METHOD FOR CONTINUOUS DETERMINATION OF THE PROPERTIES OF A FLOW OF WOOD FIBRES FOR FABRICATION OF FIBREBOARD

Abstract: In a method for continuous determination of the properties of a wood fibre flow (2) in fibreboard fabrication, a sample of the fibre flow is illuminated with light, and one or more of the properties fibre length distribution (20, 22), resin content (14), fibre moisture (16) and optical properties (18) for the sample, determined from the measured ensuing reflectance spectrum, is/are determined from predetermined relations between measured spectral values and absolute magnitudes for said properties.

Description: METHOD FOR CONTINUOUS DETERMINATION OF THE PROPERTIES OF A FLOW OF WOOD FIBRES FOR FABRICATION OF FIBREBOARD The present invention relates to a method for continuous determination of the properties of a flow of wood fibres for fabrication of fibreboard. Properties such as fibre moisture, resin content, optical characteristics and fibre characteristics, such as shives and fibre length distribution, are generally known to affect the quality properties of the finished fibreboard. Appropriate and, in particular, stable fibre moisture facilitates e.g. checks on the pressing phase of panel fabrication and creates conditions for good control of the density profile of panels. After the panel is pressed, the resin added in panel fabrication hardens and creates, with the fibre network, a strong fibre composite structure. The resin is added either by injection into the blower line after the defibering process or into a mechanical resin mixer after the drying phase. The resin content has a major impact on the finished panel's strength properties, such as bending strength and tensile strength. The resin content is often governed by the specific properties the finished panel is to have. However, an excessive amount of resin is often dispensed to ensure the desired level of quality, as there is no way to accurately control other factors, such as fibre moisture, fibre length distribution and density, factors which also affect the properties of the finished fibreboard. To date, the way in which fibre properties (such as the fibre length distribution and shives) should be specified for the fibre network to be optimal in a- chieving the desired properties for the finished fibreboard has been unclear. One reason for this is because of the previous unavailability of any simple and, not the least, rapid measurement method for determining and characterising fibre length distribution for MDF (medium density fibreboard) fibres. However, equipment is now available for determining and characterising fibre properties with the aid of image analysis. However, this is a complex technique, which is only practical in a laboratory environment and definitely unsuitable for on-line measurement in the fabrication of MDF. Since no facilities have hitherto been available for continuous on-line measurement of parameters for fibre characterisation during fibreboard fabrication, studying the way in which fibre properties should be devised in order to manufac- ture finished panels with the desired properties in an optimal fashion has been very difficult. This also applies to efforts to achieve optimal control of the defibering process so the desired fibre properties are attained. The need for resin can be minimised and production costs reduced when fibre properties are controlled. Resin accounts for about one-third of the direct cost of fibreboard fabrication. Resin coating, i.e. the distribution of resin on the wood fibres, is also affected by the distribution of fibre lengths. Fine fibre fractions require more resin than thicker fibres. As a result, increasing resin content does not produce the anticipated increase in the strength of the finished panel when the fibre length distribution of the fibres used is inappropriate. Multivariate analysis of spectral data for determining wood chip components, such as Klason lignin, extract and the total amount of carbohydrates, is described in "Dialog Information Services," File 248, PIRA, Dialog accession No. 00393878/5, PIRA accession No. 20017450, Meder R. et al.: "Prediction of wood chip and pulp and paper properties via multivariate analysis of spectral data," Melbourne, Australia, 2-6 May 1994, pp. 479-484. Here, Principal Component Analysis (PCA) and Principal Component Regression (PCR) were used for analysing near-infrared spectra (NIR), Fourier-transformed infrared (FTIR) spectra and nuclear magnetic resonance (NMR) spectra taken from examined wood chips. WO 97/04299 describes multivariate data analysis of near-infrared (NIR) spectra taken from raw materials, such as sawdust, shavings and wood chips, used for manufacturing chip board. Use of the measured values for controlling panel fabrication is also described. The objective of the present invention is to refine this technique, based on multivariate analysis of spectral data, so it can also be used for continuous determination of the properties of wood fibres for fibreboard fabrication, said properties being of decisive importance to the properties of the finished panel. This objective is achieved with a method according to claim 1. Continuous determination of fibre length distribution according to the method according to the invention greatly enhances the ability to control the fabrication process and optimise the cost of that process, primarily by always dispensing the exact right amount of resin required. In addition, continuous feedback from the measured fibre length distribution to the defibering process also becomes possible for controlling this process. On-line quality control of fibre properties, some- thing hitherto impossible in the fabrication of fibres for fibreboard, is achieved in this way. In addition, determination of the quantity of resin, in the flow of resin- coated fibres, from the reflectance spectrum supplies continuous information on both the total amount of resin dispensed, a previously per se possible determina- tion, and on the distribution of resin on fibres. The moisture content of fibres in the fibre flow, which can be obtained from the reflectance spectrum with the method according to the invention, is an important parameter in the fabrication of fibre- board. In addition, the brightness/colour of the fibre flow can be continuously determined. The brightness/colour of the fibre flow is also a measure of the effect of thermal pre-treatment on the chips or fibres and additionally supplies information on the brightness/colour of the finished product. According to one advantageous embodiment of the method according to the invention, light spectra in the 400-2500 nm wavelength range are used. Primarily, two physical processes make the study of this wavelength range especially suitable for determining said quality properties of the fibre flow, viz. energy absorption and light scattering. Near-infrared (NIR) spectroscopy is based on electromagnetic radiation in the 700 to 2500 nm wavelength range. Mainly organic substances containing C-H, O-H and N-H bonds, which absorb the radiation, lie within this range. The energy mainly excites harmonics and combinations of rotat- ing and vibrating states. Organic material absorbs less energy in the NIR range than in the UV and IR range. As a result, near-infrared light penetrates more deeply into the sample. A non-homogenous material, such as wood fibres, induces a scattering of light related to the size of the particles. This property, plus molecular vibrations and rotations, make it possible to characterise the size distribution of the fibres in the fibre flow, i.e. to determine the fibre length distribution. According to additional advantageous embodiments of the method according to the invention, a reference distribution of fibre length is established for calibration samples of the fibre flow. The same calibration samples are also used for determining fibre length distribution from the measured reflectance spectrum, and these lengths are modelled in a calibration procedure against said reference fibre length distribution with the aid of multivariate statistical regression methods for determining said predetermined relations between measured spectral values and fibre length distribution. The reference fibre length distributioTi is suitably determined by means of a standardised method. In an analogous way, the predeter- mined relations between measured spectral values and reference values for resin content, fibre moisture and optical properties are determined, the reference value for resin content then being appropriately established by determination of the nitrogen content of nitrogenous resin by means of the Kjeldahl method, the refer- ence value for fibre moisture being determined by drying and weighing the calibration samples, and the reference value for optical properties being determined according to the ISO Brightness norm. According to another advantageous embodiment of the method according to the invention, one or more of the parameters derived from the reflectance spec- trum, i.e. fibre length distribution, resin content, fibre moisture and optical properties, is/are used in a feedback procedure for on-line control of the fibre preparation process in order to impart the desired properties to the wood fibres used. This makes possible continuous, overriding quality control of the said parameters. The invention will now be explained in greater detail, referring to the at- tached sole fig. 1 , which illustrates an example of the method's implementation according to the invention. The figure depicts mechanic or pneumatic transport 2 of a flow of wood fibres intended for use in fibreboard fabrication. The wood fibre is suitably prepared in a refiner process with a specific beating energy less than 500 kWh/h. At a mea- suring point, situated after the defibering and drying process, the flow of fibres is illuminated with light of an appropriate wavelength, and the ensuing reflectance spectrum is picked up by a detector 4 in the 400-2500 nm wavelength range, i.e. in the visible (VIS) and near-infrared (NIR) range. The detected spectrum is A/D converted at 6. A typical conformation for the resulting spectrum is shown at 8 in the figure The calibration required in the method according to the invention is performed at 10 in the figure in the following manner. A number of calibration samples of fibre flow are prepared according to a test plan in which the level of the test variables is selected to produce a good spread in the values for properties of interest, viz. fibre length distribution, resin content, fibre moisture and optical properties. A calibration sample can encompass an operating period in full-scale production. Reflectance spectra of the aforementioned kind are recorded for the calibration samples, and reference values are determined, using an appropriate labo- ratory method, for each of the aforesaid properties. Thus, fibre length distribution (and shives) can be determined with e.g. image analysis. Resin content can advantageously be determined with the Kjeldahl method in instances in which a nitrogenous resin is used. If e.g. phenol resin, which does not contain nitrogen, is used, some other appropriate method must be employed for determining reference values for resin. Reference value for fibre moisture can be established using a drying and weighing procedure, and optical properties can be determined according to the ISO Brightness norm for determining light reflection. In the calculation unit 12 in the figure, mathematical correlations can be established between spectra recorded for calibration samples of fibre flow and the corresponding reference values with the aid of multivariate statistical regression methods, preferably Principal Component Analysis (PCA) and Projection to Latent Structures (PLS) regression. This type of calculation is well-known, cf. e.g. Martens H. and Naes T., "Multivariate Calibration," Wiley & Sons, New York (1989); Wold S., Johansson E. and Cocchi M., "Partial Least Squares Projections to Latent Structures, in QSAR in drug design, "Theory, Methods and Application," Ku- bini H., Ed., (1993); Hόskuldsson A., "PLS Regression Methods," Journal of Che- mometrics, vol. 2 (1988); Geladi P. and Kowalski B.R., Analytica Chimica Acta, 185 (1986) and Wold S., Esbensen K., and Geladi P., Principal Component Analy- sis, Chemometrics and Intelligent Laboratory Systems, 2 (1987). This creates a calibration model or a relation between the reference values and associated spectra, determined with appropriate laboratory methods, (quality variables (y) as a function of the spectrum (x) y = f(x)), the calibration procedure being carried out on a number of fibre flow samples with relatively good variation in the said properties or parameter values, as noted above. With the aid of relations established in this way, said parameters can be obtained from subsequently recorded reflectance spectra for the fibre flow. This achieves contactless, on-line measurement of fibre length distribution at 20, 22, resin content at 14, fibre moisture at 16 and optical properties at 18 in the figure. Thus, the four cited properties are determined in one and the same measurement procedure from a spectrum's information content. The moisture content of wood fibre is normally 5-20%, often 10-20%. The length of fibres used in fabricating fibreboard is typically 0.15-4 mm for spruce fibres, and the upper fibre length limit for wood with shorter fibres, such as beech, is about 3 mm. In the method according to the invention, fibre length distribution can be determined at a relatively large number of intervals for the lengths range in question. In the 0-7 mm length range, the method according to the invention can determine fibre length distribution in intervals as small as 0.1 mm, i.e. said length range is subdivided into 70 intervals or fractions, cf. the distribution at 22 in the figure. The three parameters fibre length distribution, resin content and fibre moisture are of major importance for controlling quality and optimising the cost of fibreboard fabrication, as discussed above. Optical properties are also of interest, as the lightest possible board is normally desirable. Thus, this continuous, on-line measurement according to the method according to the invention makes possible feedback and on-line quality control of fibre properties, something hitherto impossible in the fabrication of fibreboard.

Claims: CLAIMS 1. A method for continuous determination of the properties of a wood fibre flow for fibreboard fabrication, characterized in that a fibre flow sample is illuminated with light, and, from measured ensuring reflectance spectrum, one or more of the properties fibre length distribution, resin content, fibre moisture and optical properties for the fibre flow sample is/are determined from predetermined relations between measured spectral values and the absolute magnitudes of said properties. 2. The method according to claim 1 , characterized in that light spectra in the 400-2500 nm wavelength range are used. 3. The method according to claims 1 or 2, characterized in that a reference fibre length distribution is determined for calibration samples of the fibre flow, and a fibre length distribution, determined from the reflectance spectrum measured for the same calibration samples, is modelled in a calibration procedure against said reference fibre length distribution with the aid of multivariate statistical regression methods for determining said predetermined relations between measured spectral values and fibre length distribution. 4. The method according to any of the previous claims, characterized in that a reference fibre moisture is determined for calibration samples of the fibre flow, and reference fibre moisture, determined from the reflectance spectrum measured for the same calibration samples, is correlated in a calibration procedure against said reference fibre moisture with the aid of multivariate statistical regression methods for determining said predetermined relations between measured spectral values and fibre moisture. 5. The method according to any of the previous claims, characterized, in that a reference resin content is determined for calibration samples of the fibre flow, and spectral values, determined from the reflectance spectrum measured for the same calibration samples, are correlated in a calibration procedure against said reference resin content with the aid of multivariate statistical regression meth- ods for determining said predetermined relations between measured spectral values and resin content. 6. The method according to any of the previous claims, characterized in that reference values for optical characteristics are determined for calibration samples of the fibre flow, and spectral values, determined from the reflectance spectrum measured for the same calibration sample, are correlated in a calibration procedure against said optical reference values with the aid of multivariate statistical regression methods for determining said relations between spectral values and optical properties. 7. The method according to any of claims 3-6, characterized in that calibration samples are prepared according to a test plan with varying levels for test variables selected to produce a good spread of fibre length distribution, fibre moisture, resin content and optical properties in the different calibration samples. 8. The method according to any of the previous claims, characterized in that one or more of the parameters fibre length distribution, resin content, fibre moisture and optical properties, determined from the reflectance spectrum, is/are used in a feedback procedure for on-line control of the fibre preparation process in order to impart the desired properties to the wood fibre used. 9. The method according to claim 8, whereby the fibre is produced in a refining process, characterized in that one or more of the parameters fibre length distribution, resin content, fibre moisture and optical properties is/are fed back to control of the defibering process.

Inventors: BACKLUND ULRIKA, CARLSSON JOHAN, LINDGREN THORE, BACKLUND, ULRIKA, CARLSSON, JOHAN, LINDGREN, THORE
Applicants: BACKLUND ULRIKA, CARLSSON JOHAN, LINDGREN THORE, VALMET FIBERTECH AB, BACKLUND, ULRIKA, CARLSSON, JOHAN, LINDGREN, THORE
Application year: 2001
Main classification: G01N21
IPC codes: G01N21, G01N33