40207_02

POLYESTER AND VINYL ESTER RESINS 2 Frank A. Cassis and Robert C. Talbot

2.1 INTRODUCTION AND HISTORY sored production of styrene-butadiene rubber.

Organic polymers are divided into two types, reinforced-thermoplastic and thermoset. With thermoset polymers such as unsaturated polyesters and vinyl esters, a chemical reaction cross links the material so that it cannot be returned to liquid form. Other common thermosetting polymers include epoxy and phenolic resins. Thermoset plastics made with polyester and vinyl ester resins represent the major portion of the reinforced plastic composites industry today.

Early workers on unsaturated polyesters soon learned that despite the possession of reactive double bonds, these resins were slug- gish in reacting with themselves. Even with effective catalysts, they still required high temperatures and lengthy cure times to complete the cross linking reaction. The key to modern day application of unsaturated polyesters was the discovery by Carlton Ellis in 1937l that the addition of reactive monomers, such as styrene, gave mixtures that would copolymerize many times faster than homopolymerization. The styrene addition produced the added benefit of an easily handled liquid material that could be pumped, transported and fabricated into a finished plastic by a myriad of molding processes.

Developments during the 1940s accelerated the commercial applicability of unsaturated polyesters to the position they hold today. Styrene became readily available and lower in cost as a result of the US Government's spon-

Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7

At the same time, scientists found that styre- nated polyesters could yield high strength, light weight structures when reinforced with glass fibers. They also learned that fiberglass- reinforced polyesters had excellent electrical properties and that large structures could be molded at low pressures with low cost tooling. As a result, commercial development pro- ceeded rapidly after World War I1 with materials and molding research moving in many directions. In the 20 years that followed, polyester and vinyl ester resins matured rapidly and by the mid-l970s, the composites fabricator and end user had numerous options with these matrix systems to achieve the desired properties in the finished part.

2.2 POLYESTER RESINS

The reaction of an organic acid with an alcohol results in the formation of an ester. By using a difunctional acid and a difunctional alcohol (glycol) a linear polyester is produced (Fig. 2.1).

0 0 II II

H-(-0 - C - R - C - 0 - R -)" -OH

Fig. 2.1

Properties of polyesters can be varied by using different combinations of diacids and glycols. These products are thermoplastic polyesters and they are made with various acids and

Polyester resins 35

glycols such as the following: In the esterification reaction with maleic anhv- -

Acids GZycols Phthalic anhydride Ethylene glycol Isophthalic acid Propylene glycol Terephthalic acid Neopentyl glycol Adipic acid Diethylene glycol

The reaction product of terephthalic acid and ethylene glycol is the well known polyethylene terephthalate (PET) which is used to make polyester fibers and polyester plastics such as clear plastic bottles for soft drinks.

Unsaturated polyesters are produced by replacing part of the saturated diacid with an unsaturated diacid such as maleic anhydride or fumaric acid (Fig. 2.2). The former is vastly preferred since it is lower in cost, easily handled and produces only half the water that would be generated in the reaction when fumaric acid is used.

CH = CH t \

0 = c-0-c = 0 Maleic

anhydride

H H I1 II

HOOC- C = C - COOH

Fumaric acid

Fig. 2.2

The resultant polyester contains reactive double bonds (unsaturation) along the entire polyester chain, which becomes the site for the eventual cross linking to produce the cured plastic (Fig. 2.3).

0 0 0 H O It II II I II

HO ( C-R-C-O-R'-O-C-C=C-C-O-R'-O )n H I H

Fig. 2.3

dride, the unsaturated acid isomerizes to the fumarate structure which copolymerizes with styrene much faster than the maleate form. A high degree of isomerization to the fumarate structure is essential to produce an unsaturated polyester with high reactivity. Although the isomerization of maleic anhydride is usually from 65-95% in the esterification reaction, some commercial resins are deliberately formulated with the more expensive fumaric acid to obtain maximum reactivity with the monomer employed.

2.2.1 UNSATURATED POLYESTER CLASSES

Unsaturated polyesters are divided into types or classes depending on the structure of the basic building block. These are orthophthalic, isophthalic, terephthalic, bisphenol-fumarate, chlorendic and dicyclopentadiene.

Orthophthalic resins

These are commonly referred to as ortho or general purpose resins and are usually based on phthalic anhydride, maleic anhydride and propylene glycol. Since the acid groups in phthalic anhydride are on adjacent carbons of the benzene ring, it is very difficult to produce resin molecular weights as high as those achievable with isophthalic and terephthalic acid. Accordingly, resins made from phthalic anhydride have poorer thermal stability and chemical resistance than their iso/tere counterparts.

Isophthalic resins

These resins are produced from isophthalic acid and are characterized by greater strength, heat resistance, toughness and flexibility than their ortho cousins. In isophthalic acid, the acid groups are separated by one carbon of the benzene ring which increases the opportunity to produce polymers with greater linearity and higher molecular weight in the esterification reaction (Fig. 2.4).

36 Polyester and viny ester resins

Phthalic lsophthalic anhydride acid

0 c=o

‘OH

Terephthalic acid

Fig. 2.4

Therephthalic resins

Unsaturated polyesters can be produced from terephthalic acid with the expectation that the resin property improvement obtained in going from phthalic anhydride to isophthalic acid will be matched in going from isophthalic to terephthalic acid. This, however, is not the case and terephthalic resins appear to offer only a slight advantage in heat distortion temperature over their isophthalic counterparts. Other important resin properties such as modulus, hardness and overall chemical resistance favor the is0 resins.

Because of its lower solubility and poorer reactivity, therephthalic acid requires the use of esterification catalysts or pressure processing to produce a resin economically. Without these, processing time for a terephthalic poly-

ester can be three times longer than for an is0 resin. As a result of this, researchers have turned to polyethylene terephthalate scrap from the previously mentioned fiber and plastic operations to develop an economical source of terephthalic polyesters. This scrap can be effectively depolymerized by using different amounts of propylene glycol at elevated temperatures. The glycolyzed product is then reacted with maleic anhydride and diluted with styrene monomer to produce a cost effective terephthalic polyester.

Bisphenol A fumarate resins

These resins are unsaturated rigid polyesters made by reacting bisphenol A with propylene oxide to produce the glycol shown in Fig. 2.5.

This propoxylated bisphenol A is then reacted with fumaric acid to form an unsaturated polyester. The bisphenol structure illustrated above imparts a high degree of hardness, rigidity and thermal stability to this particular resin.

Chlorendic resins

These unique polyester resins are based on HET acid (hexachlorocyclopentadiene) or the anhydride shown in Fig. 2.6.

When reacted with an unsaturated acid and a stable glycol such as neopentyl, an extremely rigid unsaturated polyester results with outstanding thermal stability and resistance to oxidizing environments. The inherent chlorine in the resin chain imparts some fire retardancy as well.

H H . .

HO - C I - CH2 - O o l @ 0 - CH2 - C I - OH I I CH3 CH3

CI CQ?!; CI-c-CI -c=o

Fig. 2.5 Fig. 2.6

Hydrocarbon Solvents 37

FORMULA

PROPERTIES

Table 2.72: Hexene-1 (4)

cn2 = cn-cn2-cn2-cn2-cn,

RESEARCH PURE TECHNICAL GRADE GRADE GRADE

PROPERTIES

Composition. weight percent ~

Hexene-1

cis-Hexene.2

Normal Hexane lsoolefins Heptene-l

- tranr-Heptene-3 cis.Heptene.3 tranr-Heptene.2 cis.Heptene2

___-_-.___~_-- ~

tranr-Hexene-2 -

H e x e ! E r - ? p . - _ ~ ~

~~ ~

_ _ _ . _ ~ . __

Purity by freezing point, mol %

Boiling point, F ___ Distillation range, F

Freezing point, F -~ __

Initial boiling point Dry point

Specific gravity of liquid at 6076m at 2014 C

API gravity at 60 F Density of liquid at 60 F. m a l Vapor pressure at 70 F. psia

100 F, psia 130 F, psia

-___

Refractive index. 20/0 Color. Saybolt Acidity, distillation residue Nonvolatile matter. gramdl00 ml Flash point. appoximate. F

_ _ _ ._ -

-__-

*Literature values.

'Literature values.

Table 2.73: cis-Hexene-2 (4)

RESEARCH GRADE

0.1 0.2 -

~-

99.6

0.1

-~

99.28

156.00' -222.04'

-~

0.6 9 2 0 * 0.68720'

5.760' 2.4' 4.9' 9.1' 1.39761.

_________

+30

Table 2.74: Mixed 2-Hexenes (4)

Purity by freezing point. mol % Freezing point. F Boiling point, F

~

Distillation range. F Initial boiling point Dry point

Specific gravity of liquid at 60160 F at2014

API gravity at 60 F

Vapor pressure a t 70 F. psis

Refractive index, 2010 Color, Saybolt Acidity. distillation residue Nonvolatile matter, gramdl00 ml Flash point, approximate, F

Density of liquid a t 60 F. Ibdgal

100 F, psia 130 F, psia

cn3-c - c-cn2-cn2-cn, FORMULA

.~ ~~~ ~- . ~~

~ - ____ __-. ~- ~-

155.0 155.0 155.1 155.1

__.___.__ .

-. ~ 0.684 0.686 ~

c - . . ~ ~ _ _ _ - . ~ - 75.4 74.8

2.4 2.4 ____ . 5.0 5.0

~ 9.2 1.396 1.396

~- -. .- - 5.69 5.71 __ ~-.

. . . .. . -~ -~ - ~ .~~ !.2._. - -~ .-

~~ __ +30 +30 neutral neutral

_. 0.0005 .. -. 0.0005 -5 -5

_

-~ .. ~

_ _ _ _ _ _ ~

FORMULA 1 cH3-cH - cn-cH2-cn2-cn3

PURE TECHNICAL GRADE GRADE PROPERTIES

- - .. - - Composition, weight percent t r x e 0 3 . - Hexenel

tiant-Hexene-2 . i : i t 9 9 O m i n -

Normal Hexane

Heptene-l

c is Hexene-2 _ _ Hexener-3 -

lsoolefins - - -

tranr-Heptene-3 . - _ .. cis-Heptene 3 - -

0.8 2 1 -. ~ - - -- - ~ - - - - -

-_____ _

--__- -

.. tranr-Heptene 2 -

ca-Heptene2


In addition to tailoring the resin for specific applications by varying the building blocks, the properties of unsaturated polyesters can often be altered significantly by selection of the esterification process. This is particularly true with isophthalic/terephthalic polyesters which are slower reacting than phthalic anhydride. By using a two stage or modified two stage reaction with these aromatic diacids, the molecular structure of the resultant polyester can be changed to markedly improve heat distortion temperature, hydrolytic stability and chemical resistance2. In the two stage process the aromatic acid and glycol are fully or par- tially reacted before the faster reacting unsaturated acid is added to the cook. This processing method, compared to charging all ingredients at once (one stage method), also leads to a more random distribution of the unsaturation in the polymer chain which changes the character of the final cross linked network in the cured resin.

Cure plays one of the most important roles in the chemical resistance developed by unsaturated polyester resins. Theoretically, the curing reaction should go to completion at room temperature with all the double bonds converted to single bonds in the three-dimen- sional network. However, complete cross linking is rarely achieved at ambient temperatures. This then will result in reduced chemical resistance and, quite often, poorer than expected mechanical properties. In addition, unreacted diluent (styrene ) can remain in the not-so-well cured polymer leading to major problems when the polyester is used for food grade applications. Accordingly, maximum chemical resistance and certain other property improvements can most often be achieved by utilizing elevated temperatures for ‘post cure’ of the polyester resin finished product.

Unsaturated polyester resins are used in the manufacture of a broad range of plastic products. A high percentage of these products utilize reinforcing materials, particularly fiberglass. It is estimated that less than 20% of the polyester resins produced are utilized in appli-

cations which do not involve reinforcing materials. These so-called casting applications include buttons, bowling balls, putties, cul- tured marble, gel coats and decorative products. The marble industry and the more recently developed polymer concrete industry represent outstanding applications for highly filled unsaturated polyesters which offer very economical materials to the building and construction industry. Fiberglass reinforced polyesters (FRP) are used in the manufacturing of boats, automobile and truck parts, building panels, corrosion resistant equipment such as pipes, tanks, ducts, scrubbers, etc., appliances and business equipment, electrical equipment, construction products such as grating and rail- ing, sporting equipment and consumer products that are almost endless. According to the Composites Institute of the Society of Plastics Industry (SPI), automotive, construction, marine and corrosion resistant equipment are the four largest FRP markets, in that order, in the United States which produces 2.5 billion pounds of FRP annually.

Mechanical properties are most often the critical factor in the selection of a polyester resin for a specific application. Testing of mechanical properties for both resin castings and fiberglass remforced composites is carried out using stan- dardized ASTM (American Society for Testing and Materials) tests for all plastics.

ASTM D-638

ASTM D-790

ASTM D-695

ASTM D-256

ASTM D-648

ASTM D-2583

Standard Test Method for Tensile Properties of Plastics Standard Test Method for Flexural Properties of Plastics Standard Test Method for Compressive Properties of Rigid Plastics Standard Test Method for Impact Strength (IZOD) of Plastics Standard Test Method for Heat Distortion Temperature of Plastics Standard Test Method for Barcol Hardness of Plastics

Polyester resins 39

As mentioned earlier, glycol selection has a significant effect on the properties of polyesters. Ether glycols are of great value in increasing tensile elongation and impact strength which is of great importance in automotive, casting and liner applications. A principal deficiency of polyester resins is lack of alkali resistance because the ester linkages are subject to hydrolysis in the presence of caustics. Accordingly, increasing the size of the glycol has the same effect as reducing the concentration of attackable ester linkages. Thus, a resin containing neopentyl glycol, propxylated bisphenol A, or trimethyl pentanediol will exhibit improved water and chemical resistance which is highly important in gel coats, corrosion resistant equipment, construction products and many consumer products.

The major effect on polyester physical properties is, however, provided by the unsaturation content in the polyester polymer. Higher unsaturation makes for more cross linking and a stiffer cured composite. Accordingly, the formulators' selection of unsaturated acid to saturated acid ratio which determines cross linking density can move the resin flexural modulus from rigid to resilient to very flexible. In most cases, a 1 / 1 ratio will

produce a rigid polyester which tends to be hard, brittle and lower in tensile elongation. Higher unsaturation also leads to higher heat distortion temperature resins. The latter is also achieved by formulating higher molecular weight resins with the chlorendic, bisphenol A and dicyclopentadiene building blocks. As expected, all of these resin classes are more brittle and have low tensile elongation. The major exception in this scenario are the iso/terepolyesters. Using the multi-stage processing methods described earlier, these resins can be formulated with reasonably high molecular weights (more linearity) to give very tough resins having a good balance of tensile/flexural properties plus higher tensile elongation and heat distortion temperatures. Obviously then, when the end use criteria requires the 'something more' than is offered by general purpose polyesters (orthophthalics and dicyclopentadienes), the formulator turns to iso/terepolyesters which have no disadvan- tages compared to general purpose resins other than slightly higher cost.

Table 2.1 summarizes the property and application status for the various classes of unsaturated polyesters.

Table 2.1 Properties and applications of unsaturated polyesters

Class Characteristics Uses

Orthophthalics, Rigid, resistant to dicyclopentadiene crazing, light in color

Isophthalics/terephthalics Tough, good impact and overall mechanical properties, resistant to environmental elements and moderate chemical attack. Highly resistant to aromatics

highly resistant to oxidizing chemical environments Rigid, high heat distortion, highly resistant to most chemical environments particularly caustics

Chlorendic Rigid, high heat distortion,

Bisphenol A fumarates

Boats, tub/shower, spas, marble, consumer products, buttons, corrugated sheet, building panels, seating, decorative products Automotive parts, gel coats, electrical, bowling balls, trays, gasoline, tanks, septic tanks, swimming pools, tooling, aerospace products, corrosion, construction products

Corrosion resistant tanks, ducting, stacks, industrial vessels

Corrosion resistant tanks, piping, stacks, industrial vessels

40 Polyester and uiny ester resins

2.3 VINYL ESTER RESINS

Vinyl ester resins are the most recent addition to the family of thermosetting polymers. Although several types of these resins were synthesized in small quantities during the late 1950s, it was not until the mid-1960s that com- mercialization, principally by Shell and Dow Chemical led the push to establish an extremely important segment of today’s composite industry. Vinyl esters are unsaturated resins made from the reaction of unsaturated carboxylic acids (principally methacrylic acid) with an epoxy such as a bisphenol A epoxy resin. The typical structure of a vinyl ester resin is shown in Fig. 2.8.

The structure of vinyl ester resins shows several important features which account for the resultant exceptional properties of vinyl ester resins. There is an epoxy resin backbone with a high molecular weight that provides excellent mechanical properties combined with toughness and resilience. Secondly, vinyl esters display terminal unsaturation which makes them very reactive. They can be dissolved in styrene and cured like a conventional unsaturated polyester to give rapid green strength. Obviously, the vinyl ester structure also enables convenient homopolymerization which could lead to high heat distortion products. Finally, vinyl esters have much fewer ester linkages per molecular weight which combined with the acid resistant epoxy backbone, give outstanding chemical resistance (acids, caustics and solvents) to this class of resins.

Although vinyl esters have often been clas- sified as polyesters, they should be designated separately because they are typically diesters with a recurring ether linkage provided by the epoxy resin backbone.

2.3.1 VINYL ESTER RESIN TYPES

Aside from the fire retardant versions of vinyl ester resins which are discussed in the next section, there are two basic types of vinyl esters having commercial significance. These are the general purpose lower molecular weight vinyl esters and the higher heat resistant vinyl esters with greater cross link density.

General purpose vinyl esters

These are principally methacrylated epoxies made by the reaction of methacrylic acid with a bisphenol A epoxy resin. When dissolved in styrene monomer they provide a thermosetting resin with good heat resistance, excellent mechanical properties (particularly high tensile elongation) and outstanding chemical resistance to acids, bases, hypochlorites and many solvents.

Heat resistant vinyl esters

These vinyl esters have higher density cross linking sites available which leads to a more heat resistant polymer network. They are produced from novolac modified epoxy resins

OH OH H - C- I I CH2- 0 G T O O - CH2-C I I - H

7% 0

0 7H2 CH3 I c = o I C-CH3 II

CH2

I c = o I

C-CH3 II

CH2

Fig. 2.8

Vinyl ester resins 41

and methacrylic acid which provides more unsaturation sites and higher molecular weight due to the epoxy backbone. These vinyl esters increase the heat resistance by 17-27°C (30-50°F) over the general purpose types. This often translates to higher useful operating temperatures for vinyl ester based reinforced plastics even in corrosive environments. The higher-density cross linked vinyl esters are less resilient (lower tensile elongation) but still have excellent mechanical properties. Cure of the higher cross linked vinyl esters may require the use of different peroxide catalysts to reduce the peak exotherm and thereby prevent cracking/crazing in resin rich areas. In other words, resins of this type are more reactive and more caution is required in the fabrication of FRP laminates.

2.3.2 PROPERTIES/APPLICATIONS

The development of vinyl esters has led to the fastest growing segment of the thermosetting resin industry today. This is not surprising, since vinyl esters combine inherent toughness with outstanding heat and chemical resistance. In all other thermosetting resin types one has to sacrifice some heat resistance and often chemical resistance to increase resiliency and toughness. Unlike polyesters, vinyl ester resins possess low ester content and low unsaturation which results in greater resistance to hydrolysis, lower peak exotherms during cure and less shrinkage during cure. They are easily dissolved in reactive monomers such as styrene which provides easy handling and transportation to the fabrication site. As with polyesters, other reactive monomers such as vinyl toluene, chlorostyrene and f-butyl styrene can be employed with few problems.

The toughness of vinyl esters comes from the epoxy resin backbone. Since the molecular weight and structure of the epoxy resin can be varied like the polyester resin building blocks, physical properties such as tensile elongation, heat distortion temperature and key mechani-

cal properties can be 'tailored' to meet the requirements of specific applications. Another unique property of vinyl ester is the bondabil- ity of these resins to other surfaces. They are not as good as epoxy resins in this characteristic, but obviously the epoxy resin component gives them a boost over other unsaturated polyesters in this area. A case can also be made for vinyl esters providing better fiberglass wet out in FRP composites due to the backbone hydroxyl groups and their inter- action with these groups on the fiber surface. Some fabricators have reported that observ- able resin savings can be achieved with vinyl esters because of this characteristic.

However, vinyl esters such as bisphenol A polyesters and chlorendic polyesters are made from higher cost materials and often require extended process times which leads to higher finished cost. Accordingly, the specifier/fabricator turns to commercial applications where the improved performance of vinyl esters can justify the premium price of the finished composite.

The foremost application for vinyl esters is in glass reinforced laminates for corrosion resistant equipment. Because of outstanding chemical resistance combined with excellent mechanical properties, vinyl ester based FRP tanks, piping, scrubbers, fans and ductwork are being specified for waste water treatment plants, mining facilities, chemical processing and storage units, semi-conductor chip operations, pulp and paper manufacturing and odor control facilities. Since FRP corrosion resistant equipment is the fastest growing segment of the US composites industry, the future for vinyl esters looks extremely strong. They are comparable to other premium resins for chemical resistance and secondary bonding combined with a good balance of chemical resistance (acids, bases, solvents) at the same or lower cost. As a result, chlorendics and bisphenol A polyesters have been reduced to 'niche' applications where their specific property advantages such as heat resistance and resistance to oxidizing environments demand


their use. Since iso/terepolyesters also give an excellent balance of properties in corrosion applications, these unsaturated polyesters and vinyl esters now dominate the corrosion market. The bonus provided by vinyl esters is of course higher heat resistance and extended life at higher operating temperatures, but at significant additional cost compared with the iso/ terepolyesters.

The next major market area for vinyl esters utilizes the high tensile elongation characteristics of these resins to produce linings and coating with outstanding adhesion to other types of plastics and conventional materials such as steel and concrete. For example, vinyl esters are an excellent barrier coat for fiberglass boats and acrylic spas. Vinyl ester corrosion coatings are used everywhere today for steel tank linings and industrial flooring. In dual laminate structures, a vinyl ester is often the back up for exotic thermoplastics or the supe- rior corrosion barrier for lower cost polyesters in many FRP tank and pipe applications.

The growth of vinyl esters has also been boosted by their excellent handling characteristics and ease of cure. For example, vinyl esters are much preferred by FRP fabricators in filament winding operations because of excellent glass wet out and in fabrication of large structures because the resins are forgiv- ing and provide predictable curing over a wide range of temperatures. The latter situation has resulted in a virtual exclusive use for vinyl esters in field fabrication of large FRP structures.

Table 2.2 Resins for corrosion resistant applications

Table 2.2 summarizes the resin casting properties of the various resins used in corrosion resistant applications today.

The outstanding balance of properties provided by vinyl ester resins is obvious and bodes well for continued strong growth in US corrosion markets. Other significant markets for vinyl esters includes pultruded construction and electrical components, automotive structural applications, polymer concrete vessels for mining and chemical operations, grating, high performance marine applications and sporting goods.

2.4 FLAME RETARDANT VERSIONS

The need for flame retardant polymers is essential in many plastics applications today. The combustibility of plastics has drawn so much attention to the safety aspects of these materials in construction applications, that designers and specifiers have been pressured by fire officials to provide fiberglass-reinforced construction materials that exhibit low flame/low smoke characteristics. Since all plastics are based on organic constituents, they are inherently flammable and once ignited will burn until they are completely consumed. There are, however several methods available for making thermosetting resin flame retardant and these provide the capability to supply fire retardant FRP and corrosion resistant/fire retardant FRP for the numerous applications that have a need for some degree of fire retardancy.

Tensile, Flexural, Elonga f ion HDT, psi psi break, % O F

General purpose vinyl ester 12 500 20 500 6.7 221

Chlorendic polyesters 5 500 10 000 1.4 284

Resilient isopolyester 12 500 20 000 4.4 201

Heat resistant vinyl ester 13 000 20 000 5.6 248

Bisphenol A polyester 10 000 16 500 3.2 288 Rigid isopolyester 8 500 19 000 1.9 234

Flame retardant versions 43

Flame retardancy can be achieved by using numerous additives, both organic and inorganic. However, most of these have a negative effect on mechanical and/or chemical resistance properties. Accordingly, the most widely used system for achieving optimum fire retardancy will be covered here, namely, halogenated thermosetting resin systems combined with inorganic synergists.

2.4.1 CHEMISTRY AND APPLICATIONS

Flame retardancy of unsaturated polyester and vinyl ester resin is an extension of the non- flame retardant systems (as discussed above). Almost all of these resins can be reformulated to include a halogen in the chemical composition by either blending or by an in situ cook of the resin. There is an advantage to locking in the halogen in the original resin cook, in order to chemically tie in the halogen (Cl, or Br) to prevent migration of the halogen when sub- jected to thermal degradation. While flame retardancy can be achieved with additives (Dekabrom or Dechlorine), these additives have not been used for high performance applications in either the corrosion or construction industries (corrugated FRP panels).

The chlorendic resins were developed in the 1950s and were based on HET acid (hexa- chloroxyclopentadiene). Other formulations followed, based on either tetrabromo bisphe- no1 A (TBBPA) or dibromo neopentyl glycol (DBNPG). These components react with maleic anhydride or fumaric acid in the presence of a difunctional glycol, to produce flame retardant unsaturated polyesters that can be combined with styrene monomer, or other monomers used for smoke control, such as methyl methacrylate. The use of bromine as the halogen in the resin building block has proved to be the most efficient way of achieving optimum flame retardant thermosetting resins. Certainly a lower percentage of bromine than chlorine is required for satisfactory reduction of flame spread.

Brominated vinyl esters handle a wide

range of chemical environments, both acid and alkali, at operation temperatures similar to the general purpose vinyl esters. Brominated high molecular weight isopolyesters offer economic advantages and are suitable for moderate corrosion applications. These two resin types have become the workhorses for the waste water/odor control FRP market and the chemical and pulp /paper industries because they exhibit excellent impact properties combined with good overall corrosion resistance. Variations of these resins are used to meet MIL-R-21607 or MIL-R-7575 requirements.

Dibromoneopentyl glycol formulated with carefully selected chemical building blocks provides resins for exposure to severe weather- ing conditions. The construction industry uses these resin systems, which are specially formulated to meet optimum fire retardance for the continuous line products of corrugated and flat sheet panels. Such systems are formulated with ultraviolet (W) stabilizers and acrylates to achieve excellent color stability with acceptable low smoke and flame spread (FS) properties. In most cases, these formulations offer good chemical resistance for splash and spill on the exposed surfaces. Highly filled halogenated resin systems are designed to accept high filler loading with aluminum trihydrate (ATH) and other synergists to meet DOT requirements for low smoke, low flame spread properties (ASTM-E-662 and E-162 respectively). Values of <150 smoke and 10 flame spread are achieved. Highly filled resins are specified for applications where people could be exposed to indoor fires, such as underground transportation. Low smoke allows visual capability to exit an entrapped area.

Electrical applications often require the addition of halogenated base resins to achieve flame snuffing properties resulting from high voltage shorting or sparking. Such fire retardant systems are used for compression molding of complex electrical shapes, using BMC or SMC molding materials. Wet mat molding is also used to produce flat sheet for electrical insulation components.


When compared to the non-flame retardant versions, the addition of a halogen to the resin formulation has very little, if any, effect on the chemical or mechanical properties of a FRP laminate, whereas the use of additives such as aluminum hydrates, clays, carbonates or fumed silica will have a direct and adverse effect on the chemical resistance of FRP laminates. In critical corrosion applications such filler additions could result in early failure of FRP laminates.

2.4.2 TESTING AND CLASSIFICATION

Most halogenated resin systems require a synergist such as antimony usually 3-5'/0 Sb,O, by weight of resin in order to achieve a class I flame spread rating. Other proprietary synergists can be substituted, especially in translucent laminates as used for siding and roofing materials. Antimony oxide enhances the flame spread rating by forming a char on the burning surface of a laminate and effectively subdues the rate of flame progression by snuffing the flame out when the source of ignition is removed or extinguished. Such flame resistant FRP laminates will burn when sub- jected to a high temperature flame source, but the rate of burning is substantially less than for non-flame retardant systems. The use of Sb,O, will turn laminate opaque, which restricts visual inspections of a laminate in production. It is not a good idea to allow the addition of Sb,O, in the corrosion resistant barrier. Alone, Sb,O, will not improve a non-halogenated resin; its use becomes an unnecessary expensive filler with no flame retardant properties.

The most commonly used test method for evaluating flammability is ASTM Method E-84 (the tunnel test) also known as NFPA 255. This test method measures the comparative burning characteristics of a material by evaluating the flame front propagation over the surface of the test material, which is exposed to con- trolled temperatures in a forced air chamber or tunnel. A flame spread (FS) classification (FSC) is obtained which measures the ignition time

and distance of the flame front advancing down the test tunnel during a ten-minute duration test and compared to those values established for asbestos-cement board (at 0-FS) and red oak material (at 100 FS). This test method establishes the rating at 0-25FS as Class I, 25-75FS as Class I1 and 76-plus FS as Class 111. During the test procedure, the smoke emission is measured and can range from 450 to 1000 or more for unfilled laminates. When additives such as ATH are used, smoke emissions of less than 450 can be achieved.

Corrosion resistant FRP ducting exhibiting low flame/low smoke characteristics is required for waste water/odor control and semiconductor applications. This can be achieved with brominated vinyl esters or brominated isopolyesters as the base for the FRP duct which is then coated with an intu- mescent paint to reduce smoke emissions. Such systems are currently qualified by the International Conference of Building Officials (ICBO) with tunnel test ratings of <25FS and <50 smoke development.

Unfortunately, many specifying engineer- ing companies will request and specify values that are not readily achievable for most flame retardant resin systems. Ideally, a Class I system with 25FS (max) to 450 (max) smoke is acceptable for corrosion service, when the use of additives cannot be toler- ated. A FSC of <25FS is usually acceptable for most applications. However, some specifiers will claim that 15-20FS is better than 20-25FS, when under actual burning conditions there is a negligible difference in the combustibility of FRP laminates. When selecting a flame retardant resin, it is important to qualify a system to meet the properties required. Flame retardant resins are available in a wide vari- ety of formulations, including lower cost general purpose to premium grade types with better high temperature properties.

Class I flame spread thermosetting resin systems can also be achieved without the need for a synergist like antimony oxide. These

Design considerations 45

systems have a higher halogen content which obviously increases resin cost. However, these materials allow for the production of translucent FRP products that are desirable in many construction applications. Such a resin system has been specified by Disney engineers for their architectural applications at Disney entertainment centers.

2.5 DESIGN CONSIDERATIONS

Since most thermosetting resin systems are used with fiberglass reinforcements, it is important to consider material selection and the fabrication process in establishing the design of the FRP composite.

2.5.1 MATERIAL SELECTION

The type of fiberglass reinforcement, place- ment in the composite and fiberglass content determines the strength of an FRP composite and provides the mechanical properties dictated by the end use requirements. In any part made of FRP, the strength of the part will increase directly in relation to the percentage of fiberglass in the total weight of the composite. In addition, the arrangement and type of fiberglass will have important effects on the resultant physical properties since the strength obtainable in the finished part will be in the direction of the fibers.

The selection of the thermosetting resin system will determine the chemical, electrical and thermal performance of the FRP product. However, the most significant contribution by the resin relates to ‘life’ of the composite, since the resin must protect the fiberglass. Accordingly, if the resin fractures or blisters in any manner that permits an attack on the glass fibers, the composite will lose strength rapidly or delaminate. An interesting way to visualize this is to consider a FRP pipe made by the filament winding process. If the continuous fiber strands providing the hoop strength to the product are severed by chemical attack, the

product would essentially ’unzipper’ and fail. The polyester and vinyl ester resins described in this chapter offer a wide selection of materials which will accomplish the need to protect the fiberglass and, at the same time, provide optimum performance properties dictated by the end use application.

The first agenda in proper resin selection involves an analysis of the key performance requirements of the end use application. This should be very thorough as follows:

0 strength requirements; 0 thermal requirements; 0 chemical exposures; 0 electrical requirements; 0 color requirements; 0 surface requirements; 0 environmental exposures; 0 fire resistance needed; 0 smoke requirements; 0 potential upset conditions; 0 number of parts required; 0 life expectancy.

Resin selection is obviously very important in any FRP application as the above list of design criteria illustrates, but is absolutely vital in corrosion applications. Corrosive attack on a FRP laminate along with fire is the most critical situation the composite will face. A fiberglass building panel properly made can perform for an indefinite number of years, but even a properly fabricated FRP tank exposed to concentrated acids at elevated temperatures may only be good for 10-15 years. The best example of this is in the pulp and paper industry where vigorous chemical attack on FRP equipment can dictate replacement on a rou- tine basis, say every five years. In spite of this, FRP equipment may still offer the most cost effective material of construction.

Although all FRP composites will be attacked in the same manner in a particular environment, certain types (chlorendic, bisphenol A, vinyl esters and isopolyesters) are significantly more resistant. These then make up the list of corrosion and heat resistant


resins to choose from in addressing a specific application. A good rule to follow here is that no single FRP resin can handle every kind of environmental problem, so resin selection is of the utmost important. It should also be under- stood that knowledge of the molecular structure of these higher performance resins does not eliminate the need for actual testing to determine resin suitability in a given application. For example, certain vinyl esters are reasonably resistant to alkaline exposure while other types are poor. The corrosion fabricators’ guide for the suitability of a thermosetting resin in a corrosive environment is ASTM C- 581. The procedure involves complete exposure of a FRP test laminate for one year, with intermittent strength testing, to establish a curve which depicts loss of flexural strength versus time. It is absolutely essential that the resin selected for that environment form a plateau during the one year test period. Obviously, it is also important that this plateau be achieved at a satisfactory retained flexural strength. Table 2.3 summarizes a comparison of FRP properties of various thermosetting resin types versus carbon and stainless steel.

2.5.2 EFFECT OF PROCESS AND END USE REQUIREMENT

There is an old saying in the FRP business that heightens awareness of the fabrication

Table 2.3 Comparison of properties of various types of FRP

process. Simply, ’you can select the best resin and fiberglass in the world and if you don’t put them together correctly - failure will prob- ably result’. Material selection added to design and production requirements leads to a deter- mination of the fabrication process. Many methods of fabrication are used to manufacture products for the numerous FRP markets. These methods vary from hand lay-up/spray- up, filament winding and resin transfer molding which utilize low temperature curing to various high temperature molding compound (SMC), pultrusion, and continuous panel. The designer must analyze the end use property requirements such as color, surface characteristics, strength and chemical resistance requirements and then add-in cost factors, part volume, part size and finishing to finalize the selection of process. For example, transportation body panels would be a high volume application requiring outstanding surface finish and excellent strength properties. All of these can be satisfied with a isopolyester sheet molding compound that is compression molded under heat and pressure. This process can be automated and delivers the highest volume and highest part uniformity of any thermoset molding method. Lower part finishing cost is achievable because subsequent trimming machining is minimized.

Corrosion resistant equipment would be fabricated, on the other hand, by either filament

lsophthalic Orthophthalic Chlorendic Bisphenol A Vinyl Carbon Stainless fumarate ester steel steel

Corrosion resistance Acids Alkalis Peroxides Hypochlorites Solvents

Flame retardance Structural strength Thermal insulation

B C A A B C B B C C A A B B C C A B B C C C C A B A C C B C B B B A A C C A C A A A A B A A A A A A A A A A C C

A = High, B = Moderate, C = Low

References 47

winding or hand lay-up/spray-up processing. The former gives the highest strength to weight ratio of any FRP manufacturing process. However, the most important consid- eration is that these two process methods allow the easy creation of an effective resin- rich corrosion barrier which is mandatory to satisfactory FRP life expectancy in corrosion applications. The purpose of this barrier is to isolate the fiberglass reinforcements from attack that would result in wicking, blistering and delamination. The satisfactory corrosion barrier should be about 125 mils (3175 pm) thick, fabricated with glass or polyester veil on the surface, backed up by 2-3 plies of type E chopped strand mat. The resin-rich corrosion barrier should be constructed with the very best resin available in terms of chemical resistance to the expected environment. For example, the corrosion barrier for 26% hydrochloric acid should employ a vinyl ester and two layers of polyester veil. It is always wise in dual laminate construction (different liner and wall resins) to utilize the material with the higher resiliency (higher tensile elongation) in the liner portion of the laminate structure.

A final example of process effect on laminate properties should address the rapidly growing world of FRP pultrusion. This fabrication method provides very high strength due to high fiber concentration and orientation

parallel to the length of stock. Pultrusion is an automated, low labor system which can use any type of thermosetting resin. However, resilient resins such as isopolyesters and vinyl esters are much preferred because of the very high glass content in the finished part. Low cost reinforcement is adaptable to putrusion because the glass weight percentage is high. Pultrusion is used for FRP structural and electrical applications primarily, but the weight and density of the finished product does provide moderate corrosion resistance properties.

The value received from good design based on proper selection of materials and process can be very rewarding for FRP composites. The systematic analysis of end use requirements, economic requirements and competitive materials will enable the composite designer/specifier to optimize the cost/performance of FRP as a material of construction. The matrix materials available today give the fabricator sufficient opportunities to meet his final objective of providing the right product at the lowest cost.

REFERENCES

1. Ellis, C., US Patent 2 255 313; appl. August 6, 1937.

2. Amoco Chemical Company, Bulletin IP-70a, Chicago, Illinois.