Strand drying is a necessary step in OSB production, as a means to reduce furnish moisture content to an acceptable level, which in part depends on the resin used. It has been mentioned that beside wood degradation which causes fines generation and fibre surface oxidation, other problems such as VOC emissions, excess energy consumption and variability in moisture content of strands may occur as a result of strand drying. This work involved the characterisation of strands from six commercial OSB mills. The methodology adopted in this study was to characterize strands collected before and after drying, as well as information on operation conditions in order to relate the effects of drying on the overall strand degradation. Strand quality was evaluated before and after drying in terms of the following characteristics:
Strand size distribution;
Strand surface oxidation;
Strand tensile strength.
Results on classification of wet strands showed very large variability in width distribution ranging from very narrow to very wide. Results on size distribution of dry strands revealed that strand degradation characteristics appear dependent on individual mill strategy that can vary considerably from mill to mill. The study showed that conveyer dryers caused the least strand size reduction of the three dryer types included in the study. Triple and single pass dryers caused similar amounts of strand size reduction. Test results also showed that drying conditions e.g. drying temperature, drying time and final moisture content played a key role in strand size degradation. Overall strand size classification results showed no clear pattern suggesting that the drying process is not the only factor responsible for post-strander fines generation. Other factors such as strand damage incurred during strand manufacturing and mechanical damage during strand handling may also play substantial roles in the reduction of strand size observed in this study.
The drying operation had little effect on tensile strength of the strands. However, measurement of strand surface contact angles revealed that surface oxidation is taking place during strand drying. For all strand samples, the contact angles measured on strands sampled before drying were lower than the contact angles measured after drying. The study clearly indicated that thermal degradation is not related to dryer type but to other drying conditions, namely the combination of temperature and residence time. Thus it is possible that strands dried for a relatively long time at lower temperatures can incur more surface oxidation that strands dried at higher temperatures, but for a much shorter time.