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Toward Sustainable Multifunctional Coatings Containing Nanocellulose in a Hybrid Glass Matrix Farhan Ansari,† Yichuan Ding,† Lars A. Berglund,‡ and Reinhold H. Dauskardt*,† †
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Stockholm, SE-10044, Sweden
‡
S Supporting Information *
ABSTRACT: We report on a sustainable route to protective nanocomposite coatings, where one of the components, nanocellulose fibrils, is derived from trees and the glass matrix is an inexpensive sol−gel organic−inorganic hybrid of zirconium alkoxide and an epoxy-functionalized silane. The hydrophilic nature of the colloidal nanocellulose fibrils is exploited to obtain a homogeneous one-pot suspension of the nanocellulose in the aqueous sol−gel matrix precursors solution. The mixture is then sprayed to form nanocomposite coatings of a well-dispersed, random in-plane nanocellulose fibril network in a continuous organic−inorganic glass matrix phase. The nanocellulose incorporation in the sol−gel matrix resulted in nanostructured composites with marked effects on salient coating properties including optical transmittance, hardness, fracture energy, and water contact angle. The particular role of the nanocellulose fibrils on coating fracture properties, important for coating reliability, was analyzed and discussed in terms of fibril morphology, molecular matrix, and nanocellulose/matrix interactions. KEYWORDS: cellulose nanofibril (CNF), cellulose nanocrystal (CNC), sol−gel, spray deposition, fracture energy, inorganic precursors, flexible the components influence the coating properties,15 and selected nanoparticle surface functionalization is often required.12,13 In this work we demonstrate a class of nanocomposite coatings where a semicrystalline organic component, nanocellulose obtained from trees in the form of nanofibers, is incorporated in an organic−inorganic hybrid glass matrix. Such coating architectures represent an interesting matrix−nanoreinforcement combination, scalable processability, sustainability, and the potential for exciting property profiles. Nanocellulose is the primary load-bearing component in wood, forming a hierarchical composite with hemicelluloses and lignin. Depending on the extraction process, nanocellulose can be obtained as cellulose nanofibrils (CNFs) or cellulose nanocrystals (CNCs). 16 Both can have similar lateral dimensions of 3−4 nm, but differ in their length and crystallinity. While CNFs are up to a few micrometers in length and are flexible with a high aspect ratio, CNCs are short (100−500 nm), rod-like, and highly crystalline. The repeat unit
C
oatings form an important class of materials that offer surface functionality and/or protection and are increasingly used to enhance the performance and lifetimes of polymer substrates. Recent technological developments, most notably in polymer glazing and moldings for lightweight transportation systems along with flexible organic electronic devices, have prompted the design of transparent coatings with high hardness and resistance to fracture, key metrics for improved reliability.1−3 Coatings made with an organic−inorganic molecular hybrid glass structure are excellent candidates to address these property requirements4 and can be inexpensively processed using sol−gel processing of metal alkoxide and epoxy-functionalized silane precursors under mild conditions.5 The coating structure consists of intimately mixed molecular components so that the resulting cross-linked organic−inorganic network forms a continuous hybrid glassy matrix phase.6 The coating properties can be tuned by varying the precursor ratio, their structural arrangement, and/or chemical functionality.7−10 Alternatively, such hybrid coatings can also be obtained as “nanocomposites” with well-defined and discrete inorganic nanoparticles in an organic matrix.11−14 Nanoparticle dispersion and compatibility between © XXXX American Chemical Society
Received: February 7, 2018 Accepted: June 4, 2018
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DOI: 10.1021/acsnano.8b01057 ACS Nano XXXX, XXX, XXX−XXX
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hazy/cloudy solution (Figure 1c).10 The nanocellulose/water suspension was then added to the solution, and the resulting suspension retained its optical clarity (Figure 1c). Suspension stability is critical to nanostructure formation in the subsequent stages. Chemical compatibility between the hydroxyl-rich nanocellulose fibril surface and the hydrolyzed TPOZ/ GPTMS (Figure 1a) prevented phase separation and allowed intimate mixing of the components. This resulted in a homogeneous and clear suspension (Figure 1b). To further limit nanocellulose agglomeration during spraying, an ultrasonic spray nozzle was used. The processing parameters (air pressure, scan speed, nozzle power) were optimized to obtain smooth and continuous coatings (thickness ∼400 nm) on silicon, glass and flexible polymer substrates. The coatings were chemically homogeneous through their thickness, evaluated by XPS depth profiling, confirming the homogeneous dispersion and stability of the preceding suspensions. The nanoscale structure of the coatings was studied using high-resolution scanning electron (SEM) and atomic force (AFM) microscopy. The uniformly dispersed nanocellulose (CNF or CNC) fibrils were apparent as shown in Figure 1d−f and Figure S2. Tubular features, at the scale of 5 nm, were observed as a connected network on the coating surface, and the CNCs appear short and rod-like, whereas the CNFs appear as longer tubes (Figures 1d and S2a). Imaging of different areas along the surface did not show agglomerates at any length scale, for coatings up to 20 wt % nanocellulose. However, with 50 and 80 wt % nanocellulose, irregular voids at the scale of ∼10 nm in diameter could be seen throughout the nanocellulose network structure (Figures S2b and c). At these high nanocellulose content, the voids could be caused by the local nanocelluloserich regions such that the nanocellulose packing (with low amounts of matrix) was accompanied by voids, as observed for pure CNF films.29 The average roughness of the coatings, as estimated by tapping mode AFM, was ∼5−10 nm (Figure 1e and f). Transparency and Hydrophobicity. Coating optical quality was evaluated by measuring optical transmittance of the coatings on a glass substrate with glass as the baseline (assumed to have 100% transmittance). For 400 nm thick coatings, neat matrix as well as composite coatings containing nanocellulose showed very high transparency (∼95% transmittance) (Figures 2a and S3a). Pure nanocellulose films can have high transparency (>85% transmittance for a thickness of ∼50 μm), primarily due to the small size of the nanocellulose fibrils.20,30,31 Consequently, nanocellulose dispersion and film thickness32 have a strong influence on the optical transparency of composites. High transparency of the present composite coatings indicated the formation of a dense coating structure containing well-dispersed nanocellulose such that the optical losses were minimized. The presence of nanoscale voids (∼10 nm), as observed for 80 wt % nanocellulose coatings, is unlikely to have severe effects on the transmittance since these voids are much smaller than the wavelength of light in the visible spectrum. Further, thicker composite coatings of up to 2.5 μm exhibited an optical transparency of >90% with 20 wt % nanocellulose addition compared to the neat matrix coating, which had a transparency of ∼85% (Figure 2b). Thick coatings may be desired for improved wear properties but can suffer from reduced transparency, which may be caused by high surface roughness.33 For the present case, surface roughness of the composite coatings did not vary significantly with thickness,
of the constituent cellulose molecule consists of a cyclic backbone of sugar rings with three hydroxyls on each ring. The presence of hydroxyls on the nanocellulose fibril surface imparts a useful chemical functionality, while the crystalline arrangement of cellulose chains gives it excellent mechanical properties.16,17 Consequently, nanocellulose has attracted considerable interest as mechanical reinforcement for polymer matrices,16,18 including packaging applications due to high transparency and oxygen barrier properties.19,20 Polymer composites with nanocellulose (CNF or CNC) typically have improved strength and modulus over the neat polymer.18 Besides, CNF films themselves can be stiffened by silica-based inorganics, such that the modulus of CNF films increases with silica addition.21−23 An added advantage is the facile chemical reactions between cellulose hydroxyls and silanes.24−26 Prehydrolyzed silanes can be readily adsorbed on the surface of cellulose fibers and undergo condensation reactions to form covalent bonds upon thermal activation.25 Such reactions have been used to modify the fiber surface, in order to reduce their intrinsic moisture sensitivity or improve chemical compatibility with relatively hydrophobic polymers.25,27,28 Contrary to the approaches using nanocellulose as mechanical reinforcement for polymers18 or using inorganics to stiffen nanocellulose films,21,23 we investigate a class of nanocomposite coatings where the organic nanocellulose fibrils form the stiffening and toughening phase in an inexpensive organic−inorganic hybrid glass matrix. We use the intrinsically functional hydrophilic nanocellulose surface to obtain a homogeneous nanocellulose/matrix aqueous sol−gel suspension and use spraying to form nanostructured coatings with well-dispersed nanocellulose. The coating nanostructure is characterized using electron and atomic force microscopy, and the effect of nanocellulose on optical and surface properties of the coatings is investigated. We further evaluate mechanical properties in the form of coating hardness and fracture properties over a range of nanocellulose content from 5 to 80 wt %. Optimized coating optical and mechanical properties are finally related to the coating nanostructure. The optimized nanocomposite coatings exhibited a ∼3-fold increase in fracture energy and average hardness (over the pure matrix), while maintaining the optical visible transmittance at >95%. The enhanced mechanical properties are key metrics of improved coating durability and reliability, making them excellent candidates for polymer glazing applications. To demonstrate suitability for applications requiring mechanical flexibility during processing or service life, coatings were deposited on flexible polymer substrates and subjected to cyclic bending. No cracks were observed in the films even after 20 000 bending cycles (130 μm substrate, 10 mm bending radius). Furthermore, the cost-effective processing of the present coatings avoids complex chemical modifications of the nanocellulose fibril surfaces, and a single-step spray deposition can be used. The use of renewable resource-based nanocellulose to design high-performance coatings advances opportunities for high-performance protective coating design and application. Processing and Structure. The hybrid matrix (or neat matrix) was amorphous (Figure S1) and formed via polymerization of hydrolyzed (3-glycidyloxypropyl)trimethoxysilane (GPTMS) and tetrapropyl zirconate (TPOZ). The first step involved hydrolyzing a TPOZ/GPTMS solution for 24 h, which turned clear after complete hydrolysis from an initially B
DOI: 10.1021/acsnano.8b01057 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano
Figure 1. (a) Molecular structure of hydrolyzed precursors (TPOZ, GPTMS) and native cellulose. (b) Schematic of the processing steps: Nanocellulose is added to a prehydrolyzed TPOZ+GPTMS solution and sprayed. (c) Corresponding optical images showing a clear and transparent solution after precursor hydrolysis, retained transparency after adding nanocellulose, and transparency of composite coating on the glass substrate. Hydroxyls on the nanocellulose surface allow for chemical compatibility between the components, leading to a homogeneous suspension and transparent coatings. Surface analysis of composite coatings showing well-dispersed nanocellulose in the hybrid matrix, as revealed by high-resolution (d) SEM and (e, f) AFM images of coatings containing 20 wt % CNF. The edge length of an AFM image is 1.3 μm, and the average roughness is 6 nm.
over time, by as much as ∼35% over 10 s19 and with increased ambient humidity.35 For the present coatings, however, the CA was relatively stable over time and decreased by 90°. Moisture sensitivity of the coatings was further evaluated in two separate experiments where (1) the CA was measured over a period of 300 s and (2) the CA was measured after aging the coatings at a relative humidity (RH) of 90% for 10 days. The water CA of native nanocellulose films is known to decrease C
DOI: 10.1021/acsnano.8b01057 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 2. Effect of nanocellulose on coating properties. (a) Visible range transmittance of composite coatings with varying amounts of CNF and a thickness of ∼400 nm. The transmittance was measured on coatings deposited on glass and assuming glass to have 100% transmittance (b) Transmittance at a wavelength of 600 nm as a function of coating thickness. (c) Contact angle of composite coatings with CNF and CNC. (d) Hardness of composite CNF coatings measured by nanoindentation.
coatings containing highly crystalline zirconia nanoparticles,44 both of which represent coatings with much higher hardness additions. The broad distribution in hardness of the nanocellulosecontaining coatings is related to the nanoindentor tip, which for the present study probed a contact area with a 50 nm tip diameter. The nanocellulose fibrils have a much higher hardness compared to the matrix.45 Accordingly, higher hardness was measured when the tip contact was with nanofibril(s), as opposed to the matrix. With increasing nanocellulose content (15−50 wt %), the local region in contact with the tip was more representative of the global coating (Figure S3c). Therefore, the hardness distribution became increasingly narrow, while the average value remained almost constant. With 80 wt % nanocellulose, the measured hardness decreased due to the weak nanocellulose/nanocellulose bonds and voids within the coating. While both forms of nanocellulose, CNF and CNC, showed the same general trend in hardness data, the highest recorded indents for CNC-containing coatings were higher than for coatings based on CNF (Figure S3d). This was due to the higher crystallinity of CNC,16 so that the intrinsic hardness is higher than CNF, which is partially amorphous. Coating Adhesion and Cohesion. DCB tests were performed on coatings deposited on Silicon (Si) to establish their fracture behavior. The neat matrix failed adhesively at the
coating/Si interface with a fracture energy (Gc) of 26 J/m2. The addition of a small amount (5 wt %) of CNF doubled the fracture energy to 52 J/m2, which increased further to 75 J/m2 with 20 wt % CNF (Figure 3a). While the neat matrix failed adhesively at the Si interface, the failure path of the composite coatings was more complex, failing predominantly in the coating (cohesively) and occasionally meandering between coatings on either side of the epoxy (Figure 3b). This was confirmed by SEM and optical microscopy, where fracture through the epoxy layer was observed at low magnification (Figure S5). The measured fracture energy (Gcomposite) in this case is attributed to three main factors: energy spent to fracture the matrix (Gmatrix), fracture energy contributions from CNF (GCNF), and the energy spent to shear through the epoxy layer (Gepoxy). Gcomposite = GCNF + Gmatrix + Gepoxy
(1)
Gmatrix was measured by asymmetric double cantilever beam (ADCB) tests, such that crack growth occurred cohesively and close to the coating surface (at a depth of
