WO2016019420A1 - Nanocomposite adhesive - Google Patents

Abstract

The present disclosure relates to a nanocomposite epoxy adhesive film comprising an epoxy resin and less than 10 vol% elastomer nanoparticles, wherein the nanocomposite epoxy adhesive has a Young's modulus of from about 1 GPa to about 3.4 GPa when measured at 0.6mm adhesive thickness or a toughness measured by G_Ic of greater than about 3.8 kJ/m². The present disclosure also relates to a nanocomposite epoxy adhesive film comprising an epoxy resin and elastomer nanoparticles, wherein the adhesive toughness of the nanocomposite epoxy adhesive is greater than about 130% higher than the adhesive toughness of an equivalent sample of an epoxy adhesive that does not contain the elastomer nanoparticles or wherein the shear strength of the nanocomposite epoxy adhesive is at least about 49% higher than the shear strength of an equivalent sample of an epoxy adhesive that does not contain the elastomer nanoparticles.

Description

NANOCOMPOSITE ADHESIVE

PRIORITY DOCUMENT

[0001 ] The present application claims priority from Australian Provisional Patent Application No.

2014903067 titled "NANOCOMPOSITE ADHESIVE" and filed on 7 August 2014, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

10002] The present disclosure relates epoxy adhesives. BACKGROUND

[00031 Epoxy resins are prepolymers and polymers containing epoxide groups which can be cross-linked (or "cured") using suitable hardeners to form thermoset polymers with good mechanical properties, temperature and chemical resistance. Epoxy resins have a wide range of applications, such as in metal coatings, electronics and electrical components, high tension electrical insulators, fibre-reinforced plastic materials and structural adhesives.

[0004 ] Epoxy resins are used as structural adhesives or engineering adhesives in a wide range of industries and applications, such as automobile and aircraft manufacturing, civil engineering, in the construction of bicycles, boats, golf clubs, skis, snowboards, and other applications where high strength bonds are required. They can be used as adhesives for wood, metal, glass, stone, and some plastics. Epoxy adhesives have good chemical resistance, corrosion resistance and thermal properties compared to many other common adhesives.

[0005] However, epoxy resins are inherently brittle due to their highly cross-linked structure, leading to a poor resistance to crack propagation. Microcracks are readily created in cured epoxy adhesives by vibration or weathering which can, in turn, lead to high maintenance costs. In an effort to address these problems, a range of tougheners have been added to bulk epoxy resins, including clay¹'², silica''⁴ and other elastomer materials^5,6'⁷. However, the toughening effect of these materials has only been shown in bulk epoxy resins.

[0006] There is a need for epoxy adhesives that overcome one or more of the problems associated with prior art epoxy resins. SUMMARY

[0007] In a first aspect, provided herein is a nanocomposite epoxy adhesive film comprising an epoxy resin and less than 10 vol% elastomer nanoparticles, wherein the nanocomposite epoxy adhesive has a Young's modulus of from about 1 GPa to about 3.4 GPa when measured at 0.6mm adhesive thickness.

[0008] In a second aspect, provided herein is a nanocomposite epoxy adhesive film comprising an epoxy resin and less than 10 vol% elastomer nanoparticles, wherein the nanocomposite epoxy adhesive has a toughness measured by Gi_c of greater than about 3.8 kj/m².

[0009] In a third aspect, provided herein is a nanocomposite epoxy adhesive film comprising an epoxy resin and elastomer nanoparticles, wherein the adhesive toughness of the nanocomposite epoxy adhesive is greater than about 130% higher than the adhesive toughness of an equivalent sample of an epoxy adhesive that does not contain the elastomer nanoparticles.

[0010] In a fourth aspect, provided herein is a nanocomposite epoxy adhesive film comprising an epoxy resin and elastomer nanoparticles, wherein the shear strength of the nanocomposite epoxy adhesive is at least about 49% higher than the shear strength of an equivalent sample of an epoxy adhesive that does not contain the elastomer nanoparticles.

[001 1 ] In embodiments, the nanocomposite epoxy adhesive film has a thickness of less than about 1 mm. The nanocomposite epoxy adhesive film may have a thickness of 0. 1 mm, 0.2 mm. 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm or 0.9 mm. In specific embodiments, the nanocomposite epoxy adhesive film has a thickness of about 0.6 mm.

[0012] In embodiments of the first to fourth aspects, the nanocomposite epoxy adhesive film is formed by curing an adhesive composition comprising at least one diglycidyl ether of a polyhydric phenol compound and elastomer nanoparticles.

[0013 ] In embodiments, the elastomer nanoparticles are rubber nanoparticles. In embodiments, the rubber nanoparticles are about 55nm diameter particles. In embodiments, the rubber nanoparticles are present in the adhesive composition in an amount of about 5 vol%.

[0014] In embodiments, the adhesive composition is cured with a polyetheramine hardener. In specific embodiments, the polyetheramine hardener is a 0,0'-bis(2-aminopropyl)polypropylene glycol polymer. In other specific embodiments, the 0,0'-bis(2-aminopropyl)polypropylene glycol polymer has a molecular weight of about 230. In specific embodiments, the ( ,(9'-bis(2-aminopropyl)polypropylene glycol polymer has a molecular weight of about 400. 10015 J In a fifth aspect, provided herein is an adhesive composition comprising an epoxy resin, 2- 10 vol% elastomer nanoparticles having an average diameter of 40-70 nm and a hardener that can be thermally activated to cure the epoxy resin. The adhesive composition is capable of forming an adhesive film between two surfaces.

[0016J In a sixth aspect, provided herein is a process for producing an epoxy adhesive, the process comprising mixing an epoxy resin with a calculated amount of 2-10 vol% of elastomer nanoparticles having an average diameter of 40-70 nm to form an epoxy resin-nanoparticle mixture and contacting the mixture with a hardener under conditions to cure the epoxy resin.

[0017] In a seventh aspect, provided herein is a process for adhering two surfaces, the process comprising applying a film of an adhesive composition comprising an epoxy resin, 2- 10 vol% elastomer nanoparticles having an average diameter of 40-70 nm, and a hardener to at least one of the surfaces, contacting the two surfaces with the adhesive composition, and curing the adhesive to form an adhesive film in contact with the surfaces.

[0018 ] In embodiments, the epoxy resin is a diglycidyl ether of a polyhydric phenol compound.

[0019] In embodiments, the elastomer nanoparticles are rubber nanoparticles. In embodiments, the rubber nanoparticles are about 55nm diameter particles. In embodiments, the rubber nanoparticles are present in the adhesive composition in an amount of about 5 vol%.

[0020 ] In embodiments, the adhesive composition is cured with a polyetheramine hardener. In specific embodiments, the polyetheramine hardener is a 6>,0'-bis(2-aminopropyl)polypropylene glycol polymer. In other specific embodiments, the 0,0'-bis(2-aminopropyl)polypropylene glycol polymer has a molecular weight of about 230. In specific embodiments, the 0,0'-bis(2-aminopropyl)polypropylene glycol polymer has a molecular weight of about 400.

[00211 In embodiments, the curing is earned out at a temperature of 120°C for a time period of 12 hours. BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

[0022] Illustrative embodiments of the present disclosure will be discussed with reference to the accompanying drawings wherein:

[0023] Figure 1 shows a schematic of compact tension (CT) and dumbbell specimens;

[0024] Figure 2 shows a schematic of a double cantilever beam (DCB) specimen with aluminium substrates; [0025 J Figure 3 shows a schematic of a lap shear specimen with aluminium substrates;

[0026] Figure 4 shows a plot demonstrating the adhesive toughness of epoxy/rubber microcomposites using two hardeners J230 and J400, with an adhesive layer thickness of 0.6 mm;

[0027] Figure 5 shows plots demonstrating the effect of adhesive thickness on toughness of epoxy/rubber microcomposites using two hardeners (a) J230 and (b) J400;

[0028] Figure 6 shows TEM micrographs of J4(⁾0-cured epoxy/rubber nanocomposite at 5 vol%;

[0029] Figure 7 shows a plot demonstrating the adhesive toughness of epoxy/rubber nanocomposites cured by J230 and J400, respectively. Adhesive thickness: 0.6 mm;

[0030] Figure 8 shows a series of SEM micrographs of a DCB-fractured surface of the 0.6 mm-thick, 5 vol% epoxy/rubber nanocomposite cured by J230. The crack propagates from top to bottom;

[0031 ] Figure 9 shows a series of SEM micrographs of a DCB-fractured surface of the 0.6 mm-thick, 5 vol% epoxy/rubber nanocomposite cured by J400. The crack propagates from bottom to top;

[0032] Figure 10 shows plots demonstrating the effect of adhesive thickness on toughness of epoxy/rubber nanocomposites cured by (a) J230 and (b) J400, respectively;

[0033] Figure 1 1 shows a plot demonstrating the effect of particle size on surface-surface interparticle distance and total particle surface area in 1 nun³ of the 5 vol% composite;

[0034] Figure 12 shows a schematic of plane stress and plane strain around a crack-tip and its plastic zone in a finite thickness-B;

[0035] Figure 13 shows the deformation zone of CT and DCB specimens; and

[0036] Figure 14 shows a plot demonstrating the shear strength of nanocomposite-based adhesives cured by J230 and J400, respectively.

DETAILED DESCRIPTION

[0037] The present disclosure has arisen from the inventors' research into nanocomposite epoxy adhesives containing elastomer nanoparticles and, in particular, the relationship between particle size, adhesive thickness, matrix ductility and adhesive toughness. 10038 J Provided herein is an adhesive composition that can be cured to form an adhesive film, the composition comprising an epoxy resin, 2- 10 vol% elastomer nanoparticles and a hardener that can be thermally activated to cure the epoxy resin.

[0039] The epoxy resin is a monomer, pre-polymer, or polymer that can be co-polymerised with the hardener to form a solid adhesive film with good mechanical, chemical and physical properties. In embodiments, the epoxy resin is a diglycidyl ether of a polyhydric phenol compound. Suitable epoxy resins include the diglycidyl ethers of polyhydric phenol compounds such as resorcinol, catechol, hydroquinone, biphenol, bisphenol A, bisphenol AP ( l,l-bis(4-hydroxylphenyl)-l -phenyl ethane), bisphenol F, bisphenol K, tetramethylbiphenol, diglycidyl ethers of aliphatic glycols and polyether glycols such as the diglycidyl ethers of C_2-24 alkylene glycols and poly(ethylene oxide) or poly(propylene oxide) glycols; polyglycidyl ethers of phenol-formaldehyde novolac resins, alkyl substituted phenol- formaldehyde resins (epoxy novolac resins), phenol-hydroxybenzaldehyde resins, cresol- hydroxybenzaldehyde resins, dicyclopentadiene-phenol resins and dicyclopentadiene-substituted phenol resins, and any combination thereof. In preferred embodiments, the epoxy resin is a bisphenol-A based epoxy resin or a bisphenol-F based epoxy resin.

[0040 J The epoxy resin may comprise other components, such as other epoxy resins, monomers, pre- polymer or polymers or additives as required. In these embodiments, the epoxy resin may comprise up to 85 weight percent of the resin component.

10041 ] In embodiments, the elastomer nanoparticles are rubber nanoparticles. As used herein, the term "nanoparticle" means a particle having one or more dimensions of between lnm and lOOnm. In embodiments, the elastomer nanoparticles have an average diameter of between about 4()nm and about 70nm. In more specific embodiments, the elastomer nanoparticles have an average diameter of between about 50nm and about 60nm. For example, the elastomer nanoparticles may have an average diameter of 50nm, 5 lnm, 52nm, 53nm, 54nm, 55nm, 56nm, 57nm, 58nm, 59nm, and 6()nm. In specific embodiments, the elastomer nanoparticles have an average diameter of about 55nm.

[00421 Our studies have demonstrated that rubber nanoparticles show a far higher adhesive toughening effect than rubber microparticles. This effect can be explained by the surface-surface interparticle distance and total particle surface area. The interparticle distance linearly increases with particle size. For example, a distance of 1 19 nm for adjacent 55 nm particles increases to 2170 nm for 1 μηι particles at 5 vol%. This means that nanoparticles can readily interact with each other to produce more intense filler- filler networks than microparticles. Furthermore, the total particle surface area drops dramatically for particles smaller than 100 nm in diameter. In a given volume, 55nm particles produce a 7500 mm² surface area in comparison with a surface area of 300 mm² for Ι μηι particles. Thus, nanoparticles have far more interface with matrix and they can produce a much larger scale of matrix deformation to absorb fracture energy.

[0043] The elastomer nanoparticles may be present in the adhesive composition in an amount of between about 2 vol% and less than 10 vol%, such as 2 vol%, 2.5 vol%, 3 vol%, 3.5 vol%, 4 vol%, 4.5 vol%, 5 vol%, 5.5 vol%, 6 vol%, 6.5 vol%, 7 vol%, 7.5 vol%, 8 vol%, 8.5 vol%, 9 vol%, and 9.5 vol%. In some embodiments, the nanoparticles are present in the adhesive composition in an amount of about 2.5 vol%. In other embodiments, the nanoparticles are present in the adhesive composition in an amount of about 5 vol%.

[0044] In embodiments, the adhesive composition is cured with a polyetheramine hardener. The polyetheramine hardener contains a polyether backbone and has two to four primary or secondary amine groups per molecule. The polyether backbone may be, for example, a homopolymer of ethylene oxide, propylene oxide, 1 ,2-butylene oxide, styrene oxide, tetrahydrofuran and the like, or may be a copolymer of any two or more of these. Preferred polyether backbones are poly(propylene oxide), block or random copolymers of ethylene oxide and propylene oxide, and poly( tetrahydrofuran). The molecular weight of the amine-terminated polyether may be from about 200 to about 5000. A preferred molecular weight is from about 230 to about 2200. In specific embodiments, the polyetheramine hardener is a 6>,(7-bis(2- aminopropy polypropylene glycol polymer. In specific embodiments, the 0,0'-bis(2- aminopropyl)polypropylene glycol polymer has a molecular weight of about 400. In other specific embodiments, the 0,(9'-bis(2-aminopropyl)polypropylene glycol polymer has a molecular weight of about 230. Suitable polyetheramine hardeners include those sold by Huntsman Chemicals under the trade designations Jeffamine D-230, Jeffamine D-400, Jeffamine D-2000, Jeffamine D-4000, Jeffamine T-403, Jeffamine XTJ-542, jeffamine XTJ-548, and Jeffamine XJ-559.

[0045 ] In embodiments, the curing is carried out at a temperature of from about 100°C to about 150°C, such as about 100°C, 105°C, 1 10°C, 1 15°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C or 150°C. The curing can be carried out for a time period of 6 to 24 hours, such as about 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 1 1 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours or 24 hours. In specific embodiments, the curing is carried out at a temperature of about 120°C for a time period of about 12 hours.

[0046] The amine-terminated polyether contains a polyether backbone and has at least two primary or secondary amine groups per molecule. The amine groups are preferably primary amine groups. The polyether backbone may be, for example, a homopolymer of ethylene oxide, propylene oxide, 1,2- butylene oxide, styrene oxide, tetrahydrofuran and the like, or may be a copolymer of any two or more of these. Preferred polyether backbones are poly(propylene oxide), block or random copolymers of ethylene oxide and propylene oxide, and poly(tetrahydrofuran). The molecular weight of the amine-terminated polyether may be from about 200 to about 5000. In specific embodiments, the molecular weight of the amine-terminated polyether is from about 200 to about 500. Suitable amine-terminated polyethers include those sold by Huntsman Chemicals under the trade names Jeffamine D-230, Jeffamine D-400, Jeffamine D-2000, Jeffamine D-4000, Jeffamine T-403, Jeffamine XTJ-542, Jeffamine XTJ-548 and Jeffamine XJ- 559. Our results have shown that adhesives having good toughness can be formed with Jeffamine D-230 and Jeffamine D-400, with the latter producing the best results.

[0047] The adhesive composition can be produced by mixing the epoxy resin with a calculated amount of elastomer nanoparticles to form an epoxy resin-nanoparticle mixture and contacting the mixture with the hardener under conditions to cure the epoxy resin.

[0048 ] The adhesive composition can be used to adhere two surfaces (or adherends). A film of the adhesive composition can be applied to at least one of the surfaces, the two surfaces then brought into contact with the adhesive composition, and the composition cured to form an adhesive film in contact with the surfaces.

[0049] The adhesive film may have a thickness of about 0.1 mm to about 1.0 mm, such as 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm or 0.9 mm. In some embodiments, the thickness of the adhesive film is 0.6 mm.

[0050] The surfaces to be adhered may be any surface that can be bonded using an epoxy adhesive including, but not limited to, metal, ceramic, wood, plastics, glass, etc.

[0051 ] Advantageously, we have found that the adhesive formed after curing the adhesive composition has several physical properties that make it useful industrially. For example, the nanocomposite epoxy adhesive has a Young's modulus of from about 1 GPa to about 3.4 GPa when measured at 0.6mm adhesive thickness.

[0052] We have also found that the nanocomposite epoxy adhesive has a toughness measured by G_Ic of greater than about 3.8 kJ/m².

[0053 ] We have further found that the adhesive toughness of the nanocomposite epoxy adhesive is greater than about 130% higher than the adhesive toughness of an equivalent sample of an epoxy adhesive that does not contain the elastomer nanoparticles.

[0054 ] We have also found that the shear strength of the nanocomposite epoxy adhesive is at least about 49% higher than the shear strength of an equivalent sample of an epoxy adhesive that does not contain the elastomer nanoparticles. [0055] The toughening effect of elastomer nanoparticles has previously been demonstrated in bulk epoxy resins. However, the results demonstrated in bulk resins cannot be translated to epoxy adhesives because, in the latter case, the adhesive bond gap thickness constrains the deformation around a crack tip. It is generally considered that a critical adhesive thickness is needed to obtain an ideal adhesive toughness. When the actual thickness is smaller than the critical thickness, the defonnation zone at a crack tip cannot fully develop due to the restriction imposed by the substrates. When it is larger than the thickness, the zone is no longer constrained by the substrates and thus the fracture energy should plateau.^{8"1 1} In particular, a maximum adhesive toughening effect should be obtained when the thickness approximates the diameter of the plastic zone for rubber-toughened adhesives.⁹ However, our 0.6-mm thick adhesive showing superior adhesive toughness is substantially thinner than the zone diameter.

EXAMPLES

[0056] Materials

[0057] Epoxy resin, diglycidyl ether of bisphenol A (DGEBA, Araldite-F) with an epoxide equivalent weight of 182-196 g/equiv, was supplied by Ciba-Geigy, Australia. Liquid-rubber, a butadiene- acrylonitrile copolymer terminated by amine groups (Hycar 1300x35), was provided by Noveon Inc. Spherical rubber nanoparticles (Kane ACE Mx-120) were supplied as a colloidal sol (25 wt%, EEW 243 g/equiv) in epoxy by Kaneka, Japan. Hardener Jeffamine D-230 (denoted J230) and Jeffamine D-400 (J400) were kindly provided by Huntsman ( elbourne). The two hardeners are different in terms of molecular chain length (as shown below), and thus, they produce different ductility upon curing with epoxy.

[0058]

J230: n=2.5

J400: n=6.1

[0059] Adherends were manufactured in a local workshop from aluminium 6060 which has a Young's modulus of 69 GPa and a yield strength of 187 MPa. The adherends ( 150^χ 10^χ 10 mm) were fabricated according to ASTM 3433-99 and ISO standard 25217:20()9.^{12 13} 0.1-mm thick and 0.6-mm thick copper shims were used to control the adhesive thickness. 40-μιη thick, non-sticky paper was purchased locally.

[0060] Example 1 - Preparation of epoxy adhesive bonded adherends [0061 ] Epoxy resin (DGEBA) was mechanically mixed at 60°C for 30 min with stoichiometric amounts of either: (1 ) liquid mbber (for comparative testing); or (2) nanoparticle-master batch (composition of the present invention) at 2.5-5.0 vol% to produce adhesives. A calculated quantity of either J230 (weight ratio of DGEBA/J230 = 3.30: 1 ) hardener or J400 (weight ratio of DGEBA /J400 = 1.76: 1) hardener was added to the mixture and mixed 2.5 min at 40-50°C. Each mixture was carefully degassed in a vacuum oven for 15 min to remove bubbles, followed by applying adhesives on the desired surface of two substrates (adherends). Curing was then conducted at 120°C for 12 hours.

[0062] Example 2 - Adhesive toughness measurement by compact tension testing and double cantilever beam testing

[0063 ] Rubber moulds for compact tension (CT) and dumbbell testing were produced by using silicone rubber (Figure 1). After blending with hardener and degassing, the adhesive mixture was poured into the CT and dumbbell moulds, followed by curing at 120°C for 12 hours. Both sides of samples were polished with sand paper until all visible marks disappeared. Then the samples were thermally treated at 120°C for 120 min to remove local stress concentration caused by polishing. Tensile testing was performed at 0.5 mm/min at room temperature; an Instron extensometer 2630-100 was used to collect accurate displacement data to measure modulus; and all Young's moduli were calculated using 0.005-0.2% strain.

[0064] Example 3 - Preparation of a crack for adhesive toughness measurement

[0065] Since instantly propagated cracks proved sufficiently sharp for the accurate, reproducible fracture toughness measurement of CT, single-edge notched bending samples and adhesive joints,^14"16 we produced such cracks for adhesives. Specifically, a desired length of a precrack was made by embedding a non-sticky paper. After removing the brass shims from a joint, a wedge was inserted into the precrack and tapped to produce an instantly propagated crack, with a third of the double cantilever beam (DCB) length tightly clamped from the bottom by a bench vice. A layer of plaster coating was made on each side of the joint to monitor the crack growth. The propagated crack must be crescent-like and longer than the embedded crack. The sample was then mounted onto an Instron machine for measurement.^{15 16}

[0066] The adhesive toughness (G_lc) of DCB specimens was calculated according to IS025217

'corrected beam theory' ¹³

3PS F

2B(a N

Where: £ = Crack length, un

P = load measured by Instron, N

δ = displacement of the cross-head of Instron, mm

B = Specimen Width, mm

Δ = crack-length correction, mm. Only an instantly propagated crack was used.

F = large-displacement correction

N = load-block correction

/, = distance from the centre of the loading pin to the loading-block which is a tab in Fig 2, mm

/, = distance from the pin centre to the loading-block edge, mm. [0067 ] Example 4 - Adhesive performance by lap shear testing

[0068 ] Lap shear substrates (100x25x3 mm) are the same as DCB in Example 3 (aluminium 6060), and they were fabricated according to ISO standard 4587.¹⁷ 0.2-mm diameter copper wires were used to control the adhesive thickness. Figure 3 illustrates a schematic of a lap shear testing specimen.

[0069 ] Example 5- Morphology

[0070 ] Scanning electron microscopy (SEM) was used to examine the fracture surfaces (crack tip and propagation zone) of DCB and CT specimens. After coating with a thin layer of platinum, these surfaces were observed using a Philips XL30 FEGSEM at 10 kV.

[0071 ] Transmission electron microscopy (TEM) was performed to provide two dimensional images of the internal structure of epoxy/rubber nanocomposites. Ultrathin sections of 50 nm were microtomed from samples using a Leica Ultracut S microtome equipped with a diamond knife, and were collected on 400-mesh copper grids and stained with 5 wt% osmium tetroxide for 12 hours. These sections were examined with a Philips CM200 transmission electron microscope at an accelerating voltage of 200 kV. Particle sizes were analysed using an image analysis software analySIS*.

[0072 ] Results and discussion

[00731 Liquid rubber toughened epoxy microcomposiies

[0074] Liquid rubber-toughened epoxy has been extensively studied over the last 40 years.^{18 19} Although liquid rubber is miscible with epoxy and hardener producing a clear solution prior to curing, phase separation occurred during curing, and this produces rubber particles of around 1 μηι in diameter. For comparative purposes, we fabricated two cohorts of epoxy/rubber microcomposites by mixing amine- ended liquid rubber with epoxy cured by J230 and J400, respectively.

[0075] Figure 4 shows the adhesive toughness of neat epoxy and liquid rubber composites. The adhesive toughness increased in both systems, while the J400-cured system showed higher toughness and improvement. A maximum increase of 1 10% in toughness was seen for 5.0 vol% rubber-toughened adhesive. A J400 molecule is nearly 100% longer than a J230 molecule and this means that upon curing J400 can provide a more flexible network to absorb fracture energy by promoting more matrix deformation around particles than J230. Thus, the J400-cured system demonstrates higher adhesive toughness at all fractions.

[0076] It is known that adhesive toughness increases with increasing thickness since thicker adhesive can provide more space for deformation to consume energy. Figure 5 demonstrates the effect of adhesive thickness on toughness of the two liquid rubber composite systems. It can be seen that adhesive toughness of neat resins shows no visible change with increase in the thickness. However, the toughness of 5.0 vol% liquid rubber-toughened adhesive does depend on the thickness. This means that a sufficient thickness is indispensable for composite-based adhesives. Herein CT samples are assumed as an adhesive of infinite thickness, since their deformation under loading is not constrained.

[0077] Epoxy/rubber nanocomposites

[0078 ] Figure 6 shows TEM micrographs of a 5 vol% epoxy/rubber nanocomposites cured by J400. Most of the nanosized rubber particles form clusters, and these clusters are well dispersed in matrix. When a representative cluster is magnified in Figure 6b, the nanoparticles are found to be separated from each other; that is, no aggregation is observed. The particle size of the nanoparticles was measured as -55 nm in diameter.

[ 0079] Figure 7 shows the adhesive toughness of two cohorts of epoxy/rubber nanocomposites cured by J230 and J400, respectively. Although the toughness of the J230-cured system improved with increase in nanoparticle fractions, e.g. 206% improvement at 5.0 vol%, the J400-cured system demonstrated significantly higher toughness, an increase of 4908% at the same fraction. By contrast, there is no obvious difference in the toughness improvement in Figure 4 for the two sets of microcomposites cured by J230 and J400, respectively. This means that rubber nanoparticles can improve adhesive toughness, but this improvement needs to be activated in a relatively ductile system.

[0080] The fracture surface of a CT specimen was investigated to explain the improvement in toughness caused by the hardeners used. In Figures 8a-b, the 5 vol% J230-cured nanocomposite shows a clear crack tip, where a few scale-like structures are found. Narrow whitening zones are observed in higher magnification images Figure 8c and 8d, and these zones represent locally cavitated or dilatated matrix deformation. The density of the whitening zones is significantly lower than other regions, and they are more sensitive electrons and thus appear white under SEM. These whitening zones and other surface deformations consume fracture energy to prevent crack propagation.

[0081 ] Figure 9 shows micrographs of the fractured surface of the 5 vol% J40()-cured nanocomposite. A top view of the DCB fracture surface is shown in Figure 9a, where the crack propagates from bottom to top. A river line-like region of ~1 mm in height is seen in front of the crack tip, which may be caused by tapping prior to the testing. This region is followed by a very rough, whitening zone of -21 mm in height. At high magnification, this zone consists of many cross-like cracks; starting from each crack centre, deformation lines extensively grow outwards towards; nanofibrils are observed in Figure 9 (f) and (g). The formation of these phenomena can be explained as below.

[0082 ] Toughness is the resistance of a material to propagation of a sufficiently sharp crack. When crack propagation is obstructed by nanoparticles, it may bow out or deflect to produce secondary smaller cracks. It should be noted that all the cracks observed in previous studies, albeit in different directions, propagate two-dimensionally. By contrast, the cross-like cracks observed in this study propagate three- dimensionally, and this phenomenon is unique. Since these cracks were not observed in the fracture surface of CT samples, they must be caused by the constraining effect of the adherends. Under loading, a stress concentration occurs in the front of a crack tip. Since rubber nanoparticles are not as stiff and strong as the matrix, they produce voids first; with further loading, these voids grow and expand to absorb more fracture energy.⁷ Under loading, a CT specimen of rubber-toughened epoxy would produce a spherical plastic zone of a few millimetres in diameter to reduce the stress concentration. In our testing of adhesives of 0.6 mm in thickness, such a spherical zone cannot be produced to reduce the stress concentration due to the constraining effect of adherends. Therefore, there must be a higher level of stress concentration in the adhesive than the spherical zone of a CT specimen, and this may lead to the cross-like cracks that can absorb more energy than two-dimensional cracks. The constraining effect in a relatively ductile matrix causes the deformation to develop along the propagation direction, explaining the ~30 mm high deformation zone on the surface (Figure 9a). The nanofibrils in Figure 9 (f) and (g) consist of the nanoparticles and the orientated chains of matrix, which may imply that croiding (crack-tip voiding) is one of the toughening mechanisms in this work.

[0083 ] Figure 10 shows the effect of adhesive thickness on the toughness of neat epoxy resins and their nanocomposites. With increase in adhesive thickness from 0. 1 to 0.6 mm, increments of 4% and 62% in adhesive toughness are observed for the 2.5 vol% and 5.0 vol% J230-cured nanocomposites, respectively. Nevertheless, the infinite-thickness adhesives produce far higher toughness than the 0.6 mm-thick samples. This implies that elastomer nanoparticles cannot toughen the J230-cured adhesives, although they produced superior toughening effect in the bulk resin. With the adhesive thickness increasing from 0.1 to 0.6 ram, however, J40()-cured nanocomposites demonstrate obvious toughness improvements, 131% for the 2.5 vol% adhesive and 210% for the 5.0 vol% adhesive. Interestingly, the 0.6 mm-thick adhesives demonstrate similar energy release rate to their bulk nanocomposites. This means that the superior toughening effect of nanoparticles in bulk resins can be transferred from bulk CT samples to adhesives, on the condition that there is sufficient matrix ductility and a minimum adhesive thickness to activate this toughening effect. Of the matrix ductility and adhesive thickness, the former appears more important in activating the superior toughening effect of rubber nanoparticles.

[0084] Elastomer nanoparticles show far higher adhesive toughening effect than their peer

microparticles, and this can be explained from their surface-surface interparticle distance and total particle surface area. We further developed a previous model⁷ to produce Figure 1 1 where the surface- surface interparticle distance and total surface area are plotted against the particle size in a given volume and fraction composite. The interparticle distance linearly increases with particle size. For example, a distance of 1 19 nm for adjacent 55-nm particles increases to 2170 nm for l-μηι particles at 5 vol%. This means that nanoparticles can readily interact with each other to produce much more intense filler-filler network than microparticles. More importantly, the total particle surface area drops dramatically for particles smaller than 100 nm in diameter. In a given volume, the 55-nm particles produce a 7500 mm² surface area in comparison with a surface area of 300 mm² by l-μιη particles. This implies that nanoparticles have far more interface with matrix and thus, they can produce a much larger scale of matrix deformation to absorb fracture energy. Nevertheless, the results in Figure 8a indicate that these superior interparticle distance and surface area of nanoparticles need to be triggered by an appropriate matrix ductility.

[0085 ] In rubber-toughened epoxy, it is generally agreed that rubber particles cavitate first and almost simultaneously induce matrix shear yielding, producing a plastic deformation zone (Figure 12). The zone size can be calculated by:

[0086]

1 EG_{l c}

r_v = ~- Plane— strain conditions

y 6π ay

1 EG_lc

r_v = — Plane— stress conditions

^ 2.71 Oy

[0087] Where r_y is the Irwin' s fist order estimate of the plastic zone size under plane-stress and plane- strain conditions, E is the adhesive modulus, G_lc is the mode I fracture energy of the bulk adhesive specimen and a_y is the yield strength of adhesive.^9-20 The equation for plane-stress conditions is more popular for quantitative studies and direct comparison of the values of adhesive thickness. Figure 12 shows that the value of r_y is greater at the edge of an adhesive joint where plane stress conditions act. 10088] Kinlock and Shaw⁹ explained that the adhesive energy G_lc depends on the size of the plastic zone, and that the constraining effects from the two adherends control the plastic zone size. When the bond thickness is less than h,_m, the plastic zone cannot fully develop due to the restriction imposed by the substrates. When the adhesive thickness becomes larger than /?,„„ the plastic zone is no longer constrained by the substrates and hence fracture energy should plateau. The maximum G,_c is reached when the adhesive layer thickness and the plastic zone diameter 2 r_ye are approximately equal. It was shown that the adhesive thickness h_jm at the maximum G_]c value can be expressed by:

[0089] Under DCB measurement conditions featuring small adhesive thickness, the plastic zone is often smaller than the zone in a CT sample. Therefore the adhesive toughness is often smaller than a value measured by CT, unless the DCB plastic zone matches the size of the CT zone. This explains the enhancement in adhesive toughness with increase in the adhesive thickness in Figure 10a.

[0090] We calculated the deformation zone size of the 5 vol% epoxy/rubber nanocomposite cured by J400, as shown in Table 1. The actual size of the deformation zone shown in Figure 9a was measured as 21 * 10*0.6 mm corresponding to a volume of 126 mm³, which sits between the two volume numbers calculated from plane stress and plane strain in Table 1 , respectively. It is noteworthy that the actual deformation zone may be a few times larger than the measured 126 mm³, since we observed in the measurement the adhesive deforming well beyond the thickness range during the wedge-like propagation along the loading direction. This indicates that the superior toughening effect of rubber nanoparticles can be transferred into adhesive toughness as illustrated in Figure 13.

[0091 ] Table 1 Deformation zone size calculated from plane stress and plane strain, respectively

Epoxy/rubber nanocomposites, 5 vol%, Calculated from plane Calculated from plane strain cured by J400 _stress

Diameter of deformation zone, mm 8.09 2.70 Volume of deformation zone, mm' 2216.7 82.4

[0092] Lap-shear performance of nanocomposite-based adhesives

[0093 ] Adhesive toughness represents the energy that the adhesive can absorb to prevent a sharp crack propagating, which relates to its service life. The lap-shear testing of adhesives measures the shear strength of adhesive prior to service, demonstrating how strong a new adhesive bond is. In Figure 14, both nanocomposites show markedly improved strength, since nanoparticles may promote matrix deformation to absorb more shearing energy. In comparison with neat resins, 49% and 95% increases in shear strength are observed for the J230-cured and J40(⁾-cured system, respectively. The difference is caused by matrix ductility— a ductile matrix more readily facilitates the nanoparticies to absorb shearing energy than a brittle matrix.

[0094 ] We investigated the effect of particle size, matrix ductility and adhesive thickness on the toughening effect of elastomer particles. Microparticles can toughen adhesives moderately, which can be promoted by a ductile matrix and sufficient adhesive thickness. Since nanoparticies have shorter surface- surface distance and far more total interface area than microparticles, nanoparticies can produce far more toughened effect. However, this effect needs to be activated by appropriate matrix ductility and adhesive thickness. For 5.0 vol% J40()-cured adhesive, we recorded improvements of 4908% in adhesive toughness and 95% in adhesive bond shear strength. The nanoparticle toughening effect on bulk resins can be managed to completely transfer to thin adhesives. Upon loading, thin adhesive produces a higher degree of stress concentration in the most volume of adhesives rather than just around the crack tip, due to the constraining effect by adherends. New fracture phenomenon— the cross-like cracks— was observed, which may absorb more energy than conventional 2-dimensional cracks.

[0095] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

[0096] Throughout this specification the word "comprise", or variations such as "comprises" or

"comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0097] All publications mentioned in this specification arc herein incorporated by reference. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia or elsewhere before the priority date of each claim of this application. REFERENCES

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Claims

1. A nanocomposite epoxy adhesive film comprising an epoxy resin and less than 10 vol% elastomer nanoparticles, wherein the nanocomposite epoxy adhesive has a Young's modulus of from about 1 GPa to about 3.4 GPa when measured at 0.6mm adhesive thickness.

2. A nanocomposite epoxy adhesive film comprising an epoxy resin and less than 10 vol% elastomer nanoparticles, wherein the nanocomposite epoxy adhesive has a toughness measured by Gi_c of greater than about 3.8 kj/m².

3. A nanocomposite epoxy adhesive film comprising an epoxy resin and elastomer nanoparticles, wherein the adhesive toughness of the nanocomposite epoxy adhesive is greater than about 130% higher than the adhesive toughness of an equivalent sample of an epoxy adhesive that does not contain the elastomer nanoparticles.

4. A nanocomposite epoxy adhesive film comprising an epoxy resin and elastomer nanoparticles, wherein the shear strength of the nanocomposite epoxy adhesive is at least about 49% higher than the shear strength of an equivalent sample of an epoxy adhesive that does not contain the elastomer nanoparticles.

5. The nanocomposite epoxy adhesive film according to any one of the preceding claims, wherein the nanocomposite epoxy adhesive film has a thickness of about 0.6 mm.

6. The nanocomposite epoxy adhesive film according to any one of the preceding claims, wherein the nanocomposite epoxy adhesive film is formed by curing an adhesive composition comprising at least one diglycidyl ether of a polyhydric phenol compound and elastomer

nanoparticles.

7. The nanocomposite epoxy adhesive film according to any one of the preceding claims, wherein the elastomer nanoparticles are rubber nanoparticles.

8. The nanocomposite epoxy adhesive film according to claim 7, wherein the rubber

nanoparticles are about 55nm diameter particles.

9. The nanocomposite epoxy adhesive film according to either claim 7 or claim 8, wherein the rubber nanoparticles are present in the adhesive composition in an amount of about 5 vol%.

10. The nanocomposite epoxy adhesive film according to any one of the preceding claims, wherein the adhesive composition is cured with a polyetheramine hardener.

1 1. The nanocomposite epoxy adhesive film according to claim 10, wherein the polyetheramine hardener is a 0,<9'-bis(2-aminopropyl)polypropylene glycol polymer having a molecular weight of about 400.

12. An adhesive composition comprising an epoxy resin, 2-10 vol% elastomer nanoparticles having an average diameter of 40-70 nm and a hardener that can be thermally activated to cure the epoxy resin.

13. The adhesive composition according to claim 12, wherein the epoxy resin is a diglycidyl ether of a polyhydric phenol compound.

14. The adhesive composition according to either claim 12 or claim 13, wherein the elastomer nanoparticles are rubber nanoparticles.

15. The adhesive composition according to claim 14, wherein the rubber nanoparticles are about 55nm diameter particles.

16. The adhesive composition according to either claim 14 or claim 15, wherein the rubber nanoparticles are present in the adhesive composition in an amount of about 5 vol%.

17. The adhesive composition according to any one of claims 12 to 16, wherein the hardener is a polyetheramine hardener.

18. The adhesive composition according to claim 17, wherein the hardener is a 0,0'-bis(2- aminopropyl)polypropylene glycol polymer having a molecular weight of about 400.

19. A process for producing an epoxy adhesive, the process comprising mixing an epoxy resin with a calculated amount of 2- 10 vol% of elastomer nanoparticles having an average diameter of 40- 70 nm to form an epoxy resin-nanoparticle mixture and contacting the mixture with a hardener under conditions to cure the epoxy resin.

20. A process for adhering two surfaces, the process comprising applying a film of an adhesive composition comprising an epoxy resin, 2-10 vol% elastomer nanoparticles having an average diameter of 40-70 nm, and a hardener to at least one of the surfaces, contacting the two surfaces with the adhesive composition, and curing the adhesive to form an adhesive film in contact with the surfaces.

21. The process according to either claim 19 or claim 20, wherein the curing is carried out at a temperature of 120°C for a time period of 12 hours.