Researchers gauge impact of fibers in multiple applications
By Peggy Malnati
The May 12 composites session featured several talks characterizing fiber-breakage mechanisms and developing predictive models for discontinuous/short-fiber injection molding materials. It’s well established both that retained fiber length has a major impact on mechanical properties of composites and that, by its nature, injection molding provides many opportunities to break fibers.
Previous work had established that there are three fiber-damage mechanisms—fiber/fiber, fiber/machine surface and fiber/polymer interactions. Most damage occurs in the plasticization unit, and that damage is influenced by process settings like screw speed, melt temperature and backpressure.
Franziska Bürenhaus of the University of Paderborn, Germany, carried this work forward by examining the influence of process parameters on fiber length reduction in 20, 30 and 40 fiber-weight fraction (FWF) short-glass reinforced polypropylene (GR-PP) and 30 percent FWF polyamide 6 (GR-PA 6).
Working with a special press equipped with a removeable plate that allowed researchers to take samples along a 700-mm length of the screw, she documented the progression of fiber breakage in different transition zones. (Once samples were removed, the matrix was burned off, then fibers were washed, scanned and measured.) For GR-PA 6, samples were also taken at the injection nozzle tip.
The author documented interesting interactions between process parameters and FWF, including differences between the two polymer systems. Details are spelled out in her paper “Influence of Processing Parameters on Fiber Length Degradation During Injection Molding.” Bürenhaus said that future work will examine whether short-glass results translate to long-fiber thermoplastic (LFT) materials and that there are plans to develop a fiber-breakage model, including damage mechanisms seen in the mixing area of the melt pool and solid bed.
Another view of this issue was provided by Chao-Tsai Huang of Tamkang University, Taipei, Taiwan, in his talk titled “Study on the Flow-Fiber Coupling and Its Influence on the Shrinkage of FRP (fiber-reinforced plastics) Injection Parts.” A team made up of three Taiwanese universities and a machinery OEM conducted research into fiber microstructures like fiber length, orientation, and concentration and flow-fiber coupling effects using numerical simulation and physical testing on molded samples of GR-PP using three gating scenarios and with measurements made near, intermediate and far from gates. The fiber orientation behavior was verified by micro-computerized tomography scans and image analysis. Good correlation was achieved with the predictive model developed
Other interesting work presented in this session included Raveen John, University of Auckland, New Zealand, who discussed work characterizing the mechanical performance and machinability of natural-fiber PP composites reinforced with kenaf, jute or rice hulls; Jinchuan Zhao of the University of Toronto, who improved the mechanical, rheological, crystallization behavior and cell structure of structural foamed PP reinforced with nano-scale fibrils of polytetrafluoroethylene (PTFE); and Nabeel Ahmed Syed of Ontario Tech University, Oshawa, Ontario, Canada, who evaluated replacing steel cord-reinforced composites with carbon fiber-reinforced composites in a handrail to increase safety. The presentation that garnered the most questions was work by Solmaz Karamikamkar of the University of Toronto, who discussed multifunctional aerogels made with polymerized silica precursors that possessed either stiff or flexible backbones. Aerogels are used in many industries and future work with this system will include its use as the core of sandwich composites.
More composites research will be presented in a joint Composites/Building & Infrastructure session May 20.