Polypropylene blends with elastomers

Discovery of isotactic polypropylene (PP) in 1954 started a search for methods to improve its low-temperature impact strength [1, 2]. These activities commenced in the late 1950s by blending PP with poly-ethylenes (PE) or elastomers, then by copolymerizing it with ethylene into ethylene-propylene rubber (EPR) and ethylene-propylene-diene (EPDM). 

The PP/PIB blends are used for general-purpose films, boil-in bags, medical over-pouches, industrial sacks and liners. The blends compounded with carbon black served as over-wraps for the manufacture of tarpaulins, photographic films and papers, as well as in a variety of outdoor applications (weatherability of clear grades is poor). 

The butadiene-styrene rubber (SBR), or butadiene-acrylonitrile rubber (NBR) and elastomeric graft copolymers were found particularly valuable for impact modification of PP. The PP alloys (with 5-20 wt.% of an elastomer) were reported to have advantageous properties for blow molding of bottles free from brittleness and stress cracking. Blends with natural rubber (NR) require sulfur-curing [4]. Blending with NBR dramatically increased the modulus, but the material was brittle [5]. 

Numerous commercial PP/EPR blends are on the market. Reinforced and filled grades are also available [6, 7]. They have been used in automotive industry, in appliances, hardware and plumbing, medical tubing, shoe industry, sports equipment, toys, cases, etc. 

The first disclosure describing these blends dates from the 1960s. Compositions containing PP with 0.1-60 wt.% EPR (containing 2-25% ethylene) have been disclosed. These blends can be treated as polymeric emulsions - addition of EPR increases viscosity of PP well above the log-additivity line [3]. 

---

Chattopadhyay S., Chaki T.K., and Bhowmick A.K., New thermoplastic elastomers from poly(ethyle-neoctene) (engage), poly(ethylene-vinyl acetate) and low-density polyethylene by electron beam technology structural characterization and mechanical properties. Rubber Chem. TechnoL, 74, 815, 2001. Roy Choudhury N. and Dutta N.K., Thermoplastic elastomeric natural rubber-polypropylene blends with reference to interaction between the components. Advances in Polymer Blends and Alloys Technology, Vol. 5 (K. Finlayson, ed.), Technomic Publishers, Pensylvania, 1994, 161. 

During the last several years, much effort has been spent on developing new materials, based on iPP/elastomers blends. This interest is related to the fact that addition of the rubber phase improves the impact strength of the iPP, The present paper reports on a study of the isothermal crystallization and melting behaviour of thin films of isotactic polypropylene blended with an ethylene--propylene diene terpolymer and three samples of polyisobutylene with different molecular mass. 

An elastomer which upon heating turns into regularly behaving linear polymer. Polystyrene-polybutadiene block copolymers, polypropylene blends with ethylene-propylene-diene terpolymer provide examples. 

In Section 6.8, CNIS data are compared with measurements of strain energy release rates G/ obtained under the well-defined criteria of linear elastic fracture mechanics of materials under the conditions of small-scale yielding. A variety of polypropylene resins, their blends with elastomers, rigid fillers, or both are included in this study. 

Considerable amounts of EPM and EPDM are also used in blends with thermoplastics, eg, as impact modifier in quantities up to ca 25% wt/wt for polyamides, polystyrenes, and particularly polypropylene. The latter products are used in many exterior automotive appHcations such as bumpers and body panels. In blends with polypropylene, wherein the EPDM component may be increased to become the larger portion, a thermoplastic elastomer is obtained, provided the EPDM phase is vulcanked during the mixing with polypropylene (dynamic vulcani2ation) to suppress the flow of the EPDM phase and give the end product sufficient set. 

Modification ofP/astics. Many plastics, such as PVC, ABS, polypropylene, and nylon, ate blended with nitnle mbber to improve flexibiHty, toughness, or appearance. An oil-resistant thermoplastic elastomer has been prepared by blending nitnle mbber and polypropylene (24). 

Blend with Isotactic Polypropylene and Thermoplastic Elastomer.176... 

A comparatively new group of materials— thermoplastic elastomers or thermoplastic rubbers —combines the ease of processing of thermoplastics with qualities of traditional vulcanized rubbers, especially elasticity. Because of convenience in processing there is much interest too in blends of plastics with elastomers, which may be modified by the inclusion of filler or glass fibre. As an example, a rubber-like material that can be processed as a thermoplastic can be made by blending and melt-mixing an ethylene-propylene rubber with polypropylene. The use of such blends may be helpful when there are needs to reclaim and re-process material, and in order to obtain products with qualities intermediate between those of the main components of the blends. 

Among the most versatile polymer matrices, polyolefins such as polypropylene (PP) are the most widely used thermoplastics because of their well-balanced physical and mechanical properties and their easy processability at a relatively low cost that makes them a versatile material. PP has the disadvantage of becoming brittle at low temperature, however, because of its high transition temperature and high crystallinity. The best way to improve its impact strength is to blend PP with elastomers... 

It is well known that neat polypropylene (PP) is incompatible with neat polyamide. Therefore, in order to improve the compatibility between them, the maleated polypropylene is often added into the blends, which has proved to be very effective. Other attempts concentrate mainly on the improvement of impact toughness, which is generally realized by adding elastomer into the blends. The elastomer is either... 

A very important thermoplastic elastomer is comprised of a blend of polypropylene (PP) with an ethylene-propylene-diene (EPDM) terpolymer. This latter material is, of course, a crosslinkable thermoset rubber ... 

As stated previously, styrene-diene triblock copolymers are the most important category of thermoplastic elastomers. Unlike most other TPEs, they can be blended with large quantities of additives without a drastic effect on properties. In almost all applications, the actual triblock copolymer content is less than 50%. Oils are used as a processing aid and do not result in a significant loss of properties if the polystyrene domains are not plasticized. For this reason, naphthalenic oils are preferred. The use of inert fillers such as clays or chalks reduces the cost of the final material. Unlike conventional rubbers, inert fillers do not have a substantial effect on the mechanical properties of TPEs. Thermoplastics such as polyethylene or polypropylene are also used to improve the solvent resistance and can increase the upper service temperature. Polystyrene homopolymer is used as a processing aid, which also increases the hard phase weight fraction and causes the material to stiffen. 

Steller, R., Zuchowska, D., Meissner, W., Paukszta, D., Garbarczyk, J., Crystalline structure of polypropylene in blends with thermoplastic elastomers after electron beam irradiation. Radiation Physics and Chemistry 2006, 75, 259-267. 

The structure and physical properties of the thermoplastic vulcanizates (TPE-V) produced in the process of the reactive processing of polypropylene (PP) and ethylene-octene elastomer (EOE) in the form of alloy, using the cross-linking system was analyzed. With the DMTA, SEM and DSC it has been demonstrated that the dynamically produced vulcanizates constitute a t5 ical dispersoid, where semicrystal PP produces a continuous phase, and the dispersed phase consists of molecules of the cross-linked ethylene-octene elastomer, which play a role of a modifier of the properties and a stabilizer of the two-phase structure. It has been found that the mechanical as well as the thermal properties depend on the content of the elastomer in the blends, exposed to mechanical strain and temperature. The best results have been achieved for grafted/cross-linked blends with the contents of iPP/EOE-55/45%. 

Austin, J. R. and Kontopoulou, M. 2006. Effect of organoclay content on the rheology, morphology, and physical properties of polyolefin elastomers and their blends with polypropylene. Polymer Engineering and Science 46 1491-1501. 

---