WO1992010352A1

WO1992010352A1 - A method for preparing a conductive plastic product

Info

Publication number: WO1992010352A1
Application number: PCT/FI1991/000386
Authority: WO
Inventors: Toivo Kärnä; Jukka Laakso; Esko Savolainen
Original assignee: Neste Oyj
Current assignee: Neste Oyj
Priority date: 1990-12-14
Filing date: 1991-12-13
Publication date: 1992-06-25
Anticipated expiration: 1993-06-14
Also published as: FI90325B; FI90325C; FI906168A0; FI906168L

Abstract

The invention relates to a method for preparing a conductive plastic product in connection with injection moulding, in which method the product is formed from a molten plastic mixture such that the raw material is plasticized in a cylinder by means of a feeding screw (7) and injected into a mould (9). A doping agent (3) is added to a molten dopable conductor polymer (1) or to a molten mixture of a dopable conductor polymer (1) and a matrix plastic (2) after plasticization in a cylinder (5) of an injection-moulding machine (4) before injection into the mould (9) for obtaining conductivity.

Description

A method for preparing a conductive plastic product

The invention relates to a method for preparing a conductive plastic product in connection with injection moulding, in which method the product is formed from a molten plastic mixture such that the raw material is plastisized by means of a feeding screw in a cylinder and injected into a mould.

Plastics and other polymers do not by themselves conduct electricity, but they can be made conductive for various applications. Conductive polymers may be prepared from organic polymers having long chains of conjugated double bonds. The quantity of the silicon electrodes of double bonds can be affected by adding to the polymer certain doping agents, which either receive or donate electrons. Openings or extra electrons then arise in the polymer chain, which make the passage of electric current along the conjugated chain possible. The conductivity of polymers can be adjusted depending on the doping agent content so that it covers nearly the entire conductivity range from insulators to metals. Such conductive polymers have many interesting applications, e.g. EMI applications and ESD applications.

Conductive polymeric and organic conductors are generally speaking insoluble, it is not possible to melt or form them, and in certain cases, they are unstable against oxygen, moisture and high temperatures, due to which also doping at high temperatures has previously not succeeded. Until now, it has therefore not been possible to treat or form conductor polymers thermoplastically in any way. There are descriptions of the meltability of some single conductor polymers, but their conductivity has been very poor. After doping, the polymer is generally insoluble and it can no longer be formed, due to which doping has traditionally been performed subsequently after the forming of the polymer.

As known, the doping of polymers thus occurs after the treatment, or after the formation of the product, e.g. with FeCU. Such a method becomes very expens¬ ive because of special devices designed for doping, and furthermore, it is unprac¬ tical and non-pro-environmental, since toxic volatile gases spread to the environ¬ ment.

For solving the problem, attempts have also been made to develop special polymerization methods, and mixtures of conductor polymers and other polymers have been formed, which could be formed after doping. The conductivity has, however, generally been too low.

With respect to prior art related to this invention, a reference is made to the EP patent application no. 168 620, in which the object is to achieve a stable disper¬ sion of a conductor polymer into a thermoplastic polymer, the forming of which is possible such that an optimal conductivity is maintained. Another object of this publication is also the possibility of stabilization after the dispersion. In this EP publication, the conductor polymer is mixed (dispersed or dissolved) in a molten state with a thermoplastic polymer, until a homogenous mass is obtained, after which the solvent is removed. Polyether, polyester, polyvinylidene chloride, poly-aπride, etc. have been mentioned as the matrix polymer. The doping accord- ing to the publication occurs in a solution or by the action of ultrasound. Additives are also used for improving the treatability.

An earlier FI application 901632 by the applicant describes for the first time a method for preparing a conductive polymer product by doping, in which method doping occurs in connection with the treatment of the polymer or even before the treatment. The purpose of the FI application 901632 is to provide a method for doping a conductor polymer, in which method the properties of the conductor polymer can be treated as desired, and which polymer is stable.

The most general treatment method of plastic pieces is press moulding or die

_* casting, as it is also called. Dimensionally stable, complex and exactly similar products may be prepared by injection moulding. An extruder is needed for injection moulding, which extruder comprises a plastic melting and dosing device ( = an injection unit) and a die press ( = a closing unit) and a mould for each product as well as a cooling system. The machine is generally comprised of two main parts, which are an injection unit and a closing unit, which units are mounted on a common stand. In a cylinder to be heated by electric resistances a worm screw rotates in the injection unit by the power of an electric or hydraulic motor. The depth of the screw threads is at its highest at the initial end and decreases in the middle section and continues as small to the head of the screw, wherein there is located a check valve. At the rear end of the cylinder casing, there is a feeding hole and on top of it a funnel. The raw material is poured into the funnel and the rotating screw draws the raw material along with it. The plastic then melts due to the cylinder heating and friction. At the beginning of the plastizating (melting), the screw in the cylinder is in the front position. The screw may move backwards simultaneously as it allows the molten plastic mass to pass. At the front of the screw point, there is formed a chamber, to which the molten plastic turns. At the front end of the chamber or at the cylinder cam, there is a hole, which is either open or furnished with a valve. When a sufficient amount of plastic is melted, the screw stops and the cam of the injection unit is run into an injection channel hole in the mould. For this purpose, there is an opening in the middle in a fixing plate of the die press. At the rear end of the screw, there is located a hydraulic cylinder providing an injection pressure, which by means of a conventional machine amounts to max. 1500-2000 bar. The screw acts as a piston, since it is provided at its point with a check valve (a so-called ring), which prevents the molten mass from flowing back along the threads of the screw.

In the closing unit, the plastic mass is injected at a very high pressure into the mould. The maximum pressure of the injection unit is in the range of 500 bar/cm². Plastic mass is injected into the mould, and after it has sufficiently solidified, the mould is opened. The machine is then ready for the next cycle. In most machines, a free opening is arranged under the mould tables, from which opening the product may drop e.g. onto the conveyor belt. Most machines are located horizontally for facilitating the removal of the piece. The most economi¬ cal solution for a mould is a so-called natural mould, in which the division plane follows the circumference of the the largest projection of the press and the press readily in a solidified state slides out of the casing or away from the top of core. Between the cam of the extruder and the mould core a channel is needed, along which the molten plastic runs into the core. The channel may be e.g. conical, and in its most simple form, the channel continues as a cone onto the surface of the extruder and joins it. Special ejection systems are needed for the mould for ejecting the mould piece. A considerable quantity of heat enters the molten mass, which heat has to be conveyed away and cooled. In connection with the injection moulding, as an external device are needed e.g. cooling and tempering devices, since e.g. the temperature of the mould has to be maintained exactly constant and fairly high. As for the details of injection-moulding machines, a reference is made to a Finnish brochure "Properties, treatment methods and usage of plastics", Training Centre of Engineering Societies, INSKO, M.Sc.(Eng.) Mikael Boedeker).

All thermopl-asts may be injection moulded. Two groups are generally men¬ tioned: general or mass plastics and technical plastics. General plastics usually refer to plastics costing FIM 4-6,-/kg, e.g. LDPE (LD polyethylene), HPDE (HP polyethylene), PP (polypropylene), PS (polystyrene), SB (styrene butadiene) and PVC (polyvinyl chloride). These are mostly used by the packaging industry (often disposable packages, such as cans, bottles, films). They are also used for preparing cheap conveyor and preservation boxes, bottle crates, etc. They are also used for more technical products, when the properties are suitable for the application concerned.

In more detail, the invention relates to a method for preparing conductive plastic products by doping a conductor polymer or a plastic mixture containing a conductor polymer in connection with the treatment of the product, when the treatment occurs by means of injection moulding. In this application, the term "conductor polymer" is also used for a dopable polymer, although it is not yet conductive before the doping.

It is preferable to prepare the plastic product from an internally conductive polymer or from a mixture of such a polymer and a matrix plastic. The conduc- tivity of the product may then be accurately adjusted for the application desired either by changing the composition of the polymer mixture or by modifying the doping degree.

The object of the invention is to provide a method, by means of which injection moulded plastic products may be prepared from a conductive polymer.

For achieving the objects mentioned above, the inventive method is mainly characterized in that a doping agent is added to a molten dopable conductor polymer or to a molten mixture of a dopable conductor polymer and a matrix plastic after plasticization in a cylinder of an injection-moulding machine before injection into a mould for obtaining conductivity.

The preferred embodiments of the invention have the characteristics of the subclaims.

An advantage of the invention is that the inventive doping method is extremely easy to realize. An existing standard device base may be used as the equipment with minor changes.

The invention utilizes the perception of the FI application 901632 that the doping can be performed by adding a doping agent to the conductor polymer when it is in a molten state and that the doping can be timed to occur in connection with the treatment. In the FI application 901632, the doping was performed when treating the product e.g. in connection with injection moulding. In this invention, new modes have been invented for preparing conductive prod- ucts in connection with injection moulding. This invention describes a new method for doping a conductor polymer in a molten state. In accordance with the invention, the doping agent can be brought to a molten mixture either mixed with the conductor polymer, mixed with the matric plastic, or the doping agent may be added directly to the molten mixture formed by the conductor polymer and the matrix plastic in accordance with subclaims 2-4, which embodiments .are described subsequently in detail in connection with the figures. In all cases, the mixing of the doping agent in the plastic occurs before the pressing of the product.

The oxidizing and reducing doping agent can be a gas, (e.g. an iodine steam), a liquid (e.g. a liquid sulphonic acid) or a solid matter (e.g. a sulphonic acid having a suitable melting point).

The doping agent is added to the conductor polymer or to the mixture formed by the conductor polymer and the matrix plastic in connection with the melting or thereafter e.g. in accordance with subclaims 2-4, the embodiments of which are described in detail in connection with the description of the figures.

The invention can be utilized for preparing any plastic product having a conduc- tivity.

Different embodiments of the invention are described next with reference to the figures of the accompanying drawing. This is not intended to limit the invention to the details of the figures.

Fig. 1 shows a first inventive embodiment in connection with an applied injection moulding method, wherein a doping agent is added to a dopable molten conduc¬ tor polymer mixture in a mixing device before injection into the mould.

Fig. 2 shows a second inventive embodiment in connection with an applied injection moulding method, wherein a doping agent is added by means of an extruder to a molten conductor polymer mixture in a mixing device before injection into the mould.

Fig. 3 shows a third inventive embodiment in connection with an applied injection moulding method, wherein a doping agent is added to a molten conductor polymer mixture in a mould channel before injection into the mould.

In the inventive embodiment of In Fig. 1, only a conductor polymer 1 or a mixture of the conductor polymer and a matrix plastic 2 is added to a feeding funnel 6 of an injection-moulding machine 4. This mixture is pl.asticized, as previously described, in a cylinder 5 by means of a feeding screw 7. A liquid doping agent 3a and/or gaseos agent 3b is added to the molten conductor polymer mixture 1,2 in a mixing device 11, which is located in the injection- moulding machine 4 between the cylinder 5 and a channel 8 leading into a mould 9. The doping agent 3 is led via a duct 12 of the mixing device 11, and its supply is adjusted by means of a cylinder 13. The mixture is then led along the channel 8 into the mould 9, wherein it is pressed in the mould core 10. The mould piece is cooled and then removed in a manner known per se.

In accordance with the embodiment of Fig. 2, a solid doping agent 3 solely or a doping agent 3 mixed with/fixed to a material 2b is added by means of an extruder 15 to a molten conductor polymer mixture 1,2a in a mixing device 11 before injection into a mould 9. The extruder 15 has a feeding screw 16, which feeds the doping agent 3 and a possible material 2b into the mixing device 11.

The inventive embodiment according to Fig. 3 otwerwise corresponds to the embodiment of Fig. 1, but the doping agent 3 is added to the molten conductor polymer mixture 1,2 in a mould channel 8 before injection into the mould 9.

The patent claims are presented next, within the scope of whose inventive idea the details of the invention can varv.

Claims

1. A method for preparing a conductive plastic product in connection with injection moulding, in which method the product is formed from a molten plastic mixture such that the raw material is plastisized in a cylinder by means of a feeding screw (7) and injected into a mould (9), characterized in that a doping agent (3) is added to a molten dopable conductor polymer (1) or to a molten mixture of a dopable conductor polymer (1) and a matrix plastic (2) after plasticization in a cylinder (5) of an injection-moulding machine (4) before injection into the mould (9) for obtaining conductivity.

2. A method according to Claim 1, characterized in that in the method the doping agent is added to a separate mixing device, which is located in the injection-moulding device (4) between the cylinder (5) and a channel (8) leading into the mould (9) (Fig. 1).

3. A method according to Claim 1, characterized in that the doping agent is added to the mould channel (8) (Fig. 3).

4. A method according to Claim 1, characterized in that in the method a solid doping agent (3) or alternatively a doping agent (3) mixed with/fixed to the material is added by means of an extruder (15) into the mixing device (11) of the injection-moulding machine (4) to a molten dopable conductor polymer or to a molten mixture (1, 2) of the dopable conductor polymer and the matrix plastic (Fig. 2).

5. A method according to any of the Claims 1-7, characterized in that the doping agent is iodine.

6. A method according to any of the Claims 1-5, characterized in that the dopable polymer is a treatable conductor polymer, preferably poly(3-octyl thiophene).

7. A method according to any of the Claims 1-6, characterized in that the matrix polymer in the plastic mixture containing the conductor polymer is polyethylene, polypropylene, polystyrene, PVC (polyvinyl chloride) or EVA (ethylene vinyl acetate).

8. A method according to any of the Claims 1-7, characterized in that the injection of the plastic mass occurs at a temperature of ca 180°C, when the matrix polymer is polyethylene, polystyrene, PVC (polyvinyl chloride) or EVA (ethylene vinyl acetate).

9. A method according to any of the Claims 1-7, characterized in that the injection of the plastic mass occurs at a temperature of ca 195°C, when the matrix polymer is polypropylene.