Adhesive Bonding of Plastics in Medical Devices, Part 2: Material Properties of Plastics

Sterilization plays a vital role in the use of medical devices. Prior to the 1980s, most medical products were reusable and required sterilization or disinfection between uses. The advance of contagious diseases has raised some concerns over the risks of reusable medical devices, spurring the medical device manufacturing industry to develop disposable, single-use versions of many medical instruments.

CT scanner showing engine inside. (Credit: Radiological imaging/Shutterstock)

Although reusable medical devices are still the mainstay, disposable instruments using engineered plastics have become more widespread. And most significantly, adhesive products have been increasingly employed to replace mechanical fasteners for performance, cost, and compatibility with the sterilization processes that are being used today. Part 1 of this article, which appeared in the December 2024 issue, focused on the factors and requirements for selecting adhesives for use in medical devices. Part 2 delves more into the material properties of plastics for medical device assembly.

This article briefly discusses some key properties of plastics that relate to adhesive bonding. The polarity of the polymer and its resulting surface energy are important factors when assessing the ease with which adhesives will form a strong bond to the polymer surface. Generally, a greater extent of polarity will provide greater opportunity to form a strong bond between the plastic and the adhesive system. The availability of active sites or functional groups on the polymer surface such as alcohol, amine, or carboxylic acid functional groups may provide for stronger adhesion potential with certain adhesive systems. Surface treatment of polymers to improve adhesion exploit this phenomenon by chemically modified or derivatizing the surface to promote greater adhesion.

Finally, it is important to consider that plastics are complex formulations of both the polymeric material as well as additives, stabilizers, release agents, fillers, and lubricants that assist the manufacture of plastic articles as well as to promote their longterm stability. For small, migratory components present in the plastic article, it is important to consider the potential for these components to migrate to the surface of the plastic and potentially compromise adhesion by forming a weak boundary layer.

Migration of small components is enhanced at higher temperatures due to more labile diffusion; as such, this is an important factor to consider for medical devices that must undergo repeated, high-temperature stabilization. For example, common lubricants used in the extrusion of polymeric resins such as polypropylene are magnesium stearate and calcium stearate. Studies have shown that magnesium stearate offers greater resistance to migration under accelerated aging.¹ Surface migration of these small molecules may both degrade the quality of the thermoplastic material over time as well as potentially compromise adhesion at the interface between the plastic surface and the adhesive.

Theories of Adhesion

Several theories attempt to explain the complex phenomenon of adhesion. Specifically, adhesion to plastic substrates is of particular interest due to the challenging nature of forming a strong bond with the nonporous and generally chemically inert surface of plastic materials.² Further, many plastics are very hydrophobic and their low surface energy results in difficulty wetting the surface when adhesive is applied. Wetting can be generally understood in terms of contact angle — the greater the contact angle, the lower the extent of wetting — a failure to achieve appropriate wetting will result in poor adhesion at the interface. Wetting can be improved by adding active agents such as surfactants to the adhesive in order to lower the surface tension of the adhesive, although this has limited effect.

Alternatively, some kind of surface treatment is often needed when bonding plastics in order to increase the surface energy of the plastic and thus the extent of wetting when forming the bond. Common means of surface treatment include corona treatment, plasma treatment, UV/ozone treatment, or chemical surface modification. Generally, the goal of these treatments is to increase the number of oxygen-containing moieties or other polar functional groups on the surface of the polymer, thus increasing the surface energy and the resulting strength of adhesion at the interface.

Other means to improve adhesion include surface abrasion to increase the surface area present at the interface thus increasing the overall strength of adhesion. Generally, it is always critical to make sure that the surface of the polymer is clean and free of any small compounds that will interfere in the bonding process and form a weak interfacial layer between the bulk of the polymer and the adhesive. Many surface treatment properties such as plasma, corona, and flame treatment have the added benefit of ensuring that any small, organic contaminants on the plastic surface are destroyed and ablated prior to bonding.

Generally, three different models of adhesion can be used to help engineers understand the dynamics of forming a lasting bond with a plastic substrate, as well as providing insights when troubleshooting bond failures. These include mechanical interlocking, diffusional theories, and adsorption theories — electrostatic theories, most applicable to metal and glass substrates, are less applicable to plastic bonding and will not be discussed.²

Fundamentally, sufficient wetting is critical to achieving sufficient adhesion per adsorption theory because it is dependent upon the intimate contact between the plastic surface and the adhesive at the interface. The extent of wetting can be described by the contact angle, and the theoretical thermodynamic work of adhesion can be conceptualized. This theory contributes to assessing the ability to bond plastics; for example, it explains the difficulty of bonding polyolefins without surface pretreatment.

EP42HT-2Med is a two-component, high-temperature-resistant epoxy for medical device assembly. (Credit: Master Bond)

As polyolefins lack polar functional groups that would contribute to hydrogen-bonding or acid-base interactions, a corona or plasma treatment is required to yield a strong bond to polyolefinic materials. Corona, plasma, flame, and ozone treatments generally all seek to increase the surface energy of the plastic by promoting the formation of oxygen-containing functional groups or otherwise polar moieties that will increase the work of adhesion and result in a stronger adhesive bond.

Conclusion

In the coming decades, an increased demand for medical devices will provide unique opportunities for growth and innovation. Across the product hierarchy, adhesives suitable for joining the dissimilar materials used in medical device construction will continue to play a role in enabling the growth and development of this critical industry. Understanding the unique requirements of medical devices — from biocompatibility and cytotoxicity to the engineering challenges posed by various sterilization techniques — will grow in significance.

Cultivating an understanding of the material properties of plastics as well as the theories that seek to explain the complex phenomenon of adhesion further provide benefits when considering the design, manufacture, and troubleshooting of medical devices constructed with the aid of adhesives. The adhesive bonding of polymeric materials and plastics is of central importance to the design and manufacture of medical devices.

Finding an adhesive that is suitable for use in a medical device may be challenging, but the inherent flexibility and latitude of adhesive formulations coupled with their convenient and integration into both low-speed and high-speed manufacturing processes will continue to provide solutions for even the most complex engineering challenges. Master Bond, for example, offers a wide array of epoxy and silicone adhesives specially formulated for use in medical devices. Their diverse product range encompasses one-part, two-part, and UV dual-cure technologies that can meet the needs of a variety of high and low-throughput manufacturing processes. Additionally, consultation with Master Bond’s technical experts and engineers may streamline the process of adhesive selection, or, if necessary, aid in the development of a custom-tailored adhesive product that meets the rigorous performance requirements of the application while also being optimized for the manufacturing process in question.

References

Bak, M-G, Won, J-S., Koo, S-W., et al., “Migration behavior of lubricants in polypropylene composites under accelerated thermal aging,” Polymers. 2021:13, 1723.
Leeden, M., “Surface properties of plastic materials in relation to their adhering performance,” Advanced Engineering Materials, 2002:4 (5) 280-289.

This article was written by Venkat Nandivada, Manager of Technical Support, and Rohit Ramnath, Senior Product Engineer, at Master Bond, Hackensack, NJ. For more information, visit here .