Polymer Dictionary

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	Heterophasic copolymers Heterophasic copolymers (HECO) also contain ethylene (and possibly higher a-olefins) as comonomer. The production is however carried out in a two-stage process, resulting in a multiphase structure with a homopolymer matrix and inclusions consisting of amorphous EP-copolymer (rubber) and crystalline PE. Variations of molar mass and composition of the elastomeric phase in relation to the matrix allow a wide variation of properties (stiffness, toughness and transparency).
	Heterogeneous Something relating to two ore more phases, i.e. not mixed so as to be homogeneous.
	Homogeneous Something relating to only one phase, i.e. so well mixed that the mixture have the same composition throughout.
	Haze Cloudiness in polymer films or thin-walled injection-moulded parts, resulting from light scattering in the material (reducing the transparency). Measured in transmission with a well-defined light source, results are given on a percentage scale.
	Hydrogen sensitivity A property of the catalyst which indicates how sensitive the molecular weight of the polymer is to hydrogen concentration, (also: hydrogen response). The property also depends on the polymerisation conditions (mode, temperature, pressure etc.).
	Heat deflection temperature Heat deflection temperature (HDT) indicates how loaded material deforms at higher temperatures. Test bars are placed in a heating bath, resting horizontally on two supports. A constant load is applied in the center of the specimen and the bath temperature is raised at a constant rate. The temperature of the bath at which the flexural deflection of the loading point has reached a predefined level is the heat deflection temperature of the material.
	Homopolymers Homopolymers (HOMO) consist only of propylene units; their properties can be regulated via molar mass distribution and the degree of isotacticity (stereoregular structure).
	High pressure polyethylene (HPPE) The collective name given to polyethylene produced by high pressure processes. The process is using free radical polymerisation and the polymerisation is performed at pressures of 120-350 MPa (1200-3500 bar) and at temperatures of 150-3500C. Homopolymer and copolymers of ethylene is made by this process. (See more descriptions in process description).
	HDPE High density polyethylene Coming from a low pressure process based on special catalysts (Ti- or Cr-based), HDPE has a linear polymeric chain, combined with a higher density and melting point. To fit special applications, the molar mass distribution can be adjusted in a wide range by means of catalysts or polymerisation technology. New applications are opened for HDPE as well as other low pressure PEs by applying metallocene (single-site) catalysts, which allow the production of very narrow molar mass distributions and the better distribution of comonomers.
	Hardness Resistance of a (polymer) surface to deformation. The different hardness measures applied for characterising polymers are: (a) Shore hardness (two scales, A for softer and D for harder materials) (b) Ball indentation hardness (also useable on profiled surfaces because of bigger measuring device). (c) Vickers hardness (normally used for steel).