Since the food contact plastic packaging is food safety, it requires the colorant should also be food safety. Currently, too many colorants are developed for food safety plastic packagings, including a wide range of colors. These colorants are non- toxic and already applied for many years.
The following points should be considered mainly for colorants for food contact plastics, including toxicity, thermal stability, solvent resistance, color strength, etc.
This first point is the colorant should not be toxic. The general important parameter of toxicity is LD50 Oral – Rat. Normally the value of colorant for food contact plastics is more than 1,000 mg/kg at least. Some colorants values are more than 10,000 mg/kg. A compared value is LD50 Oral – Rat of Ethanol is 10,470 mg/kg. It can be simply understood that for a human with weigth 50kg, eating 50g this colorant (LD50 Oral – Rat 1,000 mg/kg) one time can cause severe poisoning. There is a genearl rule that the addition ratio of coloant to plastics is 1%. So for a 10g bottle, the total content of colorant is 0.1g. Therefore, when only taking about the colorant, if one person is poisoned, he/she need eat 500 pieces of bottles one time. The actual situation is that colorant is in the plastics and the amount leached out is at most only a few minigrams (mg). And the amount swallowed by a person is at most a few micrograms (μg) per day. Body will eliminate such tiny danger via metabolism easily. So the toxicity can be ignored.
The second point is thermal stability. Because the colorant will be mixed with plastic raw materials and heated during the plastic molding processing, the chemical must bear the temperature of melted plastics. The temperature of molding processing for different plastics is different. Example, LDPE need only 160 to 220 ℃, while PET need even 270 ℃. It means colorant for LDPE may not be used for PET. But no worry, now suppliers of commercial colorants will guide the applicable plastics for the colorants.
The third point is solvent resistance. When a colored container is filled with liquid, we don’t hope the color goes into the liquid. Normally the colorant for food contact plastics is pigment* which means these colorants are insoluble in water, in other general solvents including alcohols and hydrocarbons, and even in diluted acids and alkalis. Normally the pigment is just dispersed in the plastics. However, some special liquids can reduce the adhesion between pigment and plastics. Therefore, very little amount of pigment dissovles and then the color of liquid changes. However, we don’t need worry about this problem too much because the amount of the pigment in the liquid is still very low and will not cause poisoning (See the above calculation about toxicity). Beside the solvent resistance, the color may also migrate between solid plastic units. This parameter is called bleed resistance. Normally, solvent resistance is more important than bleed resistance because migration of color to liquid is easier than to solid. *Pigment refers the colored chemicals insoluble in solvents, while dye refers those soluble in solvents.
Last point is color strength. It includes chroma and dispersibility. If the color strength is weak, it means more colorant need be added to achieve the desired color appearance. A genearl rule that the addition ratio of coloant to plastics is 1%, so some colorants with weak color strength is not good if the plastic container requests high strength of color. The particle size of pigment can affect the tinting strength and then the opacity. Smaller particle size of pigment means better dispersibility, which increases the tinting strength and the opacity. However, if the particle size is less than 0.2 micrometer, opacity will decrease again because of decreased dispersibility due to self agglomeration of pigment. The best range of particle size is 0.2 ot 0.5 micrometer. Sometimes, not only one colorant is added in the plastic. According to the color feature, some colorants are suitable for mass tone, while some colorants are suitable for correction of tint and shade. Another paremeter is lightfastness, which means the color will not decrease along with time.
The above just includes the main points for colorants for food contact plastics. Of course there are other points but they are less important than the above points.
There are some regulatory compliances or standards for the colorant for food contact plastics. Mainly, it is 21 CFR 179.3297 ” Colorants for Polymers” in US, European Plastic Regulation (EU) No. 10/2011 and the Council of Europe Resolution AP89(1) in Europe.
Colorants are classified as inorganic and organic colorants. However, normal plastic companies don’t need pay more attendtion to the inorganic or organic types. Just know one plastic can be colored with its suitable colorants. Too many colorants are commercialized already. BASF’s brochure just introduces almost 300 his commercial colorants. I will introduce a few most general colorants in another article.