BRPI1001780B1 - carbon tubular material, composites containing carbon, uses and processes for their manufacture - Google Patents

Abstract

carbon tubular material, composites containing carbon, uses and processes for their manufacture. The present invention provides carbon tubular materials (c), as well as composites containing them, their uses and processes for obtaining them. The processes of the present invention make use of a precursor which is a polymeric material having a gradient of physicochemical property in its cross section. such a precursor is carbonized to obtain c-tubular materials while retaining the original matrix structure and reinforcement. The products of the invention are endowed with peculiar physicochemical properties and are useful in various applications including the production of composites containing tubular c. Of particular interest in the present invention are disclosed composites comprising c-reinforced hollow fibers, material which is of great applicability in the aerospace segment among others.

Description

Invention Patent Descriptive Report ‘

Tubular Carbon Material, Composites containing it, Uses and Processes for Obtaining It

Field of the Invention

The present invention relates to special materials and means for obtaining them. More specifically, the present invention relates to tubular carbon (C) materials, as well as composites containing them, their uses and processes for obtaining them. The processes of the present invention make use of a precursor that is a polymeric material with a gradient of physical-chemical property in its cross section. Such a precursor is carbonized to obtain C tubular materials, maintaining the original matrix structure and reinforcement. The products of the invention are endowed with peculiar physical-chemical properties, being useful in several applications, including the production of composites containing tubular carbon materials. Of special interest in the present invention, composites are disclosed comprising carbon reinforced with hollow carbon fibers, a material that is highly applicable in the aerospace segment, among others.

Background of the Invention

The carbon element has an atomic weight of 12.011 and is the sixth element in the periodic table. It has three known isotopes C ¹² , C ¹³ and C ¹⁴ . Of these, C ¹² is naturally used as a reference because it occurs in 99% of the cases, C ¹³ is the one that has magnetic moments and the two are stable, while C ¹⁴ is the radioactive. The carbon element is well known for its anomalous properties, including its ability to embrace the three major divisions of materials science: polymers, ceramics and metals. It forms strong bonds between its atoms (bonds between carbon CC atoms) mainly in organic chemistry and polymers; the carbon element also appears in allotropic form like diamond and graphite, which is highly refractory and remains solid above the melting point of most ceramics, allowing the

2/28 graphite crystal has electrical and thermal properties comparable to metals.

Carbon as a solid is the only substance that can be manufactured to exhibit the largest and widest variety of different structures and properties. Some structures can be extremely resistant, hard and rigid, while other shapes are soft and ductile. Many carbonaceous materials are porous, exhibiting a large surface area, while others are impermeable to liquids and gases. These variations are the result of structural effects such as: number of defects, geometry and number of phases with changes in the crystalline order.

In the form of hexagonal crystals, commonly called polycrystalline graphite, carbon finds a wide range of industrial applications. The application of this material is based on an attractive combination of properties that graphite offers. Due to its good electrical conductivity, motors are used as electrodes in electro-refinement processes and brush for electrics, as heat exchangers, due to their high conductivity in the metallurgical casting process due to their chemical and high thermal inertia, resistance at high temperatures , among others. Despite an extensive list of potential applications, graphite suffers serious restrictions for use as a structural material due to its mechanical fragility.

Currently, carbon is probably the most extraordinary chemical element known to science, being present in a wide variety of materials essential for our society such as tar, oil, diamond and, more recently, in carbon fibers and carbon fibers reinforcing carbon, also called carbon / carbon composite (C / C). The composite C / C can be defined as an engineering material composed of a carbon or graphitic matrix reinforced with carbon or graphitic material. It was first synthesized in the 1950s, but only in the 1960s was it explored in the space shuttle program for its extraordinary properties. The C / C composite has the properties of monolithic graphites, such as: high resistance to thermal shock and retention of

3/28 mechanical properties at high temperatures, in non-oxidizing environments, combined with high mechanical strength obtained by reinforcing fibers. The C / C composite exhibits excellent mechanical properties at high temperatures. The high specific resistance (ratio between mechanical resistance and specific mass) and the maintenance of resistance at high temperatures make the C / C composite extremely attractive as a structural material for high temperature applications such as: engines, aircraft and spacecraft. Other advantages of this composite include high resistance to thermal shock, high thermal conductivity and high fracture toughness. The C / C composite is stable at το temperatures on the order of 3000 ° C in vacuum or inert environments. However, this material has the disadvantage of being very reactive in oxidizing atmospheres at temperatures close to 500 ° C. This disadvantage limits use in high temperatures and oxidizing environments.

The most accepted method for protecting the C / C composite against oxidation involves covering the surface with an appropriate refractory material to prevent oxygen from attacking the substrate. Ceramic coatings used are; SiC, SÍ3N4, oxide glass layers, have been studied and shown to be somewhat successful. But two problems are associated with this technique: the failure of protection at low temperatures 20 (<600 ° C) and 0 appearance of cracks due to the stresses developed by the difference in thermal expansion between the substrate and the ceramic film, opening the way for oxygen penetration. An idea to circumvent these problems is the use of coatings with multilayers, which consist of two main layers, that is, the silicon carbide (SiC) in the inner layer and a refractory oxide in the outer layer.

C / C Composites Manufacturing

The classic method of manufacturing carbonaceous materials is similar to that used in ceramic processing. Particles of pure carbon (primary carbon) are combined with a temporary binder. This binder acts as a precursor to the secondary carbon that forms during the carbonization treatment. As a result, a material is obtained

4/28 α »’ exclusively by carbon in two distinct phases: the primary carbon, also called filler carbon, surrounded by the secondary carbon that acts as a binder for the particles of the first. In the case of the C / C composite, carbon fibers are used as primary carbon, arranged in a spatial distribution called a preform.

Two methods are used to form the matrix (secondary carbon) of the C / C composite. The first is based on the impregnation of the fiber with a liquid carbonaceous material that is converted into residual carbon via heat treatment in an inert atmosphere. In the second method, the carbon fibers are first heated and then exposed to gaseous organic compounds, usually hydrocarbons that decompose to deposit a layer of pyrolytic carbon on the surface of the fibers.

The carbon fibers that are commercially available are divided into 3 categories: general purpose; high performance; and activated carbon fibers. General purpose types are characterized by an amorphous and isotropic structure, low tensile strength, low modulus in traction and low cost. High performance fibers are characterized by high modulus and high strength. The largest modulus of these fibers is associated with the largest portion of graphite and the highest anisotropy. Activated carbon fibers are characterized by the presence of a large number of open micro porosities that act as absorption sites. The absorption capacity of activated carbon fiber is compared to that of activated carbon, but the shape of the fiber allows the material to be absorbed to reach the absorption site more quickly, thus accelerating the adsorption or desorption process. The commercial treatment of the activated carbon fiber is done with sulfuric acid followed by heat treatment above 500 ° C.

Although many materials can be used as precursors to carbon fibers, only Rayon, Tar and PAN are considered satisfactory for commercial processes. The carbon yield for these three fiber production paths are shown in Table 1.

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Table 1 - Carbon yield for several carbon fiber precursors.

Precursor Yield% of process Rayon 20-30 Tar 75-50 PAN 45-50

Rayon-based carbon fibers were the first to be produced by Union Carbide by stretching and graphitization. Isotropic tar and Rayon precursors are used to produce low modulus fibers (<60 GPa). The fibers of both types can be stretched at high temperatures to increase their modulus substantially, but the process is not operated commercially.

Over a period the conversion of polyacrylonitrile fiber (PAN) into high modulus carbon fibers was investigated and led to the large-scale production of more resistant carbon fibers. Research on a low-cost process shows the possibility of using pitch as a precursor to carbon fibers. High modulus fibers are manufactured from PAN or tar mesophase as a precursor, in both cases the oriented precursor of the fiber is stabilized by a slight oxidation to cure the fibers and carbonization at temperatures above 800 ° C for the production of the fibers. carbon. The fiber modulus is increased with an increase in the heat treatment temperature.

Synthetic polymers such as polyacrylonitrile and acrylic copolymers are considerably interesting as precursors to carbon fibers. One fact is that a large amount of high modulus fibers is produced by converting polyacrylonitrile. In this context, the material used to form carbon fibers is not the acrylonitrile homopolymer, which is quite difficult to dissolve, but the copolymer containing more than 85% acrylonitrile by weight.

Two basic processes are known for obtaining polyacrylonitrile fiber. In the dry process, the precursor is melted and extruded through ^{6 Ζ 28} ..φ ί

It is a die that contains a number of small capillaries. When leaving the die the polymer cools and solidifies in the form of fibers. In wet spinning, a concentrated solution of the polymer, dissolved in an appropriate solvent to form a solution with viscosity suitable for the spinning process, is extruded through a die in a coagulation bath. The solvent dilutes the coagulation solution better than the precursor (it is more soluble in the coagulation bath) and, thus, the polymer solution that emerges from the capillaries precipitates as fibers.

In 1961 Shindo obtained carbon fibers through the pyrolysis of pre-oxidized PAN fibers. In the same period, Watt and Johnson showed that a first step of conversion to air and temperature between 150-400 ° C was of major importance in carbon yield and in the orientation of the polymer chains that form the graphite lamellae. Consequently, in said process stabilization and orientation of the polymer chains occur simultaneously. Today 90% of the available carbon fibers are made with the PAN precursor.

Two Japanese companies, Nippon Carbon and Tokai Denkyoku started manufacturing on a small scale in 1969 with the CarbolonZ and Thermolon-S products that had a tensile strength of 1.8-2.8 GPa and a module of 350-410 GPa.

Starting from the textile fiber of specific mass 1.17-1.19 g.cnT ^3, which is white after being stabilized in air for a few hours at temperatures of 200300 ° C, color changes from white to yellow can be observed, continuing the process the fiber turns reddish-brown to black. Stabilization tests include specific mass measurements or aromatization index based on the open angle in the X-ray diffraction measurement. Correctly stabilized fibers will have a specific mass of 1.35-1.39 g cm ' ³ depending on the precursor used. Heating in an inert atmosphere, without tension and at a temperature of 1000-1200 ° C, the carbon fibers will have a module of 200-230 GPa. The subsequent heating in inert conditions results in increases in the module directly proportional to the treatment temperature.

7/28 thermal. Treatments at temperatures of 2600-2800 ° C produce fibers with ' ^k modules greater than 400GPa.

Briefly, Ozbek concludes that the specific mass of PAN precursor carbon fibers, when hot stretched, can be related to microstructural parameters such as preferred orientation and apparent crystallinity size. However, there seems to be a much more complex relationship with the mechanical properties and specific process parameters that are used to induce changes.

The mechanical properties of carbon fibers are strongly dependent on the orientation of the carbon chains. High values of modulus of elasticity and tensile strength will only be obtained when the packing arrangement has few defects. In carbon fiber, obtaining this better orientation combined with few defects depends fundamentally on the system used during the various stages of the conversion of precursor 15 (PAN). During the various stages of precursor stabilization, this material becomes highly susceptible to acquire small defects that will have a marked influence on the mechanical properties of the carbon fiber.

The first step in converting PAN fiber to carbon fiber is stretching, where better orientation of the carbon chain is obtained. In this 20 stage, the precursor is heat treated at 180 ° C and subjected to constant tension, with no chemical transformation, but only alignment I of the chains.

The second stage is pre-stabilization, where the open and already aligned carbon chains start the formation of ring chains, with PAN nitrogen being one of the components of the rings. With a temperature around 230 ° C in an oxidizing atmosphere, the density of the material increases by approximately 10%. The third phase of heat treatment is called stabilization or oxidation, and is carried out between 260 and 300 ° C, in an oxidizing atmosphere with the ring material reacting with oxygen changing its chemical composition, however maintaining the rings. Specific gravity increases by approximately 10%, which makes the stabilized precursor ideal

8/28

Ç? · <

FIs.

Φ Rub: -JL— * K. y to obtain the carbon fiber. It was agreed to call the '' thermal treatment between 200 and 300 ° C which involves the second and third stages of oxidative stabilization.

The last stage of conversion is carbonization, which occurs at temperatures of approximately 1000 ° C in an inert atmosphere, where the other chemical elements are eliminated, leaving only carbon as the graphical structure.

The manufacture of carbon fibers using PAN as a precursor can be carried out under different conditions, with different temperatures at each stage.

Oxidative stabilization of PAN

Carbon fibers that use PAN as a precursor have been extensively applied in the last two decades in composite technology. They are highly desirable for high performance composites in automotive and aerospace technologies due to their excellent physical and mechanical characteristics. During the conversion of PAN fiber to carbon fiber there is a crucial stage called oxidative stabilization. This stabilization is usually carried out at temperatures between 200 ° C and 300 ° C in an oxygen atmosphere or in air, under tension for an appropriate time 20, producing infusible and fire-resistant fibers. Temperatures below 200 ° C are not suitable from the process point of view, as they require long periods of time, while temperatures above 300 ° C cause violent exothermic reactions with significant loss of mass and formation of tar-derived by-products.

The bundles of polyacrylonitrile fibers used in the stabilization step are formed by multifilaments of the PAN copolymer fibers and the number of filaments in them depends on their application. In oxidative stabilization, the precursor fibers must be processed for one to two hours in ovens with temperatures between 200 and 300 ° C, in addition to controlling the air flow, the stresses must be carefully controlled. Since the stabilization reactions are exothermic, the temperature control of the fiber is

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fundamental. If the temperature is too low, reactions are slow and can '''·' S '' ^Λ 'result in incomplete stabilization. If the temperature is too high, the fiber can overheat and melt, or even burn.

There are alternatives for heat treatment during stabilization: the first is isothermal stabilization, which was the first method to produce carbon fibers in a continuous process. Some works have used the passage of the precursor PAN beam through an oven divided into several zones with an increasing temperature profile, the specific mass of the precursor increases continuously.

jo The continuous processes for stabilizing PAN fibers are all based on the idea of passing bundles of fibers through heated zones. The first oven sketch consists of a basic zone with multiple passages through this same zone. The beam can be oriented horizontally or vertically in the oven, air is circulated in the oven to control heat and mass transfer. Stabilization furnaces are known which contain a number of zones, at different temperatures. This concept of multiple zones with temperature stages is probably used in all business processes.

The oxidative stabilization stage is the longest and most critical step 20 in the process of transforming the PAN fiber into carbon fiber because of the need to control the (considerable) amount of heat evolution that ¹ accompanies the PAN degradation reaction. . At this stage, the initially fused thermoplastic becomes infusible by the dehydrogenation, cyclization, oxidation and cross-linking reaction, such that it can withstand the carbonization process with minimal carbon loss, while retaining its fibrillar structure and orientation. There are authors who propose that preventing the fiber from retracting during stabilization has three consequences:

- Induces the formation of smaller or smaller pores at the entrance of the pore of the fibers being stabilized, thus decreasing the speed of diffusion of the gaseous oxygen and hindering the subsequent reaction with oxygen;

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ϊ.-í FIs. Rub: ._.

- Increases the specific nitrogen removal during carbonization according to the Watt’s mechanism (partly responsible for the good mechanical properties such as the tensile strength of the resulting carbon fibers) bringing the graphite lamellas closer together; and

3 - Increase the mutual alignment of the polymer chains with respect to the fiber axis, inducing the high anisotropy in the resulting carbon fiber, which is responsible for the best elasticity module and transport properties (Thermal and Electrical Conductivity).

Appropriate conditions, such as heating rate, treatment time, heating temperature and oxygen diffusion, must be established to optimize the oxidative stabilization of each precursor.

The presence of oxygen promotes the appearance of cross-links and the formation of cyclic and aromatic sequences by eliminating water, which are essential for the formation of the basal planes in carbon fibers.

The application of tension during the oxidation of the precursor is important in preventing the relaxation of the chains and the consequent loss of orientation during the process of forming cross-links, where physical and chemical changes occur in the structure of the polymer chains.

A fiber is considered to be adequately stabilized when the oxygen content is in the range of 14% to 20%. Above these values, deterioration of the fiber may occur with a decrease in the mechanical and structural properties of the carbon fiber. The time and temperature of the heat treatment are determining factors in the stabilization process.

The temperature profile in the different regions of the oven, associated with the residence time of the fiber inside the oven, are of fundamental importance in the search for the optimization of the process of stabilization of the precursor fiber.

Several and intensive studies carried out in the 70's summarized the extensive bibliographic references, which relate the course of the stabilization reactions. Three possibilities are discussed for the temporal sequence of the course of the two most important partial reactions, cyclization and

11/28 dehydrogenation, macromolecule:

without considering the incorporation of

a) Initially cyclization and then dehydrogenation;

b) Initially dehydrogenation and then cyclization; and

c) Simultaneously cyclization and dehydrogenation.

Most authors supported the hypothetical reaction sequence postulated by Grassie and his collaborators between 1956 and 1972, that is, the primary cyclization preceding oxidative dehydrogenation, and the studies were based on the analysis of intermediate solid products.

Physical and chemical changes during the stabilization of PAN fibers inevitably bring about structural changes that can be detected by different techniques. X-ray diffraction can be used for its ease and short time for analysis, but also analyzes such as infrared spectroscopy, differential scanning calorimetry analysis and specific mass measurements can be used to assess the degree of stabilization (GE) of the fibers. PAN. X-ray diffraction and microscopy methods have been used to study the morphological changes that occur during stabilization.

The intensity of the main equatorial reflection at 2Θ = 17 ° from the plane (100) of the PAN unit cell, shows an initial increase followed by a continuous decrease during stabilization. The reflection intensity (100) in the azimuth and radial directions has been used to estimate, respectively, the size and orientation of the phase with the lateral ordering in the precursor. Subsequent changes during the precursor stabilization can be measured depending on the stabilization time or temperature. This method, however, is limited to the initial part of the stabilization, due to the complete disappearance of this reflection in the final stage of stabilization. Rose observed an initial increase followed by a continuous decrease in the orientation of the ordered phase as a function of the temperature of the heat treatment during stabilization in air, showing a correlation with the extent of the reactions.

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Carbonization and Graphitization

Carbonization is a process usually done in inert gas or in a vacuum. Two types of reactions occur mainly: the removal of the heteroatom in the polymer chain and the development of a graphitic structure. This process is carried out at temperatures between 1000-1500 ° C. A typical carbonization process for producing intermediate module fibers would carbonize at 1000 ° C followed by a heat treatment at »1300 ° C to increase the tensile strength. Some authors cite higher temperatures «1700 ° C. Carbonization often requires an inert atmosphere such as "medicinally pure" nitrogen or orderly argon; to prevent oxidation due to air access, and to dilute the extremely toxic gases wasted in the previous extraction.

Fiber loses most of its carbon element in the form of volatile gases, but some nitrogen remains in the high-resistance variety. The carbonization rate must be very well controlled to prevent the escape of gases that form defects in the fiber. Pores are created in the fiber due to the formation and perfection of the layered carbon structure. The specific gravity of the fiber first increases and then reduces because the open pores that were created first are later converted into closed pores.

Heat treatments above 1800 ° C should be considered as graphitization. Graffitiization is again a direct process, done in an inert atmosphere under tension. There is very little evolution of gas, where the main change is in the physical structure of the fibers. Microcrystals grow in size and the preferred orientation is that of the basal plane of carbon. The fiber can be considered as transforming to a graphite structure. Stretching of fibers by applying a satisfactory tension has been shown to aid in the transformation and further increase in alignment and in this way of the module. Care must be taken, however, to avoid breaking the fibers due to excessive stretching.

The high temperatures of the carbonization process of the fibers not only lead to a high modulus of the fibers, but also cause ^{13/28 / “} .4.

_Ç A RubL..J | L ----- '%>. θ \ densification of the fiber structure. The specific mass of carbon fiber with <4 · \ a PAN as a base varies between 1.7-1.9 g.cm ' ³ , depending on the final process temperature. The representation of properties for various classes of PAN fibers are given in Table 2.

Table 2 - Properties of carbon fibers from PAN as a precursor.

properties Low modulus Intermediate Module High modulus Axis Tensile strength (GPa) 3.3 4-5 2.4 Traction module (GPa) 230 270 390 Elongation at break (%) 1.4 1.7-1.9 0.6 Thermal conductivity (Wm ' ¹ K' ¹ ) 8.5 - 70 Electrical resistivity (Qm) 18 - 9.5 CTE at 21 ° C (10 ' ⁶ K' ¹ ) 0.7 - -0.5 Transversal Traction module (GPa) 40 - 21 CTE at 50 ° C (10 ' ⁶ K' ¹ ) 10 - 7 Dense body Specific mass (g m ' ³ ) 1.76 1.8 1.9 Fiber diameter (μηι) 7-8 6-7 4-6 Carbon (%) 92 96 100

Carbonaceous Matrices

The carbon matrix can be derived from tar, resin or carbonaceous gases. Depending on the carbonization / graphitization temperature, the carbon matrix can vary from amorphous to graphitic. With the highest degree of graphitization of the carbon matrix, greater is the resistance to oxidation and thermal conductivity, but greater is the fragility of the material. As the fibers used can be highly graphitic, they tend to be more resistant to oxidation. The carbon matrix limits the oxidation resistance of the composite.

The impregnation of carbon in the preform is carried out by infiltrating the preform with substances containing carbon in the form of liquids or gases, which are decomposed into carbon products in the body of the preform. The gases normally used are hydrocarbons and the conditions are in order to ensure penetration into the heart of the preform. The liquids used are piches, phenolic resin solutions, used because of their fluency.

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The technique used varies from manufacturer to manufacturer, component size and application.

The liquid impregnation can be carried out in an autoclave or by hot isostatic pressing. The characteristics to be considered when choosing the matrix precursor are: viscosity, percentage of residual carbon, matrix microstructure and the possibility of formation of crystalline structures. For a good impregnation, a good wetting of the substrate is necessary to fill the pores.

After curing, the impregnation component is pyrolysed in the process known as carbonization, resulting in a residual carbon matrix. The carbonaceous matrices whose precursors have solid materials as a starting point are represented by thermoset polymers and thermoplastic niches. Table 3 shows the different types of thermoset matrices and niches that can be used to obtain carbons, showing the carbon content in the molecule and its corresponding yield in final carbon after pyrolysis.

The carbons obtained by solid-state pyrolysis of thermoset polymers with a high degree of aromaticity and cross-linking form structures called "glassy carbon". The microstructure of the matrix produced by the 20 pyrolysis of thermoset (phenolic) resins varies according to the treatment temperature. When carbonized separately, the thermoset resin 'forms isotropic glassy carbon. On the other hand, when treated in the form of composites, the presence of the fibers, a graphical material is observed in the region of the matrix interface with the fibers.

Table 3 - Organic precursors for carbonaceous matrices.

Matrix

Carbon content

Thermosetting

Polybenzimidazole (PBI) Polyarylacetylene (PAA) Phenolic polyfurfural alcohol

1% 1

Carbon yield after pyrolysis (%)

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Epoxy-novolaca 74 55 Polyimide 77 49 Piches Coal Tar 90 56 Vegetable Tar 69 30 Oil (0.1 MPa) 88 50 Oil (10 MPa) 88 80 Mesophasic > 90 85 Synthetic Piches T ruxene 95 87 Isotruxene 95 70 Resin / Tar Blends Phenolic (60%) 78 75 Polyfurfural alcohol - 67 (60%) Novolac Epoxy (60%) - 60

The most evident mechanical characteristic of these materials is the fragile fracture. Except for adequate deformation conditions, they are not graffiti. The representative of the class of polymers most frequently used to obtain glassy carbon is phenolic resin. Table 4 shows the 5 changes in the properties of phenolic resins after pyrolysis at 1000 ° C.

Table 4 - Properties of phenolic resin and vitreous carbon obtained from resin pyrolysis.

Property Molded Pyrolyzed at 1000 ° C Specific mass (gcnT ³ ) 1.25 1.50 Tensile strength (MPa) 60 110 Modulus of elasticity 4.0 30

The thermal conversion of niches into graphical material can be briefly described as follows. At temperatures close to 55010 600 ° C, the pitch is transformed into an infusible and thermoset material called semi-coke. If pyrolysis is carried out up to 1000 ° C and atmospheric pressure, a loss of mass equivalent to 50% of the starting material occurs. Heat treatments above this temperature cause benzene chains to continuously orient themselves in a preferred direction

16/28 _v da> _Jo up to temperatures of 3000 ° C, exhibiting at that temperature a structure close to that of the ideal graphite crystal.

“Phenolic” phenol-formadehyde resin is the oldest of completely synthetic polymers. The phenolic resin class is widely used in various applications in products such as wood, molding powders, and insulators. Use in laminates is relatively less of the applications for phenolics, approximately 7% of total use. The majority of the use of phenolic resins is as fillers or reinforcements of fibrous materials where the resin serves as a binder or adhesive and a small percentage of phenolic resin is used in protective coatings. Phenolic resins are usually divided into two classes; Resol and Novolac, also known as first and second stage of resins, respectively. Resol-type resins are cured by heating or by adding acids (or both). Heating for curing between 130 and 200 ° C is the most important method used in application in composites. The healing reactions are condensation, with the release of one mole of water (or formaldehyde) for each new bond formed. The chemistry of the healing process is complex because there are many structures present and many competing reactions, each of which is influenced by the condition of the reaction and other variables.

There are two important reactions in the healing process. The first is the reaction of the methylol group with the hydrogen chains (ortho or para) with other molecules to give a bonded methylene bridge between the two rings and the second is the reaction of the two methylol groups to form the linked dibenzyl ether between the two rings. For thermal cures below 170 ° C, these two reactions occur simultaneously. At high temperatures the ether bond is converted to the methylene bond by the loss of formaldehyde. The methylene bond is a more thermodynamically stable product, which is considered the most important bond developed during curing. Resol cure can also occur at low temperatures in the presence of catalytic acid, but this route is not commercially important.

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γ, λ Rub:

Ο heat treatment of polymeric precursors for the formation of glassy carbon shows variations in specific mass, shrinks and loss of mass as a function of temperature. For a typical phenolic resin, a loss of mass equivalent to 45% / weight occurs, representing a volumetric shrinkage of 40% / volume. The thermal degradation of phenolic resins is conveniently divided into three stages, considering the loss of mass and changes in volume. In the first stage, (up to 300 ° C) mass variations occur mainly due to the loss of water and unreacted monomers, phenol and formaldehyde, which are trapped during curing, in addition to oxygen in the form of CO and CO ₂ . Between 300-600 ° C the main chemical reactions of decomposition and structural rearrangement occur with the evolution of low molecular weight hydrocarbons, ethers, alcohols and ketones. The heating rate in this stage must be low (5-15 ° C / hour), to allow the decomposition products to diffuse through the matrix without destroying the integrity of the composite. Above 600 ° C the evolution of volatile products is almost complete and the heating rate can be increased without causing structural damage to the composite. Particularly for phenolic resins, pyrolysis causes volatiles to be released with greater intensity at temperatures of 500 ° C, being dominated mainly by low molecular weight hydrocarbons 20 and oxyhydrocarbons.

Carbonization is carried out in an inert atmosphere, with a flow of nitrogen or argon gases to prevent oxidation of the fiber and the newly formed carbon matrix and allow the removal of the products from decomposition. The rate of heating depends on the shape and size of the composite. Slow carbonization cycles are necessary to minimize the effects of the evolution of the gases in the impregnating liquid. The thermogravimetric analysis of the precursor, in an inert atmosphere, is a good reference for determining the ideal heating cycle in carbonization, in order to reduce the stresses caused by the retraction of the matrix that can cause damage to the fibers.

During carbonization, the porosity is increased from = 3% to s25% and the specific gravity is decreased from 1.5 g.cnT ³ to 1.3 g.cnT ³ . So that these ^18/28 zx

-4? η Α <’<ΑιΟ ¢.

'' Sealed TVs are not maintained if heat treatments are required for * <7? · $ -Ν ' ⁵ ' a densification of the composite C / C. Densification is usually carried out in a vacuum or by impregnating the carbonized composite with resin or tar under pressure, followed by new carbonization. A mass gain occurs in each impregnation cycle followed by loss in each subsequent carbonization, according to the percentage of residual carbon in the impregnating liquid.

When a resin is heat treated in a carbon fiber composite, anisotropic graphites can be detected through X-ray diffraction and optical microscopy. At temperatures below 1000 ° C this phenomenon begins to appear in some regions of the matrix. Above 2200 ° C these anisotropic regions gradually adopt the structure of the graphite crystal, until at 2800 ° C the matrix is essentially graphitic. This is the graffiti process.

The patent literature includes examples of technologies related to the present invention, without, however, anticipating or even suggesting it.

EP 6696144 describes a modification of the conventional CVD process for carbon deposition, whereby a halogen gas or a gas of a halogen compound is introduced into the carbon gas, which must be dissociated for carbon deposition. Due to the introduction of halogen or the halogen compound, it was found to be possible to form diamond or i-carbon films and, in addition, to easily control the characteristics of the films, such as optical transmissivity, electrical conductivity and hardness, by controlling the conditions for introducing halogen. CVD devices for carrying out the process and various applications of thin carbon films are also described.

TW 583077, entitled “A manufacture method of hollow composite cyllinders with a shingle angle”, reveals a method of manufacturing composites with hollow cylinders. The composites are made of carbon fiber reinforced phenolic resin. Carbon fibers in hollow cylinder composites are arranged at an angle in the plane between 30 degrees and 60 degrees. The prepregs

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Z Rs—

Rub :.

carbon fiber reinforced phenolics are cut into fan-shaped parts and laminated to form an internal set of blades. Then, this internal set of blades is packaged in a metal mold. The material is then hot molded with a pressure greater than 140.6 kg / cm ² , and then used to solidify the set, producing hollow cylinder composites at a planned angle. Hollow cylinder composites provide excellent thermal and mechanical properties.

Document RU 2208000, entitled “Composite manufacture process”, discloses a method for preparing a specific amount of glide by mixing the dispersed filler in the polymer slurry, glide ^F which is being used more to fill the carbonized prepreg. After that, simultaneous conditions of external pressure of 100 kPa and heating of 160-200 ° C are imposed, in combination with vibrations with amplitude 110 mm and frequencies 3-10 hertz for 30-60 seconds. The prepreg is subjected to a heat treatment of 200-400 ° C for hardening or carbonization. The last operation is carried out for 1-2 hours, at temperatures from 800 to 1100 ° C, with a rate of temperature increase not exceeding 2 ° C / min. The filler for the slip is powdered fluoro-plastic added in an amount of 3 to 10% by weight, and must have the size of 20 particles not exceeding the dimensions of the prepreg tube cavities.

US 2007/132128, entitled “Method of producing carbon’ fibers, and methods of making protective clothing and a filter module, reveals a process for obtaining porous carbon fibers useful in preparing filters. This process involves spinning a mixture of two 25 polymers: one a) polymer based on polyacrylonitrile (PAN); and b) a carbonisable organic polymer that forms a smaller amount of carbonization residue than the carbonization of a).

DE 19852159, entitled “Verfahren zur Herstellung von faserverstãrkten, thermoplastischen Hohlkõrpern, discloses a process for obtaining hollow fiber-reinforced thermoplastic matrix composites. Said process comprises the integrated extrusion of the winding in which the

128 .Q 'fiber and the pre-consolidated plastic inside a conical nozzle are placed in the turning core of the mold. This forms a high quality product and the melt in the nozzle avoids any need for subsequent reheating of the product.

WO 2009/049981, entitled “Hollow carbon fibers and Method for the production thereof”, discloses a continuous method for producing hollow carbon fibers usable as reinforcement fibers for composites. Said method comprises the treatment of a carbon fiber precursor in equipment with high-frequency electromagnetic waves.

jo US 6,583,075, entitled “Dissociable multicomponent fibers containing a polyacrylonitrile component”, discloses a stabilized carbon microfilament used in the preparation of strings, filters and composites. Such microfilaments comprise at least two fusible compositions, at least one of which is a fugitive polymer. The cross section of said microfilament is wedge-shaped, multi-lobal, hexagonal or rectangular.

The article entitled “Influence of the Degree of Stabilization of PAN Fibers in Obtaining β — SiC Fibers”, by the same inventors and published in the Annals of the Brazilian Ceramics Congress, 03-06 June 2007, 20 describes the conversion of fibers of PAN on SiC fibers by reactions at high temperatures. To withstand the high temperatures, the fibers underwent a previous stabilization process. The SiC formed on the fiber surface was produced by the reaction of SiO (g) with carbon, by the decomposition of SiO molecules and the subsequent insertion of Si in the structure of the carbon lamellae, forming tetrahedral units (type SÍC-3C) . The β-SiC fibers were obtained by the PANox conversion reaction, carried out at high temperatures and an atmosphere rich in SiO. The microstructural characterization was performed by SEM and X-ray diffraction. This document, however, does not describe in detail how the process for obtaining the hollow fibers was carried out, not being sufficient for reproduction of the present invention, nor / 28

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individually or in combination with other documents and therefore does not anticipate the invention.

The co-pending patent application, from the same inventors and entitled “Polymeric material with a gradient of physical-chemical property in its cross section, its Uses, Equipment and Process for obtaining it”, reveals the means by which a polymeric material has its physicochemical properties altered, forming a gradient along its cross section. The polymeric material or a composite containing it, which are products of the said co-pending patent application, are the precursors used in the present invention for obtaining carbon matrix composites reinforced with C tubular materials. said precursor is carbonized to form C tubular materials, such as hollow C fibers and / or composites containing them.

The products of the present invention offer several technical and economic advantages. Among them, the advantage of having as a final result the conversion into C / C with the same disposition of the matrix and original reinforcement (precursor composite), being the process of obtaining simple and direct, not requiring the use of precursors with mixtures of polymers or other technical sophistications that are difficult to implement and / or high cost.

From what can be inferred from the researched literature, no documents were found anticipating or suggesting the teachings of the present invention, so that for the depositor the solution proposed here has novelty and inventive activity in view of the state of the art, also presenting industrial applicability.

Summary of the Invention

It is one of the objects of the present invention to provide tubular C materials. Such materials include C fibers and / or sheets, both of which can be presented in a ceramic and / or polymeric matrix, forming a composite.

The products of the invention are endowed with peculiar physical-chemical properties, being useful in several applications, including the production of

22/28 composites containing C tubular materials. Of particular interest in the present invention, other objects of the invention are disclosed composites comprising C reinforced with hollow C fibers.

It is another object of the present invention to provide composites containing C fibers and / or sheets that have the same layout as the matrix and original reinforcement.

It is another object of the present invention to provide uses of C tubular materials. In a preferred aspect, therefore being another of the objects of the present invention, the use of C jo tubular materials and / or composites containing them in the preparation is provided. of materials for thermal insulation and / or thermal protection. In another preferred aspect, therefore being another object of the present invention, the use of C tubular materials and / or composites containing them in the preparation of catalysts is provided. In a preferred aspect, therefore being another of the objects of the present invention, the use of C tubular materials and / or composites containing them is provided in the preparation of flow modulation systems for chemical substances, in liquid, gaseous and / or plasma. In another preferential aspect, therefore being another of the objects of the present invention, the use of C and / or 20 composites tubular materials containing them is provided in the preparation of resistive elements for equipment that operate at high temperature or high temperature gradient. , or that provide variations in temperature.

It is one of the objects of the present invention to provide processes for obtaining C tubular materials. The processes of the present invention 25 make use of a precursor that is a polymeric material with a gradient of physical-chemical properties in its cross section. It is therefore another object of the invention to provide processes for converting said precursor to C, for obtaining C tubular materials. In a preferred aspect, said processing is conducted by carbonization reactions.

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In another aspect, being, therefore, another of the objects of the invention, a process is provided to obtain tubular C and / or composites materials containing them, which comprises the steps of:

- add a precursor that is a polymeric material with a gradient of physical-chemical properties in its cross section to a reactor;

- impose temperature and / or atmosphere conditions to provide the carbonization of said precursor, forming C. Preferably, said precursor comprises polyacrylonitrile fibers (PAN) with a radial oxidative stabilization gradient, with a specific mass of 1.36 g / cm ³ to 1.42 g / cm ³ . jo Preferably, said heat treatment is carried out at temperatures ^F between 700 and 1400 ° C.

These and other objects of the invention will be better understood and valued from the detailed description of the invention.

Brief Description of the Figures

Figure 1 shows an X-ray diffractogram of a preferred precursor of the invention in the form of fiber, where the abscissae represent the Bragg diffraction angle (20) and the ordinates represent the intensity in cps.

Figure 2 shows carbonization curves in a graph in which the time in minutes (T) is indicated in the abscissa and the temperature in degrees Celsius (° C) in the ordinates. Two different heating and cooling ramp conditions are shown, where ♦ indicates curve 1 and —indicates curve 2.

Figure 3 shows a zoom photomicrograph of preferred composites of the invention, containing hollow carbon fibers.

Figure 4 shows another zoom photomicrograph, an overview of preferred composites of the invention, containing hollow carbon fibers.

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Detailed Description of the Invention

The present invention provides C tubular materials. Said materials include C fibers and / or sheets, both of which can be presented in a ceramic and / or polymeric matrix, forming a composite.

For the purposes of the present invention the terms "tubular material", "hollow fiber", "tubular microfiber", "microtube", "hollow fiber", "microtube", "tubular microfiber", "tubular silicon carbide", should be interpreted as synonyms and / or interchangeable expressions, characterized by comprising a central hollow (empty) region and a solid external region in tubular shape, from circular, elliptical or elongated section to the point of being considered a hollow blade.

The inventive concept common to the various objects of the invention consists of the complete or almost complete conversion to carbon (C) of a precursor consisting of a polymeric material with a gradient of physicochemical property in its cross section. The characteristic of the precursor provided with the said gradient makes the conversion of the precursor to C provide the obtainment of a tubular material of C.

The products of the invention are endowed with peculiar physical-chemical properties, being useful in several applications, including the production of 20 composites containing tubular carbon materials (C). Of particular interest in the present invention, composites comprising C reinforced with hollow C fibers are disclosed. The composites of the invention containing C fibers and / or sheets have the same layout as the matrix and original reinforcement.

The present invention also provides several uses for C tubular materials. In a preferred aspect, the products of the invention are useful in the preparation of materials for thermal insulation and / or thermal protection. In a preferred aspect, given their extremely high surface area, the products of the invention are useful in the preparation of catalysts. In a preferred aspect, the products of the invention are useful in the preparation of flow modulation systems for chemical substances, in liquid, gaseous and / or plasma. In a preferred aspect, the products of the invention are useful in

25/28 ςύ Rub preparation in the preparation of resistive elements for equipment that operate at high temperature or high temperature gradient, or that provide variations in temperature.

The present invention also provides processes for obtaining C tubular materials. The processes of the present invention involve the carbonization of a precursor which is a polymeric material with a gradient of physical-chemical properties in its cross section.

In a preferred embodiment, the process of the invention comprises the steps of:

- add to a reactor a precursor that is a polymeric material with a gradient of physical-chemical properties in its cross section; and

- impose temperature and / or atmosphere conditions to provide the carbonization of said precursor, forming tubular areas.

Preferably, said precursor comprises polyacrylonitrile (PAN) fibers provided with a radial oxidative stabilization gradient, with a specific mass of 1.36 g / cm ³ to 1.42 g / cm ³ . Preferably, said heat treatment is carried out at temperatures between 700 and 1400 ° C

The processes of the invention provide obtaining tubular materials and / or composites containing them, being innovative for their characteristics. Such products have their own characteristics and are useful in several applications, and are also objects of the present invention.

C-C composite materials are very useful in high temperature applications, such as in the aerospace industry and in non-fusion power generation. In the manufacture of composites, the choice of fiber is mainly based on the design requirements of the final product. The thermal stability of these fibers at elevated temperatures is a crucial factor in the performance of the composite, in view of their use at high temperatures. The behavior of the composite is the result of a combination of the behavior and 30 characteristics of the 3 components that form it: the fiber or reinforcement element; The matrix; and the fiber / matrix interface. The process of the present invention, is

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conducted through the simple carbonization of a precursor that is polymeric urrimaterial endowed with a gradient of physical-chemical properties in its cross section. Such process is characterized by maintaining the structure of the matrix and strengthening the original precursor, providing several technical and / or economic advantages.

The examples shown below are intended only to exemplify some of the numerous ways of carrying out the invention, however without limiting its scope.

Example 1 - Process for Obtaining C-C Composite Reinforced with Hollow C Fibers

In a preferred embodiment of the invention, the process of the invention is used to obtain a composite C reinforced with hollow C fibers (C hollow fiber / C), using as a precursor a composite comprising a phenolic resin as a matrix reinforced with a polymeric material with a gradient of physical-chemical properties in its cross section. Said polymeric material used in this preferred embodiment comprises polyacrylonitrile (PAN) fibers with a radial stabilization gradient, with a specific mass of 1.39 g / cm ³ (the preferred mass range of the precursors of the invention is between 1.36 g / cm ³ and 1.42 g / cm ³ ). As shown in figure 1, the precursor (fiber) used in this embodiment of the invention has a small deviation from the peak 20 = 17 ° (shoulder). This difference is confirmed by the specific mass value and indicates its partial stabilization, that is, a radial gradient of oxidative stabilization.

Said preferred embodiment of the process of the invention develops from the carbonization of the precursor composite:

The precursor composite, formed from fibers with a radial gradient of oxidation stabilization and impregnation with phenolic resin, was cured respecting conventional curing temperatures. The maximum preferred temperatures for curing the composite were 180-220 ° C for CR2830 resin, since above that temperature there is the formation of

27/28 tar-derived decomposition. The cured composite is then subjected to the carbonization process at temperatures between 700 and 1400 ° C. This process takes place at a temperature at which the phenolic resin and fiber are modified to a carbon structure. In order to obtain 5 carbon fibers it is necessary that the heating be controlled (heating rate), preferably in an oven with an atmosphere rich in an inert gas such as argon, which is the conventionally used. This non-oxidizing atmosphere is very important, as carbon and oxygen in contact at temperatures above 400 ° C react to form carbon monoxide. | 0 Therefore, it is recommended to use a controlled atmosphere so that carbonization is achieved without damage to the material.

Examples of carbonization curves are shown in figure 2. To obtain hollow fibers, it is preferable to carbonize with high heating rates at the beginning of the ramp, these rates can be up to 15 15 ° C / min to provide, through these ramps heating, the fusion of the fiber interior that was not stabilized due to the stabilization gradient of the fiber. Carbonization with high heating rates seems to cause the loss of the radial gradient of oxidative stabilization.

Example 2 - Obtaining and Characterizing the Products of the Invention in the Form of Composites

Polymeric precursors endowed with a gradient of physical-chemical properties in their cross section, in the form of polymer matrix composites (of phenolic resin) reinforced with PAN fibers endowed with radial gradient of oxidative stabilization, were converted into C 25 composites reinforced with hollow fibers of C, as shown in figure 3, which presents photomicrographs with greater details of the hollow fibers in the composite. The composites of the invention can be obtained in different configurations, depending on the precursor used and the process conditions. Preferably, the dimensions (diameter) of the hollow areas of the composite of the invention are comprised in the range of 0.1 to 10 pm.

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The composite obtained after thermal carbonization treatment was analyzed microstructurally to evaluate the morphology obtained in addition to the confirmation of the total conversion of the stabilized PAN fibers and also of the phenolic resin matrix in C.

Figure 3 shows a composite of the invention, where it is possible to observe that the fibers are hollow in the plane of the fabric and it is also possible to observe some fractured fibers in the middle of their diameter, allowing to conclude that the fiber is hollow throughout its length.

Those skilled in the art will value the knowledge presented here and will be able to reproduce the invention in the modalities presented and in other variants, covered by the scope of the attached claims.

Claims (11)

TUBULAR CARBON MATERIAL, COMPOSITES CONTAINING THE SAME, USES AND PROCESSES FOR OBTAINING IT 1. Process for obtaining tubular carbon and / or composite materials containing them, characterized by understanding the steps of: - add a precursor comprising a polymeric material with a gradient of physical-chemical properties in its cross section to a reactor; said precursor comprising polyacrylonitrile (PAN) fibers provided with a radial gradient of oxidative stabilization and the specific mass of said precursor is 1.36 g / cm ³ to 1.42 g / cm ³ ; and - varying temperature and / or atmosphere to provide the carbonization of said precursor, forming C; and said heat treatment is conducted at temperatures between 700 and 1400 ° C and comprises heating rates at the start of the ramp up to 15 ° C / min. 2. Process, according to claim 1, characterized by the fact that said heat treatment is conducted in a graphite resistive element furnace with horizontal circular section and an argon gas line. 3. Tubular material, obtained by the process defined in claims 1-2, characterized by the fact that it comprises carbon in the form of fibers and / or hollow blades; being that: - the thickness of its walls is between 0.1 and 10 pm; and - that the carbon concentration is greater than or equal to 90% by weight. 4. Tubular material according to claim 3, characterized by the fact that the carbon concentration is 100% by weight. 5. Composite, obtained from the material of claim 3, characterized by the fact that it comprises: - at least one tubular carbon material in the form of fibers and / or hollow blades; and - at least one carbon and / or polymeric matrix component. Petition 870190084653, of 08/29/2019, p. 11/10 2/2 6. Composite according to claim 5, characterized by the fact that said hollow regions have a diameter in the range of 0.1 to 10 pm. 7. Composite according to claim 5 or 6, characterized by the fact that the carbon concentration in said tubular material is greater than or equal to 90% by weight. Composite according to claim 5, 6 or 7, characterized in that the carbon concentration in said tubular material is 100% by weight. 9. Composite according to any one of claims 5 to 8, characterized in that it has the same layout as the matrix and original reinforcement. 10. Use of tubular material, defined in claims 3 and 4, comprising carbon in the form of fibers and / or hollow blades, characterized by being for the preparation of carbon and / or polymeric matrix composites. 11. Use of tubular material, defined in claims 3-4, comprising carbon in the form of fibers and / or hollow blades and / or composites, defined in claims 5 to 9, comprising said material, according to the claim 10, characterized for being for the preparation of products of the group comprising: materials for thermal insulation and / or thermal protection; catalysts; flow modulation systems for chemical substances, in liquid, gaseous and / or plasma; resistive elements for equipment that operate at high temperature or high temperature gradient, or that provide variations in temperature; or combinations thereof.