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Application of AFM on Hte Adhesion Studies of Oxygen-Plasma-Treated

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Application of AFM on Hte Adhesion Studies of Oxygen-Plasma-Treated Langmuir 1999, 15, 2985-2992 2985 Application of AFM on the Adhesion Studies of Oxygen-Plasma-Treated Polypropylene and Lignocellulosics R. Mahlberg* VTT Building Technology, Wood Technology, P.O. Box 1806, FIN-02044 VTT, Finland H. E.-M. Niemi Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, FIN-02015 HUT, Finland F. S. Denes Engineering Research Center for Plasma-Aided Manufacturing, University of Wisconsin, Madison, Wisconsin 53706 R. M. Rowell Forest Products Laboratory, USDA Forest Service, One Gifford Pinchot Drive, Madison, Wisconsin 53705-2398 Received February 5, 1998. In Final Form: December 31, 1998 Atomic force microscopy was used to study the surfaces of oxygen-plasma-treated polypropylene (PP), kraft pulp, filter paper, and wood. The effect of plasma treatment on the adhesion properties between PP film and wood was evaluated by means of a peel test. The highest adhesion to wood resulted from the shortest treatment times used. The effects of plasma on the adhesion properties were more pronounced when both the PP film and the wood surface were treated. Oxygen plasma caused changes in the morphology of the materials: the treated PP surface was covered by a nodular structure, which was not seen on the untreated film. A similar nodular structure was also seen on the lignocellulosic materials. These nodules seemed to have poor interaction with the PP surface, resulting in a weak interface between the plastic film and wood. Introduction commercially. The ultimate objective of this study was to In composites made from lignocellulosic and thermo- improve the adhesion between a polypropylene film and plastic materials, incompatibility between the components wood substrate by plasma treatment and to increase our often causes problems in the processing and utilization of knowledge of the fundamental phenomena taking place the composites. The incompatibility problems between the in the modification. As simple model materials for hydrophilic lignocellulosic material and the hydrophobic lignocellulosics, kraft pulp and filter paper (cellulose) were thermoplastics can be overcome by improving the adhesion used to study the effects of the modification. However, the using compatibilizers or coupling agents. In the paper, information related to cellulose may be expanded to wood/ electronics, and textile industries, research has focused thermoplastic composites as well. mainly on modification of surfaces with plasma, corona, Adhesion between two surfaces has been suggested to or flame treatments.1–3 The use of plasma for pretreating be a result of an interaction of two factors: an intimate molecular contact and a maximum attractive force be- polymers in order to enhance their adhesion has been 6,7 known for over 20 years.4,5 Cold plasmas affect the top tween the components. The concept of intimate molec- layers of a surface and do not cause any changes in the ular contact consists of the following theories: adsorption, bulk properties of the material. Plasma treatments are diffusion, interlocking, and weak boundary. The maximum known to form polymer radicals on the treated surfaces, attractive force theories involve the theories of chemical which react with oxygen when exposed to air after bonding, acid-base, electrostatic, and again weak boundary treatment. The polarity of the surfaces increases due to layer. While the adsorption theory is based on the rules the polar groups formed. of spreading and wetting, the diffusion and interlocking A better understanding of the chemical and physical theories explain adhesion through physical contact (en- effects of plasma treatments on cellulosic and synthetic tanglements) and through interpenetration (anchoring), materials is required before the technique can be applied respectively. In general, these theories deal with the fact that two surfaces have to come into sufficient contact with (1) Westerlind, B.; Larsson, A.; Righdal, M. Int. J. Adhesion Adhesives each other so that the premises for attractive forces are 1984, 3, 141. met. The chemical bonding theory explains that the (2) Gerenser, L.; Ehnan, J.; Mason, M.; Pochan, J. Polymer 1985, 26, covalent, hydrogen, van der Waals, ionic, and metallic 1162. (3) Briggs, D.; Kendall, C. R. Int. J. Adhesion Adhesives 1982, 2, 13. bonds are responsible for adhesion. In the electrostatic (4) Hall, J. R.; Westerdahl, C. A. L.; Devine, A. T.; Bodnar, M. J. J. Appl. Polym. Sci. 1969, 13, 2085. (6) Chung, F. H. J. J. Appl. Polym. Sci. 1991, 42, 1319. (5) Kaelble, D. H.; Dynes, P. J.; Cirlin, E. H. J. Adhesion 1974, 6, 23. (7) Marian, J. E.; Stumbo, D. A. Holzforschung 1962, 5, 134. 10.1021/la980139b CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999 2986 Langmuir, Vol. 15, No. 8, 1999 Mahlberg et al. theory, adhesion derives from the electrostatic effects at of many other types of plasma treatments, such as He, Ar, 17,18,20–22 the interface. In the acid-base theory, hydrogen bonding Xe, N2, H2, and NH3. In addition, AFM studies on is the key to adhesion. Instead of explaining why two the effects of other surface modification methods, such as 23,24 materials stick together, the weak boundary theory corona treatments, have shown the capability of AFM explains what makes adhesion fail. Lack of molecular to provide new information on the minor changes, contact and weak intermolecular attraction results in a especially, at the nanometer level for the first time. weak boundary layer which acts as a rupture center and leads to failure in adhesion.6 Experimental Section The present paper focuses on the surface morphology Materials. The nonoriented, two-layered polypropylene- of polypropylene (PP) film and the lignocellulosic materials polyamide film was supplied by Walki Pack Valkeakoski mentioned above. Correlations between the adhesion and (Finland). The polypropylene (PP) layer had a thickness of 50 the morphology of oxygen-plasma-treated PP films and µm and the polyamide (PA) layer 30 µm. The PP film reinforced wood were also investigated. The morphological features with a PA layer was used in order to be able to peel the thin film of the surfaces were studied using atomic force microscopy off the substrate without breaking it. Prior to the plasma (AFM), which is a suitable tool for observing the changes treatments, the plastic films were cut into the shape and size of in morphology on the nanoscale. A standard peel test was the plasma electrode and washed several times with acetone (Soxhlet extractions). used for the adhesion studies. Sliced birch (Betula pendula) and spruce (Picea abies) veneer The AFM measurements were made using the tapping of 0.8-mm nominal thickness (radial direction) were used as the mode developed especially for studying soft and fragile substrates for PP/PA films. The size of the veneer samples used samples. The invention of the tapping mode in atomic for the plasma treatments was 60 mm × 170 mm (tangential × force microscopy has overcome the problems of traditional longitudinal, respectively). Prior to the plasma treatments, the contact mode imaging, such as modifying the surface, veneer surfaces were smoothed by sanding them with a sand friction, adhesion, and electrostatic forces. Instead of paper of 150 grid followed by Soxhlet extraction of the veneer dragging the tip across the surface as in the conventional panels with acetone. After the extractions, the veneer samples contact mode, tapping mode imaging is done by oscillating were oven-dried for 20 h at 105 °C. Filter paper (rich in cellulose; Schleicher & Schuell) and kraft pulp with a number of 123 and the cantilever within a frequency of few hundred kilohertz originating from loblolly pine served as the model materials of near its resonance, using an amplitude normally over 20 wood veneer for the morphology studies. nm. The oscillation and the force on the sample are Plasma Installation. Oxygen plasma treatments were carried maintained constant by a feedback loop. The tip is brought out using a plasma installation from the Engineering Research close to the surface until it begins to touch the surface by Center for Plasma-Aided Manufacturing of the University of tapping it gently, which reduces the oscillation amplitude. Wisconsin (Madison). All plasma treatments of the natural and While scanning the surface, the amplitude alternates synthetic polymeric substrate surfaces were carried out in a depending on the topography, increasing when there is a cylinderical, capacitively coupled, parallel-plate, 50-kHz RF depression and decreasing above a bump. Information is plasma reactor made of stainless steel and provided with collected from the changes in amplitude. No lateral, shear, thermostating possibilities (25–500 °C) of the electrodes. Plasma treatments were preceded by cleaning procedures of the reactor or friction force is applied to the sample and no sticking in order to avoid possible contamination of the substrates from occurs, since the tip contacts the surface briefly during the reactor wall originating from previous experiments. The each oscillation. Therefore, tapping mode imaging has decontamination method involved heating the substrate holder made it possible to study samples that are easily damaged electrode (lower electrode) to 200 °C, introducing oxygen plasma or loosely bound to the substrate. Plasma-modified (230 mTorr; 200 W, 10 min) followed by argon plasma under materials with minor changes fall into this category. similar conditions, and cooling the electrode and promoting the In recent years, the application of AFM to surface plasma reactor to the low-pressure level. morphology studies of polymers has grown rapidly. It has Plasma Treatments. In a typical experiment, after the reactor already been used to study polyethylene,9 polypropylene,10 was decontaminated, the selected two identical substrates were polyaniline,11,12 polypyrrole,13,14 methyl methacrylate,15 positioned on the lower electrode, the chamber was then closed, 16 and a low pressure was created in the system. In the next step, crystallized or polymerized in situ, and nylon films, to oxygen was introduced into the reactor under the preselected mention a few.
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