Methods for Improving Moisture Resistance of LPCs

Measures to improve the moisture resistance of LPCs consist primarily of retarding the water absorption of cellulose fibers because it is virtually impossible to completely prevent their moisture absorption. A number of methods have been shown to retard moisture absorption in cellulose fibers, but they fall under two main categories water repellant treatments and dimensional stability treatments [17], as shown in Table 15.7. Whereas the water repellant treatment slows down the rate of water absorption, the dimensional stability treatment reduces the swelling in the fibers. 

Water repellant treatments involve the deposition of a thin layer of a hydrophobic substance onto external surfaces and, to some extent, internal cell lumen surfaces. Generally the equilibrium moisture content is not significantly altered by these water repellent treatments, although the absorption rate is reduced, so that it may take months or even years to reach equilibrium. For short to medium term protection, polymer coatings can be applied to reduce moisture absorption. For example, short jute fiber/PLA composites, coated with 100 pm thick polypropylene plastic adhesive tape, have been shown to be moisture resistant in humid environments, although the effects were not long lasting [34]. 

Chemical treatments improve moisture resistance by reducing fiber hydrophilicity, improving fiber/matrix bonding and/or plugging water penetration pathways in the 

Water repellent treatment Slows down the rate of water absorption Coatings, surface applied oils, chemical treatments 

Dimensional stability treatment Reduces or prevents swelling in the fibers Bulking the cell wall with polyethylene glycol, penetrating polymers, or bonded cell wall chemicals, or crosslinking cell wall polymers. Mostly used in wood fibers. 

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