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Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 282-289
LN nozzle length, cm m adjustable material parameter in eq 11, dimensionless n power law index in eq 13, dimensionless N model parameter in eq 7, dimensionless N , primar normal stress difference as defined in eq 10,
activation energy for viscous flow which in the present case can be calculated by the technique of Saini and Shenoy (1984) and is found to be equal to 8 kcal/mol. (4) The plots of 11 vs. .i.and Nl vs q2under the required conditions can now be readily obtained by substituting the correct value of MFI in the unified curves or in eq 5 and 7. In the case of the solid-state deformation behavior, the necessary data can be regenerated from Figure 9 at any temperature of interest merely by determining the comparative stress value after 24 h of deformation. The value thus obtained as uocan be used in Figure 9 to generate the necessary expected long-term behavior of the PVDF product. The method of coalescence as demonstrated here should, in principle, be applicable to all creep experiments and help greatly in curtailing experimentation time.
dyn/cm P Q flow rate, cm3/s R gas constant, kcal/mol RN nozzle radius, cm R, piston radius, cm t time scale, h T1temperature at condition 1, K T2temperature at condition 2, K Greek letters
4 shear rate, s-l p density, g/cm3 T~ zero shear viscosity, poise
q
Conclusion The present work demonstrates a very effective method for coalescing available data in the melt and solid state deformation behavior of PVDF. The resulting master curves are grade and temperature invarient in the case of the melt data and at least temperature invarient in the case of the solid-state data. The benefits of such master curves are that experimentation is simplified and the time required for experimentation curtailed. The unified curves, therefore, can act as hand tools for quick estimates of deformation behavior of PVDF necessary for design purposes.
apparent shear viscosity, poise
X relaxation time, s
stress at the end of 24 h, d n/cm2 comparative stress, dyn/cm shear stress, dyn/cm2 primary normal stress difference coefficient, dyn d / c m z Registry No. PVDF, 24937-79-9.
f
go g
T
Literature Cited Carreau, P. J. Trans. SOC.Rheol. 1972, 16, 99. Gebauer, P. DynamA Nobel AG, 0-5210, Troisdorf, FRG, private communication, 1985. Pugiia, L. Pennwait Corp., Philadelphia, PA 19102, private communication, 1985. Saini, D. R.; Shenoy, A. V. J . Mecromol. Sci., Phys. 1984,822, 437. Shenoy, A. V.; Saini, D. R.; Nadkarni, V. M. J . Appl. Polym. Sci. 1982,27, 4399. Shenoy, A. V.; Saini, D. R.; Nadkarni, V. M. Rheol. Acta 1883, 22, 209. Shenoy, A. V.; Saini, D. R. Rheol. Acta 1984a,23, 368. Shenoy, A. V.; Saini, D. R. Chem. Eng. Commun. 1984b,27, 1. Wagner, M. H. Rheol. Acta 1976, 15, 136. Wagner, M. H. Rheol. Acta 1977, 16, 43.
Nomenclature C, constant in eq 9 E activation energy for viscous flow, kcal (mol K) F force due to test load, dyne L test load, kg L , load at condition 1, kg L2 load at condition 2, kg
Receiued for reuiew April 19, 1985 Accepted November 18, 1985
Coir Fibers. 3. Effect of Resin Treatment on Properties of Fibers and Composites D. S. Varma, Manlka Varma, and I. K. Varma' Fiber Science Laboratories, Department of Textle Technology, Indian Insitute of Technology, Delhi, New Delhi 1100 IS, India
This paper describes the treatment of bristle coir fibers with a dilute solution of unsaturated polyester (USP) resin in methyl ethyl ketone (MEK). The viscosity of USP resin was significantly reduced by dilution with 50-90% (w/w) MEK. Treatment of bristle coir fibers with such diluted resin solutions resulted in an increase in weight and denier of the fibers. These resutts along with FT-IR studies confirm the deposition of USP resin on the fibers. A significant reduction was observed in moisture regain for all the resin-treated fiber samples. Hybrid composites fabricated with resin-treated coir fibers in combination with glass fibers showed improvement in flexural strength and interlaminar shear strength. A significant retention of these properties was observed when such composites were exposed to relative humidities ranging from 40 to 90% for 60 days.
Introduction Major problems associated with the applications of jute and coir fibers in composite industry are (1)poor wettability of the fibers by organic matrix resins such as unsaturated polyester resins and epoxy resins and (2) high moisture regain of natural fibers. In such composites, delamination due to moisture absorption leads to weak0196-432118611225-0282$01 .SO10
ening of the interfacial bond, thus causing a reduction in the mechanical properties of the composites. It is, therefore, necessary to modify the surface of these fibers to reduce moisture regain. Surface modification of these fibers has been reported by several workers (Sridhar et al., 1982; Prasad et ai., 1983; Mukherjee et al., 1983). In our earlier papers, we have described modification of coir fibers @ 1986 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
by chemical treatments and their applications in fibrous composites. Existence of a porous surface in coir fibers was revealed by SEM studies. A reduction in pores and their distribution on the fiber surface may lead to reduced moisture uptake by the fibers. Coating of natural fibers (e.g., palm) with polymeric materials (e.g., poly(viny1 acetate) emulsion, 2-hydroxyethyl methylacrylate) is reported to give better mechanical properties of the composites (Belmares et al., 1981). In the fabrication of a composite using coir fibers, a viscous solution of resin, catalyst, and accelerator is generally used. Such a viscous solution may not to be able to penetrate the pores. Therefore, it was of interest to study the effect of coating coir fibers by a dilute solution of unsaturated polyester resin in methyl ethyl ketone (MEK). Physical properties of modified bristle coir fibers were evaluated. Composites of coir/glass fiber as reinforcement and unsaturated polyester resin as a matrix were fabricated. The effect of subjecting such composites to humid conditions (relative humidity (RH) 4 0 4 0 % ) on the mechanical properties was also evaluated.
Experimental Methods Materials. Bristle coir fibers and coir fiber mat were obtained from Central Coir Research Institute, Coir Board, Alleppey, Kerala. Unsaturated polyester (USP) resin, Acrolite commercial grade 404 was obtained from Acropolymers Pvt. Ltd., Faridabad. Methyl ethyl ketone (BDH) was used as such. Viscosity of USP resin as received and diluted with MEK was determined at room temperature (36 "C) by using either a Brookfield SynchroLectricviscometer Model LVT (spindle 2, rpm 30 and 12) or an Ostwald viscometer. Chemical Treatment of Bristle Coir Fibers. Bristle coir fibers and coir fiber mat were treated with solutions of unsaturated polyester resin in methyl ethyl ketone. Resin solutions containing 10, 15, 25, 35, and 50% resin solution (w/w) were prepared by diluting commercially available unsaturated polyester resin with MEK. Methyl ethyl ketone peroxide (1.5%, catalyst) and cobalt naphthenate (0.75%, promoter) were dissolved in the resin solution in a beaker. Coir fibers were added to this solution and soaked for 10 min. The fibers were then taken out and held with forceps to allow the excess resin to drip off. Fibers were then spread on aluminum foil, which had been sprayed earlier with a silicone release agent. Curing of the resin coating on the fibers was done in an oven at 50 "C for 1 h. Weight gain and change in denier were evaluated for all the treated fibers. The denier of all the treated fibers was determined by taking fibers of 6-cm length and weighing accurately. An average of 60 fibers was used to calculate the denier. Bristle coir fibers having a percent weight gain (due to resin deposition) of 1.0, 4.4, 8.0, 18, and 25%, could be obtained by treatment with resin solutions of different dilutions. These fibers have been designated throughout this paper as A, B, C, D, and E, respectively. Coir fiber mat was treated only with 10 and 15% USP resin solution under identical conditions. The USP-resin-deposited coir fiber mat thus obtained have been designated as M and N, respectively. Infrared spectra of various coir fiber samples were recorded by using a Nicolet MX-5 FT-IR spectrophotometer in KBr pellets. For this purpose, the fibers were finely powdered; 6 mg of this powder was mixed thoroughly with 100 mg of KBr. The fractured surface of various samples was studied by scanning electron microscopy (SEM) using a Cambridge Stereoscan Model S4-10 instrument. The fibers were
283
fractured by holding them with forceps in liquid nitrogen for few minutes. The broken ends of the fibers were mounted between two pieces of tape such that only 1.5 mm of fiber protruded above the edge of the tape and the rest was between the pieces of the tape. The assembly of the fiber ends mounted in the tape was fixed in an upright position on a circular conducting plate. Tape was carefully coated with silver (to make it conducting) by using Polaron Sputter Coater 5000 under argon atmosphere until the coating thickness was approximately 200 A. Mechanical properties of the fibers were evaluated by using an Instron. For each sample 120 fibers were tested at the following specifications: gauge length 5 cm; crosshead speed 2 cm/min; and chart speed 10 cm/min. Moisture regain of treated coir fibers was determined at relative humidities ranging from 20 to 100%. Humidities were maintained from 20 to 50% by using concentrated sulfuric acid and from 66 to 90% by using saturated salt solutions. Casting of USP Resin Sheets. Neat polyester sheets were prepared between glass plates by using Viton rubber as a spacer and "Spreadsil" silicone mold release agent. USP resin (100 g), MEK peroxide (1.5%) as catalyst, and cobalt naphthenate (0.75%) as promoter were added in a beaker. Air bubbles were removed from the resin by a vacuum desiccator, and this resin solution was carefully poured between the glass plates having the Viton spacers. Resin was cured for 1 h at 50 "C. Composite Fabrication. Untreated bristle coir fibers and resin-coated fibers (sample A) were separately chopped to 1-cm length and used for making hybrid composites along with glass fabric (fiber glass Pilkington weighing 360 g/m2). Coir fiber nonwoven mat and resin-treated samples M and N were used as such along with glass fabric for fabricating hybrid composites. A mild steel mold 22.5 cm X 15.3 cm with interior surfaces chromium-plated was used to fabricate the composites. An 18.0-cm X 14.5-cm glass mat was uniformly coated with unsaturated polyester resin solution containing methyl ethyl ketone peroxide (1-1.5%) and cobalt naphthenate (0.5-0.75%). A higher catalyst and promoter concentration was used in coir fiber mat composites. The prepreg was placed on the female tool of the mold, which had been sprayed earlier with a silicone release agent. Chopped coir fibers and coir fiber mat were spread on this and were coated with the resin. A second glass mat coated with resin solution was then placed on top of the fibers, and a slight pressure was applied with the help of a steel roller to remove the entrapped air. The male tool of the mold was placed on this assembly, and compression molding was carried out on a flat-platen press (Carver Laboratory Press) at a pressure of approximately 30 psi. Curing was done under these conditions for 1h at 65 "C in the case of chopped coir fiber, while in the case of coir mat composites, temperature was maintained at 50 "C for 1 h. Composites were exposed to humidities ranging from 40 to 90% RH for a period of 2 months. Changes in weight and thickness of the composites were determined at regular intervals. Flexural properties and interlaminar shear strength of the composites were evaluated before and after exposure. Evaluation of Properties of USP Resin Sheets and Composites. The resin content of composites was determined by pyrolysis in a furnace. A known weight of the sample was heated in a silica glass crucible at 500 "C for 4.5 h. Residual glass fibers were weighed. From the knowledge of coir fibers taken initially the weight of resin
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
Table I. Effect of MEK on Viscosity of Unsaturated Polvester Resin viscosity of sample wt of resin, wt of MEK, resin solution," no. E g CPS ~~~
~
1 2 3 4 5 6
100 50 35 25 15 10
667.0 3.90 2.58 1.82 0.95 0.68
50 65 75 85 90
For 1-3 viscosity was determined by using Brookfield viscometer. For 4 - 6 viscosity was determined by using Ostwald viscometer.
in composites was evaluated. Density and void content were determined according to ASTM D 792-66 and ASTM D 2734-70, respectively. The tensile strength of dumbbell shaped neat polyester sheets was determined according to ASTM D 638-76 and the following specifications: gauge length 5 cm; crosshead speed 1 mm/min; and chart speed 50 mm/min. Flexural strength was determined according to ASTM D 790-71 by using Instron Model 1122. Span-to-depth ratios of 32:l in the case of USP sheets and 16:l for composites, crosshead speeds of 3 mm/min for USP sheets and 2 mm/min for composites, and a chart speed of 100 mm/min were used. Interlaminar shear strength was determined according to ASTM D 2344-76 and a span-to-depth ratio of 5:1, a length-to-depth ratio of 7:1, a crosshead speed of 1mm/ min, and a chart speed 20 mm/min.
Results and Discussion Effect of Resin Treatment on Properties of Coir Fibers. The viscosity of the USP resin was changed significantly by diluting it with an equal amount of MEK (Table I). Maximum drop in viscosity occurred after 50% dilution with MEK, beyond which only a marginal drop in viscosity took place. The mechanical properties of USP resin sheets were as follows: tensile strength 4963 psi; elongation 2%; flexural strength 13 260 psi; and flexural modulus 419 X lo3 psi. Treatment with resin-MEK solutions resulted in a linear increase in weight and denier of the coir fibers (Figure 1). The resin deposited on the fibers depended on the concentration of the unsaturated polyester resin. Significant changes were observed in the FT-IR spectra of all the resin-treated coir fiber samples (Figure 2). The IR spectra showed characteristic absorption peaks for the cured polyester resin. New absorption bands were observed at 710,745, and 800 cm-l. These could be assigned to the presence of mono- and disubstituted benzene rings arising from the styrene and phthalic anhydride residue of unsaturated polyester resin. The intensity of these peaks showed an increase with the increase in percentage
2
01 0
10
I
I
I
I
I
20
30
40
50
60
CONCENTRATION OF R E S I N SOLUTION % ( w / w )
Figure 1. Plots showing effect of concentration of resin (MEK solution) on (1)weight gain and (2) denier of coir fibers.
weight gain of coir fibers. The small absorption peak at 1740 cm-* (due to vC4 stretching) present in bristle coir fibers changed to a sharp peak for the resin-treated coir fibers. The intensity of this band increased with the increase in weight gain, thus clearly indicating the deposition of the unsaturated polyester resin in the coir fibers. The SEM micrographs of untreated and treated coir fibers are shown in (Figure 3). The bottom-right photograph indicates extensive fibrillation in the fiber. The cellulose fibrils are surrounded by the amorphous cementing material comprising lignin and hemicelluloses. Initially, when the resin solution is dilute, a significant amount of it penetrates the fibers through the pores present on the surface and deposits itself in the interfibrillar regions of the fiber. Even at higher concentrations of the resin solution (Le., 50%), the viscosity of the solution was not sufficiently high and, therefore, the extent of penetration was unchanged. However, more resin was deposited on the fiber surface (Figure 3), thereby increasing the denier. Figure 4 shows the stress-strain diagrams of the untreated and treated coir fibers. Stress and elongation at break decreased for all the treated fibers, while an increase in initial modulus was observed for samples A-C. A decrease of 9.5% in elongation at break and an increase of 10.5% in initial modulus were observed for sample A (Table 11). The decrease in elongation at break is due to the brittle nature of the resin. In addition to being deposited in the cementing material, some of the resin was deposited on the cellulosic fibrils, thus causing a reduction in the elongation at break.
Table 11. Mechanical Properties of Resin-Treated Coir Fiber Samples resin deposited on fiber, samole" untreated A B C D E
%
denier
initial modulus, a/denier
0
370.5 377.9 392.7 418.6 448.3 474.2
38.30 42.33 41.04 41.28 38.20 34.70
1.0 4.4 8.0 18 25
tenacity, g/denier (CVW) 2.26 2.30 2.15 2.07 1.97 1.68
(28.0) (29.2) (28.0) (30.0) (23.6) (27.8)
e1onga tion at break, 90 28.8 26.07 26.20 26.0 25.36 22.47
"Fiber samples A-E were obtained by treating with MEK solution of USP resin (having catalyst and promoter) for 10 min and then curing at 50 "C for 1 h.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 285
19.745
17.316
T 4.886
I
-I 2.451
E z a c
c
,
0.026 I
z Q
a
c ,5991
,1899
WAVENUMBEP
( C M - ’1
Figure 2. IR spectra of bristle coir fiber (11, bristle coir fiber treated with resin (sample A) (2), and sample E (3).
For sample E significant reduction was observed in initial modulus, tenacity, and elongation at break. This is because the concentration of the resin is sufficiently high and because it was deposited on the surface in addition to penetrating the fiber. This would make the fiber more brittle in nature. Moisture regain was determined for untreated and USP-resin-treated fibers at several relative humidities ranging from 10 to 100% at room temperature. Figure 5 shows the plots of moisture regain vs. relative humidity of bristle coir fiber and fiber sample A. Rapid increase in moisture regain was observed at low (10-30%) and high (80-100%) humidities, while in the intermediate region (30-8070)) the moisture regain increased marginally with an increase in relative humidity. Coir fiber is a lignocellulosic fiber, and lignin, cellulose, and hemicellulose contribute to its hygroscopicity. Moisture is preferentially absorbed in the noncrystalline regions of the fiber. Similar observations have also been reported by Kulkarni et al. (1983). Treatment of the coir
fibers with resin solutions results in the formation of a resin coat on the fiber surface, and the pores present on the fiber surface tend to get clogged with the cured resin. This results in a significant lowering of moisture regain at various humidities. An increase in the percentage of USP resin deposition on bristle coir fibers (samples B and C) resulted in a further decrease in moisture regain (Figure 6). For samples D and E no significant decrease in the moisture regain was observed. It may therefore be concluded that deposition of 10% resin on the coir fibers is sufficient to impart hydrophobicity and maximum possible reduction in moisture regain. Mechanical Properties of Composites. Treatment of coir fibers with dilute resin-MEK solutions (sample A) did not change the properties of the fibers significantly, while higher concentrations (samples D and E) led to deterioration of the mechanical properties by deposition of thicker coats of resin on the fiber surface. In addition, fiber sample A showed a significant reduction in moisture re-
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I d . Eng. Chem. Prod. Res. Dev., Vol. 25. No. 2, 1986
F i g u r e 3. Scanning electron micrographs of (top left) surface of untreated bristle coir fiber (259x3, (top right) surface of sample C (266x3, (bottom left) surface sample E (504X), (bottom right) fractograph of sample E (637x1. T a b l e 111. Mechanical Properties of Clan F a b r i c K b o p p e d B r i s t l e Coir Fiber Reinforced Unsaturated Polyester Resin Composites composite fabricated by usinf untr~ated fibersample property teated coir fibers A 66.0 65.1 resin content, M % 14.5 14.8 glass fabric. M % 19.5 20.1 coir fibers. M % 21380 (147.5) 26278 (181.3) flexural strength. psi (MN/mz) 604 (4.2) 568 (3.9) flexural modulua. psi x io3 (GNIm2) maximum strain, % 2.9 2.9 interlaminar shear strength, psi X 1.1 (7.5) 1.3 (9.1)
IO3 lMN/m2) density ~ ~ i p t lg/cm8 ). void content. To
1.32 3.16
1.36
1.0
OChoppd bristle coir fibers (1 cm long).
gain. Composites were, therefore, fabricated by using sample A. The tensile strength and modulus of bristle coir fibers were found to be 40 X lo3 and 686 X lo3 psi, respectively, while the tensile strength and modulus of cured USP resin sheets were 4.97 X lo? and 24.5 X lo9 psi, re-
Table IV. Mechanical Properties of Glass FabrielCoir Fiber Mat Composites coir fiber mat property tested untreated sample M sample N resin content, wt 70 51.7 57.5 57.5 12.2 12.5 12.4 glass fabric. w t ?b 30.3 30.0 coir fiber mat, wt % 29.9 flexural strength, psi 21035 (145) 21880 (151) 20991 (145)
maximum strain, % interlaminar shear strength, psi X 109 (MNIm') density (exptl), glcm3 void content, %
1.1 (7.8)
3.5 1.4 (9.6)
3.6 1.3 (8.9)
1.25
1.28
1.20
4.3
6.0
2.5
3.6
spectively. This clearly indicated that these fibers could be used as a reinforcement. In comparison to neat polyester resin sheets, the hybrid composites made with untreated coir fibers showed a considerable improvement in flexural strength and flexural modulus (Table 111). Hybrid composites made from coir fiber mat had comparable flexural strength and interla-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 287 3.21
0
5 '1.
STRAIN
Figure 4. Stress-atrain diagrams of bristle coir fibers (l), bristle coir fibers treated with resin (sample A) (2), sample B (3), and sample
10
15
20
25
INCREASE IN WEIGHT OF COIR FIBRES
Figure 6. Plots of moisture regain vs. percentage of resin deposition on coir fibers at (1)30% RH,(2)50% RH,(3)66% RH,and (4)84%
RH.
E (4). 2.52
RELATIVE
HUMIDITY
( '1. )
Figure 5. Plots of moisture regain vs. relative humidity of (1)bristle coir fibers and (2) bristle coir fibers treated with resin (sample A). EXPOSURE
minar shear strength with the composites fabricated by using chopped coir fibers, even though the weight percent of coir fibers was higher in the former composites (Table IV). However, a lower flexural modulus compared to that of composites made with chopped coir fibers was observed. Composites made with resin-treated coir fiber mat (samples M and N) showed a marginal variation in flexural properties. A -23% (sample M) and -14% (sample N) improvement in interlaminar shear strength was observed. The improvment in the mechanical properties is perhaps due to the increased interfacial bonding between the fiber and the matrix resin. Presence of a layer of resin on the
TIME
,
Days
F i g u r e 7. Plote of percent increase in weight of composites fabricated with coir fiber mat at 84% R H vs. exposure time (in days).
fiber surface results in increased interaction with the matrix resin, thereby increasing the interfacial bonding energy. In coir fiber composites the fiber and the matrix properties are expected to be affected by the presence of moisture. Moisture regain of coir fibers was described earlier. Moisture regain for USP sheets at 84,66,50,and 30% RH was found to be 1.0, 0.8, 0.5, and 0.3570,respectively.
288
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
Z
0 I-
z
t-
ii'
07.-
0 7'
I
, 1
1 0 6i - _ _ _ i ~ - - L _ _-__. _ _ lA _ _0_ . 6;0 20 40 6u e0 '30
RELATIVE
HUMIDITY
20
40
60
RELATIVE
'lo)
Figure 8. Plots of retention of flexural strength of composites fabricated by using (a) chopped coir fibers (1 cm long) and (b) coir fiber (sample A) vs. relative humidity.
I
-'
80
HUMIDITY
IO0
(V0)
Figure 10. Plots of retention of flexural strength of composites fabricated by using coir fiber mat vs. relative humidity: (1) untreated coir fiber mat; (2) resin-treated coir fiber mat M; (3) resintreated coir fiber mat N. I
181
I
I
c 4 . iii
r
4i-
LU
CI
2 4
9
1
-
A
I
1
I
20
40
60
80
RELATIVE
3
20
40 RELATIVE
60 HUMIDITY
80
100
,
Figure 9. Plots of retention of interlaminar shear strength of composites fabricated by using (a) 1-cm-long bristle coir fibers and (b) resin-treated bristle coir fibers (sample A) vs. relative humidity.
No change in thickness was observed for composites exposed to various humidities ( 5 0 4 4 % ) over a period of 2 months. Figure 7 shows the plot of weight gain vs. exposure time. The increases in the weights of composites made with untreated coir fiber mat and resin-treated coir fiber mat M were comparable, while it was higher for the composite made with sample N. A similar behavior was also observed for composites made with untreated and resin-treated chopped coir fibers. Figure 8 shows the plots of retention of flexural strength of composites fabricated by using chopped bristle coir fibers vs. relative humidity ranging from 50 to 84%. The composite made with sample
HUMIDITY,
1 1C0
o/i,
Figure 11. Plots of retention of interlaminar shear strength of composites fabricated by using coir fiber mat vs. relative humidity: (1)1-cm-long bristle coir fiber mat; (2) resin-treated coir fiber mat M; (3) resin-treated coir fiber mat N.
A showed a higher retention in flexural strength compared to that fabricated with untreated coir fibers. Significant retention in interlaminar shear strength (ILSS) of composites having resin-treated chopped coir fibers was also observed, while moisture affected the interfacial bonding in composites made with untreated coir fibers, thus causing a lowering in ILSS (Figure 9). Composites made with coir fiber mat M and N exhibited a higher retention of flexural strength in comparison to composites made with untreated coir fiber mat (Figure 10). A t lower humidities the composite made with sample M showed a better retention of flexural strength, while at higher humdities the composite made with sample N seemed to be more effective. Interlaminar shear strength
Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 289-296
of composites fabricated with resin-treated coir fiber mat showed an at for 'Omhumidities, posites based on untreated coir fiber mat a significant reduction in ILSS was observed (Figure 11). It may be concluded from these studies that a coating of USP resin on coir fibers improves the compatibility of fibers with matrix resin and improves the aging characteristics of composites. Registry No. Acrolite 404, 101374-98-5.
289
Kulkarnl, A. G.; Cheriyan, K. A.; Satyanarayana, K. G.;Rohatgi, P. K. J. Appl. folym. Scl. 1983, 28, 625. Mukherjee, R. N.; Pal, S. K.; Sanyal, S. K.; Sanyal, S. K. J. Appl. po/ym. Scl. 1983, 28, 3029. Prasad, S. V.; Pavithran, C.; Rohatgl, P. K. J. Mater. Sci. 1983, 18, 1443. SrMhar, M. K.; BasawaraJappa, G.; Kasturl. S. G.; Balasubramanlum I n d h J. Texf. Res. 1982, 7 , 87. Varma, D. S.; Varma, M.; Varma, I. K. Text. Res. J. 1984, 54, 827. Varma, D. s.; Varma. M.; Varma, I. K.. "Coir Fibers I1 Evaluation as a reinforcement in unsaturated polyester resin", J . Reinf. flast. Compos. 1985. 4 . 419.
Varma, 'D.'S.; Varma, M.; Varma, I. K., "Composite Materials from Coir Fibers", submitted for publication to folym. Mater. Scl. Eng ,
L i t e r a t u r e Cited Belmares, H.; Barrera, A.; Castilio, E.; Verheugen, E. V.; Monjaras. M.; Patfoort, G. A.; Bucquoye, M. E. N. Ind. Eng. Chem. Prod. Res. D e v . 1981, 20, 555.
Received for review August 8, 1985 Accepted January 14, 1986
Cure and Photodegradation of Two-Package AcrylicAJrethane Coatings Davld R. Bauer' and Ray A. Dlckle Scientific Research Staff, Ford Motor Company, Dearborn, Mlchigan 48 12 1
Jack L. Koenlg Department of Macromolecular Science, Case Western Reserve Unlverslty, Cleveland, Ohio 44 106
't:
The chemistry and kinetics of cure and hotodegradation of two-package acrylic/urethane coatings have been measured by infrared and magic-angle C NMR spectroscopies. The rate of disappearance of the Isocyanate band in the infrared has been found to obey simple second-order kinetics. The activation energy of the cross-linking reaction is 8.2 kcal/mol. Second-order kinetics and the relatively low activation energy lead to coatings with greater cure latitude (substantially wider cure windows) than typical acrylic/melamine coatings of similar composition. The urethane cross-links formed on cure are found to degrade on exposure to ultraviolet light. Loss of urethane cross-links has been found to be photochemical rather than hydrolytic in nature. The chemistry of cross-link degradation is a function of the composition of the acrylic copolymer and can be greatly inhibited by the addition of hindered-amine light stabilizers.
Introduction
Polyurethane coatings have been recently reviewed (Potter et al., 1984). High-solids coatings based on hydroxy functional polymers cross-linked with aliphatic isocyanates are candidates for use as top coats for automotive application particularly in base-coat/clear-coat systems. Although isocyanate cross-linking chemistry has been studied in great detail (Chadwick and Cleveland, 1981; Ozaki, 1972; Saunders and Frisch, 1962), there have been no reported studies of the kinetics of cross-linking and the implications to network formation in actual coating formulations. Studies of degradation in isocyanate cross-linked coatings have focused on hydrolytic stability of urethanes and ultraviolet (UV) induced oxidation of aromatic isocyanates (Wagner and Mennicken, 1962; Beachell and Chang, 1972; Matusak et al., 1973; Schollenberger and Stewart, 1973, 1976; Schultze, 1973; Osawa et al., 1975; Allen and McKellar, 1976). Other workers (Baker, 1980; Boch and Uerdingen, 1980) have studied the effects of different polyols on the physical durability of urethane coatings. There have been relatively few studies of the chemistry of UV-induced degradation in urethane coatings crosslinked with more durable aliphatic isocyanates (Schollenberger and Stewart, 1976). 0196-432118611225-0289$01.50/0
In this paper, the chemistry and kinetics of cure and degradation of coatings based on acrylic copolymers cross-linked with an aliphatic isocyanate (a biuret of hexamethylene diisocyanate) are studied. Infrared spectroscopy is used to follow the extent of isocyanate reaction as a function of time and temperature. The extent of reaction is used together with a network formation model (Bauer and Dickie, 1980; Bauer and Budde, 1980) to calculate a quantity that has been found to correlate well with physical measures of cure in melamine cross-linked coatings. These calculations are used to compare cure response (Bauer and Dickie, 1982) in urethane and melamine cross-linked coatings and to determine the sensitivity of network formation to the ratio of hydroxy to isocyanate. Infrared and magic-angle spinning (MAS) 13C NMR spectroscopies (Lyerla, 1980) are used to follow chemical changes in these coatings as a function of exposure to UV light and humidity. The effects of exposure conditions, coating composition, and the addition of light stabilizers are discussed. Experimental Methods Materials. The two acrylic copolymers used in this
study were prepared by conventional free-radical polym0 1986 American Chemical Society
