DESCRIPTION

TECHNICAL FIELD

The present invention relates to polymeric matrix composites and an acid etching process to modify their surface.

Specifically, thanks to the aforementioned surface acid etching, polymeric matrix composites are obtained with a coating having an improved adhesion or a joint with improved mechanical resistance.

The present invention finds advantageous applications in the biomedical, aerospace, automotive and structural fields and, in general, where a joint or a coating of a polymer matrix composite material is required.

STATE OF THE ART Add etching processes or etching of polymeric matrix composites are known to completely remove the matrix and recover the reinforcing fibers.

Examples of such processes are described in M. Nakagawa, H. Kuriya, K. Shibata "Characterization of CFRP Using Recovered Carbon Fibers from Waste CFRP", Hitachi Chemical Co., Ltd., Ibaraki - Proceedings of the 5 ^th ISFR (October 11-14, 2009, Chengdu, China).

Furthermore, joining and / or coating technologies are known in which the surface of the polymer matrix composite is attacked by acid / oxidant / base mixtures, or mechanically etched.

Examples of such technologies are described in US Patents US 3,885,071 and US 5,286,530.

US 3,885,071 describes an acid etching, with a strong oxidant, followed by a neutralization with basic agents, to be applied to two mechanically processed polymeric matrix composites, then facing each other and joined with an adhesive; however this process, having a certain complexity and duration, involves the interruption of the continuity of the fibers.

US 5,286,530 describes an "etching" carried out in two steps: a first basic step and a second acid step to improve the adhesion of a metal coating on a polymer matrix composite based on cyanate esters; however this process, having a certain complexity and duration, can lead to the interruption of the continuity of the fibers.

An acid etching process of the surface of polymeric matrix composites capable of maintaining intact the continuity of the reinforcing fibers would satisfy the need to coat and / or join high performance polymeric matrix components, suitable for industrial sectors such as aeronautics, aerospace, automotive, biomedical and the like. The present invention intends to respond to the aforementioned need.

In particular, the present invention intends to solve the technical problem of improving the adhesion of a joint and / or coating of polymeric matrix composites, without interrupting the continuity of the fibrous reinforcement which can, therefore, perform its functions in the joint or in the lining.

Furthermore, the present invention intends to solve the technical problem of maximizing the mechanical strength of joints and / or coatings between polymeric matrix composites with other similar or dissimilar materials, by surface acid etching, or "etching", of the surface to be joined or coated. .

Furthermore, the present invention intends to solve the technical problem of increasing the wettability in cases in which the value of the same is inadequate to obtain strong joints or coatings.

In summary, therefore, up to the present moment, to the knowledge of the Applicant, there are no known solutions that allow to realize, through an acid etching process, interfaces capable of being infiltrated by the joining material (adhesive) or coated more effectively.

Furthermore, there are no known polymeric matrix composites whose interface area with a joint and / or a coating is reinforced by fibers that are still intact, capable of increasing the mechanical strength of the joint or the adhesion of a coating.

Therefore the Applicant, with the acid etching process and the polymer matrix composite according to the present invention, intends to remedy this shortcoming. SCOPES AND SUMMARY OF THE INVENTION

It is the aim of the present invention to overcome the drawbacks of the known art related to the impossibility of maintaining the continuity of the reinforcing fibers of a polymer matrix composite following an "etching".

These objectives are achieved with the acid etching process and the polymer matrix composite according to the present invention which, advantageously and thanks to maintaining the continuity of the fibrous reinforcement, allows the composite to perform its functions in the joint and / or in the coating.

The acid etching process and the polymer matrix composite according to the present invention allow for the first time, according to the Applicant's knowledge, to only partially remove the matrix and therefore to create a surface where the polymer matrix composite faces only the intact reinforcing fibers and which is ready to be joined with adhesives and / or coated with paints, metals or another coating.

Specifically, the aforementioned and other purposes and advantages of the invention, which will emerge from the following description, are achieved with an acid etching process according to claim 1.

Preferred embodiments and variants of the acid etching process according to the present invention are the subject of the dependent claims 2 to 16.

Another independent aspect of the present invention relates to a polymer matrix composite and constitutes the subject of claim 17.

Preferred embodiments and variants of the polymer matrix composite according to the present invention are the subject of the dependent claims from 18 to 25.

It is understood that all the attached claims form an integral part of this description and that each of the technical characteristics claimed therein is possibly independent and can be used independently from the other aspects of the invention.

It will be immediately evident that innumerable changes can be made to what has been described (for example relating to shape, size, arrangements and parts with equivalent functionality) without departing from the scope of protection of the invention as claimed in the attached claims.

Advantageously, the technical solution according to the present invention, which provides an add etching process and a polymer matrix composite, allows to:

- creating, through a simple acid "etching", interfaces capable of being infiltrated by the joint material (adhesive) or coated more effectively (coating, painting);

- obtain an interface area between the polymer matrix composite and the joint and / or coating reinforced by still intact fibers and, consequently, increase the mechanical strength of the joint and / or the adhesion of a coating;

- increase wettability in cases where its value is inadequate to obtain strong joints or coatings.

Further advantageous characteristics will become more evident from the following description of preferred but not exclusive embodiments, provided purely by way of non-limiting example.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described hereinafter by means of some preferred embodiments, provided by way of non-limiting example, with reference to the attached drawings. These drawings illustrate different aspects and examples of the present invention and, where appropriate, similar structures, components, materials and / or elements in different figures are indicated by similar reference numerals.

FIG. 1 is a flow chart of the acid etching process according to the present invention; FIG. 2 is a schematic, perspective representation illustrating the acid etching process according to the present invention;

FIG. 3 is a lateral schematic representation illustrating the operating protocol of the add etching process according to the present invention;

FIG.4 is a schematic, perspective representation, which illustrates the surface effect on composite materials of the acid etching process according to the present invention, in particular on one face;

FIG. 5 is a schematic, perspective and lateral representation, which illustrates the surface effect on composite materials of the acid etching process according to the present invention, in particular on the two faces of the composite;

FIG. 6 is a lateral schematic representation, which illustrates the surface effect on composite materials of the acid etching process according to the present invention, in particular on the two faces of the composite;

FIG. 7 A is a scanning electron microscope (SEM, Scanning Electron Microscope) image showing the surface of the polymer matrix composite according to the present invention after acid etching at a temperature of 100 ° C and for 10 minutes;

FIG. 7B is an enlarged image of FIG. 7A;

FIG.8A is a scanning electron microscope (SEM, Scanning Electron Microscope) image showing the surface of the polymer matrix composite according to the present invention after acid etching at a temperature of 125 ° C and for 10 minutes;

FIG. 8B is an enlarged image of FIG. 8A;

FIG. 9 is a schematic representation that illustrates the effect of localized delamination caused by excessive acid infiltration between the fibers of the polymer matrix composite according to the present invention;

FIG.10 is a schematic representation that illustrates the methodology to counteract the excessive infiltration of acid between the fibers of the polymer matrix composite according to the present invention, a method that involves the addition of powders of ceramic material inert against the acid;

FIG.11 is a photographic image showing the positive effect, i.e. the absence of localized delamination, due to the application of the methodology of FIG. 10;

FIG. 12 is a schematic representation illustrating the adhesive bonding of polymeric matrix composites according to the present invention after acid etching, made to evaluate the strength of the joint itself;

FIG. 13 is a schematic, perspective view illustrating the surface of the polymer matrix composite according to the present invention after acid etching;

FIG. 14 is a diagram showing the profiling of the surface of the polymer matrix composite according to the present invention after acid etching;

FIG. 15 is a diagram showing the profiling of the surface of a comparison polymer matrix composite;

FIG. 16A is a scanning electron microscope (SEM, Scanning Electron Microscope) image showing, in section, comparative polymer matrix composites, which have not undergone the acid attack of the present invention, joined by an adhesive joint; FIG. 16B is a Scanning Electron Microscope (SEM) image showing, in section, polymer matrix composites according to the present invention, which have undergone the acid attack of the present invention at 125 ° C and for 10 minutes, joined by means of an adhesive joint;

FIG. 16C is a scanning electron microscope (SEM, Scanning Electron Microscope) image showing the detail of the interface of FIG. 16B, interface in which the infiltration of the adhesive junction is evident within the region engraved by the acid attack;

FIG. 17 is a histogram showing the results of the mechanical single-lap shear strength tests (SLO, Single Lap Offset) of the adhesive joints used according to the present invention;

FIG. 18 is a graph showing load / displacement curves of composites joined by adhesive joints after the SLO test, respectively for non-etched surfaces, etched surfaces at 100 ° C and for 10 minutes and etched surfaces at 125 ° C and for 10 minutes;

FIG. 19A is an image showing the fracture surface of comparison composites, with non-etched surfaces, joined by adhesive joint, after the SLO test;

FIG. 19B is an image showing the fracture surface of composites according to the present invention, with etched surfaces at 100° C and for 10 minutes, joined by adhesive joint, after the SLO test;

FIG. 19C is an image showing the fracture surface of composites according to the present invention, with etched surfaces at 125 ° C and for 10 minutes, joined by adhesive joint, after the SLO test;

FIG. 20A is a histogram showing the influence of the acid attack time on the single-lap shear strength (SLO, Single Lap Offset) of the adhesive joints used according to the present invention;

FIG. 20B is a histogram showing the influence of the acid concentration on the single lap shear strength (SLO, Single Lap Offset) of the adhesive joints used according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is susceptible to various modifications and alternative embodiments, some preferred ones are shown in the drawings and will be described below in detail.

It is to be understood, however, that there is no intention of limiting the invention to the specific embodiments illustrated, but, on the contrary, the invention is intended to cover all modifications, alternative constructions, and equivalents that fall within the scope of the invention as defined in the claims.

In the following description, therefore, the use of "for example", "etc.", "or / or" indicates non-exclusive alternatives without any limitation, unless otherwise indicated; the use of "also" means "including, but not limited to" unless otherwise indicated; the use of "includes / includes" means "includes / includes, but not limited to" unless otherwise indicated.

The acid etching process and the polymeric matrix composite of the present invention is based on the innovative concept of only partially removing the matrix and thus creating a surface where the polymeric matrix composite faces only the intact reinforcing fibers and which is ready to be joined with adhesives and / or coated with paints, metals or other coating.

The acid etching process and the polymer matrix composite of the present invention provide a surface ready to be joined by adhesives and / or coated with paints, metals or other coating.

The present invention can be used in all applications where joining or coating of polymer matrix composite materials is required, for example applications in the biomedical, aerospace, automotive and structural fields.

In the present description, the term "composite" refers to a material comprising a matrix and a second phase.

In the present description, the term "polymeric matrix" refers to a polymeric material. In the present description, the term "reinforcing fibers / fibrous reinforcement" refers to long or short fibers.

In the present description, the term "continuity of the reinforcing fibers / fibrous reinforcement" means that the reinforcing fibers / fibrous reinforcement are not damaged after the process.

In this description, the term "joint / joint element / joint material" means the union between two parts.

In the present description, the term "coating" means a layer that coats a substrate.

In this description, the term "mechanical strength" refers to the force required to detach the joint.

In this description, the term "adhesion" refers to the force required to detach the coating.

With reference to FIG. 1, which illustrates the preferred embodiment of the present invention, it is observed that the acid etching process of at least one surface 10, to be coated or joined, of at least one composite material 1 with a polymeric matrix 11 comprises the following steps:

- step 100: preparing at least one polymer matrix composite 1 comprising at least one surface 10;

- step 101: preparing an acid or a mixture comprising at least one acid 2;

- step 102: providing a covering 20 or a joining element 30;

- step 103: carry out the acid attack of at least one surface 10 for a predetermined period of time and suitable for leaving the fibers 12 intact in the polymer matrix 11;

- step 104: remove the polymeric matrix 11 attacked by the acid or the mixture comprising at least one acid 2 and obtain an interface 13 in which the fibers 12 face, without loss of continuity, the surface 10 of the composite 1;

- step 105: making the coating 20 or the joining element 30 adhere to the fibers 12 which face continuously the surface 10 of the composite 1, thus obtaining a composite with a coating with improved adhesion or a joint with mechanical resistance improved. Preferably, the composite is a combination of two or more polymeric materials 1 having a polymeric matrix 11.

Alter natively, the composite is a combination of at least one polymer matrix composite 1 and of at least one second material; in this case, the second material is preferably a metallic or ceramic material.

The composite material 1 having polymeric matrix 11 is chosen from the fiber reinforced polymers, or "Fiber Reinforced Polymers" FRP.

The fibers 12 of the polymeric matrix 11 are selected from carbon fibers, glass fibers, polymeric fibers and their mixtures; preferably, they are carbon fibers.

Preferably, the surface 10 has a planar or curved configuration.

Preferably, the acid or mixture comprising at least one acid 2 is sulfuric acid H2SO4 or another acid or a mixture of acids, having a composition and concentration suitable for carrying out the acid etching of the surface 10 and obtaining the interface 13 in which the fibers 12 face, without loss of continuity, the surface 10 of the composite 1. Preferably, the acid or mixture comprising at least one acid 2 has a composition varying between 30% by weight and 98% by weight, more preferably 96% by weight, and a variable concentration of 30% by weight and 98% by weight, more preferably 96 % by weight.

Preferably, the acid attack occurs by immersion or by spraying or by other technique of contacting the acid or the mixture comprising at least one acid 2 on the surface 10. Preferably, the polymeric matrix 11 attacked by the acid or the mixture comprising at least one acid 2 is removed for a thickness varying between 5 pm and 500 pm, more preferably for a thickness of 200 pm, thus obtaining the interface 13 in which the fibers 12 face, without loss of continuity, the surface 10 of the composite 1, the interface 13 being suitable for making the coating 20 or the joining element 30 adhere to the fibers 12.

Preferably, the predetermined time period varies between 300 seconds and 1,200 seconds, more preferably it is 600 seconds.

Preferably, the coating 20 is a paint, a metal or another coating and the joining element 30 is an adhesive substance.

Optionally, the acid etching process of the present invention further comprises the step of carrying out a localized heating, via a heat source (eg hot air source, lamp, hot wall and the like), of the surface 10 and/ or of the acid or of the mixture comprising at least one acid 2 to accelerate the reaction time.

Preferably, the localized heating takes place at a temperature varying between 25 ⁰ C and 150 ⁰ C, more preferably at 125 ⁰ C, and the reaction time varies between 300 seconds and 1,200 seconds, more preferably it is 600 seconds.

Optionally, the acid etching process of the present invention also comprises the step of recycling the polymer matrix 11 attacked by the acid or mixture comprising at least one acid 2.

With reference to FIG. 2 it can be seen that a polymer matrix composite 111 comprising at least one surface 10, when subjected to acid attack by an acid or a mixture comprising at least one acid 2 at a predetermined temperature and for a predetermined period of time, presents the fibers 12 which face, without loss of continuity, the surface 10 of composite 1.

With reference to FIG. 3, which illustrates the operating protocol of the acid etching process according to a preferred embodiment of the present invention, reports the use of composite materials 1 with a polymer matrix 11 reinforced by long carbon fibers 12; these composites were cut to a size of 25x25 mm ² and then tested shear after splice using Single Lap Offset (SLO) test.

Before the surface acid etching process, the composite materials 1 were cleaned with acetone in an ultrasonic bath for 10 minutes and subsequently dried with compressed air; the faces of the sample not affected by the surface modification were covered with adhesive tape to prevent acid attack.

For the surface acid etching procedure, the composite substrates 1 are placed on a heated plate 40 together with a container containing the concentrated acid solution 2 (this solution - preferably of sulfuric acid, nitric acid or other similar acids - has variable composition between 30% by weight and 98% by weight, preferably 96% by weight, and variable concentration 30% by weight and 98% by weight, preferably 96% by weight) and the whole is heated to the treatment temperature, preferably in the range variable between 25° C and 150° C, more preferably at 125° C.

The acid solution 2 was then poured onto the surface 10 of the composite 1 to be treated until the entire surface is covered with a thin film of acid; the composite 1 is left on the heated plate 40 for the acid attack treatment time, preferably in the range from 300 seconds to 1,200 seconds, preferably it is 600 seconds.

The acid attack or etching can be carried out in several steps, in each of which the composite materials 1 are subjected to acid attack; at the end of each step the composites 1 are rinsed in distilled water, before being subjected to the subsequent etching step; at the end of the entire process the composites 1 are cleaned with ethanol in an ultrasonic bath to remove the reaction products from the surface, and then dried. An example of the acid etching protocols, or "etching", with sulfuric acid at 96% by weight is shown in the following table:

The time and temperature of the acid etching process were selected so as to selectively remove the surface 10 of the polymer matrix 11 of the composite 1 and expo se the first layer of carbon fibers 12, without damaging either the fibers 12 or the matrix 11 present underneath the first layer of fibers.

With reference to FIG. 4 it is observed that the etching process highlights a first layer of fibers on the surface, well separated from the matrix and not damaged.

With reference to FIG. 5 shows the effectiveness of the etching process in obtaining a matrix-free layer with the fibers emerging also laterally.

With reference to FIG. 6 it is observed that the fibers emerging laterally can be joined by a joint material to obtain a composite joint.

With reference to FIGS. 7 A and 7B, the images under the scanning electron microscope (SEM, Scanning Electron Microscope), in a first resolution and in its enlargement, show the surfaces of the composite 1 after the surface modification occurred at 100° C and 10 minutes; the carbon fibers 12 present on the surface can be seen.

With reference to FIGS. 8A and 8B, the Scanning Electron Microscope (SEM) images, in a first resolution and in its enlargement, show the surfaces of composite 1 after the surface modification after the surface modification occurred at 125° C and 10 minutes; the carbon fibers 12 present on the surface and the total disappearance of the polymeric matrix from the surface of the fibers can be seen.

In a variant illustrated in FIGS. 9 and 10 it is observed that the acid attack process can be modified in order to avoid excessive infiltration of acid 2, for example H ₂ SO ₄ , between the fibers 12; this modification provides for the addition of powders of ceramic material inert against acid 2, for example the addition of AI2O3 alumina powders, and is made by heating on the heated plate 40 and stirring with the agitator 50; in addition, the addition of an oxidizing agent is provided, for example hydrogen peroxide H2O2, in order to accelerate the reaction.

With reference to FIG. 11 the positive effect is observed, i.e. the absence of localized delamination, due to the application of the variant illustrated above.

Furthermore, with reference to FIG. 12 which illustrates the preferred embodiment of the present invention, constitutes an independent and usable aspect independently of the other aspects of the invention a composite material 1 having a polymeric matrix 11 comprising:

- at least one surface 10, to be coated or joined;

- an interface 13 of the surface 10 overlooked by integral and continuous fibers 12 within the polymeric matrix 11;

- a coating 20 or a joining element 30 adhering to the fibers 12, where the composite has a coating with improved adhesion or a joint with improved mechanical strength when an L load is applied.

Preferably, the composite is a combination of two or more composite materials 1 having a polymeric matrix 11.

Alternatively, the composite material is a combination of at least one polymer matrix composite 1 and of at least one second material; in this case the second material is preferably a metallic or ceramic material.

The composite material 1 having a polymeric matrix 11 is chosen from the fiber reinforced polymers, or "Fiber Reinforced Polymers" FRP.

The fibers 12 of the polymeric matrix 11 are selected from carbon fibers, glass fibers, polymeric fibers and their mixtures; preferably they are carbon fibers.

Preferably, the surface 10 has a planar or curved configuration.

Preferably, the polymeric matrix 11 attacked by the acid or by the mixture comprising at least one acid 2 is removed for a thickness varying between 5 pm and 30 pm, more preferably for a thickness of 10 pm, thus obtaining the interface 13 in which the fibers 12 they face, without loss of continuity, the surface 10 of the composite 1, the interface

13 being suitable for making the coating 20 or the joining element 30 adhere to the fibers 12.

Preferably, the coating 20 is a paint, a metal or another coating and in which the joining element 30 is an adhesive substance.

The acid etching process and the polymeric matrix composite according to the present invention according to the present invention are described below in greater detail with reference to the following Examples, which have been developed on the basis of experimental data and which are intended as illustrative but not limitative of the present invention.

Example 1

Two long carbon fiber reinforced cyanate ester matrix composites (25 mm x 25 mm x 4 mm) were subjected to acid attack consisting of 96% , 76% or 33% sulfuric acid (4 mL) at 80-150 ° C for 5-20 minutes.

After etching, the composites were joined with a phenolic-based adhesive.

The mechanical strength of the jointed composites was measured by single-lap offset compression shear tests, which highlighted the effectiveness of the acid treatment at 96% and 76% compared to joints made without treatment.

The 33% acid treatment was not effective in these conditions.

Example 2

Two long carbon fiber reinforced cyanate ester matrix composites (25 mm x 25 mm x 4 mm) were subjected to acid attack consisting of a suspension of alumina powder (15 g) in 96% sulfuric acid (18 mL) and hydrogen peroxide (H2O2) at 150° C for 10-20 minutes.

After etching, the composites were butt-joined using phenolic-based adhesive and measured by four-point bending test.

The acid attack thus modified was found to be effective in facing a part of the fibers in the area to be butt-joined.

Example 3

A long carbon fiber reinforced cyanate ester "honeycomb" type composite (25 mm x 25 mm x 4 mm) was subjected to acid attack consisting of 96% sulfuric acid (4 mL) at 100 -150° C for 5-10 minutes.

After etching, the composite was joined with a phenolic -based adhesive to two sodium-calcium glass plates.

The mechanical strength of the joint was measured by means of a tensile test.

The acid attack was also effective on honeycomb structures that were perfectly adherent to the sodium -calcium glass.

With reference to FIG. 13 it is observed that the three-dimensional profilometry of the composite after chemical etching.

With reference to FIG. 14 it is observed that the profilometry of the composite before the chemical attack ("etching").

With reference to FIG. 15 it is observed that the profilometry of the composite after chemical etching

With reference to FIGS. 16A, 16B and 16C it is observed that, respectively, Figure 16A shows a section of the composite before the chemical attack; the chemical attack ("etching") exposed a first layer of fibers that was perfectly infiltrated by the adhesive (FIG. 16B), as shown by the enlargement (FIG. 16C).

The effect of surface acid attack on the strength of jointed composites was evaluated through mechanical tests; in particular, the composite substrates 1, after the surface modification, were joined with high-strength polymeric adhesives loaded with inorganic materials 30 (see FIG. 12); the shear strength of the jointed composites was evaluated by the Single Lap Offset test by applying the load L (see FIG. 12).

With reference to FIG. 17 it is observed that it is possible to obtain, by means of surface modification with sulfuric acid, a significant improvement in the shear strength of the joints.

With reference to FIG. 18 it is observed that it is possible to obtain, through the surface modification with sulfuric acid, a significant improvement in the shear strength of the joints, as shown by the curves relating to the mechanical tests on the joints.

With reference to FIGS. 19A, 19B and 19C it is observed that it is possible to obtain, through the surface modification with sulfuric acid, a significant improvement in the shear strength of the joints, as shown by the photographs relating to the mechanical tests on the joints.

With reference to FIGS. 20A and 20B it is observed that it is possible to obtain, through the surface modification with sulfuric acid, a significant improvement in the shear strength of the joints, as shown by the histograms relating to the results of the mechanical tests on the same, according to the different chemical etching treatments ("Etching").

The present invention, in the preferred embodiment, represents an innovative acid etching process capable of making interfaces capable of being infiltrated by the joint material (for example an adhesive) or covered with a coating (for example a metal or a paint) and thus provide an interface zone consisting of the joint material and / or the coating reinforced by the fibers of the polymer matrix composite capable of increasing the mechanical strength of the joint or of the adhesion of the coating.

The process according to the present invention was also suitable for CFRP carbon fiber reinforced polymers with epoxy matrix and CFRP carbon fiber reinforced polymers with honeycomb structure, "honeycomb".

As can be deduced from the above, the innovative technical solution described here has the following advantageous characteristics:

- creating, through a simple acid "etching", interfaces capable of being infiltrated by the joint material (adhesive) or coated more effectively (coating, painting);

- obtain an interface area between the polymer matrix composite and the joint and / or coating reinforced by still intact fibers and, consequently, increase the mechanical strength of the joint and / or the adhesion of a coating;

- increase wettability in cases where its value is inadequate to obtain strong joints or coatings.

From the above description it is therefore evident how with the acid etching process and the polymer matrix composite according to the present invention they allow to achieve the proposed scopes.

It is equally clear to a person skilled in the art that it is possible to make changes and further variations to the solution described with reference to the attached figures, without thereby departing from the teaching of the present invention and from the scope of protection as defined by the attached claims.