Page 133 - Advances in Textile Biotechnology

114 Advances in textile biotechnology

5.7 Surface erosion model for acrylic fibres treated with nitrilase at
alkaline pH (for simplification the comonomers are not represented).

proposed to explain the results (Fig. 5.7). The oscillation of relative K/S
values with time of treatment could be explained assuming that, above a
certain degree of conversion of nitrile groups into carboxylic groups, the
polymer chains are more stable in solution and therefore detach from the
surface of the fi bres. The modification of nitrile into carboxylic groups could

create some instability in the arrangement of PAN chains at the surface of
the fibres, mainly as a result of steric hindrance and pH-dependent charge

repulsion. Increasing concentrations of PAA in treatment solutions were
detected (Fig. 5.6). The removal of carboxylic groups from the surface of
the fabric could explain the lower points on the K/S curve. When the PAA
macromolecules leave the surface, the PAN chains underneath are exposed
to further nitrilase catalysis. This would lead to increasing numbers of car-
boxylic groups, thus, to an increase of K/S, until the threshold value for
chain solubilization would be again achieved or no more accessible nitrile
groups would be available for nitrilase. The PAA release would be depen-
dent on factors such as microstructural properties of PAN copolymers,
molar mass of the polymer chains, enzyme adsorption, nitrilase deactiva-
tion, and mechanical stress during treatment and washing. The important

consequence of these findings is that the nitrilase biomodification of acrylic

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