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A well-designed package is usually protected by a barrier film, such as polyethylene, preferably with a low moisture vapor transmission rate (MVTR). The MVTR of the barrier depends upon the type and thickness of the materials used and the external environmental conditions.
Table 1 provides the MVTR for a number of barrier materials. Knowing the MVTR (in grams of water per square foot per day), the total surface area of the barrier (package surface area) and the length of time in storage, you can calculate the amount of moisture penetrating the package over time. This moisture must also be removed by the desiccant to protect against possible condensation within the package or damage to the product due to the high humidity.
Table 1: MVTR of Common Cover Stock Materials
| MVTR* | ||
|---|---|---|
| MVTR* Material | gm/m2-day | gm/ft2-day |
| Aluminum Foil Wrapping 0.025 mm | 0.5 | 00.5 |
| Aluminum Foil Wrapping 0.009 mm | 1.0 | 0.09 |
| Cellulose Films ('Cellophane') 400's MXXT Grade (Polyvinylidene Chloride Coated) Polyvinylidene/Polyvinyl Chloride Films ('Saran') 0.005 cm (0.002 in) |
1.5 | 0.14 |
| Polyethylene Films ('Polythene') 0.0125 cm (0.005 in) Waxed Paper (45.5 kg (100 LB) per DC Ream) |
4.0 | 0.37 |
| Cellulose Films ('Cellophane') 300's MSAT Grade (Cellulose Nitrate Coated) |
7.5 | 0.70 |
| Glassine Lacquered (16 kg (35 LB) per DC Ream) |
9.0 | 0.84 |
| Polyethylene Film ('Polythene') 0.005 cm (0.002) in) |
10.0 | 0.93 |
| Polyethylene Film ('Polythene') 0.0025 cm (0.001 in) |
20.0 | 1.86 |
| Polyethylene Coated Kraft (9 kg (20 LB) per DC Ream) |
30.0 | 2.79 |
Table 2: Unit Calculation Charts
Flexible Barriers (Chart 1)
| Area in Square Feet | Area in Square Inches | Units Required |
|---|---|---|
| .1 | 30 | 1/6 |
| .3 | 45 | 1/3 |
| .6 | 90 | 1/2 |
| 1.3 | 180 | 1 |
| 1.9 | 270 | 2 |
| 2.5 | 360 | 3 |
| 3.1 | 450 | 4 |
| 3.8 | 540 | 5 |
| 4.4 | 630 | 6 |
| 5.0 | 720 | 7 |
| 5.6 | 810 | 8 |
| 6.3 | 900 | 9 |
| 7.5 | 1,080 | 10 |
| 8.8 | 1,260 | 12 |
| 10.0 | 1,440 | 14 |
| 20.0 | 2,880 | 16 |
| 50.0 | 7,200 | 32 |
| 100.0 | 14,400 | 80 |
| 120.0 | 18,000 | 160 |
| 150.0 | 22,500 | 200 |
| 180.0 | 27,000 | 250 |
Rigid Containers (Chart 2)
| Volume in Gallons | Volume in Cubic Feet | Volume in Cubic Inches |
|---|---|---|
| 1.1 | .14 | 237 |
| 2.1 | .28 | 476 |
| 3.2 | .42 | 714 |
| 6.2 | .83 | 1,428 |
| 12.2 | 1.67 | 2,856 |
| 18.7 | 2.50 | 4,284 |
| 25.0 | 3.33 | 5,712 |
| 31.2 | 4.16 | 7,140 |
| 37.4 | 5.00 | 8,568 |
| 43.6 | 5.83 | 9,996 |
| 50.0 | 6.66 | 11,424 |
| 56.1 | 7.50 | 12,852 |
| 62.3 | 8.33 | 14,280 |
| 74.8 | 10.00 | 17,136 |
| 87.3 | 11.66 | 19,992 |
| 99.7 | 13.32 | 22,850 |
| 199.4 | 26.64 | 45,700 |
| 498.5 | 66.65 | 114,250 |
| 997.0 | 133.30 | 228,500 |
| 1,240.0 | 166.00 | 285,500 |
| 1,550.0 | 207.00 | 357,000 |