GB2273063A

GB2273063A - Process for treating natural products

Info

Publication number: GB2273063A
Application number: GB9225512A
Authority: GB
Inventors: Hailin Ge
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-12-07
Filing date: 1992-12-07
Publication date: 1994-06-08
Anticipated expiration: 2012-12-07
Also published as: GB9225512D0; GB2273063B; SG52462A1

Abstract

A natural product is treated with an electrically conducting polymer by applying a layer of an electrically conducting polymer to the surface of a natural product and subsequently drying the polymer-coated product.

Description

PROCESS FOR TREATING NATURAL PRODUCTS The present invention is concerned with the treatment of natural products and more particularly to a process for applying a layer of electrically conducting polymer to a natural product.

The term natural product used herein denotes any product derived from plants or animals and includes fresh or substantially untreated products such as fruit, vegetables, flowers, eggs, furs and the like and processed products such as pastry, flour, leather and wood.

Coating the surface of such products is known for the purpose of prolonging the shelf life of the product or to modify/enhance its appearance or to enable the product to be processed further. Typical coatings applied for this in purpose include polymers, dyes, composites and other functional compounds which are applied to the surface of a product by techniques such as painting, chemical bonding or surface casting or the like.

It is also known to coat a natural product with an electrically conducting material which has to date been accomplished by a carbon - or metal - containing paint. Such methods are generally time consuming and present particular difficulties in providing a coating of uniform or substantially uniform thickness.

Electroplating techniques have also been used for this purpose but require the product to be rendered electrically conductive. Therefore this technique tends to be expensive and accordingly its industrial applicability is limited.

The present invention seeks to provide a process for applying a layer of electrically conducting polymer to a natural product which provides a uniform and smooth film over the product surface and which is quick and inexpensive and which advantageously avoids harsh process conditions which might otherwise cause damage to or perish the natural product under treatment. The conducting polymer coating provides a protective layer which limits physical deterioration of the product and further has anti-static properties.

The process according to the present invention in its broadest terms comprises applying a layer of electrically conducting polymer to the surface of a natural product and subsequently drying the polymercoated product.

The conducting polymer may be applied to the natural product in either a one - or two - step procedure. The two-step procedure requires (i) preforming the conducting polymer and (ii) applying that polymer to the natural product. The polymer of choice should be readily soluble in suitable solvents described in more detail hereinafter. Generally, the two-step procedure is not suitable for conducting polymers which are insoluble or have a particularly low solubility. The polymer may be deposited on the surface of the natural product by any suitable method and the particular method employed will depend upon the product to be treated and the properties of the polymer. Generally, preferred methods are immersion, spraying or vapour deposition.

The one-step procedure involves chemical polymerisation of the monomer in situ i.e. synthesis of the polymer on the surface of the natural product.

This may be accomplished in the following manner; (i) coating the surface of the natural product with monomer; (ii) treating the monomer-coated product with a polymerising agent; and (iii) washing the polymer-coated product to remove excess polymerising agent.

Alternatively, the natural product may first be coated with a polymerising agent and then treated with monomer to form the polymer-coated product. The polymer-coated product may then be washed as before.

Suitable conducting polymers are those which can be formed by chemical polymerisation, extensive examples of which can be found in the Handbook of Conducting Polymers V.1 and V.2, Marcel Dekker 1986.

Conducting polymers of particular utility in the present invention are those which exhibit high conductivity and stability under ambient conditions.

Particularly preferred are homopolymers of pyrrole and aniline and derivatives thereof. Synthesis of conducting polymers, particularly of polypyrrole and polyaniline are well known in the art and illustrative methods can be found in the Handbook of Conducting Polymers V.1 and V.2, Marcel Dekker 1986 and more specifically in Polymer (1989) Vol 30, December 23052311; Synthetic Metals, 22 (1988) 385-393; and Synthetic Metals, 20 (1987) 365-371.

In the two-step procedure, the polymer is preformed using well-known methods in the art as above. The concentration of the polymer solution is dependent on the solubility of the polymer and the thickness of the polymer film desired. Typically 1% wt/v of polymer in solvent is suitable. The choice of solvent depends on the solubility of the polymer and volatility of the solvent. Coating preferably carried out at ambient temperature.

In the one-step procedure the concentration of the monomer required for the coating process is dependent upon the product to be coated and the thickness of the polymer film desired. Typically, monomer concentrations in excess of 10% wt/v of solvent are suitable. The choice of solvent for the monomer will necessary depend on the monomer selected, the product under treatment and the polymerising agent employed. Where possible, volatile solvents are used to facilitate the drying step. For processes using polyaniline as the conducting polymer, aqueous-based solvents are preferred. Acetone is preferred for pyrrole.

Synthesis of the polymer is achieved preferably by oxidative polymerisation and the choice of oxidant will necessarily depend upon the monomer to be polymerised. Preferred oxidants are oxidative transition metal ions and more particularly Fe(III) and Cu(II) salts, All3, AgNO3 and also ammonium persulphate. Particularly preferred is Fell3. The solvent for the oxidant can be aqueous or organic but will depend upon the oxidant selected. The concentration of polymerising agent is typically in excess of 5% wt/v of solvent and more preferably 10% wt/v of solvent. However, the appropriate concentration of polymerising agent will depend upon the particular monomer selected and its concentration.

The monomer can be applied to the product by any method but typically immersion and spraying are used.

The temperature at which the pre-formed polymer is coated onto the product surface or chemical polymerisation in situ is carried out is dependent upon the properties of the polymer selected and the product to be coated. Typically temperatures of approximately room temperature i.e. 20 to 250C are preferred since higher temperatures tend to cause a loss in conductivity of the polymer film.

Temperatures as low as -50C may be employed also. The temperature is also critical to the solvent used thus preferably solvents which can be used at room temperature or below should be chosen. Typically the product is treated with monomer solution for a period of time sufficient to permit a liquid film to be formed on the product surface usually 1 to 5 minutes.

When using a pure liquid monomer, excess monomer can be removed by resting in air for up to 10 minutes.

When using a monomer solution, the solvent can be removed by resting at an appropriate evaporation temperature for 10 seconds to 10 minutes.

The monomer coated products are then treated with polymerising agent for a period of time sufficient to permit the polymer to form. Typically this takes between 3 to 60 minutes, periods of time in excess of 60 minutes should be avoided since degradation of the formed polymer tends to occur thereafter. The polymerisation temperature is preferably room temperature or below in order to enhance the conductivity of the conducting polymer once formed.

The effects of temperature and treatment times on the properties of the conducting polymers are well documented in the art and illustrative conditions can be found in the following references:- Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 26, 743-753 (1988); Synthetic Metals, 24 (1988) 217-221; and Handbook of Conducting Polymers V.1 and V.2, Marcel Dekker 1986, for example.

The polymer coated product is finally dried to remove residual solvent and this can be carried out at any temperature and/or in vacuo although simple evaporation at room temperature is preferred.

Prior to the above processes, the natural product is usually cleaned and/or surface treated to remove waxy deposits and dirt that would otherwise interfere with the formation of a layer of polymer of uniform thickness.

The process of the present invention will now be described by way of illustration only with reference to the following examples in which: Figure 1 shows schematically the one-step procedure with a monomer coating according to the present invention; Figure 2 shows schematically the one-step procedure with an oxidant coating according to the present invention; and Figure 3 shows schematically a the two-step procedure according to the present invention.

Referring first to Figure 1, the surface of a natural product is first cleaned with acetone to remove wax and dirt and other undesirable deposits from the product surface. The product is then dried to remove residual solvent.

The natural product is then immersed in 1008 liquid pyrrole for 2 minutes at room temperature to enable a pyrrole film to form on the product surface.

Excess pyrrole is removed by resting in air at room temperature and then immersed in 10% wt/v FeCl3(aq) for 2 minutes at 250C. The polymer-coated product is then washed in water and then acetone to remove unpolymerised monomer and/or oxidant and the residual solvent (acetone) permitted to evaporate at 250C for approximately 1 minute. Clearly the drying time and temperature employed are dependent upon the properties of the solvent used during polymerisation and subsequent washing steps. However, elevated temperatures tend to cause a loss in conductivity and therefore prolonged drying times at lower temperatures are preferred where possible.

Referring now to Figure 2, the product to be coated is immersed in 10% wt/v FeCl3 in acetone for 2 minutes at room temperature. The residual solvent is permitted to evaporate at room temperature and the oxidant-coated product treated with pyrrole vapour for 30 minutes at 250C to permit polymerisation to occur.

The polymer-coated product is washed as before.

In yet another process according to the present invention, as shown in Figure 3 the natural product is immersed in 1% wt/v polyaniline in N,Ndimethylformamide (DMF) for 2 minutes at 250C to form a conducting layer on the product surface. The product is allowed to dry in vacuo. A washing step in this process is not usually required. An alternative suitable solvent is N-methylpyrrolidinone.

Modifications to the above processes can be readily employed. For example, the natural product to be coated may be pretreated with a layer of modifier to promote adhesion between the conducting polymer and the natural product under treatment. Suitable modifiers are typically polymers which serve to change the surface tension and/or surface porosity of the product to be treated (an increase in porosity improves adhesion). Such polymers are well known in the art and are commercially available. Modifier need not be used, however, if the surface of the product to be treated is already pre-disposed to the coating process described above.

Furthermore, the temperatures, time periods, solvents and concentrations of monomer and oxidant are necessarily dependent upon the properties of the monomer and oxidant selected, the thickness of the layer of conducting polymer required and the nature of the product to be treated. Such parameters are generally well known in the art and readily ascertainable.

The process of the present invention can be employed to render a product electrically conductive so as to facilitate subsequent electroplating processes which may be carried out, for example, to coat the product with a precious metal for ornamental/aesthetic purposes. Furthermore, certain polymer coated products may find utility as adsorbents to replace polymer beads typically used in industry.

Claims

CLAIMS:

1. Process for treating a natural product comprising the steps of (i) applying a layer of electrically conducting polymer to the surface of a natural product; and (ii) drying the polymer-coated product.

2. Process according to claim 1, wherein step (i) comprises (a) coating the surface of the natural product with monomer; (b) treating the monomer-coated product with polymerising agent; and (c) washing the polymer-coated product.

3. Process according to claim 1, wherein step (i) comprises (a) coating the surface of the natural product with polymerising agent; (b) treating the coated product with monomer; and (c) washing the polymer-coated product.

4. Process according to claim 1, wherein step (i) comprises (a) pre-forming the polymer; and (b) applying that polymer to the surface of the natural product.

5. Process according to any preceding claim, wherein the polymer is polyaniline.

6. Process according to any one of claims 1 to 3, wherein the polymer is polypyrrole.

7. Process according to claim 2 or claim 3, wherein the polymerising agent is an oxidant.

8. Process according to any preceding claim, wherein the natural product surface is pre-coated with modifier.

9. A natural product coated with a layer of electrically conducting polymer.

10. Process for treating a natural product substantially as hereinbefore described with reference to the examples and the accompanying Figures 1 to 3.