Structural aspects of the thermally accelerated ageing of cellulose: effect of cellulose source and ageing conditions

摘要

Small angle X-ray scattering has been used to study the void parameters of filter paper, kraft paper and ramie after ageing in air and in vacuo at 160 deg C and 120 deg C. The cellulose fibres contain long thin voids oriented with their long axis roughly parallel to the fibre axis. The aged samples were observed both dry and fully hydrated. Hydration caused small voids to open up between the fibrils which compose a single fibre. The void parameters of dry cellulose remained largely unchanged on ageing, but the wet values of aged material approached those of dry. This effect was more profound the more extreme the ageing conditions. The conclusion is that high temperatures and chain scission encourage chain mobility and facilitate hydrogen bonding between the fibrils in a mechanism similar to that proposed for hornification. Conventional hornification, whereby intrafibre hydrogen bonding increases due to dehydration below a threshold value is also likely. The additional hydrogen bonding means that the fibre is less able to open small voids and absorb water after ageing. Measurements of water retention values of filter paper support the model of structure closure. Zero span and wide spin tensile strengths of filter paper fall by greater amounts, the more severe the ageing conditions. The effect of chain scission evidently dominates any internal structure modifications in determining the mechanical strength of aged fibres.