Polymer Surface Modification to Enhance Adhesion E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
POLYMER SURFACE MODIFICATION TO ENHANCE ADHESION This unique, comprehensive and groundbreaking book is the first on this important subject. Polymer Surface Modification to Enhance Adhesion comprises 13 chapters and is divided into two parts: Part 1: Energetic Treatments; and Part 2: Chemical Treatments. Topics covered include atmospheric pressure plasma treatment of polymers to enhance adhesion; corona treatment of polymer surfaces to enhance adhesion; flame surface treatment of polymers to enhance adhesion; vacuum UV photo-oxidation of polymer surfaces to enhance adhesion; optimization of adhesion of polymers using photochemical surface modification UV/Ozone surface treatment of polymers to enhance adhesion; adhesion enhancement of polymer surfaces by ion beam treatment; polymer surface modification by charged particles; laser surface modification of polymeric materials; competition in adhesion between polysort and monosort functionalized polyolefinic surfaces; amine-terminated dendritic materials for polymer surface modification; arginine-glycine-aspartic acid (RGD) modification of polymer surfaces; and adhesion promoters for polymer surfaces. Audience The book will be of great interest to polymer scientists, surface scientists, adhesionists, materials scientists, plastics engineers, and to those involved in adhesive bonding, packaging, printing, painting, metallization, biological adhesion, biomedical devices, and polymer composites.
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Veröffentlichungsjahr: 2024
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Table of Contents
Cover
Table of Contents
Series Page
Title Page
Copyright Page
Preface
Part I: ENERGETIC TREATMENTS
1 Atmospheric Pressure Plasma Treatment of Polymers to Enhance Adhesion
1.1 Introduction
1.2 Historical Development of APPTs
1.3 Functional Groups Produced by APPTs
1.4 Adhesion Improvement for Bonding
1.5 Targeted Adhesion for Biomedical Applications
1.6 Relevance of Adhesion in Additive Manufacturing
1.7 Summary
1.8 Acknowledgements
References
2 Corona Treatment of Polymer Surfaces to Enhance Adhesion
2.1 Introduction
2.2 Mechanism of Corona Treatment
2.3 Factors Affecting Performance of Corona Treatment
2.4 Surface Effects of Corona Treatment
2.5 Adhesion Improvement by Corona Treatment
2.6 Summary
References
3 Flame Surface Treatment of Polymers to Enhance Their Adhesion
3.1 Introduction
3.2 Chemistry of Flame Treatment
3.3 Flame Treatment Equipment
3.4 Factors Controlling Flame Plasma Surface Treatment
3.5 Measurement of Treatment Level
3.6 Safety and Other Considerations
3.7 Adhesion Improvement
3.8 Summary
References
4 Vacuum UV (VUV) Photo-Oxidation of Polymer Surfaces to Enhance Adhesion
4.1 Introduction
4.2 Vacuum UV Photo-Oxidation Process
4.3 Adhesion to VUV Surface Photo-Oxidized Polymers
4.4 Sustainable Polymers
4.5 Summary
References
5 Application-Related Optimization of Adhesion of Polymers Using Photochemical Surface Modification
5.1 Introduction
5.2 Photochemical Surface Modification
5.3 Using Photo-Addition and Photo-Grafting to Promote the Adhesion Property of Hydrophobic Substrates
5.4 Enhancing Adhesion of Hydrophobic Materials on Hydrophilic Substrates – Biobased Composites as Case Study
5.5 Biosystems: Cell and Protein Adhesion, Antifouling Surfaces
5.6 Summary
Acknowledgement
References
6 UV/Ozone Surface Treatment of Polymers to Enhance Their Adhesion
6.1 Introduction
6.2 Historical Development of UV/Ozone Surface Treatment
6.3 Parameters Controlling the UV/Ozone Surface Treatment Process
6.4 Surface Changes of Polymeric Materials by UV/Ozone Treatment
6.5 Surface Analysis of UV/Ozone Treated Polymeric Surfaces
6.6 UV/Ozone Treatment of Polymers: Improved Wetting and Adhesion
6.7 Prospects
6.8 Summary
Acknowledgements
References
7 Adhesion Enhancement of Polymer Surfaces by Ion Beam Treatment
7.1 Introduction
7.2 Ion Beam Treatment of Polymers
7.3 Analysis Techniques to Analyze Post Ion Beam Treatment
7.4 Polymer Surface Modifications for Biomedical Applications
7.5 Polymer Surface Modification for Microelectronics Applications
7.6 Summary
References
8 Polymer Surface Modification by Charged Particles from Plasma Using Plasma-Based Ion Implantation Technique
8.1 Introduction
8.2 Overview of Literature About Polymer Surface Modification by Charged Particles from Plasma Using Plasma-Based Ion Implantation [6]
8.3 Principle of PBII: Advantages and Limitations [7]
8.4 Equipment Needed
8.5 Factors Influencing the Outcome/Results
8.6 Results Showing Adhesion Improvement after PBII Treatment
8.7 Prospects
8.8 Summary
References
9 Laser Surface Engineering of Polymeric Materials for the Modification of Wettability and Adhesion Characteristics
9.1 Introduction
9.2 Methods for Measuring Wettability and Adhesion Characteristics
9.3 Laser Surface Engineering of Polymeric Materials
9.4 Summary
Acknowledgements
References
10 Competition in Adhesion between Polysort and Monosort Functionalized Polyolefin Surfaces Coated with Vacuum-Evaporated Aluminium
10.1 Introduction
10.2 Differences in Adhesion between Poly- and Monosort Functionalized Polyolefin Surfaces
10.3 Bonding of Metal Coatings to Polysort and Monosort Functionalized Polyolefins
10.4 Adhesion Results for Evaporated Aluminium Coating on Poly- and Monosort Functionalized Polyolefin Surfaces
10.5 Realization of Ideal Covalently Bonded Interface
10.6 Summary
Acknowledgements
References
Part II: CHEMICAL TREATMENTS
11 Amine-Terminated Dendritic Materials for Polymer Surface Modification to Enhance Adhesion
11.1 Introduction
11.2 Dendritic Materials
11.3 Amine-Terminated Dendritic Materials as Adhesion Modifiers
11.4 Applications of Amine-Terminated Dendritic Materials in Adhesion
11.5 Summary
References
12 Arginine-Glycine-Aspartic Acid (RGD) Modification of Polymer Surfaces to Enhance Cell Adhesion
12.1 Introduction
12.2 RGD Peptides
12.3 RGD Immobilization Techniques
12.4 Characterization
12.5 Applications
12.6 Summary
References
13 Adhesion Promotors for Polymer Surfaces
13.1 To Coat or Not to Coat Polymer Surfaces
13.2 Theory of Adhesion: Adhesion Forces
13.3 Plastics
13.4 Polymer Adhesion Mechanisms
13.5 Pretreatments
13.6 Summary
References
Index
Also of Interest
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Surface free energies of common polymers.
Table 2.2 Modification of surface free energies of polymers after corona treat...
Chapter 3
Table 3.1 Surface free energy of polymer substrates.
Table 3.2 Surface tension of liquids.
Table 3.3 Comparison of adhesion promotion techniques.
Table 3.4 Species present in flame plasma.
Table 3.5 Effect of temperature & humidity on O
2
, and other gases concentratio...
Table 3.6 Gas composition (%) from different gas fields.
Table 3.7 Surface properties of corona and flame plasma treated polypropylene ...
Table 3.8 Contact angles (degrees) of pH1 and pH 10 solutions on treated alumi...
Chapter 4
Table 4.1 Wet techniques for surface modification (Courtesy of Dr. Kash Mittal...
Table 4.2 Dry techniques for surface modification (Courtesy of Dr. Kash Mittal...
Table 4.3 Resonance emission wavelength lines for gaseous atoms [11].
Table 4.4 Emission maximum wavelength for rare gas excimers, Rg
2
*, [26–28].
Table 4.5 Cut-off wavelengths for crystalline materials [6].
Table 4.6 Percentage of sputter-deposited copper removed from PTFE surface as ...
Table 4.7 Overview of dominant examples of synthetic biopolymers or biocomposi...
Chapter 5
Table 5.1 Number of identified publications referring to adhesion-related prob...
Table 5.2 Overview of processes and applications related to adhesion to hydrop...
Table 5.3 Overview of processes and selected applications related to improving...
Table 5.4 Overview of processes and selected applications related to biosystem...
Chapter 6
Table 6.1 UV/Ozone surface treatment of polymers compared to other surface tre...
Table 6.2 Bond energies of several atomic bonds of importance during the UV/Oz...
Table 6.3 Relevant spectral lines during the UV/Ozone process in relation to t...
Table 6.4 Surface free energy including the polar and dispersion components of...
Table 6.5 Contact angle of water on PE and stainless steel as a function of UV...
Table 6.6 Surface free energy of EPDM-rubber as a function of surface treatmen...
Table 6.7 Surface free energy and coating adhesion to the EPDM-rubber as a fun...
Chapter 7
Table 7.1 Spectroscopic methods for polymer analysis. Adapted from [65].
Table 7.2 The conditions of Ar ion beam irradiation of PDMS samples. Adapted f...
Chapter 8
Table 8.1 Contact angle and surface free energy of the untreated and the modif...
Table 8.2 Spectra fitting results.
Table 8.3 Summary of the effect of PBII treatment on bond strength for the dif...
Chapter 9
Table 9.1 Typical experimentally determined water contact angles of different ...
Table 9.2 Typical experimentally determined adhesion force measurements for se...
Table 9.3 A summary of common surface engineering techniques.
Table 9.4 Typical laser surface engineering parameters with subsequent variati...
Table 9.5 Literature data detailing various laser surface engineered materials...
Chapter 10
Table 10.1 A few representative examples for O/C and C1s fitted results of pla...
Table 10.2 Physical and chemical interactions and bonds.
Chapter 11
Table 11.1 Zeta potential of surfaces by amine-terminated functionalization.
Table 11.2 The effect of surface topography modification by amine-terminated g...
Table 11.3 Interlaminar and interfacial shear strength data after modification...
Chapter 12
Table 12.1 Examples of synthetic RGD peptides used for promoting cell adhesion...
Table 12.2 Commonly used immobilization chemistries for RGD peptides.
Table 12.3 Micro/nano fabrication techniques for patterning RGD peptides.
Table 12.4
In vivo
evaluation of RGD modified polymers for tissue engineering.
Chapter 13
Table 13.1 Examples of common thermoplastic and thermoset polymer substrates t...
Table 13.2 Bond energies (kJ-mole
-1
) for different bond types. (Data from Kinl...
Table 13.3 Solvent type effect on volumetric swelling, surface area/topography...
Table 13.4 Comparison of chlorinated adhesion promoter practical adhesion stre...
Table 13.5 Dispersion and polar surface energies determined for plastic substr...
Table 13.6 Changes in surface tension and wetting properties for polypropylene...
Table 13.7 Adhesion data and failure modes for chlorinated and non-chlorinated...
Table 13.8 Adhesion data for use of silane adhesion promoters on steel, alumin...
List of Illustrations
Chapter 1
Figure 1.1 Examples of different types of plasma sources for APPTs; (a) Dielec...
Figure 1.2 Number of publications on surface functionalization with APPT with ...
Figure 1.3 Chemical species generated by a microdischarge in air (from [28]).
Figure 1.4 Important reaction steps for the activation of polypropylene surfac...
Figure 1.5 Functional groups by treatment of polypropylene surface in air (Cor...
Figure 1.6 Left: Surface free energy of biaxially oriented polypropylene (BOPP...
Figure 1.7 Overview of various treatment parameters to functionalize PP-substr...
Figure 1.8 Influence of plasma gas and the surrounding atmosphere on adhesion ...
Figure 1.9 (a) Comparison of the average peel strength on untreated, primer tr...
Figure 1.10 Bond strength between aluminum and PE depending on the surface tre...
Figure 1.11 Left: Patterned surface treatment for cell cultivation in hanging ...
Figure 1.12 Schematic representation of the deposition of polymethacrylate-bas...
Figure 1.13 Confirmation of catechol/quinone groups in plasma polymer layers c...
Figure 1.14 Scheme representation of the proposed technology and its control o...
Figure 1.15 Experimental setup for FDM 3D printing of PLA and plasma treatment...
Figure 1.16 Surface free energy of PLA as a function of treatment time with a ...
Figure 1.17 Rupture stress of FDM printed PLA test samples without and with pl...
Figure 1.18 Left: Experimental set-up for the plasma treatment of polymer part...
Figure 1.19 Water contact angle and melting enthalpy of PA samples for differe...
Figure 1.20 Water contact angles of PA samples treated in Ar/O
2
plasma as a fu...
Figure 1.21 Water contact angles of printed PEI samples with different surface...
Figure 1.22 Oxygen content and O/C ratio of plasma treated PEI samples as a fu...
Figure 1.23 Tensile stress results of bonded plasma treated PEI samples as a f...
Figure 1.24 (a) Schematic depiction of the plasma jet set-up used to treat 3D ...
Figure 1.25 Scanning electron microscopy images of untreated (A1-3) and plasma...
Chapter 2
Figure 2.1 Schematic of a basic corona-treater configuration.
Figure 2.2 (a) Simplified schematic of electrode arrangment for dielectric bar...
Figure 2.3 Schematic representation of ICP system.
Figure 2.4 Schematic representation of PDR.
Figure 2.5 Images of (a) 2D corona treatment and (b) 3D corona treatment of pl...
Figure 2.6 Shows trend curves depicting the effect of treatment level on surfa...
Figure 2.7 Comparison of surface energy stability over time for untreated and ...
Figure 2.8 Penetration of ozone reactive species through the uppermost layer o...
Figure 2.9 Comparison of contact angle before and after corona treatment of PL...
Figure 2.10 Impact of corona treatment on surface free energy and its polar an...
Figure 2.11 AFM images of PET surface (a) untreated (b) modified with corona d...
Chapter 3
Figure 3.1 Contact angle vs. surface parameters.
Figure 3.2 Different zones in a laminar flame profile.
Figure 3.3 Overview of the combustion process.
Figure 3.4 Burners used in flame treating 3D geometries.
Figure 3.5 Schematic representation of a flame treating station for treating 3...
Figure 3.6 Schematic representation of a flame treating station for treating w...
Figure 3.7 Influence of temperature and humidity on λ (1/ɸ), taken from [8].
Figure 3.8 Comparison of typical O
2
analyzer scale to plasma value scale.
Figure 3.9 Schematic of the flame plasma analyzer & air bypass flow control va...
Figure 3.10 Flame plasma analyzer.
Figure 3.11 Tape peel strength vs. Stoichiometric ratio (%).
Figure 3.12 Relationship between carbonyl and ether formation vs. treatment le...
Figure 3.13 Drilled port burner.
Figure 3.14 Ribbon burner.
Figure 3.15 Effect of Air and gas mixture flow at low line speeds (top) and at...
Figure 3.16 8-Port ribbon burner.
Figure 3.17 (a) Flame uniformity of an 8-port ribbon burner--front & side view...
Figure 3.18 Optimum distance of the substrate from the burner face.
Figure 3.19 Adhesion strength vs. distance of the substrate from the burner fa...
Figure 3.20 Surface free energy vs. film-to-flame distance (adapted from [8]).
Figure 3.21 Effect of dwell time on treatment level.
Figure 3.22 Treater stations.
Figure 3.23 Air/gas mixing station.
Figure 3.24 Electrical cabinet.
Figure 3.25 Typical Mayer rod.
Figure 3.26 Adhesion of LDPE to treated and untreated aluminum foils.
Figure 3.27 Positive ToF-SIMS spectra of untreated and flame treated LDPE [29]...
Figure 3.28 XPS spectra for Sample A untreated (lower) and flame treated (uppe...
Chapter 4
Figure 4.1 Schematic of downstream low-pressure microwave plasma VUV photooxid...
Figure 4.2 Schematic diagram of high-pressure rotating arc plasma. (A) anode, ...
Figure 4.3 Mechanism for ester formation from the reaction of ozone with sp
2
-h...
Figure 4.4 Molecular structure of FEP.
Figure 4.5 Molecular structure of PFA.
Figure 4.6 Molecular structure of protonated Nafion.
Figure 4.7 Overlapped C 1s XPS spectra for Nafion-117 after graft polymerizati...
Figure 4.8 Molecular structures of some polyimides (PIs). (a) Poly(pyromelliti...
Figure 4.9 Molecular structure of poly(etheretherketone) (PEEK).
Figure 4.10 Molecular structure of polystyrene (PS).
Figure 4.11 Molecular structure of poly(ethylene terephthalate) (PET).
Figure 4.12 Quantitative XPS (at %) and water contact angle (CA) results for P...
Figure 4.13 Poly 2,2’-m-(phenylene)-5,5’-bibenzimidazole or
meta
-PBI.
Figure 4.14 Molecular structure of poly(ethylene 2,6-naphthalate) (PEN).
Figure 4.15 Chemical structure of polyethersulfone (PES) showing three kinds o...
Figure 4.16 Water contact angle measurements after PES treated with VUV photoo...
Figure 4.17 Molecular structure of polyetherimide (PEI).
Figure 4.18 The different stages of the synthesis of poly(lactic acid) startin...
Chapter 5
Figure 5.1 Documents by year for the search word combination “UV AND photo che...
Figure 5.2 Co-occurrence network of keywords stated in papers published in the...
Figure 5.3 Number of identified publications referring to adhesion-related pro...
Figure 5.4 Fundamental reactions allowing photochemical modification of the su...
Figure 5.5 The principle of the surface modification of a polymer (here: PET) ...
Figure 5.6 Chemical structures of several reactive monomers used in various st...
Figure 5.7 UV absorption spectra of the reactive monomers PETA, TAE, DAP, and ...
Figure 5.8 Schematic representation of the layer growth for TAE (a) and PETA (...
Figure 5.9 Schematic presentation of the sequence of steps involved in a photo...
Figure 5.10 Concept for the photochemical surface modification of films or tex...
Figure 5.11 Principal factors governing biosystem-substrate interaction; illus...
Figure 5.12 Schematic of biofilm formation; illustration adapted from a scheme...
Figure 5.13 Chemical structures of (a) poly(ethylene glycol) methacrylate (PEG...
Figure 5.14 The effect on biofilm formation and adhesion (from [18]): Photogra...
Chapter 6
Figure 6.1 Low pressure mercury lamp UV-C spectral energy distribution [14].
Figure 6.2 (a) Photograph of an early UV/Ozone chamber used for the experiment...
Figure 6.3 Wavelength dependent absorption (α) of oxygen and ozone gas. Please...
Figure 6.4 Ozone concentration in a UV/Ozone exposure chamber (Figure 6.2a) co...
Figure 6.5 Relative intensity of UV-light at 172, 185, 222 and 254 nm as a fun...
Figure 6.6 Typical sample temperatures as a function of treatment time on expo...
Figure 6.7 (a) Trend lines showing the surface free energy of PP as a result o...
Figure 6.8 Measurements with added trend lines to show the course of a prolong...
Figure 6.9 Surface free energy of PP measured by test inks. Pre-cleaned by eth...
Figure 6.10 Simplified schematic models of (a) ozone generation and destructio...
Figure 6.11 Aging of a HDPE surface determined by water contact angle as a fun...
Figure 6.12 SEM (a, b, c) and AFM (d, e, f) micrographs of (a, d) the CFRP sur...
Figure 6.13 Averaged AFM data, showing: (a) The root mean square average heigh...
Figure 6.14 XPS data showing atomic concentration percentages of the elements ...
Figure 6.15 ATR-FTIR absorption spectra of CFRP surfaces at three stages of su...
Figure 6.16 OSEE output voltage as a function of an incremental exposure time ...
Figure 6.17 OSEE output voltage as a function of the UV/Ozone exposure time wi...
Figure 6.18 Surface free energy of CFRP together with its dispersion and polar...
Figure 6.19 Sketch of Hybrid Laminar Flow Control (HFLC) (top) and a simplifie...
Figure 6.20 Floating roller peel test results for acetone cleaned only and tho...
Figure 6.21 Schematic representation of chromic acid anodized aluminium with t...
Figure 6.22 Static water contact angle data on three adhesive bond primers aft...
Figure 6.23 Cross-cut tape test results on chromic acid anodized BR127 primed ...
Figure 6.24 Static contact angles of water on magnet coatings after 5 differen...
Figure 6.25 Lap shear strength together with butt joint test results for a no-...
Figure 6.26 Static contact angle (water, degrees) on PDMS surface as a functio...
Figure 6.27 SEM micrographs of a cast PDMS surface after (a) 0 minute, (b) 60 ...
Figure 6.28 Static contact angle (water, degrees) as a function of the exposur...
Figure 6.29 Average lap shear strength of POM bonded to aluminium with a 2-com...
Figure 6.30 Lap shear strength test results of adhesively bonded PE to stainle...
Figure 6.31 Static contact angle of water on HDPE as a function of the surface...
Figure 6.32 Lap shear strength of HDPE bonded to HDPE by a two-component epoxy...
Figure 6.33 Comparison of different types of surface treatment methods on the ...
Figure 6.34 Lap shear strength as a result of the exposure time of PP (a) and ...
Figure 6.35 Bar graphs representing the surface free energy of nylon including...
Figure 6.36 Double distilled water droplets (volume 3 microliters) on PPS, (a)...
Figure 6.37 Trend lines connecting the measurements of polar component of the ...
Figure 6.38 Total surface free energy (mJ/m
2
) of PMMA, including the dispersio...
Figure 6.39 Static contact angle measurements (double distilled water) on poly...
Figure 6.40 Static contact angle of water on an ABS surface “as is” and after ...
Figure 6.41 Typical example of a peel test on one of the ABS samples tested ac...
Figure 6.42 Maximum peel force at break for ABS bonded to ABS as a function of...
Figure 6.43 Test results of the static contact angle measurements (water) on S...
Figure 6.44 Surface free energy of SBS-rubber as a function of treatment time ...
Figure 6.45 Bond strength of SBS rubber bonded to SBS rubber after the surface...
Chapter 7
Figure 7.1 Schematic showing the primary interactions of an ion with a solid t...
Figure 7.2 Aging data for a PET surface modified in a N
2
(•) and an O
2
(▪) pla...
Figure 7.3 XPS spectra of biodegradable polymer (PLA) bombarded by He
+
and Ar
+
Figure 7.4 Transmittance of biodegradable polymer (PLA) irradiated with Ar
+
an...
Figure 7.5 Fluorescence intensity of pristine(control) and ion irradiated PLLA...
Figure 7.6 Mean water contact angles for pristine(unimplanted) and Au, C impla...
Figure 7.7 AFM images of the unirradiated and irradiated PDMS substrates. Adap...
Figure 7.8 Influence of He+ ion bombardment on FT-IR spectra of the polymer ma...
Figure 7.9 Arrow A (critical load for traditional deposition process such as i...
Figure 7.10 SEM morphology of the silicone rubber modified by the IBT process....
Figure 7.11 Water contact (wetting) angle of PC with varying Ar
+
dose and oxyg...
Figure 7.12 Contact (wetting) angle of PC with Ar
+
irradiation at 1 keV in oxy...
Figure 7.13 Contact angle (a) and surface energy (b) of PC as a function of Ar
Figure 7.14 Adhesion strengths as a function of N
2
+
ion beam dose of the Cu/PI...
Figure 7.15 Chemical structures of polymers (Kapton H, Teflon PFA, Tefzel and ...
Figure 7.16 XPS spectra obtained on (a) pristine and (b) irradiated polycarbo...
Figure 7.17 Relative intensities for different bond types at the polycarbonate...
Chapter 8
Figure 8.1 Publications on PBII and PBII&D of polymers vs. time.
Figure 8.2 Distribution of related publications by polymers modified.
Figure 8.3 Distribution of related publications by properties studied.
Figure 8.4 Experimental PSII apparatus.
Figure 8.5 Waveforms of the discharges with C
2
H
2
.
Figure 8.6 Characteristic I-V curves for the pulse. ▲: Strong rf discharge, △:...
Figure 8.7 Evolutions of water contact angle and total surface free energy for...
Figure 8.8 SEM pictures of various surfaces ( X100,000). (a) Untreated PET fil...
Figure 8.9 AFM photomicrographs of (a) PET control and (b) PIII-PET.
Figure 8.10 XPS spectra of carbon films. (a) C
2
H
2
with strong rf discharge. (b...
Figure 8.11 Raman shift spectrum of the thin carbon layer produced from C
2
H
2
w...
Figure 8.12 Raman shift spectra. (a) Case 1: C
2
H
2
with strong rf discharge. (b...
Figure 8.13 FT-IR spectra by reflection method. (a) Untreated PET film. (b) C
2
Figure 8.14 Ultimate tensile strength [N/mm
2
] for poly(etheretherketone) resin...
Chapter 9
Figure 9.1 Typical example goniometer images of an incident droplet on a polym...
Figure 9.2 Typical adhesion force hysteresis curve for water on Polytetrafluor...
Figure 9.3 Typical 3-D profiles of (a) CO
2
laser surface engineered polyamide ...
Chapter 10
Figure 10.1 Polysort, monosort and spacer bonded monosort functionalization of...
Figure 10.2 Poly- and monosort functionalization of polyolefin surfaces before...
Figure 10.3 Behavior of physical interactions and covalent bonds at increasing...
Figure 10.4 Gas phase oxidation of ethane.
Figure 10.5 Poly- and monosort functionalizations are not advantageous because...
Figure 10.6 Auto-oxidation initiated by chain scission in presence of oxygen (...
Figure 10.7 Methods to oxidize polyolefin surfaces.
Figure 10.8 Plasma- and post-plasma formed polysort functional groups on expos...
Figure 10.9 Development of a steady-state between introduction of oxygen funct...
Figure 10.10 Time dependence of O-insertion into polypropylene and simultaneou...
Figure 10.11 Schematic models for the formation of low-molecular weight oxidiz...
Figure 10.12 Difference in surface free energy between aluminium and polyolefi...
Figure 10.13 Peel strength of 100 nm Al on PP and polar contribution to surfac...
Figure 10.14 Wet-chemical post-plasma reduction of oxidized polyethylene [vitr...
Figure 10.15 Complexation of Cu
2+
with ethylenediamine units of poly(allylamin...
Figure 10.16 Examples of polyolefin modified with functional groups by H subst...
Figure 10.17 Poly(vinyl alcohol), poly(vinyl alcohol)-copolymer, polyethylene ...
Figure 10.18 Yields of bromination and post-plasma oxidation in polyethylene.
Figure 10.19 Loss in bromination at polyethylene surfaces by 15 min washing of...
Figure 10.20 Bond breaking along the interface because of different thermal ex...
Figure 10.21 Schematic presentation of monosort functionalized polyethylene su...
Figure 10.22 Bromination of polyethylene on exposure to a plasma of elemental ...
Figure 10.23 Models of copolymers synthesized chemically or plasma chemically.
Figure 10.24 Proposed bonding of carboxylic groups of poly(acrylic acid) to me...
Figure 10.25 Peel strength of 150 nm thermally vacuum-evaporated aluminium dep...
Figure 10.26 Peel strength in dependence on COOH concentration of the primer.
Figure 10.27 Competition for influence of mono- or polysort surface functional...
Figure 10.28 Ethylene-allyl alcohol and ethylene-acrylic acid copolymers with ...
Figure 10.29 Schematics of polysort, monosort and monosort-spacer functionaliz...
Figure 10.30 Ethylene-allyl alcohol and ethylene-acrylic acid copolymers graft...
Figure 10.31 Comparison of peel strength increments for monosort directly bond...
Figure 10.32 Maximum spacer concentration at polyolefin surface in dependence ...
Figure 10.33 A series of ethylene glycols (diols) with different chain lengths...
Figure 10.34 Water impermeable protection layer.
Figure 10.35 Principle structure of the interface between aluminium and polyet...
Figure 10.36 Survey on maximum peel strength results for differently modified ...
Figure 10.37 Scheme of the ideally designed interface for a metal-polymer lami...
Chapter 11
Figure 11.1 Various techniques for surface modification.
Figure 11.2 Dendritic characteristics affecting surface properties.
Figure 11.3 Applications of amine-terminated dendritic materials in adhesion.
Figure 11.4 Interactions between Cr ions and halloysite functionalized by amin...
Chapter 12
Figure 12.1 Binding of Integrins to a wide range of ligands. At least seven ty...
Figure 12.2 General mechanism for covalent immobilization of RGD to a substrat...
Figure 12.3 RGD immobilization on different polymers using the click chemistry...
Figure 12.4 General reaction scheme for modifying an RGD peptide with a photor...
Figure 12.5 (a) Photochemical reaction of a caged RGD peptide attached to a cu...
Figure 12.6 Agarose gel with RGD gradient on surface (Top). The shape of the g...
Figure 12.7 Schematic of multiphoton chemical patterning in the agarose hydrog...
Figure 12.8 μCP of nucleophilic inks (typically: amines) on active ester deriv...
Figure 12.9 Schematic representation of PEM film construction on a glass slide...
Figure 12.10 Seeded pre-osteoblasts at a distance of 0 mm (a), 5 mm (b), and 1...
Figure 12.11 (a) (Left) Lateral force microscopy image of an expanded region o...
Figure 12.12 Schematic representation of biodegradable oxidized alginate bioin...
Figure 12.13 Cell viability assay of density and viscosity criterion-filtered ...
Figure 12.14 Cross-section of implanted PMMA implants (staining according to G...
Chapter 13
Figure 13.1 Visualization of probability of failure at theoretical interface b...
Figure 13.2 Practical adhesion of a 50 mN/m surface tension epoxy as a functio...
Figure 13.3 A change in (a) as received polypropylene surface topography (2500...
Figure 13.4 Scanning AFM tapping mode images of polypropylene substrate (a) as...
Figure 13.5 Surface energy (mJ/m
2
) and practical adhesion (lap shear, strength...
Figure 13.6 (a) Change in surface energy produced by incorporation of poly(ole...
Guide
Cover Page
Table of Contents
Series Page
Title Page
Copyright Page
Preface
Begin Reading
Index
Also of Interest
WILEY END USER LICENSE AGREEMENT
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, 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. MittalEmail: [email protected]
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Polymer Surface Modification to Enhance Adhesion
Techniques and Applications
Edited by
K.L. Mittal
and
A.N. Netravali
This edition first published 2024 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2024 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-394-23100-3
Cover image: Pixabay.ComCover design by Russell Richardson
Preface
Polymers are used vastly in many industries for a variety of applications. However, their surfaces are intrinsically non-reactive due to their low surface free energy and absence of reactive chemical groups. Concomitantly, their surface modification (also known as surface treatment or surface activation) is a sine qua non to render them adhesionable or bondable.
Even a cursory look at the literature will evince that there is tremendous research activity in devising new and improved ways of ameliorating the existing techniques or coming up with novel ways to modify polymer surfaces. A legion of techniques ranging from mundane to sophisticated, vacuum to non-vacuum, dry to wet, and inexpensive to sumptuous have been documented for surface modification of a variety of polymer surfaces. The choice for an optimum technique depends on a host of factors, inter alia, the chemical structure of the polymer substrate, nature of mating phase, level of adhesion desired and cost. Also, the longevity of treatment is a very significant consideration. There have been different attempts to prolong the life of treatment as well.
The information on the topic of this book is scattered in many different publication media and there is no single, easily accessible, and comprehensive source available. This lacuna in the literature provides the vindication and motivation for bringing out this book.
This book containing 13 (a lucky number) chapters is divided into two parts: Part I: Energetic Treatments; and Part II: Chemical Treatments. Topics covered include: atmospheric pressure plasma treatment of polymers to enhance adhesion; corona treatment of polymer surfaces to enhance adhesion; flame surface treatment of polymers to enhance adhesion; vacuum UV photo-oxidation of polymer surfaces to enhance adhesion; optimization of adhesion of polymers using photochemical surface modification; UV/Ozone surface treatment of polymers to enhance adhesion; adhesion enhancement of polymer surfaces by ion beam treatment; polymer surface modification by charged particles; laser surface modification of polymeric materials; competition in adhesion between polysort and monosort functionalized polyolefinic surfaces; amine-terminated dendritic materials for polymer surface modification;arginine-glycine-aspartic acid (RGD) modification of polymer surfaces; and adhesion promoters for polymer surfaces.
The chapters are written by renowned researchers actively engaged in surface modification of polymers. This book is profusely referenced and copiously illustrated.
This book should of great appeal and interest to polymer scientists, surface scientists, adhesionists, materials scientists, plastics engineers, and to those involved/interested in adhesive bonding, packaging, printing, painting, metallization, biological adhesion, biomedical devices, and polymer composites.
Now it gives us great pleasure to acknowledge all those who played essential roles in giving this book a body form. Naturally, first and foremost, our profound thanks go to the authors for their keen interest, sustained enthusiasm, unwavering cooperation, and sharing their valuable research experience in the form of written accounts (which essentially provided the grist for this book), without which this book could not be materialized. Also, the steadfast interest and whole-hearted support of Martin Scrivener (publisher) in this book endeavor is highly appreciated.
Kash Mittal
Hopewell Jct., NY, USA
E-mail: [email protected]
Anil Netravali
Cornell University
December 2023
Part IENERGETIC TREATMENTS
1Atmospheric Pressure Plasma Treatment of Polymers to Enhance Adhesion
K. Lachmann*, M. Omelan, T. Neubert, K. Hain and M. Thomas
Fraunhofer Institute for Surface Engineering and Thin Films IST, Braunschweig, Germany
Abstract
In this chapter, we present an overview on recent research and development work in the area of atmospheric pressure plasma treatments (APPTs) to generate adhesion-promoting surfaces of polymers used in various applications in automotive, aerospace, packaging, and medical fields. In comparison to the classical “corona treatment” the APPTs provide access to a broader range of industrially interesting surface modifications that are normally better controlled with respect to their physicochemical nature. Thus, application of APPTs may become a superior option for preparing polymer surfaces for adhesive bonding, adhesive-free low-temperature bonding involving homogeneous and heterogeneous substrates, lacquering, or coupling of specific biomolecules, proteins, or cells. APPT technology is, however, not only flexible for tuning surface chemistry but also is flexible with respect to plasma source and equipment design. Versatility of the APPT technology facilitates its integration into a variety of process chains. An example presented here is a hybrid technology combining both APPT and additive manufacturing based on 3D printing processes using fused filament deposition. Inline treatment by quasi-simultaneous execution of printing and APPT can, for instance, increase the adhesion of 3D printed products in print direction (z-Axis) and thus increase the mechanical stability of the printed part. In the medical field such a technology may be attractive for cell growth by promoting treatment of internal surfaces of printed porous scaffolds. In future, products made from biobased or recycled polymers will become increasingly important. APPT technology could become an important enabler for meeting the technical requirements for the adhesion of such products.
Keywords:Atmospheric pressure plasma treatment, chemical groups, functional coatings, bonding, adhesive-free bonding, additive manufacturing, binding of biomolecules
1.1 Introduction
Polymers play an important role in packaging, automotive, medical, biomedical, and aerospace applications. Polymer market still experiences growth, driven by the increased use of polyolefins and also of new biobased and functional polymers. Most of these polymers exhibit inert surfaces that need to be treated to provide sufficient adhesion for lacquers, paints, and adhesive bonding. Treatment can be carried out using a wide range of surface-functionalizing processes including wet-chemical etching [1, 2], plasma [3–5], and laser modification [4, 6, 7], as well as ozone and UV treatment [4], many of which are nowadays well established in industry. In recent years, application of cold Atmospheric Pressure Plasma Treatments (APPTs) in industry has experienced very strong growth attributed to the high versatility in applications, low investment cost and the very good integration of the technology in existing process chains [8].
Changes in technical, social, and legal requirements will inevitably lead to changes in processes also in the adhesive bonding industry which are therefore the focus of this review. Adhesive bonding technology is used in a wide range of products as it offers various material combinations, longterm stability and safety while maintaining material properties and creating additional functions in the bonded product. Continuous inventions and innovations in the development of raw materials, adhesives and bonded products have enabled a dynamic development in the past decades.
Innovative bonding technology enables new, more environmentally-friendly and sustainable products and processes in many areas. For instance, improved resource efficiency can be achieved in the production of renewable energy power plant components where sealing of solar cells or joining of wind turbine rotor blades has become possible. Another example is the electromobility area with the sealing of battery cells, heat management of batteries with heat-conducting adhesives and hermetic sealing of fuel cells. In addition, adhesives with improved product properties can lead to savings, even in food packaging to increase shelf-life with resource-efficient materials.
In Europe the “European Green Deal” with its agenda responding to current socio-political changes will bring additional requirements and thus new challenges for bonding technology [9, 10]. The main point is the transition from a linear to a circular economy, minimizing the use of resources, generation of waste and emissions, and the inefficient use of energy. This will be achieved by considering resource needs, life extension, repair, refurbishment, recycling and closing energy and material loops.
The eco-friendly APPT technology with its innovations in combining suitably functionalized surfaces with bonding technology certainly has the potential to meet the new set of requirements that may result from implementation of the agenda of the European Green Deal.
These new challenges must be realized in terms of closed-loop systems along the value chain. Therefore, life cycle assessments (LCAs) enable a holistic view of adhesively bonded products with regard to ecological and economical improvements along the complete value chains [11, 12]. In this context atmospheric pressure plasma treatments (APPTs) of polymers and other materials like paper, rubber, fabrics, steel, glass and different composite materials have gained considerable importance in recent years due to their technological and economical capabilities and thus offer the potential to become the future leading technology for surface functionalization to improve adhesive bonding technology. The main advantages of APPTs are that they need no expensive equipment, are easy to handle, have a very good scalability and can simply be integrated in existing process lines.
Considering that in most applications materials are joined with other structural parts, the adhesion behavior is of great significance. The adhesion property is strongly affected by the chemistry and morphology of the surfaces. APPTs alter surface morphologies and chemical composition of different materials without affecting their bulk properties. The plasma treatment can also change the surface from hydrophobic to hydrophilic state and vice versa, depending on the type of monomer or process gas used for surface activation. Thus, plasma treatment can alter the inert polymer surface to have greater chemical/physical affinity by incorporating chemical functional groups. This leads to an increase in surface free energy and enlargement of the contact area and thus improved adhesion property. Different research groups have carried out a variety of APPT processes to improve the surface properties of materials [13–19].
These cold plasma treatments have gained increasing interest as they represent environmentally-friendly solution and can even be performed under atmospheric pressure conditions at relatively low cost [20–22]. The plasma treatment can also affect the morphology by exerting the etching effect on the surface and changing the surface roughness and wettability. This results in improved adhesion of metal/plastic compounds [23]. The cold plasma treatment of various plastics such as polypropylene, polyethylene, poly(ethylene terephthalate), poly(etheretherketone), etc. has been investigated [24–26].
In this chapter, we will focus on the use of APPTs to improve the adhesion by chemical functionalization for different applications. Firstly, we will give a short overview about the historical development of APPTs. The second section is about the establishment of functional surfaces with a high number of different chemical groups. In the third section, the adhesion improvement by APPT promoted joining of materials is discussed. In the fourth section we will show the relevance of APPTs for biomedical application for improving e.g. cell adhesion. Finally, we will give an overview on the use of APPTs to promote adhesion in additive manufacturing processes.
1.2 Historical Development of APPTs
The industrialization of APPTs started in 1951, when A. W. Eisby, a Danish engineer, invented a high-frequency treatment with a dielectric barrier discharge in air for polymer foils to improve the adhesion of inks [27]. This so-called “corona treatment” is nowadays commercially well established in the packaging sector for surface activation and cleaning. In the following decades different kinds of plasma sources for APPT working with air as process gas were developed, enabling the treatment of flat and complex surfaces [5]. In the meanwhile, APPTs have been established in the biological, pharmaceutical and medical fields, in automotive, aerospace, and electronics industries and have become important for the alternative energy sector, additive manufacturing as well as for the pretreatment of renewable and sustainable materials. Figure 1.1 shows the most common sources like dielectric barrier discharges, stabilized corona systems, and plasma jets or torches which can be operated using different types of excitations, e.g., alternating current (AC), pulsed direct current (DC), low-frequency (kHz), radio frequency (RF, 13.56 MHz) and microwaves (MW) with a frequency of 2.45 GHz.
Today APPTs can be used to functionalize a surface with various chemical groups including e.g. -OH, -COOH, -NH2, -NOx to improve the adhesion of, e.g., adhesives, lacquers and paints to polymers. Using nitrogen and nitrogen-hydrogen mixtures as process gases, for example, imines, amines or amides can be introduced on the polymer surface, frequently leading to higher surface free energy, better long-term stability and in many applications the desired high level of adhesion. DBD-based APPT using nitrogen or its mixtures with other gases like hydrogen as process gas are in the meanwhile commercially available on the market and have been recognized as a very good alternative to existing wet-chemical etching processes.
Figure 1.1 Examples of different types of plasma sources for APPTs; (a) Dielectric Barrier Discharge (DBD), (b) Stabilized Corona, (c) DBD jet, and (d) Arc jet.
Figure 1.2 Number of publications on surface functionalization with APPT with relevance to adhesion (from Web of Science – date Nov. 15, 2022).
In the last decades an increasing number of leading research groups have worked on the topic of adhesion-promoting surface modifications for coating processes with atmospheric-pressure plasmas. Depending on the precursors and processes used, they have demonstrated that high densities of chemical functionalities such as alcohols, epoxides, amines, carboxylates or silanols can be obtained on the surface. Current statistics of publications as available via the website from “Web of Science” for surface functionalization using APPTs with relevance to adhesion is shown in Figure 1.2. With presently 170 publications per year this number has increased fivefold compared to 2003.
1.3 Functional Groups Produced by APPTs
Depending on the atmospheric pressure plasma system used, process gases, precursors and process parameters, the surfaces of several materials can be cleaned, activated or equipped with a wide range of functional layers.
Figure 1.3 shows the generated chemical species by a microdischarge in air of a dielectric barrier discharge (DBD), which has been modelled by Eliasson and Kogelschatz [28]. The predominant species are electrons, ionized species, free radicals, ozone and NOx as well as UV light.
They have shown that the composition of species present varies depending on the timescale within a microdischarge of a DBD. On a nanosecond scale radicals and excited oxygen and nitrogen are dominant, whereas on a microsecond scale mostly ozone, NOx and excited nitrogen are present. In the scale of seconds, ozone and also N2O are the dominant species.
Compared to a cold DBD at room temperature, arc torches have higher temperatures of more than 600°C. At this high temperature the concentration of NOx species will increase strongly and thus lead to different surface chemistry.
With the aforementioned reactive species organic contaminants, generally present on the surface, can be decomposed and removed. At the same time the surface becomes activated by formation of functional moieties such as nitrogen- and oxygen-containing groups. This leads to significant changes in the chemical composition of the topmost layer and thus in the surface free energy of the treated polymer. Direct electrical charging of the polymer surface by trapping ions by CH2 groups of the polymer can also contribute to the increase of surface free energy [29].
Figure 1.3 Chemical species generated by a microdischarge in air (from [28]).
Figure 1.4 Important reaction steps for the activation of polypropylene surface (adapted from [30]).
In general, the surface free energy can be increased by atmospheric plasma treatment. A basic model for polypropylene was described by Dorai and Kushner and is presented in Figure 1.4 [30].
The first step is the formation of alkyl radicals which can also transform into allyl radicals. The next step is the formation of peroxide radicals by the reaction of the alkyl radicals with oxygen (diradical). The peroxide radicals abstract hydrogen from other alkyl groups and this leads to formation of new alkyl radicals. The hydroperoxides formed can react to form various stable oxygen functionalities like aldehydes, ketones or carboxylates, resulting in “active” surfaces and increased surface free energy and in many cases, this leads to an improved adhesion.
In general, the enhancement of the bonding quality is highly dependent on the material, the type of APPT system and treatment conditions used. An interesting example for future use of sustainable materials is the plasma treatment of wood. Solvent- and water-based polyurethane (PU) coatings showed deeper penetration in pre-treated wood in comparison to untreated wood substrates [31].
1.3.1 Nitrogen-Based Surface Modification
In addition to the classic “plasma pretreatment” of surfaces in air routinely used today, more and more processes with a defined oxygen-free (inert) gas atmosphere are becoming relevant. Such a defined atmosphere is necessary to establish specific chemical groups on the surface of the material and to avoid the influence of reactive oxygen groups. The easiest way is the use of nitrogen or nitrogen-hydrogen (forming gas) mixtures as process gas. Figure 1.5 shows possible functional groups on polypropylene surface generated by corona treatment and APPT using nitrogen/hydrogen as process gas. For example, when treating polypropylene with nitrogen or nitrogen/hydrogen mixtures, a relatively high density of N-containing reactive groups such as imines or amines can be generated on the surface [5, 32, 33].
In investigations studying the treatment of polypropylene surfaces with an AC Corona System stabilized by a dielectric barrier (RotoTEC, Tantec A/S, Denmark) it was found that using nitrogen and especially nitrogen-hydrogen mixtures higher surface free energy levels could be reached compared with classical air treatment and are presented in Figure 1.6. This indicates a difference in the types of surface compositions in these two cases. Further, surface free energy changes achieved with the N2/H2 mixtures were observed to be stable over a period of several weeks [34]. Moreover, in the case of adding hydrogen a long-term stability of the surface free energy for several weeks was shown.
Figure 1.5 Functional groups by treatment of polypropylene surface in air (Corona) and in nitrogen-hydrogen atmosphere (APPT) (from [32]).
Figure 1.6 Left: Surface free energy of biaxially oriented polypropylene (BOPP) in dependence of treatment times for different process gases. Right: Stability of surface free energy of BOPP in dependence of the process gas at a treatment time of 9.6 s (from [34]).
Klages and coworkers [35] found that that polyolefin surfaces plasma treated in a nitrogen atmosphere showed an amphiphilic character, i.e., the surface may react as electrophile or as a nucleophile. The polymers treated in post-discharges of nitrogen-hydrogen DBDs not only show reactivity towards the electrophilic 4-(Trifluoromethyl)benzaldehyde (TFBA) but also react as electrophiles themselves, i.e., the afterglow-plasma-treated surfaces react also with nucleophilic chemicals. In further work Klages and coworkers [36] examined polyolefin surfaces treated using nitrogen-containing plasmas. They used chemical derivatization analysis to determine the generated functional groups. Using nitrogen or nitrogen-hydrogen containing plasmas, imines are generated on the surface, which show a very varied chemical reactivity towards aromatic aldehydes, carboxylic acid anhydrides, pyrylium dyes, isothiocyanates and with fluorescent markers like fluorescamine.
Dimic-Misic et al. [37] have investigated nitrogen plasma treatment of micro/nanofibrillated cellulose films, which results in increase of wettability of the surface by both polar water and nonpolar hexadecane. The total surface free energy increases with nitrogen plasma treatment with a major increase in the polar component. The surface area per unit mass was increased by the N-plasma treatment due to increased roughness on a nanometer scale which improves the adhesion property.
Besides the treatment with nitrogen-containing gases also coating processes were investigated. Chen et al. [38] have described a partial modification of BOPP film via a hydrophilic modification using allylamine (ALA) monomer as precursor. ALA was polymerized on the surface by Ar/O2 or He/O2 plasma. The results showed a permanent hydrophilic modification of the BOPP by grafting amine groups on the surface via the ALA process. The plasma modification created many micro/nano-sized holes in the BOPP film, which increased the surface roughness dramatically and the increased roughness strengthened the adhesion to the polymer surface.
Klages and coworkers [39] have shown the potential of surface modification by barrier discharge for a pure amination of the surface using allylamine, diaminocyclohexane, and 3-Aminopropyltrimethoxysilane (APTMS) as precursors. Morand et al. [40] investigated the direct deposition of a thin film using a plasma jet system and APTMS as precursor. The quantification of NH2 showed that polymerization of APTMS and degradation of NH2 groups on the surface directly correlated with Yasuda’s parameter. NH2 concentration up to (3.7 ± 1.3) groups/nm were found on the coated surfaces.
1.3.2 Oxygen-Based Surface Modification
Using corona treatment, a mixture of oxygen-based groups such as hydroxides, aldehydes, ketones and carboxylates is established on the surface of the polymers. Wagenaars et al. [41] used an atmospheric pressure plasma jet (APPJ) to increase the wettability of polypropylene (PP) films. Reduced water contact angle of PP was observed as a function of distance between APPJ and PP surface and oxygen gas admixture (helium gas with up to 1% oxygen), suggesting that the surface reaction mechanism is related to the atomic oxygen density produced in the plasma. Hence, multiple oxygenspecies can be generated on the polymeric surface offering widespread applications especially with regard to adhesion. He-based process gas flow can also be used for hydrophilic surface modification by locally generating reactive oxygen-species, which lead to increased wettability [42].
Of high interest is the use of precursors with a single oxygen containing group to equip the surface with a monofunctional reactive group. Chen et al. [43] describe an atmospheric pressure plasma process with the capability of rapidly depositing carboxylic acid groups by using acetic acid/isopropanol solution. It was demonstrated that with this technique to functionalize polymer surface 7.8% carboxylic groups were found on poly(ethylene terephthalate) (PET) and 6.9% on poly(lactic acid) (PLA), which can be used for further applications. Thomas et al. [44] have investigated pulsed plasma processes using maleic acid anhydride (MAA) as precursor and achieved coatings with more than 10 % carboxylic groups.
Coatings with a large number of epoxy groups were also described, mostly with glycidyl methacrylate (GMA) as precursor. Using pulsed plasma processes the degree of monomer retention can be controlled. The pulsed plasma process can be divided into two phases - the plasma pulse (plasma on), where radicals will be produced by fragmenting the precursor into radicals and the pause time (plasma off), where reactions of the radicals with GMA take place with a high retention of the epoxy group of the monomer [45, 46].
1.4 Adhesion Improvement for Bonding
As described before, the variety of chemical functional groups created by plasma surface functionalization is very broad. For improving the adhesion to polymers two different ways will be presented. In the first part the classical adhesive bonding using chemically reactive surfaces combined with a subsequent adhesion process is described. By adjusting the chemical interaction between the surface and the adhesive very strong improvements of adhesively bonded joints of various materials can be achieved. In the second part an adhesive-free joining process will be presented. In this process the reactive surfaces interact with each other without any adhesive. The high surface reactivity leads to very good adhesion results between different types of materials.
1.4.1 Adhesive Bonding by Functional Groups
In recent studies [47, 48] an atmospheric pressure plasma jet was used for surface activation by plasma treatments and coatings by plasma-polymerization on flat and mechanically roughened elastomeric styrene-butadiene-rubber to improve the bond strength of SBR/leather joints based on PU-adhesive. For the coating treatment, the amine-based precursors such as 3-aminopropyltriethoxysilane (APTES) or 3-aminopropyltrimethoxysilane (APTMS) were selected to obtain functional nitrogen-groups on the surfaces.
Inoue et al. [48] demonstrated improved plasma-enhanced adhesion of conducting polymers such as poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS), polypyrrole (PPy), and polyaniline (PAni) which have been widely explored as coatings for electrodes. They report a simple method to achieve strong adhesion of various conducting polymers including PEDOT:PSS, PPy, and PAni on diverse commonly used insulating and conductive substrates including glass, polyimide, poly(dimethylsiloxane) (PDMS), indium tin oxide (ITO) and gold. Based on a hydrophilic PU-adhesive layer and an APTMS-coating, the adhesive layer strongly adheres to the substrates and moreover forms an interpenetrating polymer network with the conducting polymer.
