Thin Solid Films 500 (2006) 34 – 40 www.elsevier.com/locate/tsf
Factors affecting the adhesion of microwave plasma deposited siloxane films on polycarbonate B.W. Muir a,b,*, H. Thissen a, G.P. Simon b, P.J. Murphy c, H.J. Griesser c a CSIRO Molecular and Health Technologies, Bayview Avenue, Clayton VIC 3168, Australia School of Physics and Materials Engineering, Monash University, Clayton VIC 3168, Australia c Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia
b
Received 4 May 2005; received in revised form 21 October 2005; accepted 21 October 2005 Available online 1 December 2005
Abstract The effects of a radiofrequency oxygen plasma pretreatment and residual water content in the substrate on the adhesion of microwave plasma deposited tetramethyldisiloxane thin films on Bisphenol-A polycarbonate (BPA-PC) were investigated. Samples were characterised using a crosshatch adhesion test, optical and electron microscopy, and X-ray photoelectron spectroscopy. It was found that the use of a low power (5 W) and low treatment time (0.1 s) oxygen plasma can improve adhesion while greater treatment times (1 – 30 s) and higher oxygen plasma powers (40 W) resulted in a decreased level of adhesion. In addition, it was shown that a BPA-PC water content greater than 90 ppm resulted in rapid adhesion failure of deposited films at the substrate – plasma polymer interface during outdoor weathering. All films degraded substantially when exposed to environmental weathering, indicating ageing reactions within the plasma polymer films themselves, and at the bulk polymer – coating interface. D 2005 Elsevier B.V. All rights reserved. Keywords: Adhesion; Plasma processing and deposition; Polymers; Water
1. Introduction Optical thermoplastics such as Bisphenol-A Polycarbonate (BPA-PC) are replacing glass in a number of industries due to the fact that they are lightweight, can be mass produced, have good mechanical properties, optical transparency and generally high heat distribution temperatures [1]. However, components made from BPA-PC have negative attributes in that they are inherently soft, easily scratched and subject to wear in an abrasive environment. The use of microwave glow discharge plasmas to provide a hard coating for BPA-PC substrates using siloxane monomers is a particularly attractive method to prevent abrasion [2,3]. Advantages of this technique include the fact that the refractive index of the coating can be varied over a wide range to suit the substrate material. In addition, by varying the monomer to gas * Corresponding author. CSIRO Molecular and Health Technologies, Bayview Avenue, Clayton VIC 3168, Australia. Tel.: +61 3 9545 2452; fax: +61 3 9545 2515. E-mail address:
[email protected] (B.W. Muir). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.10.060
feed ratio during plasma deposition, a gradient coating can be produced. This can reduce or even eliminate unwanted optical interference such as Newton’s rings which can arise during the deposition of hard coatings on high index plastic lenses [1]. However, there has been one significant factor limiting the use of plasma enhanced chemical vapour deposition (PECVD) of siloxane monomers as hard coatings on BPA-PC. It is often difficult to obtain good adhesion of vacuum-coated layers on polymeric substrates [4– 6]. This work describes a study aiming to improve the adhesion of tetramethyldisiloxane (TMDSO) plasma polymer (PP) films on BPA-PC. Specifically, the use of an oxygen plasma pretreatment was investigated to provide an improvement in adhesion of the siloxane films to the polymer substrate through the creation of oxygen containing functional groups and radicals, and removal of surface contaminants. In addition, the effect of the residual water content within the bulk polymer on adhesion, which has been reported in previous studies [2,7,8], was investigated in this study with regard to its importance for the BPA-PC/TMDSO PP system. Furthermore, we investigated the performance of coated samples in outdoor weathering.
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2. Experimental details An optical grade BPA-PC (LEXAN RL7221, GE Plastics), with a refractive index (g) of 1.586 was used in this study. Injection moulding of the polymer was performed using a Meiki M-70C DF Dynameltor injection moulder to create disks of 75 mm diameter and 2 mm thickness. The melt temperature was held at 320 -C while the mould temperature was at 90 -C with an injection pressure of 110 MPa and a cooling time of 35 s. Oxygen plasma treatments were performed using a custombuilt reactor described previously [9] and ultra-high purity oxygen gas (BOC gases). Briefly, the cylindrical reactor chamber is defined by a height of 35 cm and a diameter of 17 cm. Within this sit two circular copper electrodes of 10.3 cm in diameter, spaced 15 cm apart. Samples were placed on the lower grounded electrode and a continuous radiofrequency pulse was generated at the upper electrode. The oxygen gas was supplied to the reactor chamber through a regulator and stainless steel line with a manual valve for fine control of the flow. The parameters chosen for the oxygen plasma generation were a frequency of 125 kHz, a load power of 5 W or 40 W and an initial oxygen pressure of 20 Pa for treatment times of 0.1, 1 and 30 s. Samples were stored in desiccators for 7 days prior to deposition of siloxane PP coatings in a separate microwave system. Siloxane plasma coatings were deposited using a custom designed plasma enhanced chemical vapour deposition (PECVD) reactor incorporating a remote deposition Leybold microwave source and antenna design. The microwave plasma chamber was 30 cm long by 12 cm wide. The 2.45 GHz microwave generator consists of a number of components including a magnetron to create the microwaves and a recirculator to divert any reflected power to a dummy waterload. A three-stub tuner was used to match the load impedance and source impedance. The microwave generator was operated at a forward power of 2 kW, with a reflected power of 100 W. The entire coating process was operated with a programmable logic controller. Microwave energy was delivered through a rectangular waveguide and redirected into a coaxial waveguide in which it was split and then directed down opposite ends of two antennas. The antenna system was coupled into the chamber through two aluminium oxide tubes. Oxygen was fed into the rear of the plasma chamber while the monomer was fed directly onto the lenses through two custom built gas rings designed to optimise coating uniformity. The vapour pressure of the monomer was monitored using a diaphragm gauge and the monomer and carrier gases were controlled using mass flow controllers. The custom built cavity in which the BPA-PC disks were held was circular, of a diameter of 10 cm. Gas flow was enhanced through multiple holes arranged in a circle around the cavity wall. BPA-PC disk to monomer ring distance was 6 cm and the distance from the disks to microwave tubes was 12 cm. All BPA-PC samples were pumped down in a transfer chamber for 5 min to a base pressure of less than 0.5 Pa. They were then immediately transferred into the plasma deposition
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chamber at a pressure of 0.1 Pa. Before PP deposition, the system was flushed with monomer three times for 60 s at 150 standard cubic centimetres per minute (sccm). The PECVD hard coatings were deposited from a TMDSO ((CH3)2SiH)2O, (Sigma-Aldrich)/oxygen mixture with a constant TMDSO monomer flow rate of 150 sccm and a varying TMDSO/ oxygen feed ratio. The oxygen flow during deposition was divided into 3 stages: 100 sccm for the first 5 s, then ramping from 100 to 350 sccm over the next 10 s, and 350 sccm for a further 30 s, to generate compositionally graded films with a thickness of approximately 3 Am. The pressure within the chamber increased from 20 Pa to 40 Pa during the complete deposition process and the substrate temperature increased to 45 -C. Following PP deposition, the samples were left within the chamber for 5 min before being removed. For the preparation of polycarbonate disks with different water contents, samples were stored in airtight containers containing saturated salt solutions to obtain varying humidity levels at standard temperature and pressure. The solutions and relative humidity (%) obtained were: NaI—38%, NaCl—75%, Water—100%. A sample containing 90 ppm water was produced by oven drying and another containing 2400 ppm water was produced by storage in air. To assess the water content of the polymer, extruded BPA-PC pellets were kept in the same environments as the injection moulded disks and these were measured in a Computrac 3000 (Arizona Instruments) moisture analyser. Outdoor weather testing was performed by placing samples outdoors for a period of up to 4 weeks and exposing them to the elements (UV, rain, etc.). During this time the samples were exposed to a minimum temperature of 1 -C and a maximum temperature of 23 -C. Specimens were periodically sampled and tested using a crosshatch tape adhesion test. The test was carried out using a six-blade scribe which makes 6 parallel cuts spaced 1 mm apart and transparent film tape (3 M, 25 mm wide, Adhesion; 44 N/100 mm). The crosshatch tape adhesion test was developed from ASTM D3359-87. Cuts in a crosshatch pattern are made in the PP films using the metal scriber and then a piece of pressure sensitive tape is placed over the crosshatch region. An eraser is applied lightly to the tape surface, and the tape is removed after 90 s. An optical microscope is used to determine how much of the coating has been removed from the crosshatch region. Using a template of images, the adhesion is rated on a scale of 5 to 0 (best to worst). A score of 5 corresponds to perfect adhesion of the film and no delamination, a score of 4 corresponds to less than 5% film area delamination, a score of 3 corresponds to a delaminated area of 5 – 15%, a score of 2 corresponds to a delaminated area of 15 –35%, a score of 1 corresponds to a delaminated area of 35– 65% and a score of 0 corresponds greater than 65% delamination of the film. A minimum of 4 coated samples was tested for each treatment condition, with at least 3 crosshatches performed on each specimen. Digital images of the crosshatch tests were captured using an optical microscope and a CCD camera. Samples were kept indoors for 7 days, before being placed outdoors for ageing.
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Scanning electron microscopy was used to image the crosshatched coatings using a Phillips XL30 Field Emission Scanning Electron Microscope (FESEM) in secondary electron mode at an accelerating voltage of 5 kV and various magnifications. To avoid sample charging, a thin gold film was sputtered on the PP surface. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an Axis HSi spectrometer (Kratos Analytical Ltd.) equipped with an Al –Ka monochromated source. The pressure in the analysis chamber was typically 5 10 8 mbar and survey spectra were acquired at a pass energy of 320 eV in order to obtain atomic concentrations of the elements present on the surface. In addition, high-resolution carbon 1s (C1s) spectra were obtained at a pass energy of 40 eV (yielding a typical peak width of 1.0 –1.1 eV). Processing of XPS spectra was performed using the Kratos Vision software. The precision associated with all measurements in this work, correlates to a standard deviation of less than 0.2 at.% for the elements detected. 3. Results and discussion 3.1. Surface characterisation XPS was used to characterise the effect of the oxygen plasma pretreatment and the TMDSO PP film composition on the injection molded BPA-PC disks. Results of the oxygen plasma treatment at 5 W and 40 W for 0.1, 1 and 30 s are listed in Table 1. The average O/C ratio taken from 5 regions on the treated disks is reported. As the oxygen plasma power and treatment time are increased, an increase in the amount of oxygen incorporated into the polymer surfaces is observed. After a 30 s oxygen plasma treatment the O/C ratio increases from 0.17 on the untreated control polymer to 0.22 for 5 W and to 0.26 for 40 W. In comparison, after a 5 W treatment for 0.1 s, only a small increase in the O/C ratio to 0.18 is observed. Quantitative analysis of the surface composition of the TMDSO/oxygen PP coatings enabled comparison with both the TMDSO monomer and similar, conventionally synthesised materials (e.g. polydimethylsiloxane, PDMS). Since the atomic concentration of Si did not change within experimental error during ageing, compositional data will be discussed relative to Si. The PP films were found to contain an atomic concentration of silicon of 34.2%, carbon 20.5% and oxygen 45.3%. As TMDSO contains Si 28.6%, C 57.1%, and O 14.3%, it is evident that the plasma deposition incorporates substantial Table 1 O/C ratios of BPA-PC disks exposed to oxygen plasmas under various treatment conditions Oxygen plasma treatment time (s)
Plasma power (W)
O/C
0 0.1 1 30 0.1 1 30
0 5 5 5 40 40 40
0.17 0.18 0.20 0.22 0.21 0.26 0.26
Fig. 1. Crosshatch adhesion results of TMDSO PP films after 5 W oxygen plasma pretreatments.
amounts of oxygen to create a coating whose nature is intermediate between inorganic silica and polymeric PDMS. The carbon/silicon (C/Si) atomic ratio of 0.6 is significantly lower than that of the monomer (C/Si = 2.0) while the observed oxygen/silicon (O/Si) ratio of 1.3 is significantly higher than that of the monomer (O/Si = 0.5). It is clear from the chemical composition of the film that methyl abstraction is a major process occurring during the microwave deposition of these films. This process has been reported previously for the plasma deposition of siloxane thin films [10]. 3.2. Film adhesion The crosshatch test used in this study is designed to determine the extent to which a coating adheres to a polymer surface before and after exposure to a harsh environment. The test is also suitable to assess the durability of coatings after exposure to a large range of conditions. The results of the crosshatch adhesion scores of the TMDSO films with 5 W oxygen plasma pretreatments after outdoor weathering are listed in Fig. 1. The first two bars of each treatment time refer to samples that were kept indoors prior to outdoor weathering, one for 1 day (1 day/ID) and another for 1 week (7 days/ID) after the initial plasma deposition of the film. The remaining three bars in each treatment time then describe the crosshatch rating after outdoor weathering of the remaining films for up to 3 weeks outdoors (28 days/OD). One day after deposition of the film, all of the oxygen pretreated samples exhibited perfect adhesion according to the crosshatch test method, while the untreated control sample showed some delamination of the TMDSO film at the edges of the crosshatches. An example of the excellent adhesion of the pretreated samples is shown in Fig. 2 where a sample that has been treated for 1 s and kept indoors for 1 week prior to crosshatch testing is shown. Most of the oxygen-treated samples retain a high crosshatch rating score when they are kept indoors, apart from the samples treated for 30 s. After 1 week of storage indoors, samples treated under these conditions start to delaminate at the edges of the crosshatch and therefore attain a crosshatch rating score of 4. The 5 W –0.1 s pretreated coating also performs better than the untreated control, once outdoor weathering is commenced.
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Fig. 2. 5-star adhesion of a sample kept indoors for 1 week after a 1 second, 5 W oxygen plasma pretreatment, after crosshatch adhesion testing.
The results can be readily seen in Fig. 3 where the distance between the crosshatches is 1 mm. After 1 week of outdoor weathering this sample achieved a crosshatch rating of 4 while the untreated polymer had a reduced score of 2. Interestingly, the 30 s oxygen plasma pretreated sample had almost completely failed after 1 week outdoors, scoring a crosshatch rating of 1 while after 2 weeks of outdoor weathering complete failure is observed. After 3 weeks of outdoor weathering all of the films failed substantially with the sample pretreated for 0.1 s scoring slightly better with a rating of 2 compared to the other samples. From these results it is clear that a short treatment time at a power of 5 W can provide an improvement in adhesion between the BPA-PC surface and TMDSO PP thin films. Other researchers have analogously commented on the need for brief oxygen plasma treatments on various polymers
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Fig. 4. Crosshatch adhesion results of TMDSO PP films after 40 W oxygen plasma pretreatments.
to improve the adhesion of subsequently deposited coatings [11,12]. Fig. 4 shows the crosshatch test results after 40 W oxygen plasma pretreatment and identical TMDSO PP film deposition conditions. The figure shows that samples kept indoors show a similar trend as the 5 W pretreated samples. After 1 day from the time of deposition of the PP film, all of the 40 W oxygen treated samples exhibited perfect adhesion according to the crosshatch test method. The sample that had an oxygen plasma pretreatment of 30 s is the only one to show a reduced rating of 4 after 1 week of storage indoors, which is the same as for the untreated control polymer. All films then begin to fail substantially after outdoor weathering. A comparison of the crosshatch test results in Figs. 1 and 4 suggests that higher powered oxygen plasma pretreatments are detrimental to TMDSO PP film adhesion. For the 5 W oxygen pretreatments that displayed an adhesion improvement, the O/C
Fig. 3. Crosshatch adhesion test results of TMDSO PP coatings after a 5 W oxygen plasma treatment for varying times after 1 week of outdoor weathering. Light regions indicate delamination of the plasma film.
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Fig. 5. Light microscopy images of TMDSO PP film showing interference fringes and delamination at edges of crosshatch after ageing indoors.
ratio on the BPA-PC surface did not exceed 0.2. However for the 5 W, 30 s and all 40 W pretreatments, the O/C ratio exceeds this value which suggests more pronounced oxidative surface degradation of the BPA-PC [13]. It appears that limiting the amount of incorporated oxygen into the surface is important for coating adhesion. The generation of functionalities such as hydroxyl groups or peroxide groups, to improve the adhesion of the film is important, but this must be balanced against the effect of plasma-induced chain scissions degrading the polymer chains at the surface, which may reduce the coating adhesion via the creation of a mechanically weaker PC surface layer. Outdoor weathering exacerbates the effects of oxygen plasma pretreatments on adhesion. The decrease in adhesion may be attributed to photo-oxidative reactions induced by UV exposure. While the oxygen plasma pretreatment incorporates into the BPA-PC surface adhesion-promoting functional groups, it may also incorporate chemical groups such as ketones that can undergo photochemical reactions upon UV outdoors exposure and lead to reactions such as Norrish-type reactions that can lead to polymer chain scissions. Particularly ketone groups conjugated with double bonds or aromatic rings will be susceptible to such reactions. Such reactions are, however, extremely difficult to detect on polymeric substrates that already contain oxygen before oxygen plasma treatment. The effects of such putative oxidative chain scission reactions would be a reduction in the average molecular weight of the BPA-PC surface layers accompanied by their mechanical weakening. This would lead to reduced strength of adhesion and thereby the ability of coatings containing residual tensile stress to pull away from cut edges such as those produced by the crosshatch scribe. This coupled with the generation of low molecular weight material in the extended oxygen pretreated surface layers, and possible further migration of this material to the interface with time appears to be detrimentally affecting the coating adhesion. Some researchers have reported on the difficulty of obtaining good adhesion of SiOx coatings with a thickness in the micrometer range due to the large internal stresses generated [1,6,14]. Improved adhesion is always reported for thinner films; however, these lack the surface hardness required to act as protective coatings on BPA-PC [1]. An interesting ageing phenomenon was observed on samples that had been treated with a 40 W oxygen plasma as they were stored indoors after having the crosshatch test performed on them. At the time of the crosshatch test, the films
adhered well to the surface and scored a crosshatch rating of 5. Over time, however, these films delaminated from the surface of the polymer. Images showing this effect can be seen in Fig. 5 where after 4 weeks, delamination of the film at the edge of the crosshatch can be seen via the interference fringes that have formed. This result highlights an ageing effect exhibited by the siloxane PP coated BPA-PC samples. The concentration of functional groups introduced on a polymer surface by plasma treatment may change as a function of time depending on the conditions. This can arise through diffusion of low-molecular weight oxidised compounds into or out of the bulk polymer and the migration of polar functional groups away from the surface [15,16]. The decrease in adhesion of siloxane PP films on polycarbonate with time has been observed by other researchers but the exact reasons for this effect are still unclear. Some report it to be due to the effect of changing internal stresses within the films [1, 8] while others emphasize the importance of ageing reactions from air diffusion into the film [17] which can result in a decreased level of chemical binding at the interface of the PP film and BPA-PC surface [18]. The ageing effect and poor adhesion of oxygen plasma pretreated films after extended time outdoors were further investigated by XPS to analyse the delaminated interfaces between the PP film and the substrate polymer surface. For this test, samples that had been oxygen plasma treated at a power of 40 W and weathered for 5 weeks outdoors were used. The standard crosshatch test tape was placed over the intact films and pressed down firmly for 2 min, then the tape was removed resulting in the complete delamination of the thin film. The underside of the crosshatch tape and the BPA-PC polymer surface were analysed, enabling determination of the PP film composition at the substrate interface. Table 2 displays the atomic concentrations of elements on the underside of the delaminated film and at the BPA-PC polymer interface. It can be seen that the film has completely Table 2 XPS atomic concentration of elements on the BPA-PC surface and on the underside of failed PP film after delamination
BPA-PC control BPA-PC delaminated surface PP delaminated surface
C
O
Si
85.5 83.3 47.3
15.5 15.6 27.3
0.0 1.1 25.4
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Fig. 6. SEM micrographs of TMDSO PP films after crosshatch adhesion testing showing buckling and delamination of the film and a sharp interface edge.
failed at the interface between the PP film and the polycarbonate surface, as the C/Si ratio on the polymer after delamination is 75 corresponding to 1.1 at.% Si remaining on the BPA-PC surface while the C/Si ratio of the underside of the PP film is 1.9. This finding highlights the fact that the failure occurs exactly at the interface between the polymer substrate and the TMDSO PP coating, and highlights the extremely poor bond between the PP film and the substrate after extended oxygen plasma treatments and outdoor weathering. The O/C content of the BPA-PC surface after delamination is 0.19 which is similar to that of the pretreated polymer surface of 0.18. Interestingly, however, the O/C ratio of the delaminated film is 0.58, while that of the TMDSO monomer is 0.25 and the O/C carbon content of the PP surface is 0.45. This finding highlights the fact that there is significantly more oxygen on the underside of the PP film than would be expected. It is difficult to identify the cause of this increased oxygen content but, clearly, oxidative reactions at the PP – BPA-PC interface may play a role along with the diffusion to the interface of oxidised low molecular weight material. It is possible that over time further reactions of atmospheric oxygen and water at the interface are responsible for this significant increase in the oxygen content. Scanning electron microscopy (SEM) analysis (Fig. 6) of a crosshatched surface also highlights the clean delamination at the interface between the PP film and the BPAPC polymer substrate. It can be clearly seen that the film has gradually started to detach from the polycarbonate surface. Furthermore, the SEM micrograph shows the gradient within the plasma polymer film, with the brittle, hard coating on the outside. The films may fail in outdoor weathering tests partly due to thermal cycling and the mismatch in the coefficients of thermal expansion between the BPA-PC substrate and the PP film.
BPA-PC disks containing different levels of moisture were PP coated followed by 1 week initial storage indoors and 6 weeks outdoor weathering. The results of crosshatch tests for these samples are shown in Fig. 7. The driest sample, with a water content of 90 ppm, scored 5 in the crosshatch ratings scheme after a week indoors. The edges were smooth and none of the squares detached from the lattice. On the other hand, none of the other samples displayed this quality of adhesion even when they are subject to minimal ageing and weathering. This finding highlights the fact that even a small amount of moisture (> 90 ppm) can detrimentally affect the quality and adhesion of the microwave deposited TMDSO coatings. For the two samples which had the greatest water contents (2800 and 3800 ppm), it can be seen that after 1 week indoors, the films begin to fail substantially when compared to the drier samples even before they are subject to outdoor weathering. Once the samples are placed outdoors for 1 week they all begin to fail substantially. The two driest samples, with water contents of 90 and 1800 ppm, attain a crosshatch rating of 4, while the samples with a water content over 2000 ppm score a poor adhesion rating of 2. After a period of 3 weeks outdoors, all of the samples show a crosshatch rating of 1. The rate of decline of the coatings varies in proportion to their water content. This result shows that the residual water content within the bulk of the polycarbonate substrate plays an important role. Samples must be sufficiently dry to ensure that any residual moisture within the polycarbonate will not detrimentally affect the coatings. However, the poor score of all of the samples after 3 weeks of outdoor weathering shows that there are other
3.3. Effect of residual water content Storage of the BPA-PC samples at different humidity levels allowed their measured water content to be varied between 90 ppm and 3800 ppm. The equilibrium water content of the Lexan BPA-PC when immersed in water at standard temperature and pressure is stated by the manufacturer to correspond to a level of 3512 ppm. It was found that the sample stored in the 100% humidity environment attained this level of water content, within the experimental uncertainty of the instrument.
Fig. 7. Crosshatch adhesion results of TMDSO PP films on BPA-PC samples that contained varying amounts of residual water.
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factors determining the failure of the films, in addition to the role of bulk water alone, as was the case with the investigation into the effect of oxygen plasma pretreatment. There are a number of reasons why the residual water content in the bulk polymer could affect coating adhesion, although, it is difficult to assess which factors are more important. It is possible that water diffusion to the interface of the polymer and TMDSO plasma film could result in the degradation of covalent bonds between the PC and, the plasma film, such as reactions with hydrolytically unstable silane bonds. This could result in the formation of silanol bonds, which have been shown to be detrimental to the adhesion of plasma films [18]. Another cause for the detrimental effect of water on the adhesion of these films may be due to the incorporation of water into the ignited plasma during thin film deposition. This water may affect the plasma by generating highly reactive OH radical species which interact with the surface and by adding more harmful UV radiation into the plasma, as found by other researchers [19,20]. Seidel et al. [21] found that water diffusion through the bulk of Ar etched polycarbonate to the surface leads to weak adhesion of evaporated aluminium thin films. 4. Conclusions The effect of an oxygen plasma pretreatment and the residual bulk polymer water content on the adhesion of microwave plasma deposited TMDSO/oxygen films on a commercially available BPA-PC substrate has been studied. It was found that the conditions for effective oxygen plasma pretreatment to improve adhesion require a low power (5 W) and short treatment time (less than 1 s). Longer treatment times and higher power plasmas are detrimental to coating adhesion, likely due to increased incorporation of oxygen species and degradation of the BPA-PC, as shown by XPS examination of delaminated films. It is likely that the increased oxygen content on the surface of the polymer obtained using extended treatment times and higher powers may also aid in the diffusion of atmospheric water to the interface with the PP film, substantially reducing the coating adhesion through chemical reactions of water with Si containing functionalities
at the interface. Outdoor weathering may also induce photooxidative reactions of groups incorporated by the oxygen plasma pretreatment; such reactions can lead to chain scissions and hence mechanically weaker interfaces. Acknowledgements The authors are grateful to SOLA International Inc. for financial support and for access to equipment, and the Australian government for an Australian Postgraduate Award scholarship to B.W. Muir through the ARC SPIRT program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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