Spectroscopic transmission ellipsometry enables quick measurement of pulp fiber

Abstract

The properties of a paper depend on those of the wood or pulp fibers from which it was produced. For example, individual fibers are known to vary widely even within the same tree. Nowadays, in a context of forest and limited wood supply management, the increasing demand for high-quality paper products requires more efficient use of available fiber resources. Research and analytical tools for measuring the properties of individual pulp fibers are accordingly considered essential to improve use of this resource. A wood fiber can be described by its middle layer, S2, which contains the majority (80–95%) of the cell wall material.1 The angle between the fibrillar direction and the fiber axisφ is termed the microfibril angle (MFA) (see Figure 1). A number of experimental studies have shown that the MFA is closely related to the mechanical properties of fibers, such as strength, elastic modulus, and shrinkage.2, 3 When a fiber is illuminated by polarized light, a relative phase retardation ∆ is produced between the two orthogonal components of the light traveling in the S2 layer as it emerges from a cell wall (see Figure 1). The retardation ∆ or path difference PD is proportional to the cell wall thickness (CWT) d, with ∆ = 2πd(n2 − n1)/λ or PD = ∆λ/2π = d(n2 − n1) where (n2 − n1) is the birefringence of the wall material and λ is the wavelength of the light. The CWT is related to fiber flexibility, strength, and collapsibility. In applications, it is important to know the distribution of CWTs instead of their absolute values so that ∆ or PD can be used to replace CWT. In addition, the CWT can be determined from ∆ or PD through calibration. Both MFA and CWT are difficult to measure for fibers due to their two-walled structure. Recently, a new method based on spectroscopic transmission ellipsometry (STE) was developed to Figure 1. Schematic representation of the S2 layer of a single wood fiber. φ is the microfibril angle and ∆ is the phase retardation (see text).