|NDT.net - January 2002, Vol. 7 No.01|
2.1 Fluorescent penetrants
Two main types of penetrants are in general use, water washable and post-emulsifiable. Water washable penetrants are characterised by the use of easily biodegradable surfactants that make effluent treatment easy, VOC content is eliminated and the product is odourless for the operator. The use of certain glycol ethers adds to the ease of rinsability but selection of this component needs care to avoid harmful derivatives that can cause operator discomfort, particularly headaches. Post-emulsifiable penetrants are more complex but can achieve very high sensitivities in fault detection. These products contain solvents that are selected to maintain the correct drying characteristics but which have acceptable flash points, to reduce fire risk.
2.2 Non-fluorescent penetrants
Red penetrants use intensely coloured red dyes, which can reveal surface faults in normal daylight. The use of food dyes is safer for the operator but the colour intensity is not sufficient to provide the desired sensitivity and high levels become very expensive. Rhodamines provide intense colours in smaller quantities but they have a tendency to stain the hands of operators and the colour is very difficult to remove. Diazo dyes have been used now for some years, giving very intense colours at low concentrations. However, some in vitro experiments have shown that certain enzymes will cause breakdown of the diazo linkages in the molecule to form amine derivatives, predominantly ortho-toluidines, which other work has shown may cause tumours. This mechanism has not been demonstrated with in vivo experiments and the very low solubility of the dye in water makes entry and absorption into the human body unlikely. Whereas the debate continues on the safety of this dye, certain national authorities have requested that the dye is no longer used. This has provided added impetus to look for alternatives and it is likely that this material will be phased out in the future.
2.3 Penetrant removers
Simple solvent removers are often based on hydrocarbons and therefore flammability issues need careful attention. Wherever possible, flash points should be at least 90 oC and preferably higher, although drying time can be an important negative in this case. Blends with other solvents are often used. Emulsifiers need to be highly efficient in emulsifying power and this property has been provided in the past with alkyl phenol ethoxylates. However, these materials are toxic to aquatic organisms at certain stages of biodegradation and alternatives need to be found. We are able to use much of the know-how in the detergents industry to find suitable alternatives. Another downside of aqueous emulsifiers is the tendency for bacterial contamination and the choice of suitable, safe preservatives provides another challenge to the formulator
Developers may seem, on the face of it, innocuous materials based on talc or something similar. However, the products are often dispensed via an aerosol and then flammability can become a concern. It is essential to provide a very thin and even film on the test area to provide sensitive detection of the penetrant. The dispersion of the developer in the dispensing device and controlled application onto the surface often require the use of volatile solvents and propellants. The use of CO2 as propellant may solve the flammability problem but creates others in terms of good dispersibility and maintenance of adequate pressure throughout the lifetime of the aerosol product. The solution often lies in the choice of co-solvents and propellant.
2.5 Aerosol products
The previous use of CFCs and the adverse effect on the ozone layer have been well documented. These are no longer used in NDT aerosol products. However, these propellants provided good dispersibility of particulate products such as developers. The alternative fluoro-hydrocarbon materials maintain good dispersibility without ozone damaging effects and contribute to low flammability even when highly volatile co-solvents are used such as acetone. Non-flammable product ranges in aerosol form are now available comprising penetrant, remover and developer.
2.6 The future
Operator safety and environmental issues will remain key concerns in all aspects of chemical NDT. A large part of R&D effort will be committed to maintaining continuous improvement of the products and ensuring that materials are used which are fully acceptable from these standpoints. New applications in aerospace, such as testing of composites, extending the temperatures at which NDT may be used, utilising thixotropic properties of penetrant solutions, using low concentration emulsifiers and decreasing drying times of developers will provide further challenges for the formulator who must always consider the safety of the user and the environment.
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