G Avramidis 
  
E Nothnick 
  
H Militz 
  
W Viöl 
  
A Wolkenhauer 
  
Accelerated Curing of PVAc Adhesive on Plasma-Treated Wood Veneers

In this study the effect of plasma treatment at atmospheric pressure of wood veneers on the curing behaviour of a PVAc adhesive was investigated. Time-dependent shear bond strength tests on untreated and plasma-treated maple (Acer pseudoplanatus), oak (Quercus sp.), beech (Fagus sylvatica) and teak (Tectona grandis) veneers were carried out. The results confirm that plasma treatment accelerated curing of the PVAc adhesive on the wood veneers.

Introduction

The curing of water-based coatings and adhesives such as poly vinyl acetate (PVAc) is influenced by the extraction of water. Depending on ambient conditions such as temperature, humidity, wettability and material absorptivity, these systems need a certain period of time to cure [START_REF] Zeppenfeld | Klebstoffe in der Holz-und Möbelindustrie[END_REF] and develop their full functionality or allow further industrial processing. These curing or drying times are often a limiting factor in industrial production and decrease productivity and output. Thus, there is a need to accelerate curing. An innovative method to achieve this objective is plasma treatment, which is currently the focus of increasing research interest for wood modification [START_REF] Vander Wielen | Surface modification of cellulosic fibers using dielectric-barrier discharge[END_REF][START_REF] Evans | Modification of wood using a glow-discharge plasma derived from water[END_REF][START_REF] Custódio | Activation of timber surfaces by flame and corona treatments to improve adhesion[END_REF][START_REF] Avramidis | Plasma treatment of wood and wood-based materials to generate hydrophilic or hydrophobic surface characteristics[END_REF].

In previous studies, the effect of dielectric barrier discharge (DBD) treatment at atmospheric pressure on particle board and fibre board was investigated. It was shown that surface energy, especially the polar component, is significantly increased by plasma treatment, resulting in improved wettability of the surface (Wolkenhauer et al. 2007a). This led to faster water penetration and accelerated curing of a PVAc adhesive (Wolkenhauer et al. 2007b). Since plasma treatment increases also surface energy, polarity and wettability of wood surfaces [START_REF] Topala | Dynamics of the wetting process on dielectric barrier discharge (DBD)-treated wood surfaces[END_REF][START_REF] Odrásková | Plasma activation of wood surface by diffuse coplanar surface barrier discharge[END_REF][START_REF] Lecoq | Pinus pinaster surface treatment realized in spatial and temporal afterglow DBD conditions[END_REF]Wolkenhauer et al. 2009), it is assumed that similar beneficial effects on the curing characteristics of PVAc adhesive can be obtained for plasma-treated wood veneers.

In the present study, the curing behaviour of a PVAc adhesive on plasma-treated veneers of maple, oak, beech and teak was investigated. The veneers were subjected to DBD treatment at atmospheric pressure. The curing behaviour of PVAc adhesive on untreated and plasma-treated wood veneers was measured using time-dependent shear bond strength tests.

Experimental

Materials and Methods

Sliced veneers of maple (Acer pseudoplanatus), oak (Quercus sp.), beech (Fagus sylvatica) and teak (Tectona grandis) with a thickness of 0.6 mm were stored at 20°C and 65% RH before testing. A commercial PVAc adhesive (Ponal Super3, Henkel KGaA, Düsseldorf, Germany) was used for the shear bond strength tests.

Two veneers (20×80 mm 2 ) were bonded with an overlap of 20 mm to achieve a glued area of 400 mm 2 and pressed at ~1 MPa. The pressing duration was varied and corresponds to the curing time. Shear tests were conducted using a tensile testing machine (Z010, Zwick GmbH & Co. KG, Ulm, Germany) at a traverse speed of 20 mm/min, and ten specimens were tested for each curing time.

Plasma Set-Up

The veneer was positioned centrally between two insulated (Al 2 O 3 , t=6 mm) electrodes (300×50×50 mm 3 ) to obtain a discharge gap of 2 mm between the veneer and the electrode on both sides. An alternating high-voltage (~35 kV peak) pulse generator with a pulse duration of 2 µs and a pulse repetition frequency of 17 kHz was connected to one electrode, whereas the other electrode was grounded. In this unsealed set-up, ambient air at room temperature was blown through the discharge gaps with a velocity of ~4 m/s. The treatment duration was 30 s for all experiments. Gas temperatures during plasma treatment were measured with a fibre-optic thermometer (FTI-10, FISO Technologies, Sainte-Foy, Canada) and did not exceed 50°C.

Results and Discussion

Curing of PVAc adhesives relies on the loss of water, whereas the setting rate on porous materials such as wood is a direct function of the water absorptive capacity of the wood [START_REF] Zeppenfeld | Klebstoffe in der Holz-und Möbelindustrie[END_REF]. Thus, changes in the water absorptive capacity of the wood affect the setting rate of a PVAc adhesive. Water absorption or penetration into the wood depends on the wood moisture content, absorptivity, temperature and surface energy characteristics of the wood and adhesive. In general the relatively low surface polarity of wood impedes wetting and prevents complete spreading of water [START_REF] Nguyen | The effects of aging and extraction on the surface free energy of Douglas fir and redwood[END_REF][START_REF] Mantanis | Wetting of wood[END_REF][START_REF] Nussbaum | Oxidative activation of wood surfaces by flame treatment[END_REF][START_REF] Sakata | Activation of wood surfaces by corona treatment to improve adhesive bonding[END_REF]). Thus, close and extensive contact is impeded so that water penetration into the wood is impaired. These processes can be improved by increasing the surface polarity of the wood, which can be achieved by plasma treatment. Plasma treatment increases the surface polarity by generating polar groups such as C-O and C=O [START_REF] Uehara | Effect of corona discharge treatment on cellulose prepared from beech wood[END_REF][START_REF] Belgacem | Surface characterization of cellulose fibres by XPS and inverse gas chromatography[END_REF][START_REF] Kogelschatz | Dielectric-barrier discharges: their history, discharge physics, and industrial applications[END_REF][START_REF] Klarhöfer | Investigation of pure and plasma treated spruce with surface analytical techniques[END_REF]) and thereby promotes wetting [START_REF] Topala | Dynamics of the wetting process on dielectric barrier discharge (DBD)-treated wood surfaces[END_REF][START_REF] Odrásková | Plasma activation of wood surface by diffuse coplanar surface barrier discharge[END_REF][START_REF] Lecoq | Pinus pinaster surface treatment realized in spatial and temporal afterglow DBD conditions[END_REF]. If plasma treatment positively affects water penetration into the wood by increasing its surface polarity, accelerated curing of PVAc adhesive and a faster increase in bond strength could result.

Figure 1 shows the results for time-dependent shear bond strength tests. Both untreated and plasma-treated veneers showed a continuous increase in shear bond strength with curing time, indicating the advancing setting process of the PVAcadhesive by the permanent loss of water [START_REF] Zeppenfeld | Klebstoffe in der Holz-und Möbelindustrie[END_REF]. For each curing time tested, all plasma-treated veneers had significantly higher shear bond strength than the untreated veneers, demonstrating accelerated curing of the PVAc adhesive after plasma treatment, which is mainly attributed to faster water penetration into the wood.

Similar results were obtained regarding the fracture behaviour. In Figure 1, the percentage of specimens exhibiting wood fracture is also reported. In all cases the first occurrence of wood fracture was shifted towards shorter curing times for plasma-treated specimens.

As previously stated, shorter adhesive curing times would decrease processing times and increase production throughput and productivity in the plywood industry. Furthermore, from the results presented here it can be hypothesised that plasma pre-treatment might show similar beneficial effects using other waterbased systems in applications such as bonding, painting and impregnation. Thus, the plasma technique could be a useful tool in industrial applications.

In general, the untreated veneers needed approximately double the curing time to reach the same shear bond strength as the plasma-treated veneers.

Conclusion

The results demonstrate that plasma treatment of wood veneers can considerably improve the curing characteristics of a PVAc adhesive. Curing times could be approximately halved using the plasma technique, which is mainly ascribed to faster water penetration into the wood. More research is needed to investigate the effect of plasma treatment on other water-based systems such as adhesives, coatings and impregnation agents and to evaluate its suitability in associated applications. Abb. 1 Zeitabhängige Schertests unbehandelter und plasmabehandelter Ahorn-, Eichen-, Buchenund Teak-Furniere. Die Zahlen am Fuß der Säulen zeigen den prozentualen Anteil an Proben, die Holzbruch aufwiesen.

Fig. 1

 1 Fig. 1 Time-dependent shear bond strength of untreated and plasma-treated maple, oak, beech and teak veneers. Values at the bottom of the bars give the percentage of test specimens showing wood fracture.

  

Acknowledgement

Financial support from the Federal Ministry of Education and Research (BMBF, 1708X08) is gratefully acknowledged.