Лако-красочные материалы — производство

Toluene diisocyanate (TDI) is a colorless, volatile, low-viscosity liquid commonly used in the adhesives area to manufacture low-viscosity prepolymers for flexible substrates. The structure of TDI is shown in Fig. 18. TDI is typically supplied as an 80:20 mixture of the 2,4 and 2,6 isomers, respectively, with two grades of acidity available. Type I TDI is low in acidity (10 to 40 ppm); type II TDI is higher (80 to 120 ppm). Type II TDI is generally used for prepolymer applications because the additional acidity is available to neutralize trace bases found in polyether polyols. These trace bases can cause branching reactions during prepolymer cooks, causing high viscosities and even gelations if not properly controlled (see Section V). The extra acidity present also serves to stabilize the prepolymer, extending the shelf stability. In addition, since TDI is predominately the 2,4 isomer, a reactivity difference is noted for the isocyanate groups. Since the less hindered site reacts first, the sterically hindered site is left when prepolymers are formed, leading to prepolymers that are more shelf stable. TDI prepolymers are used in adhesives for the textile and food packaging laminates industry, where a fit is found for their low viscosity and low cost. The volatility of TDI and additional handling precautions that must be taken when using TDI has limited its growth in adhesive applications.

Methylene diphenyl disiocyanate (MDI) is used where high tensile strength, toughness, and heat resistance are required. MDI is less volatile than TDI, making it

less of an inhalation hazard. The acidity levels in MDI are very low, typically on the order of 0 to 10 ppm, so the trace base levels in the polyols are much more critical in prepolymer production than with TDI. The structure of MDI is shown in Fig. 19. There are several commercial suppliers of MDI that typically supply grades with 98% or better 4,40 isomer. MDI is a solid at room temperature (melting point 38°C, 100°F), requiring handling procedures different from those for TDI. MDI should be stored as a liquid at 115°F or frozen as a solid at (—20°F) to minimize dimer growth rate. MDI reacts faster than TDI, and because the NCO groups in MDI are equivalent, they have the same reactivity, a contrast to TDI. MDI is used in packaging adhesives, structural adhesives, shoe sole adhesives, and construction adhesives.

Several MDI products have been introduced that address the inconvenience of handling a solid. They are seeing increased usage in the adhesives industry and are expected to experience a higher growth rate. Most MDI producers offer a uretonimine — modified form of MDI that is a liquid at room temperature. The uretonimine structure is shown in Fig. 20. In addition, several producers have introduced MDIs containing elevated levels of the 2,40 isomer, as shown in Fig. 21. At approximately the 35%, 2,40 isomer level, the product becomes a liquid at room temperature, greatly increasing the handling ease. A number of advantages are seen: slower reactivity, longer pot life, lower — viscosity prepolymers, prepolymers with lower residual monomeric MDI, and improved shelf stability.

Polymeric MDIs are made during the manufacturing of monomeric MDI. These products result as higher-molecular-weight oligomers of aniline and formaldehyde get phosgenated. A typical structure for these products is shown in Fig. 22. These oligo­mers average 2.3 to 3.1 in functionality and contain 30 to 32% NCO. Much of the hydrolyzable chlorides and color bodies produced in the manufacturing process of MDI is left behind in these products. The acidity levels can be 10 to 50 times the level found in pure MDI, and the products are dark brown in color. The higher acidity level decreases reactivity; however, this decrease is offset somewhat by the higher functionality.

Polymeric MDIs are typically lower in cost than pure MDI and because of the increased asymmetry have a lower freeze point (liquids at room temperature). They are less prone to dimerization, and as a consequence are more storage stable than are pure MDI and derivatives. Polymeric MDIs are used whenever the color of the finished

OCN-MDI —N. /MDI—NCO

OCN-MDr О

Figure 20 Structure of uretonimine.

NCO

adhesive is not a concern. They are generally not used for prepolymers because high — viscosity branched structures typically result. They are widely used as adhesives in the foundry core binder area, in oriented strandboard or particleboard, and between rubber products and fabric or cord. It is interesting to note that the polymeric isocyanates used commercially today are structurally very similar to the Desmodur R (trademark, Bayer) products used over 50 years ago [2].

Aliphatic isocyanates are used whenever resistance to ultraviolet light is a critical concern. Examples of aliphatic isocyanates are hexamethylene diisocyanate, hydrogenated MDI, isophorone diisocyanate, and tetramethylxylene diisocyanate. Structures for these molecules are shown in Fig. 23. The aliphatic isocyanates are usually more expensive than aromatic isocyanates and find limited use in adhesive applications. Resistance to ultraviolet light is usually not a critical concern in adhesives because the substrate shields the adhesive from sunlight.

Blocked isocyanates are also used in urethane adhesives. Blocking or ‘‘masking’’ of the isocyanates refers to reacting the isocyanate groups with a material that will prevent the isocyanate from reacting with active hydrogen-containing species at room temperature but will allow that reaction to occur at elevated temperatures. Blocked isocyanates are easily prepared and their chemistry has been developed extensively since their inception by Bayer and co-workers during the early 1940s [27-29]. As an example, the preparation of a methylethylketoxime blocked isocyanate is shown in Fig. 24.

Blocked isocyanates offer a number of advantages to unblocked isocyanates. The traditional concern for moisture sensitivity can be addressed by blocking the isocyanate. Heat activation is then required, but most commercial adhesive applications can meet this requirement. Water-based dispersions and dispersions of the isocyanate in the polyol or other reactive media become possible using blocked isocyanates. There are a number of blocked isocyanates commercially available that could be used in adhesive

applications. Miles (Bayer) produces a series of aromatic and aliphatic blocked isocyanates marketed for primers, epoxy flexibilizers, wire coatings, and automotive top­coat applications. Blocked isocyanates are widely patented for fabric laminating adhesives [30], fabric coating adhesives [31-34], and tire cord adhesives [35-40].