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Introduction to Polymers, Wood Adhesives, and Wood Finishes

1.1 Good Wood Adhesives Must Be Optimum Polymers with Optimum Secondary Forces

Many structural materials used in this world, including wood adhesives, are hard, strong solid materials, i.e. their softening points (temperatures) are appreciably higher than ambient temperatures. If they become soft or liquid at use temperatures, their structural uses would be limited. Stone, wood, plastics, metals, glass, brick, cotton, etc., are used as structural materials. Liquid adhesives that bond these materials should be transformed into structural solids when the bonding process is completed; otherwise, the bonded elements will separate under a load. Furthermore, among solid materials, certain materials such as sugar or salt have some physical strength under dry conditions, but the strength disappears quickly when they are contacted with water because they dissolve in it, i.e. the molecules become disengaged from each other. Molecular units in structural materials do not disengage from each other easily even when contacted with water or heated to high temperatures since they consist of, firstly, covalently bonded (primary bonding) atoms in the form of high molecular weight polymer molecules (macromolecules), and secondly, these macromolecules also have certain optimum molecular geometry and secondary bonding forces among the atoms and molecules, resulting in solid structures that give strength and stability against exposure to water and/or high temperatures.

For example, the basic chemical structural units in a strand of cotton or wood pulp fibers are cellulose molecules, each having about 5000 or more glucose molecules covalently bonded. It should be reminded that glucose, the basic unit of cellulose polymers, is the main constituent of corn syrup with a molecular weight of 180 and dissolves readily in water due to its low molecular weight and high affinity for water. The major reason why cellulose is cellulose is that the glucose units are bonded together covalently to a high degree of polymerization and also that the secondary forces, exerted by the many hydroxyl groups present on cellulose molecules, are optimally arranged to make the cellulose molecular strands set into crystallites (highly ordered state) that do not get easily disturbed by water. Continuation of these crystallite formation leads to the formation of cellulose fiber strands. Although these fiber strands are further reinforced by lignin and hemicellulose molecules in wood (polymeric glue of wood), much of the strength of wood is derived from the polymeric, structural nature of cellulose molecules and their secondary forces. Thus, it is readily recognized that structural adhesives have to be of (covalently bonded) polymers that have additional molecular structures and secondary bond forces that impart the strength properties under the common use conditions of moisture and temperature.

1.2 Polymeric Materials

A polymer is defined as a high molecular weight material (macromolecule) composed of many repeating units of a monomer or monomers joined by covalent bonds. Poly means "many" and "mer" means "part," i.e. "polymer" literally means "many joined parts." Polymeric materials can be organic or inorganic and natural or manufactured, as illustrated in the following:

• Inorganic: clay, brick, cement, pottery, sand, glass, glass fiber, carbon fiber, metallic carbides.
• Organic/natural: polysaccharides (cellulose, starch), proteins (hide, silk), natural rubber from rubber trees (polyisoprene).
• Organic/synthetic/semi-synthetic: polyethylene, nylon, polyester, rayon, etc. Monomer materials of synthetic polymers are derived mostly from coal, petroleum, natural gas, and some from wood, like rayon. The diverse properties and many different uses of polymers, as we will see in the next several chapters, can be traced to their high molecular weights and inter- and intramolecular secondary forces (molecular structures) present in the polymer molecules as well as different types (atoms) of monomers. Low molecular weight materials are not useful in most structural applications unless they are transformed into high molecular weight materials during the final forming (curing or molding) steps.

1.3 Synthetic Polymer Preparation Methods

Synthetic polymers are prepared by polymerization of monomers [1]. Evidently, the monomers must have two or more hands, i.e. functional groups (that can make covalent bonds), to form a repeating polymer molecular chain. When the monomer (M) has two functional groups, i.e. two free (bonding) electrons, the polymer formed will have a linear chain structure:

There are currently many different carbon-based monomers having two functional groups. When monomers having three or more functional groups are used, the polymer chain will be branched and, at the end of polymerization, the polymer molecules will be cross-linked. Also, aside from the common C-C bonds, the C-O bonds (ethers and esters), and C-N bonds (amides) are common chemical bonds formed in polymers. The end groups in polymers are not normally defined clearly in equations, but they are only slightly different groups than those in the middle of the chain, formed by some side reactions at the end of the polymerization process. The end groups are of minor importance in polymer technology because the value "n," degree of polymerization, reaches one thousand or more, and, therefore, the proportion of end groups in the entire polymer molecule becomes exceedingly small.

1.4 Typical Synthetic Polymer Materials

• Polyethylene: One ethylene molecule (gas) is added to another ethylene molecule, another ethylene molecule, etc., often called an "addition polymerization," with the help of a catalyst and under pressure. Ethylene itself is a gas at ordinary temperatures, and polyethylene is a thermoplastic solid material; the difference being the high molecular weight of the latter, attained by polymerization. Uses and properties: bottles, containers, films for packaging (shopping bags), and high-strength fiber. Polyethylene softens when heated to 95-120?°C and finally melts at higher temperatures (a thermoplastic polymer); at room temperature, it is stretchable, flexible, hydrophobic, etc. It has been the largest volume polymer among all synthetic organic polymer materials.

• Polypropylene: A thermoplastic addition polymer made by (head to tail) polymerization of propylene as monomer. Uses and properties: laboratory beakers, bottles, ice chests, rope, etc.; it softens when heated to about 150?°C (thermoplastic), similar to polyethylene, but is stiffer and the softening point is higher.

• Polystyrene: A thermoplastic addition polymer made by (head to tail) polymerization of styrene as monomer. Commercial polystyrene polymers normally have an average molecular weight in the range of 50-200?k?Da and are extensively used as expanded materials, such as cups and dishes, and for insulation and package sheeting.

• Polyvinyl acetate (PVAc): A thermoplastic addition polymer made by (head to tail) polymerization of vinyl acetate as monomer, commonly done as an emulsion polymerization process to result in latices. The glass transition (softening) temperature of commercial solid PVAc polymers is about 28?°C, and they are modified with 10-15% plasticizer (dibutyl phthalate) for use as wood binders and in coatings. Uses and properties: wood glue, interior use paint vehicle (binder). It melts when heated (thermoplastic) and has the polar characteristics needed for wood adhesive polymers due to the presence of polar acetate (-O-Ac) groups in comparison to polyethylene or polypropylene.

• Ethylene-vinyl acetate (EVA) co-polymer: A thermoplastic "co-polymer" made by co-polymerizing ethylene (5-20% by weight) and vinyl acetate monomers. The resultant co-polymer materials show high clarity, low-temperature flexibility, and crack resistance. The polymers are more flexible than PVAc. The co-polymers made with about 20% ethylene contents are used as hot-melt adhesives used in wood and paper bonding.

• Polyvinyl chloride: A thermoplastic polymer made by polymerizing (head to tail) vinyl chloride as a rigid, colorless polymer. With the addition of stabilizers and fillers, they are used extensively as water pipes, coatings, etc.

• Acrylics: Acrylic acid (i) and methacrylic acid (iv), their ester and amide derivatives (ii, iii, and v), and cyanoacrylic acid esters (vi) are the monomers for acrylic resins. Monomers are polymerized, respectively, through their double bonds to result in high molecular weight thermoplastic emulsion (latex) polymer materials, which are widely used as coatings, adhesives, and pane glasses.

• Nylon: A polyamide, thermoplastic, made by condensation polymerization of a dicarboxylic acid and a diamine as monomers: the two monomers are condensed in exactly 1.00?:?1.00?mol ratio, where a water molecule is expelled to form amide bonds (condensation polymerization reaction). Uses and properties: filament yarn (nylon, clothes) and engineering...