Plastic and nonmetallic article shaping or treating: processes – Recycling of reclaimed or purified process material
Reexamination Certificate
2000-01-28
2002-05-28
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Recycling of reclaimed or purified process material
C144S380000, C264S122000, C264S211110, C264S211000, C264S322000, C428S537100, C800S298000
Reexamination Certificate
active
06395204
ABSTRACT:
The present invention is directed to the use of trees having their lignin content and structure modified so that the wood becomes plastic or is more compatible with plasticizing agents. The wood may be molded, extruded, or otherwise treated under heat and pressure to form products conventionally useful as lumber or moldings or it may be formed into many products currently made from molded plastics, composites, or inorganic materials.
BACKGROUND OF THE INVENTION
Over the many millennia of human history and prehistory, trees have been one of natures most valuable and useful products to man. Before modern civilization, the wood that forests provided has been a material for heat, shelter, drugs, flavorings, art, weapons, and even clothing. In more recent times it has additionally had incalculable importance as a source of paper for its myriad uses in communication, personal hygiene care, packaging, and many other products.
Forest types vary over the globe but they may normally be classified as those in which conifers or deciduous trees predominate. The conifers, or so-called softwoods, are the source of most of the world's construction lumber and much of its paper. Deciduous trees, typically known as hardwoods, are much used where appearance is important, as well as for paper products where shorter fibers give advantageous properties. Many deciduous tree species have extremely beautiful wood from the standpoint of color and/or grain pattern. The term “hardwood” is actually a misnomer since actual hardness spans a wide range from extremely soft to very dense. Also, many deciduous trees from tropical or semi-tropical areas do not seasonally loose their leaves as they do in temperate zones. On the scale of evolutionary development, the deciduous trees have appeared much more recently in geologic time than conifers and are considered to be more advanced. The separation occurs at a high level on the taxonomic scale with the deciduous trees being in the botanical Class Angiospermae while the conifers are encompassed in the Class Gymnospermae. As would be expected, this wide evolutionary separation has resulted in significant differences in morphology and wood chemistry:
If the coniferous woods may be used as a model, they are composed of longitudinal fibers (tracheids) with a much lower number of thin-walled, radially oriented ray parenchyma cells. The tracheids, which are typically 2-5 mm long, are closed at the ends and have a central hollow or lumen extending most of their length. The tracheid walls have multiple layers with a multiplicity of openings (simple or bordered pits) through the walls into the lumen and in communication with similar openings in the adjacent tracheids and parenchyma cells. A layer called the middle lamella is located in the intermediate zone between adjacent tracheids. Structure of the xylem, or woody tissue, of the hardwoods has all of these features with the addition of other functional cells such as large, longitudinal thin-walled vessels.
As is well known, the primary structural and chemical constituent of the tracheids is the polymer cellulose. This occurs in the tracheid walls along with lower molecular weight polymeric sugars (hemicellulose) of somewhat different linkage and composition. In angiosperms the hemicellulose is a mixture of complex hexose and pentose polymers. Gymnosperm hemicellulose has a lower content of the pentose polymers. The tracheid walls are further reinforced with complex heterogeneous aromatic-based polymers referred to as lignin. The lignin is believed to contribute mechanical stiffness and structural integrity to the standing tree. The middle lamella between the tracheids is an amorphous zone composed primarily of hemicellulose and lignin. Middle lamella lignin may or may not be of similar composition to that in the tracheid walls. Fergus and Goring,
Holzfosrchung
24(4): 113-117 (1970) note that birch lignin in the vessel secondary wall and middle lamella is predominantly composed of one type (guaiacyl) whereas lignin in fiber and ray parenchyma secondary walls is mainly of another type (syringyl). The middle lamella around the fiber and ray cells contained both types.
Lignin is formed during cell development by the sequential expression of several known genes. Lignin formation and composition has been extensively studied and there is an extensive literature on the subject. At some point after the evolutionary separation of the gymnosperms and angiosperms the composition of the lignin followed different paths. Lignin polymers are generally classified into three groups depending on their respective monomer units. The conifers have predominantly crosslinked guaiacyl-type lignins formed as dehydrogenation polymers of coniferyl alcohol. In contrast, hardwood lignins are composed of a roughly equal mixture of guaiacyl and the more linear syringyl-types, the latter being a dehydrogenation polymer of sinapyl alcohol. A third type, guaiacyl-syringyl-p-hydroxyphenyl is found in grasses. These are believed to be the most evolutionarily advanced of the group; e.g., see T. Higuchi et al.
Wood Science Technology
11: 153-167 (1977). These authors trace the entire biosynthetic pathways of lignin formation in both gymnosperms and angiosperms. Briefly stated, phenylalanine is first deaminated to produce cinammic acid. This is then methylated and hydroxylated to produce the three acids basic to synthesis of the three lignin types. These are then reduced to aldehydes by the enzyme cinnamoyl-CoA reductase (CCR) and finally converted to alcohols (monolignols) by another enzyme cinnamyl alcohol dehydrogenase (CAD). Subsequent polymerization occurs principally at the beta carbon of the propanoid moiety and at C
5
of the aromatic ring, although other point of reactivity are also involved.
Lignin genesis and chemistry and the history of its investigation is concisely reviewed by E. Adler,
Wood Science and Technology
11 169-218 (1977). Sarkanen and Ludwig treat the subject in great depth in Lignins.
Occurrence, formation, structure and reactions
, Wiley-Interscience, New York (1971).
The difference in lignin composition is of far more than academic importance. Guaiacyl lignins tend to be far more heavily crosslinked than syringyl types. This affects their relative solubility in the various pulping liquors used for preparation of wood pulps. The syringyl lignin, being a more linear polymer due to one less available crosslinking site, is more readily solubilized and removed by the usual pulping chemicals. All other things being equal, hardwood species are more easily pulped than coniferous species.
Recent research had been directed to finding or creating both coniferous and hardwood trees that have modified lignin more amenable to removal by conventional pulping processes. This research has followed two lines. One is classic genetic selection in which trees having desirable properties are selected and reproduced. The other is genetic transformation in which new genes are introduced into a species by one of the techniques now available.
An example of the genetic selection route is detailed in MacKay et al., U.S. Pat. No. 5,824,842. In a case of serendipitous research, a scientist noted a loblolly pine (
Pinus taeda L
.) which had brownish sap wood in comparison to the usual white wood of the species. Further research revealed that the tree had a mutant gene that failed to produce the enzyme cinnamyl alcohol dehydrogenase (CAD). This enzyme lies on the critical lignin synthesis path for the guaiacyl lignins, acting to convert coniferaldehyde to coniferyl alcohol. The latter of these compounds is the primary lignin precursor in conifers. Although it had been harvested, fortunately the tree was in a research plot and had known parentage with the maternal parent still available as a seed source. Further plantings and crosses originating from the parental seed resulted in trees that were both heterozygous and homozygous in the mutant CAD null allele. Wood from both heterozygous and homozygous trees has been pulped on a micro scale using both th
Floyd Stanley L.
Neogi Amar N.
Vargot Mathieu D.
Weyerhaeuser Company
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