星期四, 七月 2, 2015
With the release of a new research report, manufacturers, processors and end users of PET (Polyethylene terephthalate) have a credible cost and time saving alternative to traditional solvent-based Intrinsic Viscosity (IV) testing. The report, by Plastic Technologies, Inc. (PTI), concludes that IV measurement performed by a solvent-free melt process provides results consistent with solvent-based viscometer testing for package evaluation.
The melt process costs less, can be completed faster, requires no special operator training and eliminates the safety and environmental hazards of solvent-based testing. In practice, this means that IV testing can be done on-site in real time instead of shipping samples to a lab and not knowing results for two weeks or more. Therefore, the assurance that its results are comparable with the older method opens significant efficiency options for producers of PET bottle pre-forms as well as resin manufacturers and end users.
Why IV Testing
The polymer chain length in PET determines the molecular weight of the material and with it the physical properties that make PET such a useful packaging material. Intrinsic Viscosity (IV) is a measure of the polymer’s molecular weight and therefore reflects the material’s melting point, crystallinity and tensile strength. The IV is used as part of the specification to select the right grade of PET for a particular application, and is measured at various points of the supply chain. Material is tested at all stages:
• Recyclers who want to control their process
• Chemical companies that draw off samples from their polymerization towers
• Processors who want to control their process and quality of finished goods
Twenty-four hour process lines and recyclers routinely test incoming resin, the dryers and extruders and finished product. With such a variety of applications and measurement locations, there has long been a need for an easy, safe, accurate and repeatable measurement method.
Intrinsic Viscosity Measurement Methods Historically, IV has been measured by either viscometer or a free blowing method, with the melt process (developed by ICI and commercialized by AMETEK Test & Calibration Instruments) added more recently.
• Viscometer methods involving the dissolution of the polymer in halogenated solvents are costly both in terms of solvent purchase and disposal. The solvents used to dissolve PET are extremely dangerous and highly toxic and include:
Phenol/tetrachlorethane, Phenol/Dichlorobenzene, Dichloroacetic Acid, TrifloroaceticAcid and ortho-Chloropehnol. The solution viscosity method requires relatively simple equipment – a glass capillary viscometer. However expensive laboratory facilities and high level laboratory staff are required to facilitate the use of dangerous solvents. Ongoing consumable costs of solvents and disposal costs are another disadvantage to the solution method. Cost and practicality of this method are seen as prohibitive for PET processors.
• The free blowing method does not actually measure the IV of the material itself. According to resin suppliers and manufacturers of injection and blow moulers, it is an indirect method showing the ability of a preform to blow to a certain size. These and other characteristics change with time and environmental conditions, limiting the usefulness of the data.
• The unique solvent-free method is based on the extrusion of the polymer through a die using a dedicated instrument. First the PET is dried under vacuum. It is then placed into a sealed barrel with a die at the base and forced through the narrow die at 295° C for 17 minutes. The free space in the barrel is filled with nitrogen gas to ensure there is no degradation due to moisture.
After extrusion, the instrument’s software performs a linear regression to obtain the polymer degradation rate and initial melt viscosity. This is a powerful alternative that has been used by the world’s major resin producers and processors for measuring the IV of PET and other hygroscopic polymers. Once the sample is dried, tests can be completed in less than 20 minutes. Routine testing carried out on the shop floor leaves skilled laboratory staff more time for other work. It can also be used in R&D laboratories, technical support departments and in PET plants to monitor the critical stages of the polymerization process.
Benefits of Rapid IV Testing
For preform manufacturers, the benefits of rapid, economical IV testing are numerous. The quality of incoming material can quickly be assessed to avoid expensive downtime in production. Manufacturers can determine if their preforms and bottles are meeting performance parameters. Similarly, evaluating blends of virgin and recycled material can be done on site to allow optimization of raw material costs. Molten polymer can be transferred from the process line to the instrument for results within 20 minutes. Manufacturers can evaluate IV directly from the production dryer prior to processing, which gives an early warning signal of issues with drying. Pre-form IV can be measured to optimize the IV drop across the process, which can be related to acetaldehyde and the dangers it poses if it migrates from the PET to the beverage.
• The free blowing method does not actually measure the IV of the material itself. According to resin suppliers and manufacturers of injection and blow moulers, it is an indirect method showing the ability of a preform to blow to a certain size. These and other characteristics change with time and environmental conditions, limiting the usefulness of the data.
• The unique solvent-free method is based on the extrusion of the polymer through a die using a dedicated instrument. First the PET is dried under vacuum. It is then placed into a sealed barrel with a die at Resin can be tested before unloading to save time or to check regrind or resin mixtures. The melt test instrument displays trends, so comparisons between suppliers and batches can be made with full statistical analysis. Processors also can monitor gradual changes in dryer operation by comparing the laboratory-dried incoming resin with plant-dried samples. This allows action to be taken before problems become serious. The economic risks of using unsatisfactory material are significant. Consider a processor with a daily production output of 20,000 preforms, each weighing 48g. If the entire day’s output is lost due to ‘out of specification’ or insufficiently dried material, 960 Kg (9.6 tons) of raw material is wasted and significant income is lost from not being able to sell the 20,000 preforms. Avoiding losses like this rapidly covers the purchase cost of a melt test instrument.
“Both systems may be relied upon”
The ability to rapidly and safely measure IV to control process parameters has been very well received by PET processors, and several major brands now specify melt testing as standard for new pre-form manufacturing lines. We expect that trend to grow as the industry learns about the PTI test showing equivalent results from the old and new methods. The report “Comparison of PETPlus Data vs Solution IV Data” concludes:
“Use of either methodology for the measurement of IV in PET gives good, consistent agreement… In the end, both systems provide consistent, reproducible results that may be relied upon for manufacturing process monitoring.”