High intensity microwaves generate steam pressure in green timber. This results in the rupturing of un-lignified ray tissue and pit membranes and the generation of micro-voids at the ray/fibre interfaces. In the case of hardwoods it also results in the rupturing of tyloses (balloon-like outgrowths from wood cells). The degree of modification varies with the intensity of microwaves, the method of microwave application and a number of other processing variables. At low levels of microwave irradiation, micro-voids are created that cannot be seen by the naked eye, but have a major impact on the drying properties of wood. The wood is made permeable so the build-up of stresses that normally lead to drying defects, such as collapse and checking, are reduced. In addition there is relatively little strength loss.
At very high intensities the micro-voids expand to become visible; the wood increases in cross-section dimension and becomes very permeable, up to 2 million times more permeable. In this instance, however, strength properties are reduced substantially. But, the high permeability of the wood is exploited by impregnating it with resins and other chemical modification agents. The wood is then pressed back to its original dimensions and cured. The resulting properties include improvements in durability, dimensional stability, strength, hardness and other aesthetic characteristics. The creep behaviour of wood, that is important in building applications, may be reduced by up to 50%.
Species behaviour
It seems that virtually every wood species, both softwoods and hardwoods, behave in the same way when exposed to intensive microwave processing. At the Co-operative Research Centre for Wood Innovations at the University of Melbourne, Australia (CRC WI), we are exploring four specific applications of the technology. These include wood drying, preservative treatment, stress relaxation of timbers susceptible to growth stresses (for example fast grown eucalypts), and the development of new materials, TORGVIN a microwave expanded solid wood product and VINTORG – TORGVIN impregnated with resin or even metal. VINTORG and TORGVIN are product names derived from their joint inventors (myself, a Cornishman, and Professor Grigori Torgovnikov, a Russian).
Primary benefits
One of the primary benefits of microwave processing is derived from the accelerated processing times and the potential for “on-line” or automated processing rather than batch processing. Microwave modification of wood for wood drying can lead to a five-fold decrease in hardwood drying times using conventional drying schedules. For example, 35mm-thick eucalyptus that might take 42 days to kiln dry from green, can be dried using the same drying schedules in a matter of six days following microwave conditioning.
Microwave conditioning also facilitates the preservative treatment of species that normally would be considered untreatable. Hardwood species such as eucalypts, oak and even softwood species like Sitka spruce and Douglas fir that are normally considered refractory or untreatable, can be readily impregnated with wood preservatives. Very high standards of treatment can be obtained in freshly peeled roundwood immediately after microwave processing. The microwave processing itself is very short in duration, literally a few seconds or minutes. The moisture lost during microwaving (up to 200 litres/m3) and improved wood permeability provide sufficient room for the preservative to achieve total penetration of the wood.
Spin-off developments
One of the spin-off technologies of the microwave research programme has been the development of a generically new method of wood treatment. The technique was developed for applying resin to wood that had been expanded using microwaves (VINTORG), where specific quantities of resin need to be impregnated into the wood. The same schedules were found to be suitable for impregnating radiata pine with conventional wood preservatives but with the added advantages of no preservative dripping after removal from the plant, preservative fixation during treatment and therefore improved environmental control and complete treatment within minutes rather than hours.
Research into the microwave conditioning of wood for preservative treatment led to the discovery that microwaves could be used to reduce stress development in roundwood. Eucalyptus posts or poles usually split longitudinally during air drying as a result of growth and drying stresses. Such behaviour was absent in poles that had first been microwaved. Research continues to see whether microwave conditioning can be used on saw logs to facilitate automated sawing.
Automated production
The production of VINTORG has the potential to totally automate production of a solid wood product. The conversion of green timber into TORGVIN can be achieved within seconds or minutes of microwave exposure. The wood becomes so permeable that moisture flashes off within minutes and is ready to be impregnated with suitable resins. The impregnation of resin can be achieved in seconds. The wood is then compressed and the resin cured. The product looks like solid wood but the properties are now engineered to specific requirements of durability, dimensional stability, strength, hardness, fireproofing, colour and texture. The latter is achieved by modifying the characteristics of the resin.
Microwaving represents the simplest part of VINTORG manufacturing. While commercially available resins can be used, there are specific requirements that need optimising such as rate of cure, resin viscosity and colour. It is anticipated that there will be some spin-off technologies developed as a result of researching suitable resin systems. For example, the best resin found experimentally for VINTORG manufacturing is a bio-resin that is low in viscosity (aids penetration into the wood), high in solids content (no solvent emission problems), low formaldehyde content (reduced environmental and health and safety issues), and low cost.
Biocidal benefits
In addition, the resins have been found to have biocidal properties that may provide protection from termite and fungal degrade. The latter properties are being tested in one of the harshest environments, the Northern Territory of Australia. In the longer term these resins may find use as alternatives to wood preservatives.
A pilot commercial plant is being developed for each of the specialist applications. This is regarded as being essential in the commercialisation process. A microwave pilot plant for wood drying has been built and is operating successfully. Our breakthrough in the development of scale up plant arose when the Centre successfully developed models for predicting microwave/wood interactions. This has helped in the design of microwave applicators rather than going through an expensive and time-consuming, iterative process of trial and error. The pilot plant also facilitated the methodology for combining microwave generators, checking in-feed and out-feed systems, the development of health and safety controls and accurate manufacturing costing details.
Microwave modification for wood drying represents the cheapest of the applications. The microwave facility is simply placed between the existing green chain in a sawmill and the mill’s traditional kilns. Technically, however, this application is the most complex because of the need to ensure the absence of any checking on the surfaces of the timber. Microwave conditioning for preservative treatment is the simplest, particularly where commodities such as railway sleepers and landscaping timbers are concerned. The new “UNI-TREAT” microwave plant technology needed for conditioning wood for preservative treatment is similar to that used for wood drying. The pilot plant needed for TORGVIN manufacture (a 300kW microwave installation) is under construction and should be operational by the end of April. Pilot plant for the production of VINTORG is awaiting the development of suitable continuous pressing technology. Hopefully this is not too far away.
