HDPE, additives Peroxides

The PP exhibits a sharp peak at the maximum on CL intensity whereas the HDPE curve shows a broad bimodal behavior that has been thoroughly described elsewhere [127]. In the CL curves of the blend all these features were observed, which may be a strong indication of the existence of a two-phase system in the molten state [128], although based on peroxide treatment of PP/PE blend melts. It appears that PP oxidises first and the oxidation sites created during this process accelerate, to some extent, the oxidation of the PE phase. The overlap between the PP and PE traces in the blend can be interpreted as the interface of these two phases where the PE starts oxidising. In addition, the shape of the curves confirms that the oxidation mechanisms of the resins are different and that this difference remains during the oxidation of the blend in the molten state. 

Modifying the molecular structure of HDPE in the extruder using free-radical initiators such as peroxides can reduce die swell (bottle weight) and broaden the MWD of the base resin to provide faster bottle fabrication rates [34,35]. The addition of 100 ppm of an organic peroxide to HDPE prepared with a Phillips catalyst lowered bottle weight 4.5%, as shown in the data in Table 6.13. 

A one step process was developed by BICC with Maillefer for power cables which eliminated the compounding stage (the Monosil process) [15], in which all the additives were mixed with the polymer and grafting performed in the cable extruder. Direct injection has been used to inject a liquid mixture of silane, peroxide and tin catalyst into an add-on mixer to make cable [4] and hot water pipe [16]. The water pipes were steam autoclaved for four hours at 110 °C, which was well below the softening point of the HDPE used and the resulting crosslinked pipes withstood 1000 hours in water at 95 °C with a wall stress of 4.4 N/mm [ ] As with peroxides there are health and safety issues. Tri-methoxy vinyl silane is both very flammable and toxic. 

Processing enhancers include mineral oils, glycerol monostearate, and pentaerythritol monooleate. These decrease melt viscosity and/or elasticity at low shear rate and result in shorter cycle times without sacrificing peak impact strength. Ultrafine monospherical silica particles and boron nitride are also used as additives for special purposes because of their high-performance mechanical properties. Combinations of liquid polybutadienes (with and without functional groups) and dialkyl peroxide have been developed as compatibilisers for polyolefin blends (LLDPE/HDPE/ polypropylene). 

TPEs of SBS (and SEBS)-type are versatile. Compared to PVC compounds, ester plasticizers cannot be used in TPE compounds. Vulcanizing agents such as accelerators, sulfur, or peroxides are not required in SBS-type TPE block copolymers. EPDM is blended with SBS to increase ozone resistance. PS, polyethylene (LDPE, LLDPE, HDPE), and polypropylene (PP) are used as additive polymers with SBS and SEBS. Polystyrene (PS) is compatible with B-S block copolymers. Polystyrene is useful to adjust properties and cost of SBS compounds. The addition of polystyrene into SBS increases hardness, modulus, tensile strength, tear strength, and abrasion resistance. Styrenic resin (a-methyl styrene) can be used as a blend with PS or as an alternative for SBS compound. This resin can be used as a blend with polystyrene as an alternative. Styrenic resin enhances flow properties and physical properties. It is more compatible compared to polystyrene and assists in adhesion and coherent processing. 

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