Polybutene and polyisobutylene are liquid oligomers widely used as plasticizers for high-molecular weight polymers, such as polyethylene, and as carriers and lubricants. They are not to be confused with the high molecular weight polymer polybutene-1.
The liquid is clear and colourless, and is sold in many different grades. The grades are determined mainly by molecular weight, with the higher molecular weight grades having increasing viscosity. The oligomer degrades at high temperature (upwards of 350 °C) to form monomer isobutene by chains unzipping from the chain ends.
Polybutene is similar to but different from polyisobutylene (PIB). Polybutene is typically made from cat cracker mixed C4s (after the stream is Merox treated to remove sulfur and amines, and contain 1-butene, 2-butene, and isobutylene. Ethylene steam cracker C4s are also used as supplemental feed for polybutene. On the other hand, PIB is produced from essentially pure isobutylene made in a C4 complex of a major refinery. The presence of isomers other than isobutylene can have several effects including: 1) lower reactivity due to steric hindrance at the terminal carbon in, e.g., manufacture of polyisobutenyl succinic anhydride (PIBSA) dispersant manufacture; 2) the molecular weight—viscosity relationships of the two materials may also be somewhat different.
The reaction is an acid catalyzed carbocationic polymerization typically using aluminum chloride or hydrogen halide acid such as HF. So called "high reactivity" polybutene or PIB is produced using purified isobutylene feedstock and strong acid catalyst such as HF or HF with promoter.
The repeat unit is in case of 1-butene:
The repeat unit in case of 2-butene is:
One of the end units in the polymer chain contains a double bond, allowing reactivity with other compounds to provide functional chemistry mainly for lubricant additives for engine oils, fuels, and greases.
But the same double bond makes the molecule sensitive to oxidation when hot liquid is exposed to air. The polymer degradation reaction produces aldehydes and ketones, which are the further oxidised to carboxylic acids. This causes loss of tack and makes the material soluble in water.
It is used in several broad areas: 1) as plasticizer, 2)as a chemical intermediate for additives used in engine oils, gasoline and greases, and 3) formulations based on its physical performance characteristics.
Industrial product applications include, sealants, adhesives, extenders for putties used for sealing roofs and windows, coatings, polymer modification, tackified polyethylene films, personal care, polybutene emulsions. Hydrogenated polybutenes are used in a wide variety of cosmetic preparations, such as lipstick and lip gloss. It is used in adhesives owing to its tackiness.
An important physical property is that higher molecular weight grades thermally degrade to lower-molecular weight polybutenes; those evaporate as well as degrade to butene monomers which can also evaporate. This depolymerization mechanism which allows clean and complete volatization is in contrast to mineral oils which leave gum and sludge or thermoplastics which melt and spread. The property is very valuable for a variety of applications. For smoke inhibition in two stroke engine fuels, the lubricant can degrade at temperatures below the combustion temperature. For electrical lubricants and carriers which might be subject to overheating or fires, polybutene does not result in increased insulation (accelerating the overheating) or conductive carbon deposits.
- Decroocq, S and Casserino, M, Polybutenes, Chapter 17 in Rudnick (Ed), Synthetics, Mineral Oils, and Bio-Based Lubricants: Chemistry and Technology, CRC Press (2005), Print ISBN 978-1-57444-723-1, eBook ISBN 978-1-4200-2718-1.