Pieter Samyn – SIRRIS
Functional bio-based additives may replace common mineral- or fossil-based fillers in polymers, adhesives, and coatings, providing better performance, biodegradation, recyclability, or non-toxicity. Novel micro- to nanocellulose grades with various dimensions, shapes, and morphologies can be recovered from various sources and replace traditional fillers such as graphite, graphene, carbon nanotubes, or silica. Recently, the industrial production of nanocellulose was enabled in parallel with a reduction in mechanical energy needed for production due to pre-processing or the use of alternative solvents. Eventual surface modification can be considered to improve dispersibility, tune interface compatibility, or provide hydrophobic surface properties. In framework of a collective research and dissemination project AddBio, a thorough screening study on the performance of different nanocellulose grades in epoxy coatings was made by Sirris, Belgium.
While adding different micro- and nanocellulose types into epoxy coating formulations with waterborne phenalkamine crosslinker, effects on processing conditions and coating performance were systematically investigated. The film formation of a waterborne epoxy is a heterogeneous process regulated through the coalescence of water-dispersed particles, comparable to the drying mechanisms of other waterborne latex coatings. Due to the hydrophilicity and high surface area of nanocelluloses, they are compatible with a waterborne epoxy and can interfere during crosslinking. The interactions between nanocellulose and the epoxy matrix can introduce reversible hydrogen bridging or permanent crosslinking.
Fig. 1 – Performance of waterborne epoxy/phenalkamine coatings with different nanocellulose additives, (a) morphology epoxy/CNC, (b) morphology epoxy/CNF, (c) abrasive wear and water contact angle for coatings with different concentrations of nanocellulose additives (numbers on X-axis in wt.%).
The variations in viscosity, thermal and thermomechanical properties, mechanical behavior, abrasive wear, water contact angles, and coating morphologies were evaluated. The selected additives include microcrystalline cellulose (MCC) at 1 to 10 wt.% and cellulose nanocrystals (CNC), cellulose nanofibers (CNF), cellulose microfibers (CMF), and hydrophobically modified cellulose microfibers (mCMF) at 0.1 to 1.5 wt.%. The viscosity profiles are determined by the inherent additive characteristics with strong shear thinning effects for epoxy/CNF, while the epoxy/mCMF provides lower viscosity and better matrix compatibility owing to the lubrication of encapsulated wax. The crosslinking of epoxy/CNF is favored and postponed for epoxy/(CNC, CMF, mCMF), as the stronger interactions between epoxy and CNF are confirmed by an increase in the glass transition temperature and reduction in the dampening factor.
The high degree of crosslinking for epoxy coatings determines a high chemical resistance, thermal stability, and better mechanical performance.
The mechanical properties indicate the highest hardness and impact strength for epoxy/CNF resulting in the lowest abrasion wear rates, but ductility enhances, and wear rates mostly reduce for epoxy/mCMF together with hydrophobic protection. In addition, the mechanical reinforcement owing to the specific organization of a nanocellulose network at percolation threshold concentrations of 0.75 wt.% is confirmed by microscopic analysis: the latter results in a 2.6 °C (CNF) or 1.6 °C (CNC) increase in the glass transition temperature, 50% (CNF) or 20% (CNC) increase in the E modulus, 37% (CNF) or 32% (CNC) increase in hardness, and 58% (CNF) or 33% (CNC) lower abrasive wear compared to neat epoxy, while higher concentrations up to 1.5 wt.% mCMF can be added.
Fig. 2 – Scratching resistance on the surface of epoxy/phenalkamine coatings with different nanocellulose additives and concentrations, as observed through optical microscopy of the scratching track
This research significantly demonstrates that nanocellulose is directly compatible with a waterborne phenalkamine crosslinker and actively contributes to the crosslinking of waterborne epoxy coatings, changing the intrinsic glass transition temperatures and hardness properties, to which mechanical coating performance directly relates. The morphological analysis of epoxy nanocomposite coatings (Fig. 1a, b) illustrates the reinforcing effect of nanocellulose (CNC and CNF) through formation of a fiber-like network, while providing a variation in the coating hardness and wear resistance together with protection against water ingress (Fig. 1c).
The differences in mechanical behaviour of the coatings with brittle or ductile fracture at the surface after scratching evaluations were investigated (Fig. 2). The epoxy/CNC mainly develops single cracking behind the tip due to more brittle fracture, the epoxy/CMF also has severe bulk fracture, while epoxy/CNF and epoxy/mCMF have more smooth and ductile scratches. The tendency for brittle and/or ductile fracture mechanisms can be related to the energy absorption of the nanocomposite coatings and obviously improves for coatings with higher hardness.
