Method for the modification of wood

Abstract

The invention is directed to a process for the continuous acetylation of wood elements. The process particularly combines a batchwise impregnation step, with a continuous reaction step. In order to realize this, a collection step is built-in, so as to allow batches of impregnated wood elements to be fed into a reactor in a continuous manner. Very high acetylation contents can be obtained, at a level that had not been achievable before in a continuous and non-catalyzed acetylation process.

1 . A process for the acetylation of wood elements comprising the following steps:
 (a) impregnating a plurality of batches of wood elements with an acetylation fluid comprising acetic anhydride and/or acetic acid so as to provide a plurality of batches of impregnated wood elements;   (b) collecting said plurality of batches of impregnated wood elements in such a manner as to provide a continuous feed of impregnated wood elements to a Reaction Chamber;   (c) subjecting a continuous flow of impregnated wood elements to acetylation reaction conditions in said Reaction Chamber so as to provide a continuous flow of acetylated wood elements exiting said Reaction Chamber.   
     
     
         2 . The process according to  claim 1 , wherein collecting the batches of acetylated wood elements is done in a Collection Chamber, preferably having a funnel-type geometry. 3 The process according to  claim 2 , wherein excess impregnation liquid is removed from the impregnated wood elements in the Collection Chamber. 
     
     
         4 . The process for the continuous acetylation of wood elements comprising the following steps:
 (a) providing wood elements impregnated with an acetylation fluid comprising acetic anhydride and/or acetic acid so as to have a Retained Weight Gain (RWG) of at least 20%, RWG being defined as [M test value (g)−M reference value (g)]/M reference value (g)×100% wherein M reference value  is the mass of a volume of wood elements contained in a glass value is cylinder, of known empty weight, internal diameter 80 mm and internal height 90 mm, said mass being determined by filling the cylinder with wood elements in a free-flowing manner and wiping off any excess wood elements with a spatula, weighing the filled cylinder and subtracting the empty weight of the cylinder, and wherein M test value  is determined by filling the aforementioned cylinder with impregnated wood elements in a free-flowing manner and wiping off any excess wood elements with a spatula, transferring the contents of the cylinder to a beaker having a diameter of 140 mm and a height of 240 mm; placing said beaker, filled with the wood elements, inside a preheated natural convection oven set at 160° C. for a period of 45 minutes; taking the beaker out of the oven and determining the mass of the wood elements by weighing the filled cylinder and subtracting the empty weight of the cylinder;   (b) introducing said impregnated wood elements into a Reaction Chamber and subjecting said impregnated wood elements to acetylation reaction conditions in said Reaction Chamber;   (c) withdrawing a continuous flow of acetylated wood elements from said Reaction Chamber.   
     
     
         5 . The process according to  claim 1 , wherein the impregnated wood elements are continuously introduced into the Reaction Chamber. 
     
     
         6 . The process according to  claim 1 , wherein the wood elements flow through the Reaction Chamber in plug flow. 
     
     
         7 . The process according to  claim 1 , further comprising a step:
 (d) drying the acetylated wood elements so as to remove unreacted acetylation fluid therefrom.   
     
     
         8 . The process according to  claim 7 , further comprising a step
 (e) withdrawing a continuous flow of acetylated wood elements from the drying apparatus of step (d).   
     
     
         9 . The process according to  claim 1 , wherein the Reaction Chamber comprises a horizontal screw conveyor, such as a double, parallel screw conveyor. 
     
     
         10 . The process according to  claim 1 , wherein the moisture content of the wood elements subjected to step (a) is less than 8% by weight, preferably 0.01% to 5%. 
     
     
         11 . The process according to  claim 1 , wherein the acetylation fluid comprises 40% by weight to 100% by weight acetic anhydride and 0% by weight to 60% by weight acetic acid. 
     
     
         12 . The process according to  claim 1 , wherein the acetylation temperature in step (c) is higher than the atmospheric boiling point of the acetylation fluid. 
     
     
         13 . The process according to  claim 1 , wherein the acetylation in step (c) is performed in two to five temperature zones. 
     
     
         14 . The process according to  claim 1 , wherein the temperature of acetylation in step (c) is controlled by a heating an environment selected from a heated gas loop containing a gas at least partially saturated with acetic anhydride and/or acetic acid, a heated mantle, a heated axis, and combinations thereof. 
     
     
         15 . A plant for the acetylation of wood elements, said plant comprising
 (i) at least one Impregnation Chamber,   (ii) a Collection Chamber having an inlet in connection with an outlet of the at least one Impregnation Chamber, said connection being such as to allow wood elements to flow from the Impregnation Chamber to the Collection Chamber;   (iii) a Reaction Chamber having an inlet in connection with the outlet of the Collection Chamber, said connection being such as to allow wood elements to flow from for the Collection Chamber to the Reaction Chamber, wherein the Reaction Chamber is designed so as to provide a continuous transport of wood elements from the inlet to an outlet thereof.   
     
     
         16 . The plant according to  claim 15 , wherein the continuous transport in the Reaction Chamber is provided by a screw conveyor. 
     
     
         17 . The plant according to  claim 15 , wherein the Reaction Chamber is a dryer, preferably selected from the group consisting of tray dryers, belt dryers, extraction trains, screw dryers and plate dryers. 
     
     
         18 . The plant according to  claim 15 , further comprising a dryer, preferably a tray dryer, the inlet of which is in connection with the outlet of the Reaction Chamber, said connection being such as to allow wood elements to flow from the Reaction Chamber to the Dryer. 
     
     
         19 . Acetylated wood elements, obtainable by a non-catalyzed acetylation process, and preferably chips, strands, slivers, or particles, wherein the acetylated wood elements have an acetyl content (AC) of at least 20.5%, and preferably of AC 21% to 26%.