ARTICLE pubs.acs.org/IECR
Structure-Surface Property Relationships of Kraft Papers: Implication on Impregnation with Phenol-Formaldehyde Resin Andreia B. Figueiredo,† Dmitry V. Evtuguin,*,† Jorge Monteiro,‡ Elvira F. Cardoso,§ Pedro C. Mena,§ and Paulo Cruz|| †
CICECO, Chemistry Department, and ‡I3N, Physics Department, University of Aveiro, Campus de Santiago, P-3810-193 Aveiro, Portugal § SONAE Industria de Revestimentos, Lugar do Espido, Via Norte, P-4470-177 Maia, Portugal || Euroresinas—Industrias Químicas, SA, Plataforma Industrial de Sines, Lote Industrial I, 7520-195 Sines, Portugal
bS Supporting Information ABSTRACT: The structural features of four different kraft papers were related to their surface properties and to the response upon industrial impregnation with phenol-formaldehyde (PF) resin. The chemical composition and the structure of paper were suggested to be important factors determining the interaction with PF resin, which was assessed by contact angle measurements and surface energy analysis. The presence of fatty matter (extractives) and inorganic fillers together with structural anisotropy of paper confers the difference in affinity of the face and backside of papers toward PF resin. This affects the resin distribution in impregnated precured kraft papers as revealed by microfluorescence spectroscopic analysis of transversal cuts.
’ INTRODUCTION High pressure laminates (HPL) are widely used on furniture work tops and floorings, being applied on horizontal and vertical surfaces that require high resistance to wear, abrasion, and stains. Laminates offer a great decorative effect, presenting surfaces with different textures, patterns, colors, and designs, without neglecting the durability, easy maintenance, and low cost compared to other materials.1-3 Conventionally, decorative laminates consist of layers of fibrous cellulosic material impregnated with resins, joined together under heat and high pressure until the resins are cured. These layers are essentially made up of three distinct phases (Figure 1): (i) a core layer consisting of an assembly of kraft paper sheets impregnated with a phenolic resin; (ii) a printed or patterned layer designated as the decorative paper treated with a melamine resin that is responsible for the attractive appearance of the laminate; (iii) a wear surface layer consisting of a translucent overlay sheet also saturated with a melamine resin, providing additional resistance to abrasion, water absorption, and moisture.4,5 Additionally, a barrier paper can be introduced between the kraft and decorative sheets to increase the opacity of the laminate. The impregnation process is a first step in the HPL production (Figure 2) when raw papers after unwinding are immersed in a bath with the corresponding resin thus promoting an intimate mixture of resin with paper sheet. Due to surface tension, the paper becomes soaked with resin, which spreads through the porous structure of the paper while the excess of resin is mechanically removed from the web by the action of squeezing rolls. After impregnation, the paper is sent to a set of dryers to evaporate the volatile compounds present in the resin in a range of 6-8%. Depending on the type of paper and resin in use, the sheet can also be subjected to a second bath, designed as coating, r 2011 American Chemical Society
Figure 1. Typical assembly of high pressure laminate.
to ensure good impregnation. The paper is then cooled by a set of rollers with internal circulation of cold water and cut to the desired size or rewound.6-12 Impregnated sheets are then laid in the sequence presented in Figure 1 with the desired number of plies and submitted to a hot pressing. The range of pressures used in the production of HPL varies from 5.5 to 10.3 MPa, with the layers consolidated at temperatures of 120-150 °C. During the pressing cycle, resin flows and cures between layers thus creating cross-linkings and consolidating the final laminate. After resin polymerization is complete and several individual impregnated sheets are transformed into a monolithic laminate, a cooling cycle occurs while the press is opened. The market share of laminates has been growing over the years. The versatility and durability of HPL make them an excellent product in the construction and furnishing industries that are becoming increasingly demanding. Therefore the producers of laminates need a Received: September 16, 2010 Accepted: January 6, 2011 Revised: December 10, 2010 Published: January 27, 2011 2883
dx.doi.org/10.1021/ie101912h | Ind. Eng. Chem. Res. 2011, 50, 2883–2890
Industrial & Engineering Chemistry Research
ARTICLE
Figure 2. Schematic representation of the impregnation process of raw paper with resin.
quick response in providing more perfect products and to overcome the accompanying technical problems. Typically, the performance of laminates and appearance of defects in the laminates (whitish blemishes, stains, surface irregularities, etc.) are associated primarily with the selection and influence of technological variables.11,12 However, according the industrial experience the quality of an obtained laminate is rather vulnerable to the raw paper used in its production. Thus, a change in the paper supplier affects the impregnation with resins, the drying of the impregnated paper, and the quality of the finished laminate. This prompted systematic research on the relationships between the structure of ingredient papers and their performance in the production of laminates. In this work a series of nonimpregnated kraft paper sheets supplied by four different producers was characterized for chemical composition and basic physical properties. These paper characteristics were correlated with the surface energies of papers, their interaction with phenol-formaldehyde resin, and their profiles of resin distribution along paper thickness, assessed by microfluorescence spectroscopy of transversal paper cuts.
’ MATERIALS AND METHODS Materials. Sheets of raw kraft papers (160 g/m2) from four different paper producers in North America and Europe (papers A, B, C, and D) which are in use by SONAE Industria de Revestimentos, were conditioned at 20 °C (60% relative humidity) for 48 h. Papers A and B were industrially impregnated with conventional phenol-formaldehyde (PF) resin and precured at Euroresinas—Industrias Químicas, SA. The main characteristics of PF resin were as follows: solids, 62.8%; pH 8.04; viscosity, 24 s. Chemical Analyses. The determination of moisture content in papers was made following the oven-drying method ISO 638:2008, and the ash content was determined by complete incineration of pieces of paper at 525 ( 25 °C during 3 h in a muffle furnace according to norm NP 3192. Extractives were removed by submitting kraft paper samples to successive Soxhlet extractions during 4 h with acetone, and their content was quantified after evaporation (TAPPI Method T204 om-88). The carboxyl content of pulp was assessed according to TAPPI Method T237 om-93. Neutral sugar analysis of papers was carried out after its Saeman hydrolysis by gas chromatography as alditol acetates.13,14 Physical Properties and Penetration Tests. The tensile strength of papers was determined according to norm NP EN ISO 1924-2, and tear resistance was determined following the norm NP EN 21 974. The determination of water absorption (capillarity) was carried out by the known Klemm method, and air permeation was measured by the Gurley method. Penetration dynamic analysis (PDA) was carried out on an Emtec Surface & Sizing Tester EST12.2. Paper samples of 75 mm 50 mm were attached by backside to a sample holder using a
Table 1. Surface Tension and Corresponding Polar and Dispersive Components of the Standard Liquids Used in the Determination of the Surface Energy of Kraft Papers
liquid
surface tension,
polar component,
dispersive component,
γl (mJ/m2)
γpl (mJ/m2)
γdl (mJ/m2)
water
72.8
51.0
21.8
formamide
58.0
20.4
37.6
diiodomethane
50.8
2.3
48.5
strip of double-sided adhesive tape and a rubber roller. The measuring cell was immersed in water, and the PDA curve was averaged from three experiments. Contact Angle Measurements. Contact angles were measured in a surface energy evaluation system (Advex Instruments) using the sessile drop method and water, formamide, and diiodomethane as probe liquids, whose corresponding total surface energy and dispersive and polar component values are listed in Table 1. The acquisition time of the images of the drops was 0.1 s. The evaluations of the surface energy of papers (γs) and its corresponding polar (γps ) and dispersive (γds ) components were carried out using the series of liquid probes based on the Owens-Wendt-Rable-Kaeble (OWRK) model:15 γs ¼ γds þγps 1þcos θ γl qffiffiffiffiffi ¼ 2 γdl
qffiffiffiffiffi p γs
sffiffiffiffiffi p qffiffiffiffiffi γl þ γds γdl
ð1Þ ð2Þ
where γl, γpl , and γdl represent the liquids' superficial tension and the corresponding polar and dispersive components, respectively. Plotting (1 þ cos θ)/2 vs (γpl /γdl )1/2 allows the calculation of the parameters γds and γps .16,17 The contact angle that phenolic resin establishes in contact with nonimpregnated core sheet (face and backside) was also evaluated. For each sample, at least 10 drops of each liquid were obtained, measured at different sites of the paper. The contact angles (θmeasured) were also corrected (θcorrected) for the surface topography using the Wenzel correction:18,19 cos θcorrected ¼ ðSdr þ1Þ cos θmeasured
ð3Þ
where Sdr is the developed interfacial ratio expressed as the proportion of additional surface area contributed by the texture compared to an ideal plane the size of the measurement region.20 This parameter Sdr was obtained from three-dimensional surface texture analysis using laser profilometry. Surface Texture Analysis. Three-dimensional surface texture parameters were obtained by microtopographic analysis using an Altisurf 500 profilometer based on white light chromatic aberration and PaperMap software. The scanning area was 2884
dx.doi.org/10.1021/ie101912h |Ind. Eng. Chem. Res. 2011, 50, 2883–2890
Industrial & Engineering Chemistry Research
ARTICLE
Table 2. Chemical Composition of Kraft Papers (160 g/m2) carboxylic acids paper
bulk (cm3/g)
ash (%)
extractives (%)
(mequiv/100)
A
1.50
0.85
1.85
11.4
B
1.39
3.03
0.28
6.2
C
1.48
12.40
0.66
8.3
D
1.43
1.06
3.20
15.2
Table 3. Results of Neutral Sugar Analysis in Kraft Papers paper Rha (%) Fuc (%) Ara (%) Xyl (%) Man (%) Gal (%) Glu (%) A B C D
0.5 0.4 1.5 0.8
0.1 0.0 0.0 0.0
0.7 0.4 0.0 0.4
12.6 5.1 5.6 17.9
4.0 9.3 8.9 2.8
0.7 0.6 9.6 0.7
81.6 84.2 74.5 77.5
4.8 4.8 mm2 with a resolution of 2 μm. For each sample, four acquisitions were made. Microfluorescence and Scanning Electron Microscopies. Fluorescence microscopy was used to examine the distribution of phenol-formaldehyde resin in bulk of kraft paper. Samples from suppliers A and B were impregnated with phenolic resin at Euroresinas, and pieces of 10 5 mm were then mounted on a glass holder, edge uppermost, in order to study resin penetration along the paper thickness (transversal direction). Fluorescence spectra were obtained on the surface of each side of paper (face and back) and in the center (paper bulk), using a Carl Zeiss microscope (Axioskop 40) with a 100 objective, employing a 532 nm laser (BENTUS) providing 500 mW at 100% power. With this arrangement, a spectrum was recorded employing a CCD camera (Hamamatsu, cooled at -10 °C, with a resolution of 2068 512 and a grating of 300) with a spectral resolution of 0.128 nm/pixel. Each spectrum was acquired on an ARC SpectraPro 300i spectrometer with an integration time of 1 s and averaging two full scans to obtain the final spectrum. Each acquisition was corrected for the camera background. Scanning electron microscopic (SEM) images were obtained on an FEG-SEM Hitachi S4100 microscope coupled with an energy dispersive spectrometer (EDS) and operating at 25 kV, using Au-Pd coated samples.
’ RESULTS AND DISCUSSION Chemical Composition and Physical Properties of Raw Kraft Papers. A set of kraft core papers used for HPL produc-
tion was supplied by four different producers (defined as A, B, C, and D) and chemically characterized in order to evaluate eventual structure-property relationships upon impregnation with PF resin. The results of the chemical analysis of papers are presented in Tables 2 and 3. Papers A and D revealed rather high amounts of carboxyl groups with simultaneous high proportions of xylose in neutral sugar analysis. This indicates that these kraft papers are composed basically of hardwood fibers since a major noncellulosic polysaccharide of hardwood is glucuronoxylan (the main xylan backbone is ramified with 4-O-methyl-R-D-glucopyranosyluronic acid residues).21 In contrast, papers B and C contained almost 3 times fewer carboxylic groups than A and D papers and showed rather significant amounts of mannose and galactose in sugar analysis. Consequently, these papers were made from softwood fibers which possess predominantly galactoglucomannan as a
major hemicellulose.21 At the same time papers A and D contained almost 10 times more extractives (fatty matter) and much less inorganic filler when compared to papers B and C (Table 2). Since kraft papers usually do not contain sizing reagents, the high amounts of extractives in papers A and D may be explained by their natural abundance in pulp. Usually, extractive content in pulp is relatively low (
