Treatment of polymer particles

Abstract

A method for treating polymer particles is disclosed. Polymer particles and a liquid are provided. The method includes the following steps (a) and (b). (a) Mixing said polymer particles with said carrier liquid to form a dispersion of said particles in said carrier liquid at a concentration of at least 0.1 g/L, based on the volume of the dispersion. (b) Subjecting the dispersion to microfluidization treatment thereby causing particle stretching, particle size reduction and increasing the surface area per unit mass of the polymer particles. Also disclosed is a particulate composition comprising polymer particles mixed with nanoplates derived from a layered material, wherein the particulate composition has a BET surface area of at least 10 m.sup.2/g. Furthermore, there is disclosed a method for the manufacture of a component formed of a composite of a polymer with a dispersion of nanoplates. The particulate composition is provided as a precursor particulate. Then the precursor particulate is formed into the component.

Claims

1. A method for treating polymer particles, the method including providing polymer particles and providing a carrier liquid, the method further including the steps: (a) mixing said polymer particles with said carrier liquid to form a dispersion of said particles in said carrier liquid at a concentration of at least 0.1 g/L, based on the volume of the dispersion; and (b) subjecting the dispersion to microfluidization treatment thereby causing particle stretching, particle size reduction and increasing the surface area per unit mass of the polymer particles.

2. The method according to claim 1 wherein the microfluidization treatment of step (b) comprises (b(i)) pressurizing the dispersion to a pressure of at least 10 kpsi; and (b(ii)) forcing the dispersion along a microfluidic channel under said pressure, to apply a shear rate of at least 10.sup.5 s.sup.−1 to said particles in the dispersion, thereby causing particle stretching, particle size reduction and increasing the surface area per unit mass of the polymer particles.

3. The method according to claim 1 further including the step of adding particles of a layered material to the dispersion, the microfluidization treatment causing exfoliation of nanoplates from said particles.

4. The method according to claim 1 further including the step of adding nanoplates derived from a layered material to the dispersion.

5. The method according to claim 4 wherein the nanoplates are added to the dispersion before step (b).

6. The method according to claim 4 wherein the nanoplates are added to the dispersion after step (b).

7. The method according to claim 1 wherein the nanoplates are selected from one or more of elemental materials such as graphene (typically derived from pristine graphite), metals (e.g., NiTe.sub.2, VSe.sub.2), semi-metals (e.g., WTe.sub.2, TcS.sub.2), semiconductors (e.g., WS.sub.2, WSe.sub.2, MoS.sub.2, MoTe.sub.2, TaS.sub.2, RhTe.sub.2, PdTe.sub.2), insulators (e.g., h-BN, HfS.sub.2), superconductors (e.g., NbS.sub.2, NbSe.sub.2, NbTe.sub.2, TaSe.sub.2) and topological insulators and thermo-electrics (e.g., Bi.sub.2Se.sub.3, Bi.sub.2Te.sub.3).

8. The method according to claim 1 wherein the layered material is graphite and the nanoplates are graphite nanoplates.

9. The method according to claim 1 wherein the layered material is pristine graphite and the nanoplates are graphite nanoplates.

10. The method according to claim 2, wherein the dispersion subjected to step (b) is subjected to steps (b(i)) and (b(ii)) repeatedly, either via the same or different microfluidic channels, according to a number of cycles, wherein the number of cycles is at least 2.

11. The method according to claim 10 wherein the particles of a layered material are added after at least one cycle of steps (b(i)) and (b(ii)).

12. The method according to claim 11 wherein the dispersion including the nanoplates is subjected to at least one further cycle of steps (b(i)) and (b(ii)).

13. The method according to claim 1 further including the step of removing the carrier liquid, to manufacture a composition comprising polymer particles mixed with nanoplates derived from a layered material.

14. A particulate composition comprising polymer particles mixed with nanoplates derived from a layered material, wherein the particulate composition has a BET surface area of at least 80 m.sup.2/g and the polymer particles have an aspect ratio defined as length/thickness of greater than 10.

15. The particulate composition according to claim 14 wherein the nanoplates are present in an amount of at least 0.1 wt % based on the mass of the particulate composition.

16. A method for the manufacture of a component formed of a composite of a polymer with a dispersion of nanoplates, the method including the steps: providing a particulate composition as a precursor particulate, the precursor particulate comprising polymer particles mixed with nanoplates derived from a layered material, wherein the particulate composition has a BET surface area of at least 80 m.sup.2/g and the polymer particles have an aspect ratio defined as length/thickness of greater than 10; and forming the precursor particulate into the component.

17. The method according to claim 16, further including mixing the particulate composition with further polymer particles to form the precursor particulate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1 shows a schematic flow diagram of suitable process for manufacturing products according to embodiments of the invention.

(3) FIG. 2 shows a schematic arrangement of a microfluidizer for use in the present invention. The interaction chamber is shown in schematic enlarged view at two scales, the most enlarged scale indicating the transition from laminar flow to turbulent flow as the flow passage is restricted. The most enlarged scale view is taken from http://digilander.libero.it/mfinotes/IVEuropeo/fluido2/fluido2.html [accessed 22 Sep. 2017]

(4) FIG. 3(a) shows an SEM image of a pristine polymer particle not yet subjected to microfluidization.

(5) FIGS. 3(b), 3(c), 3(d), 3(e) show SEM images of polymer particles after different numbers of microfluidization cycles.

(6) FIGS. 4(a) and 4(b) shows SEM images of nylon-12 nanowires.

(7) FIGS. 5(a) and 5(b) show SEM images of graphene/nylon-12 nanowire compound showing that the graphene is homogeneously dispersed.

(8) FIG. 6(a) shows an SEM image of pristine nylon 12 powder.

(9) FIG. 6(b) shows an SEM image of nylon nanowire after 20 microfluidization cycles.

(10) FIG. 6(c) shows an SEM image of nylon nanowire after 100 microfluidization cycles.

(11) FIG. 7 shows histograms of diameter distribution from the starting polymer powder material and after 5, 20, 50 and 100 cycles.

(12) FIG. 8 shows nitrogen adsorption/desorption curve for pristine nylon-12 powders and nylon-12 nanowires.

(13) FIG. 9 shows BET surface area of pristine nylon-12 powders and nylon-12 nanowires plotted against the number of microfluidization cycles.

(14) FIG. 10 shows XRD spectra of pristine nylon-12 powders and nylon-12 nanowires.

(15) FIG. 11 shows DSC plots of pristine nylon-12 powders and nylon-12 nanowires.

(16) FIG. 12 compares the thermal conductivity of pristine nylon-12 powders and nylon-12 nanowires.

(17) FIG. 13 shows thermal conductivity of nylon-12 composites processed by different methods.

(18) FIG. 14 shows various polymer compositions to which the present invention may be applied. This is taken from http://www.dupont.co.uk/products-and-services/plastics-polymers-resins/thermoplastics/products/zytel-htn.html [accessed 22 Sep. 2017].

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER OPTIONAL FEATURES OF THE INVENTION

(19) In some preferred embodiments, there are disclosed scalable microfluidic routes to fabricate polymer nanowires. These methods allow the tuning of the size and properties of the produced nanowires in a controllable way. Moreover, we demonstrate some potential applications of the PNWs which, to the knowledge of the inventors, have not been shown before. It is considered that the present disclosure will trigger new research in the polymer nanowire field.

(20) The process to fabricate polymer nanowires is shown schematically in FIG. 1. This is shown in the context of the production of either polymer nanowire or polymer/graphene compound. Polymer/graphene compounds are discussed later.

(21) Briefly, in FIG. 1, it can be seen that in step A, the polymer powder is dispersed in water/isopropanol (IPA) by magnetic stirring. Then, in step B, the dispersion is subjected to microfluidization. This is described in more detail below. The microfluidized product can then be filtered and dried (step C) or freeze dried (step D). If freeze dried, the product becomes an aerogel (step E).

(22) Thus, the product can either be polymer nanowire (also called nanofibre—step F) or polymer/graphene compound (step G). The product can be subjected to melt processing in step J (as is typical for thermoplastic materials) or may be subjected to 3D printing (step K) in order to manufacture a component at step L.

(23) In one example, DuraForm polyamide-12 (nylon-12) is used as a starting material. The average size of the nylon-12 powder is around 34 μm, which is suitable for flow in microchannels with a diameter of 87 μm. It should be noted that the powder size should be smaller than the microchannel diameter otherwise a blockage may happen. The fabrication of other polymer nanowires was also successfully demonstrated, such as polyethylene, PEEK and polyaniline. Other thermoplastic/thermoset polymers may be used. The polymer particles are pre-dispersed by magnetic stirring in water/IPA (FIG. 1 step A). The mixture is then microfluidized by using a microfluidic processor (M110-P, Microfluidics Inc.) with a Z-type geometry interaction chamber (e.g. G10Z) (see the schematic shown in FIG. 2). During microfluidization, the mixture was forced to pass through a microchannel (diameter, d<100 μm) by applying high pressure (up to 207 MPa). Here we use maximum pressure available from our system and various numbers of process cycles (1 to 100). One cycle means a complete pass through the interaction chamber. The key advantage of microfluidization is homogeneous application of high shear force across whole fluid. Subject to the applied shear force, the polymer particles dispersed in the fluid were stretched cycle by cycle, and finally the PNWs are yielded (FIGS. 3(a)-3(e)). Taking nylon-12 as an example, the original size of the particles are around 34 μm, which reduced to 45 nm after 100 cycles of microfluidization (FIGS. 3(a)-3(e)). The size of nylon nanowire can be controlled by controlling the microfluidization parameters, such as number of cycles (FIGS. 6(a), 6(b), 6(c) and 7) and pressure.

(24) In the microfluidization process (at least when applied to exfoliation of nanoplates from graphite—see below), the Reynolds number Re indicates fully developed turbulent flow:

(25) $Re=\frac{\rho \mathrm{UD}}{\mu }=2.7\times {10}^4$

(26) The pressure losses inside the channel of the interaction chamber are estimated as:

(27) $\Delta p=\frac{\mathrm{fL}\rho U^2}{2D}=3.25\times {10}^7\mathrm{Pa}$

(28) [See Ref. 14 which sets of full details of a corresponding microfluidization process.]

(29) The energy dissipation inside the channel is given by

(30) $\epsilon =\frac{Q\Delta p}{\rho V}=8.3\times {10}^9{m}^2/s^3$

(31) The turbulent shear rate is given by

(32) $\stackrel{.}{\gamma }=\sqrt{\frac{\epsilon }{v}}=9.2\times {10}^7{s}^{-1}$

(33) and in the present embodiment the turbulent shear rate is >>10.sup.4 s.sup.−1. [Paton et al (2014]

(34) As indicated in FIG. 3, in the present embodiment the Kolmogorov length is determined as:

(35) $\eta ={\left(\frac{v^3}{\epsilon }\right)}^{\left(1/4\right)}\sim 103\mathrm{nm}$

(36) Considering the two plate model for the determination of shear rate, in this model the shear rate is the speed drop across the gap, which can be obtained from the velocity v [m/s] and the plate separation y [m]:

(37) $\stackrel{.}{\gamma }=\frac{\mathrm{dv}}{\mathrm{dy}}\left[s^{-1}\right]$

(38) In the formation of polymer nanowires, the particles are stretched due to the applied shear force when passing through the microfluidic channel The particle diameter is reduced by about 1000 times from about 34 μm to about 45 nm. Thus, the particles are converted to a fibrous shape. This is shown by considering the progression of the polymer particles of FIG. 3(a) (pristine particles) as the number of microfluidization cycles increases in progression from FIG. 3(b) to FIG. 3(c) to FIG. 3(d) to FIG. 3(e).

(39) The mean surface area of the nylon nanowires are determined by nitrogen adsorption (FIGS. 8 and 9). The BET surface area significantly increased with the increasing cycles (FIG. 9). The BET surface area of pristine nylon-12 powders is about 6 m.sup.2/g. With only 5 cycles of microfluidization, the BET surface area of the polymer nanowires became about 155 m.sup.2/g, which increased to about 473 m.sup.2/g after 100 cycles.

(40) X-ray diffraction was applied to characterize the crystallization of nylon-12 nanowires. The pristine nylon-12 powders and nylon-12 nanowires exist as γ-form with two plane of [(001) and (201)] and (200) (FIG. 10 [Ref. 15]. The [(001) and (201)] plane has a spacing of 0.421 nm, while (200) has a spacing of 0.402 nm. It is clearly shown that the ratio of (200) plane to [(001) and (201)] plane decreased with the increasing number of cycles. This observation indicates that the microfluidization process alters the orientation of the nanowires.

(41) Furthermore, the melting enthalpy of nylon-12 nanowires increased to 88.99 J/g, compared to 85.72 J/g for the pristine nylon-12 nanowires, as shown in FIG. 11. This observation indicated an increase in crystallinity.

(42) Thus, the polymer nanowires produced by microfluidization have a higher degree of orientation and crystallinity compared to the starting material. The high orientation and crystallinity can result in superior performance in properties.

(43) The thermal conductivity (κ) of the polymer nanowires was investigated by using a combination of modulated differential scanning calorimetry (MDSC) and the laser flash method. The polymer powders or nanowires are pressed into a film under a pressure of 10 bar and the thermal diffusivity of the film is measured. Thermal diffusivity (D) was determined by the laser flash method (NETZSCH LFA 467 Hyper Flash). The heat capacity (C.sub.p) of the polymer powder/nanowire was measured using a TA Instruments DSC Q20 calorimeter in MDSC mode. The thermal conductivity was obtained from the relationship κ=DC.sub.pρ, where, ρ is density. After 100 cycles of microfluidization, the thermal conductivity of nylon-12 nanowires becomes 2.744 W/(m*K), which is a 900% increase compared to the pristine nylon-12 (0.304 W/(m*K)) (FIG. 12). The high thermal conductivity is considered to originate from the high orientation nature and the high crystallinity of the nanowires. It is, therefore, of interest to consider applications of the nanowires in the textiles industry, in order to enable smart textiles with much improved thermal management ability.

(44) Graphene can be incorporated into a bulk matrix to produce nanocomposites for various applications, such as mechanical reinforcement [Ref. 16], and thermal management [Ref. 17]. In automotive thermal management applications, polymers such as polyamide (PA) are predominantly used (Dupont UK). For such applications, composites containing few-layer graphene (FLG) with aspect ratio (AR) larger than 100 were reported to perform better than those containing single-layer graphene (SLG) with comparable AR [Ref. 18]. Melt compounding using screw extrusion is the most common industrial route to prepare composites [Ref. 19]. However, poor dispersion [Refs. 20, 21] of graphene limits performance [Ref. 16] and applications [Ref. 17] even if pre-mixing techniques are applied. One key reason for the poor dispersion is the limited surface area of polymer particle/pellets (<5 m.sup.2/g). Thus, there is significant mismatch between the surface of graphene (typical surface area >50 m.sup.2/g) and exposed surface of polymer, which leads to aggregation of graphene. Here, we produce polymer nanowires/graphene compound by microfluidisation [Ref. 14], which later can be melt compressed into a composite with uniform dispersion of flakes. FLG with AR>500 and PA are pre-mixed by magnetic stirring in water/isopropanol. The mixture is then microfluidized to yield a FLG/PA dispersion, then filtered and oven dried to give a FLG/PA compound. The FLG is homogenously attached onto the surface of nylon-12 nanowires (FIGS. 5(a) and 5(b)). Compare this with the corresponding SEM images of nylon-12 nanowires without FLG (FIGS. 4(a) and 4(b)).

(45) The FLG/PA compound was directly melt processed to produce a final composite. A thermal conductivity of about 1.3 W/m.sup.−1K.sup.−1 is achieved with 5 wt. % FLG, using four times less FLG compared to twin-screw extrusion (about 20 wt. %) (FIG. 13).

(46) Note that in FIG. 13, the abbreviations used are:

(47) PA=polyamide, Gr=graphene, RGO=reduced graphene oxide, f-Gr=functionalised graphene, CNT=carbon nanotube, MF=microfluidisation, and TSE=twin screw extrusion.

(48) The references listed in FIG. 13 (data for comparison to the example of the present invention) are: [Ref. 1] N. Song et. al, Compos Part a-Appl S, 73 (2015) 232-241. [Ref. 2] P. Ding et. al, Carbon, 66 (2014) 576-584. [Ref. 3] Z. Q. Shen et. al., Compos Sci Technol, 69 (2009) 239-244.

(49) Microfluidic processing can be used for masterbatch production. A masterbatch is an intermediate product containing high concentration (5 to 20 wt. %) of flakes [Ref. 19], which can later be diluted with polymer to produce the desired end-products. Current masterbatch production methodologies often require the introduction of surfactants [Refs. 22, 23] to aid graphene dispersion. However, they can have a negative impact on composite properties, such as increased interfacial resistance (i.e. reduced heat transfer) [Ref. 24]. Our microfluidic processing route is surfactant-free. This is additionally advantageous in applications where surfactants are prohibited, e.g. in bioapplications.

(50) The present invention can be applied to many different thermoplastic or thermoset materials. These polymers can be standard, engineering, or high-performance polymers, semiconducting polymers or biopolymers. For example, the present invention has been successfully applied to:

(51) Polyamide (PA)

(52) Ultra-high molecular weight polyethylene (UHMWPE)

(53) Polyether ether ketone (PEEK)

(54) Polyaniline (PANI)

(55) poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)

(56) It is considered that the present application can be applied to many different polymeric materials. In particular, any of the compositions indicated in FIG. 14 are suitable, and most particularly compositions on the left hand side of the diagram.

(57) Of particular interest are high performance polyamides HPPA: PA 11, PA 12, PA 6T, PA 9T, PA 46, PARA, PPA, PA MXD6. These materials tend to have excellent mechanical properties, chemical resistance and high heat stability.

(58) The materials produced in the present invention are considered to be of use for: Automotive thermal management components, in particular replacing metal components Engineering applications: cooling system components (in particular replacing metal components)

(59) Components in aerospace industries

(60) 3D printing

(61) Masterbatch

(62) High performance fiber filler

(63) PA12, which has been used in the illustrated examples of the present invention has particularly suitable characteristics:

(64) High mechanical performance: traction and continuous or alternate flexion

(65) High flexibility even at very low temperature

(66) Very low water absorption

(67) Excellent dimensional stability

(68) Very good resistance to chemicals and weathering and very good hydrolysis resistance

(69) Lowest density of all polyamides

(70) High impact strength down to −40° C.

(71) Biocompatibility

(72) At present, the main applications of PA12 are:

(73) Compressed air systems in car industry

(74) Aeronautic components

(75) Sanitary components (replace metallic parts)

(76) Pipes: robotic, pneumatic, tools, industrial machinery, etc.

(77) Sports and leisure goods

(78) Housings for high quality electronic devices

(79) Medical devices

(80) Optical components

(81) 3D printing

(82) Embodiments of the present invention may therefore include:

(83) Electrically conductive PA12 (compounded with a conductive material derived from a layered material, e.g. graphene) can be used in fuel systems

(84) Thermal conductive PA12 can be used in cooling systems, heat exchangers etc.

(85) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(86) All references referred to above are hereby incorporated by reference.

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