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Primary Thermal Stability

Carboxylate replaces allyl chloride and lead chloride will be produced.

Secondary Stability

They react with hydrochloric acid (released during PVC degradation) and form neutral lead chloride.

pbx.nPbo + 2n HCL → nPbCl2 + Pbx + n H2O

Note: Vinyl compounds must be stabilized beyond their capacity so that they remain stable during process condition changes (e.g., in the extruder, temperature, etc.)

or during PVC recycling.

Advantages of Lead-Based Stabilizers

• Low cost
• Provides long-term thermal stability
• Low solubility in water
• Odorless process and product
• Due to their good compatibility with PVC, the migration of these stabilizers out of the polymer is minimal.
• Lead compounds are widely used due to their good processability. They do not affect the Vicat softening temperature of rigid PVC, resulting in good mechanical properties in high-pressure pipes, gas and sewage pipes, and profiles.

Properties of Calcium Carbonate

Particle Shape:

• Due to its low aspect ratio (length-to-thickness ratio), calcium carbonate is considered a filler, not a reinforcement.
• Limestone processing affects the shape of the particles:

Dry powder: more irregular shape but higher specific surface area
Wet powder: more uniform and smoother shape due to water centrifugation
Precipitated: smaller and more uniform size

Composition of Calcium Carbonate

• Calcium carbonate is usually selected from specific grades of calcite ore (crystalline limestone).
• Typical composition of calcium carbonate used as filler:

Types of Calcium Carbonate

• Fine Powder: from calcite ore with 94% purity, particle size ranges from 0.7 microns (fine) to 20 microns (coarse)

– Hydrated

– Dry

• Precipitated: from aragonite ore with 98% purity, particle size from 0.01 microns (ultrafine) to 5 microns (coarse)

Particle Size and Size Distribution

• Equivalent Spherical Diameter (ESD)

– Diameter of a sphere having the same volume as the particle

Particle Size Distribution

• Average particle diameter d50%

– The diameter at which at least 50% of the particle mass is smaller

– ۵۰% by weight are larger than d50%, and 50% are smaller

• Top Cut d98%

– ۹۸% of the particles are smaller than d98%

– Since coarse and rough particles cause stress concentration,

   Reducing the top cut is desirable and improves the following properties:

       –   Impact Strength
U-PVC:
d98% < 5µm
d50% < 1µm

       –   Tensile Strength
       –   Surface Gloss

Titanium dioxide is found in different forms in nature. The two famous types are anatase and rutile. Rutile is one of the main ores of titanium dioxide. Its features include light weight, high strength, and resistance to corrosion.

The above image is an example of rutile crystal.

Titanium dioxide or titanium is of special importance in polymer industries, especially PVC. This is because titanium is the most important white pigment or colorant, which not only colors the PVC particles but also provides a protective effect against UV radiation.

• Mechanical, physical, and thermal properties of titanium
  Titanium and its alloys have high electrical conductivity. Pure titanium has a yield strength similar to that of low-alloy steels, but it is 45% lighter than them.

• Titanium sources:
  Currently, more than seventy types of minerals are known to contain titanium as a compound. Commercial production of titanium is primarily obtained from the ores ilmenite (FeTiO3) and rutile (TiO2). These ores also contain vanadium, scandium, tantalum, and niobium. Another titanium ore is leucoxene. Titanium can also be obtained from anatase and loparite.

• Types of titanium:
  1- Ilmenite (FeTiO3)
  This ore has a specific gravity of 4.5-5 g/cm3 and has weak magnetic properties. It contains vanadium, scandium, tantalum, and niobium.
  
  2- Rutile, the name comes from the Latin word “Rutilus” meaning red.
  This ore has a formula of TiO2 and a specific gravity of 4.2-4.3 g/cm3. It has a diamond-like luster and a hardness of 6. It is brittle and non-magnetic. Rutile powder is white, odorless, insoluble in water and dilute mineral acids but soluble in hydrofluoric acid.
  
  3- Titanite (Sphene)
  This mineral is a calcium and titanium silicate with a specific gravity of 3.45-3.55 g/cm3.
  It is mostly colorless but can occasionally be yellow, brown, or red. It is brittle and often has a prismatic shape with a strong anisotropy. It crystallizes in the monoclinic system.

Other non-economic titanium minerals include:

• Anatase
• Brookite
• Perovskite
• Magnetite or titanium-bearing hematite
• Loparite
• Ulvospinel
• Leucoxene

The titanium R838, R878 (rutile) is produced by the Blue Star factory in China.

In terms of quality, it is lower compared to European competitors such as Kronos, Zachleben, and other manufacturers. However, it is a good alternative in terms of price.

The advantage of this titanium over KA-100 titanium is its optical resistance and coating. However, the whiteness that both types of titanium provide to the product is the same.

Products that are sensitive to optical stability and exposed to sunlight and lamps should definitely use rutile titanium. However, products like PVC pipes and fittings, which are mostly used underground or in sewage, do not have particular sensitivity, and anatase titanium (such as KA-100) will provide the necessary whiteness to their products, so there is no need for rutile titanium.