Methods of Making 3-Dimensional Shaped Composite Structures

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1 Methods of Making 3-Dimensional Shaped Composite Structures Parvinder Walia, George Klumb, Jason Reese, Jack Hetzner, Dave Bank, and Keith Kauffmann The Dow Chemical Company, Midland, MI Abstract This work reports simple, cost effective methods of making shaped foam composites; eliminating the need for complex equipment, multiple molds and long cycle times. Shaped composite structures (specifically sandwich panels) are made by marrying cold forming of thermoplastic foam core and thermoset processing of skins. This combination is ideal since the thermoset processing conditions of low temperature and pressure are in a range that keeps the foamed core intact while simultaneously allowing the cold forming to be achieved. This technology affords a unique avenue to create sandwich and other composite structures that have curvilinear shape and 3-dimensionality. This is achieved in a single processing step that uses existing processing technology. Various process options are described in this paper. The reinforced thermoset skin material is either simultaneously or sequentially shaped with the foamed core. The process could be continuous or batch and can be based on typically used thermoset processing techniques like infusion, closed molding processes or continuous processes like pultrusion. The intent is to create shaped structures; while including design features and functional layers in the multilayered structure. Prototypes were prepared and basic properties of prototype sandwich panels are reported. The thermoplastic core used was IMPAXX energy absorbing foam (extruded styrenic foam) and the thermoset employed was polyurethane. Introduction The common composite sandwich structure is made up of two major elements, the skin and the core. Sandwich panel skins are constructed out of a variety of commonly used materials like wood, aluminum, and plastics. Common core materials include foam, balsa, and honeycomb. Cores are low in density and lightweight; but when combined with reinforcing fibers and resin, they become incredibly light, stiff, and strong structures. With sandwich panels, the strength and stiffness can be dramatically increased with very little added weight. Today, an endless amount of cores and skin combinations are available 1. Foam composite articles can be used for doors, wash basins, shower and bath surrounds, refrigerator and freezer panes, surfboards, pallets, doors, transformer mounting pads, automotive articles, and the like. Readily available in flat form, shaping (forming) and adding design features with these sandwich panels is relatively complex and challenging. Shaped foamed composites can be made by first foaming, shaping the foamed core and then applying the shaped skins. A foamed article may be shaped from a foam plank by various cutting methods (abrasive or hot wire, die, water jet), milling, thermoforming, continuous role forming (embossing), or combinations thereof. The shaped foam article may be foamed in the desired shape, for example by using partially foamed beads of a desired thermoplastic. The beads are then placed in a mold and heated sufficiently to expand the beads further such that they fill the mold and weld together. For example, foamed polystyrene made this way is typically referred to as expanded polystyrene (EPS). The skins can be applied in a variety of Trademark of The Dow Chemical Company Page 1

2 ways, including adhesively or expanding a resin between a pair of skin surfaces in situ. A substantially flat foamed core can be placed between formed top and bottom sheets 2. This involves a multi-step method by first thermoforming or vacuum forming an upper and lower thermoplastic sheet on their respective die faces and then inserting a pre-formed foamed article between the formed upper and lower sheets before the press closes. This method has the disadvantage that it requires multiple steps including pre-forming the skins. Alternately the thermoformed sheet can be placed within a cavity of a heated mold and the foamable polymer is injected into the cavity. However, this process is a multi-step, multi-mold process requiring a long cycle time because of the necessity to heat the final mold. In addition to producing skins by thermoforming or vacuum forming, foamed composite articles may by fabricated by blow molding or injection molding; two half-shells which are assembled with each other by gluing or welding; the hollow chamber comprised between both half-shells is then filled with foam, such as foamed polyurethane by the well known reaction injection molding (RIM) technology. These examples are illustrative of the varied techniques used to manufacture foamed composite articles. However, they suffer from a variety of drawbacks. It would be desirable to have a simple, cost effective method to make a foam composite article in which the article preferably can be shaped and the skins can be a different material than the foam core and the process does not require combinations of expensive and/or complex equipment, multiple steps, multiple molds, and/or adhesives to bond the skin(s) to the foam core. Scope of Paper This paper describes a unique (patent pending) 3 technology to making shaped foamed composite structures (specifically sandwich panels) by marrying cold forming of thermoplastic foam core and thermoset processing of skins. This combination is ideal since the thermoset processing conditions of low temperature and pressure are in a range that keeps the foamed core intact while simultaneously allowing the cold forming to be achieved. This technology affords a unique avenue to create sandwich and other composite structures that have curvilinear shape and 3-dimensionality. This is achieved in a single processing step that uses existing processing technology. Sandwich panels using low density foamed polymer core are limited in the ability to shape the foamed core. The low density is desirable for light weighting reasons. In general foams are quite resilient in compression but have little ability to withstand tensile elongation. This limits the ability to curve or bend the foamed sheet due to the tensile stresses on the convex side. In contrast, high density foamed polymer sheets are very difficult to bend or deform to any useful degree. Equally, it is very difficult to form such a foamed polymer sheet in a 3-d shape without the foam cracking or breaking on the tensile side of the sheet. Cold forming of thermoplastic polymers offer a rapid way of shaping; without heating the forming tool. This process is very economical since it can be run at high speeds and at room temperature. Various process options are described in this paper. The reinforced thermoset skin material is either simultaneously or sequentially shaped with the foamed core. The process could be continuous or batch and can be based on typically used thermoset processing techniques like infusion; closed molding like resin transfer molding (RTM), reaction injection molding (RIM), long fiber infusion (LFI); or pultrusion. The intent is to create shaped structures; while including design features and functional layers in the multi-layered structure. Page 2

3 Experimentation Prototypes were prepared and basic properties of prototype sandwich panels are reported. The thermoplastic core used was IMPAXX 5 energy absorbing foam (extruded styrenic closed cell foam) and the thermoset employed was polyurethane (PU) SPECTRIM. IMPAXX 5 energy absorbing foam is a 2.7 pcf density (ASTM D 3575) foam with a compressive strength of 79 psi at 25% compression (ASTM D1621). The sandwich panels were prepared by the procedure outlined below. A heated mold (165 F) was used to produce the prototypes. A polyethylene plastic sheet was laid down in the mold and then a sheet of non-woven glass mat was placed on top of this. A rigid polyurethane system was sprayed on to the glass mat (enough to sufficiently wet it out). IMPAXX foam was placed on top of this. Another sheet of non-woven glass mat was placed on top of the IMPAXX foam and rigid polyurethane was applied. A final polyethylene plastic sheet was laid down. Mold was closed with 25 tons of pressure applied and held for 3 minutes to allow the urethane to cure. The mold was opened and the part removed This was repeated with an insert added prior to closing to add shape to the part. Prototype samples were made 3 different ways: 1) Sequential Process: Cold form IMPAXX foam in mold; place glass mats on both side and spray PU on both sides. Place insert and Cure 2) Simultaneous Process: Start with IMPAXX foam (no forming); place glass mat on both sides and spray PU. Place insert on top and compress (form) and then cure 3) Sequential with spray skin lamination: Cold form IMPAXX foam in mold; place glass mats on both side and spray PU on both side. Place laminate and insert and cure Results and Discussion IMPAXX energy absorbing foam is a Dow developed extruded styrenic foam based on proprietary extrusion process technology. It is a strong, low-density, closed-cell foam. The process introduces a vertically oriented cell structure. This oriented foam technology is being used for improved impact energy absorption applications. However, another key advantage for IMPAXX foam is the ability to cold-form the foam 4. There are two possible options for shaping the reinforced thermoset skin. The skin can be shaped and cured either sequentially (after cold forming) or simultaneously with the shaping of the foamed core. The process could be continuous or batch and can be based on any of the typically used thermoset processing techniques. Some basic process options are outlined below: Batch Sandwich Forming Process (illustrated in Figure 1) (a) Simultaneous Processing 1. Spray thermoset resin with glass mat or glass fiber bottom layer 2. Lay IMPAXX foam 3. Spray thermoset with glass mat or glass fiber top layer 4. Compress (form) and cure 5. Paint, barrier, acoustic or other layers can be optionally added Trademark of The Dow Chemical Company Page 3

4 (b) Sequential Processing 1. Cold form IMPAXX foam by closing the mold 2. Add thermoset with glass mat or fiber formed core on either side 3. Close mold and cure 4. Paint, barrier, acoustic or other layers can be optionally added Continuous Sandwich Forming Process with cold forming and curing in simultaneous (Figure 2) or sequential manner (Figure 3). In addition, a laminating step can be added (Figure 4) 1. Start with IMPAXX foam sheet 2. Spray thermoset (PU, epoxy) on glass mat or mixed with glass fiber 3. Form and cure in heated shaped rolls 4. Paint, barrier, acoustic or other layers can be optionally laminated using the thermoset layer as adhesive or a separate adhesive layer Molding Tool Heating Optional Mold Closure (Compression & Cure) Mold Open & Ejection Finished Sandwich Part Skin IMPAXX Glass/PU Figure 1: Cold Forming/Curing Sequential Batch Process Cold Forming Thermoset resin+ Fiberglass Curing IMPAXX Foam Thermoset resin+ Fiberglass Finished Sandwich Panel Figure 2: Sequential Cold Forming and Curing Continuous Process Page 4

5 Glass Fabric Thermoset Spray Forming & Curing IMPAXX Planks Finished Sandwich Glass Fabric Thermoset Spray Figure 3: Simultaneous Cold Forming and Curing Continuous Process Thermoset resin Metal foil/ Plastic film Forming & Curing IMPAXX Planks Fiberglass Fiberglass Finished Sandwich Panel Thermoset Spray Figure 4: Simultaneous Cold Forming, Curing and Laminating Continuous Figure 5 shows examples of the prototypes made by the procedure outlined in the experimental section. Pictures are shown of a cold formed IMPAXX foam; sandwich panel prototypes made by sequential and simultaneous processes. Also shown is a prototype with a spray skin lamination added to the sequential process. The sequential process is easier to control with respect to the thickness of the reinforced thermoset layer. With the simultaneous process the resin can be more challenging to distribute evenly and requires optimum process control. Page 5

6 Cold Formed IMPAXX Foam Sequential Process Simultaneous Process Simultaneous with lamination Figure 5: Prototype Sandwich Panels made by Sequential and Simultaneous Processes PU connections in perforated foam Figure 6: Composite sandwich panel cross-sectional view Page 6

7 Force (lbs) The adhesion of the skin layers to the foam core was very good for the prototypes prepared. It needs to be pointed out that PU formulation used was not optimized with respect to the adhesion to IMPAXX foam. There is a penetration of PU resin into the surface of the foam skin that assists in mechanical adhesion. Chemical adhesion is not expected between PU and styrenic materials. The good adhesion between foam core and skin was validated in the 3-point bending test where failure mode was core breakage vs. skin delamination. An option to improve the adhesion is to use perforated IMPAXX foam (as shown in Figure 5). The perforations allow for the PU resin to flow through essentially connecting the skins together. The PU connections were confirmed by viewing the cross-sections of the prototype samples (Figure 6). The 3-point bending test data is reported in Figure 7 for flat sandwich panel samples. The plot also shows there is practically no difference in performance between the perforated vs. non perforated foam core based sandwich panels. There is a marginal improvement in stiffness and elongation to break with the perforated foam core samples. This is expected in this case since the adhesion between the skin and core is good Perforated Avg. Stiffness = 529 lb/in Non-Perf Avg. Stiffness = 58 lb/in Displacement 5 (inch) 5.3 Figure 7: Three-point bending test of prototype composite sandwich panels Page 7

8 Summary and Conclusions Marrying cold forming ability of the thermoplastic core with thermoset processing is an enabling technology (Dow patented) from a manufacturing and functional standpoint. The primary purpose of this technology would be to enable shaped and 3-d composite structures. Design features are readily incorporated with part manufacturing technologies such as edge finishing, folding, inserts and so on. Additional layers can be easily added for other functional requirements, e.g. aesthetics (paint, low gloss), structural, barrier, acoustic etc. The use of shape (e.g., ribs or corrugations) can further improve stiffness while maintaining light weight. Another desirable effect obtained was the ability to form sandwich panels without potentially utilizing any adhesive layers to bond the skin and the core. This simplifies the process and reduces cost. The bonding of skins can be further enhanced by the use of perforated foamed cores. The low viscosity thermoset resin flows into the perforations creating fibers that link the reinforced thermoset skins together. References Alesi, J. & Browning, R.L., Method of forming a plastic unit having an outer plastic shell encapsulating a foam core, USP 5,41,456 3 Patent pending 4 Patent pending Page 8