CA1152682A - Process for fracture toughening resins and resins produced therefrom - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method for fracture toughening a resin which is cured by addition polymerization through its double bonds. A fracture toughening agent comprising an esterification product of a very low viscosity hydroxy-terminated polydiene and maleic anhydride is added to the resin. A process for in situ fracture toughening a resin is also disclosed. The cured resins show increased frac-ture toughness without a reduction in other mechanical properties, chemical resistance or heat distortion temperature.

Description

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~

~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.

`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 -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 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 ~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.

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 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 ;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.

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

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 f~682 stable, that is without gellation occurring, up to periods in excess of one year.

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.

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~l~;Z682 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.

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.

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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.

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~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.

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-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 ~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~ -

Claims (42)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for fracture toughening a resin which is cured by addition polymerization through its double bonds, said process comprising adding an at least one end esterified product of a very low viscosity hydroxy-terminated polydiene and an unsaturated diacid anhydride to said resin prior to cure.

2. A process as defined in claim 1, wherein the esterified very low viscosity hydroxy-terminated polydiene comprises from about 5% to about 15% by weight of said resin.

3. A process as defined in claim 1 or 2, wherein said very low viscosity hydroxy-terminated polydiene has a viscosity of less than 25,000 poise at 30°C.

4. A process as defined in claim 1 or 2, wherein said very low viscosity hydroxy-terminated polydiene is represented by:

wherein M= 50 and the average number of hydroxyl groups per chain, fOH = 2.4 to 2.6.

5. A process as defined in claim 1 or 2, wherein said unsaturated diacid anhydride is maleic anhydride.

6. A process as defined in claim 1 or 2, wherein the esterified very low viscosity hydroxy-terminated polydiene additionally comprises a saturated anhydride selected from the group consisting of: aliphatic, cycloaliphatic and aromatic anhydrides.

7. A process as defined in claim 1 or 2, wherein said resin is selected from the group consisting of: hydrogenated bisphenol A fumarate and polystyrene.

8. A resin composition comprising a resin which is cured by addition polymerization through its double bonds and a fracture toughening agent, said fracture toughening agent comprising an at least one end esterified product of a very low viscosity hydroxy-terminated polydiene and an unsaturated diacid anhydride.

9. A resin composition as defined in claim 8, wherein said fracture toughening agent comprises from about 5% to about 15% by weight of said resin composition.

10. A resin composition as defined in claim 8 or 9, wherein said very low viscosity hydroxy-terminated polydiene has a viscosity of less than 25,000 poise at 30°C.

11. A resin composition as defined in claim 8 or 9, wherein said very low viscosity hydroxy-terminated polydiene is represented by:

wherein M = 50 and the average number of hydroxyl groups per chain, fOH = 2.4 to 2.6.

12. A resin composition as defined in claim 8 or 9, wherein said unsaturated diacid anhydride is maleic anhydride.

13. A resin composition as defined in claim 8 or 9, wherein the esterified very low viscosity hydroxy-terminated polydiene additionally comprises a saturated anhydride selected from the group consisting of: aliphatic, cycloaliphatic and aromatic anhydrides.

14. A resin composition as defined in claim 8 or 9, wherein said resin is selected from the group consisting of:
hydrogenated bisphenol A fumarate and polystyrene.

15. A process for fracture toughening a resin which is cured by addition polymerization through its double bonds and has acid or anhydride functionality, said process comprising adding up to about 10% by weight of a very low viscosity hydroxy-terminated polydiene to the reaction mixture for preparing said resin.

16. A process as defined in claim 15, wherein said very low viscosity hydroxy-terminated polydiene has a viscosity of less than 25,000 poise at 30°C.

17. A process as defined in claim 15, wherein said very low viscosity hydroxy-terminated polydiene is represented by:

wherein M = 50 and the average number of hydroxyl groups per chain, fOH = 2.4 to 2.6.

18. A process as defined in claim 15, 16 or 17, wherein said reaction mixture comprises fumaric acid and propoxylated bisphenol A.

19. A resin composition comprising a resin which is cured by addition polymerization through its double bonds and has acid or anhydride functionality and up to 10% by weight of a fracture toughening agent, said fracture toughening agent comprising a very low viscosity hydroxy-terminated polydiene.

20. A resin composition as defined in claim 19, wherein said very low viscosity hydroxy-terminated polydiene has a viscosity less than 25,000 poise at 30°C.

21. A resin composition as defined in claim 19, wherein said very low viscosity hydroxy-terminated polydiene is represented by:

wherein M = 50 and the average number of hydroxyl groups per chain, fOH = 2.4 to 2.6.

22. A resin composition as defined in claim 19, 20 or 21, wherein said resin comprises fumaric acid and propoxylated bisphenol A.

23. A process for fracture toughening a resin which is cured by addition polymerization through its double bonds and has acid or anhydride functionality, said process comprising, adding:
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 said resin prior to cure.

24. A process as defined in claim 23, wherein up to about 5% by weight of said very low hydroxy-terminated polydiene is added.

25. A process as defined in claim 23 or 24, wherein said very low viscosity hydroxy-terminated polydiene has a visco-sity of less than 25,000 poise at 30°C.

26. A process as defined in claim 23 or 24, wherein said very low viscosity hydroxy-terminated polydiene is repre-sented by:

wherein M = 50 and the average number of hydroxyl groups per chain, fOH = 2.4 to 2.6.

27. A process as defined in claim 23 or 24, wherein said unsaturated diacid anhydride is maleic acid.

28. A process as defined in claim 23 or 24, wherein said resin comprises fumaric acid and propoxylated bisphenol A.

29. A resin composition comprising a resin which is cured by addition polymerization through its double bonds and has acid or anhydride functionality, a first toughening agent comprising a very low viscosity hydroxy-terminated polydiene and a second toughening agent comprising an at least one end esteri-fied product of a very low viscosity hydroxy-terminated polydiene and an unsaturated diacid anhydride.

30. A resin as defined in claim 29 comprising up to about 5% by weight of said first toughening agent.

31. A resin composition as defined in claim 29 or 30, wherein said very low viscosity hydroxy-terminated polydiene has a viscosity of less than 25,000 poise at 30°C.

32. A resin composition as defined in claim 29 or 30, wherein said very low viscosity hydroxy-terminated polydiene is represented by:

wherein M = 50 and the average number of hydroxyl groups per chain, fOH = 2.4 to 2.6.

33. A resin composition as defined in claim 29 or 30, wherein said unsaturated diacid anhydride is maleic anhydride.

34. A resin composition as defined in claim 29 or 30, wherein said resin comprises fumaric acid and propoxylated bisphenol A.

35. An article of manufacture produced by curing the resin composition defined in claim 8, 19 or 29.

36. An article of manufacture produced by curing the resin composition defined in claim 30.

37. A process for fracture toughening a resin which is cured by addition polymerization through its double bonds, said process comprising adding an at least one end esterified product of a very low viscosity hydroxy-terminated polydiene having a Brookfield viscosity of less than 25,000 poise at 30°C and an unsaturated diacid anhydride to said resin prior to cure, and wherein the esterified very low viscosity hydroxy-terminated polydiene comprises from about 5% to about 15% by weight of said resin.

38. A resin composition comprising a resin which is cured by addition polymerization through its double bonds and a fracture toughening agent, said fracture toughening agent comprising an at least one end esterified product of a very low viscosity hydroxy-terminated polydiene having a Brookfield viscosity of less than 25,000 poise at 30°C and an unsaturated diacid anhydride, and wherein said fracture toughening agent comprises from about 5% to about 15% by weight of said resin composition.

39. A process for fracture toughening a resin which is cured by addition polymerization through its double bonds and has acid or anhydride functionality, said process comprising adding up to about 10% by weight of a very low viscosity hydroxy-terminated polydiene having a Brookfield viscosity of less than 25,000 poise at 30 C to the reaction mixture for preparing said resin.

40. A resin composition comprising a resin which is cured by addition polymerization through its double bonds and has acid or anhydride functionality and up to 10% by weight of a fracture toughening agent, said fracture toughening agent comprising a very low viscosity hydroxy-terminated polydiene having a Brookfield viscosity of less than 25,000 poise at 30°C.

41. A process for fracture toughening a resin which is cured by addition polymerization through its double bonds and has acid or anhydride functionality, said process comprising adding:
up to about 5% by weight of a very low viscosity hydroxy-terminated polydiene having a Brookfield viscosity of less than 25,000 poise at 30°C; and an at least one end esterified product of a very low viscosity hydroxy-terminated polydiene having a Brookfield viscosity of less than 25,000 poise at 30°C and an unsaturated diacid anhydride to said resin prior to cure.

42. A resin composition comprising a resin which is cured by addition polymerization through its double bonds and has acid or anhydride functionality, up to about 5% by weight of a first toughening agent comprising a very low viscosity hydroxy-terminated polydiene having a Brookfield viscosity of less than 25,000 poise at 30°C and a second toughening agent comprising an at least one end esterified product of a very low viscosity hydroxy-terminated polydiene having a Brookfield viscosity of less than 25,000 poise at 30°C and an unsaturated diacid anhydride.