Melting LLDPE

Molecular Weight. The range of molecular weights of commercial LLDPE resias is relatively narrow, usually from 50,000 to 200,000. One accepted parameter that relates to the resin molecular weight is the melt index, a rheological parameter which, broadly defined, is inversely proportional to molecular weight. A typical melt index range for LLDPE resias is from 0.1 to 5.0, but can reach over 30 for some appHcations. 

The width of molecular weight distribution (MWD) is usually represented by the ratio of the weight—average and the number—average molecular weights, MJM. In iadustry, MWD is often represented by the value of the melt flow ratio (MER), which is calculated as a ratio of two melt indexes measured at two melt pressures that differ by a factor of 10. Most commodity-grade LLDPE resias have a narrow MWD, with the MJM ratios of 2.5—4.5 and MER values in the 20—35 range. However, LLDPE resias produced with chromium oxide-based catalysts have a broad MWD, with M.Jof 10—35 and MER of 80-200. 

The technologies suitable for LLDPE manufacture include gas-phase fluidised-bed polymerisation, polymerisation in solution, polymerisation in a polymer melt under high ethylene pressure, and slurry polymerisation. Most catalysts are fine-tuned for each particular process. 

In the early 1990s, solution processes acquired new importance because of their shorter residence times and abiUty to accommodate metallocene catalysts. Many heterogeneous multicenter Ziegler catalysts produce superior LLDPE resins with a better branching uniformity if the catalyst residence time in a reactor is short. Solution processes usually operate at residence times of around 5—10 min or less and are ideal for this catalyst behavior. Solution processes, both in heavy solvents and in the polymer melt, are inherently suitable to accommodate soluble metallocene catalysts (52). For this reason, these processes were the first to employ metallocene catalysts for LLDPE and VLDPE manufacture. 

Rheology of LLDPE. AH LLDPE processiag technologies iavolve resia melting viscosities of typical LLDPE melts are between 5000 and 70, 000 Pa-s (50,000—700,000 P). The main factor that affects melt viscosity is the resia molecular weight the other factor is temperature. Its effect is described by the Arrhenius equation with an activation energy of 29—32 kj/mol (7—7.5 kcal/mol) (58). 

Processing Methods. LLDPE is easHy processed by most conventional techniques due to its low melting point and high chemical stabHity the principal techniques have been described (60—65). 

Rotational Molding. Large containers and some toys are manufactured with a specialized technique caHed rotational mol ding. A rotational mol ding machine contains several large metal molds that can be rotated in two perpendicular planes. A load of fine LLDPE particles is introduced into each mold, and the mold assembly is transferred into an oven. Inside the oven, heated molds rotate at speeds ranging from 10 to 40 rpm. The polymer powder melts and is uniformly distributed on the internal surface of each mold. After the container is formed, the mold assembly is removed from the oven and cooled at which point the plastic containers are removed. 

The molecular weight distribution of LLDPE resins is usually characterized in industry by the ratios of melt indexes measured in the same apparatus using different loads (2.16, 10.16, and 21.6 kg). The commonly used ratios are melt flow ratio, MFR) and I q/I2. Both of these ratios... 

An important property recommending the use of LLDPE in many packaging appHcations is thek sealabHity. CompositionaHy uniform resins are especiaHy attractive for such use because thek melting and softening points are 15—20°C lower than those of commodity LLDPE resins (Fig. 1). 

LLDPE with narrow molecular weight distribution exhibits a lower, a sharper melting point [3], better hot tack and heat seal properties as well as higher clarity and better impact resistance (Fig. 3), tensile strength (Fig. 4) [11], and lower levels of alkane-soluble components. The most distinguishing characteristic of metallocene-based LLDPEs is that they are not restricted by the current immutable property relationships that are... 

Figure 3 Impact strength of metallocene LLDPE (Ethylene-hexene resin, 1 melt index). (From Ref. 11.)...

Figure 5 High melt tension LLDPEs from supported metallocenes. (From Ref. 2.)...

Figure 10 shows that upon cessation of shear flow of the melt, shear stress relaxation of LLDPE is much faster than HP LDPE because of the faster reentangle-... 

When water (a Newtonian liquid) is in an open-ended pipe, pressure can be applied to move it. Doubling the water pressure doubles the flow rate of the water. Water does not have a shear-thinning action. However, in a similar situation but using a plastic melt (a non-Newtonian liquid), if the pressure is doubled the melt flow may increase from 2 to 15 times, depending on the plastic used. As an example, linear low-density polyethylene (LLDPE), with a low shear-thinning action, experiences a low rate increase, which explains why it can cause more processing problems than other PEs. The higher-flow melts include polyvinyl chloride (PVC) and polystyrene (PS). 

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