Rheology ft Mechanical Analysis

Rheology and mechanical analysis are usually familiar techniques, yet the exact tools and the far-reaching capabilities may not be so familiar. Rheology is the study of how materials flow and deform, or when testing solids it is called dynamic mechanical thermal analysis (DMTA).

During rheometer and dynamic mechanical analyses instruments impose a deformation on a material and measure the material's response that gives a wealth of very important information about structure and performance of the basic polymer. As an example stress rheometers are used for testing melts in various temperature ranges. Strain controlled rheology is the ultimate in materials characterization with the ability to handle anything from light fluids to solid bars, films, and fibers.

With dynamic testing, the processed plastic's elastic modulus (relating to energy storage) and loss modulus (relative measure of a damping ability) are determined. Steady testing provides information about creep and recovery, viscosity, rate dependence, etc,

Viscoelasticities

Understanding and properly applying the following information to product design equations is very important. A material having this property is considered to combine the features of a so-called perfect elastic solid and a perfect fluid. It represents the combination of elastic and viscous behavior of plastics that is a phenomenon of time-dependent, in addition to elastic deformation (or recovery) in response to load.

This property possessed by all fabricated plastics to some degree, indicates that while plastics have solid-like characteristics such as elasticity, strength, and form or shape stability, they also have liquid-like characteristics such as flow depending on time, temperature, rate, and amount of loading. The mechanical behavior of these viscoelastic plastics is dominated by such phenomena as tensile strength, elongation at break, stiffness, rupture energy, creep, and fatigue which are often the controlling factors in a design.

Processing-to-Performance Interface

Different plastic characteristics influence processing and properties of plastic products. Important are glass transition temperature and melt temperature.

Glass Transition Temperatures

The Tg relates to temperature characteristics of plastics (Table 1.7). It is the reversible change in phase of a plastic from a viscous or rubbery state to a britde glassy state (Fig. 1.7). Tg is the point below which plastic behaves like glass and is very strong and rigid. Above this temperature it is not as strong or rigid as glass, but neither is it britde as glass. At and above Tg the plastic's volume or length increases more rapidly and rigidity and strength decrease. As shown in Fig. 1.8 the amorphous TPs have a more definite Tg when compared to crystalline TPs. Even with variation it is usually reported as a single value.

The thermal properties of plastics, particularly its Tg influence the plastic's processability performance and cost in different ways. The operating temperature of a TP is usually limited to below its T . A more expensive plastic could cost less to process because of its Tg location that results in a shorter processing time, requiring less energy for a particular weight, etc. (Fig. 1.9).

The T generally occurs over a relatively narrow temperature span. Not only do hardness and britdeness undergo rapid changes in this temperature region, but other properties such as the coefficient of thermal expansion and specific heat also change rapidly. This phenomenon has been called second-order transition, rubber transition, or

Table 1 I Range of Tg for different thermoplastics

Plastic

°C

•F

Polyethylene

-120

-184

Polypropylene

-22

-6

Polybutylene

-25

-13

Polystyrene

95

203

Polycarbonate

150

302

Polyvinyl Chloride

85

185

Polyvinyl Fluoride

-20

-4

Polyvinylidene Chloride

-20

-4

Polyacetal

-80

-112

Nylon 6

50

122

Polyester

110

230

Polytetrafluoroethylene

-115

-175

Silicone

-120

-184

Figure 1.7 Thermoplastic volume or length changes at the glass transition temperature

Figure 1.7 Thermoplastic volume or length changes at the glass transition temperature

TEMPERATURE -

Figure 1.8 Change of amorphous and crystalline thermoplastic's volume at Tg and Tm

Figure 1.8 Change of amorphous and crystalline thermoplastic's volume at Tg and Tm

Figure 1.9 Modules behavior with increase in temperature (DTUL = deflection temperature under load). (Courtesy of Bayer)

rubbery transition. The word transformation has also been used instead of transition. When more than one amorphous transition occurs in a plastic, the one associated with segmental motions of the plastic backbone chain, or accompanied by the largest change in properties, is usually considered to be the T .

Important for designers to know that above T , many mechanical properties are reduced. Most noticeable is a reduction that can occur by a factor of 1,000 in stiffness.

Melt Temperatures

Crystalline plastics have specific melt temperatures (Tm) or melting points. Amorphous plastics do not. They have softening ranges that are small in volume when solidification of the melt occurs or when the solid softens and becomes a fluid type melt. They start softening as soon as the heat cycle begins. A melting temperature is reported usually representing the average in the softening range.

The Tm of crystalline plastics occurs at a relatively sharp point going from solid to melt. It is the temperature at which melts softens and begins to have flow tendency (Table 1.8). They have a true Tm with a latent heat of fusion associated with the melting and freezing process, and a relatively large volume change during fabrication. Crystalline plastics have considerable order of the molecules in the solid state indicating that many of the atoms are regularly spaced. The melt strength of the plastic occurs while in die molten state. It is an engineering measure of the extensional viscosity and is defined as the maximum tension that can be applied to the melt without breaking.

fable 1.8 Crystalline thermoplastic melt temperatures

Plastic

•c

°F

Low Density Polyethylene

116

240

High Density Polyethylene

130

266

Polypropylene

175

347

Nylon 6

215

419

Nylon 66

260

500

Polyester

260

500

Polyarylamide

400

755

Polytetrafluoroethylene

330

626

The Tm is dependent on the processing pressure and the time under heat, particularly during a slow temperature change for relatively thick melts during processing. Also, if the melt temperature is too low, the melt's viscosity will be high and more cosdy power required processing it. If the viscosity is too high, degradation will occur. There is the correct processing window used for the different melting plastics.

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