Laser Surface Modification and Adhesion E-Book

K. L. Mittal

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Adhesion and Adhesives - Fundamental and Applied Aspects
• Sprache: Englisch

Seitenzahl: 599

Veröffentlichungsjahr: 2014

Beschreibung

The book provides a unique overview on laser techniques and applications for the purpose of improving adhesion by altering surface chemistry and topography/morphology of the substrate. It details laser surface modification techniques for a wide range of industrially relevant materials (plastics, metals, ceramics, composites) with the aim to improve and enhance their adhesion to other materials. The joining of different materials is of critical importance in the fabrication of many and varied products.

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Preface

Part 1: Laser Surface Treatment/Modification to Enhance Adhesion

Chapter 1: Nd:YAG Laser Surface Treatment of Various Materials to Enhance Adhesion

1.1 Introduction

1.2 Methodology

1.3 Experimental

1.4 Results

1.5 Conclusions

References

Chapter 2: Effects of Excimer Laser Treatment on Self-Adhesion Strength of Some Commodity (PS, PP) and Engineering (ABS) Plastics

2.1 Introduction

2.2 Background and Literature Survey

2.3 Ultrasonic Welding of Thermoplastics

2.4 Experimental Procedures

2.5 Results and Discussion

2.6 Summary and Conclusions

References

Chapter 3: Laser Surface Pre-Treatment of Carbon Fiber-Reinforced Plastics (CFRPs) for Adhesive Bonding

3.1 Introduction

3.2 State-of-Research

3.3 Materials and Methods

3.4 Laser Sources and Principles

3.5 Results

3.6 Summary

References

Chapter 4: Laser Surface Modification of Fibers for Improving Fiber/Resin Interfacial Interactions in Composites

4.1 Introduction

4.2 Excimer Laser Treatment of UHMWPE Fibers

4.3 Excimer Laser Treatment of Vectran® Fibers

4.4 Excimer Laser Treatment of Aramid Fibers

4.5 Excimer Laser Treatment of Cellulose Fibers

4.6 Summary

References

Chapter 5: Laser Surface Modification in Dentistry: Effect on the Adhesion of Restorative Materials

5.1 Introduction

5.2 Dental Structures

5.3 Adhesion of Restorative Materials

5.4 Laser Light Interaction with the Dental Substrate

5.5 Dental Structure Ablation and Influence on Bond Strength of Restorative Materials

5.6 Summary and Prospects

References

Part 2: Other Effects/Applications of Laser Surface Treatment

Chapter 6: Fundamentals of Laser-Polymer Interactions and their Relevance to Polymer Metallization

6.1 Introduction

6.2 Impact of Laser Radiation on a Polymeric Material

6.3 Laser Selection Criteria

6.4 Surface Modification of Polymeric Materials Below Ablation Threshold

6.5 Surface Modification of Polymeric Materials Above Ablation Threshold

6.6 Application of Lasers to Polymer Metallization

6.7 Summary

Acknowledgement

References

Chapter 7: Laser Patterning of Silanized Carbon/Polymer Bipolar Plates with Tailored Wettability for Fuel Cell Applications

7.1 Introduction

7.2 Silane-based Coatings

7.3 Laser Processing of Silane-based Coatings

7.4 Fabrication and Plasma Activation of Bipolar Plates

7.5 Silanization of Bipolar Plates

7.6 Laser Processing of Bipolar Plates

7.7 Summary

7.8 Prospects

Acknowledgments

References

Chapter 8: Predominant and Generic Parameters Governing the Wettability Characteristics of Selected Laser-modified Engineering Materials

8.1 Introduction

8.2 Modification of Wettability Characteristics Using Laser Beams

8.3 Laser Wettability Characteristics Modification of Selected Ceramics

8.4 Laser Wettability Characteristics Modification of Selected Metals

8.5 Laser Wettability Characteristics Modification of a Selected Polymer

8.6 Summary and Conclusions

References

Chapter 9: Laser Surface Engineering of Polymeric Materials and the Effects on Wettability Characteristics

9.1 Introduction

9.2 Wettability Characteristics

9.3 State-of –the-Art Surface Engineering Techniques

9.4 Summary

References

Chapter 10: Water Adhesion to Laser-Treated Surfaces

10.1 Introduction

10.2 Materials, Fabrication Approaches and Results

10.3 Applications

10.4 Prospects

10.5 Summary

Acknowledgement

References

Index

Laser Surface Modifi cation and Adhesion

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106

Adhesion and Adhesives: Fundamental and Applied Aspects

The topics to be covered include, but not limited to, basic and theoretical aspects of adhesion; modeling of adhesion phenomena; mechanisms of adhesion; surface and interfacial analysis and characterization; unraveling of events at interfaces; characterization of interphases; adhesion of thin films and coatings; adhesion aspects in reinforced composites; formation, characterization and durability of adhesive joints; surface preparation methods; polymer surface modification; biological adhesion; particle adhesion; adhesion of metallized plastics; adhesion of diamond-like films; adhesion promoters; contact angle, wettability and adhesion; superhydrophobicity and superhydrophilicity. With regards to adhesives, the Series will include, but not limited to, green adhesives; novel and high-performance adhesives; and medical adhesive applications.

Series Editor: Dr. K.L. Mittal 1983 Route 52, P.O. Box 1280, Hopewell Junction, NY 12533, USA Email: [email protected]

Publishers at Scrivener Martin Scrivener([email protected]) Phillip Carmical ([email protected])

Copyright © 2015 by Scrivener Publishing LLC. All rights reserved.

Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts. Published simultaneously in Canada.

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Library of Congress Cataloging-in-Publication Data:

ISBN 978-1-118-83163-2

Preface

Surface modification (also known as treatment, pre-treatment and activation) of different materials (metals, ceramics, polymers, composites,) is sine qua non to impart surface characteristics for their applications for a legion of purposes. The beauty of surface modification is that it provides the requisite surface properties without tempering with the bulk, thus retaining the desirable attributes of bulk materials.

By using apropos surface modification process one can attain a host of surface properties (e.g. adhesion, wetting, superhydrophilicity, superhydrophobicity, omniphobicity, anti-fouling, biocompatibility, to name just a few). For example, adhesive bonding is commonly used to bond similar or dissimilar materials or components to make integral parts as it offers certain advantages vis-à-vis mechanical methods of fastening or mating. However, for adhesive bonding purpose proper surface chemistry (functional groups) and surface topography (morphology) are a desideratum. Laser surface modification of a variety of substrate materials has been shown to be effective for such purpose. Particularly, polymers are innately inert (chemically speaking) and thus it becomes imperative to activate/modify polymeric materials to generate appropriate surface characteristics, depending on the application. A plethora of techniques (ranging from wet to dry, vacuum to non-vacuum, simple to sophisticated, inexpensive to sumptuous) exist for surface modification of a variety of polymers, but the laser surface treatment provides a “cool” and “green” technique. Depending on the laser energy, there can be chemical changes (generation of functional groups) or ablation. Even a cursory look at the literature will evince that there is a flurry of research activity in laser surface modification and all signals indicate that this tempo of research and interest in laser surface treatment will continue unabated. More recently, there has been interest in laser surface modification of various reinforcements (fillers, fibers, nanotubes and graphene).

Now coming to this book (containing 10 chapters) it is divided into two parts: Part 1: Laser Surface Treatment/Modification to Enhance Adhesion, and Part 2: Other Effects/Implications of Laser Surface Treatment. The topics covered include: Nd:YAG laser surface treatment of various materials to enhance adhesion; effects of excimer laser treatment on self-adhesion strength of some commodity and engineering plastics; laser surface pre-treatment of carbon fiber-reinforced plastics for adhesive bonding; laser surface modification of fibers for improving fiber-resin interfacial interactions in composites; effect of laser surface modification on the adhesion of dental restorative materials; fundamentals of laser-polymer interactions and their relevance to polymer metallization; laser patterning of carbon/polymer bipolar plates for fuel cell applications; parameters governing the wettability characteristics of laser modified engineering materials; laser surface engineering of polymeric materials and the effects on wettability characteristics; and water adhesion to laser-treated surfaces.

This book represents the cumulative wisdom and contribution of many internationally renowned subject matter experts in the domain of utilization of lasers not only to enhance adhesion but also to achieve other surface characteristics for a host of applications. As new and more effective laser sources become available, new application vistas will emerge.

The book containing a wealth of information on fundamental and applied aspects of laser surface modification provides an easily accessible unified and comprehensive source. To our knowledge this is the first book on such a technologically important topic. With time, this emerging technique for surface modification will mature and will become a part of the surface treatment arsenal.

The book should be of interest to researchers in academia and R&D personnel in a host of industries (e.g., microelectronics, automotive, packaging, adhesive bonding, printing, metallized plastics, aerospace, dentistry, textiles, biomedical) where the success depends on suitable surface modification. Essentially, anyone interested or involved in surface modification (centrally or peripherally) should find this book useful. In our opinion, this concise treatise should serve as a primer for the neophytes and a digest of recent developments for more seasoned researchers. Also we hope this book will serve as a fountainhead for new ideas and novel approaches in the arena of laser surface modification.

Now it is our pleasure to thank those who made this book possible. First and foremost, we are beholden to the authors for their sustained interest, enthusiasm and cooperation and for sharing their knowledge (in the form of chapters) without which this book could not be materialized. Also we very much appreciate the unwavering interest and support of Martin Scrivener (Scrivener Publishing) in this book project and for giving this book a body form.

K.L. Mittal Hopewell Jct., NY, USA e-mail: [email protected]

Thomas Bahners Deutsches Textilforschungszentrum Nord-West gGmbH Krefeld, Germany e-mail: [email protected] July 18, 2014

Part 1

LASER SURFACE TREATMENT/MODIFICATION TO ENHANCE ADHESION

Chapter 1

Nd:YAG Laser Surface Treatment of Various Materials to Enhance Adhesion

A.Buchman1,*, M. Rotel2 and H. Dodiuk-Kenig3

1Rafael Ltd., Haifa, Israel

2Israel Institute of Metals, IIT, Technion City, Haifa, Israel

3Shenkar College of Engineering and Design, Ramat-Gan, Israel

*Corresponding author: [email protected]

Abstract

The quality and quantity of adhesion depends on the ability to apply proper surface treatment to the adherends. Both chemical modification and mechanical interlocking induced by surface treatment affect the strength and durability of the adhesive joint.

Various methods of surface treatment are conventionally used for plastics, metals, composites and ceramic adherends among them are abrasive treatment, blasting, chemical treatments, plasma etching, etc. Hard-to-bond adherends pose a tough problem since surface treatment is usually harsh and the adhesives used to bond these materials are especially tailored and in most cases are exotic and expensive.

Nd:YAG laser irradiation presents a new technology for surface treatment and surface modification of various adherend materials. This technology presents an alternative to the use of ecologically unfriendly chemicals involved in conventional etching and abrasive treatments.

The effect of Nd:YAG laser irradiation on polymers and metals was examined using chemical, physical, mechanical and analytical methods. The effect on adhesional strength and durability was tested on hard-to-bond materials using commercial cheap epoxy adhesive. Experimental results indicated that Nd:YAG laser surface treatment improved significantly the adhesional shear and tensile strengths compared to other conventional treatments.

Optimal Nd:YAG laser treatment parameters (intensity, repetition rate and scan velocity) depended on the substrate material and its chemical nature. The mode of failure changed from interfacial to cohesive as the optimal parameters were used. This change in failure mode is correlated with changes in morphology (uniform roughness) as indicated by Scanning Electron Microscopy (SEM), in chemical modification and removal of contamination as indicated by XPS (X-ray photoelectron spectroscopy), EDX (Energy-dispersive X-ray spectroscopy), decrease in contact angle, and FTIR (Fourier Transform Infra- Red) spectroscopy. Open time exceeded two weeks compared to other surface treatments. All Nd:YAG laser treated surfaces were able to be bonded with a commercial epoxy adhesive. It can be concluded that Nd:YAG laser has a potential as a precise, clean and simple surface modification technique for a large range of materials.

Keywords: Nd:YAG laser, surface treatment, hard-to-bond adherends, adhesive bonding

1.1 Introduction

Bonding high performance materials presents unique challenges to ensure both immediate and long term joint strengths. In addition, more manufacturing materials are becoming available from the polymer composite world, which must be adhered to metals and other substrates.

Adhesive bonding is a technology used to join similar or dissimilar materials in a wide range of applications such as automotive, aerospace, building, packaging, etc. The strength and durability of adhesive joints are affected by various factors such as nature of adhesive, nature of adherend, bondline thickness, contamination, stresses and environmental conditions but mostly by inadequate surface treatment. Adherend materials having very low surface energy such as polyolefins require unique surface treatment and especially formulated adhesives in order to bond them.

1.1.1 Surface Pretreatment for Adhesive Bonding

Surface pretreatment for adhesive bonding is required to attain joint strength as well as joint durability using the various adherends.

Surface treatment removes weak boundary layers, cleans the surface, alters the surface energy (primarily through oxidation), and improves micro- topographical characteristics. The net effect of these changes is enhanced interfacial bonding, mechanical interlocking between adhesive and adherend, and greater resistance to environmental degradation by moisture or humidity.

Different adherends require different pretreatment methods. Plastics are the most difficult adherends to be treated for the following reasons:

1. The number of different types of plastics available is very high and they differ widely in their behavior and characteristics;

2. The temperature dependence of the mechanical properties of plastics and adhesives is much stronger than it is for metals or ceramics;

3. All plastics have a low surface energy. This necessitates a strong pretreatment to make plastics wettable by adhesives;

4. Plastics contain numerous components and these can vary considerably in a single group of plastics and some of them, especially lubricants and plasticizers, hinder adhesion severely. The effect often depends on the migration of’ these components from the bulk to the surface and also on the temperature dependence of their mobility.

1.1.2 Pretreatment Processes – State of the Art

To obtain an optimum strength of adhesive bonds to adherends, it is necessary to increase the surface energy of the substrate by specific pretreatment processes. These processes can be divided into three groups [1, 2]:

1. Mechanical processes

sand-blastingSiC blastinggrinding, brushingabradingpeel ply (composite materials)

2. Chemical processes

CSA (chromic-sulfuric acid) treatment - picklingozone treatmentorganic solvent treatment - etchingcoating with chemically active substances - primingconversion coatings

3. Physico-chemical processes

low-pressure plasmaatmospheric pressure plasmacorona discharge treatmentthermal treatmentflame treatmention etchinglaserUV light.

A proper surface treatment should enable an easy spreading of the adhesive on the adherend promoting molecular interaction and physical adsorption. The roughened morphology of the treated adherend should result in mechanical interlocking to promote bonding. Various techniques are used to enhance surface energy of solid materials by modification of surface topography, and chemical activity by means of chemical or mechanical surface pretreatment. Mechanical treatment such as blasting or grinding provides a higher surface area which enables interlocking. This treatment enhances joint strength but induces inhomogeneity of the treated surface and thus the joint durability is low. An alternative chemical surface treatment such as etching or anodizing is used. Such treatment activates the surface and induces a morphology (fibrous or porous) which is homogeneous. However, both mechanical and chemical pretreatments are hazardous to human safety and to the environment. An ecologically attractive alternative to abrasive and chemical treatments is the use of lasers or plasma treatment. Plasma treatment has two disadvantages: very short open time (a few hours) which means that bonding should be performed immediately after treatment and the need for high vacuum and special gasses for performing the treatment. This also limits the size of the treated components which need to fit the size of the chamber. Atmospheric pressure plasma treatment has no limitation on chamber size, but the plasma deposited ceramic layer on the adherend is weak and peels off [2, 3]. Laser treatment has been successfully performed for more than 20 years using pulsed excimer lasers XeCl, ArF, KrF. Excimer lasers have been used on various adherends for adhesive bonding and remarkably improved adhesion strength has been obtained. The substrates tested were aluminum alloys, alumina, copper, various polymers, composites, titanium alloys, polyethylene fibers, Kevlar fibers, etc. The results showed that the improved adhesion strength was associated with improved cleaning of the surface from contaminants, removal of weak boundary layers, induced cross-linking, improved wetting, and creation of a rough and extremely uniform morphology which improved interlocking. Laser treatment did not affect the bulk of the adherend nor altered its properties.

The problems using excimer lasers is the emission of dangerous gasses to the environment and the high cost of the equipment and maintenance due to fluorine and chlorine attack. Optimal laser treatment needs selection of appropriate process parameters for each adherend material (number of pulses, intensity, wavelength, time of irradiation, and the degree of overlapping of the irradiated areas).

Solid state Nd:YAG laser has been used in this current research as an environmentally friendly alternative for pretreatment of various surfaces for adhesive bonding.

1.1.3 Solid State Nd:YAG Laser

Nd:YAG (neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12) is a crystal that is used as a lasing medium for solid-state lasers. Nd:YAG lasers typically emit light with a wavelength of 1064 nm, in the infrared. However, by using crystals of optically non-linear materials, the high-intensity pulses may be efficiently frequency- doubled to generate laser light at 532 nm, or at higher harmonics at 355 and 266 nm.

Nd:YAG lasers operate in both pulsed and continuous modes. Pulsed Nd:YAG lasers are typically operated in the Q-switching mode. Nd:YAG lasers are used in medical treatment, in engraving, etching, or marking a variety of metals and plastics. They are extensively used in cutting and welding of steel, semiconductors and various alloys for automotive applications. In aerospace applications, they are used to drill cooling holes for enhanced air flow/heat exhaust efficiency. The use of Nd:YAG lasers for surface treatment is a relatively new area of research. The main mechanism of Nd:YAG laser is based on ablation. Ablation occurs when material is ejected due to photon absorbed by the surface and this is accompanied by emission of a dense plume of gas. The intensity of the plume depends on compressibility and elasticity of the material. This plume causes removal of material from the surface, and emission of particles from the substrate. At a certain intensity threshold only absorption occurs. After removal of a few atomic layers a plume is created which behaves like a gas or plasma. Both laser wavelength and pulse duration affect the surface morphology and composition of the material.

In previous research [2] we investigated the application of ArF excimer laser irradiation for surface pretreatment of polycarbonate, polyetherimide, poly (ether–ether– ketone)/carbon fibers (PEEK/C) composite, fiberglass, aluminum, copper and fused silica. The various substrates were subjected to excimer laser irradiation using various parameters, such as: intensity, repetition rate, and number of pulses. The optimal laser treatment parameters were specific for each material for achieving maximum strength of the corresponding bonded joints. Experimental results indicated that UV laser surface treatment improved more significantly the adhesion strength compared to conventionally treated adherends. The improved adhesion resulted from roughening of the irradiated surface, chemical modification, and removal of contamination.

The use of ArF excimer laser involves relatively large amounts of poisonous fluorine gas, and produces ozone which is released to the atmosphere during surface treatment. In order to apply a more ecologically- friendly treatment we decided to investigate the feasibility of application of Nd:YAG laser surface treatment at higher wavelengths.

A group of industrial companies and university institutes [4] investigated the pretreatment of aluminum, using Nd:YAG laser to avoid the disadvantages of the current method of chromate conversion coating. Their results showed creation of an aluminum surface with good bonding performance and aging resistance following laser treatment. They noted that the newly developed laser pretreatment for adhesive application could be industrialized through the use of robotics. The researchers also showed that the main reason for the aging stability, especially for polyurethane adhesives, was the ablation of weak natural oxides and structuring of the surface for keying effect and introduction of a new oxide layer.

Rechner et al. [5] examined the Nd:YAG laser pretreatment (1064nm) of wrought aluminum alloy AW 6061 and compared with atmospheric pressure plasma pretreatment and the surface preparation based on wet-chemical deposition of a TiZr layer. Their results showed that laser pre-treatment cleaned the surface and modified the oxide layer simultaneously. These improvements resulted in higher tensile shear strength of the bonded joint before and after aging in salt –fog chamber for up to 2000h.

Alfano et al. [6] presented a preliminary investigation on the strength of Al/Mg (AA6082/AZ31B) single-lap shear joints bonded with epoxy after application of pulsed Nd:YAG laser at 1064 nm. The experimental results demonstrated the benefits of the laser treatment, in terms of both failure strength and maximum elongation at joint failure. However, the improvement depended on the type of epoxy resin used for bonding. Their results suggested that further research was necessary for the optimization of the laser process parameters. Alfano et al. [7] demonstrated that enhancement in bond toughness of laser treated Al/epoxy joints was up to 400% than that found for grit-blasted substrates.

Laser surface treatments of polymers and especially hard-to-bond polypropylene have been investigated for many years, but no published research was found using Nd:YAG laser at visible or higher wavelengths[8–11]. Most of the published results deal with irradiation with a pulse of UV light, emitted by an excimer laser, that induces chemical and physical changes on the polymer surface as well as removal of surface layers by ablation. Changes in surface morphology, roughness, surface chemistry and wettability of all polymers have been characterized after irradiation at 157nm [6]. Charbonnier and Romand [9] and other groups investigated the effect of laser pretreatment with excimer laser (at 193 and 248nm) on the enhancement of adhesion of metals deposited by the electroless process. Only one group [10] has presented some results concerning the laser-induced photochemical enhancement of adhesive bond strength between polypropylene (PP) and resin based adhesive. Under certain conditions bond strength enhancement of more than 5 times was achieved by applying pulsed excimer laser radiation with wavelengths of 248 and 308 nm and pulse duration of 30 ns.

Polyimides have broad applications ranging from aerospace to microelectronics, optoelectronics, composites, medical devices and fiber optics, due to their excellent mechanical and electrical properties, and their high thermal resistance [12, 13]. However, these applications of polyimides are limited due to their hydrophobic surface character which results in poor wettability and adhesion. Surface modification of polyimides using UV pulsed laser (both excimer and Nd:YAG at wavelengths of 266nm and 355nm) has been investigated by many researchers but no publications on this topic using Nd:YAG laser for adhesive bonding has been found except [13].

Periodic structures of sub-half-micrometer width were produced on the surface of different polymers: Poly(ethylene terephthalate), polyimide Kapton (DuPont), and polyimide- (Ciba-Geigy) by Nd:YAG laser irradiation at 266 nm wavelength [14]. Laser ablation of Upilex-S polyimide (UBE America Inc., USA) 80 μm thick was performed using a 355 nm pulsed Nd:YAG laser [15, 16]. Nanoparticles redeposited on the ablated zone. XPS results indicated that laser irradiation expelled O and N atoms, and that the nanoparticles formed in the ablated zone consisted mainly of carbon clusters. The authors suggested that these effects were probably due to the oxidation and formation of carbonyl groups. Based on their results, they concluded that the decomposition of Upilex-S polyimide irradiated by UV light occurs via a photothermal ablation process. They also observed [16] that the changes in the chemical characteristics and composition of the ablated area were found to be markedly dependent on the repetition rate. Increase in the laser repetition rate resulted in increase of the relative carbon content in the ablated area, and reduction of nitrogen and oxygen contents. After being irradiated by the laser, a new component was detected at 287.4 eV, assigned to an amide structure, as a result of breakage of the imide ring. The peak area of the C–C group also increased, while the peak areas of C-O and the amide groups decreased with increase in the repetition rate. These results are attributed to both the cumulative heat and the increase in the input energy.

In contrast, Balogh et al. [17] described the 355 nm laser ablation of polyimide as a thermal process. The experimental results presented in their paper show that the photochemical models used for excimer lasers are not applicable to the Gaussian Nd: YAG laser but a simple thermal model describes the ablation process. The simulation and experimental results presented in this paper are in qualitative agreement.

Various publications present the application of Nd:YAG pulsed laser for selective metallization of polyimides. Hanada, et al. [18] reported on micromachining of polyimide (PI) by laser-induced plasma-assisted ablation (LIPAA) using Q-switched Nd:YAG laser (1064 nm). After the LIPAA process, selective metallization of PI with excellent electrical properties was performed by subsequent electroless Cu plating. Cu line width of 40 μm, which agrees with the line width of regions ablated by the LIPAA process, is achieved using an encapsulating film. Chen et al. [19] reported that Cu was selectively deposited on PI surface catalyzed by laser-induced deposition of Ag particles. First, PI film was ablated by a focused Nd:YAG laser (266 nm) for patterning, and then the ablated film was immersed in a silver diamine solution. Ag(NH3)+2 ions were reduced to Ag in the ablated region on the film, and the Ag particles were deposited in situ. After rinsing the film, copper was successfully deposited on the seeded film by electroless plating. No articles were found on preadhesion surface treatment of PI by Nd:YAG laser.

Silicone rubber is an elastomer composed of polymer of silicon with carbon, hydrogen and oxygen. There are many kinds of commercial silicone rubbers differing in their formulations and fillers used. Literature review on laser surface treatment of silicone rubber showed that laser surface treatment was investigated only on poly(dimethylsiloxane)(PDMS) [20–32]. PDMS chemical formula is CH3[Si(CH3)2O]nSi(CH3)3, where n is the number of repeat monomer [SiO(CH3)2] units.

Although surface treatments with CO2 laser, Nd:YAG laser and excimer laser have been investigated, no one has examined the laser surface modification as a prebonding treatment.

CO2 pulsed laser surface treatment of PDMS has been carried out in order to alter the surface properties for biological applications [20–25]. The results showed that the surface had been modified by the laser treatment, and the effect depended on the number of pulses. The surface modification included increase in roughness, in O/Si ratio, and in contact angle with water (from about 105° to 175° - increased hydrophobicity). The laser treatment also produced a surface that was capable of initiating graft polymerization that reduced the contact angle (hydrophilic). The laser modified surface reduced the adherence of cells and no spreading or growth was observed in comparison to unmodified PDMS. Different surfaces with different wettabilities were prepared by grafting different polymers in order to alter blood compatibility.

Yasuda [26] demonstrated a method for fabrication of selective patterns of metal particles on self-restoring MEMS (Micro Electro Mechanical System) by surface treatment with CO2 laser. By increasing the output of CO2 laser irradiation on the surface of the silicone rubber, differently textured structures were obtained. In particular, very high hydrophilic character was observed in the case of an output power of 720 mW using a scanning CO2 laser process. The self-restoring silicone sheet exhibited hydrophobic (θH2O ≥ 100°) property at medium laser power condition, while showing a steep transition to hydrophilic (16°–40°) property at high laser power condition. The abrupt change in surface property from hydrophobic to hydrophilic coincides with the morphology.

Dupas-Bruzek and coworkers [27, 28] compared laser induced surface modification of medical grade silicone rubber, poly(dimethylsiloxane) (PDMS) by excimer laser at 248nm and Nd:YAG laser at 266nm, from the viewpoint of metallization process on the modified surface. Both treatments showed cauliflower-like surface structure and formation of nano-crystalline silicon. Carbon was found at 248nm but not at 266nm. The type of laser used as well as the laser irradiation conditions had a strong influence on the nucleation process and growth rate of platinum and on the DC resistance of Pt tracks. DC resistance was lower when tracks were irradiated using an excimer laser at 248 nm compared to Nd:YAG laser at 266 nm and when the pulse number was 30 or more, it resulted in larger and better-connected Pt particles.

Graubner et al. [29] showed that Nd:YAG pulsed laser ablation at 266nn was characterized by long incubation period of surface reaction at which O-H and Si-O groups were formed prior to the increase in roughness. A silica-like material and polycrystalline graphitic carbon with a relatively high bond angle disorder were identified as the main ablation products.

Jin et al. [30] investigated surface treated PDMS with Nd:YAG pulsed laser at 532nm. Micro, submicro and nanocomposite structures were created as a result of laser irradiation. On untreated PDMS surface a water contact angle of about 113° was measured, while on rough PDMS surface containing micro-, submicro- and nano-composite structures originating from one-step laser etching a super-hydrophobic character with contact angle higher than 160° and sliding angle lower than 5° was measured. These results mean a self-cleaning effect like the lotus leaf. The wettability of the rough PDMS surfaces can be tuned by simply controlling the size of etched microstructures.

Bremus –Kobberling and Gillner [31] have developed laser techniques for micro-structuring of silicones for application in medical implants to modify the surface properties with respect to wettability and controlled cell growth. The technique is based on excimer laser treatment of silicone surfaces using laser wavelength 193 nm (ArF) with different fluences and cumulative energies. Depending on the processing parameters and kind of polymer either hydrophobic or hydrophilic surface can be obtained. The water contact angle of poly(dimethylsiloxane)(PDMS) increased from 113° to approx. 150° with a “lotus effect”. The laser generated micro- patterns influenced the cell density and distribution.

Yoon et al. [32] treated PDMS surface with fs pulsed laser at 810nm. The roughness of the irradiated surface increased with laser fluence. The fs-laser modified surface of PDMS showed an average water contact angle of 165°, compared to untreated PDMS surface with contact angle of 105°. The contact angle of water droplet increases with increasing laser fluence, while sliding angle decreases. These observations strongly suggest that a direct surface modification based on fs-laser micro-processing resulted in super-hydrophobicity of PDMS surfaces.

1.1.4 The Aim of the Current Research

Based on the need for ecologically- friendly technique for surface treatment for enhanced adhesion, the present research aims to assess the effect of Nd:YAG laser treatment on the strength of adhesive joints. Table 1.1 presents the differences in excimer laser and Nd:YAG laser treatment parameters.

Table 1.1 Comparison of parameters between excimer laser and Nd:YAG laser.

1.2 Methodology

In order to select the proper combination of laser parameters (laser intensity, wavelength, scan speed, number of pulses, repetition rate, and line spacing), the morphological modifications, the wettability (contact angle), and the chemical composition of the laser treated surfaces were investigated. Morphology was determined using scanning electron microscopy (SEM), wettability was tested using the sessile drop technique (goniometry), and chemical composition was determined using XPS, EDX and FTIR. At this point the optimal parameters of the Nd:YAG laser were chosen. Adherends were laser treated using the optimal parameters and bonded in various modes - modified single lap shear, tensile butt or T-peel joints. Joint strength was measured and failure mode was investigated using SEM. The adherends used in this research were difficult-to-bond materials due to their low surface energy – polyolefins, silicones and polyimides compared to aluminum alloy. The main target of this research was to treat the surface in such a way that these difficult –to-bond adherends will be bonded with commercial, conventional, low price adhesives such as epoxy and polyurethanes which usually do not bond well such materials.

The durability of adhesive joints can be affected by open-time (the time between treatment and bonding). This effect was also investigated in this research.

1.3 Experimental

1.3.1 Materials

Four different kinds of adherends were investigated after irradiation with Nd:YAG laser in a pulsed mode: Aluminium 2024 T3 (as reference), polypropylene, polyimide (Kapton) film, and elastomeric molded silicone rubber SILASTIC® –TR55 (DOW Corning) containing (3.0 – 7.0wt%) dimethylhydrogen siloxane.

Adhesives used were: Epoxy two- part toughened – SW9323-2 (3M), Acrylic two- part quick cure – DP 8010 (3M) and RTV 162 (Momentive).

The thickness of the adhesive layer used was 0.15–0.20 mm. All adhesives were cured at RT for 48 h.

Prior to bonding, the samples were surface treated using Nd:YAG laser in order to enhance the adhesion between the adherend and the adhesive.

Conventional treatments used were:

Aluminum – Chromic acid anodization, non- sealed PP – Alumina grit blasting (80 μm) PI – SiC grit blasting (38μm) Silicone Rubber – Alumina paper (no. 120) abrasion.

1.3.2 Laser Parameters

A set of nonlinear lenses produced various laser wavelengths: 1064, 535, 350 and 266nm. A projection optical system directed the laser radiation onto the sample surface. Various laser parameters were investigated: laser wavelength, intensity, scan speed, line spacing, and repetition rate. The parameters considered for the experiment for each adherend are summarized in Table 1.2. About 200 samples of size 2.5cmx2.5 cm were irradiated and tested for each material. The process was carried out at ambient temperature and in atmospheric environment. Testing various atmospheres (Air, O2, N2) showed that the results were nearly identical. Based on these results the research concentrated on atmospheric environment which is also more economical.

Table 1.2 Laser parameters tested for various surface irradiations.

1.3.3 Visual Observation

All localized or continuously irradiated areas of the various adherends were observed for color changes, blisters, cracks, swelling or distortion. The irradiated samples were compared to reference untreated samples. Comparative pictures were taken.

1.3.4 SEM Observation of Treated Surfaces

A scanning electron microscope (FEI INSPECT) was used for morphological analysis of the laser treated surfaces. The polymer surfaces were coated with a thin layer of Au to avoid surface charging. The parameters examined were: surface roughness, uniformity of roughness, ridges, grooves, cracks or blisters. The elemental composition of the various surfaces prior to or after irradiation was determined using EDX. These results may indicate contamination, oxidation or degradation of the surface material.

1.3.5 XPS

A more accurate method to investigate the chemical composition of the surface is by using XPS. XPS spectra are obtained by irradiating the material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top 1 to 10 nm of the material being analyzed. The results may indicate contamination, oxidation or degradation of the surface material.

1.3.6 Contact Angle

Wettability of the surface is usually an indicator of surface adhesion. The higher the wettability, the better is the adhesion. The wetting is expressed by the Young equation [33]:

(1.1)

Where γSV and γLV are interfacial energies of solid-vapor and liquid-vapor interfaces, γSL is the interfacial energy of solid- liquid interface, and θ is the contact angle between liquid and solid. If the liquid wets the surface it will spread at a low contact angle (θ < 90) while at an angle higher than 90 the liquid does not wet the surface. Increasing the roughness of the surface will enhance wettability [33], if the contact angle on a smooth surface is < 90°. The contact angle analysis can be used to monitor the cleanliness of the surface and the effect of pretreatment. The contact angle of the laser treated (various parameters) compared to untreated surfaces was measured by a goniometer. Triply distilled water drops were placed on the surface and the contact angle was measured after the drop had attained equilibrium.

1.3.7 FTIR

The various laser treated (various parameters) surfaces were scanned by FTIR/ATR in wavenumber range of 400–4000 cm−1. The peaks in the spectrum indicate chemical groups that absorb the IR radiation at the measured wavenumber. The spectrum of untreated samples was subtracted from those of treated samples. The resulting spectra indicate contamination or change in chemical composition of the surface due to laser treatment.

1.3.8 Joint Strength

In order to assess the effect of Nd:YAG laser pretreatment on strengths of joints, the various adherends were pretreated on an area of 2.5cm × 2.5cm and bonded to each other using a toughened 2 -part epoxy adhesive SW9323–2 (3M), an adhesive that usually does not bond the low energy adherends (polyolefin, polyimide, or silicone rubber). Two types of references were used for each set of experiments: an untreated or conventionally treated adherend, and adherend bonded with a specially suited adhesive.

1.3.8.1 Shear Strength of Joints

Joint properties were determined using single lap shear (SLS) tests according to ASTM D- 1002. In case of polyimide and silicone rubber adherends (which cannot be used as rigid adherends) a modified single lap shear (MSLS) test was performed (using aluminum rigid supports).

The mode of failure was determined visually to be 100% adhesional (interfacial) in which all adhesive is located on one adherend, or interfacial divided failure (interfacial, but divided between both adherends), or mixed, or cohesive within the adhesive. Surface morphology following shear fracture was analyzed by means of scanning electron microscopy (SEM).

1.3.8.2 Tensile Strength of Joints

Though the most common failure mode of adhesive joints is the shear mode it was important to test an additional mode (tensile). Joint properties were determined using butt joint (BJ) tests according to ASTM C297 (2010). Cylindrical supports made of aluminum were bonded to the laser treated adherends. The results were compared to untreated and conventionally treated adherends. After full cure, the butt Joint samples were loaded in tension. Mechanical testing was performed using a universal testing machine (Instron 8500), with crosshead speed of 2 mm/min. To ensure statistical reliability a series of five identical replicates were tested for each condition. Fractured surface morphology following tensile test was analyzed visually and by means of SEM.

1.4 Results

1.4.1 Polypropylene (PP)

The samples of PP irradiated with various laser parameters were examined visually, by SEM, and analyzed by XPS and FTIR. Fig. 1.1 shows the irradiated PP samples. The Nd: YAG laser irradiation resulted in color change from light grey (at 1064 nm) to dark grey (at 266 nm).

Figure 1.1 Visual observation of Nd:YAG laser irradiated polypropylene samples.

Fig. 1.2 shows the morphology of the surface of PP by SEM after irradiation at various laser wavelengths. The most uniform surface morphology (with no damage) can be seen at 1064 nm. The surface is covered with small globules and a sub-structure of fine globules (Fig. 1.3) dispersed uniformly. This morphology can contribute well to the adhesion property, serving as anchoring points with lock-and-key effect. The change in velocity of laser scanning shows that the lower the velocity the larger is the damage (Fig. 1.4).

Figure 1.2 SEM pictures of the surface of PP after laser irradiation – effect of wavelength.

Figure 1.3 SEM picture of the surface of PP after laser irradiation at 1064 nm −1.1 mm/s (enlarged view).

Figure 1.4 SEM pictures of the surface of PP after laser irradiation – effect of velocity

XPS results on the chemical composition of the surface are presented in Table 1.3. It can be seen that while the untreated PP contains a lot of contaminants such as silicone, salts (NaCl), sulfur and calcium, the laser treated surfaces, especially at 1064nm, are free of all these contaminants. This means that Nd: YAG laser not only modifies the morphology at the surface but also cleans the surface of the adherend – a parameter critical for strong and durable adhesion.

Table 1.3 XPS results (in atomic % concentration) of the untreated and Nd: YAG laser treated polypropylene surfaces with different parameters.

The XPS results also show that 532 and 1064 nm irradiations cause oxidation of the surface (the oxygen concentration increases compared to carbon content). This result indicates that Nd:YAG laser treatment also causes surface activation.

1.4.1.1 Contact Angle

Table 1.4 presents water contact angles on the surface of the untreated and Nd:YAG laser treated samples at different parameters. Fig. 1.5 shows images of drops.

Figure 1.5 Images of water drops on untreated (top) and Nd: YAG laser treated PP (bottom).

Table 1.4 Contact angles on laser treated polypropylene.

The results show that treating the samples with Nd:YAG laser resulted in increased wetting (lower contact angle) which indicates an improved surface for adhesion. Wetting was only slightly affected by wavelength.

1.4.1.2 FTIR Results

PP adherends with and without Nd:YAG laser treatment were analyzed by ATR / FTIR. The spectra were not identical. The spectrum of the untreated PP was subtracted from that of treated PP, and the resulting spectrum was analyzed to determine the chemical structure of the residue. (Fig. 1.6). The residue was analyzed as fatty –acid ester, a contaminant which may prevent adhesion. This again confirms that Nd:YAG laser treatment cleans the PP adherend surface from contaminants before bonding.

Figure 1.6 FTIR spectra of untreated and Nd:YAG laser treated polypropylene surfaces (A), detailed regime of aliphatic wavenumbers (B) and the laser treated FTIR spectrum minus untreated FTIR spectrum(C) (revealing the contamination residue).

1.4.1.3 Joint Strength Measurements

Table 1.5 summarizes the results of shear testing and failure mode of laser treated PP bonded to PP at various laser parameters with a commercial structural epoxy adhesive SW9323-2. The results were compared to untreated and conventionally treated PP (abrasion with alumina) and to bonding with a special adhesive for polyolefins – Acrylic DP-8010 (3M, USA).

Table 1.5 Shear joint strength of Nd:YAG laser treated polypropylene bonded with various adhesives.

Table 1.6 summarizes the results of tensile testing and failure mode of laser treated PP bonded to PP at various laser parameters, bonded with a commercial structural epoxy adhesive SW9323-2. The results were compared to untreated and conventionally treated PP (abrasion with alumina) and to bonding with a special adhesive for polyolefins, Acrylic DP-8010.

Table 1.6 Tensile joint strength of Nd:YAG laser treated polypropylene joints bonded with various adhesives.

The results of shear and tensile joint strengths of PP bonded to PP after Nd:YAG laser treatment with structural epoxy adhesive showed an improvement of 200% in shear and 100% in tensile strengths and change of the mode of failure from an interfacial divided failure from the PP adherend to cohesive in the adhesive or in the PP adherend (Figs. 1.7, 1.8). The visual inspection of the failure mode shows that the shear joint strength after laser treatment was so high that the PP adherend failed before the adhesive was detached.

Figure 1.7 Visual failure modes of Nd:YAG laser treated samples of PP at various wavelengths compared to untreated, abrasion treated, and bonded with special adhesive, after shear test.

Figure 1.8 Visual failure modes of Nd:YAG laser treated samples of PP at various wavelengths compared to untreated, abrasion treated, and bonded with special adhesive, after tensile test.

SEM results proved these findings. The PP adherend bonds well to the structural epoxy adhesive following Nd:YAG laser treatment. Detached and pulled out fragments of the PP on the surface can be clearly seen, showing partly cohesive failure in the adherend. On the adhesive side torn pieces of PP are embedded in the adhesive (Fig. 1.9). The failure is clearly cohesive in the adhesive since glass micro-balloons are observed on the failure surface, these micro-balloons are an integral part of the epoxy adhesive.

Figure 1.9 SEM pictures of fracture surfaces of PP side x1300-(A); adhesive side: fragments of PP in adhesive x3600-(B); adhesive side: glass bead, integral part of the adhesive x1000-(C).

It can be concluded that Nd:YAG laser treatment creates a uniform micro-structure morphology on the surface of the PP which enables interlocking with the adhesive. The treatment also causes oxidation of the surface which improves the activation and enables chemical bonding to the treated PP adherend. It was also found that contaminants were removed from the PP adherend creating a surface with improved bonding strength. The laser treated PP showed a 100% improvement in both shear and tensile joint strengths and a cohesive failure in PP. This shows that PP, a difficult- to- bond surface, can be efficiently bonded using a conventional epoxy adhesive.

1.4.2 Aluminum (2024 T3)

The samples of Al 2024 irradiated with various laser parameters were examined visually, by SEM and analyzed by XPS and FTIR.

Fig. 1.10 shows the Al irradiated sample with Nd:YAG laser at 1064 nm, showing the border between irradiated and non-irradiated areas. In the non-irradiated area corrosion spots can be observed while in the irradiated area the surface is smooth. This shows that laser irradiation cleans the Al surface from corrosion products. EDX results of untreated and Nd:YAG laser treated surfaces show that corrosion elements have disappeared (Table 1.7).

Figure 1.10 Backscattered SEM picture of the surface of Al after laser irradiation (1064nm, 1.1mm/s).

Table 1.7 EDX surface composition results of Nd:YAG laser treated vs. untreated surfaces of Al.

Element

UntreatedWeight (%)

Nd:YAG laser treatedWeight (%)

O

5.2

7.9

Mg

1.4

1.7

Al

82.4

76.7

Cl

0.4

0

Mn

0.5

0

Fe

1.3

0

Cu

8.9

13.7

Fig. 1.11 shows the morphology of the surface of Al by SEM after irradiation at various laser wavelengths. At 266 and 532 nm the effect is marginal while a uniform rough surface morphology (with no damage) is formed at 1064 nm. The surface is covered with small globules inside large craters. This morphology can contribute well to the adhesion property, serving as anchoring points and lock- and- key effect. XPS results on the chemical composition of the Al surface are presented in Table 1.8.

Figure 1.11 SEM pictures of the surface of Al after Nd:YAG laser irradiation at wavelengths of 266(A), 532(B) and 1064 nm(C).

Table 1.8 XPS results (in atomic % concentration) for Nd: YAG laser treated aluminum surface l.

It can be seen that while the untreated Al contains a lot of contaminants such as carbon, silicon, salts (NaCl), sulfur and calcium, the irradiated AI surfaces are clean. This means that Nd:YAG laser not only modifies the morphology at the surface but also cleans the surface, a parameter critical for strong and durable adhesion.

The XPS results show that irradiation also causes oxidation of the surface (the oxygen concentration increases) and creation of new oxidized elements like: Al2O3, MgO and ZnO. This result indicates that Nd:YAG laser treatment causes also surface activation.

1.4.2.1 Contact Angle

Table 1.9 presents the contact angles on the surface of the untreated and Nd:YAG laser treated samples at different parameters.

Table 1.9 Contact angles of laser treated aluminum.

Wavelength nm

Contact angleθ (°)

Environment

untreated

71

–

10642 mm displacement

*

122

AIR

10644 mm displacement

107

AIR

*displacement – spacing between laser lines

The results in Table 1.9 show that treating the Al samples with Nd:YAG laser caused unexpectedly an increase in the contact angle, which indicates a lesser work of adhesion (although a stronger adhesion is observed). This may result from an unusual behavior of the surface due to different concentrations and combinations of oxides (Table 1.8).

1.4.2.2 FTIR Results

Al adherends with and without Nd:YAG laser treatment were analyzed by FTIR/ATR. The spectra of 266 and 538nm treated samples did not reveal any changes while 1064 nm indicated some change. The untreated Al surface showed a high concentration of carbohydrates (organic material) which indicates a contamination. Nd:YAG laser treatment eliminates most of such contamination, (Fig. 1.12).

Figure 1.12 FTIR spectra of untreated (lower spectrum) and Nd:YAG laser 1064 nm treated (upper spectrum) Aluminum 2024.

1.4.2.3 Joint Strength Measurements

Table 1.10 summarizes the results of shear testing and failure mode of laser treated Al adherends at various laser parameters and bonded with a commercial structural epoxy adhesive SW9323-2. The results were compared to untreated and conventionally treated Al (Chromic acid non-sealed anodization).

Table 1.10 Shear joint strength of Nd:YAG laser treated aluminum bonded with SW 9323-2 joints.

Table 1.11 summarizes the results of tensile testing and failure mode of laser treated Al adherends at various laser parameters, bonded with a commercial structural epoxy adhesive SW9323-2. The results were compared to non-treated and conventionally treated Al (Chromic acid non-sealed anodization).

Table 1.11 Tensile joint strength of Nd:YAG laser treated aluminum joints.

The results of shear and tensile joint strengths of Al bonded to Al after Nd:YAG laser treatment with structural epoxy adhesive showed an improvement of 200% in shear and 120% in tensile joint strength and change in the mode of failure from a divided interfacial failure from the Al adherend to cohesive failure in the adhesive (Figs. 1.13, 1.14). The fact that the joint strength of Nd:YAG laser treated Al was higher than chromic acid anodized Al is an unusual finding. SEM results confirmed these findings (Fig. 1.15). The Al adherend bonds well to the structural epoxy adhesive following Nd:YAG laser treatment. The failure is clearly cohesive in the adhesive since glass micro-balloons are observed on the failure surface, these micro-balloons are an integral part of the epoxy adhesive.

Figure 1.13 Visual failure modes of Nd:YAG laser treated at 1064nm compared to untreated and anodized shear samples of Aluminum 2024.

Figure 1.14 Visual failure modes of Nd:YAG laser treated at 1064nm compared to abraded and anodized tensile samples of Aluminum 2024.

Figure 1.15 SEM micrograph of fracture surface showing cohesive mode of failure in the adhesive on Al shear samples treated with Nd:YAG laser at 1064nm, 1.1mm/s (the glass beads are an integral part of the adhesive).

It can be concluded that Nd:YAG laser treatment creates a uniform micro-structure morphology on the surface of the Aluminum 2024 which enables anchoring of the adhesive. The treatment also causes a different kind of oxidation of the surface, which improves the activation and enables chemical bonding to the Al adherend. It was also found that contaminants were removed from the Al adherend creating a cleaner surface with improved bonding strength. The laser treated Al showed a vast improvement in shear and tensile joint strengths and a cohesive failure in the adhesive. This shows that the Al can be efficiently bonded with joint strengths exceeding the chromic acid anodization treatment.

1.4.3 Polyimide (Kapton)

The samples of polyimide (PI) irradiated with various laser parameters were examined visually, by SEM and analyzed by XPS and FTIR.

SEM morphology of irradiated surfaces (Fig. 1.16) shows the morphology of the surface of Pl after irradiation at various laser wavelengths. At 266nm the effect is marginal while a uniform rough surface morphology (with no damage) is formed at 532 nm and 1064 nm and the surface is wave-like and covered with small globules. This morphology can contribute well to the adhesion property, serving as anchoring points for lock – and –key effect.

Figure 1.16 SEM pictures of different morphologies of the surface of Pl after Nd:YAG laser irradiation at 266nm: no-features (A), at 532nm: wavy surface (B), and at 1064 nm: uniform roughness of the surface (C).

XPS results on the chemical composition of the surface are presented in Table 1.12. The results show that while the untreated PI contains contaminants such as silicon and calcium, the irradiated PI surfaces are clean. This means that Nd:YAG laser not only modifies the morphology at the surface but also cleans the surface, a parameter critical for strong and durable adhesion.

Table 1.12 XPS results (in atomic %concentration) for Nd:YAG laser treated polyimide surface.

The XPS results also show that irradiation reduces oxidation on the surface (the oxygen concentration decreases) while the carbon concentration increases, this may indicate crosslinking of the surface molecules which creates a stronger and more durable structure.

1.4.3.1 Contact Angle

Table 1.13 presents the contact angles on the surface of the untreated and Nd:YAG laser treated samples at different parameters. Fig. 1.17 shows images of drops.

Figure 1.17 Images of water drops on untreated and Nd: YAG laser treated PI surface at three different wavelengths: 266 nm, 532nm, and 1064nm.

Table 1.13 Contact angles of untreated and Nd:YAG laser treated PI surface.

Wavelength (nm)

Tested Location

Contact angle θ(°)

Untreated

Center

78

266

On laser line

147

266

Between laser lines

118

532

On laser line

92

532

Between laser lines

96

1064

On laser line

52

The results show an improvement in wetting (decreasing contact angle) only at 1064nm.

1.4.3.2 FTIR Results

PI adherends with and without Nd:YAG laser treatment were analyzed by FTIR/ATR.

The spectra of the surfaces treated at wavelengths of 266, 538 and 1064nm indicate the formation of hydroxyl groups (3628 cm−1) and carboxyl groups (1556 cm−1) on the surface which induces activation (better adhesion). At 266 nm, appearance of new aliphatic groups are observed 2925 cm−1(= C-H) and 2855 cm−1 (= CH2-CH), indicating breakage of aromatic bonds of the imide’s skeleton of the polymer[12]. This degradation forms a weak surface layer, which may fail after bonding (Fig. 1.18).

Figure 1.18 FTIR spectra of untreated and Nd: YAG laser treated polyimide at 1064, 532 and 266 nm.

1.4.3.3 Joint Strength Measurements

Table 1.14 summarizes the results of shear testing and failure mode of laser treated Pl bonded to Al at various laser parameters, bonded with a commercial structural epoxy adhesive SW9323-2. The results were compared to non-treated and conventionally treated Pl (abrasive treatment with SiC).

Table 1.14 Shear joint strength of Nd:YAG laser treated polyimide bonded to Al with SW 9323-2 joints.

Table 1.15 summarizes the results of tensile testing and failure mode of joints of laser treated PI bonded to Al at various laser parameters with a commercial structural epoxy adhesive SW9323-2. The results were compared to untreated and conventionally treated PI (abrasion with SiC).

Table 1.15 Tensile joint strength of Nd: YAG laser treated polyimide bonded to Al with SW 9323-2.

The results of shear and tensile joint strengths of PI treated with Nd:YAG laser bonded to Al with structural epoxy adhesive showed an improvement of 70% in shear and 600% in tensile joint strengths and modification of the mode of failure from an interfacial failure from the PI adherend to cohesive in the PI adherend (fragments and delamination of the PI film) (Figs. 1.19 and 1.20).

Figure 1.19 Visual failure modes of Nd:YAG laser treated shear samples of polyimide at 1064nm (cohesive failure in PI) and 532nm (cohesive failure in PI) compared to untreated and abraded PI samples which showed adhesional (interfacial) failure between PI and adhesive.

Figure 1.20 Visual failure modes of Nd:YAG laser treated tensile samples of polyimide at 1064nm (cohesive failure in PI) and 532nm (cohesive failure in PI) compared to untreated and abraded treated PI samples which showed adhesional (interfacial) failure between PI and adhesive.

SEM results confirmed these findings (Fig. 1.21). The PI adherend bonds well to the structural epoxy adhesive following Nd:YAG laser treatment. The failure is clearly cohesive in the polyimide adherend. The Nd:YAG laser forms an array of globules on the irradiated surface.

Figure 1.21 SEM pictures of shear fracture surfaces of the Pl after laser irradiation at 532 (A) and 1064 nm (B), showing fragments of polyimide film clinging to the rough surface of the adhesive with mirror morphology of the PI, x4500.

It can be concluded that Nd:YAG laser treatment creates a uniform micro-structure morphology on the surface of the polyimide which enables anchoring of the adhesive. The microstructure changes with laser conditions such as wavelength, number of pulses and scan velocity. The treatment also causes formation of hydroxyl and carbonyl groups on the PI surface which improves activation and enables chemical bonding to the Al adherend. The best result is achieved at 1064 nm at a velocity of 2.7 mm/s. At 266nm degradation of the surface occurs. It was also found that contaminants were removed from the Pl adherend creating a cleaner surface with improved bonding strength. The laser treated Pl showed an improvement in shear and tensile joint strengths and a cohesive failure in PI. It was shown that the Pl can be efficiently bonded with results exceeding the abrasive treatment.

1.4.4 Open Time

One of the most important requirements of pre-adhesion treatment is the “open time”. Open time is defined as the time between treatment and bonding. A long open time enables storage between these two activities (surface treatment and bonding) which is important in a production plant. Plasma and corona treatments have the disadvantage of very short open time of less than an hour. The open time of Nd: YAG laser treated samples was determined on PP samples which were irradiated and left open in air for up to 2 weeks and then bonded. These joints were tested in shear and compared to samples treated and bonded immediately.

The results are summarized in Table 1.16 and the mode of failure in Fig. 1.22.

Figure 1.22 The effect of open time on the mode of failure of Nd:YAG laser treated samples of polypropylene after shear test (immediate testing −0, after 1 week, after 2 weeks of open time). All samples showed cohesive failure in the PP adherend, indicating that adhesion was stronger than the bulk adherend.

Table 1.16 The effect of open time on shear joint strength of PP irradiated with Nd: YAG laser and bonded with epoxy.

As can be seen in Table 1.16 that even after 14 days of open time the shear joint strength is maintained and the failure mode does not change. This result is extremely important for production processes.

1.4.5 Silicone Rubber

The samples of silicone rubber irradiated with various laser parameters were examined visually, by SEM and analyzed by XPS and FTIR.

Fig. 1.23 shows the morphology of the surface of the elastomeric silicone rubber SILASTIC®–TR55 (DOW Corning) by SEM after irradiation at various laser wavelengths. At 266 and 1064 nm the effect is marginal while a uniform rough surface morphology is formed at 532 nm. The surface is covered with small globules. This morphology can contribute well to the adhesion property, serving as anchoring points and lock- and-key effect.

Figure 1.23 SEM pictures of the surface morphology of silicone rubber after Nd:YAG laser irradiation at 266–smooth surface (A), 532-rough and uniform surface (B) and 1064nm – rough and non-uniform surface (C).

XPS as well as EDX results on the chemical composition of the surface are presented in Table 1.17.

Table 1.17 XPS and EDX results for Nd:YAG laser treated silicone rubber surface.

The EDX results show that while the untreated surface of silicone rubber contains contaminants such as aluminum and calcium, the irradiated silicone surfaces are clean. This means that Nd:YAG laser not only modifies the morphology at the surface but also cleans the surface, a parameter critical for strong and durable adhesion.

The XPS results show that irradiation increases oxidation on the surface as well as silicon concentration while the carbon concentration decreases, this indicates that pure silica is exposed on the surface.

1.4.5.1 Contact Angle

Table 1.18 presents the contact angle values on the surface of the untreated and Nd:YAG laser treated silicone samples at different laser parameters. Fig. 1.24 shows images of drops.

Figure 1.24 Images of water drops on untreated and Nd: YAG laser treated silicone rubber surface at 3 wavelengths: Untreated – A, 266 nm-B, 532nm-C, 1064 nm-D.

Table 1.18 Contact angles of untreated and Nd:YAG laser treated silicone rubber surfaces.

Wavelength, nm

Velocity, mm/s

Contact angle θ (°)

Untreated

–

120

266

0.5

112

532

1.7

100

532

1.1

106

532

0.67

137

1064

0.67

103

1064

0.33

113

It can be seen from Table 1.18 that the contact angle on the irradiated silicone surfaces is slightly reduced compared to untreated silicone surface. The greatest decrease was observed at 532 nm, 1.7 mm/s.

1.4.5.2 FTIR Results

Silicone rubber adherends with and without Nd:YAG laser treatment were analyzed by FTIR/ATR. The spectra of 266, 538 and 1064nm treated samples indicate changes in the aliphatic hydrocarbon groups due to Nd:YAG laser irradiation: CH2-CH at 2922, 2910, and 2854 cm−1 of an untreated silicone rubber changes into CH3-CH at 2906 cm−1 with Nd:YAG laser treatment. Formation of Si-C group at 1076 cm−1