Melt Crystallization Process Research Articles

Crystallization of Polylactic Acid Composites with Banana and Hemp Fibres by Means of DSC and XRD Methods

This paper deals with the evaluation of crystalline structure and thermal properties of injection molded parts based on polylactic acid (PLA) matrix reinforced by banana and hemp natural fibres (NF). Using differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD) methods were evaluated thermal properties and crystalline structure of PLA/NF composites, depending on the amount of natural fibres within the weight ratio of 10% up to 30%. We observed that hemp and banana fibres work like natural nucleating agents and thus subsequently improve the material properties of PLA. We also observed process of melt (primary) and cold (secondary) crystallization of PLA/NF composites depending on cooling rate.

Morphology, crystallization and mechanical properties of biodegradable poly(butylene succinate-co-butylene carbonate)/multi-walled carbon nanotubes nanocomposites

Biodegradable poly(butylene succinate-co-butylene carbonate) (PBSC)/carboxyl-functionalized multi-walled carbon nanotubes (f-MWCNTs) nanocomposites were prepared through a solution and casting method at low f-MWCNTs loadings. Scanning electron microscopy observations revealed a fine dispersion of f-MWCNTs in the PBSC matrix. Both the nonisothermal and isothermal melt crystallization processes of PBSC were enhanced, apparently by the presence of f-MWCNTs in the nanocomposites, because f-MWCNTs acted as an efficient heterogeneous nucleation agent for the crystallization of PBSC. The crystallization mechanism remained unchanged for both neat PBSC and the PBSC/f-MWCNTs nanocomposites, regardless of crystallization temperature; moreover, the crystal structures of PBSC were not modified by f-MWCNTs in the nanocomposites. The dynamical mechanical properties were also improved in the nanocomposites. Upon incorporating only 1 wt% of f-MWCNTs, the storage modulus of PBSC improved by about 30% at −60 °C. In addition, the glass transition temperature values of PBSC were increased slightly in the nanocomposites, relative to that of neat PBSC. Open image in new window

Numerical analysis of the influence of surface-active substance in the melt on the distribution of modifying particles and crystallization at the treatment of metal surface by a laser pulse

A mathematical model is proposed for the process of modifying the metal surface layer by refractory nano-size particles with the aid of the pulse laser radiation, which accounts for the surface tension dependence on the presence of surface-active substance in the melt. Numerical modeling has been carried out, from the results of which the influence of the surface -active admixture on the character of forming flows, distribution of particles of the modifying substance in the metal, and the melt crystallization process have been estimated.

Enhanced Nucleation and Crystallization of Poly(l-lactic acid) by Immiscible Blending with Poly(vinylidene fluoride)

Effects of poly(vinylidene fluoride) (PVDF) on the non-isothermal and isothermal crystallization kinetics, crystalline structure, and spherulitic morphology of poly(l-lactic acid) (PLLA) in their immiscible blends were investigated. Both the cold and melt crystallizations of PLLA are accelerated after blending with PVDF. The crystallization rate of PLLA further increases as the PVDF content increases. PLLA does not crystallize upon cooling (10 °C/min) from the melt, while it can almost crystallize sufficiently under the same conditions after blending with 10% PVDF. PLLA crystallizes after the crystallization of PVDF in the melt crystallization process. The isothermal crystallization kinetics of PLLA in the blends was analyzed by the Avrami model. Crystallization half-time decreases and crystallization rate constant of PLLA increases after blending with PVDF. With the presence of PVDF, the spherulitic size of PLLA decreases and spherulitic number increases. Nucleation density of PLLA increases by 2–3 orders of magnitude by blending with 10 or 25% PVDF. It is found that PLLA spherulites can epitaxially grow on the PVDF crystalline domains, forming a transcrystalline structure in the PLLA/PVDF interface. It is proposed that the PVDF-promoted crystallization of PLLA in their immiscible blends is ascribed to the heterogeneously epitaxial and interface-assisted nucleation mechanism.

Effects of Preexisting Poly(butylene succinate-co-24 mol % hexamethylene succinate) Crystals on the Crystallization Behavior and Crystalline Morphology of Poly(butylene adipate) in Their Melt-Miscible Polymer Blend

The effects of preexisting poly(butylene succinate-co-24 mol % hexamethylene succinate) (PBHS) crystals on the crystallization behavior and crystalline morphology of poly(butylene adipate) (PBA) were studied in their fully biodegradable melt-miscible crystalline/crystalline blend. Regardless of crystallization conditions, PBHS crystallized first, and PBA must crystallize in the presence of preexisting PBHS crystals. The preexisting PBHS crystals may not only enhance the nonisothermal melt crystallization behavior but also favor the nucleation and growth of only α-form crystals of PBA, providing an easy method of controlling the polymorphic crystals of PBA through polymer blending; moreover, the preexisting PBHS crystals could accelerate the overall isothermal melt crystallization process of PBA in the blend. PBA must crystallize as tiny crystals at the same orientation as the crystallites of the host PBHS spherulites in the blend, unlike the spherulitic morphology in its neat state. Both components crysta...

Influence of Thermally Reduced Graphene Low-Loadings on the Crystallization Behavior and Morphology of Biodegradable Poly(ethylene succinate)

Through a solution and coagulation method, biodegradable poly(ethylene succinate) (PES) and thermally reduced graphene (TRG) nanocomposites were prepared at low TRG loadings. The nonisothermal melt crystallization peak temperature values and overall isothermal melt crystallization rates are greater in the nanocomposites than in neat PES, indicative of a nucleating agent effect of TRG; however, the crystallization mechanism of PES remains unchanged in the nanocomposites, regardless of TRG loading and crystallization temperature. The nonisothermal and melt crystallization processes of the nanocomposites are found to vary with the TRG loading, exhibiting a maximum at 0.25 wt % TRG loading. The incorporation of a small amount of TRG shows little influence on the spherulitic growth rates but obviously affects the spherulites nucleation density values of the nanocomposites. TRG does not modify the crystal structure of PES in the nanocomposites.

Largely improved crystallization behavior and thermal stability of poly(L‐lactide) via the synergistic effects of graphene oxide and carbon nanotubes

ABSTRACTGraphene oxide (GO) and carbon nanotubes (CNTs) and their compound were introduced into semicrystalline poly(l‐lactide) (PLLA) to prepare the corresponding binary and/or ternary nanocomposites, respectively. The dispersion states of nanofillers in different nanocomposites were investigated using UV‐Vis spectroscopy, scanning electron microscopy (SEM) and rheological measurement. The results showed that when GO and CNTs were simultaneously present in the PLLA matrix, good dispersion states of both GO and CNTs could be achieved and the ternary nanocomposites exhibited percolated network structure. The effects of different nanofillers on the crystallization behavior of PLLA matrix were comparatively investigated under the different crystallization conditions including melt crystallization process (nonisothermal and isothermal crystallization from the melt) and cold crystallization (crystallization occurring from an amorphous state during the annealing process). The results showed that GO and CNTs exhibited apparent synergistic effects in improving crystallization ability and enhancing crystallinity of PLLA matrix. Study on the thermal stability of nanocomposites showed that the presence of nanofillers greatly improved the thermal stability of PLLA matrix. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40143.

Effects of sweating time and cooling strategy on purification of N-vinyl-2-pyrrolidinone using a melt crystallizer

A melt crystallization process is proposed to produce high-purity n-vinyl-2-pyrrolidinone (NVP). To produce high purity products, operation strategy plays key role in the melt crystallizer. We investigated the cooling strategy and optimal sweating time using a batch-type melt crystallizer. A slow cooling followed by a slow heating was found to be an effective temperature profile to produce high purity of NVP. The optimal sweating time was found to be about 20 minutes. For industrial application, a cascade melt crystallizer which consists of four stages was constructed and the proposed crystallization/sweating scheme was applied. Using the new melt crystallizer, NVP more than 99.99% purity can be produced in semi-continuous mode.

N-vinyl-2-pyrrolidone과 2-pyrrolidone 혼합물의 고-액 상평형 및 용융결정화를 이용한 N-vinyl-2-pyrrolidone의 결정성장속도 연구

N-vinyl-2-pyrrolidone (NVP)에 포함된 불순물인 2-pyrrolidone을 제거하기 위해 용융결정화가 이용될 수 있으며, 그 기본 연구로써 두 물질의 고-액 상평형을 측정하였다. 시차주사 열량계(DSC)와 결정화기를 이용하여 얻어진 두 실험결과는 비슷한 경향을 보였으며, NVP와 2-pyrrolidone으로 구성된 2성분계 혼합물이 공융계를 형성함을 보였다. 간단한 열역학 식을 이용하여 혼합물의 상평형과 공융점(eutectic point)을 계산하였으며 실험결과와 비교적 잘 일치하였다. 결정화 공정의 설계에 중요한 요소인 결정성장속도를 알기 위해 판형 결정화기를 이용하여 시간에 따른 NVP 결정의 두께를 측정하였다. 냉각온도가 낮을수록 NVP의 결정성장속도가 증가하였다. 실험데이터로부터 상관된 열전달계수는 결정의 성장 거동을 잘 설명하였다. Solid-liquid equilibria for mixtures composed of n-vinyl-2-pyrrolidone (NVP) and 2-pyrrolidone were measured as a basic study for the melt crystallization process to remove 2-pyrrolidone as impurity included in NVP. A differential scanning calorimeter (DSC) and a crystallizer were used and the experimental results obtained from two methods were similar. The mixture showed a eutectic system which has a single composition at the minimum melting temperature. Calculation results from simple thermodynamic equations were found in general agreements with present data. To determine the growth rate of NVP crystal which is important for the design of crystallization process, thicknesses of the crystal were measured with the time using a layer melt crystallizer. The growth rates increased as cooling temperatures decreased. Heat transfer coefficient correlated from present data was found to successfully describe the crystal growth behavior.

Application of the simultaneous measurement system of WAXD, SAXS and transmission FTIR spectra to the study of structural changes in the cold- and melt-crystallization processes of trans-1,4-polyisoprene

The newly developed system of simultaneous time-dependent measurements of wide-angle X-ray diffraction, small-angle X-ray scattering and transmission-type Fourier-transform infrared spectra has been applied to study the structural changes in the melt- and cold-crystallization processes of trans-1,4-polyisoprene (TPI). For the melt-isothermal crystallization process at 40 °C, the domains of relatively higher density with approximately 220 Å radius formed at first as the intermediate state, where the chain conformation was speculated to be considerably disordered. As time passed, the correlation length ξ between these domains decreased until they joined together to form the stacked lamellar structure. In these lamellae, the crystal lattices of the regular α form were created. When the isothermal crystallization was investigated at 30 °C, the intermediate-phase regularization caused the α- and β-crystalline forms to mix. In the cold-crystallization process or the heating process that started from the melt-quenched glass below −100 °C, TPI was found to crystallize at first to the β-form at approximately −55 °C, which transformed to the α-form at 50 °C via the amorphous phase as observed above. Simultaneous time-dependent measurements of WAXSD, SAXS and Fourier-transform infrared (FTIR) spectra have been performed to study the structural changes in the melt- and cold-crystallization processes of trans-1,4-polyisoprene (TPI). In the melt-isothermal crystallization process, the domains of relatively higher density were formed at first as the intermediate state, which approached each other to form the stacked lamellar structure. In the cold-crystallization process from melt-quenched glass, TPI crystallized to the β-form at −55 °C, which transformed to the α-form at 50 °C via the amorphous phase.

Polymorphism of Racemic Poly(l-lactide)/Poly(d-lactide) Blend: Effect of Melt and Cold Crystallization

The crystallization and melting behaviors and crystalline structure of melt and cold crystallized poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blend were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD), respectively. The isothermal crystallization kinetics during the melt and cold crystallization process were analyzed using the Avrami equation. The overall crystallization rate constant (k) of cold crystallization is much higher than that of melt crystallization. Moreover, k as a function of crystallization temperature shows different trends in melt and cold crystallization, indicating different crystallization mechanisms in the melt and cold crystallization. The polymorphic crystallization of homocrystallites (the transition crystallization temperature from δ to α form) is not altered by either the equimolar blending of PLLA and PDLA or the type of crystallization procedures, while the crystallization window for exclusive stereocomplex crystallites is widened from 170 °C for melt crystallization to 170-200 °C for cold crystallization. The stereocomplex crystallites are hard to form in both melt and cold crystallization at crystallization temperatures of 90 and 100 °C, and the crystallinity of stereocomplex crystallites for cold crystallization is higher than that of melt crystallization at temperatures above 110 °C. Especially, a pure and significantly higher crystallinity of stereocomplex crystallites can be achieved at 170-200 °C by cold crystallization. The results provide a huge possibility to control stereocomplex crystallization to enlarge its applications.

Structural Differences between Cold- and Melt-Crystallized Poly(trimethylene terephthalate) Samples

Time-resolved Fourier transform infrared spectroscopy was used to investigate the structural differences between cold- and melt-crystallized poly(trimethylene terephthalate) (PTT) samples. To elucidate the arrangement of different chain segments in the crystalline phase obtained through different crystallization processes, characteristic bands associated with the phenylene rings, the CH2 segments, and the C-O-C segments were followed during the cold and the melt crystallizations. The results show that the alignment of the phenylene rings and the C=O groups in the PTT crystals produced via cold- and melt-crystallization processes are essentially the same, while the alignment of the CH2 segments depends on the crystallization condition. It was found that the CH2 segments in the melt-crystallized sample were more stable than those in the cold-crystallized sample. This was ascribed to the different crystallization environment. Melt-crystallized PTT chains have higher mobility, and the variation of flexible segments is quicker than that of the rigid segments. However, in cold crystallization, the CH2 segments are restricted by the adjacent environment in the glass state, which weakens their capability of transforming their conformation; this leads to a cooperative change of CH2 segments, phenylene rings, and C=O groups. Therefore, the stability of CH2 segments in crystalline structure is lowered accordingly.

Separation of CsCl from a ternary CsCl–LiCl–KCl salt via a melt crystallization technique for pyroprocessing waste minimization

A parametric study has been conducted to identify the effects of several parameters on the separation of CsCl from molten LiCl–KCl salt via a melt crystallization process. A reverse vertical Bridgman technique was used to grow the salt ingots. The investigated parameters were: (1) the advancement rate, (2) the crucible lid configuration, (3) the amount of salt mixture, (4) the initial composition of CsCl, and (5) the temperature difference between the high and low furnace zones. From each grown salt ingot, samples were taken axially and analyzed using inductively coupled plasma mass spectrometry. Results show that CsCl concentrations at the top of the ingots were low and increased to a maximum at the bottom of the salt. Salt (LiCl–KCl) recycle percentages for the experiments ranged from 50% to 75% and the CsCl composition in the waste salt was low. To increase the recycle percentage and the concentration of CsCl in the waste form, multiple crystallization stages were explored showing that they were practical under the optimal experimental conditions at 5.0mm/h rate with a lid configuration and temperature difference of 200°C for a total of five crystallization stages. Up to 88% of the LiCl–KCl salt can be recycled under these proposed conditions.

Effect of polyhedral oligomeric silsesquioxane (POSS) reinforced polypropylene (PP) nanocomposite on the microstructure and isothermal crystallization kinetics of polyoxymethylene (POM)

In this study, effects of small amount of methyl-polyhedral oligomeric silsesquioxanes (methyl-POSS) on the microstructure and isothermal melt-crystallization behavior of polyoxymethylene (POM) were investigated, in detail. Introducing of methyl-POSS particles in POM phase was achieved via melt blending of methyl-POSS reinforced isotactic polypropylene (i-PP) nanocomposite as POSS carrier material with POM in a twin screw co-rotating extruder. Microstructural features of the POM/PP–POSS compounds were investigated with scanning electron microscopy (SEM) analysis. SEM analysis showed that the POM/PP–POSS compounds exhibited immiscible blend morphology. The POM, continuous matrix, phase includes a significant number of POSS particles due to interfacial interactions between the Si–O bonds of POSS and C–O bonds of POM, and resulted POSS migration from PP to POM phase during the melt processing. The kinetic parameters for the isothermal melt-crystallization process of the samples were determined with the Avrami and Lauritzen–Hoffman models. The crystallization activation energies were determined by the Arrhenius method. It was found that the PP–POSS nanocomposite significantly accelerated the isothermal crystallization rate of POM. Based on the results, it has been highlighted that POM compounds including a small amount of PP–POSS nanocomposite as POSS carrier material can be successfully used in the production of injection molded POM parts because the POM/PP–POSS compounds yield much faster molding cycle thus production rate than the POM.

Thermostimulated crystallization of polylactide stereocomplex

A particular crystallization behavior of PLA stereocomplex was observed when the poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA) (50/50) was subjected to specific melting conditions. The effect of the holding temperature in the melt state of PLLA/PDLA samples on the nonisothermal melt crystallization process and on the structure have been investigated by means of DSC, and wide angle X-ray diffraction. In the moderate melting condition, residual ordered domains act as athermal nuclei and a single crystallization process via a predetermined nucleation mechanism was observed. The survived nuclei favor high temperature polymer crystallization, correspondingly, a high melting temperature (TmH) of PLA stereocomplex, of 244.9°C with the fusion enthalpy 87.0J/g were obtained. Such a Tm result of PLA stereocomplex is much higher than that of the reported previously.

Phosphoric acid purification by suspension melt crystallization: Parametric study of the crystallization and sweating steps

AbstractIn order to purify phosphoric acid, the suspension melt crystallization process was studied. The suspension crystallization experiments were carried out with 80, 84 and 88 wt% phosphoric acid melt at the cooling rates of 0.05, 0.1 and 0.2 K/min, respectively. Sweating experiments were executed for various crystals obtained in suspension crystallization step. The purification effects of the sweating parameters including sweating time, initial inclusion amount and initial impurity content were studied. The inclusion fraction increases with the increase in cooling rate. The inclusion fraction of the crystals which were formed with feed concentration of 84 wt% phosphoric acid melt is lowest among the three feed concentrations. Different impurities have different purification performances during sweating. High inclusion amount and low impurity concentration favor the purification of H3PO4·0.5H2O crystals during sweating.

Computer Analysis of Some Thermodynamic Properties of Platinum Nanoclusters by Melting-Crystallization Processes

Computer Analysis of Some Thermodynamic Properties of Platinum Nanoclusters by Melting-Crystallization Processes

Long‐Term Stability, Regeneration and Recycling of Imidazolium‐based Ionic Liquids

AbstractIonic liquids (ILs) are discussed in many current research papers extensively in terms of their potential use in the chemical industry, as process aids and novel materials. The long‐term stability of the IL is for industrial applications as important as to know which species arise during the degradation due to thermal, mechanical, chemical or electrochemical stress. The investigation of the long‐term stability of two selected ILs over several months under process‐like conditions is presented with a subsequent analysis by LC‐MS to identify the resulting decomposition products. Knowledge about the occurring species and their analytical quantification are basis for the selection of appropriate processes for the separation of the decomposition products and the development of recycling processes for ILs. Particularly melt crystallization processes are suitable for separating structurally similar decomposition products that typically occur in the IL degradation.

The Development of Modulated, Quasi-Isothermal and Ultraslow Thermal Methods as a Means of Characterizing the α to γ Indomethacin Polymorphic Transformation

While polymorphism remains a key issue within the pharmaceutical and related industries, the understanding of the transformation process itself remains relatively poorly understood. In this study we use a combination of conventional and modulated temperature differential scanning calorimetry (MTDSC), quasi-isothermal MTDSC (Qi-MTDSC) and ultraslow heating rate MTDSC as a novel means of investigating the temperature-induced α to γ transformation in indomethacin, using hot stage microscopy and variable temperature attenuated total reflectance FTIR spectroscopy as supportive techniques. In particular, we utilize the ability of MTDSC to measure subtle heat capacity changes through the transformation, we examine the use of Lissajous analysis of the modulated heating signal itself (both scanning and quasi-isothermal) and finally we investigate the use of ultraslow heating rates (down to 0.04 °C/min) so as to facilitate examination of the melt-crystallization process at a scanning rate whereby kinetic hindrance becomes negligible. Indomethacin was prepared in the metastable α and stable γ forms using standard approaches. Samples were studied using conventional DSC, Qi-MTDSC (involving holding and modulating the sample at a series of incremental temperature steps) and ultraslow MTDSC. All studies were conducted using a Q-1000 MTDSC using crimped pans, following standard calibration procedures. Conventional DSC at 10 °C/min showed the expected single melting responses for the α and γ forms, while MTDSC at slower rates indicated the presence of a melt-crystallization process. Quasi-isothermal studies allowed the heat capacity to be estimated as a function of time, while the associated Lissajous analysis demonstrated distortion of the elliptical response as a result of the kinetic events involved. Ultraslow heating resulted in superimposition of the melting and crystallization processes, resulting in a discrete thermal event that was enthalpically equivalent to the difference between the two processes. It is suggested that these combined thermal methods allow the conversion to be profiled in a manner which facilitates both kinetic and thermodynamic analysis of the transformation.

Isothermal Crystallization Kinetics and Melting Behavior of a Luminescent Conjugated Polymer, Poly(9,9-Dihexylfluorene-Alt-2,5-Didodecyloxybenzene)

A study of the isothermal crystallization behaviors of poly(9,9-dihexylfluorene-alt-2,5-didodecyloxybenzene) (PF6OC12) was carried out using differential scanning calorimetry (DSC). The crystallization kinetics under isothermal conditions could be described by the Avrami equation. The Avrami exponent n ranges from 3.43 to 3.71 for PF6OC12 at crystallization temperatures between 100.0°C and 90.0°C, indicating a three-dimensional spherical crystal growth with homogeneous nucleation in the primary crystallization stage for the isothermal melt crystallization process. In the DSC scan, after the isothermal crystallization, multiple melting behavior was found. The multiple endotherms could be attributed to melting of recrystallized materials produced originally during different crystallization processes. According to the Arrhenius equation, the activation energy was determined to be 211.29 kJmol−1 for the isothermal melt crystallization of PF6OC12.