Structural Adhesives Guide

This guide explains what design engineers need to know about selecting structural adhesives, a category of materials you can find on Gluespec. Structural adhesives withstand load-bearing forces and provide high-strength bonds, typically for the life of an assembly. They are better at distributing stress than metal fasteners and can rival welds in terms of shear strength. Structural adhesives also fill gaps for greater structural integrity and can reduce joint movement, dampen vibrations, and improve aesthetics.

Applications for structural adhesives include automotive and aerospace, manufacturing, construction, and electronics assembly – just to name a few. They can be used to join similar or dissimilar materials together, and they eliminate the need for mechanical fasteners and the hand tools that are used with them. Structural adhesives include both one-part and two-part products that cure with moisture in the air, heat, ultraviolet (UV) light, or a combination of mechanisms. Engineers can also choose different chemistries.

Types of Structural Adhesives

Gluespec divides structural adhesives into the following categories and lets you search for them online.

• Epoxy Adhesives
• Polyurethane Adhesives
• Metal Bonding Adhesives
• Plastic Bonding Adhesives

The following sections describe each type of structural adhesive in terms of material and performance properties, set and curing times, applications and industries, key technical specifications, and general advantages and disadvantages. You’ll also find links to technical resources where you can find additional information, either in the Gluespec Knowledge Center or in the Ellsworth Adhesives Literature Library.

Epoxy Adhesives

Epoxy adhesives form tough, high-strength bonds and combine good chemical resistance with excellent environmental resistance. They bond well to similar substrates and can be used with metal, plastic, rubber, glass, ceramic, or wood. Epoxies also have good void-filling capabilities and can resist creep under sustained loads. They usually cure at room temperature or with the addition of heat, and they are available in formulations that are electrically or thermally conductive.

Unless they are modified, however, epoxy adhesives are hard and brittle after curing. With two-part epoxy adhesives, optimal curing requires proper mix ratios for best results. Room temperature curing epoxies do not work well at temperatures over 120°C, so a heat-cured epoxy is required for higher temperature applications. One-part epoxies generally require heat to cure and two-part epoxies began curing after the two materials (A+B) are mixed together.

For most structural applications, two-part epoxies are used. A resin and a hardener are mixed together, and the hardener triggers polymerization, the joining of monomer molecules to create polymer chains. This bonding reaction increases the adhesive’s strength, and two-part epoxies are generally used for high-stress applications with significant temperature changes, vibrations, or mechanical shock. Cured epoxy resins are not flammable but may appear to “smoke” during curing because heat is released.

When searching for epoxy structural adhesives, design engineers can choose liquids, gels, films, pastes, or tapes. In addition to the substrate materials, it’s important to specify the application conditions and, in the case of a higher-temperature product, a specific temperature range. Designer engineers can also choose products in various colors and with a shorter or longer cure time and working time. Some cure times are short as 30 minutes but others can take up to 24 hours.

Applications for epoxy adhesives include:

• Automotive subassemblies
• Aerospace components
• Marine environments
• Building and construction repairs
• Environments with harsh chemicals.

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Tech Specs

Here are some of the technical specifications for epoxy adhesives like Parker LORD 310.

Type	Spec	Definition
Cure	Cure Time (min)	Cure time refers to the length of time needed for something to fully cure.
	Viscosity (cPs)	Viscosity is a measure of a fluid’s resistance to flow.
Bond Strength	General Bond Strength	The amount of adhesion between the bonded substrates, usually measured in pounds per square inch.
	Shear Strength (psi)	The shear strength of a material is defined as its ability to resist forces that cause the material’s internal structure to slide against itself
Hardness	Durability	Durability is the ability of a physical product to remain functional, without requiring excess maintenance or repair, when faced with the challenges of normal operations over its design lifetime.
	Elongation %.	Elongation percentage is the process of becoming or making something become longer, and often thinner.

Resources

Here are some resources about expoxy adhesives you can find on Gluespec.

• Epoxies for Flip Chips
• Epoxies in the Aerospace Industry

Polyurethane Adhesives

Polyurethane adhesives combine high-strength and durability with strong resistance to humidity and other environmental factors. They are more flexible than epoxy adhesives after curing, but uncured polyurethane is sensitive to inadequate or excess moisture. During joining, pressure may need to be applied because foaming from the release of carbon dioxide can occur. When clamping is recommended, polyurethane adhesives generally have a shorter working time (open time, pot life).

All polyurethane adhesives use a chemical reaction between an isocyanate and a polyol; however, there are still two main types of products: one-part and two-part systems. With one-part polyurethane adhesives, moisture from the atmosphere is required for complete cross-linking. Two-part systems require mixing but generally have longer shelf lives than one-part systems. If one of the substrates is non-porous, extra moisture may be required.

Two-part polyurethane adhesives also require the proper ratio of components, but their cure times tend to depend more upon the specific formulation than upon ambient conditions. All polyurethane-based adhesives are non-flammable, and they can be formulated to provide thermal conductivity. Most products are supplied as liquids, gels, or pastes and can be used with more than one type of substrate material. Heat-cured and ultraviolet (UV) light cured products are available.

Applications for polyurethane adhesives include:

• Building and construction
• Furniture and lamination
• Aerospace assembly
• Footwear and consumer products
• Automotive parts such as truck panels

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Tech Specs

Here are some of the technical specifications for polyurethane adhesives like 3M SA24 Rite-Lok.

Type	Spec	Definition
Cure	Cure Time (min)	Cure time refers to the length of time needed for something to fully cure.
	Viscosity (cPs)	Viscosity is a measure of a fluid’s resistance to flow.
Bond Strength	General Bond Strength (psi)	The amount of adhesion between the bonded substrates, usually measured in pounds per square inch.
	Shear Strength (psi)	The shear strength of a material is defined as its ability to resist forces that cause the material’s internal structure to slide against itself.
Other Properties	Specific Gravity	The ratio of the density of a substance to the density of some substance (such as pure water) taken as a standard when both densities are obtained by weighing in air

Resources

Here are some resources about polyurethane adhesives you can find on Gluespec.

• Pros and Cons of Urethanes, Epoxy, or Silicone Adhesives
• Adhesive Application Guide

Metal Bonding Adhesives

Metal bonding adhesives create strong, permanent bonds between two metal surfaces, or sometimes between a metal and a non-metal substrate. Among structural adhesives, three main chemistries are used: epoxy, polyurethane, and methacrylate. They all provide excellent resistance to environmental factors and have high shear strength but they vary in terms of other performance properties. For example, methacrylate adhesives generally lack flame resistance.

When selecting metal bonding adhesives, designer engineers can choose liquids, gels, films, pastes, or tapes. Thermally conductive formulations are available, and engineers can specify that the adhesive will be exposed to high temperatures, low temperatures, chemical or solvents, and high moisture or humidity. One-part and two-part formulations that cure at room temperature or with heat are commonly used, but some metal bonding adhesives cure with UV light.

During product selection, it’s especially important to consider the type of metal or metals to bond. For example, epoxy adhesives can bond well to most types of metals, including aluminum and aluminum alloys. With galvanized steel, however, a primer may be required. Polyurethane adhesives also bond to most metals, but cleaning and surface preparation is generally needed. Methacrylate adhesives can also bond to metals but require activators or primers.  

Applications for metal bonding adhesives include:

• Appliance manufacturing
• Aerospace components
• Electronic assembly
• Medical devices
• Industrial joining

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Tech Specs

Here are some of the technical specifications for metal bonding adhesives like Hernon Fusionbond 375.

Type	Spec	Definition
Cure	Work / Pot Time	Pot life is the length of time in which multiple part coatings or paints can be applied to a surface
	Thixotrophic	A  thixotrophic fluid is one that takes a fixed time to return to its equilibrium viscosity when subjected to abrupt changes in shear rate
Bond Strength	Peel Strength (piw)	Peel strength is a measure of the adhesive strength of two or more material that have been bonded together
	Impact Strength (psi)	The impact strength of a material is defined as its capability to resist a sudden applied force or load.

Resources

Here are some resources about metal bonding adhesives that you can find on Gluespec.

• Fatigue Resistance of Structural Adhesives
• 5 Structural Adhesives Myths Disproved

Plastic Bonding Adhesives

Plastic bonding adhesives produce strong, permanent bonds between two plastic surfaces, or sometimes between a plastic and a non-plastic substrate. Although epoxy and polyurethane chemistries are common, this category of structural adhesives can use acrylic or methacrylate chemistries instead. Acrylic adhesives aren’t as strong as epoxies, but they tend to have faster cure speeds and provide excellent adhesion to low surface energy (LSE) plastics such as polyethylene and polypropylene.

When selecting plastic bonding adhesives, design engineers can choose liquids, films, pastes, or tapes.  Thermally conducive formulations are available, and engineers can specify products that are designed to withstand high temperatures, low temperatures, chemical or solvents, and environments with high humidity. Plastic bonding adhesives come in colors such as black, blue, brown, gray or green and can have cure times that range from 10 minutes to 24 hours.

Because LSE plastics are more difficult to bond, it’s important to choose an adhesive that will adhere to these substrates, if required. Otherwise, plasma or flame treatments may be necessary. Not all plastics have low surface energy, however, and ABS and polycarbonate are examples of high surface energy (HSE) materials that are easier to bond and that support long-term adhesion. During product selection, engineers can specify a wide range of plastics, including both commodity and engineering resins.

Applications for plastic bonding adhesives include:

• Construction
• Medical devices
• Furniture and wood working
• Sporting equipment
• Transportation

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Tech Specs

Here are some the technical specifications for plastic bonding adhesives like ITW Devcon Plastic Welder.

Type	Spec	Definition
Cure	Cure Time (min)	Cure time refers to the length of time needed for something to fully cure.
	Viscosity (cPs)	Viscosity is a measure of a fluid’s resistance to flow.
Bond Strength	Peel Strength	Peel strength is a measure of the adhesive strength of two or more materials that have been bonded together.
Hardness	Elongation %	The process of becoming or making something become longer, and often thinner
Other Properties	Flash Point (°F)	The lowest temperature at which vapors above a volatile combustible substance ignite in air when exposed to flame.
	% Solids (%)	The ratio of the density of a substance to the density of some substance (such as pure water) taken as a standard when both densities are obtained by weighing in air

Resources

Here are some resources about adhesives for plastic bonding that you can find on Gluespec

• Plastic Bonding with Adhesives – Considerations Q&A
• How Plasma and Flame Surface Treating Improve Adhesive Bonding

Gluespec is Your Source for Structural Adhesives

Gluespec’s comprehensive and quality-tested database of 20,000 adhesive materials includes the structural adhesives that design engineers need. The materials and manufacturers in our database are not limited to specific suppliers, and data is quality-checked and updated as needed whenever new data sheets or product specifications are released.  

If you need deep data on structural adhesives and other materials, you’ll find what you’re looking for in our proprietary adhesives database. Along with technical data and key specifications, you’ll find best practices and comparable materials. Design engineers can also view test method information on dozens of material properties and data points.

Finally, Gluespec’s Advanced Search provides the tools you need to conduct deep filtering among all material properties. The result is a customized grid that contains materials and data points that are specific to your project. Save custom grids for quick access and share them, along with this Structural Adhesive Guide, with your colleagues.