324 Lubricant Additives: Chemistry and Applications
The intermediate ring decomposes to alpha olefi n and acid products [35]. Temperatures on the order
of 250°C are needed to initiate this reaction.
Ester pyrolysis produces volatile alpha olefi ns as well as acids or anhydrides along the polymer
backbone or perhaps monomeric acids or anhydrides should the reaction occur after depolymerization.
The fate of acid or anhydride in the polymeric backbone would still be depolymerization under the
vigorous thermal conditions present. The ultimate products are volatile under the conditions needed
to initiate the reaction.
Consequences of either thermal reaction are loss of activity as a VII or PPD and the genera-
tion of volatile small molecules. The products distill from the high-temperature reaction zone and
thus offer no further opportunity for the chemical reaction. The data in Table 11.1 indicate the
very high volatility of a PMA VII that has been exposed to extreme temperatures in air. There is
no evidence that unzipping or ester pyrolysis is important in normal lubricant applications. Most
applications generate temperatures less than reaction onset temperatures; thus, these reactions do
not appear to be an issue. A limited potential might exist in a microenvironment, such as if VII
were trapped in a piston deposit where temperatures near the combustion chamber exceed the
onset temperature [6].
11.3.1.3 Oxidative Scissioning
Like any hydrocarbon when exposed to severe oxidative conditions, PMAs can be subject to classic
oxidation reactions resulting in polymer scissioning [33]. The scissioning reaction yields two
fragments of various lengths each of which is obviously of lower molecular weight than the parent
chain, and consequently, there is some loss of viscosity contribution. The reaction takes place at
random sites along the backbone since oxidative or free radical attack may occur anywhere along
the polymer chain. Allylic, benzylic, or tertiary hydrogen are most susceptible to oxidative or free
radical attack. Methacrylates do not normally contain those structural elements; thus, the reaction
is not normally an important consideration. The pyrolysis data in Table 11.1 would seem to support
this conclusion, as scission fragments would by and large not be volatile under the conditions of the
experiment.
Proof of PMA oxidative stability and continued effectiveness has been demonstrated by
comparing viscosities of used oils exposed to the very severe Sequence IIIG oxidative engine
procedure. One oil contained a PMA PPD. The other had all the same components but had no
PPD; after the engine-aging procedure, this oil was treated with exactly the same PPD in the same
concentration as the PPD-containing oil. The resulting viscosities were essentially the same, and of
particular note, the low temperature, low shear rate viscosities did not differ in any signifi cant way,
indicating that the PMA PPD was not degraded in the severe environment of a Sequence IIIG [36].
TABLE 11.1
Pyrolysis of PMAPyrolysis
Percentage Weight Temperature (°C)
Loss After 290 315
2 min — 18.7
3 min 93.2
5 min 93.1 94.6
10 min 94.9 96.1
15 min 96.3 97.7