Adhesives for “debonding-on-demand”: Triggered release mechanisms and typical applications
Introduction
PSAs are used in nearly all industries as well as in everyday life, including household, office, personal care and medical applications. In industry, this concerns temporary surface protection (masking tape), packaging, labeling and the assembly of automotive parts, toys or electronic circuit boards, while plasters, surgical drapes, dermal dosage systems and biomedical electrodes are representatives of their application in medical field. Moreover, PSAs are applied as decorative films, double or single sided adhesive tapes and sticky notes in household and office [[1], [2], [3], [4], [5], [6], [7], [8]].
In 2015, 43.3 billion square meters of PSA tapes were produced worldwide [9]. According to market analysis, the global market of PSA tapes amounted to 50.12 billion USD in 2017 and is forecast to grow with a compound annual growth rate (CAGR) of 6.22% from 2017 to 2022, reaching 67.76 billion USD at the end of this period. While the Asia Pacific holds the largest market share of 42%, also Europe (33%) and North America (17%) are important for the PSA tape industry. The market growth is driven by the high demand in various application fields, including packaging, electrical & electronics, transportation, construction as well as the medical & healthcare sector. The highest increase is estimated for the Asia Pacific due to fast growing industrialization in this region. In terms of industry segments, the electric and electronic sector is expected to grow at the highest CAGR in 2017–2022. Here, PSA tapes are applied to mount components to printed circuit boards (PCBs), in the manufacture of consumer electronic devices (mobile phones, cameras, etc.) and for electrical interconnections and assemblies [10,11].
In this review we focus on temporary adhesives, comprising “debonding-on-demand” mechanisms. Independent on the application field, temporary tapes are required to exhibit high holding power for reliable bonding during use and simultaneously easy and clean removability in the end of the application. This can be achieved by precise adjustment of the adhesive's chemical composition (see chapter 2.1 and 2.2 for more details) or by the incorporation of a release function, which enables the reduction of the adhesive strength by exposure to external triggers, such as UV-light, heat, electric currents, magnetic fields, etc. (see chapter 3). For example, adhesive tapes with “debonding-on-demand” properties are applied in the microelectronic industry, where they serve as backgrinding or dicing tapes and in the automotive sector, where they are applied as masking tapes during painting and varnishing of cars and vehicles [6] In addition, tapes with release mechanisms serve as temporary adhesives, which support the recycling and recovery of high value materials in the automotive sector [[12], [13], [14]] or are utilized in the medical field, where they are used for painless removal of wound dressings, in dental adhesives and for drug delivery [5,[15], [16], [17], [18], [19]].
The behavior of PSAs is mainly described by three parameters: tack, adhesion (peel adhesion) and shear resistance (cohesion). Tack is defined as initial adhesion, which develops upon slight pressure and short dwell time. It enables the PSA to form a bond of measurable strength immediately after contact to the substrate and is a measure for the primary wetting of the substrate. It can be measured as the force required to separate adherend and adhesive at the interface, immediately after application. While tack describes how quickly bonds are built by a given PSA, peel adhesion is a measure of the force required to separate a PSA tape from a substrate at defined speed and peel angle (180° or 90°). Both parameters are viscoelastic properties, which are influenced by the chemical composition (choice of monomers) and structure of the adhesive (crosslinking degree, molecular weight). The detected values of tack and peel adhesion depend on the test method and conditions, including application time, temperature as well as the peel angle and speed. Moreover, the face stock material (carrier) and the coating weight have a significant impact. Compared to peel adhesion, tack is more dependent on the wetting and substrate type, as it does not require application force. High peel adhesion requires a certain level of tack for bonding and a proper cohesion for debonding. In contrast to tack and peel adhesion, cohesion (also called shear resistance or resistance to creep) describes the property of a single substance to adhere to itself (internal adhesion/strength) rather than interactions between two different materials. It is defined as the force, which holds a substance together and is thus a measure of internal molecular forces. Cohesion is influenced by the molecular weight and the crosslinking density of the base polymer. In particular, cohesion increases with rising molecular weight and high degrees of crosslinking. However, it has to be considered that a certain molecular mobility is required for adhesion build-up during bonding. In general, permanent adhesives require a higher level of cohesion than removable PSAs do. For high cohesion, the molecular weight has to exceed the entanglement molecular weight Me. On the contrary, cohesion is diminished by the addition of tackifiers, since they act as diluents and increase chain mobility. Beside these factors, also the coating weight, the face stock material, the substrate and the temperature during testing as well as the chemical composition and structure of the adhesive affect this parameter [1,2,5,[20], [21], [22]].
PSAs provide aggressive and permanent tack at room temperature, adhere firmly to a variety of surfaces upon light pressure (finger/hand) and do not need any activation such as water, solvent or heat [1,2,5,20,23]. In contrast to other adhesives, PSAs do not change their physical or chemical state during the application. Bonding and debonding processes are determined by the adhesive-substrate interface as well as by mechanical and rheological properties of the PSA. Wetting of the adherent is essential for adhesion build-up. The quality of wetting is governed by the PSAs viscoelastic properties and the interfacial energies. Good wetting and high adhesion require low surface energies of the PSA, and high surface tension of the substrate [1,2,23]. Moreover, adhesion is strongly dependent on application time and pressure [2]. The viscoeleastic behavior of the PSA provides the required flow and deformability for proper adaption to the surface topography and the establishment of molecular interactions (like liquids at low deformation rates), and simultaneously enables the adhesive to sustain loads (like solids at high deformation rates). Furthermore, the PSAs must provide sufficient inner strength (cohesion) to withstand mechanical stress and to provide clean removability in case of removable tapes. Thus, a good balance between adhesion and cohesion is demanded to meet the partially contradictory requirements of PSA tapes for temporary applications [1,2,5,23].
In general, the adhesion properties (tack, peel adhesion and cohesion) of PSAs are governed by the glass transition temperature Tg, which can be adjusted by variations of the chemical composition and structure (branching, crosslinking) of the adhesive [[1], [2], [3],21,24]. More precisely, the kind of monomers, the type and amount of initiators and crosslinking agent as well as the polymerization method play an important role. Moreover, the number average molecular weight Mn, the polydispersity Mw/Mn, the average molecular weight between entanglements Me, the molecular weight between junctions Mc of the basic adhesive polymer and the degree of branching have a strong impact on the adhesive's mobility and viscosity and hence on its mechanical and adhesive properties and the Tg. The addition of various additives, including tackifiers, antioxidants and fillers, allows further tuning of the adhesive performance [1,3,21,23,25]. Typically, the Tg of PSA is adjusted to 40–70 °C below the application temperature [1,2,24].
While the first PSA tapes were based on natural rubber and rosin, nowadays there is a huge variety of tapes which differ in their chemical base, the carrier material and their construction [2,3]. Typically, PSAs are based on visco-elastomers or elastomers such as rubbers (natural and synthetic), polyacrylates and styrenic block-copolymers. Furthermore, polysiloxanes, polyurethanes (PUR), polyesters, polyethers and ethyl vinyl acetate (EVA) polymers are used as base adhesive material [2,3,5,20,[22], [23], [24]]. Adhesives based on rubber are the most cost-efficient PSA systems on the market. Natural rubber PSAs are characterized by a high molecular weight, low Tg (−70 to −40 °C), high tack and high peel adhesion. They are usually crosslinked to achieve the required cohesion and are applied from solution or as hot melts. However, these adhesives are not stable against long-term exposure to environmental conditions, as the unsaturated polymer backbone is prone to oxidative and radiation induced degradation causing yellowing and embrittlement [1,2,5,20]. Block copolymers used in PSAs are mainly based on polystyrene in combination with isoprene or butadiene units and are composed of linear A-B-A, A-B, or branched (AB)n sequences. Tack, peel adhesion and shear strength properties are controlled by the addition of tackifiers. They have moderate molecular weights (7 x 104–2.5 × 105 Da) and high cohesion due to phase separation, where the domains act as physical crosslinkers. However, the unsaturation in isoprene or butadiene units causes oxidative instability as already discussed for natural rubber. Block copolymer based PSAs are particularly applied as hot melts or high solids and packaging tapes [1,2,20,22,23]. Another important class of PSAs is based on polyacrylates prepared from random copolymers of (meth)acrylic esters and vinyl monomers, which are inherently pressure sensitive. Adhesion properties and performance are mainly influenced by the chemical composition (choice of monomers) and the molecular weight of the acrylic polymers. Typical Tgs for polyacrylic PSAs are in the range of −70 °C to −25 °C [24]. “Soft” acrylates with long side-chains such as n-butyl acrylate, 2-ethylhexyl acrylate or iso-octyl acrylate provide low Tg corresponding to high tack and peel adhesion. In contrast, “hard” short side-chain and polar monomers (e.g. acrylic acid and methyl methacrylate) increase the Tg and thus, the peel strength and the stiffness of the adhesive. Simultaneously, the internal strength increases due to the formation of intermolecular hydrogen bonds. In addition, acrylates with functional groups can be copolymerized to tailor interactions with selected substrates. For most applications polyacrylics are crosslinked to provide sufficient cohesion and stability. They are transparent and colorless and show excellent physiological properties as well as resistance against water, chemicals and aging (light, heat, oxidation) due to their saturated polymer backbone [1,2,8,22]. Along with aqueous emulsions or 100% solid systems (hot melts or low viscosity systems), acrylic PSAs are applied as solution of the copolymers [2,4,5,26,27].
Typical silicone PSAs are based on poly(dimethylsiloxane) (PDMS) networks. In order to increase surface tension and Tg (−85 °C) and to improve thermal stability, the organofunctional groups of the polysiloxanes may be substituted with phenyl groups. Typically, the long chain PDMS is crosslinked by addition of multifunctional silicate resins [22]. Moreover, vinyl, epoxy or acrylate functionalities may be introduced for crosslinking. The high costs of these niche products are often outweight by high resistance to oxidative degradation, high chemical and weathering resistance and high thermal stability. In particular, the excellent temperature resistance relies on the Si–O–Si backbone which also provides high flexibility and thus low Tg. Hence, this class of PSAs can be applied in a wide temperature range (−70 °C–250 °C) to both high and low energy surfaces. Peel strength can be improved by compounding the elastomers with silicon resins, which are attached via reaction of some free silanol groups, while crosslinking provides improved cohesion [2,5,8,20,22,28]. However, residual siloxane monomers present in the polymer matrix often remain on the surface of the adherend after tape removal, which compromises on product quality (e.g. decreased delamination resistance of composite structures) and thus, limits the applicability of siloxane-based PSAs. PUR adhesives rely on urethane links, which are formed upon reactions of diiscocyanates and di-or polyols, and comprise excellent bond strength durability and plasticizer resistance. The viscoelastic properties of PUR adhesives are controlled by the chemical composition and structure of the monomers, whereas the structure of the diisocyanate has a strong impact on the tack. Mostly, aliphatic diisocyanates are used. Alternating blocks of high and low Tg lead to segregated structures of hard and soft segments, which allow for thermoforming. PUR PSAs are mainly applied as laminating and mounting adhesives [8,29] Another group of PSAs are based on polyvinyl (alkyl) ethers. Unlike polyacrylates, these adhesives are not inherently pressure sensitive but rather comprise “dry tack”. Consequently, blends of low and high molecular weight polymers are applied to ensure proper surface wetting as well as the required cohesion for PSA applications [5,20]. Crosslinking of di- or polyfunctional vinyl ethers provides the inner strength of the adhesive [6]. Since polyvinyl ethers display high moisture vapor permeability [5] and are non-irritating to skin, they are suitable for medical applications [6].
Section snippets
Release mechanisms in PSA tapes
Due to the wide range of applications, the adhesive properties of PSAs have to be adapted to individual requirements [25], including type and nature of the substrate as well as application conditions and purpose [30]. Especially high holding power during utilization and simultaneous easy and clean removability are desired for temporary PSA tapes, independent of the application field. Adhesive residues have to be avoided to provide reliable bonds in further processing and stainless and clean
Summary and outlook
The adhesive strength of different adhesive systems can be reduced by external triggers, such as UV-irradiation, heat or combinations thereof as well as electric, magnetic or electromagnetic fields. Moreover, shape memory polymers or electrostatic mobile carriers can be employed for temporary bonding. While some systems can be repeatedly switched between the bonding and the non-bonding state, others are only made for single use applications, since they are degraded or decomposed during
Acknowledgments
The research work of this paper was performed at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET-program of the Federal Ministry for Transport, Innovation and Technology and the Federal Ministry for Digital and Economic Affairs, with contributions by the Chair in Chemistry of Polymeric Materials (Montanuniversitaet Leoben, Austria). The PCCL is funded by the Austrian Government and the State Governments of Styria, Lower Austria and Upper Austria.
References (99)
- et al.
The mechanical performance of adhesive joints containing active disbonding agents
Int J Adhesion Adhes
(2013) - et al.
Peeling performance of a novel light switchable pressure-sensitive adhesive
Int J Adhesion Adhes
(2001) - et al.
The chemistry and applications of UV-cured adhesives
Int J Adhesion Adhes
(1991) - et al.
Electrolytically assisted debonding of adhesives: an expkerimental investigation
Int J Adhesion Adhes
(2012) - et al.
Wettability and adhesion characteristics of photo-crosslinkable adhesives for thin silicon wafer
Int J Adhesion Adhes
(2013) - et al.
UV-curing behavior and adhesion performance of polymeric photoinitiators blended with hydrogenated rosin epoxy methacrylate for UV-crosslinkable acrylic pressure sensitive adhesives
Eur Polym J
(2008) - et al.
Surface modification of thermally expandable microspheres by grafting poly(glycidyl methacrylate) using ARGET ATRP
Eur Polym J
(2009) Recent advances in polymer shape memory
Polymer
(2011)- et al.
Switchable double-sided micropatterned adhesives for selective fixation and detachment
J Mech Phys Solid
(2019) - et al.
Debonding on command of adhesive joints for the automotive industry
Int J Adhesion Adhes
(2015)
Easy removal of pressure sensitive adhesives for skin applications
Int J Adhesion Adhes
Marine-inspired polymers in medical adhesion
Eur Polym J
Fluoride-responsive debond on demand adhesives: manipulating polymer crystallinity and hydrogen bonding to optimise adhesion strength at low bonding temperatures
Eur Polym J
Adhesives and adhesive tapes
Synthesis and cross-linking of acrylic PSA systems
J Adhes Sci Technol
Pressure-sensitive adhesives: an introductory course
MRS Bull
Pressure-sensitive adhesives for medical applications
Applications of pressure-sensitive products
Removable adhesive tape
Tape market data and trends: 2016 Freedonia Study
Pressure sensitive adhesive Tapes market by resin type (acrylic, rubber, silicone), category (commodity, specialty), backing material (polypropylene, paper, PVC), type, end-use industry, and region - global forecast to 2022
Adhesive Tapes market 2018: industry review, research, statistics, and growth to 2022
Synthesis and evaluation of polypyrrole-coated thermally-expandable microspheres: an improved approach to reversible adhesion
Soft Matter
Electrochemical characterization of electrically induced adhesive debonding
J Electrochem Soc
The development of a pressure-sensitive adhesive for trauma-free removal
Int J Adhesion Adhes
Advanced debonding on demand systems for dental adhesives
Untapped potential for debonding on demand : the wonderful world of azo-compounds
Mater Horizons
Reversing adhesion: a triggered release self-reporting adhesive
Adv Sci
Adhesion and adhesives Technology - an introduction
Pressure-sensitive adhesives and applications
Pressure-sensitive adhesives
Encycl. Polym. Sci. Technol.
Photoreactivity adjustment of acrylic PSA
Rev Adv Mater Sci
Recent advances in silicone pressure-sensitive adhesives
J Adhes Sci Technol
Polyurethane based pressure sensitive adhesives (PSAs) using electron beam irradiation for medical application
J Polym Mater
Overview of disbonding technologies for adhesive bonded joints
J Adhes
Printed circuit board tapes n.d
Kleben mit Magnetfeldern
Phys J
Nanotechnology products and applications n.d
Lösbare kleberverbindungen. DE19961940A1
ElectRelease - electrically debonding adhesive n.d
Reversible adhesive-free nanoscale adhesion utilizing oppositely charged polyelectrolyte brushes
Soft Matter
Carrier techniques for thin wafer processing
Characterization of electrostatic carrier substrates to be used as a support for thin semiconductor wafers
20th Int Conf Compd Semicond Manuf Technol
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