Melting/melt viscosity/spread rate

Polyolefins, as pure hydrocarbons, are among the most combustible plastics with an oxygen index of 17 to 18 per cent and heat of combustion of 40 to 45 MJ/kg. Their flame spread rate is high both horizontally and vertically. An unpleasant side effect of their combustion is dripping of the low-viscosity burning melt, which may contribute to a further propagation of fire. The smoke production of this burning is small. 

Since n is less than unity, the apparent viscosity decreases with the deformation rate. Examples of such materials are some polymeric solutions or melts such as rubbers, cellulose acetate and napalm suspensions such as paints, mayonnaise, paper pulp, or detergent slurries and dilute suspensions of inert solids. Pseudoplastic properties of wallpaper paste account for good spreading and adhesion, and those of printing inks prevent their running at low speeds yet allow them to spread easily in high speed machines. 

The ratio of surface energies is the main factor determining the distribution of phases after a melt has formed in the material. This ratio will decide whether the melt will wet the solid surfaces and whether it will penetrate into the capillaries. The behaviour of a melt in pores of a solid system can be characterized by the contact angle O. Perfect wetting takes place when 0 = 0. The rate at which a liquid spreads over a surface or penetrates into the pores is given by its viscosity. 

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