It is the tensile force needed for elongation of an extruded filament to the point of tensile breakage and is measured as a function of the draw ratio, which is typically done with a Rheotens Tensile Tester from Göttfert. Melt strenght is important for fiber spinning, film blowing, blow molding and thermoforming. You find more detailed info for example at the following links:
- Tensile Tester
and/or you search for "Rheotens melt strength" yourself ...
Unfortunately Melt tension and Melt strength is not what was articulated to you by Rolf. He is referring to the totally formed and crystallized fibers spun. The true meaning of melt tension and strength is in the term "melt". During the melt spinning the fibers are drawn down a vertical shaft (long stack or short stack) and will undergo shear induced crystallization during the stretching process down the stack. Depending on the conditions of spinning and the nature of the polymerr i.e stereoregularity,melt flow,additives present, pigments present the fiber will start forming a frost point seen and tested by a operator that determines the tension of the fibers coming down the stack. The melt tension is again measured at the draw point on the first godet down the stack to determine the tension of the fibers going over the godet. this determines the rate of crystallization and gives the operator a sense of the temperatures needed to spin the fiber and to cool the fiber. Cooling to fast will induce premature crystallzation and cause undue stresses and whitening of the fibers. Melt strength at the top of the stack determines how well the fiber will crystallize under a particular set of conditions. A glass rod is typically used to test the melt strength at the stack during the crystallization . If it sticks to low below the die it shows that the molten polyolefin needs to cool faster.
There are no papers on the subject and Dupont spear headed the process back in the early 1970's.
Rolf is referring to the final spun fiber and the strength of the fiber after cooling and post storage and not during the melt spinning process.
In addition to the Rheotens device melted by Rolf (this test is done in the melt state). TA Instruments has an extensional viscometer attachment for their ARES rheometer
. This allows for direct measurement of melt strength and viscosity as opposed to inferring it from Rheotens data.
Melt strength is important for a number of processes. Fiber spinning has already been mentioned but other processes that require high melt strength are thermoforming, blow molding, profile extrusion and cast film. In thermoforming, blow molding and profile extrusion one is mainly concerned about sag resistance while in cast film extrusion neck in resistance is required.
No, I was referring to the "Rheotens Test", which is a melt strength test on an extruded melt stand i.e. in the melt state. This is not your typical tensile test on a cold piece of solid plastic fiber! But I admit that the term "Rheotens Tensile Tester" may be a bit misleading, but it is used by the manufacturer of the Rheotens equipment. Have a look at the info-links, which I provided in my previous note, to find out how the Rheotens test works.