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Polymer Processing PE-4104Lecture 02,03,04Parameters for Feedstock

Lecture Overview

• Important characteristics for feedstock• Hygroscopicity• Granule characteristics• Thermal Properties• Thermal Stability

• Processing Temperature• Cooling rate

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Granules Characteristics

• During melt-processing, the heat required for meltingthe granules can be supplied externally or internally

• External supplies includes- Heating coils- Or Electrical heating bands situated outside the barrel

• Internal heating- Mechanical working on material that generates frictional

heat- High frequency heating (Dielectric heating)

Granules Characteristics

• Whatever the mean of heating supply, granule sizesand shapes plays an important role

• If feed consists of different sizes of granules- Then each granule would take different time to get fluxed- Smaller sizes of granules would melt quicker than the

larger

- Adjusting the processing time would be very difficult.• If the time is fixed according to the smaller granules

- Smaller granules would attain fluidity- Larger granules would be still unmolten or semi-molten

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Granules Characteristics

• If time is fixed according to larger granules- Larger granules will melt and attain fluidity- Smaller ones would be of very low viscosity fluid or even

become volatile.

• Average particle size is not a solution in polymerprocessing

• It is desirable to have uniform granule sizes- In order to have uniform and homogeneous characteristics of

the processed item

• Solution- Polymers are granulated in a special granulator- Thin ribbon are extruded and subsequently cooled and cut into

forms called spaghetti -cut

Thermal Properties of Polymers

• Thermal properties such as- Specific heat

• Dictates the amount of heat needed for processing

- Conductivity• The amount of heat required is related to thermal

conductivity

- Viscosity• The amount of heat is also related to the melt viscosity of the

specific polymer

• Polymers by nature are bad conductor of heat

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Thermal Properties of Polymers

• For external heating the amount of heat received bythe material would depend upon- Thermal conductivity

• For internal heating the amount of heat receiveddepends upon- Viscosity of the material- Rate of mechanical working

• Greater the viscosity or mechanical work, greaterwould be the heat generated.

Thermal Stability

• An important property that has to be taken intoconsideration is the “Thermal Stability” - How stable is the compound at elevated temperatures (In

the presence as well as in the absence of oxygen)- How thermal instability affect the properties- What are the degeneration products, if any

- Any parts of the machinery or any additive is acting as acatalyst in degeneration process

• PVC does not remain stable at its processingtemperature which is >200 0 C.

• Above 200 0C PVC begins to degrade with the evolutionof HCl gas

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Effects of Cooling rate

• For crystalline polymers- The latent heat of fusion of the crystalline material- Average specific heat over the temperature range of (T p-

Tm )• Cooling rate affect the properties of the finished

products in more than one ways• Polymer molecules under thermal stress, begins to

relax on cooling.- If solidification comes before relaxation (Quenching)

• Polymer chains would freeze while still in the process of beingarranging themselves

- There would be no uniformity in the orientation of thechains

Effect of cooling rate

• The cooled mass would show anisotropic properties• The rate of cooling also effect crystalline structure of

the polymer• If solidification comes after relaxation of polymeric

chains (Annealing)- Isotropic properties- For semi-crystalline polymer crystalline structure would

build- No residual stresses in the chains

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