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Comparison of stretched polycarbonate films produced on a stretching line and a stretching frame.

INTRODUCTION

Nowadays, plastic films are widely considered the best solution for numerous applications with regard to both economic and ecological aspects. A considerable proportion of such films are used for the packaging of food, consumer goods, and medical products. Apart from the standard industrial plastics, such as polypropylene, polyethylene, and polystyrene, increasing use is being made of polycarbonates to produce plastic films, especially for the technical sector. Some of the key areas of application for polycarbonates are electrical insulating coverings, capacitor films, membrane switches, shrink coils, light diffuser films, and office transparencies [1], Because plastic products are increasingly being faced with more and more demanding quality requirements, surface finishing processes are increasingly being used to optimize the material-specific properties. One of these processes is uniaxial stretching, a procedure which produces anisotropic qualities in the stretched product; as a result, the mechanical properties of the plastic film in the stretching direction are improved. According to Retting, these mechanical properties are directly determined by the orientation; this is crucial, as orientations can relax somewhat even while being stretched. The orientation is therefore not only dependent on the stretching ratio but also dependent on the stretching temperature and stretching rate [2-4],

Previous studies on the uniaxial stretching of polycarbonate have shown that an increase in strength and stiffness can be achieved in the stretching direction. For uniaxial stretching, an increase in strength is generated not only in stretching direction but also in transverse direction (TD). Compared to conventional biaxially stretched films made of polypropylene, polyethylene terephthalate (PET), and polyamide, uniaxially stretched polycarbonate has a much higher stiffness. In addition, its optical properties, such as haze and gloss, are superior [5-7].

The focus of these studies is on the production of oriented amorphous polycarbonate film; the results are intended to compare uniaxially stretched film produced on a stretching line unit with planar stretched film produced on a stretching frame. A comparison of the two technologies should be able to establish which of the two stretching processes creates better mechanical properties. For these purposes, the mechanical properties have been determined in the machine direction (MD) and in the transverse direction (TD). Because the two stretching processes differ in how much contraction occurs in the film width, a comparison of the mechanical properties perpendicular to the stretching direction, i.e., in the TD, is of particular interest.

STATE OF THE ART

Orientation of Amorphous Thermoplastics

When a plastic is unable to arrange itself in the state of densest atomic packing, due to its irregular structure involving heavily branched molecule chains with long side chains, or due to the complex chemical structure of the monomer units, it is described as an amorphous thermoplastic [8]. Figure 1 shows the amorphous molecule structure in the unstressed state and how it changes under tensile stress. When strained, the molecule chains orient themselves in the load direction, in which direction primarily the covalent bonds are active. Consequently, the proportion of covalent bonds perpendicular to the load direction decreases, with the result that it is mostly secondary valence bonds acting in the TD. The degree of molecule orientation is dependent on both temperature and time [3, 4].

According to Ehrenstein, a distinction must be made here between reversible and irreversible molecular orientation. Reversible orientation occurs within a specific temperature range, and the molecules will revert to their original state upon removal of the load. During stretching, the reversible strains generated in the entropy-elastic ("rubbery") area above the glass-transition temperature are frozen when cooled below the glass-transition temperature (the "glassy" range), and thus become irreversible. An orientation of the molecules leads to more molecule chains per area, and thus higher strength. The strength is decreased perpendicular to the orientation. A specific orientation is achieved by stretching [10, 11]; here, a molecule orientation in the entropyelastic state is obtained and fixed by freezing.

Monoaxial Stretching

Monoaxial stretching units are generally based on rotating rollers that apply the necessary tensile stress via frictional contact. The rollers are divided between two roller units which turn at different velocities, [V.sub.1] and [v.sub.2], given in m/min. The stretching gap is between the two roller units. The stretch ratio is the quotient of the velocities [v.sub.2] and [v.sub.1] [9]:

Stretching Ratio SR = [l([t.sub.d])/[l.sub.0]] = [[l.sub.0] + [DELTA]l/[l.sub.0]] 1 + [epsilon] = [v.sup.2]/[v.sub.1] (1)

In the above equation, [t.sub.d] is the strain time, i.e., the time the film spends in the stretch gap during the stretching process, and [epsilon] is the elongation. For small elongations, the technical elongation [[epsilon].sub.t] is used, which is defined as:

[[epsilon].sub.t] = [l(t)/[l.sub.0]] - 1 (2)

For large elongations, the Hencky elongation [[epsilon].sub.h], is used and defined as:

[[epsilon].sub.h] = ln (l(t)/[l.sub.0]) (3)

Figure 2 shows a schematic illustration of the molecular orientation during cold forming of an amorphous thermoplastic below the glass-transition temperature [12]. Starting at the yield point, the orientation of the molecules changes throughout the region of plastic deformation; the alignment of polymer chains is clearly shown here.

The stress-strain curve, especially its gradient and maximum, is dependent on the strain rate. In addition to the stretching temperature, the roller velocities thus also have a major influence on the defonnation behavior. The strain rate E is defined according to [13] as follows:

[??] = [2[pi] x R x ([n.sub.2] - [n.sub.1])/s] = [v.sub.2] - [[v.sub.1]/s] where s = stretch gap length. (4)

Uniaxial Stretching on a Stretching Line Unit. The stretching line shown in Fig. 3 is from the Teach-Line series, developed by the company Dr. Collin. In this case, it is a single-gap stretching line in which the stretching is carried out in a short-gap stretching process. The unit can be used to stretch film, tapes and monofilaments. The film is heated up while passing through the first five rollers and is stretched in the gap between the sixth and seventh rollers as a result of the velocity difference between these two. Rollers 7-9 subsequently anneal the film, and Rollers 10 and 11 are used to cool the polymer down.

The rollers are separated into three heating zones; Rollers 1-3 and 7-9 are heated using water and Rollers 4-6 using oil (Marlotherm SH). Water, at a temperature of approx. 25[degrees]C, is used in Rollers 10 and 11 as the coolant medium. The stretching gap length s is 3.2 cm. With this particular stretching line, a maximum film thickness of 500 pm can be stretched with a minimum film length of 1.5 m. The maximum stretching temperature is 250[degrees]C [14]. When stretching on the stretching line, both the width and the thickness of the film contract. Due to the fact that the film is only stretched lengthwise, this process is known as uniaxial stretching.

Here, the dimension of the fluid elements changes in only one direction and, according to Ref. [15], the velocity components can be calculated as

[u.sub.x] = [??]x, [u.sub.y] = - [[??]/2]y, [u.sub.z] = - [[??]/2]z. (5)

For uniaxial stretching, there are numerous works explaining the process and its results in detail. The uniaxial deformation of PET has been studied in Ref. [16], among others. In Ref. [3], the uniaxial stretching of amorphous thermoplastics was analyzed; in that study, it was found that there is a clear association between the mechanical properties and orientation of the polymer molecules. The uniaxial stretching of polypropylene has been investigated in Ref. [17], among others.

Planar Stretching on a Stretching Frame. The structure of the KARO IV stretching frame from Brueckner Maschinenbau GmbH is shown in Fig. 4. The stretching frame consists of a stretching unit, an additional heating oven, and a stretching oven. Here, films can be stretched either monoaxially or biaxially at temperatures of up to 400[degrees]C. Monoaxial stretching is planar, because the film is held by clamps in the TD so that only the film's thickness can contract (vertical direction in Fig. 4). Before entering the oven, the film is clamped; it is then heated in the oven to the required stretching temperature. Once the stretching temperature has been reached, it is stretched at the desired stretching rate. The film temperature in the stretching oven is measured in the middle of the film by means of an IR pyrometer from the film Heitronics (KT line). In addition, the clip temperature is measured using a PT-100. With the stretching frame, specimens with a maximum thickness of 4 mm can be stretched from sizes of 90 mm X 90 mm to 140 mm X 140 mm [18].

Figure 5 shows the clamping of the film into the clips. After clamping, the entire unit is moved into the stretching oven; upon reaching the stretching temperature, the specimen is stretched by an outward pulling movement of the clips. The clips are attached to a hinged lattice, which can slide outward on rails; as this lattice expands (compare Fig. 5a and b), the clips are pulled away from each other. After stretching, the entire stretching unit leaves the oven and the stretched film can be removed.

The two stretching processes differ primarily in their strain moduli. The planar extensional flow is such that there is no deformation in one direction. According to Ref. [15], the velocity vector field is represented by

[u.sub.x] = [[??].sub.p]X, [u.sub.y] = -[[??].sub.p]X, [u.sub.z] = 0. (6)

The depicted velocity fields are applicable under the assumption of a constant volume and as long as the contraction is not restricted, for example by material sticking to the stretching rolls during uniaxial stretching. Furthermore, considerable differences exist between the two stretching processes in the strain rates and the type of heat supply and heat dissipation. The film is heated during stretching on the monoaxial stretching line via conduction. Heating of the film on the stretching frame occurs predominantly through heat radiation and forced convection.

Numerous studies have also been conducted to investigate planar stretching. The molecular orientation in planar-stretched PET films under constant load and at a constant temperature was examined in Ref. [19] by means of infrared spectroscopy; here, it was shown that the molecular orientation depends on the deformation. In Ref. [20], the planar stretching of PET films at a constant load just above the glass-transition temperature was examined. In Ref. [21], the orientation of the planar-stretched PET film was examined by measuring the refractive indices to calculate an orientation distribution. In Ref. [22], uniaxial extension was compared to planar extension by plotting the shear stresses during deformation as a function of the shear rates in all directions. In Ref. [23], an analysis of uniaxial, biaxial, and planar stretching of a high-density polyethylene melt was done and the time-dependent extensional viscosities and damping functions were compared. However, despite the plethora of information available, works in which the mechanical properties of planar and uniaxially stretched polycarbonate films have been compared are not known.

EXPERIMENTAL

Material Characterization

Polycarbonates possess a variety of desirable properties in view of today's high standards for plastics. In addition to high levels of light transmission, they possess a high gloss and are highly transparent. Table 1 gives rough values for some of the mechanical and thermal properties of polycarbonate.

The polycarbonate used for the experiments discussed in the following is Makrolon 2805 from Bayer Material Science AG. Makrolon 2805 is stabilized, contains demolding agents, has a medium viscosity, and is a non-reinforced injection molding material [24]. The material is plasticized in a single screw extruder (30 mm) with an L/D ratio of 30 and barrel wall temperatures of 240-280[degrees]C at a screw speed of 45 rpm; it is extruded through a slit die at 280[degrees]C with a gap width of 0.5 mm. The film is hauled off with a chill-roll unit at 4.5 m/min with a chill-roll temperature of 50[degrees]C and smoothed. The result is a 300 [micro]m film of the type which provided the specimens for the material characterization study. The same film was used for all of the following stretching tests.

To determine the glass-transition temperature needed for the stretching process, a dynamic differential scanning calorimetry (DSC) was carried out according to DIN EN ISO 11357-1 [25]. The DSC used here is a module of the METTLER TOLEDO [STAR.sup.e] thermoanalysis system and is performed between 20[degrees]C and 300[degrees]C, with a specimen weight of 20.83 mg and a heating rate of 10[degrees]C/min. In Fig. 6, the heat flow is plotted as a function of the temperature. Here, the glass-transition temperature of the Makrolon 2805 polycarbonate film was measured at 145[degrees]C; based on these results, the stretching temperature must therefore be above 145[degrees]C.

According to Retting [2-4], the orientation of amorphous thermoplastics produced by stretching is dependent, not only on the stretching ratio but also on both the stretching temperature and stretching rate. To measure the deformation forces, a tensile test at a defined temperature using a universal testing machine was carried out until a defined elongation had been reached. After elongation, the sample was held in the state it was in at the moment the test was completed.

The results show that during elongation, the curvature declines and becomes linear with increasing velocity. In the holding phase, the curve decays exponentially until reaching an equilibrium level. Figure 7a shows the influence of the deformation rate for an elongation of 50% at 150[degrees]C. During elongation, the curve becomes linear at higher deformation rates; additionally, the tension increases. All of curves fall to an identical equilibrium level. Figure 7b shows the influence of the elongation at a deformation rate of 300 mm/min at 150[degrees]C. During elongation, all of the curves have the same form and the tension rises with increasing elongation. In the holding phase, the curves decay in a qualitatively similar manner, but reach different equilibrium levels.

Figure 8 shows the influence of the temperature at a deformation rate of 50 mm/min and 50% elongation. As expected, the sample at a higher temperature has a much lower strength. While the tension of the 150[degrees]C curve reaches a constant value after 600 s, the 160[degrees]C curve has already reached a constant value after <300 s.

To determine the optimal temperature at which the polycarbonate can best be oriented, the reversible elongation has been determined for various temperatures. Here, the elongation at the breaking strain [[epsilon].sub.Break] at temperatures between 120[degrees]C and 170[degrees]C has determined, after which a maximum elongation [[epsilon].sub.Max] of

[[epsilon].sub.Max] = [[epsilon].sub.Break] - 5% (7)

is prescribed for the polycarbonate films at different temperatures. To allow the maximally stretched films to relax, they are held for 1 h at 140[degrees]C in a convection oven.

The residual elongation is denoted as [[epsilon].sub.Res]. The reversible elongation [[epsilon].sub.Rev] is then given by the difference between the maximum elongation and residual elongation:

[[epsilon].sub.Rev] = [[epsilon].sub.Max] - [[epsilon].sub.Res]. (8)

The reversible elongations calculated according to Eq. 8 for various temperatures are shown in Fig. 9. The reversible elongation and thus the orientation propensity of the polycarbonate is at its highest for stretching temperatures between 130[degrees]C and 150[degrees]C.

To determine the temperature resistance of the stretched polycarbonate films and thus the temperature at which the desired orientations relax, the mean coefficient of expansion according to ISO 11359-1 [26] is measured by thermomechanical analysis (SDTA841). According to Ref. [9], this is defined as

[alpha] = [[DELTA]L/[DELTA]T] x 1/[L.sub.0] (9)

and plotted as a function of temperature in Fig. 10; the figure shows the heat resistance of the stretched films up to 125[degrees]C.

Procedure

Uniaxial Stretching on the Stretching Line Unit. For

uniaxial stretching on the stretching line unit, the values shown in Tables 2 and 3 were used. The film was stretched at stretching ratios of between 1.2 and 2, and the stretching temperature was set at 150[degrees]C or 160[degrees]C, resulting in roller temperatures of 143.4[degrees]C and 154.2[degrees]C, respectively. The roller temperatures, and thus the film temperatures, were measured with a contact temperature transducer "Qtemp500 Version 2.5" from VWR International GmbFI. The stretching rate in the stretch gap was calculated according to Eq. 4, dependent on the stretch ratio; the results of the calculation can be seen in Table 2. For each setting, the films run for 5 min through the stretching line and from 6 to 10 m of the stretched film are produced depending on the stretching rate.

Planar Stretching on the Stretching Frame. For planar stretching on a stretching frame, the parameters shown in Table 4 were used. The film temperatures in the stretching oven were also 143.4[degrees]C and 154.2[degrees]C; the stretching speed was constant at 10%/s. The polycarbonate films were subjected to planar stretching with the same stretch ratios as in the case of uniaxial stretching, namely between 1.2 and 2. For the test, three (repeated) measurements were taken at each test point.

RESULTS AND DISCUSSION

The mechanical properties of the differently stretched films--uniaxial ly on the stretching line, and planar on the stretching frame--were compared with one another after completing the experiments. Figure 11 shows the resulting stress-strain curves in the MD of the uniaxially and planar stretched films. The tensile tests were performed in accordance with DIN EN ISO 527-3 [27] on a universal testing machine from Zwick AG. For each testing point, a series of six tensile specimens were examined and the mean stress-strain curves plotted. With both stretching technologies, the molecule chains orient themselves in the stretching direction, so that the acting bonds in this direction are primarily covalent bonds. One result of this (among others) is a higher tensile strength corresponding to a higher stretching ratio. However, with an increasing stretching ratio, the elongation at the breaking point decreases. Due to the high level of molecular orientation, the macromolecules cannot align in the direction of deformation and fracturing occurs. The stress-strain curves appear to remain uninfluenced by the choice of stretching technology.

When directly comparing the elongation at break as measured in the stretching direction (see Fig. 12), it can be seen that, in the case of the uniaxially stretched film at a stretching temperature of 143.4[degrees]C, there is virtually no reduction in elongation at break as the stretching ratio increases. In contrast, at both stretching temperatures, the elongation at break decreases for planar-stretched polycarbonate films as the stretch ratio increases. The strains are much higher during planar stretching in the MD, as contraction in the TD is restricted. Consequently, the elongation at break also sinks for polycarbonate films stretched below the glass-transition temperature.

Figure 13 shows the tensile strengths with standard deviation, as measured in the stretching direction of the uniaxially and planar-stretched films and at a stretching temperature of 154.2[degrees]C. Depending on the stretch ratio, the tensile strength increases as a result of both stretching technologies. Higher tensile strengths are produced for higher stretching ratios. Both with uniaxial and planar stretching, a significant increase of 100% in the tensile strength is achieved in the MD, increasing from 62 N/[mm.sup.2] to 124 N/[mm.sup.2]. When comparing the tensile strengths, it is striking that for planar stretching, even low stretching ratios lead to a higher tensile strength. In addition, for planar stretching, there is contraction only of the film thickness not of the width. With uniaxial stretching, on the other hand, contraction of both the width and thickness occurs. For this reason, planar stretching produces a higher tensile strength even with a low stretch ratio.

Figures 14 and 15 show the stress-strain curves, tensile strengths and elasticity moduli of the polycarbonate films stretched at a stretching ratio of 2 on both devices at 154.2[degrees]C. Figure 14 shows the stress-strain curves of the uniaxially and planar-stretched polycarbonate film in the machine and TDs. With the uniaxially stretched film, the stress-strain curve for the TD has risen and shows a higher maximum stress. As a result of the width reduction in TD, the macromolecules are packed more closely and secondary valence forces play a more important role. In contrast, the TD stress-strain curve does not change for the planar-stretched polycarbonate film because deformation here only occurs in the MD.

From Fig. 15a, it becomes clear that the tensile strengths after stretching are equal in MD, regardless of the stretching technology. In the TD, however, uniaxial stretching results in higher tensile strength. Planar stretching produces neither an increase nor a loss in tensile strength. It can be seen here that higher strength is achieved using a uniaxial stretching technology, in transverse as well as in MD. The elasticity moduli of the uniaxially and planar-stretched polycarbonate films are shown in Fig. 15b. In its unstretched state, polycarbonate has an elastic modulus of approx. 2300 MPa. After uniaxial stretching, this modulus remains almost unchanged in MD. Here, the highest elasticity modulus is produced by planar stretching as a result of greater elongation. In the TD, however, both stretching technologies result in a reduction in the elasticity modulus.

CONCLUSIONS

Improvements in the mechanical properties of polycarbonate can be achieved by both uniaxial and planar stretching, due to the orientation of the macromolecules produced by these technologies. With both methods, the same tensile strength can be obtained at a stretch ratio of 2; a 100% increase in tensile strength is achieved here, increasing the value to 124 N/[mm.sup.2]. The tensile strength in the TD rises by 50% for the uniaxially stretched film on the stretching line unit, but the tensile strength in this direction does not change for planar-stretched film on the stretching frame. With uniaxial stretching, contraction of the film width occurs, resulting in a denser packing of the macromolecules. Consequently, an improvement in the mechanical properties is achieved, perpendicular to as well as in the stretching direction. With planar stretching, contraction of the film width is prevented. A look at the elasticity moduli shows that the planar-stretched polycarbonate film has the highest elasticity modulus in the MD. In the TD, on the other hand, the modulus of elasticity is the same, regardless of the stretching technology.

ACKNOWLEDGMENT

Special thanks go to Brueckner Maschinenbau GmbH, who provided us with the opportunity to perform planar stretching tests on polycarbonate films on the KARO IV stretching frame. This research was carried out as part of the Collaborative Research Centre Transregio 30, sponsored by the German Research Foundation (DFG) as part of the project "Process-integrated production of functionally graded structures based on thermomechanically linked phenomena".

REFERENCES

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[8.] G. Menges, E. Haberstroh, W. Michaeli, and E. Schmachtenberg, Werkstoffkunde der Kunststoffe, 5, vollig uberarbeitete Auflage, Carl Hanser Verlag, Munchen, Wien, 32 (2002).

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[17.] S. Rettenberger, Uni- und biaxiales Verstrecken von isotaktischem Polypropylen im teilaufgeschmolzenen Zustand, Dissertation, technische Fakultat der Friedrich-AlexanderUniversitat Erlange-Nurnberg, Erlangen (2002).

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Andrea Wibbeke, Volker Schoppner

Polymer Engineering Paderborn (KTP), University of Paderborn, Paderborn, Germany

Correspondence to: Andrea Wibbeke; e-mail; andrea.wibbeke@ktp.upb.de Contract grant sponsor: German Research Foundation (DFG).

DOI 10.1002/pen.23728

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Properties of polycarbonate [9, 10, 12].

Mechanical properties

Elasticity modulus                      2300-2400 MPa
Yield stress                              55-66 MPa
Elongation                                  6-7 %

Thermal properties

Heat resistance                       36-148 [degrees]C
Glass-transition temperature          140-150 [degrees]C
Thermal conductivity (23[degrees]C)    0.20-0.21 W/(mK)

TABLE 2. Calculation of the stretching rate

                               [??] = ([v.sub.2] -
       [V.sub.1]   [V.sub.2]    [v.sub.1])/3.2 cm
SR      (m/min)     (m/min)          (1/min)         [??] (%/s)

1.2        1          1.2             6.25             10.41
1.4        1          1.4             12.5             20.83
1.6        1          1.6             18.75            31.25
1.8        1          1.8             25.00            41.67
2          1           2              31.25            52.08

TABLE 3. Settings for the uniaxial stretching line.

Parameter                                 Value

Stretching rates [??] MD (%/s)         10.41-52.08
Stretching ratio MD                       1.2-2
Stretching temperature ([degrees]C)    143.4-154.2

TABLE 4. Settings for the KARO IV stretching frame.

Parameter                                    Value

Stretching rates [[??].sub.M] MD (%/s)        10
Stretching ratio MD                          1.2-2
Stretching temperature ([degrees]C)       143.4-154.2
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