Impact Modification Of Thermoplastics
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Impact Modification Of Thermoplastics
- 1. By Salman SHAHID Gul ZEB
- 16. Rubber particles Shear band crazing
- 35. 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.
- 36. Rigid domain (physical crosslinking) Soft amorphous domain
- 41. Schematic of a styrene–butadiene–styrene block copolymer
- 42. Changes in morphology of an A–B–A block copolymer as a function of composition
- 53. Torque-time characteristics of a dynamic vulcanization process in an internal mixer. PP/EPR 40:60, peroxidic cross-linked. Points a to l: times of sampling.
- 55. Effect of mixing time on the phase morphology of Brabender-mixed EPDM/BR blends (a) 5 min (3300X), (b) 15 min (3300X), (c) 30 min (10,000X)
- 59. PP, PE, PA, SAN, ABS, PC, and PS. Diene rubber, such as NR, SBR, PBD, BR, EPDM.
- 60. Continuous PP phase Dispersed EPDM First commercial TPV: PP-EPDM ( Santoprene® ) Crystalline Tm ~ 160°C
- 61. 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
Hinweis der Redaktion
- Due to the diversity of the topic we mainly focus on the mechanism behind the impact modification and some important features of elastomeric phase. Most of the example you have seen during the course.
- It is desirable for plastics articles to be able to withstand cracking when subject to minor impact. Failure to resist cracking is a common feature of plastics and this discuss the way in which polymers can be used as toughening additives to overcome the problem. Several thermoplastics, both of the commodities kind [polystyrene (PS), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polypropylene (PP), polyvinylchloride (PVC) etc.] and engineering polymers [polyamides (PA), polyesters (PE), polycarbonates (PC), polyimides (PI), polysulfones (PSF), polyoxymethylene (POM), polyphenylene oxide (PPO) etc.] exhibit glass transition temperatures (T,) higher than or close to room temperature (R.T.). As a consequence they show, at R.T. or below it, the shortcoming of brittle impact behaviour, which limits their commercial end-uses. Three main classes of polymer matrices can be identified with respect to their failure characteristics:
- Rubber-like materials consist of relatively long polymeric chains having a high degree of flexibility and mobility, which are joined into a network structure. The flexibility and mobility allow for a very high deformability. When subjected to external stresses, the long chains may alter their configuration rather rapidly because of the high chain mobility. When the chains are linked to a network structure, the system has solid-like features, where the chains are prevented from flowing relative to each other under external stresses. As a result, a typical rubber may be stretched up to 10 times its original length. On removal of the external forces, it is rapidly restored to its original dimensions, with essentially no residual or non recoverable strain.
- In this fig a, shows the physical entanglements due to high molecular weight characteristerized by the molecular weight between the entanglements. This molecular weight is a characteristic of the structure chemistry e.g for PE> Me =4000 and for PS> Me= 35000. In fig b there shows some crosslinked elastomers and many of these links are permanent. The higher the degree of crosslinking higher the thermoset behavior of the rubber.
- In amorphous polymers with limited amounts of crosslinking , loading may cause the breaking of secondary bonds that lead to the nucleation of special type of damage called Crazing. Crazes are planner crack like defects perpendicular to direction of maximum principal stress and diffusion of light by crazes show stress whitening. Rubber particles can cause multicraze initiation and termination mechanism, capable of dissipating large impact energies. Crazes initiate within the matrix at the rubber particle equators where a very high stress concentration is built up through the application of an impulsive external load; they undergo termination during their propagation when they impinge upon neighbouring rubber particles. A craze is similar to a microcrack but differs in being bridged across by several microfibrils, made by oriented polymer chains, which can sustain a certain load. The molecular weight (M.W.) of the matrix is an important parameter for crazing: below a critical M.W., in fact, no chain entanglements and therefore no stable crazes can be formed. The fracture under a sufficiently high load is determined by the rupture of the craze fibrils, once a crack of critical size has been developed.
- The two sides of the cracks are attached to the thread like structures called fibrils. The crazing cause the dilatation of structures and induce some ductility of materix. The fig on the left shows the TEM micrograph for the crazing zone.
- Fig ( a) shows the Crazing normal to the tensile force F. Fig (b) Section of a craze with fibrils, strained by the tensile force F. Fig (c) shows the Multi crazing induced by rubber particles.
- In homogeneous polymers shear deformation consists of a distortion of the body shape without significant volume variation. In semicrystalline materials shear yielding is very localized and occurs by slip on particular planes of maximum shear stress. In non-crystalline materials the shear yielding is much more diffuse than in the previous case, requiring large co-operative chain movements. In toughened materials a diffused shear yielding is the main energy dissipation phenomenon, preceded or followed by rubber particle cavitation. Particle cavitation induces a stress whitening effect, visible along the largest deformation patterns of the body. In matrices that are inherently shear yielding, impact modifiers act as stress concentrators where shear bands are Initiated. Semiductile polymer matrices , such as PVC, ABS, PC, PA, PE, PI and PSF undergo diffuse shear yielding.
- Fig a shows the crazes in the Rubber modified sPS after deformation at room teperature. In fig b the deformation morphology of same material deformed at 110 C is shown by the formation of herringbone pattern caused by shear yielding.
- Fig shows shows the termination of crazes impinged by the rubber particles. When the polymer material tends to craze, rubber particles by blonting the neck of the crack (the sharpness reduction) prevents the large cracks that could develop instable cracks.
- Molecular orientation of within the shear band, roughly parallel to the applied stress’ is normal to the plane of the crazes. This induces a synergetic effect by which shear bands can stop the growth of crazes increasing the material toughness.
- If the molecules in the host materix are flexibel under test condition, shearing dominates like in PC and PVC at room temperature. These polymers show, below their Tg a secondary relaxation process(define below) which indicates some segmental mobility of their back bone chains between this lower temperature and Tg. On the other hand stiff polymers like PS,PSAN with out this type of sec.relaxation process preferentially deform by crazing. Secondary relaxation temperature: at which the polymer changes from brittle to ductile and is a function of intrinsic flexibility or rigidity of the coiled (Co) chains at 1 HZ frequency the corelation is given by Tb/Tg = 0.135 +0.082 Co (by Souheng Wu)
- Elastic deformation, resulting in the generation of stress concentrations around the rubber particles, and (in some cases) cavitation in the rubber particles. Plastic strain softening, characterized by local yielding of the matrix, through multiple crazing (fibrillated or homogeneous crazes), extensive shear yielding, or some combination of both . Strain hardening of the yield zone, a process to which stretching of the rubber phase to very high strains makes a significant contribution
- Figures show the typical architecture of two types of rubber particles in the matrix of nylon-6 fo same loading. Figure on the left shows the homogeneous particles and figure on right shows the core shell particle.
- Homogeneous Because they contain more rubber flexibility. Because of soft rubber phase the stiffness of material decreases. Small homogeneous particles are better for the shearing so less crazing. Small partcles on the destroyed surfaces are less distinguished than the large particles. Better gloss due to small particles Core/Shell Because they contain thin shell of rubber so low toughening. As the core of the particles contain the hard phase usually of matrix material that maintain the hardness of the matrix. By making the thickness of rubbery shell comparable to the wave length of light we can have transparent rubber toughened materials
- Figure shows the salami type particle. The intermediate architecture between the two extreams. Here the hard phase diffuse into the soft phase in case of better cohesion and chemistry. These give both properties of Homogeneous and core/ shell particle. Especially used for PS ans ABS.
- Independent of the type of deformation process, whether crazing or shearing, T application should be above T glass transition of rubber to be effective :first to generate stress concentration around the particles because of its low stiffness and second to reduce the stress intensity of the materix by the internal cavitation/stretching.
- Figure at the top shows the transition of brittle nylon to tough nylon structure at the Tg of the rubber phase. This transition is known as the BD transition. Mean the rubbers become effective after its Tg as explained in previous slides. The fig at the bottom shows temperature dependence of damping coefficient Tan delta and we can see that tan delta is max in the BD transition range.
- For the good toughness of rubber filled plastics the adhesion between the particles and Matrix should be very strong. If this compatibility is present, then the molecules at the interface can interdiffuse for good cohesion. The effective part of this grafting covers the surface of the rubber particles like a shell. If there is no grafting between the particles and matrix then the crack propagate around the particle. If there is no good adhesion between the particles and matrix the particles tends to agglomerate that reduces their efficiency as toughening agent because less mirocrazing.
- Fig at the top show the example of interdiffusion of PS into Pbu rubber in case of HIPS. These type of rubber particles are knows as salami particles. Two figures at the bottom show the difference of matrix failure because of the difference of adhesion. The figure on the left shows the fracture surfaces of rubber modified polymer. At the fracture surface, these particles can be detected very well, because the crack propagated around the particles. There is no bonding between the matrix and the particles. Figure on the right shows the material in which there is good adhesion between the rubber particles and matrix . Consequently the crack has propagated preferentially straight forward through the rubber particles. This difference in the fracture surfaces is also reflected in the impact strength of these two materials.
- It can be seen from the figs a & b that due to agglomeration some part of the matrix remained unstrained and do not play role in the energy dessipation so show lower impact strength. The figure on the right shows the matrix with good distribution of rubber particles and as it can be seen that multicrazes are produced that leads to higher impact energies.
- In brittle polymers such as PS & ABS as crazing is the dominant deforming process. Figure shows a transmission electron micrograph of a mixture of small and large particles in HIPS which was deformed only 2.6% it can observed that larger salami type particles play more better role than the smaller particles at same loading. This was also experimentally proved by Okamato.
- In pseudo ductile materials shear is the dominant mechanism of deformation. It is observed that smaller better handle the shear than the large particles. If the particles are small and the distance between the particles is low then instead of original plane strain a plane stress field is built up that is ideal for shearing. The constraint is further reduced by voids/cavitation and ductility is increased. In semi ductile materials like PMMA better results were obtained with mixed size particles in the range of 300 – 600 nm.
- In the non-annealed HIPS numerous voids and crazes have developed these deformation structures developed much less in the annealed sample thus reducing the impact strength of the sample. The embrittlement reduced the Impact strength of HIPS from 72 to 58 J/m 2 .
- The polystyrene phase, which is present as a minor part of the total volume consists of separate spherical regions (domains). These domains are attached to the ends of elastomeric chains and form in this way multifunctional junction points similar to cross-links in a conventionally vulcanized elastomer (vulcanizate). The difference is that these cross-links are of a physical nature that is in contrast to the chemical nature of cross-links in the vulcanizate and therefore considerably less stable. At ambient temperatures, this block copolymer behaves in many ways like vulcanized rubber. When it is heated, the polystyrene domains soften, the network becomes weaker, and eventually the material is capable of flowing, and when it is cooled again, its original elastomeric properties are regained as the polystyrene domains become rigid.
- The figure shows the stress strain curve for the SBS with different ratios of styrene content as it can be seen from the graph that with the increase of styrene content (hard phase) the % elongation decreases.
- Figure shows the change in morphology with time in dynamic vulcanization. Finally we get a morphology of dispersed rubber phase in the continuous phase of matrix(hard phase).
- The figure shows the change of torque as a function of time. Initially the torque goes up due to the mixing of virgin materials. After sufficient mixing the toque becomes constant. After 6 minutes the torque again increases due to the addition of POX and that enhances the crosslinking density with time.
- Figure shows the change of morphology with time during the mixing in dynamic vulcanization. The morphology is changing form co-continuous phases to dispersed rubber phase. Also it can be seen from the fig k and l that the blends show compatible behaviour with good adhesion and dispersion of rubber phase.