Polymers and plastics

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Aubrey Bond
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Determining extrusion and die swell properties of polymers Introduction Traditional methods of measuring viscosities of polymer melts include melt flow indexing (MFI) and moving die rheometers.

2 Determining extrusion and die swell properties of polymers Introduction Traditional methods of measuring viscosities of polymer melts include melt flow indexing (MFI) and moving die rheometers. However, these units only give low shear rate information. The Rosand capillary rheometer can measure across a broad range of shear rates and is able to simulate the high shear rates seen during extrusion processes. The polymer s extrusion characteristics can therefore be accurately evaluated, eliminating the need for many extrusion trials. Common problems experienced during extrusion include melt fracture/ sharkskinning, poor finish due to too much work input to the melt and excessive die swell. All of these problems can be identified with the capillary rheometer and eliminated by good formulation. This application note deals with the problems of die swell and poor finish. For further information on melt fracture testing, see the application note Eliminating Melt Fracture and Flow Instabilities. In this test, the polymer melt is subjected to a table of shear rates giving equilibrium viscometry data at each step. Comparisons of flow curves and die swell results can be a valuable tool for determining extrusion differences between samples. The die swell properties can be directly measured with a laser die swell unit, giving uni or biaxial dimensions of the extrudate at an adjustable height below the die. The die swell measured on the rheometer will therefore be an accurate indicator of the degree of die swell that could be expected during extrusion. Interpretation The results show that Sample A has a lower shear viscosity than Sample B, which will allow the material to fill small mould apertures giving a better detail definition. Discussion The die swell results show that at high shear rates Sample A displayed almost twice the die swell of Sample B, and therefore may produce an enlarged profile during extrusion. The solution to the die swell problem may be two fold. Either the formulation is adjusted so that it displays properties similar to those of sample B, or it is extruded at a lower shear rate to minimise the swell. Excessive die swell can cause a large number of part rejections and hence a considerable waste of polymer compound. Parts such as UPVC double-glazing frames, guttering and trunking need to be able to fit together tightly and so accurate dimensions are required. Polymers Die swell during polymer extrusion can vary from batch to batch in their molecular weight and molecular weight distribution. These differences might not be obvious if the quality control test used is a melt flow index, which only tests at low shear rates. The Rosand rheometers are able to simulate actual extrusion shear rates and measure the die swell by means of a laser micrometer. This is shown opposite. The polymer batches can then be blended to give acceptable properties and reduce waste. 1 Bohlin application note MRK624-01

3 Conclusion The Rosand Capillary Rheometer can be used in conjunction with the lab and pilot scale extruders to formulate high quality polymer compound melts, and determine the acceptability of batches before they are used in production. Measurement Conditions Sample: PVC Polymer Dies:16 mm x 1 mm x 180º Transducers: 10 KPSI Stage 1: 2 MPa / 50mm.min -1 hold for 3 minutes, Stage 2: 2 MPa / 50mm.min -1 hold for 6 minutes. Table of shears Range: 2-5,000 s -1, 8 steps / up / log Temperature: 180 C (or normal extrusion temperature) By volume: 60 samples/cm3, 0.1 mins max interval, no filter Pressure deviation: 0.5%, Window: 6 samples, Average: 4 samples, max samples: no limit, trip: 90%. Malvern Instruments Ltd Sales and service centers in over 50 countries for details visit more information at 2 Bohlin application note MRK624-01

4 Determining extrusion performance of rubbers Introduction When a rubber is extruded to give a certain section, it can sometimes exhibit die swell or melt fracture resulting in a poorly dimensioned part or an unattractive rough surface. The moving die rheometer and Mooney viscometer give limited information and only show a low shear rate response. However, the Rosand capillary rheometer is able to simulate the whole range of shear rates seen in extrusion. The Rosand RH7 can obtain very high shear rates due to its rigid H-frame design, which enables forces of up to 100KN to be measured. Extrusion shear rates can exceed 104s-1 and the low shear viscosity can be very different than that at high shear. Interpretation The results show that Sample A has a lower viscosity than Sample B and so might extrude more easily. By using a capillary (long) die and an orifice (zero length) die, the extensional and shear viscosity properties can be separated. The shear viscosity indicates flow properties into moulds and in pumping conditions. The extensional viscosity may indicate whether the rubber is likely to give a different shape section in thermoforming or blow moulding, etc. The extensional properties can be evaluated by plotting either the P0 value (pressure at the orifice die), or the calculated extensional viscosity. The effect of screw and barrel temperatures on the viscosity during the extrusion process can be studied by running the shear viscosity test at various temperatures. This allows us to determine the acceptable temperature limits for the process. Initially on heating, a rubber softens as could be expected. However, many rubbers become more viscous at higher temperatures due to the acceleration of crosslinking and other reactions. The optimum processing temperature and shear rate range may therefore be identified by studying the shear and extensional viscosities over a range of temperatures. The Rosand rheometers can also be configured to measure die swell with a laser micrometer system attached to the software. For more information on this please see note Determining Extrusion Performance of Rubbers. Conclusion The Rosand Capillary Rheometer can be used in conjunction with the lab and pilot scale extruders to formulate high quality polymer compounds. If the rheological data of a product can be correlated with it's performance in an extruder, any unacceptable formulations can be eliminated without the need for extrusion trials. This dramatically cuts down on wasted time and product. Results 1 Bohlin application note MRK625-01

5 Measurement Conditions Sample: EPDM Rubber Dies: 8 mm x 1 mm x 180º & 0.25 mm x 1mm x 180º Transducers: 30 KPSI 10 KPSI Stage 1: 2 MPa Pleft, 1MPa Pright / 50mm.min-1 hold for 5 minutes, Stage 2: 2 MPa Pleft, 1MPa Pright / 50mm.min-1 hold for 6 minutes. Extrusion at different temperatures Table of shears Range: 2-1,500 s -1, 8 steps / up / log Temperature: 210 C (or normal extrusion temperature) By volume: 60 samples/cm3, 0.1 mins max interval, no filter Pressure deviation: 0.5%, Window: 6 samples, Average: 4 samples, max samples: no limit, trip: 90%. Malvern Instruments Ltd Sales and service centers in over 50 countries for details visit more information at 2 Bohlin application note MRK625-01

$Eliminating melt fracture and flow instabilities Introduction A common problem encountered when extruding plastics is that they show melt fracture or shark-skinning.$

6 Eliminating melt fracture and flow instabilities Introduction A common problem encountered when extruding plastics is that they show melt fracture or shark-skinning. This is seen as a jagged surface on the extrudate. The problem is caused by the extrudate first sticking to the die wall, then slipping as the pressure builds. When the shear rate is increased the flow becomes more plug-like and the surfaces will become smooth again. Sometimes a simple adjustment of shear rate range used in the extrusion process can create or dispose of shark-skinning problems. Discussion By plotting pressure vs. shear rate the onset and cessation of melt fracture can be seen. The melt fracture can be eliminated by the use of one or two routes. Either the extrusion shear rate can be altered so that it is outside the melt fracture region, or the formulation can be modified to change the melt fracture region. It is worth noting that melt fracture is dependant on shear stress rather than shear rate, so the correlation between rheometer and process may not be exact. Measurement conditions Sample: Polyethylene Dies: 16 mm x 1 mm x 180º Transducers: 10 KPSI Stage 1: 2Pa/50mm.min-1 hold for 4 minutes, Stage 2: 2Pa/50mm.min -1 hold for 5 minutes. Shear ramp Range: ,000 s -1, autorange ramp rate, Temperature: 190ºC or extrusion temperature By speed (v6 mode), Variation on standard rate=1, no filter, trip: 90%. Results sampled data Conclusion The Rosand Capillary Rheometer can be used to help formulate polymer compounds that will not give melt fracture problems during extrusion, this can help to eliminate many pilot scale trials. The rheometer only requires small sample volumes (c.50mls) making it ideal for research and development testing. Malvern Instruments Ltd Sales and service centers in over 50 countries for details visit more information at 1 Bohlin application note MRK619-01

7 Haul-off, melt strength and fibre spinning tests Introduction Plastics compounding often gives problems with the melt breaking and needing to be repetitively fed through the pelletiser. Similarly, the fibres can break during fibre spinning, especially when a high draw-down ratio is required. The Rosand capillary rheometers can be used to measure the melt strength of the extrudate, simulate fibre spinning and calculate the maximum draw ratio of a compound or polymer. The rheometer requires only small amounts of test material and minimal cleaning, making it an attractive alternative to pilot scale extruder equipment. Figure 1. Three system configurations are available for this work: 1) Rheometer with a haul-off and a balance, 2) as 1) but also with a laser die swell attached (allowing measurement of extensional viscosity), 3) as 1) but also with an extrudate oven (allowing measurement of maximum draw ratio). NB - it is possible to use both the extrudate oven and laser die swell simultaneously if needed. Interpretation This test (Figure 1.) shows the melt strength of the polymer. The results show Sample A breaking at around 150 m/min, whereas Sample B was able to be hauled at over 500 m/min. Where long drops are used for the fibre spinning, the haul-off force at the break point may be more important to the user than the haul-off speed. Haul off Rheometer Rheometer barrel Extrudate Oven (optional) Laser Die Swell Unit (optional) PTFE wheel Balance 1 Bohlin application note MRK620-01

8 The maximum draw ratio (hauled length / original extruded length) may be determined when an oven is used to keep the extrudate hot. If an oven is not present, the fibre will cool quickly as it leaves the die and draw is restricted. In this case, the fibre is likely to break at the die. The use of an oven allows the sample to be freely drawn giving a very fine extrudate. The results also show that Sample A has a higher haul-off force than Sample B and might have a higher extensional viscosity. A comparative value of extensional viscosity can be calculated from the measured stress divided by the extensional rate, the latter being calculated from the change in length over the original length per second. Exten l stress (Pa) = Haul-off force (N) / Extrudate diameter (m 2 ) Extrudate speed(m/s) = piston speed(m/s) x (barrel radius/die radius) 2 moulding, film blowing, etc). The Rosand software is programmed to automatically control the haul-off speed as a table of discrete speed or an accelerating ramp and all variables are recorded. Measurement Conditions Sample: PP Polymer As shown above plus Dies: 20 mm x 2 mm x 180º Transducer:10 KPSI Stage 1: 2Pa / 50mm.min -1 hold for 3 minutes, Stage 2: 2Pa / 50mm.min -1 hold for 6 minutes. Threading Piston speed:10 mm.min -1 Haul-off speed: 2 m.min -1 By volume: 60 samples/ml, 0.1 mins max interval, no filter Pressure deviation: 0.5%, Window: 4 samples, Average: 4 samples, max samples: no limit, trip: 90%. (1) DM Jones et al, Rheol.Acta 26, Exten l rate(1/s) = Haul-off speed(m/s) / Extrudate speed(m/s) Exten l viscosity(ηe, Pa.s) = Exten l stress / Exten l rate The calculations depend on the hauloff angle, the sample showing uniaxial extension, the fibre being isothermal, the extrudate stretching at a constant rate throughout it s length and no localised necking occurring. Consequently, these are comparative methods. However, the results may still be valid (1). Conclusion The Rosand capillary rheometer and haul-off system is a versatile method of analysing small batches of polymers and compounds to determine their suitability for fibre spinning or other extensional viscosity dominated processes (thermoforming, blow During Test Piston speed: 10 mm.min -1 Haul-off speed: m.min -1, 7 steps Diameter axis: 1 Temperature: 190 C (or normal extrusion temperature) Malvern Instruments Ltd Sales and service centers in over 50 countries for details visit more information at 2 Bohlin application note MRK620-01

The zero length die and extensional viscosity measurement Introduction Traditional methods of measuring viscosities of polymer melts include melt flow indexing (MFI) and moving die rheometers,

9 The zero length die and extensional viscosity measurement Introduction Traditional methods of measuring viscosities of polymer melts include melt flow indexing (MFI) and moving die rheometers, however these units only give low shear rate information about shear viscosity, and give no information at all about the extensional properties of the sample. The Rosand capillary rheometer can measure across a broad range of shear rates and is able to simulate the high shear rates seen during extrusion processes. It can also measure the extensional properties directly, which can show how the polymer may behave during processes such as blow moulding and fibre spinning. In this test, the polymer melt is subjected to a table of shear rates giving equilibrium viscometry data at each step. The extensional viscosity can show comparatively how well a sample will respond to processes such as blow moulding or extrusion. P 0 is often plotted as a useful comparison of extensional properties, but it is worth remembering that the P 0 data also encompasses other properties; vortex flow, acceleration and the sample s elastic response. Where: η e = Extensional viscosity n = shear thinning index Pe = Extensional entrance pressure (Pa) γ = shear rate ė = Extensional Shear Rate (s -1 ) Interpretation By using the Bagley correction option within the software, data such as is shown in Figure 1 can be produced. The calculated extensional viscosity curves (by the Cogswell model) are shown for two polymers (2). It is probable that Sample B would show more die swell than Sample A at high shear rates, causing extruded profiles to be larger than expected, which can cause a waste of the polymer compound. The extensional viscosity can influence the bubble shape during a blow moulding process. Conclusion The Rosand Capillary Rheometer can be used in conjunction with the lab and pilot scale extruders to determine the best formulation for the process. Die swell during polymer extrusion 1 Rosand application note MRK623-01

10 Measurement Conditions Sample: PVC Polymer Dies: 16mm x 1mm x 180º and 0.25 mm x 1mm x 180º Transducers: 20KPSI and 5 KPSI Stage 1: 2 MPa Pleft, 1MPa Pright / 50mm.min -1 hold for 3 minutes, Stage 2: 2 MPa Pleft, 1MPa Pright / 50mm.min -1 hold for 6 minutes. Table of shears Range: 2 5,000 s -1, 8 steps / up / log Temperature: 180 C (or normal extrusion temperature) By volume: 60 samples/cm3, 0.1 mins max interval, no filter Pressure deviation: 0.5%, Window: 6 samples, Average: 4 samples, max samples: no limit, trip: 90%. (2) Results are illustrative only Malvern Instruments Ltd Sales and service centers in over 50 countries for details visit more information at 2 Rosand application note MRK623-01

11 Pressure volume temperature (PVT) & Injection moulding tests PVT Testing Measurement Conditions Sample: PP Polymer Dies: PVT die Transducers: 5 KPSI Stage 1: 2Pa / 50mm.min -1 hold for 3 minutes, Stage 2: 2Pa / 50mm.min -1 hold for 6 minutes. PVT Test Details Initial descent speed: 30 mm/min Initial Volume: 20 cm 3 Compression speed: 2 mm/min Record result every: 0.5 min Stop when: 20 MPa reached By volume: 60 samples/cm 3, 0.1 mins max interval, no filter Pressure deviation: 0.5%, Window: 6 samples, Average: 4 samples, max samples: no limit, trip: 90%. Introduction When samples are injection moulded it is very important to know the PVT properties of the melt, as some melts may be more compressible than others. A compressible melt is likely to require slightly more volume to fill the mould, but can then bleed from the injection point after moulding, causing unsightly die drool. Typical results - Pressure Change vs. Percentage Compression for Two Polymer Melt Samples Variations in melt compressibility can also cause parts that should have flat surfaces to be made as convex or concave surfaces. This in turn can cause problems when the parts are to be fitted together. The Rosand Capillary Rheometer is able to measure the PVT characteristics of compounds and so predict their suitability for use, or help to predict the optimum injection moulder settings. The density of the hot polymer melt can be used as an indication to processability when formulating new injection moulding compounds. The rheometer can also measure the high shear rate viscosity characteristics and simulate the injection moulding process. Interpretation The results show that Sample A is much less compressible than Sample B and so will need less volume on The Rosand PVT Testing Kit injection and is probably less likely to give drool after moulding. The compressibility of a melt is likely to be related to its filler content, the polymer chain structure, the polymer s molecular weight, molecular weight distribution and the test temperature. As Sample B is more compressible than Sample A, it would probably need a higher initial injection volume 1 Rosand application note MRK622-01

12 but then a small withdrawal (slight negative pressure) to reduce the die drool. This reduction of pressure on the die contents whilst the article solidifies for a few seconds may also help to reduce swell (causing convex surfaces). If the total weight of polymer in the barrel is measured (weight added initially - weight extruded during precompression and initial descent), the density of the melt can be calculated. Comparing the initial density value (at maximum volume) and the end density value (minimum volume / maximum pressure) can show comparative data for compressibility. Density of Melt = Wt of Polymer / Volume Injection Moulding Testing 4 samples, max samples: no limit, trip: 90%. Discussion In this test, the compounds are sheared at similar rates to those that they would undergo in injection moulding. If the sample is extremely shear thinning, the shear rate data can be corrected using Rabinowitsch, which uses the power law index (n) to adjust for non-newtonian flow through the die. The high shear rate data indicates how the easily the melt will fill small mould apertures and indicates how accurate the detail definition will be. It also shows how quickly the overall process may be carried out (within the limitations of the injection moulder). Sample A has a lower viscosity at all measured shears, indicating that it will be easily moulded and will fill mould details quickly. NB - if the melt viscosity is too low, bleeding between the two touching mould faces may also be a problem. Conclusion The Rosand Capillary Rheometer can be used in conjunction with the lab and pilot scale injection moulders to formulate high quality polymer compounds. PVT testing can show how to optimise the injection moulding settings, whilst high shear rheometry can simulate the actual injection process. Measurement Conditions Sample: PP Polymer Dies: 16 x 1 x 180º and 0.25 x 1 x 180º Transducers: 10 KPSI, 1.5 KPSI Stage 1: 2 MPa Pleft, 1MPa Pright / 50mm.min -1 hold for 3 minutes, Stage 2: 2 MPa Pleft, 1MPa Pright / 50mm.min -1 hold for 6 minutes. Test Details η c (kpas) Corrected Shear Rate (s -1 ) Shear rate (s -1 ) Viscosity During Injection Moulding Sample A Sample B Table of Shears s -1, 8 steps Temperature: 190ºC By volume: 60 samples/cm 3, 0.1 mins max interval, no filter Pressure deviation: 0.5%, Window: 6 samples, Average: Malvern Instruments Ltd Sales and service centers in over 50 countries for details visit more information at 2 Rosand application note MRK622-01

13 Low speed degradation testing Measurement Conditions Sample: PP Polymer Dies: 16 x 1 x 180º, Transducers: 5 KPSI, Stage 1: 2 MPa / 50 mm.min -1 hold for 3 minutes, Stage 2: 2 MPa / 50 mm. min -1 hold for 6 minutes. Test Details Low speed degradation test: 2 mm. min -1, samples every 30 seconds Temperature: 230ºC (or appropriate) By volume: 60 samples/ml, 0.1 mins max interval, no filter Pressure deviation: 0.5%, Window: 6 samples, Average: 4 samples, max samples: no limit, trip: 90%. Discussion In this test, the polymer is held at a high temperature and sheared at a low constant shear rate (60s -1 for a 15mm bore). As the material degrades, the shear stress measured at the transducer falls gradually τ (Pa) Time (s) Shear rate (s -1 ) Stress vs Degradation Time at 230ºC It can be seen that Sample A has better thermal stability than Sample B. Polymers degrade due to one (or a combination) of several processes. These are mainly free radical and monomer reactions, or oxidation (when air is present). Oxidation is not likely to play a large part in this test, as the air is driven out of the melt during the equilibrium time and so this simulates more closely the packing stage in an extruder or injection moulder. Sample A Sample B Conclusion The Rosand Capillary Rheometer can be used to simulate thermal degradation processing. Evaluations of sample compounds can be made to optimise the dosing of anti-oxidants, lubricants and thermal stabilisers. Malvern Instruments Ltd Sales and service centers in over 50 countries for details visit more information at 1 Rosand application note MRK621-01

14 Malvern Instruments Limited Sales and service centers in over 50 countries, for details visit Advanced technology made simple distributor details Malvern Instruments is part of Spectris plc, the Precision Instrumentation and Controls Company. MRK639-01

RHEOLOGICAL PROPERTIES ROSAND RH7 & RH10 RHEOLOGICAL INSTRUMENTS BACKED WITH RHEOLOGICAL EXPERIENCE

RHEOLOGICAL PROPERTIES ROSAND RH7 & RH10 RHEOLOGICAL INSTRUMENTS BACKED WITH RHEOLOGICAL EXPERIENCE ADVANCED, POWERFUL FLOOR STANDING CAPILLARY RHEOMETERS FOR RESEARCH AND PRODUCT DEVELOPMENT Since its