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Vinyl Ester Resins
Vinyl ester resins are similar in their molecular structure to polyesters, but differ primarily in the location of their reactive sites, these being positioned only at the ends of the molecular chains. As the whole length of the molecular chain is available to absorb shock loadings this makes vinyl ester resins tougher and more resilient than polyesters.
The vinyl ester molecule also features fewer ester groups. These ester groups are susceptible to water degradation by hydrolysis which means that vinyl esters exhibit better resistance to water and many other chemicals than their polyester counterparts, and are frequently found in applications such as pipelines and chemical storage tanks.
Figure 1 below shows the idealised chemical structure of a typical vinyl ester. Note the positions of the ester groups and the reactive sites (C* = C*) within the molecular chain.
Figure 1. Idealised chemical structure of a typical epoxy based vinyl ester.
The molecular chains of vinyl ester, represented in Figure 2 below, can be compared to the schematic representation of polyester shown previously where the difference in the location of the reactive sites can be clearly seen:
Figure 2. Schematic representation of vinyl ester resin (uncured).
With the reduced number of ester groups in a vinyl ester when compared to a polyester, the resin is less prone to damage by hydrolysis. The material is therefore sometimes used as a barrier or ‘skin’ coat for a polyester laminate that is to be immersed in water, such as in a boat hull. The cured molecular structure of the vinyl ester (Figure 3) also means that it tends to be tougher than a polyester, although to really achieve these properties the resin usually needs to have an elevated temperature post cure.
Figure 3. Schematic representation of vinyl ester resin (cured).
Source: http://www.azom.com/details.asp?ArticleID=986
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