XD-modified polychloroprene grades for mining conveyor belting.XD-modified polychloroprene grades for mining conveyor belting conveyor belt One of various devices that provide mechanized movement of material, as in a factory. Conveyor belts are used in industrial applications and also on large farms, in warehousing and freight-handling, and in movement of raw materials. There are only a few working areas where fire can create as disastrous results as in underground coal mining underground coal mining: see coal mining. . Therefore, mining authorities in all coal mining countries have introduced fire safety regulations for all organic materials and installations being used underground. One of the most important installations in this respect are conveyor belts, due to the facts that: * the belts themselves often extend one hundred meters or more; * the belts move and thereby a burning belt will spread the flames. Safety regulations for underground belts are presently more or less different in different countries. In the Federal Republic of Germany, belts have to pass the following tests: * flame resistance tests; * filter test (behavior of cumbustion products in the filters of respirator respirator /res·pi·ra·tor/ (res´pi-ra?ter) ventilator (2). cuirass respirator see under ventilator. masks); * conductivity conductivity /con·duc·tiv·i·ty/ (kon?duk-tiv´i-te) the capacity of a body to transmit a flow of electricity or heat; the conductance per unit area of the body. con·duc·tiv·i·ty n. 1. tests It is anticipated that regulations and standards in Western Europe Western Europe The countries of western Europe, especially those that are allied with the United States and Canada in the North Atlantic Treaty Organization (established 1949 and usually known as NATO). will change in the near future, due to improved understanding of flame retardancy phenomena and the economic integration of the European Community European Community: see European Union. European Community (EC) Organization formed in 1967 with the merger of the European Economic Community, European Coal and Steel Community, and European Atomic Energy Community. in 1992 (harmonization har·mo·nize v. har·mo·nized, har·mo·niz·ing, har·mo·niz·es v.tr. 1. To bring or come into agreement or harmony. See Synonyms at agree. 2. Music To provide harmony for (a melody). of different standards). Before discussing the polymer aspect for manufacturing belts The Manufacturing Belt, often referred to as the Rust Belt, is an area in parts of the Midwest and Mid-Atlantic regions of the United States of America. The region can be broadly defined as the region beginning west of the BosWash corridor and running west to eastern , also meeting tomorrow's requirements, we want to discuss briefly the present situation in flame resistance tests and its trends. Fire resistance testing in West Germany West Germany: see Germany. In West Germany several flame resistance tests are required for new belts, the most important being the "Large Gallery Test" or "Full Scale Burning Test" (figure 1). This test is specific to Germany and has also been adapted in a modified version by Poland. The burning test is carried out with a belt of 18 m length and the width of the actual belt, e.g. 1 - 1.4 m. 300 kg of wood, placed in a specified manner under and on the belt, are ignited ig·nite v. ig·nit·ed, ig·nit·ing, ig·nites v.tr. 1. a. To cause to burn. b. To set fire to. 2. To subject to great heat, especially to make luminous by heat. , and the test runs under exactly defined conditions until no flames are visible either at the belt or the wood. The condition of acceptance is that the flame cannot travel more than 10 m beyond the source of fire. This test is relatively expensive. Therefore, the German Mining Authority and the "Versuchsgrubengesellschaft," a government agency responsible for definition of tests and approval of underground mining belts, have tried to develop a simplified and more economical test. As a result the Laboratory Gallery Test (figure 2) was introduced using a conveyor belt specimen of 1200 x 90 mm. The test is similar to the Large Gallery Test, but with smaller dimensions. A lot of work has been carried out in Germany to correlate the two tests. So far, correlations have been found for textile reinforced two ply (mathematics, data) ply - 1. Of a node in a tree, the number of branches between that node and the root. 2. Of a tree, the maximum ply of any of its nodes. belts. The condition of acceptance for these belts is a residual unburnt length of 400 mm after 15 min. flame exposure at one end of the belt. If this type of belt passes this test, the somewhat less severe Large Gallery Test is no longer required. Even though good correlation for other belts has not been found yet, the Laboratory Gallery Test might gain acceptance for other belts as well, but, to be on the safe side, with very severe conditions of acceptance. Belt producers will then have the choice to test either according to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. the Laboratory Gallery Test - which is economically preferable or to the Large Gallery Test - which might be easier to pass. Up to date these are the two main tests carried out specifically in West Germany. The Mining Authority feels that they are severe enough to predict the burning behavior of a conveyor belt. But in addition there are some regulations from the European Community which are mandatory for the Federal Republic of Germany. The most important test is the "Drum Friction Test" which originated in the U.K. (figure 3). This test was installed after several disastrous fires in the U.K. around 1950, many of them caused by frictional heating of a conveyor belt. The conditions for passing this test are relatively rigorous in the U.K. and can usually only be met by PVC-belts. Rubber (polychloroprene) is used in the U.K. only for steel cord reinforced belts, which are not required to pass this test. In West Germany, where rubber belts are used as well, the conditions of acceptance with respect to the Drum Friction Test are less severe than in the U.K. so that rubber belts have a chance to pass. Polymers Although flame retardant Flame retardants are materials that inhibit or resist the spread of fire. Naturally occurring substances such as asbestos as well as synthetic materials, usually halocarbons such as polybrominated diphenyl ether (PBDEs), polychlorinated biphenyls (PCBs) and chlorendic acid SBR SBR - Spectral Band Replication compounds have been used for underground mining conveyor belts in the past, they can no longer be used in Western Europe due to insufficient flame retardancy found in underground fires in the 1950s and determined in the Large Gallery Test. For rubber belts in Western Europe today only polychloroprene (CR) is the base elastomer elastomer (ĭlăs`təmər), substance having to some extent the elastic properties of natural rubber. The term is sometimes used technically to distinguish synthetic rubbers and rubberlike plastics from natural rubber. for underground mining belts meeting the above mentioned requirements. In other countries at least the trend is similar although the test methods may be different. The base polymer polychloroprene itself contains about 38-40% chlorine which acts as a flame retarder retarder, n a chemical added to a substance to slow a chemical reaction, prolong the set of the material, and provide more working time. . Although the LOI LOI Letter of Indemnity (international trade and carriage business) LOI Letter Of Intent LOI Loss On Ignition LOI Letter of Inquiry LOI Lack Of Information LOI Lack of Interest LOI Letter of Invitation LOI List Of Items value acc. ASTM ASTM abbr. American Society for Testing and Materials D 2863 doesn't clearly correlate with the real burning behavior, the LOI of a polychloroprene gumstock of 36% gives a strong indication for the good flame retardant behavior of CR. Until recently, two main classes of CR were used in belts: * standard grades, containing mercaptan mercaptan (mərkăp`tăn) or thiol (thī`ōl), any of a class of organic compounds containing the group -SH bonded to a carbon atom. derivatives as a chain modifier (programming) modifier - An operation that alters the state of an object. Modifiers often have names that begin with "set" and corresponding selector functions whose names begin with "get". ; * sulfur modified grades, using sulfur and thiuram as a chain modifier. The standard grades are used for covers of the belts and sometimes adhesion compounds of fabric reinforced belts. The sulfur modified grades, due to their better adhesion properties and reduced viscosity through mastication mastication /mas·ti·ca·tion/ (mas?ti-ka´shun) chewing; the biting and grinding of food. mastication (mas´tikā´sh , are widely used for adhesion compounds especially for steel cord reinforced belts. Meanwhile, a third class of CR has been developed, the so-called "XD-types," which are manufactured by using xanthogendisulfide derivates (XD) as chain modifiers. These XD-grades give often improved uncured compound properties if compared with the standard M-grades with the same Mooney viscosity, and have advantages in the physical properties of the cured compounds. On the other hand, they allow a higher filler fill·er 1 n. One that fills, as: a. Something added to augment weight or size or fill space. b. A composition, especially a semisolid that hardens on drying, used to fill pores, cracks, or holes in wood, plaster, and oil level, to reduce the compound costs without disadvantages in the physical properties. The advantages of the XD-grades over the mercaptan grades are: * reduced "nerve" of the uncured compound; * easier calendering calendering, a finishing process by which paper, plastics, rubber, or textiles are pressed into sheets and smoothed, glazed, polished, or given a moiré or embossed surface. ; * the scorch time and flow time at curing temperature are in most cases slightly longer; * better penetration into reinforcing material; * tensile strength tensile strength Ratio of the maximum load a material can support without fracture when being stretched to the original area of a cross section of the material. When stresses less than the tensile strength are removed, a material completely or partially returns to its and tear strength are consistently higher; * elongation elongation, in astronomy, the angular distance between two points in the sky as measured from a third point. The elongation of a planet is usually measured as the angular distance from the sun to the planet as measured from the earth. at break, modulus See modulo. , hardness and rebound resilience resilience (r  ·zilˑ·yens),n are equal (or slightly higher). This shall be shown in the following examples. Test compound acc. to ISO (1) See ISO speed. (2) (International Organization for Standardization, Geneva, Switzerland, www.iso.ch) An organization that sets international standards, founded in 1946. The U.S. member body is ANSI. 2475 This formulation is a standard test formulation widely used for quality control. Table 1 shows the formulation, table 2 the uncured properties. It demonstrates for the XD-CR the longer scorch time and flow time [t.sub.10] at curing temperature, in general beneficial for processing and textile bonding. Table 3 demonstrates, above all, the superior tensile strength and tear resistance obtainable with the XD-CR. Table : Table 1 - test compound acc. to ISO 2475
XD-CR M-CR
XD-CR(1) 100 -
M-CR(2) - 100
Black N 762 30
Stearic acid 0.5
Magnesium oxide 4
Zinc oxide 5
ETU 0.5
Total parts 140 140
(1) - Baypren 216 polychloroprene, ML-1+4/100[degrees]C = 50 (2) - Baypren 210 polychloroprene, ML-1+4/100[degrees]C = 47 Table : Table 2 - uncured compound properties of ISO-test compound
XD-CR M-CR
ML 1+4/100[degrees]C (MU) 70 65
Density (g/[cm.sup.3]) 1.38 1.38
Mooney scorch time MS-[t.sub.5]/120 [degrees] C (min) 11 9 Vulkameter 150 [degrees] C [F.sub.min] (N) 1.7 1.6 [F.sub.max] (N) 36 32 [t.sub.10] (min) 3.7 3.2 [t.sub.80] (min) 20 16 Table : Table 3 - vulcanizate properties of ISO compound Mechanical properties (1) XD-CR M-CR TS (MPa) 22.0 19.0 EB (%) 470 415 M 100 (MPa) 2.6 2.6 M 300 (MPa) 13.0 13.2 H 23/70 [degrees] C (Shore A) 64/64 63/62 R (%) 53 51 Tear DIN 53 [507.sup.2] (N/mm) 3.8 3.3 Tear ASTM D [470.sup.2] (N/mm) 4.7 3.6 Hot air aging at 100 [degrees] C/28 days(3) (Change of initial values) TS (%) -15 -12 EB (%) -39 -33 M 100 (%) +3.8 +2.6 H (Shore A) +10 +8 (1) - 4 mm ISO ring I resp. 4mm slab; average of 20, 40, 60 min/150 [degrees] C cure (2) - 2mm sheets, 40 min/150 [degrees] C cure (3) - 4 mm ISO ring I resp. 4 mm slab; 40 min/150 [degrees] C cure Compound for conveyor belt cover Compounds for underground conveyor belt covers, based on CR, must be easy to process, i.e. they should not be excessively viscous viscous /vis·cous/ (vis´kus) sticky or gummy; having a high degree of viscosity. vis·cous adj. 1. Having relatively high resistance to flow. 2. Viscid. , despite heavy loading with flame retardant ingredients, and the vulcanizates must have high tensile strength. In addition, conveyor belt covers generally need high resistance to tear propagation The transmission (spreading) of signals from one place to another. . XD-grades of CR therefore appear particularly suitable for this application. This will be demonstrated with a typical formulation developed in our laboratory containing zinc borate borate /bo·rate/ (bor´at) a salt of boric acid. bo·rate n. A salt or ester of boric acid. borate any salt of boric acid. , aluminum trihydrate, antimony antimony (ăn`tĭmō'nē) [Lat. antimoneum], semimetallic chemical element; symbol Sb [Lat. stibium,=a mark]; at. no. 51; at. wt. 121.75; m.p. 630.74°C;; b.p. 1,750°C;; sp. gr. (metallic form) 6. oxide and chlorinated chlorinated /chlo·ri·nat·ed/ (klor´i-nat?ed) treated or charged with chlorine. chlorinated charged with chlorine. chlorinated acids some, e.g. paraffin paraffin, white, more-or-less translucent, odorless, tasteless, waxy solid. It melts between 47°C; and 65°C; and is insoluble in water but soluble in ether, benzene, and certain esters. as additional flame retarder. In this formulation, the properties of the uncured compounds do not differ significantly (table 4), but the cured properties (table 5) show the advantages of the XD-grade: higher tensile tensile, adj having a degree of elasticity; having the ability to be extended or stretched. , higher elongation, higher tear and even better abrasion abrasion /abra·sion/ (ah-bra´zhun) 1. a rubbing or scraping off through unusual or abnormal action; see also planing. 2. a rubbed or scraped area on skin or mucous membrane. resistance. Table : Table 4 - compound properties of conveyor belt cover
XD-CR M-CR
ML 1+4/100 [degrees] C (MU) 88 81
ML-1+4/100 [degrees] C after 7d/35 [degrees] C (MU) 99 91 Density (g/cm [sup.3]) 1.51 1.51 Scorch behavior MS-[t.sub.5]/120 [degrees] C (min) 16 17 MS-[t.sub.5]/120 [degrees] C after 7d/35 [degrees] C (min) 13 15 14d/35 [degrees] C (min) 10 12 Vulkameter 130 [degrees] C [t.sub.s] (min) 12.5 12.5 Vulkameter 150 [degrees] C [F.sub.min] (N) 4.7 4.9 [F.sub.max] (N) 48 51 [t.sub.10] (min) 3.4 3.0 [t.sub.80] (min) 18 18 [t.sub.90] (min) 27 27 Table : Table 5 - vulcanizate properties of conveyor belt cover Mechanical properties(1) XD-CR M-CR TS (Mpa) 20.5 18.6 EB (%) 455 405 M 100 (MPa) 13.4 13.4 H (Shore A) 70 69 R (%) 28 28 Tear (DIN 53 507) (N/mm) 11 9 Tear (DIN 53 515) (N/mm) 26 22 DIN-abrasion(2) (mm(3) 113 126 Oxygen-index (LOI) (%) 45 45 1 - 2 mmISO-dumbbell no. 2, average of 10, 20, 30 min/150[degrees]C cure for stress-strain and tear resistance test 2 - Average of 20 and 30 min/150[degrees]C cure Compound for textile reinforced conveyor belt carcass carcass, carcase 1. the body of an animal killed for meat. The head, the legs below the knees and hocks, the tail, the skin and most of the viscera are removed. The kidneys are left in and in most instances the body is split down the middle through the sternum and the vertebral The carcass compound must have excellent flame resistance as well as good adhesion to fabric. Tensile and elongation are not as important for the cover, and it may be highly loaded to reduce the cost. In the past, we used a M-grade with a high Mooney viscosity, adding in total 140 phr of filler and 40 phr of plasticizer plas·ti·ciz·er n. Any of various substances added to plastics or other materials to make or keep them soft or pliable. plasticizer or -ciser Noun . Table 6 shows the comparison of this compound based on high viscous M-grade with a similar compound based on medium viscous XD-grade. The compound viscosity of the XD-grade is significantly lower, resulting in better processing and better penetration into the fabric. Nevertheless, the cured properties are nearly the same. In addition the adhesion to different fabrics does not change. Table : Table 6 - textile reinforced conveyor belt carcass
M-CR high XD-CR
viscosity medium
viscosity
Uncured compound properties ML 1+4/100[degrees]C (MU) 88 66 MS-t(5)/120[degrees]C (mins) 26 31 Cured properties (4mm ISO-ring no. 1) Cure: 150[degrees]C/30mins TS (MPa) 10.9 9.9 EB (%) 590 605 M 100 (MPa) 2.6 2.2 M 300 (MPa) 6.4 5.3 H (Shore A) 67 66 R (%) 28 24 Tear(4) (N) 156 151 Oxygen-index (LOI) (%) 52 52 Rubber/fabric adhesion (N/25mm) Cure: 150[degrees]C/30 mins PZ-fabric(1) 340 335 EP-fabric(2) 360 350 EBPZ-fabric(3) 210 25 1 - PZ 121.02; Olbo-textilwerke, Solingen 2 - EP 150.11; Olbo-textilwerke, Solingen 3 - EBPZ 113.04; Olbo-textilwerke, Solingen 4 - Company method Compound for steel cord reinforced conveyor belt carcass For this compound in the past mainly S-types of CR have been used due to their good adhesion to galvanized gal·va·nize tr.v. gal·va·nized, gal·va·niz·ing, gal·va·niz·es 1. To stimulate or shock with an electric current. 2. steel cable. As S-types normally do not need any accelerator for curing, the formulation has to be changed when using XD-types. Furthermore, polybutadiene, often used in S-type base compounds to avoid mill sticking, is not necessary for XD-types. Therefore the formulations are not completely identical, but it will be shown that an XD-compound offers better processing, mechanical and adhesion properties. The XD-formulation in table 7 has a lower viscosity and longer scorch time than the S-type, resulting in better processability. On the other hand, the tensile is nearly the same with advantages in elongation for the XD-type. Table : Table 7 - carcass for steel cord belt (modification in curing system and processing aid) S-CR XD-CR Compound properties ML 1+4/100[degrees]C (MU) 122 120 ML-t(5)/120[degrees]C (min) 6 11 Vulkameter 150[degrees]C F(min) (N) 11 13 F(max) (min) 93 88 t(10) (min) 1.7 1.1 t(80) (min) 12.5 20.1 t(90) (min) 18.9 24.6 Vulcanize vul·ca·nize tr.v. vul·ca·nized, vul·ca·niz·ing, vul·ca·niz·es To improve the strength, resiliency, and freedom from stickiness and odor of (rubber, for example) by combining with sulfur or other additives in the presence of heat properties, ISO-dumbbell 2 (150[degrees]C/30 min) TS (MPa) 14.8 14.4 EB (%) 355 540 M 100 (MPa) 4.9 3.7 H (Shore A) 77 77 Adhesion to galvanized steel cord (N/20mm) 251 242 The adhesion level is the same within the limits of reproducibility, but the XD-type produces improved rubber coverage on the cord than the M-type, which is advantageous. Chemistry of XD-grades What is now the chemical difference making XD-CR superior to M- or S-grades? Xanthogendisulfide (XD)- or mercaptan (M)-derivatives respectively, act as a chain transfer agent during polymerization polymerization Any process in which monomers combine chemically to produce a polymer. The monomer molecules—which in the polymer usually number from at least 100 to many thousands—may or may not all be the same. . The first advantage of XD-grades is the chemical reactivity of XD-fragments at the chain ends and along the chains. This results in an improved, more homogeneous network with reduced free chain ends. Such networks exhibit improved mechanical properties, especially with respect to tensile and tear strength. Higher filler/plasticizer loadings are therefore possible with XD-grades for a certain specified tensile level. Another advantage of these XD grades is - as far as Baypren is concerned - improved processing. This can be attributed only in part to XD-chain modification. Other special production parameters are also responsible for this improvement. It results mainly in reduced elasticity ("nerve") of compounds, compared to standard M-grades of the same polymer viscosity. Guidelines guidelines, n.pl a set of standards, criteria, or specifications to be used or followed in the performance of certain tasks. for formulation Now some aspects of the formulation of conveyor belt compounds will be discussed: * polychloroprene itself is inherently flame retardant due to its chlorine content; * this flame retardancy can be further improved by addition of flame retarders. Widely used are chlorinated paraffins Chlorinated paraffins (CPs) are a complex mixture of polychlorinated n-alkanes and were introduced in the 1930s. The chlorination degree of CPs can vary between 30 and 70%. , antimony oxide, boron-compounds such as zinc borate, aluminum trihydrate and phosphate plasticizers plasticizers mostly triaryl phosphates, such as tricresyl, triphenyl phosphates, which are poisonous. See also triorthocresyl phosphate. . Each substance acts in a special way. Aluminum trihydrate splits off water, acting as an "internal extinguisher." It is said that some of these substances act with a synergetic synergetic /syn·er·get·ic/ (sin?er-jet´ik) synergic. syn·er·get·ic adj. Synergistic. effect. This may be due to the fact that each flame retarder works at a specific temperature, different from the combustion temperature of the other ingredients. By making a compound including all of these flame retarding agents there is a good chance that at each temperature there will be at least one substance acting as fire retarder. Table 8 shows the levels of flame retarders used in our underground mining conveyor belt compounds. Table : Table 8 - flame retarders in polychloroprene conveyor belt compounds
Cover Adhesion Adhesion
compound layer to layer to
fabric steel
(phr) (phr) (phr)
Chlorinated praffin wax 10-20 20-40 10-20
Antimony oxide 5-10 5-15 5-10
Zinc borate 10-20 0-10 0-10
Aluminum trihydrate 10-20 20-40 50-80
DPK (phosphate plasticizer) 0-10 5-15 0-10
The adhesion layer has less carbon black than the cover compound, but a high amount of white fillers. Due to this high loading the polymer content of the compound is not higher than 30%, indicating a relatively low cost formulation. The direct bonding compound to galvanized steel cable is very similar, but contains more A1[(OH).sub.3] and some litharge lith·arge n. A yellow lead oxide, PbO, used in storage batteries and glass and as a pigment. Also called lead monoxide. [Middle English litarge, from Old French, alteration of for improvement of adhesion. Based on these guidelines, 2-ply textile reinforced conveyor belt specimens have been produced in our laboratory, using low viscous XD-CR for the cover and a slightly higher viscous XD-CR for the adhesion layer. This belt yielded excellent mechanical and adhesion properties and passed the Laboratory Gallery Test with an unburnt length of 750-800 mm; 400 mm are required as minimum. Meanwhile, some belt producers in Western Europe use XD-CR for belt production on a regular basis, and we feel that the use of XD-CR for conveyor belts in underground mining will increase significantly in the future. Summary European regulations and flame retardancy tests of conveyor belting for mining were reviewed. The most important tests in West Germany are the "Large Gallery Test" and the "Laboratory Gallery Test." Nowadays, for rubber mining conveyor belts polychloroprene is used exclusively due to its inherent flame retardancy. A special type of polychloroprene polymer, the so called XD-grade, has been introduced. It shows, compared to the conventional mercaptan grades, improved processing characteristics and physical properties, which is due to its special polymer structure. In the highly extended compounds used in conveyor belting the advantages of the XD-grades can be seen clearly. The differences between XD- and M-type polychloroprene are shown for adhesion skim compounds as well as for cover compounds. Finally, guidelines for the formulation of cover and skim compounds were given, including fabric reinforced belts and galvanized steel cord belts. |
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