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XD-modified polychloroprene grades for mining conveyor belting.

XD-modified polychloroprene grades for mining conveyor belting

There are only a few working areas where fire can create as disastrous results as in underground 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 masks);

* conductivity tests

It is anticipated that regulations and standards in Western Europe will change in the near future, due to improved understanding of flame retardancy phenomena and the economic integration of the European Community in 1992 (harmonization of different standards).

Before discussing the polymer aspect for manufacturing belts, 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

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, 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 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 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 SBR 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 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. Although the LOI value acc. ASTM 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 derivatives as a chain modifier;

* 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, 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 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;

* the scorch time and flow time at curing temperature are in most cases slightly longer;

* better penetration into reinforcing material;

* tensile strength and tear strength are consistently higher;

* elongation at break, modulus, hardness and rebound resilience are equal (or slightly higher).

This shall be shown in the following examples.

Test compound acc. to ISO 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, 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. 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, aluminum trihydrate, antimony oxide and chlorinated paraffin 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, higher elongation, higher tear and even better abrasion 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 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. 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 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 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.

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 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, antimony oxide, boron-compounds such as zinc borate, aluminum trihydrate and phosphate plasticizers.

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 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 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.
COPYRIGHT 1990 Lippincott & Peto, Inc.
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Title Annotation:xanthogendisulfide derivates
Author:Pabst, J.
Publication:Rubber World
Date:Jun 1, 1990
Words:2940
Previous Article:Silicone usage expanding in European automotive applications.
Next Article:Developments in Rubber Technology, vol. 4.
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