DE19908920A1 - Process for the production of electrically conductive films based on starch esters - Google Patents

Abstract

The invention relates to a method for producing electroconductive films on the basis of starch acetate that has been doted with metal parts and to the environmentally friendly disposal of said films. According to the invention, films on the basis of starch acetate, water-soluble softeners and other blend components are contacted in a first process step with a metal electrode. When the electrode system so prepared is immersed in an electrolyte bath the metal electrode has no direct contact with the electrolyte solution. After electrolysis metal particles are deposited in the pores that have formed in the film and the film is finally treated with a polyethylene glycol with a molecular mass in the range of between 1,000 and 20,000.

Description

The invention relates to a method for producing electrically conductive films on the basis of starch acetate doped with metal particles and their ecological Disposal.

Due to the technical development of the past decades, electrically conductive applies attention to polymers. You will find a variety of uses as biosensors, in electrochromic windows and displays, antistatic coatings and packaging, solid-state batteries, electronic semiconductor elements and Membranes.

Most known processes for the production of such materials are based on the use of polymers such as polyacetylene, poly- (p-phenylene) and its derivatives, as well as polyaniline and polyheterocycles. The products produced in this way are characterized by good conductivities (up to 10 ³ Scm ^-1 ), but generally do not have good mechanical properties and are very sensitive to oxidative attacks. An overview of how the problem of the lack of mechanical properties and the easy oxidizability by blends with other polymers can be solved can be found in (MJ DE JESUS et. Al., Polymer Engineering and Science, 1997, Vol. 37, 1936-1946 ). However, the effort involved is justified only in special applications.

For antistatic packaging, heterogeneous composite materials are manufactured using conductive fillers (e.g. metal powder, carbon black) due to the far lower requirements with regard to conductivity (MAIR, HJ; ROTH, S .: "Electrically conductive plastics". Munich: Carl Hanser Verlag 1986 ). With the addition of 15-20% metal powder, the conductivity of conventional bulk plastics can be increased significantly so that they are suitable as packaging material with antistatic properties. The conductivities achieved are <10 ^-9 Scm ^-1 . Since such materials can be produced with relatively little effort, the high proportion of relatively expensive fillers and the difficult disposal of such composites also have clear disadvantages.

Another approach to achieving high conductivity is in the surface Chen coating described in porous membranes. So u. a. in the GB 2 254 340 porous materials by treatment with an etching solution, e.g. B. one Sulfuric acid / chromic acid mixture, oxidized on the surface and then with treated with a metal salt solution. The oxidized surface is able to part reduce and bind the metal ions. In this way it becomes a very high leader skills achieved. For the intended purpose, the above is justified. Against the use of such films. B. in Ver packaging sector speak u. a. the use of aggressive etching media and the through the production of such an intimate network of possible disposal problems.

Another approach to the production of conductive films and coatings is the use of renewable raw materials, such as cellulose and starch, which, because of free hydroxyl groups, have a much higher initial conductivity than conventional bulk plastics. This is used in DE 195 42 533, among others. There is proposed that conductive carbon black or other conductive particles are incorporated into cellulose by means of solution extrusion. The result is a highly sensitive sensor material whose conductivity in the range from 10 ^-2 to 10 ² Scm ^-1 reacts very sensitively to the effects of water and pressure. This sensitivity is a prerequisite for the intended application as a sensor material, and is less favorable for applications such as antistatic packaging. The high proportion of conductive fillers is also disadvantageous for environmentally friendly disposal.

Starting point for solving these two problems to produce an inexpensive The most easy to manufacture material with high conductivity is the use of natural product derivatives. Starch esters are particularly suitable due to their simpler nature Possibilities for their production as a starting component. With the homogeneous Synthesis of starch esters with degrees of substitution between 1.8 and 2.6 (DE 41 14 185) succeeds in materials with good thermoplastic processability with good compostability. This is connected despite Partial hydrophobization due to the even distribution of the remaining hydroxyl  groups have a relatively high conductivity. In DE 198 05 367 and DE 198 49 187 who the foundations for effective industrial production of such esters sets. DE 44 18 678 and DE 44 24 415 describe basic processing variations anten shown. It is important here that processing in particular of starch acetate without softening components is difficult. For the production of flexible foils from this material becomes the use of flexi Recommended glare components recommended.

Some of the plasticizers used for starch acetate (DE 43 26 118) are generally water-soluble. The use of such plasticizers has the disadvantage for thermoplastic applications that a large part of the plasticizing component migrates out of the thermoplastic material upon contact with water. This is also the case when using additional flexible components for film production. At the same time, a considerable water absorption can be observed for these materials, which is essentially caused by the starch acetate. For the purpose of developing a polymer electrolytic system, it is precisely this peculiarity of the starch acetate compounds and blends that is of interest, as well as the relatively low volume resistances of 10 ^-11 to 10 ^-10 ohms.

The object of the present invention is based on biological degradable starches and other biodegradable or environmentally friendly components to develop a simple process that is environmentally friendly gisch favorable production and disposal of foils with increased conductivity allowed for use u. a. suitable as an antistatic packaging material are.

The object is achieved in two steps according to the invention on the basis of starch acetate blends and compounds described in DE 44 43 539 and DE 197 57 147. Depending on the intended use, there is a third step:

• 1. By means of a special arrangement of two metal electrodes and the to behan delenden foil in an electrolysis bath metal particles in the pores of the Fo lay secluded.
• 2. Finally, a treatment of the body with a polyethylene glycol Molar masses from 1000 to 20,000, preferably from 2000 to 10,000, take place. There with a high conductivity of the film is largely independent of the moisture the environment. If this step is not done, the film can e.g. B. can be used as a moisture sensor.

The first step of the process according to the invention is based on the above finding that when water-soluble plasticizers are used for starch esters with degrees of substitution from 1.8 to 2.6 when they are stored in water, they migrate out of the material within a short time. At the same time, a considerable amount of water is absorbed (up to 30%). In this process, the migrating plasticizer components leave behind a pore system, the creation of which may also involve the amount of water absorbed. Figure 1 shows a typical electron micrograph of a blend of starch acetate (degree of substitution 2, 3), polyethylene glycol (PEG 400) and a synthetic blend component. Pores of the order of 1 µm can be seen. The number and diameter of these pores can be influenced in certain areas via the degree of substitution of the starch acetate and the molar mass and the amount of the PEG portion. This goes hand in hand with influencing the desired properties.

The structure described is generated, among other things, when the film is placed in an electrolysis bath ( Figure 2). It is essential that a metal electrode ( 1 ), the cathode, is brought into contact with the film ( 2 ). The resulting electrode system is introduced into the electrolysis bath in such a way that the coated metal electrode has no direct contact with the electrolyte solution ( 4 ). An anode made of the same material is used as the anode ( 3 ).

Copper salt solutions or nickel salt solutions with a small addition of H ₂ SO ₄ are suitable as the electrolyte solution. The concentration of the solution should be between 0.1 and 1.0 mol / l, preferably between 0.2 and 0.5 mol / l. The voltage applied must be between 1 and 8 V, preferably between 2 and 5 V. The optimal treatment time is between 3 and 6 hours.

The concentrations used in the method designed according to the invention guarantee optimal penetration of the metal ions into the pores formed in the film. At lower concentrations, too few ions penetrate into the pore system formed in the starch acetate for the intended use or the electrolytic treatment of the films would take so long that damage with greatly diminishing mechanical properties could not be avoided. At higher electrolyte concentrations, the diffusion of the metal ions into the pore system is inhibited, so that here too the desired effects cannot be obtained in reasonable periods. The specified limits for the applied voltage result on the one hand from the deposition potential of the respective metal ions (lower limit). On the other hand, too high potentials in connection with the transition metal ions used lead to oxidation phenomena and thus to irreversible damage to the films and to the deposition of metal oxides. Both counteract an inventive use of the films. The films treated in this way achieve conductivity values of 10 ^-5 Scm ^-1 to 10 ^-2 Scm ^-1 . These are still heavily dependent on the moisture of the film.

Better conductivity values and extensive independence of these values from the In a third step, the moisture contained in the foils is determined by an after treatment of the film with PEG with molecular weights of 1000 to 20,000, preferably from 2000 to 10,000. This aftertreatment can also be carried out with an aq solution of the PEG. However, a more effective dip is more effective in a melt of the respective PEG at temperatures from 70 ° C to 90 ° C. These temperatures have little mechanical effect in the test specimens Changes because after the water-soluble plasticizer has been discharged in the 1st Step the heat resistance of the material increases considerably. The ge selected molecular weight range for the PEG is a compromise between the For  the lowest possible molecular weight to ensure high system conductivity to ensure and the requirement that the selected PEG the predetermined Po ren structure if possible not changed. That is according to DE 43 26 118 for molar masses 2000 the case.

In the manner described here, films with thicknesses from 10 µm with good mechanical strength can be produced. The conductivity values achieved are 10 ^-6 to 10 ^-3 Scm ^-1 and are therefore suitable for the intended use. The type and amount of the substances and auxiliary substances used ensures the environmental friendliness of the product and the process, because the thermoplastic manufactured bodies are only treated with aqueous solutions or with pure substances. This process can be designed practically free of waste. The good values for conductivity are achieved with small amounts of metal (0.02 to 0.2%).

The disposal route proposed according to the invention for this conductive film consists of the following steps:

• 1. Treatment of the film with an approx. 1 m alkali solution. The starch saponifies Share of acetate to starch and alkali acetate. Both, and therefore also in the pores contained metal part, goes into solution.
• 2. The undissolved polymer components (blend components) are separated off and can be easily recycled, incinerated or landfilled.
• 3. The metals contained in the starch / alkali acetate solution become electrolytic deposited.
• 4. The remaining solution is fermented to methane and can be used energetically.

The procedure described is illustrated by the following examples:

Example 1 (comparison)

500 g of starch acetate (DS 2.3) are mixed in a high-speed mixer with 100 g of polyethylene lenglykol (MW 400) processed into a dry blend. Then 400 g  Polyethylene carbonate powder added. The resulting mixture is at approx. 180 ° C in a well kneading twin-shaft kneader processed into granules. The Foil production (20 µm thick) is carried out using a small extruder with a slot die se.

Example 2 (comparison)

600 g of starch acetate DS (2.3) are mixed in a high-speed mixer with 100 g of polyethylene lenglykol (MW 400) processed into a dry blend. Then 300 g Dispersion powder DLP 210 from Buna Sow Leuna Olefinverbund GmbH ben. The resulting mixture is at about 180 ° C in a well kneading two Wave kneader processed into granules. Foil production (50 µm thick) takes place with a small extruder with a slot die.

The starting foils serve as examples 1 and 2 (comparison).

The films thus produced are subjected to a treatment according to the invention The respective parameters can be found in the following table:  

Table 1

Parameters of the films produced

Post-treatment with PEG in the melt at 70 to 90 ° C, in the 20% aq solution at room temperature, duration 1 hour each. The with the aqueous Lö solution treated films were dried at 40 ° C to constant weight.

Table 2 below shows the conductivity values achieved (measured according to DIN 53 482) the samples are compiled:  

Table 2

Conductivity values

The samples treated according to the invention have specific conductivities of 10 ^-3 to 10 ^-5 Scm ^-1 . It is clear from the examples that the biodegradable blend components used to increase the flexibility of the films have only an insignificant influence on the result according to the invention. It also becomes clear that the chosen conditions guarantee optimal results. An increase in the voltage as in Example 9 brings similar results with regard to the conductivity achieved, but this is achieved with a significantly higher metal content. In addition, the beginning discoloration of the film indicates oxidative degradation, which noticeably affects the film quality.

To demonstrate the mechanical stability of the Fo treated according to the invention Here is a comparison of some mechanical characteristics of the films of Example 1 (Comparative example) and Example 5 (according to the invention) serve (see Table 3).

Table 3

Mechanical characteristics

The values show that in the treatment of the films according to the invention in Ver the elongation at break becomes slightly lower and the strength immediately compared to the original film something increases. This applies to all examined examples (except example 9, see above) to. The changes are always such that the mechanical properties are sufficient for the intended use.

The disposal of the films according to the invention is illustrated by the following example:
10 g of film (produced according to Example 5) are placed in a beaker with 100 ml of 1 M NaOH. The solution is stirred for 1 h at room temperature. After this time, the polyethylene carbonate used as the blend component remains in the glass as a shrinked residual film that is easy to separate.

The remaining solution is neutralized with 0.1 M acetic acid and 1 ml of 0.1 M H ₂ SO _{4 is} added. The copper contained in the solution can now be separated electrolytically. The residual solution is again neutralized with NaOH and can be used for fermentation, for example.

List of reference symbols for Figure 2

1

Metal electrode

2nd

foil

3rd

anode

4th

Electrolyte solution

Claims (9)

1. A process for the production of electrically conductive films based on starch acetate, water-soluble plasticizers and other blend components, characterized in that a film made of the materials mentioned is brought into contact with a metal electrode in a first process step so that when immersing the so resulting electrode system in an electrolysis bath the metal electrode has no direct contact with the electrolyte solution and that after electrolysis metal particles separate in the pores formed in the film and that it is finally treated with a polyethylene glycol with molar masses of 1000 to 20,000. 2. Process for the production of electrically conductive films based on starch keacetat according to claims 1 and 2, characterized in that the Kon concentration of the electrolyte solution between 0.1 and 1.0 mol / l, preferably between between 0.2 and 0.5 mol / l. 3. Process for the production of electrically conductive films based on starch Keacetat according to claims 1 and 2, characterized in that the ange put voltage between 1 and 8 V, preferably between 2 and 5 V. 4. Process for the production of electrically conductive films based on starch Keacetat according to claims 1 to 3, characterized in that the elec trolytic treatment between 1 and 6 hours, preferably 2 to 5 hours is. 5. A process for the production of electrically conductive films based on starch acetate according to claims 1 to 4, characterized in that copper salt solutions or nickel salt solutions with a small addition of H ₂ SO ₄ are used as the electrolytic solution. 6. Process for the production of electrically conductive films based on starch Keacetat according to claims 1 to 5, characterized in that for the night treatment of the films polyethylene glycol with a molecular weight of 1000 to 20,000, preferably from 1,000 to 10,000, is used. 7. Process for the production of electrically conductive films based on starch Keacetat according to claims 1 to 6, characterized in that the after treatment with polyethylene glycol in aqueous solution and subsequent Drying takes place. 8. Process for the production of electrically conductive films based on starch Keacetat according to claims 1 to 6, characterized in that the after treatment with polyethylene glycol in a melt at 70 ° to 90 ° C. 9. Process for the production of electrically conductive films based on starch Keacetat according to claims 1 to 8, characterized in that the waste disposal The conductive film is made by adding an approx. 1 m alkali solution is acted, the undissolved polymer components (blend components) separated, to be recycled, burned or landfilled in the starch / alkali acetate solution contained metals are electrolytically deposited and the remaining solution fermentation to methane.