Degradation mechanism of wood : 2. Outdoor exposed radial and tangential surfaces of Scots  pne and Norway spruce

The development of cracks and changes in appearance have been investigated on radial and tangential surfaces of pine (Pinus silvestris L) and spruce (Picea abies Karst) which have been exposed outdoors for 33 months. The degradation of the surfaces has also been studied at the micro-level. Untreated samples, samples impregnated with a CCA-agent and samples surface treated with linseed oil have been tested. The annual ring orientation is the most important factor for crack development on weathering. The type of wood, impregnation treatment and surface treatment with linseed oil have only a marginal effect on the crack development. No relation has been found between the density of the samples and the crack development. After 33 months of outdoor exposure, tangential surfaces of pine have 13 times more total crack length per unit area than the corresponding radial surfaces. In spruce, the total crack length on the tangential surfaces is 6 times greater than on the radial surfaces. Tangential surfaces of both pine and spruce have a greater number of cracks per unit area and wider cracks than the corresponding radial surfaces. Tangential and radial surfaces show the same colour change in the surface as a result of weathering. On the micro-level, tangential surfaces have more and deeper cracks than radial surfaces. The cracks on the tangential surfaces occur frequently in both earlywood and latewood. On radial surfaces, cracks occur primarily at the annual ring borders, but to a certain extent also in the earlywood. The radial cell wall of the earlywood has a large number of pits which are degraded at an early stage. Decomposition of the cell wall takes place on both radial and tangential surfaces. Cracks arise which follow the S2 fibril orientation in the cell-wall. Delamination in the middle lamella is especially noticeable in the latewood on tangential surfaces. No differences have been observed regarding linseed oil treatment, impregnation or type of wood.Sprickförekomst har undersökts hos radiella och tangentiella furu- och granytor som exponerats utomhus i 33 månader. Inverkan av impregnering och ytbehandling med linolja har också testats. Den exponerade ytans orientering i förhållande till årsringarna är den parameter som har i särklass största betydelsen för sprickbildningen. Träslag, impregnerings och linoljebehandling har endast marginell betydelse. Samband mellan provernas densitet och sprickförekomst har inte kunnat påvisas. Tangetiella ytor av furu har 13 gånger större spricklängd per areaenhet än radiella ytor. För gran är motsvarande värde 6 gånger. Antalet sprickor och sprickbredden är också större på tangentiella ytor än på radiella ytor. Vid undersökning av de exponerade ytorna i ESEM har tydliga tecken på nedbrytning kunnat påvisas. Det skikt av krossade fibrer som fås vid mekanisk bearbetning av trä erroderar bort i ett tidigt skede. Därefter angrips porer och ligninrika områden. Sprickor uppstår i fiberväggar och mellan fibrer