Chapter 9:Predicting Thermooxidative Degradation 379 () Oxidized surface layer 世 00 (specimens aged in a non-oxidizing environment) Average modulus in unoxidized interior of specimen 0 100 200300400500 600700 Aging time(hrs) Fig.9.11.Variation of elastic moduli with aging time The open circle data points in Fig.9.11 are the average modulus values in the interior,i.e.,in the unoxidized region,for the specimens aged in air. These measurements are in good agreement with the elastic modulus of neat resin specimens aged in a nonoxidizing environment.Because the oxidation process is diffusion limited,the interior of the aged specimens is not oxidized and has properties similar to that of the specimens aged in a nonoxidizing or inert environment.These results,therefore,validate the assumptions of Tsuji et al.[113]that the properties of the specimens aged in a nonoxidizing environment can be taken to be similar to the properties of the unoxidized core material in the air-aged specimens. Oxidation of the surface layer typically leads to a decrease in local density and a weight loss(due to outgassing of oxidation byproducts)both of which contribute to the shrinkage of the oxidized layer generating tensile stresses and possibly "spontaneous"cracks [14,29,38,63,78].For PMR-15 neat resin specimens aged in air at 343C,surface cracks in the outer oxidized layer begin to appear after 200 h of aging.These crack faces provide additional diffusion surfaces,create pathways for oxidants to penetrate deeper into the material,and accelerate the material degradation and growth of the oxidation layer [78]. Figure 9.12 shows the measured crack density as a function of aging time.The crack density is computed by dividing the total number of cracks formed on the sample edges at a given cross section by the specimen's perimeter.As one would expect,crack density increases with aging time. Beyond 800 h of aging,the severe degradation makes the crack density too complex to quantify.Figure 9.13 shows that,after 342 h of aging,crack depths have already reached 263 um into the surface of the polymer.The
Fig. 9.11. Variation of elastic moduli with aging time The open circle data points in Fig. 9.11 are the average modulus values in the interior, i.e., in the unoxidized region, for the specimens aged in air. These measurements are in good agreement with the elastic modulus of neat resin specimens aged in a nonoxidizing environment. Because the oxidation process is diffusion limited, the interior of the aged specimens is not oxidized and has properties similar to that of the specimens aged in a nonoxidizing or inert environment. These results, therefore, validate the assumptions of Tsuji et al. [113] that the properties of the specimens aged in a nonoxidizing environment can be taken to be similar to the properties of the unoxidized core material in the air-aged specimens. Oxidation of the surface layer typically leads to a decrease in local density and a weight loss (due to outgassing of oxidation byproducts) both of which contribute to the shrinkage of the oxidized layer generating tensile stresses and possibly “spontaneous” cracks [14, 29, 38, 63, 78]. For PMR-15 neat resin specimens aged in air at 343°C, surface cracks in the outer oxidized layer begin to appear after 200 h of aging. These crack faces provide additional diffusion surfaces, create pathways for oxidants to penetrate deeper into the material, and accelerate the material degradation and growth of the oxidation layer [78]. Figure 9.12 shows the measured crack density as a function of aging time. The crack density is computed by dividing the total number of cracks formed on the sample edges at a given cross section by the specimen’s perimeter. As one would expect, crack density increases with aging time. Beyond 800 h of aging, the severe degradation makes the crack density too complex to quantify. Figure 9.13 shows that, after 342 h of aging, crack depths have already reached 263 µm into the surface of the polymer. The 0 1 2 3 4 5 6 7 8 0 100 200 300 400 500 600 700 Average modulus in unoxidized interior of specimen Elastic Modulus (GPa) Aging time (hrs) Oxidized surface layer (specimens aged in a non-oxidizing environment) Chapter 9: Predicting Thermooxidative Degradation 379
380 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju 2.0 343℃ 1.5 1.0 0.5 0.0 0 20040060080010001200 Aging Time (hrs) Fig.9.12.Measurement of surface crack density for specimens aged in air photomicrograph suggests that the cracks formed in several steps(primary, secondary,and tertiary cracks).As the cracks propagate into the surface and widen,new oxidative surfaces form around the crack front and sub- sequent cracking occurs.Thus,oxidants are able to penetrate further into the sample through extensive cracking. Figure 9.14 illustrates severe oxidation damage observed in PMR-15 neat resin specimens after 670 h of aging in air at 343C.Note the appear- ance of voids in the oxidized layer with a larger concentration near the surface.The voids form because inward diffusion of oxygen is slower than outward diffusion of degradation byproducts [63]. While optical microscopy techniques are successfully used to monitor oxidation in PMR-15 resin,the same is not true for other polyimide systems, such as the recently developed AFR-PE-4 resin,because the optical characteristics of the polyimide do not change when it is oxidized.Other techniques,such as dark-field imaging [95],polarized light microscopy, Tertary Crack 262.78wm Secondary Crack Primary Crack 50 Fig.9.13.Crack penetration depth after Fig.9.14.Damaged PMR-15 from 342 h of aging in air at 343C isothermal aging at 343C for 670 h
photomicrograph suggests that the cracks formed in several steps (primary, secondary, and tertiary cracks). As the cracks propagate into the surface and widen, new oxidative surfaces form around the crack front and subsequent cracking occurs. Thus, oxidants are able to penetrate further into the sample through extensive cracking. 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 1200 Crack density (cracks/mm) Aging Time (hrs) 343o C Fig. 9.12. Measurement of surface crack density for specimens aged in air Figure 9.14 illustrates severe oxidation damage observed in PMR-15 neat resin specimens after 670 h of aging in air at 343°C. Note the appearance of voids in the oxidized layer with a larger concentration near the surface. The voids form because inward diffusion of oxygen is slower than outward diffusion of degradation byproducts [63]. While optical microscopy techniques are successfully used to monitor oxidation in PMR-15 resin, the same is not true for other polyimide systems, such as the recently developed AFR-PE-4 resin, because the optical characteristics of the polyimide do not change when it is oxidized. Other techniques, such as dark-field imaging [95], polarized light microscopy, 342 h of aging in air at 343°C isothermal aging at 343°C for 670 h G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju Fig. 9.13. Crack penetration depth after Fig. 9.14. Damaged PMR-15 from 380
Chapter 9:Predicting Thermooxidative Degradation 381 and scanning electron microscopy in backscatter mode [631,were also used without successfully being able to optically view the oxidation layer [79].This challenging problem motivated the use of surface analysis tech- niques,such as XPS and FTIR,in an attempt to detect changes in surface chemistry as a means to monitor oxidation development.Although these surface analysis techniques are able to detect increased concentrations of oxygen in the surface regions of aged specimens,the techniques are not sensitive enough to provide a quantitative measure of the extent of oxidation. Therefore,changes in the moduli,as measured by nanoindentation scans, are used to provide a quantitative measure of the degree of oxidation and monitor the propagation of the oxidation. 9.2.3 Composite Behavior The anisotropic oxidative response of PMCs was first documented by Nelson [67]when he observed that the oxidation process is sensitive to the surface area for the different test specimen geometries that he investigated He found that the dominant degradation mechanism for the graphite/ polyimides is oxidation of the matrix at the laminate edges.Additionally, the materials degraded preferentially at the specimen surface perpendicular to the fiber(axial surface)and the rate of oxidation is hastened by micro- cracks opening on the axial surface increasing the surface area for oxidation The anisotropic nature of oxidation in HTPMCs has also been observed by numerous other investigators including the works of Nam and Seferis [66] and Skontorp [95].Although it is well documented that unidirectional com- posites preferentially oxidize in the fiber direction [11,66,671,details of the rate of oxidation propagation in the orthogonal directions of unidirectional composites are not well documented.There is also substantial evidence that,for woven composites,the oxidation preferentially advances in the in- plane direction along the fiber paths [20].The oxidation in the transverse direction (normal to the specimen's top and bottom or tool surfaces)is typically constrained by the presence of the fibers. When testing a T650-35/PMR-15,8-harness,satin-weave graphite fiber material,Bowles [9]observed two types of surface degradations. Higher temperature aging(288-316C)results in the formation of a light- colored surface layer that propagates into the material causing voids and microcracks to initiate and grow within the surface layer.At temperatures lower than 288C,specimens show the same advancement of voids and microcracks into the surface but the oxidized light band of matrix material is not visible.That is,at lower temperatures,oxidation does not cause a visible change in the color of the polymer
and scanning electron microscopy in backscatter mode [63], were also used without successfully being able to optically view the oxidation layer [79]. This challenging problem motivated the use of surface analysis techniques, such as XPS and FTIR, in an attempt to detect changes in surface chemistry as a means to monitor oxidation development. Although these surface analysis techniques are able to detect increased concentrations of oxygen in the surface regions of aged specimens, the techniques are not sensitive enough to provide a quantitative measure of the extent of oxidation. Therefore, changes in the moduli, as measured by nanoindentation scans, are used to provide a quantitative measure of the degree of oxidation and monitor the propagation of the oxidation. 9.2.3 Composite Behavior The anisotropic oxidative response of PMCs was first documented by Nelson [67] when he observed that the oxidation process is sensitive to the surface area for the different test specimen geometries that he investigated. He found that the dominant degradation mechanism for the graphite/ polyimides is oxidation of the matrix at the laminate edges. Additionally, the materials degraded preferentially at the specimen surface perpendicular to the fiber (axial surface) and the rate of oxidation is hastened by microcracks opening on the axial surface increasing the surface area for oxidation. The anisotropic nature of oxidation in HTPMCs has also been observed by numerous other investigators including the works of Nam and Seferis [66] and Skontorp [95]. Although it is well documented that unidirectional composites preferentially oxidize in the fiber direction [11, 66, 67], details of the rate of oxidation propagation in the orthogonal directions of unidirectional composites are not well documented. There is also substantial evidence that, for woven composites, the oxidation preferentially advances in the inplane direction along the fiber paths [20]. The oxidation in the transverse direction (normal to the specimen’s top and bottom or tool surfaces) is typically constrained by the presence of the fibers. When testing a T650-35/PMR-15, 8-harness, satin-weave graphite fiber material, Bowles [9] observed two types of surface degradations. Higher temperature aging (288–316°C) results in the formation of a lightcolored surface layer that propagates into the material causing voids and microcracks to initiate and grow within the surface layer. At temperatures lower than 288°C, specimens show the same advancement of voids and microcracks into the surface but the oxidized light band of matrix material is not visible. That is, at lower temperatures, oxidation does not cause a visible change in the color of the polymer. Chapter 9: Predicting Thermooxidative Degradation 381
382 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju Composite oxidation is primarily a surface reaction phenomenon con- trolled by the diffusion and rate of reaction of oxygen with the material so that surfaces with different microstructural characteristics are expected to exhibit different oxidation behavior due to differences in diffusivity. Resin cure shrinkage and mismatches in the coefficient of thermal expan- sion of the fibers and matrix during the composite cure process give rise to localized micromechanical residual stresses in the fiber-matrix interphase region.The diffusion process in materials has been shown to increase with increasing stress levels even becoming nonlinear,as a function of stress, for materials with linear mechanical behavior [122].Therefore,the highly stressed fiber-matrix interphase regions tend to oxidize at an accelerated rate compared to the lower stressed matrix phase of the composite. Additionally,the local stoichiometry of the resin may be altered in the fiber-matrix interphase region by the presence of glass fiber-reinforced coup- ling agents or graphite fiber-reinforced sizing agents.Woven and braided preforms require the use of a fiber coupling or sizing agents to protect the fibers from damage during the weaving and braiding process.Owing to the significant use of woven fiber prepreg and preforms in HTPMC applica- tions,the proper selection of fiber sizing is of paramount concern.The presence of the sizing can have a strong influence on the fiber-matrix interphase or interfacial properties [17,70,114]and can ultimately affect the local diffusivity and/or thermal-oxidative stability. Using the notation of Bowles and Nowak [12],three different types of composite surfaces can be defined for unidirectional composite specimens as S1=area of nonmachined resin-rich surfaces (top and bottom or tool surfaces),S2=area of surfaces cut parallel to fibers,and S3=area of surfaces cut perpendicular to fibers.For woven composites,three different surface types can be similarly defined as i=area of nonmachined resin- rich surfaces,=area of surfaces cut perpendicular to warp fibers,and =area of surfaces cut perpendicular to fill fibers.Figure 9.15 illustrates the surface area types for both the unidirectional and woven composites. S3 Fig.9.15.Types of surface areas for unidirectional and woven composites
so that surfaces with different microstructural characteristics are expected to exhibit different oxidation behavior due to differences in diffusivity. Resin cure shrinkage and mismatches in the coefficient of thermal expansion of the fibers and matrix during the composite cure process give rise to localized micromechanical residual stresses in the fiber–matrix interphase region. The diffusion process in materials has been shown to increase with increasing stress levels even becoming nonlinear, as a function of stress, for materials with linear mechanical behavior [122]. Therefore, the highly stressed fiber–matrix interphase regions tend to oxidize at an accelerated rate compared to the lower stressed matrix phase of the composite. Additionally, the local stoichiometry of the resin may be altered in the fiber–matrix interphase region by the presence of glass fiber-reinforced coupling agents or graphite fiber-reinforced sizing agents. Woven and braided preforms require the use of a fiber coupling or sizing agents to protect the fibers from damage during the weaving and braiding process. Owing to the significant use of woven fiber prepreg and preforms in HTPMC applications, the proper selection of fiber sizing is of paramount concern. The presence of the sizing can have a strong influence on the fiber–matrix interphase or interfacial properties [17, 70, 114] and can ultimately affect the local diffusivity and/or thermal-oxidative stability. Using the notation of Bowles and Nowak [12], three different types of composite surfaces can be defined for unidirectional composite specimens as S1 = area of nonmachined resin-rich surfaces (top and bottom or tool surfaces), S2 = area of surfaces cut parallel to fibers, and S3 = area of surfaces cut perpendicular to fibers. For woven composites, three different surface types can be similarly defined as Σ1 = area of nonmachined resinrich surfaces, Σ2 = area of surfaces cut perpendicular to warp fibers, and Σ3 = area of surfaces cut perpendicular to fill fibers. Figure 9.15 illustrates the surface area types for both the unidirectional and woven composites. Fig. 9.15. Types of surface areas for unidirectional and woven composites Σ2 Σ3 S1 S2 S3 Σ1 Composite oxidation is primarily a surface reaction phenomenon controlled by the diffusion and rate of reaction of oxygen with the material 382 G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju
Chapter 9:Predicting Thermooxidative Degradation 383 Dark-field microscopy [95]is used to monitor the oxidation pro- pagation rates in both the axial direction(along the fiber)and the trans- verse direction (transverse to the fibers)of unidirectional G30-500/PMR-15 composites aged in air at 288C.Test specimens are removed from the aging oven at specified times,and a small cross section of the specimen is cut off and mounted in an epoxy plug for polishing,as illustrated in Fig.9.7.The original specimen is then placed back into the oven until the next specified aging time. Figure 9.16 shows (a)the original stitched micrograph and (b)an enhanced micrograph that clearly distinguishes the white oxidized material from the black unoxidized material.The oxidation layer appears as a frame around the composite specimen just as seen in aged neat resin PMR-15 samples in Fig.9.8.The specimen aged for 197 h is shown to have only minimal oxidation transverse to the fibers,but has moderate oxidation development in the axial direction.The method of enhancing the micro- graph consists of constructing (using Adobe Photoshop 7.0)a complete image of the entire composite by stitching together individual micrographs using standard light microscopy in the grayscale mode.Once the image is constructed,the apparent light-oxidized region is best fit in the lab mode to a pure white,specified as having a lightness value of 100,while the remaining unoxidized regions of the image are given a lightness value of zero.Thus,this image processing creates exactly two distinct grayscale colors:black and white. (a) (b) Fig.9.16.(a)Array of eight stitched photomicrographs of oxidized specimen cross section and(b)enhanced micrograph of the specimen cross section
Dark-field microscopy [95] is used to monitor the oxidation propagation rates in both the axial direction (along the fiber) and the transverse direction (transverse to the fibers) of unidirectional G30-500/PMR-15 composites aged in air at 288°C. Test specimens are removed from the aging oven at specified times, and a small cross section of the specimen is cut off and mounted in an epoxy plug for polishing, as illustrated in Fig. 9.7. The original specimen is then placed back into the oven until the next specified aging time. Figure 9.16 shows (a) the original stitched micrograph and (b) an enhanced micrograph that clearly distinguishes the white oxidized material from the black unoxidized material. The oxidation layer appears as a frame around the composite specimen just as seen in aged neat resin PMR-15 samples in Fig. 9.8. The specimen aged for 197 h is shown to have only minimal oxidation transverse to the fibers, but has moderate oxidation development in the axial direction. The method of enhancing the micrograph consists of constructing (using Adobe Photoshop® 7.0) a complete image of the entire composite by stitching together individual micrographs using standard light microscopy in the grayscale mode. Once the image is constructed, the apparent light-oxidized region is best fit in the lab mode to a pure white, specified as having a lightness value of 100, while the remaining unoxidized regions of the image are given a lightness value of zero. Thus, this image processing creates exactly two distinct grayscale colors: black and white. Fig. 9.16. (a) Array of eight stitched photomicrographs of oxidized specimen cross section and (b) enhanced micrograph of the specimen cross section (a) (b) Chapter 9: Predicting Thermooxidative Degradation 383