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Characterization of cellulose nanocrystals dispersion in varnishes by backscattering of laser light.

Abstract Cellulose nanocrystals were mixed into an aqueous UV-cured coating formulation in order to improve the mechanical properties of the coating. One of the key aspects in the technology of nanocomposites remains the dispersion of the nanoparticles within the matrix. To quantify the dispersion, efficient methods of characterization are needed. In this study, a new characterization method based on atomic force microscopy (AFM) and backscattering of laser light (HeNe 632.8 nm) is applied to characterize such nanocomposite coatings. The angular distribution of backscattered light intensity was approximated by Gaussian distribution, and its standard deviation was used for the surface roughness analyses. A strong correlation between surface nano-roughness of coatings and angular distribution (half-width of the angular spread) of backscattered laser light was found. This laser characterization is faster and may be done without direct contact over a wider surface than AFM and may give us an idea about mechanical properties of coatings. This method can advance our fundamental understanding of dispersion of the nanoparticles in coatings and could be of use in quality control in industry.

Keywords Cellulose nanocrystals, UV-water-based coatings, Optical properties. Roughness, Scattering, Dispersion

Introduction

Waterborne coatings

The overall objective of this research is to develop a nanoparticles reinforced UV-water-based coating for wood applications and to study the effect, mainly on wear properties, of the final composite coatings. The formulation of varnishes is one of the most important steps for industrial uses, together with its application and drying. Water-based UV-cured coatings are increasingly used in the wood industry, in view of their advantages, such as very low volatile organic content (VOC), hardness, and fast setting (fast curing). First of all, the photopolymerization is very fast (a few seconds) with little or no VOC emissions. Second, the use of water as single solvent reduces the viscosity of formulations to promote their ease of application in environmentally friendly and secure conditions. (1) Finally, aqueous coatings are known for their good adhesion to wood. (2) UV-aqueous technology has other advantages, compared to 100% solid UV coatings, since the polymerization of the UV-aqueous coatings is insensitive to atmospheric oxygen. Generally, coatings produced using this technology have excellent chemical and thermal resistance. (1,3) The final properties of those coating are governed by the nature of the constituents. A varnish formulation is generally composed of three component groups: binder, solvent (water in this case), and additives. Each component group is made up of a wide range of products with specific properties and compatibilities. The binder or resin is the main element of a varnish formulation. Its role is to give to the coating its main physico-chemical characteristics, and once dried, it forms a continuous dry film that adheres to the substrate. The solvent in this type of formulations is deionized water. Antifoaming agent, surfactant, dispersant, rheological agent, and photoinitiator are the additives in water-based coatings, and each one has its own important role even if they are in small quantities in those formulations. Here, the role of the photoinitiator is of special importance. It is needed for the polymerization of coatings with a UV light source.

However, mechanical performance of these aqueous coatings is generally not as good as that of UV-curable high solid content coatings. The aqueous coatings are thin (about 100 [micro]m, containing only two layers or coats), relative to commercial coatings, which for parquets for instance, may have up to 7 coats. In order to increase these properties, we use cellulose nanocrystals (CNC) as film reinforcement in our study to evaluate their efficiency as organic-based renewable nanoscale reinforcement. We also modified CNC by different organic surfactants and used these in our coatings. (4)

CNC

Cellulose is the main component of wood and, in particular, of the cell walls. Hydrogen bonds bind together the anhydroglucose repeating units of the cellulose chains, resulting in a highly crystalline material. There are many products in everyday life made of cellulose derivatives, such as cellulose acetate, nitrates, esters, and ethers, some of which are paint components. One of these products is CNC fibers, which have high aspect ratio (as opposed to many inorganic nanoparticles, which are usually spherical or plate-like) and also are bio-based, non-toxic, and recyclable. Generally, CNC is extracted from the wood by a process of acid hydrolysis ([H.sub.2]S[O.sub.4]), with strict control of conditions of time and temperature. (5) The action of the acid removes the material of amorphous polysaccharide type. There remains the highly crystalline fiber component of the original cellulose. When this level is reached, the hydrolysis is terminated by rapid dilution of the acid. A combination of centrifugation and extensive dialysis is used to completely remove the acid, and, for certain processes, ultrasonic treatments complete the process for dispersing the individual cellulose particles and produce an aqueous suspension. (6) CNC was found to be good reinforcing filler when mixed with aqueous latex matrices. (7,8) Landry et al. have shown that the addition of small concentrations of CNC particles to clear clay-based coatings improves their mechanical properties (abrasion and scratch resistance, hardness). (9) One of the reasons to choose CNC as a reinforcement agent in our study is that it is an organic nanoparticle. It is still difficult to incorporate and properly disperse inorganic nanoparticles in an organic polymer matrix. (10) The research hypothesis assumes that CNC as such, and possibly with appropriate modification of its surface, will show high dispersion and stability in the resulting nanocomposite coatings, which are, in this case, transparent coatings (varnishes). However, there are some difficulties in obtaining dispersion characteristics of the organic nanoparticles in such organic varnishes.

CNC dispersion characterizations

The choice of the mixing technique plays an important role in the dispersion state of the nanoparticles in varnishes. There are several types of equipment commonly used in the industry, such as: high-speed mixer, three-cylinder mill and ball mill. (9) In our study, a high-speed mixer was used, since it is most appropriate to our system (varnish with CNC). But whatever methods we use, efficient methods of characterization are needed to quantify the dispersion. There are many different methods to characterize nanoparticles dispersion in the matrix. The most widely used methods to analyze the dispersion state of nanoparticles in dry films are electron microscopies, scanning (SEM), or transmission (TEM). However, in this project, it is difficult to do so as the polymer matrix (resin) and the CNC particles are both organic which leads to very low contrast between the resin and the particles. Farrokhpay conducted a literature review on new assessment techniques of painted surfaces and obtained quantitative results on the surface roughness characterization by AFM. (11) Thometzek et al. also used TEM and AFM to quantify the surface quality of paintings and correlate the degree of hydrophobicity of the particles with the quality of the dispersion. (12) In our previous research, we used AFM to study the quality of CNC dispersion by measuring surface roughness. (13) Correlation between surface roughness and the light scattering is well known. (14) In this article, another characterization method, based on the backscattering (14,15) of laser light (He-Ne, 632.8 nm), is used to qualify the CNC dispersion in varnishes. This is a new method in coatings testing, and our general aim is to bring the power of such optical resources and methods to the domain of coatings.

Materials and experimental procedure

The composition of the formulation

The formulation used in this work is composed of the following components: the resin (or binder), solvent (deionized water), photoinitiator, and other additives such as defoamer, dispersant, surfactant, and thickener. These chemicals were added in order to minimize foaming, optimize the surface tension, and help CNC dispersion. The typical compositions of the UV-aqueous formulation are presented in Table 1.

The photoinitiator is an essential element in a UV curing formulation since it initiates the radical polymerization reaction by absorbing ultraviolet light. The photoinitiator used in this research is a bisacylphosphine oxide (Irgacure 819DW, BASF Resins--Inks and OPV), dispersed in water (45 w/w%).

Then, in the basic formulations, CNC was added in concentrations of 0.5%, 1%, 1.5%, and 2% (w/w).

Characteristics of CNC used in this study

CNC used in this project has been prepared by FPInnovations (Canada). To the best of our knowledge, there are no publications on the manufacturing process of this specific CNC, since it is a proprietary industrial process, with patent pending. (16) Samples of the suspension of the CNC were sonicated using a Sonics Vibra-cell 130 W 20 kHz ultrasonic processor with a 6-mm diameter probe: typically, 15 ml of a suspension of 2-3% CNC weight was placed in a plastic tube of 50 ml and sonicated at 60% of maximum power until an energy input of more than 3600 J/g CNC was reached. This was performed in an ice bath to prevent the degradation caused by the rapid heating of the suspension. (17) Following this process, the resulting CNC nanoparticles have the following properties: specific surface area of 300 [m.sup.2]/g, a Young's modulus of 150 GPa, and a tensile strength of 10 GPa. (8) Cellulose fibers that remain after this treatment are almost entirely crystalline and as such are called "crystalline." The precise physical dimensions of the crystallites depend on several factors, including the nature of cellulose hydrolysis conditions and specific origin.

In water suspension (0.05% w/w), with the specific material used in this study, the characteristic dimensions of CNC are 6-10 nm (diameter) and 100-130 nm (length). These dimensions of CNC were measured through dynamic light scattering with a Zetasizer Nano ZS (Malvern Instruments Ltd.) and TEM.

The aqueous CNC suspension was then concentrated in a Labconco RapidVap (Labconco, Kansas City, USA) up to 14% (w/w). Such high CNC concentration is required to adjust the CNC loading within the formulation and avoid excessive dilution.

Surface modification of CNC

Since CNC has an anionic surface at neutral pH, it is possible to modify its surface with hydrophobic cationic surfactants. A similar technique was used by Nypelo et al. to modify hydrophilic silica nanoparticles for coatings. (18)

CNC was chemically modified by two different cationic surfactant molecules: hexadecyltrimethylammonium bromide (HDTMA-Br) and tetramethylammonium bromide (TMA-Br), while CNC was also modified by acryloyl chloride ([C.sub.3][H.sub.3]ClO), for further use in the formulations (in later discussions, unmodified CNC will be noted as CNC). A proprietary hydrophobic grade CNC, modified by FPInnovations, was also used as a reference. Acryloyl chloride reacted with hydroxyl groups. More details about these modifications have been presented in another paper, (4) where advantage was taken of the anionic nature of the nanoparticle, as these sites were coupled to the cationic ones of the hydrophobic surfactants. Thus, the nanoparticles were coated with a hydrophobic molecular coating. The amount of cationic reactant was adjusted to give a reasonably hydrophobic surface. Tests with too much of these surfactants resulted in an intractable nanopowder akin to powdered Teflon.

In later discussions, modified CNC will be noted by the following short format:

CNC-HDTMA: CNC modified by hexadecyltrimethylammonium bromide (HDTMA-Br)

CNC-TMA: CNC modified by tetramethylammonium bromide (TMA-Br)

CNC-Acryloyl: CNC modified by acryloyl chloride ([C.sub.3][H.sub.3]ClO)

CNC-FPI: CNC modified by proprietary hydrophobic modifier by FPInnovations firm.

Preparation of the samples

Preparation of formulations; dispersion method

A high-speed mixer (Dispermat, VMA-Getzmann GMBH D-51580 Reichshof) was used to disperse CNC in the varnish formulations. This equipment can produce shear rates comparable to those of industrial mixers. The high-speed mixer consists of a vertical rotary axis with a propeller plate with tooth saws at its periphery, rotating at high speed in a vessel containing the formulation to be dispersed.

Since in our formulation there are different types of products, it is very important to follow the order of addition. The formulations should always be prepared the same way, to be comparable. Here is the protocol of formulations preparation with CNC. At the beginning, the stirring speed is 400 rpm. The protocol followed for varnish preparation with CNC is

1. Addition of defoamer to the resin (in a metal container), with 4 min mixing.

2. Addition of surfactant to the above mixture, stirring for 4 min.

3. Addition of dispersant to the above mixture, stirring for 4 min.

4. Addition of deionized water (solvent) in the above mixture, stirred for 4 min.

5. Addition of CNC gel (14% in water) gradually with 320 g of beads (glass) in the above mixture, stirring for 10 min (the chemically modified CNC was used in powder form, not gel).

6. Addition of the rheological agent, stirring for 4 min.

7. Addition of photoinitiator, stirring for 6 min.

8. Removal of beads by filtration.

An example of varnish formulation with 2% CNC is presented in Table 2.

Preparation of the samples

Formulations were applied on sugar maple (Acer saccharum) on tangential face. The dimensions of the wood samples were 96 x 96 x 15 mm (Fig. 1). Coatings were applied by spraying. After application (thickness: 127 [+ or -] 15 [micro]m in liquid state), samples were put in a convection oven for 10 min at 60[degrees]C to gradually evaporate the water. During this step, the coating passes from a milky white liquid state to a transparent solid state. Film cure is then carried out using a UV oven equipped with a medium pressure mercury lamp (600 W/cm). This is a radical polymerization-type cure or crosslinking. The intensity of incident light measured with a radiometer was in the order of 570 mJ/[cm.sup.2], and the perceived temperature during curing is between 25 and 30[degrees]C. After curing, surfaces of varnishes were slightly sanded with 150 grit sandpaper, in the direction of wood grain. These steps have to be repeated once again to get a coating of ~100 [micro]m thickness.

In Fig. 1, it may be noted that there is no visual difference between the different samples. Thus, the CNC addition does not change the optical properties (color, gloss etc.) of the varnish, at least as perceived by human eye. More details about optical properties of these varnishes will be discussed later in this article.

Optical properties of coatings

Gloss and haze

The effect of the CNC on the gloss and haze of the coatings was determined with a haze-gloss apparatus from BYK Gardner. This instrument simultaneously determines gloss and haze at three different geometries, 20[degrees], 60[degrees], and 85[degrees]. These tests were performed according to ASTM standards D523 and E430. In our research, we measured gloss and haze at 20[degrees] as shown in Fig. 2. Gloss is generally defined as the amount of light that is reflected in the specular direction (i.e., at an angle that is equal to the light incident angle, Fig. 2a). The haze is given by the ratio of the scattered intensity at an angle off the specular direction (Fig. 2 b). (19) Commercial gloss meters (like the apparatus from BYK Gardner used here) have a detector with finite dimensions in the order of 1-10 mm, which implies that the beam of light measured during a gloss measurement with a standard gloss meter will not only contain some scattered light but will also consist of purely specular reflected light. (19,20) This is the reason why we have decided to use a dedicated optical setup (see next section) to have a higher angular resolution of backscattered optical power. The experimental value for each type of coating was obtained from the average of 12 measurements.

Optical setup

The backscattering of He-Ne laser light from coatings was measured by an optical setup (Fig. 3), and the obtained results (see hereafter) were compared with the surface characterization data obtained by AFM. The same geometry of gloss and haze measurement was used (Fig. 2) to construct the optical setup (incidence angle is 20[degrees]). The beam of He-Ne laser, operating at 632.8 nm (red), was used to probe the coating sample, which was in the center of the main fixed optical table. The diaphragm and photodetector were all mounted on a rotating horizontal base. A dielectric mirror was used to obtain the desired angle of incidence. A linear polarizer and half-wave plate were used to control laser beam polarization on samples (s and p polarizations (21)). To measure the angular dependence of the scattered light, the rotating base turned over -30[degrees] to 30[degrees] from the initial position of the photodetector, which is 20[degrees] from normal of the sample's surface (position of specular reflection). Our experiments show (not presented here) that the wood fibrils' orientation in combination with laser beam polarization is important. We have chosen the vertical orientation of wood fibrils and the s-polarized laser light scattering in the horizontal plane (the photodetector's movement plane) to optimize as much as possible scattering data information. All scattering results presented in this paper correspond to the above specified configuration.

Index matching

To minimize the wood surface scattering effect (due to air-surface defects), an index matching liquid was added on the coating and covered with transparent glass. This index matching was done by using certified refractive index liquid 1-lodonaphthalene with 1.70 refractive index (Cargille-Sacher Laboratories Inc., Cedar Grove, NJ, USA), since our coatings have a refractive index of 1.69, as measured by Mline (Metricon Corporation, Pennington, NJ, USA).

AFM

AFM observations were carried out using a Nano Scope V (Veeco Instruments Inc., Santa Barbara, USA), fitted with a Hybrid XYZ scanner. AFM measurements were done under ambient air conditions in tapping mode. The sensitivity of the tip deviation and the scanner resolution was 0.3 nm. The resolution was set to 256 lines by 256 pixels for all observations. Surface roughness was calculated in 10 [micro]n x 10 [micro]m scan areas, using the classical mean surface roughness parameter [R.sub.a]. Parameters were calculated by the Research Nanoscope 7.2 software:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where [R.sub.a] is the mean roughness, the arithmetic average of the absolute values of the surface height deviations, [Z.sub.i] is the current Z value, [Z.sub.ave] is the average of the Z values within the given area, and n is the number of points within the given area: 65536 in our case. The experimental value for each type of coating was obtained from the average of 12 measurements.

Results and discussion

Characterization of coatings by AFM

We characterized the surface dispersion degree of nanoparticles in varnishes, as first studied by measuring surface roughness using AFM, since the standard common techniques such as SEM and TEM did not give useful images, for lack of contrast. (13) In Fig. 4, the surface roughness measurement results on the effect of hydrophilic CNC are shown, i.e., unmodified, addition in the varnish, as a function of CNC concentration.

Addition of 0.5% of CNC has no effect on the surface roughness. However, when 2% CNC is added into the varnishes, there is a significant increase in the surface roughness (around 5 nm). It may originate from the decreasing quality of dispersion of CNC, which can be due to formation of CNC agglomerates. To improve the dispersion of nanoparticles in varnishes, we modified CNC with 3 different molecules (4) and also used hydrophobic CNC from FPInnovations to compare results with an industrial product. The surface roughness of coatings incorporating modified CNCs is significantly lower, about 30%, than that of coatings containing same amount of hydrophilic (unmodified) CNCs (Fig. 4). Surface roughness of coatings containing modified CNCs, especially those modified by HDTMA, acryloyl chloride, and hydrophobic CNC from FPInnovations is approximately at the same level as for coatings without nanoparticles. This means that the dispersion level of these hydrophobic CNCs is better, at least at the surface, where it is more important for the final appearance of the coating. Surface roughness of coatings with 2% CNC modified by TMA is higher than that from the other hydrophobic CNCs, which means that CNC-TMA may be more or less hydrophobic than optimal and the quality of dispersion is low, which has an impact on mechanical properties as well. (22)

Characterization of coatings by haze and gloss

Haze of the different coated films on wood did not show a noticeable change with the addition of the CNC and its derivatives (Fig. 5). There appeared to be very little difference between coatings without CNC (haze value is 353.4) and nanocomposite coatings (haze values of 373-390).

The coatings with modified CNC did show a slight haze decrease, in comparison with the coatings with unmodified CNC. Since haze values are all at the same level, we cannot really resolve appearance differences between these coatings, at least with this technique. It is clear that to have more information about surface, we need other methods, that are more precise.

In Fig. 6, a slight decrease of gloss of the coating can be noticed following the addition of more and more unmodified CNC (0.5-2%). The gloss levels presented in Fig. 6 indicate that the CNC derivatives modified by HDTMA, Acryloyl and CNC-FPI conferred a better aspect to the film than the unmodified CNC.

The gloss level of coatings with CNC-TMA is equal to the gloss level of coatings with unmodified CNC. In other words, the chemical modifications (except CNCTMA) made possible the retention of the high original gloss (~90) of the coating even after the CNC addition, which is also consistent with the precedent results from surface roughness (Fig. 4). However, this weak difference is not remarkable for the human eye (Fig. 1).

Characterization of coatings by optical setup

To study in greater detail the optical characteristics of varnish coatings, we used the optical setup shown in Fig. 3. Typical measurements by using this setup provide graphs (raw data, but with same arbitrary units) like that shown in Fig. 7. As we can see, the addition of CNC (modified or unmodified) in initial varnishes increases not only the roughness of coatings (Fig. 4) but also the angular spread of light backscattering (Fig. 7).

To properly compare the angular spreads, we have fitted experimental data by theoretical distribution. We know that to accurately follow the calculations and explanations of all backscattering effects from the coatings surfaces, we need the complex functions described in many of the classical books about scattering of light. (14,15) But since the aim of our study requires much simpler mathematical representation of angular distribution, we have chosen a normal (or Gaussian) distribution:

p([theta]) = 1/[square root of 2[pi][sigma]] [e.sup.[[theta].sup.2]/2[[sigma].sup.2], (2)

where [theta] is the angle, and the parameter [sigma] is its standard deviation; its variance is therefore [[sigma].sup.2]. In Fig. 8, scattering of coating without CNC and its theoretical fit are shown. The Gaussian distribution is close enough to our experimental data.

To determine the contribution of the air-varnish interface, we have used an index matching liquid. Liquid was added on the two types of coatings: varnish without CNC and varnish with 2% CNC and covered with a transparent glass slide. Same scattering measurements were done for these samples, and the results were normalized with a mirror (we put the mirror in place of the sample in Fig. 3). In Fig. 9, it can be noticed that after index matching and normalization, the backscattering from both coatings (without and with 2% CNC) is almost the same as the backscattering of the mirror, which means that when we have eliminated the effect of surface roughness: this procedure practically stops scattering. In Fig. 9, it is very hard to distinguish the results of backscattering by index-matched coatings since they are very close to the peak of mirror (after normalization); see the inset showing a zoom on the central part of graph.

To quantitatively compare all type of coatings by using this optical setup, we calculated the half-width of peaks (angular width at half of intensity) after normalization with the mirror and theoretical fit by using equation (2). In Fig. 10, it can be noticed that the addition of CNC increases the half-width of angular spread gradually in a manner similar to the tendency we have observed with the surface roughness (Fig. 4). Addition of modified CNCs: CNC-HDTMA, CNC-FPI, and CNC-Arcyloyl, provides half-width of the same value as for the coating without CNC, this also being similar to the case of roughness studies (Fig. 4).

Correlations were built between surface roughness and half-width of angular distributions of scattering, as in Fig. 10. There is an almost linear dependence between surface roughness and half-width of angular spread, which surely, theoretically, cannot be linear, just because the half-width of angular spread has a theoretical maximum (180[degrees]) but roughness has not. The surface roughness (Ra) is much smaller than probe wavelength ([lambda]), which allows us to apply the Rayleigh scattering model. (15-21) According to this model, the half-width of angular spread ([DELTA][theta]) widens with a decrease of [lambda] A theoretical relation between [R.sub.a] and [DELTA][theta] was found based on the above-mentioned considerations, theoretical and experimental results from references 14, 15,19,20, and our empirical data from all types of coatings that we made

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (3)

where [R.sub.a] is surface roughness, [lambda] is wavelength of laser light (632.8 nm), and n is the refractive index of coatings (as already mentioned n = 1.69). The coefficient 27[[pi].sup.4]/5 was found by approximation. In our case, roughness is at nanoscopic level, and in this particular range of values (Fig. 4), this correlation (3) can be interpreted as a linear dependence (Fig. 11).

We have made coatings with modified CNC only in concentrations of 2%. The reason for this comes from mechanical properties of coatings, since it was shown that 2% is the best concentration for these coating, which can improve their mechanical resistance about 40%. (22) In a previous report (about mechanical properties of these coatings (22)), a strong correlation between mechanical resistance and surface roughness has been shown; coatings with well-dispersed CNC are more resistant and coatings with badly dispersed CNC, i.e., with high roughness, even show a decrease in the mechanical resistance (CNC-TMA). This shows the importance of the dispersion of nanoparticles in varnishes for mechanical properties optimization. Dispersion was characterized by AFM, (13) which is not a simple technique, as it takes time, is not largely available, and is not a cost-effective approach for quality control. The method described in this paper could represent the next step to characterize nanocomposite coatings in a fast, easy, and inexpensive fashion. The other advantage of this method is that it can be used in a much more mobile setup than AFM.

In other words, if we have some type of coatings and we added some nanoparticles, resulting in an increase in the backscattering from the surface, then a worse dispersion and a decrease in mechanical resistance could be inferred. This method is aimed at being a fast and easy way to perform preliminary evaluation of optical and mechanical properties of coatings, to facilitate upcoming research.

Conclusion

In this article, UV-curable varnishes containing CNC particles have been prepared. The surface roughness of coatings on wood was measured by AFM and gave good indications of the degree of dispersion of CNC in coatings. This is an efficient method to evaluate the dispersion in cases where SEM cannot be employed as there is no contrast between the particles and matrix (organic composition).

Optical properties of coatings were measured by traditional standard methods (gloss and haze) as well as by using special optical setup. Data obtained from optical setup gave us much more detailed information about the interaction of light with coating. Theoretical fit by Gaussian distribution to experimental data was done to properly calculate and compare half-width of angular distributions. Results were compared with surface roughness data from AFM, and a strong linear correlation between them was found.

Thanks to these results, it is possible to evaluate in a preliminary way the mechanical properties of coatings, and compare the influence of addition of different types of CNCs at different concentrations.

This characterization method can be used for other type of coatings with other nanoparticles too. The practical application of our results can be in the quality assurance service in industrial level by using our procedure and the analogy of our experimental setup at a smaller scale.

V. Vardanyan, T. Galstian

Center for Optics. Photonics and Laser. Department of Physics, Engineering Physics and Optics, Universite Laval, 2375 Rue de la Terrasse, Quebec. QC G1V 0A6. Canada

e-mail: vardanyan.vahe@gmail.com

T. Galstian

e-mail: tigran.galstian@phy.ulaval.ca

V. Vardanyan, B. Riedl ([mail])

Departement des Sciences du Bois et de la Foret, Faculte de Foresterie, de Geographie et de Geomatique, Universite Laval, 2425, Rue de la Terrasse, Quebec, QC G1V 0A6, Canada

e-mail: bernard.riedl@sbf.ulaval.ca

DOI 10.1007/s11998-015-9673-4

Acknowledgments We thank Arboranano, Nanoquebec, FRNTQ (Quebec, Canada) for funding this project as well as FPInnovations' pilot plant and Dr. Gregory Chauve for the production of CNC, as well as Dr. Veronic Landry and Dr. Bouddah Poaty for advice during experiments.

References

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Table 1: Typical composition of the
formulatiion (see Table 2 for details)

Component             Chemical structure         Commercial name

Resin            Polyurethane-acrylate          Bayhydrol UV 2282
Defoamer         Ether poly dimethylsiloxane    Foamex 822
Surfactant       Polyether siloxane copolymer   Byk 348
Dispersant       Solution of block copolymer    Byk 190
                   of high molecular weight
Photoinitiator   Bis-acyl phosphine oxide       Irgacure 819DW
Thickener        Polyurethane                   RM 2020
Solvent          Deionized water                --
CNC              --                             --

Table 2: List of chemicals for varnish formulation with
2% CNC (402.98 g)

                     Mass (g)   % Dry (w/w)

Resin                 366.23         94.34
Defoamer                2.48          0.41
Surfactant              1.53          0.96
Dispersant              4.96          1.29
Deionized water         1.87            --
CNC (14% in water)     22.21          2.00
Rheological agent       1.65          0.21
Photoinitiator          2.05          0.89
Total                 402.98

Fig. 4: Surface roughness of varnishes
without and with modified and unmodified CNCs

Without CNC        5.34
0.5% CNC           4.96
1% CNC             6.71
1.5% CNC           8.13
2% CNC             9.19
2% CNC-HDTMA       5.86
2% CNC-FPI         5.79
2% CNC-Acryloyl    6.43
2% CNC-TMA         7.81

Note: Table made from bar graph.

Fig. 5: Haze of varnishes without and with modified
and unmodified CNCs, measured by haze-gloss apparatus

Without CNC        352
0.5% CNC           373
1% CNC             390
1.5% CNC           384
2% CNC             388
2% CNC-HDTMA       377
2% CNC-FPI         380
2% CNC-Acryloyl    382
2% CNC-TMA         385

Note: Table made from bar graph.

Fig. 6: Gloss of varnishes without and with modified
and unmodified CNCs, measured by haze-gloss apparatus

Without CNC        87.3
0.5% CNC           93.4
1% CNC             88.8
1.5% CNC           83.9
2% CNC             75.1
2% CNC-HDTMA       85.6
2% CNC-FPI         83.7
2% CNC-Acryloyl    82.4
2% CNC-TMA         75.8

Note: Table made from bar graph.

Fig. 10: Half-width of angular spread of backscattering from
varnishes without and with modified and unmodified CNCs

Without CNC        7.5
0.5% CNC           6.0
1% CNC             9.5
1.5% CNC           13.6
2% CNC             15.0
2% CNC-HDTMA       8.5
2% CNC-FPI         7.5
2% CNC-Acryloyl    9.0
2% CNC-TMA         10.5

Note: Table made from bar graph.


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Please note: Some tables or figures were omitted from this article.
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Article Details
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Author:Vardanyan, Vahe; Galstian, Tigran; Riedl, Bernard
Publication:Journal of Coatings Technology and Research
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2015
Words:5784
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