Bonding Technology: Thermosetting Structural Adhesives

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.”

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 (T_g). 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.

Epoxies Persevere

“Epoxies are, by far, the most widely used of any structural adhesives,” notes Master Bond Inc.’s (Hackensack, N.J.) technical director Walter Brenner. First synthesized in the 1930s, epoxies are popular because of their very high tensile strength and tensile shear strength, dimensional stability, high temperature resistance, ability to adhere to many types of materials and wide range of forms. They can be room-temperature cured or cured with heat and, depending on additives, diluents and fillers, open time can vary from minutes to months. They can be tailored for many bonding requirements, notes Brenner. And while they have a reputation for being brittle and susceptible to shock loads and cracking, Cytec’s Sweet says that the current generation of epoxies have greatly improved toughness, peel strength and chemical resistance, thanks to advances in additives and formulation.

Huntsman Advanced Materials’ (Basel, Switzerland and The Woodlands, Texas) well-known and trademarked Araldite epoxy adhesives can be traced back to Dr. Pierre Castan, a Swiss epoxy pioneer, whose findings during the 1930s were originally licensed to Ciba Specialty Chemical Co., which later sold the business to Vantico, and then Huntsman. The company offers a wide range of epoxy adhesive solutions for aerospace, marine, automotive, industrial, electronics and construction applications. At the recent JEC Composites exhibition in Paris, the company showcased its Araldite 2014 two-part paste adhesive used by the Pescarolo Sport racing team to bond components of a carbon/epoxy race car chassis that, according to team leader and LeMans record holder Henri Pescarolo, will soon endure severe cyclic fatigue and cornering loads during the punishing 2007 LeMans 24-hour endurance rally, “The 2014 paste epoxy is highly resistant to moisture and automotive chemicals and can resist temperatures up to 120°C/248°F,” notes Huntsman product manager Doug Thompson.

New research by Huntsman for the wind energy market involves formulating paste epoxies with “chemical thixotropy.” The work is driven by the assembly logistics of large wind blades, which require that technicians apply significant amounts of adhesive to the blade skins and spars with automated meter/mix machines. High-shear mixing can cause some thixotropic adhesive pastes (that is, those with a tendency to decrease in viscosity under shear) to slump or even run when the skins are moved into assembly position. To prevent slump, Huntsman chemists have added special chemical fillers to both resin and hard-ener, which causes an instantaneous physical crosslinked structure during mixing that prevents sag much better than additives like fumed silica.

Pro-Set Inc. (Bay City, Mich.) is an offshoot of Gougeon Bros., a marine fabrication company started in 1969 by brothers Meade and Jan Gougeon, who were interested in building wooden boats held together with high-strength epoxy adhesive. Sold in North America and Europe through distributors like Wessex Resins and Adhesives (Romsey, U.K.), Pro-Set formulations are used by an array of marine, industrial and aerospace customers to bond not only composite parts but wood, steel, aluminum, cast iron and concrete. Alkan Shelters (Fairbanks, Alaska) uses a Pro-set epoxy to join the side, floor and top panels of its patented “Non-Expandable” advanced composite shipping containers/shelters. The 8-ft by 8-ft by 20-ft (2.4m by 2.4m by 6.1m) containers meet International Standards Organization (ISO) and International Convention for Safe Containers (CSC) certification standards, says Alkan’s R&D manager Paul Steinert. “These container/shelters have to meet very severe requirements.”

Their endurance temperatures range from -60°F/-51°C to as high as 160°F/71°C. Further, the structure must carry high corner compression loads. This structure can support more than 100 times its weight — more than 400,000 lb/181,437 kg carried by a structure that weighs about 4,000 lb/1,814 kg. Finally, to fulfill their mission, the containers/shelters must be efficiently insulated and constructed of noncorrosive materials.

During the container’s lengthy development cycle, several adhesives were tried before highly toughened Pro-Set 176 adhesive combined with 276 hardener provided peel strength and lap shear results that met Alkan’s requirements (2,880 psi/19.96 MPa measured on steel substrate, per ASTM D-1002-72). It also delivers sufficient working time — about 90 minutes for a half-inch bead — and good sag resistance, which allowed assembly of the relatively long bond lines, while accommodating slight variations in panel fit, reports Steinert.

Epoxy adhesive formulator Magnolia Plastics (Chamblee, Ga.) has 50 years of experience providing products for aerospace, industrial, marine, electronics, transportation and other markets. The company’s Magnobond 56 A&B two-part high-strength epoxy is currently used by pultruder Strongwell (Bristol, Va.) to bond its prefabricated bridge deck components. The pultruded fiberglass tubes, 4-inch or 6-inch (101 mm or 152 mm) square profiles, are bonded together after light surface abrasion; steel rods hold the tubes in place and the entire assembly is vacuum-bagged to ensure complete cure, says Strongwell’s fabrication sales manager Ed Balaban.

“We prefer the Magnolia epoxy product over other types of adhesives, primarily because of its resistance to corrosive chemicals,” notes Balaban. He also cites the long out time (more than two hours), which provides plenty of working time for assembly of the large part. Magnobond 56 delivers service temperatures ranging from -40°F/-40°C to 160°F/71°C and its lap shear strength, measured using ASTM D1002 procedures, is about 1,000 psi/6.9 MPa, exceeding the strength of the fiberglass laminate, notes Magnolia’s assistant laboratory director Greg Bunn.

Cytec is known worldwide for its range of high-performance adhesives aimed primarily at the aerospace market, which encompasses not only epoxies but bismaleimides, phenolics, polyimides, cyanate esters and silicones as well. The company does, however, sell product into high-performance automotive and marine markets. Cytec’s FM87-1HT epoxy film adhesive, for example, offers good composite-to-composite and metal-to-metal bonds after heat cure and can withstand high fatigue loads and high temperatures in severe automotive applications — service temperature is 250°F/121°C. Customers include racing teams like Honda GP Ltd., notes Sweet.

MORE EPOXIES

Gurit, formerly SP Systems, was one of the leaders in development of a structural epoxy for bonding wind turbine blade components, notes Peter George. The company’s Spabond 340LV adhesive, certified by Germanische Lloyd for turbine blade use, is toughened to avoid cracking risks that can limit the cycle times at which very large composite parts can be cured without building up stress in the laminate. Spabond 340LV also achieves a balance of properties that helps maintain a high T_g so that the adhesive doesn’t soften when blades are exposed to high temperatures in summer sunlight. The resin component can be used with three different hardeners at a 2:1 mix ratio to customize open time: three hours is the maximum, and fixture time varies from 2 to 33 minutes depending on ambient shop temperature and the selected hardener. Tensile shear strength reaches 4,800 psi/33 MPa when the adhesive is heat-cured, and its service temperature is 156°F/69°C. Gurit also offers advice to customers on joint design and adhesive selection, through its structural engineering department.

Although Master Bond sells many types of adhesives, including silicones and polyurethanes, many of the company’s epoxies are aimed at very high-performance applications. Brenner believes that a truly structural epoxy should demonstrate 4,000 to 5,000 psi (27.6 to 34.5 MPa) in a lap shear coupon test and have a service temperature approaching 500°F/260°C. In this category is the company’s EP 36, a proprietary hybrid epoxy that reportedly offers good toughness without sacrificing either heat resistance or strength. Delivered in a container as a single-component solid, the material must be softened in an oven before application, and for best results, metal adherends should be degreased, cleaned and abraded. Cure occurs at elevated temperature, generally 250°F/121°C, and open time is between 30 to 60 minutes. Suitable for use in electronic applications, the product also is used by some missile and aerospace engine manufacturers who need high performance at very high temperatures and stresses, says Brenner, who points out, “This adhesive has successfully withstood more than 1,000 hours of service at temperatures as high as 500°F.”

Cass Polymers of Michigan (Madison Heights, Mich.), manufacturer of ADTECH Plastic Systems and TCC products, says its epoxy adhesives have a very long open time — up to three hours — which allows large parts to be assembled and repositioned, or many small parts assembled, before gel. ADTECH EA-602, a high-strength, 1:1 mix ratio liquid epoxy, provides a tensile shear strength of 4,500 psi/31 MPa and a Shore D hardness of 80. Says Davy, “This type of high-strength, long-open-time epoxy is a good choice when bonding unusual materials where other fastening methods are not practical.” The company also sells vinyl ester and urethane adhesives, both paste and liquid, for a wide variety of substrates, notes Davy.

Permabond Engineering Adhesives Ltd. (Winchester, U.K. and New Hartford, Conn.) began supplying adhesives for metal bonding 50 years ago. Its core product line includes anaerobic adhesives for threadlockers (used to secure metallic fasteners) and pipe sealants as well as cyanoacrylate (known universally as “super glue”), chemically related to acrylic adhesives described below. During 2005 and 2006, Permabond added UV-curable, toughened acrylic and two-component epoxy adhesives to its line, while expanding its other offerings as well. Single-component epoxy premiered this year, for a wide range of industry applications. Shear strength of about 6,000 psi (41 N/mm²) is provided by Permabond ES578, together with a service temperature of up to 355°F/180°C.

Collano AG (Sempach-Station, Swit-zerland and Greenville, S.C.), a long-time supplier of hot-melt adhesives, films and binders for apparel and other markets, offers a unique hybrid structural adhesive, combining silyl-reactive polymers with epoxy-reactive polymers. The result is a highly elastic adhesive, with elongation of more than 200 percent and a modest bond strength, lower than epoxy, acrylic or urethane. Yet the tough adhesive, which is flame retardant, has application in niche markets like electronic enclosures or automotive interiors, says the company.

TOUGH URETHANES, URETHANE ACRYLATES

With a balance of strength, flexibility and good adhesion to many different substrates, urethane adhesives are especially noted for extremely high elongation, in some cases more than 250 percent, which means that more work is needed to break a bond despite urethane’s weaker tensile strength and lower temperature resistance when compared to epoxy. Automotive and transportation applications for urethanes abound because of the adhesives’ toughness and because the elongation accommodates differential movement when car body parts — often of different materials and with varying coefficients of thermal expansion (CTE) — move in relation to each other during use.

Ashland Specialty Polymers and Adhesives (Dublin, Ohio) is one of the major suppliers of structural urethane adhesives to the light vehicle and heavy truck markets. Its PLIOGRIP adhesive, based on methylene diisocyanate chemistry, is used to bond the Honda Ridgeline’s composite pickup truck bed components (see “Inside-the-Box Thinking ….” under “Related Content,” at left), aluminum parts to carbon fiber/epoxy components on the Aston Martin Vanquish, as well as other high-profile car applications. For its limited-edition M3 CSL sedan (see “Application” under “Related Content”), automaker BMW conducted extensive studies to validate an adhesive that could deliver sufficient stiffness, strength, torsional rigidity, crash impact resistance and vibration performance, comparing urethanes with epoxies and acrylics. Two-part PLIOGRIP urethane paste met the requirements for bonding the car’s 13-lb/6-kg carbon fiber/epoxy roof to its steel frame, at a service temperature of 176°F/80°C and with sufficient elong-ation (above 50 percent) to handle the vibration loads and differential CTEs of the two materials, says Ashland’s automotive industry manager Marc Benevento. The composite roof, half the mass of a steel roof, helped the carmaker shed weight and lower the car’s center of gravity to improve handling.

PLIOGRIP can be dispensed with automated meter/mix machines or with ordinary manual or powered dispensing guns. Open time can be tailored from several minutes to an hour, and Benevento points out that cure, which can range from several minutes to hours at ambient temperature, can be accelerated with heat to one or two minutes for production-cycle-critical applications. The company now offers two-part PLIOGRIP 5500 series epoxies as well, where higher temperature resistance is needed.

Scott Bader Co. Ltd. (Wollaston, Northamptonshire, U.K.) brings a unique product to the market — hybrid urethane/acrylate adhesives under the brand name Crystic Crestomer. Developed specifically for composites, says the company, the adhesives contain a urethane component that is reacted into an acrylic molecular backbone. This creates a thermoset adhesive with the high elongation, flexibility, toughness and gap-filling properties of a urethane combined with the impact strength and moisture resistance of acrylics, making it ideal for marine applications.

“The first applications of Crystic Crestomer were to replace titanium rivets on Sandown class naval minesweeper vessels in the late 1970s,” says Scott Bader’s global adhesives product manager Darren Coniff. “It’s now a standard method for manufacture of minesweepers in the Royal, French, Norwegian and Swedish navies.”

The adhesive also recently saw action in “freefall” lifeboats built by Norsafe AS (Arendal, Norway). Located on offshore oil platforms, these must, in the event of fire, safely convey personnel from the platform when dropped from launch points more than 115 ft/35m above the water. When a competitor’s lifeboat collapsed during such a drop, the Norwegian Oil Industry Assn. required retrofits of all freefall lifeboats used in the North Sea. Norsafe used det Norsk Veritas (DNV)-certified Crystic Crestomer Advantage 30 paste adhesive to perform an in-the-field retrofit, adding carbon composite strengthening beams to GEF-40 lifeboat hulls and roofs. The adhesive’s 30-minute working time was a help to technicians, who were not permitted to remove the boats from their highly angled launch platforms and, therefore, had to work in confined spaces. Advantage 30 delivers 3,191 psi/22 MPa tensile strength and 100 percent elongation at break. In subsequent full-scale drop tests at heights from 72 ft to 115 ft (22m to 35m), says Coniff, Norsafe’s upgraded craft showed little structural flex and no cracking in the adhesively bonded joints at the reinforcing beams.

Acrylics

Acrylics, noted for their ability to bond most surfaces with little or no preparation, are replacing tabbed fiberglass con-struction at ever-increasing rates. (Tabbing is the practice of using wet-out strips of fiberglass to attach bulkheads, stringers or other parts to the cured inner surface of a hull.) With high tensile shear strength and greater flexibility than epoxies, good impact properties and high fatigue resistance, acrylics are widely used in marine and metal-bonding applications. Methacrylate monomers are combined with a dissolved rubber polymer (a toughening agent and crack limiter), a cure accelerator and, typically, a free-radical generator, such as peroxide. Generally confined to in-use temperatures of less than 220°F/104°C in structurally loaded conditions, acrylics actually penetrate the adherend surface to form strong covalent chemical bonds. A potential drawback is its nonhazardous but often irritating sweet odor, which manufacturers are combating with new adhesive formulations.

ITW Plexus’ (Danvers, Mass.) two-component methacrylate structural adhesives for marine and transportation applications are based on a low-viscosity monomer, which solvates the surface of most thermoplastics and composites prior to cure. Because the adhesive also solvates most surface contaminants, adherend preparation prior to adhesive application typically isn’t necessary. Plexus methacrylates are formulated with a unique combination of polymers and impact modifiers that deliver high tensile strength with high elongation (over 100 percent at 74°F/23°C), making them competitive with polyurethanes and epoxies.

Plexus AO420 adhesive is used by Comptank Corp. (Bothwell, Ontario, Canada) to bond exterior fittings to the huge filament-wound fiberglass/vinyl ester tank trailers it manufactures for hauling highly corrosive chemicals and acids. An alternative to rubber-lined steel tanks, the Comptank vessels, typically 5 ft/1.5m in diameter and 40 ft/12.3m long, meet specifications of the American Society of Mechanical Engineers (ASME) and the U.S. and Canadian Departments of Transportation. The company first winds an internal “corrosion barrier,” including two dome ends, and that cured shape becomes the mandrel for winding the thicker fiberglass/vinyl ester overwrap. Total wall thickness of a finished tank is 0.5 inch/12.5 mm. According to Comptank’s Dennis Marcus, the finished vessel is ambiently cured, then postcured for several hours at about 200°F/94°C. The bonded fittings, including hose trays, walkways, safety railings and protective flanges around the tank openings, are large and substantial composite parts. “The integrity of the tank would be compromised by drilling holes into it for mechanical fasteners,” says Marcus. “An adhesive is the only option for us.”

From simple, manual dispensing cartridges, a 10:1 mix of two-component paste is hand-dispensed onto both adherends. Working time is fairly short, about five minutes, and fixture time is about 15 minutes. AO420 reportedly delivers tensile strength of approximately 3,000 psi/20.7 MPa and a lap shear value of more than 2,000 psi/13.4 MPa, with excellent resistance to acids.

Another player in the methacrylate market is IPS Structural Adhesives Inc. (Durham, N.C.). Its trademarked Weld-On series of two-component adhesives are made to work with a range of activators to provide open times up to 120 minutes, depending on bonding requirements in marine and transportation markets. Odor has been reduced and high viscosity means reduced sag on inverted surfaces, with minimal surface prep.

Grand Banks Yachts (Loyang Crescent, Singapore) uses Weld-On as part of its fabrication process. It produces about 100 high-end all-composite yachts per year, ranging from 39 ft/12m to 72 ft/22m in length. Like many boatbuilders, the company used fiberglass tabbing to attach stringers — the structural beams that stiffen hulls along their length. Grand Banks used the process to fabricate the stringers in place on the hull, a method that took as much as three days for large hulls. About three years ago, the company switched to Weld-On SS230 HVA methacrylate to adhesively bond prefabricated stringers into place in a couple of hours, significantly improving productivity, says Matt Brandli, global sales manager: “The move to a structural adhesive has allowed Grand Banks to cycle molds much more quickly, something all boatbuilders want to do.”

Grand Banks tested several adhesives, stressing parts and joints to failure. Weld-On provided the desired combination of strength, toughness and elasticity under equatorial shop conditions, with high humidity and temperatures above 90°F/32°C. The activator component, SS220B, provides the longest working time possible at shop temperature to complete the stringer bonding, says Brandli.

Henkel Corp.’s (Rocky Hill, Conn.) Loctite structural adhesives are well known in boatbuilding, automotive and industrial applications. Recently, Loctite H4800 Speedbonder adhesive was selected by Owens Woodwork and Fiberglass Manufacturing (Anderson, S.C.) to bond curved fiberglass architectural shapes. One such shape, a giant pineapple topping a decorative fiberglass column, is molded in several parts and then bonded together. The Loctite product replaced another acrylic adhesive that allowed insufficient open time (only 15 minutes) for component assembly and lacked sufficient strength along the bond line, says Owens president/owner Anthony Owens. Applied with manual cartridge-type dispensers, the 10:1, two-part structural adhesive permits 25-minute open time, adheres well to the fiberglass and achieves desired strength with no surface prep, says Mike Shannahan, Henkel’s director of marketing for industrial assembly.

A supplier of acrylics, epoxies and urethanes for composite and metal bonding, Lord Corp. recently celebrated its 25th year and passed the 50 millionth vehicle milestone for its automotive metal-bonding structural acrylics, says Scott Miller, marketing manager for industrial assembly and components. Supreme Corp. (Goshen, Ind.) manufactures an extensive line of customized truck bodies and cargo vans, including the Spartan van, configured on either a Ford or General Motors chassis. Supreme had used rivets for its body assembly, but wanted to make the switch to a structural adhesive to eliminate unsightly fasteners on the vehicle exterior. Extensive testing was initiated in 2001, including side-by-side destructive strength comparisons (between riveted and screwed doors vs. doors bonded with Lord 400 Series adhesive), environmental aging tests, and more, on both metal-to-metal and metal-to-fiberglass situations. In every case, notes Lord’s Dan Bradshaw, the adhesive joint was stronger than the substrates involved. Supreme now uses Lord adhesives not only for exterior door panels, but also as a metallic fastener replacement in the Spartan’s fiberglass front wind fairing, which attaches to both the truck’s windshield surround and roof of the cargo box. The speed of the adhesive application and cure has cut production time by nearly 75 percent, says Bradshaw, noting that body cost and weight also is reduced through elimination of the fasteners.

What does the future hold for this important segment of the composites industry? “Greener” resins and raw materials, nanoscale additives, new ways of treating surfaces prior to bonding — all will play a part in the continued growth of structural adhesives for the composites industry.