Viscosity Index Improvers

While  the change in viscosity  of lubrication oils with  change  in temperature  can  be  reduced  by  the  addition  of  certain  long  chain  polymers, such  use  in gear  oils  is not  common. Such  polymers when  subjected  to   the  shearing  action of  gear  sets  degrade  and  form  shorter  chain  compounds  which are  less effective  than  the  original  additives. These agents may also function as pour depressants. The  reason  the  polymers in  question  are  effective  is that  at  low temperatures  they  are  coiled  up  and  only  colloidally  dispersed. As  the  temperature  increases  the  polymers  uncoil and  go  into  solution  to  increase  the  viscosity  of the mixture.

Stringiness Agents

Where it  is desirable  to  increase  the  adhesiveness or  tackiness of gear  oils, high  molecular  weight  hydrocarbon  polymers or  copolymers, such  as Paratac  are  added. As little as 0.01 per cent of this compound is noticeable. Solutions of natural  crepe  rubber in oil give much the same  effect but  lose  their  tacky character upon  storage.

Pour Point Depressants

Certain  polymers when  added to wax containing lubricating oils in  proportions of 0.1 to 5 per  cent, will  alter  the  crystal  structure  of the  wax  and  thus  permit  movement  of the  oil  at a much  lower  temperature  than  before  the addition. Some  agents  used  for  other  purposes, for  example V.I. improvers, may  have a similar  influence on  pour  points.

Most  industrial  gear  oils do not  require  this  type  of  additive but  pour  depressants are  often  included in  automotive  gear  lubricants. The agents  normally  used  include  “Acryloids”  which  are  methacrylate  polymers,  and  “Paraflow”  or “ Santopour”  which  are  wax condensation  products with either  naphthalene or  phenol. 

Polymers as components of gear lubricants

Polymers find application in gear lubricants both as additives and as actual fractions of the base fluids. Specific types of polymers used as additives will be mentioned later. Since the fluids in gear lubricants consist  primarily of mineral  oils  or of  asphaltic materials, any polymers  added  should  be compatible  with  the  major  ingredients. Likewise, polymerization  of  the added compounds  should  be complete  so that  no  further  thickening  occurs  in  service  even  if  the  mixture  is  subjected  to  some  heat. Most  polymeric  materials will  break  down  under  shearing  action  of gear  to  products  of  lower  molecular   weight  and  hence  any  initial  bodying  effect  may  be  partly  lost  after  use.

Although the polymers may be miscible with lubricating oils, special procedures may be necessary to introduce them into gear oils. Thus the grade of  polyisobutylene  which  is used  to  give  tackiness  or stringiness to oils is about  the  nature  of, but tougher than, crepe  rubber. Consequently this polymer is dispersed by a rubber mill in lubricating oil and the solution known as paratac is available to compounders. Also  most  grades of  polyethylene  are  of  such  a  nature that  only  prolonged  heat and  agitation  will  disperse them  in  lubricating oils.

The following polymers have been  suggested or  actually  used as components of  fluids for  gear lubricants: polyisobutylene  of various molecular  weight; styrene-polyisobutylene;  polyethylene ;and resins recovered  from  mineral oils. With the continued  investigation and  production of new  or  modified polymers it  seems  reasonable that  increasing  use  of  such  materials can  be expected in gear  lubricants.  

Viscosity Temperature Characteristics of Gear Lubricants

Since gear and transmission oils are subjected to widely varying temperatures, particularly in vehicles, it is desirable that such lubricants have as little change in viscosity with temperature changes as is possible. In other words, high V.I. oils are desirable in many gear oil applications and absolutely necessary in certain cases. An illustration of the latter is automatic transmission fluid. Base oils with V.I. values of 90 to 95 are readily available when needed. Where it is desired to increase the V.I. of oil, additives, known as V.I. improvers, are sometimes used. These agents are generally  polymers  which act by either coiling  up or  becoming less  soluble  at  low temperatures but reverse  this action as temperatures  increase,  so as to  contribute increasing body at higher temperatures. In normal gear  operation the shearing  effect  tends to break  down such polymers  into those  of lower molecular weight  which  are  less  effective  as V.I. improvers. However, these agents do have a place in services such as ATF where they maintain their effectiveness.

Certain  high molecular weight  petroleum resins  have been found to improve  the V.I. of some lubricating  oils  and  not  have  the defect  of  breakdown with  shear.

Shear Stability of Gear and Transmission Lubricants

While most mechanisms containing gears will tolerate a considerable variation in viscosity of the gear oil used for lubrication, a radical change in viscosity at a stated temperature, while in use is not desirable. Further, if such a change is due to a partial breakdown of an additive the purpose of the agent may be defeated. Such changes may occur due to shear while in service. The components most often affected are polymers such as V.I. improvers.

At present, products used in vehicles are the lubricants most often influenced by shear. Such changes will become increasingly important as a single fluid is used for several purposes such as a hydraulic fluid, for ATF, and perhaps as an axle lubricant. Further, a multirange gear and transmission oil has advantage in cars and trucks. That is, oil which will cover two or three SAE viscosity ranges.

Under present formulations some of the lubricants offered for the above services contain polymers as V.I. improvers. However, the action of gears or even pumps tends to change the polymers by shear. The shearing action causes either a chemical or mechanical breakdown of the large polymer molecules so that their value is largely lost in the oil. In some cases a viscosity decrease in service is temporary and in such instances there may be an alignment of the polymer molecules at high rates of shear. Of course some polymers or additives are more resistant to break down with shear than others but unfortunately those of high molecular weight, which prove the most effective V.I. improvers, are also most susceptible to loss of viscosity with shear.

Where high V.I. is necessary or desirable in gear oils, tests should be made to determine the viscosity after shear tests. This is most often done by using a test where a pump forces the lubricant through a sharp edge orifice for a stated time at a given temperature. A similar breakdown of polymers occurs with sonic shear, and a method using this procedure has also been used to evaluate the shear resistance of V.I. improvers.

Klaus and Fenske^34 tested fluids containing about 7 per cent of polymer for their permanent decrease in viscosity due to shear. After 5000 cycles in a pump at 100 degree (F) and a pressure drop of 800 psi, decreases of 25.5 to 30.5 per cent occurred. At a pressure drop of 1500 psi, the decreases were 38.5 and 40.5 per cent. The time required to stabilize viscosity will vary both with the mechanism and the fluid used. 

Detergent qualities of gear and transmission lubricants

Detergent additives are not required or included in most gear oils. However, in mechanisms where the lubricant serves both gears and control devices, such as clutches, some of the moving parts will not tolerate deposits. In such cases detergent additives are included in the lubricants. A good example of such usage is in ATF.

The term detergent dose not properly describes the function of such compounds which are really dispersants or peptizers for materials that would otherwise appear as deposits on parts of mechanisms. Antioxidants which may be present in the gear oils are not completely effective in preventing formation of gum and varnish constituents, hence, the need for dispersants.

Detergents now used consist largely of phenates or sulfonates with a lesser amount of polymers. Either the phenates or sulfonates are added as metal salts, most often of barium or calcium. While neutral salts are satisfactory dispersants, the tendency is to use basic compounds since they will neutralize any acidic products formed during oxidation. A further advance is the use of non-ashing detergent additives, most of which are specific polymers, with the suggestion also of nitrogen containing soaps