Page 132 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012c-593 July 26, 2001 15:56

638 Polymer Processing

film. In terms of direction, most frequently this process As far as the mechanical properties are concerned, the
takes place vertically upward and less frequently verti- tear (test name: Elmendorf tear), impact (test name: dart
cally downward and horizontally. The major advantages drop), and tensile strengths give an indication of the me-
of this method over the first method are the economics and chanical strength of the tubular film. The amorphous as
the speed of production. well as the crystalline orientation development during the
Film blowing and fiber spinning have general similar- blowing process depend on the stretching imparted in the
ities. Both processes have free boundaries, and the flows machine and transverse directions. Finally, besides orien-
are predominantly elongational. They differ with respect tation, the amount of crystallinity as well as the size of the
to the orientation generated. The fiber spinning process crystallites may play a significant role in the mechanical
imparts orientation in the axial direction only, whereas and physical properties of the blown film.
the film blowing process imparts unequal (in general) bi- This process is not as fast as fiber spinning, which re-
axial orientation. The two axes of orientation are the axial sults in a more uniform temperature distribution in the film
(machine; MD) direction due to the drawing of the tube relative to that in the fiber. Usually, cooling is achieved by
and the circumferential (nonmachine,or transverse; TD) blowing an air stream from an axisymmetric air ring to-
direction due to the blowup of the tube. The mechanical ward the external film surface. In some cases, in addition
properties of blown film are nearly uniform in both di- to the external air ring, an internal air cooling system is
rections as a result of biaxial orientation, and this is the provided. Finally, in some other cases, especially in thick
reason for producing flat film by the film blowing process. tube and large bag production, cooling is achieved by a
The two main parameters of this process are the blow water spray or ring. Note that in the latter cases the film
ratio (or blow-up ratio), B R (or BUR), and the machine- must be dried before winding up, which leads to an addi-
direction draw (or drawdown) ratio, D R . The blow ratio tional step.
is defined as the ratio of the final tube radius, R f , to the Commercially, the film blowing process is extensively
initial tube outside radius just downstream of the annular used for the production of polyolefin (LDPE, HDPE, and
die, R 0 : PP) wrapping film. Mechanical strength, optical clarity,
which depends on the degree and type of crystallinity for
R f
B R = . (31) crystallizable polymers, and the uniformity of thickness
R 0
(variations of about 5% for films with a length scale of 10
Similarly, the draw ratio is defined as mm to 10 m is acceptable) are the three most important
and general properties of the film.
V
D R = , (32)
v 0
D. Stability
where V is the takeup speed, and v 0 is the die extrusion
rate. The final film thickness, H f , can be calculated from The rate of production in one of the three processing oper-
the blow and draw ratios and the mass conservation equa- ations just discussed, i.e., fiber spinning, film casting, and
tion as follows: film blowing, is limited by the onset of instabilities. Two
major types of instabilities are encountered in these pro-
H 0
H f = ; (33) cesses. A significant degree of drawdown is used to reduce
B R D R
the fiber diameter or film thickness. If the stresses become
where H 0 is the initial film thickness or, equivalently, too high, the polymeric material can fail by means of cohe-
the die gap thickness. Typical parameters in the film sive fracture. The filament or film merely ruptures. In the
blowing process are H 0 = 1–2mm R 0 = 2.5–25 cm; second type of instability, called draw resonance, the fiber
v 0 = 1–5cm/sec; B R = 1.5–5; D R = 5–25; P = 50 Pa diameter begins to oscillate periodically when a critical
(i.e., the internal pressure is about 0.05% of the atmo- drawdown ratio is reached. In the case of blown film, this
spheric pressure); and freeze-line height Z = 0.25–5m. is manifested in variations in the film thickness leading to
An average value of the blow and draw ratios and of the fluctuations in the bubble diameter. For Newtonian fluids
initial film thickness yields a final film thickness on the thecriticaldrawratiofortheonsetofdrawresonance, D Rc ,
order of 50 µm (i.e., about 2 mils in English units). In is about 20. For polymeric materials D Rc is significantly
terms of nomenclature, the final film is considered to be lower with values of less than 5.0 observed. The type of
thick-gauge blown film whenever its thickness exceeds instability depends on the nature of the polymer. Polymers
75 µm or equivalently 3 mils. In terms of applications, that exhibit strainhardening extensional viscosity (i.e., the
thick-gauge blown film is used in the production of extensional viscosity increases with increasing strain and
dunnage bags, heavy-duty shrink film, greenhouse film, strain rate) such as LDPE tend to exhibit cohesive fracture
lawn and garden bags, and resin and chemical packaging. but not draw resonance, whereas polymers such LLDPE