Point-of-Use Resin Heating Reduces Cost and Improves Efficiency in Pre-Impregnated Composites

High performance composites consist of carbon, Kevlar, or glass fibers impregnated with a resin system, and cured into a shape using various molding methods. They are used extensively within the aerospace industry, as well as in sporting goods, automobiles, and wind power generation equipment. One widely used resin-fiber intermediate is called “prepregs” or pre-impregnated fiber sheets. These sheets are formed when a thermoset, heat-curable resin is combined with carbon fiber and partially cured in a continuous filming impregnation process. Prepregs are produced in factories under controlled conditions to ensure the carbon fiber-to-resin ratio is tightly managed and the resin formulations have consistent quality. Prepregs have been used extensively for decades to make composite parts for the most demanding and critical applications. 
 
As demand for high performance composites grows, traditional prepreg processes must be transformed from inefficient batch operations into more efficient, and higher-capacity automated processes. Many resins used in high performance composites have molecular makeups which result in their being thick or semisolid at room temperature. As a result, they need to be heated to be mixed into formulations that can be processed into prepreg intermediates. One area of focus which addresses the need for increased capacity is speeding up the resin preheating process. The benefits of a faster heating process are many: high quality end products and improved prepreg production logistics, and reduced resin waste, which is normally attributed to lengthy heating and reheating cycles. Heating only what is needed, when it is needed, is the primary goal of point-of-use resin heating. Point-of-use resin heating is a continuous heating process that offers a high-volume, cost-saving alternative to traditional bulk heating methods. In addition to saving costs, point-of-use heating enables new formulation technologies and faster, more continuous prepreg manufacturing concepts.
 
Prepreg production process
The prepreg resin and carbon fiber are actually a formulation of different resins, modifiers, and curatives, which are mixed together and poured or pumped by hand onto a filmer or coating machine, sometimes with the assistance of a transfer pump. The mixed resin formulation, called a premix, can either be frozen to suspend its reactivity for filming at a later time, or delivered to the filming process while it’s still hot. As the resin mix is coated onto a base release paper, a continuous supply of dry carbon fiber strands are pressed into the resin film, producing prepreg.  As the prepreg continues down the coating line it is cooled, and either cut into sheets, or rolled up. When the carbon fiber and resin mix cools, the resin solidifies and enters what is known as the “b-stage”. This is an intermediate stage where the resin is partially reacted and stable for a short period of time. To ensure the highest-quality prepreg product, with the correct carbon fiber-to-resin ratio, limited volumes of the resin premixes are created several times a day and delivered to the prepreg filming process. 
 
Preparing the resins for batch mixing
Resins are usually supplied in five- to 55-gallon (20 to 200 liter) drums, and heated using an external heating source, such as an oven, or a clamshell drum heater. The heating process for a 55-gallon drum can take as long as 24-48 hours. Once the preheat process begins in the drum, higher performance multifunctional resins will begin to advance, or self-polymerize, due to their autocatalytic or reactive nature. If these resins are kept hot for any extended period of time, they will age, resulting in properties which are out-of-specification, making them no longer useable. If a 55-gallon drum of resin is heated, but only partially used, the remaining material might age beyond its useable life and may need to be disposed.
 
Even in high-volume prepreg production operations, where waste is less of an issue, bulk heating of drums can be a logistical problem. The time-consuming nature of bulk heating means production demands must be carefully planned and monitored to ensure a constant supply of appropriately heated resin. In some cases, there is simply not enough heating space to stage as many drums of material as are needed for a full day of production. This is when point-of-use heating becomes not only a high value, but a necessity. 
 
When using point-of-use drum heaters, the drums of ambient temperature resins can be moved directly from ambient temperature storage shelves, and placed one at a time into a platen heating system and the resin is melted and pumped “on-the-fly” out of the drum. This technology heats and transfers only the resin that is in close proximity with the machine’s heated platen. The rest of the drum contents remains at ambient temperatures, and safe from advancing or aging.  
 
Once the drum is emptied, a second or tandem platen heating system can be in place to continue an uninterrupted supply of hot resin to the batch mixing process. This point-of-use heating process eliminates the need for large ovens and extended wait times involved when bringing an entire container up to temperature. 
 
Improving yield by temperature controlled premix delivery
If premix temperatures vary when they are put into the coating head of the filmer, coating thickness will vary, ultimately affecting the resin-fiber ratio of the prepreg product. (Figure 4) This effect is due to viscous shear. Viscous shear is the friction that occurs between boundary layers (knife and roll) at a certain flow velocity (coating line speed). As viscosity changes so does shear, and as shear changes so does the effective deposit of coating thickness. The coating head is a doctoring blade or roll that deposits the right amount of resin on a release paper in a continuous in-line fashion. Tightly monitoring resin film thickness is key to producing a consistent resin-fiber ratio of the final prepreg.
 
If the resin temperature increases, the viscosity decreases, resulting in reduced viscous shear through the coating slot. When shear diminishes, more resin can flow through the gap (under the knife), resulting in greater resin deposition on the release paper. Assuming a constant carbon fiber feed down the line will result in a resin-rich prepreg.  
 
Inversely, as premix temperature decreases, viscosity increases, creating more coating head shear, resulting in reduced resin thickness. These variations in thickness, if large enough, will translate ultimately into higher labor costs to constantly monitor and adjust coating gaps and, if not managed, rejected prepreg materials. To counter this temperature and shear variation, point-of-use heating pumps can be used to create a constant resin temperature delivered to the coating head, leading to a more controlled prepreg manufacturing process and ultimately higher yields. 
 
Point of use heating is a technology enabler
Another major advantage for inline or point-of-use heating techniques is the enabling of inline resin metering and mixing processes. Inline meter-mix concepts leverage continuous drum platen heating by supplying accurate resin flow to a continuous in-line mixing process, eliminating batch mixing of reactive materials and the associated cleaning reactive materials from expensive batch mixing vessels. Further, by eliminating batch mixing, faster reacting resin systems can be investigated to increase prepreg production throughput by curing faster in the final molding process. The figure at left shows a typical inline meter mix system supplied by a point-of-use drum heater. The reactive hardener is metered accurately with the hot resin and the two components are mixed in line using a static mixer. This provides a constant temperature flow of freshly mixed prepreg resin to the filmer with the advantages described above.
 
With the reduced waste, improved logistics, and enabling of next generation processes, point-of-use resin heating offers great promise as the logical composite industry standard resin heating method.
 
 
Mac Larsen is director of application development at Graco Inc.
 
Graco, Inc. | www.graco.com
 
Kevlar is a registered trademark of E. I. du Pont de Nemours and Company

Volume: September/October 2015