《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 1）CHAPTER 3 EVALUATION OF REINFORCEMENT FIBERS

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers CHAPTER 3 EVALUATION OF REINFORCEMENT FIBERS 3.1 INTRODUCTION This chapter describes techniques and test methods that are generally used to characterize the chemical,physical,and mechanical properties of reinforcement fibers for application in organic matrix composite materials.Reinforcements in the form of unidirectional yarns,strands,or tows,and bidirec- tional fabrics are covered.Sophisticated experimental techniques generally are required for fiber charac- terization,and test laboratories must be well-equipped and experienced for measuring fiber properties.It is also recognized that in many cases the measurement of a fiber property that manifests itself in the rein- forced composite can best be accomplished with the composite.Sections 3.2 through 3.5 recommend general techniques and test methods for evaluating carbon,glass,organic (polymeric),and other spe- cialty reinforcement fibers.Section 3.6 contains examples of test methods that can be used for evaluating fibers. Most reinforcement fibers are surface treated or have a surface treatment(e.g.,sizing)applied during their production to improve handleability and/or promote fiber-resin bonding.Surface treatments affect wettability of the fiber during impregnation as well as the dry strength and hydrolytic stability of the fiber- matrix bond during use.Because of the direct relation to composite properties,the effectiveness of any treatments to modify surface chemistry is generally measured on the composite itself by means of me- chanical tests.The amount of sizing and its compositional consistency are significant in quality control of the fiber and measurement of these parameters is part of the fiber evaluation. 3.2 CHEMICAL TECHNIQUES A wide variety of chemical and spectroscopic techniques and test methods are available to character- ize the chemical structures and compositions of reinforcement fibers.Carbon fibers are found to range from 90-100%carbon.Typically,standard and intermediate modulus PAN carbon fibers are 90-95%car- bon,with most of the remaining material being nitrogen.Minor constituents and trace elements can be extremely important when composites containing these fibers are considered for use at elevated tempera- tures(above 500F or 260C).Organic fibers usually contain significant amounts of hydrogen and one or more additional elements (e.g.,oxygen,nitrogen,and sulfur)which can be identified by spectroscopic analysis.Glass fibers contain sulfur dioxide and usually aluminum and iron oxide.Depending upon the type of glass.calcium oxide,sodium oxide,and oxides of potassium,boron,barium,titanium,zirconium, sulfur,and arsenic may be found. 3.2.1 Elemental analysis A variety of quantitative wet gravimetric and spectroscopic chemical analysis techniques may be ap- plied to analyze the compositions and trace elements in fibers.ASTM Test Method C 169 may be used to determine the chemical compositions of borosilicate glass fibers(Reference 3.2.1(a)). A suitable standardized method for carbon and hydrogen analysis,modified to handle carbon and polymeric fibers is provided by ASTM D 3178(Reference 3.2.1(b)).Carbon and hydrogen concentrations are determined by burning a weighed quantity of sample in a closed system and fixing the products of combustion in an absorption train after complete oxidation and purification from interfering substances. Carbon and hydrogen concentrations are expressed as percentages of the total dry weight of the fiber. ASTM Method D 3174(Reference 3.2.1(c)describes a related test in which metallic impurities may be determined by the analysis of ash residue. Alternatively,a variety of commercial analytical instruments are available which can quickly analyze carbon,hydrogen,nitrogen,silicon,sodium,aluminum,calcium,magnesium and other elements in rein- forcement fibers.X-ray fluorescence,atomic absorption (AA),flame emission,and inductively coupled plasma emission(ICAP)spectroscopic techniques may be employed for elemental analysis.Operating instructions and method details are available from the instrument manufacturers. 3-1

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-1 CHAPTER 3 EVALUATION OF REINFORCEMENT FIBERS 3.1 INTRODUCTION This chapter describes techniques and test methods that are generally used to characterize the chemical, physical, and mechanical properties of reinforcement fibers for application in organic matrix composite materials. Reinforcements in the form of unidirectional yarns, strands, or tows, and bidirec￾tional fabrics are covered. Sophisticated experimental techniques generally are required for fiber charac￾terization, and test laboratories must be well-equipped and experienced for measuring fiber properties. It is also recognized that in many cases the measurement of a fiber property that manifests itself in the rein￾forced composite can best be accomplished with the composite. Sections 3.2 through 3.5 recommend general techniques and test methods for evaluating carbon, glass, organic (polymeric), and other spe￾cialty reinforcement fibers. Section 3.6 contains examples of test methods that can be used for evaluating fibers. Most reinforcement fibers are surface treated or have a surface treatment (e.g., sizing) applied during their production to improve handleability and/or promote fiber-resin bonding. Surface treatments affect wettability of the fiber during impregnation as well as the dry strength and hydrolytic stability of the fiber￾matrix bond during use. Because of the direct relation to composite properties, the effectiveness of any treatments to modify surface chemistry is generally measured on the composite itself by means of me￾chanical tests. The amount of sizing and its compositional consistency are significant in quality control of the fiber and measurement of these parameters is part of the fiber evaluation. 3.2 CHEMICAL TECHNIQUES A wide variety of chemical and spectroscopic techniques and test methods are available to character￾ize the chemical structures and compositions of reinforcement fibers. Carbon fibers are found to range from 90-100% carbon. Typically, standard and intermediate modulus PAN carbon fibers are 90-95% car￾bon, with most of the remaining material being nitrogen. Minor constituents and trace elements can be extremely important when composites containing these fibers are considered for use at elevated tempera￾tures (above 500°F or 260°C). Organic fibers usually contain significant amounts of hydrogen and one or more additional elements (e.g., oxygen, nitrogen, and sulfur) which can be identified by spectroscopic analysis. Glass fibers contain sulfur dioxide and usually aluminum and iron oxide. Depending upon the type of glass, calcium oxide, sodium oxide, and oxides of potassium, boron, barium, titanium, zirconium, sulfur, and arsenic may be found. 3.2.1 Elemental analysis A variety of quantitative wet gravimetric and spectroscopic chemical analysis techniques may be ap￾plied to analyze the compositions and trace elements in fibers. ASTM Test Method C 169 may be used to determine the chemical compositions of borosilicate glass fibers (Reference 3.2.1(a)). A suitable standardized method for carbon and hydrogen analysis, modified to handle carbon and polymeric fibers is provided by ASTM D 3178 (Reference 3.2.1(b)). Carbon and hydrogen concentrations are determined by burning a weighed quantity of sample in a closed system and fixing the products of combustion in an absorption train after complete oxidation and purification from interfering substances. Carbon and hydrogen concentrations are expressed as percentages of the total dry weight of the fiber. ASTM Method D 3174 (Reference 3.2.1(c) describes a related test in which metallic impurities may be determined by the analysis of ash residue. Alternatively, a variety of commercial analytical instruments are available which can quickly analyze carbon, hydrogen, nitrogen, silicon, sodium, aluminum, calcium, magnesium and other elements in rein￾forcement fibers. X-ray fluorescence, atomic absorption (AA), flame emission, and inductively coupled plasma emission (ICAP) spectroscopic techniques may be employed for elemental analysis. Operating instructions and method details are available from the instrument manufacturers

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers Trace metallic constituents are significant in carbon and polymeric fibers because of their possible effect on the rate of fiber oxidation.The presence of metals is usually expressed as parts per million in the original dry fiber and can be determined by analyzing the ash residue.Semi-quantitative determina- tions are generally made using flame emission spectroscopy.When quantitative values are desired, atomic absorption methods are used.With respect to oxidation of carbon fibers,sodium is usually of most concern because of its tendency to catalyze the oxidation of carbon. 3.2.2 Titration The potential chemical activity of surface groups on fibers may be determined by titration techniques. For example,the relative concentration of hydrolyzable groups introduced during the manufacture or post treatment of carbon fibers may be determined by measuring the pH (section 3.6.1).However,titration techniques are typically not used on commercial carbon fibers due to the low levels of surface functional- ity. 3.2.3 Fiber structure X-Ray diffraction spectroscopy may be used to characterize the overall structure of crystalline or semi-crystalline fibers.The degree of crystallinity and orientation of crystallites have a direct effect on the modulus and other critical properties of carbon and polymeric fibers X-ray powder diffraction using commercial power supplies and diffractometer units is used to charac- terize the structure of carbon fibers.The fiber is ground into a fine powder and then the X-ray powder diffraction pattern is taken using CuK radiation.The patterns generally undergo computer analysis to de- termine the following parameters: (a)Average graphite layer spacing:from the 002 peak position. (b)Average crystal size L:from the 002 peak width (c)Average crystal size L:from the 100 peak width. (d)Average lattice dimension a-axis:from the 100 peak position. (e)The ratio of peak area to the diffused area. (f)The 002 peak area to the total diffraction area. (g)The 100 peak area to the total diffraction area. (h)The ratio of the 100 to 002 peak areas. (i)Crystallinity index:from a comparison of the X-ray diffraction of known crystallized and amor- phous carbons. X-ray scattering of crystalline fibrous materials shows the presence of sharp and diffuse diffraction patterns which are indicative of crystal phases interdispersed with amorphous regions.The concept of the crystallinity index is derived from the fact that a portion of the scattering from a fiber is diffuse and thereby contributes to the so-called amorphous background.Thus,a simple method of estimating crystal- linity is obtained by separating the diffraction pattern into crystalline (sharp)and amorphous (diffuse) components.The crystallinity index is a relative measure of crystallinity,and not an absolute numerical result,useful for correlating with physical properties of fibers. Wide angle X-ray spectroscopy and infrared spectroscopy techniques have also been developed to determine the crystallinity and orientation of molecules in polymeric fibers.Testing and interpretation of results requires specialized equipment,sophisticated computer models,and a high level of technical ex- pertise. 3.2.4 Fiber surface chemistry Fibers generally are given a surface treatment to improve the adhesion between the fibers and resin matrix materials.Gases,plasmas,liquid chemical or electrolytic treatments are employed to modify fiber 3-2

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-2 Trace metallic constituents are significant in carbon and polymeric fibers because of their possible effect on the rate of fiber oxidation. The presence of metals is usually expressed as parts per million in the original dry fiber and can be determined by analyzing the ash residue. Semi-quantitative determina￾tions are generally made using flame emission spectroscopy. When quantitative values are desired, atomic absorption methods are used. With respect to oxidation of carbon fibers, sodium is usually of most concern because of its tendency to catalyze the oxidation of carbon. 3.2.2 Titration The potential chemical activity of surface groups on fibers may be determined by titration techniques. For example, the relative concentration of hydrolyzable groups introduced during the manufacture or post treatment of carbon fibers may be determined by measuring the pH (section 3.6.1). However, titration techniques are typically not used on commercial carbon fibers due to the low levels of surface functional￾ity. 3.2.3 Fiber structure X-Ray diffraction spectroscopy may be used to characterize the overall structure of crystalline or semi-crystalline fibers. The degree of crystallinity and orientation of crystallites have a direct effect on the modulus and other critical properties of carbon and polymeric fibers. X-ray powder diffraction using commercial power supplies and diffractometer units is used to charac￾terize the structure of carbon fibers. The fiber is ground into a fine powder and then the X-ray powder diffraction pattern is taken using CuK radiation. The patterns generally undergo computer analysis to de￾termine the following parameters: (a) Average graphite layer spacing: from the 002 peak position. (b) Average crystal size Lc: from the 002 peak width (c) Average crystal size La: from the 100 peak width. (d) Average lattice dimension a-axis: from the 100 peak position. (e) The ratio of peak area to the diffused area. (f) The 002 peak area to the total diffraction area. (g) The 100 peak area to the total diffraction area. (h) The ratio of the 100 to 002 peak areas. (i) Crystallinity index: from a comparison of the X-ray diffraction of known crystallized and amor￾phous carbons. X-ray scattering of crystalline fibrous materials shows the presence of sharp and diffuse diffraction patterns which are indicative of crystal phases interdispersed with amorphous regions. The concept of the crystallinity index is derived from the fact that a portion of the scattering from a fiber is diffuse and thereby contributes to the so-called amorphous background. Thus, a simple method of estimating crystal￾linity is obtained by separating the diffraction pattern into crystalline (sharp) and amorphous (diffuse) components. The crystallinity index is a relative measure of crystallinity, and not an absolute numerical result, useful for correlating with physical properties of fibers. Wide angle X-ray spectroscopy and infrared spectroscopy techniques have also been developed to determine the crystallinity and orientation of molecules in polymeric fibers. Testing and interpretation of results requires specialized equipment, sophisticated computer models, and a high level of technical ex￾pertise. 3.2.4 Fiber surface chemistry Fibers generally are given a surface treatment to improve the adhesion between the fibers and resin matrix materials. Gases, plasmas, liquid chemical or electrolytic treatments are employed to modify fiber

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers surfaces.Introducing surface oxidation is perhaps the most common approach to modifying fiber sur- faces. Fiber surface structure,the modifications which surfaces undergo as a result of the different fiber sur- face treatments,and the relative importance of these modifications for composite properties are not well understood.This arises because of the small surface areas involved(0.5 to 1.5 m /g)and the very low concentrations of functional groups.If 20%of the surface was covered by one particular species,this would only amount to 1 umole of chemical groups per gram of fiber.Surface characterization should be carried out on fibers which have not been sized.Residual size from solvent desized fiber can interfere with most techniques,while pyrolysis techniques may alter the fiber surface due to oxidation and char products. The following techniques have been used for characterizing fiber surfaces: (a)X-ray diffraction-provides information relating to crystallite size and orientation,degree of graph- itization,and micropore characteristics. (b)Electron diffraction-gives crystallite orientation,three-dimensional order,and degree of graph- itization.(better for surfaces since penetration is only 1000-). (c)Transmission Electron Microscopy (TEM)-provides the highest resolution of any of the micro- scopic techniques routinely available.Ultramicrotomy can be used to prepare specimens,typi- cally about 50 nanometers thick,for direct TEM analysis of the fiber surfaces.TEM provides in- formation about surface fine structure and show fibrils and needle-like pores. (d)Scanning Electron Microscopy(SEM)-Gives structural and surface features.SEM is a useful technique for determining fiber diameters and identifying morphological characteristics (scales, chips,deposits,pits)on fiber surfaces. (e)Electron Spin Resonance(ESR)Spectroscopy-gives crystallite orientation. (f)X-ray Photoelectron Spectroscopy(XPS)or Electron Spectroscopy for Chemical Analysis(ESCA) measures the binding energy of core electrons in atoms excited by low energy X-rays.Changes in the chemical environment of a surface region 10-15 nanometers thick(the first few atomic lay- ers)are revealed by slight shifts in the energy of these core electrons giving information on func- tional group types and concentrations.The surface sensitivity arises because the depth of the electrons is between 1 and 2 nanometers. The ratios of total oxygen to total carbon and of oxidized carbon(including hydroxyl,ether,ester,car- bonyl and carboxy functional groups)to total carbon may be determined in carbon fibers using XPS or ESCA. (g)Auger Electron Spectroscopy (AES)-directs high energy electrons (1-5 KeV)onto surfaces to create vacancies in the core levels of atoms.These vacancies represent excited ions which may undergo de-excitation and thereby create Auger electrons.By analyzing the characteristic ener- gies of all the back-scattered Auger electrons in the energy range 0-1 KeV,the elemental compo- sition of the first 30 or 40 atomic layers(about 30 nanometers)is possible and in some cases mo- lecular information can be obtained from analysis of data. (h)lon Scattering Spectroscopy(ISS)-uses an ion as a molecular probe to identify elements on the outermost surface layer.Only atomic information can be obtained and sensitivity depends upon the atomic element. 3-3

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-3 surfaces. Introducing surface oxidation is perhaps the most common approach to modifying fiber sur￾faces. Fiber surface structure, the modifications which surfaces undergo as a result of the different fiber sur￾face treatments, and the relative importance of these modifications for composite properties are not well understood. This arises because of the small surface areas involved (0.5 to 1.5 m2 /g) and the very low concentrations of functional groups. If 20% of the surface was covered by one particular species, this would only amount to 1 µmole of chemical groups per gram of fiber. Surface characterization should be carried out on fibers which have not been sized. Residual size from solvent desized fiber can interfere with most techniques, while pyrolysis techniques may alter the fiber surface due to oxidation and char products. The following techniques have been used for characterizing fiber surfaces: (a) X-ray diffraction - provides information relating to crystallite size and orientation, degree of graph￾itization, and micropore characteristics. (b) Electron diffraction - gives crystallite orientation, three-dimensional order, and degree of graph￾itization. (better for surfaces since penetration is only 1000S). (c) Transmission Electron Microscopy (TEM) - provides the highest resolution of any of the micro￾scopic techniques routinely available. Ultramicrotomy can be used to prepare specimens, typi￾cally about 50 nanometers thick, for direct TEM analysis of the fiber surfaces. TEM provides in￾formation about surface fine structure and show fibrils and needle-like pores. (d) Scanning Electron Microscopy (SEM) - Gives structural and surface features. SEM is a useful technique for determining fiber diameters and identifying morphological characteristics (scales, chips, deposits, pits) on fiber surfaces. (e) Electron Spin Resonance (ESR) Spectroscopy - gives crystallite orientation. (f) X-ray Photoelectron Spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA) - measures the binding energy of core electrons in atoms excited by low energy X-rays. Changes in the chemical environment of a surface region 10-15 nanometers thick (the first few atomic lay￾ers) are revealed by slight shifts in the energy of these core electrons giving information on func￾tional group types and concentrations. The surface sensitivity arises because the depth of the electrons is between 1 and 2 nanometers. The ratios of total oxygen to total carbon and of oxidized carbon (including hydroxyl, ether, ester, car￾bonyl and carboxy functional groups) to total carbon may be determined in carbon fibers using XPS or ESCA. (g) Auger Electron Spectroscopy (AES) - directs high energy electrons (1-5 KeV) onto surfaces to create vacancies in the core levels of atoms. These vacancies represent excited ions which may undergo de-excitation and thereby create Auger electrons. By analyzing the characteristic ener￾gies of all the back-scattered Auger electrons in the energy range 0-1 KeV, the elemental compo￾sition of the first 30 or 40 atomic layers (about 30 nanometers) is possible and in some cases mo￾lecular information can be obtained from analysis of data. (h) Ion Scattering Spectroscopy (ISS) - uses an ion as a molecular probe to identify elements on the outermost surface layer. Only atomic information can be obtained and sensitivity depends upon the atomic element

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers (i)Secondary lon Mass Spectroscopy(SIMS)-uses a controlled sputtering process with accelerated ions to remove surface atomic layers for direct analysis by mass spectroscopy.SIMS can be used to identify surface molecules and determine their concentrations. (j)Infrared Spectroscopy(IRS)or Fourier Transform IRS(FTIRS)-absorption vibrational spectros- copy technique to obtain molecular information about surface composition.IRS yields both quali- tative and quantitative information relating to the chemical composition of surface molecules.The quality of the IR analysis depends on the fiber composition and is directly related to the care taken during sample preparation. For fibers with diameters between 0.015 and 0.03 mm,no sample preparation is required if an IR mi- croscope is available to examine fibers directly.Organic fibers may be pressed (up to 1000/m) into a film of fiber grids. (k)Laser Raman spectroscopy-absorption/vibrational spectroscopic technique which complements IR and is relatively simple to apply.Little or no sample preparation is necessary.Fibers can be oriented in the path of the incident beam for direct analysis.Fiber sample must be stable to the high intensity incident light and should not contain species that fluoresce. (1)Contact angle and wetting measurements-provide an indirect measurement of fiber surface free energy for use in predicting interfacial compatibility and thermodynamic equilibrium with matrix materials.Contact angle and wetting measurement information can be obtained by direct meas- urement of contact angle,mass pick-up,or surface velocity.Measurement of contact angles on small diameter fibers(<10 microns)is difficult if done optically.If a fiber's dimensions are known, a simple force balance may be used to determine the contact angle by measuring the force in- duced by immersing the fiber into a liquid of known surface free energy.The apparatus usually employed for this test is the Wilhelmy balance(Reference 3.2.4(a)). Contact angles 0 also may be measured indirectly by the micro-Wilhelmy technique (References 3.2.4(b-e)).A single fiber is partially immersed in a liquid and the force exerted on the fiber due to the surface tension of the liquid is measured.The contact angle is determined from the relation- ship F CyLv cose where F is the force measured corrected for buoyancy,C is the circumference of the fiber,and YLv is the surface tension of the liquid.The results may be used to determine the fiber surface free energy and the contributions of polar and dispersive components to the free en- ergy (References 3.2.4(c)and(d)). (m)Physisorption and chemisorption measurements -adsorption of inert gas or organic molecules can be used to measure fiber surface area.To obtain accurate estimates of surface area,it is important that there is complete monolayer coverage of the surface,that the area occupied by the adsorbed gas is known and that significant amounts of the gas are not taken up in micropores. Additional complications arise when the adsorption of organic molecules is used in place of gas adsorption,since it may be necessary to know the orientation of the adsorbed molecules to calcu- late surface area.Adsorption may also occur only at specific active sites and,if solutions are used,solvent molecules may be adsorbed as well. The chemical reactivity of fiber surfaces can be determined by oxygen chemisorption and desorption measurements.Topographical changes (e.g.,pores,cracks and fissures)caused by surface treatments often can be readily detected by adsorption measurements.Flow microcalorimetry is a useful technique for directly measuring heats of adsorption(Reference 3.2.4(f)). (n)Thermal desorption measurements-desorption of volatile products from fibers by heat treatment in vacuo.Thermal gravimetric analysis (TGA),gas chromatography (GC),mass spectroscopy (MS).infrared spectroscopy(IRS)analysis or combinations of pyrolysis GC/MS or TGA/IRS may be used to identify components desorbed from fiber surfaces.Below 150C,CO,NH,CH and various organic molecules are observed depending upon the fiber type. 3-4

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-4 (i) Secondary Ion Mass Spectroscopy (SIMS) - uses a controlled sputtering process with accelerated ions to remove surface atomic layers for direct analysis by mass spectroscopy. SIMS can be used to identify surface molecules and determine their concentrations. (j) Infrared Spectroscopy (IRS) or Fourier Transform IRS (FTIRS) - absorption vibrational spectros￾copy technique to obtain molecular information about surface composition. IRS yields both quali￾tative and quantitative information relating to the chemical composition of surface molecules. The quality of the IR analysis depends on the fiber composition and is directly related to the care taken during sample preparation. For fibers with diameters between 0.015 and 0.03 mm, no sample preparation is required if an IR mi￾croscope is available to examine fibers directly. Organic fibers may be pressed (up to 1000/m2 ) into a film of fiber grids. (k) Laser Raman spectroscopy - absorption/vibrational spectroscopic technique which complements IR and is relatively simple to apply. Little or no sample preparation is necessary. Fibers can be oriented in the path of the incident beam for direct analysis. Fiber sample must be stable to the high intensity incident light and should not contain species that fluoresce. (l) Contact angle and wetting measurements - provide an indirect measurement of fiber surface free energy for use in predicting interfacial compatibility and thermodynamic equilibrium with matrix materials. Contact angle and wetting measurement information can be obtained by direct meas￾urement of contact angle, mass pick-up, or surface velocity. Measurement of contact angles on small diameter fibers (< 10 microns) is difficult if done optically. If a fiber's dimensions are known, a simple force balance may be used to determine the contact angle by measuring the force in￾duced by immersing the fiber into a liquid of known surface free energy. The apparatus usually employed for this test is the Wilhelmy balance (Reference 3.2.4(a)). Contact angles θ also may be measured indirectly by the micro-Wilhelmy technique (References 3.2.4(b-e)). A single fiber is partially immersed in a liquid and the force exerted on the fiber due to the surface tension of the liquid is measured. The contact angle is determined from the relation￾ship F = CγLV cosθ where F is the force measured corrected for buoyancy, C is the circumference of the fiber, and γLV is the surface tension of the liquid. The results may be used to determine the fiber surface free energy and the contributions of polar and dispersive components to the free en￾ergy (References 3.2.4(c) and (d)). (m) Physisorption and chemisorption measurements - adsorption of inert gas or organic molecules can be used to measure fiber surface area. To obtain accurate estimates of surface area, it is important that there is complete monolayer coverage of the surface, that the area occupied by the adsorbed gas is known and that significant amounts of the gas are not taken up in micropores. Additional complications arise when the adsorption of organic molecules is used in place of gas adsorption, since it may be necessary to know the orientation of the adsorbed molecules to calcu￾late surface area. Adsorption may also occur only at specific active sites and, if solutions are used, solvent molecules may be adsorbed as well. The chemical reactivity of fiber surfaces can be determined by oxygen chemisorption and desorption measurements. Topographical changes (e.g., pores, cracks and fissures) caused by surface treatments often can be readily detected by adsorption measurements. Flow microcalorimetry is a useful technique for directly measuring heats of adsorption (Reference 3.2.4(f)). (n) Thermal desorption measurements - desorption of volatile products from fibers by heat treatment in vacuo. Thermal gravimetric analysis (TGA), gas chromatography (GC), mass spectroscopy (MS), infrared spectroscopy (IRS) analysis or combinations of pyrolysis GC/MS or TGA/IRS may be used to identify components desorbed from fiber surfaces. Below 150°C, CO, NH , CH and various organic molecules are observed depending upon the fiber type

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers (o)Chemical identification of functional groups by titrimetric,coulometric and radiographic tech- niques. 3.2.5 Sizing content and composition The amount of sizing contained on fibers is expressed as a percentage of the dry sized fiber weight. It is generally determined by extracting the fibers with a heated solvent;then the cleaned fibers are washed,dried,and weighed.ASTM Test Method C 613(Reference 3.2.5)describes a suitable method utilizing Soxhlet extraction equipment;however,similar extractions using a laboratory hot plate and beaker are also common.The selection of a solvent which quantitatively removes all the sizing but not does dissolve the fiber is essential for accuracy in this determination. Thermal removal techniques are also utilized and are most practical for the more difficult soluble siz- ings.Time,temperature,and atmosphere conditions must be predetermined to ensure the sizing is re- moved with out seriously affecting the fiber.The precise amounts of residue from decomposition of the sizing and weight loss of the fibers due to oxidation must also be known from control tests for greatest accuracy.SACMA recommended test method SRM 14-90 "Determination of Sizing Content on Carbon Fibers"describes a pyrolysis technique for carbon fibers. Sizing compositions and lot-to-lot chemical consistency may be determined by spectroscopic and chromatographic analysis of materials isolated by extracting the fibers with a suitable solvent.Acetone tetrahydrofuran and methylene chloride are commonly used solvents for extraction.Liquid and gas chro- matography and diffuse infrared spectroscopy are used to analyze or"fingerprint"the chemical composi- tions of extracts 3.2.6 Moisture content The moisture content or moisture regain of fibers or textiles may be determined using the procedure shown in Section 3.6.3.Care must be taken when applying the procedure since volatile materials in addi- tion to moisture may be removed.If possible,tests should be performed on fibers that have not been sized.Moisture content is expressed as weight percentage moisture based upon the dry weight of the specimen. 3.2.7 Thermal stability and oxidative resistance The susceptibility of fibers and fiber surface to oxidation is measured as weight loss under given con- ditions of time,temperature,and atmosphere.This is especially important in the evaluation of carbon and organic fibers considered for use in plastics exposed to elevated temperatures since it contributes to the long term degradation of composite properties.Thermal gravimetric analysis(TGA)may be used to de- termine the thermal decomposition temperature Td of carbon and organic fibers and estimate the relative amounts of volatile,organic additives and inorganic residues. A standard method for determining the weight loss of carbon fibers is given in ASTM Test Method D 4102(Reference 3.2.7(a)).Variations in this test method regarding exposure of fibers have been stud- ied and give similar results(Reference 3.2.7(b)).In order to minimize variability in test results,proper control of gas flow rates and currents is critical when performing TGA analyses. 3.2.8 Chemical resistance This section reserved for future use. 3-5

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-5 (o) Chemical identification of functional groups by titrimetric, coulometric and radiographic tech￾niques. 3.2.5 Sizing content and composition The amount of sizing contained on fibers is expressed as a percentage of the dry sized fiber weight. It is generally determined by extracting the fibers with a heated solvent; then the cleaned fibers are washed, dried, and weighed. ASTM Test Method C 613 (Reference 3.2.5) describes a suitable method utilizing Soxhlet extraction equipment; however, similar extractions using a laboratory hot plate and beaker are also common. The selection of a solvent which quantitatively removes all the sizing but not does dissolve the fiber is essential for accuracy in this determination. Thermal removal techniques are also utilized and are most practical for the more difficult soluble siz￾ings. Time, temperature, and atmosphere conditions must be predetermined to ensure the sizing is re￾moved with out seriously affecting the fiber. The precise amounts of residue from decomposition of the sizing and weight loss of the fibers due to oxidation must also be known from control tests for greatest accuracy. SACMA recommended test method SRM 14-90 "Determination of Sizing Content on Carbon Fibers" describes a pyrolysis technique for carbon fibers. Sizing compositions and lot-to-lot chemical consistency may be determined by spectroscopic and chromatographic analysis of materials isolated by extracting the fibers with a suitable solvent. Acetone, tetrahydrofuran and methylene chloride are commonly used solvents for extraction. Liquid and gas chro￾matography and diffuse infrared spectroscopy are used to analyze or "fingerprint" the chemical composi￾tions of extracts. 3.2.6 Moisture content The moisture content or moisture regain of fibers or textiles may be determined using the procedure shown in Section 3.6.3. Care must be taken when applying the procedure since volatile materials in addi￾tion to moisture may be removed. If possible, tests should be performed on fibers that have not been sized. Moisture content is expressed as weight percentage moisture based upon the dry weight of the specimen. 3.2.7 Thermal stability and oxidative resistance The susceptibility of fibers and fiber surface to oxidation is measured as weight loss under given con￾ditions of time, temperature, and atmosphere. This is especially important in the evaluation of carbon and organic fibers considered for use in plastics exposed to elevated temperatures since it contributes to the long term degradation of composite properties. Thermal gravimetric analysis (TGA) may be used to de￾termine the thermal decomposition temperature Td of carbon and organic fibers and estimate the relative amounts of volatile, organic additives and inorganic residues. A standard method for determining the weight loss of carbon fibers is given in ASTM Test Method D 4102 (Reference 3.2.7(a)). Variations in this test method regarding exposure of fibers have been stud￾ied and give similar results (Reference 3.2.7(b)). In order to minimize variability in test results, proper control of gas flow rates and currents is critical when performing TGA analyses. 3.2.8 Chemical resistance This section reserved for future use