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Linear low density polyethylene: Microstructure and sealing properties correlation
Accepted Manuscript Linear Low Density Polyethylene: microstructure and sealing properties correlation Adriane G. Simanke, Cristóvão de Lemos, Márcia Pires
PII:
S0142-9418(12)00226-7
DOI:
10.1016/j.polymertesting.2012.11.010
Reference:
POTE 3978
To appear in:
Polymer Testing
Received Date: 1 October 2012 Accepted Date: 19 November 2012
Please cite this article as: A.G. Simanke, C. de Lemos, M. Pires, Linear Low Density Polyethylene: microstructure and sealing properties correlation, Polymer Testing (2012), doi: 10.1016/ j.polymertesting.2012.11.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Material Properties Linear Low Density Polyethylene: microstructure and sealing properties correlation Adriane G. Simanke*, Cristóvão de Lemos, Márcia Pires Braskem SA, III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, Triunfo, RS, Brazil
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Abstract: The microstructures of four commercial linear low density polyethylenes (LLDPE) were evaluated and correlated with their sealing properties. Atomic Force Microscopy (AFM), Temperature Rising Elution Fractionation (TREF), Differential Scanning Calorimetry (DSC) and Crystallization Analysis Fractionation (CRYSTAF) experiments revealed that the comonomer distribution is one of the main factors that influence the sealing properties. The superior sealing performance showed by metallocene LLDPE samples in comparison to Ziegler-Natta LLDPE samples can be attributed to their well balanced chemical composition distribution.
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Keywords: LLDPE, sealing properties, morphology, comonomer distribution 1. Introduction
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Polyethylene is the most widely used thermoplastic polymer in the world, being made into products ranging from clear food wrap and plastic bags to laundry detergent bottles and automobile fuel tanks [1]. Among the different polyethylene types, linear low density polyethylene (LLDPE) represents almost 30% of the total polyethylene global consumption [2]. The structural parameters that generally influence the ultimate properties of LLDPE are: type, amount and distribution of comonomer, average molecular weight and molecular weight distribution [3-5]. The development of new catalysts and process technologies has motivated the continuous improvement of LLDPE properties and the ability to tailor it for a wide range of applications [6-8]. LLDPE is used mainly in film application due to its toughness, flexibility and relative transparency. Product examples range from multilayer films used in food packaging to agricultural films. Among the different properties required in a material to be applied in flexible food packaging, sealing performance is one of the most important, being essential for the integrity of the package. LLDPE with good sealing performance is expected to present low sealing temperature, broad sealing window and high packaging performance, contributing to cost reduction by speeding up the automatic packaging processes. Heat sealing is a technique of sealing two materials under the combined effect of three parameters: pressure, temperature and time. The sealing process involves melting, interdiffusion and crystallization of macromolecules at the interface of two materials to be sealed. In the case of semicrystalline polymers, the sealing temperature must be near the melting temperature of the polymer to allow macromolecular mobility at the interface. The sealing strength, toughness, failure mode and appearance of these seals after cooling to room temperature are important seal variables [9,10]. Considering that there are many different commercial grades of LLDPE on the market with different sealing properties, it becomes increasingly important to determine which structural features contribute to good sealing performance. Since crystallization kinetics plays an important role in the sealing mechanism, the use of a powerful tool to understand the behavior on the film surface becomes necessary. Atomic Force Microscopy (AFM) has been widely applied to issues regarding polymer crystallization on the surface, allowing observation of crystal growth, melting and reorganization at the lamellar scale, determining how structure evolves and how local conditions influence the crystallization kinetics [11,12]. Few studies regarding polyethylene microstructure and sealing performance correlation are found at the literature [9,10,13-15]. Different from them, in this work, four LLDPE with similar comonomer content were characterized in detail in order to elucidate the influence of their molecular microstructure on the sealing performance. 2. Experimental *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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2.1 Materials Four commercial linear low density polyethylenes (LLDPE) were used in this work. Two of them are Ziegler-Natta LLDPE with 1-octene as comonomer (LLDPE ZN1 and LLDPE ZN2), and the other two are metallocene LLDPE with two different comonomers, 1-hexene and 1-octene (LLDPE M1 and LLDPE M2, respectively). Their main characteristics are described in Table 1.
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2.2 Extrusion of the films Films with average thickness of 50-55 µm were produced in a single screw extruder for casting flat film (Leonard EMF 30F equipment). 2.3 Characterization Techniques
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2.3.1 Amount of soluble polymer in ortho-dichlorobenzene (o-DCB) In order to quantify the amount of polymer soluble in o-DCB, 2g of LLDPE was dissolved in 250mL of o-DCB at 135°C, under stirring and inert atmosphere. After 30 min, the solution was allowed to cool to 25°C and kept at this temperature for 30 min. The insoluble fraction was separated by filtration and the filtered solution was precipitated in acetone and methanol, followed by filtration. After separating the precipitated solid, the filtered solution was evaporated under nitrogen flow and the residue was dried under vacuum at 90°C until constant weight was reached.
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2.3.2 Temperature Rising Elution Fractionation (TREF) Samples were fractionated according to their chemical composition distribution in a Polymer Char PREP mc2 equipment (automated preparative TREF). Samples were put into the crystallization vessels and dissolved in xylene at 130°C (at a concentration of 0.1 mg/mL). After 60 min of stabilization, the temperature of the system was decreased from 130°C to 30°C at a rate of 0.1°C/min, allowing polymer chains to crystallize in orderly fashion from higher to lower crystallinity. Next, in the elution step, preheated xylene was used as eluent to collect the fractions under heating at predetermined temperatures, ranging from 30 to 130°C with isothermal stays of 20 min. At least seven fractions were collected at different elution temperature intervals for each sample, which were isolated by precipitation in methanol, washed with acetone followed by drying to a constant weight. Fractions were characterized by DSC, FTIR, CRYSTAF and AFM.
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2.3.3 Crystallization Analysis Fractionation (CRYSTAF) The chemical composition distribution profiles of the fractions were obtained by crystallization analysis fractionation (CRYSTAF) in a Polymer Char Crystaf 200+ instrument. LLDPE was dissolved in o-DCB at 160°C (at a concentration of 0.1 mg/mL) and kept at this temperature for 60 min to ensure complete dissolution. Then, the temperature was decreased to 95°C and held for 45 min for stabilization before starting the fractionation. Polymer solution was cooled to 30°C at a constant rate of 0.2°C/min. At higher temperatures, polymer chains with low comonomer content crystallized inside the vessel. Every 5 min, aliquots of polymer solution were collected via an in-line filter and transferred to the in-line infrared detector, which monitors the changes in the concentration of polymer solution, generating the integral Crystaf curve. The Crystaf profiles showed in this paper are the differential form of the integral curve. Detailed Crystaf procedure and interpretation were presented by Monrabal [16]. 2.3.4 Infrared Spectroscopy (FTIR) Infrared spectra of the films and the fractions obtained by TREF were recorded on a Nicolet 710 FTIR in order to determine the 1-hexene and 1-octene contents. A calibration curve built with LLDPE standards was used to determine the comonomer content considering the relation between the area of methyl bands (1379-1360 cm-1) and the area related to the thickness of the sample (44823950 cm-1). Twenty four scans were performed for each sample. *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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2.3.5 Gel Permeation Chromatography (GPC) The molecular weight and molecular weight distribution were determined by GPC in a Waters GPCV 2000 equipped with viscometer and optical differential refractometer detectors. A set of four columns Toso-Hass (HT3, HT4, HT5, and HT6) and a pre-column of 500 Å were used. The measurements were carried out at 145°C using 1,2,4-trichlorobenzene as solvent, at a volumetric flow rate of 1 mL/min. The molecular weights were determined using a calibrated curve obtained from a series of monodisperse polystyrene standards and narrow molar mass LLDPE and polypropylene.
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2.3.6 Differential Scanning Calorimetry (DSC) Thermal behavior was analyzed by differential scanning calorimetry using a TA Q1000 DSC under nitrogen and connected to an intracooler that allows sub ambient temperature control. The instrument was calibrated with indium. Samples were melted at 200°C, cooled from 200°C to -20°C and heated from -20°C to 200°C at a heating rate of 10 °C/min. The melting temperature (Tm) and enthalpy of fusion (∆Hf) were taken from the second heating curve.
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2.3.7 Atomic Force Microscopy (AFM) AFM images were obtained using Veeco NanoScope V atomic force microscopy and Veeco diTAC heater system operating under heating conditions. Topography and phase images were simultaneously collected in tapping mode at 512 x 512 lines standard resolution. Veeco single side coated silicon cantilevers were used with resonant frequency at 366-401 kHz. According to the manufacturer’s specifications, the cantilevers have a spring constant of 20-80 N/m, length of 110-140 µm, width of 25-35 µm and the radius of the tip is 2-5 nm. Film samples of nominal thickness of 5055 µm and without previous surface treatment were subject to scans of 40 µm x 40 µm using a scan rate of 0.878 Hz. Sample was quickly heated to 200 °C and kept at this temperature for 5 min. In a second stage, the melted polymer was quickly cooled to the isotherm temperature of scanning to observe the crystallization step and lamellae growth.
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2.3.8 Measurement of Sealing Properties (hot tack strength and heat seal strength)
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The hot tack strength of the film was measured using a Hot Tack Tester 4000 from J&B instruments. A pair of LLDPE film ribbons with 25.4mm width was sandwiched between two layers of polyester film to avoid their adherence to the heated jaws, then, they were fitted between two heated bars and hot pressed together at 44 psi for 1 s. The hot tack strength was measured at temperatures ranging from 95° to 160°C, according to ASTM F1921 [17]. The heat seal strength of the sealed film was measured using a Teller Model EB laboratory heat sealer, 22 s after the seal jaw opening. Temperatures and pressure used were the same applied in the hot tack sealing test. The test method was carried out according to ASTM F2029 [18]. For both tests, the specimens were obtained from the central part of the films with 50-55 µm of thickness. 3. Results and Discussion
3.1 Microstructure characterization All LLDPEs studied are typical film grades with weight average molecular masses in the range of 100 kg/gmol, narrow molecular weight distribution and melt flow index between 0.75 and 1 g/10min, as shown in Table 1. TABLE 1 Comparing the samples, it is possible to observe that, excepting the LLDPE ZN1, all the other samples showed similar comonomer content. Because of its lower comonomer content, LLDPE ZN1 *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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showed higher melting temperature and degree of crystallinity than the other samples, as can be seen in Table 1. The other three samples showed more than one defined melting temperature, pointing to a heterogeneous comonomer distribution. This unexpected behavior could be caused by the polymerization conditions and industrial technology process used to obtain these samples. In order to investigate the effect of the heterogeneous comonomer distribution on the sealing properties, the samples were fractioned by chemical composition distribution. Figure 1 shows the chemical composition profiles of LLDPE ZN1 and LLDPE ZN2, where it is possible to observe remarkable differences between them. In LLDPE ZN1, around 43% of the fractions were obtained in the range of 100 – 130°C while in LLDPE ZN2 around 80% of the fractions were collected between 70 - 95°C, confirming the influence of the technology and process conditions on the microstructure of these resins. FIGURE 1
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Figure 2 shows the chemical composition profiles of LLDPE M1 and LLDPE M2 where it is possible to observe that both samples eluted in all the temperature range analyzed. In LLDPE M2, most of the fractions (about 65%) eluted at temperatures lower than 80°C, while in LLDPE M1 29% of the sample eluted at 73°C and 31% eluted at 83°C. It is expected that the high concentration of fractions that elutes at lower temperatures contributes to achieve low sealing temperature. FIGURE 2
Fractions obtained by TREF were characterized by FTIR and DSC and the results are shown in Table 2 and 3. Some fractions could not be characterized since the amount recovered was insufficient to be analyzed (all lines indicated as "nd" in Table 2 and 3). TABLE 2 and TABLE 3
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The comonomer content of the fractions was determined by FTIR, and the melting temperature and crystallinity were obtained by DSC. As expected, the lower the elution temperature, the higher the comonomer content and the lower the melting temperature and crystallinity of the fractions. No remarkable differences regarding comonomer content and melting temperature of the fractions were observed between LLDPE M1 and LLDPE M2. On the other hand, when comparing LLDPE ZN1 and LLDPE ZN2, it is possible to observe some differences in the composition of the fractions. In general, for the same elution temperature, fractions of LLDPE ZN1 showed higher comonomer content than fractions of LLDPE ZN2, and unexpected higher melting temperatures (Table 3). The differences in the comonomer content could be attributed to differences in the polymerization conditions and process which are not the subject of this work. The higher melting temperatures observed for lower comonomer contents could be attributed to differences in the comonomer distribution along the polymeric chain, as can be seen in Figure 3. The lower melting temperature observed for LLDPE ZN2 FR4 can be justified by its broader comonomer distribution compared to LLDPE ZN1 FR3. Considering the other fractions (LLDPE ZN1 FR5 and FR7; LLDPE ZN2 FR7 and FR10), no significant differences were observed regarding comonomer distribution. FIGURE 3
3.2 Morphology characterization Different AFM experiments were carried out at different temperatures in order to define the better isothermal temperature to analyze the samples. Table 4 shows the AFM phase images of samples LLDPE ZN1, LLDPE ZN2, LLDPE M1 and LLDPE M2 obtained at 25, 45 and 75 min at an isotherm between 120-125°C. At this isotherm, the molecules started to reorganize themselves at, or close to, a growth front, and the polymer molecules began passing through intermediate degrees of disorder to a more stable state with respect to the crystalline organization, so that the ordering of the *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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chain was maximized. After 25 min of isotherm, all samples indicated transformation from the molten phaseinto the solid crystalline form. In sample LLDPE M2, the bundles were equidistantly separated and showed uniform size. The sample LLDPE M1 showed bundles heterogeneously dispersed and the morphology was similar to LLDPE ZN2, that is, longer bundles with crystallization nuclei. For sample LLDPE ZN1, the morphology was completely different from the other three, because there were ellipses of crystallization poorly dispersed in the melted matrix. After 45 min of isotherm, it is possible to observe that LLDPE ZN1 showed a higher crystallization rate while LLDPE M2 showed s lower crystallization rate. This behavior could be explained considering the chemical composition distribution of these samples. In LLDPE ZN1, more than 40% of the sample eluted in the range of 100-130°C, while in LLDPE M2 around 65% of the sample eluted at temperatures lower than 80°C.The other two samples, LLDPE ZN2 and LLDPE M1, showed similar kinetic profiles and crystallization rate between LLDPE ZN1 and LLDPE M2. Regarding morphology, LLDPE ZN2 and LLDPE M1 were similar, while in LLDPE M2 smaller crystals were observed, as expected, considering its chemical composition profile. At 75 min, most parts of the samples had crystallized and the morphologies were similar to those observed at 45 min. TABLE 4
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AFM analyses were also carried out in order to investigate the morphology of the fractions eluted at 60°C, 80-83°C and 95-100°C. Table 5 shows the AFM images collected at an isotherm between 105 and 110°C for fraction 3 (FR3) of LLDPE ZN1 and LLDPE M1 and for fraction 4 (FR4) of LLDPE ZN2 and LLDPE M2 (all these fractions were eluted at 60°C). Analyzing the images, it is possible to observe that the morphology and kinetic profile observed for the LLDPEs produced with ZieglerNatta catalyst were completely different from the ones observed for LLDPEs synthesized with metallocene catalyst. Growing crystals always create or induce changes in their environment, setting up fields in, for example, concentration, density, temperature or even stress [11]. Such fields are expected to influence the growth kinetics, as well as the structure. According to the images of LLDPE ZN1, a crystal reorganization was verified from 25 min to 45 min of isotherm. It is possible that, during this period of time, there was a sequence of events, with an initial framework of fast growing ‘primary’ lamellae oriented away from the nucleus, followed by slower in-filling ‘secondary’ lamellar growth [19]. After 75 min of isotherm, more than 50% of the sample was totally crystallized. The higher amount of comonomer content in LLDPE ZN1 FR3 could explain the morphological differences when compared to the other samples. The presence of intermediated contrast, which was more pronounced in LLDPE ZN1, between light gray, melt region, and dark gray, crystalline region, is characteristic of a softer phase probably rich in comonomer content. The sequence of events observed for LLDPE ZN2 FR4, with an initial framework of fast growing primary lamellae oriented away from the nucleus and having a discrete thickness, was followed by a protracted period of in-filling growth and crystal reorganization. The crystals emerged from the melted phase, the thickness increased and the structures restarted growing and forming branches from the initial crystals (75 min). The slower kinetic growth of LLDPE ZN2 FR4 compared to LLDPE ZN1 FR3 can be justified by the broader comonomer distribution of LLDPE ZN2 FR4, as can be seen in Figure 3. Considering LLDPE M1 FR3 and LLDPE M2 FR4, the perpendicular growing and overlapping of most of the crystals can be a characteristic given for metallocene catalyst in the initial steps of the crystallization. The morphology of the LLDPE M2 FR4 was in agreement with LLDPE M2 (Table 4), possibly justified by the better distribution of the comonomer 1-octene in the polymeric chain of LLDPE. TABLE 5 Fraction 5 (FR5) of LLDPE ZN1 and LLDPE M1, and fraction 7 (FR7) of LLDPE ZN2 and LLDPE M2 (fractions that were eluted at 80-83°C) were analyzed at an isotherm between 110-115°C (Table 6) and three images of each sample were obtained in the same timeframe described previously. The morphology after 25 min of isotherm was better resolved compared to the fractions eluted at 60°C, *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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meaning there were crystals with good contrast difference and defined edge. Possibly, the lower amount of comonomer in FR5 and FR7 contributed to the better resolution of the images. Interestingly, it is apparent that the initial mechanism of crystallization of samples LLDPE ZN1 FR5 and LLDPE M2 FR7 showed all crystals having nuclei and geometric organization. Especially for LLDPE ZN1 FR5, the crystals showed equivalent size. At 45 min of isotherm, the LLDPE ZN1 FR 5 showed faster kinetic profile than the others. However, at 75 min of isotherm, the order of kinetic growth was: LLDPE ZN1 FR5 = LLDPE ZN2 FR7 > LLDPE M1 FR5 > LLDPE M2 FR7. The lower crystallization kinetics of LLDPE M2 FR7 could be justified by its chemical composition distribution profile (Figure 4). In addition, variations in the density or structure of the melt, as suggested by Ryan et al [20], could be responsible to promote lower lamellar growth rate. TABLE 6 FIGURE 4
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Fraction 7 (FR7) of LLDPE ZN1 and LLDPE M1 and fraction 10 (FR10) of LLDPE ZN2 and LLDPE M2 (fractions that were eluted at 95-100°C) were analyzed at an isotherm between 120125°C (Table 7). After 25 min, it was observed that LLDPE ZN1 FR7 showed significant amount of crystals and satisfactory dispersion. Similar behavior was observed for LLDPE M1 FR7, although the size of the crystals seemed to be smaller. The crystals of LLDPE ZN2 FR10 were larger than the others and also presented good dispersion. LLDPE M2 FR10 showed few crystals and some dots, indicating that this sample had slower crystallization kinetics. Since AFM shows hysteresis phenomena, small displacement of the area scanned can occur due to minimum distortion of the images. At 45 min, it was observed that the domains started to reorganize and to get well defined, disrupting originally, rather well ordered domain structures. In samples LLDPE ZN1 FR7 and LLDPE ZN2 FR10, it was verified that most crystals increased their size, even although some unexpected morphologies originated from secondary lamellar growth could be seen. Particularly, the image of LLDPE M2 FR10 at 45 min of isotherm showed crystals having lamellae growing from a nucleus, but the predominant morphology was lamellae groups oriented away. At 75 min, it was observed the interlacement and self organization of the lamellae of LLDPE ZN1 FR7 and LLDPE M2 FR10, and this behavior is expected to be partially responsible for the highly interwoven morphology that gives strength to these structures [11]. For LLDPE ZN2 FR10 and LLDPE M1 FR7, no significant changes were observed, excepting the increase of the crystals.
3.3 Correlation between microstructure and sealing behavior
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Figure 5 shows the hot tack strength versus sealing temperature curves of the LLDPE samples, in which the initial sealing temperature of each sample is indicated by the first triangle that appears in each curve. Analyzing the curves, it is possible to observe that LLDPE M1 showed better performance than the other samples, more specifically, higher hot tack strength and lower sealing temperature (110°C). Comparing the samples synthesized by metallocene catalyst (LLDPE M1 x LLDPE M2) and analyzing their chemical microstructure, one can conclude that, apparently, the main difference between them is the type of comonomer. According to Figure 2 and Table 2, both samples showed higher than 60% of modified polymer fractions melting at temperatures lower than 115°C, which are the fractions expected to influence the sealing process at this temperature. The chemical composition profile of the LLDPE M1 and LLDPE M2 fractions (eluted at around 60°C and 80°C) are different (Figure 4). Comparing these fractions, it can be observed that LLDPE M1 FR5 crystallized at higher temperature than LLDPE M2 FR7. This characteristic could explain the better hot tack performance, since it may have an important effect on the crystallization step of the sealing mechanism [10]. In this way, these polymer fractions are expected to have the main influence on the molecular process involved in the sealing of semicrystalline polymer films and, consequently, to be responsible for the sealing behavior. In addition to these observations and considering the *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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Considering the samples synthesized with ZN catalyst, LLDPE ZN1 and LLDPE ZN2, the better hot tack performance observed for LLDPE ZN2 can be explained by taking in account their different chemical composition profiles. Besides the higher comonomer content, around 54% of LLDPE ZN2 fractions melted at temperatures lower than 115°C, while in LLDPE ZN1 these fractions represent around 45% (Figure 1 and Table 3). These polymer fractions can melt during the sealing process, diffusing across the film interface, creating entanglements and additional crystals. The main factor that can explain the lower performance of samples produced by ZN catalyst in comparison to metallocene catalyst is their chemical microstructure, mainly because of the comonomer distribution. In Figures 1 and 2 it is possible to observe that in the case of samples LLDPE ZN1 and LLDPE ZN2, a high amount of polymer fractions was eluted at higher temperatures. The higher the temperature, the higher the crystal thickness and more energy to seal is required.
FIGURE 6 4. Conclusions
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In the case of heat seal measurements (Figure 6), no difference was observed between LLDPE M1 and LLDPE M2. This behavior can be explained by the effect of the analyses time. As explained before, heat seal experiments are carried out in a longer time than hot tack measurement; allowing the melted polymer to diffuse across the interface in the longer duration of the heat seal test. An interesting observation is that, even with more time for diffusion and crystallization, LLDPE ZN1 and LLDPE ZN2 showed lower performance than LLDPE M1 and LLDPE M2. Similar to the hot tack discussion, this behavior can be explained by the chemical composition distribution of these samples, showing how important is the presence of a polymer fraction that melts at lower temperatures to the sealing performance.
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The microstructures of commercial LLDPE resins were characterized from investigation of morphology, solubility, fractionation and thermal analyses. Metallocene LLDPE showed better sealing performance (broad sealing window and high sealing strength) than ZN LLDPE mainly because of their homogeneous comonomer distribution. In this way, an equilibrated chemical composition distribution, which means, the presence of fractions that melt at lower and at higher temperatures is the key factor to achieve a good sealing performance. Acknowledgements Authors would like to thank Braskem SA for supporting this work. We appreciate very much the work carried out by Rafael Bitello da Silva.
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4. A. G. Simanke, G. B. Galland, L. L. Freitas, J. A. H. da Jornada, R. Quijada, R. S. Mauler. Dynamic-mechanical properties of ethylene/olefin copolymers prepared by a metallocene catalyst. Macromol. Chem. Phys. 2001; 202(1): 172–179. 5. V. B. F. Mathot, R. L. Scherrenberg, T. F. J. Pijpers. Metastability and order in linear, branched and copolymerized polyethylenes. Polymer 1998; 39(19), 4541-4559. 6. R.A. Bubeck. Structure–property relationships in metallocene polyethylenes. Mater. Sci. Eng.R 2002; 39: 1–28. 7. H. Knuuttila, A. Lehtinen. A. Nummila-Pakarinen. Advanced polyethylene technologies controlled material properties. Adv. Polym. Sci. 2004; 169:13–27. 8. D. B. Malpass. An overview of industrial polyethylene processes. In: Introduction to industrial polyethylene: properties, catalysts, processes. Scrivener Publishing LLC. 2010. 9. P. Meka, F.C. Stehling. Heat sealing of semicrystalline polymer films. I. Calculation and measurement of interfacial temperatures: Effect of Process Variables on Seal Properties.; J. Appl. Polym. Sci. 1994; 51(1): 89-103. 10. F. C. Stehling, P. Meka. Heat ealing of semicrystalline polymer films. II. Effect of melting distribution on heat-sealing behavior of polyolefins.J. Appl.Polym. Sci.1994; 51(1): 105-119. 11. J. K. Hobbs, O. E. Farrance, L. Kailas. How atomic force microscopy has contributed to our understanding of polymer crystallization Polymer 2009; 50(18): 4281–4292. 12. Y. Wang, C.-M. Chan, K.M. Ng. What controls the lamellar orientation at the surface of polymer films during crystallization. Macromolecules 2008; 41(7): 2548-2553. 13. D. M. Simpson, A. D. Westwood, G. M. Brown. A study of the effects of extrusion melt temperature on the hot tack and heat seal performance of extrusion coated polyethylene. In Proceedings of Antec 2003; 3. Nashville, 2003. 14. C. Mueller, G. Capaccio, A. Hiltner, E. Baer. Heat Sealing of LLDPE: Relationships to melting and interdiffusion. J. Appl. Polym. Sci.1998; 70(10): 2021-2030. 15. J. M. Farley, P. Meka. Heat sealing of semicrystalline polymer films. III. Effect of corona discharge treatment of LLDPE. J.Appl.Polym. Sci. 1994; 51(1): 121-131. 16. B. Monrabal. Crystallization analysis fractionation: a new technique for the analysis of branching distribution in polyolefins. J. Appl. Polym. Sci. 1994; 52(4):491-499. 17. ASTM F1921-2004 – Standard test method for hot seal strength (hot tack) of thermoplastic polymers and blend comprising the sealing surfaces of flexible webs. 18. ASTM F2029-2008 – Standard practices for making heat seals for determination of heatsealability of flexible webs as measured by seal strength. 19. J. K. Hobbs. In-situ AFM of polymer crystallization. Chin. J. Polym. Sci. 2003; 21(2):129-133. 20. A, J. Ryan,J. P. A. Fairclough, N. J. Terrill, P.D. Olmsted, W.C.K. Poon. A scattering study of nucleation phenomena in polymer crystallization. Faraday Discuss. 1999; 112:13-30.
Figure Captions Figure 1 – Chemical Composition Distribution Profiles of LLDPE ZN1 and LLDPE ZN2. Figure 2 – Chemical Composition Distribution Profiles of LLDPE M1 and LLDPE M2. Figure 3 – Crystaf profiles of LLDPE ZN1 and LLDPE ZN2 fractions. Figure 4 – Crystaf profiles of LLDPE M1 and LLDPE M2 fractions Figure 5 – Hot tack strength of LLDPE samples as a function of temperature (the first indicated in each curve indicates the initial sealing temperature) Figure 6 – Heat seal strength of LLDPE samples as a function of temperature (the first indicated in each curve indicates the initial sealing temperature)
Table List Table 1 – Properties of LLDPE samples Table 2 – Characterization of the fractions of LLDPE M1 and LLDPE M2 *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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Table 3 - Characterization of the fraction of LLDPE ZN1 and LLDPE ZN2 Table 4 – AFM phase images of samples LLDPE ZN1, LLDPE ZN2, LLDPE M1 and LLDPE M2 at 120-125°C, as a function of time. Table 5 – AFM phase images of samples LLDPE ZN1 FR3, LLDPE ZN2 FR4, LLDPE M1 FR3 and LLDPE M2 FR4 at 105 -110°C, as a function of time. Table 6 – AFM phase images of samples LLDPE ZN1 FR5, LLDPE ZN2 FR7, LLDPE M1 FR5 and LLDPE M2 FR7 at 110 - 115°C, as a function of time. Table 7 – AFM phase images of samples LLDPE ZN1 FR7, LLDPE ZN2 FR10, LLDPE M1 FR7 and LLDPE M2 FR10 at 120 - 125°C, as a function of time.
*Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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Table 1 – Properties of LLDPE samples LLDPE ZN2
LLDPE M1
LLDPE M2
1-octene
1-octene
1-hexene
1-octene
2
3
3
3
MFI (g/10min)
0.75
1
1
1
Tm (°C)
124
122/120/107
117/104
122/118/100
Xc (%)
48
44
Tc( °C)
112/63
Mw (g/gmol)
Comonomer content (%mol) *
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LLDPE ZN1 Comonomer type
41
44
107/63
105/94/63
105/86/59
106200
102400
97200
102300
Mw/Mn
3.3
3.4
2.2
3.1
Polymer soluble in o-DCB (%)
4.9
3.7
0.7
2.1
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* 190°C/2.16Kg
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Table 2 – Characterization of the fractions of LLDPE M1 and LLDPE M2 LLDPE M1 Fraction (FR)
Elution Temperature (°C)
Comonomer co ntent (%mol)
Tm2 (°C)
1
30
nd
nd
nd
2
45
nd
nd
nd
nd
3
60
5.8
99
81
34
4
73
3.8
107
91
40
5
83
2.3
116
102
47
6
91
1.6
122
109
55
7
100
0.9
127
113
60
Elution Temperature (°C)
Comonomer co ntent (%mol)
Tm2 (°C)
1
30
9.1
82
2
45
5.1
92
3
50
nd
nd
4
60
4.6
95
5
70
3.7
6
75
3.2
7
80
2.3
8
85
1.6
9
90
1.2
10
95
11
100
nd
Xc (%)
84
24
79
29
nd
nd
86
32
91
33
111
97
46
115
102
52
119
107
57
123
110
61
1.0
127
113
66
0.5
128
117
61
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Tc (°C)
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Fraction (FR)
Xc (%)
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LLDPE M2
Tc (°C)
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Table 3 - Characterization of the fractions of LLDPE ZN1 and LLDPE ZN2 LLDPE ZN1 Elution Temperature (°C)
Comonomer co ntent (% mol)
Tm2 (°C)
Tc (°C)
1
30
nd
nd
nd
nd
2
45
nd
nd
nd
nd
3
60
6.8
103
88
33
4
73
4.6
109
95
43
5
83
2.4
117
103
50
6
91
1.6
122
109
58
7
100
0.9
129
114
63
Fraction (FR)
Elution Temperature (°C)
Comonomer co ntent (%mol)
Tm2 (°C)
Tc (°C)
Xc (%)
1
30
9.7
83
2
45
nd
nd
3
50
5.3
97
4
60
4.4
100
5
70
3.4
6
75
3.3
7
80
2.1
8
85
1.6
9
90
1.2
10
95
11
100
28
nd
nd
86/47
33
89/53
36
94/59
40
109
99/64
40
113
102/68
44
116
106/70
47
120
109/71
49
0.9
125
114/74
54
0.7
128
116/76
58
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106
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80/31
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LLDPE ZN2
Xc (%)
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Fraction (FR)
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Table 4 – AFM phase images of samples LLDPE ZN1, LLDPE ZN2, LLDPE M1 and LLDPE M2 at 120-125°C, as a f unction of time. 120-125° isotherm Sample 25 minutes 45 minutes 75 minutes
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LLDPE ZN1
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LLDPE ZN2
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LLDPE M2
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LLDPE M1
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Table 5 – AFM phase images of samples LLDPE ZN1 FR3, LLDPE ZN2 FR4, LLDPE M1 FR3 and LLDPE M2 FR4 at 105 -110°C, as a function of time. 105-110° isotherm Sample 25 minutes 45 minutes 75 minutes
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LLDPE ZN1 FR3
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LLDPE M2 FR4
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LLDPE M1 FR3
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LLDPE ZN2 FR4
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Table 6 – AFM phase images of samples LLDPE ZN1 FR5, LLDPE ZN2 FR7, LLDPE M1 FR5 and LLDPE M2 FR7 at 110 - 115°C, as a function of time. 110-115° isotherm Sample 25 minutes 45 minutes 75 minutes
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LLDPE ZN1 FR5
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LLDPE M2 FR7
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LLDPE M1 FR5
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LLDPE ZN2 FR7
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Table 7 – AFM phase images of samples LLDPE ZN1 FR7, LLDPE ZN2 FR10, LLDPE M1 FR7 and LLDPE M2 FR10 at 120 - 125°C, as a function of time. 120-125° isotherm Sample 25 minutes 45 minutes 75 minutes
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LLDPE ZN1 FR7
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LLDPE M2 FR 10
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LLDPE M1 FR 7
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25
LLDPE ZN1 FR7
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20
LLDPE ZN1 FR5
15 c T /d m d % 10
LLDPE ZN2 FR7
LLDPE ZN2 FR10
LLDPE ZN2 FR4
5
0 30
40
50 60 70 Solution Crystallization Temperature (\C)
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LLDPE ZN1 FR3
80
90
100
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14 LLDPE M1 FR3
12
8 c T d / m d6 %
LLDPE M2 FR4
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LLDPE M2 FR7
10
4
2
0 30
40
50 60 70 Solution Crystallization Temperature (\C)
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LLDPE M1 FR5
80
90
100
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3,00
LLDPE M1
LLDPE M2
LLDPE ZN1
LLDPE ZN2
2,50
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) 2,00 N ( th g n1,50 e rt S k c 1,00 a T t o H0,50
95
100
105
110
115
120
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Temperature (\C)
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0,00
125
130
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LLDPE M1
12,0
LLDPE ZN1
LLDPE ZN2
10,0 8,0 6,0 4,0 2,0 0,0 100
105
110
115
120
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125
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LLDPE M2
130
PII:
S0142-9418(12)00226-7
DOI:
10.1016/j.polymertesting.2012.11.010
Reference:
POTE 3978
To appear in:
Polymer Testing
Received Date: 1 October 2012 Accepted Date: 19 November 2012
Please cite this article as: A.G. Simanke, C. de Lemos, M. Pires, Linear Low Density Polyethylene: microstructure and sealing properties correlation, Polymer Testing (2012), doi: 10.1016/ j.polymertesting.2012.11.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Material Properties Linear Low Density Polyethylene: microstructure and sealing properties correlation Adriane G. Simanke*, Cristóvão de Lemos, Márcia Pires Braskem SA, III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, Triunfo, RS, Brazil
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Abstract: The microstructures of four commercial linear low density polyethylenes (LLDPE) were evaluated and correlated with their sealing properties. Atomic Force Microscopy (AFM), Temperature Rising Elution Fractionation (TREF), Differential Scanning Calorimetry (DSC) and Crystallization Analysis Fractionation (CRYSTAF) experiments revealed that the comonomer distribution is one of the main factors that influence the sealing properties. The superior sealing performance showed by metallocene LLDPE samples in comparison to Ziegler-Natta LLDPE samples can be attributed to their well balanced chemical composition distribution.
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Keywords: LLDPE, sealing properties, morphology, comonomer distribution 1. Introduction
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Polyethylene is the most widely used thermoplastic polymer in the world, being made into products ranging from clear food wrap and plastic bags to laundry detergent bottles and automobile fuel tanks [1]. Among the different polyethylene types, linear low density polyethylene (LLDPE) represents almost 30% of the total polyethylene global consumption [2]. The structural parameters that generally influence the ultimate properties of LLDPE are: type, amount and distribution of comonomer, average molecular weight and molecular weight distribution [3-5]. The development of new catalysts and process technologies has motivated the continuous improvement of LLDPE properties and the ability to tailor it for a wide range of applications [6-8]. LLDPE is used mainly in film application due to its toughness, flexibility and relative transparency. Product examples range from multilayer films used in food packaging to agricultural films. Among the different properties required in a material to be applied in flexible food packaging, sealing performance is one of the most important, being essential for the integrity of the package. LLDPE with good sealing performance is expected to present low sealing temperature, broad sealing window and high packaging performance, contributing to cost reduction by speeding up the automatic packaging processes. Heat sealing is a technique of sealing two materials under the combined effect of three parameters: pressure, temperature and time. The sealing process involves melting, interdiffusion and crystallization of macromolecules at the interface of two materials to be sealed. In the case of semicrystalline polymers, the sealing temperature must be near the melting temperature of the polymer to allow macromolecular mobility at the interface. The sealing strength, toughness, failure mode and appearance of these seals after cooling to room temperature are important seal variables [9,10]. Considering that there are many different commercial grades of LLDPE on the market with different sealing properties, it becomes increasingly important to determine which structural features contribute to good sealing performance. Since crystallization kinetics plays an important role in the sealing mechanism, the use of a powerful tool to understand the behavior on the film surface becomes necessary. Atomic Force Microscopy (AFM) has been widely applied to issues regarding polymer crystallization on the surface, allowing observation of crystal growth, melting and reorganization at the lamellar scale, determining how structure evolves and how local conditions influence the crystallization kinetics [11,12]. Few studies regarding polyethylene microstructure and sealing performance correlation are found at the literature [9,10,13-15]. Different from them, in this work, four LLDPE with similar comonomer content were characterized in detail in order to elucidate the influence of their molecular microstructure on the sealing performance. 2. Experimental *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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2.1 Materials Four commercial linear low density polyethylenes (LLDPE) were used in this work. Two of them are Ziegler-Natta LLDPE with 1-octene as comonomer (LLDPE ZN1 and LLDPE ZN2), and the other two are metallocene LLDPE with two different comonomers, 1-hexene and 1-octene (LLDPE M1 and LLDPE M2, respectively). Their main characteristics are described in Table 1.
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2.2 Extrusion of the films Films with average thickness of 50-55 µm were produced in a single screw extruder for casting flat film (Leonard EMF 30F equipment). 2.3 Characterization Techniques
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2.3.1 Amount of soluble polymer in ortho-dichlorobenzene (o-DCB) In order to quantify the amount of polymer soluble in o-DCB, 2g of LLDPE was dissolved in 250mL of o-DCB at 135°C, under stirring and inert atmosphere. After 30 min, the solution was allowed to cool to 25°C and kept at this temperature for 30 min. The insoluble fraction was separated by filtration and the filtered solution was precipitated in acetone and methanol, followed by filtration. After separating the precipitated solid, the filtered solution was evaporated under nitrogen flow and the residue was dried under vacuum at 90°C until constant weight was reached.
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2.3.2 Temperature Rising Elution Fractionation (TREF) Samples were fractionated according to their chemical composition distribution in a Polymer Char PREP mc2 equipment (automated preparative TREF). Samples were put into the crystallization vessels and dissolved in xylene at 130°C (at a concentration of 0.1 mg/mL). After 60 min of stabilization, the temperature of the system was decreased from 130°C to 30°C at a rate of 0.1°C/min, allowing polymer chains to crystallize in orderly fashion from higher to lower crystallinity. Next, in the elution step, preheated xylene was used as eluent to collect the fractions under heating at predetermined temperatures, ranging from 30 to 130°C with isothermal stays of 20 min. At least seven fractions were collected at different elution temperature intervals for each sample, which were isolated by precipitation in methanol, washed with acetone followed by drying to a constant weight. Fractions were characterized by DSC, FTIR, CRYSTAF and AFM.
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2.3.3 Crystallization Analysis Fractionation (CRYSTAF) The chemical composition distribution profiles of the fractions were obtained by crystallization analysis fractionation (CRYSTAF) in a Polymer Char Crystaf 200+ instrument. LLDPE was dissolved in o-DCB at 160°C (at a concentration of 0.1 mg/mL) and kept at this temperature for 60 min to ensure complete dissolution. Then, the temperature was decreased to 95°C and held for 45 min for stabilization before starting the fractionation. Polymer solution was cooled to 30°C at a constant rate of 0.2°C/min. At higher temperatures, polymer chains with low comonomer content crystallized inside the vessel. Every 5 min, aliquots of polymer solution were collected via an in-line filter and transferred to the in-line infrared detector, which monitors the changes in the concentration of polymer solution, generating the integral Crystaf curve. The Crystaf profiles showed in this paper are the differential form of the integral curve. Detailed Crystaf procedure and interpretation were presented by Monrabal [16]. 2.3.4 Infrared Spectroscopy (FTIR) Infrared spectra of the films and the fractions obtained by TREF were recorded on a Nicolet 710 FTIR in order to determine the 1-hexene and 1-octene contents. A calibration curve built with LLDPE standards was used to determine the comonomer content considering the relation between the area of methyl bands (1379-1360 cm-1) and the area related to the thickness of the sample (44823950 cm-1). Twenty four scans were performed for each sample. *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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2.3.5 Gel Permeation Chromatography (GPC) The molecular weight and molecular weight distribution were determined by GPC in a Waters GPCV 2000 equipped with viscometer and optical differential refractometer detectors. A set of four columns Toso-Hass (HT3, HT4, HT5, and HT6) and a pre-column of 500 Å were used. The measurements were carried out at 145°C using 1,2,4-trichlorobenzene as solvent, at a volumetric flow rate of 1 mL/min. The molecular weights were determined using a calibrated curve obtained from a series of monodisperse polystyrene standards and narrow molar mass LLDPE and polypropylene.
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2.3.6 Differential Scanning Calorimetry (DSC) Thermal behavior was analyzed by differential scanning calorimetry using a TA Q1000 DSC under nitrogen and connected to an intracooler that allows sub ambient temperature control. The instrument was calibrated with indium. Samples were melted at 200°C, cooled from 200°C to -20°C and heated from -20°C to 200°C at a heating rate of 10 °C/min. The melting temperature (Tm) and enthalpy of fusion (∆Hf) were taken from the second heating curve.
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2.3.7 Atomic Force Microscopy (AFM) AFM images were obtained using Veeco NanoScope V atomic force microscopy and Veeco diTAC heater system operating under heating conditions. Topography and phase images were simultaneously collected in tapping mode at 512 x 512 lines standard resolution. Veeco single side coated silicon cantilevers were used with resonant frequency at 366-401 kHz. According to the manufacturer’s specifications, the cantilevers have a spring constant of 20-80 N/m, length of 110-140 µm, width of 25-35 µm and the radius of the tip is 2-5 nm. Film samples of nominal thickness of 5055 µm and without previous surface treatment were subject to scans of 40 µm x 40 µm using a scan rate of 0.878 Hz. Sample was quickly heated to 200 °C and kept at this temperature for 5 min. In a second stage, the melted polymer was quickly cooled to the isotherm temperature of scanning to observe the crystallization step and lamellae growth.
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2.3.8 Measurement of Sealing Properties (hot tack strength and heat seal strength)
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The hot tack strength of the film was measured using a Hot Tack Tester 4000 from J&B instruments. A pair of LLDPE film ribbons with 25.4mm width was sandwiched between two layers of polyester film to avoid their adherence to the heated jaws, then, they were fitted between two heated bars and hot pressed together at 44 psi for 1 s. The hot tack strength was measured at temperatures ranging from 95° to 160°C, according to ASTM F1921 [17]. The heat seal strength of the sealed film was measured using a Teller Model EB laboratory heat sealer, 22 s after the seal jaw opening. Temperatures and pressure used were the same applied in the hot tack sealing test. The test method was carried out according to ASTM F2029 [18]. For both tests, the specimens were obtained from the central part of the films with 50-55 µm of thickness. 3. Results and Discussion
3.1 Microstructure characterization All LLDPEs studied are typical film grades with weight average molecular masses in the range of 100 kg/gmol, narrow molecular weight distribution and melt flow index between 0.75 and 1 g/10min, as shown in Table 1. TABLE 1 Comparing the samples, it is possible to observe that, excepting the LLDPE ZN1, all the other samples showed similar comonomer content. Because of its lower comonomer content, LLDPE ZN1 *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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showed higher melting temperature and degree of crystallinity than the other samples, as can be seen in Table 1. The other three samples showed more than one defined melting temperature, pointing to a heterogeneous comonomer distribution. This unexpected behavior could be caused by the polymerization conditions and industrial technology process used to obtain these samples. In order to investigate the effect of the heterogeneous comonomer distribution on the sealing properties, the samples were fractioned by chemical composition distribution. Figure 1 shows the chemical composition profiles of LLDPE ZN1 and LLDPE ZN2, where it is possible to observe remarkable differences between them. In LLDPE ZN1, around 43% of the fractions were obtained in the range of 100 – 130°C while in LLDPE ZN2 around 80% of the fractions were collected between 70 - 95°C, confirming the influence of the technology and process conditions on the microstructure of these resins. FIGURE 1
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Figure 2 shows the chemical composition profiles of LLDPE M1 and LLDPE M2 where it is possible to observe that both samples eluted in all the temperature range analyzed. In LLDPE M2, most of the fractions (about 65%) eluted at temperatures lower than 80°C, while in LLDPE M1 29% of the sample eluted at 73°C and 31% eluted at 83°C. It is expected that the high concentration of fractions that elutes at lower temperatures contributes to achieve low sealing temperature. FIGURE 2
Fractions obtained by TREF were characterized by FTIR and DSC and the results are shown in Table 2 and 3. Some fractions could not be characterized since the amount recovered was insufficient to be analyzed (all lines indicated as "nd" in Table 2 and 3). TABLE 2 and TABLE 3
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The comonomer content of the fractions was determined by FTIR, and the melting temperature and crystallinity were obtained by DSC. As expected, the lower the elution temperature, the higher the comonomer content and the lower the melting temperature and crystallinity of the fractions. No remarkable differences regarding comonomer content and melting temperature of the fractions were observed between LLDPE M1 and LLDPE M2. On the other hand, when comparing LLDPE ZN1 and LLDPE ZN2, it is possible to observe some differences in the composition of the fractions. In general, for the same elution temperature, fractions of LLDPE ZN1 showed higher comonomer content than fractions of LLDPE ZN2, and unexpected higher melting temperatures (Table 3). The differences in the comonomer content could be attributed to differences in the polymerization conditions and process which are not the subject of this work. The higher melting temperatures observed for lower comonomer contents could be attributed to differences in the comonomer distribution along the polymeric chain, as can be seen in Figure 3. The lower melting temperature observed for LLDPE ZN2 FR4 can be justified by its broader comonomer distribution compared to LLDPE ZN1 FR3. Considering the other fractions (LLDPE ZN1 FR5 and FR7; LLDPE ZN2 FR7 and FR10), no significant differences were observed regarding comonomer distribution. FIGURE 3
3.2 Morphology characterization Different AFM experiments were carried out at different temperatures in order to define the better isothermal temperature to analyze the samples. Table 4 shows the AFM phase images of samples LLDPE ZN1, LLDPE ZN2, LLDPE M1 and LLDPE M2 obtained at 25, 45 and 75 min at an isotherm between 120-125°C. At this isotherm, the molecules started to reorganize themselves at, or close to, a growth front, and the polymer molecules began passing through intermediate degrees of disorder to a more stable state with respect to the crystalline organization, so that the ordering of the *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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chain was maximized. After 25 min of isotherm, all samples indicated transformation from the molten phaseinto the solid crystalline form. In sample LLDPE M2, the bundles were equidistantly separated and showed uniform size. The sample LLDPE M1 showed bundles heterogeneously dispersed and the morphology was similar to LLDPE ZN2, that is, longer bundles with crystallization nuclei. For sample LLDPE ZN1, the morphology was completely different from the other three, because there were ellipses of crystallization poorly dispersed in the melted matrix. After 45 min of isotherm, it is possible to observe that LLDPE ZN1 showed a higher crystallization rate while LLDPE M2 showed s lower crystallization rate. This behavior could be explained considering the chemical composition distribution of these samples. In LLDPE ZN1, more than 40% of the sample eluted in the range of 100-130°C, while in LLDPE M2 around 65% of the sample eluted at temperatures lower than 80°C.The other two samples, LLDPE ZN2 and LLDPE M1, showed similar kinetic profiles and crystallization rate between LLDPE ZN1 and LLDPE M2. Regarding morphology, LLDPE ZN2 and LLDPE M1 were similar, while in LLDPE M2 smaller crystals were observed, as expected, considering its chemical composition profile. At 75 min, most parts of the samples had crystallized and the morphologies were similar to those observed at 45 min. TABLE 4
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AFM analyses were also carried out in order to investigate the morphology of the fractions eluted at 60°C, 80-83°C and 95-100°C. Table 5 shows the AFM images collected at an isotherm between 105 and 110°C for fraction 3 (FR3) of LLDPE ZN1 and LLDPE M1 and for fraction 4 (FR4) of LLDPE ZN2 and LLDPE M2 (all these fractions were eluted at 60°C). Analyzing the images, it is possible to observe that the morphology and kinetic profile observed for the LLDPEs produced with ZieglerNatta catalyst were completely different from the ones observed for LLDPEs synthesized with metallocene catalyst. Growing crystals always create or induce changes in their environment, setting up fields in, for example, concentration, density, temperature or even stress [11]. Such fields are expected to influence the growth kinetics, as well as the structure. According to the images of LLDPE ZN1, a crystal reorganization was verified from 25 min to 45 min of isotherm. It is possible that, during this period of time, there was a sequence of events, with an initial framework of fast growing ‘primary’ lamellae oriented away from the nucleus, followed by slower in-filling ‘secondary’ lamellar growth [19]. After 75 min of isotherm, more than 50% of the sample was totally crystallized. The higher amount of comonomer content in LLDPE ZN1 FR3 could explain the morphological differences when compared to the other samples. The presence of intermediated contrast, which was more pronounced in LLDPE ZN1, between light gray, melt region, and dark gray, crystalline region, is characteristic of a softer phase probably rich in comonomer content. The sequence of events observed for LLDPE ZN2 FR4, with an initial framework of fast growing primary lamellae oriented away from the nucleus and having a discrete thickness, was followed by a protracted period of in-filling growth and crystal reorganization. The crystals emerged from the melted phase, the thickness increased and the structures restarted growing and forming branches from the initial crystals (75 min). The slower kinetic growth of LLDPE ZN2 FR4 compared to LLDPE ZN1 FR3 can be justified by the broader comonomer distribution of LLDPE ZN2 FR4, as can be seen in Figure 3. Considering LLDPE M1 FR3 and LLDPE M2 FR4, the perpendicular growing and overlapping of most of the crystals can be a characteristic given for metallocene catalyst in the initial steps of the crystallization. The morphology of the LLDPE M2 FR4 was in agreement with LLDPE M2 (Table 4), possibly justified by the better distribution of the comonomer 1-octene in the polymeric chain of LLDPE. TABLE 5 Fraction 5 (FR5) of LLDPE ZN1 and LLDPE M1, and fraction 7 (FR7) of LLDPE ZN2 and LLDPE M2 (fractions that were eluted at 80-83°C) were analyzed at an isotherm between 110-115°C (Table 6) and three images of each sample were obtained in the same timeframe described previously. The morphology after 25 min of isotherm was better resolved compared to the fractions eluted at 60°C, *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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meaning there were crystals with good contrast difference and defined edge. Possibly, the lower amount of comonomer in FR5 and FR7 contributed to the better resolution of the images. Interestingly, it is apparent that the initial mechanism of crystallization of samples LLDPE ZN1 FR5 and LLDPE M2 FR7 showed all crystals having nuclei and geometric organization. Especially for LLDPE ZN1 FR5, the crystals showed equivalent size. At 45 min of isotherm, the LLDPE ZN1 FR 5 showed faster kinetic profile than the others. However, at 75 min of isotherm, the order of kinetic growth was: LLDPE ZN1 FR5 = LLDPE ZN2 FR7 > LLDPE M1 FR5 > LLDPE M2 FR7. The lower crystallization kinetics of LLDPE M2 FR7 could be justified by its chemical composition distribution profile (Figure 4). In addition, variations in the density or structure of the melt, as suggested by Ryan et al [20], could be responsible to promote lower lamellar growth rate. TABLE 6 FIGURE 4
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Fraction 7 (FR7) of LLDPE ZN1 and LLDPE M1 and fraction 10 (FR10) of LLDPE ZN2 and LLDPE M2 (fractions that were eluted at 95-100°C) were analyzed at an isotherm between 120125°C (Table 7). After 25 min, it was observed that LLDPE ZN1 FR7 showed significant amount of crystals and satisfactory dispersion. Similar behavior was observed for LLDPE M1 FR7, although the size of the crystals seemed to be smaller. The crystals of LLDPE ZN2 FR10 were larger than the others and also presented good dispersion. LLDPE M2 FR10 showed few crystals and some dots, indicating that this sample had slower crystallization kinetics. Since AFM shows hysteresis phenomena, small displacement of the area scanned can occur due to minimum distortion of the images. At 45 min, it was observed that the domains started to reorganize and to get well defined, disrupting originally, rather well ordered domain structures. In samples LLDPE ZN1 FR7 and LLDPE ZN2 FR10, it was verified that most crystals increased their size, even although some unexpected morphologies originated from secondary lamellar growth could be seen. Particularly, the image of LLDPE M2 FR10 at 45 min of isotherm showed crystals having lamellae growing from a nucleus, but the predominant morphology was lamellae groups oriented away. At 75 min, it was observed the interlacement and self organization of the lamellae of LLDPE ZN1 FR7 and LLDPE M2 FR10, and this behavior is expected to be partially responsible for the highly interwoven morphology that gives strength to these structures [11]. For LLDPE ZN2 FR10 and LLDPE M1 FR7, no significant changes were observed, excepting the increase of the crystals.
3.3 Correlation between microstructure and sealing behavior
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Figure 5 shows the hot tack strength versus sealing temperature curves of the LLDPE samples, in which the initial sealing temperature of each sample is indicated by the first triangle that appears in each curve. Analyzing the curves, it is possible to observe that LLDPE M1 showed better performance than the other samples, more specifically, higher hot tack strength and lower sealing temperature (110°C). Comparing the samples synthesized by metallocene catalyst (LLDPE M1 x LLDPE M2) and analyzing their chemical microstructure, one can conclude that, apparently, the main difference between them is the type of comonomer. According to Figure 2 and Table 2, both samples showed higher than 60% of modified polymer fractions melting at temperatures lower than 115°C, which are the fractions expected to influence the sealing process at this temperature. The chemical composition profile of the LLDPE M1 and LLDPE M2 fractions (eluted at around 60°C and 80°C) are different (Figure 4). Comparing these fractions, it can be observed that LLDPE M1 FR5 crystallized at higher temperature than LLDPE M2 FR7. This characteristic could explain the better hot tack performance, since it may have an important effect on the crystallization step of the sealing mechanism [10]. In this way, these polymer fractions are expected to have the main influence on the molecular process involved in the sealing of semicrystalline polymer films and, consequently, to be responsible for the sealing behavior. In addition to these observations and considering the *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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Considering the samples synthesized with ZN catalyst, LLDPE ZN1 and LLDPE ZN2, the better hot tack performance observed for LLDPE ZN2 can be explained by taking in account their different chemical composition profiles. Besides the higher comonomer content, around 54% of LLDPE ZN2 fractions melted at temperatures lower than 115°C, while in LLDPE ZN1 these fractions represent around 45% (Figure 1 and Table 3). These polymer fractions can melt during the sealing process, diffusing across the film interface, creating entanglements and additional crystals. The main factor that can explain the lower performance of samples produced by ZN catalyst in comparison to metallocene catalyst is their chemical microstructure, mainly because of the comonomer distribution. In Figures 1 and 2 it is possible to observe that in the case of samples LLDPE ZN1 and LLDPE ZN2, a high amount of polymer fractions was eluted at higher temperatures. The higher the temperature, the higher the crystal thickness and more energy to seal is required.
FIGURE 6 4. Conclusions
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In the case of heat seal measurements (Figure 6), no difference was observed between LLDPE M1 and LLDPE M2. This behavior can be explained by the effect of the analyses time. As explained before, heat seal experiments are carried out in a longer time than hot tack measurement; allowing the melted polymer to diffuse across the interface in the longer duration of the heat seal test. An interesting observation is that, even with more time for diffusion and crystallization, LLDPE ZN1 and LLDPE ZN2 showed lower performance than LLDPE M1 and LLDPE M2. Similar to the hot tack discussion, this behavior can be explained by the chemical composition distribution of these samples, showing how important is the presence of a polymer fraction that melts at lower temperatures to the sealing performance.
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The microstructures of commercial LLDPE resins were characterized from investigation of morphology, solubility, fractionation and thermal analyses. Metallocene LLDPE showed better sealing performance (broad sealing window and high sealing strength) than ZN LLDPE mainly because of their homogeneous comonomer distribution. In this way, an equilibrated chemical composition distribution, which means, the presence of fractions that melt at lower and at higher temperatures is the key factor to achieve a good sealing performance. Acknowledgements Authors would like to thank Braskem SA for supporting this work. We appreciate very much the work carried out by Rafael Bitello da Silva.
References 1. F. B. Coutinho, I.L. Mello, L. C. de Santa Maria. Polietileno: principais tipos, propriedades e aplicações. Polímeros: Ciência e Tecnologia, 2003;13:1-13. 2. V. D. Bastos. In: BNDES Setorial, Rio de Janeiro, 2009. p. 321-358. 3. A. G. Simanke, G. B. Galland, R. Baumhardt Neto, R. Quijada, R. S. Mauler. Influence of the type and the comonomer contents on the mechanical behavior of ethylene/-olefin copolymers. J. Appl. Polym. Sci. 1999; 74(5): 1194-1200. *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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4. A. G. Simanke, G. B. Galland, L. L. Freitas, J. A. H. da Jornada, R. Quijada, R. S. Mauler. Dynamic-mechanical properties of ethylene/olefin copolymers prepared by a metallocene catalyst. Macromol. Chem. Phys. 2001; 202(1): 172–179. 5. V. B. F. Mathot, R. L. Scherrenberg, T. F. J. Pijpers. Metastability and order in linear, branched and copolymerized polyethylenes. Polymer 1998; 39(19), 4541-4559. 6. R.A. Bubeck. Structure–property relationships in metallocene polyethylenes. Mater. Sci. Eng.R 2002; 39: 1–28. 7. H. Knuuttila, A. Lehtinen. A. Nummila-Pakarinen. Advanced polyethylene technologies controlled material properties. Adv. Polym. Sci. 2004; 169:13–27. 8. D. B. Malpass. An overview of industrial polyethylene processes. In: Introduction to industrial polyethylene: properties, catalysts, processes. Scrivener Publishing LLC. 2010. 9. P. Meka, F.C. Stehling. Heat sealing of semicrystalline polymer films. I. Calculation and measurement of interfacial temperatures: Effect of Process Variables on Seal Properties.; J. Appl. Polym. Sci. 1994; 51(1): 89-103. 10. F. C. Stehling, P. Meka. Heat ealing of semicrystalline polymer films. II. Effect of melting distribution on heat-sealing behavior of polyolefins.J. Appl.Polym. Sci.1994; 51(1): 105-119. 11. J. K. Hobbs, O. E. Farrance, L. Kailas. How atomic force microscopy has contributed to our understanding of polymer crystallization Polymer 2009; 50(18): 4281–4292. 12. Y. Wang, C.-M. Chan, K.M. Ng. What controls the lamellar orientation at the surface of polymer films during crystallization. Macromolecules 2008; 41(7): 2548-2553. 13. D. M. Simpson, A. D. Westwood, G. M. Brown. A study of the effects of extrusion melt temperature on the hot tack and heat seal performance of extrusion coated polyethylene. In Proceedings of Antec 2003; 3. Nashville, 2003. 14. C. Mueller, G. Capaccio, A. Hiltner, E. Baer. Heat Sealing of LLDPE: Relationships to melting and interdiffusion. J. Appl. Polym. Sci.1998; 70(10): 2021-2030. 15. J. M. Farley, P. Meka. Heat sealing of semicrystalline polymer films. III. Effect of corona discharge treatment of LLDPE. J.Appl.Polym. Sci. 1994; 51(1): 121-131. 16. B. Monrabal. Crystallization analysis fractionation: a new technique for the analysis of branching distribution in polyolefins. J. Appl. Polym. Sci. 1994; 52(4):491-499. 17. ASTM F1921-2004 – Standard test method for hot seal strength (hot tack) of thermoplastic polymers and blend comprising the sealing surfaces of flexible webs. 18. ASTM F2029-2008 – Standard practices for making heat seals for determination of heatsealability of flexible webs as measured by seal strength. 19. J. K. Hobbs. In-situ AFM of polymer crystallization. Chin. J. Polym. Sci. 2003; 21(2):129-133. 20. A, J. Ryan,J. P. A. Fairclough, N. J. Terrill, P.D. Olmsted, W.C.K. Poon. A scattering study of nucleation phenomena in polymer crystallization. Faraday Discuss. 1999; 112:13-30.
Figure Captions Figure 1 – Chemical Composition Distribution Profiles of LLDPE ZN1 and LLDPE ZN2. Figure 2 – Chemical Composition Distribution Profiles of LLDPE M1 and LLDPE M2. Figure 3 – Crystaf profiles of LLDPE ZN1 and LLDPE ZN2 fractions. Figure 4 – Crystaf profiles of LLDPE M1 and LLDPE M2 fractions Figure 5 – Hot tack strength of LLDPE samples as a function of temperature (the first indicated in each curve indicates the initial sealing temperature) Figure 6 – Heat seal strength of LLDPE samples as a function of temperature (the first indicated in each curve indicates the initial sealing temperature)
Table List Table 1 – Properties of LLDPE samples Table 2 – Characterization of the fractions of LLDPE M1 and LLDPE M2 *Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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Table 3 - Characterization of the fraction of LLDPE ZN1 and LLDPE ZN2 Table 4 – AFM phase images of samples LLDPE ZN1, LLDPE ZN2, LLDPE M1 and LLDPE M2 at 120-125°C, as a function of time. Table 5 – AFM phase images of samples LLDPE ZN1 FR3, LLDPE ZN2 FR4, LLDPE M1 FR3 and LLDPE M2 FR4 at 105 -110°C, as a function of time. Table 6 – AFM phase images of samples LLDPE ZN1 FR5, LLDPE ZN2 FR7, LLDPE M1 FR5 and LLDPE M2 FR7 at 110 - 115°C, as a function of time. Table 7 – AFM phase images of samples LLDPE ZN1 FR7, LLDPE ZN2 FR10, LLDPE M1 FR7 and LLDPE M2 FR10 at 120 - 125°C, as a function of time.
*Corresponding author. Innovation and Technology Center. Braskem SA. III Pólo Petroquímico, Via Oeste, Lote 5, Passo Raso, 95853000, Triunfo, RS, Brazil; Phone: 555137218248; Fax : 555134571084; email: [email protected]
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Table 1 – Properties of LLDPE samples LLDPE ZN2
LLDPE M1
LLDPE M2
1-octene
1-octene
1-hexene
1-octene
2
3
3
3
MFI (g/10min)
0.75
1
1
1
Tm (°C)
124
122/120/107
117/104
122/118/100
Xc (%)
48
44
Tc( °C)
112/63
Mw (g/gmol)
Comonomer content (%mol) *
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LLDPE ZN1 Comonomer type
41
44
107/63
105/94/63
105/86/59
106200
102400
97200
102300
Mw/Mn
3.3
3.4
2.2
3.1
Polymer soluble in o-DCB (%)
4.9
3.7
0.7
2.1
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* 190°C/2.16Kg
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Table 2 – Characterization of the fractions of LLDPE M1 and LLDPE M2 LLDPE M1 Fraction (FR)
Elution Temperature (°C)
Comonomer co ntent (%mol)
Tm2 (°C)
1
30
nd
nd
nd
2
45
nd
nd
nd
nd
3
60
5.8
99
81
34
4
73
3.8
107
91
40
5
83
2.3
116
102
47
6
91
1.6
122
109
55
7
100
0.9
127
113
60
Elution Temperature (°C)
Comonomer co ntent (%mol)
Tm2 (°C)
1
30
9.1
82
2
45
5.1
92
3
50
nd
nd
4
60
4.6
95
5
70
3.7
6
75
3.2
7
80
2.3
8
85
1.6
9
90
1.2
10
95
11
100
nd
Xc (%)
84
24
79
29
nd
nd
86
32
91
33
111
97
46
115
102
52
119
107
57
123
110
61
1.0
127
113
66
0.5
128
117
61
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102
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Tc (°C)
SC
Fraction (FR)
Xc (%)
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LLDPE M2
Tc (°C)
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Table 3 - Characterization of the fractions of LLDPE ZN1 and LLDPE ZN2 LLDPE ZN1 Elution Temperature (°C)
Comonomer co ntent (% mol)
Tm2 (°C)
Tc (°C)
1
30
nd
nd
nd
nd
2
45
nd
nd
nd
nd
3
60
6.8
103
88
33
4
73
4.6
109
95
43
5
83
2.4
117
103
50
6
91
1.6
122
109
58
7
100
0.9
129
114
63
Fraction (FR)
Elution Temperature (°C)
Comonomer co ntent (%mol)
Tm2 (°C)
Tc (°C)
Xc (%)
1
30
9.7
83
2
45
nd
nd
3
50
5.3
97
4
60
4.4
100
5
70
3.4
6
75
3.3
7
80
2.1
8
85
1.6
9
90
1.2
10
95
11
100
28
nd
nd
86/47
33
89/53
36
94/59
40
109
99/64
40
113
102/68
44
116
106/70
47
120
109/71
49
0.9
125
114/74
54
0.7
128
116/76
58
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106
TE D EP AC C
80/31
SC
LLDPE ZN2
Xc (%)
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Fraction (FR)
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Table 4 – AFM phase images of samples LLDPE ZN1, LLDPE ZN2, LLDPE M1 and LLDPE M2 at 120-125°C, as a f unction of time. 120-125° isotherm Sample 25 minutes 45 minutes 75 minutes
SC
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LLDPE ZN1
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LLDPE ZN2
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LLDPE M2
EP
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LLDPE M1
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Table 5 – AFM phase images of samples LLDPE ZN1 FR3, LLDPE ZN2 FR4, LLDPE M1 FR3 and LLDPE M2 FR4 at 105 -110°C, as a function of time. 105-110° isotherm Sample 25 minutes 45 minutes 75 minutes
SC
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LLDPE ZN1 FR3
TE D
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LLDPE M2 FR4
EP
LLDPE M1 FR3
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LLDPE ZN2 FR4
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Table 6 – AFM phase images of samples LLDPE ZN1 FR5, LLDPE ZN2 FR7, LLDPE M1 FR5 and LLDPE M2 FR7 at 110 - 115°C, as a function of time. 110-115° isotherm Sample 25 minutes 45 minutes 75 minutes
SC
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LLDPE ZN1 FR5
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LLDPE M2 FR7
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LLDPE M1 FR5
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LLDPE ZN2 FR7
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Table 7 – AFM phase images of samples LLDPE ZN1 FR7, LLDPE ZN2 FR10, LLDPE M1 FR7 and LLDPE M2 FR10 at 120 - 125°C, as a function of time. 120-125° isotherm Sample 25 minutes 45 minutes 75 minutes
SC
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LLDPE ZN1 FR7
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LLDPE M2 FR 10
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LLDPE M1 FR 7
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LLDPE ZN2 FR 10
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25
LLDPE ZN1 FR7
SC
20
LLDPE ZN1 FR5
15 c T /d m d % 10
LLDPE ZN2 FR7
LLDPE ZN2 FR10
LLDPE ZN2 FR4
5
0 30
40
50 60 70 Solution Crystallization Temperature (\C)
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LLDPE ZN1 FR3
80
90
100
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14 LLDPE M1 FR3
12
8 c T d / m d6 %
LLDPE M2 FR4
SC
LLDPE M2 FR7
10
4
2
0 30
40
50 60 70 Solution Crystallization Temperature (\C)
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LLDPE M1 FR5
80
90
100
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3,00
LLDPE M1
LLDPE M2
LLDPE ZN1
LLDPE ZN2
2,50
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) 2,00 N ( th g n1,50 e rt S k c 1,00 a T t o H0,50
95
100
105
110
115
120
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Temperature (\C)
SC
0,00
125
130
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LLDPE M1
12,0
LLDPE ZN1
LLDPE ZN2
10,0 8,0 6,0 4,0 2,0 0,0 100
105
110
115
120
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125
SC
95
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) N ( th g n e rt S l a e S t a e H
LLDPE M2
130