in the range of 4 2 to 5 1 and a residual bulk bl 2 in the range of 0 47 to 0 50 in general applicants have found that the use of fibers according to the present invention in batts resulted in the following bl 1 improves strongly on the addition of naphthate groups bl 2 improves strongly with the addition of naphthalate groups bl 2 is insensitive to temperature and time of exposure applicants have found that the optimum concentration for bl 1 is 10 mole pen while for bl 2 it is 20 mole this optimum behavior can be linked to the shrinkage properties of the polymer the amount of polymer chain mobility is linked to the amount the temperature is above t g as the level of napthalate in the polymer increases t g increases therefore the polymer chain mobility will decrease for a given temperature above t g with increasing naphthalate content this reduction in chain mobility could reduce the shrinkage seen in fibers at a given temperature above t g therefore applicants have found that increasing napthalate content can reduce shrinkage and improve the bulk performance alternatively increasing the level of naphthalate will reduce the crystallinity that is developed in the polymer this will reduce the number of pinning points holding the orientated polymer chains in position so on exposure to heat some of these orientated chains will relax and the polymer will shrink in this case increasing the naphthalate level will increase shrinkage and will worsen the bulk properties with these alternative mechanisms occurring at the same time one can expect to see a maximum.