Flexibility

Most commonly used standards to measure Flexural Modulus are ASTM D790 and ISO 178.

Ofcourse there exist several other methods as well as listed below, but they are not discussed here.

 

ASTM D790 and ISO 178 Test Methods

These are specifies methods for determining the flexural properties (bending properties) of reinforced and unreinforced plastics and electrical insulation materials.

The values are significantly different from the tensile modulus because the stress pattern in the specimen is a combination of tension and compression. The data is useful for comparing the strength and stiffness of different plastics when a load carrying part is subjected to bending in service.

ISO 178 standard describes a similar method for determining flexural properties.
 

• For ASTM D790, the test is stopped when the specimen reaches 5% deflection or the specimen breaks before 5%.
• For ISO 178, the test is stopped when the specimen breaks. If the specimen does not break, the test is continued as far a possible and the stress at 3.5% (conventional deflection) is reported.

Since the physical properties of many materials (especially thermoplastics) can vary depending on ambient temperature, it is sometimes appropriate to test materials at temperatures that simulate the intended end use environment.

Also, addition of fillers increases the stiffness or flexural modulus of a polymer system, especially polyolefins (PP, TPOs…) & hence decreases the flexibility. Selection of filler majorly depends on its aspect ratio and particle size. Higher the aspect ratio, high is the stiffness. For example, talc has high aspect ratio, typically 20:1, and is one of the most efficient minerals for improving flexural modulus.

Epoxy resins have excellent tensile strength and flexible modulus as well as detergent resistance, they have low resistance to gamma radiation, poor heat distortion performance, and a poor wear properties. They are also expensive and have a poor volume sensitivity and surface finish.

The addition of a thermothropic liquid crystalline aromatic polyester produces an improvement in the tensile strength and modulus of blends with polyether ketone while simultaneously producing a significant decrease in elongation at break. (Son and co-workers)