Nondestructive Evaluation of Lignin-Foam Blocks Using Acoustic Velocity Method

Abstract Structural insulated panel foams are made primarily from petrochemicals. As a biopolymer, lignin is a promising raw material substitute for foam production. Nondestructive evaluation is routinely used for structural material strength and stiffness assessment. Herein, mechanical acoustic velocity (AV) was used to nondestructively evaluate lignin-foam blocks. Overall, foam block weight was determined, and flying time was measured for various foam thicknesses ranging from 7.26 to 8.89 cm (3.0 to 3.5 in.). Additionally, for each block, AV was measured at several different positions/locations. The AV was calculated by dividing the distance (mm) by flying time (μs). Data were analyzed using two-way analysis of covariance, and the Pearson correlation and regression were also determined for foam thickness. There was no significant interaction between position and thickness for AV. Also, no significant correlation or regression was found between foam thicknesses. However, 8.38 cm (3.3 in.) thick exhibited a higher AV compared with all other thickness treatments, and AV was the lowest for 8.38 (3.3 in.) and 7.87 cm (3.1) thick; 7.26 cm (3.0 in.) remained intermediate ( P < 0.0001). Furthermore, the highest AV belonged to back positions, whereas the lowest AV was observed in the length position, and front and length positions were intermediate ( P < 0.0001). These findings indicate that lignin-foam block uniformity may not be consistent at different positions and the appropriate foam thickness should perhaps not be >8.38 cm (3.3 in.).