METHOD TO PREDICT AND/OR CONTROL THE STRENGTH PROPERTIES OF A FOIL-LIKE MATERIAL

Abstract: A method to predict and/or control the strength properties of a foil-like material in relation to machinery and processes for manufacturing this material. The measurements are performed locally on the foil-like material and with a high spatial resolution - from 20 mm and less - to obtain a mean value and variation regarding at least one local mechanical property of the foil-like material, whereby the strength value or control signal worked out from the measurement results obtained is used to achieve the strength prediction and/or the process and quality control desired.

Description: Method to predict and/or control the strength properties of a foil-like material The present invention relates to a method to predict and/or control the strength properties of a foil-like material in relation to machinery and processes for manufacturing this material. It especially relates to properties with regard to the tensile strength and tensile strength index (tensile index) of the foil-like material in question. The invention is based on the use of and access to statistical information regarding local mechanical properties, especially local tensile stiffness and local tensile stiffness index. When the foil-like material is paper, a specially interesting characteristic of the invention is that it makes it possible to achieve a suitable trade off between the tensile strength of the paper and the variation in other important measured properties of paper. Unfortunately, it is not always possible to simultaneously achieve the sought after minimum variations or the desired levels of key parameters, such as local tensile stiffness and local basis weight. For example, the tensile strength is not always greatest when variations in basis weight are minimised. In practice, this means that a compromise must be made. In many processes for manufacturing paper, it would thus be of greatest importance if the mechanical strength of the paper could be frequently predicted to allow rapid control measures to be taken and also to make it possible to achieve the desired quality in the final product. Today, the tensile strength of paper is predicted by means of models that are based on information regarding the mean value of modules of elasticity that are measured in different directions in the material. These modules of elasticity are increasingly often estimated on the basis of measurements that give the phase velocity of ultrasound in the material. Predicting tensile strength based on information about the spatial variations in basis weight of paper, such as the formation number, have also been tested in practice but have had only limited success. The lack of success regarding the desired prediction is not surprising in the light of the discoveries on which the present invention is based. The spatial resolution, which is normally 10 cm for the ultrasound measurements that are carried out on moving paper webs, is not sufficient in relation to the resolution that, according to the invention, would have been required. It has been shown that a spatial resolution in the order of millimetres gives a very good result. In addition, theoretical calculations and measurements show that the relation between the measured wave velocity for ultrasound and the module of elasticity of the material in question can vary when the process conditions change, such as variations in the beating of the pulp; see Fig. 1. It should be seen that the variation shown in Fig. 1 is significant in connection with control applications of the type referred to. When one attempts to use data that give variations in local basis weight to achieve the desired prediction, one encounters the problem that the local basis weight does not always reflect the important local mechanical properties. In accordance with the principles of the invention it now becomes possible to realize a method of the type mentioned in the introduction by the measurements being performed locally on the foil-like material and with a high spatial resolution - from 20 mm and less - to obtain a mean value and variation regarding at least one local mechanical property of the foil-like material, whereby the strength value or control signal worked out from the measurement results obtained is used to achieve the strength prediction and/or the process and quality control desired. Different advantageous embodiments of the new method are evident from the non-independent claims 2-8. In this way, the said measurements can be of the direct and/or indirect measurement type. In particular, an adjustment of the strength value and/or control signal is made as a result of structural differences in the foil-like material based on local measurements of at least one further mechanical property, preferably bending stiffness. The measurements are preferably carried out on a mm-scale with a high frequency of repetition in relation to the dynamics of the process. The foil-like material comprises paper especially. At least one of the local mechanical properties is measured in what is per se a known manner, especially by means of the arrangement described in US 5 361 638. The invention will now be described in more detail below with reference to the enclosed drawings wherein: Fig. 1 shows in diagram form the relation between a standard test and an ultrasound test on paper at different indices of tensional stiffness and beating levels for pulp; Fig. 2 shows in diagram form the relation between mean values of index of tensional stiffness and index of tensional strength for different levels of beating; Fig. 3 shows in diagram form the relation between measured index of tensional strength and, according to the invention, the predicted index of tensional strength for different levels of beating; Fig. 4 shows in diagram form the relation between strength quotient and stiffness quotient at varying conditions of orientation (81 samples, 3 forming units and 3 different pulps). The new observation on which the present invention is based is that successful prediction of the strength properties of paper, for example tensile strength, requires the use of data for both the mean value and the statistical variations in the local tensile stiffness (Its) and/or the local tensile stiffness index (ltsi) measured with a spatial resolution at the millimetre level. The statistical variations can be expressed by the measured maximum and minimum values, the standard deviation σ, the coefficient of variation CV or a number of other parameters that can be obtained from the statistical frequency function for the measured local mechanical properties. (Information regarding the local variations in tensile stiffness index can be obtained from the local tensile stiffness data if the local basis weights have also been measured at the same locations). Until very recently no measurement method has been available that has been able to provide the type of local data that are required for the said prediction. Fortunately, it has now become possible to obtain the required data by utilising a new arrangement that is described in US 5 361 638. As already mentioned, the type of predictive model that we have found to be very useful is based on information about both mean values and statistical variations in local tensile stiffness and/or local tensile stiffness index measured on a millimetre scale. In accordance with the principles of the invention, the mean value m of the tensile stiffness and the tensile stiffness index firstly give a value of the upper limit of the tensile strength that can be achieved. This upper limit can only be achieved if the paper has a completely even structure, i.e. if it lacks disturbing local variations. In accordance with the principles of the invention, variations in local tensile stiffness, secondly, give rise to a spatially variable strain in the paper that will reduce the tensile strength of the paper to below the said upper limit. There are significant variations in local tensile stiffness measured on a millimetre scale in commercially manufactured paper. To obtain a good predictive result, it is therefore very important to take account of these local variations, whose amplitude varies with varying conditions of production. The relation between measured mean values of tensile stiffness and tensile strength will vary significantly with, for example, beating level (beating of the pulp) if the local variations are not taken into account, see Fig. 2. Results of prediction experiments that are based on data obtained by means of the arrangement described in US 5 361 638 are shown in Fig. 3. The contents of the said US patent, which in this way contributes to the realisation of the present invention in an ingenious manner, are therefore intended to be included by reference in the present patent matter. By use of a prediction model according to the invention and according to what is evident in detail from the following equation, a very good prediction (r 2 = 0.997) is obtained. This result has been obtained under significantly variable process conditions, in this case varying beating levels (degree of beating). Tensile strength = f(m, σ) ltsi = const.* m K5 /(l + σ /m) It is known that structural differences in the sheet as a result of the varying degree of orientation (MD/CD ratio; where MD is the direction of the machinery and CD is the transverse direction) as well as varying tension (restraining force) during the drying of the paper affect the relation between measured tensile stiffness and tensile strength. With the aid of the arrangement described in US 5 361 638, data can be obtained that allow compensation for changes as a result of structural differences. Examples of data that can be obtained with the measurement method are local stiffness in different directions (MD, CD and ZD) in the paper, i.e. tensile stiffness and bending stiffness in MD and CD as well as compressional stiffness in the thickness direction ZD. The result that we have obtained when evaluating experimental data shows that when orientation increases (increasing MD/CD ratio), the tensile strength in MD in relation to the tensile strength in CD will increase somewhat more than the tensile stiffness in MD in relation to the tensile stiffness in CD; see Fig. 4. With knowledge of this relationship and the actual tensile stiffness in the said directions, the influence of a varying MD/CD relationship can be compensated by the introduction of a function g of the stiffness in MD and CD, which gives: Tensile strength index = f* g(stiffness-MD, stiffness-CD) It is widely known that the tensile stiffness increases to a larger extent than the tensile strength with increased drying tension. At the same time, the relation between stiffnesses measured in different directions in the paper changes. Thus, with the present invention, even this effect can be compensated for, in this case by the introduction of a function h of the stiffnesses in MD, CD and ZD, i.e. in the machine, transverse and thickness directions. When determining the tensile strength of paper with different levels of local variations in stiffness, varying MD/CD ratios and different tensions during drying the following predictive function is utilised: Tensile strength index = f * g *h(stiffness-MD, stiffness-CD, stiffness-ZD) The present invention can also lead to an improved prediction of the tensile strength of a material on the basis of indirect measurements. In these cases, the method is complemented by utilising a relation between data from a number of other local measurements and the local tensile stiffness and/or the local tensile stiffness index, which are required to obtain a better prediction. This naturally applies if a relation can be confirmed at the actual process conditions. Examples of indirect measurements that may be used (separately or in combination) to generate data for this type of prediction are local basis weight, local optical density and local thickness of the paper. Finally, it is probable that other strength properties of paper, for example tear strength and compression strength, are similarly dependent on variations in the local mechanical properties, such as the local stiffness (in the plane and/or the thickness direction) and the local rigidity to bending, i.e. properties that are measured with a high spatial resolution.

Claims: Claims 1. Method to predict and/or control the strength properties of a foil-like material in relation to machinery and processes for manufacturing this material, characterized in that measurements are performed locally on the foil-like material and with a high spatial resolution - from 20 mm and less - to obtain a mean value and variation regarding at least one local mechanical property of the foil-like material, whereby the strength value or control signal worked out from the measurement results obtained is used to achieve the strength prediction and/or the process and quality control desired. 2. Method according to claim 1, characterized in that the said measurements are direct measurements. 3. Method according to claim 1, characterized in that the said measurements are indirect measurements . 4. Method according to claim 1, characterized in that the said measurements are both direct and indirect measurements. 5. Method according to any of claims 1-4, characterized in that an adjustment of the strength value and/or control signal is made as a result of structural differences in the foil-like material based on local measurements of at least one further mechanical property, preferably rigidity to bending. 6. Method according to any of the previous claims, characterized in that measurements are carried out on a mm-scale with a high frequency of repetition in relation to the dynamics of the process. 7. Method according to any of the previous claims, characterized in that the foil- like material comprises paper. Method according to any of the claims 1 , 2 and 4-7, characterized in that least one of the local mechanical properties is measured in what is per se a known manner by means of the arrangement described in US 5361638.

Inventors: ANTILLA JORMA, PETTERSSON THORULF, ANTILLA, JORMA, PETTERSSON, THORULF
Applicants: ANTILLA JORMA, PETTERSSON THORULF, SVENSKA TRAEFORSKNINGSINST, ANTILLA, JORMA, PETTERSSON, THORULF, STFI
Application year: 1999
Main classification: G01N33
IPC codes: G01N3, G01N33