Refine
Department, Institute
Document Type
- Article (8)
- Conference Object (2)
- Part of a Book (1)
- Other (1)
- Report (1)
Year of publication
Keywords
- Camphorquinone (1)
- Complex modulus (1)
- Curing behavior (1)
- Curing kinetics (1)
- DMA (1)
- DSC (1)
- Dental composites (1)
- Dielectric analysis (1)
- Dielectric analysis (DEA) (1)
- Dimethacrylate (1)
The ongoing use of miniaturization, multi layer structure parts, and hybrid parts requires methods to determine mechanical properties on a micro scale. However, there is a gap in measuring techniques. On one hand there are the classical methods to measure hardness, e.g., Vickers, Rockwell, Universal, and IRHD, having resolutions typically above 100 µm. On the other hand, there are well-developed AFM methods that allow for the determination of mechanical properties in the nanometer range. This article describes an indentation technique that yields data of mechanical properties in the micrometer range between typically 5 and 50 µm. The measuring device and the data evaluation are presented. Results of micro-mechanical mapping are shown for NR-SBR rubber interfaces, a fuel tank, and a part manufactured by two-component injection molding. Finally, the measured micro-mechanical stiffness is compared to the Young's modulus of the corresponding materials.
Polymers are widely used in protection systems, e.g. in cars. For dimensioning those parts, crash simulation is performed. However, crash simulation programs require reliable high rate mechanical properties to provide accurate results. As polymers are viscoelastic materials Young's modulus and yield stress are rate dependent quantities. In order to determine these properties, special experimental devices and special evaluation methods have to be used. In this paper, the modification of the clamps of a servo-hydraulic tensile testing machine to reach a global strain rate of 670/s is described. Furthermore, a new evaluation method is shown to determine Young's modulus and yield stress using a curve fitting procedure. The results show that both Young's modulus and yield stress depend logarithmically on the strain rate. It is also shown that the rate dependency of the Young's modulus can be used to get rid of the stress oscillations superimposed on the stress signal.
Influence of design of extrusion blow molding (EBM) in terms of extrusion direction set-up and draw ratio as well as process conditions (mold temperature) on storage modulus of high density polyethylene EBM containers was analyzed with dynamic mechanical analysis. All three parameters - mold temperature, flow direction and draw ratio - are statistically significant and lead to relative and absolute evaluation of storage modulus. Furthermore, flow induced changes in crystallinity was analyzed by differential scanning calorimetry. Obtained data on deformation properties can be employed for more sophisticated finite element simulations with the aim to reach more sustainable extrusion blow molding production.
The ongoing miniaturization, multi-layer structure parts and hybrid parts require methods to determine mechanical properties on a micro-scale. However, there is a gap in measuring techniques. On one hand there are the classical methods to measure hardness e.g. VICKERS, ROCKWELL, UNIVERSAL, IRHD etc having resolutions typically above 100μm. On the other hand there are well-developed AFM methods that allow for the determination of mechanical properties in the nanometer range. This paper describes an indentation technique that yields data of mechanical properties in the micrometer range between typically 5 to 50 μm. The measuring device and the data evaluation is presented. Results of micro-mechanical mapping are shown for NR-SBR rubber interfaces, a fuel tank and a part manufactured by two component injection moulding. Finally, the measured micro-mechanical stiffness is compared to the YOUNG’s modulus of the corresponding materials.
Objectives
The use of a Type I photoinitiator (monoacylphosphine oxide, MAPO) was described as advantageous in a model formulation, as compared to the conventional Type II photoinitiator (Camphorquinone, CQ). The aim of the present work was to study the kinetics of polymerization of various composite mixtures (20–40–60–80 mol%) of bisphenol A glycidyl dimethacrylate/triethylene glycol dimethacrylate (BisGMA/TegDMA) containing either CQ or MAPO, based on real-time measurements and on the characterization of various post-cure characteristics.
Methods
Polymerization kinetics were monitored by Fourier-transform near-infrared spectroscopy (FT-NIRS) and dielectric analysis (DEA). A range of postcure properties was also investigated.
Results
FT-NIRS and DEA proved complementary to follow the fast kinetics observed with both systems. Autodecceleration occurred after ≈1 s irradiation for MAPO-composites and ≈5–10 s for CQ-composites. Conversion decreased with increasing initial viscosity for both photoinitiating systems. However despite shorter light exposure (3 s for MAPO vs 20 s for CQ-composites), MAPO-composites yielded higher conversions for all co-monomer mixtures, except at 20 mol% BisGMA, the less viscous material. MAPO systems were associated with increased amounts of trapped free radicals, improved flexural strength and modulus, and reduced free monomer release for all co-monomer ratios, except at 20 mol% BisGMA.
Significance
This work confirms the major influence of the initiation system both on the conversion and network cross-linking of highly-filled composites, and further highlights the advantages of using MAPO photoinitiating systems in highly-filled dimethacrylate-based composites provided that sufficient BisGMA content (>40 mol%) and adapted light spectrum are used.
During the curing process of light curing dental composites the mobility of molecules and molecule segments is reduced leading to a significant increase of the viscosity as well as the ion viscosity. Thus, the kinetics of the curing behavior of 6 different composites was derived from dielectric analysis (DEA) using especially redesigned flat sensors with interdigit comb electrodes allowing for irradiation at the top side and measuring the ion viscosity at the bottom side. As the ion viscosities of dental composites change 1–3 orders of magnitude during the curing process, DEA provides a sensitive approach to evaluate their curing behavior, especially in the phase of undisturbed chain growth. In order to determine quantitative kinetic parameters a kinetic model is presented and examined for the evaluation of the ion viscosity curves. From the obtained results it is seen that DEA might be employed in the investigation of the primary curing process, the quality assurance of ingredients as well as the control of processing stability of the light curing dental composites.
