PVC FORMULATION | ADDITIVES
Below: Calcium-based stabiliser systems are being used in foamed sheet applications in Europe
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requires good heat stability and even more importantly a very limited amount of lubrication. Tin stabilisers like DOTE or DMTE provide only limited lubrication, he says, which makes them a good stabiliser for this type of processing. Baerlocher says that for rigid PVC profile
extrusion a typical calcium-based stabiliser would be designed around inorganic acid scavengers such as zeolites or hydrotalcites. Furthermore, it would contain metal soaps, more specifically calcium and zinc soaps, to provide stabilisa- tion through the known mechanism of substitution of labile chlorine atoms by stearate moieties. Unfortunately, the company says these constituents of calcium-based stabiliser one-packs can be contrary to the requirements for rigid calen- dering. Metal soaps such as zinc stearate provide a fairly strong
external lubrication effect, which causes issues in calendering. Where formulations use the conventional calcium-zinc concept
of labile chlorine atom substitution then other, non-lubricating calcium and zinc components must be applied. Inorganic acid scavengers are non-soluble in
Right: Internal view of IKA’s additive
manufacturing facility at Bitterfeld- Wolfen in Germany
PVC and remain as inorganic particles within the PVC matrix after processing. Although finely distributed, these inorganic particles can induce haziness or opacity in transparent film applications, depending on the exact choice and amount used. Careful selection of the acid scavengers, together with appropriate dosage, can allow transparency to be achieved, according to the company. However, the tendency to appear hazy may persist, espe- cially in thick films or sheets. Where transparency is not required, the choice of acid scavengers is easier but lubrication adjustment is just as critical. Baerlocher has developed various grades, such as its Baerostab NT 1883 P and Baerostab NT 1929 P-series, which it says can cope with the demand for high transparency, good initial colour and stable calendering (due to adjusted low lubricity).
Foaming developments Another application area where tin stabilisation has been utilised is production of foamed sheets and profiles. Baerlocher says many foam profile produc- ers in the UK are now using calcium-based stabilis- ers while many European foam sheet producers are also using only calcium-based systems. In foam applications, achieving a homogeneous foam structure, smooth surface appearance, and good initial colour are the main issues to be
14 COMPOUNDING WORLD | June 2023
www.compoundingworld.com
addressed. Surface properties are, however, less important for foamed profiles because most are coextruded, enclosing the foamed core with a PVC or acrylic compact skin. Calcium-based stabiliser systems used in continental Europe for foamed applications are typically designed around the acid scavengers, metal soaps and co-stabilisers that are used in non-foamed rigid PVC profile extrusion. Baerlocher says, however, that specific care must be taken when choosing the co-stabiliser system as some of the known chemistries interfere with the blowing agents used and can lead to some yellowing from the start. Perhaps the most challenging part of a one-pack
formulation development is the lubricants. The company says adjusting melt viscosity is crucial to successfully extrude wide sheet with an even, flow line-free surface lines and the desired density. Melt viscosity adjustments are not exclusively linked to the lubricant selection either; applied acrylic processing aids can also influence the results.
Construction drivers According to German PVC additives manufacturer IKA, the building and construction sector is a key driver towards the use of calcium-based products for PVC worldwide. Chief Executive Officer Alexan- der Hofer says there is a clear trend towards more sustainable products and, as the complexity increases, so do the technical requests. Availability and sustainability of feedstock
IMAGE: IKA
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