Hardwoods extractives contents

Softwoods are generally more resistant to acids than are hardwoods because they have high lignin and low hemiceUulose contents. In general, heartwood is more resistant to acids than sapwood, probably because of heartwood s higher extractive content and slower movement of Hquid into the heartwood. For these reasons, the heartwood of certain conifers has been widely used in the chemical industry. 

The most recent publication reviewed was by Einspahr and Harder (63), who discuss the basic properties of hardwood barks that could be important in the manufacture of any fibrous product. This was a progress report showing results for 16 pulpwood species work is in progress on 16 additional species. Measured were such bark factors as specific gravity, extractives content, strength, toughness, reaction to hammermilling, and ash content. 

There have been many attempts to investigate the effect of extractives on cure chemistry and bonding to wood (33-35). For example, the effect of extractives from pressure-refined hardwood fiber on urea-formaldehyde resin was studied (34, 35) and it was found that the ethanol-soluble extractives decreased the gel time as much as 41%, and the sequentially extracted water-soluble extractives increased the gel time in excess of 65%. There was little correlation between the extractive content and gel time however, an empirical relation between the pH of the extractives and the gel time was observed (35). The effect of several species of wood on the gel time of urea-formaldehyde resin have also been studied (36). In these studies the gel time was correlated with the pH and acid buffering capacity of the extract. 

Wood extractives vary in nature and amount within and between species, and within trees there is generally a decrease in extractive content with tree height. With both softwoods and hardwoods, extractives are more abundant in heartwood and generally these differ in chemical composition from those in sapwood, although in Pinus sp. some extractives are common to both (Hillis, 1962). Wood extractives... 

An increased lifetime of wood is an essential prerequisite for sustainable outdoor applications (e.g. roofs, doors, windows type frames, bridge type structures built over rivers, roads, or railways). Wood presents a natural durability in terms of biodegradation, this feature being strongly related to the wood species (is higher in hardwoods than in softwoods) and mainly to the extractives content and composition [55]. This durability can be predicted by infrared spectroscopy [56, 57]. 

Extractives Extractives are organic chemicals that can be extracted from wood with solvents. They are generally low molecular weight compounds with the content accounting for 2-5 % of the wood. Comparatively, softwoods have a higher extractives content than hardwoods and most of them are located in the heartwood. The component of extractives is responsible for some wood properties such as the color, smell, density, hygroscopicity, flammability, and durability. 

Table 14.1. The net calorific value of wood depends on its moisture content here the values are for a typical hardwood. The reduction in available energy with moisture content is due to the need to vapourize and superheat the steam to the same temperature as the flue gases. Typically the net calorific value for a hardwood is 18.2 MJ kg that for a softwood is 19.2 MJ kg (due to its higher lignin content) and that for bark is 19.7 MJ kg (due to extractives).

Total Yield and Relative Frequencies of the Main Monomers Released by Thioacidolysis of Native Hardwood Lignins. The Data are Expressed Relative to the Lignin Content of the Extractive-Free Sample... 

---