Polyethylene improving LLDPE properties

Linear low-density polyethylene (LLDPE)440-442 is a copolymer of ethylene and a terminal alkene with improved physical properties as compared to LDPE. The practically most important copolymer is made with propylene, but 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene are also employed.440 LLDPE is characterized by linear chains without long-chain branches. Short-chain branches result from the terminal alkene comonomer. Copolymer content and distribution as well as branch length introduced permit to control the properties of the copolymer formed. Improvement of certain physical properties (toughness, tensile strength, melt index, elongation characteristics) directly connected to the type of terminal alkene used can be achieved with copolymerization.442... 

Thermoplastics used to blend with NR include PS, " polyamide 6, ethylene-vinyl acetate (EVA) copolymer, poly(methyl methacrylate) (PMMA), polypropylene (PP), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) " and high-density polyethylene (HDPE). To improve the properties of TPNR, modified NR is also used. ENR is the most frequently used modified NR. TPNR blends are prepared by blending NR and thermoplastics in various proportions. The role of rubber is to improve the impact strength and ductility of the plastic. Depending on the ratio, materials with a wide range of properties are obtained. The stiffness of the rubber is increased with the incorporation of plastic into the rubber matrix. The mechanical properties of TPNR again depend on the proportions of the rubber and thermoplastic components. The elastic properties of TPNR are considerably... 

Linear Low Density Polyethylene. Films from linear low density polyethylene (LLDPE) resias have 75% higher tensile strength, 50% higher elongation-to-break strength, and a slightly higher but broader heat-seal initiation temperature than do films from LDPE. Impact and puncture resistance are also improved over LDPE. Water-vapor and gas-permeation properties are similar to those of LDPE films. 

In order to improve the physical properties of HDPE and LDPE, copolymers of ethylene and small amounts of other monomers such as higher olefins, ethyl acrylate, maleic anhydride, vinyl acetate, or acryUc acid are added to the polyethylene. Eor example, linear low density polyethylene (LLDPE), although linear, has a significant number of branches introduced by using comonomers such as 1-butene or 1-octene. The linearity provides strength, whereas branching provides toughness. 

Metallocene-based polyethylene does not offer the lower production costs associated with LLDPE. Hence there will be a price premium for the new materials but this is felt to be justified in view of their improved property profile. 

Specifically, PVC blends with polyethylene, polypropylene, or polystyrene could offer significant potential. PVC offers rigidity combined with flammability resistance. In essence, PVC offers the promise to be the lowest cost method to flame retard these polymers. The processing temperatures for the polyolefins and polystyrene are within the critical range for PVC. In fact, addition of the polyolefins to PVC should enhance its ability to be extruded and injected molded. PVC has been utilized in blends with functional styrenics (ABS and styrene-maleic anhydride co-and terpolymers) as well as PMMA offering the key advantage of improved flame resistance. Reactive extrusion concepts applied to PVC blends with polyolefins and polystyrene appear to be a facile method for compatibilization should the proper chemical modifications be found. He et al. [1997] noted the use of solid-state chlorinated polyethylene as a compatibilizer for PVC/LLDPE blends with a significant improvement in mechanical properties. A recent treatise [Datta and Lohse,... 

Reaction with vinyl acetate [81], hydroxypropylation [82], reactions with styrene [83], with ethylene glycol and other glycols giving rise to glucosides [63,84] or with acrylamide monomer [85,86] have also been described. Reactive extrusion is also used to decrease the melt viscosity and decrease the interfacial tension of TPS-based blends [76]. Ning et al. [74] studied the effect of adding citric acid on TPS and LLDPE, via a single-step reactive extrusion. The authors showed improvements in the compatibilization and the mechanical properties and shifts of polyethylene peaks observed by FTIR. 

PDI affects application properties. Thus, stiffness of polypropylene increases as PDI increases [1]. On the other hand, the mechanical properties of the linear low-density polyethylene (LLDPE) improve when PDI decreases, but it presents a higher melt viscosity and hence a poorer processability [2]. 

The combination of HALS with a UV absorber is used in fihns of polypropylene and polyethylene as well as in thick sections. In films of LDPE, nickel quenchers were commonly used with a UV absorber, except in a very thin film, in which a higher concentration of nickel stabilizer is superior to the combination. The low-molecular-weight HALS are not sufficiently compatible with LDPE at the concentrations necessary, possibly as high as 2%, for the required protection. Incompatibility of HALS with LDPE has been overcome with the development of polymeric HALS. It is considerably better than either the UV absorber or nickel quencher or combinations of the two. For thicker films (100-200 ftm), the combination of a benzophenone-type UV absorber with polymeric HALS is significantly superior to an equivalent amount of polymeric HALS. The type of stabilizers used for linear low-density polyethylene (LLDPE) and ethyl vinyl acetate (EVA) copolymer are similar to those for LDPE. Since LLDPE has superior mechanical properties (elongation at break and tensile strength), thinner films can be used for most applications, and the loss of UV stabihty with reduction in thickness has to be compensated for by improving the stabilization system. 

For improved processability and mechanical properties of polyethylene, HOPE or LLDPE, it was blended with elastomeric polyethylene-polypropylene copolymer, EPR. For further enhancement of properties, either polypropylene, PP, ot polybutene, PB, could also be added. In ICI patent, HDPE was blended with 30-60 wt% EPR for improved processability and impact strength... 

In Esso patent, PP was blcaided with varying amount of polyisobutyltme, PIB, and polyethylene, LDPE, to give marked improvement in tear stiraigth and impact. In the Shell patent, reactor powder blending of PP with 0.1-10 wt% EPR (2—25 wt% ethyltme) was disclosed. In Farbwerke Hoechst patent, PP was blended with 5-70 wt% LLDPE, for good mechanical properties at low temperature... 

---