All ETDs from UAB

Advisory Committee Chair

Selvum Pillay

Advisory Committee Members

Haibin Ning

Peter Walsh

Mark L Weaver

Charles A Monroe

Document Type

Dissertation

Date of Award

2018

Degree Name by School

Doctor of Philosophy (PhD) School of Engineering

Abstract

The Anionic Ring Opening Polymerization (AROP) of caprolactam to form polyamide-6 (PA6) or liquid molding for polyamide-6 (LMPA6) presents the opportunity for a new class of composite materials with excellent performance and wide applicability from automotive, energy, transportation, and aerospace. Despite the inherent advantages with the process, industrial adoption is virtually nonexistent. Additionally, the growing trend of reclaiming fibers can also be coupled with this process with relative ease. The following work will evaluate the liquid molding process, particularly for the use of recycled or reclaimed carbon fibers, in the following ways. First, the adaptability of the process to use recycled carbon fiber based preform was studied. It was compared to a process currently being used for recycled fibers and thermoplastics which is wet laid or hydroentanglement process. Preforms, made from 0.25” recycled carbon fibers, were processed either by compression molding of preforms with fibers and PA6 fibers made via hydroentanglement or by liquid molding of fiber based preforms. Three weight fractions were studied i.e. 30%, 40% and 50%. Fourier-transform infrared spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC) analysis of the synthesized PA6 from caprolactam showed similar characteristic peaks thermal transitions as commercially available PA6. Tensile and impact strength characterization for LMPA6 showed an increase in properties by nearly 10% and 30% for 30% and 40% weight fractions and almost a 120% increase at 50% weight fraction when compared to hydroentanglement/compression molded (HCM) samples . Scanning Electron Microscopy (SEM) of the fractured surface revealed fiber and fiber bundle pull out at higher weight fractions for HCM whereas it was predominantly fiber fracture in the case of LMPA6 indicating a better ‘wet-out’ of fibers. Second, the versatility of LMPA6 process was evaluated by strategic introduction of additional fiber tows and bidirectional fabric, namely tow in the center of preform (TC), fabric in the center of preform (FC) and fabric on the outside as skin (FS) were looked at. These variations were compared to the baseline which had no additional reinforcements. All the variations were prepared at 40% weight fraction. Mechanical characterization showed an improved behavior in its tensile and flexural strength by 3%, 20% and 47% for TC, FC and FS as compared to baseline. DMA analysis showed the effect of reinforcement on the storage modulus and tan delta across temperatures ranging from 30 0C to 150 0C. SEM analysis showed that the fibers and the additional reinforcements were coated with PA6 which translated to consistent mechanical performance. Third, Successful testing of permeability for discontinuous recycled carbon fiber performs specifically for LMPA6 were carried out. Three volume fractions i.e. 30%, 37% and 57% were examined and results show that as the volume fraction increases, the permeability of a liquid decreases and filling time increases. In house mold was machined and successfully used to simulate infusion conditions during LMPA6. Flow front development and total fill was recorded and compared to flow simulation study done on Moldex 3D. Flow simulation was in good agreement with experimental data and there was a deviation of about 5% in the total filling time when compared to experimental data. Based on this study, flow model for the filling time was developed for the tool used to make samples for mechanical characterization. Finally, a case study was conducted to showcase the approach to utilizing LMPA6 for manufacturing a part. The seat frame commonly found in all cars was looked at. The conventional design utilizing traditional materials and an alternative design made to take advantage of composites was successfully modeled on CREO 2.0. For the processing, the front frame was specifically looked at. Finite Element Analysis (FEA) of the two designs were conducted on a variety of materials. While steel showed a deflection of 4.41 mm, the two best composite alternatives were 60% by volume of Unidirectional Carbon fibers and a sandwich structure with a skin (1mm) of 60% by volume of Unidirectional carbon fibers and a core (18mm) of 37% by volume of discontinuous recycled carbon fibers. Their respective deflections were 2.59 mm and 4.99 mm. Flow and filling simulation was conducted on the pure unidirectional carbon fibers. Results show a complete fill in 2.19 sec for a tool with 5 inlet ports and an injection pressure of 1 MPa.

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