Influence of nanoclay reinforced polyurethane foam toward composite sandwich structure behavior under high velocity impact

Materials

The sandwich panels consist of two composite facing and a foam core. Each composite facing was made from three layered E-glass plain weave woven roving (400 g/m²) obtained from Camalyaf Turkey and isophthalic unsaturated polyester resin from Busheher Chemical Co., Iran. Table 1 presents physical and mechanical properties of E-glass plain weave woven roving and polyester resin. Methyl ethyl ketone peroxide from Iran Peroxide Co and cobalt naphthenate from AkzoNobel (Netherlands) were used as the initiator and the accelerator to cure the composite facings. The resin for making composite facings was degased by placing it in a vacuum oven at 23℃ prior to laminating the composite panels. All composite panels were cured at room temperature for 24 h followed by postcuring at 80℃ for 5 h. Composite facings were constructed by the hand lay-up method. The commercial nanoclay used in this study was provided by Southern Clay Products, as Cloisite 30B (MMT-OH), a montmorillonite modified by methyl tallow bis-2-hydroxyethyl ammonium with a concentration of 90 meq/100 g clay. The PU foam core was prepared from two part system, i.e. Pluracol 735 from BASF Co., USA and Millonate MR-200 from Nippon polyurethane industry Co., Japan. Isocyanate and polyols were dehydrated under vacuum for 6 h at room temperature. The nanoclay was dehydrated in an oven at 110℃ for 3 h before use. For PU nanocomposites, clay was first mixed with isocyanate via a high shear ultrasonic mixer for 20 min. Excess isocyanate (5 w/w% NCO/OH) was used to assure the complete reaction of polyol. For rising of PU foam, pentane was used as the blowing agent at 5 wt%. Gel time and onset of foam rising time measurements were made for all samples containing different percentages of nanoclay (see Table 2). For preparation of samples, after addition of polyol to a mixture of isocyanate and nanoclay, the compound was mixed for 10 min using a mechanical mixer at 500 r/min.

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Table 1.

Physical and mechanical properties of materials used.

Name	Type	Density (kg/m3)	Tensile strength (MPa)	Tensile modulus (GPa)	Elongation at break (%)
Polyester resin	Isophthalic	1.23	56	3.6	1.2
Plain weave woven roving	E glass	2.54	2900	73	3.4

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Table 2.

Gel time, foam onset of rising time, and compressive modulus.

Nanoclay weight percentage (wt%)	Mixing time (s)	Gel time (s)	Rising time (s)	Compressive modulus perpendicular to foam rise (MPa)
0	10	55	72	6.2
0.25	10	43	56	7.1
0.5	10	40	53	8.5
1	10	37	49	11.2
3	10	33	44	10.7

All foam cores were cast by first placing the composite facings in specially designed mold prior to casting the PU foam, and the mixture of all ingredients of foam cores were mixed in a mechanical mixer at 500 r/min under controlled temperature (21℃). The foam was cast in place under free rise condition at room temperature and left for 24 h for full setting followed by 5 h postcuring at 80℃. The foam cores in the sandwich panels were prepared with 0, 0.25, 0.5, 1, and 3 wt% nanoclay. At least 10 samples were prepared for each nanocomposite and the averages are reported here. The sandwich panels were made in 12.5 × 12.5 cm² size as defined by gas gun target holder.

Characterization methods

Nanomaterials such as nanoclays tend to be agglomerate when mixed into a liquid such as polymers or resins. Effective means of de-agglomeration and dispersing are needed to overcome the bonding forces after wetting the powder. The ultrasonic breakup of the agglomerate structures in aqueous and nonaqueous suspensions allows utilization of the full potential of nano-size materials. Ultrasonic mixer (HD3200 model with KE-76 prob) was used to disperse nanoclay into isocyanate. Initially the nanoclay was dispersed in the isocynate by ultrasonic for 20 min at 35℃. The isocyanate mixture was mixed thoroughly with predetermined amount of polyol for 15 s using a mechanical mixer at 500 r/min. The clay dispersion was determined by small angle x-ray scattering (SAXS) on a Hecus (model S3-MICRO_pix) equipped with an intrinsic germanium detector system using Cu Ka radiation of a 1 mm thick sheet under operating conditions of 50 kV, 1 mA, and a Cu anode was used as the X-ray source.

The compressive modulus for different rigid foam samples containing nanoclay was determined per ASTM D 1621. The compression tests were conducted perpendicular to foam rise on universal testing machine Santam STN-150KN. The reason for reporting compressive modulus perpendicular to foam rise is the fact that the projectile contacts the side of the panel and compressive waves are generated on the side of the panel. The result for this test is presented in Table 2.

Morphology study was carried out on foam core cell via scanning electron microscopy analysis using SEM instrument (VEGA\\Tescan HV:2000KV), providing information on foam cell size, cell wall thickness, foam cell strut characteristics, and foam cell crushing inflicted by projectile perforation. The analysis was also used to investigate fracture pattern associated with front and back facings in the sandwich panel after high velocity impact.

High velocity impact test

High velocity impact tests were carried out using a gas gun device (Figure 1). The gas gun consists of 3 m long smooth barrel with inside diameter of 8.7 mm, a fast acting pressure release valve, a projectile loading unit, a supply gas vessel, a 500 ml gas reservoir for each shot release, a target holder, two projectile velocity measuring units, and ballistic paste to catch the projectile intact. The initial velocity of projectile was measured after it was propelled from the gun barrel using a chronograph F-1 model from Shooting Chrony Co., Canada. Due to yawing of projectile after exiting the target, the residual velocity for the projectile which perforated the specimen was recorded using two sets of wide screen with thin aluminum foil panels. Rectangular thin aluminum foils with a 30 µm thickness and having dimensions of 300 × 300 mm² were attached to a rigid acrylic frames. Each set consists of two pieces of the thin aluminum foils which were placed close together with a gap of 5 mm. A very thin paper was placed between the two foils to prevent premature triggering. Another pair of similar aluminum sheets was fixed at 135 mm from the first set. An electric circuit was set up between each set of thin foils which were then connected to a 20 MHz fast pulse meter from Autonics Co., to record the time of travel for the projectile between the two sets.

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Figure 1.

High-velocity impact testing device (gun).

Hemispherical nose shaped projectile was used for all impact tests. The projectile was a hardened steel (Rc60) of 25.21 mm total length, 8.7 mm diameter, and 10.7 ± 0.05 g weight (Figure 2). The initial velocity of projectile (before impact) was calibrated and measured for helium gas at various gas pressures with a chronograph. The calibration curve showed linear behavior for various gas pressures versus projectile velocities at low pressure range (see Figure 3). Velocity range of 100–140 m/s was used for all high velocity impact tests.

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Figure 2.

Projectile used in the impact tests.

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Figure 3.

Projectile velocity calibrations at various helium gas pressures.

XRD analysis

In the preparation of a polymer/nanoclay nanocomposite, it is important to know the degree of intercalation/exfoliation of the nanoclay silicate layers and its effect on the nanocomposite moderate properties. Nanoclays show a characteristic peak in XRD analysis due to their regular layered structures. The peak is indicative of the platelet separation or d-spacing in nanoclay structure. Using the peak width at half maximum height and peak position (2θ) in the XRD spectra the inter layer space can be calculated utilizing Bragg’s law (see equation (1))

When the nanoclay was first mixed with isocyanate, the reaction between the isocyanate monomers and the hydroxyl groups on alkyl chains of nanoclay caused an increase of gallery spacing of clay to facilitate nanoclay exfoliation. SAXS was used to determine how well a polymer chain penetrated the interlayer of the nanoclay in a nanometer scale and also the state of dispersion of the layered nanoclay in the polymer nanocomposite. In this investigation sonication was used for the dispersion of nanoclay layers in the polymer matrix. The results for the SAXS patterns for pure nanoclay and also different nanocomposite sandwich panels with 0.25, 0.5, 1, 3 wt% nanoclay that was prepared by the application of sonication are shown in Figure 4. The SAXS patterns for PU nanocomposites containing different percentages of nanoclay show all diffraction peaks shifted to lower 2θ degrees compared to the peak for the pure nanoclay. The SAXS spectra of PU/nanoclay nanocomposites showed the characteristic sharp peak at 4.82° for pure nanoclay and a peak shifted to lower angles after dispersion in PU. In the case of the foams with 0.25 and 0.5 wt% nanoclay, no such diffraction peak is visible within the scanned range, thus suggesting a significant increase in d-spacing between the nanoclay layers. This indicates that PU polymer chain intercalated into the nanoclay layer and that the clay plates have been dispersed effectively and the layers may have exfoliated completely. ²⁹ For the nanocomposite specimens containing 1 and 3 wt% nanoclay, SAXS results show partial exfoliations and two relatively wide shoulder at 1.12°, 1.72° on the XRD curve. The leftward shift of these characteristic peaks (i.e. toward lower angles) compared with the pure nanoclay indicates increase in interlayer spacing after sonication process and corresponding to an increase of the interlayer distance of nanoclay more than 7.88 and 5.13 Å, respectively. It is evident that with an increase in nanoclay loading from 0.5 wt% in the nanocomposites, the interlayer spacing of nanoclay reduces due to the agglomeration of excessive nanoclay. The peaks for the 1 and 3 wt% nanoclay were apparently widened and the strength decreased, indicating a diverse basal spacing distance.

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Figure 4.

SAXS patterns for pure nanoclay and sandwich panels containing different percentage of nanoclay.

Foam characterization analysis

Gel time and onset of foam rising time (Table 2) showed a significant decrease with increase in nanoclay content. The measured values for the gel time and foam’s onset of rising time for 0–3 wt% nanoclay show 22 and 28 s decrease, respectively. This change in behavior may be attributed to the fact that the nanoclay (Clositie 30B) acted as a heterogeneous catalyst for polymerization reaction and the blowing agent, which has resulted in higher speed of reaction for both cases. Additionally, both the temperature and the moisture content as the condition of the reaction may have also affected the reaction resulting in final foam core density variation. The compressive modulus for the foam core (Table 2) reveals higher stiffness for the foam core containing nanoclay. The result clearly shows increase in compressive modulus with increase in nanoclay content.

High velocity impact assessment

The projectile’s residual velocity versus impact velocity response of different sandwich panels was used to study the behavior of different panels toward high velocity impact. Figure 5 shows residual velocity of projectile after perforation versus impact velocity. The figure indicates relatively better ballistic performance for the sandwich panel containing 0.5 wt% nanoclay in its foam core in terms of lower residual velocity throughout the velocity range tested. Most panels showed a near linear behavior just above ballistic limit velocity (the velocity to just cause perforation of the panel) followed by nonlinear parabolic response at around 100–125 m/s and then again a linear increase in residual velocity with increase in impact initial velocity. Nonlinear response in the curve may be directly related to nontensile failure mode in the panels.^30,31 Similar behaviors shown by different sandwich panels are directly related to the similar facing used in terms of material, thickness, and fiber architecture. Considering similar initial impact velocity and comparison of residual velocity for sandwich panels containing different amount of nanoclay reveals the significance of nanoclay presence in the panels. Results showed reduced ballistic performance in the sandwich panels with nanoclay in the range of 1–3%.This is despite increase in nanoclay percentage. This reduced performance may be attributed to changes in the micro and macro structure of the nanocomposite containing foam core (this is to be discussed in the later part of this report). As the nanoclay content in the foam core increases, the foam nature becomes more brittle (see result for compressive modulus in Table 2). This means less resistance to foam crushing and consequently less resistance to penetration. Figure 6 presents ballistic limit velocity for different percentage of nanoclay in the foam core.

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Figure 5.

Residual velocities as a function of impact velocity.

From Figure 6, it can be noted that the ballistic limit capacity of composite sandwich panels is affected by different percentage of nanoclay in the foam core. This result shows specimens containing 0.5 wt% nanoclay have the highest ballistic limit velocity compared to other nanocomposite sandwich panels. As it can be seen from Figure 6, the addition of 0.5 wt% nanoclay to rigid PU foam results in an increase of ballistic limit velocity of the sandwich panel by as much as 15.5% whereas in the sandwich panels with 0.25, 1, and 3 wt% nanoclay content in their foam cores, this increase is much less. This change of behavior may be directly attributed to more brittle nature of foam core above 0.5% nanoclay (see compressive modulus in Table 2). Fracture study in the later part of this report will confirm more brittle behavior associated with nanoclay percentage. The figure also shows ballistic limit velocity for other nanocomposite foam core sandwich panels.

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Figure 6.

Ballistic limit velocity for different percentage of nanoclay in the foam core.

Figure 6 also presents estimated ballistic limits, which were calculated using the initial and residual impact velocity for fully perforated specimen using conservation of energy relationship (see equation (2)) ³²

Figure 7 depicts energy absorption at ballistic limit velocities for the nanoclay containing sandwich panels. The figure clearly shows the highest energy absorption at ballistic limit velocity for the 0.5 wt% nanoclay containing panel. The result indicates 27% increase in energy absorption for this panel compared with composite sandwich panel without any nanoclay.

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Figure 7.

Energy absorption at ballistic limit velocity for the nanoclay containing sandwich panels.

Data from impact initial and residual velocities for fully perforated specimens and the following equation were used to arrive at energy absorption (equation (3))

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Figure 8.

Clustered column diagram compares energy absorption at various impact velocities in sandwich panels containing different percentage of nanoclay.

Fracture assessment

Results also showed nanoclay presence increases the fracture path and consequently fracture propagation, resulting in higher energy consumption in fracturing the samples. However, at higher nanoclay percentage, agglomeration develops due to lack of dispersion and inadequate intercalation and exfoliation. This agglomeration may act as stress concentration points in the specimens and result in failure starting from these points. Also the study of the foam core structure for different amounts of nanoclay percentage indicates that with increasing the nanoclay content, the foam core becomes more brittle. It was also observed that in the fully perforated specimens, the hole developed in the back facing as a result of projectile exit was of the larger size than the projectile in the specimens containing 0, 0.25, and 0.5 wt% nanoclay, this may be attributed to projectile yawing and meeting the back composite facing side way. This explains greater energy absorption by the projectile. However, consideration of perforated hole size in the composite back facing for the 1 and 3 wt% nanoclay specimens showed the same hole size as projectile diameter. This was observed in more brittle foam core specimens. Fracture pattern in the 1 and 3 wt% nanoclay containing specimens revealed radial cracks initiating from the impact point propagating outwards in a random manner (see Figure 9(a) and (b)). A similar assessment for 0.25 and 0.5 wt% nanoclay specimens shows a few numbers of very small cracks with no particular patterns (see Figure 9(c) and (d)).

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Figure 9.

Back face failure modes in (a) PU foam core with 1 wt% nanoclay, (b) PU foam core with 3 wt% nanoclay, (c) PU foam core with 0.5 wt% nanoclay, (d) PU foam core with 0.25 wt% nanoclay.

One peculiar behavior which was noticed in all specimens is the energy absorption obtained for highest impact velocity tested (136 m/s). Figure 8 presents energy absorption in sandwich panels containing different foam core density at various impact velocities. The figure shows, with increase in impact velocity a considerable increase in energy absorption by the sandwich panels is witnessed. This behavior can be explained by the fact that, with an increase in impact velocity, the strain rate in the material increases too. This results in higher tensile and flexural modulus in the composite facings ^{33 – 35} and therefore higher resistance to fracture. Also, the higher strain rate in the foam core results in higher fracture surfaces which mean higher energy absorption to overcome while the projectile perforates the foam core. Similar findings were reported elsewhere. ^{36 – 38}

It is postulated that the behavior shown by 1 and 3 wt% nanoclay specimens may be attributable to the agglomeration of nanoclay at different sites in the foam core which leads to creation of stress concentration at these sites. Sliding of silicate platelets at agglomerated sites due to stress wave propagation initiates crack. This situation is aggravated in higher nanoclay loading, i.e. 3 wt%.

Further fracture assessment of the composite sandwich panels subjected to projectile impact showed detachment of composite facing from the foam core in 3 wt% naoclay specimens (see Figure 10); this is despite in situ foam core molding. This may also be explained by the fact that the interfacial region between the composite facing and the foam core contains sites with agglomerated nanoclay and therefore facilitating stress concentration to develop hence reducing adhesion.

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Figure 10.

Side view for failure mode in 3% nanoclay containing sandwich panel.

SEM images (shown in Figures 11 to 15) were used to better understand the morphology and microstructure of foam cores containing nanoclay. The images highlight the effect of nanoclay presence in different weight percentage of the cell structure, cell wall, and foam network.

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Figure 11.

SME morphological analyses for the foam core without nanoclay.

It is clearly evident from Figure 11 (SEM micrograph for foam core with 0 wt% nanoclay) that the foam core structure has regular polygon shaped closed cell. The number of foam cells and their average diameter per cubic centimeter together with foam core bulk density are presented in Table 3. A study of the results presented in this table show that, with increase in nanoclay content from 0 to 3 wt%, the foam core density as well as the foam cell diameter decreases. The reduction in density and cell diameter are 6.9 kg/m³ and 519 µm, respectively. The SEM micrograph also shows 165 µm increase in cell wall thickness as a result of nanoclay addition.

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Table 3.

PU foams cell morphology.

Nanoclay weight percentage (wt%)	Density (kg/m3)	Foam cell diameter (µm)	No. of cells per unit volume of the foam (×10−5 cell/cm3)	Foam cell wall thickness (µm)
0	49	773.22	0.074	2.2
0.25	48	640.52	0.109	7.6
0.5	47.1	481.36	0.126	26.0
1	44.2	379.55	0.157	32.2
3	42.1	253.3	0.058	168.1

Results indicated that nanoclay promotes rupture of cell windows and has been shown to aid the cell-opening process in rigid PU foams. ¹⁶ Figure 11 presents micrograph for the PU foam without any nanoclay having no open cell, but with the addition of nanoclay the average number of open cells increases. It is obvious that the dispersion of nanoclay in the polymer matrix plays an important role in controlling the number of open and closed cells during the foaming process. SEM images reveal that the foam cores containing 0.25, 0.5, and 1 wt% have some degree of open cell in their foam structure (see Figures 12 to 14). However, for the 3 wt% nanoclay containing foam cores, the structure becomes almost totally open cell (see Figure 15). This observation on cell structure indicated that the presence of nanoclay results in an increase of the foam cell wall thickness increase and at the same time a decrease in the cell size. Results also showed that the foam cores that were incorporated with nanoclay have a lower bulk density than neat foam. Micrographs presented in Figures 11 to 15 show that the neat foam (without any nanoclay) had a cell wall thickness of 2.24 µm, whereas in the specimens with 0.25 wt% nanoclay the foams had cell wall thicknesses of 7.67 µm. This means that the presence of 0.25 wt% nanoclay results in an increase in the cell wall thickness by 242%. This can be attributed to the fact that the nanoclay particles resisted and retarded the rising of cells in foam, thereby causing the cell sizes to decrease and hence, this leads to increase in number of cells per unit volume. Also, it was found that the addition of nanoclay up to 3 wt% results in density reduction of 49–42.1 kg/m³. It is believed that the clay particles act in two ways which results in reduction of the foam cell sizes. They can act as heterogeneous nucleation sites during cell formation producing a higher number of finer cells in the foam,^39,40 as it is well known that foaming process is strongly dependent on nanoparticles’ dispersion. ⁴¹ In a fully dispersed nanocomposites many individual clay platelets are in direct contact with the polymer matrix and foaming agent, providing a much larger interfacial area for foaming agent adsorption and cell nucleation resulting in a higher cell density. While more cells nucleate, a similar amount of gas is available for bubble growth, leading to a reduction of cell size. However, this can increase the viscosity of the medium thus reducing coalescence. A consequence of both is that cell number density increases, resulting in smaller cells.

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Figure 12.

SME morphological analyses for nanocomposite foam core with 0.25 wt% nanoclay.

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Figure 13.

SME morphological analyses for nanocomposite foam core with 0.5 wt% nanoclay.

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Figure 14.

SME morphological analyses for nanocomposite foam core with 1 wt% nanoclay.

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Figure 15.

SME morphological analyses for nanocomposite foam core with 3 wt% nanoclay.

Fracture patterns for the sandwich panels with neat 0.25 and 0.5 wt% nanoclay containing foam core show severe cell breakage at the impact point exhibiting collapse of cells. However, for 1 and 3 wt% nanoclay containing foams, the micrographs indicated that there was no collapsing of cells, instead they broke up to the lower region of the damaged area. This phenomenon proved that the cells in nanoclay containing foams were more brittle in nature than specimens with neat foam and they tended to break than collapse. This may be attributed to the agglomeration phenomenon in the specimens with high percentage of nanoclay. This agglomeration acts as a weak point resulting in easy cell breakage in high velocity test. Thus, fracture starts from these weak points and propagate through the foam structure by circumventing the struts. This resulted in lower strength and showing some area of foam crushing, fracture propagation, and back facing/foam core debonding.