聚合物多相分层流动黏弹性包围机制的数值模拟

针对黏弹性包围引起的复合共挤制品层厚均匀性控制的技术难题,通过数值模拟,研究了黏弹性流变性能参数对黏弹性包围的影响规律和机制.研究结果表明,黏弹性包围是由多相分层流动的第二法向应力差驱动的二次流动诱发,主要取决于成型流动过程中二次流动的方向与强度.熔体二次流动的方向与第二法向应力差的正负号有关,而熔体二次流动强度则与第二法向应力差大小成正比.黏弹性包围随着熔体松弛时间的增加而增大.消除黏弹性包围的理论前提是消除其二次流动,而通过使其流动的第二法向应力差趋于零方可消除其二次流动.第二法向应力差趋于零的前提条件是使无滑移黏着共挤多相分层剪切流动转化为气垫非黏着完全滑移共挤多相分层柱塞流动,而气辅共挤成型工艺的气垫壁面滑移是实现这一转变的有效技术.

Numerical simulation on mechanism of viscoelastic encapsulation in polymer multiphase stratified flows

Against the viscoelastic encapsulation induced thickness uniformity controlling technical problems of composite coextruded products, the influencing rule and mechanism of viscoelastic rheological parameter on polymer co-extrusion viscoelastic encapsulation were studied by the numerical simulation. Research results show that the viscoelastic encapsulation is induced by the second normal stress difference driven secondary flow in co-extrusion multiphase stratified flows, and depends on the direction and strength of melt secondary flow. The direction of melt secondary flow depends on the sign of secondary normal stress difference, and its strength is proportional to secondary normal stress difference. The viscoelastic encapsulation increases with increasing of polymer melt relaxation time. The theoretical premise of eliminating viscoelastic encapsulation is elimination of the secondary flow in co-extrusion multiphase stratified flows, and the secondary flow can be eliminated by means of making the second normal stress difference tend to zero. The transformation of the no-slip adhesive coextrusion multiphase stratified shear flow into the full-slip non-adhesive gas assisted coextrusion multiphase stratified plug flow is the theoretical prerequisite of making the second normal stress difference tend to zero, and the air cushion wall full slip of gas-assisted co-extrusion molding process is an effective technique to achieve this transformation.