Paschalis Antonios N Papadopoulos 
email: antonios1974@hotmail.com
  
Paschalis Tountziarakis 
  
The effect of acetylation on the Janka hardness of pine wood

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Introduction

Chemical modification is applied in order to address basic shortcomings of wood.

One of the most practical types of chemical modification is the reaction of a hydroxyl group with acetic anhydride, known as acetylation [START_REF] Rowell | Chemical modification of wood[END_REF], although there have been a few papers published on the modification of solid wood with other linear chain anhydrides [START_REF] Hill | Wood modification -chemical, thermal and other processes[END_REF]. It is well established that the chemical modification of wood is able to provide protection against fungal attack and moisture. However, limited data are available in the literature regarding the effect of acetylation on the mechanical properties of wood. Consequently, the purpose of this paper was to examine the effect that acetylation may have on the mechanical properties of wood and specifically on Janka hardness, a less studied mechanical property. 

Janka hardness test

The Janka hardness was determined according to ASTM D143 standard in both radial and tangential direction for both treated and untreated samples, where a steal ball with 11.3 mm is continuously pressed into the wood surface until a penetration depth of 5.65 mm (half of the diameter) was reached. The resistance of wood against the impression of the steal ball was directly measured and defined as Janka hardness.

Results and Discussion

Table 1 depicts the effect of chemical modification of wood with acetic anhydride on the hardness of wood. From this it can be seen that acetylation up to 14.7 WPG resulted in a slight increase of the hardness of wood, compared to the untreated wood, in both radial and tangential direction. This increase however, was not significant at 95% confidence level. It is known that acetylation of wood results in a decrease in equilibrium moisture content at a given relative humidity [START_REF] Rowell | Chemical modification of wood[END_REF][START_REF] Hill | Wood modification -chemical, thermal and other processes[END_REF] which in turn yields a concomitant increase in the mechanical properties of wood [START_REF] Dinwoodie | Timber: its nature and behaviour[END_REF]. Higher modification level of 22.8 WPG resulted in a slight reduction of the hardness, compared to the untreated wood, but again this was not ejwwp484_source.pdf significant at 95% confidence level. Overall, it can be concluded that acetylation did not affect the hardness of pine wood.

Comparable data on the effect of acetylation on hardness does not exist in the literature. However, several studies have investigated the effect of acetylation on the mechanical properties of wood. The general consensus appears to be that the strength properties of wood are not significantly affected by acetylation provided that the reaction temperature is not too high (120-140 o C) and that acetylation results in greatly improved wet strength and wet stiffness properties [START_REF] Bjorkman | Studies on solid wood. II. The influence of chemical modifications on viscoelastic properties[END_REF]. 

  Corsican pine (Pinus nigra Schneid) sapwood samples of dimensions 4 cm x 4 cm x 4 cm (radial x tangential x longitudinal) with acetic anhydride at various WPGs (weight percent gain due to modification) were performed as previously described[START_REF] Papadopoulos | Mechanical behaviour of pine wood chemically modified with a homologous series of linear chain carboxylic acid anhydrides[END_REF].

Table 1 .

 1 Effect of chemical modification with acetic anhydride at various WPG's on Janka hardness of wood (each value is a mean of six replicates -standard deviation in parentheses).Tabelle 1 Einfluss der chemischen Modifizierung mit Essigsäureanhydrid bei unterschiedlichen prozentualen Massezunahmen (WPG) auf die Janka Härte von Holz (jeder Wert ist der Mittelwert aus sechs Proben -Standardabweichung in Klammern)

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	Wood History	Janka Hardness (N/mm 2 )
	WPG (%)	Radial	Tangential
	0	25.3 (1.1)	27.4 (0.8)
	5.3	26.1 (0.9)	27.7 (1.4)
	14.7	25.5 (1.3)	27.2 (1.6)
	22.8	24.1 (1.2)	26.9 (1.8)

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