Acrylic Pressure-Sensitive Adhesives: Chemistry, Properties & Applications

What are acrylic pressure-sensitive adhesives?

Acrylic PSAs are a type of adhesive that uses acrylic polymers as a primary bonding agent. They are formulated with polymers of low molecular weight and Tg so that they are soft at ambient temperatures.

The key benefits of acrylic polymers used in PSAs are:

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

Acrylic polymers are used in laminating adhesives and construction sealants. The most common acrylic adhesive types used today are available as water emulsions in PSAs. These emulsions are thermoplastic systems that can be crosslinked depending on the requirements of the application.

Acrylic PSAs: How are they made?

Self-adhesive Labels Made from Acrylic PSAs

Acrylic PSAs are made with one or several types of acrylic monomers. These monomers have several functional groups. They are introduced during the polymerization process to form high molecular weight polymers. This is done to prepare an adhesive with various degrees of crosslinking properties.

The monomeric mixtures may be cured by ultraviolet light, heat, or chemical catalysts. The adhesives either cure in the bond line or outside the bond line.

In bond line – using thermosetting acrylic, cyanoacrylate, or anaerobic acrylic monomers
Outside bond line – using high molecular weight polymers

Acrylic adhesives are known for their versatility and scope. Hence, they are available as polymeric solvent solutions, waterborne emulsions, and monomeric mixtures.

Typical performance profile

Acrylic PSAs are widely used in several applications due to the following reasons:

Saturated nature of the polymer
Oxidation resistance surpasses most other polymers used for PSAs except for silicones
Adhere well to many polar substrates such as metals, glass, and high-energy polymeric surfaces
Good overall balance of cohesive properties
Thermoplastic and polar by nature
Soften when exposed to heat
Do not require tackifiers for pressure-sensitive adhesive properties

Polyacrylate films are water white. They do not yellow on exposure to sunlight. They are 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 get the cohesive strength. This is necessary to meet specific end-use requirements.

You can vary and fine-tune the adhesive properties by compounding with many ingredients as in the case of elastomeric pressure-sensitive adhesives. Modified acrylic adhesives incorporate tackifiers and other resinous components to improve adhesion. These modified acrylics offer improved initial tack and adhesion to low surface energy materials compared to standard acrylic formulations.

Advantages and disadvantages you should know

Understanding the pros and cons of acrylic PSAs is super important. It helps you make smart choices and avoid potential problems while selecting acrylics in PSAs. You can also ensure optimal performance, durability, and cost-effectiveness is maintained.

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

Composition Varieties of Acrylic PSAs

The acrylic PSA adhesives are available as solvent solutions, aqueous emulsions, and hot melts. However, waterborne acrylic PSAs have a dominant position in the market. Each system has its pros and cons. Let’s understand how these systems differ from each other.

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
Select 20+ Solvent-based Acrylic PSAs	Select 55+ Water-based Acrylic PSAs	Select 90+ Hot-melt Acrylic PSAs

Waterborne acrylic PSAs: What sets them apart from the rest?

Waterborne acrylic PSAs are prepared by emulsion polymerization of acrylate monomers in water using emulsifiers and water-soluble polymerization initiators. Key considerations to keep in mind while formulating waterbased arcylic PSAs incle:

Particle size of the acrylic polymer in emulsion: 0.1-0.4 microns
Viscosity: 100-500 cps
Tg: ~ -50°C to +20°C

The final emulsion generally has a solids content of 50-60% stabilized with a surfactant.

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

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 these advantages, 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.

Applications of Acrylic Pressure-sensitive Adhesives

Applications of Waterborne Acrylic PSAs

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.

Navigating the chemistry behind acrylic PSAs will ease your material selection journey. It will keep you upgraded will all the issues you might face while formulating acrylic PSAs. Let's explore the various monomers and additives involved in creating acrylic PSAs.

Acrylic & Non-acrylic Monomers for PSAs

By selecting 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, the polymer properties can be engineered for an application.

A typical acrylic polymer for PSA applications is composed of soft, hard, or functional monomer. 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.

Soft monomers

Acrylic PSAs are produced mainly from soft monomers. They have low glass transition temperature (Tg) and high level of molecular entanglement. These monomers impart pressure-sensitive tack and flexibility. They form PSAs with some degree of cohesive strength.

The primary acrylates used in PSAs are 2-ethylhexyl acrylate, butyl acrylate, ethyl acrylate, and iso-octyl acrylate.

Hard monomers

For higher peel strength, harder monomers with higher Tg are incorporated into a copolymer. They provide adhesive stiffness and high shear strength. Examples of high Tg monomer include methyl methacrylate, methyl acrylate, or vinyl acetate. They impart cohesive strength to the PSA formulation.

Functional or polar monnomers

Functional monomers such as acrylic acid to impart specific adhesion, enhance cohesion, and provide sites for crosslinking, if desired. 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.

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.

Example of monomers used in a waterborne acrylic PSA

Typical monomers used in the formulation of waterborne acrylic PSAs along with their characteristics are shown in the table below.

Monomer Type	Characteristics	Examples	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.

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.

Select from a range of 1900+ commercially available monomers

Additives for 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.

Crosslinkers

The main purpose of crosslinking is to improve the shear and creep resistance properties of a PSA. Crosslinking agents can be added to the formulation to increase crosslinking. 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.

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.

Factors You Must Consider to Improve Adhesive Performance

Formulation and Nonformulation Performance Factors

Effect of adhesion

The adhesion property is an important parameter to consider while formulating PSAs. 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)

These properties are generally derived from glass transition temperature (Tg), molecular weight and molecular weight distribution, and the viscoelastic nature of the base polymer.

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

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 depend on tack, shear resistance, and peel resistance. 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.

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.

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.

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).

Range of acrylic monomers for pressure sensitive adhesives

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How to Formulate Acrylic Pressure-Sensitive Adhesives (PSAs)?