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Evaluating the Surface Energy of Laboratory-Made Paper Sheets by Contact Angle Measurements

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Evaluating the Surface Energy of Laboratory-Made Paper Sheets by Contact Angle Measurements PEER-REVIEWED HANDSHEETS Evaluating the surface energy of laboratory-made paper sheets by contact angle measurements ISABEL MOUTINHO, MARGARIDA FIGUEIREDO AND PAULO FERREIRA ABSTRACT: Contact angle is one of the most useful tools to easily assess the surface energy of paper sheets. However, the results obtained should be treated with some caution, since these meas- urements are significantly influenced by the properties of the liquids used and of the paper surface. Our study compares the values of the contact angle and of the surface energy of laboratory hand- sheets produced with demineralized water and with white water (collected from the paper mill). In addition, and to assess the influence of paper roughness on the contact angle measurements, some 3-D topographical parameters were computed by perfilometry. Complementary measurements were also performed with commercial paper samples. The results clearly demonstrate that the kind of water used in the sheet-making process has a more pronounced influence on the surface energy of the paper sheets than whether the paper sheets were made in a laboratory sheet former or in an industrial paper machine. Application: This study clarifies factors that have influence on the practical determination of the surface energy of paper sheets by contact angle measurements. The results show the influence of the water used in the sheet-making process on the surface energy of paper sheets and are specially use- ful for experiments carried out in the laboratory. or many paper grades, particularly equations for the calculation of surface measurements obtained in laboratory Fprinting and writing papers, the energy components [3-5].Nonetheless, handsheets prepared with the same physical and chemical surface charac- the measurement of the contact angle pulp furnish, but using two different teristics are of utmost importance depends on many factors related to the kinds of water: demineralized water (as because of their influence on ink- paper structure and to the properties of is common practice in laboratory) and paper interactions, namely wetting, the liquids that are used.Although some white water collected from a paper mill adhesion, and absorption [1,2]. relevant studies can be found in the lit- circulation water system.We also tested Therefore, it is crucial to assess the erature, many related issues remain to commercial paper sheets produced surface characteristics and to relate be studied. with the same type of pulp.To separate them with these parameters. One of The motivating factor for our study the influence of the water from that of the most common procedures for was the fact that previous studies ob- the paper surface roughness on the sur- doing that involves measuring the tained opposite results for the relation- face energy,complementary perfilome- contact angle between the sample and ship between the polar and the disper- try tests were performed. some specific liquids. From the sive components of the surface free The final objective of the study was results, we can evaluate the potential energy computed from contact angle to evaluate how paper surface energy is of inks to spread or penetrate.We can measurements in commercial printing affected by the water used in the pro- also obtain the total paper surface and writing papers and in laboratorial duction of laboratory paper sheets.This energy and the corresponding polar paper sheets [6]. Both kinds of papers is an important matter since the advent and dispersive contributions. were produced with the same pulp fur- of new water-based inks has increased Contact angle methods, including nish, the only difference being the research in paper surface chemistry, the sessile drop method,are considered process of sheet formation and the with most of the experiments being the most convenient for determining water that was used. carried out at laboratory scale. In this paper sheet surface energy.They are In this work we compare and dis- scenario,it is essential to guarantee that fast, simple, and based on easy-to-use cuss the results of the contact angle both the physical and the chemical sur- 26 TAPPI JOURNAL JUNE 2007 HANDSHEETS face characteristics of laboratory paper sheets used in this re- produced in laboratory with those produced in industry,we search are as close as possible to those of the commercial also studied a sample of a commercial paper, corresponding paper sheets. exactly to the same pulp furnish of the laboratory handsheets (CP-1).However,due to process restrictions in the paper mill, EXPERIMENTAL it was only possible to collect paper samples with surface siz- Sample preparations ing.For this reason and because the laboratory handsheets did Handsheets were made in laboratory, according to the ISO not have any surface treatment, a commercial paper having 5269/1 standard, using a commercial Eucalyptus globulus- no surface treatment was analyzed (CP-2).This paper was also based kraft pulp bleached with an elemental chlorine-free produced with an E.globulus-based kraft pulp,bleached with (ECF) sequence.This pulp was collected at the headbox sam- an ECF sequence, and with identical additives. pling point of a paper mill. It therefore included all the fur- nish additives: filler (precipitated calcium carbonate [PCC]), Contact angle measurements cationic starch, retention aids, and internal sizing agents. The contact angle of a liquid over a solid surface reaches First,the handsheets were produced using demineralized equilibrium,as described by the well-known Young equation water (HS-1).Next,and to better simulate the prevailing con- [3,9,10]: ditions at the paper mill, white water collected from the paper machine was used.However,the high amount of solids (1) in the resulting fibrous suspension prevented the water from ␴ ␴ draining from the sheet former,making it virtually impossible where, s is the solid surface free energy, sl is the solid/liq- ␴ to obtain handsheets.For this reason,a second and more suc- uid interfacial free energy, l the liquid surface tension, and cessful attempt of producing handsheets was made after a ␪ the contact angle formed between the liquid drop and the previous settling of the white water overnight, so that only solid surface. On the other hand, the solid/liquid interfacial the supernatant was used (HS-2).At least five handsheets of free energy can be expressed by Dupre’s equation [3,5,9,11]: each kind (HS-1 and HS-2), with 90±5 g/m2 ovendry (o.d.) basis weight, were produced and conditioned at 23ºC ± 1ºC and 50% ± 2% relative humidity. (2) Depending on the raw materials used, the pulping and bleaching process, the washing efficiency, the paper type where Wa is the work of liquid adhesion.This work of adhe- produced, and all other production conditions, the white sion results from two types of intermolecular forces:the van water usually contains many dispersed and dissolved materi- der Waals dispersive forces, always present, and the nondis- al.This includes fillers and process chemicals,such as surface persive or polar forces, which include the dipole forces and active agents, polymers from the bleaching chemicals and the acid-base forces,present solely between polar molecules the sizing and retention aids, ions from inorganic salts, and [3,11,12]. Owens and Wendt [13] and Kaelble and Uy [14] other dispersed minerals and particles [7].Therefore,to mon- proposed that both the dispersive and the polar interactions itor the white water system, the measurement of several pa- between the two phases could be approximated by a geo- rameters is usually recommended [8]. In this study,both the metric mean expression, and therefore Eq. (2) becomes [3]: demineralized water and the white water supernatant were analyzed in terms of pH, conductivity, and calcium ion con- (3) centration (by atomic absorption spectrophotometry).The results are presented in Table I.The analysis of the settled In this equation the superscripts d and p stand respectively solids by Fourier transform infrared (FTIR) spectroscopy for the dispersive and polar components of the liquid surface confirmed that the white water contains a very high per- tension and of the solid surface energy, considering that centage of PCC. [3,10,12]: To compare the contact angle values of the paper sheets (4) Parameter Demineralized water White water Combining Eqs. (1) and (3) gives: pH 7.3 7.7 Conductivity (µS/cm) 202 820 Ca2+ content (mg/L) 7.97 36.7 (5) I. Properties of the demineralized water and of the white water. VOL. 6: NO. 6 TAPPI JOURNAL 27 HANDSHEETS Liquid Surface tension Polar Dispersive ␴ d /␴ (%) ␴ s s l (mN/m) component component ␴ p ␴ d l (mN/m) l (mN/m) Water 72.8 48.1 24.7 33.9 Water/Ethylene glycol 57.7 39.1 28.5 49.4 Formamide 58.1 25.8 32.3 55.6 Ethylene glycol 48.3 17.4 30.9 64.0 Propylene glycol 35.4 9.0 26.4 74.6 II. Surface tension and the corresponding polar and dispersive component of the liquid probes used. Thus, by measuring on the same solid surface the contact and maximum valley depth (Sv, ␮m), both measured rela- angle of different liquids whose surface tension components tively to the mean plane; and ␪ ␴ ␴ d 1/2 ␴ p are known, we can plot (1+cos )/2•( l/( l ) ) versus ( l / • interfacial area ratio (Sdr), which indicates the com- ␴ d 1/2 l ) and fit the points to a straight line in order to calculate plexity of the curvilinear surface compared with the support ␴ p ␴ d s and s , respectively from the slope and from the inter- surface [16]. section with the vertical axis [3], and then derive the total solid surface free energy (Eq. 4). RESULTS AND DISCUSSION In this work, the contact angles were measured with a Table III summarizes the results obtained for the different DataPhysics OCA20 using the sessile drop method [3].
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