n Good compatibility with RTM epoxy resin matrices

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1 Application Bulletin Automotive EPIKOTE Resin Preform Binder Systems for Mass Production of Composite Parts Hexion Inc. has developed a range of EPIKOTE Resin preform binder systems tailored to RTM and high pressure RTM. Benefits n Excellent application behavior n Good compatibility with RTM epoxy resin matrices n Re-formable at lower temperatures n Fabric permeability is preserved n Variable stiffnesses / strengths available, including cross-linkable solutions for use with high pressure injection

2 In liquid molding processes such as Resin Transfer Molding (RTM), low viscosity resin is typically injected at low or high pressure into a closed mold containing a reinforcement preform. This preform is made by assembling and forming various fabric layers of continuous or discontinuous fibers into the shape of the desired composite part. During the manufacture of fabric reinforcements, powder binders are often applied to fixate the fibers (Fig. 1). Such stabilization makes woven or nonwoven fabrics easier to handle and cut, and more resistant to fraying. Figure 1. Application of preform binder during the fabric manufacturing Weaving Powder binder application Activation by heating Stabilized weave Binder stabilized fabric Lay-up Heating and pressure Stabilized preform Automotive

3 Once cut, fabrics are positioned and heated above the powder binder s softening temperature. They can then be easily hot-formed and draped into the desired preform shape. Final shaping and consolidation is achieved by applying suitable pressure and heat to the system. Alternatively, a lay-up can first be produced from fabric plies containing no powder binder (Fig. 2). To achieve pre-formability in this situation, a powder binder can be applied between individual plies, only in areas where needed, by, for instance, a powder spray method. In the final lay-up, suitable heat and pressure are applied for definitive shaping and consolidation of the preform. Figure 2. Application of preform binder during lay-up Standard fabric Lay-up process steps with powder binder application Heating and pressure Stabilized preform Today s most economical preforming aids are hotmelt powder binders. These are typically applied to the reinforcement at the time of manufacture, or during the preforming process. Such binders chief functions are to stabilize reinforcements and facilitate the preforming process without causing adverse effects. In the case of rapid RTM processing, it is particularly important that the binder is compatible with the resin matrix and does not dimvinish the reinforcement s permeability. In high pressure RTM, it is critical to produce preforms with high dimensional stability at the processing temperature and pressure, to prevent distortion of the fibers during injection. Fiber distortion in composites is generally to be avoided as it can be detrimental during manufacturing and result in flawed finished parts. Hexion has developed specific epoxyresin-compatible preform binders to meet the requirements for RTM and high pressure RTM.

4 Typical Applications and Processing of non-reactive preform binders EPIKOTE Resin 05390, and EPIKOTE Resin binder was developed as a universal tool for a variety of preforming applications. It has a softening temperature between C, excellent application behavior, and good compatibility with RTM epoxy resin matrices. This preform binder can be conveniently applied by a powder coating process onto continuous or discontinuous fiber reinforcement. The EPIKOTE Resin is typically applied at a concentration of 8 15 grams per square meter (gsm) surface coverage; at these application rates the preform binder has almost no effect on fabric permeability. The powder is first applied as a solid material. Then, the powder coated fabric is heated to above 90 C, to melt-bond the binder onto the fabric. Once the fabrics are cut, they can be positioned and draped into the desired preform shape. During this assembly process the binder can be re-softened by heating it to C as needed for easy shaping and hot-melt bonding to the adjacent plies. EPIKOTE Resin is suitable for fabric stabilization during RTM injection. It has a softening temperature between C. The powder coated fabric is heated to above 100 C, to meltbond the binder onto the fabric. EPIKOTE Resin is a preform binder in flakes for hot melt application. It has a softening point between C. The product is compatible to EPIKOTE Resin Typical Applications and Processing of reactive preform binder EPIKOTE Resin TRAC Preform binder EPIKOTE Resin TRAC was developed for more demanding composite process applications, where strong fiber preforms are needed, or essentially no fiber deformation is allowed after preforming. This higher level of performance is achieved by means of a reactive or crosslink-able binder. EPIKOTE Resin TRAC exhibits a softening temperature between C, as well as excellent application behavior and good compatibility with RTM epoxy resin matrices. At this temperature, the preforming process is reversible. Recommended application rates are 5 10 gsm because of the system s higher intrinsic stiffness. At these application levels, there is almost no influence of the preform binder on fabric permeability. A subsequent temperature increase to above 100 C will initiate cross-linking within the EPIKOTE Resin TRAC binder. In Figure 3 it is demonstrated that EPIKOTE Resin TRAC provides higher stiffness than the conventional binder EPIKOTE Resin at typical injection temperatures, even if not fully cured.

5 Figure 3. Curing of EPIKOTE Resin TRAC and EPIKOTE Resin at different temperatures and preforming conditions. EP TRAC g/m² Preforming: 160 C / 180 sec. RT 100 C EP TRAC g/m² Preforming: 100 C / 30 sec. RT 100 C EP g/m² Preforming: 80 C / 30 sec RT 100 C

6 Maximum allowed cure conditions to maintain a fabric/preform with sufficient workability. Table 1 Temperature ( C) Max. Time (sec.) Unlimited / no significant cure Recommended cure conditions to achieve substantial cross-linking of the EPIKOTE Resin TRAC Table 2 Temperature ( C) Min. Time (sec.) Table 1 shows recommended time limits at various temperatures, for controlling the cure of the EPIKOTE Resin TRAC binder sufficiently to sustain a fabric or preform s workability. Table 2 shows the minimum allowable time to cure the EPIKOTE Resin TRAC binder and achieve permanent fiber fixation. This fixation ensures that the preform will retain its shape during resin injection, enabling the reliable production of composite parts under demanding processes such as high pressure RTM. While the conditions outlined above have achieved good results, evaluation of various cure temperatures and cycle times is recommended, to balance line speed, equipment temperature and residence time during application and curing of the preform binder and production of the reinforcement preform.

7 Impact of the Preform Binder on Fiber Positioning The efficacy of EPIKOTE Resin and EPIKOTE Resin TRAC preform binders in preventing fiber dislocation during the resin injection step during high pressure RTM is shown in Figure 4. A glass fiber, multi-axial reinforcement on top of which individual carbon fibers were placed for visualizing purposes was treated with preform binder at 12 grams per square meter (gsm) surface coverage. An identical preform without the binder was prepared as a reference sample. The entire preform assembly was consolidated in a press at 120 C. For EPIKOTE Resin TRAC 06720, this resulted in cross-linking the preform binder. A fast curing RTM resin system consisting of EPIKOTE Resin TRAC with EPIKURE Curing Agent TRAC was injected using a high pressure RTM machine. The resin was injected with an in-mold pressure of 60 bars at 120 C. As shown in Figure 4a, the fibers of the untreated preform were washed away near the injection line where the flow rates are high. Figure 4b and Figure 4c, stabilized by preform Binders EPIKOTE Resin and EPIKOTE Resin TRAC respectively, showed little or no fiber movement. In particular, EPIKOTE Resin TRAC held the carbon fibers in place even at very high fill speeds (up to 200 grams per second), as can be seen in HP-RTM processing. Both binder systems, particularly the cross-linkable epoxy compatible binder, resisted movement of the fibers during resin injection and provided maximum process robustness. The effectiveness in resisting fiber distortion with the EPIKOTE Resin will obviously depend upon resin injection pressure, as well as on infusion temperature. The high molecular weight and high melting temperature of EPIKOTE Resin will provide a temporary protection against fiber wash out, even at elevated temperatures. For RTM processes during which high temperatures and/or high pressures are applied during the injection phase, the EPIKOTE Resin TRAC may be the preferred preform binder system. Given the additional preform integrity provided by the crosslink-able binder, it may enable process developers to increase flow / mold fill rates, further reducing the time needed for the RTM injection step. Figure 4. Glass / carbon fiber hybrid preforms injected using HP-RTM and various binder materials. a) No binder. Large deformation of fibers at injection port b) EPIKOTE Resin Little fiber movement at injection port c) Cureable epoxy compatible binder EPIKOTE Resin TRAC Little/no fiber movement at injection port.

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