PB or PIB? Does It Matter?

The chemistry where I have seen most confusion over the years is that of polybutenes, where the acronym PIB is often used, but not always correctly. Search for a polybutene image on the internet, and some of the first images you will see are of polyisobutene/polyisobutylene (PIB), rather than polybutene (PB). Search for polyisobutene and you will only see polyisobutene or polybutene-1, which is a thermoplastic and not a component of lubricant additives.

At least one of the major players in the market seems to use the terms interchangeably in some of their brochures. In preparing this article, I was challenged by an academic that I was confused and that the only oligomer/polymer was polyisobutylene.

These misconceptions are unfortunate because, depending on the application, there are differences that require attention and similarities that could lead to cost savings or better supply security. Differences could affect performance; similarities could help reduce working capital by holding less inventory and taking advantage of reduced purchasing costs. Understanding the differences could also allow the development of a value proposition to support price increases or maintain customer loyalty.

Chemistry and History

Polyisobutene was patented by BASF in 1931 and is sold as Oppanol B. PIB has a regular structure, with the methyl groups derived from the isobutylene monomers arranged at every alternate carbon atom along the chain. How it gets like that involves discussion about the stability of cations and steric (repulsive) effects as the polymer grows, both of which are beyond the scope of this article. A two-dimensional stick drawing looks like a slightly spiky chain.

Reality is three-dimensional, of course, and the methyl groups repel each other, causing the molecule to form a coil. The molecular-scale coil structure looks like a spring and influences the macroscopic scale; for example, hydrogenated PIB is a synthetic rubber.

Polybutenes are derived from a mix of butenes, which consist of isobutene (also known as isobutylene or 2-methylpropene) and two linear butenes: 2-butene (consisting of cis- and transisomers) and 1-butene. The resulting polymers have random arrangements of monomers, so are relatively linear and less elastic, which is important for lubricants because they usually have a high viscosity lift.

Historically, PIBs were hydrogenated to remove the final double bond and used as antismoke additives in two-stroke oils for small engines, antimist agents in metalworking fluids, tackifiers in greases, and slideway oils and viscosity modifiers in several classes of lubricants.

The advent of large-scale production of methyl tertiary butyl ether (MTBE) as an octane booster, particularly for premium-grade unleaded gasoline, created a large demand for isobutene. This gave rise to the availability of large quantities of the two linear butenes as a raffinate from the process.

In the MTBE process, only the isobutene reacts, leaving a mixture of linear butenes and paraffins as a raffinate. As a result, producers began searching for a means to up-sell the linear butenes from fuel or alkylate production. Enter polybutenes. The problem was that PBs were not as reactive as PIBs of similar molecular weight. The double bond that remains at the end of the chain is more hindered in a PB because PIBs contain a lot of reactive vinylidene end group, whereas polybutenes do not.

During the 1990s, the solution was to make the precursors to succinimide dispersants and related molecules via the so-called chlorine route. This involved reacting PB with maleic anhydride in the presence of chlorine. The reaction creates a succinic anhydride (usually called a PIBSA, even though the hydrocarbon polymer may not be polyisobutylene) that, when reacted with an amine, produces a polybutenyl succinimide dispersant, ironically sometimes referred to as a PIB succinimide or PIBSI.

Additive companies and their suppliers were happy with the situation because they had access to PBs that were usually cheaper and readily available. However, late in the twentieth century, German automakers led the push to remove residual chlorine from lubricants, due to the potential for dioxins to form during the combustion process. A principal sources of chlorine in the lubricant at that time was the dispersant, so additive suppliers reverted to the so-called thermal process, which favored the use of PIB. But some manufacturers persisted with PB.

Because PB is much less reactive, this move resulted in some dispersants suddenly being heavily loaded with unreacted hydrocarbon polymer. While its double bond might not have been reactive enough for dispersant production, it was certainly reactive enough to undergo oxidative degradation under operating conditions. Even when steps were taken to remove the double bond, the polybutene remained. Thus, some dispersants became loaded with molecules that contributed to deposit formation and some packages became fatter to accommodate this.

Current Landscape

A significant number of additive packages on the market contain dispersants derived from PB manufactured by the thermal route and, therefore, carry plenty of free hydrocarbon. Some of these can be found by perusing Safety Data Sheets. However, because hydrocarbons are not hazardous, most legislation does not require them to be declared on the SDS. And, of course, other hydrocarbon polymers could be added to the additive package.

With additive packages becoming more heavily loaded with potentially deposit-forming hydrocarbon polymer, the chemicals industry set
about finding a way to make PBs with high reactivity. A leader in this effort was Daelim, but it also supplies the PIB market, having licensed technology to Lubrizol. BASF, on the other hand, is strongly embedded in PIB technology, with its Oppanol range in dispersants and related industries. Also, its Glissopals are hydrogenated PIBs sold into lubricants as antimist agents and viscosity modifiers.

PB or PIB is only one question to ask when looking at a polymer because it can affect viscosity index. Molecular weight and polydispersity (the molecular weight distribution) can affect pour point and flash point. While it is possible to overthink the suppliers processes and raw materials, attention to those parameters and the performance of different materials in a product will help a great deal. Also, the bromine number is a measure of the unsaturation of the polymer. This usually correlates with oxidative stability.

DTDA and DITA

Another set of initials that can cause confusion is DTDA, known for many years as an acronym for di-tridecyl adipate. DTDA was used to distinguish the ester derived from any C13 alcohol from that which made use of a rather special branched alcohol called iso-tridecyl alcohol. When reacted with adipic acid, this made DITA, or di-isotridecyl adipate.

The differences between DTDA (CAS number 16958-92-2) and DITA (CAS number 26401-35-4) are that DTDA is derived from natural alcohols and DITA is derived from a specific synthetic alcohol, which is linear with a short branch at the far end of the alcohol chain from the ester linkage.

The differences are not easily detected when examining sales specifications. However, DITA has a lower pour point – less than minus 60 degrees C vs. minus 50 degrees C for DTDA. The branching in DITA is thought to give it less biodegradability in certain tests, as the bugs in those tests dont like the branch at the end. However, it meets most biodegradability criteria while being fully saturated (some DTDAs werent) and, therefore, being more oxidatively stable.

There is potential for some confusion when considering the two esters, but think of the dialogue with suppliers when one of my colleagues decided that DTDA stood for ditridecylamine! Confusing in the highest degree.

Trevor Gauntlett has more than 25 years experience in blue chip chemicals and oil companies, including 18 years as the technical expert on Shells Lubricants Additives procurement team. He can be contacted at trevor@gauntlettconsulting.co.uk