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The backsheet is the water-resistant outer layer of the diaper and it prevents liquids from passing through the diaper and leaking onto the baby’s clothing or surrounding environment. The polyethylene backsheet can be made with a structure that allows water vapor and air (but not liquids) to pass through, reducing moisture and keeping the skin drier.
link:https://www.taiwanfabric.com/lamination-fabric/diaper-backsheet.html
Diaper backsheet