Simplifying Failure Analysis for Select Medical Device Adhesives

Cyanoacrylates are stabilized with acidic species. They cure when the acidic species are neutralized by basic species (i.e. moisture) on a substrate surface. If a surface is acidic, or has very little moisture present, it can lead to slower fixture or cure of the cyanoacrylate.

Solution:

1. Use a “surface insensitive” grade of cyanoacrylate. This family of cyanoacrylates will cure more consistently than standard cyanoacrylates, particularly on dry and acidic surfaces.
2. Apply an accelerator to one of the surfaces being bonded. An accelerator consists of a basic species dispersed in a solvent. Applied to the surface, it leaves behind the basic species that neutralizes the acidic stabilizer to initiate cure of the cyanoacrylate. Be aware that some solvents used in accelerators may cause stress cracking of some plastics. In addition, the accelerator will have a limited on-part life once it is applied to the surface, so check that this processing variable is controlled.
3. Consider the use of a light cure cyanoacrylate if the joint design allows for it. Exposing the adhesive to light of the proper wavelength and intensity will rapidly initiate cure of the cyanoacrylate.
4. Expose the parts to an environment with humidity of at least 40% RH prior to the bonding operation. This should be done long enough prior to bonding ion to allow the moisture in the air to come into equilibrium with the surface. If the parts have already been assembled, exposure to a moist environment will offer minimal benefit, as the moisture must permeate into the bonded joint to have any effect on the cure speed of the cyanoacrylate.

The cure of cyanoacrylates initiates at the substrate surfaces. If the gap between the two substrates is greater than 0.010” it will be difficult to achieve complete cure through the gap. The acceptable gap is related to the viscosity of the adhesive. (i.e. a low viscosity CA might only cure through 0.002”). Check individual TDS for guidance.

Solution:

1. Redesign the part or tighten manufacturing tolerances on assemblies to reduce the gap. Generally, with cyanoacrylates, the smaller the gap the faster the adhesive will develop fixture strength and completely cure.
2. Apply an accelerator to both surfaces being bonded. The presence of accelerator on both substrates will help drive the cure reaction through a larger gap. Be aware that even this approach will have limited success if the gap gets much larger than 0.015”. If both substrates are treated with an accelerator, “activated” surfaces will be present, and the cyanoacrylate will begin curing upon contact with the surfaces, making it necessary to mate them as quickly as possible. The potential for clogging dispense needles is also increased when cyanoacrylate is dispensed directly onto “activated” surfaces.
3. Consider the use of a light cure cyanoacrylate if the joint design allows. Exposing the adhesive to light of the proper wavelength and intensity will rapidly cure the adhesive through gaps of 0.125” or greater.

Solution:

Check the output of the lamp/wand assembly using a calibrated radiometer and compare this to values obtained when the product cured properly and/or manufacturer’s recommended curing conditions. Many light cure systems will drop in output over time due to bulb degradation, material accumulating on the light cure system reflectors and/or wand tips. Clean the system and replace bulbs before the system output drops below minimum light output values.

Solution:

Consistent cure of the adhesive requires maintaining consistent cure time and light irradiance at the adhesive joint. If the cure time has been reduced, or irradiance reduced due to the distance from the light source, unacceptable cure may result. 

Solution:

If the surface of the adhesive is tacky, it may be due to insufficient light energy at lower wavelengths (200 – 300 nm) that aid in surface cure. Consider a different light source or an adhesive that has better tack free curing characteristics. Blanketing the surface of the adhesive with an inert gas such as argon or nitrogen during cure can also eliminate tackiness.

Solution:

Consider the use of products with both UV and Visible photoinitiators, particularly if your substrate will not transmit UV light.  If you have been using a light cure product and find that it is suddenly curing poorly, check the UV transmission characteristics of your plastic to be sure that it has not changed.

Solution:

Light cure acrylic adhesives will only cure in areas that are directly irradiated with light of the proper wavelength and intensity. Be sure that no areas of the bond line are shadowed by features on the assembly during the curing operation. Additional light sources or reflectors may be required if this is the case.

Solution:

Increase the amount of adhesive being dispensed. Dispensing equipment can aid  in consistently dispensing precise amounts of adhesive and avoid problems associated with variance in adhesive dispense amounts. A wide range of dispensing equipment is available that works well with light cure acrylics and cyanoacrylate adhesives.

Solution:

If the gap in the joint is too large for the adhesive to bridge, poor contact between the adhesive and the two surfaces being bonded will result. If the gap between the two parts cannot be reduced, a higher viscosity or more thixotropic adhesive may be required.

Solution:

This problem is often seen in cylindrical assemblies where adhesive is applied to one part and then the two parts are slid over each other as they are mated. If the gap between the two parts is too small, the adhesive can be scraped from the surface as the parts are mated and not reach into the joint. In this case, designing a taper to the joint can aid in getting adhesive flow into the joint. Other potential solutions include providing a larger gap between the parts and using a lower viscosity adhesive that can be applied to the parts after they have been mated.

Solution:

It is important to provide sufficient clamp load to attain intimate contact between the adhesive and the substrate, however, excessive clamp load that deforms the adhesive joint is undesirable. If clamp load is great enough to cause deformation of the joint and then removed prior to curing the adhesive, the part may return to its original configuration and leave a joint starved of adhesive. If the adhesive is cured while the part is deformed, the resulting joint will have high amounts of residual stress that could lead to premature joint failure. It is best to supply only the clamp force needed to achieve intimate contact between the adhesive and the substrates.

Solution:

Movement of the parts after adhesive is dispensed but prior to curing may result in gap formation. Maintaining part alignment during cure without using excessive clamping force can prevent this problem.

Both adhesive systems will be affected by contamination, but cyanoacrylates have a more difficult time cutting through oils, mold release agents and other surface contaminants to develop high bond strength.

Solution:

Surface analysis of the substrates can help determine the nature of the contamination. If it is possible to perform comparative surface analysis of parts that bond well and parts that bond poorly, it will be easier to determine what species on the substrate surface may be interfering with bond strength.

Prior to spending time and money on detailed surface analysis, “quick tests” can be performed to determine if surface contamination is causing poor bond strength. Comparing the bond strength of surfaces wiped (prior to bonding) with solvents such as isopropyl alcohol to those that are not cleaned can quickly provide an indication of whether or not surface contamination is playing a role in the problem. This approach does have limitations compared to more sophisticated surface analysis techniques since it will be necessary to choose a solvent that will remove the suspected contaminant. Once the nature of the contamination is known, corrective action can be determined.

An additional “quick test”, often referred to as the water break test, utilizes the surface characteristics of a substrate to predict material “bondability”. A small drop of water that beads up on the surface (i.e. does not spread out) is often an indicator of surface contamination or a substrate’s chemical incompatibility.

Solution:

Some plastics are very difficult to bond due to their chemical composition. However, high bond strengths can be achieved to many of these plastics when using cyanoacrylates in conjunction with primers that have been specifically developed for these materials. There are currently no primers available that aid light cure acrylic adhesives in achieving structural bond strengths with these types of plastics. Surface roughening or the use of other bond joint features (i.e. chamfers, annular rings) to promote mechanical keying of the adhesive into the substrate can be helpful. Surface modification to substrates by Plasma or Corona Treatment also improves adhesion to difficult to bond plastics.

Solution:

Plasticizers used to produce flexible PVC can make it difficult to bond. In some cases, initial bond strength to plasticized PVC can be quite good but then degrade over time due to the leeching of plasticizers to the bonded surface. Heat age testing (60^oC for ~2 weeks) will accelerate plasticizer migration and provide an indication of whether long term bond strength is possible. In addition, there are light curing acrylic products that have been specifically formulated for use on plasticized PVC.

Solution:

Bonding substrates with different coefficients of thermal expansion can sometimes be problematic. The difference in movement between the two substrates (as they are heated and cooled) places stress on the adhesive. This stress may be more than the adhesive bond can withstand particularly when a rigid adhesive is used in a very thin bondline. Increasing the gap between the two substrates being bonded can aid the adhesive in handling the resulting stresses. In addition, using a lower modulus adhesive can also aid in providing a more durable joint.

Solution:

Although the cure of cyanoacrylates is initiated by the presence of imperceptible surface moisture on a substrate, the presence of a film of water, or even water droplets, can cause the cyanoacrylate to cure before it completely wets out on the substrate surface. Excess moisture should be wiped from a surface prior to application of the cyanoacrylate to avoid this problem.

Solution:

Use flexible cyanoacrylates or light cure products with a lower modulus. These products will not form the rigid “knife edge” around adhesive fillets and are less likely to cause premature failure of the assembly.

Solution:

The length of time between the dispensing and curing of a light cure acrylic adhesive can have a significant effect on bond strength when plastic parts are being bonded. It is thought that the longer time the adhesive spends on the plastic substrate in its uncured state, the greater opportunity it has to potentially solvate the plastic surface.  Evaluating the effect of this variable and establishing the proper controls can help minimize bond strength variance.