The relation between dimensional changes and elastic ... report 26.pdf · a report from STFI-Packforsk The relation between dimensional changes and elastic modulus during moisture

The relation between dimensional changes and

elastic modulus during moisture sorption and

desorption

Anne-Mari Olsson and Lennart Salmén

2005

According to Innventia Confidentiality Policy this report is public since 2011-02-04

a report from STFI-Packforsk

The relation between dimensional changes

and elastic modulus during moisture sorption and desorption

Anne-Mari Olsson and Lennart Salmén

Report no: 26 | March 2005

Cluster: Paper Mechanics Restricted distribution to: Billerud, Eka Chemicals, Mondi Packaging Frantschach, M-real,

Peterson & Son, Stora Enso, Södra, Tetra Pak, Voith, International Paper

According to STFI's Confidentiality Policy this report is assigned category 2

The relation between dimensional changes and elastic modulus during sorption and desorption Cluster Report Paper Mechanics

Acknowledgements This work has been performed within the Paper Mechanics cluster.



The relation between dimensional changes and elastic modulus during sorption and desorptionCluster Report Paper Mechanics

Contents Page

1 Summary 1

2 Background 2

3 Experimental 4 3.1 Material 4 3.2 Mechanical testing 4 3.3 Humidity generation 5 3.4 Evaluation 5 3.5 IR measurements 5

4 Results and discussion 6 4.1 Effect of measurement parameters 8 4.2 Speed of moisture change 8 4.3 Thickness of sample 10 4.4 Span of change in relative humidity 11 4.5 Fibre direction 11 4.6 The effect of temperature 12

5 Conclusion 15

6 References 16




The relation between dimensional changes and elastic modulus during sorption and desorption 1Cluster Report Paper Mechanics

1 Summary The effect of moisture sorption and desorption on the elastic modulus and dimensions was tested for papers. The speed of change in ambient relative humidity was varied, as well as the thickness of the tested papers.

It was concluded that the dimensional changes of the paper exactly reflected changes in moisture within the sample. For a thin paper a sudden change from dry to humid conditions resulted in a drop in modulus below that corresponding to the higher moisture content. Also in desorption the modulus curve fell below that corresponding to its equilibrium moisture content although the effect was much smaller. During slower moisture changes these effects were decreased. For a thick paper sample no modulus drop was seen although the change in modulus was faster than that of the moisture change. The reason may probably be related to a gradient of moisture content throughout the sample. For all measurements on samples thicker than 50-60 μm the modulus drop will be masked by this gradient.

This local phenomenon in the material subjected to sudden moisture increase might be one reason for the mechano-sorptive creep effect.



2 The relation between dimensional changes and elastic modulus during sorption and desorption Cluster Report Paper Mechanics

2 Background Moisture has a large impact on the mechanical properties of paper. Due to the strong hygroscopicity of the fibres a change in the ambient atmosphere instantly leads to changed properties, such as stiffness and dimensions. This also affects the creep properties of the paper. For a material under load mechano-sorptive creep occurring at repeated changes in climate can lead to rupture. This effect might be connected with the changes of elastic modulus during sorption (Salmén and Fellers 1996). However there are still unanswered questions regarding the relations between the dynamics of the change in moisture content, sample dimensions and elastic modulus.

The water uptake at changes in the atmosphere can instantly be measured by FTIR (Olsson and Salmén 2004), and the process of sorption and desorption of a thin paper may thus be followed in real time. Increased moisture content in the material shows up as a change in the IR spectra at several wavenumbers. In figure 1 the change compared to dry spectrum after one and 20 minutes of moistening is shown. It was shown that the peak at 130-1640 cm-1 was proportional to the moisture content (Olsson and Salmén 2004), and may thus be used to monitor the moisture changes in a paper.

Figure 1. FTIR-spectra of the moisture in a paper after 1 and 20 minutes respectively

In this report an attempt is made to establish relations between on the one hand the moisture uptake and dimensional changes, and on the other hand relations between dimensional changes and the elastic properties of the material during moisture change.

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The majority of the measurements of storage modulus presented in this report were made in a DMA, dynamic mechanical analyser, where the moisture content could not be measured. Therefore separate FTIR measurements were used to determine the relation between the physical properties and moisture content.

Different materials were tested at cyclic changes of the climate at controlled conditions. The tests were performed at low stress where no or very little creep occurs. The development of sample length and storage modulus of the sample was determined both in sorption and desorption at different speeds.




3 Experimental

3.1 Material Papers of different grammages were treated. Oriented sheets of a standard softwood kraft pulp were produced in a Formette Dynamic at (1400 rpm) with a grammage of 15 g/m2. The thickness of this paper was 25 μm. A 60 g/m2 paper made on the EUROFEX paper machine from unbleached kraft pulp (Fellers, Olsson et al. 2005) with the thickness of 96μm was also tested. Glucomannan extracted from Amorphophallus Konjac was cast into a film from a water solution to see the effect on a homogeneous material without fibres.

3.2 Mechanical testing The mechanical testing was made on a Perkin Elmer DMA 7 (Dynamic Mechanical Analyser). The mechanical loading during the test was a static load with a dynamic load added in order to be able to measure the storage modulus. The frequency of the load was 1 or 10 Hz. A schematic illustration of the loading conditions is shown in figure 2.

Figure 2. The loading conditions during the testing. The storage modulus was determined from the slope of the stress/strain loop of the dynamic loading

Throughout a test a constant dynamic deformation was maintained by controlling the dynamic load, with a static offset load of 120 % of this dynamic load. The deformation was chosen to be from 2μm up to 5μm. At the fast change during sorption and desorption the control of the deformation varied to some extent, but only for a short time, at 1 Hz for 12 s and at 10 Hz for less than 10 s.

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3.3 Humidity generation The climate surrounding the samples was controlled by a Tecnequip humidity generator. A dry stream of air was mixed with a fully water saturated stream at different ratios giving different relative humidities. The change between two humidities was either instantaneous or made as a ramp of 10 %RH/min. Cycles were mainly performed between 1 %RH and 80 %RH, some tests were also made between 30–80 %RH and 50–80 %RH. All tests were performed at 30°C.

3.4 Evaluation The storage modulus and length of the samples were determined during the tests. The load was low in order to avoid creep of the samples. Despite this some creep occurred. To be able to compare different runs, the small creep was subtracted and the height of the change in each cycle was set to 100 %, as seen in figure 3.

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3.5 IR measurements IR measurements with samples mounted in a climate conditioned stretcher were made to correlate moisture content and sample length during sorption and desorption. Absorbance spectra in transmission were registered in the IR region between 500 cm-1 and 4000 cm-1 with a Bio-Rad FTS 6000 spectrometer. Spectra with nonpolarized light were detected by an MCT detector in kinetics tests throughout the whole sorption/desorption process. The paper samples were mounted between two clamps with the possibility to control the load. A sample chamber with zinc selenide glasses and temperature control was placed in the beam path, through which air with controlled temperature and relative humidity was circulated throughout the IR measurements. During sorption and desorption the load was controlled to a constant value low enough not to affect the moisture content. The length of the sample was registered with a digital micrometer built into the stretcher.




4 Results and discussion When a paper is exposed to increasing relative humidity the paper expands as the moisture content of the paper increase. In the IR instrument the real time moisture content during moisture changes was evaluated from the peak at 1643 cm-1 in the spectra and correlated to the sample length. The development of the moisture content measured by IR and the length of the paper was found to be the same both in sorption and desorption, figure 4. This was shown for both paper and polymer films. The sorption was very quick and dependent on paper thickness. For a thin paper of 15 g/m2 only small changes in moisture content appear after 10 minutes, which is shown in figure 4. In desorption the moisture decrease was slightly slower mainly as a result of the slower speed of drying out the sample chamber itself.

Figure 4. The development of moisture content evaluated as the height of the IR peak at 1643cm-1 was the same as the development of the length of a thin paper sample both in sorption and desorption when changing the RH between 0 and 80%

In the following only the modulus and length changes were followed using a DMA, with the assumption that the length change reflects the actual moisture content.

As shown in figure 5 the storage modulus dropped very quickly below the level of the high humid equilibrium, as the moisture was absorbed in the paper. This happened despite that the moisture content curve gradually went from dry to moist as seen from the length curve. Therefore there seems to occur an additional effect to the effect of the higher moisture content on the storage modulus when the ambient relative humidity is rapidly changed. This phenomenon was not visible in desorption where the storage modulus more or less followed the moisture content development. In this experiment different speeds of the moisture change were tested why the magnitude of the drop is varying.

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Figure 5. Thin paper tested in the machine direction at 10 Hz during different ramps between 1 and 80 %RH

The effect of the moisture change is more visible when the storage modulus and length are compared on a relative basis. This is shown in figure 6 and 7 where the effect in both sorption and desorption is shown. In these diagrams the axis for the modulus is put upside down going from 100 % to 0% during sorption, while the same is made for the length axis going from 100 % to 0 % during desorption.

Figure 6. Comparison between relative values of storage modulus and sample length during sorption from 1 to 80 %RH and during desorption from 80 to 1 %RH for a thin paper

In figure 7 the relative changes in modulus as a function of the relative length changes are compared during sorption and desorption. Obviously the modulus dropped much lower in sorption than would be expected from its moisture content. During desorption the modulus change was more linearly related to the length. As the length represents the moisture content this effect of difference in sorption and desorption is not due to any hysteresis. This is quite evident when looking at the equilibrium changes of modulus and length during sorption and desorption where the modulus drops a bit more at higher moisture contents, i.e.

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at larger length of the sample due to softening but the behaviour was similar in sorption and desorption. Thus in comparing the equilibrium values with those at fast moisture changes it is obvious that both during sorption and desorption the modulus falls below its equilibrium value.

Figure 7. The relative storage modulus versus the relative length during sorption and desorption for a thin paper at immediate change and for a thick paper at a slow humidity ramp which corresponds to equilibrium values

4.1 Effect of measurement parameters At rapid changes of material properties there can occur errors due to the difficulty to control loads and to measure properties. Throughout the test the amplitude of the dynamic load was set to a constant value. At the rapid change a slight deviation from that value in amplitude can be seen as shown in figure 8 lasting for 12 s at 1 Hz and less than 10s at 10 Hz. At measurements at 1 Hz the amplitude was not as stable as at 10 Hz, but the dip in storage modulus occurred at both frequencies. Also the fact that the modulus at both frequencies reached the higher RH modulus value from below and not from the originally higher dry modulus value indicates that the phenomenon is a real one measured.

Figure 8. Amplitude and storage modulus versus time for measurements at 1 and 10Hz respectively

4.2 Speed of moisture change The speed of the change in relative humidity of the ambient atmosphere with the equipment used was different in sorption and desorption. During the

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desorption phase a lot of humidity has to be dried out from the system while at increasing humidity the capacity of generating the humid air was more effective. This is also reflected in the development of the sample length of the paper that follows the moisture content of the sample.

The modulus drop due to moisture changes was seen mainly during rapid changes of the relative humidity. In figure 9 results from measurement on thin paper at a dynamic load frequency of 10 Hz is shown. Two speeds were used; an immediate change and that of 10%RH/min. In sorption there was a modulus drop also when the relative humidity was changed at the slower rate, although not as pronounced as during the immediate change. At the ramp with 10 %RH/min the speed of sorption and desorption was similar, which means that the difference between the modulus in sorption and desorption cannot be explained by difference in speed of moisture change.

Figure 9. The relative storage modulus versus time both in sorption and desorption for different speeds of humidification for thin paper

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4.3 Thickness of sample For a thicker paper the large modulus drop during sorption was not seen as shown in figure 10, although there still was a tendency for the modulus to reach the equilibrium value from below.

Figure 10. Relative storage modulus during sorption and desorption for an immediate change in RH for papers of 15 and 60 g/m2

The change in overall moisture content of the thicker paper was slower than that of the thin paper when the RH was changed abruptly. At a speed of the humidity change of 10 %RH/min the development of moisture in the paper were similar for thin and thick samples. The difference between the modulus developments for thin and thick samples as compared to the behaviour at an immediate moisture change remained as shown in figure 11. However also in this case the modulus of the thick sample changed faster than the length in the beginning of the sorption phase. The weak modulus drop for the thick paper indicates that probably a gradient in moisture content throughout the thickness counteracts the storage modulus drop, which is a very local effect.

Figure 11. The relative storage modulus and length of the samples of thin and thick paper as a function of sorption time (1-80%RH) with the speed of 10 %RH/min

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4.4 Span of change in relative humidity When examining the effect of the magnitude of the RH drop the effect on the storage modulus of the thin paper was on a relative scale the same when the starting relative humidity was 1 %RH, 30 %RH or 50 %RH. This is shown in figure 12 for a thin paper loaded cross the fibre direction with a dynamic load with the frequency of 1 Hz. In all cases the modulus reached the equilibrium from below.

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4.5 Fibre direction To determine if the storage modulus drop is a phenomenon on the fibre level or if it is due to effects on the fibre-fibre bonds paper of different fibre direction was tested. The thin paper with a degree of anisotropy of 3.4 was tested both along and cross the fibre direction. In figure 13 the development of the storage modulus in MD and CD during sorption is shown. There was a larger dip when the sample was loaded along the fibre direction indicating that this could be an effect mainly associated to the fibre level.

Figure 13. The development of storage modulus in sorption during loading along and cross the fibre direction

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A film of glucomannan with high hygroscopicity was also tested. This sample was a homogeneous isotropic film. In order to resist the loading the film was thicker than the thin paper. The results from sorption tested at 10 Hz on thin paper in MD and the glucomannan film tested at 1 Hz is shown in figure 14.

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Figure 14. The relative storage modulus during sorption (1-80 %RH) for a thin paper in MD and for a glucomannan film

Due to the high thickness of the film the moisture sorption is much slower compared to the thin paper and thus the behaviour more resembled that of the thick paper, see figure 15. However also for the glucomannan film the modulus at the high RH was reached from below.

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Figure 15. The relative storage modulus during sorption (1-80 %RH) for glucomannan at 1 Hz and paper of 60 g/m2 in MD at 10 Hz

4.6 The effect of temperature The temperature is important for the relative humidity; where higher temperature gives a lower relative humidity at a certain absolute moisture content. For paper a higher temperature at the same moisture content would give a lower storage modulus and lower relative humidity would of course give a higher storage modulus. In the experiments the temperature was set to be constant at 30 °C. The temperature varied to some extent in the equipment as shown in figure 16.




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The overall temperature varied +/-1.3 % or in absolute temperature +/-0.15 °C. For the first minute of drying the temperature decreased slightly, at the maximum with 0.6 °C, more pronounced at rapid changes. The variations were small and due to lag in the temperature regulating system.

When moisture is absorbed into the paper material substantial amounts of energy is released due to the heat of absorption. The effect is shown in figure 17 where the temperature was measured with a 0.7 mm diameter thermocouple in contact with a thin paper during sorption and desorption at different speeds.

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The mass of thermocouple was considerably larger than the mass of the paper, therefore the measured temperature only shows some of the generated heat. At the fast changes in RH clear temperature changes were seen; a one degree cooling during desorption, and a one degree increase during sorption. This is in line with earlier measurements (Salmén 1993) where a thermocouple was isolated in-between two paper sheets showing larger temperature increase due to absorption of moisture. If calculating the heat locally emitted from the sorption of the moisture this increase would be sufficient to raise the temperature of the fibre with several tenfold in degrees. Obviously due to the radiation to the surroundings such high temperatures are not reached but probably much higher temperatures are reached than those possible to measure by the thermocouple.




5 Conclusion

For a thin paper subjected to a sudden change from dry to humid conditions there was a local drop in storage modulus below that corresponding to the higher moisture content. During slower moisture changes this effect was decreased. Foe a thick paper sample there will be a gradient of moisture content in the paper that leads to no modulus drop, although the change in modulus was faster than that of the moisture change. For all measurements on samples thicker than 40-50 μm this phenomenon will be masked by this gradient.

The reason for the modulus drop could be related to a local increased mobility of the structure either on the fibre level or in the bonding between the fibres. Such an increased mobility is probably caused by the changing moisture but may also to a large extent be due to the locally large temperature increase caused by the moisture absorption.

The implication of this phenomenon of local loss in modulus of the material during moisture sorption must be further examined, but it might have some connection to the mechano-sorptive creep effect.




6 References Fellers, C., A.-M. Olsson, et al. (2005). The effect of papermaking parameters on

the mechanical properties of paper Euro-FEX-trail No 1, STFI-Packforsk.

Olsson, A.-M. and L. Salmén (2004). “The association of water to cellulose and hemicellulose in paper examined by FRIR spectroscopy.” Carbohydr. res. 339: 813-818.

Salmén, L. (1993). Responces of paper properties to changes in moisture content and temperature. Products of papermaking, Transactions of the tenth Fundamental Research Symposium, Oxford 1993. c. f. Baker. Leatherhead, Pira International: 369-430.

Salmén, L. and C. Fellers (1996). “Moisture-induced transients and creep of paper and Nylon 6,6; a comparison.” Nord. Pulp Pap. Res. J. 11(3): 186-191.


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The relation between dimensional changes and elastic ... report 26.pdf · a report from STFI-Packforsk The relation between dimensional changes and elastic modulus during moisture