Contact adhesives: Improving performance through formulation

The mechanism by which contact adhesives adhere is based on intermolecular diffusion. Both surfaces are coated, and the carrier is allowed to evaporate until the adhesive film becomes dry (non-tacky) to the touch. When the coated substrates are brought together, autohesion occurs. Polymer chains diffuse across the interface, resulting in very high initial bond strength, which later develops into full cohesive strength as the interface disappears.

For autohesion to take place, several criteria must be met. The polymers must be capable of diffusion, which depends on the molecular free volume within the elastomer film. This free volume is influenced by:

• The nature of the polymer
• The choice of solvent

As a result, only specific polymer–solvent combinations exhibit autohesive characteristics suitable for contact adhesives.

For optimal diffusion, substrates must be in intimate contact. This is influenced by the adhesive’s rheology and mechanical behavior, including:

• Elastic stress
• Plastic flow

In addition, process conditions such as pressure, time, and temperature play an important role in achieving strong bonding.

The drying time prior to bonding is typically sufficient to remove about 90% of the solvent or water carrier. Under standard conditions, solvent-based systems require around 30 minutes, while water-based systems may need longer. This time can be significantly reduced through heat-assisted drying.

After drying, there is a defined period known as the open time, during which effective bonding can occur. During this phase, the adhesive film continues to evolve due to:

• Ongoing solvent loss
• Phase changes
• Polymer crystallization

If the substrates are joined within this open time, bond strength develops progressively. However, once this period is exceeded, diffusion and autohesion no longer occur, leading to poor adhesive performance.

At the substrate interface, standard adhesion principles apply. The adhesive must first wet the surface and have sufficient flow to fill microscopic irregularities. It must then develop adequate cohesive strength to withstand stresses during service.

Understanding this adhesion mechanism helps in evaluating the key performance characteristics of contact adhesives.

Natural rubber was the first polymer used in contact adhesive formulations, but it has been almost entirely displaced by synthetic polymers. Today, the main polymer used in contact adhesive formulation is polychloroprene. Polyurethanes, acrylic copolymers, and styrene-butadiene copolymers are also used in certain formulations. ¹

Contact adhesives based on these polymers are available in either solvent solution or water dispersion. Although the solvent based adhesives generally have greater strength and durability, environmental and legislative pressures have encouraged the development of water based systems. Water borne contact adhesives often have problems with slow evaporation rate, less tack, and microbe buildup in the wet adhesive. Often a small amount of solvent, called a coalescing aide, which is soluble in both the elastomer and water is used to aid in wetting as well as to improve the coalescence of the latex particles.

Contact adhesives can be further classified into curing and non-curing types. The curing types have a crosslinked molecular structure and provide greater resistance to heat and chemicals. However, they are generally supplied as two component systems with a limited pot life.

In addition to the base polymer and carrier, contact adhesive formulations may include:

• Tackifiers

• Stabilizers

• Pigments and fillers

• Crosslinking agents ² 

The following sections describe the formulation possibilities that are achievable with these materials.

Resins or tackifiers that are generally used for PSA formulation are also useful for contact adhesives. The concentrations of tackifiers, however, are usually greater in contact adhesives (20-60 parts per hundred of polymer). Tackifiers provide the opportunity for increasing the glass transition temperature, T_g, and the adhesion characteristics. A key function of the tackifier is to maximize interfacial bonding. Through its plasticizing effect, the resin facilitates polymer diffusion when the substrate surfaces are brought together. The tackifying resin also extends the open time of the adhesive.

The formulation of latex based contact adhesives requires tackifiers that are also latex based. Since tackification depends upon the solution of the tackifier in the elastomer, materials used to formulate a latex based contact adhesive are required to quickly fuse and dissolve as the latex coagulates. Since this is often difficult to achieve, sometimes the tackifier is added directly to the elastomer and the combination is emulsified.

Pigments or fillers are often added to contact adhesives primarily to reduce the cost of the final product. Clay and talc are often used in this respect. Another function of fillers is to provide thixotropy or sag resistance. Fumed silica is generally used for this purpose. Metal oxides have also been used to improve the performance of the adhesive and its resistance to various environments (see below Polychloroprene - Solvent Based).

By chemistry

Polychloroprene

By far, the majority of contact adhesives are formulated with polychloroprene (Neoprene) elastomers. Rapid strength development coupled with high ultimate strength is typical of polychloroprene contact adhesives. They also have autohesion characteristics, high shear strength, and resistance to oils and chemicals.

Polyurethane

Urethane polymers for solution contact adhesive are generally prepared from bulk polymerization of difunctional hydroxyl compounds (e.g., polyester diol) with difunctional isocyanates (e.g., 4,4'-diphenylmethane diisocyanate). These polymers are usually supplied in bead or granular form and can be dissolved in common ketonic solvents. It is also possible to polymerize a polyurethane polymer in solvent. This not only eliminates a step in the manufacture of the adhesive but also provides a polymer that once the solvent is removed cannot be redissolved. ¹

Solubility of polyurethane is determined primarily by the degree of crystallinity in the polyol segments of the molecule. The formulator has significant versatility through the choice of reactants, rate and degree of crystallization, and the degree of branching. Branching rapidly affects solubility (increases) and viscosity (reduces) of the polymer. A small amount of branching can also significantly improve the cohesive strength of the polymer.

Like polychloroprene, polyurethane resins are available with a wide range of crystallization, solution viscosity, and cohesive properties. Glass transition temperatures are -30°C to -40°C, and melting points range from 30°C to 80°C. Higher crystallization rates produce contact adhesives with longer open times but poorer peel strength and heat resistance.

Polyurethane contact adhesives can be applied similar to polychloroprene contact adhesives. A typical formulation for a solvent borne polyurethane contact adhesive is provided in Table 6. Polyurethane adhesives can also be applied as a heat sealable contact adhesive. The adhesive is exposed to temperatures (70°-90°C) above the crystalline melting point before joining pressure is applied. While the adhesive is in the amorphous state, the substrates are mated and the adhesive on each substrate diffuses into the other. Strength develops as the adhesive recrystallizes. This type of adhesive and processing are commonly used in the footwear industry.

Table 6: Thermoplastic polyurethane contact adhesive [Dow Chemical]

Similar to polychloroprene adhesives, polyurethane adhesives can be crosslinked with the addition of isocyanates. The isocyanates react with residual hydroxyl groups on the polyurethane, and they yield two-component adhesive systems with a limited pot life in solution. The isocyanates are usually added at a level of 5-10% based on the total polyurethane adhesive mass. Polyisocyanate cured polyurethanes will have greater heat and chemical resistance than simple solvent based thermoplastic polyurethanes.

Polyurethane adhesives are often chosen over polychloroprene adhesives when greater resistance to plasticizer migration is required or difficult to bond polymeric substrates are involved. One of the largest applications is in footwear where high bond strengths to PVC compositions are required. Polyurethane adhesives also give the formulator more freedom in "engineering" the modulus and elongation required of the adhesive film to meet specific applications.

Acrylic copolymers

Contact adhesive have been developed based on acrylic emulsions. These emulsions provide high bond strength to various surfaces, good environmental resistance, high strength, and relatively fast setting times without having to use a solvent system with resulting environmental and safety restrictions.

Acrylic emulsions are prepared by conventional emulsion polymerization processes. A blend of several monomers is used to achieve the desired copolymer characteristics such as glass transition temperature, flexibility, ultimate strength, and autohesion. These monomers are usually based on acrylate ester mixtures, most commonly the methyl, ethyl, butyl, and 2-ethylhexyl derivatives. Adjusting the ratios of the monomers allows tailoring of the polymer for a specific end-use.

Acrylic lattices used in contact adhesive formulations generally have a high solids content and are capable of self-crosslinking. They are often the same acrylic emulsions that are used in pressure sensitive adhesives or sealant formulations. Often the acrylic latex products can be used without formulation as a contact adhesive. However, heat reactive phenolic resins are sometimes added to enhance the green strength and improve the elevated temperature performance.

Table 7 provides a formulation of a typical water based contact adhesive prepared from acrylic emulsion. For suitable autohesion, the acrylic is compounded with heat reactive phenolic resin emulsions. However, there are other types of acrylic emulsions (e.g., Rhoplex CA12, Rohm and Haas) that do not require an additive for autohesion.

Specimens are canvas-to-canvas. Two coats of adhesive applied with 30 mins of dry time between coats. Bonds were assembled with a 10 pound roller.

Certain commercial water based contact adhesives are based on polychloroprene latex and acrylic / vinyl acrylic type emulsions. These blends produce desirable synergistic effects. The acrylate dispersion provides initial tack and also a destabilizing effect that accelerates coagulation.

Styrene Butadiene Rubber (SBR)

SBR elastomer is a random copolymer of styrene and butadiene formed through emulsion polymerization. As with polychloroprene, SBR can be coagulated and redissolved in a solvent or used as a latex. Since it is a random copolymer, SBR does not crystallize. The application and end-use properties are very much dependent on the ratio of the styrene to butadiene along the molecular chain. Strength is achieved by strong intermolecular forces and entanglements.

To improve the heat resistance, solution polymerized styrene butadiene polymers have terminal styrene blocks with a bloc of butadiene as the mid-chain section (SBS). These polymers have two distinct phases with two glass transition temperatures. However, SBS polymers are susceptible to oxidative degradation, especially when stressed. An antioxidant, such as a hindered phenol, is necessary for bond durability.

Styrene butadiene polymer systems have relatively poor autohesive characteristics and require compounding with a tackifying resin to provide a contact adhesive. Because of the two-phase nature of these polymers, resins can be selected which preferentially interact with either the styrene or the butadiene blocks.

Because of their good solubility in many different solvents, SBR contact adhesives have been used in applications where the substrate may be sensitive to a certain type of solvent, such as the bonding of polystyrene foam. In this application SBR contact adhesives are used where the solvents are either short chain linear hydrocarbons or alcohols.

By system

Solvent-based

A typical solvent borne polychloroprene contact adhesive formulation is shown in Table 1. There are many types of polychloroprene polymers available, and the polymer type will influence the contact adhesive properties. Table 2 summarizes the common types of polychloroprene elastomers that are available from one supplier.

The most important properties of polychloroprene polymers are molar mass and rate of crystallization. Increasing the molar mass of the polymer will increase its solution viscosity, adhesive strength, and heat resistance. Increasing the rate of crystallization improves the rate of development of bond strength; however, crystallinity will also inhibit the diffusion process and reduce autohesion. The greater the degree of crystallization, the greater will be the final cohesive strength of the adhesive film.

Thus, selection of the grade of polychloroprene will be based on a compromise between strength, strength development, and open time characteristics. The adhesive properties directly affected by the type of polychloroprene used are:

Table 1: Typical starting formulation for a polychloroprene contact adhesive ³

Table 2: Common types of polychloroprene elastomers [DuPont-Dow Elastomers]

Note that Neoprene AC and AD are the most common type used in contact adhesives. Most polychloroprene suppliers have equivalent grades: Bayer (Baypren), BP-Distugil (Butachlor), Denki-Kagaku (Denka) and Toyo-Soda (Skyprene). 
 

Chloroprene is a homopolymer of 2-chloro-1,3-butadiene usually prepared by emulsion polymerization. The polychloroprene forms most often used in solvent based adhesives are highly linear and soluble. Once polymerized, the polychloroprene is coagulated usually by the addition of salt. Lower molecular weight polymers can be dissolved by simply stirring in a solvent; however, higher molecular weight materials usually require milling before easy dissolution is possible.

Polychloroprene elastomers prepared for solvent based adhesive formulation can be dissolved in a wide variety of polar and non-polar solvents. This provides a range of solution viscosities and drying times as well as specific adhesion to many porous and nonporous substrates. The choice of solvents used in the manufacture of contact adhesive depends not only on the solvent's dissolving power, but also on its evaporation rate. Solvent selection can also influence the rate of crystallization.

Generally, the more efficient the solvent and the lower the volatility, the longer the open time for the adhesive. The selection of solvents suitable for dissolving polychloroprene elastomers is well documented by the polychloroprene suppliers. Table 3 shows the viscosity of common solvents and the solution viscosity of 10% by volume of Neoprene AD in the solvent. Often a blend of solvents is employed for optimal properties. Non-solvents are also sometimes added to solvents without impairing the dissolving action of the resulting mixture. These generally reduce the open time and influence the absorption and permeability of the adhesive into the substrate.

Table 3: Solvents commonly used for polychloroprene: Solvent viscosity and solution viscosity (10% Neoprene AD by Volume) at 24°C

Tackifiers particularly useful in polychloroprene contact adhesive formulations are t-butyl phenolic and terpene phenolic resins. Modified phenolic resins, rosin esters, and hydrocarbon and coumarone-indene resins are also employed. Figure 1 shows the effect of t-butyl phenolic additions on open time and peel strength.

Figure 1: Resin level vs. bonding range and heat resistance. ³

Reactivity of t-butyl phenolic resins can occur with metal oxides such as magnesium oxide. This is a common method to improve heat resistance of these contact adhesives. Table 4 shows the effect of various metal oxides on heat resistance. The metal oxides also stabilize polychloroprene against dehydrochlorination. (Polychloroprene degrades to liberate HCl which can attack the adhesive and metal substrates.) The metal oxides act as acid acceptors.

Table 4: Effect of Metal Oxide on Heat Resistance ³

The resins incorporated in the contact adhesive formulations may influence the aging properties of the bonds produced. Resins sensitive to oxidation, such as rosin esters and coumarone resins, can cause embrittlement or softening in adhesive films. These resins must, therefore, be used in combination with suitable antioxidants.

The thermal stability of polychloroprene contact adhesives can also be improved by adding a polyisocyanate crosslinker to the adhesive (e.g., 1-2% diphenylmethane diisocyanate). ³ The crosslinker also improves the cohesive strength and adhesion of the polychloroprene to certain substrates. The addition of isocyanate also greatly improves the solvent resistance and, in fact, makes the adhesive insoluble in most common solvents. The polyisocyanate is added to the adhesive solution immediately before use. The pot life (generally several hours) will depend on the specific adhesive formulation.

Water-dispersed

Polychloroprene lattices have not widely replaced solvent based contact cements as originally thought. It takes a water borne adhesive much longer to dry, and if force dried the energy used is an added cost. The autohesion properties of water based polychloroprene adhesives provide lower initial strength and make bonding at low pressures difficult. Also the heat and moisture resistance of these adhesive is generally lower than their solvent based counterparts because of the presence of surface active ingredients. However, significant development work has occurred over the last several years to minimize these problems. ^4-6

Latex based chloroprenes have varying degrees of gel structure and are marginally soluble in solvents. They are often stabilized with rosin acid based emulsifiers, which are anionic stabilizers. If the pH of the polychloroprene latex formulation is 10.5 or less, the latex will become destabilized as the protective surfactant layer becomes protonated and ineffective. Thus, a formulation with a relatively low pH may have mechanical stability problems, and the choice of resins and additives must be made carefully.

Polychloroprene latex contact adhesive can be prepared by blending a suitable latex with a tackifying resin emulsion. Only rosin ester resin emulsions are commercially available. Terpene phenolic and alkyl phenolic resins emulsions are limited because of poor stability. As with solvent adhesives, polychloroprene lattices need to be stabilized against dehydrochlorination. Dispersions of magnesium oxide will destabilize the anionic lattices; so only acid acceptors, such as zinc oxide and epoxide resins, can be used to protect the dry film.

Some polyurethane emulsions require an upward adjustment in viscosity for formulation. This can be achieved by adding up to 1% of water soluble cellulosic or acrylic material. Inorganic thickeners are also effective, and for low pH formulations, fumed silica and be used.

A starting formulation for two polychloroprene emulsion contact adhesives and their resulting adhesive properties are given in Table 5. The data also shows the effect of different resins on peel adhesion at room and elevated temperatures.

Table 5: Effect of two different resins on adhesive performance of a water-borne polychloroprene contact adhesive ⁴

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Next, let's look at the key markets and applications for contact adhesives.

Contact adhesives are generally applied by spraying, curtain coating, roller coating, or brushing. Among these, spraying is the most widely used commercial method as it enables rapid coverage and reduces drying time by atomizing the adhesive during application.

Roller and curtain coating are typically used for flat substrates such as panel cores and skins. These methods are generally paired with adhesives containing relatively slow-drying solvents to prevent premature drying or excessive viscosity build-up on application equipment.

Solvent removal occurs either through prolonged air drying or forced drying using heat. In some cases, highly efficient systems known as dry spraying adhesives are used, where most of the solvent is lost during application, leaving the film nearly ready for bonding immediately. These systems are particularly useful for:

• Very fast assembly processes
• Heat- or solvent-sensitive substrates (e.g., polystyrene foam)

An alternative approach is hot spraying, where the adhesive is heated during transfer through the fluid line. Upon spraying, the elevated temperature combined with high air exposure accelerates solvent evaporation, producing a near-dry adhesive film.