Physics:Melt fracture
| Continuum mechanics |
|---|
In polymer processing and rheology, melt fracture (also called extrudate distortion) is a family of viscoelastic flow instabilities that appear when a polymer melt is extruded through a die above a critical throughput, distorting the surface or the bulk of the emerging strand.[1] The Reynolds number in such flows is essentially zero, but the elasticity of the melt is sufficient to drive instability. Because the resulting defects set the upper limit on extrusion rates for many commercial polymers, melt fracture has been studied since the 1950s.[2][3]
Background
Three regimes are usually distinguished as wall shear stress is increased.[4] Sharkskin is a fine, periodic surface roughness that originates at the die exit, where the melt's surface layer is rapidly stretched as it accelerates from the no-slip wall to the free surface; when the extensional stress exceeds the cohesive strength of the entangled chains, the surface tears periodically.[5][6] Stick-slip (or spurt) flow appears at higher stresses in some linear polymers: the extrudate alternates between rough and glossy bands and the pressure oscillates, reflecting a non-monotone or multi-valued flow curve.[7] Gross melt fracture is a chaotic, three-dimensional distortion of the whole extrudate that originates upstream, in the convergent flow at the die entry, where extensional stresses and recirculating vortices destabilize at high throughput.[4][1]
Susceptibility depends strongly on molecular architecture: narrow-distribution linear polymers such as LLDPE are particularly prone to sharkskin and spurt, while long-chain branched polymers such as LDPE tend to skip these and develop gross melt fracture instead.[4] Industrially, sharkskin is most often suppressed with fluoropolymer processing aids that coat the die wall and induce slip at the polymer–polymer interface;[8] die-geometry changes, blending with branched resins, and inorganic fillers such as boron nitride are also used.[4]
See also
References
- ↑ 1.0 1.1 Denn, M. M. (2001). "Extrusion instabilities and wall slip". Annual Review of Fluid Mechanics 33: 265–287. doi:10.1146/annurev.fluid.33.1.265. Bibcode: 2001AnRFM..33..265D.
- ↑ Tordella, J. P. (1956). "Fracture in the extrusion of amorphous polymers through capillaries". Journal of Applied Physics 27 (5): 454–458. doi:10.1063/1.1722401. Bibcode: 1956JAP....27..454T.
- ↑ Petrie, C. J. S.; Denn, M. M. (1976). "Instabilities in polymer processing". AIChE Journal 22 (2): 209–236. doi:10.1002/aic.690220202. Bibcode: 1976AIChE..22..209P.
- ↑ 4.0 4.1 4.2 4.3 Vergnes, B. (2015). "Extrusion defects and flow instabilities of molten polymers". International Polymer Processing 30 (1): 3–28. doi:10.3139/217.3011.
- ↑ Cogswell, F. N. (1977). "Stretching flow instabilities at the exits of extrusion dies". Journal of Non-Newtonian Fluid Mechanics 2 (1): 37–47. doi:10.1016/0377-0257(77)80031-1. Bibcode: 1977JNNFM...2...37C.
- ↑ Kalika, D. S.; Denn, M. M. (1987). "Wall slip and extrudate distortion in linear low-density polyethylene". Journal of Rheology 31 (8): 815–834. doi:10.1122/1.549942. Bibcode: 1987JRheo..31..815K.
- ↑ Hatzikiriakos, S. G.; Dealy, J. M. (1992). "Wall slip of molten high density polyethylenes. II. Capillary rheometer studies". Journal of Rheology 36 (4): 703–741. doi:10.1122/1.550313. Bibcode: 1992JRheo..36..703H.
- ↑ Migler, K. B.; Lavallée, C.; Dillon, M. P.; Woods, S. S.; Gettinger, C. L. (2001). "Visualizing the elimination of sharkskin through fluoropolymer additives: coating and polymer-polymer slippage". Journal of Rheology 45 (2): 565–581. doi:10.1122/1.1349136. Bibcode: 2001JRheo..45..565M.
 |  |
