by Anne Forcum, Christine Marotta, Mike Williams and Nicole Laput, Henkel Corporation
As OEM and subcomponent manufacturers endeavor to design lower cost, lighter weight and more durable products, they have begun to replace metal and glass components with plastic components. As the number of plastic components continues to increase, it becomes more important for the manufacturers to be able to effectively and efficiently join these components for a complete assembly.
Surface Preparation Methods
In order to enhance adhesion, materials designated as hard-to-bond require surface preparation prior to joining. Surface preparation methods for hard-to-bond plastics include both chemical and physical treatments designed to increase reactivity and roughness on the surface of the substrate. Common methods include plasma or corona treatment, flame treatment, chemical etching or surface priming.
Plasma treatment is used on a wide variety of substrates including polyolefins and polyester. A gas such as oxygen, argon, helium or air is excited at low pressure, resulting in the production of free radicals. The ions generated bombard the substrate surface and form reactive groups that increase surface reactivity and wettability. One of the major drawbacks with plasma treatment is its potentially short shelf-life. Most substrates treated with plasma must be assembled within a very short period of time since the reactive surface is re-exposed to air and rapidly reverts back to its normal state. Plasma treatment is frequently used with cyanoacrylate, light-curing acrylic, and light-curing cyanoacrylate adhesive technologies. Since each plasma gas imparts different characteristics onto a substrate, end-users should consult their adhesive and substrate suppliers to determine the optimum gas for the materials used.
Similar to plasma treatment, corona discharge uses ionization of a gas to effect surface reactivity and roughness. Reactive groups such as carbonyls, hydroxyls, hydroperoxides, aldehydes, ethers or esters are introduced to the surface. Corona discharge has a limited shelf-life and requires parts to be assembled in a limited time period. Corona discharge is commonly used on polyolefin substrates. Both plasma and corona treatment require an investment in capital equipment or outsourcing of treatment. Like plasma exposure, corona treatment is most frequently used on polyolefin substrates bonded with cyanoacrylate or light-curing acrylic adhesives. The treatment generates reactive groups that serve as potential bonding sites for the adhesive, and results in a significant enhancement of bond strength.
Chemical treatment methods such as chromic acid etching are often used on polyolefins and acetals. Like other surface treatment methods, chromic acid etching adds reactive species to a surface. Because it can be hazardous, chemical treatment is used on a limited basis to treat a variety of substrates prior to bonding with cyanoacrylate and light-curing acrylic adhesives. Handling and storage of hazardous substances is one potential drawback of chemical treatments. Surface changes are controlled by two variables: the solvent selected and the overall exposure time.
In flame treatment, various reactive groups such as hydroxyls, carbonyls and carboxyls are introduced to bonding surfaces through an oxidation reaction when the substrate is exposed to flame. In addition, flame treatment increases the surface energy of the substrate surface, allowing for better wetting. Flame treatment is commonly used on polyolefins and polyacetals, and is most frequently used when bonding with cyanoacrylate adhesives.
Surface roughening results in mechanical interlocking sites and causes bond strength to increase dramatically. A surface roughness of approximately 63-125 micro-inches is often used as a guideline for assemblies that are to be bonded with adhesives. Surface roughening will significantly increase the bond strength of most adhesive technologies, and is highly recommended for both hard-to-bond and traditional substrates.
Primers are solvent-based systems in which a reactive species is dissolved. Applied to a surface using a brush or spray, the primer’s solvent evaporates, leaving behind the reactive species on the substrate. The reactive species acts as a linking pin or bridge between an adhesive and the substrate.
A variety of substrates exhibit enhanced bond strengths following treatment with primers, including polyolefins, acetals, fluoropolymers and TPVs. Although using a surface primer is often viewed as a two-step bonding process, primers can be applied with little to no capital expenditure and many remain active on substrates for more than eight hours. Polyolefin primers are frequently used on hard-to-bond substrates joined with traditional and/or light-curing cyanoacrylate technologies. Introducing the active species to the substrate surface generally increases bond strength three-fold for traditional ethyl cyanoacrylate adhesives.
Editor’s Note: The full article was originally published in the July-August 2010 issue of Plastics Decorating and addresses the seven adhesive families and adhesive selection for hard-to-bond plastics. Read the full article here.
Plastics Decorating would like to thank Application Engineer Anne Forcum, Medical Focus Segment Manager Christine Marotta, Application Engineer Mike Williams and Application Engineer Nicole Laput with Henkel Corporation for their input on this article. For more information, call (800) LOCTITE or visit www.henkelna.com.