**4.2 Physical properties of gamma irradiated wood**

Gamma radiation lead to significant colour changes of wood. With increasing radiation dose the darkening of the specimens increased as can be seen from Figure 4.

Fig. 4. Colour change of specimens used for mass loss determination, **3AA** – control group, **3AB** – group irradiated with 30 kGy, **3AC** – group irradiated with 90 kGy and **3AD** – group of specimens irradiated with 150 kGy.

#### **4.2.1 Decrease in mass caused by gamma irradiation**

On the question whether there is any loss of mass or density of the wood due to gamma radiation, Loos (1962) in his studies noted no significant changes in the density of wood. Seifert (1964) stated the possibility of negligible loss of CO2 from wood due to radiation induced chemical reactions. However, wood irradiated in the presence of air might absorb atmospheric nitrogen in small quantities (Seifert, 1964). Tsutomu *et al.* (1977) reported on a a very small effect of gamma radiation on specific gravity of wood and cellulose. Curling and Winandy (2008) reported that density of southern pine sapwood stayed unchanged by any tested level of irradiation dose or dose rate of gamma radiation. Hasan (2006) and Despot *et al.* (2007) measured oven dry mass of Scots pine (*Pinus sylvestris*) specimens. They determined no statistically significant changes in mass before and after irradiation at a confidence interval of 95 % although specimens were gamma irradiated in plastic bags with the presence of air (Figure 5).

Changes in Selected Properties of Wood Caused by Gamma Radiation 289

chains and the solubility of smaller cellulose fragments in water (Seifert, 1964; Tabirih *et al.*, 1977; Šimkovic *et al.*, 1991). Despot *et al.* (2007) reported that for 10 days after gamma treatment dm was significantly higher than after 150 days for each applied dose (Table 2). This finding is in contrast to the results of Fengel and Wegener (1989) and Tišler and Medved (1997), but it can easily be explained by repolymerisation reactions between the free

**[days] 10 150** 

G [kGy] 30 90 150 30 90 150 Minimum dm [%] 1.1773 1.6206 2.2598 0.9146 1.2781 1.8308 Mean dm [%] 1.3999 2.0900 3.0026 1.1471 1.5423 2.0438 Maximum dm [%] 1.9200 3.0700 4.4500 1.2916 1.8732 2.3433 Standard deviation [%] 0.2675 0.5449 0.9213 0.1126 0.1741 0.1805 Coefficient of variation [%] 19.11 26.07 30.68 9.82 11.29 8.83

T – value 6.2780 8.3589 9.0220 4.8216 8.8164 14.2067

Probability of mistake [%] 0.0020 0.0001 0.00 0.0271 0.00 0.00 Signification degree +++ +++ +++ +++ +++ +++

Panshin and de Zeeuw (1980) stated that the arrangement of the cellulose crystals in microfibrils can be observed by the existence of amorphous zones along the microfibril length, in which the crystallinity is interrupted. These zones allow the penetration of chemicals into the microfibrils. Furthermore, the gamma radiation caused break-up of cellulose to shorter chains, which are water-soluble, and it most likely leads to an "opening of additional microcracks", in which water molecules can easily penetrate. Consequently it is to expect that gamma irradiated wood swells faster but also more than non-irradiated wood. Results by Hasan (2006), Hasan *et al.* (2006b) and by Despot *et al.* (2007; 2008) showed no significant influence of radiation dose on maximum swelling, neither in radial (αR MAX) nor in tangential (αT MAX) direction (Figure 7). Leached irradiated wood showed significantly higher αR MAX compared to non-leached wood. With increasing radiation dose the difference in αR MAX between leached and non-leached specimens became more significant (Despot *et al.*, 2007). Obviously, cellulose chains and lignin (Curling and Winandy, 2008) were affected by gamma radiation and the "plywood effect" of the wood rays has been reduced and α<sup>R</sup> MAX increased. In contrast, αT MAX of leached irradiated wood significantly decreased. It can be explained by the decreasing of tangential vessel wall thickness, ray cell double-wall thickness and latewood fibre double-wall thickness, as described by Tabirih *et al.* (1977) and Cutter *et al.* (1980). Despot *et al.* (2007; 2008) found no significant influence of time after

Table 2. Results of statistical analysis on differences of decrease in mass (dm) by leaching between non-irradiated controls and with different doses irradiated specimens 10 and 150 days after the gamma treatment for 95 % confidence interval; n=15; (+ significant, ++ high

radicals caused by irradiation (Seifert, 1964; Bogner *et al.*, 1997).

**Time after gamma treatment** 

Gamma radiation dose,

significant, +++ highest significant).

**4.2.3 Maximum swelling, (αMAX)** 

gamma radiation on αMAX.

Fig. 5. Comparison of dry mass of specimens before and after gamma radiation.

#### **4.2.2 Decrease in mass by leaching, (dm)**

Leaching in water of Scots pine sapwood specimens caused a significant decrease in mass (dm). A strong linear correlation was found between gamma radiation doses (G) and dm (Despot *et al.*, 2007; Figure 6). This might be explained by random break up of cellulose

Fig. 6. Correlation between decrease in mass (dm*)* by leaching and gamma radiation dose (G) for 10 and 150 days after gamma treatment.

Fig. 5. Comparison of dry mass of specimens before and after gamma radiation.

Leaching in water of Scots pine sapwood specimens caused a significant decrease in mass (dm). A strong linear correlation was found between gamma radiation doses (G) and dm (Despot *et al.*, 2007; Figure 6). This might be explained by random break up of cellulose

0 20 40 60 80 100 120 140 160

G [kGy]

Fig. 6. Correlation between decrease in mass (dm*)* by leaching and gamma radiation dose

**dm = 0.0083×G + 0.8081 R2 = 0.9862**

10 d 150 d

**4.2.2 Decrease in mass by leaching, (dm)** 

0,0

0,5

1,0

1,5

dm [%]

2,0

2,5

3,0

3,5

**dm = 0.0145×G + 0.8312 R2 = 0.9900**

(G) for 10 and 150 days after gamma treatment.

chains and the solubility of smaller cellulose fragments in water (Seifert, 1964; Tabirih *et al.*, 1977; Šimkovic *et al.*, 1991). Despot *et al.* (2007) reported that for 10 days after gamma treatment dm was significantly higher than after 150 days for each applied dose (Table 2). This finding is in contrast to the results of Fengel and Wegener (1989) and Tišler and Medved (1997), but it can easily be explained by repolymerisation reactions between the free radicals caused by irradiation (Seifert, 1964; Bogner *et al.*, 1997).


Table 2. Results of statistical analysis on differences of decrease in mass (dm) by leaching between non-irradiated controls and with different doses irradiated specimens 10 and 150 days after the gamma treatment for 95 % confidence interval; n=15; (+ significant, ++ high significant, +++ highest significant).

#### **4.2.3 Maximum swelling, (αMAX)**

Panshin and de Zeeuw (1980) stated that the arrangement of the cellulose crystals in microfibrils can be observed by the existence of amorphous zones along the microfibril length, in which the crystallinity is interrupted. These zones allow the penetration of chemicals into the microfibrils. Furthermore, the gamma radiation caused break-up of cellulose to shorter chains, which are water-soluble, and it most likely leads to an "opening of additional microcracks", in which water molecules can easily penetrate. Consequently it is to expect that gamma irradiated wood swells faster but also more than non-irradiated wood. Results by Hasan (2006), Hasan *et al.* (2006b) and by Despot *et al.* (2007; 2008) showed no significant influence of radiation dose on maximum swelling, neither in radial (αR MAX) nor in tangential (αT MAX) direction (Figure 7). Leached irradiated wood showed significantly higher αR MAX compared to non-leached wood. With increasing radiation dose the difference in αR MAX between leached and non-leached specimens became more significant (Despot *et al.*, 2007). Obviously, cellulose chains and lignin (Curling and Winandy, 2008) were affected by gamma radiation and the "plywood effect" of the wood rays has been reduced and α<sup>R</sup> MAX increased. In contrast, αT MAX of leached irradiated wood significantly decreased. It can be explained by the decreasing of tangential vessel wall thickness, ray cell double-wall thickness and latewood fibre double-wall thickness, as described by Tabirih *et al.* (1977) and Cutter *et al.* (1980). Despot *et al.* (2007; 2008) found no significant influence of time after gamma radiation on αMAX.

Changes in Selected Properties of Wood Caused by Gamma Radiation 291

on maximum swelling (Despot *et al.*, 2007). It is probable that specimens irradiated with

Loos (1962), Ifju (1964), Shuler *et al.* (1975), El-Osta *et al.* (1985), Hasan (2006), Hasan *et al.* (2006b) and Despot *et al.* (2007; 2008) verified that gamma-radiation-induced depolymerisation causes a significant decrease in wood strength. Curling and Winandy (2008) reported that dose rate and total dose of gamma radiation differently affected bending strength of pine wood (*Pinus* sp.). They found that cumulative duration of radiation exposure period appeared to be more critical than total dose in determining overall strength loss. When total gamma radiation dose was directly compared at or near the critical doses required to achieve sterilization, shorter exposures using higher dose rates affected strength less than longer exposures using lower dose rates (Curling and Winandy, 2008). Loos (1962) comes to 1 kGy as a threshold – wood toughness had a tendency of linear increase, followed by a progressive decrease. Gamma radiation depolymerised wood had significantly reduced tensile strength, although early wood showed an initial increase in tensile strength (Ifju, 1964). Tsutomu *et al.* (1977) reported on considerable decrease in strength of wood with increasing irradiation dosage, depending remarkably on loading modes. Testing the tensile and compression strengths parallel to the grain of gamma-irradiated wood, El-Osta *et al.* (1985) have found a slight increase in tensile and compression strength till the dose of 1.4 kGy. Increasing the gamma radiation dose, tensile and compression strengths constantly decreased. Decrease in tensile strength was more pronounced than that in compression strength. Researching dynamic modulus of elasticity of spruce wood (*Picea* sp.) Shuler (1971) noted the increase of the modulus in the range of radiation doses from 0 to 1 kGy while

further increase of radiation dose decreased dynamic modulus of elasticity.

dynamic MOE was visible with high degrees of determination.

Shuler *et al.* (1975) measured bending strength of American elm (*Ulmus americana*), which was exposed to gamma radiation during growth. The results showed that small doses of radiation up to 0.22 kGy increased bending strength on average by 30 %, and further increase in dose lead to a continuous and significant decrease of bending strength. Measuring the bending strength, Shuler *et al.* (1975) noted the increase of the modulus of elasticity in the range of radiation doses from 0 to 0.22 kGy while further increase of radiation dose lead to the decrease of the modulus of elasticity and decrease of the

Csupor *et al.* (2000) irradiated wood in the range of 2 – 1400 kGy and concluded that a dose of 12 kGy was sufficient for wood sterilization while modulus of elasticity at this dose was not significantly reduced (0.2 %). Tests have also shown that an initially higher modulus of elasticity decreased more rapidly with increasing radiation dose. Divos and Bejo (2005) reported on a steady and a continuous decrease of modulus of elasticity (MOE) for all tested wood species in the range of gamma radiation dose from 130 to 770 kGy. Results also showed a difference in the intensity decrease of MOE between the wood species, and wood density had significant influence on decrease intensity of the MOE. Lower densities resulted in faster decrease in MOE. Interpolating the obtained data Divos and Bejo (2005) concluded that the dose sufficient for wood sterilization not significantly decreased MOE. In the graphs (Figure 9) an inversely proportional correlation between the gamma radiation dose and

higher radiation dose reaches faster the MCMAX.

proportionality limit.

**4.3 Mechanical properties of gamma irradiated wood** 

Fig. 7. Correlation between maximum swelling (αMAX) in radial (R) and tangential (T) direction of non-leached and leached specimens and gamma radiation dose (G).

#### **4.2.4 Maximum moisture content (MCMAX)**

Maximum moisture content, (MCMAX) of each irradiated group of specimens was equal or higher than the MC of controls (Hasan, 2006; Hasan *et al.*, 2008). It is well known, that glucose and other simple sugars make wood considerably more hygroscopic (Fengel and Wegener, 1989). No significant influence of gamma radiation dose on MCMAX was found (Figure 8). Radiation dose did not have significant influence on maximum MC, the same as

Fig. 8. Correlation between maximal moisture content (MCMAX) and gamma radiation dose (G).

on maximum swelling (Despot *et al.*, 2007). It is probable that specimens irradiated with higher radiation dose reaches faster the MCMAX.

#### **4.3 Mechanical properties of gamma irradiated wood**

290 Gamma Radiation

Fig. 7. Correlation between maximum swelling (αMAX) in radial (R) and tangential (T) direction of non-leached and leached specimens and gamma radiation dose (G).

Maximum moisture content, (MCMAX) of each irradiated group of specimens was equal or higher than the MC of controls (Hasan, 2006; Hasan *et al.*, 2008). It is well known, that glucose and other simple sugars make wood considerably more hygroscopic (Fengel and Wegener, 1989). No significant influence of gamma radiation dose on MCMAX was found (Figure 8). Radiation dose did not have significant influence on maximum MC, the same as

Fig. 8. Correlation between maximal moisture content (MCMAX) and gamma radiation dose (G).

**4.2.4 Maximum moisture content (MCMAX)** 

**MAX[%]** 

Loos (1962), Ifju (1964), Shuler *et al.* (1975), El-Osta *et al.* (1985), Hasan (2006), Hasan *et al.* (2006b) and Despot *et al.* (2007; 2008) verified that gamma-radiation-induced depolymerisation causes a significant decrease in wood strength. Curling and Winandy (2008) reported that dose rate and total dose of gamma radiation differently affected bending strength of pine wood (*Pinus* sp.). They found that cumulative duration of radiation exposure period appeared to be more critical than total dose in determining overall strength loss. When total gamma radiation dose was directly compared at or near the critical doses required to achieve sterilization, shorter exposures using higher dose rates affected strength less than longer exposures using lower dose rates (Curling and Winandy, 2008). Loos (1962) comes to 1 kGy as a threshold – wood toughness had a tendency of linear increase, followed by a progressive decrease. Gamma radiation depolymerised wood had significantly reduced tensile strength, although early wood showed an initial increase in tensile strength (Ifju, 1964). Tsutomu *et al.* (1977) reported on considerable decrease in strength of wood with increasing irradiation dosage, depending remarkably on loading modes. Testing the tensile and compression strengths parallel to the grain of gamma-irradiated wood, El-Osta *et al.* (1985) have found a slight increase in tensile and compression strength till the dose of 1.4 kGy. Increasing the gamma radiation dose, tensile and compression strengths constantly decreased. Decrease in tensile strength was more pronounced than that in compression strength. Researching dynamic modulus of elasticity of spruce wood (*Picea* sp.) Shuler (1971) noted the increase of the modulus in the range of radiation doses from 0 to 1 kGy while further increase of radiation dose decreased dynamic modulus of elasticity.

Shuler *et al.* (1975) measured bending strength of American elm (*Ulmus americana*), which was exposed to gamma radiation during growth. The results showed that small doses of radiation up to 0.22 kGy increased bending strength on average by 30 %, and further increase in dose lead to a continuous and significant decrease of bending strength. Measuring the bending strength, Shuler *et al.* (1975) noted the increase of the modulus of elasticity in the range of radiation doses from 0 to 0.22 kGy while further increase of radiation dose lead to the decrease of the modulus of elasticity and decrease of the proportionality limit.

Csupor *et al.* (2000) irradiated wood in the range of 2 – 1400 kGy and concluded that a dose of 12 kGy was sufficient for wood sterilization while modulus of elasticity at this dose was not significantly reduced (0.2 %). Tests have also shown that an initially higher modulus of elasticity decreased more rapidly with increasing radiation dose. Divos and Bejo (2005) reported on a steady and a continuous decrease of modulus of elasticity (MOE) for all tested wood species in the range of gamma radiation dose from 130 to 770 kGy. Results also showed a difference in the intensity decrease of MOE between the wood species, and wood density had significant influence on decrease intensity of the MOE. Lower densities resulted in faster decrease in MOE. Interpolating the obtained data Divos and Bejo (2005) concluded that the dose sufficient for wood sterilization not significantly decreased MOE. In the graphs (Figure 9) an inversely proportional correlation between the gamma radiation dose and dynamic MOE was visible with high degrees of determination.

Changes in Selected Properties of Wood Caused by Gamma Radiation 293

The following five fractions were separated and weighed: Fraction 1 (F1, > 5 mm), Fraction 2 (F2, 3–5 mm), Fraction 3 (F3, 2–3 mm), Fraction 4 (F4, 1–2 mm) and Fraction 5 (F5, < 1 mm).

The degree of integrity *I*, which is the ratio of the mass of the 10 biggest fragments to the

*all*

The fine fraction F5 is the ratio of the mass of fraction 5 (<1 mm) to the mass of all fractions *m*all multiplied by 100. Finally, the resistance to impact milling (RIM) was calculated from *I* 

The resistance to impact milling (RIM) decreased significantly with increasing radiation dose. Time after gamma treatment had no significant influence on RIM neither of non-

Fig. 11. Resistance to impact milling (RIM) of leached and non-leached specimens for different gamma doses (G) and different periods after gamma treatment (n = 3 × 10

not influence the linear correlation between RIM and G (Table 3).

Despot et al. (2007) and Hasan et al. (2006b) found linear dependence between RIM and G for both, non-leached and leached specimens. The elapsed time after gamma radiation did

*m I*

leached nor of leached specimens (Despot *et al.* 2007; Figure 11).

<sup>10</sup> 100 % *biggest fragments*

 (3 \* 5) 300 % 4

*<sup>m</sup>* . (1)

*I F RIM* . (2)

The following values were calculated:

mass of all fractions *m*all after crushing:

**4.3.2 Resistance to impact milling (RIM)** 

and F5 as follows:

specimens).

Fig. 9. Correlation between dynamic MOE and gamma radiation dose: a) longitudinal MOE of aspen; b) transverse MOE of spruce (taken from: Divos and Bejo, 2005).

#### **4.3.1 Structural integrity of gamma irradiated wood**

The High-energy multiple impact (HEMI) test has been developed to characterise the effect of thermal and chemical modification procedures as well as fungal and bacterial decay on the structural integrity of wood. The development and optimization of the HEMI-test have been described by Brischke *et al.* (2006a). In studies on the effect of gamma radiation onto structural integrity of gamma irradiated wood (Hasan, 2006; Hasan *et al.*, 2006b; Despot *et al.*, 2007; 2008) the following procedure was applied:

Five of ten specimens were placed in the bowl (140 mm inner diameter) of a heavy vibratory impact ball mill, together with one steel ball of 35 mm Ø, 3 of 12 mm Ø, and 3 of 6 mm Ø. The bowl was shaken for 60 s at a rotary frequency of 23.3 s-1 and a stroke of 12 mm. This crushing procedure was repeated on another five specimens (Figure 10). The fragments of the ten specimens were fractionated with slit screens on an orbital movement shaker (amplitude: 25 mm, rotary frequency: 250 min-1, duration: 10 min).

Fig. 10. Experimental set up of the High-energy multiple impact (HEMI) test. Left: Bowl with steel balls of different sizes and wood fragments. Right: Cutting scheme of the steel bowl and movement schedule.

Fig. 9. Correlation between dynamic MOE and gamma radiation dose: a) longitudinal MOE

The High-energy multiple impact (HEMI) test has been developed to characterise the effect of thermal and chemical modification procedures as well as fungal and bacterial decay on the structural integrity of wood. The development and optimization of the HEMI-test have been described by Brischke *et al.* (2006a). In studies on the effect of gamma radiation onto structural integrity of gamma irradiated wood (Hasan, 2006; Hasan *et al.*, 2006b; Despot *et* 

Five of ten specimens were placed in the bowl (140 mm inner diameter) of a heavy vibratory impact ball mill, together with one steel ball of 35 mm Ø, 3 of 12 mm Ø, and 3 of 6 mm Ø. The bowl was shaken for 60 s at a rotary frequency of 23.3 s-1 and a stroke of 12 mm. This crushing procedure was repeated on another five specimens (Figure 10). The fragments of the ten specimens were fractionated with slit screens on an orbital movement shaker

Fig. 10. Experimental set up of the High-energy multiple impact (HEMI) test. Left: Bowl with steel balls of different sizes and wood fragments. Right: Cutting scheme of the steel

of aspen; b) transverse MOE of spruce (taken from: Divos and Bejo, 2005).

**4.3.1 Structural integrity of gamma irradiated wood** 

*al.*, 2007; 2008) the following procedure was applied:

bowl and movement schedule.

(amplitude: 25 mm, rotary frequency: 250 min-1, duration: 10 min).

The following five fractions were separated and weighed: Fraction 1 (F1, > 5 mm), Fraction 2 (F2, 3–5 mm), Fraction 3 (F3, 2–3 mm), Fraction 4 (F4, 1–2 mm) and Fraction 5 (F5, < 1 mm). The following values were calculated:

The degree of integrity *I*, which is the ratio of the mass of the 10 biggest fragments to the mass of all fractions *m*all after crushing:

$$I = \frac{m\_{10\quad\text{bigges}t} \cdot \text{fragments}}{m\_{all}} \times 100 \text{[\%]}.\tag{1}$$

The fine fraction F5 is the ratio of the mass of fraction 5 (<1 mm) to the mass of all fractions *m*all multiplied by 100. Finally, the resistance to impact milling (RIM) was calculated from *I*  and F5 as follows:

$$RIM = \frac{I - \text{(3\* } F5) + \text{300}}{4} \text{[\%]} \,. \tag{2}$$

#### **4.3.2 Resistance to impact milling (RIM)**

The resistance to impact milling (RIM) decreased significantly with increasing radiation dose. Time after gamma treatment had no significant influence on RIM neither of nonleached nor of leached specimens (Despot *et al.* 2007; Figure 11).

Fig. 11. Resistance to impact milling (RIM) of leached and non-leached specimens for different gamma doses (G) and different periods after gamma treatment (n = 3 × 10 specimens).

Despot et al. (2007) and Hasan et al. (2006b) found linear dependence between RIM and G for both, non-leached and leached specimens. The elapsed time after gamma radiation did not influence the linear correlation between RIM and G (Table 3).

Changes in Selected Properties of Wood Caused by Gamma Radiation 295

level. In contrast, macroscopic defects, such as cracks and splitting, will not affect the results, because they will be masked through finer fragmentation of the wood samples (Welzbacher *et al.*, 2011). As a result of various different dynamic loads (impact bending, end grain bending, compression, cleaving, shearing, and buckling) the reduced structural integrity of the wood is reflected by the HEMI test (Welzbacher *et al.*, 2011). The significant decrease in RIM through gamma radiation is therefore considered to be also attributed to the break-up of cellulose chains (Hasan *et al.*, 2006b; Despot *et al.*, 2007; 2008). This again coincides with the higher sensitivity of the HEMI-test to cellulose break down by brown rot

Despot *et al.* (2006) and Hasan *et al.* (2006a, 2008) used pine sapwood specimens, one group steam sterilised (at 123 °C for 30 min) and second group of specimens gamma-irradiated in the range of doses of 30, 90 and 150 kGy. Specimens were subject to laboratory decay resistance tests. According to EN 113 (1996) the specimens were incubated in testing flasks with pure cultures of test fungi. They used brown- and white-rot fungi for the study. Mass loss by fungal decay (ML) was determined by weighing the oven-dry specimens before (m1)

1 3

(3)

1 100 % *m m ML m*

As gamma radiation causes break-up of cellulose to shorter chains, which are water-soluble, and that leads to an "opening of additional microcracks", in which water molecules can easily penetrate. Consequently, gamma irradiated wood is also more accessible to enzymes

Only ten days after incubation, it was clearly visible, that irradiated specimens were more overgrown, than control steam sterilised controls, which indicates a higher susceptibility to biodegradation of gamma irradiated wood compared to non-irradiated wood (Hasan, 2006a;

Fig. 13. Specimen pairs in testing flasks 10 days after exposure to brown-rot fungus *Gloeophyllum trabeum* (AC, KCC, KDC – non-irradiated controls, BC – specimen irradiated

with 30 kGy, CC – 90 kGy, DC – specimen irradiated with dose of 150 kGy).

fungi compared to white rot decay (Brischke *et al.*, 2006b; 2008).

and after (m3) incubation to the nearest 1 mg and according to equation (3):

**4.4 Biological durability of gamma irradiated wood** 

**4.4.1 Resistance against decay fungi** 

of wood decaying fungi.

Hasan *et al.*, 2008; Figure 13).


Table 3. Fitting curve equations for the relationship between RIM and G for non-leached and leached specimens at different periods after gamma treatment.

Despot *et al.* (2007) and Hasan *et al.* (2006b) calculated RIM relative to the controls (RIMr) for non-leached and leached specimens as the ratio between mean RIM of gamma treated specimens and mean RIM of non-leached controls. RIMr linearly depends on the radiation dose and decreases stronger for leached wood than for non-leached wood. Maximum reduction in RIM was 12 % at the highest gamma dose (Figure 12).

Fig. 12. Correlation between RIM relative to the controls (RIMr) of non-leached and leached specimens and gamma radiation dose (Despot *et al.* 2007).

Brischke *et al.* (2006a), Rapp *et al.* (2006), and Welzbacher *et al.* (2006) described the sensitive HEMI test, which is able to reveal fine changes in "dynamic strength properties" (increased brittleness). This has been confirmed for thermally and chemically modified timber (Welzbacher *et al.* 2007, 2011; Brischke *et al.* 2012) as well as for differently degraded timber, e.g. by different basidiomycetes, soft rot fungi and bacteria (Rapp *et al.*, 2008; Brischke *et al.*, 2009; Huckfeldt *et al.*, 2010). Also the findings in this research confirmed the suitability of the HEMI test for detecting subtle differences in the mechanical properties of wood. In particular, it is the structural integrity, which affects the Resistance to impact milling RIM, wherefore the results of the HEMI test characterise structural changes on a cellular micro

leached 10 RIM = -0.0706×G + 81.478 0.9613

Table 3. Fitting curve equations for the relationship between RIM and G for non-leached and

Despot *et al.* (2007) and Hasan *et al.* (2006b) calculated RIM relative to the controls (RIMr) for non-leached and leached specimens as the ratio between mean RIM of gamma treated specimens and mean RIM of non-leached controls. RIMr linearly depends on the radiation dose and decreases stronger for leached wood than for non-leached wood. Maximum

Fig. 12. Correlation between RIM relative to the controls (RIMr) of non-leached and leached

Brischke *et al.* (2006a), Rapp *et al.* (2006), and Welzbacher *et al.* (2006) described the sensitive HEMI test, which is able to reveal fine changes in "dynamic strength properties" (increased brittleness). This has been confirmed for thermally and chemically modified timber (Welzbacher *et al.* 2007, 2011; Brischke *et al.* 2012) as well as for differently degraded timber, e.g. by different basidiomycetes, soft rot fungi and bacteria (Rapp *et al.*, 2008; Brischke *et al.*, 2009; Huckfeldt *et al.*, 2010). Also the findings in this research confirmed the suitability of the HEMI test for detecting subtle differences in the mechanical properties of wood. In particular, it is the structural integrity, which affects the Resistance to impact milling RIM, wherefore the results of the HEMI test characterise structural changes on a cellular micro

**treatment [days] Fitting curve equation Regression** 

5 RIM = -0.0332×G + 81.989 0.9933 10 RIM = -0.0246×G + 82.006 0.9626 30 RIM = -0.048×G + 81.597 0.9206 150 RIM = -0.0462×G + 81.951 0.9769

150 RIM = -0.0484×G + 81.641 0.9763

**coefficient, R2**

**Time after gamma** 

leached specimens at different periods after gamma treatment.

reduction in RIM was 12 % at the highest gamma dose (Figure 12).

specimens and gamma radiation dose (Despot *et al.* 2007).

**Leaching procedure** 

> nonleached

level. In contrast, macroscopic defects, such as cracks and splitting, will not affect the results, because they will be masked through finer fragmentation of the wood samples (Welzbacher *et al.*, 2011). As a result of various different dynamic loads (impact bending, end grain bending, compression, cleaving, shearing, and buckling) the reduced structural integrity of the wood is reflected by the HEMI test (Welzbacher *et al.*, 2011). The significant decrease in RIM through gamma radiation is therefore considered to be also attributed to the break-up of cellulose chains (Hasan *et al.*, 2006b; Despot *et al.*, 2007; 2008). This again coincides with the higher sensitivity of the HEMI-test to cellulose break down by brown rot fungi compared to white rot decay (Brischke *et al.*, 2006b; 2008).

### **4.4 Biological durability of gamma irradiated wood**

Despot *et al.* (2006) and Hasan *et al.* (2006a, 2008) used pine sapwood specimens, one group steam sterilised (at 123 °C for 30 min) and second group of specimens gamma-irradiated in the range of doses of 30, 90 and 150 kGy. Specimens were subject to laboratory decay resistance tests. According to EN 113 (1996) the specimens were incubated in testing flasks with pure cultures of test fungi. They used brown- and white-rot fungi for the study. Mass loss by fungal decay (ML) was determined by weighing the oven-dry specimens before (m1) and after (m3) incubation to the nearest 1 mg and according to equation (3):

$$ML = \frac{m\_1 - m\_3}{m\_1} \times 100 \left[\% \right] \tag{3}$$
