Finding just the right adhesive

Oct. 10, 2002
Cyanoacrylates, light-curing acrylics, and a new hybrid, adhesive give engineers a range of fastening options.

Christine M. Salerni
Technical Market
Manager/Application Engineering Chemist
Henkel Loctite Corp.
Rocky Hill, Conn.

Edited by Stephen Mraz

Light-curing cyanoacrylate adhesives cure to depths greater than 0.25 in. with exposure to low intensity light. This far exceeds the limit of 0.01 in for traditional cyanoacrylates

Flashcure from Henkel Loctite cures in low intensity light and provides strong adhesion to a wide variety of materials including plastics, metals, and elastomers.

Traditional cyanoacrylate adhesives which are squeezed out of joints often remain tacky. With light-curing cyanoacrylates, excess material can be light cured while the adhesives in the joint cures without the need for accelerators.

Cyanoacrylate and light-curing acrylic adhesives play important roles in assembling a variety of devices. They offer designers significant manufacturing benefits, including rapid processing that is easily automated and aesthetically acceptable joints with most gaps filled. They also work on a wide variety of substrates including dissimilar materials.

However, they have several inherent limitations. A new, hybrid adhesive, light-curing cyanoacrylates, has the advantages of cyanoacrylate and lightcuring adhesives without the drawbacks.

Cyanoacrylates, one-part adhesives that cure at room temperature, undergo anionic polymerization to form thermoplastic bonds between two parts or surfaces. When applied as thin films between mating surfaces or sprayed with a chemical activator, they cure rapidly and adhere to most substrates, including most plastics. Researchers have developed a wide variety of cyanoacrylate adhesives with varying viscosities, cure times, strengths, and temperature resistances.

Typically, residual moisture on most surfaces is enough to trigger curing. They fix in about a minute and achieve full bond strength in 24 hr. This rapid cure makes them well suited for automated production.

Because they rely on surface moisture to cure, cyanoacrylates have a limited cure-through depth of approximately 0.010 in. and the size of the gap they will fill depends on their viscosities. The lowest-viscosity cyanoacrylates (~5 cP) traditionally fill gaps of only several thousandths of an inch, while gel versions fill up to 0.010 in.

In low relative humidities (30% RH or less) or with substrates having inherently low moisture levels, cyanoacrylates' ability to fill gaps decreases and cure speeds may be significantly longer. To compensate, manufacturers often add solventbased accelerators to speed the adhesive's cure and increase its gap-filling capabilities. Applying accelerators after assembly helps cure any extra adhesive squeezed out of the bond.

Primers, also solvent based, improve adhesion on difficult-to-bond substrates such as polyethylene, polypropylene, acetal homopolymer, and fluoropolymers. Manufacturers and engineers should consult adhesive manufacturers to determine if a surface primer will enhance, degrade, or have no effect on bond strength.

The downside to cyanoacrylates is that they are subject to "blooming" or "frosting" — a white haze or powdery residue near the bond line created when vaporized cyanoacrylate monomers settle to the surface. Blooming, however, is more of an aesthetic issue than a performance limitation. But it is a particular concern for manufacturers who immediately package devices after assembly. This can entrap volatiles before they polymerize and let them go on to cause blooming.

Cyanoacrylates with increased molecular weights and lower vapor pressures minimize blooming. However, these formulations can affect cure speed, physical properties, and service temperatures of the adhesive. So while traditional cyanoacrylates have maximum operating temperatures of 180 to 250°F, low-blooming adhesives weaken at approximately 160°F. Blooming is also reduced by minimizing the amount of adhesive applied, maintaining good ventilation, and using accelerators to speed the cure so the adhesive polymerizes before it can volatize.

Another drawback to cyanoacrylates is they may cause stress cracking when liquid cyanoacrylates come into contact with certain thermoplastics such as acrylic, polystyrene, polycarbonate, polysulfone, and ABS for extended periods of time. The adhesive monomers diffuse into the surface, attack the plastic, and cause stress cracks. The likelihood of stress cracking increases when there are large gaps, acidic surfaces, or low humidity, all of which prolong the cure of cyanoacrylates. Existing stresses in molded parts also increase the likelihood of adhesive-related stress cracking.

Manufacturers often anneal components with intricate designs such as sharp corners, curved components, and other places molded-in stresses are inevitable. During annealing, plastic parts are heated and cooled at a controlled rate to relieve built-in stress. To find stresses, some manufacturers use polarizing systems that display stresses.

Much like blooming, stress cracking can be limited by minimizing the amount of adhesive used, shortening cure times by using accelerators, and selecting plastics less prone to stress cracking.

Light-cure acrylics offer physical toughness, rapid cure on demand, and excellent adhesion to a variety of substrates, including heavily plasticized PVC. These one-part, solvent-free adhesives come in various viscosities and produce clear bond lines when cured in thin sections. Like cyanoacrylates, however, there are limits on the size of the gaps they can fill. In general, their gap limit exceeds that of cyanoacrylates, making them better candidates for potting connectors or housings. Cure depths in excess of 0.5 in., for example, are difficult, but not impossible.

Light-curing adhesives contain photoinitiators that convert light energy to chemical energy for curing. Once cured, they vary from hard, highmodulus resins to slightly flexible, moderate-elongation materials.

In the past, these adhesives required UV light sources to cure. But researchers have developed versions that respond to visible light sources as well.

Light-curing adhesives let manufacturers take as much time as needed to position parts without worrying about the adhesive curing. Positioning and aligning components prior to cure is critical for manufacturers of intricate devices where precision is crucial.

Unlike cyanoacrylates, light-curing acrylics form thermoset bonds with good chemical and thermal resistance. Most traditional acrylic adhesives have maximum operating temperatures of 300F, making them suitable for applications involving thermal cycling and environmental exposure.

Joints that will be created using light-curing adhesives must be designed so light reaches the entire bond line. And manufacturers must match the adhesive to the appropriate light source. Use the wrong light and the bond is unlikely to ever fully cure and could instead leave a tacky surface. Light-curing adhesives will not usually cure in shadowed areas or through opaque substrates, either. There are, however, some industrial versions that cure in shadowed areas by applying heat or chemical activators, but they also require additional processing that undermines the benefits of light-curing technology.

Introduced in 1999, light-curing cyanoacrylates were developed as an adhesive with all the advantages of cyanoacrylates and light-curing acrylics, yet none of the limitations. For example, they surface cure immediately when exposed to low-intensity UV or visible light, cure in shadowed areas, adapt to automation lines, and require no second-step accelerators or activators. They are one-part solvent-free adhesives that fix tack-free in seconds via an anionic mechanism that is not inhibited by oxygen. Because they cure so quickly, blooming is not a problem. Instead, the adhesives form a dry, glasslike surface even with short-term, low-intensity light exposure.

Any adhesive in shadowed areas cures quickly at room temperature due to the secondary moisture-cure mechanism. Light-curing cyanoacrylates also cure to depths greater than 0.25 in. within 15 sec, so they are less likely to attack plastic surfaces and cause stress cracking.

Light-curing cyanoacrylates process quickly, and are easy to use and reliable. Because they are based on cyanoacrylate chemistry, the adhesives make strong bonds to a wide variety of substrates. (Performance is similar to standard, nonphotoinitiated ethyl cyanoacrylate products.) Currently, light-curing cyanoacrylates come in two viscosity levels, low (~20 cP) and high (~900 cP).

Light-curing systems vary in cost from $1,000 for low-intensity systems to more than $10,000 for custom-designed, high-intensity systems. Tack-free surface-curing times remain the same regardless of light intensity, typically less than 5 sec. Provisions must also be made to monitor the equipment to ensure it delivers enough light to cure adhesives. Such monitoring requires the use of a radiometer.

Unlike typical light-curing acrylics, light-curing cyanoacrylates work with primers developed to improve cyanoacrylate bond strength on difficult-to-bond plastics such as polyolefins and fluoropolymers. Light-curing cyanoacrylates bond as strongly to elastomers as do traditional cyanoacrylates.

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