by Walter Brenner, PhD, Founder, Master Bond Inc.
Structural polymers offer distinct advantages over traditional fastening methods. Here, we explore the cornerstones of successful structural bonding.
Structural bonding is defined as the process of joining parts together using an adhesive as opposed to conventional mechanical fasteners or assembly methods such as soldering, brazing or welding. Structural adhesives are substances capable of holding and bonding parts together by chemical and mechanical surface attachment forces.
Structural adhesive bonding requires bonding agents that can transmit structural stresses without the loss of structural integrity. They accomplish this by transmitting stresses from one part to another evenly over the entire bonded area. This is in contrast to mechanical methods, such as screws, rivets and welding, where the structural strength is limited to the areas in contact with the fasteners or welds.
Structural epoxy advantages
The main benefit of structural bonding by epoxy resin adhesive systems is that they can produce assemblies mechanically equivalent to or stronger than conventional metal-fastened parts at a lower cost and weight. Additionally, they offer outstanding corrosion resistance, even upon prolonged exposure to aggressive solvents, high temperatures and other hostile environments. Some additional advantages of structural adhesive bonding include:
• uniform stress distribution and larger stress-bearing area
• outstanding fatigue, mechanical shock and thermal shock resistance
• contiguous contact between substrates promotes improved load-bearing and sealing properties
• can bond dissimilar substrate materials, including metals, plastics, elastomers, ceramics, glass and wood
• can bond materials with different coefficients of thermal expansion, even when subjected to low or elevated temperatures
• smooth, contour-free surfaces, without external projections and gaps
• gap-filling capability reduces required tolerances
• minimized galvanic corrosion between dissimilar metal substrates
• can provide thermal and/or electrical insulation or conductivity
• wide service temperature range capability
• long-term durability
Careful planning is the key to successful bonding. An engineer must consider several factors to maximize the performance of structural polymers including:
• style of joint to be bonded
• surface preparation options for the substrates involved
• pros and cons of the various polymer chemistry options
• necessary performance properties of the adhesive
• curing requirements
Take care with joint design
A properly designed joint should enhance bond strength and minimize stress concentrations so the load is distributed over the entire area. Popular joints used in structural bonding applications include butt, scarf, lap and offset lap. Butt joints are used when stress concentrations are located along the bond line and when forces perpendicular to the bond are minimal.
Scarf joints allow for an ample adhesive bond area, but parts joined in this way must maintain closer fits. Lap and offset joints are recommended for thin sections and rigid parts. In lap joints, the bonded parts are slightly offset causing peel and cleavage forces to develop when the joints are under load. These forces can be minimized by using the offset lap joint design instead.
Proper surface preparation
Surface preparation is critical in achieving high-strength structural bonds. To get the best adhesion, substrates must be properly prepared, cleaned and roughened. Substrates with oils, greases, dirt, moisture and other contaminants on their surfaces must be carefully cleaned prior to adhesive application. Certain forms of oxidation (notably loose rust) must be completely removed. Physical abrasive treatments and/or appropriate chemical cleaning are essential manufacturing steps to achieve the desired performance characteristics with most metallic substrates. Specially formulated primer coats are also useful in some applications.
A great deal of R&D has been carried out to develop suitable pretreatments for metallic and nonmetallic substrates, so be sure to carefully follow the adhesive manufacturer’s specific recommendations to assure optimum results. For more information on common surface pretreatments, please see Table 1.
Structural adhesive bonding offers significant technical and economic advantages over mechanical fasteners, as well as joining techniques such as soldering, brazing or welding for many demanding assembly constructions. Due to their unmatched processing versatility, high strength, low weight, and wide service temperature capabilities (even when exposed to hostile environmental conditions), epoxy polymer-based resin systems have become the driving force for the growth of structural bonding. Their strength properties can be further improved by compounding with glass, carbon or polyimide reinforcing fibers. Additionally, both thermally conductive and electrically conductive adhesive formulations are available and are widely used in the design of many electrical, mechanical and medical devices.
Fiber reinforced epoxy resin composites can compete economically with steadily increasing success against conventional metal constructions, both on the basis of a more favorable strength-to-weight ratio and enhanced corrosion resistance over the service life of the assembly. Their industrial success is demonstrated by the fact that some 60% of today’s aerospace structures are manufactured with epoxy resin adhesive systems.
The workhorses of the structural adhesives field are bisphenol A-type epoxy resins and amine-type hardeners. Epoxy resins and hardeners can be either liquid or solid, with liquids generally preferred. They are also available in supported and unsupported semi-cured films.
A polymer’s overall performance profile and cure requirements are mainly determined by the choice of base resin and hardener.
Structural epoxy adhesive systems feature the highest tensile strengths of all commercially available bonding agents. Resistance to moisture, fuels, oils, acids, bases and many other aggressive chemicals is of a very high order over a wide temperature range. They can safely be operated at service temperatures from as high as 300° C to cryogenic conditions.
Their outstanding gap-filling properties make them especially advantageous in structural designs. Generally, only contact pressure is required during curing. When bonding dissimilar metals, the epoxy adhesive bond also functions as protection against galvanic corrosion.
The core properties of an epoxy system, including its mechanical strength, temperature resistance, electrical insulation and chemical resistance, are primarily determined by the chosen combination of resin and curing agent. For example, one can choose a rubber-modified resin when formulating a toughened system with increased resistance to impact and thermal shock.
Other materials such as fillers, reactive diluents and flexibilizers are then added to alter the system’s properties. For example, adding aluminum oxide will change an epoxy from a thermal insulator to a thermal conductor while maintaining its electrically insulative features. Adding silver, nickel or graphite instead will produce an electrically and thermally conductive system. Various reactive diluents can be added to reduce viscosity, increase impact strength, promote adhesion and increase the chemical resistance of the epoxy system.
The vast number of resins, hardeners and additives available let the formulator tailor desirable processing and performance properties to an application’s requirements.
Types of cures
The most widely used epoxy adhesives are one- and two-component liquids or pastes. The two-component systems may be cured at ambient temperatures or more quickly at higher temperatures. One-component formulations require elevated temperature cures.
There are also specialty one-component epoxy systems that can be cured in seconds by exposure to UV light. Dual-cure systems that cure through exposure to UV light or heat are available for certain sensitive plastic substances, as well as for use in assemblies where the ingress of UV light in certain areas is limited.
As technology has advanced and assemblies have become smaller, traditional mechanical fastening methods have become increasingly difficult to use. For example, joining components that are small and thin, such as sheet metal less than 0.01 in. thick, proves problematic for traditional fastening methods. In addition, specialized methods such as welding, brazing and soldering require expensive skilled labor without yielding any clear advantage over adhesive bonding.
The contiguous contact between substrates afforded through the use of adhesive bonding serves as a metaphor for the incredible attention that adhesive manufacturers bring to the needs of industry. When the needs of one industry are met through the creation of new bonding systems, many others reap the spoils as well. For instance, the high-tech demands of aerospace and defense have lead to the development of lighter weight, high strength and heat resistant formulas that are now used in many other industries.
Master Bond Inc.