Coating and its Techniques used for Technical Textiles

Coatings used in the production of technical textiles are largely limited to those products that can be produced in the form of a viscous liquid, which can be spread on the surface of a substrate. This process is followed by a drying or curing process, which hardens the coating so that a non-blocking product is produced. Thus the coatings for these products are limited to linear polymers, which can be coated as a polymer melt or solution and on cooling form a solid film or form a solid film by evaporation of the solvent. There are some types of coatings that can be applied in the liquid form and then chemically crosslinked to form a solid film.

The coatings used in technical textiles are all thermoplastic polymers, which are long chain linear molecules, some of which have the ability to crosslink. The properties of these polymeric materials directly influence the durability and performance of the end product.

Coatings techniques by Components

• Polyvinyl chloride (PVC)
• Polyvinylidene chloride (PVDC)
• Polytetrafluoroethylene (PTFE)
• Rubber
• Styrene–Butadiene Rubber (SBR)
• Nitrile rubber
• Butyl rubber
• Polychloroprene (neoprene)
• Silicone rubbers

Polyvinyl chloride (PVC)

The polymer is a hard rigid solid, which if it is to be used as a coating material for technical textiles needs to be changed to a soft flexible film. This is possible because of a remarkable property of PVC, the ability of the powdered polymer to absorb large quantities of non-volatile organic liquids. These liquids are known as plasticisers and a typical plasticiser for PVC is cyclohexyliso- octylphthalate.

The polymer can absorb its own weight of this plasticiser. However, when the powdered polymer and plasticiser are first mixed, a stable paste is formed which is easily spreadable onto a textile surface. The paste of PVC and plasticiser, known as a plastisol. When this mixture is heated to 120 °C complete solution of the plasticiser and polymer occurs, which on cooling gives a tough non-blocking film. Plasticised PVC forms a clear film, which shows good abrasion resistance and low permeability. The film may be pigmented or filled with flame-retardant chemicals to produce  coloured products of low flammability. The coatings are resistant to acids and alkalis but organic solvents can extract the plasticiser, making the coatings more rigid and prone to cracking.

One great advantage of a polymer with an asymmetric chlorine atom is its large dipole and high dielectric strength. This means that the coated product may be joined together by both radiofrequency and dielectric weldings techniques..

Polyvinylidene chloride (PVDC)

PVDC is very similar to PVC. As in the case of PVC it is made by the emulsion polymerisation of vinylidene chloride. The resulting polymer forms a film of low gas permeability to gases, however, the polymer is more expensive than PVC and therefore only tends to be used where flame resistance is required. As may be seen from the formula, PVDC contains twice the amount of chlorine as PVC and this extra chlorine is used in flame-resistant coatings. When a flame heats these materials the polymer produces chlorine radicals which act as free radical traps, thus helping to snuff out the flame.

Polytetrafluoroethylene (PTFE)

PTFE is perhaps the most exotic of the polymers which occur in coated textiles. It is manufactured by the addition polymerisation of tetrafluoroethylene. Since its discovery by Du Pont in 1941, PTFE has found many uses in coating particularly in the protection of fabrics from the harmful effects of sunlight.

One remarkable feature of the polymer is its very low surface energy, which means that the surface cannot be wetted by either water or oil. Textile surfaces treated with this polymer are both water repellent and oil repellent. Hence PTFE is found on diverse substrates which range from conveyer belts used in food manufacture to carpets where stain resistance is required. In addition the polymer shows excellent thermal stability and may be used up to a temperature of 250°C.

In order to reduce the cost of fluoropolymers several less expensive compounds have been produced, such as polyvinyl fluoride (PVF) and polyvinylidene fluoride (PVDF), which are analogous to the corresponding PVC and PVDC. However, while these materials are similar to PTFE they are slightly inferior in terms of resistance to weathering.

Rubber

Natural rubber is a linear polymer of polyisoprene, found in the sap of many plants, although the main source is the tree Hevea brasiliensis. Rubber occurs as an emulsion, which may be used directly for coating, or the polymer may be coagulated and mixed at moderate temperatures with appropriate fillers.

Styrene–Butadiene Rubber (SBR)

SBR is made by the emulsion polymerisation of styrene and butadiene. The formula illustrated implies a regular copolymer but this is not the case and SBR is a random copolymer. The compounding and application techniques are very similar to those for natural rubber although the material is not as resilient as natural rubber and also has a greater heat build-up, which make SBR inferior to natural rubber in tyres.

Nitrile rubber

Nitrile rubbers are copolymers of acrylonitrile and butadiene. These materials are used primarily for their excellent oil resistance, which varies with the percentage acrylonitrile present in the copolymer and show good tensile strength and abrasion resistance after immersion in oil or petrol. They are not suitable for car tyres but are extensively used in the construction of flexible fuel tanks and fuel hose.

Butyl rubber

Butyl rubbers are copolymers of isobutylene with a small amount of isoprene to make the copolymer vulcanisable or crosslinked. Because of the low amount of isoprene in the structure, the vulcanised structure contains little unsaturation in the backbone and consequently the rate of oxidation or oxygen absorption is less than that of other elastomers except for the silicones and fluorocarbons.

When an elastomer contains double bonds, oxidation leads to crosslinking and embrittlement, whereas in butyl rubbers oxidation occurs at the tertiary carbon atom which leads to chain scission and softening. Further, the close packing of the hydrocarbon chains leads to a structure which is impermeable to gases and its main use is in tyre tubes and inflatable boats.

Polychloroprene (neoprene)

Neoprene rubber was first developed in the United States as a substitute for natural rubber, which it can replace for most applications. It is made during the emulsion polymerisation of 2-chlorobutadiene. Neoprene rubbers can be vulcanised and show tensile properties similar to natural rubber, however, they are perhaps most widely used for their excellent oil resistance. Weathering and ozone resistance is good and the polymer finds its main end-uses in the production of belts and hoses. The neoprene latex can also be used in dipping and coating.

Silicone rubbers

Silicone rubbers are polymers which contain the siloxane link Si—O—Si and are formed by the condensation of the appropriate silanol which is formed from the halide or alkoxy intermediate; the final condensation then takes place by the elimination.

The groups R1 and R2 are normally inert groups such as methyl, but they may include a vinyl group and therefore are capable of crosslinking. It is also normal to fill these polymers with finely divided silica, which acts as a reinforcing filler. These polymers show outstanding low temperature flexibility and can be used at temperatures as low as -80°C, while they retain their properties up to 250°C. They also show good resistance to weathering and oxidation. Unfortunately their price is high.

Coating Methods

The original methods of coating were largely based on various impregnating techniques based on an impregnating trough followed by a pair of squeeze rollers to ensure a constant pick-up. The material was then air dried at constant width, usually on a stenter, and rolled. However, when the coating was required on one side of the fabric then total immersion of the fabric in the coating liquor was not possible and other techniques had to be developed.

Lick roll

In this method the fabric was passed over the coating roll which was rotated in a trough of the coating liquor. There were several variations on this theme, which were developed to ensure a more even application of the coating by metering the coating onto the fabric. This was done by two main approaches, the first of which was to use a second roll on the primary coating roll, which picked up a fixed amount. The second was to use a doctor blade on the primary roll, so that again only a fixed amount of liquor was transferred to the fabric. The main disadvantage of these systems was that the amount of coating on the fabric was dependent on the surface tension and viscosity of the coating fluid and also the surface condition of the fabric. To overcome this problem knife coating was developed, which functions in basically the same way that butter is spread on toast.( Fig- 2.7)

Knife coating

In this method the coating fluid is applied directly to the textile fabric and spread in a uniform manner by means of a fixed knife. The thickness of the coating is controlled by the gap between the bottom of the knife and the top of the fabric. The way in which this gap is controlled determines the type of machinery used. The following are the main techniques used:

• knife on air
• knife over table
• knife over roller
• knife over rubber blanket

In the first of these the spreading blade is placed in direct contact with the fabric under tension and the coating compound is thus forced into the fabric. The main advantage of this technique is that any irregularities in the fabric do not affect the running of the machine. However, this is not the case with the knife over table  or knife over roll methods, for although the coating thickness can be accurately controlled, any fabric faults or joints in the fabric are likely to jam under the blade causing fabric breakage.

The problem of metering an accurate amount of coating onto the substrate was finally solved by the use of a flexible rubber blanket, which gives a controlled gap for the coating compound and yet is sufficiently flexible to allow cloth imperfections or sewing to pass underneath the blade without getting trapped and causing breakouts.(Fig. 2.9)

Knife geometry

The geometry of the coating knife and the angle of application also have an important role to play in the effectiveness and penetration of the coating. Obviously if uniform coatings are to be obtained over the width of the fabric then an accurately machined flat blade is mandatory. In addition the profile of the knife can have a marked influence on the coating weights and penetration of the coating. There are three main types of knife profile with many variations in between these three:

1. Pointed blade: the sharper the blade the more of the coating compound is scraped off and consequently the lower the coating
2. Round blade: this gives a relatively higher coating weight than a sharp
3. Shoe blade: this gives the highest coating of all the blade profiles; the longer the length of the shoe the higher the coating

In general knife coating fills in any irregularities in the fabric surface giving a smooth finish to the coated surface. The machines which use knife coating are in general simple to operate and can be used for a wide variety of thicknesses from about 1mm up to 30mm.

Air knife coating

In discussing knife coating, mention must also be made of the air knife as a method of removal of the excess coating fluid. In this technique a blast of air is used to blow off the excess coating fluid. The viscosity of the fluid is much lower than in the case of conventional knife doctoring and the coating applied follows the profile of the substrate to which it is being applied. The technique is more frequently used in the paper industry, where it is used to coat photographic paper, rather than in the textile industry.

Gravure coating

The use of a gravure roller in coating was developed from the printing industry, where it was used to print designs. The technique involves the use of a solid roller, the surface of which has been engraved with a closely packed series of small hemispherical depressions. These act as metering devices for the coating fluid, which fills the hemispheres with coating fluid from reservoirs of the fluid. The excess fluid is scraped from the roll with a doctor blade, leaving the depressions with an exact amount of fluid in each. This is then transferred to the substrate to be coated. The quantity of fluid transferred depends on the volume of the engraved depressions and the packing on the surface of the roll.

The greatest drawback to this technique is that for a fixed depth of engraving a fixed coating weight is obtained. Thus if a different coating weight is required then a new engraved roll has to be produced. Further, unless the viscosity characteristics of the coating fluid are controlled, the pattern of the printed dot can be seen on the coated substrate. What is required is a printing fluid that will flow and form a flat surface in the drying process. This formation of a flat coating can be greatly improved by the use of offset gravure printing. Here the fluid is printed onto a rubber roller before being transferred onto the substrate.

Rotary screen coating

This technique is similar to the rotary screen printing process that is used to apply coloured patterns to fabric. The applicator is a cylindrical nickel screen, which has a large number of perforated holes.The coating compound is fed into the centre of the screen, from whence it is forced through the holes by either a doctor blade or a circular metal rod. The coating weight can be controlled by the number of holes per unit area and the coating weights are very precise. However, the coatings have a dot configuration and to obtain a continuous coating a wiper blade that spreads the dots into a continuous coat must be used.

Hot-melt coating

In this technique the coating materials must be thermoplastic, so that they melt when heated and in this condition are capable of being spread onto a textile substrate. Thus, in some respects they are similar to paste coating. However, the big difference from paste coating is that the thermoplastic coating has no solvents to evaporate and no water that has to be evaporated, giving the process both economic and ecological advantages. The molten polymer is usually calendered directly onto the textile or in some cases extruded directly from a slotted die. This is followed by contact with a polished chill roller, if a smooth surface is required on the coating or a patterned roll if a patterned effect is required. One further process, the use of powdered polymers as a coating medium, needs to be mentioned in the area of hot- melt coating. In this technique the powdered polymer is sprinkled onto the substrate, followed by heating with radiant heaters to melt the thermoplastic. The coating is then compacted and rendered continuous by a compaction calender. The main materials used in this are polyethylenes and nylon and these are now being applied in the production of carpets for car interiors, where because of the mouldability of the thermoplastic, a complete car carpet may be pressed out in one operation.

Transfer coating

The final coating technique to be described in this section is transfer coating. In this the coating material is preformed into a continuous sheet which is laminated to the substrate either by the application of heat or by the use of an adhesive known as a tie coat. The great advantage of this technique over all the others is that the coating film may be made completely free of holes and defects before it is applied to the fabric. In general, transfer coating will give the softest coating of all coating techniques in terms of fabric handle and furthermore there is no possibility of the coating bleeding through onto the face of the coated fabric.

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