【正文】 E, PVC, heavily filled 3. Melt Fracture 3. Evidence of melt fracture 051015202530350 50 100 150 200 250T i m e (s )Pressure (MPa)3 0 0 0 / s4. Material degradation Test: Data obtained: Uses: Range of shear rates with wait stages Long die and/or short die Shear and extension varying with time at a range of shear rates Time dependent materials Moisture dependent materials Residence time design Troubleshooting 4. Material degradation 5. Low speed degradation Test: Data obtained: Uses: Constant low shear rate Long die and/or short die Shear and extension varying with time at a constant shear rate Time dependent materials Moisture dependent materials Residence time design Troubleshooting 5. Low speed degradation 6. Stress relaxation Test: Data obtained: Uses: Pressure decay from constant rate Long die and/or short die Decay in pressure over time indicative of elastic behaviour Time dependent materials Highly elastic materials Quality control Thermoforming 6. Stress relaxation 6. Stress relaxation results LDPE with Mg(OH)2 filler 01234567890 20 40 60 80 100 120 140T i m e (s )Pressure (MPa)5 0 % fil l e r0 % fil l e r7. Flow/No flow Test: Data obtained: Uses: Constant shear rate as temperature decreases, single die Minimum flow temperature Process design Material specification 7. Flow/No flow 8. Manual test Test: Data obtained: Uses: After initial heatup, take over manual control amp。 Rotational Rheometers Analysis of Flow Curves Onlinepressure drop ? homogeneity of your sample ? important factor for quality (influence of Mixing, preshear etc.) homogeneous: inhomogeneous: g . t Principle: Ramp in steps Analysis of inhomogeneities Pressure deviations gives approx. average length scale Equilibrium ?t ?t ?V ? mainly used for polymer blends, suspensions... 3 mon corrections in capillary rheometry 1. Bagley (entrance pressure losses) 2. Rabinowitsch (nonNewtonian flow) 3. Wall Slip (nonzero velocity at die wall) Flow curves Measurement Sequence of shear steps measurement : ? v t t Step Rate p app ? g . ? ? app ? Apparent values data has to be corrected. Advantages of Capillary Rheometry Pressure driven Mimics the process (flow through a die/nozzle) Simple to use Robust Simple to interpret Accurate drive system Accurate temperature control *engineer’s point of view Advantages of Capillary Rheometry Wide shear rate range, from 100,000/s Shear and extension Absolute Data (Bagley, Rabinowitsch, wall slip) Density, pressibility Die swell amp。C Polyethylene (HDPE, LDPE, LLDPE) Polypropylene (PP) 2. PVC: ~165180 UPVC and plasticised 3. Engineering Polymers: ~240300176。 ? 填料的作用。熔體溫度升高，粘度下降，會使松弛時間縮短，從而使擠出物外觀得以改善。 ? 1． 3． 2 擠出工藝條件和物料性質(zhì)的影響 ? 給出低密度聚乙烯在不同擠出速度（不同剪切速率）下通過同一個口模時，測得的壓力波動沿口模軸向的分布圖。而適當(dāng)改造入口區(qū)，將入口角減小變?yōu)槔瓤谛蜁r，擠出物外觀有明顯改善；且開始發(fā)生熔體破裂的臨界剪切速率（或臨界剪切應(yīng)力）增高。 ? 1． 3 影響熔體擠出破裂行為的因素 ? 一切能夠影響熔體彈性的因素，都將影響聚合物熔體的擠出破裂行為。 ? 這種過程周而復(fù)始，將造成聚合物熔體在模壁附近“時滑時粘”，表現(xiàn)在擠出物上呈現(xiàn)出竹節(jié)狀或套錐形的有規(guī)畸變。低剪切速率時，熔體流過口模壁，在壁上無滑移，擠出過程正常。主流線斷裂后，應(yīng)力局部下降，又會恢復(fù)穩(wěn)定流動，然后再一次集中彈性形變能，再一次流線斷裂。當(dāng)外力作用速率很大，外界賦予液體的形變能遠(yuǎn)遠(yuǎn)超出液體可承受的極限時，多余能量將以其它形式表現(xiàn)出來，其中產(chǎn)生新表面、消耗表面能是一種形式，即發(fā)生熔體破裂。屬于此類的材料多為線型分子聚合物，如聚丁二烯、乙烯 丙烯共聚物，線型的聚二甲基硅氧烷、聚四氟乙烯等。屬于此類的材料多為帶支鏈或大側(cè)基的聚合物，如聚苯乙烯、丁苯橡膠、支化的聚二甲基硅氧烷等。 ? 熔體的擠出破裂行為：在擠出過程中，當(dāng)熔體剪切速率超過某一臨界剪切速率時，擠出物表面開始出現(xiàn)畸變的現(xiàn)象。該問題的工程學(xué)意義是，當(dāng)工藝過程條件不合適，會造成制品外觀、規(guī)格尺寸及材質(zhì)均一性嚴(yán)重受損，直接影響產(chǎn)品的質(zhì)量和產(chǎn)率，嚴(yán)重時甚至使生產(chǎn)無法進(jìn)行。表觀拉伸黏度定義為： ? dzdzAzFTzREEa /)(/)(11?g? ?? ?IDGLR FFFFzF ????)(? 采用攝影的方法，對穩(wěn)態(tài)的絲條拍照，從照片量取纖維直徑沿軸向的變化數(shù)據(jù) )(/4)(/4 22 zdWzdQz ???? ?? Extensional Flow Analysis 5 melt fracture ? 實際成型加工及流變測量中，物料流動狀態(tài)受諸多因素影響，常常出現(xiàn)不穩(wěn)定流動情形。 176。High Pressure Capillary Rheometry 1 Introduction Polymers are used because: ? They are cheap to form into shape in molten state Therefore, ? We need to understand how they flow when molten also important in: food processing, pharmaceuticals, paints, inks, pastes, slurries etc... Very important in Polymer Processing But, molten polymers are plicated systems... ? Temperature dependent ? Rate dependent ? Time dependent ? Work dependent (and that’s before we add lubricants, fillers, plasticisers, foaming agents etc…!) ? In between a liquid and a solid Influences on Viscosity ? molecular structure of sample ? temperature ? pressure ? time ? shear rate ? ? (T, p, t, g) = g . . Shear viscosity definition g shear rate . ? shear stress ? ? = g . ? shear viscosity ?Viscosity is a measure of resistance of a fluid against the applied shear force. ?Shear viscosity is only one part of Rheology. ?It is the dominant effect for pressure in extruders, injection moulding machines and dies. 1 . E + 0 01 . E + 0 21 . E + 0 41 . E + 0 61 . E + 0 81 . E 0 5 1 . E 0 3 1 . E 0 1 1 . E + 0 1 1 . E + 0 3 1 . E + 0 5S he a r r a t e ( s1)Viscosity (Pa.s)Typical process shear rates Relaxation Coating Free surface Mixing Extrusion Injection moulding Why do we need a highpressure capillary rheometer? The application is important log g/s1 DMA Osc. / Rot. rheometer High pressure capillary rhe