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  • COMPOSITES 2011 1

    COMPOSITES 2011 American Composites Manufacturers Association February 2-4, 2011 Ft. Lauderdale, Florida USA

    The Effect of Nanosilica Matrix Modifi- cation on the Improvement of the

    Pultrusion Process and Mechanical Properties of Pultruded Epoxy Carbon

    Fiber Composites

    by

    Kristin Thunhorst, Douglas Goetz,

    Andrew Hine, and Paul Sedgwick 3M Company

    ABSTRACT

    A study was undertaken to investigate the effect of nano-scale spherical silica particle inclusion on the pul- trusion process and the mechanical properties of the resulting epoxy-carbon fiber composite pultruded ar- ticles. The trial compared a nanosilica-modified epoxy matrix resin having a loading level of 32.6% by weight with a control resin having no nanosilica. Both bis- phenol-A-based (DGEBA) epoxy resins were cured with a liquid anhydride curative (MTHPA formulated for pul- trusion) at a 0.95 ratio of anhydride equivalents to epoxy equivalents. Pultruded parts with a rectangular cross- section approximately 0.5 inch (1.27 cm) by 0.125 inch (0.318 cm) were produced with standard modulus carbon fiber (12,000 filament tows). In the pultrusion process, a significant reduction (54%) in average pull force was achieved. Carbon fiber volumes up to 68.4 volume % were enabled by the low pull force when the nanosilica was included in the epoxy. Pultruded articles with ele- vated flexural modulus (up to 21,000 ksi (144.8 GPa)) were produced due to the high carbon fiber volume enabled by the inclusion of the nanosilica. The addition of nanosilica provided significant pultrusion process im- provements as well as mechanical property improvements in the peak shear stress (Short Beam Shear Test) and flexural failure stress (3-Point Bend Test) in pultruded articles made with the nanosilica- modified epoxy resin.

    1. INTRODUCTION

    1.1 Background

    The effect of fillers on the pultrusion process and on pultruded composite properties has been studied expe- rimentally and published in the technical literature. Traditional fillers for pultrusion include a variety of clays, such as kaolin clays for corrosion resistance, elec- trical insulation and surface finish, and other fillers such as calcium carbonate as a low cost volume extender and alumina trihydrate for flame or smoke suppression (1). Fillers are commonly included in pultrusion resins to af- fect die compaction, processability, surface finish, fire retardance, electrical properties, and mechanical proper- ties, to reduce adhesion of the resin to the die and to reduce the exotherm experienced in thick pultruded parts. Frequently, the filler type, concentration, and sur- face treatment, if any, have a significant effect on the viscosity of the system and can have a profound effect on pultrusion processability.

    In many cases, formulated pultrusion resins with higher viscosity produce a greater pull force in the pul- trusion process due to the viscous drag acting between the fibers and the die wall (2). The pull force limits the line speed and fiber volumes that can be achieved. A pultrusion study of kaolin clay filler in glass fiber- reinforced unsaturated polyester resins (UPR) showed increasing pull force as the clay concentration was in- creased from 20 parts per hundred resin (phr) up to 40 phr (3). The study also showed that the increasing con- centration of low-profile additives which prevent shrinkage in the second half of the curing die also re- sulted in increases in pull force for the glass fiber-UPR composite (3).

    In addition to affecting the processing, fillers also play a role in the flexural strength and short beam shear strength of pultruded composites. In the kaolin clay – UPR pultrusion study mentioned previously, the shear strength of the pultruded composite decreased approx- imately 10% as the filler concentration was increased from 20 phr to 40 phr. In the same study, no statistically significant effect was shown on the flexural strength as the clay content was manipulated over the same concen- tration range (3). In a different study of 7 different types of fillers (including kaolin clays) in epoxy resins pul- truded with glass fiber, the sample with the lowest filler loading (5 phr) also showed the lowest short beam shear strength (4). In the epoxy study, the samples that were most highly loaded with filler (20 phr) showed the high- est short-beam shear strength (4). The composite flexural strength in the epoxy-glass composite study de- creased with increasing filler content for the two comparisons reported (4).

    Recently in the composites industry, new technolo- gy in fillers, and particularly in nanomaterials, have provided attractive property improvements in compari- son to those achieved with more traditional filler

  • COMPOSITES 2011 2

    systems. In particular, the nanomaterials are more able to produce uniform concentration and homogeneous re- sin properties in composites than the traditional fillers that can experience filtration by fibers due to their large particle size relative to fiber-fiber spacing. As one ex- ample, polymer - clay nanocomposites were shown to improve thermal stability, fire retardance, barrier proper- ties, flexural modulus strength and toughness. Additionally, recent work on prepreg-appropriate resin systems has shown that the inclusion of nanosilica in neat resin epoxy castings provides increased neat resin modulus and, correspondingly, increased compression strength in the unidirectional composite laminates made from the resin (5).

    The current paper is focused on nanomaterial ma- trix modification also, but for composite processing requiring resins with significantly lower viscosity than prepreg resins, specifically resins for pultrusion.

    1.2 Current Study and Resins

    This paper outlines the unique processing benefits and mechanical property improvements of nano-scale spherical silica-filled resin technology for pultrusion processes and products. This appears to be the first eval- uation of the effects of nanosilica on a pultrusion resin system. Comparisons are offered between composites made using a control resin with no silica nanoparticles, but having a common kaolin clay additive, and a nanosi- lica-modified epoxy containing 32.6 wt% nanosilica (but no clay) in the cured resin. Resins formulated for the current study were used for experiments run on a commercial pultrusion manu- facturing line. Pull force was monitored as the fiber volume fraction of the composite was varied. The mi- crostructure of the cured composites was examined, and key mechanical properties of the composites were com- pared.

    For the control sample in the pultrusion experiment, 5.0% by weight of a typical kaolin clay used in pultru- sion was included in a DGEBA-based epoxy in order to represent a typical epoxy pultrusion resin. No clay was added to the nanosilica-modified epoxy resin. The na- nosilica, which is less than 100 nm in diameter, had been modified to be compatible with the epoxy resin. The na- nosilica-modified epoxy resin and the control resin were each cured with the same stoichiometric amount of an MTHPA-based anhydride that had been formulated for pultrusion, resulting in a 0.95 ratio of equivalents of an- hydride to equivalents of epoxy.

    2. EXPERIMENTAL

    2.1 Resin Sample Preparation

    For the control resin sample of the pultrusion expe- riment, 94.9 parts by weight of the MTHPA-based curative (formulated for pultrusion) were blended with

    0.10 parts by weight of kaolin clay and 100 parts by weight of the DGEBA-based epoxy resin. This mixture was blended manually until homogeneous.

    Separately, 51.6 parts by weight of MTHPA-based curative (formulated for pultrusion) were blended with 100 parts by weight of the nanosilica-modified epoxy, and this mixture was blended manually until homogene- ous.

    2.2 Viscosity of Liquid Resins

    The initial viscosity of the resin-curative blends was measured immediately after the resins were mixed, before they were used in the pultrusion experiment. A viscosity measurement was taken using a Brookfield DVII (Middleboro, MA) with the RV spindle #4 and 20 rpm rate.

    The initial viscosity of the control resin was meas- ured as 1.6 Pa-sec and that of the nanosilica-modified epoxy system was 3.5 Pa-sec.

    2.3 Pultruded Carbon Fiber Composite Manu- facturing

    The pultruded carbon fiber composite parts were produced on a commercial pultrusion manufacturing line equipped with an open-bath wet-out system. A standard modulus carbon fiber with 12K tows (12,000 filaments per tow) was stocked on a creel without tensioning con- trol and was fed to the wet-out system through a series of guides and eyelets. The pultrusion die was 91 cm long and had a rectangular cross-section measuring approx- imately 0.5 inch (1.27 cm) wide by approximately 0.125 inch (0.32 cm) high. The die had a first heating zone set at 160 ºC followed by a second zone set at 182 ºC. A load cell was mounted on the pultrusion line such that the pull force exerted on the die was measured and indi- cated on a digital display, and recorded manually. Carbon fiber volume was controlled during the ex- periment by the addition or removal of individual 12K tows of the standard modulus carbon fiber. The results presented in this paper include composites made with carbon fiber volumes ranging from 60.1% to 64.3% for the control resin system, and ranging from 64.3% to 68.4% for the nanosilica-modified epoxy resin system. The fiber volumes listed here are calculated on a wet ba- sis based on the measured di