Introduction to Thermosetting Acrylic Adhesives

Acrylic Adhesives - A Unique Class of Structural Adhesives

Acrylic Adhesives - A Unique Class of Structural Adhesives

Thermosetting acrylics are reactive methacrylic adhesives that are far different than other acrylic resins that are normally used in pressure sensitive adhesives.

They are toughened systems that cure rapidly at room temperature to provide a crosslinked structural adhesive suitable for bonding metals, composites, engineering plastics, and many other substrates. In this respect, they compete for applications with two-part room temperature curing epoxy and polyurethane adhesive systems.

With higher tensile strength, greater flexibility than epoxy adhesives, and good bond strength to polymeric substrates, the key thermosetting acrylic adhesive end-users are those involved in marine, metal, and composite bonding. They have been readily accepted in the transportation industry (autos, trucks, buses, marine craft) and are making an impact in the wind turbine blade, sports equipment, and consumer products markets.

Curing Properties

These thermosetting acrylics (see starting point formulations here) are somewhat unique for a room temperature curable adhesive. They provide high tensile shear and peel strength, chemical resistance, and impact strength. Thermosetting acrylic adhesives also have the ability to bond to a wide variety of substrates with minimal surface preparation. These substrates include composites, engineering thermoplastics, and even low surface energy substrates and oily steel.

These adhesive systems also provide several unique methods of application and cure at room temperature. Acrylic adhesives cure by a free-radical addition polymerization reaction, whereas polyurethane and epoxy adhesives cure by a condensation reaction. The cure profiles for these two reaction mechanisms are shown in the figure below:

Cure profile of addition polymerization (thermosetting acrylic) and condensation polymerization (epoxy and polyurethane)
Cure Profile of Addition Polymerization (Thermosetting Acrylic) and Condensation Polymerization (Epoxy and Polyurethane)3

Curing Mechanism

Acrylic adhesives cure by a free-radical addition polymerization reaction, whereas epoxy and polyurethane adhesives cure by a condensation reaction. The unique cure profile of thermosetting acrylic adhesives has several advantages to the end-user.

  • Room temperature cure occurs relatively quickly – in minutes or hours at room temperature depending on the formulation. Room temperature curing epoxy or polyurethane takes days or weeks to reach full cure. Thermosetting acrylics generally reach 100% cure in a matter of hours.
  • Since very little polymerization occurs during the early stages of cure, parts can be positioned and re-positioned before significant strength develops.
  • Pot life is extended to longer times than with epoxy or polyurethane systems that gradually increase in viscosity.
  • The low viscosity over a long period allows the acrylic to flow into gaps, capillaries, and any micro-roughness on the substrate which may not be possible with other adhesives.

Application Methods

Thermosetting acrylic adhesives can also be applied in the same way as conventional structural adhesives or in a much different manner.

#1. They can be applied as a standard two-component meter-mix-and-dispense system

Here, two components are mixed prior to the application either by hand or with automated mix-meter-and-dispense equipment. Mix ratios of 1:1 to about 20:1 are common. These adhesives can be formulated to provide a defined induction time (minutes to hours). During the induction period, no thickening or curing takes place, followed then by a very rapid gelation and cure. Such properties provide production advantages and minimize the time in which fixturing equipment and tooling need to be committed.

#2. One component can be applied to one substrate and the second component to the other

This is called the A/B or "honeymoon" method. One component is applied to the first substrate, and the second component is applied to the second substrate. Polymerization begins when the bond is closed and the two adhesive components meet.

#3. The base component can be applied to one substrate and an activating primer to the other

This method is less commonly used. It involves a surface activator. One part of the adhesive contains the base polymer formulation (monomers and tougheners) and a portion of the free radical reactants, and the second part contains an activator solution.

The activator solution is applied to one substrate and the solvent is allowed to evaporate. These activators can dry to a non-tacky film or remain in the liquid state depending on their chemistry. In the dry conditions, they can be stored for a long period of time before the joint is actually made. The base component is then applied to the second substrate and the substrates are mated. As with the A/B method above, polymerization begins on the contact of the adhesive with the activator.

Achieving the Optimum Performance

The curing of thermosetting acrylic adhesive is favored by thin bond lines especially for the second and third methods of application as indicated above. Optimum performance is achieved in bond lines up to 0.25 mm thick. Thin bond lines enhance the curing rate because:

  • The relatively high surface area provides rapid initiation, and
  • Oxygen cannot easily permeate the adhesive to retard polymerization.

However, certain formulations have been developed that can provide bond line thickness of 2.5 mm and greater. These thicker curing adhesives find use in applications where the bond line tolerances cannot be easily controlled such as very large turbine blades and transportation vehicle bodies.

These various methods of joining along with the capability for providing cure times ranging from very short (minutes) to relatively long (hours) provide significant production advantages in assembly operations. As a result of these properties, thermosetting acrylic adhesives are competing head-on with structural epoxy and polyurethane adhesives in high volume, cost sensitive applications that have high-performance demands.

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Structural Acrylic Adhesive Formulation and Use

The table below describes some of the advantages and disadvantages of thermosetting acrylic adhesives.

Advantages Disadvantages
  • Very fast, controllable curing at room and slightly elevated temperatures
  • Tolerant to poor surface preparation (e.g., oil on metal, untreated composites and plastics)
  • High fatigue and impact resistance
  • High elongation (up to 100% at 23°C)
  • Good moisture and outdoor resistance
  • Precise mixing ratios not necessary
  • 100% solids
  • Good bond strength to many substrates including low surface energy plastics and composites
  • Several methods available for application
  • Capable of filling large gaps
  • Monomer odor and flammability
  • Inhibited somewhat by oxygen
  • May cause stress cracking of certain plastics
  • Limited resistance to polar solvents and strongly acidic or alkaline solutions
  • Zinc surfaces may require a primer
  • Limited upper service temperature (104°C continuous)
Advantages and Disadvantages of Thermosetting Acrylic Structural Adhesives

End-Use Properties

Formulation Parameters

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