Relationship between micro-fibril angle and strength of black spruce fibers

Abstract

The study of the strength properties of single wood fibers has gained momentum in recent years due to their potential exploitation as reinforcement in composite materials, and the possible selective logging of trees for lumber and paper production. The present study focuses on the relationship between the microfibril angle and the strength properties of fibers between different parts of a section of the trunk of a black spruce tree. Single fibers, approximately 2–3 mm in length, were extracted in small sections from a single trees inner growth rings (5–7) and outer growth rings (40–50) and chest height using a sharp razor blade. These sections were macerated in 20 ml glacial acetic acid and 20 ml hydrogen peroxide (100 vol.) using a reflux apparatus. This maceration process was left for 48 h after which the samples were thoroughly washed using distilled water. The fibers, now white in appearance due to the removal of lignin, were then transferred to a storage jar. At all times fibers remained wet. Fibers were removed in solution, using a pipette, and transferred to a glass slide. Under a microscope, individual fibers were observed and removed from the solution using ultra-fine tipped tweezers. These fibers were then transferred to a fiber rack and micro-droplets of DEVCONTM epoxy and hardener (in equal volumes) were applied to either end of the fibers, following a method devised by Kersavage [1] and further developed by Mott [2]. Single fibers were then transferred to a MINIMATTM system with customized fiber grips allowing tensile testing. Each fiber was tested with a cross-head speed of 0.5 mm/min and the maximum jaw separation was set at 3 mm to allow for complete fracture. Data obtained for 40 tensile tests of black spruce fibers taken from the inner and outer ring portions are shown in Table I. Typical load-elongation curves for black spruce fibers are shown in Fig. 1. It is clear that they show linear elasticity. Some fibers appeared to show “strain-hardening,” whereby the load increased non linearly with elongation. This is thought to occur in fibers which have a high S2 cell wall layer microfibril angle. This will be discussed in more detail later. In total 20 fibers were tested from each section and only from the latewood portions of the rings. The reasoning