Breakdown of polyisobutene in operation of polymer-thickened oils in a gearbox

Summary

As a result of a study of the breakdown of a 7% solution of 20,000 molecular weight polyisobutene in MK-8 oil during operation in a gearbox, the following has been established:

• 1) At temperatures of 33-65°c, the rate and extent of polyisobutene breakdown are inversely proportional to temperature and are governed solely by the action of mechanical forces on the polymer.

• 2) At temperatures of 65 to 130°C polyisobutene breakdown is at a minimum and is likewise governed by mechanical factors.

• 3) At gearbox oil temperatures above 130°C (130–175°C polyisobutene breakdown is proportional to temperature and is governed by the action of both mechanical forces and thermal movement of the polymer molecules; the relative amount of thermal breakdown is very significant at 175°C constituting 47% of the overall breakdown after 36 h of operation.

• 4) For the MK-8 oil thickened with 7% polyisobutene, the most favorable operating conditions are encountered with a gearbox oil temperature of 65–130°C.

• 5) The rate and extent of mechanical breakdown of polymer are governed mainly by agitation of the oil by the gears and passage of the oil through the narrow gaps between the contacting surfaces of the gear teeth.

• 6) Gear loading is of secondary importance in determining the mechanical breakdown of polymer.