Extrudable adhesives
In its broadest definition, extrudable adhesives are polymeric resins that can be processed by standard extrusion processes and are useful for bonding together various substrates. In practice, extrudable adhesives are commonly polyolefin materials useful in processes such as blown and cast film, blow molding, and extrusion coating.OVERVIEW
The variety of polymeric materials available to both the industrial user and the consumer and are useful as adhesives include several materials almost equal to the number of polymer types themselves (see also Adhesives). Adhesives are developed from such polymers as polyvinyl acetate, polyvinyl alcohol, polyamides, polyesters, and many others. Adhesives can be applied as solvent solutions, aqueous dispersions, pastes, spray coatings, tapes, and thermally activated films. One special subset of adhesives, called extrudable adhesives, are different in that, in their application, they are applied in an extrusion process, in which they are melted, conveyed, and inserted between the substrates that are to be bonded together. Although there are many adhesive application methods that require the melting of a polymeric adhesive, extrudable adhesives are distinguished from other adhesive types used in processes such as powder coating, flame spraying, and the thermal lamination of adhesive films and webs. As a second distinction, extrudable adhesives are also distinguished from hot-melt adhesives that generally require a resin viscosity unsuitable for traditional extrusion processes. Thus, extrudable adhesives are those materials specifically designed to function in processes such as coextrusion blown and cast film, monolayer and coextrusion coating and lamination, coextrusion cast sheet, coextrusion blow molding, and coextrusion tubing.
TYPES OF EXTRUDABLE ADHESIVE
Extrudable adhesives are most often polyolefin-based compositions. The most common use of these polyolefin extrudable adhesives is as a specific layer in a multilayer coextrusion. Coextrusion is a technique that allows the creation of a plastic composite, in a single operation, which combines the benefits of a number of different materials. The plastic composite may be, for example, a packaging film that combines the properties of an oxygen-barrier resin with a heat-sealable layer on one side and an abuseresistant layer on the other side. The purpose of the extrudable adhesive is to bond the diverse plastic materials in the construction that would not, under ordinary circumstances, bond with each other. The polyolefin adhesive is designed to bond with similar polyolefins by a diffusion process. That is, during the extrusion process, the molten extrudable adhesive resin comes in contact with the molten polyolefin. Their molecules diffuse together creating a strong bond between the materials (1). The polyolefin extrudable adhesive is designed to bond with other polymeric materials, such as oxygen-barrier resins like polyamides and ethylene vinyl alcohol, through a chemical reaction between a functional group on the adhesive and a functional group on the barrier resin. Often, the functional group on the extrudable adhesive can be chosen for bonding to specific materials of interest.
Many types of extrudable adhesive exist. Polyethylene can be considered to be an extrudable adhesive. Commonly, polyethylene is used in extrusion lamination of paper to aluminum foil. The polyethylene is extruded at very high temperatures. The melt is oxidized by contact with air creating polar functionality on the surface of the melt. This process provides chemical bonding to the aluminum oxide on the surface of the foil (2). The low viscosity of the melt and its polar nature allows for good wetting on the paper and for encapsulation of the individual fibers. Co-polymers of ethylene and vinyl acetate also are useful extrudable adhesives and capable of bonding polyethylene to polyvinyl chloride. However, the most sophisticated extrudable adhesives are polyolefins with either acid or anhydride functionality. Acids and anhydrides are particularly reactive and can create strong bonds to several different materials in extrusion processes. Examples of acid-modified polyolefins are the copolymers of ethylene with acrylic acid or methacrylic acid. Variations include the partially neutralized acid co-polymers with metal ions referred to as ionomers or terpolymers of ethylene, an acid and an acrylate such as methyl acrylate or isobutyl acrylate. Acid-containing extrudable adhesives are widely used to bond with aluminum foil (3, 4). Examples of anhydride modified polyolefins include terpolymers of ethylene, maleic anhydride, and acrylates such as ethyl acrylate or butyl acrylate and the anhydride grafted polyolefins.
The anhydride grafted polyolefins are created by combining the polyolefin with an anhydride, most commonly maleic anhydride. The anhydride is added to the polyolefin with a free-radical initiator in a solvent or the melt. This addition allows for the attachment of the highly reactive anhydride to polyolefins, such as high-density polyethylene, linear low-density polyethylene, polypropylene, ethylene vinyl acetate copolymers, or ethylene propylene rubbers. Although the polar functionality of the anhydride and the polyolefin backbone are the necessary ingredients for the extrudable adhesive to function, almost all commercially available extrudable adhesives are formulated with other polymers. Formulations of anhydride modified extrudable adhesives will generally contain two or three basic components. Two-component extrudable adhesives will contain the anhydride graft blended into a second, or matrix polyolefin (5). The purpose of the second polyolefin is to lower the overall cost of the adhesive and to control the viscosity, modulus, tensile, thermal, and other properties of the adhesive. The three-component formulation will contain the anhydride graft, the matrix polyolefin, and a modifier (6). The purpose of the modifier is to enhance the peel strength characteristics of the bonded composite. In most cases, the efficiency of the extrudable adhesive is judged by peeling the bonded composite apart in a ‘‘T’’ peel mode. The modifier affects the peel strength characteristics of the adhesive by dissipating the force at the interface where the composite is being peeled apart. The result of this process is the necessity for more work to peel apart the bonded composite (7). These extrudable adhesives also can have other additives, such as antioxidants, slip agents, or resin tackifiers.
Although the formulation of an extrudable adhesive is very important to its utility, many other factors also affect how well an extrudable adhesive will bond different materials together. How the extrudable adhesive is processed, its thickness, and whether the bonded composite is oriented, shaped, or exposed to aggressive environmental conditions can affect the performance of the adhesive (8, 9).
COMMERCIAL OFFERINGS
Many producers of extrudable adhesives around the world exist. Some of these producers will produce only a few types of extrudable adhesive. Others have a broader product line. Some manufacturers market their adhesives internationally; others only market regionally, to specific market areas, or for use in specific applications. Manufacturers of acid copolymers include Dow Chemical Company as Primacor and E. I. DuPont de Nemours as Nucrel. DuPont also manufactures acid terpolymers. Ethylene terpolymers of anhydride with acrylate are produced by Atochem under the name Lotader. DuPont manufactures extrudable adhesives based on anhydride graft technology under the name Bynel. Quantum Chemical Company also makes and sells extrudable adhesives under the name Plexar. Other manufacturers include Mitsui Petrochemical Company with Admer, Morton International with Tymor, Atochem with Orevac, and DSM with Yparex.
EXTRUDABLE ADHESIVE APPLICATIONS
Extrudable adhesives are used primarily in coextrusion processes (see Coextrusion). The major market area is food packaging. Examples include the coextrusion coating of an oxygen-barrier material, extrudable adhesive, and polyolefin onto paperboard to create high-barrier, nonscalping fruit juice cartons. Oxygen barriers, extrudable adhesives, and ionomers are coextruded by either cast-film or blownfilm processes to produce packaging films for hot dogs, bacon, and other processed meats. A third example is the coextrusion blow molding of polyolefin, extrudable adhesive, and oxygen barrier to produce high-barrier ketchup bottles. Extrudable adhesives also are used in applications that do not involve the packaging of foods. Extrudable adhesives are used to bond high-density polyethylene to ethylene vinyl alcohol in a coextrusion blow-molding process to produce automotive gas tanks (see Blow molding). Extrudable adhesives also are used to bond crosslinked polyethylene to ethylene vinyl alcohol through a coextrusion crosshead tubing process to make radiant hotwater heating pipes.
The selection of the proper extrudable adhesive for any particular application may be a complex problem. The first consideration is always the materials that need to be bonded together. The adhesive must be able to bond with these materials. When bonding a polyolefin with an oxygen barrier, such as ethylene vinyl alcohol or polyamide, extrudable adhesives with anhydride functionality are usually the adhesive of choice. The type of anhydride modified polyolefin will depend on the polyolefin being co-extruded with the barrier. For example, if polyamide is being co-extruded with polyethylene, an anhydride-modified polyethylene or anhydride modified ethylene vinyl acetate will be the resin best able to perform in the application. If polypropylene is coextruded with polyamide, then an anhydride modified polypropylene will be chosen. Second, the adhesive must be processable in the equipment that the converter intends to use. This may mean choosing a resin with a relatively lower viscosity for coating applications and a relatively higher viscosity for blow-molding applications. If the bonded composite is going to see a specific environment, such as oil or grease, then it should be resistant to that product. If the composite will be exposed to either very high or very low temperatures, then the adhesive must be functional at those temperatures. Finally, some extrudable adhesive resins may have to have a specific regulatory compliance depending on the application for the bonded composite. Most manufacturers of these extrudable adhesives are prepared to help the converter select the best adhesive for their application.