Chemical
structure of EHMA
Wood-polymer composites (WPCs) have been made from Southern pine and balsa wood by impregnating samples with a monomer solution and polymerizing in-situ. The monomers in this particular study were carefully chosen to have a chemical structure compatible with that of the cell wall material such that the majority of monomeric material is incorporated into the cell wall. Upon polymerization this yields a wood-polymer composite with the polymer and wood phases having good interactions with one another such that synergistic properties are obtained
The monomer primarily used in this particular case is ethyl a-(hydroxymethyl)acrylate (EHMA) as seen below.
Chemical
structure of EHMA
The reason for EHMA's ability to permeate and wet the cell wall so well lies in its chemical structure. The hydroxyl group yields an overall greater polarity of the molecule when compared to styrene, or methyl methacrylate. Also the hydroxyl group is able to hydrogen bond with the free hydroxyl groups of the cellulose that makes up the cell walls.
This monomer when succesfully polymerized in the wood sample either by itself or with a variety of comonomers, yielded materials with higher specific properties than the untreated wood samples. Properities such as dimensional stability, modulus, and toughness were improved.
The dimensional stability (measured by uptake of water) was improved by a factor of five with pure EHMA and less so with other polymer treatments as seen below.
Shown here is the percent uptake of water of WPCs. The less water uptake the greater the dimensional stability of WPCs. As can be seen the EHMA homopolymer yields the best results.
The specific modulus, and specific toughness of balsa was improved with incorporation of EHMA and in the table below one can see these properties of a variety of WPC systems and the important thing to note is the only improvement in specific properties is seen in the balsa/EHMA system.
Comparison of specific properties of various WPCs. It can be seen here the EHMA is the only one to exhibit greater specific properties than its control.
SEM was used to show the incorporation of the monomer into the cell wall as seen below.
Balsa/EHMA WPC. It can be seen that the cell lumens are empty and thus the polymer must be incorporated into the cell wall.
SEM was also used to show the intimate contact of the polymer and the wood matrix.
Fracture of Balsa/EHMA WPC. One can see the nonuniform failure of this sample which indicates good adhesion of the polymer to the wood matrix.