Peanut Butter Jar--the inner seal

By Richard I. Gibson

This simple little thing should be familiar to most Americans. It’s an inner safety seal from a jar of peanut butter. Many packages have them nowadays, in our society that seeks to ensure absolute safety in all aspects of food. Such hermetic barriers do help prevent contamination, extend product life, and provide a tamper-evident seal.

What’s it made of?

You’d probably say aluminum foil, and you’d be right—partly. It’s a sheet of pulp paper to which is bonded three more layers adding up to about 3.5 mils (a mil is one one-thousandth of an inch) in thickness. Only about 1 mil is aluminum. Another half-mil is a polyester that gives the sheet rigidity and flexibility. The rest, 2 mils out of 3.5, is usually a resin like DuPont’s patented and trademarked Surlyn™.

Surlyn is an ethylene copolymer, more or less ethylene vinyl acetate. Such resins make for a clean and easy peel when removing the seal, and they are also added to plastic wrap to enhance clinginess. Ethylene vinyl acetate combines ethylene and vinyl acetate. No surprises there.

Ethylene is a gas, C2H4, produced from natural gas and crude oil in the petrochemical cracking process in a refinery. It’s the largest volume organic chemical produced in the world today. Vinyl acetate comes from the chemical reaction between ethylene and acetic acid, a reaction enhanced by the presence of palladium as a catalyst. Acetic acid, essentially vinegar, was produced historically by distillation of various wood products, but today its chemosynthesis is by combining methanol (wood alcohol, CH3OH) with carbon monoxide. That’s another reaction that demands catalysts, this time including some pretty unusual metals, such as ruthenium, osmium, and iridium.

All this stuff obviously comes from somewhere. The next time you tear the seal off a new jar of peanut butter, think of it this way: you’re discarding a bit of paper, probably from pulp mills in the U.S., and some aluminum, ultimately from ores imported into the U.S. from Jamaica, Guinea, and Brazil. Processing aluminum ore (bauxite) to yield the pure metal demands a long list of chemicals, as well as a lot of electricity—which in the US is generated primarily by burning coal and natural gas.

The hydrocarbons that became the polyester and ethylene and methanol came from a long list of nations before reaching a U.S. refinery or petrochemical plant—if the stuff had “country of origin” labels, you’d have to include 86 nations that supply the U.S. with raw hydrocarbons.

Those metal catalysts are not trivial, but make the chemical reactions possible and economically feasible. While the U.S. does produce some palladium (from the Stillwater mine in Montana), more than half the palladium the U.S. consumes is imported, mostly from Russia and South Africa. Most of the iridium and ruthenium produced worldwide is mined in South Africa.