GB2036818A - Impregnating Woven Conveyor Belting - Google Patents

Abstract

Multiply woven conveyor belting is produced by a process which includes the step of subjecting the fabric carcase to a plurality of alternating changes of direction whilst immersed in a bath of impregnant material, each of said changes of direction being effected by causing the carcase to follow a path around rollers whose radius of curvature is appreciably less than 75 mm e.g. 35-40 mm. Prior to immersion, a synthetic polymeric yarn-containing carcase may be subjected to a shrinking treatment effective to shrink at least the synthetic fibre component of the carcase.

Description

SPECIFICATION Improvements In and Relating To Production of Belting This invention relates to the production of solid woven conveyor belting. The term "solid woven" in the present context denotes a fabric carcase comprised of a plurality of yarns simultaneously woven and linked together to form a thick, multiply fabric. To produce belting from such a solid woven carcase it is necessary to impregnate the carcase in order to obtain the necessary mechanical properties, the impregnant formulation being also selected to confer properties such as fire resistance and/or static electricity dissipation characteristics, as required for a particular end use. The impregnated carcase may also be given surface coatings ("covers") where necessary to meet end use requirements such as abrasion resistance and load carrying capability.

The impregnant and cover materials usually employed are plastisols of homopolymers and/or copolymers of PVC, such as PVDC and PVAc, although polyurethanes and/or other polymers available in suitable liquid or paste form may also be employed. The term "PVC" is used herein for convenience to encompass all such materials.

However, solid woven fabrics of the kind useful for the conveyor belting tend to be very difficult to impregnate due to their highly compact and dense structure. Typical carcase thicknesses would be in the range 5 to 1 5mm for final belt tensile strengths of about 400 to 1 800 KN/m.

Prior proposals to overcome this difficulty in adequately impregnating solid woven carcases have included "wet weaving", in which the warp yarn is impregnated with PVC paste during or prior to weaving. This is inconvenient because the paste tends to be displaced from the warp by the action of the loom. It has also been suggested that conventional dry weaving should be followed by impregnation under vacuum, either on a continuous or discontinuous basis.

Attempts have also been made to simply squeeze the impregnant material in liquid form into the carcase, but the latter is already so dense that application of pressure by way of squeeze rolls merely displaces the impregnant material from the surface of the carcase, without causing significant penetration. An example of such a process is found in German patent specification No. 1200782, where it is proposed to increase the effect of the treatment by driving at least some of the downstream squeeze rolls with a faster surface speed, so that slippage occurs. This technique is similar to that of an earlier German Patent Specification No. 531 910.

In its broadest aspect, the present invention provides a process for the production of a solid woven conveyor belting, the process including the step of treating a solid woven belting fabric carcase with an impregnant material by subjecting said carcase to a plurality of alternating changes in direction whilst immersed in said material, each change in direction being brought about by causing the carcase to follow a generally curved path having a radius of curvature appreciably less than 75mm. Where a roller is used to effect the change of direction, a radius in the range 35 to 40mm appears to represent a practical compromise between adequate roller stiffness and achieving satisfactory impregnation.

It has been found that the process just recited is highly effective in securing impregnation of a solid woven carcase, even at production speeds of two metres/minute or more. This is in very marked contrast to prior art processes in which simple, prolonged immersion is used at typical production speeds of up to T metre/minute.

Whilst at least two changes in direction appears to be the minimum necessary to properly impregnate a relatively thin carcase, ten or more successive changes in direction may be used. This would be for a relatively thick and dense carcase.

It is particularly preferred that in order to counteract the frictional forces generated by each change of direction, the carcase should be positively driven through each such change of direction, for example, by positively driving each guide roller about which a particular change of direction occurs. It is also preferred that the tension in the carcase be regulated by adjusting this positive drive. In particular, a progressive build-up in tension should be avoided. Minimum tension is preferred for the impregnation step.

It is presently common practice to use a carcase containing both natural fibres, e.g. cotton and also synthetic fibres, particularly cotton/polyamide and cotton/polyester yarns in both warp and weft. Such fabrics are prone to shrink on heating and this is particularly relevant where the impregnated carcase is subsequently heated in an oven to gell the impregnant.

Shrinkage tends to expel the impregnant from the carcase to form a nodular surface layer which is very difficult to treat by post-coating or pressing to obtain a surface layer having the appropriate properties. This is particularly true where a PVC material is used.

According to a particularly preferred aspect of the present invention, the process includes the step of applying a shrinkage treatment to a carcase including synthetic polymeric yarn, prior to subjecting it to passage through the impregnant material. Contrary to expectation, it has been found that this shrinking treatment, although to some extent closing the interstices of the fabric does not significantly affect the subsequent impregnation step. If necessary, a further change or number of changes in direction can be used by way of compensation for the increase in carcase density caused by shrinking.

Shrinking before impregnating has at least one further advantage in that it facilitates close control of the properties of the finished belting.

The final extensibility of the belt may be increased by shrinking the carcase before impregnation and reduced by the application of controlled tension during impregnation, during subsequent heating to gell the impregnant, and/or during any subsequent processing steps, including a final pressing treatment. It follows that the process of the invention can be used to produce belting having controlled stretch characteristics. For example, a high-stretch belting having good impact resistance can be made by deliberately avoiding undue tension after the shrinking treatment, so that the effect of the latter is not "pulled out" of the carcase.In this context, the ability to control frictional forces and the build-up of tension in the carcase during treatment in the bath of impregnant material is important since it greatly facilitates accurate control of the elongation and thickness properties of the belting, since both thickness and elongation ("stretch") are closely dependent on the tension applied in the various stages of processing. As mentioned earlier, this tension control can be accomplished in part by positively driving the carcase through each change of direction in impregnant.

Preferably the changes of direction are in vertically disposed sine-wave relation to one another. This is useful in at least one respect, namely that the level of the impregnant in the bath can be used to adjust the pick-up of impregnant by the fabric.

Although the process ofthe present invention is capable of impregnating a carcase at very significantly increased speed, the maximum speed is limited by the capability of the following oven, which is used to gell the impregnant. Using a typical PVC plastisol as impregnant, at a temperature of 200C+30C to maintain a reasonably constant viscosity, the gelling oven must be capable of raising the impregnated carcase temperature to 160--2000C, a temperature of 1 700C being particularly preferred to give adequate gelling without decomposition.

"Adequate gelling" in this present context means that the PVC resin more or less completely dissolves in the plasticiser liquid in which it is suspended, to give solid, homogeneous PVC resin to which a final, cover layer of the same, or a different PVC resin can be applied, prior to gelling this cover layer and then pressing to form the final product, which only requires minor finishing operations such as edge trimming. It will be appreciated that a complete production process according to the invention will normally comprise at least three quite discrete steps. Initially, the solid-woven carcase is subjected to a heat treatment under controlled tension effective to cause the synthetic fibre components to shrink.

This pre-shrunk carcase is then impregnated and the impregnant gelled by heating, both under controlled tension.

In order that the invention be better understood a preferred embodiment of it will now be briefly described by way of example with reference to the accompanying drawing in which the sole figure is a schematic cross-sectional side view of a complete belting production line used to carry out the process of the invention.

The production line shown in the Figure may conveniently be considered as having four discrete sections, the first being a carcase supply, tension control and pre-shrinking section, 1. This is followed by a cooling zone and speed control section, 2, which is in turn followed by an impregnation and gelling section, 3. Finally, a further cooling zone, tension control and reel-up section, 4, is provided.

In section 1, belting carcase 5 is supplied from a roll 6 through a clamp 7, an accumulator 8 and a friction brake 9, which provides control of the carcase tension entering a pre-shrinking oven 10.

In the oven, the carcase is exposed to hot air and heating sufficient to shrink the synthetic fibre component(s) of the carcase, the extent of the shrinkage being a function of temperature and tension. From the oven, the hot carcase passes into the second section, entering a cooling zone 11 in which it is exposed to jets of air at ambient temperature. This is immediately followed by speed control/drive rollers 12 which act in conjunction with the brake 9 to control the tension, both through the pre-shrinking oven 10 and into the following impregnator, 13. The latter comprises a tank 14 containing PVC plastisol 1 5 and an array of rollers 1 6 around which the carcase passes in a sinusoidal path before being led out of the tank by a guide roller 17 to squeeze rollers 18, which remove excess plastisol.The rollers 1 6 are all positively driven (by variable speed drive which is not shown, in the interests of clarity), the drive speed being regulated as will be discussed later.

The rollers 1 6 in this case were of diameter 75 mm and the guide roller 17 was 1 50 mm in diameter. From the squeeze rollers 18, the impregnated carcase enters a vertically disposed gelling oven, 19, in which it is subjected to hot air heating. The oven is followed by a cooling chamber, 20, in which jets of air at ambient temperature are used to cool the belting which is then collected on a reel, 21, after passage around a set of drive/tension rollers 22, 23. The latter cooperate with the rollers 1 6 to establish control of the tension during the gelling/cooling treatments.

The tension in the carcase is continuously monitored, both before impregnation and after the gelling oven, the outputs of the monitoring system being employed to adjust the various drive, brake and tension control rollers so as to maintain the tension both constant and at prechosen levels in each section. The drive speed of the impregnator rollers 1 6 is also controlled by the same means, being automatically adjusted to minimise the build-up of tension during actual impregnation. Because the apparatus provides for tension control in each stage of processing, it enables the elongation properties of the final product to be "tailored" to a particular end use.

Whilst the product as collected on the reel 21 will usually require the application of cover layers of the same or a different PVC resin, according to the properties required for the end use, this is not necessarily an essential feature of the process. It may only be necessary to trim the edges of the product if the surface characteristics are satisfactory. However, in this context, it should be noted that the squeeze rollers 18 may be configured to leave sufficient plastisol on the surfaces of the impregnated carcase to provide cover layers on the final product.These cover layers would normally be of the same PVC as is used in the tank 14 to impregnate the carcase, but it is to be understood that by including an ancillary coating unit between the squeeze rollers and the gelling oven, a different PVC may be applied to the surfaces and gelled at the same time as the impregnant, provided, of course, that the oven 1 9 has sufficient capacity for this purpose.

Where the PVC surface coatings are applied subsequently, they may be gelled by passage through an oven as described above or they may at least be partly gelled prior to final gelling, for example, in a conventional belting press.

However, the use of the latter is not essential if the oven 1 9 has sufficient capacity. The process of the invention may therefore replace a part or the whole of a conventional belting plant. In order that the invention be better understood, use of the apparatus of the figure is illustrated by the following Examples.

Example 1 A high strength belting was made from a carcase of thickness 8.5 mm solid woven from cotton-polyamide composite yarns to a fabric density of 4.6 Kg/metre2. This was processed into belting on the apparatus of the Figure at a speed of 1 metre/minute. The pre-shrinking oven temperature was 1 500C and the shrinkage developed was about 33%. After cooling to room temperature, the carcase was impregnated by subjecting it to ten successive alternating changes of direction in a PVC homopolymer plastisol of viscosity 1 5 poise. The pick-up was about 80% by weight. After squeezing off the excess with the rollers 18, the PVC was gelled at 1 700C for about 4 minutes before cooling and reeling.The pre-shrinking/cooling tension was approximately 100 Kg/m and the gelling/cooling tension was approximately 400 Kg/m, the impregnation tension being approximately 50 Kg/m.

Final surface coatings of a PVC plastisol were applied subsequently in this case and gelled by hot pressing. The strength of the product was 1140 KN/metre.

Example 2 A medium strength belting was produced from a similar solid woven carcase 6.8 mm thick and of density 3.3 Kg/metre2. This was pre-shrunk at 2000C at a production speed of 2 metres/minute to give a shrinkage of about 3%. Impregnation was carried out by treating the carcase as before in a PVC homopoiymer plastisol of viscosity 1 5 poise. The pick-up was about 120% by weight and gelling was carried out at 1 800C for approximately minutes. Final coatings were applied as before and the product strength was about 700 KN/m. The shrinking cooling, impregnation and gelling/cooling tensions were all held at approximately 100 Kg/m in this Example.

It will be appreciated that in both Examples, the need for subsequent cover layers to be applied was a consequence of end use requirements, since the product prior to application of these layers would have been suitable for some uses.

Claims (10)

Claims

1. A process for the production of solid woven conveyor belting, said process including the step of treating a solid woven belting fabric carcase with an impregnant material preferably comprising PVC, by subjecting said carcase to a plurality of alternating changes of direction whilst immersed in said material, characterised in that each of said changes in direction is brought about by causing the carcase to follow a generally curved path having a radius of curvature appreciably less than 75 mm.

2. A process according to claim 1 characterised in that the radius of curvature is of the order to from 35 to 40 mm.

3. A process according to claim 1 or claim 2 characterised in that the number of changes of direction is in the range of from two to ten.

4. A process according to any preceding claim further characterised by the inclusion of the step of positively driving the carcase through each change of direction.

5. A process according to claim 4 characterised in that the tension in the carcase is at least in part regulated by adjusting said positive drive so as to minimize tension in the carcase.

6. A process according to any preceding claim characterised in that the carcase contains both natural and synthetic fibres and the process includes the further step of subjecting the carcase to a preliminary heat treatment effective to shrink at least the synthetic fibre component prior to immersion in the impregnant material.

7. A process according to claim 6 characterised in that said heat treatment is carried out under controlled tension.

8. A process according to any preceding claim characterised in that the process includes the further step of subjecting the treated carcase to a heat treatment effective to gell the impregnant material therein.

9. A process according to claim 8 characterised by the inclusion of the steps of applying cover layers of the same or a different impregnant material to the treated carcase and then applying a heat treatment to gel said cover layers.

10. A process according to any preceding claim characterised in that said changes in direction have a generally sinusoidal configuration in the impregnant material with the axis of the sine wave substantially vertically disposed and the degree of impregnant material pick-up being adjusted by varying the depth of the impregnant material relative to the changes in direction.