Effect of stearate/stearic acid coating on filled high density polyethylene properties
High density polyethylene (HDPE) is a widely used plastic but it is also a combustible material. One way of reducing flammability is to add fillers, such as magnesium hydroxide (Mg(OH)2). However, this has a deleterious effect on the mechanical properties of composites. It has been found that one possible method of restoring mechanical properties is to modifY the filler particles with coating agents, such as stearic acid. In the present work, this idea was taken a stage further with the use of various metal stearates (e.g. magnesium stearate, calcium stearate, and zinc stearate) for modifying filler. The fillers examined were magnesium hydroxide and calcium carbonate. A filler loading of 40% w/w was used in all samples. Samples were moulded into a variety of shapes for mechanical testing. Such tests included, tensile, flexural, and impact testing. To obtain deeper understanding of the effect of the coating agents on the fillers, a variety of fundamental tests were carried out. These included Diffuse Reflectance FTIR (DRIFT), Thermal Analysis using a DSC cell, Xray Diffraction (XRD), contact angle measurement. Unfilled HDPE, uncoated filled-HDPE, and coated filled-HDPE were compared using uncoated filled-HDPE as a base line. Uncoated filled-HDPE is more brittle than unfilled HPDE. Surface modification of filler improves the toughness properties. Comparing coated filled-compounds, stearic acid and zinc stearate caused a small improvement, magnesium stearate improved the properties significantly with calcium carbonate while calcium stearate gave the best results for coating magnesium hydroxide. One monolayer coating gave the best compound properties compared to other degrees of coating. Although, tensile/flexural strength was not greatly affected elongation at yield, extension at maximum load, and impact properties increased significantly. DSC was used to observe the disappearance and conversion of coating agents as coating proceeded. X-ray diffraction showed the effect of injection moulding on the orientation of the filler and polymer. During coating of the filler particles, XRD and DSC were used to follow incorporation of stearate particles to produce the monolayer coverage. Surface free energy results showed that surface modification of filler resulted in the reduction of hydrophilicity of filler leading to tougher composites compared with uncoated filled-compounds.