The fibre characteristics of shives initiating web rupture

The composition of shives in various mechanical pulp grades and process sta ges were investigated. Earlywood fibres were shown to be clearly overrepres ented in shives from TMP-pulps, compared to the fibre population of the who le pulps. Conversely, grinding was found to give shives rich in latewood. I n both cases, reject refining caused a large relative decrease in the amoun t of latewood fibres in the shives. The rather stiff latewood seems to defi brate easier than earlywood in refining. Screened GW-pulps contained more t han three times as many undefibrated latewood fibres than did screened TMP- pulps. Hence, for a given total amount of shives, groundwood pulps seemed t o be of poorer quality with respect to content of shives causing web-ruptur e.