Coloring Processes.pdf

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COLORING PROCESSES

Introduction

Coloration of plastic materials is accomplished by depositing a colorant on the

surface of the plastic part or incorporating the colorant into the plastic itself.

Standard coating, dyeing, and printing techniques are used for surface coloration

(see C OATING M ETHODS ,P OWDER T ECHNOLOGY ;C OATING M ETHODS ,S URVEY ). This

article deals with colorants that are incorporated into the plastic.

The three basic forms of colorants used in plastics are raw pigments or dyes,

color concentrates, and color compounds. Dyes and pigments are typically dry pow-

ders. Some pigment suppliers also offer single pigment dispersions containing only

pigment and polymer. These products offer increased color strength, dispersion,

and ease of use. Dyes and pigments are sold into many different levels of the

supply chain.

Pigments and Dyes. The largest volume of raw pigment and dye is sup-

plied to color houses that provide value-added products and services to molders

and extruders. In turn the extruders and molders manufacture colored parts, bot-

tles, and ﬁbers. Some of the raw pigments and dyes are incorporated directly into

ﬁnished parts via molding and extrusion; however, this is not common.

Color Concentrate. Color concentrate or masterbatch consists of a carrier

resin that is highly loaded with colorants and additives. It is designed to match a

reference colored sample when let down or reduced at a speciﬁed ratio, ie 25:1, 50:1

(resin/concentrate). When let down, the concentrate colors the resin as required.

Color Compound. A color compound is a system of colorants, additives,

and resin that requires no letdown or addition of materials. It is ready to be

processed into ﬁnished parts via molding or extrusion. A compound can be colored

with dry pigment and dye or color concentrate.

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Color Houses

Color houses offer expertise in coloring of plastics. Whether a compounder or a

concentrate house, they provide a high level of service to their customers. They

supply a colorant and resin system that will match both the shade desired and

the physical properties needed for the life of a product.

The physical form of the compound or concentrate and equipment used in

manufacturing colored plastic objects can vary. Color houses can provide liquid,

powdered, or pelletized systems that can be converted using all types of equipment

(media mills, extruders, continuous mixers). The functions and equipment typical

of a color house are detailed in the following.

Color Labs. Color matching is a complicated art and/or science. Color

matchers must balance the designers’ aesthetic desires with the engineers’ re-

quired physical properties. This is not always an easy task. It is often difﬁcult to

achieve the color and physical properties in the desired resin. Interaction between

designers and engineers provides a forum for selecting colorants and end part col-

ors based on economics and/or performance. This improves the ability to achieve

a good and reproducible color match.

Color labs are outﬁtted with laboratory size equipment that simulates the

larger machines used for production internally and by their customers. Typical

processing equipment found in the lab are small extruders, two-roll mills, ban-

burry mills, and media mills. Small rotational, injection and blow molding ma-

chines are used to duplicate the customers’ process. Instruments and computers

are required for testing physical properties and color. Most labs have a computer-

controlled color measuring system and a light booth to evaluate color. The spec-

trophotometer with computer is initially used to assist in colorant formulation

and later as a quality control (QC) tool to provide certiﬁcation of the quality of

match to standard. The light booth provides a standardized set of conditions to

visually observe color and appearance.

Product literature, provided by the pigment supplier, is kept in the color lab

for reference. General information regarding cost, compatibility, FDA approval,

heat stability, lightfastness, and migration resistance is used for initial colorant

selection. Laboratory quantities of a large variety of colorants are kept on hand.

The samples are required for physical testing, QC adjustment evaluation, and

color matching.

The color lab is usually divided into two parts: color development and quality

control. The color development group works closely with the sales and marketing

departments. Anything, from a competitive color concentrate to bottles, ﬁlms, or

injection-molded parts, can be submitted to the lab for color-matching. Pantone ®

books or chips and other color standards are also used to specify or select tar-

gets. Speciﬁcations regarding resin, letdown ratios, weatherfastness, and price

are typically submitted with the target.

In the next step, a starting formulation of colorants, additives, and resin is

developed. The computer generates formulas on the basis of a database of known

spectral curves of colorants. The formulas are reviewed to ensure that colorants

meet heat stability, weatherfastness, and migration and chemical resistance re-

quirements, at a cost that is competitive. Lab samples are produced, evaluated,

and submitted to the customer for approval. The customer can be internal or

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COLORING PROCESSES

external. Internal, if they are manufacturing an end use product, or external, if

the concentrate or compound is sold to a molder or extruder.

The QC group is focused on the evaluation of the color concentrate or com-

pound made in the plant. Grab samples are taken during the production run. The

samples (molded chips, ﬁlm, ﬁber) are prepared and evaluated versus standards.

If required, adjustments are made to ﬁne-tune color and properties.

Incorporation of Colorants. There are a variety of points in the man-

ufacturing process for the introduction of colorants. All involve similar steps:

premixing, dispersion, and letdown. Colorants, and more speciﬁcally pigments,

require more attention than other additives that can be incorporated simultane-

ously, eg plasticizers, antioxidants, ﬂame retardants, ﬁllers, and impact modiﬁers.

This is true as the manufacturing process of pigments results in different particle

sizes and particle size distributions. Primary particles (true single crystals of pig-

ment) are uniform in size, shape, and distribution but they can combine to form

aggregates and agglomerates. Aggregates and agglomerates are usually created

during the drying of the pigment. The water is evaporated and the crystals come

in contact with one another. Van der Waal, electrostatic, magnetic forces, and,

at times, atomic bonding are responsible. Aggregates are primary pigment crys-

tals randomly joined at their surfaces. Their interior surfaces are not available to

polymer or plasticizers, and aggregates as a result are difﬁcult to separate. Ag-

glomerates are primary crystals joined at corners or edges with interior surfaces

available. They are more readily dispersed than aggregates.

The ﬁner the dispersion and the better the incorporation, the lower is the

impact on a polymer’s performance. Properties like impact and tensile strength

are lowered when agglomerates or aggregates are present. Surface problems such

as specks in ﬁlm and injection-molded parts are also a result of poor dispersion.

Poor incorporation can lead to other typical color problems of low strength and

inconsistency. Processing issues such as screen pack plugging and low throughput

can also be avoided if a robust process is employed.

A robust process includes several steps: wetting the pigment surfaces, break-

ing down aggregates, and agglomerates, and distribution of the particles in the

resin. The methods to accomplish this are numerous. In general, a premix is made

and milled or extruded and then is let down and extruded, calendered, or molded

into a ﬁnal part. Many of the processes and equipments are used in more than

one phase of the coloring process.

Mixing

Mixing is usually the ﬁrst step in the manufacturing of a color concentrate or

compound. The goal of mixing is to achieve a homogeneous blend of polymer and

colorants. In a dry or liquid mix, pigments are not fully dispersed. Remaining

agglomerates preclude direct use in thin cross-section parts like ﬁlm and ﬁber.

The agglomerates are less of a problem in thick cross-section parts like injection-

molded containers. In any case the undeveloped pigment provides economic mo-

tivation to process further.

High Shear Mixing Dry Powders and Resin. High shear mixing is re-

quired for achieving ﬁber and ﬁlm quality dispersion. The shear impacts the pig-

ment particle onto the surface of the resin and waxes. The air on the surface is

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partially displaced and thus the pigment is easier to “wet out” (break down) in the

dispersion phase of processing. Powdered resin and waxes are included as carriers

for the colorants in this step.

High shear mixing is accomplished in a Henschel ® or Hobart ® type mixer.

These equipment manufacturers are known throughout the food and plastic in-

dustries for jacketed kettle-type mixers with high speed impellers. The rotors have

a mixing speed of up to 3600 rpm revolutions per minute. Most blends are run on

low speed to slowly mix the ingredients and then at high speed to homogenize. A

good vortex is required. A charge of no more than 80% of the mixer’s capacity is

good practice. Mixers of this type can also be used to completely ﬂux or melt the

resin and incapsulate the colorants.

Low Shear Mixing of Powders and Resin. Low shear mixing is suit-

able for easier dispersing inorganic pigments. These pigments have a large par-

ticle size and are in turn easy to de-agglomerate. Low shear mixing is preferred

for pearlescents and metallics as these products can be destroyed in high shear

environments. Dyes are suitable for low shear mixing, as they only need to be

distributed evenly throughout the polymer.

Low shear mixing can be accomplished with anything from a drum tumbler to

a variety of planetary mixers. Planetary mixers have low speed screws or paddles

that mix the ingredients in the bin or hopper. A charge of no more than 80% of the

mixer’s capacity is good practice.

Mixing Liquids. Carriers like mineral oil and plastisol are typical. Min-

eral oil is used in a variety of liquid color application. Plastisols ﬁnd use in calen-

dered and slush-molded PVC applications. The process provides good mixing but a

low level of dispersion. Some of these blends are suitable for end-use applications.

Most are milled to provide a higher quality of dispersion.

A spindle or cowles mixer is used to mix colorants into liquids. The spindle

is equipped with a sawtooth blade. The blade turns at 1000–5000 ft/s. Average

mixing time is an hour.

Flushing. The Sigma blade mixer is used almost exclusively by pigment

manufacturers. It is often referred to as a “ﬂusher.” It is named after the ﬂush

process, used by pigment manufacturers to incorporate pigment into a polymer.

The goal of the process is to take the pigment while in its aqueous phase and

transfer it into a plasticizer or polymer. Under temperature and shear, the pigment

has a higher afﬁnity for the polymer than water. The pigment migrates, into the

polymer and the water is “ﬂushed” (displaced) to the surface and poured off. The

lid on the mixer is closed to pull vacuum and the dispersion is dried at an elevated

temperature. It is allowed to cool and is cryogenically ground.

This process avoids drying of the pigment during its manufacture and thus

there is no opportunity to form the “hard to disperse” agglomerates. The outcome

is a highly loaded, up to 60%, pigment dispersion. It is nondusting, has excellent

dispersion, and offers high throughput rates.

Concentrate houses utilize these products as single pigment concentrates

or mix them with other ﬂushes and traditional pigments to make high quality

color matches for ﬁlm and ﬁber. Flushing is also used to increase the pigment

concentration in a typical dry color concentrate. With its high loading and low

molecular weight carrier, ﬂushing aids wetting of the dry color while at the same

time increasing the pigment loading.

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COLORING PROCESSES

Dispersion Methods for Liquids

Milling is used to disperse pigment that is incorporated into a premix. The term

“milling” is generally reserved for liquid or paste systems. There are several types

of mills used to fully disperse pigment into polymer. Three-roll mills, media mills,

and ball mills are the most common. They are used in making color dispersions

for cast acrylics, epoxies, and plastisols.

Viscosity is the key to a good grind and/or dispersion in liquid systems. The

colorant’s loading and surface area are important factors. In general, higher load-

ings of low surface area pigment and lower loadings of a high surface area pigment

are desired for a good grind. High surface area pigments can cause vehicle demand

problems. Low viscosity mixes may run with little shear.

Three-Roll Mill. The three-roll mill was designed for the ink industry in

order to incorporate pigment into liquid carriers. Its main use in plastics is for the

manufacture of high quality plastisol dispersions and liquid colors. The typical

pigment levels for these concentrates range from 25 to 50%. The pigment level is

largely dependent on its surface area and the customer’s requirements.

The mill is conﬁgured as a series of three rollers horizontally positioned.

They are separated only by a small nip, which can be opened or closed to control

the level of shear. The premix is milled and agglomerates are reduced as it is

passed through the nip from roller to roller. The ﬁnished paste is scraped off the

front roll. Several passes may be required to obtain a desired level of dispersion.

The viscous paste is packed out in buckets.

Media Mill. The media mill, formerly designed for the ink and paint indus-

tries, is used in plastics, for the manufacture of liquid color or paste dispersions.

Suitable ink or paint grade colorants along with dispersion aides are useful in

solving viscosity and ﬂocculation problems. Buehler, Netzch, and Schold are a few

of the manufacturers of media mills. A premix is pumped through a cavity con-

taining steel, ceramic, or glass shot. A variety of shot sizes are used. An impeller

agitates the shot and the impact reduces agglomerate size.

Dispersion Methods and Equipment for Solids

Two-Roll Mills. Two-roll mills are key to making high quality PVC and

rubber concentrates. These mills are easily cleaned. The batch process is ideal

for producing small volumes; thus, a variety of colors and materials can be run

without much equipment down time.

The two-roll mill rollers are parallel and horizontally mounted. The speeds

and directions of rotation are different. The gap or nip between rolls can be con-

trolled. Heat is applied or can be developed by friction. The dry premix is forced

down through the gap and allowed to form a band or sheet on one of the rollers.

There is little mixing, and so the banded material is usually cut and reintroduced

manually to promote mixing (1). Leaving the material on the mill for a longer

period of time can maximize dispersion. On occasion, the majority of the resin is

added separately and allowed to band. The colorant blend is then slowly added.

This can help contain the pigments and additives. The powders quickly stick to

the banded resin and are incorporated as the material passes through the nip.

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