There  are three isomers of phthalic acid: Orthophthalic acid  (1,2  - benzenedicarboxylic acid), Isophthalic acid (1,3 -), and  Terephthalic acid  (1,4  -).  Each phthalic isomer has  particular  advantages  and        liabilities  when  used  in polyester production.  The  properties  of terephthalic  are generally better than orthopolyesters and are  often quite similar to those of isopolyesters. Terepolyesters generally have a higher heat deflection temperature. Terephthalic acid is the slowest reacting  of the three phthalic acids. Catalysts or pressure  are  required to esterify terephthalic acid within a reasonable time  period. Terephthalic  polyesters made with primary glycols such  as  neopentyl have  poor  styrene tolerance compared with ortho  and  isopolyesters.    Most  physical  properties are not substantially  different  than  1:1        isopolyesters.  The  most  dramatic differences are  the  higher  heat deflection  temperature and lower hardness of the terephthalate  polyesters.  Isopolyesters  are about 15 percent tougher  than  equivalent         terepolyesters.
       
The applications for Terephthalates are the same as for  Isophthalates but  at  a compromise with regards to lower hardness  as  compared  to Isophthalates.
       
       
        #  TEREPHTHALATES  for     *  applications requiring higher mechanical Strength and good  
                                                                chemical properties.