143

5

Dispersants

Syed Q. A. Rizvi

CONTENTS

5.1 Introduction ........................................................................................................................... 143
5.2 Nature of Deposits and Mode of Their Formation ............................................................... 144
5.3 Deposit Control by Dispersants ............................................................................................ 147
5.4 Desirable Dispersant Properties ............................................................................................147
5.5 Dispersant Structure ............................................................................................................. 147
5.6 Dispersant Synthesis ............................................................................................................. 148
5.6.1 The Hydrocarbon Group ........................................................................................... 149
5.6.2 The Connecting Group .............................................................................................. 151
5.6.3 The Polar Moiety ....................................................................................................... 1 52
5.7 Dispersant Properties ............................................................................................................ 157
5.7.1 Dispersancy ............................................................................................................... 157
5.7.2 Thermal and Oxidative Stability ............................................................................... 160
5.7.3 Viscosity Characteristics ........................................................................................... 161
5.7.4 Seal Performance....................................................................................................... 16 2
5.8 Performance Testing ............................................................................................................. 163
References ...................................................................................................................................... 166

5.1 INTRODUCTION

Lubricants are composed of a base fl uid and additives. The base fl uid can be mineral, synthetic, or
biological in origin. In terms of use, petroleum-derived (mineral) base fl uids top the list, followed
by synthetic fl uids. Base oils of biological origin, that is, vegetable and animal oils, have not gained
much popularity except in environmentally compatible lubricants. This is because of the inherent
drawbacks these base oils have pertaining to their oxidation stability and low-temperature proper-
ties. Additives are added to the base fl uid either to enhance an already-existing property, such as
viscosity, of a base oil or to impart a new property, such as detergency, lacking in the base oil.
The lubricants are designed to perform a number of functions, including lubrication, cooling, pro-
tection against corrosion, and keeping the equipment components clean by suspending ordinarily
insoluble contaminants in the bulk lubricant [1]. Although for automotive applications all functions
are important, suspending the insoluble contaminants and keeping the surfaces clean are the most
critical. As mentioned in Chapter 4 on “detergents,” this is achieved by the combined action of the
detergents and the dispersants present in the lubricant. Dispersants differ from detergents in three
signifi cant ways:

1. Dispersants are metal-free, but detergents contain metals, such as magnesium, calcium,
   and sometimes barium [2]. This means that on combustion detergents will lead to ash
   formation and dispersants will not.


2. Dispersants have little or no acid-neutralizing ability, but detergents do. This is because
   dispersants have either no basicity, as is the case in ester dispersants, or low basicity, as