Curing and pyrolysis of cresol novolac epoxy resins containing [2‐(6‐oxido‐6H‐dibenz(c,e)(1,2)oxaphosphorin‐6‐yl)‐1,4‐naphthalenediol]

This study investigates the curing kinetics, thermal properties and decomposition kinetics of cresol novolac epoxy (CNE) with two curing agents, 2‐(6‐oxido‐6H dibenz(c,e)(1,2) oxaphosphorin‐6‐yl)‐1,4‐benzenediol (ODOPN), and phenol novolac (PN). In comparison with the conventional PN system, introducing ODOPN, a phosphorus‐containing bulky pendant group, into CNE increases T_g by 33°C, char yield from 30% to 38%, and LOI from 22 to 31. The DSC curing study reveals that the E_a of the CNE/ODOPN epoxy can be obtained by Kissinger's method. The resulting E_a values indicate that the catalytic effect of EMI is insignificant on CNE/ODOPN but is marked on CNE/PN, whose E_a was reduced from 131.5 to 75.6 KJ/mole. This result may be caused by the fact that the symmetric diol attached to the 1 and 4 positions of the naphthalene ring in ODOPN sets up a steadily resonating structure and inhibits the catalytic action. Further investigating the conversion ratio with curing temperature yielded experimental data that agreed closely with Kaiser's model. The orders of the autocatalyzed reaction, m, and the crosslinking reaction, n, are close to 0.5 and 1.0, respectively, independently of the scan rate. Finally, the TGA decomposition study by Ozawa's method demonstrates that the mean E_a declines with the phosphorus content, because the easy decomposition of the phosphorus compound in the initiation stage facilitates the formation of an insulating layer. However, results in this study further reveal an increasing tendency for E_a with decomposition conversion for an ODOPN/PN mixture with the ODOPN content of over 50%, probably because of the retardation of gas diffusion by the insulating layer of phosphorus compound.     

Investigating the UV-curing performance for polyacrylated polymer in dendritic and regular conformation

In this study, PDSC was employed to monitor the UV curing process for dendritic and regular monomers with various acrylate sites. Experimental results reveal that even with only a small increase in molecular weight for regular monomers caused a large increase in viscosity. A dendritic monomer with the same number of sites has a much lower viscosity. The conversion of hydroxyl groups into acrylic groups in the dendritic monomer slightly reduces the viscosity by destroying the hydrogen bonds. The curing conversion and curing rate increased with the number of acrylate sites to maxima at five. The acceleration of the double bond reaction within a dense group of multiacrylate sites is responsible for the initial rise, but the steric effect of the branches, hindering the simultaneous free-radical propagation, causes a decline as the number of sites increases in the curing of dendrimers. The autocatalyzed reaction model was then applied to simulate the curing results from PDSC. Dendrimers with 10–19 acrylate sites were found to have lower rate constant k and smaller autocatalyzed order m than the traditional ones. Finally, the heterogeneity of the cross-linking density of dendrimers generally causes dendritic monomers to have weaker hardness and lower T _d values than the regular ones. Nevertheless, dendrimers with 19 acrylate sites yield a film having low processing viscosity, satisfactory hardness (6H), reasonable T _d (290 °C), and a superior refractive index (1.5).     

Distributive mixing in a single‐screw extruder—evaluation in the flow direction

This study investigates distributive mixing in the flow direction for a single‐screw extruder. WIth a custom‐designed transparent extruder, an Image Analysis System, and a newly defined parameter, i.e., distribution index, the distribution mechanism is thoroughly examined with respect to various processing conditions or screw designs. Experimental results indicate that the longitudinal distribution can be enhanced with an increasing RPM, a longer metering section, or a decreasing diameter of the die. However, a plateau region occurs when an optimum condition exists for the RPM and the length of the metering section. In addition, an extruder modified with a barrier, pin‐elements, or high helix angle performs better in the longitudinal mixing than the conventional one. Our results further demonstrate that leakage flow significantly enhances mixing in the flow direction.     

Analysis of dispersion of carbon black in polymeric melts and its effect on compound properties

The dispersion of carbon black agglomerates within melts of polystyrene and high density polyethylene has been investigated. Analysis of the evolution of the fragment size distribution indicates that a coarse rupturing of the agglomerates occurs during the early stages of the dispersion. This is followed by a more gradual erosion of small aggregates from larger fragments. These observations compare quantitatively with results of dispersion studies performed in fluid media having a viscosity two orders of magnitude smaller than that of the polymeric melts. The influence of dispersion quality on certain physical properties of the resulting polymeric compounds, namely UV absorption characteristics and tensile strength properties, is also quantified.     

Dynamic mechanical properties of photo resist thin films

Photo resist thin films have mainly been used and investigated for versatile applications of micro electronic mechanical systems because of its outstanding aspect ratio and attainable film thickness. An accurate structure properties derived from validated material characterization is required in engineering applications. In this work, dynamic responses of photo resist thin films are tested by a nanoindentation in association with a dynamic mechanical analysis, where the thin film is coated on a silicon wafer by spin coating. The results show that the storage modulus of the photo resist thin film remains constant at the beginning and then increases as the indentation depth increases. Meanwhile, the loss modulus increases as the indentation depth increases. Varying the film thickness shows that the substrate effect plays an important role in determining the dynamic properties of thin films. However, the results agree well with the bulk material when the amplitude of nanoindentation is relatively small. It illustrates the dynamic mechanical analysis can be an efficient method to characterize the viscoelastic properties of thin films, but proper attention on the test parameters is needed.     

Different approaches for creating nanocellular TPU foams by supercritical CO<Subscript>2</Subscript> foaming

In this study, three different approaches were applied to obtain thermoplastic polyurethane (TPU) nanocellular foams. The TPU was synthesized with a 4, 4′-methylenebis (phenyl isocyanate) and 1, 4-butanediol (MDI/BD) hard segment system using a pre-polymer method. The three approaches included increasing the hard segment content, adding a graphene nucleation agent, and replacing the soft segments. Although the synthesized TPUs had a different hardness, it was possible to obtain nanocellular structures with all of the methods. The cell structure is not a function of hardness only. Crystallinity affects the cell structure as well. The addition of graphene and replacement of the soft segments were more effective at yielding nanocellular foams. Our best results showed that after adding 0.1 wt% of graphene, the average cell size of the TPU foam decreased to 715 nm, and the cell density was improved to 4.94 × 10¹¹ cells/cm³. The relative density of the foam could be as low as 0.77. This study first reported elastomer-based nanocellular structures with such low relative density.     

Study of self‐crimp polyester fibers

Self‐crimp polyester yarns were manufactured using a conjugated spinning process involving two parallel but attached fibers with different shrinkage properties. A theoretical model proposed by Denton proved to be very useful for predicting crimp potential. Maintaining identical or very similar melt viscosities of the two components was demonstrated to be very critical for obtaining a straight interface and eliminating the dog‐legging problem. The crimp tests illustrate that the triangular shapes are superior to the round cross section. The optimum volume ratio for making a self‐crimp bicomponent skein is 50/50. Moreover, the optimal fiber thickness is 8 denier per filament. Finally, this study found that the combination of PET/PTT outperformed that of PET/PBT and PET/CD in terms of crimp potential, crimp stability, and elastic recovery. This phenomenon is primarily attributed to the markedly different thermal shrinkages of PET and PTT. POLYM. ENG. SCI. 45:838–845, 2005. © 2005 Society of Plastics Engineers     

Kinetics of UV-curing of waterborne polyurethane acrylate dendrimer

In this study, a dendrimer with 19 sites was modified to form a waterborne oligomer with hydrophilic sites and polyurethane acrylate (PUA) sites. Two PU systems hexamethylene diisocyanate (HDI) and methylene-bis (4-cyclohexylisocyanate) (HMDI) were used; each of them had 5, 9, and 13 PUA sites, respectively. Experimental results revealed that the HDI system was more thermally stable than the HMDI system. The HMDI system, however, exhibited better hardness than the HDI system, even though the final curing conversion of HMDI was low. Additionally, more PUA sites of a dendrimer were associated with greater hardness of the cured resin. In a UV-curing study, simulations revealed that the autocatalysis model could describe the UV-curing mechanism of the PUA-dendrimer system. Both the reaction rate constant k and the final conversion α were highest at a concentration of the photoinitiator of approximately 3 wt% for both HDI and HMDI systems. The reaction rates of both systems were highest when the number of PUA sites was nine. The final conversion of both the systems, however, was optimal when the number of PUA sites was 13. The final conversion in the acrylic reaction increased with temperature up to 80 °C for both the systems, owing to an increase in the ratio of potentially active sites against nonactive sites, according to Arrhenius theory. The rate constant, k, however, was optimal at 60 °C because a higher temperature damaged the photoinitiator. Finally and most importantly, HMDI always had a lower rate constant k and final conversion α than HDI under similar conditions in studies that were conducted to examine the effects of photoinitiator concentration, number of PUA sites, and reaction temperature, revealing that the steric hindrance by the cyclohexane in HMDI negatively influenced its curing kinetics.     

Shaping conjugated hollow fibers using a four‐segmented arc spinneret

This study discusses a light‐weight bicomponent hollow fiber that is formed from a low‐density material on the inside, such as polypropylene (PP), and a regularly dyeable material outside, such as polyterephthalate (PET) or nylon. Finite elements and the Optimesh‐3D remeshing approach are adopted to identify the main controlling factors of spinning the sheath‐core hollow fiber without the consideration of winding actions are performed, based on a four‐segmented arc spinneret design. The results indicate that the deflection of melt streams under the spinneret is a major factor that controls the gluing of the gap between arc segments. A greater mismatch between the viscosities of the sheath and the core causes a greater deflection and increases the likelihood of gluing events. Beyond deflection, die swelling under the spinneret is another issue of concern in the processing of bicomponent hollow fibers. Finally, the simulation results are compared with experimental data, and the most appropriate conditions for forming a PET/PP hollow fiber were obtained. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Properties of poly(ethylene terephthalate)/poly(ethylene naphthalate) blends

Blends composed of poly(ethylene terephthalate) (PET) as the majority component and poly(ethylene naphthalate)(PEN) as the minority component were melt-mixed in a single screw extruder at various PET/PEN compound ratios. Tensile and flexural test results reveal a good PET/PEN composition dependence, indicating that the compatibility of the blends is effective in a macrodomain. In thermal tests, single transitions for T_g, T_m and T_c (crystallization temperature), respectively, are observed from DSC as well as single T_g from DMA except for 50/50 blends. These results suggests that the compatibility is sufficient down to the submicron level. Moreover, isothermal DSC tests along with Avrami analysis indicate that PET's crystallization is significantly retarded when blended with PEN. Results in this study demonstrate that PEN is a highly promising additive to improve PET's spinnability at high speeds.