Polymer Additives II. Chemical and Aesthetic Property Modifiers Malcolm P. Stevens
University of Hartford, West Hatiford, CT 06117 In Part I of this series ( I )we discussed the importance of polymer additives to the chemical industry and how the chemistrv of additives offer a ranee of ~racticala ~ ~ l i c a tions thai might be discussed in thechen;istry curri&lum. tv (mechanAdditives were cateeorized as ~ r o ~ e rmodifiers ical, chemical, aestcetic, and surface) and process'ingmodifiers. Mechanical property modifiers were discussed further in greater detail. We cnntinue here with a discussion of additives that alter the chemical and aesthetic properties of polymers. Numbering of chemical compounds and tables sequentially follows the numbering in Part I. &
.
"
Antioxidant additives are of two types: primary, which intercept free radicals as they form, and secondary, which reduce hvdroueroxides. Most ~ r i m a r vantioxidants are either hinierei phenols or sec~ndary"aromaticamines. An examole of the former is 2.6-di-t-butvl-D-cresol(11).comei in monl; referred to as ~ H T ' ( b u t ~ l a t hydroxyt&&e) the polymer industry. BHT reacts with radicals by hydrogen transfer. The resultant resonance-stabilized phenoxy radical (12) undergoes a variety of reactions, including rearrangement to the benzylic radical (13)and dimerization.
Chemical Property Modifiers
Antioxidants Oxidative degradation of polymers is initiated primarily by free radicals generated in the polymer by heat or mechanical shear during processing, or by exposure to ultraviolet or ionizing radiation in the use environment (24). The radicals i n t u r n react with oxygen to form hydroperoxides and peroxy radicals that promote further free radical reactions. These various processes may be summarized as follows (RH = polymer): cH31&+y R H ~ Z E , R.+H.
+ o2'ROO. ROO. + RH + ROOH + R. ROOH + RO. + .OH 2ROOH-3 RO. +ROO. + H20 ROOR
ROOR + 2RO.
Clearly the critical factor is initial formation of the polymer radical (Re). As might be expected, polymers capable of forming relatively stable tertiary or allylic radicals such as polypropylene (10) and polybutadiene (3, Part I) are most susceptible.
Y
CHa
A
12
H
cHZcHe&'
c+ (
cy. 13
+'"
CH,
ii
12-
R.
2RO.j
Y
C(CH$a
(CH,),C 14
The dimer (14) is also an effective antioxidant. Quinonetype by-products may cause discoloration to the polymer. A major problem with BHT is that its vapor pressure is high enough that some may be lost during polymer processing at elevated temperatures. To minimize loss, higher molecular weight phenols such as 1,3,5-trimethyl-2,4,6tris(3,5-di-t-butyl-4-hydroxybenzy1)benzene (15) are used. Concentrations of hindered phenols vary between 0.01 and
Polymers containing no secondary or tertiary hydrogens such as poly(methy1 methacrylate) (5, Part I)(1)are generally resistant to oxidation and need no antioxidant additives. Volume 70 Number 7 July 1993
535
0.5 wt %, depending on processing conditions, the antioxidant's vapor pressure, and the susceptibility of the polymer to oxidation. Aromatic amine radical scavencers usually are derived es from l,4-diaminobenzeneor diph&nylamine.~ x a m ~ lare N,N-bid1,4dimethylpentyl~-1,4diaminobenzenr (161and 4,4-(up'-dimethylbenryl~diphenylamine(171. Antioxi-
Divalent sulfur and trivalent phosphorus compounds comprise the commonly used secondary antioxidants. Didodecyl 3,3-thiodipropionate (21) and tri(4-nonylpheny1)phosphite(22) are representative. Both types reduce hydroperoxides to alcohols: &S + R'OOH -+ R&O + R'OH (ROl3P+ R'OOH + (RO)3P=0+ R'OH The number of available antioxidants is considerable(4). Usually a wmbination of primary and secondary is used, often in association with &elating agents to deactivate metal ion contaminants that might catalyze polymer oxidation. Biocides
dants such as 16 also protect polymers from ozone, and are thus classified additionally as antiozonants (5). Like phenols, amine antioxidants donate hydrogen atoms to free radicals. Amine antioxidants are more effective than the hindered phenols and are used widely with unsaturated elastomers, such as polybutadiene, which readily form free radicals. However, the amines cause more staining, especially those derived from l,4-diaminohenzene, hence these are used primarily with carbon black-filled elastomers with which discoloration is not a concern. In applications where staining must be avoided, such as outdoor carpeting made of polypropylene, the so-called hindered amine antioxidants are used. Often referred to as Yight stabilizers" because their primary function is to inhibit photooxidation, the hindered amines usually are derivatives of 2,2,6,6tetramethylpiperidine (18).
Biocides are added to polymers to destroy microorganisms that cause mildew, objectionable odors, or structural degradation. Because they are used for preventing odor, biocides also qualify as aesthetic property modifiers. Microbial attack occurs commonly with natural polymers, but synthetic polymers, particularly those prepared from alkenyl monomers, often are highly resistant. Additives such as plasticizers, however, promote biological action even on the more resistant polymers. Well over 100 industrial biocides are marketed (6).They include inorganic compounds such as borax and copper (11) nitrate, metal naphthenates, and such heterocyclic compounds as 2-mercaptobenzothiazole (23) and its sodium or
zinc salts. Antimicrobial agents used to prevent odor, primarily in polyethylene trash bags and poly(viny1chloride), are organoarsenic and organoantimony compounds. The choice and concentration of biocide depends on the chemical makeup of the polymer and the polymer's use environment. Leaching of antioxidant by fluids (e.g., oil or gasoline contacting automobile gaskets and hoses) can be minimized by using more polar amine antioxidants such as Nphenyl-N-@-toluenesulfony1)-p-diaminobenene(19). The most effectiveway to avoid loss, however, is to copolymerize antioxidant monomers such a s N44-ani1inophenyl)methacrylamide (20) into the elastomer hackbone.
536
Journal of Chemical Education
Flame Retardants
Flammability, toxic by-products of combustion, and smoke formation are impediments to more widespread use of polymers in construction and transportation. One way to reduce flammability is to incorporate flame-resistant monomers into the polymer, or to rely on thermally stable polymers that have inherently low flammability (7). Another is to use flame retardant additives. Polymers burn by a candlelike process involving release ofcomhustible gases. As the burninaaasesdifuse from the polymer surface, heat radiates back-t; sustain combustion. Combustion encompasses a multitude of complex vapor phase free radical chain reactions, details of which were described previously (8).Strategies for reducing flammability with additives center on cooling the burning polymer's pyrolysis zone, inhibiting the free radical combustion reactions, and forming a protective layer on the polymer surface. Six classes of materials dominate the commercial market for flame retardants: alumina trihydrate, organochlorine compounds, organobromine compounds, organophosphorus compounds, antimony oxides, and boron compounds. Least expensive is alumina trihydrate, A1203.(H20)3,which represents about 45% by weight of total flame retardant consumption. Above 250 'C water of
Table 4. Halogen- and Phosphorus-Containing Flame Retardants
Halogen-containing Chlorinated alkanes and cydoalkanes Chlorendic acida and derivatives Bmminated aromatic hydrocarbons Brominated bisphenois Brominated phthalic anhydride Phosphoms-containing Ammonium phosphates and polyphosphates Alkyl phosphatesband phosphonatesb Triaryl phosphates Phosphonium salts PhOSDhazeneS '1,4,5,6.7,7-Hexachlor0-5-n0rbomene-2,3dicalc acid blncluding halogenated types.
Boron compounds such as boric acid and borax, although cost-effective, are the least used flame retardants. Their mechanism of retardation appears to involve formation of a glass-like coating on the polymer surface. Ultraviolet Stabilizers Because many polymers are susceptible to photooxidation or photodegradation, ultraviolet stabilizers are needed for applications requiring outdoor exposure. When a chromophore absorbs ultraviolet radiation, the molecule usually converted to an excited singlet state; i.e., the excited state contains no unpaired electrons. The singlet state may undergo intersystem crossing to an excited triplet state (containing unpaired electrons). Loss of energy from the singlet or triplet states may occur by fluorescence or phosphorescence, respectively, or thermally by molecular vibrations, a process called radiationless decay. Alternatively, the excited molecules may lose energy by bond breaking. Polymers containing carbonyl groups, for example, undergo bond scission between the a and P carbons (10):
is
-
hydration evolves endothennically from the hydrate, thereby lowering the temperature of the flame and diluting the flammable gas- ' - - Y " ' A C-R L " - q C-R eous pyrolysis products. Additionally, the II remaining alumina provides a thermal baro oI1 rier by coating the polymer surface. An added benefit with alumina is that it is a Pigments (to be discussed later) are one means of blockrelatively effective smoke suppressant. ing ultraviolet radiation. When used for this purpose the Halogenated and phosphom-containing additives are pigments are called screeners. Carbon black, for example, listed in Table 4. Chlorine- and bromine-containing comis a widely used screener. The principal nonpigment stahipounds reduce flammability by formation of hydrogen balizers are ultrauiolet absorbers, which preferentially ablide which inhibits free radical combustion reactions. The sorb ultraviolet radiation and dissipate the energy thermost important of these are chain branching reactions inmally by vibrational transitions; radical scavengers, which volving atomic and molecular hydrogen arising from the intercept free radicals arising from photochemical reacburning polymer: tions; and quenchers, which interact with photoexcited H.+O1+HO.+O. polymers and return them to their mound states. Ultraviolet absorbers, which work on the same principle O.+Hz+HO.+H. as sun screen lotions, contain chromophores that absorb in Organohalogen compounds (RX)are believed to form the 300400 nm resion of the s~ectrum.This is the anoroxHX by a chain sequence involving polymer (RH) or Hz: h a t e range of uiiraviolet w'avelengths that realh the earth's surface. To he useful. absorbers must be relntivelv R X @ R.' + X. stable; i.e., they must not react with the polymer nor decompose under the influence of radiation. Derivatives of RH+X. @ R.+HX 2-hydroxybenzophenone(24) are the most commonly used H,+x.~H.+Hx
In turn, HX reacts with radicals involved in chain branching to form the less reactive halogen atoms:
Phosphorus compounds aid in formation of a carbonaceous char on the polymer surface that helps insulate the oolvmer from the flame's heat flux. Phosoho" rous compounds are particularly effective in cellulosic oolvmers and are widelv used in cellulosic textile fibers. eh% ' phosphorus compo&ds apparently promote dehydration of the cellulose to vield unsaturated com~oundsthat subsequently aromatize to a graphite-like c h i . Antimony trioxide (Sbz03) and pentoxide (Sbz05)are used in conjunction with halogen-containing additives or halogenated polymers because of a synergistic interaction arising from formation of antimony trihalide (9)during combustion. A proposed reaction sequence is as follows:
.
Sb03 + 2HX
2SbOX + HzO
-1
5SbOX -1 Sb4O& + SbX3
ultraviolet absorbers. They undergo a photorearrangement reminiscent of keto-en01 tautomerism to a quinoidtype product that reverts to the benzophenone with loss of ~energy. Internal hydrogen bonding is a necessary element in this type of rearrangement; 3-hydroxybenzophenones, for exa m.~ l .eare , ineffective as ultraviolet absorbers. Hydroxyphenylbenzotriazoles (25) constitute the next most popular class of ultraviolet absorbers. They appear to react similarly to form quinoid-type zwitterion intermediates. Less commonly used ultraviolet absorbers include phenyl salicylates (26) and related esters and cyanoacrylates Volume 70 Number 7 July 1993
537
f d u r e s or bottles, the other white pigments are more commonly used. Titanate pigments are prepared by calciningvarious metal salts and oxides with titanium dioxide. In addition to the pigments listed in Table 5,
a > %a:a ~ ~ R
H--O
CI
-
R
R'
-
-
aine variety ofranee synthetic of colors mixed are metal available oxidesforencompasshieh tem" a wide perature polymer applications. Metal powders, particularlv aluminum and bronze. also find use a s pigments. To avoid ~roblemsin handline dves and ~iements. most polymer compounders use color concentrates in which beads. cubes. or solutions of the ~olvmercontaininc UD to 40 times the normal concentration-of coloring agentare blended in during processing.
- -
0
Ar C-C-COR
-1
II
CN
A
-
Odorants
These are fragrances (esters, terpenes, etc.) added to (27). Ultraviolet absorbers also may be copolymerized into
the polymer backbone (11). Hindered amines such as bis(2,2,6,6-tetramethyl-4piperidinyl) sebacate (28) are used as radical scavengers along with hindered phenols and phosphites of the type described previously that are used in antioxidant formulations. ~ x c & dstate quenchers are primarily salts or chelates of nickel, cobalt, and zinc. Use of quenchers is limited because they usually add color to the
Table 5. Inorganic Pigments
Pigment Aluminum sulfosilicates
Barium sulfate Cadmium sulfides Calcium silicate Chromates Ferriferrocyanides Iron oxides Molybdates Titanates Titanium dioxide Zinc oxide or sulfide
Color Range Red to blue White Yellow to blue White Yellows Metallic blue Yellow to beige, tan to brown, black Yellow to orange Yellow to beige, green to blue White White
Aesthetic Property Modifiers
Four types of aesthetic property modifiers were listed in Table 1(I):coloring agents, bioeides, odorants, and nucleating agents. Biocides are described above and nucleating agents are covered in Part 1 under mechanical property modifiers (I).
plastics for aesthetic appeal-in toys, for example-r to mask objectionable odors. Plastic garbage cans and garbage disposal units are markets for the latter. Usually odorants are added to the polymer prior to cornpounding as alcohol solutions or in encapsulated form.
ColoringAgents
Literature Cited
Although the terms dye and pigment are used interchangeably,the former oRen is reserved for soluble and the latter for i&oluble coloring agents. Pigments may be inorganic or organic; whereas, dyes are invariably organic. Organic dyes and pigments are available in a vast array of colors and encompass the various categories of complex chromophoric compounds familiar by nameif not always by structure-to most organic chemists, including the anthraquinone, azine, azo, indigo, phthaloeyanine, quinacridine, quinoline, and xanthene types, as wen as vat dyes and some specialty fluorescent and phosphorescent dyes (12, 13). Carbon black is also an important organic pigment. A partial listing of inorganic pigments is given in Table 5. Titanium dioxide is by far the most important pigment where an opaque white appearance is desired. Where higher translucency is needed, for example, in lighting
538
Journal of Chemical Education
1. Stewnq M.P.J Cham. E d w . IsBS,70,444. 2. h&d, K U.C k m . Re". 1861,61.563. 3. Luodbem, W 0.. Ed.Autrnidhfi0" .miA"tioridrid"t8.
V0k. I and II: w,lrid~-Intersd-
5. F h e , C. K.: Lattimer, R. P. I" E ~ y ~ ~ l o p wofl iPdymrr o &ma ond En&ering; Mark, H. F;Bikales, N. M.;Overbelger C.G.;Mengra. G;Krod,witz. J. I.. Ed% Wllev-Inteteience: New York. 1985:Val. 2.D 91.
i&york, iaia.' 1.J. Cham. Edae. 1974.51.481
