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Rigid polyurethane and kenaf core composite foams.

INTRODUCTION

Polyurethane foams (PUFs) are one of the most versatile thermosetting polymeric materials. They can be made into flexible and rigid PUs. Their application varies from heat and sound insulation for building structures to appliances like refrigerators and freezers. The vacuum heat insulation panels of RPUF provide energy efficiency by reducing the heat loss by 25% leading to reduced power consumption [1], PUs can also be tailored to make foams with a number of different foam cell characteristics; these foams can be open and closed cell structures and can affect their insulation properties [2-5], The closed cell foam structure is widely used, laid under floors, so as to keep a building resistant to moisture. PUs have been reinforced with inorganic and organic materials to enhance their properties [6], A recent study suggests that the incorporation of 0.1, 0.2, and 0.3 wt% of graphene nanosheets, and carbon nanotubes results in an increase in mechanical, and thermal properties [7], Much research is being carried out to substitute petroleum based polyols with a vegetable oil base like soy and castor [8] to make the PUs bio-polymers. These foams demonstrate comparable foam density and cellular morphology and an increase in compressive strength [9]. These natural materials provide encouraging avenues for their development as renewable sources for foams.

Using natural fibers such as Kenaf (Hibiscus cannabinus, L. family Malvaceae) is gaining value for reinforcing polymer composite materials. The kenaf plant is branchless and grows up to an average height of 5.5 m in 5 months and is widely grown in sunbelt regions throughout the world. Kenaf is a multipurpose plant with components that are harvestable such as stalk core, stalk bark, seeds, and leaves. The stalk core, which is the woody core, can be used as a wood-product substitute and building material [10], Most often kenaf bast fibers (these are obtained from the outer layer of the plant, i.e., phloem) are used in polymer composites. The low density of kenaf bast fiber as compared with glass fiber [11], carbon fiber, and hemp [12] has made kenaf a frequently considered reinforcement filler. Kenaf fiber is a good replacement for such current reinforcements as glass fibers, due to its competitive strength-to-weight ratio contributions [13], The advantages that natural fibers have over glass fibers are that no surface modification is required in order to provide polymer-fiber mechanical interlocking, which provides good adhesion and therefore superior properties, low cost, with renewability and biodegradability being some of the other merits over synthetic fibers [14], Low density of kenaf makes it possible for it to make automobiles more fuel efficient [15].

In our earlier work we have studied the effect of low kenaf loading of about 5% by weight giving similar benefits in mechanical properties of polylactic acid as that of 20% by weight kenaf loading reported by many researchers [16]. A well-dispersed low fiber fraction resulted in limited restriction for polymer crystal nucleation and growth, thereby increasing the mechanical properties. Further we studied the effect of kenaf fiber length and variation of bast fiber retting using chemical and enzymatic retting in poly(hydroxybutyrate-co-valerate)/poly(butylene adipate-co-terephthalate) polymer blends. The enzymatic retted fibers produced higher reinforcement than chemical retting [17]. Kenaf fibers have been reported to improve tensile and flexural strength with 30 and 40% by weight in poplypropylene [18]. Thermoforming of polyethylene, polypropylene, and polyethylene terephthalate was improved by the addition of kenaf fibers for processablity [19].

Kenaf core particles are used to make panels with easy processing and in comparison to commercially available insulation panels, making it a potential raw material for low density insulation panels [20]. The kenaf core has been investigated to reinforce unsaturated polyester to improve mechanical strength by Ishak et al. [21], By incorporating 5, 10, 20, 30, and 40% kenaf fibers and kenaf core in polyester to study the mechanical strength, they determined that kenaf fibers were required in addition to the core to provide reinforcement. Ismail et al. treated kenaf core with maleated polyethylene to improve interaction between kenaf core and HDPE to give better mechanical properties [22], The addition of montmorillonite (MMT) was found necessary in composites of kenaf core with higher filler loadings of 40-60% in composites with unsaturated polyester [23], Agglomeration of kenaf core resulted in a decrease in mechanical performance while addition of MMT led to an increase in tensile strength. Extensive work has been done with kenaf as filler with thermoplastics and thermosets, but rarely has it been used in foam. RPUFs with vegetable oil substituted for petroleum based polyol have been reinforced with natural fibers like flax and hemp to obtain better mechanical and thermal properties [24]. The plant core remains an underutilized component, as reinforcement [25, 26] Kenaf-core liquefied polyol has been used to synthesize PU adhesive [27] and used in oil spillage absorption due to its hydrophobic nature [28, 29],

EXPERIMENTAL

Materials

4,4'-Methylenebis (phenyl isocyanate) and polyester-block-polyether [alpha], [omega]-diol were procured from Fiber Glast Developments. The average molecular weight for polyester-block-polyether [alpha], [omega]-diol was 468. Kenaf-core was obtained from USDA.

Preparation of Foams and Composites

100 g polyester-block-polyether a, co-diol was mixed with 1% distilled water (i.e. 2.3 ml, which is 1% of total polymer weight considering 100 g of polyester-block-polyether [alpha], [omega]-diol and 130 g of 4,4'-Methylenebis (phenyl isocyanate)) in a disposable cup and allowed to degas for 4 min, 130 g of 4,4'-Methylenebis (phenyl isocyanate) was added to the mixture [30] and mechanically stirred using a cowless dissolver at 2500 rpm for 1 min and made to stand still. This mixture was then poured in the mold and allowed to foam freely and cure for 72 h at room temperature. Fig. 1A shows the mold with length L, width b and full depth [d.sub.f]. The mixture is poured and allowed to expand freely. The foam obtained from this free expansion is termed henceforth as [d.sub.i].

For foam composites the kenaf core was powdered using a cryomill for 30 min under liquid N2 bath. Kenaf-core had a density of 0.28 g/cc and particle size of 150 [+ or -] 20 [micro]m. The powder was preheated in a vacuum oven at 70 [degrees]C for 4 h to remove moisture. Composites were prepared with 5, 10 and 15 % kenaf-core by weight. The kenaf-core was first mixed in polyester-block-polyether [alpha], [omega]-diol which was premixed with 1% distilled water prior to foaming. Fig. IB, and 1C shows a schematic for the constrained expansion when the mold is filled with 40%, and 60% of full depth [d.sub.f] of the mold and henceforth referred respectively as [d.sub.i40], and [d.sub.i60]). The expansion ratio, i.e., the fully foamed height to the initial height, was calculated for free foam expansion which was found to be uneven with an average height of 60 mm. Introducing a constraint where the volume expansion was limited to 40% and 60% was done by setting the initial volume of unfoamed material at heights of 8 mm and 12 mm in the mold and limiting their expansion to the 20 mm full-mold height. An expansion ratio of 2.5 and 1.7 resulted from these constraints. Test samples were taken from foams cured on different days.

METHOD

Flexural Test

Samples measuring 16.5 X 10.5 X 100 [mm.sup.3] were used. The test was carried out according to ASTM C - 393-11 in displacement control at room temperature. The rate of displacement was set to 1 mm/min and maximum deflection was set to 15 mm. Flexural modulus was calculated from this test, where E is flexural modulus, L is span length, m is slope of the initial straight line, b and d are, respectively, the width and depth of the foam sample.

E = ([L.sup.3]m)/(4[bd.sup.3]) (1)

X-ray Micro Tomography ([micro]CT)

The machine used to [micro]CT was Skyscan 1172. Voltage of 61 kV and current of 163 [micro]A were used to give better contrast between the sample and background. Flat field correction was performed for dark and bright field, and later scanned with large resolution settings of 5 pm. Raw images were corrected for ring artifacts and beam hardening and analyzed for porosity before and after compression.

Compression Test

MTS. A sample measuring 25 mm in length and 27 mm in diameter was used. The MTS machine was used in displacement control with strain rate of 0.5 mm/min. Samples were compressed to 85% of the original height. Compression strength and modulus were calculated.

[micro]CT. A 10 X 10 X 10 [mm.sup.3] cubic sample was used. Machine was used in displacement control with strain rate of 0.5 mm/ min. This test gave change in foam structure and cell structure and porosity during testing. Figure 2A and B show the microCT compression stage with the foam sample. Figure 2C and D show before and after compression of the foam sample. The compression platens can be seen at top and bottom of the foam sample.

Microscopy

Environmental Scanning Electron Microscopy (ESEM). FEI Quanta Environmental Scanning Electron Microscope (ESEM; FEI Company, OR) was used. The foam was cryo-fractured using liquid nitrogen; to ensure no surface yielding; this cryofracturing kept the micro-structure intact for analysis. The average cell diameter was determined using ImageJ, and counting on an average 50 cells. Cell density ([N.sub.c]) was calculated from the micrographs using the following equation [31], where n is number of cells in ESEM image, A is the area of the micrograph in [cm.sup.2], and M is the magnification factor.

[N.sub.c] [approximately equal to] [(n[M.sup.2]/A).sup.3/2] (2)

Optical Microscopy. A Nikon Eclipse ME600 optical microscope was used to image fractured surface of the foams. These were cryo-fractured using liquid nitrogen (section 3.4.1). 5X Nikon Japan LU Plan objective was used to image surface of foams. A ruler tape was used to mark three random 1 [mm.sup.2] area for each foam sample and cells were counted to give linear cell density for 1 [cm.sup.2]. The measurements were done in triplicate (Table 1).

Foam Density

Macro-foam densities were measured using Archimedes principle of water-displacement. Densities were calculated using volume of water displaced by sample divided by weight of sample. Measurements were done in triplicate using AEA/AAA Density kit from Adam Engineering.

RESULTS AND DISCUSSION

Microstructure and Density of Foam and Composites

The ESEM images of pure PU foam for [d.sub.i] (Fig. 3) shows regular pore sizes with open cell structure. The 5% shows a foam structure that is regular as compared to 10% which shows agglomeration of kenaf-core which resulted in nonuniform distribution of foam cells with lower mechanical properties as compared to pure PU, 5, and 15% compositions, the agglomeration at higher loading is a common issue which reduces the mechanical strength of the composites [32], The 5% foam composition shows regular cell structure very similar to the pure PU foam; kenaf-core particles can be seen attached to the cell walls.

The micrograph (Figs. 4 and 5) for [d.sub.i40] and [d.sub.i60] shows the cell structure for foams when compared to 0% reinforcement; in all cases 0% shows regular foam cells. In contrast to the [d.sub.i], foams the kenaf-core for 10% can be seen inside the cells more than on the cell walls thereby reducing the mechanical properties like compression and flexural modulus and peak load. The 5, 10, and 15% for [d.sub.i40] and [d.sub.i60] shows that since there is constraint in volume expansion the packing density causes more small cells in an equivalent volume. From Figs. 4 and 5 it can be seen that for [d.sub.i40] and [d.sub.i60] the cell size of [d.sub.i60] is much smaller. Hence, it can be seen that for [d.sub.i60] the cell packing is denser than [d.sub.i40]. This increases the density for all composites for [d.sub.i60] as compared with [d.sub.i40] as shown in Table 1 and Fig. 10B. Also, it can be noted that as the kenaf-core loading increases; the density increases.

The linear cell density from Table 1 shows that the [d.sub.i60] has higher cell density followed by [d.sub.i40] and then [d.sub.i] foams. This increase in density is due to packing of higher material into same volume for [d.sub.i60] than [d.sub.i40]. It can also be noted that when the kenaf-core loading is increased, the linear cell density increases. This increase can be seen in the case of 15% kenaf-core loading in the three types of foams, which is due to kenaf-core particles acting as a barrier to foam expansion. This constraint in expansion gives a higher fraction of smaller cells. Since [d.sub.i40] has less material for occupying an equal volume as has [d.sub.i60] this initial smaller amount of material gives rise to larger cells after complete volume expansion than [d.sub.i60] in all the kenaf-core loaded samples. But, within [d.sub.i40] it shows similar trend as that of [d.sub.i60]. The 15% shows higher number of smaller cells than rest of the compositions. The same is the case with free foam volume expansion.

The average cell size for [d.sub.i], [d.sub.i40], and [d.sub.i60] for no kenaf core loading showed an increase. This might be due to the coalescing of the foam cells due to constraint in free expansion as seen from Table 1. The addition of kenaf core has acted as a nucleation site during the foaming process resulting in smaller cell sizes. Hence, 15% kenaf core loaded foams shows smallest cell sizes followed by 10 and 5% for [d.sub.i40] and [d.sub.i60].

Thus as the foam cell packing increases for [d.sub.i60] compared with [d.sub.i40] the mechanical properties are seen found to increase for [d.sub.i60]. Packing the cells in a dense manner has increased the foams resistance to compression, flexing, and shear stress. Since the kenaf-core loading influences the packing and the regularity of the cell structure. The ESEM images show that the 5 and 10% loading barely changes the cell structure in [d.sub.i40] and [d.sub.i60] volume expansion foam. It can be seen that 15% loading has disrupted the regular cell structure for both dm and [d.sub.i60] type of foams. However, this did not influence the properties in adverse ways. Even though we can see that the cell structure for 15% has changed the regular cell structure, an increase in compression and the flexural properties can be seen. This will be due to more kenaf-core particles supporting the foam cell from deforming as compressive and shear forces are applied.

It can also be noted that the kenaf-core for [d.sub.i40] and [d.sub.i60] resides on the foam walls providing reinforcement to the foam cells. Hence as the loading increases more kenaf-core particles go to the walls of the foam cells thereby increasing the resistance to deformation and thereby increasing the strength of the foam composite. This is not seen in the foams processed with no constraint. From ESEM (Fig. 3C) the 10% foam composite shows that the kenaf-core particles not only reside on the wall but also occupy the volume inside the cell, thereby decreasing the deformation and hence the strength of the composite. This can be seen from the compressive modulus and the flexural modulus and peak load for both Fig. 10A-D.

Mechanical Strength

Figure 10A and B shows compressive modulus and strength for the three types of foam volume expansion, it can be seen that the [d.sub.i40] and [d.sub.i60] shows an increase in compressive modulus and strength for the foam composites when compared with pure. With free expansion and [d.sub.i60] there was a negative impact on compressive modulus of the foam. Figure 6A and B are micrographs for d, for 10% and Fig. 6C is [d.sub.i40] for 10%. They clearly show the presence of kenaf core inside the cell of the foams marked with red open circle. Kenaf core within the cell wall is not available for reinforcement leading to lower mechanical reinforcement. From the images it can be seen that the kenaf core is attached to the cell wall, but most of it extends into the void of the cell. This reduces the resistance imparted by kenaf core while deformation, thereby reducing the compression strength by increasing the polymer mobility. Figure 6D and H for 15% kenaf core loading shows kenaf core on the cell wall. Figure 6E shows kenaf core protruding out of the fractured surface indicating good interface bonding between kenaf core and polyurethane. Figure 6F and G, it is very clear that the kenaf core is inside the pore which is formed during the foaming process. This type of kenaf core location was only seen in 15%. These loosely held kenaf core does not impart any reinforcement to the foam, even though this is the case there are kenaf cores which are deeply embedded in the foam as seen from Fig. 61 (section taken at an angle to view the embedded kenaf core). There by giving good compression strength to the foams by giving resistance for the polymer mobility during deformation. For [d.sub.i60] an increase by 2.4, 6, and 7.9% as compared with pure foam for 5, 10, and 15% kenaf core loading was seen. And for compression strength it has increased by 3.5 and 6% for 10 and 15% whereas 5% showed decreased strength by 4%.

Table 1 (Fig. 10C and D) shows flexural modulus and peak load for the three types of foam volume expansion. It can be seen that the [d.sub.i40] and [d.sub.i60] shows an increase in flexural modulus and peak load for the foam composites when compared to pure. A positive increase by 4.8, 20.7, and 49% in modulus for foams prepared with an expansion ratio of 2.5 ([d.sub.i40]) and an increase by 16.5, 23.4, and 40.4% in modulus is seen in [d.sub.i60] for 5, 10, and 15% kenaf core loading. Whereas free expansion, gave negative impact on flexural modulus.

Table 1 shows the foam density comparison between three types of foam volume expansion. Free volume expansion shows an increase in foam density with increase in kenaf-core loading to 35, 35, and 50% for 5, 10, and 15% kenaf core loading. Cell density for [d.sub.i] was 3.18 X [10.sup.4], 1.18 X [10.sup.4], 1.59 X [10.sup.4], and 1.12 X [10.sup.4] cells/[cm.sup.3] for 0, 5, 10, and 15% kenaf-core loading respectively. The [d.sub.i40] volume expansion shows an increase in foam density by 100, 160, and 304% for all composites. The cell densities for [d.sub.i40] are 1.11 X [10.sup.4], 1.03 X [10.sup.4], 8.50 X [10.sup.3] 1.28 X [10.sup.4] cells/[cm.sup.3], respectively. As compared to [d.sub.i] and [d.sub.i40], [d.sub.i60] show higher values for respective composites. The foam density increased in this case by 78, 142, and 231%. The cell densities show a steady increase too in [d.sub.i60], 7.60 X 104, 8.14 X [10.sup.4], 10.10 X [10.sup.4], and 11.42 X [10.sup.4.] respectively for 0, 5, 10, and 15% kenaf-core loading.

Linear cell densities for free foaming samples are 600 [+ or -] 81.65, 433 [+ or -] 47.14, 633 [+ or -] 125, and 700 [+ or -] 81 cells/[cm.sup.2] for 0, 5, 10, and 15% kenaf-core loading. It can be seen that the [d.sub.i40] shows higher linear cell densities compared with free expansion of 833 [+ or -] 47, 800 [+ or -] 81, 800 [+ or -] 163, and 966 [+ or -] 94 cells/[cm.sup.2] for 0, 5, 10, and 15% kenaf-core loading respectively. Whereas [d.sub.i60] shows highest linear cell densities when compared to [d.sub.i] and [d.sub.i40] with values of 1233 [+ or -] 94, 1200 [+ or -] 81,1100 [+ or -] 141, and 1266 [+ or -]47 cells/[cm.sup.2] for 0, 5, 10, and 15% kenaf-core loading.

microCT Analysis for Compressive Strength and Void Fraction

Compression strength from microCT (Table 1) shows a similar trend as that shown from the MTS test. This shows that as the expansion ratio increases the compressive properties improves. For [d.sub.i60] there is a steady increase by 1.5, 3.5, and 4% with kenaf core loading of 5, 10, and 15%. The free expansion and [d.sub.i40] show a negative reinforcement impact on compression strength but increasing kenaf core loading has reduced the magnitude of the decrease in property. The microCT compression test and the before and after the test reconstructed 3D images show that the cell structure becomes more compact as seen in Figs. 7-9. These images show the change in cells during the compression test. Table 1 shows void fraction and change in void fraction with increase in kenaf-core loading with the foam expansion as well as before and after the compression test. It can be seen that the void fraction before compression test is higher in all the three cases. The change in void fraction has reduced from [d.sub.i] to [d.sub.i40] to [d.sub.i60] shows the lowest change in void fraction of 7.93, 7.53, 4.89, and 9.43% for 0, 5, 10, and 15% respectively indicating that under compressive load [d.sub.i60] offers more resistance to deformation. [d.sub.i40] shows 8.37, 49.89, 19.85, and 9.86% while [d.sub.i] shows 20.49, 13.79, 10.74, and 9.24% for 0, 5, 10, and 15%, respectively. This contributes towards the higher compressive strength of the [d.sub.60] foam composites as compared with [d.sub.i40] and [d.sub.i]. MicroCT void fraction changes clearly shows that as the kenaf-core is increased in foam the void fraction reduces. Also it can be noted that as the volume constraint foam expansion is compared for [d.sub.i40] and [d.sub.i60], the [d.sub.i60] shows lower porosity as expected than [d.sub.i40]. This is due to higher material packed in [d.sub.i60] during foam expansion.

CONCLUSIONS

In this study we have examined kenaf core as a reinforcement. In contrast to prior efforts where kenaf core has never shown reinforcement, a new manufacturing approach of constraint during foaming enabled reinforcement to be obtained. The very low density of kenaf core 0.28 g/cc lead to the poor reinforcement when the foam was mixed and allowed to expand freely. The modulus and strength-concentration profiles show a large loss in mechanical performance in the free expansion foams in both compressive and bending modes. The nonlinear mechanical-concentration outcome correlates with the foam largely segregating and not reinforcing the foam till high concentrations of filler. Even then, the values are lower than the unreinforced rigid PU. In creating a constrained foaming environment, the kenaf core moved to the cell walls from nonreinforcement within the cell. Now gradual increase with concentration was obtained. The more the constraint the better the mechanical response. The compressive and flexural properties show increase with [d.sub.i60] foams with all kenaf core loading percentages due to dense packing of more number of smaller foam cells as seen from ESEM images. The foam densities show an increase for [d.sub.i60]. The dense packing has given rise to smaller cell sizes and increased linear cell density in [d.sub.i60]. This dense packing has contributed positively to the compressive and flexural properties. Further flexural response was better than compressive response relative to the unreinforced foam. This indicates that cell fracture under compression is the mode of failure.

ACKNOWLEDGMENTS

We acknowledge NSF-PFI 1114389 and NSF-CMM1 1031828 for financial support to fulfill this work.

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Mangesh Nar, (1) Charles Webber III, (2) Nandika Anne D'Souza (1,3)

(1) Department of Material Science and Engineering, University of North Texas, 1155 Union Circle #305310, Denton, Texas 76203-5017

(2) U.S. Department of Agriculture, Agricultural Research Service, Sugarcane Research Unit, 5883 USDA Road, Houma, Louisiana 70360

(3) Department of Mechanical and Energy Engineering, University of North Texas, Union Circle #311098, Denton, Texas 76203-5017

Correspondence to: Nandika Anne D'Souza; E-mail: nandika.dsouza@unt.edu

DOI 10.1002/pen.23868

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE l. Effect of kenaf-core loading of 5, 10, and 15 % and varying
constraint volume expansion of [d.sub.i40] and [d.sub.i60] on foam
density, cell density, linear cell density, and average cell diameter
and void fraction change during compression test in microCT for rigid
PU foam composites.

                       [d.sub.i]

                Foam                  Cell
Kenaf         density               density
core      ([[rho].sub.f])         ([N.sub.O])
(%)            (g/cc)          (cells/[cm.sup.3])

0       0.057 [+ or -] 0.063   3.18 x [10.sup.4]
5       0.077 [+ or -] 0.092   1.18 x [10.sup.4]
10      0.077 [+ or -] 0.085   1.59 x [10.sup.4]
15      0.086 [+ or -] 0.107   1.12 x [10.sup.4]

                       [d.sub.i]

                                    Average
Kenaf       Linear cell          cell diameter
core          density                 (D)
(%)      (cells/[cm.sup.3])        ([micro]m)

0        600 [+ or -] 81.65     316 [+ or -] 28
5        433 [+ or -] 47.14     650 [+ or -] 150
10        633 [+ or -] 125      250 [+ or -] 132
15        700 [+ or -] 81       293 [+ or -] 158

                       [d.sub.i40]

                Foam                  Cell
Kenaf         density               density
core      ([[rho].sub.f])         ([N.sub.O])
(%)            (g/cc)          (cells/[cm.sup.3])

0       0.008 [+ or -] 0.271   1.11 x [10.sup.4]
5       0.010 [+ or -] 0.484   1.03 x [10.sup.4]
10      0.017 [+ or -] 0.658   8.50 x [10.sup.4]
15      0.023 [+ or -] 0.899   1.28 x [10.sup.4]

                       [d.sub.i40]

                                  Average cell
Kenaf       Linear cell             diameter
core          density                 (D)
(%)      (cells/[cm.sup.3])        ([micro]m)

0         833 [+ or -] 47       350 [+ or -] 50
5         800 [+ or -] 81       300 [+ or -] 70
10        800 [+ or -] 163      283 [+ or -] 76
15        966 [+ or -] 94       168 [+ or -] 23

                       [d.sub.i60]

                Foam                  Cell
Kenaf         density               density
core      ([[rho].sub.f])         ([N.sub.O])
(%)            (g/cc)          (cells/[cm.sup.3])

0       0.197 [+ or -] 0.061   7.60 x [10.sup.4]
5       0.394 [+ or -] 0.086   8.14 x [10.sup.4]
10      0.514 [+ or -] 0.104   10.10 x [10.sup.4]
15      0.797 [+ or -] 0.131   11.42 x [10.sup.4]

                       [d.sub.i60]

                                    Average
Kenaf       Linear cell          cell diameter
core          density                 (D)
(%)      (cells/[cm.sup.3])        ([micro]m)

0         1233 [+ or -] 94      583 [+ or -] 76
5         1200 [+ or -] 81      300 [+ or -] 86
10       1100 [+ or -] 141      250 [+ or -] 50
15        1266 [+ or -] 47      150 [+ or -] 50

                                   [d.sub.i]

                         Void fraction   Void fraction
Kenaf    Compressive        before           after         Change in
core       strength       ([V.sub.f])     ([V.sub.f])    void fraction
(%)     (g/[mm.sup.2])        (%)             (%)             (%)

0            218             54.46           43.3            20.49
5             83             44.9            38.71           13.79
10           110             43.96           39.24           10.74
15           280             41.75           37.88           9.27

                                   [d.sub.i40]

                         Void fraction   Void fraction
Kenaf    Compressive        before           after         Change in
core       strength       ([V.sub.f])     ([V.sub.f])    void fraction
(%)     (g/[mm.sup.2])        (%)             (%)             (%)

0           225.05           73.92           67.73           8.37
5           187.14           74.57           37.37           49.89
10          176.83           66.21           53.07           19.85
15          196.14           57.68           51.99           9.86

                                   [d.sub.i60]

                         Void fraction   Void fraction
Kenaf    Compressive        before           after         Change in
core       strength       ([V.sub.f])     ([V.sub.f])    void fraction
(%)     (g/[mm.sup.2])        (%)             (%)             (%)

0           227.86           64.28           59.18           7.93
5           231.42           67.61           62.52           7.53
10          235.78           59.73           56.81           4.89
15          237.1            54.27           49.15           9.43
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Author:Nar, Mangesh; Webber, Charles, III; D'Souza, Nandika Anne
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jan 1, 2015
Words:5301
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