Conventional Pressure-sensitive Adhesives Research Articles

Tailoring Pressure Sensitive Adhesives with H6XDI‐PEG Diacrylate for Strong Adhesive Strength and Rapid Strain Recovery

AbstractThe development of flexible electronic technology has led to convenient devices, including foldable displays, wearable, e‐skin, and medical devices, increasing the need for flexible adhesives that can quickly recover their shape while connecting the components of the device. Conventional pressure sensitive adhesives (PSAs) can improve recoverability via crosslinking, but often have poor adhesive strength. In this study, new types of urethane‐based crosslinkers are synthesized using m‐xylylene diisocyanate (XDI) or 1,3‐bis(isocyanatomethyl)cyclohexane (H6XDI) as a hard segment, and poly(ethylene glycol) (PEG) group as a soft segment. The PSA with the synthesized H6XDI‐PEG diacrylate (HPD) demonstrates a significantly improved recoverability compared to XDI‐PEG diacrylate and a conventional crosslinker 1,6‐hexanediol diacrylate (HDDA) while maintaining high adhesion strength (≈25.5 N 25 mm−1). The excellent recovery property of the PSA crosslinked with HPD is further confirmed by 100k folding tests and 10k multi‐directional stretching tests exhibiting high folding and stretching stability. PSA with HPD also shows high optical transmittance (> 90%) even after 20% straining, suggesting its applicability in fields that simultaneously require high flexibility, recoverability, and optical clarity such as foldable displays.

Variation in adhesion properties and film morphologies of waterborne pressure‐sensitive adhesives containing an acid‐rich diblock copolymer additive

AbstractOne challenge for waterborne pressure‐sensitive adhesives (PSA)s is the typical tradeoff between cohesion and adhesion. Others have shown that waterborne PSA films with thin, hard boundaries formed from short poly(acid) segments surrounding soft particles can have simultaneous improvements in adhesion and cohesion. To study this approach in commercial‐type acrylic waterborne PSAs, a diblock copolymer (BCP) composed of poly(acrylic acid)‐block‐(n‐butyl acrylate) (PAA‐b‐PBA) is added at 1–5 wt% to PSAs composed of 100% PBA (PSA (0% acid)) or 99% BA/1% AA (PSA (1% acid)). Films of these blends prepared using industrially relevant conditions reveal complex nanodomains in atomic force microscopy (AFM) images. PSA performance tests showed that 1–2 wt% BCP additive nearly tripled adhesion to stainless steel for PSA (0% acid) while improving cohesion. Nonetheless, AFM images of PSA (0% acid) with 1–2 wt% BCP display ill‐defined morphologies, with clear percolated structures apparent primarily in films with 5% BCP. This indicates that film morphologies which enhance adhesion may not correlate with AFM features. Moreover, the balance of adhesion and cohesion of PSA (1% acid) without BCP exceeds that of any of the BCP blends, demonstrating the challenge of using designed microstructures to improve upon conventional PSAs containing copolymerized acids.

Unveiling pressure-sensitive adhesiveness of a carbonized polymer dot

Dynamic market growth for water-borne Pressure Sensitive Adhesives (PSAs) calls forth new developments over conventional PSA. In order to achieve this, we made a trade-off between the polymerization and carbonization of acrylamide (A) and N,N′-methylenebisacrylamide (BA) without adding any additives to yield a series of carbonized polymer dots (CPDs) by varying their mole ratio. Among the pyrolyzed materials, ABA4:1 outperformed others with its tacky adhesiveness in the probe tack test. The ABA4:1 exhibits noticeable blue fluorescence when irradiated with UV light that could be used for tracking the adhesive application area. Furthermore, ABA4:1 was found to generate a significant amount of reactive oxygen species (ROS) upon irradiation with natural sunlight indicating its ability for self-sterilization. Thus a water-borne pressure-sensitive adhesive (PSA) was created through the partial carbonization of the substrates for better performance including a progressive touch of fluorescence and self-sterilization.

Enhanced Adhesion and Cohesion of Bioinspired Dry/Wet Pressure-Sensitive Adhesives.

The byssus-mediated adhesion of marine mussels is a widely mimicked system for robust adhesion in both dry and wet conditions. Mussel holdfasts are fabricated from proteins that contain a significant amount of the unique catecholic amino acid dihydroxyphenylalanine, which plays a key role in enhancing interfacial adhesion to organic and inorganic marine surfaces and contributes to cohesive strength of the holdfast. In this work, pressure-sensitive adhesives (PSAs) were synthesized by copolymerization of dopamine methacrylamide (DMA) with common PSA monomers, butyl acrylate and acrylic acid, with careful attention paid to the effects of catechol on adhesive and cohesive properties. A combination of microscopic and macroscopic adhesion assays was used to study the effect of catechol on adhesion performance of acrylic PSAs. Addition of only 5% DMA to a conventional PSA copolymer containing butyl acrylate and acrylic acid resulted in 6-fold and 2.5-fold increases in work required to separate the PSA from silica and polystyrene, respectively, and a large increase in 180° peel adhesion against stainless steel after 24 h storage in both ambient and underwater conditions. Moreover, the holding power of the catechol PSAs on both steel and high-density polyethylene under shear load continuously increased as a function of catechol concentration, up to a maximum of 10% DMA. We also observed stark increases in shear and peel adhesion for the catecholic adhesives over PSAs with noncatecholic aromatic motifs, further underlining the benefits of catechols in PSAs. Overall, catechol PSAs perform extremely well on polar and metallic surfaces. The advantage of incorporating catechols in PSA formulations, however, is less straightforward for peel adhesion in nonpolar, organic substrates and tackiness of the PSAs.

Design and Fabrication of Gecko-Inspired Adhesives

Recently, there has been significant interest in developing dry adhesives mimicking the gecko adhesive system, which offers several advantages compared to conventional pressure-sensitive adhesives. Specifically, gecko adhesive pads have anisotropic adhesion properties; the adhesive pads (spatulae) stick strongly when sheared in one direction but are non-adherent when sheared in the opposite direction. This anisotropy property is attributed to the complex topography of the array of fine tilted and curved columnar structures (setae) that bear the spatulae. In this study, we present an easy, scalable method, relying on conventional and unconventional techniques, to incorporate tilt in the fabrication of synthetic polymer-based dry adhesives mimicking the gecko adhesive system, which provides anisotropic adhesion properties. We measured the anisotropic adhesion and friction properties of samples with various tilt angles to test the validity of a nanoscale tape-peeling model of spatular function. Consistent with the peel zone model, samples with lower tilt angles yielded larger adhesion forces. The tribological properties of the synthetic arrays were highly anisotropic, reminiscent of the frictional adhesion behavior of gecko setal arrays. When a 60° tilt sample was actuated in the gripping direction, a static adhesion strength of ~1.4 N/cm(2) and a static friction strength of ~5.4 N/cm(2) were obtained. In contrast, when the dry adhesive was actuated in the releasing direction, we measured an initial repulsive normal force and negligible friction.

Gecko adhesion: evolutionary nanotechnology

If geckos had not evolved, it is possible that humans would never have invented adhesive nanostructures. Geckos use millions of adhesive setae on their toes to climb vertical surfaces at speeds of over 1ms-1. Climbing presents a significant challenge for an adhesive in requiring both strong attachment and easy rapid removal. Conventional pressure-sensitive adhesives (PSAs) are either strong and difficult to remove (e.g. duct tape) or weak and easy to remove (e.g. sticky notes). The gecko adhesive differs dramatically from conventional adhesives. Conventional PSAs are soft viscoelastic polymers that degrade, foul, self-adhere and attach accidentally to inappropriate surfaces. In contrast, gecko toes bear angled arrays of branched, hair-like setae formed from stiff, hydrophobic keratin that act as a bed of angled springs with similar effective elastic modulus to that of PSAs. Setae are self-cleaning and maintain function for months during repeated use in dirty conditions. Setae are an anisotropic 'frictional adhesive' in that adhesion requires maintenance of a proximally directed shear load, enabling either a tough bond or spontaneous detachment. Gecko-like synthetic adhesives may become the glue of the future-and perhaps the screw of the future as well.

Mechanics of a Novel Shear-activated Microfiber Array Adhesive

AbstractElastic rod theory and principles of contact mechanics motivate the development of a novel, shear-activated, microfiber array adhesive. Unlike with conventional Pressure Sensitive Adhesives (PSAs), the microfiber array and backing are composed entirely of a stiff, glassy polymer (polypropylene, elastic modulus E = 1 GPa) and an externally applied shear load is required to achieve contact with a substrate. Previously, results from a Shear Power Test on glass indicated a maximum interfacial shear strength of 10 kPa over 4 sq. cm, a factor of 1000 greater than with a smooth polypropylene sheet of similar thickness. Here we present a theoretical model that describes the mechanism for shear-activated adhesion and predicts a shear strength of 27 kPa, on the order of the experimental measurement.

Frictional and elastic energy in gecko adhesive detachment

Geckos use millions of adhesive setae on their toes to climb vertical surfaces at speeds of over 1 m s(-1). Climbing presents a significant challenge for an adhesive since it requires both strong attachment and easy, rapid removal. Conventional pressure-sensitive adhesives are either strong and difficult to remove (e.g. duct tape) or weak and easy to remove (e.g. sticky notes). We discovered that the energy required to detach adhering tokay gecko setae (W(d)) is modulated by the angle (theta) of a linear path of detachment. Gecko setae resist detachment when dragged towards the animal during detachment (theta = 30 degrees ) requiring W(d) = 5.0+/-0.86(s.e.) J m(-2) to detach, largely due to frictional losses. This external frictional loss is analogous to viscous internal frictional losses during detachment of pressure-sensitive adhesives. We found that, remarkably, setae possess a built-in release mechanism. Setae acted as springs when loaded in tension during attachment and returned elastic energy when detached along the optimal path (theta=130 degrees ), resulting in W(d) = -0.8+/-0.12 J m(-2). The release of elastic energy from the setal shaft probably causes spontaneous release, suggesting that curved shafts may enable easy detachment in natural, and synthetic, gecko adhesives.

Gecko Adhesion: Structure, Function, and Applications

AbstractGeckos attach and detach their adhesive toes in milliseconds while running with reckless abandon on nearly any surface. The adhesive on gecko toes differs dramatically from that of conventional adhesives. Conventional pressure-sensitive adhesives (PSAs) are soft viscoelastic polymers that degrade, foul, self-adhere, and attach accidentally to inappropriate surfaces. In contrast, gecko toes bear angled arrays of branched, hair-like fibers (setae) formed from stiff, hydrophobic keratin that act as a bed of angled springs with an effective stiffness similar to that of PSAs. Setae are selfcleaning and maintain function for months during repeated use in dirty conditions. Setae are an anisotropic “frictional adhesive” in that adhesion requires maintenance of a proximally directed shear load. Thus, gecko setae resist inappropriate bonding and are capable of easy and rapid attachment and detachment. Engineered adhesive nanostructures inspired by geckos may become the glue of the future—and perhaps the screw of the future as well.

Microstructured Polymer Adhesive Feet for Climbing Robots

AbstractNovel insect-foot–inspired materials may enable future robots to walk on surfaces regardless of the direction of gravity. Mini-Whegs™, a small robot that uses four wheel-legs for locomotion, was converted to a wall-walking robot with compliant, adhesive feet. First, the robot was tested with conventional adhesive feet. Then a new, reusable insect-inspired adhesive was tested on the robot. This structured polymer adhesive has less adhesive strength than conventional pressure-sensitive adhesives, but it has two important advantages: the foot material maintains its properties for more walking cycles before becoming contaminated, and the feet can then be washed and reused with similar results, which is not feasible with conventional adhesives. After the addition of a tail and widening the feet, the robot is capable of ascending vertical smooth glass surfaces using the structured polymer adhesive.

Controlling the hydrophilicity of microchannels with bonding adhesives containing surfactants

In this study, a novel protocol to control the hydrophilicity of microchannels as well as to bond the microfluidic devices in all polymer-based biomedical devices is suggested. It is demonstrated that the contact angle of water in the microchannel surface can be tuned using conventional pressure-sensitive adhesives (PSA) containing surfactants. The surfactants are immersed in a hydrophobic PSA film in hydrophobic environments (air). The water–adhesive interface reconstructed quickly and became hydrophilic upon contact with water. Both the contact angle and volumetric flow rate (VFR) of water in microchannels could be systematically controlled by changing the concentration of the surfactants in the PSA film and by using different types of surfactants with different chemical structures. The VFR of water changed in a two-order range (1–102 nl s−1) when the concentration of surfactants was varied from 1% to 10%. We believe that the use of surfactants favors mainly the control of flow rate and the protocol suggested here to control the flow velocity in a broad range should find a wide application. Although the hydrophilic coating formulated with surfactants has proven effective in improving the flow rate of liquid, the surfactants located at the interface could be dissolved into the fluid samples. And the beneficial result of the use of surfactants is restricted to microfluidic devices where the liquid stays briefly in contact with the channel walls. The protocol suggested in the research can be directly applied to the development of low-cost, high-speed and reliable bonding techniques for disposable polymer-based microfluidic devices.