Polymer Additives Ill. Surface Property and Processing Modifiers Malcolm P. Stevens University of Hartford, West Hartford,CT 06117 This is the final installment of a three-part paper on the subject of polymer additives. Part I (1)provides an overview of the significant role additives play in the polymer industries. Additives of current commercial importance, as well as the function of each, are presented in Table 1of Part I. Also discussed in further detail in Part I are additives that enhance mechanical properties of polymers. Part I1 (2)deals with additives that influence chemical and aesthetic properties of polymers. We conclude here with additives that both modify surface properties of polymers and facilitate polymer processing. There is overlap between the two ;release agents are included to alter surface properties in a way that facilitates processing. Numbering of chemical structures follows in sequence from Part 11. In our earlier commentary on how plasticizers affect mechanical properties ( I ) , we noted that plasticizers also qualify as processing modifiers, because they often are added in small amounts to lower a polymer's melt viscosity. We shall not, therefore, say any more on the subject of plasticizers here except to remind the reader that the critical factor, besides volatility, that contributes to a plasticizer's permanence is its compatibility with the polymer. The same principles apply to surface property modifiers. In all instances, surface modifiers are not particularly compatible with the polymer, and consequently they tend to migrate to the polymer surface where their particular properties come into play Usually the additives are used in relatively small amounts (less than 2% by weight) because undesirable visible films might otherwise accumulate on the surface. Surface Property Modifiers AntiblockingAgents Sometimes called flatting agents, the peculiarly termed antiblocking agents prevent polymer films or sheets from sticking together, which is caused primarily by the buildup of static electricity and, to a lesser extent, by the inter-
penetration of molecules comprising the fdm (a phenomenon referred to as cold flow). Antiblocking agents may, therefore, be considered as a special type of release agent. Similar compounds are, in fact, used for both, including salts of fattyacids (soaps), waxes, or even other such as polysiloxanes or fluorinated polymers, all of whch exhibit limited solubilitv in the lilm or sheet. More details are provided below in &e section on release agents. Antiblocking agents are used in low concentrations primarily in poly(viny1 chloride) and other vinyl polymers. Frequently they are used in combination with externally applied antiblocking agents such as calcium silicate or kaolin. Note that antiblocking agents do not dissipate static charge and should not, therefore, he confused with the antistatic additives described below. Antifogging Agents These are surface property modifiers that prevent moisture condensation inside film packaging from formingminute droplets ('fog") that obscure visibility through the plastic fh.The modifers are primarily nonionic surfactants such as ethoxylated fatty acid esters that "wet" the surface and run the moisture inta a continuous film. Because they are commonly used in food packaging, antifogging agents must meet Food and Drue Administration standards. as must all additives used in>lastic food packagi&. Antistatic Agents Static charge develops on the surface of polymers because the polymers are, in eeneral, nonconducting. Surface charge a& a i a magnet fo;dirt, causes fabric to cling, and makes plastic film stick together. In the presenceofelectrical equipment static char& may cause interference, and in certain environments (grain elevators, for example) it can be an explosion hazard. Antistatic agents provide a mechanism for the surface charge to be dissipated by providing
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a hygroscopic surface layer that attracts atmospheric moisture, thereby increasing surface conduction (3). Three types of antistatic agents are in commercial use: external; internal that migrate to the surface; and fillers that provide a conducting path through the polymer. External agents applied to polymer surfaces are primarily quaternary ammonium compounds or-with textile fibers-polyelectrolytes. A typical polyelectrolyte is formulated from a poly(ethy1ene glycol) dichloride (29) and a primary amine. When equimolar amounts of the two are used, nucleophilic substitution of chloride by amine yields a linear polymer (30):
a+cH,cH,~cycH,a
RNH,
29
-+YCH,DtCH,CH,N+
30
vinyl group of 33 is copolymerized into the crosslinked network. For glass fiber-reinforced epoxy resins, coupling agents such as 3-aminopropyltrimetboxysilane (34) and 3-glycidylpropyltrimethoxysilane (351, both of which coreact with the epoxy polymer, are used.
Titanate coupling agents such as isopropyltri(dioctvluhosuhato)titanate (36) are used to imR prove-adhesion to mineral fillers, especially calI cium carbonate, in such thermoplastic polymers as high-density polyethylene, polypropylene, polystyrene, poly(viny1 chloride) and nylon, as well as in the thermosettine unsaturated oolvesters. eooxies and polyuretha& In the case df36, Ti0 b k d s are formed with the filler, while exchange reactions (transesterification, for example, in polyesters)take place with the phosphate ester. ~~~~~
Diamines and triamines also are used. An excess of the dihalide produces a crosslinked polyelectrolyte containing quaternary ammonium groups. In practice the fibers are treated with aqueous solutions of the monomers, then polymerization and crosslinking are brought about on the surface. The advantage of such a procedure is that the crosslinked polyeleetrolytes are durable; that is, they are not easily removed by laundering. Less durable external "antistats" include such materials as hydroxyetbylated starch or cellulose and crosslinked poly(viny1 alcohol). Internal agents include poly(ethy1ene glycol) (31) of molecular weight about 20.000. . . fattv . acid esters, lonechain aulfonates and phosphates, quaternary ammonium comoounds. and N-alkvldiethanolamines132).Conducting fillers, used in such applications as carpeting, linoleum,belting and footwear, including carbon black and metal filaments.
-
Coupling Agents
Coupling agents improve adhesion between polymer (usually organic) and fdler (usually inorganic) or reinforcing fiber (4).They do so by providing a chemical bridge between the two incompatible materials. The two most common types are the silanes and the titanates. Silane coupling agents, for example, vinyltrimethoxysilane (33), are used primarily with glass fiber-reinforced
plastics. Alkoxy groups bonded to silicon atoms of the coupling agent are hydrolyzed to hydroxy groups which form siloxane linkages with silicon atoms on the glass surface.
When used with unsaturated polyesters that are ordinarily crosslinked by copolymerization with styrene, the 714
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Release Agents
Release agents reduce the coefficient of friction at the polymer surface and prevent polymers from sticking to other surfaces or to themselves. As such they are important processing modifiers because they play a vital role in preventing polymers from sticking to processing machinem. Release aeents used to urevent adhesion in iniection &d compres~ionmolding Hre appropriately termed mold-release aeents: - . whereas. those used for extrusion and calendaring are called slip agents. Similar compounds are used for both. Some release agents are polymeric or oligomeric. Polysiloxanes are widely used mold-release agents for polyolefms, polystyrene, polyoxymethylene (acetall, nylon, and acr~lonitrile-butadiene-styrene(ABS) copolymers. The more expensive but very effective fluoropolymers,such as poly(viny1 fluoride) (37) and polytetrafluoroethylene (38)
are reserved mainly for the higher cost engineering plastics such as polysulfone (39) and polycarbonate (40). Polyethylene and polyethylendpolypropylene blends find occasional use as release agents for poly(viny1 chloride) and polyurethanes.
Calcium and zinc stearates, and to a lesser extent other metal soaps, are the workhorses of the nonpolymeric release agents. They are used primarily with such commodity plastics as poly(viny1 chloride), polystyrene, and polypropylene, a s well as with nylons and unsaturated
~olvesters. Fattv acid amides such as olevl Dalmitamide . . (411rank secondto themetal soaps in termb oivolume wnsumed. PoMvinvl chloride) and ~olvstvrene . - are the major outlets for ihese-amide release agents.
A newer inorganic blowing agent is sodium borohydride which, on reaction with water, yields almost 2.4 L of gaseous hydrogen per gram of hydride: NaBH, + 2 H20 + NaB02+ 4 Hz Examples of organic blowing agents that evolve nitrogen on heating are 1,l-azobisformamide (48, also called azodicarbonamide or ADC), which decomposes between 195 and 216 'C;p-toluenesulfonylsemicarbazide(43)(228235 'C); and 4,4'-oxybis(benzeuesulfonyl hydrazide) (44)
Lower volume release agents include paraffin waxes, fatty acid esters, fluorinated fatty acids and fatty alcohols, natural waxes, and such inorganics as talc and kaolin. Processing Modifiers Two processing monomers have been discussed already: plasticizers in Part I (1)and release agents (above). The remaining ones serve only to facilitate the manufacture of polymers or their fabrication into articles of commerce. Blowing agents
Blowing agents are used to generate foamed plastic or foamed rubber (6, 7).Physical blowing agents are either gases that are dissolved in the polymer under pressure, or low boiling liquids that are converted to gases by heating under processing conditions. Chemical blowing agents are compounds that decompose to form gaseous products during processing. Gas pockets produced in the viscous polymer are trapped by cooling or-in the case of thermosetting polymersby crosslinking. Compressed gas blowing agents include air, carbon dioxide, and nitrogen, the last being most important. Of the various low boiling liquids i n commercial use, chlorofluorocarbons used to be the agents of choice; however, their role in stratospheric ozone'depletion has led to r increased reliance on pentane, isopentane, and h i ~ h e liquid alkanes (most co&nonly ~ i t h ' ~ o l ~ s t y r ea nde ~ methvlene chloride lfor poly\vinsl chloride) and polyurethanes.] - In the case of pol$xethanes, the blowing agent augments the foaming action of carbon dioxide generated during curing. Typically, water is added to low-molecularweight prepolymers having terminal or both terminal and pendant isocyanate groups. The water reacts with isocyanate exothermically to yield the unstable carbamic acid that loses COz to form amine.
RNCO
II
%
RNHCOH
---L
RNH, + C02
The reaction exotherm, in turn, vaporizes the blowing agent. Build-up of molecular weight with a concomitant increase in viscosity arises from the reaction of the amine with remaining isocyanate p u p s to form urea linkages:
(158-160 'C). Of the three, 42 is most important, accounting for about 90% of all chemical blowing agent sales. Thermal decomposition of these nitrogen generators is complex and not well understood. Other gases, including CO and COz, and sometimes NHs, are formed in addition to Nz, and the residue usually consists of a mixture of solid nmducts. r--~-~-~Nitro~en-formingcom~oundsmav decom~oseat lower temperatures in thi presknce ofacceierators;42, for exam~ l edecom~oses , almost 100 'C lower in the Dresence of cadCrosslinkingAgents
The terms crosslinking and curing often are used interchangeably, although some polymer scientists associate the former with the crosslinking of high-molecular-weight linear polymers, and the latter with converting low-molecular-weight polymers or oligomers (such as epoxy resins) to thermosetting polymers. Of the numerous crosslinking methods currently available to the polymer industry, vulcanization of rubber with sulfur is the oldest, dating back to its discovery in 1839 by Goodyear in the United States and Macintosh and Hancock in Great Britain (8).The reaction is apparently ionic in nature (91,involving an intermediate sulfonium ion (45) that reacts further by hydride abstraction or proton donation to form cationic species that propagate crosslinking (see reactions in Figure 1). Peroxides constitute the most widely applied family of crosslinking agents for unsaturated polyesters, saturated and unsaturated elastomers, polyethylene, and polysiloxanes (10). The reaction is initiated by homolytic decomposition of the peroxide: ROOR
RNCO
II 5 RNHCNHR
Chemical blowing agents may be both inorganic or organic. Of the former. sodium bicarbonate is most wmmon, particularly in the manufactured foam rubber. Sodium bi: carbonate mav be decom~osedthermally at 100 to 140 'C or by additionof acid:
4 2RO.
In the case of saturated polymers such as polyethylene, hydroeen abstraction. and radical combination reactions lead to &e crosslinked product:
A
ZNaHCO,
3 Na2C03+ H20+ CO,
NaHCO, + HA + NaA + H20+ COz Volume 70 Number 9 September 1993
715
Because of its greater sensitivity, the MEKP mixture is marketed as a solution in dimetbyl phthalate or some other inert solvent, and cobalt naphthenate is normally used as a promoter to accelerate peroxide decomposition for ambient temperature curing. To facilitate dissolution in the viscous polyester-styrene solution, 48 is usually applied as a dispersion in a plasticizer. On heating, 48 decomposes to benzoyloxy radicals which, in turn, can decompose to phenyl radicals and COz:
Figure 1. Reaction for the vulcanization of rubber by sulfur Allylic hydrogens are the most susceptible to hydrogen abstraction in unsaturated polymers:
Radical combination, addition and transfer reactions all contribute to the crosslinking process:
"Methyl ethyl ketone peroxide" (MEKP) [a mixture consisting principally of %butanone peroxide (46) and its condensation product (47)l and benzoyl peroxide (48) are the peroxides of choice for curing unsaturated polyesters. Crosslinking in this instance is a free radical-initiated copolymerization of styrene (or some other solvent monomer) and the unsaturated linear polyester. W H
I CH&CH2CH3 I OOH
716
CH C
FHJ FHJ CO--OCCH,CH,
3 Y ~ I OOH
Journal of Chemical Education
OOH
Both benzoyloxy and phenyl radicals initiate cmsslinking. Ambient temperature cmsslinking is brought about by addition of a promoter such as N,N-dimethylaniline.The amine is believed to react with peroxide to form an unstable intermediate salt that decomposes to benzoyloxy radical and a radical cation, shown in Figure 2. For other crosslinkingapplications, a variety of peroxides are available. Dicumyl peroxide (49) is used to crosslink unsaturated elastomers and low-density polyethylene wirecoatings; 2,5-bi(tbutvloeroxv,-2.5-dimethvl-3-hexvne(50, is used mahiy with high-densicy polyethylene,and 2,4dichlorobenzoyl peroxide (51)with polysiloxanes. Epoxy polymers are the other major polymer type that require crosslinking additives, usually amines and anhydrides (11).Curing of epoxy resins is complex and involves, among other reactions, ring-opening of terminal epoxide groups by amine and esterification of pendant hydroxyl groups by anhydride, as shown in Figure 3. Diethylenetriamine (52) and urea- or melamine-formaldehyde resins are widely used amine crosslinking agents for ambient cure; whereas, the less reactive aromatic amines such as 1,3diamiuobenzene (53)and 4,4-diaminodiphenylmethane (54) are used for high-temperature epoxy laminating resins. The two most widely .used anhydrides are phthalic anhydride (55) and methyl-5-norbornene-2.3-dicarboxvlic anhvdride (56) (the DielsAlder adduct of mixed ~ethylcyclopentadienes and maleic anhydride, more commonly called nadic methyl anhydride, or NMA). Additionally, the ethvlamine-boron trifluoride cwrdination com~lex(57) is added tn epoxy resins as a curing agent that aflords a long shelf'life at ambient temperatures but effects rapid curing on heating. Defoaming Agents A number of polymers tend to entrap air during processing, particularly where high-speed mixing of fillers or other additives is involved. The resultant foaming can cause spill-overs,as well as problems in pumping and metering. Foaming during removal of solvents or residual monomers is a particular problem. Air bubbles also can affect properties adversely. Adhesive joints may be weakened; coatings may show surface blemishes; and molded parts may exhibit weak spots. Polysiloxane lubricants, often in combination with polyethers, are the most widely used defoamers (12). Fatty acid esters, acetylenic glycols, and anionic or nonionic surfac-
tants also are used in certain applications. The defoaming agents function by lowering the surface tension, thereby preventing bubble formation or breaking existing bubbles. Emulsifiers
igure 2The use of N.N-dimethylanilineto promote ambient temperature crosslinking.
0
I \
C H r C H y + RNH,
-
-
OH OH ,O, I CHi-CHY I RNHCH,CW RyCH&W
Emulsifying agents serve to stabilize emulsions when the emulsion method is used to manufacture polymers and to stabilize polymer emulsions for latex paint or adhesives applications. They do so by forming micelles that dissolve the monomer or polymer molecules in the hydrophobic centers of the micelles. The monomer- or polymer-swollen micelles remain dispersed because of their surface charee. Salts of fattv acids or fatty alcohols are tipical of the su;factanLs uscd. Sodium dodccvl sulfate (581. . .. for example, is a common emulsifier for polyacrylate latexes.
Heat Stabilizers
lure 3. Some of the steps in curing epoxy resins.
FHJ FH, (CH,),cooCHC~CCHooC(CH,),
CH,
CH3
Heat stabilizers are compounds added to polymers to prevent decomposition and discoloration during processing (13). By far the ereatest a~olicationis with ~olv(vinv1 chloride) which tends to evolveLh&og& chloride on heating, thus leading to highly colored conjugated units in the polymer backbone and, incidentally, to corrosion of processing equipment. Occasional allylic chloride sites in the polymer are considered responsible for the onset of dehydrohalogenation on heating: -CH=CHCHCHy
I
A
CI -CH=CHCH=CH-
+ HCI
Heat stabilizers either intercept the HCl as it evolves or renders the sensitive site inactive. A variety of stabilizers are in commercial use, the most important being mixtures of metal carboxylates, organotin compounds, phosphites, and epoxides. Less important are basic lead compounds and antimony mercaptides. Mixtures of cadmium and barium carboxylates are used most widely in plasticized poly(viny1 chloride), usually in combination with a phosphite stabilizer. Synergism between the two metal salts is believed to arise from an initial exchange of allylic chloride by carboxylate,
followed by regeneration of the cadmium carboxylate,
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Nontoxic calcium and zinc carboxylate mixtures, used in polyrvinyl chloride) food containers, behove analogously. Organotin stabilizers such a s dibutyltin dilaurate (591 are added to rieid oolvlvinvl " chloride,. The omanotin compounds stabilize the polymer either by scavenging HCI,
-
-..
fiber reinforcing is included, shrinking can weaken the polymer-fiber interface. Low-profile additives are used to counteract shrinking (14). The additives, which are polymeric in nature. are added in concentrations of 3 to 5% based on the tdtal composite weight, or 10 to 15%by weight of the polyester-styrene solution. Usually styrene solutions containing 30 to 40% by weight of additive are blended with the polyester-styrene solutions before crosslinking. Among the additives i n current use a r e polycaprolactone (62) and such widely used vinyl polymers a s polyethylene, polystyrene, and styrene-butadiene copolymers, poly(methy1 methacrylate), and poly(viny1 acetate).
or by replacing the allylic chlorine atom,
Thickening Agents
Also called antisag agents, thickening agents increase the viscosity of pol~er-solutionsor dispersions. The antis~ desimation arises from their use in thermosetting such a 9 unsaturated polyesters fur sagging while curingin a compression moldingoperation. Thickminz aeents are oanicularlv" imoonant ' (Cl,H,S)2Sn(C4H9)2 + 2HCI (C4b)2SnCI, ~ C I Z H Z ~ S H for imparting thixotropic properties to coatings and adhesives. Most thickening agents are finely divided, insoluble materials such as calcium carbonate, silica (usually in combination with a silane coupling agent), bentonite clays, and magnesium oxide (15).Partially soluble waxes and stearates are used also. Hydrogen bonding between the polymer matrix and thesurface o f t h e additive enhances thickening. Phosphite stabilizers such u s isooctyldiphenylphosphite Water-soluble polysaccharide "industrial mms" (carra(61, a160 replace the allylic chlorine atom, geenan, gum arabic, methyl cellulose, eG.1 are useful thickening agents in aqueous systems. Thickening is presumed to arise from formation of a physically-linked, continuous network.
Some organotin stabilizers, for example, dibutyltindidodecylmercaptide (601, contain tin-sulfur bonds. They function by removing HCI and subsequently adding mercaptan across double bonds in the polymer backbone:
-
--
+
Literature Cited 1. Stevens, M. P. J Chern.Educ I W 3 , 74 444 2. Stevens, M P. J Cham.Edu 1W.S. 70,535. 3. Johnson, KAntistolie &ens: 'lkchnologyend Abplimlions; Noyes: Park Ridge, NJ, 1977 A".-.
4. Pheddemann.E. PS&w
while epoxide stabilizers such a s epoxidized soybean oil react with evolved HC1:
Low-ProfileAdditives
Certain polymers, notably unsaturated polyesters, shrink during the curing cycle. Shrinking may cause warpage, internal cracking, and poor surface appearance. When
718
Journal of Chemical Education
Caupting Asenla; P l e n u m : N ~ w Y ~ 1982. ~k. 5. Owen, M. J. InEncyclopedio ofPolymorScienepondEngiiiiiig, k d e d . : Mark. H. F: Bikales, N. M.; Overbelger C. G.;Mengea. 0.:lirosehmtz, J. I., Eda.: WileyInterscience:New York, 1988; Vol 14, p 411. 6. Frkch, K C . ; Saundem, J. H. Plastic F w m ; Dekker: New York. 1972-1973. 7. Bikerman, S. J.Fwrns; Springer: NewYork, 1973. 8. Alliger. G.; Sjothun, I. J., Eds. Vvlmnvotion ofElosfomers; Van Nostrand: Nerv
Interadence:New York. 1985; Vol. 2, p 59. 13. S ~ ~ k , ~ . : ~ a o u s t , ~ . ~ d d i t i u e s / o ~ ~ l o s t i c i ~ ; s ~ r1983,pp i n ~ ~ :144-166. ~ew~ork. Scbnce and EngimnRg, 2nd ed.; 14. Carpnter, R. E. In Encyclopedia oiPoly-r
Msrk,H. F;BPales,N. M.;Ouerberger.C. 0.: Menps, G.; Kmschmiz,J.I.,Eda.; Wiley-Interscience: New York, 1989, Voi. 12, pp 286258. 15. Maaua, L.The Rok ofAdditiues in P1ostiss;Wfiey: NewYmk, 1974, pp 41-43.
