Plasma surface modification of UHMWPE fibres and its effects on adhesion and wettability

Abstract

Ultra-High Molecular Weight Polyethylene (UHMWPE) fibres are gel-spun, highly drawn polymeric fibres that offer excellent strength and modulus, chemical resistance, and impact resistance with low density. However, due to their non-polar surface character, polymer matrix composites employing these fibres have a low interfacial shear strength (IFSS), leading to a low interlaminar shear strength (ILSS). This project aims to study the surface modification of UHMWPE fibres at micro and molecular levels for improved wettability and adhesion with thermosetting matrices using a plasma treatment (reactive ion etching) route.
In this study, an in-depth characterization was conducted on UHMWPE fibres and tapes in an untreated and plasma-treated state and the effects of plasma treatment were compared. Surface modification of samples was done using the reactive ion etching (RIE) process, where Ar-O₂ plasma (1:5 standard cubic centimetres per minute) was used at 30, 50 and 100 W RF power. Samples were exposed to plasma for 10, 15, 20, 30, 60 and 300 seconds. FTIR results confirmed the presence of various oxygen-bearing functional groups such as -C=O at 2320-2220 cm⁻¹, -C=O carbonyl stretches of ketones, esters, and aldehydes at 1780-1700 cm⁻¹, and -C=C- at 1620-1520 cm⁻¹ due to chain scission of main carbon C-C bonds. XPS confirmed an increase in atomic oxygen content because of plasma treatment.
Wettability is determined by the interaction between the fibres and the resin. Untreated fibres and tapes showed mildly hydrophobic character with a water contact angle (WCA) of 93.3 ± 3.3°. Plasma treatment increased surface free energy (γSV) from 36.4 ± 3.4 mN m⁻¹ to 71.1 ± 2.8 mN m⁻¹, and decreased water contact angle (WCA) to 35.3 ± 4.7°, indicating improvements in wettability of the fibres. Specifically, the polarity (γSVᴾ) of the surface increased from 0.4 ± 0.0 mN m⁻¹ to 21.3 ± 2.2 mN m⁻¹, which proved beneficial for improved adhesion of these fibres with the epoxy matrix. Due to hydrophobic recovery, γSV and γSVᴾ decreased by 30% and 40%, respectively, within the first 24 hours and up to 40% and 59% within seven days of plasma treatment. The solid-liquid (interfacial) tension (γSL) decreased while the work of adhesion (Wa), spreading coefficient (S) and wetting tension (ΔF) increased after plasma treatment, indicating favourable wetting conditions.
The interfacial adhesion between UHMWPE and epoxy increased from 2.2 ± 0.4 MPa to 5.1 ± 1.1 MPa, when samples were exposed to plasma for 10 seconds. A longer exposure time of 60 and 300 seconds lead to an increase in IFSS to 5.7 ± 1.0 and 5.9 ± 0.5 MPa, respectively. The variation in IFSS (𝜏𝐼𝐹𝑆𝑆) was found to be independent of surface roughness, indicating that the mechanical interlocking was not the sole reason for increased IFSS. Similarly, the interfacial frictional stress (𝜏𝑓) in the post-debonded region of the microbond curve was independent of surface roughness and remained nearly constant (𝜏𝑓 = 1.3 ± 0.6 MPa) for untreated and all plasma-treated samples. Droplets cured on single fibres with different cure schedules were observed to have mean values significantly different from each other, suggesting the influence of processing parameters on measured IFSS. The micro composite samples cured at 50, 65 and 80 °C were shown to influence the 𝜏𝐼𝐹𝑆𝑆 significantly.
Novel composite laminates with UHMWPE interlayers were produced and tested in three-point (short-beam) bend tests to determine ILSS. Carbon/epoxy laminates (L1) showed an ILSS of 88.1 ± 4.1 MPa, and those with untreated UHMWPE interlayer (L2) showed a decreased ILSS of 26.0 ± 4.5 MPa. However, with plasma treatment, the ILSS of the carbon/UHMWPE/epoxy laminates (L3) increased to 42.3 ± 6.2 MPa, suggesting improved interaction between the Dyneema® and adjoining carbon/epoxy layers. Plasma treatment was shown to change the failure mechanism of Dyneema®, i.e., the untreated samples showed a clean fracture. In contrast, the plasma-treated samples showed defibrillation of the Dyneema® tape, indicating improved adhesion.
Overall, this work has furthered the knowledge of how plasma modifies UHMWPE fibre’s surface and improves its adhesion and wetting properties with epoxy matrix by increasing γSV and specifically γSVᴾ. However, this improvement was limited, and the level of adhesion achieved was significantly less than that achievable for the carbon/epoxy or glass/epoxy system. Compatibility between UHMWPE and UV curable resins was explored. With further development in processing these resins at low temperatures, the use of UHMWPE in various sectors can be exploited.

Date of Award	6 Dec 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Michael R Wisnom (Supervisor), Ian Hamerton (Supervisor) & Mark Hazzard (Supervisor)