EoW March 2009

technical article

3.3 Study of possible incorporation of nano-filler in PVC Recently there has been much interest in polymer nanocomposites (PNC), especially polymer/clay nanocomposites. Three main types of nanocomposites can be obtained when a layered silicate is dispersed into a polymer matrix. This depends on the nature of the components used including polymer matrix, layered silicate and organic cation. If the polymer cannot intercalate between the silicate sheets, a microcomposite is obtained. The phase-separated composite that is obtained has the same properties as traditional microcomposites. Beyond this traditional class of polymer-filler composites, two types of nanocomposites can be obtained: intercalated structures are formed • when a single (or sometimes more) extended polymer chain is intercalated (sandwiched) between the silicate layers. The result is a well-ordered multilayer structure of alternating polymeric and inorganic layers exfoliated or de-laminated structures • are obtained when the silicates are completely and uniformly dispersed in the continuous polymer matrix. The de-lamination configuration is of particular interest because it maximizes the polymer-clay interactions, making the entire surface of the layers available for the polymer. This should lead to the most significant changes in mechanical and physical properties

The chemical action can be distinguished in: reaction in phase gas: the radicals • generate from the flame retardant chemically to act on the combustion process reaction in condensed phase can • be carried out in two ways. The first consists in forming a protecting carbonic layer (char) on the surface of the polymer, having the characteristics of a thermal insulator and to act as a barrier between the products of pyrolysis and oxygen. The second is that this layer increases and delays the process of thermal feedback

Calcium-zinc systems, as the recent increase has shown, are a good replace- ment for lead-based stabilisers. The main application areas where Ca-Zn systems have highest penetration are wire and cable and automobile interiors, followed by pipes and profiles. The selection of metallic compounds as non-lead stabilisers was based on the fact that their effect on the human body is slight, and that there was thus little likelihood of their becoming subject to regulation and limitation in the future. Stabilisers made from these metals were combined and a PVC resin with a non-lead stabiliser was developed for use in wire insulation and sheathing. 3.2 The function of flame-retardants in PVC The process of combustion can be described in the following steps: heating • decomposition (pyrolysis) • ignition and combustion • propagation, with thermal feedback • The heating of the material by external thermal sources increases the temperature of the material, with a speed that depends on the intensity of the heat emitted, the thermal conductivity characteristic of the material, the latent heats of fusion and vaporisation and the heat of decomposition. Once reaching a sufficient temperature the material begins to degrade, forming gaseous mixtures and liquids. These mixtures are formed with a speed that depends on the intensity with which the polymer material is heated. The concentration of the decomposition products, blending with surrounding air, increases until falling back in the inflam- mability interval. The presence in this situation of a source of heat makes the ignition of the mixture. The produced heat is in part irradiated to the material (thermal feedback), so that it continues to pyrolysis. The action of a flame retardant consists in eliminating or limiting one of the factors, acting in a physical or chemical way or both, on the liquid, solid and gaseous products originated in the process. The physical action is of three types: cooling the process of thermal feed- • back, that it fails to supply the heat necessary to progress the pyrolysis of the polymer material dilution of the combustion mixture • formation of a protecting layer, where • the solid polymer material is shielded in oxygen from the rich gaseous phase, by means of a solid or gaseous protecting layer. It reduces heat to the polymer, with a consequent slowing down of pyrolysis and lessening the contribution of oxygen to the combustion process

Polymer combustion cycle,

Gas phase

Oxygen

Products

Flame

Volatiles

Heat

Dispersion

Polymer

Char

Polymer combustion cycle diagram ▲ ▲

Flame-retardants can be included in the material in several ways: reactive: react chemically with the • polymer additive: blended with the polymer • reactive and additive: present in the • material in both ways The choice of flame retardant is influenced by: toxicity • biodegradability • heat stability in the polymer • ) is normally added in order to reduce the flammability of plasticised PVC; however Sb 2 O 3 enhances the stop of the radical chain mechanism in the gas phase, and increases the amount of smoke generated in case of fire. Many PVC processors have expressed interest in alternative flame-retardant additives that provide a reduction in flammability without themselves producing toxic or corrosive components. The flame retardant should not negatively influence the specific characteristic of the PVC. It is desirable that any improvement in flame retardancy is combined with a decrease in smoke density. In a fire event, PVC releases Hydrogen Chloride (HCl), with the humidity always present in the air. Calcium carbonate is normally used in PVC as an acid scavenger and a cost saving filler. The ideal flame retardant should also possess these benefits. Antimony Trioxide (Sb 2 O 3

Layered silicate

Polymer

Phase separated (microcomposite)

Intercalated (nanocomposite)

Exfoliated (nanocomposite)

In order to characterise the structures of nanocomposites two complementary analytical techniques are used. X-ray diffraction (XRD) is used to identify intercalated structures by determination of the interlayer spacing. Nanocomposites can demonstrate significant improvements, compared to virgin polymers, with the content of the modified layered silicates in the 2-10 wt% range. There are improvements in: mechanical properties, such as tension • compression, bending and fracture • barrier properties, such as permeability • and solvent resistance Diagram showing the three main types of ▲ ▲ nanocomposites which can be obtained when a layered silicate is dispersed into a polymer matrix

optical properties • ionic conductivity •

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EuroWire – March 2009