The thermal conductivity, thermal expansion, mechanical properties at low strain rates and dynamic mechanical properties of a collection of crosslinked closed cell polyethylene foams manufactured by a high pressure nitrogen solution process have been studied as a function of the cell size. The main mechanisms that influence each property and the foam microstructure have been considered to rationalise the results. A theoretical model has been used to examine the thermal conductivity values. The results have shown the extent to which reducing the cell size could improve the insulating capabilities of these materials. The effect of cell size on the mechanical properties at low strain rates is very small, as a consequence the thermal expansion does not depend on cell size. Nevertheless, the structural characteristics are seen to influence dynamic mechanical response at temperatures below 15°C.
Zotefoams plc is a world leading manufacturer of low density, closed-cell, crosslinked block foams and is recognised globally for producing the highest performance polyolefin foams. In recent years the company has developed a new generation of low density foams based on materials where foaming has traditionally been an insurmountable technical challenge or other limitations have existed to the reduction in density possible. One of these new commercially available foam products, sold under the trademark ZOTEK® F, is based on the fluoropolymer Polyvinylidene Fluoride (PVDF). The properties of the products, not surprisingly, reflect the general properties of the fluoropolymer family however as cellular materials the combination of material attributes is unsurpassed in the industry. This paper will describe these novel foam products covering the key aspects of the current commercial grades of ZOTEK® F and the technology used to manufacture them. The paper will then focus on a specific application area in cleanroom insulation, where the product performance will be reviewed against the combination of technically challenging requirements such as low flammability, low smoke generation, low thermal conductivity, low moisture absorption, chemical inertness and resistance to fungal growth.
It is a fundamental response of any polymeric foam material to undergo non-recoverable deformation following the application of a defined compressive strain, exacerbated by temperature and humidity. This process is commonly referred to as compression set. The ability to predict recovery after the application of a compressive strain is crucial to both the manufacturers and end users of foam materials. Specific compression set test procedures have been established to quantify the extent of non-recoverable deformation in specific foam types but to date no general predictive approach exists. In this work, compression set (fixed strain) tests were undertaken on a cellular polyamide-6 material at various temperatures (-5°C to 90°C) and the foam recovery monitored over time periods in excess of those dictated by standard methods (ISO 1856 [1]). An empirical formula has been proposed to allow the prediction of recovery after compressive strain, covering recovery periods from 10 minutes to 24 hours (up to 168 hours at 23°C).
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