Versatility of Acrylic PSAs
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
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 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
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)
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
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
Non-Formulation Adhesive Performance Factors