In an effort to reduce production costs, manufacturers of PVC products are increasingly turning to highly filled formulations. Calcium carbonate (chalk) is rightly regarded as the leading filler. However, while an economist may see chalk as simply “low-cost volume,” a technologist sees a complex component that fundamentally changes melt rheology and product properties.
Chalk: from the quarry to the extruder
To understand how chalk behaves in the melt, it is important to know how it is produced. There are two main routes for producing calcium carbonate:
- Natural (ground) chalk: obtained from natural deposits of limestone, chalk or marble. The rock is crushed and then ground using a dry or wet process. The resulting particles have an irregular shape and a broad size distribution. Wet grinding produces a narrower particle size distribution and a smoother surface, improving compound flow.
- Synthetic (precipitated) chalk: a product of chemical synthesis. Limestone is calcined to obtain calcium oxide (lime), which is slaked with water; calcium carbonate is then precipitated by passing carbon dioxide through the solution. This chalk is considerably more expensive, but its particles have a uniform rhombohedral shape, high purity and a smaller surface area.
Chalk: hidden potential and less obvious risks
Calcium carbonate is more than an inert powder. In addition to the obvious cost reduction, chalk affects many parameters.
- Controlling thermal expansion: PVC itself has a high coefficient of thermal expansion. Adding chalk reduces this value by almost two orders of magnitude. This is critical for manufacturers of vinyl siding, flooring and window profiles: the finished product will not change dimensions as temperatures fluctuate.
- Acting as a secondary stabilizer: chalk has mildly alkaline properties due to Ca-OH groups on its surface. It can neutralize traces of hydrogen chloride released during PVC degradation, acting as an additional buffer for the heat-stabilization system.
- Increasing melt viscosity: adding 30, 50 or 100 parts of chalk per 100 parts of PVC fundamentally changes melt behavior.
How bonds form: chalk particles do not float in isolation in the PVC melt. As filler concentration increases, the distance between individual particles decreases sharply. Active functional groups on the chalk particle surfaces begin to interact actively with one another, with polymer molecules and with additives.
A branched network of interparticle bonds forms, a process known as structuring. The more chalk you add, the less free space remains for the polymer and the more contacts form. The particles begin to “catch” on one another, creating a strong framework.
How lubricants work: an invisible struggle of forces
Lubricants are used to break these bonds and make a highly filled compound flow. They fall into two fundamentally different types:
1. Internal lubricants (esters, oxidized waxes)
Mechanism: they are compatible with PVC and act as “molecular bearings.” They enter the spaces between polymer macromolecules and between filler particles, breaking those interparticle bonds.
Effect: melt viscosity decreases and the melt flows more readily, coating chalk particles more easily. This promotes agglomerate breakup and uniform dispersion.
2. External lubricants (esters, non-oxidized waxes)
Mechanism: they are incompatible with PVC. During heating, they “bloom” and migrate to the interface between the melt and the hot metal surfaces of the equipment.
Effect: an extremely thin lubricating film forms. This prevents the melt from burning onto or sticking to the screw and die, and reduces torque on the extruder motor.
Theory is useful, but in practice, processing highly filled PVC requires a comprehensive approach and synergy between several components. To prevent a compound with a high chalk content from turning into “concrete” in the extruder and to ensure that the finished product meets all quality requirements, the additive package must be selected carefully.
Synergy from Revada Plastic
Revada Plastic has all the tools needed to address these challenges:
- Revamod – acrylic processing aids. At high filler loadings, the polymer matrix loses strength and fusion deteriorates. Revamod processing aids provide the required melt strength, promote uniform distribution of chalk particles in the PVC matrix and significantly improve product appearance by adding gloss and smoothness.
- Revalube – oxidized and non-oxidized waxes. They address adhesion and friction. By blooming to the surface or working within the bulk material, they create conditions for filler distribution, reduce the load on the extruder motor and give the finished product high gloss and a smooth surface.
- Loxiol – ester lubricants. Working at the molecular level, they act as “bearings,” breaking those interparticle bonds between chalk and the polymer. Loxiol lubricants improve compound flow and promote better filler dispersion.
- Viscospeed – a dispersion promoter. It is a powerful auxiliary component for extremely highly filled systems, including those based on metal hydroxides. Viscospeed dispersing agents dramatically reduce compound viscosity and friction during extrusion.
Every formulation is unique: what works perfectly for a window profile may not suit a cable compound or flooring. Revada Plastic specialists will analyze your requirements, consider the specifics of your equipment and help select particular grades and optimal additive dosages from the Revamod, Revalube, Loxiol and Viscospeed ranges to achieve the ideal balance of cost savings, processability and final product quality.
