Dr. Duane Priddy, Plastic Failure Labs
Swimming pool installers commonly use 1.5 and 2 inch semi-flexible PVC hose for underground water circulation when they install swimming pools. The hoses are especially convenient to install because of its flexibility and it can be solvent cemented into standard PVC fittings. Unfortunately these hoses have a high failure rate after only a few months of use. We conducted an investigation to determine the cause of the high rate of failure. Failed hoses samples were collected from over 70 pool locations. In all, eighteen samples from the seventy plus failures were subjected to forensic analysis to determine the root cause of failure. Initially three of the eighteen hose samples were analyzed in detail to determine the material composition and the root cause of failure. The results of the analysis and testing revealed the following:
- The fracture surfaces of cracks in the wall of hose samples showed voids in both the rigid strand and flexible portions of the hose wall material;
- Mechanical strength tests of sections of hose samples where no sign of cracking was present, revealed that the hose was mechanically weak;
- Measurements of one of the hoses revealed that it did not meet IAPMO PS 33 standard specifications for 2 inch semi-flexible hose.
Since completion of that initial investigation, another fifteen selected hose samples were analyzed. Results of analysis of the remaining hose samples continued to show that the predominant root cause of failure was due to voids in both the rigid and flexible portions of the hose wall. Since PVC is a “flaw sensitive” material, the voids act as flaws or defect points in the hose wall resulting in the initiation of cracks and leaks. What is interesting about this product is that it is well known that rigid PVC and flexible PVC are not compatible with each other. The reason for the incompatibility is plasticizer migration from the flexible PVC into the rigid PVC. The migration of the plasticizer from the flexible PVC into the rigid PVC causes the rigid PVC coiled strand embedded in the flexible PVC to fail by what is commonly called environmental stress cracking (ESC). The migration process is facilitated by voids or porosity in the rigid PVC strands. Thus the quality of the manufacturing process does affect the time to failure. However, even if the fabrication conditions of the hoses had been perfectly optimized, hoses manufactured using flexible PVC containing migrating plasticizer are still inherently defective by design because they are manufactured by combining two materials that are not compatible with each other.