Identify thermoplastic and thermosetting plastics by flame test.

Experiment No :1
Identify thermoplastic and thermosetting plastics by flame test.
Materials And Equipments Required:
• Tong.                                     

• Bunsen Burner.

• Plastic samples.
• Beakers,etc.
Principle:
The experiment is based on the property of plastics.The plastic when expressed to heat if the sample is being softened and can be stretched easily,it is the thermoplastic.If the sample burns with a burning smell,it is thermosetting.
Thermoplastic:
A plastic which will soften when heated and harden when cooled.A thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces or even stacking of aromatic rings. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers such as polyethylene and polypropylene.
Plastic flow takes very readily due to heating of their structure,which breaks the secondary bonds between the individual chains.Most thermoplastic consists of very long chains of carbon atoms covalently bonded together.In thermoplastic the long molecular chains are bonded to each other by seconday bond. Thermoplastics are recyclable materials that are used frequently today to create objects such as foam cups, polyethylene squeeze bottles, acrylic lenses and safety helmets. Their unique properties set them apart from the other two types of plastic, thermosets and elastomers. Heat and pressure cause the molecular structure of a thermoplastic to change, making it pliable and more useful.
Thermoplastics are the only type of plastic that can be welded. When heated to about 120 to 180 degrees Celsius, thermoplastics become pasty or liquid, and they can be molded and formed into different shapes. Each type of thermoplastic has its own properties, characteristics and welding temperature. They can withstand multiple heating and re-shaping, which makes them recyclable. Once the thermoplastic object has cooled down, the new form is set.Types of thermoplastics include polyethylene (PE), polyvinyl chloride (PVC) and polystyrene (PS), which often are used for packaging. Other groups of thermoplastics are acrylics, fluoropolymers, polyesters, polyimides and nylons. All of these types can be melted down many times and re-shaped into different forms. For example, a foam cup is a thermoplastic material that can be heated and turned into a dish.
Thermosetting:
Thermosetting plastics are simply plastics when moulded into shape and allowed to cool will not change shape when heated again they will char or burn an example of a thermosetting plastic is urea formaldehyde.These plastics have their dimensional network structures with predominantly covalent bonding. Thermosetting, plastics are synthetic materials that strengthen during being heated, but cannot be successfully remolded or reheated after their initial heat-forming.
Thermoset plastics differ in that they are not re-mouldable. Strong cross links are formed during the initial moulding process that give the material a stable structure. They are more likely to be used in situations where thermal stability is required. They tend to lack tensile strength and can be brittle.Once the curing process is complete,these plastics cannot be softened or reworked.
Some examples of thermosets are:
• Polyester fibreglass systems: (SMC Sheet molding compounds and BMC Bulk molding compounds).
• Vulcanized rubber.
• Bakelite, a phenol-formaldehyde resin (used in electrical insulators and plasticware).
• Duroplast, light but strong material, similar to Bakelite (used for making car parts).
• Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard).
• Melamine resin (used on worktop surfaces).
• Epoxy resin (used as the matrix component in many fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic).
• Polyimides (used in printed circuit boards and in body parts of modern airplanes).
• Cyanate Esters or Polycyanurates for electronics applications with high demands on dielectric properties and high glass temperature requirements in composites.
• Mold or Mold Runners (the black plastic part in Integrated Circuits (IC) or semiconductors).
Procedure:
1. Take different samples of plastics and label them A,B,C.
2. Take sample A and clamp it in the tong.
3. Place the sample on Bunsen flame.
4. Observe the burning of sample.
5. Note the observation that whether sample are being softened or decompose.
6. Repeat procedure with other samples.
7. Record the observations.
Observations:
        Sample No.                    Nature of Plastic
       (Thermoplastic/Thermosetting)
           A        Thermoplastic
           B        Thermoplastic
           C        Thermosetting

mechanical property of polymer and ceramic

BRITTLE FRACTURE OF CERAMICS

Both crystalline and noncrystalline ceramics almost always fracture before any plastic deformation can occur in response to an applied tensile load.The brittle fracture process consists of the formation and propagation of cracks through the cross section of material in a direction perpendicular to the applied load. Crack growth in crystalline ceramics may be either transgranular (i.e., through the grains) or intergranular (i.e., along grain boundaries); for transgranular fracture, cracks propagate along specific crystallographic (or cleavage) planes, planes of high atomic density. This may be explained by very small and omnipresent flaws in the material that serve as stress raisers—points at which the magnitude of an applied tensile stress is amplified.The degree of stress amplification depends on crack length and tip radius of curvature.

Fractography of Ceramics

A fractographic study  is normally a part of such an analysis, which involves examining the path of crack propagation as well as microscopic features of the fracture surface. It is often possible to conduct an investigation of this type using simple and inexpensive equipment—for example, a magnifying glass, and/or a low-power stereo binocular optical microscope in conjunction with a light source. When higher magnifications are required the scanning electron microscope is utilized.

STRESS–STRAIN BEHAVIOR

The stress at fracture using this flexure test is known as the flexural strength, modulus of rupture, fracture strength, or the bend strength, an important mechanical parameter for brittle ceramics. A more suitable transverse bending test is most frequently employed, in which a rod specimen having either a circular or rectangular cross section is bent until fracture using a three- or four-point loading technique.

Elastic Behavior

The elastic stress–strain behavior for ceramic materials using these flexure tests is

similar to the tensile test results for metals: a linear relationship exists between stress and strain.

MECHANISMS OF PLASTIC DEFORMATION

Crystalline Ceramics

For crystalline ceramics, plastic deformation occurs, as with metals, by the motion

of dislocations. One reason for the hardness and brittleness of these materials is the difficulty of slip (or dislocation motion). For crystalline ceramic materials for which the bonding is predominantly ionic, there are very few slip systems along which dislocations

may move.This is a consequence of the electrically charged nature of the ions.

Noncrystalline Ceramics

Plastic deformation does not occur by dislocation motion for noncrystalline ceramics

because there is no regular atomic structure. Rather, these materials deform by viscous

flow, the same manner in which liquids deform; the rate of deformation is proportional

to the applied stress.

The characteristic property for viscous flow, viscosity, is a measure of a noncrystalline

material’s resistance to deformation.

MISCELLANEOUS MECHANICAL CONSIDERATIONS

Influence of Porosity

Subsequent to compaction or forming of these powder particles into the desired shape, pores or void spaces will exist between the powder particles. During the ensuing heat treatment, much of this porosity will be eliminated; however, it is often the case that this pore elimination process is incomplete and some residual porosity will remain .Any

residual porosity will have a deleterious influence on both the elastic properties and strength.. It has been observed for some ceramic materials that the magnitude of the modulus of elasticity E decreases with volume fraction porosity.

Hardness

One beneficial mechanical property of ceramics is their hardness, which is often

utilized when an abrasive or grinding action is required; in fact, the hardest known

materials are ceramics.

Creep

Often ceramic materials experience creep deformation as a result of exposure to

stresses (usually compressive) at elevated temperatures.

Mechanical Behavior of Polymers

STRESS–STRAIN BEHAVIOR

The mechanical characteristics of polymers, for the most part, are highly sensitive to the

rate of deformation (strain rate), the temperature, and the chemical nature of the environment (the presence of water, oxygen, organic solvents, etc.) Three typically different types of stress–strain behavior are found for polymeric materials, stress–strain character for a brittle polymer, inasmuch as it fractures while deforming elastically. Second one is similar to that for many metallic materials; the initial deformation is elastic, which is followed by yielding and a region of plastic deformation.Third one is totally elastic; this rubber-like elasticity (large recoverable strains produced at low stress levels) is displayed by a class of polymers termed the elastomers.

MACROSCOPIC DEFORMATION

Some aspects of the macroscopic deformation of semicrystalline polymers deserve our

attention. The tensile stress–strain curve for a semicrystalline material, which was initially undeformed, is shown in Figure 15.4; also included in the figure are schematic

representations of the specimen profiles at various stages of deformation. Both upper

and lower yield points are evident on the curve, which are followed by a near horizontal

region. At the upper yield point, a small neck forms within the gauge section of the specimen.Within this neck, the chains become oriented (i.e., chain axes become aligned parallel to the elongation direction, a condition that is represented schematically, which leads to localized strengthening. Consequently, there is a resistance to continued deformation at this point, and specimen elongation proceeds by the propagation of this neck region along the gauge length; the chain orientation phenomenon  accompanies this neck extension. This tensile behavior may be contrasted to that found for ductile metals ,wherein once a neck has formed, all subsequent deformation is confined to within the neck region.

VISCOELASTIC DEFORMATION

For intermediate temperatures the polymer is a rubbery solid that exhibits the combined mechanical characteristics of these two extremes; the condition is termed viscoelasticity.

Elastic deformation is instantaneous, which means that total deformation (or strain) occurs the instant the stress is applied or released (i.e., the strain is independent of time). In addition, upon release of the external stress, the deformation is totally recovered—the specimen assumes its original dimensions.A familiar example of these viscoelastic extremes is found in a silicone polymer that is sold as a novelty and known by some as “silly putty.”When rolled into a ball and dropped onto a horizontal surface, it bounces elastically—the rate of deformation during the bounce is very rapid.

On the other hand, if pulled in tension with a gradually increasing applied stress, the material elongates or flows like a highly viscous liquid. For this and other viscoelastic materials, the rate of strain determines whether the deformation is elastic or viscous.

Viscoelastic Relaxation Modulus

Stress is found to decrease with time due to molecular relaxation processes that take place within the polymer.We may define a relaxation modulus a time-dependent elastic modulus for viscoelastic polymers.

Furthermore, the magnitude of the relaxation modulus is a function of temperature;

and to more fully characterize the viscoelastic behavior of a polymer, isothermal stress relaxation measurements must be conducted over a range of temperatures.

Viscoelastic Creep

Many polymeric materials are susceptible to time-dependent deformation when the

stress level is maintained constant; such deformation is termed viscoelastic creep.

This type of deformation may be significant even at room temperature and under modest stresses that lie below the yield strength of the material. For example, automobile tires may develop flat spots on their contact surfaces when the automobile is parked for prolonged time periods.

FRACTURE OF POLYMERS

As a general rule, the mode of fracture in thermosetting polymers (heavily crosslinked networks) is brittle.For thermoplastic polymers, both ductile and brittle modes are possible, and many of these materials are capable of experiencing a ductile-to-brittle transition. Factors that favor brittle fracture are a reduction in temperature, an increase in strain rate, the presence of a sharp notch, increased specimen thickness, and any modification of the polymer structure that raises the glass transition temperature.

One phenomenon that frequently precedes fracture in some thermoplastic polymers

is crazing

DEFORMATION OF ELASTOMERS

One of the fascinating properties of the elastomeric materials is their rubber-like elasticity.That is, they have the ability to be deformed to quite large deformations, and

then elastically spring back to their original form.In an unstressed state, an elastomer will be amorphous and composed of crosslinked molecular chains that are highly twisted, kinked, and coiled. Elastic deformation,upon application of a tensile load, is simply the partial uncoiling, untwisting,and straightening, and the resultant elongation of the chains in the stress direction, Part of the driving force for elastic deformation is a thermodynamic parameter called entropy, , which is a measure of the degree of disorder within a system; entropy increases with increasing disorder. As an elastomer is stretched and the chains straighten and become more aligned, the system becomes more ordered.Several criteria must be met for a polymer to be elastomeric: (1) It must not easily crystallize; elastomeric materials are amorphous, having molecular chains that are naturally coiled and kinked in the unstressed state. (2) Chain bond rotations

must be relatively free for the coiled chains to readily respond to an applied force.

(3) For elastomers to experience relatively large elastic deformations, the onset of

plastic deformation must be delayed

Vulcanization

The crosslinking process in elastomers is called vulcanization, which is achieved by

a nonreversible chemical reaction, ordinarily carried out at an elevated temperature.

In most vulcanizing reactions, sulfur compounds are added to the heated elastomer; chains of sulfur atoms bond with adjacent polymer backbone chains and crosslink them

Unvulcanized rubber, which contains very few crosslinks, is soft and tacky and has poor resistance to abrasion. Modulus of elasticity, tensile strength, and resistance to degradation by oxidation are all enhanced by vulcanization.