Liquid acrylic resin-based composites for marine and renewable energy applications

Abstract

Rapid growth in the installation of wind and tidal turbines has caused increasing usage of polymer composite materials to manufacture the blades, but the current use of non-recyclable thermoset materials makes composite waste a growing problem. The development of recyclable alternatives to traditional thermoset matrices, or new techniques for recycling these materials, is therefore a necessity. While established high-performance thermoplastics such as polyether ether ketone (PEEK) are recyclable and have excellent mechanical properties, leading to their use in the aerospace industry, the high temperatures and pressures needed to process them make their use in wind and tidal turbine blades prohibitively expensive. Recyclable alternatives should therefore be processable via vacuum infusion, which is commonly used with thermoset resins in the manufacture of wind and tidal turbine blades. Room temperature infusible acrylic resins, known commercially as Elium®, are one such family of promising recyclable resins, and are currently the only commercially available ‘drop-in’ thermoplastic alternative for the marine and renewable energy sectors.

Since liquid acrylic resins are relatively new to the market, several aspects of their use remain uncertain. For example, if acrylic-matrix composites are to be used in the tidal stream energy and marine sectors, they must be able to withstand long-term immersion in seawater without significant losses in mechanical properties. While published studies on water absorption in acrylic-matrix composites often rely on multi-compatible or other non-tailored fibre sizing agents, it has been suggested that improvements to the mechanical properties and water absorption behaviour of these composites may be gained with a sizing agent tailored specifically to acrylic resins. Additionally, beyond being simply a recyclable replacement to traditional thermosets, acrylic resins create opportunities to improve the manufacturing of wind and tidal turbine blades. Since acrylic is a thermoplastic, it can be welded instead of adhesively bonded, which may result in faster manufacturing with stronger, more resilient bonds.

Several aspects of acrylic resins relevant to marine and renewable energy applications are therefore explored in this thesis. Firstly, accelerated seawater ageing is applied to glass-fibre reinforced acrylic (GF/acrylic) and PPE-modified acrylic (GF/acrylic-PPE) composites, as well as to a traditional GF/epoxy baseline. The static mechanical properties (tensile, flexural, and short beam) are compared before and after ageing, and electron microscopy of the fracture surfaces to determine the effects of water ingress on fracture propagation. The diffusion coefficients of water in GF/acrylic (1.8×10⁻¹² m² s⁻¹) and GF/acrylic-PPE (3.4×10⁻¹² m² s⁻¹) were an order of magnitude larger than that of GF/epoxy (0.16 × 10⁻¹² m² s⁻¹), and analysis showed this would correspond to greater penetration of water into acrylic-matrix tidal turbine blades.

The tension-tension fatigue resistance of 0° GF/acrylic coupons with an acrylictailored sizing is then characterised, both in dry and water-saturated conditions. Comparisons are made to the performance of GF/acrylic composites with a multicompatible sizing agent to investigate the necessity of acrylic-tailored sizing agents. Water saturation was found to decrease the low-cycle fatigue performance of the composite, but performance was more similar in the high-cycle fatigue to which wind and tidal turbine blades are subject. For example, at 2 × 10² cycles, ageing reduced the fatigue strength by 21%, but this decrease was 14% at 2 × 10⁵ cycles. Differences in static and low-cycle fatigue performance were also observed between the acrylic-tailored and multi-compatible composites—for example a 12% lower dry UTS with the tailored reinforcement—likely due to differences in the fibre diameter and tow size, but their performance was again similar in high-cycle fatigue.

A novel method of joining acrylic-matrix composites is also explored. The solubility of acrylic polymer in its own liquid monomer creates the opportunity to ‘weld’ acrylic-matrix (Elium®) composites without the application of heat. In this method, termed resin welding, acrylic monomeric resin is infused between acrylicmatrix composite parts. The resin dissolves and diffuses into the acrylic matrix and creates a continuous material, and a strong bond, when it polymerises, without the sensitivities of traditional welding methods to adherend or bondline thickness. Single lap shear testing was conducted on resin-welded and adhesively bonded coupons with varying bondline thicknesses and filling fibres, and the bonding and fracture mechanisms were investigated using scanning electron microscopy and the diffusion of dyed acrylic resin. High strengths and low sensitivity to bondline thickness were observed, with the highest bond strength of resin-welded coupons reaching 27.9 MPa, which is 24% higher than the strongest weld reported in the literature. This indicates that resin welding is a promising alternative to traditional bonding and welding methods for acrylic-matrix composites.