Effect of Cellsize Reduction on Polyurethane Foam Physical Properties

Abstract

Polymer distribution in rigid PU foams is a complex, position de pendent parameter which can be described in terms of: 1) the overall density and density distribution, 2) the average cellsize and cellsize distribution, 3) cell elonga tion and orientation and 4) the material distribution between struts and windows within the individual cells. Considering that a typical foam contains only 2-3 volume % of polymer, it is not surprising that any variation in the way this material is distributed will have a large impact on most of the physical properties of the foam. In the present paper, we evaluate what effects can be expected from changes in cellsize and/or density on the polymer distribution in foams and consequently on foam physical properties like compressive strength, initial thermal conductivity, and thermal conductivity aging. These results are obtained using a blend of modelling techniques and experimental data. Generally speaking, in PU foams a reduction in the average cellsize will also cause an increase in the amount of material which is present in the cell windows versus the cell struts. It is observed that in most systems, the window thickness remains relatively unaffected by cell size reduction. Also, for a given density, there exists a cell size at which the cells will no longer be closed simply because there is not enough material available to ensure film stability. Since the cell windows represent most of the barrier to diffusion, the rate of aging will be reduced dramatically by this shift in the material distribution as long as the cells are still closed. A reduction in cellsize will, in general, also cause a reduction in the initial thermal conductivity, mostly because of the reduction in the thermal radiation component to foam thermal conductivity. There exists a region, however, where this effect is reversed. Further reductions in cellsize will cause an increase in thermal conductivity rather than a decrease. The impact of cellsize reduction on foam compressive strength is usually negative, again because of the concomitant change in polymer distribution.

Compressive behavior of rigid polyurethane foams nanostructured with bacterial nanocellulose at low and intermediate strain rates

“…It has been reported for PUFs that the effect of cell size reduction on foam compressive strength is usually negative. 69,70 The reduction of yield stress in the BNCF could also be explained by the slight deviation of cell orientation ($10-15 ) with respect to the foam rise direction [ Figure 3(c)]. It has also been reported for PUFs that there is a decrease in yield stress with the increase of the angle between the cell orientation and the foam rise direction due to anisotropy.…”

“…This could be explained by a reduction of the cell wall thickness as a consequence of the cell size reduction in the BNCF, when the relative density is similar to that of the PUF. It has been reported for PUFs that the effect of cell size reduction on foam compressive strength is usually negative . The reduction of yield stress in the BNCF could also be explained by the slight deviation of cell orientation (∼10–15°) with respect to the foam rise direction [Figure (c)].…”

Polyurethane-graphene nanocomposite foams with enhanced thermal insulating properties

“…This could be due to a different distribution of polymer between struts and cell windows: indeed, in many PU foams, it is well known that a reduction in the average cell size will also cause an increase in the amount of material which is present in the cell windows versus the cell struts. [18] Because the cell windows represent most of the barrier to diffusion, the rate of aging will be reduced dramatically by this shift in the material distribution. [18] However, considering that 0.5 wt% filled graphene foam shows improved aging performance but not smaller cell size, also a barrier effect of the graphene shall be taken under consideration.…”

“…[18] Because the cell windows represent most of the barrier to diffusion, the rate of aging will be reduced dramatically by this shift in the material distribution. [18] However, considering that 0.5 wt% filled graphene foam shows improved aging performance but not smaller cell size, also a barrier effect of the graphene shall be taken under consideration. Graphene could create a "tortuous" path thus partly hindering, or at least delaying, the diffusion of the gases, in a similar way to what is already well-known for other nanostructured platelet-like fillers, e.g.…”

Influence of Stress Whitening Pretreatment on Cell Structure, Foaming Behavior, and Mechanical Properties of AN/MAA Copolymer Foam

“…Impact resistant application hopes the foam for extremely small cell size to prevent the crack propagation. According to the theory of foam process, the cell structure is decisively influenced by bubble nucleation, bubble expansion and bubble solidification during the foaming process [1][2][3] .…”