Field of the Invention

The present invention relates to methods and compositions for repairing, bonding or joining thermoplastic articles, and in particular, but not exclusively, the invention relates to methods and compositions for repairing, bonding or joining acrylic-based thermoplastic articles.

Background

The use of thermoplastic composites is becoming increasingly popular due to growing importance of recyclability and re-use. As a result, there is an increasing trend to replace non-recyclable thermosetting composites with recyclable thermoplastic composites. Some liquid thermoplastic resins allow the manufacture of thermoplastic composites, for example via liquid resin infusion. To enhance the recycling and re-using potential as well as to enhance the service life, joining and repairing are crucial for thermoplastic composites. This applies to a myriad of composite articles, including for example construction materials, aeronautic components, renewal energy parts such as turbine blades, etc.

Thermosetting composites are known for their high mechanical strength and thermal resistance in comparison to thermoplastic composites, which is advantageous in many applications that requires a high strength-to-weight ratio ([1] John D. Muzzy, Ancil O. Kays, Thermoplastic vs. thermosetting structural composites, Volume 5, Issue 3, 1984, Pages 169-172 https://doi.Org/10.1002/pc.750Q50302 ). However, as these resins contain covalent bonds, they tend to degrade instead of melting upon heating (Yang, Y., Boom, R., Irion, B., van Heerden, D., Kuiper, P. and de Wit, H. (2012). Recycling of composite materials. Chemical Engineering and Processing: Process Intensification, 51 , pp.53-68).

The absence of cross-linking in thermoplastic composites allows thermoplastics to be melted and remoulded without degrading, hence leading to better recyclability (Cousins, D (2018). Advanced Thermoplastic Composites for Wind Turbine Blade Manufacturing. Colorado School of Mines. [Thesis] pp.21 ). Increased recyclability, along with the superior impact resistance, and potentially faster manufacturing of thermoplastics are major driving factors for the replacement of non-recyclable thermosetting composites. The repair of composite materials includes various methods, traditionally involving mechanical fastening, which restores strength but usually at the cost of adding stress concentrations around the drilled holes, adding weight, and reducing aerodynamic quality. A lighter alternative to this method is bonded patch repairs, which involves a patch of composite material bonded to the surface of the area to be repaired. However, this method is not without limitations; extensive surface preparation is required yet the composite patch only repairs surface damage, therefore any internal cracks can remain after bonding. Other approaches include welding/melting techniques which all involve the application of heat and are therefore cumbersome, costly and hazardous, and solvent-based dissolution which involves the undesirable use of organic solvents.

Gebhardt et al, Reducing the raw material usage for room temperature infusible and polymerisable thermoplastic CFRPs through reuse of recycled waste matrix material, Composites Part B, 2021 , 216, 108877, discloses a method for recycling a thermoplastic composite based on Elium® resin. The matrix polymer is recovered from waste recyclate and is added as granules to virgin Elium® monomer in order to reduce the amount of virgin Elium® material required to manufacture an Elium-based thermoplastic resin. However, the amount of Elium polymer recyclate in the virgin monomer is limited to 7.5 wt% as the amount of waste recyclate in the virgin monomer is reported to significantly affect the viscosity of the composition.

WO 2020/163975 A1 discloses a light curable (meth)acrylate resin composition for thermoplastic elastomers bonding.

GB 1315500 A discloses forming a polymerised layer of an ethylenically unsaturated non-linear polyester dissolved in a styrene monomer, reinforced with glass fibres, on a thermoplastic resin surface.

GB 1 512 727 A discloses joining polymer articles disposing between surfaces thereof inserts formed of thermoplastic material having dispersed therein a heat- activatable cross-linking agent, and heating the surfaces and insert means under compression.

DE 102012207468 A1 discloses a method for repairing a damage area of a fiber composite material, comprising injecting a monomer of a thermoplastic matrix in the damaged area of a component, and heating the damaged area.

WO 2005/017005 A1 discloses a method to join a substrate to an object using a curable one or two part adhesive composition comprising an effective amount of a stabilized organoborane amine complex initiator and one or more monomers, oligomers, polymers or mixtures thereof having olefinic unsaturation which is capable of polymerization by free radical polymerization.

CN 112728283 A discloses a joint coating method for the anti-corrosion pipeline comprising using a mixture comprising PE or PR and an EVA monomer-olefin copolymer.

WO 2020/247549 A1 discloses a liquid, thermoplastic acrylic gel coat composition.

CN 1 10055001 A discloses a protective lacquer comprising an acrylic resin, an acrylic monomer, a photoinitiator and a solvent.

It is an object of the invention to address and/or mitigate one or more problems associated with the prior art.

Summary

The present invention is based upon the finding that it is possible to form a molecularly bound interface between the surface of a thermoplastic polymer article and a layer of material derived from a monomer being a constituent of the thermoplastic polymer. This finding led to the discovery of novel methods useful in joining and/or repairing portions of thermoplastic articles, such as reinforced composites, and/or methods useful in preparing multi-step thermoplastic multi-layer articles.

According to a first aspect, there is provided a method comprising: providing a thermoplastic polymer composite article comprising a thermoplastic polymer, wherein the thermoplastic polymer is an addition polymer derived from at least one first addition monomer; contacting a portion of the article with a solvent-free composition comprising at least one polymerisable compound, the at least one polymerisable compound comprising at least one second addition monomer; and polymerising the composition at ambient temperature, wherein the at least one polymerisable compound does not include a polymer.

Advantageously, the composition, e.g. the at least one polymerisable compound, may consist of or may consist essentially of one or more monomers comprising the at least one second addition monomer. In other words, the composition may not comprise a solvent. Thus, the one or more monomers comprising the at least one second addition monomer may act as a reactive diluent for the thermoplastic polymer. However, it will be understood that the composition may comprise one or more components required for the composition to undergo polymerisation, such as one or more initiators (e.g. peroxide), accelerators, catalysts, or the like. Typically, the composition may be free of or may not comprise a photoinitiator.

The method may not comprise applying an external source of light, e.g. UV light. It will be understood that not applying an external source of light, e.g. UV light, may not exclude exposure to natural light, e.g. sunlight.

The composition, e.g. the at least one polymerisable compound, may not comprise a polymer such as an unsaturated polymer or curable resin. Thus, the at least one second addition monomer may be the only polymerisable component(s) in the composition. The composition may comprise (the) at least one polymerisable compound consisting of the at least one second addition monomer.

The term “monomer” will be herein understood as encompassing both a true monomer and also small oligomers thereof, e.g. up to about 6, e.g. up to about 5 units.

Thus, the phrase “the at least one polymerisable compound does not include a polymer” may be construed to mean that the at least one polymerisable compound does not include either true polymers (e.g. having a number of repeat units greater than about 10) or oligomers greater than about 5 unit, e.g. greater than about 6 units.

In an embodiment, the monomer, e.g. the at least one second addition monomer, may be a true monomer.

Typically, the composition may be provided, e.g. applied, in liquid form.

Without wishing to be bound by theory, it is believed that the provision of a liquid composition including at least one addition monomer which either forms a constituent part or has a similar chemical structure to the building blocks of the addition polymer of the thermoplastic article, allows the composition to penetrate at least to some extent within the matrix of the polymer at or near a region of the polymer at the interface with the composition being applied. By such provision, upon polymerisation of the composition, the newly polymerised region, e.g. layer, forms an integral and/or molecular connection with the thermoplastic article, thus creating a much stronger interface than when using conventional methods such as melting, welding, or adhesive bonding, whilst also avoiding the use of organic solvents. The region at the interface between the thermoplastic polymer article and the polymerised composition may form or may be termed a semi-interpenetrating polymer network (“semi-IPN”). This approach fundamentally differs from the use of crosslinkable or curable polymeric resins, for example adhesives.

At least one of the at least one first addition monomer, and at least one of the at least one second addition monomer, may be the same. The at least one first addition monomer and the at least one second addition monomer may be the same.

Alternatively, at least one of the at least one first addition monomer, and at least one of the at least one second addition monomer, may be different. The at least one first addition monomer and the at least one second addition monomer may be different.

It will be understood that, so long as at least one monomer of the composition has a chemical structure which either identical to or similar to at least one monomer constituent of the addition polymer, then the composition is able to penetrate at least to some extent within the matrix of the thermoplastic polymer at or near a region of the polymer at the interface with the composition being applied.

The article may comprise a thermoplastic polymer composite, e.g. laminate.

The article may comprise a reinforced thermoplastic polymer composite, e.g. laminate.

The article may comprise a fibre-reinforced thermoplastic polymer composite, e.g. laminate.

Tthe method is carried out at ambient temperature. For example, method may be carried out at about 10-30 °C, e.g. 20-25 ^q C.

The method may not involve the application of heat.

The method may not involve the application of pressure.

Preferably, the method may be carried out at a temperature below the melting point of the thermoplastic polymer.

The method may comprise applying the composition on the portion of the article.

The method may comprise infusing the portion of the article with the composition, e.g. at room temperature or ambient temperature.

The method may comprise infusing the portion of the article with the composition under a vacuum. The method may comprise infusing the portion of the article with the composition using a Resin Transfer Moulding (RTM) tool or a Vacuum Assisted Resin Transfer Moulding (VARTM) tool.

The method may comprise providing a reinforcing element within the composition. The reinforcing element may comprise short and/or discontinuous fibre, or may comprise and long and/or continuous fibres. The reinforcing element may be a fibrous reinforcing element such as a one or more fibrous mats, weaves, braids, knits, sheets, threads, rods, or the like. Thus, the method may comprise forming a thermoplastic composite structure.

The fibrous reinforcing element may comprise or may be made from glass, carbon, metal, polymer (e.g. aramid), natural fibre or the like.

The thermoplastic polymer composite article may be a laminate.

The thermoplastic polymer article may further comprise one or more additives, such as fillers, stabilisers, lubricants, etc.

The thermoplastic polymer may comprise or may be a homopolymer derived from a first addition monomer. In such instance at least one of the at least one second addition monomer may be either identical, or similar to, the first addition monomer. For example, the composition may comprise the first addition monomer.

The thermoplastic polymer may comprise or may be a copolymer derived from at least two or more first addition monomers. In such instance at least one of the at least one second addition monomer may be either identical, or similar to, at least one of the two or more first addition monomers. For example, the composition may comprise one, or both, of the two or more first addition monomers.

The thermoplastic polymer may be a (meth)acrylic polymer. The (meth)acrylic polymer may be derived from at least one (meth)acrylic monomer. The composition may comprise at least one (meth)acrylic monomer, e.g. may comprise the at least one (meth)acrylic monomer.

The term (meth)acrylic will be understood as referring to any kind acrylic or methacrylic monomer. For example, the (meth)acrylic polymer may be a homo- or copolymer derived from acrylic acid, an acrylic ester (e.g. an alkyl acrylate), methacrylic acid, a methacrylic ester (e.g. an alkyl methacrylate), or combinations thereof.

The method described herein may be useful in repairing a thermoplastic polymer article. Thus, the method may comprise: providing a thermoplastic polymer composite article comprising a thermoplastic polymer, the thermoplastic polymer article including a damaged portion, wherein the thermoplastic polymer is an addition polymer derived from at least one first addition monomer; contacting the damaged portion of the article with a solvent-free composition comprising at least one polymerisable compound, the at least one polymerisable compound comprising at least one second addition monomer; and polymerising the composition at ambient temperature, wherein the at least one polymerisable compound does not include a polymer.

The damaged portion may include a fracture, a delamination, a cut, or the like.

The method described herein may be useful in joining two separate thermoplastic polymer composite articles. Thus, the method may comprise: providing a first thermoplastic polymer composite article comprising a first thermoplastic polymer, wherein the first thermoplastic polymer is a first addition polymer derived from at least one first addition monomer; providing a second thermoplastic polymer composite article comprising a second thermoplastic polymer, wherein the second thermoplastic polymer is a second addition polymer derived from at least one second addition monomer; providing a solvent-free composition comprising at least one polymerisable compound, the at least one polymerisable compound comprising at least one third addition monomer between the first thermoplastic polymer composite article and the second thermoplastic polymer composite article; and polymerising the composition at ambient temperature, wherein the at least one polymerisable compound does not include a polymer.

At least one of the at least one third addition monomer may be the same as, or similar to, at least one of the at least one first addition monomer and/or at least one of the at least one second addition monomer.

It will be understood that, so long as at least one monomer of the composition has a chemical structure which either identical to or similar to at least one monomer constituent of the first and/or second addition polymer, preferably of the first and second addition polymers, then the composition is able to penetrate at least to some extent within the matrix of the thermoplastic polymer(s) at or near a region of the polymer(s) at the interface with the composition being applied

Typically, the first thermoplastic polymer and the second thermoplastic polymer may be the same. In such instance, the at least one first addition monomer and the at least one second addition monomer may be the same.

Alternatively, the first thermoplastic polymer and the second thermoplastic polymer may be different. In such instance, the at least one first addition monomer and the at least one second addition monomer may be the same or may be different. The at least one first addition monomer and the at least one second addition monomer may be relatively close in chemical structure in order to allow the at least one third addition monomer in the composition to penetrate both the first thermoplastic polymer article and the second thermoplastic polymer article.

The method described herein may be useful in preparing a multilayer thermoplastic article. Thus, the method may comprise: providing a thermoplastic polymer composite article comprising a thermoplastic polymer, wherein the thermoplastic polymer is an addition polymer derived from at least one first addition monomer; contacting a portion, e.g. a first surface, of the article with a solvent-free composition comprising at least one polymerisable compound, the at least one polymerisable compound comprising at least one second addition monomer; and polymerising the composition at ambient temperature, wherein the at least one polymerisable compound does not include a polymer.

By such provision, the polymerised composition may constitute an additional layer of the thermoplastic polymer composite article, e.g. laminate.

Advantageously, upon polymerisation of the composition, the newly polymerised layer forms an integral and/or molecular connection or interface with the thermoplastic article, thus creating a much stronger interface than when using conventional methods such welding or melt-bonding together two already-formed layers.

The method may comprise repeating the above steps to form one or more further layers.

According to a second aspect of the present invention there is provided a method of repairing a thermoplastic polymer composite article, the method comprising: providing a thermoplastic polymer composite article comprising a thermoplastic polymer, the thermoplastic polymer article including a damaged portion, wherein the thermoplastic polymer is an addition polymer derived from at least one first addition monomer; contacting the damaged portion of the article with a solvent-free composition comprising at least one polymerisable compound, the at least one polymerisable compound comprising at least one second addition monomer; and polymerising the composition at ambient temperature, wherein the at least one polymerisable compound does not include a polymer.. The damaged portion may include a fracture, a delamination, a cut, or the like.

According to a third aspect, there is provided a method of joining a plurality of thermoplastic polymer composite articles, the method comprising: providing a first thermoplastic polymer composite article comprising a first thermoplastic polymer, wherein the first thermoplastic polymer is a first addition polymer derived from at least one first addition monomer; providing a second thermoplastic polymer composite article comprising a second thermoplastic polymer, wherein the second thermoplastic polymer is a second addition polymer derived from at least one second addition monomer; providing a solvent-free composition comprising at least one polymerisable compound, the at least one polymerisable compound comprising at least one third addition monomer between the first thermoplastic polymer composite article and the second thermoplastic polymer composite article; and polymerising the composition at ambient temperature, wherein the at least one polymerisable compound does not include a polymer.

Typically, the first addition polymer and the second addition polymer may be the same. In such instance, the at least one first addition monomer and the at least one second addition monomer may be the same.

Alternatively, the first addition polymer and the second addition polymer may be different. In such instance, the at least one first addition monomer and the at least one second addition monomer may be the same or may be different. The at least one first addition monomer and the at least one second addition monomer may be relatively close in chemical structure in order to allow the at least one third addition monomer in the composition to penetrate both the first thermoplastic polymer article and the second thermoplastic polymer article.

According to a fourth aspect there is provided a method of preparing a multilayer thermoplastic composite article, the method comprising: providing a thermoplastic polymer composite article including a first layer comprising a thermoplastic polymer, wherein the thermoplastic polymer is an addition polymer derived from at least one first addition monomer; contacting a portion, e.g. a first surface, of the first layer with a composition comprising at least one polymerisable compound, the at least one polymerisable compound comprising at least one second addition monomer; and polymerising the composition at ambient temperature so as to form a second layer, wherein the at least one polymerisable compound does not include a polymer.

By such provision, the polymerised composition may constitute an additional layer of the thermoplastic polymer composite article, e.g. laminate.

Advantageously, upon polymerisation of the composition, the newly polymerised layer forms an integral and/or molecular connection or interface with the thermoplastic article, thus creating a much stronger interface than when using conventional methods such welding or melt-bonding together two already-formed layers.

The method may comprise repeating the above steps to form one or more further layers.

According to a fifth aspect, there is provided a thermoplastic polymer composite article prepared using a method according to any of the first aspect, second aspect, third aspect or fourth aspect.

The article may comprise a laminate.

The article may comprise a reinforced thermoplastic composite.

The article may comprise a fibre-reinforced thermoplastic composite.

It will be understood that the features described in respect of any aspect may be equally applicable in relation to any other aspect of the invention, and are not repeated merely for brevity.

Brief Description of Drawings

Embodiments of the present disclosure will now be given by way of example only, and with reference to the accompanying drawings, which are:

Figures 1(a), 1(b) schematic drawings showing a conventional manufacturing process for preparing a reference laminate using a one-step infusion;

Figures 2(a) - 2(d) schematic drawings showing a method for preparing a laminate according to an embodiment;

Figures 3 and 4 Graphs showing respectively load-extension curve and transverse tensile strength values of the reference and of the laminate of Fig 2; Figure 5 and 6 Graphs showing respectively load-deflection curve in the SBS test and interlaminar shear strength values of the reference laminate and of the laminate of Fig 2;

Figures 7(a) - 7(c) Schematic drawings illustrating the formation of a semi- IPN at the joining interface in the laminate of Fig 2;

Figure 8 A graph showing tan delta vs temperature for the reference laminate and the laminate of Fig 2;

Figure 9 A graph showing exotherms of multi-stage laminates made using an embodiment of a method of the present invention as shown in Figure 10, and single-stage laminates made using conventional methods;

Figures 10(a)-10(c) Schematic drawings showing a method for preparing a laminate according to another embodiment.

Detailed Description

Example 1

Materials and methodology

Glass fibre reinforced acrylic matrix composites were prepared with eight plies of unidirectional E-glass fabric glass fibre mats (GF) (Ahlstrom-Munksjo), infused with Elium® 188 O resin (Arkema) with a benzoyl peroxide initiator. The laminates were infused at room temperature using a Vacuum Assisted Resin Transfer Moulding (VARTM) tool (Composite Integration).

As depicted in Figures 1(a) and 1(b), a reference laminate 12 was manufactured in a single step by a conventional method involving infusing eight plies of GF mats 10 with the Elium® resin monomer composition in the VARTM tool to produce a 4 mm-thick laminate 12.

Figures 2(a) - 2(b) illustrate a method of manufacturing a laminate 100 according to a first embodiment. This laminate 100 was manufactured in two steps, each step involving an infusion of four plies. The first infusion (illustrated by Figs 2(a) and 2(b)) of GF mats 1 10 with Elium® resin monomer composition produced a 2 mm-thick laminate 1 12. Upon completion of the polymerisation of the Elium® resin, an additional four plies 120 of dry GF reinforcement fabrics were placed in the VARTM tool underneath the previously infused laminate 1 12, as shown in Figure 2(c). A second infusion was then carried out with the Elium® resin monomer composition to wet-out the dry GF reinforcements 120 producing a 4 mm-thick laminate 100 including the 2 mm-thick laminate 120 from the first step and a further 2 mm-thick layer 122 from the second step. Mechanical and thermomechanical characterisation

In order to assess the properties of the laminates 12 and 100, transverse tensile testing was carried out (according to ASTM D3039). The Short Beam Shear test (BS EN ISO 14130) was used to determine the interlaminar shear strength (ILSS). Dynamic Mechanical Analysis (DMA) was carried out in a three-point bending mode, at a heating rate of 3 °C/min, frequency of 1 Hz and amplitude of 50 pm (according to ASTM D4065).

Results and Discussions

The representative load-extension curve and the transverse tensile strengths of the reference and bonded laminates are shown in Figure 3 and 4 respectively. An 11 % higher transverse tensile strength was observed in the bonded laminate 100, with an average value of 55 MPa, compared to 49 MPa for the reference laminate.

Both laminates exhibited non-linear curves, characteristics of amorphous thermoplastic matrix composites. The bonded laminate 100 exhibited a higher failure load at a higher failure strain than the reference laminate.

A representative load-deflection curve for the SBS test is shown in Figure 5. After the peak load, neither laminate showed sudden load drops, which are characteristics of plastic deformation. The bonded laminate 100 exhibited a higher ILSS (Figure 6), with an average value of 52 MPa, compared to an average of 42 MPa for the reference laminate 12.

Both the ILSS and transverse tensile test results have shown enhanced properties in the bonded laminate 100. This is very different to what is generally observed in a bonded thermosetting or a bonded conventional thermoplastic composite.

Without wishing to be bound by theory, this enhancement in properties in the bonded laminate 100 might be attributed to the change in molecular entanglement at the joining plane of the bonded laminate 100 during the two-step manufacturing process illustrate in Figure 2. As the Elium® acrylic composition used is a liquid monomeric mixture, during the second infusion (illustrated in Figures 2(c) and 2(d)), the monomeric composition acts as a reactive diluent for the polymerised amorphous acrylic matrix of the prefabricated laminate 112. It is believed that the low viscosity acrylic monomers diffuse through the surface of the amorphous acrylic polymeric matrix of the prefabricated laminate 112 at the interface (see Figure 7(b)) and polymerise in-situ forming a semi- interpenetrating polymer network (semi-IPN) (see Figure 7(c)). This semi-IPN formation at the interface enhances the strength of the joining layers and this is reflected in the enhanced mechanical properties of the bonded laminate 122, as discussed above.

Figure 8 shows the thermomechanical properties of the two laminates 12,100. The tan delta and glass transition (T _g ) temperatures obtained are similar, with the reference laminate showing a T _g of 107.9 °C occurring at a tan delta of 0.6, and the bonded laminate showing a T _g and tan delta of 103.5 °C and 0.62 respectively. The DMA test showed there was very little difference in the thermomechanical properties between the two laminates, both showing equivalent damping behaviour. Therefore no observed effect on T _g caused by the re-infusion of the bonded laminate 100.

Example 2

In example 2, alternative monomer composition were used, namely:

Composition (1): Elium® 188 O resin monomer;

Composition (2): mixture of Elium® resin monomer and NANOCRYL® A 210 (Evonik Industries); and

Composition (3): methyl methacrylate (MMA).

In relation to Composition (2), NANOCRYL® A 210 is a dispersion of colloidal silica in a difunctional acrylate monomer. The NANOCRYL® A 210 composition was diluted with Elium® resin monomer so that the final monomer composition included 10 wt% nano SiO ₂ particles. Thus, every 100g of the monomer composition consisted of 80g Elium and 20g Nanocryl A 210.

Each monomer composition was provided with a BP-50-FT peroxide initiator shortly before use, at a ratio of 100:3 of composition to BP-50-FT peroxide initiator.

Elium / glass fiber composites were used as substrates.

Using a wet hand lay-up technique, each of compositions (1 ), (2) and (3) above was applied onto the ends of two substrates layers and a 25 x 25 mm square of glass fibre fabric was sandwiched between the layers. Binder clips were used to apply pressure during polymerisation. Effective joining was observed with each of the three compositions. This demonstrates that the monomer composition does not require to be 100% identical to the constituents of the thermoplastic polymer, and also that certain additives, such as fillers, may be used in the composition without adversely affecting the joining process.

Example 3

This example describes the joining of two separate acrylic resin composite articles, according to another embodiment.

Two acrylic composite articles were provided, which consisted of Elium 188 O/glass fibre composites.

Elium® 188 O resin monomer was infused into a region between the two composites.

Glass fibres were sandwiched between the adherends at either 0° (parallel to test axis) or 90° (perpendicular to test axis), and specimens without glass fibres in the joint were also manufactured. The nominal thickness between the adherends was varied between 0.5 mm and 1 .0 mm.

Three joint types were tested, as described below, and their single lap shear strengths were measured as shown in parentheses:

1 . 0.5 mm thick joint, 0° glass fibres (18.2 ± 0.89 MPa)

2. 0.5 mm thick joint, no fibres (13.5 ± 3.3 MPa)

3. 1 .0 mm thick joint, 0° glass fibres (9.1 ± 2.2 MPa)

The highest average strength of 18.2 MPa was obtained with the thinner 0.5 mm joint and 0° glass fibres. Optical and scanning electron microscope analysis of the fracture surfaces show that the specimens fail at the interface between the glass fibres and the acrylic matrix, rather than at the joint between the adherend matrix and infused resin. This highlights the strength of the semi-IPN formed during joining. This strength compares favourably with single lap shear strengths of acrylic composites from the literature as summarised in the table below. The results are preliminary and further optimisation is expected to deliver improvements.

[1 ] Murray, R. E., et al. (2019). "Fusion joining of thermoplastic composite wind turbine blades: Lap-shear bond characterization." Renewable Energy 140: 501 -512.

[2] Bhudolia, S. K., et al. (2020). "Investigation on ultrasonic welding attributes of novel carbon/Elium® composites." Materials 13(5): 10-15.

Example 4

This example describes the joining of two separate acrylic resin composite articles, according to another embodiment.

Two acrylic composite articles were provided, which consisted of Elium 188 O/glass fibre composites.

The two sheets of Elium 188 O/glass-fibre composites were joined using methyl methacrylate (MMA) monomer mixed in a 100:3 ratio with a peroxide initiator. The edges of the weld region were sealed with vacuum bag attached using an adhesive and the spacing between the laminates was maintained using 1 mm diameter spacing wires. The monomer was fed into the spacing by gravity and left to polymerise for several days.

The resulting weld was found to be of good quality, and the two laminates could not be separated by hand indicating good strength.

Example 5

This example describes the joining of two clear PMMA (Perspex®) sheets, according to another embodiment.

Two clear PMMA (Perspex®) sheets were provided.

The sheets were joined using Elium® 188 O resin monomer in a 100:3 ratio with a peroxide initiator. Spacing was maintained using a 1 mm layer of glass fibre fabric.

The welding step was carried out as described Example 4.

The resulting join was found to be of good quality and strength, and the PMMA sheets could not be separated by hand.

Example 6

Example 6 relates to the investigation of the exothermic properties of multi-stage laminates made using a method of the present invention, and single-stage laminates made using conventional methods. The multi-stage laminates were made using a method broadly similar to that described above in connection with Figures 2(a)-2(d). Figures 10(a)-10(c) illustrate the structure of such multi-stage laminates.

An initial laminate (pre-consolidated laminate ‘PL’ 330) was made of a 12-ply structure of GF mats infused with Elium® resin monomer composition (including a 3% initiator) to produce a 6-mm-thick laminate.

Two multistage laminates were then formed, as represented by 331 and 332 in Figures 10(b) and (10(c).

A “base unit” 335, as shown in Figure 10(a), consisted of two plies of glass noncrimp fabric (NCF) 310 placed on each side of one pre-consolidated laminate 330, infused with Elium® resin monomer composition at room temperature, to yield a 8-mm thick base unit 335.

A first multistage laminate 331 was made by repeating this process once, thus yielding a 2-stage multilayer laminate 331 (‘MS-16-BPO3’) made of two base units 335 having a total thickness of about 16 mm, as shown in Figure 10(b)

A second, thicker, multi-stage laminate 332 was made using five iterations of the base unit structure 335, thus yielding a 5-stage multilayer laminate 332 (‘MS-40-BPO3’) made of five base units 335 having a total thickness of about 40 mm, as shown in Figure 10(c).

Comparative laminates SS-16-BPO3 and SS-16-BPO2 were single-stage infusion laminates made GF mats infused with Elium® resin monomer composition (using 3% initiator and 2% peroxide initiator, respectively) and having a total thickness of about 16mm.

As can be seen in Figure 9, the peak temperatures measured for the control (SS- 16-BPO3) and the sequential infusion laminate (MS-16-BPO3) were 89 °C and 34 °C, respectively, demonstrating that the present method significantly reduces the peak temperature of the exotherm profile for a given total laminate thickness. This is advantageous as it may help reduce or avoid the risk of boiling of damaging the resin.

T o assess the scalability of the modified infusion method, a 40-mm-thick laminate was also produced (MS-40-BPO3), reaching a much lower peak temperature (60 ^q C) than the 16-mm-thick control.

It will be appreciated that the described embodiments are not meant to limit the scope of the present invention, and the present invention may be implemented using variations of the described examples. For example, whilst the present methodology has been exemplified in relation to the 2-step preparation of a composite article, it will be understood that the same approach can be applied in the preparation of multilayer thermoplastic articles, or in other joining applications, including for example the repair of a damaged thermoplastic article, and/or the joining of two thermoplastic articles.