CH639230A5 - Method for producing an electrically conductive polymer formulation - Google Patents

Description

The invention relates to a method for producing an electrically conductive crosslinked polymer formulation according to the preamble of claim 1 and to an uncrosslinked preparation according to the preamble of claim 15.

Such polymer formulations preferably have a positive temperature coefficient of electrical resistance (PTC); For the sake of simplicity, these formulations are therefore referred to below as “PTC materials”.

In recent years, self-regulating heating systems have been used for numerous electric heating arrangements, using materials which show certain types of a PTC characteristic and are characterized by the property that a noticeable increase in resistance occurs when a certain temperature is reached. State-of-the-art heating elements based on PTC materials generally show a more or less pronounced increase in resistance within a narrow temperature range, but only relatively small changes in resistance with temperature below the temperature range. The temperature at which the resistance begins to rise sharply is often referred to as the switching or step temperature (Ts), since when this temperature is reached the heating element shows an abnormal change in the resistance and tends to switch off. Unfortunately, this shutdown occurs with the prior art PTC elements at relatively low power densities. Self-regulating heating elements based on PTC materials generally have the advantage over conventional heating elements that they do not require thermostats, fuses or series-connected electrical resistors.

The most commonly used PTC material has been doped barium titanate, which has been used for self-regulating ceramic heating elements, such as those used for heat troughs or hot plates for food and other small portable heating devices. Although such ceramic PTC materials are generally used for heating arrangements, their application possibilities are considerably restricted by their rigidity. PTC materials containing electrically conductive polymer formulations are also known, some of which have shown the particular characteristics described above. However, the use of such polymeric PTC materials has been relatively limited in the past, primarily due to their low heating capacity. Such materials generally contain one or more conductive fillers, e.g. Carbon black or powdered metal, which is dispersed in a crystalline thermoplastic polymer. PTC formulations made from highly crystalline polymers generally show a steep increase in resistance, which

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begins a few degrees below their crystalline melting point, similar to the behavior of their ceramic counterparts at the Curie temperature (Ts for ceramic materials). PTC formulations which differ from homo- and copolymers with low crystallinity, e.g. a crystallinity of less than about 50% generally show a less steep increase in resistance, which often begins in a less well-defined temperature range and significantly below the crystalline melting point. In extreme cases, polymers with low crystallinity show resistance / temperature curves that are more or less simply concave (seen from above) and that do not show a definable point at which a steep rise begins. Other types of thermoplastic polymers give resistance / temperature curves that are fairly uniform and more or less steep, but increase continuously with temperature.

Figure 1 shows the characteristic curves of different types of PTC formulations. In this figure, curve I shows the steep increase in the temperature characteristic of barium titanate and polymers with very high crystallinity; the behavior corresponding to this curve is referred to as behavior type I below. Curve II shows a gradual increase, which begins at lower temperatures according to the melting point of the polymer (behavior type II), which is characteristic of most polymers with medium to high crystallinity. Curve III shows the concave (seen from above) characteristic of many polymers with very low crystallinity (behavior type III). Curve IV shows a sharp increase in resistance without a region of more or less constant resistance (at least in a region of commercial interest), as is found for some materials (behavior type IV). Curve V shows the constant increase in resistance / characteristic as many «normal» electrical resistances show (behavior type V). Although the types of behavior mentioned above have been illustrated with reference to specific types of materials, it is clear to any person skilled in the art that the type of behavior is influenced to an extremely great extent by the type and amount of the conductive filler present. Particle size, surface quality, tendency to agglomerate formation and shape of the particles or particle agglomerates, in particular, play a role here. the tendency towards scaffolding, a crucial role, which is particularly the case when using carbon black as a filler.

Experiments have shown that the formulations described in the literature, which apparently manifest behavior that corresponds to type I, actually show behavior that corresponds to type II to type IV. These types of behavior have not yet been recognized. Furthermore, even those materials belonging to the prior art which have a certain jump point, i.e. where the resistance shows a sharp rise at Ts, a "drop", i.e. a decrease in the resistance when the temperature of the PTC element rises noticeably above the temperature Ts, which can happen in particular if there are high power densities in the element.

U.S. Patent No. 3243753 (Kohler) describes a carbon filled polyethylene in which the conductive carbon particles are in mutual contact. The product described there contains 40% by weight of polyethylene and 60% by weight of carbon particles, so that the resistance at room temperature is approximately 0.4 ohm-cm -1. As is typical for the assumed mode of operation of the materials belonging to the prior art, this PTC material is characterized by a relatively flat course of the resistance /

Temperature curve below the jump temperature, followed by a steep increase in resistance, characterized by at least 250% over a temperature range of 14 ° C, which corresponds to an approach to behavior type I. According to this author (Kohler), the steep increase in resistance is due to the different thermal expansion of the components contained in the formulation, i.e. of polyethylene and particulate carbon. It is believed that the conductive filler contained in the formulation in high concentration forms a conductive network within the polyethylene polymer matrix, resulting in an initially constant resistance at low temperatures. However, in the vicinity of the crystalline melting point, the polyethylene matrix expands rapidly, causing large parts of the conductive network to break open, which in turn leads to a steep increase in the resistance of the formulation.

Other theories that have been put forward to explain the PTC phenomenon in polymer formulations that contain conductive particles as fillers include complex mechanisms that are based on tunneling of the electrons through vacancies between the individual particles of the conductive filler tunnel-ling through inter grain gaps between particles of conductive filier) and mechanisms based on a phase transition from crystalline to amorphous areas within the polymer matrix. A comprehensive discussion of a number of proposed alternative mechanisms for the PTC phenomenon can be found in "Glass Transition Temperatures as a Guide to the Selection of Polymers Suitable for PTC Materials", J. Meyer, Polymer Engineering and Science, November 1973, Vol. 13, No. 6.

It should be noted here that the prior art polymeric PTC materials are formulations which show a transition temperature (Ts) at or below the melting point of a thermoplastic component.

As already mentioned, US Pat. No. 3,243,753 describes a formulation which essentially consists of a polymeric polyethylene or polypropylene matrix in which carbon black is dispersed, the polyolefin being polymerized in situ. Such materials show a PTC characteristic at the melting point of the polymer. Furthermore, a formulation is described there which contains polyethylene in which carbon particles are dispersed and which can either be crosslinked or can additionally contain a thermosetting resin in order to give the system strength or stability. The transition temperature Ts is not influenced by this addition, but rather remains at the crystalline melting point of the thermoplastic polyethylene, i.e. at 120 ° C.

U.S. Patent No. 3825217 describes numerous crystalline polymers with PTC characteristics. These include, for example, low-pressure and high-pressure polyethylene, as well as polypropylene, polybutene-1, poly (dodecamethylene-pyromellitimide) and ethylene-propylene copolymers. It is also proposed to use mixtures of crystalline polymers, e.g. to use a blend of polyethylene with an ethylene-ethyl acrylate copolymer to vary the physical properties of the final product. This patent also suggests a method of annealing the polymer to half the melting temperature of the polymer to achieve a decrease in resistance. Similarly, U.S. Patent No. 3 591526 (Kawashima et al.) Proposes polymer blends containing soot and exhibiting PTC characteristics. But also in this case

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the position of the transition temperature Ts, which is close to the crystalline melting point, depends on the type of thermoplastic material used, while the second component serves only as a carrier for the soot-loaded thermoplastic material. s

US Pat. No. 3793,716 (Smith-Johannsen) describes mixtures of conductive particles and polymeric materials with PTC characteristics, in which a crystalline polymer in which carbon black is dispersed in a suitable solvent above the polymer melting point is dissolved and the solvent is then evaporated off to obtain a formulation which shows a decrease in resistance at room temperature at a given concentration of the conductive filler. In this case too, the transition temperature Ts is below the melting point of the polymer matrix. Heating the polymer above the melting point only causes a decrease in resistance at room temperature and / or maintenance of a constant resistance at that temperature. 20th

Self-regulating heating elements based on PTC materials, as described above, in reality do not show extremely steep R = f (T) curves (type I), so that they practically switch off above a certain temperature and below them Temperature at 25 constant voltage deliver a relatively constant wattage. At temperatures below Ts, the resistance is relatively low and almost constant, so that the current flow is comparatively high at every voltage applied. The energy generated by this current flow is given off as heat, the PTC material being heated. The resistance remains at this relatively low value until the temperature Ts is reached. At this point the resistance increases rapidly. With increasing resistance, there is a decrease in power, which limits the amount of heat generated, so that when the temperature Ts is reached, the heating is practically stopped. If the temperature of the arrangement then drops below the temperature Ts by releasing heat to the environment, the resistance drops, as a result of which the power is increased. In the steady state there is a balance between the heat generated and the heat given off. Accordingly, the Joule heat, when a voltage is applied to a PTC heating element of type I, heats the heating element up to its transition temperature Ts 45 (the speed of such heating depends on the type of PTC element used), after which increases with resistance little additional temperature rise occurs. Due to the increase in resistance, such a PTC heating element generally reaches a steady state at approximately the temperature T $, and thus causes the heat emission of the element to self-regulate without additional aids such as fuses or thermostats.

It can be seen from the preceding explanations, 55 that materials which can be assigned to behavior type I have significant advantages over PTC materials which can be assigned to other behavior types. With materials of type II and III, it has a disadvantageous effect

that because of the much less sharp transition, the temperature of the stationary state of the heating element depends very much on the thermal load applied. In addition, such materials suffer from a so-called “current inrush problem”, as will be described in more detail below. Type IV PTC materials have not previously been considered suitable materials for heating elements to be used in practice because they lack a temperature range in which the power output is not significantly dependent on the temperature.

Although, as previously mentioned, it was recognized that having a formulation for heating elements having a resistance / temperature characteristic corresponding to Type I was recognized as a considerable advantage, many of the known formulations ascribed to Type I actually exhibit behavior which is much more similar to that of Type II or Type III. The optimal behavior (type I) is only peculiar to a limited number of formulations. There has therefore long been a need to find ways and means to modify formulations whose behavior corresponds to type II or type III and which, due to physical or other properties, appear to be suitable for use with PTC heating elements, in such a way that their behavior is more Type I corresponds. In addition, and, as already mentioned, to its great disadvantage, many of the materials belonging to the prior art undergo a "turn on" when the temperature rises slightly above the temperature Ts, although these materials increase or show less steep increase in resistance at temperature Ts. If the increase in resistance at or above temperature Ts is not large enough and / or the resistance decreases above the melting point of the formulation, as is generally the case with the prior art materials, it may cause overheating or burnout come.

Polymeric PTC formulations for heat shrinkable products have also been proposed. For example, Day's Patent Office Defensive Publication T905001 is taught how to use a heat-shrinkable plastic film with PTC characteristics. However, this film suffers from the rather serious disadvantage that, due to the fact that the temperature Ts is below the crystalline melting point of the film, only a very small recovery force can be generated. Neither Day, nor any of the other authors mentioned above address certain additional problems common to all prior art PTC heater elements, and still less provide a solution to these problems. One of these problems is that of "current inrush". This problem is particularly severe when there is a desire to create a heating element with Ts above about 100 ° C. While it is entirely possible to find a polymeric PTC material with Ts at 150 ° C, the resistance of such materials at or just below Ts can be up to 100 times greater than the resistance at room temperature. Since a PTC heating element usually works at or a little below Ts, its actual heating power is determined by its resistance a little below Ts. Accordingly, a PTC heating element, e.g. absorbs 50 amperes at 150 ° C, easily absorbs 500 amperes at room temperature.

When using heat-shrinkable material for PTC heating elements, power-on problems can occur as further defects. As is known, it is advantageous for heat-recoverable objects to carry out the shrinkage as quickly as possible. It is evident that a heating element with a flat power / temperature characteristic heats up faster and more uniformly than a heating element in which the power rises when the temperature rises to Ts, e.g. one tenth of the value at room temperature. The use of a highly crystalline polymer as a matrix for the conductive particles alleviates the "current turn-on problem" mentioned above. In addition, these highly crystalline polymers show a steep increase in

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Resistance at about 15 ° C below their crystalline Ts and above the melting point continues to rise. Under

Melting point, i.e. at this point they have a use of the formulations according to the invention e.g.

Crack temperature. Unfortunately, such poly-made heating elements have e.g. their working range at temperature Ts is still a considerable crystalline within a period of time, which operates independently within wide limits and thus shows not only a low resilience, 3 from the prevailing ambient temperature, and if they are also in the heat-recoverable state, they are also almost preferably insensitive to coverings, but also have an effect on the deformation. This insensitivity to temperature e.g. The heating of the heat-recoverable parts connected to them, dementes against thermal stress, allows the manufacture of heat-shrinkable arrangements, e.g., above their recovery temperature. Pro can oppose. If one chooses a heating resistor to construct products whose behavior is predicted i.e. a low resistance, so that the heating element can not damage the substrate to which the element switches off at a temperature applied in the vicinity of the arrangement, e.g. a thermoplastic cable

Temperature of maximum resistance (Tp) is and quite sheathing, can cause.

corresponds exactly to the actual melting point, advantageous embodiments of the method can avoid the aforementioned disadvantage. All> 5 can be achieved with the features of claims 2 to 11, show resistances belonging to the prior art. However, the resistors used in the method according to the invention either have a steep drop in resistance or in materials can e.g. Parts of a polymer, e.g. a graft-very rarely a practically constant resistance, or block copolymer, or a physical blend of if the temperature of the PTC material is above its two polymers. In addition, if a copolymer's melting point is increased. As a further disadvantage that is applied to the 20, a thermoplastic and / or ela-state of the art PTC polymer formulations are mixed with stomeric material.

it can be seen the fact that the ratio of the resistance. In the method according to the invention e.g. Under one of the temperature Tp to the resistance at the temperature- "elastomeric material" of a component of the mixture, a temperature Ts drastically decreases when these formulations mean polymeric material, which in the crosslinked state is stretched at, which is the case with the usual methods of formation 25 room temperature is elastomeric.

a heat-recoverable object is necessary. Particularly suitable graft polymers are e.g. to be named

For example, an initial ratio of 108 to: ethylene propylene with a crystalline polyolefin, e.g. 105 at 10% elongation and to 103 at 25% elongation, grafted polypropylene, e.g. fall off as block copolymers. These last-mentioned factors lead to star- especially ethylene-propylene / polystyrene and polytetra-ken enlargement of the risk of overheating or of methylene oxide / polytetramethylene terephthalate. Burning out in the known heating elements when these can be used as thermoplastic materials, in particular in heat-shrinkable arrangements. Polypropylene, polyvinylidene fluoride or polyethylene in

It would therefore represent a significant step forward, while chloro-elastomeric materials, if there was a possibility of creating a PTC material, were more closely approximated to those of type I, 35, or chlorinated or chlorosulfonated polyethylene or neoprene.

and which is not in the middle due to high current load. A formulation e.g.

is drawn. made of ethylene-propylene rubber, polypropylene and carbon black

The object of the invention is therefore the one mentioned above. The weight ratio of the components is a method for producing an electrically conductive crosslinked, preferably 40 to 90%, 5 to 40% or 5 to 35%. Polymer formulation with the characteristic features 4 "It is particularly advantageous if the formulation obtained according to the invention of claim 1 is, for example, at least in the range between

The present invention is based on the surprising see room temperature and melting point of thermo-observation that many of the plastic materials mentioned above have a positive temperature coefficient deficiency due to the creation of the polymeric, thermoplastic of the resistance, i.e. is a PTC material.

The invention also relates to a heating element in which there is a steep increase in resistance immediately below the molded body on a formulation of the aforementioned melting point of the formulation, the resistance of which is arranged between at least one pair of electrodes, but preferably continues to increase if the temperature is over. The electrodes can be connected to a suitable voltage - the melting point can be raised, can be fixed - to feed the heating element. That. Heating elements, e.g. which show this characteristic, 50 heating element can also be heat-recoverable, preferably also continue their control activity if e.g. Furthermore, the invention relates to an uncrosslinked temperature above the melting point of the thermoplastic preparation for carrying out the method according to the invention.

polymer increases, i.e. they switch e.g. not in ground; this preparation contains a) a desirable and harmful manner in a mixture with one another, whereas the heating elements belonging to a polymeric material, which in the cross-linked state in the state of the art are elastomeric under these 55 room temperature, b) a thermoplastic poly-circumstance for thermal runaway and hence the mer, whose glass transition temperature, if it is amorphous,

Tend to burn. is above room temperature, and. c) carbon black, the polymer a)

By creating PTC formulations with sufficient characteristics in the uncrosslinked state at room temperature, the method according to the invention can have strength to withstand its own weight during longer materials e.g. for heating elements, which can be worn for 60 years without noticeable deformation.

even at a resistance level, e.g. considerable Uncured (uncrosslinked) elastomers are often above the resistance at the temperature Ts, referred to as "rubber precursors" (English: gum stocks). Regulation can be used, but the risk of Mixtures of most rubber precursors and thermal runaway or burn-through significantly "match" a thermoplastic material, i.e. while diminishes. In addition, e.g. the temperature is heated for a period of time which is sufficient to achieve only a certain orientation in relation to the switched-on heating element under conditions which change this preferred molecular structure and in return from low to high thermal load, and then cools down, so it happens a lot little, e.g. the resistance above temperature when cooling very quickly to equilibrium

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in favor of a different molecular configuration attributable to the lower temperature. However, some rubber precursors show e.g. alone or in admixture with a thermoplastic polymer, a behavior which differs from this, either due to a very high molecular weight which causes distortions, to small crystallinity ranges, or to other characteristics, e.g. rigid or glass-like parts of the molecule can be attributed. With these materials e.g. If the equilibrium was shifted in favor of a configuration favored at high temperature, there is only a slow or incomplete shift of the equilibrium to another configuration when cooling to room temperature. The resulting rubber precursors show e.g. what is often referred to as “green strength”. This well known property causes e.g. Rubber precursors have a dimensional stability at room temperature, so that molded articles made therefrom do not suffer any deformation and do not flow noticeably even when uncrosslinked. Such rubber precursors, for example, have a noticeable creep resistance and, if they are in the form of granules, do not tend to cake together, whereas, by comparison, other rubber precursors which have no initial strength show this tendency. In the crosslinked state at room temperature, elastomeric materials, which preferably have a noticeable initial strength, are suitable for the production of the formulations according to the invention. It is believed that e.g. this property enables the desired configuration to be "frozen" when the formulation is crosslinked, which leads to the observed PTC behavior, which unexpectedly persists even in a temperature range in which a completely amorphous mixture of the polymers is present, i.e. above the melting point of the components.

In the present case, “elastomer” is understood to mean a polymeric material which, e.g. experiences an elastic deformation under load, is flexible and elastic and has a recovery capacity under high loads. “Thermoplastic material” is to be understood as a polymeric material which e.g. at room temperature has no recovery capacity under heavy loads, whereas at higher temperatures, i.e. above its melting point, can be reshaped to any desired new shape. The melting point is preferably the temperature above which a certain material acquires elastomeric properties when it is cross-linked or changes into a viscous liquid when it is uncross-linked.

The melting point or softening point can be determined for the material in accordance with the ASTM regulations. If the non-elastomeric component, e.g. If a graft or copolymer is at least partially crystalline, the melting point is determined in accordance with ASTM D 2117 64; the disappearance of the birefringence associated with the crystalline material is displayed with increasing temperature. If the component is not crystalline, i.e. glassy, the method according to ASTM D 2236-70 is preferably used, whereby a torsion pendulum is used. If neither method is suitable, ASTM methods D 648-72 or D 1525-70 can be used. Under «rubber preliminary stages» e.g. an uncrosslinked material that shows elastomeric properties after crosslinking. The term «significant green strength» means, if applied to rubber precursors or thermoplastic elastomers, that the material e.g. has a noticeable creep resistance and shows no tendency to cake when the individual particles come into contact with one another and also has a tensile strength of at least 0.703 kp / cm2 at 20% elongation. The following list contains a large number of examples of thermoplastic materials which can be used for the formulations according to the invention:

a) polyolefins, e.g. Polyethylene and polypropylene;

b) Thermoplastic copolymers of olefins, e.g. Ethylene or propylene with one another or with other copolymerizable olefinically unsaturated monomers, e.g. Vinyl esters, acids or esters of α, β-unsaturated organic acids;

c) Halogenated vinyl or vinylidene polymers, e.g. those derived from vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride and copolymers with one another or with other halogenated or unsaturated monomers;

d) polyester, both aliphatic and wholly or partly aromatic, e.g. Poly (hexamethylene adipate or sebacate), poly (ethylene terephthalate) and poly (tetramethylene terephthalate);

e) polyamides, e.g. Nylon-6, Nylon-6.6, Nylon-6.10 and «Versamide» (a condensation product of dimerized or trimerized unsaturated fatty acids, especially linoleic acid, with polyamines);

f) various polymers such as polystyrene, polyacrylonitrile, thermoplastic silicone resins, thermoplastic polyethers, thermoplastic modified celluloses and polysulphones.

The elastomer component is e.g. virtually any rubber precursor that has a noticeable initial strength as previously defined. Most commercially available rubber precursors for elastomers either have a noticeable initial strength or, if at all, only a very low initial strength. Accordingly, one skilled in the art can easily select from the list of elastomeric rubber precursors below that have appreciable initial strength, e.g. natural or synthetic polyisoprene, irregularly built ethylene-propylene copolymers, styrene-butadiene-copolymer rubbers, styrene-acrylonitrile-butanediene-terpolymer rubbers with or without the addition of small amounts of copolymerized a, β-unsaturated carboxylic acids, polyacrylate-based copolymers, polyurethane acrylate rubbers, polyacrylate rubbers, non-polyurethane copolymers of vinylidene fluoride and e.g. Hexafluoropropylene, polychloroprene, chlorinated polyethylene, chlorosulfonated polyethylene, polyether, plasticized polyvinyl chloride with a content of more than 21% plasticizer and practically non-crystalline irregularly structured copolymers or terpolymers of ethylene with vinyl esters or acids and esters of a, ß- unsaturated acids.

Thermoplastic-elastomeric copolymers which can be used in the process according to the invention for the preparation of the new formulations comprise both graft and block copolymers. Examples include:

a) Irregularly built ethylene and propylene copolymers grafted with polyethylene or polypropylene side chains;

b) block copolymers of a-olefins, e.g. Polyethylene or polypropylene with ethylene / propylene or ethylene / propylene / diene rubbers;

Polystyrene with polybutadiene;

Polystyrene with polyisoprene;

Polystyrene with ethylene propylene rubber;

Polyvinylcyclohexane with ethylene-propylene rubber;

Poly-a-methylstyrene with polysiloxanes;

Polycarbonates with polysiloxanes;

Poly (tetramethylene terephthalate) with poly (tetramethylene oxide) and thermoplastic polyurethane rubbers.

The formulation advantageously contained e.g. 3 to 75 wt .-%, preferably 4 to 40 wt .-% elastomer, based on

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the total weight of elastomeric and thermoplastic materials. In block copolymers which have both thermoplastic and elastomeric regions, the elastomeric fraction is preferably 30 to 70% by weight of the polymer molecule. The preparation of such block and graft copolymers and the mixing of the thermoplastic and elastomeric component can be carried out by generally known, conventional methods, e.g. by grinding or by mixing with an internal mixer (Banbury mixer). The amount of the particulate conductive filler is expediently 4 to 60% by weight, preferably 5 to 50% by weight.

In general, any particulate conductive material can be used to make the above formulations electrically conductive. In addition to particulate carbon black, preferred conductive fillers for the polymeric PTC formulations according to the invention include graphite, metal powder, conductive metal salts and oxides, and boron- or phosphorus-doped silicon or germanium.

It is self-evident to the person skilled in the art that for crosslinking the mixture of the thermoplastic material and e.g. any precursor (or block copolymer) can be used, provided that the two polymeric phases are crosslinked. These methods include e.g. chemical crosslinking with suitable reagents, e.g. Peroxides, and preferably treatment with ionizing radiation.

The following examples serve to illustrate the invention, examples 3 and 14 being comparative examples. Figures 2 through 14 show the change in resistance with temperature (R vs. T) for some of the formulations listed in the examples. The elastomers used in the examples all show a noticeable initial strength.

Unless otherwise noted, all samples for the following examples were prepared and tested in the following manner, the amounts being given in% by weight.

The polymeric components are mixed using an electrically heated two-roll mill at 200 ° C. for 5 minutes, then the carbon black is added and the mixture is mixed for a further 5 minutes.

These mixtures are pressed at 200 ° C into plates approximately 0.063 cm thick. Samples of 2.5 x 3.8 cm are cut out of these plates and two 0.63 cm wide strips are applied along the opposite edges on both sides of the cut-out plates with conductive paint.

The samples thus obtained are annealed to lower their resistance by heating them to 200 ° C. for 5 minutes several times and then allowing them to cool to room temperature. The annealing was repeated until a minimal resistance was reached. In general, a total tempering time of 15 minutes at 200 ° C. was sufficient. A more detailed description of annealing to minimize resistance is described in U.S. Patent No. 3823217 (Kampe).

The samples thus obtained are crosslinked by irradiation with a dose of 12 Mrad. To establish the resistance / temperature curves, the resistance of the samples was measured using an ohmmeter operating at low voltage (less than 1 volt), which prevented the samples from heating up themselves. The samples were heated in a forced air oven and the resistance measured at selected temperatures.

example 1

"TPR-2000" (graft copolymer from 70

Ethylene Propylene from Uniroyal Corp.)

"Vulcan XC-72" (carbon black from Cabot Corp.) 30

This polymer is believed to be grafted with a substantially crystalline polypropylene. From Fig. 2 it can be seen that this material shows a steady increase in resistance from room temperature to the melting temperature of polypropylene (165 ° C), after which the resistance remains almost constant or drops slightly to a higher temperature, where the resistance increases again begins to rise and only drops at a temperature above 220 ° C.

Example 2

"Kraton G 6521" (a block copolymer of 75 polystyrene and ethylene-propylene rubber of the ABA type from Shell Chemical Corp.)

"XC-72" (Russ) 25

As can be seen from Fig. 3, this practically amorphous polystyrene-ethylene / propylene rubber block copolymer shows a relatively strong increase in resistance, which begins immediately below the temperature Tg of polystyrene and continues to rise above the temperature Tg up to a maximum value at 2000 ° C. This state of affairs stands in stark contrast to the teachings of the prior art, such as those represented by J. Meyer in "Glass Transition Temperature as a Guide to Polymers Suitable for PTC Materials" (I.e.). In this publication it is found that practically amorphous materials, e.g. Polystyrene or ethylene / propylene rubber only show a PTC characteristic up to their glass transition temperature (Tg).

Example 3

"Vistalon 404" (ethylene / propylene rubber: 65 E / P ratio of 45:55, available from Exxon Chemical Corp.)

"XC-72" (Russ) ^ 35

It was found that an ethylene / propylene rubber with an ethylene to propylene ratio of 45:55 had no PTC characteristic. Specifically, the sample showed a relatively high resistance to room temperature, but it could not withstand repeated tempering. The material then showed a continuous and rapid decrease in resistance when heated. This property makes the material unsuitable for use as a self-regulating heating element (Fig. 4).

Example 4

"TPR-1900" (probably a 60th

EPR polypropylene graft copolymer)

"Profax 6523" (polypropylene) 20

"Vulcan XC-72" (Russ) 20

The EPR-polypropylene graft copolymer is mixed with polypropylene, after which it is ground with carbon black as in the general description. As shown in Fig. 5, this material shows a relatively uniform increase in resistance up to the melting point, after which there is a slight decrease in resistance. However, this material shows a pronounced positive temperature coefficient above the melting point.

Example 5

"Kynar 304" (polyvinylidene fluoride from Pennwalt Corporation)

"Cyanocril R" (polyacrylate from Amierican 50 cyanamide)

"Vulcan XC-72" 25

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The resistance of this formulation increases with increasing temperature up to a temperature significantly above the melting point of the polyvinylidene fluoride. This thermoplastic material is mixed with carbon black in the absence of the elastomer and then shows a maximum resistance at about 160 ° C and at high temperatures a pronounced decrease in the resistance curve.

Example 6

"TPR 1900" (probably an EPR polypropylene graft copolymer) "Vulcan XC-72" (Russ)

"Profax 6523" (polypropylene)

62.5

17.5 20

This formulation differs only slightly from Example 4. However, in contrast to the previous examples, where the general way of working was used,

In the present case, the carbon black was first mixed with the graft copolymer and then the polypropylene was added to the mixture.

In view of the prior art, it has been generally believed that a mixture of polymeric materials has an R vs. T-characteristic shows which polymer is in close contact with the carbon black, i.e. the polymer that was first mixed with the soot; all the polymers added later are not in such close contact with the conductive particles that they have a significant influence on the R vs. Exercise the T-curve. Accordingly, it could be assumed that the formulation of the present example would show a resistance / temperature curve corresponding to the graft copolymer due to the mixing order. However, as can be seen from the comparison of FIGS. 6 and 5, the polypropylene appears to have a significant influence on the resistance / temperature characteristics of the mixture.

Example 7

"CPE 3614" (chlorinated polyethylene with a 35% chlorine content of 36% Cl, available from Dow Chemical Corp.)

"Profax 6823" (polypropylene from Hercules 35 Corp.)

"Vulcan XC-72" (Russ) 30

30th

Melting point of the crystalline material until the end of the measurement at 260 ° C as shown in Fig. 8.

Example 9

"CPE 3614" (chlorinated polyethylene elastomer) 35 "Diamond PVC-35" (polyvinyl chloride, available 30 from Diamond Shamrock Chem. Co.) "XC-72"

(Soot)

Stabilizers 3

In this example, the elastomeric material is mixed with an amorphous thermoplastic polymer (PVC). As can be seen from Fig. 9, this formulation shows a PTC characteristic from room temperature to 220 ° C, especially in the range above the glass transition temperature of PVC (Tg at about 80 ° C). The decrease in resistance at temperatures above 200 ° C seen in this sample and certain other samples is likely due to thermal or oxidative degradation.

I.

Example 10

"Hypalon 45" (chlorosulfonated 35th

Polyethylene elastomer available from du Pont)

"Profax 6823" (polypropylene) 35

ì «XC-72» (Russ) 30

A chlorosulfonated polyethylene elastomer is mechanically mixed with a crystalline thermoplastic material (polypropylene). As can be seen from Figure 10, this mixture initially shows a decrease in resistance followed by an increase in resistance above the melting temperature of the polypropylene.

Example 11

35 "Neoprene TRT" (chloroprene, available from du 35 Pont)

"Profax 6823" (polypropylene) 35

"XC-72" (Russ) 30

40 An elastomeric neoprene is physically mixed with polypropylene. As can be seen from FIG. 11, this mixture shows a PTC characteristic above the melting temperature of the polypropylene.

A chlorinated polyethylene elastomer, which showed considerable initial strength, was blended with a rigid thermoplastic crystalline polypropylene. This mixture showed a continuously increasing resistance above the melting temperature of the polypropylene as can be seen from FIG. 7. Accordingly, if the elastomeric portion of the formulation itself is sufficiently structured to have a sufficiently high initial strength, mechanical mixing with the thermoplastic portion of the formulation, in contrast to graft or block copolymerization, can cause an increase in the resistance characteristic beyond the melting point of the thermoplastic component .

Example 8

"CPE 3614" (chlorinated polyethylene elastomer) 35 "Kynar451" (polyvinylidene fluoride, available 35 from Pennwalt Corp.)

"Vulcan XC-72" (Russ) 30

Similar to Example 7, the elastomer, which has significant initial strength, was mixed with an essentially hard crystalline thermoplastic material. The resulting formulation shows a continuous increase starting at room temperature over the

45 Example 12

"Valox 310" (poly (tetramethylene terephthalate), 42.5 available from General Electric Corp.)

"Hytrel 4055" (block copolymer of 42.5 poly (tetramethylene terephthalate) and thus polytetramethylene oxide, available from du Pont)

"XC-72" (Russ) 15.0

Poly (tetramethylene terephthalate) with a crystallinity of more than 50% was mixed with a block copolymer of crystalline thermoplastic and non-crystalline elastomeric polytetramethylene oxide fractions. As shown in FIG. 12, this material shows a maximum resistance at the melting temperature of the crystalline material, that is, at 180 ° C., and thereafter an increase in the resistance in the 60 amorphous range.

Example 13

"Hypalon 45" (chlorosulfonated polyethylene) 35

"Kynar 451" (polyvinylidene fluoride) 3 5

65 "XC-72" (Russ) 30

13 shows a mixture of the elastomeric "Hypalon 45" with the essentially solid

9

639 230

and crystalline thermoplastic poly vinylidene fluoride contains a 6003 »and carbon black alone. From this it can be seen that this increase in resistance up to the melting point of the polyvi mixtures which is not in accordance with the present nylidene fluoride, after which the resistance up to a temperature or the state of the art is markedly above the melting temperature of the Must formulate, the beneficial properties of the invention does not remain constant and then steadily increases. 5 show formulations according to the invention.

Example 14

"Silastic 437" (silicone rubber from Dow 60 Corning Corp.)

"Profax 6523" (polypropylene) 24

"Vulcan XC-72" (Russ) 16

Example 15 «Kynar 451»

"Viton B 50" (an elastomeric vinylidene fluoride-io copolymer available from du Pont)

"Vulcan XC-72"

30 30 40

This example describes a formulation obtained by blending a thermoplastic polymer with an elastomer that has no initial strength. As can be seen from FIG. 14, the resulting product shows no PTC characteristic above the melting temperature of the polypropylene if it was produced by mechanical mixing. A similar blend of 45.7 parts "Marlex 6003" (a 0.96 density polyethylene available from Phillips Petroleum Corp.) and 26.3 parts "Silastic 437" with 28 parts "SRF-NS" (a carbon black grade available from Cabot Corp.) showed a pronounced negative temperature coefficient of resistance above the melting point of the thermoplastic material similar to an irradiated formulation which «Marlex

The above formulation, consistent with the present invention, was irradiated at 12 and 24 Mrad. In both cases, the resistance began to rise rapidly below the melting point of the thermoplastic component and then rose continuously as the temperature increased.

20th

Examples 16-27

A high density polyethylene was mixed with various elastomers and carbon black as shown in Table I in accordance with the present invention and irradiated with a dose of 6 Mrad. The change in resistance with temperature is also shown in this table.

Table I

In the case of elastomeric resins, module of the uncrosslinked mixtures, the resistance behavior of the polymer at 20% (parts by weight) above the melting

Elongation point

No. (kp / cm2) (1) (2) (3)

16

"Texin 480"

21,090

58.2

5.8

36

PTC

17th

"Roylar E9"

21,090

58.2

5.8

36

PTC

18th

"Roylar Ed 65"

21,090

50

5

45

noticeably PTC

19th

"Royalen 502"

1,476

52.7

5.3

42

noticeably PTC

20th

"EI vax 250"

3,515

56.4

5.6

38

noticeably PTC

21st

"Neoprene WRT"

0.703

56.4

5.6

38

slightly PTC

22

"Neoprene WRT"

-

40

15

45

noticeably PTC

23

"Nysin 35-8"

1,547

40

15

45

noticeably PTC

24th

"Epsin 5508"

2,109

52.7

5.3

42

noticeably PTC

25th

"Epsin 5508"

-

40

23

37

PTC

26

"CPE 3614"

5,835

56.4

5.6

38

noticeably PTC

27th

"CPE 3614"

-

40

23

37

PTC

Footnote: Columns 1, 2 and 3 indicate the parts by weight of «Merlex 6002» (polyethylene), elastomer and carbon black (SRF / NS).

The designations given in ASTM D 1765-73a for the soot grades used in the examples “Vulcan XC-72” - N474 are: “SRF / NS” - N774.

5 sheets of drawings

Claims (15)

639 230 1. A process for producing an electrically conductive crosslinked polymer formulation, in which aj is a polymeric material which is elastomeric in the crosslinked state at room temperature, b) a thermoplastic polymer material whose glass transition temperature, if it is amorphous, is above room temperature, and c) conductive particles or ab) copolymer composed of polymer defined under a) above and polymer defined under b) above, which are chemically bonded to one another and c) conductive particles are used and the polymers or the copolymer ab) are crosslinked, characterized in that the polymeric material a) has a noticeable initial strength before the crosslinking, and in that the crosslinked polymer formulation at least in a temperature range above the transition temperatures T ( j of a) and b) has an increasing electrical resistance with increasing temperature, where Te is the crystallite melting temperature and, if the polymer is amorphous, the glass transition temperature. 2. The method according to claim 1, characterized in that the weight ratio of a) to b) in the range from 3:97 to 75:25, preferably in the range from 4:96 to 40:60. 2nd PATENT CLAIMS 3. The method according to claim 1 or 2, characterized in that a copolymer from), which can be a graft or block copolymer, is used, the component a) is an elastomer. 4. The method according to claim 3, characterized in that an ethylene-propylene / crystalline polyolefin graft polymer, preferably an ethylene-propylene / crystalline polypropylene graft copolymer, is used as the copolymer from). 5. The method according to claim 3, characterized in that an ethylene-propylene / polystyrene block copolymer or a polytetramethylene oxide / polytetra-methylene terephthalate block copolymer is used as the copolymer from). 6. The method according to claim 1 or 2, characterized in that mechanically mixing a material a) and a thermoplastic material b). 7. The method according to claim 6, characterized in that there is used as material a) chlorinated polyethylene, chlorosulfonated polyethylene or neoprene. 8. The method according to claim 6, characterized in that polypropylene, polyvinylidene fluoride or polyethylene is used as the thermoplastic material b). 9. The method according to claim 1 or 2, characterized in that one uses a copolymer ab) together with the material a) or the thermoplastic material b) or with both in the form of separate polymers. 10. The method according to claim 1, characterized in that a) ethylene-propylene rubber, b) polypropylene and c) carbon black, preferably in amounts of 40 to 90 wt .-% ethylene-propylene rubber, 5 to 40 wt. -% polypropylene and 5 to 35 wt .-% carbon black, based on the total weight of the formulation used. 11. The method according to claim 1, characterized in that carbon black is used as the conductive particles. 12. Formulation produced by the method according to claim 1. 13. Formulation according to claim 12, characterized in that it has a positive temperature coefficient of resistance in the range between room temperature and the melting point of the thermoplastic material. 14. Self-regulating heating element, characterized in that it has at least two electrodes, between which a molded body from the formulation according to claim 12 is arranged. 15. Uncrosslinked preparation for carrying out the method according to claim 1, characterized in that it is mixed with one another a) a polymeric material which is elastomeric in the crosslinked state at room temperature, b) a thermoplastic polymer whose glass transition temperature, if it is amorphous, is above Is room temperature, and c) contains carbon black, the polymer a) in the uncrosslinked state at room temperature having sufficient strength to bear its own weight without noticeable deformation for a long time.