Synthesis of HDPE

Processes for HDPE with Broad MWD. Synthesis of HDPE with a relatively high molecular weight and a very broad MWD (broader than that of HDPE prepared with chromium oxide catalysts) can be achieved by two separate approaches. The first is to use mixed catalysts containing two types of active centers with widely different properties (50—55) the second is to employ two or more polymerization reactors in a series. In the second approach, polymerization conditions in each reactor are set drastically differendy in order to produce, within each polymer particle, an essential mixture of macromolecules with vasdy different molecular weights. Special plants, both slurry and gas-phase, can produce such resins (74,91—94). 

Scheme 14 Synthesis of HDPE-A/oc/r-VLDPE diblock copolymers in a series of continuous reactors using catalyst system 4 in combination with ZnEt2. Reproduced with permission from Hustad, P. D. Kuhiman, R. L. Arriola, D. J. "m An - na Tnc/i 17 r...

The polymer synthesis was conducted on a mixture of ethylene with about 1.5% hexene comonomer in the reactor, which resulted in about 1% by weight comonomer in the polymer chain. Different catalytic systems were adopted to obtain different molecular mass distributions (Fig.l). The polymer fraction having higher molecular mass in the case of HDPE-2 is expected to produce thicker and/or more regular crystallites as compared to HDPE-1. 

Other kinds of coordination catalytic systems developed few years before the Ziegler-Natta catalysts were based on chromium and molybdenum oxides supported on SiOj AI2O3 and other supports. The catalysts were patented by Phillips Petroleum and Standard Oil companies of Indiana for the synthesis of polyolefins. Although Phillips catalysts were the first to produce a fraction of crystalline polypropylene, these systems were more useful for the production of polyethylene. In fact, the Phillips and the Ziegler-Natta catalysts are currently the two commercial systems that dominate the production of HDPE [2]. 

Undesirable chain transfer can also occur with impurities present in the reactor or from the feed stock. The most important chain transfer reactions which occur during the free radical synthesis of LDPE are shown in Figure 4. These reactions form the ethyl, 2-ethylhexyl, and butyl branches as well as the vinyl termination on the PE chains. These are the eharacteristic structural features which distinguish LDPE from LLDPE, and HDPE. These reactions all begin with the back-biting step, first proposed by Roedel at Du Pont [18]. This step forms a secondary radical which results in formation of a butyl group once the secondary radical reacts with an ethylene molecule. 

Fortunately, for Phillips Petroleum Co., John Paul Hogan and Robert L. Banks followed up the Baily-Reid investigations and applied for patents on the synthesis of linear polyolefins including polyethylene, pol3q)ropylene and polymethylpentene. In 1951, A. Zletz of Standard Oil of Indiana, working with another classmate of mine, Don Carmody, also patented a low pressure process for making HDPE. 

In the 1950s, high density PE (HDPE) was marketed. Shortly afterwards in 1953 Ziegler and Natta independently developed a family of stereospecific transition-metal catalysts that led to the synthesis and commercialisation of HDPE as well as isotactic polypropylene... 

Early Work. The history of HDPE (and polyolefins in general) actually began in the 1890s with the synthesis of polymethylene from the decomposition of diazomethane. Between 1897 and 1938, numerous reports of such polymers appeared in the literature (1-6). Catalysts such as imglazed china, amorphous boron, and boric acid esters were used for the decomposition. The empirical formula of such products was foimd to be CH2. Later reproductions of work (3,6) estabhshed... 

Polymers may be synthesized in which side-branch chains are connected to the main ones, as indicated schematically in Fignre 14.7h these are fittingly called branched polymers. The branches, considered to be part of the main-chain molecule, may result from side reactions that occur during the synthesis of the polymer. The chain packing efficiency is reduced with the formation of side branches, which results in a lowering of the polymer density. Polymers that form linear structures may also be branched. For example, high-density polyethylene (HDPE) is primarily a linear polymer, whereas low-density polyethylene (LDPE) contains short-chain branches. 

Typically a different grade of HDPE is needed to meet the requirements of each of these applications, which means frequent transitions between grades in the commercial plant. Twenty or more transitions are typically made in a month. A typical transition involves changing multiple reactor synthesis conditions, such as hydrogen, ethylene and comonomer concentrations and reactor temperatures to achieve the desired new MW and MWD. Online physical property predictive models are used to help control the MW and MWD, but hourly QC samples are still required to validate the model s output. This is especially true during product transitions where 30-minute samples are required. 

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. 

Mixed C4 olefins (primarily iC4) are isolated from a mixed C olefin and paraffin stream. Two different liquid adsorption high-purity C olefin processes exist the C4 Olex process for producing isobutylene (iCf ) and the Sorbutene process for producing butene-1. Isobutylene has been used in alcohol synthesis and the production of methyl tert-butyl ether (MTBE) and isooctane, both of which improve octane of gasoHne. Commercial 1-butene is used in the manufacture of both hnear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE)., polypropylene, polybutene, butylene oxide and the C4 solvents secondary butyl alcohol (SBA) and methyl ethyl ketone (MEK). While the C4 Olex process has been commercially demonstrated, the Sorbutene process has only been demonstrated on a pilot scale. 

We focus here on the polystyrene/high-density polyethylene (HDPE) system. We have studied this system in greater detail than any other and describe here the phase behavior of this system, the blend synthesis, and some mechanical properties of the composites. 

---