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Plasma polymerised coatings for engineered interfaces for enhanced composite performance
Composites: Part A 33 (2002) 1293–1302 www.elsevier.com/locate/compositesa
Plasma polymerised coatings for engineered interfaces for enhanced composite performance D.J. Marks, F.R. Jones* Department of Engineering Materials, School of Materials, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK
Abstract Uncoupled unsized glass fibers were subjected to a continuous treatment to coat the tows with an acrylic acid/octadiene plasma copolymer. The fibres were separated prior to coating in order to minimise shadowing effects associated with the coating of fibre bundles. The plasma polymer coating was analysed for composition and coating uniformity using high resolution XPS, which confirmed that increasing acrylic acid content in the plasma copolymer led to an increase in retained surface carboxylic acid groups (confirmed by trifluoroethanol derivatisation). The concentration of hydroxyl or ether groups in the plasma polymer was higher than expected, which affected the retained COOH functionality. The single filament strengths of the coated and uncoated, separated control fibres were comparable. However, for the plasma polymer coated fibers, there was less variability (assessed using Weibull modulus), in comparison to the separated unsized fibres. Single fibre fragmentation testing was used to identify the change in the interfacial response as a function of the fibre coating. It was noted that the presence of the plasma polymer coating altered the interfacial properties (which were assessed using the Kelly– Tyson model, and the CSTF and STE methodologies). A functionalised plasma polymer with high acrylic acid content gave rise to a strong interfacial bond, with little debonding. The unfunctionalised octadiene plasma polymer coated fibres, exhibited a poor interfacial strength, with significant levels of debonding. It is concluded that glass fiber tows can be continuously coated with plasma copolymers, leading to uniform levels of coating throughout the fibre tow, thus giving the ability to tailor the interfacial response of the composite. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Glass fibres; B. Stress transfer; C. Surface analysis; Plasma polymerisation
1. Introduction The performance of most polymer composites has long been recognized to be reliant on an optimum interface. Therefore, it is necessary to control the interfacial phenomena that occur in the material. Various methods of ‘engineering’ reinforcing fibre surfaces have been used to promote adhesion. For example, silane coupling agents for glass fibres are extensively used [1], usually in the presence of a matrix compatible size. In the case of carbon fibres, surface oxidation, usually electrolytically, has been optimised for partial debonding of unsized fibres [2]. However, these are then coated in a polymeric size, which may also contribute to the formation of an optimal bond. Drzal and Madhukar [3], have demonstrated the relationship between composite performance and interfacial quality. Plasma polymers have been used to deposit a conformal, molecularly thin functionalised coating [4,5], which can be * Corresponding author. Tel.: þ44-114-222-5477; fax: þ 44-114-2225943. E-mail address: [email protected] (F.R. Jones).
used to control the degree of interfacial adhesion because the underlying chemistry and microstructure of the surface is concealed. The coating can be functionalised to match that required of the matrix. This provides a means of tailoring the composite interface for specific requirements (i.e. off-axis bonding, or energy absorption). The use of functionalised plasma polymers provides a one-step surface treatment for adhesion and sizing. Plasma polymerisation deposits a conformal, pinhole free film to the surface of the fibres, leading to interfacial control through the chemical functionality resulting from the composition of the monomer gases employed, and not influenced by the underlying fibre surface chemistry or topography [4,5]. By modifying the deposition conditions, thicker coatings of either low or high modulus can be deposited [6]. In this work, the first stage of a scale-up study is reported. Fibre tows (as opposed to single filaments) have been coated to assess the potential for tailoring the interfacial response in high volume fraction composite materials. The success of the technique has also been examined by measuring the uniformity of the coating on the individual filaments in a bundle using high resolution X-ray photoelectron
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spectroscopy (XPS). The interfacial characteristics of individual single filaments have been assessed using the cumulative stress transfer function (CSTF) [7], and interfacial shear strength (IFSS) methodologies. This is the precursor to the examination of the properties of high volume composites. Glass fibres have been used in this study.
2. Experimental 2.1. Fibres E-glass fibres, (Owens Corning Fiberglass) without coupling agent or sizing, were utilised for this work, with a fibre diameter of 15.46 ^ 1.74 mm. 2.2. Monomers for plasma polymer deposition The monomers, acrylic acid and 1,7-octadiene, for the plasma copolymer deposition were supplied by Aldrich Chemicals (UK). Acrylic acid was used to functionalise the polymeric deposit with carboxylic acid groups. The 1,7octadiene was used as a diluent hydrocarbon, to vary the concentration of carboxyl groups and provide cross-linking, and hence mechanical stability to the coating. The monomers were used as received, with the addition of three freeze-pump-thaw cycles, to remove any gaseous impurities [8]. 2.3. Resin matrix The matrix used for these experiments was a mix of LY1556 (Vantico, UK) which is a diglycidyl ether of Bisphenol A, and a flexibilising aliphatic epoxy resin Araldite GY298 (Ciba Geigy, UK). A 60:40 ratio of the two components was utilised to give tensile properties suitable
for the single fibre fragmentation test. The curing agents were 70 phr Nadic Methylene Anhydride (Stag Polymers and Sealants, UK) and Capcure 3-800 (Henkel-Nopco, UK), a mercaptan terminated polymer. The matrix was cured at 80 8C for 4 h, followed by post-curing at 130 8C for 3 h. The samples were allowed to cool down overnight within the oven. Following the cure cycle, the dog-bone shaped specimens containing a single fibre were placed in a metal jig and heated to 130 8C, the ends of the specimens were clamped in place within the jig, and cooled slowly through the Tg (70 8C), down to room temperature, thus constraining the resin specimen from shrinkage (relative to the fibre) during cooling. This process alleviated the formation of a sinusoidal periodicity in the fibre caused by unconstrained cooling during specimen manufacture. The cured resin had the following properties. Elastic modulus was 1.2 GPa. The tensile yield strength of the resin was 50 MPa and the shear yield strength (calculated from the von Mises yield criterion) was 28 MPa. 2.4. Plasma copolymer deposition The plasma reactor, shown in Fig. 1, was designed specifically to deposit a plasma polymer continuously onto the fibres within the tow. It consisted of a glass barrel section (in which polymer deposition takes place), with a helical coil of earthing braid, wound round the outside. The plasma is generated in the barrel reactor by a 13.56 MHz radio frequency power supply and matching network supplied by Coaxial Power Systems Ltd (UK). The power used for plasma deposition was 1 W. The fibres were contained on spools, within the aluminium end-pieces, which were designed to prevent parasitic deposition of plasma polymer within the end-piece region by incorporation of a baffle between the end-piece and the barrel, so that deposition within the barrel of the reactor dictates the surface chemistry. Glass spools, which were present within
Fig. 1. Schematic diagram of plasma reactor used for continuously coating fibre tows.
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the end-pieces, facilitated the movement of the fibre tow through the plasma reactor. Glass was chosen because it should not contaminate the fibre surface. Needle valves were used to allow the metered flow of monomer into the reactor. The overall flow rate of the monomers was set at 2 sccm (cm3 (at STP) per minute) to give the required power density for retention of chemical functionality in the plasma polymer. Pumping is achieved via a two stage rotary pump, with a base pressure of 2 £ 1023 mbar. The pressure during plasma polymerisation is generally of the order of 2 – 3 £ 1022 mbar. The residence time of the fibres within the reactor barrel is 15 min. This leads to the deposition of a conformal film over the fibres that are thick enough to mask the substrate. The fibre tows were spread out using a blownair source and wound onto the glass spools, which were placed in the reactor prior to coating. This avoids shadowing effects, which could lead to a non-uniform deposit on the fibres during coating. The effect of separating the tow into a loose fibre bundle wound onto a spool was investigated with single filament strength tests. 2.5. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) measurements were taken directly from the glass fibre tows and from individual filaments. The XPS spectra were obtained with a SCIENTA 300 spectrometer operating at 2.8 kW power. Monochromated Al Ka X-rays were used, focusing the X-rays to a spot size of 6 mm £ 0.5 mm. The pressure in the spectrometer during operation was better than 2 £ 1029 mbar. The spectrometer was also equipped with an imaging mode, which enabled the analysis of the plasma deposit on single fibres. Charge compensation was achieved with an electron flood gun, with operating conditions optimised for each sample. All the XPS spectra were acquired at a take-off angle of 458 relative to the analyser. Core level spectra were taken during the XPS analysis and were curve fitted using an iterative least squares routine (a linear background was applied to the peaks in all cases), the peak shape applied was purely Gaussian, with a full width at half maximum (FWHM) of 1 –1.5 used. This enabled the component sub-peaks to be determined, therefore giving the chemical composition of the thin plasma polymer deposit on the fibre. The C1s peaks were charge corrected to 285 eV, to enable accurate assignment of the component sub-peaks [8,9]. To ascertain the concentration of carboxylic acid groups retained in the plasma polymer deposit, the labelling technique of Alexander et al. [10], was used. This enables the COOH and COOR groups to be differentiated. This technique utilised trifluouroethanol, which reacted with the surface carboxylic acid groups in the plasma polymer, resulting in the esterification of the COOH groups with fluorine containing molecules. The reaction was carried out by suspending the sample above a solution of trifluouroethanol, di-tert-butylcarbodiimide (which acted as a drying
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agent) and pyridine, which was a catalyst for the reaction. The reaction was carried out in a stoppered boiling tube, and the reactants were left for 24 h at room temperature, inside the boiling tube to ensure that the reaction ran to completion, full details of this procedure are given elsewhere [10]. Any ester groups present in the surface of the plasma polymer cannot take part in this reaction, so it is possible to compare the size of the CF3 peak directly with that of the COOR peak. If both peaks are the same size, then the surface will consist of 100% carboxylic acid groups, if the COOR peak is of greater magnitude, then ester is present in the plasma polymer film, and the level of carboxylic acid retained in the plasma polymer film can be calculated from the size of the CF3 peak. 2.6. Single fibre-strength measurement Single fibres were selected at random from the coated and separated unsized fibre tow and the strength was measured at a gauge length of 6.25 mm. The control fibres (uncoated) were also passed through the reactor, but were not coated, to give a direct comparison of the effect of the coating. The sample preparation details can be found elsewhere [4]. The specimens were tested in tension at a speed of 0.52 mm min21. Tests that exhibited slippage prior to failure were discarded. Fibre diameter measurements were made using a Camscan Series II scanning electron microscope. At least 50 samples were tested for each fibre coating. The Weibull modulus was calculated from the Weibull plot described previously. Linear regression algorithm was used for the analysis. 2.7. Single fibre fragmentation testing The single fibre fragmentation test was carried out on all of the various plasma polymer coated fibres. The unsized fibres were used as controls. Single fibres were extracted from the fibre tows and the test specimens were prepared as described elsewhere [5]. The fibres were embedded into the resin matrix, and cured as described above. The resin coupons, were capable of achieving an applied strain of . 10%, therefore allowing saturation in the fragmentation process to be achieved. The test was carried out on a custom designed testing machine, supplied by Micromaterials Ltd, Wrexham, UK. Full details are given elsewhere [11]. Test specimens with a gauge length of 33 mm, were subjected to a tensile test carried out at a speed of 0.13 mm min21. At intervals of 1% strain, the test was interrupted, and the fragmented fibres recorded digitally using an image grabbing programme. The test was considered complete when the fibre length reached a saturation value. Fibre fragment and debond lengths were measured digitally from the images taken during the test, as described elsewhere [5,11].
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2.8. Interfacial analysis and determination The quality of the interface was examined using the conventional Kelly –Tyson model, the CSTF and the stress transfer efficiency (STE) methodology. The apparent IFSS can be calculated using the former model, which assumes that the IFSS was constant over the fully debonded fragment length at saturation. This requires the measurement of fragment length (l ) and fibre-strength at the critical length ðlc ¼ 4=3lÞ: As given in Eq. (1) rs ta ¼ f fu ð1Þ lc where ta is the IFSS, rf is the fibre radius, and sfu is the tensile strength of the fibre at lc. We have utilised the analysis throughout the test despite the fact that the assumption of complete debonding did not apply. An alternative interfacial assessment is an estimation of the stress transfer capability using the CSTF model [7]. In this technique the shear stress profile in each fragment is calculated from the plasticity effect model [12], which is calculated using the variational model [13] assuming an elastic fibre and elastic matrix with perfect bonding and truncated for matrix yield using a von Mises criterion. Debonding can be incorporated using Coulomb’s law to calculate a frictional adhesion using the radial stress given in the variational model. The shear stress profile for each fragment is converted into a fibre-tensile stress profile, which can be averaged according to Eq. (2)
CSTF ¼
iX ¼N
ðLl
i¼1
0
sf ðxÞ dx
iX ¼N
ð2Þ Li
i¼1
N is the number of fragments, Li is the fragment length for i ¼ 1; 2; 3; …; N; and sf ðxÞ is the tensile stress at a distance x
from the fibre end. More details on the CSTF equation can be found in Ref. [7]. A STE [14] was also derived from the CSTF results. This involved normalizing the CSTF value with the equivalent calculation for elastic – elastic conditions and a perfectly bonded interface. Which is the ideal stress transferability of a fragment and represents the average tensile stress on a fibre with a perfect interface. Therefore the STE is defined as: STE ¼
CSTF sf
ð3Þ
3. Results 3.1. X-ray photoelectron spectroscopy The surface chemical composition of the plasma polymer deposit was obtained directly from the fibre surface. Curve fitting was used to deconvolute the spectral components of the C1s peak, to enable the chemical functionality of the polymer deposit to be quantified. Fig. 2 shows that the concentration of COOR groups on the fibre surface increases with increasing acrylic acid content in the monomer feed. Using TFE derivatisation, the concentration of COOH is less than that for COOR (Fig. 3), indicating that the efficiency of the plasma deposition process appears to be lower than for other surfaces (Table 3). The concentration of the COR groups in the plasma polymer was much higher than previously reported and cannot arise from the monomeric molecules without significant molecular fragmentation during plasma polymerisation. This is not discussed further, but is believed to result from the adsorbed water retained on the surface of the glass fibres.
Fig. 2. Functional groups present on fibre surface as a function of the concentration of acrylic acid in monomer feed.
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Fig. 3. The concentration of COOH retained in plasma polymer.
3.2. Coating uniformity and thickness The uniformity of the coating on each glass filament of 15 mm diameter, is an essential characterization requirement. Fig. 4 shows that the underlying concentration of Si 2p, which arises from the glass fibre substrate was , 1% (which is at the limit of accurate analysis by XPS). It needs to be pointed out that at an analysis angle of 458, the substrate can still contribute to the spectrum when the coating thickness is < 10 nm, as estimated from the inelastic mean free paths of carbon and silicon for the low levels of Si detected. This is not an accurate method of determining coating thickness, as there are inherent problems associated with calculating overlayer thickness using XPS, especially when the surface is not flat. A further complication is the cylindrical shape of the fibres, where photoelectrons will be emitted from round the entire circumference of the fibre, which gives a non-unique escape depth for the photoelectrons. Thus the low substrate concentration could not be attributed to a discontinuous coating. To confirm this, two single filaments were
analysed using the imaging mode on the SCIENTA 300 Spectrometer. Fig. 5(a) and (b) shows that it is possible to observe the spectrum generated from the single fibres. The spectra are of identical intensity, thus enabling it to be concluded that the coatings on randomly selected fibres from the tow are identical. The spectra generated from the single fibres, are not of sufficient intensity to quantify, but it is possible to see that the peak shape from both fibres is identical in shape to that generated from the fibre bundle (shown in Fig. 5(c)). The coating thickness has proved difficult to estimate. The fibre diameter measurements taken using scanning electron microscopy, do not show any difference between the fibre diameters of coated and uncoated fibers, within the error associated with the measurement of diameter (there was a deviation in the average diameter of the unsized fibre of 1.5 mm). The coating thickness could well be of the order of 10 nm (0.01 mm) as implied by the XPS measurements, a coating thickness of this magnitude would not have been detectable in the SEM fibre diameter measurements. Other techniques may have been used to yield more accurate
Fig. 4. Elemental composition of plasma polymer deposited onto glass fibres as a function of acrylic acid content of monomer feed.
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D.J. Marks, F.R. Jones / Composites: Part A 33 (2002) 1293–1302 Table 1 Single filament strength data Fibre type
Average failure stress (GPa)
Weibull modulus
90% Acrylic acid 0% Acrylic acid Unsized (as received fibres) Unsized (after spreading tow, and travelling through the uncharged reactor)
1.33 ^ 0.56 1.58 ^ 0.64 1.46 ^ 0.8
3.94 3.58 3.10
1.35 ^ 0.7
1.65
3.3. Single fibre-strength measurement From Table 1, it is seen that the average fibre-strength is not significantly improved by the presence of the plasma polymer coating (within the standard deviation) although the strength is reduced compared to that of a ‘virgin’ glass fibre (1.46 GPa, from Table 1) due to the fibre-separation operation. The Weibull modulus on the other-hand for the plasma polymer coated glass fibres is significantly higher, showing a reduced variability in fibre-strength in the presence of the coating. 3.4. Single fibre fragmentation testing The results of the single fibre fragmentation test are shown in Table 2. Which illustrates the average fragment length, and debonded length, for each type of fibre. From the results in Table 2, it is possible to observe that the 90% acrylic acid plasma polymer coated fibre exhibits the shortest fragment length at saturation (0.41 mm), the unsized fibre exhibits a similar fragment length of 0. 42 mm at saturation, with the 100% octadiene coated fibre giving a longer average fragment length at saturation of 0.60 mm. The 100% octadiene plasma fibre exhibits significant debonding (at an average of 44.2% of the fragment length at saturation), whereas the 90% acrylic acid coated sample only exhibited small levels of debonding (7.0% at saturation), the unsized fibre did not exhibit debonding at all. The analysis of this raw fragmentation data will be discussed further.
4. Discussion Fig. 5. (a) Imaging Spectrum Of 100% Acrylic Acid Plasma apolymer On Single Fibres. (b) Cls Peak Generated from Image Of 100% Acrylic Acid Plasma Polymer On Single Fibres. (c) Cls Peak from 100% Acrylic Acid Plasma Polymer Coated Fibre Bundle.
information about the precise thickness of the plasma polymer coating, but these were not pursued, as the uniformity of the coating was the fundamental issue, not its thickness.
4.1. Coated fibres: uniformity within a bundle Fig. 2 shows that the surface functionality of the coated fibres increases with the concentration of acrylic acid in the monomer feed. However, the concentration of COOH groups is less than reported previously [5]. The presence of other functional groups (COR and CyO) in the plasma deposit are considered to arise from residual water in the plasma reactor, as adsorbate on the walls of the reactor, or
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Table 2 Single fibre fragmentation test data Strain
Plasma polymer type Unsized
3 4 5 6 7 8 9 10 11
90% Acrylic acid
100% Octadiene
Average fragment length (mm)
Average debond (%)
Average fragment length (mm)
1.58 (0.96) 0.96 (0.61) 0.86 (0.29) 0.63 (0.30) 0.51 (0.14) 0.48 (0.07) 0.45 (0.05) 0.43 (0.04) 0.42 (0.03)
– – – – – – – – –
6.08 (3.61) 1.71 (0.76) 0.85 (0.74) 0.50 (0.10) 0.46 (0.07) 0.43 (0.03) 0.41 (0.02) 0.41 (0.02) 0.41 (0.02)
the surface of the fibres. Nishioka and Shramka [15], showed that several monolayers of water will be present on an E-glass surface, which desorb only with difficulty. With the fibre tows present, a high surface area exists. Thus it can be expected that water will be involved in the plasma polymer deposition. One effect of this would be to reduce the retained (COOH) as shown in Fig. 3. The mechanism of this is beyond the scope of this paper as it is difficult to prove that water is present on the surface of the unsized fibres using XPS, due to the variety of other surface elements that are observed, which could easily contribute to an increased O1s peak in the spectra gained from unsized fibres (the magnitude of the O1s peak would be the main indicator of surface water on the unsized fibres). TFE derivatisation suggests that the apparent retained concentration of carboxylic acid functionalities is lower than expected [10]. Table 3 shows that surface analysis of a 100% acrylic acid plasma polymer deposited onto an aluminium foil substrate in the same reactor under the same conditions. In comparison with the fibre data in Fig. 3 it can be seen that the COOH concentration is less, and the COR concentration is higher in the latter. This supports the argument that residual water on the fibre surface is the probable explanation. Further, enhanced fragmentation of the monomers during plasma polymerisation is unlikely given the low Table 3 XPS data from 100% acrylic acid plasma polymer deposited on aluminium foil substrate Component peak
% of C1s Peak
C–R B-Shift C–O– R CyO COOR % of COOR retained as COOH (from TFE derivatisation)
53 19 6 3 19 70
Average debond (%)
Average fragment length (mm)
Average debond (%)
1.65 2.00 2.76 3.28 6.98
0.82 (1.06) 1.04 (1.62) 0.93 (0.94) 0.67 (0.30) 0.64 (0.23) 0.61 (0.19) 0.61 (0.19) 0.60 (0.19) 0.60 (0.19)
3.35 12.37 22.67 27.04 32.50 35.01 40.80 44.24
powers used. However, an alternative explanation for the reduction in retained COOH groups could be that esterification of the glass surface Si –OH groups can occur, providing a chemical bond between the two components. 4.2. Effect of fibre separation process on fibre-strength The change in strength of the glass fibres during the scale-up was investigated. It was observed that the fibres have their strength reduced from 1.46 to 1.35 GPa when they are spread, wound onto spools and passed through the reactor. The results seen in Table 1 show that the plasma polymer coating does not appreciably increase the strength of the degraded glass fibre. However, the Weibull modulus for the strength distribution is at a higher value. Thus the variability in strength is improved. This appears to suggest that either the plasma polymer protects the fibre from additional damage during sampling for strength testing, or that the large flaws responsible for the premature failures are filled by the plasma polymer. 4.3. Adhesion of plasma polymer functionalised E-glass fibres Debonding of the unsized fibres did not occur during fragmentation, which is consistent with a good interfacial bond. The constant shear analysis also supports a strong interfacial bond. However, the value of IFSS (given in Fig. 6), is about twice the shear yield strength of the matrix resin (28 MPa) employed, so that the quantification can be considered doubtful. A good bond between unsized or uncoupled glass fibre has been reported previously [16,17] so the observation is not unduly surprising. Reanalysis of the data for stress transfer using the CSTF methodology (Fig. 7) still supports the view that adhesion of these unsized fibres is high. However, the adhesion is now apparently only marginally better than the 90% acrylic acid/10% octadiene plasma copolymer
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Fig. 6. Kelly–Tyson analysis of fragmentation test data.
Fig. 7. Fragmentation Test Data Analysed by Cumulative Stress Transfer Function (CSTF).
Fig. 8. STE with applied strain for fragmentation test data.
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Fig. 9. STE as a function of fragment length.
coated fibre, which exhibited only a small degree of debonding (6.98% at saturation, as shown in Table 2). These observations seem to be consistent with a strong interfacial bond of similar magnitude. The STE, has been shown [14] to be a better analysis of the stress transfer between matrix and a fragmented fibre when the degree of debonding is high, because, it was shown that for completely debonded fibres, CSTF can increase with applied strain. The STE is achieved by normalizing the CSTF value to that for an ideal fragment of equal length (Fig. 8). The fragmentation process leads to a different rate of length reduction so that comparison of the STE at constant strain is imprecise. The values of STE are compared at an average fragment length in Fig. 9, which shows that the best interface is described by the shortest fragment length with the largest STE. In this analysis, the 90% acrylic acid/10% octadiene plasma copolymer coated fibres would now appear to be slightly more efficient than the untreated glass fibres, although within the error bars the difference may not be significant. The plasma polymer coatings have a thickness of < 10 nm (estimated) and are more highly cross-linked than the epoxy resin, and therefore yield within an interphase region during fragmentation is unlikely. Support for this argument is the fact that debonding was observed in both cases. One of the most interesting observations concerns the 100% octadiene plasma polymer coated fibres, which
debond during the experiment. With carbon fibres, complete interfacial debonding was observed at low applied strains, leading to different CSTF trends [5,14]. So whereas with carbon fibres the plasma polymer completely masked the underlying adhesive ability of the fibre, in this case some adhesive ability is retained, yet the functional chemistry (shown in Fig. 2) does not appear to be significantly different. This result can only be understood from either a strong residual water effect on the fibre surface functional chemistry, or a result of the different mechanical or thermal properties of E-glass fibres in comparison with Type A carbon fibres. The good adhesion of unsized or uncoupled fibres without an obvious chemical mechanism could mean that high radial compressive thermal stresses develop during cure of the sample. Further research is needed to accurately identify this mechanism of adhesion.
5. Conclusions Plasma polymerisation was utilised to deposit a functionalised plasma polymer size continuously onto E-glass fibre tows. From XPS measurements taken directly from the fibre surface, it is possible to see that the fibres are uniformly coated within the tow, with a functionalised plasma polymer. The variability in the strength of the fibres (as measured by Weibull modulus) has been reduced by the presence of the plasma polymer coating. The presence of the plasma polymer coating has also changed the interfacial
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response of the fibre with respect to micro-mechanical testing, giving rise to interfacial debonding. Further to this work, multi-fibre composite beams manufactured from the plasma polymer sized fibres will be used to make interlaminar shear strength test specimens, to demonstrate the effect of the plasma polymer coating on the macroscopic composite properties.
Acknowledgements The authors would like to acknowledge the financial assistance of the EPSRC and the Advanced Composites Group Ltd (and Dr D. Newman for his helpful advice). The assistance of Dr G. Beamson and Dr D.S. Law at the Research Unit for Surfaces, Transforms and Interfaces (RUSTI) is acknowledged for their assistance and advice during the acquisition of the XPS spectra on the SCIENTA 300 spectrometer, housed at this facility. The helpful comments and ideas provided by Dr S.A. Hayes during this work are also acknowledged.
References [1] Plueddemann EP. Silanes and other coupling agents, 2nd ed. New York: Plenum Press; 1991. [2] Dorey G. The fibre–matrix interface and composite strength. In: Middleton DH, editor. Composite materials in aircraft structures. New York: Longman; 1990. p. 50–68. [3] Drzal LT, Madhukar M. Fiber matrix adhesion and its relationship to composite mechanical properties. J Mater Sci 1993;28(3):569 –610. [4] Kettle AP, Beck AJ, O’Toole L, Jones FR, Short RD. Plasma polymerisation for molecular engineering of carbon-fibre surfaces for optimized composites. Compos Sci Technol 1997;57(8):1023– 32.
[5] Lopattananon N, Kettle AP, Tripathi D, Beck AJ, Duval E, France RM, Short RD, Jones FR. Interface molecular engineering of carbonfibre composites. Composites, Part A 1999;30A(1):49– 57. [6] Kettle AP, Jones FR, Alexander MR, Short RD, Stollenwerk M, Zabold E, Michaeli W, Wu W, Jacobs E, Verpoest I. Experimental evaluation of the interphase region in carbon fibre composites with plasma polymerised coatings. Composites, Part A 1998;29A: 241 –50. [7] Tripathi D, Jones FR. Measurement of the load-bearing capability of the fibre/matrix interface by single-fibre fragmentation. Compos Sci Technol 1997;57(8):925–35. [8] O’Toole L, Beck AJ, Short RD. Characterization of plasma polymers of acrylic acid and propanoic acid. Macromolecules 1996;29(15): 5172–7. [9] Beamson G, Briggs D. High resolution XPS of organic polymers—the Scienta ESCA300 database. Chichester: Wiley; 1992. [10] Alexander MR, Wright PV, Ratner BD. Trifluoroethanol derivatization of carboxylic acid-containing polymers for quantitative XPS analysis. Surf Interf Anal 1996;24(3):217–20. [11] Tripathi D, Lopattananon N, Jones FR. A technological solution to the testing and data reduction of single fibre fragmentation tests. Composites, Part A 1998;29A(9– 10):1099–109. [12] Tripathi D, Chen FP, Jones FR. A comprehensive model to predict the stress fields in a single fibre composite. J Compos Mater 1996;30(14): 1514–38. [13] Nairn JA. A variational mechanics analysis of the stresses around breaks in embedded fibers. Mech Mater 1992;13(2):131– 54. [14] Lopattananon N, Hayes SA, Jones FR. Stress transfer function for interface assessment in composites using plasma copolymer functionalised carbon fibres. J Adhes 2002;78:313–50. [15] Nishioka GM, Schramka JA. Desorption of water from glass fibres. In: Ishida H, Kumar G, editors. Molecular characterisation of composite interfaces. New York: Plenum Press; 1983. p. 387. [16] Cheng TH, Jones FR, Wang D. Effect of fiber conditioning on the interfacial shear strength of glass-fiber composites. Compos Sci Technol 1993;48(1 –4):89–96. [17] Berg J, Jones FR. The role of sizing resins, coupling agents and their blends on the formation of the interphase in glass fibre composites. Composites, Part A 1998;29A(9– 10):1261–72.
Plasma polymerised coatings for engineered interfaces for enhanced composite performance D.J. Marks, F.R. Jones* Department of Engineering Materials, School of Materials, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK
Abstract Uncoupled unsized glass fibers were subjected to a continuous treatment to coat the tows with an acrylic acid/octadiene plasma copolymer. The fibres were separated prior to coating in order to minimise shadowing effects associated with the coating of fibre bundles. The plasma polymer coating was analysed for composition and coating uniformity using high resolution XPS, which confirmed that increasing acrylic acid content in the plasma copolymer led to an increase in retained surface carboxylic acid groups (confirmed by trifluoroethanol derivatisation). The concentration of hydroxyl or ether groups in the plasma polymer was higher than expected, which affected the retained COOH functionality. The single filament strengths of the coated and uncoated, separated control fibres were comparable. However, for the plasma polymer coated fibers, there was less variability (assessed using Weibull modulus), in comparison to the separated unsized fibres. Single fibre fragmentation testing was used to identify the change in the interfacial response as a function of the fibre coating. It was noted that the presence of the plasma polymer coating altered the interfacial properties (which were assessed using the Kelly– Tyson model, and the CSTF and STE methodologies). A functionalised plasma polymer with high acrylic acid content gave rise to a strong interfacial bond, with little debonding. The unfunctionalised octadiene plasma polymer coated fibres, exhibited a poor interfacial strength, with significant levels of debonding. It is concluded that glass fiber tows can be continuously coated with plasma copolymers, leading to uniform levels of coating throughout the fibre tow, thus giving the ability to tailor the interfacial response of the composite. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Glass fibres; B. Stress transfer; C. Surface analysis; Plasma polymerisation
1. Introduction The performance of most polymer composites has long been recognized to be reliant on an optimum interface. Therefore, it is necessary to control the interfacial phenomena that occur in the material. Various methods of ‘engineering’ reinforcing fibre surfaces have been used to promote adhesion. For example, silane coupling agents for glass fibres are extensively used [1], usually in the presence of a matrix compatible size. In the case of carbon fibres, surface oxidation, usually electrolytically, has been optimised for partial debonding of unsized fibres [2]. However, these are then coated in a polymeric size, which may also contribute to the formation of an optimal bond. Drzal and Madhukar [3], have demonstrated the relationship between composite performance and interfacial quality. Plasma polymers have been used to deposit a conformal, molecularly thin functionalised coating [4,5], which can be * Corresponding author. Tel.: þ44-114-222-5477; fax: þ 44-114-2225943. E-mail address: [email protected] (F.R. Jones).
used to control the degree of interfacial adhesion because the underlying chemistry and microstructure of the surface is concealed. The coating can be functionalised to match that required of the matrix. This provides a means of tailoring the composite interface for specific requirements (i.e. off-axis bonding, or energy absorption). The use of functionalised plasma polymers provides a one-step surface treatment for adhesion and sizing. Plasma polymerisation deposits a conformal, pinhole free film to the surface of the fibres, leading to interfacial control through the chemical functionality resulting from the composition of the monomer gases employed, and not influenced by the underlying fibre surface chemistry or topography [4,5]. By modifying the deposition conditions, thicker coatings of either low or high modulus can be deposited [6]. In this work, the first stage of a scale-up study is reported. Fibre tows (as opposed to single filaments) have been coated to assess the potential for tailoring the interfacial response in high volume fraction composite materials. The success of the technique has also been examined by measuring the uniformity of the coating on the individual filaments in a bundle using high resolution X-ray photoelectron
1359-835X/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 8 3 5 X ( 0 2 ) 0 0 1 6 2 - 8
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spectroscopy (XPS). The interfacial characteristics of individual single filaments have been assessed using the cumulative stress transfer function (CSTF) [7], and interfacial shear strength (IFSS) methodologies. This is the precursor to the examination of the properties of high volume composites. Glass fibres have been used in this study.
2. Experimental 2.1. Fibres E-glass fibres, (Owens Corning Fiberglass) without coupling agent or sizing, were utilised for this work, with a fibre diameter of 15.46 ^ 1.74 mm. 2.2. Monomers for plasma polymer deposition The monomers, acrylic acid and 1,7-octadiene, for the plasma copolymer deposition were supplied by Aldrich Chemicals (UK). Acrylic acid was used to functionalise the polymeric deposit with carboxylic acid groups. The 1,7octadiene was used as a diluent hydrocarbon, to vary the concentration of carboxyl groups and provide cross-linking, and hence mechanical stability to the coating. The monomers were used as received, with the addition of three freeze-pump-thaw cycles, to remove any gaseous impurities [8]. 2.3. Resin matrix The matrix used for these experiments was a mix of LY1556 (Vantico, UK) which is a diglycidyl ether of Bisphenol A, and a flexibilising aliphatic epoxy resin Araldite GY298 (Ciba Geigy, UK). A 60:40 ratio of the two components was utilised to give tensile properties suitable
for the single fibre fragmentation test. The curing agents were 70 phr Nadic Methylene Anhydride (Stag Polymers and Sealants, UK) and Capcure 3-800 (Henkel-Nopco, UK), a mercaptan terminated polymer. The matrix was cured at 80 8C for 4 h, followed by post-curing at 130 8C for 3 h. The samples were allowed to cool down overnight within the oven. Following the cure cycle, the dog-bone shaped specimens containing a single fibre were placed in a metal jig and heated to 130 8C, the ends of the specimens were clamped in place within the jig, and cooled slowly through the Tg (70 8C), down to room temperature, thus constraining the resin specimen from shrinkage (relative to the fibre) during cooling. This process alleviated the formation of a sinusoidal periodicity in the fibre caused by unconstrained cooling during specimen manufacture. The cured resin had the following properties. Elastic modulus was 1.2 GPa. The tensile yield strength of the resin was 50 MPa and the shear yield strength (calculated from the von Mises yield criterion) was 28 MPa. 2.4. Plasma copolymer deposition The plasma reactor, shown in Fig. 1, was designed specifically to deposit a plasma polymer continuously onto the fibres within the tow. It consisted of a glass barrel section (in which polymer deposition takes place), with a helical coil of earthing braid, wound round the outside. The plasma is generated in the barrel reactor by a 13.56 MHz radio frequency power supply and matching network supplied by Coaxial Power Systems Ltd (UK). The power used for plasma deposition was 1 W. The fibres were contained on spools, within the aluminium end-pieces, which were designed to prevent parasitic deposition of plasma polymer within the end-piece region by incorporation of a baffle between the end-piece and the barrel, so that deposition within the barrel of the reactor dictates the surface chemistry. Glass spools, which were present within
Fig. 1. Schematic diagram of plasma reactor used for continuously coating fibre tows.
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the end-pieces, facilitated the movement of the fibre tow through the plasma reactor. Glass was chosen because it should not contaminate the fibre surface. Needle valves were used to allow the metered flow of monomer into the reactor. The overall flow rate of the monomers was set at 2 sccm (cm3 (at STP) per minute) to give the required power density for retention of chemical functionality in the plasma polymer. Pumping is achieved via a two stage rotary pump, with a base pressure of 2 £ 1023 mbar. The pressure during plasma polymerisation is generally of the order of 2 – 3 £ 1022 mbar. The residence time of the fibres within the reactor barrel is 15 min. This leads to the deposition of a conformal film over the fibres that are thick enough to mask the substrate. The fibre tows were spread out using a blownair source and wound onto the glass spools, which were placed in the reactor prior to coating. This avoids shadowing effects, which could lead to a non-uniform deposit on the fibres during coating. The effect of separating the tow into a loose fibre bundle wound onto a spool was investigated with single filament strength tests. 2.5. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) measurements were taken directly from the glass fibre tows and from individual filaments. The XPS spectra were obtained with a SCIENTA 300 spectrometer operating at 2.8 kW power. Monochromated Al Ka X-rays were used, focusing the X-rays to a spot size of 6 mm £ 0.5 mm. The pressure in the spectrometer during operation was better than 2 £ 1029 mbar. The spectrometer was also equipped with an imaging mode, which enabled the analysis of the plasma deposit on single fibres. Charge compensation was achieved with an electron flood gun, with operating conditions optimised for each sample. All the XPS spectra were acquired at a take-off angle of 458 relative to the analyser. Core level spectra were taken during the XPS analysis and were curve fitted using an iterative least squares routine (a linear background was applied to the peaks in all cases), the peak shape applied was purely Gaussian, with a full width at half maximum (FWHM) of 1 –1.5 used. This enabled the component sub-peaks to be determined, therefore giving the chemical composition of the thin plasma polymer deposit on the fibre. The C1s peaks were charge corrected to 285 eV, to enable accurate assignment of the component sub-peaks [8,9]. To ascertain the concentration of carboxylic acid groups retained in the plasma polymer deposit, the labelling technique of Alexander et al. [10], was used. This enables the COOH and COOR groups to be differentiated. This technique utilised trifluouroethanol, which reacted with the surface carboxylic acid groups in the plasma polymer, resulting in the esterification of the COOH groups with fluorine containing molecules. The reaction was carried out by suspending the sample above a solution of trifluouroethanol, di-tert-butylcarbodiimide (which acted as a drying
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agent) and pyridine, which was a catalyst for the reaction. The reaction was carried out in a stoppered boiling tube, and the reactants were left for 24 h at room temperature, inside the boiling tube to ensure that the reaction ran to completion, full details of this procedure are given elsewhere [10]. Any ester groups present in the surface of the plasma polymer cannot take part in this reaction, so it is possible to compare the size of the CF3 peak directly with that of the COOR peak. If both peaks are the same size, then the surface will consist of 100% carboxylic acid groups, if the COOR peak is of greater magnitude, then ester is present in the plasma polymer film, and the level of carboxylic acid retained in the plasma polymer film can be calculated from the size of the CF3 peak. 2.6. Single fibre-strength measurement Single fibres were selected at random from the coated and separated unsized fibre tow and the strength was measured at a gauge length of 6.25 mm. The control fibres (uncoated) were also passed through the reactor, but were not coated, to give a direct comparison of the effect of the coating. The sample preparation details can be found elsewhere [4]. The specimens were tested in tension at a speed of 0.52 mm min21. Tests that exhibited slippage prior to failure were discarded. Fibre diameter measurements were made using a Camscan Series II scanning electron microscope. At least 50 samples were tested for each fibre coating. The Weibull modulus was calculated from the Weibull plot described previously. Linear regression algorithm was used for the analysis. 2.7. Single fibre fragmentation testing The single fibre fragmentation test was carried out on all of the various plasma polymer coated fibres. The unsized fibres were used as controls. Single fibres were extracted from the fibre tows and the test specimens were prepared as described elsewhere [5]. The fibres were embedded into the resin matrix, and cured as described above. The resin coupons, were capable of achieving an applied strain of . 10%, therefore allowing saturation in the fragmentation process to be achieved. The test was carried out on a custom designed testing machine, supplied by Micromaterials Ltd, Wrexham, UK. Full details are given elsewhere [11]. Test specimens with a gauge length of 33 mm, were subjected to a tensile test carried out at a speed of 0.13 mm min21. At intervals of 1% strain, the test was interrupted, and the fragmented fibres recorded digitally using an image grabbing programme. The test was considered complete when the fibre length reached a saturation value. Fibre fragment and debond lengths were measured digitally from the images taken during the test, as described elsewhere [5,11].
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2.8. Interfacial analysis and determination The quality of the interface was examined using the conventional Kelly –Tyson model, the CSTF and the stress transfer efficiency (STE) methodology. The apparent IFSS can be calculated using the former model, which assumes that the IFSS was constant over the fully debonded fragment length at saturation. This requires the measurement of fragment length (l ) and fibre-strength at the critical length ðlc ¼ 4=3lÞ: As given in Eq. (1) rs ta ¼ f fu ð1Þ lc where ta is the IFSS, rf is the fibre radius, and sfu is the tensile strength of the fibre at lc. We have utilised the analysis throughout the test despite the fact that the assumption of complete debonding did not apply. An alternative interfacial assessment is an estimation of the stress transfer capability using the CSTF model [7]. In this technique the shear stress profile in each fragment is calculated from the plasticity effect model [12], which is calculated using the variational model [13] assuming an elastic fibre and elastic matrix with perfect bonding and truncated for matrix yield using a von Mises criterion. Debonding can be incorporated using Coulomb’s law to calculate a frictional adhesion using the radial stress given in the variational model. The shear stress profile for each fragment is converted into a fibre-tensile stress profile, which can be averaged according to Eq. (2)
CSTF ¼
iX ¼N
ðLl
i¼1
0
sf ðxÞ dx
iX ¼N
ð2Þ Li
i¼1
N is the number of fragments, Li is the fragment length for i ¼ 1; 2; 3; …; N; and sf ðxÞ is the tensile stress at a distance x
from the fibre end. More details on the CSTF equation can be found in Ref. [7]. A STE [14] was also derived from the CSTF results. This involved normalizing the CSTF value with the equivalent calculation for elastic – elastic conditions and a perfectly bonded interface. Which is the ideal stress transferability of a fragment and represents the average tensile stress on a fibre with a perfect interface. Therefore the STE is defined as: STE ¼
CSTF sf
ð3Þ
3. Results 3.1. X-ray photoelectron spectroscopy The surface chemical composition of the plasma polymer deposit was obtained directly from the fibre surface. Curve fitting was used to deconvolute the spectral components of the C1s peak, to enable the chemical functionality of the polymer deposit to be quantified. Fig. 2 shows that the concentration of COOR groups on the fibre surface increases with increasing acrylic acid content in the monomer feed. Using TFE derivatisation, the concentration of COOH is less than that for COOR (Fig. 3), indicating that the efficiency of the plasma deposition process appears to be lower than for other surfaces (Table 3). The concentration of the COR groups in the plasma polymer was much higher than previously reported and cannot arise from the monomeric molecules without significant molecular fragmentation during plasma polymerisation. This is not discussed further, but is believed to result from the adsorbed water retained on the surface of the glass fibres.
Fig. 2. Functional groups present on fibre surface as a function of the concentration of acrylic acid in monomer feed.
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Fig. 3. The concentration of COOH retained in plasma polymer.
3.2. Coating uniformity and thickness The uniformity of the coating on each glass filament of 15 mm diameter, is an essential characterization requirement. Fig. 4 shows that the underlying concentration of Si 2p, which arises from the glass fibre substrate was , 1% (which is at the limit of accurate analysis by XPS). It needs to be pointed out that at an analysis angle of 458, the substrate can still contribute to the spectrum when the coating thickness is < 10 nm, as estimated from the inelastic mean free paths of carbon and silicon for the low levels of Si detected. This is not an accurate method of determining coating thickness, as there are inherent problems associated with calculating overlayer thickness using XPS, especially when the surface is not flat. A further complication is the cylindrical shape of the fibres, where photoelectrons will be emitted from round the entire circumference of the fibre, which gives a non-unique escape depth for the photoelectrons. Thus the low substrate concentration could not be attributed to a discontinuous coating. To confirm this, two single filaments were
analysed using the imaging mode on the SCIENTA 300 Spectrometer. Fig. 5(a) and (b) shows that it is possible to observe the spectrum generated from the single fibres. The spectra are of identical intensity, thus enabling it to be concluded that the coatings on randomly selected fibres from the tow are identical. The spectra generated from the single fibres, are not of sufficient intensity to quantify, but it is possible to see that the peak shape from both fibres is identical in shape to that generated from the fibre bundle (shown in Fig. 5(c)). The coating thickness has proved difficult to estimate. The fibre diameter measurements taken using scanning electron microscopy, do not show any difference between the fibre diameters of coated and uncoated fibers, within the error associated with the measurement of diameter (there was a deviation in the average diameter of the unsized fibre of 1.5 mm). The coating thickness could well be of the order of 10 nm (0.01 mm) as implied by the XPS measurements, a coating thickness of this magnitude would not have been detectable in the SEM fibre diameter measurements. Other techniques may have been used to yield more accurate
Fig. 4. Elemental composition of plasma polymer deposited onto glass fibres as a function of acrylic acid content of monomer feed.
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Average failure stress (GPa)
Weibull modulus
90% Acrylic acid 0% Acrylic acid Unsized (as received fibres) Unsized (after spreading tow, and travelling through the uncharged reactor)
1.33 ^ 0.56 1.58 ^ 0.64 1.46 ^ 0.8
3.94 3.58 3.10
1.35 ^ 0.7
1.65
3.3. Single fibre-strength measurement From Table 1, it is seen that the average fibre-strength is not significantly improved by the presence of the plasma polymer coating (within the standard deviation) although the strength is reduced compared to that of a ‘virgin’ glass fibre (1.46 GPa, from Table 1) due to the fibre-separation operation. The Weibull modulus on the other-hand for the plasma polymer coated glass fibres is significantly higher, showing a reduced variability in fibre-strength in the presence of the coating. 3.4. Single fibre fragmentation testing The results of the single fibre fragmentation test are shown in Table 2. Which illustrates the average fragment length, and debonded length, for each type of fibre. From the results in Table 2, it is possible to observe that the 90% acrylic acid plasma polymer coated fibre exhibits the shortest fragment length at saturation (0.41 mm), the unsized fibre exhibits a similar fragment length of 0. 42 mm at saturation, with the 100% octadiene coated fibre giving a longer average fragment length at saturation of 0.60 mm. The 100% octadiene plasma fibre exhibits significant debonding (at an average of 44.2% of the fragment length at saturation), whereas the 90% acrylic acid coated sample only exhibited small levels of debonding (7.0% at saturation), the unsized fibre did not exhibit debonding at all. The analysis of this raw fragmentation data will be discussed further.
4. Discussion Fig. 5. (a) Imaging Spectrum Of 100% Acrylic Acid Plasma apolymer On Single Fibres. (b) Cls Peak Generated from Image Of 100% Acrylic Acid Plasma Polymer On Single Fibres. (c) Cls Peak from 100% Acrylic Acid Plasma Polymer Coated Fibre Bundle.
information about the precise thickness of the plasma polymer coating, but these were not pursued, as the uniformity of the coating was the fundamental issue, not its thickness.
4.1. Coated fibres: uniformity within a bundle Fig. 2 shows that the surface functionality of the coated fibres increases with the concentration of acrylic acid in the monomer feed. However, the concentration of COOH groups is less than reported previously [5]. The presence of other functional groups (COR and CyO) in the plasma deposit are considered to arise from residual water in the plasma reactor, as adsorbate on the walls of the reactor, or
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Table 2 Single fibre fragmentation test data Strain
Plasma polymer type Unsized
3 4 5 6 7 8 9 10 11
90% Acrylic acid
100% Octadiene
Average fragment length (mm)
Average debond (%)
Average fragment length (mm)
1.58 (0.96) 0.96 (0.61) 0.86 (0.29) 0.63 (0.30) 0.51 (0.14) 0.48 (0.07) 0.45 (0.05) 0.43 (0.04) 0.42 (0.03)
– – – – – – – – –
6.08 (3.61) 1.71 (0.76) 0.85 (0.74) 0.50 (0.10) 0.46 (0.07) 0.43 (0.03) 0.41 (0.02) 0.41 (0.02) 0.41 (0.02)
the surface of the fibres. Nishioka and Shramka [15], showed that several monolayers of water will be present on an E-glass surface, which desorb only with difficulty. With the fibre tows present, a high surface area exists. Thus it can be expected that water will be involved in the plasma polymer deposition. One effect of this would be to reduce the retained (COOH) as shown in Fig. 3. The mechanism of this is beyond the scope of this paper as it is difficult to prove that water is present on the surface of the unsized fibres using XPS, due to the variety of other surface elements that are observed, which could easily contribute to an increased O1s peak in the spectra gained from unsized fibres (the magnitude of the O1s peak would be the main indicator of surface water on the unsized fibres). TFE derivatisation suggests that the apparent retained concentration of carboxylic acid functionalities is lower than expected [10]. Table 3 shows that surface analysis of a 100% acrylic acid plasma polymer deposited onto an aluminium foil substrate in the same reactor under the same conditions. In comparison with the fibre data in Fig. 3 it can be seen that the COOH concentration is less, and the COR concentration is higher in the latter. This supports the argument that residual water on the fibre surface is the probable explanation. Further, enhanced fragmentation of the monomers during plasma polymerisation is unlikely given the low Table 3 XPS data from 100% acrylic acid plasma polymer deposited on aluminium foil substrate Component peak
% of C1s Peak
C–R B-Shift C–O– R CyO COOR % of COOR retained as COOH (from TFE derivatisation)
53 19 6 3 19 70
Average debond (%)
Average fragment length (mm)
Average debond (%)
1.65 2.00 2.76 3.28 6.98
0.82 (1.06) 1.04 (1.62) 0.93 (0.94) 0.67 (0.30) 0.64 (0.23) 0.61 (0.19) 0.61 (0.19) 0.60 (0.19) 0.60 (0.19)
3.35 12.37 22.67 27.04 32.50 35.01 40.80 44.24
powers used. However, an alternative explanation for the reduction in retained COOH groups could be that esterification of the glass surface Si –OH groups can occur, providing a chemical bond between the two components. 4.2. Effect of fibre separation process on fibre-strength The change in strength of the glass fibres during the scale-up was investigated. It was observed that the fibres have their strength reduced from 1.46 to 1.35 GPa when they are spread, wound onto spools and passed through the reactor. The results seen in Table 1 show that the plasma polymer coating does not appreciably increase the strength of the degraded glass fibre. However, the Weibull modulus for the strength distribution is at a higher value. Thus the variability in strength is improved. This appears to suggest that either the plasma polymer protects the fibre from additional damage during sampling for strength testing, or that the large flaws responsible for the premature failures are filled by the plasma polymer. 4.3. Adhesion of plasma polymer functionalised E-glass fibres Debonding of the unsized fibres did not occur during fragmentation, which is consistent with a good interfacial bond. The constant shear analysis also supports a strong interfacial bond. However, the value of IFSS (given in Fig. 6), is about twice the shear yield strength of the matrix resin (28 MPa) employed, so that the quantification can be considered doubtful. A good bond between unsized or uncoupled glass fibre has been reported previously [16,17] so the observation is not unduly surprising. Reanalysis of the data for stress transfer using the CSTF methodology (Fig. 7) still supports the view that adhesion of these unsized fibres is high. However, the adhesion is now apparently only marginally better than the 90% acrylic acid/10% octadiene plasma copolymer
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Fig. 6. Kelly–Tyson analysis of fragmentation test data.
Fig. 7. Fragmentation Test Data Analysed by Cumulative Stress Transfer Function (CSTF).
Fig. 8. STE with applied strain for fragmentation test data.
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Fig. 9. STE as a function of fragment length.
coated fibre, which exhibited only a small degree of debonding (6.98% at saturation, as shown in Table 2). These observations seem to be consistent with a strong interfacial bond of similar magnitude. The STE, has been shown [14] to be a better analysis of the stress transfer between matrix and a fragmented fibre when the degree of debonding is high, because, it was shown that for completely debonded fibres, CSTF can increase with applied strain. The STE is achieved by normalizing the CSTF value to that for an ideal fragment of equal length (Fig. 8). The fragmentation process leads to a different rate of length reduction so that comparison of the STE at constant strain is imprecise. The values of STE are compared at an average fragment length in Fig. 9, which shows that the best interface is described by the shortest fragment length with the largest STE. In this analysis, the 90% acrylic acid/10% octadiene plasma copolymer coated fibres would now appear to be slightly more efficient than the untreated glass fibres, although within the error bars the difference may not be significant. The plasma polymer coatings have a thickness of < 10 nm (estimated) and are more highly cross-linked than the epoxy resin, and therefore yield within an interphase region during fragmentation is unlikely. Support for this argument is the fact that debonding was observed in both cases. One of the most interesting observations concerns the 100% octadiene plasma polymer coated fibres, which
debond during the experiment. With carbon fibres, complete interfacial debonding was observed at low applied strains, leading to different CSTF trends [5,14]. So whereas with carbon fibres the plasma polymer completely masked the underlying adhesive ability of the fibre, in this case some adhesive ability is retained, yet the functional chemistry (shown in Fig. 2) does not appear to be significantly different. This result can only be understood from either a strong residual water effect on the fibre surface functional chemistry, or a result of the different mechanical or thermal properties of E-glass fibres in comparison with Type A carbon fibres. The good adhesion of unsized or uncoupled fibres without an obvious chemical mechanism could mean that high radial compressive thermal stresses develop during cure of the sample. Further research is needed to accurately identify this mechanism of adhesion.
5. Conclusions Plasma polymerisation was utilised to deposit a functionalised plasma polymer size continuously onto E-glass fibre tows. From XPS measurements taken directly from the fibre surface, it is possible to see that the fibres are uniformly coated within the tow, with a functionalised plasma polymer. The variability in the strength of the fibres (as measured by Weibull modulus) has been reduced by the presence of the plasma polymer coating. The presence of the plasma polymer coating has also changed the interfacial
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response of the fibre with respect to micro-mechanical testing, giving rise to interfacial debonding. Further to this work, multi-fibre composite beams manufactured from the plasma polymer sized fibres will be used to make interlaminar shear strength test specimens, to demonstrate the effect of the plasma polymer coating on the macroscopic composite properties.
Acknowledgements The authors would like to acknowledge the financial assistance of the EPSRC and the Advanced Composites Group Ltd (and Dr D. Newman for his helpful advice). The assistance of Dr G. Beamson and Dr D.S. Law at the Research Unit for Surfaces, Transforms and Interfaces (RUSTI) is acknowledged for their assistance and advice during the acquisition of the XPS spectra on the SCIENTA 300 spectrometer, housed at this facility. The helpful comments and ideas provided by Dr S.A. Hayes during this work are also acknowledged.
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