ADHESIVE COMPOSTION

Abstract

An adhesive composition degradable by dielectric heating. The adhesive composition comprises a thermosetting polymer and a material sensitive to dielectric heating. The material sensitive to dielectric heating is selected from any one or more of hollow nanospheres, nanotubes, nanorods, nanofibres, nanosheets, graphene, graphene derivatives, nano/micro hybrids and mixtures of two or more nanoscale particles. The adhesive composition may be particularly useful in the assembly and disassembly of parts, particularly parts which have complicated and/or blocked joined surfaces. A method of joining at least two parts of an article together and a method of disassembling at least two parts of an article, using the adhesive composition are also provided. The adhesive composition may provide a reworkable nanocomposite adhesive. The adhesive composition maybe used to reversibly bond a biomedical or dental implant to a part of a human or animal body.

Claims

1. An adhesive composition comprising a thermosetting resin and particles susceptible to dielectric heating; wherein the particles susceptible to dielectric heating are selected from any one or more of hollow nanospheres, nanotubes, nanorods, nanofibres, nanosheets, graphene, graphene derivatives, nano/micro hybrids and mixtures of two or more nanoscale particles.

2. The adhesive composition according to claim 1, wherein the thermosetting resin comprises an epoxy resin.

3. The adhesive composition according to claim 2, wherein the epoxy resin is a modified epoxy resin selected from any one or more of polyurethane modified epoxy resin, isocyanate modified epoxy resin, polysulfone modified epoxy resin, phenolic modified epoxy resin, nylon modified epoxy resin, polysulfide rubber modified epoxy resin, nitrile modified epoxy resin, silicone modified epoxy resin and acrylic epoxy resin.

4. The adhesive composition according to claim 1, wherein the particles susceptible to dielectric heating are hollow nanospheres of Fe.sub.3O.sub.4, Co.sub.3O.sub.4, ZnO, Co/Ni alloy or carbon.

5. The adhesive composition according to claim 4, wherein the particles susceptible to dielectric heating are hollow nanospheres of Fe.sub.3O.sub.4.

6. The adhesive composition according to claim 1, wherein the particles susceptible to dielectric heating are graphene or a graphene derivative.

7. The adhesive composition according to claim 1, wherein the particles susceptible to dielectric heating are carbon nanofibres or carbon nanotubes or carbon nanorods.

8. The adhesive composition according to claim 1, wherein the particles susceptible to dielectric heating are mixtures of two or more nanoscale particles.

9. The adhesive composition according to claim 1, wherein the particles susceptible to dielectric heating are a mesoporous ferrite/carbon mixture.

10. The adhesive composition according to claim 1, wherein the particles susceptible to dielectric heating are a combination of carbon nanofibres and Fe.sub.3O.sub.4 hollow nanospheres.

11. The adhesive composition according to claim 1, wherein the particles susceptible to dielectric heating are present in the adhesive composition in an amount of from 0.01 wt % to 10 wt %.

12. The adhesive composition according to claim 1, comprising a curing agent present in the adhesive composition in an amount of from 10 to 40 wt %.

13. A method of joining at least two parts of an article together, the method comprising the steps of: a) providing a join between the at least two parts of the article with an adhesive composition according to claim 1; and b) allowing or causing the adhesive composition to cure.

14. The method according to claim 13, wherein step b) involves exposing the adhesive composition to electromagnetic radiation to accelerate curing of the adhesive composition.

15. A method of disassembling at least two parts of an article which are joined by a cured adhesive composition comprising a thermoset polymer and particles susceptible to dielectric heating, the method comprising the steps of: i) exposing the cured adhesive composition to electromagnetic energy having a frequency in the range of from 10 MHz to 20 GHz to heat the particles susceptible to dielectric heating comprised within the cured adhesive composition; and ii) separating the at least two parts of the article from each other, wherein the particles susceptible to dielectric heating are selected from any one or more of hollow nanospheres, nanotubes, nanorods, nanofibres, nanosheets, graphene, graphene derivatives, nano/micro hybrids and mixtures of two or more nanoscale particles.

16. The method according to claim 15, wherein the electromagnetic radiation has a frequency of from 10 MHz to 50 MHz.

17. The method according to claim 15, wherein the electromagnetic radiation has a frequency of from 800 MHz to 5 GHz.

18. An article comprising a join, the join provided by an adhesive composition according to claim 1, the adhesive composition having been allowed or caused to set.

19. A method of preparing hollow nanospheres of Fe.sub.3O.sub.4, the method comprising the steps of: a) dissolving an Fe salt, an ionic surfactant and a weak base in a solvent to provide a solution; and b) heating the solution produced in step a) to precipitate the hollow nanospheres of Fe.sub.3O.sub.4.

20. (canceled)

Description

EXAMPLE 1

[0156] A two-part thermosetting adhesive composition was prepared using the following weight ratios of components:

TABLE-US-00001 Part A (first part of the two-part adhesive composition) (A1) Thermosetting resin: acrylic epoxy resin 24 wt % bisphenol F epoxy resin 13 wt % (A2) Toughening agent: acrylonitrile-butadiene rubber 5 wt % (A3) Thinner: styrene 7 wt % (A4) Coupling agent: titanate coupling agent 0.5 wt % (A5) Particles susceptible to dielectric heating: Fe.sub.3O.sub.4 0.5 wt % hollow nano spheres Total of Part A: 50 wt % Part B (second part of the two-part adhesive composition) (B1) Curing agent: triethylene tetramine 23.8 wt % (B2) Toughening agent: ethylene - acrylic acid copolymer 11.9 wt % resin (B3) Thinner: phenyl glycidyl ether 8.9 wt % (B4) Accelerant: hexanediol diacrylate 4.8 wt % (B5) Stabilizer: trimethoxy boroxine 0.6 wt % Total of Part B: 50 wt % Total of Part A and Part B: 100 wt %

[0157] The Fe.sub.3O.sub.4 hollow nanospheres are used as the particles susceptible to dielectric heating in this embodiment. The thickness of shell is from 30 to 60 nm and the outer diameter is from 200 to 300 nm.

[0158] Synthesis Procedure

[0159] Part A of the two-part adhesive composition was prepared by combining the acrylic epoxy resin, bisphenol F epoxy resin, acrylonitrile-butadiene rubber, styrene and 1 titanate coupling agent and mixing mixed for 30 min at 60 C. using a high-speed mixer. The Fe.sub.3O.sub.4 hollow nanospheres were added into the above mixture with high-speed homogenization for 40 min at 60 C. The temperature was then decreased to 25 C. and the mixture further stirred at low speed under vacuum to remove any air bubbles.

[0160] Part B of the two-part adhesive composition was prepared by combining the triethylene tetramine, ethylene-acrylic acid copolymer resin, phenyl glycidyl ether, hexanediol diacrylate and trimethoxy boroxine and mixing for 30 min at 60 C. using a high-speed mixer. The temperature was then decreased to 25 C. and the mixture further stirred at low speed under vacuum to remove any air bubbles.

[0161] Bonding Procedure

[0162] Two polyimide plastic plates were used to provide the target surfaces to be bonded (the at least two parts of an article to be joined). The size of each plate was 27 mm7 mm2 mm, on which the target surface to be bonded is 5 mm7 mm. Parts A and B of the adhesive composition were mixed together with a weight ratio of 1:1 just before the bonding procedure was carried out. The mixed adhesive composition was then coated on to the surfaces of the two polyimide plastic plates. The surfaces were tightly contacted with each other and this contact was maintained for 6 minutes while the mixed adhesive composition was exposed to microwave radiation with a frequency of 2.45 GHz and a power of 20 Watts, after which the adhesive composition was cured completely to provide a join of cured adhesive composition between the two polyimide plastic plates.

[0163] Disassembly Procedure

[0164] The two polyimide plastic plates bonded by the cured adhesive composition were exposed to microwave radiation with a frequency of 2.45 GHz and a power of 100 Watts. The temperature of the cured adhesive composition between plates was monitored by an infrared temperature sensor. The temperature of the cured adhesive composition increased to 295 C. during exposure to the microwave radiation for 3 minutes. During this time the cured adhesive composition degraded and the two polyimide plastic plates were separated.

EXAMPLE 2

[0165] A two-part thermosetting adhesive composition was prepared using the following weight ratios of components:

TABLE-US-00002 Part A (A1) Thermosetting resin: silicone modified epoxy resin 35.6 wt % silicone resin 8.9 wt % (A2) Toughening agent: silicone rubber 4.4 wt % (A3) Coupling agent: silane coupling agent 0.9 wt % (A4) Particles susceptible to dielectric heating: 0.2 wt % single layered reduced graphene oxide powder Total of Part A: 50 wt % Part B (B1) Curing agent: triethanolamine 23.6 wt % ethylene diamine 8.6 wt % (B2) Toughening agent: ethylene - vinyl acetate copolymer 6.4 wt % (B3) Thinner: 2,3-epoxy-2-methyl propyl ethers of alkylene 4.3 wt % glycols (B4) Accelerant: 3-phenyl-1,1-dimethyl urine 6.4 wt % (B5) Stabilizer: thioglycolic acid 0.7 wt % Total of Part B: 50 wt % Total of Part A and Part B: 100 wt %

[0166] The single layered reduced graphene oxide powder has a sheet length of about 100 nm.

[0167] Synthesis Procedure

[0168] Part A of the two-part adhesive composition was prepared by combining the silicone modified epoxy resin, silicone resin, silicone rubber and silane coupling agent and mixing for 30 min at 80 C. using a high-speed mixer. The single layered reduced graphene oxide powder was added into the above mixture with high-speed homogenization for 40 min at 60 C. The temperature was then decreased to 25 C. and the mixture further stirred at low speed under vacuum to remove any air bubbles.

[0169] Part B of the two-part adhesive composition was prepared by combining the triethanolamine, ethylene diamine, ethylenevinyl acetate copolymer, 2, 3-epoxy-2-methyl propyl ethers of alkylene glycols, 3-phenyl-1,1-dimethyl urine and thioglycolic acid and mixing for 30 min at 50 C. using a high-speed mixer. The temperature was then decreased to 25 C. and the mixture further stirred at low speed under vacuum to remove any air bubbles.

[0170] Bonding Procedure

[0171] The bonding procedure of Example 1 was repeated using the adhesive composition of this Example 2 using two ceramic plates instead of the polyimide plastic plates. In this example the adhesive composition was exposed to radio frequency radiation with a frequency of 27 MHz and a power of 10 Watts to cure the adhesive composition.

[0172] Disassembly Procedure

[0173] The disassembly procedure of Example 1 was repeated using the joined plates of this Example 2 by exposing the cured adhesive composition to radio frequency radiation with a frequency of 27 MHz and a power of 500 Watts for 30 minutes. The temperature of the cured adhesive composition increased to 450 C. during the 30 minutes, resulting in the degradation of the cured adhesive composition and the separation of the joined plates.

EXAMPLE 3

[0174] A two-part thermosetting adhesive composition was prepared using the following weight ratios of components:

TABLE-US-00003 Part A (A1) Thermosetting resin: polysulfone modified epoxy resin 28.1 wt % bisphenol A epoxy resin 18.7 wt % (A2) Toughening agent: styrene-butadiene rubber 2.3 wt % (A3) Coupling agent: titanate coupling agent 0.5 wt % (A4) Particles susceptible to dielectric heating: 0.4 wt % graphene/ferrite mixture Total of Part A: 50 wt % Part B (B1) Curing agent: 2-methyl-1,5-pentamethylene-diamine 31.2 wt % (B2) Toughening agent: liquid nitrile rubber 4.5 wt % (B3) Thinner: diethylene glycol diglycidyl ether 11.2 wt % (B4) Accelerant: 2-ethyl-4-methylimidazole 2.2 wt % (B5) Stabilizer: 2-ethylhexyl thioglycolate 0.9 wt % Total of Part B: 50 wt % Total of Part A and Part B: 100 wt %

[0175] The graphene/ferrite mixture is used as the particles susceptible to dielectric heating sensitive in Part A. The ratio of graphene to ferrite is 6:4. The graphene is in the form of a powder with a particle size of about 200 nm and ferrite is in the form of a powder with a particle size of about 400 nm. The ferrite powder has a primary-secondary aggregated morphology in which secondary grains with a particle size of about 400 nm are aggregated by primary grains with the size of about 10 nm.

[0176] Synthesis Procedure

[0177] Part A of the two-part adhesive composition was prepared by combining the polysulfone modified epoxy resin, bisphenol A epoxy resin, styrene-butadiene rubber and titanate coupling agent and mixing for 30 min at 80 C. using a high-speed mixer. The graphene/ferrite mixture was added into the above mixture with high-speed homogenization for 40 min at 60 C. The temperature was then decreased to 25 C. and the mixture further stirred at low speed under vacuum to remove any air bubbles.

[0178] Part B of the two-part adhesive composition was prepared by combining the 2-methyl-1,5-pentamethylene-diamine, liquid nitrile rubber, diethylene glycol diglycidyl ether, 2-ethyl-4-methylimidazole and 2-ethylhexyl thioglycolate and mixing for 30 min at 60 C. using a high-speed mixer. The temperature was then decreased to 25 C. and the mixture further stirred at low speed under vacuum to remove any air bubbles.

[0179] Bonding Procedure

[0180] The bonding procedure of Example 1 was repeated using the adhesive composition of this Example 3 using two aluminium plates instead of the polyimide plastic plates. In this example the adhesive composition was exposed to radio frequency radiation with a frequency of 2.45 GHz and a power of 15 Watts for 5 minutes to cure the adhesive composition.

[0181] Disassembly Procedure

[0182] The disassembly procedure of Example 1 was repeated using the joined plates of this Example 3 by exposing the cured adhesive composition to microwave radiation with a frequency of 2.45 GHz and a power of 800 Watts for 10 minutes. The temperature of the cured adhesive composition increased to 550 C. during the 10 minutes, resulting in the degradation of the cured adhesive composition and the separation of the joined plates.

EXAMPLE 4

[0183] A one-part thermosetting adhesive composition was prepared using the following weight ratios of components:

TABLE-US-00004 (1) Thermosetting resin: polyurethane modified epoxy resin 27.11 wt % bisphenol A 9.04 wt % (2) Toughening agent: polyurethane rubber 10.85 wt % (3) Thinner: 1,2-bis(epoxyalkyl)cyclobutanes 30.73 wt % (4) Accelerant: triphenyl phosphine 1.81 wt % (5) Stabilizer: trimethoxy boroxine 0.36 wt % (6) Coupling agent: silane coupling agent 0.18 wt % (7) Curing agent: cyanuric acid modified 2-ethyl-4- 19.88 wt % methylimidazole (8) Particles susceptible to dielectric heating: 0.04 wt % mesoporous ferrite/carbon nanofibre mixture Total: 100 wt %

[0184] The mesoporous ferrite/carbon mixture is used as the particles susceptible to dielectric heating in this embodiment. The mesoporous ferrite has a pore size of about 20 nm and is coated by carbon, in which the thickness of carbon coating in the mesopores of ferrite is from 3 to 10 nm.

[0185] Synthesis Procedure

[0186] The one-part adhesive composition was prepared by combining the polyurethane modified epoxy resin, bisphenol A, polyurethane rubber, 1,2-bis(epoxyalkyl)cyclobutanes, triphenyl phosphine, trimethoxy boroxine and silane coupling agent and mixing for 30 min at 70 C. using a high-speed mixer. The mesoporous ferrite/carbon mixture and the cyanuric acid modified 2-ethyl-4-methylimidazole were then added into the above mixture with high-speed homogenization for 40 min at 50 C. The temperature was then decreased to 25 C. and the mixture further stirred at low speed under vacuum to remove any air bubbles.

[0187] Bonding Procedure

[0188] The bonding procedure of Example 1 was repeated using the adhesive composition of this Example 4 using two poly-p-oxybenzoyl plastic plates instead of the polyimide plastic plates. In this example the adhesive composition was exposed to microwave radiation with a frequency of 2.495 GHz and a power of 20 Watts for 8 minutes to cure the adhesive composition.

[0189] Disassembly Procedure

[0190] The disassembly procedure of Example 1 was repeated using the joined plates of this Example 4 by exposing the cured adhesive composition to microwave radiation with a frequency of 2.495 GHz and a power of 100 Watts for 2 minutes. The temperature of the cured adhesive composition increased to 300 C. during the 2 minutes, resulting in the degradation of the cured adhesive composition and the separation of the joined plates.

[0191] Preparation of Fe.sub.3O.sub.4 Hollow Nanospheres

[0192] The Fe.sub.3O.sub.4 hollow nanospheres used in Example 1 were prepared by dissolving 0.006 mol cetrimonium bromide (CTAB) and 0.0214 mol hexamethylenetetramine (HMTA) in 60 mL ethylene glycol (EG), then 0.016 mol FeCl.sub.3.6H.sub.2O was added under continuous stirring until it was dissolved totally. The solution was transferred to a 100 ml Teflon-lined autoclave, then sealed and maintained at 220 C. for 12 h. After the autoclave cooled down to room temperature naturally, the black precipitate was washed with deionized water and absolute ethanol for several times and separated by magnetic decantation. Finally, the product Fe.sub.3O.sub.4 hollow nanospheres were dried at 80 C. for 12 h under vacuum.

[0193] The size and morphology of Fe.sub.3O.sub.4 samples were characterized using field emission scanning electron microscopy (FESEM, Hitachi SU-70 system) at accelerating voltages of 10-20 kV. Specifically, powders of samples were mounted onto conductive copper tapes, which were then attached onto the surfaces of SEM brass stubs. The samples were then conductively coated with gold by a sputtering method to minimize charging effects under FESEM imaging conditions.

[0194] Both transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) characterizations were performed using a JEOL JEM 2100F field emission microscope equipped with a Gatan Ultrascan CCD camera and EDAX Genesis EDS facility, as well as with the potential of performing SAED. To prepare the HRTEM specimens, the powder samples were dispersed ultrasonically in anhydrous ethanol. One drop of the suspension was placed on a carbon film supported on a copper grid and allowed to dry in air before the specimens were transferred into the microscope.

[0195] X-ray diffraction (XRD) analysis was conducted using a PANalytical X'Pert PRO MRD instrument with a Cu K.sub. radiation source (=1.5418 ) and an X'celerator detector. Rietveld refinement was carried out using X'Pert High Score plus software.

[0196] The morphology and size of the as prepared Fe.sub.3O.sub.4 hollow nanospheres were characterized by FESEM and TEM. Picture a of FIG. 1 is an FESEM image of the product showing a larger amount of well dispersed spherical shapes with the diameter of about 250 nm. The further magnified FESEM image in b of FIG. 1 demonstrates a representative break sphere, indicating its hollow spherical morphology with the shell thickness and inner diameter of about 50 nm and 150 nm, respectively. TEM image in c of FIG. 1 also proves the hollow spherical construction with almost the same size and shell thickness, compared with FESEM images. he marked area is further magnified in HRTEM image of d of FIG. 1, showing the well-defined 2D lattice fringes corresponding well with the (311) planes of magnetite Fe.sub.3O.sub.4 polymorph. The corresponding SAED pattern located in the inset of d of FIG. 1 confirms the single crystallinity of as-prepared Fe.sub.3O.sub.4 hollow nanospheres.

[0197] XRD pattern in FIG. 1B confirms the product obtained in the typical procedure is an Fe.sub.3O.sub.4 polymorph, in which the diffraction peaks at 230.1, 35.4, 43.1, 56.9 and 62.5 correspond well with the (220), (311), (400), (511) and (440) lattice planes of magnetite Fe.sub.3O.sub.4 (JCPDS 19-0629) without any other impurity.

EXAMPLE 5

[0198] The Fe.sub.3O.sub.4 hollow nanospheres were dispersed into pure epoxy resin (Sigma-Aldrich) in sonication at 80 C. for 30 min. Then the hardeners methyl nadic anhydride (MNA) and dodecenylsuccinic anhydride (DDSA) (Sigma-Aldrich) and accelerator 2,4,6-tris(dimethylaminomethyl)phenol (DMP) (Sigma-Aldrich) were added into the epoxy resin with the Fe.sub.3O.sub.4 hollow nanospheres dispersed in it with stirring. A uniform mixture was then obtained, dropped onto a quartz slides, and transferred in vacuum oven at 60 C. for 3 days. Then the cured Fe.sub.3O.sub.4-epoxy resin composite sheet (25.4 mm25.4 mm1 mm) was obtained for further microwave degradation testing in single mode microwave reactor CEM Discover SP with an infrared (IR) Temperature Sensor. For comparison, the pure epoxy resin sheet was also prepared in the same procedure.

[0199] Field emission scanning electron microscopy (FESEM) was performed using Hitachi SU-70 system at accelerating voltages of 10-20 kV. Transmission electron microscopy (TEM) was performed using a JEOL JEM 2100F field emission microscope equipped with a Gatan Ultrascan CCD camera and EDAX Genesis EDS facility, as well as with the potential of performing SAED. X-ray diffraction (XRD) analysis was conducted using a PANalytical X'Pert PRO MRD instrument with a Cu K.sub. radiation source (=1.5418 ) and an X'celerator detector.

[0200] 3D X-ray microscope (XRM) analysis was performed using VersaXRM-500 employing a high-energy X-ray source (80 kV). The hardness and modulus of the cured Fe.sub.3O.sub.4-epoxy resin composites (or pure epoxy resin), obtained before and after microwave irradiation, were characterized by nanoindentation using the Nanoindenter G200 developed by Agilent Technologies. Before test, small sample pieces were mounted in clear epoxy cylinders with the fibre direction perpendicular to the top face of the cylinder in order to expose the cross sections. Mounting the samples in this way facilitated semi-automatic grinding and polishing down to a final polishing suspension particle size of 50 nm. The procedures used throughout the grinding and polishing sample preparation procedures were chosen to ensure that surface damage and reaction with the polishing agents were kept to a minimum. High depth CSM indentations were used to ensure there was no property change between that of the surface material and the material further from the surface as a result of the sample preparation procedures.

[0201] The nanoindentation experiments were carried out using the Nanoindenter G200 developed by Agilent Technologies. The continuous stiffness measurement (CSM) technique was used to carry out the indentations, which allowed the contact stiffness to be calculated throughout the indentation's loading cycle. This in turn allowed the elastic modulus to be calculated continuously as a function of the indentation depth, using the Oliver and Pharr method (M. Hardiman, T. J. Vaughan, C. T. McCarthy, Compos Part A, 2015, 68, 296-303). The load and displacement resolutions of the system are 50 nN and 0.01 nm respectively, and a Berkovich tip geometry was used. The indentations were assigned a maximum penetration depth set point of 5 m with a strain rate target of 0.05/s. This strain rate target was reached with a maximum deviation of 0.01/s for all depths deeper than 100 nm. The CSM settings were programmed to apply a harmonic displacement of 2 nm and a frequency of 45 Hz. The indentation sites were targeted using an optical microscope.

[0202] A universal tensile machine (UTM, Tinius Olsen H25KS) was used to carry out the tensile shear tests room temperature. Fe.sub.3O.sub.4-ER (ER=epoxy resin) composites (or pure epoxy resin as comparison) were used for bonding single-lap-shear (SLS) joints in this study. The substrates were high stable epoxy plastic slides (60 mm25.4 mm2 mm) with no surface coating. 0.63 ml of uncured Fe.sub.3O.sub.4-ER composites (or pure epoxy resin) was used for bonding the joints with area of 25.4 mm25.4 mm. The total length of the bonded joints is 94.6 mm. Such customized dimension of SLS specimens is designed for matching the cavity size of microwave reactor in order to further investigate the degradation behaviour by dielectric heating. Then the cured joints were exposed under single-mode microwave irradiation at fixed power of 100 W and frequency at 2.45 GHz for 0-3 min. In order to ensure that the loading direction was parallel to the bond-line, two compensation spacers were bonded with the SLS specimens after microwave irradiation. The crosshead velocity was set at a constant velocity of 2 mm/min.

[0203] FIG. 2 shows the dielectric heating performance of Fe.sub.3O.sub.4-ER composites and pure epoxy resin indicating the temperature increasing as a function of time. The pure epoxy resin shows poor dielectric heating performance as its temperature is below 180 C. after single mode microwave irradiation for 3 mins at 100 W, 2.45 GHz. Dielectric heating performance of the Fe.sub.3O.sub.4-ER composites is much better than that of pure epoxy resin over irradiation time. Interestingly, the slope of such temperature curve increases from 0 to 1.5 mins, shown in the FIG. 2. The temperature goes up to 239 C. for 2 mins dielectric heating. The slope slowly decreases from 1.5 to 3 mins and the temperature increases to 292 C. for 3 mins dielectric heating. It is noticed that an endothermic peak is shown at about 168 s, indicating that a severe structural (or morphological) collapse of Fe.sub.3O.sub.4-ER composites occurs around this period.

[0204] FIGS. 3 and 4 show the FESEM images of the cross sections of cured Fe.sub.3O.sub.4-ER composite sheet (1.0 wt % Fe.sub.3O.sub.4 HNSs in epoxy resin) before and after single-mode microwave irradiation for 0-3 mins. Part a of FIG. 3 shows the global FESEM image of the cured Fe.sub.3O.sub.4-ER composite sheet without microwave exposure in low magnification, signifying the Fe.sub.3O.sub.4 HNSs are embedded in the cured epoxy resin with good dispersity. This is also confirmed by the magnified images of b of FIG. 3, which also indicates the Fe.sub.3O.sub.4 hollow nanospheres (white dots) spread all over the fracture surface (or underneath the surface), and the distance between Fe.sub.3O.sub.4 hollow sphere clusters is about 5-10 m. Further magnified FESEM image of c of FIG. 3 shows that several hollow nanospheres are aggregated together and some of them are break hollow nanospheres, in which each of them exhibits the equivalent size of about 250 nm, compared with the results shown in FIG. 1. SEM image of d of FIG. 3 shows some tiny holes and slight cracks are generated close to the Fe.sub.3O.sub.4 hollows in the Fe.sub.3O.sub.4-ER composite sheet after 1 min single-mode microwave irradiation at 100 W. After 1.5 mins microwave exposure, more holes with the size of about 200 nm are generated in the position of Fe.sub.3O.sub.4 hollow nanospheres, as shown in e of FIG. 3, which indicates that the degradation of epoxy resin starts from the surroundings of Fe.sub.3O.sub.4 hollow nanospheres. The hollow cavities with the size of about 1-2 m are obtained after 2 mins microwave exposure, as shown in the SEM image off of FIG. 3.

[0205] The degradation performance is further improved between 2 and 2.5 minutes. SEM image of a of FIG. 4 exhibits the numerous hollow cavities with the size of about several micrometers spread all over the section surface. A vertical fracture shown in a of FIG. 4 proved that the hollow cavities were generated inside of Fe.sub.3O.sub.4-ER composite sheet uniformly. A magnified SEM image shown in b of FIG. 4 presented the hollow cavities with the size of about 5-10 m, which are much greater than the size of Fe.sub.3O.sub.4 hollow spheres. The structure of cured Fe.sub.3O.sub.4-ER composite sheet was totally changed after 3 minutes microwave exposure at 100 W, 2.45 GHz. The global FESEM images (c of FIG. 4) show that the cured Fe.sub.3O.sub.4-ER composite sheet with sponge-like, hollow network structure resulted after microwave exposure for 3 mins. The corresponding magnified FESEM image shown in d of FIG. 4 exhibits the hollow texture with the cavity size up to dozens of micrometers. It is worthwhile to note that the Fe.sub.3O.sub.4 hollow nanospheres can not be found on the surface even under 20 kV in FIG. 4, and the distance between the centre of hollow cavities is about 5-10 m in a of FIG. 4, strongly indicating that the degradation started from the surroundings of Fe.sub.3O.sub.4 hollow sphere clusters.

[0206] 3D X-ray microscopy characterization results are shown in FIG. 5 and further confirm the dispersibility and dielectric heating performance of Fe.sub.3O.sub.4-ER composites in three spatial dimensions. To be clear, the green and black colours in a and b of FIG. 5 represent Fe.sub.3O.sub.4 and epoxy resin, but the green and black colours in c and d of FIG. 5 represent epoxy resin and hollow cavities, respectively. Part of a of FIG. 5 shows the 3D internal morphologies of the cured Fe.sub.3O.sub.4-ER composites before microwave irradiation, indicating that the Fe.sub.3O.sub.4 hollow nanospheres are well dispersed in 3D epoxy resin substrates. The corresponding orthoslice shown in b of FIG. 5 demonstrates the embedding performance of Fe.sub.3O.sub.4 hollow nanospheres in three spatial dimensions. The 3D X-ray computed tomography of Fe.sub.3O.sub.4-ER composites after 3 mins dielectric heating was shown in c of FIG. 5, indicating that the 3D sponge-like morphologies with well distributed hollow cavities with the size of about dozens of microns. The corresponding orthoslice shown in d of FIG. 5 demonstrates the degraded holes with good three dimensional distributions. The results indicate that the smooth and uniform epoxy resin substance can be maintained after microwave irradiation with little damage.

[0207] The hollow spherical nature of the Fe.sub.3O.sub.4 likely improved such performance. The hollow cavities grow over time under microwave irradiation and then merge with each other to form the greater hollow cavities of micrometers size and finally a hollow network structure. This was confirmed by the FESEM results in FIGS. 3 and 4 and the 3D X-ray computed tomography in FIG. 5. Therefore, the dielectric sensitive material Fe.sub.3O.sub.4 hollow nanospheres embedded in the cured epoxy resin, acted as electromagnetic receptor, can effectively perform the energy conversion process and continuously degrade the surrounding epoxy resin from the inside of the bulk Fe.sub.3O.sub.4-ER composites, which cause the expansion of hollow cavities.

[0208] FIG. 6 shows the hardness and modulus as a function of depth (0-5000 nm) for indentations into various cured Fe.sub.3O.sub.4-ER composites (or pure epoxy resin) with different dielectric heating times, which were labelled as E0, E3, C0, C1, C2 and C3. The average hardness and modulus (1000-5000 nm) of indentation into pure epoxy resin (E0) are 0.16 and 3.3 GPa, respectively. After the microwave irradiation for 3 mins, the average hardness and modulus of pure epoxy resin (E3) maintained the almost same results, which are 0.16 and 3.4 GPa, respectively, indicating that the pure epoxy resin has no obvious degradation in 3 mins of microwave irradiation. The average hardness and modulus (1000-5000 nm) of indentation into the composites (1 wt % Fe.sub.3O.sub.4 HNSs dispersed into epoxy resin, labelled as CO) are 0.20 and 3.6 GPa, signifying that the strength and stiffness of epoxy resin are improved by the embedding of Fe.sub.3O.sub.4 HNSs. The strength and stiffness of Fe.sub.3O.sub.4-ER composites are maintained in the same performance after dielectric heating for 1 min (C1) and then decreased over dielectric heating time from 1 to 3 mins. The average hardness and modulus (1000-5000 nm) of indentation into the Fe.sub.3O.sub.4-ER composites obtained after 2 mins microwave irradiation (C2, as shown in the FIG. 6) are 0.17 and 3.0 GPa, probably due to the formation of degraded cavities with the micro size greater than the that of Fe.sub.3O.sub.4 hollows spheres shown in f of FIG. 3. The average hardness and modulus (1000-5000 nm) of indentation into the sample C3 (FIG. 6) are severely decreased into only 0.02 and 0.6 GPa, indicating that the Fe.sub.3O.sub.4-ER composites are badly damaged by the dielectric heating.

[0209] The corresponding nano-indentation sites are shown in FIG. 6B. The corresponding nanoindentation sites for pure epoxy resin exhibit the smooth flatten surface with little damage before and after dielectric heating. The relevant nanoindentation sites for Fe.sub.3O.sub.4-ER composites present the damaged surfaces that deteriorate over dielectric heating time from 0 to 3 mins. The indentation results correspond well with the FESEM and 3D X-ray microscopy results above. The greater hollow cavities further merged with each other to form sponge-like hollow network structure, which severely damaged the Fe.sub.3O.sub.4-ER composites' strength and stiffness.

Tensile Shear Test

[0210] The Fe.sub.3O.sub.4-ER composites (or pure epoxy resin as comparison) were used for bonding single-lap-shear (SLS) joints in order to test the tensile shear strength of the joins formed by the composites. The substrates are high stable epoxy plastic slides (60 mm25.4 mm2 mm) with no surface coating. 0.63 ml of uncured Fe.sub.3O.sub.4-ER composites (or pure epoxy resin) was used for bonding the joints with area of 25.4 mm25.4 mm. The total length of the bonded joints is 94.6 mm. Such customized dimension of SLS specimens is designed for matching the cavity size of microwave reactor in order to further investigate the degradation behaviour by dielectric heating. Then the cured joints were exposed under single-mode microwave irradiation at fixed power of 100 W and frequency at 2.45 GHz for 0-3 min.

[0211] A universal tensile machine (UTM, Tinius Olsen H25KS) was used to carry out the tensile shear tests at room temperature. In order to ensure that the loading direction was parallel to the bond-line, two compensation spacers were bonded with the SLS specimens after microwave irradiation. The crosshead velocity was set at a constant velocity of 2 mm/min.

[0212] FIG. 7 shows the tensile shear strength of the SLS joints affected by the microwave irradiation time. The tensile shear strength (MPa, at the failure point) of the SLS joints is increased from 7.2385 (JE0) to 8.6382 MPa (JC0), when the adhesive of pure epoxy resin is supplanted by Fe.sub.3O.sub.4-ER composites, probably due to the Fe.sub.3O.sub.4 hollow nanospheres uniformly dispersed in the epoxy resin acting as reinforcing filler. After 3 mins microwave irradiation, the SLS joints bonded by pure epoxy resin still show tensile shear strength of 5.8838 MPa (JE3), showing that the decreasing of bonding properties for joints is inefficient in this case. The tensile shear strength of SLS joints bonded by Fe.sub.3O.sub.4-ER composites is rapidly decreased after 1.5 mins microwave irradiation (FIG. 7), and decreased from 8.6382 (JC0) to 0.3126 Mpa (JC3), a decrease of 96.3% after 3 mins microwave irradiation, indicating that the bonding strength of SLS joints bonded by Fe.sub.3O.sub.4-ER composites is overwhelmingly weakened by dielectric heating.

[0213] These results show that by embedding Fe.sub.3O.sub.4 hollow nanospheres in epoxy resin, degradation of cured epoxy resin can be achieved by dielectric heating. FESEM and 3D X-ray microscopy results indicated that the dielectric sensitive material Fe.sub.3O.sub.4 hollow nanospheres, which were well dispersed in the cured epoxy resin, can effectively convert the microwave energy into thermal energy as the electromagnetic acceptor, and consequently heat and degrade the surrounding epoxy resin. Nanoindentation results also confirmed that the average hardness and modulus of Fe.sub.3O.sub.4-ER composites were decreased over dielectric heating time and they were severely decreased between 2 and 3 mins microwave irradiation, accompanied by the considerable morphological evolution as well as the swift expansion of hollow cavities from several to dozens of microns and consequently to sponge-like 3D porous architectures. The tensile shear strength of the single lap-shear (SLS) joints bonded by Fe.sub.3O.sub.4-ER composites exhibited a significant loss caused by dielectric heating, compared to that bonded by pure epoxy resin.

EXAMPLE 5MODIFIED CARBON NANOFIBRES/NANORODS

[0214] Materials

[0215] Carbon nanofibres (CNFs, diameter 130 nm, length 20-200 m), (3-Glycidyloxypropyl) trimethoxysilane (98%) employed as silane coupling agent (SCA), Sulfuric acid (H2SO4, ACS reagent, 95.0-98.0%), Nitric acid (HNO.sub.3, ACS reagent, 70%), epoxy embedding medium kit including epoxy embedding medium (epoxy prepolymer), hardener MNA, hardener DDSA and accelerator DMP 30 are purchased from Sigma-Aldrich.

Synthesis

[0216] The surface oxidation of CNFs was carried out as follows. 0.5 g of CNFs was dispersed into mixed acid with 30 ml H.sub.2SO.sub.4 and 10 ml HNO.sub.3 in a 100 mL round bottom flask. The dispersion was first stirred using a vortex mixer (Vortex Gene 3) for about 1 min, and then was put in a laboratory ultrasonic bath (37 kHz) for 10 min at room temperature. This mixing and dispersion process was repeated twice to break big CNFs aggregates. Then the dispersion was refluxed at 60 C. (also at 40 or 80 C.) for 2 hours with magnetic stirring. After the surface treatment, the CNFs were separated by filtration and washed with deionized water for several times until the pH7. Then the CNFs were dried in a vacuum oven at 60 C. for 24 hours. The oxidized CNFs at 60 C. were denoted as o-CNFs.

[0217] 0.03 g of SCA (3-Glycidyloxypropyl) trimethoxysilane was added into 15 g epoxy prepolymer in a 100 mL flask and stirred using a vortex mixer for about 1 min. Then 0.3 g of o-CNFs was added and dispersed in via sonication at 60 C. for 30 mins, and then stirred using a vortex mixer for about 1 min again. This sonication and vortex stirring process was repeated 3 times. Then 7.4 g hardener MNA and 7.3 g hardener DDSA were added into the dispersion via vortex mixing for 1 min followed by sonication at 30 C. for 30 mins. This was repeated 3 times in turn again. Consequently the dispersion was magnetically stirred at room temperature for 3 days in order to get a uniform dispersion. The total weight of the dispersion is about 30 g so that the weight proportion of o-CNFs is about 1.0 wt %.

[0218] In the curing procedure, 15 drops of accelerator DMP 30 were added into the uniform dispersion via vortex mixing for 1 min followed by sonication at 30 C. for 10 mins, and repeated 3 times in turn. The resultant nCEA was then applied to quartz slides and cured in a vacuum oven at 60 C. for 3 days. The cured nanocomposite sheet (25.4 mm25.4 mm1 mm) was obtained for analysis and dielectric degradation in single mode microwave reactor CEM Discover SP (100 w, 2.45 GHz) with an infrared (IR) Temperature Sensor. For comparison, the pure epoxy adhesive sheet was also prepared using the same curing procedure.

[0219] The resultant nCEA was also applied for bonding the joints and then cured in a vacuum oven at 60 C. for 3 days. The adhesive bonded joints with a bond area of 25.4 mm25.4 mm were obtained.

[0220] Characterisation

[0221] Raman Spectrum was performed on a Dilor XY Labram spectrometer using a 532 nm ArHe green laser. Spectra were collected in the range of 1700-1200 cm.sup.1. FESEM was performed using a Hitachi SU-70 system at accelerating voltages of 10-20 kV equipped with an energy dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) was performed using a JEOL JEM 2100F field emission microscope.

[0222] Sliced thin sections of cured nCEA with the thickness of about 100 nm, prepared by ultramicrotomy using a Leica UCT machine, were used for corresponding TEM characterisation. 3D X-ray microscopy (XRM) was performed using VersaXRM-500 employing a high-energy X-ray source (80 kV).

[0223] The hardness and modulus of the cured nanocomposites (or pure epoxy adhesive), obtained before and after microwave irradiation, were characterised by nanoindentation using the Nanoindenter G200 developed by Agilent Technologies under Continuous Stiffness Measurement (CSM) technique.

[0224] A universal tensile machine (UTM, Tinius Olsen H25KS) was used to carry out tensile shear tests at room temperature. The substrates were highly stable epoxy plastic slides (60 mm25.4 mm2 mm) with no surface coating. The total length of the bonded joints was 94.6 mm.

[0225] These customised dimensions were chosen to be as close as possible to standardised adhesive joints tests, but modified so that the joints could fit in the cavity of the microwave reactor for subsequent dielectric heating.

[0226] The cured adhesive joints were exposed under single-mode microwave irradiation at fixed power of 100 w and frequency at 2.45 GHz for 0-50 s. In order to ensure that the loading direction was paralleled to the bond-line, two compensation spacers were bonded with the SLS specimens after microwave irradiation. The crosshead velocity was set at a constant velocity of 2 mm/min.

[0227] Results and Discussion

[0228] FIG. 8 shows Raman bands of untreated CNFs (a) and o-CNFs treated at 40 (b), 60 (c) or 80 (d) C.

[0229] FIG. 8 shows the Raman spectra of the untreated CNFs and o-CNFs obtained under different temperatures. The most pronounced Raman signatures of graphite-based CNFs are collectively known as the G around 1560-1600 cm.sup.1 and D bands around 1350 cm.sup.1, corresponding to the sp.sup.2 and sp.sup.3 hybridized carbons. Oxidation process resulting from acid treatment introduces sp.sup.3 centres and other structural defects, which give rise to the intensity of D band. The integrated ratio of D/G bands indicates the degree of surface oxidized functionalization of CNFs. In examining this, the G band of each sample was tuned to exhibit the relative Raman D-to-G band intensities in FIG. 8, which were comparable to clearly signify the enhancement of D band intensity by increasing the treated temperature. A sample of o-CNFs obtained under 40 C. does not show obvious rise of D/G ratio, then the D/G ratio is considerably increased under acid treatment at 60 C., indicating that the surface of CNFs were functionalized with CO, COC, COO, and COH groups with a relative high density. Acid treatment at 80 C. can extremely further enhance the D/G ratio, but unfortunately the o-CNFs obtained at 80 C. showed poor dielectric (microwave) heating performance.

[0230] FIG. 9 shows FESEM images of untreated CNFs (a & b) and o-CNFs treated at 60 C. (c & d).

[0231] The FESEM image of untreated CNFs shown in FIG. 9a exhibits an amount of the nanofibres are intricately intertwined with each other, revealing that embedding them mono-dispersed in epoxy adhesive is challenging. The length of CNFs is more than 10 m and their diameter shown in FIG. 9b is about 100-200 nm, with smooth surface. After treated in mix acids at 60 C., the orderliness of the fibres has been much improved, the results of which are shown in FIG. 9c. The length of o-CNFs is still more than 5 m. FIG. 9d shows fibres' diameter maintains the same size comparing with the untreated CNFs, but the surface of o-CNFs is scratched due to the acid treatment.

[0232] FESEM images showed the length and surface of o-CNFs obtained at 40 C. have no obvious change morphologically (not shown). The fibres with smooth surface are still tangled with each other. While the o-CNFs obtained at 80 C. (not shown) were severely trimmed by acid corrosion and oxidation. The surface of CNFs was overwhelmingly damaged into the distorted short rods, aggregated with each other.

[0233] FIG. 10 shows FESEM images of the section fracture of cured nCEA sheet before microwave irradiation (a, b), after single-mode microwave irradiation at 100 w for 40 (c, d), 50 (e) and 60 s (f).

[0234] The nanocomposite epoxy adhesive (nCEA), was applied onto the quartz slides, obtained in the typical procedure, was employed to evaluate the dielectric heating performance as well as the degradation of nCEA under single mode microwave over irradiation time. FIGS. 10a and b show the nCEA obtained in the typical procedure, revealing that the mono-dispersed CNFs are spread throughout the epoxy substrate with even distribution, and every single carbon nanofibre is clearly seen in the FIG. 10b, indicating the successful surface modification the o-CNFs and the functionalization of silane coupling agent in the typical procedure.

[0235] Interestingly, FESEM image of FIG. 10c shows the degradation of nCEA after 40 s microwave irradiation, indicating that numerous void cavities highly dispersed over the section surface are generated surrounding the o-CNFs with the size of about 5 m. A magnified image in FIG. 10d clearly show that the degraded cavities are actually void tunnels with the corresponding shape to the CNFs located inside, suggesting that the dielectric heating degradation commenced from the border between o-CNFs and epoxy substrate. A similar result was also obtained in the untreated CNFs dispersed in epoxy although with low dispersibility, indicating that the CNFs (neither SCA nor functionalized oxide groups on the surface of o-CNFs) absorbed the microwave energy and converted it into heat, in order to further significantly degrade the composition of epoxy polymers surrounding the fibres. As shown in FIG. 10e the morphology of nCEA was significantly altered after 50 s microwave exposure resulting in a sponge-like fully voided hollow networked structure. The architecture of nCEA was overwhelmingly destroyed after 60 s microwave exposure as shown in FIG. 10f indicating that only a few of isolated ash pieces left.

[0236] FIG. 11 shows TEM images of ultra-microtomed thin cut of nCEA in global view (a) and the magnified areas (b-d); 3D X-ray computed tomography of cured nCEA under single-mode microwave irradiation at 100 w for 40 s (e) and the corresponding orthoslice (f).

[0237] FIG. 11a shows a global view of ultra-microtomed thin cut of nCEA, signifying that the cut o-CNFs are dispersed in the epoxy matrix evenly. A magnified area shown in FIG. 11b indicates the anisotropic o-CNFs are fully enwrapped with epoxy substrate. Typical images shown in FIGS. 11c and d give typical images for the tubular morphologies of carbon nanofibres dispersed paralleled or perpendicularly into the epoxy thin slice.

[0238] FIG. 11e exhibits the 3D X-ray computed tomography (3D micro-CT) for the nCEA after 40 s microwave irradiation, and the where the FIG. 11f is the corresponding orthoslice. The internal configuration in three spatial dimensions indicates that the anisotropic void tunnels run through the inside of bulk epoxy matrix, and the epoxy substrates are severely damaged after the microwave irradiation. It is noticed that the o-CNFs can not be distinguished from the epoxy substrate from 3D micro-CT, probably because carbon is the main element for both o-CNFs and epoxy substrate, so that it is very difficult to differentiate the o-CNFs by X-ray if a high dispersibility is achieved.

[0239] Ultra-microtome and 3D Micro-CT results re-confirmed the achievement of monodispersed o-CNFs in epoxy adhesive, resulting in the uniformly distributed degradation. It can thus be concluded that the dielectric sensitive material o-CNFs acted as outstandingly effective electromagnetic receptors that converted the dielectric energy into heat in the epoxy matrix and degrading it over time resulting in the formation of hollow cavities (tunnels).

[0240] FIG. 12 shows the hardness (a) and modulus (b) as a function of depth for indentations into cured nCEA and pure epoxy adhesive with different dielectric heating time. Ex=Pure epoxy resin (blank sample) for x seconds dielectric heating. Cx=nCEA for x seconds dielectric heating.

[0241] In order to test the applicability of the developed nCEA as a reworkable adhesive, a series of mechanical characterisation tests were performed before and after microwave irradiation. FIG. 12 shows nanoindentation hardness and modulus measurements as a function of depth (0-5000 nm), using the Continuous Stiffness Measurement (CSM) technique for cured pure epoxy resins (E0, E60) and nanocomposite adhesive (CO-050) with different dielectric heating time.

[0242] The indentation data is very noisy for the first 1000 nm of indentation depth due to the surface roughness on the samples and low load levels, and so average data over indentation depths of 1000-4500 nm is used as it has essentially converged. The average hardness and modulus values for pure epoxy resin with no irradiation exposure (E0) are 0.16 GPa and 3.4 GPa, respectively. After 3 mins' microwave irradiation it (E60) keeps the same average hardness and modulus results, indicating that no obvious degradation of mechanical properties has occurred. The average hardness and modulus values for the non-irradiated nCEA (CO) are 0.26 and 4.2 GPa, respectively, signifying significant improvements over the pure epoxy case by 62.5% and 23.5%, respectively. These properties were maintained for up to 20 s of dielectric heating exposure (C10 and C20), during which the hardness values are slightly decreased via 0.26 (0 s)0.24 (10 s)0.21 (20 s) GPa, and the modulus values are also slightly decreased via 4.2 (0 s)4.0 (10 s)3.7 (20 s) GPa. However, after this point the properties were observed to decrease rapidly, with hardness reduced to 0.10 GPa and modulus to 1.9 GPa at 30 s exposure time (C30), probably owing to the substantial degradation starting around 30 s as the nCEA's temperature reaches to 230 C. The properties of nCEA is still reduced rapidly from 30 to 40 s and the hardness and modulus values for the nCEA at 40 s (C40) are 0.04 and 0.7 GPa, respectively. The most likely cause of this is the formation of void tunnels with the size of 5-10 m shown in FIG. 10d. After 50 s exposure (C50), the hardness and modulus were severely decreased by 96.2% and 92.9% to only 0.01 GPa and 0.3 GPa, respectively, indicating that the nanocomposite adhesive was almost completely degraded by the dielectric heating.

[0243] FIG. 13 shows the tensile shear strength of the adhesively-bonded single lap-shear (SLS) joints as a function of microwave irradiation time at 100 w, 2.45 GHz.

[0244] The degradation in mechanical properties when the nanocomposite adhesives are used in bonded joint applications was tested and the results are shown in FIG. 13. FIG. 13 shows the ultimate strength of single-lap shear (SLS) tension loaded joints bonded by either pure epoxy adhesives (JE0, JE60) or nCEA (JC0JC50) at increasing microwave irradiation times (the number corresponds to time in second). The ultimate joint strength (MPa, at the failure point) of the SLS joints is increased from 7.24 MPa (JE0) to 11.22 MPa (JC0) by the addition of 1.0 wt % o-CNFs into the bonding epoxy adhesive, giving a adhesion rise of about 55.0%, possibly due to a toughening/reinforcing mechanism observed by introducing 1D nanomaterials into epoxy adhesives. The strength of the SLS joints bonded by the pure epoxy adhesive shows little decrease after 60 s microwave irradiation (JE60), demonstrating that microwave irradiation is not sufficient alone to disassemble these bonded joints. The strength of SLS joints bonded by nCEA remains almost constant up to 10 s microwave exposure (JC0JC10). After this point the joint strength decreases linearly (JC10JC50) and at 50 seconds of exposure time it is decreased by 97.2% to 0.31 MPa. This significant decrease in strength indicates that the bonding strength is overwhelmingly weakened by the application of dielectric heating, to the point that application of any further mechanical load (even low hand pressure) completely de-bonded the bonded components.

CONCLUSIONS

[0245] These results show that a reworkable nano-composite based adhesive can be prepared by embedding the modified carbon nanofibres (o-CNFs) as the dielectric sensitive nanomaterials in epoxy adhesive. The dispersibility of CNFs in epoxy adhesive was highly improved by the surface oxidation and silane coupling agent, so that the developed nanocomposite epoxy adhesive (nCEA) was highly sensitive to dielectric heating resulting in significant degradation of mechanical properties after an exposure time of less than 50 seconds. The dielectric heating performance of composite adhesive was investigated by microwave irradiation in fixed power over time. FESEM and 3D Micro-CT results indicated that the dielectric sensitive material o-CNFs were monodispersed in the cured epoxy adhesive and acted as electromagnetic receptors to effectively convert the microwave energy into thermal energy to significantly degrade the modulus and hardness by 96.2% and 92.9%, respectively, as a result of generating and growing of void tunnels in the adhesive surrounding the o-CNFs. Before exposure to dielectric heating, tensile loaded SLS joints bonded by nCEA were in fact 55.0% stronger than those bonded using just pure epoxy adhesive. After 50 seconds of dielectric heating exposure, the strength of nCEA joints reduced by 97.2%, thus demonstrating the excellent re-workable performance of our new composite adhesive.

[0246] In summary, the present invention provides an adhesive composition degradable by dielectric heating. The adhesive composition comprises a thermosetting polymer and a material sensitive to dielectric heating. The material sensitive to dielectric heating is selected from any one or more of hollow nanospheres, nanotubes, nanorods, nanofibres, nanosheets, graphene, graphene derivatives, nano/micro hybrids and mixtures of two or more nanoscale particles. The adhesive composition may be particularly useful in the assembly and disassembly of parts, particularly parts which have complicated and/or blocked joined surfaces. A method of joining at least two parts of an article together and a method of disassembling at least two parts of an article, using the adhesive composition are also provided. The adhesive composition may provide a reworkable nano-composite adhesive. The adhesive composition may be used to reversibly bond a biomedical or dental implant to a part of a human or animal body.

[0247] Throughout this specification, the term comprising or comprises means including the component(s) specified but not to the exclusion of the presence of other components. The term consisting essentially of or consists essentially of means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention. Typically, when referring to compositions, a composition consisting essentially of a set of components will comprise less than 5% by weight, typically less than 3% by weight, more typically less than 1% by weight of non-specified components.

[0248] The term consisting of or consists of means including the components specified but excluding addition of other components.

[0249] Whenever appropriate, depending upon the context, the use of the term comprises or comprising may also be taken to encompass or include the meaning consists essentially of or consisting essentially of, and may also be taken to include the meaning consists of or consisting of.

[0250] The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention as set out herein are also to be read as applicable to any other aspect or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each exemplary embodiment of the invention as interchangeable and combinable between different exemplary embodiments.

[0251] Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

[0252] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0253] All of the features disclosed in this specification (including any accompanying claims, and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

[0254] Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0255] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.