Today’s structural adhesives can be stronger than the parts they join.
By Kathrine Lewis
Technical Development Manager, Adhesives
Vantico A&T US Inc.
Los Angeles, Calif.
Edited by Jean M. Hoffman
An important part of product design is component assembly. Mechanical fastening is the traditional joining method. But structural adhesives are becoming more widely used thanks to recent advances in materials, thermoplastics, and dissimilar substrates for part assemblies. Toughened epoxies, resilient polyurethanes, and high-strength methacrylates can produce good bonds in metal, thermoplastic, and composite parts. End products using these materials range from surfboards, jewelry, and golf clubs to automotive and aircraft components and boat decking.
In many applications, adhesive bonding is actually preferable to mechanical fasteners (rivets and screws) or spot welding. Adhesives distribute stress evenly over the entire joined area rather than concentrating it at fastening or welding points. As a result, adhesive bonding gives the joint significant fatigue resistance and forms a much stiffer structure than mechanical fastening.
Furthermore, adhesives act as sealants, making joints leakproof and less prone to corrosion. They produce aesthetically attractive, smooth surface finishes since there are no protruding fasteners. The clear varieties help eliminate visible bondlines between mated parts. Adhesive bonds also dampen sound and/or vibration and often simplify assembly compared with mechanically fastened substrates.
There are drawbacks, however, to adhesive bonding. Most adhesives — besides heat-strippable varieties — bond assemblies permanently. It is difficult to dismantle them for repair or modification. Another factor that may limit structural adhesive use is the anticipated service temperatures that an assembly will see. This contrasts with mechanically fastened or welded assemblies that typically are impervious to heat or cold. Moreover, it may take longer to bond assemblies. The reason is that bonds aren't at maximum strength immediately as is the case with mechanical fastening or welding. And some assembly surfaces may need pretreatments for best adhesive strength and long-term durability.
It's also important to consider the substrates to be bonded along with the anticipated service conditions. Ditto for the physical properties of the adhesives as well as the bond joint design in structural applications.
There are few, if any, substrates which structural adhesives can't bond. The most challenging projects, such as joining thermoplastics or materials with different coefficients of thermal expansion, are often good candidates for bonding with a leading-edge epoxy, polyurethane, and/or methacrylate adhesive. Durable structural epoxies specifically formulated for the task also work well with all-metal assemblies.
Today's structural adhesives not only adhere to a broad range of substrates, they do so under extreme atmospheric conditions. That goes even for highly corrosive marine environments. For example, Italian yacht maker Firma Teck Pont uses a two-part epoxy for bonding preassembled teak floorboards to a fiberglass-reinforced plastic deck. The adhesive joints withstand wave stress tests, in saltwater, at extremes of temperature and humidity.
Similarly, epoxy adhesive secures hard-granite running surfaces to the base of curling stones used by the United Kingdom's gold-medal-winning 2002 Olympic team. The tough epoxy paste called Araldite 2013 stands up well to cold rink temperatures and fills gaps in the hard granite producing a seamless, smooth gliding surface.
In many cases, adhesive performance determines whether a new design is manufacturable. A case in point is Weber Aircraft Inc., Gainesville, Tex. Weber needed to replace polycarbonate (PC) tray tables for airplane cabins with a more durable ABS tray. But the adhesive used for PC assembly couldn't bond the hard-to-join ABS. Weber switched to an impact-resistant polyurethane which maintains good bond strength when tested under 150-lb loads. The adhesive also rapidly attains handling strength which cuts tray assembly time.
The availability of advanced methacrylate adhesives was also a decisive factor when Heywinkel, a German manufacturer of heavy plant vehicles, replaced metal cab components with thermoplastic panels, steering wheel columns, and hinges. The methacrylate adhesive is well suited for joining thermoplastics. Because it doesn't require surface pretreatment and has a fast curing rate. The adhesive reduces assembly cost, and improves productivity. In service, the methacrylate exhibits good aging properties and flexibility while maintaining long-term bond strength.
The majority of structural adhesives fall into distinct product families that carry specific performance properties.
Epoxies are the most commonly used structural adhesives. They are two-component systems consisting of a resin and hardener. They mix in different ratios to produce a wide range of viscosities, work lives, cure schedules, clarity, and degrees of flexibility or rigidity. Epoxies are renowned for their chemical, creep, and peel resistance. And in many applications, they're so tough that substrates fail before the bondline separates. Because epoxies produce durable bonds on most substrates, they handle a wide range of challenging applications from golf clubs and Navy ships to fire hose nozzles.
Durability under demanding conditions, for example, was key when the Norwegian Navy needed to bond fiberglass-reinforced plastic panels on minesweepers. Replacing metal with an epoxy-bonded composite hull and stiffeners makes the minesweepers a smaller target for radar. The epoxy adhesives help seal gaps between panels and retain good mechanical properties in harsh marine environments.
In France, epoxy adhesives serve in steel connector assemblies for state-of-the-art fire hose nozzles. The epoxies maintain high shear and peel strengths. They also resist dynamic loading in the presence of chemicals, water, and humidity.
Polyurethanes produce strong, impact-resistant joints with greater flexibility than epoxies. The adhesives are particularly well suited for bonding hard-to-join thermoplastics and fiberglass-reinforced plastics. Two-component polyurethanes are highly thixotropic and fill gaps up to 1-in. thick. Many of the adhesives feature convenient 1:1 or 2:1 mix ratios for precise, waste-free mixing and dispensing from dual-barrel cartridges. They have work lives ranging from fast to slow curing and most offer good T-peel and lap shear strengths. Applications range from bonding aircraft floorboard inserts to assembling pick-up truck bed covers and aircraft windshields.
United Airlines switched from epoxy to polyurethane adhesive when upgrading aircraft floorboards. The original aluminum and balsa wood floorboards had epoxybonded metal inserts. Epoxy was no longer a viable adhesive when the flooring changed to a honeycomb composite with polyamide-imide inserts. Engineers selected a flexible polyurethane (Uralane 5774-A/B) adhesive which has good pull strength between the composite panels and polyamide-imide inserts.
Polyurethane is now used by Leer Div., Truck Accessories Group Inc., Elkhart, Ind., to bond pick-up tonneau covers. Prime attributes of the adhesive include good flowability, sag resistance, and thermoplastic-bondability. The polyurethane resists run-out as panels are inverted and joined during assembly. The adhesive also effectively joins ABS to a more flexible panel made from Telene RIM polymer from Cymatech LLC, Calvert City, Ky., without read-through on cover surfaces. In service, the polyurethane produces resilient covers that maintain outstanding impact resistance and flex properties under cold and hot temperatures.
The craze resistance of polyurethane at temperature extremes is a key requirement for acrylic aircraft windshields from Sierracin/Sylmar Corp., Sylmar Calif. Although many adhesives are durable enough to assemble the acrylic panels, they stress-craze during use, cracking the windshield and compromising its structural integrity. Polyurethane, on the other hand, resists crazing and maintains its bond strength as the aircraft climbs from ground temperature to the much cooler cruising altitude temperatures.
Methacrylate adhesives cure fast and are resilient. The toughened adhesives bond ferrous metals, thermoset composites, and thermoplastics. They need little surface pretreatment and have a service temperature of up to 212°F (100°C). They also exhibit high shear and peel strengths and resist damage from chemicals and water. In addition, a 30-min cure time makes the methacrylates ideal for diverse applications including surfboard assemblies and jewelry manufacturing.
To fabricate surfboards, for example, builders attach rubber substrates to the fiberglass/epoxy composite board tail. Methacrylates cure sufficiently fast and last a long time in service, even when joints are submerged in saltwater. The same adhesive, reports Italian surfboard manufacturer Drops, withstands impacts from aerial acrobatic jumps at temperatures as high as 122°F (50°C).
Reliable bonding is equally important to Hong Kong costume jewelry manufacturer Artist Empire Jewellery Mfy. Ltd. Methacrylates securely affix semiprecious stones, ceramics, and pearls to pins, necklaces and bracelets. The adhesive gives a strong bond and, with an 18-min cure cycle, lets the company produce more than 1 million pieces/month.
Appropriate pretreatment of surfaces is critical to bond strength. So, too, is accurate weighing and thorough mixing of resin/hardener components. Because plastic and metal substrates are prone to contamination by lubricants, release agents, and plasticizers during manufacturing, they need thorough cleaning before bonding begins.
Typical pretreatment processes involve first degreasing the part, followed by abrading and subsequent removal of loose particles. Metal substrates may need chemical or electrolytic pretreatment to maximize bond strength and improve long-term resistance to deterioration. The correct acid treatment is determined by the metal substrate. Before bonding aluminum and titanium alloys the metal surface is anodized using phosphoric or chromic acid. For plastics and other materials, pretreatment depends on the substrate and varies case-by-case. It ranges from a simple isopropyl alcohol wipe to more extensive processes of abrading and cleaning.
Once an adhesive is selected, joints must be designed according to the stresses they will receive. Stresses for bonded substrates differ from those on mechanically fastened parts. For bonded joints, loading should be directed along the lines of the adhesive's greatest strength. Specifically, adhesives are strongest under shear, compression and tension loading, but perform less efficiently under peel and cleavage loading.
Proper design is critical to ensure joint durability and evenly distribute stress loads which optimizes strength.