Influence of Filler Content and Filler Size on the Curing Kinetics of an Epoxy Resin

In this research, the influences of filler content and filler particle size on the flow-hardening behavior were investigated by a measuring mixer. In order to more reliably assess the observed rheological behavior, isothermal differential scanning calorimetry (DSC) measurements were employed to study the curing kinetics of the compounds. The measured data can be fitted well with Kamal-Sourour’s model modified by the diffusion correlation according to Chern and Poehlein. After that, the influence of filler content and size on the kinetic parameters are presented discussed. The results show that the ultimate glass transition temperature is significantly lower for pure epoxy resin (EP) than for compounds filled with surface-treated glass beads, which have an essential effect on the diffusion-controlled reaction at different curing temperatures. For the surface-treated glass beads used in this study, the reaction speed in the early curing stage is accelerated by increasing filler content or decreasing of filler size. In the later curing stage, the reaction speeds of compounds with higher filler content or smaller fillers reduce more quickly. The study of reaction kinetics indicates that the activation energy Ea1, Ea2, the reaction order m, and n are affected differently by varying filler content and size

Curing Kinetic Analysis of Acrylate Photopolymer for Additive Manufacturing by Photo-DSC

In this research, the curing degree of an acrylate-based monomer using direct UV-assisted writing technology was characterized by differential photo calorimetry (Photo-DSC) to investigate the curing behavior. Triggered by the UV light, the duo function group monomer 1,6-Hexamethylene diacrylate (HDDA), photoinitiator 1173 and photoinhibitor exhibit a fast curing process. The exothermal photopolymerization reaction was performed in the isothermal mode in order to evaluate the different thermal effects that occurred during the photopolymerization process. The influences of both UV light intensity and exposure time were studied with single-factor analysis. The results obtained by photo-DSC also allow us to perform the kinetic study of the polymerization process: The results show that, for the reaction, the higher the UV intensity, the higher the curing degree together with faster curing speed. At the same time, the effect of the heat released during the exothermic reaction is negligible for the polymerization process. When increasing the exposure time, limited improvement of curing degree was shown, and the distribution is between 65–75%. The reaction enthalpy and related curing degree work as a function of time. The Avrami theory of phase change was introduced to describe the experimental data. The functions of a curing degree with light intensity and exposure time were achieved, respectively

Nanofiller materials for transparent polymer composites: Influences on the properties and on the transparency—A review

Filler materials are widely used in combination with polymer materials. Conventional filler particles generally cause light scattering and absorption because of their optical characteristic or refractive index difference. With nanoparticles (NPs) as a filler material, it is theoretically possible to manufacture transparent compounds due to their small particle dimensions reducing the interaction with light. Nevertheless, the particles tend to build agglomerates and aggregates which reduce the composite’s transparency considerably. This review gives an overview of the effect different particle materials have on the properties of transparent polymer composites with consideration of the composite’s transparency. There are very few reports on highly transparent and thick (>1 mm) polymer nanocomposites with such an amount of particles that affect other properties of the polymer significantly. In the majority of cases, NPs lead to a significant lower transparency. This indicates that the homogeneous dispersion of the particles is still a major difficulty in producing transparent nanocomposites with enhanced properties

Material Characterization of Strontium Ferrite Powders for Producing Sintered Magnets by Ceramic Injection Molding (MagnetPIM)

For this study, different strontium ferrite powders were mixed with a filling ratio of about 60 vol% in a binder system and formed into green compacts. During the process of injection molding, a magnetic field was generated in the tool via a magnetic coil, which enables magnetization and orientation of the ceramic particles. All powders were successfully processed by MagnetPIM. The investigations identified that it is impossible to extrapolate from the magnetic properties of a green compact to the magnetic properties of a sintered part. It became obvious, though, that, when producing very strong magnetic parts by MagnetPIM, the best results can be obtained by using powders with small particle sizes

Isotropy of mechanical properties and environmental stress crack sensitivity in injection‐ and injection‐compression molding of polystyrene with different mold temperature

Abstract In addition to conventional injection‐compression molding and injection molding with dynamic process temperature control, a synergistic combination of both processes has recently been applied in dynamic temperature‐controlled injection‐compression molding. The two‐dimensional holding pressure effect in combination with the long maintenance of the flowability of the melt due to high mold temperature enables particularly large flow path to wall thickness ratios. In the most cases, only the optimized molding of microstructures and aspect ratios is considered without including the changed internal structure of such manufactured components. In the course of this investigation, the influence of different process strategies under variation of the mold temperature on production‐related anisotropies in the mechanical properties and stress crack sensitivity of thin‐walled polystyrene components was examined. The determined mechanical properties are significantly below the values given in the data sheet of the material in the adapted process variant of injection‐compression molding with high mold temperature. However, the results also show a clear homogenization of the direction‐ and flow path‐dependent mechanical properties. In contrast, components produced in this way tend to show increased environmental stress crack sensitivity. This could be attributed to significantly reduced orientations as a result of the favorably proceeding orientation relaxation

Light scattering of glass-particle filled matrices with similar refractive index

If fillers can be added to transparent materials without losing transparency, then advantages like enhanced mechanical and thermal properties can be integrated. The investigated specimens consist of glass particles and refractive index oil as a model for transparent matrices with a very similar refractive index. Their optical properties and resulting limitations are described. Potential uses are also demonstrated by application-oriented optical testing. Besides a standard spectrometer, additional spectrometer setups were used. These include a diffuse as well as a collimated illumination and different sample positioning. Furthermore, the scattered light intensity was measured at different angles. This analysis reveals that composites with smaller particles transmit more light directly. In contrast, standard spectrometers indicate an increasing direct transmittance of composites with larger particles. They collect significant amounts of scattered light and, therefore, are not suitable for transmission measurements of such composites. The different positioning shows that all specimens exhibit very little scattering when placed directly on a diffuse light source. With a greater distance between specimen and light source, the scattering increases strongly. To display the composites' optical appearance, the light-dark-contrast of the diffuse white light source photographed behind the composite was analyzed. Both long and short distances between composite and light source lead to a precise image of the light source. Nevertheless, the white light source appears in the color of the wavelength with matching refractive indices at long distances

Joining laser‐sintered with injection‐molded parts made of PA12 using infrared welding

Abstract Laser‐sintering offers the possibility to produce complex and individualized components cost‐effectively. To fully exploit the advantages of laser‐sintering in assemblies with mass‐produced components, high‐performance joining processes like welding are necessary. Thus, a cost‐effective customization of products can be enabled, which allows to follow the increasing trend of individualization. Infrared welding, in particular, can also be suitable for complex laser‐sintered parts due to the reduced transverse forces during joining, compared to other welding processes. The investigations show that high strength between PA12 laser‐sintered and injection‐molded components can be achieved by infrared welding. The bond strength is mainly influenced by the welding parameters. Especially a low weld pressure leads to high achievable strengths and failure outside the weld seam. Joints between laser‐sintered parts and glass fiber reinforced injection‐molded components demonstrate the transferability of the obtained knowledge. The residual melt layer thickness of the joint decreases with increasing weld pressure, as the morphological characterization shows. Besides, the typical morphological seam structure can be seen on the side of the injection‐molded component. In the area of the laser‐sintered components, a deviating morphological structure can be observed. Distinctive flow lines can be observed, spherulitic structures can only partially be seen as well as deformed spherulites

Extension of the Application Range of Multipolar Bonded Ring Magnets by Thermosets in Comparison to Thermoplastics

To expand the range of applications of multipolar bonded magnets based on a thermoplastic matrix, the chemical and thermal resistance has to be increased and the reduced orientation in the rapid solidified surface layer has to be overcome. To meet these requirements, the matrix of multipolar bonded magnets can be based on thermosets. This paper investigates in the magnetic properties, especially in the orientation of hard magnetic fillers, the pole accuracy and the mechanical properties of multipolar bonded ring magnets based on the hard magnetic filler strontium-ferrite-oxide and compares the possibilities of thermoplastic (polyamide)- and thermoset (epoxy resin, phenolic resin)-based matrices. It was shown that the magnetic potential of the thermoset-based material can only be fully used with further magnetization. However, the magnetic properties can be increased using thermoset-based compounds compared to thermoplastics in multipolar bonded ring magnets. Further, a model of the orientation and pole accuracy is found in terms of thermoset-based multipolar magnets. In addition, the change of the mechanical properties due to the different matrix systems was shown, with an increase in E-Modulus, Et, and a reduction in tensile strength, σm, and elongation at break, εm, in terms of thermosets compared to thermoplastics

Low‐stress over‐molding of media‐tight electronics using thermoplastic foam injection molding

Abstract Due to increasing automation and the associated rising demands on electronic assemblies, a suitable manufacturing process for large‐scale production is needed to protect such products. The big challenge in this context is the low‐stress encapsulation of the assemblies to protect them from external influences. In this study, the foam injection molding process was used to encapsulate FR4 (epoxy‐based PCB) with Polyamid66 (PA66). The focus was on the production of a good assembly in terms of the quality of the bond and the media tightness. These parameters can be used to evaluate the protective effect against the surrounding. In the tests, a leakage rate of 0.025 m/min and shear stress of 6.5 MPa was achieved at low‐foaming rates. This leakage is below the maximum acceptable threshold of 0.5 ml/min. The shear stress reaches values comparable to those in injection molding In addition to the requirements for leakage and composite quality, it could be shown that the internal mold pressure is reduced from 450 bar to below 10 bar by foaming. This can be used as the first indication of a reduced shear load on electronic components during over‐molding. The suitability of the new solution concept is demonstrated

In situ curing and bonding of epoxy prepregs in epoxy thermoset injection molding

In this study, epoxy molding compounds are combined with fast-curing epoxy prepregs in thermoset injection molding using a new integrative process. The combination is carried out under the varied parameters of mold temperatures and curing times, which are dominant factors in thermoset processing. The focus of the investigations is the bond strength in the interface resulting from these parameters, as the interface is known as the weak point of hybrid components. To identify causes for possible increases and decreases of the bond strength, additional rheological and thermoanalytical analyses are done under near-process conditions. The influence of prepreg pre-crosslinking, a function of the mold temperature, is also described by means of additional tests in which specific pre-crosslinking of the prepreg is adjusted by the temperature storage and then functionalized in integrative process combination. The aim of the study is to identify and understand initial process limits for the integrative process combination for a potential process window