 View the latest medical engineering news
[April 2006] Bob Goss, Senior Technology Specialist at Henkel, discusses the bonding processes frequently used for medical applications.
Introduction
In the past, joining dissimilar materials and bonding certain plastics – both everyday situations when it comes to many medical applications – has been quite challenging. Fortunately, today, things have been made a great deal easier by the on-going development of adhesive technology. As a result, tasks which may have been considered beyond the scope of adhesives some time ago have now become commonplace. Having said that, the question still remains, which type of adhesive is best for what applications? After all, there is no such thing as a universal glue that can be used for every conceivable application. Perhaps the two most common adhesives to be considered in connection with medical equipment are instant adhesives (cyanoacrylates or CAs, for short) and UV curing acrylics. To those products can now be added Light Curing instant adhesives – a relatively new introduction that combines many of the benefits of these individual technologies. Before looking further at this innovation, it would be helpful to consider the original technologies.
Fast workers
Instant adhesives are recommended for bonding the types of plastic and rubber commonly found in medical equipment, such as ABS, PVC, latex, polycarbonate, styrene, acrylic and thermoset plastics. Typical application areas include the assembly of blood pressure transducers, endoscopes, hearing aids, IV sets, infusion pumps, catheters and diagnostic imaging equipment.
Cyanoacrylates are generally clear in colour and available in viscosities ranging from 3 mPa s to a gel consistency. The speed of the ‘instant’ cure depends on Relative Humidity – with the most effective results coming when there is 40% to 60% RH. The gap between the parts is also a determining factor, with CAs being most effective when such dimensions are less than 0.1 mm. The principle is - the thinner the bond line, the faster the cure and the stronger the joint.
Bonding ‘difficult’ plastics
Materials such as polypropylene, polyethylenes, silicone rubber, PTFE, Acetal and PEEK present a bonding challenge. While adhesives will cure on these surfaces, they cannot adhere properly. The reason lies in the fact that the surface of the plastic is such that it will not allow close contact with the liquid adhesive. For an adhesive to ‘wet’ a material (and therefore create an effective bond), it requires a lower surface tension than that of the substrate.
There are several techniques to enhance surface energy such as corona discharge, plasma etching and flame treating. But perhaps the most common method is through the use of chemical primers. Here, the primer is brushed or sprayed on the surface to be joined and, once the solvent has evaporated, the substrate is ready for bonding.
The use of a polyolefin primer when used with a cyanoacrylate on polypropylene should not be undervalued. Trials reveal that bond strengths in the order of 5 N/mm2 can be achieved - some 25 to 40 times higher than using the same adhesive without primer.
Figure 1 illustrates the surface energy values for several different substrates. For an adhesive to wet the surface, the surface energy must be greater than the surface tension of the adhesive. As the surface tension for a typical instant adhesive is between 33 to 35 milli-Newtons/metre (mN/m), the surface energy must be greater than 33 mN/m to create a useable bond. Since PTFE has a surface energy of only 15 mN/m it cannot be bonded without pre-treatment. PVC, however, has a surface energy of about 38 mN/m and can be bonded without surface pre-treatment.
| |
|
|
|
Figure 1. Surface energy
values for different substrates |
|
Making light work
The medical industry makes wide use of polycarbonate and PVC, and this has resulted in the introduction of visible light curing (400-420nm) UV adhesives. Such adhesives are very flexible and, in addition, some grades are fluorescent – which enables easy inspection of the bonded parts to verify product application.
UV curing acrylic adhesives (Figure 2) are ideal when it comes to bonding transparent substrates. Applied in liquid form, they cure in seconds when exposed to ultraviolet and/or visible light. Obviously, one of the parts must be transparent to the required wavelength of Ultra Violet – or no light will reach the face of the bond. Some grades of plastic (for example, polycarbonate) will filter out 365 nm – just the wavelength needed to cure many such adhesives. However, where they can be used, these acrylics offer the physical toughness of polyurethane, cure speedily, and will bond a range of substrates – including heavily plasticised PVC. Other benefits include a range of viscosities, solvent-free formulations, excellent gap-filling properties and high structural strength. A further benefit of UV curing acrylics is the ease of dispensing process. This can often be automated – thereby making the manufacture of medical products very efficient. They are ideal for bonding glass, stainless steel, thermoset and thermoplastics, resulting in typical applications such as cannula/hub bonding, blood filters, tracheal tubes, colostomy bags and appliances, cuff and tube assemblies and inflators. They are also to be found in the assembly of anaesthesia masks, fittings and gas outlets/inlets on oxygenators, cardiotomy reservoirs and heat exchangers, blood and centrifuge bowls, arteriograph manifolds, hypodermic and aniographic needles and many others.
| |
|
|
|
Figure 2. Manifold luer being
bonded using a UV light source |
|
The best of both worlds
Those designing for the medical industry have benefited from these two adhesive technologies. But if those two processes were combined, then the advantages would be even greater. Fortunately, that permutation is now available. Light Curing instant adhesives mix the rapid light cure of an acrylic adhesive with a secondary cyanoacrylate cure. In effect, they couple the adhesion and cure of standard CAs with the surface curing characteristics and the depth of UVs. Furthermore, the adhesives in the Loctite range are biologically tested and approved to ISO 10993 for use in medical devices. These new adhesives can be used on applications up to 80°C and require a fixture time of just 5 to 10 seconds for bonding polycarbonate. In addition, they can be used in conjunction with polyolefin primers - enabling an even wider range of substrates to be bonded. Medical applications for this new technology include balloon catheters, urinal bags and marker bands for catheters.
Making your choice
As we’ve seen, adhesive technology has progressed significantly in recent times. The bonding of materials that would have been considered difficult – if not, impossible - not many years ago is now an everyday occurrence. When choosing an adhesive it is important to look at the entire assembly process. This includes the preparation of substrates and the dispensing and curing of the adhesive. Alongside expertise on adhesive technology, your supplier should be able to offer advice in the areas of materials selection and appropriate dispensing equipment, and provide full support in process technology. Indeed, involving the adhesive supplier at an early stage will lead to the development of a reliable and efficient manufacturing process.
Bob Goss
Bob Goss gained a Mechanical Engineering Degree (Honours) at Liverpool University and then spent 9 years in the aircraft industry designing and developing cooling systems for specialised avionic equipment on military aircraft. He joined Loctite in 1985 and now gives Technical advice and support on a range of adhesives including UV, cyanoacrylate, anaerobic and epoxy adhesives to a wide diversity of customers from many different industries. Bob has presented a number of papers on adhesive bonding to many different organisations including The Institution of Mechanical Engineers, RAPRA, The Welding Institute (TWI) and The International Crystal Federation. He has also presented papers on adhesive technology to the Structural Adhesives Conference and written articles for a number of Technical magazines.
For more information please contact:
Henkel Loctite Adhesives Limited
Technologies House
Wood Lane End
Hemel Hempstead
Hertfordshire
HP2 4RQ
Telephone : +44 (0) 1442 278000
Fax: +44 (0) 1442 278071
www.loctite.co.uk
Email: technicalservice.loctite@uk.henkel.com Back to top
|