Polyisobutenylsuccinimides

Alkenyl succinic anhydrides (ASA) are modified five-membered succinic anhydrides bearing a branched iso-alkenyl chain (C₁₄ to C₂₂). They are colorless, and usually viscous liquids. They are widely used, especially in surface sizing of paper, paperboard, and cardboard, as well as in the hydrophobicization of cellulose fibers. Products treated with it show reduced penetration of aqueous media, such as inks or drinks (like milk or fruit juices).

In terms of their mode of action, the anhydride is proposed to react with the hydroxyl groups on the cellulose, forming an ester. The alkenyl side-chain modifies the surface properties of the paper product. The application is similar to that for alkyl ketene dimers. In the United States alkenylsuccinic anhydrides are the preferred paper sizing agents, whereas in Europe, alkyl ketene dimers (AKDs) predominate.

History

The reaction of maleic anhydride (MAN) with aliphatic monounsaturated ''n''- and ''iso''-alkenes was described as early as 1936 in a patent. The alkenes are obtained from the "cracked distillate", a distillate fraction with a high content of unsaturation formed by cracking of petroleum.

The patent describes the reaction of the alkenes with excess maleic anhydride at 200 °C in an autoclave. The excess alkene is removed by distillation in vacuo, the resulting alkenyl succinic anhydride hydrolyzed with dilute sodium hydroxide solution and the disodium salt reacted with an acid to achieve an alkenebutanedioic acid. However, under the "many useful applications" described for the products obtained, the use as a size has not yet been mentioned. 30% higher reaction yields were achieved with a pre-cleaned cracked petroleum distillate in an autoclave at 210 °C and it was found that the hydrolysis of the succinic anhydride can already be carried out with steam.

In the early technical applications as lubricants and rust inhibitors, the products dark brown color and black, tar-like polymer residues was unproblematic. However, for later use in cleaners and detergents, clear (meaning polymer-free) and less dark colored alkenyl succinic anhydride (ASA) were needed.

The use of alkenylsuccinic anhydrides to hydrophobize cellulose-based (cotton) textiles (first patented in 1959) and the transfer of this concept to the hydrophobization of paper using iso-octadecenyl succinic anhydrides (C₁₈-ASA) in 1963 required liquid, particle-free colours with a colour as light as possible.

After initial difficulties in using ASA for paper sizing (particularly the rapid hydrolysis to alkenyl succinic acids and the formation of poorly soluble calcium salts in hard water and deposits in the paper machine), process parameters could be adjusted to make alkenyl succinic anhydrides the most important sizing agents in the US.

Preparation

ASAs are prepared in the Alder-ene reaction of alkenes with maleic anhydride at high temperatures (> 200 °C). Thereby, competing reactions occur, such as oxidation, condensation and (alternating co-) polymerization. The undesirable secondary products are formed reduce the yield of ASA and adversely affect color, texture and processability of the final products. Unbranched 1-alkenylsuccinic anhydrides, which can be obtained from 1-alkenes, are solid at room temperature and not suited for engine sizing from aqueous emulsion.

In the SHOP process, even-numbered 1-alkenes (CH₂=CH-(CH₂)_n-CH₃, produced by oligomerization of ethylene) are isomerized on magnesium oxide catalysts with displacement of the double bond to a position along the carbon chain. Subsequently, the crude product is separated by distillation into the desired fractions, the fraction preferred for ASA production is the C₁₄ – C₂₂.

The alkenyl succinic anhydrides are prepared with an excess of isoalkene at temperatures >200 °C under nitrogen atmosphere for more than 3 hours; the excess ''iso''-alkene is distilled off at reduced pressure.

Extensive patent literature exists with regard to the suppression of side reactions in the production of ASA. The formation of polymer can be reduced by the addition of antioxidants or polymerization inhibitors (for example hydroquinone or phenothiazine). Thereby, a quantitative reaction of the maleic anhydride can be achieved. Later, it was also achieved to improve the color of the obtained solid ASA from dark brown to amber by washing with water. By combining a reducing agent (such as a trialkyl phosphite) with a phenolic antioxidant, a significant lightening and reduction of tar formation is achieved.

However, recent patents show that the problem of discoloration and tar formation in ASA synthesis is still insufficiently resolved. Recent patents include a sterically hindered phenol (BHT as a primary antioxidant), a thioether (as a secondary antioxidant) and ''N,N-disalicylidene-1,2-diaminopropane as a metal ion deactivator in a "synergistic blend". The C12-ASA formed from dodecene and MAN after six hours at 220 °C is light yellow in color but still contains significant amounts of black, tarry decomposition products. A dramatic improvement (by 600%) in sizing efficiency is found with alkenyl succinic anhydrides based on symmetrical alkenes, such as the C22 alkene docos-11-en (by alkene metathesis from dodec-1-ene).

Use

Alkenylsuccinic anhydrides - and the alkenylsuccinic acids obtainable therefrom by hydrolysis - possess reactive and surface-active properties. They find use as hardener