PET is used extensively in the packaging industry, especially for drinks bottles. With the current emphasis on recycling and reducing costs, it is important that the quality of both new and recycled material is measured. One important parameter used to determine the correct grade of PET for a particular application is intrinsic viscosity or inherent viscosity (IV). This is a measure of the polymer’s molecular weight and therefore an indicator of the material’s crystallinity, melting point, and tensile strength.
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Intrinsic Viscosity Measurement Techniques
Traditional methods for measuring IV of hygroscopic polymers involve dissolving the polymer in halogenated solvents such as chlorinated phenols and comparing the flow time of the solution with that of the pure solvent. Concern about the health problems associated with halogenated solvents, combined with their costs both of purchase and disposal, has emphasized the need to use alternative methods in the laboratory wherever possible. A different method for measuring the intrinsic viscosity of PET has been developed, which is a totally solvent-free method for the analysis of hygroscopic polymers, which involves the extruding of a molten polymer through a die using a dedicated instrument. This method mimics the extrusion process, is quicker and easier to use than the solvent method and the whole process can be computer controlled, if required. It has been proven to produce the same results as solvent-based tests.
Solvent-Free PET Intrinsic Viscosity Measurement
Molten PET is extruded through a calibrated die by nitrogen gas under pressure and the LVDT probe movement against time is recorded and displayed. The system measures the optimum flow rate and calculates intrinsic viscosity. The procedure used ensures that reliable, accurate, and repeatable results are easily obtained and checked against standard polymers. In addition to measuring the intrinsic viscosity of the polymer, this technique also measures the degradation factor of the polymer, a parameter that, although important for critical applications such as medical equipment manufacture, cannot be measured using the solvent method.
Benefits of Solvent-Free PET Intrinsic Viscosity Measurement
A dedicated solvent-free PET IV tester requires very little chemical expertise to operate correctly and can be used on-site next to the process line. The sample needs to be tested only once to obtain a value for both the intrinsic viscosity and the degradation factor, and the test takes less than 20 minutes from the dry sample. In contrast to this, the solvent method involves preparing several solutions of varying quantities of polymer and organic solvent, measuring the relative viscosity of each, and extrapolating back to zero concentration of polymer. This is time-consuming and requires trained laboratory staff to formulate the solutions accurately.
Applications of PET Testing
Raw Material for Solvent-Free PET Testing
Solvent-free PET test methods are used for monitoring the IV of incoming raw material, both for pure product and recycled material to ensure that there is no great variance from batch to batch.
Finished Products
Solvent-free PET test methods can also be used to check the injection molding process by grinding the finished product and measuring the IV again. A melt pot designed for use with the IV measurement test instrument enables molten PET to be taken directly off the process line for measurement of IV at various stages in the process. Thus not only does the instrument measure the characteristics of the raw polymer but also the integrity of all stages in the process.
Design & Development of PET Testing
The ability to accurately measure intrinsic viscosity and degradation rate is also important in the design and development of products. A particular example of this is in the PET bottle industry. For both environmental and economic reasons, it is desirable to use as little raw material as possible in the manufacture of the product. Carbonated drinks bottles need to be able to withstand an internal pressure of 100psi. The strength of a bottle is dependent on the IV of the polymer, so if a high IV polymer is used, a bottle of the same strength can be achieved with a thinner preform, using less material and creating less waste. Intrinsic viscosity measurement plays a role in measuring the IV before the bottles are blown and at the research and development stage.
Monitoring The Production Process of PET
The drying of hygroscopic polymers such as PET is an important part of the production process. If the polymer is overdried, the IV increases because the polymer starts polymerizing in an uncontrolled way. On the other hand, processing without drying properly results in an irreversible break in the polymer chain and consequent drop in IV (and therefore strength of the product). The performance of dryers tends to drift and by processing trends in IV measurement over a period of time, correction for drift can be made to the production process before the problem becomes serious.
PETPlus Intrinsic Viscosity Measurement Instrument
Lloyd Instruments’ new Davenport™ PETPlus Intrinsic Viscosity Measurement Instrument is a compact stand-alone instrument that uses the unique solvent-free method described above for routine IV measurement of PET polymers, avoiding both the costs and the environmental hazard of handling and disposal of toxic solvents. Its safe testing method has been utilized in more than 200 units installed in PET manufacturing plants worldwide. The PETPlus also measures the degradation factor and provides the mean and standard deviation for the batch.
Easy Pressure Adjustment for Testing Different Materials
It is supplied with a convenient integral nitrogen regulation valve for easy pressure adjustment when testing different materials. Requiring only power and a pressurized nitrogen supply, the instrument can be operated in stand-alone mode or used in conjunction with ‘NEXYGENPlus PET’, the company’s powerful new dedicated data analysis software package for the IV measurement of PET polymers. A user-friendly training mode is included as a standard to simplify operator instruction.
Testing Polymers at All Stages of the Production Process
Polymers can be tested by the PETPlus at virtually all stages of the production process to provide greater overall control. Tests can be completed in as little as 20 minutes on incoming resin, powder, dry granules, molten polymer, pre-forms, bottles, films and even re-ground material to eliminate scrap. Rapid testing at the different stages can be carried out using ancillary equipment for controlled sample collection and preparation. The instrument’s compact construction occupies significantly less bench space than Lloyd Instruments’ earlier models and makes it easier and quicker to load samples.
Stand Alone or Computer Controlled Configurations
The microprocessor-controlled stand-alone system provides high accuracy measurements, temperature control to + 0.1°C, and storage of up to 600 test results. A backlit LCD displays instructions for the test in a variety of languages as well as the results. An RS-232 interface allows connection to a computer for downloading of results or for fully automated software control using NEXYGENPlus PET.
Sample Preparation Equipment
A comprehensive range of sample preparation equipment including vacuum pumps and polymer dryers are available to ensure correct sample drying conditions for the instrument. By using the Quick Test Polymer Chip Grinder, the material can be ground to a fine powder to ensure drying within just 50 minutes. A Polymer Chip Transfer Vessel is available to carry dried PET directly from the process dryer to the PETPlus instrument for instant testing.
Summary
Quality testing is used throughout the polymer industry to underpin manufacturing growth and efficiency. By seeing a product through its design to completion stages, accurate test analysis with state of the art test equipment ensures only the best quality products reach the customers’ shelves.
This information has been sourced, reviewed and adapted from materials provided by Lloyd Instruments Ltd.
For more information on this source, please visit Lloyd Instruments Ltd.