Hornbeam wood is known for its high density, hardness, toughness, and wear resistance, but due to its low durability (Class 5 according to EN 350), limited wood quality, and rather small sawmill yield, it is mainly utilized as firewood today. The potential for hornbeam to be used as solid, high-quality wood material exists if its durability and dimensional stability can be increased. Hornbeam boards were acetylated under industrial conditions and tests were carried out to evaluate the treatability of this wood species by acetylation. In this study, the examination of physical, mechanical, and durability properties of acetylated hornbeam wood are described and compared to untreated hornbeam and to acetylated beech, which has a similar anatomical structure to hornbeam. Acetylated hornbeam was also compared to acetylated radiata pine, which is the main product of Accsys Technologies. These comparisons include the determination of the equilibrium moisture content, density, dimensional stability, accelerated checking, color change, water uptake, decay resistance, compression strength, modulus of rupture (MOR), modulus of elasticity (MOE), impact bending strength, Janka hardness, Brinell hardness, and impact bending strength. The aim of this project is the creation of a new product thereby widening the usage of this species.
Acetylation is a chemical modification process that has been studied by scientists around the world for almost 90 years (
During acetylation, acetic anhydride is used as a dehydrating agent. The hydroxyl groups are replaced by acetyl groups, which results in dimensionally stable wood. There are many characteristics of wood (like extractives, sample parameters, moisture, density, permeability and wood quality) and treatment settings (like catalysts, purity of the anhydride, initial moisture content, temperature, pressure, etc.) that influence the final products’ properties. After acetylation, the samples are taken for chemical quality assurance and the weight percentage gain is calculated (WPG).
The dimensional stability or anti-swelling efficiency (ASE) is produced by wood cell wall bulking. At 20% WPG, an impressive 70% ASE is found. This means that wood modified to a WPG of 20% will shrink and swell by about one-fourth of the amount exhibited with the same unmodified wood, which is a significant improvement. However, under normal service conditions the actual absolute swelling and shrinkage would be far less since wood in service is never oven dried for two days at 103 °C and then immersed continuously in water for five days (
These phenomena were experienced by many researchers. The decrease in EMC was observed in acetylated beech, poplar, Scots pine, radiata pine (
The increased WPG enables better ASE in acetylated radiata pine, southern pine, ponderosa pine, hard maple, walnut, elm, cativo, eucalyptus (
As the weight percentage increases, density also increases in acetylated beech, poplar, Scots pine, radiata pine (
In the cases of poplar and black locust wood,
Acetylated material is far more resistant against any biological attack. It is the WPG rather than OH substitution that determines the degree of decay resistance (
Acetylation increases the weight of wood, which also increases wood density giving it higher compression strength and hardness properties. This phenomenon is more prominent in saturated wood as the acetylation-induced moisture content reduction in turn increases the tensile strength, the modulus of rupture (MOR), and the modulus of elasticity (MOE). Some cell wall degradation may occur due to the enhanced heat and pressure as well as the presence of acetic acid (
Hornbeam (
The aim of this study was to perform a variety of tests on untreated and acetylated hornbeam wood (from the same area of origin) and offer conclusions and suggestions regarding the feasibility of this product. If the results are promising, the process can be optimized for hornbeam; thus, a new product group for outdoor usage can be developed and the usage of the wood species can be widened as well.
Twenty pieces of edged and air-dry boards were ordered from a Hungarian sawmill. Their dimensions were 28 × 160 × 2500 mm3 (T x W x L). The wood was from the southwest part of Hungary. Half of the boards were put aside as untreated control specimens and the other half were transported to Accsys Technologies (Netherlands).
Before acetylation, the dimensions were measured with a measuring tape (± 0.5 mm) and a caliper (± 0.01 mm). The moisture content was determined with an electrical resistance moisture meter equipped with insulated electrodes according to
There were no twists or cracks observed on the boards after acetylation. The WPG of ten boards was calculated.
The EMC and density were measured in every test according to
The maximum water uptake after 49 days was determined on 50 samples in each case according to
The color was expressed in CIE
A non-industrial accelerated checking test was performed according to a house method to determine how prone the wood is to develop checks. Before the test, 20 cm long pieces were cut from each board (10 untreated and 10 acetylated) and each was examined for splits or checks. The surfaces were marked to indicate the bark (B) and heart (H) sides. Thereafter, the samples were submerged in water at room temperature for 24 hours. Immediately after the water stage, the samples were placed in an oven at 40 °C and the temperature was increased to as high as 103 °C until constant mass was achieved. The pieces were observed for checks and splits on the surface after drying.
Fungi resistance was determined according to
The compression strength parallel to the grain was determined according to
The impact bending strength was determined according to
Janka hardness was determined in every anatomical direction, on dry and water-saturated specimens according to
Brinell hardness was determined in every anatomical direction, on dry and water-saturated specimens according to
The WPG ranged between 13.6% and 16.5%, having an average of 15.3%. The WPG level indicated that the wood was successfully impregnated.
Hornbeam was unable to absorb as much moisture in an acetylated state as it can in a natural state. EMC values decreased by 70% after acetylation and the fiber saturation point (FSP) was 35% lower. Due to weight gain, density changed by 16% in the dry state, 8% in the conditioned state, and 4% in the saturated state (
Tangential shrinkage was 41% greater than radial shrinkage in untreated samples and 43% greater in treated samples (
In connection with the moisture content reduction, the acetylated specimens had 17% lower water uptake/loss values (
Compared to acetylated beech and acetylated radiata pine, acetylated hornbeam had 18% and 80% lower water uptake after 49 days, respectively (
This change of hygroscopicity indicates that acetylated hornbeam has lower hysteresis and lower sorption isotherms than natural hornbeam (
Due to forced moisture and temperature changes, the untreated hornbeam samples discolored, warped, cracked, and developed internal checks and fiber cell collapses. None of the untreated samples remained intact; cracks appeared on both sides as well as on the end grain. On the contrary, the acetylated samples were more dimensionally stable; only a few hairline cracks appeared because of the accelerated desorption. There were no detectable differences in appearance between the heart and bark side of the board.
The darkening of acetylated wood highly depends on the reaction conditions and the catalyst used (pyridine, dimethylformamide, etc.), and of course on the wood species (chemical structure, permeability) itself (
Natural hornbeam has wavy grain because of its growing pattern, but this look is more prominent when it is acetylated. Besides this, there were stains because of condensed water on the boards as a result of drying, but these were removed by the first planing. During the acetylation process, the reagent could fully penetrate the wood. There was no envelope effect, just 1-2 mm thick, darker crusting where the acetyl content is the highest. Internal cracks were observed on the acetylated samples as they were taken from the end of the boards where wood moisture desorption is more intensive.
The test results show that going deeper into the wood, the surface color becomes more homogenous (
Acetylation greatly improved the fungi resistance of hornbeam, as can be seen in
Due to acetylation, hornbeam gained a harder and denser structure, which provided 43% higher compression strength properties. As seen in
During the bending test of the dry samples, the untreated and acetylated samples gave different fractures. Most of the samples broke on the outer side where tension stress was induced. In some cases, the sample had skewed grain, which resulted in weaker bending strength (<100 MPa). The difference in strength and flexibility is also reflected in the way the samples broke: the untreated samples had brittle fractures while the acetylated samples had stiff fractures (
Due to the increased density and hardness, hornbeam had 88% higher impact bending strength after acetylation. This is also reflected in the way the samples broke - untreated hornbeam had brittle, clean breaks while acetylated hornbeam had stiff, splintery fractures after impact. The treated material tends to have an inhomogeneous structure and sometimes lower dynamic loading properties because of the cell wall degradation caused by acetylation. Hornbeam tends to have a twisted or wavy grain, which needs to be taken into account during sample production. These wood defects are caused by natural growth and not hornbeam-specific settings used in the acetylation process, which can explain the high deviation of the results (
The Janka hardness of the tangential surface was 9% higher than in tangential direction in both untreated and acetylated samples. After saturation, this difference increased to 26% for untreated samples and decreased to 4% for acetylated samples. However, cracks tend to appear across the pith rays when the tangential surface is tested.
After acetylation, the tangential and radial surfaces’ hardness increased by 55% and 56%, respectively, whereas in the saturated samples, these increased by 154% and 111%, respectively (
During the Janka hardness test of the end grain, the acetylated samples broke; those measurements are not included in the results. As a side note,
The Brinell hardness of the tangential surface was 28% higher than the radial surface in the acetylated samples and 13% higher in the untreated samples. After saturation, this difference decreased to 25% for the acetylated samples and 7% for the untreated samples (
After acetylation, the hardness values increased by 49%, 68% and 51% on the radial, tangential and end grain surface, respectively. With the saturated samples, the hardness values were 124%, 163%, and 145% higher on the radial, tangential and end grain surface compared to natural hornbeam. The hardness of untreated samples decreased after saturation by 46%, 48%, and 55% on the radial, tangential surface and end grain, respectively. As for the acetylated samples, the same values decreased by 18%, 19% and 26%, respectively. The high deviation in the results indicates inhomogeneity within the wood, but this can be explained by not hornbeam-specific acetylation settings and the relatively small testing surface used during the Brinell hardness test (
During the acetylation process, the cell walls of hornbeam became bulked due to the reaction between the hydroxyl groups of the cell wall and the acetic anhydride. Acetyl groups replaced the OH groups, which are responsible for the swelling and shrinkage of wood. As a result, hornbeam became less sensitive to moisture than it had been before. This is confirmed by our results, as the EMC, FSP, shrinking rate, and water uptake decreased and the ASE increased.
Acetylated samples showed a lower tendency to crack than natural hornbeam, which can also be explained by the bulking effect.
Due to acetylation, hornbeam’s color darkened, became less homogenous, and developed a wavy figure. According to the color measurement, lightness (
Acetylation prevented all three fungi species from attacking hornbeam, which in its natural state is a non-durable wood species (Class 5 according to
It is difficult to state clearly how acetylation affects the mechanical properties of every wood species because there are many mechanisms that take place. On one hand, the capability of wood to absorb moisture decreases, which can influence the mechanical properties positively; also, the properties do not weaken as drastically as in the case of untreated wood during soaking. The density also increases as a result of the weight percentage gain. On the other hand, as the wood swells because of the acetyl groups, there will be fewer fibers in the cross section, which can decrease the mechanical properties. If the right settings are used for each wood species, the positive and negative effects can be kept in balance or tilted in the positive direction depending on the field of use and the properties to be improved. In the case of hornbeam wood, every mechanical property increased, which is very promising even though there were many instances of high variation in the results. Acetylated hornbeam showed even higher strength and hardness properties than those of acetylated beech and radiata pine.
Hornbeam, in its natural form, is a non-durable wood species with a strong, hard, dense, tough and wear-resistant structure. Its sensitivity to moisture and low durability has hindered its use outdoors, but with acetylation it became a denser, less moisture-sensitive, more dimensionally stable and more durable material. In addition to acetylated beech and radiata pine, it could become a raw material for many indoor and outdoor products that are exposed to varying humidity, fungi, and heat load, such as decking, marine decking, fencing, outdoor stairs, furniture, handrails, etc. After these promising research findings, further examinations that will focus on the optimization of the acetylation treatment of hornbeam are planned. In addition, other tests are being considered regarding behavior to other exposures like surface treatability and bonding, and technological properties like corrosion resistance and workability.
We thank Accsys Technologies (Arnhem, the Netherlands) for their cooperation and support in the acetylation process and for sharing their experience and ideas. We also thank the colleagues at the Institute of Wood Science in Sopron (Hungary) who helped during the measurements.
Water uptake and loss of untreated and acetylated hornbeam after 144 hours.
Color properties of acetylated hornbeam surfaces in different depths. (
Fracture diagram of control (Co) and acetylated (Ac) hornbeam samples according to bending tests.
Force-indentation diagrams of Janka hardness tests. Control (Co) and acetylated (Ac) samples.
Force-indentation diagrams of Brinell hardness tests of control (Co) and acetylated (Ac) samples.
Physical properties of untreated and acetylated hornbeam, beech and radiata pine; values in brackets are standard deviations. Literature data: (MB):
Property | Parameter | Hornbeam | Beech | Radiata pine | |||
---|---|---|---|---|---|---|---|
Acetylated | Control | Acetylated | Control | Acetylated | Control | ||
Moisture content (%) | Fiber saturation point | 24 (2.3) | 37 (1.9) | - | 32-35 MB | - | - |
Equilibrium at 20 °C 65% | 3 (0.3) | 10 (0.5) | - | - | - | - | |
Density (kg m-3) | Dried | 801 (66.3) | 689 (15.9) | - | 680 W | - | 400 W |
Conditioned | 823 (53.6) | 761 (53.5) | 800 AT | 720 W | 510 AT | 510 W | |
Saturated | 982 (58.2) | 942 (35.5) | - | 1070 W | - | 800 W | |
Shrinkage (%) | Radial | 1.17 (0.47) | 6.46 (0.67) | 1.3 AT | 5.9 AT | 0.7 AT | 2.3 W |
Tangential | 2.04 (0.83) | 10.86 (0.80) | 2.2 AT | 12.9 AT | 1.5 AT | 4.5 W | |
Longitudinal | 0.32 (0.20) | 0.42 (0.30) | - | 0.3 MB | - | 0.3 W | |
Max. water uptake (g m-2) | 4559 (1035) | 5513 (1137) | 5600 AT | - | 23442 AT | - |
Color properties of acetylated hornbeam, hornbeam and some similar wood species (
Wood species |
|
|
|
|
|
|
---|---|---|---|---|---|---|
Acetylated hornbeam | 50.05 | 6.97 | 20.42 | 71.08° | 21.58 | - |
Hornbeam | 76.78 | 3.69 | 20.44 | 79.79° | 20.77 | 26.93 |
Beech ( |
71.08 | 9.09 | 19.28 | 64.75° | 21.31 | 21.16 |
Walnut ( |
51.30 | 6.21 | 14.32 | 66.55° | 15.60 | 6.27 |
Mutenye ( |
52.00 | 9.38 | 22.99 | 67.80° | 24.82 | 4.02 |
Weight loss of untreated and acetylated samples exposed to wood-decay fungi for 16 weeks. Values are means ± standard deviation.
Fungus | Vessel | Sample | Weight loss (%) |
---|---|---|---|
|
Virulence | Control | 45.64 ± 3.68 |
Reference | Control | 18.58 ± 1.07 | |
Acetylated | 0.84 ± 0.17 | ||
|
Virulence | Control | 20.78 ± 1.54 |
Reference | Control | 21.19 ± 6.61 | |
Acetylated | 0.20 ± 0.21 | ||
|
Virulence | Control | 34.00 ± 1.91 |
Reference | Control | 32.66 ± 2.09 | |
Acetylated | 0.83 ± 0.12 |
Strength and elasticity properties of untreated and acetylated hornbeam, beech and radiata pine; values are means ± standard deviation. Literature data: (MB):
Property | Hornbeam | Beech | Radiata pine | ||||
---|---|---|---|---|---|---|---|
Acetylated | Control | Acetylated | Control | Acetylated | Control | ||
Compression strength parallel to grain (N mm-2) | 84.0 ± 6.6 | 59.0 ± 4.3 | - | 62 MB | - | 51 W | |
Modulus of rupture (N mm-2) | Conditioned | 173.0 ± 25.2 | 144.0 ± 9.8 | 114 AT | 127 AT | 39 AT | 78 W |
Saturated | 141.0 ± 15.2 | 73.0 ± 6.4 | 107 AT | 60 AT | - | - | |
Modulus of elasticity (kN mm-2) | Conditioned | 15.4 ± 1.5 | 15.4 ± 1.2 | 12.15 AT | 13.03 AT | 8.79 AT | 11 W |
Saturated | 14.1 ± 1.4 | 10.4 ± 1.0 | 11.68 AT | 7.81 AT | - | - | |
Impact bending strength (kJ m-2) | 159.0 ± 37.0 | 84.0 ± 10.8 | - | 100 MB | 50 AT | 94 W |
Hardness properties of untreated and acetylated hornbeam, beech and radiata pine. Values are means ± standard deviation. Literature data: (MB):
Property | Hornbeam | Beech | Radiata pine | ||||
---|---|---|---|---|---|---|---|
Acetylated | Control | Acetylated | Control | Acetylated | Control | ||
Janka hardness (conditioned)(N mm-2) | Radial | 114 ± 17.9 | 73 ± 4.8 | 79 AT | 71 AT | 41 AT | 28 AT |
Tangential | 124 ± 21.4 | 80 ± 6.8 | - | - | 42 AT | 28 AT | |
End grain | - | 95 ± 7.3 | 107 AT | 84 AT | 66 AT | 36 AT | |
Janka hardness (saturated)(N mm-2) | Radial | 83 ± 10.9 | 39 ± 2.9 | - | - | - | - |
Tangential | 105 ± 15.3 | 41 ± 5.7 | - | - | - | - | |
End grain | - | 46 ± 3.5 | - | - | - | - | |
Brinell hardness (conditioned)(N mm-2) | Radial | 39 ± 5.6 | 26 ± 2.4 | - | 34 MB | - | 13 W |
Tangential | 50 ± 9.1 | 29 ± 2.7 | - | 34 MB | - | 13 W | |
End grain | 101 ± 12.7 | 67 ± 10.4 | - | 72 MB | - | - | |
Brinell hardness (saturated)(N mm-2) | Radial | 32 ± 4.7 | 14 ± 1.2 | - | - | - | - |
Tangential | 40 ± 7.8 | 15 ± 1.5 | - | - | - | - | |
End grain | 74 ± 15.8 | 30 ± 3.1 | - | - | - | - |