Chapter 1
Introduction
In order to improve polypropylene competitiveness in engineering applications, it is an important objective to simultaneously increase dimensional stability, stiffness, strength and impact resistance.
This goal can be achieved either by producing PP composites containing fibre reinforcement, through special processing technology involving fibre impregnation and pre-preg formation, and by developing new grades of filled PP which is produced by means of conventional melt processing technology.
Traditional fillers for PP are calcium carbonate, talc, glass fibres, wollastonite, mica, glass beads and wood flour1.
In recent years, organic-inorganic nanometre composites have attracted great interest of researchers since they frequently exhibit unexpected ‘hybrid’ properties synergistically derived from the two components. One of the most promising composite systems would be hybrids based on organic polymers and inorganic clay minerals consisting of silicate layers.
1.1 What is a Nanocomposite?
In the context of plastics a nanocomposite is a near-molecular blend of resin molecules and nano-scale particles.
A nano-scale particle is a material with at least one dimension in the nanometre range. Conventional plastic composites can now contain functional fillers of around 0.5 µm in size. A nano-particle is 500 times smaller in at least one dimension.
In this case size does matter. Many physical and gas barrier properties are greatly enhanced when these infinitesimal particles interact at the molecular level. Achieving the near-molecular blending is one of the principal aims, at the moment, for scientists.
It is possible to have different filler dispersion levels in the composite, as Figure 1.1 illustrates: if the mineral platelets are aggregated into ‘lumps’, their behaviour is no more different from an ordinary composite material. When a surface treatment is used, the space between the mineral sheets is increased and the polymer molecules are allowed to enter more easily between them. Usually the modified filler is named Organo-Clay or Nano-Clay (NC) and the corresponding structure intercalated. In the ultimate platelet configuration, the mineral is
completely dispersed (exfoliated), the specific surface is at its maximum and the greatest advantage can be obtained from NC2.
Figure 1.1: Different order and dispersion levels in nanocomposites
Depending on the physical and chemical properties of the matrix, to completely exfoliate the mineral can be a real challenge: in many cases part of it remains intercalated or even aggregated: thus, another reason why is correct to speak about “hybrids” is that we are dealing with a family of materials which has features characteristic of both a Composite and a Nano-Composite.
1.2 Why Silicate Minerals as a filler?
It is well known that filler anisotropy, i.e. large length/diameter ratio (aspect ratio), is especially favourable in matrix reinforcement.
Due to its peculiar structure, the mineral particle thickness is only one nanometre although its dimensions in the length and width directions can be measured in hundreds of nanometres. As a result individual sheets have aspect ratios varying from 200-1000, with a majority of platelets in the 200-400 range after purification. Moreover, the very small size and thickness mean that a single gram contains over a million individual particles. This leads to average loading level which barely exceeds 5-6% on a weight basis.
In previous works different polymer-clay hybrids were studied, with matrices as nylon 6, polyimide, epoxy resin, polystyrene, polycaprolactone and acrylic polymer3.
Conventional composite
Disordered exfoliated nanocomposite Intercalated nanocomposite
PP as well was one of these since it exhibits an attractive combination of low cost, low weight and the extraordinary versatility in terms of properties, application, and recycling. On the other hand, the silicate layers of the clay have polar hydroxy groups and are incompatible with polyolefins. The most promising approach to overcome this problem seems to be the use of a functional oligomer as a compatibilizer1.
1.3 Market possibilities
The work of this thesis originates from the investigation on a very specific and, in some way, niche aspect of NCPs (namely electrical behaviour) which derives from a realistic problem strictly connected with the industrial and commercial world (see Abstract).
Nevertheless the potential use of nano-materials seems to have very many and by far wider application fields. The potential uses of NCPs based on a variety of polymeric matrices result from their improved mechanical properties (tensile strength and Young’s modulus), improved heat/dimensional stability and enhanced barrier to water, hydrocarbon and gas (particularly oxygen) permeation4.
As a consequence the packaging applications become the ‘natural’ use field for such materials. The advantage is evident: the currently adopted addition of higher barrier plastics in multilayer structures or surface coatings necessarily increases the cost of the usually cheap polymers utilized for packaging manufacturing, whereas the NCP option would be a more cost effective choice because of the easier incorporation of the filler into resin systems. In fact nylon 6 and PP are already used for packaging and injection moulded articles, whereas semi-crystalline nylon finds application for ultra-high barrier containers and fuel systems5 both in vehicle piping and in storage devices.
The enhanced optical clarity and reduced haze which NCPs films exhibit (in comparison with conventionally filled polymers) encourages even more their use in packaging: films, bottles, boil-in bags, vacuum packs and blister packs can be at the same time completely transparent and effective barrier to gas and water permeation. Indeed the use of nanocomposite formulations would be expected to enhance considerably the shelf life of many types of food and actually studies are being conducting from U.S. Army to investigate on the possibility of using NCPs for packing long life food for soldiers.
Lightness is another interesting aspect of NCPs: their reduced particle size leads to a high elements concentration allowing very low loading levels which indeed barely exceed 5% in weight. On the contrary, the currently most used glass or mineral filled systems for automotive and appliance applications have loading levels ranging from 15 to 50% in weight leading inevitably to heavier products4. A light-weight material is typically appreciated when used on vehicles because it means less fuel consumption: for this reason unsaturated polyester based NCPs are already used for watercraft lay-ups and potential utilisation can be as mirror housings, door handles, engine covers and timing belt covers.
Mobile devices can take advantage from a low density material too: covers for portable electronic equipment (mobile phones, laptops, pagers etc.) are candidates to be made with NCPs.
The possibility to improve the flame retardation of the pristine polymer is an attractive characteristic of nano-fillers: the resulting NCPs, in fact, are shown to be potent char formers and then suitable to be used in fire retardant cabling, electrical enclosures and housings5.
Other general applications currently being considered include usage as impellers and blades for vacuum cleaners, power tool housings, mower hoods and outdoor advertising panels6.
1.3.1 A tangible example
In December 1999, during a joint press conference,
General Motors (GM) Research & Development and
former Montell North America announced they have developed a new family of thermoplastic olefins which would have offered significant benefits for interior and exterior automotive applications. In the same conference two prototype parts
(a door panel and a rear quarter panel) were shown. In August 2001, GM, together with its partners Basell (former Montell),
Southern Clay Products and Blackhawk Automotive Plastics, presented the first
automotive exterior polypropylene/clay NCP application: a step assist for 2002 GMC
Safari and Chevrolet Astro vans that aids
users in stepping into and out of the vehicle (shown in Figures above, from reference 7.
According to the executive director of science for GM Research and Development, the parts made of nanocomposite material cost, on a volume basis, about as much as conventional thermoplastic olefins (because, of course, less filler is needed to manufacture them) guaranteeing a lighter-weight component and in turn, a more fuel-efficient vehicle.
GM says that the NCP and the replaced talc-filled PP share the same stiffness with a 7-8% weight saving. Besides, depending on which part is going to be replaced, the weight saving could even reach 20%.
According to GM officials, such a light weighting, together with other cost benefits, such as an improved part surface quality, can balance the higher price of Basell's nano-material8.
1.3.2 The present of the market
After the GM announcement, in early September 2001, Honeywell too presented the commercial availability of its line of nylon 6 NCP called ‘Aegis’. Its target was mainly packaging applications, nevertheless injection moulded parts for automotive, industrial and consumer applications could be produced too.
In these systems the dispersion was improved via nano-filler incorporation during polymerization and the nano-clay was supplied by Nanocor Inc.
This company has recently started a boosting alliance-policy; indeed several new partnerships were established: with Polymeric Supply Inc. for the marketing and distribution of thermoset based nanocomposites, with Clariant for the supply of its nano-filler concentrates and with the equipment manufacturer New Castle
Industries for the production of NCP dedicated devices9.
In a growing market the more the alliances the better, and so Gitto Global Corp. as well jointed Nanocor Inc. to develop flame-resistant polymers based on polypropylene and EVA using nanotechnology. As a result of this collaboration
Gitto Global has already commercialized some NCPs which not only have
improved strength and reduced weight, compared with traditional flame-retardant packages, but are also more environmentally friendly from a recycling and halogen-exposure point of view10.
1.3.3 The future of the market
The existing nanocomposites market is very small with a global use assessed only in a few thousand tonnes per annum, nevertheless the analysis published in two Bins & Associates reports11,12 forecast by 2010 an important growth of the market up to millions of tonnes annually commercialised in North America alone, with an estimated value of more than 4 billion US$.
It is important to note that in the same reports the biggest part of this cash flow is predicted to come from polypropylene based materials which, in turn, are forecast to reach by 2005 a usage in automotive applications of between 10,000 to 25,000 tonnes, in North America alone.
And it is undeniably expectable that the automotive industry is going to promote the newest applications for polypropylene based NCPs even though Bins &
Associates reports predict such applications to account only for 25-30% of the
total and fully exploited future polypropylene NCPs market9.
1.4 Health and safety
The health and safety issue regarding nano-materials has often attracted the interest of toxicologist after their recent abrupt development. Some studies have lately suggested the hazard connected with the use of such a small particulate because it was shown that, when the nano-particles are less than 100 nm in size, they can enter the body by breathing and even through the skin leading to a potential health risk. Besides it seems that the real problems this family of material can create are not necessarily related to the harmfulness of their composition, but they are derived primarily from their size.
This means that the safety question is still unsolved also for material apparently innocuous such as MMT.
In perspective of an industrial and large scale development of nano-materials it is felt there is a need of addressing issues such as the lack of long term health and safety data.
The real risk associated with each material should be determined in order to provide a way to minimise it by determining tolerable exposure levels or, in general, suitable monitoring techniques.
1 Kaempfer D, Thomann R, Mϋlhaupt R, Polymer, 43 (2002) 2909
2 Gao F, Nanocomposite Workshop, Polymer/layered Inorganic Particle Nanocomposites: A
Better Solution To Make Materials Stronger (September 2001) Risley Hall, Derby, UK
3 Hasegawa N, Kawasumi M, Kato M, Usuki A, Okada A, Journal Of Applied Polymer Science, 67
(1998) 87
4 Nanocor Technical Papers, Qian G, Cho JW, Lan T, Preparation And Properties Of Polyolefin
Nanocomposites, http://www.nanocor.com/tech_papers/Preparation_Properties_PP_nano.htm (accessed October 2002)
5 Nanocor Technical Papers, Lan T, Cho J, Liang Y, Qian J, Maul P, Applications Of Nanomer In
Nanocomposites: From Concept To Reality,
http://www.nanocor.com/tech_papers/PP_nano_to_reality.htm (accessed October 2002)
6 The Institute of Nanomaterials, Hay JN, Shaw SJ, A Review Of Nanocomposites (2000),
http://www.nano.org.uk (accessed October 2002)
7 Plastics Technology, Leaversuch R, Nanocomposites Broaden Roles In Automotive, Barrier
Packaging, http://www.plasticstechnology.com/articles/200110fa3.html (accessed October 2002)
8 PR Newswire, New Material Moves From Research To Production; Strong, Lightweight
Nanocomposites Are The Future Of Automotive Plastics, Announces GM,
http://www.scprod.com/gm.html (accessed October 2002)
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http://www.sciencedirect.com/science/journal/1464391X (accessed February 2004)
10 Elsevier Science, Additives For Polymers, August 2002, Pg. 9,
http://www.sciencedirect.com/science/journal/03063747 (accessed February 2004)
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