Since 2015, Kordsa has been working on feasible thermoplastic prepreg production technology.  The resin system used in thermoplastic prepreg can be applied in the form of powder, film  or granules. Some of the commercially available resins are PP, PA6, PA6.6, PPS, PEI, and PEEK. Glass fiber is the primary reinforcing fiber for price-sensitive markets such as the automotive sector.

When powder applications are used in thermoplastic prepreg production, homogenization problems arise in the distribution of resins. The high amount of waste occurring during production is also problematic. However, consistent and precise product quality is very important for customers and this is best provided by film application [1]. Kordsa performed experimental prepreg production trials with all the available technologies at different machine producers. Drawing on its experience and knowhow, Kordsa has started investing in automotive and industrial applications. Kordsa launched its woven based polypropylene and polyamide thermoplastic prepreg products as sheets with maximum dimensions of 1.2m x 1.2m. The resin systems were specially formulated by Kordsa.

The most important difference between thermoplastic and thermoset matrix systems is their thermal formability in terms of the reshapeability and recyclability of the thermoplastic matrix [2]. In contrast to thermoplastic materials, thermosets cure into the mold shape through thermal cross-linking, and they will not remelt after curing process [3]. Since thermoplastics do not crosslink like thermosets do, they conserve their thermal properties upon melting and reheating; subsequently, they do not degrade like thermosets unless the processing temperature is exceeded [4]. On the other hand, thermoset prepregs have enjoyed a long and glorious history in the aerospace and defense industries for more than four decades. However, there has been a growing interest in thermoplastic prepregs in the automotive, industrial, consumer goods, and sports and leisure sectors. This is due to their low cost, their lightweight and recyclable properties, and the ease of processing [5] [6].

The continuous fiber-reinforced thermoplastic prepreg market is expected to grow between 2018 and 2023 at a CAGR of 8.0% [7].  The usage of thermoplastic prepregs is currently very limited in the global prepreg market; however, the market is growing faster than the overall prepreg market. In the coming years, continued growth is expected, especially in the aerospace, defense and automotive sectors. Europe is expected to be the largest market for thermoplastic prepregs in the near future because of stringent regulations regarding fuel efficiency and carbon emissions, which are some of the key factors that are burgeoning the demand for thermoplastic prepregs. Bus bumpers, seat structures, fenders, TV tuners, front end carriers, battery cases, and underbody protection components are some of the automotive components made of thermoplastic-based continuous fiber-reinforced composites.

In thermoplastic prepreg production, reinforcing fibers such as glass and carbon fibers are completely impregnated with thermoplastic resin. Thermoplastic prepregs are used in composite part production through compression molding and over-molding production methods. In the compression molding method, thermoplastic composite parts are produced in a special press system which has heating and cooling capabilities. Compression molding is the most widespread process in the global thermoplastic prepreg market.

In the over-molding method, the woven thermoplastic prepreg is heated and shaped in the mold and then back-injected with a thermoplastic matrix in granule form. It can be used to add ribs for extra stiffness. An over-molding process can be integrated into an injection molding operation and combined with almost all special manufacturing processes. This contributes to functional integration and allows a more complicated part design. Thanks to the highly automated over-molding production method, thermoplastic composites can be produced in short cycle times of under 1 minute, which obviously brings a competitive price advantage. In addition to this, thermoplastic prepregs have many advantages such as lightweight, high fracture toughness, high service temperature, no need for cold storage, and reprocessing and recycling potential.

The biggest problem of thermoplastic composites is that the resin cannot penetrate the reinforcing fibers easily because of its high melt viscosity at melting temperatures when compared to thermosetting resins. The use of a prepreg is the most popular solution in the manufacture of thermoplastic composite parts [8]. In thermoplastic prepreg production, pressure and temperature have the greatest effect on impregnation and void content, which are key parameters affecting the mechanical behavior of the composite part. However, high temperature and pressure cannot be applied to some thermoplastic polymers, since these process conditions may affect the degradation of polymers and/or distortion of reinforcing fibers. This causes a decrease in the mechanical properties. Moreover, when using a thermoplastic resin with low melt viscosity and low molecular weight in order to improve its impregnation property, the mechanical properties of the composite part are also reduced. In the case of using a resin with a high degree of crystallization and high molecular weight in order to improve the mechanical properties of a composite part, melt viscosity increases and the impregnation decreases. In other words, the formulation of the resin system is the most important part of thermoplastic composite production in terms of adjustment of viscosity and impregnation level [9]. Specific wetting agents are added to the resin to improve the quality of impregnation.

In order to be successful in the thermoplastic composite market, design capability is a must for low-cost production, and collaboration is the most important requirement for the project-based development. Raw material suppliers, intermediate material suppliers, part producers and OEMs should work together in order to realize optimal commercial projects. It is expected that this market will grow, with increased attention to mold design and greater experience in part production.

 

Figure: The prototype of composite seat structure

Figure: Schematic representation of the overmoulding process

References

[1] Korkmaz, D., Bilge, E., Sarac, E.C., 2015, Thermoplastic prepreg production method, WO2017023225A3.

[2] Boria S. Energy absorption capability of laminated plates made of fully thermoplastic composite 2018;0:1–13. doi:10.1177/0954406218760059.

[3] Miller L, Soulliere K, Sawyer-beaulieu S, Tseng S, Tam E. Challenges and Alternatives to Plastics Recycling in the Automotive Sector 2014:5883–902. doi:10.3390/ma7085883.

[4] Feldman D. Polyamide nanocomposites. Journal of Macromolecular Science, Part A: PURE AND APPLIED CHEMISTRY 2017;54:255–62. doi:10.1080/10601325.2017.1282700.

[5] Zhao T, Palardy G, Villegas IF, Rans C. Mechanical behaviour of thermoplastic composites spot-welded and mechanically

fastened joints: A preliminary comparison. Composites Part B 2017. doi:10.1016/j.compositesb.2016.12.028.

[6] Yashas Gowda TG., Sanjay  MR., Subrahmanya Bhat K., Madhu P., Senthamaraikannan P., Yohesha B. Polymer Matrix-Natural Fiber Composites : An Overview. Cogent Engineering 2018. doi:http://doi. org/10.1080/23311916.2018.1446667.

[7] Lucintel, 2019, Market Report: Continuous Fiber Thermoplastic (CFT) Market

[8] Liu, B., Xu, A., Bao, L., 2015, Preparation of carbon fiber-reinforced thermoplastics with high fiber volume  fraction and high heat-  resistant properties, Journal of Thermoplastic Composite Materials, 1-14.

[9] Kim, J.W., Lee, J.S., 2016, The effect of the melt viscosity and impregnation of a film on the mechanical properties of thermoplastic composites, Materials, 9 (448), 1–15.