Mechanistic analysis and computer simulation of impact breakage of agglomerates: Effect of surface energy

Agglomerates are ubiquitous as intermediate or manufactured products in chemical, pharmaceutical and food industries. During handling and processing they may suffer breakage if they are weak. On the other hand, if they are too strong, their dispersion and disintegration could be difficult. The control of their mechanical strength is therefore highly desirable. However, the analysis of agglomerate strength is complex due to the large number of parameters that influence agglomerate behaviour, such as the primary particle size, density and elastic modulus, and the interparticle bond strength.A simple mechanistic model is presented here which relates the number of broken contacts in agglomerate due to impact velocity, interparticle adhesion energy and the particle properties of the particles forming the agglomerate. The model is based on the hypothesis that the energy used to break contacts during impact is proportional to the incident kinetic energy of the agglomerate. The damage ratio defined as the ratio of broken contacts to the initial number of bonds is shown to depend on the dimensionless group, Δ, in the form (ρV²D^5/3E^2/3)/Γ^5/3, where V is the impact velocity, E the elastic modulus, D the particle diameter, ρ the particle density and Γ the interface energy. This dimensionless group, Δ, incorporates the Weber number, (ρDV²/Γ), which was previously shown to be influential in agglomerate breakage, and may be presented in the form, , where I_e=ED/Γ.The predicted dependency of the damage ratio on the surface energy has been tested using distinct element method (DEM). Four different agglomerates have been formed and impacted against a target for three different values of the surface energy of the primary particles. The simulation results show that the effect of surface energy is better described by the above mechanistic model than by the Weber number alone, as previously used to characterise the impact strength of agglomerates.     

Growth and breakup of a wet agglomerate in a dry gas–solid fluidized bed

Using CFD‐DEM simulations, a wet agglomerate of particles was placed in a void region of a dry vigorously fluidized bed to understand how wet agglomerates grow or breakup and how liquid spreads when agglomerates interact with dry fluidized particles. In the CFD‐DEM model, cohesive and viscous forces arising from liquid bridges between particles were modeled, as well as a finite rate of liquid bridge filling. The liquid properties were varied between different simulations to vary Bond number (surface tension forces/gravitational forces) and Capillary number (viscous forces/surface tension forces) in the system. Resulting agglomerate behavior was divided into regimes of (i) the agglomerate breaking up, (ii) the agglomerate retaining its initial form, but not growing, and (iii) the agglomerate retaining its initial form and growing. Regimes were mapped based on Bo and Ca. Implications of agglomerate behavior on spreading of liquid to initially dry particles were investigated. This article identifies a new way to map agglomerate growth and breakup behavior based on Bo and Ca. In modeling both liquid forces and a finite rate of liquid transfer, it identifies the complex influence viscosity has on agglomeration by strengthening liquid bridges while slowing their formation. Viewing Ca as the ratio of bridge formation time to particle collision and separation time capture why agglomerates with high Ca struggle to grow. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2520–2527, 2017     

Hybridization effect of basalt and carbon fibers on impact and flexural properties of phenolic composites

In this study, the impact and flexural properties of woven basalt fiber/phenolic (BFP), woven carbon fiber/phenolic (CFP) and woven basalt/woven carbon hybrid phenolic (BCFP) composites are investigated. The hybridization effect of woven basalt and woven carbon fibers on the impact energy absorption and flexural properties is investigated for various weight ratios of basalt/carbon hybrid fibers such as 1:0, 0.83:0.17, 0.68:0.32, 0.61:0.39, 0.34:0.66 and 0:1. It is found that the impact properties of the composites are strongly improved when the basalt fiber increased. Impact energy absorption of CFP composite showed a regular trend of increase with increasing weight ratio of basalt fiber in hybrid fiber composite. The lowest impact energy absorption values are found for the composites with weight ratio 0:1 (CFP), with average of 70 kJ/m². Corresponding values for energy absorptions are obtained for 0.83:0.17, 0.68:0.32, 0.61:0.39, 0.34:0.66 basalt/carbon weight ratio in hybrid composites. The impact energy absorption of hybrid composites (BCFP) shows the highest value with an average of 219 kJ/m², when the weight ratio of 0.83:0.17 is used. Finally, the impact energy absorption of BFP composites with the weight ratio of 1:0 shows the highest value of 268 kJ/m². The experimental evidence shows that the hybrid composites based on combinations of stiff carbon fibers and tough basalt fibers have good flexural properties and therefore, they can be used as promising materials in a number of engineering sectors such as the protective structures.     

A regime map for the normal surface impact of wet and dry agglomerates

The normal surface impacts of wet and dry agglomerates are simulated in a discrete element modeling framework. While the impact behavior of dry agglomerates has been addressed previously, similar studies on wet agglomerate impact are missing. By adding a small amount of liquid to a dry agglomerate, the impact behavior changes significantly. The impact behavior of the agglomerates at different moisture contents and impact energies are analyzed through postimpact parameters and coupled to their microscopic and macroscopic properties. While increasing the impact energy breaks more interparticle bonds and intensifies damage and fragmentation, increasing the moisture content is found to provide the agglomerates with higher deformability and resistance against breakage. It is shown that the interplay of the two latter parameters together with the agglomerate structural strength creates various impact scenarios, which are classified into different regimes and addressed with a regime map. © 2018 American Institute of Chemical Engineers AIChE J, 64: 1975–1985, 2018     

The physicochemical parameters determining the size of agglomerate prepared by the wet spherical agglomeration technique

Lactose samples of five different particle sizes from 31 to 261 μm dispersed in chloroform were agglomerated with a small amount of saturated aqueous lactose solution which acted as a bridging liquid to wet the particles preferentially. The effects of raw particle size and the amount of bridging liquid on the average size of resultant agglomerate were investigated.

The agglomerate size increased with decreasing size of lactose. This effect was enhanced by increasing the amount of bridging liquid for lactose less than 79 μm. A linear correlation on a log—log plot was observed between the agglomerate size and the saturation ratio of bridging liquid. The slope increased with decreasing particle size of lactose. The size distribution of agglomerates was also determined for the particle size of lactose and the amount of bridging liquid used. The physicochemical properties of the bridging liquid, i.e. contact angle and interfacial tension, were also taken into account for interpreting the agglomerate size. The correlation between the agglomerate size and the above parameters was represented quantitatively by eqn. (10) in the text. The parameter n, which varies directly with agglomerate size, increased with increasing saturation ratio, or with decreasing lactose particle size.     

Mechanism for clogging of microchannels by small particles with liquid cohesion

We numerically investigate the effect of liquid cohesion on the clogging of microchannels induced by small wet particles. The computer simulation is performed by the discrete element method (DEM) with cohesive contact models in presence of pendular liquid bridges, which is embedded into the computational fluid dynamics (CFD). We find that liquid cohesion significantly promotes particle deposition and agglomerate growth. A clogging phase diagram, in the form of Weber number and Stokes number, is constructed to quantify the clogging-nonclogging transition. The competition between particle–particle and particle–fluid interactions is quantitatively discussed in terms of particle velocity and slip velocity. Strong cohesion can address a greater slip velocity or drag between particles and fluid, which depresses the resuspension of deposited particles and results in clogging. Finally, we compare our results with clogging induced by van der Waals adhesion of small dry particles and find that the competence of liquid cohesion is more prominent.     

Effect of the impact angle on the breakage of agglomerates: a numerical study using DEM

The study of agglomerate strength is of vital importance in several industrial applications such as pharmaceutical, detergent and food manufacturing. Agglomerates could experience a size reduction during the production and handling processes due to collisions with other agglomerates or with the moving components and walls as well as during bulk flow due to shear deformation. In this analysis, we focus on the agglomerate damage due to oblique impact on walls, as this is a common damage process during, for example, pneumatic conveying and size reduction in pin mills.

Computer simulations have been carried out using Distinct Element Analysis, where the breakage characteristics of oblique impacts and the effect of the interparticle bond strength have been analysed. The procedure adopted here provides an isotropic and spherical agglomerate (uniform mass distribution and coordination number within radial segments of the agglomerate). The results indicate that the damage ratio (i.e. the number fraction of the broken bonds) depends on the normal component of the impact velocity only, i.e. the tangential component has little effect. However, the position of the clusters produced on impact does depend on the impact angle, which influences the pattern of breakage and in turn the size distribution of the large clusters.     

Numerical modelling of the breakage of loose agglomerates of fine particles

This paper presents a numerical study of the breakage of loose agglomerates based on the discrete element method. Agglomerates of fine mannitol particles were impacted with a target wall at different velocities and angles. It was observed that the agglomerates on impact experienced large plastic deformation before disintegrating into small fragments. The velocity field of the agglomerates showed a clear shear zone during the impacts. The final breakage pattern was characterised by the damage ratio of agglomerates and the size distribution of fragments. While increasing impact velocity improves agglomerate breakage, a 45-degree impact angle provides the maximum breakage for a given velocity. The analysis of impact energy exerted from the wall indicated that impact energy in both normal and tangential directions should be considered to characterise the effects of impact velocity and angle.     

Effect of structural characteristics on impact breakage of agglomerates

The mechanical properties and evolved structure of agglomerates depend strongly on the manufacturing method. There is a great interest in finding a simple way of establishing a rank order in their processing behaviour, e.g., the ease with which they could be dispersed in fluids. For this reason, the breakage propensity of two types of detergent agglomerates produced by different processes but with the same formulation has been evaluated under different conditions by impact testing with a view to diagnose differences in mechanical properties and structure arising from their manufacturing method. The effects of impact velocity, agglomerate size, impact angle, fatigue, humidity, and temperature have been analysed. Both samples show extensive plastic deformation due to the elongation and eventual rupture of the interparticle bridges, especially for the humidified samples. Reducing the temperature increases the extent of breakage substantially. The impact test results of samples kept at −20 °C show brittle failure mode, whilst those of oblique impacts at 45° and ambient conditions show a semi-brittle failure mode by shear deformation. Drying strengthens the agglomerates presumably due to the solidification of bridges. In contrast, humidifying the granules decreases their strength. A general comparison of the impact test results of both samples for different feed sizes shows that, due to the structural differences, the breakage trend of these two types of agglomerate varies with increasing agglomerate size.     

Impact breakage of spherical, cuboidal and cylindrical agglomerates

A numerical study of the micro-mechanics of breakage of agglomerates impacting with a target wall has been carried out using discrete element simulations. Three agglomerates of different shapes are examined, namely spherical, cuboidal and cylindrical. Each agglomerate consists of 10,000 polydisperse auto-adhesive elastic spheres with a normal size distribution. The effect of agglomerate shape and impact site on the damage of the agglomerates under an impact velocity of 1.0 m/s for an interface energy of 1.0 J/m² is reported. It is found from the simulations that cuboidal edge, cylindrical rim and cuboidal corner impacts generate less damage than spherical agglomerate impacts. The cuboidal face, cylindrical side and cylindrical end impacts fracture the agglomerates into several fragments. Detailed examinations of the evolutions of damage ratio, number of wall contacts and total wall force indicate that the size of the contact area and the rate of change of the contact area play important roles in agglomerate breakage behaviour. Internal damage to the agglomerate is closely related to the particle deceleration adjacent to the impact site. However, the local microstructure may not be a decisive factor in terms of the breakage mode for non-spherical agglomerates.     

Influence of fiber orientation and fiber content on properties of sisal‐jute‐glass fiber‐reinforced polyester composites

The incorporation of natural fibers with polymer matrix composites (PMCs) has increasing applications in many fields of engineering due to the growing concerns regarding the environmental impact and energy crisis. The objective of this work is to examine the effect of fiber orientation and fiber content on properties of sisal‐jute‐glass fiber‐reinforced polyester composites. In this experimental study, sisal‐jute‐glass fiber‐reinforced polyester composites are prepared with fiber orientations of 0° and 90° and fiber volume of sisal‐jute‐glass fibers are in the ratio of 40:0:60, 0:40:60, and 20:20:60 respectively, and the experiments were conducted. The results indicated that the hybrid composites had shown better performance and the fiber orientation and fiber content play major role in strength and water absorption properties. The morphological properties, internal structure, cracks, and fiber pull out of the fractured specimen during testing are also investigated by using scanning electron microscopy (SEM) analysis. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42968.     

Effect of size ratio on the behaviour of agglomerates embedded in a bed of particles subjected to shearing: DEM analysis

This paper presents the results of analysis of the deformation and breakage of spherical agglomerates embedded in a bed of particles and subjected to shearing, a situation commonly encountered in powder granulation. The study is based on three dimensional distinct element method (DEM), in which the inter-particle interactions are governed by theories of contact mechanics. An agglomerate was first generated in a bed of particles having the same size as the primary particles forming the agglomerate. Different size ratios (i.e., the ratio of the diameter of agglomerate to the diameter of surrounding particles) in the range 3-10 were then simulated by varying the size and number of surrounding particles. The agglomerates were subjected to shearing (shear rate and strain of about and 0.3, respectively) and their breakage characteristics were analysed. The agglomerate with the size ratio 10 does not break but undergoes some structural deformation by re-arrangements of contacts. However, the agglomerates with ratio about 7 or smaller suffer breakage. For the size ratio equal or smaller than 5, the agglomerate breaks significantly leading to full disintegration. The results of stress analysis of the agglomerates suggest that the resistance to breakage for the agglomerate with size ratio of 10 is due to the nature of stresses exerted on the agglomerate. For large size ratios the stress on the agglomerate is predominantly hydrostatic. The ratio of deviatoric stress over hydrostatic pressure increases as the size ratio of the agglomerate is reduced. The nature of stresses experienced by agglomerates with smaller size ratios is predominantly deviatoric, thus causing shear deformation and breakage. The results are compared with physical experiments and a satisfactory agreement is obtained.     

Discrete particle simulation of agglomerate impact coalescence

This paper describes computer simulations of pendular state wet agglomerates undergoing pair-wise collisions. The simulation method is based upon a ‘soft’ discrete particle formulation. Each agglomerate comprised 1000 primary particles with the interparticle interactions modelled as the combination of the solid–solid contact forces and also the forces developed at discrete liquid bridges between neighbouring particles. For the range of collisional velocities implemented, the agglomerates invariably coalesced. The energy dissipated was associated primarily with the viscous resistance of the fluid and the interparticle friction rather than by liquid bridge bond rupture. The structure of the resultant coalesced agglomerate was highly disordered and depended on the impact velocity. As the impact velocity approached zero, the agglomerates behaved like two rigid bodies bonded together. When the impact velocity was increased, the size of the circumscribing sphere of the coalesced agglomerate decreased and reached a minimum value at a critical velocity above which an increase in the circumscribing sphere size occurred due to extensive flattening. An increase in the viscosity of the interstitial fluid resulted in an increase in the proportion of energy dissipated by viscous resistance and a decrease in the proportion dissipated due to interparticle friction. An increase in the fluid viscosity also resulted in an increase in the critical impact velocity at which the size of the circumscribing sphere of the coalesced agglomerate was a minimum.     

Melt-spun liquid core fibers: physical and morphological characteristics

Application of different polymers and fillers in multi-component fibers has recently emerged as an effective approach in textile industries. Recent investigations have extensively demonstrated that hollow fibers can be melt-spun and subsequently filled with liquids; however, introduction of a liquid into a fiber core and filling it up with that liquid, specifically at extended lengths, remains challenging. In this study, based on the results previously obtained for the simulation and extrusion of polymer melt and liquid co-flowing, continuous production of the liquid core bi-component filaments via melt spinning through specially designed spinneret is discussed. In fact, core/shell bi-component filaments 50 μm in diameter consisting of polypropylene sheath and complex ester core were produced undergoing 1500 m/min continuous melt spinning with drawing ratio of 5. Physical properties of the developed fibers were investigated which were in acceptable condition with those of the reference solid and hollow fibers. Successful presence of a liquid in an eccentric channel 15 μm in diameter was demonstrated by microscopic observation. Furthermore, the ester oil was retained inside the fiber due to its low contact angle against polypropylene, thereby resolving the need for sealing the fiber’s outlet. Also, TGA and FTIR analysis confirmed the presence of liquid inside the bi-component fibers. DSC tests showed a similar crystallinity for liquid core and hollow fibers, which was about 37 %, while solid fiber had 5 % more crystallinity. Due to the vast available liquids and polymers with various properties, developed liquid core fibers will provide a suitable platform for a large number of applications in future.     

Effect of the compatibilizer content on the quasi‐static and low velocity impact responses of glass woven fabric/polypropylene composites

Glass woven fabric/polypropylene laminates have been studied given their outstanding performance/cost ratio. Their flexural properties, mainly influenced by the adhesion between matrix and reinforcing fibers, have been investigated for systems containing maleated polypropylene (PP‐g‐MA) amounts ranging from 0% to 10% by weight. Results have shown that the presence of the compatibilizer improves both flexural modulus and strength, achieving plateau values approximately for 5 and 2 wt% of PP‐g‐MA, respectively. On the contrary, an inverse proportion between the compatibilizer content and the energy dissipated at perforation emerged from low velocity impact tests. The different dependence can be related to the failure mechanisms occurring at the fiber/matrix interface. These mechanisms are able to dissipate large amounts of energy through friction phenomena, and are pronounced when the fiber/matrix adhesion is weak. Pull‐out of fibers from the matrix has been detected, in particular, in systems containing low contents of compatibilizer and evidenced by the morphological analysis of fracture surfaces after failure. The large amount of energy dissipation allowed by the relative motion of fibers and matrix occurred before fiber breakage, as confirmed by the evaluation of the laminates ductility index. POLYM. COMPOS., 37:2452–2459, 2016. © 2015 Society of Plastics Engineers     

球磨法制备玄武岩短纤维的工艺参数优化

介绍了球磨法制备玄武岩短纤维的过程,对球磨工艺参数进行了正交试验优化,并对纤维断裂机理进行了分析研究。研究结果显示,大球与纤维的直接撞击、磨球与纤维之间的剪切、小球与纤维的撞击与划擦是纤维断裂的主要因素。球磨工艺参数中对纤维短化效率影响的大小的因素依次是：球料比X2,纤维长度X1,大球个数X3,球磨转速X5,小球个数X4。     

新型碳纤维用原丝——高强高模Lyocell纤维纺丝工艺研究

采用天然高相对分子质量纤维素脱脂棉为原料 ,制备了高强高模纤维素纤维 ( L yocell纤维 ) ,并用此作为碳纤维原丝 ,成功制得了强度优于粘胶基碳纤维的 L yocell基碳纤维。考察了高相对分子质量纤维素的溶解特点 ,纺丝工艺对 L yocell纤维聚集态及性能的影响 ,比较了 L yocell纤维和粘胶原丝的表面及截面形态。实验表明 :高相对分子质量纤维素溶解的静溶胀时间和温度对其溶解有明显的影响 ;纺丝过程中 ,大的气隙长度对提高纤维的性能有利 ;随着凝固浴中 N -甲基吗啉 N -氧化物( NMMO )的浓度增加 ,纤维的强度和模量增加 ,当其在凝固浴中的质量分数达到 10 %时 ,强度模量最大 ,浓度继续增加 ,纤维的力学性能开始下降 ;拉伸比增加 ,L yocell纤维的强度模量增加 ,当拉伸比大于 3.0时 ,纤维的性能略有下降     

Effect of compatibilizers on mechanical properties and morphology of in-situ composite film of thermotropic liquid crystalline polymer/polypropylene

An in-situ composite film of a thermotropic liquid crystalline polymer (LC3000)/polypropylene (TLCP/PP) was produced using the extrusion cast film technique. The compatibilizing effect of thermoplastic elastomers, styrene-ethylene butylene-styrene (SEBS), maleic anhydride grafted SEBS (MA-SEBS), and maleic anhydride grafted polypropylene (MA-PP) on the mechanical properties and morphology of the TLCP/PP composite films was investigated. It was found that SEBS provided a higher value of tensile modulus than MA-SEBS, which in turn was higher than MA-PP, despite the expected stronger interaction between the MA chain and TLCP. The observation of the morphology under optical and scanning electron microscopes suggested that all three compatibilizers helped improve the dispersion of the TLCP fibers and increased the fiber aspect ratio to a different extent. The fractured surface of the specimens showed more fiber breakage than pull-out when a compatibilizer was added, which suggested the improvement of interfacial adhesion. The surface roughness of fibers with an added elastomeric compatibilizermay also provide mechanical interlocking at the interface. It is suggested that the increase in the viscosity ratio of TLCP/PP due to the added elastomeric compatibilizer, SEBS and MA-SEBS, compared with the thermoplastic compatibilizer, MA-PP, is more effective in improving the composite mechanical properties.     

无纺布用ES纤维的生产工艺探讨

讨论了皮芯型复合纤维 (ES纤维 )生产工艺的调整对生产无纺布的影响 ,就纤维的拉伸、卷曲、抱合性等方面进行探讨。选择熔程较宽的聚乙烯 ,有助于提高热轧无纺布的强力 ,降低拉伸速度 ,可提高纤维的卷曲率 ,第一级拉伸倍数选择 3.5～ 4.5为宜 ,纤维的卷曲效果及抱合性均对无纺布的生产有影响