Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles
Reexamination Certificate
1999-12-06
2003-07-08
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
C264S122000
Reexamination Certificate
active
06589460
ABSTRACT:
The present invention relates to the use of water-soluble carbohydrates and phenolic compounds as thermosetting adhesives.
Typically, exterior grade composite and panel wood products are bonded with synthetic adhesives. Phenol-formaldehyde resin adhesives are used in more than 90% of exterior grade composite products. However, the finite supply of fossil fuels, coupled with an increasing global demand for wood products, has created a need for an alternative, environmentally sound adhesive system based on renewable resources. At the same time, the need to produce uniform and high quality composite products from lower cost raw material such as smaller diameter logs, wood processing by-products such as sawdust and bark, and agricultural wastes, has created a need for improved bonding products and processes.
Renewable resources, such as carbohydrates and phenolics, and particularly those of a lignocellulosic nature, have been investigated as a possible source of constituents suitable in exterior grade adhesives. To date, few adhesive systems based on renewable source have been successfully adapted for commercial application. Generally, such adhesives do not possess adequate properties, suffering from weak bond strength and poor handling qualities, to meet the standards required of composite products. Those renewable source adhesives capable of meeting today's composite product standards are usually too expensive to compete with existing synthetic resins, often requiring extensive chemical modification and/or extremely long curing times.
Nonetheless, the components of lignocellulosic materials have been extensively studied in hopes of finding a renewable source adhesive suitable for commercial use in composite wood products. Wood, similar to other plant resources, is composed of about 45 to 50% cellulose, 20 to 35% hemicellulose, 20 to 25% lignin, and>0 to 10% extractives. Chemically, cellulose is the main structural component of all plant cell walls, and is a linear polysaccharide built up from anhydro glucose units having the general formula (C
6
H
10
O
5
)
n
. Cellulose is the most abundant source of carbohydrates in plants. Upon acid hydrolysis cellulose converts into glucose.
Hemicelluloses are a group of heterogenous polysaccharides that, next to cellulose, constitute the most abundant sources of carbohydrates in plants. Hemicellulose, associated with the cell wall, is highly soluble in alkali and is relatively easy to degrade by an acid hydrolysis into simple sugars or sugars acids. Hemicellulose may be repeated by the general formula (C
5
H
8
O
4
)
n1
(C
6
H
10
O
5
)
a2
, representing five and six carbon sugars such as pentosan and hexosan. Upon acid hydrolysis hemicellulose converts into various reducing sugars.
The third major component of lignocellulose is lignin, which is phenolic in character and acts as the natural binder within the lignocellulose to cement cells together. The precursors of lignin and its formation in lignocellulosic material are still not completely understood, although recently, considerable progress has been made in this area of research. It is generally accepted that lignin is a polymeric material composed of phenyl propane units that are linked together by carbon to carbon as well as carbon to oxygen (ether) bonds.
The minor component of lignocellulose consists of extractives. All plants and agricultural vegetation contain a number of organic substances that can be extracted with organic solvents, or in some cases, with water. Among these belong aliphatic, aromatic and alicyclic compounds, hydrocarbons, alcohols, ketones, and various types of acids and phenol compounds. Furthermore, sterols, tannins, essential oils, resins, dyestuffs, proteins, wax, and some alkaloids are found.
Carbohydrates have been explored in the past as both coreactants with phenolic resin and as the sole ingredient in adhesive. Meigs (U.S. Pat. No. 1,593,342, 1,801,053 and 1,868,216) carried out some of the early work with a phenol-carbohydrate combination. Meigs was searching for a process to produce solid, fusible, thermoset molding compounds. The reactions used both acidic and base catalysts and often introduced coreactions with aniline or aliphatic amines. In other examples, Chang and Kononenko (Sucrose-Modified Phenolic Resin as Plywood Adhesives, Adhesives Age 5(7):36-40, 1962) developed an adhesive system by coreacting sucrose in a phenol-formaldehyde resin formulation under alkaline conditions. More recently, Gibbons and Wondolowski (Can. Pat. 1,090,026) reacted carbohydrates with phenol and urea, or a diamine, in the presence of an acid catalyst to produce a fusible resin for bonding wood products.
Other investigators have used acidic conditions only to produce carbohydrate-based resin. Mudde (Corn Starch: A Low Cost Route to Novolac Resins. Mod Plast. 57(2): 69;74, 1980) described a method that relied on the acidic conversion of starch to 5hydroxymethyl furfural—2 formaldehyde for condensing with phenol in a Novolac resin. Turner et al., (DE-A-1,905,054) investigated carbohydrates, not involving phenol as a coreactant, and produced a water-resistant adhesive. Turner et al. degraded pentose and hexose sugars with acid, while coreacting with such materials as formaldehyde, furfural alcohol, polyvinyl alcohol, or amines to produce a thermosetting adhesive suitable for particle board production. As another example, Stofko (U.S. Pat. Nos. 4,107,379 and 4,183,997) proposed formulations that used a variety of carbohydrates sources and reactions under acidic conditions with different modifiers to produce thermosetting water-resistant adhesives.
K. C. Shen (U.S. Pat. No. 5,017,319, EP-B-0,161,766 and EP-A-0,492,016) converted ligno-cellulosic material directly into both thermosetting resin adhesive and composite products by selectively hydrolyzing and decomposing hemicellulose and/or cellulose fractions, using high pressure steam, into low molecular weight water-soluble resin material including pentose and hexose sugars, sugar polymer, furfural, hydroxymethyl furfural, dehydrated carbohydrates, organic acids and other decomposition products. The water-soluble resin material alone, thus produced, can be used in liquid or powder form as a thermosetting water-proof resin adhesive.
Since lignin is believed to be the natural binder within lignocellulose and is phenolic in nature, it has been extensively studied and researched in the past hundred years as a binder for lignocellulosic composite products.
In U.S. Pat. No. 726,029, A. Classen used steam to treat saw dust with acid and then cooked the acidified saw dust under pressure at a temperature of 105 to 145° C. for 30 to 60 minutes to render the hemicellulose water-soluble. After cooking, the treated mass was washed with water to remove the water-solubles before drying and molding into composite products. U.S. Pat. No. 2,303,345 by Mason and Boehm described a process of making strong products from lignocellulosic material. Mason and Boehm employed high pressure steam to separate lignin from lignocellulosic material for bonding. In their process, the hemicellulose was hydrolysed into water-solubles and removed from the treated lignocellulose before the fibres and lignins were made into hard board. Under high temperature and pressure lignins were melted as a binder to cement fibres a into a high density hard board. The water-solubles, consisting mainly of reducing sugars, were concentrated into wood molasses commonly used in animal fodder. Although the well-known Masonite process of making hard board, using natural lignin as binder, has been in commercial production since the 1930s in USA and other countries, it is now an industry in decline due to inefficiencies and environmental concerns about the enormous quantities of waste water.
Other investigators have used lignin from the pulping industry in combination with phenol-formaldehyde resin to form copolymer resin adhesives. In U.S. Pat. No. 2,786,008, R. Herschler incorporated ammonium based spent sulfite liquor, or lignosulfonates, a by-product of sulfite pulping
Renner, Otto, Boiselle & Sklar LLP
Vargot Mathieu D.
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