Chuck Yung
EASA Senior Technical Support Specialist
Have you ever had a curing issue with your DAP monomer (diallyl-phthalate ) solventless resin (hereafter referred to as resin for simplicity)? If you haven’t, read on for guidance on preventing issues in the future. If you have, this article provides guidance on correcting the issues as well.
As expensive as resin is, all service centers should be diligent about the care of their resin dip tank and VPI (vacuum pressure impregnation) systems.
Resin maintenance
A key preventive maintenance step is to monitor resin condition. Resin samples should be drawn and sent to your resin manufacturer on a regular basis. Depending on the resin, “regular” intervals typically mean every 1 to 3 months. Check with your resin supplier for the correct frequency, and adhere to it. Until a few years ago, testing was limited to measurement of resin viscosity and gel time. Resin manufacturers now recognize that more in-depth chemistry tests should also be performed.
Older, solvent-based resins were more forgiving of poor maintenance; adding a drum or so of Xylene was a common method to reduce the viscosity of resin. Modern resins, especially DAP resins, require the use of reducers and/or inhibitors to keep the resin in balance and, in some cases, additional catalyst may be required. If neglected for too long, some resins may be beyond saving. That’s why regular sampling of your tank resin, and following the manufacturer’s corrective suggestions, is critically important.
The temperatures at which resins are stored and cured affect resin life and insulation quality. When these resins are cured at lower than the manufacturer suggested baking temperature, the electrical and bond strength characteristics are decreased. Operating at a temperature above the temperature at which they are cured, for example, can result in drastic changes in properties; a rigid resin may soften, losing much of its mechanical strength. As resin ages in the tank, a similar process takes place; “molecular clusters” (high molecular weight polymer chains) form, which could result in poor mechanical and dielectric properties.
Stirred, not shaken
Periodic stirring of the holding or dip tank is necessary to assure the complex resin is homogeneous. The mixer should be capable of stirring the resin thoroughly without introducing air bubbles. Never use compressed air to bubble or agitate the resin. This method introduces unacceptable amounts of air bubbles into the mixture, and may hasten degradation of the resin. It should be noted that the inhibitors used in the manufacture of solventless polyester resins require oxygen to remain active. Storing resin at higher than recommended temperatures will shorten the life of solventless resins. Some manufacturers recommend a resin storage temperature of 77°F (25°C). Solventless polyester VPI resins should not be stored under vacuum.
Oven basics
The recommended curing temperature for some resins has been reevaluated and increased by manufacturers in recent years, so update your procedures. Contact your resin supplier to verify the recommended baking temperature for your resin. Then calibrate the bake oven. There have been instances where the actual oven temperature was 50°F (28°C) lower than indicated on the oven instruments.
The exact meaning of certain terms is crucial; there is a difference between bake time and cure time. Bake time is the duration of the baking process; that is, the length of time the winding is in the oven. The cure time is measured from when the winding reaches the recommended curing temperature. It is the cure time that is important, not the bake time. A large stator takes several hours to reach temperature. For example, if the recommendation is to bake a 444 frame stator for 3 hours at 300°F (150°C), it will take at least 8 hours of baking time in the oven to properly cure it. If you open the oven door to add or remove other work in process, the bake and cure times increase. Smaller parts are affected more by opening the oven, because they have less mass to retain heat. If you must dip another job while other work is in the oven, close the oven door – with the other work inside – to minimize disruption of the curing process.
Preheat the winding
Random windings must be preheated to “gas off” (i.e., to boil away) the wire lubricants used to prevent scratching when the wire is manufactured. Failure to do so causes the resin to bead up on the surface of the wire, much like water beads up on the hood of a car after it is waxed. See “The Importance, Benefits of Preheating Windings Prior to Impregnation” in the May 2002 issue of Currents.
To remove moisture, preheat form coil or random windings prior to VPI or dip & bake. A typical recommendation is to preheat the winding to an adequate temperature above 212°F (100°C) and always allowing to cool to the manufacturer suggested dipping temperature. If you preheat to a typical oven set-point of 300°F (150°C), the stator (or rotor or armature) must be cooled to a safe temperature before immersing it in resin.
There is a tendency, when trying to expedite the process, to use fans to speed up the cooling process. The problem with that technique is that fans cool only the surface of the stator, but the inside core and winding temperatures remain high.
If you preheat to a temperature that exceeds the resin manufacturer’s impregnation temperature limit, you jeopardize the resin. When the stator is placed in the dip or VPI tank, the part temperature normalizes (in other words, heat migrates from the inner mass to the surface), and the surface temperature may be too hot for the resin. Processing a stator at a higher temperature than the resin manufacturer recommends will reduce the resin life.
Resin chemistry
If you experience occasional cases where a winding has very low insulation resistance to ground after impregnating and baking it, there may be chemistry issues with your resin.
The curing process of solventless polyester resins can be described as molecular bonding. In effect, the molecules “link together” like a chain mesh. In healthy resin, all of the molecules cross link to form an excellent insulation with good mechanical properties. Aging tanks contain high molecular weight chains resulting in high viscosity. If you have been adding a lot of viscosity reducer instead of fresh resin, you may have a resin that is “chemically unbalanced.” This could result in lower electrical and mechanical properties as well as low insulation resistance.
One thing that triggers this issue is introducing the stator into a dip or VPI tank while the stator is too hot. As the catalyzed resin in contact with the stator overheats, higher molecular weight chains can form in the resin. Unlike older resins, which could be thinned with solvent, these higher molecular weight chains cannot be broken apart. One manufacturer reports that special lower viscosity resins have been formulated to help lower the overall viscosity of the resin in the tank. The fact is that if the resin in the tank is allowed to deteriorate, you may face the very expensive prospect of replacing the entire tank of resin.
Aging resin
One resin manufacturer’s solution to the problem of low insulation resistance of form coil windings brought about by aging resin is to use polyester film backed mica tapes during coil manufacture. The use of polyester film backed mica tapes in conjunction with DAP monomer resins improve insulation resistance with both new resin and aging resin. Traditional VPI mica tapes, in use since the 1980s, will absorb the DAP polyester containing higher molecular weight chains, and become fully impregnated; however, they may still yield low winding insulation resistance readings. So in a small percentage of cases, a tank of deteriorating resin can result in a form coil winding that passes the insulation resistance test “green” (untreated) but fails it after processing. The film-backed mica tapes are more dielectrically compatible, and seem to prevent the occurrence of low insulation resistance. This “compatibility” may just mean the impenetrable film excludes resin penetration by the high molecular weight resin clusters. At least two major electric motor manufacturers have used the film-backed tapes exclusively, even for VPI windings, since the 1970s.
The data sheet on file for your resin may not be accurate. Be sure to contact your resin manufacturer directly to obtain the latest recommendations for storage, preheat, maximum dip temperature, and curing time as well as temperature. Do not rely on data sheets found in an Internet search; once information is on the Web, it is almost impossible for the manufacturer to retract incorrect or outdated information. For example, one resin calls for preheat temperature of 130° – 140°F (54° – 60°C), yet data sheets for the exact same resin found on the Web gave values ranging up to 150°F (66°C).
Summing up
Regular maintenance of your dip tank and/or VPI tank is critically important. Not only does it protect the substantial investment you made in the resin, it also helps you to prevent winding faults that might force a second rewind before the job can even be assembled.
Calibrate the bake oven to make sure the windings are really being cured at the temperature indicated on your oven thermometer. If you have a VPI tank, consider processing new windings for others in your area; that not only helps them, it helps you by increasing turn-over of the tank resin. Don’t process used windings in the VPI tank; they may contaminate it.
If you are in a hot region of the world, monitor the temperature of your tank. If the storage temperature is higher than 70° F (21° C), consider adding a chiller or heat exchanger to stabilize the resin temperature. It can be as simple as immersing a stainless steel tubing with cold water circulating through it. Avoid copper and galvanized pipe as materials for parts immersed in resin. According to some resin manufacturers, those materials do not react well with DAP resins.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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