Lubricant Additives

290 Lubricant Additives: Chemistry and Applications

The mechanical properties of the rubber often dictate what type of isolation and packaging pro-

cesses are the most appropriate. Amorphous Ethylene propylene (EP) copolymers are often too sticky

to successfully traverse the conventional fl occulation/drying/baling process. One way to modify

these compositions to improve their handling characteristics is by introducing long-chain branching

[5,48,73] through the use of low concentrations of nonconjugated dienes or other branching agents. For

nonfunctionalized OCPs, this is the main reason that some commercial viscosity modifi ers contain

dienes [2]. Copolymer compositions higher in ethylene content (>60 wt% [5]) are often semi-

crystalline and may be amenable to packaging in pellet form. In some cases, the pellets may contain

an anticaking or antiblocking agent to prevent agglomeration.

Another type of manufacturing process has been used to manufacture low-molecular-weight

OCP viscosity modifi ers that are diffi cult to isolate and package in conventional equipment. A

higher-molecular-weight feedstock of the appropriate composition may be fed into a masticating

extruder or Banbury mixer to break down the polymer chain to lower-molecular-weight fragments

using a combination of heat and mechanical energy [74–81]. Several patents describe the use of

oxygen [82–85] or free radical initiators [86,87] to enable this process.

10.4.4 MAKING THE OCP LIQUID CONCENTRATE

After the solid OCP viscosity modifi er has been manufactured, it must be dissolved in oil before it

can be effi ciently blended with base stocks and other additives. The fi rst stage entails feeding the

rubber bale into a mechanical grinder [2] and then conveying the polymer crumb into a high- quality

diluent oil that is heated to 100–130°C with good agitation. The rubber slowly dissolves, raising

the viscosity of the oil as shown in Figure 10.1. Certain high-intensity homogenizers can also be

used in which the entire rubber bale is fed directly into a highly turbulent diluent oil tank at high

temperature; this bypasses the pregrinding step.

When the solid polymer is supplied in pellet form, the rubber can be fed directly into hot oil, or,

if it is slightly agglomerated, it may fi rst be passed through a low-energy mechanical grinder.

10.5 PROPERTIES AND PERFORMANCE CHARACTERISTICS

10.5.1 EFFECT OF ETHYLENE/PROPYLENE RATIO ON PHYSICAL PROPERTIES OF THE SOLID

The comonomer composition of E/P copolymers has a profound infl uence on the physical properties

of the rubber. These properties, in turn, dictate the type of containers in which the product can be

stored and how it is handled during distribution and use.

(^13) C NMR has been used extensively to characterize the sequence distribution of EP copolymers
[88–93]. As the ratio of E/P increases, the fraction of ethylene–ethylene (EE) sequences (dyads)
rises, as demonstrated by the data in Figure 10.6. Concurrently, the total fraction of E/P dyads
decreases (forward and reverse propylene insertion are designated as p and p*, respectively). Thus,
the average length of contiguous ethylene increases with ethylene content. Above ∼60 wt% eth-
ylene, these sequences become long enough to crystallize, as measured by differential scanning
calorimetry (Figure 10.7) or x-ray diffraction.
When the degree of crystallinity exceeds ∼25%, EP copolymers become unsuitable as viscosity
modifi ers due to limited solubility in most mineral oils. As the propylene content approaches zero,
the copolymer takes on the physical characteristics of high-density polyethylene, which, due to its
inertness to oil, is used as the packaging material of choice for engine oils and other automotive
fl uids. Microstructural investigations of metallocene ethylene/α-OCPs by Minick et al. [94] con-
cluded that the relatively short ethylene sequences of low-crystallinity (<25%) samples are capable
of crystallizing into fringed micelle or short-bundled structures (Figure 10.8). Higher-order mor-
phologies such as lamellae or spherulites are not observed. Therefore, the physical properties of
semicrystalline OCPs fall in between those of polyethylene and amorphous EP rubber.
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