CLTE measurements

Thermal Expansion of Polymeric Materials In addition to the glass transition and melting temperature measurements, the other major application of TMA is determination of the thermal expansion of polymeric materials. The coefficient of linear thermal expansion (CLTE or a) is the slope of the relative change in the length (AL/Lo) with respect to the temperature range of interest, as defined by Eq. (4.2) (see Fig. 4.2b). CLTE measurements are important for many reasons. For example, mismatch in CLTE values may cause dimensional change or delamination in composites and adhesives due to the thermal stresses. 

All CLTE measurements were corrected for the presence of the silica fiber and the baseline behavior of the TMA sample chamber and probe. To achieve this, a fiber was heated to 750 °C in oxygen to remove the double coating. Measurements were then taken of the bare fiber when it was heated over the same temperature range as that used with the coated fibers. This stored instrumental baseline was subtracted from all subsequent tests on coated fibers. 

Not only does a properly engineered optical fiber need good adhesion at service temperatures but also, if the fiber must be stripped, the ability to reduce adhesion by heating the fiber is desirable. In this manner CLTE measurements can play an important role in engineering an optimum type and level of adhesion promoter for an optical fiber system. 

Per the recommendations of E 831, calibrations for CLTE measurements should be performed using reference specimens that are the same size and thermal conductivity as the test specimen to duplicate thermal gradients within the specimens. 

The change in length per unit length of a material per degree of temperature change is called the coefficient of linear thermal expansion (CLTE). The measurement of the CLTE is covered by several ASTM and ISO methods ... 

The CLTE is commonly measured using a silica dilatometer. A plastic specimen is placed inside a silica tube and a silica rod is inserted into the tube. A dial gauge or. similar device is attached to the rod. The end of the tube containing the test material is placed in a — 30°C constant temperature bath. When the gauge no longer changes, the test specimen has reached a temperature of — 30°C. The constant temperature bath is replaced by a 30°C constant temperature bath. After the specimen has reached a temperature of 30°C, the 30°C bath is replaced by the — 30°C bath. After the specimen has reached a... 

Expansion probes are used primarily to measure CLTE and under negligible load, 2.5-3 mm in diameter the same probe can be rounded to form a hemispherical probe some suppliers also provide a macroprobe with 6mm diameter... 

Tension probes measure tensile properties such as stress and strain of thin films and fibers in addition, creep and stress relaxation measurements can be carried out in the tensile mode CLTE and hygroscopic expansion can also be measured in this mode (Prime et al. 1974). 

The expansion response of the TMA instrument can be checked with a standard reference material of defined thermal expansion. The Polymer Handbook (Brandrup 1999) and the Handbook of Chemistry and Physics (Lide 1998) provide tables of CLTEs versus temperature for numerous materials. Prime (1997) also lists CLTE values for three calibration standards (lead, aluminum, and copper) in the temperature range from -100 C to 180 °C. ASTM E831 describes the standard test method for measuring CLTE of solid materials by TMA. 

The measured properties in thermal analysis of polymers (e.g., Tg, Tn, crystallization temperature, CLTE) are often a function of the thermal and... 

In practice TMA is mostly used for measurement of the glass transition temperature by measuring CLTE and on occasion volume expansion. Therefore, it is appropriate to review those chemical factors (effects of pendant groups, molecular mass, tacticity, crystallinity, crosslinking, etc.) that have the largest influence on the glass transition temperature and the expansion of a polymer. 

It should be noted that, in addition to traditional TMA, other techniques for measuring CLTE include capacitance change, laser interferometry, dielectric analysis, and elipsometry. These techniques, summarized by Menczel et al. (1997), are especially useful for very thin films, where the traditional TMA instruments reach their sensitivity limits. 

Once this condition is reached, the primary coating s glassy expansion curve from -50 °C to -30 °C and the secondary coating s rubbery expansion curve from 125 °C to 90 °C can be extrapolated upward and downward, respectively, until they meet the linear line extending from the expansion curve between 0°C and 75 °C (dashed line. Fig. 4.36). This process yields glass transition temperatures near -20 °C and 87 °C. DSC measurements at 15°C/min on this same optical fiber sample indicated that glass transition temperatures occur at -31 °C and 78 °C. In normal practice with the probe seated, the final stable heating run would be selected and isolated in a separate plot for determination of Tg and CLTE values. 

TABLE 4A.1. CLTE and of Commercially Available Plastic Materials as Measured by TMA... 

Thermomechanical Analysis (TMA) measures unidirectional dimensional changes in materials as functions of time, temperature and applied force. The TMA measurements are coefficient of linear thermal expansion (CLTE), glass transition temperatures (Tg) and softening points (Ts). Newer applications of TMA include elasticity, melt viscosity, and heat deflection temperature. In addition to traditional TMA instruments, many modem dynamic mechanical thermal analysis (DMTA) instruments can operate in a TMA (static force) mode. The main differences between the two types of instruments are the size of the specimens and the materials used to fabricate the measurement fixtures (stage, probe, clamps, etc.). Most TMA instruments use quartz, while DMTA instruments use larger steel components. The specimens used in these experiments are... 

Small specimens should be used for Tg and Tm measurements to minimize thermal gradients within the specimens. This may require softening points (penetration) to be used to measure Tg instead of expansion for materials with low CLTE values. 

Mechanical properties such as tensile properties (modulus, yield stress and elongation-to-break), room temperature and 0 °C impact, CLTE, weld strength and flexural modulus were measured for the TPO materials in this study. The mechanical properties of the two TPO formulations were measured using protocol outlined in appropriate ASTM standards. 

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