Acrylic Pressure-Sensitive Adhesives (PSAs): A Comprehensive Guide

Versatility of Acrylic PSAs

Acrylic adhesives are noted for their versatility and scope. They are available as:

Polymeric solvent solutions
Waterborne emulsions
Monomer mixtures, which may be cured by ultraviolet light, heat or chemical catalysts

The adhesives either cure in the bond line (thermosetting acrylic, cyanoacrylate, or anaerobic acrylic monomers) or from high molecular weight polymers that are formed outside the bond line.

Although acrylic polymers are used in laminating adhesives and construction sealants, the most common acrylic adhesive types used today are those that are available as water emulsions in pressure-sensitive adhesives (PSAs). They are thermoplastic systems that can be crosslinked depending on the requirements of the application.

General Properties of Acrylic PSAs

Acrylic PSAs are widely used in the several applications due to the polymer's saturated nature and its resulting resistance to oxidation.

Polyacrylate films are water-white and do not yellow on exposure to sunlight.
The oxidation resistance surpasses most other polymers used for PSAs except for silicones.
Monomers with various functional groups can be introduced during polymerization so that an adhesive with various degrees of crosslinking properties can be prepared.

Acrylic PSAs adhere well to many substrates and have a good overall balance of cohesive properties. Acrylic polymers used in PSAs are:

Soft
Capable of wetting the adherend surface
Capable of enough cold flow to fill any surface irregularities

Such properties are generally found in polymers of low glass transition temperature (Tg). Polyacrylates are, however, generally weak in the areas of low surface energy adhesion and tack unless a significant compromise is made in cohesive strength. As with natural rubber, acrylate-based PSAs often need to be crosslinked to obtain the cohesive strength necessary to meet specific end-use requirements.

Acrylic pressure-sensitive adhesives are made with several types of acrylic monomers that are polymerized to high molecular weight polymers. The adhesive can be made from one monomer or a mixture of several types.

Acrylic adhesives are formulated with polymers of relatively low molecular weight and Tg so that they are inherently soft at ambient temperatures. These adhesives:

Are thermoplastic by nature and soften when exposed to heat
Are polar in nature and tend to stick well to polar substrates such as metals, glass, and high-energy polymeric surfaces
Do not necessarily require tackifiers for pressure-sensitive adhesive properties

The adhesive properties, however, can be varied and fine-tuned by compounding with numerous ingredients as in the case of elastomeric pressure-sensitive adhesives. Modified acrylic adhesives commonly incorporate tackifiers and other resinous components to improve adhesion. These modified acrylics offer improved initial tack and adhesion to low surface energy materials compared with standard acrylic formulation.

The advantages and disadvantages of using acrylics as PSAs:

Advantages	Disadvantages
Crosslinkable Good resistance to varying temperature (-45 to +121°C) Good resistance to chemicals, UV, and oxidation Color stable Good shear strength Good hydrolysis resistance	Generally poor adhesion to low surface energy polymers (e.g., polyolefins) Moderate cost Initial tack is low Poor creep resistance compared to elastomer-based adhesives

The acrylic PSA adhesives are available as solvent solutions, aqueous emulsions, hot melts.

Solvent based	Waterborne	Hot-melt
Advantages
Quick drying Good adhesion to non-polar substrates Good bond strength to plastics Versatile	Easy cleaning Good adhesion to polar substrates Good heat and aging resistance High solids Ready to use	Very fast setting No solvent waste Environmentally acceptable 100% active
Disadvantages
Flammability Toxicity Relatively low solids content Less easy to clean	Slow drying Requires heat to dry Poor adhesion to non-polar substrates	High equipment cost Requires heat Thermal degradation possible Difficult to clean Temperatures can affect substrate

View a variety of acrylic polymers as per PSA system »

Waterborne Acrylic PSAs Have a Dominant Position

Waterborne acrylic PSAs are prepared by emulsion polymerization of acrylate monomers in water using emulsifiers and water-soluble polymerization initiators. The resulting polymer emulsion (50-60% solids) is generally stabilized with a surfactant.

Particle size of the acrylic polymer in emulsion is 0.1-0.4 microns
It has viscosity between 100-500 cps
The Tg is approximately -50°C to +20°C
The final emulsion generally has a solids content of 50-60 %

Many waterborne acrylic polymers provide all of the functions required for a high-quality pressure-sensitive adhesive. They are:

Permanently tacky at room temperature
Spontaneously adhere on contact with little pressure
Require no activation by water, solvent, or heat to form a strong bond.

As a result of the advantages of waterborne acrylic PSAs they are used in a wide variety of applications from transparent tapes and labels to medical bandages. Their excellent weathering properties have established the use of these adhesives in many decorative applications including exterior automotive decoration and outdoor tapes and signs.

Related Read: Emulsion Polymers Selection for Adhesives and Sealants

The primary disadvantages of waterborne acrylic PSAs are that they lack cohesive strength, water resistance, film clarity, high temperature adhesion, and adhesion to low surface energy substrates. They also have been noticed to have poor wet-out on release liners and to have foaming problems during application. However, with proper formulation these disadvantages can be minimized or eliminated.

Before understanding the complete chemistry behind acrylic PSAs and additives commonly used in the formulation, let’s first explore the basics about PSA adhesion…

Adhesion of Pressure Sensitive Adhesives

The adhesion of any PSA will depend primarily on three properties:

Tack (ability to adhere quickly)
Peel strength (ability to resist removal by peeling)
Shear resistance (ability to resist flow when shear forces are applied)

Adhesion depends on Tack, Peel Strength and Shear Resistance

These properties are generally derived from the:

Glass transition temperature (Tg)
Molecular weight and molecular weight distribution
Viscoelastic nature of the base polymer

However, the formulator also has some control over these characteristics through selection of additives that are employed in the adhesive formulation.

Related Read: Test Methods to Evaluate Tack

Acrylic Monomers: Chemistry and Properties

By selection of the monomer and polymerization conditions, many acrylic base polymers are available. Acrylic adhesives are generally prepared from copolymers or tripolymers. By changing the polymer structure in this way, the polymer properties can be engineered for an application.

Homopolymers are not often used because they cannot provide the proper balance of:

Tack
Cohesive strength
Resistance to creep or flow that is required for most commercial applications.

Acrylic PSAs are produced mainly from soft monomers that have:

A low glass transition temperature (Tg)
A high level of molecular entanglement

Let’s start by learning about different monomer used to manufacture acrylic polymers for PSAs…

Typical Monomers and Comonomers Used

A typical acrylic polymer for PSA applications is composed of the following elements:

Low Tg monomer such as butyl acrylate, 2-ethylhexyl acrylate, or iso-octyl acrylate to impart the pressure-sensitive tack
Higher Tg monomer such as methyl methacrylate, methyl acrylate, or vinyl acetate to impart cohesive strength, and
Functional monomers such as acrylic acid to impart specific adhesion, enhance cohesion, and provide sites for crosslinking, if desired.

The primary acrylates used in PSAs are 2-ethylhexyl acrylate, butyl acrylate, ethyl acrylate, and iso-octyl acrylate. The molecular weight of the polymer plays a substantial role in the performance of the PSA. Early in the 1950s research found that the performance of certain acrylate could be substantially improved by adding acrylic acid. Typical monomers used in the formulation of waterborne acrylic PSAs along with their characteristics are shown in the table below.

Type	Characteristics	Monomer	Tg (°C)
Soft	Low Tg Provides tack 2-EHA provides excellent flexibility and tack BA provides higher cohesion than 2-EHA, useful in removable PSAs	2-Ethyl hexyl acrylate	-65
		n-Butyl acrylate	-50
		Iso-octyl acrylate	-70
Hard	High Tg Provides adhesive stiffness (high modulus) Vinyl acrylate copolymers are polar, less expensive	Methyl methacrylate	105
		Methyl acrylate	9
		Vinyl acetate	32
		Styrene	100
Polar	Provides specific adhesion Improves cohesion by crosslinking or hydrogen bonding Adhesion builds up with time	Acrylic acid	106
		2-hydroxyl ethyl acrylate	-15
		n-vinyl pyrolidone	180

At room temperatures, these monomers are generally clear and colorless. In order to provide storage stability, stabilizers such as hydroquinone are added by the manufacturer.

Soft monomers such as 2-ethylhexyl acrylate and butyl acrylate provide tack and flexibility and form PSAs with some degree of cohesive strength
Polar monomers are often added in order to provide a copolymer that can form hydrogen bonds and provide a higher degree of cohesion. The polar monomers also increase wetting and improve adhesion to low surface energy substrates.
For higher peel strength, harder monomers with higher Tg are incorporated into a copolymer. They provide adhesive stiffness and high shear strength.
Crosslinkable monomers may be included to make the formulated adhesive curable by catalyst, heat, or other energy source.

Crosslinked adhesive films have improved shear strength and performance especially at higher temperatures; however, peel strength and tack are usually reduced.

A wide variety of monomer blends are employed to meet the requirements of various applications.

Most PSAs have a soft monomer content of 70-90%, a hard monomer content of 0-30%, and a polar monomer content of 3-10%. However, a simple general-purpose PSA can easily be produced from a blend of 95% soft monomer and 5% of a hard, polar monomer such as acrylic acid.

Many non-acrylic monomers, such as vinyl ethers, allyl compounds, and vinyl acetate are used as comonomers in developing acrylic PSAs. As a result, a substantial patent literature has developed regarding acrylic PSA as a function of the type of polymer monomer polymerized with non-polar, long chain acrylates. The performance of an acrylic PSA is fine-tuned by the selection of various monomers at specific ratios.

» View the Complete List of Acrylate Monomers Here!

Effect of Glass Transition Temperature

Peel adhesion can be correlated somewhat with glass transition temperature. A polymer that is too soft and fails cohesively when peeled can be improved by copolymerization with a monomer that raises its Tg. Conversely a polymer that is too hard and fails in peel can be improved by copolymerization with a monomer that will lower the Tg.

In order to possess sufficient tack required for a pressure-sensitive adhesive, the polymer must have a fairly low Tg. Therefore both 2-ethylhexyl acrylate and butyl acrylate have excellent pressure-sensitive characteristics without additives. Generally, homopolymers with high Tg are not tacky enough and require copolymerization with other monomers of lower Tg.

Effect of Molecular Weight

PSA properties as a function of molecular weight depends on:

Resistance to shear
Resistance to peel, and
Tack

Both tack and resistance to peel increase with increasing molecular weight until a maximum is reached. The maximum is at a fairly low molecular weight, and the transition of the mode of failure from cohesion to adhesion takes place in this region. A further increase in molecular weight causes a decrease and leveling of these properties. Commercial adhesives are generally offered in this range of molecular weight.

Type of Additives - Formulating Acrylic PSAs

In a formulated acrylic PSA, the additives may make up a substantial concentration of the final adhesive formulation depending on the end-use application.

It is evident that with the formulation variables offered through the choice of both the base polymer and the additives that the formulator can engineer an adhesive system with the required properties. In addition to the base polymer, tackifiers, plasticizers, and crosslinkers have the greatest effect on adhesion (tack, shear, and peel characteristics). Commonly used additives in acrylic PSAs are:

Additives	Characteristics	Example
Neutralizing agents	Added early in formulation to improve compatibility; aid in mechanical stability	NH₄OH, KOH: usually taken to neutral pH, added at room temperature
Surfactants / wetting agents	Stability for machining; lowers surface tension improves wetting; commonly used in low viscosity systems	Sulfosuccinates, acetylienic diols
Tackifiers	Increase tack and peel, lower cost; negative impact on cohesion	Rosin ester derivatives, modified C5 / C9 hydrocarbons
Plasticizers	Lowers Tg , lower peel and give smooth release from backing; tend to migrate	Phthalates (DOP, DBP), adipates
Crosslinkers	Improves shear but usually lowers tack and peel strength	Aluminum acetate, aluminum lactate, zinc oxide, zirconium ammonium carbonate, polyaziridine
Thickeners	Increase viscosity; modify rheology	Water-soluble cellulosics, water-soluble / swellable emulsions, polyacrylic acid
Defoamers	Reduce foam during formulation and processing	Dimethyl siloxanes, hydrophobic silicas, waxes, long chain alcohols and glycols, acetylenic diols
Preservatives	Reduce microorganism attack (other polymers generally more prone than acrylic)	Mercurials, phenols, formaldehydes, isothiazolone derivatives

Tackifiers

In some applications, acrylic adhesives do not require the addition of a tackifier to provide pressure-sensitive characteristics. However, in most pressure-sensitive applications, tackifiers are required to:

Increase tack and peel strength
Improve adhesion to low surface energy substrates

Tackifiers are often used in acrylic emulsion formulations for paper labels, packaging tapes. Here a high degree of adhesion is necessary for difficult to bond substrates such as plastic film, bi-oriented polypropylene (packaging) tape, and the like.

The table below shows some typical properties for an acrylic PSA tackified with 40 phr of rosin ester. Notice that the tack and peel strength is increased significantly due to the addition of the tackifier while the shear strength is reduced. This is due to the lower modulus and softening effect of the tackifier. Tackifiers are sometimes also used to lower the cost of the final adhesive and are typically used at concentrations of less than 30-40 percent by weight.

Property		No Tackifier	Tackified
Creep: shear hold (hrs), 1kg, 1" x 1"		< 200	19
Creep: shear hold (hrs), 1kg, 0.5" x 1"		50	4
Shear Adhesion Failure Temperature (SAFT), °C		137	65
Probe tack (g), initial		123	206
Probe tack (g), 72 hrs UV exposure		117	159
Peel Strength (N/25mm) after 7 days of wet-out time	Stainless Steel	15	40
Peel Strength (N/25mm) after 7 days of wet-out time	HDPE	7	20

Peel, Cohesive Strength, and Tack of Rosin Ester Tackified Acrylic PSA

Tackifiers with melt points substantially above the Tg value of the polymer can be expected to improve the strength of the adhesive at elevated temperatures but reduce the tack. Lower melting resins will impart greater tack and low temperature flexibility at the expense of creep and shear strength.

The tackifier is responsible for the balance of tack, peel, and shear properties of the final adhesive, and very often tradeoffs in properties must be made.

Impact of Tackifiers on Tg

Although tackifiers lower the modulus and provide a more flexible resin, they can often increase the Tg by reducing the rubber plateau. Since tack is measured as the force or energy to break, adhesives should have a high modulus at the strain rates and magnitudes imposed on them during rupturing of the bond.

Tackifiers raise the glass transition temperature of elastomers so that the adhesive mixture has a high modulus at high strain rates and normal ambient temperatures. Thus, a tackifier increases the modulus at low temperature, short time, and high frequencies, but decreases modulus at high temperature, long times, and low frequencies.

Tackifiers used for acrylic emulsions need to be compatible with the base polymer and the surfactant package that is used. Pre-emulsified tackifying resins are available for waterborne systems. Rosin ester derivatives and some modified C5 /C9 hydrocarbons are often employed.

Plasticizers

A plasticizer decreases both the Tg and the modulus of the adhesive. Esters, such as phthalates and adipates, are commonly used at concentrations of typically 2-5 percent. Because the adhesive is softened, tack is generally improved. However, plasticizers sometimes reduce peel strength, and this may be desirable to give a smooth release from a liner. The table below shows the results of adding a phosphate plasticizer to an acrylic PSA.

Amount of Plasticizer (phr)	Peel Adhesion (width) (g/cm)	Rolling Ball Tack (cm)
0	429	3.0
4	407	3.6
10	363	2.8

Effect of Plasticizer on Pressure-Sensitive Adhesive Properties

The primary difficulties with plasticizer are that they lower the Tg so that creep may be a concern, and they tend to migrate out of the bulk adhesive with time. Non-migrating plasticizers are commercially available. Esters, such as phthalates and adipates, are commonly used at concentrations of typically 2-5 percent.

Crosslinkers

The main purpose of crosslinking is to improve the shear and creep resistance properties of a PSA. Noticeable improvements can be achieved even at relatively low crosslink density. However, crosslinking also decreases the elasticity of the polymer film with a resulting decrease in peel strength. Tack is usually also decreased.

In the polymerization of acrylates, multifunctional monomers or reactive groups carrying monomers can be introduced into the polymer chain for crosslinking. Divinyl benzene, ethylene glycol dimethacrylate, and similar monomers are used for this purpose. However, there are many different types of crosslinking reactions that can take place. Di- or multi-variant metal ions will react with COOH to form crosslinks. Polyfunctional aziridines will crosslink via the carboxyl group at room temperature.

» View the List of Crosslinkers Commercially Available Today!

Fillers

Fillers can be used to extend the adhesive (i.e., to lower cost) or to change its properties. Clay, calcium carbonate and zinc oxide provide a moderate improvement in peel strength when added in concentrations of less than 100 phr of the base polymer. Zinc oxide, certain colloidal silicas, and other reactive pigments will significantly reduce peel strength when added at much smaller amounts.

Non-Formulation Adhesive Performance Factors

There are many adhesion variables that the adhesive formulator has no control over. These include:

The nature of the substrate
The method of application, and
The environment to which the final adhesive bond is subjected.

However, if these variables are recognizable to the formulator, modifications can be made to the PSA to maximize properties.

The PSA is designed to adhere to a substrate through the tacky nature of the adhesive and the wetting of the adhesive onto the surface of the substrate. Any external factors that prevent the adsorption of the adhesive on the substrate will result in a lower level of adhesion. The following factors have been noted to produce the greatest effect on the strength of the adhesive bond made with PSAs.

Non-Formulation Adhesive Performance Factors

Substrate Composition

Low energy substrates are not readily compatible with unmodified acrylic based adhesives and consequently may not provide good bond strength. Base polymers with a lower surface energy or additives such as tackifiers and plasticizers may improve the degree of wetting and the subsequent adhesive strength.

Substrate Texture and Shape

PSAs require good contact with the substrate to obtain adhesion. A rough surface will reduce the amount of effective contact area. Stiff adhesives may not be able to flow into the micro-roughness on the surface of the substrate. Therefore, additives may be used to lower the Tg and improve the flow properties as the adhesive is applied. Increasing the temperature of the substrate or adhesive at the time the adhesive is applied will also promote flow and wetting.

Related Read: Pressure Sensitive Adhesives that are Smart and Switchable

Cleanliness of the Substrate

Contaminants on the substrate surface can provide a weak boundary layer, which will adversely affect adhesion. Moisture and mold release are typical weak boundary layers.

Service Conditions

The environment in which the adhesive will be exposed during its life will play a critical role in bond strength. As temperature increases to near the Tg plateau of the adhesive, peel strength will generally increase and shear strength will decrease once the Tg is surpassed. Thus, peel strength may be significantly reduced at cold service temperatures (temperatures below the adhesive's Tg).

PSA Testing: Standard Tests When There Is No Standard

Acrylic Resins for Water-based Pressure Sensitive Adhesives

View a wide range of acrylic resins available today for waterborne PSAs, analyze technical data of each product, get technical assistance or request samples.

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