Sealants formulations

Besides water vapor and gas diffusion, other requirements for good edge sealants are water resistance, uv resistance, heat/cold resistance (—40 to 80°C), adhesion to glass and metal, and good characteristics for appHcation. Polysulfide sealants have maintained an exceUent record in use since the 1950s. Development of new polysulfides and sealant formulations continues in order to meet market needs of the 1990s. 

Moisture-Curing Silicones. The formulation of moisture-curing sHicones includes a sHicone polymer, filler, a moisture-reactive cross-linker, and sometimes a catalyst. The most common sHicone polymer used in sealant formulations is an alternating sHicon—oxygen backbone with methyl groups attached to the sHicon such as the sHicone polymer (1). 

Table 2. Typical One-Part Silicone Sealant Formulation ...

Two of the more recendy developed polysulftde polymers are the mercaptan-terminated polyoxypropylene urethane polymer and the polythioether polymer. The urethane-backbone-based polymer is used in many sealant formulations for insulating glass appHcations. The thioether backbone contains sulfur, but no S—S bonds, which are the weakest part of the conventional polysulftde polymer. This polymer improves the thermal stabiHty and reduces the gas—Hquid permeabiHty. 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fiUers, plastici2ers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utili2e diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, siUcones, or polysulfide, are also used. 

One-part clear acrylic latex sealant formulation, 22 42t One-part manganese dioxide-cured polysulfide formulation, 22 4 It One-part pigmented siliconized acrylic latex sealant, 22 42t One-part RTV silicones, 22 596 One-part silicone cross-linkers, 22 33t One-part silicone sealant formulation, 22 34t... 

One-part solvent-releasing butyl sealant formulation, 22 43t One-part urethane sealants, 22 36-37 One-pass clarifier, 22 61 One-photon absorption, 77 455 One-pot oxidation polymerization, 23 713 One-stage separation plants, 75 840 One-step cameras/processors, 79 276-278 One-step Ziegler process, 77 715-718 One-way memory, of shape-memory alloys, 22 341, 345-346... 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30-50% of the prepolymer the remainder consists of pigments, tillers, plasticizers, adhesion promoters, and other additives. 

Acrylic latex sealants. Basic acrylic latex sealant formulations were compounded in a sigma blade mixer, as shown in Table 3. The water-borne silanes were then incorporated into a base. Standard wet and dry peel adhesion determinations from various substrates were performed periodically in accordance with ASTM C794-8. 

The base material producers are usually large chemical or material companies that manufacture products for broad markets such as petrochemicals or plastics. When demand warrants, they will produce materials specifically for the adhesive and sealant formulators. 

An interpenetrating polymer network (IPN) consisting of an epoxy and an elastomer has been developed by Isayama.29 This is a two-component adhesive-sealant where the components are simultaneously polymerized. It consists of the MS polymer, developed in Japan by Kanegafuchi and commonly used in sealant formulations, with the homopolymerization of DGEBA using a phenol catalyst and a small amount of silane as a graft site to connect the MS polymer and epoxy homopolymer networks. 

Polymerized epoxy adhesives are amorphous and highly crosslinked materials. This microstructure results in many useful properties such as high modulus and failure strength, low creep, and good chemical and heat resistance. However, the structure of epoxy resins also leads to one undesirable property—they are relatively brittle materials. As such, epoxy adhesives tend to have poor resistance to crack initiation and growth, which results in poor impact and peel properties. In sealant formulations, epoxy resins do not often provide the degree of elongation or movement that is required for many applications. 

Several types of polymeric fibers are used as thixotropes in both adhesive and sealant formulations. These are cellulose, polyolefin, and aramid fibers. A fiber that exhibits good thixotropy usually has several key characteristics including... 

Kaohn is a common inexpensive filler used primarily as an extender in adhesive and sealant formulations. The cost of the adhesive is reduced because the kaohn addition increases the product s volume. Depending on the grade, kaohn can also... 

Epoxy-based primers are commonly used in the aerospace and automotive industries. These primers have good chemical resistance and provide corrosion resistance to aluminum and other common metals. Primer base resins, curing agents, and additives are much like adhesive or sealant formulations except for the addition of solvents or low-viscosity resins to provide a high degree of flow. 

Pyrolysis is simple thermal destruction of the molecular chain of the base polymer in the adhesive or sealant formulation. Pyrolysis causes chain scission and decreased molecular weight of the bulk polymer. This results in reduced cohesive strength and increased brittleness. Resistance to pyrolysis is predominantly a function of the intrinsic heat resistance of the polymers used in the adhesive formulation. As a result, many of the aromatic and multifunctional epoxy resins that are used as base resins in high-temperature adhesives are rigidly crosslinked or are made of a molecular backbone referred to as a ladder structure, as shown in Fig. 15.4. 

Epoxies, nitriles, and phenolics show relatively low diffusion rates and are less susceptible to moisture attack than most other polymers. As a result, these materials are often used in adhesive and sealant formulations where resistance to moisture is essential. 

Methyltrifluoropropylsiloxane Containing Elastomer. A two-part sealant formulation shown below was employed. 

A.S. Wagh, S. Jeong, and R. McDaniel, Chemically bonded phosphate ceramic sealant formulations for oil field applications, Argonne National Laboratory, Patent applied for. 

Because of their faster hydrolysis rates vinylsilanes are typically used as water scavengers in moisture-cured hybrid sealants. Vinylsilanes capture excess moisture even in the presence of other silanes added to the formulation for other purposes. Addition of vinylsilanes to a sealant formulation results in improved package stability and prevents premature cure during compounding (i.e. skin formation). 

The molecular weight of a poljrmer chosen for a sealant will influence the tensile and elongation (stress and strain) characteristics of the sealant. Therefore, a sealant formulator can... 

Non-reinforcing extending fillers are used in many silicone RTV sealant formulations. These include calcium carbonate, carbon blacks, talcs and other inorganic fillers. 

Two sealants were shown to have the most flexibility or toughness at 76 K and were subsequently used as the basis for modification studies to improve their low-temperature mechanical and physical properties. One was a polyurethane manufactured by duPont and designated Adiprene L-100. The other was a silicone manufactured by General Electric having both methyl and bulky radicals on the polymer backbone and designated RTV X-511. These sealant formulations were prepared and cured as shown in Table I. 

Table I. Unreinforced Sealant Formulations Exhibiting Best Flexibility in Bending at 76°K...

Another example of the effect of fillers on improving bond strength is reported for a silicone RTV-1 sealant formulated with Cabot Corporation s Cab-O-Sil LM-130 and TS-720 materials. Cab-O-SU fumed silicas are very effective reinforcing agents due to the very-large surface area available for polymer/filler interactions. The very low, 45 psi, tensile strength of the unfilled silicone increased to 210 psi with 14 phr of LM-130 fumed silica and to 150 psi with the same level of TS-720 fumed sUica. ... 

To illustrate the effect of sample thickness, we can compare adhesives to sealants, which in many cases can be viewed as thick adhesives. As the sample thickness is increased, the benefit of the BTZ is clearly demonstrated. In Table 3 a SEBS/hydrogenated hydrocarbon tackifier sealant formulation was prepared as a hot melt and poured into shallow petri dishes. Although both of these polymers have good inherent stability, sealant applications may require extended exposure to UV radiation. The discoloration data show that the BTZ prevents yellowing of the sealant. However, examination of the sealant surface shows surface crazing and cracking when not protected by incorporation of the HALS. The combination of the two classes of light stabilizers provides the best overall performance. 

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