Polyethylene terephthalate (PET) resin is the most common plastic used in the bottled water industry today. Water bottles produced from PET are lightweight, shatter-resistant and offer great clarity. Bottles produced with PET resin also provide exceptional package design capabilities.

PET resin is produced and sold as a chip. Most resin is bought and sold commercially in rail-car quantities and then processed through injection molding into a preform, where wall thickness and acetaldehyde (AA) regeneration are key factors.

The preform is then stretch-blow-molded into a bottle. The natural stretch ratio and the biaxial orientation are key factors in producing the highest-quality bottle possible.

PET resin is produced by combining purified terephthalic acid (PTA) and ethylene glycol (EG). Both of these chemicals are derivatives of crude oil and natural gas (diagram, this page). PET resins for bottled water applications are produced with low AA in the chip and are designed for low regeneration rates and low intrinsic viscosity (IV) for maximum throughput rates in injection molding.

Goal: Impart no taste
The bottled water market requires that the container must impart no taste to its contents. This is accomplished by control and minimization of the AA level generated during the injection molding process of bottle creation.

AA (symbol: CH₃CHO) is a colorless, volatile liquid with a distinctive fruity odor. It is generated in small amounts during the melt processing of PET resins. The acetaldehyde content of PET for water-bottle applications is stringently controlled by the resin manufacturer and is typically less than 1 part per million (ppm).

PET resin is converted into bottles by the injection molding of preforms, followed by biaxial orientation (stretching) of these preforms, either in continuous single-stage or discontinuous two-stage processes of blow molding.

Wall thickness
Secondary to minimizing AA level, lower intrinsic viscosity (IV) levels in PET resin provide advantages to the bottle itself and to converters and blow molders.

Non-carbonated water bottles can use grades of PET with lower IV levels. These lower IV resins generate less AA and need less orientation because the bottle is not under pressure from carbonation. This allows the designer to use a thinner wall on preforms, which helps reduce the thermal history of the resins, resulting in faster injection molding cycle times.

All these considerations have to be taken into account during the development of the PET resins. The properties inherent in the resins present the injection molder with a “broad window” for satisfying the AA regeneration, IV drop and clarity requirements.

Co-polymers vs. taste issue
Co-polymers using purified isophthalic acid (PIA) are typically used for water bottle applications. The crystalline melt point of PET varies inversely with PIA content, meaning PIA modification can yield resins that process at lower temperatures, thereby saving energy and shortening the time needed for cooling.

Lower temperatures also make possible a reduction in AA level generated during bottle production. The lower temperatures reduce the heat degradation of resin, which reduces the AA level created and in turn, reduces the transmission of any undesirable flavors into the contents of PET bottles. Fortified mineral waters and cola beverages are particularly sensitive to tainting.

Injection molding
To obtain the highest quality PET resin water bottles, the injection molding process must be fully optimized. AA is generated by thermal degradation of the PET molecules during melt processing, and AA is minimized by utilizing the mildest possible molding conditions.

PET resin should be injection molded at the lowest extruder stock temperatures, lowest possible shear rates and minimum residence time in the melt phase.

Higher co-polymer content and low IV (0.75 dl/g) PET resins are typically used to achieve these parameters during the injection molding process. The selection of equipment, resin processing conditions and equipment operating conditions can significantly impact the AA level. Operating the extruder at the lowest possible melt temperature while minimizing cycle times (residence times) will minimize AA regeneration in molding PET resins.

Temperature balance
The molten polymer within the mold cavity must be cooled through the maximum crystallization range as quickly as possible. Because the thermal conductivity of PET is relatively low, the heat content in the center of the preform wall is a major contributor to crystallinity problems.

The melt temperature during injection molding has a significant effect on resultant preform clarity.

As supplied, all PET resins are highly crystalline, and it is during the melt phase that this structure must be destroyed. Otherwise, residual zones of crystallinity can act as sites of nucleation during the transition from the melt-filled cavity to the solid preform. In these instances, the crystallization mechanism produces a hazy polymer matrix within the preform and the bottle.

Stretch blow molding
Stretch blow molding typically does not affect the AA in the PET bottles. Critical in the blow molding process is the reheat rate of the resin/preforms to achieve maximum cycle times on high-speed blowing equipment. Therefore, the resin should allow for rapid reheating of the preforms while maintaining good clarity and brightness in the bottle color.

The bottle’s application dictates the required grade of PET and the orientation levels required to obtain the appropriate properties. Different grades of PET have specific ranges of natural stretch ratio (NSR). This is the amount of stretching required before strain hardening occurs (i.e., before biaxial orientation takes place). This is a key property in developing the lightest container with uniform wall thickness and top load strength.

Preform clarity is directly associated with the degree of crystallinity of the PET polymer matrix. Thus, PET is transparent when amorphous (i.e., not crystallized) and opaque when crystallized.

Resolving conflicts
When considering the mechanisms of acetaldehyde formation and crystalline-associated haze, there is clearly a conflict of interest in establishing the optimum molding temperature. The acetaldehyde constraint favors low temperature and the haze aspect higher temperatures.

A similar conflict exists in terms of the optimum resin IV: crystallization rate decreases with increasing IV, yet higher IV resins require increased acetaldehyde formation. All of these considerations have been taken into account during the development of the water-grade PET resins.

The properties inherent in the PET resins present the injection molder with a “broad process window” for satisfying the acetaldehyde and clarity requirements of water bottles.

Water may be the most basic liquid on earth, but when it is in plastic bottles, there is more involved than meets the eye. Through engineering and strict manufacturing control, PET resin has proven that it is an excellent product for manufacturing water bottles.