Cross-linked PE by Dynamic Load TMA

Sample

Cross-linked polyethylene sheet

Conditions

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 

Evaluation

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*ΔL_r), where ΔF is the change in force, A, the cross-sectional area of the sample, and the ΔL_r 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

Interpretation

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)

Conclusions

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}