Failure of Flexible PVC Hose in Underground Applications
Duane Priddy, Ph.D.
President/Plastic Failure Labs
Plastics in Underground Pipes – Houston DP 11/1/11 1
Presented at “Plastics in Underground Pipes 2011” Houston, TX / November 1 & 2, 2011
Benefits of PVC Hose
• Semi-flexible PVC hose combines the attributes of rigid PVC (high modulus) with flexible PVC (high flexibility) to yield a product that has the benefits of PVC pipe (easy to install using standard glued PVC fittings) yet it is semi-flexible allowing it to be installed with flexing around underground obstructions
2 Plastics in Underground Pipes – Houston DP 11/1/11
Manufacturing Process
• The hose is manufactured using a co-extrusion process where the rigid PVC formulation is extruded into a strand along side of the flexible PVC formulation which is extruded into a narrow sheet. As the strand and the sheet co-extrude from the die, they are wound around a mandrel to form the hose. The rigid PVC strand ends up embedded in the wall of the hose as a helical coil.
Plastics in Underground Pipes – Houston DP 11/1/11 3
Plastics in Underground Pipes – Houston DP 11/1/11 4
Example Failures: Swimming Pools
These failure occurred after 1 – 2 years of service
Plastics in Underground Pipes – Houston DP 11/1/11 5
Exterior Cracking is Most Common but also may Occur on Either the Interior Wall
Plastics in Underground Pipes – Houston DP 11/1/11 6
Microscope Views of Cracks
Close-up view of failed region in outside hose wall
View of fractures surface shows repeating diagonal void pattern in flexible PVC portion
Plastics in Underground Pipes – Houston DP 11/1/11 7
Fracture surface analysis
Closer view of the voids
Plastics in Underground Pipes – Houston DP 11/1/11 8
Views of Fracture Surface
Plastics in Underground Pipes – Houston DP 11/1/11 9
SEM Analysis of Fracture Surface
Plastics in Underground Pipes – Houston DP 11/1/11 10
SEM of Fracture Surface
Plastics in Underground Pipes – Houston DP 11/1/11 11
SEM Analysis of Fracture Surface
Plastics in Underground Pipes – Houston DP 11/1/11 12
FTIR Spectra of Flexible & Rigid Components in Failed Samples
Flexible
Rigid
DOP plasticizer
Plastics in Underground Pipes – Houston DP 11/1/11 13
Conclusions
• Some pipe samples contain voids in the fracture surface and fracture propagation follows the voids = manufacturing defect
• Most fractures show evidence of ESC failure of the rigid strands
• Flex hoses that use flexible PVC containing a migrating plasticizer are inherently defective because the plasticizer migrates into the rigid PVC strands causing them to stress-crack.
Plastics in Underground Pipes – Houston DP 11/1/11 14
Concluding Remarks
• Flex hose manufactures should use only flexible PVC made using non-migrating plasticizer.
• IAPMO needs to include a requirement in the PVC flex hose standard requiring only the use of non-migrating plasticizers.
Plastics in Underground Pipes – Houston DP 11/1/11 15
Possible Technical Solutions
• Use a rigid plastic component that is compatible with flexible PVC; e.g., PP
• Place a barrier layer between the rigid and flexible components to stop the DOP migration
• Develop a low cost non-migrating plasticizer
Plastics in Underground Pipes – Houston DP 11/1/11 16