《纺织复合材料》课程参考文献（Composite materials science and applications，Second Edition）06 Tailoring Composite Materials.pdf

6.1 Tailoring by Component Selection 159 Table 6.1.Properties of thermoplastics PES PEEK PEI PPS PI T:(C) 2302 1703 2251 86 256 Decomposition temperature(C) 5502 590 5554 5271 550- Processing temperature(C) 3502 3803 3503 316 304 Tensile strength(MPa) 84d 70d 105e 66d 1170 Modulus of elasticity(GPa) 2.4d 3.8d 3.0° 33d 2.1d Ductility (elongation) 80 150d 50-65 1 Izod impact(ft lb/in.) 1.6d 1.6d 1 0.5d 1.5d Density(g/cm) 1.37 131d 1.27 1.30d 139d Data from:[2];b [3];[4];d [5] Water reducing agent-a minor additive to increase the workability of the mix Polymer(such as latex)-to decrease liquid permeability and bond strength Fine particles (such as silica fume)-to decrease liquid permeability,bond strength and drying shrinkage,and to increase the modulus and the abrasion resistance Short fiber(such as steel fiber)-to increase the flexural toughness. Continuous fibers are not suitable for inclusion in a cement mix,although they can be applied prior to cement pouring and can serve as a reinforcement.Both processing and material costs are high.In addition,penetration of the cement mix into the small spaces between microfibers is difficult.On the other hand, macroscopic steel rebars are similar in shape to continuous microfibers and are commonly used to reinforce concrete. 6.1.2.1 Polymers in Cement-Matrix Composites Polymer particles used as admixtures can take the form of a dry powder or an aqueous dispersion of particles.The latter form is more common.The inclusion of either form as an admixture results in improved joining of the mix constituents (e.g.,sand),due to the presence of interweaving polymer films.The improved joining leads to superior mechanical and durability characteristics.Aqueous dis- persions of polymer particles are more effective than dry polymer powder for the development and uniform distribution of polymer films.The most common form of polymer in aqueous dispersions is latex,particularly butadiene-styrene copolymer.The dispersions are stabilized by the use of surfactants. In polymer-modified cement-based material,polymer particles are partitioned between the interiors of hydrates and the surfaces of anhydrous cement grains.The presence of the polymer results in an improved pore structure,thereby decreased porosity.Furthermore,the workability is enhanced and the water absorption is decreased.This enhanced workability allows the use of lower values of the wa- ter/cement ratio. The rate of hydration is reduced by the presence of the polymer.The addition of a polymer tends to increase the flexural strength and toughness,but lower the

6.1 Tailoring by Component Selection 159 Table 6.1. Properties of thermoplastics PES PEEK PEI PPS PI Tg (°C) 230a 170a 225a 86a 256b Decomposition temperature (°C) 550a 590a 555a 527a 550b Processing temperature (°C) 350a 380a 350a 316a 304b Tensile strength (MPa) 84d 70d 105c 66d 117d Modulus of elasticity (GPa) 2.4d 3.8d 3.0c 3.3d 2.1d Ductility (% elongation) 80d 150d 50–65c 2d 10d Izod impact (ft lb/in.) 1.6d 1.6d 1c 0.5d 1.5d Density (g/cm3) 1.37d 1.31d 1.27c 1.30d 1.39d Data from: a [2]; b [3]; c [4]; d [5] Water reducing agent – a minor additive to increase the workability of the mix Polymer (such as latex) – to decrease liquid permeability and bond strength Fine particles (such as silica fume) – to decrease liquid permeability, bond strength and drying shrinkage, and to increase the modulus and the abrasion resistance Short fiber (such as steel fiber) – to increase the flexural toughness. Continuous fibers are not suitable for inclusion in a cement mix, although they can be applied prior to cement pouring and can serve as a reinforcement. Both processing and material costs are high. In addition, penetration of the cement mix into the small spaces between microfibers is difficult. On the other hand, macroscopic steel rebars are similar in shape to continuous microfibers and are commonly used to reinforce concrete. 6.1.2.1 Polymers in Cement-Matrix Composites Polymer particles used as admixtures can take the form of a dry powder or an aqueous dispersion of particles. The latter form is more common. The inclusion of either form as an admixture results in improved joining of the mix constituents (e.g., sand), due to the presence of interweaving polymer films. The improved joining leads to superior mechanical and durability characteristics. Aqueous dispersions of polymer particles are more effective than dry polymer powder for the development and uniform distribution of polymer films. The most common form of polymer in aqueous dispersions is latex, particularly butadiene-styrene copolymer. The dispersions are stabilized by the use of surfactants. In polymer-modified cement-based material, polymer particles are partitioned between the interiors of hydrates and the surfaces of anhydrous cement grains. The presence of the polymer results in an improved pore structure, thereby decreased porosity. Furthermore, the workability is enhanced and the water absorption is decreased. This enhanced workability allows the use of lower values of the water/cement ratio. The rate of hydration is reduced by the presence of the polymer. The addition of a polymer tends to increase the flexural strength and toughness, but lower the

160 6 Tailoring Composite Materials compressive strength,modulus of elasticity,and hardness.Furthermore,polymer addition is effective at enhancing the vibration damping capacity,the frost re- sistance,and the resistance to biogenic sulfuric acid corrosion(relevant to sewer systems).In addition,polymer addition imparts stability and thixotropy to grouts and enables control over the rheology and the stabilization of the cement slurry against segregation.Dry polymer particles used as an admixture can be water- redispersible polymer particles,such as those obtained by spray drying aqueous dispersions.Examples are acrylic and poly(ethylenevinyl acetate).Redispersibility may be attained through the use of functional monomers.The effectiveness of redispersible polymer particles depends on the cement used.One special cate- gory of polymer particles is superabsorbent particles (hydrogel),which serve to provide the controlled formation of water-filled macropore inclusions(i.e.,wa- ter entrainment)in the fresh concrete.The consequence of this is control over self-dessication.Another kind of superabsorbent polymer barely absorbs alkaline water in fresh/hardened concrete,but absorbs a great deal ofneutral/acid water and creates a gel.Thus,when neutral water is poured onto the concrete after setting, the concrete is coated with the gel and can thus be kept without drying. Organic liquid admixtures can be polymer solutions(involving water-soluble polymers such as methylcellulose,polyvinyl alcohol and polyacrylamide)or resins (such as epoxy and unsaturated polyester resin).The liquid form is attractive due to the ease with which it can be uniformly spatially distributed,and hence its effectiveness in even small proportions.In contrast to polymer solutions,particles (including particle dispersions)tend to require a higher proportion in order to be comparably effective.Polymer solutions used as admixtures can serve to op- timize the air void distribution and rheology of the wet mix,thereby improving workability with low air contents. Short fibers rather than continuous ones are used because they can be incorpo- rated in the cement mix,thereby facilitating processing in the field.Furthermore, short fibers are less expensive than continuous ones.Polypropylene,polyethylene and acrylic fibers are particularly common due to the requirements of low cost and resistance to the alkaline environment in cement-based materials.Compared to carbon,glass and steel fibers,polymer fibers are attractive due to their high ductility,which results in high flexural toughness in the cement-based material. The combined use of short polymer fibers and a polymer particle dispersion(e.g., latex)results in superior strength(tensile,compressive,and flexural)and flexural toughness compared to the use of fibers without a polymer particle dispersion. 6.1.2.2 Silica Fume in Cement-Matrix Composites Silica fume is very fine noncrystalline silica produced by electric arc furnaces as a by-product of the production of metallic silicon or ferrosilicon alloys.It is a powder with particles that have diameters that a hundredfold smaller than those of anhydrous Portland cement particles (i.e.,the mean particle size is between 0.1 and 0.2 um).The SiO2 content ranges from 85 to 98%.Silica fume is pozzolanic- it has a limited ability to serve as a cementitious binder

160 6 Tailoring Composite Materials compressive strength, modulus of elasticity, and hardness. Furthermore, polymer addition is effective at enhancing the vibration damping capacity, the frost resistance, and the resistance to biogenic sulfuric acid corrosion (relevant to sewer systems). In addition, polymer addition imparts stability and thixotropy to grouts and enables control over the rheology and the stabilization of the cement slurry against segregation. Dry polymer particles used as an admixture can be waterredispersible polymer particles, such as those obtained by spray drying aqueous dispersions. Examples are acrylic and poly(ethylenevinyl acetate). Redispersibility may be attained through the use of functional monomers. The effectiveness of redispersible polymer particles depends on the cement used. One special category of polymer particles is superabsorbent particles (hydrogel), which serve to provide the controlled formation of water-filled macropore inclusions (i.e., water entrainment) in the fresh concrete. The consequence of this is control over self-dessication. Another kind of superabsorbent polymer barely absorbs alkaline water in fresh/hardened concrete, but absorbs a great deal of neutral/acid water and creates a gel. Thus, when neutral water is poured onto the concrete after setting, the concrete is coated with the gel and can thus be kept without drying. Organic liquid admixtures can be polymer solutions (involving water-soluble polymers such as methylcellulose, polyvinyl alcohol and polyacrylamide) or resins (such as epoxy and unsaturated polyester resin). The liquid form is attractive due to the ease with which it can be uniformly spatially distributed, and hence its effectiveness in even small proportions. In contrast to polymer solutions, particles (including particle dispersions) tend to require a higher proportion in order to be comparably effective. Polymer solutions used as admixtures can serve to optimize the air void distribution and rheology of the wet mix, thereby improving workability with low air contents. Short fibers rather than continuous ones are used because they can be incorporated in the cement mix, thereby facilitating processing in the field. Furthermore, short fibers are less expensive than continuous ones. Polypropylene, polyethylene and acrylic fibers are particularly common due to the requirements of low cost and resistance to the alkaline environment in cement-based materials. Compared to carbon, glass and steel fibers, polymer fibers are attractive due to their high ductility, which results in high flexural toughness in the cement-based material. The combined use of short polymer fibers and a polymer particle dispersion (e.g., latex) results in superior strength (tensile, compressive, and flexural) and flexural toughness compared to the use of fibers without a polymer particle dispersion. 6.1.2.2 Silica Fume in Cement-Matrix Composites Silica fume is very fine noncrystalline silica produced by electric arc furnaces as a by-product of the production of metallic silicon or ferrosilicon alloys. It is a powder with particles that have diameters that a hundredfold smaller than those of anhydrous Portland cement particles (i.e., the mean particle size is between 0.1 and 0.2μm). The SiO2 content ranges from 85 to 98%. Silica fume is pozzolanic – it has a limited ability to serve as a cementitious binder

6.1 Tailoring by Component Selection 161 Silica fume used as an admixture in a concrete mix has significant positive effects on the properties of the resulting material.These effects pertain to the strength, modulus,ductility,vibration damping capacity,sound absorption,abrasion resis- tance,air void content,shrinkage,bonding strength with reinforcing steel,per- meability,chemical attack resistance,alkali-silica reactivity reduction,corrosion resistance of embedded steel reinforcement,freeze-thaw durability,creep rate, coefficient of thermal expansion(CTE),specific heat,thermal conductivity,defect dynamics,dielectric constant,and degree of fiber dispersion in mixes containing short microfibers.However,silica fume addition degrades the workability of the mix.This problem can be alleviated by using more water-reducing agent or by treating the surfaces of the silica fume particles with silane. 6.1.2.3 Short Fibers in Cement-Matrix Composites Short fibers are used as admixtures in cement-based materials in order to decrease the drying shrinkage,increase the flexural toughness,and in some cases to increase the flexural strength too.When the fibers are electrically conductive,they may also provide nonstructural functions,such as self-sensing(sensing the strain,damage, or temperature),self-heating (for deicing),and electromagnetic reflection (for electromagnetic interference shielding;i.e.,EMI shielding). Although continuous fibers are more effective than short fibers when used as a reinforcement,they are not amenable to incorporation in a concrete mix and they are relatively expensive.Low cost is critical to the practical viability of cement- based materials. Although macroscopic steel fibers that are around 1 mm in diameter are used,the most effective fibers are usually microfibers with diameters ranging from 5 to 100 um.For example,carbon fibers are typically around 10 um in diameter.Nanofibers with diameters that are typically around 0.1 um are less effective than microfibers as a reinforcement,although they are more effective than microfibers at providing EMI shielding(due to their small diameters and the skin effect,which refers to the phenomenon in which high-frequency electromagnetic radiation only interacts with the near-surface region of an electrical conductor).In general,the smaller the fiber diameter(and thus the higher the aspect ratio),the more difficult it is to disperse the fibers.Similarly,the smaller the fiber length(which relates to a lower aspect ratio),the easier it is to disperse the fibers.This is due to the tendency for fibers with small diameters or longlengths to cling to one another.The effectiveness of a fiber admixture at improving the structural or functional properties of cement- based materials is greatly affected by the degree of fiber dispersion.The attainment of a high degree of fiber dispersion is particularly critical when the fiber volume fraction is low.A low fiber volume fraction is usually preferred because the material cost increases,the workability decreases,the air void content increases,and the compressive strength decreases as the fiber content increases.The fiber dispersion is enhanced by improving the hydrophilicity(i.e.,the wettability by water)of the fibers,as the cement mix is water-based.The hydrophilicity can be controlled by treating the surfaces of the fibers prior to incorporating the fibers into the cement mix.Furthermore,the fiber dispersion is affected by the admixtures that may

6.1 Tailoring by Component Selection 161 Silica fume used as an admixture in a concrete mix has significant positive effects on the properties of the resulting material. These effects pertain to the strength, modulus, ductility, vibration damping capacity, sound absorption, abrasion resistance, air void content, shrinkage, bonding strength with reinforcing steel, permeability, chemical attack resistance, alkali-silica reactivity reduction, corrosion resistance of embedded steel reinforcement, freeze–thaw durability, creep rate, coefficient of thermal expansion (CTE), specific heat, thermal conductivity, defect dynamics, dielectric constant, and degree of fiber dispersion in mixes containing short microfibers. However, silica fume addition degrades the workability of the mix. This problem can be alleviated by using more water-reducing agent or by treating the surfaces of the silica fume particles with silane. 6.1.2.3 Short Fibers in Cement-Matrix Composites Short fibers are used as admixtures in cement-based materials in order to decrease the drying shrinkage, increase the flexural toughness, and in some cases to increase the flexural strength too. When the fibers are electrically conductive, they may also provide nonstructural functions, such as self-sensing (sensing the strain, damage, or temperature), self-heating (for deicing), and electromagnetic reflection (for electromagnetic interference shielding; i.e., EMI shielding). Although continuous fibers are more effective than short fibers when used as a reinforcement, they are not amenable to incorporation in a concrete mix and they are relatively expensive. Low cost is critical to the practical viability of cementbased materials. Although macroscopic steelfibersthat arearound1mmindiameter areused,the most effective fibers are usually microfibers with diameters ranging from 5 to 100 μm. For example, carbon fibers are typically around 10μm in diameter. Nanofibers with diameters that are typically around 0.1μm are less effective than microfibers as a reinforcement, although they are more effective than microfibers at providing EMI shielding (due to their small diameters and the skin effect, which refers to the phenomenon in which high-frequency electromagnetic radiation only interacts with the near-surface region of an electrical conductor). In general, the smaller the fiber diameter (and thus the higher the aspect ratio), the more difficult it is to disperse the fibers. Similarly, the smaller the fiber length (which relates to a lower aspect ratio), the easier it is to disperse the fibers. This is due to the tendency for fiberswithsmalldiametersorlonglengthstoclingtooneanother.Theeffectiveness of a fiber admixture at improving the structural or functional properties of cementbased materials is greatly affected by the degree of fiber dispersion. The attainment of a high degree of fiber dispersion is particularly critical when the fiber volume fraction is low. A low fiber volume fraction is usually preferred because the material cost increases, the workability decreases, the air void content increases, and the compressive strength decreases as the fiber content increases. The fiber dispersion is enhanced by improving the hydrophilicity (i.e., the wettability by water) of the fibers, as the cement mix is water-based. The hydrophilicity can be controlled by treating the surfaces of the fibers prior to incorporating the fibers into the cement mix. Furthermore, the fiber dispersion is affected by the admixtures that may