Determining the quality of a failed PVC pipe - CiteSeerX
10.1002/spepro.000056
Determining the quality of a failed PVC pipe Javier Cruz and Paul Gramann
A technique based on differential scanning calorimetry assesses the health of valuable pipes and fittings more accurately and quantitatively than current methods that use aggressive chemicals. Poly(vinyl chloride) (PVC) is among the most widely used plastics for pipes in the United States. It accounts for 66% of the water distribution market and 75% of the sanitary sewer market. Accordingly, water damage resulting from failure incurs enormous dollar losses. The breakdown of unplasticized (rigid) PVC pipes and fittings may be related to many factors, such as design, installation, abuse, manufacturing, and processing. An investigation into the cause of such an event may require detailed analysis of all these factors to successfully determine the primary fault. For instance, processing directly affects the key properties that relate to a product’s ability to withstand stress during use as well as environmental assaults. Often, design and materials are good, but manufacturing is inadequate, which can lead to failures. There are multiple techniques for determining the quality of PVC parts, the most common being the acetone immersion test1 and the dichloromethane test.2 These aggressive chemical methods require that the degree of chemical attack be observed over a certain period of time, and they only provide a pass/fail result. A much more accurate and quantitative technique uses differential scanning calorimetry (DSC) to evaluate the quality of small samples PVC.3, 4 As parts are processed, the PVC ‘grains’ melt and fuse, forming a partially gelled structure. The degree of gelation affects the quality of the components and their mechanical properties. Higher temperature and processing time increase gelation, but, due to the narrow processing window for PVC, this can easily result in material degradation. For a given part, the DSC method accurately determines both the percentage of gelled material (called the degree of gelation) and the processing temperature at the precise location tested. Using this technique, we have evaluated multiple parts that failed unexpectedly during operation and confirmed the cause to be poor processing. We were also able to establish reliable processing histories for pipes that failed prior to installation, and traced the problems directly to manufacturing practices. Figures 1– 3 show examples of PVC pipes that suffered from inadequate processing. For example, the im-
Figure 1. Internal 3in crack on a pipe that failed prior to installation.
age in Figure 3 shows fracture behavior consistent with environmental stress cracking (ESC) of the material. Under proper processing conditions, the material would have been less prone to ESC and would potentially have performed better. The DSC method for analyzing these types of failure consists of generating a series of thermograms and observing the curve behavior above the glass-transition temperature (Tg ) but prior to degradation. DSC shows an endotherm (a downshift in the curve) as a result of the heat absorption during melting of partially gelled grains. Material that did not gel during processing creates a shifted second endothermic response. Comparing the area of the two endotherms provides an accurate estimate of the degree of gelation. For properly processed parts, the percentage is high. In addition, the inflection between the two endotherms indicates the temperature at which the material was processed. Figure 4 shows an example of a thermogram obtained from a failed pipe with circumferential cracking over the interior wall. Analysis of samples near the crack location revealed improper heat transfer through their thickness. Near the center of the thickness, the Tg was found to be 84.8◦ C (as expected for PVC), the processing temperature 177◦ C, and Continued on next page
10.1002/spepro.000056 Page 2/3
Figure 2. Multiple internal cracks on a pipe in operation.
Figure 3. Axial cracks throughout a pipe that failed during operation.
the degree of gelation 55.9%. Chemical tests showed gross lifting of the surface of the samples, further confirming the DSC results. By way of comparison, Figure 5 shows a thermogram for an exemplar pipe. In this case, the Tg was estimated at 84.9◦ C, the processing temperature during manufacturing 195◦ C, and the degree of gelation 97.1%. Chemical tests for this specimen showed no lifting, consistent with the DSC findings. In summary, the specific sampling of this method enables accurate estimation of the quality of PVC parts, even as it varies throughout the thickness or length. The actual processing temperature of the samples can also be determined. Finally, the quantitative results obtained by
Figure 4. Thermogram of a failed PVC pipe showing poor quality. DSC: Differential scanning calorimetry. Tg : Glass-transition temperature. Area A: Gelled. Area B: Ungelled. exo: Exotherm.
Figure 5. Thermogram of an exemplar PVC pipe showing good quality. exo: Exotherm.
DSC can be cross-analyzed with mechanical tests for a more comprehensive evaluation of quality and performance. We are currently conducting research to extend this technique to chlorinated PVC parts.
Continued on next page
10.1002/spepro.000056 Page 3/3
Author Information Javier Cruz and Paul Gramann The Madison Group Madison, WI Javier Cruz has a PhD in materials science from the University of Puerto Rico-Mayaguez and a BS in chemical engineering from the University of Wisconsin-Madison. He has vast experience in thermoplastic and thermoset materials, polymer processing, and testing. He is currently project engineer at The Madison Group, a global leader in the plastics industry since 1993, providing consulting services and technical expertise in design, processing, and failure analysis of plastic products. Paul Gramann received his PhD from the University of Wisconsin in 1995. He is currently president of The Madison Group. He is the past chair of the Thermoset board and current chair of the Failure Analysis and Prevention board of the Society of Plastics Engineers. He is a co-editor of the Injection Molding Handbook and co-author of Compression Molding, both published by Hanser. References 1. Adequacy of fusion of extruded poly(vinyl chloride) (PVC) pipe and molded fittings by acetone immersion, ASTM D2152, 2003. 2. Unplasticized poly(vinyl chloride) (PVC-U) pipes: dichloromethane resistance at specified temperature (DCMT). Test method, ISO 9852, 1995. 3. M. Gilbert and J. Vyvoda, Thermal analysis technique for investigating gelation of rigid PVC compounds, Polymer 22, pp. 1134–1136, 1981. 4. P. Vanspeybroeck and A. Dewilde, Determination of the Degree of Gelation of PVC-U Using a DSC, Becetel VZW, Belgium, 2004.
c 2009 Society of Plastics Engineers (SPE)
Determining the quality of a failed PVC pipe Javier Cruz and Paul Gramann
A technique based on differential scanning calorimetry assesses the health of valuable pipes and fittings more accurately and quantitatively than current methods that use aggressive chemicals. Poly(vinyl chloride) (PVC) is among the most widely used plastics for pipes in the United States. It accounts for 66% of the water distribution market and 75% of the sanitary sewer market. Accordingly, water damage resulting from failure incurs enormous dollar losses. The breakdown of unplasticized (rigid) PVC pipes and fittings may be related to many factors, such as design, installation, abuse, manufacturing, and processing. An investigation into the cause of such an event may require detailed analysis of all these factors to successfully determine the primary fault. For instance, processing directly affects the key properties that relate to a product’s ability to withstand stress during use as well as environmental assaults. Often, design and materials are good, but manufacturing is inadequate, which can lead to failures. There are multiple techniques for determining the quality of PVC parts, the most common being the acetone immersion test1 and the dichloromethane test.2 These aggressive chemical methods require that the degree of chemical attack be observed over a certain period of time, and they only provide a pass/fail result. A much more accurate and quantitative technique uses differential scanning calorimetry (DSC) to evaluate the quality of small samples PVC.3, 4 As parts are processed, the PVC ‘grains’ melt and fuse, forming a partially gelled structure. The degree of gelation affects the quality of the components and their mechanical properties. Higher temperature and processing time increase gelation, but, due to the narrow processing window for PVC, this can easily result in material degradation. For a given part, the DSC method accurately determines both the percentage of gelled material (called the degree of gelation) and the processing temperature at the precise location tested. Using this technique, we have evaluated multiple parts that failed unexpectedly during operation and confirmed the cause to be poor processing. We were also able to establish reliable processing histories for pipes that failed prior to installation, and traced the problems directly to manufacturing practices. Figures 1– 3 show examples of PVC pipes that suffered from inadequate processing. For example, the im-
Figure 1. Internal 3in crack on a pipe that failed prior to installation.
age in Figure 3 shows fracture behavior consistent with environmental stress cracking (ESC) of the material. Under proper processing conditions, the material would have been less prone to ESC and would potentially have performed better. The DSC method for analyzing these types of failure consists of generating a series of thermograms and observing the curve behavior above the glass-transition temperature (Tg ) but prior to degradation. DSC shows an endotherm (a downshift in the curve) as a result of the heat absorption during melting of partially gelled grains. Material that did not gel during processing creates a shifted second endothermic response. Comparing the area of the two endotherms provides an accurate estimate of the degree of gelation. For properly processed parts, the percentage is high. In addition, the inflection between the two endotherms indicates the temperature at which the material was processed. Figure 4 shows an example of a thermogram obtained from a failed pipe with circumferential cracking over the interior wall. Analysis of samples near the crack location revealed improper heat transfer through their thickness. Near the center of the thickness, the Tg was found to be 84.8◦ C (as expected for PVC), the processing temperature 177◦ C, and Continued on next page
10.1002/spepro.000056 Page 2/3
Figure 2. Multiple internal cracks on a pipe in operation.
Figure 3. Axial cracks throughout a pipe that failed during operation.
the degree of gelation 55.9%. Chemical tests showed gross lifting of the surface of the samples, further confirming the DSC results. By way of comparison, Figure 5 shows a thermogram for an exemplar pipe. In this case, the Tg was estimated at 84.9◦ C, the processing temperature during manufacturing 195◦ C, and the degree of gelation 97.1%. Chemical tests for this specimen showed no lifting, consistent with the DSC findings. In summary, the specific sampling of this method enables accurate estimation of the quality of PVC parts, even as it varies throughout the thickness or length. The actual processing temperature of the samples can also be determined. Finally, the quantitative results obtained by
Figure 4. Thermogram of a failed PVC pipe showing poor quality. DSC: Differential scanning calorimetry. Tg : Glass-transition temperature. Area A: Gelled. Area B: Ungelled. exo: Exotherm.
Figure 5. Thermogram of an exemplar PVC pipe showing good quality. exo: Exotherm.
DSC can be cross-analyzed with mechanical tests for a more comprehensive evaluation of quality and performance. We are currently conducting research to extend this technique to chlorinated PVC parts.
Continued on next page
10.1002/spepro.000056 Page 3/3
Author Information Javier Cruz and Paul Gramann The Madison Group Madison, WI Javier Cruz has a PhD in materials science from the University of Puerto Rico-Mayaguez and a BS in chemical engineering from the University of Wisconsin-Madison. He has vast experience in thermoplastic and thermoset materials, polymer processing, and testing. He is currently project engineer at The Madison Group, a global leader in the plastics industry since 1993, providing consulting services and technical expertise in design, processing, and failure analysis of plastic products. Paul Gramann received his PhD from the University of Wisconsin in 1995. He is currently president of The Madison Group. He is the past chair of the Thermoset board and current chair of the Failure Analysis and Prevention board of the Society of Plastics Engineers. He is a co-editor of the Injection Molding Handbook and co-author of Compression Molding, both published by Hanser. References 1. Adequacy of fusion of extruded poly(vinyl chloride) (PVC) pipe and molded fittings by acetone immersion, ASTM D2152, 2003. 2. Unplasticized poly(vinyl chloride) (PVC-U) pipes: dichloromethane resistance at specified temperature (DCMT). Test method, ISO 9852, 1995. 3. M. Gilbert and J. Vyvoda, Thermal analysis technique for investigating gelation of rigid PVC compounds, Polymer 22, pp. 1134–1136, 1981. 4. P. Vanspeybroeck and A. Dewilde, Determination of the Degree of Gelation of PVC-U Using a DSC, Becetel VZW, Belgium, 2004.
c 2009 Society of Plastics Engineers (SPE)
