Synthesis and Evaluation of Amides as Slip Additives

REGULAR CONTRIBUTED ARTICLES

F. Coelho1 , L. F. Vieira1 , R. Benavides2 , M. Marques da Silva Paula1 , A. M. Bernardin1 , R. F. Magnago3 , L. da Silva1 * 1

Laboratório de Pesquisa em Materiais LAPEM, Universidade do Extremo Sul Catarinense, Criciúma, Brazil de Investigación en Química Aplicada, Saltillo, Coahuila, Mexico 3 Laboratorio de Tecnologia Química, Universidade do Sul de Santa Catarina, Palhoça, Brazil 2 Centro

International Polymer Processing downloaded from www.hanser-elibrary.com by Kungliga Tekniska Högskolan on January 14, 2016 For personal use only.

Synthesis and Evaluation of Amides as Slip Additives in Polypropylene In this study, we describe the synthesis of amides and evaluate their use as slip additive agents in polypropylene films. The additives N-isopropyl-stearamide and N,N-diisopropyl-stearamide were synthesized and characterized and then used to prepare masterbatches. Erucamide, a commercial masterbatch, was used as the reference standard. A 32 factorial experimental design was used to create the different compositions of slip additives in polymer films. The films were processed via flat-die extrusion and stored in an oven at a constant temperature of 40 8C for seven days. The following are the properties evaluated in the study: thermal decomposition temperature, fusion temperature, fusion enthalpy, coefficient of friction, surface energy, contact angle, and seal initiation temperature. The results were evaluated by analysis of variance (ANOVA) at a 90 % confidence interval. The analysis of the results showed that N-isopropyl stearamide and N,N-diisopropyl stearamide do not provide an adequate surface slip for polypropylene films at the conditions used in the study. In turn, at the same conditions, erucamide, the commercial amide, also does not provide the required surface energy for printing and lamination processes required for polypropylene films.

1 Introduction Flexible plastic packaging is currently replacing glass, paper, and metal packaging primarily because they are disposable and easy to handle and transport. However, plastic materials must exhibit excellent processing and packing mechanical performances in addition to being good barriers to gases and vapors to guarantee protection of the packaged product (Fabris et al., 2006; Lazic et al., 2010). Films used for packaging are typically printed and/or laminated to other substrates. For this reason, films require surface treatment to allow adhesion for ink and adhesives. This treat* Mail address: Luciano da Silva, Laboratório de Pesquisa em Materiais LAPEM – Universidade do Extremo Sul Catarinense, 88806 – 000, Criciúma, Brazil E-mail: [email protected]

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ment is performed after extrusion and before winding (Lahti and Tuominen, 2007). The final packaging can be performed manually in simple sealing machines or automatically in highspeed machines that pack the product without human contact (Mazzala et al., 2012). Polypropylene is a polymer commonly used to manufacture flexible plastic packaging, which is typically used for food products. Polypropylene films must have good optical properties, such as brightness and transparency, adequate surface energy for good ink anchorage and lamination adhesives, and a low coefficient of friction to allow the film to slip in equipment’s rolls, collars, and guides without affecting the packaging process. In addition, these films must have good sealing properties, low seal initiation temperature, and a wide seal curve, which are primarily required for automatic packaging. Depending on the application or product to be packaged, resistance to tear propagation and the modulus of elasticity are also important (Mazzala et al., 2012). To satisfy the processing and fabrication requirements of flexible packaging, additives must be added to polypropylene during polymerization or processing. The goal of the addition is to increase the equipment’s productivity, aid processing, improve surface properties, facilitate handling, and increase the lifetime of the final packaging. Several additives are indispensable to obtain good processing and applicability characteristics. The most important feature and the feature that defines an additive’s performance is its compatibility with the polymer. In certain cases, it is necessary that the additive migrate to the surface of the polymer by diffusion to provide surface characteristics, such as slip and antistatic properties. In other cases, the diffusion of additives to the surface may result in early failure of the material (Velasco-Ruiz et al., 2000; Drobny, 2007; Poisson, 2009). There are different types of additives with different characteristics, such as antistatic, antiblock, slip, and antioxidants. Slip additives are added to polypropylene with a migratory function and are responsible for the slipping feature of films. Without adequate slip, it would be impossible to use polypropylene film in automatic packaging processes. Slip additives that are currently used are based on fatty acid amides, silicones, polyethylene waxes, and polytetrafluoroethylene (PTFE) (Drobny, 2007). This study uses slip additives based on fatty

Carl Hanser Verlag, Munich Intern. Polymer Processing XXX (2015) 5


F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene acid amides and evaluates the structure and properties of different amides in the composition of polypropylene films. This work is important considering the difficulty in using amide-based slip additives because of their high dependence on environmental conditions during film processing. The migration speed of amide-based additives depends on the processing temperature, processing speed, cooling conditions of the film, storage ambient temperature, and time. Thermal analyses were performed in a thermogravimetric analyzer (Shimadzu, model TGA-50), in a differential scanning calorimeter (Shimadzu, model DSC-50, Kyoto, Japan), and in a DSM (DSM, model MI-3 Plastometer, São Paulo, Brazil). In addition, most of the commercial polypropylene grades are already supplied with additives, which impede the processor in controlling the additive’s concentration. The concentration of slip additives in the polymer is defined based on the main application; however, the manufacturing environment and even the type of extrusion equipment can influence diffusion process. Several studies have investigated the effects of amide migration, where the goal is to understand the migration process and to determine the most adequate additive structure for each type of polymer (Poisson, 2009; Garrido et al., 1997, 2000; Ramirez et al., 2005; Wakabayashi et al., 2007). However, at an industrial level, amides are still the most and efficient structures used. Temperature is the predominant factor in amides migration. In practical terms, temperatures greater than 30 8C significantly increase migration speed. This causes quality problems, such as low surface energy and low seal strength, and in certain cases, the coefficient of friction may be affected. These problems are a consequence of the high concentration of amides in the polymer’s surface that conceal the polar groups generated by the surface treatment, which reduces the adhesion and wettability of the film and impedes the anchorage of ink and adhesives. In addition, amides affect the seal strength because they migrate to both sides of the film. Amides could also reduce the friction coefficient (< 0.10), which may affect winding and cause slip and misalignment of the film (Ramirez et al., 2005). This work evaluates the structural effect of the amide on properties such as the surface energy, coefficient of friction, and seal strength. The study is divided into three stages. In the first stage, additives with N-isopropyl-stearamide and N,N-diisopropyl-stearamide were developed. Next, the amides were used to produce masterbatches, and finally, the masterbatches were used to process the films. The results obtained with the synthesized additives were compared with that of a commercial additive. All the experiments were performed at industrial processing conditions.

16047-BN was obtained from Cristal Master, Morro da Fumaça, Brazil. Homopolymer polypropylene H-080 (melt index of 8 g/10 min at 190 8C) was obtained from Natpet, Jeddah, Saudi Arabia. Carbon and hydrogen nuclear magnetic resonance spectra (1H and 13C NMR) were obtained using Varian Inova and Varian NMR Instruments (Palo Alto, USA), which both operated at 300 MHz. The analyses were performed with deuterated chloroform (CDCl3) and dimethyl sulfoxide (DMSO-d6) with tetramethylsilane (TMS) as the internal standard. The infrared spectra (Fourier transform infrared spectroscopy – FTIR) were obtained in a Shimadzu-IR Prestige 21 spectrophotometer (Shimadzu, Kyoto, Japan) with readings in the 4 000 to 400 cm–1 region. Thermal analyses were performed in a thermogravimetric analyzer (Shimadzu, model TGA-50), in a differential scanning calorimeter (Shimadzu, model DSC-50, Kyoto, Japan), and in a DSM Plastometer (model MI 3, DSM, São Paulo, Brazil). The TGA thermographs of the masterbatches and films samples were obtained in a nitrogen atmosphere with a 10 ml/ min flow rate and a 10 8C/min heating rate in a platinum crucible. The DSC analyses of the masterbatches and films were performed in a nitrogen atmosphere with a 50 ml/min flow rate and a 20 8C/min heating rate in an aluminum cell. The melt index of the masterbatches was measured using a 2.16 kg at 190 8C. The films’ morphology was evaluated by atomic force microscopy (AFM) using a Shimadzu SPM 9700 instrument operating in dynamic mode. The images were obtained in a 5mm2 scanning area. The coefficient of friction was measured on both sides of the film with a DSM model COF-SC1A COF meter according to standard ASTM D 1894-14. The surface energy was measured with formamide and ethyl glycol monoethylether solutions with treatment levels between 32 and 50 dyn/cm according to standard ASTM D2578-99ˆ. Contact angles on the films were determined by placing a drop of distilled water on the surface of each sample and then, capturing an image of each drop. The procedure was repeated ten times at different positions on the film. The images were taken five seconds after the drop formed to accommodate the liquid. The contact angle was calculated for each drop on the film using Image J software (Maryland, USA) with the Drop Analysis – DropSnake plugin. From the results, the averages of ten measurements of each composition were calculated. The seal tests were performed according to standard ASTM F88-00.

2.2 Preparation of the Additives 2 Experimental 2.1 Materials and Equipment Stearic acid, methanol, sulfuric acid, dichloromethane, isopropylamine, diisopropylamine, and triethylamine, all of which were of analytic purity, were obtained from Sigma-Aldrich Chemical Company, São Paulo, Brazil. Homopolymer polypropylene H-128 (melt index of 20 g/10 min at 190 8C) was obtained from Braskem S/A, Porto Alegre, Brazil. Masterbatch Intern. Polymer Processing XXX (2015) 5

2.2.1 Synthesis The additives N,N-diisopropyl stearamide is an original material and was prepared by a straight-forward and high-yield synthetic route, partially based in well known literature procedures (Garrido et al., 1997). A detailed description of the synthesis procedures and corresponding analytical data (melting point, IR, 1H, and 13C NMR spectra) are given in the Supporting Information. 575

F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene 2.3 Preparation of the Masterbatches The masterbatches were processed in an EME extruder (model EME-1, twin-screw extruder, Novo Hamburgo, Brazil) with a screw speed of 220 min–1, pressure of 20 bar, and mass temperature of 180 8C. The processing temperature was in the 150 to 200 8C range. The composition of the masterbatches was 94 % polypropylene homopolymer H-128 and 6 % amide (commercial amide or synthesized amide). The masterbatches were extruded into pellets for further use in the film processing.


2.4 Experimental Design This study used a 32 factorial experimental design. The following parameters were evaluated: masterbatch type and concentration. Nine different polypropylene films were manufactured according to the program using polypropylene Homopolymer H-080 and different Masterbatch. Masterbatch made with commercial erucamide – MC with 1 %, 3 %, and 5 % of addictive (MC1, MC2 and MC3) and masterbatch made with synthesized amide, with 1 %, 3 %, and 5 % of addictive produced with Nisopropyl stearamide (MSI1, MSI2 and MSI3) and with 1 %, 3 %, and 5 % of addictive produced with N,N-diisopropyl stearamide, (MSD1, MSD2 and MSD3) respectively. 2.5 Processing of the Films The films produced with polypropylene homopolymer H-080 and different masterbatches (film with masterbatch comercial – FMC, film with masterbatch N-isopropyl stearamide - FMSI and film with masterbatch N,N-diisopropyl stearamide – FMSD) in different concentrations. The film were processed in a single-screw extruder (Model HR 801.1.100.3R, Reifenhäuser,Troisdorf, Germany) with a screw speed of 104 min–1, pressure of 161 bar, and mass temperature of 267 8C. The temperatures in the extruder zones were in the 200 to 235 8C range. The film thickness was 0.040 mm. After extrusion, the films received corona treatment on one of their faces and were then winded. The samples for characterization were stored in an oven at 40 8C for seven days. 3 Results and Discussion Figure 1 shows a schematic of the synthesis used to prepare 3a and 3b amides. In the case of methyl stearate ester, compound 2 (Fig. S1 in the supporting information), the FTIR spectrum shows the disappearance of the axial deformation of the O–H bond of the stearic acid and the displacement of the axial deformation of the C=O bond to 1 742 cm–1 compared with the starting acid. Axial deformations of the C–H aliphatic bond are observed at 2 918 and 2 849 cm–1. The 1H NMR analysis (Fig. S2 in the supporting information) shows a singlet at 3.65 ppm, which corresponds to the hydrogen atoms of the ester’s methyl. A triplet is observed at 2.29 ppm, which corresponds to the hydrogen atoms of the CH2 group linked to carbonyl. The other hydrogen atoms linked to the aliphatic chains are observed at 1.25 ppm. A triplet corresponding to the hydrogen atoms of the CH3 terminal group is observed at 0.87 ppm. 576

Fig. 1. Synthesis schematic

The FTIR spectrum of N-isopropyl stearamide, compound 3a (Fig. S3 in the supporting information), shows the appearance of axial deformation of the N–H bond at 3,460 cm–1. There is an axial deformation of the C=O bond at 1 744 cm–1. Axial deformations of the aliphatic C–H bond are observed at 2 916 and 2 851 cm–1. In the 1H NMR spectrum (Fig. S4 in the supporting information), we observed a singlet at 3.66 ppm, which originated from the hydrogen atom linked to the amide’s nitrogen. There is a triplet at 2.30 ppm from the hydrogen atom of the methine carbon of the isopropyl group. There is a triplet at 1.62 ppm from the methylene group linked to carbonyl. The other methylene hydrogen atoms and the hydrogen atoms of the terminal methyl were observed at 1.25 ppm and 0.88 ppm, respectively. The 13C NMR spectrum (Fig. S5 in the supporting information), shows signs of carbonyl carbon and methinic carbon linked to nitrogen at 174.29 ppm and 51.37 ppm, respectively. The other carbon types are observed at 34.08, 31.91, 29.09, 29.44, 29.35, 24.94, 22.68, and 14.08 ppm. The FTIR spectrum of N,N-diisopropyl stearamide, compound 3b (Fig. S6 in the supporting information), shows axial deformations of the C–H aliphatic bond at 2 918 and 2 848 cm–1. The axial deformation of the C = O bond was observed at 1 745 cm–1. The 1H NMR spectrum (Fig. S7 in the supporting information), shows a multiplet at 3.94 ppm from the hydrogen atom of the methinic carbon of the isopropyl group. The triplet of the methylene group, which is linked to carbonyl, is observed at 2.43 ppm. The other methylene hydrogen atoms, the methyl groups in the isopropyl group, and the hydrogen atoms of the terminal methyl group are observed at 1.53, 1.29, and 0.88 ppm, respectively. The 13C NMR (Fig. S8 in the supporting information), shows signs of carbonyl carbon and methinic carbon linked to nitrogen at 170.4 ppm and 57.9 ppm, respectively. The other carbon types are observed in the region between 34 ppm and 16 ppm.

3.1 Thermal Analysis Figure 2 presents the TGA thermographs, which shows that the thermal decomposition of the MC masterbatch began at 401 8C, that of the MSI masterbatch began at 431 8C, and that of the MSD masterbatch began at 411 8C. Intern. Polymer Processing XXX (2015) 5


F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene lecules, lowering the energy required to break intermolecular interactions during melting. The melt indexes of masterbatches MC, MSI, and MSD were 13.4 g/10 min, 13.2 g/10 min, and 13.0 g/10 min, respectively, which are the average of five measurements. After processing the films, the samples were kept in an oven at a controlled temperature of 40 8C for 7 days. The characterization analyses were performed from the seventh day onwards. The 40 8C storage temperature was chosen to simulate an environmental situation recognized as a problem for the commercial slip additive (erucamide), which primarily concerns the seal strength, coefficient of friction, and surface energy. Table 1 shows the thermal decomposition and crystalline fusion temperatures and the fusion enthalpy of all the films and

The DSC analysis identified the crystalline fusion temperature (Tm) and the fusion enthalpy of the samples. The crystalline fusion points of masterbatches MC, MSI, and MSD were 167.26 8C, 165.58 8C, and 170.16 8C (Fig. 3), respectively. Previous studies showed that the amide’s fusion peak appears only at concentrations greater than 16 % (Fegley, 2013), which must be why the amides’ crystalline fusion peak was not observed in the masterbatches of the present study. The fusion enthalpies were 71.81 J g–1, 67.85 J g–1, and 65.35 J g–1 for masterbatches MC, MSI, and MSD, respectively. The lowest phase transition energy to the master MSI and MSD, compared to the value obtained for the master CM, can be attributed to the volume of isopropyl and diisopropyl groups, which one causes a higher distancing of carbon chains. The result is increased intermolecular distance of the polymer mo-

Fig. 2. Thermogravimetric analysis of the masterbatches and derivatives

Fig. 3. Differential scanning calorimetry of the masterbatches

Compositions

Master type

Master conc. %

Tonset 8C

Tm 8C

DHm J g–1

T.I.S. 8C

Contact angle h

S.E. dyn/cm

COFT

COFNT

Film without slip additive MC (6 % of slip additive) F1MC 1 % F2MC 3 % F3MC 5 % MSI (6 % of slip additive) F4MSI 1 % F5MSI 3 % F6MSI 5 % MSD (6 % of slip additive) F7MSD 1 % F8MSD 3 % F9MSD 5 %

– – MC MC MC – MSI MSI MSI – MSD MSD MSD

– – 1 3 5 – 1 3 5 – 1 3 5

379 401 409 356 402 431 402 401 415 411 409 337 401

164.6 167.3 164.7 164.61 164.7 165.6 166.7 164.9 165.2 170.2 164.7 163.8 164.8

89.7 71.8 54.5 67.0 70.2 67.8 91.9 96.2 111.2 65.3 70.2 94.2 37.1

– – 127 133 132 – 133 134 134 – 132 136 139

– – 74 90 92 – 63 61 58 – 76 65 61

– – 36 34 31 – 38 40 42 – 36 38 40

– – 0.28 0.17 0.11 – 1.15 1.15 1.15 – 1.15 1.15 1.15

– – 1.15 0.20 0.13 – 1.15 1.15 1.15 – 0.58 0.68 0.40

(S.I.T.), surface energy (S.E.), COF on the treated surface (COFT) and COF on the non-treated surface (COFNT), Tonset = onset temperature; Tm = melting temperature; DHm = melting enthalpy; S.I.T. = sealing initiation temperature; S.E. = surface energy; COF (T) = COF on the treated surface; COF (NT) = COF on the non-treated surface Table 1. Experimental design and main results for onset temperature (Tonset), melting temperature (Tm), melting enthalpy (DHm), sealing initiation temperature

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F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene masterbatches. The compositions with the lowest thermal decomposition temperature were obtained with 3 % of the slip masterbatch. Regarding the mass-loss curve, there is only one stage of mass loss, which is typical behavior of the polymer under study. The crystalline fusion temperatures are between 163.79 8C and 166.70 8C. The lowest temperature was observed in the film with 3 % of the MSD masterbatch, which is even lower than the fusion temperature of the film without the slip masterbatch. The composition with 1 % of the MSI masterbatch exhibited the highest crystalline fusion temperature. The differences between the crystalline fusion temperatures may indicate the existence of larger crystals or smaller crystalline defects or even an increase in the lamellar thickness. Although the amide concentration added to the films is small, it is suggested that the addition of different amide structures modifies the melt viscosity of the polymer mass, which modifies the pressure at the die exit, which changes mobility and alters crystallization. The fusion enthalpies of the films with the MSI masterbatch and with 3 % of the MSD masterbatch were higher than the fusion enthalpy of the film without the slip additive. This result suggests that the amide structures of the MSI masterbatch and the 3 % MSD masterbatch contribute to the molecular organization by reducing the distance between the molecules, which therefore increases the phase-change energy. The fact that there was no difference in the fusion enthalpy of the films in the cases of the MSD masterbatch at 1 % and 5 % concentrations could be related to the amides’ dispersion in the films (Peloso et al., 1998). Regarding the masterbatch concentration, the addition of the MC masterbatch (erucamide) reduced the fusion enthalpy of the films compared with that of the film without the slip masterbatch. However, the fusion enthalpy increased as the masterbatch concentration increased, which suggests a better molecular organization and orientation and, therefore, an increased crystallinity. These results indicate that when added to polypropylene films, erucamide remains restricted to the amorphous regions of the polymer and contributes to an increased degree of crystallinity. In addition, the degree of crystallinity increases with the amides’ concentration in the mixture. The same behavior was observed with the addition of the MSI masterbatch; however, in all cases, the fusion enthalpy was greater than that of the film without the slip masterbatch. However, a random behavior is observed in the case of the MSD masterbatch, where it was impossible to establish a relationship between concentration and enthalpy.

masterbatch concentration in the film increases, which reduces the COF (Schumann et al., 2000). The films with the MSI masterbatch exhibited high coefficients of friction in both sides of the films. The COF was approximately 1.15 for all the studied concentrations. The same slip behavior was observed in the films with the MSD masterbatch when the COF was measured on the side with corona treatment; however, the COF values were lower on the untreated side, down to 0.40 for the 5 % concentration. The analysis of the COF values of the films with the MSD masterbatch demonstrates the differences between the treated and untreated faces. There appears to be a tendency of additive migration to the untreated face, which justifies the significant reduction of the COF. This result implies a migration restriction to the treated face (Loop et al., 2011). Regarding the films with the MSI masterbatch, there was no change in the COF on both sides of the film. This result may be explained by the migration of the amides that do not act as a slip additive given the high COF values measured in both sides of the film for all compositions or by the storage conditions of the study, which may have influenced the migration process. In general, it was observed that at a 1 % concentration of the synthesized masterbatches, the coefficients of friction of the untreated sides are high for all compositions. The films’ surface energy is directly linked to the adhesion and wettability of the material’s surface (Mansha et al., 2011). This measurement is necessary to evaluate whether the adhesion level of the films meets the requirements for printing and lamination processes. The higher the surface energy, the larger the adhesion or wettability of the film when dealing with polar inks and polyurethane adhesives (Sellin and Campos, 2003; Strobel et al., 2003; Sun et al., 1999). Table 3 shows the surface energies of the compositions measured with formamide and ethyl-glycol-monoethyl ether. The films with the MSI and MSD synthesized masterbatches had higher surface energy values than that of the film with the MC commercial masterbatch. For a film to be suitable for printing with polar inks, its surface energy must be higher than 37 dyn/cm, and in the case of lamination with polyurethane adhesives, the surface energy must be higher than 42 dyn/cm (Mazzola et al., 2012). It was observed that for all studied concentrations, films with the MSI and MSD masterbatches exhibited adequate adhesion for these processes, with surface energies between 40 and 42 dyn/cm. In turn, none of the films with the MC (erucamide) masterbatch met this requirement because in all cases, the surface energy was less than 37 dyn/cm.

3.1.1 Coefficient of Friction (COF) 3.2 Contact Angle The static COF was greater than 1.15 for all films, which is the superior limit for the instrument and is a common reading for materials without antiblocking additives (Cáceres et al., 2012) . The results of the dynamic coefficients of friction are shown in Table 1. The lowest coefficients of friction were observed for films with 3 % and 5 % of the MC masterbatch (erucamide) in both sides of the films. The COF values measured in the films with the MC masterbatch are in accordance with literature (Poisson et al., 2009; Fink and Karl, 2010; Mesquita and Martinelli, 2010). The amides’ migration to the surface increases as the 578

The analysis of the contact angle had the same objective of the surface energy measurements: to evaluate the effect of the amides’ migration on the wettability of the films for the different compositions. Table 1 shows the values of the surface energy, contact angle, and sealing temperature of the films produced in this study. Figure 4 shows the images of the water drops applied on each composition. The image of the film without the slip additive is shown beside each composition by masterbatch type for visual comparison. Intern. Polymer Processing XXX (2015) 5


F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene

Fig. 4. Images of the analysis of the contact angles of the films produced in this study

The contact angle determined for water on polypropylene films without corona treatment was 908 (Sellin and Campos, 2003). Greater contact angles were obtained for films with the commercial masterbatch. The contact angle of the film with 5 % of MC masterbatch was greater than those of the polypropylene films without corona treatment. The film with 3 % of the MC masterbatch exhibited a contact angle of 89.58, which is similar to that of the untreated film. The film with 1 % of the MC masterbatch was the only film that showed a wettability increase with a 73.78 contact angle. The films with the MSI and MSD masterbatches exhibited excellent wettability. All of the angles were lower than that of polypropylene without corona treatment (908) and lower than that the film without the slip additive (79.58). The films with the MSI masterbatch exhibited the lowest contact angles. It was observed that the lowest contact angles and the highest surface energies occur in the films with the highest coefficients of friction, which demonstrates that amides’ deposition on the surface of these compositions is less intense. When relating the surface energy and coefficient of friction properties of the commercial masterbatch at 3 % and 5 % concentrations, the films had low coefficients of friction and low surface energies, which means that at these concentrations, the amides migrate in an out-of-control way at the studied storage condition and concealed the polar groups generated by the corona treatment. To prove this, the surface of the films was cleaned with a dry cloth, and the surface energy was 40 dyn/cm after this procedure. For the other compositions, the procedure did not change the surface energy. The seal strength was measured in cold conditions. Samples that were 25.4 mm in size were sealed at different temperatures for 0.5 s and rested for 24 h, and then, tensile tests were performed on the samples in a universal machine. The sealed faces Intern. Polymer Processing XXX (2015) 5

were those without corona treatment, which is the same as in actual packaging. The cold seal curve was created from the average of five samples for each temperature. This curve reveals important seal parameters for the industry, such as the seal initiation temperature (S.I.T.) and the size of the seal curve (DT). A similar seal behavior of the films was observed for all the analyzed compositions. The lowest S.I.T. occurred in the film with 1 % of the MC masterbatch, and the highest S.I.T. occurred in the film with 5 % of the MSD masterbatch.

3.3 Atomic Force Microscopy (AFM) The films’ morphology was analyzed by atomic force microscopy (AFM). The AFM images of both sides of the film with 5 % of the masterbatch (Fig. 5) demonstrated that the films with the commercial masterbatch contained a higher surface irregularity than the respective films processed with the synthesized masterbatches (MSI and MSD). The existence of more agglomeration or elevations in the films with the MC masterbatch are in accordance with the results of surface energy, contact angle, and COF. Poisson described a similar behavior in a study that varied the concentration of erucamide and oleamide in polyethylene films (Poisson, 2009).

3.4 Experimental Design and Statistical Analysis In order to analyze the effect of the masterbatch type and concentration on the main properties of the films, a 32 factorial design was performed, Table 1. The factors of the design were the type of masterbatch (MC, MSI and MSD) and the masterbatch 579


F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene concentration into the films (1, 3 and 5 wt.%). Regarding the concentrations, 1 wt.% corresponds to 600 ppm of amide (masterbatch) in the film, 3 wt.% corresponds to 1 200 ppm amide and 5 wt.% corresponds to 1 800 ppm amide. All results were evaluated by analysis of variance (ANOVA). Table 1 shows the compositions and main results. The ANOVA method uses regression models to compare the measured value (property) to a perfect-fit model (100 %). The model deviations are attributed to experimental error or indicate that the factor under analysis does not change the result (measured value), therefore it is not significant for the result (measured value). In this study, a confidence interval (C.I.) of 90 % was considered. For a 90 % C.I., the statistical analysis suggests that the masterbatch type and concentration are significant factors that affect the initiation temperature of sealing (S.I.T.) and the surface energy (S.E.) of the films. The type of

A)

580

B)

masterbatch significantly influences the crystalline fusion temperature (Tm), melting enthalpy (DHm), contact angle, and coefficient of friction (COF) of the films. Moreover, the masterbatch concentration is significant to the thermal decomposition temperature of the films. Statistically, the masterbatch type and concentration significantly influenced the S.I.T. of the films, where a linear model was considered for both factors, with a confidence interval greater than 95 %. Table 2 shows the analysis of variance for the S.I.T. of the films in function of the masterbatch type and concentration. It can be observed in the response surface for the S.I.T. (Fig. S9 in the supporting information) that an increase in the masterbatch concentration increases the seal initiation temperature. Regarding the effect of the masterbatch type on the S.I.T. of the films, it was observed that the films with the MC

Fig. 5. AFM images of the face with corona treatment (A) and without corona treatment (B), scan area of 5 · 5 lm2



F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene masterbatches had the lowest values of S.I.T., followed by the films with the MSI masterbatches, and finally the films with the MSD masterbatches. Probably these new amide structures, which contain isopropyl and diisopropyl groups in the chain, contribute to the S.I.T. increase and make molecular interlacement more difficult, even at a lower concentration on the surface. The analysis of variance of the surface energy on the side of the films that received corona treatment showed that both the type and concentration of the masterbatch had significant results on the combined effect of the linear and quadratic model with a C.I. close to 100 %, Table 3. As observed in the response surface (Fig. S10 in the supporting information), the increase of the masterbatch concentration in the studied films has reduced the surface energy. The same behavior was verified in previous studies with slip additives based on erucamide and oleamide (Rawls and Hirt, 2002; Neto et al., 2001). The evaluation of the effect of the type of masterbatch showed that the MC masterbatch (based on erucamide) had a negative contribution that reduced the surface energy of the films, which resulted in values unsuitable for printing and lamination applications. In turn, the synthesized masterbatches MSI and MSD do not affect the surface treatment, which had surface energies between 40 and 44 dyn/cm. The AFM images (Fig. 5) demonstrate that at the same masterbatch concentration, the highest elevation surface (film with the MC masterbatch) had the lowest surface energy, whereas the more uniform surfaces (films with the MSI and MSD masterbatches) had the highest surface energies. The films with the MSI and MSD masterbatches had surfaces similar to the film without the slip masterbatch. This result suggests two pos-

sibilities: either the amides of the MSI and MSD masterbatches do not migrate to the corona-treated face due to being restricted by the polar groups formed by the surface treatment or they migrate in a small amount and conceal the surface without interfering with the wettability. The ANOVA results show that the type of masterbatch has significant influence on the crystalline melting temperature (Tm), melting enthalpy (DHm), contact angle and coefficient of friction (COF) of the films. The temperatures of crystalline fusion of the films were statistically significant only for the type of masterbatch, with 94 % confidence interval for the quadratic model (Table 4). Figure S11 in the supporting information shows the average obtained for the crystalline melting temperature for reliability of 90 % C.I. The graph of Fig. S11 in the supporting information shows that the higher Tm temperatures are given when using the MSI masterbatch. The melting enthalpy is statistically significant only for the type of master, with 92 % C.I. for the quadratic model (Table 5). Based on the averages for the melting enthalpy to a 90 % C.I. (Fig. S12 in the supporting information), masterbatch MSI increases both the melting temperature and the melting enthalpy of the films to values higher than for films without sliders. The increase of the melting temperature may indicate that these compositions have larger crystals, lower defects and increasing lamellar thickness. Previous studies claim that the amides cannot affect the bulk property of the polymers, only affecting the surface properties due to the very small concentrations used, 2,000 ppm on average. As this study used new amide, this amide may be modifying the viscosity of the polymer in the

Factor/model

SS

df

MS

F

P

R2

Master T. (L) Master T. (Q) Master % (L) Master % (Q) Error Total SS

37.50 0.50 28.17 4.50 13.33 84.00

1 1 1 1 4 8

37.50 0.50 28.17 4.50 3.33

11.25 0.15 8.45 1.35

0.03 0.72 0.04 0.31

0.97 0.28 0.95 0.69

SS = sum of squares; df = degree of freedom; MS = mean squares; F = Fisher test; p = reliability Table 2. ANOVA for the S.I.T.

Factor/model

SS

df

MS

F

p

R2

Master T. (L + Q) Master % (L + Q) Error Total SS

130.67 10.67 2.67 144.00

2 2 4 8

65.33 5.33 0.67

98.00 8.00

0.04 0.04

0.99 0.96

SS = sum of squares; df = degree of freedom; MS = mean squares; F = Fisher test; p = reliability Table 3. ANOVA for the surface energy


581


F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene molten state, as previously reported. The fact that the amide of the MSD masterbatch (N,N-diisopropyl stearamide) shows greater molecular size than the amide of the MSI masterbatch (N-isopropyl stearamide) and does not show significant effect on this property may be related to poor dispersion of the additive into the film. The analysis of the contact angle shows statistical significance for the type of masterbatch for both the linear model (C.I. 93 %) as the quadratic model (C.I. 92 %) (Table 6). Films with MSI and MSD masterbatches had the lowest contact angles. Comparing these results with the analysis of surface energy, it is known that the higher the surface energy of the films, the smaller the contact angle measured. Similar behavior was found for these analyzes. The largest verified con-

tact angles were on the films with masterbatch MC. On the other hand, the lower surface energies were measured for these films. The opposite occurred for films with MSI and MSD masterbatches. The masterbatch MSI showed the best results, with an average of approximately 608 (Fig. S13 in the supporting information). The analysis of variance of the COF results measured on the side with the corona treatment shows that the type of masterbatch significantly affects the COF results with reliability close to 100 % for the combination of the linear and quadratic models, Table 7. Regarding the treated side of the films, the MSI and MSD masterbatches affect significantly the COF of the films, increasing the results. The masterbatch MC shows the lowest val-

Factor/model

SS

df

MS

F

p

R2


0.07 2.19 0.30 0.96 1.25 4.78

1 1 1 1 4 8

0.07 2.19 0.30 0.96 0.31

0.22 7.01 0.97 3.05

0.66 0,06 0.38 0.15

0.34 0.94 0.62 0.84

SS = sum of squares; df = degree of freedom; MS = mean squares; F = Fisher test; p = reliability Table 4. ANOVA for crystalline the crystalline melting temperature

Factor/model

SS

df

MS

F

p

R2


16.10 2 337.68 0.64 351.30 1 633.51 4 339.31

1 1 1 1 4 8

16.10 2 337.68 0.64 351.30 408.38

0.04 5.72 0.02 0.86

0.85 0.07 0.97 0.41

0.15 0.92 0.03 0.59

SS = sum of squares; df = degree of freedom; MS = mean squares; F = Fisher test; p = reliability Table 5. ANOVA for the melting enthalpy

Factor/model

SS

df

MS

F

p

R2


486.00 470.22 0.60 1.68 335.30 1 293.80

1 1 1 1 4 8

486.00 470.22 0.60 1.68 83.83

5.79 5.69 0.08 0.02

0.07 0.07 0.94 0.89

0.93 0.92 0.06 0.11

SS = sum of squares; df = degree of freedom; MS = mean squares; F = Fisher test; p = reliability Table 6. ANOVA for the contact angle

582



F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene ues for the COF results. Films containing the masterbatch MC showed low COF values, enabling their use for printing. Films with MSI and MSD masterbatches showed high COF values, and the measurement was difficult. The film without slider masterbatch, showed COF of 0.55 on the treated side, whereas the films with masterbatches MSI and MSD showed COF of 1.15, i. e., the COF increased with the addition of synthesized amides, and these amides seem to act differently on the treated side of the film. Two explanations may be assumed to this result: the polar groups generated by the corona discharge may be restricting the migration of amides to the surface of the film or the migration speed of these structures is lower due to the larger size of these molecules; therefore, the concentration of amides on these films is lower. The average results (Fig. S14 in the supporting information), for the COF by the type of masterbatch were 1.15 for films with masterbatches MSI and MSD and were 0.20 for films with masterbatch MC. Finally, the analysis of variance of the COF results measured on the side without corona treatment shows that the type of masterbatch significantly affects the COF results with a C.I. close to 95 % for the quadratic model, Table 8. Regarding the side of the films without corona treatment and analyzing the calculated averages for the results (Fig. S15 in the supporting information), it was observed that the masterbatch MSI increases the coefficient of friction, whereas the masterbatches MC and MSD decrease it, with the lowest values observed for films with masterbatch MC. As reported for the other properties analyzed in this study, the amides of the masterbatches MSI and MSD have larger molecules and, therefore, lower diffusion rate than the amide con-

tained in the masterbatch MC. The AFM analyzes showed no elevation surfaces for films with masterbatches MSI and MSD, indicating a low concentration of amides on the surface of these films, therefore resulting in higher coefficients of friction. However, for the non-treated side, films with masterbatch MSD show lower COF values than films with masterbatch MSI. Probably the differences between the masterbatches MSI and MSD are related to poor dispersion of amides in films with masterbatch MSD or that the molecules of the masterbatch MSD, due to their larger size, offer a greater sliding effect. Analyzing the results of COF on both sides of the films, the films with masterbatch MC show low COF values on both sides of the film, values lower than for films without slider/ sliding. Films containing masterbatch MSI feature high COF on both sides of the film. Films with masterbatch MSD on the side with corona treatment show high COF values; however, on the side without corona treatment, the COF is lower: for a film with 5 wt.% of masterbatch MSD, the COF was 0.40. For the printing process, under the experimental procedures and concentrations investigated in this study, the masterbatches MSI and MSD showed no coefficient of friction values ??adequate for industrial applications (0.10 to 0.30).

4 Conclusion This study evaluated the effect of adding two new amides (Nisopropylamide and N.N-diisopropylamide) as slip additives in polypropylene films. Thermal stability, mechanical strength, and surface properties studies demonstrated that the synthesized amides, at the conditions of the study, do not provide an

Factor/model

SS

df

MS

F

p

R2

Master T. (L + Q) Master % (L + Q) Error Total SS

1.86 0.01 0.01 1.87

2 2 4 8

0.92 0.01 0.01

374.53 1.00

0.01 0.44

0.99 0.55

SS = sum of squares; df = degree of freedom; MS = mean squares; F = Fisher test; p = reliability Table 7. ANOVA for the COF measured on the side with the corona treatment

Factor/model

SS

df

MS

F

p

R2


0.01 0.78 0.24 0.02 0.44 1.48

1 1 1 1 4 8

0.01 0.78 0.24 0.02 0.11

0.05 7.21 2.20 0.13

0.83 0.05 0.21 0.74

0.16 0.95 0.79 0.27

SS = sum of squares; df = degree of freedom; MS = mean squares; F = Fisher test; p = reliability Table 8. ANOVA for the COF measured on the side without the corona treatment


583

F. Coelho et al.: Synthesis and Evaluation of Amides as Slip Additives in Polypropylene adequate surface slip to polypropylene films; therefore, they cannot be used as slip additives in films that will be printed, laminated, or used in automatic packaging lines. However, erucamide, a commercially available amide, was evaluated in this study, and it was found that it also does not provide the surface energy required for printing and lamination processes; this is currently the primary problem of the industry.


For the supporting information please visit www.hanserelibrary.com, International Polymer Processing, Volume 30, Issue 5.

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Date received: March 09, 2015 Date accepted: August 25, 2015

Bibliography DOI 10.3139/217.3097 Intern. Polymer Processing XXX (2015) 5; page 574 – 584 ª Carl Hanser Verlag GmbH & Co. KG ISSN 0930-777X
