DUSSELDORF, GERMANY - PEN homopolymer can be successfully injection stretch blow molded with little or no changes to standard PET molds after careful selection of the processing conditions and molecular weight of the material, according to results of an Amoco Chemical Co. research project. Because it can offer superior performance properties, polyethylene naphthalate could become an important bottle resin that competes against PET, packaging industry observers believe. The key drawback is the cost of PEN.
PEN development has been hampered by a lack of adequate supplies of a key precursor, naphthalate dicarboxylate, or NDC.
Amoco has completed construction of the first commercial-scale NDC plant in Decatur, Ala., with initial capacity of about 54 million pounds a year.
That production will dramatically cut the price of NDC, according to James Tibbitt, research manager at Amoco Chemicals Intermediates Business Group in Naperville, Ill. Mechanical properties of bottles are said to improve with increased NDC content.
Tibbitt presented results of Amoco's research project in Dusseldorf Oct. 3 at a conference, Plastics High Performance Packaging. The laboratory study was done at Plastic Technologies Inc. in Holland, Ohio.
Researchers tested PEN homopolymer and three PET/PENblends with varying degrees of NDC, by freeblowing preforms into unheated molds. The process has been used before to generate basic data about PET homopolymer, according to the paper.
``Our intent was to provide starting-point data for the packaging engineer's use when designing preforms and containers from naphthalate polyesters,'' the paper said.
Among other findings:
PEN does not have as broad a window of processing conditions at PET. For example, PEN's stretch ratio is more sensitive to blowing conditions. Stretch ratio varies with both the composition and molecular weight. Also, optimal blow temperature increases with the level of NDC content.
PEN offers improved barrier and thermal performance.
If the proper preform temperature is selected, high-molecular-weight naphthalate polyesters develop a level of crystallinity equal to or greater than that of PET during freeblowing.
Modulus and tensile strength of PEN and high-naphthalate-content freeblow bubbles are far superior to those of PET bubbles. Mechanical properties of low-level naphthalate copolymers and blends show incremental improvement over PET.