Comparative studies of ultra high molecular weight polyethylene fiber reinforced composites.

INTRODUCTION

Historically, polyethylene (PE) was known as a low strength, low stiffness material. Conventional methods to make PE fibers, such as melt-spinning, yield only a "folded chain" molecular structure. Theory has predicted that if PE chains could be fully extended and "frozen" in a highly dense packing status, the resulting fibers would be extraordinarily strong because of the intrinsic high strength of the carbon-carbon bond. High strength and modulus of the extended chain PE fibers are attributed to their high molecular weight, high degree of orientation, and high crystallinity. After years of research in several leading universities and prompt recognition by industry, extended chain PE fibers were finally commercialized in 1985 as ultra high strength, high modulus, and high performance fibers. Currently, ultra high molecular weight extended chain PE fibers on the market are all made by the solution/gel spinning technology. They are represented by Honeywell's Spectra[R] fibers, DSM-Toyobo's Dyneema[R] fibers, and Mitsui's Tekmilon[R] fibers. The principles for solution/gel spinning of ultra high molecular weight polyethylene (UHMWPE) were developed by Pennings and his students at University of Groeningen [1].

UHMWPE fibers possess extraordinary physical and mechanical properties such as low specific weight, high modulus, high strength, high impact resistance, high cut and abrasion tolerance, excellent chemical resistance, low dielectric constant, good UV resistance, low moisture absorption, excellent vibration damping capability, low coefficient of friction and selflubricating properties, etc [2, 3]. Despite the impressive list of properties, UHMWPE fibers also have limitations. Because of their chemical inertness and lack of functional groups, UHMWPE fibers are difficult to bond to most materials, which make it difficult to produce UHMWPE fiber reinforced polymer matrix composites. There are several ways to overcome the obstacle by means of various fiber pretreatments. UHMWPE fibers can be chemically etched by chromic acid [4] or oxidized by polypyrrole [5] to increase the surface roughness. Plasma and corona treatments in [O.sub.2] or C[O.sub.2] introduce chemical groups onto the fiber surface through chain scission and substitution as well as etch and roughen the fiber surface [6, 7]. All these fiber pretreatments have been proven to significantly improve the bonding strength of UHMWPE fibers to matrices; however, the fiber properties generally degrade.

Since PE is a thermoplastic, surface melting and recrystallization can be used to fuse the fibers together by carefully applying heat and pressure. As the majority of the fiber is intact, the resulting product upon cooling is a UHMWPE fiber reinforced composite without an additional matrix; this is an example of a homocomposite. A homocomposite is a composite in which the matrix and reinforcement have the same chemical composition. The interfacial adhesion is greatly improved because the matrix and reinforcement are essentially the same material, and more importantly, there is molecular continuity throughout the composite. Homocomposites are "recycle-friendly," compared with two-component systems such as glass, fiber, and epoxy resin. The idea of homocomposites is not new; Capiati and Porter [8] presented the first "one-polymer composite" example by embedding a high modulus PE filament in a block of HDPE in 1975. Later, other researchers have extended this technique and developed the film stacking method [9, 10], powder impregnation [11], and solvent impregnation [12]. In the aforementioned cases, a secondary polymer, besides polymeric fibers, is used as binder or matrix to form the continuous phase. Ward and coworkers at University of Leeds has introduced a concept called "hot compaction." The method utilizes only one starting material, a highly oriented fiber or tape, to make homocomposites in a two-stage process: soaking and compacting. They have studied the systems of melt-spun PE, PET, PP, Spectra, and Vectran[R] LCP fibers [13-22]. Meanwhile, a parallel study conducted by Farris and coworkers at University of Massachusetts has developed a similar process. They used a one-stage press-and-heat approach to make protective coatings for optics on military airplanes using Spectra woven cloth. This research has led to several patents regarding the making of high strength and high modulus polymeric materials for impact resistant applications [23-27].

[FIGURE 1 OMITTED]

One major interest in UHMWPE fiber reinforced composites is their potential as ballistic protective materials. Today's state-of-the-art military helmets use Kevlar[R] fiber reinforcement. UHMWPE fibers have several advantages over Kevlar fiber, including: high ductility and energy absorption capability, excellent cut resistance, and damage tolerance. UHMWPE fiber based armor has light weight without sacrificing the high level of protection; composite armor produced from Spectra fiber weighs 30-50% less than Kevlar fiber composite armor for the same performance [28]. Light weight armor, especially helmets, can significantly reduce wearer fatigue and more sophisticated combat equipment to be mounted on the helmet is allowed. Nowadays, fiber reinforced composite armor is made by laying up patterned woven cloth or prepreg to produce doubly curved structures via compression moulding. Since UHMWPE fibers are thermally formable, it is possible to directly shape UHMWPE fiber materials at elevated temperature without cutting patterns or using matrices. Hence a novel processing method called high-temperature high-pressure sintering has been developed [29-31]. In this process, layers of Spectra cloth are properly confined to prevent shrinkage and exposed to an appropriate heating and stretching sequence followed by a consolidation and cool-down step. It is feasible to produce thick and rigid curved ballistic shields via this consolidation and thermoforming process. Moreover, objects with desired curvatures and shapes can be made using proper moulds by this method, such as domes and tubes (see Fig. 1). These products can be useful in performance-demanding application such as orthopedic implants, pressure vessels, high-strength tubing, sporting goods, radomes, etc.

This article describes the investigation of the process-structure-property relationship into three commercially available UHMWPE materials such as Spectra woven cloth 903, Dyneema Fraglight nonwoven felt [32], and Spectra Shield Plus PCR prepreg [33]. The high-temperature high-pressure sintering process was applied, and the thermomechanical and microstructural properties of the consolidated products as a function of the processing temperature were examined and compared with one another. It is concluded that the matrix free Spectra fiber reinforced composite has significant advantages over the other two materials as a high strength and impact resistance material.

EXPERIMENTAL

Materials

Spectra woven cloth was provided by Honeywell and designated as Spectra cloth 903. It is a plain woven cloth made of Spectra fiber 900. Spectra fiber 900 has a linear density of 1200 denier and density of 0.97 g/[cm.sup.3]. The cloth has an average thickness of 0.5 mm and areal density of 240 g/[m.sup.2]. Dyneema Fraglight, a needle punched nonwoven felt with an areal density of 190-220 g/[m.sup.2], is made from Dyneema fiber and designed for protection against fragments of exploding bombs and artillery shells. Dyneema fiber is an UHMWPE fiber commercialized by DSM with similar properties to Spectra fiber. Spectra Shield Plus PCR is a nonwoven thermoplastic composite consisting of two plies of unidirectional Spectra fiber tapes cross plied at 0[degrees]/90[degrees] and impregnated in a matrix. Spectra Shield Plus PCR is made by Honeywell and is used in applications such as rigid armor and breast plates, vehicle and architectural armor, blast containment, and shields. It has an areal density of 95 [+ or -] 15 g/[m.sup.2] and thickness of 0.13 [+ or -] 0.05 mm. The fiber weight faction of Spectra Shield Plus PCR is ~80%. The matrix is an undisclosed proprietary material. All material was sampled and tested in the as-received state without any treatments.

Material Processing and Property Measurements

Sintering Procedures. A single layer of specimen was sandwiched between two pieces of aluminum foil and placed in between two steel plates. The hot press was preheated to a desired processing temperature (148, 150, 152, or 154[degrees]C), the temperature was allowed to equilibrate, and the sandwiched specimen was placed in the hot press. The upper and lower platens of the press were closed and the pressure was raised to the desired processing pressure (7.6 MPa). The pressure and temperature were held constant for a period of processing time (30 min) to execute the sintering process. The hot press and the specimen were quenched by running tap water through the press platens while pressure was maintained. The sintered specimen was removed from the press at ambient temperature and separated from the aluminum foil; no mould release agent was used.

Measurement of Crystallinity Changes by Differential Scanning Calorimetry. A TA instrument differential scanning calorimetry (DSC) 2910 was used to observe the melting behavior of single-layer specimen consolidated under various processing conditions. Approximately 10 mg of the samples in hermetically sealed pans were heated at 10[degrees]C/min from room temperature to 200[degrees]C under nitrogen (50 ml/min).

Measurement of Molecular Orientation Changes by Wide Angle X-ray Diffraction. Wide Angle X-ray Diffraction (WAXD) patterns were collected using pin hole collimated, monochromatic Cu K[alpha] radiation and a Bruker[R] "High Star" two dimensional detector. The diffraction patterns of a series of single-layer specimen sintered under different processing conditions were collected. The specimen was oriented "flat-on" so that the incident X-ray beam was perpendicular to the fabrics. A radiation time of 300 s was used and diffraction patterns were captured digitally. The integration of the intensity was preformed using GADDS commercial software.

Measurement of Impact Properties by Puncture Test. Impact tests were performed using a dynatup[R] 8250 with the pneumatic powered shooting dart mode of puncture test since the consolidated materials are rather strong. The dart weighs 2.38 kg and is pneumatically assisted by house nitrogen. The dart speed at penetration into the specimen was ~10 m/s. This was measured by an optical sensor reading the travel distance and time of the flag attached to the dart fixture. The specimens were 100 mm by 150 mm consolidated single-layer specimens which were sintered under different processing conditions. Each specimen was secured on the sample stage by pneumatic clamps, the dart was then fired perpendicular to the surface of the specimen, and the specimen was punctured. The instrument recorded the load detected on the dart tip and the deflection of the specimen as soon as the shooting dart touched the specimen until total penetration. The total energy of impact was calculated from the integrated area under the load versus deflection curve and normalized with respect to the specimen thickness as the measure of impact resistance: normalized-to-thickness (NTT) total energy of impact.

Measurement of Interlayer Adhesion by T-Peel Test. Thermally bonded bilayers were prepared from two layers of Spectra cloth, Dyneema Fraglight felt, and Spectra Shield prepreg, respectively, and one layer of Spectra cloth and one layer of Dyneema felt. A series of bilayers were produced at different processing temperatures under constant pressure and time (7.6 MPa and 30 min) and the T-peel tests were conducted on an Instron 5500R according to ASTM D1876.

Measurement of Flexural Properties by Three-Point Bend Test. The flexural properties of multilayer consolidated specimens sintered under different processing conditions were determined by three-point bend test according to ASTM D790 with a support span of 50 mm and a crosshead speed of 1.35 mm/min. A testing limit of 5% strain was used to ensure the reliability of the data.

Thermoforming

About 250 mm by 250 mm multilayer fabrics were stacked and subject to a pressure of 6.9 MPa at 150[degrees]C for 30 min. The resulting flat panels (thickness = ~5 mm) were cut into four 125 mm by 125 mm squares, 12 holes were then drilled into the panels, and they were bolted onto the hemispherical mould (see Fig. 2). Meanwhile, the press was heated to the desired sintering temperature, for example 150[degrees]C. The punch was placed on top of the panel and carefully aligned in the press. A small initial force was applied to secure the position of the mould. Once the mould and the cloth reached the desired sintering temperature (about 10 min), the force was gradually increased up to 10 tons over a 5-min period. The cloth conformed fully to the hemispherical shape of the mould and after 5 min was consolidated. The mould assembly was rapidly cooled to ambient temperature, by running tap water through the press's platens, while maintaining pressure. The hemispherical dome could be easily removed after pressure was released and the blank holder was lifted away [29, 31].

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

Spectra cloth, Dyneema Fraglight, and Spectra Shield Plus PCR are all extended chain UHMWPE fiber products of different forms such as woven fabric, nonwoven felt, and nonwoven prepreg, respectively. Spectra cloth and Dyneema Fraglight are single component materials containing only fibers, while Spectra Shield Plus PCR is a two component material containing fibers and matrix. The load bearing component in all three materials is highly oriented UHMWPE fibers. Spectra fiber and Dyneema fiber have similar properties. The properties of the consolidated structures are largely dependent upon the structural and morphological properties of the load bearing fibers, although fiber alignment and the introduction of the matrix influence the final properties of the composite.

Spectra cloth is made of continuous long Spectra yarns in a plain weaving pattern. Each yarn consisted of 120 threads of fibers. The woven structure can be preserved after sintering, which would contribute to the impact resistance because the weaving interlocks the yarns and hinders the penetration by the dart. However, weaving causes fibers to loose straightness in the cloth thickness direction as the fibers wind sinusoidally. This would, to some extent, decrease the impact resistance and flexural properties of the sintered cloth. Moreover, the waviness of the fibers presents a nonsmooth surface and prevents a good contact between stacked layers, which would lead to relatively low interlayer adhesion in sintered cloths.

Dyneema Fraglight felt is made of continuous Dyneema fiber which has comparable mechanical properties to Spectra fiber. Dyneema Fraglight is a nonwoven felt and the fibers are unoriented and randomly aligned. It is an in-plane isotropic material and therefore it is expected that the consolidated structures of this material would have a lower rigidity and stiffness than the more oriented consolidated Spectra cloth. The needle punching process, used to manufacture the felt, introduces physical entanglements of the fibers. Good impact resistance is expected for the consolidated materials as these physical entanglements should make penetration difficult. Good interlayer adhesion is anticipated since the entanglements would hinder the separation of bonded layers.

Spectra Shield Plus PCR is a nonwoven prepreg that contains two chemically different materials: Spectra fiber and matrix. The matrix is used as a binding agent and good interfiber and interlayer adhesion is expected. In each layer of prepreg, either in the 0[degrees] or 90[degrees] direction, the fibers are aligned relative to each other. The ordered fiber alignment should be reflected in a high Hermans orientation function parameter. The unidirectional fiber alignment can also contribute positively to the impact resistance and flexural properties. However, the soft matrix would contribute negatively to the impact and flexural properties. The net outcome is difficult to predict and have to be carefully studied.

Comparative studies on three materials were carried out by monitoring the crystallinity and molecular orientation changes with processing, measuring the impact properties, interlayer adhesion and flexural properties of the consolidated products, and investigating the thermoformability.

Crystallinity Change

DSC thermographs for the single layer as-received Spectra cloth and the cloths consolidated at different temperatures are overlaid in Fig. 3. Single layer as-received and consolidated Dyneema Fraglight felt and Spectra Shield Plus PCR prepreg show essentially the same trend on their DSC trace overlays as Spectra cloth. However, the as-received prepreg contains a shoulder on the low temperature side of the melting peak (see Fig. 4) while the as-received cloth and felt show only a distinct melting peak at 150[degrees]C, suggesting that the as-received prepreg might have experienced a high temperature treatment previously in the manufacturing process of fiber impregnation. The matrix in the prepreg appears to have no thermal transition within the range of testing temperatures. For all consolidated fabrics, with increasing processing temperature, a lower temperature shoulder emerges and eventually evolves into a distinct melting peak at 135[degrees]C. The two endotherms correspond to the melting of the original highly oriented crystals and the newly formed crystals upon cooling in the fibers, respectively. With increasing sintering temperature, the higher temperature endotherm decreases and the lower temperature endotherm increases, indicating the gradual loss of the original highly oriented crystals accompanied by the gradual formation of unoriented crystals.

[FIGURE 3 OMITTED]

The degree of crystallinity was calculated by Eq. 1 using the literature value of the melting enthalpy of 100% PE crystal (293.6 J/g) [34, 35]. The melting enthalpy of the two types of crystals was calculated by deconvoluting the two overlapping melting endotherms and integrating the area under each curve. The overall crystallinity is defined as the sum of the crystallinity of the remaining original crystals and the newly formed crystals, and the original crystallinity is exclusively the amount of remained original crystals. Both types of crystallinity were determined (see Table 1). For Spectra Shield Plus PCR prepreg, attention was taken to normalize the crystallinity with respect to its fiber weight fraction (80%). The as-received Spectra Shield Plus PCR prepreg has the lowest overall crystallinity that agrees with the existence of a shoulder on DSC trace, implicating the partial loss of crystallinity due to manufacturing. Consolidated materials preserve their crystallinity up to 148[degrees]C and the degree of crystallinity decreases rapidly as sintering temperatures exceed the melting temperature of the UHMWPE fibers (150[degrees]C). Above 150[degrees]C, the difference between the overall crystallinity and the original crystallinity becomes significant, indicating that a large amount of the original crystals are melted during processing and the calculated overall crystallinity contains a great deal of newly formed crystals that are unoriented and imply lower properties for the materials sintered at these temperatures. Since a high degree of crystallinity must be preserved to assure the properties of the consolidated structures, the desired processing temperature should not exceed 150[degrees]C.

[FIGURE 4 OMITTED]

Degree of crystallinity = [Melting enthalpy of the specimen/Melting enthalpy of 100% polyethylene crystal] x 100% (1)

Extended chain UHMWPE fibers have high crystallinity, which is essential to their outstanding properties. High-temperature high-pressure sintering process induces a series of structural changes including: partial melting of the fibers, molecular reorientation due to increasing molecular relaxation at high temperatures, and orthorhombic to hexagonal crystalline transformation [36]. The amount of crystallinity change of the consolidated structures depends on the different processing conditions. For the as-received materials, Spectra cloth 903 has the highest crystallinity since Spectra fiber has an inherently higher crystallinity than Dyneema fiber and the crystallinity of Spectra Shield Plus PCR is partially destroyed during manufacturing. For the consolidated materials, Spectra cloth 903 maintains the highest crystallinity over the processing temperature range due to the fact that it has the highest initial crystallinity. The crystallinity of consolidated fabrics is extremely sensitive to the processing temperature; a two degree change in temperature results in a great difference in crystallinity.

Molecular Orientation

Hermans orientation function measures how much the average molecular chain orientation deviates from an arbitrary axis of interest and can be calculated from the WAXD results. Table 2 shows the Hermans orientation function values as a function of processing temperature for the single layer as-received as well as sintered Spectra cloth and Spectra Shield Plus PCR prepreg. As fibers in the prepreg are unidirectionally aligned, one would expect that Spectra Shield Plus PCR would have had higher Hermans orientation function than that of Spectra woven cloth due to the lack of fiber waviness in Spectra Shield Plus PCR. The results are contradictory; the Hermans orientation function of the as-received Spectra Shield Plus PCR is only 81% of that for the as-received Spectra cloth 903. All consolidated Spectra Shield Plus PCR have lower molecular orientation than consolidated Spectra cloth under the same processing conditions. This may be due to the thermal history experienced by the fibers during the manufacturing of Spectra Shield Plus PCR. The results of molecular orientation agree with the previous results of crystallinity on this perspective.

Extended chain UHMWPE fibers have high molecular orientation due to the ultra drawing process experienced during manufacturing, high orientation is a crucial factor to their outstanding properties. During the high-temperature high-pressure sintering process, the melted phase recrystallizes upon cooling to form a less oriented phase; the increase of molecular mobility at high temperatures results in the rearrangement of the molecules; and the introduction of crystallization due to pressure forcing the molecules to adopt more ordered conformation. These factors all play their roles in determining the final orientation. Macroscopic alignment of the fibers within each material also influences Hermans orientation function. Lateral movement of the fibers can happen during sintering and result in the misalignment of fibers and affects the measured orientation. In Dyneema felt, the fibers are randomly distributed and the material is inplane isotropic. Both Spectra cloth and Spectra Shield Plus PCR preserve relatively high orientation up to the processing temperature range of 150-152[degrees]C range, followed by a rapid decrease in orientation due to the excessive melting of the fibers at higher temperatures. Consolidated Spectra cloth shows a higher orientation than the original cloth at 148[degrees]C perhaps due to annealing effect under the heat and pressure. WAXD studies confirm that the desired processing temperature should not be higher than 150[degrees]C.

Impact Properties

Since our main interest in this research is to study the potential of these UHMWPE fiber materials as high impact resistant composites, optimization of the impact properties of the sintered materials was performed as a function of processing parameters [30]. Crystallinity and orientation studies indicate that the properties of each material have different temperature dependence. Spectra and Dyneema fibers are made by different companies, there are differences in structures and properties although they are quite similar. Spectra Shield Plus PCR is a preprocessed two-phase material containing both the fiber and matrix. The manufacturer's recommended temperature for its further processing is around 120[degrees]C. From this information and the appearance of the matrix, we suspect that the matrix is a Kraton[R] type polymer. As we know, Spectra fiber is essentially intact at 120[degrees]C and in order to sinter Spectra cloth and Dyneema felt, a processing temperature close to their melting peak temperature (150[degrees]C) is required. For the purpose of comparability of the data, Spectra Shield Plus PCR was processed at temperatures above 130[degrees]C despite the recommended temperature. However, if the temperature is too high, the matrix will melt and flow under the heat and pressure. Under these conditions, the matrix is squeezed out of the mould and the material is in fact destroyed. The impact resistance of single layers of each material consolidated under 7.6 MPa for 30 min at different temperatures was measured and compared with that of the as-received materials; the NTT total impact energies are shown in Table 3.

[FIGURE 5 OMITTED]

Consolidation of Spectra cloth at high temperature and pressure produces the interfiber adhesion and greatly improves the impact properties of the cloth. In the best scenarios, the impact resistance is increased by nearly 6 times compared with the as-received cloth. The maximum impact properties are achieved when the Spectra cloth was sintered at 150[degrees]C. At higher temperatures, too much crystallinity and molecular orientation are destroyed; therefore the properties of the sintered cloth deteriorate.

As-received Dyneema felt has good impact resistance because of the physical entanglements of its fibers that make it difficult for a shooting dart to penetrate. Dyneema felt consolidated at 148[degrees]C exhibits the best impact properties with a nearly 30% increase over the original felt. At sintering temperatures below 148[degrees]C, the impact resistance is hardly improved. At sintering temperatures above 148[degrees]C, the impact properties decrease rapidly due to excessive melting. The best processing temperature to achieve the maximum impact properties of the felt is ~ 148[degrees]C.

Spectra Shield Plus PCR consolidated at 140[degrees]C exhibits maximum impact energy, nearly fourfold increase over the original material due to the partial melting of the fiber surface allowing the polymer chains to penetrate into the surrounding polymeric matrix and form interlocking network with the matrix upon recrystallization. The enhanced interphase adhesion is responsible for the better impact properties. At sintering temperatures below 140[degrees]C, impact resistance is gradually improved. At sintering temperatures above 140[degrees]C, impact properties decreases gradually because of increased fiber melting and the loss of matrix due to flow under pressure as explained previously.

Two types of failure were observed in the experiments. Case 1: fibers/yarns were pushed aside and pulled out by the shooting dart but not broken, designated as failure Mode A; Case 2: fibers/yarns were broken and specimen was fractured, designated as failure Mode B (see Fig. 5). The specimens failed by Mode A absorbed higher impact energy indicating that most of the fiber strength is maintained after consolidation and the adhesion between fibers is sufficiently good. About 150[degrees]C is shown to be the critical temperature for the materials to exhibit a transition of different failure modes. This is not surprising as it coincides with the peak melting temperature of the UHMWPE fibers.

Consolidated Spectra cloth 903 has a peak NTT impact energy of 92.0 KJ/m if processed at 150[degrees]C and Dyneema felt at 76.2 KJ/m if processed at 148[degrees]C due to the random alignment of Dyneema fibers; the fibers cannot be constrained as effectively as the aligned fibers in Spectra cloth. Dyneema fibers begin to melt earlier than Spectra fibers, therefore, their maximum impact properties occur at a lower temperature. For Spectra Shield Plus PCR, the optimal processing temperature, in terms of the highest attained impact resistance of 77.5 KJ/m, is 140[degrees]C which is considerably lower than the other two materials. The matrix of Spectra Shield Plus PCR has a relatively low viscosity at high temperatures and when under a large pressure tends to flow; so the pressure cannot be effectively exerted onto the fibers. The fibers in Spectra Shield Plus PCR are not as sufficiently constrained as those in Spectra cloth and Dyneema felt and consequently lose crystallinity and orientation during sintering, resulting in lower impact properties. Consolidated Spectra cloth has the best maximum impact resistance among the three materials and proves to be the best candidate for making high strength, high impact resistant composites.

Interlayer Adhesion

T-peel test specimens of different materials for the measurement of interlayer adhesion were bonded at different temperatures. These temperatures were chosen around each material's respective optimal processing temperatures for the best impact properties.

Interlayer adhesion was characterized by T-peel strength: the average peeling load (N) per unit length (cm) of the specimen width required to separate the adherends.

T-peel strengths of three materials bonded at different temperature are shown in Table 4. Spectra cloth sintered at a temperature higher than 156[degrees]C is completely fused and cannot be separated without breaking the specimens. Interlayer adhesion of sintered Spectra cloth bilayers increases with increasing sintering temperature, especially above 152[degrees]C, since more melting occurs at higher temperatures and more recrystallized material acts as binder. Dyneema felt bilayers sintered at or above 156[degrees]C are also fused. The critical processing temperature is 152[degrees]C for a rapid increase in its interlayer adhesion. For Spectra Shield Plus PCR, interlayer adhesion increases steadily with increasing sintering temperature. At sintering temperatures between 130 and 140[degrees]C, the interlayer adhesion does not improve significantly. Above 140[degrees]C, the interlayer adhesion increases at a faster rate due to partial melting and recrystallization of the Spectra fibers, which adds to the adhesive strength in addition to that generated by the matrix. The T-peel strength for each material increases monotonically with increasing bonding temperature due to increased melting and recrystallization. Interlayer adhesion is developed at the expense of the fiber longitudinal strength. Dyneema felt has better adhesion than Spectra cloth due to its flat surface that enables better contact. Also, physical entanglements of the fibers in Dyneema felt hinder the separation of the bonded layers.

If Spectra cloth is bonded with Dyneema felt, it is anticipated that the interlayer adhesion would fall between the values of the Spectra cloth bilayers and the Dyneema felt bilayers. The experimental results agree with the anticipation, as shown in Table 5. By sintering alternating layers of Spectra cloth and Dyneema felt, an excellent method to prepare a composite with a balanced interlayer adhesion and longitudinal strength is offered which utilizes the strengths of both materials.

Flexural Properties

Flexural properties of multilayer consolidated Spectra cloth, Dyneema Fraglight, and Spectra Shield Plus PCR sintered at various temperatures are shown in Table 6. The processing temperature ranges were deliberately selected around the optimal temperatures for each material as determined previously.

Consolidated multilayer Spectra cloth panels are very stiff and rigid. The flexural modulus increases with increasing sintering temperature to 154 [degrees]C and decreases with increasing sintering temperatures above 154 [degrees]C. The maximum measured flexural modulus is 5.18 GPa, which is a good evidence of its rigidity as a polymeric composite. Consolidated multilayer Dyneema felt has relatively low flexural modulus due to random fiber alignment. Dyneema panels sintered at 145[degrees]C have the highest rigidity. For multilayer consolidated Spectra cloth and Dyneema[R] felt, no specimens showed any failure or yield within the 5% strain limit set by ASTM standard. The lower temperature sintered materials were not sufficiently consolidated and interlayer adhesion is low. At higher sintering temperatures, the fibers melt excessively and the rigidity is lost. The existence of the optimal sintering temperature regarding flexural properties illustrates the importance of the balance between the lost longitudinal strength and the gained lateral strength. The flexural modulus and strength of Spectra Shield Plus PCR flat panels sintered at different processing temperatures are shown in Table 7. The Spectra Shield Plus PCR panels are more flexible than those made from Spectra cloth 903 or Dyneema felt. Low flexural modulus results from the use of a soft matrix. Although the prepreg possesses straight and unidirectional fibers, the flexural properties of the consolidated panels are offset by the use of the soft matrix. All Spectra Shield specimens exhibited a yielding point within the 5% strain and flexural strength is reported. At higher strains, all test specimens developed delamination between the layers, indicating relatively low interlayer adhesion. The flexural modulus reaches a maximum value at a sintering temperature of 140[degrees]C and flexural strength remains predominantly constant over the entire processing temperature range.

[FIGURE 6 OMITTED]

Consolidated Spectra cloth panels have a much higher flexural modulus than the other two UHMWPE fiber materials. Consolidated Dyneema felt has about half the rigidity of Spectra cloth 903, if their achieved maxima are compared. Consolidated Spectra Shield Plus PCR has the lowest rigidity due to its soft matrix despite its better fiber alignment; this illustrates one of the advantages of eliminating the use of matrix by making matrix free Spectra fiber reinforced composite from plain woven Spectra cloth. Matrix free Spectra composite has a higher fiber volume fraction that translates to better properties; it is the best candidate among three materials to make rigid and strong composites.

Thermoformability

Spectra cloth has excellent thermoformability and can be moulded into hemispherical domes (see Fig. 6) using various shaping and consolidation sequences [31]. Spectra fiber has unique extended chain morphology with minimum chain folds and entanglements. The polymer is chemically nonpolar; there are no strong secondary forces between molecules and no strong intermolecular interactions. At higher temperatures, when chain mobility increases, the molecules slide and pass each other relatively easily under a large load. Shown on the macroscopic scale, Spectra fiber can accommodate high deformation without failing. Spectra fiber pressurized at an elevated temperature undergoes a PE crystal transformation, an orthorhombic phase to hexagonal phase transition (o-h transition) as reported by many researchers [36-39]. Hexagonal crystals possess a relatively low viscosity compared to orthorhombic crystals and chain mobility is rather high. Because of the o-h transition at high pressure and temperature, Spectra fiber is capable of undergoing large axial strains with relative ease via this so-called "mobile phase."

Multilayer Dyneema felt can be moulded to hemispherical domes using the same procedure as described for Spectra cloth. Because of the thickness of the felt, 20 layers of Dyneema felt were used to produce a dome having dimensions similar to that of 25 layers of Spectra cloth. Domes made from Dyneema felt are strong and stiff. Looseness of the fibers in the felt results in the straightening of the previously curled fibers rather than actual stretching of the fibers by the deformation of the felt in the mould. As a result, a relatively smaller force was used for moulding as compared with the Spectra cloth. Dyneema felt has good thermoformability.

Multilayer Spectra Shield Plus PCR can be moulded using 50 layers of prepreg to produce a dome with similar dimensions to that of the 25 layer Spectra cloth dome. Since adhesion between the Spectra fibers and the matrix is low, the fibers easily slip from the surrounding matrix when the load is applied to form the shape. At high moulding temperatures, the viscosity of the matrix lowers and the matrix tended to squeeze out as a result of pressure; the constraint exerted by the blank holder was questionable. The fibers are inclined to slip inside the prepreg when moulding pressure was applied and fiber stretching is not as prominent as with Spectra cloth. Spectra Shield Plus PCR is not a good material for direct shaping and would be better processed using the conventional method of cutting patterns and laying-up.

CONCLUSION

Three materials containing the extended chain UHMWPE fibers, Spectra woven cloth, Dyneema Frag-light nonwoven felt, and Spectra Shield Plus PCR prepreg, were subjected to the same high-temperature high-pressure sintering process. The physical, thermomechanical, microstructural, and morphological properties of the consolidated products were investigated and compared with one another. Differences in the materials and the influence of processing conditions are reflected in the structures and properties of the final products.

Consolidated Spectra cloth 903 achieves the highest crystallinity and better molecular orientation than consolidated Spectra Shield Plus PCR. Consolidated Spectra cloth 903 also shows the best impact resistance and the highest flexural modulus since fibers are effectively constrained during sintering and the possibility of excessive fiber melting and orientation loss is reduced. Consolidated Dyneema Fraglight is not as rigid as consolidated Spectra cloth and consolidated Spectra Shield Plus PCR suffers the lowest stiffness. In terms of interlayer adhesion, Dyneema Fraglight has an advantage that prompts the use of alternative layers of Spectra cloth and Dyneema felt to make composites having both high longitudinal and lateral strengths. The properties of consolidated materials are very sensitive to the processing temperature. All three materials are thermoformable; Dyneema Fraglight is the easiest to shape while Spectra Shield Plus PCR has difficulty in moulding due to the flow of matrix at high temperatures.

Matrix free Spectra fiber reinforced composite made of Spectra cloth is proven to be the best material to make high strength and high impact resistance composites. It has extraordinary impact and flexural properties, excellent interlayer adhesion and thermoformability as well as the simplest manufacturing processing by eliminating the need to cut patterns and add matrix.

ACKNOWLEDGMENTS

We would like to thank the US Army Soldier and Biological Chemical Command at Natick Massachusetts for supplying the Dyneema felt. Honeywell kindly provided us the Spectra materials used in this study. Thanks to MRSEC and CUMIRP at University of Massachusetts for the access to various instruments and equipment used for characterization.

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Tao Xu, Richard J. Farris

Silvio O. Conte National Center for Polymer Research, Polymer Science and Engineering Department, University of Massachusetts Amherst, Amherst, Massachusetts 01003

Correspondence to: Richard J. Farris; e-mail: [e-mail]

Contract grant sponsor: US Army Soldier and Biological Chemical Command at Natick Massachusetts.

TABLE 1. Overall crystallinity and original crystallinity of single
layer as-received and consolidated fabrics sintered at different
processing temperatures.

Specimen                  As-received  148[degrees]C  150[degrees]C

Spectra cloth
  Overall crystallinity   95.2%        97.5%          94.8%
  Original crystallinity  95.2%        92.0%          86.8%
Dyneema Fraglight
  Overall crystallinity   92.4%        92.3%          90.6%
  Original crystallinity  92.4%        90.7%          88.1%
Spectra Shield Plus PCR
  Overall crystallinity   90.3%        90.7%          85.9%
  Original crystallinity  89.6%        87.9%          83.5%

Specimen                  152[degrees]C  154[degrees]C

Spectra cloth
  Overall crystallinity   90.6%          84.1%
  Original crystallinity  76.9%          56.1%
Dyneema Fraglight
  Overall crystallinity   82.1%          75.1%
  Original crystallinity  68.7%          57.2%
Spectra Shield Plus PCR
  Overall crystallinity   80.8%          70.0%
  Original crystallinity  49.2%          27.9%

TABLE 2. Hermans orientation function of the single layer as-received as
well as sintered Spectra cloth and Spectra Shield Plus PCR prepreg
consolidated at different processing temperatures.

Specimen                 As-received  148[degrees]C  150[degrees]C

Spectra cloth            0.887        0.911          0.873
Spectra Shield Plus PCR  0.720        0.706          0.704

Specimen                 152[degrees]C  154[degrees]C

Spectra cloth            0.857          0.760
Spectra Shield Plus PCR  0.646          0.610

TABLE 3. NTT total impact energy (KJ/m) of the single layer as-received
and consolidated materials consolidated at different processing
temperatures.

Specimen           As-rec'd  148[degrees]C  150[degrees]C  152[degrees]C

Spectra cloth      16.6      78.1           92.0           13.7
Failure mode       A         A              A              B
                   As-rec'd  140[degrees]C  145[degrees]C  148[degrees]C
Dyneema Fraglight  64.5      65.8           66.7           76.2
Failure mode       A         A              A              A
                   As-rec'd  130[degrees]C  140[degrees]C  145[degrees]C
Spectra Shield     20.0      43.1           77.5           69.6
Failure mode       A         A              A              A

Specimen           154[degrees]C  156[degrees]C

Spectra cloth      3.2            2.3
Failure mode       B              B
                   150[degrees]C  152[degrees]C  154[degrees]C
Dyneema Fraglight  63.0           20.0           1.8
Failure mode       A              B              B
                   148[degrees]C  150[degrees]C  152[degrees]C
Spectra Shield     66.4           61.1           19.8
Failure mode       A              A              B

Specimen

Spectra cloth
Failure mode
                   156[degrees]C
Dyneema Fraglight  0.5
Failure mode       B
                   154[degrees]C
Spectra Shield     0.6
Failure mode       B

TABLE 4. T-peel strength (N/cm) for Spectra cloth, Dyneema Fraglight,
and Spectra Shield Plus PCR bilayers sintered under 7.6 MPa for 30 min
at different temperatures.

Sintering                 Spectra  Dyneema    Spectra Shield
temperature ([degrees]C)  cloth    Fraglight  Plus PCR

130                                           3.1
140                                           3.2
145                                           4.5
148                        1.7      2.3       6.0
150                        2.0      3.4
152                        3.5     17.3
154                       11.4     38.0
156                       13.3

TABLE 5. T-peel strength (N/cm) of Spectra cloth bilayer, Dyneema
Fraglight bilayer, and Spectra cloth-Dyneema Fraglight bilayer bonded
under 7.6 MPa for 30 min at different temperatures.

Sintering temperature  148[degrees]C  150[degrees]C  152[degrees]C

Spectra/Spectra        1.7            2.0             3.5
Spectra/Dyneema        2.2            3.8             5.5
Dyneema/Dyneema        2.3            3.4            17.3

Sintering temperature  154[degrees]C  156[degrees]C

Spectra/Spectra        11.4           13.3
Spectra/Dyneema        23.5           47.7
Dyneema/Dyneema        38.0           --

TABLE 6. Flexural modulus (GPa) of multiplayer Spectra cloth and Dyneema
Fraglight panels consolidated under 17.2 MPa for 30 min at different
temperatures.

Processing temperature ([degrees]C)  Spectra cloth  Dyneema Fraglight

130                                                 1.61
140                                                 2.23
145                                                 2.56
148                                                 2.38
150                                  3.33           2.08
152                                  4.52
154                                  5.18
156                                  4.31
160                                  0.80

TABLE 7. Flexural properties of 32 layer Spectra Shield Plus PCR panels
consolidated under 17.2 MPa for 30 min at different processing
temperatures.

Processing temperature   130[degrees]C  140[degrees]C  150[degrees]C

Flexural modulus (GPa)   0.62           0.90           0.53
Flexural strength (MPa)  3.16           3.17           3.15