EP0520058A1

EP0520058A1 - Matrix-free composite material

Info

Publication number: EP0520058A1
Application number: EP92902391A
Authority: EP
Inventors: Uwe Koser; Kuno Stellbrink
Original assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV; Deutsche Forschungs und Versuchsanstalt fuer Luft und Raumfahrt eV DFVLR
Current assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Priority date: 1991-01-10
Filing date: 1992-01-10
Publication date: 1992-12-30
Also published as: WO1992012005A1

Abstract

Il est proposé un nouveau matériau composite sans matrice comportant au moins une couche de fibres synthétiques fortement orientées, lesquelles sont disposées en plusieurs plis et en position angulaire prédéterminée les unes par rapport aux autres et liées entre elles par frittage sous l'effet de la pression et de la chaleur.A new composite material without matrix is proposed comprising at least one layer of highly oriented synthetic fibers, which are arranged in several plies and in a predetermined angular position with respect to each other and bonded together by sintering under the effect of pressure. and heat.

Description

DESCRIPTION

Matrixless composite

The invention relates to a novel matrixless composite material.

Fiber-plastic composites have been used for some time in the manufacture of specifically light and strong components and can be produced using a wide range of processing methods such as laminating, spraying or winding. The previously known composite materials consist, on the one hand, of a matrix made of thermoplastic or thermosetting plastic, which is reinforced by embedded fibers, fiber bundles, mats, fabrics, nonwovens, etc. The matrix is mainly responsible for the shape retention, the embedded fibers mainly for the improvement of the mechanical properties. In any case, the fiber-plastic composites form a so-called composite of at least two chemically or physically different materials. On the other hand, so-called "self-reinforcing" polymers, such as thermoplastic, liquid-crystalline plastics (TLCP), are known, within which one-dimensionally oriented, rod-shaped macromolecules assume a reinforcing function. Reinforcement and form-keeping functions are common tasks of a single component.

The desire of the engineer to be able to adapt the material properties to the technical requirements of his construction is met by targeted generation of anisotropy of the material. In the case of multi-component, conventional fiber-plastic composites, this is done using a spectrum of processing methods, e.g. Laminating, soaking, injection molding, fiber spraying, winding etc.

If a one-component material is to be imprinted with anisotropic properties, e.g. for thermoplastics processes such as injection molding, extrusion or fiber spinning, in which the orientation of the macromolecules of the plastic, and thus the generation of anisotropy, are generated from the melt by shear or stretching currents during the processing operation.

The former methods require the combination of several, task-specific materials, the latter contain the disadvantage that either only certain geometries can be produced, as in extrusion, or, as in injection molding, the flows producing the anisotropy are a function of the dimensions of the injection molded part, the orientation of the macromolecules is therefore not freely determinable.

In injection molding and extrusion of liquid crystalline polymers, alignment of the macromolecules is also only necessary achieved on the surfaces of the macroscopic bodies. Therefore, the specific properties of extruded films decrease with increasing layer thickness, and in the case of injection molded components, the macromolecules can only be oriented in the surface area of the plastic part. In the case of complex components, the direction of orientation and the degree of orientation depend, in particular, on the flow processes during the injection process, so that this effect can only be used to a limited extent for a targeted adaptation of the component to the loads subsequently acting on the component.

The object of the invention is to propose a material with which components or molded parts with precisely specifiable mechanical properties can be produced from a single material.

This object is achieved by a matrix-free composite material with at least one layer of highly oriented plastic fibers, which are arranged in multiple layers and in a predetermined angular position to one another and are connected to one another by sintering using pressure and temperature.

The novel material to be characterized here forms the middle between fiber-plastic composites (FRP), reinforcing fibers and unreinforced plastic matrices, without being a fiber-plastic composite in the usual sense. The term "matrixless composite material" was therefore chosen for him.

Surprisingly, it has been found that highly oriented plastic fibers can be connected to one another by using pressure and temperature without a mediating plastic matrix such that, on the one hand, their specific properties Shafts, in particular their anisotropy, for example with regard to strength, are retained and, on the other hand, a composite material is obtained which can be processed into a large number of molded parts while maintaining the anisotropic properties.

A basic prerequisite for the success of the proposed method is the following properties of the material to be processed: First of all, a high pre-orientation of the macromolecules, and thus a high anisotropy of the mechanical and other physical properties, must be present in the starting material. Here are highly oriented fibers. Furthermore, the material must be combined into one component without further additives, but the anisotropy of the properties must not decrease significantly during this time. This condition is ideally met, for example, by liquid-crystalline, thermoplastic polymers in fiber form, the TLCP fibers. The thermoplasticity enables the macromolecules to be connected to one another, while the orientation of the macromolecules is largely retained due to the special molecular structure.

The directional dependence of the mechanical properties of highly oriented, liquid-crystalline polymers in fiber form is analogous to that of conventional fiber-plastic composites, resulting from the molecular structure of the material. The oriented stiff-chain macromolecules correspond to the reinforcing fibers, the main bond strengths their tensile strength and the secondary bond strengths to the strength of the previous matrix and the boundary layer. It is thus possible to find fields of use for components made of matrix-free composite materials similar to those for conventional high-performance composites. Calculation methods which are already known for fiber-plastic composites can likewise be adopted analogously. In the case of a composite material in which the fibers are arranged essentially exclusively in parallel, the strength values of the preferred direction are obtained in the longitudinal direction of the fiber and strength properties in the transverse direction, as would roughly correspond to a normal unreinforced synthetic resin.

Since the anisotropy of the strength and rigidity properties and in particular their direction is fixed in the new matrix-free composite material and since the direction of the composite material can also be recognized in the form of a grain due to the use of fibers as the starting material, it is possible to Manufacture components, including molded components, with very specific mechanical properties.

The new matrix-free composite material is also a single-component, single-grade material that can be easily recycled.

High-strength Trevira fibers, for example, are suitable as highly oriented fibers for producing the composite material according to the invention. Care must be taken here during processing that the temperature does not rise to the melting point of the Treviva plastic, since otherwise the degree of orientation would be lost.

Liquid crystal fibers, which are particularly suitable because of their favorable fire behavior, are used as plastic fibers with a high degree of orientation, for use in composite materials which are used, for example, in aircraft interior construction. In addition, these materials generally have very good chemical resistance and weather resistance. The low toxicity should also be mentioned. In contrast to other highly oriented materials, in the case of liquid crystal fibers, the processing temperature can reach the melting point without any significant loss in the degree of orientation and thus in the anisotropy of the specific properties.

The fibers made of TLCP, which are used as examples, can be combined by sintering using pressure and temperature in such a way that, on the one hand, their specific properties, in particular also in their anisotropy, are retained and, on the other hand, a composite material is formed which remains unchanged Properties can be processed to a variety of molded parts. In principle, all of the methods customary in fiber-plastic composites are available for this purpose, with the restriction that matrix material is not used.

Depending on the processing parameters of pressure, temperature and cycle time, "the sintering of the starting material to form a uniform, homogeneous product is achieved, original interfaces are no longer distinguishable or merge into one another. The interaction forces at the interfaces are comparable to the forces that hold together of the molecular chains within the plastic. In this context, interaction forces are understood not only as Van der Waals forces, but also hydrogen bonds, chemical bonds and ionic and dipole-dipole interaction forces.

If the plastic does not flow macroscopically during the processing period, the degree of anisotropy introduced into the component with the highly pre-oriented base material is largely retained. Allowing the flow results in a molecular alignment according to the generated currents. processes, similar to injection molding, with the result of increasing isotropy.

Ultimately, in the case of a component produced in this way, for example from TLCP fibers, there is neither a fiber composite with detectable individual fibers, nor a uniform phase in which the orientation regions are arranged indefinitely. This means that a matrix-free composite material differs fundamentally from conventional fiber-plastic composites or, for example, injection molded plastics.

All specific mechanical and physical parameters of the matrix-free composite, such as Strength, elasticity, thermal expansion, etc., are closely linked to the orientation of the macromolecules forming the material. In relation to the longitudinal axis of the macromolecules, these properties differ greatly from one another in the longitudinal and transverse directions. If you now combine highly oriented fibers with the defined macromolecules in a calculated manner as a matrix-free composite material to form a component, you can e.g. tailor its strength according to requirements.

This can be done by stacking the starting material in the form of fibers in layers in the desired direction and layer thickness, corresponding to the lamination of conventional fiber-plastic composites with layers of unidirectional layer structure.

The arrangement of the starting material in the matrix-free composite material is of course not restricted to certain, for example linear, geometries. Rather, there is the possibility, for example, of adapting looped fibers to the flow of force, integrated in the component, in a targeted manner, particularly to the introduction of force. If the individual fibers are combined to form fiber strands and textile structures are formed therefrom, such as two-dimensional fabrics, fleece or three-dimensional knitted fabrics, a distribution of properties corresponding to the structure of the fiber network is already achieved within one layer. Combinations of fiber networks and unidirectional layers are possible.

In accordance with the processing method described above, the loose starting material is consolidated into a rigid component. The variation of the parameters pressure and temperature influences its flow via the viscosity of the plastic. The variation of the cycle time and the temperature control influences relaxation and recrystallization processes. This means that a component made of a matrix-free composite material can be adapted to technical conditions almost without restriction.

It is also possible to form a single layer in which several fibers are combined to form fiber strands, the fiber strands forming a woven structure with one another. Here, too, a bond can be achieved without additional matrix by sintering using pressure and temperature, the specific anisotropic properties of the fibers being retained.

The composite material is preferably produced in such a way that the individual plastic fibers are connected to one another by interaction forces which are essentially comparable to the forces which bring about the cohesion of the molecular chains in the individual fibers. With these composites, the degree of orientation disruption is extremely low, i.e. a maximum of advantageous properties of the fibers are found in the composite material.

Important benchmark for the preservation of the specific properties The highly oriented plastic fiber is the degree of orientation.

The composite materials are preferably produced in such a way that the temperature when the fibers are connected is selected in the region of the melting point of the plastic fibers or below. This temperature is also only used for a short time, so that this ensures that the degree of orientation is retained to the maximum.

The pressure used in the connection of the fibers preferably ensures that the material is essentially free of voids, which means that the composite material is obtained compactly and without greater porosity.

Although on the one hand the degree of orientation of the fibers, which is found again in the composite material, represents an important benchmark for the production of the composite materials, a mechanical strength is to be obtained in the transverse direction to the fiber running direction, which is at least comparable to the mechanical properties of a Material from the same, but unoriented, essentially isotropic plastic material.

Preferred uses for the matrix-free composite materials are in particular fire-retardant molded parts.

In particular, the specific requirements for plastic molded parts in vehicle and aircraft construction make the new composite material highly suitable in order to find very special, advantageous solutions here. The composite material according to the invention is also particularly suitable for the production of molded parts which have different predeterminable strength and stiffness values in certain areas. These and other advantages of the invention are explained in more detail below with reference to the drawings. The individual show:

Figure 1 inventive matrix-free composite material in sheet form;

FIG. 2 enlarged section A from FIG. 1 in a schematic representation;

FIG. 3 multilayer composite material according to the present invention;

FIGS. 4a, b, c composite materials according to the invention made from differently structured starting materials; and

FIG. 5 temperature and pressure curve in a typical manufacturing process of a composite material according to the invention.

FIG. 1 shows a material plate made of the matrix-free composite material according to the invention, provided with the reference number 10, the structure of which can be seen better in FIG. 2, which shows an enlarged section A of the plate 10 in FIG. 1 in a schematic illustration. The plate 10 consists of a dense, multilayered layering of liquid-crystalline polymers in fiber form. FIG. 2 shows the individual fibers 12 in the upper right half in a schematic representation and also an area 14 in which the fibers 12 are arranged close to one another. If pressure and temperature are used to connect and compress the layered individual fibers, a matrixless composite structure is obtained, as is the case in the area 16 of plate 10 is shown in section A. The temperature during pressing is preferably kept as low as possible, ie it moves in the region of the melting point of the liquid-crystalline fibers or below, so that the degree of orientation of the fibers is retained. The area 16 in FIG. 2 indicates that the individual fibers 12 adjoin one another directly and are connected to one another without a matrix. This means that the fibers are brought so close to one another by the action of pressure and temperature that interaction forces can form between the fibers as they develop themselves within the fibers for connecting the macromolecules (not shown) arranged there to have. In this context, interaction forces are understood not only to be Van der Waals forces, but also hydrogen bonds, chemical bonds and ionic and dipole-dipole interaction forces.

The material plate made of the matrixless composite material according to the invention shown in FIGS. 1 and 2 has a grain 20 on its upper side 18, as well as on the non-visible underside, which gives the user information about the preferred direction of the composite material, i.e. the direction of the maximum resilience.

FIG. 3 shows an alternative construction of a material plate made of the matrixless composite material according to the invention, layers 22 and 23 and the remaining layers belonging to plate 24 alternately having running directions which form an angle of 90 ° to the running direction of the neighboring layer.

1 has particularly high strength values in the direction parallel to the grain, which is visible on the surface 18, and only normal in the transverse direction According to FIG. 3, the plate 24 according to FIG. 3 has equally good strength values which correspond to the strength values of the highly oriented plastic fibers, proportionally related, as they have strength values as expected from an unoriented, essentially isotropic plastic material .

A similar effect can be achieved with textile surface structures, e.g. with a kind of weave structure, in a single fiber layer, as shown in the fiber layer 26 according to FIG. Even in the case of textile surface structures, there is the possibility of connecting the highly oriented fibers to one another by applying pressure and temperature to form a matrix-free composite material.

Of course, it is also possible to use the matrixless composite material, as has been described so far, in connection with the production of multilayer materials, the matrixless composite material representing only one or a few of these layers. Combinations of layers, such as the layer with the pronounced weave structure in FIG. 26, FIG. 4, are combined with others, e.g. the layers 22 or 23 of Figure 3, or with layers in which the fibers are freely arranged according to the stress, e.g. in layer 25 in FIG. 4a, or with highly oriented foils, as shown schematically in FIGS. 4b and c by 26, 27, in any way possible.

Finally, examples of the production of a matrix-free composite material using highly oriented fibers of the Vectran HS 1500 type from Hoechst Celanese (liquid-crystalline polyester fibers) are given. The polyester of the fibers has the following structural formula:

x and y are in the ratio of approximately 60:40, the molecular weight is approximately 20,000.

The production process according to the invention will be illustrated using the following two examples:

Preparation of the starting material described above

When delivered, the size-free fibers were wound in multiple layers around a plate-shaped core in parallel orientation. The plate-shaped core and the counter plates of the press mold were treated with a temperature-stable, common release agent in order to facilitate the detachment of the material from the core or the counter plates at the end of the production cycle. The roll was manufactured with a thickness of approx. 1 mm; the number of turns to achieve the mentioned thickness was calculated from the thread volume.

Before the actual sintering process, the fibers were subjected to drying at a drying temperature of 150 ° C. for one hour in a forced air oven. The temperature-time curve of FIG. 5 shows the corresponding process, the temperature profile being the same in both examples. Starting at time tn, the fiber layer on the plate-shaped core was heated to the drying temperature of 150 ° C. When the drying temperature is reached at time t- ^ WO 92/12005 PCT / EP92 / 00043

- 14 -

the drying temperature was maintained at 150 ° C for one hour ( ^~ ^ - ^).

example 1

At the beginning of the sintering process (point in time -t ^), the fiber layer was placed in a hot press heated on both sides immediately after the drying process. The heating press was already at the sintering temperature of 302 ° C selected in this case and was pressurized immediately after insertion, corresponding to a compression pressure of 3 MPa (30 bar) (cf. pressure increase in the pressure-time diagram in FIG 5 from time t2) • The holding time after reaching the sintering temperature of 302 ° C. (t ₄ - t ₃ ) was 14 minutes. After the holding time had ended, cooling of the hot press was initiated at time t _*, the pressing process only being ended after the drying temperature of 150 ° C. had been reached (time tc), and cooling was continued without pressure until room temperature (time tg) .

Example 2

During this process, the wrapped plate-shaped core was placed on a hot table at room temperature. The wrapped form was covered with aluminum foil and sealed and insulated against heat loss to the environment with an approximately 5 cm thick layer of glass wool. At time t, that is, at the beginning of the heating phase for drying to 150 ° C., the pressure was reduced to 0.001 MPa. After the drying temperature of 150 ° C. had been reached, a drying time of one hour was also observed. This was followed by heating to the sintering temperature of 320 ° C (time t3), which was kept constant for 15 minutes (until time ^). Then cooling was initiated and at time tg, in which the fiber layer on the plate-shaped core had reached room temperature, the vacuum was released and the sintered material plate was removed from the mold.

For both processes according to Examples 1 and 2, the temperature control during the sintering process or during the heating period can be found in the temperature-time diagram in FIG. 5 and the corresponding pressure conditions in the pressure-time diagram in FIG. 5. The time period within which the maximum press temperature did not change by more than ± 1 C was defined as the holding time for the sintering temperature.

Since the melting point determined for the material used is 325.5 ° C., the sintering processes were all carried out significantly below the determined melting point. The mechanical values of the plate-shaped composite materials obtained in the subsequent tensile tests are listed in the table below.

Table

* measured according to DIN EN 2561 Compared to the production of molded parts in the injection molding process, there was an increase in the tensile strength by a factor of> 7 in the case of process example 1 or> 6 in the process control according to example 2 due to the production process according to the invention receive. The modulus of elasticity improved by a factor of> 2.5 in both Example 1 and Example 2. Surprisingly, a transverse strength is obtained even at a processing temperature below the melting temperature range, that is to say a strength of the material measured in the transverse direction to the fiber direction, which almost completely corresponds to the transverse strength when processed above the melting temperature.

In contrast, the longitudinal strength is significantly higher in the production method according to the invention with a processing temperature below the melting temperature than in processing methods with a processing temperature above the melting point.

The same statements can be made for the longitudinal and transverse stiffness of the materials according to the invention in relation to materials obtained with a processing temperature above the melting point.

It is all the more astonishing that in the matrix-free fiber composite materials according to the invention, the transverse strength and transverse stiffness assume practically the same values as in a production process in which work is carried out above the melting temperature, since the use of fibers means that a large number of contact surfaces in the transverse direction is present, which are to be connected to one another in order to obtain good transverse strength and transverse rigidity values. The method according to the invention for the production of matrix-free composite materials thus opens up the possibility, with virtually unchanged transverse strength and transverse rigidity of the molded parts, of achieving a longitudinal rigidity or longitudinal strength in the molded parts which is incomparably greater than what could be achieved in the usual injection molding process with the same materials and what was possible can be achieved at a processing temperature above the melting point of the liquid-crystalline polymers.

Claims

EXPECTATIONS

1. Matrix-free composite material with at least one layer of highly oriented plastic fibers, which are arranged in multiple layers and in a predetermined angular position to one another and are connected to themselves by sintering with simultaneous application of pressure and temperature.

2. Composite material according to claim 1, characterized in that the plastic fibers are liquid crystalline fibers.

3. Composite material according to claim 1 or 2, characterized gekenn¬ characterized in that several layers are connected to each other with different running direction of the fibers.

4. Composite material according to one of claims 1 to 3, characterized in that a plurality of fibers are combined to form fiber strands in a single layer and that the fiber strands form a textile structure, in particular a tissue structure.

5. Composite material according to one of claims 1 to 4, da¬ characterized in that the individual plastic fibers are interconnected by interaction forces that are essentially comparable to the forces that cause the cohesion of the molecular chains in the original plastic fibers.

6. Composite material according to one of claims 1 to 5, da¬ characterized in that the orientation of the material substantially corresponds to that of the original plastic fibers.

7. Composite material according to one of claims 1 to 6, characterized in that a temperature in the region of the melting point of the material is used as the temperature when connecting the plastic material.

8. Composite material according to one of claims 1 to 7, da¬ characterized in that the material is substantially free of voids.

9. Composite material according to one of claims 1 to 8, da¬ characterized in that the plastic fibers are connected by a selected compression pressure and a matching compression temperature so that the mechanical properties of the material in the transverse direction to the direction of the fibers essentially compare ¬ bar with the mechanical properties of a material made of the same, but unoriented, essentially isotropic plastic.

10. Use of a composite material according to one of claims 1 to 9 for the production of fire-retardant molded parts.

11. Use of a composite material according to one of claims 1 to 9 for the production of molded parts for vehicle and aircraft construction.

12. Use of a composite material according to one of claims 1 to 9 for the production of molded parts with regions of mechanical, physical properties which can be predetermined in different areas.