Glass transition temperature testing of composites

Polymer matrix composites (PMCs) exhibit impressive stiffness and strength properties that are commonly attributed to the reinforcing fibers. However, for many important mechanical properties, the polymer matrix also plays an important role. For example, the fiber direction compression strength of a PMC is highly dependent on the polymer matrix material’s ability to support the reinforcing fibers and resist buckling under compression loading.

The role of the polymer matrix material becomes apparent when evaluating “matrix-dominated” mechanical properties of PMCs at elevated temperature conditions, at which the stiffness and strength properties of the polymer matrix are reduced. These mechanical properties gradually decrease as the test temperature increases, due to reductions in the polymer matrix material properties. As the temperature of thermosetting PMCs are further increased, the polymer matrix material undergoes a transition from the glassy state present at lower temperatures to a more rubbery state. The thermosetting polymer does not melt, but a two to three order of magnitude reduction in stiffness is produced by this transition from the glassy state¹. Although this dramatic transition takes place over a temperature range, a single temperature typically is specified for a given polymer matrix and referred to as the glass transition temperature (T_g). 

The glass transition temperature of PMCs can be determined by performing a series of tests at increasing temperatures until a sudden drop in the measured property is observed. Note, however, that a matrix-dominated property of the PMC must be measured in which a significant change occurs when T_g is reached. Since performing a series of tests at increasing temperatures is time-consuming and expensive, test procedures typically are used in which single specimens are subjected to repeated or continuous loading as the test temperature is slowly increased.

For thermosetting polymers typically used in PMCs, three material properties are commonly measured to determine the glass transition temperature: heat capacity, coefficient of thermal expansion and flexural stiffness. Standardized test methods exist for T_g measurement of polymers based on these properties. But how well do these test methods work for fiber-reinforced PMCs? 

The first method of T_g determination focuses on changes in the heat required to increase the temperature, or heat capacity, of the sample. The use of differential scanning calorimetry (DSC) provides a relatively simple and cost-effective method for determining T_g based on heat capacity changes. This method, described in ASTM D7426², is commonly used for polymeric materials because of the significant increase in measured heat capacity associated with their glass transition. Since PMCs typically are less than half polymeric material based on volume, however, the change in heat capacity associated with their glass transition is significantly smaller and more difficult to detect using DSC. Therefore, T_g determination using DSC is not a preferred method for use with PMCs.

A second property used to determine the T_g of polymeric materials is the coefficient of thermal expansion (CTE), a measure of the dimensional change of a polymer sample associated with a temperature increase. Changes in CTE are used for T_g determination of polymeric materials due to the dramatic CTE increase during their glass transition. Thermomechanical analysis (TMA) methods are used to measure the increase in length of a polymer sample with increasing temperature as described in ASTM E1545³. Although expansion TMA is most commonly used for T_g determinations, other TMA methods exist by which T_g determinations can be made while slowly increasing the sample temperature¹. Penetration TMA measures the hardness of the sample, whereas flexural TMA measures the center deflection of a beam-like sample loaded in three-point flexure. In both methods, a significant increase in displacement is measured by the TMA probe associated with their glass transition. However, for PMCs, the reinforcing fibers significantly reduce the change in CTE associated with the polymer’s glass transition. Additionally, neither penetration TMA nor flexural TMA have received much attention for PMCs. As a result, TMA methods are not commonly used for T_g determination of PMCs.

The third property used to determine the T_g of polymeric materials is the stiffness of a sample under a specified mechanical loading, a logical choice since unreinforced polymers exhibit large reductions in stiffness at their glass transition. Dynamic mechanical analysis (DMA) methods are used to apply an oscillating force to the polymer specimen and measure the resulting displacement as the test temperature is slowly increased. From the measured force and displacement, the specimen stiffness is determined and used to identify the glass transition. Standardized test methods exist for T_g determination of PMCs using DMA for a variety of loading methods, including tension (ASTM D5026⁴), compression (ASTM D5024⁵), torsion (ASTM D5279⁶), three-point bending (ASTM D5023⁷) and dual cantilever beam flexure (ASTM D5418⁸). Procedures calculating T_g values based on DMA testing of polymeric materials are provided in ASTM D4065⁹.

For PMCs, the magnitudes of stiffness reductions at T_g are reduced significantly for many of these loading methods due to the reinforcing fibers. However, significant flexural stiffness reductions are still produced, and thus DMA testing of PMCs focuses on flexural loading configurations. ASTM D7028¹⁰ recommends the use of either a dual cantilever beam or three-point bend flexural loading. The use of DMA under flexure loading is the most commonly used method for T_g determination of PMCs.

Although the measured T_g establishes a limiting use temperature for thermosetting PMCs, a somewhat lower temperature is typically chosen as the maximum allowable temperature for a particular application. Within the aerospace industry, a material operational limit (MOL) is established by reducing the T_g by a prescribed value, typically 28°C / 50°F, below the measured glass transition temperature. Additionally, it is common practice to perform two T_g measurements: one without any moisture preconditioning (dry T_g) and a second using specimens that have been moisture conditioned (wet T_g). Note that wet T_g testing of PMCs is particularly challenging because the specimen tends to lose moisture as it is heated during DMA testing. The use of thicker specimens can help minimize effects of specimen drying during T_g testing.

After determining the dry and wet T_g values for a PMC, and establishing an appropriate MOL, it is common practice to perform a series of mechanical tests at the MOL temperature, particularly with specimens that have been moisture conditioned. The resulting “hot/wet” material properties of PMCs often represent a limiting environmental condition. 

References

¹Composite Materials Handbook - 17 (CMH-17), Volume 1, Section 6.6.3: “Glass Transition Temperature,” SAE International, Rev. G, 2012.

²ASTM D7426-08(2013), “Standard Test Method for Assignment of the DSC Procedure for Determining T_g of a Polymer or an Elastomeric Compound,” ASTM International (W. Conshohocken, PA, US), 2013.

³ASTM E1545-11(2016), “Standard Test Method for Assignment of the Glass Transition Temperature by Thermomechanical Analysis," ASTM International (W. Conshohocken, PA, US), 2016.

⁴ASTM D5026-15, “Standard Test Method for Plastics: Dynamic Mechanical Properties: In Tension,” ASTM International (W. Conshohocken, PA, US), 2015.

⁵ASTM D5024-15, “Standard Test Method for Plastics: Dynamic Mechanical Properties: In Compression,” ASTM International (W. Conshohocken, PA, US), 2015.

⁶ASTM D5279-13, “Standard Test Method for Plastics: Dynamic Mechanical Properties: In Torsion,” ASTM International (W. Conshohocken, PA, US), 2013.

⁷ASTM D5023-15, “Standard Test Method for Plastics: Dynamic Mechanical Properties: In Flexure (Three-Point Bending),” ASTM International (W. Conshohocken, PA, US), 2015.

⁸ASTM D5418-15, “Standard Test Method for Plastics: Dynamic Mechanical Properties: In Flexure (Dual Cantilever Beam),” ASTM International (W. Conshohocken, PA, US), 2015.

⁹ASTM D4065-12, “Standard Practice for Plastics: Dynamic Mechanical Properties: Determination and Report of Procedures,” ASTM International (W. Conshohocken, PA, US), 2012.

¹⁰ASTM D7028-07(2015), “Standard Test Method for Glass Transition Temperature (DMA T_g) of Polymer Matrix Composites by Dynamic Mechanical Analysis (DMA),” ASTM International (W. Conshohocken, PA, US), 2015.