Characterization of mineral paper by air-coupled ultrasonic spectroscopy T.E. Gómez Álvarez-Arenas ⇑ , D.A. Soto Ultrasound for Medical and Industrial Applications (UMEDIA) Research Group, Centre for Physical Technologies, CSIC, Serrano 144, 28006 Madrid, Spain article info Article history: Received 29 November 2011 Received in revised form 15 February 2012 Accepted 16 February 2012 Available online 24 February 2012 Keywords: Mineral paper Air-coupled ultrasound Thickness resonances Plate resonances abstract A novel technology in the paper industry makes possible to produce paper by using a mineral powder and a polymer instead of cellulose fibers. This new product is called mineral paper, it presents some potential environmental advantages compared with conventional paper, while it exhibit a similar appearance and properties. The purpose of this work is to determine the possibilities of an air-coupled ultrasonic tech- nique using wide band signals and spectral analysis to study this kind of materials. As no direct contact nor coupling fluids between the paper and the transducers is required, this technique is specially well suited to this problem. It also offers good perspectives for the development of a on-line quality control system. A through transmission technique (0.15–2.3 MHz) is employed and Fourier analysis is performed to obtain both magnitude and phase spectra of the transmission coefficient. Properties in the thickness direction as well as in the paper plane has been determined by the excitation and analysis of thickness and plate resonances at several incident angles and different directions within the paper plane. Different paper grades (from 140 to 480 g/m 2 ) have been studied. Very high attenuation coefficients and very low propagation velocities (and hence elastic constant) have been obtained for most cases, this can be explained by considering the large porosity of this material (up to 50%) and the microstructure: a mixture of solid grains with a resin with a relatively large fraction of air-filled pores. Measurements show that unlike conventional cellulose machine made paper this material is transversely isotropic (isotropic in the paper plane) and that the degree of anisotropy (when in-plane directions are compared with the thickness direction) largely depends on the level of resin impregnation. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction A recent technology has been introduced into the paper indus- try that makes possible to produce paper materials by using min- eral powders instead of cellulose fibers. Therefore, this material is more environmentally friendly compared with normal paper: it is not necessary to use trees to produce it, water use is reduced as well as bleaching and it can also be easily recycled. The purpose of this work is to study this kind of materials by using air-coupled ultrasonic spectroscopy. The final goal will be to set the basis to make possible the development of an ultrasonic non-contact sys- tem useful for the characterization and/or analysis of these mate- rials (non-destructive analysis, quality control, etc.). Different ultrasonic techniques have been applied in the past to the study of conventional cellulose paper. A good review of these early developments is presented in Ref. [1]. First investigations were done in the 1960s by Craver and Taylor [2] and Jackson and Gavelin [3], who measured longitudinal and shear wave velocities in the plane of the paper using low frequencies and a point-contact technique. Later (1971), Luukkala et al. [4] presented a non-contact technique using air-coupled ultrasound generated by a dc biased electrostatic transducers at frequencies between 60 and 450 kHz to measure the propagation velocity of Lamb waves, so elastic con- stants in the plane of the paper could be obtained. This technique was later used and extended to consider the orthotropic anisotropy of the paper by Habeger et al. [5]. In this case, permanently polar- ized dielectric films were used as the active medium in the trans- ducers. In 1999, Khuory et al. measured the A 0 mode with the same technique (with some theoretical improvements in the analysis of the data), but in this case piezoelectric transducers and a frequency range between 200 and 900 kHz were employed, making it possi- ble to study thinner samples [6]. Paper properties in the paper plane directions have also been investigated using other contact techniques [7–10]. The first ultrasonic measurements of paper properties in the thickness direction were presented in 1979 by Mann [1] using direct contact (with a vacuum grease coupling agent) board sam- ples and frequencies between 1 and 5 MHz. However the modifica- tion of the paper properties due to the percolation of the coupling agent can be a serious problem for some types of paper. An im- proved system, including pressure control, thickness measurement and dry coupling was later presented by Habeger and Wink [11], frequency range was 0.5–2.0 MHz. In this case, they also obtained 0041-624X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2012.02.012 ⇑ Corresponding author. E-mail address: tgomez@ia.cetef.csic.es (T.E. Gómez Álvarez-Arenas). Ultrasonics 52 (2012) 794–801 Contents lists available at SciVerse ScienceDirect Ultrasonics journal homepage: www.elsevier.com/locate/ultras