Measuring Gels

In fiber extrusion, in particuiar PP, a screen build-up test is used in some cases. This test reflects not just the gel level in the polymer because other materials can be trapped in the screen as well. Also, not all gel particles may be captured in the 

Gels caused by contamination will have a different chemical composition from the polymer used to make the product. As a result, chemical analysis can determine whether a gel particle is contamination or if it has the same chemical composition as the polymer. Gels can be analyzed by micro-infrared analysis. The infrared spectrum can determine whether the gel is from a foreign material or if it similar to the polymer. 

Fines are created when off-spec product is shredded into regrind. If the fines are not removed they will tend to show up as gels in the extruded product. The thermomechanical action of the shredder creates fines that do not melt like regular pellets. For that reason it is important to remove fines from the regrind before it is reintroduced to the extruder. Special equipment is available to remove fines from pellets these machines are called dedusters. 

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The dynamic Weissenberg number W can be calculated from data obtained by the strain frequency sweep measurement. It s the ratio of the elastic to the viscous shares in the measured gel and leads to an objective description of the sensoric properties, representing the basis for the standardization of pectins. 

The ageing test measures the torque necessary to induce the rotation of a special spindle in the slurry. The measure must be done before the first rotation is completed in order to prevent destruction of the gel network. We measure gel after 25 seconds using a Brookfield viscometer with a special spindle (Helipath type), rotating at 1 rpm. 

For many years, the pectin industry has measured gel strength using the IFT-SAG method (Cox and Higby, 1944 IFT, 1959). In this method, pectin gels are prepared under standard conditions, and the amount of sag under the force of gravity is measured with a micrometer called a Ridgelimeter. The method is precise, reproducible and simple to operate, but it is incapable of a comprehensive evaluation of gel structure. 

Rotating rod ISO 2535-74 Measure gel time at 50 Pa s of polyesters Rotating rod at given speed, temperature and rod dimensions... 

Hand probe ASTM D2471-79 Measure gel time and exotherm temperature Wooden hand-stirred probe at given temperature and rod dimensions... 

Glass rod BS 2782 (M 835A-80) Measure gel time of phenolics Stir resin with glass rod until rubbery at given temperatures... 

Test tube (M 835B-80) Measure gel time of polyesters Resin test tube at given temperature and volume... 

Gel timer (M 835C-80) Measure gel time from restricted motion of plunger for epoxies and polyesters... 

Press (M 835E-80) Measure gel time Stir sample on hot plate with knife Measure gel time for impregnated resin Exude material from between plates with wire at given temperature and size of plates... 

The Vp/(anv) values thus calculated are plotted in Figure 5 against alb, where b represents the formal functionality spacing on the linear polymer expressed in A. In this computation, it is assumed that the activity coefficient quotient, (> Na)P/> Na, is approximately equal to unity, which has been validated by the comparison of the directly measured gel phase volume of carboxy-... 

Gel Fraction Determinations. The irradiated polyethylene samples were measured for gel content by extraction with boiling xylene for 72 h. Only one sample, NBS 1475 irradiated to 8.0 Mrad in vacuum at 300 K, showed a measurable gel content. 

This review will discuss the needs of the jelly manufacturer to measure gel strength, the theory of gel formation, factors affecting gel strength, and instrumentation used to measure gel strength. 

The quality of the gels from the different seaweeds varies considerably and different results are obtained by different observers from the same species of seaweeds. The properties of the gels depend to a considerable extent on the methods of extraction used and the methods of measuring gel strength are not necessarily comparable. 

From these equations one can show that the rate of gel or crosslink formation (assume measured gel crosslink formation) (Equation 25) exhibits a first order dependence on peroxide concentration [ROOR] which is consistent with the experimental data presented in Figure 12. 

Measure gel (from just above wells to bottom)... 

The radius of gyration can be determined experimentally by viscosity measurements, gel permeadon chromatography (GPC-), and light scattering. 

Gel Content. Gel content is a measure of extent of cure, and has implications in terms of crosslink density. The measure gel content is also can be related to mechanical properties. Gelling occurs when the substrate has affinity for the solvent, but the crosslinks present in the substrate prevent dissolution. However, when the gel content is high, swelling is decreased due to entropic limitations. The gel content data is presented in Figures 12a and 12b. The gel content corrletes well with the mechanical properties. An increase in tensile strength and modulus is reflected in the increasing gel content. 

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