Markel

plastic has no place in pyrotechnics, some pressure rated pipes can handle thousands of psi the bigger ones less and properly made paper ones the same. the internal pressure acting on the walls of the motor casing in an end burner decreases as the propellant burns away and the volume of the motor increases only the first few seconds produce enough thrust to get up, this leaves a very small surface are for the pressure to act on in the first seconds and would most likely blow the plug instead of the casing.
i dont know of plastics suriving high temps for sustained periods but the motors do look too flimsy to be anything else maybe the grain is inhibited in some way lining the walls of the pipe with a flexible fire resistant resin.
dan

Perhaps aluminium tube liners are used, but even they would give way soon enough if the burn time was 'minutes' imagine the constant heat and pressure around the nozzle end of the motor with those times!
As you say, maybe a really good inhibitor resin is used. Love to know what it is if that's the case.
Actually, it's very doubtful that any of these 'Thai' rockets burn for minutes, more like seconds (30-40) I havent seen any that do yet and we know if they did, we arent going to be able to see the proof let alone hear it!!

Yes, these are impressive to watch. But would need better than plastic or cardboard tube to handle that sort of burn time especially being end burners!!