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McGavin, R. L., Bailleres, H., Fehrmann, J., and Ozarska, B. (2015). "Stiffness and density analysis of rotary veneer recovered from six species of Australian plantation hardwoods," BioRes. 10(4), 6395-6416.

Abstract

Commercial interest in Australian hardwood plantations is increasing. The timber industry is investigating alternative supplies of forest resources, and the plantation growing industry is eager to explore alternative markets to maximize financial returns. Identifying suitable processing strategies and high-value products that suit young, plantation-grown hardwoods have proven challenging; however, recent veneer processing trials using simple veneer technology have demonstrated more acceptable recoveries of marketable products. The recovered veneers have visual qualities that are suitable for structurally-based products; however, the mechanical properties of the veneer are largely unknown. Veneers resulting from processing trials of six commercially important Australian hardwood species were used to determine key wood properties (i.e., density, dynamic modulus of elasticity (MoE), and specific MoE). The study revealed that a wide variation of properties existed between species and also within species. Simple mathematical modeling, using sigmoidal curves, was demonstrated to be an effective method to model the evolution of key wood properties across the billet radius and along the resulting veneer ribbon with benefits for tree breeders and processors.


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Stiffness and Density Analysis of Rotary Veneer Recovered from Six Species of Australian Plantation Hardwoods

Robert L. McGavin,a,b,* Henri Bailleres,b Joh Fehrmann,b and Barbara Ozarska a

Commercial interest in Australian hardwood plantations is increasing. The timber industry is investigating alternative supplies of forest resources, and the plantation growing industry is eager to explore alternative markets to maximize financial returns. Identifying suitable processing strategies and high-value products that suit young, plantation-grown hardwoods have proven challenging; however, recent veneer processing trials using simple veneer technology have demonstrated more acceptable recoveries of marketable products. The recovered veneers have visual qualities that are suitable for structurally-based products; however, the mechanical properties of the veneer are largely unknown. Veneers resulting from processing trials of six commercially important Australian hardwood species were used to determine key wood properties (i.e., density, dynamic modulus of elasticity (MoE), and specific MoE). The study revealed that a wide variation of properties existed between species and also within species. Simple mathematical modeling, using sigmoidal curves, was demonstrated to be an effective method to model the evolution of key wood properties across the billet radius and along the resulting veneer ribbon with benefits for tree breeders and processors.

Keywords: Eucalyptus; Veneer; Hardwood; Plantation; Processing; Quality; Structural; Density; Modulus of elasticity

Contact information: a: University of Melbourne, Department of Ecosystem and Forest Sciences, 500 Yarra Boulevard, Richmond, Victoria 3121 Australia; b: Queensland Department of Agriculture and Fisheries, Horticulture and Forestry Science, Salisbury Research Facility, 50 Evans Road, Salisbury, Queensland 4107 Australia; *Corresponding author: robbie.mcgavin@daf.qld.gov.au

INTRODUCTION

Forest resources in Australia, which are available to the commercial timber industry, are undergoing a substantial change. Gavran (2013) reports that over two million hectares of plantation forestry exist in Australia, of which about one million hectares are hardwood species. While the industry’s softwood sector has become well established over recent decades by relying on a plantation resource, the hardwood sector remains largely dependent on native forests for log supply, especially to provide high-value wood products. While a substantial area of hardwood plantations exists, the majority of the plantations have only been established in recent decades and only recently started to become available to the timber industry for end-uses other than pulpwood.

With a greater proportion of plantation-grown forest becoming available to the wood processing sector, combined with significant areas of native hardwood forests across Australia being progressively withdrawn from commercial harvesting and managed for conservation purposes, interest in hardwood plantations by Australia’s hardwood sector is rapidly increasing. In addition, plantation growers are continuously seeking processing streams and end-uses that can provide the highest return from the plantations.

Of the one million hectare hardwood estate, around 84% has been established for pulpwood production (Gavran 2013), meaning that the majority of the estate contains a mix of species and forest and wood qualities that are most likely not optimal for targeting higher-value products. Some small areas of plantations have been established and managed with a high-value product focus. For example, Wood et al. (2009) reported approximately 26,000 hectares of plantations, principally located in Tasmania, which have been thinned, pruned, and managed for high-value end-uses.

With rapidly growing interest in hardwood plantations by the Australian timber industry, and growers’ interest in maximising the value of their plantations, many studies have investigated solid wood processing options (i.e., sawmilling) for the plantation hardwood resource. As summarised by McGavin et al. (2014a), despite the varied approaches, mainly based on alternative technologies targeting sawn timber products, many challenges remain, resulting in excessively low recovery of marketable products and unprofitable processes.

The processing of Australian grown plantation hardwoods into veneer using relatively new small-scale spindleless veneer lathe technology has been demonstrated in research trials to produce product recoveries that are much more favourable when compared to solid wood processing techniques (McGavin et al. 2014a,b; 2015). While the technology approach is not necessarily new, recent advancements in design allow the technology to be well suited to small diameter plantation forest resources. The advancements have been quickly adopted through many Asian countries, including China and Vietnam, for successful veneer production from very small diameter hardwood billets. Arnold et al. (2013) reported well over 5000 small-scale veneer mills operating in China.

While the veneer recoveries reported by McGavin et al. (2014a,b) were high (net recoveries up to 58% of log volume), the grade recoveries were dominated by D-grade veneers (lowest visual quality) when graded to Australian and New Zealand Standard AS/NZS 2269.0:2012 (Standards Australia 2012).

With such a dominance of low appearance qualities, the veneers are potentially more suited to structural products where higher appearance traits are less relevant. However, to be acceptable in this market, the veneer is required to meet certain mechanical properties requirements (e.g., stiffness). In addition, an understanding of the variation in mechanical properties from within a species, between trees and billets and within a billet, is critical in determining the optimal processing strategy, grading and quality segregation systems, and final target products.

Within-tree radial variation (pith to bark) of wood characteristics is described by Larson (1967) (in Zobel and van Buijtenen 1989), as being very large and more variable than between trees growing on the same or on different sites. Several equation types have been reported in previous studies to describe ontogenetic variation for tree characteristics, largely on the basis of the best fit rather than on clear biological mechanisms (e.g., Koch 1972; Downs et al. 1997; Zobel and Sprague 1998). Alternatively, West et al. (2001) derived a general quantitative model based on fundamental principles for the allocation of metabolic energy between the maintenance of existing tissue and the production of new biomass. Thus, they predicted the parameters governing growth curves from basic cellular properties and derived a universal family of curves that describes the growth of many diverse species. These curves represent a classical sigmoidal shape. The model provides the basis for deriving allometric relationships for growth rates and time. Specific properties related to biomass increase, such as density or stiffness, follow the same pattern. Indeed, the specific wood density is a simple measure of the total dry mass per unit volume of wood. It is also closely related to basic wood mechanical properties such as stiffness (or Modulus of Elasticity, MoE) (Kollmann and Cote 1968; Koch 1972). While not necessarily recognized, in most experimental studies wood mechanical property trajectories have the characteristic sigmoidal shape that is observed empirically (e.g., Zobel and Van Buijtenen 1989; Zobel and Sprague 1998). Bailleres et al. (2005) reported the use of sigmoidal profiles as an effective method to describe some select key wood properties in planted Eucalyptus species.

The objective of this study was to describe, at a species level, the density and modulus of elasticity (MoE) of veneer recovered from six hardwood plantation species based on global variation from pith to bark. Sigmoidal curves were used to describe these characteristics. In addition, the trial results were to be in a format that was recognisable and directly relevant to the commercial processing industry. Veneer density and MoE are key in determining the suitability of veneer for structural veneer-based engineered wood products. The resulting analysis provides guidance on the quality of the current plantation resources for structural product end-uses, plantation management, product development programs, and marketing strategies.

EXPERIMENTAL

Materials

Veneers were sourced from processing studies conducted on billets harvested from Australian commercial plantation stands, representing the average resource available for industry to access now and in the immediate future (McGavin et al. 2014a). Six of the major commercially important Australian hardwood species were included and ranged from traditional pulp to high quality solid wood species. They include Corymbia citriodora subspecies, variegata (spotted gum), Eucalyptus cloeziana (Gympie messmate), Eucalyptus dunnii (Dunn’s white gum), Eucalyptus pellita (red mahogany), Eucalyptus nitens (shining gum), and Eucalyptus globulus (southern blue gum) (Table 1). Plantation ages ranged between 10 and 16 years for all species except Eucalyptus nitens, which was between 20 and 21 years old. These plantations were older by comparison; however, they are reflective of the E. nitens plantation resource immediately available to the wood processing sector.

Processing was undertaken using an OMECO spindleless veneer lathe, model TR4 (OMECO, Curitiba, Estado de Paraná, Brazil). The lathe is capable of processing billets with a maximum length of 1350 mm and maximum log diameter of 400 mm. The minimum diameter of the peeler core was 45 mm. A very small number of E. nitens billets were too large (> 400 mm diameter) to process on the spindleless lathe; these billets were rounded and/or partially peeled using a conventional spindled lathe before the peeling was completed on the spindleless lathe. For this study, the nominal dried veneer thicknesses were 2.4, 2.5, or 3.0 mm, depending on species and according to the thickness range mostly used by the Australian industry for structural plywood production.

The resulting veneer ribbon was sequentially clipped to target 1400 mm maximum width sheets. This target sheet size was chosen to provide 1200 mm dried and trimmed veneer sheets as per standard industry practice. These veneers sheets were used for detailed visual grade quality analyses as reported by McGavin et al. (2014b; 2015). More detailed description of the methodology regarding plantation selection, billet preparations, and processing, is described by McGavin et al. (2014a).

Table 1 provides a description for each species, age, plantation location, number, the diameter at breast height over bark (DBHOB) of the trees sampled, and the number of billets and sampling strips included in the study.

Table 1. Plantation Trial Material

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Between the clipped veneer sheets, sampling strips for density and dynamic MoE measurements were removed. Sampling in this manner ensured representation from billets in line with veneer sheet production when processed by the commercial processing industry.

The sampling strips measured 150 mm (parallel to the grain) by 1300 mm (the length of the veneer sheet) and aimed to be representative of the adjacent veneer sheets (Fig. 1). Therefore, any defects that were present were included within the sampling strip to ensure a realistic representation of the veneer qualities (i.e., sampling strips were not biased towards clear of defect veneer). The sampling strips were air-dried to 12% MC, prior to density and dynamic MoE measurement.

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Fig. 1. Schematic demonstrating clipping strategy and sample strip origin

Methods

Sample strip dimensions (length, width, and thickness) and weight were measured, allowing veneer density to be calculated. Veneer MoE measurements were followed using an acoustic natural-vibration method as described by Brancheriau and Bailleres (2002).

Sample strips were positioned on elastic supports so that the longitudinal propagation of vibration was as free as possible and could be induced by a simple percussion on one end of the sample, in the grain direction (Fig. 2). At the other end, a Lavalier type microphone recorded the vibrations before transmitting the signal via an anti-aliasing filter (low-pass) to an acquisition card, which included an analog-to-digital converter to provide a digitized signal.

A Fast Fourier Transform processed the signal to convert the information from the time to the frequency domain. The mathematical processing of selected frequencies was undertaken using BING (Beam Identification using Non-destructive Grading) software in combination with the geometrical characteristics and the weight of the specimen, to provide the dynamic MoE, among other specific mechanical characteristics (CIRAD 2009; Pico n.d).