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Voiding in glass fibre reinforced thermoplastics mouldings
Notes to the Editor Voiding in glass fibre reinforced thermoplastics mouldings M. W. Darlington and G. R. Smith Department of Materials, Cranfield Institute of Technology, Cranfield, Bedfordshire MK43 OAL, UK (Received 16 December 1974)
INTRODUCTION The tensile creep behaviour of short fibre reinforced thermoplastics has been studied in these laboratories using highly accurate equipment in which all three orthogonal strains are measured simultaneously. This has enabled volume changes associated with voiding during uniaxial tensile creep to be calculated and these have been correlated with the incidence of permanent damage (in the form of deterioration in mechanical properties) resulting from the deformation of the fibre matrix system L2. The detailed development of the voiding has not yet been studied. The nature of these composites makes the preparation of well oriented (or indeed, perfectly random) and homogeneous samples extremely difficult, and the fibre orientation distribution in typical injection mouldings of these materials is usually complex and inhomogeneous. Nevertheless, a detailed characterization of the fibre orientation distribution and all other relevant features of the composite is essential if the mechanical data are to have any real significance. Some guidance on fibre orientation distributions in the above samples has been obtained by optical examination of surfaces cut in a variety of directions through the samples and polished using standard metallographic polishing techniques. In preparation for an extension of the above studies to creep in liquid environments, a new batch of short glass fibre reinforced thermoplastics injection mouldings were obtained from a different source (referred to as moulder B) to that of the original batch (from moulder A). During an initial characterization of these mouldings a new feature became apparent. This took the form of a central white zone and was initially observed in the through-thickness cross-section of short glass fibre reinforced polypropylene (GFPP) edge-gated discs of nominal thickness 3 mm and 6 nnn. On re-exmnination, a similar, though less marked and discontinuous, central white zone was noted in the cross-section of the GFPP 3 mm thick edge-gated discs previously supplied by moulder A. The appearance of tile cross-section for the samples from moulder B is shown in Figure 1. The weight fraction of glass fibreswas 28 + 1% in all samples. Although voids were known to occur in GFPP mouldings with thick sections, enquiries and a search of the literature produced no clear statement on the nature of the white zone, or even on the occurrence of voids, in the thinner sections. In view of the possible effects of voids on mechatrical properties in air, and the proposed creep study in liquids, it was considered essential to examine the central white zone, with particular regard to any occurrence of voids. A detailed determination of fibre content in the mouldings indicated little change through thicknesS, but the central white zone was found to have a lower density than the
Figure 1
Photograph of the cross-section of 3 mm and 6 mm thickness edge-gated discs of GFPP supplied by moulder B, showing the central white zone
surrounding regions. Subsequent optical examination of the 6 mm thick GFPP disc moulding indicated the existence of voids in the central white zone, large enough to be resolved by a low power stereo-optical microscope at a magnification of 60. It was considered that the occurrence of the narrow central white zone in the 3 mm thickness moulding should be an effect similar to that observed in the 6 mm mouldings, though occurring on a smaller scale. As stereo-optical microscopy provided no evidence of the existence of voids in the 3 mm thickness mouldings, scanning electron microscopy was employed in an attempt to locate such features in the central white region. The surface preparation technique was that mentioned above for the determination of fibre orientation distributions. Although this surface preparation technique appears to be reasonably well established for the examination of glass fibre reinforced thermosets 3, and has been shown to be useful for fibre orientation distribution studies in short fibre reinforced thermoplastics, it does not appear to be readily acceptable for the study of features such as voids in the latter group of composite materials, The purpose of this note is therefore to establish the validity of the preparation technique for tile study of voids and to report some of the interesting features observed in the central white
POLYMER, 1975, Vol 16, June
459
Notes to the Editor
regions of some short glass fibre reinforced thermoplastics injection mouldings. It is concluded that (1) the polishing technique is superior to microtome methods in this context and that (2) voids occur in all the mouldings examined, but that the scale of the effect changes substantially with mould thickness. EXPERIMENTAL Examination techniques have been refined from standard metaUographic sectioning and polishing procedures in order to reduce to a minimum the number of spurious observations of surface features associated with polishing brittle glass fibres embedded in a flexible matrix. Sections were cut along planes parallel and orthogonal to the moulding surfaces from a-number of glass fibre reinforced nylon (GF nylon) mouldings, as well as from those GFPP mouldings described above. These sections were then embedded in a cold curing mounting compound prior to progressive polishing on graded emery papers followed by polishing on five graded diamond wheels down to 0.25 #m. Examination of the polished surfaces for defects by reflection optical microscopy then preceded careful extraction of the sections from the embedding medium and remounting on metal studs. These remounted, polished specimens were then coated under vacuum with a gold/palladium alloy before examination by scanning electron microscopy. RESULTS AND DISCUSSION The most severe instance of voiding observed during this study was noted in a 12 mm square cross-section GF nylon6,6 impact test bar provided by moulder A. Here holes clearly visible to the naked eye could be observed in the central white region. Figure 2 presents a low magnification scanning electron micrograph of the central region of a crosssection plane cut and polished from this impact bar. In addition to the large voids visible by eye, a wide range of smaller size voids can also be seen in this micrograph. Figure 3 then concentrates on the area around the largest void visible in
Figure 2
Scanning electron micrograph of the central zone of a section cut perpendicular to the longitudinal axis of a 1 2 mm square _ cross-section impact bar of GF nylon supplied by moulder A
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POLYMER, 1975, Vol 16, June
Figure 3 Figure 2
Detail o f a large void appearing in the micrograph of
Figure 2 and here it can be seen that void sizes range over at least two orders of magnitude (from approximately 1 mm across down to a few/am). A similar pattern of voids has been observed in the central white region of 6 mm thickness GFPP edge-gated discs. In addition to the large voids described earlier for this material many smaller voids were visible at higher magnifications by scanning electron microscopy. The largest void sizes observed in the 6 mm GFPP disc (about 0.3 mm across) were far smaller than the large void visible in Figures 2 and 3. However, a wide range of void sizes was observed and the pattern of voiding in the central white region of the 6 mm thickness GFPP disc was quite similar to that observed in the GF nylon impact bar. The incidence of a central white layer through the cross-section of 3 mm thickness GFPP edge-gated discs was far less developed than in the 6 mm GFPP disc moulding. However, this central white layer was easily observable in the 3 mm thickness disc supplied by moulder B. A wide void size range was noted in the central white zone of sections cut through the disc cross-section and in sections cut parallel to the moulding surface at half-thickness through the disc. The overall impression obtained was that void sizes in this 3 mm thickness disc (from 0.1 mm down to several/am) were smaller than those observed in the 6 mm disc provided by the same moulder. It is worth noting that sections cut at half-thickness through the disc parallel to the mould surface were found preferable to sections through the disc cross-section for the observation of these smaller holes. As the plane of these 'half-thickness' sections was the plane in which most fibres lay (the fibre distribution in the central zone of these discs being generally aligned in the plane of the disc), damage occurring at polished fibre ends was thus minimized. Examination of sections cut parallel to the surface outside the central white layer of the specimens described above produced no detectable voids (i.e. features greater than 2 or 3 ~m across). Observation of voiding in the discontinuous and narrow central white layer of the 3 mm thickness GFPP disc produced by moulder A presented a greater problem. Sections
Notes to the Editor
these laboratories by microtome techniques for the observation of voids in these composites. The polishing technique is also considered superior for the simpler problem of observing fibre orientation distributions. Figure 6 concentrates on one of the voids visible near the cut edge shown in the micrograph of Figure 4. The slight effects of fibre damage and of rounding the edge of the void, both due to the polishing technique, can be observed in this micrograph. It was possible with the void shown in Figure 6 to scan down into the void, beyond the effects of polished edgerounding, to observe the micro-structure of the void wall. This is depicted in Figure 7. It is interesting to compare
Figure 4
Voiding appearing in the central white layer of a GFPP 3 mm thickness edge-gated disc supplied by moulder A. (See t e x t for detail of sectioning.)
cut parallel to the mould surface at half thickness invariably polished through the narrow white layer during preparation. To overcome this difficulty, a section was cut through this narrow central white layer at an angle to the plane of the disc. The section was cut near to the disc edge where this central white zone had become slightly more apparent in the disc cross-section. Figure 4 presents a micrograph of this section from the GFPP 3 mm thickness disc produced by moulder A. It could be seen visually that this section had sliced through the narrow white layer and observations of this section at different magnifications under the scanning electron microscope showed that the site of the white layer coincided with the region in which the voids visible in Figure 4 occurred. Again, a large size range of voids can be seen (from 0.05 mm across to a few ~tm). It should be noted that the edge visible in this micrograph is not a moulded disc edge, but a cut edge of the section examined. Once again many sections cut parallel to the surface outside the white region in this edge-gated disc produced no voids of size greater than several/~m, Indeed very few holes even in this size range were observed. This. and the observation of holes only in the white zone, is seen as support for the contention that the holes down to this size range are inherent in the moulding and not artefacts of the preparation process. Figure 5 presents a micrograph of a small void observed in the white zone of the 3 mm GFPP disc from moulder B. This hole is approximately 5 #m across. It is thought that such a void represents the lower bound in size range which can be regarded as an inherent fault in the material when using this metallographic preparation technique. Below this size, problems of interpretation may arise due to surface damage caused by small glass particles. Figure 5 may also be used to illustrate the nature of the general polished surface at high magnification. The polished surfaces produced in the manner described above have been found to be far superior to surfaces produced in
Figure 5 Small voids appearing in the central white region of the cross-section of a 3 mm thickness GFPP edge-gated disc supplied by moulder B
Figure 6
Detail of a large void which can be observed in the scanning electron micrograph of Figure 4
POLYMER, 1975, Vol 16, June
461
Notes tO the E d i t o r
zone occurs in the cross-section of a glass fibre reinforced thermoplastic injection moulding, it is an example of the same phenomenon and involves voiding. The extent of this whitening phenomenon and the scale of voiding have been seen to be dependent on the thickness of the moulding and perhaps, more particularly, on moulding conditions. Trends with thickness in different glass fibre reinforced thermoplastics are also worth noting. While a visible white zone and voids existed in 3 mm thickness GFPP mouldings, no whitening was observed in GF nylon mouldings of a similar thickness (though it should be noted, of different fibre length distribution). However, in thicker cross-section specimens the extent of voiding appeared more pron ,unced in GF nylon, though it was accompanied by a less extensive white region than appeared in similar thickness GFPP mouldings. While this work has not endeavoured to clarify the origin of the white layer itself in these mouldings, it seems reasonable to suggest that the central white zone may arise as crazing occurs to relieve stresses created during cooling. The occurrence of this white zone may then precede the development of voids by some crazing and splitting mechanism through which built-in stress is further relieved. Figure 7
Scanningelectron micrograph of the wall of the void
shown in Figure 6
ACKNOWLEDGEMENT The authors thank Professor D. W. Saunders for helpful discussions on this work.
the fibrillar structure of this void wall with previously published 4 stereo and transmission electron micrographs of an unoriented polystyrene fracture surface in which the fibrillar structure of the fracture surface is of a similar nature to the structure observed on the void wall. Other small voids have also been observed with fibrillar material across the void suggesting that the void may have occurred as a craze parted and developed into a crack. From this work it appears that wherever a white central
REFERENCES 1 Darlington, M. W. and Clayton, D. Plastics Inst. Conf., Research Projects II1, Paper 1, London, Nov. 1971 2 Darlington,M. W. and Clayton, D. 31st Antec SPE, Montreal 1973 3 Diggwa,A. D. S. and Norman, R. H.Plastics and Polymers 1972, 40, 263 4 Haward, R. N. and Brough, l.Polymer 1969, 10, 724
Multiple melting in poly(butylene terephthalate) S. Y. Hobbs and C. F. Pratt General Electric Company, Corporate Research and Development Center, PO Box 8, Schenectady, NY 12301, USA (Received 17 December 1974)
INTRODUCTION
EXPERIMENTAL
Calorimetric investigations have revealed that a number of crystalline polymers including polyethylene l'z, polyoxymethylene 3, nylon4, and poly(ethylene terephthalate) s-8 may exhibit multiple melting peaks. This behaviour is closely tied to the thermal history of the samples and may arise from: (1) the presence of alternate crystal modifications; (2) molecular weight segregation accompanying crystallization; (3) variations in morphology; (4) orientation effects; or (5) melting, recrystallization and annealing processes taking place in the calorimeter. Identification of the specific causes of multiple melting in a particular polymer is an important aspect of its thermal characterization. We wish to report some recent calorimetric studies of multiple melting in poly(butylene terephthalate) (PBT), a polymer currently experiencing rapid growth as an injection mouldable engineering thermoplastic.
PBT samples were supplied by the GE Plastics Division (Pittsfield, Mass.) and had intrinsic viscosities ranging from 1.21 to 1.53 dl/g. All thermal measurements were carried out in a Perkin-Elmer DSC-2 calorimeter. The samples were first melted in the calorimeter for 2 min at 250°C, recrystallized by slow cooling to room temperature, and then remelted. Wide angle X-ray diffraction measurements were made on a GE XRD-5 machine using CuKa radiation.
462
POLYMER, 1975, Vol 16, June
RESULTS Since similar d.s.c, traces were obtained on all samples over the available molecular weight range only data for a sample having an intrinsic viscosity in hexafluoroisopropanol (HFIP) at 25°C of 1.31 dl/g are presented in this report. Crystallization exotherms for samples cooled at rates from
INTRODUCTION The tensile creep behaviour of short fibre reinforced thermoplastics has been studied in these laboratories using highly accurate equipment in which all three orthogonal strains are measured simultaneously. This has enabled volume changes associated with voiding during uniaxial tensile creep to be calculated and these have been correlated with the incidence of permanent damage (in the form of deterioration in mechanical properties) resulting from the deformation of the fibre matrix system L2. The detailed development of the voiding has not yet been studied. The nature of these composites makes the preparation of well oriented (or indeed, perfectly random) and homogeneous samples extremely difficult, and the fibre orientation distribution in typical injection mouldings of these materials is usually complex and inhomogeneous. Nevertheless, a detailed characterization of the fibre orientation distribution and all other relevant features of the composite is essential if the mechanical data are to have any real significance. Some guidance on fibre orientation distributions in the above samples has been obtained by optical examination of surfaces cut in a variety of directions through the samples and polished using standard metallographic polishing techniques. In preparation for an extension of the above studies to creep in liquid environments, a new batch of short glass fibre reinforced thermoplastics injection mouldings were obtained from a different source (referred to as moulder B) to that of the original batch (from moulder A). During an initial characterization of these mouldings a new feature became apparent. This took the form of a central white zone and was initially observed in the through-thickness cross-section of short glass fibre reinforced polypropylene (GFPP) edge-gated discs of nominal thickness 3 mm and 6 nnn. On re-exmnination, a similar, though less marked and discontinuous, central white zone was noted in the cross-section of the GFPP 3 mm thick edge-gated discs previously supplied by moulder A. The appearance of tile cross-section for the samples from moulder B is shown in Figure 1. The weight fraction of glass fibreswas 28 + 1% in all samples. Although voids were known to occur in GFPP mouldings with thick sections, enquiries and a search of the literature produced no clear statement on the nature of the white zone, or even on the occurrence of voids, in the thinner sections. In view of the possible effects of voids on mechatrical properties in air, and the proposed creep study in liquids, it was considered essential to examine the central white zone, with particular regard to any occurrence of voids. A detailed determination of fibre content in the mouldings indicated little change through thicknesS, but the central white zone was found to have a lower density than the
Figure 1
Photograph of the cross-section of 3 mm and 6 mm thickness edge-gated discs of GFPP supplied by moulder B, showing the central white zone
surrounding regions. Subsequent optical examination of the 6 mm thick GFPP disc moulding indicated the existence of voids in the central white zone, large enough to be resolved by a low power stereo-optical microscope at a magnification of 60. It was considered that the occurrence of the narrow central white zone in the 3 mm thickness moulding should be an effect similar to that observed in the 6 mm mouldings, though occurring on a smaller scale. As stereo-optical microscopy provided no evidence of the existence of voids in the 3 mm thickness mouldings, scanning electron microscopy was employed in an attempt to locate such features in the central white region. The surface preparation technique was that mentioned above for the determination of fibre orientation distributions. Although this surface preparation technique appears to be reasonably well established for the examination of glass fibre reinforced thermosets 3, and has been shown to be useful for fibre orientation distribution studies in short fibre reinforced thermoplastics, it does not appear to be readily acceptable for the study of features such as voids in the latter group of composite materials, The purpose of this note is therefore to establish the validity of the preparation technique for tile study of voids and to report some of the interesting features observed in the central white
POLYMER, 1975, Vol 16, June
459
Notes to the Editor
regions of some short glass fibre reinforced thermoplastics injection mouldings. It is concluded that (1) the polishing technique is superior to microtome methods in this context and that (2) voids occur in all the mouldings examined, but that the scale of the effect changes substantially with mould thickness. EXPERIMENTAL Examination techniques have been refined from standard metaUographic sectioning and polishing procedures in order to reduce to a minimum the number of spurious observations of surface features associated with polishing brittle glass fibres embedded in a flexible matrix. Sections were cut along planes parallel and orthogonal to the moulding surfaces from a-number of glass fibre reinforced nylon (GF nylon) mouldings, as well as from those GFPP mouldings described above. These sections were then embedded in a cold curing mounting compound prior to progressive polishing on graded emery papers followed by polishing on five graded diamond wheels down to 0.25 #m. Examination of the polished surfaces for defects by reflection optical microscopy then preceded careful extraction of the sections from the embedding medium and remounting on metal studs. These remounted, polished specimens were then coated under vacuum with a gold/palladium alloy before examination by scanning electron microscopy. RESULTS AND DISCUSSION The most severe instance of voiding observed during this study was noted in a 12 mm square cross-section GF nylon6,6 impact test bar provided by moulder A. Here holes clearly visible to the naked eye could be observed in the central white region. Figure 2 presents a low magnification scanning electron micrograph of the central region of a crosssection plane cut and polished from this impact bar. In addition to the large voids visible by eye, a wide range of smaller size voids can also be seen in this micrograph. Figure 3 then concentrates on the area around the largest void visible in
Figure 2
Scanning electron micrograph of the central zone of a section cut perpendicular to the longitudinal axis of a 1 2 mm square _ cross-section impact bar of GF nylon supplied by moulder A
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POLYMER, 1975, Vol 16, June
Figure 3 Figure 2
Detail o f a large void appearing in the micrograph of
Figure 2 and here it can be seen that void sizes range over at least two orders of magnitude (from approximately 1 mm across down to a few/am). A similar pattern of voids has been observed in the central white region of 6 mm thickness GFPP edge-gated discs. In addition to the large voids described earlier for this material many smaller voids were visible at higher magnifications by scanning electron microscopy. The largest void sizes observed in the 6 mm GFPP disc (about 0.3 mm across) were far smaller than the large void visible in Figures 2 and 3. However, a wide range of void sizes was observed and the pattern of voiding in the central white region of the 6 mm thickness GFPP disc was quite similar to that observed in the GF nylon impact bar. The incidence of a central white layer through the cross-section of 3 mm thickness GFPP edge-gated discs was far less developed than in the 6 mm GFPP disc moulding. However, this central white layer was easily observable in the 3 mm thickness disc supplied by moulder B. A wide void size range was noted in the central white zone of sections cut through the disc cross-section and in sections cut parallel to the moulding surface at half-thickness through the disc. The overall impression obtained was that void sizes in this 3 mm thickness disc (from 0.1 mm down to several/am) were smaller than those observed in the 6 mm disc provided by the same moulder. It is worth noting that sections cut at half-thickness through the disc parallel to the mould surface were found preferable to sections through the disc cross-section for the observation of these smaller holes. As the plane of these 'half-thickness' sections was the plane in which most fibres lay (the fibre distribution in the central zone of these discs being generally aligned in the plane of the disc), damage occurring at polished fibre ends was thus minimized. Examination of sections cut parallel to the surface outside the central white layer of the specimens described above produced no detectable voids (i.e. features greater than 2 or 3 ~m across). Observation of voiding in the discontinuous and narrow central white layer of the 3 mm thickness GFPP disc produced by moulder A presented a greater problem. Sections
Notes to the Editor
these laboratories by microtome techniques for the observation of voids in these composites. The polishing technique is also considered superior for the simpler problem of observing fibre orientation distributions. Figure 6 concentrates on one of the voids visible near the cut edge shown in the micrograph of Figure 4. The slight effects of fibre damage and of rounding the edge of the void, both due to the polishing technique, can be observed in this micrograph. It was possible with the void shown in Figure 6 to scan down into the void, beyond the effects of polished edgerounding, to observe the micro-structure of the void wall. This is depicted in Figure 7. It is interesting to compare
Figure 4
Voiding appearing in the central white layer of a GFPP 3 mm thickness edge-gated disc supplied by moulder A. (See t e x t for detail of sectioning.)
cut parallel to the mould surface at half thickness invariably polished through the narrow white layer during preparation. To overcome this difficulty, a section was cut through this narrow central white layer at an angle to the plane of the disc. The section was cut near to the disc edge where this central white zone had become slightly more apparent in the disc cross-section. Figure 4 presents a micrograph of this section from the GFPP 3 mm thickness disc produced by moulder A. It could be seen visually that this section had sliced through the narrow white layer and observations of this section at different magnifications under the scanning electron microscope showed that the site of the white layer coincided with the region in which the voids visible in Figure 4 occurred. Again, a large size range of voids can be seen (from 0.05 mm across to a few ~tm). It should be noted that the edge visible in this micrograph is not a moulded disc edge, but a cut edge of the section examined. Once again many sections cut parallel to the surface outside the white region in this edge-gated disc produced no voids of size greater than several/~m, Indeed very few holes even in this size range were observed. This. and the observation of holes only in the white zone, is seen as support for the contention that the holes down to this size range are inherent in the moulding and not artefacts of the preparation process. Figure 5 presents a micrograph of a small void observed in the white zone of the 3 mm GFPP disc from moulder B. This hole is approximately 5 #m across. It is thought that such a void represents the lower bound in size range which can be regarded as an inherent fault in the material when using this metallographic preparation technique. Below this size, problems of interpretation may arise due to surface damage caused by small glass particles. Figure 5 may also be used to illustrate the nature of the general polished surface at high magnification. The polished surfaces produced in the manner described above have been found to be far superior to surfaces produced in
Figure 5 Small voids appearing in the central white region of the cross-section of a 3 mm thickness GFPP edge-gated disc supplied by moulder B
Figure 6
Detail of a large void which can be observed in the scanning electron micrograph of Figure 4
POLYMER, 1975, Vol 16, June
461
Notes tO the E d i t o r
zone occurs in the cross-section of a glass fibre reinforced thermoplastic injection moulding, it is an example of the same phenomenon and involves voiding. The extent of this whitening phenomenon and the scale of voiding have been seen to be dependent on the thickness of the moulding and perhaps, more particularly, on moulding conditions. Trends with thickness in different glass fibre reinforced thermoplastics are also worth noting. While a visible white zone and voids existed in 3 mm thickness GFPP mouldings, no whitening was observed in GF nylon mouldings of a similar thickness (though it should be noted, of different fibre length distribution). However, in thicker cross-section specimens the extent of voiding appeared more pron ,unced in GF nylon, though it was accompanied by a less extensive white region than appeared in similar thickness GFPP mouldings. While this work has not endeavoured to clarify the origin of the white layer itself in these mouldings, it seems reasonable to suggest that the central white zone may arise as crazing occurs to relieve stresses created during cooling. The occurrence of this white zone may then precede the development of voids by some crazing and splitting mechanism through which built-in stress is further relieved. Figure 7
Scanningelectron micrograph of the wall of the void
shown in Figure 6
ACKNOWLEDGEMENT The authors thank Professor D. W. Saunders for helpful discussions on this work.
the fibrillar structure of this void wall with previously published 4 stereo and transmission electron micrographs of an unoriented polystyrene fracture surface in which the fibrillar structure of the fracture surface is of a similar nature to the structure observed on the void wall. Other small voids have also been observed with fibrillar material across the void suggesting that the void may have occurred as a craze parted and developed into a crack. From this work it appears that wherever a white central
REFERENCES 1 Darlington, M. W. and Clayton, D. Plastics Inst. Conf., Research Projects II1, Paper 1, London, Nov. 1971 2 Darlington,M. W. and Clayton, D. 31st Antec SPE, Montreal 1973 3 Diggwa,A. D. S. and Norman, R. H.Plastics and Polymers 1972, 40, 263 4 Haward, R. N. and Brough, l.Polymer 1969, 10, 724
Multiple melting in poly(butylene terephthalate) S. Y. Hobbs and C. F. Pratt General Electric Company, Corporate Research and Development Center, PO Box 8, Schenectady, NY 12301, USA (Received 17 December 1974)
INTRODUCTION
EXPERIMENTAL
Calorimetric investigations have revealed that a number of crystalline polymers including polyethylene l'z, polyoxymethylene 3, nylon4, and poly(ethylene terephthalate) s-8 may exhibit multiple melting peaks. This behaviour is closely tied to the thermal history of the samples and may arise from: (1) the presence of alternate crystal modifications; (2) molecular weight segregation accompanying crystallization; (3) variations in morphology; (4) orientation effects; or (5) melting, recrystallization and annealing processes taking place in the calorimeter. Identification of the specific causes of multiple melting in a particular polymer is an important aspect of its thermal characterization. We wish to report some recent calorimetric studies of multiple melting in poly(butylene terephthalate) (PBT), a polymer currently experiencing rapid growth as an injection mouldable engineering thermoplastic.
PBT samples were supplied by the GE Plastics Division (Pittsfield, Mass.) and had intrinsic viscosities ranging from 1.21 to 1.53 dl/g. All thermal measurements were carried out in a Perkin-Elmer DSC-2 calorimeter. The samples were first melted in the calorimeter for 2 min at 250°C, recrystallized by slow cooling to room temperature, and then remelted. Wide angle X-ray diffraction measurements were made on a GE XRD-5 machine using CuKa radiation.
462
POLYMER, 1975, Vol 16, June
RESULTS Since similar d.s.c, traces were obtained on all samples over the available molecular weight range only data for a sample having an intrinsic viscosity in hexafluoroisopropanol (HFIP) at 25°C of 1.31 dl/g are presented in this report. Crystallization exotherms for samples cooled at rates from