Ind. Eng. Chem. Res. 1994,33, 2778-2788
2778
Toughening of Unsaturated Polyesters by Reactive Liquid Polymers. 2. Processibility and Mechanical Propertied Sandeepak B. Pandit and Vikas M. Nadkarni' Polymer Science and Engineering Group, Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
Six impact modifiers were prepared using the polyester adducts formed by reaction of maleic anhydride and poly(ethy1ene glycols) (PEGS) or poly(propy1ene glycols) (PPGs) of differing molecular weights. A number of toughened resins were prepared by blending different proportions of modifiers with commercially available general-purpose unsaturated polyester resin. The impact modifiers containing PEG-based modifiers were compatible with the resin, and those based on PPG were incompatible. The tensile strengths and modulii of the toughened resin compositions containg 10% loading of the incompatible modifiers were comparable to those of the base resin. The toughened resin compositions containing incompatible modifiers exhibited better tensile strengths and moduli. The resin toughness showed u p to 5-fold improvement with the use of the impact modifiers. Resin modified with compatible modifiers exhibited higher toughness. Dynamic mechanical properties confirmed compatibility behavior of modifiers. Introduction The toughening of unsaturated polyester (UP) resins has been of interest to many material scientists for the development of impact-resistant composites. Although several studies have been carried out to understand the toughening mechanisms in epoxy resin, detailed information about toughening of UP resins is not available. The improvement in impact resistance of UP resins cannot be readily achieved without an adverse effect on the deflection temperature, heat distortion temperature, stiffness, and flexural strength. It was observed that the impact modifiers which increase the toughness of epoxy resins by 10-50-fold could not toughen the UP resins (Siebert et al. 1972; Riew et al. 1976; McGarry et al., 1978). Some researchers have suggested that the difficulty in toughening UP composites is due to the poor reinforcement -resin adhesion (McGarry, 19861, while others have attributed this behavior to the poor modifier-resin compatibility which prevents formation of the desired morphology of the dispersed phase (Grossman, 1987). A number of recent studies on toughening of UP resins have therefore been focused on increasing the resin-modifier adhesion and on increasing the compatibility of the modifier in the resin t o obtain the desired morphology. Toughened UP resins have been synthesized by addition of carboxyl- or vinyl-terminated nitrile rubbers and specially designed polyurethanes (Park, 1990). The toughening of UP resins using chemical reaction, physical blending, and block copolymer formation has been investigated (Tong et al., 1987). The addition of polymer latex as impact modifier for UP resins has been reported (Kulick et al., 1992). A detailed study on the development of elastomeric additives for impact improvement in the UP resins has been recently published (Suspene, 1989). The objective of this work is to improve the impact resistance of UP resins through the use of reactive liquid polymers (RLP). The RLP's used were UP resins based on polyether diols such as poly(ethy1ene glycols) (PEG)
* Author to whom all correspondence should be addressed. Current address: Performance Polymers F'vt. Ltd., 10 Narendra Society, Senapati Bapat Rd., Pune 411 016, India. NCL Communication No. 5890. 0888-5885/94/2633-2778$04.50/0
and poly(propy1eneglycols) (PPG) of differing molecular weights reacted with maleic anhydride. The chain length of the polyether was varied in order to investigate its effect on the compatibility of the RLP with the UP resin and hence the toughening effectiveness. The synthesis and characterization of these impact modifier resins have been discussed in our earlier paper (Pandit and Nadkarni, 1993). In this paper, we report the processibility and the tensile, flexural, and dynamic mechanical properties of the toughened resins. In a subsequent paper the fracture characteristics, impact performance and fracture surface morphology of the toughened resins will be reported.
Experimental Section Synthesis. 1. Materials. PEGs of number-average of 200, 300, and 400 were molecular weights procured from MIS NOCIL India Ltd., and PPGs of 200, 300, and 850 were procured from W s Dai Ichi Karkaria Pvt.Ltd. (India). The polyether polyols, PEG and PPG, were characterized for acid value, hydroxyl value, and viscosity for quality control and then used. The results of these tests are reported in an earlier paper (Pandit and Nadkarni, 1993). General-purpose unsaturated polyester resin (GP resin), grade HSR 8112, was procured from Ws Bakelite Hylem Ltd. (India) as a 65% (wlw) solution in styrene. Styrene monomer was procured from MIS S. D. Fine Chemicals, Bombay (India). The styrene monomer contained about 150 ppm tert-butylcatechol as inhibitor. Styrene was used without further purification. Methyl ethyl ketone peroxide (MEW) was procured from MIS Bakelite Hylem Ltd. (India). Cobalt naphthenate (Co-naphthenate) procured from MIS Fluka, A. G. (Switzerland) had a density of 0.95 g/cm3 and cobalt content of 8%. 2. Polyester Adduct Synthesis. Two series of hydroxyl-terminated polyester adducts were synthesized by reacting 1 equv of maleic anhydride (MAN) with 2 equiv of PEGs of of 200, 300, and 400 (coded as PUP-2, PUP-3, and PUP-4, respectively) and PPGs of 200, 300, and 850 (coded as IM-2, IM-3, and IM-4, respectively). The reactants, namely, the preselected diol and maleic anhydride used for the synthesis of a given unsaturated polyester, were weighed into the
(E)
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0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 11,1994 2779 Table 1. Properties of PEGBased Polyester Adducta characterization PUP-2 appearance pale yellow colored viscous liquid 3350 viscosity at 27 "C (cP) 1.96 acid value (mg of KOWg of sample) corrected hydroxyl value (mg of KOWg of sample) 196.88 570 M,by end-group analysis 480 M, calcd on basis of stoichiometry of reactants 880 M, by VPO Table 2. Properties of PPGBased Polyester Adducts characterization IM-2 appearance pale yellow colored viscous liquid 6000 viscosity a t 27 "C (cP) 2.07 acid value (mg of KOWg of sample) 480 M, based on stoichiometry of reactants 554 M, by VPO
round-bottom flask. p-Toluenesulfonic acid catalyst at a concentration of 0.3% by moles of acid was added to the reaction mixture. Hydroquinone (200-300 ppm) was also added before the start of the reaction to inhibit the free-radical reaction of the anhydride that can take place during the polyester synthesis. The reactants were heated from room temperature to 100 "C a t a rate of about 3 "C/min with continuous stirring and nitrogen purge from the start. About 30 mL of xylene was added at the start of the reaction for azeotropic removal of water of condensation. After attainment of the temperature of 100 "C, the temperature of the reaction mixture was further raised to 180 "C by adjusting the heating rate to 1 "C/min. The temperature was then kept constant a t 180 "C for 2 h. The temperature at which the water of condensation starts coming out was recorded. The reaction temperature was then raised t o 200 "C a t the rate of 1 "C/min and kept constant for 3 h. The temperature was further raised to 220 "C a t a heating rate of 1 "C/min, and the acid value of the product was monitored. When the acid value of the product reached below 5 mg of KOWg of sample, the heating was discontinued. The reaction mixture was cooled to 180 "C and the reaction was then continued under reduced pressure of about 50 mmHg, to remove xylene and traces of water of condensation till an acid value of less than 2.0 mg of KOWg was reached. The vacuum was then released and the product was allowed to cool under stirring and inert atmosphere. The polyester was allowed to cool to about 60 "C in the reactor and then it was transferred to an amber-colored bottle by filtering through muslin cloth. Properties of the polyester adducts based on PEGS are listed in Table 1, and those of PPG-based adducts are listed in Table 2. 3. Impact Modifiers. A 65% (w/w) solution of the polyester adducts was prepared in styrene. The solutions of PUP-2, PUP-3, and PUP-4 were coded as 2P, 3P, and 4P, and the solutions of IM-2, IM-3, and IM-4 were coded as 21,31, and 41. These solutions were used as reactive liquid impact modifiers for improving the toughness of the GP resin. 4. Sample Preparation. The required quantities of the base resin and modifier were added to a 500 mL beaker and mixed thoroughly using a high-speed stirrer for about 1 min. MEKP initiator (1.5% of total resin weight) was then mixed thoroughly with the resin, for about 30 s. Co-naphthenate accelerator (0.5%of total resin weight) was subsequently added to this mixture.
PUP-3
PUP-4
pale yellow colored viscous liquid 2700 2.87 159.34 704 680 715
yellow colored viscous liquid 2550 1.37 124.95 898 880 895
IM-3
IM-4
brown colored viscous liquid 4900 2.50 680 715
dark brown colored viscous liquid 650 2.78 1780 1750
Table 3. Codes and Compositions of Toughened Resins GP resid GP resid modifier modifier code modifier (wlw) code modifier (wlw) GP2Il 21 90110 10010 GP 21 80120 90110 GP212 2P GP2Pl GP213 21 70130 2P 80120 GP2P2 GP214 21 60140 2P 70130 GP2P3 GP311 31 90110 2P 60140 GP2P4 31 80120 3P 90110 GP312 GP3P1 70130 80120 GP313 31 3P GP3P2 41 90110 70130 GP411 3P GP3P3 GP412 41 80120 4P 90110 GP4P1 41 70130 4P 80120 GP413 GP4P2
As the density of Co-naphthenate is lower (0.95) than that of the reaction mixture, it was first dispersed with the help of a spatula and then mixed thoroughly using a high-speed stirrer for 2 min. The resin mixture was then cast into open molds of desired dimensions. A series of toughened resins were prepared by blending the different impact modifiers with the base resin in various proportions. The formulations of the toughened UP resins and their code numbers are listed in Table 3. The modified resins were cured in the molds at room temperature for 24 h, followed by a postcure at 80 "C for 4 h. Characterization. 1. Processibility Parameters. The gel time of the UP resins was measured by using a Tecam gel timer at room temperature. The cutoff time was recorded as "gel time". All the gel time measurements were done in a 25 mL beaker containing 15 g of resin. The peak exotherm was measured by taking about 50 g of the mixture of resin, modifier, initiator, and accelerator in a 100 mL paper cup insulated from all sides by cotton (about 25 mm thick); a thermometer was placed at the center of the pot and the exotherm temperature was recorded. 2. Tensile Properties. The tensile test specimens were cast into a dumbbell-shaped mold designed as per DIN 53455 specifications. At least four different specimens were obtained for each formulation. The specimens were tested on an Instron tensile tester, Model 1122, at 27 "C. The rate of deformation was kept constant at 5 m d m i n (12.5% elongatiodmin). In this study, Young's modulus was calculated from the slope between two fixed strain values of 0.015 and 0.030. In order to quantify the change in stiffness due to incorporation of the impact modifier, the modulus retention was calculated as follows:
2780 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
modulus retention = tensile modulus of toughened resin x 100 (1) tensile modulus of base resin
]
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The area under the stress-strain curve is a measure of the toughness of the material. The improvement in the toughness achieved by incorporation of the impact modifiers in the polyester resin was calculated from the "toughness improvement factor" (TIF). TIF was defined as the ratio of the area under the stress-strain curve of the toughened resin to that of the base resin. 3. Flexural Properties. The flexural test specimens were prepared by casting the resin into a rectangular mold of dimensions 120 mm x 15 mm x 10 mm. The flexural testing was conducted using a three point bending assembly on the Instron tensile tester, Model 1122, at 27 "C. The span was 50 mm, and the crosshead speed was 0.5 m d m i n . The flexural strengths are reported as per ASTM D 790. The changes in the flexual modulus or strength of the toughened resin with respect to the base resin were determined using the following equation: flexural property retention = [flexural property of toughened resin x 100 (2) flexural property of base resin 4. Dynamic Mechanical Analysis (DMA). The dynamic mechanical properties of the toughened resin were determined on the Rheometrics RDS-7700 using test specimens of size 60 mm x 12 mm x 2-3 mm, over a temperature range of 25-175 "C in cure mode, at a constant heating rate of 10 "Clmin (heating done in air). The torsional strain amplitude was 0.3%, and the frequency was 100 rads.
]
Results and Discussion The base GP resin was characterized for acid value, hydroxyl value, viscosity and number-average molecular (by vapor pressure osmometry (VPO)). The weight, results of these tests are summarized in Table 4. lH NMR spectrum of the GP resin is shown in Figure 1. Due to the high molecular weight of the resin, broad peaks were observed. The peak observed at 3.24 ppm exchanges with D20; hence it is assigned to a terminal hydroxyl group. The signal for vinyl protons attached to a trans double bond (fumarate esters) was observed at 6.89 ppm, and a weak signal was observed a t 6.28 ppm due to vinyl protons attached to a cis double bond (maleate esters). This indicates that most of the double bonds present in the general-purpose polyester resin are in the trans conformation. Due to the presence of propylene glycol in the resin, a broad peak was observed at 1.35 ppm. The peak due to the protons present on the methylene carbon (CH2) adjacent to the ester was observed at 4.24 ppm. Aromatic protons were observed at 7.66 ppm, confirming the presence of aromatic acid such as phthalic anhydride. The overall signals suggest that the GP resin used in this study contained a polyester composed of propylene glycol, phthalic anhydride, and maleic anhydride or fumaric acid. In order t o compare the toughening properties of the modifiers with each other, the PEG- and MAN-based polyester adducts were prepared with a carefully controlled procedure (Pandit and Nadkarni, 1993) to obtain polyesters of following ideal structures.
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0 n = 3.14 for PEG 200, 5.41 for PEG 300, 7.68 for PEG 400
Figure 1. lH NMR spectrum of GP resin. Table 4. Characteristics of General-Purpose Unsaturated Polyester Resin appearance nonvolatile matter (NVM) (96) acid value (mg of KOWg of sample) hydroxyl value (mg of KOWg of sample) viscosity at 27 "C (cP) M, from end-group analysis
-
M, by VPO
viscous, light colored liquid 64.79 9.72 85.72 600 1304 1284
This ideal structure of polyester cannot be achieved by bulk polymerization. Therefore, all the polyester adducts were synthesized using the same heating cycle and by carefully monitoring the kinetics of polymerization (Uphade et al., 1994) to obtain uniform molecular weight distribution. The adducts were therefore thoroughly characterized using various techniques such as IR, NMR, and VPO (Pandit and Nadkarni, 1993). The properties of PEG-based adducts are listed in Table 1. However, it is evident that identical molecular weight distribution could not be achieved. The polyester adduct PUP-2 showed much higher molecular weight by VPO as compared to that obtained by end-group analysis. This is attributed to the formation of branched polymer due to the side reaction between hydroxyl group and the double bond, called the "Ordelt reaction" (Pandit and Nadkarni, 1993). The PPG-based polyester adducts based on PPG 200, PPG 300, and PPG 850 were synthesized and characterized using conditions similar to those used in the case of PEG-based adducts (Nadkarni and Pandit, 1992). 0
0
n = 2.14 for PPG 200, 3.86 for PPG 300, 14.4 for PPG 850
The properties of PPG-based adducts are listed in Table 2. It was observed that the PPG-based adducts had molecular weights equivalent to the idealized structures. All the PEG- and PPG-based polyester adducts were found to be completely miscible with styrene at all proportions. Impact modifiers for GP resin were therefore prepared by taking a 65% solution of these polyester adducts in styrene. These solutions were used for further studies. The base GP resin was mixed thoroughly with the impact modifiers in a 50/50 (w/w) proportion using a high-speed stirrer for 2 min, without the addition of M E W and Co-naphthenate. It was found that the PEG-based impact modifiers, namely, 2P, 3P, and 4P, were completely miscible with the base resin. No phase separation was observed after 24 h at room temperature. In the case of the PPG-based impact modifiers, namely, 21, 31, and 41, phase separation was observed. The time required for complete phase separation, based on visual observation, was found to be 6, 4'12, and 11/2 h for the 21, 31, and 41 modifiers, respectively. Thus, as the dangling chain length of the PPG segment increased, the modifier became increasingly more incompatible with the base resin. The network structures formed by incorporation of the modifiers based on PEG
Ind. Eng. Chem. Res., Vol. 33, No. 11,1994 2781 Table 5. Processing Characteristicsof Resins Toughened with Compatible Modifiers gel time (min) peak exotherm ("C) code GP 35.8 124 2P 37.1 114 GP2Pl 22.7 115 GP2P2 23.1 23.4 106 GP2P3 104 GP2P4 23.5 42.4 3P 108 GP3Pl 32.1 GP3P2 106 28.6 104 GP3P3 27.1 4P 40.3 31.8 114 GP4P1 108 25.8 GP4P2
( A ) Network cohtoininq compatible f n s * i t k r O ( o t lenqth scale of 100-200 A )
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Figure 2. Networks formed by the resins toughened with (A) compatible and (B)incompatible modifiers.
and PPG containing UP resins with the base resin were completely different due t o the differences in the compatibility of these modifiers with the resin. The compatible polyesters present in these modifiers get hooked onto the polystyrene chains, and the polyether segments of these polyesters remain as free dangling chains. Thus, in resins toughened with compatible modifiers 2P, 3P, and 4P, the dangling chains of PEG with molecular weights of 200,300, and 400 are uniformly dispersed at the molecular level within the base resin network. These dangling chains increase the free volume of the resin network and act as flexible segments for energy dissipation. The network structure resulting from the incorporation of these modifiers is represented schematically in Figure 2A. As a result of the molecular scale dispersion and incorporation of the compatible modifiers in the resin network, the resulting toughened resin samples were completely transparent. Due to incompatibility of the impact modifiers 21,31, and 41 with the base GP resin, a multiphase network was formed with the modifier domains dispersed in the resin network, as schematically illustrated in Figure 2B. Thus, the resins toughened with incompatible modifiers form a network structure similar to the impact-resistant UP resins studied by various other researchers (Cassola et al., 1988 and 1986). Due to the formation of discrete domains of the modifiers, the resin compositions containing incompatible modifiers, namely, 21, 31, and 41, were either translucent or opaque. The morphology of the modifier domains is governed by the interfacial chemical compatibility of the modifiers with the base resin, which affects the time for phase separation, and the dispersion level achieved prior to the onset of gelation. Therefore, the key parameters influencing the domain size and domain size distribution are gel time, speed of agitation, and duration of agitation. Hence, the speed and duration of agitation were kept constant during all the experiments. In both modifier series, the polystyrene bridges were formed between the modifier and the resin. Thus, the covalent linkages were formed between the matrix and
Table 6. Processing Characteristicsof Resins Toughened with Incompatible Modifiers code gel time (min) peak exotherm ("C) GP 35.8 124 21 32.0 GP2Il 21.3 115 GP212 20.6 110 18.3 102 GP213 GP214 18.9 99 31 39.7 GP311 26.8 110 GP312 25.6 102 GP313 25.1 99 41 57.1 111 GP411 25.7 GP412 26.3 106 GP413 29.6 100
the modifier. These linkages are expected to be much stronger than the polar interactions (seconary bonding) between the matrix and the physically dispersed elastomeric modifiers. The droplets of incompatible modifiers were formed in the resin during stirring; this allowed the covalent bond formation between the modifier and the resin network t o take place only at the surface of the modifier droplets. Thus, in the resins toughened with incompatible modifiers, weaker matrixmodifier bonding and less effective stress transfer between the matrix and the modifier were expected, as compared to the resins toughened with compatible modifiers. Processing Parameters. The processing parameters of the toughened resin compositions were investigated by determining the gel time and peak exotherm. These data are summarized in Tables 5 and 6. It was observed that the gel time of the base resin decreased considerably on incorporation of both compatible and incompatible modifiers in spite of the fact that the gel times of the modifiers were generally longer than that of the base resin. This clearly indicates an increased tendency of copolymerization of the resin with the modifier. However, quantitative information about this phenomenon could not be obtained because of the presence of the ternary mixture of the resin, modifier, and styrene during the curing of the polymer. The gel times of the resins toughened with 3P modifier were higher than the resins toughened at equivalent contents of the 2P modifier (see Figure 3). This is attributed to the lower concentration of the double bonds in the polyester present in the 3P modifier as compared to the 2P modifier. In the case of resins toughened with 2P modifier, the gel time decreased considerably at 10% modifier content. The gel time remained constant on further increase of modifier. In the case of resins toughened with 3P and 4P modifiers, the gel times decreased continuously as the concentration of the modifier was increased.
2782 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
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In the case of toughened resin compositions containing incompatible modifiers, it was observed that the gel times of the toughened resins decreased sharply after addition of 10% of the modifier and remained almost constant on further increase of the modifiers due to formation of modifier droplets (Table 6). Thus, the gel time decreased sharply for GP211, containing 10%of the 21 modifier, and at higher modifier contents the gel time remained constant around 19-20 min (Figure 4). The gel time for 21 modifier was 32 min, and that for the base resin was 35.8 min. This showed that there was an increased tendency toward copolymerization of the modifier resin and the base resin. In the case of 31 and 41 modifier containing resins, the gel times were in the range of 25-30 min, confirmingthe increased tendency toward copolymerization. An increase in the gel time was observed in the case of resins toughened with 31 and 41 modifiers as compared to resins toughened with 21 modifier. This is due to the decrease in the concentration of double bonds in the polyester adducts present in these modifiers and the formation of the modifier droplets in the base resin during gelation. The formation of the modifier droplet hinders copolymerization. In the case of resins toughened with the 41 modifier, a different trend was observed (Figure 4). The gel time decreased at 10% 41 modifier content, but further increase in the modifier content increased the gel time. This is attributed to the much lower concentration of the double bonds in the modifier 41 as compared to the other incompatible modifiers and larger droplet formation due to coalescence resulting from lower viscosity and higher incompatibility of the modifier with the base resin. The peak exotherm decreased with an increase in the compatible modifier content (Figure 5). This was observed because the number of reaction sites decrease with an increase in the modifier content. Since the concentration of double bonds in the modifiers was 2P > 3P > 4P, a similar trend was expected in the peak
30
MODIFIER CONTENT (%)
MODIFIER CONTENT (%)
Figure 3. Gel times of resins toughened with compatible modifiers.
20
Figure 4. Gel times of resins toughened with incompatible modifiers.
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30
MODIFER CONTENT
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( % )
Figure 5. Peak exotherms of resins toughened with compatible modifiers.
exotherm of toughened composition. It should be noted that such a trend would be observed if all the polyester adducts had similar molecular weight distributions. However, 4P-containing resins showed marginally higher peak exotherms than 3P-containing resins. This observation is concurrent with earlier studies (Pandit and Nadkarni, 1993) that 4P modifier contained PUP-4 which contains high molecular weight species and this leads to gel formation at lower cross-linking levels.
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2783 Table 7. Tensile Properties of Resins Toughened with Compatible Modifiers code
init modulus (kg/cm2)
re1 modulus
GP GP2Pl GP2P2 GP2P3 GP2P4 GP3Pl GP3P2 GP3P3 GP4P1 GP4P2
10098 9441 3795 3169 808 3627 3596 912 4736 3889
93.5 37.6 31.4 8.0 35.9 35.6 9.0 46.9 38.5
a
yield stress (kg/cm2)
yield strain (%)
plastic deformation (%)
372 192 110
5.2 5.3 4.2
5.4 10.5 11.4
193 117
4.5 5.0
7.6 7.5
196 92
4.9 4.4
7.1 12.8
tensile strength at breaka (kg/cm2)
elongation at break (%)
TIFb
342(12) 319(19) 217(7) 144(3) 108(1) 198(1) 137(2) 102(1) 216(4) 173(3)
3.7 10.6 15.8 15.6 27.0 12.1 12.4 14.0 12.1 17.1
4.6 4.4 2.9 3.4 3.0 2.0 1.7 3.3 3.8
Figures in parentheses represent standard deviation. * Toughness improvement factor. 350 GP GPZW GP2R GPZPI GP2P4
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Figure 6. Stress-strain behavior of 2P-toughened resins.
Figure 7. Stress-strain behavior of 3P-toughened resins.
Once the gel is formed heat dissipation decreases rapidly resulting in greater entrapment of heat in the resin. The peak exotherm of the base resin decreased on incorporation of all three incompatible modifiers (Table 6). This is attributed to the reduction in the crosslinking sites due to low concentration of the double bonds in the modifiers as compared to the base resin. The cured products of resins toughened with compatible modifiers 2P, 3P, and 4P were transparent a t all modifier contents, and those containing 21 modifier were translucent at modifier concentrations up to 40%. The resins toughened with 31 and 41 modifiers were opaque at all modifier contents. This showed that 31 and 41 modifier containing resins had macrodomains (of the order of a few microns) of the modifier phase and the domain size of the modifier in the resin toughened with 21 modifier was very small. Tensile Properties. The results of the tensile properties of resin compositions toughened with compatible modifier are summarized in Table 7. It was observed that the stress at break and the tensile modulus decreased, whereas elongation a t break increased as the content of the compatible modifier was increased. The mechanical properties of the toughened resins containing 10% of 2P modifier were comparable to that of the base resin. However, at 10% loading of 3P and 4P modifiers the mechanical properties of the toughened resins were found t o be considerably lower. Figure 6 shows the stress-strain curves of the general-purpose polyester resins containing various amounts of the 2P modifier. The base resin exhibited brittle failure, and no yield point was observed. The addition of 10% 2P modifier (GPBP1) led to the occurrence of yield point at a stress value higher than the breaking tensile strength of the base resin. The elongation of the polymer beyond yield point is termed as plastic deformation. The plastic deformation provides
a mechanism for energy dissipation and thus normally leads to higher impact resistance. The plastic deformation of GP2Pl was 5.37%. This indicates that GP2Pl will exhibit better toughness and impact properties. Further increase in the modifier content resulted in a considerable decrease in the tensile yield stress and initial modulus, and an increase in elongation a t break. Although the yield stress decreased sharply at higher concentrations of 2P modifier (r20%), the yield strain remained constant around 0.04-0.05. The plastic deformation increased sharply from 5.37% for GP2Pl to 11.48% for GP2P3. Toughness improvement factor (TIF) showed a 5-fold increase in toughness of the base resin at 10% 2P modifier content. Higher modifier concentration did not result in any further increase in the toughness due to lowering of the initial modulus and stress a t break. The 3P-modified resins showed considerably lower breaking stress and initial modulus than the 2Pmodified resins at equivalent loading (Figure 7, Table 7). The plastic deformation remained constant around 7-8%. Only a 2-3-fold increase in the toughness of the base resin could be achieved with 3P modifier. It was observed that increase in TIF decreases with increase in the modifier content. Thus, the incorporation of PEG 300 dangling chains led to greater plasticization of the resin. Figure 8 shows the stress-strain behavior of the resins toughened with various amounts of 4P modifier. The 4P modifier contains a higher molecular weight polyester which led to a decrease in the number of dangling chains incorporated in the resin network. Thus, the properties of these resins were better than the resin compositions containing 3P modifier at equivalent loading. The mechanical properties of the base resin such as stress at break and modulus decreased as the compatible modifier content was increased (Figures 9 and 10).
2784 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
GP GP4PI GPLPZ
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Figure 8. Stress-strain behavior of 4P-toughened resins.
WLYETHER CHAIN IN THE NETWORK ( d h c t i v i crosslink) DANGLING CHAIN OF W L Y E T H E R
RESIN
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Figure 11. Formation of effective cross-links by higher molecular weight polyesters.
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30 MODiFlER CONTENT (%I
Figure 10. Dependence of modulus of the toughened resins on compatible modifier content.
The main difference between the modifiers, namely, 2P, 3P, and 4P, was the length of the PEG dangling chain. It was observed that the initial modulus decreased as the dangling chain length of PEG was increased from 200 t o 300, at a fixed modifier concentration. However, this effect was not observed when the PEG dangling chain length was increased from 300 to 400. This behavior was attributed to the differences in molecular
weight distribution present in the polyester PUP-4 present in the 4P modifier. The polyester PUP-4 had a higher fraction of the molecular species containing more than one double bond per molecular (Pandit and Nadkarni, 1993). This led to the incorporation of two sites of unsaturation per molecule in the unsaturated polyester matrix, schematically shown in Figure 11. This kind of tieup led to incorporation of some of the PEG chains into the base resin network, thereby decreasing the concentration of the dangling chains in the toughened resin. The 2P modifier containing resins exhibited the highest toughness improvement due to the incorporation of the shortest dangling chains of 200 molecular weight. When the increase in TIF was compared with the length of PEG dangling chains incorporated in the resin matrix, a minimum was observed at the PEG dangling chain length of 300 molecular weight (for the 3P modifier containing resins) at modifier concentrations of 10 and 20% (Figure 12). This observation could be due to the presence of higher molecular weight species in the 4P modifier. The dependence of the tensile modulus of the toughened resins on dangling PEG chain length showed a similar trend (Figure 13). The results of the tensile properties of resin compositions toughened with incompatible modifiers are summarized in Table 8. It was observed that the stress at break and tensile modulus decreased and the elongation at break increased as the incompatible modifier content was increased. However, the tensile strengths and moduli of the toughened resins at 10% loading of incompatible modifiers were equivalent to those of the base resin. The stress-strain behavior of the resins toughened with 21 modifier is shown in Figure 14. The resin toughened with 10% 21 modifier (GP211) showed a yield stress value comparable to the breaking stress of the base resin, with subsequent plastic deformation. Beyond 10% loading, the modulus and breaking stress dropped significantly with increase in the modifier content. The significant drop in the mechanical performance of the resin at modifier loading of more than
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2786
loading of the 41 modifier. This shows that the addition 5 ’ 0 of 31 and 41 modifiers did not lead to increased flexibility
t
i
200
I
l
ow
300
DANGLING CHAIN LENGTH
Figure 12. Effect of PEG dangling chain lengths on the toughness of the resin at modifier contents of 10 (A) and 20% (0).
\ 0,
5
6000
v)
3 J
2
4,500
0
I 3,000
1,x)Ol
I
2 00
I
300
I
600
DANGLING CHAIN LENGTH
Figure 13. Effect of PEG dangling chain lengths on the modulus of the resin at modifier contents of 10 (0)and 20% ( 0 ) .
20% may be due to poor dispersion resulting from coalescence of the modifier droplets. It was observed that the plastic deformation increased with an increase in the modifier content. This shows that the addition of 21 modifier in the resin led to increased flexibility. TIF of the resins toughened with 21 modifiers was around 3.5-5 for modifier concentrations between 10 and 40%. In the case of resins toughened with 31 and 41 modifiers, similar trends were observed (Figures 15 and 16). At 10% modifier concentrations, the yield stress was slightly lower than that of the base resin. However, the stiffness of the toughened resin was comparable to that of the base resin. Beyond a modifier content of lo%, the toughened resins showed considerable loss in modulus. This may be due to the poor dispersion of these modifiers in the resin which led to coalescence of the modifier droplets. The plastic deformation observed for 31 and 41 modifiers was about 1-2%, except a t 30%
of the resin matrix. At 10% loading, the toughness of 31-modified resin showed a 2.5-fold increase, while 41modified resin showed only a 1.5-fold increase. The resins toughened with higher concentrations (> 10%)of 31 and 41 modifiers did not show any appreciable increase in toughness. This observation is in agreement with the plastic deformation data. The changes in toughness of the modified resins with respect to the dangling chain lengths of the PPG at 10 and 20% modification are shown in Figure 17. The improvement in the toughness was maximum at the PPG dangling chain length of 200 molecular weight, due t o the partial solubility of the 21 modifier in the base resin which led to increase in the free volume of the base resin, finer dispersion of 21 modifier domains, and better interfacial stress transfer. Thus, a partially miscible modifier 21 is more effective than the fully incompatible modifiers 31 and 41. It was observed that the mechanical properties of resins toughened with a compatible modifier were much lower than those of the incompatible modifier toughened resins at equivalent loading. The tensile properties of resins toughened with 10% loading of 2P modifier and all the incompatible modifiers were comparable to that of the base resin. The tensile strength at break and modulus decreased with increase in both compatible and incompatible modifier concentrations. However, both these properties decreased more sharply in the case of resins toughened with compatible modifiers. The elongation a t break increased with increased loading of all the compatible modifiers and 21 modifier, while the resins toughened with 31 and 41 modifiers did not show appreciable change, at different modifier concentrations. The resins toughened with compatible modifiers and 21 modifier exhibited higher toughness improvement and higher plastic deformations as compared to the 31 and 41 modifier containing toughened resins. Flexural Modulus. The flexural properties of resins toughened with compatible modifiers are summarized in Table 9. The addition of compatible modifiers such as 2P, 3P, and 4P decreased both the flexural strength and the flexural modulus. It was observed that, beyond 10% modifier content, the flexural strength of the toughened resin was halved progressively with every 10% further addition of the modifier. A similar trend was observed for the flexural modulus of these resins. The addition of 10% of 2P, 3P, and 4P modifiers to the base resin reduced the base resin flexural strength by 34, 60, and 78%, respectively. This shows that the flexural strength of the resin is very sensitive t o the length of the dangling chain. The best properties were obtained for the resins toughened with 2P modifier which had the shortest dangling chain of the PEG. However, unlike tensile testing, the properties of resins toughened with 4P modifier showed poorer properties as compared to the resins toughened with 3P modifier. This was due to the fact that the flexural test is not very sensitive t o the degree of cross-linking, but highly sensitive to the chain length of the monomer used in unsaturated polymer backbone (Boenig, 1964). The flexural energy dissipation would be dependent on the efficiency of energy dissipation by the dangling chains of polyether, which is proportional to its length and the concentration of these chains in the resin matrix, which is proportional to the concentration of modifier in the toughened resin composition. A loss factor which represents the extent of energy dissipated by the incorporation of the modifier was therefore
2786 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 Table 8. Tensile Properties of Resins Toughened with Incompatible Modifiers tensile strength yield plastic init modulus re1 yield stress at breaka (kg/cm2) modulus strain (%) deformation (%) code (kg/cm2) (kg/cm2) 342(12) GP 10098 310(7) GP2Il 103.6 352 5.9 3.4 10488 229(6) GP212 57.9 239 6.5 9.4 5842 11.5 162(6) GP213 4247 42.1 6.8 145 130(10) GP214 12.6 1276 5.7 91.7 312 1.4 303(3) GP311 9258 190 1.6 5.0 194(12) 39.8 GP312 4018 1.8 6.9 139(4) GP313 24.4 135 2469 4.7 0.5 312 308(4) GP411 9962 98.7 1.2 191 6.5 185(8) GP412 6894 68.3 9.6 112 5.2 133(6) 16.4 GP413 1659 a
elongation at break (%I 3.7 9.3 15.9 18.3 31.0 7.1 6.6 8.7 5.2 7.7 14.8
TIFb 3.8 4.9 3.6 4.4 2.3 1.0 1.3 1.6 1.5 2.3
Figures in parentheses represent standard deviation. Toughness improvement factor.
3501 /
-.
GP - 0 GP211 GPPI2-o GP213-• GP2L4-A
I
300
300
GP
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- a
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01
$
150
100
50 50
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0.08
0.12
0 16
0 20
0 24
/
,
0 28
STR4lN
002
004
006
008
012
010
014
STRAIN
Figure 14. Stress-strain behavior of 21-toughened resins.
Figure 16. Stress-strain behavior of 41-toughened resins.
GP
- 0
-
GP 311 GP312-0
GP313-•
0 02
0 04
0 06
0 08
STR4lN
Figure 15. Stress-strain behavior of 31-toughened resins.
defined as follows:
loss factor = C x L
(3)
where C represents concentration of the modifier in the toughened resin composition and L represents the length of dangling PEG chain. The loss factor was plotted against the retentions of flexural strength and flexural modulus of the toughened resins (Figures 18 and 19, respectively). It was observed that, in spite of the presence of different modifiers with different dangling chain lengths, retentions of both flexural strength and modulus fall on a single line. This shows that the loss in the flexural property is a function of both the concentration and length of the dangling chains present in the toughened resin matrix. The flexural properties of resins toughened with incompatible modifiers are summarized in Table 10. The addition of all the incompatible modifiers decreased both the flexural modulus and the flexural strength. The flexural strengths and moduli of the resins toughened
01 0
I
200
1
I
I
400 600 800 CHAIN LENGTH OF PPG
'
Figure 17. Effect of PPG dangling chain on the toughness of the resins modified with incompatible modifiers at 10 (0) and 20% ( 0 )modifier content.
with all the incompatible modifiers were equivalent at 10%modifier loading. This clearly indicates that small particles of the incompatible modifiers were present in the GP resin matrix and they formed a discontinuous phase. The flexural strengths and moduli of the resins modified with 31 and 41 modifiers showed considerably higher standard deviation. This may be attributed to a broader droplet size distribution of these modifiers in the toughened resin. The flexural strengths and moduli of the toughened resins containing higher than 10% loading of these modifiers did not change appreciably with the increase in the dangling chain length.
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2787 Table 9. Flexural Properties of Resins Toughened with Compatible Modifiers flexural flexural flexural flexural modulusa modulus strengtha strength code (mN/m2) retention (%) (mN/m2) retention (%) GP 891(40) 51.U1.2) 631(31) 71 33.7(0.7) 66 GP2Pl 326(27) 37 16.2(0.5) 32 GP2P2 163(3) 18 8.5(0.2) 17 GP2P3 80(4) 9 4.0(0.1) 8 GP2P4 GP3P1 385(4) 43 20.4(0.4) 40 20 lO.l(O.7) 20 GP3P2 180(2) 99(1) 11 4.9(0.1) 10 GP3P3 209(15) 23 ll.l(O.7) 22 GP4Pl 108(1) 12 5.2(0.1) 10 GP4P2 a
1oc 1
2
0
k
Figures in parentheses represent standard deviation.
0 0
2000
4000 6000 LOSS FACTOR
0000
1C 30
Figure 19. Effect of loss factor on flexural modulus retention.
'I 01
0
I
I
I
I
2000
4000
6000
0000
I
10000
LOSS FACTOR Figure 18. Effect of loss factor on flexural strength retention.
The resins toughened with incompatible modifiers such as 21,31, and 41 showed higher flexural strengths and moduli than the resins containing equivalent loading of compatible modifiers. It was evident that the effect of all the modifiers on both the flexural strength and flexural modulus was comparable and thus the flexural properties depend mainly on the concentration of the continuous matrix. Thus, a significant change in the flexural properties was observed with the increase in the modifier concentration. The flexural properties of the toughened resins are less sensitive to the dispersion of the modifier than tensile properties. Dynamic Mechanical Analysis (DMA). The variation in dynamic mechanical properties like storage modulus ( G I , loss modulus (GI,and tan 6 of the toughened polyester resins were studied over a temperature range of 25-175 "C. The results are listed in Table 11 for resins toughened with compatible modifiers. The storage modulus G in the glassy region decreased as the concentration and the length of the PEG dangling chains were increased. Thus, the dangling chains of PEG work effectively as an energy-dissipating moiety. The resin toughened with 10% 2P modifier showed a shear modulus ( G ) comparable t o that of the base resin. The curve of loss modulus G did not show a maximum
Table 10. Flexural Properties of Resins Toughened with Incompatible Modifiers flexural flexural flexural flexural modulusa modulus strengtha strength code (mN/m2) retention (%) (mN/m2) retention (95) GP 891(40) 51.1(1.2) GP211 742(1) 83 42.0(0.0) 82 GP212 431(7) 48 23.5(0.4) 46 253(6) 28 12.7(0.2) 25 GP213 92(1) 10 4.3(0.0) 8 GP214 GP3Il 740(51) 83 39.1(0.7) 76 GP312 403(9) 45 22.4(0.1) 44 252(5) 28 12.4(0.1) 24 GP313 697(40) 78 37.8(3.8) 74 GP411 GP412 428(8) 48 24.6(0.9) 48 GP413 284(3) 32 8.0(0.1) 16 Figures in parentheses represent standard deviation.
Table 11. Dynamic Mechanical Properties of Resins Toughened with ComDatible Modifiers code G at 27 "C (dyn/cm2) tan d,, Tg ("C) 1.04 x 1O1O GP 103 0.94 1.07 x 1O1O 0.85 GP2Il 99 7.78 109 GP2P2 92 0.73 5.59 109 81 GP2P3 0.68 2.83 109 78 GP2P4 0.60 8.72 x 109 GP3Pl 0.83 86 5.01 109 84 GP3P2 0.73 6.77 x 109 GP4Pl 0.83 88 4.96 x 109 86 GP4P2 0.72 ~
in the selected temperature range, and hence the values of G , , could not be obtained. The tan 6 peak temperature of the toughened resins was defined as the glass transition temperature (T& The Tg of the toughened resins decreased with an increase in the compatible modifier content. Thus, the onset of main chain mobility requires less energy in the toughened resin due to an increase in the free volume and plasticization of the polyester network by incorporation of the PEG dangling chains. This effect of the modifier content on Tgbecame more pronounced as the dangling chain length of PEG was increased. The results of the dynamic mechanical testing of resins toughened with incompatible modifiers are pre-
2788 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 Table 12. Dynamic Mechanical Properties of Resins Toughened with Incompatible Modifiers code G at 27 "C (dvn/cm*) Te ("C) tan d,, 1.04 x 1O1O 0.94 GP 103 GP2Il 1.07 x 1O1O 91 0.89 0.82 GP212 89 8.64 109 6.57 109 86 0.76 GP213 80 6.04 x 109 0.66 GP214 8.44 x loy 0.91 91 GP311 92 0.86 6.92 x 109 GP312 0.83 3.96 109 93 GP313 6.52 x loy 92 GP411 0.91 6.29 x 109 89 0.86 GP412 5.01 x 109 0.83 82 GP413
to those of resins toughened with compatible modifiers. Dynamic mechanical properties showed that the Tgof the resins toughened with compatible modifiers and 21 modifier decreased gradually with increasing modifier content. This confirmed that the PEG-based adducts were completely miscible and the adduct based on PPG of molecular weight 200 was partially miscible with the base resin. The Tgof the toughened resins containing 31 and 41 modifiers remained constant around 90 "C. Thus the adducts based on PPG 300 and PPG 850 were incompatible with the base resin.
sented in Table 12. The G m a l of r these toughened resins could not be obtained in the selected temperature range. The value of storage modulus ( G ) in the glassy region a t a fixed temperature of 27 "C decreased continuously with an increase in the 21 modifier content, indicating partial solubility in the resin matrix which allowed greater energy dissipation as a result of increased free volume. A considerable decrease in the G' at 27 "C was observed at 10% loading of 31 and 41 modifiers, as compared to the 10% 21 modified resin (GP211). This is observed due to the increased segmental motion of the longer dangling chains of PPG that were incorporated by 31 and 41 modifiers. The variation of tan 6 with temperature showed that the Tg of the resins toughened with 21 modifiers decreased with increasing modifier content. This shows that the 21 modifier is partially miscible with the base resin. The Tgof the resins toughened with 31 and 41 modifiers was observed a t about 90 "C, except in the case of the resin toughened with 30% 41 modifier. This showed that 31 and 41 modifiers are not miscible with the GP resin. Thus, increasing 31 and 41 modifier content did not affect the main chain motion of the base resin.
Boenig, H. V. In Unsaturated Polyesters: Structure and Properties; Elsevier: Amsterdam, 1964; Chapter 6, 83-91. Cassola, A,; Kwok, J. C.; Robinson, K. J.; Smith, B. H. A,; Longuet, M. Elastomer Containing Thermosetting Polyester Compositions. U S . Patent Appl. 4,877,832 Oct 29, 1986. Cassola, A.; Kwok, J. C.; Robinson, K. J.;Smith, B. H. A.; Longuet, M. Impact Resistant Polyester Graft Polymer Blends. Eur. Pat. Appl. EP 265,900, May 4, 1988. Grossman, R. F. Blends of Unsaturated Polyesters with HighMolecular-Weight Elastomer Bearing Reactive Functional Groups. In Rubber Toughened Plastics; Riew, C. K., Ed.; Advances in Chemistry Series 222; American Chemical Society: Washington, DC, 1987; pp 415-426. Kulick, S. G.; Babayevsky, P. G.; Stepanova, M. I. Toughened Unsaturated PolyesteriLatex Compositions with Low Shrinkage. Polym. Prepr. 1992, 33 (11, 386-387. McGarry, F. J. In Rubber in Crosslinked Glassy Polymers; Rubber Division, America1 Chemical Society: Washington, DC, 1986. McGarry, F. J.; Rowe, E. H.; Riew, C. K. Improving the Crack Resistance of Bulk Molding Compounds and Sheet Molding Compounds. Polym. Eng. Sci. 1978, 18 (21, 78-86. Nadkarni, V. M.; Pandit, S. B. An improved process for synthesis of reactive liquid polymers. Indian Patent Application No. 6131 DEIJ92. Pandit, S. B.; Nadkarni, V. M. Toughening of Unsaturated Polyesters by Reactive Liquid Polymers. 1. Synthesis and Characterization of the Modifiers. Ind. Eng. Chem. Res. 1993, 32 (121, 3089-3099. Park, C. E. Rubber Toughening of Unsaturated Polyester Resins. Pollimo 1990, 14 (31, 266-272. Riew, C. K.; Rowe, E. H.; Siebert, A. R. Rubber Toughened Thermosets. In Toughness and Brittleness of Plastics; Deanin, R. D., Crugnola, A. M., Eds.; Advances in Chemistry Series 154; American Chemical Society: Washington, DC, 1976. Siebert, A. R.; Rowe, E. H.; Riew, C. K. Toughening of Polyester Resins with Liquid Rubbers. Annu. Conf. SPI, 1972,27th. Suspene, L. Development of an Elastomeric Additive to Improve Impact Resistance of Unsaturated Polyester Networks. Sci. Tech. Aerosp. Rep. 1989, ISAL-89-0006,ETN-90-95514,Abs. No. N90-16073, 249 pp. Tong, S. N.; Wu, P. T. K. Supertough Unsaturated Polyester and its Sheet Moulding Compound (SMC). J . Reinf. Plast. Compos. 1990, 9 (31, 299-311. Uphade, B. S.; Patil, P. S.; Pandit, S. B.; Rajan, C. R.; Nadkarni, V. M. Kinetics of Polyesterification: Effect of Diol Chain Length. J . Polym. Sci. Part A . Polym. Chem. 1994, in press.
Conclusions It was observed that the impact modifiers containing PEG-based polyester adducts were compatible and those based on PPG were incompatible with the base resin. The gel time of these resin formulations showed that there is increased copolymerization tendency between the adducts and the resin. The tensile strengths and moduli of the toughened resin compositions containing 2P modifier and all the incompatible modifiers, a t 10% loading, were comparable to those of the base resin. Due to incompatibility of the adducts based on PPG 300 and PPG 850, the resins toughened with 10% of these modifiers exhibited tensile strengths and moduli far superior than the adducts based on PEG 300 and PEG 400. Toughened resin compositions containing incompatible modifiers exhibited better tensile strengths and moduli as compared to the resins toughened with compatible modifiers. The toughness, as indicated by the area under the stress-strain curve, showed that up to a 5-fold increase in the toughness could be achieved. The flexural strengths and moduli of the toughened resins containing incompatible modifiers were superior
Literature Cited
Received for review November 16, 1993 Revised manuscript received April 13, 1994 Accepted May 10, 1994@ Abstract published in Advance ACS Abstracts, September 1, 1994. @
