’Polyester’’ word is the compound noun. Polyester word occurs "Poly" which means that ‘’a lot’’ and an organic salt "ester". It would be expressed as ‘’Many Organic Salt’’. Polymers can also be defined as the ester molecules chains. First practical application example of polyester resins were at the II. World War. However, in 1950’s after the reinforced with glass fibers, it was appeared to be very strong and lightweight material. Unsaturated polyester resins have much superior properties now compare to past.
The unsaturated polyester resins are widely use in the production of GRP, It represents approximately 75% of the resin used in Turkey and in the World. Reinforced Plastics are in the thermoset plastics group. Polyester resins are applicable simple moulding techniques from Hand-lay up to the most complex molding techniques. Polyester resins include a wide range of chemical families and they are obtained generally with the condensation reaction of polyhydric alcohols and dibasic acids.
Dibasic acids + polyhydric alcohols Polyester Resin + H2O
Unsaturated polyester resins depending on the kind of dibasic acids, are named as ‘’ortoftalik’’ or ‘’izoftalik’’ .
Polyester varieties• GRP Type Orthophtalic-Based Polyester Resin
Gelcoat is situated on the top layer of especially glass fiber reinforced plastics and composite materials. Gelcoat provides increase to surface appearance quality and external environment performance of GRP products. In general gelcoats are based on epoxy or unsaturated polyester resin. These products use in many areas as automotive, yachts and boats, water park and water slides, defense and aerospace industry, GRP furniture, frigorafik safe, kitchen countertops, building and decoration materials. Chemically gel coat is obtained by the modified high performance resins.
Gelcoats are applied to the mold surface in liquid form by brush, spray or airless spray method. It reaches from liquid form to solid form with curing through added MEKP (Methyl Ethyl Ketone Peroxide). Curing (hardening) occurs by crosslinks formed between polymers. Then reinforced with layers and occurs classical composite matrix. These reinforcing materials usually consist of polyester, epoxy and vinyl ester resins. The reinforcing fibers consist of glass fibers, carbon fibers or aramid (Kevlar) fibers. Finished part removes from the mold after curing sufficiently. When the product leave from the mold, the upper surface representes gelcoat layer. Gelcoat can be provided different colors of products surface because it contents pigment pasta. If transparent gelcoat uses at production of artificial marble, it gives feeling of depth on surface.
Today many watercrafts (yachts, catamarans), due to the light weight and corrosion resistance, are manifactured with composite materials. External layers of this tools (shells) are usually between 0.5mm-08.mm gelcoat thickness. Designing of high performance gelcoats are considered chemical resistance, UV (ultraviolet) resistance, and hydrolysis resistance.Gelcoat, which used in molding composite products, must be durable to high temperatures and mechanical stress that may occur during production.
Chemical structure (ISO NPG, acrylic, etc.) and modifications may be different types. Gelcoat species can be listed as follows in general.• General purpose gelcoats