Colorants.pdf

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COLORANTS

Introduction

Colorants for plastics can be grouped into two broad categories: pigments and

dyes. Pigments are organic or inorganic colored, white, or black materials. They

are nearly insoluble in plastic. Dyes are intended to dissolve or go into solution

in a given polymer. Physical forms for dyes and pigments can range from dry

prills or powder to liquids. These forms can be used as is or can be preincorpo-

rated into compatible dispersions. The colorant supplier or concentrate supplier

can manufacture predispersions. A concentrate supplier usually adds additional

value-added steps such as additive packages and color matching.

The main considerations when selecting colorants usually include dispersion,

migration resistance, heat stability, light stability, and cost. All are dynamic and

change with concentration of the colorant, processing conditions, part thickness,

and additives. Similarly, changes in properties can be expected for a single pigment

type depending on supplier.

Dispersion

Dispersion relates mainly to pigment. The pigment manufacturing process usually

leaves a variety of particle sizes and distributions. Primary particles are true

single crystals of pigment. Nonetheless, aggregates and agglomerates are also

present. These are single pigment particles that are joined together. To provide

good color strength and physical properties these particles must be separated. Use

of wetting agents combined with shear is the key to agglomerate and aggregate

reduction.

COLORANTS

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Migration. Migration refers to dissolved portions of a dye or pigment bleed-

ing or blooming (1). Simply, the color is transported to the surface and can be

wiped off or imparts unwanted color to an adjoining part. Dyes should be thor-

oughly evaluated, as they are naturally soluble. A pigments’ tendency to migrate

increases with the processing temperatures. When pigments are being used at

the thresholds of their heat stability and/or at very low levels, caution should

be observed. Plasticizers are known to facilitate migration; therefore plasticized

poly(vinyl chloride) (PVC) is a good medium for evaluation.

Thermal Stability. Thermal stability of a colorant is important as ther-

moplastics have different melt processing temperatures and thermosets have dif-

ferent cure temperatures. Color changes in a pigmented or dyed polymer system

usually originate from one of the following mechanisms: Thermal decomposition –

degradation occurs if a pigment is processed above its decomposition temperature.

This can be rapid as in the case of pyrazolone, Pigment Red 38; it totally decom-

poses at 218 ◦ C and turns brown. Decomposition can also be a slow gradual process

as in the case of some quinacridones. They decompose slowly at 300–350 ◦ C. Chem-

ical substitution – it is less common but can occur. In PVC some of the azomethine

pigments can shift color rapidly at temperature above 160 ◦ C. The copper complex

can be substituted by atoms from barium/cadmium and lead stabilizers. Crystal

shift – polymorphous materials may undergo crystal phase transitions. Phthalo

blue is a classic example. The alpha crystal can convert back to the more stable

beta crystal. Particle size or crystal growth – pigment particles can grow in a hot

solvent. As they grow a decrease in chroma and tinting strength can be observed.

Solubility – a pigment if processed at higher than recommended temperature

range can fully or partially dissolve. The fugitive material will generally change

color and reduce the other properties of the colorant.

Light Stability and Weatherfastness. Light stability and weatherfast-

ness are deﬁned by the ability of a pigment or dye, in plastic, to retain its color

upon exposure to sunlight and/or atmospheric impacts. If the polymer is not cor-

rectly stabilized for uv it will degrade at a rate that may negate the colorants

performance. Conversely, uv stabilizers cannot greatly improve the stability of a

colorant that has poor fastness properties. Lightfastness and weatherfastness are

largely dependent on the particle size of a colorant. This is understandable as the

job of a colorant is to absorb and reﬂect light. Failure of the pigment in most cases

is not abrupt. A darkening of color occurs when used in masstone, and a loss of

chroma and strength can be observed in the tint. The color changes layer by layer.

Cost. The broad spectrum cost for colorants is from $6.00 to $200.00/kg.

Accordingly cost is as important as the above properties. Color matching and color

formulation is critical. An effective colorant package should meet the needs of the

application, provide a slight safety factor but not signiﬁcantly exceed the proper-

ties required. As with most things, “you get what you pay for.” This is also true

for colorants. In most cases, the better the properties the higher the price. When

comparing colorants for value, do not focus on the price alone, as this method can

be misleading. Focus on the strength or amount needed for a speciﬁc application,

ie “value in use.”

As in many raw materials there can be large differences in properties among

a pigment type. A single supplier can have a variety of Pigment Blue 15:3s. The

product line can cover a range of transparency, dispersions, heat stabilities, cost,

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COLORANTS

FDA approval, shade, and physical forms. By factoring in those Pigment Blue

15:3s offered by a multitude of other suppliers the number can grow exponentially.

There are many varieties of each pigment type that is discussed.

FDA Colorants. Because of the large amount of pigment used in packag-

ing, house wares, and food-processing equipment, a colorants impact on health

and safety should be considered. The U.S. Federal Government by way of the Food

and Drug Administration (FDA) regulates food additives. Pigments that are used

to color materials that come in contact with food are deemed to be indirect addi-

tives (2). This is based on the assumption that the pigment will extract from the

plastic article and become part of the food. The FDA has published a list of sanc-

tioned colorants. The list was introduced in Title 21 of CFR 178.3297 “Colorants

for Polymers” and 175.300 for resinous and polymeric coatings. CFR 178.2600 may

also be of interest when dealing with rubber articles. In very general terms, the

pigments listed in these articles are suitable as components for plastic products

used in producing, manufacturing, packaging, transporting, or holding food. In

recent years the FDA has added new colorants or expanded usage of previously

listed colorants. Some have limitations in regard to a colorants percentage and

the resin to be incorporated. Current FDA regulations should be referenced for

details.

Inorganic Pigments

m. Most plastic grades are manufactured to minimize reactivity. Reactive

sites are masked with surface coatings of alumina, silica, and silicone ﬂuid. This

process increases dispersion and improves weatherability.

Titanium dioxide has a high Mohs’ hardness and is very abrasive. Its out-

standing importance is due to its light scattering properties, its FDA approval, and

excellent properties. Its large particle size provides ease of dispersion, excellent

heat stability, migration resistance, and lightfastness at a typical cost of less than

$2.00/kg. Titanium dioxide is used in nearly all plastics to provide pastels and to

adjust colors. The opacity is valued for ascetics and its ability to absorb uv radi-

ation. In ﬁber, titanium dioxide pigments provide a matte ﬁnish that eliminates

the undesirable oily appearance caused by translucence. Rutile titanium dioxide

is the ﬁrst choice for most plastic applications. The shade is slightly yellow. If de-

sired, very small amounts of blue or violet can be used to provide a cooler white.

Anatase titanium dioxide is less yellow, not highly recommended for outdoor use,

blocks less uv radiation and is generally more reactive.

Zinc Sulﬁde. Zinc sulﬁde pigments were developed in 1850. They still have

some use in plastic because they are less abrasive and can have a smaller particle

size. However, after the introduction of titanium dioxide in the 1950s they have

continually lost market share.

Whites.

Titanium Dioxide. Titanium dioxide is the most common white of choice

and by weight; it is actually the most widely used pigment. Product selection can

be difﬁcult as the variety can be staggering. Focus on particle size and surface

coating during the selection process. Particle sizes usually range from 0.20 to

0.35

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Zinc Oxide. Zinc oxide has lost importance as a pigment but is noteworthy

for its use as an activator for accelerating vulcanization in rubber (3).

Carbon Black. Carbon blacks are not only valued as colorants but for the

functional beneﬁts they provide. They improve weatherability of plastics by block-

ing ultraviolet, visible, and infrared radiation. They can act as free-radical traps

and provide a wide range of electrical properties. Fineness, structure, porosity,

and surface chemistry are properties used in selection of a carbon black. Typical

particle size is from 18 to 80 nm. Keep this in mind when considering loading

levels. It can require a large quantity of resin to wet out the pigment.

For practical purposes there are two basic types of carbon black, channel and

furnace. Burning enriched natural gas makes channel black . Channel black has

become nearly obsolete with increased natural gas prices. They still ﬁnd some use

in plastics for FDA and special applications. Furnace black is produced by thermal

decomposition of feedstock oil for petroleum reﬁneries. Channel black has been

all but replaced by furnace black.

Iron Oxide. Iron oxide pigments ﬁnd value in plastics as they are nontoxic,

chemically stable, and low in cost, and are offered in a variety of shades. They can

be naturally occurring, natural iron oxides , or they can be synthesized, synthetic

iron oxides . There are four basic types and thus four colors of iron oxide pigments,

yellow, red, brown, and black. Synthetic iron oxides are much purer, have better

tinting strength, and vary less in composition. As a consequence, they have nearly

replaced natural iron oxides. Natural iron oxides still ﬁnd use in cellulose and

phenolics. In cases where black is required at low levels, iron oxide is often the

pigment of choice. It has much lower strength than carbon black whereby larger

amounts can be used. This allows for a more homogenous mixture and contributes

less in weighing errors. Red, yellow, and brown can be used in most plastics. They

have good durability but are dull. Use caution with the yellow (ferric hydrate) as

water can be driven at temperatures above 175 ◦ C and shift the color red.

Chromium Oxide Green. Chromium oxide green is a dull olive green.

It has excellent heat and weatherfastness. Its large particle size offers easy dis-

persion. Its reﬂectance is similar to that of chlorophyll, making it suitable for

camouﬂage (3). However, its lack of tint strength and other suitable colorants in

this color space exclude it from signiﬁcant use.

Iron Blue. Iron Blue or Pigment Blue 27 has replaced the older names of

Paris blue, Prussian blue, Berlin blue, and Toning blue, etc. It is ferric ammonium

ferrocyanide [FeNH 4 Fe(CN) 6 ]. It ﬁnds little use in plastics at present. It is mixed

with chrome yellow to form chrome green. Chrome green offers a low cost opaque

green that ﬁnds some use in polyethylene ﬁlm. Caution should be used as it has

little alkali resistance.

Ultramarine Pigments. Ultramarine pigments are a complex of alu-

minum sulfosilicate. Pigment Blue 29 is most common though; Pigment Violet

15 and pink Pigment Red 259 are also available. The blue can be used in al-

most any polymer; the pink and violet have maximum processing temperatures

of 200 ◦ C. All have poor acid resistance. Surface-treated grades do provide better

acid resistance and dispersion. They do not promote shrinking or warpage and

are approved worldwide for coloring of food-contact plastics. The most popular is

blue. Blue has its uses in plastics but is now often replaced by phthalocyanines

that have 10 times the tint strength.

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COLORANTS

Mixed Metal Oxides. The term MMO (mixed metal oxides) denotes a pig-

ment that crystallizes in a stable oxide lattice. They are manufactured by heating

combinations of metal salts to temperatures of 800–1400 ◦ C. They are regarded

as solid solutions and so some prefer to call them “complex inorganic color pig-

ments.” The following are some of the colors produced: Cobalt blue – Pigment Blue

28 (CoAl 2 O 4 ) and Pigment Blue 36 (Co(Al,Cr) 2 O 4 ), cobalt green – Pigment Green 50

((Co,Ni,Zn) 2 TiO 4 ), zinc iron brown – Pigment Yellow 119 (ZnFeO 4 ), spinel black –

Pigment Black 28 (Cu(Cr,Mn) 2 O 4 ) and Pigment Black 22 (Cu(Fe,Cr) 2 O 4 ). nickel

rutile yellow – Pigment Yellow 53 and chromium rutile yellow – Pigment Brown

24 are commonly referred to as titanates.

Compared with organic pigments they lack clean hues and tinting strength.

Compared with most inorganics they are considered expensive and hard to dis-

perse. Nonetheless, they are indispensable in high heat applications and their

weatherfastness and chemical resistance are outstanding. Use in engineering

resins for automotive interior and under-the-hood application is common. MMOs

are also important tools for coloring rigid PVC siding.

Lead Chromates and Lead Molybdates. Lead chromates and lead

molybdates are characterized by their bright hues and good opacity. The pig-

ments of importance are chrome yellow – Pigment Yellow 34 and molybdate red

and orange – Pigment Red 104 . Because of the toxicity of lead and hexavalent

chromium these pigments are forbidden from many uses. As a result, formu-

lation into plastic has declined at a considerable rate. They are susceptible to

acids, alkalis, and hydrogen sulﬁde. Surface treating of the pigments improves

the weathering, chemical resistance, and heat stability. They are readily blended

with each other and are often shaded with quinacridones to provide a variety

of colors. Environmental regulations have caused a signiﬁcant downturn in use.

They are/have been replaced by high performance inorganic pigments where appli-

cable.

Cadmium Pigments. Among the inorganic pigments, cadmium pigments

are known for their bright shades. Their physical properties allow their use in most

plastics. They are very heat stable, weatherfast, chemical resistant, and easy to

incorporate. They also show good dimensional stability in large injection molded

parts. These pigments are sensitive to overgrinding. The colors range from yellow

through maroon (yellow, orange, red, bordeaux).

The pigments are cadmium sulﬁdes and selenides. These raw materials are

precipitated, dried, and calcined at 600 ◦ C (3). They usually undergo a dilute acid

wash to remove impurities. Cadmium orange ( Pigment Orange 20) is pure cad-

mium sulﬁde. Cadmium yellow ( Pigment Yellow 35 ) is cadmium sulﬁde mixed with

crystals of zinc cadmium sulﬁde. Cadmium red ( Pigment Red 108 ) is produced in

similar fashion to the yellow. Selenium powder is added to obtain the desired

shade. The maroon is manufactured in the same manner as the red, selenium

replacing the sulfur, at 50 mol% the shade shifts to a maroon.

Lithapones are coprecipitates with up to 60% barium sulfate. The lithapones

are offered in the full color range and provide slightly better dispersion and value

in use. Also available are a variety of mercury cadmiums in shades of orange,

red, and maroon. They contain mercuric sulﬁde to replace a quantity of sele-

nium. The advantage is slightly better economics and heat stability. Shortages

of raw materials have caused increases in the price of cadmiums. Environmental

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