WO2008146048A1

WO2008146048A1 - Method of producing a solid polymer

Info

Publication number: WO2008146048A1
Application number: PCT/GB2008/050391
Authority: WO
Inventors: Peter Robert Claiden
Original assignee: Wellstream International Limited
Priority date: 2007-05-31
Filing date: 2008-05-30
Publication date: 2008-12-04
Also published as: GB0710398D0

Abstract

According to the present invention there is provided a method of producing a solid polymer, which comprises mixing a polymer solution with a liquid, wherein the polymer is insoluble in the liquid and is dissolved in a solvent which is miscible with the liquid; and precipitating the polymer from the resulting mixture. The disclosed method is particularly useful for the production of nanocomposites.

Description

METHOD OF PRODUCING A SOLID POLYMER

This invention relates to polymeric materials and processes for the production thereof. In particular, the invention relates to nanocomposites which may be useful as thermal insulating materials.

Thermal insulating materials are employed in a variety of scenarios. In oil-carrying pipes for example, a thermal insulating layer having low conductivity is used to ensure satisfactory flow of crude oil through the pipe. The insulating layer must also be able to survive the hydrostatic pressures at service depth and the mechanical loads during manufacture and pipe-laying. The thermal insulating material typically comprises a syntactic polypropylene tape filled with hollow glass microspheres. A disadvantage with these polypropylene tapes is that they have average thermal conductivity and can fail catastrophically, causing the glass spheres to shear across the principle axis of stress.

Nanocomposite materials typically comprise a host polymer matrix with nanoparticulate inclusions having at least one dimension which is less than 100 nm. Nanocomposites present a means of enhancing the physical properties of the host polymer. For example, a nanocomposite of a host polymer may be stronger than the polymer itself. Microparticles can also be included to further modify the physical properties such as additional toughness and tensile strength. Nanocomposites have a much larger area to volume ratio and hence a greater capacity for stress-transfer. Smaller volume fractions are required, so even if thermally conducting particles are used, they have little effect on overall conductivity. Below a critical particle radius, phononic scattering increases thermal resistance still further. Generally speaking, nanoparticles have fewer defects than the microparticles, so the potential for mechanical improvement is greater. The potential for a composite to improve mechanical properties is particularly important for polymer foams. A foamed polymer is lighter than the original material, but will be inherently weaker due to the voids created by gaseous inclusions. However, when properly produced, a foamed nanocomposite will have the dual properties of lightness and strength. Moreover, due to small surface irregularities, nanoparticles adhere more closely to the polymer molecule and transfer stress better. Nanocomposites and other polymeric materials may be produced by polymer solvation processes. For example, silicon dioxide nanocomposites may be prepared by dissolving the polymer matrix material in a solvent such as N, N- dimethylformamide or toluene. The dissolved polymer is then added to particles suspended in the same solvent and mixed. Mixing continues for a considerable time, often for hours. The solvent is then evaporated by gentle heating often under vacuum. Conventional solvation processes have inherent problems even on a laboratory scale. For example, only certain combinations of polymer and solvent are possible, limiting the choice of composite material. Furthermore, evaporation of the solvent is energy intensive and the entire process including solvent recovery can be prohibitively costly. Also, the shape of the material, which constitutes the solid residue after evaporation, is limited to the shape of the containing vessel. Moreover, if inclusions are required in the resulting material, mixing of the inclusions into the host material often takes a long time and does not always result in an even and random distribution of particles.

In one aspect, the present invention provides a novel methodology for the production of polymeric materials, in particular nanocomposites, which addresses the limitations of conventional solvation processes. In particular, the invention provides a method for producing nanocomposites and other polymeric materials which obviates the need for a solvent evaporation step.

According to the invention, a solid polymer is obtained by a method which comprises: a) mixing a polymer solution with a liquid, wherein the polymer is insoluble in the liquid and is dissolved in a solvent which is miscible with the liquid; and b) precipitating the polymer from the resulting mixture.

Any suitable polymer known in the art may be used. In particular, the polymer may be a polymeric host material. Of mention are host polymers that are soluble in polar solvents but insoluble in aqueous media. Thus, the host polymer may comprise one or more polar groups. Examples of suitable host polymers include polycarbonate and ethylene vinylalcohol. The solution further comprises a solvent which is miscible with the liquid. Typically, the solvent is a polar solvent, examples including N,N-dimethylformamide (DMF), acetonitrile and dimethylsulfoxide.

In embodiments, the liquid is an aqueous liquid. In a particular embodiment, the liquid is an aqueous liquid and the solution comprises a polar polymer dissolved in a polar solvent.

In a particular process of the invention, the liquid and/or the solution are in finely divided form, e.g. droplet form. This increases the interfacial area and hence mixing of the components of the miscible mixture. This may be achieved by applying a gas (e.g. air) flow to the liquid and/or the solution. Alternatively, droplets may be produced by inducing ultrasonic vibration in the liquid and/or the solution.

Mixing of the liquid and the solution may be achieved using a nozzle assembly. The use of a nozzle assembly may be advantageous in that it allows formation and application of the solid polymer to be controlled. For example, the nozzle assembly may be used to spray the resulting material onto the surface of a substrate.

The nozzle assembly may comprise one or more inlets selected from inlets for the solution, the liquid and a gas flow, and an outlet for the resulting flow. The inlets may be arranged such that the one or more resulting flows are relatively concentric. The flow pressures may be controlled to prevent adhesion of the precipitated material to the internal surface of the nozzle.

One or more nozzles may be used to control mixing and impart shape and size to the solid polymer as it precipitates from the nozzle outlet. The shape of the nozzle outlet can be selected so that the precipitant that forms at the nozzle outlet has a predetermined shape related to the nozzle shape. For example, the nozzle may be substantially cylindrical. Furthermore, a nozzle may be capable of controlled translational movement in the x, y and z axes as well as controlled angular rotation about these axes. Thus, the precipitant can be deposited in a pattern of predetermined size and shape. A plurality of such nozzles may be used to produce a pattern of solid polymer of a predetermined shape and size. One or more nozzles may used which comprise a former placed in the flow of the precipitant. A nozzle assembly comprising a gas flow may be used to ensure fine division of the liquid and/or the solution. In this case, droplet size may be controlled by the diameter of the nozzle, and the various flows. Where a gas flow is used, the introduction of gas into the miscible mixture may also introduce porosity into the material, thereby forming a porous or foamed polymeric material. Porosity, cell size, and cellular-nature may be determined by the diameter of the one or more nozzles, and the various flows. Alternatively, fine division of the liquid and/or the solution may be achieved by using ultrasonic vibration in conjunction with the nozzle assembly. In this case, droplet size may be determined by the diameter of the nozzle, flow velocity and frequency of the induced vibration.

In one embodiment, the liquid is delivered using a nozzle assembly into a container comprising the solution. In this case, the resulting polymeric material will typically be precipitated proximal to the nozzle outlet.

In another embodiment, the liquid and the solution undergo mixing in the nozzle assembly and solid polymer is precipitated as the mixture passes out of the assembly. In a particular process, the polymer is precipitated directly onto the surface of a substrate. This may be achieved using a nozzle assembly comprising inlets for the liquid, solution and a gas flow, and an outlet for the resulting mixed flow. The liquid and the solution will undergo mixing in the nozzle assembly and the resulting precipitate will be carried with the mixed flow onto the surface of the substrate. This may be particularly useful where the material is to be applied to the surface of an oil-carrying pipe.

A composite polymeric material may be obtained by adding inclusions to the solution and/or the liquid prior to mixing. Miscibility of the mixture permits inculcation of the inclusions into the material before it solidifies. The material may comprise microinclusions, typically formed by incorporating a microparticulate into the material prior to precipitation. Examples of suitable microparticulates include rubber particles and insoluble fibres, e.g. carbon fibres.

The polymer may be porous, for example comprising micropores. Thus, included in the invention are foamed polymeric materials. Porous polymers may be obtained by applying a gas flow to the miscible mixture as described above. Porous fraction, pore size and cellular nature may be controlled by varying the gas flow. A method of the invention allows the precipitated polymer to have any predetermined size or shape. For example, the precipitant can be deposited into a mould cavity having a predetermined size or shape.

The invention is particularly relevant to the production of nanocomposites. Included in the invention is a method of producing a nanocomposite which comprises: a) incorporating nanoparticles into a host polymer by mixing a solution comprising the host polymer with a nanoparticle suspension, wherein the host polymer is insoluble in the suspension and is dissolved in a solvent which is miscible with the suspension; and b) precipitating the polymer from the resulting mixture.

Nanocomposites obtained using a method of the invention may have desirable thermal conductivity and mechanical strength. The nanocomposites may be particularly suitable for use as thermally insulators, e.g. in oil-carrying pipes. Thus, the invention provides a material comprising a substrate and a nanocomposite of the invention supported thereon. The substrate may be a pipe, for example an oil carrying pipe, and the nanocomposite may be present as a thermal insulating layer. The thermal insulating layer may be coated with an extruded polymer layer to hold the insulating layer in place, prevent water ingress and protect against scuffing.

For the production of nanocomposites, a suspension comprising nanoparticles is contacted with a solution, wherein the solution comprises a host polymer which is insoluble in the suspension and a solvent which is miscible with the suspension. Contacting of the suspension and solution results in formation of a miscible mixture is formed in which nanoparticles are incorporated into the host material. The resulting host polymer then precipitates to form the nanocomposite, which may be separated from the mixture by, for example, mechanical separation.

In a particular embodiment, the nanoparticle suspension is an aqueous suspension and the solution comprises a polar host polymer dissolved in a polar solvent. Thus, included in the invention is a nanocomposite comprising a host polymer incorporating nanoparticles, wherein the host polymer is a polar polymer which is insoluble in aqueous media. Any suitable host polymer known in the art may be used to produce the nanocomposite material. Of mention are host polymers that are soluble in polar solvents but insoluble in aqueous media. Thus, the host polymer may comprise one or more polar groups. Examples of suitable host polymers include polycarbonate and ethylene vinylalcohol.

The solution further comprises a solvent which is miscible with the nanoparticle suspension. Typically, the solvent is a polar solvent, examples including N, N- dimethylformamide (DMF), acetonitrile and dimethylsulfoxide.

In embodiments, the nanoparticle suspension is an aqueous suspension, e.g. comprising nanoparticles suspended in an aqueous medium. Nanoparticle suspensions are commercially available, e.g. from the GRACE Davison Company

The nanoparticles may comprise any suitable nanoparticulate known in the art.

Examples include inorganic nanoparticulates such as nanoparticulate silicon dioxide.

The nanoparticles will generally have at least one dimension less than 100 nm.

Thus, the nanoparticles may have a diameter ranging from about 1 to about 99 nm, e.g. from about 1 to about 50 nm. The nanocomposite may be commercially available or may be produced by, for example, a sol-gel process.

Miscible mixing of the nanoparticle suspension and the polymer solution ensures that the nanoparticles are incorporated into the host polymer before it precipitates and solidifies. The area of contact between components and hence mixing may be enhanced by finely dividing the suspension of nanoparticles, e.g. into small droplets, prior to mixing. This may be achieved by introducing ultrasonic vibration into the flow of the suspension prior to mixing. The small droplets formed increase the interfacial area between the two components of the miscible mixture. Furthermore, the interface between the two components of the miscible mixture will be more turbulent when droplets are used, thus further improving the mixing.

The nanocomposite may comprise nanoparticle loadings of from about 1 to about 10 wt %, e.g. from about 2 to about 5 wt %. Such levels of loading are desirable since they may result in improved bulk modulus, thus allowing greater porosity and hence thermal resistance. Generally, it will be preferable for the nanocomposite to comprise a substantially homogeneous dispersion of nanoparticles in the host material. In this way, the applied load that can be carried by the nanocomposite may be maximised.

The nanocomposite may comprise microinclusions, typically formed by incorporating a microparticulate into the host polymer prior to precipitation. Examples of suitable microparticulates include rubber particles and insoluble fibres, e.g. carbon fibres.

The nanocomposite may be porous, for example comprising micropores. Thus, included in the invention are nanocomposite foams. Porous nanocomposites may be obtained by applying a gas flow to the miscible mixture as described above. Porous fraction may be controlled by varying the air flow.

The following Example illustrates the invention.

Example

Polymeric solution was produced by dissolving polycarbonate in dimethylformamide or ethylene vinylalcohol copolymer in dimethylsulfoxide. Nanoparticulate suspensions were obtained from the GRACE Davison Company and comprised a colloidal suspension of silica (silicon dioxide) nanoparticles in an aqueous liquid. The dimensions of the nanoparticles in each suspension were 40 nm, 12 nm and 7 nm.

The aqueous suspension of nanoparticles was finely divided into small droplets by applying an air flow to the liquid flow prior to mixing. The aqueous suspension was supplied from an air pressurised reservoir and the air blast was supplied from a separate reservoir. The reservoirs were separately pressurised and controlled. As well as finely dividing the suspension of nanoparticles, this had the effect of introducing air into the precipitant so that a nanocomposite foam was produced. In addition, the turbulent interface further improves mixing.

Scanning electron microscopy (SEM) was used to characterise the resulting nanocomposite. Fig. 1 shows an SEM image of a polycarbonate nanocomposite incorporating 40 nm silicon dioxide nanoparticles. The SEM image depicts micropores of approximately 2 μm diameter, confirming that a nanocomposite form had been produced. Energy-dispersive X-ray microanalysis (EDXMA), which is capable of penetrating up to approximately 10 μm below the material surface, showed that there the distribution of the silicon dioxide nanoparticles in the polymer was substantially uniform. Fig. 2 shows the SEM of Fig. 1 with the results of the EDXMA superimposed. The small dots in Fig. 2 represent the silicon dioxide nanoparticles.

The thermal conductivity (k value) of the resulting nanocomposite was determined by measuring the temperature difference across a sample of know thickness and comparing values with those of a sample of pristine polymer, having a known thermal conductivity. Inclusion of nanoparticles and porosity in the polymer was found to result in approximately a 70% reduction in thermal conductivity.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

1 . A method of producing a solid polymer, which comprises: a) mixing a polymer solution with a liquid, wherein the polymer is insoluble in the liquid and is dissolved in a solvent which is miscible with the liquid; and b) precipitating the polymer from the resulting mixture.

2. A method according to claim 1 , wherein the material is a host polymer.

3. A method according to claim 2, wherein the host polymer comprises microinclusions.

4. A method according to any preceding claim, wherein the liquid is in finely divided form.

5. A method according to any preceding claim, wherein the liquid is an aqueous liquid and the solution comprises a polar polymer dissolved in a polar solvent.

6. A method according to claim 5, wherein the polymer comprises polycarbonate or ethylene vinylalcohol.

7. A method according to claim 5 or claim 6, wherein the polar solvent is selected from N,N-dimethylformamide, acetonitrile and dimethylsulfoxide.

8. A method according to any preceding claim, wherein mixing takes place in the presence of an applied gas flow.

9. A method according to any preceding claim, wherein the liquid and solution are mixed using a nozzle assembly.

10. A method according to claim 9, wherein the nozzle assembly is used to deliver one of the solution and the liquid to the other.

1 1 . A method according to claim 9, wherein mixing takes place in the nozzle assembly.

12. A method according to any of claims 9 to 1 1 , wherein the nozzle assembly comprises one or more inlets selected from inlets for the liquid, the solution and a gas flow.

13. A method according to any preceding claim, wherein the solid polymer is a nanocomposite and wherein the method comprises: a) incorporating nanoparticles into a host polymer by mixing a solution comprising the host polymer with a nanoparticle suspension, wherein the host polymer is insoluble in the suspension and is dissolved in a solvent which is miscible with the suspension; and b) precipitating the polymer from the resulting mixture.

14. A method according to claim 13, wherein the nanoparticle suspension is an aqueous suspension and the solution comprises a polar host polymer dissolved in a polar solvent.

15. A nanocomposite comprising a host polymer incorporating nanoparticles, obtainable by a method of claim 13 or claim 14.

16. A nanocomposite according to claim 15, wherein the host polymer comprises microinclusions.

17. A nanocomposite according to claim 15 or claim 16, which is in the form of a foam.

18. A nanocomposite according to claim 17, wherein the host polymer is microporous.

19. A nanocomposite according to any of claims 15 to 18, wherein the host polymer is insoluble in aqueous media.

20. A nanocomposite according to claim 19, wherein the host polymer comprises polycarbonate or ethylene vinylalcohol.

21 . A material comprising a substrate and a nanocomposite of any of claims 15 to 20 supported thereon.

22. A material according to claim 21 , wherein the substrate is a pipe.

23. A material according to claim 22, wherein the nanocomposite is present as a layer on said pipe.

24. Use of a nanocomposite of any of claims 15 to 20, as a thermal insulator.