Effect of Test Piece Size on Rheological Behavior of Wood Composites

The effect of size and the combined effect of the size and moisture sorption of test pieces on the long term creep behavior of wood composites were studied. Small-, wide-, and semisize test pieces from each of three commercial wood composites, particleboard (PB), oriented strand board (OSB), and medium density fiberboard (MDF) were tested. Three exposure regimes, constant 20°C/65%, a single change from 20°C/65% to 20°C/85%, and cyclic changes between 20°C/30% and 20°C/90% relative humidity (RH), were used. It was found that the width of test pieces had no effect while the length had a significant effect on long term behavior of wood composites, but the effect is in contrast to that of short term modulus of rupture (MOR) which ranged from 0.06 to 0.13 for the shape parameter and from 0.09 to 0.26 for the length parameter depending on the types of wood composites. The average ratio of the relative creep (kc) of small-:wide-:semisize was 1.14:1.13:1.00 for PB, 1.26:1.21:1.00 for MDF, and 1.24:1.24:1.00 for OSB, with the shape parameter ranging from 0.04 to 0.19 and the length parameter from 0.13 to 0.26. Change in RH significantly aggravated the size effect on kc with the most significant under cyclic RH, for which the ratio of kc small- to semisize was 1.45 for PB and 1.27 for OSB after 3 months’ exposure. Edge sealing on small test pieces efficiently prevented the effect of moisture sorption but the size effect on kc with a reduction of about 30 and 20%, respectively, for edge sealed PB and OSB in weekly changing climate between 20°C/30% and 20°C/90% RH compared to unsealed small-size test pieces. The findings elucidate the importance to take into account the size effects on short term strength, compensated size effect on long term creep, and the combined effects of size and moisture on long term behavior when predicting the long term load carrying capacity of wood composites in construction.     

Electrospun polymer nanofibers with internal periodic structure obtained by microphase separation of cylindrically confined block copolymers

Continuous fibers are described having concentric layer or aligned sphere microphase-separated, styrene-isoprene block copolymer morphologies. The fibers are obtained by a two-fluid coaxial electrospinning technique in which the desired block copolymer is encapsulated as the core component within a polymer shell having a high glass transition temperature (Tg). The fibers range in diameter from 300 to 800 nm, and the block copolymer core ranges from 50 to 500 nm. Subsequent annealing of the fibers above the upper Tg of the block copolymer but below the Tg of the shell polymer results in microphase separation of the block copolymer under cylindrical confinement. The resulting fibers exhibit improved long-range order. This two-step strategy creates the opportunity to fabricate continuous nanofibers with periodic internal structure.     

Structural Properties of Oriented Wood Strand Composite: Effect of Strand Orientation and Modeling Prediction

This paper first examined the effect of wood strand orientation on the flexural properties of oriented wood strand composites (OSB) under different engineering stress modes, and second, investigated the applicability of the rule of mixtures in conjunction with the theory of elasticity for predicting the properties of OSB. The results showed that the performance of OSB was closely related to the loading directions and stress modes: The flexural strength and stiffness under both flat and edgewise bending loads consistently decreased with increasing angles between the applied load and longitudinal direction of orientation of strands, but that under flat bending being much more significant. Panel shear at 45° loading angle resulted in higher strength compared to other loading angles tested, indicating an occurrence of diagonal shear stresses. In conjunction with the numerical results from image analysis of the structure of OSB, and the oriented elasticity and stress algorithms, the models for theoretically predicting the strength and stiffness of OSB under various loading angles were derived with a good estimate under bending and panel shear loads, implying that OSB can be treated as a composite and its properties may be modeled by the rule of mixtures by suitable development, even though OSB has a very high volume fraction of wood strands (0.979) and correspondingly very low volume fraction of resin (0.021).