ANNEALED ALUMINUM FOIL has a tensile strength of 8.5 pounds per inch of width per mil thickness. Tensile strength and resistance to tearing and bursting are greater for strain-hardened foil than for annealed foil of the same thickness. Aluminum foil increases in strength as the gage (thickness) is increased and temperature drops.

Aluminum foil has a low moisture-vapor transmission rate, even for thicknesses less than 0.0015 inch, which have tiny perforations called pinholes. These seem to be inevitable when metal is rolled to very thin gages. Microscopic measurements of all the pinholes in 100 square inches of 0.0004-inch foil gave an estimated area of 0.00004 square inch. A single hole of this area would transmit about 0.19 gram of water vapor per 24 hours at 100 and 100 percent relative humidity. The number and size of pinholes decrease with increasing foil thickness.

Moisture-vapor transmission also declines with increasing foil thickness. A 0.00035-inch foil will transmit approximately 0.29 gram of water vapor per 100 square inches of foil per 24 hours at 100 and 100 percent relative humidity. A 0.0005-inch foil transmits 0.12 gram of water vapor under the same conditions. Thicker foils transmit almost no water vapor at all.

Aluminum foil alone does not make good packages for seeds, but it can be bonded to other materials to produce combinations having almost any desired characteristics. Even though thin gages of aluminum foil have some pinholes, combinations with various supporting materials, such as paper or plastic films, offer effective barriers to moisture vapor and gas transfer. With the proper selection of materials, combinations can be produced that will restrict vapor transfer completely.

LAMINATIONS such as aluminum foil/glassine paper/aluminum foil/heat-sealing lacquer; aluminum foil/tissue paper/polyethylene; and the paper/ polyethylene/aluminum foil-polyethylene have been used satisfactorily.

Foil is used also as a coating and as an overwrap material for cardboard.

POLYETHYLENE, the most extensively used thermoplastic film, is made from aliphatic hydrocarbon resins. Polyethylene resins are polymers of ethylene gas.

Commercially available polyethylene resins fall into three groups on the basis of their density, which is due to differences in molecular structure. Molecular structure determines the physical structure of the resins. Resin properties and extrusion variables determine film properties, which in turn determine the utility of the film.

Such physical properties as tensile strength, tearing strength, bursting strength, moisture-vapor transmission rate, carbon dioxide and oxygen transmission rates, sealability, and elongation and folding endurance determine the usefulness of a film.

Conventional low-density films have been considered better for seed packages than medium- and high-density films because of differences in bursting and tearing strength and stretch, but a special new medium-density film shows considerable promise.

The medium- and high-density films tend to show progressively less permeability to moisture vapor and gases than conventional low-density films. A 1-mil low-density film tested at 100 and 100 percent relative humidity will permit passage of 1.4 grams of moisture vapor through 100 square inches of film in 24 hours, a medium-density film will transmit 0.7 gram, and a high-density film 0.3 gram under the same conditions. A 10-mil low-density film at 100 and 100 percent relative humidity will transmit 0.13 gram of moisture vapor per 100 square inches per 24 hours, approximately one-tenth the amount transmitted by the 1-mil film.

A polyethylene film of medium density (specific gravity 0.938) has been developed that surpasses the performance of conventional polyethylene. A 7-mil film of this special material has a moisture-vapor transmission rate of 0.10 gram per 24 hours per 100 square inches which is less than that of 10-mil conventional polyethylene. This special medium-density film has better tensile properties and greater elongation than conventional polyethylene. Because of its high percentage of stretch, this medium-density film has good resistance to puncture.

Clear conventional polyethylene and the special translucent white medium-density polyethylene films are subject to slow deterioration on direct exposure to strong sunlight and ultraviolet radiation. Deterioration can be retarded by incorporating carbon black or other pigments that absorb the ultraviolet rays. The special medium-density film has high resistance to stress cracking.

Rats and mice sometimes present a problem with conventional polyethylene, but we have had no reports of rodent attack on bags made of the special medium-density material. Perhaps this material is an answer to rodent problems.

With a tight closure, such as is produced with a heat-seal, both 10-mil conventional polyethylene and 7-mil medium-density polyethylene bags are almost completely insect-proof. Some insects may penetrate thinner polyethylene films.

Polyethylene films can be laminated to themselves, other films, foil, paper, textile fabrics, and fiberboard. Moisture barrier and other physical properties may be improved by laminations. The various properties of each film included in a laminate are more or less additive. Some laminated films are completely impervious to various gases and practically impervious to moisture vapor.

Some laminated materials handle well on automatic packaging machinery and others handle best by hand, depending on the nature of the materials used in the laminations.

METAL CONTAINERS, properly sealed, provide an absolute barrier to moisture and gas and shield the product from light. They provide complete protection against rodents, insects, changing humidity, floods, and harmful fumes, and protect the factors of physical quality, including moisture content. Metal cans can be filled and sealed automatically and quickly.

Glass containers are not used very much for packaging seeds. They provide essentially the same protection as metal, but glass breaks easily.

Glass containers are used in research and occasionally as display receptacles in stores where bulk sales are made. Some persons use glass jars for keeping seed from one season to the next.

Cardboard in the form of boxes and cans is used extensively. Cardboard cans have metal lids and bottoms. Conventional boxboard cardboard has no moisture-barrier qualities, but they are achieved by laminating polyethylene, aluminum foil, or some other barrier material to the boxboard or by overwrapping the carton with wax-paper, aluminum foil, or polyethylene.

Cardboard containers provide good protection to most physical qualities of seeds, but they protect the moisture content only when the special barrier properties are added as laminates or overwraps.

Cardboard containers are well adapted for automatic filling and sealing. Tests of all kinds of packaging materials for moisture-vapor transmission are conducted in a situation of relative humidity so high that it is seldom encountered in the marketing and storage of seeds. The rate of moisture-vapor penetration of most materials consequently is less under normal use than that indicated by tests.

LONGEVITY of seeds the maintenance of their viability is associated closely with the moisture in them. In open or porous containers, the moisture content is controlled by the relative humidity of the surrounding atmosphere and the temperature of the storage area.

Different kinds of seeds absorb different amounts of water under identical conditions. Each kind has its own equilibrium content of moisture for a given temperature and relative humidity. It drops or rises as atmospheric relative humidity goes down or up.

Seeds absorb or give off moisture according to the degree of saturation of the surrounding atmosphere not the actual amount of water vapor present in a unit volume of air.

Seeds subjected to fluctuating levels of moisture tend to deteriorate faster than seeds held at a constant level of moisture. A constant moisture content can be maintained by controlling the temperature and relative humidity of the storage area or placing the seeds in a moisture-barrier package, which may be completely or partly impervious to moisture vapor.

Hermetically sealed metal and glass containers are completely impervious to moisture vapor and gases. Containers made of flexible packaging materials resist transmission of moisture vapor and gases only to the extent that the special barrier properties are built into them.

In completely impervious packages, in which the relative humidity of the atmosphere is determined primarily by the moisture content of the seed, any rise or drop in its moisture is limited to the small effect of temperature.

The relative humidity in packages made of materials with limited permeability is determined by the seed moisture, temperature and relative humidity of the storage chamber, permeability of the packaging material, and size of the package.

Seeds in packages that are not completely impervious to moisture vapor may gain or lose moisture with time. The direction, rate, and amount of change of the moisture are controlled by the temperature and relative humidity of the storage area, moisture-vapor transmission rate of the packaging material, equilibrium moisture content of the seeds for the surrounding temperature and humidity, and the ratio of surface area of seeds to the surface area of the package.

Because the small packages contain fewer seeds than large packages per unit area of package surface, each seed in a small package gives up or absorbs a larger part of the moisture vapor transmitted through the package surface. Seeds in small packages of a given material therefore gain or lose moisture faster than seeds in large packages of the same material held under the same conditions of temperature and relative humidity. Small packages thus require better moisture-barrier materials than large packages in order to provide the same amount of moisture protection to the seeds inside.

The substitution of carbon dioxide, nitrogen, or a partial vacuum for the normal atmosphere in sealed gas-tight containers may or may not increase longevity of seed. Seeds held under unfavorable conditions may deteriorate faster than normal. Seeds under good storage conditions may not be affected at all. In the instances where carbon dioxide, nitrogen, and vacuum have prolonged the viability, the beneficial effects usually were not pronounced.