How do Oven Cooking Bags work? I know they are made of heat resistant nylon resin, but can you explain what that means? — HY, Halifax, Nova Scotia
There are two broad classes of plastics: (1) thermoplastics that can melt, at least in principle, and (2) thermosets that can’t melt under any circumstances. Thermoplastics consist of very long but separable molecules and common thermoplastics include polyethylene (milk containers), polystyrene (Styrofoam cups), Nylon (hosiery), and cellulose (cotton and wood fiber). Thermosets consist of very long molecules that have been permanently cross-linked to one another to form one giant molecule. Common thermosets include cross-linked alpha-helix protein (hair) and vulcanized rubber (car tires).
Most common plastic items are made from thermoplastics because these meltable plastics can reshaped easily. But different thermoplastics melt at different temperatures, depending on how strongly their long molecules cling to one another. The plastic in an Oven Cooking Bag is almost certainly a thermoplastic form of Nylon, but one that melts at such a high temperature that it doesn’t change shape in the oven. It’s possible that the Nylon has been cross-linked to form a thermoset, so that it can’t melt at all, but I wouldn’t expect this to be the case.