The team cut the prepreg scrap into individual rectangular chips of varying aspect ratios and geometries, and then compression molded them into flat panels and non-flat structures. Additional research investigated the conversion of chips into a continuous, flexible sheet and roll forms. Next, the team characterized the microstructure and mechanical properties of scrap-based laminates and identified feasible applications.

“We assessed different methods of converting the random shapes of uncured prepreg into chip form, first using a series of rotary and lineal cutters,” says Nilakantan. A clicker die cutting press also was trialed, which regularized the output and created what Nilakantan found to be an optimal rectangular chip size that’s 0.3 inches wide and between 1 and 2 inches long.

The team then researched the fabrication of sheets from the scrap chips since a sheet form would facilitate faster production and structural consistency through constant areal weight. In a production environment, chips are uniformly dispersed between sheets of backing paper and fed on a conveyor belt between heated pinch rollers of progressively decreasing gaps to create a uniformly thin sheet with a smooth surface. In a semi-continuous batch process, multiple stacks of chips and backing paper are compression molded under low heat and high pressure to create sheets. A low temperature was required to partially liquefy the resin film in order to bond the chips together, but ensure that the prepreg did not fully cure. High compaction pressure was used to generate a flat sheet. “Varying the parameters for chip geometry, orientation and consolidation, as well as the thickness of the final sheet, created the opportunity to customize for different applications we had in mind,” says Nilakantan.

The prepreg sheet, Infinipreg, can be processed through conventional methods such as vacuum bagging, hot pressing and autoclaving, although compression molding proved to be the most effective process for forming new shapes. Laminates fabricated from the scrap prepreg chips demonstrated excellent stiffness and strength retention compared to virgin prepreg counterparts.

In addition to their green appeal, scrap prepreg sheets can be processed with temperature ramps higher than that specified by the suppliers of virgin prepregs, which reduces overall cure cycle times. This upcycled scrap prepreg material form, which is ready for commercialization, holds the promise of reduced costs given the lower expense of the reclaimed material compared to virgin prepreg and the relative ease of manufacturing.

One of the initial prototypes manufactured by the USC research team was a composite prosthetic foot – called the Gazelle™ – made completely from uncured carbon fiber/epoxy scrap converted to sheet form. Other products manufactured included cell phone cases, skate boards, cylinders and foam-based sandwich structures for shipping containers.

The project wrapped up in July, and a technical paper will be published this fall with details on the effects of chip architecture and process parameters, an analysis of mechanical properties versus virgin prepreg and various other parameters that affect performance. “The technical publication should help commercial processors decide if upcycled scrap prepreg is right for their project platform,” says Nilakantan.

Expanding 3-D Printing Processes

Project: Digital nanocomposites
School: The University of Central Florida
Location: Orlando, Fla.
Principal Investigator: Jihua Gou

Though applications for additive manufacturing have been on the rise since the early 2000s, the method for creating objects by laying down computer-directed layers of material is still most associated with small consumer-oriented projects or larger demonstration projects. Jihua “Jan” Gou, a professor in the Mechanical and Aerospace Engineering Department and director of the Composite Materials and Structures Laboratory at the University of Central Florida, hopes to expand additive manufacturing – also referred to as 3-D printing and digital manufacturing – into large scale markets like aerospace, electronics, biomedical and automotive. To do that, Gou and a team of graduate student researchers created a new digital manufacturing process called spray deposition molding (SDM).

SDM uses multiple nozzles, contained in a vacuum chamber with a heated base, to create layers of material. One nozzle sprays nanomaterials, such as carbon nanotubes (CNT), carbon nanofibers, graphene flakes or carbon black that have first undergone sonication (applying sound energy to agitate particles), in a solvent with the aid of a dispersant. Another nozzle sprays a thermoplastic or thermoset polymer solution.