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Potentials of thermally modified beech (Fagus sylvatica) wood for application in toy construction and design
Materials & Design Materials and Design 28 (2007) 1753–1759 www.elsevier.com/locate/matdes
Potentials of thermally modified beech (Fagus sylvatica) wood for application in toy construction and design Michael Ebner b
a,*
, Alexander Johannes Petutschnigg
b
a Department of Product Design and Management, University of Applied Sciences Salzburg, Markt 136a, A-5431 Kuchl, Austria Department of Forest Products Technology and Management, University of Applied Sciences Salzburg, Markt 136a, A-5431 Kuchl, Austria
Received 27 October 2005; accepted 12 May 2006 Available online 17 July 2006
Abstract This paper deals with the potentials of thermally treated beech wood as raw material for making toys. Natural beech wood is a common toy material and the properties of treated and untreated wood are compared. Samples to analyse press fits and joints between moving components were developed and tested under varying climatic conditions. Furthermore samples to analyse the dimensional stability of wooden components were developed and observed under varying levels of humidity and room temperature. The results show that thermally modified beech wood is a promising material for the manufacture of wooden toys. It is assumed that this raw material will be used in toy industries in the future, and furthermore the design of wooden toys will be influenced as it can be proven that this raw material can be employed in diverse developments in relation to the design of functional toys. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Wooden toys; Beech wood; Thermally treated wood; Functional toys; Dimensional stability
1. Introduction In this paper, the properties of thermally treated beech wood for wooden toys are analysed. Therefore this new raw material is compared to untreated beech wood. Beech is widely used as a material for making toys and two wellknown designs are shown in Fig. 1. Wood absorbs and desorbs water if the surrounding climate changes. When water is absorbed wood swells and if water is desorbed wood shrinks. As the ambient climate in houses changes (e.g. seasonal differences), the wood in toys also swells and shrinks during the year. The joints in the various toy components have to ensure the functionality of the toy. As the wood swells and shrinks the tolerances required are usually large for wooden toys. Therefore toys with moving parts *
Corresponding author. Tel.: +43 50 2211 2012; fax: +43 50 2211 2099. E-mail addresses: [email protected] (M. Ebner), [email protected] (A.J. Petutschnigg). 0261-3069/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.05.015
or fine mechanical components are not made out of wood. If moving parts are made out of wood, the functionality of the toy can be impaired. In Fig. 2, there is on the one hand a wooden screw with a glued press fit that does not function due to swelling and shrinking, and on the other hand the plastic bearing of a tyre was employed in order to compensate for possible variations in dimension. Through thermal modification of wood the swelling and shrinking measures are influenced (see Ref. [1]). Consequently these properties of wood are changed and it can be assumed that fine mechanical moving parts can also be made of thermally modified wood, thus raising the question as to whether thermally modified wood can be used for the moving parts of toys. In the following this question is analysed and answered for beech wood. If the analysed material is suitable to make toy parts with lower tolerances, the sustainable material wood can substitute plastic materials in specific fields of application.
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Nomenclature Rel. h. relative humidity °C degree Celsius F1–F4 sample references for testing press fits
M1–M4 sample references for testing joints between moving parts S1– S6 sample references for testing the dimension stability
Fig. 1. Toys made of beech wood. (a) Baufix , and (b) Matador.
Fig. 2. (a) Screws which were stored in a cold cellar with high rel. h. and afterwards in heated room in winter (very low rel. h.). The glued press fit did not work appropriate any more. (b) A wooden toy where functional parts are made of plastic.
2. Materials and methods Through the thermal modification on the one hand the colour of the wood changes and on the other hand, the main properties of wood are influenced. For this investigation comparable beech lumber and thermally treated beech lumber were used. The raw material is shown in Fig. 3 (untreated beech is lighter in colour than treated beech). The thermal modification takes place under hot and steamy conditions (temperatures up to 200 °C over 6 h). The steam is necessary to avoid the cracking and rupturing of the wood. For the samples clear wood without disturbing structural components (e.g. knots) was used. The chemical changes and the changing properties of thermally modified wood were analysed in several investigations. For beech wood Tjeerdsma and Militz [2] used FTIR analysis to determine the chemical changes in thermally modified wood. Repellin and Guyonnet [3] analysed the wood swelling behaviour of thermally treated beech wood by differential scanning calorimetry in relation to its chemical composition. These publications only represent a small proportion of the research work relating to thermally modified wood within the last few years.
In this study, the behaviour of the following toy components was analysed: 1. Components with a press fit (fix joints). These joints are used to connect components which should not be disassembled later. 2. Components which are connected, but there should be at least one degree of freedom for movement (moveable joints). The specimens used here allow two movements, a displacement and a rotation round an axis. 3. Circular components which can be used e.g. as raw part for a gear wheel or a flywheel (dimension stability of circular components). In Table 1 the number of treated samples and reference sample is shown.
2.1. Fix joints test In order to determine whether press fits behave differently for thermally treated or untreated beech wood, two samples per group were produced. A rendering of a specimen is shown in Fig. 4. The direction of the grain of the cube is parallel in one sample and in the second sample it is perpendicular to the grain direction of the bolt.
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Fig. 3. The thermally treated and untreated lumber for the samples.
Table 1 Number of samples per test Test
Untreated wood number of specimens
Treated wood number of specimens
Fix joints Moveable joints Dimension stability of circular components
2 2 3
2 2 3
Fig. 5. The testing principle for movable joints and a sample in the strength testing machine.
Fig. 4. The testing principle of the press fit and a sample in the strength testing machine.
Table 2 Climates per test Climate 20 °C/35% 20 °C/50% 20 °C/65% 20 °C/85%
Fix joints rel. rel. rel. rel.
h. h. h. h.
a
2 1 – –
Moving parts
Dimension stability
– 1 2 3
2 1 3 4
Rel. h., relative humidity; °C, degree Celsius. a The number shows the test sequence.
The climatic conditions for the samples were selected according to DIN 52184 [4]. The samples were stored in the climates until the weight of the sample remained constant over the specified time (i.e.: there is no more sorption process). The climates used are shown in Table 2. Analysis for each type of climate was performed in order to ascertain whether the press fit still functioned, or if the components could be separated without force. For the driest climate where the joint functioned, the samples were tested. The bolt of the sample stuck on one side in the middle of the upper cube (10 mm deep, see Fig. 4) while it passed through the whole of the lower cube. During the test a force was applied on the upper cube (see Fig. 4) while the lower was fixed. The samples are referred to as follows: F1 – Untreated wood. The grain direction of the cube is parallel to the grain direction of the bolt. F2 – Untreated wood. The grain direction of the cube is perpendicular to the grain direction of the bolt. F3 – Treated wood. The grain direction of the cube is parallel to the grain direction of the bolt. F4 – Treated wood. The grain direction of the cube is perpendicular to the grain direction of the bolt.
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Fig. 6. The three different disc types for evaluating the dimension stability.
2.2. Moving parts test
3.1. Fix joint test
In Fig. 5, a rendering shows the samples produced to evaluate moving parts. The disc was fixed on the wooden axis by a press fit. The cube could rotate round its axis and furthermore the bolt can be shifted in axis direction through the cube. The direction of grain of the cube is parallel to the grain direction of the bolt. The climates used are shown in Table 2. Under these conditions, an analysis was carried out to determine whether the bolt could still be moved or if the bolt had swollen too much and a press fit had occurred. Furthermore the strength of the joint was tested at 20 °C and 85% humidity. Therefore a force was applied on the bolt as shown in the rendering. Overall 4 samples were produced. The samples are referred to as follows:
The samples were stored according to the test design and the press fit worked for all samples (no joint was released through wood shrinking). However, the strength tests showed the influence of wood modification at the driest climate 20 °C and 35% rel. h. (see Fig. 7). The force–displacement diagrams of the tests show, that the load to shift the bolt in the cube increases very fast up to a specific force (see Fig. 7). When this load is reached, the bolt is shifted in the cube and the load increase is only negligible. In order to compare the samples, the maximum load while testing was selected. One cube of sample F3 was broken at the maximum load of 300 N, but this did not affect the results, as the other samples did not reach a maximum load higher than 200 N. The strength of the fixed joint of untreated samples is much lower than the strength of the treated ones. The direction of grain is very important, as the samples F1 (respectively F3) reached a higher strength compared to the samples F2 (F4). Very often fix joints in wooden toys are made according to F2 and F4. In Fig. 8, it is shown impressively that these joints are a toy’s weak point. Through the thermal modification the negative effects of mixed grain directions in wooden toys can be significantly reduced. Note that sample F4 bears the double load compared to sample F1.
M1 and M2 – Untreated wood samples. M3 and M4 – Treated wood samples.
2.3. Dimensional stability of circular components The dimensional stability of a circular disc was analysed for three different types of discs (always one thermally treated and one untreated sample). The disc types are shown in Fig. 6, and their main properties are: Type I – The cut direction is longitudinal and the thickness of the disc is 5 mm. Type II – The cut direction is longitudinal and the thickness of the disc is 3 mm. Type III – The cut direction is perpendicular to grain and the thickness of the disc is 3 mm. The climates used are shown in Table 2. The dimension of the samples was measured per climate. Therefore the disc was divided into 12 segments and the deformation of each segment was approximated by measuring the length of the border lines (see Fig. 6). The deformation was calculated as follows: Length of a segment at the observed climate (in mm) minus the length at 20 °C and 50% rel. humidity. The samples are referred to as follows: S1 S2 S3 S4 S5 S6
– – – – – –
Untreated wood. Disc Type I. Untreated wood. Disc Type II. Untreated wood. Disc Type III. Treated wood. Disc Type I. Treated wood. Disc Type II. Treated wood. Disc Type III.
The climates for each test are shown in Table 2. The numbers in the table represent the sequence of climates per test.
3. Results All samples were stored in the different climates until their weight was constant and no further sorption processes took place.
3.2. Moving parts test The treated wood samples worked in all climates while the untreated samples did not work at 20 °C and 85% rel. h., and worked badly at 20 °C and 65% rel. h. The two untreated samples (M1 and M2) were tested after storage in 20 °C and 85% rel. h. (see Fig. 8). The load to shift the bolt increased fast until a specific load was reached and when the bolts moved, the load increased only slowly (as was observed for the press fits). The maximum load for both samples was 80 N. Although the climate of 20 °C and 85% rel. h. is not usual in residential dwellings, the functionality of moving parts could be improved by using thermally modified wood. This assumption is supported by the behaviour of the M1 and M2 samples at 20 °C and 65% rel. humidity. There the joint worked badly. The climate of 20 °C and 65% rel. h. causes moisture contents comparable to the common climatic conditions of 23 °C and 50% rel. h. (see Ref. [5]).
M. Ebner, A.J. Petutschnigg / Materials and Design 28 (2007) 1753–1759
Fig. 7. Force–displacement diagrams for the tested press fits at 20 °C and 35% rel. h.
Fig. 8. Force–displacement diagrams for the tested samples with moving parts at 20 °C and 85% rel. h.
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3.3. Dimensional stability of circular discs The samples were measured under all climatic conditions and the dimensional changes to the 20 °C/50% rel. h. climate were calculated. The percentage of the deformation for the samples is shown in Fig. 9. Please note the division of the colour scale is not equidistant for reasons of enhanced visualisation. In Fig. 9, it can be seen, that the untreated wood shrinks and swells much more than the treated wood. Furthermore the following properties are obvious: 1. It is not possible to compensate swelling and shrinking only by making parts with bigger dimensions. E.g. the samples S1 and S2 as well as S4 and S5 are behave very similarly. 2. Due to the lower shrinking in the longitudinal grain direction, the circular shape of treated samples is deformed to an ellipse.
Fig. 9. Deformation of the samples S1–S6 depending on the climate (reference climate: 20 °C/50% rel. h.).
3. The samples do not shrink symmetrically in all cases. This behaviour is due to the very heterogeneous structure of wood. 4. The samples of disc type III swell and shrink the most. Nevertheless sample S6 behaves like sample S2 and therefore the higher swelling of natural wood in the radial and tangential direction can be compensated through the thermal treatment. In the range of indoor climates both the swelling and shrinking of treated beech wood is significantly lower than that of untreated wood. The different behaviour of wood depending on the grain direction and the shape of the components must be considered when toys are designed. 4. Conclusions The ambient climate changes during the year and toys have to guarantee their functionality throughout the year. Despite the fact that thermally modified wood is more expensive than untreated wood, it can be recommended as a material for toys. The possibility to improve the functionality and reliability of wooden toys by using this new raw material will compensate the higher raw material price. The tests show, that fixed joints made of treated wood are able to bear higher loads when the wood shrinks due to climatic changes. E.g. the bolt heads of wooden screws or other wooden parts which are connected by fixed joints will be more reliable. Furthermore the results show that the modified wood is more suitable to make toys with moving parts than untreated wood. Through surface treatments also the hardness and smoothness of the wooden surface can be adapted, consequently the development of wooden toys with more functionality will be an interesting future research task. According to these findings, it can not be recommended to use untreated and treated wood together (e.g. to achieve certain aesthetic properties of the toy). The swelling and shrinking behaviour is too different to promote this combination. In addition to the development of toys made of thermally modified wood, two further research questions are of great importance. On one hand, the processing properties of thermally modified wood have to fulfil the requirements of industrial wooden toy producers. Treated wood has different properties to untreated wood, and the question arises whether common processing technologies can be used with minor adaptions, or if new processing technologies have to be developed. On the other hand the surface treatment of materials is important for toys. The regulation DIN EN 71 – 3 [6] is particularly important for coatings of wooden toys. Coatings which are applied on toys made of thermally modified wood have to fulfil those requirements. Therefore existing coatings have to be tested and, if necessary, adapted.
M. Ebner, A.J. Petutschnigg / Materials and Design 28 (2007) 1753–1759
Acknowledgement Financial support by the Austrian Science Fund under Grant P17434-N13 is gratefully acknowledged. References [1] Popper R, Niemz P, Eberle G. Investigations on the sorption of swelling properties of thermally treated wood. Holz-Roh Werkstoff 2005;63:135–48.
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[2] Tjeerdsma BF, Militz H. Chemical changes in hydrothermal treated wood: FTIR analysis of combined hydrothermal and dry heat-treated wood. Holz-Roh Werkstoff 2005;63:102–11. [3] Repellin V, Guyonnet R. Evaluation of heat-treated wood swelling by differential scanning calorimetry in relation to chemical composition. Holzforschung 2005;59:28–34. [4] DIN 52184. Testing of wood; determination of swellling and shrinkage (in German). Berlin: Deutsches Institut fu¨r Normung e.V.; 1979. [5] Kollmann F. Technology of wood and wood based products (in German). 2nd ed. Berlin: Springer; 1951. [6] DIN EN 71–3. Safety of toys – part 3: migration of certain elements (in German). Berlin: Deutsches Institut fu¨r Normung e.V.; 1979.
Potentials of thermally modified beech (Fagus sylvatica) wood for application in toy construction and design Michael Ebner b
a,*
, Alexander Johannes Petutschnigg
b
a Department of Product Design and Management, University of Applied Sciences Salzburg, Markt 136a, A-5431 Kuchl, Austria Department of Forest Products Technology and Management, University of Applied Sciences Salzburg, Markt 136a, A-5431 Kuchl, Austria
Received 27 October 2005; accepted 12 May 2006 Available online 17 July 2006
Abstract This paper deals with the potentials of thermally treated beech wood as raw material for making toys. Natural beech wood is a common toy material and the properties of treated and untreated wood are compared. Samples to analyse press fits and joints between moving components were developed and tested under varying climatic conditions. Furthermore samples to analyse the dimensional stability of wooden components were developed and observed under varying levels of humidity and room temperature. The results show that thermally modified beech wood is a promising material for the manufacture of wooden toys. It is assumed that this raw material will be used in toy industries in the future, and furthermore the design of wooden toys will be influenced as it can be proven that this raw material can be employed in diverse developments in relation to the design of functional toys. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Wooden toys; Beech wood; Thermally treated wood; Functional toys; Dimensional stability
1. Introduction In this paper, the properties of thermally treated beech wood for wooden toys are analysed. Therefore this new raw material is compared to untreated beech wood. Beech is widely used as a material for making toys and two wellknown designs are shown in Fig. 1. Wood absorbs and desorbs water if the surrounding climate changes. When water is absorbed wood swells and if water is desorbed wood shrinks. As the ambient climate in houses changes (e.g. seasonal differences), the wood in toys also swells and shrinks during the year. The joints in the various toy components have to ensure the functionality of the toy. As the wood swells and shrinks the tolerances required are usually large for wooden toys. Therefore toys with moving parts *
Corresponding author. Tel.: +43 50 2211 2012; fax: +43 50 2211 2099. E-mail addresses: [email protected] (M. Ebner), [email protected] (A.J. Petutschnigg). 0261-3069/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.05.015
or fine mechanical components are not made out of wood. If moving parts are made out of wood, the functionality of the toy can be impaired. In Fig. 2, there is on the one hand a wooden screw with a glued press fit that does not function due to swelling and shrinking, and on the other hand the plastic bearing of a tyre was employed in order to compensate for possible variations in dimension. Through thermal modification of wood the swelling and shrinking measures are influenced (see Ref. [1]). Consequently these properties of wood are changed and it can be assumed that fine mechanical moving parts can also be made of thermally modified wood, thus raising the question as to whether thermally modified wood can be used for the moving parts of toys. In the following this question is analysed and answered for beech wood. If the analysed material is suitable to make toy parts with lower tolerances, the sustainable material wood can substitute plastic materials in specific fields of application.
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M. Ebner, A.J. Petutschnigg / Materials and Design 28 (2007) 1753–1759
Nomenclature Rel. h. relative humidity °C degree Celsius F1–F4 sample references for testing press fits
M1–M4 sample references for testing joints between moving parts S1– S6 sample references for testing the dimension stability
Fig. 1. Toys made of beech wood. (a) Baufix , and (b) Matador.
Fig. 2. (a) Screws which were stored in a cold cellar with high rel. h. and afterwards in heated room in winter (very low rel. h.). The glued press fit did not work appropriate any more. (b) A wooden toy where functional parts are made of plastic.
2. Materials and methods Through the thermal modification on the one hand the colour of the wood changes and on the other hand, the main properties of wood are influenced. For this investigation comparable beech lumber and thermally treated beech lumber were used. The raw material is shown in Fig. 3 (untreated beech is lighter in colour than treated beech). The thermal modification takes place under hot and steamy conditions (temperatures up to 200 °C over 6 h). The steam is necessary to avoid the cracking and rupturing of the wood. For the samples clear wood without disturbing structural components (e.g. knots) was used. The chemical changes and the changing properties of thermally modified wood were analysed in several investigations. For beech wood Tjeerdsma and Militz [2] used FTIR analysis to determine the chemical changes in thermally modified wood. Repellin and Guyonnet [3] analysed the wood swelling behaviour of thermally treated beech wood by differential scanning calorimetry in relation to its chemical composition. These publications only represent a small proportion of the research work relating to thermally modified wood within the last few years.
In this study, the behaviour of the following toy components was analysed: 1. Components with a press fit (fix joints). These joints are used to connect components which should not be disassembled later. 2. Components which are connected, but there should be at least one degree of freedom for movement (moveable joints). The specimens used here allow two movements, a displacement and a rotation round an axis. 3. Circular components which can be used e.g. as raw part for a gear wheel or a flywheel (dimension stability of circular components). In Table 1 the number of treated samples and reference sample is shown.
2.1. Fix joints test In order to determine whether press fits behave differently for thermally treated or untreated beech wood, two samples per group were produced. A rendering of a specimen is shown in Fig. 4. The direction of the grain of the cube is parallel in one sample and in the second sample it is perpendicular to the grain direction of the bolt.
M. Ebner, A.J. Petutschnigg / Materials and Design 28 (2007) 1753–1759
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Fig. 3. The thermally treated and untreated lumber for the samples.
Table 1 Number of samples per test Test
Untreated wood number of specimens
Treated wood number of specimens
Fix joints Moveable joints Dimension stability of circular components
2 2 3
2 2 3
Fig. 5. The testing principle for movable joints and a sample in the strength testing machine.
Fig. 4. The testing principle of the press fit and a sample in the strength testing machine.
Table 2 Climates per test Climate 20 °C/35% 20 °C/50% 20 °C/65% 20 °C/85%
Fix joints rel. rel. rel. rel.
h. h. h. h.
a
2 1 – –
Moving parts
Dimension stability
– 1 2 3
2 1 3 4
Rel. h., relative humidity; °C, degree Celsius. a The number shows the test sequence.
The climatic conditions for the samples were selected according to DIN 52184 [4]. The samples were stored in the climates until the weight of the sample remained constant over the specified time (i.e.: there is no more sorption process). The climates used are shown in Table 2. Analysis for each type of climate was performed in order to ascertain whether the press fit still functioned, or if the components could be separated without force. For the driest climate where the joint functioned, the samples were tested. The bolt of the sample stuck on one side in the middle of the upper cube (10 mm deep, see Fig. 4) while it passed through the whole of the lower cube. During the test a force was applied on the upper cube (see Fig. 4) while the lower was fixed. The samples are referred to as follows: F1 – Untreated wood. The grain direction of the cube is parallel to the grain direction of the bolt. F2 – Untreated wood. The grain direction of the cube is perpendicular to the grain direction of the bolt. F3 – Treated wood. The grain direction of the cube is parallel to the grain direction of the bolt. F4 – Treated wood. The grain direction of the cube is perpendicular to the grain direction of the bolt.
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Fig. 6. The three different disc types for evaluating the dimension stability.
2.2. Moving parts test
3.1. Fix joint test
In Fig. 5, a rendering shows the samples produced to evaluate moving parts. The disc was fixed on the wooden axis by a press fit. The cube could rotate round its axis and furthermore the bolt can be shifted in axis direction through the cube. The direction of grain of the cube is parallel to the grain direction of the bolt. The climates used are shown in Table 2. Under these conditions, an analysis was carried out to determine whether the bolt could still be moved or if the bolt had swollen too much and a press fit had occurred. Furthermore the strength of the joint was tested at 20 °C and 85% humidity. Therefore a force was applied on the bolt as shown in the rendering. Overall 4 samples were produced. The samples are referred to as follows:
The samples were stored according to the test design and the press fit worked for all samples (no joint was released through wood shrinking). However, the strength tests showed the influence of wood modification at the driest climate 20 °C and 35% rel. h. (see Fig. 7). The force–displacement diagrams of the tests show, that the load to shift the bolt in the cube increases very fast up to a specific force (see Fig. 7). When this load is reached, the bolt is shifted in the cube and the load increase is only negligible. In order to compare the samples, the maximum load while testing was selected. One cube of sample F3 was broken at the maximum load of 300 N, but this did not affect the results, as the other samples did not reach a maximum load higher than 200 N. The strength of the fixed joint of untreated samples is much lower than the strength of the treated ones. The direction of grain is very important, as the samples F1 (respectively F3) reached a higher strength compared to the samples F2 (F4). Very often fix joints in wooden toys are made according to F2 and F4. In Fig. 8, it is shown impressively that these joints are a toy’s weak point. Through the thermal modification the negative effects of mixed grain directions in wooden toys can be significantly reduced. Note that sample F4 bears the double load compared to sample F1.
M1 and M2 – Untreated wood samples. M3 and M4 – Treated wood samples.
2.3. Dimensional stability of circular components The dimensional stability of a circular disc was analysed for three different types of discs (always one thermally treated and one untreated sample). The disc types are shown in Fig. 6, and their main properties are: Type I – The cut direction is longitudinal and the thickness of the disc is 5 mm. Type II – The cut direction is longitudinal and the thickness of the disc is 3 mm. Type III – The cut direction is perpendicular to grain and the thickness of the disc is 3 mm. The climates used are shown in Table 2. The dimension of the samples was measured per climate. Therefore the disc was divided into 12 segments and the deformation of each segment was approximated by measuring the length of the border lines (see Fig. 6). The deformation was calculated as follows: Length of a segment at the observed climate (in mm) minus the length at 20 °C and 50% rel. humidity. The samples are referred to as follows: S1 S2 S3 S4 S5 S6
– – – – – –
Untreated wood. Disc Type I. Untreated wood. Disc Type II. Untreated wood. Disc Type III. Treated wood. Disc Type I. Treated wood. Disc Type II. Treated wood. Disc Type III.
The climates for each test are shown in Table 2. The numbers in the table represent the sequence of climates per test.
3. Results All samples were stored in the different climates until their weight was constant and no further sorption processes took place.
3.2. Moving parts test The treated wood samples worked in all climates while the untreated samples did not work at 20 °C and 85% rel. h., and worked badly at 20 °C and 65% rel. h. The two untreated samples (M1 and M2) were tested after storage in 20 °C and 85% rel. h. (see Fig. 8). The load to shift the bolt increased fast until a specific load was reached and when the bolts moved, the load increased only slowly (as was observed for the press fits). The maximum load for both samples was 80 N. Although the climate of 20 °C and 85% rel. h. is not usual in residential dwellings, the functionality of moving parts could be improved by using thermally modified wood. This assumption is supported by the behaviour of the M1 and M2 samples at 20 °C and 65% rel. humidity. There the joint worked badly. The climate of 20 °C and 65% rel. h. causes moisture contents comparable to the common climatic conditions of 23 °C and 50% rel. h. (see Ref. [5]).
M. Ebner, A.J. Petutschnigg / Materials and Design 28 (2007) 1753–1759
Fig. 7. Force–displacement diagrams for the tested press fits at 20 °C and 35% rel. h.
Fig. 8. Force–displacement diagrams for the tested samples with moving parts at 20 °C and 85% rel. h.
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3.3. Dimensional stability of circular discs The samples were measured under all climatic conditions and the dimensional changes to the 20 °C/50% rel. h. climate were calculated. The percentage of the deformation for the samples is shown in Fig. 9. Please note the division of the colour scale is not equidistant for reasons of enhanced visualisation. In Fig. 9, it can be seen, that the untreated wood shrinks and swells much more than the treated wood. Furthermore the following properties are obvious: 1. It is not possible to compensate swelling and shrinking only by making parts with bigger dimensions. E.g. the samples S1 and S2 as well as S4 and S5 are behave very similarly. 2. Due to the lower shrinking in the longitudinal grain direction, the circular shape of treated samples is deformed to an ellipse.
Fig. 9. Deformation of the samples S1–S6 depending on the climate (reference climate: 20 °C/50% rel. h.).
3. The samples do not shrink symmetrically in all cases. This behaviour is due to the very heterogeneous structure of wood. 4. The samples of disc type III swell and shrink the most. Nevertheless sample S6 behaves like sample S2 and therefore the higher swelling of natural wood in the radial and tangential direction can be compensated through the thermal treatment. In the range of indoor climates both the swelling and shrinking of treated beech wood is significantly lower than that of untreated wood. The different behaviour of wood depending on the grain direction and the shape of the components must be considered when toys are designed. 4. Conclusions The ambient climate changes during the year and toys have to guarantee their functionality throughout the year. Despite the fact that thermally modified wood is more expensive than untreated wood, it can be recommended as a material for toys. The possibility to improve the functionality and reliability of wooden toys by using this new raw material will compensate the higher raw material price. The tests show, that fixed joints made of treated wood are able to bear higher loads when the wood shrinks due to climatic changes. E.g. the bolt heads of wooden screws or other wooden parts which are connected by fixed joints will be more reliable. Furthermore the results show that the modified wood is more suitable to make toys with moving parts than untreated wood. Through surface treatments also the hardness and smoothness of the wooden surface can be adapted, consequently the development of wooden toys with more functionality will be an interesting future research task. According to these findings, it can not be recommended to use untreated and treated wood together (e.g. to achieve certain aesthetic properties of the toy). The swelling and shrinking behaviour is too different to promote this combination. In addition to the development of toys made of thermally modified wood, two further research questions are of great importance. On one hand, the processing properties of thermally modified wood have to fulfil the requirements of industrial wooden toy producers. Treated wood has different properties to untreated wood, and the question arises whether common processing technologies can be used with minor adaptions, or if new processing technologies have to be developed. On the other hand the surface treatment of materials is important for toys. The regulation DIN EN 71 – 3 [6] is particularly important for coatings of wooden toys. Coatings which are applied on toys made of thermally modified wood have to fulfil those requirements. Therefore existing coatings have to be tested and, if necessary, adapted.
M. Ebner, A.J. Petutschnigg / Materials and Design 28 (2007) 1753–1759
Acknowledgement Financial support by the Austrian Science Fund under Grant P17434-N13 is gratefully acknowledged. References [1] Popper R, Niemz P, Eberle G. Investigations on the sorption of swelling properties of thermally treated wood. Holz-Roh Werkstoff 2005;63:135–48.
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[2] Tjeerdsma BF, Militz H. Chemical changes in hydrothermal treated wood: FTIR analysis of combined hydrothermal and dry heat-treated wood. Holz-Roh Werkstoff 2005;63:102–11. [3] Repellin V, Guyonnet R. Evaluation of heat-treated wood swelling by differential scanning calorimetry in relation to chemical composition. Holzforschung 2005;59:28–34. [4] DIN 52184. Testing of wood; determination of swellling and shrinkage (in German). Berlin: Deutsches Institut fu¨r Normung e.V.; 1979. [5] Kollmann F. Technology of wood and wood based products (in German). 2nd ed. Berlin: Springer; 1951. [6] DIN EN 71–3. Safety of toys – part 3: migration of certain elements (in German). Berlin: Deutsches Institut fu¨r Normung e.V.; 1979.