Лако-красочные материалы — производство

Loren D. Lower and Jerome M. Klosowski

Dow Corning Corporation, Midland, Michigan, U. S.A.

I. INTRODUCTION

Silicone adhesives and sealants were introduced approximately 40 years ago and many of the silicones used in the early days are still performing. Products are available in a variety of forms, from pastelike materials to flowable adhesives. Both single- and multicomponent versions are available, with several different cure chemistries. Most of the silicones of commerce are based on polydimethylsiloxane (PDMS) polymers. Other siloxane polymers may be used when resistance to ultrahigh temperature, ultralow temperature, or solvents is required.

Applications are extremely broad. A partial list includes construction, highway, automotive, appliance assembly, original-equipment manufacture, maintenance, electro­nics, aerospace, and consumer uses. In some cases, silicones compete with other materials, such as polyurethanes, polysulfides, and acrylics, whereas in applications requiring long­term durability, silicones alone are specified. Silicones are often chosen for their excellent resistance to weathering and temperature extremes, their adhesion, and their ability to accommodate substrate movement. When silicone sealants and adhesives are mentioned, the thought of excellent durability comes to most readers’ minds. Silicones [named for the similarity of the (CH3)2SiO polymer repeat unit to the analogous organic ketones, R2C=O] occupy a unique position between inorganic and organic materials. The satu­rated inorganic Si—O—Si polymer backbone provides flexibility and stability to sunlight, while the methyl groups ensure low intermolecular forces. Some of the key attributes of silicones, which are responsible for their unique properties and durability are [1]:

Low surface tension High water repellence Partially ionic backbone Large free volume

Low apparent energy of activation for viscous flow

Low glass transition temperature

Freedom of rotation around bonds

Small temperature variations of physical constants

High gas permeability

High thermal and oxidative resistance

Low reactivity

Insolubility in water

High silicon-oxygen bond energy

Selected properties of PDMS are as follows:

Critical surface tension of wetting 24mN/m

Water contact angle 110°

Glass transition temperature 150 K

Energy of rotation 0 kJ/mol

Activation energy for viscous flow 14.7kJ/mol

Si-O bond energy 445kJ/mol

Percent polar contribution 41%

The saturated backbone and high Si—O bond energy result in products that perform very well in applications involving exposure to sunlight. Since the silicone polymer does not absorb energy in the ultraviolet (UV) region of the light spectrum, one must be cautious with the use of clear silicones. The silicones need no UV absorbers to be stable (and contain none); thus the UV light from the sun can pass through clear silicones to the surface below the sealant. If the surface is sensitive to UV light, deterioration of the substrate may occur. Except for light-protected areas and unsensitive substrates (such as glass), the most judicious choice is a pigmented silicone. The pigment acts as a UV blocker and protects the substrate beneath the silicone. Because of the unparalleled stability to UV radiation, silicones are the sealants of choice for wet glazing techniques and the only generic class of sealants allowed for structural glazing (the adhering of glass and other building materials to structures with no attachment other than the silicone). Structural glazing is used in all-glass buildings and skyscrapers.

Other types of sealants often contain large amounts of filler and UV stabilizers to afford some degree of longevity in sunlight. This makes the nonsilicones satisfactory for some applications, but not in applications in which the sun shines directly on the bond line. This application is reserved for silicones. A specialty application for silicones, which further illustrates their UV-light durability, is in the sealing of accelerated UV-weathering test machines. The excellent stability to UV light is true only for pure silicones and is not true of “siliconized” organics or ‘‘modified silicones.’’ These contain very little silicone and thus have durability characteristics determined primarily by their base polymer systems.

Silicones have low intermolecular forces that result in relatively flat physical property response with temperature change. An example of this flat response is shown in Fig. 1, in which the viscosity of silicone polymers and a hydrocarbon oil are plotted as a function of temperature [2,3]. The relatively low response of silicone properties to temperature is important during sealant application (e. g., no heating needed in cold weather and no flow in hot weather). Even more important, however, is the fact that the performance of the cured sealant or adhesive will be less temperature dependent than will most organic — based products. This has practical implications: in building joints, for example. In cold weather, the building components shrink, and joint sealants must maintain elasticity to accommodate this movement. This is also fundamental to their use as a structural glazing sealant/adhesive. The sides of all-glass buildings can get very warm in the summer sun, and the silicone must not lose strength at these temperatures. While this rather constant performance is critical in some construction applications, it is also important in many industrial and appliance applications, such as steam irons, where the sealant simulta­neously prevents water leakage and acts as an assembly adhesive.

Silicone sealants are rated for their movement capability, with classes at ± 12.5%, ± 25%, ± 50%, and even higher joint movement capability. This too is quite unique, since high-movement nonsilicone sealants rarely perform for long periods of time above ± 25% joint movement.