Root Cause of Gasket Failure in a Water Valve

Dr. Duane Priddy, Plastic Expert Group

Failed icemaker valve parts were submitted for analysis. Two valve seats were analyzed. One seat is from the icemaker side of the valve assembly, the other is from the “water-in-the-door” side of the valve (referred to as “icemaker” and “water” valve seats).

FTIR and optical and SEM-EDS were used to analyze the valve seats. FTIR analysis indicates that the valve seats are made of a polybutadiene (PBD) based rubber that has been attacked by chlorine in the water. SEM-EDS shows the presence of chlorine on the degraded surface indicating that chlorine in the water was responsible for degradation of the rubber. Cracks and etching are also present on the “water” side of the valve but the affect does not appear as severe. It is well known that PBD rubber has poor chemical resistance is not stable in chlorinated water. Rubber gaskets and seals used in equipment designed for use in handling municipal water supply should be constructed of materials that are resistant to tap water containing oxidizing chemicals (i.e., chlorine and cloramines) added to the water to inhibit microbial growth.

It is well known to those in the scientific community that PBD rubber is not stable toward strongly oxidizing environments (e.g., chlorinated municipal water). Since icemakers are intended for use with municipal water supply, it is obvious that PBD rubber gaskets are NOT the correct gaskets to use for manufacture of icemakers. Therefore the icemaker was defective in its engineering/design. Several elastomeric gasket materials are commercially available that are much more chemically resistant than PBD. For example, EPDM and silicone rubber gaskets are often used in applications requiring resistance to oxidizing materials.

Failure of Extrusion Blow-Molded PVC Bottles

Dr. Duane Priddy, Plastic Expert Group

Five PVC bottles were submitted for forensic failure analysis to determine the root cause of failure of the bottles during use. Fourier transform infrared spectroscopy (FTIR) confirmed the material as PVC. Optical microscopy and scanning electron microscopy (SEM) were used to evaluate the fracture surfaces. The crack locations do not appear to be related to the mold position. The appearance of etching on the fracture indicates chemical interaction of the contents with the PVC bottle material. Thinning and orientation of the bottle wall during molding create regions that are weaker and more susceptible to failure. There are two key factors that are the root cause of failure:

1. environmental stress cracking due to chemical incompatibility with the bottle contents;
2. too great wall thickness variability.

Variation of wall thickness from 0.3 to 1.0 mm is too large (>3). When wall thickness variations are too large, the thinnest regions are under high stress and allow chemicals in the formulation inside the bottle to be absorbed into the stressed material leading to failure by a failure mode known as environmental stress cracking (ESC). Since the failures occurred where the bottle wall thicknesses were low, we suggest that the wall thickness variation specifications be decreased bottle requiring the bottle to have a more consistent wall thickness (or minimum thickness in all regions of the bottle wall). Further we suggest changing the bottle material to a material that is chemically resistant to the chemical formulation being shipped in the bottles. Chemical compatibility testing (ASTM D543) should be performed to confirm compatibility.

Above Image: View of cross-section of bottle wall showing the thin spot where the failure. occurred.

The complete 27 page article about this study is available by email at [email protected].

Failure Analysis of Glass Filled Valve and Tee Fittings

Dr. Duane Priddy, Plastic Expert Group

The valve failed on the molding weld-line because of hoop stress from the threaded connection. The lack of glass fiber reinforcement across the weld-line due to orientation results in a weak location for the failure to occur. The tee failed on the molding weld-line, because of hoop stress from a fitting that was threaded in until bottom contact (over tightened). The lack of glass fiber reinforcement across the weld-line due to orientation, and poor adhesion of the fiber to the Nylon resin resulted in a weak location for the failure to occur. In general, inside tapered pipe threads are a poor choice for thermoplastic materials as the joint relies on a tight fit between the threads to create a seal. The result is a significant amount of hoop stress on the fitting from tightening the male thread. Straight threads with a gasket seal or external threads are the better choice. If inside pipe threads are necessary, increases in wall thickness or metal reinforcement bands are potential remedies to consider.

Contamination Causes Part Failure

Dr. Duane Priddy, Plastic Expert Group

A manufacturer of a device contracted the manufacture of a frame (shown below) for the device. The client was experiencing periodic failure of the frame. Our experts analyzed the fracture surfaces of a broken plastic frame using optical microscopy, FTIR, and DSC to determine the failure mode and the root cause of failure. The results of these tests revealed that the failure mode was tensile overload and that the root cause of failure was due to contamination of the HIPS resin with ABS. HIPS and ABS are not compatible with each other resulting in significant decrease in mechanical strength of the composite material. The lower tensile strength of the composite material caused failure by tensile overload when force was applied to the frame. Plastic Expert Group recommended to the client that they work with their contract manufacturer to implement a system to insure that the HIPS raw material did not get contaminated with ABS.

The complete 7 page article about this study is available by email at [email protected]