Adhesive Bonding of Medical Plastics: An Overview

Medical Plastics and Biomaterials
Magazine
| MPB Article Index

Originally published January 1996

ADHESIVES

PATRICK J. COURTNEY AND JAMES SERENSON

Among the reasons for the popularity of plastics in medical device assemblies are a wide range of useful physical properties, low cost, and ease of processing. Adhesives are generally the method of choice for joining plastic parts because they minimize stress concentrations, can be easily applied with automated systems, and come with a variety of uncured and cured properties. The multitude of current formulations makes it possible for designers to identify adhesives and plastics whose properties should match up well

for a chosen application. Unfortunately, the wide range of plastic and adhesive types renders it unlikely that specific test data will exist concerning the performance of the chosen adhesive with the chosen plastic.

To address this need, this study conducted tests for bond strength on a representative matrix of commonly used plastics and the adhesives best suited to bond them. For many of the plastics evaluated, the effect of polymer composition on bond strength was evaluated by compounding plastic formulations with each of the most commonly used additives and fillers for that plastic; common grades were used for the remaining resins. The effect of each additive and filler was determined by comparing the bond strength achieved with the specially compounded formulations to that of the neat plastic. In addition, the effect of surface roughening and chemical treatment of the plastic surface on bond strength was examined.

The purpose of this article is to review the test methodology and provide an overview of some of the key findings of the study. The data are specifically intended to help design engineers quickly identify the most promising adhesive/plastic combinations for their needs. In a larger sense, this information should benefit anyone interested in understanding common adhesives used for medical device assembly and some of the critical variables that must be considered when evaluating their use in an application.

ADHESIVE SELECTION

A range of adhesives representative of those commonly used for bonding medical devices in high-speed, automated manufacturing processes was selected for evaluation. The adhesive types selected were light-curing acrylic and cyanoacrylate. A 300-cP light-curing acrylic was used for the study, along with a 110-cP ethyl cyanoacrylate, a 200-cP rubber-toughened ethyl cyanoacrylate, and a 100-cP surface-insensitive ethyl cyanoacrylate. In addition, the surface-insensitive ethyl cyanoacrylate was tested in conjunction with a polyolefin primer. It should be noted that, with the exception of the rubber-toughened cyanoacrylate, all adhesive systems evaluated are commercially available as USP Class VI­ qualified materials. A brief description of each type of adhesive, its chemistry, and its benefits and limitations is given below.

Light-Curing Acrylics. Light-curing acrylic adhesives are supplied as one-part, solvent-free liquids with viscosities ranging from 50 cP to thixotropic gels. Upon exposure to light of the proper intensity and spectral output, these adhesives cure rapidly to form thermoset polymers with excellent adhesion to a wide variety of substrates. Although the cure times of light-curing acrylic adhesives are dependent on many parameters, cure times of from 2 to 60 seconds are typical and cure depths in excess of 0.5 in. (13 mm) are possible. Formulations of light-curing acrylic adhesives are available that vary in cured properties from very rigid, glassy materials to soft, flexible elastomers.

Light-curing acrylic adhesives cure rapidly on demand, which minimizes work in progress and offers virtually unlimited repositioning time. In addition, their wide range of viscosities facilitates the selection of adhesives for automated dispensing. These qualities make light-curing acrylics ideally suited for automated bonding processes.

Chemistry. Light-curing adhesives are primarily composed of a blend of monomers, oligomers, and polymers containing the acrylate functionality. Photoinitiators with the appropriate light sensitivity are added to the blend. Upon exposure to light of the proper intensity and spectral output, the photoinitiator decomposes to yield free radicals, which then initiate polymerization of the adhesive through the acrylate groups to create a thermoset polymer.

When the adhesive is cured in contact with air, the free radicals created by the decomposition of the photoinitiator can be scavenged by oxygen prior to initiating polymerization. This can lead to incomplete cure of the adhesive at the adhesive/oxygen interface, yielding a tacky surface. To minimize this possibility, the irradiance of light reaching the adhesive can be increased, the spectral output of the light source can be matched to the absorbance spectrum of the photoinitiator, and/or the adhesive surface can be flooded with nitrogen during the curing process.

Benefits and Limitations. A number of benefits associated with light-curing adhesives have already been mentioned: one-part formulation, a wide range of viscosities and other physical properties, rapid curing on demand, easily automated dispensing, and Class VI status. In addition, light-curing formulations are solvent-free, offer superior gap filling with clear bond lines, and provide good environmental resistance.

Among the potential disadvantages of light-curing adhesives are the initial expense for the irradiation equipment, the fact that oxygen can sometimes inhibit curing on exposed surfaces, and the need to vent the ozone that can be created by high-intensity light sources. Another problem with certain part configurations is that light must be able to reach the bond line in order for the adhesive to cure.

Cyanoacrylate Adhesives. Cyanoacrylates are one-part, room-temperature-curing adhesives that can be provided in viscosities ranging from water-thin liquids to thixotropic gels. When pressed into a thin film between two surfaces, cyanoacrylates cure rapidly to form rigid thermoplastics with excellent adhesion to most substrates.

One of the benefits cyanoacrylates offer is a wide variety of specialty formulations with properties tailored to meet particularly challenging applications. For example, rubber-toughened cyanoacrylates feature high peel strength and impact resistance to complement the high shear and tensile strengths characteristic of cyanoacrylates. Thermally resistant cyanoacrylates are available that offer excellent bond-strength retention after exposure to temperatures as high as 250°F for thousands of hours. Moreover, "surface-insensitive" cyanoacrylates provide rapid fixture times and cure speeds on acidic surfaces--such as wood or dichromated metals--that slow the cure of typical cyanoacrylates. In some cases, the use of a general-purpose cyanoacrylate adhesive can be hindered by the appearance of a white haze around the bond line. This phenomenon is known as "blooming" or "frosting," and occurs when cyanoacrylate monomer volatizes, reacts with moisture in the air, and settles on the part. To eliminate this problem, "low-odor/low-bloom" cyanoacrylates were developed, which have a lower vapor pressure than do standard cyanoacrylates and are therefore less likely to volatize.

While advances in cyanoacrylate formulation technology have played a key role in bringing additional benefits to the end-user, there have also been important developments in cyanoacrylate primer and accelerator technology. Accelerators speed the cure of cyanoacrylate adhesives and are used to reduce cure and fixture times and to cure fillets on bond lines. Accelerators consist of an active ingredient dispersed in a solvent. The accelerator is typically applied to a substrate surface prior to the application of the adhesive. Once the carrier solvent has evaporated, the cyanoacrylate can immediately be applied and its cure initiated by the active species that the accelerator has left behind. Depending on the particular solvent and active species present in the accelerator, the solvent can require from 10 to 60 seconds to evaporate, and the active species can have an on-part life ranging from 1 minute to 72 hours. An accelerator can also be sprayed over a drop of free cyanoacrylate to rapidly cure it, a technique widely used for wire tacking in the electronics industry.

The use of primers in conjunction with cyanoacrylates enables the adhesives to form strong bonds with polyolefins and other difficult-to-bond plastics such as fluoropolymers and acetal resins. Like the accelerators, primers comprise an active ingredient dispersed in a solvent. Once the carrier solvent has evaporated, the surface is immediately ready for bonding. The primer will have an on-part life ranging from 4 minutes to 1 hour. Depending on the plastic, bond strengths of up to 20* the unprimed bond strength can be achieved.

Chemistry. Cyanoacrylate adhesives are cyanoacrylate esters, of which methyl and ethyl cyanoacrylates are the most common. Cyanoacrylates undergo anionic polymerization in the presence of a weak base, such as water, and are stabilized through the addition of a weak acid. When the adhesive contacts a surface, the water present on the surface neutralizes the acidic stabilizer in the adhesive, resulting in the rapid polymerization of the cyanoacrylate.

Benefits and Limitations. Cyanoacrylates offer many of the same advantages as light-curing adhesives: no solvents, trouble-free automated dispensing and rapid curing, multiple viscosities, and Class VI compliance. They also provide excellent adhesion to a wide range of substrates and, as mentioned, can bond normally recalcitrant materials such as polyolefins when used with a primer.

Limitations of cyanoacrylates include adhesive brittleness; limited gap cure; low durability on glass; and poor peel strength, solvent resistance, and temperature resistance. Caution is required when handling cyanoacrylates, as they bond rapidly to skin. Finally, certain polymers may exhibit stress cracking when bonded with these adhesives.

TEST METHODOLOGY

Thirty-four of the most commonly used types of plastics were chosen for testing; data are presented here on the 25 types most often selected for medical device assembly. Specialty formulations were tested for some of the plastics, and commercially available grades for others.

Specialty Formulations. Fifteen of the 25 plastics were compounded specifically to determine the effect that different additives and fillers had on the bondability of the base resin. For each of these plastics, the following procedure was used. First, a grade of the plastic containing no fillers or additives was selected and tested for bond strength with each of the adhesives. The most common additives and fillers used with the plastic were then identified. Next, a separate formulation of the plastic was compounded with a high fill level of each of the identified common additives and fillers. Adhesive bond-strength evaluations were then performed on the various compounded formulations, and results analyzed to determine if there was a statistically significant difference (within 95% confidence limits) between the bond strengths obtained with the neat resins and those observed with the compounded formulations.

Commercially Available Grades. For 10 of the 25 plastics, commercially available grades were evaluated. In these cases, an effort was made to further classify the plastic by filler type, variation in base polymer chemistry, or end use. Grades of the plastic that were representative of each of these classifications were then tested. For example, when testing ionomer, grades were evaluated for each of the major cation types.

Adhesive Test-Method Selection.The lap-shear test method (ASTM D 1002) is typically used to determine adhesive shear strengths. However, because it was designed for use with metals, it has several serious limitations when evaluating plastics. For example, because plastics have much lower tensile strength than metals, plastic lap-shear specimens are more likely to experience substrate failure than are metal specimens. This makes the comparative analysis of various adhesives on a plastic very difficult to carry out, because many of the adhesives will undergo substrate failure, rendering it impossible to identify differences in adhesive performance. Another major disadvantage to using the lap-shear test method stems from the lower moduli of plastics compared with metals. As a result, plastic specimens deform more readily during testing, which introduces peel and cleavage forces on the joint. Consequently, the lower the modulus of the plastic, the more it will deform under load, and the less representative the experimental adhesive shear strength will be of the actual adhesive shear strength.

Because of these limitations, a block-shear test method (ASTM D 4501) was chosen. Block-shear testing places the load on a thicker section of the test specimen that can withstand higher loads before experiencing substrate failure. In addition, the geometry of the test specimens and the block-shear fixture helps minimize peel and cleavage forces in the joint.

Limitations of the Test Methodology. Although the bond strengths determined in this study give a good indication of the typical bond strengths that can be achieved with many plastics, as well as showing the effect of many fillers and additives, the data exhibit several limitations. For example, whereas the additives and fillers were selected because they were believed to be representative of the most commonly used types, there are many varieties of each additive and filler produced by many different companies, and different types of the same additive or filler may not have the same effect on the bondability of a material. In addition, the additives and fillers were tested individually in this study, and thus the effect of interactions between these different substances on the bondability of materials was not gauged.

Another consideration that must be kept in mind when using these data to select an adhesive/plastic combination is how well the block-shear test method will reflect the stresses that an adhesively bonded joint will experience in real-world applications. Adhesively bonded joints are designed to maximize tensile and compressive stresses and to minimize peel and cleavage stresses, so the magnitude of the former two are generally much larger than the latter two. Thus, the shear strength of an adhesive is generally most critical to adhesive joint performance, but since all joints experience some peel and cleavage stresses, their effects should not be disregarded.

Finally, selecting the best adhesive for a specific application involves more than selecting the adhesive that provides the highest bond strength. Other factors such as speed of cure, environmental resistance, thermal resistance, suitability for automation, and price will play a large role in determining the optimum adhesive system for a given use. It is suggested that readers contact their adhesive suppliers to explore these parameters completely before selecting an adhesive for an application. For any project, it is critically important to test the adhesive with actual production parts to be sure that it will meet or exceed the requirements of the job.

RESULTS

Some general trends identified in this research program are listed in Tables I­III; more comprehensive test results are also available.1

Effect of Polymer Type. Table I shows a summary comparison of adhesive shear strengths versus plastic type. Whenever possible, a neat grade of the plastic was used. If data on a neat grade were not obtainable, a representative commercially available grade of the material was chosen.

Effect of Plastic Primer. Table II lists the plastics that showed a statistically significant improvement in bond strength when a plastic primer was used in conjunction with the surface-insensitive ethyl cyanoacrylate. This primer was initially developed to improve adhesion to polyolefins, and it is interesting to note the improvement that was found with many other plastics, particularly fluorine-containing polymers.

Effect of Antistatic Additives. For many of the plastic materials tested, the addition of antistatic agents had a beneficial effect on the bond strengths achieved with cyanoacrylates. This phenomenon could in all likelihood be exploited to produce much more robust bonded assemblies without any additional processing during the bonding step. Table III summarizes these data for the rubber-toughened cyanoacrylate, which was chosen to demonstrate this trend because it generally achieved the lowest bond strengths on most plastics and consequently demonstrates the improvement more clearly.

CONCLUSION

The rapid growth in the variety of plastic materials and adhesives available to medical device manufacturers makes it often difficult to find bond-strength data for a specific adhesive/plastic combination. By providing data on the performance of common adhesives used for bonding on a variety of plastic substrates, this study will help designers identify the most promising adhesive/plastic combination for a given application. It was found, for example, that in many plastics the inclusion of an antistatic additive substantially increased the bond strength achieved with cyanoacrylates. Consequently, selecting a grade of plastic with this additive could offer many benefits in terms of durability and assembly strength, with minimal additional cost. In addition, by determining the effect that many common additives and fillers had on bond strength, the bondability of other grades of plastics not included in this study can be estimated. For situations in which the manufacturer does not have the flexibility to select a plastic that is well-suited for bonding, there are data on other techniques for improving bond strength, such as surface roughening or the use of a primer. It is hoped that one of these techniques will allow manufacturers to improve bond strength sufficiently to meet their needs.

REFERENCE

1. Comprehensive test results for the plastics listed in Table I--including identification of specific tested adhesives, polymer substrates, grades/compounds, and additives--can be obtained from the authors or Loctite Corp. (tel. 800/562-0560). Readers should request the "Design Guide for Bonding Plastics."

Patrick J. Courtney is a senior application engineer at Loctite Corp. (Rocky Hill, CT), where he specializes in the medical industry and works with customers to qualify adhesives for manufacturing processes. An application engineering specialist at Loctite, James Serenson plastic/adhesive bonding.

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