Ind. Eng. Chem. Res. 2010, 49, 2169–2175
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Influence of Surface Chemical Composition on UV-Varnish Absorption into Permeable Pigment-Coated Paper Maiju Pyko¨nen,*,† Kenth Johansson,‡ Roger Bollstro¨m,† Pedro Fardim,§ and Martti Toivakka† Laboratory of Paper Coating and ConVerting, Center for Functional Materials, and Laboratory of Fibre and Cellulose Technology, Åbo Akademi UniVersity, Porthaninkatu 3, FI-20500 Turku, Finland, and YKI, Ytkemiska Institutet AB, Box 5607, SE-114 86 Stockholm, Sweden
Fluorocarbon, organosilicon, and hydrocarbon plasma coatings were used to modify the surface of permeable pigment-coated paper, and their impact on UV-varnish absorption was investigated. According to mercury porosimetry results, the plasma coatings had no influence on the porous structure of the paper. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) results showed characteristic surface chemical compositions for each plasma coating. The fluorocarbon plasma coating increased the UV-varnish contact angles significantly, whereas the hydrocarbon plasma coating had no clear influence. When the UV varnish was applied with a flexography unit including nip pressure, the role of surface chemical composition seemed to become minimal. The viscosity of the UV varnish was shown to impact the absorption rate with and without external pressure. 1. Introduction The absorption of a coating or fluid into a porous substrate plays an important role in many printing, painting, and coating processes, for example, in the building, tape, paper, and textile industries. Dynamic absorption of fluids into porous pigmentcoated papers has been extensively studied. Capillary forces and diffusive interactions seem to be the two major mechanisms for fluid transport. The main coating-layer factors contributing to absorption are given as the pore structure, pore size distribution, and surface chemistry.1-3 Fluid absorption into pigment-coated paper is a highly complex phenomenon because of its dynamic nature and simultaneously occurring chemical and physical phenomena. In addition, the heterogeneous and multicomponent characters of the substrate and the fluid (e.g., ink) complicate our understanding of the process. Fluid absorption into paper has typically been represented with the Lucas-Washburn equation,4,5 in which the Laplace capillary pressure relation is incorporated into the HagenPoiseuille equation of laminar flow rγLV cos R + pEr2 h2 ) t 2η
(1)
Here, h is the depth of penetration, t is the time, r is the pore radius, γ is the surface tension of the fluid, θ is the contact angle between the fluid and the substrate, η is the fluid viscosity, and pE is the external pressure gradient. The Lucas-Washburn equation assumes that all of the pores are uniform and cylindrical and that fluid continuously fills the pores in the pore network under equilibrium conditions. In practice, it is well-known that smaller pores result in faster ink setting. However, this is in conflict with the Lucas-Washburn equation, and therefore, new theories have been proposed, especially concerning ink setting. Xiang et al.6,7 developed a * To whom correspondence should be addressed. E-mail:
[email protected]. † Laboratory of Paper Coating and Converting and Center for Functional Materials, Åbo Akademi University. ‡ Ytkemiska Institutet AB. § Laboratory of Fibre and Cellulose Technology, Åbo Akademi University.
mathematical model for ink setting based on the LucasWashburn equation and Darcy’s law,8 in which the formation of an ink filter cake explains the faster filling of small pores. However, Schoelkopf et al.9 applied the absorption model of Bosanquet,10 which suggests that the inertia exerted on a mass of fluid at each capillary entry is greater in larger capillaries, which leads to preferential filling of small pores rather than the large pores at a short time scale. Rousu et al.11 considered the aspect of multicomponent transport into porous media, pointing out that models derived for single-component fluid systems do not fully apply in more complicated systems such as ink-coating interactions. Recently, Donigian12 has proposed a multiphase hypothesis for initial inkgloss development, which is based on the assumption that ink oils and resins are distributed into different phases. The ratio between the surface tension and viscosity of the fluid has been used to describe the capillary flow behavior in several studies,13 and for example, Rousu1 showed that an increased ratio of surface tension to viscosity leads to an increased rate of absorption of ink oils into a pigment coating structure. In offset ink setting, the substrate surface chemistry and ink chemical composition have been shown to play roles especially in adsorption, chromatographic,1 and diffusive interactions between the ink oil and the coating binder.3 It is generally believed that external pressure increases the relative contribution of the coating structure over the surface chemistry of the substrate for the kinetics of absorption into porous structures.2 For example, Sandås and Salminen14 showed that sorption into clay and CaCO3 coatings was equivalent in the absence of external pressure, but with external pressure added, the more closely packed structure of the clay absorbed less water, even if its higher surface energy should have promoted absorption relative to CaCO3. However, in that work, the influence of coverage of the pigment dispersant on surface energy was overlooked. Ridgway and Gane15 showed that both surface chemistry and pore structure have an impact on absorption but that the time scale is crucial: within the first few seconds, small pores absorb nonpolar liquid more quickly, whereas at longer time scales, water containing polar component is absorbed faster by larger pores. Pyko¨nen et al.16 showed that, by introducing hydrophobic character on the surface without
10.1021/ie900992t 2010 American Chemical Society Published on Web 01/20/2010
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influencing porous structure, plasma coatings are able to prevent dampening water absorption into porous pigment-coated paper under the action of nip pressure. It is well-known that narrow particle size distributions and needle-shaped particles provide permeable pigment coatings, when the pores are highly connected to each other. Permeability is the freedom of the fluid to pass under applied pressure though a porous structure, without specification of an internal driving force. This is often described by Darcy’s law.8 However, Shoelkopf et al.17 showed that the permeability of pigment coatings under certain conditions does not obey the linearity of the Darcy relation as a function of applied liquid pressure differential. In addition, they showed that there is no linear correlation between porosity and permeability. Therefore, it was stated that the high driving forces for the absorption of small liquid quantities are associated with low-porosity fine pore networks, whereas high consumption of liquid, such as varnishes, is due to drainage effects that are, in turn, a function of permeability and not necessarily porosity. Overprint varnishes are used to enhance and protect the printed product. Typical products for varnishes are covers of magazines, annual reports, brochures, catalogues, wine labels, and cosmetic and food packages.18 UV-curing inks and varnishes were introduced in the 1960s and gained popularity during the 1970s.19 The use of UV technology in printing industry is expected to expand in the near future.20 UV varnishes are based on acrylate chemistry consisting of prepolymers, monomers, and photoinitiators. The prepolymers act as the vehicle of the system and provide brilliance and mechanical and chemical resistance. The monomers are used to adjust the end properties of the varnish and act as diluents to adjust viscosity, but they also affect surface chemistry and cross-linking. The photoinitiators are selective to the light of specific wavelengths and initiate the curing. UV varnish absorbs photons of high-energy ultraviolet light, which causes the varnish to undergo chemical polymerization and thus converts the varnish from liquid to solid state.21 The aim of this work was to understand the role of surface chemistry in the absorption of UV varnish on highly porous and permeable pigment-coated paper. Whereas permeable, highporosity coatings provide excellent opacity and brightness properties for paper through high light scattering, the high permeability can lead to problems in printing related to overly fast penetration of printing liquids. For example, in the case of UV varnishing, absorption that occurs too quickly can decrease the gloss and result in uneven gloss appearance.20 In addition, uncured UV-varnish ingredients are undesirable especially in food packaging applications, as unreacted monomers and photoinitiators can cause odor, taste, or toxicity problems in the end product.22 In this work, three different plasmapolymerized coatings were deposited on pigment-coated paper substrates in order to modify their surface chemical composition. Plasma is a state of ionized gas, consisting of reactive particles such as electrons, ions, and radicals. Plasma-solid interactions can be divided into three subcategories:23 (i) etching or ablation, where material is removed from the solid surface; (ii) plasma activation, where the surface can be chemically and/or physically modified by species present in the plasma; and (iii) plasma coating, where material is deposited in the form of a thin film on the surface. Plasma coating deposition is also referred to as plasma polymerization or plasma (-enhanced) chemical vapor deposition [P(E)CVD]. The PCVD plasma technology can be used to deposit functional layers, such as hydrocarbons, hydrocarbons with polar groups, organosilicons, halocarbons
Figure 1. Pore size distribution of the untreated pigment-coated paper.
(e.g., fluorocarbons), and organometallics.24 According to previous studies,16,25 these plasma coatings can be used to change surface chemical properties of paper while maintaining its porous structure. 2. Experimental Section A fine paper base was coated with aragonitic precipitated calcium carbonate (PCC) pigment with a narrow particle size distribution to achieve a highly permeable structure in the coating. The pigment-coated paper was plasma-coated using three different chemistries. The changes in surface chemical composition were investigated by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and water and diiodomethane contact angle measurements. The pore structure of the paper was studied by mercury porosimetry. The influence of the plasma coatings on UV-varnish absorption was studied by UV-varnish contact angle measurements and gloss measurements after roll-to-roll UV varnishing. 2.1. Substrates and Sample Preparation. A 45 g/m2 fine paper base was coated using a minipilot-scale roll-to-roll blade coater (web width 300 mm, speed 15 m/min, IR and hot-air drying). The target coat weight was 15 g/m2. The coating formulation contained 100 parts of PCC pigment (Opacarb A40, Specialty Minerals), median weight 0.40 µm particle size (Sedigraph) with a narrow particle size distribution and aragonite particle shape, and nine parts of styrene-butadiene latex (DL966, Dow Chemical Company) with a glass temperature of 20 °C and a particle size of 140 nm. Figure 1 shows the pore size distribution of the coated paper measured with mercury porosimetry (PASCAL 140/440, ThermoElectron). The average pore size of the coating layer was approximately 0.14 µm, and the pore volume was found across the pore size range of 3-500 nm, amounting to 14%. The laboratory-scale plasma depositions were performed at the Institute for Surface Chemistry (YKI), Stockholm, Sweden. The plasma reactor used was an in-house-constructed reactor consisting of a glass vessel connected to a double-stage rotary vacuum pump (Leybold-Heraeus D 65 B). Two externally wrapped, capacitively coupled copper electrode bands were powered by either a low-radio-frequency (125-375 kHz) power generator (ENI, model HPG-2) or a 13.56 MHz radio-frequency power generator (ENI, model ACG-3) connected to an automatic matching network (ENI, model MW-5D). The A4 paper sheets were mounted in the lower part of the chamber. The chamber was evacuated to a base pressure below 10 mTorr before the precursor (monomer) was introduced from the top of the reactor. Ethylene was used as the monomer for the hydrocarbon coating, hexamethyldisiloxane (HMDSO, >98.5%) for the organosilicon coating, and perfluorohexane (C6F14, 98%) for the fluorocarbon coating (Table 1).
Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010 Table 1. Plasma Parameters HMDSO frequency discharge power, W pressure during treatment, mTorr flow rate of precursor treatment time a
ethylene
C6F14
13.56 MHz 30 ∼25
135 kHz 20 ∼49
13.56 MHz 40 ∼140
∼5 cm3/min 2 min
10 cm3/min 2 min
a
1 min
Value out of range of the instrument scale.
Table 2. Total Surface Tension (γtot) and Its Nonpolar (Dispersion or Lifshitz-van der Waals Forces, γLW) and Polar (Acid-Base, γAB) Components for the Liquids Used for Contact Angle Measurements liquid
γtot (mN/m)
γLW (mN/m)
γAB (mN/m)
water DIM
72.8 50.8
21.8 50.8
51 ∼0
Table 3. Density, Surface Tension, and Viscosity of UV Varnishes Used in Contact Angle Measurements density (g/cm3)
surface tension (mN/m)
viscosity (mPa s)
UV varnish
average
std dev
average
std dev
average
std dev
low η high η
0.977 0.990
0.043 0.073
21.00 21.13
0.03 0.01
72.5 147.8
1.3 1.2
The UV-varnish application was carried out using a homebuilt roll-to-roll mini-pilot-scale printer. The printing method was flexography using a commercial OHKAFLEX (Shore A 64-66°) photopolymer plate. The ceramic anilox cylinder was manufactured by Cheshire Engraving Services Ltd. and had a cell angle of 60° with 120 lines/cm and a cell volume of 12 cm3/m2. The speed was set to 4 m/min, and the amount of UV varnish applied was approximately 2 g/m2 (dry). The UV curing unit was a Bluepoint 4 Ecocure apparatus supplied by Ho¨nle UV Technology and equipped with a six-piece light guide. The light guide dividing the light on an area of 2 × 10 cm2 was installed 2 cm after the nip, and the intensity used was 240 mW/cm2 measured at a wavelength of 370 nm. 2.2. Analyses. The plasma-coated samples were characterized by XPS using a Physical Electronics Quantum 2000 ESCA instrument, equipped with a monochromatic Al KR X-ray source and operated at a power of 25 W. The pass energy for the survey spectra was 184 eV, and the measurement time 5 min. Three different spots were measured on each sample. ToF-SIMS analyses were carried out using a PHI TRIFT II spectrometer. Mass spectra and images of paper surfaces in positive ion mode over the mass range of 2 - 2 000 Da were acquired using a Ga primary source with an area of 2.5 mm × 2.5 mm, when the voltage of the primary ion source was 15 kV with applied voltage of 25 kV. Primary ion current was 600 pA. Acquisition time was 10 min and minimum of three different spots were analyzed on each sample. Wetting, drop spreading, and sorption were investigated using a DAT 1100 (Fibro System AB) contact angle meter applying the following liquids: water, diiodomethane (DIM), and two high-gloss UV varnishes with different viscosities (Ultra King Overprint Varnish). The drop volume was adjusted to 4 µL for water and the UV varnishes and to 1.7 µL for DIM. A minimum of six parallel measurements were carried out on each sample. The properties of the test liquids are presented in Tables 2 and 3. The viscosities of the UV varnishes were measured with a Gemini-Advanced Rheometer (Malvern Instruments) with shear rates of 1-800 s-1, where the varnishes showed Newtonian behavior. Viscosity measurements were performed at room temperature (22 C°). Surface tension was determined using the ring method (KSV Sigma70).
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Table 4. Relative Surface Composition (Atom %) and Elemental Ratios Measured by XPS untreated element average C O Ca Na P Si F O/C Si/C F/C
56.6 33.1 8.5 1.5 0.4 0.59 -
CH plasmaa
HMDSO plasma
CF plasmab
std dev
average
std dev
average
std dev
average
std dev
0.5 0.6 0.1 0.2 0.1 -
76.6 18.8 4.0 0.5
5.1 4.0 1.3 0.2
54.6 26.3 2.3 16.6 -
0.2 0.8 0.4 1.1 -
36.0 1.7 62.3
1.0 0.5 0.4
0.02 -
0.25 -
0.07 -
0.48 0.30 -
0.01 0.03 -
0.05 1.73
0.02 0.07
a One parallel measurement clearly deviated, with reduced amounts of calcium (1.3 atom %) and oxygen (11.5 atom %) and was therefore not included in average and standard deviation values presented in the table. b One parallel measurement clearly contained more oxygen (10.2 atom %) and less fluorine (49.9 atom %) and was therefore not included in average and standard deviation values presented in the table.
Paper gloss (75°) for untreated, plasma-coated, and UVvarnished samples was measured with a Zehntner ZLR 1050M glossmeter. 3. Results and Discussion According to the mercury porosimetry results, the plasma coatings had no measurable influence on the porous structure of the paper in the range of 3-500 nm. This is in agreement with previous studies,16,25 where plasma coatings were used to modify the surface chemical composition of the paper while maintaining its porous structure. The surface chemical composition and changes thereof created by plasma coatings were determined by XPS. Survey spectra of the untreated sample gave typical peaks for pigment-coated paper containing calcium carbonate and styrene-butadiene latex (Table 4). Sodium and phosphate in the untreated sample are probably ingredients of the calcium carbonate pigment’s dispersion chemicals, and part of the oxygen (7.6 atom %) also seems to originate from a source other than CaCO3. The XPS results showed characteristic plasma coating chemistry for each sample. For the hydrocarbon-plasma-coated sample, the calcium signal indicated that the plasma coating did not cover the whole surface, or that the plasma coating was thinner than 10 nm, which is the escape depth of the XPS photoelectrons. Subtracting signals coming from CaCO3 and taking into account the oxygen and carbon coming from sources other than CaCO3, the pure hydrocarbon coating would contain 93.9 atom % hydrogen and 6.1 atom % oxygen (O/C ) 0.06). Small amounts of fluorine in the coating can be a common finding due to contamination from the fluorocarbon O-rings used in the plasma chamber. The survey spectra for the organosilicon plasma coating also gave signals for calcium in addition to the expected carbon, oxygen, and silicon. Subtracting the CaCO3 signals and the oxygen and carbon coming from other sources, the pure HMDSO plasma coating would contain 55.2 atom % carbon, 21.5 atom % oxygen, and 23.3 atom % silicon (Si/C ) 0.42). Compared to the HMDSO structure, Si(CH3)-O-Si(CH3), the HMDSO plasma coating clearly contained more oxygen and less carbon and silicon. It is typical that no repeated units are recognizable in the structure of the plasma-polymerized thin film, as plasma polymerization differs from conventional polymerization in that it does not follow the pattern of initiation, propagation, and termination steps.24 With the fluorocarbon plasma coating, no signals from the pigment coating were detected, and pure
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Figure 2. ToF-SIMS images illustrating the distribution (2.5 mm × 2.5 mm) of the calcium on (A) untreated and (B) hydrocarbon, (C) organosilicon, and (D) fluorocarbon plasma-coated samples.
Figure 3. Change of (left) water and (right) DIM contact angles (deg) with time.
Figure 4. Change of (left) low- and (right) high-viscosity UV varnish contact angles (deg) with time.
fluorocarbon plasma coating contained 36.0 atom % carbon, 1.7 atom % oxygen, and 62.3 atom % fluorine (F/C ) 1.73). The deviations between parallel measurements with the hydrocarbon and fluorocarbon plasma coatings suggest that the chemical composition of the plasma coating was not completely uniform. The uniformity and coverage of the plasma coatings can be illustrated using chemical mapping in ToF-SIMS images. Calcium mapping confirms that the plasma coating coverage was not complete in the cases of the hydrocarbon- and organosilicon-plasma-coated samples, but it seems that the signals of calcium were evenly distributed across the surface (Figure 2). With the fluorocarbon sample, only a few signals from calcium could be detected. Contact angle measurement is a useful technique for quantifying interfacial and intermolecular phenomena. Figure 3 shows that the dynamic contact angles of water (polar) and nonpolar diiodomethane were higher for the fluorocarbon- and organosilicon-plasma-coated samples compared to the untreated sample, which indicates that they reduced both polar and dispersion interactions. The hydrocarbon plasma coating increased the water contact angles, whereas it decreased the diiodomethane contact angles slightly. All of the plasma coatings, excluding the hydrocarbon plasma coating with DIM, reduced spreading and absorption of droplets, which can be seen as the decay of the contact angle values during the first seconds
after drop application. In contact angle measurements, the standard deviation for all samples was between 1° and 4°. Contact angle measurements were also used to study the impact of plasma coatings on the wetting of UV varnishes with two different viscosities without the influence of external pressure. It must be emphasized that UV varnish is typically applied under the influence of nip pressure as forced wetting, and therefore, the results are not directly comparable to the UVvarnish process. Figure 4 shows that the fluorocarbon plasma coating resulted in the highest increase in UV-varnish contact angles. The organosilicon plasma coating also increased the UVvarnish contact angles, but the change was clearly smaller. The hydrocarbon plasma coating resulted in unchanged or even slightly reduced UV-varnish contact angles. These results indicate that the organosilicon plasma coating clearly had more interactions with the UV varnishes than the fluorocarbon plasma coating; however, polar interactions cannot explain that result, as the fluorocarbon and organosilicon plasma coatings had similar water contact angles. One explanation could be the amount of hydrocarbons: the fluorocarbon plasma coating did not contain any hydrocarbon groups, whereas the organosilicon plasma coating contained at least methyl groups. UV varnishes are based on polyacrylate chemistry containing a hydrocarbon chain with vinyl and ester bonds. The results also showed that the high-viscosity UV varnish gave higher contact angles in
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Figure 5. Drop base diameter with time for (left) low- and (right) high-viscosity UV varnishes.
Figure 6. Drop volume (µL) with time for (left) low- and (right) high-viscosity UV varnishes. Table 5. Gloss for Pigment-Coated Paper without and with UV Varnishes without UV varnish
untreated CH-plasma-coated HMDSO-plasmacoated CF-plasma-coated
with UV varnish low η
high η
average (%)
std dev
average (%)
std dev
average (%)
std dev
38.6 38.4 39.1
0.2 1.3 0.8
41.6 40.9 44.8
1.1 1.7 2.5
48.1 47.0 46.9
0.5 1.5 0.7
38.2
0.5
40.9
2.0
47.4
0.7
comparison to the low-viscosity UV varnish. This is in agreement with, for instance, the surface tension-to-viscosity ratio and the Lucas-Washburn equation, as presented in the Introduction. In addition to contact angles, drop volume and drop base diameter were investigated to show the influence of the plasma coatings on sorption (drop volume) and spreading (drop base). Figure 5 shows that, with the untreated and hydrocarbon- and organosilicon-plasma-coated samples, the drop base increased with time whereas, with the fluorocarbon-plasma-coated sample, the drop base hardly changed at all. Therefore, the results indicate that the fluorocarbon plasma coating reduced drop spreading. The drop volume of the UV-varnishes was 4 µL, which was calibrated on a nonabsorbing smooth surface. The measured drop volume of the low-viscosity varnish on paper was stable on all samples, whereas the drop volume of the highviscosity varnish increased slightly. This implies some swelling of the paper when the drop wet the surface (Figure 6). However, the fact that the drop volume on the fluorocarbon-plasma-coated sample was closest to the true drop volume of 4 µL suggests that the fluorocarbon plasma coating also reduced sorption. Overly fast penetration of the UV varnish into highly permeable pigment-coated paper typically produces a low gloss level and uneven gloss appearance. Slower absorption of the high-gloss UV varnishes should lead to a gloss increase, when a thicker layer of UV varnish would be cured on the surface and not absorbed into a coating. When the UV varnishes were applied with a roll-to-roll flexographic unit, the paper gloss was only slightly increased (Table 5), and the gloss appearance was visually equally uneven for all of the samples. However, the fluorocarbon and organo-
silicon plasma coatings had no influence on the gloss of the roll-to-roll UV-varnished samples. Therefore, it seems that, with highly permeable pigment-coated paper, the chemical interactions between the fluid and the substrate surface have no influence or a minimal influence on the absorption rate when an external pressure is applied. In addition, the results suggest that forced wetting cannot be well-described by contact angle measurements. However, in our previous study,16 similar plasma coatings reduced dampening water absorption into pigmentcoated paper under the influence of nip pressure. The hydrophobicity level determined by contact angle measurements correlated well with the amount of absorbed dampening water. The pigment coating in the previous study was a typical coating used in commercial offset printing papers containing ground calcium carbonate (GCC) and kaolin. The permeability for this type of coating can be assumed to be lower than that for the 100% PCC coating studied here. Considering also the results from the previous study, it seems that, without external pressure, the surface chemical composition has a significant impact on the absorption rate aside from the degree of permeability or porosity. When the external pressure is present, however, the role of the surface chemical composition seems to diminish in the case of highly permeable pigment coatings. With the less permeable GCC- and kaolin-containing pigment coating, the surface chemical composition retained a significant impact on the absorption rate of fluid. Direct comparison of these studies is hampered, however, because the type of fluid, the amount of fluid, and the application method differed. It is possible that the amount of fluid is also crucial. Shoelkopf et al.17 stated that the absorption of high quantities of liquid, such as in the case of varnishes, is more associated with permeability and not porosity. One should also note that the modification created by the plasma coatings is commonly believed to be very surface specific.26,27 Therefore, once the liquid has penetrated through the top layer, it would be free to absorb further into the porous structure. However, it has also been shown that, with porous surfaces, plasma modification can extend along the pores in the bulk material depending on the plasma parameters used and choice of monomers.28 In this study, the high-viscosity varnish provided higher gloss than the low-viscosity varnish, which indicates that the increased viscosity decreases the absorption rate even if the external pressure is present. This is in agreement with the Lucas-Washburn equation (eq 1).
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Table 6. Penetration Depth (h) and Capacity for UV Varnish Absorption (g/m2) Calculated According to Lucas-Washburn Equation, for a Pore Volume of 14% and a Nip Pressure of 0.5 MPa sample low-η varnish
with nip pressure (0.5 MPa, 0.015 s) without nip pressure (0.3 s), speed 4 m/min without nip pressure (0.012 s), speed 100 m/min
high-η varnish
with nip pressure (0.5 MPa, 0.015 s) without nip pressure (0.3 s), speed 4 m/min without nip pressure (0.012 s), speed 100 m/min
In the application nip, the penetration of the UV varnish occurs under the influence of an external pressure, and then, during the time delay from the nip to the UV lamp, without an external pressure. Figure 7 shows the penetration depth (h) with time calculated according to the Lucas-Washburn equation (eq 1) for untreated and fluorocarbon-plasma-coated samples, with and without external pressure. Theoretically, in the absence of external pressure, the change from low-viscosity to highviscosity UV varnish and the contact angle change caused by the fluorocarbon plasma coating have a similar impact on the absorption rate. In the presence of external pressure, the influence of contact angle decreases, and the influence of viscosity increases. With increasing time, the impacts of both contact angle and viscosity increase. The low-viscosity UV varnish on the untreated sample with the higher contact pressure has the highest absorption rate. The slowest absorption is obtained with the high-viscosity UV varnish on fluorocarbonplasma-coated sample in the absence of external pressure. The plate cylinder diameter was 7 cm, and therefore, the estimated nip length was 1 mm, and the dwell time in the nip 15 ms. Because the UV lamp was set 2 cm away from the printing nip and the printer was running at 4 m/min, it took 0.3 s to reach the UV lamp. Table 6 shows the calculated UVvarnish penetration depths and capacities for UV-varnish absorption (g/m2) with and without external pressure. Because the typical thickness for the pigment coating is some tens of micrometers, the results indicate that, after the nip, all of the applied UV varnish (2 g/m2) could have been absorbed into the coating near the base paper. These results suggest that UV varnishing should have been performed with a higher amount of UV varnish or/and with a higher line speed (example given in Table 6 with 100 m/min). Therefore, all of the UV varnish would not have penetrated into the coating, which could have improved the possibility of detecting differences between the untreated and plasma-coated samples. The surface chemical
h (µm)
capacity for UV varnish absorption (g/m2)
19.9 17.9 53.2 36.5 10.6 7.3 13.8 12.2 36.4 22.0 7.3 4.4
2.7 2.4 7.3 5.0 1.5 1.0 1.9 1.7 5.0 3.1 1.0 0.6
untreated CF-plasma-treated untreated CF-plasma-treated untreated CF-plasma-treated untreated CF-plasma-treated untreated CF-plasma-treated untreated CF-plasma-treated
modification could also have had a more significant impact on the absorption rate if the UV varnish could be applied without the presence of an external pressure, such as by using spray coating. 4. Conclusions The surface chemical composition of permeable and highly porous pigment-coated paper was modified by using hydrocarbon, organosilicon, and fluorocarbon plasma coatings. The XPS results showed the characteristic chemical composition for each plasma coating. Although the coverage of the hydrocarbon and organosilicon coatings was not complete, the ToF-SIMS images suggested that the plasma coatings were evenly distributed over the pigment-coated samples. Contact angle measurements with water and DIM indicated that the fluorocarbon and organosilicon plasma coatings reduced both polar and dispersion interactions, whereas the hydrocarbon plasma coating slightly increased the dispersion interactions and reduced the polar ones. The aim of this work was to understand the role of surface chemistry on UV-varnish absorption. The amount of hydrocarbon groups in the plasma coatings seemed to have a strong influence on the wetting of the UV varnishes. This might explain why the fluorocarbon coating gave the highest increase in contact angles, and the hydrocarbon coating did not significantly influence the UV-varnish contact angles. Even if the change in wetting was significant (Figures 5 and 6) when measured by contact angles, the gloss measurements indicated that the plasma coatings had no influence on the absorption rate when the UV varnish was applied using a flexography printing process. The results suggest that the UV-varnish absorption under the influence of the printing nip pressure, which leads to forced wetting, cannot be well described by contact angle measurements. Taking into consideration also the results from our previous study,16 it is possible that the influence of surface chemical composition might diminish in the case of highly permeable and porous pigment coatings. Calculations according to the Lucas-Washburn equation also suggested that the applied amount of UV varnish was so small and had such a long time available for absorption that all of the varnish was absorbed into the paper before UV curing, which might have been the reason that no differences could be seen. Acknowledgment
Figure 7. Penetration depth (µm) with time calculated according to the Lucas-Washburn equation without and with (1 MPa) external pressure for untreated and fluorocarbon plasma-coated samples.
This work was funded by the Finnish Funding Agency for Technology and Innovation (Tekes). The authors acknowledge Specialty Minerals for pigment supply, Dr. Tapio Ma¨kela¨ for advice in roll-to-roll UV varnishing, and Laboratory Technician Pauliina Saloranta for her contributions to laboratory measurements at Åbo Akademi University.
Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010
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ReceiVed for reView June 19, 2009 ReVised manuscript receiVed November 9, 2009 Accepted January 4, 2010 IE900992T
