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Atmospheric Pressure Plasma As a Method for Improving Adhesive Bonding Authors THOMAS S

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Atmospheric Pressure Plasma As a Method for Improving Adhesive Bonding Authors THOMAS S TP14PUB43 Atmospheric Pressure Plasma as a Method for Improving Adhesive Bonding authors THOMAS S. WILLIAMS, HANG YU, RICHIE WOO, MIKHAIL GRIGORIEV, DICK CHENG, and ROBERT F. HICKS Aerospace Materials Processing LLC Redondo Beach, CA abstract Atmospheric pressure plasma treatment is an alternative surface preparation method for plastics and organic matrix composites prior to structural adhesive bonding. The atmospheric pressure plasmas are a promising technique for replacing traditional methods of surface preparation such as manual sanding, grit blasting, peel ply, or wet chemistry. All surfaces studied were converted from a hydrophobic state into a hydrophilic state with a water contact angle of <40°. Atomic force microscopy confirmed that plasma activation resulted in only minor changes to the composite surface structure. Characterization of the composite surfaces by X-ray photoelectron spectroscopy revealed an increase in oxygen content following plasma treatment. Lap shear testing showed that after plasma activation, the strength can be increased from ~50% up to several-fold over that achieved by either solvent wiping or abrasion depending on the material used. terms Plasma treatment, Surface preparation, Adhesive bonding, Plastics, Matrix composites conference Composites Manufacturing 2014 SME April 8-10, 2014 Covington, KY SME TECHNICAL PAPERS This Technical Paper may not be reproduced in whole or in part in any form without the express written permission of SME. By publishing this paper, SME neither endorses any product, service or information discussed herein, nor offers any technical advice. SME specifically disclaims any warranty of reliability or safety of any of the information contained herein. SME Technical Paper TP14PUB43 ATMOSPHERIC PRESSURE PLASMA AS A METHOD FOR IMPROVING ADHESIVE BONDING Thomas S. Williams, Hang Yu, Richie Woo, Mikhail Grigoriev, Dick Cheng and Robert F. Hicks Aerospace Materials Processing LLC 2631 Manhattan Beach Blvd. Redondo Beach, CA 90278 ABSTRACT Atmospheric pressure plasma treatment is an alternative surface preparation method for plastics and organic matrix composites prior to structural adhesive bonding. The atmospheric pressure plasmas are a promising technique for replacing traditional methods of surface preparation by manual sanding, grit blasting, peel ply, or wet chemistry. All surfaces studied were converted from a hydrophobic state into a hydrophilic state with a water contact angles of <40°. Atomic force microscopy confirmed that plasma activation resulted in only minor changes to the composite surface structure. Characterization of the composite surfaces by X-ray photoelectron spectroscopy revealed an increase in oxygen content following plasma treatment. After plasma activation, the composite surfaces displayed specific chemical changes, i.e., the formation of alcohols, ketones or aldehydes and carboxylic acid groups. Lap shear testing showed that after plasma activation, the strength can be increased from ~50% up to several fold over that achieved by either solvent wiping or abrasion depending on the material used. The trends in adhesion with plasma exposure time do not correlate well with surface wetting or roughness; instead they correlate with the fraction of the polymer surface sites that are converted into carboxylic acid functional groups. The data to be presented will include and detailed bond failure mode evaluations in hot-wet environmental conditions. 1. INTRODUCTION The adhesion of bonded joints is of great concern to the aerospace and automotive industries [1]. Metal-polymer composites can produce structures that are significantly lighter, but with the same strength and durability as traditional metal structures. Adhesively bonded metal-plastic composites offer advantages over overmolding, riveted structures and other mechanical joining techniques because it yields a continuous bond between the two substrates, minimizes stress, and acts as a buffer between the metal and plastic to absorb impact [2-7]. Additional technical challenges still exist for bonded structures and repairs before widespread implementation can be achieved. This includes qualification of the adhesive bonding process in terms of bond strength and durability [8]. An understanding of the mechanism responsible for attaining proper surface preparation and adhesion is also needed. Atmospheric pressure plasma activation is rapidly gaining acceptance as a desirable method of surface preparation prior to bonding [1]. The use of this method in the joining of composite surfaces has been the subject of several publications [1,9,10]. This technique provides an alternative to traditional methods of surface preparation by wet chemical cleaning and mechanical abrasion [1,11]. While these traditional procedures are well tested and reliable, they 1 SME Technical Paper TP14PUB43 are also time consuming manual processes. Additionally, when not properly administered, these techniques can yield poor bond strengths, and can even result in damage to composites that are fabricated with high modulus fibers [1,11]. Furthermore, abrasion methods generate dust which is an environment, health, and safety concern. Consequently, there is a need to explore alternative, environmentally friendly approaches that activate the surface without damaging the bulk material. Atmospheric pressure plasmas show promise for fulfilling this need. In this paper, we review recently published work using atmospheric pressure, radio frequency, capacitive discharge plasmas to activate metal, polymer and composite surfaces. These particular plasmas have been fully characterized, so that we know the concentrations of the reactive species that come in contact with the surface [12-15]. The mechanism of adhesion improvement by atmospheric pressure plasmas has been a matter of debate. A number of factors have been proposed to contribute to the higher bond strengths observed. Among these are higher surface energy as measured by the water contact angle (WCA), greater surface roughness, and a desirable change in the chemical composition of the surface. These concepts are examined in this paper. 2. EXPERIMENTAL The composites used by Zaldivar et al. [10] were prepared from 8 prepreg plies made with Nelcote E765 epoxy and AS4 PAN-based carbon fibers, and laid up in a unidirectional configuration. The resulting coupons were bonded together and cured using Hysol EA 9394 adhesive paste. The lap shear specimens had a nominal width of 25.4 mm with a 15.2 mm overlap. Previous work has also been performed on carbon-fiber-reinforced epoxy (CFRE) composites and stainless steel treated using atmospheric pressure plasma [16]. Wear-resistant stainless steel type 410 was purchased from McMaster-Carr. Grade 410 stainless steel (11.5- 13.5% Cr, <1.0% Mn, <1.0% Si, >0.75% Ni, <0.15% C, <0.04% P, <0.03% S, balance Fe) is common in automotive parts. The stainless steel sheets were 30.5 cm × 61.0 cm × 0.23 cm thick. These sheets were later cut down into smaller sample sizes as per ASTM specifications. A rigid carbon-fiber/epoxy composite was purchased from McMaster-Carr. This composite had a tensile strength of 120,000 psi. Additional studies have been performed on many other substrates, but will not be discussed in detail here. Hicks et al. [17] have studied the surface preparation of carbon fiber reinforced polyetheretherketone (PEEK) composites. Water contact angle analysis was conducted using polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and high density polyethylene (HDPE) [18,19]. A number of adhesives are reported as being used for bonding including urethane (Hardman, Inc. 04022), epoxy (Hardman, Inc. 04005), acrylic (3M Scotch-Weld DP805), and silicone (NuSil Technology MED1-4013) [12,18]. The study performed by Hicks et al. [17] used 3M AF563 film adhesive to bond PEEK samples. The atmospheric pressure plasma used for surface preparation was a Surfx Technologies AtomfloTM system. This machine was fed with helium and up to 3.0 volume% oxygen. The discharge was struck with radio frequency power at 13.56 MHz. Zaldivar et al. [1,10] treated their composite samples with a 25-mm linear beam plasma at 96 W of RF power, 0.45 L/min oxygen, 15.0 L/min helium, 1.0 mm source-to-sample distance, and a 25.4 mm/s scan speed. Work perfomed on CFRE and stainless steel as well as the work performed by Gonzalez et al. 2 SME Technical Paper TP14PUB43 [12, 16, 18-20] treated their polymer samples with a 50-mm linear beam plasma operated at 200 W RF power, 0.8-0.9 L/min O2, 30.0 L/min He, 3.0-5.0 mm source-to-sample distance, and 10 mm/s scan speed. Some stainless steel samples were abraded with 3M Stikit 300D aluminum oxide stearate-free 180 grit sanding disks attached to a random orbital sander. The stainless steel and CFRE composite coupons were bonded together using Cytec FM300-2 film adhesive. The film adhesive had a shear strength of >5000 psi. In most cases, the stainless steel was coated with Cytec Industries BR6747-1 metal bond primer prior to applying the adhesive. A Krüss EasyDrop goniometer was used to measure the water contact angles on the surface of the plasma activated surfaces. For each test, the mean and standard deviation of ten droplets, 2 μL in volume, were obtained. Surface roughness values were measured using a VEECO DI3100 atomic force microscope (AFM). The root-mean-square (RMS) surface roughness and total surface area of the samples were obtained by using the software program, Nanoscope V6.12r2. Surface composition was analyzed by X-ray photoelectron spectroscopy (XPS). Core level photoemission spectra of the C 1s and O 1s lines were collected with a PHI 3057 spectrometer using Mg Kα X-rays at 1286.6 eV, and a 23.5 eV pass energy. The detection angle with respect to the surface normal was 25°. All spectra were referenced to the C 1s peak of the graphitic carbon atom at 285.0 eV. The integrated areas of the C 1s and O 1s photoemission peaks, divided by their sensitivity factors, 0.30 and 0.71, respectively, were used to determine surface atomic percentages. Double-notch lap shear strengths were measured following the ASTM D-3165 procedure [21].
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