I n d . Eng. C h e m . Res. 1988,27, 2056-2060
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MATERIALS AND INTERFACES Unsaturated Polyester Resins from Poly(ethy1ene terephthalate) Waste. 2. Mechanical and Dynamic Mechanical Properties? Utpal
R. Vaidya and Vikas M.Nadkarni*
Polymer Science and Engineering Group, Chemical Engineering Division, National Chemical Laboratory, Pune 41 1 008, India
Poly(ethy1ene terephthalate) (PET) waste was depolymerized using different amounts of propylene glycol. The oligomeric diols thus obtained were reacted with maleic anhydride to get unsaturated polyesters. These polyesters were miscible with styrene and could be cured using peroxide initiators. The mechanical and dynamic mechanical properties were compared with the conventional general purpose resins. The mechanical properties of these resins were found to be comparable to the general purpose resin. The tensile strength was about 24 N/mm2. The flexural strength was between 20 and 40 N/mm2. The impact strength ranged between 1.5and 2.5 kJ/m2. The hardness was measured to be 80 on a Shore D durometer. These resins exhibited better heat distortion temperatures (80-85 "C) compared to the general purpose resin (65 "C). The dynamic mechanical properties showed marginal dependence on frequency ranging between 0.1 and 1000 rad/s. The viscoelastic behavior was also investigated over a temperature range of 30-150 "C and was found to be comparable to that of general purpose resin. Unsaturated polyester (UP) resins represent the most widely used polymer matrix in glass fiber reinforced composites. The main reason for the widespread use of UP resins is the broad spectrum of properties offered by them through proper selection of the monomeric building blocks. The general purpose resins are prepared by reacting maleic anhydride and phthalic anhydride with propylene glycol (Parkyn et al., 1967). The other common monomers include neopentyl glycol, bisphenol A, isophthalic acid, adipic acid, etc. The effects of different monomers on the properties of the resins are well studied (Boenig, 1969). The possibility of synthesizing UP resins having terephthalic moiety in the chain backbone is discussed in an earlier publication by us (Vaidya and Nadkarni, 1987a). It was demonstrated that incorporation of the terephthalic acid moiety can be achieved by synthesis of UP resins from glycolyzed PET waste. The processing characteristics of these novel polyester resins, such as viscosity and gelation behavior, were also investigated and discussed in that publication. The kinetics of the polyesterification reactions have been reported in another publication by us (Vaidya and Nadkarni, 1987b). In the present work, the static mechanical properties of the resins, prepared from PET waste, are compared with those of the existing grades of general purpose UP resins. In a number of engineering applications, the material may be subjected to dynamic stress conditions. The mechanical properties of the material under static loading may not fully describe its performance in such applications. Therefore, the cured samples of the resins were also investigated for their dynamic mechanical behavior. There is limited published literature on the viscoelastic or dynamic mechanical characteristics of unsaturated polyester
* To whom correspondence should be addressed. NCL Communication No. 4295.
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resins, although the viscoelastic behavior has been known to be sensitive to changes in the chemical structure. The results of the present investigation are thus relevant to assessing the application potential of the PET waste based UP resins. Experimental Section The fiber-grade PET waste, obtained from M/s Century Enka Ltd. (India), was glycolyzed with propylene glycol at three different weight ratios using 0.5% w f w zinc acetate, based on the amount of PET waste, as the catalyst. The reaction was carried out at about 200 "C under reflux for 8 h in a nitrogen atmosphere. The reactor used was a 2-L round-bottom glass reactor. Thus, three grades of the glycolyzed waste, coded, GPET-1, -2, and -3, were prepared by glycolyzing PET with 37.5%, 50%, and 62.5% w/w propylene glycol. These oligomeric diols, GPET-1, -2, and -3, were reacted with maleic anhydride at the hydroxyl-to-carboxyl group ratio of 1.1 in order to produce three grades of the unsaturated polyesters, coded UVMW-57, UVMW-58, and UVMW-59, respectively. These resins were then blended with styrene monomer. The experimental details of the glycolysis, polyesterification of glycolyzed PET, and characterization of unsaturated polyesters are reported in the earlier publication by the authors (Vaidya and Nadkarni, 1987a). The control resins used for comparison were an improved grade of the general purpose (GP) resin, UPM-121, supplied by Dr. Beck and Co. (India) and an ordinary grade GP resin obtained from a local supplier. The resins were analyzed for the amount of nonvolatile matter by evaporating the styrene under vacuum at 80 "C. The molecular weights of the resins, without styrene monomer, were determined by using a Knauer vapor pressure osmometer. The temperature of the chamber was 50 "C. Ethyl acetate was used as solvent and benzil as a standard. 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2057 Table I. Characterization of Polyesters styrene '70, polyester M,,(VPO) w/w UVMW-57 1045 36 UVMW-58 1325 36 UVMW-59 1269 36 G P ordinary 1300 32 G P improved 1977 30
and 1 X lo3 rad/s, and the temperature sweep was taken at 100 rad/s over a temperature range of 30-150 OC.
viscosity a t 31 O C , CP 320 790 575 540 810
The viscosities of the unsaturated polyesters were determined at 31 "C by using a Brookfield viscometer. It is well-known that, during the high-temperature polyesterification reaction, the maleate moiety isomerizes into fumarate moiety. The relative concentrations of the maleate and fumarate moieties were determined by FT NMR analysis of the samples. The samples for NMR were prepared in acetone-d6. The test specimens for mechanical and dynamic mechanical testing were prepared by casting the resin into the molds, shaped as per the relevant specifications. The curing was done using methyl ethyl ketone peroxide (MEW)and cobalt naphthenate. MEKP was in the form of 50% solution and cobalt naphthenate contained about 8% cobalt. The amount of MEKP used ranged between 0.5% and 1.5% w/w, whereas the amount of cobalt naphthenate used ranged between 0.2% and 0.5% w/w. The amounts of the initiator and accelerator used were carefully chosen so as to generate a comparable exotherm profile (peak temperature about 110 "C). The specimens were cured for 16 h a t room temperature and 4 h at 80 "C. The mechanical testing of the resins was done at Dr. Beck and Co. (India). The measurements were carried out under identical conditions. The test methods used for static mechanical testing of the unsaturated polyester resins are listed as follows: tensile strength, DIN 53455; impact strength, DIN 53453 (unnotched); flexural strength, DIN 53452; heat distortion temperature, DIN 53458 (Martin's); and hardness, DIN 53505 (Shore D). The test specimens for dynamic mechanical testing were cast to conform to the following dimensions: length = 60 mm, breadth = 12 mm, and thickness = 2 mm. All the specimens were tested on the Rheometrics dynamic spectrometer, Model RDS-7700. Besides the PET waste based resins, the improved and ordinary grades of the general purpose resin were also tested. The effect of the amount of styrene on dynamic mechanical properties was studied by casting the specimens with UPM-121 (improved grade GP resin) having different amounts of styrene. The mean profile curves were obtained from the test curves, run in quadruplicate for each resin grade. The frequency sweep was taken at room temperature (25 "C) between 1 X lo-'
Results and Discussion The data on styrene amounts, resin viscosities, and molecular weights of the UP resins are summarized in Table I. The improved grade GP resin contained 30% styrene and the ordinary grade contained 32% styrene, whereas the PET waste based resins had 36% styrene for cross-linking. The molecular weight of the improved grade resin is significantly higher than those of ordinary grade and the PET waste based resins. It is well-known that, during polyesterification of maleic anhydride at high temperature, a part of maleic (cis) unsaturation isomerizes into fumarate (trans) form. The extent of isomerization is a function of temperature and time of the polyesterification reaction (Vancso Szmercsanyi et al., 1961; Curtis et al., 1964). The isomerization has a profound effect on the mechanical properties of the unsaturated polyesters (Boenig, 1969). The fumaric double bond reacts more readily with styrene than the maleic double bond, thereby giving a higher cross-linking density than the corresponding maleic unsaturation. Therefore, it is essential to determine the extent of the cis-trans isomerization. The extent of isomerization can be determined by NMR analysis (Curtis et al., 1964). The FT NMR of the unsaturated polyester resins show signals for maleate hydrogens (cis) and fumarate hydrogens (trans) at 6 values of 6.4 and 6.8, respectively. In all the resins, the fumarate form was found to be above 96% of the total unsaturation. It is interesting to note that in spite of the shorter reaction times for PET based resins (Vaidya and Nadkami, 1987b)the extent of isomerization is comparable to that in the GP resins. The static mechanical properties of the resins are summarized in Table 11. Although the properties of the PET waste based resins were comparable to those of the ordinary GP grade, the improved GP grade supplied by Dr. Beck and Co. (India) exhibited better mechanical properties. The heat distortion temperatures of the PET waste based resins are much higher than those for both grades of the GP resin. Table I11 shows the composition of resins. Among the three grades of the PET waste based resins, the flexural strength seems to increase with increasing molecular weight. The trend for impact strength and hardness is as per the trend found in the literature (Boenig, 1969). However, the published data deal with a comparison of the orthophthalic polyester with isophthalic acid based polyester and not with the terephthalic acid based systems. The heat distortion temperatures of the PET waste based resins are much higher than those of the GP
Table 11. Static Mechanical ProDerties of the Unsaturated Polyesters GP test tensile strength, N/mm2 impact strength, kJ/m2 flexural strength, N/mm2 hardness (Shore D) heat distortion temp, O C
UVMW-57 23.3 1.6 24.5 80 85
UVMW-58 23.4 2.1 38.4 80 82
UVMW-59 23.9 2.4 27.2 80 81
improved 51 4 83 83 65
ordinary 19.6 2.2 40.1 78 55
Table 111. Chemical Constituents of the UP Resins polyester UVMW-57 UVMW-58 UVMW-59 GP resin
terephthalic acid 32 23 16
percentage of moiety in the chain phthalic acid maleic acid ethylene glycol 26 14 10.6 32 39 7 34 (approx.) 22 (approx.)
propylene glycol 28 34.4 38 44 (approx.)
2058 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
resins. This was expected because of better packing of the polymer chains having para-para linkages. It is reported that the amount of styrene has a pronounced effect on the properties of the polyester (Boenig, 1969). The flexural and tensile strengths go through an optimum value with the concentration of styrene monomer. The relatively low impact, tensile and flexural strengths of the P E T waste based polyesters, compared to those of the improved grade GP resin, may be attributed to their lower molecular weight and the higher amount of styrene. The properties of these resins may be significantly improved by optimizing the amount of styrene and by increasing the molecular weight. Since polymers are viscoelastic in nature, their mechanical properties exhibit a pronounced dependence on temperature and rate of deformation. In a number of FRP applications, the material is subjected to complex dynamic stress conditions over a broad temperature range. The viscoelastic response of the material can be studied either in a temperature sweep a t a fixed frequency (rate of deformation) or in a frequency sweep a t a required temperature. The viscoelastic behavior of the polyester resins was studied on a dynamic mechanical spectrometer to obtain data on the viscous and elastic components of the shear modulus. The complex shear modulus, G*, can be represented as (1) G* = G'+ iG" where G'and G"are termed as the storage modulus and loss modulus, respectively. G ' represents the elastic material response related to the potential energy stored by the material under deformation, whereas the viscous response of the material is characterized in terms of G", which signifies the dissipation of energy as heat during deformation. The ratio of the two moduli is represented as tan 6 = G"/G' (2) Tan 6 represents internal friction or damping and is termed as the loss factor. The mathematical relationships between the parameters are well described in the literature (Murayama, 1978). All the parameters described above are directly related to the molecular motions under stress, and therefore they yield useful information about the polymer structure. All three parameters, G', G", and tan 6, are sensitive to the molecular structure of the material. The value of the storage modulus, G') signifies the rigidity of the material. A t any fixed frequency, corresponding to the deformation rate in the end use, the temperature a t which this value starts falling rapidly indicates the heat distortion temperature of the material. In general, the temperature at which G'falls below 1O1O dyn/cm2 may be considered as the maximum service temperature for that frequency (Murayama, 1978). The variations of Gr'and tan 6 with temperature provide information about the energy dissipated in molecular motions and thereby the impact resistance of the material. The greater the area under the G" or tan 6 peak, the greater is the energy dissipation, signifying better impact strength. The tan 6 peak temperature corresponds to the glass transition temperature of the material. The shift in the position of the tan 6 peak to higher temperatures would indicate improved thermomechanical performance. Thus, the dynamic mechanical properties are useful in predicting the end use performance of the material. The dynamic mechanical parameters are dependent on the frequency and temperature. The conventional way of checking the performance of any new material is to compare with the existing material under
A-GP IMPROVED .-GP ORDlNLRY 0-UVMW- 57 0-UVHW-18 1-UVMW-59
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Figure 1. Effect of temperature on the value of storage modulus, G', at 100 radfs. Table IV. Viscoelastic Behavior of Polyesters: Elastic ComDonent temp, O C , a t temp when G'= G'= G'= dev in G' 8 X lo9 5 X lo9 2 X loB polyester starts, "C dyn/cm2 dyn/cm2 dyn/cm2 90 112 UVMW-57 57 77 57 77 96 >150 UVMW-58 126 >150 UVMW-59 62 87 62 91 115 133 GP improved 103 118 GP orindary 57 90
identical conditions. This comparison would help in giving directions for chemical modification of the system to achieve the desired properties. In the present work the PET based polyesters are compared with the existing grades of general purpose resin. The samples were subjected to a frequency sweep between 0.1 and 1000 rad/s a t room temperature. The dependence of G', G", and tan 6 on frequency was found to be insignificant. The samples were then subjected to temperature sweeps between 30 and 150 "C a t 100 rad/s. Figure 1 shows the effect of temperature on the storage modulus, G'. It is seen from the plot that the storage moduli of the PET waste based resins are marginally lower than that of the GP resin up to 65 "C. However, a rapid drop in the value of G'is observed in the case of conventional GP resin beyond 100 "C. It is interesting to note that the temperature dependence of the G' is less in the case of the PET waste based resins. Table IV represents the temperatures a t which the deviation in the value of G'starts appearing. The temperatures at which the value of G'drops to 8 X lo9, 5 X lo9, and 2 X lo9 dyn/cm2 are also represented in the same table. It may be noted from Figure 1 and Table IV that even though the initial value of G'is higher for GP resin it falls rapidly a t higher temperatures. In the case of improved grade GP resin, the value of G'drops to 2 X lo9 dyn/cm2 at 133 "C, whereas in the case of UVMW-58 and -59 the value does not drop down to that extent even a t 150 "C. The higher value of the storage modulus a t high temperatures may be attributed to the higher extent of cross-linking. The aromatic contents of UVMW-57 are comparable to that of GP resins. However, since para-para linkages offer better packing, which restricts the movement of molecules, the storage factor of UVMW-57 is lower than that of the GP resin. Figure 2 shows the variation of G"with temperature at 100 rad/s. The energy dissipation displays a maximum when the frequency of operation corresponds to the re-
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2059 1.o.d' e - G P ORDINARY 0-UVHW-57 0-UVMW-58 x - u V HW - 5 9
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Figure 2. Effect of temperature on the value of loss modulus, G", a t 100 rad/s.
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Figure 3. Effect of temperature on the value of loss factor, tan 6, a t 100 rad/s.
Table V. Viscoelastic Behavior of Polyesters: Viscous Component temp range for peak G", polyester peak G"value, dyn/cmz O C UVMW-57 9.0 x 108 80-97 80-88 UVMW-58 7.5 X 10" (small hump) UVMW-59 6.0 X lo8 (no peak) GP improved 12 X lo8 97-109 GP ordinary 11 X lo8 90-101 Table VI. Viscoelastic Behavior of Polyesters: Loss Factor values of tan 6 a t different temp, dvn/cm2 polyester 35 O C 65 O C loo "C UVMW-57 3.4 X 5.2 X 2.6 X lo-' 5.2 X 1.6 X lo-' UVMW-58 3.7 X UVMW-59 4.3 X 5.1 X 7.7 X GP improved 1.9 X 3.5 X 1.4 X lo-' GP ordinary 3.9 X 2.0 X lo-' 2.5 X
laxation times of the molecular movements, at that temperature. The value of G"goes through a maximum for GP resin. The value of G"at the maximum decreases for UVMW-57 and -58. This peak almost disappears in the case of UVMW-59 and the value of G"is practically unchanged over the temperature range. Table V indicates the peak values of G"and the corresponding temperatures. It is seen that the peak value of G" for the improved grade GP resin is the highest. The peak is gradually shifted to lower temperatures for UVMW-57 and UVMW-58 and almost disappears in the case of UVMW-59. The drop in the value of G" at the peak and the shift of the peak to lower temperatures, for PET waste based resins, may be attributed to the increase in the aliphatic contents of the polymeric chain between the cross-linking points (Table 111). Figure 3 shows the variation in the dissipation factor, tan 6, with temperature, at 100 rad/s. Again, only a marginal change is observed in the case of UVMW-59, whereas a steep increase is observed in the case of the GP resin and UVMW-57. The behavior of UVMW-58 was intermediate to that of UVMW-59 and -57. Table VI gives the values of tan 6 at 35, 65, and 100 OC. The relative insensitivity of UVMW-59 to temperature under the dynamic testing is observed in this table too. The dynamic mechanical performance of the PET waste based resins is thus found to be comparable to that of the GP resin. The performance of the PET based resins can
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Figure 4. Effect of temperature, a t 100 rad/s, on the dynamic mechanical properties of the improved grade GP resin at various amounts of styrene.
be further improved, as stated earlier, by increasing the molecular weight and optimizing the amount of styrene. The effect of the amount of styrene on the properties of polyester was studied. The improved grade GP resin with varying amount of styrene was investigated for dynamic behavior. It is seen from Figure 4 that the dynamic properties are affected by the amount of styrene. The value of G' is affected by the amount of styrene, and the temperature at which G' drops below 10'O dyn/cm2 was found to be reduced by about 12 "C with increasing amount of styrene. Similarly, with increased amount of styrene, one extra peak is observed in the tan 6 curve. This may be attributed to the presence of longer polystyrene chains cross-linking the polyester chains, arising from a higher concentration of styrene. The impact strength also appears to decrease with the increased styrene amount, making it more brittle, as indicated by the reduction in the area under the tan 6 peak. Thus, the composition of polyester can be optimized using dynamic mechanical data to get the desired behavior of G'and tan 6 by varying the molecular weight, chemical structure, and amount of styrene. Conclusions The PET waste can be glycolyzed into low molecular weight, oligomeric diols, with propylene glycol. These diols can be reacted with maleic anhydride to get unsaturated polyesters. These polyesters are compatible with styrene
Ind. Eng. C h e m . R e s . 1988,27, 2060-2063
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and can be cured using peroxide initiators. During the polyesterification reaction, most of the maleate unsaturation is converted into the fumarate form for both the o-phthalic and PET based resins. The mechanical behavior of the PET waste based resins was found to be comparable with the ordinary grade of general purpose resin. However, the impact and tensile strengths of these resins were lower than those of the improved grade GP resin. The lower properties may be attributed to the lower molecular weights and higher styrene contents of the PET waste based resins, compared to the improved GP resin. The heat distortion temperatures of the PET based resins were found to be higher than the improved grade GP resin. The viscoelastic behavior of the PET based resins is comparable to that of the GP resin. Sample UVMW-59, containing the least amount of terephthalic moiety (Table 111), showed almost no change in G’ and G ” for a wide range of temperatures. The initial values of G‘ of PET waste based resins were marginally lower than that of the GP resins, yet the shear modulus of the GP resins falls much more rapidly with temperature than that of the PET waste based resins. The loss modulus of the improved grade GP resin showed a higher maximum than the PET based systems. The molecular weight and the amount of styrene in the UP resin affect their mechanical properties. Therefore, these parameters need to be optimized to achieve better mechanical performance with the PET waste based resins.
Thus, the PET waste based resins can be used, as such, in applications that require a higher heat distortion temperature. Acknowledgment
The assistance rendered by Dr. J. P. Jog (National Chemical Laboratory, Pune, India) in evaluation of dynamic mechanical performance and by S. Barve (Dr. Beck and Co., Pune, India) in mechanical testing is gratefully acknowledged. Registry No. PET,25038-59-9. Literature Cited Boenig, H. V. In Encyclopedia of Polymer Science and Technology; Wiley: New York, 1969; Vol. 11, pp 129-168. Curtis, L. G.; Edwards, D. L.; Simons, R. M.; Trent, P. J.; Von Bramer, P. T. Znd. Eng. Chem. Prod. Res. Dev. 1964,3(3)218-21. Murayama, T.In Dynamic Mechanical Analysis of Polymeric Materials; Elsevier Scientific: Amsterdam, 1978. Parkyn, B.; Lamb, F.; Clifton, B. V. Polyesters; American Elsevier: New York, 1967; Vol. 2, Chapter 2. Vaidya, U. R.; Nadkarni, V. M. Znd. Eng. Chem. Res. 1987a,26(2), 194. Vaidya, U. R.; Nadkarni, V. M. J. Appl. Polym. Sei. 1987b,34(1), 235. Vancso Szmercsanyi, I; Maros Greger, K.; Makay Bodi, E. J.Polym. S C ~1961, . 53,241-8.
Received for review September 21, 1987 Revised manuscript received April 7, 1988 Accepted May 5 , 1988
PROCESS ENGINEERING AND DESIGN Optimum Controller Settings for Processes with Dead Time: Effects of Type and Location of Disturbance Peter Harriott School of Chemical Engineering, Cornel1 University, Ithaca, N e w York 14853
Optimum controller settings were calculated for processes with dead time or an effective time delay using the integral of the absolute error as the criterion. For proportional integral control, the optimum gain was close to half the maximum gain for all cases, but the optimum reset time relative to the ultimate period varied considerably with the type and location of the disturbance as well as with the ratio of the dead time to the major time constant. With small dead times, the optimum reset time was as much as 3 times the value calculated by using the Ziegler-Nichols continuous-cycling tuning procedure. Many processes include a dead time or time or time delay caused by the flow of material through a pipe or piece of equipment or the flow of fluid through a sample line to an analyzer. Other processes with a number of first-order lags appear to have a time delay because of the very slow initial response to a step change at the start of the process, and such processes are often modeled with an “effective time delay” and one or two first-order lags. The effects of the time delay on recommended controller settings for feedback control have been discussed in previous studies, but different disturbances have been used to test the systems and different criteria chosen to determine the 0888-5885/88/2627-2060$01.50/0
best settings. In this study, the integral of the absolute error, .flelmdt, or IAE is used as a measure of performance, since for small errors the penalty for poor control is generally a linear function of the error. The optimum settings are those which give a minimum IAE for a given disturbance. The effects of the type and location of the disturbance are considered as well as the ratio of the time delay to the largest time constant. For the first case, the system included a time delay, a single first-order lag, and an ideal proportional integral controller. The valve and measurement lags were assumed to be negligible. The block diagram is the same as in 0 1988 American
Chemical Society
