A comparative study on hydrophobic properties of wood functionalized by phthalic and acetic anhydrides

Hygroscopic nature of wood is one of the limiting factors for some outdoor applications. Coating, impregnation with various hydrophobic chemicals or modification methods can be applied to protect the wood against moisture absorption. In this study, the effect of chemical modification of wood by substituting hydroxyl groups with long and cyclic functional group (phthalate) and short group (acetyl) on its hygroscopic properties was investigated.

Sapwood specimens of poplar (Populus deltoides) and Norway spruce (Picea abies) with 48 mm in diameter and 5 mm in thickness were cut. The specimens were extracted for 8 hours prior to modification using acetone/ethanol (2:1, v/v) solution. The specimens were then pressure treated with acetic anhydride and phthalic anhydride at 120 °C for 1, 2, 4 and 24 hours. After modification, the specimens were first washed with acetone for 48 hours and then washed with distilled water for 48 hours. After leaching, the specimens were dried in an oven at 103 ±2 °C for 24 h and weighed. Weight percent gain (WPG), physical properties of the modified wood including reduced equilibrium moisture content (EMC R), moisture exclusion efficiency (MEE) and wettability by Sessile Drop method were measured and the chemical changes were also evaluated by infrared spectroscopy test (ATR-FTIR).

ATR-FTIR studies showed that the peak observed at 1732-1746 cm-1 which is related to the stretching vibration of carbonyl group (C=O) confirms the formation of ester bonds within both types of modified woods. The results showed that modification with acetic anhydride in comparison with phthalic anhydride at the same reaction time led to a higher WPG and lower MEE. Less WPG obtained by modification with phthalic anhydride can be due to the larger size and more non-polarity of the phthalate group compared to the acetyl group, as well as the hydrolysis of the ester bond between the phthalate and hydroxyl groups caused by leaching. However, there was no difference between the anti-moisture exclusion efficiencies of the woods modified by two types of anhydrides at the same WPG. In the acetylation process, modification of Norway spruce was more successful than that of poplar, whereas the wood species showed no difference in the modification with phthalic anhydride. The results of the wettability test showed that the contact angle of the Norway spruce increased after modification by both types of the anhydrides. However, in poplar wood, only the specimen modified by phthalic anhydride at high WPG had a higher contact angle than the control specimen. Results also showed that compared to the acetylation process, WPG of the wood modified by phthalic anhydride was less affected by increasing the reaction time. This may be due to the lower reactivity of the phthalic group with wood compared to the acetyl group.

There was no difference between the anti-moisture exclusion efficiencies of woods modified by short-chain anhydride (acetic anhydride) and the cyclic anhydride (phthalic anhydride) at the same WPG, which indicates that the type of anhydride does not affect this physical property of wood. Our results showed that wettability of the modified woods can be a function of other factors such as surface quality and permeability in addition to the amount of hydroxyl group substitution.