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The Importance of Intrinsic Viscosity Measurement

Learn about intrinsic viscosity (IV) measurement methods, uses of PET, resin processors, final product testing, recycling and the intrinsic viscosity measurement instruments.
The Importance of Intrinsic Viscosity Measurement
By: Toby Rogers, Director of Lloyd Instruments Materials Testing Equipment and Chatillon Force Testing Instruments


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 polymers 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 from the R&D laboratories who develop new polymers and chemical companies who draw off samples from their polymerisation towers to the 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, the need for an easy, safe, accurate and repeatable measurement method is of paramount importance. 

Intrinsic Viscosity (IV) Measurement Methods 
One technique that fulfils these needs uses a unique solvent-free method based on the extrusion of the polymer through a die using a dedicated instrument such as the Davenport PETPlus Intrinsic Viscosity Measurement Instrument. This is a powerful alternative to both solvent-based techniques and the free blowing method. Methods involving the dissolution of the polymer in halogenated solvents are costly both in terms of solvent purchase and disposal. 

Other factors such as health and safety requirements for working with solvents, consideration for the environment and the need for specialist personnel to carry out these tests (which must be carried out in a laboratory) add to the high popularity of the solvent-free method, which has been used by the world’s major resin producers and processors as a tried and tested method for measuring the IV of PET and other hygroscopic polymers. 

The free blowing method does not measure the IV of the material itself. According to the resin suppliers and manufacturers of injection and blow moulders, 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, whereas the IV of the material does not. The PETPlus Intrinsic Viscosity Measurement Instrument is robust and simple enough for use on the shop floor. 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 polymerisation process.  

Uses of PET
By far the largest application for PET is bottles for colas and carbonated drinks, mineral water and edible oils, cosmetics and personal care products. High IV PET is used for these products. Polyester used in the fibre and textile industries is the amorphous or low IV form of PET. Out of specification high IV PET waste can be recycled or reused to make fibre or strapping tape. Amorphous and crystalline grades of PET are extruded as sheet or film mostly for the packaging industry. The application dictates the IV. The higher the tensile, burst , impact  or temperature specification, the higher the IV. Applications include strapping tape, photographic film, video tape and food trays. PET sheet is thermoformed into trays for freezer to oven or microwave meals, pre-prepared snacks or airline meals and also packaging for medical devices. The medical devices themselves may also be made from PET or similar polyesters, injection moulded into implants and other devices. 

Resin Processors
PET is difficult to process correctly so international processors take no risks and check their process throughout. Resin can be tested before unloading to save time or to check regrind or resin mixtures. The PETPlus allows trends in IV measurements to be displayed, making comparisons between suppliers and batches simple and quick with full statistical analysis and search facilities. Processors can also monitor gradual changes in dryer operation by comparing the laboratory dried incoming resin with plant-dried samples, allowing action to be taken before problems become serious. A polymer chip transfer vessel is available for the PETPlus which can store up to 10 dry samples from the dryer for several days. The drop in IV is proportional to moisture content which can be derived from the results. 

The PETPlus test method is a physical test method which mimics the extrusion process. Since all polymers degrade during processing, the degradation rate is quoted at the end of every test. The lower the degradation rate, the more consistently the resin will process. The solvent method does not offer this capability. Molten polymer can be transferred from the process line to the PETPlus for results within 20 minutes.

The economics of using unsatisfactory material are significant. Consider a processor with a daily production output of 20,000 preforms weighing 48g. If the entire day’s output is lost due to ‘out of specification’ or insufficiently dried material, then 960 Kg (9.6 tonnes) of raw material is wasted, in addition to the loss of income from not being able to sell the 20,000 preforms. Avoiding losses like this rapidly covers the cost of purchase of a PETPlus instrument, and by being used by non-specialist staff at the point of use, saves on expensive and time-consuming laboratory or test-house expenses.

Final Product Testing
Although resin of the required IV should be used at the processing stage, the processing itself can affect the IV of the final product. Therefore the final product should also be tested in case adjustments need to be made to the processing conditions. Virtually any product can be ground up, dried and then tested to measure the drop in IV after processing. An industrial grinder can be used to reduce the product to small pieces, and then a polymer grinder can be used to reduce it either to granules or powder ready for drying. There are two standard drying methods: 150˚C for 16-24 hours for PET granules between 0.5 and 1 mm diameter or 170˚C for 50 minutes for powdered PET. The IV can then be measured using the PETPlus.

Waste has become a thing of the past for many processors. After testing to find the Ideal mixture, process waste can be re-used or a cheaper recycled resin added before further processing. For processors who produce more waste than they can re-use or for waste from critical applications, such as the medical device industry, the waste can be ground up and re-extruded into granules for non critical applications such as strapping tape. Recyclers can grade the incoming material and test the resulting product using the PETPlus.

PETPlus Intrinsic Viscosity Measurement Instrument
The PETPlus Intrinsic Viscosity Measurement Instrument uses a well-established unique, solvent-free method for routine measurement of PET. 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 instrument offers an IV measurement range of 0.43-1.43 depending on die selection. The PETPlus also measures the degradation factor and provides the mean and standard deviation for the batch. It can be operated in stand-alone mode or used with the powerful data analysis software package NEXYGENPlus PET dedicated for IV measurement. 

The PETPlus has common electronics and software used with other Lloyd Instruments material test machines providing the perfect partner in the laboratory with such benefits as operator familiarity, ease of use and integration of results from both the IV test instrument and tensile test machines.