Optimisation of anionic polyamide-6 for vacuum infusion of thermoplastic composites: choice of activator and initiator

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Abstract

A process for the manufacturing of large thermoplastic composite structures through vacuum infusion is currently being developed at Delft University of Technology. This article discusses the choice of activator, initiator and polymerisation temperature for reactive processing of the anionic polyamide-6 resin. It will be shown that this manufacturing process requires another activator–initiator combination than conventional casting. The concentration of the reactive species and the polymerisation temperature, however, determine the polymerisation rate and the physical properties of the polymer. For use in a vacuum infusion process, a hexamethylene-1,6-dicarbamoylcaprolactam activator and a caprolactam magnesium bromide initiator are suggested to allow for longer infusion times and to increase the compatibility with reinforcing fibres.

Introduction

Following international agreements on reducing CO2-emission, the Dutch government has formulated the ambitious target to have a 6000 MW wind power park installed offshore in 2020. A pilot program is initiated by the WE@SEA foundation (Wind Energy at Sea [1]), which reflects the combined effort of public and private interest towards realizing this target. In the next decades, more than 1000 turbines need to be installed in the North Sea. The two main requirements formulated by WE@SEA for the 3000 blades to be constructed are durability and sustainability. The blades should be more fatigue resistant to increase the current lifetime of 20 years and to reduce preventive and corrective maintenance visits, which are currently required 4–6 times a year. In the light of producing greener energy, considerable effort is made on the development of economic and environmentally friendly manufacturing processes, in procedures for installation and dismantling, and in the destruction and re-use of materials.

In that respect, a process for manufacturing large thermoplastic composite (TPC) blades is being developed at Delft University of Technology. Due to the higher toughness of the matrix, TPCs potentially offer a higher resistance to fatigue than their thermoset-based counterparts that are currently used, such as fibre-reinforced epoxies and vinylesters. In addition, TPCs can be remoulded upon melting, enabling re-use of the blade material. In order to produce blades with lengths in excess of 50 m, the currently most widely applied manufacturing process for turbine blades, vacuum infusion, is the preferred choice. This process utilizes a low viscosity resin that is injected into a mould with pre-placed dry fibres, followed by a curing step. Since the viscosity of thermoplastic polymer melts is too high, reactive processing is required: a low viscosity monomer melt is injected between the fibres together with an activating system, if required, followed by in situ polymerisation of the monomer to form a linear polymer with a sufficiently high molecular weight.

This article discusses the use of anionic polyamide-6 (APA-6) for the manufacturing of large composite structures through vacuum infusion. Since 1970's, this resin system has been extensively used for (monomer) casting of stock-shapes. After introducing the basic chemistry, the influence of the concentration and combination of activator and initiator, as well as the influence of polymerisation temperature on the reaction mechanism, the reaction rate and the final material properties is assessed. Finally, the choice of a suitable activator–initiator combination for two processes is discussed: (i) monomer casting of unreinforced polyamide-6 products and (ii) vacuum infusion of fibre-reinforced polyamide-6 composites.

Section snippets

Monomer

Anionic polymerisation grade caprolactam (AP-Caprolactam, see Fig. 1) supplied by DSM Fibre Intermediates, The Netherlands, was used in this study for its low moisture content (<100 ppm). The monomer was stored at 50 °C under atmospheric conditions to keep it dry without causing the monomer flakes to fuse together due to sublimation and recrystallisation.

Activators

Two types of activators were used in this study, as shown in Fig. 1. The first is monofunctional N-acetylcaprolactam (100%, ‘Activator0’,

Anionic ring-opening polymerisation of polyamide-6

The anionic ring opening polymerisation of caprolactam into high molecular weight polyamide-6 (PA-6), see Fig. 3, is a catalysed reaction performed at 130–170 °C [2]. Final conversions of up to 99.3% can be obtained [3] in 3–60 min, depending on the type and amount of activator and initiator added. Due to the anionic nature, the reaction is easily terminated by proton donating species, such as for instance moisture [4]. Therefore, storage and processing have to be conducted in an absolutely

Conclusions

The effect of different activator–initiator concentrations and combinations, and the polymerisation temperature on the anionic ring-opening polymerisation of polyamide-6 has been assessed. It was shown that, whereas the reaction mechanism is more or less determined by the activator–initiator combination, the concentration and polymerisation temperature have a larger influence on the reaction rate and the physical properties of the polymer.

Linking these observations to manufacturing of anionic

Acknowledgements

The authors thank B. Norder for assisting with the viscosity measurements, D.P.N. Vlasveld, A.A. van Geenen, M. Houben and A. Kerssemakers for the many useful discussions and S. Lindstedt for developing the processing equipment. Furthermore, DSM Fibre Intermediates (The Netherlands) and Ten Cate Advanced Composites (The Netherlands) are acknowledged for kindly supplying the materials.

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