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From:Hongliang  Click:  time:2015-04-23 11:37:32

PVC is one of the world's most widely used polymers because of its versatility, excellent inherent properties and cost effectiveness, and yet its sensitivity to the processes used to convert it into end products make it a challenge for any PVC compound producer. Bob Goulding, technical manager of Dugdale, explains why and discusses what is involved in developing new grades and alloys of PVC to meet challenging applications.

Developing tailored grades of PVC compound to suit specific performance requirements calls for a wealth of formulation knowledge, technical experience and an in-depth understanding of processing. Success in the laboratory has to transfer not only to the process but also to the individual equipment on which the compound is converted into usable product. For example, the processing equipment could be an extruder, the extrusion screw configuration – single, twin (conical or parallel) - will change the way either the dry blend or the pelletised compound behaves. For these reasons an in-depth understanding of each stage of manufacture is absolutely crucial.

The difficulty is that unlike most other thermoplastics, PVC, as a base polymer, is inherently unstable. When subject to energy PVC can deteriorate rapidly without the addition of key additives. Perhaps not surprisingly then the first consideration when processing PVC is how to minimise the degradation process. This is achieved with the addition of stabilisers, the choice of which is critical. Predominantly, “heavy metals” such as cadmium and lead have traditionally been used, however, due to environmental concerns, the former has now been eliminated and lead is voluntarily being phased out. Alternative stabilisers based on calcium zinc, barium zinc, organic stabilisers including some organotins, that are not classified as such heavy metals, are finding increasing use because they are more environmentally acceptable.

The conversion away from lead-based stabilisers in rigid PVC and in flexible PVC is a key focus area for most compounders.

Key factors when formulating
Key factors to consider when formulating PVC compounds include:

What is the intended application? 
What environmental conditions will the end products experience? 
Is it for use indoor, outdoor (specific climate conditions), what are the service temperatures? 
Physical appearance? – Colour, clarity, surface finish, surface feel. 
What physical properties does the compound need to have? 
For example, does it need to be high impact, is it going to move during its service life, come into contact with corrosive liquids or other plastics? Does it require electrical properties, flame retardancy or does it need to bio-stable? 
How will the material be processed? 
For example, with regard to machinery is it extrusion, injection moulding or blow moulding? 
Specifically, what type and make of machinery will be used to process the compound? 
How fast does the product need to be produced? 
What form is the material? Pellet or dryblend 
Are there any cost constraints on the product or processing?

Base material
Polyvinyl Chloride is derived from salt and oil. It is the electrolysis of salt water to produce chlorine, and the subsequent combination with ethylene, extracted from oil, which forms vinyl chloride monomer (VCM). VCM is then polymerised to form PVC resin.

PVC resin, which is the base material for all grades of PVC compounds and its polarity makes it extremely compatible with a plethora of different additives. This makes the material highly versatile; it can be clear or coloured, rigid or flexible. The material’s ability to blend with varying amounts of fillers, additives and stabilisers, and in so doing completely change its properties, is what makes PVC so incredibly adaptable. PVC can be formulated to produce clear flexible films through to rigid window profiles, coloured roofline products, sheet, blown bottles, tubing and injection mouldings for the packaging, building, medical, cable and automotive industries. However, the sheer versatility of the material makes the task of formulating compounds quite complex with the formulator needing to draw information from a library of experiences and resources.

Additives
With PVCP grades, a plasticiser is added to give PVC flexibility and elasticity. In combination with the plasticizer, a filler is normally included which enhances the mechanical properties, with the added advantage of cost efficiency. As the PVC resin content in PVCP can vary between 30 to 60%, the balance, which is invariably plasticiser and filler, is adjusted to suit. With PVCP, the most important parameters are softness and specific gravity (SG) – the higher the level of filler used the higher the SG will be. The softness is achieved predominantly by altering the level of plasticiser. The specific gravity of PVCP compounds can range from 1.13 to over 2g/cm3.

Rigid PVC compounds also include the necessary stabilisers and some lubricants – this tends to be far less than with PVCP as the plasticiser also has inherent lubricity.

While stabilisation protects the raw material from the heat required to process it, lubricants are also critical to aid processing because PVC has a significant tendency to adhere to metal. It is essential therefore to form a barrier between the metal of the processing equipment and the PVC melt itself. This is achieved by using external lubricants such as paraffin based products and polyethylene waxes. Internal lubricants are also necessary. These reduce the melt viscosity of the material and so prevent internal friction generating too much heat during compounding and subsequent processing into the final product. The addition of lubricants has a further function in that they control the “fusion” or “gel” time of the material. This is essential to ensure that by the time the PVC compound reaches the end of the processing equipment, the material is sufficiently homogenised to disperse the various additives and hence achieve optimal physical properties.

Colours and toners are commonly included in formulations. For white or pastel coloured products, the incorporation of relatively high levels of the appropriate grade of titanium dioxide into products such as window profile enhances physical characteristics such as weatherability.

Ultra violet (UV) light can actually break the polymer chains in PVC and so cause degradation, which initially manifests itself as a colour change, for example, fading and/or yellowing. If the end product is not opaque or is to be used in an outside environment, protection against UV light is essential. UV stabilisers are complex chemical compositions and have to be chosen to be compatible with other ingredients in a formulation.

Processing aids are also commonly included in formulations. In a typical PVCU formulation, a process aid – usually an acrylic type high molecular weight polymer - is added to assist the melting process and enhance melt strength. This is useful, for example, when pulling a profile out of a die as good melt strength will minimise any tendency for the extrudate to tear. The processing aid also improves surface finish and helps eliminate the so called ‘shark skin’ effect that can occur when the material is stretched while in a molten state.

Modifiers that enhance impact and tensile strength are also employed where a material is likely to be subjected to any kind of physical force. For example, rubber type materials counteract the inherent brittleness of PVC. Again, they have to be chosen with great care to meet service conditions. Typical modifiers used for outdoor applications are acrylic based or chlorinated polyethylene. Styrenic modifiers such as MBS (methacrylate-butadiene-styrene terpol), should only be used inside because they can be sensitive to UV.

Finally there might be some other specialist additives required to suit eventual use, for example blowing agents in cellular products and nitrile rubber in gasket grades.

Manufacture is generally a two stage process. Once a formulation has been defined on paper the ingredients are accurately weighed and brought together in a high-speed mixer. During high-speed mixing, the blend is heated by friction, which allows the various additives to become dispersed amongst the PVC resin with some bonding or absorbed into its surface. The blend is then transferred to a cooler mixer which subsequently cools the blend which then becomes a free-flowing dryblend. Customers can use this product or opt for a pelletised version, which is made by a subsequent compounding process, i.e. producing a melt and extruding it through a pelletising die.

Material developments
During its 50 years in the PVC compounding business Dugdale has developed literally thousands of PVC compounds for as many applications and is creating a new breed of innovative, high performance, materials.

For some time the company has been investing in research and development in alloying PVCU with other materials to try to overcome deficiencies in certain areas, for example resistance to weathering, especially in coloured grades. PVCU traditionally performs well in light shades particularly, however in darker colours it requires the selection of specific raw materials to prevent the surface appearance from prematurely deteriorating. Dark colours particularly can be adversely affected by heat and UV which can lead to physical distortion and a detrimental appearance. With many building products now manufactured in both dark and pastel shades as well as white these problems have been addressed to enable market penetration and growth. Various options have been investigated including the use of alternative materials to PVC and composites that are subsequently extruded as a layer onto PVCU profiles, but the company’s focus is on developing compounds that can perform equally as effectively without the cost or complexity of extruding non-PVC based materials.

ASA (acrylate-styrene-acrylonitrile) copolymer materials that enhance the weathering properties of compounds could provide a solution as they process at temperatures compatible with PVCU. Adding ASA copolymer also raises the heat distortion temperature of the compounds.

Lead by a shift to lead-free stabiliser formulations material developments also focus on developing core stabiliser technology that delivers cost competitive calcium zinc stabiliser systems as well as creating products with greater durability than is currently available. These new materials will also possess strong environmental credentials and will offer a combination of excellent colour and UV resistance, low migratory characteristics and even anti-microbial performance.

Dugdale believes it has established a lead over its competitors by offering lead-free versions of all its grades ahead of the 2015 voluntary commitment to phase out the use of lead stabilisers in PVC compounds.

Recent developments include new nitrile PVC gasket compounds with attractive cost performance profiles aimed at co-extrusion sealing applications in the PVC window systems sector. Branded Ducaseal, these PVC/nbr (acrylonitrile butadiene rubber) materials are suitable for static and dynamic applications. Formulated to offer a deflection recovery of 27% the static compound provides a durable seal between PVC profiles and sealed glazing units. Conversely, the dynamic compound features a deflection recovery of 43% and is designed to provide weatherproof door and window closure seals.

Both compounds are easy to process on co-extrusion and post co-extrusion processes.

Compounds of this type are highly specified and must meet the performance requirements of BPF 345/2. They require considerable technical formulation skills as well as mixing and compounding capabilities to enable processors to manufacture end products that will adhere to PVC and offer consistently good surface finish – an important characteristic that minimises the ingress of dust and dirt and so increases product service life.

As companies move towards both the PVC industry voluntary commitment which stipulates the phasing out of lead by 2015, and the forthcoming implications with regard to REACH compliance, purge and freeze compounds have tended to be overlooked due to the efficiency of ‘heavy metal’ based systems with regard to thermal stability. However, Ducafreeze, a high performance PVC compound, developed by Dugdale, has rewritten the script.

Using new generation stabiliser technology this freeze compound has elevated temperature resistance, making it suitable for use as a freeze or purge compound in rigid and flexible PVC materials across a wide range of applications.

Conventional non-lead purge and freeze compounds have thermal stability limitations which render them unsuitable for use in high end rigid PVC applications where processing temperatures can exceed 200°C.

Ducafreeze can be used to purge material in the die while processing plant temperatures are being reduced. It can be left in the tooling and machine for extended periods. On start up the temperature is increased allowing Ducafreeze to be purged through using standard material – saving both time and resources as the die does not need to be stripped down and cleaned. This has the knock-on benefit of reducing wear and the risk of damage to tooling caused by continuous disassembly, cleaning and reassembly.

Alternatively Ducafreeze can be used as a conventional purge material that when cooled is easily removed from metal surfaces and intricate die flow paths – again minimising the potential for tooling damage.

Suitable for use on extrusion and injection moulding equipment Ducafreeze also has the flexibility to provide a purging solution for all rigid and flexible PVC processors while its robust temperature performance will appeal to trade extruders and producers of window profile, pipe and products for the building and construction industries.

Several new injection moulding and extrusion materials, for both flexible and rigid application areas have been added to Dugdale’s portfolio of high performance compounds. Offering “simple solutions to surface hygiene” is the Ducavin ABC range of PVC compounds developed in partnership with SteriTouch™ and which incorporate this company's antimicrobial technology. These compounds reduce the growth of harmful organisms such as bacteria, mould and fungi, while remaining entirely safe for sensitive applications. A comprehensive development program has resulted in an extensive suite of both rigid and flexible antibacterial PVC compounds. All the materials have been independently tested and proven to meet the necessary criteria.

Ducavin ABC rigid PVC compounds include high impact, UV resistant, standard and easy-flow moulding, general purpose and profile extrusion products. The Ducavin ABC flexible PVC compound range consists of conventionally plasticised and phthalate-free anti-microbial materials, including products designed for antifungal, flame retardant and non-marring applications. Both product lines are available in clear or opaque as standard and custom colours can be formulated upon request.

As target applications for these materials are for products used in hospitals, washrooms and food preparation areas, the main organisms of concern are pathogenic bacteria such as methycillin-resistant Staphylococcus aureus (MRSA), E.coli and other enteric organisms. Typical end products include sanitary ware, hospital furniture, food preparation worktops, flooring, fittings and other applications where surface hygiene is paramount.

Other developments include Ducavin DEHP-free and phthalate-free flexible PVC compounds for moulding and extrusion. These materials have been created to satisfy following customer concerns over REACH legislation. Dugdale’s entire range of flexible compounds is available with these alternative plasticiser systems.