CA2051977A1 - Method for measuring the fibre orientation anisotropy in a fibrous structure - Google Patents

Abstract

ABSTRACT

A method and a device for the non destructive measurement of the fibre orientation anisotropy in a fibrous structure. The method relies on the difference of refractive indices for light beams polarized in two perpendicular directions to measure the value of the anisotropy on the surface of a fibrous structure. Because the use of an infrared light, with a wavelength in the 9 to 11 µm spectral range, the penetration depth of the light is of the order of a few tenths of microns and the fibrous structure shows a large birefringence directly associated with the fibre orientation anisotropy. This birefringence can be measured as a difference in the refractive indices of the fibrous structure in two different directions. A light beam produced with an infrared light source, such as a CO2 laser, is modulated by a chopper, linearly polarized and sent to a photoelastic modulator which varies the state of polarization of the light at a given frequency. After directing the modulated light towards the surface of the fibrous structure, the light is reflected and polarized again. The intensity of the second harmonic and the total intensity of the reflected polarized beam is measured according to a plurality of angles of incidence. These angles are obtained by the rotation of the sample platform parallel to the plane of incidence. The angles of incidence at the minimum ratio between the intensity of the second harmonic and the total intensity (Brewster's angle) are calculated for two different orientations and the fibre orientation anisotropy is determined by subtracting the two Brewster's angles: the higher is the difference between the Brewster's angles, the higher is the fibre orientation anisotropy. The average refractive index is obtained from the mean value between the Brewster's angles.

Description

20~1977 TITLE OF THE INVENTION

Method for measuring the fibre orientation anisotropy in a fibrous structure FIELD OF THE INVENTION

The present invention is concerned with a method for determining the fibre orientation anisotropy in a fibrous structure, such as paper and textile. More particularly, the invention is concerned with the non destructive measurement of fibre orientation anisotropy of a surface of a fibrous structure and with a device for carrying out this method.

DESCRIPTION OF PRIOR ART

Several techniques have been proposed so far to determine the distribution of fibre orientations in paper. Some of the more common are: image analysis, ultrasound propagation, zero-span tensile strength ratio, X-ray diffraction, light diffraction and light diffusion. Recent techniques are the microwave attenuation technique, the holography interferometry and the far infrared or submillimeter attenuation technique.
The microwave and far infrared attenuation techniques are both non destructive and rely on the dichroism of a fibrous structure: the absorption is stronger with the light polarized in the direction of maximum fibre alignment.
The far infrared approach is much faster and offers high spacial resolution. However, the microwave approach is a better known technique with cheaper sources. Both are unable to distinguish the top and wire side and only provide an averaged value over the thickness of the structure.
In the far infrared technique, the wavelength normally used is 70 ~m which is of the same order as the thickness of many papers. This makes it impracticable for surface studies.
Going to much shorter wavelengths like near infrared brings about serious problems with diffusion of light and does not allow straight forward analysis of the data.
Only three techniques can provide a surface measurement. Those techniques are the insertion of coloured fibres in the fibrous structure, the tear test and the diffusion of a vlsible laser beam on the surface of the fibrous structure. However, these techniques have many drawbacks.
The insertion of coloured fibres in the fibrous structure re~uires the making of special samples and is very tedious because the fibres must be counted one by one. These samples are also no longer good for retail.
The tear test is a destructive method and is very random. A great precision cannot be obtained with this technique.
Finally, the diffusion of a visible laser beam on the surface of the fibrous structure requires the use of an elaborated data processing in order to get the pertinent information. The measuring of the diffusion is also dependent upon the quality of the surface. Such a device must be able to detect weak signals since the diffusion of a visible light on the surface of a fibrous structure is not very strong.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for the non destructive measurement of fibre orientation anisotropy on the surface of a fibrous structure by measurement of the refractive indices of light beams polarized in two perpendicular directions. Since the absorption and scattering properties of a fibrous structure, such as paper, is such that the refractive index cannot be determined from measurement of the angle of minimum reflectivity for a light beam polarized parallel to the plane of incidence (Brewster's angle), the method had to be modified.
The present invention uses an ellipsometry technique to measure complex refractive indices of thin films or bulk materials. The ellipsometry technique is based on the change 20~1977 of the polarization state of light as it is reflected from a surface.
The use of an infrared wavelength makes surface probing possible. Because of the presence of a cellulose absorption band, the penetration depth is of the order of a few tenths of microns and the fibrous structure shows a large birefringence directly associated with the fibre orientation anisotropy. This birefringence can be measured as a difference in the refractive indices of the fibrous structure in two directions 90~ apart, preferably the machine direction (MD~, which is the direction of manufacturing, and the cross-direction (CD) which is perpendicular to the machine-direction.
In accordance with a preferred embodiment of the invention, the invention may provide a method for measurement of a fibre orientation anisotropy in a fibrous structure, comprising the steps of:
(a) producing a light beam with an infrared light source, (b) producing a modulated interruption of the light beam at a given frequency, so a lock-in amplifier, referred to as the DC lock-in, can measure the total intensity of said light beam, (c) polarizing linearly the infrared light beam in a first polarization plane, (d) producing a modulated phase retardation of the polarization of the polarized light beam produced in step (c) before it reaches the fibre structure, (e) guiding the modulated beam produced in step (d) towards the surface of the fibrous structure in a plane of incidence parallel to the first polarization plane in order to have the beam reflected by the surface, (f) polarizing linearly the reflected beam in a second polarization plane, (g) measuring the intensity of a second harmonic and total intensity of the pGlarized reflected beam at different angles of incidence of the modulated beam on the 20~1~77 fibrous structure in the plane of incidence parallel to the first polarization plane, (h) calculating the angle of incidence at the minimum ratio between th~ intensity of the second harmonic and the total intensity measured in step (g), the angle being hereinafter called "first Brewster's angle", (i) changing the direction of the fibrous structure by turning the fibrous structure 90~, (j) measuring again the intensity of the second harmonic and the total intensity of the polarized reflected beam at different angles of incidence, in that other direction of the fibrous structure, (k) calculating the angle of incidence at the minimum ratio between the intensity of the second harmonic and the total intensity maasured in step (j), the angle being hereinafter called "second Brewster's angle", (l) calculating an average value between the first and the second Brewster's angles, the average value being hereinafter called "average refractive index", and (m) determining the fibre orientation anisotropy by mathematically comparing the first and the second Brewster's angles.
Preferably, the first and the second Brewster's angles are subtracted, the higher is the difference between the Brewster's angles, the higher is the fibre orientation anisotropy.
According to a preferred embodiment, the infrared light is produced with a C02 laser. The wavelength of the infrared light is preferably between 9 and 11 ~m and the phase retardation is preferably between 0 and 3 radians. The power level of the light may be kept below 100 mWatt before it reaches the fibrous structure.
The Brewster's angles values may be calculated by a least square fitting routine. In this case, it is possible to calculate the Brewster's angle in the machine direction of the fibrous structure and in its crosæ-direction.

2 0 ~ 7 Advantageously, the first and the second polarization planes are so apart.
This method may be applied to a fibrous structure which comprises natural. cellulose, such as paper This method also proposes a device for measuring the fibre orientation ln a fibrous structure, the device comprising:
ta) an infrared light source which produces a light beam, (b) a chopper which produces a modulated interruptlon of said liqht beam at a given frequency, so a lock-in amplifier, referred to as the DC lock-in, can measure the total intensity of said light beam~
(c) a linear polarizer for linearly polarizing the modulated light beam in a first polarization plane before it reaches the fibrous structure, (d) a photQelastic modulator which creates a modulated phase retardation of the polarized light beam, ~e) a rotating sample platform to which the fibre structure, on which the modulated beam is directed and then reflected, is rigidly attached, said rotating sample platform being provided to vary the angle of incidence of said modulated beam on the fibre structure in a plane parallel to said first polarization plane, (f) a detector arm comprising:
- means to follow the reflected beam, - a linear polarizer to polarize said reflected beam in a second polarization plane, - a detector connected to ~eans to measure the intensity of a second harmonic and total intensity of said polarized reflected beam;
(g) means to memorize said intensity of the second harmonic and total intensity of said reflected beam as a function of the angle of incidence, (h) means to calculate, for two fibrous structure directions 90 apart, the anqle of incidence at the 2 ~ 7 7 minimum ratio between the intensity of the second harmonic and the total intensity measured with said detector on said detector arm, said angles being hereinafter called "first and second Brewster's angles", (i) means to calculate an average value between ~aid first and said second Brewster's angles, said average value being hereinafter called "average refractive index", and (j) means to calculate the fibre orientation anisotropy with Brewster's angles.
According to a preferred embodiment, the device is further comprising a chopper to modulate the light beam before it reaches the fibrous structure. The device may also comprise a beam splitter and a reference detector to check for light source fluctuations.
Means to control the size of a probed area, such as an iris, may be provided.
Preferably, the means to follow the reflected beam is con rolled by a comp~ter. A computer may also provide a means to calculate the Brewster's angles. Moreover, the computer may perform a least square fitting routine to yield the Brewster's angle from the data.
Another embodiment is that the device is further comprising a lens located on the detector arm in order to concentrate the reflected polarized beam on the detector.
The ellipsometer technique using the Brewster's angles is not the only technique that can be used to measure the complex refractive indices. For example, the perpendicular incidence ellipsometry technique can be used instead.
A non restrictive description of a preferred embodiment of the invention will be given with reference to the appended drawings.

2~51~77 BRIEF DESCRIPTION OF THE DRAW:CNGS
Figure 1 is a schematic representation of a device according to the invention.
Figure 2 is a schematic representation of the electronic detection and control system.
Figure 3 is a graph showing the topography profile of a paper sample along a line.
Figure 4 is a graph showing the reflectivity of paper measured on a FTIR spectrometer.
Figure 5 is a graph showing the approximate penetration dep~h of infrared radiation in the paper sample.
Figure 6 is a graph showing the high anisotropy side of a non homogeneous sample oriented in the MD (0) and the CD
t90) .
Figure 7 is a graph showing the low anisotropy side of a non homogeneous sample oriented in the MD (0) and the CD
(90) .
Figure 8 is a graph showing side 1 of an homogeneous sample oriented in the MD (0) and the CD (90).
Figure g is a graph showing side 2 of an homogeneous sample oriented in the MD (0) and the CD (90).

DESCRIPTION OF TH~ PREPERR~D RMBO~IHFNT

~he device according to the invention as shown in figure 1 comprises an infrared light source 1 which produces a light beam 3. By example, use can be made of a CO2 infrared laser. This laser may produce a light beam with a wavelength preferably between 9 and 11 ~m, such as 10.6 ~m which has shown the best results in the initial trials.
If desired, mirrors 5 may be provided to deviate the light beam 3 in order to ~duce the size of the device.
The device has a chopper 7 which modulates the light beam at a given frequency, before it reaches the fibrous structure 23, so a lock-in amplifier, referred to as the DC
lock-in, can measure the total intenslty of the light beam. The frequency is preferably 3 KHz.

~051977 The device may further comprise a beam splitter 9 and a reference detector 11 to check for light source fluctuations. Such fluctuations may lead to a lack of precision. It may also be provided with an attenuator 13 in order to maintain the power of the light beam 3 below 100 mWatt before it reaches the fibrous structure 23 and thus prevent damaging the sample or the other components of the device.
The light beam 3 is polarized linearly by the polarizer 15. The polarized beam 17 is then guided to a photoelastic modulator (PEM) 19 which purpose is to create a modulated phase retardation of the polarized beam 17 at a given frequency. The retardation is a modification of the state of polarization, more specifically a transformation from linear to elliptical polarization. This retardation may be between 0 and 3 radians, that value being the maximum limit of the retardation. The frequency of the modulated phase retardation is preferably 37 KHz.
The modulated beam 21 is guided ~o the fibrous structure 23 fixed on the sample platform with an angle of incidence e which depends on the position of the rotating sample platform 25. The plane of incidence must be parallel to the first polarization plane.
The modulated beam 21 is reflected onto the surface 24 of the fibrous structure 23 and thus produces a reflected beam 27.
Since the light beam 21 reaching the surface 24 has a modulated phase retardation, the reflected beam 27 can be Fourier analyzed to yield parameters proportional to the standard ellipsometric parameters. This Fourier analysis is accomplished by electronic detection of the total intensity tVw) and the intensity of the second harmonic (Vz~) of a repolarized reflected beam 35. This detection is taking place in a detector arm 29 which is having means to follow the reflected beam 27 since its orientation depends on the angle of incidence e, thus of the angular position of the rotating sample platform 25. The total and the second harmonic intensities are measured preferably with a lock-in amplifier 20~77 tuned at the frequency of the chopper 7, referred to as the DC
lock-in 41, and a second lock-in amplifier tuned at a frequency of 2w, referred to as the 2w lock-in 43, respectively (Fig. 2).
The means to follow the reflected beam is preferably controlled S by a computer via a GPIB bus (Fig. 2) The terms V~ and V~ are related to a reflection property that can be expressed by the Bessel function:
V1~ r/// r = - 4 Jz (~) cos V~ 1 + (r~ / r 1)2 where J2 iS the second Bessel's function, ~ is the maximum phase retardation, ~ is the phase difference between polarization and reflection, r~/ and r 1 are respectively the amplitudes for parallel and perpendicular polarization. When the above equation s plotted versus the angle of incidence e, the curve has a narrow minimum at Brewster's angle.
A means to control the size of the probed area, such as iris 31, may be used to concentrate the measures on a specific region of the surface 24. The reflected beam is repolarized in a second polarization plane, preferably 9o apart from the first polarization plane. The polarized reflected beam 35 is guided onto the detector 37. Preferably, the beam 35 passes through a lens 39 which concentrates it on a specific point on the detector.
The intensities V~ and VL~ are measured for a fibrous structure direction according to a plurality of angles of incidence e. Preferably, the values are directly transmitted to a computer 45 via the GP~B bus 47 (Fig. 2). The computer 45 then calculates the first Brewster's angle, preferably with a square fitting routine. If wanted, the computer 45 may print the curve of the ratio between the intensities versus the angle of incidence.
After, the fibrous structure is turned 90, by hand or automatically with the rotating sample platform. When completed, the intensities V~ and Vz~ are measured again but 20~1~77 ~or that other direction. A second Brewster's angle is calculated after. An average value of the Brewster's angles is giving the average refractive index and the difference between the first and the second Brewster's angles is giving a value proportional to the fibre orientation anisotropy, wherein the higher is the difference bPtween the Brewster's angles, the higher is the fibre orientation anisotropy.

EXPERIMENTAL R~SULTS
Experiments with paper was done on specially prepared samples made of two layers. In one sample, the layers are identical with an orientation anisotropy of 2.5 (=EHD/ECD)~ The other sample has a top layer with an anisotropy of 3~5 and a bottom layer with an anisotropy of 1.5. The grammage for both samples is 80 g/mZ and the averaged thickness is 140 ~m. The surface roughness was estimated by profiling a sample along a line (Fig. 3). Normally, ellipsometric measurements require that the surface be relatively smoothed to avoid polarization scrambling. The surface of our samples cannot be considered smoothed even for a 10 ~m wavelength, but as it will be seen, it did not impede this approach for surface anisotropy measurement.
The next step was to characterize the general optical properties of paper in the 9 to 11 ~m spectral region using a FTIR spectrometer. Fig. 4 and 5 show the reflectivity and penetration depth as a function of wavelength. The penetration depth is defined as the inverse of the power absorption coefficient and Fig. 5 is perfectly consistent with the spectral characteristics of natural cellulose in this spectral region. However, the vertical scales on both graphics are only approximate as surface scattering makes precise measurements of penetration depth and reflectivity difficult. Nonetheless, the averaged values of the refractive indices of paper at different wavelengths, as shown in Fig. 4 with the square, are in good agreement with the general features of the reflectivity curve. T~ese refractive indices were measured with the laser 20~ 77 ellipsometer and adds credibility to the data obtained with the instrument.
The surface anisotropy is observed by measuring the Vz~/V~ versus angle of incidence curve with the MD in the vertical and horizontal positions. The results presented here were obtained with an effective area of 3 mm in diameter. The angles at which these curves reach a minimum are then calculated by a least square fitting routine. The averaged value of the two angles is a direct measure of the average refractive index while the difference is an indication of anisotropy. Some initial trials were completed at four different wavelengths and the best results were obtained at 10.6 ~m and shown in Figs. 6, 7, 8 and 9. For the non homogeneous sample, Fig. 6 is the high anisotropy side and Fig.
7 is the low anisotropy side. The sample is oriented in the MD
(0~) and the CD (90~). As it can be seen, it was clearly possible to distinguish the two different sides. The homogeneous sample (Figs. 8 and 9) had very similar results for koth sides.

Claims (38)

1. A method for measurement of a fibre orientation anisotropy on a surface of a fibrous structure, wherein said fibre orientation anisotropy is determined by comparing refractive indices of said fibrous structure anisotropy in two perpendicular directions, said refractive indices being measured with an ellipsometry technique.

2. The method of claim 1, wherein said infrared light beam is produced by a CO2 laser.

3. The method of claim 1 or 2, wherein said infrared light beam has a wavelength between 9 and 11 µm.

4. The method of claim 1, wherein the phase retardation is between 0 and 3 radians.

5. The method of claim 1, wherein said two perpendicular directions are the machine direction (MD) and the cross-direction (CD).

6. A method for measurement of a fibre orientation anisotropy on a surface of a fibrous structure, comprising the steps of:
(a) producing a light beam with an infrared light source, (b) producing a modulated interruption of said light beam at a given frequency, so a lock-in amplifier, referred to as the DC lock-in, can measure the total intensity of said light beam, (c) polarizing linearly said infrared light beam in a first polarization plane, (d) producing a modulated phase retardation of the polarization of the polarized light beam produced in step (c) before it reaches the fibre structure, (e) guiding the modulated beam produced in step (d) towards said surface of said fibrous structure in a plane of incidence parallel to the first polarization plane in order to have said beam reflected by said surface, (f) polarizing linearly the reflected beam in a second polarization plane, (g) measuring the intensity of a second harmonic and total intensity of the polarized reflected beam at different angles of incidence of said modulated beam on said fibrous structure in said plane of incidence parallel to said first polarization plane, (h) calculating the angle of incidence at the minimum ratio between the intensity of the second harmonic and the total intensity measured in step (g), said angle being hereinafter called "first Brewster's angle", (i) changing the direction of said fibrous structure by turning said fibrous structure 90°, (j) measuring again the intensity of the second harmonic and the total intensity of said polarized reflected beam at different angles of incidence, in that other direction of the fibrous structure, (k) calculating the angle of incidence at the minimum ratio between the intensity of the second harmonic and the total intensity measured in step (j), said angle being hereinafter called "second Brewster's angle", (l) calculating an average value between said first and said second Brewster's angles, said average value being hereinafter called "average refractive index", and (m) determining the fibre orientation anisotropy by mathematically comparing said first and said second Brewster's angles.

7. The method of claim 2, wherein said first and said second Brewster's angles are subtracted, the higher is the difference between the Brewster's angles, the higher is the fibre orientation anisotropy.

8. The method of claim 6, wherein the infrared light source is a CO2 laser.

9. The method of claim 6, 7 or 8, wherein in step (a), said light beam has a wavelength between 9 and 11 µm.

10. The method of claim g, wherein said wavelength is 10.60 µm.

11. The method of claim 6, wherein in step (d), said given frequency of said modulated phase retardation is 37 KHz.

12. The method of claim 6, 7 or 8, wherein in step (c), the phase retardation is between 0 and 3 radians.

13. The method of claim 6, wherein in step (b), said given frequency of said modulated interruption is 3 KHz.

14. The method of claim 6, wherein said light beam has a power level below 100 mWatt before it reaches the fibrous structure.

15. The method of claim 6, wherein in steps (h) and (k), said Brewster's angles are calculated by a least square fitting routine.

16. The method of claim 6, wherein the two directions are the machine direction (MD) and the cross-direction (CD).

17. The method of claim 6, wherein said first and said second polarization planes are 90° apart.

18. The method of claim 6, 7, 3, 10, 11, 13, 14, 15, 16 or 17, wherein the fibre structure comprises natural cellulose.

19. The method of claim 18, wherein the fibre structure is paper.

20. A device for measuring a fibre orientation anisotropy in a fibrous structure, said device comprising:
(a) an infrared light source which produces a light beam, (b) a chopper which produces a modulated interruption of said light beam at a given frequency, so a lock-in amplifier, referred to as the Dc lock-in, can measure the total intensity of said light beam, (c) a linear polarizer for linearly polarizing the modulatad light beam in a first polarization plane before it reaches the fibrous structure, (d) a photoelastic modulator which creates a modulated phase retardation of the polarized light beam, (e) a rotating sample platform to which the fibre structure, on which the modulated beam is directed and then reflected, is rigidly attached, said rotating sample platform being provided to vary the angle of incidence of said modulated beam on the fibre structure in a plane parallel to said first polarization plane, (f) a detector arm comprising:
- means to follow the reflected beam, - a linear polarizer to polarize said reflected beam in a second polarization plane, - a detector connected to means to measure the intensity of a second harmonic and total intensity of said polarized reflected beam;
(g) means to memorize said intensity of the second harmonic and total intensity of said reflected beam as a function of the angle of incidence, (h) means to calculate, for two fibrous structure directions 90° apart, the angle of incidence at the minimum ratio between the intensity of the second harmonic and the total intensity measured with said detector on said detector arm, said angles being hereinafter called "first and second Brewster's angles", (i) means to calculate an average value between said first and said second Brewster's angles, said average value being hereinafter called "average refractive index", and (j) means to calculate the fibre orientation anisotropy with Brewster's angles.

21. The device of claim 20, wherein the light source is a CO2 infrared laser.

22. The device of claim 20 or 21, wherein said light beam is provided with a wavelength between 9 and 11 µm.

23. The device of claim 22, wherein said wavelength is 10.6 µm.

24. The device of claim 20, wherein said given frequency of said modulated phase retardation is 37 KHz.

25. The device of claim 20 or 24, wherein the phase retardation is between 0 and 3 radians.

26. The device of claim 20, wherein said given frequency of said modulated interruption is 3 KHz.

27. The device of claim 20, wherein said means to measure said total and said second harmonic intensities are respectively DC lock-in means and 2w lock-in means.

28. The device of claim 20, further comprising an attenuator in order to maintain the power of said light beam below 100 mWatt before it reaches the fibrous structure.

29. The device of claim 20, further comprising a beam splitter and a reference detector to check for light source fluctuations.

30. The device of claim 20, further comprising means to control the size of a probed area.

31. The device of claim 30, wherein said means to control the size of said probed area is an iris.

32. The device of claim 20, further comprising a lens located on said detector arm, said lens being provided to concentrate the reflected polarized beam on the detector.

33. The device of claim 20, further comprising mirrors to deviate said light beam in order to reduce the size of said device.

34. The device of claim 20, wherein said means to follow said reflected beam is controlled by a computer.

35. The device of claim 20, wherein said means to calculate said Brewster's angles is a computer.

36. The device of claim 35, wherein said computer is performing a least square fitting routine.

37. The device of claim 20, 21, 23, 24, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35 or 36, wherein said fibrous structure comprises natural cellulose.

38. The device of claim 37, wherein said fibrous structure is paper.