polymer additives and reinforcements
iv. alloys and blends
the traditional method of enhancing properties by adding fillers and reinforcements, while still effective in some applications, has been inadequate for coping with the increasing performance requirements of design problems and the changing material specifications. development of new resin systems to meet demands for high performance materials would undoubtedly take too long and would certainly be too expensive since it would require huge investments in totally unexplored technologies and new plant facilities. an alternative to the development of new polymers is the development of alloys and blends that are a physical combination of two or more polymers to form a new material. the basic objective
is to combine the best properties of each component in a single functional material that consequently has properties beyond those available with the individual resin components and that is tailored to meet specific requirements. another goal is to optimize cost/performance index and improve processability of a high-temperature or heat-sensitive polymer.
although the terms alloys and blends are used interchangeably, they differ in the levels of inherent thermodynamic compatibilities and in resulting properties. in general, a necessary though not sufficient condition for thermodynamic compatibility (miscibility) is a negative change in the free energy of mixing, ?gm, given by equation 9.2.
?g m ???h m ??t ?sm (9.2)
where ??h and ??s are, respectively, the changes in enthalpy and entropy of mixing at temperature t.
m m
generally because of their large size and unfavorable energy requirements, chains from a given polymer
prefer to intertwine among themselves than with those of another polymer. consequently, the magnitude
of ??s is usually small. therefore, for two polymers to be thermodynamically miscible, ??h must be
m m
negative, zero, or, at most, slightly positive. if ??h is strongly positive, the components of a physical
m
mixture separate into different phases resulting in a blend. however, some polymers tend to be mutually soluble, at least over a limited concentration range. such are called alloys. in other words, alloys represent the high end of the compatibility spectrum. individual polymeric components in alloys are intimately mixed on a molecular level through specific interactions such as donor–acceptor or hydrogen bonding between the polymer chains of the different components.
the composition dependence of a given property, p, of a two-component polymer system may be described by equation 9.3:10
p ??p 1 c 1???p 2 c 2??i p 1 p2 (9.3)
where p and p are the values of the property for the isolated components and c are, respectively,
1 2 1 , c 2
the concentrations of the components of the system. i is the interaction parameter that measures the magnitude of synergism resulting from combining the two components. if i is positive, then the magnitude of the property for the system exceeds that expected for a simple arithmetic averaging of the two component properties. the system in this case is referred to as synergistic. if i is negative, then the mixture has a property value less than that predicted from the weighted arithmetic average. this is known as nonsynergistic. polymer systems for which i is either zero or nearly zero are called additive blends. they have properties that are essentially arithmetic averages of the properties of their components (figure 9.2).
compatible polymer blends form single-phase systems and have a single property, like the glass transition temperature, the value of which is generally the weighted arithmetic average of the values of the components of the blend. at certain compositions, some compatible polymer blends exhibit strong intermolecular attraction and hence have a high level of thermodynamic compatibility. this results in properties superior to those of the individual components alone. such blends display synergistic prop¬erties (e.g., tensile strength and modulus). only a few commercially available polymers are truly compatible. some of these are shown in table 9.8. examples of the most significant commercial engi¬neering alloys are polystyrene (ps)-modified poly(phenylene oxide) (ppo) and polystyrene (ps)-mod¬ified poly(phenylene ether) (ppe).
incompatible polymer blends consist of a heterogeneous mixture of components and exhibit discrete polymer phases and multiple glass transition temperatures corresponding to each of the components of the blend. a polymer blend with completely incompatible components has limited material utility because the components separate during processing due to lack of interfacial adhesion, which is required for optimum and reproducible polyblend properties. improvement in adhesion in such blends can be effected by the addition of compatibilizers. compatibilizing agents provide permanent miscibility or compatibility between otherwise immiscible or partially immiscible polymers creating homogeneous materials that do not separate into their component parts. the most effective compatibilizing agents are generally block and graft copolymers whose polymer blocks or segments are the same as the components of the polyblend. the inherent compatibility between the segments of the compatibilizer and each component
figure 9.2 composition dependence of the property of a two-component polymer system: alloy properties better than arithmetic averages blend properties equal to or less than arithmetic averages.
table 9.8 representative miscible polymer blends
polymer 1 polymer 2
polystyrene poly(2,6-dimethyl-1,4-phenylene oxide)
poly(methyl vinyl ether) tetramethyl bpa polycarbonates
poly(vinyl chloride) polycaprolactone
poly(butadiene-co-acrylonitrile)
chlorinated polyethylene poly(ethylene-co-vinyl acetate)
poly(methyl methacrylate) poly(vinylidene fluoride)
poly(styrene-co-acrylonitrile) poly(vinylidene fluoride)
from fried, j.r., plast. eng., 39(9), 37, 1983. with permission.
of the blend effectively ties the discrete polymer phases together. it is useful to visualize the function of a compatibilizing agent as akin to that of the emulsifier in water–oil emulsified systems.
as we said earlier, the primary objectives in blending are to enhance material processing character¬istics and optimize product performance while minimizing costs. a common strategy for achieving these objectives is to combine a crystalline polymer with an amorphous polymer. the aim is to exploit the strengths of each component while deemphasizing their weaknesses. crystalline materials such as nylon, poly(butylene terephthalate) (pbt), and poly(ethylene terephthalate) (pet) offer excellent chemical resistance, processing ease, and stiffness, but suffer from poor impact strength and limited dimensional stability. on the other hand, amorphous polymers such as polycarbonate (pc) and polysulfone have outstanding impact strength and dimensional stability but poor chemical resistance. polycarbon¬ate–poly(ethylene terephthalate) blends combine the desirable properties of polycarbonates with the
table 9.9 major properties and applications of some commercially available alloy/blends
trade name/
alloy/blends manufacturer major properties applications
pc/abs cycoloy/borg-
warner chemicals
abs/nylon elemid/borg-warner
chemicals
ppe/nylon gtx/general
electric
improved no-load heat resistance, impact strength
chemical resistance, low-load warpage resistance, toughness processing high heat and chemical resistance, dimensional stability, ductility
switches, power tools, food trays, lighting fixtures, automotive interior trim, connectors business machine housings and components, industrial and mechanical parts, switches, terminal blocks, food trays automotive interior trim, instrument panel, appliance and equipment housings electroplated parts, grilles, wheel covers, appliances, instrument panels, telecommunications power tools, automotive and agricultural components automotive exterior body panels, wheel covers, mirror housings
polysulfone/abs mindel-a631/union platability, processability, toughness fda- and nsf-recognized for
carbide food processing and food service
systems
ppe/ps noryl/general
electric
pc/pbt, pc/pet xenoy/general
electric
improved processing, combination of heat resistance, excellent dimensional stability and toughness
balance of chemical resistance, toughness, low-temperature impact strength and high temperature rigidity
computer and business equipment housings, automotive instrument panels, interior trim, connectors, electrical housings, medical
components
automotive, lawn and garden materials handling, sporting goods, military
from wigotsky, v., plast. eng., 42(7), 19, 1986. with permission.
excellent chemical resistance of pet and have been specified for use in many applications, for instance, as replacements for metal, including automotive, lawn and garden appliances and electrical/electronic, consumer, industrial/mechanical, sporting and recreation, and military equipment.11 a blend of ppo and nylon is used for fenders and rocker panels of some automobiles, applications demanding chemical-resistant performance under high impact and high heat. table 9.9 lists some alloys and blends, their characteristic properties, and areas of application.
v. antioxidants and thermal and uv stabilizers
polymers, during fabrication or storage or in service, may be exposed, sometimes for long periods, to the separate or combined effects of moderate or high temperatures, ultraviolet radiation, and air or other potential oxidants. under these environmental conditions, polymers are susceptible to thermal, uv, and/or oxidative degradative reactions initiated, in most cases, by the generation of free radicals. polymer stabilization, therefore, involves incorporation of antioxidants and thermal and uv stabilizers to mini¬mize, if not avoid, such degradative reactions.
a. polymer stability
polymers deteriorate through a complex sequence of chemical reactions resulting from the separate or combined effects of heat, oxygen, and radiation. in addition, polymers may be susceptible to attack and mechanical failure on exposure to water (hydrolysis) or a variety of chemical agents. molecular weight is changed considerably in most of these reactions by chain scission and/or cross-linking. however, deterioration can also occur without significant change in the size of the polymer molecules.
1. nonchain-scission reactions
nonchain-scission reactions resulting, for example, from the application of heat, involve elimination of a
small molecule — usually a pendant group — leaving the backbone essentially unchanged (equation 9.4).
ch2 ch ch2 ch ch2
r r r
vinyl polymers are particularly susceptible to thermal degradation. a typical example is rigid pvc, which is impossible to process under commercially acceptable conditions without the use of thermal stabilizers. unstabilized pvc undergoes dehydrochlorination near the melt processing temperature. this involves liberation of hydrochloric acid and the formation of conjugated double bonds (polyene forma¬tion). the intense coloration of the degradation products is due to polyene formation. a second example of a polymer that undergoes nonchain-scission reaction is poly(vinyl acetate) or pvac. when heated at elevated temperatures, pvac can liberate acetic acid, which is followed by polyene formation.
2. chain-scission reactions
the chemical bonds in a polymer backbone may be broken with the generation of free radicals by heat, ionizing irradiation, mechanical stress, and chemical reactions (equations 9.5).
h
ch2 ch ch2 ch ch2 c + •
• ch2 ch
r r r r
in the thermal degradation of polyethylene and polystyrene, for example, chain scission occurs through the homolytic cleavage of weak bonds in the polymer chain. this results in a complex mixture of low-molecular-weight degradation products. in some cases, particularly 1,1-disubstituted vinyl polymers, the long-chain radical formed from initial chain scission undergoes depolymerization resulting in the reduc¬tion of molecular weight and formation of monomers. in a few cases, e.g., pmma, the initial chain scission occurs at one end of the molecule and the subsequent depolymerization results in a gradual decrease in molecular weight. in other cases, e.g., poly(?-methylstyrene), the initial chain scission occurs at random sites and as such there is an initial rapid reduction in molecular weight.
3. oxidative degradation
in the presence of oxygen or ozone, as soon as free radicals form, oxygenation of the radicals gives rise to peroxy radicals, which through a complex series of reactions result in polymer degradation. oxidative degradation may occur at moderate temperature (thermal oxidation) or under the influence of ultraviolet radiation (photooxidation). unsaturated polyolefins are particularly susceptible to attack by oxygen or ozone (equation 9.6).
ch3 o ch3 o
4. hydrolysis and chemical degradation
in addition to the separate or combined effects of heat, oxygen, and radiation, polymers may deteriorate due to exposure to water (hydrolysis) or different types of chemical agents. condensation polymers like nylons, polyesters, and polycarbonates are susceptible to hydrolysis. structural alteration of some poly¬mers may occur as a result of exposure to different chemical environments. most thermoplastics in contact with organic liquids and vapors, which ordinarily may not be considered solvents for the polymers, can undergo environmental stress cracking and crazing. this may result in a loss of lifetime performance or mechanical stability and ultimately contribute to premature mechanical failure of the polymer under stress.
b. polymer stabilizers
the two main classes of antioxidants are the free-radical scavengers, (primary antioxidants, radical or chain terminators) and peroxide decomposers (secondary antioxidants or synergists). free-radical scav-engers, as the name suggests, inhibit oxidation through reaction with chain-propagating radicals, while peroxide decomposers break down peroxides into nonradical and stable products. commercial antioxi¬dants include organic compounds like hindered phenols and aromatic amines, which act as free-radical scavengers, as well as organic phosphites and thioesters that serve to suppress homolytic breakdown.
thermal stabilizers may be based on one or a combination of the following classes of compounds: barium/cadmium (ba-cd), calcium/zinc (ca-zn), organotin, organo-antimony, phosphite chelates, and epoxy plasticizers. ba/cd stabilizer systems, which represent the largest share of the pvc stabilizer market, are available as liquids or powders. the liquids normally employ one or more of the following anions combined with the respective metal: nonyl phenate, octoate, benzoate, naphthenate, and neoda¬canoate. conventional ba/cd powder stabilizer systems have one or more of the anions of the following acids: stearic, palmitic, and lauric. the powder ba/cd stabilizers, which act both as thermal stabilizer and lubricant, are normally employed in rigid and semirigid products. ca/zn stabilizers represent a small volume of the stabilizer market. the best known ca/zn stabilizer systems are associated with fda-sanctioned products, including blown films, medical and beverage tubing, blood bags, blister packs, and bottles. the organotin compounds are butyl-, methyl-, and octyltin and are available in liquid and solid forms. most organotin stabilizers are used in rigid extrusion and injection molding processes for such products as pvc pipes, profiles, and bottles as well as fittings, siding, and clear pvc bottles.
uv radiation in the range 290 to 400 nm has potentially degradative effects on polymers since most polymers contain chemical groups that absorb this radiation and undergo chain scission, forming free radicals that initiate the degradative reactions. uv stabilizers are employed to impede or eliminate the process of degradation and, as such, ensure the long-term stability of polymers, particularly during outdoor exposure. light stabilizers are typically uv absorbers or quenchers. the former preferentially absorbs uv radiation more readily than the polymer, converting the energy into a harmless form. quenchers exchange energy with the excited polymer molecules by means of an energy transfer mech¬anism. other uv stabilizers deactivate the harmful free radicals and hydropingeroxides as soon as they are formed. pigments offer good protection for polymers by absorbing uv radiation. carbon black, used widely in tire manufacture, absorbs over the entire range of uv and visible radiation, transforming the absorbed energy into less harmful infrared radiation. pigments and carbon black cannot be used, unfortunately, in applications where transparency is required. in these applications, stabilizers that contribute minimal color or opacity are used. for example, transparent polymers like polycarbonate can be protected against yellowing and embrittlement from uv light (photolysis) by incorporating com¬pounds like benzophenone derivatives (e.g., 2-hydroxybenzophenone) and 2-hydroxybenzotriazoles. these highly conjugated compounds are able to convert absorbed uv radiation to less harmful heat energy without chemical change by forming a transient rearranged quinoid structure that changes back to the initial form. other uv stabilizers include acrylic and aryl esters, hindered amines, and metal salts. unlike conventional uv stabilizers, the metallic complexes interact with photoexcited polymer mole¬cules, deactivating them by dissipating excess energy as infrared radiation.
vi. flame retardants13,14
most materials, synthetic or natural, burn on exposure to temperatures that are sufficiently high. the response of polymers to high temperatures depends on their formulation and configuration in the end-use situation. the essential goal of flame retardancy is to preserve life and prevent or at least minimize damage to property. therefore, the function of flame retardants in a resin formulation is ideally the outright inhibition of ignition where possible. where this is impossible, a flame retardant should slow down ignition significantly and/or inhibit flame propagation as well as reduce smoke evolution and its effects. the presence of flame retardants also tends to cause substantial changes in the processing and ultimate behavior of commercial resins.
the burning characteristics of polymers are modified by certain compounds — including alumina trihydrates bromine compounds chlorinated paraffins and cycloaliphatics phosphorus compounds, notably phosphate esters and antimony oxides, which are used basically as synergists with bromine and chlorine compounds. the halogens are most effective in the vapor phase they act in the flame zone by forming a blanket of halogen vapor that interferes with the propagation of the flame by interrupting the generation of highly reactive free radicals, thus tending to quench the flame. others such as phosphorus or boron operate in the condensed or solid phase, minimizing the availability of fresh fuel. they form a glaze that limits the heat and mass transfer necessary for flame propagation and/or lowers the melt temperature of the polymer causing it to flow away from the flame. table 9.10 shows the characteristics and end use of various flame retardants.
flame retardants may be classified as additives, reactives, intumescents, and nonflame-retardant systems based on their method of incorporation in the resin formulation or their mode of action. additive flame retardant systems can be further classified essentially as fillers, semiplasticizers, or plasticizers depending on their melting points and area of application. typical additive flame retardants are haloge¬nated additives used alone or in synergistic combinations with antimony oxide (with ps, pp, polyester, and nylons), phosphate esters, mineral hydrates, boric acid, sodium tetraborate, and ammonium bromide. additive flame retardants are used with both thermoplastics and thermosets. the use of additive flame retardants involves the addition of very high-melting inorganic materials that will reduce combustibility. this is based on the rationale that the fewer the combustibles, the less burning will occur. alumina trihydrate, which dominates the flame retardant market in terms of sales volume, serves this function. in addition, it liberates water at high temperatures and this further dampens flame energy. alumina trihydrate is a typical dual function filler additive (like calcium carbonate and clays), which provides noncombustible filling plus gas or moisture release at elevated temperatures. alumina trihydrate is used as a primary flame-retardant additive in carpet backing, in electrical parts, and in applications employing glass-fiber-reinforced unsaturated polyester. it is also used in sheet and bulk molding compounds for electrical enclosures in business machine and computer housings, and in laminated circuit boards. alumina trihydrate offers low-cost flame retardance in epoxy systems and for wire and cable insulation, cross-linked pe and ethylene vinyl acetate copolymers and for flexible polyurethane foams.
flame retardants that melt or flux at or near the polymer processing temperature are known as semiplasticizing additives, while those that truly flux during processing are referred to as plasticizing additives. these types of flame retardants can be tailor-made for a specific polymer. plasticizing flame retardant filler additives provide beneficial effects in the processing of rigid polymers and quite often improve the physical properties of the polymer. commercial examples of plasticizing filler additives include phosphate systems in polyphenylene oxides, phosphates or chlorinated paraffins in vinyl chloride and vinyl acetate polymers, and octabromobiphenyl oxide and brominated aromatic compound in abs.13
in reactive flame retardants, retardancy is built into the polymer chain through appropriate polymer¬ization or postpolymerization techniques. for example, the current additive method of producing flame retardant high-impact polystyrene involves blending decabromobiphenyl oxide and antimony oxide to produce a blend with 8 to 10% combined bromine content. reactive flame retardants with the same bromine content can be obtained either by brominating the resin or copolymerizing styrene and bromi¬nated styrene. reactive flame retardants include halogenated acids such as tetrabromophthalic anhydride and tetrachlorophthalic anhydrides, halogenated or phosphorated alcohols (e.g., dibromoneopentyl gly¬col, dibromopropanol) and halogenated phenol bisphenol a. these materials are usually incorporated into the polymer molecular structure through copolymerization with other reactive species.
the use of reactive flame retardants has a number of advantages over that of additive flame retardants. with flame retardants as an integral part of the polymer chain rather than incorporated as an additive,
table 9.10 (continued) characteristics and applications of flame retardants
phosphorus compounds antimony oxide
resin end use resin end use
markets
electrical modified polyphenylene oxide (ppo) connectors, terminal strips, etc. nylon 6/6, 6, pbt, pet connectors, switches, circuit boards, terminal strips
electronic housing and enclosures modified ppo business machines abs, hips business machines
wire and cable pvc communications cable pvc, xlpe, ldpe, epdm pvc building wire, similar uses as for bromine and chlorine compounds
appliances modified ppo kitchen appliances, tv cabinetry hips, abs, polypropylene tv cabinetry, power tools, kitchen appliances
building and construction rigid polyurethane foam, pvc, unsaturated polyesters thermal insulation, wall coverings, flooring, sanitary ware, laminates unsaturated polyester, rigid pvc, flexible pvc building panels, panels, windows, insulation covering
transportation
(automotive) pvc, flexible polyurethane foam, rigid polyurethane foam upholstery, seat cushioning, thermal insulation (trucks)
characteristics phosphorus promotes char formation to protect substrate, and halogen
acts in vapor phase. good thermal stability. process with modified ppo up to 550–600°f. flame-retardant mechanism: condensed phase. flame retardant induces reactions in host resin that lead to charring and insulation against further burning. inert material by itself. must be used with halogen-type compound. used in all polymers where halogen compounds are the selected flame-retardant system. as a synergist, enhances efficiency.
from wigotsky, v., plast. eng., 41(2), 23, 1985. with permission.
the loss of performance of the base polymer due to diluents (other constituents) in the resin formulation is minimized. in addition, migration and/or bloom are completely eliminated while compounding costs are substantially reduced. leaching of reactive flame retardants by solvents and corrosives is relatively more difficult than leaching additive retardants.
reactive flame retardants are used mainly with thermoset resins, particularly unsaturated polyesters, both reinforced and unreinforced. a technique for introducing flame retardants into unsaturated polyesters involves the use of compounds with residual unsaturation such as diallyl tetrabromophthalate, which is used as a cross-linking agent alone or with styrene.
intumescent flame retardance is based on the formation of a char on the surface of the resin on the application of heat, consequently insulating the substrate from further heat and flame. phosphorous compounds such as inorganic or organic phosphates, nitrogen compounds such as melamine, and poly¬hydroxy compounds — usually pentaerythritol — are used in intumescing formulations. high loadings, are usually required to achieve the required level of fire retardancy. this tends to degrade the physical properties of the base polymer. intumescent flame retardant systems are generally limited to polymers with low processing temperature (e.g., pp) because they expand considerably on application of heat.
nonflame-retardant systems are polymeric systems that inherently have some level of flame retardance and therefore do not require additive or reactive flame retardants. examples include pvc and its compounds, poly(vinylidene chloride) films and compounds, phenolic foams, amide-imides, polysul¬fones, and poly(aryl sulfides).
vii. colorants15,16
as we said earlier, very few polymers are used technologically in their chemically pure form it is generally necessary to incorporate various additives and reinforcements to assist processing and achieve desired properties. unfortunately, these components also often produce a significant amount of undesirable color and opacity in the resin. each resin itself has its color that may vary from grade to grade or batch to batch. for example, polystyrene crystal is transparent, whereas high-impact polystyrene has a white, somewhat translucent, appearance while the common grade flame-retardant polystyrene is opaque. the color of general-purpose abs is off-white and opaque. glass fibers, the most common reinforcements added to nylons and polyesters, darken the color of these resins.17 the marketability of a polymer product quite frequently depends on its color therefore the purpose of adding a colorant to a resin is to overcome or mask its undesirable color characteristics and enhance its aesthetic value without seriously compro¬mising its properties and performance.
colorants are available either as organic pigments and dyes or inorganic pigments. they may be natural or synthetic. by convention, a dye is a colorant that is either applied by a solution process or is soluble in the medium in which it is used, while pigments are generally insoluble in water or in the medium of use. dyes are generally stronger, brighter, and more transparent than pigments. as a result of the intrinsic solubility, dyes have poor migration fastness and this restricts their use as polymer colorants. inorganic pigments are largely mixed metal oxides with generally good-to-excellent light fastness and heat stability but variable chemical resistance. organic pigments and dyes are generally transparent and possess good brightness. the heat stability and light and migration fastness of organic pigments range from poor to very good. table 9.11 shows some colorants, their characteristics, and their applications.
colorants are used in polymers either as raw pigments (and dyes), concentrates (solid and liquid), or precolored compounds. precolored resins, solid and liquid concentrates, are all offsprings of the basic dry pigments. colorants are available in a variety of forms, including pellets, cubes, granules, powder and liquid, and paste dispersions. raw pigments are generally supplied as fine particles, which require dust control measures. to optimize color development when raw pigments are used, the size of pigment particles or agglomerates must be reduced and coated with appropriate resin. most finished colors use multiple pigments. this requires a homogeneous mixing of all the pigments in the formula in high-shear mixing equipment to produce a uniform color. precise metering into the processing machine is required to produce consistent colors since some components of the pigment system, though present in relatively small quantities, have strong color characteristics. raw colorants or pigments generally cost less than other forms of colorants, but they can be more difficult to disperse and may result in inconsistent master batches.
table 9.11 characteristics and applications of some colorants
pigment characteristics applications
reds
good to excellent heat stability in 500–525°f range, excellent high fastness, expensive, some grades difficult to disperse
excellent fastness, and heat stability in 325–500°f range, expensive, some grades difficult to disperse
very good heat stability, light fastness, easily dispersable, relatively expensive
heat stability in the 450–500°f range, light fastness range from poor to good, economical, limited light fastness in tint economical light red shades, good heat stability and light fastness in mass tone and near mass tone tendency to plate out, limited light fastness in tints with white
oranges
transparent, excellent heat stability and light fastness, relatively expensive
heat stability in 450–500°f range, fair light fastness, bright, clear, high tinting strength, moderate price, limited light fastness, some tendency toward bleeding
yellows
very good to excellent heat stability (500–575°f), good to very good light fastness, transparent or opaque
moderate light fastness, good to excellent heat stability tint light fastness limited
metal oxides
iron oxides (synthetic), good heat stability, inexpensive, inert, poor tinting strength,
red-maroon dull
zinc ferrite tan good heat stability, inert, light fast, good weatherability,
more expensive than other iron oxides
iron oxides (natural), inexpensive, color uniformity can contain impurities limited use in plastics,
siennas polyethylene film bags
chromium oxide green good heat stability and light fastness, excellent
weatherability and chemical resistance, inexpensive, dull, poor tinting strength
mixed metals oxides
nickel tinuium yellow excellent weatherability, inert, easy to disperse, good
chemical resistance, poor tinting strength, weak color
pigment characteristics applications
inorganic browns heat and light stable, good chemical resistance, good color most thermoplastics and
uniformity, relatively expensive thermosets
from wigotsky, v., plastic. eng., 42(10), 21, 1986. with permission.
color concentrates are intimately mixed dispersions of pigments in a base carrier resin. the pigment content of concentrate is usually in the 2 to 30% range, but higher loadings are being developed to enhance versatility and cost/performance benefits. the pigments and resin are subjected to high stress during processing to promote thorough dispersion of the colorant. the concentrate is blended with the resin material being colored in a predetermined proportion by weight known as the let-down ratio to produce the desired color and opacity in the master batch or end product. to ensure compatibility between the concentrate and the let-down resin, the color concentrate is generally made from the same generic polymer as the let-down resin but with a higher melt index so as to promote ready and even mixing. a uniform blend of concentrate must be fed either continuously by a metering device or by weight on a batch basis into the processing equipment, which must be able to convert the blend of pellets into a uniform melt. color variation is produced if the melt is nonuniform or the concentrate is not completely incorporated into the resin.
concentrates are available in solid or liquid forms. solid color concentrate, the major form in which colorants are manufactured and sold, comes in pellet, cube, granulated, and powder forms. they usually consist of dry pigments, additive components encapsulated in a base resin carrier. dispersion and color control are excellent, weighing is less critical, and flowability is good for easy feeding while cleanup is easier because of the need to process color with the carrier resin.15 it is, however, necessary to ensure that the pigment carrier and other components of the concentrate do not compromise the resistance to heat, light, basic physical properties, and rheological compatibility of the resin. liquid concentrates possess many of the advantages of solid concentrates but are usually more costly. they allow lower pigment loading and can sometimes be used at slightly lower concentrations because they cover more surface. liquid concentrates require less material handling and floor space for inventory, and their production does not involve a previous heat history. some resins, however, are unable to absorb a high percentage of liquid.
there is currently a trend to increase the multifunctional nature of color concentrates so that color as well as other desired properties are added to the resin system. this increases the processor’s flexibility in meeting user’s needs and simplifies user inventory by reducing the need to stock large quantities of tailored resins.
precolored compounds provide the processor with a single source of color and resin. this eliminates the need for mixing during processing and provides highly accurate color control, particularly in cases where the base resin is subject to color variations arising from lot-to-lot color differences. in complex part designs where resin flow may not be uniform or where equipment is unable to provide uniform mixing, precolored compounds offer the best method of making a uniformly colored part. however, processors sometimes tend to have preference for dry pigments and concentrates for greater flexibility of inventory and color changeover. for example, from an inventory standpoint, it is much simpler to stock a supply of basic natural resin to satisfy diverse, changing requirements with smaller supplies of dry pigment or color concentrate. a processor who prefers to compound his color from pigment must, of course, have the proper equipment and an in-depth knowledge of the process. one of the growth areas for precolored resin is in specialized engineering applications where not only colorants but a variety of other modifiers must be included in the compound. for example, fiber-reinforced resins that may also contain mold-release agents, flame retardants, and other additives are best made from fully compounded precolored resin.
certain fundamental criteria must be considered in selecting a colorant for a particular application. these include the ability of the colorant to provide the desired color effect and withstand processing conditions and whether or not the fastness will satisfy end-use requirements. therefore, the initial step in the selection of a colorant is to determine whether it will provide the coloristic properties desired, alone or in combination with others. the performance properties of a colorant are generally of two types — those related to processing and those related to the ultimate end use. the most important processing-related property is the heat stability of the colorant. it must be able to withstand not only the process temperature encountered during manufacture but also, for possibly prolonged periods, the temperature in the end-use situation.
migration fastness is related to the solubility of the colorant in the polymer. color migration is manifested through bleeding, blooming, or plate-out. bleeding is the migration from a colored polymer film to an adjacent uncolored or differently colored material, while blooming involves colorant migration, recrystallization, and formation of a dustlike coating on the surface of the polymer. plate-out is charac¬terized by the building of a coating on the metallic surfaces of processing equipment. inadequate light fastness of a colorant is usually manifested in the form of fading or, in the case of some colorants, darkening. the severity and rapidity of color change depend on the chemical structure of the colorants, its concentration in the part, and part thickness.
the compatibility of a colorant is assessed not only on the basis of the ease with which it can be mixed with the base resin to form a homogeneous mass but also on the requirement that it neither degrades nor is degraded by the resin. in relation to product functional properties, incompatibility of a colorant can affect mechanical properties, flame retardancy, weatherability, chemical and ultraviolet resistance, and heat stability of a resin through interaction of the colorant with the resin and its additives. flame retardancy, for example, may impinge directly on the performance of a colorant. pressure to produce materials with lower levels of toxic combustion products can involve organic fire retardant additives that interact with the colorant either to negate the effect of the additives or affect the color.
viii. antistatic agents (antistats)
most synthetic polymers, unless specially treated, are good electrical insulators. they are therefore capable of generating static electricity, which can be potentially costly and dangerous. static-induced accumulation of dust reduces the attractiveness and thus saleability of products displayed on store shelves. the attraction of a formed part to the charged surfaces of a processing mold prevents proper ejection of the formed part and consequently slows down production. electrostatic charges can cause problems when textile, films, or powders join up in automatic machinery. sparks, and possibility explosions or fires, can occur when static electricity is induced from plastics on nearby conductor. damage of sensitive semiconductors and similar complex microelectronic devices can also occur from either the direct discharge from the conductive skin of personnel or by exposure of such devices to the close approach of a static-charged polymer material.20
when two surfaces that are in intimate contact are rubbed together or pulled apart, static electricity is generated. this is due to the transfer of electrons from the surface of the donor material, which consequently becomes positively charged, to the surface of the acceptor material, which then becomes negatively charged. for materials that are nonconductive, these static charges do not flow easily along the surfaces and therefore remain fixed or static. whether a material behaves as a donor or an acceptor depends on its position in the triboelectric series (table 9.12). for example, if nylon and propylene are rubbed together, nylon is the donor, while polypropylene is the acceptor.
the generation of static charges is not confined strictly to nonconductors only conductors also generate static charges, but since they dissipate the charge quickly the level of static charges developed by these materials is difficult to measure. when a conductor and nonconductor are separated or rubbed against each other, the nonconductor develops a measure of static charge. the nonconductor will not lose its charge to the ground. consequently, the charge is removed by employing other techniques. the use of air-ionizing bars and blowers provides an atmosphere of ionized air capable of neutralizing the charged objects or surfaces. however, this does not provide lasting protection since it does not prevent another charge from forming once the object is removed from the ionized air atmosphere. to ensure an extended removal of static charges from the surface, the nonconductor must be made sufficiently conductive to carry charge to the ground. a layer of water, even a few molecules thick, will do this
table 9.12 the triboelectric series
negative [–] end
1. teflon 9. rubber 17. polyester
2. pvc 10. brass, stainless steel 18. aluminum
3. polypropylene 11. nickel, copper, silver 19. wool
4. polyethylene 12. acetate fiber 20. nylon
5. saran 13. steel (carbon) 21. human hair
6. polyurethane foam 14. wood 22. glass
7. polystyrene foam 15. cotton 23. acetate
8. acrylic 16. paper
positive [+] end
1.
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