Pvc Handbook

References: on Page 361] metal stabilizers are sufficient to cause phase separation and serious loss of properties by hydrolyzing some of the phosphite and adding to a portion of the epoxide. Numerous calcium-zinc mixed metal stabilizers are sanctioned by FDA for use in flexible PVC food contact films. Regulated phosphites and polyols are used as synergists in these stabilizers, some of which are sold as one-pack systems. New calcium-zinc stabilizers were described by Bacaloglu [33]. The compositions of most lead-replacement stabilizers are proprietary because of unresolved patent and technical issues. They are reported to contain combinations of primary and secondary metals, metallic chloride deactivators, inorganic acid acceptors, metal coordinators, and antioxidants. Some of these use hydrotalcites similar to the well-known antacid Maalox®, which has aluminum, magnesium, hydroxyl, and carbonate functionalities. β-diketones, such as Rhodiastab 83® or Rhodiastab 50®, are recommended to prevent early discoloration in some lead-replacement stabilizer systems. A novel approach using “latent mercaptides” was described by Conroy [34]. Promising early work on the stabilization of PVC by “plasticizer thiols” was described by Starnes [35]. Stabilizer technology is covered in Chapter 4. Organotin stabilizers are very successful in the United States in rigid PVC, but are only used in specialty flexible applications. When foaming flexible PVC with azodicarbonamide blowing agents, it is advisable to use a stabilizer recommended by the blowing agent manufacturer. For satisfactory foaming, the stabilizer needs to be matched to the desired temperature range for foam formation. For instance, some lead stabilizers are good “kickers” for blowing in the range 160 to 180 °C (320 to 356 °F). Some zinc-containing stabilizers are effective kickers for blowing above 180 °C. 10.6.4 Fillers Generically, filler may be any low cost solid, liquid, or gas which occupies volume in a part and reduces its volume-cost. The flexible PVC industry uses the term “fillers” to refer to inert particulate solids incorporated into formulations for various reasons, including hardening, stiffening, and reduction of volume-cost. Functional fillers are added to improve specific properties. Examples are calcined clays added to wire insulation formulas to raise electrical volume resistivity, fumed silica or bentonite clay added to plastisols to increase their yield value, and hollow microspheres used to lower specific gravity while achieving other desired filler effects. Particulate solids called fillers must not dissolve in the flexible PVC matrix. Since many flexible vinyl products are sold by volume rather than weight, their volume-cost is the dominant economic parameter. For use in volume-cost calculations, the specific gravity of calcite is 2.71; that of true dolomite is 2.85 and that of aragonite is 2.95 The most widely used fillers in flexible and semi-rigid PVC are grades of dry-ground, wetground, or precipitated calcium carbonate derived from limestone or marble, which are predominantly calcite. This is the stable crystal structure of CaCO3 at ordinary temperatures and pressures. Marble consists of small, interlocking crystals of calcite. Calcite is soft, having a Mohs hardness of 3. Therefore, pure calcium carbonate fillers are low in abrasivity to processing equipment. Grades which contain significant fractions of hard silicates are much more abrasive. Recent work carried out in a PE carrier resin confirms this long-accepted fact and shows that coarser grades are more abrasive than fine particle size fillers [36]. 10.6 Additives Used in Flexible PVC Compounds 335 Considerations in selecting a particular grade of calcium carbonate filler include the purity of the original ore, whether it has been dry-ground or wet-ground or precipitated, the average particle size and size distribution, and whether the particles have had a surface treatment. The “packing fraction” (PF) is a measure of how efficiently finer particles fill the voids between coarser particles. Presence of iron oxides such as Fe2O3 in the filler tends to color a compound yellow-brown and will compromise its heat stability unless it is stabilized to withstand the presence of the iron oxide. The average size of filler particles is usually defined in terms of an equivalent spherical diameter (esd). The ratio of the average lengths of the major to minor axes of filler particles is called the “aspect ratio”. The most used fillers have aspect ratios of less than 4 : 1. Reinforcements such as glass or metal fibers generally have aspect ratios in excess of 10 : 1. Vinyl floor tile made by calendering tolerates filler particle sizes up to 99% through a U.S. Standard 50 mesh screen having 297 micron openings (11.7 mils). Typical electrical insulations and cable jackets, which are extruded, require fillers with an average esd of 3 microns or less and coarsest particles of 12 microns diameter (0.47 mils). Cable jackets designed to give low HCl emission on burning generally use precipitated calcium carbonates having 0.6 micron esd. The best filler particle sizes for most flexible PVC applications are determined by experience, in optimizing end-use properties and minimizing cost. The softest non-carbonate filler used in flexible PVC is talc represented as 3 MgO · 4 SiO2 · H2O. Zero or very low content of asbestos-related minerals is specified for talcs used with PVC. Talc is often added to calendering formulations to reduce plate-out on the rolls and to extrusion formulations to reduce plate-out on screws and dies. Talc may also be dusted at 0.1 to 0.25% onto PVC compound cubes or pellets to improve flow in bulk handling systems and hopper cars. Mica is added to PVC compounds to impart a non-blocking surface and to provide stiffening when that is also desired. Typical grades used in non-blocking calendered films are fine-ground so that > 99% passes a 325 mesh screen (with openings of 1.7 mils or 44 microns). Diatomite (amorphous silica) is added to PVC plastisols to increase viscosity and yield value and to reduce surface gloss after fusion. Fumed silica may be added to hot-processed compounds as a scrubbing agent and to plastisols to increase viscosity and yield value. The refractive index (RI) of flexible PVC matrices usually ranges between 1.51 and 1.53 because the RI of PVC is 1.55 and that of typical phthalate plasticizers ranges between 1.48 and 1.50. TiO2, with an RI of 2.76 for rutile, is a strong pigment, which contributes a high degree of opacity. Calcium carbonate (calcite), with an RI of 1.65, is a weak pigment as well as a filler for flexible PVC. Barium sulfate (Barytes), with a slightly lower RI (1.6) than calcite, may be used in translucent flexible vinyl compounds, but allowance must be made for its high specific gravity (4.5). The high gravity is an advantage for use in sound-absorbing and visco-elastic damping compounds. Clear vinyl compounds are generally unfilled. The principal advantages of inorganic fillers in flexible PVC include cost reduction, stiffening, reducing coefficients of thermal expansion, and contributing to better flammability behavior. Specific heats per unit volume are comparable for most fillers and many polymers. The disadvantage of using high levels of fillers in flexible PVC is the reduction of tensile and tear strength, elongation at failure, toughness at low temperatures, abrasion resistance, and resistance to attack by moisture and chemicals. High filler levels also compromise processability by increasing melt viscosity.