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OAR Box 1413 

Prepped by Ollie Stewart 

Document Number: 

78) 11-1-79 

Docket Number: 

A-94-52 

Printed 7/9/2008 3:11:23 PM Sheet 78 of 114 
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HK Al l l L E Nv G 

A-?4-52 

Strategies for Low 

Resins 

Purposes of Styrene 

| tyrene is added to unsaturated po 

£PAAIRD.OC,KET . . }«st.er..resins.^d..vm^^ two major 

'purposes — (1) to dilute the mixture for effective viscosity control so that rhe fibers 

| can be wetted; and (2) to serve as a crosslinking agent and promote the ease of crosslinking of the resin. Materials, like styrene, that simultaneously accomplish both of these 

| purposes are called "reactive diluents." Other active diluents exists, such as methyl 

methacrylate, but because of the predominance of styrene in the unsaturated polyester 

and vinyl ester resin market, our focus will be almost exclusively on styrene. 

The ability of styrene to accomplish these two purposes has resulted in lower costs 

£ overall and improved performance and manufacturing efficiencies. Imagine, for instance, 

a diluent that was non-reactive. This would require that the diluent be removed during 

molding. Several epoxy and other high performance resins have this problem and, as a 

| result, are difficult to use and are more cosdy. 

The excellent reactivity of styrene means that the styrene is generally used up in the 

crosslinking reaction. Styrene forms a part ofthe cured polymer, generally at a lower cost 

K than the backbone resin system itself. When it has been crosslinked, styrene imparts stiffness to most cured systems (and also some brirdeness), but generally enhances the physical properties ofthe finished product. 

Styrene is also an effective and relatively inexpensive diluent. It is able to easily 

dissolve most of the imponant resins and, dius, allow easy wetout of the reinforcement. 

"s»s 

By A. Brent Strong 

Brigham Young University 

(Jowposiies F.ibri,;uionl)n\K 19l>8 
P.3 

nee, styrene is nearly ideal from a 

ctional standpoint. However, as you 

know, styrene is listed as a hazardous 

pollutant (HAP). Therefore, the 

ount of styrene emitted in the 

nufacture of FRP parts must be 

uced. We will now examine some of 

strategies being pursued by the resin 

nufacturers to reduce the styrene 

itent of their products. 

The chemistry of making these 

nges to reduce styrene content is 

nplex. Therefore, the discussion which 

ows on the various strategies to low 

ene content will be simplified. The 

us will be on the direction and 

icepts behind the changes rather than 

details. 

ategy One — Modifications 

existing polymer systems 

: initial effort by almost all ofthe resin 

nufacturers, unsaturated polyesters 

• vinyl esters, was to modify their existresin systems so that less styrene 

ild be needed. This strategy was not 

y prudent commercially, it also gave 

ck results. 

The resin manufacturers realized that 

ie viscosity ofthe base resin were lower, 

amount of styrene needed to achieve a 

d viscosity for fiber wet out could be 

iced. The first method to reduce the 

osity of the base resin was to shorten 

molecular chains, that is, to reduce 

ecular weight of the resin. Shorter 

ins have less intermolecular entangleu and, therefore, flow more easily, 

s means that the viscosity is lower. 

The amount of sryrene needed in 

;e lower molecular weight resin 

:ms has been successfully reduced, 

t reduced level of styrene, which was 

ated by viscosity control requireits, has also reduced the amount of 

ene available for crosslinking. 

refore, the nature of the crosslinking 

changed. The question is, "How has 

slinking changed the resulting 

•mer properties?" 

Two different crosslink characteristics 

'er overall styrene content and higher 

slink density) occur simultaneously 

i these low molecular weight resin 

:ms. The lower amount of styrene 

lable for crosslinking results in a 

•ved" situation during crosslinking. 

; starving means that the number of 

:ne molecules in each of the crosslinks 

is reduced. (Under excess styrene conditions, multiple styrene molecules join 

together to form long crosslinks. Under 

starved conditions, each crosslink would 

be formed with only one or two styrene 

molecules.) 

The reduction in the total amount of 

styrene contained in the crosslinks, and 

therefore, in the cured polymer, would 

normally lead to lower overall britdeness 

since styrene is inherently brittle. 

However, the shorter polymer chains 

Some resin manufacturers 

have found that new resin 

systems requiring lower 

styrene concentrations can 

be made by varying the 

sequence of addition of 

the di-acid and the 

glycol components. 

means that the carbon-carbon double 

bonds occur more frequendy and this 

results in a higher number of crosslinks 

being formed overall. This higher 

crosslink density would normally increase 

brittleness. Therefore, the effects on 

brittleness from the reduction in styrene 

content and from the increase in crosslink 

density approximately cancel and the 

britdeness is about where it would be in 

the unmodified resin system. 

Generally heat distortion temperature 

and solvent resistance will be increased by 

higher crosslink density. Other properties, 

such as blister resistance, ultra-violet light 

resistance, chalking, cracking, and surface 

smoothness, may also be affected but the 

extent of these effects must be evaluated 

on a case by case basis with each resin and 

application. These effects are much more 

1 

important in gel coats than in laminating 

resins and should, therefore, be examined 

closely before any change in gel coat resin 

is made. 

Another modification in current resin 

systems is a change in molecular weight 

distribution. This is not just a shift to 

lower molecular weights but, rather, a 

change in the statistical spread of the 

molecular weights of the polymers. Even 

when rhe average molecular weight is still 

quite high, the presence of a significant 

quantity of low molecular weight 

molecules has a tendency to make all the 

molecules slide over each other more 

easily, thus lowering the viscosity. This 

lower viscosity allows less styrene to be 

used. 

Strategy Two — Developing 

new polymer systems 

Polyesters are made from the reactions of 

di-acids and glycols. Resin producers 

have found that by varying the types and 

quantities of di-acids and glycols, the 

resin properties can be significantly 

changed. (For instance, the differences 

between ortho and iso resins are just in 

the choice of acid that is reacted.) 

Therefore, many possibilities exist for 

changing the nature of rhe base polyester 

resin. 

In addition to different combinations of previously used components, 

some completely new di-acids and glycols 

are being tried. (Some resin manufacturers call this the clean-slate approach.) 

The resin manufacturers are reluctant to 

disclose the details of these changes 

because of competitive reasons, but it is 

safe to say that all of the major resin 

manufacturers are working on innovative 

new resin structures that will enable the 

sryrene content to be reduced. We 

should anticipate that within the next 

few years, many new products will be 

announced. 

Without disclosing any proprietary 

information, a few basic concepts can 

illustrate some of the directions that the 

resin producers are investigating. Some of 

• the acids and glycols (like neopentyl 

glycol) are known to be more soluble in 

styrene than the other common components. This increased solubility comes 

from both molecular shape and chemical 

polarity effects. At the same time, these 

alternate components might give 

additional benefits such as increased 
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toughness or heat distortion temperature. 

Therefore, the new resins might have 

both reduced styrene content and 

improved physical properties, or at least 

no major reduction in properties as a 

result ofthe decrease in styrene. 

Some resin manufacturers have found 

that new resin systems requiring lower 

styrene concentrations can be made by 

varying the sequence of addition ofthe diacid and the glycol components. These 

changes occur because mixtures of multiple types di-acids and glycols are often 

used in making the polyester resins which 

have different reactivities. An example of 

such a change would occur under these 

two scenarios. If cwo di-acids are mixed 

together and then added to the glycols, the 

distribution ofthe acids could be random. 

However, if one ofthe di-acids is added to 

the glycols and then the second added 

later, the result could be a preferential 

positioning of the second di-acid at the 

ends of the polymer. Hence, two very 

different polymers could be formed. 

The sequence of components along 

the backbone resin can also be altered by 

changing the addition sequence because 

of differences in reactivities ofthe components. A particular component could be 

forced to group together or to be 

dispersed to the ends depending on its 

relative reactivity with respect to other 

components and when they are added. 

Some have called these changes one-stage 

and two-stage additional polymers. 

Using known principles and new 

technologies, the resin manufacturers are 

attempting to create shapes and chemical 

natures that favor lower styrene contents. 

Occasionally a polyester is made 

using a component other than a di-acid 

and glycol, such as dicyclopentadiene 

(DCPD). This material can be used to 

react with maleic acid and glycols to form 

a low molecular weight chain of only a 

few units. Such units are often called 

"adducts" because a few units are "added 

together." The net result of mixing 

DCPD with maleic acid and ethylene 

glycol is an adduct of DCPD—maleic— 

glycol—maleic—DCPD. This adduct 

has low molecular weight and, therefore, 

low styrene requirement. The net result is 

a resin with fast cure cycles, good 

reinforcement print hiding, good fiberglass wetout, good moisture resistance, 

good stiffness, and good air bubble elimination when compared with many 

conventional resins. 

Vinyl esters properties can also be 

changed significantly by modifying the 

nature of the backbone. Vinyl esters tend 

to have more options available for these 

modifications of the resin backbone and, 

of course, the specific natures of the 

changes are different, but the concepts 

discussed in this section are similar 

between vinyl esters and unsaturated 

polyesters. 

Another major area of focus is the 

blending of unsaturated polyesters and 

vinyl esters with other polymer types. 

These blends can be done as monomers 

(in which case they arc often referred to as 

copolymers) or as blends after full 

polymerization has been done (in which 

case they are referred to as mixtures). 

Some of the interesting blends with 

unsaturated polyesters include: 

polyurethanes, melamines, phenolics, 

epoxies, and vinyl esters. In general, these 

blended polymers are higher in cost than 

the basic polyester, but the properties of 

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)lend can be substantially better, thus allowing for the use of 

resin or of entering new markets. 

itegy Three — 

er monomers 

:ne is not the only monomer that acts as a reactive diluent 

insaturated polyesters and vinyl esters. Therefore, some 

nate monomers which have lower vapor pressures are being 

stigated as styrene alternatives. One approach is to choose 

omers that are similar to styrene in chemical reactivity but 

a higher molecular weight and, therefore, a lower vapor 

;ure. For instance, vinyl toluene is very similar to styrene 

ing just an additional methyl group on the benzene ring) 

has a lower vapor pressure and, therefore, lower air pollupotential. Another example is t-butyl styrene. The major 

lem is the higher cost of these alternate monomers. 

Another approach is to simply combine two or three 

ne molecules together and then use these dimers and 

srs as the reactive diluents. (Very short chains such as these 

ometimes called oligomers.) Performance of these materialmost the same in the cured product, and wet out of the 

glass is still excellent since only enough of the dimers and 

:rs is added to achieve the same viscosity used with the 

omer. However, the cost of producing the dimers and 

:rs is significant. 

some have been looking at using completely new monomers 

are not like styrene at all. The prime example is methyl 

;acrylate (MMA), ("Strategies..." continued on p. 38) 

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"Strategies..." continued from 23) 

Daietimes known as acrylic. This materd also has a high vapor pressure and so it 

; also considered a hazardous air polluuit (HAP) but may be more compatible 

drh some polymer systems and, theresre, allow lower concentrations to be 

sed to achieve acceptable viscosities. 

•4MA can also give some improved 

roperties (such as lower smoke generaon). Again, cost is a problem. 

itrategy Four — Suppressants 

et another strategy is to form a surface 

lyer to suppress the emission of styrene. 

ayers such as this, often formed by 

elding wax to the resin mixture, have 

een used for many years to exclude 

xygen so that cures can be improved, 

fence, diis concept is really using an old 

ichnology for a new purpose. Th e 

roblem with this method has always 

een the potential for poor bonding 

hereve r the wax is present. This 

dhesion problem has been alleviated 

jmewhat by creating adhesion-promotlg films. These films contain carbonirbon double bonds, thus allowing 

crosslinking to occur through the film. 

Adhesion between materials on both sides 

of the film is thus promoted. 

These film-forming materials work 

best on horizontal surfaces and when rhe 

materials are quiescent (that is, nonturbulent). Hence, if sprayed, they will 

form best if allowed to sit for some time. 

Non-horizontal surfaces will form some 

films, but they are not as effective. To 

alleviate these problems, some of rhe film 

forming systems contain excess 

thixotropic agents to ensure that the resin 

stays on the vertical surfaces and becomes 

quiescent. 

T h e use of highly thixotropic materials can, in themselves, reduce solvent 

escape. Th e paint industry has successfully used this technology for some time. 

(That is why some paints are so thick 

when in the can but smooth and spread 

easily when brushed, rolled, or sprayed.) 

Perhaps there are other lessons that can be 

learned from the paint industry. 

Summary 

T h e resin manufacturers are working very 

hard to reduce the amount of styrene in 

their resin systems. Some excellent 

success has already been achieved and 

more success is shortly to be announced. 

Some important new scientific principles 

are being discovered and, hopefully, these 

will lead to even greater successes in the 

future. 

Undoubtedly, these new resins will 

require some changes in product design, 

manufacturing and molding. Along with 

the improved techniques in manufacturing, the amount of styrene moving into 

the atmosphere will surely be significantiy 

reduced. 

Acknowledgments 

T h e author thanks Larry Craigie and Do n 

Kelley of Dow Chemical, Mark Callicutt 

of Reichhold Chemicals, Glenn 

Wigington of Neste Polyester America, 

Brent Baker of Cook Composites, Keith 

Johnson of ARCO , Bern Brody of 

Ashland Chemical, and Steve McNally of 

CFA for their assistance in this article. 

Contributing Editor Brent Strong, Ph.D. is 

a Professor of Manufacturing Engineering 

Technology at Brigham Young University 

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