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Polymer erosion front speed is the rate at which a surface-localized degradation boundary advances into a specimen. In the IUPAC sense of polymer erosion, the process progresses from the surface inward. A decrease in specimen mass alone does not establish that such a front exists.^[ref]

For an original spherical model, let $R(t)$ be the radius of the surviving uniform material, assume constant density $\rho$ and define positive inward speed $v=-dR/dt$. ThenThe local speed is therefore $v=-M^{\prime }/(4\pi \rho R^2)$. A declining mass-loss rate can arise from shrinking surface area even at constant front speed; it need not mean the surface reaction slowed.

Writing the retained mass fraction as $f=M/M_0$ gives, under these assumptions,Geometry-dependent erosion-rate inference matters: for a sheet attacked on two faces of fixed area $A$, the corresponding relation is $-M^{\prime }=2\rho Av$ before the fronts meet. Substituting the spherical relation for a sheet would misassign the same observed mass rate.

An original radius measurement finds $R_1=1.00\mathrm{mm}$ and $R_2=0.94\mathrm{mm}$ six days apart. The average inward speed is $(R_1-R_2)/6=0.01\mathrm{mm}\mathrm{da}{\mathrm{y}}^{-1}$. Independent standard uncertainties of 0.005 mm in the radii give speed uncertainty about $0.00118\mathrm{mm}\mathrm{da}{\mathrm{y}}^{-1}$ when timing uncertainty is negligible. A shared additive radius offset cancels from the difference, but a shared scale error does not generally cancel from the speed. Uncertainty propagation must retain that distinction.^[ref]

To test the front interpretation, pair dimensional or cross-sectional measurements with mass and chemical observations. Swelling, nonuniform density, fragment detachment and a broad reaction zone violate parts of the sharp-front model. Numerical differentiation of noisy mass measurements can also magnify scatter.

A manufacturer's timed water-exposure test for polyglycolic acid weighs filtered dry residue.^[ref] It supplies a retained-mass observable, not by itself a measured front position. Report a front speed only with the spatial evidence and geometric assumptions needed to give that speed a physical meaning.