There  are  three isomers of phthalic acid: Orthophthalic  acid  (1,2- benzenedicarboxylic  acid), Isophthalic acid (1,3-)  and  Terephthalic acid  (1,4-). Orthophthalic acid , the only one of the three that  can form an anhydride, is normally used in that form. Each phthalic isomer has  particular  advantages  and liabilities when  used  in  polyester production.  The ortho isomer is normally used as the anhydride  which reacts  faster  initially and releases one, rather than two  moles  of esterification  water. Orthophthalic resins  are low molecular  weight materials having relatively higher water solubility and reduced corrosion resistance. These resins are rarely processed to the same molecular  weight as isophthalic and terephthalic based resins. Those  that are specially processed to the molecular weight range easily  attained with  isophthalic acid, are susceptible to premature gelation and  may contain low molecular weight species. Several factors cause the  lower resistance  of  orthopolyesters to attack by  aqueous  solutions.  The orthophthalate ester linkages are more visible, hence are more susceptible to attack than the ester linkages of isopolyesters. The hydrolytic  stability  of orthopolyesters is far inferior to that  of  isopolyesters. Orthopolyesters are cheaper to produce than isopolyesters or tere-polyesters due to raw material costs, processing costs and ancillary  costs. This is one single large reason why  orthopolyesters  are favourites  in the moulding industry. However, customer oriented  cost analysis  must include not only resin purchase price  and  fabrication        costs,  but life-cycle cost factors such as maintenance,  useful  life before replacement and performance in multiple environments.     
       
Orthophthalates  are used for a wide variety of applications. Some of the applications for which Dipol Orthopolyesters are used, are as follows:
        
#  ORTHOPHTHALATES  for    *  Potting and Encapsulation of Electrical & Electronic components
                                             *  General Purpose Laminating resins
                                             *  Resilient Cultured Marble grade
                                             *  Quick setting Button grade Casting resins
                                             *  Optically clear Casting resins
                                             *  Translucent UV stabilized Rooflight grade
                                             *  Neo Pentyl Glycol modified Marine grade  
                                             *  Low viscosity Resin Transfer Moulding
                                                and Spray Up resins
                                             *  Medium to high reactive Pultrusion grade
                                             *  High reactive resin for Low shrink
                                                Sheet Moulding / Dough Moulding compounds