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Additives: elastomeric modifiers toughen polyolefin films.

Polyolefin and olefin copolymer films, which were first introduced in the late 1940s, are used in larger volume than any other class of films in packaging. Some of the features of polyolefins that have led to their strong growth are low odor and good chemical and environmental resistance, resulting from their saturated molecular backbones, and high yield and moderate cost, resulting from their low densities. Limitations in impact strength and tear properties may restrict their broader application, especially in the case of polypropylene films. However, synthetic elastomeric modifiers can improve the toughness of films and, in many cases, impart significant special properties.

Olefinic elastomers, such as polyisobutylene and ethylene-propylene rubber, have high molecular weights-in the 300,000 to 900,000 range-and are important components of the modifiers. These elastomers are normally available in the form of bales or crumb-but not pellets, because they agglomerate during long-term storage-and are therefore not suitable for in-line modification of films. However, when olefinic elastomers are melt mixed with polyolefin plastics in an intensive mixer, such as a Banbury, elastomeric modifiers or concentrates are formed that can be pelletized into stable pellets.

Olefinic elastomers have a high degree of affinity for polyolefin thermoplastics such as polyethylene (PE) and polypropylene (PP). This compatibility does not result in molecular mixtures, but it is sufficient to produce blends with micronsize morphologies that impart useful engineering properties. As the use of this family of materials continues to grow, the technology for compounding and effective melt mixing continues to evolve. In view of the diversity of ingredients employed and the economic incentives to achieve efficient melt mixing, considerable effort has been made to formulate the best possible elastomeric systems for on-line modification. This article describes the performance of three elastomeric modifiers-EM 23, EM 30, and EM 50-used to enhance the performance of high-density polyethylene (HDPE) and PP films.

The Modifiers

EM 23 is a soft, low-modulus elastomeric modifier used in all polyolefinic films, and PE and PP films in particular. EM 23 imparts high melt strength to PP, facilitating PP blown-film manufacture on conventional LDPE film equipment. Its relatively low viscosity facilitates good in-line dispersion and film processability. When converted by itself, it produces a soft, elastic film.

EM 30 is designed to modify both PP and HDPE films. It imparts exceptional impact strength, strong heat seals over a broad temperature range, and a translucent matte finish to either blown or cast films.

EM 50 is designed for in-line modification of HDPE blown films. It improves film toughness without sacrifice of moisture barrier properties. During processing, the elastomer in EM 50 undergoes controlled melt fracture, imparting a translucent matte finish to the film.

Elastomeric concentrates of the modifiers were prepared in a high shear mixer and pelletized for easy dispersion with the base resin by either a simple tumble blend process or direct feed into the extruder hopper.

PE Film Modification

Polyethylene films constitute over 75% of the many thermoplastic films used in packaging. End-use markets include food, industrial, and retail packaging. For most of these applications, toughness, high tear resistance in both directions, and low moisture permeability are important requirements. Use of elastomeric concentrates can enhance the performance characteristics and expand the applications for PE films-particularly for HDPE films.

The properties of 2-mil HDPE films modified with various levels of EM 50. Toughness properties-as measured by dart drop impact, puncture, and tear strength-increase significantly with increasing elastomer content. For example, at 40% modifier concentration, improvements in these values exceed 100%. Modifier addition reduces stiffness, making the film softer and enhancing its flexibility.

In a transmission electron micrograph of an EM 50 dispersion in HDPE film, the elastomeric particles appear to range in domain size from 0.5 to 3 microns. The EM 50-modified and unmodified HDPE films have similar water vapor transmission rates (WVTR). Rates for linear low-density PE (LLDPE) and medium-density PE (MDPE) films are shown for comparison. Hence, addition of EM 50 to HDPE allows production of a family of intermediate stiffness films with an outstanding balance of toughness and WVTR characteristics.

PP Film Modification

Polypropylene films can be divided into oriented and non-oriented categories. While the oriented film markets are larger, good growth is predicted for non-oriented film, which is used in textile and food packaging, labels, and disposable diapers. Non-oriented blown films are also finding increasing application in medical packaging, where the high-temperature performance of PP permits their use at the autoclave temperatures commonly used for sterilization or safe disposal of medical wastes. Another rapidly growing market is paper protector sheets, where PP cast film is replacing vinyl and polyester films. Heat sealability is critical for this application. Some of the inherent limitations of PP films are poor tear and impact properties.

The properties of unmodified and 50% EM 30,3-mil cast PP films. Enhancements in dart impact by a factor of 2 and in Elmendorf tear strength by a factor of 5 are shown for the modified film. Addition of EM 30 substantially reduced the secant modulus (or stiffness), yielding a softer film. The matte finish imparted to the film by EM 30, as expected, dramatically reduced gloss. For concentrations above 50%, modifier EM 30 can be dry blended with the PP or fed directly into the extruder hopper. However, for concentrations below 50%, proper modifier dispersion requires either a good mixing screw in the extruder or melt mixing in high shear equipment, such as a Banbury, prior to film processing.

Use of modifier EM 23 at a 40% concentration in a 2-mil non-oriented blown film to significantly improve impact and tear properties. The modifier reduces secant modulus to a value closely matching that of LDPE films, and softens the films. Heat seal range is also broadened. The particular elastomer in EM 23 makes the modified film more elastic, as shown by its lower percent set values. In Fig. lb, the domain size of the elastomer in the EM 23/PP dispersion appears to range from 0.5 to 2 microns. The apparent viscosities of EM 23 and PP homopolymer at 230 [degrees] C Fig. 3) are closely matched at a shear rate of 100 sec-1, a typical operating shear rate in a film line extruder, thereby facilitating good dispersion.


The primary end uses of the elastomeric concentrates are as modifiers of polyolefin films used in food packaging, heavy duty bags, drum and box liners for moisture-sensitive products, personal care packages, and medical overwraps. Other applications are in retort and medical pouches, surgical sleeves, disposable gloves, and paper protector sheets. The translucent matte finish imparted to the film surface by modifiers EM 30 and EM 50 is useful in several specialty applications. The wider heat sealing range of the modified films makes them suitable for the more demanding packaging applications.
TABLE 1. Properties of
Elastomeric Modifiers.
Property EM 23 EM 30 EM 5
Resins to PP PP HDPE,
Melt index, 2.7 0.15 0.22
 g/10 min @ 190 [degrees] C
Melt Flow 7.2 0.50 0.65
 Rate, g/10 min @ 230 [degrees] C
Density, g/cc 0.90 0.91 0.92
Vicat softening 60 154 123
 point, [degrees] C, 200g
Hardness 65 85 78
 @ 5 sec, Shore A
Flex 8 42 41
 modulus, MPa
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Title Annotation:Engineering resins: innovation, development, realization
Author:Dharmarajan, N.; Hazelton, D.R.; Kaltenbacher, E.J.
Publication:Plastics Engineering
Date:May 1, 1990
Previous Article:Engineering resins; breaking the rules.
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