Modification of wood by radio-frequency discharge plasma
Authors: Novák Igor 
Published: November 2013

MODIFICATION OF WOOD BY RADIO-FREQUENCY DISCHARGE PLASMA aIgor Novák1, a,bAnton Popelka1, aMarian Valentin, aIvan Chodák1, a,cIgor Krupa dVladimír Vanko, dJozef Preťo aPolymer Institute, Slovak Academy of Sciences, Bratislava, Slovakia bCenter for Advanced Materials, Quatar University, Doha, Quatar cCenter for Advanced Materials, QAPCO Polymer Chair, Quatar University, Doha, Quatar dVIPO, Partizánske, Slovakia Email: igor.novak@savba.sk Abstract The modification of wood surfaces is of considerable interest with the respect to construction of the strongest wood adhesive joints. Great efforts have been made in developing various kinds of furniture using wood or plastics veneers in adhesive joints wood-adhesive-veneer. The radio-frequency (RF) discharge plasma is currently an efficient method for modification of surface and adhesive properties of wood, and is considered as the 'green' ecologically friendly method. For a common industrial wood application various woods have to possess a large set of various surface characteristics, including polarity (hydrophobicity or hydrophilicity), dyability, scratch resistance, tailored adhesive properties, antibacterial resistance etc. In experimental work we have been focused to study of surface and adhesive properties of beech (Fagus sylvatica) wood modified by RF-discharge plasma. There are two reasons why in the case of wood to apply discharge plasma modification. Firstly, discharge plasma in air itself significantly increases hydrophilicity of the wood, because of formation various polar groups (e.g. hydroxyl, carbonyl, carboxyl, etc), and, the wood macromolecules are also cross-links (up to a few microns) what leads to the increase in scratch resistance and to the improvement in barrier properties of the wood material. Second reason for the plasma use is an increase of adhesion in adhesive joint between wood substrate, that is important for industrial applications. The contact angle of re-distilled water drop on the beech wood surface was measured immediately after drop deposition. The contact angles showed a steep decrease from 66 deg (pristine sample) to 40 deg after 120s activation of wood by RF plasma in air. The decrease of the contact angles of polar testing liquid (water) can be explained by an increase of the hydrophilicity of beech wood surface due to the treatment by RF plasma in air. The hydrophilicity of the surface depends on the formation of polar oxygenic functional groups on wood surface during the plasma modification in air. The surface energy and its polar component of beech wood increased with time of plasma activation. The surface energy of beech wood treated 120s by RF plasma in air increased from 66 mJ.m-2 (pristine sample) to 78 mJ.m-2, and the polar component of the surface energy increased from 17.4 (pristine sample) to 27.2 mJ.m-2 (120s). The shear strength of adhesive joint beech wood modified by RF plasma in air-epoxy adhesive vesrus activation time increased non-linearly from 5.2 MPa (pristine wood) up to 7.8 MPa (120s activation by CCRF plasma). Acknowledgements This contribution was supported by project No. 26220220091 by Research & Development Operational Program funded by ERDF, as well as the project „Application of Knowledge-based Methods in Designing Manufacturing Systems and Materials", project No. MESRSSR 3933/2010-1.