The nature of foam
Polymeric foam is a dispersion of a gas in a polymeric material. The solid, cellular structure of the polymer is filled with one or more gases. Foams can be made from nearly every polymer.
The polymer selection for foam applications mainly depends on the properties of the polymer, the foam production process and the economics of the process. Polyurethane, polystyrene and polyethylene (PE) foams are three popular choices for industrial applications due to their excellent physical and mechanical properties.
The choice of polymer determines whether the resulting foam will be rigid or flexible. Rigid foams are not suitable for energy absorption applications. Flexible materials are available in two categories: elastomeric or plastic. Elastomeric foams show high levels of recovery. The recovery process can either be instantaneous or over a period of time. Flexible elastomeric foams are the most desirable material in foam fender applications.
Foams are characterized by density and cell structure.
Density is one of the most important properties of foam materials. Foams are available in the form of low density, high density, extra high density and super high density, in which the ratio of the gas volume (inside the cell) to polymer volume varies.
The cellular structure (cell) inside the foam can be either open or closed. Individual cells are interconnected in open cell foams.
Stress and strain behavior of foam material under load
The above figure shows a typical compressive stress and strain curve for foam. In principle, the compressive behavior of foam is characterized by three different regions.
Firstly, there is a linear elastic region which is represented by an initial increase in stress. The initial high slope is associated with the elastic modulus (stiffness) of the foam sample.
Subsequently, with more compression, the cells gradually get closed and elongated; the stress no longer increases with strain. This is manifested by a wide stress plateau region. The nature of this region is highly dependent on the degree of porosity (density) as well as the deformation behavior of the cell walls under compression.
After the plateau region, the stress curve gradually comes to a region where the cell walls touch each other leading to a steep increase in stress. The behavior of the graph at this region varies with the type of polymeric material used in the foam’s production (polystyrene, polyethylene, and polypropylene).
Change of microstructure of PE foam under compression
The behavior of closed-cell PE foam under compression has been examined by many researchers in the past (1990). These studies shed light on changes in the microstructure of foam under compression, as described below:
- Microscopic studies of uncrushed foams show that the closed structure has faces with uniform wall thickness
- Most faces have five sides; however there are four and six sided faces
- When closed cell foams are compressed most of the cell faces are bent. Under Scanning Electronic Microscope (SEM), cell deformation can be observed during compression and recovery.
- PE in bulk does not yield under compression until the strain exceeds 10%
- The yield stress increases as foam density increases.
The cell size plays an important role in controlling the rate at which the gas exits the foam when it is compressed.
In most cases, open cell foams possess a high gas and vapor permeability and a high compression modulus. They are most suitable for packaging applications.
On the other hand, the flow of gases between cells in closed cell foams is comparatively low, as the gas needs to flow through extra layers of cell walls.
Foam fenders are usually manufactured by closed cell, cross-linked polyethylene (PE) foam.
Production process of closed-cell PE foam
In order to make PE foams, a blowing agent is needed, which expands within the base PE polymer during heat treatment process. Chemical blowing agents are materials with relatively low decomposition temperatures. Decomposition of these materials results in the release of a large amount of gas.
In the chemical foaming process, the blowing agent is mixed with PE. Subsequently, heating the mixture will decompose the blowing agent inside the PE matrix. At the same time, PE will melt and its viscosity (resistance to flow) will decrease. The combination of these two processes results in bubble nucleation and the formation of cellular structure.
Cross-linking of PE is necessary to provide mechanical stability to the foam, and is achieved through a chemical cross-linking agent. The density and strength properties of PE are not influenced by cross-linking.
- At low compression, cells shear but walls don’t buckle
- The cells narrow in the direction of compression and elongate in the lateral direction
- It is clear to see in Fig 2 that cell A starts to buckle at 25% compression, whereas cell B buckles at 30% compression
- It can be observed from Fig 2 that cells A and B recover unevenly
- Another example of uneven recovery is observed in Cell P, which recovers fully and Cell S, which never recovers its shape
Recovery dynamics after compression of PE foam
The rate of recovery is independent to the density of the foam.
- Foams do not exhibit complete recovery of their original thickness
- The final recovery of the thickness after compression varies with the deformation of the foam
- One can expect a low energy impact to almost recover to the original thickness
- Recovery occurs by the viscoelastic straightening of the buckled faces, but it is incomplete due to some plastic deformation in the structure, leaving the faces remaining slightly buckled
- There is a possibility that the compressed gas inside the cell has a role in the recovery process. However, experimental evidence shows it is unlikely that gas pressure is the main driving force in the recovery process
- Foam creeps under static loading, which is another example of viscoelastic behavior. The recovery after impact compression is a prolonged process
Effect of multiple impacts on PE foam behavior
- The first compression shows an irregular graph with a very high yielding load
- After the first compression’s yield stress peak disappears, the graph normalizes into a plateau shape, for each of the succeeding compressions
- With each successive compression the yield stress is slightly reduced
- If the foam is allowed to fully recover in storage for a long time, one can expect the stress curve to recover to the shape of the second compression graph
- The yield stress, however, is still reduced from its first compression value. This implies that some potential damage occurs during the first compression
References: 1 ‘The Mechanism of the Recovery of Impacted High Density PE Foam’ by P.Loveridge & N.J Mills, University of Birmingham, UK