The self-wiping co-rotating twin-screw extruder as a polymerization reactor for methacrylates

The self-wiping co-rotating twin-screw extruder was studied as a reactor for two polymerizations in bulk: the homopolymerization of n-butylmethacrylate and the copolymerization of n-butylmethacrylate with 2-hydroxypropylmethacrylate. The influence of the extrusion parameters on the product was analyzed. With both reactions, conversions up to 95% were obtained. Nevertheless, a significant difference was observed in the working domain of both polymerizations, in which a stable reactive extrusion process could be attained wherein the discharge rate is constant and equals the feed rate. In the case of the relatively fast copolymerization, both the throughput and the screw rotation rate could be raised without endangering the stability of the process. This was not the case for the homopolymerization studied. It was determined that the stability of the process depends on the reaction velocity and the product viscosity. Within the boundaries of the working domain, the molecular weight could be influenced by adjustments of the extrusion parameters.     

用同向双螺杆挤出机制备液体脱硫橡胶

采用双螺杆挤出机将废胶粉进行连续脱硫以制备溶胶含量较高的脱硫橡胶,脱硫橡胶可呈流体状态。影响脱硫效果的工艺因素主要有螺杆转速、机筒温度、废胶粉的粒径以及添加剂的用量。对脱硫橡胶进行了溶胶含量、门尼黏度、凝胶渗透色谱及差示扫描量热等的测试分析,结果表明,提高螺杆转速会降低橡胶的溶胶含量,升高机筒温度可以有效提高橡胶的溶胶含量,胶粉粒径越大制得的脱硫橡胶的溶胶含量越低,增加脱硫剂的用量可以提高橡胶的溶胶含量。通过温度、螺杆转速和添加剂用量的选择可以得到脱硫程度不同的脱硫橡胶。     

Melt-mixing by novel pitched-tip kneading disks in a co-rotating twin-screw extruder

Melt-mixing in twin-screw extruders is a key process in the development of polymer composites. Quantifying the mixing performance of kneading elements based on their internal physical processes is a challenging problem. We discuss melt-mixing by novel kneading elements called “pitched-tip kneading disk (ptKD)”. The disk-stagger angle and tip angle are the main geometric parameters of the ptKDs. We investigated four typical arrangements of the ptKDs, which are forward and backward disk-staggers combined with forward and backward tips. Numerical simulations under a certain feed rate and screw revolution speed were performed, and the mixing process was investigated using Lagrangian statistics. It was found that the four types had different mixing characteristics, and their mixing processes were explained by the coupling effect of drag flow with the disk staggering and pitched-tip and pressure flows, which are controlled by operational conditions. The use of a pitched-tip effectively controls the balance of the pressurization and mixing ability.     

Measurement of three-dimensional velocity field in the translational region of a co-rotating twin-screw extruder

The velocity field in the screw channels of a co-rotating twin-screw extruder was measured using laser Doppler anemometry. Velocity distributions were measured for two screw elements having pitches of 14 and 28 mm, respectively. The magnitude of radial velocity component for both elements was no more than 10% of the magnitude of total velocity. The radial and the axial velocity components were similar for both screw elements. Wider range of tangential velocity values and steeper gradients near the flights were observed for smaller pitch screw element.     

Morphology development of in situ compatibilized semicrystalline polymer blends in a co-rotating twin-screw extruder

This paper concerns the morphology development of in situ compatibilized semicrystalline polymer blends in a co-rotating, intermeshing twin-screw extruder, using polypropylene (PP) and polyamide 6 (PA-6) blends as model systems. The morphology of in situ compatibilized blends develops much faster that of mechanical ones. The size of the dispersed phase (PA-6) undergoes a 10⁴ fold reduction from a few millimeters to sub-micron during its phase transition from solid pellets to a viscoelastic fluid. The final morphology is reached as soon as the phase transition is completed, which usually requires only a small fraction of the screw length in a co-rotating twin screw extruder. Screw profiles and processing conditions (screw speed, throughput and barrel temperature) control the PA-6's melting location and/or rate, but do not have significant impact on the ultimate morphology and mechanical properties of in situ compatibilized blends. The finding that morphology of PP/PA-6 reactive blend develops rapidly makes it possible to produce compatibilized PP/PA-6 blends by the so-called one-step reactive extrusion. It integrates the traditionally separated free radical grafting of maleic anhydride onto PP and the compatibilization of PP/PA-6 into a single extrusion step.     

同向双螺杆挤出机的停留时间分布及填充度

引 言 双螺杆挤出机在高分子材料加工中已被广泛地应用于聚合物共混改性、反应挤出及高分子可控降解等各个方面[1].但先前对挤出过程研究较少,一般仅停留在"黑箱"型经验操作的层面,主要以定性的机械设计为主.     

Resin distribution along axial and circumferential directions of self-wiping co-rotating parallel twin-screw extruder

A self-wiping co-rotating twin-screw extruder (TSE) is operated in a starved state in which the screws are partially filled with resin. Understanding the resin distribution on the screw surface of a TSE in this state is essential for the design, operation, and maintenance of the twin-screw extrusion process. Accordingly, in this study, the circumferential and axial distribution of resin in a TSE were simulated using a novel method combining the mathematical formulation of Hele–Shaw flow, the finite element method, and a newly developed down-wind pressure updating scheme. The results of the simulation were found to be in good agreement with experimental measurements. The proposed simulation method enables the detailed visualization of resin distribution in the entire axial and circumferential directions over the length of a TSE, improving the ability to determine both the devolatilization and fiber attrition during the extrusion process.     

Computation of the morphological changes of a polymer blend along a twin-screw extruder

The control of the morphology of an immiscible polymer melt is of vital importance for the mastering of the final properties of the product. As polymer blends are produced using corotating twin-screw extruders, understanding and modeling the changes experienced by the blend during this process is of great interest. In the present study, starting from Ludovic software, developed for computing flow parameters in the twin-screw extrusion process, we present a computation of the droplet morphology development, based on the basic mechanisms of break-up and coalescence. Depending on the value of a local capillary number and on local flow conditions, different changes may occur: affine deformation, drop splitting, break-up by capillary instability, and coalescence. It is thus possible to follow, all along the screws, the changes in morphology, either for a single particle or for a particle distribution. Examples of these different computations are presented and compared with experimental results. Generally speaking, orders of magnitude of droplet size and tendencies when modifying processing conditions are correctly described, but the model still suffers from the absence of descrption of the melting process.     

Polymer blend mixing and dispersion in the kneading section of a twin-screw extruder

This paper examines the mechanisms by which a polymer is dispersed in a co-rotating twin-screw extruder. An experimental investigation of the morphological evolution has been carried out on a 45-mm co-rotating twin-screw extruder. Polyethylene/polystyrene (PE/PS) blends in the low concentration range (i.e., 5–15 wt% of PE) were used as a model system. The following general trends were observed. First, the minor phase right after melting is predominantly in a fibrillar form. Secondly, droplet and fiber diameter at this early stage of compounding are already in the micron or sub-micron range. Even though a wide variety of mixing section configurations were used, the fibers created in the early compounding stages were relatively stable throughout extrusion. Morphological evolution after melting must therefore be discussed in terms of variation in the fiber fraction (i.e., fiber to droplet transition) rather than in a change in particle diameter. A control volume model for the flow in kneading blocks is used to interpret the morphological results and to predict the deformation and breakup of dispersed phase fibers under shear and in absence of coalescence. Theoretical results indicate that fiber breakup under shear is not likely in the kneading block under the normal processing conditions, which is confirmed by morphological observations made at the mixing section exit. The influence of several geometrical parameters on mixing and pumping in kneading blocks is also discussed with the use of flow model results.     

新型双螺杆塑木挤出机

辛辛那提挤出公司新推出了一种牌号为Cincinnati Milacron TC96型锥形双螺杆挤出机,适合用于塑木复合材料的挤出加工,挤出加工时木屑纤维含量呆高达70%,产量最高可达到每小时2600磅,加工得到混有天然纤维,热塑性塑料（回收料或新料）及其它助剂（如成核剂、着色剂等）的均匀塑化熔体。     

A mass transfer model for a twin-screw extruder

A mass transfer model was developed to represent the desorption of a volatile species from a molten polymer in an extruder consisting of co-rotating twin screws in a figure-eight bore. The melt undergoes a series of alternating exposure and mixing processes while being transported axially through the device. The model relates the ratio of entrance to exit concentrations of the volatile species to the dimensions of the screws, the rate of their rotation, the rate of polymer flow, and the diffusion coefficient of the volatile species in the polymer. Experimental data, obtained with a halocarbon-polybutene system in a twin-screw extruder, were compared with predictions of the model. For the observed performance, the model predicts a process screw length about 14 percent below the actual screw length in the experimental extruder.     

Scale-up rules for mixing in a non-intermeshing twin-screw extruder

Various scale-up rules and theories have been presented for extrusion, including both single- and twin-screw extruders. Until now, however, most of these theories have concerned fully-filled channels, not only for twin screw extruders of the co-rotating fully intermeshing type (COTSE) or non-intermeshing counter-rotating type (NITSE), but single screw extruders as well. As the demand for distributive mixing and devolatilization devices increases, more and more nonintermeshing twin screw extruders with regions of partially-filled channels are being used. Therefore, developing scale-up rules for such screw extruders is imperative. In this paper, scale-up rules are developed, theoretically and experimentally, by examining the relationship between distributive mixing and important flow parameters. Two partially-filled NITSE's, with screw diameters of 0.8 and 2 inches, have been studied by using a flow visualization technique entailing a dye tracer to study the effects of distributive mixing by varying such parameters as: percentage of drag flow, screw stagger, and screw velocity. Qualitative evaluation of the spread of the dye with the number of screw revolutions was obtained from videotape of the experiments. Factorial experimental design method has been applied for evaluating these results. Finally, new scale-up rules were developed and compared with rules in the literature.     

Modeling melt conveying and power consumption of co-rotating twin-screw extruder kneading blocks: Part B. Prediction models

Intermeshing co-rotating twin-screw extruders are very versatile because their screw configurations can be tailored both to the application and to the properties of the materials used. Finding the best screw configuration is one of the main purposes of twin-screw extrusion modeling, and requires models that accurately predict conveying and power consumption. The better the process can be predicted, the better the requirements of the final product can be met. We present novel prediction models of the conveying and power-consumption behaviors of intermeshing co-rotating twin-screw extruder kneading blocks for Newtonian fluids. These are based on numerical simulations and therefore consider the complex three dimensional (3D) geometry of this element type without the need for common simplifications. Our models are thus capable of including all leakage flows and gap influences, which are usually ignored, for example, by the flat-plate model. Since our models are derived by symbolic regression based on genetic programming, they consist of algebraic functions and are low-threshold. They can be used to calculate various process parameters for individual kneading blocks or entire screw configurations, as illustrated by a use case.     

Development of polymer blend morphology during compounding in a twin-screw extruder. Part II: Theoretical derivations

In Part II of the work, the intermeshing twin-screw extruder is briefly described and the theoretical procedures used to model its operation are summarized. Based on the microrheological considerations discussed in Part I, a predictive procedure of the morphology evolution during compounding of two immiscible polymers is proposed. In this first generation model, only the shear flow effects are considered. Furthermore, to avoid complications due to coalescence a low concentration of the dispersed phase was assumed. In the procedure, two drop breakup mechanisms are discussed. The first assumes that the drops do not break under flow while the second postulates that breakup occurs under flow. Two dispersion mechanisms are considered, the first postulating continuously increasing polydispersity of drop size and the second postulating that drop polydispersity is inversely proportional to deformation strain. The influence of the screw configuration and operating conditions on blend morphology evolution is studied. It is expected that the computed drop size distribution provides limiting values for the experimental data. Dependency of predicted morphology on operating conditions is also investigated. Increasing screw rotating speed (resulting in increasing energy consumption) and decreasing throughput (resulting in decreasing productivity) lead to prediction of finer drop size. In practice, therefore, a compromise would be required. The proposed procedure is limited to melt flow (excluding the die region) within the region of large capillary parameter values, k > 4k_crit.     

Transport in a twin-screw extruder for the processing of polymers

The fluid flow and heat transfer in polymer extrusion in a twin-screw extruder was studied numerically by using the finite volume method. In the mathematical model, the coordinate system is fixed to the screw so that it is held stationary and the barrel is moved to simplify the complicated geometry. The screw channel of a twin-screw extruder is approximated as two regions: translation and intermeshing. The flow in the translation region is similar to that in a shallow single screw extruder and is treated by the numerical methods given in the literature. In the nip or intermeshing region, strong mixing effects are expected, along with the diffusion of energy and momentum. The full governing equations are solved in this region to determine the velocity components in all the three coordinate directions. The energy equation is coupled with the equations of motion through viscosity, since the viscosity of the polymeric, non-Newtonian, fluids considered here is dependent upon the shear rate and temperature. There is no clear physical demarcation between the nip region and the translation region. Therefore, a domain matching was employed at an arbitrary location that was varied numerically to ensure that the results were independent of this location. The variation of pressure and bulk temperature along the helical channel of the twin-screw extruder is obtained, along with the shear rate. An experimental investigation of the velocity profiles in the translation region of a self-wiping twin-screw extruder, which is often used in practical applications, was carried out using a Laser Doppler Anemometer. The numerically predicted velocity profiles are compared with those from the experiments, yielding fairly close agreement.     

Residence time distribution in a commercial twin-screw extruder

The residence time distribution of poly(vinyl chloride) (PVC) polymers in a counterrotating twin screw commercial extruder was determined and analyzed. The experimental technique involved the use of manganese dioxide as a tracer after being neutron activated and was injected into the extruder during normal operation without interrupting the poly(vinyl chloride) compound production. The experimental results enabled us to better understand the flow and mixing conditions in the extruder.     

Influence of a compatibilizing agent on the phase morphology of a polyethylene-polyamide 6 blend in a modular intermeshing co-rotating twin screw extruder

A study of the influence of a compatibilizing agent on phase morphology development in a 75/25 polyethylene/polyamide-6 blend in a modular co-rotating twin screw extruder is presented. The development of phase morphology along the axis of the modular screw was observed by cooling the extruder and removing the polymer from the screw channels. Changes in phase morphology due to the addition of a compatibilizer have been investigated using scanning electron microscopy. Sufficient quantities of compatibilizing agent produce significant increases in the rate of mixing and also reduce the scale of the phase morphology. Much larger quantities (5%) than actually required for interface coverage are needed for rapid mixing. This seems to be due to the high viscosity of the matrix.     

The copolymerization of methacrylates in a counter-rotating twin-screw extruder

The copolymerization of n-butylmethacrylate with 2-hydroxypropylmethacrylate was studied in a closely intermeshing counterrotating twin-screw extruder. The average molecular weight of the product can be increased by increasing the screw rotation rate or the die resistance or by decreasing the throughput or the barrel temperature. The conversion can be improved by decreasing the throughput, increasing the die resistance, and (within limits) increasing the barrel temperature, as well as through post-initiation. Compared with various classical polymerization processes, this situation requires that particular attention be paid to the occurrence of a gel effect, the existence of a thermodynamic ceiling temperature, and the reactivity ratio of the monomers used.     

Development of polymer blend morphology during compounding in a twin-screw extruder. Part III: Experimental procedure and preliminary results

Currently, selection of screw configurations as well as the operating conditions for compounding polymer blends with desired morphology in a co-rotating twinscrew extruder is an art based on experience. In this paper a quenching section of a twin-screw extruder is described. The section may replace any segment of the extruder barrel. It allows, on the one hand, a regular operation of the machine, and on the other, a rapid quenching and removal of blend specimens for morphology analysis from any place along the extruder barrel and at any time of the blending. The experimental observation of development during compounding of polymer blends enables verification and improvement of the theoretical model, aimed at predicting and controlling the size and polydispersity of the minor phase. Development of the predictive model for blend morphology will provide a valuable guide to the polymer processing industry. The preliminary data were collected using polystyrene/high density polyethylene (PS/HDPE) blends at low concentration of the dispersed phase, 5 wt% of either PS or HDPE. It was observed that the viscosity ratio, blend composition, screw configuration, temperature, throughput, and screw speed significantly influence the blend morphology.