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2009 International Nuclear Atlantic Conference - INAC 2009 Rio de Janeiro,RJ, Brazil, September27 to October 2, 2009 ASSOCIAÇÃO B RASILEIRA DE E NERGIA N UCLEAR - ABEN ISBN: 978-85-99141-03-8

INFLUENCE OF GAMMA RADIATION ON MECHANICAL AND

THERMAL PROPERTIES OF CEDRELLA FISSILISAND

OCOTEA POROSA USED IN WORKS OF ART

Lucio C. Severiano1*

, Francisco A. R. Lahr2, Marcelo A. G. Bardi

1and

Luci D. B. Machado1

1*Instituto de Pesquisas Energéticas e Nucleares, IPEN - CNEN/SPAv. Professor Lineu Prestes, n. 2242

05508-000 São Paulo, [email protected]

2 Departamento de Engenharia de Estruturas de Madeira – EESC-USPAv. Trabalhador Sancarlense, n.400

São Carlos-SC

ABSTRACT

Woods, as other materials, are susceptible to alterations in their internal structure because of physical, chemicalor biological agents. Wood can be considered a natural composite with high strength capacity provided bycellulose and hemicellulose agglutinated by lignin, substances with very distinct structures. In severalapplications, the use of radiation can be interesting, once it turns wood more resistant to biological demand. Theapplication of gamma radiation in work of arts and archeological artifacts preservation began in 1970, in France.

By other side, no changes in wood properties and no remaining radioactive waste weredesirable. Gamma radiation from a cobalt-60 source usually is applied as a tool to the decontamination of insects and microorganisms, as well as to provide resins cure in impregnated wood. In this way, the aim of this paper is to evaluate gamma radiation effects on some physical, thermal and resistance mechanical of Brazilian wood species used in carving, as Cedro Rosa (Cedrella fissilis) and Imbuia (Ocotea porosa). Gammaradiation process considered different doses (25kGy, 50kGy and 100kGy). Results showed that no gammaradiation influences were detected in the studied wood properties in the dose range applied. This is a

relevant conclusion that will improve safety on arts conservation around the world.

1. INTRODUCTION

Wood species have had major roles in many ancient and modern cultures due to thepossibility of manufacturing furniture, sculptures and pieces of art, which can be foundespecially in churches, such as images, altar decorations, frames and others [1].

Essentially, the wood fibers are composed of cellulose and lignin. As cited by Zickler et al.[2], the crystalline cellulose microfibrils represent the fiber reinforcement and the amorphoushemicelluloses and lignin represent the composite matrix.

The elementary cellulose microfibrils are aligned parallel to each other and wind around thewood cell in a helical manner at a characteristic microfibril angle with respect to the cell axis.Two polymorphs of cellulose I exist, triclinic cellulose Iα and monoclinic cellulose Iβ.

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INAC 2009, Rio de Janeiro, RJ, Brazil.

Cellulose produced by primitive organisms were said to have the Iα component dominant,while those produced by the higher plants have the Iβ form dominant [2].

Native lignin is an amorphous, three-dimensional copolymer of phenylpropanoid units linkedthrough ether and carboncarbon bonds [3-4]. It provides mechanical support for plants, as

well as facilitating transport of nutrients and providing defense against attack frommicroorganisms due to, as discussed by Lawako et al. [5], covalent linkages between ligninand carbohydrates in native lignin in spruce (Picea abies L.) wood.

Besides the content of lignin, the degree of resistance is greatly dependent on the quality andquantity of extractives in wood [6]. As described by Beek et al. [7], these constituents arebasically resin acids, free fatty acids, sterols, triglycerides and sterol esters, which are presentin small concentrations (1-3% wt.).

Besides the extractives, there are other factors that contribute to increase the naturalresistance of wood, such as: shape, size and arrangement of the cells (anisotropy) and reserve

of materials such as crystals of silica and calcium oxalate [8-9]. There are also someimportant differences between the cell wall structures of hardwoods and softwoods. Nativesoftwood tracheid tend to be longer (3−4 mm) than hardwood tracheids (0.5−1.5 mm), aswell as somewhat wider (35 µm vs 20 µm, respectively) [10].

Xu and collaborators [11] investigated, by means of dual-axis electron tomography, the cellwalls of radiata pine early wood, and noticed that individual cellulose microfibrils measureapproximately 3.2 nm in diameter, and appear to consist of an approximate 2.2 nm unstainedcore and an approximate 0.5 nm thick surface layer that is lightly stained. Both individualand clustered cellulose microfibrils are seen surrounded by more heavily stained andirregularly shaped residual lignin and hemicellulose.

In Brazil, the specie Cedrella fissilis (locally denominated as cedro-rosa) has greateconomical importance because of its huge application in furniture, naval and aerospatialeindustries. It can be generally found at southeast Brazil in Atlantic-pluvial forest regions [12-13]. Another wood greatly used for luxury furniture and civil construction is Ocotea porosa (regionally known as Imbuia), which is a plant largely found at southern Brazil and it canreach 30 m height [14-15].

Wood, as well as all other materials, are able to undergo modifications that occur over timewith varying pace [16]. These changes are produced by physical, chemical or biologicalagents [9]. The climate of some tropical countries which present both high temperature and

air relative humidity, like Brazil, encourages the development of insects, termites, fungi andmicroorganisms that attack and deteriorate trees and wood made artifacts, as well asderivative materials such as paper. Silva et al. [17] said that fungi are able to hydrolyse awide variety of polymers, including cellulose, as a result of their efficient degradativeenzymes.

In this context, some studies have been performed to evaluate the use of ionizing radiation inthe recovery and preservation of wood-made art works. This process has been highlightedsince 1970’s when the gamma rays were applied to the destruction of living organismspresent in woods without radioactive waste production. Moreover, gamma radiation from60Co can be used to cure resins that are impregnated into wood pieces to keep its form [18-

19].

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Gamma radiation as sterilizing treatment causes direct damage to cell DNA throughionization inducing mutation and killing the cell. It also has an indirect effect as a result of radiolysis of cellular water and formation of active oxygen species, free radicals andperoxides causing single and double strand DNA breakages [17].

Gamma rays, electromagnetic waves with high penetrating power, pass through materialswithout leaving any residue. Fungi have been successfully inactivated from differentmaterials, such as paper, wood and soil with radiation doses ranging from 6 to 15 kGy.However, in a Brazilian study some fungi from books could not be completely eliminatedafter irradiation with doses of 20 kGy [20].

The insect infestation control can be performed by submitting wooden objects to lowerradiation doses, e.g. 0.2 – 0.5 kGy whereas the fungi and microorganisms infestation can beprevented by doses of 3 – 8 kGy and 15 – 20 kGy, respectively [21]. It is asserted to mentionthat irradiation technique for desinfestation of cultural heritages, frames and wooden-madeartifacts has shown to be very efficient but this process does not protect the material to a re-

infestation [22].

By this way, this work aims to investigate the effects of gamma radiation on disinfestationand decontamination processes of Cedrella fissilis (cedro-rosa) and Ocotea porosa (imbuia)by accompanying their thermal and mechanical properties.

2. MATERIALS AND METHODS

2.1. Sample Preparation

Samples of Cedrella fissilis and Ocotea porosa were prepared according to standard NBRABNT 7190, Annex B [23] for the compression parallel to the grain.For thermal analysis, each wood was cut in several pieces in order to obtain homogeneoussamples.

2.2. Irradiation

The samples were irradiated in a multipurpose irradiator by gamma rays from Co-60 sourceat a dose rate of 10 kGy·h-1, and absorbed doses of 25kGy, 50kGy and 100kGy. These doseswere performed after considering that the gamma irradiation is a very effective treatment for

recovery biodeteriorated artifacts [24]. This irradiator is located at Centro de Tecnologia dasRadiações (CTR/IPEN/CNEN-SP).

2.3. Thermogravimetry (TG) and derivative thermogravimetry (DTG)

The thermogravimetry was performed using a TGA50, Shimadzu Co. TG results wereobtained applying mass samples of around 5 mg, heating rate of 10°C·min-1 from roomtemperature up to 700°C, in dynamic atmosphere of compressed dry air with flow rate of 50mL·min-1.

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2.4. Mechanical Tests

The compression tests of the parallel fibers were performed in equipment for universal testingmachine, AMSLER. The samples were subjected to different loads to verify the limits of resistance for different wood species. The strength is calculated by:

,á

(1)

where 0,á. is the applied force for rupture and A is the cross section area.

3. RESULTS AND DISCUSSION

Table 1 presents the average values for tensile strength at break for compression parallel tothe fibers of samples of imbuia (Ocotea porosa) and cedro-rosa (Cedrella fissilis) withdifferent radiation doses: 0kGy, 25kGy, 50kGy and 100kGy.

Table 1. Average values of tensile strength at break for

irradiated and non-irradiated Imbuia and Cedro-rosasamples

Radiation dose(kGy)

Strength (MPa) Imbuia Cedro-rosa

0 51.5 ± 1.0 31.2 ± 0.425 52,4 ± 1.6 32.6 ± 0.350 50.6 ± 4.4 31.9 ± 0.6

100 50.7 ± 1.1 31.2 ± 0.9

By Table 1, it can be seen a similar mechanical behavior when different irradiation doses were applied to the samples of imbuia and cedro-rosa, although the latter can be consideredsofter than the former.

25kGy-irradiated samples showed an increasing of 2% on the tensile strength at breakaverage values when compared to the non-irradiated imbuia samples (Table 1). Moreover, thesame results were observed to cedro-rosa irradiated samples. By this way, neither higher dose(100kGy) nor lower dose (25kGy) in the studied dose range has meaningfully modified themechanical properties of the studied wood species.

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These results are corroborated by Magaudda [21] because the longer are irradiation time andirradiation doses, the greater is the possibility for oxygen in air interacts with radicalsoriginated from material irradiation processes. Although the moisture, weather and otherfactors had interfered on the mechanical properties of woods, it is possible to observe nomeaningful alterations on the reproducibility of the performed tests for the different

irradiation doses.

The TG data allow the quantitative determination of main components of the two woodspecies studied. Fig. 1 to Fig. 6 and Table 2 and Table 3 show TG/DTG curves of samples of imbuia (Ocotea porosa) and cedro-rosa (Cedrella fissilis) non-irradiated and submitted todifferent irradiation doses of 25kGy, 50kGy and 100kGy.

In all TG curves, the first weight loss step refers to volatile components of the samples. Thesecond one (around 150ºC to 430ºC) corresponds to cellulose weight loss and the third step(starting around 430ºC) is attributed to lignin thermal decomposition [25].

Figure 1. TG/DTG curve for non-irradiated Imbuia.

-0 100 200 300 400 500 600 700Temp [C]

-0

20

40

60

80

100

%TGA

-0.0

%/minDrTGA

-9.704%

-48.789%

-37.857%

-97.091%

Figure 2. TG/DTG curve for 100kGy-irradiated Imbuia.

-0 100 200 300 400 500 600 700T e m [ C]

-0

20

40

60

80

100

%T GA

-10

0

10

%/minDrTGA

-10.657%

-54.291%

-32.540%

-98.0

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-0 100 200 300 400 500 600 700Temp [C]

-0

20

40

60

80

100

%TGA

IMBUIANI.D00IMBU100I.D00IMBUI25E.D00IMBUI50F.D00

TGATGATGATGA

Figure 3. TG curves for non-irradiated, 25kGy-, 50kGy-,100kGy-irradiated Imbuia.

-0 100 200 300 400 500 600 700 800Temp [C]

-0

20

40

60

80

100

%TGA

-20.0

0.0

20.0

%/minDrTGA

-9.538%

-59.460%

-25.640%

-94.655%

Figure 4. TG/DTG curve for non-irradiated cedro-rosa.

-0 100 200 300 400 500 600 700Temp [C]

-0

20

40

60

80

100

%TGA

-10

0

10

%/minDrTGA

-10.580 %

-59.980 %

-27.024 %

-98.969 %

Figure 5. TG/DTG curve for 25kGy-irradiated cedro-rosa.

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Figure 6. TG curves for non-irradiated, 25kGy-, 50kGy-,100kGy-irradiated cedro-rosa.

Table 2. TG weight loss data for non-irradiated, 25kGy-, 50kGy-, 100kGy-irradiated

Imbuia

Radiation dose (kGy)Volatile

components (%)Cellulose (%) Lignin (%) Weight loss (%)

0 11 54 35 98

25 10 52 35 10050 9 48 38 96

100 8 48 38 97

Table 3. TG weight loss data for non-irradiated, 25kGy-, 50kGy-, 100kGy-irradiated

cedro-rosa

Radiation dose (kGy)Volatile

components (%)

Cellulose (%) Lignin (%) Weight Loss (%)

0 9 59 26 9525 10 60 27 9950 12 57 28 100

100 11 57 29 100

The data from table 2 and table 3 allow observing that ratio between cellulose: lignin ishigher for cedro-rosa than for imbuia, which agree with the evaluated mechanical properties

and suggest that cedro-rosa is softwood meanwhile imbuia is a hardwood, comparatively.

-0 100 200 300 400 500 600 700T e m [ C]

-0

20

40

60

80

100

%T GA

CR25AME1.D00CR50-F.D00CR100-I.D00CR--NR-1.D00

TGATGATGATGA

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According to Campanella et al. [25] the cellulose thermal degradation occurs in many steps,starting by its depolymerization, generating glucose and oligosaccharides. Later on, duringthe pyrolysis, water and acids are also produced from cellulose derivatives. Additionally, thethermal degradation of lignin is attributed to α- and β-aryl-alkyl-ether bonds breakage,followed by aromatic ring bonds breakage and finally a rupture of carbon-carbon bonds

between the structural units of lignin [26].

By this way, after irradiation of samples, as presented in table 2 and table 3, cellulosecomponent of both wood studied species slightly undergo to chain breakage when absorbedradiation dose reaches 50 kGy or higher whereas the lignin structure is less affected bygamma radiation. Some samples have presented traces of non-volatile inorganic residualcomponents, which are derived from wood extractives.

4. CONCLUSIONS

The applied gamma radiation range dose does not promote significant alterations onmechanical and thermal behavior of the studied wood samples.

In other words, gamma rays can be used for decontamination and disinfestation purposeswithout damaging the art work artifacts even if the treated material is once more exposed toradiation source, i.e. due to a re-infestation by biodeteriorating agents.

ACKNOWLEDGMENTS

The authors acknowledge M.Sc. Marcia Mathias Rizzo and M.Sc. Danilo Bras dos Santos forsuggesting wood species, Alex Correia dos Santos for helping with TG analysis and LAMEM(Laboratório de Madeiras e Estruturas de Madeira, USP-SC) for supporting somecharacterization tests, as well as CNEN for fellowship, M.Sc. Yasko Kodama and Mr. Paulode Souza Santos for samples irradiation.

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