Polyethylenes in blown films: effect of molecular structure on sealability and crystallization kinetics.
Polyethylene is one of the most interesting polymers available in the marketplace. The variety of commercial grades is huge because of the vast number of variables like catalysts systems, production process, technologies, and type and amount of comonomers normally used. The combination of these variables can produce different classes of polyethylene according to their macro characteristics: density and crystallinity; or even according to their microcharacteristics: short chain branching (SCB)-type, SCB distribution, long chain branching (LCB) amount, LCB distribution, molecular weight, and molecular weight distribution. These possibilities allow finding polyethylene in almost all areas of application. A special case, linear low density polyethylene (LLDPE) is responsible for almost 30% of the total world consumption of polyethylene. The major use for this type of polyethylene is flexible packaging [1, 2].
Flexible packaging is used in our daily life, in packages for rice, bean, and sugar, for example. These packages are produced by roll films, in which the gauge is about 30-100 [micro]m. If the weight of package is below 5 kg, the thickness of the film is about 30-50 [micro]m. To keep the integrity of the package, some characteristics are extremely important, such as ultimate seal and heat seal. Other mechanical properties, like tensile strength, tear and impact resistance are also important, but the sealing performance is a determining factor for a flexible package. The seal will keep the product safe from outside contact and will keep the atmosphere inside the package.
Many researchers have evaluated the correlation between microstructure and seal properties [1, 3-5], but this assay has not been completely exhausted. The development of new catalyst systems or production processes add new variables into polyethylene. Such is the case of metallocene polyethylene grades. LLDPE was conventionally produced by Ziegler Natta catalyst (ZN), where the comonomer and monomer combination was not very controllable. ZN is known as a multisite catalyst, so the comonomer distribution was not homogenous . The polymerization process in more than one reactor can increase the ability of comonomer incorporation even when ZN catalyst is used . In the case of the metallocene system, there is more than one generation of catalyst. The first metallocene generation, in the 1990s, was known to produce a highly homogeneous SCB distribution and an extremely narrow molecular weight distribution [5, 7]. These polyethylenes were barely introduced to the market, because the elevated viscosity of these polymers forced to reduce the output of extruders. The converters were not interested in this type of polyethylene because of the reduction on the processability. For this reason, a new generation of metallocene was developed, and it became known as easy process metallocene. The reason for that is the introduction of LCB on metallocene LLDPE. This new metallocene LLDPE was introduced to the market in 2000.
Gabriel and Luge  evaluated ZN and metallocene LLDPE in relation to the SCB distribution using three fractionation/crystallization methods. They concluded that all types of LLDPE could be analyzed by the three methods, including differential scanning calorimetric (DSC) method. It was possible because the fact that the crystallization step was basically the same for all methods . Nevertheless, the aim of this study was only the comparison of the three methods for the investigation of short chain branching distribution of linear low-density polyethylene. There is no discussion about microstructure between LLDPE, which is the main reason of this article. As we know from the literature was not usually compare two metallocenes LLDPE with exactly the same SCB distribution and different amount of LCB.
This article discusses seal properties and processes of different polyethylene samples. The basic equation for sealing flexible packaging film is heat + time + pressure = seal. These three factors are closely related--an increase or a decrease in any of these factors will have an impact, positive or negative, on the others. For example, if line speed is increased (and dwell time reduced), then an increase in heat or pressure is often required to maintain seal integrity. The molecular process postulated for heat sealing of semicrystalline polymers is briefly described as a complex relation between melting + self-diffusion + solidification/crystallization of polymer [3, 4]. The first step occurs when heat and pressure are applied to the film surface leading to melting of the crystalline fraction of the polymer. In commercial heat sealing processes of flexible films, heat is applied for a second or less. As mentioned before, fast line sealing brings on short dwell time. On the second step of the process, polymer-chain segments from opposite sides of the interface may self-diffuse across the interface and create molecular entanglements between polymer molecules in the interfacial zone. The third and final step is cooling and crystallization of the polymer, yielding a heat-sealed film.
Sadeghi and Ajji  also studied the sealability and crystal properties of cast films from three mPEs and one LLDPE. Two mPEs were octene copolymers and containing sparse LCB. They observed similar crystal lattice with wide-angle X-ray diffraction (WAXD) for mPEs although they demonstrated different behavior on seal properties. The explanation for that was the presence of LCB on some mPEs. LCB was related to slow diffusion of molecules during sealing process, which provoke a lower strength on that samples. However, there were a huge difference in relation to molecular weight between the mPEs, what could be also responsible for different sealing properties.
We believe that sealing properties of mLLDPE was extremely discussed on the literature [1,5, 8], although we intend to analyze mLLDPE with similar SCB distribution, but with high differences on crystallizability because the presence of LCB. In this article, we develop a new method of sealing properties. We perform the conventional seal test and observe light differences between mLLDPE, but when we convert these data on welding power, it was possible to identify huge differences between them.
In this article, to deeply investigate heat sealing processes, some microstructure characterizations were done. Samples were analyzed to investigate molecular weight and molecular weight distribution, short and LCB content, short chain branch distribution (SCBD), thermal characteristics, and crystallization kinetics. The sealing properties were obtained by the hot seal strength test, and a correlation between microstructure and seal performance was determined.
Two metallocene LLDPE (mPE) and one conventional LLDPE were used in this work and are listed in Table 1. mPE1 and mPE2 are based on 1-hexene comonomers produced from different metallocene systems by Braskem. The former is a traditional mPE with only SCB and the latter is an easy flow metallocene with sparse LCB. The LLDPE is a 1-octene grade from Dow obtained from solution process.
Carbon Nuclear Magnetic Resonance. Samples were analyzed by carbon nuclear magnetic resonance [sup.13]C NMR on a Varian 400 wide-bore spectrometer, with a 5 mm probe. Samples were prepared by dissolving 50 mg of polymer in 0.7 mL of orthodichlorobenzene and 0.2 mL of tetrachloroethane-[d.sub.2].
For LCB quantification, Eq. 1 was used, where [alpha] is the average area of LCB carbon and [T.sub.tot] is the total carbon intensity.
Branches/10,000 carbons = [(1/3) x [alpha]/[T.sub.tot]] x [10.sup.4] (1)
Chemical shifts of 1-hexene and 1-octene comonomer were assigned as reported in the literature .
GPC. The molecular weight and molecular weight distribution were determined by GPC in a Viscoteck-Malver HT-GPC triple detector equipped with a light scattering detector. A set of three Shodex columns were used. The measurements were carried out at 150[degrees]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.
DSC. Thermal behavior was analyzed by differential scanning calorimetry using a TA Q1000 DSC under nitrogen rate and connected to an intracooler, that allows subambient temperature control. The instrument was calibrated with Indium. Samples were melted at 200[degrees]C, cooled from 200[degrees]C to -20[degrees]C and heated from -20 to 200[degrees]C at a heating rate and cooled rate of 10[degrees]C/min. The melting temperature (Tm) and enthalpy of fusion ([DELTA][H.sub.f]) were taken from the second heating curve (only this curve is reported).
Crystallization kinetics studies were carried out in a Q20 TA Instruments DSC. A heating rate of 10[degrees]C/min, from 25 to 200[degrees]C, was used. Samples were maintained at 200[degrees]C for 5 min and then a cooling rate of 10[degrees]C/min was used to achieve the temperatures of interest. For each sample, three temperatures were chosen for isothermal crystallization, based on the peak observed during dynamic crystallization from 200 to 25[degrees]C with a cooling rate of 10[degrees]C/min. The results obtained were used to determine the parameters of Avrami's Equation.
X(t) = 1 - exp(-[k.sub.a][t.sup.na]) (2)
where X(t) is the relative crystallinity at time t; [n.sub.a], the Avrami index (crystal geometry information); and [k.sub.a], the isothermal crystallization rate constant containing the nucleation and growth rates [10-12].
The film samples were produced using a semi-industrial Carnevalli CHD blown film single extruder. All samples were obtained with a temperature profile of 180-200[degrees]C, 80 rpm screw speed, 600 mm frost line high, 1.8 mm die gap, 200 mm die diameter, 2.2:1 blow up ratio and 40 [micro]m nominal gauge film. LDPE was used for all samples to replicate the commercial practice formulation (LL 80/LD 20 wt%).
Hot Seal Strength
Hot seal strength (hot tack) experiments were carried out using a Hot Tack Tester 4000 from J&B Instruments. The standard method for hot tack is ASTM F-1921 , which presents conditions A and B, dependent on sample thickness, that are shown in Table 2. The film samples were cut to a width of 25 mm and taped with a Mylar[R] film to prevent the polyethylene from sticking to the heated platens. The cooling time used is in the range 0.2-0.8 s. The dwell time used is in the range 0.25-2 s. The other parameters were fixed as the standard method indicates for condition B.
Hot tack measures the strength of heat seals formed between thermoplastic surfaces of flexible webs, immediately after the seal has been made and before it cools to room temperature . This property is important since this situation frequently occurs on vertical form, fill, and seal packaging (FFS). In this process, the contents are dropped into the bag immediately after the horizontal seal bars have opened. Since the content can be heavy, the hot seal must be able to withstand high loads, thus requiring a high hot tack force.
The film sample is fixed in the upper specimen grip, which is connected to a load cell, and the lower specimen grip, which is connected to a peeling actuator. The film sample is inserted between the sealing bars by means of a specimen insertion mechanism. A seal is made under defined conditions of temperature, contact time and pressure. At the end of the sealing time, and after a preset delay time, the peeling actuator moves down with a preset speed and peels the hot seal totally apart. The maximum force required to peel or break the seal is the hot tack force.
Welding Power--Modified Hot Tack Test
To evaluate the sealing power of different materials, the dwell time was changed from the standard conditions. The dwell time used was in the range 0.2-2 s. Hot tack curves were plotted as force versus time for each temperature. Data were collected at the beginning of the curves (0.2-0.5 s), as shown in Fig. 1. In this range of dwell time, the relationship is linear and it was possible to calculate the linear equation of force as a function of time.
As the film length is constant and equals to 25 mm, the resultant force was plotted as a function of time. The new variable obtained was named "welding power," because power is a physical relation obtained from force in a fixed length divided by time (power = force length/time). Finally, welding power was plotted as a function of temperature for each polymer studied.
Crystallization Analysis Fractionation
The chemical composition distribution (CCD) profiles of the samples were obtained by crystallization analysis fractionation (Crystaf) in a Polymer Char Crystaf 200 instrument. LLDPE was dissolved in o-DCB at 160[degrees]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[degrees]C and held for 45 min for stabilization before starting the fractionation. Polymer solution was cooled to 30[degrees]C at a constant rate of 0.2[degrees]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 article are the differential form of the integral curve. Monrabal  described procedure and interpretation for Crystaf.
RESULTS AND DISCUSSION
Molecular Weight and Molecular Weight Distribution
Resins were selected because of their large use on flexible films and exhibit melt flow index of about 1 g/10 min, as shown in Table 3. Samples were characterized with size-exclusion chromatography, to assure that the molecular weight profile was not significantly different.
LLDPE samples are not exactly equal, but they may be equivalent in relation to molecular weight and molecular weight distribution. mPE1 and LL present very similar molecular weight distribution, although the former is produced with a metallocene catalyst system, which is expected to narrow molecular weight distribution. mPE2 presented larger molecular weight distribution even with a metallocene catalyst system . It is important to remember that both metallocene resins were produced using gas phase technology and LL was produced by solution process. It is well known that the polymerization system (catalyst system + process technology) has a huge impact on polymer properties such as molecular weight distribution .
Short and LCB Content
[sup.13]C NMR analyses were done to determine sample microstructures, which means short chain branch content and distribution, LCB content and molecular sequences of monomer and comonomer. LCB presence was possible to be detected in 1-hexene copolymers, mPE1 and mPE2. In the case of 1-octene grade, LL sample, LCB detection was not possible because of the superposition of the signals for 1-octene comononer and LCB. The [sup.13]C NMR chemical shift assignments are given in Table 4 for all samples.
The chemical shifts usually observed when ethylene is polymerized using 1-hexene as comonomer are present on both mPE samples. However, the two characteristic LCB chemical shifts (32.32 ppm and 22.92 ppm), are present only for mPE2. These two peaks were observed only for the mPE2 sample and correspond to the insertion of a branch with six or more carbon atoms. As the polymer was produced using 1-hexene as comonomer, the branching formed by the comonomer insertion cannot have more than four carbon atoms. The LCB level on sample mPE2 is about 4.7 LCB/10,000C [16, 17]. To obtain the level of LCB, Eq. 1 was used.
The CCD, also referred to as SCBD, is conventionally analyzed by temperature rising elution fractionation (TREF) or Crystaf [8, 14]. The fractionation mechanism of Crystaf and TREF relies on differences of chain crystallizabilities in dilute solution: polymer chains with high crystallizabilities are fractionated at higher temperatures, while chains with low crystallizability are fractionated at lower temperatures . As expected, Crystaf peak temperatures are dramatically influenced by the comonomer composition of the copolymer chains. Moreover, the Crystaf profiles become broader with an increase in comonomer content, which means that higher temperature peaks correspond to lower comonomer composition, "almost linear fraction," and lower temperature peaks correspond to higher comonomer composition, "completely copolymeric fraction." To obtain a more reliable SCBD, two different techniques were used: [sup.13]C NMR and Crystaf, and it was possible to observe many more details about SCBD.
Crystaf curves for the three samples were obtained and are shown in Fig. 2. The two metallocene samples (mPE1 and mPE2) were produced with 1-hexene as a comonomer and the one ZN-LLDPE sample (LL) with 1-octene as a comonomer. With respect to the CCD of the LL, a bimodal distribution is expected. Such a bimodal distribution reflects the heterogeneity of the incorporation of the comonomer into the growing polyethylene backbone during polymerization, which is clearly observed in Fig. 2, for the LL sample. The LL Crystaf curve begins earlier than for the other samples and presents a bimodal shape, as expected. In the meantime, one of the metallocene samples (mPE1) also exhibited a slightly bimodal distribution. Nevertheless, the two models are similar to each other, indicating a more homogenous distribution. A completely different shape was observed for the mPE2 sample, in which a narrow peak in the middle of the curve is present. If we have in mind only the SCB crystallization behavior, we should affirm that mPE2 is the most homogeneous PE in comparison with the others. Indeed, we already knew that mPE2 had SCB and LCB, and we are not able to confirm the presence or absence of LCB in the LL sample. Some researchers reported that both SCB and LCB have a huge effect on Crystaf analysis . So, we cannot directly compare samples with and without LCB. For this reason, we decided to determine SCBD using the NMR results.
[sup.13]CNMR analyses allow to determine the molecular structure from chemical signals (Table 4). These molecular parameters are extremely important to find out the molecular configuration of LLDPE straight away. Two of them are widely used: SCB/ 1000C and comonomer content in mol%. The monomer and comonomer average sequence lengths, nE and nC, were obtained as reported by Randall .
The tendency of the comonomer (1-hexene or 1-octene) units to form contiguous series or "clusters" can be described by the "monomer dispersity," MD (the inverse of the sequence length), as shown by Randall [9, 18]. An MD value of 100 would indicate that the comonomer units are all "isolated" as ethylene-comonomer-ethylene sequences. Any value below 100 is an indicative of a propensity toward blockiness to form contiguous comonomer-comonomer sequences. This means that MD is a real indicative of the SCBD for polymers.
Observed triad distributions, SCB content, comonomer mole fractions, the MD, and average sequence lengths are given in Table 5 for the studied polymers.
All samples exhibit similar comonomer content (about 3 mol% and 13 SCB/1000C), although LL has 1-octene as comonomer. In relation to the monomer average sequence length (nE), it is easy to notice that LL exhibits a smaller sequence of ethylene than the mPE samples (nE = 24.6). Also, the comonomer sequence is smaller than the others (nO = 0.66). This is an indicative that the comonomer sequences are short and very close to each other, because the sequence of ethylene is also short. They are the two striking features for these data. Adding the fact that MD is equal to 100%, which means a completely random copolymer or highly homogenous comonomer distribution [9, 18], it is possible to conclude that LL was produced in a bimodal process, which permits to better adjust and control the molecular parameters. mPE samples exhibit very similar characteristics in relation to monomer and comonomer sequences and also in relation to the MD parameter. It is possible to conclude that both should exhibit a very close crystallization behavior and final properties. However, they were not observed in the Crystaf analysis, where huge differences between mPE1 and mPE2 were clearly observed in relation to their crystallizabilities. Indeed, NMR analysis was able to identify SBCD and the LCB presence individually, whereas Crystaf analysis was able to evaluate the tendency of each sample to crystallize, which is a response of SCB and LCB.
We can conclude that LL is more homogenous than both mPE and that it was a combination of at least two polymerization reactors. Because of this molecular architecture, this resin has shown special properties in the flexible packaging market. Metallocene samples are exactly equal with regard to SCBD, and only differ concerning LCB presence, which provokes a completely different crystallization process, as evidenced by Crystaf analysis .
Sealability Performance--Hot Tack Analysis
One of the main applications of mPEs, as mentioned previously, is in multilayer flexible packaging, where they can be used as sealant layer . Sealability performance of films have been determined by hot tack or strength analysis [1, 3]. A typical hot tack curve was obtained by force versus temperature, as shown on Fig. 3. Parameters related to sealability were obtained from the hot tack curve:
a. Maximum force = related to the highest force obtained during the test.
b. Seal window = usually a minimum force of 2 N/25 mm was necessary to maintain the seal closed. The seal window was obtained by calculating the difference between the temperatures crossing this force. This means that between these temperatures, the minimum force to seal the packaging will be reached.
c. Seal initiation temperature (SIT) = related to the beginning of sealing. SIT directly affects the speed and quality of sealing, which, in turn, controls packaging speed .
Typical hot tack curves and sealing parameters were obtained for all samples and are shown in Table 6.
It is sometimes hard to correlate industrial or real packaging properties with controllable laboratorial analysis. Hot tack analysis is one of the techniques used for evaluation of sealability of films to estimate the real packaging film performance. The important question is which of the three parameters are better correlated with an actual situation? In general, hot tack presented a good correlation with FFS packaging. Maximum force will only be a parameter to indicate if a polymer or film were able to be sealed. There is a minimum force necessary to maintain the films sealed. Seal window is a good indicative of operating window for industrial sealers. A narrow window is a weak parameter, because it means that any variation on the operating conditions could cause failure in the sealing process. In the end, if the film exhibits a very low initial temperature, it means that the filling packaging should be faster. It is probably the most important parameter obtained from the hot tack curves.
Observing all data in Table 6, it is possible to conclude that mPE1 is one of the best products to produce flexible packaging. It will be possible to increase filling speed during the sealing process, because the film will be sealed at a lower temperature, which allows a wide operation window. This packaging will keep the products safe during the sealing process, and will also guarantee a perfect seal, because of the higher force on the sealing curve. Similar results were observed by Simanke et al. for a metallocene grade . mPE2 exhibits a higher initial temperature, which brings down the value of high speed sealing process for this product. LL also exhibits a very low initial sealing temperature, but the maximum force is lower than the others and the operating window is narrower, which would cause failure during the sealing process.
Molecular parameters which correlate very well with the initial temperature of sealing should be monomer average sequence and also MD. The higher the MD, the lower the temperature of initial sealing. Also, the lower the monomer sequence, the lower the temperature of initial sealing. At the beginning of the sealing process, the correlation is clear, because it is easier to melt polymer chains with short ethylene backbones. This is also true for random copolymers, because the crystal is not perfectly formed with a homogenous distribution of comonomer , and it was observed for LL and mPE1, but it was not so clear for mPE2. The monomer average sequence and MD for mPE2 is almost the same as mPE1, but the sealing performance is slightly different. Parameters show that mPE2 seems to have a delayed sealing process, and because of that, a higher temperature is necessary to start the process.
The answer to this observation, which is also the final step of the sealing process, is the crystallization, or solidification of the film. For this reason, some crystallization kinetics analyses were done.
An isothermal crystallization kinetics was chosen because the sealing process has been studied as a function of temperature. Samples were analyzed in relation to their specific isothermal temperatures. Figure 4 shows the crystallization speed (ka) obtained from the Avrami model [16, 20].
The crystallization delay for the mPE2 sample is evident. At 113[degrees]C, it is clearly noticeable that for mPE2 the crystallization speed is almost zero, but for the other samples (LL and mPE1), crystallization occurs very fast (ka ~ 0.2).
Another important parameter obtained from the crystallization kinetics study is t (1/2), which means the time necessary to achieve 50% of crystal formation (Fig. 5). At the same temperature, a shorter time equals to faster crystallization.
In the same way, mPE2 corresponds to the lowest crystallization behavior in comparison with the other samples. LL crystallization seems to be faster than mPE1 at lower temperatures, and both happen more than three times faster than mPE2 .
The hindrances to crystallization should result from the chain structure of the macromolecules on mPE2: their large length and mutual entanglement restrain their mobility to a great extent. Chain motion occurs mainly through reptation. Large-scale conformational chain rearrangements are required to shift small fragments of chains to suitable positions and incorporate them into the growing crystal. Therefore, the mobility of a macromolecule with respect to its neighbors is crucial in polymer crystallization and requires a careful consideration in the description of nucleation and growth of polymer crystals [22, 23]. Taking into consideration all data presented, it is possible to conclude that a higher content of LCB (~5/10,000C) on mPE2 affects the crystallization and also the sealability in a negative way.
Hot Tack Analysis--Dwell Time Variation. There is another way to perform the hot tack test. This time, the dwell time was changed along the test, to simulate the increase in the speed line of the packaging production. The dwell time was used in the range 0.2-2 s. Hot tack curves were plotted as force versus time and data were collected at the beginning of the curves (0.2-0.5s). At this point, it was observed a region of linearity between force and time. At each temperature, the welding power was obtained from the angular coefficient, reported as force/dwell time (N/25 mm s). A final plot was drawn as welding power versus temperature, as shown in Fig. 6.
LL and mPE1 showed a rapid crystallization rate, which cooperates with a higher sealing power at low temperatures (<105[degrees]C). Before the complete melting of the samples (as shown by the DSC in Fig. 7), with temperatures in the range 110-115[degrees]C, the sealing process is governed by the ability to crystallize. This means that the final step of sealing (solidification) becomes more important and will drive the process. Part of polymer chains were melted and are able to diffuse across the film interface. At temperatures lower than 105[degrees]C the most important step will be the solidification. Because the film was submitted to tensile stress right after heating. Thus, the faster crystallization and solidification of film more resistant is the sealing. After the melting temperature (T > 115[degrees]C), fusion becomes the most important factor, because the crystallization naturally becomes a slower process at higher temperatures.
This time, mPE2 takes over the sealing power since this material is completely melted and allows the increase of molecular mobility. The first and second steps of sealing, fusion and diffusion, are more prone to occur on mPE2. At this point, the high amounts of LCB contribute with large entanglements of molecules, leading to very good sealing on the packaging . This could explain the delayed sealing performance of mPE2 at lower temperatures.
We obtained some photographs at 115[degrees]C with 0.4; 0.6 and 0.8 s dwell time for the three samples after hot tack test. It is important to remember that only 0.8 s dwell time is sufficient to achieve a seal strength plateau. Figure 8a and b exhibit similar behavior for LL and mPE1. Films were sealed and a region of plastic deformation results from tensile stress. The other metallocene sample, mPE2, present film rupture near to seal zone without plastic deformation (Fig. 8c). The study shows the complexity of the sealing process and the importance of evaluating the materials through the graph of sealing power.
Blown films were produced with 40 [micro]m gauge with three different types of PE. The PE were different in microstructure, SCB and LCB presence, and CCD. The films were tested by hot seal strength test, in standard conditions and with dwell time variation to evaluate the power of sealing. The main findings of this work are summarized below:
* [sup.13]C NMR is a powerful technique to evaluate the microstructure of polyethylene, especially if a 1-hexene comonomer is used. In this case, it was possible to identify a LCB on the mPE2, and to quantify it. It was also possible to obtain the comonomer distribution by average comonomer sequences and MD. It was evident the similar behavior of comonomer sequence distribution for both metallocene polymers.
* The correlation between heat SIT and SCB distribution is true if a comparison is made between polymers without LCB. In the presence of LCB, the SIT is shifted to higher temperatures even with a homogenous SCB distribution.
* The variation of dwell time in each temperature allows another way to evaluate the sealing performance of the samples. The new variable, welding power, was determined. Observing the welding power as a function of temperature, it was possible to observe the huge difference during the seal process for polyethylene with LCB.
* The crystallization kinetics of the samples was fundamental to understand the differences between samples in relation to welding power and the standard seal test. LCB is responsible for reducing the ability of the polyethylene backbone to crystallize. It shifts the crystallization process to lower temperatures and longer times.
The authors wish to thank Braskem for allowing publishing of this study. We would like to thank all technicians of the Innovation and Technology Center at Braskem.
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Ana Cristina Fontes Moreira, (1) Paula Cristina Dartora, (2) Francisco Paulo dos Santos (3)
(1) Instituto Politecnico/UERJ, Nova Friburgo, Brazil
(2) LAPOL--Laboratorio de Materiais Polimericos/UFRGS, Porto Alegre, Brazil
(3) LAMOCA--Instituto de Quimica/UFRGS, Porto Alegre, Brazil
Correspondence to: A.C.F. Moreira; e-mail: email@example.com
TABLE 1. List of the resins. Name in MFI Density Polymerization this paper (190[degrees]C/2.16 kg) (g/[cm.sup.3]) process mPE1 1.0 0.917 Gas phase mPE2 0.6 0.918 Gas phase LL 1.0 0.918 Solution LD 2.3 0.923 High pressure mPE, metallocene grade; LL, conventional LLDPE; LD, conventional LDPE used in film formulation. TABLE 2. Standard conditions for hot tack analysis. Parameters Condition A Condition B Thickness ([micro]m) [less than or equal to] 25 >25 Platen pressure 0.304 0.304 (N/[mm.sup.2]) Dwell time (s) 0.5 1 Cooling time (s) 0.2 0.2 Strain rate (mm/s) 200 200 Width (mm) 25.4 25.4 Temperature range 95-160 95-160 ([degrees]C) TABLE 3. Molecular weight and molecular weight distribution data. [M.sub.n] [M.sub.w] [M.sub.z] Samples (kg/mol) (kg/mol) MWD (kg/mol) mPE1 50 140 2.90 350 mPE2 31 121 3.90 371 LL 45 133 2.95 334 TABLE 4. Chemical shift assignments in the [sup.13]C NMR spectra of the samples. Chemical shift (ppm) Carbon Triads/sequences 38.14 Methine EOE or EHE 34.55 [alpha][[delta].sup.+] EOEE + EEOE or EHEE + EEHE 34.17 4[B.sub.4] EHE 32.22 3[B.sub.6] or EOE or LCB 3[B.sup.b.sub.n] 30.94 [gamma][gamma] HEEH 30.49 [[gamma]sup.[delta]+] OEEE + EEEO or HEEE + EEEH 30.00 [[delta].sup.+] (EEE)n [[delta].sup.+] 29.56 3[B.sub.4] EHE 27.28 5[B.sub.6] or EOE or EHEE + EEHE [beta][[delta].sup.+] 27.08 [beta][delta] HHEE + EEHH 23.42 2[B.sub.4] EHE 22.92 2[B.sup.0.sub.6] or EOE or LCB 2[B.sup.b.sub.n] 14.24 Methyl EOE or EHE LCB, long chain branching; H, hexene; O, octene; E, ethene , TABLE 5. Molecular parameters obtained from [sup.13]C NMR analyses. NMR data mPEl mPE2 LL ECE 2.96 2.57 2.62 ECC + CCE 0 0.46 0 CCC 0 0 0 CEC 0 0 0 CEE + EEC 5.57 6.87 10.59 EEE 91.47 90.10 86.78 SCB/1000C 13.4 12.9 13.2 Comonomer (%mol) 2.9 3.0 2.6 Monomer sequence (nE) 33.8 35.5 24.6 Comonomer sequence (nH or nO) 1.03 1.02 0.66 MD (%) 96.98 97.18 100 Prediction of molecular structure/process Monomodal Monomodal Bimodal H, 1-hexene; O, 1-octene; and C, comonomer. Monomer comonomer TABLE 6. Sealing parameters. Max. force Initial Seal Samples (N/25 mm) temperature ([degrees]C) window ([degrees]C) mPE1 3.9 95.5 34 (135-101) mPE2 3.2 98 34 (141-107) LL 2.8 93 29 (133-104)
Please note: Some tables or figures were omitted from this article.
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|Author:||Moreira, Ana Cristina Fontes; Dartora, Paula Cristina; dos Santos, Francisco Paulo|
|Publication:||Polymer Engineering and Science|
|Date:||Jan 1, 2017|
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