Colorants change a plastic part's color but may also modify polymer chemistry, physical properties, manufacturability, and cost.
Director of Technology
Clariant Masterbatches, Americas
Edited by Jean M. Hoffman
It's no secret that today's consumers are crazy for colorful products, from toothbrushes to cell phones. But, the speed at which color preferences change can be a challenge for plastic part designers if its not addressed during the initial design phase.
A dramatic example of new and ever-changing color schemes is the colorful iMac from Apple Computer. Historically, the business machine market's color palette spanned the gamut from beige to gray. Apple Computer shocked the public during its phase two launch of the iMac by offering striking color options of grape, tangerine, lime, strawberry, and blueberry. Auxiliary equipment suppliers and vendors were caught off guard leaving no option but for them to quickly convert existing product lines to these bold new hues to meet consumer demand.
Some may say the dust had barely settled a year or so later when Apple switched yet again to a distinctly different palette — indigo, ruby, sage, and snow. The auxiliary equipment suppliers and vendors were suddenly left with vibrantly colored, out-of-date resins and part inventories and again had to scramble to reformulate materials to match the new resin pigments.
However, improvements in color concentrates or masterbatch technologies and close working relationships with colorant houses let many peripheral-equipment designers switch colors quickly and smoothly. Masterbatches mix with neat or natural colored resins at the molding machine and can generally be formulated and produced in less than a week.
In contrast, lead times for precolored resin are typically 4 to 16 weeks and often require larger material inventories. The result is a greater risk of being stuck with obsolete materials when product demand or color preferences shift. The automotive industry realized this in the late 70s, when it successfully moved to masterbatches from precolored materials.
Maintaining repeatable performance
Designers sometimes consider color issues a nuisance, treating it as an afterthought and something for the marketers to worry about. Colors and aesthetics may trigger a purchase, but if a product breaks because the resin system is adversely changed by a new or alternate colorant additive, the designer will likely be held accountable.
Changing colorants without proper testing may lead to unexpected results. Designers should consider many factors including:
Masterbatch carrier selection: Here cheap is not always best. For example, from a cost perspective, polyethylene (PE) might first appear as a good carrier resin for colorants and additives, but it is prone to migrating to part surfaces where it may peel or stick to the mold.
Pigment dispersion: Colorants are akin to a contaminant when added to a resin. Thus, particle size, consistency, and the colorant dispersion are critical in helping maintain desired physical properties of each molded product, no matter how or when they are molded.
Interfacial adhesions: Strong hydrogen bonds between base resins and pigments, Van der Waals forces, or dipole-dipole interactions are crucial to help eliminate microvoids that may act as crack initiation sites leading to part failure under applied stress.
Dispersion aid selection (functional waxes): Functional waxes modify interfacial adhesion. The wrong one will leave voids or air pockets that may compromise base resin tensile strength and elongation properties.
Dehalogenation of substituted dyes and pigments: Chlorine and bromine in dyes and pigments can react with amines in the resin, eliminating halogen and creating an acid, which may degrade materials by overpowering compounding in stabilizers.
Colorant selection: Dyes maintain base resin properties better than pigments and special effect colorants. However, each has pros and cons. For example, using pigment instead of a dye may drastically decrease material toughness.
Consumers easily spot even subtle shifts in color on display shelves and may associate color variation with poor design. And signature colors must be highly repeatable in order to maintain brand awareness by consumers. To ensure consistent color consider these factors:
Base resin variations: Base resins usually have other, potentially reactive trace chemicals such as antioxidants or antistats. Base resins from different manufacturers may look the same, but a small amount of antistat reacting with a dye or pigment may change resin color. Other color variations can be caused by flame-retardants, butadiene-based impact modifiers, surfactants, catalyst residue, stabilization types and levels, and the use of rework/regrind.Dye migration: Dyes at the correct dilution produce beautiful colors. But when they migrate to the surface, the color turns dull and muddy. In hydrophilic resins, such as nylon, water can act as a plasticizer, opening channels in the resin matrix through which the dye may migrate.
Pigment solubility in the resin: Some pigments can dissolve in certain resins causing them to act as dyes, potentially producing dramatic color shifts, such as from orange to yellow. Understanding how to match the correct pigments and resins is critical.
Pigment/resin interactions at high temperatures: Certain resin/pigment combinations can trigger acid/base or oxidation reactions. This may not significantly impact color at lower processing temperatures, but a melt temperature increase as little as 50°C — to help ease mold fill while processing glass or mineral-filled resins, for example — could change an orange part into a brown one.
TYPES OF MASTERBATCHES
A masterbatch is a premeasured mixture of concentrated additives, such as color, stabilizers, or lubricants, in a resin carrier. Masterbatches come in solid, powder, semisolid, or liquid forms.
Solid masterbatches are either resin-specific masterbatches or universal systems. In resin-specific masterbatches, colorants are compounded into a carrier resin at a high concentration and cut into micro or standard pellets. Prior to melt processing, masterbatch pellets are mixed with the base or letdown resin, which is the same polymer as the carrier. Resin-specific masterbatches are an excellent all-around choice, but may require the use of mixing screws or nozzles to maximize color incorporation.
Universal systems are similar to resin-specific masterbatches except that the carrier resins are broadly compatible with a variety of letdown resins. This makes high colorant loadings that mix and disperse easily in the letdown resin. The larger volume of letdown resin to the relatively small amount of colorant/carrier helps overshadow any incompatibilities between carrier and letdown resin. Universal carrier systems are a good compromise when molding a variety of letdown resins. One concentrate might suit multiple resins provided they all have the same base opacity and color tone.
With powder masterbatches, pigments and dyes are preblended and added to the resin directly at the processing equipment. Since there is no predispersion step, agglomerated pigments can be a problem. However, this is not a bad choice for tinting transparent resins with a dye. Be prepared to deal with a dusty mess, however.
Semisolid masterbatches call for colorants to be fluxed in a high intensity mixer with wax, forming small bearing-ball-shaped prills, flattened particles called pastilles, or conventional pellets. The concentrate generally is added separately at the feed throat of the conversion equipment. Wax-based concentrates have good incorporation characteristics, but they can cause screw slippage and their crayon effect can complicate clean up.
Liquid forms are produced by milling colorants with surfactants and a liquid carrier until they are adequately dispersed. A pump accurately meters liquid-color concentrates into molding or extrusion equipment. These colorants have high incorporation although the metering equipment is maintenance intensive and best suited for long runs of standard colors.
Highly loaded masterbatches, with base resin to concentrate (letdown) ratios ranging from 50:1 to 200:1, minimize the amount of concentrate being added to a letdown resin. A well-engineered, highly loaded system also will function as an impact modifier. This helps the letdown resin compensate for the colorant contamination. Minimizing the amount of carrier going into the letdown resin maximizes additive effectiveness.
Resin-specific carriers and precolored resins have the most consistent physical and thermal properties of colored parts. High-volume, long-running standard colors lend themselves to liquid color.
Equipment and part size may also dictate appropriate masterbatch form. It can be difficult to get good incorporation using older and general purpose machinery as well as large molding machines that operate with over-capacity barrels and large extruders running at low rpm. In these cases, use lower loaded resin-specific concentrates or universals at medium loadings.
ONE OEM'S EXPERIENCE
Engineering managers like Tom Roetker from HewlettPackard's All-In-One Div., San Diego, develop manufacturing processes that consistently produce plastic components in dozens of company-owned and contract assembly facilities and molders on four continents. The Div. designs and builds HP's OfficeJet and fax products as well as its popular multifunction machines that combine printing, copying, faxing and scanning capabilities.
According to Roetker, color consistency is critical for part-to-part match-ups on each machine and for ensuring the HP products match other OEMs' computers and peripherals regardless of origin. While HP uses precolored resins from certain producers, it began in the mid-90s using natural resin and color concentrates from Clariant Masterbatches for molded parts. HP rigorously checks and certifies every colorant lot before production.
It's far more cost efficient to use color concentrates and natural resin, Roetker says. The approach lets the company buy large amounts of natural resin and smaller batches of colorants. The natural resin is suitable for a number of different product lines and color combinations. This helps HP balance material needs with consumer demands. Using color masterbatches also helps reduce obsolete inventories as colors change.
Natural resin with color added at the machine gives more flexibility, letting HP easily switch between colors or from part to part, cutting time to market.
Roetker finds designers are varying colors to differentiate their products. Even with beige and gray, each OEM has its own particular shade. And HP must be prepared to supply its products to that OEM in a perfectly matching color. "Consumers are demanding more variety and, increasingly, are basing their decisions on product appearance. It's not just a gray box anymore."