Converting PSAs to Near Structural Adhesives

TAGS:  Reactive Adhesives 

Reactive liquid adhesives are normally used for structural applications due to the strength and durability that these adhesives provide. Pressure-sensitive adhesives are used primarily for their ease of application, where structural capabilities are not important.

Ideally, one would like an adhesive that can be applied like a pressure-sensitive adhesive but can easily convert from the pressure-sensitive state to a state approaching high strength, structural adhesives. Such adhesives are currently being explored using cross-linking reaction mechanisms that can be activated after the pressure-sensitive adhesive is applied, and the substrates are bonded.

In these formulations, a curing system is incorporated beforehand so that the bond strength increases after activation and cure. In this way, strong and durable pressure-sensitive adhesives can be considered as potential replacements for liquid adhesives.

An assessment makes the state-of-the-art and the progress to the ultimate goal of obtaining high permanence and structural strength (approximately 1000 psi).

Let's review the expected growth of structural adhesive market and development strategy used to convert PSAs into near structural adhesives...

Market Value

Convertible PSAs can be used in many applications from tapes and labels to the automotive accessory attachment. However, the most likely near-term application would be as a structural tape. They would be a part of what is considered the “structural tape” market. However, two-stage cross-linkable products that approach structural potential are currently a small but growing part of this market.

Structural adhesive tapes are made up of specialized adhesives which provide high strength bonds between two surfaces. This tape offers:

• High bonding strength
• Fast fixture
• Durability
• Impact resistance, and
• Versatility features

» Grasp existing technologies, future trends & requirements to ensure durability & debonding requirements for your structural adhesives!

Structural adhesive tape offers various advantages over other alternatives such as welding, rivets and bolts as the former gives better bond strength.
The global structural adhesive tape market is expected to reach USD 15.3 billion by the end of 2024¹. Major applications are in markets such as:

• Transportation
• Building and construction, and
• Electrical and electronic

Increasing application in these market areas and expected evolution in performance through materials and formulation (e.g., convertible PSAs) will drive the growth. Major players in this market include:

• 3M
• Nitto Denko
• Henkel
• Sika AG
• Adhesives Research, and
• Avery Dennison

The unmet need provided by convertible PSAs, either as a structural tape or in more conventional applications, is that they not only bond and hold substrates together, but they also provide additional performance benefits that are similar to structural adhesives. In addition, they will provide fast, near-instantaneous bond strength without the need for mixing and dispensing multiple liquid components.

The key benefits of this technology are:

• Activation can be made to occur at the most appropriate time and on-demand
• Eliminates the need for fixturing while curing
• Forms strong, flexible bonds (ideal for bonding dissimilar structural substrates)
• May be formulated with desirable functional attributes
• No mixing, mess, or clean-up
• Consistent bond line thickness and uniform stress distribution, and
• Die-cuttable

Development Strategy

The primary strategy used in developing convertible PSAs is to start with a cosslinkable PSA formulation, and then, once a bond is made, expose the adhesive to a form of energy that activates the cross-linking reaction. Convenient forms of energy are thermal and radiation (visible or UV light), although other forms are under investigation (electron beam and even ultrasonics²).

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


A basic model for the strategy in formulating convertible PSAs has been developed by Wigdorski et. al.³

• The initial strategy was to treat these materials like conventional PSAs and modify standard base polymers with compatible tackifiers and plasticizers to adjust the glass transition temperature and modulus of the base polymer to produce the initial properties required. These systems can then be converted by some means, at a point in the future, to produce the desired end result.
• Further, it was felt that the base polymer itself should not only have the capability to react with the modifiers when necessary but also have the ability to react with itself. This would be beneficial if the modifiers used did not have the functionality to chemically bond to the base polymer during the curing phase of the adhesive.
• Additionally, the base polymer should be capable of very high levels of modification to produce pressure sensitivity, at least 200 parts of modifier to 100 parts of the base polymer, to take advantage of the capabilities of the modifying ingredients during the curing stage. The base polymers should be selected so that cross-linking can be triggered by the appropriate thermal or energy reaction mechanism.

In their development work, Wigdorski et. al. investigated a PSA produced from a glassy acrylate with epoxy functionality built into the backbone. The simple formulation consisted of the following:

• Base polymer: 100 part by weight solids
• Epoxy modifier: 250 parts by weight solids
• Latent amine hardener: 100% of the theoretical amount needed to cure the epoxy functionality
• Solvent: sufficient to provide coatability

The cure was achieved after the substrates were coated and joined together. Full cure occurred in approximately 150 mins at 80°C or 30 mins at 100°C. The final property data are shown in the table below.

Property	Substrates	Test Methods	Results
			Before Thermal Cure	After Thermal Cure
Peel strength	Polyester to stainless steel	12 inch/min after 5 min residence time at RT	4 pli	Polyester rupture
Peel strength	Aluminum to aluminum		5.5 pli	Aluminum rupture
Shear strength	Polyester to stainless steel	500 gm load over a ¼ inch test area	60 mins to failure (cohesive)	>2000 psi*

*Exceeds load cell capability

This performance data suggests that the convertible pressure-sensitive adhesive has peel and shear which should be sufficient to hold parts in place prior to cure and after cure performance comparable to what one would expect from a two-part epoxy adhesive designed for high-performance applications. The figure below illustrates strength development in a typical two-stage PSA.


This example is only one-possible variation of a convertible PSA. Formulation and curing parameters are sufficient to design a workable PSA that can meet various end-use requirements on different substrates.

It should also be noted that the same strategy of cross-linking a pre-applied PSA can also be used to cleanly remove the adhesive from a substrate after its service life. In this case, the ultimate strength is controlled by the formulation and the degree of cross-linking or the energy input to the applied adhesive.

For removable adhesives, the aggressive PSA is converted to a non-tacky state with minimal bond strength.

Now, let's discuss further the potential evolution of this type of convertible adhesives...

Thermally Convertible PSAs

Due to the novel and relatively recent introduction of two-stage convertible PSAs on the market, the formulation of commercial products is considered proprietary in most cases. As a result, little information exists on starting formulations except for the patent literature. Information presented in this section is meant to provide the reader with a general foundation of the materials and types of thermally convertible PSAs. It is, by far, a non-inclusive representation of the technology.

Many base polymers can be used in thermally cross-linkable PSAs. These include primarily acrylic, styrene-butadiene copolymers, silicone, and other polymers that have reactive functional groups in the molecule. Widely used cross-linkable base polymers and their cross-linking agents are summarized in the table below. The type of cross-linking agent will, of course, be dependent on the functionality of the groups in the base polymer.

Base Polymer	Monomers, Oligomers, or Polymers	Cross-linking Agents
Acrylic	Methyl acrylate Ethyl acrylate Butyl acrylate 2-EHA MMA Acrylic acid Methacrylic acid Other	Metal salts and metal chelates Metal acid esters Amino resins Polycarbodiimides Isocyanate cross-linking agents Ethylene imines and propylene imines
Styrene butadiene copolymers	Styrene butadiene styrene Styrene isoprene styrene Styrene ethylene butadiene styrene Others	Aluminum acetylacetonate Peroxoide
Silicone	Polydimethylsiloxane Others	Peroxides

The cross-linkable acrylic polymers have been the main source for near-structural PSA due to its:

• High strength
• Moisture and UV resistance
• Wettability to various substrates, and
• Formulation flexibility

Acrylic monomers can cross-link with epoxy resins, amines, reactive resins, as well as alkyd urea derivatives. In the polymerization of acrylates, multifunctional monomers or reactive groups carrying monomers can be introduced into the polymer chain for cross-linking. Divinyl benzene, ethylene glycol dimethacrylate, and similar monomers are used for this purpose. However, there are many different types of cross-linking reactions that can take place.

In spite of the advantages of waterborne acrylic PSAs, the solvent-borne counterparts have been preferred in high-performance applications because of their superior adhesive performance at high temperatures and chemical resistance.

The functional monomers, such as acrylic acid and acrylic amide, are incorporated for specific adhesion to diverse substrates, solubility, dispersibility, etc. and to provide reactive sites for cross-linking reaction. From a large number of monomers and cross-linking agents available, one can easily see why acrylic monomers are generally the first choice in formulating a structural PSA.

Historically, the main component for a curable pressure-sensitive adhesive layer is a high-performance, pressure-sensitive adhesive, prepared in a solvent, which contains acid and epoxy functionality. A typical acrylic pressure-sensitive adhesive has the following composition:

• 67.0% 2-ethylhexyl acrylate
• 23.7% methyl acrylate
• 7.0% acrylic acid
• 2.0% vinyl pyrrolidone, and
• 0.3% glycidyl methacrylate (epoxy functionality)

A permanent PSA is obtained after drying the solvent and cross-linking the polymer, utilizing the acid functionality with metal chelate ionic cross-linkers. The subsequent cross-linking of the epoxy functionality during the bake cycle yields an ultra-high-adhesion (near structural) adhesive.

Latest Developments in Structural Adhesives

#1. 3M's Acrylic-Epoxy Structural PSA Tapes

A series of commercial acrylic-epoxy structural PSA tape with varying thicknesses (0.25mm to 1.0mm) has been developed by 3M (Structural Bonding Tapes 9244, 9245, 9246).⁴

• They are formulated with a blend of acrylate copolymers and epoxy resins. Dicyandiamide and triazine are used as thermal-curing agents.
• These tapes are capable of bonding glass, ceramics, and most metals, and will find use in many interior and exterior industrial applications.
• In many situations, they can replace screws, rivets, spot welds, liquid adhesives, and other permanent fasteners.
• In the pressure-sensitive stage, the substrates do not need to be fixtured or clamped during cure, although overall performance can be improved by providing pressure during the thermal cure.
• Cure temperatures range from 121°C to 176°C with cure times ranging from 10 mins to 90 mins. In addition to oven curing, these tapes can also be quickly cured at 218°C using direct contact with a hot lamination bar.

Physical properties of adhesive joints made with the 3M structural tapes are shown in the table below.

Test method	Substrate	Uncured Value
Peel adhesion	Stainless steel	7-12 lb/in
	Glass	8-14 lb/in
Static shear	Stainless steel (0.5 in2)	10,000 mins at 22°C and 250 gm load
Test method	Substrate	Cured Value
Overlap shear @ 23°C	Stainless steel	850-1400 psi
	Etched aluminum	800-1600 psi

Properties of Cured and Uncured 3M Structural Tapes 

Similar products have also been developed by several other companies.

#2. Avery Dennison's PSA Structural Adhesives

• In a novel, early approach, Avery Dennison developed a PSA structural tape by applying a thin layer of PSA to one or both sides of a core layer of partially cured structural adhesive.⁵
  • The pressure-sensitive adhesive provides a tacky surface allowing a temporary bond at room temperature.
  • Upon curing, the bond properties change to that of a structural adhesive, providing a strong permanent bond.
  • On cure, the skin layer or layers of pressure-sensitive adhesive are absorbed into the layer of structural adhesive.

• More recently, Avery Dennison has developed PSAs which can be converted thermally to form structural adhesives. These adhesives include a blend of acrylic PSA resins in combination with one or more particular urethane (methacrylate) oligomers.⁶

#3. L&L’s Epoxy-based and Heat-activated Structural Tapes

L&L Products (Romeo, MI, USA) has developed a range of tacky structural adhesives in tape form that are principally epoxy-based and heat-activated.

• L&L’s tapes have the ability to bond areas of the vehicle where there is limited weld accessibility, varying design gaps and requirements to join dissimilar substrates.
• When bonding dissimilar substrates, these tapes can manage the coefficient of thermal expansion differences.⁷

#4. Adhesives Research Inc's Transformable PSA Platform Technology

Adhesives Research Inc. has also developed transformable PSA platform technology. These PSAs incorporate chemistries that are triggered on-demand when exposed to a form of energy such as thermal, radiation (visible light, ultraviolet, electronic beam, etc.), or sound (ultrasonics).⁸

Although the current state-of-the-art for curable structural adhesives employs solvent-borne technology, waterborne products are promising to enter the field as well. Waterborne PSA uses a base polymer system with particles dispersed in water, which makes the cross-linking a bit complicated. However, new waterborne acrylic technology has been developed, claiming excellent adhesion performance, high-temperature resistance, and high levels of performance after immersion in different fluids.⁹

Radiation-curable PSAs

The primary disadvantage of thermally activated PSAs is that they may not be appropriate for temperature-sensitive substrates such as plastics. This has limited the potential market for structural PSA adhesives and tapes. As a result, researchers have looked to radiation-curable PSAs as an alternative for heat activation.

Radiation by light, UV or electron beam sources has been used to cross-link PSAs since the 1970s. The primary advantage of UV-cured PSAs is that they offer non-flammable, solvent-free formulations with lower VOCs and better environmental compliance. However, UV-curable resins have several disadvantages for use in manufacturing near-structural PSAs or tapes.

• UV cross-linking is generally done immediately after the liquid monomers are applied to the carrier. Thus, there is no cure activation “on-demand” after bonding, and the degree of cross-linking is generally not sufficient for the adhesive to attain structural properties.
• The UV must be able to penetrate the substrates to the adhesive layer. Many industrial substrates are not transparent to UV, and the adhesive, therefore, cannot be cross-linked after the substrates are joined.
• UV-cured PSAs have a maximum curing thickness of about 100µm due to the filtering of UV light even in transparent substrates.

Radiation cationic-curable PSAs have been developed to overcome most of these drawbacks. However, structural strength is generally less than that can be obtained with thermal-curing technology. As a result, the primary applications for UV-cured structural PSAs or tapes today include those with UV transparent and relatively light weight substrates. These applications include:

• Labels
• LED displays, and
• Protective films

One of the major applications for a curable PSA is optical devices.¹⁰ The choice and amount of monomer, oligomer, photo-initiator, and additives are all critical to the liquid UV/EB formulation. The source and energy provided by the radiation source is also a significant parameter in achieving specific performance targets.

It has been suggested that even though substrates may be opaque, they could be joined with curable tapes activated by UV. To achieve this type of “dark” cure, a UV-curing technology based on cationic polymerization can be used. The adhesive was prepared by binding epoxy resins and a UV cation catalyst (triarylsulfonium) with methacrylate copolymers.


In a more recent investigation, EB curing was found to provide higher shear strength and unprecedented high SAFT value compared to UV curing of the same base formulation.

• Free radicals are generated from a photo-initiator with a UV cure, and with EB cure, the free radicals are generated on the reactive function sites of the polymer.
• The electron beam is a much higher energy source than UV light and promotes a higher degree of cross-linking.

The table below shows the performance comparison of UV and EB cured PSAs. However, the difficulty remains in activating these adhesives through opaque substrates, and the cost of EB radiation is much greater than UV.

Property	UV cured	EB cured
180° peel on stainless steel after 1 day, lb/in	7.6	4.2
180° peel on polypropylene after 1 day, lb/in	4.8	4.5
Probe tack, lb	2.3	2.1
Room temperature shear (2kg), hr	34	>167
Room temperature shear (1kg), hr	84	>167
SAFT, °C	85	224

Performance Comparison for UV and EB Cured PSAs using the Same Formulation

Dual-Cure Technology

The disadvantages of UV/EB curable PSAs noted above have prompted the study of dual-curable PSAs that combine UV-radiation and thermal-curing processes. These systems contain two types of reactive functions:

1. A UV-curable functional group (a photo-initiator), and
2. A thermally curable functional group

In dual-curable PSA tape, it is possible to advance the adhesive to a strong pressure-sensitive state sequentially. UV energy is applied after the adhesive is coated on a substrate, and then further advancement of cross-linking can occur by thermal means after the bond is made. Thus, the process is more appropriately defined as a “two-stage” cure rather than a “dual-cure”. The two-stage approach generally avoids disadvantages caused by opaque substrates.

The combination of UV- and thermal cross-linking enables the manufacturing of acrylic PSA adhesives having different adhesion/cohesion properties and offers opportunities to develop innovative structural tapes with new unique features.

A typical UV-curable acrylic PSA contains the following main components such as:

• Photoreactive polymers with photoreactive groups in polymer chains
• Photoreactive diluents in the form of monomers, oligomers or prepolymers, usually acrylates, polyester acrylates, and polyurethane acrylates
• Multifunctional monomers such as acrylates and methacrylates
• Unsaturated photo-initiators, and
• Thermal cross-linking agents such as amino resins, acid anhydrides, epoxide resins, etc.

Certain amino resins, such as melamine derivatives, have shown interesting properties when added to solvent-borne acrylic PSAs.¹² Amino resins are characterized by their reactive end groups and enable a controlled cross-linking reaction and a precise adjustment of the required adhesive properties. The cross-linking rate is practically zero at room temperature while it increases exponentially above 100°C.¹³

Dual-curable acrylic pressure-sensitive adhesives (PSA) were synthesized by the radical polymerization of acrylic monomers containing benzophenone, hydroxyl, and alkyl groups. The coated pressure-sensitive adhesives were cured either by short UV exposure to induce the grafting of acrylic polymers, or by heating for 6 h at 60°C to promote the reactions between the polyisocyanates and hydroxyl groups.¹⁴

In an early example of a dual-cure PSA, Avery Dennison has shown that they can have good peel adhesion and high SAFT.¹⁵ This was achieved using a tackifed acrylic copolymer with glycidyl and carboxy functionality. Initial curing of the adhesive can be completed by heat or UV, and this provides an adhesive with excellent tack and peel. Secondary curing at higher temperatures gives greater cross-linking to yield structural properties.

Adhesives Research has developed a commercial dual-stage thermal or UV-curable adhesive platform which cures to a structural bond.¹⁶ In addition, the system is able to handle opaque substrates since the exposure to UV happens in an “open” state which initiates the curing process that is then completed upon bonding followed by exposure to 90°-110°C. This type of adhesive is currently used for producing polishing pads and templates for use in the wafer polishing industry. A variation of this technology is also used in splicing multiple large screens for display applications.

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