Handbook of Plastics Technologies Part 10 potx

When two polymers are immiscible and form two separate phases, themajor polymer will form the continuous matrix phase and retain most of its original prop-erties, while the minor polymer

and quinacridones produce reds; disazos, isoindolines, and isoindolinones produce reds toyellows; anilines produce orange; monoazos produce orange to yellows; anthraquinones,diarylides, and nickel azos produce yellows; and phthalocyanines produce greens to blues.Overall, there is a trend to give up inorganics of suspected toxicity and replace them byorganics, but the organics must be chosen carefully to retain the heat and light stability re-quired in processing and using plastics.

5.8.4 Criteria in Choosing Colorants

A typical checklist includes dispersability, rheology, plate-out, thermal stability, ance, light fastness, weathering, migration, and toxicity in both processing and use, partic-ularly in leaching from solid waste

carbon black 13 percent, organic pigments 5 percent, and dyes 2 percent But recent trendshave probably favored the replacement of inorganic by organic colorants.

Prices for inorganic colorants are mostly $1 to $3 per pound, and $3 to $30 for organiccolorants

5.8.6 Compounding Techniques

5.8.6.1 Powdered Color Pigments. The primary particles are individual crystals These

are firmly bonded into tight clumps called aggregates These are further bonded into loose clumps called agglomerates It takes skill and energy to disperse these into molten plas-

tics, and this is best done by experts The average compounder/processor may waste muchtime looking for the optimum technique

5.8.6.2 Colored Compound. Processors can buy the plastic compound already ored This is commonly done with specialty plastics It is expensive and leaves the proces-sor with inventory problems

precol-FIGURE 5.11 Organic colorants

5.8.6.3 Color Concentrate (Masterbatch). Expert compounders disperse colorants at

20 to 60 percent concentration in a carrier polymer, using high shear to break down glomerates and produce uniform dispersion of maximum coloring efficiency This colorconcentrate is used by processors, simply blending it with virgin (natural color) resin (“let-down with natural”) Typical ratios of concentrate/natural are 1/20 to 1/100 This tech-nique is low in cost, does not create inventory problems, and is most commonly used withcommodity resins

ag-5.8.6.4 Liquid Color. The colorant is predispersed in a liquid carrier, hopefully ible with the resin It is metered into the base of the hopper or the beginning of the screw

compat-in extrusion or compat-injection moldcompat-ing and blends uniformly with the rescompat-in by the time itreaches the exit from the screw Although originally billed as a universal technique, it hasrather found applications in certain processes where it is the optimum technique

5.8.6.5 Color Infusion. This immerses the finished plastic product in a hot aqueous persion of colorant + dispersant In several minutes, the color diffuses into the plasticproduct, giving it permanent coloration The length of time determines the depth of thecolor

dis-5.8.7 Special Colorants

Fluorescent colors are used to produce brighter reds and yellows Phosphorescent colorsare used to produce brighter yellows-greens-blues Pearlescent colors combine internaland external reflections; they are made by techniques such as coating titanium dioxide onmica

Metallic flakes are added to colorants to give them a metallic sheen Aluminum flakesgive a silvery sheen and also improve UV stability and impermeability Bronze flakes can

be formulated into a range of colors from green to red to gold

5.8.8 Fluorescent Whiteners

Most polymers tend to form conjugated unsaturation during aging, absorbing blue lightfrom the visible spectrum and therefore turning somewhat yellow One way to mask this is

to add fluorescent whiteners

These are primarily bis-benzoxazoles, triazines and triazoles of phenyl coumarins, andbis-styryl biphenyls (Fig 5.12) They absorb invisible UV light, dispose of part of the en-

FIGURE 5.12 Fluorescent whiteners

ergy, and re-emit the rest as visible light at the blue-violet end of the visible spectrum Thisneutralizes the yellowness in the polymer, and emits a brilliant white They are frequentlyused in polyolefins, polystyrene, ABS, PVC, polycarbonate, and polyurethanes Concen-trations are typically 0.01 to 0.1 percent.

Conventional structural materials are conductive enough to bleed off the charge toground Organic polymers are nonconductors, and may hold the charge for a long time The charge on plastics may develop during separation from the mold or roll, from fric-tion during manufacture or use, or simply from evaporation of water from the surface Thissometimes causes problems in processing, particularly in handling thin films and fibers Itcauses a much greater range of problems in the use of the product: collection of dust; un-sightly packaging; cling and discomfort of clothing and upholstery; shock; occasional dustexplosions; oxygen hazard in hospitals; “noise” in sound recordings and photography andmagnetic tapes and discs, computer chips, military electronics; and electromagnetic inter-ference (EMI) of electronic equipment in general

These are arranged more or less in order of increasing need for static dissipation Theyare generally classified in terms of electrical resistance For example, over 1012Ω-cm isnonconductive insulation, 1010–12 is antistatic, 106–10 is statically dissipative, 102–6 isslightly conductive, 101 is EMI shielding, 100 to –3 is semiconductive, and 10–3 to –5 is me-tallic conductivity

Various techniques are used to minimize these problems In manufacturing, it is ble to ionize the air and thus neutralize static charges In textile manufacturing, it is com-mon to humidify the air to make fiber surfaces more conductive Organic additives canmake plastics fairly conductive to dissipate a static charge In more extreme cases, highloading with carbon black makes rubber and plasticized PVC fairly conductive And load-ing with carbon fibers and metallic fillers (particularly aluminum flakes and fibers) makesplastics conductive for EMI shielding

possi-5.9.1 Mechanisms of Antistatic Action

When organic antistats are used to reduce static charge on plastics, several theories are fered to explain their action Most commonly, it is assumed that the additive is polarenough to exude to the surface of the plastic, where it absorbs moisture from the air, per-mitting ionic impurities to conduct current electrolytically The most effective antistats ac-tually contain ionic groups that are free to migrate and conduct Some theorists believethat simple passage of water vapor over the surface of the plastic may be enough to carryaway the static charge From a different point of view, static charge is created by friction;the antistat acts as a surface lubricant, reducing friction and therefore reducing the buildup

of-of a static charge

5.9.2 Commercial Antistats

Quaternary ammonium soaps, R4N+ X–, have the most powerful antistatic action tunately, they tend to decompose in high-temperature processing, so they are sometimespost-applied as a 1 to 2 percent solution They also encounter objections from the FDA.Ethoxylated amines, RNH(CH2CH2O)nH, approach quaternary ammonium soaps,both in effectiveness and in problems

Unfor-Ethoxylated esters, RCO2(CH2CH2)nOH, are the most widely used class By balancingthe organic acid portion (R) against the polyoxyethylene portion, it is possible to controlpolarity and therefore semicompatibility and rate of migration to the surface of the plastic,thus making it self-renewable over the lifetime of the product They adsorb water to thesurface, making it conductive and lubricating it to reduce friction They are usually non-toxic and stable enough for melt processing Ethoxylated alcohols, RO(CH2CH2O)nH, arealso used

Glycerol mono- and di-esters perform fairly similarly to ethoxylated esters and areused for this reason Being derived from natural products, they are easily acceptable toFDA

Organic phosphate esters are also reported in similar use

More recently, alkali sulfonates have been reported in PS and PVC

5.9.3 Use in Commercial Plastics

LDPE typically uses 0.05 to 1.0 percent, HDPE 0.2 to 0.3, PP 0.5, and PS 2 to 4 percent.Rigid PVC uses 1 to 2 percent, and plasticized PVC 2 to 5 percent

5.9.4 Test Methods

5.9.4.1 Dust Attraction. Dust attraction is the oldest and crudest method The cian rubs the plastic sample against his clothing, and then lowers it toward a dish of dust,and notes the height at which the dust jumps up to the charged plastic A more sophisti-cated test uses a sooty flame to deposit soot on the plastic, and then measures the amount

techni-of soot collected

5.9.4.2 Surface Conductivity. Determining the surface conductivity of the plastic ple is a popular, simple measurement that is often assumed to correlate with antistatic be-havior Practical proof would be more reassuring

sam-5.9.4.3 Electrostatic Decay. A high static charge is applied to the sample electrically.Then the rate of decay is measured instrumentally

In all these methods, relative humidity is the most treacherous variable that must beconsidered This can produce a 104 range in electrical resistivity over the normal range ofambient humidity

5.9.5 Market Analysis

Ethoxylated fatty amines are 48 percent of the market, aliphatic sulfonates 25 percent,fatty acid esters 16 percent, quaternary ammonium compounds 2 percent, others 9 percent.For use in individual plastic materials, styrenics used 39 percent of the market, LDPE/LLDPE 20 percent, HDPE 13 percent, PVC 12 percent, PP 11 percent, and others 5 per-cent

5.10 ORGANIC PEROXIDES

The O:O bond in peroxides is quite unstable

RO:OR → RO. + .ORWhile they are difficult to make, ship, store, and handle, the radicals they produce are veryuseful in vinyl free-radical polymerization, cure of unsaturated polyesters, cross-linking ofthermoplastics, grafting, and compatibilization of polymer blends

Stability/reactivity is generally measured by the temperature at which the half-life ofthe peroxide is 10 hr, called “the ten-hour half-life temperature.” It is controlled by choice

of the R groups and accelerated by raising the temperature, radiation, catalysis by cobaltsoaps, amines, or redox reaction with reducing agents

5.10.1 Major Classes of Peroxides

Major classes of peroxides are shown in Fig 5.13

5.10.1.1 Acyl Peroxides

• Benzoyl peroxide is the longest-established and most widely used With 10-hr half-life

at 71°C, it is used to polymerize styrene and other vinyl polymers, for ature cure of unsaturated polyesters, and for a variety of grafting and compatibilizationreactions

medium-temper-• Lauroyl peroxide (61°C) is used for somewhat higher reactivity Its aliphatic structurealso gives lighter color in polymers than can be obtained with the aromatic benzoyl per-oxide

• Decanoyl peroxide is used to a lesser extent

5.10.1.2 Ketone Peroxides. MEK peroxide is used for room-temperature cure of urated polyesters Typical concentrations are 0.5 to 2.0 percent It may be catalyzed by0.05 to 0.3 percent of cobalt naphthenate and also further catalyzed by amines

unsat-5.10.1.3 Peroxy Esters. These cover a wide range of reactivities and uses

• t-butyl peroxy pivalate is a typical low-temperature peroxide

• t-butyl peroctoate (70°C) is a typical medium-temperature peroxide

• t-butyl perbenzoate (101°C) is a typical high-temperature peroxide, useful in izing styrene and in cure of BMC and SMC unsaturated polyesters

polymer-5.10.1.4 Dialkyl Peroxides. These are typically high-temperature materials

• Dicumyl peroxide (dicup or DCP) (104°C) is useful in cross-linking LDPE, EVA, EPR,and EPDM

• Di-t-butyl peroxide (125°C) is useful for the high-temperature finish of styrene merization to reduce residual styrene monomer content and thus improve modulus,HDT, taste, and odor

poly-5.10.1.5 Hydroperoxides. Hydroperoxides such as cumene hydroperoxide are used marily for low-temperature emulsion polymerization of butadiene to make “cold rubber.”

pri-They are catalyzed by redox systems consisting of reducing sugars, iron soaps, and phates

phos-5.10.1.6 Peracetic Acid (CH 3 CO 3 H). This is used mainly in epoxidizing olefins such assoybean oil for vinyl stabilizers and in synthesis of aliphatic epoxy resins

5.10.1.7 Peroxyketals. These are particularly popular for cure of BMC and SMC urated polyesters

unsat-5.10.1.8 Peroxydicarbonates. These are the least stable class, often too unstable forshipment, in which case they must be synthesized where they are going to be used Theyhave become the leading initiator for vinyl chloride polymerization

FIGURE 5.13 Peroxides

5.10.1.9 2,5-Dimethyl-2,5-di-t-Butyl Peroxy Hexyne-3. This, having a 10-hr half-lifetemperature 135°C, was developed specifically for the higher temperature processing re-quired in the cross-linking of HDPE.

5.10.3 U.S Market Analysis

Table 5.33 provides an analysis of peroxides used in plastics in the United States

the ratio of the two polymers in the blend Miscibility depends on equal polarity or mutualattraction such as hydrogen bonding or cocrystallization This is not very common, butthere are several important examples of such completely miscible blends It gives the com-pounder simple straightforward control over balance of properties.

5.11.2 Practical Compatibility

Most polymer pairs are too dissimilar for complete miscibility They reject each other andseparate into two or more phases Generally, the major polymer forms a continuous matrixphase and retains most of its original properties The minor polymer separates into dis-persed “domains” and may affect certain specific properties When the domains are ex-tremely fine, sensitive properties may detect the phase separation, but many practicalproperties may resemble homogeneous single-phase blends When the domains are larger

in size, they will have distinct effects on certain specific properties; when these effects are

beneficial, the blend is described as theoretically immiscible but practically compatible.

When the domains are too coarse, most properties will suffer, and the blend is described as

incompatible.

5.11.3 Interface/Interphase

In multiphase polyblends, a critical factor is the interface between the phases If the twopolymers reject each other and separate into phases, they are likely to reject each other atthe interface as well Such a weak interface will fail under stress, and most properties willsuffer Thus, most polymer blends are practically incompatible Yet, most successful com-mercial polyblends are multiphase systems This means that there must be a mechanism tostrengthen the interface

In some cases, the two polymers have some partial miscibility, so the interface is not asharp separation of one polymer from the other but, rather, a modulating solution of the

two polymers in each other, offering a gradual interphase rather than a sharp interface.

Such an interphase can modulate properties gradually from one phase to the other and thusreduce the stress

5.11.4 Compatibilizers

In most cases, it is necessary to add a compatibilizing agent to strengthen the interface Inbasic research, the preferred compatibilizing agent is a diblock copolymer, with one blocksoluble in one phase and the other block soluble in the other phase The block copolymertends to locate at the interface This creates primary covalent bonds across the interfaceand thus strengthens it In commercial practice, the compatibilizing agent is usually a graftcopolymer, with a backbone soluble in one phase and side-chains soluble in the otherphase; this is not as theoretically satisfying, but it is usually easier and more economical tomake and appears to work perfectly well in practice In some cases, the graft copolymer ismade separately and then added to the polyblend during compounding; in other cases, itmay be formed directly during compounding by reactive processing

5.11.5 Effect of Polyblend Ratio on Polyblend Properties

When two polymers are blended in ratios from 100/0 to 0/100, and the effect on properties

is measured, we may observe one of four types of behavior (Fig 5.14)

5.11.5.1 Type I. If the two polymers are completely miscible down to the molecularlevel and form a single homogeneous phase, properties are generally proportional to theratio of the two polymers in the blend Even if the two polymers are immiscible and formfine phase separation, many property tests are relatively insensitive to fine-phase separa-tion and may still show such “homogeneous behavior.” Practically, this is useful to com-pounders who want the ability to produce a spectrum of balance of properties at low cost.

5.11.5.2 Type II. When two polymers are immiscible and form two separate phases, themajor polymer will form the continuous matrix phase and retain most of its original prop-erties, while the minor polymer will form finely dispersed domains and contribute certainspecific properties Thus, high A/B ratios will have properties similar to poly-A, and highB/A ratios will have properties similar to poly-B Obviously, at fairly equal ratios of A and

B, there will be a phase inversion with a rapid change of properties from one plateau to theother

This explains the two leading uses of polymer blends (1) When rigid plastics sufferfrom brittleness, dispersion of fine rubbery domains in the rigid matrix can add great im-pact strength with little sacrifice of rigidity (2) Rubber molecules must be tied together togive them strength, creep resistance, and insolubility; while this is usually done by ther-moset vulcanization, it can also be done by dispersion of fine rigid thermoplastic domains,either glassy or crystalline, to form thermoplastic elastomers

5.11.5.3 Type III. When two polymers are immiscible and separate into two phases,there may be so little attraction between them that the interface between the phases is ex-tremely weak and will fail under stress This is most often seen in ultimate tensile strengthand ultimate elongation In most products, this would be labeled “incompatibility.” How-ever, there are occasional examples where such behavior is actually beneficial For exam-ple, adding an immiscible polymer may decrease melt viscosity and thus improve meltprocessing Or it may decrease breaking strength, producing a package that is easier toopen and therefore more customer friendly Thus, it is safer to label Type III behavior “U-shaped” or “trough-shaped,” rather than simply incompatible

5.11.5.4 Type IV. Once in a while, the polymer blend may exhibit properties greaterthan either of the individual polymers, a major synergistic improvement in practical utility.The leading example of this phenomenon is the use of finely dispersed rubbery domains toincrease the impact strength of a brittle glassy matrix polymer Commodity examples are

FIGURE 5.14 Properties vs polymer/polymer ratio in a polyblend

high-impact polystyrene, ABS, rigid PVC, and high-impact polypropylene; more ized examples are toughened epoxy resins and super-tough nylon.

special-5.11.5.5 Modulus vs Temperature. When a rigid polymer and a rubbery polymer arecompletely miscible, blending them in rigid/rubbery ratios from 100/0 to 0/100 simply

shifts the log modulus versus temperature curve horizontally along the temperature axis

(Fig 5.15, Type I) This makes it easy for the processor to adjust balance of properties tosuit the individual customers’ needs

On the other hand, when the two polymers separate into separate phases, this adds anintermediate plateau to the original curves (Fig 5.15, Type II) Here, the height of the in-

FIGURE 5.15 Modulus vs temperature for polyblends

termediate modulus plateau is proportional to the ratio of the two polymers in the blend,and the useful temperature range extends from the glass transition of the rubber to theglass transition or melting point of the rigid polymer This explains the successful use ofimmiscible, compatible polymer blends to make both high-impact rigid plastics and ther-moplastic elastomers.

5.11.6 Major Commercial Polyblends

• Low-density polyethylene is added to linear-low-density polyethylene to produce

non-Newtonian shear sensitivity needed for blown-film production

• EPDM rubber is added to high-density polyethylene and polypropylene to provide

en-vironmental stress-crack resistance It is grafted into polypropylene to increase temperature impact strength It is grafted with maleic anhydride (maleated) and thengrafted onto nylon to increase its impact strength Recent news releases suggest that thistechnique is also being applied to other engineering thermoplastics

low-• Polybutadiene is grafted into polystyrene and SAN to produce high-impact polystyrene

and ABS

• Nitrile rubber is used to increase impact strength of epoxy resins It is made as a

low-molecular-weight liquid oligomer with carboxy end-groups (CTBN) and used as a ing agent for the epoxy resin

cur-• Polystyrene is added to polyphenylene ether to improve melt processability and

de-crease cost When impact styrene is used, it also inde-creases impact strength

• ABS is added to rigid polyvinyl chloride to increase melt processability and impact

strength It is added to polycarbonate to increase melt processability and environmentalstress-crack resistance and to decrease notch sensitivity and cost

• Polytetrafluoroethylene is added to acetal, polycarbonate, and nylon to decrease friction

and increase abrasion resistance

• Polyvinyl acetate is added as low-profile resin in unsaturated thermosetting polyester to

decrease shrinkage and prevent reinforcing fibers from protruding It thus gives proved surface

im-• Polyethyl acrylate is added to rigid polyvinyl chloride to improve melt processability It

is compatibilized by grafting polymethyl methacrylate onto it

• Polybutyl acrylate is added to rigid polyvinyl chloride to increase impact strength It is

compatibilized by grafting polymethyl methacrylate onto it

• Polyethylene terephthalate and polybutylene terephthalate are added to polycarbonate

to provide environmental stress-crack resistance

• Nylon is added to high-density polyethylene to make it impermeable for use in gasoline

tanks

5.11.7 Other Uses of Polyblending

• Melt flow may be increased by adding a more fluid polymer, or sometimes an

immisci-ble polymer

• Melt strength and elasticity may be increased by adding a high-MW polymer or one

with long-chain branching

• Modulus of rubber may be increased by adding a miscible rigid plastic Modulus of a

rigid plastic may be decreased by adding a miscible rubber to act as a polymeric cizer

plasti-• Strength of rubber can be increased by adding a rigid plastic.

• Abrasion resistance can be increased by adding PTFE powder or by blending with

5.12.1 Polymerization, Cross-Linking, and Curing Agents

These additives create reactive (unstable) systems, so they are usually added by the cessor just before final plastic processing In most cases, the chemistry of these systems isvery precise and sophisticated, so these additives are best specified by the polymer pro-ducer, not casually chosen and used by the average compounder/processor They are bestdescribed according to the polymer system in which they are used (See also Chap 3.)

pro-5.12.1.1 Cross-Linking of Thermoplastics (Polyethylene, Saturated Elastomers, Acrylic Ester Polymers). These are most often cross-linked by peroxides, choosing the peroxideaccording to the processing temperature Peroxide forms free radicals, which abstract less-stable hydrogen atoms from the polymer, leaving polymer radicals When two polymerradicals join, this forms a cross-link The degree of cross-linking is low—not enough tocause rigidity, but enough to improve strength, creep-resistance, hot strength, and insolu-bility Thus, polyethylene is cross-linked by 2 to 10 percent of peroxide during reactive ex-trusion to form piping and wire and cable insulation Conversely, polypropylene radicalstend to cleave rather than cross-link, so peroxide is used to decrease melt viscosity for eas-ier processability

Several companies have experimented with the use of vinyl organosilanes as linking agents For example, 2 percent of vinyl trimethoxy silane is first activated by0.1 percent of dicumyl peroxide and grafted onto the thermoplastic polymer The system iskept dry to stabilize the methoxy groups After melt processing, the solid product is ex-posed to moisture to hydrolyze the methoxy groups, which then condense with each other

cross-to form cross-links

5.12.1.2 Cure of Epoxy Resins. The reactivity of the epoxy ring permits tion and cross-linking reactions with many types of additives Polyamines are most com-monly used, particularly polyethylene polyamines Since these are often too volatile,allergenic, and reactive, they are usually reacted into epoxy adducts or polyamides ofsomewhat higher molecular weight, lower volatility, greater safety, and more controlledreactivity Polybutadiene oligomers with acid end-groups, and polysulfide oligomers withmercaptan end-groups, are curing agents that are used to reduce the inherent brittleness ofcured epoxy resins Polyanhydrides are used for higher heat resistance, because they canreact further with the hydroxyl groups formed during the polymerization of the epoxyresin For solid molding compounds and impregnated tapes, aromatic amines, and high-

polymeriza-temperature catalysts like BF3:amine adducts and dicyandiamide are often pounded, giving systems that are fairly stable at room temperature For flame retardance,halogenated anhydrides are often used in place of normal anhydrides.

precom-5.12.1.3 Unsaturated Polyesters. Copolymerization and cure of the fumarate esterswith styrene monomer is initiated by peroxides, choosing the peroxide appropriate to theprocessing temperature chosen For room-temperature cure, MEK peroxide is generallyused; it can be accelerated by cobalt naphthenate and further catalyzed by tertiary amines.For higher-temperature cure reactions such as BMC and SMC, higher-temperature perox-ides are chosen (Sec 5.10)

5.12.1.4 Polyurethanes. The two major types of catalysts, for the polyol-polyisocyanatereaction to form polyurethanes, are tertiary amines and/or organotin compounds such asdibutyl tin dilaurate For delayed reactions, the amines and/or the isocyanates can be tem-porarily blocked by adducts, which are removed and liberated during the cure reaction

5.12.1.5 Furfuryl Alcohol Resins. Polymerization and cure, and copolymerization/curewith urea-formaldehyde and phenol-formaldehyde, are generally catalyzed by acids such

as p-toluene sulfonic acid and zinc chloride

5.12.2 Surface Properties

A variety of additives are used primarily to modify surface properties, either during cessing or during use of the finished product They are collected here for this general pur-pose

pro-• Hyperdispersants are low-molecular-weight block copolymers designed to separate

filler and pigment particles from each other, disperse them more readily in liquid tems, and stabilize these dispersions for more efficient use of the solid particles Oneblock is designed to be attracted to the surface of the solid particle, the other block to beattracted into the plasticizer, polyolefin, or other matrix being used for the masterbatch,and also into the final matrix polymer in the finished product The exact nature of theseblock copolymers is still a secret of their developers and producers

sys-• Corrosion inhibitors are commonly included in coatings on steel These include

phos-phates of iron, manganese, and zinc; chromates of zinc and strontium; soaps of calcium,lead, sodium, and zinc; lead oxide, carbonate, and sulfate; ferric complexes; and zincdust Some newer types include organic phosphates and sulfonates Due to worriesabout toxicity and the environment, this entire field is in a state of change

• Prebonding etch is needed to activate the perfluoro surface of PTFE before it can be

bonded with adhesives This is typically a solution of sodium in naphthalene, which isextremely alkaline It pulls some fluorine atoms off the surface, or even carbonizes it,leaving a surface which is more ready to accept adhesives such as epoxy resins

• Antiblocking agents are often fine filler particles that roughen film surfaces enough to

prevent them from coming into good contact with each other, and thus reduce adhesionbetween them These are typically 0.1 percent of chalk or 1 percent of amide wax

• Antislip agents are sometimes needed to overcome excessive lubrication For example,

lubricants are added to films to keep them from sticking together during handling If thefilms are converted into bags, filled with heavy solids, and stacked on a pallet, they may

be so slippery that they slide off the stack and fall all over the floor In such cases, an tislip agent may be added to the formulation to create enough friction/adhesion to pre-

an-vent such sliding Antislip agents for polyolefin films are often materials such asoleamide in PE or erucamide in PP, used at about 0.05 percent.

• Antifog agents are added to films to keep moisture condensation from clouding them

and preventing the passage of light Typical uses are in polyolefins, polystyrene, PVC,and polyester films, as an aid in marketing refrigerated and frozen foods, and in plasticfilm for greenhouses These are hydrophilic organic compounds such as mono- and dig-lycerides, higher-polyol partial esters, and ethoxylated phenols and fatty alcohols andacids, used at 0.5 to 4.0 percent, at a cost of $0.80 to 4.00 per pound They adsorb mois-ture and spread it into a continuous transparent surface film rather than the opacifyingdroplets that normally form on a low-polarity plastic surface

• Water repellants are sometimes used in surface treatments such as isobutyl trimethoxy

silane

5.12.3 Degassing Agents

Compounding and mixing of liquid systems often traps air, or volatile liquids that ize during processing, forming bubbles and other flaws in the finished products Severaltypes of additives are used to remove these volatiles before they cause trouble

volatil-• Humidity eliminators are used to absorb moisture from PVC plastisols and other liquid

systems, to prevent blistering, bubbles, and craters A typical system would be a nation of calcium oxide plus a wetting agent

combi-• Air-release agents are added to liquid epoxy, polyester, and polyurethane systems to

re-move air bubbles before cure These function by their surface activity They are mostlyproprietary compositions

• Antifoam agents are added to latexes to prevent air from producing foam that would ruin

the dried final coatings Typical antifoams are octyl alcohol and liquid silicones Theymust be chosen with care to avoid negative effects on adhesion and decoration of thefinished products

5.12.6 Fragrances

Chemical fragrances are added to products either to produce a desired odor (“decorative”)

or to mask an undesirable odor (“functional”) in a material or an environment They aregenerally perfume oils that are masterbatched into thermoplastics, often combined with

colorant and sold in powder or pelletized form to processors who blend the masterbatchwith natural resin in conventional molding, extrusion, and foam processes

They may be used at concentrations from 10 ppm (0.001 percent) (food packagingfilm) up to 40 percent (air fresheners) The concentrates are sold at $2 to $8 per pound Lifetime of the odor is controlled by vapor pressure of the odorant, surface/volume ra-tio of the product, temperature, and controlled air flow Lifetimes may range from severaldays to many years Shelf life, properly packaged, is “almost infinite.”

Major developed uses are in garbage bags, films in general, room and auto air ers, toys, and housewares Developing markets include textiles, hospital supplies, con-sumer packaging to stimulate sales, wall tile, air conditioning, and enhancement of workefficiency In most of these uses, the odor is perfectly apparent However, in some uses, it

freshen-is kept at a subliminal level for subtle psychological effect

5.12.7 Masterbatches

Compounders are always free to buy individual additives and combine them to their cific needs In some cases, particularly when the additive is difficult to compound, it is bestmasterbatched by specialists and then sold to the processor who simply “lets it down withnatural.” Many processors prefer not to become involved in the chemical details and buycombined masterbatches containing several additives all together in the same masterbatch.This is particularly common in stabilizer packages, surface-treated fillers and fibers, lubri-cant packages, colorants + odorants, polyblends, and antifog/antistat/lubricant additives.While this saves the processor a lot of detail formulating effort, it makes it much more dif-ficult for him to identify and solve problems when the compound does not perform prop-erly

spe-5.13 GENERAL REFERENCES

1 Jesse Edenbaum, Plastics Additives and Modifiers Handbook, Van Nostrand Reinhold, 1992.

2 J T Lutz and R F Grossman, Polymer Modifiers and Additives, Dekker, 2000.

3 Hans Zweifel, Plastics Additives Handbook, Hanser, 2001.

5.14 SPECIALIZED REFERENCES

1 H S Katz and J V Milewski, Handbook of Fillers for Plastics, Van Nostrand Reinhold, 1987.

2 J W Lyons, The Chemistry and Uses of Fire Retardants, Wiley-Interscience 1970.

3 J V Milewski and H S Katz, Handbook of Reinforcements for Plastics, Van Nostrand

Rein-hold, 1987

4 D R Paul and C B Bucknall, Polymer Blends, John Wiley & Sons, 2000.

5 E P Plueddemann, Silane Coupling Agents, Plenum, 1982.

6 J K Sears and J R Darby, The Technology of Plasticizers, John Wiley & Sons, 1982.

NANOMANUFACTURING

WITH POLYMERS

Daniel Schmidt, Joey Mead, Carol Barry

Department of Plastics Engineering University of Massachusetts Lowell, Massachusetts

Julie Chen

Department of Mechanical Engineering University of Massachusetts Lowell, Massachusetts

6.1 INTRODUCTION

Nanotechnology offers the promise of unique and wonderful products as a result of thedramatic changes that occur when structures approach nanoscale dimensions At thesesizes, on the molecular level, the surface or interfacial properties play a more significantrole These effects are used to produce more effective drug delivery systems, higher-per-formance plastic parts, and faster and lighter-weight memory devices—products that af-fect our everyday life Many of the current nanotechnology products and reports arefocused on semiconductor processing and ceramic-based materials; however, these materi-als have limitations due to high density and rigidity Polymer-based products offer an ad-vantage because of their lighter weight, flexibility, and biological compatibility Inaddition, polymers also provide the benefit of ease of fabrication using high-rate and con-tinuous processing As a result, it is anticipated that polymeric materials will play a moreimportant role in the future of the nanotechnology revolution

One of the critical issues in advancing the field of nanotechnology is the need to velop economic and robust manufacturing methods Since polymers can be fabricated in awide array of shapes and forms, their manufacturing approaches can be utilized to developnumerous products Some of the potential new products include extruded multicomponentthin films for conformable, high-density data storage or displays, injection-molded low-cost calibration standards, and electrospun nanotextiles for energy storage The issue fordeveloping these products is the need to develop commercially viable nanomanufacturingmethods Nanomanufacturing of polymers is likely to look quite different from currentmacroscale processes; however, we can make modifications to existing equipment to pro-

de-duce a host of new products Although it is anticipated that many new products willemerge, we can explore four basic geometries: nanocomposites, nanofibers, nanolayeredfilms, and nanofeatured polymers This chapter briefly covers these current technologies,some already appearing in commercial products.

6.2 NANOCOMPOSITES

Nanocomposites consist of a nanometer-scale phase in combination with another phase.While this section focuses on polymer nanocomposites, it is worth noting that other im-portant materials can also be classed as nanocomposites—super-alloy turbine blades, forinstance, and many sandwich structures in microelectronics Dimensionality is one of themost basic classifications of a (nano)composite (Fig 6.1) A nanoparticle-reinforced sys-tem exemplifies a zero-dimensional nanocomposite, while macroscopic particles produce

a traditional filled polymer Nanofibers or nanowhiskers in a matrix constitute a mensional nanocomposite, while large fibers give us the usual fiber composites The two-dimensional case is based on individual layers of nanoscopic thickness embedded in a ma-trix, with larger layers giving rise to conventional flake-filled composites Finally, an inter-penetrating network is an example of a three-dimensional nanocomposite, while co-continuous polymer blends serve as an example of a macroscale counterpart

one-di-Well before the term nanocomposite was coined, researchers, especially those in the

rub-ber industry, were working toward the use of nanoscopic filler particles (carbon black, fumed

FIGURE 6.1 A dimensionality-based classification system for nanocomposites, ing nanoparticles (0-D), nanorods/nanofibers (1-D), nanolayers/nanodiscs/nanoplatelets(2-D), and interpenetrating networks (3-D)

cover-silica, and so on) In the prenanometer age, their size was measured in either angstroms or

“millimicrons,” but these were some of the first true nanocomposite systems Even then,some of the basic truths of nanocomposite research had already been explicitly stated:

Fine particle size does not necessarily lead to good reinforcement In practice, the tion is complicated by the fact that very finely divided fillers tend to agglomerate and are ex-tremely difficult to disperse The use of organic or other coatings in filler surfacessometimes promotes dispersion, and increases the effective use of fillers of very fine particlesizes.1

situa-Why were nanometer dimensions so important? Again, the same questions raised days by nanocomposite research were beginning to be answered far earlier, in the siliconeindustry:

nowa-The factor common to all reinforcing fillers is high specific surface area, though whetherthis is the only—or even the principal—requirement has not yet been demonstrated with cer-tainty.”2

This issue of specific surface area hints at how one might change the nature of forcement In typical micro- and macrocomposites, the properties are dictated by the bulkproperties of both the matrix and the filler This relationship between the properties of thecomposite and the properties of the filler is what leads to the stiffening and degraded elon-gation mentioned earlier In the case of nanocomposites, the properties of the material are

rein-instead tied to the interface Terms like bound polymer, bound rubber, and interphase have

been used to describe the polymer at or near the interface, where significant deviationsfrom bulk structure and properties are known to occur (Fig 6.2)

In polymers filled with fillers with high specific surface areas (that is, hundreds ofmeters squared per gram), most of the polymer present is near an interface (and thus

bound polymer), even with only a small weight fraction of filler Such fillers are

necessar-ily nanoscopic, as this is the only way to achieve such a high specific surface area If theinteraction at the interface is a strong one, or if the structure of the interfacial polymer isvery different from the bulk, one can expect to see markedly different properties in the ma-terial as a whole These changes have a fundamentally different origin from those found inmicro- and macrocomposites, where the volume of the interphase is only a small fraction

of the overall volume of the material Therefore, nanocomposites are often referred to as

FIGURE 6.2 Bound polymer The thickness of this boundary layer is

typi-cally described as being in the range of nanometers to tens of nanometers

being “different” from other reinforced systems Some specific examples of these ences follow.*

differ-6.2.1 1-D Nanocomposites

The most readily cited examples of this class of nanocomposite are systems based on bon nanotubes Carbon nanotubes have many interesting properties, including exception-ally high mechanical strength and remarkably versatile electronic properties They occur

car-in two distcar-inct forms: scar-ingle-walled nanotubes (SWNTs) and multiwalled nanotubes(MWNTs) Compared with multiwalled nanotubes, single-walled nanotubes are expensiveand difficult to obtain, but they have been of great interest due to their superior electronic,mechanical, and gas adsorption properties

The use of carbon nanotubes in polymer composites has attracted much attention, duenot only to their interesting mechanical and electronic properties but also to their very highaspect ratio These properties make them ideal reinforcing fibers in nanocomposites How-ever, carbon nanotubes do not spontaneously disperse in polymers, making filler disper-sion a major issue Nanotubes aggregate easily to form bundles that are very difficult todisrupt In addition, such bundles or ropes are often heavily entangled with one another.With high shear, these ropes can be untangled, but dispersion at the single-tube level is dif-ficult to achieve, since the attractive forces are large, and the percolation threshold is low.Likewise, high concentrations of carbon nanotubes are difficult to work with due to vis-cosity issues Because of these difficulties, the potential advantages of these fillers havebeen difficult to realize in practice

Generally, polymer/nanotube nanocomposites have been fabricated by direct mixing or

in-situ polymerization In-situ polymerization is more effective in producing homogenous

dispersions, because the nanotubes more readily disperse monomers than in the polymers

Direct melt mixing has many advantages over in-situ polymerization when it comes to

practical application However, methods aimed at improving dispersion in such systemsinclude surface functionalization, acid treatment, and the use of special surfactants In ad-dition to these methods, which often involve harsh conditions that may degrade the prop-erties of the carbon nanotubes, techniques of specific interest due to milder conditions andthe potential for application in polymer nanocomposites include vapor-phase amination togive good solvent compatibility3 and the use of highly charged nanoparticles to allow forthe dispersion down to the single tube level in water.4 Epoxies,5–7 poly(vinyl alcohol),8and poly(methyl methacrylate)9–11 have all been used in the production of carbon nano-tube nanocomposites, to give just a few examples Much of the work, however, has fo-cused on conducting polymers related to poly(phenylene vinylene),12–16 due not only tobetter polymer/nanotube interactions but also the possibility for interesting electronicproperties in the nanocomposites thus formed