Polyolefin blends

Polyolefins constitute the largest volume class of polymeric materials. Polyethylene, polypropylene, and ethylene-propylene rubber are major products in this family, with many subset variations with each material. Polyethylene variants include high density polyethylene (HDPE), low density polyethylene (LDPE), ultrahigh molecular weight polyethylene (UHMWPE), linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE) and various ethylene copolymers (including comonomers of vinyl acetate, ethyl and methyl acrylate, acryhc acid and methacryhc acid and their metal salts (ionomers)). Polypropylene has fewer variations, of which low amounts of ethylene are included while maintaining crystaUinity. More recently, ethylene-styrene copolymers have been introduced. 

Generally fair to good mechanical compatibiUty exists with various combinations of the noted polyolefins, as may be expected due to the similarity of structure. This has resulted in a large number of combinations achieving commercial status (as wih be discussed in Chapter 7). The blends are often prepared via extrusion melt mixing however, the utiUzation of multiple catalysts or variations in monomer feed compositions can produce polyolefin blends during 

IPP/EPR blends have been the subject of coimtless papers and phase separation in the melt with as low as 8% ethylene observed [350], based on SANS results. Seki et al. [351] noted that earlier studies were based on Ziegler-Natta catalyst prepared EPR and metallocene catalyst yield more random structures. Their results on iPP blends with metallocene prepared EPRs with perdeuterated ethylene contents of 19 and 47 mol% showed single phase behavior, as determined by SANS. 

Elends of the polyethylene variants (HDPE, LLDPE, LDPE, VLDPE) and ethylene copolymers with polar monomers (EAA, EMAc, EEA, EVA)) are employed in many variations in commercial apphcations, particularly involving film apphcations. LLDPE/LDPE blends were commercially introduced in film apphcations shortly after the introduction of LLDPE in the late 1970s. The earher version of LLDPE required LDPE addition to achieve good processabihty and reduced haze to take advantage of the mechanical properties offered by LLDPE. Several more recent papers wih be briefly discussed, showing the continuing interest in this 

PP EVA Phase separated, mechanical properties enhanced with PP containing low ethylene levels 366 

Polyolefin blends are of critical importance to the success of the material. Ethylene propylene diene rubber (EPDM) immiscibly blends with PP as an impact modifier. It is the most common and most commercially utilised blend of polyolefins. High-density polyethylene (HOPE) can be added to this blend to achieve maximum toughness [11-13]. Applications include wire and cable insulation, automotive 

HDPE immiscibly blends with hutyl ruhher [14] to provide improved chemical resistance, compression set and high-temperatnre mechanical properties versus nnvnlcanised hlends. LDPE and HDPE blend immiscibly with ethylene copolymers to improve environmental stress crack resistance, tonghness, filler acceptance, film tear resistance, improved flexibility and so on. In polyolefin, polybntene-1 forms miscible blends with PP [15,16]. The addition of PP to polybutene-1 increases the crystallisation rate of polyhntene-1 and would have utility as a nucleation additive. 

Name (other name) Chemical structure (repeat unit) 

Polyolefin terpolymers contain three olefinic monomers. A well-known example is ethylene propylene diene monomer (EPDM). The diene (double bond) monomer is 

Commercial polyolefins often contain additives such as colorants, flame retardants, antioxidants, light stabilizers, nucleating agents, antistatic agents, lubricants (microcrystalline waxes, hydrocarbon waxes, stearic acid, and metal stearates), and so on. These additives aid the processing and fabrication of products from polyolefins. Detailed treatments about specific polyolefins, polymerization systems/ mechanism/processes, structures, properties, processing, and applications may be found in References 2-9. 

Polymer blends (mixtures of structurally different polymers (10-19)) are of interest because synthesized polymers have not satisfied increasing application demands. Polyolefin blends are a subset of polymer blends and may be classified into two groups. The first group contains polyolefins only, which are formulated to broaden the range of structures, properties, and applications offered by polyolefins. The second group contains polyolefins and nonpolyolefins, which are formulated to mitigate some of the property drawbacks of the polyolefin or the nonpolyolefin. For a blend to be classified as a polyolefin blend, it is presumed that the polyolefin component is of significant composition in the blend. 

In terms of miscibility, polyolefin blends may also be classified as miscible and immiscible blends (10, 11). Polyolefin blending requires knowledge of the miscibility and crystallinity of the blend, in addition to the contributions of the components of the blend. Miscibility depends on molecular structure, blend composition, and mixing temperature. To characterize miscibility, a phase diagram is needed. 

Blending within the family of PO has, however, been more common [Plochocki, 1978]. Although they are usually immiscible with each other, there exists some degree of mutual compatibihty between them. The similarity of their hydrocarbon backbones and the closeness of their solubility parameters, although not adequate for miscibility, accounts for a relatively low degree of interfacial tension. Eor example, the solubility parameters of polyethylene, polyisobutylene, ethylene-propylene rubber and polypropylene are estimated to be 16.0, 15.4, 

and 17.0 J cm respectively, all very close to each other [Van Krevelen 1990]. Similarly, the inter- 

1 Polyethylene Blends (LLDPE/LDPE or HOPE Blends) 

Over 60% of the LDPE and LLDPE produced are used in film applications. Because of the continuous nature of the film extrusion, traditionally this process has been more amenable for blending different types of polyolefins to achieve specific improvements in film properties. For example, by blending 30% LLDPE with LDPE, thinner gage films with improved tensile and tear strengths could be produced [Forger, 1982]. Similarly, by blending about 30% LLDPE with 

the toughness, puncture resistance and heat sealability of the HDPE films could be improved. It is estimated that in the USA over 30% of PE consumed in films, is usually blended with LLDPE [Kosoff, 1987]. 

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A route to compatibility involving ionomers has been described recently by Eisenberg and coworkers [250-252]. The use of ionic interactions between different polymer chains to produce new materials has gained tremendous importance. Choudhury et al. [60] reported compatibilization of NR-polyolefin blends with the use of ionomers (S-EPDM). Blending with thermoplastics and elastomers could enhance the properties of MPR. The compatibility of copolyester TPE, TPU, flexible PVC, with MPR in aU proportions, enables one to blend any combination of these plastics with MPR to cost performance balance. Myrick has reported on the effect of blending MPR with various combinations and proportions of these plastics and provided a general guideline for property enhancement [253]. 

Roy Choudhury N., De P.P., and Bhowmick A.K., Thermoplastic elastomeric natural rubber-polyolefin blends. Thermoplastic Elastomers from Rubber Plastic Blend (De S.K. and Bhowmick A.K., eds.), Ellis Horwood, London, 1990, 11. 

Roy Choudhury N. and Bhowmick A.K., Strength of thermoplastic elastomers from rubber-polyolefin blend, J. Mat. Sci., 25, 161, 1990. 

It appears that the main determinant in polyolefin miscibility is the way the chains pack together, a feature that controls the intermolecular interaction between molecules. The solubihty parameter approach is predictive in most cases and can be useful in designing polyolefin blends. 

Examples of Irregular Contribution to Interaction Strength for Polyolefin Blends... 

Incorporation of a perfume into PCL or a PCL-polyolefin blend is reported to be a useful method of retaining the fragrance in soaps and detergent bars (81). 

The increase in scientific production on polymer deformulation is considerably higher than the inflation in scientific papers (3.4% as from 1989). Bourgeois [111] has recently described reverse engineering of polyolefins and polyolefin blends. 

ARA Palmans, M Eglin, A Montali, C Weder, and P Smith, Tensile orientation behavior of alkoxy-substituted BA(phenylethynyl)benzene derivatives in polyolefin blend films, Chem. Mater., 12 472-480, 2000. 

Other uses of blends include controlled rate of fertilizer release(77) based on ethylene/vinyl acetate/carbon monoxide polymers which is U.V. sensitive, polyolefin blends with any biodegradable polymers,(78) and polyolefins blended with metals and autoxidizable substrates. (79) Doane and co-workers(80) at the U.S.D.A. have used grafted starches in many applications, including soil stabilization. 

Patent Number US 6096793 A1 20000801 FOAM COMPRISING POLYOLEFIN BLEND AND METHOD FOR PRODUCING SAME Lee S-T Baker J J Sealed Air Corp. 

Fig. 11.4 Comparison of (a) the POX decomposition rate in dodecane at 180°C with (b) the rate of reduction of the batch mixer torque, that is, reduction of the viscosity of the reacting PP, indicates that the two are identical the POX decomposition is rate controlling and the CR-PP reactions are practically complete after 6-7 POX half-lives. [Reprinted by permission from D. W. Yu, Polyolefin Blends Modified through Peroxide Initiated Reactions, Ph.D Dissertation, Department of Chemical Engineering, Stevens Institute of Technology, Hoboken, NJ, 1991.]...

The use of borane-containing monomers clearly presents an effective and general approach in the functionalisation of polyolefins, which has the following advantages stability of the borane moiety to coordination catalysts, solubility of borane compounds in hydrocarbon solvents (such as hexane and toluene) used as the polymerisation medium, and versatility of borane groups, which can be transformed to a remarkable variety of functionalities as well as to free radicals for graft-form polymerisations. The functionalised polymers are very effective interfacial modifiers in improving the adhesion between polyolefin and substrates and the compatibility in polyolefin blends and composites [518],... 

Xie, R.C. and Qu, BJ. 2001. Synergistic effects of expandable graphite with some halogen-free flame retardants in polyolefin blends. Polymer Degradation and Stability 71(3) 375-380. 

PVC can be blended with numerous other polymers to give it better processability and impact resistance. For the manufacture of food contact materials the following polymerizates and/or polymer mixtures from polymers manufactured from the above mentioned starting materials can be used Chlorinated polyolefins blends of styrene and graft copolymers and mixtures of polystyrene with polymerisate blends butadiene-acrylonitrile-copolymer blends (hard rubber) blends of ethylene and propylene, butylene, vinyl ester, and unsaturated aliphatic acids as well as salts and esters plasticizerfrec blends of methacrylic acid esters and acrylic acid esters with monofunctional saturated alcohols (Ci-C18) as well as blends of the esters of methacrylic acid butadiene and styrene as well as polymer blends of acrylic acid butyl ester and vinylpyrrolidone polyurethane manufactured from 1,6-hexamethylene diisocyanate, 1.4-butandiol and aliphatic polyesters from adipic acid and glycols. 

Silicone Polydimethylsiloxane Works by producing a char surface Polyolefins, polyolefin blends, EVA, polycarbonate, polyurethane Dow Corning, GE Silicones... 

In addition to the polyolefin blends designed for thermoplastic elastomer applications, a great deal of interest also has centered on other kinds of blends of polyolefins as has been reviewed recently (see chapter 21 of Ref. 10 by Plochocki). In a recent paper (84), we showed that blends involving polypropylene-high density polyethylene-low density polyethylene in various proportions and combinations exhibit additivity of tensile strength however, there are serious losses in ductility in some cases such that the blends are less ductile than either pure component. It is interesting to note, however, that these losses in ductility can largely be restored by addition of rather small amounts of an amorphous ethylene-propylene rubber (84). 

This is often conducted to make impact-resistant polyolefin blends. For this purpose, isotactic polypropylene, which is tough but somewhat brittle, is produced on the catalyst pellet in a first reaction step, using highly active stereospecific... 

Growing interest in polymer blends is due to the feasibility of varying properties of the materials within wide limits which may modify tensile strength, impact strength, etc. Numerous papers have appeared in connection with the preparation and mechanical properties of polyolefin blends. However, there appears to be a lack of information regarding oxidative stability of this type of blend. 

There have been recent efforts to predict, or at least rationalize, the x parameters of these and other polyolefin-polyolefin blends. Bates et al. (1992) and Fredrickson et al. (1994) suggest that the x parameter is correlated to a difference in statistical segment length of the polymer molecules, on a volume-normalized basis. The volume normalization is required because the definition of the statistical segment length depends on how the monomer unit... 

Graessley et al, (1994,1995) have proposed an alternative scheme that correlates the x values for polyolefin blends with differences in inferred values of the solubility parameters of the pure components [see Eq. (2-31)]. For the series of polyolefins they studied, values of for the pure components can be assigned in such a way that the x parameters of the mixtures computed from Eq. (2-31) are in reasonable qualitative agreement with measured values. Direct estimates of the (5 s can be obtained from the following formula derived from the van der Waals cohesive energy density (Hildebrand and Scott 1950 Krishnamoorti et al. 1996) ... 

K. W. Foreman and K. F. Freed (1998) Lattice cluster theory of multicomponent polymer systems Chain semiflexibility and speciflc interactions. Advances in Chemical Physics 103, pp. 335-390 K. F. Freed and J. Dudowicz (1998) Lattice cluster theory for pedestrians The incompressible limit and the miscibility of polyolefin blends. Macromolecules 31, pp. 6681-6690 E. Helfand and Y. Tagami (1972) Theory of interface between immiscible polymers. 2. J. Chem. Phys. 56, p. 3592 E. Helfand (1975) Theory of inhomogeneous polymers - fundamentals of Gaussian random-walk model. J. Chem. Phys. 62, pp. 999-1005... 

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