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Advances in VOC-Free, Renewable Adhesive/CoatingTechnology for Radiation Dosimetry and Elevated Temperature Applications

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posted on 2023-04-29, 22:24 authored by Wen JiangWen Jiang

  

The research and development for this dissertation focused on the VOC-free, biodegradable “green” polylactic acid (PLA) polymer for deriving tailored properties via thermal and ionizing radiation and targeted for the following three areas: (i) Deriving a first-of-kind wear-resistant distortion-free coating for luxury vinyl flooring tiles (to replace PVC), (ii) Deriving an ultra-low cost, rapid turnaround, and versatile gamma-neutron solid state detector-dosimeter (PLAD), and, (iii) for studying the possible use general purpose adhesion of various substrates at elevated temperatures. Collectively, the PLA based technology is referred to as PLATech. 

Polylactic acid (PLA) bio-renewable green polymer films were studied for their use as alternate to polyvinyl chloride (PVC) for deployment as the wear layer (WL) for luxury vinyl tiles. The WL is to be adhesively bonded to a composite layer comprising a thin ~0.08mm print layer (PL) bonded onto a relatively thick ~2.5mm ethyl vinyl acetate (EVA) backing layer (BL). Challenges included studying for: optimizing for the combinations of PLA polymer film types and thicknesses that are wear resistant enough, joined under hot press temperature, compression loading and time duration of compression for attaining key metric of adhesion related peel strength of over 354 N/mm (7 pli), with minimal (< 2% thickness) distortion of the PL-cum-BL for producing spatially uniform strength sizes ranging from 4”x4” towards 12”x12” LVT; additional challenges involved ensuring adequate WL abrasion resistance commensurate with residential and industrial uses, along with tensile strength and shrinkage similar to that for PVC. Abrasion resistance for the PLA WL varied with film thickness ranging from 450 cycles to over 4,500 cycles for film thicknesses of 0.075 mm and ~0.5mm, respectively. PLA WL tensile strength at failure was found to be 7x higher than that for PVC films of equal thickness (0.5 mm); however, the tensile elongation for PLA film at failure is ~12% versus ~200% for PVC film. High temperature-pressure (93-110℃; 1-3 MPa) combinations with press times ranging from 5 to 20 min. resulted in peel strengths ranging from 177 N/m (~3.5 pli) towards 810 N/m (16 pli). However, these parametric combinations resulted in significant PL disfigurement, BL melting and distortion with over 0.4mm thickness reductions. Peel strength variations were highly non-linear versus compression time, platen temperature/pressure and film thickness. Distortion-free, high peel strength 455-610 N/m(9-12 pli) adhesion and industrial grade WL abrasion resistance (>4,000 cycles) LVTs were successfully produced using 500 mm thick crystalline PLA film compressed onto PL-BL composite layers, with platens heated between 90.5-93℃, under 1 MPa loading, applied for ~5 min. duration. This combination of parameters also allowed scaling of sizes of the composite tiles from 0.1mx0.1m (4”x4”) to ~0.3mx0.3m (12”x12”). The onset of unacceptable distortion and thickness reduction occurs at/beyond ~ 95℃ and is attributed mainly to plastic deformation for the EVA-based base layer, and affirmed via DSC test results. Effects of PLA WL surface pre-treatment were studied for impact on peel strength of PLA-LVTs. Silicone layer coating nominally used by suppliers (and used for the bulk of the studies reported herein) did not lead to noticeable changes in bond strength when using a single PLA WL, but, surprisingly close to 100% increase was noted for PLA-LVTs produced using a double lap layer of non-silicone coated PLA films. Corona plasma discharge treated PLA WL surprisingly resulted in a significant (~90%) degradation of peel strength. Finally, sand paper roughened PLA WL showed no significant effect on resulting peel strength.

PLAD related studies were motivated from observations that PLA resin responds well in terms of mechanical, thermal and rheological property variations when subject to ionizing radiation in the 1–100 kGy (100-10,000 kRad) range–of significant interest in biomedical and general nuclear industry applications. Co-60 was used as the mainstay source for gamma irradiation. It was found that PLA resin responds well in terms of rheology and porosity metrics with an absorbed gamma dose (Dg). In this work, rheological changes were ascertained via measuring the differential mass loss ratio (MLR) of irradiated PLA placed within PTFE-framed (40 mm × 20 mm × 0.77 mm) cavities bearing ~0.9 g of PLA resin and pressed for 12–16 min in a controlled force hot press under ~6.6 kN loading and platens heated to 227 °C for the low Dg range: 0–11 kGy (0-1,100 kRad); and to 193 °C for the extended Dg range: 11–120 kGy (1,100-12,000 kRad). MLR varied quadratically from 0.05 to ~0.2 (1σ ~0.007) in the 0–11 kGy (0-1,100 kRad) experiments, and from 0.05 to ~0.5 (1σ ~0.01) in the 0–120 kGy (0-12,000 kRad) experiments. Rheological changes from gamma irradiation were modeled and simultaneously correlated with void-pocket formations, which increase with Dg. A single PLA resin bead (~0.04 g) was compressed 5 min at 216 °C in 0–16 kGy (0-1,600 kRad) experiments, and compressed 2 min at 232 °C in the 16–114 kGy (1,600-14,000 kRad) experiments, to form sturdy ~100 µm thick wafers in the same press. Aggregate coupon porosity was then readily measurable with conventional optical microscope imaging and analyzed with standard image processing; this provided complementary data to MLR. Average porosity vs. dose varied quadratically from ~0 to ~15% in the 0–16 kGy (0-1,600 kRad) range and from ~0 to ~18% over the 16–114 kGy (1,600-14,000 kRad) range. These results provide evidence for utilizing “green”/renewable (under $0.01) PLA resin beads for rapid and accurate (+/-5–10%) gamma dosimetry over a wide 0–120 kGy (0-12,000 kRad) range, using simple to deploy mass and void measuring techniques using common laboratory equipment. In addition to Co-60 gamma irradiations alone, irradiation studies were also initiated using the Purdue University Reactor (PUR-1) irradiation capabilities in order to further develop and characterize PLAD technology. It was found that the afore-mentioned MLR and porosity metrics did not offer the ability to monitor for neutron dose; this was overcome in scoping studies with an alternate metric, Ratio of Mass Dissolved (RMD) of irradiated PLA in acetone at elevated temperatures. It was found that below ~54℃, PLA dissolution correlated very well with gamma irradiation dose. At and above ~54℃ the combined effect of neutron and gamma irradiation dose appears feasible to determine. 

The effect of gamma radiation induced crosslinking the adhesion bonding strength at room temperature and elevated temperatures were studied. For room temperature applications, IngeoTM 4043D semi-crystalline PLA was first used to bond steel dowels. Bonding strength enhancement of ~28% was obtained with the addition of 10wt% Triallyl Isocyanurate (TAIC) and cross-linked under 20 kGy (2,000 kRad) gamma irradiation dose. For elevated temperature assessments, IngeoTM 10361D amorphous PLA was then used to bond aluminum lap-joint shear samples as a control. For studying for application for elevated temperatures, tests were conducted with addition of 3wt% TAIC and 50 kGy (5,000 kRad) gamma dose. Under shear loading, an enhancement of ~30% working temperature improvement was attainable raising the temperature at failure from ~70℃ towards ~100℃. Alternative crosslinking agent Dicumyl peroxide (DCP) was also explored and indicated little to no benefits. 

History

Degree Type

  • Doctor of Philosophy

Department

  • Nuclear Engineering

Campus location

  • West Lafayette

Advisor/Supervisor/Committee Chair

Rusi Taleyarkhan

Additional Committee Member 2

Ahmed Hassanein

Additional Committee Member 3

Hany S. Abdel-Khalik

Additional Committee Member 4

Janelle Wharry

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