Advancements in UV Cure Pressure Sensitive Adhesives: Entering the Circular Economy

Tags:   Sustainability / Natural Solutions    

UV cured pressure sensitive adhesives (PSAs) are gaining traction in the marketplace due to advances aimed at their sustainability characteristics. They are also seen as contributors to the circular economy model. A circular economy takes a step forward from the traditional take-make-waste linear production cycle toward one that promotes products emphasizing resource reduction, extended use, reuse, and recycling while meeting performance, safety, and health requirements.

This article will focus on advancements in the sustainability of UV cured PSAs and how they contribute to the circular economy. These advancements are primarily in the use of renewable resources, reuse or recycling of the end-product, and increased performance to extend the life of the product to which they are applied.

UV Cured PSAs: The Basics

Radiation curing is a production technique for polymerizing and curing of adhesives through the use of radiant energy. One of the most popular uses of radiant curing is the advancement or curing of pressure sensitive adhesives with ultraviolet (UV) radiation. This form of radiation has the energy necessary to initiate polymerization of low molecular weight, unsaturated resins.

High productivity, good processing flexibility, relatively low processing temperature, low or no VOC emissions, and properties approaching high performance solvent bone PSAs products are potential advantages brought by UV cured PSAs.

Examples of radiation cured PSA adhesive applications include:

• Tapes and labels
• Graphics and vinyl applications
• Laminating adhesives
• Assembly of medical, LCD, electronic components
• High coating weight applications (e.g., duct and damping tape, PSA more tolerant of large or variable joint gaps)

Although UV curable adhesives are a relatively small segment of the overall adhesive market, they represent a fast growing segment. Their market penetration is expected to increase due to stricter environmental regulations and the availability of a greater variety of products and UV curing technology.

Currently, there are primarily two polymer systems that are used as base polymers for UV curable PSAs:

• Acrylic polymers with photoreactive groups that are polymerized into the acrylic backbone, and
• Styrene-block-copolymers with free vinyl groups. Systems based on styrene-block-copolymers require an additional photoinitiator system.

These chemistries can be formulated into two main types of UV curable PSA products: room temperature coatable and hot melt coatable.
Hot melt UV PSAs require heating to facilitate the coating process. UV sensitive chemistry is built into the hot melt UV PSAs, aiding crosslinking, and giving the PSA improved shear and high-temperature properties. Room temperature coatable UV PSAs are liquid with liquid viscosity at room temperature.

Recent years have seen a surge in the number of research publications on UV curable PSAs, and manufacturers now recognize the benefits of UV cured PSAs in sustainable products. New polymers are being developed, new formulation strategies are being put forth, and improvements are being made in the lamp technology. Many of these advancements are summarized and referenced in this review article.

If the reader requires additional information on the basics of UV cured PSA raw materials, formulations, and applications, he or she is referred to several articles¹ in the SpecialChem technical library (https://adhesives.specialchem.com/tech-library).

Sustainability and Circularity

The major attraction to UV cured PSAs has been due to the importance these products have on achieving sustainability and characteristics that fit into the circular economy model. UV cured PSAs provide added value in both industry models.

Sustainability has been defined as "meeting the needs of the present generation without compromising the ability of future generations to meet their own needs."² It includes environmentally friendly concepts that have become recognized over the years such as low VOCs and toxicity, efficient use of raw materials, reduced energy usage and transport, reduced processing costs and time.

Circularity or the circular economy is a newer concept. It also incorporates the practice of sustainability in consumption, manufacturing, and use. However, the three distinctive principles required for the transformation to a circular economy are: (1) eliminating waste and pollution, (2) using circular products and materials, and (3) the regeneration of natural resources. Circularity focuses more on the front and back ends of the value chain – primarily the use of renewable raw materials and more efficient end-of-life solutions by reuse, recovery, or added product lifetime.

In the more common linear economy, Natural resources are turned into products which are ultimately destined to become waste because of the way they have been designed and manufactured. This linear process is often summarized by "take, make, waste”. Differences in the linear and circular economies are illustrated in Figure 1.³




The reuse-remake-recycle aspect of the circularity model is an important subject for adhesives and sealants. Due to their chemistry and method of use, adhesives and sealants are difficult to recycle. However, they do have properties that provide end-of-life improvements.

• Certain adhesive systems are capable of being debonded by external stimuli to allow for recycling or reuse of the substrates.
• Compostable and biodegradable adhesives have been developed and are being used in packaging for added sustainability at the end of life. Biodegradable adhesives eventually break down naturally and safely in the environment, and compostable adhesives break down in industrial processes to allow recycling of substrates.
• Resealable PSAs are now used in packaging to provide added value and to avoid the need for a secondary fastener.
• Finally, increased durability (e.g., improved resistance to high temperature, oxidation, and moisture) can provide longer product life that results in reduced replacement costs.

Key Value Propositions Being Re-thought

The terms “eco-friendly”, “sustainability”, and “circularity” are often incorrectly used together and somewhat interchangeably, which is confusing and dilutes the importance and value of the actions related to either one. Sustainability is a marathon not a sprint. Every individual innovation must be valued whether it be defined as eco-friendly, sustainable, or circular.

UV cured PSAs are eco-friendly because they contain no solvents and release little or no amount of volatile organic compounds (VOC) into the atmosphere. They are also sustainable because of their cost savings, part quality, and production improvement. UV cured PSAs are now becoming recognized for their circularity because they are beginning to use renewable raw materials in their formulations, and they impart significant durability (service life extension) in comparison to other PSAs. Table 1 lists several UV cured PSA characteristics that support eco-friendly, sustainability, and circularity models.

The remainder of this article will describe advancement in UV cured PSAs that are compatible with the circular economy model. These advancements are primarily in the areas of renewable raw material, improved durability, and less wasteful end-of-life scenarios.

Benefit	Eco-Friendly Characteristics
Low VOCs	There are no solvents and therefore no or extremely low VOCs in UV cured PSA coatings. There is no solvent waste.
Low carbon footprint and cost	The results of the eco-efficiency study4 demonstrate the advantages of UV cured hotmelt adhesives as compared to both solvent and dispersion adhesives in both a reduced environmental footprint and lower total cost of ownership.
No hazardous waste	There is little or no toxic waste generated during the application process, and there is no need to remove hazardous solvents.  UV cured PSAs have no effluent disposal problems as do waterborne coatings which sometimes are placed directly into a drain or allowed to settle out in sludge tanks.
No hazardous surface treatment	UV PSAs do not require environmentally unfriendly chrome-based primers or surface treatment to insure adequate adhesion or durability.  In fact, UV cured PSAs adhere well to low surface energy substrates and no primer is needed over properly pretreated aluminum.
Benefit	Sustainable Characteristics
FDA approval	Since March 2008, Ultraviolet (UV) and Electron Beam (EB) ink, coating, and adhesive formulations have been used in direct contact with food products.
Low cure temperature and energy costs	UV activated PSAs cure at room temperature and are applied at lower temperatures than conventional hot melt PSAs.  This means lower energy costs. There is also less heating of air, and the oven air turnover rate is significantly reduced as there is no buildup of potentially explosive solvent.  Lower cure temperatures also enable coating of heat sensitive substrates.
Less processing time	Processing times are shorter than those for water or solvent borne conventional PSAs.  There is no solvent so no flash off period is required. This gives substantial savings in facility space and processing time.  Total processing time can be reduced from hours to minutes.
Lower facility costs and space	A UV cured PSA coating facility is simple to operate,  less costly than other systems, and has a reduced footprint.
LEED points	Using UV cured PSAs helps gain LEED points.  LEED certification is a globally recognized symbol of sustainability achievement and leadership.
Benefit	Circularity Characteristics
Single coat required	UV cured PSA coatings achieve the same or better levels of chemical, mechanical, and weathering performance than solvent and waterborne PSAs coatings with a single coat. This means savings on product, time, and energy.  Thicker coatings of water or solvent borne PSAs require multiple coats increasing drying time and energy cost.
Reuse	UV cured PSA have tunable adhesion properties which allow resealing of packaging.  This avoids the need for secondary fasteners after the package is opened.
Removability	UV cured PSAs can be formulated so that they have high strength and yet can be cleanly removed from a substrate.  This is important for debonding and recycling substrates, in masking tape, and in temporary bonding of elements.
Durability	When compared to water or solvent borne coatings, UV cured PSAs can maintain or increase durability by providing enhanced resistance to weather and moisture.  This increases the life of the end-product saving replacement or maintenance costs.
Enhanced Performance	The performance is considered equal or better than solvent borne PSAs.  High and low temperature resistance, chemical and moisture resistant, low migration potential are possible.
Recycling	Certain UV cured PSA formulations have been designed so that they do not adversely affect composting processes.  Substrates can be debonded for recycling.  Release liners, once a significant source of waste can be designed to be recycled and / or biodegradable
Renewable resource usage	Several developments have indicated that UV cured PSAs will be synthesized from renewable raw materials in the future.  Energy that is required will also be supplied by renewable sources (solar, agriculturally based fuels, etc.)

Table 1  Environmental Benefits of UV Curable PSAs as Characterized by Eco-Friendliness, Sustainability, and Circularity

Advancement in Renewable Raw Materials

The first UV curable PSAs were limited to acrylate and styrene block copolymers synthesized from petroleum-based sources. Several UV cured PSA systems are now being developed that are synthesized from renewable resources such as agricultural products. Bio-based PSA adhesives have been commercialized in several conventional cure PSA products, and these advancements are also being targeted by UV cured PSA adhesive manufacturers.

A major restraining factor for the conventional cured PSA market growth is the volatility in raw material prices. The raw materials used for manufacturing PSAs are petrochemicals and are derived from crude oil. Most of the world's crude oil is located in regions that have been prone historically to political upheaval or have had their oil production disrupted due to political events. These events create uncertainty about future supply, which can lead to higher prices and price volatility.
As a result, an emerging field of research in UV curable PSAs is renewable base polymers.⁴ This involves combining bio-sourced molecules with an inexpensive and rapid curing method that avoids any emission of volatile organic compounds. Emerging naturally occurring raw materials that are being developed for UV cured PSAs are detailed below.

Industrial pulp and paper processing represent an industry that can provide by-products for renewable polymers and at the same time limit waste generation. Lignin represents a readily available by-product that can be used to produce a bio-based monomer. Chemically, lignins are polymers made by cross-linking phenolic precursors. The main compounds derived from pulp and paper manufacturing that are suitable for UV curing have been detailed in a review article⁵. Particular focus was given to developments concerning lignin, rosin, and terpenes and their possible applications in UV curing chemistry.



A solvent-free PSA with 97% bio-content was successfully prepared using epoxidized soybean oil (ESO), dehydroxylated soybean oil (DSO), and rosin ester through UV initiated cationic polymerization. Functionalized plant oils such as ESO are widely used in plastic industries as additives and are available for more value-added applications. A PSA with an optimized ratio of ESO : DSO : rosin ester with photoinitiator showed peel and tack properties required of a PSA tape. On aluminum substrates, the UV cured PSA tape showed much stronger peel strength than commercial conventionally cured tapes. It also exhibited significantly stronger shear strength than commercial PSA tapes. The bio-based PSA was also thermally stable in the temperature range from −10° to 150°C.⁶

Among the various UV curable resins, polyurethane acrylate (PUA) has attracted great interest due to unique properties such as outstanding adhesion, excellent flexibility, and good chemical resistance. A novel bio-based UV curable PUA oligomer has been synthesized by modifying cardanol with a polyfunctional acrylate precursor. Cardanol is an agricultural byproduct obtained from cashew nutshell liquid. The optimized UV cured coating showed excellent performance, including a glass transition temperature of 74°−123 °C, maximum thermal degradation temperature of 437°C − 441°C, tensile strength of 12.4−32.0 MPa, tensile modulus of 107.2 −782.7 MPa, and excellent adhesion. These PUA resins show great potential as UV curable adhesives and coatings.⁷

Another UV curable PUA resin was successfully synthesized from polyol based on a sustainable resource originated from renewable itaconic acid, isophorone diisocyanate, and 2-hydroxyethyl methacrylate. Historically, itaconic acid was obtained by the distillation of citric acid, but currently it is produced by fermentation. The synthesized resin was incorporated in varying concentrations in a conventional PUA coating system. The effects on rheology, crystallinity, thermal, and coating properties were evaluated.

The UV curable itaconic based PUA coatings show better mechanical, chemical resistance, and solvent resistance properties as compared to the conventional PUA. It was concluded that itaconic acid based UV curable PUA is a good alternative to conventional PUA coatings and possibly adhesives. The coatings show 100% adhesion to metal substrates as measured by the cross-cut adhesion method. The UV curable PUA coatings have polar N–H groups present in the molecules that create hydrogen bonds with metal to improve the adhesion properties.⁸

A novel bio-based UV curable PSA with approximately 50% biomass content was developed from alkali lignin, cardanol, and linseed oil. Bio-based prepolymers of cardanol diol acrylate and acrylated epoxidized linseed oil were synthesized to prepare PUA-based PSA systems. Liquid alkali-lignin-based acrylates (LA) were incorporated into the system at 10-30 weight percent loading to tune the functional PSA properties. The Fourier transform infrared spectroscopy (FTIR) analysis showed weakened cross-linking in the PSA systems on LA addition, which is desirable for removable PSA applications.

The single glass-transition temperature noticed in all of the formulations revealed good miscibility among the oligomers/prepolymers. The viscoelastic window also confirmed that the incorporation of 10-20% LA could improve the viscoelastic properties effectively to be used as removable PSAs. The addition of 20% LA into the PUA-based PSA system showed reasonable tackiness, lap shear adhesion (166 kPa), and 180° peel strength (∼2.1 N/25 mm) for possible nonstructural or semistructural applications. Lignin improved the thermal stability by hindering the degradation rate even at higher temperatures. Therefore, lignin-based PSAs with a high bio-based content can pave the way of replacing petrol-sourced PSA by proper tuning of the lignin content and modifications.⁹

Advancements in Performance and Extending Applications

As was indicated in Table 1, UV cured PSAs have a number of characteristics that relate to a circularity model. UV cured PSAs can offer enhanced performance, higher productivity, and an improved environmental footprint versus well-established water and solvent borne adhesives. UV curing adhesives can be formulated to be 100% solids. As a result, no water or solvents are released to the environment, which is better for the environment (no emissions) and safety (no flammable fumes) as well as water or solvent sensitive parts.

Certain properties that provide circular qualities are described below. They apply particularly to the end-of-life use in the circular economy model by improving performance to provide longer service life and by employing technologies that allow reuse or recycling. These and other advanced properties are expected to open the market opportunities for UV cured PSAs even further. Most importantly, UV cured PSA materials provide the potential to achieve performance on par with solvent borne PSAs. The ability of UV adhesives to rapidly cure without heat provides energy savings and productivity gains. Energy savings also occur because capital expense and facility floor space required for costly thermal curing ovens are eliminated. Thermally cured adhesives can take hours to cure compared to UV adhesives, which can cure in seconds to minutes.

Balancing of Properties

By controlling the design of the polymer and the additives employed, conventional PSAs with different balances of adhesion and cohesion can be developed for various applications. For example, PSAs can combine a unique balance in tack, peel, and shear properties to optimize products in a specific application. UV cured PSAs add another parameter to balance properties – UV dosage for crosslinking. Of particular interest are specialty applications demanding high temperature stability and solvent resistance.

An example of this balancing process has been reviewed by Jamaluddin, et. al. ¹⁰ They studied the effects of methyl methacrylate (MMA), trimethylolpropane triacrylate (TMPTA) , difunctional silicone urethane acrylate oligomer, and UV dose on the adhesive properties of UV curable PSAs. The investigation showed how properties can be optimized to meet the requirements of high holding power and low peel strength. It was determined that with increasing MMA content peel strength decreases and the holding power improves. It was further demonstrated that with an increase in TMPTA content, the gel fraction increases enhancing the holding power because of the crosslinking of TMPTA. Also, by increasing the oligomer content to more than 40% by weight, peel strength reduces to zero, whereas the holding power is significantly enhanced. Aa a result, UV curable PSAs with low peel strength and high holding power were successfully synthesized, and they possessed desirable features which could be fabricated to meet specific requirements for industrial applications.

Thermal Stability

The development of adhesive tapes that can be applied at high temperature is a major challenge for pressure-sensitive adhesives. In a study directed to the relationship between curing structures and properties, a series of acrylic UV curable PSAs with excellent heat resistance were prepared. Commercial zirconium acetylacetonate, Desmodur L75, and a diamine were employed as heat-curing agents. Trimethylolpropane triacrylate (TMPTA) was used as a UV curing agent to form semi-interpenetration polymer network structures after UV exposure. The influences of different curing agents on the thermal stability, adhesion performance, gel fraction, and viscoelastic of PSAs were explored. The results showed that the optimized PSAs exhibited excellent heat resistance. The PSAs could be peeled off substrates without residues after treatment at 170°C for 4 h, while the nonmodified acrylic PSAs provided residues after 110°C treatment. The cured PSAs adhesive provided 180° peel strength (16.7 N/25 mm) which is comparable to current PSAs. These resulting UV cured PSAs also showed high heat resistance, and they are suitable for a broad range of special fields.¹¹

Hybrid UV cured resins were prepared by using bisphenol A epoxy resin and bisphenol A epoxy acrylate as the main raw materials with monomers and photoinitiators. The structure and properties of the hybrid UV cured material were characterized. The influence of the content of epoxy resin on the curing extent and structure was discussed. The results showed that the curing extent of the hybrid UV cured material was not correlated with the content of the epoxy resin. The hybrid UV cured material had two glass transition temperatures providing good thermal stability and damping properties.¹²

UV curable PSAs with higher thermal stability were successfully prepared by forming composites with silica nanoparticles and modified via reaction with 3-methacryloxypropyltrimethoxysilane. The acrylic copolymer was synthesized as a base resin for PSAs by solution polymerization. The acrylic copolymer was modified to have the vinyl groups available for UV curing. The peel strength decreased with the increase of gel content which was dependent on both silica content and UV dose. Thermal stability of the PSAs was improved noticeably with increasing silica content and UV dose mainly due to the strong and extensive interfacial bonding between the organic polymer matrix and silica.¹³

Improving Processing Speed

Epoxidized block copolymers were used as the base polymer to formulate UV curable hot melt pressure sensitive adhesives. The ratio of epoxy to mono-alcohol groups was shown to be critical to optimizing the balance of peel, tack, and shear. Low levels of mono alcohol produce cured adhesives with low extensibility and low peel. Too high a level results in a weak adhesive with poor shear resistance. The optimal level of epoxy/mono-alcohol groups is 1.5 - 2.5 for the systems studied. Properly cured formulas can be (1) cured at high line speeds (>500 fpm with a 3 bulb system), (2) provide high heat/shear resistance, and (3) produce excellent adhesion to both polar and non-polar surfaces.¹⁴

Reduced Energy Costs

While mercury-based bulbs have long been a staple of the UV curing process, UV light-emitting diodes (LEDs) have made vast strides in becoming an excellent alternative due to their lower operating costs and superior lifetime over conventional bulbs. Mercury bulbs require a large amount of energy to operate the system, which leads to high energy costs during operation. Mercury is also a toxin and must be disposed of properly. With the proper design, UV LEDs can last tens of thousands of hours and require only a fraction of the energy and maintenance.¹⁵ A successful LED cure bonding process requires the pairing of LED equipment with adhesives and sealants that are specifically formulated for lower intensity, isolated wavelength curing.

End of Life Properties

Recently, increasing attention has been paid to the recovery or recycling of polymeric materials due to environmental regulations. Crosslinked polymers are insoluble and infusible; therefore, it is very difficult to remove crosslinked materials from substrates. Polymers with both crosslinkable and degradable properties are new materials that differ from conventional network polymers.

To this end, several types of photocrosslinkable polymers with degradable properties have been studied. Multifunctional methacrylates and epoxides with degradable properties were prepared, and their UV curing and degradation profiles were studied. Furthermore, polymers with crosslinkable units and degradable side chains were prepared and characterized in a similar manner. Some potential applications of these monomers and polymers were also described. These polymers and monomers are environment-friendly materials and novel photopolymers can be utilized in applications (e.g., negative-type photoresists, adhesives, inks and coatings) and then removed after use.¹⁶

Advancements in Dual Cure Systems

An essential requirement of UV curing is that the adhesive has to be transparent to the UV light in order to be cured. Filled or pigments or UV resins that are in shadowed areas may pose a curing challenge. However, this challenge can be overcome by the use of dual (UV / heat) cured PSAs. With these cationic or “dark-cure” systems, UV light initiates the cure mechanism, and cure is completed after heat exposure.

Also, a drawback of UV curable PSAs has been that the curing thickness was limited to approximately 100 μm due to filtering of the UV light. Attempts to overcome this problem have also introduced a cationic crosslinking mechanism into the UV curable PSA. Cationic curable PSAs can be cured for thicknesses of 1-2,000 μm, but they still exhibit problems such as slow curing and high cost.

The limitation of “dark cure” and high coating thickness has prompted the study of “dual-curable” PSAs that combine UV irradiation and thermal curing processes. This system contains two types of reactive functions: (1) a UV curable functional group and (2) a thermally curable functional group (e.g., polyols to react with polyisocyanate).¹⁷

Prepolymers have been synthesized through UV initiation, subsequently followed by thermal curing to prepare polyacrylate PSAs. The developed dual-curable acrylate PSAs based on both UV and thermal processes overcame the insufficient UV curing in thick coatings. These results showed that UV and thermal dual curable technologies provided a good balance of tack, peel, and shear with excellent pressure-sensitive properties.¹⁸

Smart Adhesives

“Smart” or “intelligent” materials are materials that can be significantly changed in a controlled fashion by external stimuli.¹⁹ The pressure sensitive adhesive (PSA) industry is exploring some of these new technologies to provide added value to their products. UV cured PSAs can provide a valuable contribution to this technology. Some of the more obvious examples of smart PSAs’ adjustable properties and typical applications are provided below:

• Tack or adhesion properties (switchability)
• Electrical or electromagnetic properties
• Thermal properties
• Viscoelastic properties
• Light or color properties
• Permeability properties
• Shape

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.

Several examples are provided in Table 2 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 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.

Table 2  Several Examples of PSA Products with Adherence Switchability

Removability and Resealing

A dismantlable, photo-curable adhesive has been reported. The adhesive is composed of a multi-acrylate oligomer with a tertiary ester, a photo-radical initiator, a photo-acid generator, and a phosphate-substituted acrylate as an adhesion promoter. Photo-exposure to the adhesive sandwiched between two glass slides induces crosslinking and causes the slides to bond to each other. Post-exposure baking then promotes an acid-catalyzed decomposition of the hardened adhesive, which results in significantly decreased adhesive strength. An alkaline developer can easily remove the residual adhesive on the debonded glass plates, thus recovering glass transparency. By using the dismantlable adhesive, a small glass plate attached to a glass slide can be successfully transferred to another glass slide.²¹  UV cured PSAs based on this technology can be used for temporary bonding of electronic substrates. 

Microsphere pressure sensitive adhesives are regarded as a niche specialty in the field of pressure sensitive adhesives. They are commonly used for production of removable products such as notes, labels, etc. The adhesive properties of microsphere PSAs can be modified using different procedures. The most common technique is the variation of glass transition temperature using different types and amounts of comonomers. Microsphere adhesive properties can also be modified by UV crosslinking via initiator concentration or by addition of chain transfer agent.  UV induced crosslinking is more efficient than crosslinking of polymer molecules in the polymerization phase by addition of multifunctional acrylic monomers.²²

Conclusions 

To deliver the maximum sustainability benefit, it is necessary to adopt an approach that examines the entire lifespan of a product, from the raw materials and processes used to manufacture it, through the use phase, to the end-of-life processing which includes ease of disassembly and the capability to re-use or recycle materials and components.  As a result, the concept of circularity or the circular economy has been introduced.  Linear consumption is reaching its limits. A circular economy has benefits that are operational as well as strategic, on both a micro- and macroeconomic level. It is expected that circularity will eventually be a trillion-dollar opportunity, with huge potential for innovation, job creation. and economic growth.²³ 

UV curable PSA can be designed to meet most of the applications where solvent borne PSAs are found.  They are solventless products that offer significant advantages in line speed, efficiency, and coating thickness.  It has become well known that UV curable PSAs meet many of the criteria for a sustainable product.  This article reviews recent advancements in UV cured PSA technology that also meet criteria for a circular product, namely renewable raw materials, longer product life, and better end-of-life scenarios.