Abstract
The effect of ethylene oxide (EO) sterilization and/or accelerated ageing on impact bending strength (IBS), tensile strength along and across the fiber (TSALF and TSACF), ultimate flexural strength along and across the fiber (FSAL and FSAC), and splitting resistance (SR) was tested in bulk and veneer wood. The IBS was greatly increased in beech but decreased in some oak samples. The EO treatment followed by ageing caused the TSACF to increase in bulk beech and to decrease in bulk oak. The EO treatment decreased the TSALF values in bulk oak, beech, and elm veneer. However, beech veneer samples exhibited increased TSALF. The ageing of EO treated samples decreased the TSALF values for veneers (oak and beech), while both bulk samples increased. A notable decline was observed due to EO treatment of oak and beech, along with post ageing decline for oak samples in FSAC values only. The FSAL values decreased after ageing of EO treated and untreated samples (oak and beech), while a post EO treatment decline was observed in oak samples only. The SR (Rw) values decreased in EO-only treated oak wood and increased in EO treated and aged samples (oak and beech).
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Effect of Ethylene Oxide Sterilization and Accelerated Ageing on the Physical and Mechanical Properties of Beech, Oak, and Elm Wood: Part 2
Daniela Tesařová,a Petr Čech,a Eva Jeřábková,a Jiří Stádník,a Josef Hlavatý,a Adam Ekielski,d Nora Rapavá,c and Pawan Kumar Mishra b,*
The effect of ethylene oxide (EO) sterilization and/or accelerated ageing on impact bending strength (IBS), tensile strength along and across the fiber (TSALF and TSACF), ultimate flexural strength along and across the fiber (FSAL and FSAC), and splitting resistance (SR) was tested in bulk and veneer wood. The IBS was greatly increased in beech but decreased in some oak samples. The EO treatment followed by ageing caused the TSACF to increase in bulk beech and to decrease in bulk oak. The EO treatment decreased the TSALF values in bulk oak, beech, and elm veneer. However, beech veneer samples exhibited increased TSALF. The ageing of EO treated samples decreased the TSALF values for veneers (oak and beech), while both bulk samples increased. A notable decline was observed due to EO treatment of oak and beech, along with post ageing decline for oak samples in FSAC values only. The FSAL values decreased after ageing of EO treated and untreated samples (oak and beech), while a post EO treatment decline was observed in oak samples only. The SR (Rw) values decreased in EO-only treated oak wood and increased in EO treated and aged samples (oak and beech).
Keywords: Ethylene oxide; Wood sterilization; Surface characterization; Oak wood; Elm wood; Beech wood
Contact information: a: Department of Furniture, Design and Habitat, Mendel University, Zemědělská 1665/1, 613 00 Brno-sever-Černá Pole, Czech Republic; b: Department of Wood Processing Technology, Mendel University, Zemědělská 1665/1, 613 00 Brno-sever-Černá Pole, Czech Republic; c: Slovak National Library, Námestie Jozefa Cígera Hronského 1, 036 01 Martin, Slovakia; d: Department of Production Management and Engineering, Warsaw University Of Life Sciences, Nowoursynowska 166, 02-787 Warszawa, Poland; *Corresponding author: Pawan.mishra@mendelu.cz
INTRODUCTION
Ethylene oxide (EO), also known as oxirane, is the simplest epoxide compound, and it is most commonly used as a fumigant for sterilization purposes. The advantage of EO as a sterilizing agent for porous materials (such as wood) is its gaseous nature, which helps its quick diffusion into pores and easy removal after sterilization. A number of studies have been reported on the sterilizing action of EO for wood based materials (Smith and Sharman 1971; Tohmura et al. 2012). A comparative study on the sterilizing actions of EO and propylene oxide (PO) on spruce heartwood blocks reported that in the presence of moisture bacteria was more resistant to PO than fungi, and ascomycetes were more resistant to PO than EO (Smith 1965; 1968). In another study, EO was studied as a sporicidal sterilizing agent for pine wood, archival paper, and painted canvas, and has also been studied for B. anthracis and B. atropheaus under optimal conditions (Whitney et al. 2003; US EPA 2013). Along with sterilization of wood, the tendency of EO to react with various chemical components of wood (cellulose, lignin, and hemicellulose) is due to the highly strained cyclic structure of EO, which favors a nucleophilic attack by hydroxyl groups, leading to the generation of an additional hydroxyl group that is available for further polymerization reaction (Kumar 1994; Cetin and Hill 1999; Kral et al. 2015). These epoxides have also been studied as a wood modifying agent because the chemical reaction between wood components and EO leads to polymer formation (Kumar 1994). Ageing (normal and storage) has been widely studied as an effect on the property of wooden material since it affects the mechanical properties of wood (Ganne-Chédeville et al. 2012; Kránitz 2014; Mishra et al. 2018). However, the effect of ageing on EO treated wood is relatively less studied. Ethylene oxide is the fumigant of choice for libraries and museums due to its ease of application and practicality.
This manuscript represents the second part of a two-stage study. The first part reported the effect of EO and/or accelerated ageing on the surface characteristics of beech, oak and elm wood species (Tesařová et al. 2018). In this study, beech, oak, and elm species were studied to gauge the impact of EO treatment and/or ageing (accelerated testing) on mechanical properties. Two different types of thicknesses (veneer and solid wood) were used as samples. The goal was to understand the immediate and long-term effects (using accelerated ageing methods) of EO treatment on the properties of wood. The derived aim of this study was to understand the mechanical performance of library and museum furniture (immediate and long term) following EO treatment.
EXPERIMENTAL
Materials
Three different species of wood were considered, beech, oak, and elm. Due to Dutch elm disease, samples of solid wood/bulk wood in the required thickness were not available in the central European market, and they were not used in this study. Instead, elm samples of a thickness of 0.6 mm were used. Different samples with specifications, treatments, and sample codes are presented in Table 1. All wooden boards were supplied Potrusil s.r.o (Brno, Czech Republic) and sampling was done was at university laboratories.
Methods
Ethylene oxide treatment
The effect of the EO treatment was similar to what was reported in Part 1 of this study. A sterilization chamber (DeLama, San Martino Siccomario, Italy) with a usable volume of 1.8 m3was used to sterilize the samples. As a sterilization medium, the device employs a gaseous mixture of ethylene oxide and carbon dioxide in a ratio of 10:90. During the sterilization process, the P5 sterilization program (Inbuilt in the instrument) was used that involved an operating temperature of 25 °C, an overpressure of 3.8 bars, and a 50% wetting of the material (by EO).
The whole process lasted 720 min. After completion of the sterilization program, wood samples were subjected to the P98 aeration program (Inbuilt in the Instrument) to remove ethylene oxide residues. With the P98 venting program, the air was exchanged 88 times in 22 h.
Accelerated ageing
Accelerated ageing was performed using a modified method based on ČSN 673098 (1988). The accelerated ageing process was done for 25 cycles. Each cycle consisted of two stages: one at 50 °C and a relative humidity (RH) of 75% for 12 h, followed by the second stage at -30 °C and a RH of 0% for 12 h. The Memmert HPP 108 air conditioning chamber (Swabach, Germany) was used at an operating temperature range of 5 °C to 70 °C ± 1 °C and RH range of 10 to 90% ± 1.5%. The freezer used was an Elcold (HPP108, Hobro, Denmark) with operating range of 0 °C to -40 °C ± 0.5 °C, with setting gradations of 1 °C.
Table 1a. Explanation of Different Sample Codes Used in the Study
Impact bending strength (IBS)
Impact bending strength tests (Charpy impact test) were completed to compare the effects of EO treatment and ageing on the fracture resistance of the wood samples. All tests were done using an Instron CEAST 9050 Impact Pendulum (Norwood, MA, USA) per ČSN 490117 (1982) standards. The shape of test body was a rectangular prism. The IBS (Aw) was calculated using the formula,
(1)
where Q is the work done in splitting/disturbing the sample in joules, and b and h are dimensions (cm) of the sample in radial and tangential directions. Usually, IBS is calculated at fixed humidity but the effect of humidity was ignored due to comparative nature of study (all samples were measured at same humidity level).
Tensile strength along and across the fibers (TSALF and TSACF)
For the tensile strength measurements an Instron UTM 3365 (Norwood, MA, USA) was used. ČSN 490113 (1992) and ČSN 490114 (1992) standards were employed for the tensile strength measurement along and across the grain, respectively. The modulus of elasticity parallel to grain was calculated from the linear part of the load curve using the strain gauge. Test bodies for both tests were prepared according to ČSN 490123 (2002). TSALF (σtw) was calculated using the following formula,
(2)
where Fmax is the break force in newtons (N), and a and b are width and thickness (mm) of test body/samples.
Ultimate flexural strength along and across the fibers (FSAL and FSAC)
Ultimate flexural strength was calculated using a three-point bending test. Dimensions of the test specimens were 20 × 20 mm in cross section, with a length of 300 mm. All measurements were done using an Instron UTM 3365 device (Norwood, MA, USA) per the ČSN 490115 (1979) standard. Fractural strength ( was calculated using the following formula,
(3)
where Fmax is the fracture load in N, l is the distance between the centers of the supports in mm, (h) is the height in mm, and (b) is the width of the test body in mm.
Splitting resistance
Splitting resistance was calculated using a ZWICK Z 0500 / TH3A Splitting Testing Device (Ulm, Germany) per the ČSN 490119 (1984) standard. In this test, the sample is placed in clamping jaws to firmly secure the sample in grooves. Maximum force is measured for splitting the sample; two different coefficients can be calculated with respect to width of test body and nominal area of splitting. Splitting resistance (Rw) was calculated using the following formula,
(4)
where Fmax is the maximum force in N, and A is the area of the test body in mm2 (A=bl, where b is the width of test sample, and l is the length of the split in mm.)
RESULTS AND DISCUSSION
Ageing generally can be assumed to involve the decomposition of hemicellulose and cellulose, cross-linking of lignin, rearrangement of polysaccharides, and hornification (Obataya 2017). However, all artificial ageing cannot be assumed similar to the natural ageing, as different ageing parameters (maximum temperature and presence or absence of UV light) affect different ageing processes differently. The impact bending strength of all tested samples is listed in Table 2, and Fig. 1 shows a mean interaction plot. In bulk beech samples, ageing and EO treatment followed by ageing showed a significant increase of 17.4% and 9.1% respectively, while EO-only treatment reflected an insignificant increase in IBS when compared with the control. In the case of beech and oak veneer samples, EO treatment and ageing caused an insignificant decrease in IBS values as compared to the control.
Table 2. IBS (Impact bending strength) Values Measured in J ×cm-2
Fig. 1. Mean interaction plot of measured IBS (Impact bending strength) values
A significant increase in IBS by ageing of EO treated oak veneer samples was observed; however this effect was insignificant in the case of beech veneer samples. The bulk oak wood samples showed an insignificant increase in IBS due to EO treatment and ageing separately; however a considerable decrease (significant) in IBS of the EO treated sample was observed because of ageing. The reason for the decreased IBS is unclear. The elm samples also behaved similarly to the oak and beech veneer samples, where an insignificant increase in IBS values was observed. However, a decrease in IBS and fungal attack on heat treated wood was also reported (Henningsson 1967; Kubojima et al. 2000). The decline in bending properties caused by ageing has also been reported in eastern white pine (Attar-Hassan 1976). The effect of ageing on Oak wood was also generally reported as negative (IBS value decrease with age) by previous reports (Sonderegger et al. 2015); however an inherent variability in wood has precluded a definitive conclusion (Cavalli et al. 2016).
The TSACF of the tested samples is presented in Table 3 with a mean interaction plot in Fig. 2. Measurements for the oak and elm veneers were not possible due to sample fragility. In case of bulk samples (beech and oak), the ageing caused a significant decline (12.13 and 31.5% respectively) in TSACF values with respect to control. The decline in TSCAF values after EO treatment was great 14.09% for bulk beech and insignificant for bulk oak samples as compared to the control.
Table 3. TSACF (Tensile strength across the fibers) Measured in N × mm-2
Fig. 2. Mean interaction plot of the measured TSACF (Tensile strength across the fibers) values
The tested samples of beech veneer showed an effect, which was mostly insignificant for every treatment. The EO treatment followed by ageing caused a significant increase in TSCAF values for beech and a significant decline for oak wood samples. The difference in behavior of these two types of wood can be attributed to the difference in relative chemical composition, and variability with in wood samples makes it difficult to generalize behavior as both type of effects have been reported in literature (Cavalli et al. 2016).
The TSALF values of measured samples are presented in Table 4 with a mean interaction plot in Fig. 3. In the case of beech samples (both bulk and veneer), ageing showed a significant increase (12.8 and 1.95%, respectively) in TSALF values; however, the effects of the EO treatment caused a significant decrease in bulk beech, and this effect was reversed in veneer samples (significant decrease). The EO treatment followed by ageing led to an increase and decrease (both significant) in TSALF values for bulk and veneer samples, respectively.
It was noticeable that sample thickness and EO treatment reversed the behavior of beech samples. For oak samples, the ageing caused a decrease and increase (both significant) in TSALF values for bulk and veneer samples, respectively. However, the decline (significant) in TSALF values by EO treatment was noticeable in bulk oak but insignificant in veneer oak. The EO treatment followed by ageing led to a visible rise in TSALF values of bulk oak samples but veneer samples showed a decline (both significant). In elm samples, three of the treatments led to a decline in TSALF values, out of which in EO treatment followed by ageing reflected an insignificant decline.
Table 4. TSALF (Tensile strength along the fibers) Measured in N × mm-2
Fig. 3. Mean interaction plot of estimated TSALF (Tensile strength along the fibers) values
The FSAL values of the tested samples can be found in Table 5 with a mean interaction plot in Fig. 4. In both cases of bulk samples (oak and beech) the ageing caused a significant decline in FSAL values. The effect of EO treatment was insignificant in beech samples; however, a significant decrease in FSAL value was observed in oak samples. The EO treatment followed by ageing led to a significant decrease in FSAL values, but these values were 10.6 and 10.9%. When compared to the EO treated beech and oak sample (4.15 and 22.6%), respectively. These decrease in FSAL value showed similar values as the result of ageing, which was relatively in only EO samples.
The FSAC values for the tested samples can be found in Table 6, with their mean interaction plot in Fig. 5. Like the FSAL values, the ageing caused a decline in FSAC values (insignificant in the case of beech). However, the decline shown in FSAC values by EO treatment was significant in both (beech and oak) types of samples. In the samples that underwent EO treatment followed by ageing the decline in FSAC values was visible (significant) in the beech samples and insignificant in the oak samples.
Table 5. FSAL (Ultimate flexural strength along the fibers) Measured in N × mm-2
Fig. 4. Mean interaction plot of measured FSAL (Ultimate flexural strength along fibers) values
Table 6. FSAC (Ultimate flexural strength across the fibers) measured in N×mm-2
Fig. 5. Mean interaction plot of measured FSAC (Ultimate flexural strength along the fibers) values
Splitting resistance (Rw) values of the tested samples are shown in Table 7, and their mean interaction plot is shown in Fig. 6. There was no significant effect of ageing on Rw values in either (beech or oak) samples. However, EO treatment caused a significant increase and insignificant decrease in Rw values of beech and oak samples, respectively. The EO treatment followed by ageing, showed a significant increase in Rw values for both types of samples. The post ageing Rw values of EO treated samples, when compared to EO-only treated samples, showed an insignificant increase in the case of oak samples and significant decrease in case of Bulk beech samples.
Table 7. Rw (Splitting resistance) measured in N × mm-2
Fig. 6. Mean interaction plot of the measured Rw (Splitting resistance) values
CONCLUSIONS
- In impact bending strength (IBS) measurements, the veneer samples for all three wood species did not show any significant effect of ageing, ethylene oxide (EO) treatment and EO treatment followed by ageing. However, in bulk beech showed a significant increase in IBS upon ageing and EO treatment followed by ageing samples. In oak samples (bulk), the effect of ageing and EO treatment were found to be insignificant; however, a significant decrease was observed in aged EO treated samples.
- In the measured values of tensile strength across the fiber (TSACF), the bulk samples of both types (oak and beech) showed a significant decrease with ageing. Those subjected to EO treatment followed by ageing reflected different behaviors: a significant increase in TSACF values in beech samples and a significant decrease in oak samples. In tensile strength along the fiber (TSALF) values, ageing caused a significant increase for beech (bulk and veneer samples) and oak veneer samples, while a significant decrease was registered in bulk oak and elm veneer samples. The EO treatment reflected a significant decrease in bulk (oak and beech) and elm veneer samples. However, a significant increase was registered for beech veneer samples. The ageing of EO treated samples caused a significant decline in the TSALF values for veneers (oak and beech), while both bulk samples showed a significant increase.
- In the ultimate flexural strength across the fiber (FSAC) value measurements, a significant decline was observed with EO treatment of both (oak and beech) samples, along with post ageing decline for oak samples only. In measured values of ultimate flexural strength along the fiber (FSAL), with ageing of EO treated and untreated, both samples showed a significant decline for both types (oak and beech) except ageing of EO treated oak (significant increase) and insignificant decrease ageing of EO treated Beech.
- In the measured values of splitting resistance (Rw), EO treatment followed by ageing led to a significant increase in splitting resistance (Rw) values for both samples with exception to the EO treated bulk oak.
ACKNOWLEDGMENTS
The authors are grateful for the support of the Technology Agency of the Czech Republic and IGA; grant number TH02020984 and LDF_VP_2018003 respectively along with OPIS (Operational Program Information Society) grant number ITMS – 21120120007, Bratislava, Slovakia for the financial support.
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Article submitted: April 20, 2018; Peer review completed: June 15, 2018; Revised version received and accepted: September 20, 2018; Published: September 26, 2018.
DOI: 10.15376/biores.13.4.8464-8476