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Compendium
„Composites“
ASM Handbook / extraction
Polyester Resins
Prof. H. Hansmann
Hochschule Wismar
FB MVU, Werkstofftechnologien / Kunststofftechnik
Oct. 2003
Polyester Resins
Tim Pepper, Ashland Chemical Company
Introduction
UNSATURATED POLYESTER (UPE) RESIN is used for a wide variety of industrial and
consumer applications. In fact, more than 0.8 billion kg (1.7 billion lb) was consumed in the
United States in 1999. This consumption can be split into two major categories of
applications: reinforced and nonreinforced. In reinforced applications, resin and
reinforcement, such as fiberglass, are used together to produce a composite with improved
physical properties. Typical reinforced applications are boats, cars, shower stalls, building
panels, and corrosion- resistant tanks and pipes. Nonfiber reinforced applications generally
have a mineral “filler” incorporated into the composite for property modification. Some
typical nonfiber reinforced applications are sinks, bowling balls, and coatings. Polyester resin
composites are cost effective because they require minimal setup costs and the physical
properties can be tailored to specific applications. Another advantage of polyester resin
composites is that they can be cured in a variety of ways without altering the physical
properties of the finished part. Consequently, polyester resin composites compete favorably in
custom markets.
Polyester Resin Chemistry
Polyesters are macromolecules that are prepared by the condensation polymerization of
difunctional acids or anhydrides with difunctional alcohols or epoxy resins. Unsaturated
polyester resins, commonly referred to as “polyester resins,” are the group of polyesters in
which the acid component part of the ester is partially composed of fumaric acid, a 1,2ethylenically unsaturated material. Maleic anhydride is the predominant source of this
fumarate. Maleic anhydride is incorporated into the polyester backbone and then isomerized
to provide fumarate esters (commonly referred to as unsaturated polyesters). In most cases,
the polymer is dissolved in styrene to provide a solution that will typically have a viscosity in
the range of 0.2 to 2 Pa · s (200–2000 cP). Other reactive vinyl monomers, such as vinyl
toluene, diallyl phthalate, or methyl methacrylate may be used to obtain specific properties.
The resin viscosity is tailored for specific fabrication processes in which the resin is
ultimately “cured” via a free-radical process. A final formulation may include resin, inorganic
filler, fiberglass reinforcement, and a free-radical initiator, such as an organic peroxide. This
final formulation is formed against a mold prior to the cross-linking reaction between the
unsaturated polymer and the unsaturated monomer. The “curing” is a cross-linking chain
reaction, converting the low-viscosity solution into a three- dimensional thermoset plastic
(Ref 1). This is referred to as the cure. Terminology has developed to distinguish between
various types of UPEs.
General-purpose resins are a group of resins generally used because of their low cost. While
processing parameters can affect this, the cost of the raw materials tends to limit the type of
unsaturated polyesters selected. The resins used in this area are commonly referred to as PET,
DCPD, and ortho resins. While PET (polyethylene terephthalic) resin and orthophthalic
anhydride are sources of low-cost saturated acids, DCPD (dicyclopentadiene) is typically
coupled with maleic anhydride during an initial step, prior to both the isomerization and the
condensation polymerization. Dicyclopentadiene improves the solubility of the polyester resin
in styrene. As one drives toward lower-cost resins, solubility in styrene is a serious concern.
Dicyclopentadiene offers a low-cost solution. Due to government regulations aimed at
minimizing styrene emissions in open molding, a subset of general-purpose resins, “lowstyrene” resins, has been developed. These resins tend to have a higher dependency on DCPD
and are cooked to a lower molecular weight. This allows less styrene to be used to achieve the
desired viscosity, and performance generally suffers.
Isophthalic resins are based on isophthalic acid and maleic anhydride. The incorporation of
isophthalic acid creates a high-molecular-weight resin with good chemical and thermal
resistance and good mechanical properties. The use of nonpolar glycols contributes to
improved aqueous resistance, which is required to protect the fiberglass.
Bisphenol A (BPA) fumarate resins are prepared by the reaction of propoxylated BPA with
fumaric acid. The result is a relatively nonpolar polyester with a reduced number of ester
linkages. The reduced number of ester linkages contributes to excellent corrosion resistance.
Bisphenol A fumarates were once the workhorse of the corrosion-resistant composite
industry. In recent years, they have been replaced by isophthalic resins for mildly corrosive
applications and bisphenol A epoxy based vinyl esters in more aggressive environments.
Currently, BPA fumarates are used almost exclusively in applications requiring exceptional
corrosion resistance to caustic environments.
Chlorendic resins are prepared using either chlorendic anhydride or chlorendic (HET) acid
(Ref 2) reacted with maleic anhydride. Composites made from these resins have excellent
chemical resistance and some fire retardancy because of the presence of chlorine. Chlorendic
resins are used in applications requiring exceptional resistance to acidic or oxidizing
environments. In many of these environments, metals are attacked quite aggressively, while
chlorendic resin composites generally perform much better.
Vinyl ester resin is the common name for a series of unsaturated resins that are prepared by
the reaction of a monofunctional unsaturated acid, typically methacrylic acid, with an epoxy
resin. The epoxy resin is “end-capped” with an unsaturated ester to form the vinyl ester resin.
The resulting polymer, which contains unsaturated sites only in the terminal positions, is
mixed with an unsaturated monomer, generally styrene. At this point, the appearance,
handling properties, and curing characteristics of vinyl ester resins are the same as
conventional polyester resins. However, the corrosion resistance and mechanical properties of
vinyl ester composites are much improved over standard polyester resin composites. These
improved properties have enabled vinyl ester resins to become the workhorse of the polyester
custom corrosion industry. However, the properties of vinyl ester resins are not as easily
tailored to a specific application as are standard unsaturated polyester resins. This combined
with the use of higher-cost raw materials has somewhat limited the ability of vinyl ester resins
to penetrate the unsaturated polyester resin market.
Within each of these five resin classifications, specific polyesters can be formulated by
varying the starting materials. Specific properties such as flexibility, thermal properties, fire
retardancy, and hydrophobicity can be altered (Ref 3) by varying the type of dihydric
alcohol/epoxy resin used or by varying the fumaric/saturated acid ratio.
Low-profile additives (LPAs) are a class of saturated resins that are used for dimensional
stability. Although these materials are not UPEs, they play a very important role in the use of
UPEs. One of the problems associated with the use of UPEs is that they shrink volumetrically
about 6 to 8% upon curing. This creates challenges in fabricating a high-quality surface and/
or maintaining the dimensional stability of a part. Low-profile additives offer a unique
solution to this problem when processing at elevated temperatures. Addition of an LPA to a
UPE formulation reduces or eliminates shrinkage. Some formulations using LPAs can
actually have expansion greater than the original mold or form size. Since the LPA does not
chemically react with the UPE or styrene, care should be taken in determining the type and
amount of LPA used.
Mechanical Properties
Mechanical properties are often the critical factor in selecting a polyester resin for a specific
application. Table 1 lists the common test methods of the American Society for Testing and
Materials (ASTM) that are used to characterize the mechanical properties of polyester resin
composites.
Table 1 ASTM test methods for characterizing mechanical properties of polyester
resins
Properties
ASTM Test Method
Tensile strength, modulus, and % elongation
D 638
Flexural strength and modulus
D 790
Compressive strength, modulus, and % compression on break D695
Izod impact
D256
Heat distortion
D 648
Barcol hardness
D 2583
While the physical properties of polyester composites are predominately controlled by
reinforcement, the physical properties of the polyester resin do affect the durability and
thermal performance. Representative examples of clear- cast polyester resin data are shown in
Table 2. It should be noted that within each class of resins, modifications are made to the
polymer. These modifications effectively trade off thermal performance for increased
toughness (Table 2). Table 2 highlights the differences among the classes of polyesters.
Isophthalic resins tend to show higher tensile and flexural properties than orthophthalic resins.
This may be because isophthalics usually form more linear, higher- molecular-weight
polymers than orthophthalics. In contrast, the BPA fumarate and chlorendic resins are
formulated for service in aggressive corrosive conditions and consequently are much more
rigid. This results in clear castings that are brittle and have low tensile elongation and
strength. The vinyl ester, because of its bisphenol diepoxide content, exhibits excellent tensile
and flexural properties as well as high elongation.
Table 2 Mechanical properties of clear-cast (unreinforced) polyester resins
Material
Barcol
hardness
Tensile
strength
MPa
Tensile
modulus
Elongation,
%
ksi GPa 10 psi
6
Flexural
strength
MPa
ksi
Flexural
modulus
Compressive
strength
GPa 106 psi
MPa
Heat-deflection
temperature
ksi
°C
°F
Orthophthalic …
55
8
3.45 0.50
2.1
80
12
3.45
0.50
…
…
80
175
Isophthalic
40
75
11
3.38 0.49
3.3
130
19
3.59
0.52
120
17
90
195
BPA
fumarate
34
40
6
2.83 0.41
1.4
110
16
3.38
0.49
100
15
130
265
Chlorendic
40
20
3
3.38 0.49
…
120
17
3.93
0.57
100
15
140
285
Vinyl ester
35
80
12
3.59 0.52
4.0
140
20
3.72
0.54
…
…
100
212
In this article, it is impossible to discuss thoroughly the effects that dihydric alcohols, acids,
levels of unsaturation, monomer types and amounts, and cure temperatures have on
mechanical properties; however, Ref 4 provides an excellent review. In general, increasing the
chain length of the dihydric alcohol increases the flexibility of the cross-linked resin. The
same occurs with saturated acids. Aromatic groups, in either the dihydric alcohol or acid
component, increase stiffness and hardness.
Using a reinforcing fiber to produce a polyester composite dramatically improves both the
tensile and flexural properties. Table 3 lists the same five samples as Table 2; however, Table
3 shows the mechanical properties of the fiberglass reinforced polyester resin composites. In
Tables 4 and 5, the properties obtained were dependent on the amount and type of glass fiber
used. The last two entries in Table 5 show the influence of fiberglass orientation. Both
contained 70 wt% glass fiber, but the unidirectional composite showed much higher tensile
properties and flexural modulus when tested in the glass fiber direction. The properties of
unidirectional composites are even more anisotropic than typical polyester laminates.
Mechanical properties measured transverse to the glass direction will approach those observed
for clear castings. The pultrusion or filament-winding process commercially produces
polyester composites employing unidirectional reinforcements. They can be used in structural
applications where strength or stiffness is required in only one direction. In Table 6, physical
properties of fiberglass reinforced polyester resin composites are compared to those of various
metals.
Table 3 Mechanical properties of fiberglass-polyester resin composites (glass content,
40 wt%)
Tensile Tensile
strength modulus
Material
Flexural Flexural Compressive
Izod impact
strength modulus strength
Barcol
106
106 Elongation,
MPa ksi GPa
MPa
%
hardness MPa ksi GPa psi
psi
ksi J/mm
ft ·
lbf/in.
Orthophthalic …
150 22 5.5
0.8 1.7
220 32 6.9
1.0 …
…
…
…
Isophthalic
45
190 28 11.7 1.7 2.0
240 35 7.6
1.1 210
30
0.57
10.7
BPA
fumarate
40
120 18 11.0 1.6 1.2
160 23 9.0
1.3 180
26
0.64
12
Chlorendic
40
140 20 9.7
1.4 1.4
190 28 9.7
1.4 120
18
0.37
7
Vinyl ester
…
160 23 11.0 1.6 …
220 32 9.0
1.3 210
30
…
…
Table 4 Effect of glass content on mechanical properties of fiberglass reinforced
polyester composites
Material
Glass
content,
wt%
Flexural
strength
Flexural
modulus
MPa ksi GPa
106
psi
Tensile
strength
MPa
ksi
Tensile
modulus
GPa
106
psi
Compressive
strength
MPa
ksi
Orthophthalic 30
170
25
5.5
0.80 140
20
4.8
0.70 …
…
40
220
32
6.9
1.00 150
22
5.5
0.80 …
…
30
190
28
5.5
0.80 150
22
8.27 1.20 …
…
40
240
35
7.58 1.10 190
28
11.7 1.70 210
30
25
120
17
5.1
12
7.58 1.10 170
24
35
150
22
8.27 1.20 100
14
10.3 1.50 170
24
40
160
23
8.96 1.30 120
18
11.0 1.60 180
26
24
120
17
5.9
11
7.58 1.10 140
21
34
160
23
6.89 1.00 120
18
9.65 1.40 120
18
40
190
28
9.65 1.40 140
20
9.65 1.40 120
18
25
110
16
5.4
12.5
6.96 1.01 180
26.5
35
260
37.3 9.52 1.38 153.4 22.25 10.8 1.56 230
34
40
220
32
30
Isophthalic
BPA
fumarate
Chlorendic
Vinyl ester
0.74 80
0.85 80
0.79 86.2
8.89 1.29 160
23
11.0 1.59 210
Table 5 Effect of glass type and amount on mechanical properties of fiberglass reinforced polyester composites
Type of glass fiber
reinforcement
Glass
content, wt%
Density,
g/cm3
Tensile
strength
MPa
ksi
Tensile
modulus
Elongation,
%
GPa 10 psi
6
MPa
ksi
Flexural
modulus
GPa 106 psi
Compressive
strength
MPa
ksi
Neat cured resin
0
1.22
59
8.6
0.783
2.0
88
12.8
3.90
0.565
156
22.6
Chopped-strand mat
30
1.50
117
17.0 10.80 1.566
3.5
197
28.6
9.784 1.419
147
21.3
50
1.70
288
41.8 16.70 2.422
3.5
197
28.6
14.49 2.102
160
23.2
Roving fabric
60
1.76
314
45.5 19.50 2.828
3.6
317
46.0
15.00 2.175
192
27.8
Woven glass fabric
70
1.88
331
48.0 25.86 3.750
3.4
403
58.4
17.38 2.520
280
40.6
70
1.96
611
88.6 32.54 4.720
2.8
403
58.4
29.44 4.270
216
31.3
Unidirectional roving
fabric
Source: Ref 1
5.40
Flexural
strength
Table 6 Comparative properties of fiberglass reinforced polyester composites and
various metals
Flexural
strength
Glass
content,
wt%
Material
Flexural
modulus
Tensile
strength
Tensile
modulus
Density,
106
106
3
MPa
ksi
GPa
MPa
ksi
GPa
g/cm
psi
psi
Unidirectional glass
70
roving, rod and bar
2.0
690
100 40
6
690
100 40
6
Unidirectional glass
50
roving, sheet
1.6
205
30
2
140
20
12
1.8
Chopped-strand
mat
30
1.5–1.7
110– 16– 6.9– 1.0– 60–
190 28 8.3 1.2 120
9–
18
6–12
0.8–
1.8
Aluminum sheet
…
2.8
140
10
40–
190
6–
27
70
10
Stainless steel
…
8.0
210– 30–
190
240 35
28
210– 30–
190 28
240 35
Low-carbon steel
…
8.0
190
30
200– 29–
210 30
230 33
Compressive
strength
Material
Elongation, MPa
%
20
28
15
70
210
Impact
strength
Thermal
Specific heat Coefficient
conductivity
of thermal
Btu ·
expansion,
ft · W/m
kJ/kg Btu/lb
ksi J/mm
in./h ·
10–6/K
lbf/in. · K
·K
·°F
2
ft ·°F
Unidirectional
glass roving, 1.5
rod and bar
410
60
2.6
49
0.70 5
1.0
0.24
5
Unidirectional
glass roving, 1.5
sheet
140
20
0.96
18
0.60 4
1.2
0.28
9
Choppedstrand mat
1–1.2
100–
170
15–
25
0.2–
0.64
4–12
0.20– 1.2–
0.25 1.6
1.3–
1.4
0.31–
20–35
0.34
Aluminum
sheet
30–40
…
…
…
…
120– 810– 0.92– 0.22–
20–25
130 1620 0.96 0.23
Stainless steel 40–50
210
30
0.45– 8.5–
0.59 11
14–
25
96–
185
Low-carbon
steel
190
28
…
35–
65
260– 0.42– 0.10–
10–15
460 0.46 0.11
38–50
…
0.50
0.12
15–20
As mentioned earlier, different types of reinforcement affect mechanical properties. While Eglass is the most commonly used reinforcement in polyester resins, S-glass, aramid, and
carbon fibers can also be used. Table 7 compares a variety of reinforcements in both
orthophthalic polyester and vinyl ester resins. While the tensile strength of the orthophthalic
polyester was much improved with aramid, no such improvement was seen with the vinyl
ester.
Table 7 Effect of reinforcement on mechanical properties of polyester-matrix
composites
Tensile strength Tensile modulus
Material
MPa
ksi
GPa
106 psi
Elongation, %
E-glass-ortho polyester
Ambient temperature 157
22.8
11.0
1.59
1.7
50 °C (125 °F)
148
21.5
8.41
1.22
2.4
65 °C (150 °F)
140
20.3
7.31
1.06
2.6
Ambient temperature 159
23.0
10.3
1.49
1.9
50 °C (125 °F)
165
24.0
8.55
1.24
2.6
65 °C (150 °F)
157
22.8
7.38
1.07
2.6
Ambient temperature 212
30.7
12.5
1.82
2.0
50 °C (125 °F)
208
30.2
11.5
1.67
2.1
65 °C (150 °F)
200
29.0
9.52
1.38
2.4
Ambient temperature 206
29.9
12.6
1.83
2.1
50 °C (125 °F)
192
27.9
11.4
1.66
2.2
65 °C (150 °F)
201
29.2
11.6
1.68
2.4
Ambient temperature 198
28.7
11.4
1.66
2.2
50 °C (125 °F)
172
25.0
9.6
1.39
2.3
65 °C (150 °F)
199
28.8
10.3
1.49
2.5
Ambient temperature 189
27.4
12.1
1.76
1.8
50 °C (125 °F)
221
32.0
11.7
1.70
2.2
65 °C (150 °F)
218
31.6
11.7
1.69
2.2
S-glass-ortho polyester
Aramid-ortho polyester
E-glass-vinyl ester
S-glass-vinyl ester
Aramid-vinyl ester
Inorganic fillers are commonly used in polyester resin composites. While they do improve
stiffness, as shown by an increase in modulus (see Table 8), they have little effect on other
strength characteristics. They are used primarily to reduce cost.
Table 8 Effect of filler and glass fiber reinforcement on mechanical properties of
polyester resins
Flexural
strength
Flexural
modulus
Tensile
strength
Tensile
modulus
Reinforcement
106
MPa
ksi
GPa
MPa ksi GPa
material
psi
Neat resin
casting (A
material)
106
psi
Heat
deflection
Barcol
temperature(a)
Elongation,%
hardness
°C
°F
129 18.7 3.60 0.522 70.0 10.1 3.50 0.507 3.0
80 pph A; 20
pph CaCO3 (B 109 15.8 4.30 0.623 52
material)
7.5 5.60 0.812 1.26
74 pph B; 26
pph 2.5 cm
long chopped 183 26.5 6.10 0.884 116 16.8 9.694 1.406 1.72
strand (C
material)
pph, parts per hundred.
(a) For 250 µm (10 mil) deflection at 1.82 MPa (0.264 ksi).
Source: Ref 1
30
58
135
45
67
150
45
>260
>500
As expected, mechanical properties at elevated temperatures vary significantly among the
general classifications of polyester resins (Fig. 1). The extreme rigidity and high glass
transition temperature (Tg), of BPA fumarate and chlorendic polyesters result in high flexural
strength retention up to 120 °C (250 °F). Vinyl esters also show performance advantages over
isophthalic polyesters in fatigue studies (Fig. 2, Ref 5). The advantage of vinyl ester was
evident at elevated temperatures (Ref 6). At 105 °C (220 °F), vinyl ester and isophthalic
polyester composites (60% glass) were cycled to a stress level of 60 to 70 MPa (9–10 ksi).
After 200,000 cycles, the drop in flexural modulus was only 5% for the vinyl ester, compared
to 12% for the isophthalic polyester. The thermal performance of vinyl esters, combined with
their excellent mechanical properties and toughness, explains why they are often chosen for
structural resin composites.
Fig. 1 Flexural strength versus temperature, glass-polyester composites of 40% glass
Fig. 2 Flexural fatigue strength
References cited in this section
1. M. Grayson and D. Eckroth, Ed., Encyclopedia of Chemical Technology, 3rd ed., Vol
18, John Wiley & Sons, 1982, p 575
4. H.V. Boenig, Unsaturated Polyesters: Structure and Properties, Elsevier, 1964
5. B. Das, H.S. Loveless, and S.J. Morris, Effects of Structural Resins and Chopped
Fiber Lengths on the Mechanical and Surface Properties of SMC Composites, 36th
Annual Conference of the Reinforced Plastics Composites Institute, The Society of the
Plastics Industry, 1981
6. P.K. Mallick, Fatigue Characteristics of High Glass Content SMC Materials, 37th
Annual Technical Conference, Society of Plastics Engineers, 1979, p 589
Thermal and Oxidative Stability
Polyester resins are commonly used in elevated-temperature applications, especially in the
electrical and corrosion-resistance areas. At temperatures above 150 °C (302 °F), the polymer
begins to slowly dissociate chemically. The temperature at which this decomposition occurs
depends on the structure of the polyester used. Regardless of the polymer composition, at
temperatures near 300 °C (570 °F), the cured polyester resin will undergo spontaneous
decomposition. This is characteristic of vinyl polymers and is caused by their
depolymerization to form monomeric species.
Among polyesters, high-molecular-weight UPEs and epoxy vinyl esters show better stability
above 150 °C (302 °F) than low-molecular- weight UPEs. High-molecular-weight isophthalic,
PET, DCPD, and BPA fumarate resins, when formulated properly, can perform very well in
thermal applications (Fig. 3). In thermal stability, nonhalogenated resins outperform
halogenated resins. While some chlorinated resins have respectable thermal stability,
brominated resins universally have poor thermal stability with aromatic bromides performing
somewhat better than aliphatic bromides. Low-styrene (general-purpose) resins also exhibit
poor thermal stability. The same is true for orthophthalic resins, as they tend to be lowmolecular-weight resins. In spite of their poor thermal stability, orthophthalic resins perform
well in moderate- to low-temperature applications and are often preferred in such applications
because of their low cost.
Fig. 3 Thermal stability of glass-polyester composites at 180 °C (355 °F)
Monomer type also plays an important role in the thermal stability of a polyester resin. For
example, a polyester resin in vinyl toluene shows superior thermal stability when compared to
the same resin in styrene. This is attributed to the strong copolymer bonds formed with the
fumarate unsaturation. Even after aging at 200 °C (390 °F), an isophthalic resin in vinyl
toluene has better flexural strength retention than an isophthalic resin in styrene (Fig. 4).
While vinyl toluene versions of an isophthalic polyester and a BPA fumarate showed
comparable performance at 200 °C (390 °F), the BPA fumarate had much better flexural
strength retention at 220 °C (430 °F) (Fig. 5) and 240 °C (465 °F) (Fig. 6).
Fig. 4 Flexural strength retention of glass-polyester composite when aged at 200 °C (390
°F), tested at room temperature
Fig. 5 Flexural strength retention of glass-polyester composite when aged at 220 °C (430
°F), tested at room temperature
Fig. 6 Flexural strength retention of glass-polyester composite when aged at 240 °C (465
°F), tested at room temperature
Chemical Resistance
Polyester resins have been used for many years in applications requiring resistance to
chemical attack. Parts made from UPEs have corrosion-resistant properties that complement
most metals. As an environment increases in polarity, it becomes less aggressive toward parts
made from UPEs. Numerous applications for corrosion-resistant tanks, pipes, ducts, and liners
can be found in the chemical process and pulp and paper industries. Resin selection depends
on the specific chemical environment to be contained. Table 9 shows how four different
classes of polyester resin perform in several environments. Isophthalic resins perform well in
both mild aqueous and mild organic environments. Generally, they are the most economic
resin choice. Vinyl ester resins are typically used in more aggressive environments. A
properly selected vinyl ester resin will perform well in many applications. When the
environment becomes more aggressive, premium polyesters are preferred. Premium polyester
resins offer improved corrosion resistance for specific applications. Chlorendic resins are
chosen for strong acid and oxidizing environments, especially at elevated temperatures, while
BPA fumarate resins are better in caustic environments. Using glass fiber does not improve
the corrosion resistance of polyester resins and, in some cases, actually reduces performance.
This is especially true in hydrofluoric acid or strong caustic environments where the
chemicals actually attack and dissolve glass. In this and other special cases, reinforcing
materials, such as carbon fibers, may be preferred (Ref 7).
Table 9 Corrosion resistance of glass fiber polyester resin composites
Resin
75%
H2SO4 80
°C (175
°F)
15%
5.25%
Xylene
NaOH 65
NaOCl 65
Ambient
°C (150
°C (150 °F)
°F)
Deionized
Seawater 80
water 100 °C
°C (180 °F)
(212 °F)
Isophthalic –
–
–
+
–
–
Chlorendic +
–
–
+
+
+
BPA
fumarate
–
+
–
+
–
+
Vinyl ester –
–
+
–
–
+
Reference cited in this section
7. H.S. Kliger and E.R. Barker, A Comparative Study of the Corrosion Resistance of
Carbon and Glass Fibers, 39th Annual Conference of the Reinforced
Plastics/Composites Institute, The Society of the Plastics Industry, 1984
Ultraviolet (UV) Resistance
Polyester resins are used in many outdoor applications. They can survive exposure to the
elements for periods exceeding 30 years, although some discoloration and loss of strength will
occur. The onset of surface degradation is marked by a yellow discoloration that becomes
progressively darker as erosion and surface stress crazing occur. In translucent systems, this
UV radiation causes yellowing of the composite as a whole, although the color is usually
more intense on the surface. The negative effects of UV exposure can be effectively
eliminated with the addition of UV stabilizers to the outermost resin layer. Monomer selection
also affects UV stability. Styrene and other aromatic vinyl monomer derivatives are more
susceptible to oxidative degradation and are usually supplemented with more resistant
acrylate or methacrylate monomers. Of the acrylate monomers, methyl methacrylate (MMA)
is the most common. When MMA is copolymerized with styrene, the cured polyesters have
superior durability, color retention, and resistance to fiber erosion (Ref 8).
The improved UV resistance with MMA is evident in Table 10, which compares four
polyester resins composed of identical polymers and four different monomer systems. These
resins were used to prepare glass fiber reinforced panels that were exposed to outdoor
weathering for five years. The gloss retention for styrene/MMA monomer blends was much
greater than for either monomer alone. The refractive index of MMA is also lower than that of
styrene, allowing the formulation of polyester resins with a refractive index matched to the
glass fibers. This, combined with improved UV resistance, has resulted in the use of MMA
polyesters to fabricate glass reinforced transparent building panels that can be used in
greenhouses, skylights, and other applications. Using UV-absorbing chemical additives, such
as the substituted benzophenones or benzotriazoles, can further reduce the effect of radiation.
Table 10 Effect of methyl methacrylate on the gloss retention of weathered polyester
panels
Panel number
Components
3
4
5
75 75 60
60
60
Methyl methacrylate, % 25 … 20
10
0
Styrene, %
0
30
40
Gloss retention, %
Source: Ref 8
5.2 48 83.5 56.5 11.8
Polyester, %
1
2
25 20
Reference cited in this section
8. A.L. Smith and J.R. Lowry, Long Term Durability of Acrylic Polyesters versus 100%
Acrylic Resins in Glass Reinforced Constructions, 15th Annual Conference of the
Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, 1960
Electrical Properties
Most organic polymers have medium to excellent electrical properties. Wide ranges of
thermoset and thermoplastic materials are used in the electrical and electronics industries.
Applications in which polyester resins have been used include the insulation of motor
windings, encapsulation of electrical components, fabrication of printed circuit boards, highvoltage standoff insulators, switch boxes, and miscellaneous equipment for high-voltage line
work. Typical electrical properties of polyester resins are shown in Table 11. Tables 12 and
13 show specific data on an isophthalic polyester and a BPA fumarate as well as the influence
of filler type.
Table 11 Electrical properties of glass- polyester composites
Volume resistivity, 50% relative humidity, Ω· m 1010–1012
Dielectric strength, kV/mm (kV/in.)
Short-time, 3.2 mm (⅛ in.)
13.6–16.5 (345–420)
Step-by-step, 3.2 mm (⅛ in.) increments
10.8–15.4 (275–390)
Dielectric constant
60 Hz
5.3–7.3
1 kHz
4.68
1 MHz
5.2–6.4
Dissipation factor
60 Hz
0.011–0.041
1 MHz
0.008–0.022
Arc resistance
120–200
Table 12 Electrical properties of isophthalic polyester 3.2 mm (⅛ in.) laminates with
various fillers
Filler
Dielectric
Volume Dielectric Dissipation Dielectric Dissipation Dielectric Dissipatio
strength short
resistivity, constant, factor, 1 constant, factor, 1 constant, factor, 6
time
60 Hz
kHz
1 kHz
MHz
10–13Ω· m 1 MHz
Hz
kV/mm kV/in.
Calcium
15.0
carbonate
380
7.8
4.10
0.007
4.18
0.005
4.19
0.003
Gypsum
CaSO4
14.4
365
2.1
3.69
0.011
4.04
0.023
4.19
0.027
Aluminum
15.4
trihydrate
390
2.6
3.67
0.009
3.81
0.010
3.89
0.011
Clay
14.4
365
6.4
4.08
0.018
4.61
Note: Isophthalic polyester resin based on a vinyl toluene monomer
0.040
5.10
0.057
Table 13 Electrical properties of BPA fumarate polyester 3.2 mm (⅛ in.) laminates with
various fillers
Filler
Dielectric
Volume Dielectric Dissipation Dielectric Dissipation Dielectric Dissipatio
strength short
resistivity, constant, factor ,1 constant, factor, 1 constant, factor, 6
time
60 Hz
kHz
1 kHz
MHz
10–13Ω· m 1 MHz
Hz
kV/mm kV/in.
Calcium
6.1
carbonate
155
1.6
3.94
0.005
4.00
0.004
4.03
0.004
Gypsum
CaSO4
150
3.3
3.72
0.009
4.03
0.024
4.24
0.029
300
3.3
3.64
0.008
3.81
0.015
3.93
0.025
0.043
5.11
0.053
5.9
Aluminum
11.8
trihydrate
Clay
12.6
320
3.5
4.08
0.023
4.68
Note: BPA fumarate polyester resin based on a vinyl toluene monomer
Because many electrical applications require performance at elevated temperatures, polyester
resin composites must have good thermal stability. Thermal stability and electrical
performance at elevated temperatures are directly related, as can be seen by comparing the
retention of dielectric strength at 200 °C (390 °F), shown in Fig. 7, with the retention of
flexural strength at 200 °C (390 °F), shown in Fig. 4. As with flexural strength, a vinyltoluene-based polyester outperforms a styrene-based polyester. At temperatures above 200 °C
(390 °F), vinyl-toluene- based BPA fumarates outperform vinyl-toluene- based isophthalic
polyesters (Fig. 8, 9).
Fig. 7 Dielectric strength retention of glass-polyester composite when aged at 200 °C
(390 °F), tested at room temperature
Fig. 8 Dielectric strength retention of glass-polyester composite when aged at 220 °C
(430 °F), tested at room temperature
Fig. 9 Dielectric strength retention of glass-polyester composite when aged at 240 °C
(465 °F), tested at room temperature
Large electrical equipment, such as high-voltage motors or generators, often operate at
elevated temperatures. In such applications, the electrical property of greatest concern is the
dissipation factor, especially the dissipation factor versus the temperature. Polyester resins can
be formulated for a low dissipation factor at elevated temperatures (Fig. 10). They are used as
electrical varnishes at continuous-use temperatures up to 180 °C (355 °F).
Fig. 10 Isophthalic polyester at 2.4 kV/mm (60 kV/in.)
Flame-Retardant Polyester Resins
All organic materials, including polyesters, will burn in the presence of a flame. In many
applications, polyester resins are required to have some degree of resistance to burning. This
can be accomplished by using either a filler or a specially formulated flame-retardant
polyester resin, depending on the degree of resistance required. The addition of filler is the
more economical route to achieving flame retardancy in parts made from UPEs. However, the
addition of filler increases weight and compromises tensile properties.
Incorporating halogen into a polyester resin is an effective way of improving flame
retardance. This can be accomplished using a halogenated dibasic acid, such as chlorendic
anhydride or tetrabromophthalic anhydride, or a halogenated dihydric alcohol, such as
dibromoneopentyl glycol or tetrabromobisphenol A. At equivalent concentrations, bromine is
much more effective than chlorine. Additives such as antimony oxides and ferrous oxide act
as synergists with halogenated polyesters and improve their flame-retardancy properties (Ref
9).
Burning rate and smoke generation are measured using the Steiner Tunnel Test (ASTM E 84).
In this test, a gas burner is placed at one end of a 53 cm by 7.6 m (21 in. by 25 ft) section. The
distance the flame travels and the amount of smoke generated are measured (by the
obscuration of a photoelectric beam). These are compared to two standards: red oak board,
which is given a rating of 100 for flame spread and smoke generation, and asbestos cement,
which is given a rating of 0 for both flame spread and smoke generation. Smoke generation is
also measured with an NBS smoke chamber, which uses a photoelectric cell to measure
smoke buildup in a closed chamber. The sample is burned either with or without a direct
flame. When NBS smoke chamber testing is used, it is often common to use ASTM E 162,
Flame Spread Index, to measure this variable.
Table 14 compares the performance of several resins using the above-mentioned fire and
smoke tests. In each of these tests, the halogenated resin clearly outperforms the orthophthalic
resin. Ferrous oxide also reduces smoke generation in the NBS chamber when compared to
antimony oxide.
Table 14 Performance of selected polyester composites in fire tests
System
Material or test
I
II
III
Material
Resin
100(a) 100(b) 100(c)
Alumina trihydrate
100
Antimony oxide
Ferrous oxide
100
100
… 57
…
…
…
5
Test method and property
ASTM E 162
Flame spread index 75
7
7
ASTM E 84
Flame spread
120
23
25
Smoke emission
608
270
268
NBS chamber: flaming mode
System
Material or test
I
II
III
Maximum density 203
433
264
90 s density
2.5
18
11
240 s density
162
245
128
NBS chamber: nonflaming mode
Maximum density 481
400
350
90 s density
1
5
1
240 s density
16
45
50
(a) Orthophthalic resin.
(b) HET acid resin A, 26% Cl.
(c) HET acid resin B, 26% Cl.
Source: Ref 10
Table 15 compares various filled and unfilled polyester resin composites. The improvement of
an orthophthalic resin by the incorporation of alumina trihydrate (ATH) is dramatic, the flame
spread is reduced from 350 to 64. However, halogenated resins reach this level of
performance without the incorporation of any filler. Halogenated resins with a synergist show
only a slight reduction in flame spread when ATH is added, but smoke emission is greatly
reduced. When comparing brominated and chlorinated resins, 26% Cl gave comparable flame
and smoke results as 18% Br.
Table 15 Flame spread and smoke emission characteristics of unfilled and filled
polyester systems
Glass reinforced laminates 3.2 mm (⅛ in.) thick, containing 30% glass, tested in accordance
with ASTM E 84
System formulation and property value
Component or property 100(a) 100(b) 100(b) 100(c) 100(d) 100(d) 100(e) 100(e) 100(f) 100(f)
Unfilled system
Antimony oxide
…
…
5
…
…
5
…
5
…
5
Flame spread
350
60
25
…
60
15
67
20
69
18
Smoke emission
1100 780
450
…
747
731
980
1043 837
838
100
100
100
100
100
100
100
100
Filled system
Alumina trihydrate, phr
Antimony oxide
…
Ferrous oxide
…
Flame spread
120
100
5
…
…
23
… 5
5
5
20
25
…
…
10
Smoke emission
608 270 242 168 364
(a) Orthophthalic resin.
(b) HET acid resin A, 26% Cl.
(c) HET acid resin B, 26% Cl.
(d) Dibromotetrahydrophthalic resin, 18% Br.
(e) Dibromotetrahydrophthalic glycol resin, 18% Br.
(f) Tetrabromophthalic resin, 18% Br.
5
5
100
… 5
… 5
…
… 5
25
18
10
10
12
260
450
400
761
620
Source: Adapted from Ref 10
References cited in this section
9. E. Dorfman, W.T. Schwartz, Jr., and R.R. Hindersinn, “Fire-Retardant Unsaturated
Polyesters,” U.S. Patent 4,013,815, 1977
10. J.E. Selley and P.W. Vaccarella, Controlling Flammability and Smoke Emissions in
Reinforced Polyesters, Plast. Eng., Vol 35, 1979, p 43
Acknowledgment
This revised article is largely built around the original article “Polyester Resins” by Charles
D. Dudgeon, Ashland Chemical Company, from Composites, Vol 1 of the Engineered
Materials Handbook.
References
1. M. Grayson and D. Eckroth, Ed., Encyclopedia of Chemical Technology, 3rd ed., Vol
18, John Wiley & Sons, 1982, p 575
2. P. Robitschek and C.T. Bean, Flame Resistant Polyesters from
Hexochlorocyclopentadiene, Ind. Eng. Chem., Vol 46, 1954, p 1628
3. E.N. Doyle, The Development and Use of Polyester Products, McGraw-Hill, 1969, p
258
4. H.V. Boenig, Unsaturated Polyesters: Structure and Properties, Elsevier, 1964
5. B. Das, H.S. Loveless, and S.J. Morris, Effects of Structural Resins and Chopped
Fiber Lengths on the Mechanical and Surface Properties of SMC Composites, 36th
Annual Conference of the Reinforced Plastics Composites Institute, The Society of the
Plastics Industry, 1981
6. P.K. Mallick, Fatigue Characteristics of High Glass Content SMC Materials, 37th
Annual Technical Conference, Society of Plastics Engineers, 1979, p 589
7. H.S. Kliger and E.R. Barker, A Comparative Study of the Corrosion Resistance of
Carbon and Glass Fibers, 39th Annual Conference of the Reinforced
Plastics/Composites Institute, The Society of the Plastics Industry, 1984
8. A.L. Smith and J.R. Lowry, Long Term Durability of Acrylic Polyesters versus 100%
Acrylic Resins in Glass Reinforced Constructions, 15th Annual Conference of the
Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, 1960
9. E. Dorfman, W.T. Schwartz, Jr., and R.R. Hindersinn, “Fire-Retardant Unsaturated
Polyesters,” U.S. Patent 4,013,815, 1977
10. J.E. Selley and P.W. Vaccarella, Controlling Flammability and Smoke Emissions in
Reinforced Polyesters, Plast. Eng., Vol 35, 1979, p 43
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