Gel coatings

Gelation characteristics Gelation point Gelcap Gel coats Geldan aldehyde Geldanamycin... 

The weatherabihty and hydrolytic stabiUty of unsaturated polyesters based on neopentyl glycol have made it a popular intermediate for use in formulations exposed to severe conditions, eg, in gel coats for cultured marble and marine appHcations (see Coatings, marine) (13). 

Unsaturated resias based on 1,4-cyclohexanedimethanol are useful ia gel coats and ia laminating and molding resias where advantage is taken of the properties of very low water absorption and resistance to boiling water (6). Thermal stabiHty is imparted to molding resias, both thermoplastic (71,72) and thermoset (73—76), enabling retention of physical and electrical properties at elevated temperatures (77). Additionally, resistance to chemical and environmental exposure is characteristic of products made from these resias (78). 

The hand lay-up or spray-up process, used universally for the production of laminar composites incorporating glass fiber reinforcement, is most efficient for the manufacture of large parts, such as boats, bathtubs, tanks, architectural shapes, and recreational accessories. Resins intended for spray-up processes are usually modified with thixotropic additives, such as fumed siHca (1%), to reduce the risk of drainage when appHed over large vertical mold surfaces. Molds are also made from ERP for short-mn products usually surfaced with a tooling gel coat to provide consistent surface quaHty and appearance. 

Gel coats are pigmented polyester coatings appHed to the mold surface and are an integral part of the finished laminate. Gel coats are used widely on hand lay-up and spray-up parts to enhance surface aesthetics and coloration as weU as to provide an abrasion-resistant waterproof surface that protects the underlying glass-reinforced stmcture. 

Thermoformed acryHc sheet is displacing gel coats in some bathtub appHcations spas have converted almost exclusively to formed acryHc sheet reinforced with glass-reinforced polyester laminations due to higher temperatures and higher stmctural requirements. 

Gel coats are typicaHy used to provide a part with a finished surface directly from the mold. Various inserts, stiffeners, and mechanical attachments can be incorporated in the mol ding step, thereby further reducing secondary operations. Final edge trimming is accompHshed with a variety of tools such as... 

A special attribute of these processes is the abiHty to pre-position reinforcement, inserts, and core materials for stiffening ribs. Gel coatings can be apphed to the mold surface to eliminate post-mold finishing. Because both surfaces of the part are formed in a mold to close tolerances, accurate assemblies are possible, which is a requirement for many automotive or tmck body appHcations. 

For spas, shower stalls, bath tubs, etc, a gel coat containing no fiber reinforcement is appHed first to the mold. It forms a smooth, strong, impervious, durable chemical, weather, and wear-resistant surface. The bulk of the resin, which may be reinforced with glass fiber, is appHed by hand lay-up or by spray gun. The article is then cured at or near ambient conditions. 

Sanitary ware, including tubs, showers, combined units, basins, and toilet tank, may be made of thermoformed ABS or acryHc sheet, molded glass-fiber-reinforced polyester, or cast acryHc resins. The glass-polyester type dominates the tub/shower market. It is possible to install the units as a two-component system, assembled in place. Gel coats may be of thermoformed decorative acryHc skins. To reduce the smoke generated by fire, methyl... 

In addition to electrical uses, epoxy casting resins are utilized in the manufacture of tools, ie, contact and match molds, stretch blocks, vacuum-forrning tools, and foundry patterns, as weU as bench tops and kitchen sinks. Systems consist of a gel-coat formulation designed to form a thin coating over the pattern which provides a perfect reproduction of the pattern detail. This is backed by a heavily filled epoxy system which also incorporates fiber reinforcements to give the tool its strength. For moderate temperature service, a Hquid bisphenol A epoxy resin with an aHphatic amine is used. For higher temperature service, a modified system based on an epoxy phenol novolak and an aromatic diamine hardener may be used. 

Isophthalic acid (m.p. 347°C), made by oxidation of /w-xylene, has also been introduced for resins. The resins have higher heat distortion temperatures and flexural moduli and better craze resistance. They are also useful in the preparation of resilient gel coats. 

Where a flexible resin is required adipic and, rarely, sebacic acids are used. Whereas the phthalic acids give a rigid link these materials give highly flexible linkages and hence flexibility in the cured resin. Flexible resins are of value in gel coats. 

Impactor 1. Koni meter Impaction on gel-coated disc 60-100... 

Sol-gel. A limited evaluation of sol-gel coatings on stainless steel has been reported with the goal of reducing hazardous wastes and other environmentally undesirable materials [22]. In this study, the GTMS/TPOZ sol-gel discussed... 

Gel coat Specially formulated polyester resin that is pigmented and contains fillers. Provides a smooth, pore-free surface for the plastic article. 

For the separation of D,L-leucine, Ding et al. [62] used poly(vinyl alcohol) gel-coated microporous polypropylene hollow fibers (Fig. 5-11). An octanol phase containing the chiral selector (A-n-dodecyl-L-hydroxyproline) is flowing countercur-rently with an aqueous phase. The gel in the pores of the membrane permits diffusion of the leucine molecules, but prevents convection of the aqueous and octanol phase. At a proper selection of the flow ratios it is possible to achieve almost complete resolution of the D,L-leucine (Fig. 5-12). 

Fig. 5-11. Fractional extraction with gel-coated hollow fibers [57].

This process is carried out at low pressure and at ambient temperature. The fiber is placed in the tool in the form of a mat and the resin is poured in. The mold is closed, and the resin spreads through the reinforcement thereby impregnating and fully wetting out the fiber strands. The process is done on an hydraulic press. Pressure levels required for molding are typically around 10 MPa. Gel coat can be applied to the mold faces before the molding operation commences. 

To a stirred solution of 4 mmol of the diamide in 40 mL of dioxane (distilled from LiAlH4) and 13 mL of water is added 1 mg of osmium tetroxide. When the solution turns brownish (after about 10 min) 2.06 g (9.2 mmol) of sodium metaperiodale are added at 25 26 "C. The progress of the reaction is monitored by TLC on silica gel coated plastic sheets with CHCI,/diethyl ether/methanol (3.3 0.1) as eluent. When the reaction is complete, the precipitated solid is filtered and the filtrate concentrated in vacuo at 1 Torr. The residue is dissolved in 50 mL of CHC13, dried over MgSO,. and evaporated in vacuo to leave a residue, which is crystallized from a suitable solvent. 

Quadraxially oriented (four directional layer) glass fabric-TS vinyl ester polyester RP sheet panels with a foam core and gel coating are used. Most of the panels are 3 mm thick with molded-in rib structure supports. Body skins are bonded to the chassis with a double-stick acrylic tape developed by 3M Co. as well as mechanical fasteners. Unlike most steel designs, no B-pillar structural component between the front and rear doors is required thus providing more interior space and easy entry since doors open in opposite directions. 

Gel coat In RP processing a gel coat on the outer surface can be used to ensure a smooth surface appearance and a tough surface. It could contain a thin synthetic fiber veil to improve performance of the gel coat and/or a surfacing mat. It is a quick setting plastic and gelled prior to reinforcement layup. The gel coat becomes an integral part of the finished RP product. 

Surface appearance Gel coated Very good Very good... 

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