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Characterization of filled powders for powder coating of paper
Powder Technology 279 (2015) 127–133
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Characterization of filled powders for powder coating of paper Mika Vähä-Nissi ⁎ KCL Science and Consulting, P.O. Box 70, FI-02151 Espoo, Finland
a r t i c l e
i n f o
Article history: Received 23 February 2015 Received in revised form 31 March 2015 Accepted 4 April 2015 Available online 15 April 2015 Keywords: Cohesion Electrical properties Powder Powder coating Paper
a b s t r a c t The purpose of this study was to evaluate the feasibility of using highly filled powders with 100 parts per weight of calcium carbonate and 10 parts per weight of polymer, acting as binder, as electrostatically applied paper coatings. Such powders prepared by different methods were evaluated as being suitable for electrostatic coating. Powders compacted at elevated temperatures demonstrated cohesion superior to that of calcium carbonate, and at best similar to that of aqueous coatings. Cohesion, porosity and thus absorption properties of compacted powders were affected by the powder preparation method, compaction process, and powder composition. Powders containing semicrystalline polymer turned out to be the most interesting for further testing. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Electrostatic powder coating has a potential as an innovative and waterless coating process for paper [1–3]. Further innovations are needed as far as high-speed application, thin coatings and optimized coating compositions are concerned. This study focused on evaluating and predicting the properties and performance of highly filled powders prior to actual deposition and coating trials. There are several methods for preparing powders for coating, such as extrusion followed by grinding. The filler addition level is typically low. With aqueous formulations the process can include different drying methods, and significantly more fillers can be added. Koishi et al. [4] have presented several methods for preparing modified powders. Powder particle size and size distribution affect electrostatic charging, transfer, attachment and adhesion as well as the general coating properties [5,6]. It is favourable to have a narrow size distribution, and small particles tend to carry larger charge, adhering easier to surfaces to be coated at high velocities [5,7,8]. In addition, gravity, air drag, ion wind, external electric field, and deposition tool design all have an effect on particle transfer onto a surface [6,9–12], and interactions between the particles, and between the particles and the surface, are therefore caused by a variety of forces [6,13]. Charging of powder particles is a combination of ion entrapment and polarization [6]. Charge to mass profile of the powder is important, and maximum charging can actually ⁎ VTT Technical Research Centre of Finland P.O. Box 1000, FI-02044 VTT, Finland. Tel.: +358 40 530 8472. E-mail address: mika.vaha-nissi@vtt.fi.
http://dx.doi.org/10.1016/j.powtec.2015.04.008 0032-5910/© 2015 Elsevier B.V. All rights reserved.
be detrimental due to back-ionization [5,6,14]. A lower limit, below which the particles may not adhere to the grounded substrate is stated to be 0.1–0.2 μC/g [5,11]. The adhesion of the outer layer is reduced when the charge exceeds 3.5 μC/g [15]. Resistivity of 1012 Ωm is often regarded as the lower limit for good adhesion [14]. These electrical properties depend on the structure and composition of powder particles. The attachment of powder particles on the surface prior to fusing can be improved by increased moisture content or by using a grounded backing [6]. For printing papers ink absorption and transfer are dependent on the coating pore structure and surface chemistry [16]. The effect of porosity, and more particularly pore network structure, has been highlighted; increased porosity decreases mechanical stresses during drying, but depending on the ratio of the finest and largest pores can either increase the rate at which the ink sets or detrimentally increase the rate of pressure-driven permeation of fountain solution or drainage of a low viscosity varnish, for example. Porosities of 28–31% and average pore sizes of 43–125 nm have been reported for paper coatings and considered to be within a suitable range for offset printing [17]. Loosely packed powder layers, therefore, obviously need densification for an optimal pore structure, and to develop cohesion and adhesion against mechanical stresses. Various models for powder packing exist [18–20]. There is a correlation between the mechanical properties and structure of compacted powders. The smaller the void fraction the higher the cohesive strength, and the smaller the particle diameter the higher is the tensile strength [20]. Bridge formation between polymer binder particles is affected by particle size, polymer and strain rate [21]. Heat transfer is crucial and can be improved by pressing [22,23], and elevated
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temperatures have a positive effect on packing. Compaction is a combination of inter- and intra-granular deformations in the case of porous agglomerates [24].
samples. The behaviour of the powder coating in a hot nip, typical for paper calendering, was also approached theoretically in this way. 2.3. Sample characterization
2. Experimental details 2.1. Powder preparation Table 1 lists the powders prepared and the processes used. 100 parts per weight of calcium carbonate (CaCO3) was used as the total filler in all the experimental powders. The latex binders A, B and C used in this study were the same or similar to those used in conventional paper coatings. These together with the semicrystalline polymers were chosen by the collaborating companies. Three processes were used successfully for preparing the powders: (i) low solids mixing, referring simply to mixing together ready-made aqueous slurries of pigment and binder, (ii) pseudo-fluidized bed mixing, allowing significantly higher solids content, thus significantly reducing the amount of water required to be removed, and (iii) preferential absorption, utilizing the finding that freshly ground CaCO3 readily adsorbs certain chemical compounds. In this latter case CaCO3 was wet ground without dispersant together with the polymer, and the polymer was adsorbed in-situ in colloidal form onto the CaCO3 particles. The final agglomerate size was also affected by spray drying. Ground CaCO3 and a commercial powder were also used as the reference materials. Reference coating formulations were also produced containing 100 parts per weight of Setacarb® (Omya) and 10 parts per weight of styrene-acrylate latex B (BASF). These were partly dewatered in order to control the packing porosity and to slow down polymer and filler segregation. The partly dewatered samples were carefully mixed and then placed in Teflon tablet moulds. The filled moulds were kept in an oven at 60 °C for 4 h to dry, followed by several days at room temperature. The latex rich top layer and the filler rich bottom layer of each sample were removed by wet surface grinding, and the remaining tablets were dried again at room temperature.
To estimate resistivity and dielectric constant for the powders and compacted samples, a simple test chamber was constructed. This was connected to an HP 4339A (high resistivity meter) and an HP 4284A (precision inductance, capacitance and resistance (LCR) meter) both supplied by Hewlett Packard (later Agilent and Keysight Technologies). Powders were also evaluated at John Chubb Instrumentation in the UK for charging and charge decay using their JCI 155v5 test unit. Dielectric constants and charge to mass ratios of powders were also estimated theoretically. Differential scanning calorimetry (DSC) was used to evaluate possible polymer bonding to CaCO3 surfaces. This was assumed to cause peak shifts as the mobility of polymer molecules would become restricted. Tests were carried out at Aalto University (formerly Helsinki University of Technology) and BASF with equipment supplied by Mettler Toledo and Netzsch. Thermal properties of powders were also estimated theoretically. Compacted powders were evaluated for radial compression (cohesive) strength at the University of Helsinki, Department of Pharmacy. The measurement system was a Lloyd LRX with a load cell of 2.5 kN and Nexygen 4.1 analysis software from Lloyd Instruments (now part of AMETEK). A deformation rate of 5 mm/min was used. Mercury porosimetry measurements were performed for compacted powders at the University of Maine, Micromeritics Corp., and at VTT Technical Research Centre of Finland with Micromeritics AutoPore IV 9500 and Pore Sizer 9310. Images were taken with a high resolution scanning electron microscopy (HR-SEM from JEOL) at TopAnalytica Oyj in Turku, Finland. Compacted powders were also tested for silicone oil (50 MPas) absorption at the University of Maine adopting their micro-probe test. The force–time response of a small drop of oil on a probe was analyzed as it contacted a surface. 3. Results
2.2. Powder compaction Compaction tests at room temperature were performed with a laboratory press (MTS Systems Corporation) and a mould normally used for preparing tablets for infra-red (IR) spectroscopic analysis. Some of these cold compacted powders were post heat treated for 3 min at 200 °C in order to evaluate possible changes in porosity and the role of polymer in bonding. Compaction at elevated temperatures was performed at Tampere University of Technology, Department of Material Science, with a PC-operated Instron and a heated chamber. Compaction behaviour was observed during the compaction tests from the movement of the punch and/or from the dimensions of the compacted powder Table 1 Experimental powders, their composition and preparation processes. All powders contained 100 parts per weight of CaCO3 and 10 parts per weight of polymer. Powder
Polymer
Process
1 2
10 parts semicrystalline polymer 1 0.5 parts semicrystalline polymer 1, 9.5 parts starch stab. SA latex A 0.5 parts semicrystalline polymer 1, 9.5 parts SA latex B 10 parts SB latex C 10 parts semicrystalline polymer 1 10 parts semicrystalline polymer 2 10 parts semicrystalline polymer 1 0.5 parts semicrystalline polymer 1, 9.5 parts SA latex B 0.5 parts semicrystalline polymer 1, 9.5 parts starch stab. SA latex A
Low solids mixing Low solids mixing
3 4 5 6 7 8 9
Low solids mixing Low solids mixing Pseudo fluidized bed mixing Pseudo fluidized bed mixing Preferential adsorption Preferential adsorption Preferential adsorption
Differences in the structure of the powder particles were observed in SEM images, indicating the role of the powder preparation method used. The final powder particle size was largely determined by the drying. Fig. 1 demonstrates hybrid particles of powders 4 and 7. Particles were porous, and the binder was evenly distributed covering some of the CaCO3 particles. 3.1. Compaction behaviour of powders The behaviour of powders under compaction is important, as this determines to a large extent the porous structure of the final coating, and thus also the mechanical and absorption properties. The relative change in porosity for cold compacted CaCO3 was small and porosity decreased almost linearly, indicating that the agglomerates in dry CaCO3 powders were easily broken (Fig. 2). The experimental powders were compacted already at low pressures mainly due to inter-granular packing. However, these granules were stronger than in the case of CaCO3 alone, and intra-granular deformations required therefore often high pressures. Powders 5 and 6 demonstrated less profound compaction due to the differences in the particle structure. The porosities of cold compacted samples were high, and an increased rate of compaction increased the final porosity further. Powders 1 and 7 with semicrystalline polymer provided the highest and powders 2 and 9 with latex A the lowest porosities at specific pressures. The particle size distribution was also wider for powder 9 than for powder 7. Elevated compaction temperatures led to decreased porosity as the granules became more deformable. However, the final porosity was often even lower than expected. This could be explained by additional
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Fig. 3. Porosity (%) as function of speed (100 vs. 1000 m/min) through a 0.02 m wide nip and maximum nip pressure (MPa) at room and elevated (N125 °C) temperatures. This model is based to predict the compaction behaviour of powder 7 and other findings from the compaction tests.
The first task was to define the approximation for cold compaction (powder 7) in a nip of which the first half is critical. Porosity (φ) of the powder layer was estimated to increase by at least 2.5% for each tenfold increase in the compaction rate (dP/dt in MPa/s). For cold compaction this resulted in Eq. (1) and for elevated temperatures (≥125 °C) in Eq. (2). Fig. 3 presents the porosities of samples (100 and 1000 m/min) and different pressures (P in MPa). This simple approach does not include the rheological behaviour of the polymer or that of the fibre substrate. It seems necessary to use elevated temperatures and several (long) nips as far as the coating porosity is concerned.
Fig. 1. Hybrid granules of powder 4 (A) and powder 7 (B) prepared using two of the processes. White indicates pigment and dark grey/black polymer, while light grey is resin used for sample preparation (porosity).
compaction of tablets during tablet removal. Secondly, the difference between the compaction temperatures of 125 and 150 °C was small. Compaction at elevated temperature was the most effective with powders containing semicrystalline polymer, while the powder with latex A demonstrated the smallest relative change compared to cold compaction. A theoretical porosity after a single hot nip was calculated. It was assumed that the width of the nip is 0.02 m, and that the pressure increase to the maximum in the middle of the nip can be considered to be linear. It was observed earlier that a tenfold increase in compaction rate resulted in a higher porosity. This was estimated to be the case also at much higher compaction rates. On the other hand, hot compaction decreased porosity. A step-wise change was assumed in compaction between 100 and 125 °C.
Fig. 2. Porosity (%) as function of compaction pressure (MPa) for cold compacted experimental powders and one CaCO3 reference (Setacarb® from Omya AG).
φ ¼ 1:09 ln ðdP=dt Þ−9:52 ln ðP Þ þ 79:43
ð1Þ
φ ¼ 0:9698 ln ðdP=dt Þ−8:47 ln ðP Þ þ 62:11
ð2Þ
3.2. Electrical properties of powders Charging of powder particles is essential for both transfer and attachment of powder particles on the surface to be coated. Charge to mass ratio is one of the key parameters. A powder composition consisting of 100 parts per weight of CaCO3 (dielectric constant ε of 6.1–8.5) and 10 parts of polymer (e.g. polystyrene with ε = 2.2) should have a dielectric constant of 5.1–6.8 based on Eq. (3), where γ is the volume fraction of embedded polymer, and p and pol refer to pigment and polymer, respectively [25]. In this paper dielectric constant (ε) refers to relative permittivity. For this equation to be valid the dielectric constant of embedded particles should be lower than that of the matrix. If
Fig. 4. Charge to mass (q/m) ratio as a function of particle diameter for both non-porous and porous (20%) powder particles (E = 300 kV/m) versus the operating window provided in the literature. Minimum q/m calculated also based on a loosely packed coating consisting of porous granules and additional 70% inter-granular porosity.
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this is then reapplied for particles with 20% air (ε = 1) embedded in the composite the dielectric constant of such porous particles would be 4.0–5.3. 1 εpol þ 2εp þ 2γ εpol −ε p A ε ¼ εp @ ε pol þ 2εp −γ ε pol −ε p 0
ð3Þ
Charge to mass ratio (q/m) can be calculated using Eq. (4), where E is the electric field, ε0 is the permittivity of free space (0.89 × 10−11 C/V m), d is the diameter and ρ is the density of powder particles [14]. Fig. 4 demonstrates the theoretical charge to mass ratio for powder particles. ε−1 6ε0 1 þ 2 E q εþ1 ¼ ρd m
ð4Þ
The dielectric constant of a highly porous coating layer (εc) can be estimated using Eq. (5), where ρc is the density of the porous coating and ρ is the density of the coating material itself [11]. This can then be used to calculate the minimum constant charge density required for a homogeneous powder coating layer (Eq. (6)). Example of such a limit close to 0.1–0.2 μC/g is included in Fig. 4 for a coating with porous particles and inter-particle porosity of 70%. Although the charge to mass ratio of charged particles is likely lower than calculated, the theoretical values are within the window stated to be optimal for powder transfer and attachment. lnε c ¼ q mmin
Table 3 Dielectric constant, i.e. relative permittivity (ɛ), for nonporous powders 1, 3 and 4 calculated from dielectric constants measured from powders/compacted tablets at 1 kHz and known porosities using Eq. (5). Sample
ε (at 1 kHz)
Porosity (%)
ε (non-porous powder)
Powder 1 Powder 3 Powder 4
1.8 3.2 and 4.2 2.7 and 3.7
41 and 57 53 and 38 48 and 36
2.6–3.7 10–12 6.9–7.7
material is exposed to the electric field and/or ion bombardment. In the case of high speed coating this will eventually be short. Therefore, the value of the dielectric constant at 0.5 kHz was of interest. The dielectric constant increased or remained more or less constant when frequency was decreased, which indicates more efficient charging at lower speeds. Table 3 shows the behaviour powders 1, 3 and 4 as examples. Semicrystalline polymer 1, as such, had ε of 0.72 and latex B 16. By using Eq. (3), the calculated dielectric constants are 4.6–6.3 for powder 1 and 7.9–9.9 for powder 3. The experimental powders can be charged and they should attach to the paper substrate surface. Small particles charge easier, while higher electric field intensity is required for large particles. Powder 3 was easy to charge, but due to lower resistivity this effect was lost faster. Powders with semicrystalline polymers had only slightly lower charging tendency but their resistivity was higher, leading to slow charge decay similar to commercial powders. 3.3. Thermal properties of powders
ρc ln ε ρ
ð5Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ε c ε0 g ¼ ρc d
ð6Þ
Proper electrical properties are essential for the transfer and attachment of powder particles on the paper surface. CaCO3 powder, a commercial dry ground calcium carbonate powder, and experimental powders 3 and 5–9 were tested (Table 2) for charge decay. CaCO3 powder and powder 3 had short charge decay times, while powders 8 and 9 had moderate values. For the commercial reference and powders 5–7, charge decay was too slow for direct measurements and values were estimated based on the initial charge decay. Semicrystalline polymer had longer charge decay than the lattices, while latex A provided slower decay than latex B. In the case of a fast process, such as paper coating, the time from the charging zone to the fusing nip is short and the rate of charge decay was probably slow enough to prevent powder detachment. In the initial resistivity measurements, CaCO3 had clearly the lowest volume resistivity (105–106 Ωm), while with powders 1, 2, 5 and 6 the resistivity was high, contributing to long charge decay. Powders 3 and 4 were between these two extremes. Dielectric constants were determined by measuring the capacitance and parallel resistance as a function of frequency. Charging efficiency depends also on the time the Table 2 Peak voltage relative to ground and charge decay for a commercial powder, CaCO3 powder, and powders 3 and 5–9. Sample
Average peak voltage (V) Time (s) to 10% of original voltage
Commercial powder Plain CaCO3 Powder 3 Powder 5 Powder 6 Powder 7 Powder 8 Powder 9
−960 −280 −1700 −1100 −1000 −1000 −730 −910
~80 000 0.31 4.6 ~60 000 ~40 000 ~35 000 140 1700–2000
Elevated temperature during compaction is a precondition for decreased porosity, cohesion of powder coating, and adhesion to the surface. Thermal analyses of powders as such turned out to be challenging. Latex A indicated no observable crystallinity in the case of powder 9 (Table 4). The increase in glass transition temperature, Tg, was small for the lattices and the corresponding powders 8 and 9 containing them. Semicrystalline polymer, as such, showed relatively high crystallinity. However, in powder 7 crystallinity was lower and Tg was higher indicating decreased mobility of polymer chains and increased bonding to CaCO3 surfaces. This also supports other findings concerning the polymer dependent behaviour of powders. The whole powder layer on paper has to achieve a temperature high enough for optimal cohesion and adhesion. This was approached theoretically. Specific heat (C) of a powder is the sum of the specific heats of its components based on their weight fractions (excluding air). This was approximately 1 J/g °C for a typical powder in this study. When the thermal conductivity of each component and the volume fractions are known, it is then possible to estimate the conductivity of a nonporous powder using Eq. (7) [26], where a is the orientation coefficient (0.5), γ is the volume fraction of polymer (0.213), Kp is the thermal conductivity of the pigment GCC (2.4–5.5 J/s m °C), and Kpol is the thermal conductivity of that of the polymer (for low density polyethylene 0.29–0.42 J/s m. C). Eq. (7) was reapplied to a porous coating based on the calculated thermal conductivities of a nonporous powder and that Table 4 DSC analyses (BASF) from polymers used in powders 7–9 and corresponding powders, where Tg, is the glass transition temperature, Tm the melting point, and ΔH is the change in enthalpy. 1. Cooling
2. Heating
Material
Tg (°C)
Tg (°C)
Tm (°C)
ΔH (J/g)
Semicr. polymer Powder 7 Latex B Powder 8 Latex A Powder 9
3 20 20 19 12 12
2 22 22 25 13 17
76 Ca. 74 83
38 0.5 0.4
80
0.7
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of air (0.026 J/s m °C). It was then possible to calculate the heat conductivity as a function of porosity in the powder layer. Decreasing the porosity from 70 to 40% increases its thermal conductivity from 0.07 to 0.12 J/s m °C. Thermal diffusivity (D) can be calculated using Eq. (8) where ρ is density [27]. 1
K¼
a ð1−γ ÞK P þ γK pol
D¼
þ ð1−aÞ
1−γ γ þ Kp K pol
!
K ρC
ð7Þ
ð8Þ
Simplifications had to be made to continue the calculations. The base paper is assumed to be of constant thickness in the nip and has a basis weight of 86 g/m2, density of 1040 kg/m3, specific heat of 2.89 J/g °C, and thermal diffusivity of 2.6 × 10−7 m2/s. There were 15 g/m2 of coating with initial and final porosities of 70 and 40%, respectively. The coating porosity and thermal diffusivity of coated paper were expected to be more or less linearly dependent on the position from the nip entrance to the centre of the nip. Thermal losses between the hot roll and the coating were excluded, and the maximum temperature was expected to occur at the nip exit. The thermal conductivity of the coated paper Kcpap was calculated using Eq. (9), where L is thickness, and c, pap and cpap refer to coating, base paper and coated paper, respectively [26]. This was then used to calculate the thermal diffusivity of the coated paper (Dcpap). The nip was divided into sections. A steady state solution of Fourier's law (Eq. (10) [28]) was used to estimate the temperature increase at a depth equal to coating thickness during the first section of the nip. This was then the input for the next section with different coating porosities and Dcpap. When there is a strong temperature gradient across the coated paper, i.e. hot (T1 = 200 °C) and cool (T2 = 25 °C) rolls, initial web temperature (T0) of 25 °C, nip length of 0.02 m and web speed of 1000 m/min, the temperature at the coating/paper interface would reach approximately 107 °C, using the given parameters. In the case the initial web temperature is increased to 55–60 °C, the interfacial temperature would reach approximately 125 °C. Reaching an adequate interfacial temperature can be a challenge at such speeds if using only one hot nip. K cpap ¼
Lcpap Lc Lpap þ K c K pap
T ðxÞ ¼ T 1 þ ðT 2 −T 1 Þ
ð9Þ
x Lcpap
−
∞ 2X π 1
! −n2 π2 D ! cpap t ðT 1 −T 0 Þ−ðT 2 −T 0 Þð−1Þn nπx 2 sin e Lcpap n Lcpap
ð10Þ
3.4. Cohesion and structure of compacted powders Deformation energy increased with increasing compaction pressure with all the powders, and post heat treatment increased further the cohesive strength of the compacted samples. However, the type of failure varied. Porous samples were deformable, and compacted powders with semicrystalline polymer and produced by pseudo fluidized bed were weak (Fig. 5). However, increased compaction pressure and/or post heat treatment increased specifically the brittleness of the compacted powders with semicrystalline polymers compared to the more deformable samples with latex. Compacted samples prepared using fast compaction rate indicated no obvious differences due to the post heating treatment, which indicates the importance of small porosity and interparticle distance prior to the thermal fusing step. Performing compaction at elevated temperatures increased further the force needed to deform and break the compacted powder samples,
Fig. 5. Force of rupture normalized to the thickness (N/mm) of compacted powder samples as function of porosity (%) for cold and hot compacted samples, cold compacted samples after heat treatment, and the references (plain CaCO3 and tablets prepared from aqueous coating colours).
indicating strong and brittle samples. Samples prepared by preferential adsorption were generally displayed more deformability prior to breakdown than the other samples. This is likely due to different polymer coverage and granular structure. Reference tablets from aqueous coatings were strong and brittle. Also compacted pure CaCO3 powder tablets were tested, but the cohesive strength was extremely low. Fig. 6 shows surface SEM images of cold and hot compacted powders. There were obvious differences between the powder particles due to the different processing. In the case of hybrid granules (such as powder 4) the initial increase in density was likely due to intergranular packing, while further deformation of smaller pores within the granules occurred at higher compaction pressures. In the case of powders prepared by pseudo fluidized bed mixing no similar granules could be identified. Powder 7 had some agglomerates, while with powders 8 and 9 these were already significantly deformed by cold compaction. There were indications of polymer softening and improved packing with polymer bonds between the pigment particles and even across the granular boundaries in the case of hot compacted powders. Hot compaction could also cause redistribution of polymer to the surfaces and thus create potential sites for adhesion between powder and the surface to be coated. However, this could not be confirmed in this case. Mercury porosimetry measurements were performed for compacted powders 3 and 5–9. The presence of small pores (30–200 nm) was clearly observed for the cold compacted powders, and powders indicated a small but detectable dual pore size distribution. The correlations between porosities calculated from tablet geometry and intrusion volumes were similar, indicating interconnected porosity accessible for mercury even after hot compaction. Powders 5 and 6 had more small pores, and powder 7 had a greater contribution of large pores than powders 8 and 9. Contribution of pores larger than 150 nm to the measured pore volume was negligible after hot compaction, as presented in Fig. 7, and the average size of the pores is similar to conventional paper coatings. Large inter-granular pores were typically affected already at lower compaction pressures.
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Fig. 6. Surface SEM images of powder 4 cold compacted at A) 8 MPa and B) 80 MPa, powder 5 compacted at 50 MPa and C) room temperature and D) 150 °C, and powder 7 compacted at 50 MPa and E) room temperature and F) 150 °C.
Silicone oil absorption was quite similar for the cold compacted powders 7–9. Absorption was fast (e.g. 5–7 μN/s) due to high porosity and large pores. Coated paperboard with less viscous oil (5 MPas) has provided absorption slower than 1 μN/s. However, hot compaction decreased absorption rate close to 1 μN/s. In addition, hot compacted powder 7 had slightly slower tack build up and absorption than powders 8 and 9. This could indicate different spreading behaviour or chemical interactions between the test liquid and hot compacted powder 7 as porosities and pore size distributions were quite similar between powders 7 and 9. The latex binders are amphiphilic in nature. Whereas the core of the latex particles consists of a nonpolar polymer, the surface is polar to provide compatibility with the aqueous medium. This polar surface also enables the interaction between binder and pigment. The hydrophobic core, on the other hand, will render some hydrophobicity to the coating layer, and control the interaction of the coating layer
with hydrophobic printing inks. It is also known that the latex can absorb oil and solvents from ink by swelling, which can then contribute slightly to the rate of oil and solvent absorption [29,30]. Swelling of polymer is typically affected by its chemical nature, density and molecular mobility. A limited molecular mobility due to crystallinity and the interactions with the pigment surface are the likely reasons for a low swelling tendency of the semicrystalline polymer. Anyhow, it can be concluded that the powder has to be fused using elevated temperatures to bring the absorption behaviour closer to that of conventional coatings. 4. Summary and conclusions Powder coating is an innovative coating process for paper. It could enable cost savings, quality improvements, and new coated paper
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References
Fig. 7. Pore volume divided into very large (N250 nm), large (150–250 nm), medium (100–150 nm) and small (b100 nm) pores based on mercury intrusion into powders compacted at 50 MPa and 25/150 °C.
grades. However, this requires development of highly filled powders, and there have to be easy to use methods supporting the development work. The goal of this study was to test the feasibility of using highly filled powders for paper coatings prior to actual electrostatic deposition and coating trials. Powders with a similar polymer to CaCO3 ratio than in conventional paper coatings were characterized. Particle size and morphology were largely affected by the preparation process. Powder particles were typically granules with internal porosity and evenly distributed polymer. Powders containing semicrystalline polymer as binder turned out to be the most interesting. Charging of the tested designs of powder particles was ranked, and confirmed to fall into a region possible for transfer and attachment. However, formulation, especially the polymer, affected the charging and discharging behaviour of the particles. Some powders had decay times typical for commercial polymeric powders, while some latex containing powders lost their charge only slightly slower than plain CaCO3. Compaction at elevated temperature was essential to decrease porosity and to improve cohesion. Porosity and average pore size were obtained similar to those in conventional paper coatings depending on the compaction process, particle structure, size distribution and composition. At low pressures, the increase in density was mainly due to the inter-granular packing, while at high pressures the compaction was due to the intra-granular deformations. Similar cohesive properties as in conventional paper coatings were achieved after compaction at elevated temperatures, and semicrystalline polymers indicated both interactions with pigment particles and response to heat. However, no unambiguous binder migration favourable for adhesion between powder and paper could be observed. Fast coating processes typical for the paper industry may, therefore, require the use of paper surface pretreatment techniques, and the application of several hot nips in order to optimize coating porosity and to provide suitable conditions for adhesion. There are only few studies about such highly filled powders for electrostatic coating of paper, their composition and key properties. Although the approach has been quite different, some of the results are in line with those obtained from other studies (e.g. Putkisto and Maijala [2,3]) dealing with powder coating of paper. Acknowledgements This paper is based on the Dry Paper Coating project carried out at KCL during 2001–2003. The author thanks the collaborating companies (BASF SE and Omya International AG), the financing companies of KCL, the Finnish Funding Agency for Technology and Innovation (TEKES) and all the project members. The production of the powder samples and some of the analyses were carried out by Omya International AG and BASF SE.
[1] M. Vähä-Nissi, J. Kekkonen, H. Wessman, Review of waterless paper coating processes, Paper Timber 85 (2003) 450–453. [2] K. Putkisto, Dry Surface Treatment of Paper—Possibilities and Demands from a Material Perspective, Tampere University of Technology, Publication, 4692004. [3] J. Maijala, Evaluation of a dry surface treatment technique for pigment coating of printing papers, Tampere University of Technology, Publication, 4712004. [4] M. Koishi, Surface modification, in: K. Iinoya, K. Gotoh, K. Higashitani (Eds.), Powder Technology Handbook, Marcel Dekker, New York 1991, pp. 453–459. [5] M.K. Mazumder, D.L. Wankum, R.A. Sims, J.R. Mountain, H. Chen, P. Pettit, T. Chaser, Influence of powder properties on the performance of electrostatic coating process, J. Electrost. 40–41 (1997) 369–374. [6] A.D. Zimon, Adhesion of Dust and Powder, Plenum, New York, 1982. [7] D.M. Howell, Vol. 1 powder coatings; the technology, formulation and application of powder coatings, in: J.D. Sanders (Ed.), WILEY/SITA Series in Surface Coatings Technologies, John Wiley and Sons, 2000. [8] P. Barmuta, K. Cywinski, Electroseparation and efficiency of deposition during electrostatic powder coating, J. Electrost. 51–52 (2001) 239–244. [9] K. Adamiak, Numerical modelling of tribo-charge powder coating systems, J. Electrost. 40–41 (1997) 395–400. [10] G.D. Cheever, Electrostatic charge acceptance and decay of powder coating particles, J. Appl. Polym. Sci. 19 (1975) 147–163. [11] H. Bauch, Untersuchungen zur Schichtbildung beim elektrostatischen Auftragen von Plastpulvern, Plaste Kautsch. 21 (1974) 853–859. [12] H. Bauch, Einfluss der Anlagenparameter und der Pulvereigenschaften bei der elektrostatischen Plastbeschichtung mit pistolenförmigen Sprühgeräten, Plaste Kautsch. 22 (1975) 280–287. [13] Y. Kawashima, Adhesion and cohesion of powder, in: K. Iinoya, K. Gotoh, K. Higashitani (Eds.), Powder Technology Handbook, Marcel Dekker, New York 1991, pp. 99–115. [14] J.F. Hughes, Electrostatic Powder Coating, Research Studies Press, Baldock, 1984. (121 pp.). [15] H. Stötzel, H.J. Schmidt, D. Auerbach, B. Makin, K. Dastoori, H. Bauch, Adhesion measurements for electrostatic powder coatings, J. Electrost. 40–41 (1997) 253–258. [16] D.L. Hensel, D.M. Lee, D.H. Philip, A latex binder approach to tomorrow's printing demands, PaperAge 112 (1996) 32–33. [17] S. Rousu, P. Gane, D. Spielmann, D. Eklund, Separation of off-set ink components during absorption into pigment coating structures, Nord. Pulp Pap. Res. J. 15 (2000) 527–535. [18] M. Suzuki, Description of particulate assemblies, in: K. Iinoya, K. Gotoh, K. Higashitani (Eds.), Powder Technology Handbook, Marcel Dekker, New York 1991, pp. 73–87. [19] F.M. Thomson, Particulate polymer handling and storage engineering, in: M. Narkis, N. Rosenzweig (Eds.), Polymer Powder Technology, John Wiley & Sons 1995, pp. 71–112. [20] M. Suzuki, Powder mechanics, in: K. Iinoya, K. Gotoh, K. Higashitani (Eds.), Powder Technology Handbook, Marcel Dekker, New York 1991, pp. 65–71. [21] S. Mazur, Coalescence of polymer particles, in: M. Narkis, N. Rosenzweig (Eds.), Polymer Powder Technology, John Wiley & Sons 1995, pp. 157–216. [22] D.-W. Sun, Heat transfer in particulate systems, in: M. Narkis, N. Rosenzweig (Eds.), Polymer Powder Technology, John Wiley & Sons 1995, pp. 126–156. [23] M. Narkis, Sintering of compacted thermoplastic powders, in: M. Narkis, N. Rosenzweig (Eds.), Polymer Powder Technology, John Wiley & Sons 1995, pp. 279–299. [24] J.S. Reed, Principles of Ceramics Processing, John Wiley & Sons, New York, 1995. 418–449. [25] B. Sareni, L. Krähenbühl, A. Beroual, C. Brosseau, Ab initio simulation approach for calculating the effective dielectric constant of composite materials, J. Electrost. 40–41 (1997) 489–494. [26] D. Guérin, V. Morin, D. Chaussy, J.-L. Auriault, Thermal conductivity of handsheets, papers and model coating layers, 12th Fundamental Research Symposium, Fundamental Research Society, Bury 2001, pp. 927–945. [27] K. Niskanen, S. Simula, Thermal diffusivity of paper, Nord. Pulp Pap. Res. J. 14 (1999) 236–242. [28] K.E. Duckett, J. Cain, Finite element methods applied to thermal calendaring, Nonwoven Conference Proceedings, TAPPI Press, Atlanta 1991, pp. 383–391. [29] R.L. Van Gilder, R.D. Purfeerst, Latex binder modification to reduce coating pick on six-color offset presses, Tappi J. 77 (1994) 230–239. [30] Y. Xiang, D. Desjumaux, D. Bousfield, M.F. Forbes, The relationship between coating layer composition, Ink setting rate, and offset print gloss, Pan-Pacific and International Printing and Graphic Arts Conference Proceedings, Quebec, Canada 1998, p. 85. Dr. Mika Vähä-Nissi has over 20 years in research and development. Mika received his M.Sc. in 1994 and D.Sc. in 1998, both from Tampere University of Technology in Finland. In 2000 he moved to Oy Keskuslaboratorio – Centrallaboratorium Ab (KCL) – a research company owned by the Finnish forest industry. Since 2010 Mika has been working at VTT Technical Research Centre of Finland. He is attributed with roughly 120 scientific papers, presentations and other publications.
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Characterization of filled powders for powder coating of paper Mika Vähä-Nissi ⁎ KCL Science and Consulting, P.O. Box 70, FI-02151 Espoo, Finland
a r t i c l e
i n f o
Article history: Received 23 February 2015 Received in revised form 31 March 2015 Accepted 4 April 2015 Available online 15 April 2015 Keywords: Cohesion Electrical properties Powder Powder coating Paper
a b s t r a c t The purpose of this study was to evaluate the feasibility of using highly filled powders with 100 parts per weight of calcium carbonate and 10 parts per weight of polymer, acting as binder, as electrostatically applied paper coatings. Such powders prepared by different methods were evaluated as being suitable for electrostatic coating. Powders compacted at elevated temperatures demonstrated cohesion superior to that of calcium carbonate, and at best similar to that of aqueous coatings. Cohesion, porosity and thus absorption properties of compacted powders were affected by the powder preparation method, compaction process, and powder composition. Powders containing semicrystalline polymer turned out to be the most interesting for further testing. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Electrostatic powder coating has a potential as an innovative and waterless coating process for paper [1–3]. Further innovations are needed as far as high-speed application, thin coatings and optimized coating compositions are concerned. This study focused on evaluating and predicting the properties and performance of highly filled powders prior to actual deposition and coating trials. There are several methods for preparing powders for coating, such as extrusion followed by grinding. The filler addition level is typically low. With aqueous formulations the process can include different drying methods, and significantly more fillers can be added. Koishi et al. [4] have presented several methods for preparing modified powders. Powder particle size and size distribution affect electrostatic charging, transfer, attachment and adhesion as well as the general coating properties [5,6]. It is favourable to have a narrow size distribution, and small particles tend to carry larger charge, adhering easier to surfaces to be coated at high velocities [5,7,8]. In addition, gravity, air drag, ion wind, external electric field, and deposition tool design all have an effect on particle transfer onto a surface [6,9–12], and interactions between the particles, and between the particles and the surface, are therefore caused by a variety of forces [6,13]. Charging of powder particles is a combination of ion entrapment and polarization [6]. Charge to mass profile of the powder is important, and maximum charging can actually ⁎ VTT Technical Research Centre of Finland P.O. Box 1000, FI-02044 VTT, Finland. Tel.: +358 40 530 8472. E-mail address: mika.vaha-nissi@vtt.fi.
http://dx.doi.org/10.1016/j.powtec.2015.04.008 0032-5910/© 2015 Elsevier B.V. All rights reserved.
be detrimental due to back-ionization [5,6,14]. A lower limit, below which the particles may not adhere to the grounded substrate is stated to be 0.1–0.2 μC/g [5,11]. The adhesion of the outer layer is reduced when the charge exceeds 3.5 μC/g [15]. Resistivity of 1012 Ωm is often regarded as the lower limit for good adhesion [14]. These electrical properties depend on the structure and composition of powder particles. The attachment of powder particles on the surface prior to fusing can be improved by increased moisture content or by using a grounded backing [6]. For printing papers ink absorption and transfer are dependent on the coating pore structure and surface chemistry [16]. The effect of porosity, and more particularly pore network structure, has been highlighted; increased porosity decreases mechanical stresses during drying, but depending on the ratio of the finest and largest pores can either increase the rate at which the ink sets or detrimentally increase the rate of pressure-driven permeation of fountain solution or drainage of a low viscosity varnish, for example. Porosities of 28–31% and average pore sizes of 43–125 nm have been reported for paper coatings and considered to be within a suitable range for offset printing [17]. Loosely packed powder layers, therefore, obviously need densification for an optimal pore structure, and to develop cohesion and adhesion against mechanical stresses. Various models for powder packing exist [18–20]. There is a correlation between the mechanical properties and structure of compacted powders. The smaller the void fraction the higher the cohesive strength, and the smaller the particle diameter the higher is the tensile strength [20]. Bridge formation between polymer binder particles is affected by particle size, polymer and strain rate [21]. Heat transfer is crucial and can be improved by pressing [22,23], and elevated
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temperatures have a positive effect on packing. Compaction is a combination of inter- and intra-granular deformations in the case of porous agglomerates [24].
samples. The behaviour of the powder coating in a hot nip, typical for paper calendering, was also approached theoretically in this way. 2.3. Sample characterization
2. Experimental details 2.1. Powder preparation Table 1 lists the powders prepared and the processes used. 100 parts per weight of calcium carbonate (CaCO3) was used as the total filler in all the experimental powders. The latex binders A, B and C used in this study were the same or similar to those used in conventional paper coatings. These together with the semicrystalline polymers were chosen by the collaborating companies. Three processes were used successfully for preparing the powders: (i) low solids mixing, referring simply to mixing together ready-made aqueous slurries of pigment and binder, (ii) pseudo-fluidized bed mixing, allowing significantly higher solids content, thus significantly reducing the amount of water required to be removed, and (iii) preferential absorption, utilizing the finding that freshly ground CaCO3 readily adsorbs certain chemical compounds. In this latter case CaCO3 was wet ground without dispersant together with the polymer, and the polymer was adsorbed in-situ in colloidal form onto the CaCO3 particles. The final agglomerate size was also affected by spray drying. Ground CaCO3 and a commercial powder were also used as the reference materials. Reference coating formulations were also produced containing 100 parts per weight of Setacarb® (Omya) and 10 parts per weight of styrene-acrylate latex B (BASF). These were partly dewatered in order to control the packing porosity and to slow down polymer and filler segregation. The partly dewatered samples were carefully mixed and then placed in Teflon tablet moulds. The filled moulds were kept in an oven at 60 °C for 4 h to dry, followed by several days at room temperature. The latex rich top layer and the filler rich bottom layer of each sample were removed by wet surface grinding, and the remaining tablets were dried again at room temperature.
To estimate resistivity and dielectric constant for the powders and compacted samples, a simple test chamber was constructed. This was connected to an HP 4339A (high resistivity meter) and an HP 4284A (precision inductance, capacitance and resistance (LCR) meter) both supplied by Hewlett Packard (later Agilent and Keysight Technologies). Powders were also evaluated at John Chubb Instrumentation in the UK for charging and charge decay using their JCI 155v5 test unit. Dielectric constants and charge to mass ratios of powders were also estimated theoretically. Differential scanning calorimetry (DSC) was used to evaluate possible polymer bonding to CaCO3 surfaces. This was assumed to cause peak shifts as the mobility of polymer molecules would become restricted. Tests were carried out at Aalto University (formerly Helsinki University of Technology) and BASF with equipment supplied by Mettler Toledo and Netzsch. Thermal properties of powders were also estimated theoretically. Compacted powders were evaluated for radial compression (cohesive) strength at the University of Helsinki, Department of Pharmacy. The measurement system was a Lloyd LRX with a load cell of 2.5 kN and Nexygen 4.1 analysis software from Lloyd Instruments (now part of AMETEK). A deformation rate of 5 mm/min was used. Mercury porosimetry measurements were performed for compacted powders at the University of Maine, Micromeritics Corp., and at VTT Technical Research Centre of Finland with Micromeritics AutoPore IV 9500 and Pore Sizer 9310. Images were taken with a high resolution scanning electron microscopy (HR-SEM from JEOL) at TopAnalytica Oyj in Turku, Finland. Compacted powders were also tested for silicone oil (50 MPas) absorption at the University of Maine adopting their micro-probe test. The force–time response of a small drop of oil on a probe was analyzed as it contacted a surface. 3. Results
2.2. Powder compaction Compaction tests at room temperature were performed with a laboratory press (MTS Systems Corporation) and a mould normally used for preparing tablets for infra-red (IR) spectroscopic analysis. Some of these cold compacted powders were post heat treated for 3 min at 200 °C in order to evaluate possible changes in porosity and the role of polymer in bonding. Compaction at elevated temperatures was performed at Tampere University of Technology, Department of Material Science, with a PC-operated Instron and a heated chamber. Compaction behaviour was observed during the compaction tests from the movement of the punch and/or from the dimensions of the compacted powder Table 1 Experimental powders, their composition and preparation processes. All powders contained 100 parts per weight of CaCO3 and 10 parts per weight of polymer. Powder
Polymer
Process
1 2
10 parts semicrystalline polymer 1 0.5 parts semicrystalline polymer 1, 9.5 parts starch stab. SA latex A 0.5 parts semicrystalline polymer 1, 9.5 parts SA latex B 10 parts SB latex C 10 parts semicrystalline polymer 1 10 parts semicrystalline polymer 2 10 parts semicrystalline polymer 1 0.5 parts semicrystalline polymer 1, 9.5 parts SA latex B 0.5 parts semicrystalline polymer 1, 9.5 parts starch stab. SA latex A
Low solids mixing Low solids mixing
3 4 5 6 7 8 9
Low solids mixing Low solids mixing Pseudo fluidized bed mixing Pseudo fluidized bed mixing Preferential adsorption Preferential adsorption Preferential adsorption
Differences in the structure of the powder particles were observed in SEM images, indicating the role of the powder preparation method used. The final powder particle size was largely determined by the drying. Fig. 1 demonstrates hybrid particles of powders 4 and 7. Particles were porous, and the binder was evenly distributed covering some of the CaCO3 particles. 3.1. Compaction behaviour of powders The behaviour of powders under compaction is important, as this determines to a large extent the porous structure of the final coating, and thus also the mechanical and absorption properties. The relative change in porosity for cold compacted CaCO3 was small and porosity decreased almost linearly, indicating that the agglomerates in dry CaCO3 powders were easily broken (Fig. 2). The experimental powders were compacted already at low pressures mainly due to inter-granular packing. However, these granules were stronger than in the case of CaCO3 alone, and intra-granular deformations required therefore often high pressures. Powders 5 and 6 demonstrated less profound compaction due to the differences in the particle structure. The porosities of cold compacted samples were high, and an increased rate of compaction increased the final porosity further. Powders 1 and 7 with semicrystalline polymer provided the highest and powders 2 and 9 with latex A the lowest porosities at specific pressures. The particle size distribution was also wider for powder 9 than for powder 7. Elevated compaction temperatures led to decreased porosity as the granules became more deformable. However, the final porosity was often even lower than expected. This could be explained by additional
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Fig. 3. Porosity (%) as function of speed (100 vs. 1000 m/min) through a 0.02 m wide nip and maximum nip pressure (MPa) at room and elevated (N125 °C) temperatures. This model is based to predict the compaction behaviour of powder 7 and other findings from the compaction tests.
The first task was to define the approximation for cold compaction (powder 7) in a nip of which the first half is critical. Porosity (φ) of the powder layer was estimated to increase by at least 2.5% for each tenfold increase in the compaction rate (dP/dt in MPa/s). For cold compaction this resulted in Eq. (1) and for elevated temperatures (≥125 °C) in Eq. (2). Fig. 3 presents the porosities of samples (100 and 1000 m/min) and different pressures (P in MPa). This simple approach does not include the rheological behaviour of the polymer or that of the fibre substrate. It seems necessary to use elevated temperatures and several (long) nips as far as the coating porosity is concerned.
Fig. 1. Hybrid granules of powder 4 (A) and powder 7 (B) prepared using two of the processes. White indicates pigment and dark grey/black polymer, while light grey is resin used for sample preparation (porosity).
compaction of tablets during tablet removal. Secondly, the difference between the compaction temperatures of 125 and 150 °C was small. Compaction at elevated temperature was the most effective with powders containing semicrystalline polymer, while the powder with latex A demonstrated the smallest relative change compared to cold compaction. A theoretical porosity after a single hot nip was calculated. It was assumed that the width of the nip is 0.02 m, and that the pressure increase to the maximum in the middle of the nip can be considered to be linear. It was observed earlier that a tenfold increase in compaction rate resulted in a higher porosity. This was estimated to be the case also at much higher compaction rates. On the other hand, hot compaction decreased porosity. A step-wise change was assumed in compaction between 100 and 125 °C.
Fig. 2. Porosity (%) as function of compaction pressure (MPa) for cold compacted experimental powders and one CaCO3 reference (Setacarb® from Omya AG).
φ ¼ 1:09 ln ðdP=dt Þ−9:52 ln ðP Þ þ 79:43
ð1Þ
φ ¼ 0:9698 ln ðdP=dt Þ−8:47 ln ðP Þ þ 62:11
ð2Þ
3.2. Electrical properties of powders Charging of powder particles is essential for both transfer and attachment of powder particles on the surface to be coated. Charge to mass ratio is one of the key parameters. A powder composition consisting of 100 parts per weight of CaCO3 (dielectric constant ε of 6.1–8.5) and 10 parts of polymer (e.g. polystyrene with ε = 2.2) should have a dielectric constant of 5.1–6.8 based on Eq. (3), where γ is the volume fraction of embedded polymer, and p and pol refer to pigment and polymer, respectively [25]. In this paper dielectric constant (ε) refers to relative permittivity. For this equation to be valid the dielectric constant of embedded particles should be lower than that of the matrix. If
Fig. 4. Charge to mass (q/m) ratio as a function of particle diameter for both non-porous and porous (20%) powder particles (E = 300 kV/m) versus the operating window provided in the literature. Minimum q/m calculated also based on a loosely packed coating consisting of porous granules and additional 70% inter-granular porosity.
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this is then reapplied for particles with 20% air (ε = 1) embedded in the composite the dielectric constant of such porous particles would be 4.0–5.3. 1 εpol þ 2εp þ 2γ εpol −ε p A ε ¼ εp @ ε pol þ 2εp −γ ε pol −ε p 0
ð3Þ
Charge to mass ratio (q/m) can be calculated using Eq. (4), where E is the electric field, ε0 is the permittivity of free space (0.89 × 10−11 C/V m), d is the diameter and ρ is the density of powder particles [14]. Fig. 4 demonstrates the theoretical charge to mass ratio for powder particles. ε−1 6ε0 1 þ 2 E q εþ1 ¼ ρd m
ð4Þ
The dielectric constant of a highly porous coating layer (εc) can be estimated using Eq. (5), where ρc is the density of the porous coating and ρ is the density of the coating material itself [11]. This can then be used to calculate the minimum constant charge density required for a homogeneous powder coating layer (Eq. (6)). Example of such a limit close to 0.1–0.2 μC/g is included in Fig. 4 for a coating with porous particles and inter-particle porosity of 70%. Although the charge to mass ratio of charged particles is likely lower than calculated, the theoretical values are within the window stated to be optimal for powder transfer and attachment. lnε c ¼ q mmin
Table 3 Dielectric constant, i.e. relative permittivity (ɛ), for nonporous powders 1, 3 and 4 calculated from dielectric constants measured from powders/compacted tablets at 1 kHz and known porosities using Eq. (5). Sample
ε (at 1 kHz)
Porosity (%)
ε (non-porous powder)
Powder 1 Powder 3 Powder 4
1.8 3.2 and 4.2 2.7 and 3.7
41 and 57 53 and 38 48 and 36
2.6–3.7 10–12 6.9–7.7
material is exposed to the electric field and/or ion bombardment. In the case of high speed coating this will eventually be short. Therefore, the value of the dielectric constant at 0.5 kHz was of interest. The dielectric constant increased or remained more or less constant when frequency was decreased, which indicates more efficient charging at lower speeds. Table 3 shows the behaviour powders 1, 3 and 4 as examples. Semicrystalline polymer 1, as such, had ε of 0.72 and latex B 16. By using Eq. (3), the calculated dielectric constants are 4.6–6.3 for powder 1 and 7.9–9.9 for powder 3. The experimental powders can be charged and they should attach to the paper substrate surface. Small particles charge easier, while higher electric field intensity is required for large particles. Powder 3 was easy to charge, but due to lower resistivity this effect was lost faster. Powders with semicrystalline polymers had only slightly lower charging tendency but their resistivity was higher, leading to slow charge decay similar to commercial powders. 3.3. Thermal properties of powders
ρc ln ε ρ
ð5Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ε c ε0 g ¼ ρc d
ð6Þ
Proper electrical properties are essential for the transfer and attachment of powder particles on the paper surface. CaCO3 powder, a commercial dry ground calcium carbonate powder, and experimental powders 3 and 5–9 were tested (Table 2) for charge decay. CaCO3 powder and powder 3 had short charge decay times, while powders 8 and 9 had moderate values. For the commercial reference and powders 5–7, charge decay was too slow for direct measurements and values were estimated based on the initial charge decay. Semicrystalline polymer had longer charge decay than the lattices, while latex A provided slower decay than latex B. In the case of a fast process, such as paper coating, the time from the charging zone to the fusing nip is short and the rate of charge decay was probably slow enough to prevent powder detachment. In the initial resistivity measurements, CaCO3 had clearly the lowest volume resistivity (105–106 Ωm), while with powders 1, 2, 5 and 6 the resistivity was high, contributing to long charge decay. Powders 3 and 4 were between these two extremes. Dielectric constants were determined by measuring the capacitance and parallel resistance as a function of frequency. Charging efficiency depends also on the time the Table 2 Peak voltage relative to ground and charge decay for a commercial powder, CaCO3 powder, and powders 3 and 5–9. Sample
Average peak voltage (V) Time (s) to 10% of original voltage
Commercial powder Plain CaCO3 Powder 3 Powder 5 Powder 6 Powder 7 Powder 8 Powder 9
−960 −280 −1700 −1100 −1000 −1000 −730 −910
~80 000 0.31 4.6 ~60 000 ~40 000 ~35 000 140 1700–2000
Elevated temperature during compaction is a precondition for decreased porosity, cohesion of powder coating, and adhesion to the surface. Thermal analyses of powders as such turned out to be challenging. Latex A indicated no observable crystallinity in the case of powder 9 (Table 4). The increase in glass transition temperature, Tg, was small for the lattices and the corresponding powders 8 and 9 containing them. Semicrystalline polymer, as such, showed relatively high crystallinity. However, in powder 7 crystallinity was lower and Tg was higher indicating decreased mobility of polymer chains and increased bonding to CaCO3 surfaces. This also supports other findings concerning the polymer dependent behaviour of powders. The whole powder layer on paper has to achieve a temperature high enough for optimal cohesion and adhesion. This was approached theoretically. Specific heat (C) of a powder is the sum of the specific heats of its components based on their weight fractions (excluding air). This was approximately 1 J/g °C for a typical powder in this study. When the thermal conductivity of each component and the volume fractions are known, it is then possible to estimate the conductivity of a nonporous powder using Eq. (7) [26], where a is the orientation coefficient (0.5), γ is the volume fraction of polymer (0.213), Kp is the thermal conductivity of the pigment GCC (2.4–5.5 J/s m °C), and Kpol is the thermal conductivity of that of the polymer (for low density polyethylene 0.29–0.42 J/s m. C). Eq. (7) was reapplied to a porous coating based on the calculated thermal conductivities of a nonporous powder and that Table 4 DSC analyses (BASF) from polymers used in powders 7–9 and corresponding powders, where Tg, is the glass transition temperature, Tm the melting point, and ΔH is the change in enthalpy. 1. Cooling
2. Heating
Material
Tg (°C)
Tg (°C)
Tm (°C)
ΔH (J/g)
Semicr. polymer Powder 7 Latex B Powder 8 Latex A Powder 9
3 20 20 19 12 12
2 22 22 25 13 17
76 Ca. 74 83
38 0.5 0.4
80
0.7
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of air (0.026 J/s m °C). It was then possible to calculate the heat conductivity as a function of porosity in the powder layer. Decreasing the porosity from 70 to 40% increases its thermal conductivity from 0.07 to 0.12 J/s m °C. Thermal diffusivity (D) can be calculated using Eq. (8) where ρ is density [27]. 1
K¼
a ð1−γ ÞK P þ γK pol
D¼
þ ð1−aÞ
1−γ γ þ Kp K pol
!
K ρC
ð7Þ
ð8Þ
Simplifications had to be made to continue the calculations. The base paper is assumed to be of constant thickness in the nip and has a basis weight of 86 g/m2, density of 1040 kg/m3, specific heat of 2.89 J/g °C, and thermal diffusivity of 2.6 × 10−7 m2/s. There were 15 g/m2 of coating with initial and final porosities of 70 and 40%, respectively. The coating porosity and thermal diffusivity of coated paper were expected to be more or less linearly dependent on the position from the nip entrance to the centre of the nip. Thermal losses between the hot roll and the coating were excluded, and the maximum temperature was expected to occur at the nip exit. The thermal conductivity of the coated paper Kcpap was calculated using Eq. (9), where L is thickness, and c, pap and cpap refer to coating, base paper and coated paper, respectively [26]. This was then used to calculate the thermal diffusivity of the coated paper (Dcpap). The nip was divided into sections. A steady state solution of Fourier's law (Eq. (10) [28]) was used to estimate the temperature increase at a depth equal to coating thickness during the first section of the nip. This was then the input for the next section with different coating porosities and Dcpap. When there is a strong temperature gradient across the coated paper, i.e. hot (T1 = 200 °C) and cool (T2 = 25 °C) rolls, initial web temperature (T0) of 25 °C, nip length of 0.02 m and web speed of 1000 m/min, the temperature at the coating/paper interface would reach approximately 107 °C, using the given parameters. In the case the initial web temperature is increased to 55–60 °C, the interfacial temperature would reach approximately 125 °C. Reaching an adequate interfacial temperature can be a challenge at such speeds if using only one hot nip. K cpap ¼
Lcpap Lc Lpap þ K c K pap
T ðxÞ ¼ T 1 þ ðT 2 −T 1 Þ
ð9Þ
x Lcpap
−
∞ 2X π 1
! −n2 π2 D ! cpap t ðT 1 −T 0 Þ−ðT 2 −T 0 Þð−1Þn nπx 2 sin e Lcpap n Lcpap
ð10Þ
3.4. Cohesion and structure of compacted powders Deformation energy increased with increasing compaction pressure with all the powders, and post heat treatment increased further the cohesive strength of the compacted samples. However, the type of failure varied. Porous samples were deformable, and compacted powders with semicrystalline polymer and produced by pseudo fluidized bed were weak (Fig. 5). However, increased compaction pressure and/or post heat treatment increased specifically the brittleness of the compacted powders with semicrystalline polymers compared to the more deformable samples with latex. Compacted samples prepared using fast compaction rate indicated no obvious differences due to the post heating treatment, which indicates the importance of small porosity and interparticle distance prior to the thermal fusing step. Performing compaction at elevated temperatures increased further the force needed to deform and break the compacted powder samples,
Fig. 5. Force of rupture normalized to the thickness (N/mm) of compacted powder samples as function of porosity (%) for cold and hot compacted samples, cold compacted samples after heat treatment, and the references (plain CaCO3 and tablets prepared from aqueous coating colours).
indicating strong and brittle samples. Samples prepared by preferential adsorption were generally displayed more deformability prior to breakdown than the other samples. This is likely due to different polymer coverage and granular structure. Reference tablets from aqueous coatings were strong and brittle. Also compacted pure CaCO3 powder tablets were tested, but the cohesive strength was extremely low. Fig. 6 shows surface SEM images of cold and hot compacted powders. There were obvious differences between the powder particles due to the different processing. In the case of hybrid granules (such as powder 4) the initial increase in density was likely due to intergranular packing, while further deformation of smaller pores within the granules occurred at higher compaction pressures. In the case of powders prepared by pseudo fluidized bed mixing no similar granules could be identified. Powder 7 had some agglomerates, while with powders 8 and 9 these were already significantly deformed by cold compaction. There were indications of polymer softening and improved packing with polymer bonds between the pigment particles and even across the granular boundaries in the case of hot compacted powders. Hot compaction could also cause redistribution of polymer to the surfaces and thus create potential sites for adhesion between powder and the surface to be coated. However, this could not be confirmed in this case. Mercury porosimetry measurements were performed for compacted powders 3 and 5–9. The presence of small pores (30–200 nm) was clearly observed for the cold compacted powders, and powders indicated a small but detectable dual pore size distribution. The correlations between porosities calculated from tablet geometry and intrusion volumes were similar, indicating interconnected porosity accessible for mercury even after hot compaction. Powders 5 and 6 had more small pores, and powder 7 had a greater contribution of large pores than powders 8 and 9. Contribution of pores larger than 150 nm to the measured pore volume was negligible after hot compaction, as presented in Fig. 7, and the average size of the pores is similar to conventional paper coatings. Large inter-granular pores were typically affected already at lower compaction pressures.
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Fig. 6. Surface SEM images of powder 4 cold compacted at A) 8 MPa and B) 80 MPa, powder 5 compacted at 50 MPa and C) room temperature and D) 150 °C, and powder 7 compacted at 50 MPa and E) room temperature and F) 150 °C.
Silicone oil absorption was quite similar for the cold compacted powders 7–9. Absorption was fast (e.g. 5–7 μN/s) due to high porosity and large pores. Coated paperboard with less viscous oil (5 MPas) has provided absorption slower than 1 μN/s. However, hot compaction decreased absorption rate close to 1 μN/s. In addition, hot compacted powder 7 had slightly slower tack build up and absorption than powders 8 and 9. This could indicate different spreading behaviour or chemical interactions between the test liquid and hot compacted powder 7 as porosities and pore size distributions were quite similar between powders 7 and 9. The latex binders are amphiphilic in nature. Whereas the core of the latex particles consists of a nonpolar polymer, the surface is polar to provide compatibility with the aqueous medium. This polar surface also enables the interaction between binder and pigment. The hydrophobic core, on the other hand, will render some hydrophobicity to the coating layer, and control the interaction of the coating layer
with hydrophobic printing inks. It is also known that the latex can absorb oil and solvents from ink by swelling, which can then contribute slightly to the rate of oil and solvent absorption [29,30]. Swelling of polymer is typically affected by its chemical nature, density and molecular mobility. A limited molecular mobility due to crystallinity and the interactions with the pigment surface are the likely reasons for a low swelling tendency of the semicrystalline polymer. Anyhow, it can be concluded that the powder has to be fused using elevated temperatures to bring the absorption behaviour closer to that of conventional coatings. 4. Summary and conclusions Powder coating is an innovative coating process for paper. It could enable cost savings, quality improvements, and new coated paper
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References
Fig. 7. Pore volume divided into very large (N250 nm), large (150–250 nm), medium (100–150 nm) and small (b100 nm) pores based on mercury intrusion into powders compacted at 50 MPa and 25/150 °C.
grades. However, this requires development of highly filled powders, and there have to be easy to use methods supporting the development work. The goal of this study was to test the feasibility of using highly filled powders for paper coatings prior to actual electrostatic deposition and coating trials. Powders with a similar polymer to CaCO3 ratio than in conventional paper coatings were characterized. Particle size and morphology were largely affected by the preparation process. Powder particles were typically granules with internal porosity and evenly distributed polymer. Powders containing semicrystalline polymer as binder turned out to be the most interesting. Charging of the tested designs of powder particles was ranked, and confirmed to fall into a region possible for transfer and attachment. However, formulation, especially the polymer, affected the charging and discharging behaviour of the particles. Some powders had decay times typical for commercial polymeric powders, while some latex containing powders lost their charge only slightly slower than plain CaCO3. Compaction at elevated temperature was essential to decrease porosity and to improve cohesion. Porosity and average pore size were obtained similar to those in conventional paper coatings depending on the compaction process, particle structure, size distribution and composition. At low pressures, the increase in density was mainly due to the inter-granular packing, while at high pressures the compaction was due to the intra-granular deformations. Similar cohesive properties as in conventional paper coatings were achieved after compaction at elevated temperatures, and semicrystalline polymers indicated both interactions with pigment particles and response to heat. However, no unambiguous binder migration favourable for adhesion between powder and paper could be observed. Fast coating processes typical for the paper industry may, therefore, require the use of paper surface pretreatment techniques, and the application of several hot nips in order to optimize coating porosity and to provide suitable conditions for adhesion. There are only few studies about such highly filled powders for electrostatic coating of paper, their composition and key properties. Although the approach has been quite different, some of the results are in line with those obtained from other studies (e.g. Putkisto and Maijala [2,3]) dealing with powder coating of paper. Acknowledgements This paper is based on the Dry Paper Coating project carried out at KCL during 2001–2003. The author thanks the collaborating companies (BASF SE and Omya International AG), the financing companies of KCL, the Finnish Funding Agency for Technology and Innovation (TEKES) and all the project members. The production of the powder samples and some of the analyses were carried out by Omya International AG and BASF SE.
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Auriault, Thermal conductivity of handsheets, papers and model coating layers, 12th Fundamental Research Symposium, Fundamental Research Society, Bury 2001, pp. 927–945. [27] K. Niskanen, S. Simula, Thermal diffusivity of paper, Nord. Pulp Pap. Res. J. 14 (1999) 236–242. [28] K.E. Duckett, J. Cain, Finite element methods applied to thermal calendaring, Nonwoven Conference Proceedings, TAPPI Press, Atlanta 1991, pp. 383–391. [29] R.L. Van Gilder, R.D. Purfeerst, Latex binder modification to reduce coating pick on six-color offset presses, Tappi J. 77 (1994) 230–239. [30] Y. Xiang, D. Desjumaux, D. Bousfield, M.F. Forbes, The relationship between coating layer composition, Ink setting rate, and offset print gloss, Pan-Pacific and International Printing and Graphic Arts Conference Proceedings, Quebec, Canada 1998, p. 85. Dr. Mika Vähä-Nissi has over 20 years in research and development. Mika received his M.Sc. in 1994 and D.Sc. in 1998, both from Tampere University of Technology in Finland. In 2000 he moved to Oy Keskuslaboratorio – Centrallaboratorium Ab (KCL) – a research company owned by the Finnish forest industry. Since 2010 Mika has been working at VTT Technical Research Centre of Finland. He is attributed with roughly 120 scientific papers, presentations and other publications.