Hierarchical structures of fine fibrils on insect's feet induce strong molecular forces and provide extraordinary adhesive strength, enabling them to support large loads. Such dry adhesion allows, for example, the gecko's locomotion on slippery surfaces against gravity as well as firmly attach onto and detach with ease from rough substrates. Presently, polymer micro/nano fibrillar dry adhesives are known to be made using micro/nano fabrication processes. These techniques are common in microfabrication and often require expensive equipment and are costly to implement on a commercial scale.
This invention makes use of a drastically different approach to manufacture flexible, fibrillar, and adhesive microprotrusion and adhesive laid-flat membranes by scalable nozzle-free electrospinning, which presents novel approaches to fabricating dry, removable, reusable adhesives on substrates. The invention differs drastically from other disclosed technologies, which are based on the use of aligned carbon nanotubes, photolithography, chemical etching, and time-consuming batch micro/nano molding processes. The new methodology offers advantages of significantly lower costs and easier scale-up, which will replace the more expensive approaches such as micro/nano fabrication process. Introduction of nozzleless electrospinning polymer blending and equal-channel angular pressing will combine processing stages such as solution preparation, spinning and spin-finish in one production line, significantly reducing processing time and costs.
Although some of these spinning methodologies are known in the art, one of the inventors, Shing-Chung Wong, led the first group to examine electrospun nanofibers and membranes for their adhesion strength and energy and thus their applications as dry adhesives. Measurement of Adhesion Work of Electrospun Polymer Membrane by Shaft-Loaded Blister Test, Langmuir Vol. 28 (2012) 6677-6683; A Nano-Cheese-Cutter to Directly Measure Interfacial Adhesion of Freestanding Nano-Fibers, Journal of Applied Physics Vol. 111, (2012); Mechanism of Adhesion Between Polymer Fibers at Nanoscale Contacts, Langmuir Vol. 28 (2012) 4663-4671; and Do Electrospun Polymer Fibers Stick?, Langmuir, Vol. 26 (2010) 14188-14193.
An adhesive is a kind of material that can bond items together. Adhesives are typically liquid or semi-liquid, with the earliest adhesives being made of natural materials, such as tree sap, beeswax, tar and etc. Advances in the science and understanding of adhesive mechanisms have led to more and more adhesive formulations. Today's adhesives can be classified in many different ways, generally by their bonding mechanism, with three major categories including: physically hardening adhesives, chemically curing adhesives and pressure sensitive adhesives.
Physically hardening adhesives are non-reactive adhesives, and they are in their final chemical state before applying to surface. Only polymers that can be liquefied, either melt or dissolved, can be used for physical hardening adhesives. Physically hardening adhesives provide a wide range of adhesive properties, generally good bond flexibility, and are used in a variety of applications.
There are three major types of hardening adhesives: (i) hot melts, (ii) solvent based adhesives and (iii) polymer dispersion adhesives. Most of the hot melt adhesives are thermoplastics, which can be applied in molten form in the range of 65° C.-180° C. They can be solidified in room temperature to form strong bonding with various materials. Ethylene-vinyl acetate (EVA) is a particularly common hot melt adhesive. EVA possesses good physical properties, such as good clarity, low-temperature toughness, stress-crack resistance, water resistance, UV resistance etc. Solvent based adhesives build strength through the evaporation of the solvent. The performance of solvent-based adhesives is largely determined by the polymer system in the formulation. The choice of adhesive type depends on the specific substrates and environmental resistance needed—temperature resistance, oil and plasticizer resistance, etc. Polymer dispersion adhesives are typically formulated from compounds including vinyl acetate polymers and copolymers (PVAC), ethylene vinyl acetate (EVA), acrylics, styrene-butadiene rubber (SBR), natural rubber latex and synthetic elastomers, and polyurethane (PUR). These adhesives are heterogeneous systems comprising a solid polymer phase dispersed in an aqueous phase.
One of the major advantages is the absence of VOC's. For many water based adhesives, it is a requirement that at least one of the substrates be permeable to allow water to escape from the system. It is not surprising, then, that these materials have found wide use in bonding wood, paper, fabrics, leather and other porous substrates.
Chemically curing adhesives are reactive materials that require chemical reaction to convert them from liquid to solid. Generally they can be classified in to single component adhesives and two component adhesives. Single component adhesives have pre-mixed adhesive components which are blocked normally. Only when the required condition was met, they will activate the hardener. These conditions could be heat, moisture, radiation, etc. Two component adhesives have two reactive components which can form solid systems after mixing them. The most widely used two component adhesives include epoxies, methyl methacrylates (MMA), silicones, etc.
Cyanoacrylates are known for their “instant” bonding to most surfaces. When a drop of cyanoacrylate adhesive is put on the surface of a part, the acid stabilizer molecules react with the water molecules present on the surface of the part from the relative humidity in the air. The reaction of the water and acid causes the acid stabilizer to be neutralized. The cyanoacrylate molecules then react with each other and form polymer chains without cross-linking. Cyanoacrylates can bond most types of glass, plastics and metals, and has broad application in optics, microelectronics, transportations and medical industries, etc.
Single-component epoxy adhesives include solvent-free liquid resins, solutions in solvent, liquid resin pastes, fusible powders, sticks, pellets and paste, supported and unsupported films, and preformed shapes to fit a particular joint. Two-component epoxy adhesives are usually composed of the resin and the curing agent, which are mixed just prior to use. The components may be liquids, putties, or liquid and hardener powder. They may also contain plasticizers, reactive diluents, fillers, and resinous modifiers. The processing conditions are determined by the curing agent employed.
Typical cure conditions range from 3 h at 60° C. to 20 min at 100° C. Epoxy adhesives form strong bonds to most materials, in addition to excellent cohesive strength, but are not reusable and irremovable, and residue is left on cohesively fractured surfaces. Epoxies yield good to excellent bonds to steel, aluminum, brass, copper, and most other metals but are brittle and fracture usually with cohesive failure mode.
Pressure sensitive adhesives (PSA's) are most used in tape and label industry. PSA's are typically formulated from natural rubber, certain synthetic rubbers, and polyacrylates. PSA's form a bond simply by the application of pressure to marry the adhesive with the adherend. Once the adhesive and the adherend are in proximity, there are also molecular interactions such as van der Waals forces involved in the bond, which contribute significantly to the ultimate bond strength. PSA's exhibit viscoelastic (viscous and elastic) properties, both of which are used for proper bonding. Pressure sensitive adhesives are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow the adherend. The bond has strength because the adhesive is hard enough to resist flow when stress is applied to the bond. Since these adhesives are not true solids, the strength of pressure sensitive adhesives decreases when the temperature is increased. PSA's also exhibit a tendency to undergo creep when subjected to loads.
There is a growing demand in the art of removable, reusable and dry adhesives. Typically, dry reusable adhesives produce substantial shear adhesion strengths but significantly lower normal lifting forces, giving rise to high anisotropic adhesion properties. Dry adhesives with anisotropic force distribution may find potential use in several applications such as tapes, fasteners, treads of wall-climbing robots, wall-climbing suits, microelectronics, and medical and space applications.
Dry adhesives have been fabricated using polymers, which has resulted in high shear adhesion strength, by forming tens of millions of contact points between adhesive and adherend. Carbon nanotube (CNT) based arrays possess a shear adhesion strength as high as 100 N/cm2 on a glass slide. These arrays have high aspect ratios and mechanical strengths. However, a significant normal preload of 50 N/cm2 is required to achieve this shear adhesion force. CNT arrays have considerable limitations including being electrically conductive for applications necessitating electrically insulating adhesives. These arrays also possess a low ratio of shear adhesion strength to normal detachment strength (V) which limits their range of applications.
Hierarchical pillars have been fabricated in polydimethysiloxane where the top rounded pillars are 10 μm in diameter with aspect ratios ranging between 0.5 and 2. The base pillars are 200 μm in length and 50 μm in diameter. Hierarchical nanofiber-based structures have been developed using polymethylmethacrylate by chronologically employing two porous alumina templates. The adhesion strength in this hierarchical structure is also severely low. The heavily packed pillars cause clumping in the structure, which is suggested to lead to this deteriorated adhesion. None of the prior art made use of fiber spinning methodologies to form the adhesives.
High aspect ratio (AR) structures exhibit significant shear adhesion strength compared to ones with low AR's. Various techniques have been employed to fabricate moderate AR structures including nanomolding, e-beam lithography, and replication of nanoporous membranes with polymers. The methodologies are costly to be scaled up for mass production.
Thus, there is a need in the art for improved methods of forming dry adhesives from electrospinning spinnable materials. There is also a need in the art for dry adhesives with high shear adhesion strength and low normal detachment.