PHYSICAL CHARACTERIZATION OF WOOD AND WOOD POLYMER COMPOSITES - AN UPDATE

New and reliable test methods have been developed, and are under devel opment, for the physical characterization of wood and whole wood-polym er composites (WPC is used in this article to refer to polymer-impregn ated whole wood). The methods described here have been designed for sm aller samples than are required for most ASTM tests. It should be stre ssed that, when comparing treated samples to untreated samples in any type of testing, the initial density or specific gravity (density befo re treatment) of the treated sample should be the same as the untreate d control sample. If possible, measurements should be made on a given sample before and after treatment; on a split sample, half should be t reated and compared with the untreated half. If there is much variatio n in density between samples within a group, the effectiveness of the treatment cannot be determined with an acceptable degree of accuracy, since whole wood varies greatly between specimens and density is a maj or factor contributing to property variability. For example, swelling (due to moisture uptake), modulus, toughness, surface hardness, and co mpressive strength of wood all increase dramatically with increasing d ensity for both untreated whole wood and WPCs. Scanning electron micro scopy, coupled with x-ray energy analysis, indicated the presence or a bsence of good interaction between wood components and in situ formed polymer. For example, poly(EHMA) (the homopolymer of ethyl alpha-hydro xymethylacrylate) and wood components were seen to be strongly bonded, and x-ray activation elemental analysis confirmed the presence of pol y (EHMA) and its copolymers within the wood cell walls. On the other h and, proton spin-lattice relaxation in the rotating frame (T1rho) meas urements (by C-13 solid-state NMR) for balsa/EHMA WPCs gave two separa te sets of relaxation times, one each for unique peaks corresponding t o either the polymer or the wood components. It is probable that the r egion of interaction between poly (EHMA) and the wood component in the balsa/EHMA WPC (the interphase region) is small, as compared to the i ndividual components, and is not observed. This result is consistent w ith a two-parameter relaxation process for the peak at ca. 61 ppm, whi ch includes overlapping peaks for the hydroxymethyl carbons of poly (E HMA) and cellulose.