BARRIER FILM COMPOSITION

Abstract

Barrier films are prepared from a composition comprising 1) a blend of two high density polyethylene HDPE blend components, 2) zinc glycerolate and 3) a dispersion aid/synergist. The two high density polyethylene blend components have substantially different melt indices. It is difficult to properly mix the zinc glycerolate into this HDPE blend. The use of the dispersion aid/synergist improves the water vapor transmission rate (WVTR) of polyethylene film (in comparison to barrier films made with zinc glycerolate, in the absence of the dispersion aid/synergist). The resulting barrier films are suitable for the preparation of packaging for dry foods such as crackers and breakfast cereals.

Claims

1. A barrier film comprising at least one extruded polyethylene layer, wherein said at least one extruded polyethylene layer comprises: I) zinc glycerolate; II) a high density polyethylene blend composition comprising: II-i) from 5 to 60 weight % of at least one high density polyethylene blend component a) having a high melt index, I.sub.2; and II-ii) from 95 to 40 weight % of at least one high density polyethylene blend component b) having a low melt index, I.sub.2, wherein: a) said zinc glycerolate is present in an amount of from 100 to 3000 parts per million based on the weight of said high density polyethylene blend composition; b) each of said blend component a) and blend component b) has a density of from 0.950 to 0.975 g/cc; c) the melt index, I.sub.2, of said high density polyethylene blend composition is from 0.5 to 10 grams/10 minutes; and d) the I.sub.2 ratio, obtained by dividing the I.sub.2 value of said blend component a) by the I.sub.2 value of said blend component b) is greater than 10/1; and III) a synergist, wherein said synergist comprises at least one fatty acid salt of a metal chosen from calcium and zinc.

2. The barrier film of claim 1 wherein said high density polyethylene blend composition comprises from 10 to 40 weight % of said component a) and from 90 to 60 weight % of said component b).

3. The barrier film of claim 1 wherein said high density polyethylene blend composition comprises from 20 to 40 weight % of said component a) and from 80 to 60 weight % of said component b).

4. The barrier film of claim 1 wherein said blend component a) is further characterized by having a molecular weight distribution, Mw/Mn, of from 2 to 4.

5. The barrier film of claim 1 wherein said high density polyethylene blend composition has a density from 0.955 to 0.965 g/cc.

6. The barrier film of claim 1 wherein said high density polyethylene blend composition has a melt index, I.sub.2, of from 0.8 to 8 grams/10 minutes.

7. The barrier film of claim 1 wherein said synergist is present in an amount of from 20 to 90 weight % of the weight of said zinc glycerolate.

8. The barrier film of claim 7 wherein said zinc glycerolate is present in an amount of from 200 to 2000 ppm, based on the weight of said high density blend composition.

9. The barrier film of claim 1 wherein said high density blend composition has a molecular weight distribution, Mw/Mn, of from 6 to 12.

10. The barrier film of claim 1 wherein said synergist comprises zinc stearate.

11. A process to prepare a barrier film for food packaging, said process comprising the film extrusion of a composition comprising: I) zinc glycerolate; II) a high density polyethylene blend composition comprising: II-i) from 5 to 60 weight % of at least one high density polyethylene blend component a) having a high melt index, I.sub.2; and II-ii) from 95 to 40 weight % of at least one high density polyethylene blend component b) having a low melt index, I.sub.2, wherein: a) said zinc glycerolate is added in an amount of from 100 to 3000 parts per million based on the weight of said high density polyethylene blend composition; b) each of said blend component a) and blend component b) has a density of from 0.950 to 0.975 g/cc; c) the melt index, I.sub.2, of said high density polyethylene blend composition is from 0.5 to 10 grams/10 minutes; and d) the I.sub.2 ratio, obtained by dividing the I.sub.2 value of said blend component a) by the I.sub.2 value of said blend component b) is greater than 10/1; and III) a synergist, wherein said synergist comprises at least one fatty acid salt of a metal chosen from calcium and zinc.

12. The process of claim 11 wherein said high density polyethylene blend composition comprises from 10 to 40 weight % of said component a) and from 90 to 60 weight % of said component b).

13. The process of claim 11 wherein said blend component a) is further characterized by having a molecular weight distribution, Mw/Mn, of from 2 to 4.

14. The process of claim 11 wherein said dispersion aid comprises at least one fatty acid salt of a metal chosen from calcium and zinc.

15. The process of claim 11 wherein conducted at a blow up ratio of from 1.5/1 to 4/1.

16. The process of claim 11 wherein said barrier film has a water vapor transmission rate that is from 15 to 40% lower than the water vapor transmission rate of a control film that is prepared in the absence of said synergist.

Description

EXAMPLES

[0083] The HDPE blend composition used in these examples was prepared in a dual reactor solution polymerization process in accordance with the disclosure of published U.S. patent application 20060047078 (Swabey et al.). The HDPE blend composition had a melt index, I.sub.2, of 1.2 grams/10 minutes, a density of 0.967 g/cc and a molecular weight distribution, Mw/Mn, of 8.9. The HDPE blend composition had two distinct fractions which varied according to molecular weight. The low molecular weight fraction (or component a)) was about 55 weight % of the total composition and had a melt index, I.sub.2, which was estimated to be greater than 5000 grams/10 minutes. The high molecular weight fraction was about 45 weight % of the total composition and had a melt index which was estimated to be less than 0.1 grams/10 minutes.

[0084] As noted above, melt index (I.sub.2) is generally inversely proportional to molecular weight for polyethylene resins. This was confirmed for homopolymer hdpe resins having a narrow molecular weight distribution (of less than 3) by preparing a plot of log (I.sub.2) versus log (weight average molecular weight, Mw). In order to prepare this plot, the melt index (I.sub.2) and weight average molecular Mw) of more than 15 different homopolymer hdpe resins was measured. These homopolymer hdpe resins had a narrow molecular weight distribution (less than 3) but had different Mwranging from about 30,000 to 150,000. (As will be appreciated by those skilled in the art, it is difficult to obtain reproducible I.sub.2 values for polyethylene resins having a molecular weight which is outside of this range).

[0085] A log/log plot of these I.sub.2 and Mw values was used to calculate the following relation between I.sub.2 and Mw for such homopolymer hdpe resins:

I.sub.2=(1.77410.sup.19)(Mw.sup.3.86).

[0086] Extrapolation (based on the above relation) was used to estimate the I.sub.2 values of component a) and component b) of the HDPE resin. That is, the molecular weight of component a) and component b) was measured and the Mw values were used to estimate the I.sub.2 values. It will be appreciated by those skilled in the art that it can be difficult to physically blend these hdpe blend components (due to the very different viscosities of these hdpe blend components). Accordingly, solution blending or an in-situ blending (i.e. prepared by a polymerization process) are exemplary methods to prepare such HDPE blend compositions.

[0087] Water Vapor Transmission Rate (WVTR, expressed as grams of water vapor transmitted per 100 square inches of film per day at a specified film thickness (mils), or g/100 in.sup.2/day) was measured in accordance with ASTM F1249-90 with a MOCON permatron developed by Modern Controls Inc. at conditions of 100 F. (37.8 C.) and 100% relative humidity.

Example 1Screening Study

[0088] While not wishing to be bound by theory, it is believed that some nucleating agents improve the WVTR of films made from HDPE resins by altering the crystal structure of the HDPE in the films. Accordingly, a screening study was undertaken to investigate the effect of adding various levels of zinc stearate on the peak melting point (T.sub.m) of HDPE blend compositions that also contain zinc glycerolate. Peak melting point is regarded as being an indication of the type/level of nucleation of the crystalline part of the HDPE resin composition. Results are shown in Table 1.

[0089] The first comparative experiment (1-C) shows that the T.sub.m of the HDPE blend composition is 132.7 C. and that the addition of 1000-1500 ppm of zinc glycerolate actually decreases T.sub.m slightly to 132.5 C. (see comparative experiments 2-C and 3-C). Inventive compositions are shown in experiments 4 to 7. The combination of zinc stearate and zinc glycerolate is shown to increase T.sub.m to between 132.9 to 133.1 C. The best result shown in Table 1 was observed when using 1500 ppm of zinc glycerolate and 500 ppm of zinc stearate (experiment 6, Tm=133.1). Interestingly, Tm actually decreased somewhat when the zinc stearate level was increased to 1000 ppm (while keeping the zinc glycerolate level at 1500 ppm)T.sub.m went down to 132.9 C. (although this is still better than the T.sub.m observed using 1500 ppm of zinc glycerolate alone in comparative experiment 3-C). Tm measurements were made using a conventional Differential Scanning calorimetry (DSC) instrument.

[0090] It is not particularly important to use a specific DSC instrument, or a specific test method to measure T.sub.m (provided that the same instrument and test method are used for all samples). This is because we believe that relative differences in measured T.sub.m values are helpful for ranking compositions in this screening test.

In Table 1 and the remaining tables, ZnSt means zinc stearate and Zn Gly means zinc glycerolate.

TABLE-US-00001 TABLE 1 Experiment Zn Gly (ppm) ZnSt.sub.2 (ppm) T.sub.m ( C.) 1-C 132.7 2-C 1000 132.5 3-C 1500 132.5 4 650 350 133.0 5 1000 350 132.9 6 1500 500 133.1 7 1500 1000 132.9

Example 2

Blown Film Study

[0091] The formulations shown in Table 2 were converted into blown films on a conventional blown film line manufactured by Macro Engineering. The extruder was equipped with an annular die having a die gap of 35 mils. The line was operated using the following conditions (aiming points are shown): [0092] 1) mass flow rate=40 pounds/hour (18.2 kilograms/hr); [0093] 2) Blow Up Ratio (BUR)=2/1; [0094] 3) Film thickness=1.5 mils; [0095] 4) Frost Line Height (FLH)=7 inches (15.4 centimeters). [0096] WVTR of the films was measured in accordance with ASTM F 1249-90.

[0097] A control film (not shown in Table 2) was prepared on this blown film line using the same HDPE blend composition but without containing either zinc glycerolate or zinc stearate and was observed to have a WVTR of about 0.16 g/100 square inches/day at a film thickness of 1.5 mils.

[0098] As shown in Table 2, the addition of 1600 ppm of zinc glycerolate (in the absence of zinc stearate) provided some improvement, with WVTR of the film measured at 0.118 g/100 square inches/day (experiment 2.1-C).

[0099] Inventive film 2.2i had a WVTR of 0.0821 g/100 square inches/day, which is a further improvement of 0.035 gor, alternatively stated, an improvement/reduction of 30.4% in comparison to the control film (2.2 c) that was made without the synergist.

TABLE-US-00002 TABLE 2 ZnGly ZnSt2 WVTR (g/100 Thickness Experiment (ppm) (ppm) in.sup.2/24 h) (mil).sup.3 2.1-C 1600 0.118 1.55 2.2-i 1600 500 0.0821 1.52

Example 3

Blown Film Study

[0100] A second blown film study was conducted on a higher capacity blown film line manufactured by Gloucester Engineering. The extruder on this line was also equipped with a die having an annular gap of 35 mils. The line was operated using the following conditions (aiming points as shown): [0101] 1) mass flow rate=100 pounds/hour (45.5 kilograms/hr); [0102] 2) Blow Up Ratio (BUR)=2/1; [0103] 3) Film thickness=1.5 mils; [0104] 4) Frost Line Height (FLH)=14 inches (35.6 centimeters).

[0105] Two inventive films were prepared in this study, one with zinc stearate and the other with calcium stearate. The actual film thickness of the films was higher than the aiming points (and higher than the thickness of the films in the first example) and WVTR values were also bettersee Table 3.

TABLE-US-00003 TABLE 3 ZnGly ZnSt.sub.2 CaSt.sub.2 WVTR (g/100 Thickness Experiment (ppm) (ppm) (ppm) in.sup.2/24 h) (mil).sup.3 3.1 1600 500 0.067 1.63 3.2 1600 500 0.077 1.60