Sheet materials with fire-retardant, anti-static properties and ventilation ducting manufactured therefrom

Abstract

Laminate comprises core layer 204 between outer layers 202a, 202b. An outer layer comprises polymer, fire-retardant and electrically conductive additive. Preferably the sheet is coextruded at 160-19

Description

FIELD This disclosure relates to a fire-retardant, anti-static (FRAS) sheet material. Without limiting the generality of its applications, the material can be used in the manufacture of ducts suitable for ventilation in mining operations. BACKGROUND Fire-retardant, anti-static (FRAS) materials are used in areas with a high risk of fire or static buildup. FRAS requirements exist worldwide in numerous industries and are intended to limit flammability and static charge buildup that may cause arcing and possible ignition. To provide their anti-static properties, these materials must be static-dissipative as a minimum and preferably highly conductive. They must also prevent propagation of a fire (fire-retardant). In underground mining operations, FRAS requirements are applied to specific uses for non-metallic materials, such as ventilation equipment, conveyor belts, pipes or hoses, and bags. FRAS properties of are particular importance in ventilation ducts used in underground mines, especially coal mines, in which ignitable coal dust travelling through ventilation ducts may create sparks. Apart from applications in mining, FRAS materials have numerous other uses including, but not limited to, tunnel ventilation ducting and automotive, electronic and electrical applications. A type of FRAS sheet material is made from a modified styrene-butadiene rubber (SBR) polymer. However, products made from this material have drawbacks such as poor resistance to chemicals (especially hydrocarbons where it swells and weakens) and low temperatures. It further exhibits poor strength without reinforcement and has a high toxicity index. Other common shortcomings of existing FRAS sheet materials include high cost and low conductivity. The preceding discussion of the background is intended only to facilitate an understanding of the present disclosure. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY In accordance with an aspect of the disclosure there is provided a fire-retardant, anti-static (FRAS) sheet material having a layered, laminar or stratified structure comprising a core layer fused on opposed sides thereof to a pair of outer layers, wherein at least one of said outer layers comprises an outer layer polymer blend comprising a first (outer layer) polymer, a fire-retardant additive and an electrically conductive additive. The core and outer layers may be co-extruded. The FRAS sheet material may accordingly be provided as a single extrusion combining the layers. The disclosed technology extends to a duct assembled from at least one panel comprising the disclosed FRAS sheet material. The duct may be a ventilation duct. The duct may comprise a plurality of said panels. The panels may be joined to one another along mutual seams or welds. At least one of the panels may define an arcuate cross-section. A further aspect of this disclosure relates to a method of manufacturing a fire-retardant, anti-static (FRAS) sheet material, the method comprising steps of: blending a first (outer layer) polymer, at least one fire-retardant additive and at least one electrically conductive additive together, thereby to produce an outer layer polymer blend; co-extruding said outer layer polymer blend and a core polymer blend comprising a second (core) polymer, thereby to produce an extrusion comprising: a core layer comprising said core polymer blend; and a pair of outer layers fused to said core layer on opposite sides thereof, wherein at least one of said outer layers comprises said outer layer polymer blend. The method may include feeding the core polymer blend and the outer layer polymer blend into a combining manifold prior to the coextrusion step, thereby to combine said blends into a compact layered material; and feeding said material to a co-extrusion die configured to extrude said material as a sheet. The method may include feeding the extrusion to a set of cooling rollers to solidify and shape it as a sheet. A further aspect of this disclosure relates to a co-extrusion apparatus for manufacturing a fire-retardant, anti-static FRAS sheet material, the apparatus comprising: a source of an outer layer polymer blend comprising a first (outer layer) polymer, a fire-retardant additive and an electrically conductive additive; a source of a core polymer blend comprising a second (core) polymer; a combining manifold having separate channels configured to receive each of said melted polymer blends respectively and to combine them into a compact layered material; a co-extrusion die configured to receive said material from the combining manifold and to extrude it, thereby to produce an extrusion comprising: a core layer comprising said core polymer blend; and a pair of outer layers fused to said core layer on opposite sides thereof, wherein at least one of said outer layers comprises said outer layer polymer blend. The co-extrusion apparatus may further include a set of cooling rollers configured to solidify and shape the extrusion as a sheet. For parallel aspects of the technology described and parallel subsets of claimed subject matter, corresponding embodiments or modes of performance described are likewise applicable to each of the parallel aspects or subsets. Embodiments and modes of performing the technology will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a schematic three-dimensional illustration of a sheet manufactured from the FRAS sheet material according to the present disclosure; Figure 2 is a schematic detail drawing of the indicated portion of the FRAS sheet shown in Figure 1, illustrating its core and outer layers; Figure 3 is a block diagram providing a synopsis of exemplary materials and additives that may be used to make up the layers of an embodiment of the disclosed FRAS sheet material; Figure 4A is a schematic three-dimensional illustration of a duct assembled from panels made from the disclosed FRAS sheet material; Figure 4B is a schematic three-dimensional illustration of an elongate arcuate panel of the duct of Figure 4A made from the disclosed FRAS sheet material; Figure 4C is a schematic illustration of a side view of the duct shown in Figure 4A, the duct having been assembled from two of the elongate panels shown in Figure 4B bonded to each other along opposed seams or welds; Figure 5 is a flow diagram which illustrates, schematically, an example of a method for manufacturing the FRAS sheet material according to the present disclosure; Figure 6 is a schematic three-dimensional illustration of an apparatus for manufacturing the disclosed FRAS sheet material; Figure 7 is a schematic three-dimensional illustration of a first polymer feed subassembly for use in manufacturing the disclosed FRAS sheet material; Figure 8 is a schematic three-dimensional illustration showing internal detail of the indicated portion of the first polymer feed subassembly of Figure 7 which includes a screw barrel; Figure 9 is a schematic three-dimensional illustration showing a portion of the apparatus of Figure 6 with detail of a supplementary polymer-feed subassembly provided in addition to the first and second polymer-feed subassemblies; Figure 10 is a schematic three-dimensional illustration of a portion of the apparatus shown in Figure 6, focusing on a combining manifold where extruded layers from the first, second and supplementary polymer-feed subassemblies come together and proceed to a co-extrusion die; and Figure 11 is a schematic three-dimensional illustration of a portion of the apparatus shown in Figure 6, focusing on a set of cooling rollers. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS This disclosure presents a fire-retardant anti-static (FRAS) sheet material having a layered, laminar or stratified structure comprising a core (or central) layer and a pair of outer layers. On opposed sides of the core layer, the core layer is fused, melded, bonded or at least partially integrated with the two respective outer layers along generally planar zones or interfaces between adjacent layers. Each of the outer layers may be made from a polymer blend. This blend may comprise a polymer, a fire-retardant additive and an electrically conductive additive. The FRAS sheet material may comprise co-extruded component layers. The sheet material is engineered for fire-retardant and safety-critical applications. The sheet material features a three-layer sandwich structure, with the two outer layers acting as a coating or capping to enhance durability and to provide the required functionality. The electrically conductive additive (for example a carbon based compound or a carbon-filled composition) and the fire-retardant additive may provide essential fire-retardant properties as well as rendering the sheet static-dissipative and electrically conductive. These properties are advantageous environments where static control is crucial. The core layer may form a main body of the sheet material and may be thicker than each of the outer layers. The core layer may comprise a polymer resin combined or blended with a fire-retardant masterbatch. The design in the present disclosure may promote fire resistance and conductivity while providing structural stability. These properties are important for achieving safety and performance standards. The disclosed sheet material presents an antistatic, conductive product that complies with safety standards related to electrical and surface resistance, without the need to use 100% of a conductive compound. This approach may significantly reduce costs while maintaining desired performance characteristics. At least one of the outer layers may have a measurable resistance not exceeding 100 kQ. It may have a measurable resistance not exceeding 10 kQ. It may have a measurable resistance not exceeding 5 kQ. Both of said outer layers may each independently include a fire-retardant additive and an electrically conductive additive. The electrically conductive additive may comprise a conductive carbon material, for example, carbon black. The core layer may comprise a second (core) polymer which may be provided in a core polymer blend. The second polymer (of the core layer) may be the same as the first (core) polymer or the first and second polymers may differ from each other. The core layer may further comprise a fire-retardant additive. This may comprise a halogen-based fire-retardant substance such as, but not limited to, decabromodiphenyl ethane. The fire-retardant additive may include antimony trioxide. The fire-retardant additive may be provided by way of a masterbatch blended with the second (core) polymer, the masterbatch comprising the additive and a polymeric carrier material. The masterbatch blended into the core polymer blend may comprise the polymeric carrier material in a proportion ranging from about 50% to about 55% w / w and the fire-retardant additive in a proportion ranging from about 40% to about 50% w / w. The core polymer blend may comprise the second (core) polymer in a proportion ranging from about 85% to about 95% w / w and the masterbatch in a proportion ranging from about 5% to about 15% w / w. At least one of the first and second polymers may have a melt flow index in a range from about 0.1 g / 10 min to about 2.0 g / 10 min (190°C 15 kg). At least one of the outer layers may comprise the first (outer layer) polymer in a proportion ranging from about 60% to about 70% w / w, the fire-retardant additive in a proportion ranging from about 10% to about 13% w / w, and the electrically conductive additive in a proportion ranging from about 20% to about 25% w / w. At least one of the pair of outer layers may have a thickness in a range from about 0.25 mm to about 1.30 mm. The core layer may have a thickness in a range from about 1.5 mm to about 6.5 mm. At least one of the first and second polymers may be a thermoplastic polymer, for example, polyethylene. Properties of the disclosed FRAS sheet material may include the following: (i) when undergoing combustion, the material may release gases having a toxicological index (WLC50M) not exceeding about 4.0; and (ii) a surface of at least one of the outer layers may have an Atkinson friction factor not exceeding about 0.0022 Ns2 / m4 Sheets made from the disclosed FRAS sheet material may be sufficiently bendable or pliable to be deformed in order to produce generally cylindrical ventilation ducting. For example, the sheets may be bendable into elongate section lengths each having an arcuate or semi-circular crosssection, so that two or more of these sections may be connected to one another along seams to form cylindrical tubular ducts. It will be appreciated, however, that ducts having any required cross-sections may be manufactured from the sheet material, for example, square or rectangular cross-sections. The disclosed FRAS sheet material may feature a smooth finish. This can be achieved through the incorporation of a carbon material into the outer layer, for example. The smooth surface of the resultant ventilation duct may reduce friction encountered by moving air when compared to other ducts manufactured from metals, for example. This may result in increased airflow efficiency, with a higher volume of air due to reduced pressure drop within the system, enabling air to travel further and improving overall ventilation performance. Consequently, less energy is required by the blowers or compressors supplying the air to the ventilation ducts. In Figure 1, an example of a sheet (100) comprising the disclosed FRAS sheet material is shown. The sheet has three layers. It may be manufactured through an extrusion process, which may involve co-extrusion of the three layers. The FRAS sheet may be used to assemble ducts used for ventilation systems in mining operations which will be discussed later. Figure 2 shows detail of a corner region (102) of the FRAS sheet (100) of Figure 1. The three layers as presented in the current disclosure are shown, namely the core layer (204) and the two outer layers (202a, 202b) on either side of the core layer (204). At least one of the outer layers (202a, 202b) may comprise a polymer blend comprising an outer layer (first) polymer, a fire-retardant additive and an electrically conductive additive. The core layer (204) may comprise a core polymer blend. This blend may in turn comprise a second (core) polymer mixed with a masterbatch. It will be appreciated that the first and second polymers (of the outer layer and core layers respectively) may be the same polymer, although in other embodiments the polymers of the outer layers and core layer may differ from one another. The polymers of the respective outer layers may likewise differ from each other or be the same polymer. The second polymer, used for the core layer, may be a virgin resin capable of being used as a feed in extrusion operations. In an exemplary embodiment discussed in the present disclosure, the second polymer comprises a thermo-plastic polymer of the polyethylene group of polymers, more specifically High-density Polyethylene (HDPE). It should be appreciated, however, that any suitable polymer may be chosen which is capable of being used as a feed in an extrusion process. The masterbatch may be used to introduce one or more additives to the core layer (204) blend, thereby promoting the required properties of the layer. In the present disclosure, the masterbatch may be used to promote fire-retardant properties in the core layer (204). The masterbatch may therefore comprise a fire-retardant additive along with a polymeric carrier material. The polymeric carrier material may be a polymer capable of being used as a feed in extrusion operations. The second polymer and the polymeric carrier material may be the same polymer or differing polymers. In an exemplary embodiment, the polymeric carrier material is a thermo-plastic polymer of the polyethylene group of polymers, more specifically Linear Low-density Polyethylene (LLDPE) and High-density Polyethelene (HDPE). According to the current disclosure, however, different polymers with a similar melt flow index (MFI) may be used when manufacturing the FRAS sheet material. MFI is an indication of the molecular weight of the polymer, and polymers with a similar MFI can be more easily used together (mixing, fusing and flowing at similar rates) in the extrusion process. In the current disclosure, an MFI in a range from about 0.1 g / 10 min to about 2.0 g / 10 min (190°C 15 kg) in accordance with ISO 1133 may be used. The fire-retardant additive may comprise an element or compound or a mixture of various compounds or elements. By way of example, the fire-retardant additive may comprise a halogenbased fire retardant. It may optionally also include an agent effective to act as a synergist. In some embodiments the fire-retardant additive may, for example, comprise an antimony bromide masterbatch which combines antimony trioxide with a bromine compound to inhibit combustion chain reactions. The halogen-based fire retardant may be decabromodiphenyl ethane and the agent acting as a synergist may be antimony trioxide. The purpose of the synergist is to increase the effectiveness of the fire-retardant. It should however be appreciated that any suitable fire-retardant may be used while the addition of an agent to act as a synergist may be optional. The core layer (204) may have a thickness in a range from about 1.5 mm to about 6.5 mm. The thickness of the core layer may depend on operational requirements and economic considerations. A thinner core layer (204) may be able to bend and be moulded more easily but may be less durable. A thinner core layer (204) may further reduce the cost when compared to a thicker core layer (204). A thicker core layer (204) may be more durable but less easily assembled into a ventilation duct while also driving up the cost. The core layer (204) thickness may therefore be altered depending on the specific requirements. The outer layers (202a, 202b) may each be made from an outer layer polymer blend. This blend may comprise an outer layer polymer (first polymer), a fire-retardant additive and an electronically conductive additive. The fire-retardant additive may be as described above; however, it should be appreciated that any suitable fire-retardant additive may be used. The composition of the fire-retardant additive in the outer layers (202a, 202b) may be the same as the fire-retardant additive in the core layer (204) or it may be different. The electrically conductive additive may comprise a conductive carbon material, for example carbon black. It should be appreciated, however, that any suitable electrically conductive additive may be used. Advantageously, the outer layer polymer (first polymer) may be a polymer capable of being used as a feed in an extrusion process. In an exemplary embodiment of the present disclosure the outer layer polymer is a thermo-plastic polymer of the polyethylene group of polymers. It may be High-density Polyethylene (HDPE), for example. The first polymer (in the outer layers), the second polymer (in the core layer), and the polymeric carrier material of the various masterbatches may all be the same polymer or each may respectively be a different polymer. The outer layers (202a, 202b) may each have a thickness in a range from about 0.25 mm to about 1 mm. The thickness of the outer layers (202a, 202b) may depend on operational requirements, cost and the like. A thicker outer layer may have a higher measurable surface conductivity, meeting the requirements according to South African National Standard SANS 1287: 2007 and SANS 60079-0: 2019, but manufacturing costs will increase. Thinner outer layers (202a, 202b) may be less durable and have a lower measurable surface conductivity. However, it can be beneficial to keep the outer layers thin for cost effectiveness while retaining adequate conductivity (advantageously above about 300 microns). For the examples discussed in the present disclosure, the aim was therefore to manufacture a FRAS sheet with sufficiently durable outer layers (202a, 202b) while meeting conductivity standards according to SANS 1287: 2007 and SANS 60079-0: 2019, taking costs of manufacturing into account. The disclosed technology permits relatively thin conductive outer layers to be provided which do not peel or rip after extrusion. According to the present disclosure, the FRAS sheet presents a fire-retardant and antistatic, conductive product that complies with all safety standards related to electrical and surface resistance when used to manufacture ventilation ducting for mining operations. An example of a manufactured FRAS sheet, as exemplified in Figures 1 and 2 may achieve a measurable surface resistance of about 1.62 kQ while the upper limit according to SANS 1287: 2007 and SANS 60079-0: 2019 is 1 000 kQ. The example as shown in Figures 1 and 2 may further have a toxicity index, tested and calculated according to Defence Standard 02-713 to be around 2.0. According to Defence Standard 02-713, the toxicity index shall have a maximum value of 5.0 for sheaths which is the category a ventilation duct will fall under. Figure 3 is a block diagram which provides an overview (300) of an exemplary embodiment of the disclosed FRAS sheet material and the various polymers, additives and compounds which can be used to manufacture its three layers. The core polymer layer (204) may be manufactured from a core polymer blend (312). This blend may comprise a core layer (second) polymer (314) in a proportion ranging from about 85% to about 95% w / w, preferably 90% w / w, and a masterbatch (318) in a proportion ranging from about 5% to about 15% w / w. Advantageously, the masterbatch may be present in a proportion of about 10% w / w. The masterbatch (318) may itself comprise a polymeric carrier material (320) in a proportion ranging from about 50% to about 55% w / w, and a fire-retardant additive (322) in a proportion ranging from about 40% to about 50% w / w. The outer layers (202a, 202b) may be manufactured from an outer layer polymer blend (332) which may comprise an outer layer (first) polymer (334) in a proportion ranging from about 60% to about 70% w / w, a fire-retardant additive (336) in a proportion ranging from about 10% to about 13% w / w, and an electrically conductive additive (338) in a proportion ranging from about 20% to about 25% w / w. Figure 4A shows an example of a duct (400) assembled from panels of the FRAS sheet material discussed above. The duct may be used for ventilation systems in mining operations but also has numerous other applications, including but not limited to tunnel ventilation. The duct (400) may be assembled from one or more elongate panels made from the disclosed FRAS sheet material. When multiple FRAS panels are used for assembling the duct (400), the panels may be joined to one another along mutual seams or welds (not shown). The duct (400) may have additional accessories for ease of transportation and installation. For example, the duct may have ring (404) attachments for lifting and transporting the duct (400) as well as installation of the duct (400) and fixing it in place. The duct (400) may also have handles (406) for manually lifting it and transporting it to another location. In Figure 4B an elongate FRAS panel having an arcuate cross-section (420) is shown. The panel is made from the disclosed FRAS sheet material. The panel which is illustrated has a semi-circular cross-section, but other cross-sections are feasible depending upon the intended use and required configuration of the duct. A plurality of the panels may be used for assembling the duct. For example, two of the panels (420) as shown in Figure 4B may be joined along mutual seams or welds formed between edges (422, 424) of the panels to form the duct (400) as shown in Figure 4A. Figure 4C shows a side view (440) of the duct illustrated in Figure 4A. Duct hoop clamps (410) are provided to strengthen the overall structure of the duct. A further aspect of this disclosure relates to a method of manufacturing a fire-retardant, anti-static (FRAS) sheet material. The method may include blending a first (outer layer) polymer, at least one fire-retardant additive and at least one electrically conductive additive together, thereby to produce an outer layer polymer blend; co-extruding the outer layer polymer blend and a core polymer blend comprising a second (core) polymer, thereby to produce an extrusion. The extrusion may comprise the following: a core layer comprising said core polymer blend; and a pair of outer layers fused to said core layer on opposite sides thereof. At least one of said outer layers may be made of or comprise the outer layer polymer blend. The method may include feeding the core polymer blend and the outer layer polymer blend into a combining manifold prior to the coextrusion step, thereby to combine said blends into a compact layered material. The method may then include feeding this material to a co-extrusion die configured to extrude the material as a sheet. The method may also include feeding the extrusion to a set of cooling rollers to solidify and shape it as a sheet. The method may include heating the first (outer layer) polymer blend to a temperature in a range from about 220°C to about 240°C and heating the second (core) polymer blend to a temperature in a range from about 190°C to about 240°C. The co-extrusion die may operate in a temperature range from about 190°C to about 260°C. The cooling rollers may operate in a temperature range from about 75°C to about 115°C. In some modes of performing the method, the FRAS sheet material may be extruded at a rate in a range from about 160 kg / h to about 190 kg / h. Figure 5 shows a block flow diagram (500) which provides an overview of an exemplary mode of performing the disclosed method of producing the FRAS sheet material through co-extrusion. The diagram (500) shows the overall process flow, while more detail the co-extrusion process itself is given below. As shown in the diagram (500), the second polymer (314), from which the core layer (204) is made, comprises a virgin resin (316) which is mixed with a masterbatch (318) to provide a core polymer blend (312). The masterbatch (318) comprises at least one fire-retardant additive (322) and a polymeric carrier material (320). In the exemplary process, an outer layer (first) polymer (334), at least one flame retardant additive (336) and at least one electrically conductive additive (338) are mixed to produce an outer layer polymer blend (332). The core polymer blend (312) and the outer layer polymer blend (332) may be routed to a combining manifold (510) where the core polymer blend (312) may be routed through a core layer channel (514) and the outer layer polymer blend (332) may be split into at least two stream and each routed to either the bottom outer layer channel (512) or the top outer layer channel (516) in the combining manifold (510). Alternatively at least two mixtures of the outer layer polymer blend (332) may be produced separately and routed to the bottom outer layer channel (512) and the top outer layer channel (516) without the need to split one routing to two separate channels. The combining manifold (510) with the core layer channel (514) as well as the bottom outer layer channel (512) and the top outer layer channel (516) may route the core polymer blend (312) and the outer layer polymer blend (332) to a co-extrusion die (520). The co-extrusion die may be configured to shape and produce a layered, laminar or stratified sheet having a sandwich configuration comprising of a core layer (204) fused with two outer layers (202a, 202b), one on either side of the core layer. The co-extrusion die (520) shapes the material into the sheet as the material exits. A set of downstream rollers, discussed elsewhere herein, may also be provided to assist with the shaping of the sheet. The disclosed technology also provides a co-extrusion apparatus for manufacturing a fire-retardant, anti-static FRAS sheet material. The apparatus may comprise the following: a source of an outer layer polymer blend comprising a first (outer layer) polymer, a fire-retardant additive and an electrically conductive additive; a source of a core polymer blend comprising a second (core) polymer; a combining manifold having separate channels configured to receive each of said melted polymer blends respectively and to combine them into a compact layered material; and a co-extrusion die configured to receive said material from the combining manifold and to extrude it, thereby to produce an extrusion comprising: a core layer comprising said core polymer blend; and a pair of outer layers fused to said core layer on opposite sides thereof, wherein at least one of said outer layers comprises said outer layer polymer blend. The co-extrusion apparatus may further include a set cooling rollers configured to solidify and shape the extrusion as a sheet. Figure 6 shows an example (600) of such a co-extrusion apparatus which may be used for manufacturing the disclosed FRAS sheet material through co-extrusion. The main individual sections of the apparatus are discussed in more detail below. The sections of the illustrated apparatus (600) include a first polymer-feed subassembly (602), a second polymer-feed subassembly (604), a combining manifold, a co-extrusion die and cooling rollers (608). Figure 7 shows detail of the first polymer-feed subassembly (602). Two hoppers are shown (702, 704) where the second polymer (314) (for the core layer) and the masterbatch (318) are respectively fed into each of them. It is possible instead to have only one hopper, into which the second polymer (314) and masterbatch (318) are both fed together. The second polymer (314) and the masterbatch (318) may be mixed together in a combination zone (706) before being routed to a first screw barrel (708) of the first co-extruding subassembly (602). By way of example only, the core polymer blend (312) may be produced from the first screw barrel (708) at the first co-extruding subassembly (602) by feeding the second polymer in a proportion ranging from about 85% w / w to about 95% w / w and the masterbatch in a proportion ranging from about 5% w / w to about 15% w / w. Figure 8 shows internal detail (800) of the first screw barrel (708) forming part of the first polymerfeed subassembly (602). During operation, the first screw barrel (708) may receive the second polymer (314) and the masterbatch (318) in the proportions discussed above to melt and combine the mixture into the core polymer blend (312). The first screw barrel (708) may have separate zones (802, 804, 806, 808, 810, 812, 814) that operate at different temperatures in order to achieve the required flow in the first screw barrel of the core polymer blend (312). By way of example, the temperature range for the first screw barrel (708) over all zones may range from about 190°C to about 240°C. Seven zones are indicated in Figure 8 by way of example, but more may be advantageous, e.g. 20 or more zones. In such embodiments, the first screw barrel (708) may be divided into 20 zones of which 14 zones may be operational. An exemplary set of temperature setpoints which can be used in such embodiments to produce the core polymer blend (312) is shown in Table 1 below. Table 1: Temperatures in Zones of the First Screw Barrel Zone 1 2 3 4 5 6 7 8 9 10 Temp (°C) 190 195 200 200 200 200 200 200 200 N / A Zone 11 12 13 14 15 16 17 18 19 20 Temp (°C) 200 200 200 200 210 N / A N / A N / A N / A N / A At least one supplementary polymer-feed subassembly may be provided in addition to the first and second polymer-feed subassemblies. Figure 9 shows a portion (900) of the example apparatus (600) viewed from the rear, providing detail of a supplementary polymer-feed subassembly indicated by reference numeral 904. This supplementary subassembly (904) performs a similar function to that of the second polymer-feed subassembly (604), that is, it feeds extrusion of the outer layers of the sheet material. The subassembly (604) feeds one of the outer layers and the supplementary assembly (904) feeds the other outer layer. The apparatus (600) therefore includes two polymer-feed subassemblies arranged to feed the respective outer layers of the sheet material. However, it will be appreciated that other configurations (not shown) of the apparatus can be employed in which, for example, there is just a single (second) polymer-feed subassembly with a splitter to feed both outer layers, or in which there are more than two such subassemblies so that multiple polymer-feed subassemblies feed each outer layer. Any required number of supplementary polymer-feed subassemblies may be added as required for each additional material added to the sheet material produced. The second- and supplementary polymer-feed subassemblies (604, 904) are substantially similar to the first polymer-feed subassembly (602) in their configurations and operations. However, they may be smaller because each of the outer layers (202a, 202b) requires less volume of material compared to the core layer (204). The supplementary polymer-feed subassembly (904) may include one or more hoppers (902) for feeding the outer layer (first) polymer (334), the fire-retardant additive (336) and the electrically conductive additive (338) to a screw barrel (906) for melting and mixing the material to produce an outer layer polymer blend (332). The temperature range for the screw barrel (906) on the supplementary polymer-feed subassembly (904) over all zones may range from about 220°C to about 240°C. In the illustrated example, the screw barrels of the second and supplementary extruding subassemblies are each be divided into seven zones. Exemplary temperature setpoints which may be used in the screw barrels to produce an outer layer polymer blend (332) are shown in Table 2 below. Table 2: Screw Barrel Operating Temperatures of Co-Extruding Subassemblies Second co-extruding subassembly Zone 1 2 3 4 5 6 7 Temp (°C) 220 235 235 235 235 235 235 Supplementary co-extruding subassembly Zone 1 2 3 4 5 6 7 Temp (°C) 230 235 235 235 235 235 235 The outer layer polymer blend (332) may be routed to the combining manifold together with the core polymer blend (312) from the first polymer-feed subassembly (602) and subsequently to the co-extrusion die which will be explained in more detail below. Figure 10 shows detail of a portion (1000) of the apparatus (600) which includes a combining manifold (1006) and a co-extrusion die (1008). The core polymer blend is routed through a terminal portion (1010) of the first screw barrel while the outer layer polymer blend is routed through terminal portions of the screw barrels (906, 1004) to the combining manifold (1006). The combining manifold (1006) may be configured with three channels, one for each layer of the FRAS sheet material. The combining manifold (1006) is configured to combine the three separate feeds of melted polymer blends from the respective screw barrels and to deliver them through outlets from the channels. The blends emerge from the channels and combine to form a compact layered feed material. This melted feed material is then fed to the downstream co-extrusion die (1008). The combining manifold (1006) may be provided with internal gauges to adjust volumes flowing through the outlets of the channels, thereby to permit individual layer thicknesses to be controlled. The co-extrusion die (1008) of the illustrated example is elongate and horizontally orientated and is aligned transversely to the direction of flow of the layered feed material produced by the combining manifold (1006). The co-extrusion die (1008) is configured to shape the material into a sheet-like extrusion comprising the three layers. The layers become fused or integrated with one another along mutual interfaces as the material is extruded. The core polymer blend (312) forms the core layer of extruded sheet while the outer layer polymer blend (332) forms the top and bottom outer layers (202a, 202b). The co-extrusion die (1008) is heated and temperature-controlled in order to maintain a required flow rate of the melted material in opposite lateral (transverse) directions so that the sheet can be extruded. The co-extrusion die (1008) has an extrusion outlet which can be adjusted in order to further control the thickness of the extruded sheet material. Temperatures within the co-extrusion die (1008) over all zones may be set to range from about 190°C to about 260°C. In the illustrated example, the co-extrusion die (1008) is divided into nine zones. Exemplary temperature setpoints used to achieve the required lateral flow are shown in Table 3 below. Table 3: Exemplary Temperatures in Zones of the Co-Extrusion Die Zone 22 23 24 25 26 27 28 29 30 Temp (°C) 195 229 200 150 190 205 230 233 260 Figure 11 shows detail of an exemplary set (1100) of cooling rollers (1102, 1104, 1106). During the manufacturing process, the formed FRAS sheet material comprising the three layers exits the co-extrusion die (1008) and passes through the set of cooling rollers which solidify and shape the sheet to further spread the material and provide a desired thickness and surface finish. In various exemplary embodiments, the thickness of the core layer may be in a range from about 1.5 mm to about 6.5 mm while the thickness of each of the outer layers may be in a range from about 0.25 mm to about 1.30 mm. It will be appreciated that other thicknesses may be chosen according to requirements and the intended use of the final product, however. The thickness of the outer layer is important for maintaining a resistance measurement of less than 1 000 kQ in order to comply with standards relating to electrical resistance and surface resistance according to SANS 1287-1: (2007) (Clauses 6.10) and SANS 60079-0: (2019) (Clauses 26.13) respectively. The cooling rollers can be configured to operate at different temperatures in order to achieve required flow rates and spread for further shaping of the sheet material, and for providing a smooth surface finish. If the cooling roller temperature is too high, there is a risk that the sheet may get stuck to the cooling roller during the process. In the illustrated example, the rollers operate according to the roller temperature ranges shown in Table 4 below. Table 4: Roller Temperatures Roller Top Centre Bottom Temp (°C) 100-115 100-115 75-95 A smooth finish can be advantageous for reducing energy usage requirements, for example when pumping air (via a blower, compressor or induction fan) through ducts assembled from the FRAS sheet material. A smooth surface compared to other materials will have a lower Atkinson friction factor. A ventilation duct with a lower Atkinson friction factor will have a lower pressure drop across the system which means either less energy is required to move the air, or the same volume of air can be moved over a longer distance for the same energy consumption. After the FRAS sheet material has been extruded and further shaped by the cooling rollers, excess edges of the extruded sheets may be trimmed for uniformity of size. The finished product may be a sheet or panel of the FRAS sheet material as disclosed and illustrated herein. It will be appreciated that the configuration of the disclosed FRAS sheet material is not restricted to having three layers only. While three layers can be advantageous from the point of view of providing the required functionality while maintaining cost effectiveness, it will be appreciated that further layers may be added without departing from the scope of the disclosed technology. For example, additional outer claddings, coatings or further polymer layers may be considered depending on requirements. Such additional layers may also be considered for ducts and other products manufactured or assembled from the disclosed FRAS sheet material. The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the technology to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the present disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the present disclosure is intended to be illustrative, but not limiting, of the scope of any accompanying claims. Finally, throughout the specification and any accompanying claims, unless the context requires 5 otherwise, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. A fire-retardant, anti-static (FRAS) sheet material having a laminar structure comprising a core layer fused on opposed sides thereof to a pair of outer layers, wherein at least one of said outer layers comprises an outer layer polymer blend comprising a first (outer layer) polymer, a fire-retardant additive and an electrically conductive additive.

2. The FRAS sheet material of claim 1, wherein the core and outer layers are co-extruded.

3. The FRAS sheet material of claim 1 or claim 2, wherein at least one of the outer layers hasa measurable resistance not exceeding 100 kQ.

4. The FRAS sheet material of claim 3, wherein at least one of the outer layers has a measurable resistance not exceeding 10 kQ.

5. The FRAS sheet material of any one of the preceding claims, wherein both of the outer layers each independently includes a fire-retardant additive and an electrically conductive additive.

6. The FRAS sheet material of any one of the preceding claims, wherein the electrically conductive additive comprises a conductive carbon material.

7. The FRAS sheet material of any one of the preceding claims, wherein the core layer includes a fire-retardant additive.

8. The FRAS sheet material of any one of the preceding claims, wherein the fire-retardant additive comprises decabromodiphenyl ethane and antimony trioxide.

9. The FRAS sheet material of any one of the preceding claims, wherein the core layer comprises a core polymer blend comprising a second (core) polymer mixed with a masterbatch comprising a fire-retardant additive in a proportion ranging from about 40% to about 50% w / w, and a polymeric carrier material in a proportion ranging from about 50% to about 55% w / w.

10. The FRAS sheet material of any one of the preceding claims, wherein the first polymer has a melt flow index in a range from about 0.1 g / 10 min to about 2.0 g / 10 min (190°C / 5 kg).

11. The FRAS sheet material of any one of the preceding claims, wherein at least one of the outer layers comprises the first (outer layer) polymer in a proportion ranging from about 60% to about 70% w / w, the fire-retardant additive in a proportion ranging from about 10% to about 13% w / w, and the electrically conductive additive in a proportion ranging from about 20% to about 25% w / w.

12. The FRAS sheet material of any one of the preceding claims, wherein at least one of the pair of outer layers has a thickness in a range from about 0.25 mm to about 1.30 mm.13 The FRAS sheet material of any one of the preceding claims, wherein the core polymer layer has a thickness in a range from about 1.50 mm to about 6.50 mm.

14. The FRAS sheet material of any one of the preceding claims which, when undergoing combustion, releases gases having a toxicological index (WLC50M) not exceeding 4.0.

15. The FRAS sheet material of any one of the preceding claims wherein a surface of at least one of the outer layers has a measurable Atkinson friction factor not exceeding about 0.0022 Ns2 / m4.

16. A ventilation duct assembled from at least one panel comprising the FRAS sheet material of any one of the preceding claims.

17. The duct of claim 16, which comprises a plurality of said panels.

18. The duct of claim 17, wherein at least one of the panels defines an arcuate cross-section and the panels are joined to one another along mutual seams.

19. A method of manufacturing a fire-retardant, anti-static (FRAS) sheet material, the method comprising steps of:blending a first (outer layer) polymer, at least one fire-retardant additive and at least one electrically conductive additive together with one another, thereby to produce an outer layer polymer blend;co-extruding said outer layer polymer blend together with a core polymer blend comprising a second (core) polymer; thereby to produce an extrusion comprising:a core layer comprising said core polymer blend; anda pair of outer layers fused to said core layer on opposite sides thereof, wherein at least one of said outer layers comprises said outer layer polymer blend.

20. The method of claim 19, which includes feeding the core polymer blend and the outer layer polymer blend into a combining manifold prior to the coextrusion step, thereby to combine said blends into a compact layered material; and feeding said material to a co-extrusion die configured to co-extrude said material as a sheet.

21. The method of claim 20, wherein the co-extrusion die is operated at a temperature in a range from about 190°C to about 260°C.

22. The method of any one of claims 19 to 21, which includes feeding the extrusion to a set of cooling rollers to solidify and shape the extrusion as a sheet; and wherein the cooling rollers operate in a temperature range from about 75°C to about 115°C.

23. The method of any one of claims 19 to 22, wherein the FRAS sheet material is extruded at a rate in a range from about 160 kg / h to about 190 kg / h.

24. A co-extrusion apparatus for manufacturing a fire-retardant, anti-static FRAS sheet material, the apparatus comprising:a source of an outer layer polymer blend comprising a first (outer layer) polymer, a fire-retardant additive and an electrically conductive additive;a source of a core polymer blend comprising a second (core) polymer;a combining manifold defining separate channels configured to receive each of said melted polymer blends respectively and to combine them into a compact layered material;a co-extrusion die configured to receive said material from the combining manifold and to extrude it, thereby to produce an extrusion comprising:a core layer comprising said core polymer blend; anda pair of outer layers fused to said core layer on opposite sides thereof, wherein at least one of said outer layers comprises said outer layer polymer blend.

25. The co-extrusion apparatus of claim 24, which further includes a set cooling rollers configured to solidify and shape the extrusion as a sheet.

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