Doctor of Philosophy
Chemical and Biochemical Engineering
Andrew N. Hrymak
Musa R. Kamal (McGill University)
There has been increasing demand for microparts in the areas of electronics, automotive, biomedical and micro-electro-mechanical systems. Microinjection molding (μIM) is becoming an important technology to fabricate miniature products or components to satisfy the ever-increasing needs of the above industries. Polymers and polymeric composites are ubiquitously adopted as molding materials due to their weight advantage, good processability and excellent resistance to corrosion.
Earlier studies have been primarily focused on the μIM of unfilled thermoplastics; however, microparts with multi-functionalities, such as electrical, thermal and mechanical properties are always accommodated by using multi-functional filler loaded polymer composites. Recently, μIM of carbon nanotubes (CNT) filled polymer composites has received much attention due to its commercialization potential. However, a comprehensive study is necessary to understand the properties of carbon filled micromoldings. Thus, it is important to understand the effects of the extreme shearing and cooling conditions which are common features of μIM, due to the very high molding temperatures and molding pressures as well as injection velocities involved and the very large surface area to volume ratio of microparts. These characteristics of the μIM process significantly determine the microstructure of the parts, thereby affecting the properties of final micromoldings.
In the present research, the influence of process parameters (i.e. melt temperature, mold temperature, backpressure and injection velocity), the types of carbon fillers (i.e. CNT, carbon black, graphite nanoplatelets and graphite) and the host polymer matrices (i.e. polypropylene, polystyrene, polyamide 6 and polycarbonate), the adoption of CNT filled immiscible polymer blends, and the hybrid carbon fillers loading on the electrical and morphological properties of carbon-containing microparts was systematically investigated. To this end, a rectangular mold insert which has three consecutive zones with step decreases in thickness along the melt flow direction was used to explore the effect of abrupt changes in mold geometry on the distribution of carbon fillers within subsequent moldings. To facilitate characterizations, the microparts were divided into three sections based on part thickness, namely thick section, middle section and thin section. The volume electrical conductivity for each section of carbon filled microparts was measured using a two-probe method, which was correlated with the development of internal microstructure, as characterized by morphology observations. Therefore, the study was carried out according to the sequence described below:
Firstly, the effect of process parameters (i.e. melt temperature, mold temperature, backpressure and injection velocity) on the properties of 10 wt% CNT filled polypropylene (PP) composites was explored by employing the design of experiments method. The distribution of maximum shear rates along the melt flow direction was simulated via Moldflow (Autodesk), and the state of distribution of CNT within each section of the microparts was examined using a high-resolution scanning electron microscope (SEM). The melting and crystallization behavior of microparts molded from unfilled PP and PP/CNT 10 wt% composites at different sampling positions along the flow direction was studied using differential scanning calorimetry (DSC). Results showed that there is no significant difference between the melting behavior for sections taken from both microparts. However, the crystallization process of unfilled PP taken from different regions of the microparts is temperature dependent (i.e. sections taken from high shear regions crystallize first), and such behavior is not significant for CNT-containing counterparts. In addition, the crystallization of PP was significantly accelerated with the presence of CNT. For example, the crystallization temperature for sections taken from the PP/CNT 10 wt% microparts is at least 8oC higher than respective sections taken from pure PP microparts.
Secondly, the influence of host polymer matrix selection and the loading concentration of CNT on the electrical and morphological properties of nanotubes-containing microparts was studied by adopting various polymer matrices based on their polarity and crystallinity. The adopted host polymers were PP, polystyrene (PS), polycarbonate (PC) and polyamide 6 (PA6), respectively. Results suggested that the choice of host polymer(s) could significantly influence the microstructure development within the microparts, thereby affecting the electrical conductivity. Additionally, the percolation threshold of polymer/CNT microparts shifted to higher filler concentrations, when compared with that observed for the compression molded counterparts. Furthermore, the thermal stability of PP/CNT and PC/CNT composites, as well as subsequent microparts, was studied using thermogravimetric analysis (TGA). Moreover, the melting and crystallization behavior of CNT filled PP and PA6 counterparts were evaluated using DSC.
Thirdly, the concept of selective localization of CNT in immiscible polymer blends was studied by adopting poly(lactic acid)/poly[(butylene succinate)-co-adipate] (PLA/PBSA) blend as the host matrix. The weight ratio of PLA to PBSA is 70:30 and the concentration of CNT is fixed at 5 wt%. Four different types of compounding procedures were employed to study the effect of compounding sequence of various components on the electrical conductivity of subsequent microparts. Results indicated that the electrical conductivity of PLA/PBSA/CNT microparts is invariably higher than that of CNT loaded mono-PLA counterparts, regardless of the component blending sequence. The prevailing high shearing conditions in µIM led to the coalescence of CNT-enriched PBSA domains, which is beneficial to the formation of conductive pathways along the flow direction, as confirmed by SEM observations. The thermal properties of CNT-containing blends and sections of associated microparts were characterized using TGA and DSC, respectively.
Fourthly, the influence of different types of carbon fillers on the electrical and morphological properties of microparts was carried out by adopting PP as the model host matrix. Different types of carbon fillers such as CNT, high structure carbon black (CB), graphite nanoplatelets (GNP) and graphite (i.e. synthetic graphite and low temperature expandable graphite) were employed as the conductive fillers. Results suggested that the evolution of microstructure is strongly dependent on the type of carbon filler used in µIM, which is crucial to the enhancement of electrical conductivity for resulting microparts. For example, the utilization of high structure CB and CNT is beneficial to the formation of conductive pathways within the molded samples. In addition, the adoption of large size low temperature expandable graphite (LTEG) is essential to the formation of intact conductive network, thereby enhancing the electrical conductivity moldings for subsequent moldings.
Lastly, the influence of hybrid carbon filler (i.e. CB and CNT) loading on the electrical and morphological properties of microparts was studied by systematically changing the weight ratio of CB to CNT in the PS matrix. A series of polymer composites with three different filler concentrations (i.e. 3, 5 and 10 wt%) at various weight ratios of CNT/CB (100/0, 30/70, 50/50, 70/30, 0/100) were prepared by melt blending, then followed by μIM under a defined set of processing conditions. Results showed flow-induced orientation of the carbon fillers increased with increasing shearing effect along the melt flow direction. High structure CB is found to be more effective than CNT in terms of enhancing the electrical conductivity. This was attributed to the good distribution of CB particles in PS and their ability to form conductive pathways via self-assembly. Furthermore, it was found that an annealing treatment is advantageous for enhancing the electrical conductivity of CNT-containing microparts, whereas the electrical conductivity of only CB filled counterparts decreased.
Zhou, Shengtai, "Microinjection Molding of Carbon Filled Polymer Composites" (2018). Electronic Thesis and Dissertation Repository. 5579.