WO2020110038A1

WO2020110038A1 - Prepreg for decorative components which are highly heat-resistant, transparent, colorless and free of defects, such as spots and dots, and manufacturing method thereof

Info

Publication number: WO2020110038A1
Application number: PCT/IB2019/060242
Authority: WO
Inventors: Federico Becagli; Massimiliano Becagli; Luca Calamai; Silvio Campigli
Original assignee: Microtex Composites Srl
Priority date: 2018-11-27
Filing date: 2019-11-27
Publication date: 2020-06-04
Also published as: IT201800010629A1

Abstract

A method for obtaining prepreg - fiber-reinforced composite materials intended for making "Carbon Look" decorative components, characterized by transparency and total absence of color, and free of white spots and/or dots, provides the use of a Lewis acid with latency properties, such as BCl₃.(N,N-dimethyl octylamine), as polymerizing catalyst of the epoxy resins used as matrix. The decorative components, after the curing process, in addition to being transparent, colorless and free of defects, such as white spots and/or dots, are also highly heat-resistant, because the cured composite obtained from prepreg manufactured with an appropriately formulated matrix, reaches high glass transition temperature values.

Description

PREPREG FOR DECORATIVE COMPONENTS WHICH ARE HIGHLY

HEAT-RESISTANT, TRANSPARENT, COLORLESS AND FREE OF DEFECTS, SUCH AS SPOTS AND DOTS, AND MANUFACTURING METHOD THEREOF

k k: k: k: k:

DESCRIPTION

Field of application

The present invention relates to a method for the production of prepreg (or pre-preg) - fiber-reinforced composite material of the "Carbon Look" type - for decorative components which, in addition to being free of defects, such as spots and dots, are characterized by total transparency and total absence of color, unlike the prepreg materials currently on the market.

The invention further relates to the product obtained by such a method, which provides, in particular, the use of a Lewis acid, such as BCI₃. (N,N- dimethyl octylamine) as a catalyst for polymerization of the epoxy resins used as a matrix. A further benefit of the use of this catalyst is that of obtaining highly heat-resistant decorative components because the cured composite obtained from prepreg manufactured with a suitably formulated matrix can reach high glass transition temperature values.

Prior art

The term "prepreg" refers to pre-impregnated, fiber-reinforced composite materials in which a thermosetting resin, such as an epoxy resin, is present as a matrix.

The fibers form a weave, while the matrix fixes the fibers and determines the thermo-mechanical properties of the resulting composite. The matrix, after the weave impregnation process, is in a B-Stage state, i.e. in the liquid state having high viscosity at ambient temperature in a desired configuration which can be successively cured because the curing agent with latency properties is present in the resin matrix itself .

Low-temperature storage is required to maintain the handling properties of the prepreg.

The prepreg is therefore always stored in cooled areas because heat triggers the polymerization process.

The manufacturing of composite products from prepreg takes place through the use of autoclaves, heated presses or ovens.

Prepregs are used for manufacturing components with high thermo-mechanical resistance properties. The product manufacturing process provides the lamination of one or more layers of prepreg in a mold which determines the shapes and dimensions of the component. Prepregs can be worked to obtain components having complex shapes.

The prepregs used for decorative applications are part of a manufacturing process which is commonly used to obtain the classic "Carbon Look" effect (i.e. in which the structure of the weave is visible and the cured resin matrix is transparent and almost colorless) .

For example, a prestigious application of "Carbon Look" panels was that of the entire bodywork of the super sports car Pagani Zonda R (Italy 2007) . The use of prepreg makes it possible to achieve excellent surface finishes and to create the "Carbon Look" effect which is highly sought after by car manufacturers to enhance their products.

Decorative fiber-reinforced composites of the "Carbon Look" type are used for manufacturing decorative products, which use carbon fiber prepregs, prepregs containing thermosetting matrices, e.g. based on appropriately formulated epoxy resins, as raw materials .

After the curing process, the components must be transparent, colorless and free of defects, such as white spots and/or other types of imperfections, e.g. white dots.

Epoxy matrix prepregs typically contain dicyandiamide (DiCy) which acts as a cross-linker in the polymerization process.

DiCy is a powder which can be dispersed in the formulation as-is or in pasty form, i.e. previously dispersed in epoxy resin. It is insoluble in resin, and the matrix polymerization/cross-linking process leads to obtaining a polymer which contains DiCy in its structure, therefore in an ideal situation DiCy as-is, not present in the fully cross-linked composite.

Small amounts of unreacted dicyandiamide are sometimes are present during the manufacturing process of carbon-fiber reinforced composite components, i.e. in the polymerization of the matrix contained in the prepreg. It is a white powder and can cause defects, such as white spots and/or the presence of highly visible small spots on the black surface of the "Carbon Look" decorative composite. Components displaying these types of defects are discarded because they are not suitable for the application itself. The use of small quantities of DiCy, which however allow the polymerization of the matrix leading to achieving the required thermo-mechanical properties, is a strategy used to reduce the probability of occurrence of the aforesaid defects, without giving an absolute guarantee of obtaining defect-free parts.

The most obvious solution to the problem of defects such as white spots and/or small white dots on decorative "Carbon Look" products is to use cross linkers other than DiCy.

For example, Patents W02014020072A2, Alzchem GmbH, and W02012113879A1 , Alzchem GmbH describe the use of cyanamide . The use of cyanamide, however, displays some formulation difficulties, e.g. the preparation of matrices with viscosity higher than 25 Pa.s at 60 °C which maintain colorless properties. Furthermore, at present, cyanamide bears H361 labeling (Suspected of damaging fertility or the unborn child) , see https : //www . echa . europa . eu/web/guest/registration- dossier/-/registered-dossier/15823/2/1 .

It is the task of the present invention to provide a method for obtaining fiber-reinforced composites with an epoxy resin-based matrix for making "Carbon Look" decorative components, characterized by transparency and total absence of color, and which are free of white spots and/or dots.

Such an object was achieved by using a Lewis acid with latency properties, such as BCI₃. (N, N-dimethyl octylamine) as a polymerization catalyst, which surprisingly showed that the decorative components obtained after a curing process developed by the inventors, are not only free of defects, such as white spots and/or dots, but are also perfectly transparent and colorless, and thus able to meet the most sophisticated demands of the market.

It is worth noting that a Lewis acid (named after Gilbert Lewis) is any molecule or ion which is able to form a new coordination bond by accepting a pair of electrons (electrophile or electron acceptor) .

BCI₃. (N, N-dimethyl octylamine) is known as a catalyst for the polymerization of epoxy resins but its use has never been described to obtain decorative components made of fiber-reinforced composites of the "Carbon Look" type which are free of the defects mentioned above, and also have the property of being transparent and colorless.

Reinforced composite materials, intended for structural elements in which the emphasis is on heat resistance and durability, consisting of a layer of carbon fibers impregnated with an epoxy resin composition, where BCL . (N, N-dimethyl-optilamine) , is the hardening agent, are known from patents such as EP 2484715 Al, EP 857428 A1 and JP H05 239317. Epoxy resin can be a mixture of bisphenol type A, phenol-novolaks and/or thermoplastic polymers. The peculiar property of such composites is their mechanical strength and stability. In EP 2484715, which is precisely aimed at the production of fiber-based reinforced composite materials for industrial use, BCI3. (N, N-dimethyl octylamine) is suggested instead of imidazole which, as an alternative to Dicy, is used very often, in a fiber- reinforced composite material, as a transparent curing agent, to avoid white spots. This is because imidazole, like DiCy, cannot maintain the same heat resistance as epoxy resin. No suggestion is provided to the person skilled in the art about the use of BCI₃. (N, N-dimethyl octylamine) to achieve transparency and lack of color in the prepreg.

In the course of experimentation, the present inventors also verified that the use of this catalyst provides the further advantage of obtaining decorative components with high heat resistance, through the appropriate choice of epoxy resins as components of the formulation, because the cured composite, obtained from prepreg made with a suitably formulated epoxy resin matrix, can reach high glass transition temperature values, Tg, up to 170 °C, intended as the value of DMA- tand, (the tangent of the phase shift between stress and deformation in dynamic-mechanical analysis, DMA) .

Summary of the invention

It is thus a first object of the present invention to provide a method for obtaining prepregs for decorative components made in fiber-reinforced composite of the "Carbon Look" type, which are perfectly transparent and colorless, using a Lewis acid with latency properties, such as BCI₃. (N, N-dimethyl octylamine) as a catalyst for polymerization of the epoxy resins used as a matrix.

It is a second object of the present invention the use of BCI₃. (N, N-dimethyl octylamine) as a catalyst for the polymerization of epoxy resin matrices suitably formulated for manufacturing transparent, colorless "Carbon Look" decorative components, completely free of white spots and/or dots.

It is a third object of the present invention is the use of BCI3. (N, N-dimethyl octylamine) as a catalyst for the polymerization of epoxy resins impregnating the carbon fiber fabric, appropriately formulated to obtain transparent and colorless decorative components with high heat resistance.

It is a fourth object of the invention the formulation of epoxy resin matrices which, polymerized through the use of BCI₃. (N, N-dimethyl octylamine), can provide transparent and colorless composites with high heat resistance.

It is a fifth object of the invention the formulation of epoxy resin matrices which, polymerized through the use of BCI₃. (N, N-dimethyl octylamine), can provide composites with a low thermal expansion coefficient, can reduce the so-called fiber print- through phenomenon after prolonged exposure to accelerated aging cycles, and are therefore suitable for the making "Class A" parts for the automotive sector .

Indicatively, the linear CTE (coefficient of thermal expansion) range of the resin is less than 50 pm/m C .

Finally, it is a sixth object of the invention the formulation of epoxy resin matrices which, polymerized by using BCI₃. (N, N-dimethyl octylamine), can provide composites with high resistance to yellowing caused by prolonged exposure to UV radiation and/or heat, and which are therefore suitable for making "Class A" parts for the automotive sector.

Brief description of the figures

Figure 1 shows the DMA trace - tand curve - of the epoxy matrix fiber-reinforced composite containing BCI₃. (N, N-dimethyl octylamine) cured in autoclave 90 min @ 135 °C, pressure 6 bar. Carbon fiber weave 400g/m², resin content 36%. DMA according to ASTM D7028 performed on DMA Q850- TA Instruments.

Figure 2 shows the complex viscosity curve according to temperature ramp, 2 °C/min, the viscosity value at 60 °C and the minimum viscosity value. The values were determined using a DHR2 (TA Instruments) rheometer equipped with parallel plates in oscillatory mode. The viscosity values at 60 °C are in the range between 3 and 30 Pa.s, preferably between 15 and 25 Pa . s .

Figure 3 shows the transparency of the components in an epoxy matrix fiber-reinforced composite containing BCI₃. (N, N-dimethyl octylamine), determined in after the curing process in autoclave. After having evaluated several curing cycles ₍2 h at 110 °C, 1 h at 120 °C, and 90 min at 135 °C) , two images of the same component cured for 90 min at . 135°C are shown, taken under different lighting conditions.

Figs. 4 and 5 compare portions of fiber-reinforced composite according to the invention with a traditional prepreg under different magnification, which show both the presence of white spots and the transparency which makes it possible to see the reinforcement fibers. Prepregs obtained by impregnation with epoxy matrices containing the catalyst Bcl3.₃. (N, N-dimethyl octylamine) have good tack and good workability. The prepregs have been made by means of an impregnation process using the following techniques: Solvent impregnation - resin solution in methyl ethyl ketone, or hot-melt .

Detailed description of the invention

The invention described herein is aimed at a method for obtaining prepregs for decorative components in fiber-reinforced composite of the "Carbon Look" type using BCI₃. (N, N-dimethyl octylamine) as a catalyst for polymerization of the epoxy resins used as a matrix. The catalyst is in solid form at room temperature and is made liquid by heating in an oven at 38-40 °C.

As mentioned, epoxy resins containing dicyandiamide (DiCy) as a cross-linking agent can generally lead to defects, such as white spots and/or white dots on the cured decorative components which lead to the rejection of the components themselves.

The choice of the cross-linking agent of the epoxy resin according to the invention derives from the need to overcome such a technical problem, thereby eliminating the presence of white spots and/or white dots on the decorative "Carbon Look" components.

Indeed, according to the present invention, by using BCI₃. (N, N-dimethyl octylamine) as a catalyst for the polymerization of epoxy resins, a prepreg is made constituted by carbon fiber weaves impregnated with epoxy resins which are completely free of DiCy, and provides a transparent coating which make it possible to obtain "Carbon Look" decorative components which are colorless and free of white spots and/or white dots.

Furthermore, according to another aspect of the invention, an appropriately formulated epoxy resin matrix is provided for a coating characterized by high thermal resistance because the cured composite obtained from prepreg made with such an appropriately formulated matrix can reach high glass transition temperature values, Tg, up to 170 °C, understood as the value of

DMA-tand .

In a particularly preferred embodiment, the method according to the invention provides the use of BCI₃. (N, N-dimethyl octylamine) as a catalyst for polymerization of epoxy resins appropriately formulated in quantities between 4.0 and 11.0%, preferably between 5.0 and 8.0% by mass to obtain decorative components of the "Carbon Look" type which are transparent and colorless .

Furthermore, in combination with BCI₃. (N,N- dimethyl octylamine) , different formulations of the matrix of curable epoxy resins have been identified which can guarantee high heat resistance in the decorative components obtained through the application of the invention, such as:

- mixtures of bisphenol-A epoxy resins, solid and liquid,

- epoxy resin mixtures from phenol-novolaks,

- mixtures of epoxy resins containing thermoplastic polymers,

All the formulations identified fall within the ranges in percentages by mass shown in the following table .

The formulation of epoxy resin matrices when polymerized using BCI₃. (N, N-dimethyl octylamine) can provide composites with a low thermal expansion coefficient, in order to reduce the so-called fiber print-through phenomenon after prolonged exposure to accelerated aging cycles, the previous formulation is modified as follows:

Finally, as mentioned, it is a further object of the invention the formulation of epoxy resin matrices which, when polymerized by using BCI₃. (N, N-dimethyl octylamine) can provide composites with high resistance to yellowing caused by prolonged exposure to UV radiation and/or heat and therefore suited to making "Class A" parts for the automotive sector.

The following formulation was defined for this purpose:

The CIELab colorimetric analysis on carbon fiber composite laminate, after prolonged exposure to 140 °C for 7 days in a ventilated stove, is provided by way of example :

The visual grayscale evaluation on carbon fiber composite laminate, after exposure according to SAEJ2020 for 100 hours is provided by way of example

The method according to the present invention can be used by means of a solvent impregnation process, with epoxy resin solution in methyl ethyl ketone (Process A) or hot melt (Process B) according to production requirements. Both processes, which are described below, make it possible to obtain prepregs with good tack and good workability.

Prepreg production by solvent impregnation (Process A)

Step 1 : Resin preparation

In a steel container, a solution of poly (phenyl glycidyl ether) -co-formaldehyde) in solution in methyl ethyl ketone or acetone, a solution of solid poly (bisphenol A-co-epichlorohydrin) in the form of solution in methyl ketone or acetone, and a solution of liquid poly (bisphenol A-co-epichlorohydrin) is added; b) the mixture is maintained under stirring for 20 minutes ;

c) additives between 0 and 15% and then BCI₃. (N,N- dimethyl octylamine) between 4% and 11% by mass are adding under stirring, leaving the mixture under stirring for 20 min;

d) Brookfield viscosity is measured and, if necessary, further solvent, methyl ethyl ketone or acetone, is added to reach the Brookfield viscosity value (typically between 70 and 400 cP) which is optimal for the solvent impregnation process.

Step 2 : Solvent impregnation

e) the resin is poured into the impregnation tank in which the carbon fiber weave will be impregnated by immersion;

f) after the passage in the impregnation tank, the excess of resin solution is eliminated by means of squeezing rollers;

g) the resin-wetted weave passes through one or more ovens at temperatures varying between 80 and 115 °C, depending on the weight of the weave being impregnated and of the solvent used;

h) at the exit from the last oven the weave is rolled together with a separating film, above and below the prepreg, around a cardboard core.

Step 2 : Solvent impregnation

The resin is poured into the impregnation tank in which the carbon fiber weave will be impregnated by immersion. After the passage in the impregnation tank, the excess of resinous solution is eliminated by means of squeezing rollers. The resin-wetted weave passes through one or more ovens at temperatures varying between 95 and 115 °C, depending on the weight of the weave being impregnated. At the exit from the last oven, the weave is rolled together with a separating film, above and below the prepreg, about a cardboard core .

Therefore, in short, the method according to the invention is based on the use of a catalyst BCI₃. (N,N- dimethyl octylamine) for the polymerization of epoxyresins in order to obtain decorative components in fiber-reinforced composite of the "Carbon Look" type.

Prepreg production by hot melt impregnation (Process B)

Resin preparation - a) Poly (phenyl glycidyl ether) -co-formaldehyde) , previously heated to 80-100 °C, solid poly (bisphenol A- co-epichlorohydrin) , and liquid poly (bisphenol A-co- epichlorohydrin) are added in a container;

b) the obtained mixture is subjected to hot stirring, typically at 90-140°C, until complete dissolution of the solid resin;

(c) one or more additives and then BCI₃. (N,N- dimethyloctylamine) , previously made liquid by heating to 40 °C, with maximum temperature during mixing equal to 60-80 °C are added to said homogeneous solution, again under agitation.

Resinous film preparation

f) The resinous film is prepared by pouring the mixed resin so obtained between calenders heated between 40 °C and 90 °C, after setting up the gap between said calenders to obtain the desired film weight on silicon paper;

g) the silicon paper is rolled up around a cardboard core .

Prepreg preparation

h) The hot resin film (temperature between 55 °C and 65 °C) is coupled with the carbon fiber weave of the desired weight.

Experimental part

The invention will now be further described by means of the following non-limiting, exclusively indicative examples.

Example 1 - Formulation 1

The reactivity of the epoxy matrix relative to formulation 1 was determined by differential scanning calorimetry (DSC) on DSC Q 2000 - TA Instruments according to ASTM E2160. For the purpose of the test, the formulation was prepared free of solvent, methyl ethyl ketone. The values measured are: Entalpic onset = 123,95 °C, Entalpic peak = 152.43 °C and Enthalpy =

371.5 J/g. The analysis of complex viscosity values at 60 °C, determined by the DHR2 rheometer (TA

Instruments) equipped with parallel plates in oscillatory mode. These values allow the correct impregnation process to be carried out and prepregs to be obtained for the making of "Carbon Look" type decorative components free of defects by means of polymerization, after lamination in special molds in an autoclave . Example 2 -

In a particularly preferred embodiment, the method according to the invention provides the use of BCI₃. (N, N-dimethyl octylamine) as a catalyst for polymerization of epoxy resins appropriately formulated in quantities between 4.0 and 11.0%, preferably in the narrower range between 5.0 and 8.0% by mass to obtain decorative components of the "Carbon Look" type which are transparent and colorless.

Example 3 -

The cured composite obtained from prepregs made with an appropriately formulated matrix can reach high glass transition temperature values, Tg, up to 170 °C, intended as a value of DMA-tand. The DMA curve is shown in Fig . 1.

Example 4 -

Formulations containing mixtures of bisphenol-A epoxy resins, solid and liquid, phenol-novolaks, thermoplastic polymers, e.g. phenoxylic resin in combination with BCI₃. (N, N-dimethyl octylamine) . The complex viscosity values at 60 ^c determined using a DHR2 rheometer (TA Instruments) equipped with parallel plates in oscillatory mode range from 3 to 30 Pa.s.

Example 5 -

The transparency of the components was determined after curing in autoclave. Several curing cycles were evaluated, e.g. 2 h at 110 °C, 1 h at 120 0, 90 min at 135 °C. The prepregs containing the epoxy matrices contain the catalyst BCI₃. (N, N-dimethyl octylamine) . The transparency properties of the components are shown in fig. 3.

Claims

1) A prepreg for "Carbon Look" decorative components, comprising at least one layer of carbon fibers and a thermosetting resin matrix, the thermosetting matrix at least partially impregnating the at least one carbon fiber layer, wherein the matrix comprises at least one epoxy group and a curing agent (polymerizing agent) to cure the curable thermosetting resin or resins, characterized in that, in order to prevent defects on said decorative components, such as spots and dots, and to ensure that the components themselves are completely transparent and colorless, the curing agent present dissolved in the matrix consists of a Lewis acid having latency properties.

2) A prepreg according to claim 1, characterized in that the curing agent (polymerizing agent) is BCI₃. (N, N-dimethyl octylamine) . 3) A prepreg according to any one of the preceding claims, characterized in that the curable thermosetting resin matrix is constituted by a mixture selected from at least one mixture of: epoxy resins from bisphenol - A, solid and liquid, epoxy resins from phenol-novolaks, and resins from thermoplastic polymers.

4) A prepreg according to any one of the preceding claims, characterized in that the thermosetting matrix has the following formulation:

5) A prepreg according to any one of the preceding claims, characterized in that the BCI₃· (N, N-dimethyl octylamine) is present in amounts ranging from 4.0 to 11.0% by mass of the epoxy resins used.

6) A prepreg according to any one of the claims from 1 to 4, characterized in that, to have a low thermal expansion coefficient, in order to reduce the fiber print-through phenomenon after prolonged exposure to accelerated aging cycles, the thermosetting matrix has the following formulation:

7) A prepreg according to any one of the claims from 1 to 4, characterized in that, to provide composites with high resistance to yellowing caused by prolonged exposure to UV radiation and/or heat and therefore suitable for making "Class A" parts for the automotive sector, the thermosetting matrix has the following formulation:

8) A cured composite obtained from prepreg manufactured with matrices according to any one of the preceding claims, characterized in that it reaches glass transition temperature (Tg) values up to 170 °C, intended as the value of DMA-tand.

9) A method for making a carbon fiber-reinforced composite material, of the "Carbon Look" type, transparent, colorless and free of white spots, by polymerization of the impregnation matrix comprising one or more thermosetting resins, with at least one epoxy group, characterized in that it comprises the steps of :

a) adding, in a container to a solution of poly (phenyl glycidyl ether) -co-formaldehyde) in solution in methyl ethyl ketone or acetone, a solution of solid poly (bisphenol A-co-epichlorohydrin) in solution form in methyl ethyl ketone or acetone, and a solution of liquid poly (bisphenol A-co-epichlorohydrin) ;

b) maintaining the mixture under stirring for 20 minutes ;

c) adding under stirring additives between 0 and 15% and then BCI₃. (N, N-dimethyl octylamine) between 4% and 11% by mass, leaving the mixture under stirring for 20 min;

d) measuring Brookfield viscosity and, if necessary, further solvent, methyl ethyl ketone or acetone, is added to reach the Brookfield viscosity value (typically between 70 and 400 cP) which is optimal for the solvent impregnation process;

10) Use, in a prepreg comprising at least one layer of carbon fibers and a thermosetting resin matrix comprising at least one epoxy group, the thermosetting resin matrix at least partially impregnating the at least one carbon fiber layer, of BCI₃· (N, N-dimethyl octylamine) as a catalyst (polymerizing agent) of the epoxy resins used as an impregnation matrix, to ensure the transparency and avoid the presence of defects, such as white spots and dots in the decorative components manufactured with such prepreg.

11) Use of BCI3. (N, N-dimethyl octylamine) according to the preceding claim, wherein the curable thermosetting resin matrix is constituted by a mixture selected from at least one mixture of: epoxy resins from bisphenol - A, solid and liquid, epoxy resins from phenol-novolaks, and resins from thermoplastic polymers . 12) A method for making of a carbon fiber- reinforced composite material, which is transparent, colorless and free of white spots, by polymerization of the impregnation matrix comprising one or more thermosetting resins, with at least one epoxy group, characterized in that it comprises the steps of:

a) adding in a container poly (phenyl glycidyl ether) -co-formaldehyde) , previously heated to 80-100 °C, solid poly (bisphenol A-co-epichlorohydrin) , and liquid poly (bisphenol A-co-epichlorohydrin);

b) subjecting the obtained mixture to hot stirring, typically at 90-140°C, until complete dissolution of the solid resin;

(c) adding to said homogeneous solution, always under agitation, one or more additives and then

BCI₃. (N, N-dimethyloctylamine) , previously made liquid by heating to 40 °C, with a maximum temperature during mixing equal to 60-80 °C.

f) preparing the resinous film by pouring the mixed resin thus obtained between calenders heated between 40 °C and 90 °C, after setting up the gap between said calenders to obtain the desired film weight on silicon paper;

g) rolling the silicon paper about a cardboard core;

h) coupling the hot resin film (temperature between 55 °C and 65 °C) with the carbon fiber weave of the desired weight.