James P. Nokes

Effect of Processing Parameter Changes on the Adhesion of Plasma-treated Carbon Fiber Reinforced Epoxy Composites

Atmospheric plasma treatment for the surface preparation of adhesively bonded composite joints appears promising as a replacement to current surface preparation techniques. However, questions remain regarding the sensitivity and optimization of various plasma processing parameters on final composite bond properties. In this study, we continue to investigate how plasma surface treatment processing variables ultimately affect the surface chemistry and bonding behavior of a graphite-epoxy composite. The plasma power level, the working distance of the plasma head, the carrier gas (helium) flow rate, the duration of plasma exposure, and the active gas (oxygen) concentration within the plasma were varied and correlated to surface chemistry variations using X-ray photoelectron spectroscopy (XPS). The carboxyl concentration on the surface was then measured as a function of these changes and correlated to lap shear strengths. In addition, samples were monitored using XPS to evaluate the decay behavior of the surface treatment as a function of time. Treated specimens in both inert and air environments exhibited similar decay profiles. Large changes were not observed until after 24 days of out-time. The effects of plasma treatment, duration of plasma exposure, and out-time on the crack delamination resistance (GIC) of bonded parts were assessed. G IC measurement indicated that solvent wiped bonded specimens exhibited a purely adhesive failure with unstable crack growth. Specimens with abrasion treatment exhibited reduced performance with cracks initiated in the adhesive traveling through both the adhesive-composite interface as well as the outer surface plies of the composite substrate. We believe damage to the composite substrate due to surface preparation caused this failure mode. On the other hand, plasma-treated specimens exhibited consistent failure modes for all treatments above 12 passes. The failures were entirely cohesive with the very high bond strength promoting crack propagation only within the adhesive. The GIC values indicated that the plasma-treated composites were two times as resistant to fracture as conventionally prepared specimens.

The Effect of Atmospheric Plasma Treatment on the Chemistry, Morphology and Resultant Bonding Behavior of a Pan-Based Carbon Fiber-Reinforced Epoxy Composite

A study was undertaken to evaluate the effect of atmospheric plasma treatments on the surface chemistry, morphology, and mechanical properties of graphite/epoxy composites. Characterization included contact angle measurements, XPS, FTIR, SEM and AFM. Treatment was shown to increase strength by as much as 50% relative to untreated specimens. The improvement was related to the number of passes and can be attributed to chemical surface modifications. While the total amount of oxygen on the surface stabilized quickly after a few plasma passes, the concentration of the carboxyl groups was shown to continuously increase, and correlated well with observed increases in strength.

Bonding Optimization on Composite Surfaces using Atmospheric Plasma Treatment

The effect of atmospheric plasma treatment (APT) on the bonding performance of a cyanate ester and an epoxy carbon fiber reinforced composite fabricated with a polyester peel ply was evaluated. A room temperature (RT) cured epoxy, an elevated temperature cured epoxy, and a cyanate ester resin, were used as the bonding adhesives. Only small increases in the carboxyl species concentration were observed for both composite systems as a function of increasing plasma treatment. Lap shear (LS) tests of the bonded composites showed that the APT resulted in a 30% strength improvement for the RT cured epoxy bonded specimens while the cyanate ester composite exhibited negligible increases due to the formation of a highly oxidized, weakly bonded ash. Contact angle measurements indicated that the temperature exposure associated with the curing of the elevated temperature adhesives also reduced the efficacy of APT. Modifications of the bonding surface of these composites by the incorporation of a plasma responsive (PR) layer resulted in significant LS improvements. After incorporating the PR layer, the improvement in adhesive strength was over 225% that of an untreated specimen and approximately 190% that of the equivalently treated unmodified system. Bond strengths correlated with corresponding increases in carboxyl concentrations after APT.The effect of atmospheric plasma treatment (APT) on the bonding performance of a cyanate ester and an epoxy carbon fiber reinforced composite fabricated with a polyester peel ply was evaluated. A room temperature (RT) cured epoxy, an elevated temperature cured epoxy, and a cyanate ester resin, were used as the bonding adhesives. Only small increases in the carboxyl species concentration were observed for both composite systems as a function of increasing plasma treatment. Lap shear (LS) tests of the bonded composites showed that the APT resulted in a 30% strength improvement for the RT cured epoxy bonded specimens while the cyanate ester composite exhibited negligible increases due to the formation of a highly oxidized, weakly bonded ash. Contact angle measurements indicated that the temperature exposure associated with the curing of the elevated temperature adhesives also reduced the efficacy of APT. Modifications of the bonding surface of these composites by the incorporation of a plasma responsive (PR)...

Bondability of TC410 composites: the surface analysis and wetting properties of an atmospheric plasma-treated siloxane-modified cyanate ester composite

Atmospheric plasma treatment (APT) has been used on a number of composite systems as an effective surface preparation technique for adhesive bonding. Recently, a new class of siloxane-modified polycyanurate resin systems (TC410) has been developed for use as a matrix material in composites utilized for space applications. The effect of APT on a TC410/M55J fiber composite was evaluated. Contact angle measurements exhibited a sharp reduction in wetting angle with exposure. An increase in the polar surface energy component with treatment coincides with Fourier transform infrared and X-ray photoelectron spectroscopy (XPS) results, which verify the formation of an oxidized carbonate species formed on the composite surface. A solvent rinse removes the majority of the oxidized species and results in an increase in contact angle, due to the removal of a fine ash. XPS and energy-dispersive x-ray spectroscopy verify the distribution of a siloxane component throughout the resin, which was oxidized during APT and forms a silicate. A 24% increase in bond strength was realized after six passes in comparison to abraded specimens. Higher treatment levels resulted in a decrease in strength; however, a solvent rinse of the APT surface gave rise to some recovery in strength due to the removal of the weakly bonded interlayer.

NDE of composite seismic retrofits to bridges

Composite materials are being used for bridge column seismic retrofits and to rehabilitate other concrete structures. There are three different manufacturing methods for applying composites to concrete columns which are outlined in this paper. Each method has the potential for creating debonds at the composite-concrete interface and within the composite itself. Thermography is a non-destructive evaluation technique which can be used to image debonds below the composite surface. Data from thermographic tests of a variety of retrofit applications, which include examples for each of the three aforementioned manufacturing processes, are presented.

Mechanical enhancement of graphite nanoplatelet composites: Effect of matrix material on the atmospheric plasma-treated GnP reinforcement

Graphite nanoplatelets (GnPs) are currently employed to manufacture a new class of carbon nanomaterial composites with unique electrical and thermal as well as mechanical properties. However, due to their unreactive graphitic structure, surface activation of GnPs is critical to promote bonding to the matrix material. In a previous study, the effect of atmospheric plasma treatment (APT) on the mechanical performance of GnP epoxy composites was evaluated where the GnP surface activation resulted in a significant increase in composite strength. The current investigation evaluates the effect of GnP plasma activation when using a polycyanurate (PCN) resin as the matrix material. GnPs (5, 25 microns in particle size) were surface treated as a function of plasma exposure durations and then used to manufacture composites. Flexural strengths of these plasma-treated PCN composites increased by 25%, for both the 0.5 wt.% loaded M25 and M5 composite systems. The higher loaded systems (1.0 wt.%) exhibited smaller increases in strength (11%) with APT, due to increased particle-to-particle interactions. The glass transition temperature (Tg) of surface-treated GnP PCN composites exhibited little variation with APT, which was in sharp contrast to APT-treated GnP epoxy composites that exhibited Tg increase up to 20℃. This suggests that the oxygen functional groups formed on the surface of GnPs are less chemically reactive toward the PCN than epoxy resins, translating to relatively limited composite strength improvements when utilizing oxygen APT-treated GnPs in PCN matrices.

Effect of isopropanol rinse on adhesion of plasma-treated carbon-fiber reinforced epoxy composites

In this article, we present a comparative assessment of adhesion in plasma-treated carbon fiber reinforced epoxy specimens subject to a post plasma isopropanol rinse. We find that plasma-treated and rinsed samples have better average adhesion than otherwise comparable unrinsed samples and attribute this finding to removal of a thin layer of ash from the plasma-treated surface. Using SEM, XPS, SIMS, and FTIR as diagnostic techniques, we are able to monitor the effects of the rinse on surface chemical functionalization.

Quantitative Evaluation of Silicone Contamination Effect on Composite Bonding

Quantitative assessment of adhesive bond strength on composite surfaces with respect to silicone contamination is presented and discussed. By using X-ray photoelectron spectroscopy, the precise surface contamination level was determined. When correlated with adhesive bond strength measurements, low-, medium-, and high-risk contamination levels were identified.

Characterization of atmospheric plasma as a surface preparation process for the bonding of space composite materials

There is a growing interest in the use of atmospheric plasma treatment techniques for the surface preparation of fiber-reinforced composite hardware before bonding. In this article, we will discuss the effects of atmospheric plasma treatment on the chemical and physical properties of two types of composites (cyanate ester and epoxy) and the resultant mechanical properties. Atomic force microscopy and X-ray photoelectron spectroscopy were used to investigate the surface morphology and chemistry of the treated composites and were correlated to lap shear bond strengths. We observed a strong correlation between the increases in lap shear strength with the surface carboxyl concentration of the composite after treatment as verified by X-ray photoelectron spectroscopy. Changes in surface roughness appeared to be secondary in nature. As a result, we have separated the mechanical contributions caused by etching-induced surface roughness from chemical contributions due to formation of specific reactive surface functional groups. The mechanical and chemical mechanisms that govern surface properties and contribute to the observed enhanced bonding will be discussed.

Detection of manufacturing flaws in composite retrofits

Composite materials are being used for bridge column seismic retrofits and to rehabilitate other concrete structures. There are three different manufacturing methods for applying composites to concrete columns which are outlined in this paper. Each method has the potential for creating debonds at the composite-concrete interface and within the composite itself. Thermography is a non-destructive evaluation technique which can be used to image debonds below the composite surface. Background fundamentals of the thermographic technique are discussed. Data from thermographic tests of a variety of retrofit applications, which include examples for each of the three aforementioned manufacturing processes, are then presented. The paper concludes with a list of issues which need to be addressed when performing a thermographic inspection in the field.