Handbook of Plastics Technologies Part 7 pps

Over theyears, much of the research on accelerated-sulfur vulcanization was done by using natural FIGURE 4.24 Cross-link types and chain modifications... As in the case of natural rubber

The contours for flex fatigue life are complex The test is run such that the specimensare about equally strained; however, there is some question as to whether the tests should

be run at equal strain or at equal strain energy For some cases, where strain is restricted byfabric reinforcement, fatigue test data should be compared at equal strain amplitude Forother applications, where the strain is not limited, the tests should be run at equal strain en-ergy The contours as presented here can be interpreted in terms of either constant strain orconstant strain energy All points on the chart can be compared at an approximately equalstrain per cycle; however, if we interpolate between the flex-life contours but only along aconstant modulus contour, we can extract values corresponding to approximately equalstrain energy per cycle By choosing higher modulus contours, we are considering higherstrain energies

The low values for fatigue life at low levels of sulfur, but high levels of accelerator,have been attributed to high concentrations of accelerator-terminated appended groups andhigh concentrations of monosulfidic cross-links Monosulfidic cross-links are not able toexchange, rearrange, or break to relieve stresses without the breakage of main chains

On the other hand, polysulfidic cross-links are able to rearrange under stress The rangement of a cross-link occurs in two steps: (1) breaking and (2) reforming Recent dataindicate that only the breaking of the weak polysulfide cross-links is required for thestrengthening of the vulcanizate network It is better to relieve the stress by the breaking of

rear-a cross-link threar-an by the brerear-aking of rear-a polymer chrear-ain

When even higher concentrations of sulfur are used (with the maintenance of constantmodulus), flex life decreases It is possible that this is due to the large amount of cyclicchain modification associated with high levels of sulfur As always, there are compro-mises

4.5.1.7 Accelerated-Sulfur Vulcanization of Various Unsaturated Rubbers. Over theyears, much of the research on accelerated-sulfur vulcanization was done by using natural

FIGURE 4.24 Cross-link types and chain modifications

rubber as a model substrate Natural rubber was the first elastomer and, therefore, thesearch for understanding of vulcanization originated with work on natural rubber Most of

what we have discussed so far vis à vis vulcanization has been related to natural rubber

The chemistry of the accelerated vulcanization of BR, SBR, and EPDM appears tohave much in common with the vulcanization of natural rubber Before the formation ofcross-links, the rubber is first sulfurated by accelerator-derived polysulfides (Ac-Sx-Ac) togive macromolecular, polysulfidic intermediates (rubber-Sx-Ac), which then form cross-links (rubber-Sx-rubber) As in the case of natural rubber, the average length of a cross-link (its sulfidic rank, the value of x in the cross-link, rubber-Sx-rubber) increases with theratio of sulfur concentration to accelerator concentration (S/Ac) used in the compoundedrubber mix However, in the case of BR or SBR, the cross-link sulfidic rank is not nearly

as sensitive to S/Ac as it is in the case of natural rubber Model compound studies of thevulcanization of EPDM (e.g., wherein ethylidenenorbornane was used as a model forEPDM) indicate that the polysulfidic rank of the EPDM cross-links probably responds tochanges in S/Ac in a natural rubber-like fashion

Reversion (when defined as the loss of cross-links during nonoxidative thermal nizate aging) is a problem associated mainly with natural rubber or synthetic isoprenepolymers It can occur only under severe conditions in butadiene rubber; in SBR, instead

vulca-of the svulca-oftening associated with the nonoxidative aging vulca-of natural rubber, one can observe

FIGURE 4.25 Vulcanizate properties, 300 percent modulus (MPa); ,

De Mattia flex fatigue life (kHz × 10–1); -O-O-O-O-, percent retention of ultimate gation after two days at 100°C

elon-hardening (the so-called marching modulus) during extensive overcure In natural rubberand synthetic isoprene-polymer rubbers, the cross-links tend to be more polysulfidic than

in the case of BR or SBR The highly polysulfidic cross-links are more heat-labile thantheir lower rank cousins in BR and SBR; they are more likely to break and then form cy-clic chain modifications

The effect of zinc is much greater in the vulcanization of isoprene rubbers than it is inthe vulcanization of BR and SBR Again, the reason for the difference is not known, but astrong speculation is that this difference is also related to the presence of methyl groupsonly in the case of the isoprene rubbers

Curing-System Recipes for Accelerated-Sulfur Vulcanization. Recipes for onlythe curing-system part of formulations are given in Table 4.6

4.5.1.8 Vulcanization by Phenolic Curatives, Benzoquinone Derivatives or ides. Diene rubbers such as natural rubber, SBR, and BR can be vulcanized by the action

Bismaleim-of phenolic compounds, which are (usually di-substituted by -CH2-X groups where X is

an -OH group or a halogen atom substituent A high-diene rubber can also be vulcanized

by the action of a dinitrosobenzene which forms in situ by the oxidation of a

quino-nedioxime, which had been incorporated into the rubber along with the oxidizing agent,lead peroxide

The attack upon rubber molecules by the vulcanization system can be visualized in away similar to that which was postulated for the sulfurization of the rubber molecules by

TABLE 4.6 Recipes for Accelerated Sulfur Vulcanization Systems*

2,2´-Note: conditions change depending on other aspects of the compositions.

the action of accelerated-sulfur vulcanization systems Reaction schemes for these twotypes of vulcanization can be written as shown in Schemes 10 and 11.

As shown, the chemical structural requirements for these types of vulcanization arethat the elastomer molecules contain allylic hydrogen atoms The attacking species fromthe vulcanization system must contain sites for proton acceptance and electron acceptance

in proper steric relationship This will then permit the rearrangement shown in Scheme 12,where A is the proton acceptor site and B is the electron acceptor site

This is an explanation for the fact that this type of vulcanization is not enabled by

dou-ble bonds per se, without allylic hydrogens in the elastomer molecules (It should be

pointed out that the phenolic curative can also act by a slightly different mechanism togive cross-links that contain chromane structural moieties, the allylic hydrogens still beingrequired.)

SCHEME 10 Vulcanization by phenolic curatives

SCHEME 11 Vulcanization by nedioxime

benzoquino-Another vulcanizing agent for high-diene rubbers is m-phenylenebismaleimide A alytic free-radical source such as dicumyl peroxide or benzothiazyl disulfide (MBTS) isusually used to initiate a free-radical reaction Although a free-radical source is frequentlyused with a maleimide vulcanizing agent, at high enough vulcanization temperatures, themaleimides react with the rubber without the need for a free-radical source This could oc-cur as shown in Scheme 13.

cat-This is similar to the reaction written for the attack of rubber molecules by phenolic

cur-atives or the in situ formed nitroso derivative of the quinoid (e.g., benzoquinonedioxime)

vulcanization system It is also closely related to the sulfurization scheme written for erated-sulfur vulcanization Comparisons between accelerated sulfur, phenolic, quinoid,and maleimide vulcanization can then be visualized as shown in Scheme 14

accel-Selected recipes for vulcanization by phenolic curatives, benzoquinone-dioxime, or phenylenebismaleimide are given by Table 4.7 Vulcanizates based on these types of cura-tives are particularly useful in cases where thermal stability is required

m-4.5.1.9 Vulcanization by the Action of Metal Oxides. Chlorobutadiene, i.e., prene rubbers (CR), also called neoprene rubbers, are generally vulcanized by the action

chloro-of metal oxides CR can be represented by the following structure:

SCHEME 12

SCHEME 13

TABLE 4.7 Recipes for Vulcanization by Phenolic Curatives, Quinone Derivatives, or Maleimides*

Zinc oxide is the usual cross-linking agent It is used along with magnesium oxide Themagnesium oxide is used for scorch resistance The reaction is thought to involve the al-lylic chlorine atom, which is the result of the small amount of 1,2-polymerization:

A mechanism that has been written for the vulcanization of CR by the action of zincoxide and magnesium oxide is shown in Scheme 15

Most accelerators for accelerated-sulfur vulcanization do not work for the metal oxidevulcanization of neoprene rubbers An exception to this is in the use of the so-called mixedcuring system for CR, in which metal oxide vulcanization is combined with accelerated-sulfur vulcanization In this case, along with the metal oxides, accelerators such as tetram-ethylthiuram disulfide(TMTD) or N,N´-di-o-tolylguanidine (DOTG) are used with sulfur.This may be desirable for high resilience or for good dimensional stability

The accelerator that has been most widely used with metal oxide cures is urea (ETU), N,N´-diphenylthiourea or 2-mercaptoimidazoline The use of ETU in the vul-

ethylenethio-SCHEME 15

canization of CR is somewhat in doubt since it is a suspected carcinogen A mechanismfor ETU acceleration is shown in Scheme 16.

Examples of recipes for metal oxide vulcanization are given in Table 4.8 It should benoted that, in one case, calcium stearate was used instead of magnesium oxide to obtainbetter aging characteristics

4.5.1.10 Vulcanization by the Action of Organic Peroxides. Peroxides are vulcanizingagents for elastomers, which contain no sites for attack by other types of vulcanizingagents They are useful for ethylene-propylene rubber (EPR), ethylene-vinylacetate copol-ymers (EAM), certain millable urethane rubbers, and silicone rubbers They are not gener-ally useful for vulcanizing butyl rubber, poly(isobutylene-co-isoprene) Elastomersderived from isoprene and butadiene are readily cross-linked by peroxides, but many of

SCHEME 16

the vulcanizate properties are inferior to those of accelerated-sulfur vulcanizates ever, peroxide vulcanizates of these diene rubbers may be desirable in applications whereimproved thermal ageing and compression set resistance are required.

How-Peroxide Vulcanization of Unsaturated Hydrocarbon Elastomers. The initiationstep in peroxide-induced vulcanization is the decomposition of the peroxide to give freeradicals If the elastomer is derived from butadiene or isoprene, the next step is either theabstraction of a hydrogen atom from an allylic position on the polymer molecule or the ad-dition of the peroxide-derived radical to a double bond of the polymer molecule In eithercase, polymeric free radicals are the result (Scheme 17)

For isoprene rubber, the abstraction route predominates over radical addition Twopolymeric free radicals then unite to give a cross-link Cross-links could also form by achain reaction that involves the addition of polymeric free radicals to double bonds

In this case, cross-linking occurs without the loss of a free radical, so that the processcan be repeated until termination by radical coupling Coupling can be between two

TABLE 4.8 Vulcanization Systems for Chloroprene Rubber*

*.Concentrations in parts by weight per 100 parts of neoprene W.

†.Conditions change depending on other aspects of the compositions.

SCHEME 17

polymeric radicals to form a cross-link or by unproductive processes A polymeric cal can unite with a radical derived from the peroxide Also, if a polymeric radical de-composes to give a vinyl group and a new polymeric radical, a scission of the polymerchain is the result

radi-Few monomeric radicals are lost by coupling with polymeric radicals when dialkylperoxides are used as the curative Also, if the elastomer is properly chosen, the scissionreaction is not excessive For dicumyl peroxide in natural rubber, the cross-linking effi-ciency has been estimated at about 1.0 One “mole” of cross-links is formed for each mole

of peroxide; cross-linking is mainly by the coupling of two polymeric radicals One ide moiety gives two monomeric free radicals that react with rubber to give two polymericradicals, which couple to form one cross-link

perox-In the case of BR or SBR, the efficiency can be much greater than 1.0, especially if allantioxidant materials are removed A chain reaction is indicated here One might expectthat nitrile rubber would also be vulcanized with efficiencies greater than 1.0; however,though the double bonds in nitrile rubber are highly accessible, the cross-linking effi-ciency is somewhat less than 1.0

Peroxide Vulcanization of Saturated Hydrocarbon Elastomers. Saturated carbon polymers are also cross-linked by the action of organic peroxides, though the effi-ciency is reduced by branching Polyethylene is cross-linked by dicumyl peroxide at anefficiency of about 1.0, saturated EPR gives an efficiency of about 0.4, while butyl rubbercannot be cured at all For polyethylene, the reaction scheme is similar to that of the unsat-urated elastomers However, branched polymers undergo other reactions Though the per-oxide is depleted, no cross-links may be formed between polymer chains, and the averagemolecular weight of the polymer can even been reduced by scission Sulfur or the so-

hydro-called coagents can be used to suppress scission Examples of coagents are

m-phenyleneb-ismaleimide, high-1,2 (high-vinyl) polybutadiene, triallyl cyanurate, diallyl phthalate, ylene diacrylate, and others

eth-Peroxide Vulcanization of Silicone Rubbers. Silicone rubbers weigh polydimethylsiloxanes) can be represented by

(high-molecular-where R can be methyl, phenyl, vinyl, trifluoropropyl, or 2-cyanoethyl Silicone rubbersthat contain vinyl groups can be cured by dialkyl peroxides such as dicumyl peroxide Sat-urated silicone rubbers require diacyl peroxides such as bis-(2,4-dichlorobenzoyl)perox-ide In the case of saturated siloxane rubbers, the mechanism is hydrogen atom abstractionfollowed by polymeric radical coupling to give cross-links The incorporation of vinylgroups in the rubber molecule improves the cross-linking efficiency

Vulcanization is frequently done in two steps After a preliminary vulcanization in amold, a high-temperature (e.g., 180°C) postcure is carried out in air The high-temperaturepostcure removes acidic materials that can catalyze hydrolytic decomposition of the vulca-nizate Also, the high temperature enables the formation of additional cross-links of thefollowing type:

Peroxide Vulcanization of Urethane Elastomers. Urethane elastomers suitable forperoxide vulcanization are typically prepared from an hydroxyl-group-terminated oligo-

meric adipate polyester and 4,4´-methylenediphenylisocyanate (MDI) A typical structuralrepresentation is as follows:

Hydrogen atoms can be abstracted from arylated methylene groups, but hydrogen oms may also be abstracted from alpha-methylene groups of the adipate moieties Thoughthey are usually sufficient, vulcanization efficiencies can be increased by the incorporation

at-of urea structures into the polymer chain

Recipes for Peroxide Vulcanization. Examples of starting-point recipes are given inTable 4.9 Outstanding characteristics of peroxide vulcanizates are low permanent set andthermal stability of the network

4.5.1.11 Other Types of Vulcanization. There are still other types of vulcanization tems based on other types of chemistry These are applied to elastomers such as acrylates,fluoroelastomers, chlorosulfonylpolyethylene, and epichlorohydrin type elastomers Theseare very specific curing systems Some of them will be dealt with later sections of thischapter

sys-TABLE 4.9 Recipes for Peroxide Vulcanization*

*.Concentrations in phr.

Siliconerubber

Millableurethane

4.5.2 Preservation of Vulcanizates

As with other polymers, elastomers are subject to degradation due to elements of their ronment These elements include oxygen, atmospheric ozone, and ultraviolet light as fromsunlight Molecular chain scission or cross-linking can be the result, giving rise to substan-tial losses in performance properties (ultimate elongation, strength, flexibility, fatigue life,and so on) Thus, it is generally necessary to include additives in rubber recipes or formula-tions to protect elastomeric or rubber compositions from damage by the environment

envi-4.5.2.1 Oxidation of Polymers. The need for stabilization of organic polymers is tial, because they are exposed to oxygen throughout phases of their lifetime: the polymerproduction phase, the fabrication phase, and the application stage Thus, antioxidants areadded to polymers just prior to isolation, before exposure to oxygen Such a stabilizer isexpected to maintain polymer properties and to suppress gel formation or changes in vis-cosity This protection is then expected to continue during storage before fabrication Fabrication involves shear and thermal energy, e.g., by Banbury mixing or mill mixing,extrusion, and calendering Additional antioxidants are frequently added The end-useproduct is expected to survive the environmental stresses throughout its service life Theamount and type of stabilizer chosen to protect a product will depend on the type of poly-mer and its use

essen-The oxidative degradation of the polymer proceeds by a free-radical chain reactionmechanism Initiation usually occurs by exposure to heat, light, or mechanical stress Theprocess is sometimes catalyzed by certain transition metal-ion impurities The oxidation ofhydrocarbon or related polymers by oxygen is an autocatalytic process with primary prod-ucts being hydroperoxides

Autoxidation occurs in three mechanistic phases: initiation, propagation, and tion steps:

Also, radicals also differ in relative lifetime

Strongest C-H bonds Weakest C-H bondsPrimary

Different radical types exist in different diene polymers; e.g., polyisoprene producestertiary alkyl radicals, polybutadiene gives secondary alkyl types, and SBR gives allylicand benzylic types, while a nitrile polymer gives allylic and tertiary cyano types Theseradicals and their corresponding peroxy radicals interact with parts of their own polymerchains, giving shielding effects, rearrangements, cleavages, and internal additions toneighboring double bonds Also, free radicals undergo disproportionation, addition to car-bon-carbon double bonds, and coupling

Polyisoprene softens on oxidative aging, while rubber polymers, which are based onbutadiene, harden on aging The tertiary allylic radical in polyisoprene, being stericly pro-tected, does not easily undergo cross-linking with other radicals or neighboring chain dou-ble bonds It reacts primarily with oxygen and subsequently leads to polymer cleavage.The secondary allylic system is more reactive It can undergo cross-chain reactions by rad-ical addition; this can cause hardening of the composition

4.5.2.2 Antioxidants. Factors affecting the performance of antioxidants include the trinsic activity of the antioxidant system, the solubility of the antioxidant system in thepolymer matrix, and the volatility of the antioxidant For rubber, unfortunately, the mosteffective antioxidants are the staining and discoloring derivatives of aryl amines; however,the need for nondiscoloring antioxidants has been filled by phenolic nonstaining antioxi-dants Amines find application as both raw polymer stabilizers and also as final vulcani-zate stabilizers On the other hand, phenolics can be used in nonblack reinforcedvulcanizates where nonstaining and nondiscoloration is desired

in-Amine and phenolic antioxidants are considered free-radical scavengers They bly work by the direct abstraction of amine hydrogen by the RO• group Increased sterichindrance at the 2 and 6 position of phenolic antioxidant has resulted in improved antioxi-dant performance There may be an optimum amount of steric hindrance of the phenolicgroup, which should be matched to the oxidizable matrix polymer This will allow a bal-anced interference of the radical chain process so that both propagating species, R• and

proba-RO•, are effectively neutralized

Amine and phenolic type antioxidants, acting as free-radical scavengers, are illustrated

below, where general structures for hindered phenol and p-phenylenediamine types are

shown (AH is the antioxidant, and RH is the rubber molecule.) The relative rates of

free-radical quenching and free-free-radical-chain propagation are indicated by k Q and k P

Aromatic amines (e.g., N,N´-alkyl- or aryl-substituted p-phenylenediamines) find

ap-plication as both raw polymer stabilizers and also as final vulcanizate stabilizers On theother hand, phenolics (i.e., 2,6-dialkyl substituted) can be used in nonblack reinforced vul-canizates where nonstaining and nondiscoloration are desired

Examples of amine and phenolic-type antioxidants are given in Table 4.10

In addition to the use of the above phenolic and amine type antioxidants, peroxide composers are used to harmlessly decompose the peroxides, which otherwise could de-compose to give free radical propagating species, e.g., R-O• or H-O• Examples of suchperoxide decomposers, which act synergistically with the phenolic or amine antioxidants,are dilauryl-β,β-thiodiproprionate and tris(p-nonylphenyl)phosphite These and others are

de-also listed in Table 4.10, with their chemical structures being listed in Table 4.11 We notethat some of the peroxide decomposers are also accelerators for sulfur vulcanization

4.5.2.3 Degradation by the Action of Ozone. The degradation of polydiene rubbers bythe action of atmospheric ozone is characterized by the appearance of cracks on the sur-face of a finished rubber product This degradation is caused by direct ozone attack and re-action with the double bond sites of unsaturation in a polydiene rubber

Early work established the following:

1 Ozone absorption occurs at a linear rate

2 The absorption of ozone is proportional to its concentration

3 Rubber that is not strained undergoes reaction with ozone to form oxidized film, but

it does not show the characteristic “ozone cracking.”

This last result was a key observation and has come to be known as the critical strain effect,

i.e., no crack growth occurs unless a specific strain for the particular polymer is exceeded Examples of rubbers that are thus affected include natural rubber (NR), synthetic cis-polyisoprene, styrene-butadiene rubber (SBR), polybutadiene (BR), and nitrile rubber(NBR) Rubbers with highly saturated backbones, such as ethylene-propylene-diene rub-ber (EPDM) or halobutyl rubber (XllR), react very slowly with ozone and do not show thiscracking phenomenon Typically, these ozone cracks develop in the polydiene elastomers

in a direction that is perpendicular to an applied stress They are the result of rubber chainscission and lead to the formation of several oxygen-containing decomposition products.Ozone reacts with the main-chain double bonds of an elastomer molecule to give thethree-oxygen-atom-containing ozonide structure, which decomposes to give chain scissionproducts (peroxy zwitterion- and carbonyl-terminated), which can reform the ozonide, un-less the zwitterion (>C+-O-O-) and the carbonyl (>C=O) chain-end groups are removedfrom one another due to strain in the rubber composition The reformation of the ozonide,

in the absence of strain, is essential a repair of the chain breakage or scission This isshown below

TABLE 4.10 Examples of Chemicals that Have Been Used as Antioxidants and Chemical

diamine

Styrenated and alkylated phenol SAPH

Phenyl-β-naphthylamine PBN thio-β-naphthol

2-mercaptobenzothiazole MBT

Diphenylamine derivatives (strongly

discoloring)

benzothiazyl disulfide MBTSOctylated diphenylamine ODPA

Styrinated diphenylamine SDPA tris(p-nonylphenyl)phosphite TNPP

Acetone/disphenylamine

conden-sation product

ADPA zinc dimethyldithiocarbamate ZDMC

Benzimidazole derivatives

For molecular layers of the rubber polymer, this is shown schematically in Scheme 18.

If there is sufficient strain in the vulcanized rubber, even if the ozonide reforms, it does so

in a manor to permit a crack to form and grow

Effective chemical antiozonants share certain common functions as follows:

1 They react directly with ozone

2 They migrate to the surface of the rubber product to react with ozone

3 They decrease the rate of cut crack growth

TABLE 4.11 Peroxide Decomposers

4 Antiozonants such as N,N´-dialkyl-p-phenylenediamines and

N-alkyl-N´-aryl-p-phe-nylenediamine raise a polymer’s apparent critical stress; i.e., polymers containingthese materials require greater elongation for ozone cracks to occur

4.5.2.4 Protection against the Effects of Ozone

Waxes. Paraffinic waxes function by blooming to the rubber surface to form a thin ert protective film Since the wax is unreactive toward ozone, this film is a physical but not

in-a chemicin-al bin-arrier to ozone The number of cin-arbon in-atoms per molecule of win-ax vin-aries from

18 to 50 Microcrystalline waxes are heavier and less crystalline They have between 37and 70 carbon atoms per molecule The migration rate of waxes is dependent on severalfactors These include the type of rubber or blend, the amount and type of reinforcingfiller, the concentration and structure of the wax, and the temperature range that the prod-uct will experience in use

Unfortunately, waxes do not protect against ozone under dynamic conditions, e.g., for

a rolling tire Under such conditions, rupture of a barrier wax film can occur and cause

SCHEME 18

fault points on the rubber surface Instead of total surface involvement, the ozone attackoccurs at these relatively few fault areas, causing rather large cracks to develop Thus,other types of additives are needed for protection against the effects of ozone under dy-namic conditions.

Chemical Antiozonants. The first effective chemical antiozonant was a

dihydroquin-oline type, 1,2-dihydro-6-ethoxy-2,2,4-trimethylquindihydroquin-oline (DETQ) However, ised quinoline derivatives provide only slight ozone protection, although they are good

polymer-antioxidants DETQ provides protection against the action of ozone, but it is highly ing and discoloring and is lost from rubber compounds because of its volatility

stain-p-Phenylenediamine antiozonants such as N,N´-di-sec-alkyl-p-phenylenediamines

were then introduced They surpassed the dihydroquinolines in their ability to protect ber from ozone attack

rub-The success of the dialkyl-p-phenylenediamines led to the development of related ozonants, i.e., alkyl/aryl-analogs, N-isopropyl-N´-pheny-p-phenylenediamine (IPPD) and N-cyclohexyl-N´-phenyl-p-phenylenediamine, and mixed N,N´-diaryl-p-phenylenedi-

anti-amine mixtures

The longer-chain alkyl substituents served to reduce the volatility However, the

fugi-tive nature of the protection offered by the di-octyl-p-phenylenediamine was attributed to

the fact that it reacts directly with oxygen (O2) as well as ozone Alkyl/aryl- or

diaryl-p-phenylenediamines are less subject to depletion by reaction with oxygen and are lasting in rubber compounds

longer-The antiozonants, N-(1,3-di-methylbutyl)-N´-phenyl-p-phenylenediamine (6PPD) and

the C7 and C5 alkyl-analogs were later introduced In addition to their use as antiozonants,

p-phenylenediamines, primarily the more persistent mixed diaryl-derivatives, have

re-placed N-phenyl-β-naphthylamine as antioxidants flex crack inhibitors and synthetic mer antioxidant stabilizers

poly-All of these antiozonants are staining and discoloring This has limited their use rily to carbon black-loaded compounds We also note that the antiozonants and amine-based antioxidants cause a reduction in scorch resistance

prima-Multiple Functions. The N,N´-disubstituted-p-phenylenediamines (PPDs) are unique

stabilizers Many of them simultaneously are potent antioxidants as well as antiozonants

(especially the sec-alkyl-p-phenylenediamines) They are also good flex-crack inhibitors.

In general, the best antioxidant protection is afforded at levels slightly less than 1 phr.Higher levels of use actually become detrimental, as the system can become pro-oxidative.Nevertheless, to provide antiozonant protection, antiozonant levels in excess of 2 phr or

more may be necessary The use of di-aryl-p-phenylenediamines with di-alkyl or alkyl/

aryl derivatives is beneficial, since it reduces the pro-oxidative effect Their low volatilityand low extractability provide for long-term protection

Differences between Polymers. The degree of required ozone protection varies withthe type of rubber Saturated elastomers need no antiozonant protection, because they have

no sites for reaction with ozone Rubbers such as EPDM, which have a low olefin tration, need essentially no protection against the effects of ozone Styrene-butadiene rub-ber (SBR) requires antiozonant, while NR and synthetic polyisoprene (IR) may requiresomewhat increased dosages of antiozonant Nitrile rubber (NBR) is very difficult to pro-tect against ozone attack

concen-Antiozonant activity changes with time For short periods of aging time, the

dialkyl-p-phenylenediamines are the most effective antiozonants, very closely followed by the alkyl/aryl analogs, with the diaryl being less effective However, with increased aging time, theorder of effectiveness is completely reversed, as oxidation and reaction with ozone oc-cur—another reason for using mixtures of antiozonants, i.e, to provide protection for anextended period of aging

Mechanisms for Protection against the Effects of Ozone Attack. The mechanism

of antiozonant protection is still not fully understood However, there are several theories,which detail the mechanism of protection by chemical antiozonants: inert barrier, compet-itive reaction, reduced critical stress, and polymer back-bone chain repair

The inert barrier theory says that a material that is nonreactive migrates to the surfaceand forms a physical barrier that prevents the ozone from reaching the reactive doublebonds in the polymer Waxes are thought to behave in this manner

According to the competitive-reaction or “scavenger” model, the antiozonant migrates

to the surface of the rubber and then selectively reacts with ozone so that the rubber is notharmed until the antiozonant is consumed A protective-film theory suggests, that after theantiozonant has done the above and behaved as a “scavenger,” the reaction products be-come an inert film Any chemical antiozonant might function in both of these ways There

is much evidence to support the “scavenger” mechanism as the dominant one There isalso good support for the formation of a protective film Surface films on rubber have beenseen visually and microscopically With partial removal of the film and reexposure toozone, only the cleaned surface is degraded

According to the reduced critical stress theory, certain materials migrate to or near therubber surface and modify the internal stress of the polymer such that cracks do not ap-pear Although this phenomenon is poorly understood, it is easy to observe The use of in-creasingly higher levels of antiozonant raises the critical stress level required for cracks toform

The chain repair theories suggest that severed polymer chains (terminated by carboxy

or aldehyde groups) can be relinked by reaction with the antiozonant or that the nant reacts with the ozonide or zwitterion (carbonyl oxide) to give a low-molecular-weight, inert, self-healing film Either way, the antiozonant would be chemically linked tothe rubber However, the chain repair or self-healing film theories do not appear to be asstrongly supported as the other theories

antiozo-Ideal Antiozonants. An ideal antiozonant should be competitively reactive withozone in the presence of carbon-carbon double bonds in the rubber-molecule backbone.However, it should not too reactive with ozone (or even oxygen) lest it not persist to givelong-term protection It should not react with sulfur accelerators or other ingredients in thecure package It should be nonvolatile and persist at the surface of the rubber In addition,the ideal antiozonant should not discolor the rubber Unfortunately, an ideally active non-staining chemical antiozonant has not yet been found

4.5.3 Types of Vulcanizable Elastomers and their Applications

4.5.3.1 Natural Rubber (NR). Natural rubber, as stated above, was the first elastomer to

be used in commercial applications Although the polymer (cis-1,4-polyisoprene) occurs

in over 200 plants, the rubber tree, hevea brasiliensis, is the source of essentially all that is

used The chemical structure of the polymer is given here:

There are two possible structures for poly-1,4,polyisoprene Natural rubber structure is

of the cis form The trans forms (the structure of guta perch or balata gum) have higher

melting point and higher glass transition temperatures (see below)

The rubber is harvested from the tree in the form of a latex, the aqueous emulsion tained from the tree by tapping into the inner bark and collected in cups attached to thetrees The latex itself can be used for the fabrication of rubber articles, but most of the NR

ob-is used as a dry raw rubber taken from the coagulated latex There are many types andgrades of the dry rubber However, the Malaysian rubber industry produces standard NRgrades that correspond to technical specifications Their system is being followed by otherproducer countries, thus the designations SMR (Standard Malaysian Rubber), SIR (Stan-dard Indonesian Rubber), SSR (Specified Singapore Rubber), SLR (Standard Lank Rub-ber), TTR (Thai Tested Rubber), and NSR (Nigerian Standard Rubber) Within a nationalstandard type, there are grades that differ with respect to color, viscosity, molecularweight, and other qualities

NR contains small amounts of highly important nonrubber constituents, e.g., proteins,sugars, and fatty acids Some of these nonrubber components are vulcanization activators,antidegradants, and, unfortunately, allergens

NR can be vulcanized by using any of a number of vulcanizing systems, e.g, ated sulfur, peroxide, phenolic (resole), quinonedioxime, and others However, by far, ac-celerated sulfur systems are the most used

acceler-Properties. NR vulcanizates have a range of interesting properties Individual ties of NR can be surpassed by those of synthetic rubbers, but the combination of high ten-sile strength, high resilience, good low-temperature flexibility, and low hysteresis and heatbuildup is unique In addition, the building tack and green strength of NR are unsurpassed

proper-by synthetic rubbers Building tack is the ability for unvulcanized pieces of rubber to sticktogether during the building process, for example, for a tire, where plies and other compo-nents must adhere and “become one” before vulcanization Green strength is the mechani-cal strength of the uncured polymer It is high in the case of natural rubber because, evenbefore vulcanization, natural crystallizes during straining This property is likely also re-lated to building tack, wherein there would be crystallization at the autoadhesive interfacedue to high local strains as one attempts to pull apart one component from the other

NR vulcanizates can be produced in a wide hardness range (Shore A 30 to that of hardrubber or ebonite) Due to its crystallization during strain, NR has high tensile strengtheven without reinforcing fillers (e.g., carbon black) Also, because of strain-induced crys-tallization, the tear resistance of NR vulcanizates is quite high The ultimate elongation of

a NR vulcanizate is generally between about 500 and 1100 percent Also, NR vulcanizateshave very good fatigue resistance (resistance to repeated strains, each one alone less thanultimate) With respect to elastic rebound, NR vulcanizates are surpassed only by those of

BR

The heat resistance of NR is not good enough for many uses, and it is exceeded bymany synthetic rubbers It is affected by the choice of vulcanization system, vulcanizationconditions, choice of protective agents, and even choice of filler To obtain good aging re-sistance of NR vulcanizates, one must use protective agents in the compound and use rela-tively short curing cycles at relatively low temperatures

Because of its main-chain double bonds, unstabilized NR exhibits extremely poor sistance to atmospheric ozone Its light-colored vulcanizates have poor resistance to