Structural Thermosetting Acrylic Adhesives - Basics & Formulation Practices

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

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.

Be an early adopter & capitalize on the full potential of this technology to meet your customer needs better. Join our exclusive course 'Structural Acrylic Adhesive Formulation and Use' and go deeper into formulating and selection of thermosetting acrylic adhesives (core chemistry, base materials, formulation principles…)

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

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

The most important components of acrylic adhesives are:

Monomers
Catalyst systems, and
Tougheners

The composition of a typical thermosetting acrylic structural adhesive is shown in the table below. The adhesive is generally formulated as a two-component system. The first component (sometimes defined as the "base") contains the monomers and initiators, and the second component (sometimes defined as the "activator") contains the curative.

Composition	Percent by Weight
Base Component:
Methyl methacrylate (vinyl monomer)	54
Chlorosulphonated polyethylene (toughening elastomer)	35
Methacrylic acid (adhesion promoter)	10
Ethylene dimethacrylate (crosslinking agent)	1
Cumene hydroperoxide (initiator)	0.5
2,6, Di-t-butyl-4-methylphenol (stabilizer)	0.15
Activator Component:
Aniline butyraldehyde (e.g., Vanax 808, R.T. Vanderbilt Company, Inc.)	100

Composition of a Reactive Acrylic Adhesive

Monomers

A wide variety of polymerizable monomers have been claimed to be used in thermosetting acrylic adhesive including methyl methacrylate, ethyl methacrylate, 2-ethylhexyl methacrylate, cyclohexyl methacrylate, and ethyl acrylate. Methyl methacrylate is generally considered to be the most cost-effective choice. Longer chain monomers increase flexibility and toughness. Acidic monomers such as methacrylic acid can be added to other monomers and are claimed to be beneficial both in terms of faster cure, increased bond strength, and temperature resistance. View several methacrylate monomers available today »

Lower volatility monomers have been used to reduce the odor problem, but they are more expensive and have lower performance in terms of curing consistency, plastic adhesion, and temperature resistance. Odor can also be improved by the addition of a paraffinic wax to reduce volatility. The wax can also extend the open time by occluding atmospheric ozone.

Catalyst Systems

A typical catalyst system uses organic peroxides, organic hydroperoxides, peresters, peracids, or azo compounds. Other combinations disclosed in the literature are shown in table below. A common catalyst system is benzyl peroxide as the initiator or oxidizing component, and a tertiary amine (typically N,N-dimethylaniline) as the curative or reducing component. Diethyl aniline, N,N-dimethyl-p-toludiene and aldehyde amine reaction products are other typical activators.

The type and concentration of the activator will impact the cure speed, exotherm, and stability of the formulation. The free radical source (e.g., benzyl peroxide) is added later in the mixing process. Often a non-reactive carrier liquid such as DGEBA or an organic plasticizer is used to preserve stability.

Initiator	Curative	Characteristics
Hydroperoxides	Aniline / n-butyraldehyde condensate	Original second generation acrylic adhesives
Cumene hydroperoxide	Tetramethylthiourea
Copper saccharinate	p-toluene sulphinic acid
Benzoyl peroxide	Ferrocene
Aromatic perester (e.g., t-butyl perbenzoate) and a soluble transition metal compound (e.g., ferric sulfate)	Amine / aldehyde condensation product	Low odor formulation
Saccharin	Amine / aldehyde condensation product	Curable hot melt formulations

Combination of Initiators and Curatives for Thermosetting Acrylic Adhesives

In most cases the free radical initiator is a constituent of the base adhesive component while the curative is applied separately either neat or as a solution in a volatile solvent to one or both of the surfaces to be bonded. However, with the benzoyl peroxide - tertiary amine combination, the amine can be a component of the adhesive and the benzoyl peroxide mixed in with the adhesive immediately prior to use.

Acrylic monomers are inherently unstable and are generally supplied with free radical stabilizers to ensure a reasonable storage life. This concentration is generally not sufficient to provide adequate storage life to formulated thermosetting adhesives, especially those containing peroxide catalyst. Thus, further additions of free radical stabilizers such as hydroquinone, methyl hydroquinone (MHQ), 2,6-di-t-butyl-4-methylphenol (BHT), and p-benzoquinone are required.

Often a compromise must be made between the storage stability and cure speed. However, BHT has been found to increase shelf life without affecting cure speed.⁴ Since the stabilizers are mostly solids, it is advantageous to add them early in the mixing cycle to allow them to completely dissolve.

To speed cure rate at room temperature, accelerators based on organic salts of transition metals (e.g., copper or cobalt naphthenate, octoates, acetylacetaonates, and hexoates of transition metals) can be added to the curative. Similar to the activators, the efficiency of these catalysts will impact the cure speed, exotherm, and stability of the adhesive.

Tougheners

Various tougheners have been used in thermosetting acrylic adhesives. The toughener is an essential ingredient in providing the thermosetting acrylic with its structural performance properties. The choice of toughener is generally made by considering its glass transition temperature and its solubility in the monomer. The toughener is generally first dissolved in monomer.

Below is the list of some of the tougheners generally added to the thermosetting acrylic adhesives:

Chlorosulfonated polyethylene
Core shell fillers
Chlorinated polyethylene
Polychloroprene
Polybutadiene
Polybutadiene / acrylonitrile copolymers

Urethane elastomers
Acrylic elastomers
Polybutadiene / isocyanate adduct
Vinyl-terminated butadiene / acrylonitrile
Polyurethane methacrylates

Thermosetting Acrylic Adhesive Tougheners

Vinyl monomers are grafted onto the chlorosulphonated polyethylene to establish a chemical link between the brittle matrix and the elastomeric phase. This chemical bonding enhances toughening and durability and is the essence behind the newer acrylic formulations.

Be an early adopter & capitalize on the full potential of this technology to meet your customer needs better. Join our exclusive course 'Structural Acrylic Adhesive Formulation and Use' and go deeper into formulating and selection of thermosetting acrylic adhesives (core chemistry, base materials, formulation principles…)

Get Started with Acrylic Polymer Selection for Adhesives

View all the commercially available acrylics & acrylic copolymers for adhesives, analyze technical data of each product, get technical assistance or request samples.

References

Briggs, P.C. and Jialanella, G.L., "Advances in Structural Adhesives", Chapter 6 in Advances in Structural Adhesive Bonding, D. Dillard, ed., Woodhead Publishing, Oxford, 2010.
DeCato, A., "Formulating (Meth)Acrylic Structural Adhesives for Modern Vehicle Assembly", Thermoses Resin Formulators Association Meeting, Montreal, Quebec, September 6-12, 2006.
Damico, D.J., "Reactive Acrylic Adhesives", Chapter 39 in Handbook of Adhesive Technology, A. Pizzi and K.L. Mittal, eds., Taylor & Francis, New York, 2003.
Morganelli, P. and Cheng, H., "New Methacrylates Match Two Part Urethane and Epoxy Performance", Adhesives Age, June 1998, pp. 22-25.
Stampler, D.J., "Toughened Acrylic and Epoxy Adhesives", in Synthetic Adhesives and Sealants, edited by W.C. Wake, John Wiley and Sons, New York, 1987.

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