Magnetocuring of temperature failsafe epoxy adhesives

Highlights

• One-pot epoxy adhesives are designed for contactless on-demand curing with alternating magnetic fields (AMF).

• Curie nanoparticles (CNP) additives within commercial epoxy adhesives cured the thermoset resins within minutes.

• Resins based coating of CNP enhances the colloidal stability and thermoset initiation within epoxy/adhesives.

• Curie nanoparticles provide precise temperature control with no evidence of scorching during magnetocuring.

• Magnetocuring is demonstrated on wood, ceramics, and plastics with exceeded lap shear adhesion strength by 6.5 MPa.

Abstract

Adhesive technology is of high and increasing interest in a wide variety of conventional and emerging applications. One-component adhesives typically cure using moisture, heat and light. These approaches limit applications to specific substrates, inefficient handling in manufacturing, and can only be indirectly activated. Hence, we developed a method for remote, wireless, contactless curing of adhesives using alternating magnetic fields (AMF). This approach (“magnetocuring”) offers energy efficient, on-demand adhesion. Exposure of Mn_xZn_1-xFe₂O₄ Curie temperature tuned magnetic nanoparticles (CNP) additives within commercial epoxy adhesives to an AMF cured thermoset resins within min with minimal rise in substrate temperature. The heating of the CNP “switches off” above its Curie temperature offering failsafe heating. The in-situ heating of the CNP can be controlled by CNP composition, CNP loading, and AMF strength. Internal temperatures of 160 °C could be reached in 5 min, allowing curing of most commercial epoxy adhesives without resin scorching. The maximum lap shear adhesion strength exceeded 6.5 MPa. Magnetocuring is demonstrated on wood, ceramics, and plastics, which is of considerable interest in sports, automotive, and aerospace industries.

Introduction

Chemical curing adhesives (CCA) are preferred over mechanical fixation due to their light weight and stress distributed bonding which is free of substrate damage. The global market of instant curing adhesive is expected to be more than $ 3 billion USD [1] by 2023 and is dominated by two-part, thermosetting structural adhesives. [2] Structural adhesives require mixing of epoxy/hardener resins or thermal activation of one-pot epoxy/hardener blends (thermocuring), which leads to energy losses and stress/strain mismatches due to non-uniform temperature cycling of the substrates and resin. Attempts to overcome these impediments have led to alternative methods such as snap-cure epoxy [3], [4], [5], photocuring [6,7], electron beam curing [8], and electrocuring [9], [10], [11], [12].

Snap-cure thermosets are one pot adhesives that rapidly cure within min. However, the rapid curing nature is of limited benefit to insulating or heat-sensitive materials (e.g., wood, ceramics, or plastics) [13]. Photocuring offers non-contact activation, but is dependent on UV transparent materials and free radical initiators, which contribute to manufacturing problems of dermal sensitivity [14]. Electron beam curing works by impinging high speed electrons that initiate free radicals within the polymer-initiator. The high energy of the electron beams/ radiation penetrates offers uniform curing, but requires high capital and infrastructure investments. All parts must be electron irradiated, which requires shielded rooms and advanced technical personnel [15,16]. Surface-curing adhesive is methyl/ethyl-cyanoacrylate, also known as ‘Superglue’. It has the unique property of either forming strong substrate bonds or not bonding at all. The inability to bond rough/acidic surfaces, the difficulty in handling brittle materials, and unsatisfactory temperature stability (cured bonds must be kept < 70 °C) limits surface curing to do-it-yourself home repairs [17].

Alternating magnetic field (AMF) mediated adhesive curing (‘magnetocuring’) occurs by in situ activation of thermoset adhesives. It is a non-contact method of bonding non-metal materials. Previous studies have investigated magnetocuring based on FeCo epoxy composties [18]. Induction curing of thiol-acrylate and thiol-ene composite systems using cobalt and nickel particles has been demonstrated [19]. Polymerization of cyanate ester using Fe₃O₄ as an internal heat source through induction heating was studied [20]. Induction curing was also studied with nickel nanoparticles for bonding of composites [21] and polymerization using iron oxide nanochains [22]. However these formulations were never successfully commercialized due to the follwing limitations: (i) absence of surface functionalization leads to poor colloidal stability of the magnetic nanoparticles. This would prevent adequate shelf stability due to the formation of large aggregates (ii) nanoparticle aggregates lead to thermal hotspots and localized resin/epoxy pyrolysis [18], (iii) high power input (3–32 kW) [19] paired with inefficient metallic Co (2 μm) and Ni (3 μm) particles[20] or Fe₃O₄ particles [22], (iv) use of high frequency (>2 MHz) and broad particle size distribution (70 nm – 22 μm) [21].

To overcome these limitations, colloidal stability and aggregate induced hotspots needs to be addressed. Herein, it is hypothesized that surface functionalized magnetic nanoparticles (CNP) will serve as magnetocuring additives within thermoset resins. This allows one-pot adhesive formulations that activate substrate bonding and adhesive crosslinking upon exposure to AMF. The bonding initiation can be precisely tuned to the Curie nanoparticle cutoff temperature with optimized heating, allowing bonding to heat-sensitive substrates while eliminating scorching. In order to support the hypothesis, following steps were carried out: (1) Mn_xZn_1-xFe₂O₄ CNP will be synthesized by a facile hydrothermal method with controlled particle size (< 20 nm) and Curie temperature (Tc). The Curie temperature of Mn_xZn_1-xFe₂O₄ ferrites can be tuned by changing the ratio of Mn to Zn content. (2) Organic coatings and surface functionalization on CNP with oleic acid and bisphenol A diglycidyl ether was used to overcome previous laboratory failures with long term CNP colloidal stability in liquid epoxy/adhesives. (3) Incorporation of CNP into adhesives and optimal AMF induction (low power system) that allows snap-curing formulations while preventing substrate hotspots and scorching. (4) Lastly, the loading of CNP, thermal and physical properties of adhesives, selection of substrates will allow tuning of mechanical properties and shear adhesion strength.

CNP offers a prime advantage over other magnetic nanoparticles due to its failsafe temperature limits. This is the major rationale for choosing them for magnetically induced heating and activation of thermoset epoxy adhesives. For the first time, CNP have been employed to cure one component epoxy adhesives through a non-contact modifier methodology. The modifier methodology allows its incorporation into already commercialized thermoset adhesive formulations. Magnetocuring offers a more cost-effective activation method, since the adhesive is heated directly without substrate thermal conduction. Here, curing of one-component epoxy adhesives through AMF activation or ‘magnetocuring’ is demonstrated on wood, ceramics, and plastics, which is of significant interest in sports, automotive, and aerospace industries.

The novelty of the present work includes i) development of several temperature fail safe magnetoadhesives using commercial one component adhesives, ii) proof of concept to join a range of materials using magnetoadhesive under AMF, these materials are close to impossible to join using a conventional oven method, iii) Our approach of curing is remotely controlled, rapid and localized heating, reduced processing cost and energy and absence of scorching.