Chapter 15

Basic Polymer Chemistry:Thermosets

In the previous chapter we talked about linear polymers and havementioned the concept of cross-linking only in passing. Linear polymers areusually thermoplastic', they soften or melt when heated and will dissolve insuitable solvents. They can be remelted and shaped into their finishedproduct with no further chemical reactions. Thermoset resins, those havingelaborately cross-linked three-dimensional structures, set or harden byundergoing a chemical reaction during the manufacture of finished products.They decompose on heating and are infusible and insoluble. Their chemistryand physical properties are quite different from thermoplastic polymers. Theimportant ones are now discussed.

1. PHENOL-FORMALDEHYDE POLYMERS(PHENOLIC RESINS)

1.1 One-Stage Resins

These copolymers of phenol and formaldehyde were the first fullysynthetic polymers made. They were discovered in 1910 by Leo Baekelandand given the tradename Bakelite.® They may be prepared in two ways,both involving step growth polymerization. A "one-stage" resin may beobtained using an alkaline catalyst and excess formaldehyde to form linear,low molecular weight resol resins. Slight acidification and further heatingcauses the curing process to give a highly cross-linked thermoset. This

complex reaction is summarized here in steps. First we see the alkalinecatalyst with phenol and formaldehyde reacting to form methylolphenols.

(+ para attack)

omethylolphenol (+/?)

The o- and /7-methylolphenols are more reactive toward formaldehydethan the original phenol and rapidly undergo further reaction to give di- andtrimethylol derivatives.

trimethylolphenol

The methylolphenols will react to form di- and trinuclear phenols at stillfree ortho and para positions.

Although these resols can be cross-linked under basic conditions,acidification and further heating is preferred. The mechanism of poly-merization under acidic conditions involves carbocation chemistry.

resol

The final structure of the product is very highly branched. Most linkagesbetween aromatic rings are -CH2- groups, though some -CH2-O-CH2-linkages are present.

®a phenolic or Bakelite resin

1.2 Two-Stage Resins

The second method, called a "two-stage" process, uses an acid catalystand excess phenol to give a linear polymer that may be stored or sold. Theseare called novolacs and have no free methylol groups for cross-linking.

novolac

Thus in a separate second part of this "two-stage" process a cross-linkingagent is added and further reaction occurs. Although formaldehyde may beadded, quite often hexamethylenetetramine is used, which decomposes toformaldehyde and ammonia. Occasional nitrogen bridges occur in the finalstructure of some phenolics made by this method.

a phenolic

Other modifications in making phenolics are the incorporation of cresolsor resorcinol as the phenol and acetaldehyde or furfural as the aldehyde.

cresols resorcinol furfural

2. UREA-FORMALDEHYDE POLYMERS (UREARESINS)

Urea will give cross-linked resins with formaldehyde. Methylolureas areformed first under alkaline conditions.

Continued reaction under acidic conditions gives a fairly linear, lowmolecular weight intermediate polymer.

Heating for an extended period of time under acidic conditions will givea complex thermoset polymer of poorly defined structure including ringformation, of which the following may be typical.

3. MELAMINE-FORMALDEHYDE POLYMERS(MELAMINE RESINS)

Melamine, having three amino groups and six labile hydrogens, will formthermoset resins with formaldehyde. The chemistry is similar to that for theurea resins.

(R)

a melamine or Formica resin

4. EPOXY RESINS

This type of thermoset polymer is typically made first by reaction of thesodium salt of bisphenol A and excess epichlorohydrin, which forms a lowmolecular weight polymer with terminal epoxy groups; n is between 1 and 4.

This reaction is best summarized in steps:

(1) Formation of the sodium salt of bisphenol A

HO—Y-OH + OH' *> 'O-Y-OH + H2O

(2) Reaction of the epoxy group of epichlorohydrin with the phenoxideanion

(3) Elimination of chloride ion

(4) Reaction of the new epoxy group with the phenoxide ion

(5) Formation of a hydroxy group by protonation

(6) Reaction with the excess epichlorohydrin causing the low molecularweight polymer to have terminal epoxy groups

These low molecular weight compounds with epoxy end groups arecross-linked by adding a curing agent such as ethylenediamine. Primaryamines react with epoxides to form tertiary amines and branches.

Thus a cross-linked polymeric structure is the final result when a primarydiamine and a diepoxide react and all four N-H bonds on the two aminegroups are broken.

5. POLYURETHANE FOAMS

Most useful polyurethanes are cross-linked. Those commonly used infoams start with a diisocyanate like toluene diisocyanate (TDI) and a lowmolecular weight polyether such as poly(propylene glycol). Recall that thebasic reaction of an isocyanate plus an alcohol gives the urethanefunctionality.

isocyanate alcohol urethane

poly(propylene glycol)

a linear polyurethane

One way of obtaining the more useful cross-linked polyurethanes is byusing a trifunctional reagent. Thus either the TDI can react with a triol orthe propylene oxide can be polymerized in the presence of a triol. Then theisocyanate-alcohol reaction would of course give a cross-linked urethane.

trimethylolpropane TDI

glycerol propylene oxide

Remember that (!!functional molecules give a linear polymer.Trifunctional molecules yield a cross-linked polymer.

In the urethane process a small amount of water is added to convert someisocyanate functionalities into CO2 gas and amines. The degree of foamingcan be controlled by the amount of water added.

6. UNSATURATED POLYESTERS

An unsaturated polyester resin consists of a linear polyester whose chaincontains double bonds and an unsaturated monomer such as styrene thatcopolymerizes with the polyester to provide a cross-linked product. Themost common unsaturated polyester is made by step growth polymerizationof propylene glycol with phthalic and maleic anhydrides. Subsequenttreatment with styrene and a peroxide catalyst leads to a solid, infusiblethermoset.

Unsaturated polyesters are relatively brittle and about 70% are used withfillers, of which glass fiber is easily the most popular. Glass fiber-reinforcedpolyester for small boat hulls consumes one quarter of unsaturatedpolyesters. Automobiles, furniture, and construction also make use of thisthermoset resin.

7. ALKYD RESINS

This is a very broad class of compounds commonly used in coatings.Over 400-500 different alkyd resins are commercially available. They arepolyesters containing unsaturation that can be cross-linked in the presence ofan initiator known traditionally as a "drier." A common example is thealkyd formed from phthalic anhydride and a glyceride of linolenic acidobtained from various plants. Cross-linking of the multiple bonds in thelong unsaturated chain R produces the thermoset polymer by linking Rgroups of separate molecules with each other.

8. NATURAL POLYMERS

Mention has already been made of two polymers that can be obtainednaturally from living animals: silk (from the silkworm) and wool (fromsheep). They are proteins made of various amino acids; both are used intextiles. Other biologically derived polymers are also familiar such as wood,starch, and some sugars. We will not cover these in detail here. However,certain cellulosics we will discuss briefly since they are compared tosynthetic fibers later. Cellulose is the primary substance of which the wallsof vegetable cells are constructed and is largely composed of glucoseresidues. It may be obtained from wood or derived in very high purity fromcotton fibers, which are about 92% pure cellulose.

cellulose

The important fiber rayon is simply "regenerated" cellulose from woodpulp that is in a form more easily spun into fibers. Cellophane film isregenerated cellulose made into film. One method of regeneration isformation of xanthate groups from selected hydroxy groups of cellulose,followed by hydrolysis back to hydroxy groups.

cellulose cellulose regeneratedxanthate cellulose

+ CS2

Cellulose acetate and triacetate may be used as plastics or spun into fibersfor textiles. They are made by the reaction of cellulose with aceticanhydride.

cellulose cellulosetriacetate

Finally, one last type of natural polymer is natural rubber, obtained fromthe rubber tree and having the all c/s-l,4-polyisoprene structure. Thisstructure has been duplicated in the laboratory and is called "syntheticrubber," made with the use of Ziegler-Natta catalysis.

natural rubber

The biosynthesis of synthetic natural rubber has been completelydetermined and appears in Fig. 15.1. Many plants and animals use this samebiosynthetic pathway to make hundreds of terpenes and steroids from theircommon isoprenoid building blocks.

isopentenyl dimethylallyl

geranyl (Ci0)

geranyl isopentenyl

farnesyl(C15)

naturalrubber

squalene (€30)

steroidssex hormonescholesterolcortisonebile acidsvitamin Aterpenescamphor

Figure 15.1 Biosynthesis of natural rubber.

9. POLYMER PROPERTIES

9.1 Molecular Weight

The detailed treatment of the molecular weight analysis of polymers isleft to other texts. We should be aware that there are two types of molecularweights, number average and weight average.

2N M Mn = number average molecular weightMn = —— N1 = number of molecules with a molecular weight of MI

^N N = total number of molecules

_ SwjMi _ SNiMi Mw = weight average molecular weightW Swi SNjMi wi = weight of molecules with a molecular weight of MI

Because the weight average is biased toward molecules with highermolecular weight, it is larger than the number average. Boiling pointelevation, freezing point depression, osmotic pressure, and end groupanalysis give the number average molecular weight. Light scattering andsedimentation give the weight average. Viscosity measurements give avalue somewhere between the two. Molecular weight and mechanicalstrength are related since strength increases rapidly as the degree ofpolymerization (or the number of repeating units, n) increases from 50 to500. Further increases in molecular weight have a smaller effect.

9.2 Crystallinity

This is a key factor in governing polymer properties. If the polymermolecules can align themselves with a high lateral order and the chains lieside by side, we say that the polymer is highly crystalline. Bulky groups orbranching prevent the polymer from being highly crystalline. Another thingaffecting crystallinity is the magnitude of attractive forces betweenneighboring polymer molecules. Strong intermolecular forces, such ashydrogen bonding in the nylons, promote greater crystallinity. Isotacticpolymers are always more crystalline than atactic polymers because of theregularity of any large groups. Ziegler-Natta catalysts promote isotactic,crystalline polymerizations.

Crystalline polymers tend to have greater mechanical strength, highermelting points, and higher densities than amorphous polymers. On the otherhand, they are usually much less transparent (more opaque) because light isreflected or scattered at the bour dary between the crystalline ana amorphous

portions of the polymeric structure. Amorphous polymers are transparentand glasslike.

Examples of crystalline polymers are nylons, cellulose, linear polyesters,and high-density polyethylene. Amorphous polymers are exemplified bypoly(methyl methacrylate), polycarbonates, and low-density polyethylene.The student should think about why these structures promote more or lesscrystallinity in these examples.

The crystallinity of a specific polymer may be altered by orientation orstretching the polymer mechanically in a certain direction. On stretching,the molecules align themselves and become more crystalline. Did you evernotice that a rubber band becomes more opaque on stretching? However, ifa crystalline polymer is biaxially oriented, as with a drawn nylon sheet, thenthe whole sheet is in effect a single crystal and is very transparent! Thestretching or drawing of fibers causes greater crystallinity and gives thelongitudinal strength required in fibers.

9.3 Temperature Dependency of Polymers

Polymers usually do not have a single, sharp melting point like a purechemical might. Then too each polymer is a little different in its reaction totemperature changes, and the same polymer but with different molecularweights will have different observable changes when heated or cooled. Theglass transition temperature, Tg, and the crystalline melting point, Tm, aremost often used to describe the rather nebulous changes of a polymer withtemperature, shown here with a graph of volume versus temperature.

relativevolume

amorphous

crystalline

temperature

An amorphous material such a polystyrene does not solidify sharply. Itgoes from a viscous liquid to a rubbery solid, then to a leathery solid.Finally, it becomes a glassy solid. This last change is a sharper one and thetemperature at which it occurs is called the glass transition temperature, Tg.Or upon heating a polymer, it is the temperature at which the polymer loses

its hardness or brittleness and becomes more flexible and takes on rubbery orleathery properties. At this transition temperature noticeable changes in thespecific volume, thermal conductivity, refractive index, stiffness, heatcontent, and dielectric loss are apparent.

More crystalline polymers have a glass transition temperature because allpolymers have amorphous regions between the crystalline regions.Crystalline polymers also have a crystalline melting point, Tm. It is thetemperature at which a molten polymer changes from a viscous liquid to amicrocrystalline solid. It is accompanied by more sudden changes indensity, refractive index, heat capacity, transparency, and similar properties,but it is still not so sharp as a nonpolymeric melting point. Usually Tg isabout one half to two thirds of Tm for most polymers if expressed in degreesKelvin.

Table 15.1 gives the appropriate Tg's for a few selected polymers. Note

Table 15.1 Approximate Glass Transition Temperatures (Tg)for Selected Polymers

Polymer Tg

(0K)Cellulose acetate butyrate 323Cellulose triacetate 430Polyethylene (LDPE) 148Polypropylene (atactic) 253Polypropylene (isotactic) 373Poly(ethyl acrylate) 249Poly(methyl acrylate) 279Poly(butyl methacrylate) (atactic) 339Poly(methyl methacrylate) (atactic) 378Polyacrylonitrile 378Poly(vinyl acetate) 301Poly(vinyl alcohol) 358Poly(vinyl chloride) 354c/,s-Poly-1,3 -butadiene 165trans-Poly-1,3 -butadiene 255Poly(hexamethylene adipamide) (nylon 6,6) 330Polyethylene adipate) 223Polyethylene terephthalate) 342Poly(dimethyl siloxane) (silicone) 150Polystyrene 373

Source: Seymour/Carraher

that the Tg values are low for elastomers and flexible polymers (low-densitypolyethylene, cj's-poly-1,3 -butadiene, silicone) and relatively high for hardamorphous plastics [polypropylene-isotactic; polyacrylonitrile; poly(vinylalcohol); nylon 6,6; poly(ethylene terephthalate); polystyrene]. Notice alsothat Tg varies with even slight changes in structure. For instance, Tg

decreases as the size of the ester groups increases in polyacrylates andpolymethacrylates. Tg increases when aromatics are added [poly(ethyleneadipate) vs. poly(ethylene terephthalate)].

Even a single polymer can vary significantly in its amorphous orcrystalline nature depending on how it was made and the degree ofbranching. Low-density polyethylene (LDPE) is amorphous because themany branches prevent crystallinity. Linear low-density polyethylene(LLDPE) has a few branches and is semi-crystalline. High-densitypolyethylene (HDPE) has no significant amount of branching and is highlycrystalline.

LDPE LLDPE

HDPE

Many different test methods can be used to study polymers and theirphysical changes with temperature. These studies are called thermalanalysis. Two important types of thermal analysis are called differentialscanning calorimetry (DSC) and differential thermal analysis (DTA). DSCis a technique in which heat flow away from a polymer is measured as afunction of temperature or time. In DTA the temperature difference betweena reference and a sample is measured as a function of temperature or time.A typical DTA curve easily shows both Tg and Tm.

endo

ther

mic

ex

othe

rmic

change intemperature

temperature

9.4 Tensile Properties

Many of the quoted physical properties of a polymer are derived from astress-strain experiment. The polymer is cut into an appropriate shape. Forinstance, plastics are cut into the shape shown here (sometimes called adogbone). They are placed in two jaws of a special instrument (Fig. 15.2).

Figure 15.2 Tensile testing instrumentation. Polymer samples are stretched undercontrolled conditions and the tensile properties are evaluated. (Courtesy of Du Pont)

Breakpoint

Elongation at break

YieldpointElongation

at yield

Alternativeyield point

Ultimate ortensile strength

Stre

ss (

psi)

Yieldstress

Strain (% elongation)

Figure 15.3 Common stress-strain curve for a thermoplastic. (Source: Wittcoff and

Reuben, Industrial Organic Chemicals, John Wiley & Sons, 1996. Reprinted by

permission of John Wiley & Sons, Inc.)

The ends are pulled apart at a certain speed and the distance pulled is plottedversus pounds per square inch of tension placed on the sample. A typicalstress-strain curve for a thermoplastic is given in Fig. 15.3.

In the initial stages of the extension the graph is sometimes linear andobeys Hooke's law. The slope of this section is called Young's modulus.This portion of the curve is reversible. Because many polymers do not obeyHooke's law the modulus is frequently expressed as pounds per square inchat a certain elongation or extensibility. The 2% modulus is a commonquotation. Some elastomers are better described as a 100% modulus. Thestiffer the polymer is that is tested, the higher will be the modulus value thatis recorded.

After the initial stress a yield point is reached, beyond which permanentdeformation and nonreversible extension occur. Then the stress andelongation gradually increase until the plastic is broken. The stress at thispoint is called the ultimate tensile strength (or tensile strength) and the strain

Relative areas under curves

Fiber <1Elastomer 15Thermoplastic 150

Thermoplastic

Elastomer

Stre

ss (p

si)

Strain (% elongation)

Modulus (psi)Percent ElongationCrystallinityExample

Elastomers

15-150100-1000

LowNatural rubber

Plastics

1,500-200,00020-100

ModeratePolyethylene

Fibers

150,000-1,500,000<10High

Nylon

Figure 15.4 Stress-strain diagrams for typical polymers. (Source: Wittcoff and Reuben,Industrial Organic Chemicals, John Wiley & Sons, 1996. Reprinted by permission ofJohn Wiley & Sons, Inc.)

is the percent elongation (at break) where 100% would mean it could bestretched to twice its original length before breaking.

Figure 15.4 gives the stress-strain diagrams for a typical fiber, plastic,and elastomer and the average properties for each. The approximate relativearea under the curve is fiber, 1; elastomers, 15; thermoplastics, 150.Coatings and adhesives, the two other types of end-uses for polymers, willvary considerably in their tensile properties, but many have moduli generallybetween elastomers and plastics. They must have some elongation and areusually of low crystallinity.

9.5 Other Properties

There are many other properties of polymers that can become veryimportant depending on the type of processing and final use. Rheological or

Table 15.2 Estimated Use Percentage for Polymers

Plastics 71%

Fibers 11

Elastomers 7

Coatings 6

Adhesives 5

melt properties are especially critical as a polymer is processed into a certainshape and type of material (plastic, fiber, etc.). Viscosity and melt strengthare important here. Optical clarity, refractivity, and water vapor and gaspermeability are important in packaging. Abrasion and wear resistance forclothing, weatherability and UV stability for outdoor use, hardness, electricalproperties, chemical resistance, flammability, toxicity, and blendability areall properties that can be specifically tested using a detailed methodpublished by the American Society for Testing and Materials (ASTM).

With this brief introduction into polymer chemistry, let us now turn ourattention to specific studies of the five major applications of polymers:plastics, fibers, elastomers, coatings, and adhesives, with the approximateuse percentages as shown in Table 15.2.

Suggested Readings

Carraher, Seymour/Carraher's Polymer Chemistry: An Introduction, pp. 19-164.

Wittcoff and Reuben, Industrial Organic Chemicals, pp. 445-467.