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The effects of thermoform molding conditions on polyvinylchloride and polyethylene double layer package materials.

INTRODUCTION

Polymer processing for production of all forms of polymeric articles has found a great place in chemical industries (1). Thermoform mold process is one of the most popular techniques in this field. It applies to thermoplastic sheet as a film-forming technique for various packaging applications such as medical devices, food containers, and pharmaceuticals (2), (3). Wide applications of thermoforming are due to its high performance, simplicity, compactness, and relatively low cost equipment. In thermoforming, a heated plastic sheet is stretched into a mold cavity by applying pressure and eventually direct mechanical loading are used (4). Thermoforming consists of warming a plastic sheet and forming it into a cavity or over a tool using vacuum, air pressure, and mechanical means. The process begins by heating a thermoplastic sheet slightly above the glass transition temperature, for amorphous polymers, or slightly below the melting point, for semi-crystalline materials. As the final thickness distribution of the part is drastically controlled by the initial temperature distribution inside the sheet, it is very important to optimize the heating stage (5). Thermoforming of film having deep draws is usually done using plug forming to achieve part shape and thickness uniformity. However, the design of the plug shape and material selection are important during the production stage. Hsiao and Chuang (6) demonstrated that shape re-engineering from digitized part is successful for general product form design. However, thermoforming mold also have special features which are different from general products. Thompson et al. (7) suggested a feature based reverse engineering approach to model machining features for mechanical parts. This concept is adopted to model features in thermoforming features that are modeled based on the digitized data instead of general surface fitting in a conventional reverse engineering process. Freeform surface can be found very often in thermoforming mold. A classification of freeform features in thermoforming mold facilitates thermoforming feature modeling. Both the price and performance characteristics could be improved generally with elastomers, particularly ethylene-propylene copolymers, ethylene-propylene-diene terpolymers, and ethylene vinyl acetate (EVA). Although the toughness characteristics are improved by the addition of an elastomer to the polymeric structures, this causes a decrease in the elongation and impact strength and an increase in the elongation and impact strength properties. For this reason, multipurpose fillers and reinforcements could be used to overcome this problem because they would ensure better dimensional stability and rigidity (8-10).

Over the past several years, many studies have been conducted on the material behavior and the forming process of thermoplastic composites (TPCs). Many investigations of the TPCs are based on the thermoformed hemispherical geometry, where the highly deformed edges and corners are considered as a waste (11). It has been observed that irrespective of the underlying weave geometry, the fabric undergoes nearly pure shear deformation during draping, with the constituent tows rotating with a material specific resistance at the crossover points (12), (13). The drape behavior is, therefore, often simply modeled by mapping these crossover points over the curved mold shape (14). Material test methods for woven fabrics reported in the literature include bi-axial tensile tests, bias extension tests, and shear frame tests (15). These test methods are restricted to components with uniformly curved surfaces.

Many types of homopolymers and copolymers have been previously studied by chemical and electrochemical methods (16-18). Electrochemical impedance spectroscopic measurements and morphological analysis of electrocoated thin films of pyrrole derivatives (19), poly(N-vinylcarbazole) (20), polyaniline (21), and poly(N-methyl pyrrole) (22), (23) were studied previously.

There is no systematic study on the interaction of the effects of thermoform mold process conditions on polyvinylchloride (PVC) and polyethylene (PE) double layer package materials in literature. Hence, the goal of this work was to form mechanical properties of the package materials, which were examined by tensile strengths and tear resistances and SEM. Package materials are tested by thermal aging process at 60[degrees]C in first, third, and seventh days.

EXPERIMENTAL

Mechanical Characterization

In this study, the package material was prepared via extrusion process, used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross section. The bottom side of PE-PVC double layer package material was used due to the regular structure of the material. It has preventive properties against to oxygen, moisture, and oil. Upper part of laminate cover of the material is made up of PE. Therefore, PE and PVC are produced in two different portions by extrusion process.

The obtained packaging material was evaluated through the mechanical properties such as tensile strength tear resistance and SEM respectively. The tensile testing of the packaging material was carried out on a Zwick Z010 10 kN universal tensile test machine (Ulm-Einsingen, Germany), according to the TS 138 EN 1002-1 test procedure at a crosshead speed of 100 mm [min.sup.-1] at room temperature. Samples were pulled until them come off, and so tensile test results were obtained in various depths (25, 35, and 75 mm) and temperatures as shown in Figs. 1-3. The tear resistance results were also made on Zwick Z010 universal tensile test machine (Ulm-Einsingen, Germany) (Fig. 4). A computer was connected to the Zwick load cell, and a data-acquisition program recorded the force measured by the load cell. At least 10 specimens were tested for each condition and average values are reported. Before the mechanical tests, all specimens were kept at room temperature for at least 72 h. The fracture surfaces of tensile test specimens were evaluated with Jeol SEM (5410LV, JSM) (Tokyo, Japan). The surfaces of the specimens were coated with a thin gold in 30 [Angstrom]. SEM investigations were performed at 20 kV after the samples were coated with gold for better conductivity. TFZ 1000/2000 AISI 304 stainless-steel packing machine was used to perform samples as shown in Fig. 5. Aging processes was evaluated by BINDER 115 hot air oven (Tuttlingen, Germany) at 60[degrees]C for holding times first, third, and seventh days. In this study, PVC and PE double layer package material was used for tensile properties, tear resistances and SEM results.

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RESULTS AND DISCUSSION

Tensile Test Results

The changes in the tensile strengths of the PVC and PE double layer packaging materials are shown in Fig. 6(a--c), respectively. In tensile strength tests results shown before thermoforming process, tensile strength was ~45 MPa and maximum elongation amount 35%. During the thermoforming, process temperature (150[degrees]C, 165[degrees]C, and 175[degrees]C) and mold depth (25 mm, 35 mm, 75 mm) of the samples were taken in different parameters for PVC-PE double layer package material. The other parameters (vacuum and thickness, etc.) were taken constant. The tensile strengths for 600 [micro] thickness and 25 mm mold depth were obtained as ~35 MPa and ~37 MPa for 165[degrees]C and 175[degrees]C, respectively. Elongations were 35% at 165[degrees]C and 175[degrees]C, respectively. The maximum tensile strength of ~45 MPa, thermoform temperature at 150[degrees]C and the maximum elongation 58% at 150[degrees]C thermoform temperature were shown in Fig. 6a. The tensile strengths for 600 [micro] thickness and 35 mm mold depth were obtained as ~42 MPa both for 165[degrees]C and 175[degrees]C, ~45 MPa for 150[degrees]C, respectively. Elongation 10%, 50%, and 80% were obtained for 175[degrees]C, 150[degrees]C, and 165[degrees]C, respectively. The maximum tensile strength ~45 MPa at 150[degrees]C were shown in Fig. 6b. The tensile strengths for 600 [micro] thickness and 75 mm mold depth were obtained as ~55 MPa at 150[degrees]C, and ~50 and ~42 MPa at 175[degrees]C and 165[degrees]C thermoform temperatures, respectively (Fig. 6c). Maximum elongation was obtained ~60% at 175[degrees]C and 55% at both for 150[degrees]C and 165[degrees]C. As a result of the tensile test results, mold depth of the material increases by the increase of tensile strength test that results during the thermoform mold process of PE-PVC double layer packaging materials. For example, tensile strength results of thermoform temperature at 150[degrees]C were obtained as ~38 MPa for 25 mm mold depth; ~42 MPa for 35 mm mold depth; ~55 MPa for 75 mm mold depth. This situation was caused by the amorphous and crystalline structure of the PE-PVC double layer packaging material. Together with increasing of mold depth from 25 mm to 75 mm, orientation of the polymer segments and the crystalline structure of the material increase in parallel with tensile strength. Because polymer segments or axis of crystalline chains formed a line in parallel position under certain tensile strength.

[FIGURE 6 OMITTED]

Because of reasons such as packaging strength and sales appeal, the shape of a thermoforming mold often deviates considerably from the shape of the corresponding packed part. Thermoforming molds can contain both regular and freeform surface patches, which can be interpreted as shape features because they bear characteristics that represent the shape elements of the thermoforming mold; able to map to same generic shape patterns; and possess functional significance. It was observed that thermoforming mold makers also intuitively use this feature concept to identify and select regions of interest when they manipulate the thermoforming clay model. Based on a function viewpoint, four basic types of functional features are identified in a thermoforming mold: a) packaging, b) draw ratio, c) demolding, and d) strengthening (24).

Effect of the thermoform dept (25 mm, 35 mm, 75 mm) and thermal aging (at 60[degrees]C) on the tensile strength properties of PVC and PE double layer packaging materials are shown in Fig. 7(a-c), respectively. With dependence of aging time (day), tensile strength of the material decreases due to the difference of thermoform mold temperatures (Fig. 7a-c). Tensile strength of thermoform mold temperature at 165[degrees]C is higher than temperature at 150[degrees]C and 175[degrees]C. Because process temperature at 165[degrees]C is very effective in this work, which causes more orientation of the segments of the material. At 175[degrees]C, tensile strength of the material decreases slightly due to degradation of the material on this condition (Fig. 7a). There are important changes with 35 mm mold thermoform temperature in similar to 25 mm mold depth of the material. There was an increase in depth of the material during mold process, and so orientation of chains or segments of the double layer package material increased significantly. As a result of rising up the orientation of material, tensile strength increased up to third day. After that time, it remained constant for three different samples at 150[degrees]C, 165[degrees]C, and 175[degrees]C (Fig. 7b). As there is a significantly increase of orientation of the material, it causes an increase in tensile strength at 75 mm mold depth sample (Fig. 7c). Although there is no so much change in tensile strength after aging time of third day, structure of material was easily broken down by various thermoform mold temperatures. Because both PVC and PE have enough temperature at 165[degrees]C to break the structure easily. As we know from literature that melting points of PE and PVC are at 135[degrees]C and 80[degrees]C, respectively. Therefore, it causes the structure hard and also reduces the elongation at 165[degrees]C. So, the higher tensile strength values of samples were obtained at 165[degrees]C. Mechanical properties of the material were obtained by different thermoform mold temperatures.

[FIGURE 7 OMITTED]

Tensile strength-aging time graphs were given in similar results via elongation-aging time graphs (Fig. 8a-c). Elongations were obtained in the maximum value as 162% of thermoform molding temperature at 165[degrees]C (Fig. 8a). It decreases gradually by increasing of aging time at the same behavior at 150[degrees]C and 175[degrees]C. The elongations increase dramatically (until 125-150%) after first day and then decrease slightly (Fig. 8b). Elongation of the material decreases especially after aging time of third day with dependence on orientation of the segments of the material because of the hardness of the double layer package material (Fig. 8c).

[FIGURE 8 OMITTED]

Multi-layer PE-PVC double layer package materials continue to capture numerous food packaging applications because of their performance and economic advantages. In the majority of these applications, the packaging film's permeation or barrier properties are the specific HDPE (high density PE) resins or resin blends used, processing conditions and the relative position of the resins in the multi-layer structure.

Samples indicate that the maximum tear strength (30 N) at 165[degrees]C for 25 mm and 35 mm mold depths (Fig. 9a), and 25 N at 165[degrees]C for 75 mm mold depth (Fig. 9b), respectively. Temperature and sample depth were changed during heat process in PVC-PE double layer package material. The other parameters were kept constant. As a result of tear tests, while thermoform molding temperature increases, tear resistance of material decreases (Fig. 9a-c).

[FIGURE 9 OMITTED]

Morphological Analysis

Double layer package materials are especially used in an increase of self-life of food. In this work, effects of thermoform mold temperature and mold depth of the material were examined in detail. According to SEM images, there is a phase difference between layers with dependence on thermoform mold temperatures. The reason of the phase difference may be caused by different thermoform mold temperatures. The effects of different high-pressure processing (HPP) treatments EVOH-based packaging materials were studied, and they were compared with the morphological effects produced by a more traditional food preservation technology, i.e., sterilization. The results proved that HPP scarcely affects packaging materials, especially when compared with the detrimental consequences of retorting. It is essential to be able to assure food safety during pressure treatment and storage. A slight increase in crystalline morphology resulting in better barrier properties could be found after pressure treatment (25). Therefore, it may cause degradation of the double layer package material (Fig. 10a-c).

[FIGURE 10 OMITTED]

CONCLUSION

In this study, the effects of thermoform molding process conditions on PVC-PE double layer package materials were performed via tensile, tear test properties and SEM. With dependence on thermoform mold depth, tensile strength increases up to ~55 MPa at 150[degrees]C. However, elongations (%) decrease slightly. According to tear resistance results, with increasing in mold depth, it causes tear resistance of double layer package material. Although we observed difference in tensile strength experiment results, there was a decrease in resistance of the material according to the tear test experiments. The reason of this decrease is to reduce intern tensions of the chains. With the changing of thermoform mold temperature effect, generally thermoplastic structures depend on to heat and cold conditions, structural molecules oriented easily. The more ordered structures were obtained by the help of orientation, the longer shelf life was obtained in food package sector. Because structure was obtained more crystalline, the oxygen permeability was so low that there was an increase of shelf life of food.

ACKNOWLEDGMENTS

PAK-FORM Company (Istanbul) supports this work.

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Mustafa Oksuz, (1) Cafer Alsac, (1) Murat Ates (2)

(1) Plastic Division, Metal Education Department, Faculty of Technical Education, Marmara University, 34722, Goztepe, Istanbul, Turkey

(2) Department of Chemistry, Faculty of Arts and Sciences, Namik Kemal University, Namik Kemal Street, Number: 14, 59100, Tekirdag, Turkey

Correspondence to: Mustafa Oksuz: e-mail: moksuz@marmara.edu.tr.

Tel: +90 (216) 336 57 70/358.

DOI: 10.1002/pen.2l471

Published online in Wiley InterScience (www.interscience.wiley.com).
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Author:Oksuz, Mustafa; Alsac, Cafer; Ates, Murat
Publication:Polymer Engineering and Science
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Date:Nov 1, 2009
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