WO2011086354A2

WO2011086354A2 - Sonication method and apparatus

Info

Publication number: WO2011086354A2
Application number: PCT/GB2011/000041
Authority: WO
Inventors: Bala Kandasubramanian
Original assignee: Pera Innovation Ltd
Priority date: 2010-01-13
Filing date: 2011-01-13
Publication date: 2011-07-21
Also published as: GB201000527D0; WO2011086354A3

Abstract

A method for dispersing a sample of entities which are prone to agglomeratio comprises the steps of (i) sonicating said entities to disperse them temporarily, and (ii) attaching polar moieties to said entities in order to prolong the dispersal. Apparatus for sonicating a sample comprises a container for the sample and a source of sonic waves, wherein the dimensions of the container are selected to maximise the efficiency of the sonication.

Description

Sonication Method and Apparatus

The present application relates to a method of chemically modifying multi-walled carbon nanotubes, for example by using a sonication apparatus. It also relates to apparatus for carrying out sonication of a sample (for example by transmitting ultrasound waves through the sample) and in particular to an apparatus for sonicating multi-walled carbon nanotubes.

Multi-walled carbon nanotubes (MWCNTs) have been considered for use as a reinforcing agent in the polymer matrix of structural materials as they have very high strength, stiffness and flexural modulus and a large aspect ratio. However, this application critically depends on efficient interfacial molecular adhesion between the MWCNTs and the polymer component and on effective molecular level dispersion of the MWCNTs. This has been seen as a huge challenge in most of the industries because MWCNTs tend to tightly bundle together as a result of van der Waals forces. The resulting bundle structure affects the thermal, electrical and mechanical properties of the reinforced polymer.

Attempts have been made to prevent such bundling by carrying out exfoliation and surface functionalisation of the MWCNTs to disrupt or reduce the van der Waals forces. However, up to now this has required special physical or chemical treatments such as modification with surfactants, treatment with organic solvents etc. In particular, the reactions in question have employed highly toxic and corrosive chemicals like peroxytrifluoroacetic acid, mixtures of concentrated sulphuric acid and nitric acid, mixtures of sulphuric acid and hydrogen peroxide, potassium permanganate in the presence of strong sulphuric acid etc. These reactions are reported to take place at temperatures ranging from 500°C to 1000°C for a minimum period of 6 hours and up to a couple of days. Clearly such reactions are unfavourable from both an environmental and economic perspective.

As an alternative, exfoliation of MWCNTs by sonication has been reported by some researchers but that was carried out in the presence of acidic chemical agents and other unfavourable additives (see for example Hong et al., Composites Science and Technology 67 (2007), 1027-1034). Industries have reported exfoliation of MWCNTs with sonication with additional high energy shear mixing involving high temperature and pressure. With such exfoliation processes improved exfoliation was achieved but due to the harsh chemicals and thermal conditions involved in the process, the MWCNTs in question lost most of their reinforcing and mechanical properties.

It would be desirable to develop a method of chemically modifying MWCNTs without having to use the toxic chemicals of the prior art. Chemical modification of MWCNTs is an important step to improve their surface functionality. It shows a great potential in improving the physical and chemical properties for speciality applications like electrochemical storage, hybrid solar cells, photovoltaic devices, electroactive polymer sensors, replacement of metallic additives for antistatic coatings and conducting composites.

In accordance with a first aspect of the present invention, there is provided a method for dispersing a sample of entities which are prone to agglomeration comprising the steps of (i) employing bubbles to disperse said entities temporarily, and (ii) attaching polar moieties to said entities in order to prolong the dispersal.

The entities are present in a liquid medium and the bubbles are gaseous particles within the liquid medium. In a preferred embodiment, the bubbles are produced by means of a sonication apparatus, and in a particularly preferred embodiment the sonicating step is carried out by using an apparatus as defined below.

Preferably, the bubbles have an average diameter of up to lOOnm, more preferably from lnm to lOOnm and most preferably from lOnm to lOOnm. In the shorthand known to the skilled addressee, such bubbles are known as 'nanobubbles'.

An aspect of the invention is based on highly energy efficient sonication reaction of

MWCNTs in the presence of viscous solution of semicrystalline polymer like poly (vinyl alcohol) in specially designed reactor vessel. In this research deionised water was used as a dispersing media which proves the process to be highly industrially viable and economical. Various forms of sonication apparatus are known in the art, and examples can be found in the following publications: JP 58163425 A; US 6515030 B2; WO 00/57756 Al; WO 02/079751 A2; JP 59199025 A; JP 2004033948 A; US 3957252 A; and EP 1552879 A. In a second aspect of the present invention, there is provided apparatus for sonicating a sample, comprising a container for a sample, said container having a notional maximum level to which sample can be filled, and a source of sonic waves positioned within the container below said level, wherein the ratio of (i) the shortest distance from the centre of said source to said level to (ii) the distance from the centre of said source to the furthest point from the centre of the source within the container is from 1 :2 to 1 :5, preferably about 1 :3. This is the ratio G to E in Figure 3.

In a third aspect of the present invention, there is provided apparatus for sonicating a sample, comprising a container for a sample, said container having a base and a notional maximum level to which sample can be filled, and a source of sonic waves positioned within the container below said level, wherein the ratio of (i) the shortest distance from the centre of said source to said level to (ii) the shortest distance from the centre of said source to said container base is from 1 : 1 to 1 :5, preferably about 1 :3. This is the ratio G to D in Figure 3. In a fourth aspect of the present invention, there is provided apparatus for sonicating a sample, comprising a container for a sample, said container having side walls and a notional maximum level to which sample can be filled, and a source of sonic waves positioned within the container below said level, wherein the ratio of (i) the shortest distance from the centre of said source to said level to (ii) the shortest distance from the centre of said source to one of the side walls is from 1 :0.5 to 1 :4. This is the ratio G to B in Figure 3.

Any one of the containers defined above may have a base and side walls tapered towards the base to result in additional internal corners within the container. In a fifth aspect of the present invention, there is provided apparatus for sonicating a sample, comprising a container for a sample, said container having four side walls, two of which on opposite sides of the container have tapered sections, and a source of sonic waves positioned within the container, wherein the ratio of (i) the shortest distance from the centre of said source to one of said tapered sections to (ii) the shortest distance from the centre of said source to said container base is from 1 :08 to 1 :1.05. This is the ratio C to D in Figure 3.

In a sixth aspect of the present invention, there is provided apparatus for sonicating a sample, comprising a container for a sample, said container having a base and side walls tapered towards the base, and a source of sonic waves positioned within the container, wherein the ratio of (i) the shortest distance from the centre of said source to one of said tapered sections to (ii) the distance from the centre of said source to the furthest point from the centre of the source within the container is from 1 : 1 to 1 :1.1. This is the ratio C to E in Figure 3.

In a seventh aspect of the present invention, there is provided apparatus for sonicating a sample, comprising a container for a sample, said container having a base and side walls tapered towards the base and a notional maximum level to which sample can be filled, and a source of sonic waves positioned within the container below said level, wherein the ratio of (i) the shortest distance from the centre of said source to said level to (ii) the shortest distance from the centre of said source to one of said tapered sections is from 1 :2 to 1 :5. This is the ratio G to C in Figure 3.

In an eighth aspect of the present invention, there is provided apparatus for sonicating a sample, comprising a container for a sample, said container having a base and side walls tapered towards the base and a notional maximum level to which sample can be filled, and a source of sonic waves positioned within the container below said level, wherein the ratio of (i) the shortest distance from the centre of said source to said level to (ii) the distance from the centre of said source to the highest point of one of the tapered sections is from 1 :2 to 1 :5. This is the ratio G to F in Figure 3.

In accordance with a ninth aspect of the present invention, there is provided apparatus for sonicating a sample comprising one or more of the features of the inventions defined above in any combination. In accordance with the invention, ultrasonic cavitation in the presence of a water soluble polymer PVOH in a specially designed reactor (see Figure 2) serves the dual function of exfoliation and surface functionalisation. The ultrasonic cavitation of the low power system helps in the formation of small localised bubbles with diameters on the nanometer (nm) scale which are unstable and collapse violently creating some temperature increase in the system. With continuous sonication thousands of such nano bubbles penetrate between the layers of MWCNTs (see Figures 4 and 5) carrying the active hydroxyl group of the carrier polymer. The collapse of functionalised bubbles between the nanometer layers of MWCNTs results in grafting of the active hydroxyl group on the individual layers of M WCNT as shown in Figures 4 and 5. Formation of such nanometric bubbles acts as a individual nanoscale reactor and the energy released during the collapse is used to break the van der Waals forces ( dipole- dipole or dipole induced forces). The increase in intrinsic temperature during the collapse of the bubbles aids in the surface functionalisation of the nanotubes.

The formation of nano or microbubbles/cavities is an important step in controlling the exfoliation and surface activation with functional groups of multiwall carbon nanotubes. The smaller the diameter of these bubbles or cavities, the higher the surface area and so bubbles burst dissipating high energy. The bursting of such small bubbles/cavities has the dual functional of assisting exfoliation and activating surface functionality. These bubbles are extremely unstable with high energy and so they collapse violently, momentarily creating intense temperatures and pressures. The presence of sharp corners in the reactor further helps in increasing the internal stress on the bubbles thereby increasing their internal energy.

Smooth corners as are found in a round bottom flask will need more power to create unstable cavities/bubbles with high internal stress. So the more corners the reactor vessel has, the better.

Such a batch scale reaction can be scaled up to continuous process by optimising the cross sectional area and the power of a continuous reactor line as shown in Figure 6.

A number of preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic comparison between the treatment of MWCNTs in an ultrasonic bath and the exfoliation of MWCNTs in a reactor with an ultrasonic probe;

Fig. 2 shows further details of a batch reactor with an ultrasonic horn in accordance with the invention;

Fig. 3 shows a generalised batch reactor with an ultrasonic horn in accordance with the invention;

Fig. 4 is a schematic representation of the sonication of bundles of MWCNTs in accordance with the invention;

Fig. 5 is a schematic representation of the method of the present invention;

Fig. 6 is a schematic representation of a continuous reactor line with ultrasonic horns in accordance with the invention; and

Fig. 7 is a comparison of the dispersion of MWCNTs in a PVOH carrier when carried out under conventional magnetic stirring and under the sonication of the present invention.

Figure 1 shows bundles 1 of MWCNTs being treated in an ultrasonic bath 2 to produce un- exfoliated MWCNTs 3 or being treated in a reactor 4 with an ultrasonic probe to produce exfoliated MWCNTs 5.

Figure 2 shows a reactor 4 with an ultrasonic probe 20 in accordance with the invention wherein the depth of probe 20 within the vessel is 15mm, the diameter of the probe is 3.175mm, the inner diameter of the vessel is 25mm, the slant height 21 of the vessel is 12mm and the baseof the vessel is 5mm square.

Figure 3 shows a generalised batch reactor with an ultrasonic horn in accordance with the invention with various dimensions labelled A-G.

Figure 4 shows bundles 1 of MWCNTs being sonicated to result in nanocavities 40 inside the bundles 1 of MWCNTs and then to result in layers 45 of MWCNTs which have been exfoliated and activated with hydroxyl groups.

Figure 5 shows a schematic representation of a method in accordance with the invention in which bundles 1 of nanotubes are sonicated 50 with polyvinyl alcohol to form nanobubbles/nanoreactors 51 which then act 52 to break the van der Waals forces between the nanotubes to result in surface activated and exfoliated nanotubes 53.

Figure 6 is a schematic representation of a continuous reactor line 60 with polymeric solution having a series of ultrasonic horns 61, the reactor 60 having cross-sectional area 62.

Figure 7 shows a comparison of the dispersion of MWCNTs in a PVOH carrier when carried out under conventional magnetic stirring 70 and under the sonication 71 of the present invention.

EXAMPLE

The process in question involves the sonication of a semicrystalline water soluble polymer system which in this case was poly (vinyl alcohol) (PVOH) with a low weight fraction of multiwall carbon nanotubes (MWCNTs).

0.5 g of PVOH was mixed with 0.005 g (lwt%) of MWCNTs and dispersed in 20 ml of deionised water. Sonication was carried out with Misonix Microson XL2000™ Ultrasonic Processor with the horn diameter of 3.175mm (0.125inch) for 15 minutes (residence time) in a batch reactor vessel as shown in Figure 2. The internal diameter of the cylindrical profile was 25 mm and the slant height of the conical profile of the reactor was 12 mm. The sonicator had a known output power of 5 W-25W and operating frequency of 22.5 kHz.

The precise measurements of the sonicator used are given below with reference to Figure 3:

A: 3.175mm

B: 10.9125mm

C: 42mm

D: 43mm

E: 44mm F: 43.1mm

G: 15 mm

After 15 minutes of residence time of sonication, dispersion of MWCNTs was tested by coating on glass plate. For comparison a similar reaction was carried out by conventional agitation (magnetic stirring) for the same period of time. It clearly proved that ultrasonication of MWCNTs with PVOH and deionised water aided in exfoliation, surface activation and dispersion as shown in Figure 7.

Advantages

• This invention doesn't involve the use of hazardous and corrosive chemicals.

• The use of deionised water as a dispersing media proves to highly economical and industrially viable.

• The batch scale reaction can be scaled up to continuous scale by optimising the cross sectional area and power of the ultrasonic horns.

• The residence time of 15 minutes proves to be very economical compared to the time consumption with high energy thermal and sonication processes.

• Formation of functionalised bubbles with a very high surface area acts like a

nanoscale reactor aiding in the activation of any nano surfaces.

• Formation of such nanosized bubbles or cavities can help in dispersion of additives in highly viscous systems.

Claims

1. A method for dispersing a plurality of entities which are prone to agglomeration comprising the steps of

(i) employing bubbles to disperse said entities temporarily, and

(ii) attaching polar moieties to said entities in order to prolong the dispersal.

2. A method as claimed in claim 1, wherein the bubbles have an average diameter from lnm to lOOnm.

3. A method as claimed in claim 1 or 2, including the additional step of producing the bubbles by means of a sonication apparatus.

4. A method as claimed in any preceding claim, wherein said sample comprises multi- walled carbon nanotubes or graphene.

5. A method as claimed in any preceding claim, wherein the polar moieties for step (ii) are provided by lactic acid, a hydroxy butyrate, a caprolactone, ethylene oxide, propylene glycol or by poly (vinyl alcohol).

6. Apparatus for sonicating a sample, comprising

a container for a sample, said container having a notional maximum level to which sample can be filled, and

a source of sonic waves positioned within the container below said level, wherein the ratio of

(i) the shortest distance from the centre of said source to said level to

(ii) the distance from the centre of said source to the furthest point from the centre of the source within the container

is from 1 :2 to 1 :5.

7. Apparatus as claimed in claim 6, wherein said ratio is about 1 :3.

8. Apparatus as claimed in claim 6 or 7, wherein said container has a base, wherein the ratio of

(i) the shortest distance from the centre of said source to said level to

(ii) the shortest distance from the centre of said source to said container base is from 1 : 1 to 1 :5.

9. Apparatus as claimed in claim 8, wherein said ratio is about 1 :3.

10. Apparatus as claimed in any of claims 6 to 9, wherein the container has side walls and wherein the ratio of

(i) the shortest distance from the centre of said source to said level to

(ii) the shortest distance from the centre of said source to one of the side walls is from 1 :0.5 to 1 :3.5.

1 1. Apparatus as claimed in any of claims 6 to 10, wherein the container has a base and side walls tapered towards the base to result in additional internal corners within the container.

12. Apparatus as claimed in claim 11, wherein the container has four side walls, two of which on opposite sides of the container have tapered sections, and wherein the ratio of

(i) the shortest distance from the centre of said source to one of said tapered sections to

(ii) the shortest distance from the centre of said source to said container base is from 1 :08 to 1 : 1.05.

13. Apparatus as claimed in claim 11 or 12, wherein the ratio of

is from 1 : 1 to 1 : 1.1.

14. Apparatus as claimed in any of claims 11 to 13, wherein the ratio of

(i) the shortest distance from the centre of said source to said level to

(ii) the shortest distance from the centre of said source to one of said tapered sections is from 1 :2 to 1 :5.

15. Apparatus as claimed in any of claims 11 to 14, wherein the ratio of

(i) the shortest distance from the centre of said source to said level to

(ii) the distance from the centre of said source to the highest point of one of the tapered sections

is from 1 :2 to 1 :5.

16. . Apparatus as claimed in any of claims 6 to 15, . wherein the source of sonic waves is a sonic horn having an output power from 5 to 25 watts.

17. Apparatus as claimed in any of claims 6 to 16, wherein the internal surface of the container has a plurality of internal edges.

18. Apparatus as claimed in any of claims 6 to 17, wherein the internal surface of the container has a plurality of internal corners.

19. Apparatus as claimed in any of claims 6 to 18, wherein the internal surface of the container is roughened.

20. A method as claimed in any of claims 1 to 5 wherein the bubbles are produced by using an apparatus as claimed in any of claims 6 to 19.

21. A method as claimed in claim 20, wherein the fill level of sample in the container is between the centre of the sonic source and the maximum level to which sample can be filled.