Low Density Polyethylenes

also known as high pressure polyethylene, is produced at pressures ranging from 82—276 MPa (800—2725 atm). Operating at 132—332°C, it may be produced by either a tubular or a stirred autoclave reactor. Reaction is sustained by continuously injecting free-radical initiators, such as peroxides, oxygen, or a combination of both, to the reactor feed. 

The mechanical properties of LDPE fall somewhere between rigid polymers such as polystyrene and limp or soft polymers such as polyvinyls. LDPE exhibits good toughness and pHabiUty over a moderately wide temperature range. It is a viscoelastic material that displays non-Newtonian flow behavior, and the polymer is ductile at temperatures well below 0°C. Table 1 fists typical properties. 

LDPE homopolymers in the 0.2—150 melt index or 100, 000 20,000 mPa s(= cP) viscosity range. Specialty polymers such as greases and waxes or highly cross-linked polymers are not included. 

Structure. The physical properties of LDPE depend on the molecular weight, the molecular weight distribution, as well as the frequency and distribution of long- and short-chain branching (2). 

The short side chain branching frequency is inversely proportional to polymer crystallinity. Short branches occur at frequencies of 2—50 per 1000 carbons in chain length their corresponding crystallinity varies from 35 to 75%. Directiy proportional to the polymer density, crystallinity can be calculated by the following formula, 

Applications include film for packaging manufactured by the tube-blowing process, which consumes some 70% of LDPE production a miscellany of uses takes the remaining 30%, including  

Bottles Mustard jars Bowls, buckets Cold water tanks Paper coating 

Extrusion blow moulding Injection blow moulding Injection moulding Rotational moulding Extrusion. 

Low-density polyethylene is still one of the cheapest thermoplastics, possesses excellent impact performance, but is very flexible. A grade of LDPE was selected to give the best overall properties, viz. a density of 0.92 Mgm and MFI of 12-14g per lOmin (2.16kg at 190°C). 

The archetypical example of a branched polymer is low density polyethylene (LDPE), the product of radical polymerization at high temperature and pressure. That LDPE is significantly branched was first suspected because of the influences of polymerization conditions upon the crystallinity of the polymer and upon its rheological behaviour in the melt and in solution. Confirmation was provided by IR analysis which indicated a considerable excess of methyl groups ideally, the maximum number of such groups would be two per molecule, corresponding to the end-groups of a linear alkane. 

A typical LDPE contains 18.9 branches for 1000 backbone carbons [74], made up thus methyl, 0 ethyl, 0.7 n-propyl, 0 n-butyl, 13.1 n-amyl, 2.9 n-hexyl and longer, 2.2. The preponderance of C4 branches is consistent with the view that back-biting occurs mainly through the thermodynamically favoured six-membered cyclic transition state as indicated in equation (3.18). 

There is evidence that some of the ethyl branches occur in 1,3-pairs and that 

Xiao et al. [59] carried out a detailed study of the mechanical and rheological properties of low-density polyethylene reinforced by the incorporation of multiwalled carbon nanotubes. It was found that the Young s modulus and tensile strength of the composites can increase by 89% and 56%, respectively, when the nanotube loading reaches 10 wt%. The curving and coiling of multiwalled carbon nanotubes play an important role in the enhancement of the composite modulus. It was also found that the materials experience a fluid-solid transition at the composition of 4.8 wt%, beyond which a continuous multiwalled carbon nanotube network forms throughout the matrix and in turn promotes the reinforconent of the multiwalled carbon nanotubes. 

Apart from the different t) es of reactors used, the autoclave and tubular reactor processes are very similar. The two types of reactors produce, however, products which have a different molecular structure and are, therefore, used in different product applications. 

Modem crackers produce ethylene of sufficient purity to be used in the high pressure process without the need for additional purification. The fresh ethylene is normally delivered to the high pressure plant by a pipeline grid. If the high pressure plant is located on the same site as the cracker, the ethylene can be delivered directly from the cracker. 

The supply pressure can range between 1 and 10 MPa. A first compressor (primary or medium pressure compressor) increases the ethylene pressure to 20-30 MPa. The number of compression steps depends on the pressure of the ethylene which is supplied to the plant. If this pressure is above 3 MPa, the primary compressor typically has two compression stages. Because the ethylene gas is used as a heat sink for the heat generated by the exothermic reaction, the ethylene gas is not totally converted to a polymer in the reactor. The unreacted gas is recycled back into the process. This recycled ethylene is combined with the fresh ethylene at the outlet discharge of the primary compressor. The combined gas streams are fed to the suction of the high pressure compressor. This compressor increases the pressure of the reactor up to 150 - 350 MPa in two steps. The process gas is cooled with cooling water and/or chilled water between the two compression steps. 

To tailor the application properties of the polymer, different initiation systems and chain transfer agents (modifier) are used. Typical initiators are oxygen or organic peroxides. To control the molecular weight distribution of the polymer produced, polar modifiers (aldehydes, ketones or alcohols) or aliphatic hydrocarbons are fed into the monomer stream. 

The reactor is protected by pressure relief devices which guarantee an immediate release of the reactor content in case a runaway reaction occurs. The runaway reaction of ethylene causes a sharp increase in pressure and temperature. These sharp increases cause the activation of the emergency relief system. Because of the last response required, the emergency relief systems of the reactor vent the content of the reactor to the air. 

The chain branching in homopolymer LDPE gives this polymer a number of desirable characteristics such as clarity, flexibility, heat sealability, and ease of processing. The actual values of these properties depend on the balance between the molecular weight, molecular weight distribution, and branching. 

LDPE is also versatile with respect to processing mode, and is adaptable to blown film, cast film, extrusion coating, injection molding, and blow molding. Film is the 

LDPE is characterized by its excellent flexibility, good impact strength, fair machi-nability, good oil resistance, fair chemical resistance, good heat sealing characteristics, and low cost (about 1.60/kg). Its transparency is better than HDPE because of its lower percent crystallinity. For the same reason, while it is a good water vapor barrier, it is inferior to HDPE. Similarly, it is an even poorer gas barrier than HDPE. A summary of the properties of LDPE is presented in Table 4.1. 

Medium density polyethylene (MDPE), 0.925-0.940 g/cm, is sometimes listed as a separate category, but usually is regarded as the high density end of LDPE. It is somewhat stronger, stiffer, and less permeable than lower density LDPE. MDPE processes similarly to LDPE, though usually at slightly higher temperatures. 

The major competitor to LDPE is LLDPE (discussed in Section 4.2.1), which provides superior strength at equivalent densities. However, LDPE is still preferred in applications demanding high clarity or for extrusion coating a substrate. 

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Circulatory flow o low-density polyethylene melt in a rectangular domain 932 1,000 0.000... 

One of the mam uses of the linear a olefins prepared by oligomerization of ethylene is in the preparation of linear low density polyethylene Linear low density polyethylene is a copoly mer produced when ethylene is polymerized in the presence of a linear a olefin such as 1 decene [H2C=CH(CH2)7CH3] 1 Decene replaces ethylene at random points in the growing polymer chain Can you deduce how the structure of linear low density polyethylene differs from a linear chain of CH2 units ... 

Low-density polyethylene (LDPE) Poly(phenyl sulfone)... 

Branching occurs to some extent and can be controlled. Minimum branching results in a high-density polyethylene because of its closely packed molecular chains. More branching gives a less compact solid known as low-density polyethylene. 

OLEFIN POLYTBRS - POLYETHYLENE - LINEAR LOW DENSITY POLYETHYLENE] (Vol 17)... 

Raman spectrum [INFRARED TECHNOLOGY AND RAMAN SPECTROSCOPY - RAMAN SPECTROSCOPY] (Voll4) Low density polyethylene pDPE)... 

Low density polyethylene has been pyrolyzed at 800°C to produce ethylene, propylene, and other light olefins ia 75% yield (43). 

HDPE, high density polyethylene PP, polypropylene EVA, ethylene—vinyl alcohol SMC, sheet-molding compound ERP, fiber-reinforced plastic LDPE, low density polyethylene PE, polyethylene BMC, bulk mol ding compound TPE, thermoplastic elastomer. 

In the sheeting market, the low density polyethylenes are less important than the high density resins. The high density resins have excellent chemical resistance, stress-crack resistance, durabiUty, and low temperature properties which make them ideal for pond liners, waste treatment faciUties, and landfills. In thicker section, HMW-HDPE sheet makes good containers, trays, tmck-bed liners, disposable items, and concrete molds. The good durabiUty, abrasion resistance, and light weight are critical elements for its selection. 

Table 6 shows the sales estimates for principal film and sheet products for the year 1990 (14). Low density polyethylene films dominate the market in volume, followed by polystyrene and the vinyls. High density polyethylene, poly(ethylene terephthalate), and polypropylene are close in market share and complete the primary products. A number of specialty resins are used to produce 25,000—100,000 t of film or sheet, and then there are a large number of high priced, high performance materials that serve niche markets. The original clear film product, ceUophane, has faUen to about 25,000 t in the United States, with only one domestic producer. Table 7 Hsts some of the principal film and sheet material manufacturers in the United States.

Economics. Rigid foam systems are typically in the range of 32 kg/m (2 Ibs/fT) and, in 1992, had a foam price of about 3.63/kg ( 1.65 per lb) with hquid foam systems at about 2.75/kg. Unit prices for pour-ia-place polyurethane packaging systems fall between the competitive expandable polystyrene bead foam at 3.30/kg and low density polyethylene foams at 5.80/kg. 

There are three basic types of polyethylene foams of importance (/) extmded foams from low density polyethylene (LPDE) (2) foam products from high density polyethylene (HDPE) and (J) cross-linked polyethylene foams. Other polyolefin foams have an insignificant volume as compared to polyethylene foams and most of their uses are as resia extenders. 

Poultry, susceptible to microbiological deterioration, is an excellent substrate for Salmonella. Therefore, the temperature is reduced as rapidly as possible after slaughter. Packagiag at factory level is in soft film, ie, low density polyethylene or plastici2ed PVC, which retards water-vapor loss and permits oxygen entry. 

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