Plasma etching and plasma polymerization coating of carbon fibers. Part 2. Characterization of plasma polymer coated carbon fibers

Unsized AS-4 carbon fibers were subjected to RF plasma etching and/or plasma polymerization coating in order to enhance their adhesion to vinyl ester resin. Ar, N₂ and O₂ were utilized for plasma etching, and acetylene, butadiene and acrylonitrile were used for plasma polymerization coating. Etching and coating conditions were optimized in terms of plasma power, treatment time, and gas (or monomer) pressure by measuring the interfacial adhesion strength. Interfacial adhesion was evaluated using micro-droplet specimens prepared with vinyl ester resin and plasma etched and/or plasma polymer coated carbon fibers. Surface modified fibers were characterized by SEM, XPS, FT-IR, α-Step, dynamic contact angle analyzer (DCA) and tensile strength measurements. Interfacial adhesion between plasma etched and/or plasma polymer coated carbon fibers and vinyl ester resin was reported previously (Part 1), and characterization results are discussed is this paper (Part 2). Gas plasma etching resulted in preferential etching of the fiber surface along the draw direction and decreased the tensile strength, while plasma polymer coatings altered neither the surface topography of fibers nor the tensile strength. Water contact angle decreased with plasma etching, as well as with acrylonitrile and acetylene plasma polymer coatings, but did not change with butadiene plasma polymer coating. FT-IR and XPS analyses revealed the presence of functional groups in plasma polymer coatings.     

Enhanced adhesion of steel filaments to rubber via plasma etching and plasma-polymerized coatings

Zinc-plated steel filaments were coated with RF plasma polymers of acetylene or butadiene in order to enhance adhesion to rubber compounds. Plasma polymerization was carried out as a function of the plasma power, deposition time, and gas pressure. In order to maximize adhesion, argon plasma etching was performed and carrier gases such as argon, nitrogen, and oxygen were used. Plasma polymer coatings were characterized by FT-IR, Alpha-Step, and a dynamic contact angle analyzer. The adhesion of steel filaments was evaluated via a tire cord adhesion test (TCAT). The best results were obtained from a combined process involving argon etching (90 W, 10 min, 30 mTorr) and acetylene plasma polymer coating (10 W, 30 s, 30 mTorr) with argon carrier gas (25/5, acetylene/argon). These samples exhibited a pull-out force of 285 N, which is comparable to that from the brass-plated steel filament (290 N).     

The influence of plasma gas species on the adhesion of thermoplastic to organic fibers

Adhesion between organic fibers and thermoplastics in a composite material is usually weak because of the lack of chemical and mechanical bonding. This study investigated the influence of plasma gas species in a novel technique to improve interfacial adhesion of Kevlar 49 fibers to polycarbonate. Single Kevlar fibers were exposed to radio-frequency plasma discharge in atmospheres of argon, oxygen, and ammonia. Plasma treatment has been shown to generate reactive free radicals on polymer surfaces. The fibers that were coated immediately with a polycarbonate melt while still in the vacuum environment of a plasma reactor showed an increase in interfacial shear strength that was maximum at 4.1 s exposure and decreased at longer exposure times. At 4.1 s exposure, Ar and O₂ plasma treatment increased shear strength by 20 and 18%, respectively. NH₃ plasma treatment produced only a statistically insignificant increase at 4.1 s of exposure and a significant decrease in adhesion at 8.3 s of exposure. Fibers that were exposed to air for 1 h before coating showed no statistically significant increase or decrease in adhesion. The increase in adhesion of the directly coated fibers was attributed to free radical-initiated formation of covalent bonds between the Kevlar and the polycarbonate. © 1993 John Wiley & Sons, Inc.     

Enhanced interfacial adhesion of carbon fibers to vinyl ester resin using poly(arylene ether phosphine oxide) coatings as adhesion promoters

Poly(arylene ether phosphine oxide)s (PEPO) were prepared and utilized to coat carbon fibers to enhance the interfacial adhesion with vinyl ester resins. For comparison, poly(arylene ether sulfone) (PES), Udel^® P-1700, and Ultem^® 1000 were also used. The interfacial shear strength (IFSS) of thermoplastic polymer-coated fibers was measured via microbond pull-out tests. The interfacial adhesion between thermoplastics and as-received carbon fibers was also measured in order to investigate the adhesion mechanism. Thermoplastic polymer-coated fibers exhibited a higher IFSS than the as-received fibers with vinyl ester resin, and with thermoplastic polymers. PEPO-coated fibers showed the highest IFSS, followed by Udel^®, PES, and Ultem^®-coated fibers. The high IFSS obtained with PEPO coating could be attributed to the phosphine oxide moiety, which provided a strong interaction with functional groups in the vinyl ester resin and also on carbon fibers. A diffusion study revealed the formation of a clear interphase not only between PEPO and the vinyl ester resin, but also between Udel^® (PES or Ultem^®) and the vinyl ester resin, although the morphology of the two interphases differed greatly.     

Effect of Halloysite Nanotube Incorporation in Epoxy Resin and Carbon Fiber Ethylene/Ammonia Plasma Treatment on Their Interfacial Property

Abstract

Effects of halloysite nanotube (HNT) loading of up to 2% in epoxy resin on its mechanical properties were characterized. The interfacial property of the resin with carbon fiber nanocomposite was also studied. Single fiber composite (SFC) technique was used to characterize the carbon fiber/epoxy resin interfacial shear stress. Carbon fibers were also coated with ammonia/ethylene plasma polymer to obtain a thin coating of the polymer with amine groups that could react with the epoxy and thus improve the interfacial property. The results indicated that the Young’s modulus of HNT containing nanocomposites increased slightly up to a loading of 0.25% after which it started to decrease. The tensile strength, however, steadily decreased with increasing of HNT loading although the fracture strain did not change significantly. This might be related to the nanotube shape, size and clustering. The interfacial shear strength (IFSS) was also increased slightly with HNT loading. The ethylene/ammonia plasma polymer coated fibers exhibited significantly higher IFSS by over 150%, independent of the HNT loading. The highest IFSS obtained was almost 79 MPa for plasma treated fibers. The results suggest that the carbon fiber/epoxy interface is not affected by the incorporation of up to 1.5% of HNT. Furthermore, the fiber surface modification through plasma polymerization is an effective method to improve and control the IFSS.     

Plasma surface treatment of poly(p-phenylene benzobisthiozol) fibers

Poly(p-phenylene benzobisthiozol) (PBZT) fibers were subjected to radio-frequency (RF)-induced, glow-discharge plasma treatments using argon and carbon dioxide gases in order to modify the adhesion of the fibers to bisphenol-A epoxy. The interfacial shear strength (IFSS) was used as a measure of the adhesion and was determined using the microbond technique. Scanning electron photomicrographs revealed no visible surface etching at magnifications of up to 10 000 x . Slight, but statistically significant, improvements in IFSS were noted with the CO₂ plasma-treated fibers as compared with control fibers, but Ar plasma-treated fibers showed no improvement.     

Effect of solubility and miscibility on the adhesion behavior of polymer‐coated carbon fibers with vinyl ester resins

Interfacial shear strength (IFSS) of carbon fibers with vinyl ester resin was investigated as a function of the structure of the polymer coating on carbon fibers via microdroplet tests. For coating carbon fibers, high‐performance polymers such as poly(arylene ether phosphine oxide) (PEPO), Udel® P‐1700, and Ultem® 1000, water‐soluble poly(hydroxy ether ethanol amine) (PHEA), water‐dispersed carboxy‐modified poly(hydroxy ether) (C‐PHE), and water‐insoluble poly(hydroxy ether) (PHE) were utilized. Adhesion of polymers to carbon fibers was also evaluated and the failure surface of the fibers was analyzed by SEM to understand the adhesion mechanism. Diffusion between polymers and vinyl ester resins was investigated and the solubility parameters of the polymers were calculated, with the results being correlated to the IFSS. A highly enhanced IFSS was obtained with PEPO coating, while marginally improved IFSS resulted from PHE, Udel®, and C‐PHE coatings. However, PHEA and Ultem® coatings showed no improvement. Such results were attributed to the extent of solubility and/or miscibility of polymer coatings in vinyl ester resin, with better solubility and miscibility leading to a higher IFSS. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1042–1053, 2001     

Electrochemical properties of polypropylene membranes modified by the plasma polymerization coating of SO2/acetylene

Polypropylene membranes were modified by the plasma etching of SO₂, SO₂? O₂, or SO₂? H₂O, followed by the plasma polymerization coating of SO₂/acetylene. The conditions for SO₂ plasma etching were optimized by the measurement of the ion‐exchange capacity (IEC) as a function of the plasma‐etching power (10–30 W), gas pressure (40–60 mTorr), and treatment time (15–120 s). For the plasma etching of SO₂? O₂ and SO₂? H₂O, only the pressure ratio (SO₂/O₂ and SO₂/H₂O) was optimized under the optimized conditions determined from SO₂ plasma etching. Plasma etching was then combined with the plasma polymerization coating of SO₂/acetylene, for which the conditions were again optimized by the measurement of the IEC as a function of the plasma power (10–40 W), chamber pressure (50–200 mTorr), SO₂/acetylene ratio (15/135–60/90), and treatment time (0–10 min). Next, the electrical resistance and water uptake were evaluated. The modified membranes were also analyzed with scanning electron microscopy, whereas plasma polymer coatings were characterized with Fourier transform infrared/attenuated total reflection. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3692–3699, 2006     

Effect of surface preparation on electroless Ni‐P plating on carbon fiber‐Reinforced Cyanate Ester Resin Laminate

The effect of surface preparation on electroless Ni‐P plating on carbon fiber reinforced cyanate ester resin laminate has been investigated. Three different surface chemical etching ways were performed, namely: (i) hexanediamine etching, (ii) hexanediamine followed by aniline etching, and (iii) aniline etching. The results indicated that Ni‐P coating on hexanediamine etching or hexanediamine followed by aniline etching pretreated carbon fiber reinforced cyanate ester resin laminates usually tended to be cracked, veined, and low adhesion, while aniline etched laminates showed a homogenous roughing surface which is suitable for subsequently uniform, cellular, and good adhesion Ni‐P plating. POLYM. COMPOS., 37:1161–1166, 2016. © 2014 Society of Plastics Engineers     

Surface modification of spectra™-900 polyethylene fibers using RF-plasma

RF-plasma polymerization and bonding of allylamine onto ultrahigh molecular weight polyethylene (UHMWPE) “Spectra™-900” is described using an inductively coupled plasma reactor. This process was found to enhance the interfacial strength between the fibers (Spectra-900) and room-temperature-cured epoxy matrix up to fivefold. Fibers covalently coated with allylamine plasma showed no loss in tensile strength, while argon gas plasma pretreatment of the same fibers caused up to 10% reduction in tensile strength depending on the energy and duration of the treatment. Optimum treatment was attained through a short argon plasma etching (15 s), followed by allylamine polymerization and coating for 3 min. The coating process was found to protect the fiber surface from etching by plasma ion bombardment. A loss of 19% of the original diameter was found during the 15 s precoating etching with argon plasma, indicating the sensitivity of the fiber structure to plasma etching.     

Characterization of ramie fiber/soy protein concentrate (SPC) resin interface

Ramie fiber/soy protein concentrate (SPC) polymer (resin) interfacial shear strength (IFSS) was measured using the microbond technique. To characterize the effect of plasticization, SPC resin was mixed with glycerin. Fibers were also treated with ethylene plasma polymer to reduce fiber surface roughness and polar nature to control the IFSS. Fiber surfaces after ethylene plasma polymerization, and fracture surfaces of specimens before and after the microbond tests were characterized using a scanning electron microscope (SEM). Some specimens were also characterized using electron microprobe analyzer (EMPA) to map the residual resin on the fiber surface after the microbond test. Effects of glycerin concentration in SPC and ethylene plasma fiber surface treatment time on the IFSS were investigated. Preparation of SPC resin requires a large amount of water. As expected, during drying of SPC resin, the microdrops shrank significantly. The high IFSS values indicate strong interfacial interaction in the ramie fiber/SPC resin system. This strong interfacial interaction is a result of a highly polar nature of both the ramie fiber and the SPC resin and rough fiber surface. Ethylene plasma polymerization was used to control the IFSS. The plasma polymer imparted a polyethylene-like, non-polar polymer coating on the fiber surface. As a result, the fiber surface became smoother compared to the untreated fiber. Both fiber smoothness and non-polar nature of the coating reduced the ramie fiber/SPC resin IFSS. Plasticization of the SPC resin by glycerin also decreased the adhesion strength of the ramie fibers with the SPC resin. The load-displacement plots for IFSS tests obtained for different resin and fiber combinations indicate different interfacial failure modes.     

Effect of Organic Gas Plasmas on the Adhesion of Matrix Resins to Carbon Fibers

IM7 carbon fibers were surface treated in methane, ethylene, trifluoromethane and tetrafluoromethane plasmas. The surface chemical composition of the fibers was determined by X-ray photoelectron spectroscopy (XPS). The adhesion between as-received and plasma-treated carbon fibers and polyethersulfone (PES) and an epoxy resin was measured by the microbond pull-out test. XPS showed that the methane and ethylene plasmas deposited a thin layer of hydrocarbon on the fiber surface. The trifluoromethane plasma deposited a layer of fluorocarbon on the surface of the fibers. The tetrafluoromethane plasma etched the fibers and introduced a significant amount of fluorine on the surface. The microbond pull-out test results indicated that an etching plasma, such as the tetrafluoromethane plasma, improved the adhesion between carbon fibers and PES. These results are consistent with earlier work performed with ammonia plasma. The adhesion is believed to be due primarily to the differential thermal shrinkage between the fiber and the matrix. It was shown that in the case of a reactive matrix such as an epoxy resin, the fiber chemical composition plays a role in the fiber-matrix adhesion. However, this chemical effect is secondary to the cleaning effect of the surface treatment.     

Surface coating protective against oxidative plasma etching of polypropylene

Polypropylene (PP) sheets were coated with the ultrathin polymer layers by plasma polymerization of hexamethyldisiloxane and two other Si-containing monomers, and the protection effects from oxidative plasma etching were investigated. Etching was evaluated by the weight loss of PP sheets after the exposure to an oxidative plasma of O₂ or air. The effects of plasma polymer coating on the etching resistance were investigated with respect to the type of plasma polymer, thickness of a coating layer, oxidative plasma etching conditions, etc. Weight of the coated PP sheets was less changed and the substrates remained stable after a certain period of oxidative plasma treatments, during which time the original PP film had prominently lost weight. The importance of the crosslinked network with —Si— MPO components in plasma polymers on the etching resistance was suggested from the results. Infrared spectra were taken and analyzed with the plasma polymers after O₂-plasma treatments, and the increase in the Si—O structure was indicated by the increase in the peak intensity at 1023 cm⁻¹. Stabilization against oxidative etching was attributed to the crosslinked Si—O structure on the surface layer. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1049–1057, 1997     

Enhanced adhesion of silica for epoxy molding compounds (EMCs) by plasma polymer coatings

Silica for epoxy molding compounds (EMCs) was coated via plasma polymerization using an RF plasma (13.56 MHz) as a function of the plasma power, gas pressure, and treatment time. The monomers utilized for the plasma polymer coatings were 1,3-diaminopropane, allylamine, pyrrole, 1,2-epoxy-5-hexene, allyl mercaptan, and allyl alcohol. The EMC samples were prepared from biphenyl epoxy resin, phenol novolac, triphenyl phosphine, and plasma polymer-coated silica, and the loading of silica was controlled to 60 wt%. The EMC samples were cured at 175°C for 4 h and subjected to T_g, CTE, and water absorption measurements. The adhesion of silica to epoxy resin was evaluated by measuring the flexural strength of EMC samples and the fracture surfaces were analyzed by SEM. Plasma polymer coatings were also characterized by FT-IR and coating thickness measurements. The plasma polymer coating of silica with 1,3-diaminopropane and allylamine enhanced the flexural strength of EMC samples (167 and 165 MPa), compared with the control sample (140 MPa), and exhibited a higher T_g, a lower CTE, and lower water absorption. The enhanced properties with 1,3-diaminopropane and allylamine plasma polymer coatings can be attributed to the amine functional groups in the plasma polymer coatings.     

Plasma‐graft polymerization of a monomer with double bonds onto the surface of carbon fiber and its adhesion to a vinyl ester resin

Double bonds reactive with active radical species were introduced onto the surface of carbon yarn by the plasma‐graft polymerization of adipic acid divinyl ester and ethylene glycol dimethacrylate monomers to increase the adhesive strength in the interface between the carbon yarn and a vinyl ester resin. The degree of grafting increased with increasing polymerization time and polymerization temperature. The degree of grafting depended on both the solvent and the monomer species used in the polymerization, and a high degree was obtained with ethylene glycol dimethacrylate as the conjugated monomer and in a mixture of methyl isobutyrate and water. The grafted yarn, whose surface layer contained double bonds, was reacted with a vinyl ester resin containing benzoyl peroxide and N,N‐dimethylaniline. The pull‐out force of the yarn embedded in the resin increased with increasing degree of grafting. The failure in pulling out the yarn was cohesive. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2415–2419, 2003     

Plasma surface treatments on carbon fibers. II. Mechanical property and interfacial shear strength

Carbon fiber are surface treated by oxygen, argon, and styrene plasma to study the effects on fiber strength and interfacial shear strength with PPS resin. Interfacial shear strength between carbon fiber and high melting temperature thermoplastic resins is successfully measured with the microbond pull-out test with the help of scanning CO₂ laser beam which solved the difficulties in preparing PPS microspheres. Tensile tests show that etching by oxygen plasma and deposition with plasma–PS increase strength of the fibers in some cases. ESCA spectra deconvolutions demonstrate that the improved interfacial strength is strongly related to the hydroxyl, ether, or aromatic groups on the surface. On the other hand, hydrocarbon segments are detrimental to the interface. Surface area and roughness have little influences on the interfacial strength of carbon fiber/PPS composites.     

Coating of carbon fibers with adhesion-promoting thin poly(acrylic acid) and poly(hydroxyethylmethacrylate) layers using electrospray ionization

Thin coatings of poly(acrylic acid) (PAA) and poly(hydroxyethylmethacrylate) (PHEMA) were deposited onto carbon fibers by means of the electrospray ionization (ESI) technique in ambient air. These high-molecular weight polymer layers were used as adhesion promoters in carbon fiber–epoxy resin composites. Within the ESI process, the carbon fibers were completely enwrapped with polymer in the upper 10 plies of a carbon fiber roving. As identified with scanning electron microscopy also shadowed fibers in a bundle as well as backsides of fiber rovings were pinhole-free coated with polymers (‘electrophoretic effect’). Under the conditions used, the layers have a granular structure. Residual solvent was absent in the deposit. PAA and PHEMA films did not show any changes in composition and structure in comparison with the original polymers as analyzed by X-ray photo-electron spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Single-fiber pullout tests of coated fibers embedded in epoxy resin showed significantly increased interfacial shear strength. It is assumed that chemical bonds between carbon fiber poly(acrylic acid) and epoxy resin contribute significantly to the improved interactions.     

Improvement of adhesion of PET fibers to rubber by argon-oxygen plasma treatment

Polyethylene terephthalate fibers cords were modified with argon, oxygen, and successive argon/oxygen cold plasmas as a function of treatment time. Plasma treated cords were coated with resorcinol formaldehyde latex, then tested as rubber reinforcing materials. The peel strength was discussed with respect to the polar component of the surface energy and the etching of the fibers. An increased adhesion of ∼ 280% was obtained with 30 min argon plasma followed by 30 min oxygen plasma, at 75 W power and 40 Pa pressure without altering the traction strength of the fibers cords. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2321–2330, 1998     

Organometallic polymer-derived interfacial coatings on carbon and SiC fibers

Organosilicon and organogermanium polymers containing unsaturated carbon–carbon bonds were used as precursors for the SiC-based interfacial coatings on commercially available carbon and silicon carbide fibers and fabrics. The approach based on usage of the organometallic polymer solutions allowed to obtain uniform, adherent, crack-free and non-bridging SiC-based interfacial coatings on carbon and SiC fibers. The coated fibers retain their tensile strength. The morphology, composition, structure of coated fibers were evaluated by various analytical techniques. The drop-like germanium-containing phase was detected in the organogermanium polymer-derived coating on carbon and SiC fibers.     

Impact behavior of carbon fiber/ultra-high modulus polyethylene fiber hybrid composites

Carbon fiber (CF)/ultra-high modulus polyethylene (UHMPE) fiber hybrid composites were fabricated using vinyl ester resin as a matrix. Interfacial adhesion of carbon fiber/vinyl ester composites and UHMPE fiber/vinyl ester composites as model composites was optimized using low temperature plasma treatment. Interlaminar shear strengths of carbon fiber/vinyl ester and UHMPE fiber/vinyl ester homocomposite were greatly increased by plasma and silane coupling agent treatment. From the result of the impact test, total absorbed energy of carbon fiber/UHMPE fiber hybrid composites was correlated with laminating sequences at optimized interfacial adhesion between the reinforcing fiber and matrix resin. UHMPE fiber layers of hybrid composites played an important role in absorbing energy. Elastic and plastic deformation of UHMPE fiber layers also played a key role in improving the impact properties of carbon fiber/UHMPE fiber hybrid composites.