【正文】 完全不同的流體，可以預見，它們擠出時的彈性松弛行為也完全不同，由此造成口模出口處擠出物的無規(guī)畸變。主流線斷裂后，應力局部下降，又會恢復穩(wěn)定流動，然后再一次集中彈性形變能，再一次流線斷裂。當剪切速率較低時，流動是穩(wěn)定的，死角處的渦流也是穩(wěn)定的，對擠出物不產生影響。當外力作用速率很大，外界賦予液體的形變能遠遠超出液體可承受的極限時，多余能量將以其它形式表現出來，其中產生新表面、消耗表面能是一種形式，即發(fā)生熔體破裂。隨著剪切速率的提高，流變曲線出現大幅度壓力振蕩或剪切速率突變，曲線不連續(xù)，有時使流變測量不能進行 ? 造成熔體破裂現象的機理十分復雜，肯定地說，它與熔體的非線性粘彈性、與分子鏈在剪切流場中的取向和解取向（構象變化及分子鏈松弛的滯后性）、纏結和解纏結及外部工藝條件諸因素有關。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。熔體破裂的特征是先呈現粗糙表面，而后隨著剪切速率的提高逐步出現有規(guī)則畸變，如竹節(jié)狀、螺旋型畸變等。屬于此類的材料多為帶支鏈或大側基的聚合物，如聚苯乙烯、丁苯橡膠、支化的聚二甲基硅氧烷等。 ? 從現象上分，擠出破裂行為可歸為兩類： ? 一類稱 LDPE（低密度聚乙烯）型。 ? 熔體的擠出破裂行為：在擠出過程中，當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畸變的現象。熔體在管壁發(fā)生滑移與此類現象密切相關。該問題的工程學意義是，當工藝過程條件不合適，會造成制品外觀、規(guī)格尺寸及材質均一性嚴重受損，直接影響產品的質量和產率，嚴重時甚至使生產無法進行。一旦超越該臨界值，就會發(fā)生從層流到湍流，從平整到波動，從管壁無滑移到有滑移的轉變，破壞了事先假定的穩(wěn)定流動條件。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據 )(/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，常常出現不穩(wěn)定流動情形。 C 。 176。 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 rheometer Viscosities: (low), middle to highviscous Quantities: Shear and extensional viscosity, wall slip relaxation, PVT, Flow instabilities 101 103 Levelling 104 101 Extrusion, Injection Moulding 100 102 Mixing 103 106 Roll Coating, Spraying Viscosities: low viscous to solidlike Quantities: Shear viscosity, yield, viscoelastic properties, relaxation etc. Nondestructive Oscillarory shear ? rot. rheometer: structural/low shear measurements – highpressure capillary: processing flow behaviour ? Single point test (generates one number) MFI die ( diameter) ? Defined by standards (ISO1133) ? Simple ? Cheap ? Easy to use ? Pressure driven Melt Flow Indexer (MFI) Melt Flow Indexer (MFI) ? Single point test ? Does not generate engineering units (grams per 10 minutes) ? Mainly a shear flow measurement neglects extension ? Very low shear rate test (gapp = MFI) But, 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 MFI test Capillary Rheometer Measure: Pressure drop Piston Capillary die Pressure transducer Polymer melt Set: Piston speed Die Dim’s Capillary rheometry We set: Temperature, piston speed, die geometry We measure: Melt pressure (long amp。 short dies) Giving us: Shear stress (at a range or rates) Extensional stress (at a range or rates) Capillary rheometry Long die: shear Short die: extension 3 Shear viscosity ? 完全發(fā)展區(qū)剪切應力的計算 ? 管壁處 22 rPrL ??? ???L2Pr???LPR2???3 Shear viscosity ? 不可壓縮性流體剪切速率的計算 )zv( r00rz ???? ?g?? ?2rzp11rv0rz0r????????? )rR(zp41v 220r??? ?zp8Rr d r)rR(zp21drvr2Q04220R0rR0 ????????? ???????30Nw R4Q???g ? ???L 2R 3 Shear viscosity Given quantity: piston speed ? wall shear rate Measured quantity: pressure drop ? wall shear stress Entrance pressure drop Shear pressure drop v Measured pressure drop = + P L BARREL PL Pl Pw ENTRANCE LENGTH FULLY DEVELOPED FLOW REGION 0 Z 0 L ? small ram extruder Measuring Principle Pressure drop through a capillary/slit die N o n Ne w to n ia n flu id rh e o lo g ic a l equ a tio n o f sta te :( p o w e r la w ) ? = K ( a )n ? = K . a n 1n = ? lo g ? / ? lo g aNewtonian fluid shear rate , = 4Q / ? r3 shear stress, ? = ?Pr