M. Arroyo

Composites based on PP/EPDM blends and aramid short fibres. Morphology/behaviour relationship

Abstract The effect of both a thermoplastic elastomer (EPDM) and short aramid fibres on polypropylene (PP) crystallization kinetics and tensile behaviour has been investigated. The results have shown that PP/EPDM blends are non-miscible in the melt, and, at low EPDM percentages in the blend (

Crystallization kinetics of polypropylene: 1. Effect of small additions of low-density polyethylene

In order to analyse the effect of small additions (up to 20 wt%) of low-density polyethylene (LDPE) on the crystallization kinetics and thermodynamics of an isotactic polypropylene (iPP), morphological studies of iPP/LDPE blends have been carried out. The results have shown that at LDPE percentages above 10% in the blend, the LDPE aggregates pass over the spherulitic limits and go into the intraspherulitic domains of the PP. The half-crystallization times (τ12) as well as the Avrami exponent (n) and the chain folding energy for PP crystallization (σe) increase markedly at LDPE percentages above 10% in the blend. The isothermal radial growth rate, G, of the PP in the blends is one-third of the plain polymer, although it remains constant at any blend composition. However, the overall kinetic rate constant seems to be influenced by the presence of the melt LDPE phase. In all cases, and according to the obtained results, an instantaneous nucleation takes place.

Crystallization kinetics of polypropylene: II. Effect of the addition of short glass fibres

The effect of short glass fibres on the polypropylene (PP) cyrstallization kinetics and thermodynamics has been investigated. Although at high fibre percentages in the composite the spherulitic limits are not clearly defined, the presence of crystalline entities is evident. A marked decrease of the half time of PP crystallization, τ12, as well as a sensible increase of the overall crystallization rate, Kn, has been observed in the presence of fibres. However, at any crystallization temperature, a maximum of τ12 is reached at 20% glass fibre content in the composite followed by a continuous decrease as fibre percentage increases. A slight increase in the Avrami exponent as temperature increases is obtained although it is relatively independent of composite composition. In all cases an instantaneous nucleation takes place, a nucleant effect of fibres is evident, and no transcrystallinity has been observed with these short glass fibres. Fibres give rise to a marked increase in the crystallinity of the samples. The chain folding energy for PP crystallization (σe) decreases with fibre content, and a minimum is observed at 20% fibre content.

Carbon nanotubes provide self-extinguishing grade to silicone-based foams

This article examines the feasibility of using multiwall carbon nanotubes (MWCNTs) as a fire retardant in silicone-based foams. MWCNTs were mechanically blended with polydimethylsiloxane reactants without any previous functionalisation treatment. The flammability of the materials was characterised as a function of the filler content using a standard fire testing method. The limiting oxygen index of the composite increased as a function of nanotube content, attaining the self-extinguishing grade even at low mass fraction of the carbon nanotubes (0.5 wt%). Further thermal characterisation revealed increases in thermal dissipation efficiency and thermal degradation temperature. The different thermal and flame retardancy results were attributed to the formation of a CNT network throughout the sample. The formation of this network was attributed to a good dispersion of the CNTs via non-covalent CH–π interactions and to the surface tension of the gas–polymer interface during foaming.

Crystallization kinetics of polypropylene. III. Ternary composites based on polypropylene/low density polyethylene blend matrices and short glass fibres

Abstract The effect of, simultaneously, both short glass fibres (SGF) and low density polyethylene (LDPE), on PP crystallization kinetics and thermodynamics has been investigated. Glass fibres tend to initially compensate the delaying effect of LDPE on the PP crystallization as is shown by the values of the crystallization half time, τ 1/2 , as well as by the overall crystallization rate, K n . However, at high glass fibre percentages in the composite (30–40%) a considerable increase of τ 1/2 values is observed although they remain always below the corresponding values of the plain polymer. The isothermal radial growth rate of PP spherulites is hardly affected by both the fibres and the LDPE. According to the values of the Avrami exponents, it can be established that the spherulitic development arises from an athermal, instantaneous and heterogeneous nucleation with two-directional diffusion controlled growth. No transcrystallinity has been observed.

Effect of the incorporation of pet fibers on the properties of thermoplastic elastomer based on PP/elastomer blends

Abstract Ternary composites based on isotactic polypropylene (iPP), thermoplastic elastomer, such as ethylene–octhene copolymer (EOC) and poly(ethylene-terephtalate) (PET) textile fibers, have been processed and their properties analyzed. The effect of matrix composition and fiber content on the final properties of the composite has been investigated by means of an experimental design based on a Doehlert Uniform Net. The results have shown that PET fibers act as an effective reinforcing agent, giving rise to a sensible improvement in the tensile and flexural behavior, mainly in matrices with high copolymer percentages. It must be pointed out that the analyzed mechanical properties depend more on the matrix composition than on the fiber percentage. So, as PP content increases, the blend becomes more rigid and stable and a noticeable increase in tensile and flexural modulus and strength, as well as a sensible decrease in tensile elongation are observed. The dynamic-mechanical analysis strongly supports the assumption that the PET fibers behave as a reinforcing agent, showing a sensible increase of the storage modulus in the presence of the fibers. Moreover, this increase is more noticeable as fiber content in the composite is increased. The morphology of the composites has been also analyzed through scanning electron microscopy (SEM).

New developments in dynamically cured PP-EPDM blends

ifferent procedures for preparing thermoplastic vulcanizates (TPVs), based on isotactic polypropylene (iPP) and ethylene-propylene-diene terpolymer rubber (EPDM), are used and analyzed in this work. In order to determine the effect of the vulcanization method on material properties, a rheological study, dynamic-mechanical analysis, mechanical properties and morphological study have been carried out. In all cases, the sulfur is used as crosslinking agent of the elastomeric phase. It has been shown that the dynamically cured blends (referred in the work as V2 and V3) present better properties in relation to those statically cured (V I) and uncrosslinked (V). Thus, the elastic ability, mechanical properties and rheological characteristics of these systems sensibly increase when the samples are dynamically vulcanized. Morphological analysis performed by scanning electron microscopy (SEM) is clearly in agreement with the analyzed properties, showing a better dispersion between both polymeric chains, when the blends are dynamically cured. These results seem to indicate that the dynamic vulcanization process gives rise to the formation of a thermally stable three-dimensional network, and as a consequence of it, a sensible increase of the properties is obtained.

Optimisation of nanocomposites based on polypropylene/polyethylene blends and organo-bentonite

The effect of the organophilisation of a Na-bentonite, through cationic exchange reaction with a commercial quaternary benzalkonium salt, on the morphology and mechanical behaviour of polypropylene (PP)/low density polyethylene (LDPE) blend matrices has been investigated employing a statistical experimental design. The results show that the tensile modulus and strength of the composites increase (up to 760 and 360%, respectively) as both PP percentage in the matrix and clay content in the composite increase. The properties are more dependent on matrix composition than on clay content. In all cases, the organo-bentonite gives rise to more resistant materials in comparison with the Na-bentonite. Increments of 50–35% in stiffness and strength, respectively, have been observed. The elongation at break mainly depends on matrix composition and a minimum is observed when phase inversion has taken place. The organophilized bentonite gives rise, in some formulations, to an increase in elongation up to 400%. Theoretical equations to predict the tensile behaviour of composites based on PP/LDPE blends with Na-bentonite and benzalkoniun chloride modified bentonite, inside the investigated limits, have been deduced. A morphological study has been carried out by means of scanning electron microscopy.

Effect of interface on the morphology and properties of composites comprising poly(propylene) and short organic fibers

The effect of the fiber surface modification with an azide derivative on the morphology and properties of composites based on poly(propylene) (PP) and short poly(ethylene terephthalate) (PET) and nylon 66 (PA) fibers, has been investigated. Both organic fibers act as reinforcement of the PP, and the reinforcing effect increases with the introduction of azide groups on the chemical structure of the fibers. This effect is more sensible in PP/short PET fiber composites although PA fibers gives rise to higher improvements in toughness. Scanning electron microscopy (SEM) has shown that the azide treatment of PET fibers gives rise to a better wettability and adhesion at the fiber/matrix interface. A good correlation between SEM and mechanical behavior of the composites has been observed. Der Einflus der Oberflachenmodifizierung von Fasern mit einem Azidderivat auf die Morphologie und die Eigenschaften von Verbundwerkstoffen aus Polypropylen (PP) und kurzen Fasern aus Polyetylenterephthalat (PET)- oder Nylon 66 (PA) wurde untersucht. Beide organischen Fasern wirken als Verstarker von PP, und der Verstarkungseffekt nimmt mit der Einfuhrung von Azid-Gruppen in die chemische Struktur der Fasern zu. Dieser Effekt ist bei Verbundwerkstoffen aus PP/PET-Kurzfasern deutlicher ausgepragt, obwohl PA-Fasern eine starkere Verbesserung der Festigkeit bewirken. Rasterelektronenmikroskop-Aufnahmen zeigen, das die Behandlung von PET-Fasern mit einem Azidderivat eine bessere Benetzbarkeit und Adhasion an der Grenzflache Faser/Matrix bewirkt. Es wurde eine gute Ubereinstimmung zwischen den Ergebnissen aus der Rasterelektronenmikroskopie und dem mechanischen Verhalten beobachtet.

Preparation and Characterization of Thermoplastic Vulcanizates-Organoclay Nanocomposites

Thermoplastic vulcanizates nanocomposites based on polypropylene (PP) and ethylene–propylene-diene terpolymer rubber (EPDM) blends reinforced with organoclay modified montmorillonite have been prepared via melt intercalation. The silicate layers of the clay were intercalated and dispersed at a nanometer level in the matrix blends. The nanocomposites exhibit improved mechanical properties, this effect being more evident at high EPDM contents in the blend (above 80%). This behavior is attributed to the fact that the EPDM chains are more easily inserted into the galleries silicate, giving rise to a more intercalated structure as was observed by X-ray diffraction.