Polypropylene was first introduced in tea bags during the late 1960s and became more widespread in the 1970s. The timeline breaks down as follows: 1950s: - Polypropylene was first commercially produced (1957) - Initial development for various commercial applications Late 1960s/Early 1970s: - Tea companies began incorporating polypropylene as a heat-sealable material - It replaced the traditional stapled or sewn closures - Offered advantages of: - More efficient manufacturing - Better sealing properties - Lower production costs 1970s-1980s: - Widespread adoption by major tea manufacturers - Introduction of the "heat sealed" edge became industry standard - Typically comprised about 20-25% of the tea bag material The use became so standard that many consumers were unaware of its presence until recent years (2010s-2020s) when environmental concerns brought it to public attention. Before polypropylene, tea bags were typically sealed by: - Metal staples - String ties - Folding and sewing - Natural fiber knots It's worth noting that the exact date of first use varies by manufacturer and region, as different companies adopted the technology at different times, and documentation from this period isn't always precise about material specifications.