Plasma modification
The purpose of wood plasma treatment is to enhance its surface wettability and the penetration of finishing agents into the wood, to improve adhesive bonding or to achieve a hydrophobic coating from suitable precursors. Two main factors drove the current interest in plasma activation of wood surfaces:
- a demand to replace organic solvent-based formulas for indoor wood appliances with water-based ones;
- availability of new types of plasma generators, allowing surface treatment even for thick pieces of wood.
The most common types of plasma generators are used for wood modification.
- Volume dielectric barrier discharge (VDBD) is widely employed in the printing industry, where it is used for surface activation of polymer foils and paper. It is sometimes nicknamed as the “industrial corona” there. In a typical arrangement of VDBD, the treated material is fed into the discharge gap formed by two plane parallel or coaxial cylindrical electrodes, at least one of which is covered by appropriate dielectric insulation (glass or ceramics). At sufficiently high electric field strength (~40 kV/cm for dry air) created by alternating, an electrical breakdown of gas occurs between the insulated electrodes and tiny (0.1–0.2 mm in diameter) short-lasting (~10-30 ns) plasma filaments are formed. Due to the presence of dielectrics, an alternating high voltage has to be supplied to the VDBD reactor. When the plasma filament hits the wood material, it propagates along its surface, forming a characteristic circular footprint. The footprint defines the plasma-modified region of the wood surface. The drawback of VDBD lies in the excessively high voltage needed when used to treat thick wood samples (more than 1 cm thick).
- Surface dielectric barrier discharge (SDBD) benefits from enhancing the tangential component of the applied electric field caused by aptly positioned power feed electrodes. As a result, discharge plasma filaments are forced to propagate along the dielectric surface. Such an open-type geometry of surface DBDs allows unobstructed access to the layer of generated plasma. It makes it possible to treat thick, panel-like wood materials without any restrictions to their actual thickness. Since the filaments are confined in a rather narrow space above the dielectric surface (0.2-0.3 mm), a high power density of the generated plasma can be achieved. This contributes to generally shorter treatment times needed compared to VDBD. At the same time, SDBD suffers from poor treatment quality when used to treat samples with surface roughness larger than 0.2 mm.
- Non-thermal plasma jets: Similar to SDBD, nonthermal plasma jet treatment is not constrained by the thickness of the wood material. Typically, an abnormal glow or arc plasma is generated inside a hollow tube, and an intense flow of working gas is used to transport reactive species in the form of a luminous plume toward the treated material. The hot ionised gas that leaves the space of intense ionisation is cooled down by mixing with ambient air. This cooling allows control of the thermal load delivered to the treated material. Plasma jets can be easily employed to also activate complex 3-D objects. Nevertheless, the area of treated surface offered by most of the plasma jet systems is limited to a few square millimetres due to the small size of exhaust plume. The problem can be addressed either by rapid scanning of plasma jet over the surface, by assembling multiple jets into 2-D arrays, or by adopting a more complex electrode design (such as a narrow slit to form a linear plasma jet source) in combination with a rare operation gas.
- Low-Pressure Plasma Systems generate plasma in a vacuum environment, unlike previous types of plasma generators. At low pressure, it is substantially easier to fill the whole reactor chamber with uniform, low-temperature plasma. It allows for more straightforward control over the chemical kinetics and safer handling of toxic or flammable chemical precursors in the working gas. Low-pressure plasma wood modifications are mainly aimed at depositing water-repellent (hydrophobic) coatings. Less work is dedicated to improving wood wettability because high-pressure plasma generators are economically more favourable. For hydrophobic coatings, treatment times of tens of minutes are needed. This may seem like a lot compared to times of standard liquid solvent-based spray application. But plasma-deposited films do not require any additional drying steps, unlike the solvent-based chemical coating. The principal shortcoming that hampers the large-scale use of low-pressure plasma for wood processing treatment is the necessity to expose wood to a vacuum of less than 1 mbar. Reaching a required vacuum level in a technologically feasible time requires a powerful vacuum pumping system. In addition, when the wood dries below the fibre’s saturation point, shrinkage or even warping can be expected. Dimensional changes will be more pronounced for large pieces of wood.
Practical Application of Plasma Wood Modifications
For practical use of wood surface plasma activation, one has always bear in mind two main constraints: the generally low cost of wood products and the large surface/volume of wood that has to be processed in the shortest time possible. These constraints make plasma treatments done at atmospheric pressure a preferable choice. The list of verified plasma-mediated wood modifications includes:
- Wettability Improvement: Plasma operating in an oxidising atmosphere such as air efficiently introduces polar functional groups to the wood surface. These are consequently responsible for the improved surface wettability. Plasma treatment in an inert gas (Ar, N2, He) can deliver a similar increase in surface wettability due to secondary processes involving adsorbed humidity and ambient oxygen-containing gas on the wood surface.
For the actual evaluation of wettability improvement, two main techniques are used: (1) a sessile liquid droplet method, based on measuring the contact angle of a small testing liquid droplet (1–2 μl) of known free surface energy laid on the wood surface; (2) a water uptake time, based on measuring the time interval needed for complete penetration of larger droplet (eg, 50 μl) into the wood surface.
- Adhesion Improvement: The functional chemical groups introduced by plasma can also participate in ongoing chemical reactions to give rise to paints or glues adhesion improvement. Consequently, new material combinations, elimination of primers, and cost savings are conceivable benefits. An essential advantage of plasma activation toward the adhesion strength is its ability to achieve substantial surface modification without creating a so-called weak boundary layer (WBL), as is frequently the case when standard chemical or mechanical pretreatment is used. WBL contributes to a cohesion failure in a layer between the bulk material and an adhesive-adherent interface. The adhesion to a diverse spectrum of wood-based materials can be improved, including wood particles, fibres, or strands.
- Hydrophobic and Superhydrophobic Coating: To protect the wood from swelling or even biological degradation, water-repellent or hydrophobic surface finishing of the wood is often carried out. In this respect, plasma activation can be of use in two ways: either as a pretreatment to enhance the adhesion of the liquid phase chemical formula (varnish, paint) or by depositing the hydrophobic film through a plasma enhanced chemical vapour deposition (PECVD) process. For PECVD, low-pressure plasma systems are usually better suited. Precursors from organosilicon compounds [hexamethyldisiloxane (HMDSO)], fluorine-containing compounds (CF4, C2F2, SF6), or simple unsaturated hydrocarbons (ethylene, acetylene) are most commonly used to create the hydrophobic surface.
- Sterilisation: Plasma sterilisation carries several advantages compared to standard sterilisation processes. Unlike other types of sterilisation, which rely solely on the effect of heat, ultraviolet radiation, and biocide substances, plasma sterilisation offers synergistic effects to reactive particle chemistry, ultraviolet radiation, heat, and electric field. Plasma sterilisation of wood surfaces can be intentional (e.g., for preserving wooden historical artefacts) or unintentional, but it is a welcomed side-benefit of plasma activation during wood processing. A plasma-mediated reduction in the number of viable microorganisms colonising the wood structure before applying the coating system can lead to avoidance and slowdown of the biological degradation by the growth of microorganism colonies (fungi and moulds) underneath the paint film.
General steps involved in plasma wood modification
- Evaluate the surface wettability and the adhesive strength of the pristine wood sample.
- Determine the appropriate method of plasma treatments. This should usually accommodate the constraints given by the size and shape of the treated specimen and their variations.
- Survey for optimum plasma treatment conditions: treatment time, energy dose, type of operation gas/gas admixtures, absolute humidity of treatment gas, and moisture of treated wood piece.
- Evaluate the magnitude of the ageing effect of plasma treatment caused by secondary chemical processes initiated by plasma treatment.
- Reach sufficient process control by identifying all possible external factors that may negatively affect the outcome of plasma treatment.