Canoe, a leading French Research & Technology center, has unveiled an innovative, in-situ, and non-destructive methodology for characterizing the crystallinity rate of semi-crystalline polymers using ultrasonic measurements. This breakthrough offers a crucial alternative to conventional, pseudo-destructive techniques like Differential Scanning Calorimetry (DSC), providing a significant leap forward for quality control in polymer manufacturing.
Traditionally, the crystallinity rate—a pivotal parameter directly influencing mechanical resistance, stiffness, thermal, and gas barrier properties—is determined via DSC, a method that requires destructive sampling of the final product. This limitation has historically precluded its use for in-situ or continuous process control. Canoe's new ultrasonic methodology addresses this challenge, enabling a complete cartography of properties across an entire part without compromising its integrity.
The methodology leverages the well-established principle that ultrasound speed and material density are directly related to mechanical stiffness. Building on this, Canoe has developed a technique that precisely determines ultrasound velocity within the material, thereby establishing a direct and accurate relationship to the crystallinity rate. This approach can be oriented to account for material anisotropy, ensuring robust measurements.
To validate this correlation, Canoe conducted a detailed study using Polyamide 11 (PA11) semi-crystalline polymer specimens, which were subjected to controlled heat treatment to progressively increase their crystallinity. Ultrasound velocity measurements were then meticulously performed across the entire surface of these specimens using an immersion technique, ensuring consistent acoustic coupling. A focused 0.5-inch diameter, 5 MHz single-element transducer was employed to map the surface with a 1 mm step, yielding a comprehensive cartography of ultrasound speeds.
Following the ultrasonic inspection, samples from each specimen underwent traditional DSC analysis to determine their crystallinity rates. The comparison between the two techniques revealed a strong, nearly linear correlation (R²=0.92) between the crystallinity rate and the averaged ultrasound velocity, demonstrating a correlation coefficient of approximately 95%. Further analysis confirmed excellent linearity (R²=0.93) between the geometrical density and the corresponding elastic tensor coefficient (C33), reinforcing the methodology’s robustness.
This study underscores the feasibility of accurately estimating crystallinity rates through a straightforward measurement of ultrasound velocity. While the immersion technique proved effective for automated surface monitoring, the ultrasound velocity can also be determined using portable devices, allowing for in-situ measurements directly on manufactured structures. This NDT approach is fast, reproducible, and accurate, applicable across a wide range of materials where geometry and attenuation properties allow for correct wave propagation.
While the methodology's efficiency in composite materials—where the presence of fibers or loads might impact ultrasound wave speed—still requires further demonstration, Canoe's innovation represents a major advancement in non-destructive testing for quality control of semi-crystalline polymer components.
