Gel Time Measurement

In order to determine the influence of these differences in UV spectra, we used a "chemical filter" to absorb the radiation between 200 and 240 nm. This chemical filter A is a complex non-reactive molecule that has been especially designed for this purpose. It absorbs only between 200 and 240 nm with a molar extinction coefficient close to that of the photoinitiator. If the active range of fis above 240 nm, there must be no difference in reactivity under UV whether the chemical filter A is present or not. Several formulations with the photoinitiator 1 and the filter A have been prepared with different molar ratios of the two components. These formulations have then been used to catalyse the epoxy-silicone UV200, and the gel times measured using the instrument described above. 

The exact value of the index n is not critical for our purposes and the choice of n = 2 fits the data well. Note the very difference activation energy compared to the high temperature expression (Equation A). Certainly this discrepancy is beyond any experimental error, indicating the presence of a different curing mechanism at these temperatures. The value of = A6A0 K compares well with values quoted by others for epo -amine systems studied by isothermal gel time measurements at low temperatures... 

The adhesive mixes were prepared by addition of 10 mass% of wheat flour as extender and 0.05 mass% of Safranin (Superlab, Belgrade) as marker, both based on solid resin. Preliminary tests in the laboratory showed that Safranin did not segregate from the resin mix at higher temperatures. Also there was no influence of Safranin on the gel times measured. In order to keep the same gel time of the adhesive mixes the addition of ammonium sulphate as hardener was 0.5% for UF 1 and 0.3% for UF 11 and UF III, both values expressed as solid ammonium sulphate... 

The hardening of MUF resins can be enhanced by the addition of formaldehyde-based accelerator mixtures and monitored via rheology, gel time measurements, as well as the so-called ABES tests (98,99). Analyses indicate that cured MUF resins are mainly composed of separate MF and UF networks. Thus particleboards glued with an MUF/accelerator mixture exhibit improved mechanical properties compared to boards produced with commercially used MUF adhesives. The swelling properties of particleboards glued with an MUF/accelerator mixture are comparable to boards made from a commercial MUF resin (100). 

Thermomechanical analysis (TMA) provides a convenient and more reproducible, scientific approach to gel time measurement. Using a specific TMA probe configuration (parallel plate rheometer), TMA-measured dimensional changes can be converted to gel time and viscoelastic values. 

Gel point, evaluation, 51-64 Gel preparation, 51, 98-117 Gel time measurement method, 60 Gelatin, 113, 114... 

Resoles. Like the novolak processes, a typical resole process consists of reaction, dehydration, and finishing. Phenol and formaldehyde solution are added all at once to the reactor at a molar ratio of formaldehyde to phenol of 1.2—3.0 1. Catalyst is added and the pH is checked and adjusted if necessary. The catalyst concentration can range from 1—5% for NaOH, 3—6% for Ba(OH)2, and 6—12% for hexa. A reaction temperature of 80—95°C is used with vacuum-reflux control. The high concentration of water and lower enthalpy compared to novolaks allows better exotherm control. In the reaction phase, the temperature is held at 80—90°C and vacuum-refluxing lasts from 1—3 h as determined in the development phase. SoHd resins and certain hquid resins are dehydrated as quickly as possible to prevent overreacting or gelation. The end point is found by manual determination of a specific hot-plate gel time, which decreases as the polymerization advances. Automation includes on-line viscosity measurement, gc, and gpc. 

Gel time values of the three systems measured as abrupt change in the slope of G (t) under isothermal curing conditions show that gelation occurs earlier in PWE system at all temperatures considered as shown in Table 11.27. ETPI behaves like a catalyst for the primary epoxy-amino reaction which dominates the cure until vitrihcation occurs. Dynamic mechanical analysis and dielectric spectroscopic analysis carried out by the authors also confirm the above conclusions. 

The gel time of a 2000 ppm Flocon 4800 (a Pfizer xanthan polymer) in 2% NaCl solution was measured with various Cr(III) crosslinkers at room temperature (Table II). In this series of experiments Cr(III) concentration was 90 ppm. The most reactive Cr(III) species were dates derived from Cr(N0 )g with one and two equivalents of NaOH. Gels formed within 5 minutes and the reaction rate appeared to be diffusion-controlled. Cr(N03)3 without NaOH required 48 hours to gel the polymer solution. This reflects the time needed to hydrolyze CrCNOg) in Equation 3. 

Fig. 6. Protein identification using a peptide map measured with a matrix-assisted laser desorption time-of-flight mass spectrometer. All the peptide extracted from the gel is measured and the set of masses is used in the database search. The mass resolution is in the order of 10,000. Individual isotopes of a 2.5 kDa peptide are clearly resolved.

The solvent mobility in atactic polystyrene-toluene solutions has been studied as a function of temperature using NMR. The local reorientation of the solvent was studied using deuterium NMR relaxation times on the deuterated solvent. Longer range motions were also probed using the pulsed-gradient spin-echo NMR method for the measurement of diffusion coefficients on the protonated solvent. The measurements were taken above and below the gel transition temperatures reported by Tan et al. (Macromolecules, 1983. 16, 28). It was found that both the relaxation time measurements and the diffusion coefficients of the solvent varied smoothly through the reported transition temperature. Consequently, it appears that in this system, the solvent dynamics are unaffected by gel formation. This result is similar to that found in other chemically crossed-linked systems. 

On the other hand, if the cure rate is much faster than the phase separation, then the morphology is controlled by the cure rate through a chemical pinning process. In this system, phase separation is mainly controlled by the cure rate of the epoxy matrix. Faster curing rates and shorter gel times lead to smaller PEI-rich particles with an increasing cure temperature. The temperature effect on the viscosity of reaction mixture is relatively small (i.e., the complex viscosities measured by Physica are 7 and 4 Pa.s at curing temperatures of 150 and 190°C, respectively). 

PNIPAM microsphere gels with diameter of 100-200 jim were prepared by emulsion polymerization [21]. The gel containing 12 mole % benzo[18]crown-6 was immersed in water and the diameter change of the gel was measured during heating at a rate of 0.3 °C/min. The gel was swollen below 25 °C. In the absence of metal ions, it started to shrink at 26 °C and showed a sharp volume change at 28.4 °C. Finally, the volume decreased by as much as 10 times the original volume. 

In a mixture of two alkoxide precursors a mutual influence on the hydrolysis and condensation processes can be expected. How the molar mass develops in Si(0R,)4/(R 0)3Si—X—A mixtures is a question which was hardly investigated. For the MEMO/Si(OEt)4 system this was studied by measuring the gel times (/g)108. The addition of MEMO to an acidic mixture of Si(OEt)4, ethanol and water resulted in a considerable increase of tg. Under basic conditions the increase of tg was less dramatic. The influence of MEMO is probably due to steric hindrance by the methacrylate group during formation of the gel network and the decrease of the average degree of crosslinking [partial replacement of Si(—O—)4 by RSi(—O—)3]. 

Various techniques are available for determining the effective diffusivity of solute in gel (Itamunoala, 1988). One of the most reliable techniques is the thin-disk method which uses a diffusion cell with two compartments divided by a thin gel. Each compartment contains a well-stirred solution with different solute concentrations. Effective diffusivity can be calculated from the mass flux verses time measurement (Hannoun and Stephanopoulos, 1986). A few typical values of effective diffusivities are listed in Table 3.2. 

The NMR results are, in general, not directly comparable with the above results. This is because of the relatively slow time scale of an NMR scan compared with many of the observed gel times. The aluminium spectra could be measured fairly quickly (in about ten minutes) and were less of a problem, but the silicon spectra took longer (up to half the gel time) and at best give a blurred snapshot of the solutions. 

This proved extremely informative. Aluminosilicate complexes were observed in every solution studied. Although the spectra measured are of solutions undergoing change, the time taken to collect the data was generally small compared with the gel time. Figure 5 shows spectra for the nine representative potassium aluminosilicate solutions. A summary of the shifts is given in Table II. 

The purpose of this work was to correlate catalytic activity with chemical structure for various aromatic and heterocyclic carbonyl compounds. Gel time, gel time as a function of initiator concentration, and gel time as a function of temperature were used to measure catalytic activity for the unsaturated polyester styrene systems used throughout this work. 

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