Clay Based Nanocomposites - Production and Major Benefits of Clay Based Nanocomposites by MERI

Topics Covered

Background
Introduction
The Holy Grail for a Clay Based Nanocomposite
Making Nanocomposites
Major Benefit of Clay Based Nanocomposites

Background

MERI (formerly MRI) has been a centre for research excellence in materials since 1990. This excellence was recognised in 2001 when the Research Assessment Exercise (RAE) awarded a 5 rating to the research carried out at MRI. This makes MERI the highest rated department of its type in the new university sector and rated alongside the materials departments of universities such as Liverpool and Queen Mary's.

MERI's research and consultancy activities are supported by a large advanced equipment base ranging from the latest electron microscopes to high performance computing hardware.

Introduction

Clays have been used as low cost fillers and extenders, at the 20-30 weight percent (wt%) level since Adam was a lad. These phase separated microcomposites (Figure 1, label A) offered improved rigidity but less strength, elongation or toughness than the pure polymer. Now the latest trick is to coax polymer between the individual clay sheets (Figure 1, label B) or, ideally, to get the sheets to disperse completely into the polymeric matrix (Figure 1, label C).

Figure 1

Generally, it is necessary to replace the naturally occurring inorganic (Na, Ca, Mg) cations with positively charged organic species such as hexadecyltrimethylammonium ions which render the clay surface more organophilic and therefore more compatible with polymers of low polarity.

Get it right and at clay loadings of 2-5wt%, these hybrid materials can give you nanocomposites with higher thermal stability, better mechanical properties, less warpage or shrinkage, increased barrier properties, as well as increased resistance to scratches, solvents and fire.

The Holy Grail for a Clay Based Nanocomposite

The Holy Grail for a clay based nanocomposite is a disordered exfoliated structure in which the individual clay platelets (ca 1nm thick) are randomly dispersed in the polymer matrix and no longer display any of their original face to face ordering (Figure 1, label C). This configuration maximises the amount of clay surface and hence the area available for interaction with the polymer leading to the largest improvement in physical and mechanical properties. However, this is often difficult to achieve because the hydrophilic clay surface is not compatible with the hydrophobic polymer matrix, making it a real challenge.

Scientists at Toyota overcame these hurdles by replacing the naturally occurring exchange cations (Na+, Ca2+ and Mg2+, which are necessary to balance the negative charge on the clay layer) with positively charged organic species such as the hexadecyltrimethylammonium ion. The positive head group attaches itself to the negatively charged clay surface and the long hydrophobic tail, which contains 16 carbon atoms, renders the clay surface more organophilic (inset, Figure 1). The Toyota researchers found that just 4 weight % (1.6 volume %) of this organoclay in a nylon-6 composite doubled the tensile modulus, increased the strength by 50% and increased the heat distortion temperature by 80°C (compared to the pristine polymer).

Making Nanocomposites

There are three major routes which can be used to synthesise nanocomposites. The most straightforward is to pre-swell, or exfoliate, the clay in a solvent such as N,N-dimethylformamide or toluene and then add this to the polymer suspended in the same solvent. The polymer enters the clay gallery and an intercalated nanocomposite (Figure 1, label B) is formed from which the solvent is removed by gentle heating or under vacuum. The large quantities of volatile solvent necessary for this approach make it less attractive as an industrial process.

The second route, called in-situ polymerisation, is particularly effective for epoxy and nylon-6 systems. The organomodified clay is swollen with the monomer or a monomer/solvent mixture and then initiated using heat, radiation or an appropriate initiator. If the rate of reaction equals or exceeds that in the bulk then this is an effective route to a disordered, exfoliated nanocomposite (Figure 1, label C).

Melt intercalation, the third route, is particularly attractive to industry because a molten thermoplastic is blended with the organoclay. In some instances a compatabilising agent is required and the use of maleated polypropylene oligomers can lead to disordered, exfoliated polypropylene/clay nanocomposites. Some reports suggest that further processing of beads of polypropylene clay nanocomposites, prepared without the maleated oligomer, can lead to reaggregation of the clay nanolayers. Nonetheless, a wide range of thermoplastics such as polyamide-6, ethylvinyl acetate and polystyrene have been made using melt intercalation and the method is now being used successfully with polymers from renewable resources including starch, polylactic acid and polyhydroxybutryrate. The complete absence of solvent makes melt compunding attractive from an environmental perspective.

Major Benefit of Clay Based Nanocomposites

The incorporation of only 4 wt% of dispersed organoclay in polypropylene gives a Young's Modulus value that would require 30-60 wt % of talc. The heat deflection temperature of a polypropylene nanocomposite containing 6 wt % dispersed clay is 30°C higher than for the unfilled material. The heat release rates are also significantly reduced when delaminated clays are dispersed in polymer matrices. Polyamide-6 containing 2 wt % dispersed organoclay exhibited a 32% reduction in the peak of the heat release rate (HRR) curve. An organoclay content of 6 wt% caused a 63% reduction in the HRR peak. Indeed 3 wt% of organoclay in polystyrene reduces the HRR peak by 48%. An equivalent reduction in the HRR peak requires a mass fraction of 30% of the brominated flame decabromodiphenyl oxide.

A major benefit of clay based nanocomposites is that in addition to the mechanical and flame properties they also offer enhancement of other properties such as optical clarity and scratch resistance. However another very significant feature is their ability to enhance the barrier properties of both petroleum based and renewable polymers.

Figure 2

The permeability of polypropylene to liquids and gases is halved when 4 wt% of organoclay is added and the solvent uptake decreases accordingly. When 5 wt% of organoclay is dispersed in polycaprolactone the relative to water is reduced almost tenfold. Hence the potential for these materials in the packaging industry is clear and under intense scrutiny.

Source: MERI - Materials and Engineering Research Institute

For more information on this source please visit MERI - Materials and Engineering Research Institute.

Date Added: Oct 12, 2008 | Updated: Jun 11, 2013
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