Cure monitoring techniques are used to improve the efficiency of processing, for quality assurance and to study the curing process. Such cure studies can prevent wastage due to failure of resin to react, use of incorrect proportions of resin components, poor mixing of resin, or incorrect processing conditions. This review focuses on in-line cure monitoring as a key way of optimising production.
Composite manufacturing methods vary from labour intensive techniques such as hand lay-up to capital intensive techniques such as autoclaving. The basic curing process is the same in each case: the liquid resin first gels and then becomes a glassy solid. If the curing process carries on for too long, degradation of the material can occur. On the other hand, if it does not proceed for long enough or at too low a temperature, insufficient curing takes place and the material properties are inadequate.
It is critical that the material remains in a more fluid state during the initial stages so that it can be readily manipulated, for example, in mould filling. Thus it is useful to know when gelation occurs and viscosity increases. Property measurement is a basis of many key techniques for monitoring cure. As well as viscosity, the glass transition temperature increases with the degree of crosslinking of the material. It is important that whatever is measured as a degree of cure relates to the final properties and thus quality of the end material.
Difficulties arise when cure is not uniform across a curing product. In this instance, some sections may be overcured and degrade whilst others are still undercured. This can typically happen when the curing reaction is strongly exothermic - local heat degrades the cured material. The solution is to undertake the main cure cycle using a relatively low temperature. This situation highlights the importance of good siting of cure monitoring sensors - a single location may not detect variations across a part.
The different methods used to monitor cure in-line are discussed in this review, from temperature measurement, through ultrasound, to fibre optics. Laboratory analysis is also briefly described, but the emphasis of this work is on practical application.
The review is accompanied by over 300 abstracts from the Polymer Library database on cure monitoring of thermosets and adhesives. This allows the reader to study the subject in greater depth. The abstracts are fully indexed with both subject and
1.1 Aims and Scope
1.2 Cure of Composites and Adhesives
1.3 Benefits of Cure Monitoring
2 Techniques Monitoring Thermal Properties
2.2 Thermal Conductivity
3 Techniques Monitoring Mechanical Properties
3.3 Fibre Optic
3.3.1 Extrinsic Fabry-Pérot Sensor
3.3.2 Fibre Bragg Grating Sensor
4 Techniques Monitoring Electrical Properties
4.1 Electrical Techniques
4.2 Dielectric Sensors
4.3 Interpretation of Dielectric Data
5 Techniques Monitoring Optical Properties
5.1 Refractive Index
5.2.1 Infrared Spectroscopy
5.2.3 Raman Spectroscopy
5.2.4 Comparison of Optical Sensors
6 Implementation of Cure Monitoring
6.1 Process Modelling and Control
6.2 Off-line Cure Assessment
6.2.1 Physical Property Measurements
6.2.2 Chemical Property Measurements
6.2.3 Comparison of Off-line Techniques
6.3 Quality Assurance
6.4 Comparison of Techniques
6.4.1 Technical Considerations
6.4.2 Practical Considerations
7 The Way Ahead for Cure Monitoring
Dr David Mulligan is currently Project Manager in the Materials Centre of the National Physical Laboratory. His current work includes a Department of Trade and Industry sponsored study of 'Cure Monitoring for Shorter Cycle Times'. David holds a doctorate in structure-property relationships in short-fibre materials and has worked as an applications scientist in industry.
NPL is a world leading centre in the development and application of highly accurate measurement techniques. As the UK's national standards laboratory, NPL underpins the national measurement system, ensuring consistency and traceability of measurements throughout the UK. Other areas of expertise include the design and characterisation of engineering materials, and mathematical software, especially its application to measurement and instrumentation