Лако-красочные материалы — производство

The polyamide hot melts are high-performance systems and are used selectively where good heat resistance is required. Their high cost relative to EVA types makes them rather unattractive for general use. Polyamide resins offer high tensile strengths and high initial tack, often without the need for additional formulating. Their higher melt points ensure good heat-resistance qualities and are responsible for rapid setting on cooling. Their two main drawbacks are cost and the tendency to char easily if kept at high temperatures.

Hot-melt polyamide resins are obtained by the reaction of diamines with diacids. While in their simplest form polyamides are the reaction of a particular diamide with a particular diamine, most of the polyamides used in adhesive formulations are complex reaction products obtained by combining several diacids and diamines to obtain the particular properties required. The most common diacid used is a dibasic acid obtained by polymerizing oleic or linoleic acid or other unsaturated fatty acids. This acid can be represented as HOOC—R—COOH, where R is a hydrocarbon residue of 34 carbon atoms and of indeterminate configuration. Commercial forms of this dimeric diacid also contain preparations of products obtained by polymerization of three or more molecules of unsaturated fatty acids and thus contain varying quantities of trimeric acids and of higher homologs. Monomeric forms are also present. The most used diamine for this type of adhesive is ethylenediamine, H2N—(CH2)2—NH2, but other diamines are also used, responding to the general formula

X Y

I I

HN — R— HN

where X and Y can be H or other chemical groups. Polyamides are then formed according to the schematic reaction

X Y X Y

HOOC — R— COOH + HN — R’—NH — HOOCRCONR’—NH + H20 or simply

X Y X Y

II II

иНООС—R—COOH + «HN—R’—NH—«HOOCRCONR’—NH + «H20

The reaction occurs with the elimination of water of form amide groups. The high polarity of the amide groups contributes to give, by formation of interchain hydrogen bonds, the characteristic polymer strength and adhesive properties to the polyamides.

The basic resins need some form of modification to achieve (1) suitable application viscosities, (2) flexibility, and (3) reduction in costs if possible. Suitable polyamide resins (those of the more flexible variety) are thus frequently modified by the addition of EVA copolymer (high-viscosity, high-melt-point grade). The amount of EVA that can be added is restricted to a maximum of 25% in most cases because of compatibility problems. The blend is then further modified with selected tackifying resin addition and small quantities of filler, to reach an optimum balance of performance properties. To achieve maximum adhesion, it is common for polyamide hot melts of this type to be used in conjunction with a polyamide resin solution primer system for edging material. The primer is invariably a dilute solution of the base polyamide resin.