Moisture-sensitive smart hot-melt adhesive from polyamide 6
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Moisture‑sensitive smart hot‑melt adhesive from polyamide 6 Masayuki Yamaguchi1 · Ryo Takatani1 · Yusuke Sato1 · Shuichi Maeda2 Received: 23 March 2020 / Accepted: 20 August 2020 © Springer Nature Switzerland AG 2020
Abstract The bonding strength of a hot-melt adhesive composed of polyamide 6 (PA6) and lithium bromide (LiBr) with a metal plate was evaluated. PA6 containing LiBr showed strong lap shear strength with a metal plate immediately after hot-melt adhesion above the melting point of PA6, e.g., 1.0 MPa between PA6/LiBr (90/10) and aluminum plates. Owing to the high adhesive strength, the cohesive failure and/or plastic deformation of PA6 was detected during lap shear strength measurements. In contrast, separation was achieved without any difficulty after moisture absorption with a low strength, e.g., 0.1 MPa between PA6/LiBr (90/10) and aluminum plates. Furthermore, there was no trace of the adhesive on the metal plate after peeling. Therefore, this smart hot-melt adhesive provides an easy-peeling nature only through exposure to humid conditions as a trigger, which can be employed for various applications such as high-performance masking films. Keywords Hot-melt adhesion · Polyamide 6 · Lithium salt · Adhesive strength · Smart polymer
1 Introduction The bonding strength of hot-melt adhesives needs to be precisely controlled for all applications. Furthermore, the durability of bonding strength is a critical factor in choosing the appropriate adhesive system in general. However, if bonding strength is controlled by a specific trigger, its applications will be widened greatly. For example, a masking film used to protect a product surface should have a high bonding strength only during transit and be removed from the product without difficulty when the stimulus loses bonding strength. This is a desirable property, especially in the semiconductor industry. A glue comprising a water-based adhesive, such as starch, might be one such example, because the bonding strength becomes weak after immersing in water. However, this technique is not desirable because a clean surface is hard to obtain after separation. Herein, a new type of a hot-melt adhesive has been proposed that shows high bonding strength with a metal
plate under dry conditions. However, after absorbing moisture, this bonding strength is lost without remaining in the bonding area. The adhesive employed in this study was polyamide 6 (PA6) containing lithium bromide (LiBr). Polyamides have long been widely employed as hot-melt adhesives and are known to have high softening temperatures and good adhesive strength, even with metals [1–3]. Recently, we found that adding LiBr greatly enhanced the glass transition temperature (Tg) of PA6 (48 °C for PA6 compared with 130 °C for PA6 containing 20% LiBr) [4]. Furthermore, the modulus in the glassy region was enhanced. Similar phenomena have been detected also for other polar polymers, which are attributed to the strong ion–dipole interaction between cations and polar functions in polymers such as amide group
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