Note: Descriptions are shown in the official language in which they were submitted.
1 1~Z6~2
This invention relates to a process for fracture
toughening unsaturated polyester resins, vinyl ester resins and
like resins which are cured by addition polymerization through
their double bonds. More particularly, an additive or in situ
prepared modifier comprising an ester of a very low viscosity
hydroxy-terminated polydiene which enhances the fracture
toughness of the above-identified resins when cured, while
maintaining or enhancing chemical resistance, mechanical strength
and modulus, heat distortion temperature and desirable cure
properties is utilized. This invention also relates to a resin
comprising the additive or in situ prepared modifier and to
articles of manufacture prepared by curing the resin.
The following disclosure is primarly directed to
unsaturated polyester resins, however, it should be noted that
the teachings apply equally well to all resins which, as defined
above, are cured by addition polymerization through their double
bonds.
A number of methods are known for improving the
fracture toughness of an unsaturated polyester. These methods
include, for example:
(i) incorporating a totally compatible, reactive, flexible
additive into the unsaturated polyester;
(ii) modifying the unsaturated polyester backbone by the
incorporation of a longer chain saturated diacid and/or
glycol therein; and
(iii) by utilizing liquid or solubilized solid rubbers.
The success of each of the above methods has been
limited by certain disadvantages. Methods (i) and (ii) reduce
the heat distortion temperature of the unsaturated polyester
3~
-- 1 --
~t52682
and, by extension, the chemical resistance thereof, particularly
at elevated temperatures. This is a significant disadvantage
since premium chemical resistant unsaturated polyester resins
are not only characterized by their rigidity, low elongation
modulus and brittleness but also by their high heat distortion
temperatures, generally greater than 100C, and their resistance
to attack by a wide variety of chemicals over a broad range of
temperatures.
Method (iii) not only suffers from the deficiencies of
methods (i) and (ii), as described above, but further, the
rubbers, even in the liquid state, are incompatible with the
unsaturated polyesters and, therefore, mixing and separation
problems arise.
The recognition of the above-described problems has
led to a number of attempts in the prior art to enhance the
fracture toughness of unsaturated polyesters without
deteriorating other inherent and desirable properties of
unsaturated polyesters.
Thompson et al, in U. S. Patent 3,733,370, teach an
additive for fracture toughening unsaturated polyesters
composed of an ester of a hydroxy terminated polybutadiene with
allylic positioned, ethylenically-unsaturated acyloxy groups.
Bonnington, in U. S. Patent 3,989,769, teaches the use of
polybutadienes and preferably hydroxy terminated ones to modify
thermosettable polymers including polyesters. Kajuiva et al,
in U. S. Patent 3,806,490, Roberts et al, in U. S. Patent
3,998,909, Curtis Jr. et al, in U. S. Patent 3,793,400, and
Nowak et al, in U. S. Patent 3,857,812, all teach the use of
various polybutadienes with polyesters. Baum et al, in Canadian
Patent 968,496, teach a polyester of an addition product of a
~15Z6~2
hydroxy terminated-polybutadiene with an ethylenically
unsaturated dicarboxylic acid or anhydride and a vinyl cross-
linking agent.
The above prior art attempts, however, have not
completely succeeded in overcoming all of the previously
mentioned problems.
Accordingly, it is an object of this invention to
provide an improved method for fracture toughening resins which
are cured by addition polymerization through their double bonds.
According to an aspect of the invention there is
provided a process for fracture toughening a resin which is
cured by addition polymerization through its double bonds which
comprises adding an at least one end esterified product of a
very low viscosity hydroxy-terminated polydiene and an unsaturated
diacid anhydride to the resin prior to cure.
According to a second aspect of the invention there is
provided a process for fracture toughening a resin which is
cured by addition polymerization through its double bonds and has
acid or anhydride functionality, the process comprising adding
up to about 10% by weight of a very low viscosity hydroxy-
terminated polydiene to the reaction mixture for preparing the
resin.
According to a third aspect of the invention there is
provided a process for fracture toughening a resin which is
cured by addition polymerization through its double bonds and
has acid or anhydride functionality, the process comprising
adding up to about 5% by weight of a very low viscosity hydroxy-
terminated polydiene; and an at least one end esterified product
of a very low viscosity hydroxy-terminated polydiene and an
unsaturated diacid anhydride to the resin prior to cure.
26~3Z
According to a further aspect of the invention there
is provided resin compositions prepared according to the above
described methods.
Articles of manufacture produced by curing the above
fracture toughened resins are also provided by the invention.
Rubbers with reactive functional end groups have been
used to improve fracture toughness of thermosets, particularly
epoxide resins by the above-mentioned method (iii). In
practice the rubbers used are high molecular weight liquid
rubbers which are incompatible with uncured unsaturated
polyesters and, therefore, mixtures of the rubbers and the
unsaturated polyesters readily separate both on standing
and during cure. Their usefulness, thus, is restricted to such
high viscosity systems as bulk and sheet molding compounds.
Even in these systems some exuding occurs during storage and
handling. Further, these rubber additives, generally, prolong
gel and cure times and reduce cure exotherms.
The necessary properties of a desirable fracture
toughening agent are as follows. The fracture toughening agent
should be compatible or resistant to phase separation when
incorporated into an uncured resin but should form discrete
microscopic domains upon curing and these domains should be
chemically bonded to the surrounding matrix along their
interface. In so doing, the fracture toughness of the cured
material will be enhanced. By not plasticizing the continuous
matrix, the desirable mechanical strength, chemical resistance
and heat distortion properties of the system will be maintained
and in some cases enhanced. It is also possible, by the proper
choice of materials, to e~sure that the cure properties of the
system are not affected adversely.
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`Z682
It has been found that an ester of a very low
viscosity hydroxy-terminated polydiene can serve as a fracture
toughening agent for unsaturated polyesters and like resins
without the above-described disadvantages.
The precursors of the esters are a very low viscosity
hydroxy-terminated polydiene (e.g. less than 25,000 poise at
30C), and maleic anhydride or like reagents.
The term "like reagents" as used herein means any
unsaturated diacid anhydride, e.g. chloromaleic anhydride or
citraconic anhydride. These unsaturated (copolymerizable) diacid
anhydrides may be used alone or in combination with any
saturated aliphatic, cycloaliphatic or aromatic anhydride as
long as at least one end of the ester chain is reacted with the
unsaturated (copolymerizable) diacid anhydrides. For example,
1 to 2.4 - 2.6 moles of maleic anhydride and 0 to 1.4 - 1.6 moles
of phthalic anhydride, tetrachlorophthalic anhydride, chlorendic
anhydride, t~trahydrophthalic anhydride, succinic anhydride
and the like per ester chain could be used. Maleic anhydride
is the preferred reagent and is exclusively but not limitatively
taught in the following. Diacids such as, for example, fumaric
acid or mesaconic acid may also be used but then the simple
bulk polymerization process carried out at or below 90 C, as
taught below, could not be used.
One example of a commercially available, very low
viscosity hydroxy-terminated polydiene is R 45 HT from ARCO.
The structure of R 45 HT may be schematically represented as
follows:
~ ~2682
/ CH2) ~ OH
/ CH=CH ~ CH=CH
HO - -(CH2 2~0.2 ~CH2-~cH ~ --~CH2
CH=CH2 M
wherein M = 50
and the average number of pendant hydroxyl groups per chain,
fOH= 2.4 to 2.6. On reaction with maleic anhydride R 45 HT
will produce an ester.
Although the following disclosure teaches the
invention with the use of R 45 HT, it will be understood that
any very low viscosity hydroxY-terminated polydiene could be
utilized.
The esters useful as fracture toughening agents as
described herein are those which have at least one esterified
end. For example, the two end esterified product of R 45 HT
and maleic anhydride may be schematically represented as
follows:
- 5a -
Z682
o o _ o o
Il 11 11 11
HO-C CO _ _ OC C-OH
CH=CH _ _ CH=CH
M
The ester toughening agent disclosed herein can be
utilized in two ways, i.e. as a previously prepared additive to
the polymerization mixture or as an in situ prepared modifier
for the polymerization reaction.
The ester additive can be prepared by reacting the
previously noted precursors at a temperature up to about 90C
in a suitable apparatus equipped with a nitrogen sparge and an
agitator. Inhibitors may be used to prevent premature gellation,
and the half-ester may be stored without further treatment.
However, preferably, for convenience, the half-ester is
dissolved in styrene to give a 30% to 70% solids solution.
To ensure reactivity of the additive with an
unsaturated polyester resin through free radical initiated
addition polymerization each polydiene molecule must contain
at least one pendant maleate half-ester group. Thus as a
minimum one-half mole of maleic anhydride is required per
hydroxyl equivalent and the preferred composition is one mole
of maleic anhydride per hydroxyl equivalent. Under the above
reaction conditions maleic anhydride in excess of one mole per
hydroxyl equivalent may not react in the desired manner and
may precipitate out with time.
Without wishing to limit the invention to a specific
rnechanism it is believed that the reaction proceeds through a
ring opening-addition of the polydiene hydroxyl groups to the
2682
maleic anhydride to produce half-esters.
As disclosed above the novel and advantageous half-
ester is based on a very low molecular weight precursor and its
activity is obtained through half-ester formation using maleic
anhydride, which results in pendant carboxyl groups. In fact,
the level of maleic anhydride can be adjusted within the
previously described limits to provide for a combination of free
pendant carboxylic and hydroxylic groups.
Again without limiting the invention to any specific
theory it is believed that the combination of low molecular
weight, half-ester linkages and pendant carboxylic groups
provides for the enhanced compatability of the present half-
ester over prior art rubber additives.
The preparation of the half-ester is by a "bulk"
reaction of maleic anhydride and a hydroxy terminated polydiene,
i.e., without using solvents and thus not requiring isolation
or purification procedures. However, it should be noted that
attempts to prepare the half-ester at temperatures greater than
about 90C resulted in gellation.
The in situ method of preparing and using the ester
toughening agent described herein will now be explained. The
very low viscosity hydroxy-terminated polydiene, e.g. R 45 HT
may be incorporated into an unsaturated polyester during its
preparation through the esterification of the hydroxyl groups
of the polydiene with the acid or anhydrid~ functionality of
the growing polyester. The time of addition of the polydiene
is critical in that too early an addition will result in an
excessive viscosity increase and too late an addition will
result in insufficient reaction between the polyester carboxylic
components and the hydroxyl groups of the polydiene. This can
be done at normal reaction temperatures for preparing polyesters,
i.e. 180C to 225C. _ 7 _
2~i8Z
Upon tl~ g the unsaturated polyester/very low
viscosity hydroxy-terminated polydiene system with styrene a
hazy solution is obtained, demonstrating that miscibility has
not been achieved. The components, however, do not separate
into discrete layers on standing but rather form a stable,
homoyeneous mixture. The cured castings and laminates are
o~aque demonstrating the desired formation of discrete rubbery
domains. The cure properties of the modified resin are not
significantly altered from those of the base resin and the
chemical resistance, mechanical properties and heat distortion
temperature of the modified resin are not deteriorated.
Comparing the two methods of using the ester fracture
toughening agent described herein, i.e. the additive and the
in situ methods, the following conclusions can be drawn.
The primary advantage of the additive approach is that
the half-ester can be prepared separately and added to any resin
at the required level. The half-ester will gradually separate
from the resin on standing, particularly for systems which
require long gel times, but can readily be re-dispersed. The
primary advantage of the in situ approach is that separation
does not occur, while the primary disadvantage is that a
practical limit of approximately 10% by weight based on the
total weight of the resin exists on the amount of polydiene
that can be incorporated into the resin. Larger amounts
result in excessive viscosity levels for conventional polyester
alkyd reactors and also result in higher than preferable
polyester viscosity.
The additive and in situ methods can be combined. It
is possible to prepare a resin by the in situ approach to prefer-
ably about 5~ by weight polydiene content and thereafter tointroduce
-- 8 --
~52682
the half-ester additive to achieve the desired level of reactive
liquid polydiene.
The ester fracture toughening agent disclosed herein
having the properties of low viscosity, storage stability both
by itself and when incorporated into a resin and compatability
and reactivity with the resin can be used to produce resins
with improved fracture toughness and with minimal loss of, and
in some instances, with improvements in other desirable
properties such as mechanical strength and moduli, chemical
resistance and heat distortion temperature. Additionally, the
ester fracture toughening agent can decrease surface defects
in cured parts and when incorporated into suitable polyesters
decrease shrinkage and improve surface profile.
As noted above with the in situ approach up to a
maximum of about 10% by weight of a polydiene can be incorporated
into a resin. With the additive approach the optimum level of
polydiene added to a resin is dependent on the resin but is
generally from about 5% to about 15% by weight based on the
total weight of the resin.
The pendant carboxylic and hydroxylic groups of the
fracture toughening agent disclosed herein can react with metal
oxide and hydroxide or isocyanate thickening agents respectively
to improve the compatability and mouldability of the system.
If desired, the fracture toughening ester described herein can
be used in combination with other thermoplastic low shrink
additives.
The invention will now be further described by means
of examples in which:
Figure 1 is a graphical representation of viscosity
changes with time for the additive of Example 3.
~15268;~
Bigure 2 is a graphical representation of normalized
flexural strength changes with immersion time in 10% NaOH
for the resins of Examples 5 and 7.
Figure 3 is a graphical representation of normalized
flexural strength changes with immersion time in distilled water
for the resins of Examples 5 and 7.
The resins used in the Examples were as shown in
Table 1.
TABLE 1
Description of Resins
Resin
A - hydrogenated bisphenol A polyester
B - semi-rigid isophthalic polyester
C - rigid isophthalic polyester
D - rigid modified-bisphenol A polyester
E - propoxylated bisphenol A polyester
FLEX - internally flexibilized version of Resin D
PE 1 - hydroxyl terminated polyether
PE 2 - hydroxyl terminated polyether (higher
molecular weight)
VTR - vinyl terminated liquid rubber
EXAMPLE 1
This example demonstrates the effect of adding a
low molecular weight, hydroxy-terminated polydiene to a resin.
A liquid, low molecular weight, hydroxy-terminated
butadiene polymer (referred to as polymer hereinafter) was
incorporated into a hydrogenated-bisphenol A fumarate (referred
to as resin A hereinafter). Rapid separation of the polymer and
resin A occurred and was generally noticeable in less than one
hour. The separation was complete after a few hours. The
-- 10 --
;8Z
polymer produced a marked detrimental effect on the gel and
cure of resin A and particularly in its suppression of the
cure exotherm. Post cured castings exhibited a strong
reduc~ion in mechanical properties over similarly treated
controls.
The results obtained above were not unexpected as no
mechanism for chemical or strong physical interaction between
the polymer and resin A was available.
EXAMæLE 2
This example demonstrates the effect of adding an
esterified, low molecular, hydroxy-terminated polydiene to a
resin.
Utilizing the terminal hydroxyl functionality of the
polymer of Example 1, ester linkages were introduced primarily
at the chain ends by reacting the polymer with maleic anhydride.
The esterification reaction was ch~osen to provide for reactivity
with styrene and/or unsaturated moieties in a resin backbone
through free radical initiated chain growith polymerization.
Noticeable separation of the modified or esterified
low molecular weight, hydroxy-terminated polybutadiene polymer
(referred to as MR hereinafter) from resin A, after mixing,
required from several hours up to a day. Low shear agitation
rapidly redispersed the components.
Castings prepared from polymerized solutions of
styrene and the MR were rubbery solids. Neither polystyrene
nor the MR could be extracted from these castings, suggesting
that copolymerization had taken place effectively. ~hese
castings were clear, suggesting that phase separation had not
taken place on cure. Free unpolymerized styrene levels and
water absorptions determined for cured castings demonstrated
`Z68Z
that cure was complete. The water absorptions for castings
containing the MR additive were generally lower than for the
controls, while free styrene levels were the same.
Castings and laminates prepared from mixtures of the
MR additive with various unsaturated polyesters were invariably
translucent to opaque, demonstrating separation of the
components into discrete phases.
The effect of the MR additive on the uncured properties
of resin A is shown in Table 2. The MR additive resulted in
an increase in viscosity and a moderate decrease in peak
exotherm temperature.
TABLE 2
Uncured Resin Properties
Resin A
Property Control 10% MR
BrookOield Viscosity 650 850
t25 C, c.p.)
Refractive Index (25 C) 1.540 1.538
Specific Gravity 1.03 1.01
SPI Gel 13'10";16'19";379 F 11'12";15'13";350F
EXAMPLE 3
This example illustrates the storage stability of the
MR. Figure 1 shows the aging characteristics of the MR
depicted by Brookfield Viscosity increase as a function of
storage time at ambient temperature. The study was done both
for the material alone and its solution at 70 per cent solids in
styrene. Following preparation, a rapid rise in viscosity
occurs over approximately a two week period followed by a much
slower but continued increase. The reason for this behaviour
has not been explained, but the material has been shown to be
- 12 -
f~682
stable, that is without gellation occurring, up to periods in
excess of one year.
EXAMPLE 4
This example illustrates the mechanical properties of
resins comprising the MR additive.
All castings and laminates were prepared using a
"room temperature" cure system based on cobalt and dimethyl or
diethylaniline with MEKP as the initiator. This was followed by
a postcure cycle.
Table 3 depicts the results which were obtained when
the MR was incorporated with three premium chemical resistant
resins. Generally, use of the additive, resulted in significant
increases in the strength and elongation of cured castings over
their respective controls. Some decreases in the stiffness
(moduli) and heat distortion temperature also were noted. It
is likely that the increases in strength were due to a reduction
in defects in the castings brought about by the "toughening"
or stress relaxation properties of the MR additive. The
toughening effect was readily noted during the preparation of
castings. The number of control castings which had to be
discarded due to cracking on removal from the moulds far exceeded
those which contained the MR additive. This was especially
evident for castings prepared from the most brittl~ polyester,
propoxylated bisphenol A polyester (referred to as resin E
hereinafter). Resin D (referred to as such hereinafter) in
Table 3 was a rigid modified-bisphenol A polyester.
~;2682
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EXAMPLE 5
This example illustrates the corrosion resistance of
resins comprising the MR additive.
A typical set of laminate properties for Resin A is
shown in Table 4. The measured properties were similar for the
control and the MR additive containing sample. Laminate
coupons were immersed in caustic solution and in distilled water
at 90C and their ability to withstand the environments as
determined by flexural strength retention was followed over a
nine-week period. The results are graphically depicted in
Figures 2 and 3. The laminates which contained the MR additive
were probably better, but certainly at least as resistant as
their respective controls over the duration of the testing. The
control coupons were much more prone to surface blistering
than coupons which contained the MR additive. Fiber "show" was
also more prevalent for the controls, although the lack of
effect for the modified resins was presumably due to the
opacity of the laminates.
TABLE 4
TYPICAL LAMINATE PROPERTIES
-
RESIN A
PROPERTYCONTROL 10~ MR
Tensile Strength (p5i) 12,300 13,100
Tensile Modulus (psi) 1,070,000 1,060,000
Elongation (~) 1.4 1.2
Flexural Strength (psi) 17,600 18,400
Flexural Modulus (psi) 750,000 710,000
Heat Distortion
Temperature ( C)304 300
2 plys, 1.5 oz. mat, surface veils, 25% glass
~152~82
In another series of tests, a study was conducted to
determine the suitability of various isophthalic polyesters for
use in gasoline-storage-tank structures. Laminates prepared
from these resins were exposed to "Super-Unleaded" gasolines
from two suppliers for six months at 100F. The MR additive
was incorporated in the resin expected to be the more durable,
a rigid isophthalic polyester (referred to as resin C herein-
after). The results are shown in Table 5. Laminate coupons
prepared from a semi-rigid isophthalic polyester (referred to as
resin B hereinafter) underwent a large decrease in modulus and
a large increase in "lst crack strain" after six months of
exposure. Coupons based on the more rigid Resin C withstood
the exposure over the length of the test period. Incorporation
of the MR additive into Resin C resulted in laminates with
increased strength and improved "lst crack strain" over the
control along with some decrease in stiffness. This method for
improving the toughness of the rigid isophthalic resin
proved superior to the alternative of using a semi-rigid resin
for this environment.
- 16 -
~52682
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2682
EXAMPLE 6
This example compares alternative fracture toughening
agents to the MR additive.
Although the MR additive effectively "toughened" a
number of isophthalic and premium chemical resistant resins
as judged primarily by increases in elongation and "lst crack
strain" along with increased mechanical strength, some reduction
in stiffness and heat distortion temperature was apparent.
Table 6 contains the results obtained when a higher level of
MR additive was incorporated with Resin A. For comparison,
results obtained with a commercial additive, vinyl terminated
liquid rubber (referred to as VTR hereinafter), are also
included. For this system, and in fact what generally has been
shown to occur, increasing the level of additive results in
further increases in elongation, countered by decreasing
stiffness and heat distortion temperature. Improvements in
tensile and flexural strength may also be surrendered at higher
levels of addition. The VTR additive also improved the
elongation to break, in fact to a greater degree at comparable
levels. Reductions in strength, modulus, and heat distortion
temperature, however, were much more severe.
- 18 -
~i~2682
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-- 19 --
~1~268;~
The effects of a higher level addition of MR on Resin
D are shown in Table 7. Also included, are the results obtained
for similar levels of addition of commercially available
hydroxyl terminated polyethers of differing molecular weights,
(referred to as PE 1 and PE 2 hereinafter and wherein the
molecular weight of PE 2 is greater than that of PE 1) and for
an internally flexibilized version of Resin D (referred to as
FLEX hereinafter) of the totally compatible reactive type. In
this system, the increased concentration of MR has resulted in
only a moderate gain in elongation with a large decrease in
heat distortion temperature. Flexural and tensile strengths
and moduli have also decreased. Obviously, an optimum level of
addition exists, dependent on the desired end properties and
the individual base resin characteristics. In this system, the
MR additive is a superior alternative to the FLEX due to its
greater effect on elongation combined with its smaller effect
on the heat distortion temperature. Examination of the effects
of the PE 1 and PE 2 additives, also shows these materials
too present viable candidates. The polyethers, although capable
only of forming physical rather than chemical bonds in the
cured system, can also improve "toughness" while maintaining
or increasing mechanical strength. They have an added advantage
of superior compatibility with the uncured liquid resin. In
this system, however, they show a greater effect on the lowering
of the heat distortion temperature than the MR additive at the
10 per cent level.
- 20 -
~Z682
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-- 21 --
268Z
EXAMPLE 7
-
This example illustrates the in situ method of
fracture toughening a resin.
A propoxylated bisphenol A fumarate was prepared by
a conventional method at a temperature of 200-220C using
reactor equipped with nitrogen sparge, partial and total
condensers and an agitator. The resin was prepared from fumaric
acid and propoxylated bisphenol A. The hydroxy-terminated
polydiene, R 45 HT, was added at the point in the reaction
wherein approximately one-half to all of the hydroxyl groups
had reacted upon reaching the final desired acid number. The
reactor contents were cooled, inhibitors were added and the mass
was thinned with a desired amount of styrene, usually 45 to 50
per cent by weight of the total weight of the mass. This
procedure has been successful with a propoxylated bisphenol A
and with a hydrogenated bisphenol A polyester based on
hydrogenated bisphenol A, fumaric acid, dipropylene glycol and
isophthalic acid prepared in a "two-stage" cook.
The final properties of the above systems were similar
to corresponding resins obtained by the additive method.
Mechanical properties of the resins produced by the in situ and
additive methods were virtually the same for equal levels of
addition. The chemical resistance of the in situ prepared and
fracture toughened resin A is shown in Figures 2 and 3 by the
Resin A (8~ R) plots.
From the above examples it can be concluded that the
MR additive effectively fracture toughens premium and
isophthalic chemical resistant resins. Additionally, the MR
additive maintains the chemical resistance of the resins and
usually increases their strength. Tradeoffs with stiffness
- 22 -
~1~2~82
and heat distortion temperature are apparent, but the MR
additive provides a superior overall alternative to internal
flexibilization. In comparison wi~h other fracture toughening
agents the MR additive was clearly superior to vinyl terminated
additives and lower molecular weight polyethers, while
comparable in its effect to high molecular weight polyethers
but with less effect on the heat distortion temperature of the
resin.
- 2~ -
