DESCRIPTION

Field of application The present invention relates to a high energy absorption laminated composite material.

Furthermore, the present invention relates to a process for manufacturing such a laminated composite material.

Prior art It is known that in the most varied fields of application there is a strong demand for new materials capable of having a number of complementary properties.

In particular, especially in the automotive industry, in the aeronautical and nautical field, the use of composite materials that feature additional advantages over the use of traditional materials is rapidly spreading.

Particularly, a material that finds widespread application is carbon, which in the form of reinforcing fibres, is employed for a variety of new composite materials, especially for its lightness and resistance properties.

A problem of the composite materials of the prior art, such as those that involve the use of carbon reinforcing fibres, is their low capacity of absorbing impulsive, pulsating or fatigue energy.

In practice, the materials of the prior art although extremely resistant especially with reference to their specific weight, by contrast exhibit a brittleness that in some fields of application is entirely unwanted, so that their use in such fields is to be excluded.

The technical problem underlying the present invention is, therefore, to provide a laminated composite material that presents on the one side the strength and ductility of conventional metallic materials such as steel and aluminium, and at the same time which presents the properties of lightness and resistance that can be found in prior art composite materials, without however having the typical brittleness of these materials, but rather by adding the ability to ensure a high resilience, so as to ensure high resistance to impacts, in the context of a simple and rational constructive solution.

The object of the present invention is to satisfy the aforesaid requirement and to remedy at the same time the drawbacks mentioned above with reference to the known technique.

Summary of the invention The object is a high energy absorption laminated composite material, in accordance with claim 1 of the present invention.

The dependent claims outline preferred and particularly advantageous embodiments of the material according to the invention.

Additionally, this object is also achieved by a process for the manufacturing such a laminated composite material.

Further features and advantages will become more apparent from the detailed description given hereinafter of a preferred, but not exclusive, embodiment of the present invention.

Detailed description With reference to a preferred embodiment of the present invention, the manufacturing of a high energy absorption laminated composite material in accordance with the present invention is described in the following.

The laminated composite material generally comprises:

- at least one carbon fibre layer on both outer faces; - at least one layer of fabric of continuous-filament natural basalt fibre or a layer of glass fibre;

- at least one layer of fabric selected from flax, cotton or hemp fibre. Preferably the layers are arranged between them in a specular manner with respect to a plane of symmetry parallel to the layers and passing through the centre.

Below, the present description will make explicit reference to the use of carbon, flax and basalt layers, without, however, limiting the scope of the present invention.

The use of basalt and flax with carbon is preferable.

In particular the use of basalt interposed between flax and carbon enables an optimal lamination of three materials, since the use of flax directly in contact with the carbon would facilitate the delamination. Moreover, the use of basalt in the transition layer promotes optimal cohesion of the carbon with the linen, significantly improving the shock absorption properties of the laminated composite material according to the present invention, as well as preventing an easy detachment of the carbon layer following the effects of the loads resulting in delamination and detachment of the various types of sheets between them respectively.

In essence, the carbon fibres remain more cohesive to a much more flexible support being able to continue to respond to the imposed loads even in case of structural impairment due to impact.

This effect is surprisingly achieved through the use of the layer of basalt, as a third material in addition to carbon and to flax.

It is also noted that a similar effect, although with a less obvious effectiveness, is also achieved with the use of glass fibre, preferably lightweight.

As aforementioned, the laminated composite material presents on the two end faces, that could be defined as internal and external, respectively, one layer of carbon fibre, while in the inner part of these carbon layers according to the needs of the flax and basalt layers are placed.

In particular, the choice regarding the arrangement of layers will be done based on the requirements of use. In fact, the carbon has the effect of ensuring the desired strength to the laminated composite material, having the task of withstanding high loads to which the material is subjected; the flax instead has the purpose of withstanding the vibrations and the energy of the impacting loads; the basalt serves as a bridge between the rigid material (carbon) and the flax. The laminated composite material thus produced allows resisting beyond the breaking limit for a value of approximately 80% of the value recorded immediately before the break.

In essence, a high-energy absorption material is obtained.

The arrangement of layers and their thickness will be chosen based on the desired properties.

For example, a succession of layers can be employed, with the carbon fibre on the outside, and a plurality of layers of basalt / flax alternating between them.

Alternatively, flax and basalt can define a respective single layer, with possible specularity with respect to the central axis.

By way of example, the following arrangement may be used:

• 2 carbon fibre sheets;

• 2 fabric sheets of natural basalt fibre;

• 2 fabric sheets of flax fibre; · 2 fabric sheets of flax fibre;

• 2 fabric sheets of natural basalt fibre;

• 2 carbon fibre sheets.

In practice, we have a six-layer laminated with carbon layers on the outer faces, all the layers of linen inside, and basalt layers interposed between the carbon layers and the flax.

Similarly, the direction of the fibres in the various layers will be chosen depending on the use of the material, favouring for example the arrangement of the carbon fibres parallel to the anticipated load lines. Of course it is also possible to arrange the fibres so as to obtain a final material featuring a behaviour as much as possible isotropic.

The layers are bound to each other by means of a resin-based matrix in chemical cure which involves an exothermic reaction, wherein the resin has a percentage by weight on the finished product comprised between 40 and 60, preferably of about 45%.

In this manner a finished product can be obtained which has a final specific weight extremely reduced and entirely comparable with the weight of the carbon fibre of the technique resulting in many cases also more lightweight, yet avoiding the fragility typical of carbon fibre-based composite materials of prior art.

With reference to the procedure for obtaining a laminated composite material in accordance with the present invention, an illustrative example is described below. A mould is prepared, which can be male or female depending on the desired product.

The abovementioned arrangement is symmetrical in nature with the four edges of each divided material two by two in a mirror image.

The carbon fibre sheets are arranged so as to have the carbon fibres oriented at -90°, + 45°, respectively, on one face, and + 90°, - 45° on the other face; the sheets of fabric of continuous filament natural basalt fibre have the fibres oriented at + 90° , -90°, respectively, on a layer and + 90°, - 90° on the other layer; the four sheets of flax fibre fabric have the fibres oriented at -90°, + 45°, + 90°, - 45°, respectively; all sheets must be suitably impregnated with a resin-based matrix adapted to act as binder between the sheets, and with a relative catalyst that allows catalysis.

The set of foils in the mould is brought to the temperature required to make the catalysis take place for the duration necessary to complete hardening. In the case where the reaction is of exothermic type, heat will not be necessary, unlike in the presence of endothermic reactions when the temperature must be controlled, as in the case of use of pre- impregnated sheets, as we shall see below. During catalysis the excess matrix is drained.

Although reference has been made to the use of four sheets of homogeneous material, any number of sheets may be employed, and the orientation of the fibres will be chosen based on the end use of the product.

The arrangement of the layers can vary as well.

As a resin-based matrix, a resin chosen from polyester resin, vinyl ester and epoxy can be used. An epoxy resin is preferable.

The phase in which the various sheets are impregnated with the resin can be performed with rollers or by immersion in the resin. Alternatively, the infusion technique can be used, consisting in laying the various sheets on the dry mould and then injecting the resin through an artificially created pressure difference.

During the infusion technique, an area with a lower pressure than the surrounding atmosphere is created inside the mould, able to attract the resin for which the inflow channels were prepared moving from resin storage tanks to the mould.

This depression, commonly called "vacuum," is obtained by covering the mould with a film of plastic material, commonly called "vacuum bag", effectively connected to the mould to prevent air infiltration.

The area between the vacuum bag and the mould is sucked through an electromechanical pump and suction pipes, thus obtaining a vacuum bag adhering strongly to the mould by crushing the fibres and creating the necessary depression to inject the resin. Once the resin begins to flow into the mould, such must be able to cover the entire surface of the mould, that is, by impregnating all the fibres, in a time shorter than the curing. The resin, in fact, must remain in the liquid state during the entire infusion process to allow complete impregnation of all areas only at this point can the curing process of the resin commence, which then passes to the solid state through an exothermic reaction. In general, the resin passes from the storage tanks to the mould by means of suitably dimensioned and positioned pipes. Alternatively, the use of pre-impregnated sheets is also possible, i.e. sheets that already contain within them the resin needed for the hardening phase and which, therefore, do not require manual lamination. These pre-impregnated sheets should be simply spread on the mould and cut, then placed in "vacuum" and brought to the temperature required to initiate the catalysis of the resin.

In order to avoid damage to the mould during the forming operation, due to friction, a release agent material is employed, which is spread in a homogeneous layer on the entire mould making sure to use products compatible with the resins used to infuse.

Before applying the fibre sheets, one can move to the laying out of an eventual gel coat, in case of a female mould and if the item requires it. The gel coat is spread regularly with rollers on the mould and effects a good surface finish of the composite once removed from the mould. Then the following are laid in succession over the fibre strips:

- a peel ply film in direct contact with the blade farthest from the mould;

- a micro-perforated fabric that can allow air and resin to pass;

- a cloth to absorb the residual resin during the drainage of the matrix.

Finally, the vacuum bag is prepared and effectively fixed to the mould along its entire perimeter. The vacuum bag is effectively fixed to the mould by means of a high double-sided thickness tape commonly called "tacky tape".

A side of the tape adheres to the mould and the other to the vacuum bag, the high thickness of the tacky tape and its workability allow the creation of a barrier against entry of air from outside.

The drafting of the vacuum bag must of course incorporate suction channels connected to vacuum pumps.

When the positioning of the empty bag is finished, the vacuum pump connected to the suction channels can be turned on. This operation must be accomplished with the resin flow stop valves closed to prevent the infusion from beginning prematurely. When the vacuum pump is in operation, the air present between the vacuum bag and the mould flows outside and the bag is pressed onto the mould.

The time to create the right depression varies according to the size of product and power of the pump.

The vacuum formed during the reaction of the resin catalyst is of approximately 0.9 bar.

The next phase is that of the resin infusion: once certain that the previous operations were successful, it is possible to open the stopcocks along the infusion lines and to allow the resin to pass from the storage tanks to the mould. Prior to infusion, the resin must of course be catalysed to allow the hardening process to start.

The resin curing depends essentially on its temperature, the type of resin, and the catalyst. By way of example, an epoxy resin is employed at room temperature (22- 25°C), for approximately 12 hours.

At the end of curing, the manufactured item is extracted from the mould and exposed to thermal treatment in the oven for approximately 9- 13 hours, at a temperature between 70°C and 90°C. This treatment allows degassing and dehumidification of the item in order to achieve the mechanical characteristics of the resin project.

As can be noted from the description, the laminated composite material with high energy absorption and the method for obtaining it according to the present invention allow to satisfy the requirements and overcome the drawbacks outlined in the introductory part with reference to the prior art.

In fact, products are obtained that can be widely used in all areas where a high-energy absorption is required mainly as a result of impacts.

The material according to the present invention is an alternative to a basis of the prior art carbon-based laminar materials, since it allows to preserve the typical resistance of the laminar carbon and their desired lightness, however, avoiding the typical problems of fragility of prior art materials. It is also possible to combine the three fibre materials forming the layers of the present invention according to various combinations so as to enhance specific desired characteristics.

The material proposed in the present invention has the following advantages:

- Simple production also for intricate details, and its inherent low cost compared to materials that exhibit similar mechanical characteristics;

- Specific lightness;

- High impact resistance;

- High resistance to aggressive environments;

- Fire resistance.

Obviously, a skilled person, in order to satisfy contingent and specific requirements, may make numerous modifications and variants to the invention described above, all however contained within the scope of the invention as defined by the following claims.