Introduction to Thermosetting Acrylic Adhesives
Introduction to Thermosetting Acrylic Adhesives
Thermosetting acrylic adhesives are structural adhesives that often compete with epoxy and urethane adhesives but have very unique characteristics of their own. Although they have been commercially available since the late 1960s and relegated to rather niche markets, thermosetting acrylic adhesives are increasingly finding favor in a number of market sectors due to a unique combination of performance and cure properties.
Do you know? The original acrylic adhesives were known as "modified acrylic structural adhesives". These adhesives were crosslinkable acrylic monomers reinforced with elastomers and reactive polymers to achieve adhesion and durability. A major breakthrough, however, occurred around 1974 with the introduction to the market of "second generation acrylics" by DuPont under the trade name Cavalon.
The second generation acrylics utilize chlorosulfonated polyethylene and related materials as tougheners. This leads to increased peel and impact strength and greater durability and reliability due to more efficient transfer of stress to the rubbery phase.
These adhesives are referred to by many names including: acrylics, first generation acrylics, second generation acrylics, modified acrylics, surface activated acrylics and "honeymoon" adhesives. They also have chemistry and physical properties that are very similar to anaerobic adhesives, which are used primarily as an interference adhesive or as a sealant. As a result, confusion often exists, and the thermosetting acrylic adhesives are not very well understood.
In this guide, you will get in-depth understanding of thermosetting acrylic adhesives, their performance characteristics, and their expanding opportunities in the market place.
Comparison with Traditional Structural Adhesives
Comparison with Traditional 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)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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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
End Use Properties
An outstanding feature of reactive acrylic adhesives is that many of them possess excellent bond strength as measured by tensile-shear, peel, and impact tests. They are also capable of bonding to many different types of substrates. In the case of plastic substrates, the bond strength is normally high enough that substrate failure occurs before bond failure.
Bond Strength
Substrates to which these adhesives bond well include metals such as steel, aluminum, and copper. Most metals, due to their catalytic effect on the free radical cure mechanism, accelerate the rate at which thermosetting acrylic adhesives cure. Zinc surfaces, however, sometimes present a problem, and primers or surface treatments are necessary to enhance adhesion.
Low energy plastics are difficult to bond; however, thermosetting acrylic adhesives can bond many low surface energy plastics, including polypropylene, polyethylene, and thermoplastic polyolefins without special surface preparation. This characteristic is believed to be due to the diffusion of the acrylic monomer into the substrate before cure.
Tensile Lap Shear Strength
Plastics such as ABS, acrylic, polycarbonate, rigid PVC, nylon, phenolic, reinforced plastic, epoxy, and melamine can be easily bonded with thermosetting acrylic adhesives. Tensile lap shear strength of thermoplastics and thermosets bonded with epoxy, urethane, and thermosetting acrylic adhesives are shown in table below.
Substrate
|
Epoxy
|
Acrylic
|
Urethane
|
ABS (Cycolac)
|
374 psi (a)
|
822 psi (s)
|
632 psi (a)
|
Polycarbonate (Lexan)
|
287 psi (a)
|
1136 psi (s)
|
1054 psi (s)
|
Acrylic
|
347 psi (a)
|
1260 psi (s)
|
960 psi (s)
|
FRP
|
890 psi (a)
|
1714 psi (s)
|
356 psi (a)
|
Gelcoat
|
760 psi (s)
|
770 psi (s)
|
790 psi (s)
|
Xenoy
|
500 psi (a)
|
1200 psi (c)
|
1115 psi (a)
|
Tensile Lap Shear Strength of Thermoplastics and Thermosets Bonded with
Epoxy, Urethane and Thermosetting Acrylic Adhesives4
Certain elastomers as well as low energy plastics have been found difficult to bond. However, there have been significant advancements in thermosetting acrylic adhesives that can bond many low surface energy plastics, including many grades of polypropylene, polyethylene, and thermoplastic polyolefins without special surface preparation. Certain plastics such as acrylic and ABS are susceptible to stress cracking if exposed to excess quantities of solvent, activator, or monomer. These parts may have to be thermally annealed before application of the adhesive.
Tolerant of Surface Contamination
Another very interesting advantage of thermosetting acrylic adhesives is that they are particularly tolerant of surface contamination. They provide good bond strength on unprepared metal substrates such as oily steel. It is suspected that the oil diffuses into the polymer and acts as a plasticizer. This is a distinct advantage for bonding large area steel surfaces, where the oil (used to prevent corrosion) must be cleaned from the substrate prior to bonding and then reapplied after bonding, depending on the application.
Maximum operating temperatures of 120°C short term and 104°C continuous are usually recommended for thermosetting acrylic adhesives. High-temperature resistance can be extended to 175°C by the addition of epoxy resin. 5 Excellent low-temperature performance can be achieved down to -40°C.
Durability in Extreme Conditions
Another distinct advantage of acrylic chemistry is the moisture and outdoor weather resistance. Thermosetting acrylic adhesives exhibit durability in aggressive environments. Tests have also shown a 90% retention of the bond strength of steel joints after 100 hours exposure to 95% relative humidity and 40°C. Thermosetting acrylic adhesives are also moderately resistant to attack from many industrial chemicals. Exceptions are polar solvents, such as acetone, and strongly acidic or alkaline solutions.
Formulation Parameters