Strength stiffness

Extmsion of polyethylene and some polypropylenes is usually through a circular die into a tubular form, which is cut and collapsed into flat film. Extmsion through a linear slot onto chilled rollers is called casting and is often used for polypropylene, polyester, and other resins. Cast, as well as some blown, films may be further heated and stretched in the machine or in transverse directions to orient the polymer within the film and improve physical properties such as tensile strength, stiffness, and low temperature resistance. 

Fig. 3. Effects of composition on physical properties. A, acetyl B, butyryl C, cellulose. 1, increased tensile strength, stiffness 2, decreased moisture sorption 3, increased melting point 4, increased plasticizer compatibiUty 5, increased solubiUties in polar solvents 6, increased solubiUties in nonpolar...

Polymers of this type have exceptional good values of strength, stiffness and creep resistance (see Table 18.13). After 100 h at 23°C and a tensile load of 70 MPa the creep modulus drops only from 4200 to 3(K)0 MPa whilst at a tensile load of 105 MPa the corresponding figures are 3500 and 2500 MPa respectively. If the test temperature is raised to 150°C the creep modulus for a tensile load of 70 MPa drops from 2400 to 1700 MPa in 100 h. 

Aromatic resins. Fully aromatic resins are used in block copolymer and ethylene copolymer systems. In the former they are soluble in the styrenic end blocks upon cooling where they serve to increase the strength, stiffness, and creep... 

Glass fibres are the principal form of reinforcement used for plastics because they offer a good combination of strength, stiffness and price. Improved... 

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

Polyamides (nylon). There are several different types of nylon (e.g. nylon 6, nylon 66, nylon 11) but as a family their characteristics of strength, stiffness and toughness have earned them a reputation as engineering plastics. Table 1.3 compares the relative merits of light metal alloys and nylon. 

Acrylonitrile-butadiene-styrene (ABS). ABS materials have superior strength, stiffness and toughness properties to many plastics and so they are often considered in the category of engineering plastics. They compare favourably with nylon and acetal in many applications and are generally less expensive. However, they are susceptible to chemical attack by chlorinated solvents, esters, ketones, acids and alkalis. 

Laminated composite materials consist of layers of at least two different materials that are bonded together. Lamination is used to combine the best aspects of the constituent layers and bonding material in order to achieve a more useful material. The properties that can be emphasized by lamination are strength, stiffness, low weight, corrosion resistance, wear resistance, beauty or attractiveness, thermal insulation, acoustical insulation, etc. Such claims are best represented by the examples in the following paragraphs in which bimetals, clad metals, laminated glass, plastic-based laminates, and laminated fibrous composite materials are described. 

We could go through that same kind of problem analysis for many different structures, and, in the process, design requirements could be expressed for each of them. The most common (Jesign requirements would be expressed in some manner in terms of strength, stiffness, and life, but there are many other issues as well. Whether the material will corrode, for example. Whether it will provide the proper insulation or just the opposite, sufficient conductivity, and so on. There are many, many different types of tasks that we ask a structure to perform. 

Hardness is closely related to strength, stiffness, scratch resistance, wear resistance, and brittleness. The opposite characteristic, softness, is associated with ductility. There are different kinds of hardness that measure a number of different properties (Fig. 5-5). The usual hardness tests are listed in three categories (a) to measure the resistance of a material to indentation by an indentor some measure indentation with the load applied, some the residual indentation after it is removed, such as tests using Brinell hardness,... 

High strength Stiffness at elevated temperatures Mineral reinforced Most economical Low warpagc Polyester (glass reinforced) High stiffness Lowest creep Excellent electrical properties properties... 

The following information is a continuation of what has been reviewed earlier in this chapter (PROCESSING AND PROPERTY, Orientation). Orientation consists of a controlled system of stretching heated plastic material (molecules) to improve their strength, stiffness, optical, electrical, and other properties. This process, which has been... 

The isotactic polypropylene is an essentially linear, highly crystalline polymer. The density of polypropylene is 0.905. It has high tensile strength, stiffness and hardness due to its high crystalline character. 

Product Thickness Weight Bulk Density Porosity Tensile Strength Stiffness Through-Plane Air Permeability In-Plane Air Permeability Compressibility Through- plane Resistivity In-Plane Resistivity... 

Moisture absorption. Directly related to the atoms making up the polymer. The more moisture-absorbing the molecule, the less dimensional stability strength, stiffness, electrical properties are also adversely affected. 

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