49 CFR § 178.521 - Standards for paper bags.
(a) The following are identification codes for paper bags:
(1) 5M1 for a multi-wall paper bag; and
(2) 5M2 for a multi-wall water-resistant paper bag.
(b) Construction requirements for paper bags are as follows:
(1) Bags must be made of a suitable kraft paper, or of an equivalent paper with at least three plies. The strength of the paper and the construction of the bag must be appropriate to the capacity and intended use of the bag. Seams and closures must be sift-proof.
(2) Paper bags 5M2: To prevent the entry of moisture, a bag of four plies or more must be made waterproof by the use of either a water-resistant ply as one of the two outermost plies or a water-resistant barrier made of a suitable protective material between the two outermost plies. A 5M2 bag of three plies must be made waterproof by the use of a water-resistant ply as the outermost ply. When there is danger of the lading reacting with moisture, or when it is packed damp, a waterproof ply or barrier, such as double-tarred kraft paper, plastics-coated kraft paper, plastics film bonded to the inner surface of the bag, or one or more inner plastics liners, must also be placed next to the substance. Seams and closures must be waterproof.
(3) Maximum net mass: 50 kg (110 pounds).
(4) UN5M1 and UN5M2 multi wall paper bags that have paper wall basis weights that vary by not more than plus or minus 5% from the nominal basis weight reported in the initial design qualification test report.