Polyethylene resins, characteristic

Polyethylene. Traditional melt spun methods have not utilized polyethylene as the base polymer because the physical properties obtained have been lower compared to those obtained with polypropylene. Advances in polyethylene technology may result in the commercialization of new spunbonded stmctures having characteristics not attainable with polypropylene. Although fiber-grade polyethylene resin was announced in late 1986 (11,12), it has seen limited acceptance because of higher costs and continuing improvements in polypropylene resin technology (see Olefin POLYMERS, POLYETHYLENE). 

The mechanisms described above tell us how heat travels in systems, but we are also interested in its rate of transfer. The most common way to describe the heat transfer rate is through the use of thermal conductivity coefficients, which define how quickly heat will travel per unit length (or area for convection processes). Every material has a characteristic thermal conductivity coefficient. Metals have high thermal conductivities, while polymers generally exhibit low thermal conductivities. One interesting application of thermal conductivity is the utilization of calcium carbonate in blown film processing. Calcium carbonate is added to a polyethylene resin to increase the heat transfer rate from the melt to the air surrounding the bubble. Without the calcium carbonate, the resin cools much more slowly and production rates are decreased. 

The physical characteristics of polyethylene resins vary widely as a function of their density. The density of polyethylene is highest when it has very few branches to impede the crystallization process. A 3 mm thick plaque of high density polyethylene is an opaque white solid that can... 

Raw polyethylene resin is melted and shaped into pellets. This increases bulk density, improves handling characteristics and reduces shipping costs. Pellet size is typically 3 mm (or 0.1 in). 

Incorporation of chlorine atoms onto the polyolefin backbone then causes sufficient molecular irregularity to break up crystalline chain segments of the base resin. As the chlorine content is increased, the crystallites gradually disappear and, eventually, the thermoplastic material becomes amorphous and behaves as an elastomer because of the inherent flexibility of the polyethylene chain. Chlorosulfonated polyethylene resins made in slurry or fluidized beds generally have a more blocky chlorine distribution, both intramolecularly and intermolecularly, so that the same degree of amorphous characteristic is not always achieved. The increase in molar cohesion, by the addition of chlorine atoms, increases the polymer solubility parameter, and thus decreases its miscibility with paraffinic and aromatic oils. So, as chlorine content of the polymer increases, resistance to swelling effect of oil increases. 

Various additives can also be mixed in with the polyethylene resin by the material suppliers to enhance other material properties, as needed, to resist the demands of a specific type of environment. Carbon black (or furnace black) is the most common additive and is used to improve the jacket material s resistance to the effects of solar radiation. Adding carbon black also gives outside plant cables their characteristic black color. Other UV stabiUzers in limited use allow for jacket colors other than black but may not be as effective. 

Permanent internal antistatic agent for use in polyethylene, polypropylene, polystyrene, ABS, HIPS and SAN resins. Characteristics ... 

With ELITE enhanced polyethylene resins, you can overcome this problem, realise faster packaging line speeds and fewer leakers on vertical form-fill-seal equipment, and achieve better handling characteristics. The lower seal initiation temperature also creates a broader sealing window, while providing extra flexibility in various bag-making processes. 

Polyethylene resins consist of molecules that exhibit a distribution of molecular lengths and branching characteristics. The characteristics of a polyethylene resin could be uniquely described if each of its component moleeules were defined... 

The various types of polyethylene exhibit a wide range of properties, the speciflc attributes depending on the moleeular and morphological characteristics of the polyethylene resin. Each variant of polyethylene has its own characteristics, and within each type there is a spectrum of properties. There is much overlap between the ranges of properties available for the different variants of polyethylene. The relationships linking moleeular strueture and physical properties are discussed in Chapter 5. 

The molecular characteristics of a polyethylene resin control its melt rheological properties. These characteristics include the distribution of molecular lengths and the number and type of branches (if any). Except in the case of polar copolymers, there is very little interaction between adjacent polyethylene chains in the melt. The combination of limited chain interaction and a flexible backbone of carbon-carbon bonds results in polymer melts that are highly mobile on a local scale. 

In this chapter no attempt is made to list the mechanical properties of all the polyethylene resins available. It is more important to understand the basic relationships that govern such properties. The nature of a specimen s response to applied stress can be correlated with its morphological and molecular characteristics it is these relationships that are emphasized. The mechanical properties of a specimen are controlled by its processing history within the limits imposed by its molecular characteristics. The nature of the molecular mechanisms involved in the physical deformation of polyethylene is discussed in Chapter 8. 

The molecular characteristics of the polyethylene resin from which an article is fabricated play a role secondary to the roughness of the counterface with which it come in contact [53]. 

Under less stringent conditions the choice of polymers available for insulation is much broader. In low voltage usage, considerations regarding a polymer s ultimate electrical properties may be outweighed by other physical characteristics and the cost of resin. Thus at low voltages polyethylene resins of all types come into competition with a variety of other polymers including polystyrene, poly (vinyl chloride), and polypropylene. 

The melt index (Ml)—also known as the melt flow index (MFl)—of a polyethylene resin refers to the rate at whieh it extrudes from a capillary die under a standard set of conditions. The method by which it is determined is described in Chapter 6. The melt index of a polyethylene resin depends on its molecular characteristics, primarily average molecular weight, molecular weight distribution, and branching characteristics—short chain versus long chain, concentration, and distribution. The melt index reflects the average dimensions of the molecules in a resin and their entanglements with one another. From a commercial point... 

---