Abstract
Nucleating agents cause an increase in crystallization temperature, making the production of highly oriented films much more difficult. In this study, a nucleating system composed of 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS) and tetrasilanolphenyl silsesquioxane (phPOSS) was used to modify isotactic polypropylene (iPP). Oriented films characterised by different draw drown ratios (4.8, 9.7, 14.5) were prepared and analysed. Their mechanical properties, coefficient of friction, thermal properties, and structure as well as optical properties such as gloss, haze, and transparency, were also determined. The macromolecular orientation of films was tested by polarised light Fourier transform infrared spectroscopy. The proposed nucleating system provides films with superior mechanical and optical properties, which are accompanied by high orientation ability. The application of DMDBS and phPOSS allowed a 26% increase in penetration load at break and Young modulus by 70% compared to iPP. It can be assumed that DMDBS/phPOSS system is an efficient modifier for polypropylene film production.
Introduction
Isotactic polypropylene (iPP) is the second most processed commodity polymer. It is characterised by excellent chemical resistance, a barrier towards moisture and water, low price, and good mechanical properties. Moreover, it has been widely applied in the packaging industry due to its physiological indifference. As a semicrystalline polymer, it is particularly susceptible to modification in the process of heterogeneous nucleation with the use of organic and inorganic additives. One of the most often used polypropylene modification methods, utilised in the case of packaging products, is a targeted change of the crystal structure resulting in obtaining ultrafine crystallites, leading to clarifying effect [1-3].
As a semicrystalline polymer, isotactic polypropylene reveals a high tendency to create crystalline structures and exhibit polymorphic behaviour. It can crystallize by creating different crystalline forms, namely monoclinic (α), hexagonal (β), orthorhombic (γ), and in the case of very high super-cooling – the mesomorphic (smectic) phase [4, 5]. Most inorganic fillers such as talc, calcium carbonate, or silicates increase the degree of iPP crystallinity [6-8]. Their introduction into the polymer matrix causes the formation of additional active nucleation centres and an acceleration of the spherulite growing rate. Unfortunately, the incorporation of most of the additives that increase the crystallinity of polypropylene results in a reduction of transparency of the final product. The second group of nucleants, which simultaneously reduce the haze of iPP, are clarifiers [9]. Most of the currently used nucleants are sorbitol derivatives that are soluble in molten polypropylene [10]. Upon cooling, they form a finely dispersed three-dimensional network of fibrils which serves as substrates for the growth of the polypropylene monoclinic (α) phase [11]. The effectiveness of 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS), described as the third-generation sorbitol-based clarifier, depends on the dispersion degree, concentration in the polymer matrix, and the maximum melt temperature. The preparation of a highly saturated solution of iPP–DMDBS allows the use of eutectics from which, during cooling, submicron-sized structures of polymer-rich liquid are released, resulting in the development of a large number of active nucleation sites on a finely crystallized additive [10]. As a result, a so-called fibrillar structure of DMDBS is formed, the molecules of which are joined together by forming π–π interactions and hydrogen bonds [11, 12]. Polyhedral oligomeric silsesquioxanes (POSS) are one of the most intensively studied nanosized materials. Their versatility comes from many possible ways of their modification with functional groups [13-16]. This feature determines silsesquioxanes as potential tailorable polymer additives designed for special applications as self-contained modifiers [17-19]. At the same time, many studies showed that silsesquioxanes with cage structure reveal limited nucleating ability on isotactic polypropylene [13, 18, 20-22] compared to commercially used additives. However, simultaneous incorporation of hybrid organic–inorganic nanosized modifiers with commercial additives, such as fillers or nucleating agents, allows extending their application range as the synergistic influence of even partially reciprocally reactive functionalities often results in increased modification ability [23-28]. This effect, in turn, yields the reduction of an additive amount sufficient to modify polymer properties. While the possibilities of modifying functional groups of silsesquioxanes extend the application potential of this group of materials, the high price excludes their commercial use. Based on the research results, a new direction of work was undertaken, assuming the maximum use of the resulting potential from the properties of silsesquioxanes by the aforementioned synergistic effects of interaction with other modifiers, which also allows limiting their quantitative share in the polymeric matrix.
In the literature to date, two approaches related to the use of POSS for the development of nucleating systems have been described. The first one is based on the functionalization of silsesquioxanes using compounds with confirmed polypropylene nucleation activity, both systems promoting crystallization of iPP in α and β phases [23, 29, 30]. The second concept, much often applied, is compounding non-reactive or reactive mixtures of POSS with nucleating agents. Roy et al. [24, 25] proposed the use of reactive compositions in the form of mixtures of POSS containing hydroxyl groups with the di(benzylidene)sorbitol (DBS) in order to obtain a comprehensive interaction with iPP. The reactive DBS-POSS system allowed higher degrees of fibre orientation and increased the mechanical strength of material samples formed in the injection molding process. The use of a non-reactive nucleating system consisting of the physical mixture of octamethyl silsesquioxane simultaneously with sodium 2,2′-methylene-bis(4,6-di-tertbutylphenyl)phosphate (NA40) was incorporated into iPP by Zhang et al. [31]. The investigations showed the beneficial influence of well-distributed POSS on the surface of NA40 and the nucleating efficiency of the composition. Chen and co-workers [26] compared the effectiveness of chemical combinations of various POSS with DMDBS. The studies revealed a different interaction with synthesised (3-mercapto)-propyl-heptaisobutyl silsesquioxane with DMDBS and physical mixtures of trisilanolisobutyl-POSS and octaisobutyl-POSS. Physical mixtures showed different nucleating efficiency on iPP, but the effects were insufficient to consider them as notable DMDBS activity. However, the reactively coupled POSS-DMDBS system was characterised by a complete limitation of the sorbitol-based nucleant interaction on iPP. The presented rheological tests confirmed a lack of an increase in complex viscosity during cooling, characteristic of sorbitol derivatives, associated with the release of submicron-sized structures of polymer-rich liquid. Summing up, research on the coupled effects of silsesquioxanes and nucleants remains the current subject of scientific consideration.
In our former studies, an effect of silsesquioxane incorporation into isotactic polypropylene nucleated with the sorbitol-based nucleating agent was described [32, 33]. It was assumed that simultaneous application of double-decker silsesquioxane, mainly tetrasilanolphenyl silsesquioxane (phPOSS) and 1,3:2,4-bis(3,4-dimethyl benzylidene)sorbitol (DMDBS) into isotactic polypropylene would enable controlling the crystallization of the polymeric matrix. An addition of a nucleating agent itself anticipates polypropylene crystallization due to the formation of a so-called fibrillar network upon cooling, which hosts a large number of nucleation sites for the growth of iPP spherulites [34, 35]. However, upon the addition of phPOSS, the creation of the DMDBS network is suppressed, and polymer crystallization shifts towards lower temperatures. This phenomenon allows suspecting that polypropylene may be stretched more during extrusion, i.e. higher draw ratios and longer stretching times may be applied during the production of films and fibres. As an effect, the production of iPP products with reduced haze and enhanced mechanical properties may be possible.
The research presented herein is an attempt to simultaneously use tetrasilanolphenyl silsesquioxane (phPOSS) and sorbitol-based nucleating agent as a reactive modifying system for polypropylene films incorporated to improve their transparency, extend processing range, and achieve sufficient mechanical properties. Previously, the high modification efficiency of silsesquioxanes was attributed to their high content in the polymeric matrix. The presented case study proves that incorporating silsesquioxanes, as a part of the reactive modifying system, extends their application possibilities. This study evaluates the mechanical and optical properties of iPP cast films modified with phPOSS and DMDBS as a function of processing conditions and modifier concentrations.
Experimental
Materials
The following materials were used in the investigations: isotactic polypropylene Moplen HP500N (iPP) produced by Basell Orlen Polyolefins, Poland, with the melt flow rate of MFR = 10 g/10 min (2.16; 230°C); 1,3:2,4-Bis(3,4-dimethylbenzylidene) sorbitol, Millad 3988 (DMDBS), which is the third generation of sorbitol derivative, offered by Milliken Chemical Company, USA and tetrasilanolphenyl-POSS (C44H44O14Si8), abbreviated phPOSS, which acts as a nucleating agent modifier, synthesised by Department of Organometallic Chemistry UAM, Poznań, Poland. The chemical formulas of both additives are depicted in Figure 1.
Chemical structures of (a) phPOSS and (b) DMDBS.
Sample preparation
Isotactic polypropylene (iPP) was mixed with 0.25 wt-% DMDBS (iPP/DMDBS), in the case of a reference sample, and additionally with 0.25 or 0.5 wt-% of phPOSS (iPP/DMDBS/phPOSS), in case of the modified samples, in the molten state according to the procedure described in the literature [28]. Cast film samples were formed in a single screw extruder Fairex with a screw diameter d = 30 mm, screw length and diameter ratio l/d = 25, equipped with a flat slit die and chill roll device with a drum diameter of 0.265 m (Proma, Poland), as detailed in work [36]. The technological process was realised under die temperature of 230°C, constant screw rotating speed of 25 rpm, drum temperature of 80°C, and chill roll rotational speeds of 5, 10, and 15 rpm. The draw-down ratio (λ) values were determined from the velocity of film take-up and the velocity of the polymer melt exiting the die and calculated as 4.8, 9.7 and 14.5, respectively.
Methods
The morphology of samples was investigated by polarised optical microscopy (POM) using a Nikon model Eclipse E400 polarised optical microscope equipped with the Linkam model THMS 600 heating/cooling stage. A small piece of each film was sandwiched between two cover glasses. The samples with a thickness of less than 10 μm were inserted in the hot stage. The following temperature program was applied: heating to 220°C, melting at 220°C for 5 min, then cooling down to 156°C with the highest possible cooling rate and finalised by isothermal crystallization at 156°C until impingement of iPP spherulites. All photographs presented herein were taken with a cross-polariser.
Thermal properties of polypropylene were evaluated in non-isothermal conditions by differential scanning calorimetry (DSC) using Netzsch DSC 204 F1 apparatus. Samples of 2.0 ± 0.05 mg were placed in aluminium crucibles with pierced lids and subjected to measurements in an inert nitrogen atmosphere. The samples were heated from 20°C to 230°C at a rate of 10°C min−1, held in this temperature for 5 min to erase the thermal history, and cooled back to the initial temperature at a rate of 10°C min−1. After a 5 min isotherm at 20°C, the materials were heated again to 230°C at a rate of 10°C min−1. The crystallinity degree (XC) was calculated according to the following equation:
Fourier transform infrared spectra (FT-IR) of the pure and modified iPP cast films were obtained using a Thermo Scientific Nicolet 5700 spectrometer equipped with a polariser in order to examine the orientation of the cast film. The spectra were recorded wavelength (ν) from 400 to 4000 cm−1 with a resolution of 2 cm−1. Measurements were conducted at an ambient temperature (23°C). To evaluate the extent of uniaxial orientation in the extended iPP films, the dichroic ratio (D) was calculated using the following equation [38]:
is the parallel-polarised infrared absorbance intensity and
is the perpendicular-polarised infrared absorbance for a specified vibration at a defined wavelength of the measured FT-IR spectra. Based on dichroic FT-IR spectra obtained for iPP films, Hermans orientation factors were calculated for crystalline (fc) and amorphous (fa) phases. The calculations are based on a determination of dichroic ratio, as in Equation (2), by the following equation:
A static tensile test was conducted to determine the influence of iPP modification on the mechanical behaviour of the composites. The following parameters were determined: elastic modulus and yield strength. The tests were performed per ISO 527 with Zwick/Roell Z010 tensile tester, at room temperature with a 100 mm min−1 crosshead speed.
The puncture resistance of iPP films was determined using Zwick/Roell Z2.5 testing machine according to ASTM F1306 standard. The films were cut into 12 mm wide strips for the puncture resistance test. The testing speed of the needle was 10 mm min−1. At least four replicated samples for each series were tested, and the parameters with standard deviations were calculated with the TestExpert software.
Friction coefficient (FC) evaluation was performed according to ASTM D 1894-87 standard with an adequately equipped Zwick/Roell Z2.5 universal testing machine. The sliding speed was 10 mm min−1. Static (µS) and dynamic (µD) friction coefficient values were determined according to the following equations:
The UV-Vis spectra analysis (transmittance and absorbance in the region 190–800 nm) of the samples was conducted using UV-Vis Thermo Scientific Evolution 220 spectrophotometer.
The optical behaviour of the cast film samples was analysed by a Murakami Color Research Lab HM-150 hazemeter as per PN-84/C-89100 standard.
Gloss was evaluated using Anticorr TestAN DT-268 glossmeter with 60° test angle; minimum of 40 measurements were taken for each series.
Results and discussion
Polarised optical microscopy measurements were performed to gain information on the morphology and structure of iPP composites containing the DMDBS nucleating agent and tetrasilanolphenyl-POSS. The results are presented in Figure 2. Figure 2(a) represents pure polypropylene sample crystallized isothermally at 156°C and reveals quite large and uniform iPP spherulites as expected. The morphology of the iPP/DMDBS sample, presented in Figure 2(b), differs significantly from pure iPP as much smaller spherulite size is observed due to precipitated sorbitol derivative, which serves as a substrate for the growth of iPP spherulites, whose number is enhanced and size reduced. An addition of phPOSS into the composition results in a nonuniform crystal structure since regions of spherulites with larger dimensions, not visible in the case of nucleated polypropylene, occur. This is due to the suppression of sorbitol-based additive interactions during its gelling process upon the addition of a silsesquioxane modifier [28], as described earlier. In the effect of π–π interactions forming and hydrogen bonds between dispersed in polymeric matrix phPOSS and sorbitol derivative, the so-called DMDBS fibrillar network building in a polymeric melt during cooling is characterised with less density, which results in the lowered nucleation sites for polypropylene spherulitic growth [34, 35].
POM micrographs for iPP (a), iPP/DMDBS (b) and iPP/DMDBS/0.5phPOSS (c) crystallized in controlled cooling conditions.
The structure and thermal properties of polypropylene films of various modifications were assessed via the DSC method. The melting and cooling curves for the first and second heating cycles are shown in Figure 3. At the same time, Table 1 summarises the thermal properties, taking into account crystallization temperature (Tc), melting temperature during the first and second heating cycles (Tm 1 and Tm 2), and the crystallinity degree (Xc 1) calculated according to Equation (1). Typically, an increase in melting point is observed during the first heating cycle for highly stretched polypropylene products, including fibres and tapes. It is related to the occurrence of an oriented structure of the polymer resulting from solidification with the simultaneous impact of significant tensile forces. Referring to the research by Kościuszko et al. [39], in the presence of the oriented phase for the unmodified polymer, the melting temperature may be higher than the isotropic phase of unmodified polypropylene, even by about 15°C. In the case of unmodified iPP, a change in the course of the melting curve was noted, comparing the first and second heating. Hence, the analysis of the two DSC heating runs was relevant for the qualitative assessment of the draw effect on the structure of the polypropylene films. The observed increase in the melting temperature (1st heating) and the sharpening of the course of the DSC melting curve is related to the increased amount of the crystalline phase in the form of fibrils compared to lamellae. The results obtained for the iPP series are in agreement with those reported by Tabatabari and co-workers [40]. The iPP series produced with higher draw-down ratios (λ = 9.7 and 14.5) reveal that the contribution of fibrils to the crystals is more important than in iPP made with λ = 4.8. Simultaneously, there are only series showing the negative value of ΔT, which is consistent with the dominant effects induced by stretching processes discussed above.
DSC heating and cooling curves of pure and modified iPP films manufactured with different draw down ratios. Thermal properties of pure and modified iPP films obtained by DSC. ΔTm = Tm 2 − Tm 1.
The DMDBS-modified compositions showed the opposite tendency; the curves obtained from the first heating showed a melting point at a lower temperature than the second heating. This is due to the presence of a highly effective DMDBS. Therefore, the criterion of qualitative evaluation of the degree of orientation based on the ΔTm parameter is not a proper evaluation method for systems containing highly efficient nucleant. However, it should be noted that the Tm1 values are higher for iPP containing DMDBS, and gradually, albeit to a small extent, increase with the addition of phPOSS. This can be interpreted as a simultaneous influence of nucleant and macromolecular orientation caused by a partial limitation of the initial increase in viscosity caused by the formation of precipitates of highly saturated DMDBS domains in the polymer bulk [25, 28].
The addition of DMDBS to iPP resulted in a predicted increase in the crystallization temperature relative to the iPP, consistent with the literature data [41, 42]. The additional introduction of the hybrid nucleating system containing phPOSS did not significantly change the crystallization temperature of the nucleated composition. Therefore, it can be stated that despite larger spherulite size in the case of iPP/DMDBS/0.5phPOSS obtained in the isothermal crystallization process, as well as confirmed by other changes in crystallization behaviour [28, 32], the presence of phPOSS in case of rapid cooling has a low impact on final Tc of nucleated iPP.
It should also be underlined that different solidification conditions for cast films made of iPP did not result only from the introduced tensile stresses and modifications but also from changes in the heat transfer conditions during forming. According to the observations of Aniunoh and Harrison [43], the iPP films shaped with a high draw down ratio had a lower thickness, which resulted in a higher cooling rate. This effect is due to the dominant share of conduction in heat transfer during the film solidification process.
The second method used to assess the influence of the DMDBS addition and the DMDBS–phPOSS hybrid nucleating system on the formation of oriented structures during cast film manufacturing was the polarised light Fourier transform infrared spectroscopy. Referring to the methodology presented in studies [44, 45], the FT-IR spectra of the investigated films longitudinally and perpendicularly to the extrusion direction in the wavelength range of 700–1400 cm−1 are presented in Figure 4. The presented considerations evaluated absorbance intensities at four wavenumbers: 809, 841, 973 and 988 cm−1. The wavenumber of 809 cm−1 was associated with the α-crystalline presence, while 841 cm−1 contained information about both α-crystalline and mesophase microstructure changes. The evaluation of the amorphous phase was based on absorbance changes at 973 cm−1. Variations in the absorbance at 988 cm−1 allowed determining the influence of film stretching on iPP α-phase. Characteristic infrared absorbance peak at 809 cm−1 responded to C–C stretching and coupled C–H deformation, while 841, 973 and 988 cm−1 were connected with the following vibration modes: C–C stretching, CH2 rocking, CH3 rocking. The most significant differences between the spectra obtained for the foil in the field of α-crystal orientation increase were noted for the iPP/DMDBS/0.25phPOSS series. This is in line with the assumptions about a partial limitation of the impact of DMDBS on iPP while increasing its ability to form oriented structures during the cast extrusion process.
FT-IR spectra of film samples made with a draw-down ratio of 14.5 measured in two orientations.
The dichroic ratio of different bands of the iPP cast film produced at different draw ratios.
Note: |ΔD| = |D 15rpm − D 5rpm|
The Herman orientation factors for selected vibrations calculated for crystalline (fc) and amorphous (fa) phases are presented in Figure 5 as a function of draw down ratio. The lowest values of Herman orientation factors were achieved for pure iPP. For all the series in question, predictably, along with an increase in λ, both orientation parameters fa and fc gradually increased. For all materials, higher orientation values were recorded for the crystalline phase. The application of the nucleant caused the difference between fa and fc to widen. Interestingly, the introduction of phPOSS at both concentrations did not significantly change the Herman orientation factors. However, based on the obtained results, it can be concluded that despite the introduction of the nucleant, the nature of the course of the orientation effects did not change, with the crystalline phase orientation predominating.
Crystalline (fc) and amorphous (fa) orientation function vs. draw drown ratio.
To assess the influence of modifiers on the mechanical properties of polypropylene matrix, a static tensile test was performed. Results in a function of polypropylene film draw down ratios are summarised in Figure 6. A qualitative change in the stress–strain behaviour was noted for all the compositions since the samples containing DMDBS only or coupled with phPOSS do not undergo yielding. The composition dependence of elastic modulus reveals an increase in E for the nucleated sample compared to pure iPP, from 810 to 1030 MPa, and further increase to a value of 1270 MPa for the sample containing the highest amount of phPOSS. In addition, an increase in draw down ratio values, from 4.8 to 14.5, causes further improvement in elastic modulus independently of the sample composition. A similar tendency is observed for the tensile strength parameter of all tested samples, i.e. an increase in a value from 42 MPa for pure iPP up to 58 MPa, for iPP/DMDBS, and 57 MPa, for iPP/DMDBS/phPOSS, followed by its further increase with chill-roll rotational speed. Other authors have also reported the decreased ductility of polypropylene nucleated with sorbitol derivatives [47-49]. Crack propagation in nucleated samples is promoted by an increased total length of spherulite boundaries due to their smaller size [50]. Our former studies showed that there was a further improvement in brittleness upon the addition of phPOSS caused by POSS aggregates behaving as voids in the structure that facilitated premature fracture upon deformation [28]. Figure 6(c,d) additionally presents the results of the penetration measurements. The highest force values were observed for the samples made with the lowest draw ratio (Figure 6(c)), when compared to values normalised to their thickness (Figure 6(d)), it can be seen that the best results occurred for the samples produced with the highest draw ratio, made with iPP/DMDBS and iPP/DMDBS/0.25phPOSS.
Mechanical properties of pure and modified polypropylene cast films.
Figure 7 compares the gloss values measured at an angle of 60° for pure and modified iPP cast films produced with different draw ratios. The highest gloss was noted for the samples of unmodified polypropylene. The addition of nucleant caused a slight reduction in gloss with small degrees of stretching and a significant deterioration of this quality parameter in the case of λ=14.5. Interestingly, the introduction of phPOSS resulted in a slight reduction of gloss with the increasing proportion of the nano-modifier, but for the highest draw-down ratios, the values were comparable to the iPP/DMDBS series. If the limited compatibility of phenyl substituted POSS with polypropylene is considered [51], the presence of the agglomerated domains resulted in the formation of clusters deteriorating the quality of the film surface. It should be emphasised that from the application point of view where the exposition of food products comes into play, too high gloss value is not recommended as the excessive reflection of artificial light may deteriorate product presentation.
Gloss of polypropylene films.
UV-Vis spectroscopy allows for the quantitative assessment of the optical properties of polymer films, enabling the determination of selected ways of product applications, particularly in the case of obtaining the barrier effects in selected wavelength ranges [52]. Figure 8 shows UV-Vis spectra made for pure and modified iPP films manufactured with different draw down ratios. Additionally, Figure 6(d) shows the transmittance value at 750 nm, used for the film transparency comparison, and the haze value determined in a separate test for all materials. In the case of all draw ratios, the iPP/DMDBS series showed the highest transmittance value in the entire wavelength range, while the 0.5 wt-% phPOSS modified samples were by far the highest wavelength absorption. The exception is the series made at the 14.5 draw down ratio where all the materials showed similar spectra. Decreased haze values for the nucleated sample indicate better transparency compared to pure polypropylene – the addition of phPOSS causes deterioration of matrix transparency. As proved in the literature, spherulite size is the parameter that influences the optical properties of polymeric materials the most, wherein the smaller crystal size, the more transparent film [53]. As evidenced in microscopic micrographs presented in Figure 1, the addition of DMDBS into formulation causes a significant decrease in spherulite size; hence the haze values for the sample are the lowest. An addition of phPOSS causes suppression of sorbitol derivative network formation, resulting in the formation of spherulites with higher dimensions reflected in worsened optical properties. Additionally, filler agglomerates visible in the structure may interfere with visible light and reduce transparency. However, formulations containing both modifiers reveal enhanced transparency compared to pure polypropylene matrix. According to Wilchinsky [54], a considerable loss of film clarity may occur due to refraction from surface irregularities. Therefore, UV-Vis spectroscopy results may be correlated with gloss evaluation. The highest amounts of the irregularities were observed on the iPP/DMDBS/0.5phPOSS cast film surface. While the visible inclusions in the polymeric film were noted in all modified series, the improved clarity of the nucleated iPP balanced the measured spectroscopic characteristics in the case of iPP/DMBS and iPP/DMDBS/0.25phPOSS.
UV-Vis spectra of iPP-based films (a–c), and transmittance at 750 nm reflecting transparency of the films (d).
Considering various operations performed on films, especially with automatic packaging machines, the friction coefficient becomes an important utility parameter [55, 56]. Figure 9 presents the static (µS) and dynamic (µD) friction coefficient values. Taking into account the changes in the static friction coefficient, it can be assumed that its higher values will be more favourable from the application point of view. This is because greater force is needed to set the products packed in motion, which translates into a more stable position of the packaged products [57]. The highest values of the friction coefficient were noted for the iPP/DMDBS and iPP/DMDBS/0.25phPOSS compositions; in the case of films manufactured with the highest draw down ratio, made of iPP modified with a lower content of phPOSS, a clear difference was observed concerning the other series. In the series containing 0.25 and 0.5 wt-% phPOSS, a different trend in µS was noted for the highest draw down ratio (λ = 14.5). Considering the comparable gloss, which consists of the roughness of both samples, the changes may be related to different thickness of the samples or different surface physical characteristics. The lowest µS values were recorded for the composition containing 0.5% phPOSS at the highest draw down ratio. In the case of the dynamic coefficient of friction, a specific trend can be observed. Samples of unmodified iPP showed the lowest value of this parameter. While in the case of the minor draw down ratio, only the iPP/DMDBS samples showed significantly higher µD parameter, in the case of higher draw down ratios, the differences between the samples modified with phPOSS were comparable. For λ = 14.5, the composites modified with POSS filler had the highest values µD. In the case of the dynamic coefficient of friction, it is advantageous to obtain low values to limit the potential stick–slip phenomena, increase speed in the manufacturing processes, and enhance the packing machine operations [58]. Summing up, the phPOSS addition changed the physical properties of the film, which translated into different friction coefficient values. The iPP/DMDBS/0.25phPOSS series showed favourable performance characteristics in final applications; unfortunately, the incorporation of POSS modifiers may limit the high-performance production of films with higher draw ratios.
The friction coefficient of polypropylene films.
Conclusions
This study details how a combination of the sorbitol derivative-based nucleating agent and tetrasilanolphenyl-POSS influences the properties of isotactic polypropylene. The research showed that the incorporation of complex nucleating system into polypropylene films results in improved transparency, extended processing range, and good mechanical properties. This behaviour can be explained by the fact that the presence of phPOSS disrupts the formation of the sorbitol microfibrillar network and consequently – the increase of the polymeric melt viscosity during cooling, but also provides nucleating sites for the formation of polypropylene spherulites. The presented case study proved that the use of silsesquioxanes as a part of the reactive modifying system extends their application possibilities. The proposed modification results in the stiffening of the samples as revealed by an increased elastic modulus upon the addition of DMDBS only or the nucleating agent coupled with phPOSS.
Moreover, transparency is improved upon the addition of DMDBS and only slightly worsened by incorporating 0.5% phPOSS. However, the composition containing phPOSS still reveals improved optical properties compared to pure polypropylene matrix. Therefore, an advantageous nucleating system that can be applied to produce packaging materials with enhanced mechanical and optical properties was developed.
Footnotes
Acknowledgements
The authors are grateful to Milliken & Company for supplying the nucleating agent samples for research.
Disclosure statement
No potential conflict of interest was reported by the author(s).