(356d) Additive Manufacturing of Polypropylene/Hydrogenated Resin Blends: Effect on Crystallinity, Morphology and Mechanical Properties

Additive Manufacturing of Polypropylene/Hydrogenated Resin Blends: Effect on Crystallinity, Morphology and Mechanical Properties
Arit Das^1,+, Alexandra Marnot¹, Eugene Joseph¹ and Michael J. Bortner^1,*
1Department of Chemical Engineering, Virginia Tech, Blacksburg, VA 24061
*Corresponding Author: mbortner@vt.edu (Dr. Michael J. Bortner)
⁺Presenting Author: darit7@vt.edu (Arit Das)
Co-author 1: malex15@vt.edu (Alexandra Marnot)
Co-author 2: sboppfilm@yahoo.com (Dr. Eugene Joseph)

Abstract:
Additive manufacturing (AM) is one of the fastest growing processing techniques in recent times and is fast becoming an alternative to subtractive manufacturing or milling. The most commonly used method is the fused filament fabrication (FFF) due to its operational flexibility, facile maintenance, low energy demands, low material wastage and low cost¹. However, in spite of notable development in the FFF process, there is still a need to develop and study new materials that can be used for the FFF based AM process. Since becoming commercially available in 1958, isotactic polypropylene (iPP), a semicrystalline polymer, has become a commodity with widespread use throughout nearly all commercial and consumer industries. The crystallization behavior of iPP creates a significant challenge during extrusion based additive manufacturing because it crystallizes relatively rapidly and traps stresses in the layered parts, which in turn causes warpage². Therefore, it is of paramount importance to control the crystallization process in order to successfully 3D print materials using iPP. It has been reported in literature that the addition of low molecular weight hydrocarbon compounds to polypropylene shifts the onset of crystallization to lower temperatures, decreases the rate of crystallization and reduces the overall amount of crystallinity when cooling it from the melt³.

In this work, blends of iPP with two types of hydrocarbon resins having different degree of hydrogenation have been prepared and the morphology, thermal properties and crystallinity the blends have been studied. In addition, the effect of the amount of resin present in the blend on the above-mentioned properties has been evaluated. Non-isothermal DSC results showed that with increasing resin content the crystallization temperatures of the blends decreased from 121ºC (pure iPP) to 117ºC. A similar trend was observed for the melting temperature as well which decreased slightly from 165ºC to 161ºC. The degree of crystallinity decreased from 57% for pure iPP to 42% for the blends due to the addition of partially miscible and non-crystalline resins to pure iPP. The growth of crystal nuclei during the crystallization process was monitored using polarized optical microscope coupled with a hot-stage. The addition of partially hydrogenated resin was found to slow down the crystallization of the blends more effectively than the blends of completely hydrogenated resin. This may be attributed to the presence additional nucleating sites in the fully hydrogenated blends as compared to the partially hydrogenated ones. It is expected that by analyzing the change in morphology and slowing down the crystallization process, we might be able to improve self-diffusion within the blends prior to crystallization, since diffusion is hindered in the crystalline state. Understanding the thermal behavior and morphology of the blends at different temperatures will play a pivotal role in facilitating filament extrusion and subsequent extrusion based additive manufacturing.

References:
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2. Coxon, L. D. & White, J. R. Polym. Eng. Sci. 20, 230–236 (1980).
3. Cimmino, S., Silvestre, C. & della Vecchia, G. J. Appl. Polym. Sci. 92, 3454–3465 (2004).