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Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 121-126
Magic Angle Spinning Carbon-I 3 Nuclear Magnetic Resonance of Acrylic-Melamine Coatings Davld R. Bauer' and Ray A. Dlckle Research Staff, Ford Motor Company, Dearborn, Michigan 48 12 1
Jack L. Koenlg Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44 106
Magic angle sample spinning 13C NMR has been used to obtain a molecular characterization of cure and photodegradation processes in styrene acrylic copolymer-melamine formaldehyde coatings. Two different pulse sequences have been employed: cross polarization and gated high-power decoupling. Cross-polarization spectra yield data on coating structure as a function of such variables as coating composition and extent of cure and degradation. The resutts have been compared with previous studies by infrared spectroscopy. The gated high-power decoupling spectra are sensitive to local motion in the coating. Changes observed in the local motion of specific groups on exposure to ultraviolet light and humidii have been interpreted in terms of changes in network structure that occur on degradation. It is concluded that acrylic-melamine cross-links formed during cure are hydrolyzed and that melamine-melamine cross-links are formed. For coatings containing fully alkylated melamines, melamine-melamine bond formation occurs only after extensive exposure. Photoinduced free-radical cross-linking of the main chain of the acrylic copolymers is also observed.
Introduction Coatings based on acrylic copolymers cross-linked with melamine formaldehyde resins, in common with other polymeric materials, undergo significant changes in physical properties and presumably in molecular structure upon prolonged exposure to high levels of ultraviolet (UV) light, humidity, and extremes of temperature. There are numerous techniques available to measure the effects of such exposure on the physical properties of coatings, but study of the chemistry of the degradation processes that occur is hampered by the difficulty of obtaining a molecular characterization of intact coating films. Infrared spectroscopy has been used to characterize cure (Dorfel and Biethan, 1976; Bauer and Dickie, 1980, 1982; Bauer and Budde, 1981) and degradation (Bauer, 1982; English and Spinelli, 1984; Bauer and Briggs, 1984) in acrylicmelamine coatings. Their cure chemistry has been found to depend primarily on the structure of the melamine cross-linker. Fully alkylated melamines form ether cross-links between the acrylic chain and the melamine triazine ring. Partially alkylated melamines form both acrylic-melamine cross-links and melamine-melamine cross-links involving a N-CH,-N linkage between triazine rings. The degradation studies indicate that hydrolysis of acrylic-melamine cross-links is an important degradation process. Subsequent formation of melamine-melamine cross-links has been observed in coatings crosslinked with partially alkylated melamines (Bauer, 1982; Bauer and Briggs, 1984) but not in coatings cross-linked with fully alkylated melamines (English and Spinelli, 1984). The nature of these processes, and of other structural changes that occur on degradation, is not well understood. Magic angle sample spinning carbon-13 nuclear magnetic resonance (MASSJ3CNMR) has been employed to obtain high-resolution NMR spectra from solid polymers (Lyerla, 1980; English et al., 1983; Havens and Koenig, 1983). Initial MASS-NMR results on a series of acrylic copolymers cross-linked with a partially alkylated melamine have been 0196-432118511224-0121$01.50/0
Table I. Coating Formulations acrylic" melamine* coating no. copolymer cross-linker la low Mel-D lb low Mel-D IC low Mel-D Id low Mel-D le low Mel-D 2 low HMMM 3a high Mel-D 3b high Mel-D 4 high
bake schedule, "C (min) 90 (20) 110 (20) 130 (20) 150 (20) 180 (20) 130 (20) 130 (20) 110 (15)
"Low = low molecular weight acrylic copolymer; high = high molecular weight acrylic copolymer. Mel-D = partially alkylated melamine cross-linker; HMMM = hexamethoxymethylmelamine.
*
discussed elsewhere (Bauer et al., 1984). In this paper we describe a MASS-NMR study of cure and degradation processes in acrylic copolymer coatings cross-linked with partially and fully alkylated melamines. Quantitative compositional information and information about changes in local motion in the coatings are presented.
Experimental Section Materials. The two styrene acrylic copolymers used in this study were prepared by conventional free-radical polymerization. Both copolymers contained 30% by weight hydroxyethyl acrylate for cross-linking and 25% by weight styrene. The number average molecular weights of the copolymers as determined by gel permeation chromatography were 2200 and 6400. The GPC was calibrated with a series of fractionated styrene acrylic polymers whose number average molecular weight was determined by vapor phase osmometry. Two melamine formaldehyde crosslinkers were used-a fully alkylated melamine (hexamethoxymethyl melamine) and a partially alkylated melamine (Mel-D of Bauer and Budde, 1980). Both cross-linkers were obtained from American Cyanamid Co. Coatings were formulated with a 70:30 ratio of acrylic polymer to melamine cross-linker. Coating formulations 0 1985
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Res Dev.,
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No. 1, 1985
and cure schedules are described in Table I. The coatings were cast and baked on aluminum panels. Coatings were exposed in a UV weathering chamber (Q-Panel Co.) using a cycle consisting of 4 h of LJV light at 60 "C followed by 4 h condensing humidity at 50 "C. Coatings were removed from the chamber and delaminated from the aluminum by cooling with dry ice and rapidly bending the panel to crack off the coating. The coating was then packed in the spinner of the NMR. Samples for infrared spectroscopy were ground in KBr; spectra were obtained with a FTS 20E Digilab FTIR spectrometer. MASS-NMR. Solid-state I3C NMR spectra were obtained using a Nicolet Technology NT-150 (Bauer et al., 1984). The polyoxymethylene spinner containing the cured coating was spun at a rate of 3.3-3.8 kHz. Two pulse sequences were used in this study-spin lock cross polarization (CP-MASS) and gated high-power decoupling (GHPD-MASS). For the CP-MASS spectra, a contact time of 1 ms was used. The GHPD-MASS spectra were obtained using a 90' carbon pulse followed by high-power decoupling (450 W) of the protons. A 2-s delay between pulse sequence repetitions was used for both experiments. For the coatings studied, the two pulse sequences yield spectra which are complementary. In general, the intensity of a given resonance using a given pulse sequence is a complex function of the concentration of carbons a t the resonance and a variety of relaxation processes (Lyerla, 1980). The CP-MASS spectra are generally more sensitive to rigid carbon species since these are more easily crosspolarized. For highly cross-linked polymers such as these, the carbons are sufficiently immobile that the intensity of individual resonances is not very sensitive to differences in mobility. The intensities of individual resonances of the coatings studied are reproducibile to h5-10% and the measured intensities agree with the known stoichiometry of the coating to within f10-20% (Bauer et al., 1984). In contrast to the CP-MASS spectra, the intensities of the resonances in the GHPD-MASS spectra are very sensitive to the mobility of the groups. In the GHPD-MASS pulse sequence the carbon and hydrogen spins are not locked, and the carbon polarization decays with the 13C spinlattice relaxation time, T,. 13C T,'s are determined by the magnitude of the spectral density function at frequencies related to the carbon and hydrogen resonances (37.7 and 150 MHz, respectively). The lower the value of the spectral density function, the longer the relaxation time. 13C T,'s can be quite long in rigid polymers (e.g., 13C T , for the aromatic carbons of polystyrene is 26 s) (Kaplan, 1984). If T , is much longer than the delay time (in our case 2 s), then the magnetization does not relax back to equilibrium before the next pulse (Waugh, 1970), and it can be shown that the signal intensity is decreased by the ratio of the delay time to T I . Thus, the more rigid the group, the longer its T I ,and the more the signal intensity is reduced. The value of T I is also affected by the number of nearby hydrogens. By measuring the ratio of intensities for the GHPD-MASS and the CP-MASS spectra, it is possible to compare qualitatively the mobility of specific groups in different polymers or to follow how the mobility of those groups changes with exposure. Changes in the GHPD to CP intensity ratio can be caused by changes in the frequency or amplitude of motion or by changes in the distribution of motions. These changes will be related to changes in the network structure of the cross-linked coatings. Since changes are followed, it is not necessary to have absolute values of the GHPD and CP intensities. For comparison purposes, the GHPD t o CP ratio of the ppm has been assigned a value of methyl resonance at 1 ,i
PPM
Figure 1. CP-MASS-NMR spectra of coating IC after cure (A) and after 1000 h exposure (B). SSB indicates a spinning side band.
1.0. In principle, more detailed information could be obtained from direct measurements of TI and Tip. However, a measurement of T , even using a cross-polarization pulse sequence (Torchia, 1978) would have required a 10-fold increase in data accumulation time. Considering the number of coatings studied and the number of transients required to obtain an adequate signal-to-noise ratio, direct measurements of the relaxation times was felt to be impractical. Results and Discussion CP-MASS Spectra. Typical CP-MASS spectra of undegraded and degraded coatings are shown in Figure 1. In both cases, high-resolution spectra (2 ppm) are obtained and at least 12 resonances can be identified. The assignment of most of the resonances is straightforward. The ester carbonyl resonance is at 176 ppm; the styrene resonances are at 129 and 145 ppm; the acrylic side chain alkyl groups are between 12 and 32 ppm; the ether carbon is at 65 ppm; the melamine triazine ring is at 167 ppm; the melamine methylol group is at 72 ppm; and the melamine methoxy group is at 56 ppm. The region between 38 and 48 ppm consists solely of carbons in the main chain of the acrylic copolymer. After exposure in the UV weathering chamber, several changes in the spectrum can be observed. There are subtle changes in the acrylic main chain carbon resonances and in the styrene resonances. There is a small (10-15%) loss of intensity in the side chain carbons resonances relative to the main chain resonances and weak shoulders appear near the carbonyl resonance. These changes reflect some of the photooxidative chemistry that is occurring, but the interpretation of these changes is not straightforward. Similar changes can be observed in the spectra of uncross-linked acrylic copolymers similarly exposed (Figure 2). In addition to these changes a weak band at 169 ppm appears on extended exposure. This resonance, which is hidden by the triazine ring resonance at 167 ppm in the cross-linked coatings, has a chemical shift which is consistent with either a perester carbonyl or with a lactone. An infrared band (1780 cm-') which could be attributed to either of these species has also been observed following ultraviolet exposure of acrylic polymers (Dickenson et al., 1982) and acrylic-melamine coatings (Bauer and Briggs, 1984). The most dramatic change in the spectra of cross-linked coatings on degradation is the disappearance of the methoxy resonance at 56 ppm. The methoxy groups react during bake with hydroxy groups on the copolymer to form acrylic-melamine cross-links. Since coatings are generally
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 123
- - - -5cc 0
400 800 l20C EXPOSURE TIME ( H R S )
Figure 4. Fraction of methoxy groups remaining vs. exposure time coating l b (O), coating IC (A),coating Id (v),and for coating l a (O), coating l e ( 0 ) . The data are an average of the IR and NMR data. 150
200
I00
0
50
PPM
Figure 2. CP-MASS-NMR spectra of coating 4 after 500 h exposure (A) and after 1440 h exposure (B). I t was not possible to obtain a CP-MASS spectrum for undegraded coating 4 due to the fluidity of the uncross-linked polymer. I
c-
005 0
7
400
I
800 I200 EXPOSURE TIME ( H R S I
Figure 3. Fraction of methoxy groups remaining in coating ICvs. exposure time. The 0 indicate IR data while the indicate CPMASS-NMR data.
formulated with more methoxy groups than hydroxy groups, there is always unreacted methoxy functionality after cure. The disappearance of the methoxy resonance during exposure has been shown to be due in large part to hydrolysis of the methoxy group (Bauer and Briggs, 1984). A similar hydrolysis reaction cleaves the acrylicmelamine cross-links. The reaction during cure and the subsequent hydrolysis of methoxy groups can be followed by both infrared and NMR. The decrease in intensity of the methoxy band (915 cm-l) can be followed as a function of cure and hydrolysis. Uncured coatings are too fluid to obtain a CP-MASS spectrum. Thus it is not possible to measure directly the decrease in methoxy functionality on baking by NMR. However, an indirect estimate of this decrease can be obtained by comparing the ratio of the area under the methoxy resonance to that under the triazine ring resonance with the known (Bauer and Budde, 1981) stoichiometry before cure. A comparison of the measured decrease in the methoxy group with cure and exposure as determined by infrared and NMR is shown in Figure 3. The two techniques yield identical results within experimental error. This lends support to the use of CPMASS-NMR as a quantitative tool for monitoring structural changes in coatings. The effect of bake schedule on the methoxy reaction and subsequent hydrolysis is shown in Figure 4 for coating 1. The data presented are an average of the NMR and infrared data. The reaction of the methoxy group during bake corresponds well with the disappearance of hydroxy groups as measured by IR (Bauer and Dickie, 1980; Bauer and Budde, 1981) except a t the highest bake temperature,
where it was found that the methoxy group decreased more than the hydroxy group. This may be due to a small amount of methoxy-methoxy self-condensation which is known to occur a t high bake temperatures (Saxon and Lestienne, 1964). The measured extents of reaction agree well with those previously determined for this cross-linker (Bauer and Budde, 1981) except at the lowest bake temperature. Since the samples were stored at room temperature for a period of time before spectra were taken, it is possible that some additional cure took place in the sample cured at low temperature. The rate of disappearance of methoxy functionality during exposure in the UV weathering chamber decreases with increasing bake temperature. Within experimental error, the rate of disappearance is inversely proportional to the cross-link density in the coating. Similar results have been found for the rate of hydrolysis of acrylic-melamine cross-links in coatings subjected to condensing humidity (Bauer, 1982). Hydrolysis of methoxy groups and acrylic-melamine linkages produces melamine methylol groups and the parent alcohol. The melamine methylol resonance in these spectra is broad and difficult to quantify. Its intensity is roughly fairly constant for coating 2 and decreases steadily for coatings 1 and 3 relative to the melamine triazine ring resonance. The methylol groups produced on hydrolysis undergo further reactions: deformylation to amine and self-condensation with another methylol group to yield a melamine bridge (Bauer and Briggs, 1984). The methylene bridge resonance has been found (Bauer et al., 1984) to have a chemical shift between 47 and 55 ppm. The resonance is difficult to see in the CP-MASS spectra both because it is broad (due to the several different substituent patterns) and because it is partially obscured by the main chain acrylic resonance. Formation of melamine-melamine cross-links has been inferred by the appearance of a band in the IR at 1340 cm-', but this assignment has not been confirmed (Bauer, 1982; Bauer and Briggs, 1984). The band is seen on hydrolysis of partially alkylated melamines (Bauer, 1982; Bauer and Briggs, 1984) during accelerated exposure, but it was not observed on hydrolysis of fully alkylated melamines (English and Spinelli, 1984) during natural exposure. The question of whether or not melamine-melamine self-condensation occurs is important particularly since the acrylic-melamine bonds are hydrolyzing so rapidly. The CP-MASS spectra cannot by themselves confirm or deny the formation of melamine methylene bridges. Further discussion concerning the formation of melamine-melamine cross-links is presented below. GHPD-MASS Spectra. Typical GHPD-MASS spectra of cured and degraded coatings are shown in Figure 5 . Comparison of the intensities of the different resonances
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
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I
I
In
00
1000 EXPOSURE TIME (HRS)
0
2000
Figure 7. Ratio of GHPD-MASS intensity to CP-MASS intensity vs. exposure time for the melamine triazine ring carbons of coating and coating 3b (0). 3a (0) 200
150
50
100
0
30
r---
1
PPM
Figure 5. GHPD-MASS-NMR spectra of coating ICafter cure (A) and after 1000 h exposure (B).
o
0
o
d
-
1
1000 1500 EXPOSURE TIME ( H R S ) 500
Figure 8. Ratio of GHPD-MASS intensity to CP-MASS intensity vs. exposure time for the melamine triazine ring carbons of coating IC(0) and coating 2 (0).
0
100
I40
I80
BAKE TEMPERATURE “C
Figure 6. Ratio of GHPD-MASS intensity vs. cure temperature for and the acrylic main chain the melamine triazine ring carbons (0) of coating 1. For a given resonance, a higher ratio imcarbons (0) plies increased mobility.
in the GHPD-MASS and CP-MASS spectra clearly reflect the effect of mobility on the GHPD-MASS spectra. In our previous study (Bauer et al., 1984), i t was found that the mobilities of the side-chain acrylic carbons were independent of the glass transition temperature of the acrylic copolymers or of the extent of degradation. It was found that the main chain acrylic carbons in polymers with a high glass transition temperature were less mobile than those in polymers with a low glass transition temperature. The mobility of the melamine triazine ring was independent of the glass transition temperature of the acrylic copolymer. As shown in Figure 6, the GHPD to CP intensity ratio (and thus the mobility) of both the acrylic main chain carbons and the melamine triazine ring carbons decreases with increasing bake temperature (and cross-link density). The mobilities of the acrylic main chain and the melamine triazine ring are clearly affected by cross-link density, and changes in cross-link density on exposure should be reflected in changes in mobility of these resonances. On exposure to ultraviolet light and water, the acrylicmelamine cross-links hydrolyze. If no melamine-melamine cross-links are formed, the triazine ring should become more mobile. In fact, as shown in Figure 7, the opposite is true for coatings cured with partially alkylated melamines. As degradation proceeds, the melamine triazine ring becomes less mobile. The triazine ring in the coating cured a t the lower bake temperature is initially more mobile but loses mobility more rapidly than the identical coating cured at a higher temperature. The rate at which the triazine ring loses mobility is similar to the rate of
hydrolysis in these coatings. As shown in Figure 8, the behavior of the coating cross-linked with the fully alkylated melamine is much different from that observed for the coating cross-linked with the partially alkylated melamine. Instead of decreasing rapidly on exposure, the mobility of the triazine ring of the fully alkylated melamine remains roughly constant for a substantial period of time (750 h) before becoming abruptly decreasing. The initial (just after cure) mobility of the fully alkylated melamine is greater than that of the partially alkylated melamine even though the number of cross-links per triazine ring is slightly greater for the fully alkylated melamine. Intuitively, it seems likely that the N-CH2-N melamine-melamine cross-link is less mobile than the N-CH2-0-Racrylic-melamine cross-link. Fully alkylated melamines form only acrylic-melamine cross-links while partially alkylated melamines form both acrylic-melamine as well as melamine-melamine cross-links. This could explain why the triazine rings of partially alkylated melamines are more rigid after cure than fully alkylated melamines. It could also explain why the partially alkylated melamine loses mobility after hydrolysis. As outlined below, hydrolysis can convert a substantial fraction of the acrylicmelamine cross-links (as well as unreacted methoxy functionality) to melamine-melamine cross-links. -N
/H
H20
-N
‘CH20R
+
/H
ROH
(1)
\CH20H
-
-N
-NHz
-I- HzC=O
(2)
4- H&=O
-t HzO ( 3 )
‘CH~OH
-
2-N ‘CH~OH
H H
-N
/ \
N ‘-
/
CH2
The formation of a large number of melamine-melamine
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 125
i \ f V
0
500
1000
1500
EXPOSURE TIME (HRSI
IO
Figure 9. Ratio of GHPD-MASS intensity to CP-MASS intensity vs. exposure time for the acrylic main chain carbons of coating IC(0) and coating 2 (0). Also shown ( 0 )is data from Bauer et al. (1984) for a coating consisting of a low molecular weight styrene-free copolymer crosslinked with Mel-D.
cross-links is consistent with the increase in intensity of the IR band a t 1340 cm-l. This band can be used to estimate the number of melamine-melamine cross-links formed on cure and hydrolysis. The number of acrylicmelamine cross-links formed on cure and subsequently broken on hydrolysis can be estimated from measurements of the fraction of methoxy functionality in the film. From these data and the mobility data it can be estimated that a melamine-melamine cross-link decreases the mobility of the triazine ring some 3-4 times as much as does an acrylic-melamine cross-link. The fully alkylated melamine remains mobile during the initial part of the exposure, suggesting that little or no melamine-melamine cross-linking occurs. This is consistent with the fact that over this time period no IR band at 1340 cm-I is observed. After long exposure time, the triazine ring abruptly becomes more rigid and the band at 1340 cm-’ can be observed, suggesting that melaminemelamine cross-linking does occur in fully alkylated melamines after an extended period of exposure. The lack of melamine-melamine cross-link formation in the initial stages of exposure is also consistent with the lack of formation of the 1340-cm-I band in a coating cross-linked with a fully alkylated melamine and exposed outdoors (English and Spinnelli, 1984). As outlined below, the lack of melamine-melamine cross-linking in the fully alkylated melamine cross-linked coating may be caused by steric hindrance which prevents the formation of melaminemelamine cross-links until substantial deformylation has occurred.
-N
/CH20H
-
-N
‘CH20R
\CH,OR
’
CH
$(-
+
H,C=O
(5)
t 0
500
1000
1500 2000
EXPOSURE T I M E ( H R S )
Figure 10. Ratio of GHPD-MASS intensity to CP-MASS intensity vs. exposure time for the acrylic main chain carbons (O),the protonated styrene carbons (o),and the nonprotonated styrene carbon (A)of coating 4.
of the main chain carbons in the coating cross-linked with the fully alkylated melamine is less than that for the coating cross-linked with the partially alkylated melamine. This is due to the fact that the fully alkylated melamine forms more acrylic-melamine cross-links than does the partially alkylated melamine. The mobility of the acrylic main chain depends primarily on the number of acrylicmelamine cross-links and is almost independent of the number of melamine-melamine cross-links. In both coatings the mobility of the acrylic backbone first increases, and then after about 500 h exposure it begins to decrease steadily. The initial increase is likely due to hydrolysis of acrylic-melamine cross-links. The subsequent decrease in mobility is a function of the composition of the acrylic copolymer. Previous work (Bauer et al., 1984) has shown that the decrease in mobility is much greater in coatings in which the acrylic copolymer contains styrene than in coatings which are styrene free. This suggests that the decrease in mobility of the acrylic main chain is due to photochemical cross-linking of the acrylic chain. In order to investigate this possibility in the absence of other changes in the cross-linked structure (e.g., hydrolysis), the mobility of the main chain of an uncross-linked acrylic copolymer has been determined as a function of exposure time. Due to excessive fluidity it was not possible to obtain a MASS spectrum of the undegraded polymer. As can be seen in Figure 10, the mobility of the main chain of the polymer decreases steadily on exposure. In addition, the mobilities of the styrene resonances (particularly the nonprotonated styrene resonance) also decrease on exposure. This suggests that the following acrylic-acrylic cross-link formation may be occurring.
‘CH20R
,CHzOH
2-N
/H
2 0 1
-N
I CHzOR
‘\N-
I CHzOR
+ H2C=0 + H20
(6)
It was found above that the mobility of the main chain acrylic carbons also depends on the extent of cross-linking. Since the cross-link structure of the coating is changing drastically during degradation, the mobility of the main chain carbons should also be a function of exposure time. The ratio of GHPD to CP intensity of the main chain acrylic carbons is plotted in Figure 9 for coatings ICand 2 as a function of exposure time. After cure, the mobility
Such a mechanism is consistent with known degradation processes in polystyrene. Some cross-linking a t acrylic (as opposed to methacrylic) sites may also be occurring. The formation of acrylic-acrylic cross-links together with the previously discussed hydrolysis chemistry converts the condensation network formed on cure to one resembling an interpenetrating polymer network.
Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 126-129
126
Conclusion MASS-13C NMR has been shown to be an extremely versatile tool for characterizing cure and degradation processes in coatings. The combination of cross-polarization and gated high-power decoupling provides a convenient method for studying both composition and mobility of the coating. In particular, these studies have led to a better understanding of changes in network structure during photodegradation. It has been found that in acrylic melamine coatings, the acrylic-melamine bonds hydrolyze and that melamine-melamine bonds are formed. For coatings with partially alkylated melamines, this bond formation occurs concurrently with hydrolysis. For coatings cross-linked with fully alkylated melamines, melamine-melamine bond formation occurs only after extensive exposure. Photooxidative cross-linking of the main chains of styrene acrylic copolymers is also observed. The implications of these changes in structure for the physical properties of the coating are as yet unknown. The formation of acrylic-acrylic cross-links was not previously detected in either IR spectra or the CP-MASS spectra,
demonstrating the contributing that mobility information can make to the study of degradation processes. Literature Cited Bauer, D. R. J. Appl. Polym. Sci. 1982, 2 7 , 3651. Bauer, D. R; Budde. G. F. Ind. Eng. Chem. Prod. Res. Dev. 1981, 2 0 , 674. Bauer, D. R.; Briggs, L. M. I n "Characterization of Highly Crosslinked Polymers", Labana, S. S.;Dickie, R. A,, Ed.; American Chemical Society: Washington, DC, 1984; pp 271-284. Bauer. D. R.; Dickie, R. A. J. Polym. Sci., Polym. Phys. 1980, 18, 1997. Bauer, D. R.; Dickie, R. A. J. Coat. Techno/. 1982, 54 (685), 57. Bauer, D. R.; Dickie, R. A.; Koenig, J. L. J. Polym. Sci., Polym. Phys. 1984, 2 2 , 2009. Dickenson, H. R.; Rogers, C. E.; Simha, R. Polym. Prepr. 1982, 2 3 , 217. Dorffel, J.; Biethan, U. Farbe L a c k 1978, 8 2 , 1017. English, A. D.; Chase, D. B.; Spinelii, H. J. Macromolecules 1983, 16, 1422. English, A. D.; Spinelii, H. J. I n "Characterization of Highly Crosslinked Polymers", Labana, S. S.; Dickie, R. A,, Ed.; American Chemical Society: Washington, DC, 1984; pp 257-269. Havens, J. R.; Koenig, J. L. Appl. Spectrosc. 1983, 3 7 , 226. Kapian, S. Polymn. Prepr. 1984, 25, 356. Lyerla, J. R. I n "Method of Experimental Physics", Fava, R. A,, Ed.; Academic Press: New York, 1980; Vol. 16A, pp 241-369. Saxon, R.; Lestienne, F. C. J. Appl. Polymn. Sci. 1984, 8 , 475. Torchia, D. A. J. Magn. Reson. 1978, 30, 613. Waugh, J. S. J. Mol. Spectrosc. 1970, 3 5 , 298.
Receiued for reuiew June 28,1984 Accepted September 26,1984
Screening To Identify Chemical Markers of Plant Resistance to Pests and Plant Stress P. A. Hedln' Boll Weevil Research Laboratory, U.S.Department of Agriculture, Agricultural Research Service, Mississippi State, Mississippi 39762
F. M. Davls, W. P. Wllllams, J. C. McCarty, R.
L. Shepherd, and A. Ben Porath
Crop Science Research Laboratory, U.S. Department of Agriculture, Agricultural Research Service, Mississippi State, Mississippi 39762
Plants can express resistance to pests by biosynthesizing allelochemicals. They may be normally present, or their biosynthesis may be elicited or enhanced by injury. This work was initiated to develop a more rapid, integrated approach to identify chemicals that are responsible for resistance to pests, or that are associated with stress. Intact tissues were subjected to histochemical evaluations with a spectrum of class specific reagents. Treatments gave chromophores in solution that could be semiquantitated. Stains of the tissues were also obtained in this process and evaluated. Alternately, surface chemicals of the plant tissues were selectively removed by dipping in solvents and chromatographed in appropriate TLC systems. Characteristic colors were developed with diagnostic spray reagents. Plants investigated were cotton Gossypium hirsutum (L.) lines with varying levels of resistance to boll weevil Anthonomus grandis (Boh.) oviposition, cotton roots with resistance to the root knot nematode Meloidugyne incognita (Chitwood and Otiefa), and corn Zea mays (L.) lines with resistance to southwestern corn borer Dietraea grandioselk (Dyar) larval leaf-feeding. The effects of root pruning and water stress on cotton root chemicals were also investigated in relation to carbohydrate partitioning.
The development of crop plant varieties with resistance to insects, diseases, and other stresses has several advantages compared with other approaches to pest control. There is no complicated technology for the grower to understand, it is inexpensive, since increased seed costs are likely to be more than offset by decreased costs for pesticides and their application, and it is desirable for environmental purposes. Until now, resistant varieties have been selected from field evaluations by plant breeders, often in cooperation with entomologists, plant pathologists, and/or other biologists. In recent years, chemists have This article not subject to
identified a number of resistance compounds (so-called secondary plant compounds or allelochemicals) that had previously or subsequently been shown to be expressed by single genes or combinations of genes. Investigations in which a relatively large number of available susceptible and resistant lines or varieties have been analyzed for content of allelochemicals to determine whether correlations between the allelochemical and resistance are statistically significant have only recently been carried out on a substantive scale. The search for chemical factors of resistance has been complicated by evidence that resistance
U.S.Copyright. Published
1985 by the American Chemical Society
