Лако-красочные материалы — производство

To resist to dynamic loading and corrosive conditions, the fibers and the elastomer matrix must build up chemical bonds. This is achieved by treating textile fibers with an adhesion promoter in a separate processing step. Mechanical anchorage of the fiber is sufficient for products subjected to minor stress (low-pressure hoses, the typical ‘garden hose’). No adhesion promoter is needed for certain articles with open — weave nettings owing to ‘strike-through’ adhesion — that is, the matrix is bonded

through the spaces between the textile reinforcement. Short-fibril fibers such as cotton yarns are anchored in the elastomer via anchorage fibrils.

The following production steps are required for treatment with an adhesion promoter to enhance the adhesion of rubber to textile fibers: The textiles are unrolled under predefined tension, dipped (put through an impregnating bath) with the depth of penetration depending on the tension, the direction of the twist, and the viscosity of the adhesion promoter (dry uptake: tire cord: 4-6%, conveyor belt fabrics: 6-8%, glass cord (toothed belt): up to 20%), and then dried (convection/hot air, infrared, microwave, HF). This is followed by condensation of the adhesion-promoting resin as well as heatsetting and winding of the yarn onto precision winders.

The following treatments are possible to have reinforcements build up chemical bonds to elastomers [57]:

• soaking in solutions containing bifunctional adhesion promoters (cement)

• impregnation of yarns with aqueous resin adhesion promoters (RFL systems)

• addition of the adhesion promoter to the rubber mixture (direct adhesion)

Polyfunctional Adhesion Promoters Di — and triisocyanates are used as adhesion promoters for fibers that have only a few reactive groups, or that have only low reactivity such as PET (polyester) or polyphenylene terephthalamide (aramid). Adhesion promoters are applied either by soaking the fibers or the fabrics in a solution, or in the form of a paste. Due to high reactivity, fibers or fabrics coated with isocyanates must be processed rapidly or covered with a rubber layer. For polyester, epoxy resins are also suitable as adhesion promoters.

Resorcinol Formaldehyde Latex (RFL) Dip Yarns and fabrics are usually impregnated with a mixture composed of latex and a condensate of resorcinol and formaldehyde. Generally, styrene vinylpyridine-butadiene terpolymer or chloroprene latex is used. Natural latex has a high tack and is only rarely employed. The latex component is added to resorcinol-formaldehyde precondensate; occasionally, waxes or tackifiers are also contained in RFL dips.

The reaction product ofthese substances determines the specific characteristics ofthe dip, such as viscosity, durability, adhesive strength and flexibility. The latex in the RFL is selected for good bonding to the solid rubberthatwill beused inthe final product. Details of the compatibility between the rubber types and latex types are listed in Table 8.8.

RFL is produced in a two-step process, starting with the generation of a resorcinol-formaldehyde precondensate composed of mono-, di — and trihydroxy- methylated resorcinol, followed by the addition of latex. Alternatively, the precon­densate can be obtained commercially. The rubber component is a blend of SBR latex with vinyl pyridine latex. After a waiting period of approximately 6 h, the aqueous dip is applied to the fibers, predried at 100-130 °C, and finally allowed to react with the fibers in the main dryer at temperatures of 150-230 °C. The binding reaction takes place as a condensation reaction between the methylol groups ofthe resin and active hydrogen atoms of the fiber. At the same time, the resin reacts with the latex rubber component. Binding to the matrix takes place by covulcanization with the latex component. Nitrile rubber lattices are well suited for producing polar rubbers.

Polyester fibers have only a few reactive OH groups, and are predipped at manufacture; they can then also be coated with RFL dip. Predips for polyester filaments are composed of a water-soluble epoxy resin (e. g. glycerol polyglycidyl ether) and a blocked isocyanate (e. g. phenol-blocked methylenebis(4-phenyl isocya­nate)) that only reacts at elevated temperatures. The components are applied as dispersion and allowed to react at approximately 220 °C.

Aramid filaments are also slow-reacting, and are provided with a predip composed of epoxy resin and e-caprolactam-blocked isocyanate. Glass fibers are typically coated with an adhesion promoter that is already applied during manufacture.

8.10.4.2 Effect of the Fiber Type on the Adhesive System

Although the treatment of textile fibers to enhance their adhesion to rubber is a typical process, some particularities must be noted. First, the adhesive system must be matched to the type of the fiber, and the basic formulation is frequently modified to some extent to achieve optimum enhancement with regard to special elastomers. The application of adhesive is generally combined with heatsetting.

Rayon With the advance in manufacture processes aiming at the production of high-strength rayon fibers, it was found that SBR lattices did not confer sufficiently adhesive properties to rayon; hence, vinyl pyridine (VP) lattices were developed. The typical formulations of RFL systems with VP contents up to 80% for the preparation of high-strength rayon fibers are shown in Table 8.9.

These formulations can contain either resol resin produced by basic catalysis, or novolak resin produced by acid precondensation. As acid catalysis activates ortho protons, novolak is a one-dimensional, noncrosslinked resin, whereas resol contains crosslinks (both activated in ortho and para positions; see Figure 8.66). Therefore, in order to allow crosslinking a novolak resin-based formulation must contain a higher amount of formalin compared to resol resin-based formulations (see Section 5.4).

Although, usually, comparable results are obtained with these formulations, for articles exposed to high dynamic stress, formulations without sodium hydroxide solution as the basic catalyst are better suited because the sodium hydroxide solution increases the modulus of the resulting film, which impairs dynamic performance.

Polyester As mentioned above, standard RFL systems do not confer satisfactory adhesive properties to polyester. In view of the environmental aspects and fire protection, aqueous rather than solvent-containing systems have therefore been developed which are based on dispersed blocked isocyanates that split offat heat (ca. 230 °C) to yield free isocyanate groups that are allowed to react. The solvent-contain­ing systems are applied as a predip before the RFL dipping and allowed to react. The adhesion-promoting effect is almost as good as with solvent-containing systems. Alternatively, the predip is added to the RFL dip. Both systems are used widely for soft cords and conveyor belts. In contrast, stiffcords for the belt industry (raw-edge belts) must not fray at the cut edges, and therefore a high, interfilamentary adhesion must

be produced. To date, this has only been achieved by using solvent-containing isocyanate systems. Owing to their high penetration capacity, they build up a high-strength polymer matrix between the filaments. To date, no waterborne system could be established commercially, despite intense developmental activities.

Aramid Although aramid (aromatic polyamide) is chemically closely related to nylon (aliphatic polyamide), no satisfactory results have been obtained with standard RFL systems. Whilst the established methods applied to the preparation of poly­ethylene-styrene (PES) can be used here, they sometimes impair the dynamic resistance of textiles. When using dispersed glycerol polyglycidyl ether as predip, followed by standard RFL dip in a second stage, it must be noted that optimum results are only obtained when the elevated temperature is maintained over a sufficiently long period (for about 60 s at 240 °C).

Special adhesion-promoted aramid fibers that no longer depend on a two-stage process are also available commercially, at least from one supplier. However, for complex systems, a two-bath dipping process performed in-house usually gives better results. As with PES, minor modifications in the rubber mixture can have major effects on adhesion, although in some particular cases the addition of carbon black to the dip can level this out.

Particularly with aramid, the latex component of the dip should be carefully adjusted to the rubber with regards to miscibility, but this problem does not occur with commercially available latex components (CR, NBR, etc.). Care should be taken, however, when using butyl rubber (IIR) or EPDM, for example. In this case, only polymer emulsions are available, which may not produce the same results.