HDPE, additives

In pure HDPE, addition of both thermoplastic elastomers results in little increase in the impact energy (6.4% and 3.2% for the SEES and MA-modified SEES, respectively), although ductility is enhanced. In the 40 wt% GRT composites, however, the increases in impact energy are 58% and 62%, for the SEES and MA-modified SEES, respectively. These larger increases compared to those... 

Results obtained for two mixed plastics are summarized in Table 4. A balance exists between process temperature, plastics feed rate, and product yields (67). For example, lower temperatures increase wax formation due to incomplete depolymerization. Slower feed rates and increased residence times reduce wax formation and increase the yield of Hquids. The data summarized in Table 4 illustrate that the addition of PET to a HDPE PP PS mixture changes the performance of the Conrad process. Compared to the reference HDPE PP PS mixture, increased amounts of soHds ate formed. These are 95% terephthahc acid and 5% mono- and bis-hydroxyethyl esters. At higher temperatures, apparentiy enough water remains to promote decarboxylation. 

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

SASOL. SASOL, South Africa, has constmcted a plant to recover 50,000 tons each of 1-pentene and 1-hexene by extractive distillation from Fischer-Tropsch hydrocarbons produced from coal-based synthesis gas. The company is marketing both products primarily as comonomers for LLDPE and HDPE (see Olefin polymers). Although there is still no developed market for 1-pentene in the mid-1990s, the 1-hexene market is well estabhshed. The Fischer-Tropsch technology produces a geometric carbon-number distribution of various odd and even, linear, branched, and alpha and internal olefins however, with additional investment, other odd and even carbon numbers can also be recovered. The Fischer-Tropsch plants were originally constmcted to produce gasoline and other hydrocarbon fuels to fill the lack of petroleum resources in South Africa. 

In broad tonnage terms the injection moulding markets for high-density polyethylene and polypropylene are very similar. The main reasons for selecting polypropylene have been given above. In favour of HDPE is the inherently better oxidation and ultraviolet resistance. Whilst these properties may be greatly improved in polypropylene by the use of additives these may increase the cost of polypropylene compounds to beyond that which is considered economically attractive. It is for this reason that HDPE has retained a substantial part of the crate market. 

Because of the excellent gas barrier properties, EVOH is of interest as a packaging material. However, because of its high water absorption it is usually used as an internal layer in a co-extruded film, sheet, bottle or tube. For example, the system HDPE-EVOH-EVA may be used as a barrier film for packaging cereals, and the system polystyrene-EVOH-polystyrene for packaging coffee and cream, whilst the system polystyrene-EVOH-polyethylene has the additional advantage of heat scalability. 

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... 

Elliot [38] has reported that interfacial adhesion in the NR-PP blend can be enhanced by the addition of small amounts of HOPE. Addition of HDPE does give some improvement in the notched Izod impact strength of NR-PP blend (Fig. 7). The effect of HDPE on the impact modification of NR-PP is associated with the improved crystallinity of PP, enhanced by HDPE. During the mill mixing of NR and PP, chain scission may occur to give polymeric radicals that, on reaction with... 

Figure 7 Effect of addition of HOPE on Izod impact strength of NR-PP blend. (O) 20 80 NR-PP homopolymer, (V) 20 67 13 NR-PP-homopolymer-HDPE, ( ) 15 85 NR-PP copolymer grade, and (x) 15 75 10 NR-PP-copo-lymer-HDPE.

Low-density polyethylene (LDPE) is produced under high pressure in the presence of a free radical initiator. As with many free radical chain addition polymerizations, the polymer is highly branched. It has a lower crystallinity compared to HDPE due to its lower capability of packing. 

Reaction conditions are generally mild, but they differ from one process to another. In the newer Unipol process (Eigure 12-1) used to produce both HDPE and LLDPE, the reaction occurs in the gas phase. Ethylene and the comonomers (propene, 1-butene, etc.) are fed to the reactor containing a fluidized bed of growing polymer particles. Operation temperature and pressure are approximately 100°C and 20 atmospheres. A single-stage centrifugal compressor circulates unreacted ethylene. The circulated gas fluidizes the bed and removes some of the exothermic reaction heat. The product from the reactor is mixed with additives and then pelletized. New modifications for gas-phase processes have been reviewed by Sinclair. ... 

Applications Caceres et al. [114] compared various methods for extraction of Tinuvin 770 and Chimas-sorb 944 from HDPE pellets, namely room temperature diffusion in CHC13 (20 % extraction), ultrasonica-tion (20% extraction), Soxtec extraction with DCM (nonsolvent) (50 % extraction), dissolution (dichloroben-zene)/precipitation (2-propanol) (65-70% recovery) and boiling under reflux with toluene (solvent) at 160 °C (95 % extraction). By changing conditions (nature of solvent, T, t) similar comparisons do not have much added value. Table 3.6 compares the results of reflux extraction and MAE for additives in LDPE [115]. 

Figure 3.16 Influence of time on microwave-extraction efficiencies for additives in HDPE. After Jassie et al. [454]. Reprinted with permission from L. Jassie et al., in Microwave-Enhanced Chemistry (H.M. Kingston and S.J. Haswefl, eds), American Chemical Society, Washington, DC (1997), pp. 569-609. Copyright (1997) American Chemical Society...

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