Surface Aspects of New Fibers, Boron, Silicon Carbide, and Graphite

A review of the literature for surface characteristics of boron, silicon carbide and graphite fibers was made. Some of the physical and chemical characteristics of the surfaces of these fibers are described. Surface properties that are unique as well as common to each fiber are discussed. Comparisons of these fiber surface properties to those of glass fibers are made in efforts to bring out differences and similarities. This report is an attempt to summarize the strength of the “bond” at the fiber-resin interface in high modulus fiber reinforced composites. The problems associated with defining the chemical and physical nature of the fiber surfaces which are most relevant to the development of composites with optimum mechanical properties are pointed out.     

Adhesion of Graphite Fibers to Epoxy Matrices: II. The Effect of Fiber Finish

Reinforcing fibers are available from various manufacturers with matrix compatible “finishes” applied to them. Usually these finishes or coatings are 100-200 nm thick resin layers applied after surface treatment. Their function has been hypothesized as being to enhance adhesion through either protecting the fiber from handling damage, protecting the fiber surface reactivity, or improving fiber wettability. This study of finished and unfinished graphite fibers concludes that the mechanism by which an epoxy compatible finish operates is different from what has been hypothesized to date. The finish layer creates a brittle interphase layer between the fiber and matrix which increases the interfacial shear strength but at the expense of changing the failure mode from interfacial to matrix.     

Surface Silanization of Polyethylene for Enhanced Adhesion

Low density polyethylene has been treated using a novel surface treatment process “SICOR” (“SIIane-on-CORona” treated polymer) in order to enhance adhesion with a range of adhesives including polyure-thane, methacrylate and cyanoacrylate. The process comprises two steps, i.e corona discharge followed by application of an organo-functional silane. The incorporation of surface hydroxyl groups onto the polymer surface enables organo-silane to create the hydrogen or covalent bonds with the oxidized polymer surface. The possibility of the creation of these bonds has been investigated using FTIR, XPS and wettability studies. The adhesion enhancement due to the new process is significant. Frequently, the strength increase exceeds 200% compared with the corona discharge treatment and more than 300% compared with LDPE priming using the “Loctite 770” polyolefin primer. The process is shown to be as good as, or better than, plasma treatment in terms of the strength increase following substrate treatment prior to adhesive bonding.     

The Strength of Adhesive Joints

The main rules pertaining to the strength of adhesive joints are: (1) This strength is a mechanical (or rheological) property. The local stress which causes the extension of a pre-existing crack can be determined only if the stress pattern in the whole adhint is known and the intensification of stress at flaws is taken into account. (2) The rupture occurs in a material, not between two materials. Consequently, the molecular forces across the adhesive-adherend interface are irrelevant, and the “adhesion tension” does not determine the adhint strength.     

Foreword

The effect of surface treatments and fiber sizings on the stress transfer characteristics and composite properties of AS-4 carbon/epoxy materials has been determined. Fiber surface chemistry was systematically varied from acidic to basic with RF glow discharge plasmas of CO₂ and NH₃ and characterized with ESCA techniques. Sizings applied to some of the treated fibers consisted of diglycidyl ether of bisphenol-A(DGEBA). Single fiber tension tests were used to measure the interfacial shear strength of samples made with DGEBA/metaphenylene diamine resin. Short beam shear and transverse flexure tests were used to examine the composite properties of modified materials.

Results showed that the plasma treatments were effective in altering the surface chemistry of the fiber but that changes in surface chemistry had surprisingly little effect on the critical stress transfer length. Sizing had a more significant effect on the transfer length. The interlaminar shear strength of the composites were unaffected by the treatments. Transverse flexure tests were more sensitive to the changes in surface characteristics. The work indicates that the interface properties of AS-4 fibers are close to optimal but that improvements in composite performance are possible through interphase formation.     

Effect of Interphase Structure on the Debonding of Polycarbonate from S-2 Glass Fibers

The effect of interphase structure on the debonding of polycarbonate from S-2 glass fibers has been studied. The shear strength, fracture toughness and hydrolyic stability of the interphases were measured in a single fiber composite of a continuous S-2 glass fiber embedded in a polycarbonate matrix. Polycarbonate oligomers were chemically grafted onto the glass fiber surfaces through use of a silicon tetrachloride intermediary and the properties of the resulting interphases were compared with those of two commercial sizings and ozone-cleaned surfaces. Evaluation was accomplished by measuring the stress transmission across the interphase, τ, by carrying the embedded single fiber fragmentation test to saturation and by using computer simulations and a finite element analysis to calculate the strain energy release rate, G, of the observed fiber-matrix debonding accompanying the first fiber fracture. The oligomer-grafted interphase exhibited improved stress transmissibility and toughness, after 24 hours in boiling water. The tenacity of the tightly bound oligomers was confirmed via DRIFT, TGA and GC/MS experiments on Soxhlet-extracted fibers.

The grafting reaction was modeled on a high surface area silica and studied using solid state NMR to determine reasons for the greater stability of the oligomer-treated surfaces. Measurements of chemical shifts and spin-lattice relaxation times indicate that the oligomers are chemically attached to the surfaces, providing for a well bonded, water resistant interphase. Parallel experiments on a monomeric Bisphenol A-primed silica surface provided evidence that chemical bonding was primarily responsible for the greater hydrolytic stability.     

Adhesive Bonding to Galvanized Steel: I. Lap Shear Strengths and Environmental Durability

Initial (i.e., unaged) adhesion, as well as adhesion after seven day, 60°C water immersion and six week scab corrosion accelerated environmental exposures, has been assessed for five different one and two-part epoxy adhesives, bonded to three different types of galvanized steel substrates. We have shown that adhesion, as measured by lap shear strength, is specific to the galvanized substrate type. In general, for a given adhesive, adhesion to “hot-dipped” galvanized substrates is harder to achieve and maintain under accelerated environmental exposure than is adhesion to “electroplated” galvanized. Also, for a given type of galvanized steel, the one-part epoxies evaluated generally showed higher initial strengths, as well as better strength retention under environmental exposure than did the two-part epoxies.     

Influence of the fiber surface properties on the mechanical strength of unidirectional fiber composites

The influence of the thermodynamic adhesion between fibers and matrix on the mechanical properties of a continuous fiber reinforced composite is studied for two systems: carbon fiber reinforced poly(ether ether ketone) and glass fiber reinforced poly(ether imide). The fibers are modified chemically and characterized by measuring the contact angle formed by molten resin on the fibers. Various fiber treatments yield a wide range of contact angles, which are determined optically. Unidirectional fiber reinforced laminates are manufactured and transverse flexural strength is measured with the values reported as a function of the specific work of adhesion. It is shown that adhesion at the fiber-resin interface correlates with both the composite strength and the void morphology within the laminate after consolidation.     

New and Improved Tests for Adhesion

Three new methods are discussed for measuring the work G_a, required to detach unit area of an adhering material from a substrate. The first is a simple modification of the Outwater double-torsion test for long rectangular plates, bonded together. This method is suitable for evaluating aluminum-epoxy bonds, for example, or the transverse strength of fibrous composites. The second is a pull-off test for long strips adhering to a rigid surface. It seems suitable for adhesive tapes and laminates. The third is a reconsideration of the “blister” test for films and coatings, in which a circular debond at the interface is made to grow by internal pressure. The relation obtained between pull-off force F for a strip, or blow-off pressure P for a layer, takes the unusual form:

F⁴ (or P⁴) ∞ KG³_a

where K is the tensile stiffness of the detaching layer. This dependence arises from the non-linear (cubic) relation between load or pressure and deflection in these configurations. Nevertheless, the product Fθ, where θ is the angle of detachment of a strip, or Py, where y is the height of a “blister”, give direct measures of the strength of adhesion G_a, independent of the stiffness of the adhering material and of the extent of detachment.     

A highly fluorinated epoxy resin. III. Behavior in composite and fiber-coating applications

The behavior of a highly fluorinated epoxy resin used as a composite matrix material with AS-4 fibers and as an AS-4 fiber coating was studied. The composite mechanical properties were obtained, and the adhesion of the matrix to the fibers was evaluated. Comparisons of uncoated and fluoropolymer coated AS-4 fibers using single fibers embedded in an Epon 828 matrix were made. Substantial improvement in fiber critical length, and therefore fiber-matrix adhesion, was observed.     

Adhesion Studies of Fluoropolymers

A comparative study of the treatment of polytetrafluoroethylene (PTFE) and poly(vinyl fluoride) (PVF) with “Tetra-Etch” has been carried out. The treatment of PTFE resulted in extensive changes in surface chemistry and topography, whereas with PVF there was no significant change in topography and the chemical changes were much less marked. However, treatment of both polymers resulted in large increases in bond strength.

Multiple bonding experiments in which samples are repeatedly fractured and re-bonded were carried out with untreated PTFE and PVF. These resulted in moderate increases in bond strength with PTFE and large increases with PVF. The results indicate that weak boundary layer (WBL) removal is a key element in adhesion improvement by “Tetra-Etch” on PVF. With PTFE, WBL removal also improves adhesion, but the chemical and/or topographical changes introduced by the “Tetra-Etch” are required for optimum performance.     

COMPREHENSIVE STUDY OF THE GAS HOLD UP AND BUBBLE SIZE DISTRIBUTION IN HIGHLY VISCOUS LIQUIDS I. GLYCEROL SOLUTIONS

The overall gas hold up, E_G, and bubble size distribution were separated into the particular gas hold up, E_GK, and Sauter diameter. d_SG. due to “small bubbles” as well as E_GG and d_SG, due to “intermediate to large bubbles.” Bubbles are defined to be “small” if they remain in the bubbling layer 15 seconds after the gas flow is turned off. The bubbles which leave the layer during this time are considered to be “intermediate to large bubbles.” The time dependences of E_G E_GK and E_GG, as well as of bubble size distribution after initiating the aeration of the liquid, is investigated. The steady state E_G, E_GK and E_GG, Sauter diameter and specific geometrical surface area of “small” and “intermediate to large” bubbles as well as of the entire bubble population were determined in bubble columns employing 50, 70, 90 and 95% glycerol solutions and perforated plates with different hole diameters (d_H = 0.5. 1.0 and 3.0 mm) respectively. In highly viscous media the “small” and “very large” bubble fractions are high. A comparison of the specific geometrical bubble surface areas with the corresponding volumetric mass transfer coefficients, k_La's, measured earlier indicate that the “small” bubbles do not contribute to k_La. The influence of the “small” bubbles on the fluiddynamics of the two phase system is discussed.     

Adhesion Mechanisms at Amine-Cured Epoxy/Aluminium Interfaces

Because the structure and the chemical composition of the interface can have a large effect on the adhesion properties of polymeric materials to metallic surfaces, many investigations have concentrated on the study of the interphase region. However, the complexity of the materials often leads to the use of model compounds to mimic the interfacial reaction. We have presented a critical discussion of three different approaches which have been used to understand the adhesion mechanism at amine-cured epoxy/aluminium interfaces: i) fracture of “real world” joints; ii) deposition of model (amino-alcohol) molecules on “real world” substrates; i) deposition of model (amino-alcohol) molecules on clean, oxidised and hydroxylated Al (100) surfaces. We have shown that model compounds can adequately duplicate the interface chemistry observed in “real world” joints. However, a detailed understanding of the exact nature of the interactions and of the role of the different reactive sites can only be achieved through studies performed on a model surface under controlled ultrahigh vacuum conditions.     

The Mechanical Behaviour of Polyimide/Copper Laminates Part 2: Peel Energy Measurements

The mechanical peel behaviour of laminates consisting of polyimide films adhered to copper foil using a modified acrylic adhesive has been studied over a wide range of test rates and temperatures. The laminates were prepared from polyimide films which had been subjected to either a “high-thermal history” or a “low-thermal history” treatment during the production of the film. The measured peel energies of the laminates could be superimposed to give a master curve of peel energy versus the reduced rate of peel test, Ra_T, where R is the rate of peel test and a_T is the time-temperature shift factor. The appropriate shift factors were a function of the test temperature and were mainly deduced from tensile tests conducted on the bulk adhesive. The “high-thermal history” laminates gave higher peel energies and the locus of failure of the laminates was mainly by cohesive fracture through the adhesive layer. At low values of log₁₀ Ra_T, i.e. Low rates of peel and high test temperatures, the “low-thermal history” laminates also failed in the adhesive layer and possessed similar peel energies to those measured for the “high-thermal history” laminates. However, at high log₁₀ Ra_T values, the peel energies measured for the “low-thermal history” laminates were lower and showed a wider scatter. These arose from a different locus of failure occurring in these “low-thermal history” laminates when tested under these conditions. Namely, it was found that most of these laminates failed in a weak boundary layer in the outer regions of the “low-thermal history” polyimide film.     

Fracture Mechanics of aV-peel Adhesion Test - Transition from a Bending Plate to a Stretching Membrane

A linear elastic solution is proposed for a “V-peel” adhesion test for a thin film adhered to a rigid substrate. The mechanical responses of a stiff plate-like coating under pure bending, a semi-flexible film under mixed bending and stretching, and a flexible membrane-like film under pure stretching are discussed. For delamination to occur, the mechanical energy release rate is shown to be G = χ(Fw₀/2bl) with χ a numerical constant varying from 3/2 for a plate-like disc to 3/4 for a thin flexible membrane.     

ISOTHERMAL AND NON-ISOTHERMAL MULTICOMPONENT ADSORPTION KINETICS

The model equations in the relaxation form for the multicomponent kinetics of isothermal and non-isothermal adsorption, taking into account all major distinctive features of the interphase heat and mass exchange inside porous grains and at their surface (see points 1 to 4 below) for P (“pore”) and S (“solid”) models of mass transfer within porous grains of the adsorbent, have been obtained.

First for isothermal and non-isothermal kinetics in the mixed kinetics region of mass and heat exchange in the absence natural mutual diffusion and natural thermal-diffusion the essential influence effective mutual diffusion and effective thermal-diffusion is shown.

Recommendations on the use of model equations of adsorption kinetics for describing isothermal and non-isothermal adsorption dynamics of multicomponent mixtures in the inner-diffusion and mixed (outer- and inner-diffusion) kinetic region of heat and mass exchange are made.     

Recent Aspects of Glass Fiber-Resin Interfaces

The recent developments in Auger spectroscopy have been used to define the composition of two glass fiber surfaces. The effect of fiber surface area on the interlaminar shear strength was also investigated. The chemistry of several silane “coupling agents” has been studied from the standpoint of its chemical form when it is applied to the glass fibers, and has in part been determined using a gas chromatographic technique. The relative thermal stability of some silanes in high temperature resin matrices was determined. A comparison of a treatment of glass fibers with aqueous and non-aqueous systems is made.     

The breaking strength of imperfect (real) polymer fibers

The thermodynamic fusion theory of strength of perfect polymer fibers of finite molecular weight is extended to include imperfect (i.e. real) fibers of incomplete crystallinity and orientation. Approximate equations for failure strength, strain, and work of failure are derived by extracting from the real visco-elastic fiber an equivalent reversible component suitable for thermodynamic analysis. This is facilitated by an explicit relationship between fiber breaking stress, σ_*, and breaking strain, _*, which is shown to be σ_*=0.632K_* (K=modulus) for constant strain-rate deformations. It is shown that fiber breaking time is equivalent to the fiber visco-elastic mechanical relaxation time. Experimental data shows that the activation energy of rupture of polyethylene fibers is not the activation energy of covalent bond rupture. Instead it agrees with the activation energy expected of crystal melting in accordance with the fusion theory of rupture. The activation volume of the polyethylene fibers also agrees with the value expected from this theory.     

The Effect of Etching on Ti6A 14V Interfacial Chemistry and Adhesion to Evaporated Gold and a Commercial Adhesive

Titanium 6-aluminum 4-vandium alloys were etched for varying periods of time in aqueous solution of hydrofluoric acid and ammonium dihydrogen phosphate. Following etching, one-half of the specimens were covered with vacuum evaporated gold while the other half were bonded with a commercial adhesive. Gold adhesion to the alloys was evaluated by pressure sensitive tape peel tests and lap shear tests using a commercial adhesive. The uncoated specimens were evaluated by lap shear tests. Adhesion of Ti6A14V to gold and to the commercial adhesive was directly proportional to the length of time the specimen was etched. Although etching in HF/NH₄H₂PO₄ resulted in a rather rough surface, there were only subtle differences with increasing time. Surface chemistry changes suggest selective etching of the alpha phase and increasing exposure of the beta phase. Heating of the gold on Ti6A14V resulted in improved adhesion, probably by diffusion mechanisms. Exposure to steam resulted in bond degradation in both gold/Ti6A14V and in adhesive/Ti6A14V systems. The adhesive bonding results for the etched specimens were compared to performance based on “attachment site” theory. Excellent agreement for both gold on Ti6A14V and commercial adhesive on Ti6A14V was observed. Degradation of the bond due to steam followed the same form in both systems, suggesting H₂O transport along the interphase.     

Adhesion Between Metals and Polymers as a Three-Dimensional System

Adhesion science in a technical sense is the study of reactions in boundary layers. From a macroscopic point of view the result is the adhesive joint strength dependent on the magnitude of the adhesion forces without hints on the nature of these forces. So the question of the nature of adhesion has at least to be answered for technical applications by using other measurement techniques. From the microscopic point of view adhesion is of interdisciplinary nature, where molecules or atoms act with each other across the interface. Mainly adhesive bonds are based on these interactions of different bodies like metals and polymers or other material discontinuities. So far we can speak about a “chemical adhesion”. But in practice there we realize a “technical adhesion” with more or less sharp discontinuities.