Syntactic foams as building materials consisting of inorganic hollow microspheres and starch binder

Syntactic foams as building materials consisting of inorganic hollow microspheres and starch binder

Syntactic foams as building materials are studied. Various manufacturing parameters contributing to syntactic foam composition in relation with the ‘pre-mould’ method were identified and inter-related. An equation based on lattice unit cell models with the minimum inter-microsphere distance concept for a relation between volume expansion rate of bulk microspheres in aqueous starch and microsphere size was derived and successfully used to predict experimental data. A simple method for estimation of syntactic foam density prior to completion of manufacture was suggested. Shrinkage of syntactic foam precursor was discussed in relation with different stages such as slurry, dough and solid. Also, the ‘post-mould’ buoyancy method involving mixing starch particles and ceramic hollow microspheres in water is discussed in relation with composition and properties. It was found starch particles tend to adhere to hollow microspheres during mixing, forming agglomerations. A transition in the formation of mixture volumes in water was found to take place at a calculated relative density value of 1 for an agglomerate consisting of multiple starch particles and one microsphere. A Simple Cubic cell model for the starch-microsphere inter-distance was adopted to quantitatively explain various effects on starch content in agglomeration such as hollow microsphere size, initial bulk volume of hollow microspheres and water volume. Further, the following were found for syntactic foams: (a) volume fraction of starch in foam is of linear relation with starch content in binder for a given experimental data range and (b) shrinkage of syntactic foam precursor is relatively high for small hollow microspheres with high starch content. Compressive failure behaviour and mechanical properties of the manufactured foams were evaluated. Not much difference in failure behaviour or in mechanical properties between the two different (pre- and post-mould) methods was found for a given binder content in syntactic foam. Compressive failure of all syntactic foams was of shear on plane inclined 45° to compressive loading direction. Failure surfaces of most syntactic foams were characterised by debonded microspheres. Compressive strength and modulus of syntactic foams were found to be dependant mainly on binder content but independent of microsphere size. Some conditions of relativity arising from properties of constituents leading to the rule of mixtures relationships for compressive strength and to understanding of compressive/transitional failure behaviour were developed. The developed relationships based on the rule of mixtures were partially verified.

Novel sandwich composites made of syntactic foam core and paper skin were developed. Interface bonding between syntactic foam core and paper skin was controlled by varying starch content. Two different microsphere size groups were employed for syntactic foam core manufacturing. Properties of skin paper with starch adhesive on were found to be affected by drying time of starch adhesive. Skin paper contributed to increase up to 40% in estimated flexural strength over syntactic foams, depending on starch content in adhesive between syntactic foam core and paper skin. Small microsphere size group for syntactic foam core was found to be advantageous in strengthening of sandwich composites for a given starch content in adhesive. This finding was in agreement with calculated values of estimated shear stress at interface between paper skin and foam core. Failure process of the sandwich composites was discussed in relation with load-deflection curves. Hygroscopic behaviour of syntactic foam panels was investigated. Moisture content in syntactic foam was measured to be high for high starch content in syntactic foam panels. No significant moisture effect on flexural strength syntactic foam panels after being subjected to moisture about two months was found. However, substantial decrease in flexural modulus was found for syntactic foam panels made of large microspheres although not much moisture effect was found on that of small microspheres.

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