Higher cost of polymers has reduced processor margins. The present razor thin margins, coupled with high polymer prices over the past two years, have led processors to increase loadings of calcium carbonate (CaCO3) by about 10%. Processors are successfully making good quality bags containing 15-20% CaCO3. Evaluations have been done at optimum conditions using 100% prime virgin indicate that filler levels in bags could go as high as 30%. In non-bag applications, films contain up to 30% filler, but they must meet lower specs than bag films. To impart breathability, oriented and cavitated hygienic blown films contain 40-50% CaCO3. In Europe, blown film used to wrap blocks of butter that is filled up to 60% with CaCO3 use a highly refined filler costing two or three times more than standard film grade. Film masterbatches contain 70-80% CaCO3 by weight are supplied by Ampacet.

The filler is generally dosed through a concentrate. The carrier resin in concentrates is invariably LLDPE. Tests showed that using a carrier with close to the same viscosity as the base resin in the bag: 0.7 MI HDPE or 0.9-1.5 MI LLDPE improves physical properties of the final product. The two main mineral suppliers: Omya and Imery, have been responsible for the recent biggest improvements in concentrates, from which have developed new grades with narrower particle-size distribution and surface coatings specifically for film. Omya also selects mineral deposits with brighter and bluer color for use in film. In North America , ground CaCO3 for film comes from marble; in other parts of the world it is derived from chalk, limestone or marble. All three are chemically identical, but chalk and limestone are geologically younger and must be pretreated to remove moisture before being converted into concentrate. For bag films, the CaCO3 particle size should be 1-2 microns and coated with 1.0-1.2% stearic acid to make it hydrophobic so the mineral disperses and wets out in PE. Agricultural films and tarpaulins, which are heavier gauge, may use CaCO3 with up to 3 micron median size. Larger particles may not need surface treatment to aid wet-out, thereby cutting the cost. On the other hand, films containing larger CaCO3 particles can be scratchy. CaCO3 use in agricultural films is also limited because it causes thermal degradation.

The increased density of filled film results in increased weight at the same bag thickness and fewer bags for a given weight. Both factors limit filler use. CaCO3 also affects gloss and clarity, making it's use redundant for glossy high-end merchandise bags or clear packaging. Because it adds opacity, less filler can be used in natural films than in pigmented ones. Coextrusion can add gloss to filled film. More CaCO3 can be added to LLDPE (14-20%) than to HMW-HDPE (8-15%) and more to thicker films than thinner ones. Some types of LLDPE can take higher loadings than others, depending on the bag properties required. Certain combinations of base resin, blow-up ratio and mineral loading produce higher dart impact without a loss in tensile yield strength.

Tensile strength at yield, which is critical for grocery and trash bags, is affected by CaCO3 content, but the effect depends on the loading and resin type. Adding 5% CaCO3 improves the tensile yield strength of butene LLDPE in both MD and TD. But at 20% CaCO3, butene LLDPE's tensile yield is pretty much the same as that of neat resin. Tensile yield for octene and hexene LLDPE also show the biggest gains at 5% CaCO3 and increase only slightly more at 20% filler. Interestingly, at these filler levels, the tensile yield never drops below that of neat LLDPE resin.
On the other hand, LLDPE film properties like puncture and tear resistance improve at higher loadings of CaCO3 and deteriorate at lower loadings. At 11-25% CaCO3, dart impact improves over neat resin, but it drops below neat resin at filler levels up to 10%. Dart impact can also improve dramatically with higher loadings, depending on the resin type. At 20% CaCO3, dart impact for a 15-micron butene-copolymer LLDPE is 100 gm, slightly higher than that of unfilled film at 75 gm. Hexene copolymer, however, gets a big boost in dart impact with 20% CaCO3�rising to nearly 500 gm from 150 gm unfilled. Octene LLDPE also jumps to 500 gm dart impact with 20% CaCO3, from 200 gm unfilled.

A higher density of filled film will have an obvious impact on productivity measured in kg/hr. CaCO3 has a specific gravity of 2.71 gm/cc, approximately three times that of PE, which has a density of 0.92 to 0.97. But output of linear meter/hr of film also increases because the CaCO3 raises heat transfer� it heats and cools faster than PE. CaCO3 has five times the thermal conductivity of PE, so compounds with CaCO3 melt and solidify faster than unfilled resin. One can anticipate for every 1% increase in CaCO3, one can get 1% more lineal meter/hr output. Faster cooling means the frost line comes down, so the bubble is more stable, a further advantage for LLDPE processors, whose output is often limited by bubble stability. Because of this effect, adding 25% CaCO3 can boost output with certain LLDPE by as much as 50%. Output increases more with a standard smooth bore on the extruder than with a grooved feed throat because the grooved-feed extruder is already feeding at a higher rate. In the smooth-bore machine, pumping occurs after the material has melted, so faster-melting material pumps more efficiently.

For LLDPE, output improvement depends on the comonomer. Adding 20% CaCO3 raises extrusion output 22% for hexene copolymer, 39% for octene, and 47% for butene. Though output improves the most with butene, properties improve most with hexene. Volumetric expansion in the extruder and die swell are different with higher loadings of CaCO3, which doesn't expand much when heated. Neat LLDPE swells from a solid density of 0.920 gm/cc to 0.70 gm/cc in the melt. LLDPE with 25% CaCO3 is 1.06 gm/cc when cold and 0.85 gm/cc when melted.
PE with higher loadings of CaCO3 also extrudes at lower pressure and uses lower motor amperage, which means screw speed can be raised to increase lineal output of film without using more energy. There also are fewer amp and pressure spikes with more CaCO3, so processing is easier.

Test results indicate that as CaCO3 content goes up from 10% to 20% in HMW-HDPE film, screw speed can be increased while motor amps either remain the same or actually decrease. At screw speeds of 70 and 115 rpm, amperage was nearly identical with 10% and 20% CaCO3. Output shoots up with the higher loadings at both screw speeds and the same or lower amperage.

Converting processes like high-speed printing and bag making will run faster with more highly loaded film because CaCO3 evens out film gauge. Increased coefficient of friction of filled HDPE bags also makes them easier to stack.
Adding higher levels of CaCO3 to film can reduce the amount of slip and antiblock additives and colorant needed, cutting costs even more. A processor adding 20% CaCO3, for example, can reduce the slip level, depending on gauge and application. Using less slip also improves the plant atmosphere because it generates less blue haze and dust in bag making. Blocking resistance improves so much with higher levels of CaCO3 that no antiblock may be needed at all. Octene LLDPE bags open readily with 5% or more CaCO3, while hexene LLDPE bags open easily with 10% CaCO3 and higher, and no antiblock.

CaCO3 is a natural whitener, so it extends and brightens pigments. With 7-10% CaCO3 in HDPE, TiO2 levels can be reduced by about 25%, a substantial saving for white T-shirt bags. Some pigments, however, may require higher levels when used with CaCO3. Red can liners for medical waste, for example, will turn pink with CaCO3, so they may require more red pigment or a darker red to mask the CaCO3.

Processors say blown film lines require no modifications to run CaCO3, other than an additional additive feeder. Screw wear increases, but not much at lower CaCO3 loadings. CaCO3 is mildly abrasive, but much less so than the antiblocks (silica and diatomaceous earth) or TiO2 that it replaces. CaCO3 has a Mohs hardness of 3 vs. 7 for diatomaceous earth or 5.5 for TiO2.

Some processors using 10-15% filler say they see increased wear; others say they don't. The effect on dies is a mild scrubbing and purging. Machine suppliers say higher filler loadings may require longer L/D (25 to 30:1) and barrier screws with better mixing. Some HDPE film companies have extended their L/D and added barrier screws to facilitate mixing. Extruders, for filled PE film are typically 25:1 L/D. Some film producers even use screw with L/d as high as 30:1.
More rapid heating and cooling with CaCO3 means bags seal at lower temperatures and behave differently in sealing machines. Operators are used to turning up the heat to correct sealing problems, but with high CaCO3 levels, they may have to turn the temperature down instead. CaCO3 improves ink adhesion so that highly filled bags may not need corona treating for simple printing jobs where a Scotch tape adhesion test isn't required.

How much calcium carbonate goes into a given bag film depends on application, resin, gauge, color, and, more than anything else, on how the bags are sold�by gauge, weight, or unit count.