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Crystallization kinetics of polyamide 66 at processing-relevant cooling conditions and high supercooling
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Thermochimica Acta xxx (2014) xxx–xxx
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Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Crystallization kinetics of polyamide 66 at processing-relevant cooling conditions and high supercooling Alicyn Marie Rhoades a, *, Jason Louis Williams a , René Androsch b a b
Pennsylvania State University, Behrend College, School of Engineering, 4701 College Drive, Erie, PA 16563, USA Martin-Luther-University Halle-Wittenberg, Center of Engineering Sciences, 06099 Halle/Saale, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 August 2014 Received in revised form 30 September 2014 Accepted 2 October 2014 Available online xxx
Processing of polyamide 66 (PA 66) by injection molding includes cooling of the melt at rates between 100 and 103 K/s and its solidification at high supercooling. The kinetics of crystallization at such conditions is unknown and has been evaluated in this work using fast scanning chip calorimetry. Slow cooling of the melt of PA 66 leads to formation of crystals, with the maximum crystallinity being about 30%. It has been found by analysis of the crystallinity as a function of the cooling rate that crystallization is suppressed on cooling faster than about 300 K/s. Isothermal analysis of the crystallization rate revealed a bimodal temperature dependence, with maxima obtained at about 165 and 110 C. It is suggested that the observation of two distinct crystallization-rate maxima is related to a change of the nucleation mechanism on temperature variation or the formation of different crystal polymorphs exhibiting different growth rate. The findings provide a fundamental step towards accurately predicting the solidification behavior, the skin–core morphology, and properties of injection moldings of PA 66. ã 2014 Published by Elsevier B.V.
Keywords: Polyamide 66 Crystallization kinetics Fast scanning chip calorimetry
1. Introduction Polyamide 66 (PA 66) is a versatile and highly utilized engineering thermoplastic homopolymer. The polymer exhibits high strength over a broad temperature range, has excellent impact and chemical resistance, and can withstand high wear. Besides its use for production of fibers, applications include automotive, aerospace, and industrial replacements for metal gears, bearings and engine components [1–4]. The material is typically injection molded and is particularly prone to shrink and warpage problems. The material is known to be very sensitive to changes in melt processing, presenting a challenge for engineers that design manufacturing processes for high-end part production[5]. Both the excellent physical properties and the challenging design drawbacks of PA 66 are due to the inherent semi-crystalline microstructure. During injection molding, this semi-crystalline microstructure forms under shear and thermal gradients, typically leading to the development of skin–core morphologies with implications on the property profile [6]. In addition, the molten polymer volume densifies to a greater extent in crystallizing regions than in areas which remain macroscopically amorphous, resulting in problematic variations in shrinkage across a part [7]. To
* Corresponding author. Tel.: +1 814 898 6287. E-mail address: [email protected] (A.M. Rhoades).
control and maximize the benefits resulting from a uniform microstructure it is critical to develop an understanding of the crystallization process, at conditions relevant in processing. The process of injection molding can subject PA 66 to a wide range of cooling rates. Conventional injection molding employs a steel mold held at a constant temperature, and a mold temperature in the range between 0 and 90 C is often used [8]. Oftentimes, lower mold temperatures will be used to “quench” the melt and to allow for more rapid production rates. Melt temperatures range from 280 to 305 C, and the material is also subject to injection pressures between 35 and 140 MPa [8]. After the mold cavity is filled, time to solidification is dictated by the heat transfer out of the mold and can vary greatly with mold and melt temperature, and the wall thickness. Cooling rates in injection-molding of PA 66 range from about 3 K/s in the center of a molded tensile test bar to more than 1200 K/s at the interface of the polymer/mold surfaces [9]. Despite the potential usefulness of such information, there are no published resources available that describe the crystallization kinetics of PA66 at processing-relevant high cooling rates, nor is there published information regarding the crystallization rate of PA 66 at temperatures experienced during the cooling processes of injection molding, extrusion, or thermoforming. Prior studies of the crystallization behavior of PA 66 were mainly performed by differential scanning calorimetry (DSC) which, however, only allows analysis of crystallization processes at cooling rates lower
http://dx.doi.org/10.1016/j.tca.2014.10.020 0040-6031/ ã 2014 Published by Elsevier B.V.
Please cite this article in press as: A.M. Rhoades, et al., Crystallization kinetics of polyamide 66 at processing-relevant cooling conditions and high supercooling, Thermochim. Acta (2014), http://dx.doi.org/10.1016/j.tca.2014.10.020
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than about few 100 K/min, or at temperatures at which the meltcrystal phase transition is longer than several ten seconds, [10,11] both being insufficient to derive conclusions regarding the materials behavior at conditions present in polymer processing. Therefore, we have utilized a fast scanning calorimetry technique [12,13] to study the crystallization process of PA 66 at rapid cooling and at low temperature. The primary chemical structure of the PA 66 macromolecule consists of amide groups separated by methylene sequences. PA 66 is a polymorphic polymer and forms different supermolecular structures depending on the condition of crystallization. Slow cooling of the quiescent melt or crystallization at rather high temperature leads to formation of triclinic a-crystals with sheetlike arrangement of hydrogen bonds at room temperature, connecting amide groups of neighbored chain segments [14]. The equilibrium melting temperature and bulk heat of fusion of the a-phase, as listed in the ATHAS data base, are 301 C (574 K) and 255.4 J/g, respectively [15–17]. The triclinic a-form converts on heating at the Brill transition temperature into the high-temperature a0 -form with a pseudo-hexagonal unit cell and a threedimensional network of hydrogen bonds [18,19]. The Brill transition is reversible, that is, the a0 -phase reverts on cooling into the a-phase. The formation of a/a0 -crystals on slow cooling/ low supercooling typically is connected with formation of lamellae and spherulites [20–22]. Regarding the kinetics of formation of a0 -crystals it is known that on cooling at rates lower than 20 K/min, as is typically applied in DSC analyses, crystallization occurs at 220–240 C [23–29]. Isothermal DSC experiments revealed that at a temperature of 230 C crystallization is finished after about 1– 2 min, with the crystallization time rapidly increasing with crystallization temperature [29–31]. Besides formation of the a-polymorph as a result of slow cooling or crystallization at high temperature, there has been reported the development of a pseudohexagonal mesophase with non-planar arrangement of hydrogen bonds upon quenching of the melt [32]. The mesophase is metastable at room temperature and converts irreversibly into a/a0 -crystals on heating [33]. Details of the morphology and superstructure of the mesophase of PA 66, and their exact conditions of formation are not known yet. In case of PA 6, which exhibits a similar cooling-rate/supercooling controlled crystal/ mesophase polymorphism as PA 66 [32–35], it has been proven that the mesophase domains are of non-lamellar habit and not spatially organized in a higher-order superstructure [36,37]. Summarizing the scope of the present study, we attempt to gain information about the crystallization kinetics of PA 66 at conditions being typical in injection molding, for example on cooling at rates between 1 and 1000 K/s, and at temperatures lower than 200 C, not yet reported. The observed data may then be used to predict the solidification behavior of injection moldings, their skin–core morphology and, with that, their properties. In a wider view, the study is also considered as a continuation of our research efforts to obtain information about a possible change of the mechanism of crystal nucleation on variation of the supercooling, as has been suggested for the cases of isotactic polypropylene (iPP) [38,39], polyamide 11 [40], or poly(e-caprolactone) [41].
2.2. Sample preparation The as-received, dry pellets were injection-molded to standard tensile test bars using a 100 t Sodick Plustech injection molding machine. The molded bars were then microtomed parallel to their thickness direction, to obtain sections with a thickness of 13 mm. These thin sections were then further prepared for subsequent analysis of the crystallization behavior using FSC by reduction of their lateral size to 50–100 mm, using a scalpel and a stereomicroscope. 2.3. Fast scanning calorimetry FSC analysis was performed using a power-compensation Mettler-Toledo Flash DSC 1, attached to a Huber intracooler TC100. Prior positioning of the specimen on the heatable area of the sample calorimeter, the UFS 1 FSC sensor was conditioned and temperature-corrected according to the specification of the instrument provider. The calorimeters were purged with dry nitrogen gas at a flow rate of 35 mL/min. As will be outlined below, experiments were partly performed using different, but identically configured instruments, with the sample preparation and instrument handling done by different operators, for the sake of estimation of errors. The sample mass was between 120 and 300 ng, and was estimated by comparing the measured heatcapacity increment on heating a fully amorphous sample at the glass transition temperature Tg in units of J/K with the expected mass-specific heat-capacity increment of 0.51 J/(g K) [15]. Further details about the instrument/sensor are reported in [13,44,45]. 2.4. Differential scanning calorimetry DSC was used to obtain information about the non-isothermal crystallization behavior on cooling at rates of 2, 5 and 10 K/min. DSC data were collected with a calibrated heat-flux calorimeter DSC-1 from Mettler-Toledo. The sample mass was about 5 mg, and the furnace was purged with nitrogen gas at a flow rate of 30 mL/ min. Calibration and operation of the instrument was performed as described in textbooks [46]. 3. Results and discussion 3.1. Non-isothermal crystallization Fig. 1 shows the temperature–time profile of FSC experiments for analysis of non-isothermal crystallization of PA 66. The sample was melted by heating to 300 C, and kept at this temperature for 0.5 s to achieve equilibration of the melt, and then cooled at the
2. Experimental 2.1. Materials For the analysis of the crystallization kinetics of PA 66 we used a commercial general purpose injection-molding grade Zytel 101 L from DuPont (USA) with a number-average molar mass of 17 kg/ mol [42,43]. This grade of PA 66 contains a small amount of additive that functions as a lubricant during injection molding.
Fig. 1. Temperature–time profile for analysis of non-isothermal crystallization of PA 66.
Please cite this article in press as: A.M. Rhoades, et al., Crystallization kinetics of polyamide 66 at processing-relevant cooling conditions and high supercooling, Thermochim. Acta (2014), http://dx.doi.org/10.1016/j.tca.2014.10.020
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cooling rate of 1 K/s to 60 C. Immediately after cooling, the sample was re-melted by heating at a rate of 1000 K/s. After data collection the method was repeated on the same sample, after systematically increasing cooling rate from 1 K/s to 3000 K/s. The cooling scans were analyzed regarding the temperature and enthalpy of crystallization while the subsequently recorded heating scans were employed to gain additional information about the fraction of crystals formed on cooling. It is worthwhile noting that crystallization enthalpies on cooling slower than about 10 K/s are difficult to evaluate, due to the low signal-to-noise ratio of the FSC heat-flow rate data. For this reason, crystallinities were also determined by analysis of the total enthalpy change of the sample during heating, considering both cold-crystallization and melting. Regarding the conditions of equilibrating the melt prior to cooling, to ensure an identical structural state of the melt independent of the thermal history, both the melt temperature and residence time were systematically varied for analysis of an effect on the crystallization kinetics. In detail, in a first set of experiments, the temperature of the melt was varied between 280 and 320 C while keeping the residence time of 0.5 s constant. In a second set of experiments the melt-residence time was varied between 0.01 and 1 s while keeping the temperature of the melt constant at 300 C. Subsequent analysis of the crystallization kinetics at 180 C did not reveal any effect of variation of these parameters within the reported temperature and time limits on the crystallization half-time. As such, a melt temperature of 300 C with a residence time of 0.5 s was deemed sufficient for further reliable kinetic studies. Fig. 2 shows two sets of FSC cooling curves for PA 66, plotting heat-flow rate as a function of temperature. The bottom set of redcolored curves represents measurements performed at rates between 20 (bottom curve) and 100 K/s (top curve). The top set of curves was obtained on cooling PA 66 between 200 and 1000 K/s (blue curves), and on cooling between 1200 and 2,000 K/s (black curves). Scaling of the heat-flow rate axis is different in the bottom and top sets of curves, as is indicated with the scaling bars at the left-hand side of the curves. All curves are corrected for instrumental asymmetry by subtraction of a baseline recorded on cooling at a rate of 1 K/s. The data of Fig. 2 reveal that PA 66 crystallizes on slow cooling at rates lower than about 30 K/s at
Fig. 2. Sets of baseline-corrected FSC cooling curves of PA 66, recorded at different rates as is indicated at the right hand-side of the curves. From bottom to top, the cooling rate increment between the various curves is 10, 100, and 200 K/s for the cooling rate ranges of 20–100 K/s (red curves), 200–1000 K/s (blue curves), and 1200–2000 K/s (black curves), respectively. Note that the scaling of the heat-flow rate axis is different in the bottom and top sets of curves. The approximate positions of the glass transition temperature (Tg) and crystallization temperature (Tc) are indicated with the dashed lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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temperatures higher than 200 C. As expected, due to the thermodynamically irreversible nature and time dependence of the crystallization process, there is observed a decrease of the crystallization temperature (Tc) with increasing cooling rate, as is indicated with the dashed line in Fig. 2. Cooling faster than about 100 K/s leads to a distinct decrease of the area of the crystallization peak, indicating that the system achieved less crystallization than possible at slower rates. If the cooling rate exceeds about 500 K/s then crystallization is completely absent. In particular the curves of the top data set, due to the high cooling rate and absence of crystals, reveal with the step-like change of the heat-flow rate the glass transition of the amorphous phase at a temperature around 50 C, slightly decreasing with increasing cooling rate. It is worthwhile noting that for dry PA 66 a glass transition temperature (Tg) of 50 C is reported in the ATHAS data base [15], suggesting absence of moisture in the investigated sample. The wiggles in the curves recorded on cooling at rates faster than 1000 K/s at temperatures lower than about 20 C (see vertical arrow) indicate loss of instrument control, that is, the programmed cooling rate cannot be achieved by the system due to the chosen experimental setup with a sample-stage temperature of 90 C and sample mass of 120 ng. This is of no importance in the context of the present work. This experiment, repeated on a different but identically configured instrument using a different sample, sample size, and operator yielded nearly identical results. Fig. 3 shows a set of FSC heating curves, heat-flow rate as a function of temperature, obtained on PA 66 which has been solidified at different rate in the prior cooling experiment, with exothermic heat flow directed upward. The front curve was measured on PA 66 which was cooled at a rate of 3000 K/s, and reveals with the step-like decrease of the heat-flow rate signal devitrification of the amorphous phase at the glass transition temperature of about 50–60 C, exothermic cold-crystallization around 160 C, and endothermic melting of crystals which have been formed during cold-crystallization at about 250 C. The areas of the cold-crystallization and melting peaks are identical which indicates that the sample was amorphous before measurement. Qualitatively, with decreasing rate of cooling, up to 500–1000 K/s, the FSC heating curves are unchanged; the areas of the coldcrystallization and melting peaks approximately are identical and confirm absence of crystals prior heating. However, if the cooling rate is further decreased to below about 500 K/s then the enthalpy of cold-crystallization decreases, as represented by the area of the cold-crystallization peak, and gets smaller than the area of melting peak. This observation is interpreted such that prior the heating experiment crystals are present which formed on rather slow cooling, reducing the amount of cold-crystallization proportional to their fraction. If the cooling rate was lower than 100–200 K/s then cold-crystallization is not observed on heating.
Fig. 3. Set of FSC curves, heat-flow rate as a function of temperature, obtained on heating PA 66 at a rate of 1000 K/s. The various curves were measured after prior cooling at rates between 3000 (front curve) and 1 K/s (back curve). Exothermic heat flow is directed upward.
Please cite this article in press as: A.M. Rhoades, et al., Crystallization kinetics of polyamide 66 at processing-relevant cooling conditions and high supercooling, Thermochim. Acta (2014), http://dx.doi.org/10.1016/j.tca.2014.10.020
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Regarding the heating curves of Fig. 3, it is emphasized that the cold-crystallization kinetics depends on the thermal history of the amorphous phase since it affects the formation and survival of crystal nuclei [38,47–50]. Furthermore, for samples with a predefined number of crystal nuclei formed on cooling, annealing of the glass, and subsequent heating, cold-crystallization may be suppressed with increasing heating rate due to hindered growth. In the present work, the samples were held in the melt, and then quenched in order to achieve complete vitrification. Upon subsequent heating, cold-crystallization is observed. This was achieved by cooling at 2000 K/s to 60 C, with a dwell time of 0.1 s at 60 C. Heating these vitrified samples yields cold crystallization at all heating rates below 1000 K/s, and this process begins to taper off only on heating faster than 1000 K/s, to be completely absent if the heating rate exceeds 8000–10,000 K/s. It is worthwhile noting that the critical heating rate required so suppress cold-crystallization is distinctly lower in case of PA 6 of similar thermal history [13,51]. Analysis of the effect of the thermal history of amorphous structure on cold-crystallization is subject of an independent study and will be presented elsewhere. Fig. 4shows in the top and bottom parts temperatures and enthalpies of crystallization of PA 66 as a function of the cooling rate, respectively. Crystallization temperatures were collected by DSC and by FSC as is indicated in the legend. For the sake of demonstration confidence in our measurements we included additional DSC data available in the literature, collected on the same PA 66 grade [24]. Enthalpies of crystallization were estimated from the area of the crystallization peak in the FSC cooling experiments (see Fig. 2), or by analysis of the total change of enthalpy on subsequent heating (see Fig. 3). Related to uncertainties in estimation of the sample mass and the correct definition of a baseline for integration, crystallization enthalpies were adjusted such that the low-cooling rate plateau fits the enthalpy of crystallization obtained on slow cooling using DSC. For the specific PA 66 grade used in this work, it has been reported in the literature that the crystallization enthalpy on cooling at 10 K/min is about 80 J/g (blue diamond symbol) [25], being equivalent to a crystal fraction of 32%.
The crystallization temperature of PA 66 on slow cooling, at rates typically applied in DSC analyses, is 230–240 C, and decreases due to the kinetics of the crystallization process with increasing cooling rate. For example, if the melt is cooled at a rate of 800 K/s, as may be evident in the skin layer of injection moldings [6], then crystallization only occurs at a temperature of about 140 C. Crystallization peaks in FSC scans were detected on cooling at rates up to about 1000 K/s though then being very low in intensity. If the melt is cooled faster to a temperature lower than Tg then crystals are not forming and the material remains amorphous. It is worthwhile noting that there is not observed any indication of a change of the mechanism of nucleation, or of formation of a different crystal polymorph on variation of the cooling rate. This notwithstanding, a qualitatively similar cooling-rate dependence of the crystallization temperature has been observed for PA 6, however, X-ray data showed a replacement of monoclinic a-crystals by pseudo-hexagonal mesophase on cooling at high rates [34,35]. Regarding the enthalpy of crystallization, the data of Fig. 4 suggest a plateau value of 70–80 J/g if the melt is cooled slower than 100–200 K/s. At such cooling conditions there is observed an only minor decrease of the crystallinity with increasing cooling rate. While on slow cooling at a rate of 10 K/min (0.167 K/s) the enthalpy of crystallization amounts 80 J/g, cooling at 100 K/s still allows considerable crystallization as is suggested with the observed transition enthalpy of about 60 J/g. However, if the melt is cooled faster than 100–200 K/s then the crystallinity decreases in a rather narrow cooling-rate range, approaching zero if cooling at a few 100 K/s. For PA 6, in former work, a critical cooling rate, required to be suppress crystallization of about 150 K/s has been reported [13,34,35,51]. In case of PA 66, as suggested by the data of Fig. 4, slightly faster cooling is necessary to inhibit crystallization, confirming an earlier report that the maximum crystallization rate of PA 66 is slightly higher than that of PA 6 [52]. It should be emphasized that the data of Fig. 4 only hold for crystallization experiments in which cooling is extended to temperatures below Tg, leading to vitrification of the amorphous phase. 3.2. Isothermal crystallization Fig. 5 shows the temperature–time profile of FSC experiments for analysis of the isothermal crystallization kinetics of PA 66 at temperatures between 75 and 230 C. As in case of non-isothermal crystallization experiments, samples were melted by heating to 300 C, equilibrated at this temperature for a period of time of 0.5 s, and then cooled at a rate of 2000 K/s to the crystallization temperature. After crystallization, the sample was first cooled and then re-melted. Subsequently, the same sample was used for the
Fig. 4. Cooling-rate dependence of the crystallization temperature (top, red data points) and enthalpy of crystallization (bottom, blue data points) of PA 66. Crystallization temperatures have been collected by DSC and FSC as is indicated in the legend. Enthalpies of crystallization were estimated from the area of the crystallization peak in the FSC cooling experiments (dark-gray filled circles), or by analysis of the total change of enthalpy on subsequent heating (light-gray filled circles). The enthalpies of crystallization, estimated by FSC in this work, were adjusted such to fit the enthalpy of crystallization obtained on slow cooling using DSC (blue diamond symbol). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Temperature–time profile for analysis of isothermal crystallization of PA 66.
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analysis of crystallization at a 5 K lower temperature. The cooling rate of 2000 K/s, from the melt to the crystallization temperature, has been selected based on the data of Figs. 2 and 4, to ensure absence of crystallization during the cooling segment. Fig. 6 is a plot of FSC curves, heat-flow rate as a function of the time, obtained during isothermal crystallization of PA 66 at temperatures between 200 C (front curve) and 75 C (back curve), with exothermic heat flow directed upward. The peak in each of the curves is due to crystallization, with the corresponding crystallization time, in the following named peak-time of crystallization, being a measure of the crystallization rate. The shorter the peak-time of crystallization the faster is the crystallization process. For a given mechanism of nucleation, including the nature of the nucleating species in case of heterogeneous nucleation, it is expected that the rate of crystallization/formation of a specific crystal polymorph increases with decreasing temperature due to increasing thermodynamic driving force for the melt-crystal phase transformation, passes through a minimum and then decreases on the approach of Tg due to decreasing mobility of chain segments [53,54]. The data of Fig. 6, however, reveal the occurrence of two minimum peak-times at high and low crystallization temperature pointing to a qualitative change of the crystallization mechanism upon variation of the crystallization temperature. Taking into account prior research in this field, it is speculated that the observation of two qualitatively different crystallization events, controlled by temperature, is due to different mechanisms of nucleation in combination with the formation of different crystal polymorphs. More specific, it is assumed that a0 /a-crystals are forming at rather high crystallization temperatures while there is mesophase formation at rather low temperatures, near Tg, as it has been suggested by quenching experiments reported in the literature [32]. Similar kinetically controlled formation of mesophases at high supercooling has been evidenced for PA 6 [32–35], PA 11 [40,55,56], for PBT [57,58] or iPP [59–62], also showing a bimodal temperature dependence of the crystallization rate as was observed in the present study for PA 66. For these polymers, that is, for PA 6, PA 11, and iPP, tailored study of the mesophase morphology by microscopy revealed absence of lamellae and spherulites, and indicated a distinctly higher nucleation density than on crystallization at low supercooling of the melt [36,40,59], ultimately leading to the suggestion that heterogeneous nucleation, evident at high temperature, is replaced by homogeneous nucleation at low temperature. For PA 66 such analysis of the mesophase morphology and superstructure is still not available, however, is required to explain the origin of the lowtemperature crystallization process shown in Fig. 6. Fig. 7 is a plot of the peak-time of isothermal crystallization of PA 66 as a function of the crystallization temperature. The blue and red circles represent peak times of crystallization obtained by FSC in this work while the other symbols represent half-times of crystallization collected by DSC on different PA 66 grades, available in the literature [29–31]. As discussed above, the peak-time of crystallization decreases with increasing supercooling due to
Fig. 6. Set of FSC curves, heat-flow rate as a function of the time, obtained during isothermal crystallization of PA 66 at temperatures between 200 C (front curve) and 75 C (back curve). Exothermic heat flow is directed upward.
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Fig. 7. Peak-time of isothermal crystallization of PA 66 as a function of the crystallization temperature. Blue and red circles represent peak times of crystallization obtained by FSC in this work while the other symbols represent half-times of crystallization collected by DSC, reported in literature [29–31]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
increasing thermodynamic driving for the crystallization, passes through a minimum at 160–170 C, and begins then to increase because of decreasing mobility of chain segments (see red data points and arrow) [53,54]. With further increasing supercooling, at temperatures below about 130 C, however, there is observed again an increase of the crystallization rate which leads to a lowtemperature crystallization-peak-time minimum around 110 C. This increase of the crystallization rate at temperatures below 130 C may be due to an increase of the nucleation density/change of the nucleation mechanism, or increase of the crystal growth rate due to formation of a different crystal polymorph/mesophase, or both. The data of Fig. 7 reveal furthermore that the maximum rate of the high-temperature crystallization process (red data points) is faster than the maximum rate of the low-temperature crystallization/mesophase-formation process (blue data points). This observation implies that in linear cooling experiments the lowtemperature transition can only be observed if cooling is interrupted at a temperature between 130 C and Tg. Otherwise, either crystallization will occur at higher temperature, or the sample will completely vitrify. Only if the low-temperature ordering formation process is distinctly faster than crystallization at high temperature then a low-temperature exothermic event would be detected in cooling experiments. Such result has been observed in the case of iPP [58–62]. 4. Conclusions Processing of polyamide 66 (PA 66) by injection molding includes cooling of the melt at rates between 100 and 103 K/s and its solidification at rather high supercooling, depending on processing parameters like the temperatures of the melt and of the mold, and upon the thermal gradient created in the polymer across the wall thickness of the product. Detailed and quantitative information about the crystallization behavior of PA 66 at such conditions are not yet available. Former studies in the field of analysis of the crystallization kinetics of PA 66 were mainly conducted by DSC which is restricted to the analysis of crystallization at cooling rates lower than few hundred K/min, or at temperatures higher than about 200 C; at lower temperatures the crystallization rate of PA 66 is too high to be quantified by DSC. For this reason we employed in the present work the novel
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tool of fast scanning chip calorimetry which, in the used setup, allowed cooling at rates up to few 1000 K/s, and recording of isothermal crystallization processes which are completed in less than a second. Slow cooling of the melt of PA 66 leads to formation of a semicrystalline structure, with the maximum crystallinity being about 30%. Analysis of the crystallinity as a function of the cooling rate showed that the crystallinity remains at rather high level if the cooling rate is lower than about 100 K/s. If the cooling rate exceeds 100–200 K/s then crystallization is suppressed, to be completely absent on cooling at 500–1000 K/s. The data suggest that PA 66 crystallizes slightly faster than PA 6 since in this case a cooling rate of only 100–200 K/s is sufficient to inhibit crystallization. Isothermal analysis of the peak time of crystallization as a function of temperature revealed two distinct maxima at about 165 and 110 C. It is suggested that this observation is related to a change of the nucleation mechanism on temperature variation, or to the formation of different crystal polymorphs, or a combination of these effects which exhibit different growth rates. For several polymers including PCL, iPP, PA 6 and PA 11, which show a similar bimodal temperature-dependence of the crystallization rate, it has been proven by analysis of the nucleation density that heterogeneous crystal nucleation, evident at rather low supercooling, is replaced by homogeneous nucleation pre-dominant at high supercooling. Such analysis of the nucleation density as a function of the crystallization temperature is still lacking for PA 66, however, is needed to confirm the above assumption. In addition, we propose to analyze the structure of the ordered phase forming at different temperatures, in order to shed further light into the crystal/mesophase polymorphism of PA 66. At present, it can only be speculated that crystallization at temperatures lower than about 125 C (see blue circles in Fig. 7) is related to formation of mesophase. Such condition, however, may often be found in the skin layer of injection moldings. There, formation of mesophase at temperatures between Tg and 125 C would only require cooling of the melt at a rate faster than about 500 K/s (see Fig. 4), in order to suppress the high-temperature crystallization process. In summary, it is expected that the observed data about the crystallization kinetics are useful to predict the solidification behavior, the skin–core morphology, and with that properties of injection moldings of PA 66. Acknowledgment The authors wish to thank General Motors Corporation for partial funding and encouragement of the described research.
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Thermochimica Acta xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
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Crystallization kinetics of polyamide 66 at processing-relevant cooling conditions and high supercooling Alicyn Marie Rhoades a, *, Jason Louis Williams a , René Androsch b a b
Pennsylvania State University, Behrend College, School of Engineering, 4701 College Drive, Erie, PA 16563, USA Martin-Luther-University Halle-Wittenberg, Center of Engineering Sciences, 06099 Halle/Saale, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 August 2014 Received in revised form 30 September 2014 Accepted 2 October 2014 Available online xxx
Processing of polyamide 66 (PA 66) by injection molding includes cooling of the melt at rates between 100 and 103 K/s and its solidification at high supercooling. The kinetics of crystallization at such conditions is unknown and has been evaluated in this work using fast scanning chip calorimetry. Slow cooling of the melt of PA 66 leads to formation of crystals, with the maximum crystallinity being about 30%. It has been found by analysis of the crystallinity as a function of the cooling rate that crystallization is suppressed on cooling faster than about 300 K/s. Isothermal analysis of the crystallization rate revealed a bimodal temperature dependence, with maxima obtained at about 165 and 110 C. It is suggested that the observation of two distinct crystallization-rate maxima is related to a change of the nucleation mechanism on temperature variation or the formation of different crystal polymorphs exhibiting different growth rate. The findings provide a fundamental step towards accurately predicting the solidification behavior, the skin–core morphology, and properties of injection moldings of PA 66. ã 2014 Published by Elsevier B.V.
Keywords: Polyamide 66 Crystallization kinetics Fast scanning chip calorimetry
1. Introduction Polyamide 66 (PA 66) is a versatile and highly utilized engineering thermoplastic homopolymer. The polymer exhibits high strength over a broad temperature range, has excellent impact and chemical resistance, and can withstand high wear. Besides its use for production of fibers, applications include automotive, aerospace, and industrial replacements for metal gears, bearings and engine components [1–4]. The material is typically injection molded and is particularly prone to shrink and warpage problems. The material is known to be very sensitive to changes in melt processing, presenting a challenge for engineers that design manufacturing processes for high-end part production[5]. Both the excellent physical properties and the challenging design drawbacks of PA 66 are due to the inherent semi-crystalline microstructure. During injection molding, this semi-crystalline microstructure forms under shear and thermal gradients, typically leading to the development of skin–core morphologies with implications on the property profile [6]. In addition, the molten polymer volume densifies to a greater extent in crystallizing regions than in areas which remain macroscopically amorphous, resulting in problematic variations in shrinkage across a part [7]. To
* Corresponding author. Tel.: +1 814 898 6287. E-mail address: [email protected] (A.M. Rhoades).
control and maximize the benefits resulting from a uniform microstructure it is critical to develop an understanding of the crystallization process, at conditions relevant in processing. The process of injection molding can subject PA 66 to a wide range of cooling rates. Conventional injection molding employs a steel mold held at a constant temperature, and a mold temperature in the range between 0 and 90 C is often used [8]. Oftentimes, lower mold temperatures will be used to “quench” the melt and to allow for more rapid production rates. Melt temperatures range from 280 to 305 C, and the material is also subject to injection pressures between 35 and 140 MPa [8]. After the mold cavity is filled, time to solidification is dictated by the heat transfer out of the mold and can vary greatly with mold and melt temperature, and the wall thickness. Cooling rates in injection-molding of PA 66 range from about 3 K/s in the center of a molded tensile test bar to more than 1200 K/s at the interface of the polymer/mold surfaces [9]. Despite the potential usefulness of such information, there are no published resources available that describe the crystallization kinetics of PA66 at processing-relevant high cooling rates, nor is there published information regarding the crystallization rate of PA 66 at temperatures experienced during the cooling processes of injection molding, extrusion, or thermoforming. Prior studies of the crystallization behavior of PA 66 were mainly performed by differential scanning calorimetry (DSC) which, however, only allows analysis of crystallization processes at cooling rates lower
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than about few 100 K/min, or at temperatures at which the meltcrystal phase transition is longer than several ten seconds, [10,11] both being insufficient to derive conclusions regarding the materials behavior at conditions present in polymer processing. Therefore, we have utilized a fast scanning calorimetry technique [12,13] to study the crystallization process of PA 66 at rapid cooling and at low temperature. The primary chemical structure of the PA 66 macromolecule consists of amide groups separated by methylene sequences. PA 66 is a polymorphic polymer and forms different supermolecular structures depending on the condition of crystallization. Slow cooling of the quiescent melt or crystallization at rather high temperature leads to formation of triclinic a-crystals with sheetlike arrangement of hydrogen bonds at room temperature, connecting amide groups of neighbored chain segments [14]. The equilibrium melting temperature and bulk heat of fusion of the a-phase, as listed in the ATHAS data base, are 301 C (574 K) and 255.4 J/g, respectively [15–17]. The triclinic a-form converts on heating at the Brill transition temperature into the high-temperature a0 -form with a pseudo-hexagonal unit cell and a threedimensional network of hydrogen bonds [18,19]. The Brill transition is reversible, that is, the a0 -phase reverts on cooling into the a-phase. The formation of a/a0 -crystals on slow cooling/ low supercooling typically is connected with formation of lamellae and spherulites [20–22]. Regarding the kinetics of formation of a0 -crystals it is known that on cooling at rates lower than 20 K/min, as is typically applied in DSC analyses, crystallization occurs at 220–240 C [23–29]. Isothermal DSC experiments revealed that at a temperature of 230 C crystallization is finished after about 1– 2 min, with the crystallization time rapidly increasing with crystallization temperature [29–31]. Besides formation of the a-polymorph as a result of slow cooling or crystallization at high temperature, there has been reported the development of a pseudohexagonal mesophase with non-planar arrangement of hydrogen bonds upon quenching of the melt [32]. The mesophase is metastable at room temperature and converts irreversibly into a/a0 -crystals on heating [33]. Details of the morphology and superstructure of the mesophase of PA 66, and their exact conditions of formation are not known yet. In case of PA 6, which exhibits a similar cooling-rate/supercooling controlled crystal/ mesophase polymorphism as PA 66 [32–35], it has been proven that the mesophase domains are of non-lamellar habit and not spatially organized in a higher-order superstructure [36,37]. Summarizing the scope of the present study, we attempt to gain information about the crystallization kinetics of PA 66 at conditions being typical in injection molding, for example on cooling at rates between 1 and 1000 K/s, and at temperatures lower than 200 C, not yet reported. The observed data may then be used to predict the solidification behavior of injection moldings, their skin–core morphology and, with that, their properties. In a wider view, the study is also considered as a continuation of our research efforts to obtain information about a possible change of the mechanism of crystal nucleation on variation of the supercooling, as has been suggested for the cases of isotactic polypropylene (iPP) [38,39], polyamide 11 [40], or poly(e-caprolactone) [41].
2.2. Sample preparation The as-received, dry pellets were injection-molded to standard tensile test bars using a 100 t Sodick Plustech injection molding machine. The molded bars were then microtomed parallel to their thickness direction, to obtain sections with a thickness of 13 mm. These thin sections were then further prepared for subsequent analysis of the crystallization behavior using FSC by reduction of their lateral size to 50–100 mm, using a scalpel and a stereomicroscope. 2.3. Fast scanning calorimetry FSC analysis was performed using a power-compensation Mettler-Toledo Flash DSC 1, attached to a Huber intracooler TC100. Prior positioning of the specimen on the heatable area of the sample calorimeter, the UFS 1 FSC sensor was conditioned and temperature-corrected according to the specification of the instrument provider. The calorimeters were purged with dry nitrogen gas at a flow rate of 35 mL/min. As will be outlined below, experiments were partly performed using different, but identically configured instruments, with the sample preparation and instrument handling done by different operators, for the sake of estimation of errors. The sample mass was between 120 and 300 ng, and was estimated by comparing the measured heatcapacity increment on heating a fully amorphous sample at the glass transition temperature Tg in units of J/K with the expected mass-specific heat-capacity increment of 0.51 J/(g K) [15]. Further details about the instrument/sensor are reported in [13,44,45]. 2.4. Differential scanning calorimetry DSC was used to obtain information about the non-isothermal crystallization behavior on cooling at rates of 2, 5 and 10 K/min. DSC data were collected with a calibrated heat-flux calorimeter DSC-1 from Mettler-Toledo. The sample mass was about 5 mg, and the furnace was purged with nitrogen gas at a flow rate of 30 mL/ min. Calibration and operation of the instrument was performed as described in textbooks [46]. 3. Results and discussion 3.1. Non-isothermal crystallization Fig. 1 shows the temperature–time profile of FSC experiments for analysis of non-isothermal crystallization of PA 66. The sample was melted by heating to 300 C, and kept at this temperature for 0.5 s to achieve equilibration of the melt, and then cooled at the
2. Experimental 2.1. Materials For the analysis of the crystallization kinetics of PA 66 we used a commercial general purpose injection-molding grade Zytel 101 L from DuPont (USA) with a number-average molar mass of 17 kg/ mol [42,43]. This grade of PA 66 contains a small amount of additive that functions as a lubricant during injection molding.
Fig. 1. Temperature–time profile for analysis of non-isothermal crystallization of PA 66.
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cooling rate of 1 K/s to 60 C. Immediately after cooling, the sample was re-melted by heating at a rate of 1000 K/s. After data collection the method was repeated on the same sample, after systematically increasing cooling rate from 1 K/s to 3000 K/s. The cooling scans were analyzed regarding the temperature and enthalpy of crystallization while the subsequently recorded heating scans were employed to gain additional information about the fraction of crystals formed on cooling. It is worthwhile noting that crystallization enthalpies on cooling slower than about 10 K/s are difficult to evaluate, due to the low signal-to-noise ratio of the FSC heat-flow rate data. For this reason, crystallinities were also determined by analysis of the total enthalpy change of the sample during heating, considering both cold-crystallization and melting. Regarding the conditions of equilibrating the melt prior to cooling, to ensure an identical structural state of the melt independent of the thermal history, both the melt temperature and residence time were systematically varied for analysis of an effect on the crystallization kinetics. In detail, in a first set of experiments, the temperature of the melt was varied between 280 and 320 C while keeping the residence time of 0.5 s constant. In a second set of experiments the melt-residence time was varied between 0.01 and 1 s while keeping the temperature of the melt constant at 300 C. Subsequent analysis of the crystallization kinetics at 180 C did not reveal any effect of variation of these parameters within the reported temperature and time limits on the crystallization half-time. As such, a melt temperature of 300 C with a residence time of 0.5 s was deemed sufficient for further reliable kinetic studies. Fig. 2 shows two sets of FSC cooling curves for PA 66, plotting heat-flow rate as a function of temperature. The bottom set of redcolored curves represents measurements performed at rates between 20 (bottom curve) and 100 K/s (top curve). The top set of curves was obtained on cooling PA 66 between 200 and 1000 K/s (blue curves), and on cooling between 1200 and 2,000 K/s (black curves). Scaling of the heat-flow rate axis is different in the bottom and top sets of curves, as is indicated with the scaling bars at the left-hand side of the curves. All curves are corrected for instrumental asymmetry by subtraction of a baseline recorded on cooling at a rate of 1 K/s. The data of Fig. 2 reveal that PA 66 crystallizes on slow cooling at rates lower than about 30 K/s at
Fig. 2. Sets of baseline-corrected FSC cooling curves of PA 66, recorded at different rates as is indicated at the right hand-side of the curves. From bottom to top, the cooling rate increment between the various curves is 10, 100, and 200 K/s for the cooling rate ranges of 20–100 K/s (red curves), 200–1000 K/s (blue curves), and 1200–2000 K/s (black curves), respectively. Note that the scaling of the heat-flow rate axis is different in the bottom and top sets of curves. The approximate positions of the glass transition temperature (Tg) and crystallization temperature (Tc) are indicated with the dashed lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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temperatures higher than 200 C. As expected, due to the thermodynamically irreversible nature and time dependence of the crystallization process, there is observed a decrease of the crystallization temperature (Tc) with increasing cooling rate, as is indicated with the dashed line in Fig. 2. Cooling faster than about 100 K/s leads to a distinct decrease of the area of the crystallization peak, indicating that the system achieved less crystallization than possible at slower rates. If the cooling rate exceeds about 500 K/s then crystallization is completely absent. In particular the curves of the top data set, due to the high cooling rate and absence of crystals, reveal with the step-like change of the heat-flow rate the glass transition of the amorphous phase at a temperature around 50 C, slightly decreasing with increasing cooling rate. It is worthwhile noting that for dry PA 66 a glass transition temperature (Tg) of 50 C is reported in the ATHAS data base [15], suggesting absence of moisture in the investigated sample. The wiggles in the curves recorded on cooling at rates faster than 1000 K/s at temperatures lower than about 20 C (see vertical arrow) indicate loss of instrument control, that is, the programmed cooling rate cannot be achieved by the system due to the chosen experimental setup with a sample-stage temperature of 90 C and sample mass of 120 ng. This is of no importance in the context of the present work. This experiment, repeated on a different but identically configured instrument using a different sample, sample size, and operator yielded nearly identical results. Fig. 3 shows a set of FSC heating curves, heat-flow rate as a function of temperature, obtained on PA 66 which has been solidified at different rate in the prior cooling experiment, with exothermic heat flow directed upward. The front curve was measured on PA 66 which was cooled at a rate of 3000 K/s, and reveals with the step-like decrease of the heat-flow rate signal devitrification of the amorphous phase at the glass transition temperature of about 50–60 C, exothermic cold-crystallization around 160 C, and endothermic melting of crystals which have been formed during cold-crystallization at about 250 C. The areas of the cold-crystallization and melting peaks are identical which indicates that the sample was amorphous before measurement. Qualitatively, with decreasing rate of cooling, up to 500–1000 K/s, the FSC heating curves are unchanged; the areas of the coldcrystallization and melting peaks approximately are identical and confirm absence of crystals prior heating. However, if the cooling rate is further decreased to below about 500 K/s then the enthalpy of cold-crystallization decreases, as represented by the area of the cold-crystallization peak, and gets smaller than the area of melting peak. This observation is interpreted such that prior the heating experiment crystals are present which formed on rather slow cooling, reducing the amount of cold-crystallization proportional to their fraction. If the cooling rate was lower than 100–200 K/s then cold-crystallization is not observed on heating.
Fig. 3. Set of FSC curves, heat-flow rate as a function of temperature, obtained on heating PA 66 at a rate of 1000 K/s. The various curves were measured after prior cooling at rates between 3000 (front curve) and 1 K/s (back curve). Exothermic heat flow is directed upward.
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Regarding the heating curves of Fig. 3, it is emphasized that the cold-crystallization kinetics depends on the thermal history of the amorphous phase since it affects the formation and survival of crystal nuclei [38,47–50]. Furthermore, for samples with a predefined number of crystal nuclei formed on cooling, annealing of the glass, and subsequent heating, cold-crystallization may be suppressed with increasing heating rate due to hindered growth. In the present work, the samples were held in the melt, and then quenched in order to achieve complete vitrification. Upon subsequent heating, cold-crystallization is observed. This was achieved by cooling at 2000 K/s to 60 C, with a dwell time of 0.1 s at 60 C. Heating these vitrified samples yields cold crystallization at all heating rates below 1000 K/s, and this process begins to taper off only on heating faster than 1000 K/s, to be completely absent if the heating rate exceeds 8000–10,000 K/s. It is worthwhile noting that the critical heating rate required so suppress cold-crystallization is distinctly lower in case of PA 6 of similar thermal history [13,51]. Analysis of the effect of the thermal history of amorphous structure on cold-crystallization is subject of an independent study and will be presented elsewhere. Fig. 4shows in the top and bottom parts temperatures and enthalpies of crystallization of PA 66 as a function of the cooling rate, respectively. Crystallization temperatures were collected by DSC and by FSC as is indicated in the legend. For the sake of demonstration confidence in our measurements we included additional DSC data available in the literature, collected on the same PA 66 grade [24]. Enthalpies of crystallization were estimated from the area of the crystallization peak in the FSC cooling experiments (see Fig. 2), or by analysis of the total change of enthalpy on subsequent heating (see Fig. 3). Related to uncertainties in estimation of the sample mass and the correct definition of a baseline for integration, crystallization enthalpies were adjusted such that the low-cooling rate plateau fits the enthalpy of crystallization obtained on slow cooling using DSC. For the specific PA 66 grade used in this work, it has been reported in the literature that the crystallization enthalpy on cooling at 10 K/min is about 80 J/g (blue diamond symbol) [25], being equivalent to a crystal fraction of 32%.
The crystallization temperature of PA 66 on slow cooling, at rates typically applied in DSC analyses, is 230–240 C, and decreases due to the kinetics of the crystallization process with increasing cooling rate. For example, if the melt is cooled at a rate of 800 K/s, as may be evident in the skin layer of injection moldings [6], then crystallization only occurs at a temperature of about 140 C. Crystallization peaks in FSC scans were detected on cooling at rates up to about 1000 K/s though then being very low in intensity. If the melt is cooled faster to a temperature lower than Tg then crystals are not forming and the material remains amorphous. It is worthwhile noting that there is not observed any indication of a change of the mechanism of nucleation, or of formation of a different crystal polymorph on variation of the cooling rate. This notwithstanding, a qualitatively similar cooling-rate dependence of the crystallization temperature has been observed for PA 6, however, X-ray data showed a replacement of monoclinic a-crystals by pseudo-hexagonal mesophase on cooling at high rates [34,35]. Regarding the enthalpy of crystallization, the data of Fig. 4 suggest a plateau value of 70–80 J/g if the melt is cooled slower than 100–200 K/s. At such cooling conditions there is observed an only minor decrease of the crystallinity with increasing cooling rate. While on slow cooling at a rate of 10 K/min (0.167 K/s) the enthalpy of crystallization amounts 80 J/g, cooling at 100 K/s still allows considerable crystallization as is suggested with the observed transition enthalpy of about 60 J/g. However, if the melt is cooled faster than 100–200 K/s then the crystallinity decreases in a rather narrow cooling-rate range, approaching zero if cooling at a few 100 K/s. For PA 6, in former work, a critical cooling rate, required to be suppress crystallization of about 150 K/s has been reported [13,34,35,51]. In case of PA 66, as suggested by the data of Fig. 4, slightly faster cooling is necessary to inhibit crystallization, confirming an earlier report that the maximum crystallization rate of PA 66 is slightly higher than that of PA 6 [52]. It should be emphasized that the data of Fig. 4 only hold for crystallization experiments in which cooling is extended to temperatures below Tg, leading to vitrification of the amorphous phase. 3.2. Isothermal crystallization Fig. 5 shows the temperature–time profile of FSC experiments for analysis of the isothermal crystallization kinetics of PA 66 at temperatures between 75 and 230 C. As in case of non-isothermal crystallization experiments, samples were melted by heating to 300 C, equilibrated at this temperature for a period of time of 0.5 s, and then cooled at a rate of 2000 K/s to the crystallization temperature. After crystallization, the sample was first cooled and then re-melted. Subsequently, the same sample was used for the
Fig. 4. Cooling-rate dependence of the crystallization temperature (top, red data points) and enthalpy of crystallization (bottom, blue data points) of PA 66. Crystallization temperatures have been collected by DSC and FSC as is indicated in the legend. Enthalpies of crystallization were estimated from the area of the crystallization peak in the FSC cooling experiments (dark-gray filled circles), or by analysis of the total change of enthalpy on subsequent heating (light-gray filled circles). The enthalpies of crystallization, estimated by FSC in this work, were adjusted such to fit the enthalpy of crystallization obtained on slow cooling using DSC (blue diamond symbol). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Temperature–time profile for analysis of isothermal crystallization of PA 66.
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analysis of crystallization at a 5 K lower temperature. The cooling rate of 2000 K/s, from the melt to the crystallization temperature, has been selected based on the data of Figs. 2 and 4, to ensure absence of crystallization during the cooling segment. Fig. 6 is a plot of FSC curves, heat-flow rate as a function of the time, obtained during isothermal crystallization of PA 66 at temperatures between 200 C (front curve) and 75 C (back curve), with exothermic heat flow directed upward. The peak in each of the curves is due to crystallization, with the corresponding crystallization time, in the following named peak-time of crystallization, being a measure of the crystallization rate. The shorter the peak-time of crystallization the faster is the crystallization process. For a given mechanism of nucleation, including the nature of the nucleating species in case of heterogeneous nucleation, it is expected that the rate of crystallization/formation of a specific crystal polymorph increases with decreasing temperature due to increasing thermodynamic driving force for the melt-crystal phase transformation, passes through a minimum and then decreases on the approach of Tg due to decreasing mobility of chain segments [53,54]. The data of Fig. 6, however, reveal the occurrence of two minimum peak-times at high and low crystallization temperature pointing to a qualitative change of the crystallization mechanism upon variation of the crystallization temperature. Taking into account prior research in this field, it is speculated that the observation of two qualitatively different crystallization events, controlled by temperature, is due to different mechanisms of nucleation in combination with the formation of different crystal polymorphs. More specific, it is assumed that a0 /a-crystals are forming at rather high crystallization temperatures while there is mesophase formation at rather low temperatures, near Tg, as it has been suggested by quenching experiments reported in the literature [32]. Similar kinetically controlled formation of mesophases at high supercooling has been evidenced for PA 6 [32–35], PA 11 [40,55,56], for PBT [57,58] or iPP [59–62], also showing a bimodal temperature dependence of the crystallization rate as was observed in the present study for PA 66. For these polymers, that is, for PA 6, PA 11, and iPP, tailored study of the mesophase morphology by microscopy revealed absence of lamellae and spherulites, and indicated a distinctly higher nucleation density than on crystallization at low supercooling of the melt [36,40,59], ultimately leading to the suggestion that heterogeneous nucleation, evident at high temperature, is replaced by homogeneous nucleation at low temperature. For PA 66 such analysis of the mesophase morphology and superstructure is still not available, however, is required to explain the origin of the lowtemperature crystallization process shown in Fig. 6. Fig. 7 is a plot of the peak-time of isothermal crystallization of PA 66 as a function of the crystallization temperature. The blue and red circles represent peak times of crystallization obtained by FSC in this work while the other symbols represent half-times of crystallization collected by DSC on different PA 66 grades, available in the literature [29–31]. As discussed above, the peak-time of crystallization decreases with increasing supercooling due to
Fig. 6. Set of FSC curves, heat-flow rate as a function of the time, obtained during isothermal crystallization of PA 66 at temperatures between 200 C (front curve) and 75 C (back curve). Exothermic heat flow is directed upward.
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Fig. 7. Peak-time of isothermal crystallization of PA 66 as a function of the crystallization temperature. Blue and red circles represent peak times of crystallization obtained by FSC in this work while the other symbols represent half-times of crystallization collected by DSC, reported in literature [29–31]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
increasing thermodynamic driving for the crystallization, passes through a minimum at 160–170 C, and begins then to increase because of decreasing mobility of chain segments (see red data points and arrow) [53,54]. With further increasing supercooling, at temperatures below about 130 C, however, there is observed again an increase of the crystallization rate which leads to a lowtemperature crystallization-peak-time minimum around 110 C. This increase of the crystallization rate at temperatures below 130 C may be due to an increase of the nucleation density/change of the nucleation mechanism, or increase of the crystal growth rate due to formation of a different crystal polymorph/mesophase, or both. The data of Fig. 7 reveal furthermore that the maximum rate of the high-temperature crystallization process (red data points) is faster than the maximum rate of the low-temperature crystallization/mesophase-formation process (blue data points). This observation implies that in linear cooling experiments the lowtemperature transition can only be observed if cooling is interrupted at a temperature between 130 C and Tg. Otherwise, either crystallization will occur at higher temperature, or the sample will completely vitrify. Only if the low-temperature ordering formation process is distinctly faster than crystallization at high temperature then a low-temperature exothermic event would be detected in cooling experiments. Such result has been observed in the case of iPP [58–62]. 4. Conclusions Processing of polyamide 66 (PA 66) by injection molding includes cooling of the melt at rates between 100 and 103 K/s and its solidification at rather high supercooling, depending on processing parameters like the temperatures of the melt and of the mold, and upon the thermal gradient created in the polymer across the wall thickness of the product. Detailed and quantitative information about the crystallization behavior of PA 66 at such conditions are not yet available. Former studies in the field of analysis of the crystallization kinetics of PA 66 were mainly conducted by DSC which is restricted to the analysis of crystallization at cooling rates lower than few hundred K/min, or at temperatures higher than about 200 C; at lower temperatures the crystallization rate of PA 66 is too high to be quantified by DSC. For this reason we employed in the present work the novel
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tool of fast scanning chip calorimetry which, in the used setup, allowed cooling at rates up to few 1000 K/s, and recording of isothermal crystallization processes which are completed in less than a second. Slow cooling of the melt of PA 66 leads to formation of a semicrystalline structure, with the maximum crystallinity being about 30%. Analysis of the crystallinity as a function of the cooling rate showed that the crystallinity remains at rather high level if the cooling rate is lower than about 100 K/s. If the cooling rate exceeds 100–200 K/s then crystallization is suppressed, to be completely absent on cooling at 500–1000 K/s. The data suggest that PA 66 crystallizes slightly faster than PA 6 since in this case a cooling rate of only 100–200 K/s is sufficient to inhibit crystallization. Isothermal analysis of the peak time of crystallization as a function of temperature revealed two distinct maxima at about 165 and 110 C. It is suggested that this observation is related to a change of the nucleation mechanism on temperature variation, or to the formation of different crystal polymorphs, or a combination of these effects which exhibit different growth rates. For several polymers including PCL, iPP, PA 6 and PA 11, which show a similar bimodal temperature-dependence of the crystallization rate, it has been proven by analysis of the nucleation density that heterogeneous crystal nucleation, evident at rather low supercooling, is replaced by homogeneous nucleation pre-dominant at high supercooling. Such analysis of the nucleation density as a function of the crystallization temperature is still lacking for PA 66, however, is needed to confirm the above assumption. In addition, we propose to analyze the structure of the ordered phase forming at different temperatures, in order to shed further light into the crystal/mesophase polymorphism of PA 66. At present, it can only be speculated that crystallization at temperatures lower than about 125 C (see blue circles in Fig. 7) is related to formation of mesophase. Such condition, however, may often be found in the skin layer of injection moldings. There, formation of mesophase at temperatures between Tg and 125 C would only require cooling of the melt at a rate faster than about 500 K/s (see Fig. 4), in order to suppress the high-temperature crystallization process. In summary, it is expected that the observed data about the crystallization kinetics are useful to predict the solidification behavior, the skin–core morphology, and with that properties of injection moldings of PA 66. Acknowledgment The authors wish to thank General Motors Corporation for partial funding and encouragement of the described research.
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