Polymer Additives Part I. Mechanical Property Modifiers Malcolm P. Stevens University of Hartford, West HarLford, CT 06117
In recent years a variety of topics dealing with polymer chemistry have been reviewed in this Journal, including elastomers (11, flammability (21, fluoropolymers (31, glass transition temperature (4, 51, history (6-9), ionomers (101, kinetics and mechanism (11, 12), mechanical properties (13),molecular weight (141, morphology (151, polymeric insulators (161, polymers in lithography (171, rheology (18). rubber elasticity (19, 201, and sealants and caulking compounds (21). In this paper we discuss materials, both inorganic and organic, that are added to polymers to modify their properties, thus making them more suitable for industrial and consumer applications. Several monographs (22-251, handbooks (26,271, and occasional survey articles (28,29) have been published on polymer additives. There Table 1. Polymer Additives
Function Mechanicalproperly modifiers Fillers Increase strength,reduce wst Impact modifiers Improve impact strength Nucleating agents Promote crystallinity Plasticizers Increase flexibility Increase strength and stiffness Reinforcing fibers Surfacepropedy modifiers Prevent film or sheet from sticking Antiblocking - agents together Prevent moisture from obscuring Antifogging agents film clariiv Prevent Static charge build-up Antistatic agents Improve bonding to filleror Coupling agents reinforcing fiber Release agents Prevent sticking Chemical properly modifiers Antioxidants Prevent oxidative degradation Biocides Prevent microbial attack and mildew Flame retardants Reduce flammability Prevent degradation by sunlight Ultraviolet stabilizers Aesthetic properly modifie~s Coloring agents Impart color Prevent development of odor Biocides Add fragrance,mask Odorants objectionable odors Nucleating agents Improve light transmission Processing modfiers Blowing agents Manufacture foams Crosslinking agents Promote crosslinking (curing) Reduce foaming, remove trapped Defoamingagents air Emulsifiers Stabilize polymer emulsions Heat stabilizers Prevent thermal degradation Prevent shrinkage and warpage Low profile additives Plasticizers Reduce melt viscosity Prevent sticking to processing Release agents machinely lncrease viswsity of polymer Thickening agents solutions or dispersions TYpe
444
Journal of Chemical Education
also is a periodical newsletter (30) and a comprehensive annual updating of the field in a plastics encyclopedia (31). Additives serve to alter the polymer's properties and to enhance the polymer's processability. They provide a remarkably broad range of functions, as may be seen from the listing in Table 1.As such, polymer additives present an exceptional opportunity to illustrate a variety of practical applications in courses dealing with inorganic and organic materials. They also represent an important element of the industrial or polymer chemistry curriculum. Some additives play dual roles; for example, plasticizers may be used to impart flexibility to the finished product or to reduce the polymer's melt viscosity during processing. Additives may he completely miscible with the polymer or-in the case of fillers and fiber reinforcers-eompletely immiscible. Most surface property modifiers have limited solubility; hence, they migrate to the polymer surface where their properties are manifested. Some additivesant~oxidants'or~ultravioletstabilizers, for e x a m p l e a r e used in relatively low concentration, while others, notably fillers, reinforcing fibers, and plasticizers, usually represent a significant percentage of the finished polymer formulation. Rarely do polymers contain a single additive. Usually a combination of ingredients is desirable. For illustration, Table 2 lists various polymer types and a representative set of additives that find use in certain applications of those polymers. It should be kept in mind, however, that some additives may be used almost exclusively in only one or two polymers. Plasticizers, for example, are used in large volumes in poly(viny1 chloride), hut in relatively small amounts in the other indicated polymers. Approximately 5.0 x lo9 kg of plastics additives are produced annually in the United States, which amounts to about 19 kg of additives per 100 kg of polymer consumed. Most important in terms of consumption are fillers, which represent about 58% of all additives used on a weight basis, followed by plasticizers (16%), reinforcing agents (lo%), flame retardants and coloring agents (5%). Total production is projected to rise to about 6.6 x lo9 kg by the end of the century (28); however, the amount of additive used per unit weight of polymer is gradually declining because of continuing improvements in performance. It is estimated that more than 4000 individual products are used as polymer additives. The additives listed in Table 1 are those currently in widespread commercial use. Future years will see other types of additives as products now in development gain market acceptance. Dopants that increase the electrical conductivity of conjugated polymers (32, 331 are a case in point. Additives that promote polymer degradation for controlled-release applications (34, 35) or microlithography (171, or for reducing the volume of plastics waste also are likely candidates for new developments and growth. Furthermore, increasing concern over possible toxicological effects of polymer additives in the environment, worker exposure, and problems arising ,during polymer recycling
Table 2. Applications of Representative Additives In Polymers
Polymer type
Anti- Blowing Flame Impact Plasticizer oxidant agent retardant modifier
UV stabilize!
Thermoplastic ABS~
Acetalb Acrylic Cellulosics Nylon (polyamide) Polycarbonate Polyester polyethyleneC Polypmpylene Poly(pheny1ene oxide) Polystyrene Poly(viny1acetate) Poly(viny1chloride)
.
Ca(OH)z().
+ COZ(g) --t CaC4 ca) + HzOa,
Colloidal silicas are prepared by treating sodium silicate solution with mineral acids,
-
Phenolic ~o~yesterd Polvurethane
or by burning a mixture of tetrachlorosilane with natural gas to generate a silica "smoke": SiCL 0, + CH4 (,I
.
+ 0% (,I
+
SiOz (8, + coz (8, + 4HC1 I, Addition of fillers to plastics usually results in stiffening of the latter with a subsequent increase in tensile strength. Exactly how the filler and nolvmer interact is not known with certainty, but it is presumed that dioolar attractions between oolvmer and the filler surface'and, in some instances, ch.&ial bonding with the su,.face are important. phenol-formaldehyde and ureaformaldehyde resins bond chemically t o wood flour and other cellulosic fillers by electrophilic substitution of methyl01 groups on the polymer with the phenolic or phenolic ether rings of the lignin component of the filler.
a A c ~ l o n i t r i l ~ t a d i e n ~ ~copolymer rene b~oly~xymethylene 'High- and IowdensW
dunsaturated
..
could well foster changes in the array of substances incurrent use. In this part of the series we discuss additives used to enproperties. Surface, and aesbe thetic property modifiers and processing covered in subsequent installments. Mechanical Property Modifiers Fillers
organic fdler, especially in phenol-formaldehyde and urea-formaldehyde polymers. Other organics include ground nut shells and corncobs. Cellulose is commonly used in light-colored melamine-formaldehyde dinnerware. Mineral fillers are manufactured most commonly by quarrying, crushing, and milling; however, some are synthesized to effect a more uniform, finer particle size. One type of calcium carbonate filler, for example, is made by bubbling carbon dioxide through an aqueous sluny of calcium hydroxide (slaked lime)
F t
Although fillers (or extenders, as they often are called) are used primarily for improving strength and reducing cost (more often the latter), they serve a variety of other functions,especiallyin the coatings and paper industries where flow properties, gloss, and hiding power are imoortant considerations. With thermosetting polymers additives serve to reduce shrinkage during crosslinking. Metallic fillers also are used in plastics to increase conductivity and heat transfer or to facilitate metal plating. Inorganic fillers, which are used primarily with thermoplastic polymers, include compounds such as calcium carbonate; silicates such as kaolin; talc or mica; silicas such as diatomite or silica gel; alumina trihydrate; and barium sulfate (barite). Each comes in a variety of mesh sizes and morphologies (36). Some are aerated to reduce their densities. Other fillers include fly ash, a by-product from coalburning power plants, pulverized anthracite, and perlite, a volcanic ash. Carbon black is used primarily as a reinforeing filler for rubber. Glass beads fmd occasional use, especially in traffic paints, because of their reflecting properties. Metallic fibers include aluminum, iron, and bronze. Of the various inorganic fillers, calcium carbonate is used in the greatest amount. Wood flour is the most widely used
--r
%OH + M
Impact Modifiers
Plastics frequently are classified as either tough or brittle. Toughness is a measure of the material's impact strength, or its ability to withstand a blow without fracturing (13).To reduce brittleness (improve impact strength) rigid polymers may be blended with more flexible polymers that dissipate the energy of the impact through molecular motions. It is important, therefore, that the polymeric impact modXier have a glass transition temperature (4, 5) well below ambient temperature. Much of the impact modifier production is used with poly(viny1chloride) (11, but other polymers also use significant quantities (See Table 2). More than half of all the polystyrene produced, for example, is impact polystyrene, which is an immiscible blend of polystyrene (2) and 5 to Volume 70 Number 6 June 1993
445
Table 3. Commonly Used External Plasticizers
10%by weight of the elastomeric polybutadiene (3)modifier. Acrylonitrile-butadienestyrene (ABS) and methacrylate-butadiene-stvrene (MBS) cooolvmers represent another class of imp& modifier.Theie consist of polystyrene and polyacrylonitrile (4) or poly(methy1 methacrylate) (5)
grafted to a polybutadiene main chain. Other impact modifiers include chlorinated polyethylene, ethylene-vinyl acetate copolymers (6), poly(isoocty1 acrylate) (7), and "nitrile rubber" (a copolymer of butadiene and acrylonitrile.) The acrylic and chlorinated polyethylene modifiers exhibit good weatherability, hence they find use in siding, gutters, and other outdoor applications.
Nucleating Agents Nucleation is the onset of crystallinity in an amorphous polymer matrix (15).Crystalline polymers have highly ordered, aligned polymer chains. The consequent strong intermolecular forces imoart hieh ., streneth to the oolvmers: . however, optimum mechanical properties necessitate a uniform distribution of crvstallites. Such a momholoev results from the introduction of nucleating agentsAwhi*provide a surface for nucleation. Finely divided, thoroughly mixed nucleating agents thus promote formation of large numbers of uniformly distributed, small crystallites. An added benefit with translucent plastics of the type used in lamp covers or decorative panelling is that uniform crystallinity provides enhanced light transmission, which is why nucleating agents also are listed in Table 1as aesthetic property modifiers. Anv finely divided inert material is appropriate as a nucleat& agent. Most commonly silica o;day& having particle sizes of 40 nm or less are used. Salts such as sodium benzoate also find occasional use.
-
Plasticizers Plasticizers are used either to make polymers more flexible (i.e., reduce the glass transition temperature (4,5)),or to lower a polymer's melt viscosity (15)to facilitate processing. One of the classic examples of plasticizer innovation occurred in the 19th century when the Hyatt brothers treated cellulose nitrate with camphor to create a viable substitute for ivory. Softening of leather with neat's-foot oil is another example of plasticization. 446
Journal of Chemical Education
Aromatic estem Dibutyl phthalate Dimethoxyethyl phthalate DiP-ethylhexyl phthalate Di-moctyl phthalate Di-Coctyl phthalate Di-Cdecyl phthalate Di-mundecyl phthalate Di-mtridecyl phthalate Butyl benzyl phthalate Butyl octyl phthalate Tri-2-ethylhexyltrimellitate Aliphatic esters Di9-ethylhexyladipate Di-Cbutyl sebacate Di-Bethylhexyl sebacate Di-Coctyl sebacate DiP-ethylhexyl azelate Epoxidized oils Epoxidized soybean oil Epoxidized linseed oil Epoxidized tall oil fatty adds Flame retardant Aryl and atyllalkyl phosphates Chlorinated paraffins Polymeric Poly(alky1eneadipates, sebacates,and azelates Acrylonitrile-butadiene copolymers Polymer scientists make a distinction between ezternal plasticizers that are dissolved in the polymer, and internal plasticizers that are chemically bonded to the polymer. Between 80 and 90% of all external plasticizer production goes into one polymer, poly(viny1 chloride). By itself, poly(viny1 chloride) is rigid and dimcult to process. It is unaffected by temperatures as high as 160 'C. If small amounts of plasticizer are added, however, the polymer softens a t 160 'C and can be press-rolled into semirigid sheets for such applications as floor tile. At plasticizer levels of about 50 wt %, poly(viny1chloride) is flexible enough for use in shower curtains and vinyl upholstery. There are a tremendous number of plasticizers available commercially (37, 381, the most important of which are listed in Table 3. Phthalate esters account for more than two-thirds of U.S. plasticizer production, with di-2ethylhexyl phthalate (8) (also called dioctyl phthalate or DOP) being the most widely uaed. Also available, besides those listed, are a variety of citrate, isophthalate, terephthalate, and fumarate esters, carboxylate esters of diols and polyols, sulfonamides, and some miscellaneous polymeric plasticizers.
A number of hypotheses have been advanced to explain the phenomenon of plasticization. One suggests that the plasticizer acts as a lubricant, reducing intermolecular friction and allowing the polymer chains more freedom of motion. Another pr6posei that the plasticizer "solvates" polar sites on the polymer chains, especially a t high plasticizer levels. therebv reducine intermolecular attraction. p r o p ~ ~ a r t hthermal at motions in the Most prevaient is low-molecular-weieht ~lasticizermolecules increase the polymer's free volume, thus providing room for longer range segmental movement of the chains. Of particular importance is the plasticizer's permanence or its resistance to loss bv evanoration or bv leaching when the plastic comes into contact' with solvenki or lub&ants. Two factors contribute to permanence: the plasticizer's moand its comlecular weight (and hen& its vapor patibilitv with the polvmer. For vinyl car interiors, for example,-where high temperatures a r e a common occurrence, the less volatile di-i-decyl and di-n-undewl phthalates, and tri-2-ethylhexyl trimellitate, as well as polymeric plasticizers, are preferred. In older automobiles it was not uncommon on-hot days to find the interior coated with a fme film of plasticizer condensate. Compatibility is governed by the free energy of mixing, AG, which is related to the heat (AZ% and entropy (AS)of mixing by the familiar relationship,
tee -
A
Polymer and plasticizer are miscible when AG is negative. Because A S is invariably positive, the magnitude of Alf determines the sign of AG. According to Hilderbrand's theory of solutions (391,AHis related to concentration and the molar energies of vaporization of solvent and solute (Eland AE2,respectively),by the expression,
where V is the total volume.. V,. and V.- are the molar volumes of solvent and solute, respectively, and ol and $2 are the volume fractions. The term EN is called the cohesive energy density, which is the energy needed to remove a molecule from its nearest neighbors. By replacing (AEN)" with the symbol 0,defined as the solubility parameter, the above relationship can be simplified to
A
The closer the two solubility parameters are to each other, the smaller will be AH, and the more com~atiblethe solvent and solute. For lowmolecular-weight~om~ounds, solubility parameters can be calculated from the latent heats of vaporization. Polsmers, however, have no vapor pressure-for all practical purposes, and the vapor pressures of plasticizers are gen&aily too low to yield reliable energy-values. lnstead;approximate values of S1 and & may be obtained from group molar attraction constants (GI, which assign numerical values to molecular groupings based on intermolecular forces. The G values defmed in the literature (40, 41) are related to S by the expression,
where d is density and M is molecular weight.
Reinforcing Fibers The combinationof fiber (or filler) and polymer is termed a composite. The great advantage of fiber composites lies in their strength and toughness, which are usually greatly improved over those of the individual components. Because of their high strength-to-weight ratio, fiber-reinforced plastics are increasingly replacing metals in engineering (42). - applications .. Three reinforcing fibers dominate the market: glass, aramid, and carbon (graphite). Each is available in a variety of forms depend& on the application, including continuous fdament (roving), chopped fiber, woven cloth, mat, and pulp. Usually the fibers are treated with surface property modifiers called coupling agents (to be discussed later) to promote interfacial attraction between fiber and polymer. Glass is the most widely used and least expensive of the reinforcing fibers. Aramid (i.e., aromatic polyamide fiber) consists almost entirely of poly@-phenylene terephthalamide) (91, more commonly known by its du Pont trade name Kevlar. which is svnthesized from terephthaloyl chloride and 1.4 benzenediamine by interfacial polymerization. The aramid fibers are manufactured by kxtruding a lyotropic liquid crystal solution of the polyamide through the holes of a spinneret into an aqueous bath, called uet spiining. The very high tensile a strength of the fiber is due in large measure to the molecular alignment (and consequent strong intermolecular forces, in the liquid crystal phase. Carbon fiber is manufactured bv the controlled ovrolvsis longitudinally stressed polyac~onitrilefiber~.'6~cfization is a multiste~. .. free radical Drocess leading.. to loss of hydrogen cyanide and nitrogen, although some residual nitrogen is usually present in the fin~shedproduct.
&m /
/
/
/
Both aramid and carbon fibers are found primarily in high-strength composites for aerospace and military applications, as well as in sporting equipment, while glass fiberreinforced plastics are widely used in automobile components, boat hulls, corrosion-resistant industrial equipment, circuit boards, and household items. Aramid also is used as a reinforcing agent in automobile tires. Literature Cited 1. Ksufman, G. B.; Seymymym.R B. J. Chem. Edur IssO,67,422: 11)81,68,217. 2. Fador. A. J. Cham. Educ. 1974,51,453. 3. WaUeq E J. J Chm. Edm. 1888.66.481. 4. Beck K R.; h e m e y e r , R.; Kunz,R. J. J Chm. Edm. 1984,61,668. 5. Bufeld, D.R. J. Cham. Edm. 1887.64,875. 6. Mark, H.J. Cham. Edue. 1881,56,527; 5.ChemEdue. 1987,64,858. I. Marvel, C. S. J. C h . Edm. 1881,58,535. 8. Seymour.R. B. J. Cham Edvc 1988.65.321. 9. Kauffman,G. B. J. Chrm. Educ. 1988,66,803.
Volume 70 Number 6 June 1993
447
10. Dieh1,J. W J.Chom.Educ 1383,66.901. 11. McGrath, J. E. J. C h m . Educ. lWl.58.844 12. Stille, J. K J Chom Educ. 1981.58.662. 13. Aklonis. J. J. J Chom E d v c 1981.58.802. 14. Ward,? C. J. Chom. Edue. 1981,58,867. 15. Geil,P.H.J C h m E d u c 1981,58.879. 16. Wang, S. F.;Grossmsn, S. J. J Chem. E d u c 1987,M,39. 17. Turner, S. R.: Daly, R. C. J C h m . Edue. l w ,65,3B. 18. Wilkea, G.L.J. C h m . E d u r 1981,58,867. 19. Mar*, J. E. J. C h m . Edvr 1981,58,898. 2 0 h w s l z - 1 .I. P: Mark. J E. J Cham. Educ. 1987.64.491.
NewYmk 1990. h d u s h i o l Gui&,Nayes: WrkRidge, NJ, 1986. 27. ~ k kE, w ~1~trcsAdditiues:An 2%Greek, B.t? Chem. Engin. News. 1988,66(28),35. 29. Seymour, R. B.InEnr/clopPdin ofPolymr S e k m o n d E i n o r R n g , 2nd ed.: Mark, H.F;Bikales. N. M.: Ouerberger, C. G.; Mengea, G . : K m s c h ~ h ,J. I., E*.; WileyInterscience: New York, 1985;Voll,p 100.
448
Journal of Chemical Education
30. Shelton. J., Ed.Additiw8JorPdymom; Elaeviar: New York. 31. Agrsnoff, J., Ed. M&m Plorfles E w c l o p d l o : M&mw-Hin: NewYork, published snnllsll~ -. .* 32. Dirk,C.W.;Inabe,T.;Lyding,J.W.;Sehoeh,K.F.;Ka~ewurk,C.W.;Marks,TJ.J. Pdym. Sci.. P d y m Symp. 1985,70,1. 33. Skotheim,T A , Ed. Handteak ofconduetkg P d y w m , Vols 1 and 2;Dekker: New
.".",,a%
""&
34. Fan, L.T.;Smgh, S. K Contm1MRPlew:AQuonflfatiurAppmaeh;Springring: New
.".-,.""".
""-b
.O*O
35. Paul,D. R.; Harris, F. W., Eds. Contmllsd R P k Pobmorie Fwnulotions, ACS Symp Sex 33: American Chemical Sadety: Washington, DC, 1976. 36. Katz. H.S.: Milewslo, J. Y Handbook of Fillers and Reinfbobomnts for Ploaties: Reinhold: New York, 1978:w 79182. 37. Melien, I. Indualrivl ~lorfier&m:Macmillan:NmYork, 1 W . 38. Sears, J. 8.;Darby, J. R. The lkchnology of P l o r t k i m : Wiley-Interaeienee: New NewYor*, 1950. 40. Small, P M. J A p p l . CnDm 1853.3,71. 41. Hw. K. L.J h i n l W h . 1970,42,76.
