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Novel composites by hot compaction of fibers.

INTRODUCTION

Research at Leeds University and a number of other academic and industrial institutions has led to several routes for the production of high stiffness and high strength oriented polymers. The methods developed include melt spinning and gel spinning followed by high draw for high modulus polyethylene fibers (1, 2) ram extrusion (3) and hydrostatic extrusion (4) for highly oriented polyethylenes and other flexible polymers, and more recently die-drawing (5) and roller drawing (6) for a wide range of polymers, including polypropylene, polyvinyl chloride, and poly(ethylene terephthalate).

A recent development in this area is the hot compaction of highly oriented fibers which is now being extensively studied at Leeds University and will now be reviewed. There are two major aspects to the recent work. First, hot compaction is of considerable scientific interest in terms of throwing light on the structure and properties of highly oriented fibers by providing definitive compacted structures that can be more easily studied in some respects than the fine fibers. Second, it appears that this is an alternative technology for making stiff and strong sheets and shapes in one chemical composition; moreover, the sheets are thermoformable.

THE COMPACTION PROCESS

The essence of the compaction process is to melt a fraction of the surface of each fiber under a comparatively low contact pressure, to apply a substantially higher pressure for a short time to achieve excellent consolidation of the structure, so that on cooling the recrystallizing polymer acts to bind the fibers together like the resin matrix in a fiber/resin composite.

The hot compaction process was first demonstrated for the melt spun - high draw polyethylene fibers (710) manufactured initially by SNIA FIBRE, Italy, with the trade name TENFOR. These fibers are now produced commercially by Hoechst Celanese USA with the new trade name CERTRAN. Typically the fibers are aligned in a parallel array in a matched metal mold, which is placed in a heated compression press. Under a low contact pressure of 0.7 MPa, the fibers are heated to the required temperature and allowed to soak for 10 min. A higher pressure of 21 MPa is then applied for 10 s, and the mold removed from the press and cooled to room temperature. Although the choice of compaction pressure is important, it has been found that the compaction temperature is critical in determining the final compacted structure, so the effects of changing this have formed the subject of several publications.

For optimal final mechanical properties, a balance between the transverse flexural strength and the loss of axial stiffness and strength in the fibers can be achieved by melting the CERTRAN fiber at a temperature close to the fiber melting point [ILLUSTRATION FOR FIGURE 1 OMITTED]. Here only [approximately] 10% of the initial fiber has been melted, close to that required to fill the gaps in a close packed hexagonal arrangement of cylinders, which is (1 - [Pi]/(2[-square root of 3]) = 9.3%, and the density of the compacted material is close to that of the original fiber. Morphological studies show a remarkable degree of coherence [ILLUSTRATION FOR FIGURE 2 OMITTED], where the recrystallized material is only just visible in sections cut perpendicular to the fiber axis. Longitudinal sections show that the melted material recrystallizes on the fibers as nuclei. The lamellae so formed therefore show the c-axis orientation of the fibers, which explains the good lateral properties of the resulting composite compacted at temperatures in the range 136 [degrees] C to 140 [degrees] C, where a substantial fraction of the original fiber modulus has been retained [ILLUSTRATION FOR FIGURE 3 OMITTED].

If the fibers are compacted at lower than the optimum temperature (e.g. 134 [degrees] C), there is a comparatively small degree of surface melting and the compaction pressure produces a cross section showing a somewhat irregular hexagonal network, where the original circular fibers evidently deform by shear into irregular hexagons. For compaction at 140 [degrees] C, above the optimum temperature of 138 [degrees] C, a significant amount of melting occurs, but a circular outline of the original fibers can still be clearly distinguished. Nucleation of the recrystallization occurs on the surface of the fibers, the lamellae sharing the chain-axis orientation of the fiber, creating a giant row nucleus. At 142 [degrees] C melting also occurs in some internal regions of the fiber, as well as on the surface. It has been suggested that these are longitudinal regions of lower density or lower constraint, which melt before and recrystallize within the surrounding rigid cage of material, possibly associated with a rigid stretched molecular network of entangled molecules. Support for this network concept comes from measurements of shrinkage, shrinkage force, and thermal expansion behavior.

In a recent investigation (11), the hot compaction of the gel spun polyethylene fiber (SPECTRA 1000, manufactured by AlliedSignal Corporation) has been studied, using a combination of DSC, scanning electron microscopy (SEM), and broad-line NMR to examine the structure of the compacted materials. It was shown that the two stage hot compaction procedure, with a significantly higher initial contact pressure to restrain fiber shrinkage, can produce a homogeneous product with a longitudinal modulus similar to that of the melt spun fiber compacted material and transverse strength sufficient for useful applications. The fiber to fiber bonding developed does not, however, result from melting and recrystallization at the surface of the fibers, as for the melt spun fiber, but is a combination of mechanical interlocking and fiber to fiber fusion. The morphology of the compacted material shows well-packed irregular polygons with some welding points where melting and recrystallization occur. The broad-line NMR measurements of crystallinity show that a substantial amount of material is transformed from the rigid crystalline structure into oriented mobile chains so that much more of the original fiber stiffness is lost during successful compaction than in the case of the melt spun fiber.

HOT COMPACTION OF OTHER FIBERS

The two stage hot compaction technique (low pressure at the hot compaction temperature for several minutes, followed by a higher pressure for a few seconds before cooling to room temperature), has now been applied successfully to several other fibers, including poly(ethylene terephthalate) (PET), polypropylene (PP), and Vectran liquid crystalline polyester fibers.

In PET (12), successful compaction was achieved with compaction temperatures in the range 252 [degrees] C to 256 [degrees] C, employing a rather higher contact pressure (1.85 MPa) than for PE, to prevent fiber shrinkage. Transmission electron micrographs [ILLUSTRATION FOR FIGURE 4 OMITTED] of cut but unetched samples showed surface melting and recrystallization, confirming that the compaction process is similar to that in melt spun PE. It appears that the recrystallized lamellae again grow from the original fiber backbones in row structures. DSC measurements show a considerable rise in the melting point for compacted fibers, which was attributed to super-heating effects. The low-temperature peak that was clearly seen in the compacted PE fibers was only just discernible. It was concluded that this is because recrystallized PET has a similar melting point to the fibrillar crystalline regions, so that its endotherm is subsumed into that of the original fiber which remains.

The hot compaction results for PET and melt-spun PE have been usefully compared by a normalization [TABULAR DATA FOR TABLE 1 OMITTED] [TABULAR DATA FOR TABLE 2 OMITTED] procedure where the tensile moduli of the compacted fibers are normalized as a proportion of their initial fiber moduli, and the compaction temperatures as a fraction of the total melting temperature interval of the fibers, i.e.,

Normalized temperature

[T.sub.compaction] - [T.sub.onset]/[T.sub.end of melting endotherm] - [T.sub.onset] (1)

where [T.sub.onset] means the temperature at which melting commences. Figure 5 shows this comparison, which emphasizes the similarity in behavior between the two fibers.

In the case of PP (13), the compaction of undirectionally arranged high-tenacity fibers has been reported. The two stage compaction process was explored for compaction temperatures in the range [TABULAR DATA FOR TABLE 3 OMITTED] 164 [degrees] C to 174 [degrees] C. It was found to be rather more difficult to achieve surface melting of the fibers, although clear evidence was again found for lamellar recrystallization at the fiber surface. DSC measurements showed that there were considerable structural changes during compaction with significant recrystallization. Comparatively low lateral strengths of [approximately]10 MPa were observed for the undirectional compacted fiber samples, which could be due to the ready tendency of the high tenacity PP fibers to fibrillate, rather than to unsatisfactory compaction. This explanation is supported by evidence that compacted woven PP fabrics have very adequate mechanical properties (see next section).

Finally, it should be mentioned that very satisfactory results have been obtained for the compaction of Vectran LCP fibers (14). Under optimum conditions, lateral strengths up to 20 MPa were obtained for unidirectional hot compacted composites, with similar reductions in axial tensile modulus to those reported for compacted melt spun PE fibers.

PROPERTIES OF COMPACTED PRODUCTS

It is of immediate interest to compare the elastic properties of hot compacted sheets with those obtained by rival technologies, especially for fiber reinforced composites and oriented polymers produced by die-drawing. To make these comparisons, the elastic constants have been determined by the ultrasonic immersion method (15). The fiber elastic constants have also been obtained by determining the elastic constants of sheet made by compacting at different temperatures, and hence different amounts of melted polymer and extrapolating to zero melted polymer.

The results of this comparison (Table 1) show that the hot compaction process is very effective in retaining the fiber properties. The stiffness constants, especially [C.sub.33], are close to the extrapolated values. Hot compaction is more effective in this respect than the conventional fiber (epoxy composite) and comes remarkably close to the die-drawn sheet. Similar results were obtained for hot compacted polypropylene sheet.

A further interesting comparison is shown in Table 2, where the compacted sheet elastic constants for PE are compared with those predicted theoretically by Lacks and Rutledge (16). The measured pattern of anisotropy agrees well with that calculated for a uniaxially oriented sheet, based on taking the mean of averaging the stiffness constants and the compliance constants respectively obtained for the predicted orthorhombic elastic constants. The lower value of [C.sub.33] is to be expected, because the melt spun and drawn fibers do not possess a perfectly aligned crystalline structure. In particular there is limited crystal length, which limits the stiffness [C.sub.33] similar to the shear-lag effect in short fiber composites. Similar results have been obtained for hot compacted PP fibers.

For most practical applications of the hot compaction technology, where a better balance between stiffness and strength is required, it is more appropriate to use a woven fabric rather than to attempt to align fibers. Table 3 shows collected results for hot compacted sheets produced from plain weave fabrics in PE, PP, PET, and Vectran LCP. It can be seen that high values of flexural and tensile moduli and strengths are obtained in all cases, together with good impact strengths.

An additional important attribute of the hot compacted woven sheets is their ability to be thermoformed, so that quite complex shapes can be readily produced. There are also some special attributes, which are summarized in Table 4.

COMMERCIAL APPLICATIONS

The present indications are that the hot compacted products offer commercial opportunities in both specialty applications such as radomes and medical imaging screens, and larger commodity applications such as protective clothing for sporting goods, loudspeaker cones, and automotive products.

CONCLUSION

Hot compaction of highly oriented fibers offers a new entry to basic polymer science studies, in addition to providing exciting new materials for a wide range of practical applications. This technique appears to be in the interesting situation of bridging the gap between conventional fiber/resin composite technology and the new methods for fabricating oriented polymers in solid section, hydrostatic extrusion, die-drawing, and roller-drawing.

Table 4. Advantages of Compacted Fiber Sheets.

General

* High retention of original fiber properties (stiffness and strength)

* Lightweight

* Energy absorbing

* Single phase system, therefore recyclable

* Low thermal expansion

* Thermoformable

Special

* Microwave transparent (PE)

* Good insulator (LCP)

* Biocompatible (PE)

* High thermal conductivity (LCP and PE)

* Inert (PE)

REFERENCES

1. G. Capaccio and I. M. Ward, Polym. Eng. Sci., 15, 219 (1975).

2. P. Smith and P. J. Lemstra, J. Mater. Sci., 15, 505 (1980).

3. N. E. Weeks and R. S. Porter, J. Polym. Sci., Polym. Phys. Ed., 12, 635 (1974).

4. A. G. Gibson, B. N. Cole, B. Parsons, and I. M. Ward, J. Mater. Sci., 9, 1193 (1974).

5. P. D. Coates and I. M. Ward, Polymer, 20, 1553 (1979).

6. P. E. Burke, G. C. Weatherly, and R. T. Woodhams, in High Modulus Polymers: Approaches to Design and Development, Chap. 14, A. E. Zachariades and R. S. Porter, eds., Marcel Dekker Inc., New York (1988).

7. P. J. Hine, I. M. Ward, R. A. Olley, and D. C. Bassett, J. Mater. Sci., 28, 316 (1994).

8. R. H. Olley, D. C. Bassett, P. J. Hine, and I. M. Ward, J. Mater. Sci., 28, 1107 (1993).

9. M. A. Kabeel, D. C. Bassett, R. H. Olley, P. J. Hine, and I. M. Ward, J. Mater. Sci., 29, 4694 (1994).

10. M. A. Kabeel, D. C. Bassett, R. H. Olley, P. J. Hine, and I. M. Ward, J. Mater. Sci., 30, 601 (1995).

11. R. J. Yan, P. J. Hine, I. M. Ward, R. H. Olley, and D. C. Bassett, J. Mater. Sci., (submitted for publication).

12. J. Rasburn, P. J. Hine, I. M. Ward, R. H. Olley, D. C. Bassett, and M. A. Kabeel, J. Mater. Sci., 30, 615 (1995).

13. M. I. Abo El-Maaty, D. C. Bassett, R. H. Olley, P. J. Hine, and I. M. Ward, J. Mater. Sci., 31, 1157 (1996).

14. J. Rasburn, P. J. Hine, and I. M. Ward (to be published).

15. P. J. Hine and I. M. Ward, J. Mater. Sci., 31, 371 (1996).

16. D. J. Lacks and G. C. Rutledge, J. Phys. Chem., 98, 1222 (1994).
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Title Annotation:First Symposium on Oriented Polymers
Author:Ward, I.M.; Hine, P.J.
Publication:Polymer Engineering and Science
Date:Nov 1, 1997
Words:2337
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