The large amount of different and highly specialized products in the paper and paper processing industry need tools to ensure a constant product quality. From teabags, tissue paper, beyond photo-copying paper to heavy cardboards, there is a huge variety. One thing in common is the need for a quick analysis technique in order to determine the physical parameters like thickness, grammage, wet extension and moisture content.

Determination of Absolute Moisture in Paper

Each porous material like wood, textile, leather and paper contains water in the form of vapor in larger pores and in the form of liquid in the narrow capillaries of the structure. The absolute moisture content is expressed as the percentage of water in the paper related to the mass of the material. Naturally the absolute moisture is dependent on the surrounding climate conditions such that equilibrium between the moisture content from the paper and the humidity of the environment is achieved. Under norm climate for papers this value varies between 1 to 7% in different papers.

The typical reference analysis is a Karl-Fisher titration, where the sample is heated up to 105°C and the use of special reagents is required. The preparation of this titration plus the measurement time takes about 30 minutes and exceeds the measurement time of approximately 20 to 30 seconds for a NIR spectrum by far. The absolute moisture of papers could be correlated with their respective NIR-spectra, records with an MPA spectrometer independent whether the NIR measurement was made from up-side or down-side of the paper.

Determination of Paper Grammage and Thickness

As a rule of thumb it is said that the ratio between grammage and thickness of paper is close to one. Nevertheless the mass for a surface unit of measure from paper or cardboard is to be determined by an exact testing procedure. It is designated as basic weight or grammage and expressed in g/m². This value is significant for the specification of a kind of paper (< 300 g/m²) or cardboard (ca. 600 g/m²). The paper was analyzed with an MPA FT-NIR spectrometer using the integrating sphere.

The thickness of paper or cardboard is decisive for the further conditioning, especially the choice of coating and furthermore printing method. It is a typical number to specify the paper type. Conventionally the thickness can be determined by a special testing procedure with a precision micrometer. The thickness values acquired from conventional methods could be correlated well with the NIR spectra from the same papers.

Determination of Ash from Paper

The ash content is a conventional expression for the residue on ignition from a solid sample according to certain testing procedures. As a result an empirical number will indicate a clue but not a quantitative measure for the quantity of mineral salts and other inorganic substances in the sample because it will be subject to not exactly defined thermal decomposition and volatility. Both phenomenons are dependent among other things on the temperature and duration of ignition, the overall atmosphere and of course on the composition of the residue itself. NIR spectroscopy has proven to be a good method for the analysis of ash content in various types of paper.

On-line Monitoring of the Silicone Coating Process

The FT-NIR spectrometer MATRIX-F duplex, developed for contactless measurements, can be used for monitoring the grammage of silicone coatings directly at the coating machine.  The NIR spectrometer MATRIX-F for contactless measurements.  The coating machine works at a speed of 600 m/min and every ten seconds the MATRIX-F carries out a measurement. In the first step of the analysis the basic paper is identified and dependently on this result the quantification model is chosen to determine the grammage of the silicone coating. The results are transferred to the control system to adjust the silicone addition.