Do It Right the First Time

Airtight cosmetic packaging poses unique challenges.

By Victor Suben, P. E.

Recent advances in formulation technology have led to the incorporation of water droplets in such color cosmetic products as lipstick, eye shadow and blush. To control the high evaporation rate of water in these products, the cosmetics/personal care packaging manufacturer has had to develop sealing systems that will eliminate or at least reduce the rate of water vapor loss.

Airtight Compacts

The sealing systems that have been developed for pressed powder products have taken two forms—a gasket system or a valve seal system. Each has its own level of success in controlling the rate of evaporation.

In a gasketed system, either an elastomeric or thermoplastic material is utilized to make the seal. Elastomers are materials that are natural or synthetic rubber. Whether the gasket material is an elastomer or a thermoplastic, it needs to have a considerable amount of flexibility and the thermoplastic material is usually low density polyethylene. By their nature, elastomers are very flexible.

To eliminate the possibility that the gasket will fall out when the compact is opened and to maintain security of the gasket within the compact, it is usually best to place the gasket in the compact’s base rather than in the cover. With this type of system, provisions will need to be made in the cover to enable compression of the gasket when the compact is closed.

The second type of airtight compacts incorporate what is known as a valve seal type system. In this system, the pan in the compact’s base is made of a rigid thermoplastic such as polypropylene. The cover would contain a matching piece constructed of a more flexible material such as low density polyethylene. The cover piece is manufactured with a taper from top to bottom. At its bottom, the taper is designed to fit over the top of the pan well. When the compact is closed, there is line contact between the cover piece and the pan well. This line contact is what makes the seal effective.

Sealing systems that employ gaskets (regardless of material) are generally not as effective in keeping the evaporation rate at acceptable levels as valve seal systems. The disparity in performance between gasket sealing systems and valve seal systems can probably be attributed to the fact that the compact geometry does not allow uniform compression of the gasket. As the compact is closed, the gasket near the hinge is compressed first, while the gasket near the clasp is not held as tightly. With the gasket retained in a groove, it is not free to move to a position in which a better seal might be created. In a valve seal system, on the other hand, the flexibility of the cover piece allows it to conform to the contour of the pan well.

Keep It Sealed

Regardless of whether a gasket or a valve seal is used to control weight loss, secure closing of the compact is required. Compacts that contain products of this type cannot have the least amount of “smile.” (A “smile” in a compact is a gap between the base and the cover that gradually widens as it progresses from the hinged side to the clasp side.) Moreover, these compacts need a positive locking device. Many compacts incorporate an interrupted screw thread such that positive closure occurs when the cover is rotated 90° relative to the base. Other designs incorporate lugs that ride on an inclined plane. A slide handle on the outside of the compact moves the plane. As the plane moves, it pulls the lugs down to make a tight seal.

With either type of system, pans are secured to the compact with either an anaerobic adhesive, double-faced tape or hot melt adhesive. The reason for this is that without a freely moving supply of air to cure the adhesive by allowing the solvent to evaporate, the adhesive must either be of the pressure-sensitive type, or the type that can cure when there is little or no air present.

Airtight Packaging for Lipsticks

Water-based lipsticks also need to have airtight packaging to prevent excessive loss of water, the volatile ingredient. To accommodate the need to control solvent (water in this case) loss, the most commonly used, cartridge-type case needed to be modified.

Probably the most common type of case for this sort of application is one in which the A-shell is tightly fitted to the outer base (fashion part). On the upper part of the A-shell there are usually one or more annular rings. There is also an inner cap seated inside the fashion part cap. As in the compact, the inner cap is usually made of low density polyethylene.

As the lipstick mass is inserted into the holder cup, it displaces the air in the cup. As it is displaced, the air moves through one or more holes in the bottom of the cup. To prevent pressure buildup inside the base of the case as the holder cup is retracted, a means of venting the air to the outside of the case must be provided.

Similarly, as the cover is placed on the base, the air inside the cover must be vented to the outside of the case. Unless the air is vented, the cover will not stay in place but will move up until the pressures inside and outside the case are equalized.
Different manufacturers have their own proprietary systems for venting the air from the lipstick cases.

Other Airtight Packaging

There are other air-tight systems commonly used in the cosmetics industry. One of these is the ubiquitous packette. Basically packettes are small pouches that are most commonly utilized as promotional items or to introduce new products. Packettes are usually constructed of several layers of different materials. Typically, packettes include a thin layer of aluminum foil, two or more layers of polyethylene, adhesive and other materials, depending upon the application. Air permeation is prevented by the layer of aluminum.

Another class of airtight containers is the flexible tube. Tubes are typically made of polyethylene or polypropylene and are often co-extruded with EVOH or other barrier materials. In addition, there are tubes known as glaminate tubes in which the polyolefin is laminated with aluminum. As with packettes, the aluminum layer prevents the passage of air and other gases through the tube’s wall. Once compressed to dispense product, glaminate tubes do not return to their original shape. Although glaminate tubes do find some application in the cosmetics industry, tubes co-extruded with EVOH are more commonly used when a barrier for gas is needed. Unlike glaminate tubes, polyethylene, polypropylene and co-extruded tubes do return to their original shape regardless of the number of times they are squeezed.

By itself, polyethylene is a good liquid barrier. However, at the molecular level, it can be seen that it has holes through which gases can either escape or enter the tube (depending upon the partial pressure of the contents). The inclusion of EVOH, on the other hand, provides a barrier for the transmission of gases. Thus, the combination of polyethylene with EVOH produces a package that is resistant to both moisture and vapor transmission. The typical construction of a co-extruded tube is polyethylene, adhesive, EVOH, adhesive, polyethylene. By itself, polyethylene is easily recycled as either pre-consumer or post-consumer material. However, when it is combined with EVOH, the tubes can no longer be recycled.

The selection of type of airtight container is mandated by the type of product that is to be packaged. Clearly, a pressed powder cannot be marketed in a tube, and a lotion cannot be sold in a compact. Selecting an airtight tube or packette is pretty straight forward, and rules-of-thumb can be applied when determining whether the vapor barrier is adequate. Compacts and lipstick cases, on the other hand, must be evaluated in terms of the product and its expected shelf life.