Semicrystalline plastic

The question whether hardness is a property related to modulus (E) or yield stress (Y) is a problem which has been commented before by Bowman and Bevis 13). These authors found an experimental relationship between microhardness and modu-lus/yield-stress for injection-moulded semicrystalline plastics. According to the clasical theory of plasticity the expected indentation hardness value for a Vickers indenter is approximaterly equal to three times the yield stress (Tabor s relation). This assump-... 

Fig. 5.20 The PED for PP is apparently smaller in magnitude than that for PS it is also more temperature dependent, decreasing with increasing initial temperature. Semicrystalline plastics are weaker and their amorphous phase in the region T > Tg becomes more mobile, rapidly lowering the needed deformation stresses. [Reprinted by permission from M.H. Kim, Ph.D. Thesis, Department of Chemical Engineering, Stevens Institute of Technology, Hoboken, NJ (1999).]...

To achieve consistent properties and morphology of semicrystalline plastics, nucleating agents are added to trigger a heterogeneous nucleation of the plastic melt and to give crystals of regular size. Nucleation can occur on the surface of some impurities in... 

TPEs are blends of various amorphous rubbers such as ethylene-propylene and of polyolefin semicrystalline plastics such as PP. They are part of the family of TP olefins (TPOs). TPOs are mechanical blends consisting of a hard plastic and softer rubber. They are considered different from blends that are dynamically thermoplastic vulcanized (TPV) a process in which the elastomer phase is cured during mixing of the polymers.84 94... 

Amorphous Phase - See Amorphous Polymer and Semicrystalline Plastic. 

Semicrystalline Plastic - A plastic (polymeric) material characterized by localized regions of crystallinity. See also Amorphous Polymer. 

Here we will briefly discuss two major topics related to thermoplastic starch physical chemistry the starch granule destructuration or gelatinization during TPS production and the macromolecular properties of the semicrystalline plasticized starch which include indistinctly thermoplastic starch, destructurized starch and plasticized starch. 

Significant aftershrinkage is observed mainly in semicrystalline plastics as a result of postcrystallization. It can be reduced by selection of suitable mold temperatures. 

The conceptual overlapping of the two ideal damping curves to obtain the real curve for a semicrystalline plastic clearly shows the two-phase nature of semicrystalline thermoplastics. 

Below the glass transition temperature (Tg), long range motions are frozen out. The materials behave like semicrystalline plastics. Above the Tg, the amorphous phase becomes liquid, but structure is imparted to the melt by whatever crystallinity exists. 

For instance, the Tg of PP depends strongly on its tacticity. Atactic PP has a Tg—19 C and is used in bitumen and hot-melt adhesive formulations, while for isotactic PP, it is approximately -8°C. The latter is a tough, semicrystalline plastic used in packaging films. The Tg of syndiotactic form of PP can be as high as ISO C Sophisticated catalyst systems even aUow synthesis of polymers that have ordered blocks of atactic materials and isotactic materials on the polymer chains. 

The behavior of a thermoplastic material above its glass transition temperature depends on its level of crystallinity. As a noncrystalline (amorphous) polymer is slowly heated from a temperature below its Tg, it displays a large decrease in modulus as the glass transition temperature is reached. As one heats a semicrystalline plastic from a temperature below its Tg, it displays a relatively small modulus change at the glass transition temperature, followed by a plateau and then a decreasing modulus as the temperature increases and approaches the crystalline melting point. 

Plastics have thermal conductivity much lower than metals and therefore are poorer heat conductors. Thermal conductivity for plastics ranges from 3 X lO cal/-(cm s °C) for polypropylene to 12 X lO cal/(cm s °C) for HDPE. For aluminum the thermal conductivity is 0.3 cal/(cm s °C), and for steel it is 0.08 cal/(cm s °C). For closed-cell plastic foams, values of thermal conductivity are much lower than those of the unfoamed plastic because gases, and air in particular, have a lower thermal conductivity than polymers. Plastic fillers may increase the thermal conductivity of plastics. Semicrystalline plastics generally have a higher thermal conductivity than do amorphous plastics. 

The above outlined microstructural efficiency proved to be a valuable tool for upgrading the fracture toughness of short fibres reinforced semicrystalline plastics (SFRPs) by means of evaluation both of matrix and fibres - related failure mechanism [1267]. 

Typical permeability data are shown in Table 4.5 (70) for a range of elastomers, amorphous plastics, and semicrystalline plastics. In general, permeabihty decreases from elastomers to amorphous plastics to semicrystalline plastics. 

Semiciystalline Polymers There are two cases to be considered in the stress-strain relationships of semicrystalline plastics. If the amorphous portion is rubbery, then the plastic will tend to have a lower modulus, and the extension to break will be very large see polyethylene. Table 11.1. If the amorphous portion is glassy, however, then the effect will be much more like that of the glassy amorphous polymers. Orientation of semicrystalline polymers is also much more important than for the amorphous polymers. A special case involves fibers, where tensile strength is a direct function of the orientation of the chains in the fiber direction. 

Semicrystalline plastics such as polyamide 610 and polyethylene are particularly tough. In this case the amorphous portions are well above Tg, the —CH2— groups being highly flexible. The crystalline portions are hard and act to hold the entire material together. 

Crazing. This develops in such amorphous plastics as acrylics, PVSs, and PCs as creep deformation enters the rupture phase. Crazes start sooner under high stress levels. Crazing occurs in semicrystalline plastics, but in those its onset is not readily visible. It also occurs in most fiber-reinforced plastics, at the time-dependent knee in the stress-strain curve. 

A general comparison of some of the processing properties of amorphous and semicrystalline plastics is given in Table 3.1. 

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