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Instant Bonding of Plastics Using Adhesive Technologies

by Diva Evans, applications engineer

Henkel Corporation

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Figure 1. Traditional cyanoacrylate cure mechanism


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Figure 2. Surface-insensitive cyanoacrylate cure mechanism


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Figure 3. Elongation percentage of various cyanoacrylates2


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Figure 4. Heat exposure strengths of standard cyanoacrylates and thermally resistant cyanoacrylates


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Figure 5. Electromagnetic spectrum


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Figure 6. Wavelength absorbance of UV curing products


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Figure 7. Wavelength absorbance of UV/visible light products

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In a market where innovative technology is expanding at an exponential rate and production speeds continue to increase, design engineers must exploit new joining options to maintain a competitive advantage and provide new benefits to their customers. Different adhesive options have been designed to help increase production and reduce work in process. New advances in cyanoacrylates and light cure technologies have made it even easier to achieve these processing benefits in production.

Introduction to adhesive instant bonding

Increased throughput and shortened cycle times are goals that manufacturers continuously strive to reach. To hit aggressive manufacturing targets, adhesives are relied upon frequently to efficiently and permanently join plastic components. Adhesives resist vibrational stresses and distribute joint stresses evenly over a broad area, resulting in increased joint strength and long-term durability. Many adhesive chemistries provide high bond strength to a wide variety of plastics. Three of them provide instant bonding of assemblies – cyanoacrylates, light cure acrylics and light cure cyanoacrylates.

Cyanoacrylates

Cyanoacrylates (also known as CAs or superglues) are fast-fixturing, single-component adhesives. Cyanoacrylates begin the curing process once the product is confined between two mated components in the presence of surface moisture (see Figure 1).

Cyanoacrylates maintain their liquid form in the package due to acidic stabilizers. Once in contact with the surface, these stabilizers are neutralized by surface moisture, allowing the cyanoacrylate to begin its curing reaction.

Cyanoacrylate cure is completely reliant on surface moisture and greatly affected by humidity. These products typically have an average cure-through depth of 0.010" and perform best in environments with a controlled relative humidity in the range of 40 percent to 60 percent.

Although cyanoacrylates require 24 hours to reach full cure, fixture strength occurs within seconds, and accelerators can be used to increase the curing speed of cyanoacrylates. Many newer generation products offer additional benefits from classic cyanoacrylates, such as surface insensitivity, low odor and blooming, flexibility and increased thermal resistance.

Surface insensitive
Many older-generation cyanoacrylates faced curing issues when encountering acidic or dry surfaces. Newer surface-insensitive cyanoacrylates are able to overcome both of these issues to provide consistent and reliable curing. Figure 2 demonstrates the curing of surface-insensitive cyanoacrylates.

Surface-insensitive cyanoacrylates contain an additive called crown ether, which helps neutralize the inhibiting acidic surface and requires less moisture for the curing reaction than traditional cyanoacrylates. This upgrade allows for an increase in the variety of substrates that cyanoacrylates can be used to bond while maintaining the quick fixture speed of the adhesive.

Low odor/low bloom
Small, volatile molecules of cyanoacrylate monomer in open beads or bondline squeeze out can result in outgassing during the curing process. When these volatiles fall back to the surface of an assembly, they react with the present surface moisture and cause a hazy white color around the bondline. This is known as blooming. Not only can blooming result in scrap as parts are rejected for not passing visual inspection, but blooming also can affect the overall performance of final assemblies. Blooming can cause malfunctions of sensitive parts, such as electronics, by blocking internal sensors and connections.

Low odor/low bloom grade products are formulated with higher molecular weight monomers, which minimize outgassing. These products reduce the smell of cyanoacrylate and any manufacturing difficulties associated with blooming.

Flexible cyanoacrylates
A limiting factor of cyanoacrylates has been their brittleness and lack of flexibility. Historically, lack of flexibility caused cyanoacrylates not to be considered when one or both substrates are flexible for fear of brittle fractures as a result of concentrated forces from part deformation, resulting in inadequate bond strength. To meet these requirements, new products have been developed in the adhesive industry2.

Figure 3 demonstrates elongation percentage of various types of cyanoacrylates and the brittleness of traditional cyanoacrylates (typically one to two percent), as well as the increased flexibility of the latest generation of products (>100 percent).

Thermally resistant
Thermally resistant formulations offer high bond strengths at temperatures above 180°F. Figure 4 demonstrates average strength characteristics of traditional cyanoacrylates in comparison to thermally resistant formulations. At 1,000 hours at 100°C, a standard cyanoacrylate loses approximately 50 percent of its room temperature strength. Thermally resistant cyanoacrylates not only retain their strength but increase in strength after 1,000 hours at 100°C.

Light cure acrylics

Light cure acrylics are single-component, solvent-free adhesives that cure rapidly upon exposure to the proper wavelength of light and can provide a maximum depth of cure of 0.5". Because light cure acrylics remain liquid until exposed to the proper wavelength and intensity, they provide a nearly unlimited open time for positioning of parts. While cyanoacrylates cure to form thermoplastic resins with limited temperature resistance, light cure acrylics cure to thermoset resins, allowing for superior temperature and chemical resistance.

Light cure acrylics are formulated with photoinitiators that are activated upon exposure to specific wavelengths and intensities of light. These photoinitiators then form free radicals, which initiate the polymerization of the adhesive. Upon full polymerization, the adhesive is finished curing. There are three major classifications of light cure acrylics: UV curing, UV and visible light curing, or visible light-only curing. Figure 5 is a visual representation of the electromagnetic spectrum.

The type of light cure acrylic being used will determine the best light source and the intensity to maximize cure time as each of the three categories will react at different light wavelengths. Figures 6 and 7 demonstrate a typical absorbance graph of a UV curing and a UV and visible light curing product, respectively.

While light intensity and wavelength are major considerations when using a light cure acrylic, they aren’t the only important parts of using this chemistry. Because light cure acrylics can cure only in the presence of specific light wavelengths, it’s important to ensure the necessary wavelengths are not absorbed by the substrate being used, as many varieties of plastics contain UV blockers. The simplest way to check is to place the substrate between the curing light and a radiometer to see if the radiometer is able to detect the necessary wavelength through the substrate.

Light cure cyanoacrylates

Light cure cyanoacrylates are a unique chemistry among instant bonding. These adhesives cure rapidly upon light exposure (similar to light cure acrylics) and also quick fixturing via moisture cure, just like traditional cyanoacrylates. This dual-curing mechanism allows light cure cyanoacrylates to be used where neither parent chemistry can be.

Light cure cyanoacrylates allow for shadowed area curing, unlike a light cure acrylic, and provide surface curing abilities, unlike a cyanoacrylate. The dual-cure mechanisms allow this chemistry to eliminate blooming, as well as cure through large gaps, while providing high bond strength on a large variety of plastics.

Light cure cyanoacrylates have enhanced impact and fracture resistance in comparison to traditional cyanoacrylates. They also cure more quickly than traditional light cure acrylics upon exposure to light.

Surface treatment

Some plastic families are frequently used in manufacturing for their price and durability, despite being known as “difficult-to-bond.” These plastics are typically characterized by linear or branched carbon chain polymers, low surface energies, low porosity and non-polar or non-functional surfaces. These plastics include polyolefins, fluoropolymers, acetal resins and thermoplastic vulcanizates1. When using these hard-to-bond plastics, surface treatment is essential and beneficial to bond strength in nearly all instant bonding methods.

Plasma treatment
When a plastic undergoes plasma treatment, a gas (such as oxygen, argon, helium or air) is excited at a low pressure, resulting in the production of free radicals. The ions generated bombard the surface of the plastic and form reactive groups, which increase surface reactivity and wettability, resulting in a better surface environment for adhesive bonding to occur.

Manual abrasion
Manual abrasion of a plastic surface results in mechanical interlocking sites, which allow for an increase in bond strength. A surface roughness of approximately 63-125 micro-inches is a typical guideline used to maximize adhesive strength on plastic components3.

Primer
Primers are typically solvent-based chemicals that possess in their formulation a dissolved reactive species. Once the primer is applied to the plastic surface, the solvent carrier deposits the reactive component onto the surface and evaporates. The reactive components then act as bonding sites for the adhesive.

References

1. Courtney, Patrick. Designing with Plastics: A Tutorial on Adhesive Assembly. Tech. N.p.:n.p., 2011. Print.

2. Lavoie, Nicole. Time is Money: High-Speed Adhesives Solutions for Instant Bonding. Tech. N.p.:n.p., 2015. Print.

3. Salemi, Christine. Adhesive Technologies for the Assembly of Hard to Bond to Plastics. Tech. N.p.:n.p., 2012. Print.

Diva Evans has been an application engineer with Henkel Corporation since 2014, with a technology focus on cyanoacrylates. She graduated from the University of Connecticut with a bachelor’s degree in chemical engineering and is currently pursuing an MBA at the University of Southern California. In her current position, Evans supports Henkel’s customers with production locations along the US/Mexico border, as well as in the Caribbean by providing engineering support and technical know-how on adhesive use, part design and equipment integration. She can be contacted at diva.evans@henkel.com.