There have recently been a number of incidents involving DuPont Aldyl-A medium-density polyethylene (MDPE) pipe. This article will first review the history of Aldyl-A pipe and the various resins used in its production, and then review the Avista Utilities program to systematically remove selected portions of Aldyl A pipe from its natural gas distribution system in Washington, Oregon and Idaho.
The first thermoplastic resin the DuPont Company used to manufacture plastic pipe for the industry was polyacetal, with the trade name Delrin[R]. Manufacturing began at the plant in Tulsa, OK in 1960. Because the polyacetal material was brittle, DuPont made a co-extruded pipe with a polyacetal core and an outer layer of polyethylene (PE).
The trade name was Alathon[R], with DuPont taking the "al" from Alathon and "del" from Delrin to arrive at the trade name "Aldel." Apparently, this was too close to an existing trade name, so they changed the "e" to a "y" and came up with Aldyl[R] as the new trade name for its co-extruded thermoplastic gas pipe.
In 1965, DuPont eliminated the polyacetal layer and began to make gas pipe as a solid wall PE pipe. The name of this PE pipe was Aldyl-A pipe.
The PE resin DuPont initially used for the production of Aldyl-A pipe from 196570 was Alathon 5040. This PE resin used a butene comonomer and had a base resin density of 0.935 g/cc and a melt index (MI) of 2 g/10 min.
These two properties of melt index and density control many of the other physical properties for PE materials. Most of the other PE materials used for the gas industry at that time had an MI of about 0.2 g/10 min, so Aldyl-A was not fusion-compatible with these other materials. With this relatively low molecular weight (high MI), the recommended butt fusion temperature for Aldyl-A pipe was 310[degrees]F (154[degrees]C), compared to 400[degrees]F (204[degrees]C) to 500[degrees]F (260[degrees]C) for the other PE materials.
Because some of the small tubing sizes made from the Alathon 5040 resin did not consistently meet the ASTM D1599 quick burst minimum stress requirement of 2520 psi, DuPont decided to use a higher density PE resin. DuPont changed to Alathon 5043 resin in 1970.
This was also a butene comonomer, but with a higher base resin density of 0.939 g/cc to increase the yield strength and more consistently meet quick burst stress requirements. In order to maintain a balance of molecular parameters, the molecular weight was increased when the density was increased, and the corresponding melt index was 1.2 g/10 min. With this higher molecular weight (lower MI), the butt fusion joining temperature was increased to 340[degrees]F (171[degrees]C).
Alathon 5043 was the primary PE resin that DuPont used for Aldyl-A pipe from 1970-83. It was also during this time that the LDIW (low-ductile inner wall) phenomenon occurred.
In the late 1970-71 era, DuPont had a manufacturing issue that resulted in a brittle inside surface. This was detected during some elevated-temperature, stress-rupture testing, resulting in premature failures in which multiple slits were observed as opposed to the normal single slit failure.
It was also noted that the spherulites on the inside surface were extremely large (30-40 microns). Because of these large spherulites on the inside surface, this pipe was called "large-bore spherulite" pipe, or the term more commonly used is LDIW.
The brittle inside surface resulted from the manufacturing process that degraded the inner surface. The premature failures were due to an oxidized inner surface that dramatically reduced the initiation time and thus the overall failure time.
The effect of this LDIW surface on long-term pipe performance has been determined using the rate process method (RPM). In early 1972, DuPont changed the manufacturing process to prevent these large spherulites from forming. DuPont estimated about 30-40% of the pipe it produced in 1970-71 had an LDIW inner surface, and it was primarily in pipe sizes 1 1/4-inch to 4-inch IPS.
Alathon 5043 was a good PE resin at the time it was manufactured. However, by today's standards, the slow crack growth (SCG) resistance of Alathon 5043...