Resins for the Fast Zone

There are some realities of life that you can’t change, like taxes. Another is that an automotive supplier must have the ability to produce a composite part in a minute, or at least in minutes — not hours or days, the norm for composites in most other industries.

Meeting that part-per-minutes goal is a huge hurdle that a few composite fabricators have cleared, and many more are involved in development efforts, with OEM or Tier partners. Wider use of advanced composite parts — those made with continuous fiber —is being driven by fuel economy rules, and the penalties for failing to meet those rules, which have effectively reduced composites’ perceived high cost. Lightweight composite designs have been proven to meet safety standards. And, as consumers’ tastes change, OEMs are looking to offer more model variations on a basic platform, where composites’ lower tooling costs and design flexibility compared to legacy metals can be an advantage. Over time, automotive composite economies of scale will grow, and issues such as recycling and repair will be resolved.

Dale Brosius, chief commercialization officer of the newly formed Institute for Advanced Composites Manufacturing Innovation (IACMI, Knoxville, TN, US), is in the midst of tackling these issues: “We need grades of carbon fiber specific to automotive, faster cycles for orienting fibers in preforms or stacks to match the speeds of the resin systems, both thermoplastic and thermoset, and improved modeling and simulation for end-to-end simulations of design and processing, that are easily accessed by engineers and manufacturers.”

IACMI, as well as many other groups, are working to improve confidence among the automotive OEM community that composite materials (and processes) are ready for implementation on a wide scale, that is, vehicle platforms greater than 100,000 per year. The value of thermoplasticsnfor fast automotive production is well established, but new versions of thermoset epoxies are proving to process nearly as quickly. But, that said, part-per-minute cure demands are taking epoxies into unknown territory, and not all parameters are well-understood yet, says automotive industry consultant Gary Lownsdale of Trans Tech International (Loudon, TN, US), formerly the of Plasan Carbon Composites (Wixom, MI, US): “As we develop faster processing machines to match  faster cure times, process parameters become critical for characterizing resins.   Too many resin formulations are being rushed to the market before they are completely characterized and validated for available processes, and it is essential that machine suppliers are consulted early in the development process phase to ensure that  process requirements are captured and validated before resins are selected for production of CFRP parts.  Product-Process-Machines-Environment must all be considered by the product designer.”

Pushing the limits?

Fast  resins go hand in hand with speedy processing. Currently, the fastest processing methods for thermoset composites include resin transfer molding (RTM), which includes high-pressure RTM, wet compression molding — in which dry reinforcements are wet out with a resin and quickly transferred to a hot compression press — and prepreg compression molding, with and without a vacuum assist. Plasan Carbon Composites uses a fast compression molding technology called RapidClave that incorporates fast mold heating with hot oil. Overmolding of thermoset parts by injection molding methods can be included as well, for hybrid part approaches.  

Automotive composite suppliers are for the most part focusing on epoxy, because of superior adhesive strength and modulus, no creep, high toughness and good fatigue performance. (Snap-cure polyurethanes are another option, and can provide strength similar to epoxy, but are not covered here.) Resin chemists have gone way beyond traditional autoclavable aerospace epoxies, which were formulated for stable storage and long outtimes. Those require  temperature ramps, with dwells, to reduce viscosity and induce flow under slowly-increasing pressure in order to remove volatiles and mitigate thermal stress, to achieve very low void content. In contrast, the cure technology of fast-cure epoxies has been greatly accelerated by changing the “protonation” or catalytic amine reaction at the terminal carbon atom sites with proprietary additives, which results in a “thermolatent” system — that is, one that exhibits very low viscosity for a specified length of time, delaying the onset of cure and allowing easy flow to fully wet out the fiber reinforcement, coupled with fast reaction with the hardener as process temperature reaches a specific target.

Lownsdale asserts that while a 30-second cure sounds great, there is much to be considered before actually making parts. “All of the process parameters need to be taken into account, as to how you achieve your part T_g, to ensure the resin is compatible with your process.” In addition to the degree of cross-linking, he also points out that little is known about “dynamic viscosity,” that is, how does the resin viscosity change under all of the temperature and pressure conditions of the process: “This is a critical issue, and there’s a lot of confusion in the industry with very little information available because standardized testing is lagging behind the new processes.” nHe cautions composite processors to investigate these unknowns with their resin suppliers, to ensure that parts produced match OEM requirements and expectations. Part of the IACMI’s charter is to carry out such investigations of resin rheology and process, adds Brosius, who points to several projects already planned for fast-cycle thermoset parts.

So how to decide on a snap-cure resin, and whether it’s a good fit with your process? It comes down to working with a supplier, and trusting them to guide you through a selection process, as well as asking the right questions, says Lownsdale. We checked in with the makers of snap-cure epoxy, for both liquid molding and prepreg processing, for their perspectives on the industry, their products and where they see the industry going. Peter Cate, global strategic marketing manager for composite structures at Dow Automotive Systems (Horgen, Switzerland), says that “snap-cure” is triggered by the material’s exposure to tool temperature, but adds, “Snap cure alone is relatively easy to achieve but is only useful only if the resin has fully infused the fiber preform. The key is to know the latency, or the gel time before the material will no longer flow through the preform, to differentiate products.”

Dow introduced its VORAFORCE fast-cure automotive epoxy infusion resin platform at JEC Europe 2014. It includes the “workhorse” 5300 grade and several developmental grades, to suit customer needs. Cate asserts that VORAFORCE offers cure time as fast as 30 seconds, and the resin’s latency characteristics coupled with low viscosity (as low as 10 MPa/sec), maximizes infusion speed. The resin is intended for high-pressure RTM-processed structural composites, to replace steel and aluminum, although it can be used for body panels. Cate points out that cost-reduction benefits come from part consolidation applications: “The low viscosity and latency of our resin enables much larger parts to be infused in one shot.” He adds that the “thermal and chemical stability” of epoxy enables optimized part designs, with less material, whereas less able polymers require overdesign.

Working with equipment from KraussMaffei (Munich, Germany) or Cannon (Peschiera, Italy), Dow reports that wet compression molding separates resin application from the curing step, enabling molding cycle times as low as 30 seconds. Complexity is a limiting factor of the technology, Cate says, and it is best for relatively flat or lightly contoured parts. For more complex parts, he explains, RTM is the next best alternative. Dow currently has a part on a European car model. Several more will be commercialized in the near future, reports Cate, including projects for North American and Asian car models.

Keeping fiber architecture intact

Hexion (Columbus, OH, US) has offered a full range of optimized fast-cure epoxies, curing agents, and preform binders for several years, and continues to work with customers as structural composite programs progress, says Francis Defoor, global segment leader transportation. “The performance benefits of epoxy can meet safety demands as well as lightweighting objectives. Suspension systems such as leaf and coil springs are good candidates.”

Specifically, Hexion’s trademarked Epikote Resin TRAC 06170 with Epikure curing agent TRAC 06170 are targeted to structural parts made using RTM or wet compression molding. The combination results in a part-to-part cycle time of less than one minute, depending on part size and complexity. Epikote Resin TRAC 06400 series  is available for fast-cure prepregs as well, with cure time as low as 90 seconds, he adds, when cured at 130°C:

 “The result is a fast cycle, fast demoldnand excellent adhesion to glass, carbon and aramid. The secret, he adds, was a complete redesign of the resin chemistry, achieved by “rigorous control” of processing conditions and an optimized balance of viscosity and reactivity.

Hexion’s Epikote TRAC 06720 is a curable preform binder developed to minimize fiber deformation before or during injection. Fully compatible with RTM matrix epoxies, it co-cures with the part and, says Defoor, allows less preform material to be used, for faster layup and faster filling rate, since the fibers are firmly fixed.

A premium German automaker is using Hexion fast-cure epoxies for several structural parts on a sports car model, both flat and more complex shapes.

Balancing properties for mass production

“Rapid cure materials are a fundamental part of the solution, but the automotive industry needs far more than just a fast resin,” says Alexander Aucken, global automotive director  at Cytec Industries Industrial Materials (Heanor, Derbyshire, UK). Mass production requires  a balance of technical and commercial aspects. Materials, design for manufacture and processing must come together to create the most affordable solution. Technical factors that affect the issue  include not only the resin’s processing time, toughness and T_g, but also preforming needs such as highly drapable fabrics, affordable carbon fiber, automation,  recyclability and sustainability. “We’re working with key alliance partners to develop technologies that address all of these automotive cornerstones,” adds Aucken.

In terms of resin development, Mark Steele, R&D director at Cytec Industries Industrial Materials, reports that an 18-month R&D program has resulted in several sub-3-minute-cure epoxy resin chemistries for primary and secondary vehicle body-in-white structures.  “Our new multi-million dollar Application Centre, scheduled to open later this year, is being equipped to support these technology developments and will be used to showcase to OEMs and their supply chain ways to integrate carbon fiber part manufacture within their current infrastructures,” concludes Aucken.

“Instead of pushing new processes into an established industry, we are trying to work with automotive partners to determine the best way to support their existing manufacturing processes, and manufacturing speeds,” asserts Adam Harms, marketing manager for automotive at Huntsman Advanced Materials (The Woodlands, TX, US). Specifically, the speed of wet compression molding allows Tier 1 fabricators to more closely mimic existing sheet molding compound (SMC) molding or metal stamping processes. The company has worked for the past seven years with BMW on its i3 production car program, which uses Huntsman’s trademarked Araldite LY 3585 resin, coupled with Aradur 3475 hardener in a high-pressure RTM process. “The Aradur resin chemistry allows us to adjust the chemistry and the cure time for an optimized reaction, for a specific part,” adds Harms. “As the robustness of the resin system and processes continue to evolve, OEMs can now begin targeting more parts to convert into composites.” Several European OEMs are currently “aggressively” evaluating the Huntsman product, he adds, on programs potentially starting in 2017.

Adapting prepregs to automotive demands

Hexcel (Stamford, CN, US) offers a snap-cure prepreg, trademarked HexPly M77, that has a 2-minute cycle at 150°C (80 bar pressure) for a 5-mm-thick part. The low tack of HexPly M77 enables the prepreg to be cut into precise shapes by laser cutter, then robotically oriented, assembled and consolidated into flat preforms. Its T_g of 125°C enables the cured parts can be demolded while hot for a faster production cycle, says the company. M77’s long shelf life at room temperature helps companies meet lean manufacturing goals, as well.

HexMC, the company’s trademarked carbon/epoxy molding compound material — made with HexPly M77 — has also been adopted by the automotive industry. At 150°C, a complete cycle takes just 120 seconds from mold close to mold opening, for a 4 mm-thick part, but low viscosity and gel time allows the resin to flow and conform to mold contours while air is evacuated, for complex part geometries. M77 prepregs and HexMC (based on M77) are targeted for structural body-in-white parts and for aesthetic parts requiring a carbon fiber look. HexPly M77 can be used for body panels that are subsequently painted.While Lamborghini is already a Hexcel customer, using 90-minute cure products for the Aventador LP700-4 roof part, Hexcel says that its faster-curing products are certainly contenders for series production applications.

Looking down the road

Fast-curing thermosets for automotive apps will continue to be improved, says Dow’s Cate: “We see automotive customers becoming bolder in their plans for composites, and both process and chemistry have to adapt to keep up.” He adds that epoxy systems can be corrosive and tough on metal molds, so suppliers are working to try to produce more benign formulations, easier on equipment, which will help keep production rate high, without having to stop for maintenance issues. Hexion is confident that composites will be adopted in applications where they bring greater functionality, such as better fatigue properties, while maintaining fast cycle times.

According to Cytec, new technologies must include automated preforming systems, scrap reduction strategies, material recycling and affordable large-tow carbon fiber. Huntsman’s Harms says that a collaborative focus with composites machine manufacturers to jointly develop chemistry and processes for even faster cycle times is key, in order to achieve sub-1-minute cycle times and compete with existing automotive manufacturing processes. Hexcel believes prepregs and molding compounds will be developed with even shorter cure cycles and higher T_g together with reduced shrinkage and better bonding properties. And watch this space for announcements ofnew prepreg snap-cure forms for Class A and non-Class A structure.

Concludes Brosius, “Composites suppliers don’t have to follow the example of the BMW i3 and i8, and perhaps the way BMW is using carbon fiber on its new 7-Series platform offers a more practical path to reducing weight, in a multi-material vehicle.” Even small composite parts, used in high-volume production, obviously represent a huge opportunity for the composites industry. The prospects for more automotive composites are looking pretty good.