Rigid packaging is defined as injection-molded plastic containers, ranging in size from 1 pint to 7 gallons. The containers can be round, square or rectangular and are typically made of 90-millimeter thick high-density polyethylene (HDPE) or polypropylene. This type of packaging will typically provide the lowest cost option for lubricant suppliers in North America, for a 5-gallon fill capacity for most applications.

Rigid plastic packaging provides many other advantages to the lubricant supplier, as well. For example, it will not rust or dent, and it is seamless, minimizing the potential for leaks. The lightweight material reduces shipping costs, and the nestable containers reduce required storage area.

Plastic containers also minimize noise during the filling process, and round containers, especially, provide secure stacking capability, again reducing storage space after filling. Finally, both HDPE and polypropylene can be recycled.

Typically, the petroleum industry uses a 20-liter container for a fill capacity of 5 gallons. This taller size provides sufficient head space to prevent sloshing and spillage during filling and conveying. This is an important consideration due to the low viscosity of lubricants at the typical fill temperatures of 45 to 55 degrees C (110 to 120 degrees F).

Materials

As noted above, plastic containers are typically molded from high-density polyethylene or polypropylene. Both provide certain benefits and advantages for lubricant suppliers.

HDPE is more widely used and is more often the lower priced option compared to polypropylene. Because it is softer, HDPE provides better impact resistance and is less brittle at lower temperatures.

HDPE also resists static charge build up better than polypropylene, so does not attract dust and debris. The material is a commonly recycled plastic, carrying recycling symbol 2. It is typically recycled into plastic lumber, tables, roadside curbs, benches, truck cargo liners, trash receptacles, stationery and other durable plastic products and is usually in demand.

Polypropylene, on the other hand, provides better sidewall stiffness and has a higher melting point than HDPE, so it better suited for use with liquids processed at higher temperatures. The material is rugged and resistant to different chemical solvents, acids and bases. However, while it carries recycling symbol 5, according to production and recycling figures provided by the American Chemistry Council, polypropylene is one of the least recycled post-consumer plastics, at a rate below 1 percent.

Both materials are available as injection grade (also known as prime or virgin), regrind (from scrap) and post-consumer resin. Injection grade is new material that has never been molded into a product and, therefore, has fewer impurities and more predictable performance characteristics. As a result, injection grade resins are generally preferred for lubricant packaging.

A number of factors affect resin pricing. HDPE is derived from ethane, and in the United States, ethane comes primarily from natural gas. In the rest of the world, ethane is made from crude oil. The difference can have a large impact on resin prices around the world. When the cost of natural gas is low in relation to crude oil, resin prices in the U.S. will be lower than in the rest of the world

Other factors affecting resin prices include currency fluctuations, export market demand and geopolitical issues. Natural disasters also have a significant effect. For example, when Hurricane Rita shut down ethane production on the U.S. Gulf Coast in 2005, resin prices increased by 20 percent.

Container Performance

The performance of a plastic container depends on how it is made and handled. Performance attributes include stack strength, impact resistance, dynamic top load strength (compressive), environmental stress crack resistance and chemical permeation resistance.

When stacked, pails can fail in several ways – lid collapse or sidewall buckling being the most prominent. The recommended stack height is three containers per pallet, and no more than two pallets high.

Stacking tests are performed in accordance with 49 CFR Section 178.606, depending on whether it is a design qualification or a periodic retest. In a design qualification test, a calculated top load is applied for 28 days at 20 degrees C and is followed by a stack stability test. A periodic retest comprises a dynamic compression test to a calculated required load with less than 1-inch of deflection.

Plastic pails generally have high impact resistance, but again it depends on how they are stored and handled. Top load strength is a function of sidewall thickness and other design features, and is measured by compressing the container between two steel plates.

Environmental stress crack resistance is a mechanism of chemical attack that is highly dependent upon the test reagent, resin, container manufacturing or processing history, exposure temperature, applied stress and other factors. The combination of these factors may result in eventual stress-crack failure.

Stress crack resistance is measured in accordance with ASTM D 1975 and indicates what happens when a container is exposed to surface active solutions while under environmental stress. This test is especially important for automatic transmission fluid, which can attack the plastic when stored for a long period of time.

Chemical compatibility or permeation resistance is evaluated in accordance with 49 CFR Part 173, Appendix B. Compatibility and rate of permeation are determined by testing the container under one of three conditions – 180 days at a temperature no lower than 18 degrees C, 28 days at a temperature no lower than 50 degrees C, or 14 days at a temperature no lower than 60 degrees C.

The container fails if there is visible evidence of permanent deformation due to vapor pressure build-up or collapse of walls, deterioration, swelling, crazing, cracking, excessive corrosion, oxidization, embrittlement, leakage, rupture or other defects likely to cause premature failure or a hazardous condition. Maximum permitted permeation rate is 2.0 percent.

Handling & Storage

Lubricants are typically filled on automated packaging lines. Here, the best closing method is a pneumatic lid press because it applies an even force on the center of the lid, snapping it into place. Roller conveyors, which apply force from one edge to the other, are more convenient for mass production under continuous conveying. However, roller conveyors can place undue pressure on the container and are not adequate to close U.N. Certified containers.

U.N. Certified Packaging is required for materials that absolutely cannot leak. It must meet more severe lid design and sealing requirements. U.N. lids are more difficult to close but stay closed under more adverse conditions, such as being dropped.

The use of rubber mallets is an acceptable means of closing plastic containers. But hammers and hand pounding are not.

In stacking and storing plastic containers, it is important to avoid point loads; that is, pressure at a single point on the bottom of the container. Point loads can cause sidewalls to collapse, dent, bulge or buckle; and chimes to bend.

A frequently overlooked factor in preventing uneven loads on containers is pallet design. Pallets should have no knots, broken boards or protruding fasteners. Deck boards should be even, with narrow board spacing. Slip sheets should be placed between pallets, if possible.

Important environmental factors are fill temperature and storage time. Containers filled with hot liquids (in excess of 65 degrees C) deform more easily under high capping forces, and the vacuum seal that results from hot filling can cause sidewall buckling. The stiffness of HDPE at 60 degrees C is one-half that at 25 degrees C. The maximum recommended fill temperature for HDPE pails is 65 degrees C. This is one reason that polypropylene is a preferred material for hot-fill requirements.

Shelf life depends on the material being stored. Lubricants generally have a 5-year shelf life; therefore, an HDPE package is more than adequate for the application.

A variety of methods can be used to apply graphics to plastic pails. These include screen and dry offset printing, heat transfer labels, in-mold labels and glued or pressure sensitive labels.

Factors to consider when selecting a decorating option are the number of containers to be handled, the number of different product types, inventory turns, required resolution, number of colors and the container color. Another consideration is whether the container is to be U.N. Certified Packaging. U.N. Certified Packaging of 15 liters or larger must carry an infant drowning label. ο

Tom Bishop is Director – National Accounts for Letica Corp., based in Rochester, Michigan, United States. Contact him at tbishop@letica.com.