Torsional braid method

The curve of tan8 vs. o) is very useful, since it peaks at characteristic polymer transitions. Certain methods of dynamic analysis, such as the torsional braid method, yield loss tangents readily, but are difficult to analyze separately for the storage and loss moduli. (See also Chapter 5 by Chu.)... 

Free- Vibration Methods. Free-vibration instmments subject a specimen to a displacement and allow it to vibrate freely. The oscillations are monitored for frequency and damping characteristics as they disappear. The displacement is repeated again and again as the specimen is heated or cooled. The results are used to calculate storage and loss modulus data. The torsional pendulum and torsional braid analy2er (TBA) are examples of free-vibration instmments. 

Major methods involved with the generation of information about thermal property behavior of materials include thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), differential thermal analysis (DTA), torsional braid analysis (TBA), thermal mechanical analysis (TMA), and pyrolysis gas chromatography (PGC). 

One of most popular techniques for dynamic mechanical analysis is the torsion pendulum method. In a modification of this method designed to follow curing processes, a torsion bar is manufactured from a braid of fibers impregnated with the composition to be studied this is the so-called torsional braid analysis (TBA) method.61 62,148 The forced harmonic oscillation method has been also used and has proven to be valuable. This method employs various types of rheogoniometers and vibroreometers,1 9,150 which measure the absolute values of the viscoelastic properties of the system under study these properties can be measured at any stage of the process. The use of computers further contributes to improvements in dynamic mechanical analysis methods for rheokinetic measurements. As will be seen below, new possibilities are opened up by applying computer methods to results of dynamic measurements. 

Dynamic testing DMTA, DMA, torsional braid analysis (Enns and Gillham, 1983) is first used as a thermal analysis method to detect the transitions, using dissipation peaks. Certain commercial DMTA instruments have a relatively low accuracy in measuring forces and/or strains. In contrast, they give relatively accurate values of the damping factor tan 5, so that dissipation spectra tan 8 = f (oo, T), are very useful analytical tools. 

The changes which occur in thermosetting resins undergoing cure have been studied by a variety of techniques. Supported sample methods have been used to monitor the relative changes modulus and damping behavior from the initial low viscosity liquid to the fully cured rubbery or glassy state. Torsional braid analysis (TBA) employs a resin sample impregnated on a... 

The automation of the torsion pendulum utilizing a desktop computer eliminates the tedious data analysis previously associated with that technique. Any one of four data reduction methods can be used the experimental conditions will determine which is the optimum one to employ. The torsion pendulum technique provides quantitative values of shear modulus and logarithmic decrement and in the torsion braid mode provides a qualitative analysis of materia i, especially in the liquid-to-solid transition region. 

An alternative method for examining the dynamic mechanical properties of liquids is to coat them onto an inert support (typically a glass fibre braid). This measurement is termed Torsional Braid Analysis and does not provide quantitative modulus measurements since it is difficult to decouple the response of the substrate from that of the sample. 

Of course, such dynamic mechanical test methods are only suitable for such probes as can support their own weight. In examining paints, lacquers, and non-self-supporting films, a glass fiber cord is impregnated with a solution of the test material and the solvent is evaporated. The impregnated cord is then subjected to periodic oscillations (torsional braid analysis). 

Torsional braid analysis n. A method of performing torsional tests on small amounts of materials in states in which they caimot support their own weight, e.g., hquid thermosetting resins. A glass braid is impregnated with a solution of the material to... 

The methods most commonly used at the present time (9) include direct-recording DSC units, the Rheometric Scientific ARES System, and the torsional braid. The special value of the mechanical units lies in the fact that loss and storage moduli are frequently of prime engineering value. Thus the instrument supplies basic scientific information about the transitions while giving information about the damping and stiffness characteristics. 

The imidization process, either thermally or chemically induced, may be followed by a variety of means. It has been traditionally studied on poly(amic acid)s, as well as with molecular models, by IR and NMR spectroscopy [47,48]. But many other analytical methods have been used, for instance TGA [41,49,50], DSC [42,51], polarizing microscopy [41], gas chromatography [52,53], microdielectrometry [54], or torsional braid analysis [55]. From the numerous contributions on this topic some conclusions can be drawn. Among other features, we remark that a rate reduction of the imidation and the rate constant occurs as the conversion increases, so that it can not be considered as a classical first order reaction. This phenomenon has been explained by considering entropic factors [56]. Since the kinetic data could not be unequivocally assimilated to a determined reaction order, they were interpreted as if the imidization reaetion could be divided into rapid and slow first order cyclization steps. The retardation in... 

In the so-called torsional braid analysis,the liquid is absorbed in a long strip of soft braided thread or filter paper which has very slight torsional stiffness, and torsion pendulum measurements are made by the methods of Chapters 6 and 7. Both storage and loss components of the dynamic torsional response may be attributed primarily to the viscoelastic liquid, and examined on a relative basis, although absolute measurements are not possible. Care must be taken that a contribution from the braid does not lead to erroneous conclusions about the properties of the viscoelastic liquid.25a. a flexural braid method has also been utilized. 

Free vibration and forced vibration methods have been employed to determine the respective data. Free vibration was common earlier in polymer blend studies, but forced vibration data is much more common today due to the availability of more sophisticated testing equipment. Free vibration methods include the torsion pendulum, the vibrating reed and the torsional braid analyzer. The torsion pendulum is comprised of an inertial source (disk or rod), which can freely vibrate and is attached to a specimen, which is rigidly fixed at one end. Upon angular deformation of the inertial source and releasing, a damped sinusoidal curve depicts the resultant deformation of the sample [18,19]. Tan S can be calculated from... 

Dynamic mechanical analysis is quite useful to observe the result of chemical reactions of polymer chains (e.g., transesterification) as evidenced by Figs. 3.12 and 3.13 [26]. The DMA method can be applied isothermally to determine crystallization kinetics (modulus versus time measurements) [13, 27] and reaction rate of thermosetting materials (e.g., epoxy) [28]. For reaction rate determination of liquid systems, the torsional braid analyzer is most appropriate as the braid can be saturated with the prepolymer liquid. A cellulose blotter could be used for the torsion pendulum, and a section of nylon hosiery could be used for forced vibration studies (both supports saturated with liquid prepolymer). 

Analytical methods to probe adhesion have also advanced. Dynamic mechanical analysis methods have been found especially useful in investigating the cure process of epoxy resins and in research on pressure-sensitive formulations. Widely used is torsional braid analysis Surface investigation avails itself of electron spectroscopy for chemical analysis (ESCA). Other investigative approaches to following the cure of epoxies include dielectric spectroscopy and viscosity-dependent fluorescent probe. 

In torsional braid analysis (TBA) the sample is impregnated into a multistranded glass braid which acts as support. The method allows analysis of mechanically weak materials, and also materials which are available in very small amounts. Since the specimen is a composite the properties determined are only relative. The main use of TBA is in locating the temperatures of transitions and relaxations. The temperature range available is similar to that given above for conventional torsion pendulums. Manufacturers include Plastics Analysis Instruments Inc., Princeton, NJ, USA. 

In dynamic thermomechanometry the dynamic modulus and/or damping of a substance under an oscillatory load is measured as a function of temperature while the substance is subjected to a controlled temperature. The frequency response is then studied at various temperatures. Torsional braid analysis is a particular case of dynamic thermomechanometry in which the substance is supported on a braid. These are all sophisticated versions of thermomechanical methods. The word dynamic here, as noted above, means oscillatory and this term can be used as an alternative to modulation. In DMA the sample is oscillated at its resonant frequency, and an amount of energy equal to that lost by the sample is added in each cycle to keep the sample oscillation at a constant amplitude. 

Tifi of similar numerical values have been obtained by other methods such as torsional braid analysis and dynamic melt viscosity. ... 

Softening and cure is examined with the help of a torsional pendulum modified with a braid (65), which supports thermosets such as phenoHcs and epoxies that change from a Hquid to a soHd on curing. Another method uses vibrating arms coupled to a scrim-supported sample to measure storage and loss moduH as a function of time and temperature. An isothermal analytical method for phenoHc resins provides data regarding rate constants and activation energies and allows prediction of cure characteristics under conditions of commercial use (47). 

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