Smart Pressure Sensitive Adhesives – A Current Review

TAGS:  Reactive Adhesives 

This article was first published in 2017 and was revised in 2020.

“Smart” or “intelligent” materials are materials that can be significantly changed in a controlled fashion by external stimuli. Today, there are a number of smart materials (e.g., shape memory alloys, piezoelectric crystals, ferrofluids, self-healing coatings, etc.), and new ones seem to appear every day. The pressure-sensitive adhesive (PSA) industry is exploring some of these new technologies to provide added value to their products.

Not only do smart PSAs perform a primary function of providing an adequate bond between substrates, but they also can accomplish numerous additional functions. This often requires intelligence being integrated into the material.

Let’s review smart material technologies with a focus on PSAs. This article explores how PSAs can be infused with intelligence to provide added value, and provides examples of recent and developing smart PSA products. The emphasis is on PSAs since these are very adaptable, well-accepted, and a potentially large market for smart technologies.

Advancement of Smart Materials to PSAs

Smart materials¹ are materials that respond to certain stimuli such as stress, temperature, chemicals, electric fields, and so forth. The response should be at an appropriate time and preferably reversible. Smart materials can be characterized by the stimulus to which they respond.

Smart PSAs may not have the full range of capabilities as earlier intelligent materials; however, they can provide intelligent actions and bring added value to a product. Some of the more obvious examples of smart PSA properties and typical applications are provided in the table below.

Change Induced by Smart PSAs	Example Applications and Value
Tack or adhesion (switchability)	Debonding for disassembly or recycling On-demand bonding Bonding to specific substrates (e.g., hydrophobic/hydrophilic) Ouch-less bandages
Electrical or electromagnetic properties	On-demand conductivity Debonding Resistance measurement, electrical sensors Viscosity change
Thermal properties	On-demand thermal conductivity Safety sensors, resistors Thermal management On-demand debonding On-demand change in dimensions or geometry
Viscoelastic properties	On-demand vibration damping and noise abatement On-demand deformation Pressure transducer Vibration sensor Quality control (cure monitoring, age, and failure) Leak sealing and repair Ability to be machined, die-cut, etc.
Light or color properties	Heat detection Full-cure detection Detection of hazardous gases, aged food, pharmaceuticals, etc. Security detection (packaging, tape, etc.)
Permeability properties	Drug delivery On-demand absorbency Packaging (internal moisture and gas control) Health care products
Shape	Shape-memory articles Sensors responsive to temperature, chemicals, stress, etc Constant stress devices (orthodontic appliances) Actuators, resistors, etc.

Many adhesives are useful for providing secondary functions as well as their primary function of holding substrates together. Product designers feel that this “multitasking” feature is one of the most valuable characteristics of adhesives in general and PSAs specifically. In addition to performing a mechanical fastening operation, a smart product may also be used as a:

• Vibration damper,
• Electrical conductor,
• Switchable adhesive (strong adhesion and fast detachment), and
• Many other applications.

The objective is to develop an adhesive that will bond to any substrate, offer all the performance characteristics required, and provide a “nerve center” for feedback on the environment so that the material’s properties can change accordingly. To a great extent this “nerve center” mimics how the human body responds to external stimuli.

A few examples of developments that could be the forerunners of truly intelligent PSA materials are described below and in a number of publications.

Smart PSA Materials and Constructions

With pressure-sensitive products, the adhesive polymer itself or the backing/carrier can provide the necessary intelligence. Like any other material, they are made up of reinforcing components, fillers, additives, and resin matrices. However, the smart PSA polymer or backing must be synthesized specifically to be responsive to a certain stimulus.

Linear and matrix smart polymers exist with a variety of properties depending on reactive functional groups, side chains, and cross-links. These groups might be responsive to:

• pH,
• Temperature,
• Ionic strength,
• Electric or magnetic fields, or
• Light

Some polymers are reversibly cross-linked by noncovalent bonds that can be made to break and reform depending on external conditions.

Nanotechnology has also been fundamental in the development of certain smart polymers. Their intelligence is obtained by using specific nanoparticles capable of inducing the desired response to the material in which they are embedded.

Traditional encapsulation technology has also been used to form lattice-like matrices that can hold drugs or other chemicals of interest between polymer molecules. The encapsulants release their contents on the action of a stimulus (e.g., stress, age, moisture).

Another commonly adopted strategy to obtain smart devices consists of connecting an arrangement of several elements into a predefined pattern. The transition between the two stable conformation states is triggered by environmental stimuli and enables the development of molecular-level responsiveness.

• Liquid crystalline polymers can produce a reversible mechanical actuation thanks to the spontaneous order embedded in their microstructure arrangement.
• A graft-and-block copolymer is two different polymers grafted together. The product exhibits properties of both individual components which adds a new dimension to a smart polymer structure and may be useful for certain applications.
• Cross-linking hydrophobic and hydrophilic polymers results in the formation of micelle-like structures that can protect a chemical medium until conditions at the target location cause the simultaneous breakdown of both polymers and delivery of the medium.
• Interactive thin polymer layers within an adhesive or as a backing can be constructed so that the layers differ in size, axis, or physical property. This can result in predefined interactions with their environment. Some of their potential applications include water shrinkable and pressure indicating products.

Smart and Switchable PSA Applications

Several examples are provided in the table below to show the breadth of smart technology that is available to the pressure-sensitive adhesive formulator and manufacturer. Although only a few have resulted in commercial products to date, they point to a significant future.

Application	Description
Labels, posters, or notices	Labels comprising an adhesive can be used in product tags, pricing tags, advertisement posters, etc. There will result in a strong bond and an easy removal without any adhesive residue left on the surface.
Protective films	Goods may get scratches during transportation, storage, etc., and by using a removable PSA in combination with protective films, the goods will be protected.
Fixation of sensitive parts during manufacture or transport	Products that are very fragile can be adhered to a supporting substrate using an adhesive. The product can then be detached from the adhesive when desired.
Wallpaper	A removable adhesive can be used to fix wallpaper, markers, or labels on a wall. When debonded, the wall surface is left without any damage or residues.
Masking tapes	Very strong fixation of masking tapes to a substrate is possible. When debonded the surface is left without any residue.
Packaging	Debonding on-demand applications exist for opening packages. Potential exists for the package to be open and reclosed for several cycles with suitable adhesives.
Recycling	Recycling of different materials in a product is made possible by detaching them after the end of service life.

Most of the current and future applications are designed for “switchability” or the process of going from one state to another. This is perhaps the most notable accomplishment in smart PSAs today. Switchable PSAs can go from:

• Strong adhesion to fast detachment,
• Insulator to conductor,
• Hydrophobicity to hydrophilicity,
• Rigid to elastic,
• One color to another color, etc.

It is important that the adhesive properties of the materials are not sacrificed for the additional switchable function.

Adhesion or another property is switched by controlling molecular interactions using chemical functionality. An exceptional feature article by Kaperman and Synytska reviews the design principles to induce switchable adhesion. Various means of deactivating PSA’s have been published, including UV curing, application of heat or cold, or the use of water-dispersible components which can be activated by immersing in water.

The commercial possibilities for removable adhesives appear endless.

• Removable labels, temporary signs, and banners, attachment of credit cards to mailers, and postable notes are only a few of the current applications.
• Another potential high-volume application is the use of removable adhesives in medical devices that should stick firmly to a substrate such as skin and then be easily and cleanly removed when desired.
• Future uses of switchable PSAs may be for applications such as:
• Easy disassembly of parts for reuse or recycling,
• Removable subcomponents (e.g., preparing modular assemblies for test and then disassembly for diagnostics), and
• Other advanced delatching and deployment mechanisms.

Thermally Responsive

Several thermally responsive switchable adhesives have been developed based on crystalline polymers. They operate through the lower critical solution temperature (LCST) which is the critical temperature below which the components of a mixture are miscible for all compositions.

• Switchable PSA adhesion has been developed based on a crystallizable side chain which results in detackification on cooling below its melting point. The melting point of the side chain, and hence the deactivating temperature, can be tailored to suit the specific application².
• A novel PSA was found to adhere to the hydrophilic substrates at room temperature and detach reversibly at temperatures above LCST, while their behavior towards hydrophobic substrates follows the opposite pattern. At room temperatures, the thermo-switchable PSAs have a low contact angle with water, which then becomes large at temperatures above LCST. After temperature elevation, the complex turns hydrophobic inside out and loses hydrophilicity³.
• A shape switchable PSA has been developed that includes a continuous crystalline polymer component and a discrete crosslinked elastomeric polymer component. The PSA has at least one distinct crystalline melting point and is capable of transitioning in a predetermined way between a secondary shape and a primary shape upon an increase in temperature⁴.
• A novel pressure-sensitive adhesive has been developed by employing a binary mixture of small hard and large soft latexes. This new formulation (a collaboration of research between Cytec, the University of Sheffield, and Surrey University) allows adhesion to be turned off on demand within thirty seconds of near-IR heating, leading to irreversible loss of PSA behavior. Since this desirable effect was achieved simply by blending existing latex formulations, it is potentially attractive for commercial applications⁵.
• Another strategy has been developed to switch off the tack in a nanocomposite adhesive in which temperature is the trigger. The nanocomposite comprises hard methacrylic nanoparticles blended with a colloidal dispersion of soft copolymer particles. At relatively low volume fractions, the nanoparticles accumulate near the film surface, where they pack around the larger soft particles. When the nanocomposite is heated above the glass transition temperature of the nanoparticles (Tg = 130°C), they sinter together to create a rigid network that raises the elastic modulus at room temperature and, therefore, the tackiness is switched off. With intense infrared radiation, tack can be switched off within 30 seconds⁶.
• Thermally responsive oligomers (TROs) appear to be a unique oligomer class that will pave the way for dynamically tunable properties⁷. The use of TROs has been demonstrated in smart UV curing PSAs. In one example, investigators were able to make a more permanent/stronger adhesive while in another example, the adhesive debonds or became easier to remove (in both cases after a thermal treatment).
• The University of Alicante has developed an innovative polyurethane PSA that presents a controllable degree of tack. The tack is changed abruptly in specific short ranges of temperature. The adhesive is potentially biocompatible and has excellent properties for use in medical applications such as dressings or bandages. By applying a slight temperature variation, the adhesive can be easily removed without solvents or causing discomfort to the patient. It can also be used in other applications such as labeling for the transport of goods or refrigerated food⁸.

Chemical / Moisture Responsive

• The switchable properties of hydrophilic pressure-sensitive adhesives have been developed by blending poly(N-vinyl caprolactam) (PVCL) with short-molecular weight poly(ethylene glycol) (PEG). It was found that the adhesive properties of these blends are the result of an extensive hydrogen-bonding network formed between PVCL and PEG. The adhesion is considerably diminished when the amount of absorbed water exceeds 30% or at elevated temperature, but it can be easily recovered by drying or cooling the sample⁹.
• Water-absorbent adhesive compositions composed of blends of polymers, possessing lower LCST, will debond from a substrate at temperature elevation above the LCST or cloud point. This type of product can be useful when requiring an adhesive composition that adheres to a substrate surface at ambient conditions, and at a different relative humidity loses tack reversibly upon temperature elevation¹⁰.
• Several studies have determined that it is possible to formulate PSAs (e.g., from copolymers based on n-isopropyl acrylamide and acrylic acid) which display an LCST and can be swollen into a gel below a critical temperature. The purpose is to produce a product that will deactivate in water by swelling, but only when it is exposed to water below a certain temperature. The switching effect is completely reversible. On heating above the critical temperature, the composition releases absorbed water and regains its pressure sensitivity¹¹. Pressure-sensitive adhesives like these may be of interest as de-tackifying adhesives or potentially as drug reservoirs that release their aqueous content on warming.
• A number of water detachable two-phase compositions based on hydrophobic PSAs have also been developed which rely on at least one dispersed water-soluble or swellable component (polymer filler, absorbent of water, tackifier, or plasticizer). Upon exposure to water and swelling of the hydrophilic dispersed phase, the phase separation occurs that inhibits the adhesion. For example, smart gels are engineered to shrink or swell by a factor of 1,000 and are designed to absorb or release fluids in response to the presence of any chemical or physical stimuli. These gels are being applied in such areas as agriculture, food, drug delivery, and cosmetics.
• A chemical engineering team at Virginia Commonwealth University has created a novel polymer that is hydrophilic (water-loving) when dry and hydrophobic (water-resistant) when wet. This is the opposite of most polymeric materials. When exploring new antimicrobial coatings, the researchers found a modified fluorocarbon that exhibited hydrophobic, nonwetting properties – but only when water was placed on the surface. They conclude that the change was caused by the rearrangement of the polymer’s side chain. Water-induced hydrophobic surfaces may lead to new medical devices, switching devices, drag-reducing coatings, and many other products¹².

Light Responsive

A series of novel photosensitive polyacrylates have been designed and synthesized to have switchability. These are copolymers of n-butyl acrylate, 2-carboxyethyl acrylate and a chromophore contained diacrylate crosslinker. The photo-responsive crosslinker achieves the removal of the adhesive since the photo-responsive linkage can be cleaved upon photo-irradiation¹³.

Random block copolymers with a photoacid generator have shown the ability to debond in response to dual external stimuli of photoirradiation and post-baking. The simple use of a LED was able to achieve a rapid dismantling process during UV irradiation within several minutes¹⁴.

Researchers in the UK have developed a novel light-curing PSA composition which can be removed cleanly from the skin. The photoinitiator is built into the polymer chain to make it safe for skin contact. The tape can be removed by removing an opaque liner and exposing it to ambient light.

pH-Responsive

A diblock copolymer, composed of polyacrylic acid (PAA) and poly(n-butyl acrylate) (PBA), was synthesized and adsorbed onto soft acrylic latex particles prior to film formation. The thin adsorbed shells on the particles create a percolating network that raises the elastic modulus, creep resistance, and tensile strength of the final film. When the film formation occurs at pH 10, ionomeric crosslinking occurs, and high tack adhesion is obtained in combination with high creep resistance. The results show an effective means of adjusting the mechanical and adhesive properties of the resulting composite film¹⁵.

A double-network hydrogel of poly(methacrylic acid) and poly[oligo(ethylene glycol)methyl ether methacrylate] exhibits pH-switchable loss of adhesion. The surfaces are attached at pH 6 and detached at pH 1. This switchable adhesive system can be reused repeatedly. The adhesive strength does not deteriorate after each adhesion cycle, and reversing the adhesion is a relatively rapid process¹⁶.

Vibration Responsive

Because their viscoelastic properties can be varied, polymers can provide a practical solution for vibration control. Passive vibration control using blocks of viscoelastic materials with macro- and microscopic inclusions has been widely investigated. Significant changes in the vibration response have been observed with such inclusions¹⁷. This can lead to the use of these materials as shock and vibration absorbers and as vibration sensors.

Piezoelectric Responsive

Piezoelectrics can be made from polymer-based composite materials that operate via the conversion of applied stress into usable electrical charge or they can convert an applied electrical potential to mechanical strain. Because of their piezoelectric response, polymer films are already widely used as sensors such as:

• Ultrasonic transducers,
• Strain gauges, and
• Microphones

In the form of PSA tapes, the materials have potential applications in the structural monitoring of bridges, transportation composites, and utility poles.

Stress Responsive

One of the more effective ways of achieving self-healing is to add encapsulated healing materials (often a cyanoacrylate adhesive) into another material. Applications include leak-preventing seals, ballistic proof tanks, etc.

A PSA powder consisting of particles with an adhesive polymer core and a hard nanoparticle shell morphology have been synthesized based on liquid marble technology. The PSA shows no adhesion in its original form but shows its adhesive nature only after the application of shear stress¹⁸.

Conclusion

Adhesive developers and PSA manufacturers are generating new products that are sophisticated, multi-functional, and intelligent. Activity is brisk indicating that newer commercial switchable pressure-sensitive adhesives may soon enter the market and provide significant added value.

There appears to be virtually no limit to the properties that can be achieved from the large range of chemical families, advanced processes, and technical information available. For example, there is the potential invention of a toilet that uses these polymers to detect health issues in urine, or even in eco-friendly fertilizing methods, where smart polymers identify the necessary functions of water use and ultimately mitigate waste. It is apparent that the designers of the future will have smarter adhesives that do considerably more than just stick.

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