By Salman SHAHID

Gul ZEB

Toughness ?Types of matrices.Mechanism of tougheningArchitecture of Rubber particlesInfluence of Structure and properties of

rubberThermoplastic elastomersStyrenic block copolymersThermoplastic vulcanizates Salman SHAHID

Gul ZEB

Toughness is the deformation energy dissipated up to the beginning of failure.( by the frame work of fracture mechanics)

How to measure:

Most accessible measurements are the notched Izod and Charpy protocols

1. Brittle amorphous polymers, such as PS and SAN, with low impact strengths

2. Pseudo-ductile engineering polymers, such as PC, PA, PI, PE, PP and PSF.

3. Polymers, such as PMMA, POM, and PVC, exhibiting fracture behavior intermediate between types 1 and 2.

The main aim of the rubber modification of thermoplastic homopolymers is to improve their toughness. Methods to increase the toughness1.Copolymerization2.Incorporation of a second phase like other thermoplastics 3.Inorganic materials4.Very small voids and spherical rubber particles.

The last mechanism is mostly used.

Rubber-like materials have long chains with higher flexibility and mobility which are joined in network.

Due to higher mobility the chain alter their configuration rather fast so able to bear higher loads.

On removal of the external forces, it goes back to original dimensions with non recoverable strain.

(a) Molecular entanglements in a high molecular weight polymer.

(b) Molecular entanglements locked by cross-linking.

A network is obtained by the linking of polymer chains together, and this linkage may be either chemical or physical. Physical linking can be obtained by (1) Absorption of chains onto the surface offinely divided particulate fillers;(2) Formation of small crystallite (3) Coalescence of ionic centers; and(4) Coalescence of glassy blocks.

Well dispersed rubber particles are able to induce in the thermoplastic matrix different mechanisms of toughening:

1. crazing;2. shear yielding and rubber particle

cavitation;3. combined crazing and shearing yielding.

Breaking of secondary bonds along the planes normal to the maximum tensile axis.

◦ Planner crack like defects ◦ Stress whitening of material ◦ High stress concentration

Toughening particles > multiple craze Formation elastomeric nature > prevents

the growth of large crazes

TEM micrograph of crazing zone

Crazing: Crazing is a brittle mechanisms leading to the small ductility of most base polymers

Figure: Schematic representation of the crazing phenomenon. (a) Crazed specimen subjected to a tensile force F. (b) Section of a craze with fibrils, strained by the tensile force F. (c) Multi craze mechanism induced by the presence of rubber particles in a rigid matrix.

In homogeneous polymers shear deformation consists of a distortion of the body shape without significant volume variation. In toughened materials> diffused shear yielding,

followed by rubber particle cavitation.

PVC, ABS, PC, PA, PE, PI and PSF undergo diffuse shear yielding.

TEM image showing the prevention of growth craze due to filled rubber particles.

Rubber particles

Shear band

crazing

The choice between both deformation mechanisms depends on:

Matrix’s chemistry (Secondary relation temp> see foot note)

Rubber phase

Generally, crazing prevails at low temperatures high deformation rates .

Shearing Above the glass transition Low deformation rates

Wu criteriaPolymers with a critical entanglement density above 0.15

mmol/cc should deform by shearing and below this critical level by crazing

Crazing because the chains have not enough time to rearrange under the stress field. If have time then shear yielding

1.Elastic deformation

2.Plastic strain softening

3. Strain hardening of the yield zone

There are two extremes for architecture of Rubber particles.

Bulk or pelletized elastomeric compounds (homogeneous particles)

core/ shell particles

In Core/shell particles, cores is often formed from matrix material and is covered with a thin layer rubbery shell, which is grafted with an outer second shell.

Homogeneous Particles1. Good toughening agent2. Deleterious effects on matrix stiffness 3. Less stress whitening.4. Good weatherability.5. Better gloss

Core/shell particles1. less efficient as toughening agents2. Less deleterious effect on matrix stiffnes.3. For transparent toughened polymers.

Comparison

Glass transition temperature.Independent of the type of deformation process, whether crazing or shearing,

Tapplication > Tglass transition of rubber

(mean should be in the BD transition).

In this context two thing are very important. 1. Adhesion

Good adhesion: Interdiffusion of phases thus good toughening

2.Agglomeration large agglomerates are ineffective in

toughening

In addition to adhesion grafting is also important for the dispersion of rubber particles.

In brittle homopolymers as in PS, crazing is the dominating deforming process.

It was observed experimentally that larger salami type particles play more better role than the small particles (at same composition) for same conditions.

In Pseudo-ductile materials shear is dominant mechanism of deformation. It is observed that smaller particles better handle the shear than large particles.

In semi ductile materials like PMMA better results were obtained with mixed size particles 300nm to 600nm.

Generally saying, polymers are inherently brittle and crazing is always dominant but if chemistry of the structure allows shearing (through secondary relaxation process) then shear yielding is dominant.

Rubber particles must be at least slightly cross-linked, otherwise the rubber phase loses its individual particular structure in processing and is transferred, e.g., to an interpenetrating network. But with increase of the degree of crosslinking the brittle strength of the material decreases.

A thermoplastic elastomer (TPE) is generally considered a bimicrophasic material that exhibits rubber elasticity over a specified service temperature range but at elevated temperature can be processed as a thermoplastic

Thermoplastic Elastomers (TPe)Thermoplastic Elastomers (TPe)

Tailor made properties by varying the ratio of two phases(hard and soft).

Upper service temperature softening point

of the hard phase.

Low temperature properties controlled largely by the soft

segments.

Rigid domain (physical crosslinking)

Soft amorphous domain

(1) Simple processing. (2) Shorter fabrication times.(3) There is little or no compounding. (4) reusing scrap as with thermoplastics.

Advantages of TPesAdvantages of TPes

1. They melt at elevated temperatures

2. They may require drying before processing

3. There is a limited number of low modulus compounds

Disadvantages of TPeDisadvantages of TPe

1- Styrenic block copolymers(SBCs)2- Dynamic vulcanizates (TPVs) 3- Polyamide TPEs 4- Copolyester elastomers (COPEs) 5- Thermoplastic polyolefins (RTPOs); and 6- Ionomeric TPEs

A substantial portion of industrially produced TPEs is represented by block copolymers, consisting of two or more polymer chains attached at their ends. Most block copolymers are prepared by Anionic polymerization and controlled polymerization .

Styrenic block copolymers (SBCs) are based on simple molecules of the type A–B–A, where A is

polystyrene and B is an elastomeric segment The most common structure of SBCs is that

where the elastomeric segment is a polydiene Polybutadiene Polyisoprene Example SBS, SEBS, SIS S : Styrene B : Butadiene EB : Hydrogenated butadiene

Schematic of a styrene–butadiene–styrene block copolymer

Changes in morphology of an A–B–A block copolymer as a function of composition

Two types of compatibility1. Thermodynamic 2. Technological

If polymers are thermodynamically compatible, i.e. miscible, their intimate mixture exists as a single phase. For this case

Unlike the case of monomeric materials, the entropy of mixing of polymers is very low.

it would be best that the enthalpy of mixing, be negative (i.e., that mixing be exothermic).

It would be required that unlike polymer molecules associate with one another more strongly than do like polymer molecules

Polymers are rarely Thermodynamically compatible

Ideal elastomeric rubber-plastic blend would comprise finely divided rubber particles dispersed in a relatively small amount of plastic

Practically the “ideal” case proposed above could arise as a result of the polymers being thermodynamically incompatible

low Tg of the rubber phase would be maintained because of the relative purity of the rubber phase; yet the high Tg of the hard phase could be retained for structural integrity over a useful temperature range

If two polymers are said to be technologically compatible, it merely means that their blends are technologically useful.

Technological compatibilization, then, is any process that improves the properties of a blend to make it more useful.

Compatibilization techniques for improving such mixtures may be mechanical or chemical in nature

Such techniques generally do not make the mixtures become miscible, i.e compatible in thermodynamic sense

We can improve the properties of blends prepared by simple melt blending by Dynamic vulcanization

(the process of crosslinking the rubber phase during its melt-mixing with the plastic material)

Technological compatibilization(by addition (or in situ formation) of

small amounts of block copolymers, which contain blocks of each of the polymers to be compatibilized)

Mixing as well as selective crosslinking of the rubber are superimposed processes that happen in the melt-mixing process called dynamic vulcanization.

Rubber particles cross-linking Embedded in less viscous

thermoplastic component

• Lower permanent set• Improved mechanical properties (tensile• strength, elongation at break)• Better fatigue resistance• Lower swelling in fluids, such as hot oils• Higher melt strength• Improved utility at elevated temperatures• Greater stability of phase morphology in the• melt• Greater melt strength• More reliable processing characteristics in• melt processing.

Torque-time characteristics of a dynamic vulcanization process in an internalmixer. PP/EPR 40:60, peroxidic cross-linked. Points a to l: times of sampling.

Effect of mixing time on the phase morphology of Brabender-mixed EPDM/BR blends(a) 5min (3300X), (b) 15 min (3300X), (c) 30 min (10,000X)

Rubber-plastic blends have generally been prepared by melt-mixing techniques.

Melt-mixing has been accomplished by

various mixing devices1. Two-roll mills2. Twin-screw extruders

• In some blends, the rubber can be slightly cross-linked by the action of an organic peroxide.

• A disadvantage of the process of vulcanizing rubber before mixing it with polyolefin is that the compositions generally contain rather large rubber particles.

After sufficient melt-mixing to form a well-mixed blend, vulcanizing agents (curatives, crosslinkers) are added.

Vulcanization then occurs while mixing continues.

The more rapid the rate of vulcanization, the more rapid the mixing must be to ensure good fabricability of the final blend composition.

PP, PE, PA, SAN, ABS, PC, and PS.

Diene rubber, such as NR, SBR, PBD, BR, EPDM.

Continuous PP phase

Dispersed EPDM

First commercial TPV: PP-EPDM ( Santoprene® )

CrystallineTm ~ 160°C

Stress-strain behavior of a non- reactive and a dynamic vulcanized PP-EPR blend: PP/EPR 40:60.

Effect of polypropylene concentration of EPDM/PP thermoplastic vulcanizate

Modes of reuse :

Use as generic plastic > recycled TPE + Virgin

Use of mixed plastic > e.g improve properties of

TPOs

Use in energy recovery > little sulfur, better incineration

Properties equal to thermoset elastomers Improved processing & increasing fabrication methods Tailor made properties > transparency, adhesion and

compatibility Struggling for potential application > Artificial implants > Biological

adhesives > Soft tissue replacements.

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2. Mechanical properties of polymers based on Nanostructure and Morphology edited by G. H. Michler F. J. Baltá-Calleja

3. Handbook of Thermoplastic Elastomers by Jiri George Drobny Drobny Polymer Associates.

4. Micro and Nanostructured Multiphase Polymer Blend Systems Phase Morphology and Interfaces Edited by Charef Harrats, Sabu Thomas and Gabriel Groeninckx .

5. Polymer Blends Handbook 3rd volume edited by L. A. Utracki

6. Current Topics in Elastomers research edited by Anil K. Bhowmick.

7. Modern Styrenic Polymers: polystyrenes and styrenic copolymers edited by Jhon Scheirs ExcelPlas.