NO312903B1 - Plastic material and conductive plastic article - Google Patents

Abstract

A plastic material to produce a conductive plastic object, especially to form a shielding profile (3) in situ on part (1) of a shielding housing, comprising a liquid or paste-like, cross-linkable or highly viscous plastic resin matrix (5) and gas-filled, compression-elastic, especially compressible micro hollow bodies (7) dispersed in said plastic resin matrix.

Description

The invention relates to a plastic material according to the introduction to claim 1, as well as a conductive plastic object produced from this material, in particular a sealing and shielding profile.

There are already known electrically conductive silicone-based sealing materials which are filled with conductive filler for the on-site production ("mold-in-place gaskets" = MIPG or "form-in-place gaskets" = FIPG) of apparatus housing seals with an electromagnetically deforming effect .

Until the beginning of the 1990s, they were particularly used for adhesive seals of individual parts in shielding apparatus housings or for sticking completed shielding seals during the assembly of apparatus housings, and with properties adapted accordingly. For similar products, reference is made to data sheet CS-723 "Conductive Caulking Systems" (1972) from the company Tecknit, USA, Technical Bulletin 46 "CHO-BOND 1038" (1987) from the company Comerics, USA, as well as DE-A-39 36 534.

From DE-A-39 34 845, a shielding seal in several parts is known which consists of an elastic carrier and a highly conductive cover layer, and an embodiment of apparatus housing parts with seals for assembly which also allows repeated opening of the housing after the first closure. With this structure, the inhomogeneity problems that arise due to the high specific weight of the metal parts that make the material conductive are to be counteracted. However, the production of the multi-component seal is cumbersome.

In the method for mass production described in EP-B-0 629 114, the conductive material is brought to a pasty state by means of pressure and with a needle or nozzle placed on a housing part where it sticks to the surface and becomes elastically stuck so that (without mould) a conductive, elastic shielding profile is formed. The profile design is determined through suitable choice of cross-sectional shape and size and the application speed with the needle or nozzle, as well as through adaptation of material properties such as viscosity, thixotropy and curing or cross-linking speed.

As a filler, particularly massive noble metal particles, for example silver, noble metal-coated particles with a non-noble core, such as e.g. Ag- or Pt-coated Cu or Ni particles, composites of base metals, such as e.g. Ni-coated Cu particles, or of precious metal coated glass or ceramic particles. The similarly known carbon-based fillers do not meet today's requirements for conductivity.

As a result of the ongoing mass use and the falling prices of electronic equipment that only functions safely with highly efficient shielding, the production of shielding apparatus housings will be exposed to great price pressure which makes it necessary to search for cost-effective and easily processed fillers that allow the production of sealing materials with properties that can be variably set as desired so that they provide reliable shielding profiles.

The task underlying the invention is thus to provide an improved plastic material and an electrically conductive plastic object.

The invention provides a plastic material for the production of a conductive plastic object, in particular for forming a shielding profile in situ on part of a shielding apparatus housing, with a liquid or pasty, crosslinkable or highly viscous plastic matrix, and where gas-filled microhole cells with a metallic conductive shell, characterized by the fact that the microhole cores are elastically compressible.

The invention also relates to a conductive plastic object of a plastic material according to one of claims 1-12, in particular in the form of a free-supporting shielding profile formed by applying such a plastic material to a surface on part of a shielding housing, characterized by the fact that the microholes are homogeneously distributed in the plastic matrix.

The invention is based on the basic idea that the starting material for a conductive plastic object is produced in the form of a plastic matrix with incorporated form-elastic microhole cells and which consequently has low density and high elasticity. The material in particular contains a conductive filler. In an alternative embodiment, the manufactured plastic object of thermoplastic or thermosetting plastic can be made conductive or have increased conductivity through an at least partially conductive coating.

In a preferred embodiment, the wall in the microhole cells is made of plastic and it is essentially gas-tight, which together gives the filling cells the desired high degree of form elasticity, particularly elastic compressibility. In an alternative embodiment, the hole bodies are essentially formed exclusively of metal which, together with gas filling during the production of the metal wall, also means that a certain elasticity is achieved.

In a preferred embodiment, at least a part of the metallic filler is present as a metallic shell or coating on the microhole cores, so that the conductivity of the entire material (matrix with filling) is achieved at least to a significant extent through contact between the conductive walls of the hole cores .

If the metallic filler is essentially present alone as a shell on microhole cells, then this mechanism requires a relatively high degree of filling with hole cells so that in a material cross-section there is sufficiently large contact between the walls to achieve a sufficient density of current paths. The degree of filling must therefore be made higher the smoother the surface of the conductive coating.

However, when the metallic coating has a rough surface, especially with a substantially radial, outwardly directed crystallite structure, one can advantageously get away with a moderately high degree of filling and also achieve a particularly stable relationship between matrix and filler cells.

When the metallic filler, admittedly in another further development of the idea behind the invention, is usually substantially present as metal powder dispersed in the matrix, the degree of filling with the microhole bodies should not be too high (and non-conductive, but consequently insulating areas are then formed).

The microhole bodies suitably have a diameter in the range from 5 µm to 100 µm, in particular between 15 µm and 50 µm.

In a technologically simple realizable variant, where an electrically and mechanically isotropic plastic mass is obtained, at least part of the microhole cores are in the form of hollow spheres. Special properties such as thixotropy and electrical and mechanical anisotropy can be achieved when at least part of the microhole cores have an approximately ellipsoidal, cylindrical or prismatic design (also a briquette-like design or flake shape), where the ellipsoid's largest main axis has a length of at least 1.5 times the second largest major axis, the cylinder height is at least 1.5 times the radius, or the height of the prism is at least 1.5 times the length of the largest side in the base. Especially when such a material is fed out through a needle or nozzle, but also when applied with a squeegee, the dynamic interface orientation effect can lead to an orientation of the elongated microhole bodies, and a subsequent attachment to the substrate preserves this orientation.

The micro-hole bodies can also be internally divided by intermediate walls, or made up of several separate cavities.

The metallic shell preferably comprises the entire surface of the microhole body and thus achieves particular gas density and consequently elastic compressibility with a high and practically constant return to original shape over a long period of time. The shell has an average thickness in the range between 0.1 µm and 5 µm and is adapted to the size of the microhole cores so that the effective density of the microhole cores is not significantly greater than that of the plastic matrix.

In the described manner, two major disadvantages of plastic masses filled with solid metal powder are very effectively remedied, namely great tendency to sedimentation and limited "permanent compression", and excellent storage stability and good mechanical workability are achieved.

A corresponding plastic object, in particular a shielding profile which is formed free-standing by applying a sealing material of this type to a surface on part of a shielding apparatus housing, consequently has an almost ideal distribution of the microholes in the plastic matrix and a uniform, high elasticity and dimensional stability.

The surface of the metallic shell of the microhole cores is preferably rough or porous, or it is a strongly structured coating with crystallite-like, far-reaching radially outward extensions that provide a good connection with a matrix, and which, through a regular interlocking of neighboring hole cores in the matrix, ensures a high volume conductivity also at relatively low filling levels.

Particularly high savings in material costs are possible with an embodiment where the metallic shell consists of at least two layers, where only the outermost layer is a noble metal layer.

The conductive shell, which has the aforementioned preferred characteristics, on the microhole cores is obtained with an electrodeless ("electroless") or galvanic method known per se, or with a vacuum design, especially with vaporization or spraying.

In order to form relatively soft shielding profiles that have a certain plasticity, a silicone matrix is preferably used with a proportion of non- or weakly cross-linked siloxane of more than 3% by weight.

Incorporation of such a non-crosslinking, long-chain siloxane gives, after curing of the crosslinkable silicone component (through humidity, heat or irradiation), a large-meshed, crosslinked structure in the matrix with a certain plasticity, where the degree of crosslinking can be determined through the mixing ratio. In order to form highly plastic seals for special applications, which, however, due to the incorporated form-elastic hollow spheres do have sufficient elasticity, the proportion of non-crosslinkable components can be increased to many times the proportion of crosslinkable components.

An optional additional addition of an organic solvent serves to optimize the material's processing properties and can also positively affect the usage properties of the finished profile. The addition does indeed cause a "swelling" of the matrix material, but makes it particularly easier to mix the components, and improves cross-linking. Good results are achieved with a proportion of between 5 and 20% by weight of petrol and/or toluene.

A corresponding shielding profile can easily have a degree of deformation of over 30%, in particular over 50%, based on the height of an unloaded U-shaped sealing profile of solid material.

However, the matrix can also be formed from a two-component epoxy resin or polyurethane mixture, or from special cross-linking or thermoplastic plastic materials.

Through the aforementioned material-related and possibly additional construction-related measures, in particular specification of a suitable flexurally elastic or compressible cross-sectional shape of the profile, it is also possible to achieve satisfactory sealing of apparatus housings with longitudinal joints with considerable variation in width. This allows, for example, cost-effective greater tolerances when manufacturing apparatus housings.

Within the scope of the invention, embodiments shall be indicated in the light of the attached drawings: figure l schematically shows a cross-section through a shielding profile according to a first embodiment,

Figure 2 schematically shows a cross-section through a shielding profile according to a further embodiment, and

figure 3 schematically shows a cross-section through a shielding profile according to a further embodiment.

A first embodiment of the invention is an electrically conductive material for the matrix, indicated in the following table as mixture 1, which is a thermosetting one-component system. This material, which after curing is elastic and relatively soft, is suitable for the production of shielding profiles for the edge of equipment housings, such as resealable EMI housings, with medium design tolerances.

As can be seen from Figure 1, this material is filled in the matrix 5 with a volume ratio of 1:1, and it contains PVDC (polyvinylidene chloride) hollow spheres 7 with a mean diameter of approx. 20 µm and with a strongly crystalline structured Ag coating 7a with an average thickness of approx. 1 µm applied to part of the apparatus housing 1 in the form of a shielding seal 3.

A second design example includes mixture 2 which is an electrically conductive plastic material where the matrix material consists of a two-component system that hardens at room temperature. The shielding profile formed from this material has a high degree of deformation, shows clear plasticity and is particularly suitable for sealing gaps in EMI shielding housings with significant design tolerances.

As Figure 2 shows, this matrix material 15 is for a shielding seal 13 on part of an apparatus housing 11, filled with approximately parallelepiped-shaped ACN (acrylonitrile) hole cells 17 with a coating of a Ni layer 17a and an overlying porous Ag layer 17b with a total thickness of approx. 2 um in a ratio of approx. 1 part by volume matrix to 2 parts by volume fillers, and the whole mixture has anisotropic mechanical and electrical properties due to the anisotropic design of the fillers.

In a third embodiment, mixture 3 is used, which is a thermosetting one-component system corresponding to that indicated in the first example, where the matrix contains the conductive component in the form of powder of crystalline, stem-like or flake-like particles with a relatively large surface area, or as a mixture of particles with this shape in the matrix.

As Figure 3 shows, this matrix material 25 with Ag particles 25a, for a shielding seal 23 on part of an apparatus housing 21, is filled with spherical and ellipsoidal thermoplastic hollow cores 27 in a ratio of approx. 2 parts matrix by volume to 1 part filler, to lower the average density of the profile and improve the elasticity properties, especially the so-called permanent compression. With this degree of filling, a sufficiently dense distribution of the silver particles 25a is achieved in the entire volume, and thereby (in conjunction with the branched shape of the particles) the formation of continuous current paths is achieved. The special shape of the metal particles shown binds them as well as the hole cores, which are anchored securely with the help of the metal particles, in the matrix.

In a fourth embodiment, the hole cores consist exclusively of a Ni or Ni/Ag metal shell with a wall thickness of 4 to 5 µm which is formed by applying a metal layer to plastic particles of suitable diameter and then removing the plastic core by pyrolysis.

The formation of a shielding profile of the plastic material takes place in a manner known per se, such as e.g. basically stated in EP-B-0 629 114, and depending on the setting of the physico-chemical properties, it is possible to carry out the application to the apparatus housing parts to be shielded, by spraying, showering or pressing.

Depending on the requirements for a specific embodiment, it is particularly possible with differentiated forms of mixing between introducing the metal into the material exclusively as a coating on microhole cells, or introducing it exclusively as powder, in addition to non-conductive microhole cells. As a metal filling, as a result of the reduced amount of use, it is possible to use, in addition to silver, also gold, platinum or platinum alloys.

Further optimization is possible by a combination with a conductive, particularly pure metallic surface coating on the manufactured plastic object.

As matrix material, in addition to silicone mixtures, polyurethanes, epoxy resins, MDK or other proven plastic materials can be used, which can be provided in a specific way and processed as one or multi-component mixtures.

The conductive plastic object of the material according to the invention does not need to be manufactured only as "form-in-place" or "mold-in-place" shielding, but also as pre-manufactured seals and for other applications in the field of electrical engineering and electronics.

Claims (16)

1. Plastic material for the production of a conductive plastic object, in particular for forming a shielding profile (3,13,23) in situ on part of a shielding apparatus housing (1,11,21), with a liquid or pasty, crosslinkable or highly viscous plastic matrix (5,15,25), and where gas-filled microhole cells with a metallic conductive shell are dispersed in the plastic matrix, characterized in that the microhole bodies are elastically compressible. 2. Plastic material according to claim 1, characterized in that the wall of the microhole body (7,17,27) consists of a plastic material and is essentially gas straight. 3. Plastic material according to claim 1 or 2, characterized in that the microhole bodies (7,17,27) have an average density that is not significantly greater than the density in the plastic matrix (5,15,25). 4. Plastic material according to one of the preceding claims, characterized in that the microhole bodies (7,17,27) have a diameter in the range between 5 µm and 100 µm, in particular between 15 µm and 50 µm. 5. Plastic material according to one of the preceding claims, characterized in that at least part of the microhole bodies (7,27) are in the form of hollow spheres. 6. Plastic material according to one of the preceding claims, characterized in that at least a subset of the microhole bodies (17,27) are shape anisotropic and in particular have an approximately ellipsoidal, cylindrical or prismatic design, where the largest major axis of the ellipsoid has a length that is at least 1.5 times the length of the second largest major axis, the cylinder height is at least 1.5 times the radius, or the height of the prism is at least 1.5 times the length of the longest side in the base surface, and that the plastic material has anisotropic mechanical and/or electrical or thixotropic properties. 7. Plastic material according to claims 4-6, characterized in that the metallic shell (7a, 17a, 17b) comprises essentially the entire surface of the microhole body (7,17). 8. Plastic material according to claims 4-7, characterized in that the metallic shell on the microhole body consists of at least two layers (7a, 17b). 9. Plastic material according to claims 4-8, characterized in that the microhole body's metallic shell (7a, 17b) has a rough surface, and in particular with a mainly radially outwardly oriented crystallite structure. 10. Plastic material according to claims 4-9, characterized in that the microhole body's metallic shell (7a, 17a, 17b) has an average thickness in the range from 0.1 µm to 5 µm, where the average thickness is matched in relation to the dimensions of the microhole body so that the coated microhole body has an effective density that does not is significantly greater than the density of the plastic matrix. 11. Plastic material according to one of the preceding claims, characterized in that the plastic matrix (5, 15, 25) is silicone-based with a proportion of non- or weakly cross-linking siloxane of more than 3% by weight. 12. Plastic material according to one of the preceding claims, characterized in that the starting material for the plastic matrix contains more than 3% by weight of an organic solvent. 13. Conductive plastic object of a plastic material according to one of claims 1-12, in particular in the form of a free-standing shielding profile (3,13,23) formed by applying such a plastic material to a surface (1,11,21) on a part of a shielding housing, characterized in that the microhole cores (7,17,27) are homogeneously distributed in the plastic matrix (5,15,25). 14. Plastic material according to claim 13, characterized in that it has a reversibly achievable degree of deformation of over 30%, in particular over 50%, based on the height of an unloaded U-shaped sealing profile of homogeneous material. 15. Plastic object according to claim 13 or 14, characterized in that it has an at least partially conductive coating, particularly in connection with a non-conductive filling. 16. Plastic object according to claims 13-15, characterized in that it has anisotropic mechanical and/or electrical properties as a result of filling with oriented, aligned form anisotropic microhole cells.