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Stick Together: Choose the Right Adhesive

9/19/22

Adhesives are incredibly useful. They can attach things where there's no room for bolts or screws. They can bond items that are soft or brittle, like fabric and glass. They can stick to stuff, and they can stick that stuff to other stuff. But before you start sticking stuff together, there are a few basic questions that will help you choose the right adhesive for your project.

What do you need it to stick to? Many adhesive products are designed to bind to specific materials. A glue that forms a strong bond to glass or metal may attach poorly to a more porous material like fabric or wood. 

How strong do you need it to be? There are some extremely strong adhesives that can support structural loads, but most are meant for lighter duty. Many adhesives are rated by shear strength, which is the amount of lateral, sliding pressure (in pounds per square inch) that bonded materials can withstand before breaking apart. 

How quickly do you need it to cure? Quick-setting super glues form a strong, nonstructural bond in seconds, while wood glues and white glues can require that pieces be clamped together for hours as the bond strengthens. Keep in mind that longer curing times can be helpful in some situations, giving you more opportunity to reposition pieces before they're permanently bonded.

How large are the surfaces or areas where you're applying it? It's easier to apply an adhesive to a small, targeted area if you're extruding it from a tube or cartridge. But if you want to apply it evenly to a large surface, you'll be better off with something you can spray, brush or roll on. It's also worth thinking about the viscosity or thickness of the adhesives. Thin liquids can fit in tighter spaces, while thicker liquids and gels are easier to spread over large areas. Thick pastes are best for vertical applications.

What kind of conditions does it need to withstand? Adhesives like urethane can survive a comparatively high degree of vibration and flexing. Others, like cyanoacrylate super glues, form a stiff but brittle bond, prone to cracking when flexed or impacted.

Considering Your Options

Once you have a basic idea of what the adhesive needs to do, there are many kinds to consider.

Load-Bearing Structural Adhesives

Structural adhesives form extremely strong bonds and are appropriate for load-bearing applications. There are three kinds of structural adhesives: 

  • Epoxy is the strongest and most common structural adhesive. It sticks well to a wide variety of materials, creating a durable bond that can withstand high temperatures. 
  • Acrylic cures quickly, withstands high temperatures and adheres strongly to plastics and oily or unprepared surfaces, which can be problematic for other structural adhesives.  
  • Urethane creates a more flexible bond than acrylic or epoxy and holds up well in areas that experience impact and vibration.

Construction Adhesives

Construction adhesives are designed to permanently bond building materials together, but they're not meant for load-bearing applications.

In addition to general purpose construction adhesives, there are products for special applications, including:

Spray Adhesives

Aerosolized spray adhesives can quickly cover large surfaces with a thin, even coating of glue. They're ideal for bonding large surface areas like layers of insulation or fabric upholstery.

In addition to general purpose spray adhesives, there are products for special applications, including:

Contact Adhesives

Rubber cement and other contact adhesives are liquid glues with relatively long drying times, which allows for precise application and repositioning of the materials as the bond strengthens. Contact adhesives can be spread with a brush or roller, creating a thick, even coating.

In addition to general purpose contact adhesives, there are products for special applications including:

Instant Adhesives

These super glues are strong and cure almost instantly. They come in bottles and tubes with applicator tips that allow you to squeeze beads of the adhesive directly onto the materials. The bonded materials are ready to use quickly, but you don't have the chance to reposition materials once the glue is applied. 

In addition to general purpose instant adhesives, there are products for special applications including:

Hot Melt Adhesives

Sticks of solid, thermoplastic adhesive are melted inside a glue gun to create a hot liquid that flows into gaps and binds tightly to a wide range of materials. Hot melt adhesives have the advantage of a long shelf life.

In addition to general purpose hot melt adhesives, there are products for special applications including:

Wood Glues

Wood glues create a thin layer of glue that penetrates and bonds to wood, particleboard, and composite materials.

In addition to general purpose wood glues, there are more specialized wood glues that are suitable for outdoor use and others designed for laminates, veneers and other high-precision applications involving extremely thin bond lines. 

General Purpose Glues

Don't forget the classic white glue in a squeeze bottle. General purpose glues and glue sticks are perfect for everyday tasks. They stick well to porous surfaces, and their residue is easy to wash away from harder surfaces.

Frequently Asked Questions

Q: What's the difference between reactive and nonreactive adhesives?

A: All adhesives can be divided into two basic categories&emdash;reactive and nonreactive&emdash;depending on what happens when the adhesive cures, creating the bond.

  • When reactive adhesives cure, they undergo a chemical change. Sometimes this requires mixing an adhesive resin with a hardener, causing a chemical reaction. In other cases, the adhesive undergoes a chemical change when it absorbs energy from heat or ultraviolet (UV) light, or when it absorbs moisture from the air. Reactive adhesives include epoxy, acrylic, silicone, urethane and cyanoacrylate.
  • On the other hand, nonreactive adhesives cure without undergoing a chemical change. Most commonly, these adhesives are sticky substances dissolved in water or another solvent. After application, the solvent evaporates, allowing the adhesive to harden and form the bond. Hot melt adhesives are also nonreactive, remaining solid until they're heated in a glue gun and then returning to their solid state as they cool. Nonreactive adhesives include construction adhesives, hot melt adhesives, contact adhesives like rubber cement and polyvinyl acetate (PVA) glues like wood glue and white glue.

Q: What are VOCs, and why are they in some adhesives?

A: VOCs are volatile organic compounds. Many household products and building materials contain VOCs and emit them as gasses over time through a process called off-gassing. According to the EPA, VOC can contribute to poor indoor air quality and can cause headaches and irritation of the eyes, nose and throat, among other symptoms. Some adhesives contain VOCs as a solvent, and they release VOCs into the air as they harden. When indoor air quality is a concern, look for adhesives that contain a lower number of grams of VOC per liter, or no VOCs at all. In general, there's little or no VOC content in hot melt adhesives, reactive adhesives and water-based adhesives.

Q: What's the strongest adhesive?

A: Epoxies are the strongest adhesives for structural, load-bearing applications. The strongest epoxies can typically withstand more than 5,000 pounds per square inch (psi) of shearing force. While super glues and instant adhesives form extremely strong bonds, they're also brittle, which means they can't withstand strong shearing forces and aren't suitable for structural applications.

Q: Why doesn't glue stick to the inside of the bottle?

A: Traditional white glue doesn't stick to the inside of its bottle because it contains a solvent that keeps the glue in liquid form until the solvent can evaporate. Instant super glues are packed in air-tight tubes and start to stick when they start absorbing moisture from the air, causing the chemical change that forms the bond.

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The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.