Comparison of techniques to measure the fibril angle

The different techniques available for the measurement of fibril angle are reviewed. A number of comparative sets of data in the literature are evalua ted for information on the overall accuracy of fibril angle measurements'. Recently, several new techniques for the measurement of fibril angle, which take advantage of the optical sectioning power of a confocal microscope, h ave been published. These techniques are reviewed and the relative advantag es and disadvantages of each technique are discussed. The cellulose polymer molecules in wood fibres are aligned in long, highly crystalline microfibrils wound helically around the lumen. The S2 layer of the fibre wall contains as much as 90 % of the fibre mass, and therefore la rgely determines the fibre properties. The pitch of the helix in the S2 lay er defines the microfibril angle. As the fibrils in the S2 layer are tightl y packed and the S2 layer is itself surrounded by the S1 and S3 layers and is not exposed, it is not possible to simply place: a fibre under a microsc ope and measure the fibril angle from visual observation, except in a limit ed number of special cases (eg. softwood compression wood). There is a wide variation in the properties of fibres from different parts Of a tree and b etween different species of trees. Theoretical and experimental work (1-3) have shown that while the elastic m odulus and strength of fibres are approximately constant at very low fibril angles, they fall rapidly for fibril angles beyond 5 or 10 degrees. At fib ril angles of 40 degrees and above the tensile strength is reduced to about a third (1) and the stiffness is reduced to about a fifth (2) compared to fibres with fibril angles between 0 and 10 degrees. The fibril angle also s trongly affects the mechanical and shrinkage (4) proper ties of wood. Despite the importance of the fibril angle as a variable, influencing wood, fibre and paper properties, it is a quantity that is relatively rarely mea sured.