Barrier film for food packaging
Barrier films are prepared from a blend of two high density polyethylene blend components and a high performance organic nucleating agent. The two high density polyethylene blend components have substantially different melt indices. Large reductions in the moisture vapor transmission rate of the film are observed in the presence of the nucleating agent when the melt indices of the two blend components have a ratio of greater than 10/1. The resulting barrier films are suitable for the preparation of packaging for dry foods such as crackers and breakfast cereals.
1. A method for improving the barrier properties of a polyethylene film, said method comprising the steps of converting into a film a mixture comprising;
I) from about 5 to about 60 weight percent, based on the total weight of said mixture, of a blend component a), comprising a first high density polyethylene having a melt index, I 2 ;
II) from about 95 to about 40 weight percent, based on the total weight of said mixture, of a blend component b), comprising a second high density polyethylene having a melt index, I 2 ′, and;
III) from 100 to 3,000 ppm of a nucleating agent, comprising a calcium salt of 1,2 cyclohexanedicarboxylic acid;
wherein, a melt index ratio, defined by dividing said melt index, I 2 , of said blend component a) by said melt index, I 2 ′, of blend component b), is from about 12 to about 19;
wherein each of said blend component a) and said blend component b) have a density from about 0.950 to about 0.975 g/cc;
wherein said mixture has a melt index from about 0.5 to about 4 grams/10 minutes;
wherein the water vapor transmission rate of said polyethylene film is improved, by being from about 19 percent lower to about 25 percent lower relative to a control polyethylene film of the same composition but not containing said nucleating agent, and;
wherein melt index is measured according to ASTM D1238 (2.16 kg load and 190° C.), density is measured according to ASTM D1505 and water vapor transmission rate is measured according to ASTM F1249-90.
2. The method of claim 1 , wherein said mixture comprises; from 10 to 50 weight % of said first high density polyethylene and from 90 to 50 weight % of said second high density polyethylene, based on the total weight of said mixture.
3. The method of claim 1 , wherein said first high density polyethylene is further characterized by having a molecular weight distribution, Mw/Mn, of from 2 to 4.
4. The method of claim 1 , wherein said mixture has a density from 0.955 to 0.965 g/cc.
5. The method of claim 1 , wherein said mixture has a melt index, I 2 , of from 0.8 to 2.5 grams/10 minutes.