Coating, treating, rolling
Textile fabrics are subject to numerous factors influencing the ageing process and thus the quality requirements. In addition to load-dependant influences like alternating loads, extent of loading and creep, other factors and above all load – independent effects like ageing, natural climatic and atmospheric effects caused by pollutants have to be taken into account with textile fabrics. Textile fabrics, in addition to undertaking structural functions, have to be resistant against chemical and biological influences and also be almost non-flammable.
Fig. 49: Internal construction of a polyester fabric coated on both sides and with topcoating
The resistance of materials against external effects is achieved through a wide range of measures during production. Textile fabrics used in construction for wide-span lightweight structures are therefore designed for specific purposes as composite materials. With various combinations and thickness of materials for base fabric, coating and finish, various deleterious effects can be resisted. With coated membranes, the fabric undertakes the load bearing function. The coating protects the fabric from damage. Together with the surface treatment (topcoating, finishing), it ensures the sealing of water and air.
The coating material for polyester fabrics is PVC-P (Polyvi – nylchloride-Plastisol with plasticizer and additives), a thermoplastic made of polymers with a linear, unbranched molecule chain arrangement. With the application of heat, it can be formed and becomes flowing. Coatings are applied to PVC-coated polyester fabrics in order to delay the escape of plasticizer from the coating and thus protect the fabric coating from becoming brittle. If the protection against UV rays failed, this would lead to a rapid decrease of strength in the short term.
This surface treatment also hinders the growth of sedimentation and the formation of microbes with the resulting impairment of the optical properties.
PVC-coated polyester fabrics are mostly coated using a knife coating system in Europe. An exception is the process for coating pretensioned polyester fabrics in the Precontraint process (Ferrari). In a process used in the USA (Seaman process), the weft thread is laid on the warp thread and then the coating compound is laid on top.
In the painting process, the first stage is to apply the adhesion coat on the raw fabric. The adhesion coat is applied in the form of a paste with an air knife. An adhesion compound (containing isocyanate) with the addition of plastisol (PVC-P, amorphous thermoplastic) guarantees the necessary bond between the raw fabric and the coating (adhesion strength) for subsequent processing. The adhesion coat is heated to approx 180 °C in a fusion tunnel. This makes the plasticizer penetrate into the PVC granules very quickly, swell them up and fuse them together. In the jelly-like soft PVC layer, the PVC and plasticizer molecules combine to a homogeneous mass. In order to be able to unroll the coated fabric leaving the fusion tunnel, it is cooled by passing through a number of water-cooled rollers.
After cooling, the adhesion layer lies like a film on the raw fabric. In a second process, the top coat is applied with an air knife. This contains the same components as the adhesion layer, but without adhesion compound. This is followed by another partial gelling and cooling. Because textile fabrics are normally coated on both sides, the entire process has to be repeated for the reverse side.
One criterion for the long-term preservation of the mechanical properties of polyester/PVC fabrics is the coating thickness on the thread ridge. The fabric protection is directly proportional to the coating thickness: "The higher the thickness of the covering, the better the protection of the thread." The usual coating thickness is from 0.08 mm to 0.25 mm. For fabric type 1 (plain weave) with a total coating weight of 500 g/m2, 200 g/m2 are applied to the fabric underside
and 300 g/m2 to the upper side. The base coat on each side of the fabric is approx. 100 g/irb The remaining weight is represented by the top coat. For raw fabrics with higher area weights (basket weave), the base coat layer is about 200 g/ m2. The number of layers of coating depends on the fabric type. Depending to the number of layers, the coating speed is approx. 10-15 m/min.1
Some manufactures possess a "4-coat" coating plant, where the PVC coating is applied to both sides of the fabric in one production pass. The first three layers are applied by vertical coating equipment and the last coat, normally the top coat of the upper side, is applied with a horizontal roller knife.
Fig. 52: Top coat with roller knife 1 Graf, W. (2004)
During the production steps of weaving and coating, the warp is held under tension. The coating plants are controlled so that a uniform tension can be applied to the warp. Along the edge, the fabric acting as substrate for the coating is held in the tension frame with needles or clips during the gelling process. Because of the shrinking forces in weft direction, there is the danger of the needles not holding, which is why pre-shrunk thread is used, with a shrinkage in hot air of max. 2.5 %.2
The surface treatment of the coating, called topcoating or finishing, gives an additional protection against the fabric becoming dirty too rapidly or moisture penetrating. The treatment also delays the loss of plasticiser from the coating.
Two processes are available for the application of the top coating, which affect the properties of the finished membrane in different ways: the lamination of films and the application of paints. Lamination achieves protection against external influences through the application of a 0.03 – 0.08 mm thick, solid and transparent film to the PVC coating. An absorber contained in the primer makes the film resistant against UV effects and a white pigment additionally shuts out and reflects the UV light. This technique does, however, have disadvantages for the manageability of the fabric. The lamination with film additionally hinders the flexibility of the warp and weft threads at an angle (see section 2.4.1).
When applying paint based on PVDF (fluoroplastic) and acrylates, the fabric is only negligibly stiffened. The paints, which are applied from solvent systems, are applied to the inner and outer sides in a layer thickness of approx. 5-10 pm. PVDF paints can be non-weldable paints, which have to be scraped off before welding, or else weldable derivatives made of PVDF paints and acrylates. PVDF paint is applied to the outer side with at least 2 coats. Top coat coatings consist-
ing of PVDF paints ensure a surface of unaltered appearance in the long term, but acrylic paints cannot prevent discolouration in the long term.
The importance of treating coated fabrics can be seen from the various effects resulting from moisture, which can affect fabrics with damaged coating.
One example of this is the wicking effect, a by-product of osmotic pressure. At damaged locations in the coating, water can be transported along the thread and damage the fabric permanently (Fig. 53).
In addition to the alterations caused by the effects of temperature, light and natural media, material weakening through attack by microorganisms and chemicals also has to be expected. Fungi, lichens and the tips of roots exude metabolic products in the form of acids (oxalic acid, sulphuric acid), enzymes and dyes, and these can can alter the properties of the material permanently. The biogenic alteration caused in this way can lead to the coating coming loose (Fig. 54). With
Glass/PTFE fabrics, the good anti-adhesive capability of the PTFE coating on both sides has the effect of preventing deposits on the coating.
In the last step of the process, the coated and treated fabric is rolled on the outer side. This process produces a flatter surface with the result that the fabric stays cleaner. The rolling is done with steel rollers, which press the fabric against rubberfaced opposing rollers.
If there are special requirements for durability or fire behaviour, then one of the group of PTFE-coated glass fabrics will be used.
The stiffness and strength of these fabrics comes mostly from the glass fabric. The protective PTFE coating consists of fluor – inated plastic, one of the strongest bonds in organic chemistry. PTFE is a less stiff, but very strong plastic. It is to a high degree resistant to chemicals, temperature, light and weather-resistant and has very good anti-adhesive properties. PTFE is also, without the addition of stabilisers and plasticisers,
Section in warp direction
Section in weft direction
Fig. 55: top:Glass fibre, glass fabric and coated fabric; bottom: Internal construction of fabric
non-flammable and can be used over a temperature range of -270 °C to +300 °С.1 The glass fibres forming the base fabric are UV-resistant. If the coating is damaged, the penetration of water is mainly a problem on account of the loss of strength.
Glass fibres, because they are brittle, are spun in single filaments, which are then twisted together to form single threads. During the spinning and twisting together to form yarn, the threads are dipped in a size. This reduces the mechanical loading and the resistance of the filaments and acts as lubrication.2
The glass fabric is coated in 5 m wide rolls in 6 – 10 passes through a watery PTFE emulsion and then subjected to infrared rays, which fully evaporate the water. After the first stage of coating, the PTFE has to be sintered at 370-380 °С on account of its low flowing ability after fusing, which causes a part of the applied material to caramelise. This process is repeated until the coating has the desired thickness. The sintering on of PTFE particles in layers results in a fissured surface, which can be levelled with a finish of FEP The warp direction
is tensioned with a defined force during the entire coating process. In the weft direction, it is not possible to hold the fabric fast on account of the strong heating during sintering. The crimping of the weft thread resulting from the weaving remains and is made still stronger by the tensioning in the warp direction.
After completion of the coating process, the fabric is cooled. The different coefficients of thermal expansion of glass fabric and PTFE and the more rapid cooling of the outer layers lead to the PTFE expanding before the glass fabric. The crystal transition at 19 °С is reinforced by this effect at low processing temperatures.
Because of the caramelisation during coating, the coating initially discolours to light brown. This colouration disappears through bleaching after a few months of UV exposure. Apart from this colour change from brown to white, there are no appreciable effects on Glass/PTFE membranes from longterm radiation intensity.3
One disadvantage of PTFE-coated fabrics is the susceptibility to kinking, which requires a special edge detail. The preparation work for erection (packing and transport) and the erection have to include suitable measures to prevent the occurrence of breakages in the coating. Such breakages could be the starting point for further tearing. These fabrics are therefore not suitable as structural elements in temporary buildings and convertible construction. In addition, they are nearly rigid in shear and hard, are difficult to handle and can only be erected without damage at temperatures over 5 °С. On account of the strong creep, considerably more time is required for the erection.
A group of materials, which are seldom used, are silicone – coated glass fibre plastics. The carrier material for this fabric is interwoven filaments of silicate glass. The coating material is a clear to opaque silicone, treated with additives.
The advantage of these fabrics is being especially flexible and not prone to kinking. They fulfil all strength requirements and have favourable tearing behaviour. They are largely resistant against chemical effects and can be used at temperatures from -60 ° to +180 °С. They are also UV resistant, do not become brittle and can be coloured as required. One disadvantage that should be mentioned is the behaviour of the coating in getting dirty. Silicones are resistant against chemical attack, but their surface charges up statically and attracts dirt.4
To prepare it, the silicone rubber has to be kneaded, dissolved in solvent and mixed. The silicone is applied to the
glass fabric with a knife coating system. The silicone mixture in paste form is applied to both sides of the glass fabric with many passes of an air knife and then a top coat of silicone mixture is applied. Weathering tests are constantly carried out by the manufacturers to investigate and evaluate the behaviour with regard to attracting dirt.
Silicone cannot be welded thermally. It has no pronounced melting point and hardens to an elastomer with spatially networked molecules. The roll material up to 2 m wide is, according to information from the manufacturers, sewed, stapled or vulcanised.
The vulcanisation of the strips is carried out with beam presses. A silicone adhesive tape is laid between the parts to be welded and heated to 150 – 170 °C. The resulting vulcanisation reaction between the adhesive made of synthetic polymers with organic silicon compounds and the rubber molecules of the silicone coating leads to a high-strength joint. The welding duration with this process is between approx. 30 sec and 2 min. With this jointing method, care has to be taken with the temperature resistance of the glued seam. In order to achieve form stability and freedom from shrinkage, the product has to be stored at particular temperatures for a particular time, which ensures mould striking by the end of the reaction.