Thermal measuring and testing – Transformation point determination
Reexamination Certificate
2000-06-09
2002-07-30
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Transformation point determination
C374S022000, C374S023000, C374S049000, C374S051000, C073S078000, C073S081000
Reexamination Certificate
active
06425686
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of material properties. More particularly, the present invention relates to the testing of the glass transition property of composite glass materials.
BACKGROUND OF THE INVENTION
Composite materials having desirable mechanical properties have been used to structurally reinforce and repair thousands of highway columns and bridges. Epoxy matrix composites are the material-of-choice, usually with carbon and/or glass fiber reinforcement. Quality control techniques are desirable to validate and ensure the soundness of these new structures. Reliable and efficient quality control techniques are essential for cost effective field testing. The degree of cure of the resin matrix used will have a pronounced effect on the final mechanical and thermal properties of the composite. Factors that may affect the degree of cure, such as lower-than-expected thermal exposure, excessive post-cure temperatures, contamination, moisture or solvent exposure, improper component mixing, and nonstoichiometric epoxy hardener formulations will also have a direct effect on the glass-transition temperature (Tg) of the composite. The resin material undergoes a solid to a semisolid phase transition at Tg. The elastic modulus of some polymers may decrease by over a thousand times as the temperature is raised through the Tg. For this reason, Tg can be considered the most important material property of a polymer. By identifying the Tg of a composite material after fabrication, validation of a complete cure can be verified. In addition, the Tg of the composites can be monitored in conjunction with other non-destructive evaluation techniques over the lifetime of the structure to provide reliability.
Many different composite systems and fabrication test techniques may be used. The composites can be fabricated by infiltrating the liquid resin into the reinforcement and then hoop winding the tows/fabric onto the large concrete columns. The composites are also being applied to the tensile side of support beams. The composites can also be laid-up by hand and/or wrapped as infiltrated tape. Most of the epoxy resins used in these composites require only room-temperature cure to fully crosslink and reach a suitable degree of cure, greater than 85% for their required application. However, the thermal and mechanical limits of these composites are often much lower than that of resins that require elevated temperature cure processing. This upper thermal limit is controlled by the glass-transition temperature, Tg, of the resin. The glass transition temperature Tg is the temperature at which the material undergoes a solid to semisolid phase transition. At this temperature, the elastic modulus may decrease by over 1000 times as the temperature is raised through this region. The Tg of the resin is controlled by the inherent chemical structure of the resin, the degree of crosslinking or cure the resin has experienced, and whether it has been exposed to any environmental and/or contamination species that may affect the primary polymerization reaction. Therefore, the contractors must ensure that they use resin materials in their composites that have sufficiently high glass-transition temperatures for the application, and that the selected material reaches a sufficiently high degree of cure or polymerization to achieve this Tg upper limit. A lower than expected Tg will signal problems associated with processing or environmental exposure.
Quality control techniques must be developed and are necessary to validate and ensure the soundness of these new composite structures. These techniques are especially essential for field testing. A low Tg can be the result of a lower-than-expected thermal cure treatment, excessively high post-cure temperature exposure, contamination, moisture or solvent exposure, improper component mixing, and non-stoichiometric epoxy to hardener formulations. This report discusses some promising preliminary tests that have been performed to quickly verify the glass-transition temperature of these structural composites after processing and throughout their lifetime.
The ultimate Tg of a material is controlled by the chemical structure, the degree of polymerization, and-contamination or plastization prior to or after cure. The most important factor is chain stiffness or flexibility of the polymer. Long aliphatic groups increase flexibility and lower the Tg. Rigid groups, such as aromatic structures and pendant tertiary butyl groups tend to raise the Tg. The Tg is effected by decreasing the molecular. flexibility by the substitution of bulky side groups onto a polymer chain, for example, polyethylene has a Tg of −120 C., polypropylene has a Tg of −10° C., polystyrene has a Tg of 100° C., and two-six polydichlorostyrene has a Tg of 176° C. A second factor is the backbone symmetry of the polymer. Unsymmetrical polymers are more likely to have a higher Tg than the symmetrical polymers. The greater the impedance to bond rotation for unsymmetrical polymers increases their glass transition temperature compared with symmetrical structures. The Unsymmetrical composite is illustrated by the pairs of polymers, for examples, polypropylene has a Tg of 10° C. and polyisobutlylene has a Tg of −70°, and polyvinylchloride has a Tg of 87° C. and polyvinylidene chloride has a Tg of −19° C. However, within the same resin system, the greatest effect on Tg will be related to the degree of cure or crosslinking the resin has experienced. As the crosslinking sites increase, the polymer introduces restrictions on the molecular motion of the chains. These restrictions cause an increase in the resultant Tg. For example, in the case of an epoxy resin, if the resin material in the composite is poorly cured or has not experienced sufficient crosslinking during processing, a lower than expected Tg will indicate processing problems.
The elastic modulus of the resin material provides variations across the glass-transition temperature range. Typical standard Tg analysis tests usually take twenty to thirty minutes. However, other physical properties have also been observed to change rapidly. The thermal expansion, the heat capacity, the mechanical damping behavior, the electrical properties, the nuclear magnetic resonance behavior, and the refractive index all change abruptly through the Tg. Therefore, there are numerous techniques that have been developed to identify the Tg of polymer resins. However, there are disadvantages to many of these techniques, especially with respect to testing composites in the field. Dynamic mechanical analysis is one of the most commonly used. Dynamic mechanical analysis measures the response of a material to sinusoidal or other periodic stress. Because the stress and strain are usually not in phase, two quantities, a modulus and phase angle or damping term can be determined. Because the material usually undergoes a large drop in modulus through the glass-transition temperature, the instrument can identify the Tg point. In addition, the damping variable or viscous component of the material reaches a maximum through this transition and is also easily identified.
The samples for testing may be cut to some specified dimensions and held between two grips while tested under a torsional shear mode. The tests are performed from tag ends or by destructive evaluation of the part to be tested. The test is usually not suited for out-of-laboratory testing. The advantage of this testing mode is that tests can be performed at a range of frequencies and temperatures. Depending on the frequency used, very subtle secondary transitions can also be identified. These secondary transitions are related to the physical and chemical structure of the resin and have been correlated with properties such as impact strength and toughness. A disadvantage of the Dynamic mechanical analysis method is the required use of a gripping apparatus that cannot be easily used in the field.
A second commonly used test is differential scanning calorimetry. Differential scanning calorim
Hawkins Gary F.
Nokes James P.
Zaldivar Rafael J.
Gutierrez Diego
Reid Derrick Michael
The Aerospace Corporation
Verbitsky Gail
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