The extent to which this can be accomplished without fundamentally changing the nature of the polymer in its definition or other properties is debatable. Some basic criteria must be met in order for the material to be satisfactory: the co-monomer must be stable during polymerization and extrusion and not lead to a polymer with markedly inferior mechanical properties.
Thus, for example, challenges in dyeing polyester have been answered in part by the development and commercialization of PET that includes a proportion of 5-sulfoisophthalic acid as a co-monomer: the resulting sulfonic acid groups provide "sites" for dyeing with cationic ("basic") dyes . Co-polyesters of PET have been examined for obtaining deep dyeing material . For flame retardance, bromine-containing co-monomers have been developed, such as 2,5 dibromophthalic acid . More recently, phosphorus-based co-monomers have been examined .
In general, the use of a co-monomer as opposed to an additive in the melt results in reduced leaching tendency in later use. This may be an advantage or disadvantage. Related to the use of a co-monomer is the inclusion of some agent that affects the DP or linearity of the polymer. Thus, the inclusion of a depolymerization agent in the melt produces a PET with a lower MW (narrower weight distribution) that pills less , and the addition of 6-16% (by weight) of polyethylene glycol together with ca. 0.1% pentaerythritol branching agent increases the wetting and wicking behavior of the polyester fiber .
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