Fundamentals of Structural Adhesives for Device Assembly

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI July 1997 Column

Selecting the proper adhesive and curing process can be crucial to developing safe and effective devices.

The wealth of adhesive technologies that are available for device assembly can make adhesive selection a daunting task. The best material for an application will depend as much on processing considerations as on performance issues. This is particularly true in the medical manufacturing field, where process control is critical in meeting regulatory requirements. Substrate preparation, dispensing options, curing method, and equipment needs are just some of the factors that manufacturers must consider. Selection of a medical adhesive is further complicated by the need to assess biocompatibility and the effects of sterilization on joint performance.

At left: In high-volume applications such as hub-to-cannula syringe bonding, light-cure adhesives provide one- to two- second cure. Photos courtesy of LOCTITE CORP. (Rocky Hill, CT).

COMMON CURING PROCESSES

Adhesives are liquids that must be converted to a solid state. Those that are commonly used in medical device manufacture achieve this change through polymerization, also known as "curing." There are many curing processes available, and many adhesive systems use more than one; therefore, it is best to consider each process individually. A summary of bonding processes and the trade-offs associated with each are shown below.

Table I. Summary of trade-offs for common joining methods and structural adhesive curing processes.

Reactive-Component Mixing. Generally, two-part adhesive systems are made up of a base polymer resin and a catalyst. When mixed, the adhesive starts to polymerize, forming a thermoset polymer. Reactive components of the adhesive are kept separate prior to use and generally demonstrate good shelf life without refrigeration. Large areas can be filled for potting without an external activator such as moisture or light. Depth of cure is restricted by the heat that is naturally generated during cure reaction; in some cases, the temperature can rise high enough to burn the adhesive or damage the substrate.

The biggest drawbacks to these systems are the problems and costs associated with mixing the components. Automated mixing requires proper equipment, with its associated capital and maintenance costs, whereas manual mixing introduces greater labor costs and the potential for human error. For consistent adhesive performance, the mix ratio must remain constant so that the actual adhesive used in manufacturing has the same properties as the adhesive that was specified at design.

The cure of multicomponent adhesives can be accelerated using heat, although cure temperature can adversely affect the cross-link density and modulus of the cured adhesive. Adhesives selected and qualified for room-temperature curing should be requalified if a heat-cure step is added. The cure rate of multicomponent systems can also be controlled by varying the type and amount of catalyst.

Heat Curing. A relatively simple process, heat curing is easily controlled by maintaining consistent cure times and temperature profiles. The process usually aids adhesion and yields thoroughly cross-linked polymer networks.

Rapid polymerization of light-curable acrylics allows for immediate on-line quality checks.

The disadvantages of heat curing include the initial capital outlay for ovens and the long-term expenses for their maintenance and operation. Also, maximum cure temperature is dependent on the temperature limitations of the bonding substrates. For example, thermoplastics will require low cure temperatures and long curing cycles. Moreover, adhesive viscosity decreases as temperature increases, causing the adhesive to run out of the joint at higher temperatures, particularly on assemblies that have large gaps. Even when such gaps are not present, fixturing may be necessary to prevent parts from moving when the adhesive starts to flow.

Moisture Curing. Moisture-cure systems polymerize when moisture from the atmosphere diffuses into the adhesive. In general, curing will succeed when relative humidity is above 25%, with the cure rate increasing as the relative humidity increases. Because moisture must permeate throughout the polymer, depth of cure is limited to between 0.25 and 0.5 in.

Moisture curing takes longer than other processes. Cure time varies with adhesive depth and relative humidity, but typically ranges from 12 to 72 hours. Increasing the relative humidity can accelerate the cure to some degree, but adding heat won't. Moisture-cure adhesives generate by-products, which can be corrosive or bad smelling and therefore undesirable for medical device applications. Even formulations that generate relatively inert by-products can cause fogging or other aesthetic problems. Although moisture curing is limited as a primary cure process for medical device assembly, it works well as a secondary cure mechanism for light-cure adhesives.

Light Curing. Light-cure adhesives contain photoinitiators that absorb light energy to begin polymerization. Generally supplied as stable one-component systems, light-cure adhesives allow the user to take as much time as necessary to position parts. Upon exposure to the appropriate light source, the adhesive cures fully in less than a minute, minimizing work in progress. Traditionally, light-cure adhesives have required UV light; however, new formulations can be cured with visible light. Some thermoplastics, including many grades of polycarbonate, contain additives that block UV radiation and cannot be used with UV-cure adhesives. The new visible light-cure adhesives have solved this problem.

A light-cure adhesive joint must be designed so that light can reach the bond line. There is a limit to how far light will penetrate; in general, cure depths in excess of 0.5 in. are difficult to achieve. In addition, the cost of purchasing and maintaining the necessary curing equipment can vary from $1000 for a low-intensity system to tens of thousands of dollars for custom, high-intensity systems.

COMMON ADHESIVE TYPES

Adhesive science has progressed sufficiently to provide medical manufacturers with a range of materials tailored to their specific needs. Diverse adhesive formulations are available to address a variety of assembly requirements, whether they involve bonding dissimilar substrates, filling large gaps between mating surfaces, or forming a hermetic seal between two substrates. The most important adhesive types include epoxies, polyurethanes, silicones, acrylics, and cyanoacrylates. A summary of adhesive types, along with their respective advantages and disadvantages, is provided in Table II.

Table II. Summary of trade-offs for common medical device structural adhesives by chemistry.

Epoxies. In medical device manufacturing, epoxies are often used for bonding cannulae to needle hubs and for deep-section potting of devices such as heat exchangers. Available as two-component systems or frozen premixes, epoxies cure at room temperature or with heat to form thermoset polymers. They bond well to a wide variety of substrates, generate no by-products, and shrink minimally upon cure. Cured epoxies typically have excellent cohesive strength, very good chemical resistance, and good heat resistance. A wide variety of specialty epoxy formulations are available to meet different application needs, including thermally conductive varieties and formulations optimized for gap filling.

Light-curable acrylic adhesives are ideal replacements for solvent welding in disposable polycarbonate or acrylic devices.

Epoxies tend to be very rigid and generally exhibit low peel strength. These characteristics can become a problem in the bonding of flexible substrates. Toughening agents can be compounded into the epoxies to improve peel strength to some extent. Epoxies usually produce large amounts of heat upon curing, causing problems with heat-sensitive substrates, particularly if large volumes of material are used. The yellow color of most epoxies can be aesthetically undesirable; however, specially formulated water-white materials are available. Medical manufacturers must be sure that the hardener and catalyst used are acceptable for bodily fluid contact.

Tracheal or endotracheal tubes, often made of extruded silicone rubber, are best assembled with silicone adhesives/sealants.

Polyurethanes. Polyurethanes are tough polymers that offer greater flexibility, better peel strength, and lower modulus than epoxies. Typical applications include bonding tips on catheters and optical scopes and sealing large devices such as heat exchangers and blood oxygenators. Polyurethanes are available as two-part systems, one-part frozen premixes, and one-part moisture-cure systems. The cured adhesive consists of soft regions, which add flexibility to the joint, and rigid regions, which contribute cohesive strength, temperature resistance, and chemical resistance. By varying the ratio of hard and soft regions, a range of physical properties can be achieved. Polyurethanes are available in water-white formulations for applications where look is important.

Like epoxies, polyurethanes bond well to a variety of substrates, including heavily plasticized PVC. Sometimes, however, a primer is required to prepare the surface. These primers are solvent based and require extra time to evaporate. Although polyurethanes do not present much of a stress-cracking hazard, the solvents used in the primers often do. Polyurethanes have good chemical and temperature resistance, though long-term exposure to high temperatures will degrade them more rapidly than epoxy. For a disposable medical device that will undergo only a few sterilization cycles, this weakness would probably not differentiate the two systems.

When bonding is being done with polyurethane systems, moisture must be excluded from the adhesive components, since it can impair both performance and appearance. Some polyurethanes contain toxic heavy-metal catalysts that can pose serious problems in medical device applications. Additionally, isocyanates present in polyurethanes can cause sensitization reactions after repeated exposure; although a finished formulation will typically have little free isocyanate, common-sense safety precautions should be observed to avoid any long-term problems.

Silicones. Silicones are predominantly elastomeric polymers exhibiting excellent flexibility but low cohesive strength, which makes their status as a structural adhesive debatable. However, being unique thermoset elastomers with much lower durometer and modulus than many other adhesive materials, silicones fit a particular niche worth reviewing. Silicones used in medical device assembly are typically one-part moisture-cure systems or UV-cure systems, which require secondary moisture cure. Formulations are available in a range of viscosities, from self-leveling liquids to thixotropic pastes. Silicones offer good resistance to polar solvents, flexibility over a wide temperature range, and good adhesion to many substrates. Typical applications include bonding and sealing cuffs to endotracheal and tracheotomy tubes, where the material's soft elastomeric qualities help eliminate burrs that could cause irritation.

Silicone's low cohesive strength, typically 300­900 psi, minimizes a joint's load-bearing capabilities. Moisture-cure silicones have a limited cure depth of approximately 0.375 in. and require lengthy cure cycles. UV-cure silicones minimize process time by permitting rapid fixturing; however, complete cure still requires a moisture-cure reaction. UV-cure silicones also require high-intensity UV lighting systems, which cost a minimum of $3000­$5000.

Acrylics. Most acrylic adhesives used in medical device assembly are light cured, although they can be formulated to cure using heat or activators as well. Light-cure acrylic adhesives offer physical toughness, rapid cure on demand, and excellent unprimed adhesion to a variety of substrates, including heavily plasticized PVC. The one-part solvent-free systems can be specified in various viscosities, ranging from 100-cp fluids to thixotropic gels. They adhere well to most substrates and give a clear bond line when cured in thin sections. Light-cure acrylics are typically used to bond polycarbonate and acrylic cases for devices such as blood oxygenators and heat exchangers and to join plasticized PVC to tube sets.

When working with light-cure acrylics, manufacturers must be careful to match the adhesive type with the appropriate light source. If the wrong light is used, the material is unlikely to cure fully, resulting in a tacky surface after processing.

Cyanoacrylates. Cyanoacrylates, also known as instant adhesives, are one-part systems that cure rapidly when pressed into thin films to form strong bonds between two substrates. Typically, the residual moisture present on most surfaces is sufficient to initiate polymerization. Cyanoacrylates offer excellent adhesion to many substrates, including most plastics used in disposable medical devices. Available in a range of viscosities, they achieve fixture strength in seconds and full strength within 24 hours. This rapid cure makes cyanoacrylates well suited for use in automated production environments.

Unmodified cyanoacrylates are extremely rigid and have very low peel strength; to overcome these limitations, a number of rubber-toughened formulations exhibiting greater peel strength have been developed. Because cyanoacrylates cure in the presence of surface moisture, manufacturing environments with low relative humidity or substrates that are hydrophobic can retard cure. The process can be hastened using accelerators or specialty "surface-insensitive" formulations, which are engineered for rapid curing even on dry or slightly acidic surfaces.

Cyanoacrylates are subject to blooming, a condition that results when the cyanoacrylate monomer volatilizes and settles on the part, leaving a white residue. Blooming is more an aesthetic than a performance problem, but where medical devices are concerned poor aesthetics can be just as damaging as poor performance. The white residue is easily removed with common cleaning solutions, but such a step is difficult to accommodate in an automated manufacturing environment. Specialty low-odor/low-bloom formulations have been developed with a higher molecular weight and a lower vapor pressure than unmodified cyanoacrylates. Blooming can also be reduced by minimizing the amount of excess adhesive applied, maintaining good ventilation, or using an accelerator or surface-insensitive cyanoacrylate to speed the cure so the adhesive polymerizes before it can volatilize.

Cyanoacrylates have a limited cure-through gap of about 0.010 in. Cyanoacrylates used on plasticized PVC can lose bond strength over time and therefore require heat aging and testing to determine whether the bond will withstand the effect of plasticizer leaching to the substrate surface. Cyanoacrylates can also induce stress cracking if left uncured on a thermoplastic; minimizing the bond gap and using an accelerator or surface-insensitive cyanoacrylate can eliminate this problem. The low temperature resistance of cyanoacrylates is especially pertinent with medical devices that must withstand sterilization processes such as autoclaving. Joints bonded with cyanoacrylates should not bear loads when autoclaved, because high temperatures will soften the adhesive and possibly lead to joint failure. Bond strength should be tested before and after sterilization to ensure that the bond design is adequate for the adhesive and the sterilization method to be used.

CRITICAL ISSUES

Quality. Once the proper adhesive has been chosen, it is important to establish incoming quality inspections with the adhesive manufacturer to make sure the adhesive consistently performs acceptably in the bonded assembly. Most adhesive manufacturers will provide test reports detailing the results of their quality testing for each batch of adhesive. Often, medical device manufacturers will perform more limited testing to confirm the acceptability of the adhesive. Spot checking critical parameters such as viscosity or bond strength can be valuable for ensuring that adhesive quality levels are maintained.

Biocompatibility. Adhesive manufacturers generally qualify their materials using the same biocompatibility testing regimens that are used to qualify plastics for medical applications. While specific testing varies slightly from standard to standard, most testing is done in the following manner: extractions of cured adhesive are tested against control solutions to determine if any harmful extractable chemicals are present. This testing involves injecting the extract into animals or exposing it to cell cultures. Implantation testing is generally also done with strips of the material to determine if irritation or other undesirable effects occur. Another approach is to submit an actual device assembled with the adhesive for biocompatibility testing; if the device itself passes, the adhesive supplier can claim the same results for the adhesive.

A few key factors must be considered before accepting an adhesive supplier's claims of compliance with a biocompatibility standard. First, did preparation and testing of samples reflect how the adhesive will be used in real life? Often, for example, samples submitted for biocompatibility testing are cured between two sections of a clear thermoplastic to mimic the material's performance in a bond line; if the OEM is going to cure the material as a coating, the evaluation of the material in a bond line may be meaningless. Also, if a biocompatibility claim is based on certification of a finished device, the claim can be considered valid only if the manufacturer uses the adhesive in a similar manner on a similar device.

Sterilization. The need for sterilization places unique demands on adhesives used in medical device assemblies. Many adhesives can only be used for disposable devices because they cannot tolerate repeated sterilization. The high humidity and temperatures used in autoclaving, for example, are very aggressive toward most adhesives, but there are a few precautions designers can take. Minimizing the bond gap and maximizing the bond area, for instance, will help mitigate any detrimental effects. In general, the chemical and thermal resistance of an adhesive will serve as a good indicator of its ability to withstand sterilization. Naturally, adhesive joints must be tested before and after sterilization to make sure they will perform to satisfaction.

Achieving fixture strength in seconds, cyanoacrylates are well suited for manual to fully automated production lines.