Managing a VPI Resin for Maximum and Build Control

(Shake and shake the ketchup bottle
None will come, and then a lot'l - Ogden Nash)

The term thixotropy describes materials that liquify or flow when stirred and return to a thick consistency when allowed to rest. Many everyday products are thixotropic. Probably the most common is ketchup which stays stubbornly in the bottle until it is well shaken or stirred and then - whoosh! You've known about ketchup since age two - now we have a name for it.

A thixotropic material can be used at both high and low viscosities. If used immediately after mixing, thixotropic materials display low viscosity and so have the ability to penetrate into the windings. Table 1 shows the effect of mixing and resting on the viscosity of a typical thixotropic resin.

Table1

Viscosity in Centipoise @ 80°F

 

Spindle	Immediately	After
Speed	After Mixing	1 Hour	Overnight
1 RPM	16,000	65,000	168,000
2 RPM	15,000	52,800	124,000
5 RPM	13,440	38,080	85,000
10 RPM	11,680	29,440	63,200

The coating thickness will vary with the viscosity of the resin. A simple, yet careful, treatment cycle utilizes the thixotropic properties of the resin system. After processing, a soak or rest before removing will produce the desired build up and minimize run off in the oven. Table 2 shows build up on a copper strip after various periods of time in the resin.

Table II

Build Up, Mils per Side*

Length of Soak	Build Up
Removed Immediately	2 - 2 1/2
After 1 Hour	4
Overnight	9 - 10

The John C. Dolph Company has developed a series of unique solventless varnishes. Dolphon® CC-1115, CC-1118LV and CC-1152 are VPI epoxy resins. These products offer extraordinary tank stability (at least one year without cooling jackets), and exceptional physical and electrical properties. These are thixotropic resins for formulated to give high retention (fill), and moderate to high surface coating. Thus, they give superior protection for motors in rigorous conditions and for high voltage applications. Recommend Dolphon CC-1115, CC-1118LV and CC-1152 for VPI applications especially those used exclusively for rigorous environments or total submergence for sealed motors under MIL-I-17060E.