EP0191026A1

EP0191026A1 - New polymer compositions, processes for the production thereof and applications thereof

Info

Publication number: EP0191026A1
Application number: EP85903193A
Authority: EP
Inventors: Jean-Paul Issi; Gordon Derek Allen
Original assignee: Universite Catholique de Louvain UCL
Current assignee: Universite Catholique de Louvain UCL
Priority date: 1984-07-11
Filing date: 1985-07-08
Publication date: 1986-08-20
Also published as: LU85462A1; WO1986000629A1

Abstract

Compositions de matière constituées d'un matériau polymère de base et de charges spécifiques orientées conférant au matériau dans une direction prédéterminée par ladite orientation des propriétés de conductivité de l'ordre d'au moins un facteur 10 supérieur à celle du matériau polymère de base. Avantageusement, la composition présente dans une direction prédéterminée des valeurs de conductivité thermique de l'ordre de 500 Wm-1K-1 et de préférence de l'ordre de 1000 Wm-1K-1 dans une plage de température d'approximativement 50oK à la température de décomposition du polymère de base, éventuellement combinées avec une bonne conductivité électrique. Application à la réalisation d'échangeurs ou de dissipateurs de chaleur travaillant en milieux corrosifs et application dans le secteur électronique ou électrique ou le secteur automobile, aéronautique ou spatiale ou dans les secteurs où une bonne conductivité doit être associée à un poids réduit.Compositions of matter consisting of a base polymer material and specific oriented fillers giving the material in a direction predetermined by said orientation conductivity properties of the order of at least a factor of 10 greater than that of the base polymer material. Advantageously, the composition presents in a predetermined direction thermal conductivity values of the order of 500 Wm-1K-1 and preferably of the order of 1000 Wm-1K-1 in a temperature range of approximately 50oK to decomposition temperature of the base polymer, possibly combined with good electrical conductivity. Application to the production of heat exchangers or heat sinks working in corrosive environments and application in the electronic or electrical sector or the automotive, aeronautics or space sector or in sectors where good conductivity must be associated with reduced weight.

Description

NOVEL POLYMER COMPOSITIONS, PROCESSES FOR THEIR PRODUCTION AND THEIR APPLICATIONS. Metals that are good conductors of heat have been widely used in applications requiring heat transfer, mainly in heat exchangers or heat sinks.

However, in particular applications of heat exchangers intended to be brought into contact with corrosive gases or liquids, we have been led to recommend the use of resins which are essentially fluoropolymers (PTFE or FEP) despite the fact that their thermal conductivity is very significantly lower than that of metals. Expressed in the units Wm ^-1 k ^-1 , the thermal conductivity of pure copper is of an order of magnitude of 500 and the equivalent of 250 if a layer of oxides is formed in contact with water by example; around 20 for titanium and 15 for stainless steel, while the corresponding values are 0.3 for PTFE. Despite unattractive characteristics, fluorinated resins have undergone significant development and various techniques for their implementation have been developed for their applications, and above all to compensate for their lack of intrinsic thermal conductivity. These techniques essentially involve increasing the exchange surface and it is understandable that they are expensive and that they involve a considerable bulk of the industrial installations in which they are brought to intervene. There is therefore a currently unmet need for a material which is both good conductor of heat, easy to use, in particular which is easily formable with a weight less than the usual metals and which moreover avoids a series disadvantages linked to the use of metals, including in particular the high sensitivity to the action of corrosive agents, such as gases or acid or oxidizing liquids. The object of the present invention is to provide a composition. news to resolve these difficulties. The invention also extends to the processes which can be used for the production of such compositions and to the numerous applications, in particular industrial ones, which are opened thanks to the composition of the invention.

According to a particular form of application, the invention also aims to provide a new composition having anisotropy characteristics of the thermal conductivity, if necessary associated with an anisotropy of the electrical conductivity.

The various objectives thus targeted by the invention can be achieved thanks to a composition of new material - consisting of a polymer material forming a matrix and of oriented organic fibers conferring on the material in a predetermined direction by said orientation of the conductivity properties of the material. order of at least a factor 10 greater than that of the polymer material constituting the matrix.

Advantageously, the composition has, in a predetermined direction, thermal conductivity values of the order of 500 Wm ^-1 k ^-1 and preferably of the order of 1000 Wm ^-1 K ^-1 in a temperature range of approximately 50ºK at the degradation temperature of the base polymer.

This composition can also preferably have an electrical conductivity greater than 5.10 ⁵ ohm ^-1 cm ^-1 in the same temperature range, which is particularly advantageous for applications where a high electrical conductivity must be associated with a high thermal conductivity for quickly dissipate or transfer calories.

These values, which are clearly different from those known for resins and which compare very favorably with the aforementioned values for different metals, offer a wide field of applications to materials. rials of this type.

According to the invention, thermal energy is essentially transmitted along the axes of organic fibers maintained in a strongly oriented manner and parallel to the desired direction of energy transfer.

As a result of its polymer matrix, the material of the invention which can be classified as composite materials, can be transformed and in particular be shaped by most manufacturing techniques, such as those used for plastics, rubbers and the metal processing industry, as long as any significant breakage of organic fibers is avoided.

Said fibers can be anchored in a base which can be a woven or a nonwoven of plastic materials, in a parallel arrangement with one another and substantially perpendicular to said base.

The base can be produced by conventional techniques known for textiles, namely weaving, knitting or the production of a nonwoven using the usual fibers or fibrils such as for example cotton, polyamides etc. It is also possible to use fibers that are little or rarely used for textile production, such as glass, carbon, polyaramides and polyethylene or polyethylene terephthalate fibers which more often serve as reinforcing fibers in composite materials. These fibers of different or identical nature to those intended for thermal transmission can be added to the base (with an orientation along the axis of the thermal transmission fibers or different from that, in particular in the plane of the base) in order to improve physical properties. The base can also be made of a sheet of paper or plastic.

In the case where the base consists of a woven material, the interstices of the textile structure can be filled with a coherent material which solidifies and / or which exists in the solid state at least at the processing temperature of the material by forming a continuous phase which includes at least the base and at least a portion of the fibers.

Advantageously, in order to maintain the state of orientation of the organic fibers oriented perpendicular to the base and to prevent their deterioration, in particular mechanical, said fibers are impregnated and completely enclosed in a continuous mass of plastic material of the matrix, which solidifies or which exists in the solid state at least at the processing temperature of the material, forming a solid phase which includes the assembly constituted by the base and the fibers intended for thermal transmission.

This technique is particularly recommendable because certain types of carbon fibers, in particular those of the VDF (vacuum deposited fibers) type, are sensitive to the effect of ions contained in media with which they are in contact (CaCl ₂ , copper salts) and risk to lose their thermal conductivity as a result of intercalation phenomena. The fibers included in the matrices are thus better protected.

In this regard, it may also be useful to provide protection for the ends of the carbon fibers in order to protect them, whether or not they are included in the matrix.

The deposition of a nickel film produced electrochemically is particularly recommendable in this regard.

One conceivable technique for producing such materials comprises the steps consisting in implanting on a basis in particular, according to a technique related to needling, flocking or tufting, of organic fibers oriented in the desired direction of heat transfer and in the coating. of these without substantial modification of said orientation. Said coating can be carried out by immersion in a liquid binder resin drawn in a narrow die and transformed by vulcanization or thermal or catalytic hardening into a solid structure. In another way, it is possible to carry out a lamination under the effect of heat, and under pressure, of the textile structure into a thermoplastic film or sheet which may possibly subsequently undergo crosslinking by radiation or by another technique. It is also possible to arrange a bundle or cable of fibers in a plane, and to bind this bundle by an elastomer which is then subjected to vulcanization.

The excess binding agent can be removed if necessary from both sides of the material obtained by any physical and / or chemical means, thereby exposing the two ends of the organic fibers ensuring thermal conductivity. In this case, it may be particularly advisable to protect these ends from organic fibers. The use of an electrochemical deposit of nickel has been found to be particularly effective in this regard.

Said thermally conductive organic fibers are preferably carbon fibers and are advantageously produced by pyrolysis of suitable organic compounds such as benzene or methane and the deposition of carbon generally from the gas phase in the form of whiskers on a cooler surface. , generally consisting of a metal, the growth being initiated by a nucleating agent. These fibers have mechanical properties similar to those of conventional carbon fibers derived from pitch and polyacrylonitrile but are further characterized by coefficients of thermal conductivity which are generally 2 to 20 times greater.

However, it is also possible to use conventional carbon fibers derived from pitch and polyacrylonitrile or other origins as long as the desired properties are achieved.

The compounds of intercalation with electron donors / acceptors in carbon fibers of the aforementioned type are particularly advantageous when it is desired to obtain electrical conductivity properties in addition to thermal conductivity properties.

Because the thermal conductivity and possibly electrical conductivity properties are due mainly to organic fibers, while the matrix of polymeric material is essentially non-conductive or not very conductive, care is taken to charge the matrix with at least an amount of organic fibers sufficient for the intended use. Advantageously, the fibrous component represents at least 50% by volume and preferably approximately 70% by volume or more of said composite material, the balance being constituted by a polymeric material forming a continuous phase possibly including fillers or inorganic materials involved in particular in the impregnation material of the textile structure used or in the mechanical reinforcement of the base used or constituting this base itself.

Advantageously, the non-fibrous material used either to form the base for implanting the organic fibers or to constitute the so-called impregnation material consists of preferably thermosetting materials such as unsaturated polymers (epoxy resins, polyesters, etc.). Thermoplastic resins are generally less advantageous, considering that composite materials are generally intended to be used at relatively high temperature. However, they offer better possibilities for thermoforming. The invention also relates to composite materials in which both the fibrous material and the continuous phase or phases consist of identical chemical materials, especially resins.

As has been said previously, the composite material resulting from the new composition of the invention offers wide possibilities of implementation. In particular, it becomes possible to form plates or tubes of heat exchangers which combine, as no material known until now has permitted, excellent thermal conductivity characteristics (and where applicable electrical conductivity) with a excellent resistance to aggressive agents. The field of application of these materials is therefore naturally found among heat exchangers or heat sinks exposed to aggressive media or in fields of application where good conductivity must be associated with reduced weight. In the case of a combination of thermal conductivity property associated with good electrical conductivity and taking into account simple techniques of implementation, in particular of forming, the material of the invention can find a wide field of use in installations. electronic and / or electrical, or in the automotive, aeronautical or aerospace sector.

Although particular embodiments of the invention have been described, it should be understood that many embodiments other than those described are possible and that these fall under the invention.

The invention will be described with reference to the attached examples made on a laboratory scale and which are intended to illustrate it.

The examples relate to tests carried out using different types of carbon fibers and two systems. Examples 1 and 2

The choice of the matrix used was essentially dictated by the search for great fluidity in order to to allow a good impregnation of the fibers, an easy evacuation of air bubbles and, in general, a relatively simple preparation.

A commercial resin "POLYSET ^® " was used, which is a styrene-based resin used with a hardener.

PITCH type fibers supplied by Union Carbide Corporation, Parma Technical Center, were used. The fibers are of the THORNEL ^® P-55, P-75, P-100 and P-120 type and their essential characteristics are listed in Table 1.

The preparation of the composite was carried out in the laboratory according to the following stages:

- preparation of a bundle of fibers kept parallel by means of a flexible link,

- preparation of the resin mixed with its hardener,

- immersion of the fiber bundle in the resin so as to carefully impregnate all of the fibers,

- spin the beam in order to remove as much excess resin as possible,

- exposure of the beam to ultra violet rays to initiate polymerization,

- mechanical shaping of the sample to obtain the appropriate geometric shape (here a cylinder), - elimination of the edges of the sample where the distribution of the fibers is less homogeneous,

- the samples obtained have approximate dimensions: diameter 1 cm, length 4 or 8 cm.

The measurement of the thermal conductivity of the samples was carried out using the quasi-axial thermal flux method in the stationary state. The sample is thermally anchored to a variable temperature thermal reservoir at one of its ends, the other end being in contact with a heating resistor which serves to generate the thermal power P. A differential thermocouple measures the temperature drop ΔT over a length L along the sample (Fig. 1). The thermal conductivity κ is given by

where S is the cross section of the sample. The measurement is carried out in an enclosure evacuated at 10 ^-5 Torr to avoid thermal losses by conduction and gas convection.

In order to ensure that all the thermal power P passes through the sample and therefore to minimize the thermal losses which occur mainly by radiation, a first measurement was made on a standard sample available in the laboratory, machined to close dimensions samples to be measured. This allows to calibrate the system-

In the series of tests, the heat flux was always parallel to the axis of the fibers contained in the composite, these fibers themselves being parallel to the length of the sample.

In these measurements, it is necessary to eliminate or reduce to a negligible value the thermal contact resistance to measure κ (contact resistance between the sample and the thermal reservoir and with heating resistance).

This was obtained by electrochemically depositing a layer of nickel on the two faces on which the contact welds are to be made. This layer with a typical thickness of 10 ^-4 m adheres very well to the composite, makes it possible to weld easily and does not pose any problem of differential contraction when the sample is cycle in temperature.

During an industrial transposition involving the use of the composite as a heat exchanger, it will also be essential to ensure good thermal contact between it and the medium from which it is desired to dissipate the calories. The use of an electrolytic deposit of nickel will make it possible to satisfy this constraint.

The electrochemical nickel plating method and the appropriate bath have been adapted to the case of our composites according to a scheme developed in the laboratory for this purpose.

The deposition was carried out using an electrolytic bath of the following composition (% by weight)

- SO ⁴ Ni, ₇ H ² O 100 to 150 g

- S0 ⁴ NI, SO ⁴ (NH ⁴ ) ² , 6H ² O - CINa 10 g

- BO ³ H ³ 15 g

- SO ⁴ Na ² 0 to 100 g

- SO ⁴ Mg, ₇ H ² O

- Water 1 liter The electrolysis conditions are as follows: temperature: around 20ºC current density: <1A / dm ²

The samples are washed and dried. Examples 3 to 9 The conditions of Examples 1 and 2 were repeated.

A commercial resin RELOPAL-GK ^{R was used} which is a resin based on polyesters and a hardener. The preparation of the composite was carried out in the laboratory according to the following steps; with different types of fibers mentioned above.

- preparation of a polyethylene mold with the desired dimensions (v 8 x 1 x 1 cm ³ or 4 x 1 x 1 cm ³ ), - cutting of fiber bundles to a given length (4 or 8 cm),

- weighing of the fibers so as to have a first estimate of their future concentration in the composite,

- preparation of the resin mixed with its hardener, - placement in the mold of alternate layers resin fiber,

- pressurizing the assembly during hardening by means of a comb designed for this purpose. - placing the mold and its charge in an oven for 24 hours at a temperature of 70 ° C,

- mold release,

- mechanical shaping of the pure sample to obtain the appropriate geometric shape (here a parallelepiled),

- elimination of the edges of the sample where the distribution of the fibers is less homogeneous,

- the samples obtained have approximate dimensions: 4 x 1 x 1 cm ³ or 8 x 1 x 1 cm ³ .

The values measured in the various examples 1 to 9 are given in the attached Table 2.

Claims

CLAIMS.

1. Composition of matter characterized in that it consists of a polymer material forming a matrix and of oriented fibers imparting to the material in a direction predetermined by said orientation of the conductivity properties of the order of at least a factor of 10 higher than that of polymer material constituting the matrix.

2. Composition according to claim 1 characterized in that it has, in a predetermined direction, thermal conductivity values of the order of 500 Wm ^-1 κ ^-1 and preferably of the order of 1000 Wm ^-1 κ ^{- 1} in a temperature range of approximately 50 ° K at the decomposition temperature of the base polymer.

3. Composition according to claims 1 or 2 characterized in that it has an electrical conductivity greater than 5.10 ⁵ ohm ^-1 cm ^-1 .

4. Composition according to any one of claims 1 to 3 characterized in that the thermal energy is essentially transmitted along the axes of organic fibers maintained in a strongly oriented manner and parallel to the desired direction of the energy transfer.

5. Composition according to claim 4 characterized in that said fibers are anchored in a base constituted by a woven or a nonwoven of plastic material, in parallel arrangement therebetween and substantially perpendicular to said base.

6. Composition according to claim 5 characterized in that the interstices of the basic textile structure are filled with a coherent material which solidifies and / or which exists in the solid state at least at the processing temperature of the material in forming a continuous phase which includes at least the base and at least a portion of the fibers.

7. Composition according to claim 5 or 6 ca characterized in that said fibers are impregnated and completely enclosed in a continuous mass of plastic material of the matrix, which solidifies and / or which exists in the solid state at least at the processing temperature of the material, forming a phase solid which includes the base and fibers for thermal transmission.

8. Composition according to any one of claims 4 to 7 characterized in that said thermally conductive organic fibers are produced by pyrolysis of suitable organic compounds such as benzene or methane and the deposition of carbon from the gas phase in the form of whiskeys on a cold surface, generally consisting of a metal, the growth being initiated by a nucleating agent.

9. Composition according to claim 8 characterized in that said fibers have mechanical properties similar to those of conventional carbon fibers derived from pitch or polyacrylonitrile but are further characterized by coefficients of thermal conductivity which are generally 2 to 20 times higher .

10. Composition according to claims 8 or 9 characterized in that intercalation compounds with electron donors / acceptors in the carbon fibers provide, in addition to thermal conductivity properties, electrical conductivity properties.

11. Composition according to any one of claims 4 to 10 characterized in that the non-fibrous material used either to form the base for implantation of the organic fibers or to constitute the so-called impregnation material consists of preferably thermosetting materials such as unsaturated polymers or by thermoplastic resins.

12. Composition according to any one of claims 4 to 10 characterized in that the material as well as the continuous phase or phases consist of identical chemicals.

13. Composition according to any one of claims 4 to 12 characterized in that the fibrous constituent represents at least 50% by volume, preferably about 70% by volume or more of said composition, the balance being constituted by a polymeric material forming a continuous phase possibly including fillers or inorganic materials.

14. A method for the production of a composition according to any one of claims 1 to 13 characterized in that it comprises the steps consisting in implantation on a base of organic fibers oriented in the desired direction of heat transfer and in the coating thereof without substantial modification of said orientation.

15. The method of claim 14 characterized in that said coating is carried out by immersion in a liquid binder resin stretched in a narrow die and transformed by vulcanization or thermal or catalytic hardening into a solid structure.

16. The method of claim 14 characterized in that one proceeds to a lamination under the effect of heat and under pressure of the textile structure in a thermoplastic film or sheet which may possibly subsequently undergo crosslinking by radiation or by another technique .

17. Method according to any one of claims 14 to 16 characterized in that the excess binding agent is removed if necessary from the two faces of the material obtained by any physical and / or chemical means thereby exposing the two ends of the organic fibers ensuring the thermal conductivity, these then being preferably protected by a protective deposit, advantageously of a metal such as nickel electrodeposited.

18. Application of the compositions according to any one of claims 1 to 13 to the production of plates or tubes or other elements of heat exchangers. heat exposed to aggressive media.

19. Application of the compositions according to any one of claims 1 to 13 in electronic and / or electrical installations or in the automotive, aeronautical or aerospace sector.