MARKET PLACE
The advantages of change
Many manufacturing businesses have adopted techniques such as lean and just in time manufacturing to increase production and reduce downtime. Increasingly, they are also using modern adhesives for fixing parts together.
Almost everything that is made by industrial processes has component parts that must be fixed together and often traditional mechanical devices, clips or fixings such as screws or welding are used. Some manufacturers have not changed the design or the way they produce an end product in years. This may be because they subscribe to the belief that “if it ain’t broke don’t fix it.” However, using adhesives has numerous benefits.
Less components and assembly
Money can be saved by reducing the inventory of components required to make an end product. In addition, time and money can also be saved by improved assembly processes.
Durability and fewer operations
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Figure 1: PVC anaesthesia mask.
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A more uniform distribution of stress over the bonded area is achieved with adhesives. This means fewer and less severe concentrations of stress, which results in a more robust structure in which the risk of fatigue cracks and corrosion are significantly reduced.
Smooth and sleek appearance
With adhesive bonding there are no protruding fasteners such as screws and rivets. Adhesives can also provide the ideal solution to join together materials that may differ in composition, moduli and thickness. Furthermore, because adhesive bonding does not require high tempera-tures, it is a suitable means of joining together heat-sensitive materials such as plastics, films or thin sheet aluminium, which are prone to distortion or melting from heat sources such as curing ovens.
Reduced energy consumption and capital equipment expenditure
Some medical disposable manufacturers still use old solvent based adhesive technology to bond plastic substrates. Solvents are dangerous and the Solvent Emissions Directive (SED) 1999/13/EC is in place to reduce, and in time ban, the use of solvents in manufacturing. The SED mandate is, “To prevent or reduce the direct or indirect effects of volatile organic compounds into the environment, mainly into air and the potential risks to human health.” For more information: www. healthandsafety.co.uk or www.envirowise.gov.uk
Many solvents contain harmful chemicals and volatile organic compounds that are damaging to health and the environment. High performance fume extraction systems must be employed at individual workstations or along a production line to prevent harmful vapours from affecting the workforce. These systems consume huge amounts of energy that can add a significant sum to the annual energy bill. In addition, solvents are wasteful. Typically a container of solvent based adhesive will contain 65% solvent and 35% adhesive. As the adhesive is poured or applied, the solvent evaporates leaving the adhesive in place to bond two substrates; in effect, 65% of the adhesive cost is disappearing as waste product!
Case study
One leading global brand manufacturer of medical disposable products decided in 2007 to dramatically change and improve its manufacturing processes. For many years it had manufactured poly(vinyl chloride) (PVC) anaesthesia masks (Figure 1), respiratory equipment and tubing products using outdated, environmentally unfriendly solvent adhesive technology. Production was slow, manual labour was intense and health and safety issues using solvent adhesives were real and costly.
The company underwent a technical review and changed to using a medical grade USP Class VI ultra violet (UV) curing adhesive to bond its rigid PVC mask shell to the flexible PVC inflated mask cushion. The medium viscosity adhesive, which is also suitable for bonding chest drain bag sets, blood pressure transducers, stopcocks, surgical masks and arteriograph manifolds, exhibited good peel, tensile strength of typically 35 N/mm2 on rigid PVC and resistance to most chemicals and gases; it also allowed production to be semiautomated. The subsequent reduction in manual labour eliminated the need to maintain an air extraction system and therefore an energy cost saving was achieved here too.
Preproduction trials were performed over a four week period. A 60% increase in production was proved, a reduction in the quantity of adhesive required to fulfill the orders was achieved and an improved health and safety environment for the production workers was realised.
Types of modern adhesive
Modern adhesive technology encompasses cyanoacrylate adhesives and UV light curing adhesives. Cyanoacrylate is kept in a liquid form by an acidic stabiliser molecule and only cures to a solid state when the stabiliser is neutralised by surface moisture on the substrate to be bonded. When the moisture overcomes the stabiliser, adhesive molecules form creating a thermoplastic solid. Best results are usually achieved with a relative humidity of 40–60%. Some cyanoacrylates give off an odour that can cause irritation to mucous membranes and can be a prob- lem to production workers. In addition, they can cause “blooming” or “stress fractures” on some substrates such as polycarbonate and acrylics. Specially formulated nonfume and nonbloom adhesives are available to overcome this problem. Typical applications for these adhesives include tubing sets for infusion and blood testing, catheter sets, latex balloons, insulin sets and winged infusion sets.
UV light curing adhesives are different from solvents and cyanoacrylates. When applied, the adhesive will remain completely inert until it is exposed to a high intensity UV light source. Upon exposure, a photoinitiator or catalyst reacts and cross links with the adhesive molecule and the substrate molecule; it cures in just a few seconds and forms a resilient bond. UV light curing adhesives are 100% solid, that is, nothing evaporates away during the curing phase. They are available in a number of grades, ranging from thin viscosities that can be used to bond substrates by capillary attraction to thick viscosity adhesives. Typical applications include bonding tubing to fittings, glass syringes for insulin, cannulae adaptors, plastic to metal bonding, lumbar puncture needles, chest drain assemblies and catheters. The “cure on demand” properties of UV adhesives make them attractive to designers and production managers alike. The final product is of a much higher quality finish and the adhesive is easier and safer to use.
Looking ahead
The future holds many advances, partic-ularly in the field of nanotechnology. Recent innovations include microscopically altering the surface of a substrate to replicate that of Velcro and thereby eliminate the use of adhesives and mechanical fixings altogether. This innovation is revolutionary, but a long way from mainstream use.
In terms of assessing current practices, the use of adhesives in manufacturing may be a difficult move for some manufacturers who have rigidly maintained traditional methods of joining two substrates. But those companies that have embraced this technique have benefited enormously from better designed products, increased production, better health and safety benefits for their workers and even an improved bottom line.
Simon Dearing is Managing Director at Eurobond Adhesives Ltd, Bonham Drive, Eurolink Industrial Estate, Sittingbourne ME10 3RY, UK, tel. +44 1795 427 888, e-mail: simon@eurobond-adhesives.co.uk, www.eurobond-adhesives.co.uk
