Materials analysis and image-based modelling of transmissibility and strain behaviour in approved face mask microstructures

Abstract

Comparisons are made between six different approved face masks concerning their particle transmissibility allied to mechanical properties. The latter involves material testing and stretch or strain behaviour under load. SEM and X-ray elemental analyses showed contrasting structures between random and ordered fibre orientations. These constitute the mask designs where transmissibility is to be minimised. Airflow velocity measurement enabled filtration to be measured between the different mask designs, from two to six layers of different fabrics in combination. SEM provided the fibre diameter and pore size of each mask layer, up to a maximum of six. Stretching each complete mask showed its elasticity and recovery behaviour on an energy basis. The energy conversion involved in mask straining involves areas enclosed within steady and cyclic load-extension plots. Thus, the work done in extending a mask and the energy recovered from its release identified a hysteresis associated with an irrecoverable permanent stretch to the mask fabric. Failure of individual layers, which occurred successively in extended stretch tests, appeared as a drop in a load-extension response. That change is associated with permanent damage to each mask and friction contact within the rearrangement of loose fibre weaves. Masks with the greatest number of layers reduced particle transmissibility. However, woven or ordered mask fabrics in two layers with different orientations provided comparable performance. Simulation of each mechanical response, velocity streamlining and fibre distribution within the mask layers are also presented.

Figure 1

SEM for each of the three layers of a surgical mask. (A–C) front layer. (D–F) second layer. (G–I) third layer.

Figure 2

SEM for each of the four layers of a reusable cotton mask. (A–C) front layer. (D–F) second layer. (G–I) third layer. (J–L) Filter holder layer or back layer.

Figure 3

SEM for each of the five layers of a FFP2 mask. (A–C) front layer. (D–F) second fibrous layer. (G–I) third layer. (J–L) forth layer. (M–O) fifth layer.

Figure 4

SEM for each of the six layers of a FFP3 face mask. (A–C) front layer. (D–F) second layer. (G–I) third layer. (J–L) forth layer. (M–O) fifth layer. (P–R) six layer.

Figure 5

SEM for each of the three layers of an antiviral face mask. (A–C) first or front layer. (D–F) second layer. (G–I) third layer.

Figure 6

SEM for each of the two layers of a silk face mask with nose wire- pure mulberry silk breathable. (A–C) front layer. (D–F) the back of front layer. (G–I) second layer.

Figure 7

EDX elemental analysis of the six approved face masks. (A) surgical. (B) reusable cotton. (C) FFP2. (D) FFP3. (E) antiviral. (F) silk.

Figure 8

The pore size for the six masks was derived from individual layer measurements. (A) for each layer. (B) average pore size of each mask.

Figure 9

The fibre diameter for the six masks was derived from their individual layer measurements. (A) for each layer. (B) average fibre diameter of each mask.

Figure 10

Strength test (stretch to failure) for multi-layered mask materials. Key: (A) surgical. (B) reusable cotton. (C) FFP2. (D) FFP3. (E) antiviral. (F) silk.

Figure 11

Mechanical properties of approved face masks. (A) maximum load. (B) stretch to failure.

Figure 12

(A) Surgical and (B) FFP2 experimental hysteresis plots. (C–D) Simulation of the area under the curve (unloading-loading cycles) for (C) surgical and (D) FFP2 face masks. [Area Unit: N x mm = mJ (millijoule)].

Figure 13

Simulation of fibre distribution of all layers for face masks: (A, B) surgical. (C, D) FFP2. (E, F) FFP3. (G, H) reusable cotton. (I, J) antiviral. (K, L) silk. The velocity streamline appears in the z-direction for the layers of two face masks: (M) surgical and (N) FFP2.