Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1995-03-06
2002-09-03
Reddick, Judy M. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C427S316000, C427S372200, C427S393500, C427S536000, C427S569000, C428S502000, C428S515000, C428S516000, C428S524000, C524S503000, C525S417000
Reexamination Certificate
active
06444750
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to PVOH-based coatings and, more particularly, to improved PVOH-based coating solutions which provide improved viscosity stability, and improved crosslinking and oxygen barriers characteristic upon drying.
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 crosslink the poly(vinyl alcohol). For example, a crosslinking agent, e.g., glyoxal, urea formaldehyde, melamine formaldehyde, may be applied along with the poly(vinyl alcohol) such that the agent interconnects and thereby crosslinks the poly(vinyl alcohol) molecules as such coating is dried. An acid catalyst is typically employed with the formaldehyde-containing crosslinkers to facilitate the crosslinking process. The ph of the solution including the acid catalyst is generally about 4.5 to 6.5.
However, it can be difficult in the prior art films to ensure that the layer of poly(vinyl alcohol) crosslinks fully throughout itself. In this regard, it is believed that the degree of crosslinking is indicative of the oxygen barrier characteristics of the resultant film. Stated differently, poly(vinyl alcohol) layers that are fully crosslinked tend to exhibit better barrier characteristics than those layers which are less than fully crosslinked. Of course, fully crosslinked PVOH layers are also more resistant to rubbing.
Glyoxal, although generally providing good crosslinking, even without the need of a catalyst, has been found to cause yellowing of the resultant film when such film is printed with white acrylic water-based ink systems, while PVOH coatings employing formaldehyde-containing crosslinkers have generally suffered from slow curing and short pot life.
Thus, films employing formaldehyde-containing crosslinkers, in an attempt to achieve maximum crosslinking, 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 crosslinking. The aforementioned storage period increases the production time and cost for manufacturing polymeric films having crosslinked PVOH coatings thereon. Attempts to facilitate the crosslinking process with films employing formaldehyde-containing crosslinkers have generally been unsuccessful.
There is therefore a need in the art for a PVOH-based coating solution employing a formaldehyde-containing crosslinker which exhibits improved viscosity stability, and which after application to a polymeric substrate and upon drying, is substantially 100% crosslinked thereby eliminating or, at the minimum, greatly reducing the storage period. Finally, the resultant coating should exhibit improved oxygen barrier characteristics.
SUMMARY OF THE INVENTION
The present invention, which addresses the needs of the prior art, relates to an improved coating solution for application to a polymeric substrate. The solution exhibits improved viscosity stability and provides improved crosslinking and oxygen barrier characteristics upon drying. The coating solution includes poly(vinyl alcohol), formaldehyde-containing crosslinking agent and crosslinking-promoting acid catalyst. The solution has a pH of about 3.5 or less. In one preferred embodiment, the solution has a pH of about 3 or less and the acid catalyst is selected from the group consisting of phosphoric acid, nitric acid, hydrochloric acid and maleic acid. In one particularly preferred embodiment, the solution includes a second acid catalyst selected from the group consisting of citric acid, maleic acid and lactic acid.
The present invention also relates to a polymeric film structure having an enhanced oxygen barrier produced by the process of coating at least one side of a polymeric substrate adapted for receipt of an oxygen barrier with a solution of polyvinyl alcohol, formaldehyde-containing crosslinking agent and crosslinking-promoting acid catalyst wherein the solution has a pH of about 3.5 or less. In one preferred embodiment, the solution has a pH of about 3 or less and the acid catalyst is selected from the group consisting of phosphoric acid, nitric acid, hydrochloric acid and maleic acid. In one particularly preferred embodiment, the solution includes a second acid catalyst selected from the group consisting of citric acid, maleic acid and lactic acid.
The present invention also relates to a method of producing a film structure having an enhanced oxygen barrier layer adhered thereto. The method includes the step of coating at least one side of a polymeric substrate adapted for receipt of an oxygen barrier with a solution of polyvinyl alcohol, formaldehyde-containing crosslinking agent and crosslinking-promoting acid catalyst wherein the solution has a pH of about 3.5 or less. In one preferred embodiment, the solution has a pH of about 3 or less and the acid catalyst is selected from the group consisting of phosphoric acid, nitric acid, hydrochloric acid and maleic acid. In one particularly preferred embodiment, the solution includes a second acid catalyst selected from the group consisting of citric acid, maleic acid and lactic acid.
As a result, the present invention provides a coating solution wherein the resultant PVOH layer is substantially 100% crosslinked upon drying thereof. The resultant crosslinked PVOH layer exhibits improved oxygen barrier characteristics. The use of a formaldehyde-containing crosslinking, rather than a crosslinker such as glyoxal, eliminates any drawbacks associated with the use of white acrylic water-based ink systems for printing. Finally, the coating solutions of the present invention exhibit improved viscosity stability.
DETAILED DESCRIPTION OF THE INVENTION
The films of the present invention are produced by coating at least one side of a substrate with a solution of poly(vinyl alcohol), a formaldehyde-containing crosslinking agent and a crosslinking-promoting acid catalyst. The solution has a pH of about 3.5 or less and, preferably, about 3 or less,. The poly(vinyl alcohol) thereafter crosslinks to provide an oxygen barrier, i.e., a polymeric layer which resists the transmission of oxygen therethrough.
The improved oxygen barrier layer of the present invention may be adhered to any number of substrates, including polymeric films, boxboards, metallic films and paper. The oxygen barrier layer is preferably adhered to a polymeric film such as a polyolefin. One particularly preferred polyolefin is polypropylene.
To ensure that the oxygen barrier layer of the present invention properly adheres to the substrate, the substrate preferably includes at least one side that is adapted for receipt of the layer. Particularly, the side of the substrate to be coated should have surface properties which facilitate the securing of a polylvinyl alcohol) layer thereto. For example, the side to be coated may be treated with corona discharge and/or a primer such as poly(ethyleneimine). Of course, other suitable primers may also be utilized. The side to be coated may also be adapted for subsequent receipt of a poly(vinyl alcohol) layer during formation of the substrate itself. For example, a polymeric substrate, e.g., polypropylene, may include a material such as maleic acid which improves the ability of poly(vinyl alcohol) to bond thereto. Finally, the substrate may include a side which is naturally adapted for receipt of a poly(vinyl alcohol) layer, i.e., the side to be coated requires no treatment.
Although polylvinyl alcohol) provides a barrier to the transmission of oxygen, it is soluble in water and therefore susceptible to attack by moisture. As a result, poly(vinyl alcohol) layers which will be exposed to moisture are typically crosslinked. The crosslinking of the layer substantially reduces its susceptib
ExxonMobil Oil Corp.
James Rick F.
Reddick Judy M.
Santini Dennis P.
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