composition for conveyor belts of any one of claims 7 to 14.