Attaching Hard-to-bond Construction Materials for Innovative Performance
- Jan 10, 2012
It's hard to deny hybrid composite beams' impressive performance. They are one-tenth the weight of concrete, one-third the weight of steel, yet strong enough to carry railroad locomotives and freight cars.1 As fibreglass composite boxes filled with a concrete and steel arch, covered by composite tops secured using a two-part methacrylate adhesive, they don't corrode, crack or chip. Consequently their useful life is at least 100 years, during which they need less maintenance than existing materials. Their reduced mass and resilient, energy absorbing, materials also let them claim excellent earthquake resistance. Finally, they can lower construction projects' carbon footprint thanks to their reduced use of concrete, and the lower fuel usage needed to ship these lighter weight products.
Figure 1: By encasing a steel-reinforced concrete arch in a fiber-reinforced plastic shell, hybrid composite beams deliver impressive performance. Credit: HCB.
The choice of methacrylate polymer to secure the composite top illuminates the properties that formulators must increasingly deliver in bonding structures that contain different materials. The three common classes of two-part room temperature curing reactive adhesives used in structural applications are epoxies, polyurethanes, and acrylics.
Epoxy adhesives are the longest-used, best known and among the most common structural adhesives in general use. They typically consist of epoxy resin adhesive components that are hardened with amines and/or polyamides. Polyurethanes normally consist of one isocyanate-terminated polyol component, paired with polyol and/or amine curative components. Methacrylate adhesives are usually a polymer-in-monomer solution component, hardened by an amine or peroxide free-radical initiator.
Epoxies are typically safe and relatively easy to mix and apply. They are strong and rigid, with high shear strengths and an affinity for metals that often sees them used with this kind of substrate. However, they need clean substrates, have a limited ability to bond thermoplastic materials, and their rigid nature makes them poorly suited to applications that demand flexibility. Polyurethanes are much more flexible, tough and elastic, which helps when adhesive bonds are subjected to impact or peeling forces and subjected to dynamic fatigue stresses. However, they are not as useful as epoxies for bonding metals, and are generally more suitable for bonding plastic materials in applications that are subjected to bending and impact stresses. They are also sensitive to contaminated surfaces, moisture and humidity. Due to the linear addition polymerisation that both types of polymer system undergo fast-curing formulations usually have very short open working time after mixing. For the same reason, adhesives with longer open times have very long cure times.
Methacrylate adhesives are much more tolerant of unclean or unprepared surfaces, and have a more user-friendly open working time and cure rate. They also provide equal or better affinity with metal and plastic surfaces than either epoxies or polyurethanes. But they struggle to bond composite materials like gel coats that form the outer or "show" surface of fiberglass reinforced polyester in the condition they're originally received in. These materials generally resist the solvating effect of the methacrylate monomers that normally soften or penetrate the bonding surface prior to the adhesive hardening. Additionally, many of these materials use processing aids that yield smooth surfaces for painting, which can also interfere with the bonding process.
Figure 2: Technicians at dispense a bead of methacrylate adhesive on a hybrid composite bridge beam. The adhesive is used to structurally bond a composite top to the beam. Credit: IPS Corporation/Scigrip
Epoxy and polyurethane adhesives have both effectively been used to bond some composite materials, but these adhesives do not completely cure at room temperature, and generally require thermal post-curing to develop full physical strength. However, methacrylates have recently been developed that display excellent adhesion to such difficult-to-bond composite substrates without needing post-curing or extensive surface preparation.2 These blend chlorinated polymers such as polychloroprene, chlorinated polyethylene and chlorosulfonated polyethylene with butadiene-acrylonitrile elastomers and methacrylate monomers and free-radical catalysts to form polymerizable methacrylate adhesives. Such adhesives exhibit a high degree of elasticity that they retain following exposure to heat, allowing engineers to reliably predict their physical characteristics when planning their designs.
That ability has enabled methacrylate adhesives exploitation in HCBs, even though they only play a comparatively small role in producing them. But it would be appropriate if their use in HCBs - which are currently employed in multimillion dollar bridge projects3 - paves the way for adhesives to play an even larger part in major construction projects.
What are the most impressive projects you've seen structural adhesives enable recently?
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posted by Hartwig Lohse, Consulting/ Training/ Education at Klebtechnik Dr. Hartwig Lohse e.K
Let me please comment on the comparision between the three adhesives chemistries, PU, epoxy and MMA.
It is not true that PU and epoxy adhesives in general require a thermal post cure. There are quite some adhesives on the market, specially designed for ambient cure. Quite a few composite bridges have been buildt using epoxy or PU adhesives.
It is correct that MMA adhesive show the shortest cure time as you compare adheisves with similar open times. On the other hand for applications referred to in the article you need adhesives with a long open time to safely apply the long beads before viscosity has reached a state that the adhesive will no longer wet the second part. As (at least for today) no adhesive cures on command you need to fixture parts anyway for some time which is related to the open time of the adhesive. There are some 2-part PU adhesives providing about 0,8 MPa after three times the open time.
Regarding bonding to contaminated surfaces please let me warn everybody to allow this as an argument for the use of any adhesive, especially in construction industry. How would you define the degree of a still acceptable contamination not having an adverse effect on the bond? Keep in mind that the degree of conamination need to be checked prior to bonding if you take quality assurance serious. As a result e.g. a solvent wipe will be done anyway and there is no longer an advantage for MMAs.
It is correct that PU adhesives do not in general stick very well to metals. If steel is used there is in many cases a coating applied to the bonded part to prevent corrosion. PU in general does bond well to such coatings.
It is also correct that epoxy adhesives have been traditionally quite brittle. New developed products, so called toughend epoxies have overcome this issue. Epoxy adhesives have proven their ability to withstand vibration and impact forces in many automotive applications bonding the crash relevant body parts. Epoxies are also used to bind rotor blades in the wind energy market, an application with very high requirement on the adhesive. On the other hand PU adhesives are not neccessarily low in strength. Tensile strength of > 20 MPa at about 50 % elongation is quite common.
Selecting the best adhesive for a particular application is for sure not an easy job and requires to consider a lot of different parameters. An adhesive working fine for one application must not neccessarily work fine for a similar one. You have to decide case by case. That's why adhesive manufacturers do support their customers by recommending suitable adheasives. If an independent statement is wanted there are consultants like myself who are able to support.