HDPE melt, additives

MW fraction increases the melt flow, thus improving the processability but at the cost of toughness, stiffness, and stress crack resistance. In addition, the improvement in performance through narrowing the MWD is restricted by the catalyst, the process hardware, and the process control limitations. Dow has developed a reactor grade HDPE of optimized breadth, peak, and shape of MWD... 

Polyolefins are produced in a variety of forms HDPE and PP are produced as powders, while LDPE emerges from the melt preferably in the form of lenticular granules. All types, however, are supplied primarily as granules. As a rule, any thermoplastic transformation of a polymer powder to a granulate is carried out in the presence of additives. This is also partly true for pigments. 

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. 

Upon exiting the die, the sheet extrudate will swell to a level determined by the polymer, the melt temperature, the die length-to-opening ratio, and the shear stress at the die walls. Additionally, flow instabilities will occur at values of the corrected shear stress at the wall, of the order of, but higher than 105 N/m2, as found by Vlachopoulos and Chan (58), who also concluded that, for PS, HDPE, and LDPE, the critical Sr in slits is 1.4 times higher than in tubes of circular cross section. Aside from these differences, the information presented in Section 12.1 and 12.2 applies to slit flow. 

Tomboulian et al. (2002) has reported that butylated hydroxytoluene (BHT) can impart a "burnt plastic" odor and is an additive in HDPE pipes. Quinone may be derived from BHT due to interactions with residual chlorine in pipes (Anselme et al., 1985). Yam et al. (1996) reported that antioxidants, such as vitamin E, Irganox 1010, and BHT, contributed to off-flavors in water. Vitamin E yielded less off-flavor, possibly due to lower aldehyde and ketone concentrations. Extrusion temperatures over 280 °C and exposure time for melt contributed to more oxidation of LDPE films and higher intensities of off-flavors in water in contact with LDPE with different antioxidants (Andersson et al., 2005). 

Xue et al. (16) prepared UHMWPE-HDPE and UHMWPE-HDPE-CNT blends by mixing the melt in a kneader and then hot pressing into plates. The addition of HDPE reduced the melt viscosity of... 

Blends of three PP and two HDPE resins having different values of Hq were studied in a capillary viscometer ( 35). Independently of the viscosity ratio, X, at 200 C all blends showed small negative deviation from the log-addltlvlty rule, NDB. The non-equilibrium extrudate swell at low stress (measured after quenching) showed small positive deviation from the additivity rule while at higher stresses the additivity. The NDB tendency for the viscosity combined with the constant critical shear stress for melt fracture, oxp " 2MPa, indicate a better extrudablllty of the PP/HDPE blends than that of the neat resins. 

As illustrated in Fig. 4, the course of changes in MWD for the additive free HDPE are very similar to those for LDPE at temperatures above its melting range. This is also the case for both the degree of LCB and the formation of insoluble material. However, compared to LDPE the formation of low MW material is more rapid for HDPE. This might be due to catalyst residues, influencing the way of decomposition of hydroperoxide groups. 

Polyethylene and polypropylene blended with iron carboxylate complexes, for example, acetylacetonate (FeAcAc) and stearates (FeSt), and irradiated by UV light under accelerated aging conditions were shown to act as effective phtoactivators giving rise to rapid photoxidation as shown from the rapid rate of carbonyl formation without any induction period (see Fig. 16.4a for FeAcAc in HDPE) and with a reduction in molar mass (see Fig. 16.2a for FeSt in LDPE). However, these complexes have been shown to cause considerable oxidation to both PE and PP during processing reflected in a sharp increase in the polymer s melt flow index (reflecting chain scission and drop in molar mass) (Fig 16.4b) and act, therefore, as thermal prooxidants and cannot be used without the use of additional antioxidants in the system [2,3,17-19,48,49]. 

High density (HDPE), 52 Irregularities, 52 Linear low density (LLDPE), 52 Low density (LDPE), 52 Molecular weight, 52 Melt flow index, 53 Melting temperature, 51 Moisture absorption, 51 Polymeric forms, 52 Resistance to chemicals, 52 Resistance to oxidation, 52 Shrinkage, 54 Unsaturations, 54 a-transition, 51 P-transition, 51 y-transition, 51 Polyisocyanate, 79 Polylactic acid, 79, 91 Polymer alloys, 48 Polymer processing additives, 646 Polymer rheology, 619 Polymeric forms, 52 Polyphase PlOO, 451 polypropylene (PP), 2, 11 Polypropylene homopolymer, 70... 

Chen et al. [1988] reported about blends of polyamides with a polyolefin. PA-ll/LDPE blends and PA/HDPE blends both showed an increase of the melting temperature of the PA-11 matrix due to the addition of the polyolefin. No further attention was paid to this phenomenon. 

More extensive investigations have been performed on HDPE/PP blends by Martuscelli et al. [1980] and Bartczak and Galeski [1986]. From the isothermal crystallization experiments, it was found that the rate of crystallization of the HDPE matrix was markedly reduced upon addition of small amounts of PP (10 wt%). The authors attributed this phenomenon to the increased melt-viscosity of the sample caused by the presence of solidified PP domains. Moreover, Plesek and Malac [1986] have calculated from the surface tensions of the homopolymers at T, that PP crystallization will not cause the nucleation of the HDPE phase, while in the reverse case HDPE crystals will induce the nucleation of PP. 

The main particularity is that in PA6/g-LDPE blends the crystallinity of PA6 phase grows against its calculated (additive) value while that of the g-LDPE phase drops. In PA6/g-HDPE blends, the crystallinity of both components is noticeably lower than the calculated amount (Table 18.5). Probably owing to a lack of compatibility of the components, the effects observed are explained by the specific character of interphase events in the blends, which depend on the molecular mobility as well as on the specific interactions of grafted carboxyl groups in g-PE and functional groups of PA6, and also on the melting and crystallization temperatures of blended components, and on thermal properties of their melts. 

The results presented here illustrate the general feasibility of this technique. They relate primarily to the behavior of thermal and current noise in the glass transition (Tg) or melting (Tm) region of an amorphous (polystyrene) and a crystalline (HD-polyethylene (HDPE)) polymer rendered conductive by the addition of minor amounts of carbon black, and further they relate to the noise of aqueous solutions of certain polymers during Couette flow. Because of experimental diflBculties, noise measurements on solid polymers during deformation and flow have not yet produced useful results. 

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