Method and system for pulsed light debonding of metal-transparent substrates and transparent-transparent substrates bonded with an adhesive

Abstract

A method and system for the separation of an adhesive layer are disclosed. The method includes irradiating a pulsed light source through a first substrate toward the adhesive layer. The first substrate is an optically transparent substrate. The adhesive layer is provided between the first substrate and the second substrate.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of U.S. Patent Application No. 63 / 392,537, filed on July 27, 2022, and U.S. Patent Application No. 63 / 500,459, filed on May 5, 2023, the entire contents of both applications are incorporated herein by reference.

[0002] Government Clause This invention was made with government support under Grant No. N00014 - 18 - 1 - 2624 awarded by the Office of Naval Research. The government has certain rights in this invention.

[0003] Technical Field This disclosure generally relates to pulsed light, for example, methods and systems for pulsed laser debonding of metal - transparent substrates and transparent - transparent substrates adhered with an adhesive.

Background Art

[0004] Background Many manufacturing and construction processes rely on joining materials using mechanical fasteners or adhesives. The relatively high adhesive strength combined with the low weight of chemical adhesives has made the manufacture of chemical adhesives a billion-dollar industry. Ideally, an adhesive like a bolt can be used for a reversible joint that forms a strong bond and can be broken later in a relatively quick and convenient way. Adhesive delamination has received less attention, while adhesive formation has been the main focus of research in this field. In the case of relatively weak adhesions, delamination can be achieved by physical separation, but when the adhesive strength of the adhesive is comparable to that of the substrate, physical separation is likely to break not only the adhesive layer but also the substrate. Strategies for peeling strong adhesives usually rely on including thermally expandable materials or other elements that can generate heat internally in response to an external electric or magnetic field. In most cases, the peeling process is relatively slow (seconds to minutes), and a large secondary device such as a heater or magnet needs to be used in close proximity to the adhered object.

[0005] Light provides a non-thermal input that can be used for the delamination of adhesives over relatively large distances. There are multiple strategies for photoinduced delamination, all of which require the use of custom-made photosensitive adhesives. Photoinduced polarity changes in photochromic molecules such as spiropyran, diarylethene, donor-acceptor Stenhouse adducts can be used to modify the relatively weak adhesion between a doped polymer film and another solid surface. By modifying the molecule-guest-host interaction with light, a switchable adhesion between functionalized surfaces can be created. Photoisomerization and / or photothermal heating can cause softening or liquefaction of the adhesive layer, which can then be peeled off. Using this strategy, the adhesive layer can be composed of a photosensitive polymer, a molecular film, or a gel. In most cases, specially designed molecules used as adhesives have not demonstrated the strength and reproducibility of commercial adhesive formulations. Furthermore, delamination usually requires a chemical reaction and generally takes several minutes using a standard lamp light source. Additionally, since photoreactions are usually initiated by the wavelengths of ultraviolet or visible light, at least one of the adhered pieces must be transparent at those wavelengths. SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

[0006] Photoinduced delamination usually relies on a special photosensitive adhesive that undergoes a melting transition. However, when a high-power light pulse is absorbed by a thin interfacial layer, rapid phase changes such as melting, thermal decomposition, or vaporization may be induced by local heating. From the above, a system and method capable of generating delamination accompanied by a photoinduced interfacial phase change are desired.

Means for Solving the Problems

[0007] According to one embodiment, a method for delamination of an adhesive layer is disclosed. This method includes irradiating a pulsed light source through a first substrate toward the adhesive layer. The first substrate is an optically transparent substrate. The adhesive layer is provided between the first substrate and the second substrate.

[0008] According to one embodiment, a system for delaminating an adhesive layer from a substrate is disclosed. This system includes a first substrate that is an optically transparent substrate and an adhesive layer provided between the first substrate and the second substrate. The first substrate and the adhesive layer are configured to receive a pulsed light source that is irradiated through the first substrate and the adhesive layer toward the second layer.

Brief Description of the Drawings

[0009]

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DETAILED DESCRIPTION OF THE INVENTION

[0010] Detailed Description Many manufacturing, construction, or some everyday processes rely on the joining of components, parts, or materials using adhesives / adhesive pastes. Due to their high adhesive strength and low weight, adhesives often serve as a suitable alternative to mechanical fasteners in many cases. In addition to being used as an alternative to mechanical fasteners, they are also applied to the retention of lightweight materials such as artificial nails in beauty care products or the assembly of internal components of mobile devices. However, there is no convenient way to easily and quickly peel off the high - strength adhesives used in such applications.

[0011] Conventional peeling methods include, but are not limited to, methods using thermally expandable materials or elements that respond to external stimuli such as electric or magnetic fields, heat, harsh solvents, mechanical forces, etc. These methods rely on the use of large devices such as heaters or electromagnets or solvents that are not environmentally friendly. Also, they tend to take time to act (from several seconds to several minutes). The application of such conventional methods is often not suitable for delicate systems. Therefore, there is a desire for methods and systems that can reverse the adhesion of strong adhesives in less than 1 second without exposing the system to heat or other solvents.

[0012] According to one embodiment, the methods and systems as disclosed can further peel the interface between one of the first substrate and the second substrate and the adhesive layer using a pulsed light source. For example, the interface may have a width of 10 microns or less.

[0013] According to another embodiment, the second substrate is optically transparent, and the method and system further include disposing an absorption layer on one of the optically transparent first substrate or the optically transparent second substrate before applying the adhesive layer. Further, the method and system may include separating the adhesive layer from one of the optically transparent first substrate or the optically transparent second substrate having the absorption layer by inducing a phase change in the absorption layer. The phase change is one of melting, thermal decomposition, or vaporization. Further, the absorption layer retains the cohesive properties and adhesion strength of the adhesive layer while the pulsed light source irradiates the first substrate.

[0014] According to one embodiment, the second substrate may be a metal substrate, and the method and system may further include peeling the adhesive layer from the metal substrate by melting the surface of the metal substrate. The pulsed light source may be, for example, one or more high-energy nanosecond laser pulses of 355 nm, 532 nm, and 1064 nm. Further, the adhesive layer may be, for example, cyanoacrylate, two-component epoxy resin, or adhesive paste.

[0015] According to another embodiment, the adhesive layer may be an adhesive paste, the first substrate may be polymethyl (methacrylate) (PMMA), and the second layer may be aluminum. The first substrate or the second substrate may be selected from, for example, one or more of mechanical fasteners, beauty care products, transparent circuit boards, or plastic parts.

[0016] According to one embodiment, the methods and systems as disclosed herein can be implemented by irradiating a light source of pulsed light with one or more of a single-shot pulsed laser or a high-intensity light source. The high-intensity light source is a flash lamp or a pulsed light-emitting diode. Further, the single-shot pulsed laser may have, for example, a 1 nanosecond (ns) pulse width, a 5 ns pulse width, a 10 ns pulse width, or a duration less than the thermal diffusion characteristic time across the absorption layer.

[0017] According to another embodiment, pulsed light delamination can provide immediate delamination of a transparent substrate from a metal or other transparent substrate, regardless of the type of adhesive used. For example, the disclosed methods and systems preferably include an optically transparent substrate that allows transmission of pulsed light. The delamination mechanism depends, for example, on rapid and local heating at an ultrathin (e.g., on the order of 10 microns or less) interface between the adhesive and one of the substrates. Since only a small volume is heated in a relatively short time, large structures do not experience a large temperature rise and are not damaged.

[0018] According to one embodiment, to peel the metal from the transparent substrate adhered by the adhesive, the pulsed light is absorbed at the metal surface, leading to surface melting of the metal and loss of adhesion. For other materials, an ultrathin absorption layer can be disposed on the substrate before attaching the second piece with the adhesive paste. One of the keys to the disclosed approach is to select an absorption material that maintains the high adhesive strength of the adhesive paste. Instead of doping the adhesive with a light-absorbing species, the bulk properties of the adhesive can be maintained by maintaining a thin layer separated from the adhesive. Another important factor is to use high-energy pulsed irradiation to impulsively heat this light-absorbing layer before other components of the structure are heated by heat diffusion. This rapid heating causes selective thermal decomposition and / or melting and / or vaporization in the absorption layer, resulting in the destruction of the adhesive bond between the adhesive paste and the coated substrate.

[0019] According to another embodiment, the light pulse characteristics can be determined by the characteristics of the absorption substrate layer and the adhesive. This idea is to induce very local heating at the substrate-adhesive interface. When a thick substrate (metal, semiconductor) is the absorber, the depth of heating denoted as L heat is given by the following formula.

Equation

[0020] Here, l pen is the penetration depth of the light into the absorption substrate, κ is the thermal diffusivity, and τ pulse is the pulse duration. To localize the heating, it is necessary to make L heat as small as possible. This means selecting the wavelength of the light to minimize l pen and using short pulses to minimize the heat transport term κτ pulse . The temperature rise ΔT in the L heat zone can be estimated from the following relationship.

Equation

[0021] Here, C P is the volumetric heat capacity of the interface, and E pulse is the pulse fluence (in units of energy / area or J / cm 2 ). From this equation, the ideal case seems to be to maximize the intensity E pulse / τ pulse , but since this quantity is limited by the damage threshold of most transparent materials, the following constraints arise.

Equation

[0022] These three equations provide general theoretical guidelines for the design of optical pulses that can selectively heat the adhesive interface. Note that to optimize delamination, three independent parameters need to be considered: the wavelength of light, the pulse fluence, and the duration.

[0023] When the adhesive substrate itself is an absorber, it is typically a metal or semiconductor. According to one embodiment, a new technique using aluminum metal as the absorbing substrate was exemplified, but other metals can also be peeled from the transparent substrate. Another example is the metal body of an electronic component from a transparent circuit board. In this case, the adhesive is an electronic epoxy. This application enables the recycling of specific electronic components such as reworkable circuit boards or chips.

[0024] When the adhered substrate is a semiconductor, it can also be impulsively heated. The adhesion of a silicon wafer and ethylene vinyl acetate (EVA) is an example, which is used for the encapsulation of silicon solar cells. In this application, the EVA can be peeled from the silicon by an optical pulse, and the semiconductor can be recycled after removing the top glass plate adhered with EVA.

[0025] When the absorption layer is an ultrathin layer between two non-absorbing substrates, l penIt is replaced by the layer thickness, but all of the above three equations are still applicable as they are. This case applies to two pieces of plexiglass that can be instantaneously peeled off with a laser pulse like an explosion bolt.

[0026] The transparent substrate can be any material, including biological tissues. As an example, an absorber is thinly coated on a fingernail and then a manicure is applied or an artificial nail is adhered. In this case, the light pulse irradiates the upper nail and breaks the bond between the nail and the adhesive without damaging the lower nail.

[0027] Furthermore, since this method does not depend on the type of adhesive to be applied, it can be used for various applications. For example, it can instantaneously peel off structural parts adhered with a strong adhesive such as cyanoacrylate or two-component epoxy resin. For example, applications include optically controlled transparent separation bolts, peeling of any plastic parts inside mobile devices, artificial nails, beauty care or cosmetics.

[0028] According to one aspect, a pulsed light source, for example, high-energy nanosecond laser pulses at 1064 nm and 532 nm, can peel off an aluminum (Al) piece adhered to a polymethyl (methacrylic resin) (PMMA) surface by a commercially available cyanoacrylate (CA) adhesive. The dependence of peeling on both the laser fluence (energy per unit area) and the applied load is further disclosed herein. For example, single-shot peeling occurs at a fluence of 0.4 J / cm 2 at 1064 nm and 0.2 J / cm 2 at 532 nm. From the characterization of the Al surface before and after the laser impact, it was confirmed that the peeling occurred from the melting of the surface and did not damage large Al pieces. A model of the peeling process was derived to explain the relatively weak load dependence of the peeling. The ability of a single laser pulse to instantaneously generate a relatively clean break at the interface between the adhesive and the metal can be applied when rapid peeling is desired. Also, the disclosed method and system are compatible with commercially available adhesives that are transparent at the laser wavelength, thereby enabling laser peeling to be applied to a wide range of material systems.

[0029] High-power lasers, for example, can accumulate energy with relatively high spatial and temporal resolutions and provide a method for selectively heating an adhesive interface. When a laser pulse interacts with a solid surface, various processes can occur, including melting, ionization, and ablation. The relative importance of these processes depends on parameters such as the laser intensity, wavelength, duration, and chemical structure of the solid. Much of the research in this field has focused on using very high-energy pulses to physically remove material by ablation in order to refine the surface prior to adhesion. Pulsed lasers can also be used to generate shock waves to evaluate the adhesion of laminated composites, but this usually involves surface damage. In contrast, lower pulse energies can be used to break the interfacial bond between an adhesive and a metal in one rapid step via surface melting, as shown in FIG. 1. As a result, for example, assuming that the bond is not rapidly reformed, a clean cut between the metal and the adhesive occurs within one pulse. According to one aspect, it would be desirable to identify conditions under which a laser pulse breaks the adhesion without damaging the adhesive surface.

[0030] According to one embodiment, in order to implement this concept, it was chosen to focus on metal-plastic adhesion. This is because metal-plastic adhesion has the highest potential for structural applications while meeting the requirement that one of the adherends is transparent at the laser wavelength. In the present disclosure, it is disclosed to use high-energy nanosecond laser pulses of 1064 nm and 532 nm to peel off polymethyl (methacrylate) (PMMA) and Al surfaces adhered with a commercially available adhesive having a high (>1 MPa) adhesion strength. According to one aspect, the light irradiation is limited to a single laser shot, and relatively rapid debonding was demonstrated. Also disclosed is the pulse fluence (energy per area) and applied load dependence of the peeling. In order to confirm the surface melting mechanism, the characteristics of the Al surface were measured before and after the laser impact. As shown in FIG. 1, a model of the separation process is shown to explain the load dependence of the results. The ability of a single laser pulse to generate an instantaneous and relatively clean break at the interface between the adhesive and the metal can be applied in cases where rapid load peeling is desired, such as in certain aerospace fields where explosive bolts are used for joining objects.

[0031] [Experiment] A. Plastic-Metal Peeling (Sample Preparation) Using a cyanoacrylate adhesive (3M Scotch-Weld Instant Adhesive CA8) and a two-component acrylic adhesive (3M Scotch-weld DP810 Black), a 6061 aluminum cylinder was adhered to a transparent polymethyl methacrylate (PMMA) substrate (part number 8531K23) purchased from McMaster-Carr. The aluminum cylinder has one flat end and one threaded end with a thread that can be attached to a pulley system. To improve adhesion, the surfaces of the aluminum and PMMA were roughened before bonding. The flat aluminum surface was first polished with clean abrasive paper (Norton TufBak 220-A sandpaper), then polished again with fine 30-micron SiC polishing paper (3M), and finally wiped with pure acetone. The surface of the PMMA was wiped with a solution of 30% isopropyl alcohol dissolved in water. Then, the glue was placed on the surface of the PMMA, and the Al cylinder was pressed with moderate hand pressure. Excess glue was carefully wiped off, and it was cured in air for more than 24 hours without applying a load. As a result, as shown in Figures 8 and 9, the thickness of the adhesive line was typically 100 microns.

[0032] (Characteristic Evaluation) Scanning electron microscope (SEM) images were obtained using a Nova NanoSEM (NNS450 SEM) and a Hitachi TM4000PlusE II SEM. A Leica DM2700 M microscope was used to measure the thickness of the adhesive line. A stereo microscope (Amscope SW-2T13) combined with a camera (Amscope MU900) was used for imaging the metal surface. A quartz cell with a path length of 1 cm and an Agilent Cary 60 UV-vis spectrophotometer were used for absorption measurements in solution. For measurements in the solid state, the adhesive was cured on a slide glass, and the absorption spectrum was measured using an Agilent Cary 60 UV-Vis spectrophotometer.

[0033] (Delamination by Nanosecond Pulse Laser) According to one aspect, for the debonding based on pulsed laser irradiation, a single-shot pulsed laser (Amplitude Surelite II-10) with a pulse width of 5 ns was used. The wavelength could be changed by frequency doubling from the fundamental wave of 1064 nm (maximum pulse energy ~685 mJ) to the second harmonic of 532 nm (maximum pulse energy ~285 mJ). To reduce the power, the laser fluence was attenuated by delaying the Q-switch, and the pulse energy was measured with a power meter (Newport 843-R). By changing the delay of the Q-switch, the beam profile also changed and became approximately Gaussian. To maintain the uniformity of the beam at each laser fluence, negative lenses from Thorlabs (LF1015-C for 1064 nm, LF1547-A for 532 nm) were used, and for all laser outputs, the beam diameter was made slightly larger (~7 mm) than the diameter of the aluminum (5 mm). A custom-made two-pulley system (Figure 10) was used for the measurement of the peel adhesion strength and the laser ablation experiment. This two-pulley system was calibrated with standard weights and a RoMech digital hanging scale. The variable load on the adhesive sample was released slowly using an adjustable-height stage to avoid sudden force changes. At least five samples were tested for each laser fluence under each load condition to collect statistical data. For example, in more than 95% of the shots, the debonding occurred within 1 second (s) from the pulse arrival, but in a few cases, it took several seconds for the debonding. According to one embodiment, an upper limit of 60 seconds can be used for a laser shot to be eligible as a debonding event.

[0034] An experimental setup for measuring laser lift-off is shown in FIG. 2. The PMMA substrate was adhered to an Al post having a threaded end that allows it to be screwed into an eyelet attachment. This eyelet was tied to a rope passing through a pulley system. A variable weight was attached to the end of the pulley system. As shown in FIG. 2, this system tests the mechanical strength of the Al-adhesive-PMMA structure by attaching different loads to the end of the rope. It was found that the maximum adhesion strength varied with time. Most companies report the maximum load, but do not report how long the adhesive can sustain this load. For example, a cyanoacrylate adhesive was able to withstand a load of over 10 MPa, but only for a few minutes before breaking. In accordance with one aspect, the test was limited to the maximum load level that the adhesive could withstand for at least 12 hours. For example, in the case of a cyanoacrylate adhesive paste, the maximum load was limited to about 5 MPa, slightly below the maximum loads reported by the manufacturer for Al-Al adhesion (14.48 MPa) and polyvinyl chloride (PVC)-PVC adhesion (6.89 MPa).

[0035] According to one embodiment, stable Al - adhesive paste - PMMA samples were obtained and subjected to one laser shot. Adhesive failure was observed by looking at the drop in weight due to the loss of Al - PMMA adhesion. Generally, as judged by video analysis, the adhesion failed within a few milliseconds from the pulse arrival. To determine the percentage of samples peeled under this condition, the experiment was repeated multiple times with a constant pulse energy and load. Typically, multiple trials (more than 5 times) were conducted for each condition. Next, the incident laser fluence was varied by delaying the Q - switch of the laser to lower the output, and the experiment was repeated with the same load. Since changing the delay of the Q - switch also changes the beam profile, for each pulse energy, the optical system was adjusted to keep the beam radius constant. In the case of laser pulse energy, for example, the intensity drop from the center to the edge of a 5 - mm Al surface was in the range of about 60% to about 95% before optical adjustment, but was kept constant at 65% after adjustment. An example of the fluence - dependent ablation data at 1064 nm for different loads is shown in FIG. 3A. Similar ablation behavior was also observed for a two - component acrylic adhesive having a low maximum adhesion strength (FIG. 16).

[0036] To extract quantitative information from FIGS. 3A - 3B, the fraction (f debond ) of the broken adhesion was fitted to a sigmoid function of the following form.

Equation

[0037] Here, E debond is the threshold pulse fluence at which 50% of the adhesion fails, and k is the stiffness parameter that measures the steepness of the sigmoid transition near E debond . The linear least - squares fit using Equation (1) is shown superimposed on the data of FIGS. 3A and 3B. FIG. 7 shows k and E debondThis is a table summarizing the values. According to one embodiment, when the adhesion was subjected to a higher load, peeling could be initiated at a lower laser fluence. However, this dependence was relatively weak, and even when the load changed by 25 times, the peeling threshold only changed by 2 times. As shown in FIG. 3B, laser-induced peeling also depended on the laser wavelength. E debond was 2 to 3 times lower for light at 532 nm. The variation in the k value was not systematic at 1064 nm, E debond and it is likely that this reflects the uncertainty of the fit due to the sparse data points near E debond However, the sigmoid transition was always gentler at 532 nm, and the k value was at least 10 times smaller than that at 1064 nm. The smaller the k value, the more E debond it indicates that the variation in the peeling process around the periphery is large. The small k value at 532 nm may reflect the large variation in energy transfer during the absorption process.

[0038] To confirm the physical image of the laser-induced separation process shown in Fig. 1, the following control experiments were conducted. First, an experiment was carried out to confirm that the laser interaction with the Al surface is the cause of the separation. The laser pulse was expected to pass through both the PMMA and the acrylate adhesive paste, with some loss due to scattering but negligible absorption. The cyanoacrylate adhesive paste showed negligible absorption in the range of 450 nm to 1100 nm in solution (Fig. 12). After polymerization, the solid cyanoacrylate was slightly turbid and showed a long scattering tail in this wavelength range. The polymerized adhesive paste was redissolved in CHCl3 and showed no true absorption in this spectral region, as shown by taking an absorption spectrum without a scattering background (Fig. 17). When two transparent PMMA substrates were adhered using cyanoacrylate, laser-induced separation was not observed, confirming that separation is not initiated by light absorption in the adhesive paste or PMMA. It was also investigated whether nanosecond pulse excitation is required for separation. When a sample under a load of 0.44 MPa was irradiated with a 2 W continuous wave (cw) laser at 532 nm, no separation or weakening was observed even after 2 hours of irradiation. The total amount of energy imparted by the cw laser to the adhesive interface was approximately 105 times the energy imparted by a single 532 nm nanosecond pulse that easily separates the sample. Therefore, it was concluded that non-equilibrium heating of the metal by the laser pulse is necessary for separation.

[0039] Further evidence of the role of laser-induced morphological changes on the metal surface was obtained by examining the Al surface before and after separation. A photograph of the polished Al surface before adhesion is shown in Fig. 4A. After the Al piece was adhered to the PMMA surface, there were two ways to separate the Al piece. Simply increasing the load caused cohesive failure, where breakage occurred within the adhesive, and a significant amount remained attached to the surfaces of both Al and PMMA. This can be seen from the fragments in the microscopic image in Fig. 4B. On the other hand, when separation was carried out using a high-power laser pulse, the separated Al surface appeared smooth and clean, with no visible residue (Fig. 4C). Detailed observation of these surfaces using SEM revealed that the surface morphology changed at the nanoscale before and after laser-induced separation.

[0040] Figure 5A shows the Al surface polished manually before bonding. The microscale ridges caused by sanding were necessary for the acrylate to form a strong adhesive bond as recommended by the manufacturer. After laser shock, obvious signs of melting were seen on the exposed Al surface in Figure 5B, and the ridges seen in Figure 5A had completely disappeared. When the Al surface was exposed to the same laser fluence as part of the bonding surface with PMMA, the melted Al surface was covered with a thin layer of insulating organic matter that appeared black in the SEM image (Figure 5C). This organic layer was not visible in the optical microscope images of Figures 4A - 4C and was less than 1 micron thick. This organic layer is thought to be due to the residue of the decomposed cyanoacrylate adhesive paste.

[0041] According to one embodiment, this result shows that even with a very light load, single - shot peeling can be achieved with a fluence of 0.5 J / cm 2 or higher. Observation of the peeling at the interface between the adhesive and Al suggests that, along with clear signs of melting of the Al surface, the peeling is mainly driven by the melting of the metal that breaks the adhesion between the adhesive and Al. For example, the fluence required for peeling is in the range of Al melting (0.2 - 0.4 J / cm 2 ), but below the fluence thresholds of more destructive processes such as vaporization (1.5 J / cm 2 ), plasma formation (1.5 - 3.6 J / cm 2 ), and phase explosion (7 J / cm 2 ). Surface melting will break all organic - metal adhesion interactions, whether they depend on mechanical linkage or chemical bonding. Due to the large absorption of Al at 532 nm, this wavelength is effective in heating and melting the solid metal, which explains the low E debond value. A similar enhancement factor for 532 nm has also been observed in the formation and ablation of Al plasmas, which also depends on the efficient absorption of the laser wavelength.

[0042] An interesting question is why E debondis whether it depends on the load. Once surface melting occurs, it may be expected to separate under any load. However, as is clear from the low-magnification SEM images (Figs. 18A to 18C), there is a possibility that the adhesion at the peripheral part is not completely broken. In addition, due to surface variations, there are local regions where the adhesion is strengthened, and some adhesion may withstand the laser pulse there. Regarding the influence of the residual adhesion on the separation energy E debond two simple physical models were considered for the influence on. Before the laser pulse is irradiated, the Al-adhesion interface is assumed to be held by the effective adhesion density given by N b 0 . After the laser interacts with the Al surface, the adhesion density decreases to N b . Assuming that the load (L debond ) required to break the residual adhesion depends linearly on the number N b of the residual adhesion, it becomes as follows.

Equation

Equation

[0043] Alternatively, an inverse dependence on E can also be assumed.

Equation

[0044] Substituting equations (3) and (4) into equation (2) and setting E = E debond , and by rearranging, Edebond of L debond Two possible expressions for the dependency were obtained. From Equation (3) [Number] is obtained, and from Equation (4) [Number] is obtained.

[0045] Figure 6 shows E for the 1064 nm pulse, together with the fit using Equations (5) and (6), debond versus L debond plotted. The fit using Equation (6) does a very good job of reproducing the rapid initial decay and the stability at high load values. From a physical point of view, when the pulse energy exceeds E0, Equation (4) suggests that the residual adhesion decreases very rapidly as E increases, and it is certain that fluences above 0.5 J / cm 2 will peel off except for the lightest loads. However, it should be noted that this model does not take into account complex factors such as the spatial variation of the laser intensity across the adhesive surface or the possible role of chemical decomposition and vaporization of the organic layer. Nevertheless, it is reassuring that this simple model provides a reasonable fit to the data.

[0046] According to one embodiment, laser pulse peeling of a strong adhesive was characterized. The laser ablation process produces a relatively clean break with minimal damage to the Al surface, and the process is fairly insensitive to the properties of the adhesive and the applied load. High-power nanosecond pulses can propagate with low loss and negligible dispersion in optical fibers and transparent solids, and it is relatively easy to supply laser energy to the adhesive interface. The main limitation of this technique is the requirement that one of the adhesive materials must be transparent at the laser wavelength and the other must be an absorbing metal. According to one embodiment, to implement nanosecond laser peeling, it is necessary to identify transparent materials that can be used as structural elements in combination with metals.

[0047] Peeling of Si-EVA Solar cells are at the forefront of the shift away from fossil fuels. A solar cell includes a crystalline silicon (Si) layer sandwiched between a plastic substrate and a protective ethylene vinyl acetate (EVA) layer adhered to a glass plate. This encapsulation gives the solar panel a lifespan of about 25 years, but recycling is difficult because of the high adhesion strength between the substrate and the EVA adhesive. Current separation methods include grinding, heating, and solvent immersion. Localized impulse heating by pulsed laser irradiation has the potential to lead to the separation of the two interfaces to separate the glass / EVA layer from the Si wafer.

[0048] As a proof-of-concept test, an EVA film with a thickness of 0.3 mm (vinyl acetate content 33%, eBay) was heat-treated on a hot plate (Fisher Scientific, USA) at 160 °C for 15 minutes, sandwiched between a microscope slide glass (Eisco Labs, USA) roughened by hand and a commercially available silicon (Si) wafer (AOSHIKE 0.5V 400mA polycrystalline silicon solar cell, Amazon), and then the heating of the hot plate was stopped and the sample was left on the heater for 15 minutes to cool slowly. According to this method, a laminated area of EVA with an area of 1 cm × 1 cm was produced. Next, the back surface of the Si wafer was adhered to another slide glass in a cross shape with an ultraviolet-curable adhesive paste (Ultrabond 721, Hernon). Due to this cross-shaped structure, an external force could be applied while irradiating the adhesion area. For pulsed laser irradiation at different wavelengths (355 nm, 532 nm, 1064 nm), a pulsed laser (Amplitude Surelite II-10) with a pulse width of 5 nanoseconds and a repetition frequency of 10 Hz was used (Figs. 10A - 10D). Due to different laser fluences (pulse energy on the area), the EVA / glass slide was peeled off from the Si wafer with a certain external force. In order to irradiate the laser beam over an area of 1 square centimeter, a computer-controlled electric X-Y translation stage (Zaber, Canada) was used for the variable-speed raster scan of the self-made Si-EVA-glass assembly. The external load on the solar panel and the scanning speed of the electric stage were changed to optimize the parameters (Figs. 11A - 11D). Optical images and SEM images were taken to examine the surface morphology of the Si wafer before and after laser irradiation.

[0049] C. Metal stripping with black electronic epoxy for reworkable electronic devices Expensive electronic devices / components such as integrated circuits can be reused if they can be removed from the circuit board on which they were originally mounted. The peeling method using a pulsed laser can, if properly designed, assist in the disassembly of used electronic devices / printed circuit boards (PCBs). One issue with this application is that electronic epoxy is generally black and is expected to prevent light from reaching the absorption interface.

[0050] As a proof-of-concept test, as shown in FIGS. 12A to 12D, a transparent PMMA sheet (McMaster-Carr, USA) was attached to an aluminum (Al) piece using a two-component black industrial adhesive (DP270, 3M, USA) commonly used for bonding electronic and electrical components. The adhesive was cured between the aluminum piece and the PMMA board over 24 hours under light pressure. When a single laser pulse (Amplitude Surelite II-10) with a wavelength of 1064 nm and a duration of 5 nanoseconds was irradiated, the metal part could be peeled off from the transparent PMMA board despite being bonded with a non-transparent black epoxy adhesive.

[0051] D. Transparent-Transparent Separation by a Thin Absorbing Layer The metal / opaque substrate is opaque to visible light, while the transparent substrate transmits light. When two transparent substrates are bonded with a transparent adhesive (such as cyanoacrylate), most transparent adhesives are also transparent to visible light. To separate two transparent substrates bonded with an adhesive, a thin layer of an absorbing material must be introduced at the interface.

[0052] Three types of dyes, Rose Bengal (Aldrich Chem. Co.), Malachite Green (Exciton), and Fluorescein 27 (Lambda Physics), were coated (spin-coated / drop-cast) onto a transparent PMMA substrate with a machined rough surface. All three dyes strongly absorb at 532 nm. Next, another transparent PMMA post with a rough surface was adhesively bonded to the coated substrate using a commercially available cyanoacrylate adhesive (CA-8, 3M). The tensile strength of the bonded joint was tested using a self-made calibrated tensile strength testing apparatus to measure the bonding strength. Also, the sample joint was laser irradiated with a 5 ns, 532 nm laser pulse to examine the debonding behavior. All absorbers showed photo-induced delamination, but Fluorescein 27 was the optimal dye that enabled delamination with a single-shot laser pulse while the joint could hold a higher load. In all experiments, the adhesive area was smaller than the beam spot size (0.65 cm). In the remaining experiments, only Fluorescein 27 was studied for its excellent performance.

[0053] Fluorescein-27 dye at different concentrations (0.011, 0.022, 0.055, 0.114, 0.228 M) was prepared in ethanol / NaOH aqueous solution. 60 μL of the sample solution was spin-coated (1000 rpm, 30 s) onto the PMMA substrate using a spin coater (WS-400-6 npp / lite, Laurell, USA) and dried for 1 minute. The thickness of the spin-coated layer was calculated to be on the order of several hundred nanometers. The machined PMMA post was attached to the substrate coated with the commercially available cyanoacrylate adhesive and left to cure under light pressure for at least 24 hours following the manufacturer's instructions of the adhesive.

[0054] The subsequent bonding strength was tested under different load conditions using a custom-made two-pulley setup capable of laser irradiation. Even when an absorption layer was introduced at the interface, the cohesive properties of the adhesive did not change. However, compared to joints fabricated similarly without the absorption layer, the strength of the entire joint decreased (Figs. 13A - 13D). When examining the peeling behavior under two different external pressures (0.5 MPa and 3.5 MPa) and different laser fluence conditions at a wavelength of 532 nm, it was found that average fluences of 0.038 J / cm 2 and 0.227 J / cm 2 were required for peeling under the conditions of 3.5 MPa and 0.5 MPa, respectively.

[0055] F. Debonding of Artificial Nails FIG. 14 shows a method of bonding an artificial nail according to an embodiment. The upper 3 mm of the surface of a plastic-based artificial nail (AN: Artificial Nail) was passed through fine-grained sandpaper to make the "bottom nail". This process was repeated for the second AN, but the lower side of the AN was made the "top nail". The upper part of the AN resembles the tip of a finger nail and is different from the root of the AN that resembles the part closest to the nail bed. Subsequently, the sanded surface of the AN was wiped with a Kimwipe, and a primer layer that absorbs light was applied once to the upper 3 mm of the bottom nail and dried for 3 minutes. The second primer was applied and dried for 3 minutes. Several different light-absorbing undercoat layers were prepared, all of which were suspensions of absorbents (carbon (Alfa Aesar, #45537), iron(II,III) oxide powder (Sigma-Aldrich, #518158), iron(II,III) oxide powder <5 mm 95% (Sigma-Aldrich, #310068)) mixed into a colorless and transparent nail polish (Orange Beauty Supply, Riverside, California). The concentration range was 0.1 - 10 wt%. The test nails were also painted with a nail polish containing gel polish so that the entire test nail was completely covered.

[0056] On the upper 1 / 8 inch of the primed bottom nail, an amount (<2 microliters) of a cyanoacrylate-based adhesive was applied so as to be evenly distributed over that area. The top nail was adhered to the bottom nail in the same manner as performed by an individual or a nail technician, and the top nail extended from the bottom nail to lengthen the length of the bottom nail. The shape of the adhesion between the top nail and the bottom nail is shown in Fig. 14. Two nails (test nails) were brought into close contact with each other by applying moderate pressure by hand for 10 seconds and then dried for 2 minutes. The experiment was conducted within 24 hours after the test nails were prepared.

[0057] The test nails were fixed to an X-Y scanning stage (manufactured by Zaber) so that the top nails faced upward, and the lower surface of the test nails was adhered to the stage with Earthquake Putty. Using Zaber's software, the directions (X, Y) of the scanning stage and the step size (2 mm) of each step were controlled. A Q-switched Nd:YAG laser (Surelite, Continuum, Milpitas, Canada) with a wavelength of 1064 nm and a pulse width of 10 ns was used as an irradiation source with pulse energies of 650 mJ, 300 mJ, 150 mJ, and 100 mJ. The diameter of the laser beam was 6.7 mm, and it was moved 2 mm at a time until the top nail peeled off from the bottom nail.

[0058] At each step on the test nails, the samples were irradiated using one laser shot. The laser beam was directed at the test nails by a mirror, and the test nails were moved using the scanning stage and irradiated with a new spot. The irradiated area was at the adhesion interface between the two ANs, and its approximate dimensions were 3 mm × 8 mm. For all concentrations and primers tested, when the energy was 650 mJ, it was observed that the top nail popped out after scanning 1 to 3 irradiation spots. When the primer concentration was decreased and the output was decreased, more irradiation spots were often required (2 to 4 rows in 2-mm increments). A hypothesis was proposed that the number of irradiation spots and irradiation rows depends on the area where the adhesive to which the two ANs are adhered extends and whether all the adhered areas are irradiated.

[0059] G. Removal of Nail Polish The surface of the artificial nail (AN) was buffed using fine-grained sandpaper. After that, the sanded surface of the AN was wiped with a Kimwipe, and a light-absorbing primer was applied once. The light-absorbing primer was a suspension of the test absorbent, and carbon (Alfa-Aesar, #45537), iron (II,III) oxide powder (Sigma-Aldrich, #518158), iron (II,III) oxide powder <5mm 95% (Sigma-Aldrich, #310068) were each mixed into a colorless and transparent nail polish (Orange Beauty Supply, Riverside, California). The concentration range was 0.1 wt% to 10 wt%. Then, nail polish (red, white, black, blue; OPI and Sally Hansen brands; Target, Moreno Valley, Canada; or red gel, Orange Beauty Supply, Riverside, Canada) was applied to the AN. The nail polish was applied 2 - 3 times. Before curing with UV light, the gel was applied once according to the product instructions. Each sample was dried for 2 hours. Each experiment was conducted within 24 hours after preparing the samples. The test nails were fixed to an X-Y scan stage (Zaber) with the top nail facing up, and the bottom surface of the test nails was adhered to the stage with Earthquake Putty. The direction (X, Y) of the scanning stage and the step size (2 mm) of each step were controlled using Zaber software. A Q-switched Nd:YAG laser (Surelite, Continuum, Milpitas, Canada) with a wavelength of 1064 nm and a pulse width of 10 ns was used as the irradiation source with pulse energies of 650 mJ, 300 mJ, 150 mJ, and 100 mJ. The diameter of the laser beam was 6.7 mm.

[0060] For the irradiation of the samples at each step on the test nail, one laser shot was used. The laser beam was directed onto the test nail by a mirror, and the test nail was moved using a scanning stage and irradiated with a new spot. At the spot where the laser irradiated the test nail, after one laser pulse, the top paint layer or gel layer peeled off.

[0061] [Experimental setup] Figure 15 is a diagram of an experimental setup with a two-pulley system for measuring adhesion strength and debonding by pulsed laser.

[0062] Figure 16 shows the percentage of peeled samples plotted against the laser pulse fluence of 1064 nm pulses. The samples were Al - adhesive - PMMA, the adhesive was a two - component acrylic adhesive from 3M and was basically a standard mixed epoxy resin. The samples were held under a load of 1.5 MPa. The blue line is a fit to the data using the sigmoid function given by Equation (1).

[0063] Figure 17 is a diagram of the UV - Vis absorption spectra of 3M ethyl cyanoacrylate (CA) instant adhesive (black) dissolved in chloroform solution before polymerization, the solid polymerized form on a slide glass (blue), and the polymerized adhesive redissolved in chloroform (black). This adhesive has no measurable absorption at laser wavelengths of 1064 nm and 532 nm.

[0064] Figures 18A - 18C are respectively, the SEM reflection images of the Al surface with a diameter of 5 mm before adhesion, the Al surface after physical peeling by applying a load of 10 MPa with residual adhesive and PMMA fragments remaining on the surface, with minimal residue remaining around the edge, and the Al surface after laser peeling (0.78 J / cm 2 )

[0065] [Physical model for debonding] According to one embodiment, two physical models with several assumptions shown below were proposed to understand the peeling phenomenon. (Assumption) i. N b o = Effective adhesion density of the Al-PMMA interface before irradiation N b = Adhesion density remaining after pulsed laser irradiation ii. Adhesion is only destroyed after the fluence exceeds the threshold fluence E0. iii. The load (L debond ) required to break the remaining adhesion is linearly dependent on the number N b of the remaining adhesion. Therefore, the load at separation is L debond ∝ N b Or, the load is L debond = AN b (7) where A = proportionality (fluctuating) constant

[0066] (Model 1) In the first model, it was assumed that when the laser fluence E exceeds the threshold E0, N b decreases exponentially. Here, E0 represents the energy required to break the adhesion of the adhesive. Therefore,

Equation

Equation

Equation

[0067] (Model 2) In the second model, an inverse dependence of N b on E was assumed. [Mathematics] E = E debond Let it be so, and inserting Equation (10) into Equation (7), as the load, [Mathematics] is obtained. Rearranging both sides gives the following. [Mathematics] Here, C1 and C2 are fitting parameters.

[0068] The fluence value (E debond ) was fitted for different loads using Equation 9 and Equation 12.

[0069] In the above detailed description, embodiments of a method and system for debonding pulsed light from a metal-transparent substrate and a transparent-transparent substrate attached with an adhesive were described. However, the present invention is not limited to the exact embodiments and variations described. Various changes, modifications, and equivalents can occur to those skilled in the art without departing from the spirit and scope of the present invention as defined in the appended claims. It is expressly intended that all such changes, modifications, and equivalents that fall within the scope of the claims are included in the claims.

Claims

1. A method of peeling an adhesive layer, comprising: irradiating a pulsed light source through a first substrate toward the adhesive layer, wherein the first substrate is an optically transparent substrate, and the adhesive layer is provided between the first substrate and a second substrate. A method.

2. The method according to claim 1, further comprising peeling an interface between one of the first substrate and the second substrate and the adhesive layer with the pulsed light source. The method according to claim 1.

3. The method according to claim 2, wherein the interface has a width of 10 microns or less. The method according to claim 2.

4. The second substrate is optically transparent, and the method further comprises disposing an absorption layer on one of the optically transparent first substrate or the optically transparent second substrate before applying the adhesive layer. The method according to claim 1.

5. The method according to claim 4, further comprising separating the adhesive layer from one of the optically transparent first substrate or the optically transparent second substrate having the absorption layer by inducing a phase change in the absorption layer, wherein the phase change is one of melting, thermal decomposition, or vaporization. The method according to claim 4.

6. The method according to claim 4, wherein the absorption layer retains the cohesive properties and adhesive strength of the adhesive layer during irradiation of the pulsed light source through the first substrate. The method according to claim 4.

7. The second substrate is a metal substrate, and the method further comprises separating the adhesive layer from the metal substrate by melting a surface of the metal substrate. The method according to claim 1.

8. The pulsed light source is one or more high-energy nanosecond laser pulses of 355 nm, 532 nm, or 1064 nm. The method according to claim 1.

9. The adhesive layer is cyanoacrylate, a two-component epoxy resin, or an adhesive paste. The method according to claim 1.

10. The adhesive layer is an adhesive paste, the first substrate is polymethyl (methacrylate) (PMMA), and the second layer is aluminum. The method according to claim 1.

11. The method further comprises irradiating the pulsed light source with one or more of a single-shot pulsed laser or a high-intensity light source, wherein the high-intensity light source is a flash lamp or a pulsed light-emitting diode. The method according to claim 1.

12. The single-shot pulsed laser has a pulse width of 5 nanoseconds (ns) or a duration less than the thermal diffusion characteristic time across the absorption layer. The method according to claim 11. Claim 13 The first substrate or the second substrate is selected from one or more of a mechanical fastener, a beauty care product, a transparent circuit board, or a plastic part. The method according to claim 1. Claim 14 A system for peeling an adhesive from a substrate, comprising: a first substrate that is an optically transparent substrate; an adhesive layer provided between the first substrate and a second substrate. The first substrate and the adhesive layer are configured to receive a pulsed light source that irradiates the second layer through the first substrate and the adhesive layer. System Claim 15 The interface between the adhesive layer and the second substrate is peeled off by the pulsed light source. The system according to claim 14. Claim 16 The interface has a width of 10 microns or less. The system according to claim 15. Claim 17 The second substrate is optically transparent. The absorption layer is disposed on one of the optically transparent first substrate or the optically transparent second substrate before applying an adhesive to form the adhesive layer. The system according to claim 14. Claim 18 The adhesive layer is configured to be separated from one of the optically transparent first substrate or the optically transparent second substrate having the absorption layer by inducing a phase change in the absorption layer. The phase change is melting, thermal decomposition, or vaporization in the absorption layer. The system according to claim 17. Claim 19 The absorption layer maintains the cohesive properties and adhesive strength of the adhesive layer during irradiation of the pulsed light source through the first substrate. The system according to claim 17. Claim 20 The second substrate is a metal substrate. The adhesive layer is configured to be separated from the metal substrate by melting the surface of the metal substrate. The system according to claim 14. Claim 21 Further comprising a pulsed light source that is one or more high-energy nanosecond laser pulses of 355 nm, 532 nm, and 1064 nm. The system according to claim 14. Claim 22 The adhesive layer is cyanoacrylate, a two-component epoxy resin, or an adhesive paste. The system according to claim 14. Claim 23 The adhesive layer is an adhesive paste. The first substrate is polymethyl (methacrylic resin) (PMMA), The second layer is aluminum, The system according to claim 14.

24. Further comprising a pulse light source irradiated with a single-shot pulse laser, The system according to claim 14.

25. The single-shot pulse laser has a pulse width of 5 nanoseconds (ns), The system according to claim 24.

26. The first substrate or the second substrate is selected from one or more of a mechanical fastener, a beauty care product, a transparent circuit board, or a plastic part, The system according to claim 14.

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