Лако-красочные материалы — производство

Silicones are available in one — and multicomponent forms. The one-component types are commercially the most important and will be the focus of most of this discussion. These products, which generally cure by reaction with atmospheric moisture, are called RTV (room-temperature vulcanizing) sealants or adhesives. The surface cure rate of these pro­ducts is a function of the cure system, but the rate of cure in depth depends on the ability to transmit water vapor through the mass of sealant. Silicones are highly permeable to moisture vapor, and generally the one-component types cure at a rate of about 0.3 cm/day. Due to this high vapor permeability, the one-component silicones typically cure faster than do their nonsilicone counterparts.

The multicomponent products generally do not rely on moisture penetration for cure. Their chief attribute is fast cure in very deep sections. Thus many industrial produc­tion lines that demand fast cure use a two-component sealant (including the use of silicone encapsulants for electrical components). Cure of these two-part systems can be accelerated further by additional catalyst or exposure to elevated temperatures.

One of the more common two-part cure chemistries is based on the addition reaction of Si—H cross-links with vinyl functional polymers using platinum catalysts. This chemistry is shown below. One advantage of this addition chemistry is that it produces no cure by-products. Another common two-part chemistry involves condensation cure with alkoxysilane cross-linkers using Sn(IV) catalysts.

-Si(Me)2OSi(Me)2CH = CH2 + HSi — ! — Si(Me)2OSi(Me)2CH2CH2Si-

A simplified cure mechanism for the one-component silicone RTV sealants or adhesives is shown below.

Reaction of cross-linker with polymer ends:

2RSiX3 + HOSi(Me)2O[(Me)2SiO]nSi(Me)2OH!

X2 (R)SiO[(Me)2SiO]n Si(R)X2 + 2HX

Reaction of cross-linker with polymer ends:

X2(R)SiO[(Me)2SiO]nSi(R)X2 + H2O ! OH(X)(R)SiO[(Me)2SiO]nSi(R)X2 + HX (A) ()

Reaction of resultant polymer end with another polymer:

A + B! X2(R)SiO[(Me)2SiO]nSi(R)(X) — O — Si(X)(R)O[(Me)2SiO]nSi(R)X2 + HX

As indicated, the X groups above are hydrolyzable. Repeated hydrolysis and reac­tion of resultant polymer end groups leads to full cure, with elimination of HX as the leaving group. Examples of leaving groups, cross-linkers, and the common cure system names are given in Table 1.

Numerous other cross-linkers may be used. For the trifunctional cross-linkers, the R group may be methyl, ethyl, vinyl, and several other groups, with methyl the most common. In some cases tetrafunctional and higher-functionality cross-linkers or polymeric cross-linkers may also be employed. The acetic acid cure system should be avoided where substrates are subject to acid corrosion.

Two other classes of silicones deserve mention. These are the water-based silicones that are used in sealant and coating applications and the silicone pressure — sensitive adhesives. Water-based silicones can be prepared by anionic polymerization of siloxanes in water using a surface-active catalyst such as dodecylbenzenesulfonic acid [4]. The resulting emulsion can then be cross-linked in several ways, including the use of alkoxysilane copolymerization or tin catalysts in conjunction with colloidal silica. The result is essentially an emulsion of cured PDMS in water. Various fillers and other components are added, resulting in a sealant composition. Upon evaporation of water,

a silicone elastomer results. These sealants have the advantages of low odor, ease of installation, and easy cleanup. Their properties are rather close to those of their conventional silicone counterparts.

The components of the silicone pressure-sensitive adhesives (PSAs) are analogous to their organic counterparts [5]. Generally, a silicate resin and a silicone polymer or gum are dissolved in solvent. Both the resin and the polymer typically contain silanol (Si—OH) groups that are reacted during processing of the PSA, leading to a cross-linked network. Additional reactions can be accomplished through the use of free-radical catalysts. The extent to which these cross-linking reactions occur, the resin/polymer ratio, as well as the respective molecular weights of these components, are important in setting the properties of the PSA.

Silicone PSA products are used in a number of medical and industrial applications, ranging from a variety of PSA tapes and transfer films to automotive bonding. Advantages for the silicone PSA products include resistance to temperature extremes, chemical resistance, conformity to irregular surfaces, and electrical properties. They are also unique to most PSAs in their ability to adhere to difficult low-energy substrates, such as polytetrafluoroethylene and other silicones.