- Open Access
Control of adhesive strength of acrylate polymers containing 1-isobutoxyethyl and isobornyl esters in response to dual stimuli for dismantlable adhesion
Applied Adhesion Science volume 5, Article number: 6 (2017)
To develop an adhesion system satisfying both constant adhesion strength during use and quick debonding ability during a dismantling process.
Adhesive properties were investigated for the random and block copolymers consisting of 1-isobutoxyethyl acrylate (iBEA), 2-ethylhexyl acrylate (2EHA), and 2-hydroxyethyl acrylate (HEA) as the dismantlable pressure-sensitive adhesives in the presence of a photoacid generator in response to dual external stimuli of photoirradiation and post baking.
The use of LED combined with a new photoacid generator SIN-11 was enable us to achieve a rapid dismantling process during UV irradiation within several minutes. The protection of the ester alkyl group in the iBEA repeating unit to give an acrylic acid unit was suppressed by the introduction of isobornyl acrylate (IBoA) as the additional unit into the copolymer of iBEA, 2EHA, and HEA. While IBoA‐containing block copolymer showed a constant adhesive strength during photoirradiation as the single external stimulus, deprotection was immediately induced by the subsequent heating, leading to a significant decrease in the adhesive strength.
The copolymer including the iBEA and IBoA units was revealed to function as the highly sensitive adhesive materials for dual-locked dismantlable adhesion.
Dismantlable adhesion systems are smart technology and materials, which satisfy both a sufficient bonding strength during use and a quick debonding process on demand. They have attracted attention because of saving materials and energy in various application fields, such as housing, electronics, medical and dental applications as well as manufacturing processing for industrial parts and machines [1, 2]. For the design of dismantlable adhesive materials, the adhesive property needs to instantaneously change in response to any external stimulus as a trigger for dismantling, for example, heating, UV irradiation, induction heating, electricity, and chemicals [3,4,5,6,7,8,9,10,11,12,13,14,15]. A change in the chemical structures of the adhesives by external stimuli was expected to induce a significant change in the adhesive properties. We previously reported a dismantlable adhesive system using degradable polyperoxides as curable and pressure-sensitive adhesives and the control of bonding strength by the radical chain degradation of the polyperoxide adhesives [16,17,18]. More recently, we developed an advanced system using acrylic polymers containing t-butyl acrylate (tBA) unit in order to overcome the dilemma of reliable adhesion property during use and the subsequent quick debonding [19,20,21,22,23,24]. The tBA-containing polymers were demonstrated to function as the dismantlable adhesive materials due to a facile transformation to polymers including acrylic acid repeating units, accompanied by the elimination of isobutene gas, under the appropriate photo irradiation conditions followed by postbaking at a desired temperature. It was previously revealed that the tBA-containing block copolymers showed excellent dismantling properties compared with the corresponding random copolymers [19, 22]. The validity of the dual-locked adhesion system in the presence of a photoacid generator (PAG) was also reported. In this system, an acid was formed by the photoreaction of PAG, and then chemically amplified deprotection proceeded during postbaking, in which a large number of repeated chemical reactions were induced by a single photochemical event, resulting in the efficient transformation of protected functional groups.
In order to develop adhesives more sensitive to external stimuli, we investigated the dismantlable adhesion behavior of the acrylic copolymers consisting of 1-isobutoxyethyl acrylate (iBEA), 2-ethylhexyl acrylate (2EHA), and 2-hydroxyethyl acrylate (HEA) units . Reactive polymers with functional groups protected with vinyl ethers have been synthesized and used for various applications, as reported in the literatures [26,27,28,29,30,31]. We found that the polymers containing the iBEA units were readily deprotected under single-stimulus conditions, such as hydrolysis without an acidic catalyst or acidolysis at room temperature under photoirradiation in the presence of PAG . The use of the iBEA repeating unit as the reactive groups was suited to the construction of a quick debonding system, but the iBEA-containing copolymers were too reactive against the external stimuli such as heating in water and photoirradiation in the presence of PAG and consequently they were not applied as the dual-locked adhesive polymers. Previously, we reported that the deprotection conditions significantly depended on the stability of the ester groups of the adhesive polymers . For example, the deprotection of the isobornyl ester proceeded under the conditions at a higher temperature for a longer reaction time in the presence of a larger amount of PAG. In this study, we investigated the dismantlable adhesion properties of the acrylic copolymers including an isobornyl acrylate (IBoA) unit in order to modify the responsibility of the iBEA-containing copolymers during a debonding process under the photoirradiation and subsequent heating conditions. We examined the dismantling properties of the random and block copolymers containing the iBEA, 2EHA, and HEA repeating units in the presence or absence of the additional IBoA repeating unit.
The 1H NMR spectra were recorded on a JEOL ECX-400 spectrometer using chloroform-d at room temperature. The number- and weight-average molecular weights (M n and M w) were determined by size exclusion chromatography (SEC) in tetrahydrofuran as the eluent at 40 °C using JASCO PU-2086 Plus equipped with UV-2075 Plus and 830-RIS detectors and Shodex A-800P columns. The molecular weights were calibrated with standard polystyrenes. The thermogravimetric (TG) and differential scanning calorimetry (DSC) were performed using Shimadzu DTG-60 and DSC-60, respectively, at a heating rate of 10 °C/min in a nitrogen stream. The 180° peel test was performed using a Shimadzu universal testing machine AGS-X with a 1 kN load cell according to ASTM D3330 at room temperature and a peel rate of 30 mm/min.
2EHA (Nacalai Tesque, Inc., Japan), HEA (Nacalai Tesque, Inc., Japan), and IBoA (Tokyo Chemical Industry Co., Ltd., Japan) were distilled under reduced pressure before use. 2,2′-Azobis(isobutyronitrile) (AIBN) and 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMVN) were purchased from Wako Pure Chemicals Co., Ltd., Japan and recrystallized from methanol. Acrylic acid (Nacalai Tesque, Inc., Japan), isobutyl vinyl ether (Tokyo Chemical Industry Co., Ltd., Japan), 10-campharsulfonic acid (Tokyo Chemical Industry Co., Ltd., Japan), and diphenylditelluride (DPDT, Tokyo Chemical Industry Co., Ltd., Japan) were used as received. Other reagents and solvents were used without further purification. iBEA was synthesized according to the method described in the literature . All copolymers were synthesized by organotellurium mediated radical polymerization (TERP) using binary azo initiators [20, 32]. SIN-11 [33,34,35] was supplied from Sanbo Chemical Industry, Ltd., Sakai, Japan, and used as received.
Synthesis of iBEA
To acrylic acid (17.98 g) and 10-campharsulfonic acid (6.0 mg) in 100 mL of n-hexane, isobutyl vinyl ether (25.28 g) was dropwise added at 0 °C under an argon atmosphere with stirring. After the addition, the stirring of a reaction mixture was maintained at room temperature for 3 h. Added was a small amount of calcium hydroxide then stirred for 30 min. After filtration, the solvent was removed under reduced pressure. The obtained crude product was distilled under reduced pressure. The pure iBEA was obtained in 97% yield.
Liquid; 1H NMR (300 MHz, CDCl3) δ 6.36 (dd, J = 17.4 and 1.5 Hz, CH 2=CH (trans), 1H), 6.05 (dd, J = 17.4 and 10.5 Hz, CH2=CH, 1H), 5.92 (q, J = 5.4 Hz, OCH(CH3), 1H), 5.78 (dd, J = 10.5 and 1.5 Hz, CH 2=CH (trans), 1H), 3.39 − 3.16 (m, OCH2, 2H), 1.84-1.71 (m, CH2CH(CH3)2, 1H), 1.19 (d, J = 5.4 Hz, OCH(CH 3), 3H), 0.82 (d, J = 6.6 Hz, CH(CH 3)2, 6H).
Synthesis of copolymers
A typical polymerization procedure was as follows  (Fig. 1). To a glass tube, iBEA (1.81 g), 2EHA (3.04 g), HEA (0.35 g), AIBN (1.6 mg), AMVN (3.5 mg), and DPDT (4.1 mg) in 1.38 g of anisole were added. The solution was degassed by a freeze–thaw technique three times, and then N2 was purged. The polymerization was carried out at 60 °C for 7 h. The conversions of iBEA, 2EHA, and HEA were 52, 54, and 86%, respectively. The copolymer was separated using a methanol/water mixture (90/10 in volume ratio) as the precipitant. The yield was 1.79 g (37.8%). The M n and M w/M n values were 1.41 × 105 and 1.59, respectively.
The block copolymers were synthesized according to the similar method  (Fig. 1). The homopolymerization of iBEA and the random copolymerization of iBEA and iBoA were carried out during the first stage of polymerization, and then 2EHA and HEA were further added to the polymerization systems to synthesize the corresponding block copolymers without isolating the precursor polymers produced at the first-step polymerization. To AIBN (1.6 mg), AMVN (3.5 mg), and DPDT (4.1 mg) in 1.5 g of anisole in a glass tube, was added 1.36 g of iBEA or a mixture of 0.95 g of iBEA and 0.62 g of IBoA. The solution was degassed by a freeze–thaw technique three times, and then N2 was purged. After the polymerization was carried out at 60 °C for 9 or 22 h, the determined amount of 2EHA and HEA were added and the copolymerizations were continued in order to synthesize the corresponding block copolymers. The block copolymers were separated using a methanol/water mixture (90/10 in volume ratio) as the precipitant. The results of the copolymerization are summarized in Table 1.
A SUS430 (150 × 50 × 0.5 mm3) plate was cleaned by ultrasonication in acetone for 15 min, then in 2-propanol for 15 min. The 15 wt% acetone solution of the polymer was applied on a poly(ethylene terephthalate) (PET) film (50 mm thickness) by a film applicator (200 μm gap). The film was dried in vacuo for 12 h in the dark, and then cut to a 20-mm wide. The film was pressure bonded on a SUS plate using a 2-kg hand roller. After UV irradiation (and the subsequent heating if needed), the 180º peel test was carried out after the specimen was left to stand for over 30 min at room temperature.
UV irradiation was carried out using an LED lamp HLDL-50UV365-FN (365 nm, CCS Inc., Japan) at room temperature. For the UV irradiation, the test piece was placed at a distance in a range of 5.9–14.5 cm from the LED lamp. For the thermal treatment after the UV irradiation, the test piece was placed in a preheated oven for a determined time.
Results and discussions
The random (polymer code R1) and block (B1 and B2) copolymers were synthesized using the TERP method as one of the living radical polymerization techniques. The results of the characterization of the obtained copolymers are summarized in Table 2. The structures of the obtained copolymers were determined based on the results of the NMR and SEC measurements, as shown in Fig. 2 and Table 2. The M n values were high as 8.8–14.1 × 104 and enough for the use as the adhesive polymer materials. The copolymers included the reactive iBEA units in a range of 21–34 mol% while the content of the HEA unit was 10–15 mol%. The latter segment acts for enforcing the cohesive force of adhesives. The contents of the 2EHA repeating units as the major components were in a range of 53–64 mol%. The B1 and B2 copolymers included 0 and 5 mol% of the IBoA units, respectively. The IBoA content in the reactive segment was calculated to be 17 mol% for the B2 copolymer.
The T g values of the homopolymers of iBEA, HEA, and IBoA were reported to be −10, −15, and 94 °C, respectively, being much higher than that of the homopolymer of 2EHA (−85 °C) [19, 25]. The T g value was determined to be −19 °C for the random copolymer R1, which consisted of 2EHA unit (53 mol%) as the low T g repeating unit and iBEA (32 mol%) and HEA (15 mol%) units as the moderate T g repeating units. Because the introduction of IBoA into the copolymer increased the T g values of the copolymers, we carefully controlled the copolymer compositions in order to exhibit similar T g values for the copolymers with and without the IBoA unit. For the block copolymers synthesized in this study, the reactive segments showed the constant T g values at −18 °C due to the small contribution of the IBoA unit introduced with an only 5 mol% into the reactive segment, as is shown in Table 2. The T g values of the random copolymer R1 and the hard segment of the block copolymer B1 containing no IBoA unit were similar to each other. The effect of the M n values (14.1 × 104 and 8.8 × 104 for R1 and B1, respectively) should be considered to discuss the T g values of these copolymers. As a result, we successfully prepared three types copolymers containing a segment with T g value. The adhesive segments consisting of 2HEA as the major component exhibited T g values lower than −50 °C for the block copolymers. These T g values were enough for the use as the pressure-sensitive adhesive materials. In this study, the adhesive segment including the mainly 2EHA units was produced during the second-step block copolymerization without isolation of the prepolymers produced during the first-step polymerization of iBEA or a mixture of iBEA and IBoA (See Fig. 1 for the accurate sequence structures of the block copolymers). Therefore, the second adhesive sequences produced during the second-step polymerization were confirmed to include not only the 2EHA and HEA repeating units but also small amounts of iBEA and IBoA repeating units as a result of the participation of the residual monomers after the first-step polymerization. The observation of two T g values for the block copolymers (B1 and B2) undoubtedly indicated the microphase separation structure of the reactive segment produced during the first-step polymerization and the adhesive segment produced during the second-step polymerization.
In the previous study, we reported that the random copolymer consisting of iBEA, HEA, and 2EHA with the 73 mol% of iBEA contents readily deprotected and the drastic reduction of the adhesive strength was observed under the photoirradiation using N-hydroxynaphthalimide triflate (NIT) as the PAG and a high-pressure mercury lamp (0.5–0.7 mW/cm2 at 330–390 nm) at room temperature . NIT is one of the most popularly used i-line (365 nm) sensitive PAGs . In this study, we used an LED lamp (2–4 mW/cm2 at 365 nm) as the photoirradiation source and a new type of PAG, SIN-11 [34-36], which showed excellent optical properties as follows: λmax = 293 and 317 nm, εmax = 1.35 × 104 and 1.62 × 104 L/mol cm, ε365 = 2.45 × 103 L/mol cm. The ε365 value of SIN-11 was much higher than that of NIT (λmax = 335 nm, εmax = 1.01 × 104 L/mol cm, ε365 = 3.30 × 102 L/mol cm), as shown in Fig. 3.
First, the adhesion test was carried out using R1 as the random copolymer in the presence of SIN-11 (0.5 wt% against the polymer) under the irradiation intensity of 4 mW/cm2 for 3 min (the irradiation dose was 720 mJ/cm2). The relative value of the adhesion strength after photoirradiation was 1% of the original strength (Table 3). This indicated the validity of the copolymer containing a small amount of iBEA (32 mol%) for the quick dismantlable adhesion within a short time. We also investigated the effect of the polymer sequence structure on the dismantling behavior using the block copolymer (B1) under similar UV irradiation conditions for dismantling. As summarized in Table 3, quicker dismantling was achieved when the block copolymer was used. The 0.5-min irradiation (the irradiation dose 120 mJ/cm2) of the block copolymer resulted in a decrease in the adhesive strength (0.042 ± 0.034 N/20 mm) similar to that after the 3-min irradiation (the irradiation dose 720 mJ/cm2) of the random copolymer (0.035 ± 0.012 N/20 mm). As the previous results using the iBEA-containing adhesive polymers under the dismantling conditions in a hot water in the absence of an acid, the random copolymers provided preferred dismantling performance rather than the block copolymers . The result obtained in this study was opposite to the previously reported one, being probably due to the difference in the dismantling conditions and the effects on the surface interactions, especially difference in the presence and absence of water.
Because the introduction of the IBoA unit was expected to suppress the deprotection, we tested the peel strength during the UV irradiation of the B2 copolymer, which consisted of the composition of the IBoA and iBEA units with 17/83 molar ratio in the reactive segment. It should be noted that a copolymer with a higher IBoA content (ca. 30 mol%) exhibited poor adhesive property. The irradiation dose of 120 mJ/cm2 was insufficient for the decrease in the adhesive strength for the test using B2 as the block copolymer containing the IBoA unit, while the same irradiation conditions were enough to significantly change the adhesive strength of the B1 sample. The fashion of a decrease in the adhesion strength as a function of the irradiation dose for the two kinds of block copolymers, B1 and B2, is shown in Fig. 4. The retardation of a decrease in the adhesion strength for B2 is clearly seen in this figure. We further investigated the postbaking effect of the B2 adhesives under the photoirradiation conditions at 60 and 120 mJ/cm2. As a result, a drastic decrease in the adhesive strength was observed [Fig. 4(c)]. This result indicated that the block copolymer containing the IBoA units successfully acted as dual-locked adhesive polymers under the appropriate conditions used in this study, while the copolymers containing iBEA units were stable at 100 °C in dry conditions in the absence of an acid . The chemically amplified reaction mechanism includes the photoirradiation process which produces a small amount of protons and the subsequent heating process which accelerates the deprotection reaction. This reaction mechanism is important for the dismantling adhesion system in response to the dual stimuli.
It was also found that the failure mode diversely changed depending on the photoirradiation conditions, as shown in Table 3. For the failure mode of the copolymers with different sequence structures, i.e., R1 and B1, the mode for the both copolymers changed depending on the photoirradiation dose as follows: cohesive failure, failure at the PET/adhesive interface, and then failure at the SUS/adhesive and PET/adhesive interfaces. In the case of R1, the failure occurred at the PET interface due to the considerably high cohesive force rather than the PET/adhesive interface interaction. The high molecular weight of R1 was favorable for increasing the cohesive interaction. The cohesive force further increased along with the photoirradiation because of the formation of polar and functional groups as well as the occurrence of cross-linking. The interactions at the SUS/adhesive and PET/adhesive interfaces competed with each other and determined the failure mode, but the both interactions seemed to be weak and finally the isolation of the adhesive layers was often observed. In fact, it was considered that the transesterification and/or esterification of the HEA unit took place with the deprotection of the iBEA unit in the presence of an acid catalyst causing cross-linking . In the previous study, the adhesive layer stuck on the PET film was easily peeled off and consequently the cross-linked adhesive layer was removed from both the SUS and PET without any adhesive deposit . The cross-linking caused by the reactions of the HEA units temporarily increased the modulus of the adhesives. In this study, the SUS or PET interface failure observed after dismantling was accounted for by similar cross-linking and transesterification reactions. The heating process for the dismantling using dual external stimuli led to the interfacial failure between the PET and adhesives because of an increase in the interaction between the SUS surface and the adhesives containing a carboxylic acid. However, it was hard to fully explain the all failure modes of B2 including hydrophobic IBoA, hydrophilic HEA, and reactive iBEA units as well as adhesive 2EHA unit under various irradiation and heating conditions for dismantling, because they kaleidoscopically changed according to the dismantling conditions.
We synthesized the random and block copolymers consisting of iBEA, 2HEA, and HEA repeating units in the absence and presence of IBoA by the TERP method as the living radical polymerization technique, which was valid for the synthesis of high-molecular-weight acrylate polymers including polar and functional groups in the side chain. The adhesion property of the obtained iBEA copolymers were investigated as the dismantlable adhesives responsible to photoirradiation and heating. It was demonstrated that the use of LED combined with a new photoacid generator SIN-11 was enable us to achieve a rapid dismantling process during UV irradiation within several minutes. The introduction of the IBoA unit into the copolymer resulted in the enhanced resistance to photoirradiation, while the subsequent heating at 100 °C for 3 min immediately led to a significant decrease in the adhesion strength. Thus, the copolymer including the iBEA and IBoA units was revealed to successfully function as the highly sensitive adhesive materials for dual-locked dismantlable adhesion.
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AM and ES designed the study and prepared the manuscript. YF carried out the experiments for polymer synthesis and measurements. HO and HH designed a photo acid generator and the related experiments. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.