Nanocrystalline cellulose (CNC) is a renewable material with high potential in many applications. Due to its unique self-assembly and optical properties, CNC tends to behave as an iridescent pigment. The aim of this research was to explore the potential of CNC as an effect pigment in wood coatings. CNC-based coatings were developed from an aqueous CNC solution, a UV-curable water-based clear coating formulation, several colorants, and specialized additives. In this paper, the morphology of the resulting CNC films was investigated through circular dichroism and optical microscopy under polarized light. The effect of the CNC surface charge changes was monitored through zeta potential measurements. Color changes, or travel, and flop index were used to assess the iridescent effect of the coatings containing CNC. The experimental wood coatings contained CNC showed that the enhancement of the iridescent effect depends on the distribution and alignment of the CNC rod-like particles in order to generate the right pitch in the helical structure and their interaction with the polymer matrix as well with the additives. In conclusion, CNC could be successfully used as effect pigment in finishing systems, which can enhance the attractiveness and bring out the special grain of various types of wood.
Nanocrystalline cellulose (CNC) is a renewable material with high potential in many applications. CNC is generally produced by controlled acid hydrolysis from bleached wood pulp [
Favier et al. [
Gray and coworkers discovered that a unique self-assembled ordered liquid crystal phase was formed when the cellulose crystallites were sufficiently short and uniform (always less than 100 nm) and had a high degree of sulfates esterified onto the surface [
CNC films having the optical properties of a chiral nematic liquid crystal could be prepared by simple casting from aqueous suspensions. Circular dichroism (CD) can be used to measure the difference in apparent absorption of left-handed (AL) and right-handed (AR) circularly polarized light:
Previous studies had established that, by adding an electrolyte or providing ultrasonic energy to the CNC suspension, one could control the reflection wavelength of iridescent solid CNC films [
Cholesteric liquid crystals can be used as polymerized platelets (polysiloxanes, commercially available in thicknesses over 4
In recent years, the possibility to use special effect pigment in wood coatings for furniture or flooring has attracted the attention of architects and designers. Effect pigments can enhance the attractiveness of wood surfaces. For example, they are increasingly used in kitchen furniture coatings having a silky effect, which cannot be produced with conventional pigments [
The aim of this research was to explore the potential of CNC as an interference pigment in wood coatings. CNC-based special-effect coatings were developed from an aqueous CNC solution, a UV-curable water-based clear coating formulation, several colorants, and additives. The colorants used in this study are already widely used in the paints and coatings industry. In this paper we investigated the morphology of the resulting CNC films. More specifically, we considered the effect of these additions on the chiral nematic phase at a macroscopic level using circular dichroism and optical microscopy under polarized light. The effect of the CNC surface charge variation (hence colloidal stability) was monitored through measurements of the zeta potential of the suspension.
Color travel and flop index were used to assess the iridescent effect of the free films and wood coatings containing CNC. This characterization approach is often used by industries that consume effect pigments.
The CNC aqueous suspension (5.3% w/w) used in this study was provided by FPInnovations (Pointe-Claire, Quebec, Canada). The CNC was prepared at the FPInnovations pilot plant by acid hydrolysis of a commercial bleached softwood kraft pulp. The pH of the suspensions was adjusted to 7 for improved compatibility with the coating formulations. According to Beck et al. [
Prior to use, 500 mL batches of the aqueous suspension were homogenized for 10 minutes in an ultrasonic homogenizer (Cole Parmer 750 W, Illinois, USA) set at 40% of maximum power.
A water-based UV-curable varnish was prepared in accordance with formulas listed in Table
UV-curable waterbased varnish formulation.
Additive | Commercial name | Role | % wt. |
---|---|---|---|
Urhetane acrylate dispersion | Bayhydrol UV 2282 (Bayer) | Resin | 92.14 |
Polyetherdimethylsiloxane copolymer | Byk 025 (Byk-Chemie) | Defoamer | 0.5 |
Polyethersiloxane copolymer | Byk 348 (Byk-Chemie) | Surfactant | 0.46 |
Bisacylphospine oxide | Irgacure 819DW (BASF) | Photoinitiator | 0.92 |
Polyurethane | Acrysol RM2020 (Dow Corning) | Thickening agent | 1.38 |
Water | — | Solvent | 4.6 |
Industrial colorants.
Colorant | Pigment | Color | Pigment content in colorant % |
---|---|---|---|
Coltec C LS Blue (CPS Color) | Alpha isomer of copper phthalocyanine | Blue (PB 15:1) | 8 |
Temacolor W CH9 (CPS Color) | Carbon black | Black strong (PBk7) | 16 |
Coltec C BS Magenta (CPS Color) | 2,9 dimethyl quinacridone | Magenta (PR122) | 30 |
Additives for improving orientation of effect pigments in waterborne systems.
No. | Additive | Commercial name |
---|---|---|
1 | Polyethersiloxane solution in dipropyleneglycol monomethylether | Byk 346 (Byk-Chemie) |
2 | Non-ionic emulsion of a modified ethylene-vinyl-acetate (EVA) copolymer wax | Aquatix 8421 (Byk-Chemie) |
Boards of sugar maple, that were sawn in tangential sections, were planned and conditioned in a climate controlled room at a temperature of 20°C and a relative humidity of 50% until they reached a constant mass. Prior to cutting the wood in samples, the boards were sanded in sequence with 120, 150, and 180 grit papers. The dimensions of the samples used to applying the prepared formulations were 101.6 mm × 101.6 mm × 19 mm (
The formulations shown in Table
Formulations.
Formulation |
Description | g CNC/g varnish | % Colorant (w/w) | % Additive (w/w) |
---|---|---|---|---|
A | Non-sonicated CNC suspension | — | — | — |
B | Sonicated CNC suspension | — | — | — |
C | Varnish in B suspension | 1.25 | ||
D | Blue colorant in B suspension | — | 0.2 | — |
E | Black colorant in B suspension | — | 0.2 | — |
F | Magenta colorant in B suspension | — | 0.2 | — |
G | Mixture of varnish, blue colorant and B suspension | 1.25 | 0.2 | — |
H | Mixture of varnish, blue colorant, additive no. 1 and B suspension | 1.25 | 0.2 | 0.5 |
I | Mixture of varnish, blue colorant, additive no. 2 and B suspension | 1.25 | 0.2 | 0.5 |
J | Mixture of varnish, black colorant and B suspension | 1.25 | 0.2 | — |
K | Mixture of varnish, black colorant, additive no. 1 and B suspension | 1.25 | 0.2 | 0.5 |
L | Mixture of varnish, magenta colorant and B suspension | 1.25 | 0.2 | — |
M | Mixture of varnish, magenta colorant, additive no. 1 and B suspension | 1.25 | 0.2 | 0.5 |
N | Mixture of varnish, magenta and black colorants (1 : 1), additive no. 1 and B suspension | 1.25 | 0.4 | 0.5 |
O | Mixture of varnish, blue and black colorants (1 : 1), additive no. 1 and B suspension | 1.25 | 0.4 | 0.5 |
A goniospectrophotometer Model GSP-1B with GCMS-3B optical measurement unit (Murakami color Research Laboratory, Tokyo, Japan) was used to quantify reflection color differences in free films containing CNC. Following instrument calibration against a standard white tile, the films were placed against a matte black background and held flat with removable tape. The color parameters, that is, L* (lightness), a* (green-red coordinate) and b* (blue-yellow coordinate), were measured under 45° incident illumination over a reflection range of 0 to 80° in 5° steps. A standard D65 illuminant was used.
A portable multiangle spectrophotometer, MA98 from X-Rite (Manutrol Inc., Canada), was used to measure the color parameters L*, a*, and b* at 45° incident illumination for the coatings applied onto wood. The aspecular viewing angles were: 15°, 25°, 45°, 75°, and 110°. A standard D65 illuminant was used. Five measurements of color parameters L*, a*, and b* at different viewing angles were obtained from each sample.
The color differences obtained against the black standard were further employed to illustrate color travel and to generate flop index values.
CD spectra were measured with a Jasco J815 Circular Dichroism Spectropolarimeter. The specimens were analyzed in a 1 mm path length rectangular cell that was set perpendicularly to the incident cross-polarized light and scanned at 100 nm/min with a step resolution of 0.2 and 1 nm bandwidth.
Photomicrographs were taken at the surface of the free films at 20X magnification with an Axio Imager 2 (Zeiss) optical microscope equipped with a camera and crossed-polarizers.
The zeta potential of CNC suspensions, mixtures of CNC with colorants, and coating formulations was determined with a Zetasizer Nano from Malvern (Worcestershire United Kingdom), which has a zeta potential analyzer based on electrophoretic light scatting. A 4 mW He-Ne laser source with 633 nm wavelength was used as light source. All measurements were performed at 25°C.
The films prepared with the CNC suspension, colorants, varnish and additives are shown in Figures
Films prepared from CNC suspension (A), sonicated suspension (B), and sonicated suspension with varnish (C).
Films prepared from sonicated CNC suspension (B) with addition of blue (D), black (E), and magenta (F) colorants.
Films prepared with CNC suspension, varnish, colorants, and additives (formulation identification as per Table
Micrographs of optical microscopy with polarized light at 20X magnification (formulation identification as per Table
Pitch calculation was based on the distance between three successive dark (or light) bands associated with the retardation lines that create the fingerprint patterns [
Effect of adding varnish, colorants and additives on the chiral nematic.
Formulation | Chiral nematic pitch, |
---|---|
A | 4.7 (0.45) |
B | 5.0 (0.80) |
C | 3.8 (0.25) |
D | 4.2 (0.56) |
E | 4.1 (0.62) |
F | 4.9 (0.52) |
G | 5.9 (0.78) |
H | 6.5 (0.62) |
I | 5.9 (0.77) |
J | 4.7 (0.77) |
K | 4.9 (0.30) |
It can be observed that sonication of the CNC suspension has not increased significantly the chiral nematic pitch. Beck et al. [
The addition of blue and black colorants to the sonicated CNC suspension reduced pitch in the films cast from formulations D and E, while the addition of a magenta colorant did not affect the pitch of the film cast from the sonicated CNC suspension (formulation F). With the addition of 0.8 g of varnish per gram of CNC, the chiral nematic pitch of the film cast from formulation C became tighter (3.8
In the case of the formulations involving the varnish, the black colorant, and the two additives (J and K formulations) the pitch has not increased significantly in comparison with the sonicated CNC suspension. When the blue colorant, varnish, and the two additives were added (G, H, and I), the pitch has significantly increased in comparison with the formulation of CNC with varnish (C formulation). In the formulation H, for example, the addition of siloxane and the blue colorant caused the pitch to reach 6.5
CD spectra were measured for aqueous suspensions (Figure
Circular dichroism (CD) spectra of formulations with CNC suspension including blue and black colorants (a) and with varnish, colorants, and additives (b).
Circular dichroism (CD) spectra of films containing CNC, varnish, and colorants.
The CD signal of the CNC suspension was found to be negative in the UV region (Figure
The solid films based on CNC showed strong positive CD signals, off scale in some cases, which indicated that all the formulations containing CNC had undergone an isotropic to nematic phase transition having a left-handed structure (Figure
The film produced from the aqueous nonsonicated CNC suspension exhibited strong absorption in the UV and visible regions. The film produced from the sonicated aqueous CNC suspension exhibited a red shift by comparison with that produced from the non-sonicated suspension, which suggests elongation in the cholesteric pitch of the helical structure. The addition of the varnish led to a broader CD band than with the sonicated CNC suspension [
The CD band of film cast from a mixture of a sonicated CNC suspension with the black colorant (formulation E) follows almost the same path as the film cast from the sonicated CNC suspension. This behavior was confirmed by almost same pitch found for those two formulations displayed in Table
The film cast from a mixture of a sonicated CNC suspension and blue colorant showed the smallest positive CD signal. The two peaks, at 490 and 689 nm, are due to the color appearance of
The stability of a suspension is governed by the degree of surface charge. Measurements of the zeta potential indicate the surface charge on colloidal particles. Suspensions are stable; that is, particles do not aggregate (or unstable, particles aggregate) depending on the magnitude of the zeta potential of the particles [
In an CNC suspension the degree of surface charge is determined by sulfate ester groups. The degree of sulfation is known to affect the reflection color of CNC films [
Effect of adding varnish, colorants and additives on zeta potential of CNC suspension.
Formulation | Zeta potential, mV |
---|---|
A |
|
B |
|
C |
|
D |
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E |
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F |
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G |
|
H |
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J |
|
K |
|
The mixture of the CNC suspension with the varnish based on an urethane-acrylate dispersion changed the surface charge of the CNC particles. The zeta potential was found to be −22 mV as the mixture became unstable. The formulations with varnish and colorants, G and J, showed an increase in zeta potential in a different way. The siloxane additive gave rise to critical instability (the zeta potential was 0 mV) in formulation K with the black colorant containing carbon black. The additive triggered flocculation of the rod-like CNC particles, and agglomeration of the carbon black pigment, possibly leading to a separation phase at any time.
The polarity of the polyether chains of the siloxane additive proved to have more impact on the formulation containing carbon black, as this kind of pigment orients itself in a plate-like structure.
Angle-dependent spectrophotometric testing was required to characterize special effect pigments in free films. The color dynamics resulting from a combination of geometries and pigments can be illustrated by color changes (color travel). As CNC acts as an interference effect pigment, the a*b* diagram illustrates color changes through a color travel.
Figures
Color travel through a*b* diagrams of free films with CNC suspension and colorants.
Color travel through a*b* diagrams of free films with CNC suspension, varnish, colorants, and additives.
As observed in Figure
When the varnish was mixed with the CNC suspension, the color travel of the film decreased (Figure
Color travel through L* changes of free films based on CNC suspension and colorants.
Color travel through L* changes of free films based on CNC suspension, varnish, colorants, and additives.
Prior to application of the coating formulations onto sugar maple wood specimens, several tests were conducted. The special effect appearance of the coatings was investigated with a portable multiangle spectrophotometer.
The flop index (FI) with standard deviation was automatically calculated with the X-Color QC software of the X-Rite equipment. The following formula was used:
Effect of wet thickness on the flop index of coating C.
Images of the coatings applied onto sugar maple wood are shown in Figure
Wood coatings prepared with formulations of varnish, C, H, K, N and O.
Color travel through a*b* diagrams of finishing systems based on varnish, CNC, blue and black colorants (the arrows shows increasing of viewing angle).
Color travel through L* changes of finishing systems based on varnish, CNC, blue and black colorants.
In the a*b* chromaticity plane color travel was more intense for the coatings prepared with varnish, CNC, and black colorant, where the color had traveled through three quadrants (formulations K and O) (Figure
Oriented CNC in the colored coat produced a light to dark color travel appearance that could be observed on the L* axis (Figure
The flop indexes of finishing systems applied to wood are shown in Table
Flop index of finishing systems for wood with standard deviation.
Finishing system | Flop index |
---|---|
2 coats of varnish (control) | 1.92 (0.25) |
1 coat of varnish + 2 coats of C formulation | 7.36 (0.32) |
1 coat of varnish + 2 coats of H formulation | 9.84 (2.39) |
1 coat of varnish + 2 coats of K formulation | 18.82 (2.20) |
1 coat of varnish + 2 coats of M formulation | 8.96 (0.7) |
1 coat of varnish + 2 coats of N formulation | 17.04 (0.68) |
1 coat of varnish + 2 coats of O formulation | 12.87 (2.06) |
Experimental wood coatings were developed using CNC as effect pigment. As thick layers of the originally dilute aqueous CNC suspension had to be applied to retain the iridescent effect, practical difficulties were experienced due to insufficient workability. The iridescent effect of the sonicated CNC-containing films depended on the wet thickness of the layer, which was correlated with CNC concentration in the dry film. Chiral nematic pitch changes, broadening of CD spectra, fingerprint patterns degradation, and CNC surface charge changes were correlated with the iridescent effect. CNC orientation in a proper position and formation of the stacked planes are essential to generate the right pitch in the CNC helical structure, locking in the chiral nematic structure. The distribution and alignment of the CNC rod-like particles, and their interaction with the polymer matrix as well as with the additives were shown to determine optical effect of the new coatings. The changes in chiral nematic pitch observed through optical microscopy with polarized light were confirmed by the CD spectra.
The iridescent effect of coatings and free films was explained through color travel and flop index. Enhancement of the iridescent effect was shown to depend on the nature and concentration of the colorant/pigment used and the additives selected to improve CNC orientation in the dry film.
In conclusion, CNC could be successfully used as effect pigment in wood coatings. It offers an original prospect to this renewable resource at a time where novel applications of the pulp and paper are sought. Finishing systems contained CNC, varnish, and colorants can accentuate the attractiveness of wooden surfaces, by bringing out the special grain of various types of wood.
The authors are grateful to ArboraNano and Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support. The authors would like to thank also Stephanie Beck from FPInnovations for technical support (Pulp, Paper and Bioproducts division, Pointe Claire, Québec, Canada).