Mechanism of veneer dyeing for production of multilaminar veneer from plantation-grown Eucalyptus globulus
AffiliationSchool of Ecosystem and Forest Sciences
Document TypePhD thesis
Access StatusOpen Access
© 2018 Dr Ngoc Nguyen
Large plantations of Eucalyptus globulus wood established in Australia have mainly been grown to produce pulpwood. This resource is not suitable for the production of decorative products, principally because of the low wood quality grades and unattractive colour. The manufacture of veneer-based products has recently been identified as an unprecedented opportunity to promote higher value utilization of plantation resources. However, many uncertainties remain regarding the impacts of the inferior wood quality of young plantation trees on product recovery and value, as well as the optimal processing techniques. Moreover, the quality of veneers and veneer-based products is far from optimal because these trees are young and have small diameters. The veneers also have substantial colour variation that affects the added value of the final products. Developing production methods that can enhance the appearance of low-quality veneers produced from young, small-diameter logs has great potential. An innovative method for enhancing the appearance of low-quality veneers has been developed by ALPI, a company in Italy that is involved in the production of multilaminar veneers, which are also called “reconstructed veneers”. One of the most important stages in multilaminar production is dyeing the veneer throughout its entire thickness, which can be achieved by using dyes of different colours depending on the appearance of the products, their design, and market demand. In the current multilaminar veneer manufacturing process, veneer dyeing is normally applied to give veneers a colour similar to that of another timber or to minimise variations and make the veneers more homogeneous in colour. Basically, veneer dyeing methods are based on the deep colouration of wood, which is known as wood dyeing, as opposed to surface colouration (varnishing and painting). The method used depends on the applications of the veneers. The benefit of dyeing is that the colour cannot be removed by sanding because the veneer is dyed throughout its entire thickness. Although veneer dyeing technology is well advanced in Italy, it has focused on poplar (Populus) veneers, using plantation wood that is characterised by a low density, even colour, small number of defects, and high permeability. Conversely, the majority of plantation eucalypts have a medium to high density, many defects, uneven colour, and low permeability. Therefore, a detailed study is required to investigate the veneer dyeing process for eucalypt veneers. The aim of this PhD study was to investigate the enhancement of veneer appearance through veneer dyeing technology for the production of high value veneer-based products from plantation grown Eucalyptus globulus. The specific objectives were to: • investigate the dyeing methods and types of dyes that would be the most suitable for dyeing Eucalyptus globulus veneers; • investigate the effect of dyeing parameters such as dye concentration, dyeing time and temperature on the dye uptake, dye penetration and colour difference of dyed veneer; • investigate Ultraviolet (UV) - driven discolouration of dyed wood veneers. The study investigated dyeing methods (soaking and vacuum-pressure), types of dye (direct dye and reactive dye), and dyeing parameters (dye concentration, dyeing time, and temperature) in the veneer dyeing process for Eucalyptus globulus grown on plantations in Australia. Two different reactive dyes were used: Procion H-EXL and Procion Brown P2RN. One direct dye was used, which was a mixture of Sirius Red F-4BL, Sirius Yellow K-GRL, and Sirius Grey K-CGL. Veneers with two different moisture content (MC) levels were used: green veneer (80% ± 5% MC) and dried veneer (12% MC). The study showed that the reactive dye at a concentration of 2% resulted in a significantly higher dye penetration than other dyes. Soaking was not recommended as the dyeing method for this species because the dyed samples were severely damaged by the pre-treatments and high temperatures. Therefore, an alternative method, the vacuum pressure method, was investigated. The results showed the vacuum-pressure method had a significant influence on the dye uptake, dye penetration, and colour change; in particular, a reactive dye penetration of 100% was achieved with a dyeing time of 120 minutes, pressure of 1000 kPa, and the addition of 20 g/L of sodium chloride. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM) analyses were conducted. The results indicated that the reactive dye was able to react with the wood compounds (cellulose, hemicellulose, and lignin), a large amount of the dye molecules diffused into wood lumens, and various dye molecules aggregated around the surface of wood fibres. A comparison of two dye uptake measurement methods (the liquid uptake-based method and the spectrophotometry method) has been conducted and analysed. The new technique for the measurement of dye penetration of the wood veneer using ImageJ program has been developed. UV-driven discolouration of veneers dyed at the optimal dyeing parameters was investigated. Results showed that there was no significant difference in the colour change of the tight and loose sides of sapwood and heartwood of the dyed green and dry veneer samples after 504 hours of UV light exposure.
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