Surface modification of wood and wood-based panels

Native wood species are very attractive in many application fields, but they do have some disadvantages on the technological side, such as low dimensional stability with changes in humidity or temperature, ultraviolet sensitivity as well as low natural durability. In contrast to tropical woods, many European species are generally regarded as unsuitable for outdoor service requiring long-term stability, such as terrace decking or furniture. 

The further development of wood and wood-based materials as construction materials as well as evolve new methods of production is a permanent goal of material research. As a natural material, wood is exposed to various natural degradation and aging processes, which place restriction on later improvement and machining operations and thus reduce the scope of applications. Atmospheric-pressure plasma treatments successfully address this problem, enabling coating and treatment processes which offer long-term stability and defect-free, safe and efficient coatings and treatments. Surface treatments with atmospheric pressure plasma processes are opening up new areas of application while simultaneously improving the quality of various woods species. 

Effects of plasma treatment

• Increasing and setting of the surface energy level
• Increasing the uptake of modifying agents
• Modifying the surface by plasma polymerization

Advantages of plasma treatment at atmospheric pressure

• Increasing the adhesive strength of paints and coatings
• Homogenous distribution of liquid coating agents on surfaces
• Improved penetration of modifiers within the wood
• Increasing weathering resistance
• Creating functional coatings on wooden surfaces

General advantages of atmospheric pressure plasma technology

• Low emission
• Easiliy scalable
• High ecological efficiency
• Low operating costs

Our offers

• Analytical services: contact-angle measurements, adhesion tests, artificial weathering
• Proof of concepts in optimizing the properties of wooden surfaces by plasma treatment
• Hydrophilization and hydrophobization by means of plasma
• Customized process optimization of joining and impregnation processes
• Development, design and implementation of plasma treatment equipment, prototypes and system components
• Experimental coatings on wooden surfaces by plasma based particle deposition and plasma polymerization

Our fields of application

Wood

Sanding is a standard method of pretreating wooden surfaces for further production operations. Compared to unsanded or aged wooden surfaces, they do have a higher wettability. Analyses have shown that plasma treatments have a greater influence on surface-energy characteristics than sanding the surfaces does. Plasma treatment thus can replace the sanding processes which is a standard procedure for ‘freshening up’ wooden surfaces before subsequent coating processes.

Impregnation of wood veneers

Many native wood species with low natural durability cannot be successfully treated with liquid modification or impregnation reagents. Improvement and long-term protection coatings do however require plain and deeply distribution of the reagents in the wood body. A plasma treatment enables an accelerated impregnation of the wood veneers, thereby facilitating a much more homogeneous distribution of the modification reagents on the surface while the liquid uptake of the modification reagents used can be increased by up to 60 percent. 

Wood materials

After a plasma treatment a significant increase in surface energy is observed on the surfaces of particle boards and fiberboards. This results in an accelerated penetration of water into plasma-treated particle boards and fiberboards. An accelerated removal of water out of the PVAc adhesive leads to faster curing and an increase in the strength of glued components. The rapid hardening of water-based adhesives can result in shorter processing times and an increasing of the production.

Mobile plasma sources

By means of particular arrangements of the dielectric barrier discharge, items to be treated can be plasma-treated even independently of the thickness of the material. With these arrangements, hand-held plasma tools are developed at the Application Center for Plasma and Photonics of the Fraunhofer IST. They allow the treatment of small areas of highly profiled surfaces. The battery-powered units ensure maximum flexibility and mobility; as well, they can easily be used in many different fields of application, such as crafting and hobbies where smaller surfaces have to be prepared for subsequent gluing or painting.

WPCs: Wood-plastic components

When glued or coated, wood-plastic composites do often show insufficient adhesive properties due to the plastic components mainly determining the materials surface characteristics. The surface energy can be significantly increased by a plasma treatment of, for example, polypropylene- and polyethylene-based WPC. This increase in surface energy results in a greater adhesive strength. 

Experimental plasma coating

To improve the durability of wood, usually lacquers and glazes are applied. The Application Center of the Fraunhofer IST aims to develop alternative protective coatings using atmospheric-pressure plasmas: for example, metal particles can be melted or activated in the plasma and applied to wood surfaces. Another technique is the plasma-assisted polymerization in which a precursor is polymerized in the plasma to create long-chain molecules which are then deposited on the surface of the wood. With these methods, layers can be generated which e.g. prevent water penetrating the wood.