Лако-красочные материалы — производство

Animal glues are derived by the hydrolysis of the protein constituent collagen of animal hides and bones. Collagen in its natural state is water insoluble and must be conditioned to solubilize the protein. Collagen molecules are triple helices of amino acid sequences and contain both nonpolar and charged acidic and basic side chains. The conversion of collagen to the soluble protein of animal glue (gelatin) involves breaking the intra — and intermolecular polypeptide bonds through the use of acid or alkali and heat. The collagen — glue (gelatin) transition has been described as a stepwise process involving the melting of the trihelical network to an amorphous form, followed by the sequential hydrolysis of various types of covalent bonds [2,3].

Glues and gelatins are described as hydrolyzed collagen with the following formula [4,5].

C102H149O38N31 + H2O! C102H151O39N31

The approximate chemical composition of glue (gelatin) protein is

Carbon 50.3%

Hydrogen 6.2%

Oxygen 25.6%

Nitrogen 17.8%

Animal glues are composed of a-amino acids joined in polypeptide linkages to form long — chain polymers [5-8]. A typical chain fraction with three amino acids:

In aqueous solutions of animal glues, the polypeptide chains take up random configura­tions of essentially linear form. Studies have indicated that most glue molecules consist of single chains terminated at one end by an amino group and at the other end by a carboxyl group [7]. The molecules may also have side chains and contain cyclic structures. They may in part conform to the oriented chain in the original collagen. The polypeptide chains are of varying lengths and consequently, widely different molecular weights. A wide range of average molecular weights has been reported, ranging from approximately 10,000 to over 250,000. Molecular weight distribution is of equal importance in studying animal glue (gelatin) protein systems [3].

Amino acid studies corroborated by various analyses indicate that there are 18 different amino acids present in collagen and animal glue (gelatin) in varying amounts (see Table 1). The acidic and basic functional groups of the amino acid side and terminal groups confer polyelectrolyte characteristics to the protein chains. The chains contain both amine and carboxylic groups which are reactive and ionizable. These electrically charged sites affect the interactions among protein molecules and between protein molecules and water. These polar and ionizable groups are believed to be largely responsible for the gelation and characteristic rheological properties of animal glues. Cross-linkage between protein molecules is possible through hydrogen, ionic, and covalent bonds.

Animal glues are amphoteric because the amine and carboxyl groups contained in the polypeptide protein chain are reactive and ionizable. In strongly acid solutions, the protein is positively charged and acts as a cation. In strongly alkaline solutions it is negatively charged and acts as an anion. The intermediate point, where the net charge on the protein is zero, is known as the isoelectric point (IEP) and is designated in pH units.

The isoelectric point varies, depending on pretreatment of the collagen, whether acidic or alkaline. During processing, the acidic or alkaline treatments used hydrolyze the amide groups in the collagen to a greater or lesser extent, liberating the acid functions. Acid — processed glues (little amide group modification) have an isoelectric point near 9.0, and alkaline-processed glues (low residual amide groups) have an isoelectric point close to 4.8.

Gelation of aqueous solutions of animal glues upon cooling is an important char­acteristic. Gelation involves both intra — and intermolecular reorientation upon cooling of the solution. It is caused by the formation of random primary and secondary bonds. Intermolecular network formation is primarily the result of a cross-linking mechanism between molecular chains by hydrogen bonds [11].