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Measuring fiber/matrix adhesion in thermoplastic composites.

Measuring Fiber/Matrix Adhesion in Thermoplastic Composites

Interfacial shear strengths in various fiber/thermoplastic systems were measured by a simple single-fiber-pullout test and the effects of cooling rate and plasma treatment quantified.

Interfacial coupling of the reinforcing fiber to the polymer matrix is recognized as one of the keys to optimizing properties of high-performance, fiber-reinforced plastics. Despite important advances in the science and technology of composite materials, our understanding of molecular-level interactions between fiber and matrix remains incomplete. This is particularly the case for newly emerging thermoplastic matrices, where the absence of reactive functionality precludes the possibility of covalent bond formation at the interface. Interfacial adhesion in crystallizable thermoplastic matrices is further complicated by the possibility of a cyrstalline morphology near the fiber that may be different from the bulk polymer morphology.

Optimization of composite performance and increased understanding of fiber/matrix adhesion in thermoplastic composites require measurements of interfacial bond strengths. Several techniques have been developed to measure fiber/matrix interfacial shear strength. The single-fiber-pullout test used in this investigation, developed by Miller, Muri, and Rebenfeld, provides a relatively easy and direct method. It is called the "microbond" technique because it uses only a very small amount of resin in the form of a microdroplet (30 to 100 microns long) deposited on a single filament. The force required to displace the microdroplet is recorded and used to compute the interfacial shear strength. Because the interfacial area is very small, the level of force required for pullout is also small. Otherwise, the filament may rupture in tension before pullout. Therefore, this technique is particularly suited for dealing with very fine filaments ([is less than] 20 microns in diameter).

In a recent paper, the experimental parameters of the microbond technique were examined in detail, and measurements made on aramid, E-glass, and carbon fibers embedded in epoxy resin were analyzed to show that reliable and reproducible values of shear strength can be obtained. The method has also proven to be sensitive enough to quantify the effects of fiber surface modification and environmental aging. In this article, the technique is described and its use reported in the investigation of carbon and aramid fibers embedded in four thermoplastic resins: polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polycarbonate (PC), and polybutylene terephthalate (PBT). Examples are also given of the effects of resin thermal history and of plasma treatment on interfacial adhesion.

Microbond Technique

Microdroplets of resin are deposited on single filaments held horizontally on a mounting plate. To form droplets of thermoplastics, a small piece of polymer film is placed on the single fiber as follows: A longitudinal cut is made in the center of a piece of film (about 2 x 30 mm) for almost its entire length, to form two strips joined at one end for a distance of 50 to 100 microns. The slit film is suspended on the horizontal fiber as illustrated in Fig. 1. This limits the contact area between the fiber and the film to the thickness of the film. Upon melting, nearly uniform-sized droplets are obtained; their length is controlled by the film thickness. A photomicrograph of a typical droplet on a filament is shown in Fig. 2.

The fiber/droplet specimen is suspended at one end from a force gage. The filament is gripped by the microvise as diagrammed in Fig. 3. The microvise is attached to an elevator that moves downward at a rate of 1 mm/min. The microvise slit width is adjusted to achieve a small but specific level of frictional resistance (initial tension) with the fiber surface before contacting the drop and exerting a downward force on it. After contact, the shearing force is transferred to the fiber through the fiber/matrix interface. When the shearing force reaches a critical value, pullout occurs, and the droplet is displaced downward along the axis of the fiber. Initial tension is subtracted from the recorded force to obtain the pullout force. Interfacial shear strength (IFSS) is calculated using the equation: IFSS = Pullout force/Embedment area

The recorded force gage reading serves to distinguish true bond shearing from the other possible consequences of the experiment. As illustrated in Fig. 4, shearing will produce a distinctive force peak followed by a constant force reading that represents the frictional resistance as the droplet is pushed along the filament. If the matrix material exhibits a cohesive failure under the applied force, the recorded force will not show a single peak value. If the filament ruptures, the force drops to zero. Therefore, true bond shearing can be identified without having to examine each specimen after the experiment.

In a typical experiment, 20 to 70 specimens are measured. The collection of individual shear strength values forms a Gaussian type of distribution. Considerable evidence has been obtained to establish that these shear strength distributions are real variations in bond strength and not a reflection of systematic experimental errors. The distributions are believed to be due to fiber surface heterogeneity. The distributions may be used to provide comparisons between fiber/resin systems, or average shear strengths may be calculated from the aggregate of data points.

Carbon/PEEK Systems

PEEK droplets on single carbon fibers (unsized AS-4 and HMS, both from Hercules) were heated above the melting temperature of PEEK to 370 [degrees] C and 420 [degrees] C, simulating two commonly used processing temperatures, and cooled in each case at the rate of 2-3 deg/min. Interfacial shear strength measurements were found to be independent of the processing temperature for the two carbon fibers. Consequently, the data for both temperatures were combined; they are shown in Fig. 5.

The data, typical of results obtained using the microbond technique, show that the interfacial shear strength for the HMS/PEEK system, 29.7 [+ or -] 4.7 MPa, is lower than that for the AS-4/PEEK system, 38.9 [+ or -] 4.7 MPa, its entire distribution being shifted to lower values. The variances represent 95% confidence limits. Preliminary studies indicate that the lower bond strength value for the HMS/PEEK system may be related to differences in interfacial morphologies.

Effect of Cooling Rate

PC and PBT droplets were kept at 25 [degrees] C above their melting temperatures for two hours on aramid single fibers, then solidified using fast or slow cooling rates of about 50 deg/min and 1 deg/min, respectively. Shear strength data, summarized in Table 1, show that for the aramid/PC system, the average shear strength value of 51.9 MPa for slow-cooled specimens is significantly higher than the 42.0 MPa for fast-cooled specimens. Although PC is generally considered to be an amorphous resin, we feel that the slower cooling rate may have produced a more ordered structure, which has better stress transfer characteristics and, therefore, would show greater interfacial strength. Alternatively, the slow cooling may have caused more shrinkage, which would increase the compressive force on the fiber, also resulting in increased shear strength. The average shear strengths for the fast- and slow-cooled aramid/PBT samples were essentially identical. Apparently, the fast cooling rate is not rapid enough to affect PBT crystallinity, and both systems attained essentially the same crystallinity.

Effect of Fiber Surface Treatment

Unsized Hercules IM-6 carbon fiber, subjected to surface oxidation but with no subsequent size deposition, was compared with the same fiber treated with RF glow discharge acid plasma. The plasma-treated fiber was characterized using inverse gas chromatography and was found to have an increased number of surface acidic sites. Interfacial shear strengths for the fibers in PPS and epoxy matrices, summarized in Table 2, show that the plasma treatment produced a significant increase in bond strength with the PPS resin, presumably a Lewis base. The plasma treatment had no such effect with the epoxy resin. While the epoxy could be classified as a Lewis acid, this does not necessarily explain the ineffectiveness of the plasma treatment. Since the epoxy is a reactive matrix, other bonding mechanisms may control shear strength. [Figure 1 to 5 Omitted]
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Author:Gaur, Umesh; Desio, Glenn; Miller, Bernard
Publication:Plastics Engineering
Date:Oct 1, 1989
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