Cellulose absorbs water. This is not a flaw — it is a fundamental property of a material whose structure contains abundant hydroxyl groups that form hydrogen bonds with water molecules. For nanocellulose composites used in controlled indoor environments, moisture absorption is manageable. For composites used in outdoor, high-humidity, or wet conditions, it is the primary design concern. Here is what our data shows and what we recommend.
Equilibrium Moisture Absorption Data
We conditioned injection-molded test bars (3mm thick per ASTM D638) in four environments and measured weight gain over 90 days until equilibrium was reached. Materials tested: NC-200 at 22 wt% in PP, NC-200 at 22 wt% in PA6, and unfilled PP and PA6 controls.
At 23°C / 50% RH (standard laboratory conditions): NC-200/PP absorbed 0.42 wt% at equilibrium. Unfilled PP absorbed 0.03 wt%. NC-200/PA6 absorbed 2.1 wt%. Unfilled PA6 absorbed 1.6 wt%. The nanocellulose adds approximately 0.4 wt% moisture uptake above the neat matrix in both cases. PA6 absorbs more total moisture because the polyamide matrix is itself hygroscopic.
At 23°C / 95% RH (tropical humidity simulation): NC-200/PP absorbed 1.8 wt%. Unfilled PP absorbed 0.08 wt%. NC-200/PA6 absorbed 5.4 wt%. Unfilled PA6 absorbed 4.2 wt%. At high humidity, the cellulose phase absorbs significantly more moisture. The NC-200/PP result — 1.8 wt% versus 0.08 wt% for neat PP — shows that the cellulose reinforcement dominates the moisture behavior of the composite even in a hydrophobic matrix.
At 23°C immersed in water (worst case): NC-200/PP absorbed 3.2 wt% at 90-day equilibrium. NC-200/PA6 absorbed 7.8 wt%. These are unacceptably high for structural applications. Water immersion is the boundary condition that defines where nanocellulose composites should not be used without additional protection measures.
Effect of Moisture on Mechanical Properties
We tested flexural properties of conditioned specimens immediately after removal from the conditioning environment (wet testing) and after re-drying at 80°C for 24 hours (recovery testing). The question is twofold: how much property loss occurs when the material is wet, and how much recovers when it dries out?
NC-200/PP at 50% RH equilibrium (0.42 wt% moisture): flexural modulus dropped from 5.3 GPa (dry) to 5.0 GPa (conditioned) — a 5.7% reduction. Flexural strength dropped from 78 MPa to 74 MPa — a 5.1% reduction. After re-drying, both properties recovered to within 2% of original values. At standard indoor humidity, the property loss is minor and reversible.
NC-200/PP at 95% RH equilibrium (1.8 wt% moisture): flexural modulus dropped to 4.2 GPa — a 20.8% reduction. Flexural strength dropped to 62 MPa — a 20.5% reduction. After re-drying, modulus recovered to 5.1 GPa (96% of original) and strength recovered to 75 MPa (96% of original). At high humidity, the property loss is significant but still largely reversible.
NC-200/PP after 90-day water immersion (3.2 wt% moisture): flexural modulus dropped to 3.4 GPa — a 35.8% reduction. Flexural strength dropped to 48 MPa — a 38.5% reduction. After re-drying, modulus recovered to 4.8 GPa (91% of original) and strength recovered to 70 MPa (90% of original). Prolonged water immersion causes permanent property loss — approximately 10% — attributable to partial debonding of the cellulose-matrix interface by water molecules. The damage is not fully reversible.
Why Moisture Causes Property Loss
Water in a nanocellulose composite occupies three locations. First, free water in microvoids between the cellulose phase and the matrix — this water is absorbed quickly and released quickly. It acts as a plasticizer, reducing modulus reversibly. Second, bound water hydrogen-bonded to cellulose hydroxyl groups — this water is absorbed more slowly and causes the cellulose phase to swell. Swelling introduces internal stress at the cellulose-matrix interface. Third, water at the cellulose-matrix interface — this water directly displaces the secondary bonds (and can hydrolyze covalent bonds in silane-treated grades) between cellulose and matrix polymer.
The first two mechanisms are reversible upon drying. The third is partially irreversible — once interface bonds are broken by water, not all of them reform when the water is removed. This explains the approximately 10% permanent property loss after prolonged water immersion.
For NC-200/PA6, the situation is more complex because the PA6 matrix is itself moisture-sensitive. Water plasticizes PA6, reducing its glass transition temperature from approximately 75°C (dry) to approximately 15°C (wet). At room temperature, a wet PA6-based composite is operating above its glass transition — the matrix is essentially rubbery, and flexural modulus drops dramatically. This effect is inherent to PA6 and has nothing to do with the cellulose reinforcement. However, the cellulose adds additional moisture uptake that pushes the PA6 further into the plasticized state.
Mitigation Strategies
Strategy 1: Choose a hydrophobic matrix. NC-200/PP composites absorb less than half the moisture of NC-200/PA6 composites under identical conditions. If the application environment is humid and the temperature range permits (PP is limited to approximately 100°C maximum service), PP is the better matrix choice. Polypropylene's near-zero moisture absorption means the only moisture in the composite is associated with the cellulose phase — and the PP matrix acts as a moisture barrier that slows water diffusion to the cellulose.
Strategy 2: Silane surface treatment. Our NC-200S (silane-treated) grade reduces equilibrium moisture absorption by 25-30% compared to untreated NC-200 in both PP and PA6 matrices. The silane coating reduces the number of exposed cellulose hydroxyl groups available to bind water. In NC-200S/PP at 95% RH, equilibrium moisture is 1.3 wt% versus 1.8 wt% for untreated — a meaningful improvement for borderline applications.
Strategy 3: Coating or encapsulation. For parts that will experience occasional wet conditions but are not continuously immersed, a surface coating (paint, clear coat, or conformal coating) provides an effective moisture barrier. A 25-micron polyurethane clear coat on an NC-200/PP test bar reduced 95% RH moisture uptake from 1.8 wt% to 0.6 wt% over 90 days. The coating must fully encapsulate the part — exposed edges or scratches create fast diffusion paths.
Strategy 4: Design for moisture-conditioned properties. Rather than trying to eliminate moisture absorption, design the part using the conditioned mechanical properties. If the flexural modulus at 50% RH is 5.0 GPa, design to 5.0 GPa, not the dry value of 5.3 GPa. This approach is standard practice for PA6-based composites (glass fiber reinforced PA6 data sheets routinely list both dry and conditioned properties) and should be applied equally to nanocellulose composites.
What We Recommend
For indoor applications (automotive interiors, consumer electronics, furniture, HVAC components): NC-200/PP or NC-200/PA6 without special moisture precautions. The 50% RH property retention is within 6% of dry values — well within typical design safety factors.
For partially exposed applications (automotive underbody, outdoor equipment housings, marine accessories): NC-200S/PP with surface coating. This combination keeps moisture uptake below 1 wt% under most conditions and maintains mechanical properties within 12% of dry baseline.
For continuously wet applications (submerged parts, water treatment equipment, outdoor structural elements without coating): do not use nanocellulose composites. Use glass fiber or carbon fiber reinforced materials that are inherently moisture-resistant. We are direct about this limitation because selling material into applications where it will fail damages both the customer and our reputation.
Full moisture conditioning data for all NC-200 grades is available in our technical data package. Contact info@soarceusa.org to request the moisture sensitivity supplement.