Technical Field of the Invention

The present invention relates generally to the field of fusion welding. In particular, but not exclusively, the invention concerns a composition for high efficiency fusion or laser welding of polymers.

Background to the Invention

Fusion welding of two plastic parts requires heat energy at their interfaces to increase the polymer above its melting temperature.

There are various techniques of applying heat energy to the interfaces of two thermoplastic polymers to be welded. For fusion welding, it is important that the melting temperatures of the polymers overlap.

The use of laser welding is being favoured in many engineering and medical applications as it provides an effective, clean solution to provide localised heating and melting of polymers above their melting point. This allows for very accurate welding with the minimal disruption to the homogeneity of the polymer structure, thereby avoiding isotropic effects.

Laser plastic welding, also often referred to as through-transmission welding, is a process of bonding thermoplastics using focused laser radiation. It involves passing a focused laser beam through an upper, laser transmissive thermoplastic to the interface of the two parts to be joined. The laser light is turned into heat energy as it is absorbed by the lower joining partner. The heat created at the interface creates a weld seam and the two plastics are fused once the laser is no longer active and the temperature of the thermoplastic falls, in the case of semi-crystalline polymers, below its crystallisation temperature or for amorphous polymers below their glass transition temperature.

A high-powered laser is typically required in order to generate sufficient heating at the interface between the two plastic parts.

Embodiments of the invention seek to at least partially overcome or ameliorate any one or more of the abovementioned disadvantages or provide the consumer with a useful or commercial choice. Summary of the Invention

According to a first aspect of the invention there is provided a fusion welding polymer composition comprising a one or more polymers in a polymer matrix and graphene dispersed homogeneously and encapsulated within the polymer matrix.

According to a second aspect of the invention there is provided a method of fusion welding for polymers, the method comprising the steps of placing a first polymer matrix comprising a one or more polymers and graphene dispersed homogeneously and encapsulated within the polymer matrix into contact with a second polymer and using a laser at a contact point or interface.

The laser is typically focussed at the contact point or interface between the polymer matrix containing the graphene and the second polymer.

The second polymer may include graphene dispersed homogeneously and encapsulated within the polymer matrix of the second polymer.

According to a third aspect of the invention there is provided a method of forming a fusion welding polymer matrix comprising the steps of dispersing graphene homogeneously within a base polymer melt to encapsulate the graphene within the polymer matrix.

Providing a polymer matrix composition including graphene homogenously encapsulated therein can substantially increase the efficiency of the fusion welding process such that lower powered equipment and lower energy is required to achieve a weld.

The polymer composition may be utilised for any fusion welding process in which heating of one or more polymers is utilised, but is particularly effective for use in a laser welding process.

In an embodiment, the graphene present in the polymer matrix composition is selected based on a number of parameters. These parameters may include amount of graphene, type of graphene, and/or size of graphene and/or the type of base polymer melt. Typically, the graphene will be selected based on interaction between all three of these parameters. Typically, the amount of graphene is based on the type of graphene and/or size of graphene. The amount of graphene is preferably optimised to provide the sought- after increase in efficiency of the laser welding, without adversely affecting the material properties of the polymer matrix.

In an embodiment, the amount of graphene will be between 0.1% and 5% on a weight basis. More preferably, the amount of graphene will be 0.4%-0.8% of graphene on a weight basis.

The amount of graphene to achieve the benefits of the invention, may be able to be reduced if one or more additives are also added to the polymer matrix. In an embodiment, one or more additives such as carbon (in a different allotropic form to graphene), TiCh, aluminium powder or other metal powder or metal-based powder can be added to the polymer matrix to enhance the laser welding capabilities. Additives such as these may have a synergistic effect with the graphene allowing a reduction in the amount of graphene used whilst maintaining the enhancement of the laser welding capabilities of the polymer matrix.

In one embodiment, 0.5% graphene and 0.1% aluminium powder (on a weight basis) can be used.

Typically, the type of graphene used is related to the amount of graphene and/or size of graphene.

Without wishing to be limited by theory, graphene family members have been categorised based on three fundamental attributes of carbon-based two-dimensional materials: number of layers, the C/O ratio and the lateral dimension. Based on those three properties, the classification of graphene-based materials is as follows according to ISO/TS 80004-13:2017:

• Monolayer graphene is the one-atom-thick material in which sp2- bonded carbon atoms are hexagonally-arranged.

• Few-layer graphene is the material consisting of 2-5 sheets of graphene.

• Multi-layer graphene consists of 5-10 layers of graphene. • Graphite/graphene nanoplatelets also consist of graphene sheets but their lateral dimensions/thickness are higher than those of multi-layer graphene (more than 10 layers, less than 100 nm thickness).

• Exfoliated graphite is a multi-layer material that can be prepared by partial exfoliation of graphite and retains its 3D crystal stacking.

• Graphene oxide (GO) is chemically modified graphene prepared by oxidation and exfoliation that is accompanied by extensive oxidative modification of the basal plane. Graphene oxide is a monolayer material with high oxygen content.

• Reduced graphene oxide (rGO) - graphene oxide (as above) that has been reductively processed by chemical, thermal, microwave, photochemical, photo-thermal or microbial/bacterial methods to reduce its oxygen content.

The graphene used in an embodiment is a multi-layer graphene. More preferred is use of graphene nanoplatelets.

One or more pigment materials may be added to the base polymer melt to provide a desired aesthetic property to the polymer matrix after the application of the laser energy. The application of the laser energy to the graphene will typically couse colour change in the graphene (known in the field of laser marking using graphene) which may be undesirable depending upon the nature of the polymer matrix.

The laser will typically be focussed during the welding process. Focussing the laser at or around the interface between the materials to be welded, will typically apply the energy from the laser to the graphene at the focal length of the laser at the point at which the welding is to be performed.

In one embodiment, the graphene and the one or more additives (if present) typically absorb the laser energy at the focal length and heat the plastic where the laser is focused. The presence of graphene typically increases the rate at which the welding can be done and with lower power equipment. The homogeneous dispersion of the graphene within the polymer matrix will preferably allow the increased heat absorption efficiency to be realised at any location within the polymer matrix. The polymer matrix may be of any type. More than one polymer may be present in the polymer matrix. Where more than one polymer is present in the polymer matrix, the more than one polymer may be present in any structural organisation. For example, more than one polymer may be present in one or more blocks, randomly, alternating or as graft copolymers.

The polymer matrix may comprise one or more thermoplastic polymers and/or one or more thermosetting polymers.

The enhancement of the laser welding capabilities of the polymer matrix is dependent, not on the polymer matrix, but the polymer matrix with the graphene dispersed homogeneously and encapsulated within the polymer matrix when compared to the same polymer matrix without the graphene dispersed homogeneously and encapsulated within the polymer matrix.

The presence of the graphene preferably results in localised melting of the polymer matrix at or around the focal length of the laser.

The graphene may be added to the polymer matrix or vice versa. The graphene may be added to the polymer matrix whilst the polymer matrix is molten in order to obtain homogeneous dispersion of the graphene within the polymer matrix.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only.

An illustrative embodiment of a polymer composition comprises a one or more polymers in a polymer matrix and graphene dispersed homogeneously and encapsulated within the polymer matrix.

The graphene enhanced polymer matrix is normally used in a method of fusion welding for polymers, the method comprising the steps of placing a first polymer matrix comprising a one or more polymers and graphene dispersed homogeneously and encapsulated within the polymer matrix into contact with a second polymer and using a laser at the contact. The laser is typically focussed at the contact point or interface between the polymer matrix containing the graphene and the second polymer.

The second polymer may also include graphene dispersed homogeneously and encapsulated within the polymer matrix of the second polymer.

In an embodiment, the graphene present in the polymer matrix composition is selected based on a number of parameters. These parameters include amount of graphene, type of graphene, and/or size of graphene and/or the type of base polymer melt. Typically, the graphene will be selected based on interaction between all three of these parameters.

Typically, the amount of graphene is based on the type of graphene and/or size of graphene. The amount of graphene is preferably optimised to provide the sought- after increase in efficiency of the laser welding, without adversely affecting the material properties of the polymer matrix.

In an embodiment, the amount of graphene is be 0.4%-0.8% of graphene on a weight basis.

The amount of graphene to achieve the benefits of the invention, may be able to be reduced if one or more additives are also added to the polymer matrix. In an embodiment, one or more additives such as carbon (in a different allotropic form to graphene), TiCh, aluminium powder or other metal powder or metal-based powder can be added to the polymer matrix to enhance the laser welding capabilities. Additives such as these may have a synergistic effect with the graphene allowing a reduction in the amount of graphene used whilst maintaining the enhancement of the laser welding capabilities of the polymer matrix.

In one embodiment, 0.5% graphene and 0.1% aluminium powder (on a weight basis) can be used. A preferred form of the invention can be explained with reference to the following example in which a graphene enhanced polymer matrix is used in a method of fusion welding for polymers.

Example 1 :

Equipment: 6W Diode Pumped solid state laser, wavelength 1064nm.

Method:

Test plaques for the [IR] transparent top layer were moulded in general purpose polystyrene, approximate thickness of 2mm. The IR transmission for the top layer used is almost 100%.

Test plaques for the [IR] absorbing base layer were moulded in polypropylene at approximately 2mm thickness.

The base layer and top layer for each trial are placed on top of one another and put inside the laser enclosure.

A high intensity laser pattern was created and used throughout the testing of all the welding formulations. In this example, no focussing of the laser was required due to the use of the [IR] transparent top layer on the graphene-enhanced [IR] absorbing base layer.

Testing:

The welding was tested by the operator, and the result was classified as either Bonded or Not bonded

Results:

The formulations as per the results were 0.5% graphene and 0.1% aluminium powder dispersed homogenously in a polypropylene matrix.

The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.