Methylolation Crosslinking

Cotton and other cellulosic fabrics, and blends, usually require reactive cross-linking finishes to provide improved bending properties (i.e., crease behavior) and shrinkage. The products of choice for crosslinking cellulose are N-methylol compounds, which are made by reacting urea with formaldehyde and other additives. In application, storage, and use, these reactive A-methylol crosslinkers release formaldehyde to the air. 

Other crosslinking reactions may be triggered by a catalyzed reaction between different units of a copolymerized functional monomer, such as A -methylol acrylamide or a copolymerized silane compound [86]. 

Possible formaldehyde crosslinkers are paraformaldehyde [16,17,144,145], methylolurea mixtures such as urea-formaldehyde precondensate or formurea [145], or urea and phenol methylols with longer chains to overcome steric hindrance. 

The second step is the condensation reaction between the methylolphe-nols with the elimination of water and the formation of the polymer. Crosslinking occurs hy a reaction between the methylol groups and results in the formation of ether bridges. It occurs also by the reaction of the methylol groups and the aromatic ring, which forms methylene bridges. The formed polymer is a three-dimensional network thermoset ... 

The acid-catalyzed reaction occurs by an electrophilic substitution where formaldehyde is the electrophile. Condensation between the methylol groups and the benzene rings results in the formation of methylene bridges. Usually, the ratio of formaldehyde to phenol is kept less than unity to produce a linear fusible polymer in the first stage. Crosslinking of the formed polymer can occur by adding more formaldehyde and a small amount of hexamethylene tetramine (hexamine. 

Since a small amount of water is always present in novolac resins, it has also been suggested that some decomposition of HMTA proceeds by hydrolysis, leading to the elimination of formaldehyde and amino-methylol compounds (Fig. 7.15).42 Phenols can react with the formaldehyde elimination product to extend the novolac chain or form methylene-bridged crosslinks. Alternatively, phenol can react with amino-methylol intermediates in combination with formaldehyde to produce ortho-or para-hydroxybenzylamines (i.e., Mannich-type reactions). 

There are reactive softeners, some of which are N-methylol derivatives of long-chain fatty amides (10.241) while others are triazinyl compounds (10.242). The N-methylol compounds require baking with a latent acid catalyst to effect reaction, whereas dichloro-triazines require mildly alkaline fixation conditions. The N-methylol compounds are sometimes useful for combination with crease-resist, durable-press, soil-release and water-repellent finishes. In this context, the feasibility of using silane monomers such as methyltri-ethoxysilane (10.243), vinyltriethoxysilane (10.244), vinyl triace tylsilane (10.245) and epoxypropyltrimethoxysilane (10.246) in crosslinking reactions to give crease-resist properties and softness simultaneously has been investigated [492]. 

Formaldehyde fixes proteins in tissue by reacting with basic amino acids— such as lysine,5 7—to form methylol adducts. These adducts can form crosslinks through Schiff base formation. Both intra- and intermolecular cross-links are formed,8 which may destroy enzymatic activity and often immunoreactiv-ity. These formaldehyde-induced modifications reduce protein extraction efficiency and may also lead to the misidentification of proteins during proteomic analysis. 

The methylol groups in reaction M12 can either deformylate (reaction M7) to amine or self condense (reaction M9) to yield melamine-melamine crosslinks. 

Cellulosic fibers (cotton, rayon) are crosslinked by reaction of the hydroxyl groups of cellulose with formaldehyde, diepoxides, diisocyanates, and various methylol compounds such as urea-formaldehyde prepolymers, /V, /V -di tnethylol-A(A -dimethy lene urea, and trimethyl-olmelamine [Marsh, 1966]. Crosslinking imparts crease and wrinkle resistance and results in iron-free fabrics. 

Acrylonitrile N-methylol acrylamide Acrylamide Ethylene sulfonic acid Allyl sulfonic acid Crosslinker (3) Semicrystalline polymers (2)... 

Crosslinking monomers include N-methylol acrylamide, acrylamide, further, acrylamidobutyraldehyde, dimethyl acetal, diethyl... 

Dase-catalyzed phenol-formaldehyde resins polymerized with a mole ratio of formaldehyde to phenol greater than one pose an interesting molecular weight characterization problem. This system is a dynamic one with active methylol end groups. Branched and crosslinked structures are formed, and in general, the separation of the resin from the reaction mixture is difficult. Figure 1 illustrates the chemical nature of a resole resin. 

Based on these results, it can be concluded that phenol-formaldehyde resins modified with 0.6 to about 1.0-mole of carbohydrate per mole of phenol and cured at neutral conditions can bond wood with acceptable dry- and wet-shear strengths, and wood failures. Also, reducing as well as non-reducing carbohydrates can be used as modifiers for neutral phenol-formaldehyde resins. It was found that the resins formulated under neutral conditions are very light in color and would thus be useful in the preparation of decorative products. Carbohydrate modifiers are incorporated into the resin via ether linkages between the hydroxyls of the carbohydrate and the methylol groups in the resin. Apparently carbohydrates, at least in theory, can participate in a crosslinked network. 

The possibility of crosslinking natural macromoleculcs, such as cellulose and proteic materials, via the Mannich reaction has also been inve.stigatcd. Cellulose derivatives, however, are mostly subjected to the analogous amidomcthylation reaction, " usually employing bis-methylol amides (urea, oxalyl, adipoyl derivativas, etc.) capable of reacting with the cellulose hydroxy groups. 

Anticrease treatments consist in the controlled functionalization and crosslinking of the protcic or cellulosic macromolecules of fibers. This topic has been already discussed (Sec. A.2, Chap. Ill, C.2, and Fig. 179, Chap. IV), but a very accurate study" of N,N -bis-methylol ureides (594 and 595), which stresses the importance of a well-balanced hydrophilic power among these additives, is worth mentioning. The appropriate choice of either of the above products, or the use of a mixture of both, in fact permits a. satisfactory optimization of the results. 

Reactive softeners Some softeners have functional groups that can react with the corresponding groups of some fibres, for example A-methylolated amines with the hydroxyl groups of cellulose (compare the mechanism of the crease resistance finish). The result is a very durable finish, combined with the typical advantages and disadvantages of this crosslinking chemistry, as discussed in Chapter 5. 

Typically, polymers of these acrylic and methacryUc esters are produced as copolymers with other acrylic and vinyl monomers. For example, acrylonitrile is often added to impart additional water and solvent resistance. Other features that can be improved include abrasion resistance, adhesion, elasticity, flexibility and film hardness. Enhanced durability to laundering can be achieved by incorporating reactive, especially crosslinking, monomers such as A -methylol acrylamide, hydroxyethyl acrylate, acrylamide, acrylic and methacrylic acid. Optimisation of polymer properties with the large variety of available monomers leads to near endless combinations of copolymers that are limited only by the imagination of the chemist and by the reality of the cost-efficiency ratio. 

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