Heat seal properties of polymer–aluminum–polymer composite films for application in pouch lithium-ion battery

Abstract

Polymer–metal–polymer composite films are widely used in packaging, building, and cooling, and more recently as envelopes for pouch lithium-ion batteries (LIBs). The influences of heat seal temperature and dwell time on the heat seal strength (HSS) of five different multilayer films were investigated by T-peel testing. The failure modes were observed and analyzed by digital optical microscopy. Heat-sealing temperature and dwell time interacted and simultaneously influenced the HSS. Temperature was confirmed as the primary factor while dwell time was secondary. Failure modes included interfacial failure, cohesive failure, material break (root) alone or combined with partial delamination, material break (remote) or combined with partial cohesive failure, material necking alone or combined with partial cohesive failure were closely related to the HSS. The optimum combinations of temperature and dwell time for each multilayer film were obtained in the respective process windows correlating to different failure modes. The sealant layer thickness and material processing also affected the HSS.

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Introduction

Polymer–metal–polymer composite films are widely used in packaging^1–10 and more recently in construction and cooling as the envelope material for vacuum insulation panels.^7,11–15 Most recently, these composite films have been used in soft-packaged or pouch lithium-ion batteries (LIBs).^16–19 For LIB packaging, composite films must provide high barrier levels to gas, light, water, and electrolyte corrosion during the service life of the LIB. As a result, trilayer materials are widely used as envelopes for LIB packaging. A nylon (polyamide, PA) layer provides mechanical durability, an aluminum layer blocks gas and water vapor, and a third polypropylene (PP) layer adds stability and sealability.

Many studies concerning heat sealing properties and the characteristics of sealant materials have been reported. Meka and Stehling¹ calculated and measured the interfacial temperature of various polyethylene (PE) films. They determined that higher pressures had no beneficial influence on the seal properties, but determined the seal appearance at temperatures above the final melting point of the polymers. Stehling and Meka² studied the effect of sealing temperature on the seal strength of various PE films. They found that seal strength as a function of sealing temperature changed with the fraction of amorphous PE in the film. Mueller et al.³ studied the effects of platen temperature and dwell time on the seal strength of linear low-density polyethylene (LLDPE) and examined the peel fracture surface by scanning electron microscopy (SEM). They found that the strong influence of the seal temperature was related to the heterogeneous composition of the LLDPE. Yuan et al.⁴ studied the heat sealability of polymer films with LLDPE and low-density polyethylene (LDPE) as sealants. The heat seal strength (HSS) was determined primarily by the platen temperature and secondarily by dwell time. Su et al.⁵ studied the influence of heat seal temperature on the heat seal properties of soy protein isolate/poly(vinyl alcohol)-blend films. The mechanical properties were affected by the interfacial microstructure between the laminated films. Mihindukulasuriya and Lim⁶ studied the effects of liquid contaminants on the HSS of LDPE, determining that the jaw pressure was necessary to displace liquids from the seal area to form intact seals. These studies focused on optimizing the heat-sealing parameters for single-layer semicrystalline polymer films and understanding the molecular mechanisms involved in seal formation.

Planes et al.⁷ investigated the heat-sealing properties of composites of one PE layer and one or three aluminum-coated polyethylene terephthalate (PET) layers, optimizing the heat sealing parameters of such composite polymeric films. Tsujii et al.⁸ studied the effect of the crystallinity of oriented PA- and LLDPE-laminated films on the strength of the heat seal, finding profound effects of crystallinity on the mechanical properties. Tetsuya et al.⁹ studied the effect of the heat-sealing temperature on the mechanical properties and morphologies of oriented polypropylene (OPP)/cast polypropylene (CPP) laminated films. A minimum seal initiation temperature of 120 °C was identified for OPP/CPP laminate heat sealing. Three failure types were observed during the peel test. Hashimoto et al.¹⁰ investigated the effects of heat-sealing temperature on the failure criteria of OPP/CPP heat seals; the failure criteria were sensitive to the heat-sealing temperature. Andreasson et al.¹¹ studied the micro-mechanisms of fracture in a laminate composed of an aluminum foil and a polymer film, determining that failures occurred through localized plasticity. Vacuum insulation panels (VIP) with envelopes made of composite polymer films have been reviewed by many researchers.^12–15 The performance of the composite polymer films, however, was studied only briefly. The mentioned studies focused on the heat-seal properties of multilayer films, omitting the influence of metal layers.

The direct study of the heat-seal properties of polymer–metal–polymer composite films has received little scientific attention,^16–19 especially for the packaging and storage of pouch LIBs.^17–19 Devisme et al.¹⁶ studied the influence of processing parameters on adhesion in grafted PP/aluminum laminates made by extrusion coating. They found that high-temperature conditions improved adhesion by increasing the open time for the reaction and the rate of the chemical reaction between the grafted polymer chains and aluminum. Xu et al.^17–19 studied the influence of molybdate-treated aluminum foil and PP grafted with glycidyl methacrylate (GMA), acrylic acid (AA), or maleic anhydride (MAH) on the peeling strength of polymer–aluminum laminated films. They found that the peeling strength of the molybdate-treated foil was approximately 10 times greater than that of the untreated foil. The interfacial peel strength of the GMA-, AA-, or MAH-modified PP was significantly improved compared to that of pure PP. They only studied the surface morphologies of Al foils after peeling by SEM and didn't mention the failure mode.

This study targets the heat-seal properties of five different polymer–aluminum–polymer composite films, which are popular packaging materials of commercial pouch LIBs, by identifying and analyzing the principle sealability parameters. The HSS from varying processing conditions is investigated: the temperature and the dwell time are varied, while the pressure, which has no measureable effect on HSS,^1–4,7–10 is fixed at 0.4 MPa. The influence of the heat seal temperature, heat seal dwell time, thickness of sealant material layer, and processing of the sealant materials on the HSS are discussed. The failure behavior and the morphology of sealed zones are investigated through tensile peel testing and digital optical microscopy (DOM).

Experiments

Materials

Five different polymer–aluminum–polymer composite films S-1, S-2, S-3, S-4, and S-5 which were used as package materials of LIBs, supplied by different manufacturers, are studied. The structure and material of each sample are summarized in Table 1 and Fig. 1. The outermost layer is PA, which is mainly decorative but has some anti-piercing capability to prevent leaking. The intermediate layer is Al foil, the carrier of the heat-sealing material, which prevents the penetration of moisture and gas. The innermost layer of CPP or PP is the heat-sealing layer, which has the most influence on the heat-sealing mechanical properties.

Table 1 Composition and thickness of each layer of composite film

Sample	Total thickness (μm)	Nylon (μm)	Al foil (μm)	CPP (μm)	Adhesive (μm)
S-1	150	25	40	80	5
S-2	115	25	40	45	5
S-3	110	25	40	40	5
S-4	155	25	40	85	5
S-5	155	25	40	30 (CPP) + 50 (PP)	10

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Fig. 1 Schematic illustrating the structure of composite films.

Specimen preparation

To characterize the sealed zones, samples were cut and superposed to be sealed, as shown in Fig. 2. According to the ASTM F88/F88M-09 (ref. 20) and GB/T 22638.7-08 (ref. 21) standards, the composite films were cut into strips of specific sizes, also shown in Fig. 2. Two strips of the composite films of the same length are heat-sealed together to form one heat-sealing specimen. The length of the heat-sealed zone is 6.5 mm.

Fig. 2 Fabrication progress of heat-sealed specimens and dimensions.

Heat-sealing and heat sealing conditions

Sealing technologies include heat sealing, hot air welding, ultrasonic welding, and chemical adhesives. Among these methods, the heat sealing technique is the most common. The prepared film strips were heat-sealed on a PX-KF-02 top/side heat-sealing machine. The strips were placed face-to-face between the upper and lower heat-sealing pliers of the seal-testing machine. In order to study and optimize the heat-sealing temperature and dwell time for each composite film, we investigated the mechanical properties and morphologies at varying sealing temperatures from 160 to 230 °C and dwell times from 1 to 12 s. The pressure was set to 0.4 MPa throughout the heat-sealing procedure. After heat-sealing was performed, the films were cooled to 25 °C at ambient conditions.

T-peel test

The HSSs were determined by tension peel testing on a ZWICK universal testing machine, as shown in Fig. 3. The specimens were aligned with seal lines perpendicular to the direction of tension. All tests were conducted at about 25 °C with a strain rate of 45 mm min⁻¹. During the test, the maximum load was recorded. The HSS was defined as the maximum tensile load divided by the sample width, in units of N/15 mm. The HSS was averaged over five samples for every heat-sealing condition. In addition, the failure mode of each test was carefully examined for further investigation later in the study.

Fig. 3 T-peel test.

Tensile tests

Uniaxial tensile tests were also performed on the raw composite films using the universal testing machine to fully characterize the performances of the films. The tensile tests were conducted on strip-form specimens measuring 15 mm in width and 100 mm in length at the speed of 45 mm min⁻¹. For each material, five specimens were tested.

Digital optical microscopy

The seal morphologies were studied through images obtained by DOM (Huabai H800X, China). The microscopic observation was intended to elucidate the influence of the sealability parameters on the heat seal morphology and to provide a deeper insight into the failure mechanisms experienced by the specimens.

Results and discussion

Load-displacement curves of composite films before and after heat-sealing

The load-displacement curves of five different composite films both before and after heat-sealing were studied. Fig. 4 shows the typical load-displacement curves of S-1 under different heat-sealing conditions. The load-displacement curves change with the raw materials because of the heat sealing temperatures, dwell times, and pressures applied to the heat-sealed zone during fabrication. The heat-sealed zone should be treated as a new material with unique properties generated during the fabrication process. The highest load and elongation values of each curve vary based on the corresponding failure mode. The HSS and elongation have the maximum values under remote material breakage or necking failures.

Fig. 4 Typical load-displacement curves of S-1 tensile specimens.

Influence of heat-sealing temperature on HSS

The effect of the sealing temperature on the HSS of the five different composite films was examined. The HSS values of the heat-sealed samples under different heat-sealing temperatures are shown in Table 2 for S-1 and in Fig. 5 for the four other materials. For low sealing temperatures, HSS values approaching or equal to zero are observed (see Table 2). This may relate to the insufficient melting of the CPP or PP layer, preventing molecular interdiffusion between layers of the seal. These phenomena were observed previously for both PE^1,2 and LDPE.^3,4,6,7 In the following discussion, we omit HSS values of zero or very small magnitudes. The minimum heat seal temperature and dwell time are defined as 170 °C and 3 s, respectively. This initiation temperature was confirmed by differential scanning calorimetry (DSC) results, which determined a melting temperature of 164.5 °C for CPP or PP.²²

Table 2 HSS of S-1 material under different heat-sealed processing conditions

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Fig. 5 HSS versus heat temperature for different materials at various dwell times.

From Table 2 and Fig. 5, the HSS changes similarly to the changes in heat-sealing temperature for all five composite films. The HSS increases with increased heat-sealing temperature, particularly in a certain temperature range. The increase of HSS with the sealing temperature gradually slows, and the HSS value stabilizes. Intuitively, the HSS is expected to decrease under higher temperatures and longer dwell times. The HSS initially increases with the heat-sealing temperature because the heat-sealing temperature permits more thorough fusing of the sealant layer material, allowing its hot tack capacity to be utilized more fully. In addition, the complete melting of the sealant layer material fills the sealing region thus causing the HSS to reach a maximum value and stabilize. After this point, the increasing temperature causes the melt sealant materials to overflow the heat-sealing area, especially at longer dwell times. Additional improvement of HSS is impossible because the melt decreases in viscosity; combined with the applied pressure, this causes the sealant material to thin.⁹ In a certain regime, the HSS is reduced because of crack formation caused by the enhanced quench cooling effect.⁸

Influence of dwell time on HSS

Fig. 6 shows the effect of dwell time on the HSS at different temperatures for all five composite films. Similarly to the influence of the heat-sealing temperature on the HSS, the HSS initially experiences notable increase with increased heat-sealing dwell time, before becoming stable. Sufficient dwell time is required to obtain a certain HSS at lower temperatures. When the heat seal temperature is below the melt point of the sealant material, no extension of dwell time can improve the sealing capacity. When the temperature is sufficiently high, for example, exceeding 210 °C for S-1, HSS becomes independent of dwell time. The sealant material layer of the heat-sealing zone is melted thoroughly above a certain temperature; longer sealing times cannot aid sealability.

Fig. 6 HSS versus dwell time for different materials at various heat temperatures.

Based on the analysis of Table 2, Fig. 5 and 6, the heat temperature and dwell time are understood to be interrelated in obtaining the HSS. However, the heating temperature is more important, while the dwell time is of secondary importance. Previous researchers^1–3,5 have also found that the dwell time had less influence on the seals than the sealing temperature. The pressure, applied to create close contact between the heat seal layers and prevent deconsolidation during melting, was necessary for producing high-quality adhesion.^1–4,7–10 Higher pressures had no beneficial influence on seal properties.^1–3

Tensile failure modes and mechanisms

After each T-peel test, the failure mode was carefully examined and summarized. The failure modes and the ultimate strength of the heat seal were interrelated. Tensile failure modes of the heat-sealed zones of the composite films involved at least five failure mechanisms. Fig. 7 shows a schematic of the different failure modes, as well as photographs of failed films obtained by DOM. Interfacial failure (failure mode A) occurs when the heat seal temperature is low, dwell time is short, or both. The two sealant films are fully de-bonded, which is not acceptable during service. Cohesive failure (failure mode B-1) occurs when the heat seal temperature, dwell time, or both are intermediate. The two sealant films are stretched gradually, resulting in high elongation at failure. This is an acceptable failure mode in service, representing the good quality of the heat seal. Peel failure, either alone or combined with delamination (failure mode B-2), also occurs under heat-seal fabrication parameters similar to those of failure mode B-1. The sealant materials both neck and experience cohesive failure, which causes delamination between the sealed film and aluminum layer. For simplification, we have combined failure modes B-1 and B-2 into cohesive failure mode B. Material break alone at the root or combined with partial cohesive failure (failure mode C) occurs at higher temperatures, longer dwell times, or both. In this mode, HSS is high and elongation is low (see Fig. 4). Remote material failure, either alone or combined with partial cohesive failure (failure mode D), occurs when the HSS exceeds the strength of the composite films. This mode indicates that the HSS quality is very good. Material necking failure (failure mode E) occurs when the HSS is much higher than the strength of composite films. No significant fracture phenomena are observed, except for the severe necking of the composite film wings. Materials in the heat-sealing area change slightly, remaining tightly bonded. Failure occurring outside the heat-sealed zone indicates that the HSS exceeds the strength of the raw composite films. Tetsuya et al.⁹ found three patterns of failure (fracture at heat seal, peeling at sealed area, and fracture at edge of heat seal) when they studied the mechanical properties of OPP/CPP heat seal. Three failure modes were also found by other scientists^1–3,5,7 which studied the heal seal properties of other one-layer films. Planes et al.⁷ also found new failure mode (peeling and tearing add delamination between PE and PET layers) for multilayer films. That is to say, a larger variety of failure modes would be appeared for multilayer LDPE-PET films than for simple films. Our results also demonstrate more complex relationships between failure modes and heat-seal processing of polymer–metal–polymer composite films.

Fig. 7 Typical failure modes of five studied composite films.

The morphologies, obtained by DOM, revealed that the seals were partially formed at lower temperatures, shorter dwell times, or both, while the CPP or PP were fused completely at high temperatures, long dwell times, or both. Intermediate temperatures or times created seals showing intermediate properties. Tetsuya et al.⁹ observed that the boundary between the CPP–CPP heat seal OPP/CPP laminated films wasn't distinguishable by SEM when temperature was much higher than the melting temperature of both OPP and CPP films.

In the T-peel tests, the location of the failure sometimes shifted from the heat-sealing zone to the non-heat-sealed zone, accompanied by necking and yielding of the composite films (see C, D, and E mode). The film strength could not be evaluated by the T-peel test alone, so the tensile testing of the raw composite films was performed: special tensile specimens should be fabricated in order to solve this problem in further studies.

The failure modes and mechanisms of the heat-sealed zone were strongly influenced by the processing parameters. The relationship between failure modes and heat seal conditions is summarized in Table 3. Obviously, containers of composite films can be no stronger than the seals holding the containers together.²³ Acceptable heat seals are defined as those which have strengths greater than the strength of the packaging material. This is true of films experiencing failure modes C, D, or E. The optimum combination of heat temperature and dwell time for each composite film can be obtained in the respective processing windows that created films experiencing these failure modes. The optimum sealing temperature is a range in which certain HSS values can be produced. The optimum dwell time ensures that sufficient heat is applied to the sealant material, without wasting time and reducing the production speed.

Table 3 Failure mode distribution of each specimen for S-1 composite films with different temperature–time combinations

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The tensile failure mode distribution of specimens under a variety of processing conditions can be represented by a rectangular area graph (Fig. 8). The vertical axis represents the heat-seal dwell time, the horizontal axis represents the heat-sealing temperature, and each region represents the corresponding main failure mode. The main tensile failure mode is interfacial failure for heat-sealing temperatures of 160 °C, 170 °C, and 180 °C and dwell times of 3, 6, and 9 s. According to the failure modes provided by the figure, the region of cohesive or material necking failures (regions B and E) is the optimized heat-sealing processing window. The material necking failure results from higher HSS values, and therefore the best combination of heat-sealing temperatures and dwell times for the S-1 composite should be located around the boundary lines of regions B and E. Based on the discussion in the above sections, optimal heat-sealing parameters can be inferred to exist, considering either high HSS or simple processing. The area surrounded by the dash line shown in Fig. 8 contains these optimal parameters. To achieve a high strength, the area above the red line produces the correct processing parameters, while areas under the red line allow for process simplification.

Fig. 8 Failure mode distribution of S-1 composite film.

Influence of sealant layer thickness on HSS

Fig. 9 shows the HSS as a function of dwell time for S-1 and S-2 at 170 °C and 210 °C, respectively. S-1 and S-2 are identical in all aspects other than sealant layer thickness (see Table 1). The HSS values of these two films are similar at lower temperatures (170 °C) and shorter dwell times (3 and 6 s). A large difference in HSS value is observed at higher temperatures for all dwell times or lower temperatures at longer dwell times. At lower temperatures and shorter dwell times, CPP melting is limited, which causes similar HSS values for the two films. At higher temperatures or longer dwell times, the sealant layer is melted more thoroughly. The influence of sealant layer thickness on HSS is significant: the seal temperature lower limit increases with increasing sealant layer thickness. Above the minimum temperature, the HSS is greatly increased with increases in sealant layer thickness. This is similar to the results of van Malsen et al.,¹⁴ who studied the HSS of LLPDE.

Fig. 9 HSS versus dwell time for S-1 and S-2 at two temperatures.

Influence of different sealant layer processing parameters on HSS

The influence of different sealant layer processing parameters on HSS was studied at constant total sample thickness. Fig. 10 shows the HSS as a function of dwell time for S-4 and S-5, which had identical sealant layer thicknesses but different compositions (see Table 1). The HSS of S-4 is significantly higher than that of S-5, regardless of the heat-sealing temperature for all dwell times. This indicates that CPP sealant is better than PP sealant from a mechanical perspective.

Fig. 10 HSS versus dwell time for S-4 and S-5 at two temperatures.

Conclusion

The influence of the heat-seal temperature and dwell time on the HSS of five polymer–aluminum–polymer composite films was investigated. With increasing temperature, dwell time, or both, the HSS increased, reached a maximum value, and plateaued. With further increases in temperature or dwell time, the HSS was decreased. Though temperature and dwell time interacted and simultaneously influenced the HSS, temperature was the most important factor while time was secondary. When the temperature was sufficiently high, HSS became independent of dwell time.

Interfacial failure, cohesive failure, material break at root alone or combined with partial cohesive failure, remote material failure alone or combined with partial cohesive failure, and material necking failure occurred. Failure occurring outside the heat-sealed zone indicated that the HSS was superior to the strength of the raw composite films. The HSS and failure mode were closely related.

The optimum combination of heat temperature and dwell time for each polymer–aluminum–polymer composite film was obtained in the respective processing windows correlating to different failure modes. The optimum sealing temperature was located in a range of settings at which certain HSS values were produced. The optimum dwell time setting ensured that sufficient heat was applied to the sealant material, without wasting time or sacrificing production speed.

The HSS also depended on the sealant layer thickness and material processing. With increasing sealant layer thickness, the minimum heat-sealing temperature increased, and the HSS also increased. For different processing of sealant material, common PP decreased the HSS compared with CPP.

This study may benefit the understanding of the performance of polymer–aluminum–polymer composite films. The results may be important for choosing more mechanically stable polymer–aluminum–polymer composite films for packaging LIBs. The moisture and gas permeation rates are also of great importance, and should be used as complementary criteria for the selection of heat-seal fabrication conditions.

Acknowledgements

The authors gratefully acknowledge the financial support by the National Science Foundation of China (No. 11472165 and 11332005).

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