Medical Device Link.

MANUFACTURING

Designing for adhesives

The increasing demand for disposable medical devices means that manufacturers are constantly reviewing their choices of materials, designs and methods of production. Correct choices in adhesives can make an important difference in manufacturing procedures and costs, because no one adhesive technology can fulfil all the requirements when bonding disposable medical devices. Indeed, a balance of adhesive properties is needed to ensure successful bonding and this is especially true with plastics.

A vast array of plastics is used in the manufacture of medical devices including, polycarbonate (PC) and flexible PVC (polyvinyl chloride), but some of these plastics, for example polypropylene (PP) are traditionally difficult to bond. However, this is no longer the case.

Adhesives offer an optimum, and often the only, assembly method. Yet, all too often, the adhesive is not fully considered at the design stage and this results in much time and trouble for engineers when taking prototypes into production.

Adhesive benefits

Various grades of cyanoacrylate (CA) adhesives are available for bonding medical devices. CAs (also known as instant adhesives or super glues) have undergone considerable technological advances and there is now a range of products on offer. The benefits include

• the low cost of capital equipment
• good adhesion to a wide range of plastics
• the ability to bond different substrates
• complex 3D shapes can easily be bonded without the need for special tooling
• they gap fill up to 0.15 mm
• they are solvent free.

Curing CAs

CAs are generally available as colourless liquids in grades that vary in viscosity from a thin liquid to a thixotropic gel. They contain an acidic stabiliser that prevents the adhesive from polymerising. However, when the adhesive comes into contact with surface moisture, this stabiliser is neutralised and the bonding process is initiated. This moisture, which is normally found on surfaces exposed to the atmosphere, is sufficient to initiate curing within a few seconds.

Figure 1. (click to enlarge) Cure speed of a cyanoacrylate.

For joints that are to be bonded with a CA, the gap between parts should ideally be less than 0.1 mm. The optimum cure condition for CAs is when the relative humidity (RH) is between 40–60%. Lower RH, for example, 20%, will result in a slower cure; high RH (80%) results in a faster cure. It should be noted, however, that high RH can be detrimental because it allows the CA to work so fast that the adhesive cures before it has properly adhered to the surface and the result is poor bonding. The strength of a CA develops with time. Generally, CAs will gain handling strength within the first minute or so, but they continue to cure over the following 24 hours (Figure 1).

Recently, ultraviolet- (UV) curable CAs have been introduced. These combine the benefits of a CA with light-curing adhesives; unshaded surfaces are cured through exposure to UV light and any shadowed areas are cured with surface moisture. UV-curing CAs are used for bonding catheters and small-bore tubing.

Figure 2. A polycarbonate cover is bonded to an acrylic housing with a low-bloom cyanoacrylate.

Heavy-molecular-weight CAs are best for applications where bonds must be cosmetically perfect. The additional low-odour characteristic of these adhesives is ideal when operators are required to work in confined unventilated spaces (Figure 2).

Bonding difficult plastics

Figure 3. (click to enlarge) The effects of using a primer with cyanoacrylate. The use of a primer on polycarbonate can be detrimental to the strength of the bond.

PP, polyethylene (PE), silicone rubber, polytetrafluoroethylene, acetal and polyetheretherketone are common materials that always represent a bonding challenge. Adhesives will cure on these surfaces, but will not adhere properly to them. This is because the substrate has a lower surface energy than the surface tension of the adhesive. It is necessary, therefore, to prepare (modify) the surface to achieve adhesion by means of a primer. In fact, the effect of a polyolefin primer when used with a CA on, for example, PP should not be underestimated. Bond strengths are often 25 to 40 times higher than those achieved when using the same adhesive without primer (Figure 3). Note however that this primer can be detrimental when used on PC.

Acrylic adhesives

Other adhesives that use UV light as a source for curing are UV-curing acrylic adhesives. Their speed of cure and the ability to cure on demand means they are increasingly being used in high-volume assembly processes. This ability means that parts can be positioned with the adhesive in place and curing only takes place when the components are exposed to light, once the operator is satisfied the alignment is correct. Depending on the product and cure system, bonding is usually achieved within a few seconds, which allows fast throughput and reduced work-in-progress levels.

These adhesives are generally single-part products that allow easy dispensing. In addition to assembly of medical devices, typical applications for UV adhesives include bonding glass-to-glass, joining clear and semi-translucent plastics, and potting electronic and electrical components.

The curing process

Many UV-curing products cure at a wavelength of 365 nm (UVA). However, a number of plastics, including PC, can act as a UV filter and inhibit the cure of some grades of UV adhesive. The wide use of PC and flexible PVC in the medical industry has resulted in the development of a range of adhesives that cure at the visible end of the UV spectrum (400-450 nm), thereby overcoming the problem of absorbance at shorter wavelengths.

Figure 4. A UV-wand system.

Light sources can range from a simple bench-top open unit through to a fully automated conveyor system with several lamps. Where small components are involved and a high intensity light is required over a diameter of approximately 10 mm, UV light guides, known also as “wand” systems, can be utilised to ensure that light is directed to the precise area for curing (Figure 4).

Figure 5. Blood-collection units are bonded together with a UV-curing acrylic to give a clear bond line.

To achieve optimum cure strength, it is essential that the output spectrum of the selected UV source matches the photoinitiator system used in the adhesive. Low intensity would be 5–10 mW/cm²; high intensity is 100–1000 mW/cm². In general, the higher the intensity, the faster the cure, and for most production applications the UV intensity should not be less than 50 mW/cm² (Figure 5).

UV-curing acrylics bond exceptionally well to PVC and PC, but for applications involving PP or PE, a UV cyanoacrylate should be used in conjunction with a primer.

Ideal design attributes

As already noted, no single adhesive will satisfy all needs. Therefore, designers and engineers must balance a variety of adhesive properties to obtain the required bond strength and ease of use in production. A common application for adhesives involves the joining of co-axial and cylindrical parts. The engagement length (L) to diameter (D) ratio of ideally 1:5 is an important factor in terms of the final strength of the bond.

Figure 6. (click to enlarge) Tube bonding with long engagement lengths.

CAs and UV-curing acrylics require, if possible, a bond line diametral gap (D-d) of approximately 0.04 mm (Figure 6). It is important that the design incorporates a small chamfer to allow the adhesive to be wicked in after assembly. Where the assembly process is horizontal, the chamfer is not always necessary, but the male part may need to be rotated to ensure full joint coverage.

When selecting the adhesive, it is necessary to consider the application technique and the cure system. A CA will cure quickly, but may be inappropriate if the engagement length is long. This is because the adhesive may cure before the parts are fully assembled. In flat surface bonding applications, the joint should be designed to minimise peel and cleavage loads and, where possible, the joint width should be increased rather than the overlap area.

Dispensing

The dispensing capability of the adhesive can often be an important factor in its successful application during production. Both CAs and UV-curing adhesives can be dispensed precisely, cleanly and reliably using a selection of equipment, ranging from simple hand-held dispensers to fully automatic systems incorporated into purpose-built special production equipment. The type of application and workplace conditions such as production rate, parts handling and cure speed will determine whether manual, semiautomatic or fully automatic dispensers will be required.

Help with selection

The selection at the design stage of the correct adhesive is important so that the objectives of manufacturability and end-product performance are achieved. To help in that process and to ensure the right product is used for bonding, adhesives suppliers are happy to become involved early in design discussions and provide recommendations and testing. In addition to expertise on bonding technology, they can offer knowledge of materials selection, specially adapted dispensing equipment and support in process technology.

Bob Goss is Senior Technology Specialist at Henkel Loctite Adhesives Ltd, Wood Lane End, Hemel Hempstead HP2 4RQ, UK, tel.+44 1442 278185, e-mail: bob.goss@uk.henkel.com, www.loctite.co.uk.