Heat Deflection Temperature of Plastics

The heat deflection temperature (HDT) is a measure of the polymer's resistance to distortion under a given load at an elevated temperature. In other words, it is the temperature at which a given polymer test bar will be bent by 0.25 mm under a given load. It is one of the two basic methods for assigning value to the performance of plastics at high temperatures. The value of 0.25 mm is an arbitrary value and has no specific significance.

The deflection temperature is also known as the:
 

• Deflection Temperature Under Load (DTUL),
• Heat Deflection Temperature Under Load (HDTUL), or
• Heat Distortion Temperature (HDT).

Note: Although tests performed at higher loads such as 5.0 MPa (725 psi) or 8.0 MPa (1160 psi) are occasionally encountered. They are not discussed here.

 

1. HDT @0.46 MPa (67 psi)

In the case of the HDT @0.46 MPa, the load value is 0.46 MPa (67 psi). Practically, the test bar is immersed in an oil bath that is heated. It leads to a continuous increase in the bath temperature (usually 2°C per minute).

HDT @0.46 MPa is often more accurate than HDT @1.8 MPa. The latter is useful when final parts are used under high mechanical loads.

 

2. HDT @1.8 MPa (264 psi)

In the case of the HDT @1.8 MPa, the load value is exactly 1820 kPa. It is commonly referenced as 1.8 MPa (264 psi). Practically, the test bar is immersed in an oil bath. It is heated in such a way that the increase of the bath temperature is continuous (usually 2°C per minute).

As compared to HDT @ 0.46 MPa, the HDT @1.8 MPa is often useful when predicting the maximum service temperature of parts submitted to high mechanical loads.

 

1. Thermoplastics with High HDT — View All Products
2. Thermosets with High HDT — View All Products
3. Rubbers with High HDT — View All Products

HDT measures the effect of temperature on stiffness. However, this is only an estimate and cannot predict how the final part will perform. Several factors influencing HDT are:
 

• Extrusion conditions — Higher orientation levels generally lead to higher HDT values.

• Morphology — Higher crystallinity levels mean higher HDT values.

• Filler loadings — An increase in filler loadings leads to an increase in the HDT. For e.g., reinforced and filled grades have high HDT (harder and stiffer under the heat).

• Presence of plasticizers — They decrease HDT by making the polymer softer and more flexible.

• Other factors that significantly influence the final thermal performance of an application are the:
  • Time of exposure to elevated temperature
  • Rate of temperature increase
  • Part geometry
  • Presence of base resin

• It is used to screen materials for injection molding developments. Higher HDT means a faster molding process.
• It is used in many aspects of product design, engineering, and manufacture of products. This uses thermoplastic components.
• It is used for defining the use of materials in heating elements.
• It represents a value that can be used to compare different materials with each other.
   

Limitations that are associated with the determination of the HDT are that:
 

• the sample is not thermally isotropic and
• thick samples will contain a temperature gradient.

• ASTM D 648 — It determines the deflection temperature for plastics under flexural load in the edgewise position. ASTM D 648 is equivalent to ISO 75.
• ISO 75 — It determines the temperature of deflection under load.

Procedure
 

• A test bar of a specific thickness and width is molded.
• The test sample is submerged in oil for which the temperature is raised at a uniform rate (usually 2°C per minute).
• The load is applied to the midpoint of the test bar that is supported near both ends.
• The temperature at which a bar of material is deformed 0.25mm is recorded as the HDT.

The test is performed using an apparatus as shown below.
 

Source: The Effect of Long-Term Thermal Exposure on Plastics and Elastomers by Laurence W. McKeen

Click to find polymer you are looking for: