The moisture of wood during the drying process measured using NMR

The report

The entire report.pdf (in Dutch) is public. Note that most the illustrations were designed with Maya and code was written for CVI/LabWindows.

Topic

Many building materials are damaged by moisture and minerals. Porosity of materials influences the maximum moisture content of a material. Materials with many and large pores take up more water and more quickly. In concrete or brick, a high moisture content can lead to the growth of mildew or moss. In winter time, the freezing water in the pores can crack the material.

Wood is also a porous material and is highly influence by its surroundings, such as temperature and air moisture content. A warm and moist surrounding stimulates the growth of mildew and moss and causes wood rot.

Research into the moisture dynamics of wood has more applications than just the sustainability of the material. Wood needs to be dried before it can be used for biomass energy. Wood as a building material is dried, just as wood that is used to produce electricity. Research is conducted into drying the wood as efficiently as possible, which requires a better insight into the processes in the wood during drying.

Using NMR techniques, the drying process can be followed. Information about the pore distribution and drying profiles are obtained without destroying the sample.

Abstract

For many applications, wood is ideally dried as efficiently as possible. Using Nuclear Magnetic Resonance (NMR) techniques, it is tried to obtain more insight into the drying process. A wet, pinewood cubic sample was dried for 35 hours using a dry, unheated airflow. By taking 1D and 2D profiles of the drying sample it was found that it is hard to determine the exact macroscopic diffusion of water in wood.

The NMR relaxation of the water in the pores of the samples was investigated using a Carr-Purcell-Meiboom-Gill pulse train (CPMG) and the pore size distribution could be determined. It was attempted to measure the total moisture content of the sample by means of 1D and 2D Hahn spin-echo measurements and a CPMG pulse train. Next to the temprature of the sample, also this moisture content was followed during the drying process. We found lower Fiber Saturation Point than reported in literature. This can be attributed to the fast relaxation of the water in the smaller pores, which hardly contribute to the 1D and 2D profiles that were measured at relatively large echo times. Also from the results of the CPMG measurements, accurate moisture values could not be determined by this fast relaxation.

Reflection

I wrote the report in Dutch and in Word. This would be my last report to be mistreated in that way. While I enjoyed the practical work on a lab NMR machine, designing a sample holder and doing hourly measurements. I think I realized here that I would be more interested in theoretical work.