Handbook of Plastics Technologies Part 9 ppsx

Dispersion of polar fillers and fibers in the molten polymer quires special care to produce interfacial wetting and shear mixing to produce dispersion.Fillers and fibers rubbing against

ers (iron particularly), or from corrosion of process equipment, or as adjacent surfaces onfinal products (insulation on copper wire), they may aggravate the attack of atmosphericoxygen and the resulting degradation of the polymer One way to remove the metal fromthe system is to tie it up in an inactive complex, in which form it is no longer able to cata-lyze the oxidation reaction These complexing agents are usually organo nitrogen com-pounds or polyols They are not used alone but are added as synergists to a system thatalready contains primary antioxidants.

5.1.1.4 Acid Scavengers. Oxidation of polymers produces organic acids Chlorine andbromine, from catalyst residues and flame-retardants, produce stronger acids These cancause hydrolysis of polymers and corrosion of process equipment Therefore, it is fairlycommon practice to add acid scavengers to neutralize them These are mildly alkaline sub-stances such as calcium and zinc stearates, hydrotalcite, hydrocalumite, and zinc oxide

5.1.1.5 Use in Commercial Plastics. LDPE is usually stabilized by 0.005 to 0.05 cent BHT DLTDP and nonylphenyl phosphite may be added as well For wire and cableinsulation, metal deactivator is also needed

per-LLDPE and HDPE use higher-molecular-weight phenols and higher concentrations.Cross-linked polyethylene, containing carbon black, permits use of thiodiphenols and dia-ryl amines, since their discoloration is masked by the carbon black For wire and cable,hydrazides and triazines are common metal deactivators to protect against copper catalysis

of oxidation

Polypropylene contains less-stable tertiary hydrogens and processes at higher tures, so it requires higher concentrations (0.25 to 1.0 percent) of higher-molecular-weightphenols and more vigorous use of aliphatic sulfides and aromatic phosphites Poly-1-butene is similar

tempera-ABS contains 10 to 30 percent of butadiene rubber, whose C=C bonds are very tive to oxidation, producing embrittlement and discoloration Triaryl phosphites are used

sensi-as primary antioxidants, in concentrations up to 2.5 percent, producing excellent tion

stabiliza-“Crystal” polystyrene is resistant to oxidation, but most “polystyrene” is actually pact styrene containing 2 to 10 percent of butadiene rubber Like ABS, it requires similarstabilization, but lower concentrations are sufficient

im-Acetal resins are sensitive to oxidation and are generally stabilized by weight phenols Polyesters and polyurethanes are commonly stabilized by phosphites.Polyamides are stabilized by phosphites and also (surprisingly) by copper and manganesesalts, presumably through complex formation with the amide groups themselves

of additives are used: physical and chemical

5.1.2.1 Physical Antiozonants. Saturated waxes are added during rubber compounding.Being immiscible, they migrate to the surface (bloom), forming a barrier coating that

keeps ozone from reaching the rubber Paraffin waxes bloom rapidly but are too brittle.Microcrystalline waxes bloom more slowly but are less brittle Mixture of the two typesgives broader protection These are adequate for static performance but are too brittle fordynamic stretching and flexing.

A saturated rubber can be coated on the surface to provide a barrier against ozone ylene/propylene, plasticized PVC, and polyurethane are typical coatings However, theseinvolve problems of adhesion, elasticity, and cost, so they are not commonly used

Eth-5.1.2.2 Chemical Antiozonants. These are mostly secondary alkyl aryl amines

R-NH-Ar and related compounds They give excellent protection Most of them discolor badly,but several are recommended for nonstaining applications

Most compounders use a combination of physical and chemical antiozonants andachieve excellent protection in this way For more severe ozone-resistance problems, thereare, of course, a number of specialty elastomers that are saturated and therefore com-pletely ozone-resistant: ethylene/propylene rubber, chlorinated and chlorosulfonated poly-ethylene, ethylene/vinyl acetate, ethylene/acrylic esters, butyl rubber, SEBS, plasticizedPVC, butyl acrylate copolymers, polyepichlorohydrin and copolymers, polyetheresterblock copolymer, polyurethane, and silicone

5.1.3 PVC Heat Stabilizers

PVC is very heat sensitive When it is heated during processing, or even during use, itloses HCl, which is toxic and corrosive; forms C=C bonds which cause discoloration; andcross-links, causing clogging of process equipment and embrittlement of products(Fig 5.2) The problem is caused by an occasional unstable Cl atom that is destabilized bybeing adjacent to a branch point, a C=C group, a C=O group, or an oxygen atom It re-quires strong and precise stabilization for practical use There are three major classes ofheat stabilizers for PVC, as described below

5.1.3.1 Lead Compounds. These were the earliest in commercial practice “Normal”lead salts included sulfate, silicate, carbonate, phosphite, stearate, maleate, and phthalate

“Basic” lead salts combined these with lead oxide, giving greater stability They were cost, efficient, and gave excellent electrical resistance Disadvantages were opacity, sulfur-staining, and toxicity Due to worries about toxicity, their use has been restricted to electri-cal wire and cable insulation

low-TABLE 5.6 World Consumption of Antioxidants

5.1.3.2 Ba/Ca Soap + Cd/Zn Soap + Epoxidized Fatty Ester + Organic Phosphite. Thissynergistic combination has always been unnecessarily secretive, sold under vague namessuch as “mixed metal,” “synergistic,” and so on It is universally used for plasticized PVC,because it is soluble, economical, and effective The metal soap may be phenate, octoate,neodecanoate, naphthenate, benzoate, laurate, myristate, palmitate, or stearate.

The Group IIB metal soap (Cd or Zn) is the primary stabilizer It replaces an unstable

Cl atom by a stable ester group,

Polymer-Cl + M(O2CR)2→ Polymer-O2CR + MCl2

Cd is more reliable, but worries about toxicity have practically eliminated its use Zn ismore powerful but tricky, so compounders have had to learn how to handle it very care-fully

The Group IIA metal soap (Ba or Ca) is a reservoir to regenerate the essential GroupIIB metal soap:

Ba(O2CR)2 + ZnCl2→ BaCl2 + Zn(O2CR)2

Ba works best, but there is some worry about toxicity Ca is less effective but completelynontoxic, so it is used when there is worry about toxicity

The epoxidized fatty ester may be epoxidized soybean oil for compatibility and toxicity, or epoxidized tall oil esters for low cost and low-temperature flexibility It is gen-erally believed to function by neutralizing HCl It may also replace unstable Cl on thepolymer or complex ZnCl2 to keep it from degrading the PVC

non-The organic phosphite is generally believed to function by complexing ZnCl2 to keep itfrom degrading the PVC

The synergistic effect is clearly seen by comparing the individual ingredients with thetotal system (Table 5.7) Typical concentrations are about 2 percent metal soap, 5 percentepoxidized fatty ester, and 1 percent organic phosphite

5.1.3.3 Organotin Salts. The most powerful and expensive stabilizers for PVC are notin compounds, most generally of the type R2SnX2 The R group is most often butyl, but

orga-FIGURE 5.2 Thermal degradation of PVC

sometimes is octyl for food packaging or methyl for higher efficiency The most powerful

X group is –SCH2CO2C8H17, which is called isooctyl thioglycollate or isooctyl

mercap-toacetate For greater lubricity or UV stability, the X group may be maleate or laurate

(Table 5.8) The relative amounts of R and X are sometimes varied for subtle reasons Inrigid PVC, where high melting point and high viscosity cause the most serious instabilityproblems, organotin is always used Concentrations range from 2 to 3 percent down to onetenth as much, depending on the equipment and process

5.1.3.4 Miscellaneous Stabilizers. A variety of other stabilizers are vaguely mentioned

in the literature, mainly by vendors Polyols and organo-nitrogen compounds may beadded to complex iron impurities in fillers and keep them from catalyzing degradation ofPVC Other additives are more secretive and their benefits less clear Bisphenol is added towire and cable insulation to stabilize the plasticizer rather than the PVC UV stabilizersmay be added for outdoor use, and biostabilizers are important to protect the plasticizer

5.1.3.5 Other Organohalogens. Thermal instability is also a problem in other polymerssuch as chlorinated polyethylene, chlorinated PVC, polyvinylidene chloride, chlorinatedrubber, and chlorinated and brominated flame-retardants PVC heat stabilizers may helphere, too, but require careful adjustment for optimum performance in each system

TABLE 5.7 Synergistic Stabilization of PVC: Gardner Color After Aging in 150°C Oven

Aging time, minutes 0 50 200

1 percent barium laurate 1 13 14

1 percent cadmium laurate 1 3 3

1 percent zinc laurate 1 18 18

5 percent epoxidized soybean oil 2 10 13

1 percent alkyl diaryl phosphite 1 17 18

TABLE 5.8 Organotin Stabilization of PVC: Gardner Color After Aging in 175°C Oven

Aging time, minutes 30 60

3% dibutyl tin dilaurate 2 5

3% dioctyl tin bis-octylthioacetate 1 23% dibutyl tin bis-octylthioacetate 1 2 3% dimethyl tin bis-octylthioacetate 1 1

5.1.3.6 Market Volume. The total market for PVC heat stabilizers may be about 100million pounds in the United States and 1 billion pounds worldwide, half for organotin andhalf for metal soap-epoxidized fatty ester-organic phosphite systems.

5.1.4 Ultraviolet Light Stabilizers

Five percent of the sunlight that penetrates the ozone layer and reaches the Earth is energy short-wavelength ultraviolet (UV) radiation, 290 to 400 nm When polymers areused out of doors, absorption of this UV energy raises the electrons of primary covalentbonds from their low, stable energy level up to higher unstable energy levels that lead todegradation (Tables 5.9 and 5.1) Polymer structures that can absorb UV include benzenerings, C=C, C=O, OH, ROOH (Table 5.10), and especially conjugated groups of suchstructures Even polymers that do not contain such groups may still degrade, and theblame is then placed on impurities or complex-formation UV degradation can lead tocleavage to lower molecular weight or cross-linking to higher molecular weight, unsatura-tion, photooxidation, and photohydrolysis, all of which result in weathering deterioration

high-There are a number of ways to protect plastic products for use outdoors, as described low

be-TABLE 5.9 UV Wavelengths and Energy Levels

UV wavelength, nm Energy level, kcal

5.1.4.1 UV Reflectors. If a UV-resistant material will reflect UV light away from thepolymer, this can increase its lifetime tremendously A metallized surface can give suchprotection, and, if it is made extremely thin, it may be able to combine UV stability andvisible transparency Pigmented fluoropolymer and acrylic coatings can be applied to thepolymer, either by coextrusion of capstock or by post-coating, and provide such stability.More simply, dispersion of TiO2 and especially aluminum flake in the polymer can reflectaway most of the UV before it reaches more than a few surface molecules of the polymer,and this technique has been very popular.

5.1.4.2 UV Absorbers. Certain classes of additives absorb UV so efficiently that there isvery little UV left to attack the polymer They also have the little-understood ability to dis-pose of the excess energy harmlessly o-hydroxy benzophenones, and especially o-hydrox-yphenyl benzotriazoles, are quite successful, even in concentrations below 1 percent(Fig 5.3, Tables 5.11 through 5.13) Salicylic esters are less effective at lower cost Car-bon black is the most effective additive for stabilizing against UV degradation(Table 5.14), but, of course, it limits color to opaque black; also, it may generate so muchheat that it can cause thermal degradation Zinc oxide is the most efficient inorganic UV

FIGURE 5.3 Ultraviolet light stabilizers

absorber and is useful especially when combined with organic synergists, typically inHDPE and polypropylene.

5.1.4.3 Quenchers. When a polymer absorbs UV energy, it may be able to dispose of itharmlessly by intermolecular transfer to certain additives that can then carry the energy

away and dispose of it harmlessly These additives are referred to as energy quenchers

Or-TABLE 5.11 Polypropylene UV Stabilization:

Laboratory-Accelerated UV to 50 Percent Loss of Tensile Strength

TABLE 5.12 ABS UV Stabilization: Retention of

20 kg/m2 Impact Strength After Lab-Accelerated

0.5 percent UVA + 0.5 percent HALS 2000

TABLE 5.13 Polycarbonate UV Stabilization:

Laboratory-Accelerated UV Aging to Yellowness Index +5

0.25 percent UV absorbers 2800 hr

TABLE 5.14 ABS Stabilization by Carbon Black Impact Strength Retained After Five Years Outdoor Weathering

gano-nickel compounds are often useful as quenchers Carbon black probably functionspartly as a quencher

5.1.4.4 Hindered-Amine Light Stabilizers (HALS). Most UV degradation is actuallyphotooxidation—UV-accelerated free-radical attack by atmospheric oxygen The most re-cent and most popular way of stabilizing polymers against it is by addition of hinderedamines to interfere with the free-radical chain reaction

R2NH + O2→ R2NO.This nitroxide radical reacts with a degrading polymer radical R´.

lead-5.1.4.6 Prodegradants. When plastics accumulate in solid waste, it might be desirable

to accelerate their UV degradation This has been accomplished semicommercially by corporating enough C=O groups to absorb UV energy and initiate photodegradation pro-cesses It has also been demonstrated experimentally by adding transition metalcompounds such as ferrous laurate to catalyze photooxidation of the polymer (Table 5.16)

in-TABLE 5.15 Leading UV Stabilizers

Benzotriazoles 27 percentBenzophenones 20 percent

Use in polymers

Polypropylene 45 percentPolyethylene 29 percent

Engineering plastics 7 percentStyrenics 5 percent

These techniques do not destroy the polymer, but they embrittle it enough to crumble, andoxidize it enough to promote biodegradation later (Table 5.17).

5.1.5 Biostabilizers

Microorganisms such as bacteria, actinomycetes, and fungus can attack plastics, ing discoloration and degradation of mechanical and electrical properties They thrive pri-marily at 20 to 30°C and high humidity, whenever they can find a source of food Naturalpolymers such as cellulose and protein are a good source of food Animal fats and vegeta-ble oils are a good source of food; when they are used in paints, alkyds, and urethanes,these polymers are biodegradable Synthetic polymers that contain aliphatic hydroxyl andester groups may be a good source of food; these include polycaprolactone, polyester ure-thanes, and the new purposely biodegradable polylactic acid, polyhydoxybutyrate, andpolyhydroxyvalerate Fairly sensitive polymers include polyvinyl acetate, polyvinyl alco-hol, and ethylene/vinyl acetate Most other polymers are not inherently biodegradable.However, monomeric additives are often an excellent source of food and primary focus ofbiological attack: ester plasticizers, epoxy ester stabilizers, and natural esters used in poly-urethanes and fatty ester lubricants are the most common problems (Starch fillers have ac-tually been used to incorporate biodegradability in plastics.) A variety of chemicals can beused to stabilize plastics against biological attack

produc-TABLE 5.16 Accelerated UV Embrittlement of Polypropylene

Time to embrittlementFerrous laurate, % Unstabilized, hr Heat-stabilized, hr

TABLE 5.17 Fungus Growth* on Molded Plastics: Effect of UV Degradation

*.Trace = barely noticeable, slight = 10–30% of surface, moderate = 30–60% of surface, heavy = 60–90% of surface.

UV degradation before fungus test None 4 months

90 percent PS + 10 percent styrene/vinyl ether copolymer Trace Slight

50 percent PS + 50 percent styrene/vinyl ether copolymer Trace Moderate

Testing usually begins by placing plastics samples in Petri dishes, injecting ganisms, and observing whether they grow Further testing may include humidity, soilburial, and other natural exposures A major problem is that species of microorganismsvary from one geographic region to another, so it is hard to design reliable broad-spectrumlaboratory tests and to recommend successful additives from one region to another.The greatest problem is differential toxicity Any chemical that is toxic to microorgan-isms will probably be toxic to macroorganisms such as ourselves Thus, it is necessary todistinguish those additives that offer maximum toxicity toward microorganisms alongwith minimum toxicity toward macroorganisms, and to define the critical balance for dif-ferent plastic products.

microor-5.1.5.1 10,10´-oxy-bis(phenoxarsine) (OBPA). This (Fig 5.4-I) is the leading cial antimicrobial It is very efficient, so it can be used at very low concentration (0.04 per-cent) and can be synergized by bis(trichloromethyl) sulfone

commer-5.1.5.2 2-n-octyl-4-isothiazoline-3-one. This (Fig 5.4–II) is a newer antimicrobial that

is nontoxic to humans and is used at 3 percent in vinyls and paints

5.1.5.3 Trichloromethyl Thio Phthalimide. This (Fig 5.4–III) is harmless to humansand is useful at 0.25 to 0.50 percent to control actinomycetes, which cause pink staining ofplasticized vinyls

5.1.5.4 Diphenyl Antimony 2-Ethylhexoate. This (Fig 5.4–IV) is approved for use invinyl shower curtains, wallpaper, upholstery, and rug underlay

5.1.5.5 Copper Quinolinolate. This (Fig 5.4–V) is relatively harmless to humans Used

at 0.5 percent, it controls mildew Because of its deep yellow-green color, it is used mainlyfor military purposes

FIGURE 5.4 Biostabilizers

5.1.5.6 Tributyl Tin Oxides. These (Fig 5.4–VI) have been useful in vinyls, thanes, and marine paints Use is decreasing because of worry about toxicity.

polyure-5.1.5.7 Copper Powder. At high loading (70 percent), copper powder has been mended for control of fouling in marine paints

recom-5.1.5.8 Alkyl Amines. Alkyl amines have been grafted onto polymer surfaces in recentresearch to make them bactericidal

5.1.5.9 Use in Commercial Plastics. The major use is in plasticized PVC to protect theester plasticizers Other wide uses are in polyester urethanes and in oil paints Typicalproducts include shower curtains, wall and floor coverings, carpet underlay, marine uphol-stery, awnings, refrigerator gasketing, weatherstripping, swimming pool liners, waterbeds, and hospital sheeting

5.2 FILLERS AND REINFORCEMENTS

When large amounts of solid materials are finely dispersed in a polymer matrix, we call

these materials fillers or reinforcements In terms of total tonnage, these are the leading

type of additives in plastics Some of their effects are quite general Many of their specificeffects are so different that it is best to study them in four distinct classes

5.2.1.1 Packing. Many of these properties are proportional to the volume fraction offillers or fibers added Maximum packing fraction can be calculated geometrically andconfirmed experimentally For spherical particles, maximum packing fraction can go ashigh as 85 percent For conventional fibers, it can go as high as 91 percent Man-made fi-bers with rectangular or hexagonal cross sections are easy to make and theoretically can bepacked neatly to approach 100 percent!

5.2.1.2 Processability. Dispersion of polar fillers and fibers in the molten polymer quires special care to produce interfacial wetting and shear mixing to produce dispersion.Fillers and fibers rubbing against screws and channels produce frictional heating, and theyadd thermal conductivity; both effects can speed the processing cycle They do increaseviscosity considerably, which makes processing more difficult, and they are so hard thatabrasion of process equipment requires more frequent replacement

re-5.2.1.3 Mathematical Modeling. Mathematical modeling can attempt to predict and tionalize effects on properties but requires so many assumptions that it leaves quite a gapbetween theory and practice

ra-5.2.1.4 Modulus. Modulus is increased greatly, because, when flexible polymer cules bump against the hard surface of inorganic particles, they lose much of their inherentflexibility The effect is most pronounced for fibers in the axial direction, because, even ifthe polymer is willing to respond, the high-modulus fibers absolutely refuse to respond at

mole-all Creep Resistance correlates with modulus, both theoretically and practically This can

bring performance of plastics much closer metals and ceramics

5.2.1.5 Breaking Strength. Breaking strength is increased greatly by continuous fibers;when the polymer is ready to fail, the high-strength fibers absolutely are not Short fibersmay or may not increase strength somewhat, depending on stress-transfer across the fiber/polymer interface; they may actually decrease it, because the fiber ends act as stress con-centrators, causing premature failure Particulate fillers usually decrease strength due tostress concentration at sharp edges and corners of the filler particles

5.2.1.6 Impact Strength. This is increased tremendously by continuous fibers(Fig 5.5); they seem to distribute the shock over the entire length of the fiber so that thestress at any one point is very small Short fibers are unpredictable; they may increase im-pact strength moderately or not at all, or even decrease it, their ends acting as stress con-centrators Particulate fillers almost always decrease impact strength, again due to stressconcentration at their sharp edges and corners Impact strength theory is seriously handi-capped by the assumption that the same failure mechanisms operate at both low speed andhigh speed; it would be much better to recognize that high-speed impact failure is a com-pletely different phenomenon that deserves its own theoretical analysis

5.2.1.7 Friction and Abrasion Resistance. These qualities are increased by the sharpedges of filler particles and the sharp ends of fibers that protrude from the surface of thepolymer matrix

FIGURE 5.5 Unbreakable plastics

5.2.1.8 Coefficient of Thermal Expansion (CTE). CTE is inverse to the attractiveforces holding the molecules together The weak secondary attractions between polymermolecules permit a high rate of thermal expansion, whereas the strong primary forces ininorganic materials restrict them to a much lower rate of thermal expansion For simpleextender fillers, the expansion rates of polymer and filler are simply additive, so the CTEsimply decreases in proportion to volume fraction of simple extender fillers (Fig 5.6) Re-inforcing fillers are more effective, and reinforcing fibers are most effective in reducingthermal expansion, because they restrict the molecular motion of the polymer molecules.This brings plastics closer to the performance of metals and ceramics.

5.2.1.9 Heat Deflection Temperature. This is increased slightly in amorphous mers, because the fillers or fibers reduce the mobility of the polymer molecules It may beincreased tremendously in crystalline polymers, because fillers and especially fibers raisethe plateau of the modulus versus temperature curve just enough to extend the pass/faillimit of the standard test by hundreds of degrees (Fig 5.7, Table 5.18) The practical sig-nificance of this obviously depends on the judgment of the product designer

poly-5.2.1.10 Thermal Conductivity. The thermal conductivity of inorganic fillers and fibers

is higher than organic polymers, so adding them does increase conductivity in proportion

to volume fraction (Sec 5.2.5.2)

5.2.1.11 Flame Retardance. Flame retardance is increased somewhat, because fillersand fibers increase both viscosity and thermal conductivity (Secs 5.2.5.3 and 5.7)

5.2.1.12 Dielectric Constant and Loss. These are much higher in highly polar ganic materials, so fillers and fibers generally increase them proportionally in plastics

Volume Fraction of Filler

FIGURE 5.6 Effect of fillers on coefficient of thermal expansion

5.2.1.13 Opacity. Opacity results from the fact that inorganic fillers and fibers aredenser than organic polymers, so the speed of light is slower, so their refractive index ishigher, so light waves are scattered and dispersed as they pass through the interface Fill-ers are often used to produce opacity Conversely, to seek transparency in filled and rein-forced polymers, one must either match the refractive indices of the two phases or reducethe particle size below the wavelength of visible light; both of these approaches are verydifficult.

TABLE 5.18 Effect of Fillers on Heat Deflection Temperature

Polymer Unfilled HDT, °C Glass fiber, % Filled HDT, °C

5.2.1.14 Swelling and Permeation. These are reduced, because fillers and fibers restrictfree volume and mobility of the polymer matrix, making it harder for small molecules ofliquids and gases to dissolve and diffuse through the polymer, and because the small mol-ecules must permeate around the impervious particles—a “tortuous” route that further im-pedes permeability On the other hand, the high polarity of most fillers and fibers mayattract moisture to penetrate along their interface with the polymer, weakening stresstransfer across the interface, and often plasticizing and even hydrolyzing the polymer; this

is particularly noticeable in outdoor weathering

5.2.1.15 Cost. The cost of simple extender fillers may be lower than polymers on aweight basis, but their higher density, more difficult processability, and decrease instrength properties may eliminate any overall economy Fillers should be chosen primarilyfor their beneficial effects on technical properties; if they also decrease cost, this is simply

an added benefit Reinforcing fibers increase the cost of commodity plastics, but they mayactually reduce the cost of some high-end engineering thermoplastics

5.2.2.2 Calcium Carbonate. This is the most widely-used economic extender for mers Benefits commonly reported include processability, hardness, dimensional stability,whiteness, opacity, gloss, and mar resistance Particle sizes range from 0.125 in for groundmineral grades down to submicron sizes for chemically precipitated grades that may evenreinforce strength and impact strength; price is generally inverse to particle size Calciumstearate surface treatment improves most of these properties

poly-5.2.2.3 Titanium Dioxide. This is the leading white pigment in coatings and is alsowidely used in paper and plastics (Relative market volumes are coatings 50 percent, paper

25 percent, and plastics 25 percent.) Its high refractive index produces opacity, and itschemical and UV stability produce weather resistance (It is important to use the rutilegrade for weather resistance; the less-stable anatase grade is strictly for paints that erodegradually, producing a chalky surface that is self-cleaning, washing away easily to sheddirt and mold.)

5.2.2.4 Clays. Clays such as kaolin are finer than calcium carbonate, typically 0.2 to

10 µm, providing more reinforcement They are used to increase the viscosity of polyesterbulk molding and sheet molding compounds; to give hardness, opacity, and whiteness invinyl flooring; and to increase heat and electrical resistance in wire and cable insulation.They are improved by calcining and by silane surface treatment New delamination treat-ments to produce extremely small particle size are the basis of current developments innanotechnology (Sec 5.2.3.6)

5.2.2.5 Silica. Silica is a naturally occurring mineral that is ground down to particlesizes of 2 to 10 µm and used as a low-cost, stable, white filler

5.2.2.6 Talc. Talc is a magnesium silicate mineral, often used in polypropylene to prove processing, rigidity, creep resistance, and heat deflection temperature

im-5.2.3 Reinforcing Fillers

Fibers increase strength but make melt processing much more difficult Reinforcing fillersare fine particles that permit fairly normal melt processing but do increase strength; they

are often referred to as mineral reinforcement When they are examined under a

micro-scope, they are generally plate-like or fiber-like in appearance Theoretically, the strength

of reinforced plastics depends on the force required to pull a fiber out of the polymer trix: if the fiber is embedded far enough into the matrix, it must break before pulling out.Model calculations often conclude that, when the aspect (L/D) ratio is greater than 20/1,the fiber will not pull out before it breaks Many reinforcing fillers appear to have morethan the critical aspect ratio of 20/1

ma-5.2.3.1 Wood Flour. Wood flour is made by controlled attrition of wood and containsmicroscopic cellulose fibers It was first used in phenolic plastics to increase their strength,and it remains the basis of general-purpose phenolic moldings It was occasionally used inother plastics and is currently gaining popularity in vinyl and other thermoplastic wood/plastic composites for processability and durability superior to wood alone At high load-ings in HDPE and PVC, these “wood/plastics composites” look like wood but are moredurable, and they are finding growing use as “plastic lumber” in outdoor construction andfurniture

5.2.3.2 Wollastonite. This is a calcium silicate mineral, acicular, with aspect ratios of 3

to 20/1 It is of interest for reinforcement of strength and as a safe replacement for tos

asbes-5.2.3.3 Franklin Fiber. This is a calcium sulfate crystal with aspect ratios of 60/2 µm It

is an easy-processing reinforcement but suffers from water sensitivity

5.2.3.4 Mica. Mica is a potassium aluminum silicate mineral that occurs as flakes withaspect ratio up to 50/1 The best grades offer good processability, reinforcement, and im-permeability

5.2.3.5 Asbestos. Asbestos is a low-cost magnesium silicate mineral that occurs as veryshort, fine fibers of high modulus, strength, and thermal and chemical resistance It was apopular filler until it was noticed that it collected in the lungs and caused serious healthproblems Its use has been discontinued except in critical applications such as brake lin-ings Since then, a number of other promising short, fine fibers have been abandoned forfear that they may cause similar problems

5.2.3.6 Nanofillers. Nanofillers are extremely fine particles, under a micron in size Themost successful ones have been made by intercalating quaternary ammonium surfactantsbetween the layers of montmorillonite clay, followed by fluid polymer, to exfoliate themdown to 1-nm platelets with aspect ratio of 1000/1 When these are dispersed in nylon atlow concentrations of 2 to 10 percent, the tremendous numbers of plate-like particles canproduce easy processing, high modulus and strength, heat deflection temperature, trans-

parency, and impermeability Typical studies report flexural modulus increased 126 cent, flexural strength increased 60 percent, HDT increased 87°C, and impermeabilityincreased fourfold Since they are smaller than the wavelength of visible light, they do notreduce transparency The technology is being extended into commercial practice, includ-ing a variety of other fillers and polymers

per-5.2.3.7 Carbon Black. Carbon black is made by cracking organic oils in a ature furnace, producing particle sizes in the 10 to 100 nm range The best grades, seen un-

high-temper-der an electron microscope, are clusters of particles, referred to as high-structure The

aromatic carbon rings are attracted to the C=C bonds in rubber and may graft to them ing vulcanization They give such high-strength reinforcement of rubber that their use isalmost universal For some reason, they do not reinforce the strength of plastics but arevery useful for UV stabilization and electrical semiconductivity

dur-5.2.3.8 Fumed Aerosil Silica. This is produced by mixing SiCl4 with steam Hereagain, the particle size is in the nanometer range, and high-structure clusters give good re-inforcement to silicone rubber They also give extreme viscosity and thixotropy to liquidsystems such as vinyl plastisols and epoxy resins

5.2.4 Reinforcing Fibers

Fibers have much higher modulus and strength, and much lower thermal expansion, thanbulk polymers, so dispersing them in a polymer matrix can produce an excellent increase

in modulus, strength, and dimensional stability

5.2.4.1 Glass. Continuous glass fibers are typically sium silicate, melt spun at 2400°F (1316°C), and 9 to 18 µm in diameter When they areincorporated into plastics, they produce the highest modulus, strength, and impact strengthever achieved (Table 5.19) However, processing of continuous fiber is limited to special-ized techniques such as filament winding, pultrusion, and compression molding Forbroader application, glass fibers are chopped 1 to 2 in (~25 to 50 mm) long for sheet mold-ing compound, 0.5 to 1 in (~13 to 25 mm) for bulk molding compound, and 0.125 to 0.5 in(~3 to 13 mm) for thermoplastic molding and extrusion This does permit fairly conven-tional melt processing, but it certainly sacrifices a good portion of the potential properties,whether processors admit it or not (Table 5.20) Optimum performance depends on stresstransfer between polymer and fiber, and fiber ends act negatively as stress concentrators.Furthermore, fiber breakage during melt flow severely reduces the final length of the fi-bers, reducing properties even further Nevertheless, it is still possible to improve thermo-plastic properties considerably by adding glass fibers, so the technique is very popular(Table 5.21)

calcium/aluminum/boron/magne-5.2.4.2 Mineral Wool. Mineral wool is a low-cost silicate fiber spun from molten slag insteel refineries It is widely used as thermal insulation in housing and appliances Since itscomposition and structure are not well controlled, it is not comparable with chopped glassfibers; however, it is sometimes used as a partial replacement for them Jim Walters Pro-cessed Mineral Fiber (PMF) in particular has been reported for such applications

5.2.4.3 Specialty Fibers. Specialty fibers offer benefits, but, because they are expensive,they are only used in special high-performance products Carbon fibers are made by pyro-lyzing polyacrylonitrile, producing amorphous carbon reinforced by crystalline graphitefibrils; they offer high strength, lubricity, and electrical conductivity Aramide fibers arearomatic polyamides; they offer low density, impact strength, vibration damping, and wearresistance

5.2.5 Specialty, or “Functional” Fillers

Aside from their use for economics or mechanical reinforcement, a number of fillers areused to improve a variety of specific properties, as described below

5.2.5.1 Lubricity and Abrasion Resistance. In plastic gears and bearings, these ties are improved by adding solid powders such as brass, molybdenum sulfide, graphite,polyethylene, and especially polytetrafluoroethylene

quali-TABLE 5.19 Maximum Properties of Reinforced PlasticsEpoxy/glass Modulus to 5,500,000 psi

Flexural strength to 70,000 psiImpact strength to 10 fpiHDT to 600°FThermoset polyester/glass Modulus to 3,000,000 psi

Flexural strength to 80,000 psiImpact strength to 30 fpiHDT to 500°F

TABLE 5.20 Impact Strength of Reinforced Nylon 6,6

Fiber length, inches fpi

5.2.5.2 Thermal Conductivity. Thermal conductivity can be increased to shorten ing cycles and to avoid overheating of electrical equipment Silver, copper, and aluminumhave conductivities 1000 times that of unfilled plastics; loading them into plastics can in-crease conductivity considerably, in proportion to their volume fraction (Table 5.22) Be-ryllium oxide, boron nitride, aluminum oxide, aluminum nitride, and graphite are alsoquite effective.

mold-5.2.5.3 Flame Retardance. Flame retardance is commonly produced by adding solidpowders of organo-bromine, organo-chlorine, antimony oxide, and inorganic hydrates(Sec 5.7) It is also reported that fillers and reinforcements in general can contribute toflame retardance by increasing melt viscosity and heat transfer

5.2.5.4 Electrical Conductivity. This quality is important to bleed off static charge and

to avoid electromagnetic interference (EMI) (Sec 5.9) It can be produced by adding bon black, graphite, and especially metallic fillers (Table 5.23) This requires particle-to-particle contact, so flakes are more efficient than simple powders, and fibers are most effi-cient of all (Table 5.24)

car-5.2.5.5 Magnetism. Magnetism can be produced by magnetic fillers such as barium rite This produces moldable magnets that are nonconductive and rust resistant

fer-5.2.5.6 Color and Opacity. These features are, of course, produced by fillers, both ganic and organic (Sec 5.8), in much lower concentrations than are normally considered

inor-“fillers.”

5.2.5.7 Ultraviolet Light Stabilization. UV light stabilization is produced by fillersthat reflect UV, particularly aluminum flake and TiO2, and by fillers that absorb UV ra-diation and reduce it to harmless wavelengths, particularly carbon black and zinc oxide(Sec 5.1.4)

TABLE 5.22 Thermal Conductivity of Filled Plastics

Material Thermal conductivity, Btu/[(ft2-hr-ºF)/ft]

5.2.5.8 Impermeability. Impermeability (barrier performance) is produced by plate-likeflakes, which increase the tortuous path that permeating molecules must seek.

5.2.5.9 Controlled Degradability. This has been produced by use of biodegradable ers such as starch powder Once the filler has disappeared, the polymer crumbles, and thehigh surface area accelerates oxidative and biodegradation

fill-5.2.5.10 Carbon Nanotubes. These are tiny hollow fibers made up of carbon atoms ranged in a hexagonal pattern, in flat sheets that roll up into seamless tubes Diametersrange from 1 to 200 nm and aspect ratios up to 10,000! Their modulus, strength, and ther-mal and electrical conductivity are superior to graphite and carbon fiber Used at 1 to 5percent in plastics, they provide very high modulus, strength, and thermal and electricalconductivity Processing is difficult, and cost is extremely high, but researchers are opti-mistic about their future

ar-5.2.6 Technical Summary

The relative effects of fillers and reinforcements on plastics may be clarified by ing them in tabular form (Table 5.25) In the table, (+) means an increase in the property,(++) means a great increase, (–) means a decrease, (– –) means a great decrease, and (±)means the effect varies depending on the specific filler, fiber, polymer, or test

summariz-TABLE 5.23 Electrical Conductivity of Filled PolymersMaterial Log volume resistivity (Ω-cm)

Graphite-filled coatings 1 to 2Nickel-filled epoxy 0 to –2

Aluminum, copper, silver –6

TABLE 5.24 Electrical Conductivity of Reinforced Plastics: 40 Percent by Weight of Fiber

Fiber Log volume resistivity (Ω-cm)