Advances in Cyanoacrylate Technology for Device Assembly

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI September 1999 Column

ADHESIVES TECHNOLOGY

Patrick J. Courtney and Christopher Verosky

Cyanoacrylates are solvent-free, one-part adhesives that cure rapidly when pressed into a thin film between two surfaces. Their ease of use and availability in USP Class VI–qualified formulations have made them attractive to manufacturers of a wide variety of medical devices. Unfortunately, earlier generations of cyanoacrylates had significant performance limitations, such as poor thermal resistance and peel strength. Consequently, many medical device manufacturers were unable to take advantage of the processing advantages that these adhesives offer. Subsequent developments in cyanoacrylate technology have greatly expanded the performance of these adhesives and led to products that offer the ease of use of a cyanoacrylate coupled with the performance of a true structural adhesive.

Cyanoacrlate adhesives provide benefits that make them advantageous for medical device manufacturing.

The intent of this article is to provide an overview of cyanoacrylate technology, with an emphasis on the technical developments that have led to these performance improvements. This overview comprises a brief explanation of cyanoacrylate chemistry, a summary of the common types of cyanoacrylates and the key developments in cyanoacrylate technology that have led to performance enhancements for these products, and descriptions of the critical adhesive process and performance properties of interest to end-users. Finally, the introduction of rubber-toughened, surface-insensitive, thermally resistant cyanoacrylates—which offer advantages over earlier generations of toughened products—is highlighted.

CYANOACRYLATE ADHESIVE CHEMISTRY

Cyanoacrylates are one-part, room-temperature-curable adhesives that are available in a wide range of viscosities. When confined in a thin film between two surfaces or sprayed with a chemical activator, cyanoacrylates cure rapidly to form rigid thermoplastics with excellent adhesion to most substrates. Cyanoacrylates typically fixture within 1 minute and achieve full bond strength in 24 hours. These types of adhesives are familiar to the general public under brand names such as Super Glue, Quick-Tite, and Krazy Glue.

Figure 1. Common cyanoacylate monomers.

Cyanoacrylate adhesives are cyanoacrylate esters, of which methyl and ethyl cyanoacrylates are the most commonly used in adhesive formulation (Figure 1). 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, trace amounts of water or other species present on the surface neutralize the acidic stabilizer in the adhesive, resulting in the rapid polymerization of the cyanoacrylate (Figure 2).

Figure 2. The cyanoacrylate cure process (cyanoacrylate anionic polymerization).

TYPES OF CYANOACRYLATES

The relative benefits and limitations of various cyanoacrylates make them appropriate for different applications.

Methyl Cyanoacrylates. Methyl cyanoacrylates are based on the methyl cyanoacrylate monomer (Figure 1). Formulations based on this monomer were the first commercially available cyanoacrylate products. This monomer has the lowest molecular weight of any cyanoacrylate monomer, and consequently leads to the most rigid polymer matrix. While generally superior to ethyl cyanoacrylates for metal-bonding applications, they do not offer the durability of rubber-toughened products on metallic substrates.

The latest generation of cyanoacrylate offers the ease of use of a one-part formulation and the performance of a structural adhesive.

Ethyl Cyanoacrylates. Ethyl cyanoacrylates are based on the ethyl cyanoacrylate monomer, the most commonly used monomer in cyanoacrylate adhesives (Figure 1). In general, ethyl cyanoacrylates are considered to offer superior performance on plastics and elastomeric substrates compared with methyl cyanoacrylates.

Low-Odor, Low-Bloom Cyanoacrylates. Cyanoacrylates have a sharp, irritating odor that can be unpleasant if proper ventilation is not used. The volatility of cyanoacrylate monomer can also lead to the formation of a white haze, known as blooming, around the cyanoacrylate bond line. Blooming occurs when cyanoacrylate monomer volatizes and settles around the bond line in the form of minute crystals of polycyanoacrylate resin. Although this is purely an aesthetic problem and does not affect the performance of the adhesive in the bonded joint, it is unacceptable in many manufacturing processes.

Blooming can be minimized through adequate ventilation, removal of excess adhesive, and rapid curing via proper adhesive selection and/or the use of an accelerator. Another solution involves the use of adhesives based on cyanoacrylate alkoxy ester monomers, which because of their higher molecular weight are less volatile than standard ethyl and methyl cyanoacrylates (Figure 1). Consequently, they do not have as strong an odor and are less likely to cause blooming.

Surface-Insensitive Cyanoacrylates. Cyanoacrylates are stabilized through the addition of trace amounts of acidic species: it is the neutralization of the acidic stabilizer by trace amounts of moisture or other species that leads to the polymerization of the cyanoacrylate adhesive. On acidic surfaces, such as wood or dichromated metals, or in low-humidity environments, the neutralization of the acidic stabilizer may be hindered, leading to long fixture times. To address this limitation, surface-insensitive cyanoacrylates were developed by adding agents such as silacrowns, crown ethers, and calixarenes to ethyl cyanoacrylates. By minimizing the effect of surface acidity and dryness, these products generally offer the most rapid fixture times of all cyanoacrylates.

Toughened Cyanoacrylates. Given their rigidity, cyanoacrylates generally have low peel and impact strength. Not surprisingly, a great deal of effort has gone into addressing this limitation through the development of toughened cyanoacrylates. These efforts are described later in the article, in the subsection devoted to peel and impact strength.

Thermally Resistant Cyanoacrylates. As a result of their thermoplastic nature, cyanoacrylates typically offer very limited high-temperature performance. Generally, at 188°F and above they soften considerably, and the bond strength drops precipitously. Early attempts to alter this characteristic involved the development of cyanoacrylates based on allyl cyanoacrylate esters (Figure 1). These materials could be cross-linked through the allyl group, resulting in a thermoset polymer matrix. Unfortunately, such systems had to be fixtured during the heat-cure step, as the cyanoacrylate would soften well below the temperatures required to initiate cross-linking. Subsequent developments in thermally resistant cyanoacrylate technology have involved the use of unique additives to produce products that provide long-term bond strength at temperatures as high as 250°F for thousands of hours without a heat-cure step. These developments are explored more fully in the subsection on thermal resistance.

Thermally Resistant, Rubber-Toughened, Surface-Insensitive Cyanoacrylates. The latest developments in cyanoacrylate technology combine the performance advantages of earlier generations of thermally resistant, rubber-toughened cyanoacrylates with additional features. These are described in detail following the review of the benefits and limitations of current cyanoacrylate technologies.

ACCELERATORS AND PRIMERS

The capabilities of cyanoacrylate adhesives can be expanded through the use of specialty accelerators and primers.

Cyanoacrylate Accelerators. Cyanoacrylate accelerators are made up of an active species dispersed in a solvent such as acetone or isopropyl alcohol. The active species is generally a base that is capable of initiating the cure of the cyanoacrylate adhesive. Accelerators can be applied to the substrates being bonded prior to adhesive application in order to minimize fixture time. Generally, the accelerator is applied to one surface while the adhesive is applied to the mating surface, which prevents curing of the adhesive by the accelerator before the parts can be mated. Accelerators can also be sprayed over cyanoacrylates to cure fillets of adhesive or unconfined drops. This technique is widely used for wire tacking on printed circuit board assemblies.

Polyolefin Primers. Low-surface-energy plastics such as polyolefins, fluoropolymers, and acetal resins are typically difficult to bond with any adhesive. Special primers have been developed that allow improved bond strengths to be achieved with cyanoacrylates on these types of materials (Figure 3). These primers are made up of an active species—typically a tertiary amine—dispersed in solvent. The primer is applied to the substrate, which is ready for bonding once the solvent has evaporated.

Figure 3. Effect of polyolefin primers on bond strength of surface-insensitive ethyl cyanoacrylate to plastics. All assemblies tested in accordance with ASTM D 4501 (block shear method). (ETFE = ethylene tetrafluoroethylene copolymer, LDPE = low-density polyethylene, PFA = polyperfluoroalkoxyethylene, PBT = polybutylene terephthalate, PMP = polymethylpentene, PPS = polyphenylene sulfide, PP = polypropylene, PS = polystyrene, PTFE = polytetrafluoroethylene, PU = polyurethane.)

Investigations into the specific method by which the primers enhance bond strength have identified two likely mechanisms. One theory is that the accelerating effect of the amine on the curing reaction enhances bond strength. It is thought that the heat released during the rapid exothermic reaction of the accelerated polymerization raises the temperature of the polymer layers in intimate contact with the adhesive and leads to improved diffusion of the cyanoacrylate polymer chains into the polymer substrate. While this mechanism may account for some of the effectiveness of the primers, it does not explain why primary and secondary amines—which also act as accelerators—are not very effective polyolefin primers.

The second phenomenon, which is thought to play a more critical role in bond-strength development, is diffusion of the alkyl chains of the tertiary amine into the polymer surface. This leaves the nitrogen with the free-electron pair at the surface to act as an initiation site for the cyanoacrylate polymerization. In this way, the tertiary amine is believed to promote bond strength by acting as an "anchoring site" for the cyan-oacrylate. This explanation would be consistent with the observation that the effectiveness of polyolefin primers increases as the crystallinity of the plastic they are used on decreases, since the more amorphous substrates would facilitate diffusion of the alkyl chains.¹

KEY PROPERTIES OF UNCURED (LIQUID) CYANOACRYLATES

In the adhesive's uncured state, the most important characteristic of interest to the end-user is the viscosity of the cyanoacrylate. Cyanoacrylates are available in viscosities ranging from water-thin liquids to thixotropic gels. The driving force behind the development of products with such a wide range of viscosities has been the need to meet end-users' varying application requirements. For example, the low-viscosity products, also known as wicking-grade adhesives, can be applied to assemblies after the components have been fitted together. The cyanoacrylate will then rapidly fill the bond area through capillary action. In some cases, the low viscosity of wicking-grade products is a disadvantage because it makes the placement of the cyanoacrylate difficult to control. Given the propensity of the adhesive to fill gaps and rapidly polymerize, the ability to control its placement is critical.

To address this need, higher-viscosity products and thixotropic gels were developed through the addition of thickeners such as polymethylmethacrylate resin and silica. While the wicking-grade-viscosity products will tend to run out of any gap larger than 0.001–0.002 in., the higher-viscosity products can be used on much larger gaps. The gels can even be applied to inverted or vertical surfaces and will not drip or run. It is still best to keep joint gaps no larger than 0.010 in., even though these products can be used in still-larger gaps without flowing. The limiting factor in joint-gap design for the higher-viscosity products is the ability of the adhesive to cure through the entire gap. One must remember that the cyanoacrylate curing process is surface initiated, and consequently it is difficult to achieve complete cure when a large volume of the adhesive is not in close proximity to a surface.

Most types of cyanoacrylate are commercially available in the entire range of viscosities, with the exception of the rubber-toughened cyanoacrylates. The addition of the rubber to the formulation raises the viscosity such that wicking grades of the rubber-toughened cyanoacrylates cannot be produced.

KEY PROPERTIES OF CYANOACRYLATES DURING CURING (LIQUID TO SOLID)

Cyanoacrylates are known for their rapid fixture and cure speed. Figure 4 shows typical fixture speeds of three types of cyanoacrylates on a variety of substrates. When reviewing such data, it is important to understand what is meant by fixture speed. Adhesive manufacturers will cite the time required to develop a certain shear strength as the fixture speed of the adhesive. In the testing done to generate the data shown in Figure 4, the fixture time was defined as the time required for the adhesive to achieve a shear strength of 14.5 psi when tested in accordance with ASTM D 1002.

While such testing is valuable for making comparisons between adhesives, it should be noted that the actual fixture time for an adhesively bonded joint of an assembly on the manufacturing floor may differ significantly from the value found on a product data sheet. In a manufacturing process, the fixture time of an adhesively bonded joint is the time required for the joint to develop sufficient strength for the assembly to proceed to the next step in the manufacturing process. For example, if the joint has a large bond area and bears very little load, the fixture time required will be much shorter than for an assembly in which the bond area is limited and needs to bear a significant load in subsequent operations.

Figure 4. Fixture time for several types of cyanoacrylates. In these tests, the fixture time was defined as the time required to develop a shear strength of 14.5 psi by ASTM D 1002.

From the data in Figure 4, some interesting trends can be noted. As would be expected, the surface-insensitive cyanoacrylate generally fixtured fastest, followed closely by the ethyl cyanoacrylate, while the rubber-toughened cyanoacrylate had the longest fixture times. The difference between the ethyl cyanoacrylate and the surface-insensitive cyanoacrylate was most significant on the acidic surface of zinc dichromated steel, where the surface-insensitive cyanoacrylate had a fixture time of 15 seconds compared with 60 seconds for the ethyl. The longer fixture time of the rubber-toughened product is characteristic of these products and represents one of the trade-offs associated with earlier-generation rubber-toughened cyanoacrylates. Time to complete cure generally follows the trends seen in fixture speeds, with most cyanoacrylates reaching full strength in 24 hours while the rubber-toughened products require a week to develop ultimate bond strength.

Since the cure of cyanoacrylates is initiated by the presence of surface moisture, it is not surprising that relative humidity can have a strong effect on cure speed. Generally, if the relative humidity falls below 30%, the cure rate will drop dramatically. It is best to use a surface-insensitive product in this situation or an accelerator if the humidity cannot be maintained at an acceptable level.

KEY PROPERTIES OF CURED (SOLID) CYANOACRYLATES

In its cured (solid) state, the cyanoacrylate properties of concern to end-users are those associated with performance of the adhesive in a bonded joint when subjected to different environmental and physical stresses. To help evaluate the suitability of various cyanoacrylates for device applications, the following properties are discussed in this section: initial and long-term shear strength, resistance to humidity and moisture, resistance to solvents, peel and impact strength, and the ability to withstand exposure to elevated temperatures.

Figure 5. Typical bond strengths of several types of cyanoacrylates on common metals. Testing done according to ASTMD 1002.

Figure 6. Typical bond strengths of several types of cyanoacrylates on common plastics. All samples tested according to ASTM D 4501

Initial Shear Strength. Once the cyanoacrylate adhesive has cured, one of the most important properties is the bond strength it has to various substrates. Figures 5 and 6 show typical shear strengths achieved with a variety of metallic and polymeric substrates. The rubber-toughened cyanoacrylates generally show the best bond strength to metallic substrates, whereas the ethyl and surface-insensitive cyanoacrylates demonstrate better performance on polymeric substrates. It should be noted that the difference in performance on metallic substrates between the various types of cyanoacrylates is not as distinct as the difference on polymeric substrates. On plastics, rubber-toughened cy-anoacrylates generally tend to have bond strengths that are from 25 to 50% of those achieved by surface-insensitive and ethyl cyanoacrylates.²

One substrate that poses a unique challenge for cyanoacrylates is glass. While the initial bond strength will often be high enough to break the glass, it drops dramatically after a number of weeks, resulting in a bond of negligible strength. This phenomenon demonstrates in a drastic way the importance of considering long-term bond strength when evaluating an adhesive for an application.

Long-Term Shear Strength. Once cured, cyanoacrylates form thermoplastic polycyanoacrylate, which is a stable, inert material at room temperature. Assemblies bonded with cyanoacrylate have shown acceptable long-term performance in a variety of applications. Typically, accelerated testing is done at high temperatures and extreme conditions to predict whether long-term bond performance will be acceptable. While these techniques are valuable, it is desirable to see actual long-term data in order to substantiate predicted values.

An investigation of the durability of the long-term bond strength of cyanoacrylates on electroless-nickel-plated steel was performed to address this need. This material was selected as the test substrate because its smooth surface and lack of reactivity make it particularly difficult to bond; consequently, it would represent a worst-case scenario for maintaining long-term bond strength. The results of this evaluation are shown in Figure 7. As can be seen, after 3 years the bond strength has yet to show any significant change.

Figure 7. Long-term bond durability of rubber-toughened cyanoacrylate on electroless nickel. Testing done according to ASTM D 1002.

Figure 8. Effect of water immersion on bond strength of rubber-toughened cyanoacrylate to steel and aluminum. Testing done according to ASTM D 4501. Bond area 0.5 x 1.0 in., no induced gap. Humid = 120°F, condensing humidity.

Humidity and Moisture Resistance. Because cyanoacrylates are extremely polar, a reasonable concern is that a polar solvent such as water could detrimentally affect their long-term bond strength. Testing has shown that the resistance of a cyanoacrylate-bonded joint to long-term exposure to humidity or immersion in water is dramatically affected by the type of substrate. For example, on metallic substrates such as aluminum and steel, exposure to humidity leads to a drop-off in bond strength for all types of cyanoacrylates. Figure 8 shows the effect of humidity and immersion in water for rubber-toughened cyanoacrylates on aluminum and steel. Data for rubber-toughened cyanoacrylates is shown because they typically demonstrate the best performance in water or humid environments. Consequently, using these products with no induced gap should represent the best-case scenario for a cyanoacrylate bond. Even under these conditions, the trends are clear. On steel, there was a decline in bond strength over time; on aluminum, the strength dropped off more abruptly, after just 2 weeks of immersion. On polymeric substrates such as ABS and polycarbonate, however, even standard ethyl cyanoacrylates showed good bond-strength retention after 4 weeks of immersion in water (Figure 9). Similar results have also been seen with neoprene rubber.

Figure 9. Effect of water immersion on bond strength of ethyl cyanoacrylate to ABS and polycarbonate. Testing dome according to ASTM D 4501. Bond area 0.5 x 1.0 in., no induced gap. Humid = 120°F, condensing humidity.

The ability of cyanoacrylates to resist attack from moisture when bonded to polymeric substrates can be most drastically tested by subjecting bonded assemblies to autoclaving. The autoclaving process combines the environmental stresses of high temperature, high-pressure, and humidity. As such, it provides a good indicator of the ability of adhesives to withstand exposure to moisture.