Influence of Surface Chemical Composition on UV-Varnish Absorption into Permeable Pigment-Coated Paper

1

Influence of Surface Chemical Composition on UV-Varnish

Absorption into Permeable Pigment-Coated Paper Maiju Pykönen

1*), Kenth Johansson

2, Roger Bollström

1, Pedro Fardim

3 and Martti Toivakka

1

1

Åbo Akademi University, Laboratory of Paper Coating and Converting and Center for Functional Materials,

Porthaninkatu 3, FI-20500 Turku, Finland 2 YKI, Ytkemiska Institutet AB, Box 5607, SE-114 86 Stockholm, Sweden

3 Åbo Akademi University, Laboratory of Fibre and Cellulose Technology, Porthaninkatu 3, FI-20500 Turku,

Finland *)

Corresponding author; [email protected]

Abstract

Fluorocarbon, organosilicon and hydrocarbon plasma coatings were used in order to modify the surface of

permeable pigment-coated paper, and their impact on UV-varnish absorption was investigated. According to

mercury porosimetry results, 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 composition for each plasma coating. The fluorocarbon plasma coating

increased significantly the UV-varnish contact angles, whereas the hydrocarbon plasma coating had no clear

influence. When the UV-varnish is applied with a flexography unit including nip pressure, the role of surface

chemical composition seems to become minimal. Viscosity of the UV-varnish was shown to impact the

absorption rate with and without external pressure.

Keywords: Plasma Coating; Surface Chemistry; Absorption; Permeability; Pigment-Coated Paper; UV-varnish

1. Introduction

The absorption of a coating or fluid into a porous substrate plays important role for many printing, painting, and

coating processes for example in building, tape, paper and textile industries. Dynamic absorption of fluids into

porous pigment-coated papers has been extensively studied. Capillary forces and diffusive interactions seem to

be identified as the two major mechanisms for fluid transport. Main coating layer factors contributing to the

absorption are given as pore structure, pore size distribution, and surface chemistry [1-3]. Fluid absorption into

pigment-coated paper is a highly complex phenomenon due to its dynamic nature and simultaneously occurring

chemical and physical phenomena. In addition, the heterogeneous and multi-component character of the

substrate and the fluid (e.g. ink) complicate our understanding of the process.

Fluid absorption into paper has been typically presented with the Lucas-Washburn equation [4,5], in which the

Laplace capillary pressure relation is incorporated into the Hagen Poiseuille’s equation of laminar flow.

2

cos 22 rpr

t

h ELV (1)

Here h is the depth of penetration, t time, r the pore radius, γ surface tension for the fluid, θ the contact angle

between fluid and substrate, η the fluid viscosity, and pE the external pressure gradient. The Lucas-Washburn

equation assumes that all the pores are uniform and cylindrical, and fluid continuously fills the pores in the pore

network under equilibrium conditions.

In practice, it is well known that smaller pores are found to 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 mathematical model for ink setting based on Lucas-

Washburn equation and Darcy’s law [8], in which 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 at short time-scale rather than the large pores.

Rousu et al. [13] considered the aspect of multi-component transport into porous media pointing out that models

derived for the single-component fluid system do not fully apply in more complicated systems like ink-coatings

Submitted for publication in Industrial and Engineering Research Journal

2

interactions. Recently, Donigian [14] has proposed a multiphase hypothesis for initial ink-gloss development,

which is based on the assumption that ink oils and resins are distributed into different phases.

The ratio of surface tension and viscosity of the fluid has been used to describe the capillary flow behavior in

several studies [15], and for example, Rousu [1] has shown that an increased ratio of surface tension and

viscosity leads to 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 their role 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 coating structure over surface

chemistry of substrate for absorption kinetics into porous structures [2]. For example Sandås and Salminen [16]

have shown that sorption into clay and CaCO3 coatings were equal 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 this paper the influence of

coverage of the pigment dispersant on surface energy has been overlooked. Ridgway and Gane [17] showed that

both surface chemistry and pore structure have an impact on absorption, but the time scale is crucial: within the

first few seconds, small pores absorb faster non-polar liquid, whereas at longer time-scales water containing

polar component absorbs faster by larger pores. Pykönen et al. [18] have shown that by introducing hydrophobic

character on the surface without influencing porous structure, plasma coatings are able to prevent the dampening

water absorption into porous pigment-coated paper under the action of nip pressure.

It is well-known that narrow particle size distribution and needle-shaped particles provide permeable pigment

coatings, when 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. [19] have shown that permeability of pigment coating

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 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 which 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 [20].

UV-curing inks and varnishes were introduced in the 1960s, and gained popularity during the 1970s [21].

Nowadays, the use of UV technology in printing industry is expected to expand [22]. UV-varnishes are based on

acrylate chemistry consisting of prepolymers, monomers and photo-initiators. The prepolymers act as a vehicle

of the system and give brilliance, mechanical and chemical resistance. The monomers are used to adjust the end

properties of the varnish, and they act as diluents to adjust viscosity, but also affect surface chemistry and cross-

linking. The photo-initiators are selective to the light of specific wavelengths and initiate the curing. UV-varnish

absorbs photons of high energy ultraviolet light, which undergoes chemical polymerization, and thus converts

the varnish from liquid to solid state [23].

The aim of this work was to understand the role of surface chemistry on the absorption of UV-varnish with

highly porous and permeable pigment-coated paper. While the permeable, high porosity coatings provide

excellent opacity and brightness properties for paper due to high light scatter, the high permeability may lead to

problems in printing related to too fast penetration of printing liquids. For example, in the case of UV-varnishing,

too fast absorption may decrease the gloss and result also in uneven gloss appearance [22]. In addition, uncured

UV-varnish ingredients are undesirable especially in food packaging applications, since unreacted monomers

and photoinitiators may cause odor, taste, or toxic problems in the end product [24]. In this work, three different

plasma-polymerized coatings were deposited on the 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 sub-categories [1]: (i) in etching or ablation,

where material is removed from the solid surface, (ii) in plasma activation, where the surface may be chemically

and/or physically modified by species present in the plasma, and (iii) in 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 vapour deposition process (P(E)CVD). The PCVD plasma

technology can be used to deposit functional layers, such as hydrocarbons, hydrocarbons with polar groups,

organosilicons, halocarbons (e.g. fluorocarbons) and organometallics [25]. According to previous studies [18,26],

these plasma coatings can be used to change surface chemical properties of paper while maintaining its porous

structure.

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2. Experimental

A fine paper base was coated with aragonitic precipitated calcium carbonate (PCC) pigment with 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 with X-

ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and

water and diiodomethane contact angle measurements. Pore structure of the paper was studied with mercury

porosimetry. The influence of the plasma coatings on UV-varnish absorption was studied with 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) weight median particle size (Sedigraph) 0.40 μm with

narrow particle size distribution and aragonite particle shape, and nine parts of styrene-butadiene latex (DL966,

Dow Chemical Company) with Tg 20ºC and particle size 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 is approximately 0.14 μm, and the pore volume is found across the pore size range of 3–500

nm, amounting to 14 %.

0

20

40

60

80

100

120

140

0.01 0.1 1 10

Mean pore size, um

Po

re v

olu

me

, d

V/d

Lo

gD

, m

l/g

Figure 1. Pore size distribution of the untreated pigment-coated paper.

The laboratory scale plasma depositions were performed at the Institute for Surface Chemistry (YKI), Stockholm.

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 by 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 down to a base pressure below 10 mTorr before introducing the precursor

(monomer) from the top of the reactor. Ethylene was used as monomer for the hydrocarbon coating,

hexamethyldisiloxane (HMDSO, >98.5 %) for the organosilicon coating and perfluorohexane (C6F14, 98 %) for

the fluorocarbon coating (Table 1).

Table 1. Plasma parameters.

Laboratory scale

HMDSO Ethylene C6F14

Frequency 13.56 MHz 135 kHz 13.56 MHz

Discharge power, W 30 20 40

Pressure during treatment, mTorr ~25 ~49 ~140

Flow rate precursor ~5 cm3/min 10 cm

3/min *)

Flow rate of carrier gas (N2) – – –

Treatment time/ line speed 2 min 2 min 1 min *) The value was out of range of the instrument scale.

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

4

ceramic anilox cylinder is manufactured by Cheshire Engraving Services Ltd. and has a cell angle of 60° with

120 lines/cm and a cell volume of 12 cm3/m

2. 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 is a Bluepoint 4 Ecocure supplied by Hönle 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 used intensity was 240 mW/cm² measured at 370 nm wavelength.

2.2 Analyses

The plasma-coated samples were characterized by XPS using a Physical Electronics Quantum 2000 ESCA

instrument, equipped with a monochromatic Al Kα 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 five minutes. 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 x 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 UV-

varnishes and 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 viscosity of the UV-varnishes was measured

with a Gemini-Advanced Rheometer (Bohlin Instruments) with the shear rate 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).

Table 2. Total surface tension (γtot) and its non-polar (dispersion or Lifshitz-van der Waals forces, γLW) and

polar (acid-base, γAB)) components of the liquids used at the contact angle measurements.

Liquid

tot

LW

AB

(mN/m) (mN/m) (mN/m)

Water 72.8 21.8 51

DIM 50.8 50.8 ~ 0 Table 3. Density, surface tension and viscosity of UV-varnishes used in contact angle measurements.

Density, g/cm3

Surface tension, mN/m Viscosity, mPas

Average Std. Average Std. Average Std.

UV varnish, low η 0.977 0.043 21.00 0.03 72.5 1.3

UV varnish, high η 0.990 0.073 21.13 0.01 147.8 1.2

Paper gloss (75º) for untreated, plasma-coated and UV-varnished samples was measured with Zehnter ZLR

1050M glossmeter.

3. Results and Discussion

According to mercury porosimetry results, plasma coatings had no influence on porous structure in the range of

3–500 nm. This is in agreement with previous studies [18,26], where plasma coatings have been used to modify

the surface chemical composition of the paper while maintaining its porous structure.

The surface chemical composition and changes in it created by plasma coatings were determined by XPS.

Survey spectra for untreated sample gave typical peaks for pigment-coated paper containing calcium carbonate

and styrene-butadiene latex (Table 4). Sodium and phosphate in untreated sample are probably ingredients of the

calcium carbonate pigment’s dispersion chemicals, and also part of oxygen (7.6 at.%) 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 plasma coatings did not cover the

whole surface, or that the plasma coatings were thinner than 10 nm, which is the escape depth of the XPS

photoelectrons. Subtracting signals coming from CaCO3 and taking into account of the oxygen and carbon

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coming from sources other than CaCO3, the pure hydrocarbon coating would contain 93.9 at.% of hydrogen and

6.1 at.% of oxygen (O/C = 0.06). Small amounts of fluorine in the coating can be a common finding due to

contamination from fluorocarbon o-rings used in the plasma chamber. The survey spectra for organosilicon

plasma coating gave also 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 at.% of carbon, 21.5 at.% of oxygen and 23.3 at.% of silicon (Si/C = 0.42). Comparing with

the HMDSO structure, Si(CH3)-O-Si(CH3), the HMDSO plasma coating contained clearly more oxygen and less

carbon and silicon. It is typical that no repeated units are recognisable in the structure of the plasma polymerized

thin film, since plasma polymerization differs from conventional polymerization as it does not follow the pattern

of initiation, propagation and termination steps [25]. With the fluorocarbon plasma coating no signals from the

pigment coating were detected, and pure fluorocarbon plasma coating contained 36.0 at.% of carbon, 1.7 at.% of

oxygen and 62.3 at.% of fluorine (F/C = 1.73). The deviation between parallel measurements with hydrocarbon

and fluorocarbon plasma coatings suggests that the chemical composition of the plasma coating is not

completely uniform.

Table 4. Relative surface composition in atomic % and elemental ratios measured by XPS.

Element Untreated CH plasma*) HMDSO plasma CF plasma**)

Average Std. Average Std. Average Std. Average Std.

C 56.6 0.5 76.6 5.1 54.6 0.2 36.0 1.0

O 33.1 0.6 18.8 4.0 26.3 0.8 1.7 0.5

Ca 8.5 0.1 4.0 1.3 2.3 0.4 – –

Na 1.5 0.2 – – – – – –

P 0.4 0.1 – – – – – –

Si – – – – 16.6 1.1 – –

F – – 0.5 0.2 – – 62.3 0.4

O/C 0.59 0.02 0.25 0.07 0.48 0.01 0.05 0.02

Si/C – – – – 0.30 0.03 – –

F/C – – – – – – 1.73 0.07 *) One parallel measurement deviated clearly with reduced amount of calcium (1.3 at.%) and oxygen ( 11.5 at.%); not included into average and standard deviation values presented in the table.

**) One parallel measurement contained clearly more oxygen (10.2 at.%) and less fluorine (49.9 at.%); not included into average and

standard deviation values presented in the table.

Uniformity and coverage of the plasma coatings are illustrated using chemical mapping in ToF-SIMS images.

Calcium mapping confirms that plasma coating coverage was not complete in the cases of the hydrocarbon and

organosilicon plasma-coated samples, but it seems that signals of calcium were evenly distributed across the

surface (Figure 2). With the fluorocarbon sample, only a few signals from calcium could be detected.

Figure 2. ToF-SIMS images illustrating the distribution (2.5 mm x 2.5 mm) of the calcium on untreated (A),

hydrocarbon (B), organosilicon (C) and fluorocarbon (D) plasma-coated samples.

Contact angle measurement is a useful technique in quantifying interfacial and intermolecular phenomena.

Figure 3 shows that the dynamic contact angles of water (polar) and non-polar 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 water

contact angles, whereas diiodomethane contact angles decreased slightly. All the plasma coatings, excluding

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 angles measurements,

the standard deviation for all samples was between 1 and 4 degrees.

A) B) C) D)

6

40

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60

70

80

90

100

110

120

130

140

0 1 2 3 4 5

Time, s

Wa

ter

co

nta

ct

an

gle

Untreated CH plasma HMDSO CF plasma

0

20

40

60

80

100

120

0 1 2 3 4 5

Time, s

DIM

co

nta

ct

an

gle

Untreated CH plasma HMDSO plasma CF plasma

Figure 3. Change of water (left) and DIM (right) contact angles (º) with time.

Contact angle measurement was also used to study the impact of plasma coatings on wetting of the UV-varnishes

with two different viscosities without influence of external pressure. It must be emphasized that the UV-varnish

is typically applied under the influence of nip pressure as forced wetting, and therefore the results are not directly

comparable to the UV-varnish 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 UV-varnish

contact angles, but the change was clearly smaller. The hydrocarbon plasma coating resulted in unchanged or

even slightly reduced UV-varnish contact angles. The results indicate that the organosilicon plasma coating had

clearly more interactions with UV-varnishes compared to the fluorocarbon plasma coating, however the polar

interactions can not explain that, since 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 organosilicon plasma coating contained at least methyl groups. UV-varnishes

are based on polyacrylate chemistry containing a hydrocarbon chain with vinyl and ester bonds. Results show

also that the high viscosity UV-varnish gives higher contact angles in 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.

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5

Time, s

UV

-va

rnis

h (

Lo

w η

) c

on

tac

t a

ng

le

Untreated CH plasma

HMDSO plasma CF plasma

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5

Time, s

UV

-va

rnis

h (

Hig

h η

) c

on

tac

t a

ng

le

Untreated CH plasma

HMDSO plasma CF plasma

Figure 4. Change of low (left) and high (right) viscosity UV-varnish contact angles (º) with time.

In addition to contact angles, drop volume and drop base diameter were investigated to show influence of plasma

coatings on sorption (drop volume) and spreading (drop base). Figure 5 shows that with untreated, hydrocarbon

and organosilicon plasma-coated samples the drop base increased with time, whereas with 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 non-

absorbing smooth surface. The measured drop volume of the low viscosity varnish on paper was stable on all

samples, whereas the drop volume of the high viscosity varnish increased slightly. This implies some swelling of

the paper when the drop wets 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 fluorocarbon plasma coating

also reduced sorption.

7

0

1

2

3

4

5

6

7

0 1 2 3 4 5

Time, s

Dro

p b

as

e, m

m

Untreated CH plasma

HMDSO plasma CF plasma

0

1

2

3

4

5

6

7

0 1 2 3 4 5

Time, s

Dro

p b

as

e, m

m

Untreated CH plasma

HMDSO plasma CF plasma

Figure 5. Drop base diameter with time for low (left) and high (right) viscosity UV-varnishes.

3

3.5

4

4.5

5

5.5

6

0 1 2 3 4 5

Time, s

Dro

p v

olu

me

, u

L

Untreated CH plasma

HMDSO plasma CF plasma

3

3.5

4

4.5

5

5.5

6

0 1 2 3 4 5

Time, sD

rop

vo

lum

e, u

L

Untreated CH plasma

HMDSO plasma CF plasma

Figure 6. Drop volume (μL) with time for low (left) and high (right) viscosity UV-varnishes.

Too fast penetration of the UV-varnish into highly permeable pigment-coated paper typically produces 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 the samples. However, the

fluorocarbon and organosilicon 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 fluid and substrate surface have no or minimal influence on the absorption rate, when an external

pressure is applied. In addition, the results suggest that forced wetting can not be well-described by contact angle

measurements. However, in our previous study [18], similar plasma coatings reduced dampening water

absorption into pigment-coated 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. For this type of coatings, the permeability can be assumed to be lower

when compared to 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 absorption

rate aside from the degree of permeability or porosity. When the external pressure is present, however, the role

of 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, since

the type of fluid, fluid amount and application method differed. It is possible that also the amount of fluid is

crucial. Shoelkopf et al. [19] state 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 [27,28]. 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 may extend along the pores in the bulk material

depending on the plasma parameters used and choice of monomers [29]. In this study, the high viscosity varnish

provided higher gloss compared to the low viscosity varnish, which indicates that the increased viscosity

decreases absorption rate even if the external pressure is present. This is in agreement with the Lucas-Washburn

equation (Equation 1).

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Table 5. Gloss for pigment-coated paper without and with UV-varnishes.

Without UV varnish UV varnish, low η UV varnish, high η

Average, % Std. Average, % Std. Average, % Std.

Untreated 38.6 0.2 41.6 1.1 48.1 0.5

CH plasma coated 38.4 1.3 40.9 1.7 47.0 1.5

HMDSO plasma coated 39.1 0.8 44.8 2.5 46.9 0.7

CF plasma coated 38.2 0.5 40.9 2.0 47.4 0.7 In the application nip, the penetration of the UV-varnish occurs under influence of external pressure and then

during the time delay from the nip to the UV lamp, without external pressure. Figure 7 shows the penetration

depth (h) with time calculated according to Lucas-Washburn equation (Equation 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 high viscosity 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, impact of both contact angle and viscosity increases. 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 fluorocarbon plasma-coated sample in the absence of external

pressure.

0

20

40

60

80

100

120

140

160

0 0.1 0.2 0.3 0.4 0.5

Time, s

Pe

ne

tra

tio

n d

ep

th,

um

UT, low visc., 0 MPa

UT, high visc., 0 MPa

UT, low visc., 1 MPa

UT, high visc., 1 MPa

CF, low visc., 0 MPa

CF, high visc., 0 MPa

CF, low visc., 1 MPa

CF, high visc., 1 MPa

Figure 7. Penetration depth (μm) with time calculated according to Lucas-Washburn equation without and with

(1 MPa) external pressure for untreated and fluorocarbon plasma-coated samples.

The plate cylinder diameter was 7 cm, and therefore the estimated nip length was 1 mm and dwell time in the nip

15 ms. Since 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 seconds to reach the UV-lamp. Table 6 shows the calculated UV-varnish penetration depths and

capacities for UV-varnish absorption (g/m2) with and without external pressure. Since the typical thickness for

the pigment coating is some tens of micrometers, the results indicate that after the nip, all the applied UV-

varnish (2 g/m2) could have been absorbed into the coating near to 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 the UV-varnish would not have penetrated into the

coating, which could have improved the possibility to detect differences between the untreated and plasma-

coated samples. The surface chemical 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.

9

Table 6. Penetration depth (h) and capacity for UV-varnish absorption (g/m2) calculated according to Lucas-

Washburn equation, when the pore volume is 14 % and nip pressure 0.5 MPa. Sample h [μm] Capacity for UV-varnish absorption [g/m

2]

Low η varnish With nip pressure (0.5 Mpa, 0.015 s) Untreated 19.9 2.7

CF plasma treated 17.9 2.4

Without nip pressure (0.3 s) Untreated 53.2 7.3

Speed 4 m/min CF plasma treated 36.5 5.0

Without nip pressure (0.012 s) Untreated 10.6 1.5

Speed 100 m/min CF plasma treated 7.3 1.0

High η varnish With nip pressure (0.5 Mpa, 0.015 s) Untreated 13.8 1.9

CF plasma treated 12.2 1.7

Without nip pressure (0.3 s) Untreated 36.4 5.0

Speed 4 m/min CF plasma treated 22.0 3.1

Without nip pressure (0.012 s) Untreated 7.3 1.0

Speed 100 m/min CF plasma treated 4.4 0.6

4. Conclusion

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 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, can not be well described by the contact angle measurements. Taking into

consideration also the results from our previous study [18], it is possible that the influence of surface chemical

composition may diminish in the case of highly permeable and porous pigment coatings. Calculations according

to 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 the varnish was absorbed in the paper before UV-curing, which might

have been the reason that no differences could be seen.

Acknowledgements

This work was funded by the Finnish Funding Agency for Technology and Innovation (Tekes). The authors

would like to acknowledge Specialty Minerals for pigment supply, Dr. Tapio Mäkelä for advice in roll-to-roll

UV-varnishing and Laboratory Technician Pauliina Saloranta for her contributions to laboratory measurements

at Åbo Akademi University.

Literature Cited

(1) Rousu, S. Differential Absorption of Offset Ink Constituents on Coated Paper. Doctoral thesis, Åbo Akademi

University, Painotalo Gillot Oy, Turku, 2002.

(2) Aspler, J. Ink-Water-Paper Interactions in Printing: An Updated Review. 9th

TAPPI Advanced Coating

Fundamentals Symposium 2006, Turku, Finland, February 8–10 2006, pp. 117–146.

(3) Ström, G. Interaction between Offset Ink and Coated Paper – A Review of the Present Understanding. 13th

Fundamental Research Symposium 2005, Cambridge, September 2005, pp. 1101–1137.

(4) Lucas, R. Ueber das Zeitgesetz des Kapillaren Aufstieges von Flüssigkeiten. Kolloid Zeitschricht 1918, 23,

15.

(5) Washburn, E.W. The Dynamics of Capillary Flow. The Physical Review XV11:3 1921, 273.

(6) Xiang, Y.; Bousfield, D.W. The Influence of Coating Structure on Ink Tack Development. TAPPI

International Printing and Graphic Arts Conference 1998, Tappi Press, Atlanta, GA.

(7) Xiang, Y.; Bousfield, D.W.; Haynes, P.C.; Kettle, J.A. A Model to Predict Ink-Setting Rates Based on Pore-

Size Distribution. Journal of Pulp and Paper Science 2004, 30(5), 117.

(8) Darcy, H. Les Fontaines Publiques de Ville de Dijon. Dalmont: Paris, 1856.

10

(9) Schoelkopf, J.; Gane, P.; Ridgway, C.; Matthews, G. Influence of Inertia on Liquid Absorption into Paper

Coating Structures. Nordic Pulp and Paper Research Journal 2000,15(5), 422.

(10) Bosanquet, C. On the Flow of Liquids into Capillary Tubes. Phil.Mag. 1923, S6:45:S25, 525.

(11) Kadoya, Y.; Usuda, M. The Penetration of Nonaqueous Liquids, Chapter 19 in R.E. Mark (ed.) Handbook

of Physical and Mechanical Testing of Paper and Paperboard; Marcel Dekker Inc., NY, 1984.

(12) Zang, Y.H.; Aspler, J.S. The Influence of Coating Structure on the Ink Receptivity and Print Gloss of Model

Clay Coatings. Tappi J. 1995, 78(1), 147.

(13) Rousu, S.M.; Gane, P.A.C.; Eklund, D.E. Influence of Coating Pigment Chemistry and Morphology on the

Chromatographic Separation of Offset Ink Constituents. 12th

Fundamental Research Symposium of the Pulp &

Paper Fundamental Research Society 2001, England, September 17–21 2001.

(14) Donigian, D.W. A New Mechanism for the Setting and Gloss Development of Offset Ink. Journal of Pulp

and Paper Science 2006, 32 (3), 163.

(15) Geiss, F. Fundamentals of Thin Layer Cromatography; Hüthig, Germany,1987.

(16) Sandås, P.E.; Salminen, P.J. Water Penetration in Coated Papers. Tappi Coating Conference 1987, Tappi

Press, Atlanta, GA, pp.97–101.

(17) Ridgway, C.J.; Gane, P.A.C. Correlating Pore Size and Surface Chemistry during Absorption into a

Dispersed Calcium Carbonate Network Structure. Nordic Pulp and Paper Research Journal 2006, 21, 563.

(18) Pykönen, M.; Johansson, K.; Dubreuil, M.; Vangeneugden, D.; Ström, G.; Fardim P.; Toivakka, M. An

Attempt to Reduce Dampening Water Absorption Using Hydrophobic Plasma Coatings. TAGA 61st Annual

Technical Conference 2009, New Orleans, March 15-18 2009.

(19) Schoelkopf, J.; Gane, P. A. C.; Ridgway, C. J. Pigment coating permeability : measurement and

correlation with wetting front penetration. 8th TAPPI Advanced Coating Fundamentals Symposium 2003,

Chicago, IL, United States, May 8-10, 2003, pp. 215-228. (20) Kipphan, H. (Ed.). Handbook of Print Media, Technologies and Production Methods; Springer, 2001.

(21) Thompson, B. Printing Materials, Science and Technology; Pira International, 1998.

(22) Rousu, S.; Gustafsson, J.; Preston, J.; Heard, P. Interactions between UV Curing, Hybrid-UV and Sheetfed

Offset Inks and Coated Paper – Commercial Print Trials. TAGA Journal 2006, 2, 174.

(23) Leach, R.H.; Pierce, R.J. The Printing Ink Manual; Kluwer Academic Publishers; Dordrecht (Netherland),

1999.

(24) Kariniemi, M.; Ilmonen, A.; Kenttä, E.; Sundqvist, S.; Aurela, B. Curing of UV Ink and product Safety. 35th

International Research Conference, Advances in Printing and Media Technology 2008, September 7-10 2008,

Valencia, Spain, pp.237–244.

(25) Roth, J.R. Industrial Plasma Engineering. Vol. 2: Applications to Nonthermal Plasma Processes; IOP:

Bristol and Philadelphia, 2001.

(26) Tan, H.I.; da Silva, M. L.; Demarquette, N.R. Paper Surface Modification by Plasma Deposition of Double

Layers of Organic Silicon Compounds, J. Mater. Chem. 2001, 11 (6), 1019.

(27) Sahin, H.T.; Manolache, S.; Young, R.A.; Denes, F. Surface Fluorination of Paper in CF4-RF Plasma

Environments. Cellulose 2002, 9(2), 171.

(28) Navarro, F.; Davalos, F.; Denes, F.; Cruz, L. E.; Young, R. A.; Ramos, J. Highly Hydrophobic Sisal

Chemithermomechanical Pulp (CTMP) Paper by Fluorotrimethylsilane Plasma Treatment. Cellulose 2003, 10(4),

411.

(29) Mukhopadhyay, S.M.; Joshi, P.; Datta, S.; Macdaniel, J. Plasma Assisted Surface Coating of Porous Solids,

Applied Surface Science 2002, 201, 219.