Microwave Wood Processing

Grigory Torgovnikov

Innovative Microwave Technology for Wood Modification and its Applications

The method of MW wood modification was invented and developed by G. Torgovnikov and P. Vinden in the University of Melbourne

 

  The low permeability of many wood species causes problems during timber manufacturin. The shortcomings include: very long drying times, large material losses after drying, expensive drying processes. Impregnating low permeability timber with preservatives and resins is extremely difficult. Furthermor, growth stresses in wood and collapse lead to drying defects and high losses in recovery of sawn timber. In the pulp and paper industry low permeability wood results in shallow chemical penetration, requires the use of small sized chips, a high chemical usage and high energy consumption. This new microwave technology provides solutions to many of these problems.

  Green wood moisture content varies between species and between sapwood and heartwood in the range from 30 to 300%. Green wood readily absorbs microwave energy because of its high moisture content. The passage of MW energy through wood results in a very high release in energy from within the material. When intense levels of MW energy is applied to wood, steam is generated within the wood cells. Under high internal steam pressure the pit membranes in cell walls, tyloses in vessels and the weak ray cells rupture to form pathways for easy transportation of liquids and vapors. Increased applied MW intensity further increases internal steam pressure, resulting in the formation of narrow voids in the radial-longitudinal planes. The number cavities, their dimensions and distribution depend highly on wood structure and can be controlled through variation of MW intensity. A several thousand-fold increase in wood permeability in the radial and longitudinal directions can be achieved in species previously found to be impermeable to liquids and gases.

  A study of microwave (MW) wood modification showed significant changes to wood structure and a dramatic increase in wood permeability. Physical and mechanical characteristics of wood also change. MW process parameters to provide different levels of wood modification were developed which provide a scientific and engineering base for the new technology. The following commercial applications have been developed:

  • the treatment of refractory wood species with preservatives
  • rapid drying of hardwoods
  • growth and drying stress relief in timber
  • manufacturing of MW modified wood "Torgvin"
  • production of new wood-based material "Vintorg"
  • microwaving logs, sawn timber and chips for pulping
  Research results indicate that the technology can be applied to any wood species. The cost of MW conditioning varies from AU$ 28 - 69/m³ depending on the wood species, its moisture content and the degree of structural modification required. These costs are acceptable to the industry and provide good opportunities for the commercialization of this MW technology, which can significantly change the face of the timber industry.