Plastics,Rubber and Composites: Macromolecular Engineering

The market of wood–polymer composites in the building and construction sector is focused on external long term applications such decking, siding and window sills. The current PVC market share of 14% is described and the reasons for its sharp predicted growth in the next few years are discussed. Recent technical studies and developments are reviewed. The influence of wood fibre size on the processing of the composites and their mechanical properties is reported. The water absorption of wood–polymer composites, including the capillarity effect, and the evolution of mechanical properties with water content has also been investigated. A special resin with rapid gelation has been developed, giving improved properties with increased productivity. Finally, the progress made in European standardisation and initial work to establish a quality label are described.

A standard rigid PVC profile formulation has been optimised to better accept the wood fibre presence in terms of compatibility, dispersion level, rheology, extrusion output, physical and mechanical properties of extrudates. Several different additives have been evaluated at different use levels. Process aids and lubricants appear to be the most critical ones. The effects of wood flour type and content, PVC resin K value, as well as specific wood–PVC compatibilisers have also been investigated. It is concluded that PVC is the most suitable polymer matrix for wood–plastic composites applications.

Impact modified rigid PVC formulations for the building and construction industry have evolved since their introduction in the 1960s. Formulations mainly consist of stabilised PVC with low levels of inorganic filler, pigment, and impact modifier. For outdoor applications n-butyl-acrylic grafted poly-methyl-methacrylate core/shell modifiers (AIM, a thermoset) and chlorinated polyethylene (CPE, a thermoplastic) modifiers are commonly used. It is known that CPE, especially the newer high molecular weight CPE products, are very efficient at high levels of filler (i.e. CaCO₃>8 phr). The high molecular weight CPE impact modifier has a synergistic effect with high CaCO₃ filler levels, boosting the impact strength of the final formulation. This effect is also seen at low temperatures, important in cold regions. This advantage has been exploited in the North American rigid PVC siding market; however, the European window profile market has kept CaCO₃ levels at 5–8 phr. There is a lack of knowledge around high molecular weight CPE in comparison with newer AIM in highly filled formulations, especially on how the impact modifier affects toughness and plate-out. A range of impact modifiers has been studied, focusing on their effects on impact strength (at room and low temperatures) and plate-out in formulations with high filler levels.

The multiple functions of titanium dioxide (TiO₂) in exterior PVC durability are reviewed. TiO₂ absorbs UV light to protect the polymer from direct UV degradation, and provides light scattering for visible light for opacity and to mask the discoloration of PVC. In addition, TiO₂ photocatalytically degrades the PVC surface which is detrimental to gloss retention and causes colour fade. The chalking effect of TiO₂ to PVC surface degradation was further analysed and the acid solubility test was introduced to differentiate the chalking grades and durable grades. A two year outdoor weathering study of rigid PVC at representative exposure sites included exposures in the USA, France and Belgium. The gloss loss and colour change due to weathering and UV exposure was discussed with the influence of different exposure climates, stabiliser types, TiO₂ loading and chalking effects.

The ‘eco-profiles' (cradle to grave life cycle inventories) of the major thermoplastics published by PlasticsEurope since the early 1990s provide a recognised reference for environmental impacts of plastics up to and including resin production. The suspension and emulsion PVC eco-profiles were fully updated in 2005, on the basis of 2004 averages collected in the plants of ECVM members and sister companies. The results are very robust, covering more than 90% of West European production of VCM and PVC. The results show a decrease in both energy consumption and CO₂ emissions. Many other emissions, related to energy production (SO₂, NO_x) or directly related to VCM and PVC processing (e.g. VCM), also decreased very significantly. The data obtained have been aggregated into environmental declarations for suspension and emulsion PVC, the first such declarations available for polymers. These documents provide a compact summary of the main impact categories as well as other relevant information for downstream users.

The European Resilient Floorcovering Manufacturers Institute includes manufacturers of floorings from vinyl, linoleum, rubber and synthetic thermoplastic and takes a leading role in the development of new European and international standards. The increased pressure to supply environmental as well as performance data has led European Resilient Floorcovering Manufacturers Institute to produce a set of environmental product declarations for a range of product types. The implications of drawing together data for 12 different types of flooring, with four different material types, using data from 13 separate companies in nine different countries, are discussed. The study has provided what is believed to be the most comprehensive survey available of the environmental impact of flooring, being derived from actual industry data. The end result of the project is, first, a document that can be used by designers to assess the impact of their choice of flooring and, second, a tool to assist member companies to improve the environmental performance of their products.

It has been found that N-alkyl (C₈ to C₁₈) pyrrolidones are highly efficient, strong solvating performance plasticisers which decrease gelling temperatures substantially. Higher alkyl pyrrolidones also exhibit very low volatility. Solubility temperatures (DIN 53408) are between 52°C (N-octyl pyrrolidone) and 80°C (N-octadecyl pyrrolidone). Plasticising efficiency tests using comparative of Shore A values showed that N-alkyl pyrrolidones are up to 40% more efficient than the standard plasticiser diisononylphthalate. Cold flexibility as assessed by the folding test DIN EN495-5 and impact test DIN 53372 found sample failure to begin in the temperature range from −55 to −65°C. Further trials in filled systems showed that N-alkyl pyrrolidones are highly compatible with calcium carbonate and allow very high filler loads. Manufacturing tests with a highly filled system using an extruder resulted in increased output while significantly reducing plasticiser levels and processing temperatures, and thus reducing energy requirements.

A PVC nanocomposite has been obtained by exploiting the exfoliation of Na–bentonite in water and the fact that vinyl chloride is usually polymerised as a water suspension or emulsion. An exfoliated clay aqueous dispersion is prepared and blended with PVC latex before the latter is dried, leading to intimate mixing and the desired morphology. The nanocomposite has useful properties in PVC paste applications, exhibiting shear thinning rheology, improved thermal stability and transparency. Possible applications are mainly for plasticised PVC but also rigid applications such as pipes, profiles or films. The PVC nanocomposite also shows good dispersion in plasticiser and may be used instead of a paste resin, so extending the range of PVC pastes towards very high plasticiser ratios. These plastisols have been shown to be more stable against sedimentation or decantation during storage and very soft articles can be produced by coating technologies. The very high low shear viscosities make it possible to produce thick films in only one coating step.

PVC paste formulations are defined on the one hand by the required properties of the finished product, and on the other hand, by the essential processing properties. It is often difficult to reconcile both demands. The use of extender resins as viscosity modifiers and for gloss control is discussed. Extender resins (also named blending resins or filler resins) decrease the paste viscosity in general and also suppress flow anomalies like dilatancy. They increase the matt effect of the surface of the finished article. The strength of these effects depends on the grades used. Special grades avoid the impact some extender resins can have on the mechanical properties. Special copolymeric blending resins have the additional benefit of accelerating fusion of the paste. These features make extender resins a powerful component in paste formulations, as is illustrated by the examples given.

Environmental concerns will lead to the elimination of lead based heat stabilisers over the next 5 years, forcing pipe producers to re-evaluate their formulations as they strive to remain cost competitive. A new stabilisation system is described based on a derivative of the mercaptan chemistry that is well established for conventional tin stabilisers. The patented technology includes organic based, heavy metal free compositions that contain blocked thiols, which under vinyl processing conditions generate active stabilising components. These innovative systems are shown to offer wide processing windows, to give excellent early colour and to meet the technical requirements of the pipe market. An additional benefit is reduced cross-linking to facilitate recycling. Results concerning the stabilisation mechanism and performance attributes (stabilisation, rheology and recycling data) of the new stabiliser on both laboratory and industrial equipment are presented.

Polymer composites, filled with ultra fine particulate fillers, are alternatives to the conventional filled polymers. The reinforcement of the mechanical properties occurs to a greater extent when ultra fine particulate fillers are used in comparison with the conventional microdimensional fillers. To achieve all the benefits that the ultra fine fillers can provide, optimal dispersion as primary particles is essential. To achieve better dispersion of the inorganic particles in a polymer matrix, the ultra fine particles (UFP) are added to the polymerisation reactor so that they are dispersed in the monomer before polymerisation. Hence, the monomer is polymerised in the presence of the UFP (in situ). In this paper the effects of the UFP on the initial monomer dispersion are examined. The presence of the inorganic UFP in the polymerisation reactor influences the properties of the monomer phase and affects the drop size distribution. This in turn influences the grain sizes as well as their distribution, which influence the processability of the resin.

A comprehensive multiscale, multiphase dynamic model is developed to simulate heterogeneous vinyl chloride (VCM) suspension polymerisation in industrial batch reactors. From the numerical solution of the proposed integrated model, the evolution of the molecular (i.e. molecular weight distribution, long chain branching, short chain branching, terminal double bonds) and morphological (i.e. particle size distribution, porosity) polymer properties in a PVC batch suspension polymerisation reactor can be predicted. In particular, the polymer molecular properties are determined by employing a kinetic mechanism which describes the VCM free radical polymerisation in both monomer and polymer rich phases. Semi-empirical and phenomenological expressions are used to describe the breakage and coalescence rates of dispersed monomer droplets in terms of the type and concentration of suspending agent, quality of agitation and evolution of the physical, thermodynamic and transport properties of the polymerisation system. The dynamic discretised particle population balance equation (PBE) is solved to calculate the dynamic evolution of the particle size distribution of the produced PVC. Furthermore, the primary particle size distribution (PPSD) inside the polymerising monomer droplets, which influences to a large degree the porosity of the final PVC grains, is determined by the solution of a PBE governing the nucleation, growth and aggregation of the primary particles. Finally, dynamic mass and energy balances are derived to assess the dynamic behaviour of the PVC batch suspension polymerisation reactors. The theoretical model predictions show good agreement with a comprehensive series of experimental data provided by the PVC industry. This verification of the proposed model makes it a powerful tool for the simulation of large scale PVC batch suspension polymerisation reactors.

The standard production of glass fibre reinforced PVC involves mixing and granulating the fibre and matrix before extrusion, which allows the use only of very short fibres. More recently a technique has been developed in which long or medium size fibres are arranged in a thin layer into which a dry blend of PVC resin with additives is dispersed using a high voltage electrical field. The product is then heated and pressed in a roll mill to obtain a stiff, impact resistant mat. Trials using a specially developed PVC resin that provides good gelation even without the application of shearing are reported. Significant improvements in flexural modulus and impact strength have been achieved. The sheet also shows good fire resistance and has attracted interest for applications including construction, transport (to provide weight saving) and furniture.

Hydrotalcites, compounds of magnesium–aluminium–hydroxycarbonate, are promoted as environmentally safe materials for costabilisation of PVC products. Commercial grades of hydrotalcites have been added to rigid PVC formulations, containing a range of different stabiliser types, to evaluate their contribution to heat stability and their effect on mechanical properties. Hydrotalcites are confirmed to be effective costabilisers for rigid PVC: static thermal stability (through oven testing) and dynamic thermal stability (via torque rheometry) of PVC compounds are modified by their presence. The extent of change is determined by the primary stabiliser type and the grade of the hydrotalcite. Detailed analysis of mechanical properties has been carried out on compression moulded samples and on extruded compounds. With the former, no significant change in tensile or impact performance was observed, for hydrotalcite levels up to 5 phr. Charpy impact data on extruded PVC have shown significant increase in performance of compounds containing an acrylate modifier when hydrotalcites are used.

The industry's commitment under Vinyl 2010 to phase out lead stabilisers by 2015 has generated market potential for more environmentally friendly stabiliser systems. The market volume for calcium/zinc and zinc free organic stabilisers is expected to increase strongly in the next 3–5 years. Most of these stabiliser formulations need costabilisers such as mineral acid scavenging additives. A new mineral additive, calcium aluminium hydroxyl carbonate, is described which is suitable for use as an acid scavenger for many PVC end applications. The performance of the new additive is shown to be acceptable in both zinc and zinc free systems using an organic core stabiliser. Screened applications include flexible PVC end uses such as sheeting and wire and cable, as well as rigid PVC applications including sheeting and profiles.

PVC compounding with renewable materials is reviewed, including raw materials production technology, market dynamics and technical aspects of the finished products compared with current solutions. PVC resins produced from renewable ethylene and plasticisers obtained from vegetable oils are described and evaluated. An overview of the market changes that have enabled the competitiveness of biobased materials is also considered, outlining the agricultural technology and challenges and the state of the art of chemical processes to obtain renewable components for PVC formulating. Options to formulate with renewable components are discussed on the basis of a review of the literature. It is concluded that a PVC compound formulated with renewable resin and plasticiser is not only viable, but represents a natural evolution towards a more sustainable PVC supply chain. As a sample calculation, 1 ton of a clear flexible vinyl formulation with 100 phr resin and 60 phr plasticiser would remove 2·3 tons of CO₂ from the atmosphere.