Composition and structure of balsa (Ochroma pyramidale) wood

Abstract

Balsa, with its low density and relatively high mechanical properties, is frequently used as the core in structural sandwich panels, in applications ranging from wind turbine blades to racing yachts. Here, we describe both the cellular and cell wall structure of balsa, to enable multi-scale modelling and an improved understanding of its mechanical properties. The cellular structure consists of fibers (66-76%), rays (20-25%) and vessels (3-9%). The density of balsa ranges from roughly 60 to 380 kg/m 3 ; the large density variation derives largely from the fibers, which decrease in edge length and increase in wall thickness as the density increases. The increase in cell wall thickness is predominantly due to a thicker secondary S2 layer. Cellulose microfibrils in the S2 layer are highly aligned with the fiber axis, with a mean microfibril angle (MFA) of about 1.4°.The cellulose crystallites are about 3 nm in width and 20-30 nm in length. The degree of cellulose crystallinity appears to be between 80-90%, considerably higher than previously reported for other woods. The outstanding mechanical properties of balsa wood in relation to its weight are likely explained by the low MFA and the high cellulose crystallinity.

Comparison of the structure and flexural properties of Moso, Guadua and Tre Gai bamboo

“…Previous results on samples of oak wood from the Swedish warship Vasa (Svedström et al, 2012) showed similar crystallinity values while those of balsa (Ochroma pyramidale) (Borrega et al, 2015) were much higher. Small differences in the sample crystallinity values between the bamboo species can also be due to the differences in the orientation distribution of the cellulose microfibrils and might not be an independent result.…”

Mechanics of balsa (Ochroma pyramidale) wood

“…The thickness of the double cell wall is about 4 µm in vessels, 0.9 µm in rays, and between 0.8 µm and 3 µm in fibers, increasing with density (Easterling et al 1982;Da Silva & Kyriakides 2007;Borrega et al 2014). …”

“…The cellular structure in balsa wood consists of fibers (66-76%), rays (20-25%) and vessels (3-9%) (Borrega et al 2014). The vessels are long tubular structures that run axially along the trunk of the tree and transport fluids from the roots to the crown.…”

“…The large density variations in balsa derive predominantly from the fibers (Borrega et al 2014), long prismatic cells that act as the main load-bearing elements in wood.…”

“…To date, there are no engineered materials suitable for sandwich panel cores with a similar combination of mechanical properties and low density. With a view towards guiding the design of engineering materials inspired by balsa, we recently conducted a detailed characterization of its structure and composition (see Borrega et al 2014). In this paper, we investigate its mechanical behavior over a…”