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Performance of multilayer films using maleated linear low-density polyethylene blends.

INTRODUCTION

Coextrusion is a process in which two or more polymers are extruded simultaneously and joined together to form a single structure with multiple layers. This technique has become an attractive means to economically produce multilayer sheet, blown film, cast film, tubing, wire coating, and so on (1). This is a widely used process in the food packaging industry because it allows the design of multilayered structures with different functional properties associated to each layer.

Polyethylene (PE) and polyamide (PA) are two important classes of polymers used in coextrusion. PE is widely employed because of its low cost, high barrier properties to moisture, good optical properties, and ease of processing, but its high permeability to oxygen and many organic solvents limits its potential (2, 3). On the other hand, PA is a good barrier material for oxygen and organic compounds but is relatively expensive, hygroscopic, and thus a poor barrier for water vapor (4, 5).

PE and PA are immiscible across the whole range of compositions (6-8). Various compatibilization studies for PE/PA blends are well documented in the literature (9-11). In a number of studies a graft copolymer is used as a compatibilizing agent, where the copolymer consists of a polyolefin backbone grafted with functional groups, such as butyl acrylate and maleic anhydride, that can react with terminal amine groups in PA (5, 7, 12-15). The formation of a block polyolefin-polyamide copolymer was recognized as the key point to improve the interfacial adhesion between these polymers (16, 17). The addition of block copolymers has been used successfully to improve the mechanical properties of binary blends, in which the block copolymer reduces the particle size of the dispersed phase and forms a physical link between the dispersed and the continuous phases (11). In a recent study by Bidaux et al. (17), bond formation between maleic anhydride grafted polypropylene and polyamide-6 by in situ block copolymer formation was investigated.

Typical commercial multilayer barrier films, for food packaging, contain EVOH or PA as the oxygen resistant layer and polyolefin resins as the moisture resistant layer. This involves a multilayer structure, where outer PE layers protect an inner PA layer from exposure and subsequent attack by water. Because of the incompatibility of these layers, an extrudable adhesive or "tie" layer must be incorporated into the structure to hold these two materials together (18-20). These additional adhesive layers in the coextruded film make the manufacturing process more complex and more expensive; a special design for the feed block is required, and sometimes an additional extruder in the coextrusion system.

In this work, blends of linear low-density polyethylene (LLDPE) and linear low-density polyethylene-grafted maleic anhydride (LLDPE-gMA) were used to promote the adhesion to polyamide (PA) in a three-layer coextruded film without using an additional adhesive or tie layer. Our goal was to determine if the maleated PE in the blend would improve the interfacial adhesion between layers. This particular film could be an option when equipment for a five-layer system is not available. Blends of LLDPE/LLDPEgMA were prepared by melt mixing and then coextruded as external layers, with PA as a central layer on a three-layer coextruded flat film. The T-peel strength, oxygen permeability, and water vapor permeability were determined. The surface of the peeled films was analyzed by FTIR and SEM.

MATERIALS AND PROCEDURE

Materials

The materials used in this work were an LLDPE grade MJA-202 from Mobil, an LLDPE-gMA grade Bynel Cxa-E-409 from DuPont and a polyamide-6 (PA) grade Ultramid B-4 from BASF. All are commercially available products whose main characteristics are reported in Table 1. The LLDPE-gMA contains 0.25% maleic anhydride (MA) by weight, as determined by acid-basic titration.

[TABULAR DATA FOR TABLE 1 OMITTED]

Preparation of Blends

Prior to processing, PA was dried in a vacuum oven for 12 hours at 90 [degrees] C. The PE were also dried, but for 5 hours. Blends of LLDPE with different content of LLDPE-gAM were prepared by melt mixing in a Werner and Pfleiderer twin-screw mixer (ZSK 30) operating at 220 [degrees] C and 60 rpm.

Preparation of Multilayer Films

The extruded blends were then coextruded as external layers, With PA as a central layer on a three-layer 0.075-mm-thick fiat film, using three Killion single-screw extruders (KTS-100, L/D = 24) connected to a three-layer feedblock and a fiat die, operating at 260 [degrees] C on the die. The three-layer films were prepared With an overall 17 wt% of PA and blends of LLDPE with 0, 5, 8, 10, 12, 15, 20, 35 and 55 wt% of LLDPE-gMA (PE blends) in the external layers. Five-layer (LLDPE/adhesive/PA/adhesive/LLDPE) 0.075-mm-thick film was used as a reference film for the permeability tests. Of this film, 77 wt% was LLDPE, 17 wt% PA and 6 wt% adhesive.

Peel Tests

Prior to the preparation of peel strength specimens, a monolayer film was obtained from the extrusion of the PA resin and each of the PE blends using the single-screw extruder described above, operating at 210 [degrees] C for PE blends and 260 [degrees] C for the PA, on the die. The peel strength specimens were prepared by laminating layers of a 0.03-mm-thick PE blend film and a 0.03-mm-thick PA film, between two 2-mm-thick vulcanized rubber plaques (to avoid bubble formation), using a hot press and applying bonding temperatures of 180, 220 and 250 [degrees] C and a pressure of 1.9 Mpa for 5 minutes, at which point the heat was turned off and cooling initiated. Both mold halves were heated to the same temperature ([+ or -]1 [degree] C) and allowed to equilibrate to the same temperature for approximately 15 min. The pressure was maintained until the mold reached room temperature. A 10-mm-wide strip of DuPont Teflon tape, 0.05 mm thick, was placed between the PE/PA layers to serve as a delamination initiator. Film samples were then cut into 2.54 cm x 14 cm strips. Peel strength was measured at 23 [degrees] C by 180 [degrees] C peel test using a tensile tester with a cross-head speed of 25.4 cm/min, as described in the ASTM D 1876-93 standard. The average force after the initial peak load was taken as the peel strength.

Surface Characterization

The chemical composition of the surface of the peeled layers was investigated by attenuated total reflection infrared spectroscopy (FTIR-ATR) using a Nicolet 710 spectrometer; the incident beam angle was 45 [degrees]. The surface topography of the peeled films was analyzed; the surface was coated with a thin layer of carbon and examined in a scanning electron microscope (SEM), Jeol 840 at 15 kV.

Crystallinity

Thermograms of fusion for the LLDPE/LLDPE-gMA blends were measured using a DuPont DSC 910s differential scanning calorimeter. The rate of temperature increase was 10 [degrees] C/min under [N.sub.2].

Crystallinity was calculated by the heat of fusion of the LLDPE peak on the polymer blend, on the assumption that [Delta]H of linear polyethylene was 279.8 J/g (21) when it was all in the crystalline state.

Permeability Tests

[O.sub.2] permeability was determined on a Mocon Ox-Tran 100-A apparatus (Modem Controls, Inc.), operated at 25 [degrees] C (ASTM D-3985). A polyester film at 25 [degrees] C was used as a standard. Its permeance value was 60 [cm.sup.3] [multiplied by] mm/([m.sup.2][multiplied by]d[multiplied by]atm). [H.sub.2]O vapor transmission was determined at 38 [degrees] C and 90% relative humidity (R.H.) according to ASTM E-96. Permeability results are the average of three specimens per sample.

RESULTS AND DISCUSSION

The dependence of the peel force of the multilayer joint on the bonding temperature, as a function of the concentration of LLDPE-gMA, is shown in Fig. 1. A sharp increase in peel strength was observed when the temperature increased from 180 [degrees] C to 250 [degrees] C, for the whole range of compositions. All the specimens bonded at 180 [degrees] C (T [less than] Tm of PA) exhibited an apparent interfacial failure and the peel strength showed the lowest values. The specimens bonded at higher temperatures of 220 [degrees] C and 250 [degrees] C (T [greater than] Tm of PA) showed quite similar behavior. An apparent adhesive failure through the interface, at these temperatures, yields to much higher strength values, especially for LLDPE-gMA contents above 10%. A sharp increase in peel force, at these temperatures, was observed when using near 10 wt% of the maleated polymer. All the specimens with content lower than 10% exhibited a low value of peel strength, while the ones with higher content showed much higher strength values, related to an apparent adhesive failure through the interface. In films with concentrations above 20%, the film is first elongated and then broken, but not peeled off. This break is in the PE film and is not due to interface delamination, indicating a much higher interfacial adhesion between layers. This behavior appears to be related to an intimate contact between layers when the bonding temperature is near Tm of PA, satisfying the requirements for wetting and increasing the interfacial interactions.

The interaction of the PA with the polyethylene blend was characterized by FTIR-ATR spectroscopy. The spectra of the PE films' surfaces, separated from the PA film, are summarized in Fig. 2 as a function of maleated PE content. The peak at 1645 [cm.sup.-1] is related to the C = O stretching of amide, which was used to differentiate one polymer from another. Defining a carbonyl index (C.I.) as the ratio between the peak at 1645 [cm.sup.-1] and a reference peak height at 1461 [cm.sup.-1], it was noted that this index increased with the maleated PE content and with the bonding temperature (Table 2). This increase on C = O stretching of amide absorbance implies that the PE film surface contains certain amounts of a mixture of PE and PA.

The films bonded at 180 [degrees] C show the lowest value in carbonyl index for the whole range of maleated PE composition. This indicates that only small amounts of PA were present on the PE film surface, while the other films, bonded at 220 [degrees] C, present higher values of C.I., which increase with the maleated PE content, indicating that the film's surface is a mixture of both polymers. It can be concluded that the interface of the multilayer specimens consists of both PE and PA. especially when bonding was made at temperatures sufficiently high for both polymers to be in a molten state.

The surfaces of the delaminated films with different LLDPE-gMA contents, bonded at 220 [degrees] C, were characterized by SEM, as shown in Fig. 3. The film surface for 0% LLDPE-gMA is smooth, implying poor adhesion, as shown in Fig. 3a. With 10% and 15% of LLDPEgMA, the film surface micrographs are remarkably different. The film surface with 15% of LLDPE-gMA [TABULAR DATA FOR TABLE 2 OMITTED] [ILLUSTRATION FOR FIGURE 3C OMITTED] shows a rough surface with continuous striations as a result of extensive plastic deformation. This surface deformation increases with the peel force as well as with the LLDPE-gMA content.

In summary, the behavior of peel strength with bonding temperature is in agreement with similar findings in multilayer systems studied by Bidaux et aL (17), Yamakawa (22), and Sung (23). The implication of these results is that to achieve a good joint strength in a compatible polymer/polymer system, bonding must be made at a condition where both polymers have sufficient molecular mobility to develop an interdiffused layer at the interface. This condition is frequently met during coextrusion in which the plastic melt flows, establishing an intimate contact between layers and satisfying the requirements for wetting. However, in an incompatible system, as PE-PA, this is not sufficient to achieve good adhesion. In addition to this intimate contact between incompatible polymers, specific interactions as chemical covalent bonding of maleic anhydride with PA end groups, to create a block copolymer at the interface, played a key role in the improvement of the interfacial adhesion across the interface of PE and PA films. This finding has been extensively reported for polyolefin/PA blends (12-16, 24-28) and more recently in a similar multilayer system studied by Bidaux et al. (17), in which the formation in situ of a block polyolefin-polyamide copolymer at the interface was investigated. Molau tests, thermograms of fusion and infrared spectroscopy of soxhlet extracted film residues and atomic force micrographs (AFM) of the peeled surfaces are under study on this direction. The dependence of peel strength on bonding time and molecular weight of the modified polymer for this system and for others using EVOH as the central layer is also under study.
Table 3. Heat of Fusion and Crystallinity for LLDPE/LLDPE-gMA
Blends.

LLDPE-gMA Content [Delta]Hm Crystallinity
(wt %) (J/g) (%)

0 79.72 28.48
10 62.37 22.30
15 63.55 22.71
20 56.64 21.84
55 52.82 18.98


Heat of fusion and crystallinity of LLDPE on the LLDPE/LLDPE-gMA blends are shown in Table 3. It can be observed that LLDPE crystallinity shows a continuous decrease as the maleated PE content increases. This decrease in crystallinity could be caused by a difficulty in polymer chain arrangement induced by the modified polymer in the blend.

[O.sub.2] permeability and [H.sub.2]O vapor transmission values, for the three-layer films and the five-layer reference film, are given in Table 4. [O.sub.2] permeability values of the three-layer films show no significant differences among them and are quite similar to those of the five-layer film. This could be related to the minimus differences of the oxygen barrier film (PA) in each film. On the other hand, [H.sub.2]O vapor transmission [TABULAR DATA FOR TABLE 4 OMITTED] values for the three-layer films show an increase with the LLDPE-gMA content. A 60% increase in water vapor transmission was observed when the LLDPE-gMA content in the film increased from 5% to 55%. This seems to be related to a different morphology and a reduction in the degree of crystallinity induced for the polar groups of the MA on the LLDPEgMA, as was observed in the crystallinity results. The [H.sub.2]O vapor transmission values of the three-layer films are quite close to that of the five-layer film only when LLDPE-gMA contents are lower than 15%, even though better adhesion performance was achieved with higher LLDPE-gMA content.

CONCLUSIONS

* The peel strength of a joint consisting of two layers of incompatible polymers such as PE and PA can be increased using maleated PE blended in the PE layer.

* The peel strength of a joint consisting of films of PA and LLDPE-LLDPE-gMA blends shows a sharp increase when LLDPE-gMA content is increased above 10 wt%. A good interfacial adhesion is achieved when using 15% of maleated PE in the external layers.

* The peel force of this joint shows a sharp increase when the bonding temperature is increased above the melting temperature of both polymers.

* The sharp increase in peel strength appears to be influenced by the polymer/polymer interdiffusion at the interface as well as specific interactions between maleated PE and PA, such as the development of covalent bonds through the reaction of the anhydride with the polyamide end groups, across the interface.

* The use of higher contents of LLDPE-gMA in a three-layer film, even though it increases the adhesion performance, also increases the water vapor transmission rate by a reduction in the degree of crystallinity.

ACKNOWLEDGMENT

The authors wish to thank the Mexican National Council of Science and Technology (CONACyT) for partly financing this study.

NOMENCLATURE

PE Polyethylene.

PA Polyamide.

EVOH Ethylene vinyl alcohol copolymer.

LLDPE Linear low-density polyethylene.

LLDPE-gMA Linear low-density polyethylene grafted maleic anhydride.

MA Maleic anhydride.

FTIR Fourier transform infrared spectroscopy.

FTIR-ATR Fourier transform intrared-attenuated total reflection spectroscopy.

SEM Scanning electron microscopy.

[Delta] Hm Enthalpy of fusion.

RH Relative humidity.

T Temperature.

Tm Melting temperature.

C.I. Carbonyl index.

Mw Weight-average molecular weight.

Mn Number-average molecular weight.

MFR Melt flow rate.

A Absorbance.

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Author:Sanchez-Valdes, S.; Orona-Villarreal, F.; Lopez-Quintanilla, M.; Yanez-Flores, I.; De Valle, L.F. Ra
Publication:Polymer Engineering and Science
Date:Jan 1, 1998
Words:3090
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