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Application of polymer nanoparticle coating for tuning the hydrophobicity of cellulosic substrates.

Abstract Nanoparticles of partially imidized poly(styrene-maleic anhydride) were applied from an aqueous dispersion as a one- or two-layer coating onto paper substrates, for controlling the paper surface hydrophobicity and improving the water barrier resistance. The effect of deposition conditions and thermal treatments on the topography and properties of the coating was studied by scanning electron microscopy, atomic force microscopy (AFM), contact angle measurements, and friction measurements. The wettability of paper surfaces with adsorbed nanoparticles can be controlled by tuning the chemical and topographical surface parameters: the water contact angles were found to increase at higher imide content as determined by Raman spectroscopy (depending on synthesis and thermal treatment), and higher average surface roughness determined by AFM (depending on the deposition method). The present technique may serve as a unique replacement for chemical treatments hydrophobizing fibrous substrates.

Keywords Nanoparticle, Coating, Paper, Cellulose, Wetting, Hydrophobicity

Introduction

Paper, paperboard, and linerboard are often used as packaging materials with good strength, flexibility, and low cost. The cellulose libers are natural materials with biodegradability and recyclability, but their hydrophilic nature results in high water and moisture penetration. The wetting and absorption mechanisms of water droplets onto a paper surface depend on the chemical and physical heterogeneities of the porous substrate.1-3 Additional surface modifications are needed to increase the surface hydrophobicity, create effective barrier properties, induce unidirectional liquid transport, and enhance self-cleaning properties. The wettability and water resistance are in general controlled by sizing treatments or coatings. Sizing reduces the surface energy, while it also improves certain physical properties of the paper sheet such as surface strength and internal bond.

Internal sizing is usually done by wet-end fiber modifications, using either nonreactive agents (e.g., rosin) or synthetic agents that chemically react with the cellulose hydroxyl groups to form stable ester linkages (e.g., alkene ketene dimers, alkenyl succinic anhydride). Scientifically proven, but commercially less attractive, techniques for modifying cellulose fiber surfaces include graft polymerization, (4-10) oxygen-plasma treatment," plasma-assisted coating deposition, (12-20) chemical vapor deposition, (21) pentafluorobenzoylation, (22) esterification, or silylation. (24) Surface sizing is commonly applied on the size press, using a gelatinized solution of starch in cooked or modified form, with wax emulsions or additives. (25) Although sizing treatments increase the hydrophobicity, the waterproof efficiency for food and drink packages should be further enhanced by barrier coatings containing modified starches, (26), (27) polyvinyl alcohol or polyvinylidene chloride emulsions, (28) styrene-butadiene latex, (29) acrylic latexes, (30) sol-gel layers, (31) biopolymers, (32-34) styrene-acrylale core-shell latexes," or acrylate microlatexes, (36) Conventional barrier coatings have been developed by extrusion of polymers such as wax, polyethylene, fluorocarbons, and polyethyleneterephthalate. In combination with specific inorganic fillers (e.g., clay, delaminated talc, aluminum-hydroxide), the entire weight of traditional paper coatings takes about 5-20% of the paper sheet.

The introduction of nanotechnology in papermaking coatings is driven by the need to further enhance surface functionalities, while reducing the coating weight and replacing fluorocarbons, (37-39) or silanes. (40-42) Recent nanotechnological advancements in papermaking focused on cellulose nanocrystals, (43) and fiber or pulp treatments: the latter include fluorosiloxane-modified silica nanoparticle depositions for creating superhydrophobic fibers, (44) or cationic nanoparticle adsorption for photoluminescence. (45) Nanostructured coatings are under development for controlling conductivity (carbon nano-tubes loaded in sprayed silica sols, (46) polyaniline, (47) or multilayer deposition (48)), magnetism (magnetite nanoparticles (49)), antibacterial properties (zinc-oxide nanoparticles (50)), or water repellency (starch/cellulose nanofibers, (51) nanoclays, (52) or ceramic nanoparticles (53)). Specific interactions at the paper surface can be achieved by applying amphiphilic copolymers with a micelle structure (leading to a hydrophilic surface) and hydrophilic blocks attached to the surface (leading to a hydrophobic surface). The wettability depends on the deposition method, the length, and the amount of the hydrophilic block copolymer. (54) An interesting amphiphilic copolymer is poly(styrene-maleic anhydride) (SMA), that was introduced as bulk-polymer in the dry-end paper technology because of good ink absorption, (55), (56) or that was used for synthesis of microcapsules containing oil, ink, and pigments adsorbed on carbon-less copy paper. (57), (58) According to Malardier-Jugroot et al., (59) the copolymers show a linear conformation with high chain rigidity at neutral pH, which allows for self-assembly and strong association into nanosized structures regardless of the chain chirality. The copolymer was found to act more effectively as a hydrophobizing sizing agent when applied at neutral pH, indicating that not only the number of functional groups but also their presentation to the outer surface is important.

In this study, high-molecular weight SMA will be imidized into thermally and mechanically stable nanoparticles that can be adsorbed as a coating onto paper. The unique surface structure with macropores (cellulose or textile nonwovens) and nanoscale functionality (roughness and active sites) will offer a combination of controllable wettability, barrier resistance, and printability.

Experimental details

Materials

Two grades of SMA copolymer were used as ground pellets: (i) SMA-1 has a relative molecular mass of 80,000 and contains 26 mol% maleic anhydride or MA (74 mol% styrene), and (ii) SMA-2 has a relative molecular mass of 65,000 and contains 28 mol% maleic anhydride or MA (72 mol% styrene). Ammonium hydroxide was used as a 25% aqueous solution.

The SMA copolymers reacted with ammonium hydroxide into partially imidized poly(styrene-maleimide) (SMI) nanoparticles according to Fig. 1. The imidization was done in a double walled, oil-heated reactor of 1 L with stirrer. About 140 g of SMA was charged together with an equivalent amount of ammonium hydroxide as such that the molar ratio of MA to ammonium hydroxide was 1:1.01. Water was added until a total volume of 700 mL was obtained. When adding the ammonium hydroxide, the temperature was raised to about 90[degrees]C at a reaction pressure of 1 bar. The reaction mixture was subsequently heated up to 150[degrees]C and the reaction pressure was increased to 6 bar to prevent boiling. The viscosity profile was monitored as the electric power needed to drive the stirrer at a constant speed of 50 rpm. As the reaction mixture attained 100-120[degrees]C, the viscosity increased significantly through gel formation. The viscosity increase lasted until 160[degrees]C and suddenly dropped after a reaction time of about 5 h. After 6 h reaction time, the reactor was cooled down to room temperature and partially imidized nanoparticles were obtained in stable aqueous dispersions.

[FIGURE 1 OMITTED]

Preparation of nanoparticle-coated substrates

Using a laboratory bar coater (K303 Multi-coater from RP-Print Coat Instruments, Ltd.), the aqueous dispersions of SMI nanoparticles were spread onto paper substrates under different conditions: either a one-layer coating (using a bar speed 4 or 6 mm/s) or a two-layer coating (using a bar speed 6 mm/s with intermediate drying) was applied. The coatings were dried for 2 min at 100[degrees]C and further stabilized for 1 day in air. Supplementary heat treatments were applied by placing the coated papers in a hot-air oven at temperatures between 100 and 250[degrees]C for 6h.

Characterization

The morphology of the paper coatings was investigated by optical microscopy (Olympus BX 51) and scanning electron microscopy, using an FEI SEM XL30 (LaB6 filament) and FEI Dualbeam (FEG-SEM). The topography of the nanoparticle coatings was studied with an atomic force microscopy (AFM) in tapping-mode, using PicoScan 2500 PicoSPM II Controller (PicoPlus, Molecular Imaging) with a silicon probe of k = 40 N/m and 300 kHz resonant frequency. The average surface roughness ([R.sub.a]) was calculated on a 2 x 2 [micro][m.sup.2] standard scan size that was flattened by image processing procedures. The variation on roughness values is about [+ or -]2 nm. The chemical composition of the nanoparticle coatings was determined from FT-Raman spectroscopy, using Perkin Elmer Spectrum GX equipment. The spectra were collected with an Nd:YAG laser power of 500 mW at a 4 [cm.sup.-1] resolution and averaged over 64 scans. The spectra for coated paper surfaces were corrected with a spectrum of uncoated paper surfaces.

The static and dynamic contact angles of deionized water were measured on a Kruss drop shape analysis system (DSA 10 Mk2) by placing a constant drop volume of 4 [micro]L (static) or by increasing the drop volume from 0 to 7 [micro]L at a rate of 6 [micro]L/min (dynamic). The water drop shapes were geometrically determined from Laplace-Young (static) or tangent (dynamic) fitting procedures. The variation on contact angles is about [+ or -]2[degrees]. The water absorptiveness of the substrates was determined from a Cobb test on paper samples cut with the size 14 x 14 [cm.sup.2] and clamped inside a ring of diameter 10 cm. A 50 mL sample of water was poured into the ring and remained in contact with the paper for 2 min. The wet paper was dried afterward dried between two pieces of blotting paper. The amount of absorbed water (g/[m.sup.2]) is calculated by weighing the samples before and after the test. The coefficients of friction are determined by placing two coated surfaces in contact with each other on a Solatronic inclinometer under a weight of 100 g, and recording the tilting angle at sliding (accuracy 0.1[degrees]). The gloss was measured with a BYK Gardner microgloss meter under 85[degrees] over an area of 9 x 21 mm2, in directions parallel and perpendicular to the coating bar (variation [+ or -] 0.25).

Test results

Morphology and topography

The macroscopic morphology of one-layer SMI-1 and SMI-2 coatings on paper substrates is illustrated by optical microscopy in Fig. 2. For uncoated paper, [CaCO.sub.3] fillers are recognized in between the bundles of cellulose fibers with diameter 10 urn (microfibrillar structure 500-50 nm) and pores with 10-50 [micro]m diameter. The coated papers have a noncontinuous, transparent film with irregular macrodomains created during the drying process. A similar structure was observed for one- and two-layer coatings. The transparency results from the nanometer-sized elementary particles, which are smaller than the wavelength of visible light, and their amorphous nature. The macro-cracks are independent of the fiber location underneath (see location A), but some fine cracks within the macrodomains are perpendicular to the fiber direction due to local stress concentrations (see location B). The coating of SMI-1 shows some larger defects than SMI-2, related to coating strength and imide content. The scanning electron microscopy in Fig. 3 also reveals a macro-to-nanostructured coating with good adhesion to the cellulose fibers. This structure seems independent of sample preparation and changes the capillary forces at the surface offering good printing properties.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The nanoscale morphology of SMI nanoparticles adsorbed on cellulose surfaces is illustrated by AFM in Fig. 4. The nanoparticle application significantly reduces the roughness in respect to uncoated papers (Table 1). The SMI-2 coatings are smoother than SMI-1, because of a somewhat broader particle diameter distribution measured by dynamic light scattering. The diameter of adsorbed nanoparticles is about 90-105 nm for SMI-1 and about 80-100 nm for SMI-2. A detailed observation on 500 x 500 n[m.sup.2] surface areas in Fig. 5 more clearly indicates the effect of deposition conditions on topography of multilayer coatings. At low bar speeds, the coating is rough and has a more open structure compared to high bar speeds. For one-layer nanoparlicle coatings, enrichment of larger nanoparticles on top of the coating has been observed while the smaller nanoparticles penetrate in between the cellulose fibers. For two-layer coatings, a more homogeneous distribution of small and large nanoparticles at the surface results in smoother surfaces.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]
Table 1: Characterization of SMI-1 and SMI-2 coatings on paper surfaces
under various deposition conditions

Coating       Dry      [R.sub.a]  [[theta].sub.stat]  [[theta].sub.adv]
 type       coating    roughness     ([degrees])         ([degrees])
           weight (g/    (nm)
           m.sup.2])

Blanco        -            60            104                 100
paper

Deposition of coating at 4 mm/s bar speed

 SMI-1,       5.7          20            115                 132
 1 layer

 SMI-2,       7.5          13            127                 140
 1 layer

Deposition of coating at 6 mm/s bar speed

 SMI-1,       5.0          15            102                 123
 1 layer

 SMI-1,       6.7          8.2            92                 113
 2 layers

 SMI-2,       5.2          10            120                 128
 1 layer

 SMI-2,       6.8          6.5           113                 120
 2 layers

  Coating type    [[theta].sub.rec]  [DELTA][theta]    Friction
                     ([degrees])                     coefficient

Blanco paper              20                80           0.62

Deposition of coating at 4 mm/s bar speed

 SMI-1, 1 layer           21               111           0.82

 SMI-2, 1 layer           28               112           0.78

Deposition of coating at 6 mm/s bar speed

 SMI-1, 1 layer           22               101           0.75

 SMI-1, 2 layers          24                89           0.77

 SMI-2, 1 layer           28               100           0.71

 SMI-2, 2 layers          38                82           0.73

  Coating type                 Gloss (a)    Water absorption Cobb-test,
                                                  2' (g/[m.sup.2])
                  [perpendicular to]   //

Blanco paper              6.1          6.9               90

Deposition of coating at 4 mm/s bar speed

 SMI-1, 1 layer          10.5         13.6               30

 SMI-2, 1 layer           5.4          6.7               36

Deposition of coating at 6 mm/s bar speed

 SMI-1, 1 layer          11.6         14.8               27

 SMI-1, 2 layers         22.8         26.4               27

 SMI-2, 1 layer           7.2          9.2               26

 SMI-2, 2 layers         14.6         17.5               27

(a) Directions perpendicular ([perpendicular to]) and parallel (//) are
relative to the coating bar


Raman spectroscopy

The Raman spectra for uncoated and coated paper are shown in Fig. 6a. The imide content is calculated from the ratio of imide-related bands (1765 [cm.sup.-1]) to styrene-related bands (1602 [cm.sup.-1]), after calibration with a fully imidizcd sample (obtained after drying the dispersion at 200[degrees]C). As such, the maximum theoretical imide content (relative to styrene groups) is 35% for SMI-1 and 39% for SMI-2. After reaction in aqueous environment, the nanoparlicles have an imide content of about 24% for SMI-1 and 26% for SMI-2, as some MA groups remain in the ammonolyzed state: it is obvious that not all MA groups imidized into ring-closed moieties as the polycondensation reaction only happens partially in water and further develops during subsequent drying. Moreover, the nonimidized moieties stabilize the nanoparticle dispersions by charge repulsion (zeta potential -60 mV). The imide content for SMI nanoparticles adsorbed and dried on paper is somewhat lower (26% for SMI-1 and 29% for SMI-2) compared to the fully imidized nanoparticles onto inert substrates, indicating that a number of ammonolyzed SMA has not transformed into imide and remains in the ring-opened configuration after drying in contact with paper substrates. These probably allow for additional interactions between the nanoparticles and the cellulose fibers. A detail of the 1100 [cm.sup.-1] absorption band region, assigned to the CO stretching of cellulose C-OH groups, is shown in Fig. 6b after normalization on the 1370 [cm.sup.-1] cellulose band (invariant). The intensity enhancement in C-OH stretching band at the lower wavenumber suggests the interaction of the SMI nanoparticles with the cellulose fibers through hydrogen-bonding, providing strong adhesion between the coating and the substrate without the need of additional binding components.

[FIGURE 6 OMITTED]

Physical coating properties

Among the physical properties of coated paper substrates, the roughness, contact angles, water absorption, and coefficients of friction were studied as a function of the coating conditions (Table 1) or after additional heat treatments (Table 2). The dry coating weight depends on the roughness of the used bars and the bar speed, being very reproducible, and indicating rheological compatibility of the nanoparticle dispersions with the paper coating process (Brookfield viscosity 175 cP for SMI-1 and 660 cP for SMI-2). The deposition at low bar speeds results in a higher coating weight and more viscous dispersions may offer somewhat higher coating weight. The Cobb-test value for the original uncoated paper (100 g/[m.sup.2], 125 [micro]m) is 90 g/[m.sup.2] and depends on the internal sizing of the paper (present paper is only slightly sized). The Cobb-value reduces to about 30 g/[m.sup.2] for coated paper, indicating better resistance against water absorption. The barrier performance of coated papers seems to depend mainly on the used bar coating speed. The Cobb-value for coatings deposited at low speed is higher than for coatings deposited at high speeds, which compares to the rough and open structure as observed in Fig. 5 for coatings at 4 mm/s. At higher coating speed, the SMI type and the number of deposited layers seem to have inferior influence on the water absorption. This means that the quality and porosity of the first deposited coating layer is important and can determine further barrier performance.
Table 2: Characterization of SMI-1 and SMI-2 coatings on paper surfaces
after various heat treatments

   Curing                            SMI-1 coating
temperature

               [R.sub.a]   Imide   [[theta].sub.adv]  [[theta].sub.rec]
                 (nm)     content     ([degrees])        ([degrees])
                            (%)

Noncured         15.1       25            120                 22

125[degrees]C    13.2       27            122                 38

135[degrees]C    15.6       29            133                 39

150[degrees]C    15.4       27            130                 39

180[degrees]C    10.8       24            117                 30

200[degrees]C     1.96      23             98                 20

250[degrees]C     0.89      20             90                 25

Theoretical                 35
Mximum (a)

   Curing                            SMI-2 coating
temperature

               [R.sub.a]   Imide   [[theta].sub.adv]  [[theta].sub.rec]
                 (nm)     content     ([degrees])        ([degrees])
                            (%)

Noncured         11.2       33            127                 31

125[degrees]C     7.6       35            132                 39

135[degrees]C    10.2       37            144                 46

150[degrees]C     9.7       34            136                 43

180[degrees]C     8.2       29            134                 38

200[degrees]C     0.7       26            108                 32

250[degrees]C     0.5       23             91                 20

Theoretical                 39
Mximum (a)

(a) Assuming that all anhydride moieties are reacted into imide form


The evolution of static contact angles ([[theta].sub.stat]) with time is shown in Fig. 7 for uncoated and SMI-coated papers. For uncoated papers, the contact angle decreases from 104[degrees] to 70[degrees] over a sampling time of 20 s, as a result of water penetration into the substrate. The static contact angles and drop volumes on SMI-1 and SMI-2 coatings remain constant over the sampling time, representing an equilibrium situation: the influence of water penetration into the coated cellulose fibers is minimized and the fibrous structure underneath the coating is protected against water penetration. In general, the SMI-2 coatings present higher static contact angles than SMI-1 coatings and a one-layer coating has higher static contact angles than a two-layer coating. The dynamic contact angles (advancing [[theta].sub.adv], and receding [[theta].sub.rec]) and contact angle hysteresis ([DELTA][theta] = [[theta].sub.adv] - [[theta].sub.rec]) are significantly higher for coated than for uncoated papers. The advancing contact angles ([[theta].sub.adv]) for SMI-1 and SMI-2 coatings are higher than the static contact angles and indicate high hydrophobicity. Repeatable trends are observed between the different coating types: (i) one-layer coatings show higher advancing and lower receding contact angles than two-layer coatings, (ii) the SMI-2 coating has higher advancing and receding contact angles than SMI-1. The contact angle hysteresis is significantly higher for a one-layer compared to two-layer coatings. While the chemical surface composition is similar, the rough one-layer coating favors sticking of the contact angle to the surface while the smoother two-layer coating favors sliding.

[FIGURE 7 OMITTED]

The coefficients of friction for nanostructured paper coatings are higher than for uncoated paper and are within the ranges required for fluent paper processing. The coefficients of friction decrease at low roughness toward a minimum value and further increase at high roughness, as a result of the energy dissipation according to two sliding mechanisms near the contact asperities: i.e., the adhesion component is important at low roughness and decreases for high roughness while the viscoelastic deformation component is low in contact with smooth surfaces and becomes high for rough surfaces. Under adhesive conditions, the coefficients of friction are lower for SMI-2 compared to SMI-1, in agreement with the lower surface energy of SMI-2 revealed from contact angle measurements. After the sliding experiments, AFM of the worn surfaces reveals good adhesion of the nanoparticles and good mechanical resistance of the coatings with only a slight decrease in nanoscale roughness ([R.sub.a] = 15.3 nm before, [R.sub.a] = 11.2 nm after sliding for SMI-1, or [R.sub.a] = 11.0 nm before, [R.sub.a] = 10.3 nm after sliding for SMI-2), only caused by leveling off the top asperities.

The gloss values are anisotropic with lower values perpendicular to the coating bar direction. However, the applied coatings were visually homogeneous over the entire coating area (no striations) and the anisotropy rather represents the variation in gloss on the uncoated paper. The SMI-1 coating has superior gloss compared to SMI-2, likely due to the higher amount of styrene. The two-layer coatings have higher gloss compared to the one-layer coatings, likely due to higher smoothness.

After heat treating, the coated papers below the glass transition temperature [T.sub.g] ([T.sub.g] = 181[degrees]C for SMI-1, and [T.sub.g] = 190[degrees]C for SMI-2), the average surface roughness remains constant within a range of 1 nm. The imide content increases upon heating and shows a maximum value of 29% for SMI-1 or 37% for SMI-2 at 135[degrees]C. While the imidization reaction was done in an aqueous environment, some anhydride moieties remain hydrolyzed and further transform by thermal curing. In parallel, maximum advancing and receding contact angles of [[theta].sub.adv] = 133[degrees], [[theta].sub.rec] = 39[degrees] for SMI-1 or [[theta].sub.adv] = 144[degrees], [[theta].sub.rec] = 46[degrees] for SMI-2 were measured after heating at 135[degrees]C. The advancing and receding contact angles follow similar trends, not significantly changing the contact angle hysteresis. After heating above [T.sub.g], the coating starts to flow at the edges of the macroscopic domains, thereby still exhibiting nanoscale roughness, while it completely flows into a smooth and continuous polymer film upon heating at 250[degrees]C.

Discussion

A relationship between the forces acting at a solid/liquid interface was first proposed by Young, (60) assuming that the energy of the system reaches a global minimum surrounded by infinitesimally close nonequilibrium states in the energetic field. However, this condition is not realistic for real substrates because of the presence of surface heterogeneities. The experiments described in this article indicate that the wettability of a nanostructured organic coating deposited onto cellulose substrates can be controlled by varying the degree of imidization (determined by synthesis or thermal curing) and the roughness (determined by the application method). The interactions of a liquid drop with heterogeneous substrates can be described by equations taking into account the physical and chemical surface heterogeneities, according to the models of Wenzel (61) (formula 1) and Cassie-Baxter (62) (formula 2):

Wenzel: cos [theta] = r cos [[theta].sub.s] (1)

Cassie-Baxter: cos [theta] = [Florin] cos [[theta].sub.s] - (1 - [Florin]) (2)

where [theta] is the apparent contact angle on a rough nanostructured surface, [[theta].sub.s] the equilibrium Young's contact angle, r the ratio between the actual surface area to the geometrically projected surface area, and [Florin] the solid-liquid area fraction. Dettre and Johnson (63) combined both equations to model the transition from a Wenzel-type to a Cassie-type of wetting. They concluded that for a hydrophobic surface, the contact angle hysteresis increases with roughness if the wetting is governed by the Wenzel state, and the contact angle hysteresis decreases with roughness if the wetting is governed by the Cassie state. As shown in Table 1, the advancing contact angle increases and the receding contact angle decreases at higher surface roughness, thereby increasing the contact angle hysteresis. Therefore, we assume that the coated papers are most likely wetted in the Wenzel state, and verify the model as follows.

* First, the influence of roughness was examined to determine the equilibrium contact angle on a virtually nonstructured surface of SMI. Therefore, we use the contact angle data presented in Table 1, including coatings with similar chemical composition and different surface roughness controlled by the deposition conditions. Plotting the contact angle values against the average surface roughness [R.sub.a] (Fig. 8) for SMI-1 and SMI-2 indicates that the advancing contact angles on nanostructured organic coatings decrease at lower roughness, in agreement with the Wenzel model. The equilibrium Young's contact angle [[theta].sub.s] on a nonstructured (virtually flat) surface can be determined by extrapolating the contact angle data toward a surface with virtual roughness [R.sub.a] = 0 nm, resulting in a value [[theta].sub.s] = 100[degrees] for SMI-1, or [[theta].sub.s] = 107[degrees] for SMI-2. The receding contact angles are less sensitive to the chemical composition and uniquely increase at lower roughness for both SMI-1 and SMI-2, thereby reducing the contact angle hysteresis. The relations with roughness data obviously only apply under the present conditions as determined from a 2 x 2 urn AFM scan. It is known that the surface roughness value varies greatly with the sampling size (64): the larger the sample size the higher the roughness value. We first studied a wider range of sampling sizes by combining AFM measurements and contactless profilometry, (65) relating the nanoscale roughness to a microscale roughness.

[FIGURE 8 OMITTED]

* Second, the combined influences of topographical and chemical parameters on the contact angle measurements should be determined. Therefore, we use the contact angle data presented in Table 2, including coatings with different chemical composition and roughness controlled by the thermal treatment. The effect of the surface roughness is expressed by relating the roughness parameter r to the average roughness ([R.sub.a]) determined from AFM scans (Fig. 9a). The roughness parameter was determined from the ratio of the contact angles on nanostructured surfaces (experimental measurements cos 6) to the extrapolated data (theoretical flat surface cos [[theta].sub.s]), according to formula (1). It is obvious that this relation is valid for the roughness determined on 2 x 2 [micro][m.sup.2] scans. For hydrophobic surfaces, an increase in surface roughness results in an intensification of the hydrophobic surface nature. The effect of chemical surface composition is expressed by relating the equilibrium contact angle [[theta].sub.s] to the degree of imidization determined from Raman scans (Fig. 9b). The increase in contact angle with higher degree of imidization is further evidenced by experimental contact angle measurements on nonimidized SMA coatings ([[theta].sub.s] = 58 [+ or -] 2[degrees]). The increase in contact angle with higher imide content is due to the more hydrophobic nature of imide compared to the ammonolyzed maleic anhydride moieties (see Fig. 1). The wettability of SMI nanoparticle coatings turns from hydrophobic into hydrophilic for imide contents below 20%. For the hydrophilic state, an increase in surface roughness results in an intensification of the hydrophilic surface nature.

[FIGURE 9 OMITTED]

Although the contact angle is a macroscopic parameter, the dynamic measurements were demonstrated to be very sensitive to local variations in topography and surface chemistry, and they may give important indications on the presentation of the outermost chemical groups to the surface. Gietzelt (66) studied the dependency of contact angles on the molecular structure of spin-cast SMI copolymers, concluding that side groups such as alkyl-imides with a small alkyl side chain (less than 3 -[CH.sub.2]- groups) did not significantly alter the contact angle. This was explained by the bulkiness of the styrene groups that are organized in a star-like molecular conformation, thereby shielding the polar side chains. Only larger side groups with four to six carbon atom chains penetrate outside the radius of the star-shaped styrene conformation and significantly increase the contact angle. Our work demonstrated that, despite the relatively small imide sequence, the contact angles of SMI nanoparticle coatings significantly increase with higher imide content. It is therefore very reasonable to assume that the hydrophobic styrene groups are oriented to the inner side of the nanoparticles while the more hydrophilic imide groups are oriented outwards after the reaction in aqueous medium. This observation illustrates the importance and ability for good control over the presentation of functional chemical moieties to the surface by organization at nano-level, thereby both providing good adhesion to fibrous substrates and controlling the hydrophobicity.

Conclusions

Nanoparticles synthesized by partial imidization of styrene maleic anhydride in the presence of ammonium hydroxide can be coated onto cellulosic (paper) substrates and form a noncontinuous film in order to successfully improve the water repellency and hydrophobicity of the fibrous surface. Good adhesion to the cellulosic fibers was demonstrated by hydrogen bonding, providing mechanical resistance under sliding. The friction coefficients of coated paper are higher than that for uncoated paper. The gloss of the paper surface both depends on the surface roughness and amount of styrene. The wettability of coated papers can be controlled by tuning the chemical and topographical surface parameters according to the Wenzel model: the water contact angles were found to increase at higher imide content (determined by synthesis and thermal treatment) and higher average surface roughness (determined by application methods). The coatings become hydrophilic for imide contents below 20%. As the contact angle measurements are very sensitive to local variations in surface chemistry, it is important to have good control over the presentation of functional chemical moieties to the surface by organization at nano-level. These nanoparticle coatings may serve as a unique and ecological replacement for chemical treatments hydrophobizing fibrous substrates.

Acknowledgments Pieter Samyn acknowledges the Research Foundation Flanders (F.W.O.) for a Postdoctoral Research Fellow Grant. Henk Van den Abbeele acknowledges the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT) for a 3-year funding program. Contact angle measurements were done at The Particle and Interfacial Technology Group (Ghent University) and AFM studies were done at The Physics and Chemistry of Nanostructures Group (Ghent University). Peter Mast performed scanning electron microscopy studies.

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P. Samyn (**), G. Schoukens

Department of Textiles, Ghent University, Technologiepark

907, 9052 Zwijnaarde, Belgium

e-mail: Pieter.Samyn@UGent.be;

Pieter.Samyn@fobawi.uni-freiburg.de

G. Schoukens

e-mail: Gustaaf.Schoukens@UGent.be

H. Van den Abbeele, L. Vonck, D. Stanssens Topchim N.V., Nijverheidstraat 98, 2160 Wommelgem, Belgium

H. Van den Abbeele e-mail: info@topchim.be

[C]ACA and OCCA 2010

DOI 10.1007/s11998-010-9309-7
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Author:Samyn, P.; Schoukens, G.; Abbeele, H. Van den; Vonck, L.; Stanssens, D.
Publication:JCT Research
Date:May 1, 2011
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