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The Use of Calcium Carbonate In Molded Containers
Many producers of blow molded articles have wondered what the effect would be of adding calcium carbonate (CaCO3) to their product. When resin prices are high, the addition of minerals like calcium carbonate can reduce their overall cost. It has also been reported that the addition of calcium carbonate can improve cycle time. However, what are the trade-offs to using this mineral? How is container performance affected through the addition of relatively high levels of calcium carbonate?
Ampacet has recently concluded a study of blow molded bottles where the effect of high levels of calcium carbonate on bottle properties is measured. Variables measured in this study were demold temperature (to ascertain improvement in cycle time), bottle weight, calcium carbonate loading, and diameter swell. The properties measured were environmental stress crack resistance (ESCR), column crush, drop impact, and bottle dimensions. Two bottle styles were tested - a 16 oz Boston Round and a 1-liter Round. Bottles were produced with 7.5%, 15%, and 25% CaCO3 incorporation. For each iteration, the bottle was produced to the same target weight as the control (0.954 density, 0.3 MI HDPE copolymer) with no mineral incorporated.

Krupp-Kautex, Continuous Extrusion
6 Extruder/6-Layer Capable

 

Cycle Time Improvements
Demold temperature was measured for both the 15% and 25% loadings. Cycle times were decreased until an equivalent demold temperature to the control was achieved. The standard reduction in bottle temperature for both sets of bottles was approximately 25°F, which allowed for a 20% reduction in cycle time.

Since a high specific gravity mineral was being added to the bottles, Ampacet wanted to determine whether or not the improved cycle time was due to the thermal conductivity of the mineral, or whether it was due to the thinner bottle sidewalls (13% thinner) now needed to achieve the desired bottle weight. When the bottle wall thickness was increased to be equivalent to the control bottle, no cycle time improvement was noted. All of the cycle time efficiency that is seen from the use of CaCO3 is due to the production of a bottle with a thinner sidewall.

1-liter Round
Cycle Time (sec)

 

Environmental Stress Crack Resistance

Internal pressure ESCR (5 psi) and top-load ESCR (15 lbs top-load) were run on the 16 oz bottles produced. For both tests, performance was 75-80% worse than was seen with the control bottles.

 

Column Crush

Contrary to popular belief, the presence of calcium carbonate will act to reduce column crush in many blow molded containers. This is because crush failures will occur earlier in the thinner sidewall of a bottle possessing the mineral. Therefore, while CaCO3 can act as a stiffener in injection molded parts where wall thickness has not changed, it will not do so in blow molded articles (with the possible exception of injection blow molded bottles, which were not tested in this study). The amount of column crush loss depends heavily on bottle style, bottle weight, parison programming, and mineral loading. In this study, column crush (at the 15% CaCO3 loading level) was decreased by 30% for the 16 oz bottle, but was only slightly worse for the 1-liter bottle

Column Crush
16 oz Boston Round

 

Drop Impact
Generally, drop impact decreases as CaCO3 loading increases in the bottle. The bottles show a marked decrease in resiliency, with correspondingly lower drop impact values. Again, depending on bottle type and loading, drop impact varied from no reduction to 55% property loss.

 

Processing
Compared to the control, calcium carbonate modified samples have lower flare (diameter) swell and higher weight swell. Extruder speeds, and therefore back-pressure, were higher due to the faster cycle times made possible by running CaCO3. Fill volumes and thread dimensions were unaffected by the faster cycle times since the bottle was demolding at the same temperature as before.

 

Use of Calcium Carbonate
Obviously, molders have to be careful when utilizing calcium carbonate in containers where bottle properties are critical (which is most applications). While using calcium carbonate can potentially reduce container cost from both a cycle time and raw material cost standpoint, it cannot be done without consideration to performance properties. ESCR, column crush, and drop impact can be negatively affected, but there may be ways to minimize or negate the performance drop. Burying the calcium carbonate on the inside layer of a multi-layer bottle may reduce the issues seen with ESCR and drop impact. In addition, stiffness may not be reduced (it may increase!) on an injection blow molded container. Ultimately, the decision to use calcium carbonate will come down to a cost/benefit analysis for each container being considered for use with calcium carbonate.
One area where calcium carbonate is successfully used is as a part of current color concentrate formulations. Calcium carbonate can be added to color concentrates at levels up to 25%, and acts to replace an equivalent amount of more expensive carrier resin in the formulation. By doing this, a concentrate supplier can deliver all of the functionality intended for the concentrate, but at a lower cost for the end-user. Even at a 25% loading, calcium carbonate will be present in the final article at only 1% with typical let-down ratios. Ampacet's studies have shown that this amount of calcium carbonate will have absolutely no effect on bottle properties, either positively or negatively.

 

Calcium Carbonate Concentrates
If a molder decides to proceed with evaluations of calcium carbonate in select product lines, several products can be obtained from Ampacet. These low cost products offer high loading, high quality, and affordability. Contact your Ampacet Salesperson if you are interested. Many products are available for sampling, including 10847 and 101870, 70% and 75% CaCO3, respectively, carried in LLDPE resin. Many low-cost color concentrates are also available where calcium carbonate has been utilized to reduce formulation cost.
For more information, please contact Ampacet’s technical support team at 888-822-7546 or 812-466-9828.
Disclaimer:  The information and recommendations contained in this document are based upon data collected by Ampacet and believed to be correct.  However, no warranty of fitness for use or any other guarantees or warranty of any kind, expressed or implied, is made to the information contained herein, and Ampacet assumes no responsibility for the result of the use of the products and processes described herein.  This is an uncontrolled document and information may be out of date.