Structure and Physical Properties of Polyethylene and Polypropylene
As explained in the previous section, the most suitable grade of polyethylene
or polypropylene is selected from among a variety of kinds, molded, and
finally made into a product. It is important to understand the relationship
between the structure and the physical properties of polyethylene or polypropylene,
or in other words, how the structure affects not only the formability of
raw resin but also the physical properties, performance and quality of
a product. This section will look at polyethylene in more detail.
There are two typical indices to indicate the basic properties of polyethylene: MFR (Melt Flow Rate) and density. Raw material suppliers offer a number of grades of different indices. MFR represents the ease of the flow of molten resin. MFR affects the extrusionability of raw resin and the mechanical properties of a product. MFR is determined according to the molecular weight and long chain branching. On the other hand, density affects the stiffness, transparency and melting point of a product. Density is determined according to short-chain branching. Both MFR and density are very important indices to indicate the basic properties of polyethylene. But in order to design and develop an excellent product, the nature of these indices, formability and the physical properties of a product should be understood in depth.
The diagram below shows the flow from raw resin (polyethylene) to a product
and controlling or key factors.
Correctly understanding the relationship between these factors leads to superior design and development of a product. Below is a brief description of entanglement and solid structures. Entanglement significantly affects formability, while the solid structure effects the physical properties of a product.
Polyethylene, which is characterized by flexibility, has entanglements
or physical cross-links when it is in a molten state. Entanglements make
polyethylene elastic. On the other hand, modular motion causes disentanglement
of molecular chains. When molecular chains are deformed, friction between
them causes viscosity.
As a result, polyethylene has viscoelasticity or both viscosity and elasticity characteristics when it is in a molten state. Viscoelasticity varies depending on a degree of entanglement. To be more specific, the strain rate varies the degree of disentanglement, and that eventually varies the viscosity and elasticity. The figure below shows the dependence of viscosity h* (complex viscosity), moduli of elasticity G’ (storage modulus) and G” (loss modulus) on the shear rate. Viscosity h* and moduli of elasticity G’ and G’’ are obtained by viscoelastic measurement. As the shear rate becomes lower (or as shearing takes a longer time), elasticity becomes lower, but viscosity becomes higher due to disentanglement. In contrast, as a shear rate becomes higher (or as shearing takes a shorter time), elasticity becomes higher, but viscosity becomes lower because entanglement is maintained.
In the actual molding process, it is important to identify an appropriate
shear rate range to obtain the desired viscoelasticity. If the extrusionability
or formability is to be improved, the molecular weight, molecular weight
distribution, and long-chain branching, all of which are the key determinants
of viscoelasticity, have to be well controlled so that the desired viscoelasticity
can be obtained within the proper shear rate range.
Molded polyethylene is cooled and solidified. When solidified due to short-chain
branching and entanglement, polyethylene consists of a number of molecular
chains folded regularly as well as amorphous regions, as shown in the schematic
model below. The ratio of amorphous to crystalline structures significantly
affects stiffness and flexibility while the thickness of lamellae determines
the melting temperature (or the melting point). As described here, polyethylene
is a semicrystalline polymer composed of crystals and amorphous regions.
The physical properties of a product are determined by the solid structure
of a product, which is affected by the structure and molding of polyethylene.
Next Page. 4.THP's Design and Development
Technical information, Other pages
1.Polyethylene, Polypropylene and THP's Products 2.Kinds of polyethylene and polypropylene
3.Structure and Physical Properties of Polyethylene and Polypropylene 4.THP's Design and Development