We have prepared and tested a series of polymer-filled syntactic foams. Foams with a high proportion of polymer microspheres showed excellent recovery and damage resistance in compression tests.
The first syntactic foam was developed in 1955 by the Bakelite Company of New York, and hailed as “A plastic foam, which promises to cut the partial cost of boat and airplane construction as much as 50%” [1]. It offered strength, insulation and tuneable properties in a lightweight material.
Syntactic foams are materials made up of hollow microspheres (commonly of glass, ceramic or plastic) held in a polymer matrix. The strength and buoyancy of syntactic foams has led to their widespread use in marine applications, their sound absorbing properties find uses in acoustic applications, and they have even been used in World Cup footballs due to their low density and elastic recovery [2,3].
The first syntactic foam was developed in 1955 by the Bakelite Company of New York, and hailed as “A plastic foam, which promises to cut the partial cost of boat and airplane construction as much as 50%” [1]. It offered strength, insulation and tuneable properties in a lightweight material.
Syntactic foams are materials made up of hollow microspheres (commonly of glass, ceramic or plastic) held in a polymer matrix. The strength and buoyancy of syntactic foams has led to their widespread use in marine applications, their sound absorbing properties find uses in acoustic applications, and they have even been used in World Cup footballs due to their low density and elastic recovery [2,3].
 Syntactic foam sphere used as a subsurface float in oceanographic mooring. Photograph by Z22, distrubuted under a Creative Commons Attribution-Share Alike 4.0 International license |  Adidas Fevernova 2002 World Cup Football containing a syntactic foam layer. Photograph by Warrenski, distributed under a Creative Commons Attribution-Share Alike 2.0 Generic license |
Glass vs plastic
The mechanical behaviour of materials can be defined in terms of stress (the force applied to the material per unit area) and strain (the deformation in the material in response to stress).
Viscoelastic materials combine the behaviours of viscous fluids and elastic solids when deformed. A plot of stress vs strain shows hysteresis, where the unloading (reverse) curve follows a different path to the loading (forward) curve, because some energy is lost to the system as heat:
The mechanical behaviour of materials can be defined in terms of stress (the force applied to the material per unit area) and strain (the deformation in the material in response to stress).
Viscoelastic materials combine the behaviours of viscous fluids and elastic solids when deformed. A plot of stress vs strain shows hysteresis, where the unloading (reverse) curve follows a different path to the loading (forward) curve, because some energy is lost to the system as heat:
Stress (σ) vs strain (ε) for an elastic (a) and a viscoelastic (b) material. The red shaded hysteresis loop shows the energy dissipated as heat in the viscoelastic case. Image credit: Gene Settoon This work has been released into the public domain by its author at English Wikipedia.
Under increasing compression, syntactic foams with glass or ceramic microspheres typically respond in three stages:
Stages 2 and 3 correspond to catastrophic damage to the microspheres, so glass microspheres are inappropriate for applications where foams will be under high strain.
Where plastic microspheres are used, the response of the material to compression also has three stages, but with less definition in-between:
The mechanical properties of syntactic foams with glass microspheres are well documented, but less work exists on plastic microsphere syntactic foams.
- Elastic behaviour.
- Crushing of the microspheres (a region of low stiffness).
- Densification: the cavities fill up with debris from the crushed microspheres.
Stages 2 and 3 correspond to catastrophic damage to the microspheres, so glass microspheres are inappropriate for applications where foams will be under high strain.
Where plastic microspheres are used, the response of the material to compression also has three stages, but with less definition in-between:
- A small region of elastic behaviour.
- Buckling of the microsphere walls (a low stiffness region).
- Densification: the microsphere walls begin to touch.
The mechanical properties of syntactic foams with glass microspheres are well documented, but less work exists on plastic microsphere syntactic foams.
Our study
In this study we manufactured and tested polyurethane (PU) syntactic foams containing two grades of polymer microsphere. The foams contained a 2%, 10% or 40% volume of microspheres.
In this study we manufactured and tested polyurethane (PU) syntactic foams containing two grades of polymer microsphere. The foams contained a 2%, 10% or 40% volume of microspheres.
Scanning electron microscope images of syntactic foams with microsphere grades 551 and 920.
The syntactic foams were tested up to medium (25% and 50%) strain alongside unfilled polyurethane. Samples were compressed and unloaded five times and the response measured.
The foams showed viscoelastic behaviour, with hysteresis in the stress-strain curves. Samples with a low volume of microspheres showed similar behaviour to unfilled PU, but increased proportions of microspheres led to some different results. The 10% and 40% samples exhibited stress softening, where a smaller force is needed to achieve the same deformation in successive loadings. This may be due to the buckled microspheres not fully recovering between load cycles.
After 1 week, the samples were re-tested. They showed the same behaviour and little or no change in thickness, indicating that they had fully recovered from the previous testing.
After 1 week, the samples were re-tested. They showed the same behaviour and little or no change in thickness, indicating that they had fully recovered from the previous testing.
Schematic showing the pattern adopted for cyclic compression testing.
Testing up to high strain (70%) revealed interesting behaviour. Unfilled PU and foams with low concentrations of microspheres were damaged, but samples with higher concentrations of microspheres showed damage resistance. Foams with 10% microsphere content were damaged only in the last of the 5 load/unload cycles, and foams with 40% appeared intact after test.
Stress-strain curves for unfilled PU and syntactic foams tested up to 70% strain. Inset: Images of the foams after testing. Damage appears reduced in foams with increasing content of polymer microspheres.
Microscopy reveals cracks in the polyurethane in the damaged samples. The microspheres mitigate the damage by presenting a barrier to crack propagation. This is the opposite result to the case with glass and ceramic microspheres, where foams with higher volume fractions are less resistant to damage, due to the brittle nature of the microspheres.
SEM images of unfilled PU and syntactic foams with 2%, 10% and 40% polymer microspheres, after testing to 70% strain. The damage is significantly reduced in syntactic foams containing higher proportions of microspheres.
These results indicate that polymer-filled syntactic foams containing higher volume fractions of microspheres can show good elastic recovery and significant damage resistance. They may provide an excellent alternative to glass and ceramic containing syntactic foams for applications that require low density materials with high resistance to damage under strain.
Read the full article here: doi.org/10.1016/j.compositesb.2020.107764
[1] Plastic Foam Developed for Boats and Planes, The Science News-Letter, 1955, 67(14), 213
[2] N. Gupta, S.E. Zeltmann, V.C. Shunmugasamy, D. Pinisetty Applications of polymer matrix syntactic foams, JOM, 2014, 66(2), 245-254
[3] For explanation of the anatomy of the 2014 Adidas Brazuca World Cup football: www.livescience.com/46299-microscopic-analysis-brazuca-world-cup-ball.html
Read the full article here: doi.org/10.1016/j.compositesb.2020.107764
[1] Plastic Foam Developed for Boats and Planes, The Science News-Letter, 1955, 67(14), 213
[2] N. Gupta, S.E. Zeltmann, V.C. Shunmugasamy, D. Pinisetty Applications of polymer matrix syntactic foams, JOM, 2014, 66(2), 245-254
[3] For explanation of the anatomy of the 2014 Adidas Brazuca World Cup football: www.livescience.com/46299-microscopic-analysis-brazuca-world-cup-ball.html
RSS Feed