Rapid manufacturing of thermoset polymers and composites processed by frontal polymerization

Parikh, Nil Alpesh

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https://hdl.handle.net/2142/120530 Copy

Description

Title

Rapid manufacturing of thermoset polymers and composites processed by frontal polymerization

Author(s)

Parikh, Nil Alpesh

Issue Date

2023-04-27

Director of Research (if dissertation) or Advisor (if thesis)

• Sottos, Nancy R
• Geubelle, Philippe H

Doctoral Committee Chair(s)

• Sottos, Nancy R
• Geubelle, Philippe H

Committee Member(s)

• Tawfick, Sameh
• Baur, Jeffery

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Composite
• Manufacturing
• Polymer
• Frontal Polymerization
• Additive Manufacturing

Abstract

Lightweight thermoset fiber-reinforced polymer composites (FRPCs) excel as structural materials for high-performance applications in the aerospace, automotive, marine, and energy industries. However, conventional bulk-curing of FRPCs is an inherently costly, energy-demanding, and time-consuming process, which motivates exploration into more efficient curing schemes that produce economically sustainable composite components. Current solutions to this problem include the use of rapid-cure resins and time-temperature optimized cure profiles to reduce composite cure time from hours to minutes. However, these current solutions still require a large energy expenditure from using of autoclaves, ovens, and heated molds to facilitate thermal curing conditions. A solution that has garnered significant interest is frontal polymerization (FP) which permits a reduction in energy and time required to produce polymeric and composite materials by using the heat of polymerization to drive curing, effectively eliminating the need for a sustained external energy source. Frontal ring-opening metathesis polymerization (FROMP) of dicyclopentadiene (DCPD) has been previously utilized to fabricate neat polymers and FRPCs. Inclusion of fiber reinforcement and fillers reduces the exothermic energy produced by the system and quenches the reaction front at high loadings necessitated by structural applications. The research described in this dissertation is focused on integration of FP for the fabrication of carbon fiber-reinforced polymer composites (FRPCs) and polymers through conventional composite manufacturing and additive manufacturing (AM). The limit of FP used with carbon fiber reinforcement is driven by sustaining the FP reaction at increased fiber volume fraction (Vf) without debit to final composite properties. Therefore, we proposed a homogenized thermo-chemical model to simulate FP-based manufacturing of unidirectional carbon/pDCPD FRPCs. Front characteristics, front velocity (vf) and maximum front temperature (Tmax), were extracted from the model and validated experimentally. The model was then solved using a nonlinear finite element solver resulting in a maximum Vf =55%. To further gauge the limitations of FP cure, high Vf > 60%, carbon/pDCPD FRPCs are prepared using a single bag vacuum-assisted resin transfer molding (VARTM) technique combined with an isostatic press and cured using a linear lengthwise FP cure (4200J). The FP cure results in a cured 30 cm x 30 cm composite panel in 5 mins. The debits associated with increase in Vf are quantified. The DCPD resin system is modified to eliminate solvent and systematically vary the catalyst loading leading to a 28% increase in Vf (ca. 64%) while increasing the degree of cure (α) to 89%. An alternative triggering configuration was explored where the front transverses through the thickness (shortest distance) of the carbon FRPC. Additional energy was used to supplant the heat loss experienced for highly filled systems and subsequent characterization of the impact of energy input on resulting carbon/pDCPD FRPC properties. We imposed target temperatures, Ttarget, ranging from the onset of the reaction (Ttarget = 77°C) to temperatures observed during polymerization of the monomer system (Ttarget = 193°C). High energy inputs resulted in favorable final composite and thermomechanical properties (Vf=65%; Void Content (Vv) = 0%; α= 89%; Glass Transition Temperature (Tg) =157°C; Onset of Storage Modulus (E’onset)=155°C). A computational study complimented the work aimed towards determining the minimal energy required for a cure while attaining similar properties. The optimal curing of the composite resulted in energy savings of 25% while retaining similar performance. The amenable nature of the DCPD monomer permits the inclusion of various co-monomers and additives aimed at improving the performance and functionality of the resulting FRPC. For example, the limitation in interface strength is addressed in traditional and FP-cured carbon/pDCPD FRPC through inclusion of adhesion promoters in the DCPD to react with the surface functionality of the carbon fibers. In addition, the maximum operating temperature of carbon/pDCPD composites is addressed by inclusion of a norbornene-based crosslinking co-monomer resulting in a of Tg = 172°C. We extend the inclusion of FP to another FRPC fabrication method by developing a rapid-cure thermosetting carbon-fiber reinforced prepreg (RTP). We first investigated the impregnation of the FP resin within an AS4C (oxidized) and AS4C-GP (general purpose) fiber system. Using the liquid absorption coefficient (Ks), we determined the optimal parameters for the resin surface energy, contact angle, and viscosity to promote the fibers' impregnation while retaining the resin system's reactivity. A sweep of varying inhibition schemes aimed at reducing the inhibitor's accelerated decomposition and absorption into the reinforcement was explored. A formulation resulted in a prepreg material with more than 8 h of reactivity at room temperature and 45 days at -20°C. We expand the fabrication FP Curing to AM of continuous fiber-reinforced thermoset composites. We developed a novel printer head that facilitates the inclusion of a continuous reinforcing element by miniaturizing the prepregging process to impregnate, consolidate, deposit, and cure the material. The printed FRPC requires no post-curing or postprocessing steps to attain the full mechanical and thermomechanical properties. The AM system was designed to be amenable to any reinforcement and resin formulation that undergoes FP. Systematic variation of the processing parameters that facilitate printing and their effects on the mechanical and thermomechanical properties was explored.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

© 2023 Nil Alpesh Parikh. All rights reserved.

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