**4. The influence of gamma radiation on wood**

Sterilisation by gamma radiation is very easy, fast and effective, but at doses higher than disinfestation doses it changes the molecular structures not only in wood decaying organisms but also in wood cell walls. Although Severiano *et al.* (2010) reported no influence of gamma radiation on some wood physical, thermal and mechanical properties in the radiation dose range between 25 and 100 kGy, the majority of other studies reported significant influence of gamma radiation on wood properties.

#### **4.1 Chemical properties of gamma irradiated wood**

The random break-up of cellulose chains in gamma irradiated wood is a typical reaction (Seaman *et al.*, 1952; Kenaga and Cowling, 1959). Seifert (1964), Tabirih *et al.* (1977) and Cutter *et al.* (1980) found that the holocellulose portion of cell walls was degraded by gamma irradiation. After exposure of cypress wood (*Pseudotsuga mensziessi*) and tulipwood (*Liriodendron tulipifera*) to gamma rays, Lhoneaux *et al.* (1984) confirmed the occurrence of ultra structural changes in the cell walls of tested wood species. Following the above research results, Fengel and Wegener (1989) reported that gamma irradiation changes the anatomical and chemical structure of wood, but also the physical and mechanical properties. Seifert (1964) also found that small doses of gamma radiation lead to a destruction of hemicelluloses' pentose creating new compounds and new chemical bonds. Chawla (1985) reported that up to the dose of 500 kGy the increase of wood solubility occurs primarily because of hemicelluloses depolymerisation and destruction. The influence of gamma radiation was also investigated on compounds of cellulose-acetate and cellulose nitrate (Fadel and Kasim, 1977; Zamani *et al.*, 1981; Subrahmanyam *et al.*, 1998). They came to the conclusion that also modified cellulose chains broke up. Seifert (1964) came to the conclusion that, an average increase of 25 kGy of gamma radiation caused a loss of 1 % cellulose in the dose range of 0 – 1 MGy. This proportional loss in cellulose crystallinity with increasing radiation dose was confirmed by Zamani *et al.* (1981) in a narrower interval of doses from 0 to 0.5 MGy. Fedel and Kasim (1977) successfully used cellulose-acetate as an indicator of the amount of absorbed energy of gamma irradiated objects, which reaffirms the proportionality between the reduction of cellulose crystallinity and gamma radiation dose.

Changes in Selected Properties of Wood Caused by Gamma Radiation 285

treatment. Curling and Winandy (2008) gamma irradiated southern pine sapwood specimens (*Pinus* spp.) at a range of nine total irradiation doses (from a 60Co source) applied at two different dose rates: 8.5 kGy/h (total doses of 15.0, 25.0, and 50.0 kGy) and 16.9 kGy/h (total doses of 15.0, 20.0, 25.0, 37.5, 50.0 and 75.0 kGy). Gamma radiation at higher (16.9 kGy/h) dose rate clearly had more negative effect on Mason lignin levels than did either the steam sterilization or gamma radiation at lower (8.5 kGy/h) dose rate. Gamma radiation at the 16.9 kGy/h dose rate reduced Klason lignin content from 29.5 % to 28.1 and to 27.5 %, but seemed to have little on-going effect related to dose accumulation. They also found that galactans, which are side-chain constituents of the hemicelluloses, are affected sooner and to a greater extent than xylans or mannans that represent the primary backbone of the hemicellulose polymers. Furthermore arabinan was probably unaffected by various radiation regimes. The authors suspected that the β-(13) linkage of the arabinan-xylan bond was less affected by radiation than the β-(16) linkage of the galactan-mannan bond.

Hasan (2006) and Despot *et al.* (2007; 2008) measured the total amount of water-soluble carbohydrates (TSC) according to Rapp *et al.* (2003). They used the following procedure: Five oven-dried specimens of each group were milled together for 60 s in a heavy vibratory disc mill (Herzog Maschinenfabrik, Osnabrück, Germany). 400 mg of wood powder were mixed up with one drop of detergent and 20 ml distilled water in 50 ml flasks. Afterwards, the flasks were shaken at 20 ºC for 60 min with a vibration of 120 min-1 and filtered through

The reagent for optical spectrophotometry was prepared by dissolving 2 g of dihydroxytoluol (Orcin) in 1l of 97 % sulphuric acid. For analysing the carbohydrate content 1 ml of dilution was mixed with 2 ml reagent in a test-tube and heated for 15 min at 100 ºC in a Thermo-block (Merck TR205). After cooling to room temperature, the dilution was analysed at 540 nm with a Merck SQ 115 filter-photometer. The TSC was calculated after calibration with a glucose

Hasan (2006) and Despot *et al.* (2007; 2008) found a strong influence of gamma irradiation on TSC. As gamma radiation causes destruction of cellulose chains and the resulting smaller cellulose fragments are easily soluble, increasing of TSC with increasing radiation dose was expected. They found no significant influence of time after gamma treatment on TSC

The sensitiveness of the TSC content to reveal changes in the carbohydrate structure of wooden materials was shown in earlier studies with thermally modified timber (TMT) where TSC was significantly reduced with ascending heat treatment temperature and increasing heat-induced loss of mass (Welzbacher *et al.,* 2004; 2009). Thus, the extraction of soluble carbohydrates appeared to be a sensitive method to display degradation of carbohydrates (Hasan, 2006; Despot *et al.*, 2007; 2008) and is therefore also considered as a

According to Fengel and Wegener (1989) and Tišler and Medved (1997), the highest TSC should be observed 150 days after gamma radiation. In contrast to mass loss by leaching, TSC did not decrease over time. Accordingly, the radical recombination is probably limited

glass filter paper. The filtrates were diluted with distilled water in proportion 1:5.

standard between 20 and 200 ppm leading to extinction between 0.12 and 1.32.

suitable tool to characterize the treatment intensity of gamma radiation.

to higher mol mass fragments but not to lower fragments, which appear as TSC.

(Figures 2 and 3).

**4.1.1 Total amount of water-soluble carbohydrates, (TSC)** 


Summary of data collected from the literature on chemical changes of wood caused by gamma radiation are presented in Table 1 and in Figure 1.

Table 1. Summary of data on the influence of various doses of gamma radiation onto main wood compounds.

Fig. 1. Different ranges of polymerisation degree (DP) of cellulose depending on gamma radiation dose (according to Struszczyk *et al.* (2004)).

Seifert (1964) found an increasing amount of free radicals in wood after gamma radiation, which could be effective after irradiation for wood modification in terms of their repolymerisation. Fengel and Wegener (1989) and Tišler and Medved (1997) discussed that the cellulose chains continue breaking down during 100 days after finishing the gamma

Summary of data collected from the literature on chemical changes of wood caused by

**Change Dose Reference** 

cellulose cross linking up to 1×10-3 kGy Seifert (1964)

6 to 12 % degradation 31,6 kGy Seifert (1964) 82 % degradation 1778,3 kGy Seifert (1964)

10 % degradation 1778,3 kGy Seifert (1964)

Table 1. Summary of data on the influence of various doses of gamma radiation onto main

Fig. 1. Different ranges of polymerisation degree (DP) of cellulose depending on gamma

Seifert (1964) found an increasing amount of free radicals in wood after gamma radiation, which could be effective after irradiation for wood modification in terms of their repolymerisation. Fengel and Wegener (1989) and Tišler and Medved (1997) discussed that the cellulose chains continue breaking down during 100 days after finishing the gamma

radiation dose (according to Struszczyk *et al.* (2004)).

Cellulose no change up to 31,6 kGy Seifert (1964)

Lignin no change up to 31,6 kGy Seifert (1964)

50 % reduction 0,95×106 kGy Cutter & McGinnes (1980) unchanged up to 300 kGy Tsutomu *et al.* (1977) rapid reduction above 1000 kGy Tsutomu *et al.* (1977) 100 % reduction 1,9×106 kGy Cutter & McGinnes (1980)

strong decrease of DP above 10 kGy Fengel & Wegener (1989)

complete degradation 6,55×103 kGy Fengel & Wegener (1989)

15 % degradation 19×103 kGy Cutter & McGinnes (1980)

Tabirih *et al.* (1977)

gamma radiation are presented in Table 1 and in Figure 1.

**Characteristics of compounds** 

Degree of crystallinity of cellulose

Degree of

cellulose

polymerisation of

wood compounds.

treatment. Curling and Winandy (2008) gamma irradiated southern pine sapwood specimens (*Pinus* spp.) at a range of nine total irradiation doses (from a 60Co source) applied at two different dose rates: 8.5 kGy/h (total doses of 15.0, 25.0, and 50.0 kGy) and 16.9 kGy/h (total doses of 15.0, 20.0, 25.0, 37.5, 50.0 and 75.0 kGy). Gamma radiation at higher (16.9 kGy/h) dose rate clearly had more negative effect on Mason lignin levels than did either the steam sterilization or gamma radiation at lower (8.5 kGy/h) dose rate. Gamma radiation at the 16.9 kGy/h dose rate reduced Klason lignin content from 29.5 % to 28.1 and to 27.5 %, but seemed to have little on-going effect related to dose accumulation. They also found that galactans, which are side-chain constituents of the hemicelluloses, are affected sooner and to a greater extent than xylans or mannans that represent the primary backbone of the hemicellulose polymers. Furthermore arabinan was probably unaffected by various radiation regimes. The authors suspected that the β-(13) linkage of the arabinan-xylan bond was less affected by radiation than the β-(16) linkage of the galactan-mannan bond.
