Laser Surface Modification and Adhesion
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Kashmiri Lal Mittal was employed by the IBM Corporation from 1972 through 1993. Currently, he is teaching and consulting worldwide in the broad areas of adhesion as well as surface cleaning. He has received numerous awards and honors including the title of doctor honoris causa from Maria Curie-Sklodowska University, Lublin, Poland. He is the editor of more than 110 volumes dealing with adhesion measurement, adhesion of polymeric coatings, polymer surfaces, adhesive joints, adhesion promoters, thin films, polyimides, surface modification, surface cleaning, and surfactants. Dr. Mittal is also the Founding Editor of the journal Reviews of Adhesion and Adhesives.
Thomas Bahners is a scientist at the German Textile Research Center at Krefeld, Germany.
Content
Preface xv Part 1: Laser Surface Treatment/Modification to Enhance Adhesion 1 Nd:YAG Laser Surface Treatment of Various Materials to Enhance Adhesion 3 A. Buchman, M. Rotel and H. Dodiuk-Kenig 1.1 Introduction 4 1.2 Methodology 13 1.3 Experimental 13 1.4 Results 17 1.5 Conclusions 49 References 51 2 Effects of Excimer Laser Treatment on Self-Adhesion Strength of Some Commodity (PS, PP) and Engineering (ABS) Plastics 55 Erol Sancaktar, Hui Lu and Nongnard Sunthonpagasit 2.1 Introduction 56 2.2 Background and Literature Survey 56 2.3 Ultrasonic Welding of Thermoplastics 65 2.4 Experimental Procedures 71 2.5 Results and Discussion 74 2.6 Summary and Conclusions 94 References 97 3 Laser Surface Pre-Treatment of Carbon Fiber-Reinforced Plastics (CFRPs) for Adhesive Bonding 103 F. Fischer, S. Kreling and K. Dilger 3.1 Introduction 103 3.2 State-of-Research 105 3.3 Materials and Methods 110 3.4 Laser Sources and Principles 112 3.5 Results 121 3.6 Summary 134 References 135 4 Laser Surface Modification of Fibers for Improving Fiber/Resin Interfacial Interactions in Composites 139 Anil N. Netravali 4.1 Introduction 140 4.2 Excimer Laser Treatment of UHMWPE Fibers 143 4.3 Excimer Laser Treatment of Vectran Fibers 154 4.4 Excimer Laser Treatment of Aramid Fibers 159 4.5 Excimer Laser Treatment of Cellulose Fibers 160 4.6 Summary 161 References 162 5 Laser Surface Modification in Dentistry: Effect on the Adhesion of Restorative Materials 167 Regina Guenka Palma-Dibb, Juliana Jendiroba Faraoni-Romano and Walter Raucci-Neto 5.1 Introduction 167 5.2 Dental Structures 173 5.3 Adhesion of Restorative Materials 180 5.4 Laser Light Interaction with the Dental Substrate 186 5.5 Dental Structure Ablation and Influence on Bond Strength of Restorative Materials 190 5.6 Summary and Prospects 196 References 196 Part 2: Other Effects/Applications of Laser Surface Treatment 6 Fundamentals of Laser-Polymer Interactions and their Relevance to Polymer Metallization 205 Piotr Rytlewski 6.1 Introduction 205 6.2 Impact of Laser Radiation on a Polymeric Material 208 6.3 Laser Selection Criteria 215 6.4 Surface Modification of Polymeric Materials Below Ablation Threshold 220 6.5 Surface Modification of Polymeric Materials Above Ablation Threshold 233 6.6 Application of Lasers to Polymer Metallization 241 6.7 Summary 251 Acknowledgement 252 References 252 7 Laser Patterning of Silanized Carbon/Polymer Bipolar Plates with Tailored Wettability for Fuel Cell Applications 263 Martin Schade, Steffen Franzka, Anja Schr"ter, Franco Cappuccio, Volker Peinecke, Angelika Heinzel and Nils Hartmann 7.1 Introduction 264 7.2 Silane-based Coatings 269 7.3 Laser Processing of Silane-based Coatings 271 7.4 Fabrication and Plasma Activation of Bipolar Plates 272 7.5 Silanization of Bipolar Plates 276 7.6 Laser Processing of Bipolar Plates 278 7.7 Summary 282 7.8 Prospects 283 Acknowledgments 283 References 284 8 Predominant and Generic Parameters Governing the Wettability Characteristics of Selected Laser-modified Engineering Materials 289 Jonathan Lawrence, David Waugh and Hao Liang 8.1 Introduction 290 8.2 Modification of Wettability Characteristics Using Laser Beams 291 8.3 Laser Wettability Characteristics Modification of Selected Ceramics 296 8.4 Laser Wettability Characteristics Modification of Selected Metals 307 8.5 Laser Wettability Characteristics Modification of a Selected Polymer 316 8.6 Summary and Conclusions 329 References 331 9 Laser Surface Engineering of Polymeric Materials and the Effects on Wettability Characteristics 337 D.G. Waugh, D. Avdic, K.J. Woodham and J. Lawrence 9.1 Introduction 337 9.2 Wettability Characteristics 338 9.3 State-of-the-Art Surface Engineering Techniques 345 9.4 Summary 366 References 367 10 Water Adhesion to Laser-Treated Surfaces 377 Athanasios Milionis, Despina Fragouli, Ilker S. Bayer and Athanassia Athanassiou 10.1 Introduction 377 10.2 Materials, Fabrication Approaches and Results 381 10.3 Applications 395 10.4 Prospects 404 10.5 Summary 406 Acknowledgement 406 References 407
Chapter 2
Effects of Excimer Laser Treatment on Self-Adhesion Strength of Some Commodity (PS, PP) and Engineering (ABS) Plastics
Erol Sancaktar*, Hui Lu and Nongnard Sunthonpagasit
Department of Polymer Engineering, University of Akron, Akron, Ohio, USA
*Corresponding author: erol@uakron.edu
Abstract
This Chapter presents the effects of KrF excimer laser irradiation on the self-adhesion (weld) strength of commodity (polypropylene (PP), polystyrene (PS)), and engineering (acrylonitrile butadiene styrene (ABS)) thermoplastics. after laser irradiation, the polymer samples were welded using the ultrasonic welding method. The tensile stress-strain behaviors of the welded samples were obtained subsequently. It was found that laser irradiation increased the weld strength of ultrasonically welded samples. Increases in the weld strength values, in comparison to the untreated weld samples, were as high as 810% for PS. For ABS, the maximum increase in weld strength for increases in pulse frequency was 350%. The maximum increases in weld strength corresponding to increases in pulse energy and pulse number were 400% and 460%, respectively. For PS, the maximum increase in weld strength for increases in pulse frequency was 690%. The maximum increases in weld strength corresponding to increases in pulse energy and the pulse number were 690% and 810%, respectively. For PP, the increase in weld strength was as high as 191% due to laser treatment.
Keywords: Excimer laser surface treatment, ultrasonic welding, weld strength, polypropylene, polystyrene, acrylonitrile butadiene styrene
2.1 Introduction
Due to their high speed and precision, excimer lasers find applications in semiconductor processing, optical communications and medical fields, in which polymers have extensive applications. Furthermore, it has been shown that excimer lasers can be used to enhance adhesion strength [1], as well as in evaluating effects of process conditions such as injection molding [2] and nanoclay exfoliation [3]. In addition to desirable electrical and optical properties, polymers also have high strength to weight ratio, good corrosion resistance, and low processing cost. As an important joining method for plastics, ultrasonic welding plays an important role in processing of thermoplastics because it is easily automated, and it is the most rapid way to weld thermoplastics with low costs. In this Chapter, we present results utilizing ultrasonic welding to assess any improvements we may have in self-adhesion behavior of PS, PP and ABS polymers as induced by excimer laser treatment of their surfaces to be welded.
Successful welding often requires a suitable surface treatment of material prior to bonding. Selection and application of appropriate surface treatments are major factors in good weldability and durability. There is a wide range of surface treatments available for removing contaminants and weak boundary layers from polymer surfaces. These methods include mechanical methods, such as abrasion, grit and shot blasting, as well as chemical methods such as solvent degreasing, acid etching, and compatibilization by adhesion promoters in the form of primers. The disadvantages of these methods include environmental hazards due to undesirable emission, and poor controllability of surface finish.
A relatively recent technique for altering the surface properties of polymers is the use of excimer lasers [4]. Therefore, the relationships between weld strength and the parameters of excimer laser used to treat the welding surfaces are especially important.
In this Chapter, the influences of pulsed UV laser parameters on the self-adhesion (weld) properties (weld strength and break strain) of commodity polymers polystyrene (PS) and polypropylene (PP), as well as an engineering polymer, ABS (acrylonitrile butadiene styrene), are described. The laser parameters studied include pulse number, frequency, and energy.
2.2 Background and Literature Survey
2.2.1 Excimer Laser Surface Treatment
2.2.1.1 Overview of Excimer Laser Processing
The term laser is an acronym for “Light Amplification by the Stimulated Emission of Radiation”. Helium or neon buffer gas containing a halogen atom and rare gas binary complex produces the lasing plasma under high voltage in an excimer laser [5]. Several complexes which produce characteristic emission wavelengths [6] are commonly used. Recombination of electrons and ionized rare-gas ions in the plasma yields electronically excited rare gas atoms. These atoms react with halogen atoms and produce excited molecules, which relax to their ground states by emitting a ultraviolet (UV) photon.
The pulse durations for excimer lasers are in the nano-to pico-second range, thus providing the capacity to deliver high peak power output at several UV wavelengths. This makes them attractive for a wide range of applications such as micro-machining, surface modification including surface treatment for adhesion [1], corneal sculpting and marking, and process evaluation [2, 3, 7].
By subjecting the polymer surface to UV-laser light in atmospheric environment, some part of its chemical structure in its hydrocarbon group (CHx) chain can be altered to form intensely polar groups, such as carbonyl (-C=O) and hydroxyl- (-OH). The presence of these polar groups on the surface can enhance adhesion. High energy flux can also cause instant fragmentation of polymeric chains without any oxidation [8–11]. The increased surface roughness produced in this manner (ablation) can serve to enhance adhesion [8, 12–15]. Enhancement of polymer surface conductivity has also been reported as a result of laser irradiation [8, 16–18].
Thermal-oxidation and photo-oxidation generally cause main chain scission and crosslinking. The presence of oxygen typically propagates thermal oxidation [19]. Both thermal and/or photo-oxidation can also be initiated in the presence of free radicals (R•) formed by the thermolysis and/or photolysis of impurities, additives or photoinitiators. Photon absorption can initiate stepwise degradation of macromolecules. Typically, photo-oxidation and thermal-oxidation involve reactions between the polymer, polymer alkyl radical (P•), polymer oxy radical (polymer alkoxy radical), polymer peroxy radical (polymer alkylperoxy radical), polymer hydroperoxide (POOH), and the hydroxy radical. Hydroxy (OH) and hydroperoxy (OOH) groups are formed in reactions between polymer oxy radicals (PO•) and polymer peroxy radicals (POO•) with the same and/or neighboring polymer molecule (PH), respectively. Both groups can be formed along the polymer chain, or its ends.
Surface modification by laser irradiation can be carried out in variety of ways depending on the purpose of the surface modification (etching, ablation, deposition, evaporation, surface functionalization, etc.), the type of the laser used, the ambient conditions, and the materials to be treated. The desired level of surface modification can be achieved by the choice of an appropriate type of laser, by considering the optical and thermal properties of the material to be treated, matched with the wavelength, pulse energy, and pulse frequency of the laser to be used.
2.2.1.2 Mechanism of Thermal-oxidation by Laser Irradiation
The theory originally developed by Bolland and Hughes [20] to explain the thermal oxidation of olefins and rubber can also be applied to explain thermal oxidation of other polymers.
For ABS and PS, hydroperoxide and acetophenone groups have been identified on the polymer chain as a result of thermal oxidation. The volatile products are phenol, benzaldehyde and acetophenone. The major physical change due to thermal oxidation of PS is chain scission. Sequences of neighboring hydroperoxide groups are formed through intermolecular hydrogen abstraction. The main chain scissions observed on thermal oxidation of PS are generally attributed to the decomposition of tertiary alkoxy radicals [21]. The chemical structure of excimer laser irradiated PP shows that carbonyl (-C=O) or hydroxyl (-OH) groups are formed. It is known that the presence of these strongly polar groups on a polymer surface can improve the adhesion properties [13].
2.2.1.3 Mechanism of Photo-oxidation by Laser Irradiation
Incident light will either be reflected from the surface or scattered or absorbed in the bulk of the polymer. The absorption of light by polymers is related to their structure. For example, saturated hydrocarbons do not absorb above 250 nm, but if double bonds (chromophores) are present, longer wavelength laser light can be absorbed [22].
Laser light above 290 nm wavelength may also degrade polymers such as polyolefins, which do not contain chromophores in their repeat units. This is due to structural irregularities of polymers or is caused by traces of impurities left over from manufacturing e.g. catalyst residues or oxidation products. The first may absorb in the UV range, leading to photochemical transformation. Moreover, in semicrystalline polymers, scattering of light by the crystallites likely increases its path in comparison to amorphous materials, causing semicrystalline polymers to absorb rather high quantities of energy at low concentrations of chromophoric groups.
Subsequent to photo-absorption, the chromophores are raised to excited states at higher energy levels. This excitation energy is dissipated by several processes such as fluorescence, phosphorescence, and radiation decay. Energy can also transfer from the excited state to a suitable acceptor molecule.
In the presence of oxygen, polymers which simultaneously...
