A method to determine thickness and wave height of a thin substrate

A METHOD TO DETERMINE THICKNESS AND WAVE HEIGHT OF A THIN SUBSTRATE

Technical field

The present invention relates to a method to measure thickness and wave height of a thin substrate.

The expression thin substrate is in this context a film or barrier substrate such as microfibrillated cellulose (MFC) or thin barrier paper such as greaseproof paper, glassine or coated kraft paper which substrate has a grammage of less than 100 gsm, preferably less than 80 gsm and most preferred 15-60 gsm.

In the following the expression MFC or microfibrillated cellulose will be frequently used.

Microfibrillated cellulose (MFC) shall in the context of the patent application mean a cellulose particle, fiber or fibril having a width or diameter of from 4 nm to 1000 nm.

Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp used when producing MFC may thus be native or pretreated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such

545 420 groups include, among others, carboxymethyl (CM), aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example TEMPO), or quaternary ammonium (cationic cellulose) . After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC.

MFC can be produced from wood cellulose fibers, both from hardwood or softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It can be made from pulp, including pulp from virgin fiber, e.g. mechanical, chemical and/or thermomechanical pulps . It can also be made from broke or recycled substrate.

Thin barrier substrates made from cellulose such as barrier comprising MFC or for example highly refined pulp, can be efficient barrier materials in packaging or in various laminate structures .

Prior art - Problem

A thin substrate is in this context a film or barrier substrate such as microfibrillated cellulose film or barrier substrates such as microfibrillated cellulose (MFC) or thin barrier paper such as greaseproof paper, glassine or coated kraft paper which substrate has a grammage of less than 100 gsm, preferably less than 80 gsm and most preferred 15-60 gsm. A problem with these substrates is to determine their thickness and especially thickness variations i.e. wave height. This includes micro-wrinkles or small-scale unevenness in thickness profile. Variations in thickness can be harmful for

545 420 convertibility, printing, lamination, gluing, vacuum coatability, or post-coatability, etc.

Standard methods are not sensitive enough and might not reveal protruding fibers or fiber flocs or micro-buckles or wrinkles due to drying or defects in the sheet.

Profilometry or other contact methods could reveal some roughness variations, although these methods are also very much dependent on how the substrate is adhered to a carrier medium.

One method is to apply Bristow wheel method to extrapolate the surface volume, whereas this is challenging and very dependent on liquid-substrate interaction (surface and base can swell upon contact with the liquid)

It is also essential that base substrates and substrates should have good surface smoothness in order to gain low barrier coating uptake and hence good substrate formation at low coating amounts - in order to gain improvements in barrier properties (with low coating amounts).

Object of invention

An object with the invention is to provide a method to determine the thickness and thickness variations i.e. wave height of thin substrates, which method solves, or at least reduces, the above-mentioned problems.

Summary of the invention

In accordance with the invention the method comprising the steps of:

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a) placing a first electrode against a first side of the substrate and a second electrode against a second side of the substrate,

b) fixing the position of the electrode and removing the substrate from the electrodes,

c) measuring the electrical capacitance between the electrodes,

d) using the electrical capacitance value from step (c) to determine the distance, dAc_z between the first electrode and the second electrode, i.e., the distance between the substrate ^z s wave tops,

e) measuring the thickness, dwECH/ of the substrate with a micrometer, and

f) combining the results from step (d) and step (e) to calculate the wave height of the substrate.

Detailed description of the invention and examples

Fi gure 1 discloses a schematic view of a parallel plate capacitor to measure the thickness between the wave tops of a substrate.

Fi gure 2 discloses a thin substrate between the plates of a plate capacitor.

Fi gure 3 discloses the thickness dAc_z capacitance and effective dielectric constant dependence on pressure.

Fi gure 4 discloses a schematic view of a micrometer for mechanical measurement of substrate thickness.

Fi gure 5 discloses the profile of a substrate thickness, wherein a mechanical measurement of the thickness is obtained dwECH.

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Fi gure 6 discloses the thickness ratio between the thickness from capacitor measuring dAc and mechanical measuring dwECH.

In the following, the invention will be described more in detail with reference to Figures 1 to 6. The inventive method for measuring thickness the wave height of thin substrates comprising three method steps: A) Capacitor measuring of substrate with a parallel plate capacitor, d_Ac, B) Mechanical measuring of substrate thickness with a micrometer, dwECBo and C) Combining the thickness results from A) and B) to determine waviness / wave height of the substrate .

A) Capacitor measuring of substrate with a parallel plate capacitor, dAc.

Figure 1 discloses a parallel plate electrical capacitor 1 comprising a supporting frame 4, an upper, flat plate, first electrode 2a and a lower, flat plate, second electrode 2b, where electrodes 2a, 2b are connected to a standard RLC meter, e.g. Wayne Kerr 6440B, precision component analyzer. The plate capacitor 1 further comprising a first insulator 5a and a second insulator 5b which insulators 5a, 5b insulate the first and second electrodes 2a, 2b respectively from the frame 4. A thin substrate 3 with an upper, first surface 3a and a lower, second surface 3b is placed between the first and second electrodes 2a, 2b. The first electrode 2a is then pushed against the first surface 3a of the substrate 3 such that it contacts the first surface 3a. The clamping pressure can be selected by selecting the weights. The clamping pressure is in the range 3-20 kPa, preferably 5-10 kPa and

545 420 most preferred 5-7 kPa. Figure 3 discloses the thickness d_Ac, capacitance and effective dielectric constant dependence on pressure. Then position of the first electrode may be fixed by using a screw. Distance between electrodes is the substrate thickness dAc. The substrate is then pulled out and the electrical capacity Co is measured. The thickness (distance between electrodes) is calculated according formula for the electrical capacitance of a flat capacitor:

, £_nS <D where ^=8.85-10¹² F/m is an electrical constant, S electrode area and Co - measured capacitance of the capacitor without substrate. Using Cac [mm], GSM [g/m²] the substrate density Pac [kg/m³] is calculated:

Substrates are not flat and the thickness Cac may depend sometimes very strongly on the clamping pressure from the electrodes, as disclosed in Figure 2. Hence, the calculated substrate density is depended on the clamping pressure. This density is not real substrate density and can be named as density of substrate with air gaps.

B) Mechanical measuring of substrate thickness with a micrometer, dMECH.

The substrate thickness Cac is determined testing large areas and it is difficult to make the substrate flat. The similar situation is in the ISO method although the testing area is smaller but large enough comparing with the wave dimension. Therefore, we use the testing with a small contact area point by using a micrometer.

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A schematic view of a micrometer 7 for mechanical measurement of substrate 3 thickness dwECH is shown in Figure 4. The micrometer 7 comprising a cylinder 8 and a measuring rod 9, which moves freely inside the cylinder 8. The micrometer further comprising a spring 11 which generates a compressive force of the rod 9 against the substrate 3. A round head 10 is arranged at the end of the rod, which head faces against the substrate. The head 10 is rounded and the diameter of the head is 1 mm and the contact area (measuring area) is ^0.8 mm². The compressive force of the rod is 90 g and the pressure against the substrate is 0.5-1 MPa during measuring.

The substrate thickness is measured in many (12-20) points and the mathematical average is calculated and the average of substrate thickness (mechanical thickness) is obtained. The accuracy of the thickness measurement depends on the measurements number. To increases the determination accuracy substrate thickness can be scanned. Substrate is dawned with the constant speed and thickness is recorded. The result is thickness profile and average thickness can be calculated, see Figure 5.

C) Combining the thickness results from A and B to determine waviness / Wave height

When substrate is flat and smooth, its real surface area is equal to the geometric area. When substrate is wavy, the real surface area (surface volume) can be much larger depending on the wave height and frequency component. Wave height can be determined using electrical capacitor and mechanical point thickness determination methods: the difference between dAc and mechanical point thickness is wave height. Some information about the wave frequency

545 420 can be obtained analyzing (spectral analysis) thickness profile. Information about the waves can be obtained also analyzing the photo of the substrate cross section - the computer program will calculate the surface area S of the 5 substrate with pixel accuracy.

Waviness is detected applying both methods together. Capacitance is measured at minimal pressure 7 kPa and from the capacitance value is calculated distance between both side wave tops. Then real substrate thickness is measured 10 mechanically. The wave height is the distance between wave tops minus substrate real thickness, see table 1 below.

The capacitance is measured at minimal (kiss) pressure.

It is 7 kPa. So, it can be said that the waviness is determined almost without pressure.

<td> Substrate</td><td> Grammage [gsm]</td><td> Capacitor thickness, Öac (distance between wave tops), [pm]</td><td> Mechanical thickness, dwECH [pm]</td><td> Wave height [pm]</td>

<td> DI</td><td> 39.9</td><td> 49.2</td><td> 30.9</td><td> 18.3</td>

<td> D2</td><td> 49.4</td><td> 59.6</td><td> 38.6</td><td> 21.0</td>

<td> D3</td><td> 21.3</td><td> 34.6</td><td> 17.5</td><td> 17.1</td>

<td> D4</td><td> 27.0</td><td> 40.6</td><td> 22.2</td><td> 18.4</td>

<td> D5</td><td> 41.4</td><td> 68.5</td><td> 31.2</td><td> 37.3</td>

<td> D6</td><td> 30.9</td><td> 55.3</td><td> 25.3</td><td> 30.0</td>

Table 1

The samples in the figure 6 are:

SI: A substrate comprising MFC and softener.

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S2: A substrate comprising MFC which has been coated with UV varnish.

S3: A substrate comprising MFC with modified rougnhness.

S4: A substrate comprising MFC with 1-side coating of PVOH.

S5: A substrate comprising MFC with 1-side coating of PVOH and applied with higher amount.

S6: A substrate comprising MFC - uncoated (ca 30 gsm).

S7: A substrate comprising MFC with slightly higher grammage (ca 32 gsm).

S8: A substrate comprising MFC with higher grammage (ca 40 gsm).

S9: A substrate comprising MFC with higher grammage (ca 50 gsm).

S10: A substrate comprising MFC with higher grammage (ca 60 gsm).

Sil: A regular office paper substrate.

Figure 6 discloses a ratio between the thickness from mechanical measurement with a micrometer 7 dwECH and capacitor measurement with a capacitor dAc. This ratio could be defined as macroroughness. The ratio should have a value higher than 0.4, preferably higher than 0.5 and most preferred higher than 0.6.

The benefit with the inventive method is that it reveals more efficiently micro buckliness and protruding fibers in the substrate that can cause problem with air entrapment etc .

The method can be applied than 100 gsm, preferable preferred 15-60 gsm. The on very thin substrates less less than 80 gsm and most density can vary, but is

545 420 preferably between 100-2000 kg/m3, more preferred 200-1800 and most preferred 600-1400 kg/m3.

The method can also be made at different relative humidity as disclosed in Figure 6. The relative humidity during the measuring may vary in the range 0-90 %RH, preferably in the range 0-85 %RH. The most preferred is in the range 10-60 %RH.

Moreover, the temperature during the measuring may vary in in the range 0-100 °C, preferably 10-70 °C and most preferred 20-50 °C.

The moisture content in the substrate may vary in the range 0-30 %. The most preferred moisture content in the substrate in the range 1-25 %.

In the foregoing, the invention has been described based on some specific embodiments. However, a person skilled in the art realises that other embodiments and variants are possible within the scope of the following claims. For example, it is obvious that the clamping pressure and arrangement of the capacitor measuring may be conducted in several different ways e.g. springs, weights etc. Also, the clamping pressure with the mechanical measurement may be conducted in other ways e.g. weights, springs etc.

Moreover, the moving and the fixing of the electrode during the measuring may be done in many various ways such as screws, clamp arrangement, tape etc.

The substrate can be various coated on uncoated cellulose or fiber-based substrates such as substrate comprising MFC

545 420 or higher refined pulp, greaseproof paper, glassing paper, kraft paper, translucent paper, transparent paper, tracing paper, capacitor paper, label paper or regenerated cellulose. Furthermore, the substrate can be coated on one or both sides with various polymers or materials. The substrate is preferably a substrate for barrier application in packaging. Moreover, it can be used as freestanding substrate and/or in various forms of laminates.