The present invention relates to cross-linked PVOH coatings having enhanced barrier characteristics and, more particularly, to PVOH coatings which exhibit improved processing characteristics resulting in cross-linked coatings providing improved oxygen barriers especially at high relative humidities.
Poly(vinyl alcohol) ("PVOH") coatings have been applied to various substrates in the past, and are known to provide a barrier to the transmission of oxygen. PVOH, however, is soluble in water and therefore is susceptible to attack by moisture. Various attempts have been made to decrease the sensitivity of PVOH to moisture.
One known method of decreasing the sensitivity of the PVOH coating to moisture is to cross-link the poly(vinyl alcohol). For example, a cross-linking agent and catalyst may be applied along with the poly(vinyl alcohol) such that the agent interconnects and thereby cross-links the poly(vinyl alcohol) molecules as such coating is dried. The catalyst is present to facilitate the cross-linking process.
Commonly owned co-pending application Ser. No. 08/080,602 discloses a polymeric substrate having a cross-linked layer of PVOH adhered thereto. The PVOH layer includes a cross-linking agent such as melamine or urea formaldehyde, which is cross-linked in the presence of a sulfuric acid catalyst. The resultant film exhibits enhanced oxygen barrier characteristics, as compared to barrier films of the prior art.
However, it can be difficult in the prior art films, including the film disclosed in commonly owned co-pending application Ser. No. 08/080,602, now U.S. Pat. No. 5,380,586, to ensure that the layer of poly(vinyl alcohol) cross-links fully throughout itself. In this regard, it is believed that the degree of cross-linking is indicative of the oxygen barrier characteristics of the resultant film. Stated differently, poly(vinyl alcohol) layers that are fully cross-linked tend to exhibit better barrier characteristics than those layers which are less than fully cross-linked.
The prior art films, in an attempt to achieve maximum cross-linking, are often dried and/or stored for an extended period of time. For example, it may be necessary to store a film for weeks, or even months, before such film has reached its point of maximum cross-linking. The aforementioned storage period increases the production time and cost for manufacturing polymeric films having cross-linked PVOH coatings thereon.
There is therefore a need in the art for a method of providing a PVOH coating on a substrate, such as a polymeric film, which upon drying is substantially 100% cross-linked thereby eliminating or, at the minimum, greatly reducing the storage period. The resultant coating should exhibit improved oxygen barrier characteristics and improved rubbing resistance especially at high relative humidities. Moreover, the resultant coating should bond firmly to the underlying substrate.