Solventless laminating adhesive with barrier properties

Details Solventless laminating adhesive with barrier properties Solventless laminating adhesive with barrier properties

2001-03-14

2002-10-08

Gorr, Rachel (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From reactant having at least one -n=c=x group as well as...

C528S905000, C525S440030

Reexamination Certificate

active

06462163

ABSTRACT:

The present invention is directed to laminating adhesives useful for joining polymeric films, particularly in providing polymeric film laminates for food products, drug products, and the like. The laminating adhesive is “solventless”, meaning it is both water-free and organic solvent-free. The adhesive provides excellent oxygen and moisture barrier properties.
BACKGROUND OF THE INVENTION
Oxygen, moisture, and other gases tend to degrade many food and drug products. Many polymeric films useful in food packaging, drug packaging, or the like are relatively permeable to both oxygen and moisture. To enhance barrier properties, polymeric films may be bonded together to form laminates using adhesive layers having high barrier properties.
Currently, barrier laminating adhesives are chiefly water-borne or solvent-borne polyvinylidine chloride (PVDC) formulations. If the solution is water-borne, energy needs to be expended to evaporate the water. Organic solvent-based formulations also involve some energy expenditure in removing the solvent, and solvents represent a workplace and environmental detriment. Packagers who have only solventless laminating machines must use films that are pre-coated with PVDC. Also, the chlorine in PVDC represents an environmental contaminant.
Another high barrier material is ethylene vinyl alcohol (EVOH), typically applied as a water-based adhesive formulation. While EVOH has no chlorine, the water in the formulation represents an energy expenditure. EVOH has a high melting temperature, i.e., greater than 150° C. At such high melting temperature, most polymeric films would also melt; thus EVOH cannot be applied as a solventless adhesive. While EVOH has excellent barrier properties with respect to oxygen and other gases, it has poor moisture barrier properties. Also, EVOH is relatively expensive.
The present invention is directed to solventless urethane adhesives useful for laminating and which provide high barrier properties, both with respect to oxygen and with respect to moisture. Most commercial urethane solventless adhesives have very poor oxygen barrier properties because such urethane adhesives tend to be amorphous polymers.
SUMMARY OF THE INVENTION
Solventless urethane adhesives in accordance with the invention comprise the reaction product of A) a hydroxyl terminated polyester formed from a single linear aliphatic diol having between 2 and 10 carbon atoms, preferably between 3 and 6 carbon atoms, and a single linear dicarboxylic acid with B) a single species of liquid diisocyanate at an NCO/OH ratio of between about 1 and about 1.1. The polyester A) is in crystalline form at ambient temperature and has a melting point of about 80° C. or below, preferably about 70° C. or below, more preferably about 60° C. or below, and most preferably about 55° C. or below. The number average (M
n
) molecular weight of the polyester is between about 300 and about 5000, preferably between about 500 and about 2000. Because the resulting urethanes are formed from a single linear diol, a single linear dicarboxylic acid and, a single diisocyanate species, the polyurethanes have high crystallinity with attendant high oxygen and high moisture barrier properties. The low melting points of the polyesters allows the adhesives to be applied at low temperatures without any solvent, where by the polymer films being laminated are not degraded and not deformed.
It is within the scope of the invention, and is in some cases preferred, that a portion A′) of the polyester A) be reacted with all of or a portion B′) of the isocyanate B) at an NCO/OH ratio of between about 2 and about 8 to form a urethane prepolymer C), and the remaining portion A″) of the polyester A) subsequently admixed with the prepolymer C) and any remaining portion B″) of the isocyanate B) to form an adhesive mixture suitable for laminating the adhesives. In such case, the NCO/OH ratio of B), including in such case B′) and B″), to A), including both A′) and A″), is, nevertheless, between about 1 and about 1.1.
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
Preferred linear aliphatic diols for forming the polyester chains are C
3
-C
6
diols, n-butanediol and n-hexanediol being the preferred diols, both from the standpoint of forming an adhesive with good barrier properties and from an availability and economic standpoint.
Adipic acid is the preferred dicarboxylic acid for forming the polyesters of the present invention, although other suitable dicarboxylic acids may be used, including, but not limited to azelaic acid and sebacic acid.
Currently, the most preferred polyesters are the reaction products of either 1,6-hexanediol or 1,4-butanediol with adipic acid.
Diol is reacted with dicarboxylic acid so as to form hydroxyl-terminated polyesters having OH numbers between about 20 and about 350, preferably between about 100 and about 250.
Preferred diisocyanates are aliphatic diisocyanates, particularly linear diisocyanates, including polymeric hexamethylene diisocyante (HDI).
However, other diisocyanates, including methylene diphenyl diisocyante (MDI), dicyclohexylllmlelthane 4,4′-diisocyanate (H
12
MDI) and toluene diisocyanate (TDI) may also be used. The diisocyanate generally comprise a smaller portion of the linear polyurethane chain than does the polyester; thus the choice of diisocyanate is generally less critical than the nature of the polyester. From the standpoint of forming a very high barrier, polymeric HDI is currently preferred. However, polymeric HDI is a relatively expensive material, and less expensive diisocyanates, such as MDI, may be used where the barrier property demands are less stringent.
The diisocyanates useful in the invention are liquids at ambient temperatures, i.e., 20-25° C. The low molecular weight polyesters of the invention are highly crystalline, but melt at relatively low temperatures due to their low molecular weights. It is necessary that the polyesters melt at a low temperature only somewhat above ambient temperatures so that the adhesives may be applied at a temperature only somewhat above ambient temperatures, whereat the polymer films forming the laminate are not degraded or deformed. This, of course, depends upon the particular nature of the polymeric film(s), but generally it is desired that the polyester melt below about 80° C. and even lower for the more heat-sensitive films.
For application using high speed laminating machines, e.g., 600 ft/min (183 m/min) or higher, it is preferred that the viscosity of the urethane/polyester mixture range from about 300 to about 2000 cps at the laminating temperature, preferably between about 400 to about 1000 cps at the laminating temperature. Preferably, the laminating temperature is 80° C. or below, more preferably 70° C. or below, even more preferably 60° C. or below, and most preferably 55° C. or below. Accordingly, for reference purposes, depending upon the particular application, the above-described viscosities should apply at least one of the above-listed temperatures, i.e., 80°, 70°, 60°, or 55°C.
The adhesives of the present invention are useful for adhering a wide variety of polymer film types, including polypropylenes, polyethylenes, polyesters, polyamide, and coextruded films. Of course, the adhesives are most useful in adhering films having poor barrier properties to provide high barrier laminates. Adhesion between two films is very good, and laminates formed from the urethane adhesives are inseparable.
Generally, the two adhesive components are mixed just prior to lamination, i.e., within about 1 min. of lamination or less, molten polyester being mixed with liquid diisocyanate to form an adhesive mixture composition. The adhesive mixture composition is applied to one of the polymer films and the films nipped together and the resulting laminate rolled into a reel. Coating weights typically range from 1 to 3 pounds per ream (1.6-4.9 g/m
2
), preferably around 1.5 pounds per ream (2.4 g/m
2
). A slow urethane-forming reaction is most desirable for achieving high bar

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