Article pubs.acs.org/IECR
Effect of Crystallization Conditions on Spherulite Size and Melt Spinning Performance of Polyester Industrial Yarn by Solid-State Polymerization Bum-Seok Kim†,‡ and Hee-Woo Rhee*,† †
Department of Chemical and Biomolecular Engineering, Sogang University, 35 Baekbeom-ro, Mapogu, Seoul, 121-742, Korea Technical Yarn Team, Textile Research Group, R&D Business Laboratory, Hyosung Corporation, 74, Simin-daero, Dongan-gu, Anyang, 431-080, Korea
Downloaded by CENTRAL MICHIGAN UNIV on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 8, 2015 | doi: 10.1021/acs.iecr.5b01924
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ABSTRACT: Polyester industrial yarn has high tenacity by increasing the degree of orientation and crystallinity via the melt spinning of high-molecular-weight (high-Mw) polymer. Solid-state polymerization (SSP) is very important to get higher Mw; it is composed of a crystallization step, which prevents sticking among the chips, and a polycondensation step at solid state. Spherulites in the chip were observed using polarized optical microscopy (POM) in the crystallization step. We studied the effect of crystallization conditions on spherulite size by SSP and determined that the higher the crystallization temperature, the larger the size and deviation of the spherulite. We also observed relatively uniform sizes in the batch process but various sizes in the continuous process. The latter difference in the size and shape of the spherulite would cause differences in the diffusivity of both the end-group and the byproduct, thus resulting in large deviations in Mw and viscosity. Thus, when the crystallization temperature in the continuous process was reduced by 15 °C to decrease the differences of thermal and physical impact history among chips, the sizes and shapes of spherulites consequently became uniform and the difference of Mw decreased because of the reduced crystallization rate difference among chips by decreased temperature deviation in the crystallization reactor. We could get more uniform polymer with reduced variations of Mw, intrinsic viscosity (IV), and melt viscosity (MV). Finally, the max draw ratio, which represents the drawability in melt spinning, increased from 6.28 to 6.71. It indicated that fluffs in yarn could be reduced, and downstream processabilities such as warping and weaving could be enhanced by increased drawability.
1. INTRODUCTION Polyester (PET) industrial yarn is used in a wide variety of applications such as seat belts, airbags, tire cord, broad-woven fabrics, sewing thread, and conveyor belts,1 and it is manufactured through a series of melt polymerization, solid state polymerization, and melt spinning processes. It has high tenacity by increasing the degree of orientation and crystallinity via the melt spinning of high-molecular-weight polymer. Such properties require multistep drawing and heat treatment at high temperature. High intrinsic viscosity (IV) can be achieved through the solid-state polymerization (SSP) process. The purpose of the SSP process is to increase low IV values of polymer chips from 0.6 to 0.8−1.2.2−5 The polycondensation reaction in the melt state occurs above the melting temperature (Tm). This means that the activation energy for diffusion is very low; thus, the end-group does not need more energy for diffusion, quickening the reaction rate.6,7 Meanwhile, the SSP reaction begins at below Tm, and the crystallinity increases during the SSP process (diffusion of the end-group and byproduct takes longer, hence, the longer reaction time).6,7 Figure 1 shows four steps of SSP reaction: diffusion of end-group, chemical reaction invoked by end-groups, transport of byproduct by diffusion, and transport of byproduct by convection.8,33 The main factors that influence the reaction rate of SSP include reaction temperature,3,4,8−10 initial end-group concentration,3,9,11−17 chip size, transport rate of byproduct (gas flow rate),3−5,8,12,15,16,18−21 crystallinity,3,8,15,17,19,22−26 and so on. © XXXX American Chemical Society
Figure 1. Solid-state polymerization reaction: (a) diffusion of endgroup, (b) chemical reaction, (c) transport of byproduct by diffusion, and (d) transport of byproduct by convection.
SSP has two different methods: batch processes and continuous processes.7,34−37 Batch processes can easily adjust temperature (between 110 °C and 150 °C) and the time Received: May 28, 2015 Revised: August 11, 2015 Accepted: August 24, 2015
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DOI: 10.1021/acs.iecr.5b01924 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Downloaded by CENTRAL MICHIGAN UNIV on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 8, 2015 | doi: 10.1021/acs.iecr.5b01924
Industrial & Engineering Chemistry Research
Earlier studies have not done research on spherulite growth inside of the chip during cold crystallization. However, we used polarized optical microscopy (POM) to identify spherulites inside the chip directly for the first time. The intent of this paper is to study how crystallization conditions in SSP affect the size and shape of spherulites inside the chip, according to physical impact and thermal history, via different SSP methods (laboratory, batch, and continuous processes), and attempt to determine the correlations between the size and shape of the spherulite and the growth of molecular weight (viscosity). The latter is closely related to the drawability in the melt spinning process. Finally, we focused on the effect of crystallization conditions on spherulite size and drawability stands for melt spinning performance.
needed for crystallization, but it results in high manufacturing cost. On the other hand, the continuous process is a cost-saving method with high productivity, which is now widely used by the majority of yarn manufacturers. This process has two crystallization reactors and goes through crystallization at a relatively high temperature (between 170 °C and 220 °C) for higher productivity in general. Different physical impact and thermal history in each process seem to be the cause of differentiation in the size and shape of the spherulite. The SSP process consists of a crystallization step and a polycondensation step. The crystal created on surface of the PET chip prevents agglomeration, which is known to require more than 40% crystallinity.15 Crystallinity is directly related to the reaction rate of SSP, because it has correlation with the control of diffusion of end-group and byproduct.3,6,15,17,23−27 Zimmerman suggested a “two-phase model”, according to which end-groups and low-Mw substances excluded from crystallization are in amorphous region where polycondensation occurs.6,7,24 In terms of chemical reaction, an increased crystallinity gathers end-groups in the amorphous region, and thus increases the reaction rate. Most of the end-groups in the crystallized region are largely inactive, while those in the amorphous area are active. On the other hand, from the perspective of mass transfer, crystallinity escalates as crystallization and polycondensation progresses, reducing chain mobility, hindering diffusion of low-Mw byproduct (ethylene glycol (EG), acetaldehyde (AA), water, etc.).22,25 Therefore, the diffusivity decreases as the crystallinity advances. For PET, when the transport of byproduct is improperly carried out meaning low diffusion ratereverse reaction causes depolymerization to occur and then the reaction rate slows.28−31 This shows that transport of the byproduct is crucial to the reaction rate. For removal of the SSP byproduct, N2 gas is used in the continuous process. Chang et al. studied the number-average molecular weight (Mn) at different temperatures over time, and they found that the higher crystallinity, the lower the increase in Mn.32 However, if the crystallization conditions in SSP decide the spherulite size of the chip, there may be some differences in the diffusivity of the end-group and byproduct. These differences in diffusivity may differentiate molecular weight (Mn, Mw) and intrinsic viscosity (IV) of the chip, causing deviation in the melt viscosity (MV) in the melt spinning process. In other words, if the size and shape of the spherulite made in the crystallization step are different, the growth of the molecular weight and the MV of the SSP chip will vary. Variation of the MV seems to harm melt spinning performance (drawability), which is important for the manufacture of the PET industrial yarn. Therefore, chips with irregular MV can create defects, such as broken monofilaments, that harm downstream processability in the weaving process for the main applications of the industrial yarn, such as seat belt, broad woven fabric, and others.1 Irregularity in the molecular weight and viscosity in the SSP chip can increase the difference in the degree of orientation of monofilament of undrawn yarn in the melt spinning process. Potentially, this uneven quality could cause more frequent monofilament breakages at a high draw ratio and a high-temperature heat treatment. Meanwhile, the low-IV chip for textile yarn has uniform molecular weight and viscosity, because the yarn-producing process does not involve SSP; thus, the correlation between the drawability of the yarn and the uniformity of Mw and IV of the chip is less important.
2. EXPERIMENTAL SECTION 2.1. Solid-State Polymerization (SSP). 2.1.1. Polymer Chip and Reagent. A prepolymer chip (PET) with IV = 0.63 dL/g was prepared by Hyosung Corporation. A solution of solvent 1,1,2,2-tetrachloroethane (Duksan Pure Chemical Co., Korea) and phenol (Duksan Pure Chemical Co., Korea) was used to measure the viscosity. All reagents were first-grade without purification. Spin finish used in the melt spinning process was a 25% solvent of TN-2004 for jet guide oiling from ICEI Woobang Corp. 2.1.2. Crystallization Conditions in SSP. Thirty grams (30 g) of prepolymer chip was allowed to sit in glass tubes in a nitrogen atmosphere (laboratory test), at different temperatures of 90, 110, 130, and 160 °C for 3 h before sampling. No physical impact was involved. In the batch process at pilot scale, 100 kg of prepolymer chip was placed in 1-ton-scale SSP equipment, to tumble the chip at a speed of 2 rpm for 3 h to ensure that physical impact was applied. In the continuous process at production scale, crystallization proceeded at various temperatures and times in the first and second crystallization reactors. 2.1.3. SSP. In the batch process at pilot scale, SSP was allowed as the temperature increased to 237 °C after crystallization. In the continuous process at production scale, SSP was processed in the hopper reactor at 220 °C. The length of time was set by IV = 1.05 dL/g. Figures 2 and 3 clarify the details. 2.2. Melt Spinning. 2.2.1. Drying. SSP chips were dried and tumbled for 24 h at 130 °C under vacuum to remove moisture. By doing so, the chip’s surface would have
