Side chain crystallinity enhances polymeric lubricant additives

3 June 2013
Ante Jukić, Tomislav Karažija, and Elvira Vidović
Increasing the proportion of long-chain alkyl methacrylates in a copolymer from 4 to 7 mol% lowers the pour point of its mineral oil solutions from −9°C to below −30°C.

In formulating lubricants, polymeric modifiers are added to achieve a requisite set of rheological properties, which typically include improved cold flow performance (a lower ‘pour point’) and reduced thinning at high temperatures (a better ‘viscosity index’). Multi-component (copolymeric) systems are often used to obtain other mechanical or physicochemical properties at the same time. For both economic and performance reasons, it is desirable to limit the amount of polymeric additive that needs to be used in a lubricant, which calls for designing efficient copolymers. Also, even small differences in the composition and structure of commercial copolymeric additives significantly influence their flow-modifying behavior. Therefore, understanding how the composition and structure of such additives affect their application characteristics is important for achieving optimal performance.

Alkyl-methacrylate based polymers (PAMA) are widely used as rheology modifiers for hydrocarbon liquids, and more specifically as viscosity index improvers (VII) and pour point depressants (PPD), to optimize the viscosity-temperature behavior of lubricating mineral oils. For PPD applications, the side chain crystallinity of the polymeric additive is of particular interest because of its role in preventing the wax crystals that form in lubricants from agglomerating or fusing together at low temperatures. Without adequate PPDs, the flow characteristics of a lubricant can be impaired, with a negative impact on engine performance and protection.

In our work, we investigated the thermal properties and side chain crystallinity of lubricating oil additives based on terpolymers (copolymers made up of three different co-monomers) of either styrene (ST) or methyl methacrylate (MMA) with two long-chain PAMAs.1 Styrene is a component that improves the thermal and oxidation stability of an additive, which is an especially important feature for new, advanced lubricant formulations due to the higher operating temperatures of modern engines. However, the homopolymers of ST and MMA—polystyrene and poly(methyl methacrylate)—are both insoluble in mineral-based oil, which limits the amount of these co-monomers that can go into a copolymeric additive. Naturally, the overall composition of the copolymer has to be optimized with an eye to the multiple tasks that lubricant additives are called upon to perform. These include not just improving the flow behavior at high (viscosity index) and low (pour point) temperatures, but also assuring oxidation, thermal and shear stability, and good solubility in oil.

We employed a differential scanning calorimetry (DSC) method to investigate the thermal properties of terpolymers composed of two long-chain PAMAs—dodecyl methacrylate (DDMA) and octadecyl methacrylate (ODMA)—and either ST or MMA. The results show that a higher MMA or ST content in the terpolymers corresponded to higher glass transition temperatures and a decreased ratio of crystalline relative to amorphous phase: see Figure 1. We also found a direct correlation between the low-temperature flow behavior of terpolymer solutions in mineral oil (pour point) and the terpolymers' side-chain crystallinity, which in its turn is related to long-chain alkyl methacrylate content: see Figure 2. Thus, by increasing the proportion of crystalline phase, i.e., of the long-chain alkyl methacrylates in the terpolymer, the pour point of their oil solutions can be lowered.


Crystalline phase content (xc) in terpolymers of dodecyl methacrylate and octadecyl methacrylate (DDMA/ODMA) with styrene (ST) or methyl methacrylate (MMA).


Pour point temperature (T) of the dilute solutions of ST/DDMA/ODMA terpolymers in mineral base oil depending on the proportion of crystalline phase (xc) in the terpolymer.

In addition to the above effects, homopolymers of alkyl methacrylates with long alkyl side chain groups exhibit a comb-like structure that makes them of great fundamental research interest, prompting authors to investigate aspects such as their multiple glass transitions and other structural features. In particular, the nanophase separation between incompatible main and side-chain parts of PAMAs has been confirmed by DSC, dielectric spectroscopy, and neutron-diffraction methods. The investigators found that alkyl groups of different monomeric units aggregate in the melt and form self-assembled alkyl nanodomains with a typical size of 0.5–3nm.2–4

Corroborating these observations, Figure 3 shows a micrograph of an MMA/ODMA statistical copolymer (one whose monomer sequence distribution follows a known statistical law), in which the pattern of the phase separated structure, with a polar copolymer backbone and nonpolar segments of long-chain alkyl groups of octadecyl methacrylate, can be clearly seen. The resulting structural pattern fully matches that reported in the literature for amphiphilic block copolymers, with a linear arrangement of alternating monomer sequences (blocks) featuring hydrophilic (polar) and hydrophobic (nonpolar) groups.5 Block copolymers, particularly in the form of thin films, have been extensively used to pattern substrates at the mesoscopic and nanoscopic scale.


Scanning electron micrograph showing the phase separation in the bulk of the MMA 70 / ODMA 30 copolymer.

In summary, our work showed how the composition of a copolymeric additive (the amount of alkyl methacrylates) can result in structural features (namely side chain crystallinity) which in their turn impact on applications characteristics such as the pour point in solution with a mineral oil. Understanding such composition-structure-properties dependencies is important for achieving optimal lubricant additive formulations, and offers further insights into the properties of alkyl methacrylates with alkyl side chain groups. To follow up this work, we plan to investigate incorporating functional co-monomers with detergent properties into a polymeric additive, and determine the effects on lubricants.


Authors

Ante Jukić
Faculty of Chemical Engineering and Technology (FKIT) University of Zagreb (UNIZG)

Ante Jukić is a professor in the FKIT of the UNIZG, where he received his PhD in 2004. He leads projects related to optimization of copolymer properties in radical polymerization processes, and nanostructured and functional polymer materials.

Tomislav Karažija
Faculty of Chemical Engineering and Technology (FKIT) University of Zagreb (UNIZG)

Tomislav Karažija is a PhD student in the FKIT. His PhD is focused on the development of nanostructured polymer composites.

Elvira Vidović
Faculty of Chemical Engineering and Technology (FKIT) University of Zagreb (UNIZG)

Elvira Vidović is an associate professor at FKIT. She received her PhD in 2006 at DWI-RWTH in Aachen, Germany, and has worked in the area of polymer science and engineering.


References

  1. T. Karažija, E. Vidović and A. Jukić, Thermal properties and side chain crystallinity of styrene and n-alkyl methacrylate terpolymers, Polym. Eng. Sci., 2013. (accepted)

  2. M. Beiner, K. Schröter, E. Hempel, S. Reissig and E. Donth, Multiple glass transition and nanophase separation in poly(n-alkyl methacrylate) homopolymers, Macromolecules 32 (19), pp. 6278, 1999.

  3. A. Arbe, J. A.-C. Genix S. Arrese-Igor and D. Richter, Dynamics in poly(n-alkyl methacrylates): a neutron scattering, calorimetric, and dielectric study, Macromolecules 43 (6), pp. 3107, 2010.

  4. S. Hiller, O. Pascui, H. Budde, O. Kabisch, D. Reichert and M. Beiner, Nanophase separation in side chain polymers: new evidence from structure and dynamics, New J. Phys. 6, 2004.

  5. C. Fodor, B. Iván, G. Kali, P. Mezey, S. Szabó, R. Thomann and R. Mülhaupt, Nanophasic amphiphilic polymer conetworks as a new material platform for organic-inorganic nanohybrids, Front. Polym. Sci., Mainz, 2009. abstract P3-162

DOI:  10.2417/spepro.004910