Anatomy of chemically and mechanically isolated bark sclereids

Graduation date: 1975Sclereids are sclerenchyma cells having thick lignified\ud secondary walls. Douglas-fir sclereids, also called bast fibers,\ud are long (1 mm in length), sharply pointed, spindle-shaped fibers\ud of a red-brown color. Sclereids from Douglas-fir bark and nine\ud other western coniferous species were examined for their morphology.\ud The external surface characteristics of sclereids isolated from\ud Douglas-fir bark by mechanical disk refining (F858 Fiber and Steam\ud Refined Fiber) and chemical treatment through digesting in Kraft\ud liquor (WEF-313, 606, and 900) were studied with both light and\ud scanning electron microscopy.\ud For these ten western coniferous species studied, only Douglas-fir\ud sclereids can be an adequate plastic reinforcing agent. There\ud are two reasons for this: 1) Sclereids in Douglas-fir bark differentiate\ud individually and usually not in groups. Individual\ud sclereids can be separated because of this arrangement, while the\ud sclereids from nine other conifers studied were in groups. 2) The\ud Douglas-fir sclereid has sufficient length and stiffness, while the\ud sclereids from nine other conifers studied do not.\ud WEF is a chemically treated Douglas-fir bark fiber. It has a\ud clean surface and has been used as a reinforcing fiber in the plastics\ud industry and gave satisfactory results to improve impact\ud strength of plastic. However, the cost and inconvenience in production\ud of WEF has limited the market. An acceptable quality bast\ud fiber produced through mechanical processing is needed.\ud Both F858 Fiber and Steam Refined Fiber are mechanically\ud processed fibers; but, E858 Fiber has a surface heavily encrusted\ud with surrounding parenchyma fragments and extractives, and often\ud several sclereids are still together in a group. The Steam Refined\ud Fiber is well separated and free of parenchyma fragments on its\ud surface like the WEF fiber, but the Steam Refined Fiber appears to\ud have a smoother surface than much of WEF fiber.\ud The comparison of surface cleanliness among F858 Fiber, WEF,\ud and Steam Refined Fiber indicates that steam pressurized disk\ud refining has accomplished the objective of producing bark sclereids\ud with clean surfaces, and, therefore, Steam Refined Fiber would\ud appear acceptable for reinforcement of plastics