Calcium Carbonate for HDPE Blow Molding

 

Reasons why the HDPE Blow Molding Industry is using Calcium Carbonate

• Improved ESCR even with reduced wall thickness.
• Cycle time reduction due to the higher thermal conductivity of calcium carbonate.
• Reduced raw material cost, as the calcium carbonate displaces the more expensive resin.
• Reduced wall thickness without increasing bottle weight.
• Lower carbon footprint, due to the replacement of resin with non-fossil derived calcium carbonate.
• Increased flexural and compression moduli.
• Suitable top load and drop impact performance.
• Lower machine energy usage.
• Maintains neck-finish, thread dimensions, bottle volume and weight.

Results from a joint Ampacet/Omya study

By incorporating Omyacarb® FT calcium carbonate into High Density Polyethylene (HDPE) blow molded containers, cycle time can be reduced and environmental stress crack resistance (ESCR) improved. When maintaining bottle weight, wall thickness is reduced, generally reducing crushing strength under top load while the drop impact strength is typically unchanged or slightly reduced. The bottle’s neck finish dimensions and bottle volume are maintained.

In this one of many case studies, 16 oz. Boston round bottles were consistently made to 30 +/- 0.5 grams, using Co-HDPE (0.3MI / 0.953 d).

These results represent the effects of Omyacarb® FT calcium carbonate on high-density polyethylene (HDPE homopolymer and copolymer) containers for household industrial chemicals (HIC) and other similar applications.

Calcium Carbonate and Loading Levels

Two Omya calcium carbonate products were testedjOmyacarb® FT (1.4 micron median size) and Omyacarb® 3T (3 micron median).

In these tests, the loading of calcium carbonate represents the weight percent of actual mineral particles in the bottle, not the weight percent of the masterbatch pellets (concentrate).

For example, this test used 9% of a masterbatch containing 70% of the mineral grade Omyacarb® FT in order to achieve 6% of calcium carbonate mineral in the bottle, known below as “6% FT.” Masterbatches (concentrates) are available from compounders like Ampacet.

Bottle ESCR

Omya calcium carbonate can dramatically improve ESCR. Environmental stress cracking is the number one cause of failure for many containers molded from HDPE polymer, even with mild chemicals.

Chemical agents, by diffusion, can accelerate the extension of cracks through a polymer under stress. Calcium carbonate and its effects on morphology blocks this crack propagation, creates a longer (torturous) path for crack growth, and thus can greatly prolong the time until failure. The relatively finer sized Omyacarb® FT (1.4 micron) tends to improve ESCR more than the relatively coarser sized Omyacarb® 3T (3 micron) at the same loading level.

This is because for a given loading weight, there are more particles of the smaller sized Omyacarb® FT (1.4 micron) than in the same weight of the larger particles of Omyacarb® 3T (3 micron). The higher particle count (more particles) of Omyacarb® FT creates more opportunities to block the stress crack propagation. This means further delay of ESC-failure and extended useful bottle life.

Reduced Cycle Time

Calcium carbonate allows a reduction of overall cycle time of roughly 1% for each 1% of mineral added by way of reduced cooling (blow) time. At higher loadings the cycle can be reduced by up to 2% for each 1% mineral. The main reason for this effect is the much higher thermal conductivity of calcium carbonate, as compared to the polymer.

The brighter colors in the image (above left) indicate higher temperatures for the typical “Control” bottle without calcium carbonate. Incorporating 7% of Omyacarb® FT reduces the bottle temperature profile (above right). This reduction in post-mold bottle temperature allows a reduction of cooling time (blow time) until the bottle comes out of the mold at the same temperature as the Control, before adding calcium carbonate. In this case, the blow time was reduced by ~9%, an overall cycle time reduction of 6.2%. Incorporation of ~11% of Omyacarb® FT reduced the post-mold temperature even more, allowing a further reduction of cycle time.

Dimensional Stability

Thread dimensions measured by an optical comparator showed that with or without calcium carbonate, “T” and “E” dimensions were within the error of measurement.

Shrinkage was not measurably reduced for bottles containing calcium carbonate.

The bottle volumes were within 1%, a relatively minor difference, especially for bottles filled by weight. The bottle wall thickness was as expected. A slightly reduced wall thickness comes from increasing levels of calcium carbonate, while holding the bottle weight constant.

The measured values were as predicted by calculation.

Drop Impact Resistance

Drop impact performance remained the same within experimental error, despite the reduced wall thickness of the bottles containing calcium carbonate.

All bottles passed the normal (Bruceton stair step) single drop method, each surviving a single 15 foot drop. In order to produce failures for comparison purposes, each bottle was dropped repeatedly at increasing heights until the accumulated multi-drop abuse created a leak failure.

The containers resisted about 50 to 70 lbs of crushing force. The Crush Resistance for the recommended range of up to 5.5% calcium carbonate for HDPE-homopolymer and up to 8% for HDPE-Copolymer is only slightly reduced. Because all of the bottles were made to the same target weight, the bottles containing calcium carbonate (of higher specific gravity) had slightly reduced sidewall thickness, which is responsible for reduced crushing strength. The reduction in strength is due to the reduced wall thickness, not the calcium carbonate. This also highlights the natural advantage of material savings, due to both reducing wall thickness and the partial replacement of HDPE resin by calcium carbonate.

Omya recommends that fitness for use testing be performed for containers of customer-specific size, shape and material, in order to determine the actual effects of incorporating calcium carbonate on all physical properties of interest.