Understanding ASTM D4974: Hot Air Shrinkage Test for Yarn and Cord

Reinforcement Yarn and Thread for Industrial Hoses

Synthetic yarns, such as nylon and polyester, are known for their relatively high shrinkage when exposed to heat. To maintain consistent physical and mechanical properties after extrusion, these yarns are commonly subjected to heat setting. This process involves stretching the yarn under controlled tension at elevated temperatures, permanently altering the fiber’s crystallinity and internal structure. The process ensures that the yarn maintains its dimensional stability and mechanical properties during subsequent high-temperature processing.

By selecting precise heat-setting conditions—including temperature, tension, and exposure time—manufacturers can adjust the yarn’s shrinkage, strength, and elasticity to meet specific performance requirements. Properly heat-set yarns resist deformation during downstream processes, such as rubber vulcanization, and provide long-term stability in demanding industrial applications.

ASTM D4974: Purpose and Applications

The ASTM D4974 standard is widely used for heat-resistant yarns, including nylon, polyester, and aramid fibers. These materials are commonly utilized in tire cords, belt reinforcements, and other mechanical rubber goods (MRG) that undergo vulcanization. During vulcanization, rubber is exposed to high heat to achieve its final mechanical and physical properties. If a yarn has high thermal shrinkage, it can constrict components such as hoses or belts during this process, potentially compromising the final product’s dimensions and performance.

Low-shrinkage yarns, carefully heat-set according to proper parameters, exhibit minimal dimensional change when exposed to heat. This makes them more suitable for industrial applications requiring high thermal stability, consistent performance, and reliable integration with rubber or other heat-sensitive materials.

 

Hot Air Thermal Shrinkage Tester: Testing Procedure

그리고 Hot Air Thermal Shrinkage Tester is the primary tool for performing ASTM D4974 testing. This instrument exposes a specimen of yarn or cord to controlled dry heat while maintaining a precise tension. Shrinkage is measured directly as a percentage, while the sample remains under tension at the specified temperature.

The ASTM D4974 procedure generally heats the yarn to 177°C (350°F) for 2 minutes with a pretension load of 0.05 grams per denier (gpd). For yarns with very low shrinkage, a lower pretension of 0.01 gpd is recommended for more accurate measurements. Adjustments to pretension can influence the shrinkage reading, with higher pretension values generally reducing the measured shrinkage.
This testing method allows manufacturers to confirm that the yarn or cord maintains its physical stability under high-heat conditions, ensuring predictable performance during vulcanization and other thermal processes.

Variations in Testing and Considerations

It is important to note that not all manufacturers follow ASTM D4974 exactly. Differences in testing temperature, duration, or applied tension can significantly affect the measured shrinkage. For instance, a yarn tested at 190°C for 10 minutes under a specific load may yield different results compared to a yarn tested at ASTM D4974 conditions of 177°C for 2 minutes. Despite these differences, when both materials are tested under the same conditions, their thermal shrinkage may be nearly identical.
Materials with lower melting points or reduced heat resistance require adjusted testing conditions to avoid fiber damage. Therefore, shrinkage test parameters are often explicitly stated in material specifications, allowing end-users to compare results accurately and select the most suitable yarn for their application.

Ensuring Quality and Reliability

Using a Hot Air Thermal Shrinkage Tester is essential for verifying the thermal stability of yarns and cords. By performing ASTM D4974 testing, manufacturers can ensure that materials will retain their dimensions and mechanical properties during high-temperature processes, such as vulcanization of rubber products.
Understanding the testing setup, procedure, and results interpretation is crucial for accurate measurements. Industry professionals can assist in implementing ASTM D4974 testing, helping manufacturers and end-users achieve consistent, high-quality materials that perform reliably in demanding industrial applications.