Two of the most important properties of structural adhesives and methyl methacrylate (MMA) adhesives without exception are the “working time” or “open time” and the “fixture time” or “handling strength.”
Working Time/Open Time
The working or open time seems to be the easiest value to grasp. This is the amount of time you have to apply the structural adhesive to the first substrate material and successfully join the second substrate material. Effectively, the amount of time you have to work with the adhesive or that the bond is “open.” Pretty simple.
For moisture-curing adhesives e.g. polyurethanes or silicones, this time starts as soon as the structural adhesive is exposed to the air. For two-component adhesives like epoxies or MMA, this time begins when the adhesive and activator are mixed.
In both the case of the one and two-component materials, this time is affected by temperature. Colder temperatures result in longer open times while warmer temperatures result in shorter open times. This is simply due to the nature of chemical reactions – which are faster when heated. In the case of humidity-curing materials, the amount of water-vapor in the air is also a factor. Slower in dry conditions, faster in humid weather.
Once we go beyond the working time of the material we begin to talk about handling & fixture time. Structural adhesive technical data sheets usually quote one or the other (or sometimes both). Plexus® brand MMA’s always show a range of values for fixture time.
Effectively, an MMA adhesive’s handling time is the amount of time that elapses after the mixing of the two-components until it’s generally possible to “handle,” which is to say gently move or pick-up an assembly. Of course, this is dependent on the size of the assembly! What would be the handling time for, say, brackets on a molded composite part would be different than a large steel plate bonded to concrete! The handling time of your application will be unique to your process, your product, and your adhesive.
Fixture time is generally a value that defines when an adhesive has achieved a given tensile strength, or a percentage of its final strength, or sometimes for a two-component material, when it has passed its peak exotherm (heat of reaction). As with the handling time, the fixture time for your application will depend on its specific characteristics.
Structural adhesive manufacturers usually pick a definition for fixture time that they apply and note it on their technical data sheets. When you are selecting a structural adhesive, it’s important to keep an eye on the definition of the fixture time used so that you can compare your choices in an apples-to-apples fashion.
The Plexus Difference
ITW Performance Polymers, makers of the Plexus brand of adhesives, is the leader in formulating methacrylate adhesives and has always held itself to the highest standards of quality and technical expertise. In that way, Plexus’ philosophy for reporting product data has always been to hold itself to a higher level. That’s not going to change. We will continue to lead the market – tell the story of our products in a way that helps our customer achieve the best result.
Do you have questions about open/working time, handling time, fixture time, or bonding in general? Contact Technical Support for more information.