Surface and bulk erosion

[1] In bulk erosion, degradation occurs throughout the whole material equally.

[2] Therefore, surface and bulk erosion can be thought of as a spectrum instead of two separate categories.

For very thin materials, the surface area remains relatively constant when the material degrades, which allows surface erosion to be characterized as zero order release since the rate of degradation is constant.

[3] Due to degradation, the volume of the material decreases during bulk erosion causing the erosion rate to decrease over time.

Therefore, bulk erosion rates are difficult to control since it is not zero order.

[1] To determine whether a polymer will undergo surface or bulk erosion, the degradation rate of the polymer in water (how fast the polymer reacts to water) and the rate of diffusion of water penetrating through the material must be considered.

If the degradation process is faster than the diffusion process, surface erosion will occur since the material's surface will quickly degrade before water has time to diffuse and penetrate through the material.

If the diffusion process is faster than the degradation process bulk erosion will occur because water penetrates through the material before significant erosion occurs on the surface.

On the other hand, decreasing the dimensions of a material will allow water to travel to the center of the material more quickly, which speeds up the diffusion process and causes bulk erosion.

[4] Where is the mean length of the material and D is the diffusion coefficient of water inside the polymer.

[4] Where M = molecular weight of polymer, NA = Avogadro constant, N = degree of polymerization, p = density of polymer, k = degradation rate The ratio between diffusion time and degradation time gives us a dimensionless parameter ε called the erosion number.

[4] From the model above, it is clear that certain changing certain parameters can determine what kind of erosion a polymer goes through by either increasing or decreasing the rate of the degradation process or the diffusion process.

[2] However, bulk erosion can be useful in situations that do not require controlled release, such as plastic degradation.