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12/05/2011
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Tom Shelley reports on a new technology for confirming the quality of adhesive bonds, particularly appropriate for the medical sector.
 A new system applies adhesive, cures it with ultraviolet light, checks whether it has been cured properly, and then if necessary, cures it some more, all within the space of a very few seconds. The initial application is with an acrylic adhesive specifically developed for bonding disposable medical devices, but the plan is to extend it other light curable adhesives used in products ranging from aerospace and automotive to consumer durables.
'AssureCure' has been developed by Henkel Loctite in the USA and was demonstrated to us at the recently held MEDTEC exhibition in Birmingham. The curing is initiated when light in the range 200 to 500nm, delivered by an optical fibre, is absorbed by photoinitiator molecules in the adhesive which fragment into free radicals. These free radicals react with the monomers to produce polymerisation.
The AssureCure system then sends another pulse of light down the optical fibre and uses FTIR – Fourier Transform Infrared Spectroscopy, to assess whether the photoinitiator has been used up or is still present and able to absorb some of the incident test light at certain wavelengths. The FTIR spectral response has previously been calibrated against mechanical needle pull tests, so the relationship between spectrum and cure is known empirically, rather than being based on assumptions.
As well as ensuring the mechanical strength of joints, it also ensures that the bonded joints are biochemically inert, reducing cytotoxicity from grade 4 (severe reactivity) to grade 0 (no reactivity). Alternative methods of verifying adhesive bonds include fluorescence, which can only confirm that adhesive is present and in what locations, and colour changing adhesives, which can only confirm that the adhesive has been exposed to light.
Mechanical testing of bonded components is normally destructive and so can only be applied to a proportion of bonded parts, all of which have to subsequently be discarded as scrap. Ultrasonic testing is appropriate for aerospace, but too slow and expensive for use in mass production. In practical terms, the AssureCure system is very compact, works with any PC or most PLCs, and sits on a bench top. It is also extremely rapid.
The verification typically takes about 20ms. Hence the system is highly compatible with high speed production. The first and so far the only adhesive it has been applied to is Loctite 3924AC, the AC referring to AssureCure system compatibility. It is a single component acrylic adhesive of medium viscosity which cures in a few seconds under the action of 365nm ultra violet light.
Glass transition temperature of the cured material is 61°C, Shore Hardness, ISO 868 Durometer D is 60. Elongation at break is 280%, tensile strength is 18 MPa and tensile modulus, 283 MPa. The adhesive is compatible with sterilisation using gamma rays, ethylene oxide or autoclaving. It is not, however, recommended to use with pure oxygen, chlorine, or other strong oxidisers. Cooling should be provided for temperature sensitive substrates such as thermoplastics.
The uncured adhesive has to be kept in the dark and dispensed using black feed lines. It has been tested bonding polycarbonate to polycarbonate, polyvinyl chloride to glass, and steel to glass. Test results all seem to refer to cannulas, tubes that can be inserted into the body, often for the delivery or removal of fluid or data. However, one of the other target applications is bonding automotive headlight assemblies.
UK marketing manager Colin Chapman says that the plan is to extend the technology to a wide range of the company's other adhesives.
Design Pointers• Testing takes as little as 20ms to establish that cure has been completed
• Bonded components can be 100% tested without risk of damage to products
• The system is compact enough to sit on a bench top and is used in conjunction with either a PC or PLC
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Author
Tom Shelley
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