DE10336473A1 - Molding composition and moldings used for dissipating electric charge, e.g. antistatic packaging, structural part in zone protected from explosion or electromagnetic screening, contains electroconductive cellulose fibers in polymer matrix - Google Patents

Abstract

Molding composition has electroconductive cellulose fibers in a polymer matrix. An independent claim is also included for moldings made from this composition.

Description

The The present invention relates to the production of electrically conductive plastics for charge dissipation, as required for example in the ESD sector become. The plastics are made by the addition of electrically conductive cellulose fibers while modified compounding.

The industry demands new materials for the development of high-quality products. An important area is represented by electrically conductive plastics. The plastics industry already provides plastic compounds, ie mixtures with conductive substances, in a wide range of variations. For the specific volume resistivity values between those of the pure plastics (> 10 ¹⁴ Ω cm) and about 0.1 Ω cm can be obtained.

Polymer Materials are usually insulators. This property will be for many Applications used in electrical engineering and electronics. The solution of different However, technical issues with polymer materials requires the presence of a defined electrical conductivity.

A defined and reproducible setting of a sufficiently low conductivity a carbon black filled material within the percolation threshold succeed so far only inadequately and requires a precise filler dosage as well as very high consistency in compound processing. It exists therefore great interest the reproducibility in the setting of the desired conductivity by adding fillers to improve.

A Electrostatic discharge occurs when a non-conductive surface against another is rubbed and the surfaces are then separated. Electrostatic discharges can damage or destroy sensitive electronic components, magnetic Change disk or Clear or cause explosions and fire in a flammable environment. each Year alone in the electronic industry estimated 40 Billion euros damage only by electrostatic discharges.

• – Antielektrostatische Ausrüstung für viele technische Bereiche wie z. B. Elektronikarbeitsplätze, Geräte und Anlagen in Reinsträumen, sehr dünne Beschichtung von Filmen, kälteschlagzähe, benzin- und ölfeste Teile im Verkehrsmittelbau usw.;
• – Antistatische Verpackungen aller Art dort, wo man elektrostatische Aufladung vermeiden muss;
• – Elektromagnetische Abschirmung von elektrischen Geräten und von elektronischen Baugruppen und Systemen;
• – Anwendungsbereiche, die aus Sicherheitsgründen einen niedrigen Oberflächenwiderstand der Werkstoffe verlangen, z. B. für elektrische Betriebsmittel in explosionsgeschützten Bereichen.

The following applications are in the foreground for the electrically conductive plastics:

• - Anti-electrostatic equipment for many technical areas such. As electronic workstations, equipment and equipment in clean rooms, very thin coating of films, cold weather resistant, petrol and oil-resistant parts in transportation, etc .;
• - Anti-static packaging of all kinds where it is necessary to avoid electrostatic charging;
• - Electromagnetic shielding of electrical equipment and electronic assemblies and systems;
• - Applications requiring a low surface resistance of materials for safety reasons, eg. B. for electrical equipment in explosion-proof areas.

Of the classic way to equip polymers electrically conductive is to isolate the Matrix conductive Add additives. Examples are polymer-carbon black compounds or the addition of hygroscopic additives.

The Addition of hygroscopic additives has disadvantages. So is their effectiveness is linked to the presence of (air) moisture and they are removable by wiping the finished parts, as they are to the plastic surface migrate. The stored in them "reservoir" of active substance is also finite.

In many cases is the desired, defines limited conductivity even difficult to set by the incorporation of conductive carbon black, since the steep percolation threshold already at low Rußkonzentrationsschwankungen critical resistance changes causes.

• – höchstgenaue Dosierung des Additivs im Anstiegsbereich der Leitfähigkeit oder
• – Verwendung eines Additivs mit niedriger Leitfähigkeit im Plateaubereich.

To set permanent, defined surface resistivities between 10 ¹⁰ and 10 ⁶ Ω, as required for a number of antistatic applications, there are two ways:

• - High-precision metering of the additive in the rise range of the conductivity or
• Use of a low conductivity additive in the plateau region.

Of the Path for setting defined conductivities in the specified range is very problematic from the material development side, because already slight fluctuations of the filler concentration in polymers, which can easily occur during processing, too strong variations to lead.

Another disadvantage of using Leitrußen or similar substances in the plastic is that they by their presence, depending on the required concentration, lead to a negative influence on the mechanical properties. Thus, with a carbon black concentration of 10 to 40% by weight, as is relevant for such an application, a significant increase in the viscosity of the system occurs. Furthermore, an increase in stiffness, indentation hardness and heat resistance, associated with a mostly noticeable decrease in the flexibility, ductility and toughness of the carbon black-filled plastic is observed with increasing carbon black concentration. To a critical extent, the impact strength and its temperature dependence are also often influenced. Similarly, this statement may apply to other conductivity-promoting pigments.

Surprisingly, it has been found that these deficiencies can be eliminated by the targeted introduction of conductive cellulose fibers which have been produced, for example, by a lyocell spinning process. Such a process for producing these fibers is described, for example, in US Pat DE 44 26 966 A1 and DE 195 37 726 A1 described.

object The invention is therefore an electrically conductive thermoplastic molding material with a polymer matrix and incorporated therein electrically conductive cellulose fibers.

The Polymer matrix may contain any polymer, for example a polyamide (PA), a polyester, a polystyrene, an ABS or a polyolefin.

suitable Polyamides are, for example, PA46, PA66, PA6, PA610, PA612, PA810, PA 1010, PA 1012, PA 1212, PA 11, PA 12, higher polyamides of the type PAXY with X and / or Y> 12 such as B. PA614, PA618 or PA1815, each as homopolyamide or as corresponding copolyamides.

suitable Polyesters are, for example, polyethylene terephthalate, polybutylene terephthalate, Polyethylene-2,6-naphthalate or polyeuylene-2,6-naphthalate.

suitable Polystyrenes are, for example, clear polystyrene or rubber-modified Polystyrene CHIPS), also copolymers with z. For example, methyl methacrylate or acrylonitrile.

SECTION is an acrylonitrile-butadiene-styrene terpolymer, usually in the form of a with a styrene-acrylonitrile copolymer grafted polybutadiene, optionally with further units Comonomers can be included.

suitable Polyolefins are, for example, polyethylene, polypropylene or polybutene (1).

These Polymers are all state of the art. You can in the polymer matrix too exist as a mixture, for. B. from one or more polyamides as well as an impact-modifying Rubber.

Furthermore the polymer matrix may be the usual Auxiliary and aggregates contain such. As plasticizers, flame retardants, Dyes, pigments, stabilizers and / or processing aids.

The Cellulose fibers get their conductivity by being incorporated therein conductive Additives, for example, from carbon black (especially conductivity soot), from Graphite powder consist of graphite fibrils or carbon nanotubes can. This conductive Additives are usually present in quantities in the cellulose fibers 0.1 to 70 wt .-%, preferably from 2 to 60 wt .-% and particularly preferably from 5 to 50 wt .-%.

It is known that by modifying a Lyocell process cellulosic fibers with adjustable electrical conductivity can be produced ( DE 44 26 966 A1 and DE 195 37 726 A1 ). This method is based on the dissolution of cellulose in an organic solvent, extrusion of the solution through a die through an air gap into a coagulation bath, subsequent removal of the solvent and drying of the shaped article, especially the fiber. A modification of the method is based on the addition of conductive additives in the stage of dissolution of the cellulose.

The conductive additives, such. As conductivity, modified cellulose fibers have, depending on the concentration of the additives used, a resistivity of 10 ¹³ to 0.2 Ω cm, and preferably from 10 ⁸ to 40 Ω cm.

The fibers retain, due to a high reason specific to the manufacturing process strength, largely their textile properties.

The explained, electrical and mechanical properties of the fibers provide a Excellent condition, by incorporation in conventional plastics to influence their electrical properties such that a reproducible dissipation of electrical charges, coupled with sufficient Insulation properties (contact protection), can be achieved because a flattened percolation threshold at smaller Fluctuations in the fiber concentration have no significant effect on the resistance values. Another advantage of introducing conductive fibers is the fact that the same electrical effect due to the fiber geometry and the conductive structures of the fibers in the plastic much less conductive additive requires, as with the addition of conductivity additive in the plastic according to the prior art.

Farther bear the textile properties of the fibers (tear resistance, Elongation) with, that in contrast to pure Leitrußen or Filled with graphite Plastics the decrease of flexibility, ductility and toughness of the filled one Plastic is largely prevented.

• – Gehalt an Leitfähigkeitsadditiv in der Faser,
• – Einsatz verschiedener Arten von Leitfähigkeitsadditiven,
• – Faserfeinheit ( Querschnitt),
• – Faseranteil im Polymer sowie
• – Faserlänge im Compound.

Electrical and mechanical properties of the compound of plastic and electrically conductive cellulose fibers can optionally be varied by:

• Content of conductivity additive in the fiber,
• Use of different types of conductivity additives,
• - fiber fineness (cross-section),
• - Fiber content in the polymer as well
• - Fiber length in the compound.

By selecting suitable parameters of the fibers, the type of plastic matrix, the proportions of plastic to fiber, surface resistivities of between 10 ¹ and 10 ¹² Ω and preferably between 10 ⁶ and 10 ¹⁰ Ω, as required for use in potentially explosive atmospheres, can be achieved. Resistance values below 10 ⁶ ohms can be realized by choosing a higher percentage of conductivity additive in the fiber and a higher fiber content in the compound. With such compounds, a targeted dissipation of electrical charges and an attenuation of electromagnetic waves are possible.

The Introducing the conductive Fibers in the polymers occur in the context of a compounding in Single or twin screw extruders. This is the matrix material (Plastic) together with the conductive fiber in the extruder given and the fiber at temperatures above the melting point incorporated into the matrix.

Surprisingly showed that at processing temperatures of up to 270 ° C and the for the Needed extrusion Residence time of the fibers at these temperatures no significant damage the fibers occurs.

The Matrix material can be in granular, powder or fiber form available. The fineness of the compounded fibers is between 0.1 and 20 tex.

The Fibers can as a short fiber, but also as a fiber roving or as a sliver ordered staple fibers are added to the extruder (pull-trending method).

Farther it is possible, Matrix fibers and conductive To mix fibers in a textile process and then to to handle a hybrid tape (carding). This sliver can then over one Preheating, nozzle with following cooling and deduction with twisting a granulator are supplied. In this pull-drill process Long fiber granulate is produced. This is characterized by the gentle nature Processing by a narrow distribution of fiber lengths, where individual fiber lengths much longer could be as the granule cut length.

The produced granules can be pressed into plates or with injection molding machines be splashed. The processing can be in pure form or as admixture (Masterbatch). As a rule, the manufactured ones include Articles 1 to 50, preferably 2 to 20 wt .-% and particularly preferably 3 to 10 wt .-% conductive Fibers, with the average fiber length in the finished article more as 100 μm, preferably more than 800 microns should be sufficient to dissipate electrical charges to guarantee. If a masterbatch granulate is produced, can this up to 50 wt .-% conductive Contain cellulose fibers.

The Forming can, instead of granules, also directly from the after Compounding of the electrically conductive fiber in a single or twin screw extruder obtained melt done.

Appropriate Moldings are also the subject of the invention.

The moldings according to the invention can find use for the derivation of electrical charges, for example as antistatic packaging, such as in the form of a film, or as Component in an explosion-proof Area. You can about that In addition to the electromagnetic shielding of electrical equipment, electronic Assemblies or electronic systems are used.

Examples:

Example 1:

It corresponding amounts of matrix polymer polyamide 12 (Vestamid L 1700 of Degussa AG) and weighed cellulose fibers and weighed mixed. After mixing, the samples were generally at 60 ° C and a Vacuum of <1 mbar dried within 18 h.

The Compounding of the samples was done on a single-screw laboratory extruder made by Randcastle Extraction Systems Inc. The used Snail was a ½ '' snail (12.5 mm); it was worked with electrical resistance heaters. All temperatures from feed zone to slit nozzle were set equal. The throughputs amounted to 350 to 400 g / h. The average residence time of the goods in Extruder was 3 to 4 min.

The resulting extrudate was then pressed into plates. The specific surface electrical resistance was measured at the Plates using a milli and tera ohmmeter (Milli TO3 the Fa. Fischer Elektronik, Berlin) in a 1 cm annular gap at a measuring voltage measured from 100V. Below are the settings and readings reproduced:

Used fiber:

• Fiber fineness: 0.3 tex
• fiber length 2 mm

Example 2:

It was proceeded as in Example 1. The matrix material used was polypropylene (ATOFINA PPH 9060).

Used fiber:

• Fiber fineness: 0.3 tex
• fiber length 2 mm

Example 3:

It was proceeded as in Example 1. The matrix material used was polyethylene.

Used fiber:

• Fiber fineness: 1 tex
• fiber length 5 mm

Example 4:

Fiber slivers of a blend of 30% conductive lyocell fiber (Printex L content of 28.5%, 60 mm, 0.5 tex) and 70% polypropylene fiber were pulped using a pull-drill process to a masterbatch long fiber granulate. The technological conditions were as follows:

The Long fiber granulate masterbatch was subsequently mixed with various admixtures of standard PP granules mixed and those in the following table injection molded into plates. The admixtures were chosen that fiber contents of 3.5, 5 and 7.5% are set in the injection molded article were.

The electrical resistivity was measured on the injection-molded plates using a milliand terahohm meter (Milli TO3 from Fischer Elektronik, Berlin) in a 1 cm annular gap at various measuring voltages. The following are the measurements:

Example 5:

Maschinenparameter: Drehzahl: 100 l/min Drehmoment: ca. 60 % Granulierung: 2,5 mm Kaltabschlag The preparation of the compound granules of vestamide and cellulose fiber (with 37.5% Printex L carbon black, fineness 0.3 tex) was carried out on a co-rotating twin-screw extruder ZSK 25 WLE from Werner & Pfleiderer with the following settings:

Machine parameters: Rotation speed: 100 l / min torque: about 60% granulation: 2.5 mm cold cut

For the production of pressed test specimens granules were melted in a tempered mold under pressure at the following parameters. Plate Dimensions: 250 ∙ 250 ∙ 3 Press temperature: 200 ° C Pressing time: 6 min Press pressure: 350 N / cm ²

The following resistance values were determined for different variants:

Claims (18)

Molding composition with a polymer matrix and embedded therein electrically conductive Cellulose fibers. Molding composition according to claim 1, characterized in that the polymer matrix is a polyamide, a polyester, a polystyrene, an ABS or a polyolefin. Molding composition according to one of the preceding claims, characterized in that the electrically conductive cellulose fibers have a volume resistivity of 10 ¹³ to 0.2 Ω cm. Molding composition according to one of the preceding claims, characterized in that the electrically conductive cellulose fibers have a volume resistivity of 10 ⁸ to 40 Ω cm. Molding composition according to one of the preceding claims, characterized in that it has a surface resistivity between 10 and 10 Ω. Molding composition according to claim 5, characterized in that it has a surface resistivity between 10 ⁶ and 10 ¹⁰ Ω. Molding composition according to one of the preceding claims, characterized in that the conductive cellulose fiber is a fineness from 0.1 to 20 tex. Molding composition according to one the claims 2 to 7, characterized in that the polyamide is selected from the group PA46, PA66, PA6, PA610, PA612, PA810, PA1010, PA 1012, PA 1212, PA 11 and PA 12. Molding composition according to one claims 2 to 7, characterized in that the polyolefin is selected from the group polyethylene, polypropylene and polybutene (1). Molded part made of a molding compound according to a of the preceding claims. Molded part according to claim 10, characterized in that the conductive cellulose fiber has an average fiber length of more than 100 μm has. Molded part according to claim 10, characterized in that the conductive cellulose fiber has an average fiber length of more than 800 μm has. Molded part according to a the claims 10 to 12, characterized in that it contains 1 to 50 wt .-% of conductive cellulose fibers contains. Molded part according to a the claims 10 to 12, characterized in that it contains 2 to 20 wt .-% of conductive cellulose fibers contains. Molding according to one of claims 10 to 14, characterized in that the shaping directly from the obtained after the compounding of the electrically conductive fiber in the single or twin screw extruder melt takes place. Use of a molding according to one of claims 10 to 15 for the discharge of electrical charges. Use according to claim 16, characterized in that the molded part is an antistatic Packaging is. Use according to claim 16 as a component in an explosion-proof area.