Cross-linked polyethylene sheet
Measuring Cell: TMA40 with 3-mm ball-point probe.
Pan: The sample is measured between fused silica disks of 6-mm diameter and 0.5-mm thickness
Sample preparation: A rectangular sample of about 4X4 mm is cut off with a knife. The height of the sample, 5.1 mm, is the same as the thickness of the original sheet
TMA measurement: A first heating run to 150°C at 10 K/min eliminated thermal history. After uncontrolled cooling to 25°C, the measurement was performed at a heating rate of 10 K/min. Note: the pretreatment was also done under dynamic load.
Load: Periodically changing every 6 s between 0.01 and 0.19 N
Atmosphere: Static Air
The elastic deformation of 1.9% in the amorphous state allows the complex Young's modulus, E, to be calculated as follows:
E=ΔF/(A*ΔLr), where ΔF is the change in force, A, the cross-sectional area of the sample, and the ΔLr the relative change in length (1.9%).
E=0.18 N/(4*4mm2 * ).019)= 0.59 N/mm2
The onset of the difference of the envelopes at 99°C is related to the crystallite melting.
The expansion coefficients derived from the slope of the upper envelope (load 0.01 N) are:
between 40 and 50°C, semicrystalline 275 ppm/K
between 120 and 130°C, amorphous 353 ppm/K
Cross-linked polyethylene below the crystallite melting range is rigid, just like standard PE. When the crystallites melt the volume increases and the properties become rubbery-like. There is an elastic deformation of approx. 1.9% caused by the varying force. The distance between upper and lower employees is proportional to the compliance (1/E). The overall slope of the curve envelope reflects the expansion of the sample
Note: PE that is not cross-linked would be squeezed out between the disks after melting (plastic deformation)
The thermomechanical properties of cross-linked polyethylene above the crystallite melting range are completely different to those of normal PE. Instead of viscous flow there is rubbery elastic behavior. The cross-linked macromolecules prevent plastic deformation.
Cross-linked PE by Dynamic Load TMA | Thermal Analysis Application No. HB 210 | Application published in METTLER TOLEDO TA Application Handbook Thermoplastics