Originally posted by julian_doe
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Ok, now for the long winded science-y explanation. This non-fastness of dyes seen in plant stalk derived cellulose based fibres (linen, hemp, ramie) comes from them having a higher crystallinity at the molecular level when compared to other cellulose fibres, notably cotton, which hold their colour better. The higher crystallinity results from the fibre being a longer, more closely packed polymer than imparts the stalk based cellulose fibres with their marked stiffness (compared to cotton) but that tighter packing also results in fewer locations for the dye molecules to react and become attached (covalently bond) to the polymer. The polymer is often so tight that dye molecules can't ever penetrate in between the parallel lined up long polymers to just "sit there" (ionically bond, H-bond) and give the fabric a colour that remains in place after repeated use.
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