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Natural weathering effects on some properties of CPVC pipe material
Journal of Materials Processing Technology 191 (2007) 198–201
Natural weathering effects on some properties of CPVC pipe material Nesar Merah ∗ Mechanical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
Abstract Polyvinyl chloride (PVC) and more recently chlorinated-PVC (CPVC) pipes are extensively used for water distribution, wastewater and in limited applications for gas distribution. More and more cities are using CPVC pipes and pipefittings in their water distribution networks. Chlorinated-PVC is a material designed to withstand higher service pressures and temperatures than PVC. Its utilization in harsh environmental conditions, such as those of the Arabian Peninsula requires understanding of weathering effects, including UV, moisture and high ambient temperature on its mechanical properties. In this work, tensile and single-edge-notch bending (SENB) specimens prepared from locally manufactured CPVC commercial pipes have been naturally weathered for different periods (1–9 months) in harsh Saudi weather conditions. Standard tensile and SEN fracture toughness tests were performed after natural exposure periods of 1, 2, 3, 6 and 9 months. The tensile test results show that exposure for periods up to 9 months, including summer season, had limited effects on the tensile strength and modulus of elasticity of the material. This was expected given that the damage due weathering is mainly a surface phenomenon. However, the deterioration of the ultimate elongation is noticeable for exposure periods as low as 1 month. Analysis of fracture toughness results reveals a general trend that indicates a deterioration of this property at the beginning of the exposure time. © 2007 Elsevier B.V. All rights reserved. Keywords: CPVC; Natural weathering; Tensile; Fracture toughness
1. Introduction Most thermoplastic pipes with sufficient good quality UV screens and heat stabilizers have sufficient weather resistance to withstand normal exposure that occurs before installation. This resistance depends on the ambient temperature, humidity and geographical location. The high intensity of solar radiation and the long period of sunlight exposure coupled with temperatures attaining 50 ◦ C in summer are ingredients that can cause the breakdown of PVC and CPVC pipes used in Saudi Arabia. The service life of plastics under such harsh conditions is reported to be dramatically reduced. For instance, the tensile strength of white poly(vinyl chloride), PVC, pipes exposed for 24 months in Dhahran (Saudi Arabia) decreased by 43% while an exposure of the same duration in Florida resulted in only a 26% decrease in the property [1]. Gonzalez et al. [2] reported that one of the major problems associated with PVC is its sensitivity to weathering and UV irradiation. Furthermore, the presence of oxygen and humidity induces changes in its mechanical properties. Summers and Rabinovitch [3] found that when the air temperature varied in
∗
Fax: +966 3 860 2949. E-mail address: [email protected].
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.03.031
the range of 26–36 ◦ C during the period of exposure the surface temperature of plastics exposed to sunlight can be much higher (by as much as 60 ◦ C for common plastics depending on color and thickness) than that of the surrounding air due to heat buildup. A large number of investigations on natural and artificial weathering of PVC were performed by a number of researchers [4–11]. Puterman [4] found that while the ultimate percentage elongation of PVC specimens under tensile testing deteriorates within a short time of accelerated and natural weathering, the yield strength is hardly affected by weathering. Ragab and Alawi [5] found that the yield strength of PVC pipe material increases with natural exposure period. Testing of sharply notched specimens reflected higher susceptibility of PVC to brittle fracture due to weathering. Rabinovitch et al. [6] reported that yield strength and modulus of elasticity of PVC pipe material generally rise slightly with weathering. El Raghi et al. [7] found that the yield strength of PVC stabilized with lignin is enhanced with artificial weathering exposure while the percentage elongation experienced a drastic reduction. Discoloration of the PVC pipes is also observed by these authors [7]. Similar results were reported by Real et al. [8] for PVC under accelerated weathering. Davis and Sims [9] showed that the relative impact strength of PVC and uPVC deteriorates with weathering time. This
N. Merah / Journal of Materials Processing Technology 191 (2007) 198–201
degradation is dependent on the geographical location because of the variation of sunlight radiation intensity and sunlight hours. The relative impact strength of uPVC reduced by about 45% after 1 year exposure in Singapore and 20% in Arizona. Ollick and El-Amri [10] found that the fracture toughness of the exposed side of PVC pipe showed a 40% reduction as compared to the side in the shadow. Ragab and Alawi [11] concluded that the general trend of their experimental results showed that natural weathering of PVC material for 2 years resulted in lowering the fracture toughness of the material. Weathered pipes displayed susceptibility to brittle fracture. From the above review, it can be concluded that the study of the effect of natural and artificial weathering on the performance of PVC material had its fair share. Weathering influence on the properties of chlorinated-PVC however has not been investigated. This paper presents results on the effect of Saudi Arabian outdoor environment on tensile, fracture toughness and colorimetric properties of CPVC extruded pipe material.
199
Fig. 1. Effect of natural weathering on yield strength of CPVC pipe material.
2. Experimental procedure The specimens for tensile and fracture toughness testing programs were prepared from commercially available 4 in. schedule 80 extruded pipes manufactured by a local Saudi Company. Following the specimen preparation procedure adopted by different researchers [12,13], where rings were cut from the pipe, slit and straightened between metal plates after heating for 20 min at 105 ◦ C in an electric oven. The specimens for fracture toughness and tensile tests were then machined from the straightened plates. The tensile specimens were prepared according to the ASTM Standard D638-01 [14] method of test for tensile properties of plastics. The fracture toughness specimens were produced as per ASTM D5045-93 Standard test methods for plain-strain fracture toughness and strain energy release rate of plastic materials [15]. An initially sharp flaw (1 mm deep) was made by pressing a sharp razor blade mounted on a specially manufactured fixture. The specimens were exposed in Dhahran, Saudi Arabia in accordance with ASTM D1435-99 standard practice for outdoor weathering of plastics [16]. The fracture toughness specimens were exposed with the notched surface facing south at an angle of 45◦ . The exposure period of the specimens extended from March 2005 to November 2005. To evaluate the continuous weathering effects on the CPVC pipe material the samples were collected after 1, 2, 3, 6 and 9 months. The as-received samples were tested earlier and the results are used as baseline data. An Instron 8501 material testing frame was used for testing. The machine was equipped with a hydraulically actuated self-aligning gripping system. The tensile tests were performed at room temperature and at a strain rate of 6 × 10−4 s−1 (5 mm/min). Three point bend fracture toughness tests were conducted at room temperature and at a crosshead speed of 5 mm/min. During testing, the load P and the crack opening displacements (COD) are recorded. The latter was measured using an Instron clip gage mounted across the notch. Three to four tests were performed for each exposure condition and a reasonable data scatter was observed.
Fig. 2. Effect of natural weathering on modulus of elasticity of CPVC pipe material.
3. Results and discussion 3.1. Tensile results Table 1 illustrates the average values of the tensile properties of CPVC pipe material for different exposure periods. From these results, it can be concluded that the yield strength, the fracture strength and the modulus of elasticity are hardly affected by weathering. The percent fracture strain however, shows an abrupt decrease in average value after 1 month of natural exposure. A more general picture of weathering effect on the tensile properties is obtained when all the experimental results obtained for these properties are plotted as a function of weathering time in Figs. 1–3. The effect of natural exposure time on the yield
Table 1 Average values of CPVC tensile properties obtained at different outdoor exposure periods Exposure time (months)
Average yield strength (MPa)
Average elastic modulus (MPa)
Average fracture strain (%)
Average fracture strength (MPa)
0 1 2 6 9
46.5 46.6 46.7 46.2 48.2
2883 2956 3084 2861 3006
12.5 9.9 10.4 8.0 6.8
43.2 43.0 41.7 42.6 45.9
200
N. Merah / Journal of Materials Processing Technology 191 (2007) 198–201
Fig. 3. Effect of natural weathering on fracture strain of CPVC pipe material.
strength is depicted in Fig. 1. As reported for PVC by several researchers [6–8] weathering effect on the yield strength is minimal. This was expected given that the damage due weathering is mainly a surface phenomenon. But despite the data scatter a trend line showing a slight increase in the yield strength with exposure time can be drawn. The trend indicates that there is an increase of about 3.5% in strength over a period of 270 days. Similarly, the modulus of elasticity is slightly enhanced by weathering as shown in Fig. 2. This increase in yield strength and modulus of elasticity is mainly caused by the process of cross-linking occurring during long period of natural exposure. The fracture strain, however experiences a reduction in value that is more pronounced during the 1st month of exposure (Fig. 3). Despite the scatter in the experimental results, on the average around 20% decrease in the fracture strain is caused by a single month of weathering. Following the 1st month, the reduction is gradual reaching 45% after 9 months of exposure. The decrease in percent elongation is due to the predominant process of chain scission mainly on the specimen surface, making it more brittle. As fracture strain is generally thought of as measure of material toughness, its reduction is an indication of lower toughness.
f (a/W) =
Fig. 4. Load vs. COD for CPVC Pipe material after 1-month natural exposure. Table 2 Weathering effect on average values of critical load and fracture toughness √ Exposure time (months) Average PQ (N) Average KQ (MPa m) 0 1 3 6
488 396 382 373
3.96 3.21 3.10 3.03
3.2. Fracture toughness Fig. 4 illustrates the typical variation of applied load with crack opening displacement (COD) for fracture toughness tests performed on SENB specimens after 1 month of exposure. Table 2 presents the average values of the critical load, PQ and the fracture toughness, KQ. The critical load is obtained from the applied load versus crack opening displacement curves using the 5% secant line and the value of fracture toughness is computed from the following relationship according to ASTM D5045-93: KQ =
PQ Sf (a/W) BW 3/2
where S is the span, a the crack length, W the specimen width, B the specimen thickness and f(a/W) is a dimensionless function of a/W calculated by:
3(a/W)1/2 [1.99 − (a/W)(1 − a/W)(2.15 − 3.93(a/W) + 2.7(a/W)2 )] 2(1 + 2(a/W))(1 − (a/W))3/2
The combination of high ambient Dhahran temperatures reaching 50 ◦ C in summer, extra hours of UV exposure and humidity are responsible for the reduced CPVC toughness. Visual inspection of the exposed surface of as-received and weathered specimens revealed a clear change in color from bright grey to tan after 2 months of exposure. The colorimetric evolution does not seem to be dependant on longer exposure time; the effect of UV light seems to have reached its peak after 60 days. The change in color and surface degradation are mainly due UV radiation promoted chemical reaction and the formation of heat.
(1)
(2)
In this work, KQ is referred to as fracture toughness because CPVC is a material that fails by crazing and the fracture toughness of materials that craze is relatively insensitive to specimen thickness. The effect of natural weathering period on fracture toughness of CPVC is illustrated in Fig. 5. Similarities can be drawn between the effect of weathering on fracture strain and fracture toughness. A sudden drop of about 18% in KQ value occurs after 1 month of exposure followed by a slow gradual decrease in subsequent months. This degradation is expected since PQ is measured at the onset of crack and given that the root of the crack is exposed to the sunlight. The direct exposure of crack tip to UV light, high temperature and humidity made the crack
N. Merah / Journal of Materials Processing Technology 191 (2007) 198–201
201
Acknowledgements The author thankfully acknowledges the support of King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, Saudi Arabia. The author would like to thank Turki Al-Qahtani and S. Zainualdin for their support in running the tests. References
Fig. 5. Variation of CPVC fracture toughness with natural exposure time.
tip more brittle resulting in lower resistance to crack initiation. Here again, chain scission is the controlling process. Like PVC, CPVC seems to be affected by the short exposure time. These results are similar to those reported by Davis and Sims [9] for relative impact strength of PVC. 4. Conclusions The effect of natural exposure to harsh Saudi environment on CPVC pipe material was investigated by studying the evolution of its tensile, fracture toughness properties and surface color with weathering time. Among the tensile properties that are most affected by weathering is the tensile strain at break. It was found to decrease by about 20% in the 1st month. The degradation in subsequent months was gradual. The yield strength and the modulus of elasticity are slightly enhanced by weathering. The initial effect of weathering on the fracture toughness of CPVC was similar to that observed for the strain at fracture. The value of fracture toughness experienced a sudden drop in the 1st month of exposure. Longer exposure periods resulted in a slow gradual decrease of material toughness. A clear change in specimen color from bright grey to tan was observed after 2 months of exposure. The colorimetric evolution does not seem to be dependant on longer exposure time.
[1] I. Hussein, S.H. Hamid, J.H. Khan, Poly(vinyl chloride) pipe degradation studies in natural environments, J. Vinyl Addit. Technol. 1 (1995) 137–141. [2] A. Gonzalez, J.M. Pastor, J.A. De Saja, Monitoring the UV degradation of PVC window frames by micro hardness analysis, J. Polym. Sci. 38 (1989) 1879–1882. [3] J.W. Summers, E.B. Rabinovitch, Weather ability of plastics compared at different exposure locations, J. Vinyl Technol. 1 (1983) 55–60. [4] M. Puterman, Natural and accelerated weathering of PVC and polypropylene waste water pipes, J. Mater. Struct. 22 (1989) 170–175. [5] A.R. Ragab, H. Alawi, Weathering effects on some mechanical properties of polyvinyl chloride pipes, J. Test. Eval. 18 (1990) 45–52. [6] E.B. Rabinovitch, J.W. Summers, W.E. Northcott, Changes in properties of rigid PVC during weathering, J. Vinyl Technol. 15 (1993) 214–218. [7] S. El Raghi, R.R. Zaharan, B.E. Gebril, Effect of weathering on some properties of polyvinyl chloride/lignin blends, Mater. Lett. 46 (2000) 332–342. [8] L.P. Real, A.P. Rocha, J.L. Gardette, Artificial accelerated weathering of polyvinyl chloride for outdoor applications: the evolution of mechanical and molecular properties, Polym. Degrad. Stab. 82 (2003) 235–243. [9] A. Davis, D. Sims, Weathering of Polymers, Appl. Sci. Publ., London, 1983. [10] A.M. Ollick, A.M. El-Amri, The effect of outdoor weathering conditions on the fracture toughness of PVC Pipes, Alexandria Eng. J. 39 (2000) 837–846. [11] A.R. Ragab, H. Alawi, Weathering effects on the fracture properties of polyvinyl chloride pipes, J. Test. Eval. 29 (2001) 44–49. [12] N. Merah, M. Irfan-ul-Haq, Z. Khan, Temperature and weld effects on mechanical properties of CPVC, J. Mater. Process. Technol. 142 (2003) 247–255. [13] M. Irfan-ul-Haq, N. Merah, Effect of temperature on fatigue crack growth in CPVC, ASME J. Pressure Vessel Technol. 125/1 (2003) 71–77. [14] Standard Test Method for Tensile Properties of Plastics, American Society for Testing and Materials, 2001 (Designation: ASTM D638-01). [15] Standard Test Methods for Plain-strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials, American Society for Testing and Materials, 1993 (Designation: ASTM D5045-93). [16] Standard Practice for Outdoor Weathering of Plastics, American Society for Testing and Materials, 1999 (Designation: ASTM D1435-99).
Natural weathering effects on some properties of CPVC pipe material Nesar Merah ∗ Mechanical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
Abstract Polyvinyl chloride (PVC) and more recently chlorinated-PVC (CPVC) pipes are extensively used for water distribution, wastewater and in limited applications for gas distribution. More and more cities are using CPVC pipes and pipefittings in their water distribution networks. Chlorinated-PVC is a material designed to withstand higher service pressures and temperatures than PVC. Its utilization in harsh environmental conditions, such as those of the Arabian Peninsula requires understanding of weathering effects, including UV, moisture and high ambient temperature on its mechanical properties. In this work, tensile and single-edge-notch bending (SENB) specimens prepared from locally manufactured CPVC commercial pipes have been naturally weathered for different periods (1–9 months) in harsh Saudi weather conditions. Standard tensile and SEN fracture toughness tests were performed after natural exposure periods of 1, 2, 3, 6 and 9 months. The tensile test results show that exposure for periods up to 9 months, including summer season, had limited effects on the tensile strength and modulus of elasticity of the material. This was expected given that the damage due weathering is mainly a surface phenomenon. However, the deterioration of the ultimate elongation is noticeable for exposure periods as low as 1 month. Analysis of fracture toughness results reveals a general trend that indicates a deterioration of this property at the beginning of the exposure time. © 2007 Elsevier B.V. All rights reserved. Keywords: CPVC; Natural weathering; Tensile; Fracture toughness
1. Introduction Most thermoplastic pipes with sufficient good quality UV screens and heat stabilizers have sufficient weather resistance to withstand normal exposure that occurs before installation. This resistance depends on the ambient temperature, humidity and geographical location. The high intensity of solar radiation and the long period of sunlight exposure coupled with temperatures attaining 50 ◦ C in summer are ingredients that can cause the breakdown of PVC and CPVC pipes used in Saudi Arabia. The service life of plastics under such harsh conditions is reported to be dramatically reduced. For instance, the tensile strength of white poly(vinyl chloride), PVC, pipes exposed for 24 months in Dhahran (Saudi Arabia) decreased by 43% while an exposure of the same duration in Florida resulted in only a 26% decrease in the property [1]. Gonzalez et al. [2] reported that one of the major problems associated with PVC is its sensitivity to weathering and UV irradiation. Furthermore, the presence of oxygen and humidity induces changes in its mechanical properties. Summers and Rabinovitch [3] found that when the air temperature varied in
∗
Fax: +966 3 860 2949. E-mail address: [email protected].
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.03.031
the range of 26–36 ◦ C during the period of exposure the surface temperature of plastics exposed to sunlight can be much higher (by as much as 60 ◦ C for common plastics depending on color and thickness) than that of the surrounding air due to heat buildup. A large number of investigations on natural and artificial weathering of PVC were performed by a number of researchers [4–11]. Puterman [4] found that while the ultimate percentage elongation of PVC specimens under tensile testing deteriorates within a short time of accelerated and natural weathering, the yield strength is hardly affected by weathering. Ragab and Alawi [5] found that the yield strength of PVC pipe material increases with natural exposure period. Testing of sharply notched specimens reflected higher susceptibility of PVC to brittle fracture due to weathering. Rabinovitch et al. [6] reported that yield strength and modulus of elasticity of PVC pipe material generally rise slightly with weathering. El Raghi et al. [7] found that the yield strength of PVC stabilized with lignin is enhanced with artificial weathering exposure while the percentage elongation experienced a drastic reduction. Discoloration of the PVC pipes is also observed by these authors [7]. Similar results were reported by Real et al. [8] for PVC under accelerated weathering. Davis and Sims [9] showed that the relative impact strength of PVC and uPVC deteriorates with weathering time. This
N. Merah / Journal of Materials Processing Technology 191 (2007) 198–201
degradation is dependent on the geographical location because of the variation of sunlight radiation intensity and sunlight hours. The relative impact strength of uPVC reduced by about 45% after 1 year exposure in Singapore and 20% in Arizona. Ollick and El-Amri [10] found that the fracture toughness of the exposed side of PVC pipe showed a 40% reduction as compared to the side in the shadow. Ragab and Alawi [11] concluded that the general trend of their experimental results showed that natural weathering of PVC material for 2 years resulted in lowering the fracture toughness of the material. Weathered pipes displayed susceptibility to brittle fracture. From the above review, it can be concluded that the study of the effect of natural and artificial weathering on the performance of PVC material had its fair share. Weathering influence on the properties of chlorinated-PVC however has not been investigated. This paper presents results on the effect of Saudi Arabian outdoor environment on tensile, fracture toughness and colorimetric properties of CPVC extruded pipe material.
199
Fig. 1. Effect of natural weathering on yield strength of CPVC pipe material.
2. Experimental procedure The specimens for tensile and fracture toughness testing programs were prepared from commercially available 4 in. schedule 80 extruded pipes manufactured by a local Saudi Company. Following the specimen preparation procedure adopted by different researchers [12,13], where rings were cut from the pipe, slit and straightened between metal plates after heating for 20 min at 105 ◦ C in an electric oven. The specimens for fracture toughness and tensile tests were then machined from the straightened plates. The tensile specimens were prepared according to the ASTM Standard D638-01 [14] method of test for tensile properties of plastics. The fracture toughness specimens were produced as per ASTM D5045-93 Standard test methods for plain-strain fracture toughness and strain energy release rate of plastic materials [15]. An initially sharp flaw (1 mm deep) was made by pressing a sharp razor blade mounted on a specially manufactured fixture. The specimens were exposed in Dhahran, Saudi Arabia in accordance with ASTM D1435-99 standard practice for outdoor weathering of plastics [16]. The fracture toughness specimens were exposed with the notched surface facing south at an angle of 45◦ . The exposure period of the specimens extended from March 2005 to November 2005. To evaluate the continuous weathering effects on the CPVC pipe material the samples were collected after 1, 2, 3, 6 and 9 months. The as-received samples were tested earlier and the results are used as baseline data. An Instron 8501 material testing frame was used for testing. The machine was equipped with a hydraulically actuated self-aligning gripping system. The tensile tests were performed at room temperature and at a strain rate of 6 × 10−4 s−1 (5 mm/min). Three point bend fracture toughness tests were conducted at room temperature and at a crosshead speed of 5 mm/min. During testing, the load P and the crack opening displacements (COD) are recorded. The latter was measured using an Instron clip gage mounted across the notch. Three to four tests were performed for each exposure condition and a reasonable data scatter was observed.
Fig. 2. Effect of natural weathering on modulus of elasticity of CPVC pipe material.
3. Results and discussion 3.1. Tensile results Table 1 illustrates the average values of the tensile properties of CPVC pipe material for different exposure periods. From these results, it can be concluded that the yield strength, the fracture strength and the modulus of elasticity are hardly affected by weathering. The percent fracture strain however, shows an abrupt decrease in average value after 1 month of natural exposure. A more general picture of weathering effect on the tensile properties is obtained when all the experimental results obtained for these properties are plotted as a function of weathering time in Figs. 1–3. The effect of natural exposure time on the yield
Table 1 Average values of CPVC tensile properties obtained at different outdoor exposure periods Exposure time (months)
Average yield strength (MPa)
Average elastic modulus (MPa)
Average fracture strain (%)
Average fracture strength (MPa)
0 1 2 6 9
46.5 46.6 46.7 46.2 48.2
2883 2956 3084 2861 3006
12.5 9.9 10.4 8.0 6.8
43.2 43.0 41.7 42.6 45.9
200
N. Merah / Journal of Materials Processing Technology 191 (2007) 198–201
Fig. 3. Effect of natural weathering on fracture strain of CPVC pipe material.
strength is depicted in Fig. 1. As reported for PVC by several researchers [6–8] weathering effect on the yield strength is minimal. This was expected given that the damage due weathering is mainly a surface phenomenon. But despite the data scatter a trend line showing a slight increase in the yield strength with exposure time can be drawn. The trend indicates that there is an increase of about 3.5% in strength over a period of 270 days. Similarly, the modulus of elasticity is slightly enhanced by weathering as shown in Fig. 2. This increase in yield strength and modulus of elasticity is mainly caused by the process of cross-linking occurring during long period of natural exposure. The fracture strain, however experiences a reduction in value that is more pronounced during the 1st month of exposure (Fig. 3). Despite the scatter in the experimental results, on the average around 20% decrease in the fracture strain is caused by a single month of weathering. Following the 1st month, the reduction is gradual reaching 45% after 9 months of exposure. The decrease in percent elongation is due to the predominant process of chain scission mainly on the specimen surface, making it more brittle. As fracture strain is generally thought of as measure of material toughness, its reduction is an indication of lower toughness.
f (a/W) =
Fig. 4. Load vs. COD for CPVC Pipe material after 1-month natural exposure. Table 2 Weathering effect on average values of critical load and fracture toughness √ Exposure time (months) Average PQ (N) Average KQ (MPa m) 0 1 3 6
488 396 382 373
3.96 3.21 3.10 3.03
3.2. Fracture toughness Fig. 4 illustrates the typical variation of applied load with crack opening displacement (COD) for fracture toughness tests performed on SENB specimens after 1 month of exposure. Table 2 presents the average values of the critical load, PQ and the fracture toughness, KQ. The critical load is obtained from the applied load versus crack opening displacement curves using the 5% secant line and the value of fracture toughness is computed from the following relationship according to ASTM D5045-93: KQ =
PQ Sf (a/W) BW 3/2
where S is the span, a the crack length, W the specimen width, B the specimen thickness and f(a/W) is a dimensionless function of a/W calculated by:
3(a/W)1/2 [1.99 − (a/W)(1 − a/W)(2.15 − 3.93(a/W) + 2.7(a/W)2 )] 2(1 + 2(a/W))(1 − (a/W))3/2
The combination of high ambient Dhahran temperatures reaching 50 ◦ C in summer, extra hours of UV exposure and humidity are responsible for the reduced CPVC toughness. Visual inspection of the exposed surface of as-received and weathered specimens revealed a clear change in color from bright grey to tan after 2 months of exposure. The colorimetric evolution does not seem to be dependant on longer exposure time; the effect of UV light seems to have reached its peak after 60 days. The change in color and surface degradation are mainly due UV radiation promoted chemical reaction and the formation of heat.
(1)
(2)
In this work, KQ is referred to as fracture toughness because CPVC is a material that fails by crazing and the fracture toughness of materials that craze is relatively insensitive to specimen thickness. The effect of natural weathering period on fracture toughness of CPVC is illustrated in Fig. 5. Similarities can be drawn between the effect of weathering on fracture strain and fracture toughness. A sudden drop of about 18% in KQ value occurs after 1 month of exposure followed by a slow gradual decrease in subsequent months. This degradation is expected since PQ is measured at the onset of crack and given that the root of the crack is exposed to the sunlight. The direct exposure of crack tip to UV light, high temperature and humidity made the crack
N. Merah / Journal of Materials Processing Technology 191 (2007) 198–201
201
Acknowledgements The author thankfully acknowledges the support of King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, Saudi Arabia. The author would like to thank Turki Al-Qahtani and S. Zainualdin for their support in running the tests. References
Fig. 5. Variation of CPVC fracture toughness with natural exposure time.
tip more brittle resulting in lower resistance to crack initiation. Here again, chain scission is the controlling process. Like PVC, CPVC seems to be affected by the short exposure time. These results are similar to those reported by Davis and Sims [9] for relative impact strength of PVC. 4. Conclusions The effect of natural exposure to harsh Saudi environment on CPVC pipe material was investigated by studying the evolution of its tensile, fracture toughness properties and surface color with weathering time. Among the tensile properties that are most affected by weathering is the tensile strain at break. It was found to decrease by about 20% in the 1st month. The degradation in subsequent months was gradual. The yield strength and the modulus of elasticity are slightly enhanced by weathering. The initial effect of weathering on the fracture toughness of CPVC was similar to that observed for the strain at fracture. The value of fracture toughness experienced a sudden drop in the 1st month of exposure. Longer exposure periods resulted in a slow gradual decrease of material toughness. A clear change in specimen color from bright grey to tan was observed after 2 months of exposure. The colorimetric evolution does not seem to be dependant on longer exposure time.
[1] I. Hussein, S.H. Hamid, J.H. Khan, Poly(vinyl chloride) pipe degradation studies in natural environments, J. Vinyl Addit. Technol. 1 (1995) 137–141. [2] A. Gonzalez, J.M. Pastor, J.A. De Saja, Monitoring the UV degradation of PVC window frames by micro hardness analysis, J. Polym. Sci. 38 (1989) 1879–1882. [3] J.W. Summers, E.B. Rabinovitch, Weather ability of plastics compared at different exposure locations, J. Vinyl Technol. 1 (1983) 55–60. [4] M. Puterman, Natural and accelerated weathering of PVC and polypropylene waste water pipes, J. Mater. Struct. 22 (1989) 170–175. [5] A.R. Ragab, H. Alawi, Weathering effects on some mechanical properties of polyvinyl chloride pipes, J. Test. Eval. 18 (1990) 45–52. [6] E.B. Rabinovitch, J.W. Summers, W.E. Northcott, Changes in properties of rigid PVC during weathering, J. Vinyl Technol. 15 (1993) 214–218. [7] S. El Raghi, R.R. Zaharan, B.E. Gebril, Effect of weathering on some properties of polyvinyl chloride/lignin blends, Mater. Lett. 46 (2000) 332–342. [8] L.P. Real, A.P. Rocha, J.L. Gardette, Artificial accelerated weathering of polyvinyl chloride for outdoor applications: the evolution of mechanical and molecular properties, Polym. Degrad. Stab. 82 (2003) 235–243. [9] A. Davis, D. Sims, Weathering of Polymers, Appl. Sci. Publ., London, 1983. [10] A.M. Ollick, A.M. El-Amri, The effect of outdoor weathering conditions on the fracture toughness of PVC Pipes, Alexandria Eng. J. 39 (2000) 837–846. [11] A.R. Ragab, H. Alawi, Weathering effects on the fracture properties of polyvinyl chloride pipes, J. Test. Eval. 29 (2001) 44–49. [12] N. Merah, M. Irfan-ul-Haq, Z. Khan, Temperature and weld effects on mechanical properties of CPVC, J. Mater. Process. Technol. 142 (2003) 247–255. [13] M. Irfan-ul-Haq, N. Merah, Effect of temperature on fatigue crack growth in CPVC, ASME J. Pressure Vessel Technol. 125/1 (2003) 71–77. [14] Standard Test Method for Tensile Properties of Plastics, American Society for Testing and Materials, 2001 (Designation: ASTM D638-01). [15] Standard Test Methods for Plain-strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials, American Society for Testing and Materials, 1993 (Designation: ASTM D5045-93). [16] Standard Practice for Outdoor Weathering of Plastics, American Society for Testing and Materials, 1999 (Designation: ASTM D1435-99).