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How Adhesives Work

Adhesives hold different materials together by surface attachment. Adhesives are called "glues" by DIYs, but technically they should be called "adhesives".

Adhesive products are made into different application forms: low viscosity liquids, thick paste, films, powders, pre-applied on tapes, or solids that must be melted.

Different types of adhesives are designed for a large variety of applications. Some have very weak strength for holding papers in place, others are so strong that they can used for bonding cars and airplanes.

Adhesives are valid alternatives to mechanical fastening systems, such as nuts and bolts, rivets and welding, etc. Mechanical engineers often need to consider the pros and cons of both methods.

Benefits of Using Adhesives

Adhesives have their unique advantages when compared to mechanical fasteners. Here is a brief list of the benefits.

• Often faster than mechanical fasteners
• Most of the time, adhesives are of low cost
• Adhesives can distribute loads across the entire joint area
• Adhesives usually have very good fatigue properties
• Adhesives can minimize mechanical vibrations and sound
• Adhesives can reduce galvanic corrosion between dissimilar metals

General Properties of Adhesives

When you are choosing an adhesive, you must know some of the most important properties of adhesives. These properties are discussed in detail in the following section.

1. Rate of Cure

Rate of cure is the speed at which an adhesive provides handling strength and then proceeds to full cure.

Fast curing time is desired in many applications, such as automotive or industry assembly, where fast curing means productivity.

Some adhesives, like cyanoacrylates or hot melts, give handling strength in seconds, while other adhesives often require clamping to build up strength.

In some applications, controlled rate of cure is very desirable. In this case, reactive acrylic adhesives are designed to give an induction time where no polymerisation takes place and allow assembly of large complex parts, followed by a rapid polymerisation to give quick handling strength.

2. Tensile Shear Strength

The tensile strength is the strength of  an adhesive bond when it is pulled apart, parallel to the bond line. Adhesive types like epoxies, cyanoacrylates, reactive acrylics and polyurethanes can often give tensile shear strength of 20 to 35 MPa (3000 – 5000 psi).

3. Peel Strength

Peel strength is a method of testing a bond of two flexible materials, or a flexible and a rigid material, whereby the flexible material is pulled from the mating surface at 90º or 180º angle to the plane onto which it is adhered.

This has been traditionally been the mode in which structural adhesives are the weakest. Some adhesive bonds used in the automotive industry have frequently been augmented with rivets at their ends to prevent failure from peel forces.

Some adhesive, like reactive acrylics and polyurethanes, have very good peel strengths associated with their elastomeric backbones. Others like anaerobics, cyanoacrylates and epoxies have intrinsically low peel strengths, although developments such as toughening with elastomers have improved their properties substantially.

4. Impact Resistance

Impact resistance is a key requirements for components operating under dynamic stresses and is a characteristic associated with elastomeric adhesives or those toughened with elastomers.

ASTM D950 – 03 Standard defines the test method for Impact Strength of adhesive bonds. This test method can be used to compare the sensitivity of various adhesives to suddenly applied loads.

5. Load Bearing Capability

Adhesives used in structural applications, such as in the construction or automotive industry must have a high modulus, but must not creep under operating loads. Load bearing capability is associated with highly crosslinked thermoset systems and epoxies have been the material of choice for most applications involving metal bonding.

6. Heat and Cold Resistance

High temperatures can cause a loss of strength of adhesives and ultimately degradation of the polymers. Thermoplastic polymers such as polycyanoacrylates and hot melts soften and fail at temperatures close to their glass transition temperature (Tg), and can rarely be used above 85-90ºC, whilst highly crosslinked thermosets such as anaerobics and epoxies have very good hot-strengths and can retain tensile shear strength, often to around 200ºC.

While at low temperatures, adhesives become glassy and hard and tensile shear strengths are usually very high. However, impact resistance and peel strengths become very low. Adhesives designed for operating at low temperatures use monomers and polymers with glass transition temperatures way below room temprature.

7. Fluid Resistance

Most adhesives are not exposed continuously to fluids in the same way that sealants are, but resistance is sometimes important both during processing and in service.

Typical examples are under-the-bonnet adhesives in automotive that are exposed to oils and gasoline vapours, and adhesives used to bond needles in pre-filled medical syringe, which have continuous exposure to water.

Adhesives can fail in two ways when exposed to aggressive fluids, either an interfacial adhesive failure, or dissolving or softening of the organic matrix. Additives such as silane adhesion promoters have been shown to be very beneficial in preventing adhesive failures in systems like epoxies and acrylics that are exposed to water.

8. Gap Filling Capability

Although adhesives do not typically handle very large gaps as compared to sealants, the ability to fill gaps is a very important property of adhesives, particularly when they are used to bond uneven surfaces or parts with large manufacturing tolerances.

Cyanoacrylate and anaerobic adhesives, which have the convenience of being one-component systems do have the distinct weakness that their polymerisations are initiated from the surface being bonded and lack of molecular diffusion as polymerisation proceeds prevents curing through large gaps.

In order to improve this situation, surface primers must be used, thus eliminating the very desirable one-component nature of the adhesives. Two-component systems like epoxies and polyurethanes have their components thoroughly mixed before application and can normally be used for very large gaps.

9. Bonding Dirty Surfaces

Except in electronics or medical applications, adhesives are often expected to deal with surfaces that are contaminated with process oils, rust preventatives or mould release agents. The ability to deal with contaminants without extensive surface cleaning is an important factor in determining the economic viability of an adhesive for a particular application. Mechanical fasteners do not have this limitation. Adhesive like reactive acrylics are particularly good at bonding to contaminated surfaces.

10. Health and Safety Issues

Over the last several decades, there has been a dramatic shift away from the use of organic solvents in adhesives because of flammability and toxicity concerns. Although total emissions from adhesives is quite low compared to that from the coatings industry, legislation is in place in most parts of the world to limit the use of solvents.

This has led to a dramatic increase in the use of latex adhesives, hot melts and 100% solid adhesives. However, even some of these systems have toxicity concerns, notably the amine hardeners in epoxies and isocyanates in polyurethanes.

There are still some adhesives where the use of solvents is beneficial to performance. For example, polychloroprene adhesives are available in both solvent and latex form but the solvent version still give the best overall performance for most applications.

General Properties of Hot Melt Adhesives

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1. Fast Setting

Hot melt adhesives are 100 percent solid one-component systems that are applied in a molten state and solidify by cooling. Hot melt adhesives do not have a carrier material such as a solvent or water, they are fast-setting materials, most of the time even faster than instant adhesives such as cyanoacrylates.

2. How to Dispense Hot Melt Adhesives

Hot melt adhesives are dispensed in stick form from handguns or in bulk volume from semi-automatic or automatic application systems.

3. Hot Melt Adhesive Formulation

Most hot melt adhesives are made of the following materials

• Base polymer. This is the main ingredient and it determines overall properties.
• Tackifying resins to improve tack and adhesion.
• Waxes and oils. Lower viscosity, affect surface melting.
• Plasticisers. Lower viscosity, soften polymers.
• Fillers. Affect viscosity and bond strength, lower cost.
• Antioxidants. Maintain hot stability of adhesive.

The base polymers determine most of the overall properties of the adhesive. These base polymers can include ethylene-vinyl acetate copolymers (EVA), low-density polyethylene (LDPE) and styrenic block copolymers (SBCs).

The most common base polymer is EVA:

Amorphous polyolefins (APOs) are also very important base polymers. Atactic polypropylene is a particularly useful polymer that is a by-product of the polymerisation of crystalline polypropylene.

4. The Advantages and Disadvantages of Hot Melt Adhesives

:: Loctite 454 Cyanoacrylate Adhesive Characteristics

Loctite 454 is designed for the assembly of difficult-to-bond materials which require uniform stress distribution and strong tension and/or shear strength. The product provides rapid bonding of a wide range of materials, including metals, plastics and elastomers.

Loctite 454 is particularly suited for bonding porous or absorbent materials such as wood, paper, leather and fabric. The gel consistency prevents adhesive flow even on vertical surfaces.

The following table lists Loctite 454′s characteristics.

:: Cure Speed vs. Bond Gap

The rate of cure will depend on the bondline gap. Thin bond lines result in high cure speeds, increasing the bond gap will decrease the rate of cure.

:: Cure Speed vs. Humidity

The rate of cure will depend on the ambient relative humidity. The following graph shows the tensile strength developed with tim eon Buna N rubber at different levels of humidity.

:: Cure Speed vs. Activator

Where cure speed is unacceptably long due to large gaps, applying activator to the surface will improve cure speed. However, this can reduce ultimate strength of the bond and therefore testing is recommended to confirm effect.

:: Typical Properties of Cured Material

:: Typical Performance of Cured Material

1. Adhesive Properties

After 24 hours @22ºC
Lap Shear Strength, ISO 4587:
Steel	N/mm2 (psi)	18 to 26 (2,610 to 3,770)
Aluminum (etched)	N/mm2 (psi)	11 to 19 (1,595 to 2,755)
Zinc Dichromate	N/mm2 (psi)	4 to 10 (580 to 1,450)
ABS	N/mm2 (psi)	6 to 20 (870 to 2,900)
PVC	N/mm2 (psi)	6 to 20 (870 to 2,900)
Polycarbonate	N/mm2 (psi)	5 to 20 (725 to 2,900)
Phenolic	N/mm2 (psi)	5 to 15 (725 to 2,175)
Neoprene	N/mm2 (psi)	5 to 15 (725 to 2,175)
Nitrile	N/mm2 (psi)	5 to 15 (725 to 2,175)
Tensile Strength, ISO 6922:
Steel (grit blasted)	N/mm2 (psi)	12 to 25 (1,740 to 3,625)
Buna-N	N/mm2 (psi)	5 to 15 (725 to 2,175)
After 30 seconds @22ºC
Tensile Strength, ISO 6922:
Buna-N	N/mm2 (psi)	> 6.90 (> 1,000)

2. Typical Environmental Resistance

After 1 week @22ºC

Lap Shear Strength, ISO 4587:

Mild Steel (grit blasted)

3. Hot Strength

4. Heat Aging

Aged at temperature indicated and tested @22ºC

5. Chemical / Solvent Resistance

Aged under conditions indicated and tested @22ºC

6. General Information

Loctite 454 is not recommended for use in pure oxygen and/or oxygen rich systems and should not be selected as a sealant for chlorine or other strong oxidizing materials.

7. Direction to Use Loctite 454

a) For best performance, bond surfaces should be clean and free from grease

b) Loctite 454 performs best in thin bond gaps (0.05mm).

c) Excess adhesive can be dissolved with Loctite cleanup solvents, nitromethane or acetone.

8. Storage

Store Loctite 454 in the unopened container in a dry location. Storage information may be indicated on the product container labeling.

Optimal Storage: 2ºC to 8ºC. Storage below 2ºC or greater than 8ºC can adversely affect product properties.