Polypropylenes crosslinked

Here pp denotes fractional chains of polypropylene. Crosslinking of PP can also occur by reactions analogous to (R-30), and (R-37), and by the analogues of the cationic reactions in PE [Singh, 1995]. 

Low effidency of the radiation crosslinking of polypropylene is a reason why irradiation cannot be used for improving the thermomechanical properties of this polymer as it is commonly practiced with pofyethylene. On the other hand, this fact has stimulated a large amount of research on polyfunctional monomers aimed at a deliberate increase of radiation effidency d"polypropylene crosslinking [76]. 

When a polymer is irradiated below the melting temperature of crystallites, the polyfunctional monomer remains in the amorphous regions. Gels of polypropylene crosslinked in the presence of butadiene are crystalline which is a manifestation of the formation of crosslinks at the lamellar surfaces [79]. The presence of hexadecane and hexadecene-1 in isotactic polyiroiylene increases the rate of termination of radicals formed in the irradiated polymer and the oxidative post-effect is reduced as well as the deterioration of mechanical properties is less than in a polymer without additives [80]. 

New potential possibilities in polypropylene crosslinking which have not been elaborated very much are those from the use of azobisdicarbonamide as a crosslinking and blowing agent. This was attempted at temperatures ranging fi-om 200 to 260 °C [129],... 

TPEs from blends of rubber and plastics constitute an important category of TPEs. These can be prepared either by the melt mixing of plastics and rubbers in an internal mixer or by solvent casting from a suitable solvent. The commonly used plastics and rubbers include polypropylene (PP), polyethylene (PE), polystyrene (PS), nylon, ethylene propylene diene monomer rubber (EPDM), natural rubber (NR), butyl rubber, nitrile rubber, etc. TPEs from blends of rubbers and plastics have certain typical advantages over the other TPEs. In this case, the required properties can easily be achieved by the proper selection of rubbers and plastics and by the proper change in their ratios. The overall performance of the resultant TPEs can be improved by changing the phase structure and crystallinity of plastics and also by the proper incorporation of suitable fillers, crosslinkers, and interfacial agents. 

Whereas PVA fleeces are used only in primary cells polyamide fleeces compete with polyolefin, preferably polypropylene fleeces. The latter are more stable at higher temperatures and do not contribute to electrolyte carbonation, but they wet only after a pretreatment either by fluorination [131] or by coating and crosslinking with hydrophilic substances (e.g., polyacrylic acid [132]) on the surface of the fiber. 

The rapid decomposition of a peroxide in the presence of MAH results in the excitation and homopolymerization of MAH. When the latter is conducted in the presence of polypropylene, the latter undergoes degradation ( 10) while polyethylene is crosslinked under the same conditions (11). This has been attributed to the presence of excited MAH which increases the radical generation on the polymer beyond that due to the radicals from the peroxide. 

Stearamide is one of many electron donors which donate an electron to the cationic moiety in excited MAH or in propagating -MAH chains. This results in the inhibition of the homopolymerization of MAH and decreases the crosslinking of polyethylene and the degradation of polypropylene which accompany the peroxide-catalyzed reaction of MAH with these polyolefins (8,9). ... 

Unlike polyethylene, polypropylene is not crosslinked and deteriorates. The action of high-energy radiation depends in a complex way on the power, dose and wall thickness of polypropylene. Thus, it is often advised to avoid doses greater than 1 kJ/kg (0.1 Mrad). However, special grades are marketed for radiation sterilization but doses of the order of 25kJ/kg (2.5 Mrad) can lead to a certain brittleness after storage of several months. 

RECYCLING TECHNOLOGY FOR LAMINATES COMPOSED OF THERMOPLASTIC POLYOLEFIN ELASTOMER AND CROSSLINKED POLYPROPYLENE FOAM... 

By modifying the functional groups they can be used,for example, as crosslinkers in high solid or powder coatings and in thermosets. Because of their good miscibility and low melt viscosity, they find applications as melt modifiers and as blend components. Modified hyperbranched polymers, like alkyl chain substituted poiy(ether)s and po-ly(ester)s sometimes exhibit amphiphilic behavior.They can, therefore, be used as carriers for smaller molecules,for example, dyestuff into polypropylene. 

The possibility of modifying polypropylene by extrusion at 220° C in the presence of alkaline sulphate lignin and a plasticizer to increase the low temperature resistance was also studied (96). The modifying effect of the lignin appears to be due to its rupture into free radicals which may cause crosslinking of the polypropylene macromolecules. The polypropylene which was modified (Propolin) is used to make films which have increased low temperature and light resistance. 

From industry s viewpoint, the action of nitrous oxide should be useful to cut down the dose required for the crosslinking of polyethylene or polypropylene and to keep polyisobutylene from degradation. However, it is a question whether the complexity of processing caused by using nitrous oxide would pay economically. 

A related class of polymers is the crystalline, thermoplastic materials. These also are fabricated by heating to a high temperature so diat they flow but when they are cooled ordered regions develop within them, which makes them translucent. They have much tendency to flow because of these mechanical crosslinks" and have good dimensional stability. Polyediylene and polypropylene belong to this class. Here the chain is simple and regular so that different polymer molecules, or different parts of the same molecule, can pack next to each other. The same situation exists with Teflon" (polytetrafl uoroethylene). 

The support membrane is soaked in the prepolymerisation mixture consisting of functional monomer, crosslinker and template. An MIP layer is formed on or in the porous support by in situ photo or thermal polymerisation of the monomer mixture. This was first demonstrated by Haupt and coworkers on an amino acid specific MIP synthesised in the pores of a polypropylene membrane. Composite membranes... 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

Crazing inhibition by crosslinking explains the better fatigue resistance of thermosets compared with thermoplastics (this is not valid for semicrystalline thermoplastics such as polypropylene or poly (ether ether ketone). 

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