652
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 652-657
Lubin, G. "Handbook of Fiberglass and Advanced Composites"; Society of Plastics Engineers: Van Nostrand-Reinhold Co.: New York, 1969;Appendix, p 873. McGarry, F. J.; Mandeli, J. F. "Fracture Toughness of Fiber Reinforced Composites", M.I.T. Civil Eng. Report R70-79,Dec 1970. Park, W. J. J. Compos. Mater. 1979, 73, 219. Piggott, M. R. J. Mater. Sd.1970, 5 , 669. Rieke, J.; Bhateja, S.; Andrews, E. Ind. Eng. Chem. prod. Res. Dev. 1980,
Sines, G. "Recent Advances in Composite Materials": UCLA course 853.15, Mar 1972. WhRe, J. L.: Czarnecki, L. J. Appl. Pdym. S d . 1980, 25, 1217. Whittaker, R. E. J . Appl. folym. Sd.1974, 78, 2339. Yang, J. N.; Liu, M. D. J. Compos. Mater. 1977, 1 7 , 176.
Received for reuieu, December 14, 1982 Accepted June 6, 1983
19, 601.
New Composite Materials from Natural Hard Fibers. 3. Biodeterioration Kinetics and Mechanism Hector Belmares, Arnold0 Barrera, and Margarlta MonJaras Centro de Investigacion en Quimica Aplicada, Saltillo, Coahuila, Mexico
The effect of soil burial biodeterioration on natural hard fibers, natural hard fiber composites, and glass fiber composites was studied. First-order kinetics of the tensile strength degradation rate was found. A comparison of tensile strength half-life values was made for a variety of composites. An inverse relation between the tensile strength half-life values and the hydrophilicityof the matrix was found. The mechanism of biodeteriorationinvolves water transport to the natural hard fiber, fiber swelling, slight matrix cracking, and finally, massive microorganism penetration to the fiber. Means to avoid water penetration and to protect the fiber from biodegradation were developed. Therefore, important disadvantages of the natural hard fiber composites have been overcome. Additionally, the wide angle X-ray diffraction pattern of palm fibers is essentially due to a crystalline lattice of cellulose I, in agreement with reported work on other lignocellulosic vegetable fibers.
Introduction Renewable resources are regaining the interest of many nations in a programmed search to find economically useful industrial applications. Natural hard fibers are one example of such a programmed search (Belmares et al., 1979a,b). A recent paper by us (Belmares et al., 1981, with references therein) describes the use of such fibers to develop composite materials with a favorable strength/price ratio. The studies were done with palm fibers (Yucca carnerosana) which were demonstrated to be similar in physical, mechanical, and chemical properties to other natural hard fibers in the world. Those findings could help to alleviate the worldwide scarcity of low-cost building materials and the market slump in which the natural hard fibers have been for several decades. The present paper describes the biodeterioration of natural hard fiber composites, its kinetics and mechanism, and ways to avoid it. This work is a continuation of our programmed studies of the mentioned composite materials. The behavior of the composites under cyclic stress fatigue degradation has already been reported (Belmares et al., 1983). Experimental Section Sample Composition and Preparation. Table I shows the composition of samples. A recent paper (Belmares et al., 1981) describes the natural hard fibers obtainment, procedures and conditions for polyester (PER) laminates manufacture, and their testing to determine tensile properties. Additional Equipment a n d Wide Angle X-ray Diffraction (WAXS) Pattern. The WAXS palm fiber patterns were made with nickel filtered Cu K a radiation. The diffraction patterns were recorded on flat film using a Rigaku-General Electric rotating anode X-ray generator. The sample-to-film distance was 3.00 cm. The X-ray unit was operated at 30 kV and 20 mA. Figure 1shows a typical
WAXS palm fiber pattern. In agreement with reported work on other lignocellulosic vegetable fibers including sisal (Barkakaty, 1976, with references therein), the X-ray pattern of the palm fiber is essentially due to a crystalline lattice of cellulose I. The pattern includes the (101), (lOT), and (002) reflections (Ray, 1968; Ray and Montague, 1977). Measurements and Test Methods. The soil burial test is a severe test; it exposes the materials to a broad spectrum of destructive organisms and it is a reasonable simulation of the field use (Turner, 1972). This test was performed by burying the samples (composite materials or fibers) in a matured compost of loam and grass clippings. The soil was always kept at 80% (by wt) of its water saturation point. This was done by weighing the soil each week and adding any missing water. Incubation conditions were 30 f 2 "C and 80-90% relative humidity. Before the start of a burial test, the matured compost was statistically "calibrated" in its biodeteriorating effects by the use of palm fibers. Figure 2 shows a typical result. Less tensile strength than the lowest point makes the fibers unmanageable. The effectivity of the compost is comparable to other more complex preparations that include manure (Kulkarni, 1963). We believe that the present soil preparation minimizes variations in the repeatability of the burial test. Turner (1972) has shown statistically the importance of controlling moisture content and apparent density of the soil bed. Considerable local variations in both parameters will give place to local intensification or abatement of attack, thus damaging the repeatability of the soil burial test. For the present work, the samples used for the burial test were 30 X 30 cm laminates with the edges coated with a resin formulation PER/Sty = 60:40. The coating was dried for 5 days and the test laminates were then immersed in the soil with care being taken that they did not touch each other. At each specified time, one laminate per
0196-4321/83/1222-0652$01.50/00 1983 American Chemical Society
I d . Eng. Chem. Rod. Res. Dev.. VOI. 22. NO. 4, 1983 653
Table I. Composition of Samples sample
no. of
composite fiber fiber type per ply content,e wt % 30 1 1 NFM 34 2 A 2 NFM/NFM 34 3 B 1 NFM 35 4 B 2 NFM/NFM I A 1 GWR 51 8 A 2 GWRIGWR 56 20:22 9 A 2 GWR/NFM 32 11 D 1 NFM 12 D 2 NFM/NFM 34 30 19 C 1 NFM 33 20 C 2 NFMINFM 12:18:12 21 A 3 GMINFMIGM 15:14:14 22 A 3 GM/NFM/NFM 36 23 A 3 GM/GM/GM 42 24 A 3 NFMINFMINFM 25 A 4 GWR/NFM/NFM/GWR 14:12:12:14 26 A 3 GWRINFMINFM 18:15:15 13:11:11:13 21 A 4 GWR/NFM/NFM/GM 28 A 3 GWRIGWRIGWR 54 a Mixing ratio in weight; formulation A, PER/Sty = 60:40; formulation B,PER/Sty/HEMA (2-hydroxyethyl methacrylate) = 60:35:5; formulation C, PER/Sty/MAA (methacrylicacid) = 60:35:5; formulation D,0.055% (by wt, fiber basis) of zinc chloride sprayed on fibers, PER/Sty = 60:40. GM, chopped glass strand mat of 458 glm' made with randomly distributed strands (5 cm long); NFM. natural fiber mat (palm fiber mat) of 450 glm' made with 9 cm long randomly distributed fibers and 6% (by wt, fiber basis) of polyvinyl alcohol as the binder and interfacial agent; GWR, glass woven roving of 500 glm'. For example, sample no. 9 has 20% and 22% (by wt) of GWR and NFM, respectively (that is, 58% of the sample weight corresponds to the PER polymer matrix). no.
PER/Sty formulationa A
plies in composite
1
I
Figure
1.
Typical WAXS pattern of palm fibers.
1
r , l
\
TIME. hour3
Figure 2. Least-squares fit of In (residual strength)"9. biodeteriw ration time for palm fibers. Each data point is the average of 30 single-fiber determinations. Errors were calculated at a 90% con(.i.l^-.-
.IYC..CC
.=.-._
In..-,
sample was taken out of the soil, the soil was given a good mixing, and the laminate was washed free of growth with a palm fiber brush and water that contained a small
amount of detergent, washed again with pure water, and then air-dried for 5 days. Specimens were cut, conditioned, and tested following the ASTM D638 method. The edges of the laminate from where the specimens had been taken were coated with PER/Sty = 6040, the coating dried for 5 days, and the laminate buried again in the soil. However, generally, one laminate was consumed for the obtention of one data point. For the fibers biodeterioration test, the fibers were spread inside the soil. At each specified time some were taken out, washed carefully with water, air-dried a t room temperature, conditioned, and tested following a published method (Belmares e t al., 1979a). Results and Discussion Bidetenoration Kinetics of Natural Hard Fibers. The natural hard fibers are composed of ultimate cells cemented together with lignin, hemicellulose, and other natural adhesives. Their cellulose, hemicellulose, and lignin contents are in the ranges of 5C-80%, 1C-2570, and 8-13%, respectively; the number-average degree of polymerization (DP) of cellulose for palm and henequen fibers is in the range of 50C-900 (Belmares e t al., 1981), which is relatively low when compared with the DP of cotton usually falling in the range of 2400-2500 (Hebeish et al., 1981);WAXS patterns for palm fibers (Figure 1)and for many other lignocellulosic vegetable fibers (Barkakaty, 1976) are essentially due to a crystalline lattice of cellulose I; the holocellulose fraction of jute fibers has a degree of crystallinity of 58% and 41% for the dry and moist states, respectively (Ray, 1968); the monosaccharides arabinose, xylose, mannose, and galactose have been identified in the hemicellulose fraction of palm fibers (Belmares et al., 1981) and in the Same fraction of sisal fibers, arabinose, galactose, glucose, xylose, mannose, and rhamnose have been identified among other components, all of these forming hemicellulose with a number-averace molecular weieht of about 25000 with a DP of 165-177 (Das Gupta andhukherjee, 1967). From these facts and considerations we conclude that the biodeterioration process of natural hard fibers is a rather complex one due to their complex morphology and
654
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
I I
-A-3
i-
8 .
I'
sz
B
0
n w
; r
-
71
;i-
E6&,
, , , , , , , , , , , , , , ,
40
120 160 200 240 280 320 360
80
T I ME days
Figure 4. Least-squares fit of In (residual strength) vs. biodeterioration time for composite samples no. 7-9.
" & I r I =0.34
.
.
.
1
.
.
40
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
1
-0-11 -A-lZ
.
80 120 160 200 240 280 320 36C T I M E . days
Figure 3. Least-squares fit of In (residual strength) vs. biodeterioration time for composite samples no. 1-4. Plot for sample no. 1 shows the typical error in tensile strength (*12%, at a 90% confidence level) for each data point of Figures 3-7.
composition. Figure 2 shows that a first-order kinetics of the tensile strength degradation rate of palm fibers is a good fit for the data. This means that the rate equation and its integrated form are given by --dat - kat (1) dt at = a. exp(-kt) (2) where ut and uo are the fiber tensile strengths at time t and time t = 0, respectively, and k is the first-order rate constant (in this case a t 30 " C ) . For cotton, it has been demonstrated (Hebeish et al., 1981) that there is a direct relation between DP and the tensile strength. An inverse relation between the fraction of bonds broken, B, and DP was also found and is given by B = l/Pt - 1/P, (3) where Po and Pt are the DP of the original cellulose and the degraded cellulose, respectively. Therefore, in principle, eq 1 and 2 could be expressed in terms of the DP or the fraction of bonds broken. In general, for the natural hard fibers or even for cotton the accessibility of cellulose to extracellular enzymes is limited in part by its distribution within the cell wall and the nature of structural relationships among the cell wall components. The presence of lignin throws another variable in the biodeterioration of natural hard fibers. I t is noteworthy that simple expressions such as eq 1 and 2 are found for the biodeterioration kinetics of palm fibers. I t must be pointed out that in the present work no inhibition of the biodegradation of cellulose due to cellobiose (the latter one produced during the initial steps of cellulose biodegradation) was observed. The inhibition by cellobiose is reported to decrease significantly the rate of cellulose I biodegradation at an early stage (Focher et al., 1981). These authors report that the inhibition dissappears when the enzymes cellulase (from Trichoderma uiride) and cellobiase (from Aspergillus niger) were placed
-C
1
1
80
40
1
1
1
1
/
1
l
l
1
1
1
1
120 160 200 240 280 320 360 T I ME, days
Figure 5. Least-squares fit of In (residual strength) vs. biodeterioration time for composite samples no. 11 and 12.
?
A
i
l
, 80
4C
1
I
1
'20
,
,
,
,
I
,
I
,
,
, ,
160 200 240 280 320 360
-0-2 20
-A-
I
b
-
i L p 5.4
,
,
40
,
,
80
,
/
,
,
,
,
,
,
/
/
,
,
,
,
,
120 160 200 240 280 320 360 T I ME, days
Figure 6. Least-squares fit of In (residual strength) vs. biodeterioration time for composite samples no. 19 and 20. Samples no. 1 and 2 are shown as a comparison.
together with the cellulose (in vitro). For the present work, the soil used for the biodeterioration of natural fibers must contain the two fungi mentioned and many more. Actually, these two fungi and more than 120 other species of fungi have been isolated from soil samples (Apinis, 1972). Biodeterioration Kinetics and Mechanism of Composite Materials. Figures 3-6 show the data obtained from a variety of samples. For some samples the shape
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 655
Table 11. Tensile Strength Half-Life t,,, and Strength Degradation Slope m for the First and Second Parts, Respectively, of the Biodeterioration Plots of Composite Materials ~~
sample no.
t,,,, daysa,b
1 2 3 4 7 8 9 11 12 19 20
66 i 3 82 i 4 35t 2 61 t 3 >lO0OC
>1000d >lOOoe 134 -c 20 333 ?- 25f 30 * 2 66 t 3
-m x lo4,day-’ 16.19 5.33 16.00 5.33 no second part no second part
no second part -0 no second part 31.58 28.57
a The experimental error was calculated at a 90%confidence level. Natural fibers without polyester polymer matrix have t,,, = 1.854 days under the same conditions Slope = (from Figure 2). Slope = -4.44 X -4.44 x e Slope = -1.33 X f Slope = -20.63 x 10-4.
Table 111. Tensile Strength of Natural Fiber Composite Materials After a Room Temperature Water Immersiona and a 45-day Biodeterioration tensile strength? ka/cmZ dry
sample sample no. controld 1 2 11 12 24
372 437 324 388 512
%day
water immersion 355 4 54 374 417 473
45-day
45-day
338 397 304 369 480
235 299 255 358 ND e
water biodeterioimmersion ration
Laminates were immersed in room temperature water for the indicated time and then air-dried for 5 days (same air-drying period as for biodeteriorating samples) at room temperature. Specimens were cut and then tested Data obtained following the ASTM D638 method. from corresponding biodeterioration plots. Error is *12%at a 90%confidence level. Untouched by water. e ND, not determined. of strength degradation can be represented reasonably well by two intersecting straight lines that we will call first and second parts (for an example, see Figure 3a) of the biodeterioration curve. For some other compositions the shape of strength degradation can be represented reasonably well by only one straight line (for an example, see Figure 4). Table I1 shows the tensile strength half-life and strength degradation slope for the first and second parts, respectively, of the biodeterioration curve for each sample studied. The data indicate that there is a relatively fast tensile strength degradation followed by a slower one for some samples. Before making any correlations from Table 11, let us study Table 111. This table shows the results of immersing different natural fiber reinforced polyester samples in water a t room temperature for a prolonged time. The tensile strength is not affected by a 45-day immersion. However, the effects of soil biodeterioration on these samples, for the same period, significantly reduced the initial tensile strength, except for sample no. 12, which has a microbicide agent (zinc chloride). The microorganisms are expected to attack preferentially the natural fibers instead of the styrene polyester matrix. In fact, styrene polyester matrix without any fibers was essentially unaffected in tensile strength by soil exposure for at least eight years and was comparable to acrylics and fluorocarbon polymers (Connolly, 1972). Therefore, the leveling off of
the tensile strength degradation for samples no. 1-4, 11, 19, and 20 (Table I1 and corresponding figures) is inferred to happen after the natural fibers have been practically biodegraded through first-order kinetics of the tensile strength degradation rate. This leaves the styrene polyester matrix practically without reinforcement and with a great deal of voids. Further proofs that this inference is correct are obtained, firstly, by observing that the tensile strengths of the composites are very close to each other at the point of leveling off. Secondly, the slopes of the leveling off lines in some cases (samples no. 2,4, and 11) are similar to the slopes of samples that do not biodeteriorate readily (samples no. 7 and 8, glass reinforced polyester composites). Interestingly enough, although samples no. 3 and 4 contain 2-hydroxyethyl methacrylate (hydrophilic monomer) in the resin formulation, this does not affect the slope of the second part region of the biodeterioration curves. However, when HEMA is replaced by methacrylic acid (Figure 6, samples no. 19 and 20), the slope of the second part region becomes negatively greater, giving the impression that even the matrix is being rapidly deteriorated. This is most possibly due to the action of the hydrogen ions from the carboxylic acid introduced, which catalyses the hydrolytic scission of the polyester chains to produce even more carboxyl end groups, rendering the polyester matrix more hydrophilic, weaker, and more susceptible to biodegradation. Thirdly, sample no. 12 which contains zinc chloride as a microbicide agent does not present levelling off, which means that the natural fibers are very slowly biodegraded and even after 300 days have not been totally consumed (Figure 5). Fourthly, a test of the burial soil with palm fibers without matrix a t the end of the biodeterioration experiments (to get again a plot similar to the one of Figure 2) speaks against an inhibition by cellobiose or a drying of soil microorganisms to explain the leveling off (second part regions) of the biodeterioration curves. Coming back to Table 11, the tensile strength half-life for the first part of the biodeterioration curves is very revealing. For natural fiber reinforced materials and for any resin formulation, the change from one to two plies is beneficial (samples no. 1,2; 3,4; 19,20). This is due to the increased protection against attack of the composite inner fibers. Keeping the number of plies constant, but making the polyester matrix more hydrophilic, the tensile strength half-life is reduced drastically (samples no. 1, 3, 19; 2, 4, 20). Here, the action of HEMA or MAA upon biodeterioration is about equivalent. The effect of reduced half-life with increased matrix hydrophilicity shows the wrong direction to go to solve the problem of biodeterioration and can be explained in the following way. HEMA and MAA make the matrix to be of increased capacity for water absorption and therefore weaker to stress resistance. The natural fibers, on the other hand, having a greater capacity for water absorption, increase much more in volume than the matrix. In the direction parallel to the fiber this differential expansion produces axial compression in the fiber and axial tension in the polyester matrix. This results in large axial and tangential shear stresses and causes matrix cracking allowing the microorganisms to enter the fibers rapidly. This points out the importance of protecting the natural fiber reinforced composites from water penetration. Lastly, when the natural fibers contain a relatively small amount of zinc chloride, the tensile strength half-life increases up to fourfold (samples no. 11 and 12). Zinc chloride is a known deodorant, disinfectant, and astringent agent (Stecher, 1968). It is also an inhibitor (even in small concentrations) for the growth of Rumino-
050
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
Table IV. Boiling Water Absorption of Reinforced Materials Protected by Two Different Coatings"
__
water absorption, wt %
-q 18
9 10 11 In iEYDWLTENYLE MODULUS, kgicm'
Figure 7. Least-squares fit of In (residual strength) vs. In (residual tensile modulus) for progressive biodeterioration of composite sample no. 12. Tensile modulus error is &lo% at a 90% confidence level.
coccus albus, a cellulolytic anaerobe (Taya et al., 1981).
When fiber glass reinforced polyester is subject to biodeterioration in soil, the loss of tensile strength proceeds a t a slow rate (Table 11, samples no. 7 and 81, even when natural fiber plies are included (sample no. 9). This latter case shows the importance of protecting at least one side of the natural fiber ply. With these results it should be possible to propose a mechanism for the biodeterioration of natural fiber reinforced polyester and by extension apply it to matrices other than polyester. Initially, these samples have a resin-rich surface with essentially no fibers exposed. With the absorption of water the thin surface layer of polyester resin is fractured by a mechanism already mentioned. The initial resin breakdown is expected to occur in the fiberresin interface due to the axial tension placed on the resin because of the high amount of water absorbed by the natural fibers. The amount of water absorption will be discussed later on. After some time fiber prominence (fiber pop out) is observed. Indeed, for glass fiber reinforced polyester, breakdown in the glass-resin interface, which always results in fiber prominence, has been observed during exposure of glass fiber reinforced polyester to moist-dry cyclic stress in the presence or absence of actinic radiation. Apparently the cyclic stress fatigue is necessary due to the relatively small amount of water (discussed later on) absorbed by the glass fiber composites. Now, in the presence of actinic radiation, the fiber prominence was followed by general microcracking on the surface after fiber prominence had become relatively extensive. Later on, when the deterioration was severe, mechanical and other physical properties were adversely affected (Blaga and Yamasaki, 1973, 1978). These authors believe that water is the most detrimental environmental factor. Therefore, by analogy with the glass fiber composites, after the breakdown of the resin-interface and fiber prominence occurrence, the natural fiber composites may go through a further cracking of the resin with the consequent rupture of the superficial resin layer. This may occur within the first hours or days of exposure but does not cause a significant loss of bulk tensile strength (Table 111). Finally, massive microorganism penetration to the fiber occurs and from then on the process continues until all the fibers are biodegraded. During the biodeterioration of the composites, not only the tensile strength decreases but also the tensile modulus does the same. This is shown in Figure 7. Surface Protection of Composite Materials. It has been reported (Blaga and Yamasaki, 1978) that gel-coated glass fiber reinforced polyester is beneficial when the composites are subject to natural or artificial weathering. The explanation given by those authors is that with the
sample no.
uncoated control
coating AC
coating B d
21 22 23 24 25 26 27 28
0.31 i 0.04 9.3 c 3.0 0.23 r 0.03 23.7 f 7.1 0.46 i 0.14 16.1 i 8.3 0.32 i 0.05 0.27 i 0.04
0.43 i 0.09 3.7 i 0.9 0.29 i 0.14 5.8 i 2.1 0.27 i 0.09 3.6 * 1.1 0.23 i 0.04 0.23 i 0.06
0.47 i 0.94 t 0.56 i 1.26 i 0.56 i 1.00 i 0.44 i 0.64 i
0.06 0.12 0.14 0.30 0.31 0.19 0.08 0.01
" Two-hour boiling water soak, ASTM D570-77 test method. The results are given as the mean c standard deviation of t h e percentage increase in weight during immersion. Only the specimens edges were coated (0.3 m m thick layer) with formulation PER/Sty = 60:40. The coating was allowed t o dry for 20 days. Formulation PER/Sty = 60:40 was used to coat (0.3 m m thick layer) surfaces and edges of the specimens. The coating was allowed t o dry for 20 days. Specimens (surfaces and edges) were coated (four times to obtain a 0.3 m m thick layer) with polyurethane varnish Marvethane A67VA01 of Sherwin-Williams Co. The coated specimens were allowed t o dry for 20 days. gel-coat, the reinforcing glass fibers are no longer in the surface layer. The resulting resin-rich surface thus consists of a more homogeneous component. As a result, the environmentally induced stresses normally operating in the surface region and, more particularly, in the glass-resin interface are reduced, thus preventing formation of fiber prominence. Table IV shows the results of boiling water absorption for several protected and unprotected composites. Glass-reinforced materials have a relatively low water absorbance. This justifies the protective use of outer plies of glass reinforced polyester for natural fiber polyester composites (Belmares et al., 1981). This is also seen for samples no. 21,25, and 27. In Table IV it is shown that unprotected natural fiber composites can absorb around 24% of water (by wt) under the conditions of the test. This water absorption can give an idea of the amount of swelling that goes on. When only one protective outer ply of glass reinforced polyester is used, the water absorption of the composite decreases somewhat (samples no. 22 and 26). This protective effect toward water absorption by a single ply of glass-reinforced polyester is related to an improved resistance to biodeterioration mentioned before for a similar unbalanced type of composites (Table 11, sample no. 9). Lastly, the best coating found was the polyurethane varnish which by itself can effectively block the water penetration of the natural fiber composites. Table V shows the tensile strength correlation with the data of water absorption given in Table IV. Sample no. 24 lost more than 50% of its original tensile strength due to a 24% increase in weight due to water absorption and to the thermal treatment. Apparently the combination of water and heat accelerates the damage of the composite to the point of abatement of mechanical properties. This severe damage does not occur if the natural fiber composites are placed in water at room temperature as discussed before. Samples protected with the polyurethane varnish did not have any loss of tensile strength. Other possible choices for coating natural fiber composites include acrylic and fluorocarbon films. Conclusions Natural hard fibers and their respective polyester composites are biodeteriorated following first-order kinetics.
657
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 657-661
Table V. Tensile Strength of Reinforced Materials after 2-h Boiling Water Soaka
a
~~
tensile strength, 2-h boiling strength, water soak, kg/cm2 dry specsample imens,b controlC coating coating no. kg/cm2 (uncoated) AC B C 21 22 23 24 25 26 27 28
949 810 1042 477 1422 942 1114 2513
878 591 1440 203 1586 862 1229 2680
984 681 1537 391 1398 753 1191 2331
997 727 1462 597 1501 927 1130 2048
a ASTM D570-77 and ASTM D638 test methods for the boiling water s o a k and the tensile strength determination, respectively. Specimens untouched by water. Tensile strength error is * 12%at a 90%confidence level. See Table IV for specimens preparation and description of the type of coatings used. Tensile strength error is i 1 2 % at a 90%confidence level.
For the composites, water transport to the natural fibers must be avoided or greatly minimized if biodeterioration is to be overcome. Protective coatings have been studied and have been found to greatly decrease water transport in natural fiber reinforced polyester, but the findings can be extrapolated to other matrices different from polyester. Acknowledgment
The authors wish to thank Professor James L. White and Mr. Hernan Menendez of the Polymer Engineering Department, The University of Tennessee, for the X-ray diffraction photographs. We also thank the Consejo Na-
cional de Ciencia y Tecnologia (CONACYT) and the Comision Nacional de Zonas Aridas (CONAZA) for the grants that supported this work. Registry No. Water, 7732-18-5. L i t e r a t u r e Cited Apinis, A. E. “Mycological Aspects of Stored Grain”; I n Walters A. H.; Hueck-Van der Plas, E. H. “Biodeterioration of Materials”, Volume 2; Applied Science Publishers Ltd.; London, 1972; p 493. Barkakaty, B. C. J. Appl. Polym. Sci. 1978,2 0 , 2921. Belmares, H.; Castillo, E.; Barrera, A. Textile Res. J. 1979a,49, 619. Belmares, H.; Barrera, A.; Castillo, E.; Mojaras, M.; Tristan, M. E. Interclencia, 1979b,4(6), 320. Beimares, H.; Barrera, A.; Castillo, E.; Verheugen, E.; Monjaras, M.; Patfoort, G. A.; Bucquoye, E. N. Id.Eng. Chem. Prod. Res. Dev. 1981,20, 555. Belmares, H.; Barrera. A.; Monjaras, M. Ind. fng. Chem. Prod. Res. D e v . 1983,preceding article in this issue Biaga, A.; Yamsaki, R. S. J. Meter. Sci. 1973,8,654. Biaga, A.; Yamasaki, R. S. Mater. Constr. (Paris) 1978, 71(63), 175. Connoily, R. A. “Soil Burial of Materials and Structures”; I n Walters, A. H.; Hueck-Van der Pias, E. H. “Blodeterioration of Materials”, Vol. 2; Appiled Science Publishers Ltd.; London, 1972; p 168. Das Gupta, P. C.; Mukherjee, P. P. J . Chem. SOC. C 1987, 1179. Focher, B.; Marzetti, A.; Cattaneo, M.; Beltrame, P. L.; Carniti, P. J. Appl. Polym. Sci. 1981, 2 6 , 1989. Hebeish, A.; Abou-Zeid, N. Y.; El-Kharadly, E. A,; El-Aref, A. T.; Allam, E.; Shaiaby, S.; ECAlfy, E. A. J. Appl. Polym. Sci. 1981,2 6 , 2713. Kuikarni, A. Y.; Chltale, A. G.; Vaidya, B. K.; Mehta, P. C. J. Appl. Povm. Sci. 1963, 7, 1581. Ray, P. K. J. Appl. Polym. Sci. 1968, 12, 1787. Ray, P. K.; Montague, P. E. J . Appl. Polym. Sci. 1977, 21, 1267. Taya, M.; Honma, K.; Ohimya, K.; Kobayashi, T.; Shimizu, S. J. Chem. fng. Jpn. 1981, 74(4), 330. Turner, R. L. “Important Factors in the Soil Burial Test Applied to Rotproofed Textiles”; I n Walters, A. H.; Hueck-Van der Phs, E. H. “Biodeterioration of Materials”, Volume 2; Applied Science Publishers Ltd.: London, 1972; p 218. Stecher, P. G. “The Merck Index, an Encyclopedia of Chemicals and Drugs”; Merck and Co.; Rahway, NJ, 1968; p 1128.
Received for review December 14, 1982 Accepted June 6, 1983
Characterization of Radiation-Cross-linked, High-Density Polyethylene for Thermal Energy Storage Ruth B. Whltaker,” Stephen M. Craven, Donald E. Etter, Eugene F. Jendrek, and Allan B. Nease MRC-Mound,
Monsanto Research Corporation, Miamisburg, Ohio 45342
Electron beam cross-linked highdensity polyethylene (HDPE) pellets (DuPont Alathon, 0.93 MI) have been characterized for potential utility in thermal energy storage applications, before and after up to 500 melt-freeze cycles in ethylene glycol. Up to 95% of the HDPE’s initial DSC AH, value (44.7 cal/g) (at 1.25 OC/min cooling rates) was retained up to 9.0 Mrad radiation dosage. Form-stability after 500 melt-freeze cycles was very good at this dosage level. Xqay diffraction measurements showed little difference between irradated HDPE’s and the unirradated control, indicating that cross-linking occurred primarily in the amorphous regions. FTIR spectroscopy showed the pellets to be uniformly reacted. The ratios of the 965-cm-’ absorption band (trans RCH-RH‘) to the 909-cm-’ band (RCH=CH,) increased with increasing radiation dosage, up to 18 Mrad. Gel contents reached a maximum of 75 % at the 13.5 Mrad dosage, indicating that other reactions, in addition to cross-linking, occurred at the highest (18 Mrad) dosage level.
Introduction
The development of crystalline, form-stable, high-density polyethylene (HDPE) pellets for thermal energy storage (TES) applications by Monsanto Research Corporation under U.S. Department of Energy Contract CY-76-C-05-5159 and ORNL Sub7398 has been described (Salyer et al., 1978,1980; Whitaker et al., 1979). This work primarily focused on chemical cross-linking of the HDPE to produce the form-stable pellets, by either peroxide-in-
duced cross-linking or silane-grafting and cross-linking. The properties of these cross-linked HDPE’s were determined in relation to their utility in thermal energy storage applications. Takahashi et al. (1981) have also demonstrated that surface cross-linking of polyethylene by ion bombardment has potential for thermal energy storage applications. One important potential application area is that of solar absorption air conditioning, since the melting temperature
0 196-4321/83/1222-0657$01.50/00 1983 American Chemical Society
