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Polyester resin is an unsaturated, thermosetting resin produced by a reaction between several organic acids and polyhydric alcohols. It is most commonly used in the construction of molded reinforced fiber and composite products. The polyester resin used in most molding applications is a viscous liquid requiring the addition of catalysts and accelerators to complete the curing process. Polyester resins are contact products which require no pressure to cure and can be cured from a fluid or solid state. Although these products have several distinct disadvantages when compared to other commonly used composite resins, they still offer an attractive balance of ease of use, low cost, and positive physical characteristics.
Most commonly used polyester resins are pale colored, fairly viscous liquids consisting of a polyester and styrene solution. Styrene, added to reduce the viscosity of the resin, makes it flow better and plays a vital role in the curing process of the product. Resin manufacturers include several other additives in polyester resin products depending on their intended uses. These include pigments, UV stabilizers, fillers, and fire or chemical resistant substances. These additives are available in off-the-shelf resin formulations or can be added in varying amounts to produce custom products tailored to consumer needs.
Polyester resin has a short shelf life and will gel out completely after an extended period. Gel inhibitors are often added to the resin to slow this process but the resin will eventually completely harden with time. The curing process is basically no more than an accelerated adaptation of this natural gelling action. A catalyst is added to initiate the curing process; an accelerator speeds up the gel time and allows the process to proceed at room temperatures and with no applied pressure. During this process, the styrene is released or flashed off resulting in the hardening of the resin.
One of the most extensive single uses of polyester resins is in the production of glass fiber reinforced moldings. In these applications, the resin is used to bind layers of glass fiber matting or machine applied chop strand fibers to form a tough laminate. One of the most common of these is the construction of marine products such as yacht and powerboat hulls and internal structures. Glass reinforced plastics (GRP) are also commonly used to produce roof sheeting, cladding, automotive parts, surf boards, and many other molded domestic and industrial items. As noted, the addition of fire and chemical retardants and UV stabilizers greatly extends the scope of use for these products.
Apart from their many uses, polyester resin products do have several disadvantages. By comparison they lack the same mechanical properties and are less water resistant than vinyl ester and epoxy resins. The flash off of styrene during the curing process causes a fairly large degree of volumetric shrinkage, the full extent of which is often difficult to predict. Styrene emissions are also harmful and may necessitate the institution of specific, often expensive, environmental safety measures. Apart from these negative points, polyester resin moldings are still among the most cost effective and generally efficient of all the molding resin types.
The resin does not harden because the styrene is "flashed off" as you so eloquently put it. The styrene is incorporated into the resulting cross-linked polymer.