Diffusivity in Nylon 6.66 as function of Humidity  

Thread by Drew Peterson on 13 Nov 2008 at 00:08:10. 
I go to school at the University of Minnesota Twin Cities and am currently working with a group on a design project to age a plastic part made from Nylon 6.66. In my research, I have found that this plastic absorbs moisture which leads to a decrease in the strength properties of the part. I have used the Arrhenius equation to determine the moisture uptake in the part as a function of the diffusivity coefficient and time but it is only for 100% relative humidity. I fail to draw the conclusion to moisture uptake and relative humidity. I believe the diffusion coefficient is affected by relative humidity but do not know if the relationship is linear, exponential, or some other type of relationship. Any help??

Thank you

    Comment by Amberose on 07 Apr 2010 at 05:47:14  | |responses: 0|
    I'm currently working on a group project to change the properties of biodegradable plastic to make it have a higher self life and was looking at adding nylon because it can degrade with the correct enzyme additives. I was wondering if you have any information on your project and if you would be willing to share it. Thank you.

    Comment by Rodney on 13 Nov 2008 at 16:06:53  | |responses: 0|
    Dear Drew:
    For the diffusion coefficient of moisture in Polyamide (Nylon) you may assume an exponential dependence on concentration. Note that this factor is only included to account for plasticization of the matrix due to presence of Water.

    Only for a ball park estimation the above described diffusivity is sufficient. To obtain an exact diffusion rate as function of humidity, one must include:

    - The swelling of the matrix (which makes the matrix move in the opposite water diffusion direction),
    - The chemical activity of water in the matrix (chemical activities drive diffusion process instead of concentrations).

    Fick's laws are insufficient for this. One must use the Maxwell - Stefan equation for the diffusion balance and the Flory Huggins theory for the water solubility in Polyamide.