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Optimization of melt blending process of nylon 6-POSS: improving mechanical properties of spun fibers.

INTRODUCTION

Polyhedral oligomeric silsesquioxanes (POSS) are a hybrid materials (organic-inorganic) which have been heavily studied in recent years [1-11]. POSS contains a basic polyhedral silicon-oxygen nanostructured skeleton or cage, with versatile chemistry, which makes possible to attach different functional organic groups (-R) to the corner silicon atoms to enhance interactions with a matrix material. The POSS molecule can thus be synthetically modified to contain functional groups which allow copolymerization, light sensitization, and improved solubility in organic solvents, distinguishing it from other nanofillers. The potential advantages gained from using POSS come from its hybrid organic-inorganic nature whose inorganic core potentially provides molecular reinforcement, while its variety of functionalization schemes allows reaction or compatibilization with the host polymer. The incorporation of POSS cages into polymeric materials may potentially result in dramatic improvements in polymer properties, including increase in upper use temperature, oxidative resistance, and surface hardening, leading to improved mechanical properties as well as a reduction in flammability [12].

While improvements due to the incorporation of POSS by copolymerization or through grafting is well-known [13, 14] polymer-POSS systems using nonchemically bonded POSS have not been studied consistently. Polymer systems containing nonchemically bonded POSS are usually prepared by melt blending, such as with a twin-screw extruder [15, 16]. Successful nanocomposite melt compounding requires very careful design of the processing conditions to obtain a proper dispersion and distribution of nanoparticles [8, 17, 18]. Processing parameters such as temperature, time, and shear forces need to be optimized, as well as the chemical attributes, such as is the case with the selection of compatibilization agents and functionalization of nanoparticles [8]. The chemical-physical interaction between POSS and the matrix is also an important element in the preparation of polymer-POSS melt blends, with POSS having a precisely controlled chemical structure with organic groups covalently attached to the Si-0 framework, thus allowing tunable miscibility into organic polymers [19]. Aggregation of POSS due to self-interactions between geometrically regular POSS cages often results in inferior properties of the blend [20-22].

The reinforcement of melt-spun fibers with POSS additive was recently reported [23, 24]. Prior work showed that there exists an optimal loading in terms of POSS solubility in nylon 6 at 2.5 to 3.0 wt%, where a significant increase in the Young's modulus and yield stress occur in highly elongated melt-spun fibers [23]. At low POSS concentrations, small, elongated aggregate domains were formed in the axial direction, although individually dispersed POSS molecules were also present. As the concentration of POSS was increased past this window, larger spherical aggregates begin to form resulting in POSS acting primarily as a processing aid and a decrease in the blend's mechanical properties. A multitude of different suggestions have been presented to explain the reinforcement mechanism of POSS particles within the nylon 6 matrix. One suggestion is that the solubility of the POSS in the matrix increases due to the shear associated with fiber melt-spinning. With this increased solubility, the POSS particles are able to act as anchors to the amorphous regions of the nylon 6, resulting in a more oriented amorphous phase [23].

Our previous publication established a mechanism for optimal reinforcement at approximately 2.5% POSS loading [25]. Above the POSS melting temperature of 267[degrees]C, POSS becomes partially soluble in the nylon 6 matrix, leading to smaller domains upon posterior cooling. The data suggest a maximum acceptable size of approximately 4 pm in diameter for agglomerates before a decrease in mechanical properties can be seen. The present work builds on the previously proposed mechanism by preparing nylon 6 blends with 2.5 wt% POSS using different melt-blend processing conditions. The aim of the present work is to improve and optimize the mechanical properties of spun fibers. Blend morphology will be correlated with rheological and tensile properties to determine the mechanical reinforcement derived from POSS addition. The effects of temperature and screw speed on the melt-mixing process are correlated with the final morphology and tensile properties of the spun fibers.

[FIGURE 1 OMITTED]

EXPERIMENTAL

Materials

Polyamide 6 (nylon 6) resin (Grilon[R] FG40-NL) was obtained in the form of pellets from EMS-Grivory with MVR 20 g/10 min (ISO 1133), density 1.14 g/[cm.sup.3] (ISO 1183). Aminopropyl isobutyl-POSS (POSS, AM0265) was acquired from Hybrid Plastics Inc. in the form of a white powder and was used as received. Figure 1A and B presents the chemical structures of nylon 6 and POSS, respectively.

Preparation of POSS Blends

All materials were dried in a vacuum oven at 60[degrees]C for 24 h before blending. POSS-polymer compositions were weighed and dry-mixed before blending. The mixing consisted of nylon 6 blended with 2.5 wt% POSS. Mixtures were melt-bended using a Thermo Scientific EuroLab XL 16 mm co-rotating twin-screw extruder, and L/D ration 40:1. Different processing conditions, namely screw speed and temperature, were tested, as described in the Table 1. The extrudate was collected and pelletized in preparation for further analysis.

Preparation of Nylon6/POSS Fibers

Nylon 6/POSS blend fibers were produced using the same conditions of previous work [23]. A Malvern Instruments Rosand RH-7 advanced capillary rheometer fitted with a rotating drum was used to spin and collect the fibers. The draw down ratio (DDR) of the process was controlled by varying the plunger speed, die diameter, and take-up velocity (drum RPM). In this study, the DDR was held constant at 950 using a die diameter of 2 mm, a plunger speed of 0.5 mm/min, and a drum speed of 100 rpm, all the same conditions used in our previous work [23].

Tensile Properties

Molded specimens with 0.5 mm thickness were molded at 240[degrees]C for 5 min using a Carver 25 Ton Hydraulic Units compression molder. Once specimens were obtained, ASTM T23055 dog bones tensile bars were die cut and tested on an Instron 5565, equipped with a 1 KN load cell and a strain rate of 10 mm/min. For each blend, 10 samples were tested. For the fiber samples tensile testing was carried out on an Instron Model 5965 using a 50 N load cell and a strain rate of 1000%/min. Before the experiments, all samples were vacuum dried for 12 h at 80[degrees]C.

Image Analysis

The morphology was evaluated through scanning electron microscopy (SEM). The composites were compression molded, as described above, then cryofractured. The fibers were embedded in epoxy and cryo-ultramicrotomed at -90[degrees]C using a Leica EM UC6 ultramicrotome to produce a surface revealing a cross-section of the fibers. The upper surface of the prepared samples were sputter-coated with gold and analyzed in a JEOL JSM-6510 microscope operated at an acceleration voltage of 10 kV.

Thermal Analysis

The thermal behavior of the POSS-polymer blends was investigated by differential scanning calorimetry (DSC) using a TA Instruments Q100 DSC under continuous nitrogen purge, at a flow rate of 50 mL/min. The thermal properties such as melting temperature ([T.sub.m]), glass transition temperature ([T.sub.g]), and enthalpy change ([DELTA][H.sub.m]) in melting were determined at a heating rate of 10[degrees]C/min from 0 to 290[degrees] C.

Rheological Analysis

A MARS III rheometer (Thermo Fisher Scientific), operated with a 25-mm parallel plate setup was used to measure the dynamic rheological properties of the nylon 6 and nylon 6/ POSS blends. Disk-shaped specimens of 25 mm diameter and 1 mm thickness were molded at 240[degrees]C in a compression molder. The specimens were subject to 240[degrees]C for 5 min during molding. The polymer compound in the rheometer was first heated and kept at the desired temperature for 2 min to reach equilibrium. All rheological experiments were performed under a nitrogen atmosphere. Before the experiments, all samples were vacuum dried for 12 h at 80[degrees]C.

Frequency sweeps at constant temperature 265[degrees]C with oscillatory shear frequency between 1 and 100 rad/s were carried out. Temperature sweep experiments were also conducted, from 225 to 285[degrees]C. These experiments were performed at constant frequency at 1 Hz. Storage modulus (G'), loss modulus (G"), and damping factor (tan [delta]) were measured in all rheological experiments described above. All experiments were performed in the linear viscoelastic regime, a fact confirmed independently by regular stress sweeps.

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

Extruded Blends

Nylon 6 was melt blended with 2.5 wt% POSS under different processing conditions, and these samples were characterized as a function of the processing parameters. SEM images cross-sections of the polymer blends show POSS forms spherical, phase-separated aggregates (Fig. 2). It is notable here and in Fig. 3 that all the samples show a considerable decrease in particle size when compared with previous results [25], which it was approximately 4 pm in diameter, indicating a better dispersion in the matrix. All of the particles are at or below 16 [micro][m.sup.2] in area, after they were melt-blended around or above the POSS melting point, which is in agreement with our recent hypothesis. However, at this point no major conclusions are possible to make regarding the processing conditions effect, even though there are some differences in POSS domain size dispersion.

[FIGURE 3 OMITTED]

Thermal analysis of the nylon 6/POSS samples shows no real evidence of interactions between the POSS and the polymer matrix, since there is little change in the nylon 6 melting behavior (Fig. 4). From our previous work [23], it is known that this grade of POSS contains two distinct endothermic transitions upon heating, one at 50[degrees]C and other at approximately 270[degrees]C. It is also known that the POSS remains a solid after the first transition and becomes a liquid after the second transition. It was shown for POSS concentrations above 1% in the polymer, a residual peak that increases with the POSS concentration around 267[degrees]C can be observed [25]; this peak appears to be the second endothermic transition of POSS, but is not observed in the present work, suggesting that POSS molecules are dispersed and in an amorphous state.

[FIGURE 4 OMITTED]

Rheological experiments were conducted in order to analyze the blends' viscoelastic properties at different temperatures. The stability of the nylon 6 at different temperatures was determined previously and that information was used to set the time interval for the frequency sweeps [25]. In keeping with these findings, all the rheological temperature sweeps were performed by heating the samples from 225 to 285[degrees]C, i.e., above the melting point of both the nylon 6 and POSS, at 10[degrees]C/min. Frequency sweeps were performed at temperatures no higher than 265[degrees]C and over no more than 6 min, in order to avoid nylon 6 degradation.

The variation of G', G" and tan [delta] as a function of frequency at 265[degrees]C, are shown in Fig. 5 for nylon 6 and nylon 6/POSS. Only a very small, if any, influence of processing conditions on blends can be detected, consistent with the literature [25], where at this concentration only very small variations were observed from the nylon 6. It is nonetheless important to show that the same rheological behavior is kept for all samples.

The rheological data collected during the temperature sweeps are shown in Fig. 6, demonstrating that there is a small decrease in the modulus for some of the samples, due to the good dispersion of the POSS [25], indicating that there is only a small variation in POSS dispersions across the samples. It is important to note how sensitive tan [delta] is to the onset of the POSS melting point (second endothermic transition) around 267[degrees]C, which it was not able to be detected by the DSC measurements during the present work. At this temperature, it is possible to observe a decrease in the slope of the tan [delta] curve for nylon 6/POSS blends for all the samples, suggesting that the melting of the POSS is changing the physical structure of the blend and, consequently, its viscoelasticity. The tensile properties of nylon 6 and nylon 6/POSS blends are shown in Fig. 7; the data suggests that the state of dispersion is good enough in all samples that they do not show significant differences in tensile properties. This is not surprising, since the processing conditions (defined based on the knowledge gained from our previous works [23, 25]) were set from the beginning to achieve good POSS dispersion upon extrusion. The mechanical properties only confirm that was indeed achieved.

Melt Spun Blends

SEM imaging of the melt spun fiber cross sections show differences between the samples (Fig. 8). Samples A and B formed smaller agglomerates since they were melt-blended at higher temperatures (285[degrees]C) than samples C and D, as is shown in Fig. 9 as well. Interestingly, all samples show an increase in the maximum domain size, which is an indication of poor POSS/matrix bonding that leads to at least some coalescence, at least to some degree, upon remelting before fiberspinning. Samples B and C, in particular, are very sensitive to this effect. For example, Sample C now exhibits domains of up to 140 [micro][m.sup.2] in area. Samples C and D formed agglomerates, which are larger in sample C due the lower shear (screw speed) used in the melt-blend process. Again, both samples exhibited poor adhesion between the POSS and the nylon 6 matrix, as shown by the formation of gaps/voids between the POSS domains and the matrix. Clearly, the sample that shows less debonding and POSS coalescence is Sample A, i.e., the one subject to lower stresses (higher temperature and lower screw speeds) upon extrusion. In it, it is possible to see that there are no voids between the POSS domains and the matrix and that it is the POSS domains that have started to crack instead of debonding upon fiber-spinning. The question then is whether these differences are reflected in the final mechanical properties.

[FIGURE 5 OMITTED]

The results obtained for the tensile properties are shown in Fig. 10. First, it can be observed that within the elastic limit of the tensile test, the modulus and the yield stress have an improvement about 75% when compared with the neat nylon 6. As per the conclusions of our previous work [25] this is likely caused by the fact that most of the small quantities of agglomerates in all samples have a diameter of around 4 pm (despite the fact that there are some larger ones, as was discussed before). By optimizing the melt-blending process conditions, tensile moduli 20% higher than those achieved by Milliman et al. were obtained [23]. The effectiveness of stress transfer between the matrix and the POSS can be evaluated by analyzing the tensile strength results. Sample A is the system that shows the most significant improvement in tensile strength, which is consistent with the fact that it is the one with smaller overall POSS domains and lower POSS/matrix debonding. It is known that the composite strength increases with decreasing particle size due to a higher total surface area and a more efficient stress transfer mechanism [26]. From the SEM images (Fig. 8) and particle distribution (Fig. 9), and as was pointed out before, it is clear that sample A displays better POSS dispersion and higher adhesion between the matrix and POSS, with the absence of voids. A sine qua noncondition for good reinforcement upon fiberspinning is that the dispersed, nonagglomerated POSS, act as a processing aid. This new result suggests that, in addition to that, the screw speed should not be so high that the mechanical energy imparted on the system leads to debonding between the POSS that remains in aggregates and the nylon 6 matrix. In fact, even though high shear may break the aggregates more effectively (see Fig. 2C and D), it may also lead to debonding of the aggregates from the matrix, which is detrimental to the final properties. In conclusion, too much shear in the system upon extrusion is detrimental to the final morphology and mechanical properties.

[FIGURE 6 OMITTED]

CONCLUSIONS

It has been proposed that in PA-6/POSS blends: (1) above the POSS melting temperature of 267[degrees]C in blends with nylon 6, the POSS becomes partially soluble in the nylon 6 matrix, leading to smaller domains upon cooling, and (2) the remaining nondiffused POSS agglomerates must have an acceptable size distribution, with most aggregates being 16 pm2 or less in area, so as not to negatively impact the mechanical properties. The present work used these assumptions to optimize the mechanical properties of melt-spun PA-6/POSS fibers. By meltcompounding a series of nylon 6/POSS blends with different process conditions, but always at temperatures close to or above the POSS melt temperature, we aimed to optimize particle dispersion and maximize bonding between the particles and the matrix.

The results show extruded blends with POSS existing as phase-separated aggregates relatively smaller size than in previous works [23, 25], which is not surprising, since the processing conditions were set specifically to achieve small and well dispersed POSS aggregates. Thus, as expected, there were no significant differences in the morphology and mechanical properties between the extruded samples.

The results from the spun fibers, however, show significant improvements in tensile properties relative to previous reports, and also show significant differences between samples, namely in terms of the tensile strength. The sample processed under the milder conditions, i.e., higher temperature and lower screw speed, showed the lowest debonding between POSS particles and the matrix and also smaller average particle size, which explains the better mechanical properties. Thus, the main conclusions to be drawn from the present work is that in order to maximize the mechanical properties of spun fibers, the processing conditions upon compounding need to be set in such a way that: (a) The POSS is molten and well dispersed; (b) The thermomechanical environment in the extruder is relatively mild, so as not to cause debonding of the remaining POSS agglomerates under the very harsh melt-spinning conditions.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

ACKNOWLEDGMENTS

The authors also acknowledge Professor Sadhan Jana for his help with fiber preparation and discussion of the topic. POSS[R] is a registered trademark of Hybrid Plastics, Inc.

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Ricardo J. Andrade, Zachary N. Weinrich, Creusa I. Ferreira, David A. Schiraldi, Joao M. Maia

Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland Ohio 44106-7202

Correspondence to: Joao M. Maia; e-mail: joao.maia@case.edu

Contract grant sponsor: NSF Center for Layered Polymeric Systems; contract grant number; DMR 0423914; contract grant sponsor: Foundation for Science and Technology (FCT), Portugal; contract grant number: SFRH/BD/ 62152/2009.

DOI 10.1002/pen.24089
TABLE 1. Processing parameters for all the samples.

                           Max. temp
           Percent (wt)    in profile    Flow rate   Screw speed
             APOSS(%)     ([degrees]C)    (kg/h)        (RPM)

Sample A       2.5            285           3.4          250
Sample B       2.5            285           3.4          400
Sample C       2.5            265           3.4          250
Sample D       2.5            265           3.4          400
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Author:Andrade, Ricardo J.; Weinrich, Zachary N.; Ferreira, Creusa I.; Schiraldi, David A.; Maia, Joao M.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jul 1, 2015
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