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A new process for aligning chopped fibers in composites.

Thermoplastic composites manufactured using electric fields to align the chopped-glass fibers have properties approaching those of unidirectional continuous-fiber composites while providing superior processability.

Fiber-reinforced thermoplastic composites are gaining popularity because of their good toughness properties, high processing speeds, excellent chemical resistance, and the potential for recyclability and reparability of the material forms Short-fiber or random-fiber-mat-reinforced composites form a major share of these thermoplastic composites because of ease in processability. The lack of suitable techniques to control fiber orientation and constraints in processing long fibers at high volume fractions have set limitations in the performance capabilities of these materials, restricting their utilization to non-structural applications. Because of expensive and labor-intensive fabrication techniques, the usage of high-performance, continuous-fiber composites is limited, restricted mostly to aerospace and other niche applications.

Micromechanics models[1,2] predict that the performance of discontinuous-fiber composites approaches that of continuous-fiber composites when the reinforcing length of the fibers far exceeds the critical length, and when the fibers are aligned in the direction of the applied stress. These principles provided the impetus to develop an Aligned Discontinuous Fiber Composite (ADF) process.

Recent advances in the area of powder coating of fibers at Michigan State University,[3] in combination with the phenomenon of aligning fibers in electric fields, have paved the way to conceptualize and develop a novel high-speed processing methodology that can manufacture aligned discontinuous-fiber composites. Full realization of the stiffness-to-weight benefits of these composites is possible because of effective fiber alignment combined with the ability to pack and process at higher volume fraction of fibers. The absence of solvents or liquids in the ADF process greatly improves the speed of processing and makes the process environmentally friendly. The fiber-alignment technique of the ADF process is simple in operation and design, with the scope for retrofitting this technique to an existing composite sheet or lamination processing unit. Unlike continuous-fiber, random-mat-reinforced composites, which have poor drapability and problems with delamination under compression, aligned discontinuous-fiber composites are flexible and can be molded or stamped into complex parts. It is envisaged that ADF composites, with their unique performance and processability capabilities, will expand the range of the applicability of discontinuous-fiber thermoplastic composite materials to many areas in the automotive, infrastructure, and durable-goods sectors.

ADF Process

The schematic of the ADF process is shown in Fig. 1. Dry, powder-coated fibers of lengths greater than the critical fiber length are generated and fed into the electric-field orientation chamber in a controlled manner using a vibratory feeder. The mode of fiber delivery and the orientation chamber dimensions have been designed such that the fibers settle in a predominantly planar orientation before coming under the influence of electric fields.[4] The aligned fibers then are deposited on a moving veil, which is heated to retain the integrity of the ADF mat. Plies of this material are then layered together and compression molded to form a composite part.[5]

It is important to note that the uniform distribution of polymer particles around the fibers helps in reducing polymer melt-flow times during the compression-molding step, since the polymer has to flow only locally over small distances.[6] This improves matrix impregnation, minimizes void formation during consolidation, and improves the mechanical performance of the part. Powder impregnation makes compression molding of the ADF mat into a final composite part a rapid step. The ADF process can operate continuously to make ADF preforms or in a batch mode to fabricate a composite part.

The study of the behavior of the fibers in electric fields led to the key development of the process - the ability to control the orientation of fibers with and without powder impregnation in a dry state. Fiber orientation is controlled by a combination of factors, including the electric-field intensity and the hydrodynamics of fiber motion. Fiber geometric dimensions and the fiber dielectric constant are the two most important material properties that affect the degree of orientation and the alignment time in air. The process has the potential to align dielectric fibers (e.g., glass, aramid, and other polymeric fibers) as well as conductive fibers (e.g., carbon) without any pretreatment.

Fabrication

For the components of the fiber/matrix system used, chopped E-glass fibers of four different nominal lengths of 3.2, 6.4, 12.7, and 25.4 mm were supplied by Owens-Corning Fiberglas and Vetrotex CertainTeed; and nylon 12 (Orgasol) powder matrix with a mean particle size of 10 microns was supplied by Elf Atochem. Fibers with two different sizings, polyester- and nylon-compatible, and in two different bundle sizes, 200 and 400 filaments per bundle approximately, were used.

For each fiber length, ADF panels were fabricated under two modes: random fiber orientation, which provided the base-line case for comparison; and aligned fibers using a 400-kV/m electric field. Measured quantities of fiber and the matrix powder were used to make ADF panels with 40 vol% fiber.

The consolidation cycle consisted of heating the plies to 200 [degrees] C with vacuum and a 0.7-MPa pressure applied when the temperature reached 100 [degrees] C. The part was held at 200 [degrees] C for approximately 10 min to ensure complete consolidation, and then rapidly cooled to room temperature. The whole cycle took about 30 min. This consolidation cycle gave consistently good quality parts with minimal resin bleeding and voids. Manufacturing implementation of this process would, after optimization, yield much shorter cycle times.

Microstructure

Three principal features of the microstructure that dictate the final mechanical properties of a discontinuous-fiber/matrix composite system are (i) fiber-orientation distribution; (ii) fiber aspect ratio; and (iii) fiber-matrix interaction. Burnout tests were conducted on the panels to verify the volume fraction of the composites, and typically all the panels had a fiber-volume fraction of 40 [+ or -] 2%. Void fractions of all the panels were under 3%. Polished cross sections were observed under an optical microscope for microstructural details and the presence of voids, if any.

Fiber-Orientation Distribution

One of the objectives in developing this process was to demonstrate the feasibility of making ADF composites with controllable fiber-orientation distributions (FODs). Typically, once the processing for the manufacture of an ADF composite has been completed, its FOD is obtained in an indirect way by image analysis The method adopted in obtaining the FOD consisted of running the ADF process once again under identical conditions, not with the intention of making a composite, but recording a series of images using a Panasonic CCTV [ILLUSTRATION FOR FIGURE 2 OMITTED]. A number of images of the fibers, deposited on polytetrafluoroethylene release film, are taken to give a statistically significant FOD. Global Lab image-analysis software is used to identify fibers and determine their orientations. It has also been carefully verified that the FOD of the ADF mat is not disturbed during the subsequent processing of the composite. Therefore, the FOD obtained by this method was concluded to be a true representation of the FOD in the ADF composite.

Using the above method, FODs were obtained for each of the chopped-glass-fiber mats that were produced under the two conditions: randomly oriented and aligned in a 400-kV/m E-field [ILLUSTRATION FOR FIGURE 3 OMITTED]. In the case of aligned fiber mats, for all four fiber lengths, the orientations of about 70% of the fibers lie between [+ or -]20 [degrees] and nearly 80% to 90% of the fibers between [+ or -]30 [degrees], indicating a high degree of fiber alignment in the direction of the E-field. The orientations of the fibers in the random case for all fiber lengths indicate a uniform orientation distribution, without any significant bias in any direction.

Aspect Ratio

A statistically significant number of fiber lengths were measured for each type of fiber used for making ADF composites The fiber-length distribution in each case was very narrow, and hence the nominal length was used in all subsequent computations. In the chopped-glass-fiber systems, the aspect ratio of the reinforcing bundle is not just the ratio of bundle length to bundle width, nor does it always turn out to be the ratio of the length of the bundle to the diameter of individual filaments. It depends on the degree of resin impregnation into the fiber bundle. Good impregnation is achieved when there is a good wet-out between the fiber and the matrix, and when an optimum time-temperature-pressure consolidation cycle is used.

The compatibility of the sizing and the resin impregnation was investigated by optical microscopy for the polyester-sized and the nylon-sized cases. The fiber bundles distinctly remain as bundles in the ADF composite fabricated with polyester-sized fibers because of poor wet-out [ILLUSTRATION FOR FIGURE 4 OMITTED], while the nylon-sized fibers show good fiber dispersion, with the nylon 12 matrix impregnating the individual fiber bundles [ILLUSTRATION FOR FIGURE 5 OMITTED]. Thus, the reinforcing-fiber aspect ratio is closer to the bundle aspect ratio in the polyester-sized case, and closer to the filament-aspect ratio for the nylon-sized case.

Performance of ADF Composites

Tensile properties were determined by ASTM D638 using dogbone specimens cut from ADF panels (approximately 40 vol% fiber) using a C[O.sub.2] laser source of 360 W operated at a cutting speed of 76.2 cm/min.

Effect of Fiber Alignment

The tensile properties follow expected trends as a function of fiber length. This is very clearly reflected by the performance of ADF composites compared with the random base-line cases for all fiber lengths [ILLUSTRATION FOR FIGURE 6 AND 7 OMITTED]. Improvements in stiffness range from 61% to 97%, while improvements in strengths range from 58% to 86%. The polyester-sized glass fibers were used in the fabrication of this series of ADF composites.

Effect of Fiber Length

The modulus vs. fiber-length data [ILLUSTRATION FOR FIGURE 6 OMITTED] indicate that the modulus values increase with fiber length. The properties of random composites seem to taper off around a fiber length of 25 mm, while the improvements due to fiber alignment have an increasing trend. The effect of fiber length is very dramatic in the case of strength values [ILLUSTRATION FOR FIGURE 7 OMITTED], increasing about 300% as the length of the fibers increases from 3 mm to 25 mm. This is because composites with smaller-length fibers have a higher density of fiber ends, which act as stress concentrations, resulting in composite failure at low stresses.

Fiber-Matrix Interaction

The tensile-modulus values of the panels that were prepared using the polyester-sized fibers (OCF 227P) and the nylon-sized fibers (VCT 211N) of nearly the same number of filaments per bundle were compared for both the "perfectly" random and "perfectly" aligned cases. Improvements in the modulus values have been obtained in both extremes of fiber orientation by using the more compatible sizing between the matrix and the fiber [ILLUSTRATION FOR FIGURE 8 OMITTED].

Subsequently, the effect of fiber bundle size was investigated by comparing the properties of composites made from nylon-sized fibers (VCT 211N and VCT 418N) under identical conditions. A drop in modulus occurred when the number of filaments in the bundle was increased. The efficiency of resin impregnation decreased with an increase in the number of filaments in the bundle because of a decrease in the permeability of the bundle. It may be concluded that fiber sizing, as well as the number of filaments in a bundle, has a direct bearing on the effective aspect ratio of the reinforcing fiber, which controls the modulus and strength values of the discontinuous-fiber composites.

Conclusion

A novel process has been developed to manufacture aligned discontinuous-fiber composites using electric fields. The orientation of dielectric fibers, such as glass fibers (with and without polymer powder coating), was shown to be effectively controlled using electric fields. ADF composites made of glass fibers and a nylon 12 matrix provide significant improvements in stiffness and strength with fiber alignment. The effect of the microstructure on the performance of ADF composites has been established.

Acknowledgements

Financial support provided by NSF through the Center for Low-Cost, High-Speed Polymer Composite Processing at Michigan State University and the Michigan Materials and Processing Institute is gratefully acknowledged. The authors thank Owens-Corning Fiberglas and Vetrotex CertainTeed for supplying the chopped glass fibers.

References

1. B.D. Agarwal and L.J. Broutman, Analysis and Performance of Fiber Composites, John Wiley 8: Sons (1993).

2. M. Piggott, Load Bearing Fibre Composites, Pergamon Press (1980).

3. M.N. Vyakarnam and L.T. Drzal, U.S. Patent 5,310,582 (1994).

4. M.N. Vyakarnam and L.T Drzal, filed for patent (1995).

5. M.N. Vyakarnam and L.T Drzal, Proc. 11th Ann. ASM Advanced Composites Conf. & Exp., Dearborn, Mich. (1995).

6. S. Padaki and L.T Drzal, Proc. 10th Ann. ASM/ESD Advanced Composites Conf. & Exp., Dearborn, Mich. (1994).
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Author:Vyakarnam, Murty N.
Publication:Plastics Engineering
Date:Jan 1, 1997
Words:2099
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