To understand why Vinyl ester resins perform better than polyesters in corrosive environments, you have to look at the molecular structure of  the resins. A polyester molecule is made up of many ester linkages and         vinyl  groups  (carbon-to-carbon double bonds). Chemical attack  of  a polyester occurs at the most reactive sites - the ester linkages  and the unreacted vinyl groups. Ester linkages are susceptible to hydrolytic  attack by strong acids like hydrochloric acid and strong  alkalis like  caustic soda. Unreacted vinyl groups in the polymer can  be  attacked by oxidation and halogenation.
       
Vinyl  ester  resins,  however, have no ester linkages  in  the  epoxy backbone.  So they are less prone to chemical attack. Also, the  ester linkages  that  do occur at the ends of the molecule are  shielded  by       methyl  groups which inhibit the hydrolysis of the esters. As for  the reactive  vinyl groups, they only occur at the ends of  the  molecular chain  in vinyl ester resins. Here, they react more completely when  a  fabricator  catalyses the resin. So, the vinyl ester polymer also  has fewer unreacted vinyl groups than polyesters, resulting in a much more chemically  resistant structure. Crosslinking in a molecule  of  vinyl         ester  resin is confined to the ends of the molecule. This leaves  the entire  length of the molecular chain freee to elongate  under  stress and  absorb  mechanical and thermal stress or shock.  Therefore  vinyl        ester  resin  have better strength and  flexibility  than  polyesters. Structures  fabricated  with vinyl ester resins will provide  for  improved  stress fatigue resistance especially in hot-cold  thermal  cycling. Vinyl ester resins combines the excellent mechanical properties of  epoxy resins and ease of processing of Unsaturated polyester  resins. They are notable for high temperature properties, low  shrinkage, good adhesion and high tensile elongation.
       
       
#  VINYL ESTERS - General Purpose for severe chemical resistance with maximum limiting
                                    temperature of 100°C
       
 - Superior grade for more severe chemical resistance with limiting temperature
of 130°C combined with higher tensile and elongation properties