Лако-красочные материалы — производство

A large variety of melamine resins are commercially available, with important variables being type of amino compound, molar ratio of formaldehyde to amino compound, degree of polymerization, type of alcohol used for etherification and degree of etherification.

Based on the type of amino compound, the aminoplasts used in coatings are classified as four main types: urea formaldehyde (UF), melamine formaldehyde (MF), benzoguanamine formaldehyde (BF) and glycouril formaldehyde (GF) resins. The majority of commer­cially used amino cross-linkers are MFs and UFs, with others used only for specialty applications. These resins vary in their characte­ristics as summarized below:

• Reactivity: UF > MF > BF > GF

• Adhesion: UF > GF > BF > MF

• Flexibility: GF > MF > BF > UF

• Resistance properties: GF > BF > MF > UF

• Exterior durability: GF > MF > UF > BF

• Cost: UF > MF > BF > GF

Commercially, each of these types of amino resin is available in different subtypes and degrees of alkylation. Based on alcohol type, the main grades are methylated, butylated, isobutylated and mixed ethers (using mixed alcohols). The type of alkylating agent (alco­hol) significantly affects the properties of the final coatings; that
is, longer chain alcohols tend to reduce curing rate and increase weight loss on curing but give better flow and leveling and stronger intercoat adhesion.

Depending upon the degree of alkylation and methylolation (Figure 2.25), the above four categories of aminoplasts may be further divided into three subtypes: highly alkylated aminoplasts (alkoxy being the main functional group), partially alkylated aminoplasts (with a con­siderable number of methylol groups) and high imino content amino­plasts (with relatively more secondary amine groups). For the coating industry, generally only GF is marketed in non-alkylated grades.

Figure 2.25: Representative structures of aminoplasts with different degrees of alkylation and methylolation

Highly alkylated aminoplasts have very low methylol and imino con­tent with high monomer content. Due to their very low tendency for self-cross-linking, they improve the flexibility and toughness of cured films. They are also known for their excellent compatibility with co­resins, low viscosity and excellent stability. A strong acid catalyst and higher curing temperatures are needed while using highly alkylated aminoplasts in coatings. Due to availability of more functional sites for cross-linking, they can produce a higher cross-linkage density.

The partially alkylated aminoplasts are characterized by a consi­derable number of methylol groups with a polymeric structure and lower monomer and imino contents. MW is an important factor in deciding compatibility of such cross-linkers with solvents and other resins, with higher polymeric aminoplasts having limited compati­bility. Higher MW also results in higher viscosity. During curing of coatings, these types of aminoplasts undergo a noticeable degree of self-cross-linking. The cross-linking requires a weak acid catalyst and lower temperature compared to the highly alkylated version, but these aminoplasts suffer from poor storage stability.

High imino aminoplasts are characterized by a lower methylol con­tent and high imino content but high levels of alkylation, an oligo­meric structure and higher viscosity. These types of products are predominantly derived from melamine. They are highly reactive with some tendency for self-cross-linking. Curing them with a co­resin can also be carried out without a catalyst or with a weak cata­lyst. These types of amino cross-linkers release less formaldehyde than partially alkylated or conventional butylated versions. Parti­ally alkylated high imino aminoplasts are available commercially that are even more reactive.