CORE-SHELL NANOPARTICLES AND THEIR USE IN ADHESIVE FORMULATIONS

Abstract

The present invention relates to magnetic core-shell a nanoparticles, as well as to their use in the initiation of redox-triggered polymerisation processes. The invention also relates to a formulation comprising the magnetic core-shell nanoparticles, and particularly an adhesive formulation, as well as to associated methods of controlling the adhesion of at least two adherends utilising the adhesive formulation. Further embodiments of the invention relate to methods of controlling redox-triggered polymerisation processes, and particularly redox radical-triggered adhesive polymerisation.

Claims

1. A pre-polymerisation formulation comprising: (i) a nanoparticle comprising a core comprising a magnetic material, and a shell comprising a polymerisation-inhibiting oxidising agent; (ii) at least one monomer capable of undergoing redox-initiated polymerisation; and (iii) a redox radical initiator system.

2. A formulation as claimed in claim 1, wherein the at least one monomer is a methacrylate ester monomer.

3. A formulation as claimed in claim 2, wherein the at least one methacrylate ester monomer is triethyleneglycol dimethacrylate.

4. A formulation as claimed in claim 1 wherein the redox radical initiator system comprises copper (II) tetrafluoroborate hydrate and tert-butyl peroxybenzoate.

5. A formulation as claimed in claim 1, wherein the magnetic material is selected from the group consisting of magnetite, a ferrite, iron, nickel, cobalt, and magnetic alloys.

6. A formulation as claimed in claim 5, wherein the magnetic material is a ferrite.

7. A formulation as claimed in claim 6, wherein the ferrite is CoFe.sub.2O.sub.4.

8. A formulation as claimed in claim 1, wherein the oxidising agent is selected from the group consisting of MnO.sub.2, CeO.sub.2, ReO.sub.2, OsO.sub.4, ammonium cerium (IV) nitrate, an inorganic peroxide, an organic peroxide and an organic oxidant.

9. A formulation as claimed in claim wherein the oxidising agent is MnO.sub.2.

10. A formulation as claimed in claim 1 for use as an adhesive.

11. A nanoparticle comprising: a core comprising a magnetic material, wherein the magnetic material is CoFe.sub.2O.sub.4; and a shell comprising a polymerisation-inhibiting oxidising agent.

12. A nanoparticle as claimed in claim 11 wherein the oxidising agent is as defined in claim 8.

13. Use of a nanoparticle comprising a core comprising a magnetic material and a shell comprising a polymerisation-inhibiting oxidising agent as an additive in a pre-polymerisation adhesive formulation.

14. (canceled)

15. A method of initiating a redox-triggered polymerisation process comprising at least the steps of: (a) providing a pre-polymerisation formulation comprising (i) at least one monomer capable of undergoing redox-initiated polymerisation; (ii) a redox radical initiator system, and (iii) nanoparticles comprising a core comprising a magnetic material and a shell comprising a polymerisation-inhibiting oxidising agent dispersed within the formulation; and (b) applying a magnetic field to the formulation such that at least a portion of the nanoparticles are attracted thereto.

16. A method as claimed in claim 15, wherein the step of applying a magnetic field to the formulation comprises bringing the formulation into contact or proximity with a permanent magnet.

17. A method of controlling the adhesion between at least two adherends, comprising: applying a pre-polymerisation formulation as claimed in any of claims 1 to 9 claim 1 between the at least two adherends, and applying a magnetic field to the formulation such that at least a portion of the nanoparticles in the formulation are attracted thereto.

18. A method as claimed in claim 17 comprising bringing a magnet into proximity or contact with the adherends such that at least a portion of the nanoparticles in the formulation are attracted thereto.

19. A container for an adhesive comprising: a reservoir for containing an adhesive pre-polymerisation formulation as defined in claim 1; and a dispenser; wherein the container comprises a magnetic material positioned such that the formulation is brought into proximity or contact with the magnetic material when the formulation is dispensed through the dispenser.

20. A container for containing an adhesive as claimed in claim 19, wherein the reservoir comprises an adhesive pre-polymerisation formulation comprising nanoparticles comprising a core comprising a magnetic material.

21. A method of applying an adhesive to a surface, the method comprising: providing a container as claimed in claim 20, and dispensing the adhesive pre-polymerisation formulation through the dispenser such that at least a portion of the nanoparticles in the formulation are attracted to the magnetic material.

Description

FIGURES

[0067] FIG. 1: a) Anaerobic adhesive formulation. Schematic representation of b) inhibition of adhesion by CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles and c) subsequent triggering of polymerisation/adhesion following magnetic removal of CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles.

[0068] FIG. 2: TEM images of a) MnO.sub.2 nanoparticles, b) CoFe.sub.2O.sub.4 nanoparticles, c) and d) CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles. Scale bar is 100 nm.

[0069] FIG. 3: X-ray powder diffraction patterns of MnO.sub.2, CoFe.sub.2O.sub.4 and CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles. * represents the -MnO.sub.2 phase and # represents CoFe.sub.2O.sub.4 cubic spinel phase present in the core/shell particles, as labelled.

[0070] FIG. 4: Raman spectra of a) CoFe.sub.2O.sub.4 nanoparticles and b) CoFe.sub.2O.sub.4/MnO.sub.2 core/shell nanoparticles.

[0071] FIG. 5: FTIR spectra of a) CoFe.sub.2O.sub.4 nanoparticles and b) CoFe.sub.2O.sub.4/MnO.sub.2 nanoparticles.

[0072] FIG. 6: Magnetisation curves of CoFe.sub.2O.sub.4 and CoFe.sub.2O.sub.4/MnO.sub.2 nanoparticles.

[0073] FIG. 7: Schematic of adhesion procedure and testing of anaerobic adhesive formulations.

[0074] FIG. 8: Plot of normalised transmittance of the CC bond of TRIEGMA (1637 cm.sup.1) for unmodified adhesive formulation with optimised CoFe.sub.2O.sub.4/MnO.sub.2 nanoparticles present and after they had been removed.

[0075] FIG. 9: Change in gelation time with increasing concentration of CoFe.sub.2O.sub.4/MnO.sub.2 nanocomposites in adhesive formulation at 82 C.

[0076] FIG. 10: Screen shots of a demonstration video showing anaerobic adhesive formulations with CoFe.sub.2O.sub.4/MnO.sub.2 nanocomposite additives used for adhering glass to metal. Image A shows the formulation deposited on glass in the presence of a permanent magnet and the nanocomposites being moved in the presence of the magnetic field, and image B shows the glass bound to a stainless steel plate after magnetic separation of the nanocomposites from the formulation.

EXAMPLES

[0077] The following examples are intended to be illustrative only.

[0078] Materials

[0079] All chemicals were used as obtained from Sigma-Aldrich. Steel plates for adhesion testing (Q-Panel RS-14) were purchased from q-lab and were cleaned thoroughly with acetone prior to use.

[0080] Methodology

[0081] A JEOL JEM-2100, 200 kV LaB.sub.6 transmission electron microscope operated at 120 kV with a beam current of 65 mA was used to image nanoparticle samples. Aqueous suspensions were drop-cast onto a formvar coated copper grid for imaging.

[0082] Size analysis was carried out using ImageJ software. X-ray powder diffraction was performed using a Siemens-500 X-ray diffractometer. Powder samples were adhered on silica glass using silica gel and overnight spectra were run for all samples. Diffractograms were compared to the JCPDS database. FTIR spectroscopy was performed using a Perkin Elmer Spectrum One NTS FTIR spectrometer. CC bond monitoring experiments were conducted using a Perkin Elmer Spectrum 100 FTIR spectrometer using a diamond ATR attachment. Micro Raman spectra were recorded using a Renishaw 1000 micro-Raman system fitted with a Leica microscope and Grams Research TM analysis software. The excitation wavelength was 633 nm from a Renishaw RL633 He-Ne laser. Vibrating sample magnetometry (VSM) was carried out at room temperature with field applied up to 1 Tesla using a home-built machine. VSM was calibrated using a nickel sample of known mass; magnetisation values are representative of the total mass of the sample.

EXAMPLE 1

Preparation of MnO.SUB.2 .Nanoparticles

[0083] MnO.sub.2 nanoparticles were prepared by reaction of KMnO.sub.4 (0.5 g, 3.14 mmol) in Millipore water (250 mL) with oleic acid (5 ml, 0.016 mol) using ultrasonication (40 mins), followed by stirring (2.5 h) at room temperature (H. M. Chen and J. H. He, Chemistry Letters, 2007, 36, 174-175). The resulting dark brown precipitate was washed with ethanol using centrifugation and dried under vacuum at 80 C.

EXAMPLE 2

Preparation of CoFe.SUB.2.O.SUB.4 .Nanoparticles

[0084] CoFe.sub.2O.sub.4 nanoparticles were prepared by the basic co-precipitation of cobalt (II) nitrate hexahydrate (0.58 g, 2 mmol) and iron (II) chloride tetrahydrate (0.80 g, 4 mmol) in deoxygenated Millipore water (100 mL) using ammonium hydroxide solution (28 v/v %, to a pH of 11) under heating at 80-90 C. for 1 h (G.-L. Davies, S. A. Corr, C. J. Meledandri, L. Briode, D. F. Brougham and Y. K. Gun'ko, ChemPhysChem, 2011, 12, 772-776). Particles were washed with Millipore water, isolated using centrifugation and dried under vacuum at room temperature.

EXAMPLE 3

Preparation of CoFe.SUB.2.O.SUB.4./MnO.SUB.2 .Core-Shell Nanoparticles

[0085] CoFe.sub.2O.sub.4/MnO.sub.2 core/shell nanoparticles were produced through controlled deposition of MnO.sub.2 on the surface of the CoFe.sub.2O.sub.4 nanoparticles prepared in Example 2 above. CoFe.sub.2O.sub.4 nanoparticles (0.15 g, 0.64 mmol) were dispersed into degassed Millipore water (153 mL) in a round bottomed flask. KMnO.sub.4 (0.31 g, 1.95 mmol) was then added and stirred to mix. A thermometer was inserted into the flask and the suspension was heated with vigorous stirring to 80 C. Oleic acid (3.06 mL, 9.7 mmol) was added, and the solution was stirred for 1 h at 80 C. and then allowed to stir at room temperature overnight. The resulting nanomaterials were isolated using magnetic separation, washed four times with ethanol and the isolated material was dried under vacuum.

EXAMPLE 4

Preparation of Adhesive Formulation

[0086] Adhesive formulations were prepared by mixing a solution of triethyleneglycol dimethacrylate (TRIEGMA, 10 mL, 0.037 mol), copper (II) tetrafluoroborate hydrate (Cu(II), 0.088 g, 0.37 mmol) and tert-butyl peroxybenzoate (98%, peroxide, 1 mL, 5.26 mmol) with nanoparticles (MnO.sub.2, CoFe.sub.2O.sub.4, CoFe.sub.2O.sub.4/MnO.sub.2 or control reagents) in various ratios, as described in Tables 1 and 2.

TABLE-US-00001 TABLE 1 Ratios of MnCO.sub.2 and CoFe.sub.2O.sub.4/MnCO.sub.2 samples tested for adhesion inhibition properties and their adhesion capabilities ( for successful adhesion, x for unsuccessful adhesion). Deactivation Adhesion Mass ratio of adhesion in post of par- presence of Magnetic magnetic Sample ticles:Cu.sup.II particles.sup.[a] removal.sup.[b] removal.sup.[c] MnO.sub.2 1:1 x x .sup.[d] 2:1 x .sup.[d] 3:1 x .sup.[d] CoFe.sub.2O.sub.4/MnO.sub.2 4.5:1.sup. x 4:1 x 3.5:1.sup. 3:1 x 1.3:1.sup. x .sup.1:1.3 x .sup.[a]Deactivation of adhesion defined as prevention of reduction of Cu.sup.II to Cu.sup.I, polymerisation and hence no adhesion of metal plates using adhesion tests (FIG. 7). .sup.[b]Ability to attract the nanocomposite sample system from the bulk of the formulation using an external magnetic field. .sup.[c]After removal of nanoparticles from the bulk of the formulation by the magnetic field, assessment of supernatant adhesion as per [a]. .sup.[d]Not applicable due to lack of magnetic characteristics.

TABLE-US-00002 TABLE 2 Summary of the conditions required to achieve polymerisation (and hence metal substrate adhesion) in the absence and presence of nanoparticles (optimised ratios) and control systems ( for successful adhesion, x for unsuccessful adhesion). Deactivation of Magnetic Adhesion post Sample adhesion.sup.[a] removal.sup.[b] magnetic removal.sup.[c] MnO.sub.2.sup.[e] x .sup.[d] Oleic acid.sup.[f] x x .sup.[d] KMnO.sub.4.sup.[g] x x .sup.[d] CoFe.sub.2O.sub.4.sup.[h] x CoFe.sub.2O.sub.4/MnO.sub.2.sup.[i] .sup.[a]Deactivation of adhesion defined as prevention of reduction of Cu.sup.II to Cu.sup.I, polymerisation and hence no adhesion of metal plates using adhesion tests. .sup.[b]Ability to attract the sample system from the bulk of the formulation using an external magnetic field. .sup.[c]After removal of nanoparticles from the bulk of the formulation by the magnetic field, assessment of adhesion as per [a]. .sup.[d]Not applicable due to lack of magnetic characteristics. .sup.[e]2:1 mass ratio of particles:Cu.sup.II. .sup.[f]5:1 mass ratio of oleic acid:Cu.sup.II. .sup.[g]1:1 mass ratio of KMnO.sub.4:Cu.sup.II. .sup.[h]5:1 mass ratio of particles:Cu.sup.II. .sup.[i]3.5:1 mass ratio of particles:Cu.sup.II.

EXAMPLE 5

Characterisation of Nanoparticles

[0087] 5.1 Transmission Electron Microscopy (TEM)

[0088] CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles were imaged using Transmission Electron Microscopy (TEM) and TEM images of uncoated MnO.sub.2 nanoparticles (46.78.1 nm diameter), uncoated CoFe.sub.2O.sub.4 nanoparticles (41.015.1 nm diameter) and CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles (52.819.6 nm diameter) are shown in FIGS. 2a (uncoated MnO.sub.2 nanoparticles), 2b (uncoated CoFe.sub.2O.sub.4 nanoparticles) and 2c and 2d (CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles). The images showed CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles with an MnO.sub.2 shell of 73 nm thickness.

[0089] 5.2 X-Ray Diffraction (XRD)

[0090] X-ray diffraction (XRD) analysis confirmed the presence of both cubic inverse spinel CoFe2O4 and -MnO.sub.2 phases (FIG. 3). The presence of both of these oxide phases in the nanostructures was also confirmed by Raman Spectroscopy (FIG. 4), in which Raman modes at 680 cm.sup.1, 617 cm.sup.1, 475 cm.sup.1 and 300 cm.sup.1 are characteristic of the cubic inverse spinel structure of CoFe.sub.2O.sub.3 (Davies, G.-L.; Corr, S. A.; Meledandri, C. J.; Briode, L.; Brougham, D. F.; Gun'ko, Y. K., NMR relaxation of water in nanostructures: Analysis of ferromagnetic cobalt-ferrite polyelectrolyte nanocomposites. ChemPhysChem 2011, 12, 772-776) and peaks at 560 cm.sup.1 and 620 cm.sup.1 (increased intensity compared to CoFe.sub.2O.sub.4 alone) are characteristic of -MnO.sub.2 (Julien, C. M.; Massot, M.; Poinsignon, C., Lattice vibrations of manganese oxides: Part i. Periodic structures. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2004, 60, 689-700).

[0091] 5.3 Fourier-Transform Infrared Spectroscopy (FTIR)

[0092] FTIR spectra are shown in FIG. 5. FIG. 5a) is the FTIR spectrum of CoFe.sub.2O.sub.4 nanoparticles showing FeO stretching vibrations (530 cm.sup.1), a small shoulder representing CoO stretching vibrations (720 cm.sup.1) and hydroxyl group stretching vibrations (3400 cm.sup.1), as labelled and b) CoFe.sub.2O.sub.4/MnO.sub.2 nanoparticles showing MnO stretching and MnOH bending vibrations (725 and 490 cm.sup.1), characteristic FeO vibrations, hydroxyl groups stretches (3400 cm.sup.1) as well as distinctive peaks for oleic acid, including asymmetric and symmetric CH.sub.2 stretches (2925 and 2850 cm.sup.1), CO and CO stretches (1712 and 1340 cm.sup.1), OH (1410 cm.sup.1) and asymmetric COO.sup. stretches (1535 cm.sup.1), as labelled (Julien, C. M.; Massot, M.; Poinsignon, C., Lattice vibrations of manganese oxides: Part i. Periodic structures. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2004, 60, 689-700; and Limaye, M. V.; Singh, S. B.; Date, S. K.; Kothari, D.; Reddy, V. R.; Gupta, A.; Sathe, V.; Choudhary, R. J.; Kulkarni, S. K., High coercivity of oleic acid capped CoFe.sub.2O.sub.4 nanoparticles at room temperature. The Journal of Physical Chemistry B 2009, 113, 9070-9076).

[0093] 5.4 Magnetisation Studies

[0094] Magnetisation curves of CoFe.sub.2O.sub.4 and CoFe.sub.2O.sub.4/MnO.sub.2 nanoparticles are shown in FIG. 6. Samples demonstrate hysteresis loops indicative of ferromagnetism. Saturation magnetisation values (as labelled, based on total mass of sample) decrease upon MnO.sub.2 coating, as expected due to the presence of the non-magnetic MnO.sub.2 layer.

[0095] The magnetisation measurements additionally demonstrated that the core/shell nanostructures retained strong magnetisation behaviour despite the presence of the MnO.sub.2 shell (FIG. 6), indicating their ability to be manipulated with ease using an external magnet.

EXAMPLE 6

Removal of Nanoparticles

[0096] 6.1 From Adhesive Formulation

[0097] Activation of the adhesive formulation was performed by placing a permanent magnet (0.05 T) alongside a container containing the adhesive formulation and holding the magnet in close proximity to the container for approximately 1 minute. After this time, the CoFe.sub.2O.sub.4/MnO.sub.2 nanoparticles were removed to the surface of the container in proximity to the magnet, and the remaining adhesive formulation was free of nanoparticles. Aliquots of the nanoparticle-free formulation were taken and tested for adhesive capability as in Example 7 below.

[0098] Control experiments demonstrated that MnO.sub.2 nanoparticles could not be removed from the adhesive formulation by the application of an external magnetic field.

[0099] 6.2 In Situ

[0100] The adhesive formulation comprising CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles was applied to surfaces to be adhered. A magnet was placed at the joint to be adhered, and held in place for 1 minute. Adhesion was subsequently tested as in Example 7 below.

EXAMPLE 7

Adhesion Studies

[0101] Adhesive capability was assessed by applying the adhesive (in the absence or presence of nanoparticle samples) to stainless steel test panels (see Schematic shown in FIG. 7). The panels were held together using a clip for a set period of time (30 sec) and then tested for adhesion by connecting one of the panels to a 3 kg weight and holding for 20 seconds. The adhesives were also tested after magnetic removal of the nanoparticles (1-3 min beside a permanent magnet) and subsequent testing of the supernatant, or by placing the magnet next to an adhesive joint between steel and glass plates. A strong adhesive joint was deemed to have been formed if the joint remained intact for a minimum of 20 s.

[0102] 7.1 MnO.sub.2

[0103] The amount of MnO.sub.2 nanoparticles added to the adhesive formulation was initially varied to determine the optimal amount required to prevent polymerisation and hence plate adhesion. It was found that at a mass ratio of 2:1 or higher of MnO.sub.2:Cu(II) no adhesion was observed, indicating that the MnO.sub.2 nanoparticles inhibited polymerisation. However, as noted in Example 6.1 above, MnO.sub.2 nanoparticles could not be removed by the application of an external magnetic field.

[0104] 7.2 CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles

[0105] The procedure outlined above was repeated using the CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles prepared in Example 3. The CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles successfully inhibited polymerisation of adhesive formulations and substrate fixing at mass ratios of 3.5:1 of CoFe.sub.2O.sub.4/MnO.sub.2:Cu(II) and above.

[0106] Ratios below this amount were not sufficient to deactivate the polymerisation process and resulted in adhesion of the metal substrates in the presence of the nanocomposites.

[0107] 7.3 Adhesive Following Removal of Nanoparticles

[0108] The CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles were removed from the adhesive formulation as detailed in Example 6 above. The remaining adhesive successfully fixed the metal substrates within 30 s post-removal. FIG. 10 shows screen shots of a demonstration video showing anaerobic adhesive formulations with CoFe.sub.2O.sub.4/MnO.sub.2 nanocomposite additives used for adhering glass to metal. Image A shows the formulation deposited on glass in the presence of a permanent magnet and the nanocomposites being moved in the presence of the magnetic field and B the glass bound to a stainless steel plate after magnetic separation of the nanocomposites from the formulation.

[0109] 7.4 Control Experiments

[0110] In order to ensure that none of the individual nanocomposite components caused the deactivation/activation behaviour observed, a number of control experiments were performed. Oleic acid, KMnO.sub.4 and non-coated CoFe.sub.2O.sub.4 nanoparticles were tested in the adhesive formulation and showed no deactivation of adhesive capability in their presence and no capability of magnetically triggered adhesion.

[0111] 7.5 Adhesion after Three Weeks

[0112] The CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticle-containing adhesive formulation prepared in Example 4 was allowed to stand at ambient temperature conditions for three weeks. After this time, the anaerobic adhesion texts outlined in Example 7 were repeated. Similar results to those of the original experiments were obtained, i.e. the colloidal mixture comprising the CoFe.sub.2O.sub.4/MnO.sub.2 core-shell nanoparticles still successfully deactivated polymerisation and substrate adhesion and, upon magnetic removal of the nanoparticles, provided an adhesive which was capable of triggering the polymerisation reaction and adhesion of the metal substrates. This clearly demonstrates the long-term stability and activity of the adhesive formulation.

EXAMPLE 8

Characterisation of Adhesion

[0113] Real-time FTIR spectroscopy can be used to analyse the progression of polymerisation of adhesive, through monitoring of the CC stretch (band maximum at 1637 cm.sup.1), whose increase in transmittance intensity is characteristic of vinyl polymerisation on metal and glass surfaces (Yang, D. B., Direct kinetic measurements of vinyl polymerization on metal and silicon surfaces using real-time FT-IR spectroscopy. Applied Spectroscopy 1993, 47, 1425-1429) and is therefore indicative of the polymerisation of TRIEGMA in the adhesive formulation in contact with a steel substrate with respect to time.

[0114] Real-time FTIR spectroscopy was carried out through deposition of a droplet of the adhesive formulation either with or without nanoparticles on the diamond of the ATR system followed by attachment of a steel plate substrate. FTIR spectra were recorded every 30 seconds for 30 minutes in total. The change in the transmittance intensity of the 1637 cm.sup.1 band (corresponding to CC bond in TRIEGMA) was monitored and behaviour analysed.

[0115] The degree of vinyl monomer consumption is directly related to the increase of the IR transmittance band at 1637 cm.sup.1; the % conversion of the system can be calculated from equation 1:

[00001]$\begin{array}{cc}\%.Math..Math.\mathrm{conversion}=\left(\frac{A_0-A_t}{A_0}\right)*100& \left(1\right)\end{array}$

[0116] Where Ao represents the absorbance at 1637 cm.sup.1 at time 0 and A.sub.t represents the absorbance at 1637 cm.sup.1 at time t. Absorbance values were calculated from the transmittance data collected using equation 2:

A.sub.t=lnT.sub.t (2)

[0117] Where T.sub.t is transmittance intensity at time t.

[0118] The adhesive formulation, the adhesive formulation comprising CoFe.sub.2O.sub.4/MnO.sub.2 nanoparticles and the adhesive formulation after the CoFe.sub.2O.sub.4/MnO.sub.2 nanoparticles had been magnetically removed were analysed by FTIR as outlined above.

[0119] The transmittance intensity of the 1637 cm.sup.1 band of the unmodified adhesive formulation showed an initial rapid increase as CC bonds were converted to CC bonds during polymerisation, which then slowed over time (FIG. 8). This behaviour is typical for a rapid polymerisation process which reduces in rate as available monomer species are consumed during the fast curing process. The formulation containing CoFe.sub.2O.sub.4/MnO.sub.2 nanoparticles showed similar behaviour, but with considerably less overall change in transmittance, indicating that only very limited polymerisation occurred under bonding conditions and the lack of significant changes in transmittance profile are indicative of remaining CC bonds in the unpolymerised TRIEGMA monomer. Most importantly, after magnetic removal of the core/shell nanoparticles from the adhesive formulation, the trend in transmittance intensity was almost identical to that of the unmodified formulation. The restoration of the polymerisation behaviour to that of the initial unmodified adhesive formulation clearly demonstrates that the polymerisation behaviour is not negatively affected by the inclusion of the particles and remains equivalent to native, unmodified samples after nanoparticle removal.

[0120] Analysis of the conversion of the monomer with respect to curing time supports this, with the unmodified formulation showing 17.9% conversion after 30 minutes cure time. In the presence of the core/shell nanoparticles on the other hand, conversion only reaches 8.0%. This degree of conversion is not enough for a strong adhesive bond to be formed, as demonstrated in previous adhesion tests. After magnetic removal of the particles, the formulation reaches a 17.2% conversion after 30 minutes cure time, similar to the unmodified formulation, demonstrating that the formulation can still be applied as an efficient adhesive, forming strong bonds.

EXAMPLE 9

Stability Studies

[0121] Adhesives were tested for long-term stability using an industry-standard accelerated ageing testing protocol (Henkel Loctite STM-08, with alterations). The adhesive formulation (1 mL) was placed within a 12 mm diameter test-tube to which was then placed an applicator stick. Each test tube was then placed into an aluminium heating block set to 82 C. At set times, the agglutination of each formulation (minimum 2 replications) was tested by pulling the applicator stick out of the test tube. If the adhesive formulation offered resistance to removal of the applicator stick the sample was determined to have polymerised (gelled). As expected, the stability of the active formulation increased with increasing concentration of CeFe.sub.2O.sub.4/MnO.sub.2 nanoparticles (FIG. 9).

[0122] The inventors have thus demonstrated that magnetic core shell nanoparticles can be successfully used for magnetically-triggered reaction initiation, and particularly for controlling and initiating adhesion in an anaerobic adhesive.