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polyester composites
Accepted Manuscript Continuous surface modification of glass fibers in a roll-to-roll plasma-enhanced CVD reactor for glass fiber/polyester composites Vladimir Cech, Ales Marek, Antonin Knob, Jan Valter, Martin Branecky, Petr Plihal, Jiri Vyskocil PII: DOI: Reference:
S1359-835X(19)30112-5 https://doi.org/10.1016/j.compositesa.2019.03.036 JCOMA 5390
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
14 November 2018 21 March 2019 23 March 2019
Please cite this article as: Cech, V., Marek, A., Knob, A., Valter, J., Branecky, M., Plihal, P., Vyskocil, J., Continuous surface modification of glass fibers in a roll-to-roll plasma-enhanced CVD reactor for glass fiber/polyester composites, Composites: Part A (2019), doi: https://doi.org/10.1016/j.compositesa.2019.03.036
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Continuous surface modification of glass fibers in a roll-to-roll plasma-enhanced CVD reactor for glass fiber/polyester composites
Vladimir Cech a,*, Ales Marek b, Antonin Knob c, Jan Valter b, Martin Branecky a, Petr Plihal b, Jiri Vyskocil b a
Institute of Materials Chemistry, Faculty of Chemistry, Brno University of Technology, Purkynova 118,612 00
Brno, Czech Republic b
HVM PLASMA, spol. s r.o., Na Hutmance 2, 158 00 Prague, Czech Republic
c
Remarkplast s.r.o., Luka 152, CZ-783 24 Luka, Czech Republic
Plasma-enhanced chemical vapor deposition (PECVD) is a cost-effective and green process of plasma polymer synthesis that can be used as a gentle but powerful tool for the surface modification of fibers and retains their bulk properties. This paper introduces the notion of a compact and modular device consisting of low-pressure reactor segments that have a specialized function and are adapted for the continuous modification of fiber surface. A roll-to-roll device, designed on an industrial scale, with a PECVD reactor was used for continuous plasma pretreatment and plasma polymerization coating on 1,600 glass fibers in a bundle, with the fibers used as reinforcements in a glass fiber/polyester composite. Optimization of the plasma-processing conditions allowed for the improvement of interfacial adhesion between the glass fibers and polymer matrix, resulting in an increase in the shear strength of the polymer composite by 14%, compared to industrially sized fibers coated by wet chemical processes.
Keywords: A. Glass fibers; A. Polymer-matrix composites (PMCs); B. Interface/interphase; E. Chemical vapour deposition (CVD)
*
Corresponding author. Tel.: +420 541149304; Fax: +420 541149361 E-mail: [email protected] (V. Cech) 1
1. Introduction Many applications require materials in fiber form. These fibers must be surface-treated or surface-coated so that their surface properties (be they physical, chemical, or topographic) suit their application. Low-temperature plasma is a non-invasive and solvent-free tool that can be very effective in the surface treatment (the chemical and/or physical altering of the fiber surface) or coating (thin film deposition) of fibers. Reactive species (such as radicals, ions, and electrons) in plasmas can usually affect only the surface layer of fibers up to a depth of no more than a few nanometers, depending on the structure of the material and its density, and therefore do not affect the bulk properties of the fibers. Plasma treatment and plasma coating are used for the surface modification of high-performance fibers (carbon [1–6], glass [7–10], aramid [11–13], polyethylene [14]), and even natural fibers (flax [15], coconut [15], cellulose [16,17], and sisal [18]) that are applied as reinforcements in fiber-reinforced polymer composites. O2 [2,3,5,11], CO2, H2O [6,11], or air [4,12,13,15,16,18] plasmas are mostly employed for plasma treatment to improve compatibility between the fiber surface and polymer matrix, which can be achieved due to the increased wettability of the fiber surface by incorporating polar groups such as hydroxyl, carbonyl, and carboxyl into the surface layer. Like oxidative reactions, argon (Ar) plasma [3,17] leads to increased fiber surface roughness due to chemical or physical etching, which enlarges the fiber contact area with the polymer matrix and increases interfacial adhesion. NH3 plasmas [1,11] are suitable for incorporating amine groups into the surface of the fibers, allowing chemical bonding with the epoxy resin. When using plasma-enhanced chemical vapor deposition (PECVD) technique [8–10,14], plasma coating - unlike plasma treatment - makes it possible to significantly change the physical, chemical, and topographic properties of the fiber surface. The gas molecules of organic, inorganic, or organic-inorganic precursors are activated and fragmented in the plasma discharge, and the reactive and radical species form a thin film material that is applied directly to the surface of the fiber. The chemical composition and structure of the deposited films may differ significantly from the composition and structure of the precursor molecules. The variability of the physical and chemical properties of these thin films can be enormous, depending on the precursor molecules and deposition conditions. The plasma synthesis (PECVD) is called plasma polymerization [19–21] when organic or organosilicon precursors are used, and the deposited film has the character of a polymeric material. Plasma polymerization coating is a cost-effective and green process that uses a minimum amount of input precursor and produces a minimal amount of waste products, and therefore does not require an environmental remediation process [22].
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Plasma surface treatment can also be applied to textile fibers (like cotton, linen, silk, and wool) for effective disinfection. O2, N2, and Ar plasmas inhibited the growth of microorganisms, increased the fiber strength, and did not alter the molecular structure of the fibers [23]. Air plasma was successfully employed to improve the absorption of wool and dye fixation in the process of coloring the wool fibers [24], and it was also found that air plasma is useful for the surface cleaning of raw cotton fibers, increasing their wettability by removing only the surface layer containing non-cellulosic components [25]. Oxide nanoparticles can be synthesized in a plasma discharge (PECVD) at higher process pressures. Zhang et al. [26] deposited TiO2 nanoparticles on wool fibers, resulting in photocatalytic activity that improved the self-cleaning and antibacterial properties of the fibers. Conductive polymers (such as polyaniline, polypyrrole, ethylenedioxythiophene, and polythiophene) can be deposited (PECVD) on the textile fibers used for smart textiles (like medical textiles, protective clothing, touchscreen displays, flexible fabric keyboards, and sensors) [27] or smart biomaterials for tissue engineering [28]. Plasma-polymerized acrylic acid can also be deposited on different fibers to improve cell growth, proliferation, and differentiation for use in regenerative medicine and tissue engineering [29]. Electrospun nanofibers can be assembled into bundles and yarns [30,31] that have a remarkably high specific surface area that improves chemical absorption and flexibility, compared to bundles of conventional fibers. These bundles of nanofibers can also be surface-modified by plasma [32], and as such, they are promising for new applications in medicine, filtration, and the reinforcement of polymer composites. Carbon nanotubes can now be produced as carbon nanotube fibers using floating catalyst chemical vapor deposition (CVD), and these macroscopic filaments are plasma treated [33] to serve as reinforcements for composite structures. Both commercial production and laboratory fiber processing require continuous surface modification using a roll-to-roll plasma device. Simple roll-to-roll devices, mostly for the plasma treatment of fibers using atmospheric-pressure plasmas, were constructed on a laboratory scale, employing a dielectric barrier discharge or a plasma jet [34–36]. The plasma coating in low-pressure plasmas allows for a much wider variability of physical, chemical, and topographic properties for the functionalization of the surface of the fibers compared to atmospheric-pressure plasmas, and therefore a larger range of applications. Roll-to-roll devices with PECVD reactors that operate at low pressure and specialize in the surface modification of fibers are lacking not only in industry, but also in research laboratories. The idea of a roll-to-roll system comprising six segments of reactors for the surface modification of single fiber or bundle of fibers is shown in Fig. 1. This is an example of a compact and modular device that can be operated as a batch processing vacuum equipment in which the fiber supply and take-up spools are in a vacuum,
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or as an air-to-air vacuum equipment featuring a fiber transition between atmospheric pressure and the medium vacuum (0.1–100 Pa), using differential pumping. The device with reactor segments is a variable system, where the number of segments can be adapted to the desired application. Each reactor can be specified for various operations (like plasma pretreatment, plasma coating, or the post-treatment of fibers), or several reactors can use the same operation to increase the fiber winding speed. Based on the idea outlined in Fig. 1, a roll-to-roll device with two plasma reactors for the continuous surface modification of fibers was designed on an industrial scale, constructed, put into operation, and successfully tested. Process scale-up, from laboratory to industrial scale, is a considerable challenge for any application of low-temperature plasma, because the geometry of the reactor and its hydrodynamic characteristics affect the performance of plasma processes [37]. This study focused on optimization of the process of plasma surface modification for glass fibers (GFs) used as a reinforcement for polymer composites to demonstrate the usability of the device. GF bundles with 1,600 filaments of 19 µm in diameter were pretreated with oxygen plasma and coated with a plasma polymer film of tetravinylsilane, using precursor deficient conditions to functionalize the GF surface. The plasma reactor was characterized in terms of the operation conditions, along with the physical, chemical, and surface properties of the deposited films, while surface-modified fibers were tested as reinforcements in polyester composites. Using a mixture of tetravinylsilane precursor and oxygen gas, it was found that the incorporation of oxygen atoms into the plasma polymer network and the formation of polar groups (hydroxyl, carbonyl) on the coated GF surface improves the composite performance.
2. Experiment
2.1 Materials Bundles with unsized and industrially sized GFs (E-glass, 1,200 tex, diameter 19 µm, 1,600 filaments in a bundle) were provided by Saint-Gobain Adfors CZ s.r.o. (Czech Republic). Unsized GF means that the fibers are untreated and uncoated, whereas sized GF indicates that the fibers are surface-modified by sizing (a wet chemical process) based on silane coupling agents that are optimized for polyester resin by the GF manufacturer. Flat glass and silicon wafers were used as substrates for the thin film analysis. Soda-lime float glass with dimensions of 1.0×26×76 mm3 was supplied by Knittel Glaeser (Germany), while double-side polished wafers (100) measuring 0.8×10×10 mm3 were obtained from ON Semiconductor (Czech Republic). The silicon wafers, which were transparent for infrared (IR) radiation, were covered with a 3 nm-native-SiO2 layer. Argon gas (purity 99.999%) and gaseous oxygen (purity 99.99%) were delivered by Linde Gas (Czech Republic), while the
4
tetravinylsilane (TVS) precursor (Si–(CH=CH2)4) had a purity of 97%, and was supplied by Sigma Aldrich (Czech Republic).
2.2 Model PECVD reactor
A batch processing vacuum equipment with one model reactor was developed to test its functionality for various operations such as the plasma pretreatment of fibers, plasma coating of fibers, and subsequent treatment of coated fibers. A schematic view of this equipment is shown in Fig. 2. We used the SIMAX glass tube as a reactor, with a length of 100 cm and an outside diameter of 4.6 cm (Merci, Czech Republic). The glass tube was equipped with seven powered and six grounded ring electrodes connected to the radio-frequency (RF) power supply, Cesar 1310 (13.56 MHz, 1000 W), and a VM 1000A Matching Network (Advanced energy, U.S.A.). Both ends of the tube were closed by a plasma limiter that prevented the plasma from spreading out from the tube. The cylindrical shape of the reactor was important for the axially symmetrical plasma formed in the tubular reactor by the RF glow discharge, so that the fiber that is disposed or movable along the axis of the tubular reactor is evenly surrounded by plasma, resulting in a uniform surface modification of the fiber. The plasma reactor was placed in a Faraday cage with five opening windows, blocking electromagnetic fields and preventing heating-induced damage to the glass tube. The batch processing vacuum equipment was pumped by a HiCube 80 Eco pumping station (Pfeiffer Vacuum, Germany), which consists of a turbo pump HiPace 80 (pumping speed 66 dm3 s-1) and a backing pump MVP 015 (pumping speed 0.7 m3 h-1). The required pressure in the plasma reactor was controlled by a butterfly valve 615 DN (VAT, Switzerland) that restricts the pumping speed of the pumping station. Inlet gases, argon, and oxygen were fed into the reactor via the mass flow meters F-201CV (Bronkhorst, the Netherlands), enabling the mass flow rate to be controlled. The liquid precursor was placed in a glass tube immersed in a water bath, whose temperature was controlled to a range of 8–40 °C by means of a connected Peltier thermostat PT 31 (A.KRÜSS Optronic, Germany). The mass flow rate of precursor vapors was controlled by a mass flow meter F-201DV (Bronkhorst, the Netherlands). The pressure in the vacuum equipment was measured using a Pirani gauge TPR 280, a full-range gauge PKR 251, and capacitance gauges CMR 362 and CMR 364 via a MaxiGauge TPG 256 A controller, all supplied by Pfeiffer Vacuum, Germany. The typical basic pressure in the reactor was 5×10-4 Pa. The vacuum equipment was equipped with manual, venting, and pneumatic valves from Pfeiffer Vacuum, Germany, and the samples were inserted into the plasma reactor from the left chamber connected to the tubular reactor. Flat glass and silicon substrates can be set in any position along the axis of a tube reactor, using special glass stages. The single fiber or bundle of fibers is placed along the
5
tube axis, using a special glass frame. The model PECVD reactor was tested for the plasma pretreatment of GFs using argon and oxygen plasma, and for plasma polymer deposition using a pure TVS precursor or its mixtures with oxygen (O2) gas. After the application of plasma pretreatment or plasma coating, the fiber surface is activated containing free radicals, which can be used for the subsequent surface grafting, achieved by covalent bonding, of suitable vapor molecules that were introduced into the plasma-free reactor. This process is known as plasma-induced grafting.
2.3 Roll-to-roll device with two PECVD reactors Using the experience from the model PECVD reactor, a roll-to-roll device with two plasma reactors for the continuous surface modification of fibers was developed on an industrial scale. The main reason for designing the device was to improve the functionality of the surface of the fibers to reinforce the polymer composites. However, it can be used much more widely, and is suitable for any fiber that is not too brittle, but sufficiently strong to prevent damage by rewinding. The roll-to-roll device was designed as a batch processing vacuum equipment, the scheme of which is shown in Figure 3. A spool with fibers with a maximum diameter of 120 mm and width of 300 mm is inserted into the left chamber that is evacuated by auxiliary scroll pump nXDS6i (pumping speed 6.3 m3h-1, Edwards, UK) to remove the desorbed water molecules from the fiber surface. The typical basic pressure in reactors was < 0.1 Pa. The fibers were guided from the fiber-supply spool through a Teflon roller system into the first reactor, which was designed for fiber pretreatment with Ar and/or O2 plasma, with the fibers moving along the axis of the tubular reactor. Thereafter, the fibers moved through an aperture, the size of which could be changed, into the second reactor; the size of the aperture can be adapted to the diameter of the fiber to prevent gas permeation between reactors. An open aperture allows for the distribution of gases between the reactors, and thus the deposition of gradient films. A second reactor that allows the precursor and its mixtures with the Ar, and O2 gases to be introduced was used for the thin film deposition using the PECVD technique. After passing through the second reactor, the fibers were wound onto the take-up spool in the right chamber (Fig. 3). The fiber rewinding was electronically controlled by stepping motors SX34-2740N (Microcon, Czech Republic) in a range of 0.1–8 m min-1 (these too can be adapted). Both reactors were independently pumped by two HiPace 300 turbopumps (pumping speed 255 dm3 s-1, Pfeiffer Vacuum, Germany), with one auxiliary scroll pump, and powered by two RF generators with matchboxes. The other vacuum components were similar to those of the model reactor. These plasma reactors were also enclosed in Faraday cages to prevent the spread of electromagnetic radiation, and a roll-to-roll device was pre-prepared for expansion into the four reactor segments. 6
2.4 Thin film characterization The thickness of the films deposited on flat glass and silicon substrates was evaluated by the DektakXT stylus profiler (Bruker, U.S.A.). A 25-μm-diameter stylus, to which a force corresponding to 2 mg was applied, was used to measure vertical steps in the deposited film that were prepared by mechanically removing a portion of the film from the substrate. Low-force measurements result in negligible deformation of the film, and hence the height of the step, determined from the sample profile, corresponds to the thickness of the film. Five steps in the film were used to evaluate the average film thickness, and the deposition rate was determined as the ratio between the film thickness and deposition time. Nanoindentation measurements were performed using a 2D TriboScope (Hysitron, U.S.A.) attached to an NTegra Prima Scanning Probe Microscope (SPM) (NT-MDT, Russia) to determine the Young’s modulus, E, and hardness, H, of the deposited films. The thickness of the analyzed films of 1.0 µm was sufficiently high to reduce the influence of the substrate on the measured data. The depth profiles of mechanical parameters (E, H) were determined using a cyclic mode [38] of a nanoindentation technique of up to 30% film thickness, applying 20 load cycles at an unload fraction of 0.8. A Berkovich diamond indenter (Hysitron, U.S.A.) with a radius of 50 nm was used for instrumented nanoindentation, and unload-displacement curves were used to calculate mechanical parameters by the Oliver-Pharr method [39]. The unload curve was used to evaluate the sample stiffness, S, according to the equation
S
2
Er A, (1)
where A is the projected contact area, and Eris the reduced modulus that relates to the Young’s modulus, E, and Poisson’s ratio, ν, of the sample, through the relationship
1 (1 2 ) (1 vi2 ) , Er E Ei
(2)
where the index i corresponds to the parameters of diamond: Ei = 1,141 GPa, νi = 0.07. The Poisson’s ratio, ν, for the polymeric material was 0.35. The hardness of the sample was determined from the maximum applied load, Pmax, as
H
Pmax . A
(3)
7
The depth profiles of the mechanical parameters were constructed from the mean values determined from five nanoindentation measurements, and extrapolated to zero contact depth (film surface) in order to evaluate the correct mechanical parameters (E, H) of the measured film, which are not affected by the substrate [40]. The surface topography of deposited films was analyzed using SPM, which operated as atomic force microscopy (AFM) using semicontact mode and silicon probes NSG03 (Rc<10 nm, NT-MDT) of a resonant frequency of 90 kHz and a force constant of 1.1 N m -1. The root-mean-square (RMS) roughness as a characteristic of the surface topography was determined using AFM images of a scanned area of 5×5 μm2. The composite short beams were observed using a scanning electron microscope (SEM) (Philips XL 30/EDAX/Microspec). The IR measurements of the thin films deposited on the IR-transparent silicon wafers were performed by a VERTEX 80v, vacuum Fourier transform infrared (FTIR) spectrometer (Bruker Optics, U.S.A.) to characterize the chemical structure of the deposited films. The samples were measured in a vacuum at a pressure of 160 Pa using a mid-IR source, a KBr beamsplitter, and a DLaTGS D301 detector. Transmission spectra were obtained in a wave number range of 750–4000 cm-1 using 256 scans and a spectral resolution of 4 cm-1. Due to the eliminated atmospheric moisture absorptions, the sensitivity and stability of the spectrometer was sufficiently high to measure the deposited films with a thickness of only 0.10µm. The IR spectrum of the bare silicon wafer was subtracted from that of the specimen with the deposited film to extract the IR peaks corresponding to the film.
2.5 Fabrication and shear test of GF/polyester composites Short beams (18×10×3 mm3) of unidirectional GF/polyester composites were manually manufactured and cured for 30 min at 100 °C and then for 1 h at 140 °C, using an unsaturated polyester resin (Viapal HP 349 F, Sirca S. p. A., Italy). The beams were polished with sandpaper with a particle size of 220 μm until reaching the desired size. The composite short beams were characterized by a short-beam-shear (SBS) test (ASTM D 2344 [41]) using a universal test machine Z010/TH2A (Zwick, Germany). The ASTM standard requires a span-tothickness ratio of 4.0 and a length-to-thickness ratio of 6.0. The diameter of the load nose must be 6 mm, and the support nose diameter 3 mm, while a rate of crosshead movement of 1.0 mm min-1 is required. The beam was loaded until a fracture occurred, and the fracture load was used to characterize the shear strength of the composite. The short-beam strength, Fsbs, is calculated as [41]
F sbs 0.75
Pm , bh
(4)
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where Pm is the maximum load observed during the test, b is the measured specimen width, and h is the specimen thickness measured. For each surface modification of the GFs, eight composite beams were tested.
3. Results and discussion
3.1 Deposition process characteristics The device with the model PECVD reactor was very useful for a basic characterization of plasma operations using Ar and O2 gases, and TVS precursors such as a determination of the plasma distribution along the reactor, the evaluation of suitable process parameters (flow rate, process pressure, RF, or effective power), consumption of the precursor, and distribution of deposition rates. This characterization would be more complicated for a more complex roll-to-roll device. The plasma distribution along the reactor, which was estimated based on the intensity of the radiation in visible light, was affected by the number of electrodes, their distribution along the tube, the used gas, its mass flow rate, the pressure in the reactor, and whether continuous wave or pulsed mode power was in place. As shown in Fig. 2, 13 equidistant electrodes (seven powered, six grounded) along the tubular reactor were chosen as optimal for a wide range of process parameters. Although the configuration of the electrodes was optimized for Ar discharges, the distribution of the plasma along the tube was less uniform, using O2 and a TVS precursor. The TVS plasma was tested for RF power in the range of 1 to 500 W for a continuous wave discharge at a flow rate of 1.4, 4.0, and 7.0 sccm (standard cubic centimeters per minute). The process pressure in the left chamber (at the downstream end of the tubular reactor) was preset to 3.8 Pa for all flow rates before the discharge was ignited. The process pressure as a function of RF power for different flow rates is given in Fig. 4a. In the case of the two lower flow rates of 1.4 and 4.0 sccm, the TVS plasma was burning for all powers (1–500 W). However, at a flow rate of 7.0 sccm, the TVS plasma was ignited from 5 W. The intensity of the radiation was higher in the left side of the tube for low powers, but expanded to the right with enhanced power, and was almost constant along the tube from 30 W. The process pressure decreased to a minimum and then slightly increased for 1.4 and 7.0 sccm, but dropped to a saturated value in the case of 4.0 sccm. A reduced process pressure after plasma ignition is characteristic for a precursor-deficient regime [20]. The difference between the preset pressure, poff, when the plasma is switched off, and the process pressure, pon, when the plasma is switched on, can be used to calculate the precursor consumption, MC, as in
MC
( poff pon ) poff
100%.
(5)
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The precursor consumption characterizes the plasma process efficiency for the deposition of thin films, and is an important industrial parameter for calculating operating costs. The minimum process pressure corresponds to the maximum consumption of the precursor, which increased from 56 to 73% for reduced flow rates from 7.0 to 1.4 sccm. When the plasma discharge was operating in a pulsed regime, the process pressure decreased with enhanced effective power (2–150 W) and the plasma process efficiency was lower than that of the continuous wave, and therefore the precursor consumption was lower, as shown in Fig. 4b, for flow rates of 1.4 and 4.0 sccm. Effective power, Weff, is determined by the equation
Weff
t on Wtotal , (t on t off )
(6)
where ton and toff is the time when the plasma is switched on and off, respectively, and Wtotal is the total power. The TVS molecules are fragmented in the plasma discharge due to inelastic collisions with the high-energy electrons, and therefore lead to the formation of free radicals, which are a highly reactive species that result in the synthesis of plasma polymer due to a radical recombination at the surface of the growing film. When the degree of ionization increases with enhanced RF power, the higher concentration of free electrons leads to an increase in the number of free radicals, and thereby to a higher degree of fragmentation. If the process pressure decreases with enhanced power, it means that there is a higher number of recombinated than generated fragments in the plasma discharge. The process pressure is not constant along the tubular reactor and is higher at the precursor inlet, and decreases in the direction of pumping. The increase in process pressure with enhanced power (Fig. 4a) can be explained by the locally reduced pressure, plocal, at a certain position along the tube, which in our case was too low to locally ignite the discharge (similarly to Paschen’s law [42]). At a steady-state flow, the local pressure, plocal, corresponds to the concentration of molecule fragments generated per unit of time, ngen, reduced by recombined fragments per unit of time, nrec, plus the concentration of by-products per unit of time, nbypr, and the concentration of unfragmented TVS molecules, nunfr, according to the equation
plocal (ngen nrec nbypr nunfr )kT,
(7)
where T is the gas temperature and k is the Boltzmann constant. This trend of increased pressure with enhanced RF power observed for continuous wave was not observed in the pulsed plasma regime (Fig. 4b), where a sufficient amount of unfragmented precursor was supplied to the position of low pressure during toff, when the plasma was switched off.
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The process pressure dependent on the RF power for a constant flow rate of 1.4 sccm but different preset pressure values of 3.8, 7.6, and 15.2 Pa is shown in Fig. 4c. The plasma discharge was ignited at a power that increased with increasing preset pressure. The precursor consumption could be optimized for a preset pressure of 7.6 Pa, and reached 81% at the minimum process pressure. In the case of the highest preset pressure of 15.2 Pa, the intensity of radiation was very low on the right side of the tubular reactor for powers < 10 W. The range of flow rates 1.4–7.0 sccm, preset pressures 3.8–15.2 Pa, and RF powers 1 (5)–200 W represent suitable operation conditions that provide a stable plasma for the batch processing vacuum equipment with one model reactor. The plasma was distributed along the reactor (Fig. 5) for the above deposition conditions, but this does not mean that the deposition rate was constant along the axis of the tube. The deposition rate along the reactor for selected powers of 10, 30, and 100 W (1.4 sccm TVS, 3.8 Pa) is plotted in Fig. 6a. The distance along the reactor was measured from the end of the tubular reactor with the precursor input. Five silicon wafers were distributed along the tube axis for each RF power, and the thickness of the deposited film for each sample was measured by a mechanical profilometer to evaluate the deposition rate as the ratio between the film thickness and the deposition time. The trend of the deposition rate as a function of distance from precursor input was similar for all the RF powers used (Fig. 6a). The deposition rate is affected by the concentration of free radicals, their chemical reactivity (such as monoradicals and multiple radicals), and their mass. We can see that the highest deposition rate is located near the precursor inlet, where a sufficient number of heavy radicals can be expected. The concentration of unfragmented TVS molecules decreases towards the pumping system, resulting in lower deposition rates. The minimum deposition rate at the 75 cm position can be assumed to correspond to the position of locally reduced pressure, as mentioned in the previous paragraph. The effect of different preset pressures (1.4 sccm TVS) on the deposition rate for a selected RF power of 10 W is given in Fig. 6b. The trend for all pressures is very similar in the first half of the tube at the precursor inlet, but the deposition rate in the second half significantly increased at a higher preset pressure. In Fig. 6c, the precursor flow rate was increased to 4.0 sccm, compared to Fig. 6a, but the preset pressure was the same. The deposition rate increased to 570 nm min-1 at the precursor inlet for 100W, but for 10 W ranged from 112 to 219 nm min -1 along the tube. The distribution of the deposition rate along the tubular reactor for the mixture of TVS precursor (4.0 sccm) with oxygen gas (4.0 sccm) is shown in Fig. 6d. In the case of 10 W, the deposition rate gradually decreased from 353 to 31 nm min-1 as the distance from the precursor input increased. However, for 60 W, the deposition rate was extremely high (689 nm min -1) at the precursor inlet, followed by an almost constant value at about 74 nm min -1. The four examples provided in Fig. 6 show that the distribution of the deposition rate along the tubular reactor is
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different, and depends on the deposition conditions, but the predominant trend is for the highest deposition rate to be determined at the precursor inlet. The average deposition rate, vdp, can be calculated as the arithmetic mean of the five rates determined along the tube. In the roll-to-roll plasma device, where the fiber moves along the axis of the tubular reactor, the thickness of film, l, deposited on the fiber will be the same along the fiber, and estimated as
l
v dp v wind
L,
(8)
where vwind is the fiber winding speed and L is the length of tubular reactor; L = 100 cm for both plasma devices used in this study.
3.2 Thin film properties The Young’s modulus and hardness (Eq. 1–3) of the deposited films from pure TVS precursor (1.4 sccm), depending on the RF power and preset pressure for a selected RF power of 10 W, are shown in Fig. 7a. By enhancing the RF power, the mechanical properties, i.e., Young’s modulus and hardness, were slightly increased from 9.6 to 12.5 GPa and from 0.98 to 1.87 GPa, respectively. These values are typical of plasma polymer films, and provide evidence of a weakly crosslinked polymer network. In the case of increased preset pressure at a constant RF power of 10 W, the Young’s modulus slightly decreased from 9.6 to 8.0 GPa, but the hardness was almost the same, in the range 0.98–1.09 GPa. The mechanical properties of the films deposited along the tubular reactor were identical for the given deposition conditions. Plasma polymeric films, unlike conventional polymers, have a sufficiently low Young’s modulus, along with a high yield strength [10], to be used as functional interlayers in GF/polyester composites, as can be seen from model simulations [10]. Examples of infrared spectra for plasma polymer films deposited at different flow rates, preset pressures, and RF powers, but of the same thickness of 0.10 μm, are shown in Fig. 7b. The assignment of absorption bands [43,44] is displayed directly at the spectra. We can see that the infrared spectra are very similar, despite the different deposition conditions. The only significant difference is in the concentration of CHx species, which correlates with the process pressure during the film deposition. It is important that the vinyl group is incorporated into the plasma polymer network, and is not completely fragmented during the plasma process. The vinyl groups are important for chemical bonding with the polyester resin, when the plasma polymer film is used for the surface modification of the glass fibers employed as reinforcements in polymer composites. The infrared spectra were measured one month after the film deposition and reveal the post-deposition oxidation of the films, because the oxygen atoms were partly incorporated into the plasma polymer network, forming Si-O-C species 12
and partly forming hydroxyl and carbonyl groups. This is an aging effect that results from a weakly crosslinked polymer network and a high affinity of oxygen with silicon, where oxygen from the ambient atmosphere (O2, H2O) can diffuse into the film. Polar groups (hydroxyl and carbonyl) on the surface of the deposited film are beneficial for improving surface wettability and the surface free energy can be increased up to 58 mJ/m 2, which is higher compared to 34 mJ/m2 for polyester resin [45]. The infrared spectra of the films deposited along the tubular reactor were approximately the same for the given deposition conditions, indicating that the chemical composition of these films is independent of their position in the tube. It follows that the mass distribution of the TVS fragments along the tube is approximately the same, but their concentration varies locally, which is related to the different deposition rate along the tube. Detailed chemical analysis of deposited films for different deposition conditions can be found in previous studies [8 –10]. The surface topography of the deposited films was characterized by AFM and the AFM images for films of the same thickness of 1.0 µm that were deposited (10 W) from a pure TVS precursor (4.0 sccm TVS) and a TVS precursor (4.0 sccm) mixed with oxygen gas (4.0 sccm), which are given in Fig. 8a and Fig. 8b, respectively. The RMS roughness of the deposited films increased from 1.8 nm (TVS precursor) to 4.7 nm (TVS/O2 mixture), showing that the incorporation of oxygen atoms into the plasma polymer network leads to a slightly rougher surface. The RMS roughness of plasma polymer films increased with increased film thickness, which corresponds to the most relevant previous study [46].
3.3 Shear strength of GF/polyester composites The experience and basic characterization of the plasma device with the model PECVD reactor were used for the roll-to-roll device, with two plasma reactors used to continuously modify the surface of the GF bundle. The GF bundle was pretreated with oxygen plasma in the first reactor (Fig. 9), using an oxygen flow rate of 10 sccm, a preset pressure of 6.0 Pa, and an RF power of 50 W. The second reactor (Fig. 9) was used to deposit the plasma polymer film on a cleaned and activated GF surface from a TVS precursor or a TVS/O2 mixture. The size of the aperture between the two reactors was adjusted to the diameter of the fiber to minimize gas permeation between the reactors, and the winding speed of the fibers ranged from 0.16 to 0.34 m min -1, depending on the average deposition rate in the second reactor. The unsized, industrially sized, and plasma-coated GFs were used to reinforce GF/polyester composites in the form of short composite beams that were tested by the SBS test. The fiber volume fraction was 35%, meaning that 24 bundles were embedded into the polyester resin longitudinally along the beam; i.e., the short beam contained 38,400 individual fibers. The load-displacement curves for the unsized, industrially sized, and 13
plasma-coated GFs are shown in Fig. 10a. The curve for the unsized GFs is characterized by a low maximum load of 0.858 kN at a high displacement of 0.94 mm, which is responsible for a short-beam strength of only 21 MPa (Eq. 4) due to the weak mechanical bonding and weak van der Waals forces between the GF surface and polymer matrix. The significantly higher maximum load of 1.90 kN, with a shorter displacement of 0.72 mm for industrially sized GFs, corresponds to a short-beam strength of 49 MPa, indicating increased adhesion between the surfaces due to siloxane bonding at the GF/polyester interface [7]. The optimized plasma coating resulted in a maximum load of 2.36 kN at a 0.71 mm displacement, demonstrating an increased short-beam strength of 56 MPa, together with an increased composite toughness (the area below the load curve, from the beginning to the maximum load). SEM micrographs were used to observe fractured short beams for unsized and plasma-coated GFs on the same scale of 50 µm in Fig. 10b and Fig. 10c, respectively. Fig. 10b is an example of weak interfacial adhesion between the GF and the polyester resin. It can be seen that the GF fibers are bare on their surface, without the residues of the adhered resin, and that the delaminated resin residues are completely separated from the fibers. Differently, the plasma-coated GFs are entirely covered with a polyester resin bonded to the plasma polymer film on the GF surface, and a character along with a large amount of resin debris onto GFs and between prove the high interfacial adhesion and cohesive failure of resin (Fig. 10c). Some GFs were broken due to the high shear stress transferred from the polymer matrix to the fiber, as a result of the strong interfacial adhesion. A film thickness of 0.15 µm (Eq. 8) on a GF bundle is sufficient for the proper surface modification of GFs [8–10], which are used as reinforcements in GF/polyester composites. When coating a fiber bundle, the shielding effect caused by adjacent fibers must be analyzed. The free radicals diffuse into the fiber bundle and the fiber bundle does not have to be spread for the coating. The required film thickness of 0.15 μm refers to fibers at the edge of the bundle; however, the deposition rate decreases toward the center of the bundle due to multiple shielding, as [10]
l n l s q n1 ,
(9)
where ls is the thickness of the film on the fiber at the edge of the bundle, ln is the film’s thickness on the n-th fiber in the direction to the bundle center shielded by n-1 fibers, and q is the shielding factor. For the GF bundle of circular cross-section and the number of fibers 1,600, where the fiber in the bundle center is the 22 nd, and for ts = 0.15 µm and q = 0.9 [10], the film thickness is t22 = 16 nm on the central GF. It was found that the interfacial shear strength is independent of the coating thickness for a thickness greater than 0.1 µm corresponding to the fibers at the edge of the bundle [8,9]. Therefore, the interfacial properties of the composite do not change across
14
the bundle. The short-beam strength for GF/polyester composites reinforced with plasma-coated GFs was compared to the strength of unsized and industrially sized GFs in Fig. 11. The shear strength of composite beams, with plasma-coated GFs using a pure TVS precursor (4.0 sccm, 3.8 Pa) at 10 and 60 W, was significantly increased to 39 MPa (10 W) and 37 MPa (60 W), compared to the value for unsized GFs. This increase is associated with the covalent bonding of the interlayer on both interfaces with the fiber and the polyester resin [8]. We have previously proven [9,10] that the incorporation of oxygen atoms into the plasma polymer network is favorable for improving interfacial adhesion, not only at the film/GF interface due to the increased number of Si–O–C species, but also at the polyester/film interface due to the carbonyl and hydroxyl groups that improve the wettability of the plasma polymer film with a polyester resin. For this reason, for the surface modification of the GFs, a TVS precursor (4.0 sccm) in a mixture with oxygen gas (4.0 sccm) at an RF power of 10 and 60 W was used, and the short-beam strength was further increased to 50 MPa (10 W) and 42 MPa (60 W). A higher power of 60 W resulted in lower strength due to the lower interfacial adhesion at the film/GF interface, which corresponds to an excessively high concentration of carbonyl groups [9,10] because the shear strength of the composite was associated with the film adhesion [46]. The optimization of the oxygen fraction (0.5) in the TVS/O2 mixture resulted in a short-beam strength of up to 56 MPa at an RF power of 10 W, using continuous wave plasma. This optimization makes it possible to reduce the plasma polymer cross-linking and hence the Young’s modulus, and to optimize the concentration of carbonyl groups in the plasma coating, and thus the interfacial adhesion at the film/GF interface, both of which are responsible for the increased shear strength of the composite [9,10]. A further decrease in 5 W power, using pulsed plasma, was done to reduce the Young’s modulus, but did not lead to any further increase in the shear strength.
4. Conclusion Many fibers are produced such as high-performance fibers (carbon, glass, aramid, and polyethylene), natural fibers (flax, coconut, cellulose, and sisal), textile fibers (cotton, linen, silk, wool), electrospun nanofibers assembled into bundles or yarns, and carbon nanotube fibers. They have to be surface treated or coated with material in the form of thin films to improve or alter their surface properties for the desired application; plasma pretreatment of fibers and plasma-polymerization coating are the right tools for modifying the fiber surface only, with respect to its physical, chemical, and topographic properties. For the continuous surface modification of a single fiber or bundle of fibers, we introduced the idea of a compact and modular device that consists of segments whose number and functions (plasma pretreatment, plasma coating, or post-treatment of fibers) can be adapted for the desired application. Based on this idea, a 15
plasma device with a model PECVD reactor was developed and tested to determine the range of operation conditions (1.4–7.0 sccm, 3.8–15.2 Pa, 1 (5)–200 W) and characterization of thin film properties (E = 8.0–12.5 GPa, H = 0.98–1.87 GPa). This basic characterization of model plasma reactor was used to construct and test a roll-to-roll device with two plasma reactors for the continuous surface modification of fibers. As an example, the device was successfully used for the surface modification of GF bundles that were used as reinforcements in GF/polyester composites to demonstrate the usability of the device. Our study shows that the shear strength of GF/polyester composites with plasma-coated GFs can be controlled in the range of 37 to 56 MPa by processing conditions, and therefore significantly higher than 21 MPa for unsized GFs. The optimized plasma coating resulted in a shear strength that was 14% higher than that of industrially sized GFs, and 167% higher than that of unsized GFs. This being so, the plasma processing of GFs provides a sufficiently wide range of shear properties for the selected composite application. The roll-to-roll device was operated as a batch processing vacuum equipment but can be adapted for air-to-air vacuum equipment suitable for the production line. Plasma devices with model and roll-to-roll PECVD reactors are suitable for any fiber that is not too brittle, and sufficiently strong to prevent damage by rewinding. Plasma coating can be used to deposit protective, barrier, biocompatible, or multifunctional films with tailored physical, chemical, and surface properties on the fiber surface with controlled adhesion. Thin films can be deposited as single-layer, multilayer, or gradient nanostructures, depending on the number and functions of reactor segments.
Acknowledgements: This work was supported by the Czech Science Foundation, grant no. 16-09161S, and the Technology Agency of the Czech Republic, grant no. TA01010796. The authors would like to thank T. Plichta for nanoindentation and AFM measurements, and Dr. M. Sirovy and Saint-Gobain Adfors CZ s.r.o. for providing the glass fibers.
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[37] Nobrega PA, Gaunand A, Rohani V, Cauneau F, Fulcheri L. Applying chemical engineering concepts to nonthermal plasma reactors. Plasma Sci Technol 2018; 20:065512 (11 pp). [38] Trivedi R, Hoferek L, Cech V. Depth profile of mechanical properties of plasma-polymerized tetravinylsilane films evaluated by cyclic nanoindentation. Surf Coat Technol 2011;205:S470–S474. [39] Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7:1564–1583. [40] Fischer-Cripps AC.The IBIS Handbook of Nanoindentation. Forestville: Fischer-Cripps Laboratories, 2005. [41] ASTM D2344. Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates. American Society for Testing and Materials, West Conshohocken, 2001. [42] Raizer YP. Gas Discharge Physics. Berlin: Springer-Verlag, 1991. [43] Lin-Vein D, Colthup NB, Fateley WG, Grasselli JG. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules. San Diego: Academic Press, 1991. [44] Tolstoy VP, Chernyshova IV, Skryshevsky VA. Handbook of Infrared Spectroscopy of Ultrathin Films. New Jersey: John Wiley & Sons, 2003. [45] Cech V, Lichovnikova S, Sova J, Studynka J. Surface free energy of silicon-based plasma polymer films. In: Mittal KL, editor. Silanes and Other Coupling Agents. Vol. 5, Leiden: VSP, 2009, p. 333–348. [46] Cech V, Xu L, Vanek J, Drzal LT. Deposition single plasma-polymerized vinyltriethoxysilane films and their layered structure. Jpn J Appl Phys 2006;45:8440–8444. [47] Palesch E, Knob A, Plichta T, Cech V. Functional interlayers with controlled adhesion developed for polymer composites. Thin Solid Films 2018;656:37–43.
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Figure Captions Fig. 1. Schematic view of a compact and modular device for the surface modification of fibers.
Fig. 2. Schema of a plasma device with a model PECVD reactor.
Fig. 3. Schema of a roll-to-roll plasma device for the continuous surface modification of fibers.
Fig. 4. Process pressure as a function of (a) RF power for different flow rates, (b) effective power for different flow rates, and (c) RF power for different preset pressures.
Fig. 5. Low temperature plasma distributed along the model PECVD reactor.
Fig. 6. Deposition rate along the tubular reactor for (a) selected RF powers 10–100 W and a TVS flow rate of 1.4 sccm (preset pressure 3.8 Pa), (b) different preset pressures 3.8–15.2 Pa at 1.4 sccm TVS and 10 W, (c) selected RF powers 10–100 W and a TVS flow rate of 4.0 sccm (preset pressure 3.8 Pa), and TVS precursor (4.0 sccm) in a mixture with oxygen gas (4.0 sccm) at 10 and 60 W. Fig. 7. (a) Young’s modulus and hardness of the deposited films from pure TVS precursor (1.4 sccm) dependent on RF power (upper image) and preset pressure at a selected RF power of 10 W (lower image), (b) infrared spectra for plasma polymer films deposited from pure TVS precursor at different flow rates, preset pressures, and RF powers but the same thickness of 0.10 µm.
Fig. 8. AFM images for plasma polymer films of the same thickness (1.0 µm) deposited (10 W) from (a) pure TVS precursor (4.0 sccm TVS) and (b) TVS precursor (4.0 sccm) mixture with oxygen gas (4.0 sccm).
Fig. 9. A roll-to-roll device with two plasma reactors. The first reactor in the left figure with a glow discharge was used for the pretreatment of the fibers, and the second reactor in the right figure with plasma was used for the plasma polymer deposition.
Fig. 10. (a) Load-displacement curves for unsized, industrially sized, and plasma-coated GFs, and SEM micrographs of fractured composite beams for (b) unsized GFs and (c) plasma-coated GFs.
Fig. 11. Short-beam strength for GF/polyester composites reinforced with plasma-coated GFs, compared to that of unsized and industrially sized GFs (marked by dashed line).
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S1359-835X(19)30112-5 https://doi.org/10.1016/j.compositesa.2019.03.036 JCOMA 5390
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
14 November 2018 21 March 2019 23 March 2019
Please cite this article as: Cech, V., Marek, A., Knob, A., Valter, J., Branecky, M., Plihal, P., Vyskocil, J., Continuous surface modification of glass fibers in a roll-to-roll plasma-enhanced CVD reactor for glass fiber/polyester composites, Composites: Part A (2019), doi: https://doi.org/10.1016/j.compositesa.2019.03.036
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Continuous surface modification of glass fibers in a roll-to-roll plasma-enhanced CVD reactor for glass fiber/polyester composites
Vladimir Cech a,*, Ales Marek b, Antonin Knob c, Jan Valter b, Martin Branecky a, Petr Plihal b, Jiri Vyskocil b a
Institute of Materials Chemistry, Faculty of Chemistry, Brno University of Technology, Purkynova 118,612 00
Brno, Czech Republic b
HVM PLASMA, spol. s r.o., Na Hutmance 2, 158 00 Prague, Czech Republic
c
Remarkplast s.r.o., Luka 152, CZ-783 24 Luka, Czech Republic
Plasma-enhanced chemical vapor deposition (PECVD) is a cost-effective and green process of plasma polymer synthesis that can be used as a gentle but powerful tool for the surface modification of fibers and retains their bulk properties. This paper introduces the notion of a compact and modular device consisting of low-pressure reactor segments that have a specialized function and are adapted for the continuous modification of fiber surface. A roll-to-roll device, designed on an industrial scale, with a PECVD reactor was used for continuous plasma pretreatment and plasma polymerization coating on 1,600 glass fibers in a bundle, with the fibers used as reinforcements in a glass fiber/polyester composite. Optimization of the plasma-processing conditions allowed for the improvement of interfacial adhesion between the glass fibers and polymer matrix, resulting in an increase in the shear strength of the polymer composite by 14%, compared to industrially sized fibers coated by wet chemical processes.
Keywords: A. Glass fibers; A. Polymer-matrix composites (PMCs); B. Interface/interphase; E. Chemical vapour deposition (CVD)
*
Corresponding author. Tel.: +420 541149304; Fax: +420 541149361 E-mail: [email protected] (V. Cech) 1
1. Introduction Many applications require materials in fiber form. These fibers must be surface-treated or surface-coated so that their surface properties (be they physical, chemical, or topographic) suit their application. Low-temperature plasma is a non-invasive and solvent-free tool that can be very effective in the surface treatment (the chemical and/or physical altering of the fiber surface) or coating (thin film deposition) of fibers. Reactive species (such as radicals, ions, and electrons) in plasmas can usually affect only the surface layer of fibers up to a depth of no more than a few nanometers, depending on the structure of the material and its density, and therefore do not affect the bulk properties of the fibers. Plasma treatment and plasma coating are used for the surface modification of high-performance fibers (carbon [1–6], glass [7–10], aramid [11–13], polyethylene [14]), and even natural fibers (flax [15], coconut [15], cellulose [16,17], and sisal [18]) that are applied as reinforcements in fiber-reinforced polymer composites. O2 [2,3,5,11], CO2, H2O [6,11], or air [4,12,13,15,16,18] plasmas are mostly employed for plasma treatment to improve compatibility between the fiber surface and polymer matrix, which can be achieved due to the increased wettability of the fiber surface by incorporating polar groups such as hydroxyl, carbonyl, and carboxyl into the surface layer. Like oxidative reactions, argon (Ar) plasma [3,17] leads to increased fiber surface roughness due to chemical or physical etching, which enlarges the fiber contact area with the polymer matrix and increases interfacial adhesion. NH3 plasmas [1,11] are suitable for incorporating amine groups into the surface of the fibers, allowing chemical bonding with the epoxy resin. When using plasma-enhanced chemical vapor deposition (PECVD) technique [8–10,14], plasma coating - unlike plasma treatment - makes it possible to significantly change the physical, chemical, and topographic properties of the fiber surface. The gas molecules of organic, inorganic, or organic-inorganic precursors are activated and fragmented in the plasma discharge, and the reactive and radical species form a thin film material that is applied directly to the surface of the fiber. The chemical composition and structure of the deposited films may differ significantly from the composition and structure of the precursor molecules. The variability of the physical and chemical properties of these thin films can be enormous, depending on the precursor molecules and deposition conditions. The plasma synthesis (PECVD) is called plasma polymerization [19–21] when organic or organosilicon precursors are used, and the deposited film has the character of a polymeric material. Plasma polymerization coating is a cost-effective and green process that uses a minimum amount of input precursor and produces a minimal amount of waste products, and therefore does not require an environmental remediation process [22].
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Plasma surface treatment can also be applied to textile fibers (like cotton, linen, silk, and wool) for effective disinfection. O2, N2, and Ar plasmas inhibited the growth of microorganisms, increased the fiber strength, and did not alter the molecular structure of the fibers [23]. Air plasma was successfully employed to improve the absorption of wool and dye fixation in the process of coloring the wool fibers [24], and it was also found that air plasma is useful for the surface cleaning of raw cotton fibers, increasing their wettability by removing only the surface layer containing non-cellulosic components [25]. Oxide nanoparticles can be synthesized in a plasma discharge (PECVD) at higher process pressures. Zhang et al. [26] deposited TiO2 nanoparticles on wool fibers, resulting in photocatalytic activity that improved the self-cleaning and antibacterial properties of the fibers. Conductive polymers (such as polyaniline, polypyrrole, ethylenedioxythiophene, and polythiophene) can be deposited (PECVD) on the textile fibers used for smart textiles (like medical textiles, protective clothing, touchscreen displays, flexible fabric keyboards, and sensors) [27] or smart biomaterials for tissue engineering [28]. Plasma-polymerized acrylic acid can also be deposited on different fibers to improve cell growth, proliferation, and differentiation for use in regenerative medicine and tissue engineering [29]. Electrospun nanofibers can be assembled into bundles and yarns [30,31] that have a remarkably high specific surface area that improves chemical absorption and flexibility, compared to bundles of conventional fibers. These bundles of nanofibers can also be surface-modified by plasma [32], and as such, they are promising for new applications in medicine, filtration, and the reinforcement of polymer composites. Carbon nanotubes can now be produced as carbon nanotube fibers using floating catalyst chemical vapor deposition (CVD), and these macroscopic filaments are plasma treated [33] to serve as reinforcements for composite structures. Both commercial production and laboratory fiber processing require continuous surface modification using a roll-to-roll plasma device. Simple roll-to-roll devices, mostly for the plasma treatment of fibers using atmospheric-pressure plasmas, were constructed on a laboratory scale, employing a dielectric barrier discharge or a plasma jet [34–36]. The plasma coating in low-pressure plasmas allows for a much wider variability of physical, chemical, and topographic properties for the functionalization of the surface of the fibers compared to atmospheric-pressure plasmas, and therefore a larger range of applications. Roll-to-roll devices with PECVD reactors that operate at low pressure and specialize in the surface modification of fibers are lacking not only in industry, but also in research laboratories. The idea of a roll-to-roll system comprising six segments of reactors for the surface modification of single fiber or bundle of fibers is shown in Fig. 1. This is an example of a compact and modular device that can be operated as a batch processing vacuum equipment in which the fiber supply and take-up spools are in a vacuum,
3
or as an air-to-air vacuum equipment featuring a fiber transition between atmospheric pressure and the medium vacuum (0.1–100 Pa), using differential pumping. The device with reactor segments is a variable system, where the number of segments can be adapted to the desired application. Each reactor can be specified for various operations (like plasma pretreatment, plasma coating, or the post-treatment of fibers), or several reactors can use the same operation to increase the fiber winding speed. Based on the idea outlined in Fig. 1, a roll-to-roll device with two plasma reactors for the continuous surface modification of fibers was designed on an industrial scale, constructed, put into operation, and successfully tested. Process scale-up, from laboratory to industrial scale, is a considerable challenge for any application of low-temperature plasma, because the geometry of the reactor and its hydrodynamic characteristics affect the performance of plasma processes [37]. This study focused on optimization of the process of plasma surface modification for glass fibers (GFs) used as a reinforcement for polymer composites to demonstrate the usability of the device. GF bundles with 1,600 filaments of 19 µm in diameter were pretreated with oxygen plasma and coated with a plasma polymer film of tetravinylsilane, using precursor deficient conditions to functionalize the GF surface. The plasma reactor was characterized in terms of the operation conditions, along with the physical, chemical, and surface properties of the deposited films, while surface-modified fibers were tested as reinforcements in polyester composites. Using a mixture of tetravinylsilane precursor and oxygen gas, it was found that the incorporation of oxygen atoms into the plasma polymer network and the formation of polar groups (hydroxyl, carbonyl) on the coated GF surface improves the composite performance.
2. Experiment
2.1 Materials Bundles with unsized and industrially sized GFs (E-glass, 1,200 tex, diameter 19 µm, 1,600 filaments in a bundle) were provided by Saint-Gobain Adfors CZ s.r.o. (Czech Republic). Unsized GF means that the fibers are untreated and uncoated, whereas sized GF indicates that the fibers are surface-modified by sizing (a wet chemical process) based on silane coupling agents that are optimized for polyester resin by the GF manufacturer. Flat glass and silicon wafers were used as substrates for the thin film analysis. Soda-lime float glass with dimensions of 1.0×26×76 mm3 was supplied by Knittel Glaeser (Germany), while double-side polished wafers (100) measuring 0.8×10×10 mm3 were obtained from ON Semiconductor (Czech Republic). The silicon wafers, which were transparent for infrared (IR) radiation, were covered with a 3 nm-native-SiO2 layer. Argon gas (purity 99.999%) and gaseous oxygen (purity 99.99%) were delivered by Linde Gas (Czech Republic), while the
4
tetravinylsilane (TVS) precursor (Si–(CH=CH2)4) had a purity of 97%, and was supplied by Sigma Aldrich (Czech Republic).
2.2 Model PECVD reactor
A batch processing vacuum equipment with one model reactor was developed to test its functionality for various operations such as the plasma pretreatment of fibers, plasma coating of fibers, and subsequent treatment of coated fibers. A schematic view of this equipment is shown in Fig. 2. We used the SIMAX glass tube as a reactor, with a length of 100 cm and an outside diameter of 4.6 cm (Merci, Czech Republic). The glass tube was equipped with seven powered and six grounded ring electrodes connected to the radio-frequency (RF) power supply, Cesar 1310 (13.56 MHz, 1000 W), and a VM 1000A Matching Network (Advanced energy, U.S.A.). Both ends of the tube were closed by a plasma limiter that prevented the plasma from spreading out from the tube. The cylindrical shape of the reactor was important for the axially symmetrical plasma formed in the tubular reactor by the RF glow discharge, so that the fiber that is disposed or movable along the axis of the tubular reactor is evenly surrounded by plasma, resulting in a uniform surface modification of the fiber. The plasma reactor was placed in a Faraday cage with five opening windows, blocking electromagnetic fields and preventing heating-induced damage to the glass tube. The batch processing vacuum equipment was pumped by a HiCube 80 Eco pumping station (Pfeiffer Vacuum, Germany), which consists of a turbo pump HiPace 80 (pumping speed 66 dm3 s-1) and a backing pump MVP 015 (pumping speed 0.7 m3 h-1). The required pressure in the plasma reactor was controlled by a butterfly valve 615 DN (VAT, Switzerland) that restricts the pumping speed of the pumping station. Inlet gases, argon, and oxygen were fed into the reactor via the mass flow meters F-201CV (Bronkhorst, the Netherlands), enabling the mass flow rate to be controlled. The liquid precursor was placed in a glass tube immersed in a water bath, whose temperature was controlled to a range of 8–40 °C by means of a connected Peltier thermostat PT 31 (A.KRÜSS Optronic, Germany). The mass flow rate of precursor vapors was controlled by a mass flow meter F-201DV (Bronkhorst, the Netherlands). The pressure in the vacuum equipment was measured using a Pirani gauge TPR 280, a full-range gauge PKR 251, and capacitance gauges CMR 362 and CMR 364 via a MaxiGauge TPG 256 A controller, all supplied by Pfeiffer Vacuum, Germany. The typical basic pressure in the reactor was 5×10-4 Pa. The vacuum equipment was equipped with manual, venting, and pneumatic valves from Pfeiffer Vacuum, Germany, and the samples were inserted into the plasma reactor from the left chamber connected to the tubular reactor. Flat glass and silicon substrates can be set in any position along the axis of a tube reactor, using special glass stages. The single fiber or bundle of fibers is placed along the
5
tube axis, using a special glass frame. The model PECVD reactor was tested for the plasma pretreatment of GFs using argon and oxygen plasma, and for plasma polymer deposition using a pure TVS precursor or its mixtures with oxygen (O2) gas. After the application of plasma pretreatment or plasma coating, the fiber surface is activated containing free radicals, which can be used for the subsequent surface grafting, achieved by covalent bonding, of suitable vapor molecules that were introduced into the plasma-free reactor. This process is known as plasma-induced grafting.
2.3 Roll-to-roll device with two PECVD reactors Using the experience from the model PECVD reactor, a roll-to-roll device with two plasma reactors for the continuous surface modification of fibers was developed on an industrial scale. The main reason for designing the device was to improve the functionality of the surface of the fibers to reinforce the polymer composites. However, it can be used much more widely, and is suitable for any fiber that is not too brittle, but sufficiently strong to prevent damage by rewinding. The roll-to-roll device was designed as a batch processing vacuum equipment, the scheme of which is shown in Figure 3. A spool with fibers with a maximum diameter of 120 mm and width of 300 mm is inserted into the left chamber that is evacuated by auxiliary scroll pump nXDS6i (pumping speed 6.3 m3h-1, Edwards, UK) to remove the desorbed water molecules from the fiber surface. The typical basic pressure in reactors was < 0.1 Pa. The fibers were guided from the fiber-supply spool through a Teflon roller system into the first reactor, which was designed for fiber pretreatment with Ar and/or O2 plasma, with the fibers moving along the axis of the tubular reactor. Thereafter, the fibers moved through an aperture, the size of which could be changed, into the second reactor; the size of the aperture can be adapted to the diameter of the fiber to prevent gas permeation between reactors. An open aperture allows for the distribution of gases between the reactors, and thus the deposition of gradient films. A second reactor that allows the precursor and its mixtures with the Ar, and O2 gases to be introduced was used for the thin film deposition using the PECVD technique. After passing through the second reactor, the fibers were wound onto the take-up spool in the right chamber (Fig. 3). The fiber rewinding was electronically controlled by stepping motors SX34-2740N (Microcon, Czech Republic) in a range of 0.1–8 m min-1 (these too can be adapted). Both reactors were independently pumped by two HiPace 300 turbopumps (pumping speed 255 dm3 s-1, Pfeiffer Vacuum, Germany), with one auxiliary scroll pump, and powered by two RF generators with matchboxes. The other vacuum components were similar to those of the model reactor. These plasma reactors were also enclosed in Faraday cages to prevent the spread of electromagnetic radiation, and a roll-to-roll device was pre-prepared for expansion into the four reactor segments. 6
2.4 Thin film characterization The thickness of the films deposited on flat glass and silicon substrates was evaluated by the DektakXT stylus profiler (Bruker, U.S.A.). A 25-μm-diameter stylus, to which a force corresponding to 2 mg was applied, was used to measure vertical steps in the deposited film that were prepared by mechanically removing a portion of the film from the substrate. Low-force measurements result in negligible deformation of the film, and hence the height of the step, determined from the sample profile, corresponds to the thickness of the film. Five steps in the film were used to evaluate the average film thickness, and the deposition rate was determined as the ratio between the film thickness and deposition time. Nanoindentation measurements were performed using a 2D TriboScope (Hysitron, U.S.A.) attached to an NTegra Prima Scanning Probe Microscope (SPM) (NT-MDT, Russia) to determine the Young’s modulus, E, and hardness, H, of the deposited films. The thickness of the analyzed films of 1.0 µm was sufficiently high to reduce the influence of the substrate on the measured data. The depth profiles of mechanical parameters (E, H) were determined using a cyclic mode [38] of a nanoindentation technique of up to 30% film thickness, applying 20 load cycles at an unload fraction of 0.8. A Berkovich diamond indenter (Hysitron, U.S.A.) with a radius of 50 nm was used for instrumented nanoindentation, and unload-displacement curves were used to calculate mechanical parameters by the Oliver-Pharr method [39]. The unload curve was used to evaluate the sample stiffness, S, according to the equation
S
2
Er A, (1)
where A is the projected contact area, and Eris the reduced modulus that relates to the Young’s modulus, E, and Poisson’s ratio, ν, of the sample, through the relationship
1 (1 2 ) (1 vi2 ) , Er E Ei
(2)
where the index i corresponds to the parameters of diamond: Ei = 1,141 GPa, νi = 0.07. The Poisson’s ratio, ν, for the polymeric material was 0.35. The hardness of the sample was determined from the maximum applied load, Pmax, as
H
Pmax . A
(3)
7
The depth profiles of the mechanical parameters were constructed from the mean values determined from five nanoindentation measurements, and extrapolated to zero contact depth (film surface) in order to evaluate the correct mechanical parameters (E, H) of the measured film, which are not affected by the substrate [40]. The surface topography of deposited films was analyzed using SPM, which operated as atomic force microscopy (AFM) using semicontact mode and silicon probes NSG03 (Rc<10 nm, NT-MDT) of a resonant frequency of 90 kHz and a force constant of 1.1 N m -1. The root-mean-square (RMS) roughness as a characteristic of the surface topography was determined using AFM images of a scanned area of 5×5 μm2. The composite short beams were observed using a scanning electron microscope (SEM) (Philips XL 30/EDAX/Microspec). The IR measurements of the thin films deposited on the IR-transparent silicon wafers were performed by a VERTEX 80v, vacuum Fourier transform infrared (FTIR) spectrometer (Bruker Optics, U.S.A.) to characterize the chemical structure of the deposited films. The samples were measured in a vacuum at a pressure of 160 Pa using a mid-IR source, a KBr beamsplitter, and a DLaTGS D301 detector. Transmission spectra were obtained in a wave number range of 750–4000 cm-1 using 256 scans and a spectral resolution of 4 cm-1. Due to the eliminated atmospheric moisture absorptions, the sensitivity and stability of the spectrometer was sufficiently high to measure the deposited films with a thickness of only 0.10µm. The IR spectrum of the bare silicon wafer was subtracted from that of the specimen with the deposited film to extract the IR peaks corresponding to the film.
2.5 Fabrication and shear test of GF/polyester composites Short beams (18×10×3 mm3) of unidirectional GF/polyester composites were manually manufactured and cured for 30 min at 100 °C and then for 1 h at 140 °C, using an unsaturated polyester resin (Viapal HP 349 F, Sirca S. p. A., Italy). The beams were polished with sandpaper with a particle size of 220 μm until reaching the desired size. The composite short beams were characterized by a short-beam-shear (SBS) test (ASTM D 2344 [41]) using a universal test machine Z010/TH2A (Zwick, Germany). The ASTM standard requires a span-tothickness ratio of 4.0 and a length-to-thickness ratio of 6.0. The diameter of the load nose must be 6 mm, and the support nose diameter 3 mm, while a rate of crosshead movement of 1.0 mm min-1 is required. The beam was loaded until a fracture occurred, and the fracture load was used to characterize the shear strength of the composite. The short-beam strength, Fsbs, is calculated as [41]
F sbs 0.75
Pm , bh
(4)
8
where Pm is the maximum load observed during the test, b is the measured specimen width, and h is the specimen thickness measured. For each surface modification of the GFs, eight composite beams were tested.
3. Results and discussion
3.1 Deposition process characteristics The device with the model PECVD reactor was very useful for a basic characterization of plasma operations using Ar and O2 gases, and TVS precursors such as a determination of the plasma distribution along the reactor, the evaluation of suitable process parameters (flow rate, process pressure, RF, or effective power), consumption of the precursor, and distribution of deposition rates. This characterization would be more complicated for a more complex roll-to-roll device. The plasma distribution along the reactor, which was estimated based on the intensity of the radiation in visible light, was affected by the number of electrodes, their distribution along the tube, the used gas, its mass flow rate, the pressure in the reactor, and whether continuous wave or pulsed mode power was in place. As shown in Fig. 2, 13 equidistant electrodes (seven powered, six grounded) along the tubular reactor were chosen as optimal for a wide range of process parameters. Although the configuration of the electrodes was optimized for Ar discharges, the distribution of the plasma along the tube was less uniform, using O2 and a TVS precursor. The TVS plasma was tested for RF power in the range of 1 to 500 W for a continuous wave discharge at a flow rate of 1.4, 4.0, and 7.0 sccm (standard cubic centimeters per minute). The process pressure in the left chamber (at the downstream end of the tubular reactor) was preset to 3.8 Pa for all flow rates before the discharge was ignited. The process pressure as a function of RF power for different flow rates is given in Fig. 4a. In the case of the two lower flow rates of 1.4 and 4.0 sccm, the TVS plasma was burning for all powers (1–500 W). However, at a flow rate of 7.0 sccm, the TVS plasma was ignited from 5 W. The intensity of the radiation was higher in the left side of the tube for low powers, but expanded to the right with enhanced power, and was almost constant along the tube from 30 W. The process pressure decreased to a minimum and then slightly increased for 1.4 and 7.0 sccm, but dropped to a saturated value in the case of 4.0 sccm. A reduced process pressure after plasma ignition is characteristic for a precursor-deficient regime [20]. The difference between the preset pressure, poff, when the plasma is switched off, and the process pressure, pon, when the plasma is switched on, can be used to calculate the precursor consumption, MC, as in
MC
( poff pon ) poff
100%.
(5)
9
The precursor consumption characterizes the plasma process efficiency for the deposition of thin films, and is an important industrial parameter for calculating operating costs. The minimum process pressure corresponds to the maximum consumption of the precursor, which increased from 56 to 73% for reduced flow rates from 7.0 to 1.4 sccm. When the plasma discharge was operating in a pulsed regime, the process pressure decreased with enhanced effective power (2–150 W) and the plasma process efficiency was lower than that of the continuous wave, and therefore the precursor consumption was lower, as shown in Fig. 4b, for flow rates of 1.4 and 4.0 sccm. Effective power, Weff, is determined by the equation
Weff
t on Wtotal , (t on t off )
(6)
where ton and toff is the time when the plasma is switched on and off, respectively, and Wtotal is the total power. The TVS molecules are fragmented in the plasma discharge due to inelastic collisions with the high-energy electrons, and therefore lead to the formation of free radicals, which are a highly reactive species that result in the synthesis of plasma polymer due to a radical recombination at the surface of the growing film. When the degree of ionization increases with enhanced RF power, the higher concentration of free electrons leads to an increase in the number of free radicals, and thereby to a higher degree of fragmentation. If the process pressure decreases with enhanced power, it means that there is a higher number of recombinated than generated fragments in the plasma discharge. The process pressure is not constant along the tubular reactor and is higher at the precursor inlet, and decreases in the direction of pumping. The increase in process pressure with enhanced power (Fig. 4a) can be explained by the locally reduced pressure, plocal, at a certain position along the tube, which in our case was too low to locally ignite the discharge (similarly to Paschen’s law [42]). At a steady-state flow, the local pressure, plocal, corresponds to the concentration of molecule fragments generated per unit of time, ngen, reduced by recombined fragments per unit of time, nrec, plus the concentration of by-products per unit of time, nbypr, and the concentration of unfragmented TVS molecules, nunfr, according to the equation
plocal (ngen nrec nbypr nunfr )kT,
(7)
where T is the gas temperature and k is the Boltzmann constant. This trend of increased pressure with enhanced RF power observed for continuous wave was not observed in the pulsed plasma regime (Fig. 4b), where a sufficient amount of unfragmented precursor was supplied to the position of low pressure during toff, when the plasma was switched off.
10
The process pressure dependent on the RF power for a constant flow rate of 1.4 sccm but different preset pressure values of 3.8, 7.6, and 15.2 Pa is shown in Fig. 4c. The plasma discharge was ignited at a power that increased with increasing preset pressure. The precursor consumption could be optimized for a preset pressure of 7.6 Pa, and reached 81% at the minimum process pressure. In the case of the highest preset pressure of 15.2 Pa, the intensity of radiation was very low on the right side of the tubular reactor for powers < 10 W. The range of flow rates 1.4–7.0 sccm, preset pressures 3.8–15.2 Pa, and RF powers 1 (5)–200 W represent suitable operation conditions that provide a stable plasma for the batch processing vacuum equipment with one model reactor. The plasma was distributed along the reactor (Fig. 5) for the above deposition conditions, but this does not mean that the deposition rate was constant along the axis of the tube. The deposition rate along the reactor for selected powers of 10, 30, and 100 W (1.4 sccm TVS, 3.8 Pa) is plotted in Fig. 6a. The distance along the reactor was measured from the end of the tubular reactor with the precursor input. Five silicon wafers were distributed along the tube axis for each RF power, and the thickness of the deposited film for each sample was measured by a mechanical profilometer to evaluate the deposition rate as the ratio between the film thickness and the deposition time. The trend of the deposition rate as a function of distance from precursor input was similar for all the RF powers used (Fig. 6a). The deposition rate is affected by the concentration of free radicals, their chemical reactivity (such as monoradicals and multiple radicals), and their mass. We can see that the highest deposition rate is located near the precursor inlet, where a sufficient number of heavy radicals can be expected. The concentration of unfragmented TVS molecules decreases towards the pumping system, resulting in lower deposition rates. The minimum deposition rate at the 75 cm position can be assumed to correspond to the position of locally reduced pressure, as mentioned in the previous paragraph. The effect of different preset pressures (1.4 sccm TVS) on the deposition rate for a selected RF power of 10 W is given in Fig. 6b. The trend for all pressures is very similar in the first half of the tube at the precursor inlet, but the deposition rate in the second half significantly increased at a higher preset pressure. In Fig. 6c, the precursor flow rate was increased to 4.0 sccm, compared to Fig. 6a, but the preset pressure was the same. The deposition rate increased to 570 nm min-1 at the precursor inlet for 100W, but for 10 W ranged from 112 to 219 nm min -1 along the tube. The distribution of the deposition rate along the tubular reactor for the mixture of TVS precursor (4.0 sccm) with oxygen gas (4.0 sccm) is shown in Fig. 6d. In the case of 10 W, the deposition rate gradually decreased from 353 to 31 nm min-1 as the distance from the precursor input increased. However, for 60 W, the deposition rate was extremely high (689 nm min -1) at the precursor inlet, followed by an almost constant value at about 74 nm min -1. The four examples provided in Fig. 6 show that the distribution of the deposition rate along the tubular reactor is
11
different, and depends on the deposition conditions, but the predominant trend is for the highest deposition rate to be determined at the precursor inlet. The average deposition rate, vdp, can be calculated as the arithmetic mean of the five rates determined along the tube. In the roll-to-roll plasma device, where the fiber moves along the axis of the tubular reactor, the thickness of film, l, deposited on the fiber will be the same along the fiber, and estimated as
l
v dp v wind
L,
(8)
where vwind is the fiber winding speed and L is the length of tubular reactor; L = 100 cm for both plasma devices used in this study.
3.2 Thin film properties The Young’s modulus and hardness (Eq. 1–3) of the deposited films from pure TVS precursor (1.4 sccm), depending on the RF power and preset pressure for a selected RF power of 10 W, are shown in Fig. 7a. By enhancing the RF power, the mechanical properties, i.e., Young’s modulus and hardness, were slightly increased from 9.6 to 12.5 GPa and from 0.98 to 1.87 GPa, respectively. These values are typical of plasma polymer films, and provide evidence of a weakly crosslinked polymer network. In the case of increased preset pressure at a constant RF power of 10 W, the Young’s modulus slightly decreased from 9.6 to 8.0 GPa, but the hardness was almost the same, in the range 0.98–1.09 GPa. The mechanical properties of the films deposited along the tubular reactor were identical for the given deposition conditions. Plasma polymeric films, unlike conventional polymers, have a sufficiently low Young’s modulus, along with a high yield strength [10], to be used as functional interlayers in GF/polyester composites, as can be seen from model simulations [10]. Examples of infrared spectra for plasma polymer films deposited at different flow rates, preset pressures, and RF powers, but of the same thickness of 0.10 μm, are shown in Fig. 7b. The assignment of absorption bands [43,44] is displayed directly at the spectra. We can see that the infrared spectra are very similar, despite the different deposition conditions. The only significant difference is in the concentration of CHx species, which correlates with the process pressure during the film deposition. It is important that the vinyl group is incorporated into the plasma polymer network, and is not completely fragmented during the plasma process. The vinyl groups are important for chemical bonding with the polyester resin, when the plasma polymer film is used for the surface modification of the glass fibers employed as reinforcements in polymer composites. The infrared spectra were measured one month after the film deposition and reveal the post-deposition oxidation of the films, because the oxygen atoms were partly incorporated into the plasma polymer network, forming Si-O-C species 12
and partly forming hydroxyl and carbonyl groups. This is an aging effect that results from a weakly crosslinked polymer network and a high affinity of oxygen with silicon, where oxygen from the ambient atmosphere (O2, H2O) can diffuse into the film. Polar groups (hydroxyl and carbonyl) on the surface of the deposited film are beneficial for improving surface wettability and the surface free energy can be increased up to 58 mJ/m 2, which is higher compared to 34 mJ/m2 for polyester resin [45]. The infrared spectra of the films deposited along the tubular reactor were approximately the same for the given deposition conditions, indicating that the chemical composition of these films is independent of their position in the tube. It follows that the mass distribution of the TVS fragments along the tube is approximately the same, but their concentration varies locally, which is related to the different deposition rate along the tube. Detailed chemical analysis of deposited films for different deposition conditions can be found in previous studies [8 –10]. The surface topography of the deposited films was characterized by AFM and the AFM images for films of the same thickness of 1.0 µm that were deposited (10 W) from a pure TVS precursor (4.0 sccm TVS) and a TVS precursor (4.0 sccm) mixed with oxygen gas (4.0 sccm), which are given in Fig. 8a and Fig. 8b, respectively. The RMS roughness of the deposited films increased from 1.8 nm (TVS precursor) to 4.7 nm (TVS/O2 mixture), showing that the incorporation of oxygen atoms into the plasma polymer network leads to a slightly rougher surface. The RMS roughness of plasma polymer films increased with increased film thickness, which corresponds to the most relevant previous study [46].
3.3 Shear strength of GF/polyester composites The experience and basic characterization of the plasma device with the model PECVD reactor were used for the roll-to-roll device, with two plasma reactors used to continuously modify the surface of the GF bundle. The GF bundle was pretreated with oxygen plasma in the first reactor (Fig. 9), using an oxygen flow rate of 10 sccm, a preset pressure of 6.0 Pa, and an RF power of 50 W. The second reactor (Fig. 9) was used to deposit the plasma polymer film on a cleaned and activated GF surface from a TVS precursor or a TVS/O2 mixture. The size of the aperture between the two reactors was adjusted to the diameter of the fiber to minimize gas permeation between the reactors, and the winding speed of the fibers ranged from 0.16 to 0.34 m min -1, depending on the average deposition rate in the second reactor. The unsized, industrially sized, and plasma-coated GFs were used to reinforce GF/polyester composites in the form of short composite beams that were tested by the SBS test. The fiber volume fraction was 35%, meaning that 24 bundles were embedded into the polyester resin longitudinally along the beam; i.e., the short beam contained 38,400 individual fibers. The load-displacement curves for the unsized, industrially sized, and 13
plasma-coated GFs are shown in Fig. 10a. The curve for the unsized GFs is characterized by a low maximum load of 0.858 kN at a high displacement of 0.94 mm, which is responsible for a short-beam strength of only 21 MPa (Eq. 4) due to the weak mechanical bonding and weak van der Waals forces between the GF surface and polymer matrix. The significantly higher maximum load of 1.90 kN, with a shorter displacement of 0.72 mm for industrially sized GFs, corresponds to a short-beam strength of 49 MPa, indicating increased adhesion between the surfaces due to siloxane bonding at the GF/polyester interface [7]. The optimized plasma coating resulted in a maximum load of 2.36 kN at a 0.71 mm displacement, demonstrating an increased short-beam strength of 56 MPa, together with an increased composite toughness (the area below the load curve, from the beginning to the maximum load). SEM micrographs were used to observe fractured short beams for unsized and plasma-coated GFs on the same scale of 50 µm in Fig. 10b and Fig. 10c, respectively. Fig. 10b is an example of weak interfacial adhesion between the GF and the polyester resin. It can be seen that the GF fibers are bare on their surface, without the residues of the adhered resin, and that the delaminated resin residues are completely separated from the fibers. Differently, the plasma-coated GFs are entirely covered with a polyester resin bonded to the plasma polymer film on the GF surface, and a character along with a large amount of resin debris onto GFs and between prove the high interfacial adhesion and cohesive failure of resin (Fig. 10c). Some GFs were broken due to the high shear stress transferred from the polymer matrix to the fiber, as a result of the strong interfacial adhesion. A film thickness of 0.15 µm (Eq. 8) on a GF bundle is sufficient for the proper surface modification of GFs [8–10], which are used as reinforcements in GF/polyester composites. When coating a fiber bundle, the shielding effect caused by adjacent fibers must be analyzed. The free radicals diffuse into the fiber bundle and the fiber bundle does not have to be spread for the coating. The required film thickness of 0.15 μm refers to fibers at the edge of the bundle; however, the deposition rate decreases toward the center of the bundle due to multiple shielding, as [10]
l n l s q n1 ,
(9)
where ls is the thickness of the film on the fiber at the edge of the bundle, ln is the film’s thickness on the n-th fiber in the direction to the bundle center shielded by n-1 fibers, and q is the shielding factor. For the GF bundle of circular cross-section and the number of fibers 1,600, where the fiber in the bundle center is the 22 nd, and for ts = 0.15 µm and q = 0.9 [10], the film thickness is t22 = 16 nm on the central GF. It was found that the interfacial shear strength is independent of the coating thickness for a thickness greater than 0.1 µm corresponding to the fibers at the edge of the bundle [8,9]. Therefore, the interfacial properties of the composite do not change across
14
the bundle. The short-beam strength for GF/polyester composites reinforced with plasma-coated GFs was compared to the strength of unsized and industrially sized GFs in Fig. 11. The shear strength of composite beams, with plasma-coated GFs using a pure TVS precursor (4.0 sccm, 3.8 Pa) at 10 and 60 W, was significantly increased to 39 MPa (10 W) and 37 MPa (60 W), compared to the value for unsized GFs. This increase is associated with the covalent bonding of the interlayer on both interfaces with the fiber and the polyester resin [8]. We have previously proven [9,10] that the incorporation of oxygen atoms into the plasma polymer network is favorable for improving interfacial adhesion, not only at the film/GF interface due to the increased number of Si–O–C species, but also at the polyester/film interface due to the carbonyl and hydroxyl groups that improve the wettability of the plasma polymer film with a polyester resin. For this reason, for the surface modification of the GFs, a TVS precursor (4.0 sccm) in a mixture with oxygen gas (4.0 sccm) at an RF power of 10 and 60 W was used, and the short-beam strength was further increased to 50 MPa (10 W) and 42 MPa (60 W). A higher power of 60 W resulted in lower strength due to the lower interfacial adhesion at the film/GF interface, which corresponds to an excessively high concentration of carbonyl groups [9,10] because the shear strength of the composite was associated with the film adhesion [46]. The optimization of the oxygen fraction (0.5) in the TVS/O2 mixture resulted in a short-beam strength of up to 56 MPa at an RF power of 10 W, using continuous wave plasma. This optimization makes it possible to reduce the plasma polymer cross-linking and hence the Young’s modulus, and to optimize the concentration of carbonyl groups in the plasma coating, and thus the interfacial adhesion at the film/GF interface, both of which are responsible for the increased shear strength of the composite [9,10]. A further decrease in 5 W power, using pulsed plasma, was done to reduce the Young’s modulus, but did not lead to any further increase in the shear strength.
4. Conclusion Many fibers are produced such as high-performance fibers (carbon, glass, aramid, and polyethylene), natural fibers (flax, coconut, cellulose, and sisal), textile fibers (cotton, linen, silk, wool), electrospun nanofibers assembled into bundles or yarns, and carbon nanotube fibers. They have to be surface treated or coated with material in the form of thin films to improve or alter their surface properties for the desired application; plasma pretreatment of fibers and plasma-polymerization coating are the right tools for modifying the fiber surface only, with respect to its physical, chemical, and topographic properties. For the continuous surface modification of a single fiber or bundle of fibers, we introduced the idea of a compact and modular device that consists of segments whose number and functions (plasma pretreatment, plasma coating, or post-treatment of fibers) can be adapted for the desired application. Based on this idea, a 15
plasma device with a model PECVD reactor was developed and tested to determine the range of operation conditions (1.4–7.0 sccm, 3.8–15.2 Pa, 1 (5)–200 W) and characterization of thin film properties (E = 8.0–12.5 GPa, H = 0.98–1.87 GPa). This basic characterization of model plasma reactor was used to construct and test a roll-to-roll device with two plasma reactors for the continuous surface modification of fibers. As an example, the device was successfully used for the surface modification of GF bundles that were used as reinforcements in GF/polyester composites to demonstrate the usability of the device. Our study shows that the shear strength of GF/polyester composites with plasma-coated GFs can be controlled in the range of 37 to 56 MPa by processing conditions, and therefore significantly higher than 21 MPa for unsized GFs. The optimized plasma coating resulted in a shear strength that was 14% higher than that of industrially sized GFs, and 167% higher than that of unsized GFs. This being so, the plasma processing of GFs provides a sufficiently wide range of shear properties for the selected composite application. The roll-to-roll device was operated as a batch processing vacuum equipment but can be adapted for air-to-air vacuum equipment suitable for the production line. Plasma devices with model and roll-to-roll PECVD reactors are suitable for any fiber that is not too brittle, and sufficiently strong to prevent damage by rewinding. Plasma coating can be used to deposit protective, barrier, biocompatible, or multifunctional films with tailored physical, chemical, and surface properties on the fiber surface with controlled adhesion. Thin films can be deposited as single-layer, multilayer, or gradient nanostructures, depending on the number and functions of reactor segments.
Acknowledgements: This work was supported by the Czech Science Foundation, grant no. 16-09161S, and the Technology Agency of the Czech Republic, grant no. TA01010796. The authors would like to thank T. Plichta for nanoindentation and AFM measurements, and Dr. M. Sirovy and Saint-Gobain Adfors CZ s.r.o. for providing the glass fibers.
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Figure Captions Fig. 1. Schematic view of a compact and modular device for the surface modification of fibers.
Fig. 2. Schema of a plasma device with a model PECVD reactor.
Fig. 3. Schema of a roll-to-roll plasma device for the continuous surface modification of fibers.
Fig. 4. Process pressure as a function of (a) RF power for different flow rates, (b) effective power for different flow rates, and (c) RF power for different preset pressures.
Fig. 5. Low temperature plasma distributed along the model PECVD reactor.
Fig. 6. Deposition rate along the tubular reactor for (a) selected RF powers 10–100 W and a TVS flow rate of 1.4 sccm (preset pressure 3.8 Pa), (b) different preset pressures 3.8–15.2 Pa at 1.4 sccm TVS and 10 W, (c) selected RF powers 10–100 W and a TVS flow rate of 4.0 sccm (preset pressure 3.8 Pa), and TVS precursor (4.0 sccm) in a mixture with oxygen gas (4.0 sccm) at 10 and 60 W. Fig. 7. (a) Young’s modulus and hardness of the deposited films from pure TVS precursor (1.4 sccm) dependent on RF power (upper image) and preset pressure at a selected RF power of 10 W (lower image), (b) infrared spectra for plasma polymer films deposited from pure TVS precursor at different flow rates, preset pressures, and RF powers but the same thickness of 0.10 µm.
Fig. 8. AFM images for plasma polymer films of the same thickness (1.0 µm) deposited (10 W) from (a) pure TVS precursor (4.0 sccm TVS) and (b) TVS precursor (4.0 sccm) mixture with oxygen gas (4.0 sccm).
Fig. 9. A roll-to-roll device with two plasma reactors. The first reactor in the left figure with a glow discharge was used for the pretreatment of the fibers, and the second reactor in the right figure with plasma was used for the plasma polymer deposition.
Fig. 10. (a) Load-displacement curves for unsized, industrially sized, and plasma-coated GFs, and SEM micrographs of fractured composite beams for (b) unsized GFs and (c) plasma-coated GFs.
Fig. 11. Short-beam strength for GF/polyester composites reinforced with plasma-coated GFs, compared to that of unsized and industrially sized GFs (marked by dashed line).
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