Butyl Sealant Formulation

Butyl rubber is a copolymer of a diene such as butadiene and an olefin such as isobutylene. The primary difference between butyl rubber and polyisobutylene is that butyl is a copolymer containing a minor amount (1.5 to 4.5 percent) of isoprene and polyisobutylene is a homopolymer. Adhesives based on polyisobutylene have been addressed in a previous SpecialChem article. ¹

Butyl rubber molecules (Figure 1) contain a small amount of unsaturation (i.e., double bonds in the molecular chain). This provides very good stability and inertness to the effects of weathering. The double bonds introduced into the molecule by isoprene permit the butyl polymer to be crosslinked. Crosslinking can be accomplished by the classical sulfur cure.

Figure 1: Butyl rubber and polyisobutylene molecule
 

Since the butyl molecule is very long and linear, it provides good barrier properties to the passage of moisture and air. The butyl polymer is very amorphous and as a result imparts good flexibility, vibration resistance, and resistance to shock or impact.

Since the molecule is composed entirely of hydrocarbon groups, it also provides very low moisture absorption and it is soluble in typical hydrocarbon solvents. However, the hydrocarbon nature also provides nonpolarity and difficulty in bonding to low surface energy substrates without modifications.

The physical, chemical, and compounding properties of butyl polymers will vary significantly with molecular weight, degree of unsaturation (number of isoprene units per hundred monomer units in the molecular chain), and minor constituents in the polymer (stabilizers, etc.). The butyl polymer can also be chemically modified such as by chlorinating, and it can be crosslinked. The properties of several commercial grades of butyl rubber are summarized in Table 1.

Table 1: Characteristics of Commercial Grades of Butyl Rubber ²

Partially crosslinked butyl rubbers, depolymerized butyls, butyl rubber solutions or cutbacks, butyl rubber latex systems, and other forms are also available. These products are used by many adhesives and sealant formulators who cannot process the raw butyl stock which is generally available as solid bales.

Because of their low price and excellent stability, butyl sealants are used in caulks and sealants for the construction industry. Butyl sealants are normally low strength and tend to creep under load. As a result, butyl rubber can be formulated with polyisobutylene to provide additional cohesive strength and creep resistance (due to butyl) and degree of flexibility, tack, and peel strength (due to polyisobutylene).

Butyl can also be crosslinked to improve its otherwise poor cohesive properties. They can also be crosslinked by agents such as p-quinone dioxime and an oxidizer. Curing butyl sealants, although weather resistant, have generally poor adhesion and cohesion. Introduction of small amounts of chlorine or bromine into the molecules enhances reactivity and crosslinking capability.

Butyl polymers are characterized by high elongation, but they are somewhat susceptible to atmospheric oxidation due to their low degree of unsaturation. In addition to its good resistance to moisture, butyl rubber has good resistance to vegetable and animal oils and to attack by chemicals. Due to their hydrocarbon nature, butyl compounds possess very good electrical insulating characteristics. This combined with their low water absorption properties provide significant value in sealants for wire and cable, splicing, and void-filling during cable construction.

Butyl rubber is generally used in solvent solution, but it can also be used as a hot melt. The tacky nature and auto-adhesion characteristics of butyl also enable it to be used as a dry sealing tape or rope. Tack free time could be quite long for butyl sealants, ranging from six hours to several days, and for curable systems, complete cure can range for one to six months. Butyl sealants are considered to be stringy and more difficult to apply neatly than other conventional construction sealants.

Most butyl based sealants are non-curing or slow curing and not recommended for large joint movement applications. The joint movement capabilities of butyl sealants are generally considered to be less than +/- 10-15 percent. This limits certain applications in the construction market. Commercial sealants contain fillers such as carbon black, zinc oxide, or silica; tackifiers, such as esters of rosin and solvents, such as cyclohexane. Carbon black or other reinforcement is used to improve tear resistance.

When compared to other sealants used in the construction industry, butyl sealants are considered to be a "medium performance" sealant, which generally indicates 5-12% movement capability, 5-15 years of service life, and medium cost. Comparisons to other conventional sealants are provided in Table 2.

Butyl sealants are used in a variety of new construction and home repair applications. Common applications in the building and construction industry are general caulking and sealing, roofing, and structural glazing. Skylights, roofs, and gutters are commonly sealed with butyl compounds.

One of the most common uses for butyl sealants is in glazing window and wall panels. The use of 100% solids preformed tape is preferred in this application because it does not shrink nor emit volatile materials on aging. Butyl is also valued in this application because of its good moisture and air barrier characteristics. Butyl sealants are also used in the construction industry to seal single-ply ethylene-propylene elastomer based roofing sheets. The sealants serve in flashing, patching, seaming, and bonding applications. Butyl sealants have also been used in precast concrete applications for drainage and sewer piping.

ASTM C1085 is a standard specification for a butyl rubber based solvent release sealant with a maximum of 10% joint movement capability. These have a service life of 5-15 years depending on the application. Butyl sealants are known for excellent adhesion to most substrates. However, they can stain adjacent surfaces and have relatively poor recovery from extension. The polymeric compositions along with the other additives will affect both the application and the ultimate performance properties of the joint. Typically solvent based butyl sealants have the following characteristics:³

• Excellent adhesion to many common substrates
• Limited movement capabilities, generally up to +/- 10%
• Excellent weathering
• Good use as both adhesives and sealants
• Sometimes used in curtain wall applications where adhesion to rubber compounds is needed
• Most are stringy and difficult to apply neatly
• May show some shrinkage after cure; may harden and crack over time on exposed surfaces.

Butyl sealants usually require relatively large amounts of solvent to achieve a gun-grade consistency. This high level of solvent contributes to shrinkage on aging. Generally large amounts of filler and permanent plasticizers such as polyisobutylene are also commonly used in these formulations. A good quality gun-grade butyl sealant will typically contain only about 15% butyl rubber. A typical butyl rubber caulking compound has the following composition shown in Table 3.

Table 3: Formulation for a Butyl Rubber Caulking Compound⁴

Butyl rubber compositions are available as one and two component solvent release sealants. One component butyl caulks are superior to oleoresinous caulks for outdoor exposure, and they are impermeable to gases and vapors. They do not harden or oxidize. They can be used in joints with movement up to 10-15%. Two component, chemically curing butyl systems are crosslinked with p-quinine dioxime activated with a peroxide. They have lower viscosity and are used in applications such as insulated glass and electrical components. Curing butyl sealants have generally poor adhesion and cohesion, but they can perform with +/-10% movement.

Butyl rubber sealants can be formulated as well as rope caulks and extruded tapes. These preformed butyl sealants are 100% solids so that there is no shrinkage on cure. Noncuring butyl sealants with no solvent have long life if used between building panels and in sound deadening applications. They do not harden or oxidize and are not affected by weather.

Preformed butyl sealants often contain significant quantities of polyisobutylene. Polyisobutylene as a homopolymer is permanently tacky, and it has the characteristic of being self-healing. Polyisobutylene is a glass-clear polymer with elastomeric properties although showing much greater cold flow than most unvulcanized rubbers. Most of its use is as an additive for other sealants or tapes, primarily of the butyl type.

Preformed butyl tapes are used extensively in the glazing industry between glass and aluminum. These tapes have 20-30% compression and a 10-year service life. A typical dry rope caulk formulation is shown in Table 4.

Table 4: Formulation for a Dry Rope Caulk Sealant⁵

Another significant application for butyl tape is corrosion protection. The backing is generally polyethylene or flexible polyvinyl chloride filled with carbon black. In these applications the adhesive is applied in a thick mass generally greater than 0.010" thick. Table 5 describes a formulation for a polyethylene backed, anticorrosion tape. Free standing butyl tapes (no backing or carrier) are also used as sealants or glazing in the building and construction industry.

Table 5: Formulation for an Anticorrosion Tape Manufactured with a Polyethylene Backing and Butyl Rubber Pressure Sensitive Adhesive⁶

Butyl tapes can also be applied as a hot melt at temperatures between 150-200°C with an appropriate heated applicator. The compound is heated and extruded into a joint opening to form a smooth joint that cools rapidly with excellent adhesion, even to oily and otherwise poorly prepared surfaces.

Usually high levels of thermoplastic resins such as tackifier resins are added to the butyl to provide good melt flow properties. Plasticizers are often added as well to reduce melt viscosity. Even with these modifications, the melt viscosity of hot melt butyl sealants is generally high and temperatures of 175°C are commonly used.

Butyl thermoplastic sealants can be supplied either in bulk (pellets or block) form or as rope. The bulk material requires typical hot melt application equipment. The rope can be heated in hand-held guns or extrusion equipment and applied continuously to joints. In this way it provides easy and efficient field application.

Butyl polymer is covered under a broad range of Food and Drug Administration regulations (FDA) for food contact purposes. Because of the FDA approvals, their relative purity, and their good resistance to fatty foods, gun-grade butyl sealants are used to seal can ends and are used in certain cap and closure sealant systems in the food packaging industry.

Butyl sealants are used in the automotive industry mainly because of their good chemical resistance and their excellent damping characteristics. Butyl tapes had been the primary means to seal windshields until the advent of polyurethane sealants. However, they are still widely used for replacement parts in body shops. Hot melt butyl sealants are also used in the auto industry because they can flow and seal in paint-baking ovens.

Butyl sealing tapes are also used in appliance manufacture of refrigerators, freezers, washers, etc. Damping, vibration absorption, and resistance to moisture are valued attributes of butyl sealants in these applications.