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Assessment of moisture content in nylon using differential scanning calorimetry
Polymer Testing 10 (1991) 189-194
Assessment of Moisture Content in Nylon Using Differential Scanning Calorimetry C. Y. Y u e School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, 2263, Singapore
& C. Y. Chan Department of Mechanical Engineering, University of Hong Kong, Pokfulam Road, Hong Kong (Received 20 November 1990; accepted 23 December 1990) ABSTRACT It is shown that the absorbed water moisture content in both nylon 6 and nylon 66 can be detected and assessed by differential scanning calorimetry. The absorbed water gives rise to a lower endothermic peak below 100 °C. There is evidence that the absorbed water is not present solely in the amorphous regions. The amount of absorbed water moisture present could be estimated from the endotherms.
INTRODUCTION The assessment of moisture content in nylon granules is important since the absorbed water will affect the processibility and the appearance and properties of the moulding. The absorbed water also affects the properties of the moulding through its influence 1-3 on the glass transition temperature. Absorbed water is believed 4 to be present as firmly bound water, loosely b o u n d water and capillary condensed water in the polymer. To date, only dilatometric and coulometric techniques have been utilised to detect the absorbed moisture. In the present work, a m e t h o d for detecting and assessing the absorbed water in nylon using the differential scanning calorimetry (DSC) is outlined. The nature of water retention in nylon 6 and nylon 66 is also considered. 189 Polymer Testing 0142-9418/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
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C. Y. Yue, C. Y. Chan
EXPERIMENTAL The nylon 6 and nylon 66 granules used were Bayer Durethan B 30 S (Bayer AG, FRG) and Du Pont Zytel 101 (Du Pont, USA), respectively. The granules were compressed into thin platelets about 5 cm long either between two glass slides or steel plates on a hot-plate at 10 °C above the respective melting points of the resins. Silicon oil was used as the mould release agent. The molten granules were either cooled in air (AC), quenched in water (WQ) or quenched in liquid nitrogen (LNQ), immediately after they were compressed into platelets to yield specimens with different degrees of crystallinity. The platelets were used to maximise the surface area of the specimen to promote water absorption/desorption and to allow accurate thermal analysis through maximising the contact area of the sample and the DSC pan. The thermal analysis of the platelets was conducted on a Du Pont 9900 DSC system. Sample weights of about 10 mg were used and the weight of each sample weight was recorded. Unless otherwise stated, a heating rate of 10 °C/min was used. However, heating rates of 1 to 80 °C/min were utilised in the study of the effect of scanning rate on the endothermic peaks. The heat of fusion of both 100% crystalline nylon 6 and nylon 66 was assumed to be 44 cal/g(184-2 J/g). Samples of the LNQ specimens were scanned immediately after preparation. In contrast, the AC and WQ specimens were washed with water and then dichloromethane to remove the silicone before they were scanned. Other AC and WQ specimens were scanned after one of the following three treatments: oven-dry for 2 days at 100 °C, immersion in boiling water for 3 h or immersion in boiling water for 3 h followed by oven-drying for up to 75 min at 70 °C. The immersion of the platelets in boiling water facilitated rapid attainment of the equilibrium concentration of water in the specimens. The treatment of the platelets in boiling water did not cause any microstructural changes in the nylons. Thermal analysis of the as-received resin granules and of resin granules which had been immersed in water for 25 days under room temperature conditions (23 °C) were also carried out. Slices of the granules obtained using a knife were used as samples.
RESULTS AND DISCUSSION Typical melting endotherms for LNQ-nylon 66 and AC-nylon 6 are as shown in Fig. 1. The shape of the melting endotherm for the LNQ-nylon 6 sample was similar to that for the LNQ-nylon 66
Assessment of moisture content in nylon
191
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1 Temperature ( "C ) Fig. l. Melting endotherms for nylon 6 and 66 samples: (A) LNQ nylon 66 sample, 20 °C/rain; (B-D) AC nylon 6 samples scanned at 10, 20 and 60 °C/min, respectively.
specimen, while the shape of the melting endotherms for AC-nylon 66, WQ-nylon 6, and WQ-nylon 66 specimens were similar to that for the AC-nylon 6 specimen. The only difference between the nylon 6 and nylon 66 results was in the location of the melting peaks of the polymers. No cold crystallization peaks were observed in the thermograms. From Fig. I(A) it can be seen that the LNQ nylon samples only exhibit one endothermic peak. This peak can be attributed to the melting of crystallites. The AC and WQ samples for both nylons exhibited an additional lower temperature peak (see Fig. I(B-D)). This suggests that the lower endothermic peak is associated with the presence of absorbed water moisture since only the LNQ samples would be free of water. This is substantiated by the fact that all the oven-dried AC nylon samples, almost all oven-dried WQ nylon samples, and all the boiled-oven-dried nylon samples exhibited endotherms similar to that for the LNQ samples. Therefore, the above results indicate that very little moisture is present in the oven-dried samples and that the lower temperature endotherm is associated with the presence of absorbed moisture. There was no difference in the size and area (in J/g) of the lower
192
c. Y. Yue, C. Y. Chart
water moisture endothermic peaks between the boiled AC and WQ samples and the as-prepared WQ nylon samples of the same crystallinity. This is surprising since the as-prepared WQ platelets were not in contact with water for as long as the boiled AC and WQ samples. The as-prepared WQ samples were only in contact with water when the WQ platelet was quenched and when the silicone was being washed off, while the diffusion of water into the latter samples occurred for 3 h at an elevated temperature. There is little doubt that the boiled nylon samples had reached their equilibrium concentration of absorbed water. This is because the size and area of their lower temperature endothermic peaks were similar to that for a sample taken from the surface of the respective resin granule that had been immersed in water for 25 days. It was further observed that for samples of the same degree of crystallinity, the area of the moisture endotherms in the as-prepared AC samples was always smaller than that for the as-prepared WQ samples (see Fig. 2). This is to be expected since such AC samples were only in contact with water during washing of the platelets. The above observations suggest that the equilibrium concentration of absorbed water is attained fairly rapidly and easily in the nylon platelets due to their large surface-to-mass ratio. The absorbed moisture content of each sample could be calculated from the area of moisture endotherm by taking the heat of vapourisation of absorbed water from the nylon sample to be equal to that of free water (40.7 kJ/mol). This assumption is reasonable since most of the absorbed water in nylon exists as loosely bound water and capillary absorbed water but not as firmly bound water. Moreover, the heat of
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lqg. 2. Plot of equilibrium water absorption against the degree of crystallinityin nylon 66: (O) samples with equilibrium water absorption; ( x ) as-prepared AC samples.
Assessment of moisture content in nylon
193
vapourisation for firmly bound water is much lower and is only 8.4 kJ/mol, while that for loosely bound water and capillary absorbed water is believed 4 to be similar to that for free water. On this basis, the equilibrium water absorption in nylon 66 (30% crystallinity) was calculated to be 10% (see Fig. 2) which is similar to that reported elsewhere. 3 The above method for calculating the absorbed moisture content is therefore valid. The plot of equilibrium water absorption against the crystallinity of nylon 66 is as shown in Fig. 2. A similar plot was obtained for nylon 6. It can be seen from Fig. 2 that although the equilibrium water absorption increased with decreases in the crystallinity, their relationship was not linear. This is in contrast to results reported elsewhere. 3 However, the difference can probably be attributed to the different routes used to attain the equilibrium water concentration. In earlier work, 3 water absorption of the nylon occurred via diffusion of airborne water from the controlled humid atmosphere. In contrast, in the present work, diffusion of water into the nylon occurred during immersion and direct contact of the platelets with water. The nonlinear relationship in Fig. 2 implies either that the absorbed water is not all contained only within the amorphous regions or that the assumed linear relationship between equilibrium water absorption and crystallinity only applies if the retained moisture in the nylon exists predominantly as firmly and loosely bound water but not capillary water as may be the case with the latter samples. However, more work is necessary before a firm conclusion can be reached. Since the lower endothermic peak is associated with the vapourisation of water moisture, the position of this peak is expected to vary with the scanning rate. This is due to the temperature lag and larger temperature difference between the DSC sample pan and the nylon sample at the higher scanning rates. This is as shown in Fig. 1(B-D). It is apparent that an increase in the scanning rate shifts the lower moisture endotherm to higher temperatures such that at above 20 °C/min, the melting of the crystallites occurs before all the absorbed water had evaporated (see Fig. I(C)). The change in scanning rate has a much larger effect on the lower moisture endothermic peak than on the melting peak of the crystallites. The change in location of the peak of the moisture endotherm for nylon 66 with scanning rate is as shown in Fig. 3. It can be seen in Fig. 3 that the peak temperature was moved to 170 °C at a scanning rate of 80 °C/min. Extrapolation of the plot to a very slow scanning rate close to 1 °C/min gives a peak temperature of 75 °C which is reasonable. Similar results were obtained for nylon 6. In contrast, a change in
194
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Plot of lower endothermic peak temperature against DSC scanning rate.
scanning rate from 1 to 80 °C/min caused the position of the crystallite melting peak to increase by only 16 °C.
CONCLUSIONS The absorbed water moisture in nylons gives rise to the presence of a lower endothermic peak at around 100 °C. The actual position of the endothermic peak is d e p e n d e n t on the scanning rate. The a m o u n t of absorbed water can be reliably calculated from the area of this endothermic peak. Equilibrium water absorption is attained rapidly in nylon samples which are in direct contact with water. The present work shows that the absorbed water in nylon 6 and nylon 66 can be detected and assessed using differential scanning calorimetry.
REFERENCES 1. Jin, X., Ellis, T. S. & Karasz, F. E., The effect of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water. J. Polym. Sci., Polym. Phys. Edn, 22, (1984) 1701-17. 2. Kettle, G. J., Variation of the glass transition temperature of nylon-6 with changing water content. Polymer, 111 (1977) 742-3. 3. Starkweather, H. W., Moore, G. E., Hansen, J. E., Roder, T. M. & Brooks, R. E., Effect of crystallinity on the properties of nylons. J. Polym. Sci., 21 (1956) 189-204. 4. Puffr, R. & Sebenda, J., On the structure and properties of polyamides--The mechanism of water sorption in polyamides. J. Polym. Sci., Part C, 16 (1967) 79-93.
Assessment of Moisture Content in Nylon Using Differential Scanning Calorimetry C. Y. Y u e School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, 2263, Singapore
& C. Y. Chan Department of Mechanical Engineering, University of Hong Kong, Pokfulam Road, Hong Kong (Received 20 November 1990; accepted 23 December 1990) ABSTRACT It is shown that the absorbed water moisture content in both nylon 6 and nylon 66 can be detected and assessed by differential scanning calorimetry. The absorbed water gives rise to a lower endothermic peak below 100 °C. There is evidence that the absorbed water is not present solely in the amorphous regions. The amount of absorbed water moisture present could be estimated from the endotherms.
INTRODUCTION The assessment of moisture content in nylon granules is important since the absorbed water will affect the processibility and the appearance and properties of the moulding. The absorbed water also affects the properties of the moulding through its influence 1-3 on the glass transition temperature. Absorbed water is believed 4 to be present as firmly bound water, loosely b o u n d water and capillary condensed water in the polymer. To date, only dilatometric and coulometric techniques have been utilised to detect the absorbed moisture. In the present work, a m e t h o d for detecting and assessing the absorbed water in nylon using the differential scanning calorimetry (DSC) is outlined. The nature of water retention in nylon 6 and nylon 66 is also considered. 189 Polymer Testing 0142-9418/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
190
C. Y. Yue, C. Y. Chan
EXPERIMENTAL The nylon 6 and nylon 66 granules used were Bayer Durethan B 30 S (Bayer AG, FRG) and Du Pont Zytel 101 (Du Pont, USA), respectively. The granules were compressed into thin platelets about 5 cm long either between two glass slides or steel plates on a hot-plate at 10 °C above the respective melting points of the resins. Silicon oil was used as the mould release agent. The molten granules were either cooled in air (AC), quenched in water (WQ) or quenched in liquid nitrogen (LNQ), immediately after they were compressed into platelets to yield specimens with different degrees of crystallinity. The platelets were used to maximise the surface area of the specimen to promote water absorption/desorption and to allow accurate thermal analysis through maximising the contact area of the sample and the DSC pan. The thermal analysis of the platelets was conducted on a Du Pont 9900 DSC system. Sample weights of about 10 mg were used and the weight of each sample weight was recorded. Unless otherwise stated, a heating rate of 10 °C/min was used. However, heating rates of 1 to 80 °C/min were utilised in the study of the effect of scanning rate on the endothermic peaks. The heat of fusion of both 100% crystalline nylon 6 and nylon 66 was assumed to be 44 cal/g(184-2 J/g). Samples of the LNQ specimens were scanned immediately after preparation. In contrast, the AC and WQ specimens were washed with water and then dichloromethane to remove the silicone before they were scanned. Other AC and WQ specimens were scanned after one of the following three treatments: oven-dry for 2 days at 100 °C, immersion in boiling water for 3 h or immersion in boiling water for 3 h followed by oven-drying for up to 75 min at 70 °C. The immersion of the platelets in boiling water facilitated rapid attainment of the equilibrium concentration of water in the specimens. The treatment of the platelets in boiling water did not cause any microstructural changes in the nylons. Thermal analysis of the as-received resin granules and of resin granules which had been immersed in water for 25 days under room temperature conditions (23 °C) were also carried out. Slices of the granules obtained using a knife were used as samples.
RESULTS AND DISCUSSION Typical melting endotherms for LNQ-nylon 66 and AC-nylon 6 are as shown in Fig. 1. The shape of the melting endotherm for the LNQ-nylon 6 sample was similar to that for the LNQ-nylon 66
Assessment of moisture content in nylon
191
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1 Temperature ( "C ) Fig. l. Melting endotherms for nylon 6 and 66 samples: (A) LNQ nylon 66 sample, 20 °C/rain; (B-D) AC nylon 6 samples scanned at 10, 20 and 60 °C/min, respectively.
specimen, while the shape of the melting endotherms for AC-nylon 66, WQ-nylon 6, and WQ-nylon 66 specimens were similar to that for the AC-nylon 6 specimen. The only difference between the nylon 6 and nylon 66 results was in the location of the melting peaks of the polymers. No cold crystallization peaks were observed in the thermograms. From Fig. I(A) it can be seen that the LNQ nylon samples only exhibit one endothermic peak. This peak can be attributed to the melting of crystallites. The AC and WQ samples for both nylons exhibited an additional lower temperature peak (see Fig. I(B-D)). This suggests that the lower endothermic peak is associated with the presence of absorbed water moisture since only the LNQ samples would be free of water. This is substantiated by the fact that all the oven-dried AC nylon samples, almost all oven-dried WQ nylon samples, and all the boiled-oven-dried nylon samples exhibited endotherms similar to that for the LNQ samples. Therefore, the above results indicate that very little moisture is present in the oven-dried samples and that the lower temperature endotherm is associated with the presence of absorbed moisture. There was no difference in the size and area (in J/g) of the lower
192
c. Y. Yue, C. Y. Chart
water moisture endothermic peaks between the boiled AC and WQ samples and the as-prepared WQ nylon samples of the same crystallinity. This is surprising since the as-prepared WQ platelets were not in contact with water for as long as the boiled AC and WQ samples. The as-prepared WQ samples were only in contact with water when the WQ platelet was quenched and when the silicone was being washed off, while the diffusion of water into the latter samples occurred for 3 h at an elevated temperature. There is little doubt that the boiled nylon samples had reached their equilibrium concentration of absorbed water. This is because the size and area of their lower temperature endothermic peaks were similar to that for a sample taken from the surface of the respective resin granule that had been immersed in water for 25 days. It was further observed that for samples of the same degree of crystallinity, the area of the moisture endotherms in the as-prepared AC samples was always smaller than that for the as-prepared WQ samples (see Fig. 2). This is to be expected since such AC samples were only in contact with water during washing of the platelets. The above observations suggest that the equilibrium concentration of absorbed water is attained fairly rapidly and easily in the nylon platelets due to their large surface-to-mass ratio. The absorbed moisture content of each sample could be calculated from the area of moisture endotherm by taking the heat of vapourisation of absorbed water from the nylon sample to be equal to that of free water (40.7 kJ/mol). This assumption is reasonable since most of the absorbed water in nylon exists as loosely bound water and capillary absorbed water but not as firmly bound water. Moreover, the heat of
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lqg. 2. Plot of equilibrium water absorption against the degree of crystallinityin nylon 66: (O) samples with equilibrium water absorption; ( x ) as-prepared AC samples.
Assessment of moisture content in nylon
193
vapourisation for firmly bound water is much lower and is only 8.4 kJ/mol, while that for loosely bound water and capillary absorbed water is believed 4 to be similar to that for free water. On this basis, the equilibrium water absorption in nylon 66 (30% crystallinity) was calculated to be 10% (see Fig. 2) which is similar to that reported elsewhere. 3 The above method for calculating the absorbed moisture content is therefore valid. The plot of equilibrium water absorption against the crystallinity of nylon 66 is as shown in Fig. 2. A similar plot was obtained for nylon 6. It can be seen from Fig. 2 that although the equilibrium water absorption increased with decreases in the crystallinity, their relationship was not linear. This is in contrast to results reported elsewhere. 3 However, the difference can probably be attributed to the different routes used to attain the equilibrium water concentration. In earlier work, 3 water absorption of the nylon occurred via diffusion of airborne water from the controlled humid atmosphere. In contrast, in the present work, diffusion of water into the nylon occurred during immersion and direct contact of the platelets with water. The nonlinear relationship in Fig. 2 implies either that the absorbed water is not all contained only within the amorphous regions or that the assumed linear relationship between equilibrium water absorption and crystallinity only applies if the retained moisture in the nylon exists predominantly as firmly and loosely bound water but not capillary water as may be the case with the latter samples. However, more work is necessary before a firm conclusion can be reached. Since the lower endothermic peak is associated with the vapourisation of water moisture, the position of this peak is expected to vary with the scanning rate. This is due to the temperature lag and larger temperature difference between the DSC sample pan and the nylon sample at the higher scanning rates. This is as shown in Fig. 1(B-D). It is apparent that an increase in the scanning rate shifts the lower moisture endotherm to higher temperatures such that at above 20 °C/min, the melting of the crystallites occurs before all the absorbed water had evaporated (see Fig. I(C)). The change in scanning rate has a much larger effect on the lower moisture endothermic peak than on the melting peak of the crystallites. The change in location of the peak of the moisture endotherm for nylon 66 with scanning rate is as shown in Fig. 3. It can be seen in Fig. 3 that the peak temperature was moved to 170 °C at a scanning rate of 80 °C/min. Extrapolation of the plot to a very slow scanning rate close to 1 °C/min gives a peak temperature of 75 °C which is reasonable. Similar results were obtained for nylon 6. In contrast, a change in
194
C. Y. Yue, C. :1I. Chan 200-
~" 150100-
oi o 8
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2b
4b
6'0
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rate
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Plot of lower endothermic peak temperature against DSC scanning rate.
scanning rate from 1 to 80 °C/min caused the position of the crystallite melting peak to increase by only 16 °C.
CONCLUSIONS The absorbed water moisture in nylons gives rise to the presence of a lower endothermic peak at around 100 °C. The actual position of the endothermic peak is d e p e n d e n t on the scanning rate. The a m o u n t of absorbed water can be reliably calculated from the area of this endothermic peak. Equilibrium water absorption is attained rapidly in nylon samples which are in direct contact with water. The present work shows that the absorbed water in nylon 6 and nylon 66 can be detected and assessed using differential scanning calorimetry.
REFERENCES 1. Jin, X., Ellis, T. S. & Karasz, F. E., The effect of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water. J. Polym. Sci., Polym. Phys. Edn, 22, (1984) 1701-17. 2. Kettle, G. J., Variation of the glass transition temperature of nylon-6 with changing water content. Polymer, 111 (1977) 742-3. 3. Starkweather, H. W., Moore, G. E., Hansen, J. E., Roder, T. M. & Brooks, R. E., Effect of crystallinity on the properties of nylons. J. Polym. Sci., 21 (1956) 189-204. 4. Puffr, R. & Sebenda, J., On the structure and properties of polyamides--The mechanism of water sorption in polyamides. J. Polym. Sci., Part C, 16 (1967) 79-93.