Polycarbonate (PC) Water Bottles

Polycarbonate has high light-transmittance (88-91% as compared to 92% for clear plate glass) and high resistance to stains by tea, coffee, lipstick, inks, soap, detergent, and many other household materials. Its relatively dense composition makes it resistant to odors, and its hard, smooth surface facilitates easy removal of foodstuffs (Milhalick and Baccaro, 1986). For these advantages, PC is the material of choice for returnable bottles.

Common bottle sizes are 3 and 5 gallons. The rectangular 3-gallon, an exclusive bottle design of McKesson Water Products Company, is stackable.

Interlocking grooves on the sides allow the user to side-stack five 3-gallon bottles vertically to save space. These bottles, which have identical non threaded finishes, are sealed by press-on polyethylene caps. The 5-gallon round bottles is the basic bottle for the water bottling industry.

Another popular PC bottle group includes bottles which are available for sale to consumers at water-vending machine locations. These bottles are designed with built-in handle, screw cap, and dispensing valve. Available sizes are 1, 2, 3, and 5 gallons. The rectangular 2-gallon container with its dispensing valve is a popular portable container. The 3 and 5-gallon round bottles have integral handles and softer rib design to simplify handling.

The two most important quality requirements for the returnable bottles are aesthetic and sanitary conditions.
Aesthetic Conditions
To optimize the bottle appearance, emphasis on the handling of the raw resin and the regrind is essential. PC resin will absorb moisture upon exposure to the air. To prevent silvery streaks, "chicken tracks", or air bubbles, the resin feed stock has to be dried to the manufacturer's specification. Regrind, the reusable plastic which is obtained from the trimming of the flash from the pinch-off, handle, and neck finish areas, should also be kept dry.

Both resin and regrind have to be processed within 20 minutes to be effective. When using regrind in processing, the maximum allowable ratio should not be more than 30 percent regrind to 70 percent virgin resin (General Electric, 1992).

The manual trimming (around the spout and along the mold line) and the logo silk- screening (on the side wall) should be done carefully.

Silk Screen Processing
Decorating process employing screen stencils. Silk screens were actually once used in the methodology, hence, the name commonly used for the process. Now, however, the process uses stencil screens of various materials-especially fine wire mesh (The Packaging Institute International, 1988).

Blemishes are not permitted on the sealing surface of the bottle spout to prevent leakage when full bottles are stored (3 and 5 gallon bottles are stored sideways). A smooth spout helps the press-on cap to seal properly.

Delamination, degradation, and weak spots can be prevented by periodic cleaning of the blow molding machine and mold (General Electric, 1990).

In the rectangular 2 and 3 gallon bottles, where thin wall corners are a problem, the machine's mold vent should be clean at all times. An unobstructed vent will permit uniform plastic stretch at the bottle corners during blow molding.

Sanitary Condition
To satisfy the federal Food and Drug Administration's (FDA) requirement for a food product, all good manufacturing practices outlined in the Code of Federal Regulations have to be followed. The returnable bottles have to be cleaned and sanitized before the drinking water is bottled.

Bottles returned to the bottling plant are sorted before entering the washer. Only the serviceable ones are reused. Others, with physical damages or hardened foreign matter adhering to bottle walls, are discarded to the recycling center. Inspections are performed at both ends of the washer.

At the entrance, an electronic vision system and inspection personnel examine both the interior and exterior bottle walls for any foreign matter. While the electronic "sniffer" detects bottles which were contaminated with hydrocarbons (gasoline or petroleum products), alcohol, or nitrogen-based contaminants (ammonia or solvents'. On the exit side of the washer, the vision system and the inspection personnel again scrutinize the bottles before allowing them to enter the bottling room. Daily calibration of all inspection instruments is necessary to ensure a constant supply of clean, sanitized bottles.

At the bottle washer, washing solutions from 49C (120F) to 60C (140F) contact the bottles for five minutes. Although this high temperature is required for effective cleaning, it will shorten the bottle's life. Tests show that continuous exposure in hot 65C (149F) water causes gradual embrittlement in PC bottles (Machine Design, 1991). Hourly verification of the washer solution's temperature, therefore, is essential during bottling. Beside the temperature, the concentration of the cleaner and sanitizer solutions are also tested hourly (bottles are washed with the cleaning solution first, then rinsed with the sanitizer) to insure a thorough washing cycle.

Container and closure bacteriological tests are conducted quarterly to verify the clean, sanitized condition of the washed bottles and their caps. Randomly selected bottles and caps are rinsed with sterile water. A measured sample of this solution is tested for the presence of bacteria.

Packaging the PC Water Bottles
The filled PC water bottles are packaged by two methods: crate and rack systems. In the crate system, bottles enter the bottling room in their individual plastic crates. The crated full bottles are stacked sideways onto wood pallets for transport. A standard load is a 30-crate bottle pallet (two rows of five-crates high, three-crates wide). The polyvinyl crate is designed to cradle the bottle throughout the distribution cycle. The bottle separates from its crate during two occasions: in the washing operation and in the dispensing mode when the bottle is installed onto the water cooler. Crates require maintenance and inspection on the return trip and they occupy additional space in transport and in storage. As a result, modern bottling plants are converting to the rack system.

In the rack system, bottles are filled without crates, then inserted into metal racks for distribution. A standard size rack carries 30 bottles in the same side-stacking arrangement as in a full pallet of crated bottles. The rack system eliminates the need for plastic crates and simplifies handling. However, a glossy and durable coating will be required and maintained on the rack's supporting bars to reduce scratches on the bottles and to withstand the outdoor exposure. Caring for the racks includes special truckbeds to protect the rack and the periodic checking for the rack's structural integrity.