WO2020159409A1

WO2020159409A1 - Functional sheath-core bicomponent fiber comprising hexagonal boron nitride

Info

Publication number: WO2020159409A1
Application number: PCT/SE2019/050081
Authority: WO
Inventors: Björn BOSTRÖM; Erik Nilsson
Original assignee: Saab Ab
Priority date: 2019-01-31
Filing date: 2019-01-31
Publication date: 2020-08-06

Abstract

The present disclosure relates to sheath-core bicomponent fibers, wherein the sheath comprises a first thermoplastic polymer and the core comprises a second thermoplastic polymer and hexagonal boron nitride. The first thermoplastic polymer and the second thermoplastic polymer may be the same or different.The present disclosure further relates to a method of manufacturing such a sheath-core bicomponent fiber, as well as yarns or textiles comprising such sheath-core bicomponent fibers.

Description

FUNCTIONAL SHEATH-CORE BICOMPONENT FIBER

COMPRISING HEXAGONAL BORON NITRIDE

TECHNICAL FIELD

The present invention relates to functional fibers and methods of producing such fibers. The present invention further relates to textiles or yarns comprising such fibers.

BACKGROUND ART

Adding functionality to technical fibers increases the value of the fibers and facilitates the development of new products utilizing the added functionality. By adding the functionality during the fiber manufacturing stage, the requirement for post-processing is decreased and the functionalised fiber may be produced more cheaply as compared to a fiber requiring a separate post-production step in order to attain the desired functionality.

Hexagonal boron nitride is a substance possessing a number of interesting properties. For example, hexagonal boron nitride has excellent chemical, photochemical and thermal stability, is a dielectric and has an excellent anisotropic thermal conductivity. Therefore, a fiber comprising hexagonal boron nitride and displaying such properties is potentially of interest.

Document CN104178840 describes a polyester fiber comprising from 0.5 to 5 wt% of boron nitride. The boron nitride may be hexagonal boron nitride.

There remains a need for technical fibers having added functionality.

SUMMARY OF THE INVENTION

The inventors of the present invention have identified a number of shortcomings with prior art fibers comprising hexagonal boron nitride. The amount of hexagonal boron nitride filler used in such prior art fibers is insufficient to adequately impart many of the favourable properties of hexagonal boron nitride to the fiber. For example, a fiber comprising only 5 wt% hexagonal boron nitride filler may not demonstrate significantly improved thermal conductivity as compared to the unfilled fiber. However, the inventors have discovered that at least some polymer composites comprising hexagonal boron nitride have poor spinnability, and increasing the amounts hexagonal boron nitride filler renders the composites difficult or impossible to spin. It would be advantageous to achieve a fiber overcoming, or at least alleviating, at least some of the above mentioned shortcomings. In particular, it would be desirable to enable a fiber having improved spinnability, even with greater hexagonal boron nitride loadings. To better address one or more of these concerns, a fiber having the features defined in the independent claims is provided.

The fiber is a sheath-core bicomponent fiber. The sheath comprises a first thermoplastic polymer. The core comprises a second thermoplastic polymer and hexagonal boron nitride (h- BN). The first thermoplastic polymer and the second thermoplastic polymer may be the same or different. By providing a sheath-core bicomponent fiber comprising h-BN in the core, a fiber having good spinnability, even at high h-BN loads, is obtained. Thus, fibers may be produced that have h-BN loads sufficient to bequeath desirable properties upon the fiber.

The core may comprise from about 10 wt% to about 50 wt% of the hexagonal boron nitride, preferably from about 30 to about 50 wt%. Such h-BN loads may be sufficient to impart the fiber with altered or improved functionality.

The hexagonal boron nitride may have a particle size of from about 0.1 pm to about 1 pm, preferably from about 0.4 pm to about 0.6 pm. This provides good coverage when the h-BN is used as a pigment, and is an appropriate size for inclusion in fibers.

The second thermoplastic polymer (i.e. the core polymer), may be a polyolefin, preferably a polypropylene homopolymer. Polyolefins are readily stabilized by addition of a single hindered amine additive (NOR HALS flame retardant) during production, without negatively impacting other properties of the fiber. Moreover, polypropylene homopolymers have a relatively high maximum operating temperature, good tensile strength, modulus and heat deflection temperature (HDT).

The core further comprises a coupling agent, preferably a maleic anhydride grafted polyolefin, even more preferably a maleic anhydride grafted polypropylene. The addition of a coupling agent improves the compatibility of the h-BN filler with the polymer and allows higher h-BN loadings, especially whenever the polymer matrix is a relatively non-polar polymer such as a polyolefin homopolymer. The mass ratio of hexagonal boron nitride to coupling agent may be from about 50:1 to about 5:1, preferably from about 15:1 to about 7:1, even more preferably about 10:1. This provides a suitable compatibility between the polymer matrix and the h-BN filler.

The second thermoplastic polymer may have a melt flow rate greater than 500 g/lOmin, preferably greater than 1000 g/min, as measured at 230°C under a standard weight of 2.16 kg using the standard method of ISO 1133-1:2011. Using a polymer matrix having a high MFR may in part be used to compensate for the increase in viscosity caused by high h-BN filler loadings.

The sheath may comprise at least 70 wt% of the first thermoplastic polymer. This provides a sheath with excellent mechanical properties that may withstand the stresses arising during melt spinning. The first thermoplastic polymer may be a polyamide, polyolefin, or combination thereof, preferably a polypropylene homopolymer. Polyolefins are readily stabilized by addition of a single hindered amine additive (NOR HALS flame retardant) during production, without negatively impacting other properties of the fiber. Moreover, polypropylene homopolymers have a relatively high maximum operating temperature, good tensile strength, modulus and heat deflection temperature (HDT).

The sheath and/or the core may further comprise a flame retardant, wherein the flame retardant is preferably an alkoxyamine hindered amine light stabilizer (NOR HALS) flame retardant. Incorporating a flame retardant directly into the fiber reduces the need for post processing. NOR HALS flame retardants may be added during production without negatively impacting other desirable properties of the fiber.

The fiber may have a diameter from about 30 pm to about 100 pm, preferably from about 40 pm to about 80 pm, more preferably from about 50 pm to about 70 pm. This provides a fiber having a suitable dimension for further processing to gam or textile.

The volumetric ratio of the core to the sheath may be from about 1:4 to about 4:1, preferably from about 1:2 to about 3:1, more preferably from about 1:1 to about 3:1. This assists in providing a fiber having an abundant h-BN content while maintaining sufficient mechanical properties for spinning and further processing.

The sheath-core bicomponent fiber may have a thermal conductivity of greater than 0.3 W-m^_1-K^_1 , preferably greater than 0.4 W-m^_1-K^_1 measured as specified in the method of ISO 22007-2:2015. This may be a thermal conductivity that is higher than an unfilled reference fiber. Improved thermal conductivity improves the utility of the fiber in many applications.

According to a further aspect of the invention, a method is provided, as defined the appended independent claims, for manufacturing a sheath-core bicomponent fiber.

The method comprises the steps:

- providing a first thermoplastic polymer;

- compounding hexagonal boron nitride and a second thermoplastic polymer to provide an h- BN composite; and

- melt spinning the first thermoplastic polymer and the h-BN composite through a

bicomponent sheath-core spinneret to provide a sheath-core bicomponent fiber, wherein the core comprises hexagonal boron nitride.

The method facilitates the production of a sheath-core bicomponent fiber wherein the core comprises hexagonal boron nitride.

According to another aspect of the invention, a yarn or textile comprising a sheath-core bicomponent fiber as defined in the appended independent claims is provided.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which:

Fig la is a microscopy image of a fiber comprising h-BN in the fiber sheath;

Fig. lb is an inset of the image of Fig. la having a higher magnification;

Fig. 2a is a microscopy image of a fiber cross-section comprising h-BN in the fiber

sheath;

Fig. 2b is an inset of the image of Fig. 2a having a higher magnification;

Fig. 3 is a microscopy image of a yarn bundle comprising fibers comprising h-BN in the fiber cores; and

Fig. 4 is a plot of the complex viscosity vs. angular frequency for a number of h-BN/PP composites.

DETAILED DESCRIPTION

The present invention relates to a sheath-core bicomponent fiber, wherein the sheath comprises a first thermoplastic polymer and the core comprises a second thermoplastic polymer and hexagonal boron nitride (h-BN). By a sheath-core bicomponent fiber it is meant a bicomponent fiber comprising a sheath component and a core component, where the sheath component and core component differ in composition. For example, the sheath component and core component may be of different polymers, differ in polymer blend ratios, or may differ in filler content or composition.

Bicomponent spinning with one of the components being essentially unfilled allows fibers with a greater amount of h-BN filler to be produced, since the unfilled component is capable of withstanding the tensile stresses arising during the spinning process. However, the inventors of the present invention have found that the presence of hexagonal boron nitride filler in the exterior portion of the fiber contributes to a rough surface structure on the fiber and may cause increased breakage of the fiber during melt spinning, spinning to a yarn or knitting to a textile. Therefore, by confining the hexagonal boron nitride filler to the core of the fiber instead of the sheath, sheath-core bicomponent fibers may be provided that allow the production of fibers, yarns and textiles having even greater h-BN filler content.

The sheath component of the fiber comprises, consists essentially of, or consists of a first thermoplastic polymer. The sheath component is preferably free of hexagonal boron nitride. By free of hexagonal boron nitride it is meant that no hexagonal boron nitride has been added to the sheath component during manufacture of the fiber, although some hexagonal boron nitride may migrate across the component boundaries during fiber spinning and manufacture. The first thermoplastic polymer is a polymer suitable for melt-spinning, and may for example be a polyamide such as PA6 or PA66, a polyolefin such as polyethylene or polypropylene, a polyester such as PET, or any combination thereof. It is preferred that the first thermoplastic polymer is a polyolefin, more preferably polypropylene, most preferably a polypropylene homopolymer. This is because polyolefin fibers may easily be flame-retarded and light- stabilized by addition of a single hindered amine additive (NOR HALS flame retardant) during production. Incorporation of this additive in relevant quantities into the does not negatively impact other properties of the fiber, such as the fiber's electromagnetic spectral properties. Despite having poorer compatibility with h-BN filler due to their low polarity, PP

homopolymers are preferred over PP copolymers or other more polar polymers because they have a relatively higher maximum operating temperature, better tensile strength, modulus and HDT. The heat deflection temperature or heat distortion temperature (HDT) is the temperature at which a polymer or plastic sample deforms under a specified load. The sheath component preferably comprises at least 70 wt% of the first thermoplastic polymer, such as at least 80 wt% or at least 90 wt%. The sheath component may further comprise other constituents known in the art, such as pigments, fillers, plasticizers, flame retardants, antioxidants, UV- and light-stabilizers, etc.

The core component of the fiber comprises, consists essentially of, or consists of a second thermoplastic polymer and hexagonal boron nitride.

Boron nitride is a compound of boron and nitrogen having the chemical formula BN. It exists in various crystalline forms (polymorphs). The boron nitride used in the present invention is hexagonal boron nitride (h-BN), a polymorph having a graphitic (graphite-like) 2-D layer structure. Hexagonal boron nitride has a number of interesting properties for functional fibers. It is exceptionally stable: thermally, chemically and photochemically, meaning that fibers manufactured using h-BN may be very robust. It is an excellent anisotropic thermal conductor, and this property may for example be used to tailor the thermal conductivity of the fiber. The relatively low density of h-BN in relation to other known solid polymer additives (such as pigments and fillers) means that functional fibers utilizing h-BN may be lighter than functional fibers utilizing other solid polymer additives. Hexagonal boron nitride is also non-toxic and is not classified as hazardous in any manner according to standards such as GHS (Globally Harmonized System of Classification and Labelling), CLP (EU Classification, Labelling and Packaging regulation) and OSHA (Occupational Safety and Health Administration). This facilitates safe production and handling of the fibers, and makes materials comprising such h- BN functionalised fibers an environmentally sound choice.

Besides the properties listed above, hexagonal boron nitride is a white powder and is suitable for use as a white pigment. This is because it has a refractive index in the visible range that differs sufficiently from common polymer matrices. In order to ensure that a white pigment is reflective and not transparent when incorporated into a matrix, it is essential that the pigment has a refractive index that differs sufficiently from the matrix at the relevant wavelength. For example, the common fillers calcium carbonate and barium sulfate in isolated powder form appear white. However, they have a refractive index (circa 1.59 for calcium carbonate and 1.64 for barium sulfate) that is similar to common polymer matrices (usually circa 1.5), and thus they appear transparent when incorporated in a binder, and cannot be used as white pigment. In contrast, hexagonal boron nitride has a refractive index within the visual range of approximately 2. This means that h-BN has good reflectivity within the visual range when incorporated in a common matrix, and thus may be utilized as a white pigment.

In order to ensure good reflectivity and obtain optimal pigment coverage, the particle size of the pigment is important. The particle size providing optimal light scattering may be estimated using Weber's formula: l

2.1 (rip - n_M) wherein D is the particle diameter in microns; l is the incident light wavelength in microns (taken to be 0.55 miti for visible light in this case);

P_M is the refractive index of the matrix (circa 1.5); and np is the refractive index of the pigment (circa 2 for h-BN in the visible range).

Using this formula, the particle size giving optimal pigment coverage is determined to be approximately 0.5 pm. A small particle size is also beneficial for incorporation of h-BN filler into fibers which typically have a diameter on the micron scale. It therefore is preferred that the h-BN pigment used in the present invention has a particle size of from about 0.1 pm to about less than 1 pm, preferably from about 0.3 pm to about 0.8 pm, even more preferably from about 0.4 pm to about 0.6 pm. Varying grades of hexagonal boron nitride having different particle sizes are readily commercially available. The particle size distribution of h-BN powders may be measured using for example laser diffraction methods as defined in standard ISO 13320:2009 "Particle size analysis - Laser diffraction methods".

In order that the various properties of hexagonal boron nitride may be manifested in the produced fiber, this may require differing amounts of hexagonal boron nitride, depending on the property in question. The inventors of the present invention have found that in some circumstances incorporation of h-BN in amounts in excess of 30 wt% in the fiber core are required in order to markedly impact the thermal conductivity of the fiber. Due to the excellent light scattering property of h-BN and the optical properties of common polymer matrices, the white pigment properties of the h-BN filler may manifest at relatively low amounts of h-BN incorporated in the fiber core, such as about 10 wt% h-BN or greater. On account of this, it may in some cases be desirable to provide a core component comprising 10 wt% h-BN or greater h-BN, such as from about 10 wt% to about 50 wt% of the hexagonal boron nitride, preferably from about 20 to about 50 wt%, even more preferably from about 30 wt% to about 40 wt%.

The second thermoplastic polymer is a polymer suitable for melt-spinning, and may for example be a polyamide such as PA6 or PA66, a polyolefin such as polyethylene or

polypropylene, a polyester such as PET, or any combination thereof. It is preferred that the second thermoplastic polymer is a polyolefin, more preferably polypropylene, most preferably a polypropylene homopolymer. This is because polyolefin fibers may easily be flame-retarded and light-stabilized by addition of a single hindered amine additive during production.

Incorporation of this additive in relevant quantities into the does not negatively impact other properties of the fiber, such as the fiber's electromagnetic spectral properties. The core component preferably comprises at least 40 wt% of the second thermoplastic polymer, such as at least 50 wt% or at least 60 wt%.

The core component may further comprise a coupling agent in order to increase the compatibility between the second thermoplastic polymer and the hexagonal boron nitride filler. This is advantageous if greater h-BN filler loading are desired in the core, such as in excess of 10 wt%. It is also particularly advantageous if the second thermoplastic polymer is a relatively non-polar polymer such as a polyolefin, especially a polyolefin (polyethylene, polypropylene) homopolymer, since polyolefins may otherwise have poor compatibility with the h-BN binder and high h-BN loadings may otherwise be difficult or impossible to achieve. The coupling agent may be any coupling agent known in the art. However, for use with polyolefins, a maleic anhydride grafted polyolefin, such as a maleic anhydride grafted polypropylene, are preferred. The coupling agent may be added such that a ratio of h-BN to coupling agent in the core component is from about 50:1 to about 5:1, preferably from about 15:1 to about 7:1, even more preferably about 10:1.

The core component may further comprise other constituents known in the art, such as pigments, fillers, plasticizers, flame retardants, antioxidants, UV- and light-stabilizers, etc.

The constituents of the core component are compounded using any technique known in the art, such as by twin screw or multi-screw extrusion. The load of h-BN filler added to the composite may be confirmed by thermogravimetric analysis. Appropriate feed rates, screw speeds and temperature profiles may readily be determined by the skilled person. If necessary, the constituents of the sheath component may also be compounded prior to spinning.

The sheath-core bicomponent fiber is produced by bicomponent melt-spinning using a bicomponent sheath-core spinneret. The skilled person may readily determine appropriate spinning temperatures, component flow rates and winding rates. The spinneret is preferably of the concentric type, although an eccentric type, trilobal type or hollow concentric type may also be used. Melt-spinning is a rapid and highly cost-effective means of producing fiber. Since no solvent is required during fiber production, no coagulation or wash baths are required for the produced fiber.

In order to obtain an even, well-defined fiber it is desirable to ensure that the viscosities of the core component and sheath component are well matched during spinning. Addition of hexagonal boron nitride increases the viscosity of the core component. In order to

compensate for this increase in viscosity, it may therefore be desirable to choose a second thermoplastic polymer that has a higher melt flow rate that the first thermoplastic polymer. For example, the first thermoplastic polymer may have a melt flow rate of from about 15 to about 40 g/lOmin, whereas the second thermoplastic polymer may have a melt flow rate of from about 35 to in excess of 1000 g/lOmin depending on the loading of h-BN filler to be used in the core component. For example, the second thermoplastic polymer may have a melt flow rate greater than 500 g/lOmin, preferably greater than 1000 g/min, such as 1200 g/lOmin. Melt flow rate is determined using the standard method of ISO 1133-1:2011, at a temperature of 230 °C and under a standard weight of 2.16 kg.

In order to maximize the amount of hexagonal boron nitride in the fiber, the fiber should preferably be spun using as high a core:sheath ratio as possible. The spun fiber preferably has a volumetric core:sheath ratio of from about 1:4 to about 4:1, preferably from about 1:1 to about 3:1. The spun fiber may have a diameter from about 30 pm to about 100 pm, preferably from about 50 pm to about 70 pm.

Fibers with a sufficiently large hexagonal boron nitride content may display enhanced thermal conductivity. For example, the thermal conductivity of the fiber may be 0.2 W-m^_1-K^_1 or more larger than a reference fiber not comprising h-BN (e.g. a reference fiber where the h-BN content in the core is replaced by an equivalent amount of the second thermoplastic polymer). The fiber may for example have a thermal conductivity of greater than 0.3

W-m^_1-K^_1 , preferably greater than 0.4 W-m^_1-K^_1 . The thermal conductivity may be measured as specified in ISO 22007-2:2015 "Plastics— Determination of thermal conductivity and thermal diffusivity— Part 2: Transient plane heat source (hot disc) method". The fibers described herein may be spun to yarn and/or be incorporated in textile products. For example, the fibers may be used to produce textile sheets or nets by methods known in the art.

The invention will now be described in more detail with reference to certain exemplifying embodiments and the figures. However, the invention is not limited to the exemplifying embodiments discussed herein and/or shown in the figures, but may be varied within the scope of the appended claims.

Examples

Production of h-BN containing composites Composites comprising polypropylene, hexagonal boron nitride and coupling agent were prepared by compounding in a twin screw extruder, using settings as detailed in Table 1 below. Polypropylene homopolymers having a melt flow rate of 38 g/lOmin (Borealis HL 512 FB) or 1200 g/lOmin (Borealis 450 FB) were used. The hexagonal boron nitride was obtained commercially (3M Grade SCP 1) and had a D50 particle size of 0.5 pm as determined by SEM. The coupling agent used was a maleic anhydride grafted polypropylene (Orevac CA 100) and was used in a ratio of 1:10 relative to the amount of h-BN. The feeding limit of the compounder was 30 wt% filler, so in order to obtain composites comprising in excess of 30 wt% filler, the compounded mixture was re-compounded with additional filler. The filler load was confirmed using thermogravimetric analysis. Table 1

Composite no. 1 2 3 4 (run twice) 5 (run twice) Polymer PP HL512FB PP HH450FB PP HL 512FB PP HH450FB PP HL 512FB

(MFR=1200) (MFR=38) (MFR=1200) (MFR=38) (MFR=1200)

Filler 1 h-BN h-BN h-BN h-BN h-BN

Filler 2 OREVAC OREVAC OREVAC OREVAC OREVAC

CA100 CA100 CA100 CA100 CA100

Filler 1 amount 30 30 15 40 50

(wt%) Filler 2 amount 3 3

(wt%)

Feeding rate

(g/h)

Polymer 5600 5600 8500 5600 5600

Filler 2400 2400 1500 2400 2400

Total 8000 8000 10000 8000 8000

Main screw 230 230 230 230 230 speed (rpm)

Side screw speed 350 350 350 350 350

(rpm)

Screw Filler Filler Filler Filler Filler configuration

Temperature

profile

T1 (°C) 165 165 165 165 165

T2 (°C) 210 210 210 210 210

Production of sheath-core fiber

After compounding, the produced composites were used to produce sheath-core fibers by bicomponent melt spinning. Table 2 (below) outlines the samples which successfully provided bicomponent melt-spun fibers. Fiber samples were produced comprising hexagonal boron nitride either in the sheath (samples 1-2), or in the core (samples 4-6). For samples 5 and 6, PA6 (BASF B33L) was used as the sheath material. A reference sample (sample 3) was produced which comprised only PP and did not comprise any hexagonal boron nitride. Table 2

Sample no. Core Sheath Sheath/core Volume flow Volume flow material material pump (rpm) rate core rate sheath

(cm³/min) (cm³/min)

1 PP HF420FB 30 wt% h-BN 5/10 24 12

PP HL512FB

2 PP HH450FB 30 wt% h-BN 5/10 24 12

PP HL512FB

3 (reference) PP HH450FB PP HH450FB 5/10 24 12

4 PP HH420FB 30 wt% h-BN 5/10 24 12

PP HH450FB

5 40 wt% h-BN PA6 B33L 10/5 24 12

PP HH450FB

6 50 wt% h-BN PA6 B33L 12/4 28.8 9.6

PP HL512FB

Figure la is a microscopy image of a core-sheath fiber wherein the sheath comprises 30 wt% h-BN (sample 4). Figure lb is an inset of Figure 1 with greater magnification. The approximate location of the inset is indicated by the white circle in Figure la. It can be seen that particles of the h-BN protrude from the surface of the fiber, giving a rough surface structure. Figure 2a shows a cross-section image of the same fiber obtained by SEM, and Figure 2b is an inset of the image of Figure 2a, but with greater resolution. The approximate location of the inset is indicated by the white circle in Figure 2a. From these images it can also be seen that some h- BN particles sit at the surface of the fiber.

The fibers obtained by melt spinning have been knitted to a textile tube. It was confirmed during spinning and knitting to a textile that fibers comprising h-BN in the sheath had a greater tendency to snag and break as compared to fibers comprising h-BN in the core.

Figure 3 is a microscopy image of a yarn bundle in cross-section, wherein each yarn is spun using 24 fibers, and each fiber in turn is melt-spun using a core component comprising 50 wt% h-BN in PP and a PA6 sheath (sample 6). The material observable between the fibers is a binder added to fix the bundle for microscopy. The high-flow polypropylene PP H512FB (MFR=1200 g/10 min) was used in the fiber core in order to compensate for high h-BN loading and provide a core component with suitable viscosity for melt spinning. Polyamide PA6 was chosen for use as a sheath material in the high h-BN load samples (samples 5 and 6) due to its ready availability and appropriate melt and viscosity characteristics. Attempts to spin a fiber using 50% h-BN core and a PP sheath were unsuccessful with the PP grades at hand, most probably due to an imbalance in viscosity between the core and sheath. Note however that a sheath material of polypropylene or another polyolefin is viable providing that an appropriate polymer of suitable viscosity and melt characteristics is chosen. It can be seen from Figure 3 that the obtained fiber diameter and ratio of core to sheath is somewhat uneven. This may be ascribed to a remaining imbalance of flow in the spin pack (spinneret), due to the viscosities of the core component and sheath component still being non-optimally balanced. It can be seen that some of the obtained fibers contain more core material than others. From this, it can be inferred that it should be possible to produce a fiber comprising at least 50% core component with a sheath thickness of approximately 20 pm. The total diameter of such a fiber is estimated to be approximately 55 pm. Therefore, fibers having a linear density of

approximately 20-30 dtex should be able to be produced upon process optimization.

Viscosity of h-BN/PP composites

The complex viscosities of a number of h-BN/PP composites were investigated as a function of angular frequency (dynamic shear). The composites investigated were Composite 2 (30 wt% h- BN in PP MFR=38); Composite 4 (40 wt% h-BN in PP MFR=38) and Composite 5 (50 wt% h-BN in PP MFR=1200). Pure PP (MFI=38) was also tested as a reference. The obtained plots are shown in Figure 4. It can be seen that for composites using the same PP matrix, increasing load of h-BN in the composite increases viscosity, especially at low shear rates, but that these differences become less pronounced at higher shear rates. It can also be seen that by using a high MFR PP as the matrix, Composite 5 (50 wt% h-BN in PP MFR=1200) attains a similar viscosity profile as to Composite 4 (40 wt% h-BN in PP MFR=38).

Thermal conductivity of h-BN composites

Table 3 (below) shows the thermal conductivity of a number of h-BN composite materials, measured using the method of ISO 22007-2:2015 "Plastics— Determination of thermal conductivity and thermal diffusivity— Part 2: Transient plane heat source (hot disc) method". The reference fiber comprising only PP (Sample 3) was also tested. It can be seen that the thermal conductivity of the fiber comprising 30 wt% h-BN in the sheath (Sample 4) does not differ markedly from the reference PP fiber (Sample 3). It can be seen however that the fiber having a core comprising 50 wt% h-BN (Sample 6) has a significantly improved thermal conductivity. A plaque produced from Composite 5 comprising 50 wt% h-BN has a much larger thermal conductivity than any of the fiber samples.

Table 3

Sample No. of Average thermal Standard

tests conductivity (W-m^_1-K^_1 ) deviation

Sample 3 (ref) 5 0.235 0.001

Sample 4 5 0.239 0.004

Sample 6 5 0.529 0.037

Composite 5 (Injection moulded 5 2.169 0.004 plaque, 2mm thick)

Claims

1. A sheath-core bicomponent fiber, wherein the sheath comprises a first thermoplastic polymer and the core comprises a second thermoplastic polymer and hexagonal boron nitride, wherein the first thermoplastic polymer and the second thermoplastic polymer are the same or different.

2. A sheath-core bicomponent fiber according to claim 1, wherein the core comprises from about 10 wt% to about 50 wt% of the hexagonal boron nitride, preferably from about 30 to about 50 wt%.

3. A sheath-core bicomponent fiber according to any one of the preceding claims, wherein the hexagonal boron nitride has a particle size of from about 0.1 pm to about 1 pm, preferably from about 0.4 pm to about 0.6 pm.

4. A sheath-core bicomponent fiber according to any one of the preceding claims, wherein the second thermoplastic polymer is a polyolefin, preferably a polypropylene homopolymer.

5. A sheath-core bicomponent fiber according to any one of the preceding claims, wherein the core further comprises a coupling agent, preferably a maleic anhydride grafted polyolefin, even more preferably a maleic anhydride grafted polypropylene.

6. A sheath-core bicomponent fiber according to claim 5, wherein the mass ratio of hexagonal boron nitride to coupling agent is from about 50:1 to about 5:1, preferably from about 15:1 to about 7:1, even more preferably about 10:1.

7. A sheath-core bicomponent fiber according to any one of the preceding claims, wherein the second thermoplastic polymer has a melt flow rate greater than 500 g/lOmin, preferably greater than 1000 g/min, as measured at 230°C under a standard weight of 2.16 kg using the standard method of ISO 1133-1:2011.

8. A sheath-core bicomponent fiber according to any one of the preceding claims, wherein the sheath comprises at least 70 wt% of the first thermoplastic polymer.

9. A sheath-core bicomponent fiber according to any one of the preceding claims, wherein the first thermoplastic polymer is a polyamide, polyolefin, or combination thereof, preferably a polypropylene homopolymer.

10. A sheath-core bicomponent fiber according to any one of the preceding claims, wherein the sheath and/or the core further comprise a flame retardant, wherein the flame retardant is preferably an alkoxyamine hindered amine light stabilizer (NOR HALS) flame retardant.

11. A sheath-core bicomponent fiber according to any one of the preceding claims, wherein the fiber has a diameter from about 30 pm to about 100 pm, preferably from about 50 pm to about 70 pm.

12. A sheath-core bicomponent fiber according to any one of the preceding claims, wherein the volumetric ratio of the core to the sheath is from about 1:4 to about 4:1, preferably from about 1:1 to about 3:1.

13. A sheath-core bicomponent fiber according to any one of the preceding claims, having a thermal conductivity of greater than 0.3 W-m^_1-K^_1 , preferably greater than 0.4 W-m^_1-K^_1 measured as specified in the method of ISO 22007-2:2015.

14. A method of manufacturing a sheath-core bicomponent fiber, wherein the core comprises hexagonal boron nitride, the method comprising the steps:

providing a first thermoplastic polymer;

compounding hexagonal boron nitride and a second thermoplastic polymer to provide an h-BN composite; and

melt spinning the first thermoplastic polymer and the h-BN composite through a bicomponent sheath-core spinneret to provide a sheath-core bicomponent fiber, wherein the core comprises hexagonal boron nitride.

15. A sheath-core bicomponent fiber obtained by the method according to claim 14.

16. A yarn or textile comprising a sheath-core bicomponent fiber according to any one of claims 1-13 or 15.