The present invention relates generally to polyolefin foams and, more particularly, to extruded foam sheets comprising a blend of low density polyethylene and a higher density, high melt index ethylene polymer.
Polyolefin foams, particularly those made from polyethylene, and methods of manufacturing polyolefin foam sheets are well known in the art. See, e.g., U.S. Pat. Nos. 5,462,974 (Lee), and 5,667,728 (Lee), the disclosures of which are incorporated herein by reference thereto. One of the most common polyolefins used to produce foam is polyethylene and, specifically, low density polyethylene (LDPE). While LDPE possesses a number of beneficial physical and chemical properties when used to produce a foamed sheet, a disadvantage of LDPE is that extruded foam sheets made therefrom have mechanical properties, such as tear resistance, tensile strength and creep resistance, that are lower than would otherwise be desired for certain applications.
U.S. Pat. No. 4,738,810 (Cheng-Shiang) teaches that the mechanical properties of an LDPE foam sheet can be improved by blending linear low density polyethylene (LLDPE) with the LDPE when the LLDPE has a melt index (MI) below 10 g/10 minutes, and preferably between 1 and 7 g/10 min.
The inventors hereof have found, however, that when an LLDPE, a higher density ethylene/alpha-olefin copolymer, or a higher density ethylene homopolymer with a MI of less than 10 g/10 min. is blended with LDPE, the shear forces exhibited by the blend increase sharply during extrusion, relative to extruding LDPE alone, resulting in the generation of excess heat. This heat generation was found to degrade the appearance of the resultant foam and lead to inferior mechanical properties due to a high percentage of open cells and non-uniform cell-size and cell-wall thickness. Also, the excess heat reduces the foaming efficiency, i.e., the foam has less void volume and therefore higher density for a given amount of blowing agent.
The foregoing problems are believed to occur because the heat build-up reduces the melt strength of the polymer melt and increases the volatility of the blowing agent, resulting in less void volume and more open cells as the blowing agent escapes from the polymer melt before it can cool sufficiently to form an adequate number of closed cells within which the blowing agent would otherwise be trapped. The increased heat is also believed to reduce the beneficial tendency of the LDPE to exhibit strain hardening during extrusion through a foaming die and during expansion outside of the die. As is known, strain hardening during extrusion and expansion promotes uniform cell-size distribution. One of the reasons that LDPE is commonly used in the production of foams is its strain-hardening capability and resultant promotion of cell uniformity. However, this capability decreases with increasing temperature, as caused, e.g., by the high shear forces exhibited by low MI ethylene/alpha-olefin copolymer (less than 10 g/10 min.) during extrusion foaming.
Accordingly, a need exists in the art for a polyolefin blend capable of producing an extruded foam sheet with improved mechanical properties but without the drawbacks of using low MI (less than 10 g/10 min.) ethylene/alpha-olefin copolymer or ethylene homopolymer.
That need is met by the present invention, which provides a foam, comprising a blend of a low density polyethylene and an ethylene polymer having a density ranging from greater than 0.94 to about 0.97 grams/cubic centimeter and a melt flow index of greater than 10 g/10 minutes, the ethylene polymer comprising at least one member selected from ethylene/alpha-olefin copolymer, ethylene homopolymer, and blends thereof
In another aspect of the present invention, a method is provided for making a foam, comprising:
a. blending a low density polyethylene and an ethylene polymer having a density ranging from greater than 0.94 to about 0.97 grams/cubic centimeter and a melt flow index of greater than 10 g/10 minutes, the ethylene polymer comprising at least one member selected from ethylene/alpha-olefin copolymer, ethylene homopolymer, and blends thereof;
b. adding a blowing agent to the blend; and
c. causing the blowing agent to expand within the blend, thereby forming a foam.