Лако-красочные материалы — производство

Polymer. The polymer determines the properties of the hot melt. Variations are possible in molar mass distribution and in chemical composition (copolymers). The polymer is the main component and backbone of the hot melt adhesive blend as it gives strength, cohesion, and mechanical properties (filmability, flexibility). Ethylene vinyl acetate (EVA) is the most used type (approximately 80%). It can be varied in viscosity (melt index) and content of acetate within a broad range of values, and once hardened it presents a predominantly amorphous structure. The vinyl acetate groups impart good adhesion ability towards many materials. The low heat stability, however, limits its areas of application. With increasing content of the vinyl acetate comonomer the adhesion ability, the wetting behavior, and the flexibility increase, but also the setting time and the price. Heat resistance and cohesion properties become worse. The higher the average molar mass of the polymer, the worse its wetting behavior, but the better the cohesion properties, the heat resistance and temperature resistance, and the higher the melting viscosity at a given temperature.

Ethylene-acrylic acid ester copolymers show high heat resistance and high elasticity at low temperatures. Amorphous poly-a-olefins (APAOs) are also used as the basic poly­mer and their main component monomer is propylene. They present better heat resistance than EVA. APAO shows good adhesion properties to nonpolar surfaces, good flexibility, and a high resistance to temperature and moisture.

Polyamides give the fastest setting speed, good cohesion and very high heat resis­tance. They are oil and solvent resistant. Due to the narrow melting region (sharp transition between the elastic and plastic areas) a short setting time during cooling is allowed. Depending on their type the melting temperature is between 105 and 190°C. Advantages are the low melt viscosity, high bond strength, and a high green tack. Disadvantages are the high price and the susceptibility for carbonization at high tem­peratures in the presence of oxygen. Thermoplastic polyurethanes have no reactive iso­cyanate groups and cannot crosslink. Thermoplastic, linear, and saturated polyesters give, depending on their chemical composition, hard or elastic and tacky bondlines. They have relatively high melt viscosities, and the bondlines are resistant against moist­ure, water, and ultraviolet (UV) radiation.

Tackifiers. Tackifiers usually are hydrocarbon resins (aliphatic C5, aromatic C9) or natural resins (polyterpenes, rosin and rosin derivatives, tall oil rosin ester). They improve hot tack, wetting characteristics and open time, and enhance adhesion. The content of tackifiers in a hot melt can be in the region of 10-25% of total material.

Other Components. Waxes increase the resistance against water and moisture (hydrophobization) and improve flow and lubricate during application. Inorganic fillers (CaCO3 and/or BaSO4) improve cohesion (small particle size) and adhesion, decrease sagging, and improve the price of the product. Pigments are also used, often in the case of white colored hot melts, the most common pigment being TiO2. Plasticizers decrease the viscosity and the heat resistance; they ameliorate the wetting behavior and the flexibility of the bondline; however, cold flow can occur. Stabilizers improve the thermooxidative behavior of the hot melt (heat and aging stability).

2. Curing Hot Melts

Curing hot melts are easily meltable polyurethane prepolymers (polyaddition of polyva­lent alcohols and isocyanate) with reactive isocyanate end groups (-N = C = O), which react with the moisture content of the wood under hardening. This leads to the formation of a crosslinked polyurethane network. Therefore, as thermoplasticity is no longer present, they cannot melt and are insoluble and show good mechanical and chemical resistance. During application a two-step bonding process takes place, the two steps running in parallel, but at different rates:

(i) quick physical solidification due to cooling: high green strength for further rapid processing

(ii) slower chemical hardening by crosslinking: the reaction of the free isocyanate groups is initiated by the moisture content of the surrounding air and of the adherend.

The advantages of the curing hot melts are:

Higher resistance against heat, moisture and steam, good aging and long term stability.

Higher mechanical bond strength.

Lower application temperatures: lower molar masses and lower softening and melting temperatures. Processing of heat susceptible adherends, e. g., PVC foils, is possible, for example at a processing temperature of 70°C. The heat resistance of the bondline is up to 120°C.

Good aging resistance The disadvantages of curing hot melts are:

They contain monomeric isocyanate, which is toxic, and thus working safety must always be taken into account.

They have stricter requirements concerning packaging and application, namely preventing the access of water during storage and application is necessary.

They are expensive.

Two component curing hot melts consist, for example, of (i) polyamide + epoxy, or (ii) a polyol component + isocyanate. After the mixing of the two components, they possess only limited pot life.