[0001] This invention relates to an adhesive suitable for, but not limited to, use in repairing aircraft and aircraft parts. The invention further relates to a method of manufacture of the adhesive.

BACKGROUND TO THE INVENTION

[0002] Thermoset plastic materials such as, but not limited to, epoxy and acrylic resins, acrylonitrile-butadiene-styrene (or ABS), polystyrene, polyethylene, polyvinylchloride (PVC), polycarbonate, cellulose acetate butyrate & polyurethane, tend to be repaired with multi-part adhesives falling into the broad category of structural or engineering adhesives. Such multipart adhesives harden by mixing two or more components which, individually, are not adhesive in nature, but when come into contact with one another, chemically react, via a cross-link polymerisation reaction, to crosslink the polymer units or chains into thermoset plastics. The hardening process via the cross-link reaction is known as curing.

[0003] Several combinations of multi-component adhesives are commercially available, including, for example, the following two-part systems:

• Polyester resin - polyurethane resin

• Polyols - polyurethane resin

• Acrylic polymers - polyurethane resin

• Acrylic polymers - acrylic resin

[0004] Such two part adhesive systems have the advantage over traditional glues in that they are fast curing at room temperature, they provide high strength, are easy to use, require minimal surface preparation and can be used to repair or bond a variety of different materials. [0005] Aircraft interior plastic components tend to be fabricated from thermoset plastics such as epoxy and acrylic resins. Whilst the above two- part adhesives are able to adequately adhere and provide the necessary strength in the repair of such thermoset plastics, they do not conform to the required flammability requirements as imposed by most civil aviation authorities. Furthermore, whilst the cross-linking reaction during the curing process tends to produce adhesives with a 3-dimensional network of bonds thereby resulting in a material of high strength, such materials tend to be brittle due to the rigid bonding structure between the cross-linked polymer units or chains.

[0005] The aviation industry is highly regulated to promote safe aviation, protecting passengers, pilots, flight attendants and the general public from risk associated with aviation activities. In particular, the design of aircraft and aircraft components are regulated and as an example, Part 25 of the US Federal Aviation Regulations (FARs) specify the airworthiness standards of Transport Category Airplanes. These regulations set down the standards required to be met in the maintenance and repair of aircraft internal components including, but not limited to, instrument panel facings and interior window, door and seat trim panels. In particular, these regulations require minimum flammability requirements of all aircraft components and parts, including materials used to repair interior components, such as adhesives.

[0006] Given the stringent regulatory requirements for the repair of the internal components of aircraft, damaged components tend to be replaced rather than repaired. Exceptions to this are the use of epoxy and other conventional adhesives limited to temporary or very small repairs. For larger or permanent repairs, however, the damaged part must be replaced which is both time consuming and expensive. SUMMARY OF THE INVENTION

[0007] In one aspect, the present invention provides a two-part adhesive, comprising a first part and a second part, the first part comprising a polymeric resin base composition comprising a polymer and, optionally, an elastomer, a flame-retardant composition, and alumina hydrate, the second part comprising a curing agent wherein the two-part adhesive further comprises multi-walled carbon nanotubes added to either the first part or the second part and wherein the first part and the second part are mixed prior to use.

[0008] The term "flame-retardant composition" will be understood to mean within the context of the invention a composition that when added to a polymeric material, effectively blankets or "chokes" the combustion process. One possible mode of action is via intumescence where the flame retardant material foams on exposure to the flame. In this mode, a carbon foam layer forms that protects the polymeric material through its heat-insulating effect and reduces oxygen access and hence combustion.

[0010] According to embodiment of the invention, the carbon nanotubes may be about 5μιη to about 15μητι in length and about 4nm to about 12 nm in diameter.

[0011] Furthermore, the carbon nanotubes may be added in an amount of 2 g/500 ml of curing agent or 2 g/500 g of polymeric resin base composition.

[0012] In one embodiment, the carbon nanotubes may be added to the curing agent in the second part by ultrasonic dispersion at a range of about 24khz to about 40khz.

[0013] In an embodiment, the carbon nanotubes are added to the curing agent in the second part by ultrasonic dispersion at about 30 khz.

[0014] The ultrasonic dispersion may be carried out for at least about 15 minutes to about 75 minutes. [0015] In an embodiment, the ultrasonic dispersion is carried out for at least about 70 minutes.

[0016] It will be appreciated that the use of ultrasound to achieve the mixing of the multi-walled carbon nanotubes within the curing agent generates alternating low-pressure and high-pressure waves in the liquid curing agent, leading to the formation and violent collapse of small vacuum bubbles known as cavitation. This cavitation phenomenon results in strong hydrodynamic shear-forces and high speed movement of the liquid within the mix and hence results in high rates of dispersion of the multi-walled carbon nanotubes within the liquid curing agent.

[0017] In another embodiment, the carbon nanotubes may be added to the polymeric resin base in the first part by high energy ball milling at a range of about 100 rpm to about 190 rpm.

[0018] In an embodiment, the high energy ball milling may be conducted at about 145 rpm.

[0019] It will be appreciated that through the use of high energy ball milling, high shear rates are desirably achieved within the polymeric resin base and multi-walled carbon nanotube mix. It will also be appreciated that mixing via high energy ball milling is in accordance with one embodiment of the invention. In this embodiment, mixing times of at least about 5 minutes have been found to be sufficient, however, depending on the method of mixing, longer times may be required. In this regard, mixing of the polymeric resin base and multi-walled carbon nanotubes may be achieved by any other conventional mixing methods such as manual mixing or mixing with a stirrer, and longer mixing times may be required using such techniques compared to high energy ball milling.

[0020] In an embodiment, the first part and the second part of the two- part adhesive are mixed in a 1 :1 ratio.

[0021] It will be appreciated, however, that other ratios in which the first part and the second part are mixed are possible. In an embodiment, the ratio of the first part to the second part may be about a 2:1 . In another embodiment, the ratio of the first part to the second part is about 1 :2.

[0022] In an embodiment, the polymeric resin base composition comprises no less than about 75%(w/w) of the polymeric base and no more than about 15%(w/w) of the elastomer.

[0023] The alumina hydrate may be added to the first part in a ratio of about 1 :10 (alumina hydrate : polymeric resin base composition and flame- retardant composition).

[0024] In an embodiment, the polymeric resin component is an acrylic.

[0025] In an embodiment, the acrylic is polymethyl methacrylate.

[0027] In an embodiment, the elastomer is a dialkyi phthalate. The dialkyi phthalate may be selected from any one dimethyl phthalate, diethyl phthalate, dipropyl phthalate.

[0028] In an embodiment, the curing agent is an acrylic monomer.

[0030] In an embodiment, the acrylic monomer is ethyl methacrylate.

[0031] In an embodiment, the flame-retardant composition may comprise a potassium salt. The potassium salt may be selected from any one of, but not limited to, KCI, KF, KBr or K ₂ C0 ₃ .

[0032] In another embodiment, the flame-retardant composition may comprise diphenylacetylene.

[0033] In yet another embodiment, the flame-retardant composition may comprise a polyphosphonate.

[0034] In yet another embodiment, the flame-retardant composition comprises b/s-hydroxy deoxybenzoin. [0035] In an embodiment, the flame-retardant composition is added to the first part at a ratio of about 2:1 (polymeric resin base composition : flame- retardant composition).

[0036] In an embodiment, the fire retardant composition comprises: about 20%(w/w) of the potassium salt; about 15% (w/w) diphenyl acetylene; about 40% (w/w) the polyphosphonate; and about 25% (w/w) b/s-hydroxy deoxybenzoin.

[0037] It will be appreciated that the flame-retardant composition in accordance with the invention desirably comprises only non-halogenated fire retardants. In this regard, the use of flame-retardant additives provide a convenient means for reducing flammability of some materials, but these additives may compromise material properties due environmental problems associated with toxicity, bioaccumulation, and recycling. In particular, halogenated flame retardants such as polybrominated biphenyl (PBBs) have been banned throughout various parts of the world and other halogenated flame retardants have likewise been identified as environmental hazards and may face future legislative restrictions. The use of non-halogenated polymers that decompose primarily to water and carbon dioxide upon heating are desired as the decomposition effectively precludes the combustion process.

[0009] In a second aspect, the invention also provides a method of formulating a two-part adhesive comprising:

(a) preparing a first part of the two-part adhesive comprising mixing a polymeric resin base composition, a flame retardant composition and alumina hydrate; and

(b) preparing a second part of the two-part adhesive comprising uitrasonically dispersing multi-walled carbon nanotubes within a curing agent, wherein the ultrasound is conducted at about 24khz to about 40khz for at least about 15 minutes; and

(c) mixing the first part and the second part to produce the two-part adhesive.

[0039] In an embodiment, mixing of the polymer, the flame retardant composition and the alumina hydrate of the first part occurs by high energy ball milling. In this embodiment, the mixing occurs at about 145 rpm for at least about 15 minutes.

[0040] In a third aspect, the invention provides a method of formulating a two-part adhesive comprising:

(a) preparing a first part of the two-part adhesive comprising mixing a polymeric resin base composition, multi-walled carbon nanotubes, a flame retardant composition and alumina hydrate; and

(b) mixing the first part and the second part comprising a curing agent to produce the two-part adhesive.

[0041] In an embodiment, mixing of the polymer, the multi-walled carbon nanotubes, the flame retardant composition and the alumina hydrate of the first part occurs with high energy ball milling. In this embodiment, the mixing occurs at about 145 rpm for at least about 15 minutes.

[0042] In a fourth aspect, the invention provides a method of repairing an object comprising:

(a) preparing a first part of a two-part adhesive in accordance with the second aspect of invention;

(b) preparing a second part of a two-part adhesive in accordance with the second aspect of the invention;

(c) preparing an adhesion promoter solution;

(d) applying the adhesion promoter solution to the surface areas of the object to be repaired prior to application of the mixed two-part adhesive; mixing the first part and the second part to produce the two-part adhesive immediately prior to its use;

applying the mixed two-part adhesive to the surface areas of the object to be repaired; and

allowing the two-part adhesive to cure.

[0010] In a fifth aspect, the invention provides a method of repairing an object comprising:

(a) preparing a first part of a two-part adhesive according to the third aspect;

(b) preparing an adhesion promoter solution;

(c) applying the adhesion promoter solution to the surface areas of the object to be repaired prior to application of the mixed two-part adhesive;

(d) mixing the first part and a second part comprising a curing agent to produce the two-part adhesive immediately prior to its use;

(e) applying the mixed two-part adhesive to the surface areas of the object to be repaired; and

(f) allowing the two-part adhesive to cure.

[0044] In an embodiment, the adhesion promoter solution is a blend of about 80% (v/v) tetra hydrofluoride and about 20% (v/v) methyl ethylketone. Whilst this is one possible adhesion promoter solution, it will be appreciated that there are many promoter solutions that used in accordance with the present invention. Examples of some other promoter solutions suitable for use in the invention include, but are not limited to, acetone, hydrochloric acid or nitric acid.

[0045] In an embodiment, the method of repairing an object according the fourth or fifth aspects of the invention further includes the step of grinding any damaged areas of the object to be repaired, prior to application of the adhesion promoter solution. [0046] In a sixth aspect, the invention further provides an object repaired using the two-part adhesive as herein described according to the first aspect of the invention and embodiments.

[0011] It should be noted that any one of the aspects mentioned above may include any of the features of any of the other aspects mentioned above and may include any of the features of any of the embodiments described throughout as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate various embodiments of the invention.

[0013] Figure 1 is a flowchart representing the steps in manufacturing the adhesive according to one embodiment of the invention.

[0050] Figure 2 is a flowchart representing the steps in manufacturing the adhesive according to an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The following detailed description of the invention refers to the accompanying drawings in which two different embodiments of the invention are illustrated in figures 1 and 2.

[0052] Referring to figure 1 , the manufacture of the adhesive in accordance with an embodiment of the invention occurs in two parts. In Part I, the multi-walled carbon nanotubes are added to the curing agent, ethyl methacrylate, in an amount of 2 g/500 ml of curing agent. In order to disperse of the multi-walled carbon nanotubes within the ethyl methacrylate, the mixture is subjected to 70 minutes of ultrasound at 40khz. This forms the first part of the adhesive.

[0053] Still referring to figure 1 , Part II of the process involves mixing a polymeric resin base composition comprising 85% (w/w) polymethyl methcrylate and 15% (w/w) dialkyl phthalate with a flame retardant composition comprising 20% (w/w) potassium salt, 15% (w/w) diphenylacetylene, 40% (w/w) polyphosphonate and 25% (w/w) bis hydroxyl deoxybenzoin. The polymeric resin base composition and flame- retardant composition are mixed in a 2:1 ratio (polymeric resin base composition : flame-retardant composition) and to this mixture is added alumina hydrate powder in a 10:1 ratio (polymeric resin base composition/flame-retardant composition : alumina hydrate). Mixing of these components is achieved by high energy ball mixing at 145 rpm for 20 minutes. This forms the second part of the adhesive.

[0054] The first and second part are then mixed in a 1 :1 ratio to form an adhesive paste which may be used to repair cracked or otherwise damaged objects.

[0055] Referring to figure 2, the manufacture of the adhesive in accordance with an alternative embodiment of the invention is illustrated wherein the multi-walled carbon nanotubes are added to the second part instead of the first part as described in the previous embodiment. In this embodiment, Part I comprises ethyl methacrylate as the curing agent.

[0056] Still referring to figure 2, Part II of the process involves mixing a polymeric resin base composition comprising 85% (w/w) polymethyl methcrylate and 15% (w/w) dialkyl phthalate with multi-walled carbon nanotubes in an amount of 2 g/ 500 g of polymeric resin base composition. To this is added a flame retardant composition comprising 20% (w/w) potassium salt, 15% (w/w) diphenylacetylene, 40% (w/w) polyphosphonate and 25% (w/w) bis hydroxyl deoxybenzoin. The polymeric resin base composition and flame-retardant composition are mixed in a 2:1 ratio (polymeric resin base composition : flame-retardant composition) and to this mixture is added alumina hydrate powder in a 10:1 ratio (polymeric resin base composition/flame-retardant composition : alumina hydrate). Mixing of these components is achieved by high energy ball mixing at 145 rpm for 20 minutes. This forms the second part of the adhesive. [0057] The first and second part are then mixed in a 1 :1 ratio to form an adhesive paste which may be used to repair cracked or otherwise damaged objects.

[0058] The following example ^! describes, more specifically, how the adhesive is manufactured in accordance with an embodiment of the invention.

Example

Preparation of Polymeric Resin Base composition

[0059] 100 grams of polymethyl methacrylate (PMMA) powder was dispensed into clean a container to which 50 grams of Exploit AP740 (Clariant) (a non-halogenated flame retardant based on ammonium polyphosphate) was added. To this composition, 5 grams of alumina hydrate was then added.

[0060] The polymeric resin base composition was mixed for at least 15 minutes with high energy ball mixing. After mixing the composition was ready to dispense into smaller containers.

Preparation of Curing agent

[0061] 1 litre of ethyl methacrylate (EMA) liquid was dispensed into an ultrasonic tank. To this, 4 grams of multi-walled carbon nanotubes (MWCNT) were added and it was noted that no dispersion of the multi- walled carbon nanotubes occurred prior to ultra-sonication. Sonification was activated for a minimum of two 7 minute cycles, and on the third cycle, the EMA-MWCNT solution was dispensed into a small dispensing bottle while the solution was still active.

Testing

[0062] After the products were mixed, the adhesive was applied to various specimens and tested according to the US Federal Aviation Regulations (FAR) Part 25 Appendix F Part 1 (a) (5) - Horizontal Test (Table 1 ) and FAR Section 25.853 - 60 second ignition (Table 2). These results show all specimens tested were found to be compliant in terms of the requirements of the US Federal Aviation Regulations.

Table 1 : Results of FAR Part 25 Appendix F Part 1 (a)(5) - Horizontal burn test

Type of Burn: SE - self extinguishing

Table 2: Results of FAR Section 25.853 - 60 second ignition test

Sample Average* Burn Average* Average* Type of Result Description Length (mm) Flame Time Drip flame Burn**

(seconds) time

(seconds)

Plastic/fibreglass 132 0 0 SE Compliant patch repair on

1mm plastic

Plastic patch repair 103 0 0 SE Compliant on cement board

Plastic/fibreglass 37 0 0 SE Compliant patch repair on

cement board

Plastic repair on 98 0 0 SE Compliant 3mm ridged board

Plastic/fiberglass 90 0 0 SE Compliant patch repair on

3mm ridged board

*The average resu t of three test specimens

^** Type of Burn - self extinguishing

[0015] 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 before the priority date of each claim of this application.

[0016] 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, step or group of elements, integers or steps.

[0017] 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.