Polyethylene foams prepared from low density polyethylene resins (“LDPE” resins) have been widely accepted for industrial uses. Typically, these foams have light weight and a high degree of uniform enclosed fine cells. LDPE foams can be produced with densities in the range of from about 1 to 30 pcf (16 to 480 kg/cubic meter). Polyethylene foams generally have low water vapor transmission properties and are resistant to mechanical and chemical deterioration. Polyethylene foams are particularly suitable for use in thermal insulation, flotation, cushioning, and packaging. LDPE resins exhibit good melt strength desirable for foaming by conventional methods.
LDPE is made by the so-called “high pressure” process by polymerization of ethylene in the presence a suitable catalyst. LDPE typically has a relatively low density of from about 0.91 g/cc to less than about 0.94 g/cc, typically about 0.92 g/cc. The good melt strength of LDPE is usually attributed to the long-chain and short-chain branches that are distributed along and extend from the polymer backbone. These branches make it more difficult for the individual molecules to slide over each other, which increases the resistance of the molten polymer to stretching during elongation. Increased resistance to stretching is sometimes referred to as “extensional viscosity” and is indicative of the melt strength of the resin and of the ability of a polymer to produce stable, high quality foams of low density. The cell walls formed by nucleation of bubbles during the foaming process offer sufficient resistance to expansion and do not become thin and collapse.
Efforts have been made to produce foams from so-called “linear” polyethylene resins. Generally speaking, linear resins have poor melt strength and are considered unsuitable for making lower density foams. Whereas LDPE is relatively highly branched with widely spaced chains and can be compared to dead tree branches piled together, linear resins are characterized by long, straight chains with less branching and so the molecules are more closely aligned in the manner of carefully folded rope.
Polyethylene resins become more difficult to foam as density, and linearity, increase. For example, unlike LDPE, high density polyethylene (“HDPE)” is produced in a low pressure process and has a relatively high density of from about 0.94 g/cc to 0.96 g/cc. HDPE molecules are among the most linear of the polyethylenes and have a small, controlled number of short-chain branches and normally have essentially no long chain branching. HDPE usually has a higher degree of crystallinity than LDPE and is physically a stiffer, stronger substance than LDPE. HDPE is typically about 70% crystalline at room temperature while LDPE may be as low as about 30 to 45% crystalline. HDPE has a higher flexural modulus and increased thermal stability as compared to LDPE as indicated in part by its higher melting point, and these properties would be useful in expanded cellular products. However, the individual molecules in an HDPE melt can slide over each other easily, and thus HDPE generally exhibits poor melt strength and low extensional viscosity. When HDPE polymers are used in foaming processes, these drawbacks frequently result in a large faction of open cells, foam collapse, and process instability. The cell walls of the HDPE foam normally do not have sufficient resistance to expansion and become thin and collapse.
Mixing branched and linear resins has been attempted in the production of extruded foams and other products, including films. Unlike films, which can be produced from such a mixture, foams require a uniform crystallization of the polymer molecules upon cooling of the expanded resin. The different crystallization characteristics of linear and branched polyethylene resins in physical admixture typically produce foams having large voids.
There are a large number of variables that impact whether a given resin is useful for foam production, including melt index, extensional viscosity, the presence of a cross-linking agent, and other parameters. Good quality foams have been made in which linear polyethylenes are a component under certain circumstances. For example, a cross-linking agent can be activated after extrusion to assist the foam is holding its extruded shape.
Many methods in the art employ cross-linked polyethylenes in the foaming process. Cross-linking enables the extruded foam to retain its shape. For example, HDPE resin can be extruded to the desired shape, cross-linked, and then expanded, normally by a chemical blowing agent that is activated after extrusion and cross-linking in a process called the “two stage process.” The resin is extruded prior to cross-linking because the shape of the product is fixed after cross-linking and cross-linking strengthens the resin to withstand expansion by a blowing agent. The two-stage process is in contrast to single stage extrusion foaming, in which a physical blowing agent, including, for example, a volatile organic compound, is mixed under pressure with a molten LDPE resin and then the mixture is extruded into a zone of lower pressure so that the blowing agent expands upon extrusion to produce the foam.
Cross-linking can lead to gelation of the ethylene polymers, which are undesirable localized concentrations of polymer more highly cross-linked than the surrounding areas, and can decrease the melt extensibility of the polymers. As a result, foams made with cross-linked HDPE generally have relatively high density, which is undesirable in many applications.
Methods have also been proposed for increasing long chain branching in the absence of cross-linking, typically by application of radiation. These methods can require steps that increase the complexity of processing the polymer. For example, U.S. Pat. No. 5,508,319 describes a process for improving strain hardening elongational viscosity in linear polyethylene polymers such as HDPE and LLDPE in the absence of cross-linking. The polyethylene is irradiated with high energy ionizing radiation at a radiation absorbed dose of 2.0 megarads or less in an environment having an oxygen content of less than 15% by volume. The irradiated polyethylene is maintained in the environment for a period of time and is then treated to deactivate the free radicals present in the irradiated material. The resulting ethylene polymer is said to have a substantial amount of long chain branches without cross-linking and to exhibit improved melt strength and elongational viscosity.
U.S. Pat. No. 4,598,128 describes a method for making a polyethylene composition having enhanced temperature sensitivity and high low-shear viscosity. The composition is a blend of a linear polyethylene and a long chain Y-branched polyethylene. The Y-branched polyethylene is prepared by irradiating a polymer comprising molecules having at least one vinyl end group per molecule under non-gelling conditions in the absence of oxygen. It is disclosed that the vinyl end group can be created by heating an ethylene polymer under non-gelling, non-oxidizing conditions. The irradiation process is purported not to cause cross-linking.
Mobil Oil Company has recently marketed a group of HDPE resins designated as the HFE-03X series that are said to have sufficient melt strength to produce stable foams. While not wishing to be bound by theory, the Mobil resin is believed to be a “reactor” resin that is a linear resin, but is produced with some degree of branching during the polymerization process that is favorable for producing a foam. The Mobil HFE-03X series resins are among the highest melt strength high density polyethylene resins available and have among the highest extensional viscosities available. However, stable lower density foams comparable in density to foams that can be made from LDPE, are not believed to have been achieved with these resins.
It would be desirable to produce stable, closed cell polyethylene foams of the lowest possible density from linear polyethylene resins in the absence of the drawbacks and disadvantages of complex processing steps and special environments and to increase the available options for producing high quality foams.