Hygiene products such as incontinence pads bring nonwoven fabrics into contact with users' skin, which can cause damage in various ways, including the nonwoven abrading the skin by friction. The aim of the work described here was to develop and use methods for understanding the origin of friction between nonwoven fabrics and skin by relating measured normal and friction forces to the nature and area of the contact (fibre footprint) between them. The method development work reported here used a skin surrogate (Lorica Soft) in place of skin for reproducibility. The work was primarily experimental in nature, and involved two separate approaches. In the first, a microscope with a shallow depth of field was used to determine the length of nonwoven fibre in contact with a facing surface as a function of pressure, from which the contact area could be inferred; and, in the second, friction between chosen nonwoven fabrics and Lorica Soft was measured at a variety of anatomically relevant pressures (0.25-32.1kPa) and speeds (0.05-5mms(-1)). Both techniques were extensively validated, and showed reproducibility of about 5% in length and force, respectively. Straightforward inspection of the data for Lorica Soft against the nonwovens showed that Amontons' law (with respect to load) was obeyed to high precision (R(2)>0.999 in all cases), though there was the suggestion of sub-linearity at low loads. More detailed consideration of the friction traces suggested that two different friction mechanisms are important, and comparison with the contact data suggests tentatively that they may correspond to adhesion between two different populations of contacts, one "rough" and one "smooth". This additional insight is a good illustration of how these techniques may prove valuable in studying other, similar interfaces. In particular, they could be used to investigate interfaces between nonwovens and skin, which was the primary motivation for developing them.
Friction is important across a wide range of applications. In particular, in health care, friction is thought to be the cause of some pressure ulcers in largely immobile patients, and abrasion due to friction contributes to the deterioration of skin health in incontinence pad users, especially in the presence of liquid. Some of these frictional forces are due to stress in materials wrapped around curved anatomical surfaces, which are often complicated shapes. The little work to date that has considered friction arising by this mechanism has assumed very simplified geometries (prisms, or even cylinders), which have enabled coefficients of friction to be extracted from laboratory tests on arms, but which are certainly not applicable to, for example, the diaper region. This work describes the development of a much more general mathematical model for friction between a draped, stressed sheet and the substrate, relating geometry, material mechanical properties and stress for essentially any convex surface. A general, wide, class of frictional interfaces is described (which includes those which obey Amontons’ law), and the model is presented in differential form for a generic member of this class. Finally, an analytical solution is developed for convex, instantaneously rigid substrates isomorphic to the plane draped with a low-density sheet exhibiting no Poisson contraction, a fair approximation to some anatomical situations. The solution is explicitly calculated for a general prism and a general cone, producing expressions consistent with previous published models and with limited new experimental data.
A new method for measuring the coefficient of friction between nonwoven materials and the curved surface of the volar forearm has been developed and validated. The method was used to measure the coefficient of static friction for three different nonwoven materials on the normal (dry) and over-hydrated volar forearms of five female volunteers (ages 18–44). The method proved simple to run and had good repeatability: the coefficient of variation (standard deviation expressed as a percentage of the mean) for triplets of repeat measurements was usually (80 per cent of the time) less than 10 per cent. Measurements involving the geometrically simpler configuration of pulling a weighted fabric sample horizontally across a quasi-planar area of volar forearm skin proved experimentally more difficult and had poorer repeatability. However, correlations between values of coefficient of static friction derived using the two methods were good ( R = 0.81 for normal (dry) skin, and 0.91 for over-hydrated skin). Measurements of the coefficient of static friction for the three nonwovens for normal (dry) and for over-hydrated skin varied in the ranges of about 0.3–0.5 and 0.9–1.3, respectively. In agreement with Amontons' law, coefficients of friction were invariant with normal pressure over the entire experimental range (0.1–8.2 kPa).
Despite the wide usage of isotropic fibrous composites with a viscoelastic polymer matrix, no analytic model for their mechanical behaviour is known. This paper develops such a model for time-dependent Young's modulus, showing that for typical constituents the time constants of composites are up to about 6% greater than the matrix shear time constant. Viscoelasticity is strongly suppressed for stiff fibres even at modest fibre volume fractions. Comparison with known results for particle and oriented fibre composites confirms isotropic fibrous composites as between the two in terms of viscoelastic behaviour, but more similar to the latter.
An analytical mathematical model for friction between a fabric strip and the volar forearm has been developed and validated experimentally. The model generalizes the common assumption of a cylindrical arm to any convex prism, and makes predictions for pressure and tension based on Amontons' law. This includes a relationship between the coefficient of static friction ( μ) and forces on either end of a fabric strip in contact with part of the surface of the arm and perpendicular to its axis. Coefficients of friction were determined from experiments between arm phantoms of circular and elliptical cross-section (made from Plaster of Paris covered in Neoprene) and a nonwoven fabric. As predicted by the model, all values of μ calculated from experimental results agreed within ±8 per cent, and showed very little systematic variation with the deadweight, geometry, or arc of contact used. With an appropriate choice of coordinates the relationship predicted by this model for forces on either end of a fabric strip reduces to the prediction from the common model for circular arms. This helps to explain the surprisingly accurate values of μ obtained by applying the cylindrical model to experimental data on real arms.
Various hygiene products, notably incontinence pads, bring nonwoven “topsheet” fabrics into contact
with individuals’ skin. This contact can damage the skin in various ways, including abrading it
by friction, a mechanism enhanced by the presence of moisture. In recent years skin-nonwoven friction
has been the subject of significant experimental study in the Continence and Skin Technology
Group, UCL, in the course of which methods have been developed which can detect differences
in friction between a chosen nonwoven and equivalent skin sites on different individuals under
fixed conditions. The reasons for these differences are unknown; their elucidation is one focus of
this work. The other is to establish the influence of coarse geometry on the dynamics of a tense
nonwoven sheet sliding over a substrate and interacting with it by friction.
The first part of this work (“microfriction”) is primarily experimental in nature, and involves two
separate experiments. The first involves using a microscope with a shallow depth of field to determine
the length of nonwoven fibre in contact with a facing surface as a function of pressure; the
second consists of measuring friction between chosen nonwovens and a skin surrogate at a variety of
pressures and speeds whilst simultaneously observing the behaviour of the interface down a microscope.
Both techniques were extensively validated, and the data from the two experiments were then
compared. It had originally been intended to conduct the friction experiment on skin (the other
experiment does not require it), and though all equipment was developed with this in mind and all
relevant permission was sought and obtained, it was not eventually possible. Instead, a skin friction
surrogate (Lorica Soft) established in the literature was used. Data from this show that Amontons’
law (with respect to load) is obeyed to high precision (R2 > 0.999 in all cases), though there is the
suggestion of sublinearity at low loads. Detailed consideration of the friction traces suggests that
two different friction mechanisms are important, and comparison with the contact data suggests
tentatively that they may correspond to adhesion between two different populations of contacts,
one “rough” and one “smooth”.
Further work applying these techniques to skin is necessary.
The second aspect of the work is “geometric friction”; that is, the relationship between the geometry
of a surface (on the centimetre scale and upwards) and the friction experienced by a compliant
sheet (such as nonwoven topsheet) laid over it in tension. A general equation of motion for slippage
between sheet and surface has been derived which in principle allows for both objects to deform
and interact according to any plausible friction law. This has then been solved in integral form for Amontons’ law and a low density strip exhibiting no Poisson contraction sliding over any surface
with zero Gaussian curvature; closed form solutions for the specific cases of a prism and a circular
cone have then been derived and compared. Experimental verification has been provided by a colleague,
which shows very good agreement between theory and experiment. It has also been shown
that, taking a naive approach, the classic model for a rigid cylinder can be applied even to a quite
extreme cone with experimentally negligible error.
NB All prior copyrighted material (diagrams in all cases) has been removed from this edition to
facilitate electronic distribution. They have been replaced with boxes of the same size, so pagination
is identical with the complete version.
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