Curing, thermomechanical analysis

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) were used to measure the glass transition temperatures (Tgs) of the uncured and cured AT-resins respectively (Figure 6). 

A glass mold treated with release agent was used to prepare 0.125" thick cured plates of the resin compositions. These cured plates were used to provide samples for tensile, differential scanning calorlmetery and thermomechanical analysis. 

For comparative isothermal aging studies, all the samples of pure BCB and dlcyanate monomers, as well as their 1 1 molar mixtures, were cured in a single batch at 200-220°C for 40 hours under nitrogen atmosphere. The cured samples of BADCy, METHYLCy, and THIOCy were all transparent and yellow/amber, and their blends with BCB were also transparent but dark red in color. The cured sample of BCB was translucent and yellow. The Tg s (cure) of the thermosets derived from the dlcyanate monomers are relatively high, 224°-261°C as determined by thermomechanical analysis (TMA). There Is an increase of 10-31°C in Tg (cure) values in their blends with BCB. The ITGA results of the cured samples of BCB,... 

The complex sorption behavior of the water in amine-epoxy thermosets is discussed and related to depression of the mechanical properties. The hypothesized sorption modes and the corresponding mechanisms of plasticization are discussed on the basis of experimental vapor and liquid sorption tests, differential scanning calorimetry (DSC), thermomechanical analysis (TMA) and dynamic mechanical analysis. In particular, two different types of epoxy materials have been chosen low-performance systems of diglycidyl ether of bisphenol-A (DGEBA) cured with linear amines, and high-performance formulations based on aromatic amine-cured tetraglycidyldiamino diphenylmethane (TGDDM) which are commonly used as matrices for carbon fiber composites. 

The thermal characterisation of elastomers has recently been reviewed by Sircar [28] from which it appears that DSC followed by TG/DTG are the most popular thermal analysis techniques for elastomer applications. The TG/differential thermal gravimetry (DTG) method remains the method of choice for compositional analysis of uncured and cured elastomer compounds. Sircar s comprehensive review [28] was based on single thermal methods (TG, DSC, differential thermal analysis (DTA), thermomechanical analysis (TMA), DMA) and excluded combined (TG-DSC, TG-DTA) and simultaneous (TG-fourier transform infrared (TG-FTIR), TG-mass spectroscopy (TG-MS)) techniques. In this chapter the emphasis is on those multiple and hyphenated thermogravimetric analysis techniques which have had an impact on the characterisation of elastomers. The review is based mainly on Chemical Abstracts records corresponding to the keywords elastomers, thermogravimetry, differential scanning calorimetry, differential thermal analysis, infrared and mass spectrometry over the period 1979-1999. Table 1.1 contains the references to the various combined techniques. 

Thermomechanical analysis (TMA). In this technique, information on changes in the size of a sample is obtained, e.g. thermal expansion and coefficient of thermal expansion, cure shrinkage, glass transition, thermal relaxations, any phase transformation involving volume change in the material. We describe the measurement of the coefficient of thermal expansion in detail later in this section. 

CHARACTERIZATION. Melting points were determined on an E. I. DuPont Series 99 Thermal Analyzer at 20°C/min. Inherent viscosities of polyamic acid solutions were obtained at a concentration of 0.5% (w/w) in DMAc at 35°C. Glass transition temperatures (T ) of the fully cured polymer films were measured by thermomechanical analysis (TMA) on a DuPont 943 Analyzer in air at 5°C/min. Films fully-cured at 300°C were tested for solubility at 3-5% (w/w) solids concentration in DMAc,N,N-dimethylformamide (DMF), and chloroform (CHCl-j). Solubilities at room temperature were noted after periods of 3 hours, 1 day and 5 days. Refractive indices of 1 mil thick films were obtained at ambient temperature by the Becke line method (11) using a polarizing microscope and standard immersion liquids obtained from R. P. Cargille Labs. 

Thermomechanical Analysis. A thermomechanical analyzer (Perkln-Elmer TMS 2) was used to measure glass transition temperature (Tg) and coefficient of thermal expansion (a ) of completely cured samples In the penetration and expansion modes respectively. Experimental conditions are listed In Table I. 

Properties relating to performance of completely cured adhesive were determined by mechanical spectroscopy and thermomechanical analysis. Measurement of glass transition temperature and coefficient of thermal expansion was obtained from temperature scanning. 

Thermomechanical analysis (TMA) is a very useful technique for determination of the properties of coatings and thin films. In many cases it can be used to determine the Tg of a film which is too thin or too adherant to the substate to permit analysis by DSC. However, interpretation of TMA results can be complicated by the effects of sanqple curl during curing coupled with volume relaxation effects. 

Duske [6] discussed the use of thermomechanical analysis (TMA) in assessing the quality of wire insulation materials. This is particularly important for characterization of the polymer glass transition temperature, Tg, and for curing process control. TMA has been known to be a very sensitive technique for the... 

DSC has been used to establish optimum cure schedules for rapid curing (snap cure) adhesives and adhesives requiring longer cures. Results showed that the snap-cured adhesive was 98% cured in 3 min, compared with over 15 min for the conventional cured adhesive. The cure for the latter adhesive was verified by thermomechanical analysis (TMA) by measuring the Tg as a function of cure schedules where it was shown that up to 150 °C, the Tg had not yet reached a plateau after 30 min. At 165 °C, a plateau was reached in 30 min, and at 170 °C, the Tg peaked in 10 to 15 min. 

Figure 1. Results of thermomechanical analysis of resins A, B and C. Note the higher ultimate value of MOE for resin C and the increase in MOE at lower temperatures for both resins C and A, evidencing them as faster curing resins.

This cure temperature region was independently confirmed by differential scanning calorimetry and thermomechanical analysis studies of another comonomer system, m PDM in EBIPMA. First, Fig. 3 shows the DSC analysis of neat m-PDM which indicated the neat m-PDM appears to first melt and then spontaneously polymerize (large exotherm) at 200 C. This is shown by curve (1) and region (3) in the Figure. When the just-polymerized m-PDM is itself scanned, a smooth, "flat" curve (2) is obtained in the regions which earlier showed the endotherm melt and polymerization exotherm behavior of the neat monomer. This shows the material is now virtually fully cured. 

A duplicate comonomer mixture, pre-cured at 150 C, demonstrated no exotherm with the DSC but rather a smooth baseline indicating an already apparently fully cured, cross-linked system (Fig. 5). Confirmation of the DSC data was essentially provided by Thermomechanical Analysis instrumentation and actual adhesive strength tests. 

The coefficient of expansion of a polymer is very much greater than that of a metal. Such a mismatch of expansion will introdnce stresses within an adhesive joint, especially if the joint is cured at an elevated temperatnre. The measnrement of the expansion coefficient can be carried out by a variety of techniques, and one of the simplest is thermomechanical analysis (TMA), where the dimensions of a small sample are measured over a range of temperatures. The change in the sample dimension gives the variation in expansion coefficient with temperature. 

This system measures dimensional changes as a function of temperature. The dimensional behavior of a material can be determined precisely and rapidly with small samples in any form— powder, pellet, film, fiber, or as a molded part. The parameters measured by thermomechanical analysis are the coefficient of linear thermal expansion, the glass-transition temperature (see Figs. 9-10 and 9-11), softening characteristics, and the degree of cure. Other applications of TMA include the taking of compliance and modulus measurements and the determination of deflection temperature under load. 

---