Wood adhesive casein

Adhesives from Diisocyanates and Proteins. Protein-based adhesives have been used as traditional binders for wood since the beginning of wood products manufacture. Glutin and casein binders can provide interior-grade wood products but with modified casein binders, even panels for exterior use are possible. For economic and technical reasons, protein-based wood adhesives have been replaced more and more by synthetic adhesives since the beginning of the century. 

In the context of a book on adhesives from renewable resources, blood- and casein-based adhesives are worthy of mention because of their great historical importance to the adhesives industry. Caseins are particularly important because they were the first adhesives recognized for forming structurally capable bonds in wood products. They have been used since 1880 and they are still the preferred materials for many types of bonding due to their excellent adhesive properties and ease of cure. Despite their animal origins, these materials are far from plentiful. This makes them quite expensive in terms of wood adhesives, which is the viewpoint of this discussion. The review of the history, nature, manufacturing, and uses of these materials that follows may help to develop a perspective on their future usefulness to the adhesives industry. 

Casein adhesive glue Adhesive on animal basis, made of acid casein accruing from milk processing. Application as wood adhesive and in the packaging industry (label adhesive). 

Uses Source of protein for feedstuffs organic filler casein-based wood adhesives binder for paper coatings plastics microbial fermentation nutrient medium for bacitracin prod. 

Uses Emollient, skin conditioner in cosmetics Soybean (Glycine soja) meal Synonyms Soyabean meal Soybean meal Definition Crushed residue obtained from soybeans after expressing the oil Uses Source of protein for feedstuffs organic filler casein-based wood adhesives binder for paper coatings plastics microbial fermentation nutrient medium for bacitracin prod. 

A wide range of adhesive types and chemistries are used to bond wood elements to one another (Table 2), but relatively few adhesive types are utilized to form the composites themselves. The vast majority of pressed-wood products use synthetic thermosetting adhesives. In North America the most important wood adhesives are the amino resins (qv), eg, urea-formaldehyde (UF) and melamine-formaldehyde (MF), which account for 60% by volume of adhesives used in wood composite products, followed by the phenolic resins (qv) eg, phenol-formaldehyde (PF) and resorcinol-formaldehyde (RF), which account for 32% of wood composite adhesives (12,13). The remaining 9% consists of cross-linked vinyl (X-PVAc) compounds, thermoplastic poly(vinyl acetates) (PVA), soy-modified casein, and polymeric diphenylmethylene diisocyanate (pMDI). Some products may use various combinations of these adhesives to balance cost with performance. 

Soybean flour first came into use as an adhesive in the 1920s, when a method was developed for extracting the oil from the soybean meal without altering the solubility of the protein. Soybean flour was widely used as a wood adhesive before World War II. It is still an important adhesive in many phases of the wood utilization industry, with much of its use in combination with casein or blood or both (see Lambuth in Chapter 10 of the second edition of this Handbook). 

Casein is utilized in diverse ways, as an adhesive, as a binder, and as a protective colloid and stabilizer for emulsified adhesive systems. Casein is sold as a raw material, and casein adhesives are sold in both the dry and liquid forms. The useful wood adhesive known as casein glue, which is formulated with lime and sodium salts, will be addressed later. This section will cover other uses of casein, for which the solubilization of the casein is generally accomplished with the use of heat, and at lower pH s than for the lime-containing glues. 

CASEIN AND PROTEIN BLEND GLUES AS WOOD ADHESIVES... 

The wood adhesives discussed here will be the casein-lime-sodium salt glues and similar glues formulated with combinations of casein with soy flour, blood, or both. The pH of these adhesives is 11-12, due to the lime, sodium hydroxide, and sodium salts used for dispersion. 

Canadian Specifications. In Canada, CSA Standard for Wood Adhesives No. 0112.3-M 1977 covers the specifications for casein glue. Glue line requirements for wood doors are covered in CSA Standard 0132.2-M 1977. (Copies may be obtained from Canadian Standards Association, 178 Rexdale Blvd., Rexdale, Ontario, Canada, M9W 1R3. 

The first applications of adhesives for bonding aircraft structure probably date back to about 1920, when light-weight airframes were fabricated with plasticized nitrocellulose-impregnated fabric stretched over a framework of wooden stringers. The wood adhesives used for these applications were first based on casein, then urea formaldehyde resins, and finally phenol formaldehyde resins. 

Developments in glued laminated structures and panel products such as plywood and chipboard raises the question of the durability of adhesives as well as wood. Urea-formaldehyde adhesives are most commonly used for indoor components. For exterior use, resorcinol adhesives are used for assembly work, whilst phenolic, tannin and melamine/urea adhesives are used for manufactured wood products. Urea and casein adhesives can give good outdoor service if protected with well-maintained surface finishes. Assembly failures of adhesives caused by exudates from some timber species can be avoided by freshly sanding the surfaces before glue application. 

The major synthetic adhesives used for bonding wood include urea, phenol, and melamine formaldehyde resorcinol formaldehyde, phenol resorcinol, and polyvinyl acetate emulsions. More recently one-component, moisture cured polyurethane adhesives have become popular for bonding wood. Natural adhesives such as casein and animal glues are also often used for general-purpose wood bonding. Epoxies have been used for certain specialized wood joining applications such as when wood is bonded to metal substrates. 

Adhesives made from blood and casein were some of the first used in the wood products industry. They have been replaced in many applications by petroleum-based adhesives, which show improved performance or better economics. Despite this competition, they remain important in certain specialty areas due to their unique curing and bonding characteristics. 

There is archeological evidence to indicate that early Egyptians used casein adhesives 5,000 years ago (i). Caseins have been used in Germany since about 1880 and in the United States since around 1900 (2). Impetus for the use of blood and caseins in wood gluing was generated by their extensive use in airplane construction during World War I (i-3). This led to broader commercial... 

Despite the many consumer and wood gluing uses mentioned here, an estimated 75% of casein production went into paper coatings until World War II (4). Casein use for adhesives reached its peak in 1973, at about 121 million dry pounds (5). Today, very little casein glue is used by household consumers, paper coaters, and plywood manufacturers. In 1986, nearly all U.S.-casein adhesive production was used by the door manufacturing industry, where the total volume was less than 10 million pounds. 

The casein content of the dry adhesive varies from 30% to 70% depending on its use. Extenders and fillers such as soy and wood flours are added to obtain an economical adhesive. One or more alkalis such as sodium hydroxide, lime,... 

Results of a 2-year paint study indicate acetylated wood is a better painting surface (37) than untreated wood. UV radiation darkens unacetylated wood, but there is no change or a slight bleached effect with acetylated wood (37). In general, acetylation reduces the adhesive strength of wood (48). Adhesive strength is reduced with urea-formaldehyde resins (54, 55) and casein glues (55), but there is very little effect with resorcinol-formaldehyde resins (55). 

Ethers. Methylation. The simplest ether is the methyl ether. Reaction of wood with dimethyl sulfate and NaOH (54, 55), or methyl iodide and silver oxide (54) are two systems that have been reported. Methylation up to 15% weight gain did not affect the adhesive properties of casein glues. The mechanical properties of methylated wood are greatly reduced because of the severe reaction conditions required. 

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