| Fundamental problems overcome, but practical issues remain Better performance on the surface The specific working time, substrate compatibility and weatherability needs present in wind turbine blade production push existing adhesives to their limit. Today epoxy compositions are used, but other materials, including acrylates and polyurethanes, are being formulated to challenge their dominance and extend the promise of wind power further. Much wind turbine technology - like the fibre-reinforced plastics (FRPs) that make the blades - is inherited from aircraft manufacturing. Consequently manufacturers have also generally taken on epoxy adhesives1. These must safely bear large loads in the final blade, for example delivering an adhesive strength above 9.6 MPa in a single lap shear testing. They must also withstand a range of environmental conditions, in particular exposure to moisture and temperature cycling. One critical parameter in this regard is the adhesives glass transition temperature (Tg) for which one target is 70°C.
To give adequate power output blades must be as long as possible, but turbine blades that are too long cannot be economically transported to wind farm sites. Until recently blades have been made from two large moulded pieces, bonded together with aircraft-like structural adhesives, at sizes restricted by transportation limits. But in the interests of greater power output, there is a shift to making blades in two sections that are then transported to wind farm sites for assembly. Depending upon the precise method, it can be desirable for adhesives to become thixotropic very soon after being dispensed. These typically must quickly develop non-slump properties to allow them to be applied on vertical surfaces. Alternatively, the adhesive should be able to fill all the gaps in 20 mm blade bondlines, which requires viscosities less than 500 centipoise. In these cases, a mat of material can be used in the bondline to wick up the adhesive and hold it in place before curing. The adhesive must also allow blade sections to be precisely positioned, but also not delay manufacturing and installation processes.
To date, high bond strength two-component epoxies have been used that fully cure over as much as a week at room temperature, but can be accelerated by heating. Conventional adhesives based on epoxy resins alone did generally exhibit very high tensile shear strength, but were hard and brittle when cured3 The adhesive bonds obtained flake on exposure to peel, impact or impact/peel stress, particularly at low temperatures, causing the adhesive joint to fail. To tackle this they are typically toughened by dispersing rubber into their formulations.
Likewise, two-part epoxy resins can be comparatively slow to cure, but this is resolved by increasing the proportion of accelerator functional groups such as acrylic esters, mercaptans, and hydroxyl groups in the curing agent4. But even with these advances, commercial epoxies that take a week to fully cure offer a working time of just 20 minutes, when wind turbine producers might like an hour or more. Some epoxy systems cure so rapidly at temperatures above 32°C that they become almost unusable, whereas below 16°C slow curing poses corresponding problems. This is a major problem if assembly is to move to the field, where turbines are being erected.
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