There are a number of reasons why you might require a chemical storage tank of a certain color. Sometimes it is a question of chemical identification or a particular brand’s colors, and yet other times color can provide a functional benefit.
Small tanks or tanks stored in higher temperatures can benefit from a lighter color, since an increase of a few degrees can result in off-gassing issues in certain chemicals. Black might be the best color option for a tank stored outside in the elements, since UV exposure can negatively impact the chemical structure of polyethylene.
There are three ways to color a tank. Painting the polyethylene tank is one option, but paint can chip and applying it can be time-consuming, since it is necessary to apply many thin coats of paint to create the right barrier, and because the surface tension of polymer makes it difficult to get the paint to bond to the material.
The other two options are compounded color, where the resin is colored at the time it is made, and pigmented color, where dry color is mixed in with the resin to create color. Let’s explore five key differences between the two.
Uniformity of Color in Polyethylene Resin
Because the dry color pigment method involves adding certain amounts of dry pigment to existing resin material, it is difficult to achieve the same shade from tank to tank. Additionally, even when the dry color is mixed well into the resin, the process can create swirls or spots where the color is more concentrated.
Tanks colored through a compounding process have the color added as the resin is being made. The color is added with other additives like UV stabilizers, heat stabilizers and peroxides so that the color is part of the cross-linked resin itself. This results in a uniform color without swirls or spots.
Cost and Complication in Manufacturing
Dry color pigments tend to stain molds, which causes costly rework and cleaning of the molds. In some cases, molds have to be sandblasted to remove the pigment and return the mold to a neutral color. There are also issues involved in storing the dry pigments, which can be hazardous materials themselves.
Colored resin that has been compounded can be stored as normal resin, eliminating the need to sandblast molds or store dry pigment color. Because the extra steps are not needed, compounded resin allows the manufacturer to pass savings and efficiencies on to the customer.
Impact on Cross-linking Speed
A commonly used test to check end-processing measures of the material is the Brabender Rheology test, which measures the resistance of the materials as it goes through two opposing screws. There is a range that the resin has to fall between in order to ensure the crosslinking speed, and thus quality, of the tank’s material. A study done between Poly Processing and the Pennsylvania College of Technology (PCT) proved that material colored with dry color pigment failed to perform as well as compounded material.
Inconsistencies in Percent Gel Tests
An ASTM gel test gives a count of how much of the material is actually cross-linked after the sample is boiled in xylene for 16 hours. In cases where pigments were added to the virgin resin, the samples did not perform as well as samples colored using the compound method. The gel percentage should determine a crosslink percentage of 60% to 70%. Outside of that range a tank’s material can be brittle on the high end, unstable on the low end.
Low Temperature Impact Test Differences
The Gel Test and the Impact Test are the two ASTM D-1998 tests that properly evaluate the integrity of the plastic in regards to its cure time and correctness. In the ASTM impact test, a sample is placed in a -20 degree freezer for a minimum of two hours. The sample is then placed in a machine that drops a weighted dart onto it to measure how the material will withstand impact stress under extreme temperatures. In Poly Processing’s study with PCT, impact tests on dry color pigmented resin underperformed compared to compounded color resin.
Dry color pigment has been an accepted method of coloring plastic storage tanks for a long time. However, in addition to the points discussed above, there are other structural and chemical problems with the method that make it problematic. Adding dry color to virgin resin introduces a contaminant at the outset. The color can clump, creating lumps, and later weaknesses in the cross-linked material.
Since compounding adds color as the resin itself is created, the resin is pure and there are no concerns as to the tank’s color uniformity, and, most importantly, there are no risks to the integrity of the plastic itself.