Plasma treatment of polymers for surface
and adhesion improvement
Dirk Hegemann *,Herwig Brunner,Christian Oehr
Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB,Nobelstrasse 12,70569Stuttgart,Germany
Abstract
Different plasma treatments in a rf discharge of Ar,He,or N 2are used to etch,cross-link,and activate polymers like PC,PP,EPDM,PE,PS,PET and PMMA.Due to the numerous ways a plasma interacts with the polymer surface,the gas type and the plasma conditions must be adjusted on the polymer type to minimize degradation and aging effects.Wetting and friction properties of polymers can be improved by a simple plasma treatment,demonstrated on PC and EPDM,respectively.However,the deposition of ultra-thin layers by plasma enables the adjustment of wetting prop-erties,using siloxane-based or fluorocarbon films,and further reduction of the friction coefficient,applying siloxane or a-C:H coatings.Nevertheless,the adhesion of plasma-deposited coatings should be regarded,which can be enhanced by depositing a graded layer.
Ó2003Elsevier B.V.All rights reserved.
Keywords:Polymers;Rf plasma;Aging;Wettability;Adhesion;Friction
1.Introduction
Polymers are distinguished by their low density,flexibility,ease of manufacture,and cost-effec-tiveness.However,their surface properties often do not meet the demands regarding scratch-resis-tance,wettability,biocompatibility,gas transmis-sion,adhesion,or friction.Hence,an additional surface modification is required to achieve the desired properties,while maintaining the charac-teristics of the volume [1].Primarily,a plasma treatment provides manifold possibilities to refine a polymer surface,enabled by the adjustment of
parameters like gas flows,power,pressure and treatment time.
Depending on the gas composition and plasma conditions,ions,electrons,fast neutrals,radicals and VUV radiation contribute to the polymer treatment,resulting in etching,activation and/or cross-linking [2,3].
Degradation of polymer surfaces mainly occurs when a rapid interaction with radicals or ions takes pla
ce.Here,intensity and duration of the plasma treatment are of importance.While a short treatment of PP causes only outer chain scissions,which may lead to improved surface properties,a more intense treatment also yields inner chain scissions,supported also by VUV radiation [4].Plasma treatment of polymer surfaces causes not only a modification during the plasma expo-sure,but also leaves active sites at the surfaces
*Corresponding author.Tel.:+49-711-970-4028;fax:+49-711-970-4200.
E-mail address:hegemann@igb.fraunhofer.de (D.Hege-mann).
0168-583X/03/$-see front matter Ó2003Elsevier B.V.All rights reserved.
doi:10.1016/S0168-583X(03)00644-X
reactor debug mode is enabled
Nuclear Instruments and Methods in Physics Research B 208(2003)
281–286
which are subject to post-reactions;this is also called aging[5].Aging effects depend on external influences like adsorption or oxidation,and on the internal tendency to attain an energetically favor-able state(thermodynamic equilibrium)by re-structuring processes and diffusion.Especially,a high-energy surface is prone to adsorb contami-nants from the atmosphere in order to lower its surface energy[6].
The investigations in this work aim for the im-provement of wettability and adhesion,as well as for the reduction of friction of polymer surfaces.
2.Experimental
The plasma treatment of different polymers (PMMA,PET,PC,PC/ABS,PP,PS,PE and EPDM)were carried out in a symmetrical,ca-pacitively coupled rf discharge reactor.The mea-surement of the maximum excitation voltage V0ðV rfðtÞ¼V0sinðx tÞÞshows that well-defined con-ditions over a wide parameter range are present (Fig.1).The plasma reactor is described in more detail elsewhere[7].Hardness was obtained using a TriboScopeâfrom Hysitron Inc.with a calibrated Berkovich indenter.Wettability was examined by measuring the water contact angle with the sessile drop method.To measure the peel strength of the plasma coatings,an adhesive-tape peel test was performed according to ASTM D3359.By using tapes with different adhesion to SiO x,the mea-sureme
nt of the practical adhesion is limited be-tween1and8N/cm.Friction was measured according to the German standard DI N53375. 3.Results and discussion
A plasma interacts in numerous ways with a polymer surface.At rather severe conditions,the polymer surface is subject to etching effects, whereby the polymer is continuously degraded. While chemical etching is promoted by radicals as in O2,CF4and SF6discharges,physical etching occurs under strong ion bombardment,as for ex-ample in Ar plasmas[8].As can be seen in Fig.2, different polymers exhibit different etch rates under nominally equal plasma conditions(Ar plasma, 350W,0.2mbar),where O-containing polymers are more degradable.However,the plasma–sur-face interaction leads to a competition between chain scission and cross-linking in the near-surface layer[9].By varying the plasma gases used(2min, 350W,0.2mbar),a polypropylene surface,for example,is not only degraded by high-energy
ions
as in Ar discharges,but can also be reinforced by cross-linking,promoted mainly by VUV radiation as in He or N 2(Fig.3).3.1.Wettability
The wettability of polymer surfaces can be im-proved when oxygen functionalities are generated,which can be achieved directly in O-containing plasmas or via post-plasma reactions.However,the new functionalities can be lost again to the atmosphere,resulting in a reduced surface energy [10].On polycarbonate,the lowest aging effects were observed when a N 2plasma treatment was used (5min,350W,0.2mbar),compared to O 2,Ar and mixtures with N 2.However,an
ultra-thin
plasma-deposited layer like SiO x yields a more stable hydrophilic treatment(Fig.4).Withfluo-rocarbon or siloxane layers,a hydrophobic surface can also be attained.Nevertheless,to minimize aging effects,a stable and well-adherent plasma-deposited layer is required[11].
3.2.Adhesion
Especially SiO x coatings on polycarbonate, which is considered as a glass substitute,are sub-ject to aging due to water diffusion into the inter-face[12]and resulting hydrolytically unstable siloxane bonds[13].To improve the adhesion,a plasma pre-treatment(5min N2plasma)of the PC isfirst carried out to clean,activate and cross-link the polymer surface[14].Since SiO xfilms are ra-ther brittle and usually possess compressive stres-ses of about100MPa,thefilm properties should be adapted to the polymer,which can be achieved by the deposition of a graded layer[15].By con-tinuously increasing the O2/HMDSOflow in the plasma,the residual carbon content in thefilms is gradually reduced,thereby forming an interphase of about200nm thickness.Thus,the peel strength measured directly after deposition can be increased from about2.5N/cm to about8N/cm for1.5l m thick SiO x coatings on PC(Fig.5).Nevertheless,this useful adhesion value is subject to peel strength decreases with time of storage at ambient conditions in air.The analysis of the failed surface by XPS proved that thefilm always detached in the interfacial region of PC and SiO x.
A reducedfilm thickness reveals less aging,which can be explained by a lowered force exerted by the internal stress of thefilm[16];however,storing in deionized water shows a more rapid aging(Fig.5). With SiO xfilms on a polymer blend of PC and ABS(50:50),on the other hand,no aging effects could be observed over one year storage in air. Hence,aging offilm adhesion depends strongly on the polymer,the internal stresses in the plasma-deposited coating,and the storage conditions. 3.3.Friction
Friction on an atomic scale is based on dissi-pative mechanisms,due to the‘‘plucking’’action of one atom on another[17,18].On a microscopic scale,friction is determined by the true contact area of two interacting surfaces,the shear strength of the interface and the plowing of one surface into another[19,20].These terms can be analyzed by scanning force microscopy,for example[21,22].
Macroscopically,the friction coefficient is de-fined as the quotient of friction force and load.
In
particular,rubber surfaces exhibit high coefficients of friction due to their high tackiness.Therefore,a pla
sma treatment can be suitable to reduce the friction coefficient of a rubber like EPDM,due to cross-linking of the near-surface layer.In Fig.6it can be seen that a He or Ar plasma(20min,300W, 0.2mbar)reduces friction of EPDM rubber against steel,where Ar appears to be more effec-tive.Moreover,it was found that longer plasma treatments are required to reduce the friction. Repeating the measurements one month after treatment shows no aging of the modified surface. However,it is also clear from Fig.6that ultra-thin plasma-deposited layers can further improve the friction behavior of EPDM rubbers.About40nm thickflexible siloxane and a-C:H layers
were

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