Polyisobutylene as a Base Polymer and Modifier for Adhesives and Sealants

Polyisobutylene (PIB) is a glass-clear, hydrocarbon elastomer. It is a linear polymer (Figure 1) of isobutylene of varying average molecular weights. Commercially there are three grades of polyisobutylene resins as shown in Table 1. This paraffinic hydrocarbon varies from a soft, tacky, viscous liquid to a tough elastomeric solid depending on its molecular weight.

Figure 1: Polyisobutylene

Table 1: Commercial Grades of Polyisobutylene Resin

The lower molecular weight grades will flow at elevated temperature and can be handled as a liquid at temperatures in the range of 150-175°C. However, molten PIB will oxidize rapidly when exposed to air at their melt temperatures. These low molecular weight grades are used predominantly as permanent tackifiers in cements, pressure sensitive adhesive, hot melt adhesives and sealants. PIB adhesives have much greater cold flow than most other elastomers.

Higher molecular weight grades are rubber-like solids that have good physical properties and they are more resistant to flow than lower molecular weight grades. PIB adhesives have much greater cold flow than most other elastomers. They are generally used as both base polymers and additives in pressure sensitive adhesives, such as tapes and labels, as well as in sealants.

Low molecular weight PIB resins are very tacky and the tack is permanent. As a result they are often used in pressure sensitive adhesive formulations. The tack decreases substantially with increasing molecular weight.

PIB is an amorphous polymer that relies mainly on molecular chain entanglement for cohesive strength. As a result, PIB resins have good tack and flexibility but a relative low degree of cohesive strength. These characteristics are used to advantage in formulating pressure sensitive adhesives that need to perform at low temperature or that need to be removed from the substrate after use.

The straight chain hydrocarbon structure of polyisobutylene also contributes excellent moisture and barrier properties. PIB resins are resistant to water and have low permeability to many gases. They also have low color and odor. As a result of these characteristics, many grades of PIB have FDA clearance for food contact application.

PIB based adhesives are susceptible to atmospheric oxidation and attack by ozone although they are better than natural rubber in this respect. They have moderately good environmental resistance and elasticity. However, polyisobutylene adhesives have poor thermal resistance, and they are attacked by many solvents.

Depending on the molecular weight, the mechanical properties of PIB resins remain unchanged up to about 100°C. It gradually becomes plastic at higher temperatures and begins to flow at 180-200°C. The polymer starts to degrade rapidly at this temperature in the presence of air.

Because of its low glass transition temperature PIB resins are elastic at temperatures down to -50°C and gradually become harder at lower temperatures. Because of this, PIB adhesives and sealants are particularly useful where low temperature flexibility and impact resistance is a requirement.

PIB resins have a relatively low surface energy and, as a result, they can have a high degree of specific adhesion to various hard-to-bond substrates such as polyolefin. They are particularly useful for improving adhesion to polyolefin plastic surfaces. Because of their low polarity, PIB has very good dielectric properties and electrical stability. As a result, they are often found in electrical and electronic applications (e.g., electrical insulating tape, cable sealant).

Due to its chemical inertness, PIB resins generally cannot be cured or vulcanized using standard rubber technology (sulfur or peroxide cure). PIB is very compatible with butyl rubbers, and although the PIB will not cure, the butyl component can be cured. Butyl rubber is simply polyisobutylene with a very low degree of crosslinking. Thus, butyl additions can be used to bring about improved cohesive strength and creep resistance.

Polyisobutylene adhesives are generally available as solvent solutions or as a pressure sensitive film on a carrier. PSA compositions generally consist of a blend of high and low molecular weight polymers, where the low molecular weight component is used as a plasticizer.

Polyisobutylene adhesives are commonly formulated using a variety of plasticizers and modifiers. Typical modifiers include petroleum oils, waxes, hydrocarbon resins, rosin esters, phenolics and so forth. Modifiers are selected on the basis of desired end use properties (i.e., specific adhesion, melt or solution viscosity, and cost).

Tackifying resins are often added to PIB adhesives to adjust the balance between tack and cohesive strength. The most commonly used resins are hydrocarbon resins, alkylated phenols, terpenes, terpene-phenolics, ester gums, and hydrogenated esters. Preferred tackifiers are C5 hydrocarbons and polyterpenes.

Plasticizers are sometimes used to increase flexibility, although generally blends of lower molecular weight PIB resins will accomplish the same goal. Common plasticizers are paraffinic petroleum oils, polybutenes, coal-tar distillates, chlorinated hydrocarbons, and esters.

PIB resins have a high degree of affinity for all types of fillers. There is no difficulty in preparing films consisting of one part PIB and ten parts of inorganic filler such as clay, chalk, or talc. Fillers are often used to reduce formulations cost. They also reduce tack and blocking characteristics of the resin.

Many polymeric additives can be used in PIB adhesive and sealant formulations. The most notable of these are butyl rubber, amorphous polypropylene, and polymeric waxes. Butyl rubber can be used to provide enhanced cohesive strength and a means for crosslinking the adhesive formulation. Amorphous polypropylene additives can reduce cost and improve processing as well as impart a thermoplastic character to the final product. Waxes function similarly to amorphous polypropylene and can also be used to adjust the viscosity of the product.

Antioxidants are often used in PIB adhesive to protect against severe environmental aging caused by UV light and heat. Antioxidants are also used in hot melt formulations to protect the PIB polymer during melt processing.

PIB resins are soluble in several common hydrocarbon and chlorinated solvents such as benzene, toluene, xylene, cyclohexane, and methylene chloride. They are also soluble in paraffin and mineral oils. PIB resin is not soluble in ketones, alcohols, or methyl acetate. The presence of minor amounts of stabilizers in PIB may cause solvent cements to be slightly clouded, even though the polymer is completely dissolved.

In general, PIB adhesives for spraying contain 5-10% solids, for dipping 10-30%, for spreading 25-50%, and for applications by trowel or spatula 50-70%. The relationship between solids and viscosity is logarithmic so a small change in solids can make a large difference in solution viscosity, especially at higher levels. Viscosity is also dependent on polymer molecular weight, and a small change in molecular weight can result in a relatively large change in the viscosity of polymer solutions.

The choice of equipment for the production of solutions or mastics from PIB resins is based primarily on the viscosity of the final product. Resins and other soluble ingredients are also often incorporated at this point. The solution processing guidelines in Table 2 may be helpful.

Table 2: Guidelines for Dissolving PIB Resins in Solvent

Due to the low permeability of PIB polymers, adhesive coatings should be applied in thin coats to facilitate solvent release and avoid porosity. Adequate drying time must be allowed between coats when multiple applications are needed to increase bond thickness. Adhesive containing appreciable volumes of filler are less sensitive in this respect.

PIB resins are very functional base polymers for hot melt pressure sensitive adhesives and they impart many of the same characteristics that are noted for solvent-based systems. PIB resins are easy to melt mix and result in low viscosity blends due to their low molecular weight. PIB grade choice and concentrations are important considerations, however, in that these elastomeric products become more difficult to melt mix, significantly increasing in melt viscosity as the PIB molecular weight increases.

For hot melt adhesives, PIB resins typically have a viscosity of 30,000 to 60,000 cps at 177°C depending on grade. Melt viscosity can readily be tailored to meet specific requirements by incorporation of thermoplastic ingredients including resins and waxes. Petrolatum and waxes are most effective as viscosity reducers. Resins, waxes, and thermoplastics such as low-density polyethylene, ethylene vinyl acetate, and crystalline and amorphous polypropylene increase hardness and cohesive strength. LDPE and crystalline polypropylene particularly raise the blend's softening point.

Waxes and thermoplastics must be used in moderation, however, since they reduce the tack of the finished blend. However, hydrocarbon oils and petrolatum plasticize and bring out aggressive tack in most PIB resin mixtures.

Since PIB is an amorphous polymer that relies mainly on molecular chain entanglement for cohesive strength, adhesives are characterized by their high degree of flexibility and tack and by their low cohesive strength. PIB adhesives and sealants have property profiles that are unique with respect to other products (Table 3). As a result they find a high degree of utilization in many products.

Table 3: PIB Property Profile in Adhesives and Sealants

PIB resins are used in adhesive formulations generally because of their resistance to aging and to attack by chemicals. They are used in pressure sensitive adhesives for technical and medical applications, such as surgical tapes, sealing tapes, insulating tapes, masking tapes, and anticorrosion tapes.

Many pressure sensitive tape products are formulated with polyisobutylene resins as the base polymer. These products remain permanently tacky. Table 4 provides formulations for a pressure sensitive label adhesive.

Table 4: Formulation for a Polyisobutylene Based Pressure Sensitive Label Adhesive

PIB pressure sensitive adhesives can either be applied via solvent solution or hot melt application. Several formulations illustrating PIB used in hot melt pressure sensitive adhesive formulations are provided in Table 5. The low molecular weight PIB functions as an age resistant tackifier, adhesion promoter, and flexibilizer.

Table 5: Polyisobutylene Hot Melt Pressure Sensitive Adhesive Formulation

The lack of age hardening characteristics has made PIB adhesives one of the first choices for removable pressure sensitive adhesive products. These are often used for bonding credit cards, product handouts, etc. to mailing brochures. A typical formulation for a removable label PSA is shown in Table 6.

Table 6: PIB Based Pressure Sensitive Adhesive for Removable Label Stock

Polyisobutylene is permanently tacky and as such has a self-bonding characteristic. It can therefore be used in many cold-sealing applications such as envelopes, food packaging, etc. A simple paper cement can be achieved by using unmodified medium molecular weight polyisobutylene (10%) in hydrocarbon solvent. This cement is similar to that sold for home and office use.

PIB resins are also used in sealant formulations because of their good aging and moisture resistance. In sealants, PIB resins are most often used as a modifier (see below) especially in butyl type sealants but in certain applications, where a high degree of cohesive strength is not necessary, PIB is also used as the base polymer.

In sealant formulations the PIB resin is often extended with inorganic fillers such as chalk, zinc oxide, china clay or barites. Bitumen may also be included in the formulation to provide for specific physical or applications properties and / or cost reduction purposes. The ease of melt mixing PIB resins makes them particularly valuable for hot melt sealants.

Because of their good aging characteristics, moisture resistance, and low out-gassing tendency, polyisobutylene hot melt sealants are used in double glazed windows. Table 7 shows the characteristics of two formulations where hot melt PIB sealants are used in the building and construction industry for window applications.

Table 7: Properties of Hot Melt Sealant Formulations Based on PIB Resins

Many types of polyisobutylene are used as modifiers and tackifiers to other elastomers and polymers. They are used as a permanent flexibilizer and tackifier in pressure sensitive and hot melt adhesives. They are used in many adhesive, sealant, wax, and asphalt formulations due to their tackiness, flexibility and adhesive-cohesive properties. As modifiers they impart:

PIB additions are also used as adhesion promoters to adhesive and sealant formulations. PIB modified adhesives are often used in packaging applications where adhesion to polyethylene and other difficult to bond plastic films is desired. Table 8 describes a starting formulation for a modified EVA hot melt that has good bond strength to many packaging films.

Table 8: Hot Melt Packaging Adhesive

The cohesive strength of many solvent-based pressure sensitive adhesives can be improved by the incorporation of high molecular weight PIB. The modification also provides a degree of age resistance for long-term service. Table 9 provides several formulations for a PIB modified butyl PSA.

Table 9: Formulations for PIB Modified Butyl Pressure Sensitive Adhesives

PIBs in sealant formulation are often used to improve non-drying (non-hardening) properties. They can also be used to improve adhesion and elongation, reduce slump and sag, reduce seepage and staining, and provide UV and color stability.

In styrene butadiene rubber based sealants, PIBs provide better resistance to aging at elevated temperature, weathering, and flex cracking. In acrylic emulsion based sealants (Table 10), PIBs have been used as a plasticizing resin to increase the solids content and thereby reduce shrinkage.

Table 10: Aqueous Acrylic Sealant Formulation Modified with PIB

Properties of common paraffin and microcrystalline waxes are considerably improved by the addition of PIBs. These additions result in increased tensile strength, flexibility at low temperatures, scuff resistance, and heat sealing strength. They also improve the resistance to penetration by water vapor. The bonding strength of heat sealing wax coatings can be relatively low, but addition of PIB gives bonds of considerable strength.

PIBs can also be added to bitumen to provide lower brittle point, better adhesion, higher melt viscosity and decrease in oil exudation. Once set, the modified asphalt exhibits improvement in toughness, abrasion resistance, weathering, and low temperature properties.