If you're a tea drinker, this one is for you. Standard square tea bags – used for teas such as Earl Grey, English breakfast and green tea – are "heat sealed", meaning that a thin film of polypropylene is applied to seal the two sides together. Their "string and tag with sachet" range also contains polypropylene and a small amount of acrylic co-polymer emulsion, a plastic-based glue that bonds the bags together.
The way tea bags are manufactured varies depending on the brand but about 70 to 80 per cent of bags are made from compostable paper while the remaining 20 to 30 per cent contains heat-resistant polypropylene. This is to prevent the bag breaking mid-dunk, but it does mean that small pieces of plastic mesh are left behind in the soil when you compost the bags.