The curliness of fibres and he degree of microcompression in the fibre wall strongly influence the properties of pulp suspensions, wet-webs, and dry sheets. In mill operation, curl and microcompression can be induced accidentally or intentionally, by shearing at high consistency. Some pulps are highly susceptible to curling; others are more resistant. Curl is not necessarily stable; it is readily removed from some pulps but not from others. Curl can be stabilized by certain treatments, notably by heat treatment at high consistency. This can be deliberate, or it can occur accidentally during mill operation, when a pulp is stored at an elevated temperature. Both curl and microcompression are often disregarded because they cannot be easily measured. Yet in practice their effects often dominate the properties of pulp suspensions, wet webs, and dry sheets. Ignoring these effects has led to costly surprises both in research and mill operation.
In this paper the literature is reviewed and new data are introduced, illustrating the importance of curl and microcompression for mechanical, chemi-mechanical, and chemical pulps.