The work by Plastic Technologies Inc. of Holland, Ohio, looks at preforms for both 20-ounce and 2-liter bottles stored for more than a year.
The bottom line is that PET preforms, because they absorb moisture while being stored, create more problems as time pass, the research and development firm said.
New preforms have close to 0 parts per million of moisture, but preforms stored over time or in what PTI calls "extremely humid conditions" can contain up to 10,000 ppm of moisture, PTI said in a new white paper examining the issue.
PTI studied preforms with 3,000 ppm, which is typical in preforms that are stored from three to six months in moderate humidity conditions.
"Our plant support personnel were frequently being asked what storage duration was acceptable for preforms and what problems can present themselves during longer storage periods," Sumit Mukherjee, PTI chief technology officer, said in a statement.
The company's goal was to lower the moisture level to 200 ppm with "relatively low temperature to minimize preform dimensional deformation" using vacuum oven drying.
Research found that this approach takes about 50 days to lower the moisture content using 22° C (71.6° F) for the first 15 days and then increasing to 40° C (104° F). That's not a viable time frame, PTI said.
There also is not equipment available to dry large quantities of preforms in a vacuum oven simultaneously. Using temperatures higher than 40° C also might deform preforms that would then cause blow molding problems.
PTI undertook the research after frequently receiving questions about how long preforms can be stored.
"The older the preform, the more difficult it will be to process so that the container meets performance criteria," PTI said in its white paper. "The time of year the preforms were injection molded, along with the environmental conditions in which they are kept, also will impact performance."
Buyers might think they are making a good economic decision by buying and storing preforms in larger quantities to save money, PTI said.
But, it added, "the downside of this strategy is that older preforms do not process the same. This means that the blown containers don't meet performance specs unless the processing conditions are tweaked sufficiently or have a reasonable factor of safety."
Over-engineering preforms to withstand the rigors of storage also does not make sense because of the added weight or orientation needed compared to when preforms is used in a timely manner, the white paper reports.
Moisture in preforms, PTI said, "acts as a plasticizer allowing for more stretching and less strain hardening." The added moisture also allows preforms to absorb more heat during processing.
The work also found that larger-diameter preforms are impacted more by moisture than smaller-diameter units.
PTI's white paper, "Preforms: what you need to know about storage time/conditions," is available at https://s27935.pcdn.co/wp-content/uploads/2019/07/The-Impact-of-Time-and-Environmental-Conditions-on-Inventoried-Preforms-06.28.2019.pdf.
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