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Preparation and Properties of Polyolefin Nanocomposites
by
Guoqiang Qian, Jae Whan Cho, and Tie Lan
Nanocor, Inc.
1350 W. Shure Drive, Arlington Heights, IL 60004

ABSTRACT
Quick jump to:
Introduction
and Background

Experimental
Results and Discussion
Conclusion
Figure 1
Figure 2
Table 1
Table 2
Table 3
Table 4
Table5
Acknowledgements
References

Polyolefin nanocomposites have been successfully manufactured using commercially available surface modified montmorillonite (Nanomer®). These nanocomposites are notable for their improved mechanical properties, such as tensile strength and modulus and for their dimensional stability. They also demonstrate enhanced barrier to gas permeation, particularly oxygen. Nanocomposite processing uses twin screw technology, coupled with standard processes for injection molding, film casting and blow molding. In addition to packaging applications, polyolefin nanocomposites are of particular interest for automotive and industrial applications where wall down-gauging is desirable. Nanomer® chemistry, nanocomposite processing, and properties will be discussed in the presentation.

Polyolefins 2001, Houston, TX, February 25~28, 2001

INTRODUCTION
Polyolefins are one of the fastest growing classes of thermoplastics due to their good balance of physical and chemical properties, their low cost, light-weight, favorable processing and recycle characteristics. Some polyolefins, such as polypropylene, find extensive use in both packaging and engineering applications, especially automotive. Like all plastics, polyolefins demonstrate some performance shortcomings. In packaging for example, they are poor gas barriers. In automotive on the other hand, dimensional and thermal stability limit their range of application.

Most schemes to improve polyolefin gas barrier involve either addition of higher barrier plastics via a multilayer structure or high barrier surface coatings. Although effective, the increased cost of these approaches negates one big attraction for using polyolefins in the first place-economy.

Currently, automotive and appliance applications employ glass or mineral-filled systems with loading levels ranging from 15-50% by weight. This approach improves most mechanicals ie, dimensional and thermal stability, but it carries two drawbacks. First, polyolefin's easy processing is compromised and second, high filler loadings inevitably lead to heavier products.

The emerging field of polymer-layered nanocomposites is unique in that it addresses shortcomings of polyolefins for both packaging and engineered applications, and it does so with favorable cost, processing and weight profiles. Polymer-layered silicate nanocomposites are plastics containing low levels of dispersed platey minerals with at least one dimension in the nanometer range. The most common mineral is montmorillonite clay. Its aspect ratio exceeds 300, giving rise to enhanced barrier and mechanical properties. In general, every one weight-percent of these "nanoclays" creates a 10% property improvement. Their interaction with resin molecules alters the morphology and crystallinity of the matrix polymer, leading to improved processability in addition to the benefits to barrier, strength and stability.

Polyolefin nanocomposites have attracted significant research interest[1~4]. Nanocomposite research activities are mainly focused on the use of chemically modified montmorillonite. Recently, we successfully synthesized polypropylene-layered silicate nanocomposites employing a masterbatch route[5]. Improved gas barrier and mechanical properties were achieved for different grades of polymers at low nanoclay loading (typically 4~6 wt%). Our findings provide one opportunity for end-users to down-gauge finished products and maintain equivalent or enhanced performance properties.

EXPERIMENTAL

Materials:

Nanomers® I.30P, I.31PS, and I.44PA are commercial products manufactured by Nanocor, Inc. Each is an onium ion modifed montmorillonite, designed for maximum compatibility and dispersion in a polyolefin matrix. Available as free-flowing powders with a mean size of 15-25 microns, they are capable of dispersing to nanoscale in twin screw compounders. In addition to the typical onium treatment, Nanomer I.31PS contains a silane-coupling agent to promote higher tensile properties. I.30P and I.31PS are film grade Nanomers and I.44PA is an engineering grade.

Formation of polyolefin nanocomposites:

Polyolefin nanocomposite was formed using a two-step process: Nanomer® masterbatching and subsequent letdown into neat polyolefin. A typical masterbatch contains 50~60 wt% Nanomer® and the remainder a combination of compatibilizer (such PP-g-MA) and standard polyolefin. Masterbatching pre-disperses Nanomers, promoting full nanocomposite formation while minimizing heat history during letdown. The final Nanomer loading after letdown is typically 2-7% by weight.

For our work a Lestritz co-rotating twin screw extruder was used to produce both the masterbatches and nanocomposites. The extruder has a diameter of 27 mm and a L/D ratio of 36:1. There are three shear-mixing zones in the extruder set-up to maximize dispersive energy. Screw speeds ranged from 300-500 rpm and extrusion temperatures ranged from 170~190 ºC.

A detailed screw configuration is shown in Figure 1.

Figure 1. Extruder Screw Element

Materials testing:

Nanocomposite formation and the degree of Nanomer dispersion can be monitored using wide-angle x-ray diffraction (XRD) and transmission electronic microscopy (TEM). Both techniques were used in this study.

Polyolefin nanocomposite films were made using cast and blown processes. Oxygen transmission rates (OTR) of nanocomposite film samples were measured using a Mocon 2/60 at room temperature and 65% relative humidity. Mechanical properties were determined on injection molded specimens according to ASTM testing methods.

RESULTS and DISCUSSION

Nanoclay dispersion

Both XRD and TEM confirm good Nanomer® dispersion in the polyolefin matrices. XRD measures the degree of dispersion by estimating the distance between individual platelets after compounding. Table 1 presents d-spacing data for Nanomers® before and after dispersion. The increases confirm that polyolefin host polymers have successfully interacted with Nanomer® surface treatments, promoting nanocomposite formation. TEM confirms the results

Table 1. Formation of Polyolefin Nanocomposite: Compositions and Dispersion

Polyolefin MFI
(gm/10 min)
d-spacing (Å)
Nanomer® Nanocomposite
PP-PE random co-polymer 2.0 22 29
Homopolymer PP 0.45 22 28
Homopolymer PP 4.0 22 32
TPO 12 22 32

Film properties

Table 2 provides tensile properties for the PP/PE co-polymer nanocomposites cast into monolayer films. Reinforcing effects can be easily observed in modulus with minimal impact on strength. Assuming gas barrier improves commensurate with tensile modulus, the film can be down-gauged without compromising integrity. Barrier for this film improved by 45%, indicating that significant down-gauging can be done.

Figure 2. TEM Images of HPP Nanocomposite (Homopolymer PP- 6wt% I.31PS)

Barrier improvement is explained using tortuous path theory[5], as it relates to alignment of the nanoclay platelets. It follows, then, that film processing conditions can influence nanocomposite barrier improvement. Table 3 compares cast and blown films using homopolymer polypropylene as the matrix. Blown film, with its greater stretching ratio, creates more uniform nanoclay platelet orientation, leading to a more tortuous path and better barrier.

Table 2. Properties of PP Copolymer Nanocomposite Films

Nanomer Loading
Level (wt %)
Tensile Modulus
(MPa)
Improv. OTR
(cc.mil/100in2.day)
Improv.
Control 0 909 205 /
I.30P 6.0 1751 + 93 % 137 1.5x
I.31PS 6.0 1495 + 64 % 108 2x

Table 3. OTR of HPP Nanocomposite Blown and Cast Films

Processing Nanomer Grade Loading Level
(wt %)
OTR
(cc.mil/100in2.day)
Improv.
Blow film Control 0 2520 /
Blow film I.31PS 7.0 1019 2.5x

Cast film Control 0 1823 /
Cast film I.31PS 7.0 1272 1.4x

Mechanical properties

We have previously reported the mechanical properties of homopolymer PP (HPP) and TPO nanocomposites using film grade Nanomers I.30P and I.31PS.[6] These nanocomposites showed improved mechanical properties, thermal properties (HDT), and coefficients of linear thermal expansion (CLTE). Recently Nanocor developed Nanomer I.44PA, a grade designed specifically for non-film applications. Tables 4 and 5 summarize the performance of this new grade for HPP and TPO nanocomposites. A standard grade HPP with a melt flow of 4 g/10min and a standard grade TPO with a melt flow of 12 g/10min were used as representative matrix polymers.

Table 4. Tensile Properties of PP Nanocomposites

Nanomer
Grade
Loading
Level (wt %)
Resin Tensile Strength
(MPa)
Improv. Tensile Modulus
(Mpa)
Improv.
Control 0 TPO 19.6 / 957 /
I.44PA 6.0 TPO 23.5 +20 % 1458 +53 %

Control 0 HPP 31.3 / 1388 /
I.44PA 6.0 HPP 35.5 +13 % 2180 +57 %

Table 5. Flexural Properties of PP Nanocomposites

Nanomer
Grade
Loading
Level (wt %)
Resin Flexural
Strength
(MPa)
Improv. Flexural
Modulus
(MPa)
Improv. HDT (ºC) Improv.
Control 0 TPO 22.4 / 811 / 72.8 /
I.44PA 6.0 TPO 29.5 +32 % 1295 +60 % 93.3 +28 %
Control 0 HPP 34.6 / 1181 / 88.3 /
I.44PA 6.0 HPP 46.0 + 33 % 1777 +50 % 109.1 +24 %

Grade I.44PA creates the “nano-effect” in both HPP and TPO as evidenced by improved mechanical properties and HDT.

Conclusion

Nanocomposites have been successfully prepared in a range of polyolefins for both film and engineering applications. Significantly improved gas barrier properties, as well as mechanicals, are obtained at low Nanomer® additions. This provides down gauging opportunities for films and rigid containers. A new Nanomer® grade expands options for improving performance in the engineered plastics arena, especially for automotive and industrial applications. This more cost-effective grade improves mechanical, thermal and dimensional properties, while preserving two inherent advantages of the technology—low weight and easy processing.


Acknowledgements

The authors wish to thank the Nanocor’s laboratory technicians and administrative personnel for their assistance in data generation and manuscript preparation.

 

References

  1. M. Kawasumi, N. Hasegawa, M. Kato, A. Usuki, and A. Okada, Macromolecules, 1997, 6333.
  2. N. Hasegawa, M. Kawasumi, M. Kato, A. Usuki, and A. Okada, J. Appl. Polym. Sci., 1998, 67, 87.
  3. J. Heinemann, P. Reichert, R. Thomann, R. Mulhaupt, Macromol. Rapid Commun., 1999, 20, 423.
  4. P. Reichert, H. Nitz, S. Klinke, R. Brandsch, R. Thomann, and R. Mulhaupt, Macrocol. Mater. Eng., 2000, 275, 8.
  5. L. E. Nielsen, J. macromol. Sci. A1, 929, 1967
  6. T. Lan and G. Qian, Proceeding of Additive’00, Clearwater Beach, FL, April 10~12, 2000.


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