7 LIQUID IMPREGNATION TECHNIQUES FOR CARBON-CARBON COMPOSITES R.Menendez,E.Casal,and M.Granda 1 Introduction Carbon fiber reinforced carbon composites (C-C composites)are unique in providing materials with the highest possible specific thermal-mechanical properties.They have an additional value in that at high temperatures their excellent properties are not only retained but also improved as a result of a better structural order.However,the main problem with C-C composites,apart from their reactivity to oxygen at high temperatures,is their high cost due to the complexity and length of the fabrication processes.Consequently,C-C composites are currently used only in high-technology applications where no other materi- als are available.In recent years the search for ways to improve properties,reduce costs,and increase the efficiency of the processes involved in the preparation of C-C composites has been intensified.Unfortunately,easily available information is very limited because of con- fidentiality agreements,most of it being in patent form(Plotzker et al.,1988;Hocquellet and Chenier,1993). The type and architecture of the fibers and the microstructure of the supporting matrix determine the physical properties of C-C composites.It is,therefore,important to under- stand the mechanisms that control the formation of the microstructure and its interaction with the fibers in order to be able to achieve the desired composite properties at the lowest possible cost and make full use of the electrical,thermal,and mechanical properties of the fibers (Meyer,1994). Two methods are at present used for the preparation of C-C composites:(i)liquid impreg- nation;and(ii)carbon vapor infiltration (CVI).A combination of the two methods is also used.Both have been described in detail in the literature(Savage,1993a).Liquid impreg- nation,the topic covered in this chapter,is a two-stage method.Basically,in the first stage the fibers are impregnated with an organic precursor,giving rise to what is called a prepreg. Afterwards,the precursor is transformed into a carbonaceous matrix by thermal treatment (650-1,000C)in an inert atmosphere via a carbonization process.Depending on the appli- cation of the material,graphitization (>2,500C)may also be required.These stages have to be repeated until the desired density is obtained (Fig.7.1).The other method,CVI,implies the cracking of a gas and the deposition of carbon onto the fiber substrate (Chapter 7 of this book).In general,liquid impregnation is considerably quicker and cheaper than CVI pro- cessing,and the equipment required,in an ambient pressure fabrication route,is not limited ©2003 Taylor&Francis
7 LIQUID IMPREGNATION TECHNIQUES FOR CARBON–CARBON COMPOSITES R. Menéndez, E. Casal, and M. Granda 1 Introduction Carbon fiber reinforced carbon composites (C–C composites) are unique in providing materials with the highest possible specific thermal-mechanical properties. They have an additional value in that at high temperatures their excellent properties are not only retained but also improved as a result of a better structural order. However, the main problem with C–C composites, apart from their reactivity to oxygen at high temperatures, is their high cost due to the complexity and length of the fabrication processes. Consequently, C–C composites are currently used only in high-technology applications where no other materials are available. In recent years the search for ways to improve properties, reduce costs, and increase the efficiency of the processes involved in the preparation of C–C composites has been intensified. Unfortunately, easily available information is very limited because of confidentiality agreements, most of it being in patent form (Plotzker et al., 1988; Hocquellet and Chenier, 1993). The type and architecture of the fibers and the microstructure of the supporting matrix determine the physical properties of C–C composites. It is, therefore, important to understand the mechanisms that control the formation of the microstructure and its interaction with the fibers in order to be able to achieve the desired composite properties at the lowest possible cost and make full use of the electrical, thermal, and mechanical properties of the fibers (Meyer, 1994). Two methods are at present used for the preparation of C–C composites: (i) liquid impregnation; and (ii) carbon vapor infiltration (CVI). A combination of the two methods is also used. Both have been described in detail in the literature (Savage, 1993a). Liquid impregnation, the topic covered in this chapter, is a two-stage method. Basically, in the first stage the fibers are impregnated with an organic precursor, giving rise to what is called a prepreg. Afterwards, the precursor is transformed into a carbonaceous matrix by thermal treatment (650–1,000 C) in an inert atmosphere via a carbonization process. Depending on the application of the material, graphitization ( 2,500 C) may also be required. These stages have to be repeated until the desired density is obtained (Fig. 7.1). The other method, CVI, implies the cracking of a gas and the deposition of carbon onto the fiber substrate (Chapter 7 of this book). In general, liquid impregnation is considerably quicker and cheaper than CVI processing, and the equipment required, in an ambient pressure fabrication route, is not limited © 2003 Taylor & Francis
Fiber Organic matrix precursor Liquid impregnation Prepreg Carbonization (650-1000C) Carbonized Graphitization composite (>2500C) Liquid impregnation or CVI Densified Graphitized composite composite Figure 7.I Flow diagram of C-C composite preparation by liquid impregnation. by size or by the need for a large initial capital investment.The main advantage of the liq- uid impregnation procedure,however,concerns the great variety of matrix microstructures which can be produced,with the subsequent impact on composite properties. The aim of this chapter is to offer a general view of the fundamentals of the liquid impreg- nation procedure and to point out the technical problems which may arise through eyes of the authors concerned.Special emphasis is given to the role of matrix precursors and the selection of the correct processing conditions for producing the desired microstructural fea- tures and composites with optimum properties.At the same time care is taken not to under- estimate the important role of fiber in the properties of the composite.Ways of enhancing these properties through the carbon matrix precursor by means of preliminary treatment or by acting on the prepreg are also considered. 2 Impregnation technology The technologies available for the impregnation of carbon fibers using a carbonaceous matrix precursor are similar to those used for fiber-reinforced plastics.Depending on the matrix precursor and the type of reinforcement(discontinuous,linear,planar,or n-D)many alternatives are available.Essentially,the procedures include:(i)passing continuous fibers through a solution of the matrix precursor;(ii)infiltration of a liquefied matrix precursor in a fibrous skeleton assisted by vacuum or pressure;and(iii)hot pressing of sandwiched fiber/matrix precursor mixtures. 2003 Taylor Francis
by size or by the need for a large initial capital investment. The main advantage of the liquid impregnation procedure, however, concerns the great variety of matrix microstructures which can be produced, with the subsequent impact on composite properties. The aim of this chapter is to offer a general view of the fundamentals of the liquid impregnation procedure and to point out the technical problems which may arise through eyes of the authors concerned. Special emphasis is given to the role of matrix precursors and the selection of the correct processing conditions for producing the desired microstructural features and composites with optimum properties. At the same time care is taken not to underestimate the important role of fiber in the properties of the composite. Ways of enhancing these properties through the carbon matrix precursor by means of preliminary treatment or by acting on the prepreg are also considered. 2 Impregnation technology The technologies available for the impregnation of carbon fibers using a carbonaceous matrix precursor are similar to those used for fiber-reinforced plastics. Depending on the matrix precursor and the type of reinforcement (discontinuous, linear, planar, or n-D) many alternatives are available. Essentially, the procedures include: (i) passing continuous fibers through a solution of the matrix precursor; (ii) infiltration of a liquefied matrix precursor in a fibrous skeleton assisted by vacuum or pressure; and (iii) hot pressing of sandwiched fiber/matrix precursor mixtures. Figure 7.1 Flow diagram of C–C composite preparation by liquid impregnation. Carbonized composite Densified composite Graphitized composite Liquid impregnation Liquid impregnation or CVI Graphitization (>2500 °C) Carbonization (650–1000 °C) Prepreg Fiber Organic matrix precursor © 2003 Taylor & Francis
2.1 Wet-winding technique This is the most common technique used for preparing unidirectional C-C composites at laboratory scale(Fig.7.2a).Basically,a continuous fiber bundle is made to pass through a bath which contains a liquefied matrix precursor.The impregnated fibers are then wound onto a mandrel under controlled tension.The winding processes differ from each other in the impregnation conditions chosen and the reinforcement scheme and type of winding path used (Borokh et al.,1995).Depending on the impregnation conditions,winding can be wet (a) Wet-winding 1D-Prepreg (b) Pultrusion Pulling device Heat 1D-Prepreg (c) Injection Pressure, vacuum etc. nD-Prepreg (d) Hot-press molding △ Pressure 2D-Prepreg Figure 7.2 Impregnation techniques. ©2003 Taylor&Francis
2.1 Wet-winding technique This is the most common technique used for preparing unidirectional C–C composites at laboratory scale (Fig. 7.2a). Basically, a continuous fiber bundle is made to pass through a bath which contains a liquefied matrix precursor. The impregnated fibers are then wound onto a mandrel under controlled tension. The winding processes differ from each other in the impregnation conditions chosen and the reinforcement scheme and type of winding path used (Borokh et al., 1995). Depending on the impregnation conditions, winding can be wet Figure 7.2 Impregnation techniques. Wet-winding ∆ Pressure Pressure, vacuum etc. Injection nD-Prepreg 1D-Prepreg Pultrusion Heat source Pulling device 1D-Prepreg 2D-Prepreg Hot-press molding (a) (b) (c) (d) © 2003 Taylor & Francis
(impregnation is carried out during winding)or dry(winding is carried out on a preimpreg- nated,dried tape).Winding can also be performed with non-impregnated fibers followed by impregnation. When using wet winding techniques,the impregnation conditions vary depending on the matrix precursor(Hirano et al.,1993),i.e.a mixture of polymers and a plasticizer,melt pitch, liquid resins,pitch,or resin dissolved in an organic solvent,or fine particles suspended in the form of an emulsion(Hocquellet and Chenier,1993).Two fundamental factors which must be controlled in this step and borne in mind when selecting the optimum conditions are:the alignment of the fibers and the homogeneous distribution of the matrix precursor.Correct fiber alignment is very important because small variations in the orientation of the fibers can mean additional stresses,which may lead to premature failure of the material.As an exam- ple,impregnation is relatively easy with solutions at low concentrations of the organic pre- cursor.However,complete removal of the solvent is hard to achieve and it may produce fiber misalignment,as well as the generation of porosity during solvent removal. The next step,after the impregnation process,involves curing and/or hot-pressing (depending on the matrix precursor),and then carbonization/graphitization.After this,other techniques(CVI,injection)are required in order to reduce the final porosity and obtain a high density material. 2.2 Pultrusion This technique is derived from wet-winding.In this case,after being impregnated in a bath, the fibers are pulled through a heated die specifically shaped to produce the desired cross- section(Fig.7.2b).Partial or complete curing occurs during the passage through the die. 2.3 Hand lay-up Fibers are placed by hand into a mould covered by a released substance and then impreg- nated with a liquid precursor by means of painting or spraying and rolling.Rolling compacts the material and ensures the effective impregnation of the fiber by the matrix precursor, homogeneous distribution and the removal of air.The fibers may be in the form of random mats or woven fabrics.Layers of impregnated fibers are stacked up(lay-up)until the desired thickness is achieved.Carbonization is then carried out to obtain a stable porous structure. Again additional techniques are needed to obtain the desired final density. 2.4 Injection technique This method(Fig.7.2c)is generally used to impregnate preforms of carbon fibers produced by dry winding or braiding and also as a way to increase the density of the composite pre- pared by other techniques.The preform is dipped into the liquefied precursor or alterna- tively the precursor is forced into the pores of the preform by means of surface tension, gravity,atmospheric pressure or external forces such as gas pressure,centrifugal force, mechanical vibration,etc.(Borokh et al.,1995).In some cases this is achieved with the help of a vacuum.Pore distribution in the preform and the rheological properties of the matrix precursor are the main factors that control this process. 2.5 Hot-press molding This technique is frequently used at laboratory scale to prepare bidirectional C-C composites (Arianoutsos et al.,1990).Woven mats of carbon fibers are stacked in a mold and sandwiched ©2003 Taylor&Francis
(impregnation is carried out during winding) or dry (winding is carried out on a preimpregnated, dried tape). Winding can also be performed with non-impregnated fibers followed by impregnation. When using wet winding techniques, the impregnation conditions vary depending on the matrix precursor (Hirano et al., 1993), i.e. a mixture of polymers and a plasticizer, melt pitch, liquid resins, pitch, or resin dissolved in an organic solvent, or fine particles suspended in the form of an emulsion (Hocquellet and Chenier, 1993). Two fundamental factors which must be controlled in this step and borne in mind when selecting the optimum conditions are: the alignment of the fibers and the homogeneous distribution of the matrix precursor. Correct fiber alignment is very important because small variations in the orientation of the fibers can mean additional stresses, which may lead to premature failure of the material. As an example, impregnation is relatively easy with solutions at low concentrations of the organic precursor. However, complete removal of the solvent is hard to achieve and it may produce fiber misalignment, as well as the generation of porosity during solvent removal. The next step, after the impregnation process, involves curing and/or hot-pressing (depending on the matrix precursor), and then carbonization/graphitization. After this, other techniques (CVI, injection) are required in order to reduce the final porosity and obtain a high density material. 2.2 Pultrusion This technique is derived from wet-winding. In this case, after being impregnated in a bath, the fibers are pulled through a heated die specifically shaped to produce the desired crosssection (Fig. 7.2b). Partial or complete curing occurs during the passage through the die. 2.3 Hand lay-up Fibers are placed by hand into a mould covered by a released substance and then impregnated with a liquid precursor by means of painting or spraying and rolling. Rolling compacts the material and ensures the effective impregnation of the fiber by the matrix precursor, homogeneous distribution and the removal of air. The fibers may be in the form of random mats or woven fabrics. Layers of impregnated fibers are stacked up (lay-up) until the desired thickness is achieved. Carbonization is then carried out to obtain a stable porous structure. Again additional techniques are needed to obtain the desired final density. 2.4 Injection technique This method (Fig. 7.2c) is generally used to impregnate preforms of carbon fibers produced by dry winding or braiding and also as a way to increase the density of the composite prepared by other techniques. The preform is dipped into the liquefied precursor or alternatively the precursor is forced into the pores of the preform by means of surface tension, gravity, atmospheric pressure or external forces such as gas pressure, centrifugal force, mechanical vibration, etc. (Borokh et al., 1995). In some cases this is achieved with the help of a vacuum. Pore distribution in the preform and the rheological properties of the matrix precursor are the main factors that control this process. 2.5 Hot-press molding This technique is frequently used at laboratory scale to prepare bidirectional C–C composites (Arianoutsos et al., 1990). Woven mats of carbon fibers are stacked in a mold and sandwiched © 2003 Taylor & Francis
by the matrix precursor powder (Figure 7.2d).The mold,matrix precursor and fibers,are heated together until the matrix precursor is thoroughly melted.At this point mechanical pressure is applied in order to place the melted precursor inside the mats and so ensure an effective impregnation.During pressurization,trapped air and excess precursor are forced out of the mold.After cooling,carbonization is required.Infiltration/carbonization cycles are repeated several times to achieve the desired density. Unidirectional C-C composites are prepared in a similar way,using a powder coating process(Klett and Edie,1995).Fiber bundles are separated by a vacuum spreader and then the individual fibers are coated with fine polymer particles in a fluidization chamber.The coated tow thus formed (tow-preg)is heated until the particles are fused to the individual filaments.Next the tow-pregs are cut,stacked and hot-pressed to form the final composite. This procedure may be specially desirable when using pitches with a high softening point and low solubility as matrix precursors. Hot-press molding is also used as part of the process of preparation of unidirectional C-C composites by wet-winding.Laminated prepregs are stacked and then hot-pressed at spe- cific temperatures,depending on the characteristics(chemical composition and rheological properties)of the precursor(Figueiras et al.,1995;Casal et al.,2001). 3 Densification efficiency The preparation of C-C composites with the right density currently requires several impregnation/carbonization cycles,as mentioned above.This is because in the initial stages of the carbonization process of the organic precursor there is a major weight loss of volatiles which continues,to a lesser extent,throughout the process as the temperature increases, giving a porous material.The pores are produced,either in the initial stages of the prepara- tion of the composite (impregnation process and molding),or during subsequent carboniza- tion.Pores of the first type are usually open (Fig.7.3a),whereas those developed on carbonization may be either open or closed(Fig.7.3a).Carbonization pores may be devel- oped either while the matrix precursor is still plastic or as a result of stresses(cracks)due to structural rearrangements or thermo-mechanical effects (Fig.7.3b).Several studies have been carried out in relation with the characterization of porosity in C-C composites (Jortner, 1986;Granda et al.,1998).All of these structural imperfections have an adverse effect on the bulk properties. A knowledge of the parameters which control the efficiency and effectiveness of densifi- cation is essential for the optimization of the process.Experience shows that the first step of impregnation is not enough to ensure that all of the empty pores in a C-C composite are filled by the matrix precursor(Fig.7.3c and d).An impregnation efficiency parameter,Y, can be defined as the relationship between the volume of impregnant introduced into the pores of the composite and the volume of the open porosity of the composite (eq.(1)).This parameter can be calculated from the increase in weight gain of the composite during the impregnation,AWi,and the volume fraction of open porosity of the composite before impregnation,0: Y= Volume ofimpregnant introducted AW p(0) Volume ofinitial empty pores Pp 0 (1) ©2003 Taylor&Francis
by the matrix precursor powder (Figure 7.2d). The mold, matrix precursor and fibers, are heated together until the matrix precursor is thoroughly melted. At this point mechanical pressure is applied in order to place the melted precursor inside the mats and so ensure an effective impregnation. During pressurization, trapped air and excess precursor are forced out of the mold. After cooling, carbonization is required. Infiltration/carbonization cycles are repeated several times to achieve the desired density. Unidirectional C–C composites are prepared in a similar way, using a powder coating process (Klett and Edie, 1995). Fiber bundles are separated by a vacuum spreader and then the individual fibers are coated with fine polymer particles in a fluidization chamber. The coated tow thus formed (tow-preg) is heated until the particles are fused to the individual filaments. Next the tow-pregs are cut, stacked and hot-pressed to form the final composite. This procedure may be specially desirable when using pitches with a high softening point and low solubility as matrix precursors. Hot-press molding is also used as part of the process of preparation of unidirectional C–C composites by wet-winding. Laminated prepregs are stacked and then hot-pressed at specific temperatures, depending on the characteristics (chemical composition and rheological properties) of the precursor (Figueiras et al., 1995; Casal et al., 2001). 3 Densification efficiency The preparation of C–C composites with the right density currently requires several impregnation/carbonization cycles, as mentioned above. This is because in the initial stages of the carbonization process of the organic precursor there is a major weight loss of volatiles which continues, to a lesser extent, throughout the process as the temperature increases, giving a porous material. The pores are produced, either in the initial stages of the preparation of the composite (impregnation process and molding), or during subsequent carbonization. Pores of the first type are usually open (Fig. 7.3a), whereas those developed on carbonization may be either open or closed (Fig. 7.3a). Carbonization pores may be developed either while the matrix precursor is still plastic or as a result of stresses (cracks) due to structural rearrangements or thermo-mechanical effects (Fig. 7.3b). Several studies have been carried out in relation with the characterization of porosity in C–C composites (Jortner, 1986; Granda et al., 1998). All of these structural imperfections have an adverse effect on the bulk properties. A knowledge of the parameters which control the efficiency and effectiveness of densification is essential for the optimization of the process. Experience shows that the first step of impregnation is not enough to ensure that all of the empty pores in a C–C composite are filled by the matrix precursor (Fig. 7.3c and d). An impregnation efficiency parameter, Yi, can be defined as the relationship between the volume of impregnant introduced into the pores of the composite and the volume of the open porosity of the composite (eq. (1)). This parameter can be calculated from the increase in weight gain of the composite during the impregnation, Wi , and the volume fraction of open porosity of the composite before impregnation, : Y (1) i Volume of impregnant introducted Volume of initial empty pores Wi p (0) © 2003 Taylor & Francis