THE EFFECTS OF COLD WINTER CONDITIONS ON FISHING GEAR MATERIALS .2. STRENGTH EXAMINATION OF FREEZE-THAWED POLYAMIDE AND POLYETHYLENE MULTIFILAMENT NETTINGS

This paper continues the research into the wet mesh breaking strengths of commercial fishing nets exposed to freezing and thawing. In the fi rst paper, it was discovered that the strengths generally increase dur ing exposure. However, in this paper the strengths of polyamide multif ilament nettings were found to decrease, when tensile measurements wer e conducted in a frozen state in an environmental chamber. The changes in the netting yarns caused by freeze-thaw cycling were analysed by u sing mechanical and thermoanalytical methods (DSC and DMTA) and wide a ngle X-ray diffraction scans, (WAXS). DMTA revealed changes In transit ion temperatures typical of the early stages of degradation. More deta iled interpretation of the analyses was not possible due to a lack of information concerning the production and thermomechanical background of the yarns. WAXS patterns of polyamide monofilament nets were resolv ed by a curve fitting method to distinguish between the crystalline an d amorphous phase. Changes in crystallinity did not alone explain the changes in the wet mesh breaking strengths although they may have cont ributed to them. In polyethylene nettings, the increase of the wet mes h breaking strengths fan also be explained as a result of crosslinking reactions at the early phase of degradation. In both polyamide and po lyethylene nets, the rising of wet mesh breaking strengths was probabl y partly due to settling and tightening of knots caused by relaxation and creep. Differences in the heat setting processes of the yarns and nets provide a possible explanation for the different dispositions to degradation. Low service temperatures, in general, are favourable to f ishing gear because the strengths of the yarns were higher in a frozen state than at room temperature.