Bonding Technology: Thermosetting Structural Adhesives
A wealth of epoxies, polyurethanes and acrylics speed assembly and cut manufacturing cost in an ever-wider range of composites applications.
By Sara Black, Technical Editor | June 2007
Source: IPS Corp.
Boatbuilder Grand Banks Yachts uses IPS Weld-On structural acrylic to bond stringers to hulls on this and other craft it builds, a practice that replaces fiberglass tabbing methods and, as a result, saves considerable labor and production time (see next photo).
As in life, the formation of lasting bonds requires care and understanding — not to mention the right chemistry. While mechanical fasteners have their place, good reasons abound for adhesively bonding composites to like or unlike materials: weight reduction, cost savings, elimination of localized stress points, no drilling and subsequent reduction of composite part strength by cutting fibers, and a smooth, cosmetic finished surface. Dan Bradshaw, senior account manager for industrial assembly and components for Lord Corp. (Cary, N.C.), points out, “When compared to mechanical fasteners and welding, structural adhesives distribute stresses over wider areas, act as sealants and also reduce noise and vibration.”
A structural adhesive must have strength enough to transfer or share loads between highly stressed components. A general rule of thumb is that a force of at least 1,000 psi/6.9 MPa should be required to separate a bonded joint in a lap shear coupon test, says Cytec Engineered Materials’ (Havre de Grace, Md.) adhesives product manager David Sweet. The structural adhesive’s strength comes from a combination of several phenomena. First, the adhesive’s surface energy (that is, the energy inherent in the disrupted chemical bonds that occur when the surface was created) should closely match that of the substrates, notes Sweet. If it doesn’t, the adhesive will be repelled. Second, abrasion increases the adherend’s surface area, which augments adhesion as the adhesive flows into and around microscopic crevices in the abraded surface, forming an actual mechanical interlock. Third, in some composite-to-composite bonds, the adhesive dissolves some of the adherend’s polymer molecules, allowing molecules in the adhesive to form covalent chemical bonds with molecules on the adherend, helping to chemically lock the two together.
“To achieve ‘effective adhesion’ or an optimum adhesive joint, all of these factors must work together,” says Sweet. “Over the past 50 or 60 years, adhesive science has advanced through several generations of improvements to create today’s materials that can be tailored for very specific niches.”
Source: IPS Corp.
A view of the bonded stringers on a Grand Banks Yachts hull.
Gurit UK’s (Newport, Isle of Wight, U.K.) global marketing manager Peter George points out that despite advances, formulation is still a challenge. For example, tougheners added to improve elongation and impact resistance can reduce adhesive strength and glass transition temperature (Tg). The correct selection by end users is a challenge as well.
Three basic thermosetting resin chemistries dominate the structural adhesives market: epoxy, polyurethane and acrylic. In transportation, marine and industrial applications, all three types are in common use, while in the aerospace sector, epoxy dominates, due mainly to very high strength and temperature resistance and compatibility with the sector’s predominately epoxy-based laminates. Lord’s Bradshaw notes that acrylics excel at bonding unprepared metals and composites; epoxies give the highest strength and highest temperature resistance and are a good choice for prepared metals and composites in high-stress environments; and urethanes offer good resilience and flexibility for a range of materials. The most common structural adhesive forms include one- and two-component pastes and solid films. One part, premixed forms, including films, generally require heat for cure, while two-part adhesives generally cure at ambient temperatures. Open time refers to the time available between mixing (or exposing the adhesive to air) and gel or cure initiation, and fixture time is the time required for complete cure once the two adherends are placed in contact with the adhesive. While most of the companies highlighted below manufacture a wide range of adhesives, the following focuses on case studies gathered from suppliers that best illustrate the unique benefits of select products.
Source: Jeff Sloan
Chassis components on this Pescarolo LeMans race car, shown here at the recent JEC Composites Show, are bonded together with Huntsman Araldite epoxy structural adhesive.
