Tuesday, July 1, 2008

Keeping It Green: Advances in Textiles for Vacuum Infusion Processing

Wind power’s portrayal as a 100% clean source of renewable energy bodes well for those seeking a power source with little environmental impact. Nevertheless, to stay true to this green promise, we mustn’t lose sight of the carbon footprint laid down prior to the generation of electricity. Composite materials, acknowledged as an enabler of the green promise, can contribute greatly to this footprint. In recognition of this reality, manufacturers’ have turned from open mold processing to closed mold vacuum infusion processing.

Closed-mold vacuum infusion processing of composites, is recognized as offering reduced volatile emissions, higher fiber content, and improved laminate quality relative to the baseline open mold wet lay-up process. Additional benefits include a low capital investment and an easily manageable learning curve. Accordingly, examples of its use in wind turbine construction abound, from rotor blades to nacelles. Recent advances in infusion specific textiles present an evolutionary step towards further reductions in manufacturing costs while reducing the carbon footprint for wind turbine manufacturers. The advantages these products pose extend beyond consideration of cost and carbon footprint, however, as they offer superior post-fabrication adhesive joining of composite structures.

Process Explanation
Vacuum infusion processing was born from the desire to mate the value proposition of aerospace closed molding to the needs of the commercial manufacturer engaged in open mold processing. In closed mold processing, liquid resin is injected using a pressure gradient. In vacuum infusion processing, negative pressure provides the gradient needed to motivate resin flow.

There are two basic vacuum infusion-processing techniques: surface infusion and inter-laminar infusion. In both practices, a flexible bag or membrane is sealed to a rigid mold to form the “closed” mold. As the closed mold is evacuated by vacuum, the bag collapses against the pre-form, consolidating it against the mold. While this consolidation promotes high fiber content in the final laminate, it does so at the expense of in-plane resin flow. Hence both practices employ a distribution medium designed to facilitate in-plane resin flow, allowing out-of-plane (through thickness) resin infusion to occur. The terms surface infusion and inter-laminar infusion denote the location of the distribution medium relative to the laminae pre-form.

In conventional surface infusion (Figure 1), a removable layer, commonly referred to as a peel-ply, is placed on top of the pre-form before applying the flexible bag and the distribution medium and/or perforated injection tubing is placed on top of the peel-ply. Once the bag is in place, vacuum is applied and resin is drawn through feed-lines into the mold across the distribution medium and through the pre-form. Upon resin cure, the bag is removed, as are the peel-ply and distribution medium, which are subsequently disposed. The peel-ply facilitates removal of the distribution medium while leaving a textured surface on the part for improved secondary bonding. To date, the greatest drawback of surface infusion has been the high waste and cost associated with the application, removal, and disposal of peel-plies and distribution media.


Figure 1. General Surface Infusion Illustration

In inter-laminar infusion (Figure 2), the distribution medium is integrated with other laminae in the ply stacking sequence, typically in the center or neutral axis, and maintains an open porosity while the laminae pre-form is being compressed under vacuum. Because the distribution medium remains within the laminate, the need for peel-ply can be reduced to areas where a textured surface is desired. Since the composite becomes the infusion pathway, placement of vacuum and resin feed lines is simplified and the post-process waste stream is reduced. Inter-laminar infusion is particularly proficient at infusing thick composites because its placement in the center of the ply stack halves the out of plane distance the resin needs to travel. To date, the non-structural nature of available distribution media has been cause for concern, even when used in the neutral axis.



Figure 2. General Inter-laminar Infusion Illustration


Advances in textile design have led to the development of a new class of distribution media known as Infusion Flow Reinforcements™ (IFR). Aptly named, this class of textiles facilitates infusion flow while contributing to the laminate as a constituent material. This shift in thinking broadens process control for inter-laminar infusion and affords the opportunity to eliminate the waste stream associated with conventional surface infusion. In inter-laminar infusion, ply stack placement of an IFR wouldn’t necessarily be limited to the neutral axis. Placement of IFR would be driven by optimum flow considerations (i.e. off-neutral axis or in multi layers in a thick composite). In surface infusion, IFR could be used as the last ply in the laminae, replacing the disposable medium, and potentially eliminating the need for peel-ply altogether.

Application Study

Recently, Polynova Composites participated in a test program to evaluate a commercially available IFR, HIFLUX-90™, as the last ply in surface infusion.
Constructed of high tenacity polyester fiber, the open nature of this textile, shown in Figure 3, assures a high degree of out-of-plane permeability, while its periodically raised or ribbed members (knops) lend a third-dimensional prominence to separate adjacent layers and ensure bi-lateral in-plane resin flow. Engineered with preferential in-plane flow in the weft [90°] direction, the product is well suited for high aspect ratio applications, such as wind blades, where the width presents the shortest infusion path. Further, its good hand and drape eases lay-up of complex parts.



Figure 3. Example IFR


The test program investigated the lap shear and cross peel adhesive performance of ITW Plexus® MA530, MA560 and MA590 methyl methacrylate adhesives. The adhesives were used to bond composite substrates: (i) identical peel-ply textured surface coupons; and (ii) identical HIFLUX-90™ surface coupons.

Both composite substrate panels were fabricated at Polynova Composites, of Milford, Massachusetts, USA, by surface infusion using Ashland Aropol 63301-10 INF polyester resin initiated with 2 wt./% Norox CHP. Both resin and room temperature were 23°C, and both composite panels were infused under 91.43 kPa vacuum. Table 1 identifies the substrate laminate schedules.



Table 2 compares the measured lamina and laminate weights and highlights for both panels.



Table 3 presents the measured post-fabrication waste stream associated with the use of the peel-ply, disposable distribution medium, and associated resin.




Tensile lap shear testing was performed in accordance with ASTM D 5868 and cross peel testing was performed in accordance with SAE J1553. For each adhesive, test specimens were created by dry rag wiping the substrates then bonding coupons at room temperature. The bond line was 0.0762 cm in each case.

Study Results
Table 4 contains all lap shear data for this study and Table 5 contains all cross peel data for this study.










Plexus® MA530, MA560 and MA590 adhesives produced quality bonds to both the peel-ply textured surface and the HIFLUX-90™ IFR surface, with a favorable Fiber-Tear failure mode noted for the HIFLUX-90™ surface.
Figure 4. Light-Fiber-Tear Failure

For the peel-ply textured surface (07501-3), the mode of failure was Light-Fiber-Tear (Figure 4) which occurs when resin and fibers are pulled free from the surface of one coupon while the adhesive bond-line remains intact on the other coupon. Such failures indicate good adhesion between the adhesive and the substrate, and point to substrate integrity as the limiting factor of obtained strength values.

For the HIFLUX-90™ surface, the mode of failure was Fiber-Tear (Figure 5). Fiber-Tear occurs when resin and fibers are pulled free from within the laminate of one coupon, while the adhesive bond-line and laminate remain intact on the other coupon. Such failures indicate good adhesion between the adhesive and the substrate, and point to substrate integrity as the limiting factor of obtained strength values; examination of the specimens’ reveal that the failure occurred within the E-2LTI 3600 laminae. Also there appears to be a strong correlation between the adhesive gel time and the depth of fiber tear, with longer gel times corresponding to greater depth. This observation holds true for both the lap shear and cross peel sample. This may be attributed to the resin rich surface provided by the HIFLUX-90™ and the extent to which the adhesive is allowed to etch into this surface as a function of time.

Conclusion

Social sentiment has undeniably shifted toward an emphasis on reducing industry’s environmental impact. Governmental policies increasingly reflect such interests. Composite materials are widely recognized as key enablers of the green promise. Ultimately, the wind energy industry must adopt composite processing techniques that truly fulfill the green promise by minimizing the carbon footprint created prior to generation of the first watt. Using Infusion Flow Reinforcements™ as the last ply in a surface infusion process enables the elimination of disposable waste streams, while enhancing post-fabrication adhesive joining of the composite structures in the intended application. These attributes pose a winning combination for the environment and manufacturers of wind turbines. While the Plexus® adhesives present a strong choice for adhering the two composite substrates in this study, it is recommended that customers prepare a testing protocol to determine the adhesives’ suitability for their particular applications and processes.


Lamina Legend:

E-BXM 1708 (Vectorply Corporation)
Fiber Type: E-Glass
Architecture: +45°/-45° Double Bias w/chopped strand mat
45°: 304 g/m2
-45°: 304 g/m2
Chopped Mat: 275 g/m2

E-2LTI 3600 (Vectorply Corporation)
Fiber Type: E-Glass
Architecture: 0°/90°/0°/90° Biaxial
0°: 627 g/m2
90°: 608 g/m2

HIFLUX-90™ (Polynova Composites)
Fiber Type: High Tenacity PET Fiber
Architecture: Proprietary
Areal Weight: 363 g/m2
Knop: Surface projection.
Knop down: projection facing tool surface

Econo Ply E (Airtech International):
Peel ply designed for use in more difficult environments or when a more textured surface is required for secondary bonding.

Resin Flow 75 (Airtech International):
High flow rate disposable surface infusion media

Affiliation:

Patrick Mack is Polynova Composites’ Chief Technologist.

Polynova Composites
229 East Main Street, Ste. 204
Milford, MA 01757 USA

Phone: 508-634-8181
Fax: 508-634-2922

pmack@polynovacomposites.com

http://www.polynovacomposites.com/