The impact strength of polymers

For line A the ratio Hb /hy 2 rather than unity, but the difference may be accounted for by the measurement of Hb at very low temperatures and possibly by the measurement of aB in flexure rather than in tension. (The latter may reduce the possibility of fracture at serious flaws in the surface.) It is encouraging that even an approximate relationship holds along the lines of the Ludwig-Davidenkov-Orowan hypothesis. Even more encouraging is the fact that line B has a slope Hb/hy 6, which is three times that of A, as expected on the basis of the plastic constraint theory. 

The principal value of the aB- Y diagram is that it may guide the development of modified polymers or new polymers. Together with the ideas of the previous section on the influence of material variables on the brittle stength and yield stress, it can lead to a systematic search for improvements in toughness. 

The ability of a structural part to maintain its integrity and to absorb a sudden impact is often a relevant issue when selecting a suitable material. Impact testing of polymers is thus a subject of some importance and is extensively employed, although many of the results obtained are of an empirical and hence comparative 

The two major types of impact test are categorized as flexed beam and falling weight. 

---

This new theory of the non-equilibrium thermodynamics of multiphase polymer systems offers a better explanation of the conductivity breakthrough in polymer blends than the percolation theory, and the mesoscopic metal concept explains conductivity on the molecular level better than the exciton model based on semiconductors. It can also be used to explain other complex phenomena, such as the improvement in the impact strength of polymers due to dispersion of rubber particles, the increase in the viscosity of filled systems, or the formation of gels in colloids or microemulsions. It is thus possible to draw valuable conclusions and make forecasts for the industrial application of such systems. 

The size and number of sphemhtes in the polymer tends to affects the physical properties. Thus, the impact strength of polymer hhns or their flex life usually increases as the sphemlite size decreases. On the other hand, there does not appear to be any correlation between the yield stress and ultimate elongation and the size of the sphemhtes. 

As with calcimn carbonate, the colour of talc is variable, but it can sometimes be white. Its hydrophobic surface makes it readily compatible with hydrocarbon polymers such as polypropylene, and it is used to increase the stiffness and heat distortion temperature of automotive parts, being more effective in this respect than calcium carbonate. Talc lowers the impact strength of polymers, although the effect can be mitigated to some extent by a stearate coating. Wax coatings are also used, but silanes do not react with the surface of talc. 

Mechanical Properties of Solid Polymers 13.9 The Impact Strength of Polymers... 

One unfortunate characteristic property of polypropylene is the dominating transition point which occurs at about 0°C with the result that the polymer becomes brittle as this temperature is approached. Even at room temperature the impact strength of some grades leaves something to be desired. Products of improved strength and lower brittle points may be obtained by block copolymerisation of propylene with small amounts (4-15%) of ethylene. Such materials are widely used (known variously as polyallomers or just as propylene copolymers) and are often preferred to the homopolymer in injection moulding and bottle blowing applications. 

Thermoplastic polymers, such as poly(styrene) may be filled with soft elastomeric particles in order to improve their impact resistance. The elastomer of choice is usually butadiene-styrene, and the presence of common chemical groups in the matrix and the filler leads to improved adhesion between them. In a typical filled system, the presence of elastomeric particles at a level of 50% by volume improves the impact strength of a brittle glassy polymer by a factor of between 5 and 10. 

The radius of the notch is quite important, particularly for plastics. For example, polyvinyl chloride (PVC) is a notch-sensitive material. If the notch is blunt (2-mm radius), the impact strength is higher than that for ABS. If the notch is sharp (0.25-mm radius), the impact strength of PVC is lower than that for ABS. Other polymers that are notch brittle are high-density polyethylene (HDPE), polypropylene (PP), polyethylene teraphthalate (PET), dry polyamides (PAs), and acetals. 

Polycarbonate is blended with a number of polymers including PET, PBT, acrylonitrile-butadiene-styrene terpolymer (ABS) rubber, and styrene-maleic anhydride (SMA) copolymer. The blends have lower costs compared to polycarbonate and, in addition, show some property improvement. PET and PBT impart better chemical resistance and processability, ABS imparts improved processability, and SMA imparts better retention of properties on aging at high temperature. Poly(phenylene oxide) blended with high-impact polystyrene (HIPS) (polybutadiene-gra/f-polystyrene) has improved toughness and processability. The impact strength of polyamides is improved by blending with an ethylene copolymer or ABS rubber. 

The impact strength of a polymer is strongly dependent on the molar mass, in particular on (number average, number of chain ends)... 

There is a much smaller variation in impact strength of polymers, since it is not directly related to either tensile or tear strength. Once again PET has a high impact strength relative to the other polymers, which is important for soda bottles (have yon ever dropped a full, 2-liter bottle of soda ). PVC also has excellent impact strength. 

Blends of butadiene-styrene block polymers with polyolefins, particularly polypropylene are mentioned in literature to improve the impact strength of the latter. Since similar improvements can be realized from the use of polyolefin block polymers, the blends have not gained much recognition. However, butadiene-styrene radial teleblock polymers are blended into polyethylene film, to increase the tear resistance and tensile impact. ... 

The use of a plasticizer helps to improve the impact strength of the resultant polymer. The plasticizer may be added at any point in the process so as to ensure that it is mixed well with the polymer. Preferred plasticizers include mineral oil, poly(butene)s, or a combination of both mineral oil and poly(butene)s. The amount of plasticizer used is 2- 4%. 

A secondary glass transition is, in general, important for the impact strength of a polymer it creates the possibility to dissipate energy in situations of shock loading, so that the polymer is less brittle. 

Besides by motions of chain parts or side groups (see 3.4), a secondary transition can also result from the presence of a second polymer, which has been added in a small quantity by blending, or which has, as tails , be copolymerized with the main polymer. Such a second polymer then has its glass transition at Tsec. Both cases occur when the impact strength of a polymer has been improved, either by blending with some rubber, or by copolymerization. A similar situation is found with block copolymers. In 3.5 these cases are dealt with in more detail. 

The impact strength of brittle polymers is often improved by the incorporation of small rubber particles, either by blending or by copolymerization (e.g. PP copolymer). 

The impact strength is temperature-dependent near the glass temperature the impact strength of glassy polymers increases dramatically with temperature. Secondary transitions play an important role-, a polymer with a strong low-temperature secondary transition in the glassy state is nearly always much tougher than a polymer which has no such transition. 

The impact strength of thermosetting polymers varies little with temperature over a wide range. 

---