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Reply to Jimmy's question
March 11, 2008
Question asked by Jimmy in comments:
Is it true that cast pp is stiffer than blown PP? Consider the case, where everything is the same including the PP material and gauge. I know that a more oriented film will make the crystalline structure uniform thus making it’s density higher.
Hello Jimmy, this is hard to answer in any exact way because the casting and blowing conditions used will change the stiffness (modulus) a great deal. The film stiffness will depend on the percent crystallinity (density), the average crystal size and the molecular orientation in the film.
In general the blown film might be stiffer due to the haul off rate or draw down from the die (ratio of film thickness to die gap or melt velocity at the die to the film speed) and the slower cooling rate of the blown film relative to the cast film. Blown film properties will also depend greatly on the balance between draw down and blow up ratio used to produce the film. The higher the blow up ratio used, the higher the TD stiffness and the lower the MD stiffness (at constant draw down). However, in cast films the film is formed by drawing the film from the die and rapidly quenching which can lock in some MD orientation, but there is no appreciable TD orientation. Also the quenching rate is high so the film is generally clearer due to smaller (and perhaps fewer) crystals. This combines to make the cast film potentially less stiff than the blown film. If I want to increase the stiffness of the cast film I could increase the casting temperature optimizing the crystallization rate and increase the crystal size and perhaps percentage and this would generally increase the film stiffness and haze. So you can see that there is no real single answer (figures) to fit all circumstances. In general the blown film will have higher levels of residual molecular orientation and will on average be stiffer due to its manufacturing conditions. This also shows up as increased thermal shrinkage.
In regards to the blown film crystalline structure and higher density of the blown film, the stiffness will depend on the density as it is a measure of the amount of crystallinity in the film which correlates to stiffness (modulus) But the blown film also tends to be more oriented as well giving a higher modulus relative to the cast films.
Posted by Eldridge M. Mount on March 11, 2008 | Comments (2)
In response to: Reply to Jimmy's question
Dilwyn Jones commented:
The "perceived" stiffness of the cast film may be higher as a result of its greater thickness. Bending stiffness is modulus times the cube of thickness. So if you handle cast and finished film from the same line, the cast is much more difficult to bend.
In response to: Reply to Jimmy's question
Eldridge Mount commented:
The relation ship between thickness and stiffness is as you say related to the modulus times the cube of the thickness, my comments were directed to a comparison at an equivalent film thickness so that the film morphology would control the changes. But this is an important thing to keep in mind as there will be some properties such as the perceived stiffness, or bending which are extensive properties (i.e. related to the thickness as here) and intrinsic properties, such as the modulus which are independent of the size of the sample.