The packaging industry, the plastic industry as a whole and society is becoming more concerned with the environmental impact of raw material resources and product disposal. Polylactic acid (PLA) is biodegradable and advantageously can be derived from renewable raw material sources. PLA films are biodegradable under some natural conditions and under more controlled industrial composting conditions. Use of PLA has enabled industry to addressed environmental concerns so successfully that it is one of the largest commercially successful plastics based on a renewable material source.
One disadvantage of polylactic acid, particularly as a packaging film or of films made from similar polyhydroxy acid materials, is its relatively poorer resistance to moisture vapor transmission compared to oil based plastics. The relatively high moisture permeability leads to shorter shelf life of certain packaged goods. To improve the moisture barrier resistance of hydroxy acid polymer films and thus improve their packaging performance, these films are often coated with a very thin layer of a good moisture barrier material. The resulting coated film has dramatically improved moisture vapor transmission resistance when such coatings as inorganic barrier materials of aluminum or the oxides of aluminum or silicon are used.
Notwithstanding the great strides taken to improve moisture vapor transmission by adding the barrier material coating, further problems remain. It is desirable to further improve the moisture barrier properties of packaging films so that shelf life of the packaged product can be extended for greater convenience and economy of the consumer. Moreover, a need exists to provide the further improved barrier properties without significantly adding to the mass, cost or reduction in biodegradability or renewability of the packaging film. It is also desired to achieve the production of an improved moisture vapor transmission resistant biodegradable film with least disruptive modification of existing equipment and techniques currently in place throughout the packaging film industry.
U.S. patent application Ser. No. 11/464,331 describes multi-layer packaging film structures bio-based films, adhesive, and a barrier film layer wherein the bio-based film comprises polylactic acid and/or polyhydroxyalkanoate. The application mentions that PET can be coextruded with PLA but no details or enablement is given as to what type of PET should be used (i.e. crystalline or amorphous), formulations, or expected improvements, benefits, or properties of such a coextrusion.
U.S. patent application Ser. No. 12/332,153 describes high moisture barrier polylactic acid films coextruded with polyolefins such as polypropylene and/or tie-resin blends as a metal receiving layer. However, the application does not contemplate using amorphous polyesters as a metal receiving layer coextruded with PLA for improving moisture barrier properties.
U.S. Pat. No. 7,368,160 discloses a co-extruded BOPLA film displaying improved winding and manufacturability and end-user performance. U.S. Pat. No. 6,600,008 discloses an aliphatic polyester film and gas barrier film having specific surface, thermal and orientation properties for packaging applications. U.S. Pat. No. 7,390,558 discloses a primer-coated biaxially oriented aliphatic polyester film having an amorphous skin layer and its inorganic deposited film having about 3 to 5 cc/m2/day of oxygen gas permeability.
U.S. patent application Ser. No. 12/542,428 describes using a unique sputtering type of treatment on PLA substrates to “seed” the PLA surface with Cu or Ni sites for subsequent metal deposition of aluminum for improving metal adhesion and barrier properties. This reference is incorporated in this application.
However, these prior arts do not disclose enough nor are sufficient to achieve the objectives of this invention as described below.