Monitoring of Birch Veneer Moisture Content During the Drying
Process by Single-Sided NMR technique
Anti Rohumaa
1, Ekaterina Nikolskaya
1, Jyrki Pesonen
2and Yrjö Hiltunen
11FibreLaboratory,
South-Eastern Finland University of Applied Sciences, 57200 Savonlinna, Finland
2Raute Oyj,
Mecano Business, 87400 Kajaani, Finland
ABSTRACT
The evaluation of veneer moisture content before and after drying is an important issue for veneer-based product industry. Moisture content affects many veneer properties, e.g. veneer dimensional changes, bonding quality. Controlling moisture content is critical in order to be able to produce a high-quality product with the required properties and quality. However, traditionally evaluation of moisture content of wood bases on oven-drying method, which is time-consuming and sample-destructive.
Moisture content measurement in wood and wood-based materials is a process that could be handled by nuclear magnetic resonance (NMR) technique. In present study, single-sided NMR was used in order to evaluate the moisture content of rotary cut birch (Betula pendula Roth.) veneer during drying process at ambient conditions. The results show that the NMR signal correlates well with moisture content of veneer obtained by oven-drying method. Hence, the single-sided NMR technique can be used for the robust and rapid determination of water content in veneer during the drying process, and that it can be considered as being a routine instrument for testing veneer quality in the production of veneer-based products.
1.INTRODUCTION
The moisture content of wood material is an important factor affecting many properties of wood-based products and its processing. Hence, proper drying of wood veneers plays a crucial role in successful production of veneer and veneer-based products. The control of drying process and its parameters is critical in order to produce a high-quality product.
Traditionally in research, oven drying method has been used in order to measure the total moisture content of wood and wood-based products as the most accurate method. However, it is time-consuming and sample-destructive. In veneer-based products industry, several methods are in use (mainly based on microwave techniques or measurement of the electrical resistance of wood) for online measurement of veneer moisture content. Unfortunately, the described methods do not give information where the moisture locates, in cell wall or lumens. This information could provide understanding in order to optimise industrial drying process and veneer behaviour during drying.
Several researchers have shown that water in wood and wood-based products can be measured with nuclear magnetic resonance (NMR) technique, e.g. from untreated, acetylated and furfurylated wood (Labbé et al. 2002, Thygesen and Elder 2008, Thygesen and Elder 2009). NMR can distinguish water-bound hydrogen located in different physical and chemical environments and allows measuring the total wood moisture content and the distribution of water in cell walls and lumens (Labbé et al. 2002). According to Menon et al. (1987) NMR evidence indicates, that the 1H
NMR signal from water in wood may be separated on the basis of spin-spin T2 relaxation times into the bound water
(associated with wood cell walls) and lumen water (associated with free water in the cell lumens). Hence, the T2
relaxation times provide detailed and quantitative information. However, time-domain NMR has shown to be very robust approach for measuring water content in wood (Nikolskaya et al., 2012). These NMR methods are non-invasive
preparation, there is no limitation towards the sample size and it could measure water content at different depths inside the sample.
In this study the moisture content of green and rewetted veneer was measured with the help of single-sided NMR in order to evaluate the natural variability of moisture content inside of veneer prepared under the industrial conditions. Obtained information will be utilised in the future by veneer processing industry and technology companies in order to develop optimized process control systems.
2.MATERIALS AND METHODS
2.1.WOOD MATERIAL
Industrially prepared birch (Betula pendula Roth) veneer sheets were used in the present study. Logs were felled during winter and spring season. The storage of logs was conducted for approximately 1-2 months in log yard before soaking process at 40 C was performed. After peeling the five wet veneer sheets by 100 x 100 mm and 1.5 mm thickness were obtained for NMR measurements. Additionally, five similar 100 x 100 mm pieces were directly sent to the oven for drying in order to verify the initial moisture content.
NMR measurements were made for all five samples continuously one after other by placing the NMR sensor above the sample, so the deformations of veneers that might occur during drying would not affect the measurements. NMR measurements of samples were done immediately after preparation and afterwards in every 30 min during the first 3 h of air-drying. The last NMR signals were obtained in 4 h of air-drying. Between the NMR measurements samples were kept in the special rack, in order to secure even drying process for each veneer sheet.
After 4 hours of drying, the samples were photographed and oven dried at 103 C in order to obtain the moisture content 0%. This procedure was conducted four times at different days in order to obtain information about the natural variability of material used in plywood mill during springtime. Additionally, three oven-dried samples from first, second and third experiments were rewetted and then measured during air-drying again.
2.1.NMR TECHNIQUE
NMR measurements were made by a single-sided NMR sensor from Resonance Systems Ltd (Fig. 1). Single-sided NMR sensor consists of a set of magnet bars of special geometry and organization, that a magnetic field is generated in nearly parallel plane to the surface of the sensor (X-Y plane in Figure 1). The maximum depth of penetration for this sensor is approximately 3 mm. The sensitive slice is about 20 x 20 mm2 in the central area of the sensor. The NMR
sensor was tuned and calibrated so that the maximum sensitivity was at the thin slice, at about 1.3 mm over the surface. This corresponds to the frequency of 1H protons of 19.4 MHz. A standard pulse sequence CPMG was used to measure
NMR magnetisation decays. After removing of first obtained 10 signal points, measured decays were fitted by mono-exponential functions, and the spin-spin relaxation time T2 values were solved.
Average amplitude (Amean) was calculated between 8th and 12th points with subtracting the noise
(1) The moisture contents (mc) of the veneers were calculated:
(2) where Wwet is the original weight of wet sample, Wdry is the weight of dried sample, determined by drying for 24 h
at 103 C in an oven. Wwet was measured before and after each NMR measurement, and mc for each experimental point
Figure 1: Single-sided NMR sensor with wood sample on top of it. 4.RESULTS AND DISCUSSION
The results of the study show that there are variations in the initial mc of veneer measured from the separate sheets (Fig 2). This is mainly caused by moisture evaporation during the sample preparation process. However, the results clearly show that this process was quick since the initial mc was still around 65-75%. According to the literature, the average moisture content of freshly felled birch varies between 71 - 80% (Jalava 1932, Hakkila 1962, Kärkkäinen 2007)
Figure 2: Air-drying curves of veneers delivered from the industry to laboratory on four different days (Experiments 1 to 4) and curve received from oven-dried and rewetted veneers samples.
The results also show (Fig 2.), that the final mc of veneers after air-drying at ambient conditions for 4 h reached 10%. Rewetted samples had higher initial moisture content, but after 4 h drying quickly reached similar values as initially wet samples.
in the porous system, and this water is highly mobile, so the observed T2 is longer. When the sample is dried, water
remains in the smaller pores, and its mobility is restricted, so T2 is shorter.
Interestingly, different sets of samples obtained at different days act differently at the same testing and measurement conditions. Fig 3 shows, that Exp 1 and Exp 4 data shows similar trends. However, Exp 2, Exp 3 and rewetted samples had different trend. This is related to mobility of free water in veneers. In Exp 1 and Exp 4 water is more mobile.
Figure 3: Dependence of T2 on moisture content for different experiments.
Prior to the drying tests, a calibration between water contents and NMR parameter (spin-spin relaxation time) was performed. Several NMR pulse sequences and measurement settings were tested for finding the best measurement conditions. The spin-spin relaxation time T2 showed linear correlation with water content with very high correlation
coefficient (R² = 0.9172). The calibration between water content and relaxation time T2 is given in Fig 4.
Figure 4: Dependence of average signal magnitude Amean on moisture content. All points from all conducted experiments are
For the development of a real application based on this NMR technology to monitor the drying process precise sensor calibration, finding of a proper measurement and fitting conditions giving minimal errors, and accurate models between NMR parameters and water contents of different wood types would be required.
6.CONCLUSIONS
The acquired preliminary results show, that single-sided NMR is a promising technique for analyzing water content in wood veneer and observation its drying process. However, more studies should be conducted using different wood materials to get more general calibration models between water content and NMR parameters.
The results show that the NMR could detect differences in moisture behavior in different sets of veneers. This is hardly possible to measure with traditional methods used in veneer-based products industry. Understanding veneer moisture relation more accurately allows developing new and optimized processes for veneer-based product industry. ACKNOWLEDGEMENTS
This research was funded by The Regional Council of South Savo (Finland), European Regional Development Fund (ERDF), and partner companies.
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