Influence of Microfibril Angle on Within-Tree Variations in the Mechanical Properties of Chinese Fir (Cunninghamia Lanceolata)

IAWA Journal, Vol. 32 (4), 2011: 431–442 INFLUENCE OF MICROFIBRIL ANGLE ON WITHIN-TREE VARIATIONS IN THE MECHANICAL PROPERTIES OF CHINESE FIR (CUNNINGHAMIA LANCEOLATA) Yafang Yin1, Mingming Bian1,2, Kunlin Song1, Fuming Xiao3 and Jiang Xiaomei1,* SUMMARY Radial variations in microfibril angle (MFA) and their effect on the mechanical properties of plantation-grown Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) were investigated with the aim of achiev- ing an effective utilization of the wood. Correlations between MFA and mechanical properties, including longitudinal modulus of elasticity (MOEL), static bending strength (MOR) and compression strength parallel-to-the-grain (CS), were analyzed for predicting the quality of timber. The results indicated that MFA had a greater variation in juvenile wood than in mature wood. The biggest change occurred close to the pith in Chinese fir. The outer-rings (rings 9–30 from the pith) have a relatively low MFA, together with high mechanical properties and high density, when compared with the inner-rings (rings 1–8 from the pith). The MFA had significant negative curvilinear correlations with all the mechanical properties (MOEL, MOR and CS) of Chinese fir, with the value of r2 being 0.88, 0.69 and 0.74 respectively. The correlation between the MFA and basic density (BD) was strong in certain consecutive rings (rings 5–30 from the pith), but this did not apply across the whole billet, i.e. from the pith to the bark. Key words: Cunninghamia lanceolata, MFA, mechanical properties, basic density. INTRODUCTION Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is one of the most important, fast growing and major commercial plantation species that mainly occurs in 17 prov- inces of the central and southern regions of temperate China. The wood of Chinese 1) Wood Anatomy and Utilization Department, Chinese Research Institute of Wood Industry, Chinese Academy of Forestry, No.1 Dongxiaofu, Beijing, CN 100091, China. 2) State Academy of Forestry Administration, No. 8 Linxiao North Road, Beijing, CN 102600, China. 3) Jiangxi Academy of Forestry, No. 1629 Fenglin Street, Nanchang, Jiangxi Province, CN 330032, China. *) Corresponding author [E-mail: [email protected]]. Associate Editor: Lloyd Donaldson Downloaded from Brill.com10/01/2021 02:19:26PM via free access 432 IAWA Journal, Vol. 32 (4), 2011 fir has traditionally been used for furniture making, bridge and boat building, general carpentry and timber constructions. However, plantation-grown Chinese fir has not been effectively and fully utilized, due to a high proportion of juvenile wood and large variations in wood properties. With a decrease in natural wood resources and an increase in the demand for wood materials, the wood supply from this tree species is undergoing a recent increased demand in China. As a result, there is an urgent need for much more detailed information on wood quality to ensure the production of high value wood products from this species. As is generally known, when wood properties are analyzed transversely across the radius, around the perimeter or longitudinally along the stem, the wood of Chinese fir varies in its chemical, mechanical, and anatomical properties (Bao & Jiang 1998; Savidge 2003). Therefore, in order to achieve the best utilization of this wood, it is essential that research work should be carried out on the radial and longitudinal variations in wood properties of the tree. Correlations among wood properties may be of help in effectively predicting the quality of the wood. A few studies have been conducted on the variation of MFA within or between trees (Donaldson 1992; Donaldson & Burdon 1995; Zhang et al. 2007; Yin et al. 2011). The variation of MFA between growth rings indicates a consistently decreasing trend from pith to bark at all heights. MFA between heights shows greater values at breast height of the stem and a trend of general decline within the stem with height (Zhang et al. 2007; Donaldson 2008; Yin et al. 2011). Significant variation among trees and growth rings was also indicated in Pinus radiata D. Don (Donaldson & Burdon 1995). The MFA of the S2 layer in the cell wall is known to be one of the main determi- nants affecting the mechanical wood properties in different species (Cave & Walker 1994; Walker & Butterfield 1996; Treacy et al. 2000; Deresse et al. 2003). A strong correlation between MFA and MOEL (longitudinal modulus of elasticity) was obtained, when the distribution along the stem of butt logs of Pinus radiata was studied (Xu & Walker 2004). The MFA was also significantly related to the modulus of rupture (MOR) in small clear wood samples from different species (Bendtsen & Senft 1986; Treacy et al. 2000; Deresse et al. 2003). Using clear wood samples of Pinus radiata, high correlations among MOEL, MFA and density (r = -0.78 and 0.69 respectively) were obtained, while path analysis showed that MFA was the only significant causal factor for MOE per unit of mass (Booker et al. 1998). Cown et al. (2004), in a study of the relative effects of MFA and basic density on MOEL in boards of Pinus radiata clones, found only a low contribution from MFA, when compared to other factors, such as spiral grain and knot area ratio. The correlation between MFA and CS of small clear wood has rarely been investigated (Donaldson 2008). Nevertheless, micromechanical properties parallel-to-the- grain have been obtained using microtomed sections, single fibers or the nano-indentation of cell walls. A high correlation was observed between MFA and the extensibility of wood microtomed sections (Reiterer et al. 1999), whereas single fibers with a larger MFA showed greater extensibility (Mott et al. 2002). Hardness obtained using the nano- indentation of cell wall regions displayed no strong correlation with MFA (Gindl et al. 2004). Downloaded from Brill.com10/01/2021 02:19:26PM via free access Yin, Bian, Song, Xiao & Jiang — Microfibril angle in Cunninghamia 433 Previous research work has indicated that there is still uncertainty regarding the relationship between MFA and mechanical properties in different species, or at different positions in the tree, and the level of correlation between MFA and specific mechanical properties. Only a few reports on the wood properties of Chinese fir are available and, consequently, correlations between MFA and mechanical properties still need to be ascertained. The aim of this paper is to assess the radial variations in MFA and mechanical properties of plantation-grown Chinese fir, in order to achieve an effective utilization of this wood, and to investigate whether and to what degree the mechanical properties (bending modulus of elasticity (MOE), the bending modulus of rupture (MOR) and the compression strength parallel-to-grain (CS)) of this species are affected by its MFA properties. MATERIALS AND METHODS Sample preparation Twenty vigorous sample trees were randomly collected from a 36-year-old Chinese fir plantation at the Shanxia Forestry Centre (114° 30' E, 27° 30' N), in Dagangshan in the Jiangxi Province, which is located at the Experimental Centre of Subtropical For- estry of the Chinese Academy of Forestry. The plantation was cultivated from seedlings, planted at a spacing of 4 by 6 m. Small clear specimen 10 mm (R) × 10 mm (T) a L/15 mm South North b L/20 mm c L/30 mm L/300 mm 8 ... 1 1 ... 8 a - MFA d L/150 mm b - CS c - BD d - MOEL and MOR A B C Figure 1. A schematic diagram of the sampling. – MFA = microfibril angle; CS = compression strength; BD = basic density; MOE = modulus of elasticity; MOR = modulus of rupture. A stem disk (billet) sample with a thickness of 300 mm was taken from each tree, at a height of 1.3 to 1.6 m from ground level (Fig. 1A). After marking the north and the south facing stem surfaces, a radial section 10 mm thick and including the pith, was taken from each billet in the north-south direction of the tree stem. A series of longitudinal wood strips were then sawn from the pith to the bark in each section (Fig. 1B). Each strip was subsequently cut into four blocks for measuring MFA, basic density (BD) and mechanical properties (MOE, MOR and CS) respectively (Fig. 1C). No significant variations of selected properties were observed between north and south Downloaded from Brill.com10/01/2021 02:19:26PM via free access 434 IAWA Journal, Vol. 32 (4), 2011 sections. Therefore, it was assumed that the average value of samples that had been collected from both the north and south sections at the same growth ring from the pith would represent the average properties. X-ray diffraction for MFA measurement Tangential sections, approximately 1 mm thick and containing latewood, were obtained from wood blocks using a stereo microscope. The method for measuring MFA using X-ray diffraction was similar to that described in an earlier paper (Yin et al. 2011). In brief, a tangential section was attached to a holder in an X-ray scattering system (Dmax-3BX). When the sample was rotated, the intensity curve was measured as a function of the rotation angle φ at a step of 0.5° and a measuring time of 180 s per point. The radiation source was a nickel-filtered radiation of 40 kV, and 30 mA. The mean MFA of the tracheid cell wall layer was then calculated, according to the measuring method of the 002 peak made in transmission, developed by Cave (1966) and Meylan (1967), using 0.6T, where T is the half-width of the peak, taken from the tangents drawn at the points of inflection (Anderssonet al. 2000). Mechanical testing The mechanical properties, i.e. bending MOE, MOR and CS, were used for evalu- ating the quality of the wood using small clear wood samples without any defects.

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