Thermoplastic Matrix Properties

The ductility of GRT-polyethylene blends drastically decreases at ground rubber concentration in excess of 5%. The inclusion of hnely ground nitrile rubber from waste printing rollers into polyvinyl chloride (PVC) caused an increase in the impact properties of the thermoplastic matrix [76]. Addition of rubber powder that is physically modihed by ultrasonic treatment leads to PP-waste ethylene-propylene-diene monomer (EPDM) powder blends with improved morphology and mechanical properties [77]. 

ABA triblock copolymers of the styrene-diene type are well known, and owe their unique properties to their heterophase morphology. This arises from the incompatibility between the polystyrene A blocks and the polydiene B blocks, leading to the formation of a dispersion of very small polystyrene domains within the polydiene matrix. This type of elastic network, held together by the polystyrene "junctions", results in thermoplastic elastomer properties. 

In multiphase filled polymer compositions, which may contain mixed filler types, combinations of fillers and fibres, or proportions of filler and a secondary modifying polymer, such as an elastomer, the spacial distribution of the phases has a direct bearing on the properties of the composite. In the case of the last mentioned system, the rubber may encapsulate the filler, be present as discrete droplets within the thermoplastic matrix or co-exist in both structural forms [80,81]. 

In addition to the nature of particulate platelet orientation induced during injection moulding, the associated consequences on molecular orientation and crystalline order of the host thermoplastic matrix have also been reported with particular regard to various flake-filled polypropylenes [174], together with an attempt to interrelate these higher order structural parameters with physical properties of the composites [175]. 

The major trend observed is a modest increase in KIc or GIc by the introduction of initially miscible thermoplastics. This improvement is obtained without any loss in stiffness and thermal properties. Some very high improvements in KIc, claimed by some authors, are due to phase inversion, leading to a thermoplastic matrix with thermoset particles. 

Precrosslinked particles with low crosslink density exhibit elastic properties and ean be applied for toughening thermoplastics or thermosets. The size of the elastic domains in blends consisting of elastic particles and a polymer matrix can be adjusted precisely, provided that the particles are dispersible. Via functional groups, microparticles can be covalently attaehed to a (thermoset) matrix. The grafting of polymer shells onto elastic microparticles improves the compatibility with the polymer matrix to be modified [3]. Thus, after processing of the polymer alloy discrete elastic particles can be observed as disperse phase in a continuous thermoplastic matrix. 

Poly(aryl ketones) (PEEK, PEK, and PEKK) are commercial high temperature polymers offering an excellent combination of properties combined with thermoplastic behavior. Poly(aryl ether ketone) PAEK blends have been reviewed by Harris and Robeson [1989]. Miscibility with PEI (Ultem 1000 GE) and other PI containing isopropylidene bridging units was noted. Arzak et al. [1997] reviewed the performance ofPEEK/PEI blends and noted a synergistic behavior in ductility and impact strength as reported earlier. Utility of these blends as a thermoplastic matrix candidate for advanced composites has been proposed [Harris and Robeson, 1989 Davis et al., 1992]. 

Following this, in a joint project lasting several years, we investigated over a broad concentration range, the conductivity of polyaniline dispersed in a thermoplastic matrix. The results were confirmed, in quantitative terms as well as the metallic properties and the dimensions (9.6 nm) of the PAni primary particles, which contain a metallic core of 8 nm [68]. 

The microstructure and stereoblock distribution peculiar of polypropenes produced with this class of catalysts imparts thermoplastic elastomeric properties to the polymers. Thermoplastic elastomers or elastoplasts (TPEs) owe their elastomeric properties of resiliency and high tensile strength to physical cross-linking (formation of hard domains in a soft matrix) due to the presence of short, crystallizable... 

All TP or TS matrix property can be improved or changed to meet varying requirements by using reinforcements. Typical thermoplastics used include TP polyesters, polyethylenes (PEs), nylons (polyamides/ PAs), polycarbonates (PCs), TP polyurethanes (PURs), acrylics (PMMAs), acetals (polyoxymethylenes/POMs), polypropylenes (PPs), acrylonitrile butadienes (ABSs), and fluorinated ethylene propylenes (FEPs). The thermoset plastics include TS polyesters (unsaturated polyesters), epoxies (EPs), TS polyurethanes (PURs), diallyl phthalates (DAPs), phenolics (phenol formaldehydes/PFs), silicones (Sis), and melamine formaldehydes (MFs). RTSs predominate for the high performance applications with RTFs fabricating more products. The RTPs continue to expand in the electronic, automotive, aircraft, underground pipe, appliance, camera, and many other products. 

PROPERTIES OF SPECIAL INTEREST Mostly synthesized as flexible semicrystalline thermoplastic, PBT has outstanding resilience and toughness. High toughness and resilience is due to improved chain flexibility derived from the four methylene units. Used in thermoplastic matrix composites for gears, machine parts, small pump housings, and insulators. 

Recently, bacterial cellulose, produced by Acetobacter Xylinum, was used as reinforcement in composite materials with a starch thermoplastic matrix [230]. The composites prepared with bacterial cellulose displayed better mechanical properties than those with vegetable cellulose fibers. 

Unfortunately, both these modifications are complicated processes to carry out. There is also a negative side effect of using modified cellulose whiskers. It has been shown that modified whisker have less reinforcing effect than unmodified whiskers. Grunert and Winter [74] prepared nanocomposites with a hydrophobic thermoplastic matrix using trimethylsilylated cellulose whiskers. They found that unmodified whiskers showed a better reinforcing performance than the trimethylsilylated whiskers. Similarly, the mechanical properties of nanocomposites containing chemically modified chitin whiskers from crab shell were found to be inferior to the unmodified nanocomposites [75]. 

It is common practice to add fillers, such as talc or glass fibers, to a thermoplastic matrix to achieve cost reduction and mechanical reinforcement, as well as to enhance various properties such as electrical conductivity, thermal properties, and dimensional stability. Large amoimts of conventional micron-size fillers are typically required in these formulations, which results in deterioration of processability and surface appearance. 

More recently nanoscale fillers such as clay platelets, silica, nano-calcium carbonate, titanium dioxide, and carbon nanotube nanoparticles have been used extensively to achieve reinforcement, improve barrier properties, flame retardancy and thermal stability, as well as synthesize electrically conductive composites. In contrast to micron-size fillers, the desired effects can be usually achieved through addihon of very small amounts (a few weight percent) of nanofillers [4]. For example, it has been reported that the addition of 5 wt% of nanoclays to a thermoplastic matrix provides the same degree of reinforcement as 20 wt% of talc [5]. The dispersion and/or exfoliahon of nanofillers have been identified as a critical factor in order to reach optimum performance. Techniques such as filler modification and matrix functionalization have been employed to facilitate the breakup of filler agglomerates and to improve their interactions with the polymeric matrix. 

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