Resources, Conservationand Recycling 14 (1995) 91-101
conservation and recycling
Recycling of glass fiber reinforced thermo-plastic composites. I. Ionomer and low density polyethylene based composites C.D. Papaspyrides *, J.G. Poulakis, C.D. Arvanitopoulos Laboratory of Polymer Technology, Department of Chemical Engineering, National Technical Universityof Athens, Zographou, Athens 157 80, Greece
Accepted 18 January 1995
Abstract Ionomer and low density polyethylene (LDPE) based composites were prepared containing 40% w/w glass fibers in a random in-plane orientation. These composites were then dissolved, and consequently the polymer/fibers solution was separated by filtration to recover the reinforcing agent and the polymer matrix. Different amounts of hot solvent were employed for washing during filtration to vary the polymer content remaining on the fibers. The recycled fibers were used to prepare 'new' composites, i.e., they were incorporated in the same polymer matrix, but of virgin quality. This paper investigates the effect of such an interphase alteration on the tensile performance of these materials. A significant improvement of the tensile modulus was found, namely after one washing of the fibers for the ionomer based composites and two washings for the LDPE based ones. This behavior is discussed in terms of degree of fiber dispersion. Keywords: Glass fiber; Plastic; Thermoplasticcomposite;Solvent
1. Introduction In modem society the concern for the environmental problems leading to the destruction of nature becomes increasingly intense. Humans find it difficult to control the environmental pollution due to old ingrained habits and to already established manufacturing processes. Trying to find solutions recycling appears to be a logical way of saving both money and energy from 'useless' materials. Focusing on plastic materials, although they represent only 8% (approx.) of domestic wastes [ 1,2 ], their recycling processes are being thoroughly examined by scientists all over * Corresponding author.
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the wodd. This is due to their high cost, their high volume percentage (almost 30%) in domestic wastes and their being environmentally non-degradable [3-5]. Moreover, in modem societies thermoplastic composites gradually take the place of thermoset composites due to their specific potential advantages: the manufacturing time is reduced, the storage of intermediate materials for a long time is less expensive, sensibility to impact damage is greatly reduced and, most importantly, molding and recycling are in principle possible [ 6]. This is the reason that nowadays special attention is being paid to the recycling of reinforced plastics; not only because they represent a large amount of the total plastic waste , but also because of their high cost due to the presence of the reinforcing agent [ 8,9]. The disadvantages of existing methods of plastics recycling make clear that the perfect solution has not yet been found. Except when plastics wastes are non-recoverable by other means, incineration and pyrolysis - two destructive techniques aiming at the utilization of plastics wastes as energy sources -appear to be wasteful, as the value added during polymerization stage is lost [ 10,11 ]. Chemical recycling-the breakdown of polymeric waste into reusable fractions for reincarnation as polymers, monomers, fuels or chemicals -is yet in its early stages [ 12]. Reprocessing in the melt phase is a simple, flexible, inexpensive and most popular recycling technique but it leads to downgraded final products due to chemical changes which occur during reprocessing [ 13,14]. A solvent based technique has been thoroughly investigated in our laboratory for recycling plastic waste and was found in our opinion to be very promising [ 15-21 ]. The main steps followed in this method are: 1. Cutting the wastes into smaller pieces and washing with water if necessary. 2. Dissolution into a polymer solvent to the highest possible concentration. 3. Filtration of the solution under pressure to separate any insoluble ingredients. 4. Reprecipitation of the polymer still in solution by means of non-solvents. 5. Filtration, washing the polymer obtained and drying. 6. Recovery of solvent and non-solvent employed from their mixture by fractional distillation. The attractiveness of this route is based on the following points [ 15-25]: 1. The dissolution of the plastics will cause a massive decrease in the bulk volume of the plastic waste. 2. The polymer can be precipitated in a suitable form (granules, fibrils or powder) for further processing. 3. Ingredients or additives can be removed and recycled. 4. Any defective material, such as gelling lumps, due to the previous degradation history are removed during the dissolution stage. 5. The value added during the polymerization stage is maintained intact and the recycled polymers, free of any contaminants, can be used for any kind of application, since the final product is of competitive quality compared with the virgin material. 6. Solvent based processes have the potential to deal with mixtures ofpolymers or polymers alloys, based on the principle of the 'selective dissolution'. According to the latter one polymer could be dissolved at a time from a plastics mixture with evident impact on recycling municipal solid waste plastics. Based on the third of the aforementioned advantages this paper deals with a model investigation to recycle glass fiber reinforced thermoplastic composites. The concept is
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material separation so that both polymer and glass fibers can be obtained and reused. Ionomer and low density polyethylene (LDPE) are considered as matrix materials, comprising one typical polar and one typical non-polar thermoplastic polymer, most commonly used with glass fibers [26-28]. Since the recycled fibers may contain traces of polymer resistant to wash out during the separation cycle, it was attempted to understand the effect of such a coating and even more to control it and investigate its impact on the mechanical'properties of the second generation composites, made out of these modified fibers.
1. PE: low density polyethylene (grade 2202 F, Alcudia Co., Spain). 2. Ionomer: copolymer of ethylene and methacrylic acid partially neutralised with sodium cations (Surlyn 8528, E.I du Pont de Nemours and Co. Inc.). 3. Glass fibers: glass fibers type E. 4. Solvents: all solvents used were general reagent grade chemicals. 2.2. Preparation of composites containing glass fibers
The experimental procedure followed for preparing composites containing glass fibers, unused or recycled as described below, was the same for ionomer and LDPE based composites. The glass fibers were mixed with polymer of virgin quality at a content level of 40% w/ w (Vf = 0.13). A Plastograph B rabender was employed at 1600C for 10 min. Torque levels of 600 m.g. and 400 m.g. were required for the ionomer and the LDPE based composites, respectively. Specimens for the tensile tests were prepared by compression molding and subsequent cutting. An hydraulic thermopress was employed under the following conditions: temperature 230°C; pressure 7 MPa; time 6-8 min. The same cooling rate was always followed to exclude interference from processing history effects. 2.3. Composites recycling
Two different experimental procedures were employed to recycle ionomer or LDPE based composites: lonomer based composites After cutting it into small pieces the composite material was dispersed at 120°C in a 4:1 mixture of xylene and n-butyl alcohol (n-butanol). A 2-1 reaction vessel, fitted with stirrer, reflux condenser and thermometer was employed. The solution containing the glass fibers was filtered through a Seitz Merkur EF 6/03 device with a volume capacity of 300 ml, using 5500 Seitz-T-Filter sheets of exclusion size in the range of 20-70/xm. After filtration,
C.D, Papaspyrides et al. / Resources, Conservation and Recycling 14 (1995) 91-101
the fibers were washed with hot solvent (4:1, xylene/n-butyl alcohol) to remove any amounts of remained ionomer. Up to three successive washings were applied, using each time 100 ml of solvent, resulting in fibers with different ionomer contents. The dissolved ionomer was precipitated by pouring slowly the filtrate into a large amount of methanol (non-solvent) under stirring, so as to brake the precipitated polymer lumps to fine polymer fibrils. A volume ratio (solvent/non-solvent) equal to 1:2 was applied. The dispersion was filtered in a centrifuge and the polymer fibrils separated were washed with methanol and were dried in a vacuum chamber at 950C for 10 h. Finally, the ternary solvent/ non-solvent mixture collected was separated for reuse in a distillation column (0.85 m height) filled with Raching rings and equipped at the top with a side condenser.
LDPE based composites After cutting it into small pieces the composite material was dispersed at 900C in toluene. A 2-1 reaction vessel, fitted with stirrer, reflux condenser and thermometer was employed. The LDPE solution (which contained the glass fibers) was then filtered as previously described for the ionomer based composites. Eventually, by varying the number of washings after filtration, recycled glass fibers were obtained, containing different amounts of polyethylene. The filtrate was transferred into a 2-1 vessel, acetone was properly added to and the polymer contained was precipitated, washed, filtrated under vacuum and dried overnight in a vacuum oven at 70°C. The solvent/non-solvent mixture collected (of a volume ratio equal to about 1:7) was separated for reuse in the aforementioned distillation column (0.85 m height). Characterization aspects of the recycled polymer have already been discussed elsewhere [ 17]. 2.4. Characterization of the recycled fibers Differential scanning calorimetry (DSC) was applied to determine the amount of polymer that remained on the recycled glass fibers. A Perkin-Elmer DSC-4 calorimeter was employed and all runs were carried out between 50°C to 2000C in a nitrogen atmosphere and using an empty capsule as reference. Sample weights varied from 4.0 to 15.4 mg and the heating rate was fixed at the level of 10*C/min. The amount of the polymer contained was calculated according to the following equation:
w / w % ffi (A/B) × 100 where A is the heat of fusion (cal/g) of the recycled glass fibers and B is the heat of fusion (cai/g) of the virgin polymer under identical test conditions. B was determined to equal 8.9 cal/g for the ionomer and 11.36 cal/g for the low density polyethylene.
2.5. Tensile breaking tests Tensile tests were carded out using an Instron tensile machine in standard laboratory atmosphere. The specimens were tested according to the D638-76 ASTM method at a rate of extension equal to 50 × 10-3 m/min for the determination of tensile strength at yield and break and 1 × 10 -3 m/rain for the modulus of elasticity.
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3. Results and discussion 3.1. Some theory on composites
In discontinuous fiber-reinforced composites the performance and properties are generally a function of fiber type, fiber content, fiber aspect ratio, fiber orientation, fiber dispersion, fiber-matrix adhesion, processing methods and properties of the matrix [29-34]. In these factors, the fiber-matrix interface plays an essential role in determining the mechanical properties, but also it has proven to be the 'Achilles heel' of these materials. Good adhesion between matrix and fibers depends mainly upon the following three factors : (i) the area of contact (aspect ratio) of the fiber; (ii) the frictional properties arising from the geometry of the fiber, the nature of the surface and the degree of gripping of the fiber in the matrix; and (iii) the positive bonding, which depends upon the chemical nature of the matrix and fiber, and the surface treatment of the fiber to achieve coupling. According to Ehrburger and Donnet , the above classification is equivalent, by distinguishing: (i) the 'mechanical bonding model' which involves the wetting of the fibers and the mechanical anchoring of the matrix into the pores and the unevenness of the fibers; (ii) the 'physical coupling model' which involves the development of secondary or Van der Waals forces; and (iii) the 'chemical coupling model' in which the absorption of chemical groups and the formation of true covalent bonds between fiber and matrix are assumed. Glass fibers, as manufactured, have poor adhesion to polymer matrices . Considerable effort has been expended to change their surface properties with a view to improve the i nterfacial bond. For glass fibers the surface is treated mainly with coupling agents [ 36,37]. Typical treating materials are vinyl and methacrylate silanes and organic chromium compounds, such as the methacrylate chromyl chloride [ 38 ]. The chemical bonding mechanisms that involve silane coupling agents or other bifunctional molecules apply generally to thermosetting polymers because the organo-functional group is chemically locked into the cross-linked structure of the resin during the chemical curing reactions which change the resin from a liquid to a rigid solid. This type of chemical coupling cannot occur with glass fibers introduced into thermoplastic matrices because the molecules are already fully polymerised . On the other hand, the smooth surface of glass fibers implies that direct mechanical anchoring of the matrix to the pores and the unevenness of the fibers will be inadequate. An alternative approach will be the ease of coating the reinforcing fibers with an organic polymer, i.e., inserting an interlayer promoting the full and continuous contact between the resin and fiber surfaces and even more, a functional interlayer interacting chemically with both resin and fiber to enhance bonding between the fibers and the matrix. This ensures good stress transfer and reduces the stress concentration in the vicinity of the interface, where it is highest [34,39--42]. Such a type of interface modification includes several techniques, namely coating with a thermoplastic polymer by electropolymerization , coating by interfacial polymerization , coating by in situ polymerization of monomer reactants  and dip coating . As already mentioned, in this work advantage is taken of the recycling procedure itself to produce glass fibers containing different amounts of polymer matrix. The flow diagram, shown in Fig. 1, describes the conception of the experimental procedure applied so far.
C.D. Papaspyrides et al. / Reso,rces, Consert,ation and Recycling 14 (1995) 91-101 i
Cutting of the composite I into smaller pieces
I Dissolution of the
I Filtration under pressure Washing
Recycled glass fibers
IVirgin polymer INon-solvent ~ - - - Mixing Compression molding Precipitation of the polymer
I Recycled polymer I l"Recycled" compositel Fig. l. Flow diagram of the experimental procedure.
1 2 NUMBEROF WASHINGS
Fig. 2. Plots of polymer deposited on the glass fibers recycled vs. the number of washings. Open bars, ionomer; closed bars, LDPE.
C.D. Papaspyrides et al. /Resources, Consen,ation and Recycling 14 (1995) 91-101
300 / 2oo / 100 I/ OV
1 2 3 4 5 Fig. 3. Ionomer based composites: Modulus of elasticity vs. different fiber grades. I = unused glass fibers; 2=recycled glass fibers, no washings; 3=recycled glass fibers, one washing; 4=recycled glass fibers, two washings; 5 = recycled glass fibers, three washings.
Based on the aforementioned discussion it will be attempted now to approach the data obtained from tensile tests on composites filled with recycled glass fibers.
3.2. lonomer based composites In Fig. 2 the amount of polymer remaining on the recycled fibers vs. the number of washings after filtration is plotted. Within our experimental conditions, it is readily observed that washing the fibers with increasing volumes of hot solvent leads to a gradual reduction of the polymer traces, until zero for three washings. Fig. 3 presents the modulus of elasticity of the ionomer based composites containing unused glass fibers in comparison with the modulus of similar composites containing, however, recycled fibers after none, one, two and three washings. As the figure indicates, the so achieved incorporation of the ionomer phase prior to embedding the recycled glass fibers in the new ionomer matrix improves significantly the modulus, but the phenomenon
20/ t5- / ILl
t0- / ,..,.,
Fig. 4. Ionomer based composites: Tensile strength at yield vs. different fiber grades. 1 = unused glass fibers; 2 = recycled glass fibers, no washings; 3 = recycled glass fibers, one washing: 4 = recycled glass fibers, two washings; 5 = recycled glass fibers, three washings.
C.D. Papaspyrides et aL /Resources, Conservation and Recycling 14 (1995) 91-101
Fig. 5. lonomer based composites: Tensile strength at break vs. different fiber grades. I = unused glass fibers; 2 = recycled glass fibers, no washings; 3 = recycled glass fibers, one washing; 4 = recycled glass fibers, two washings; 5 = recycled glass fibers, dwee washings. is not valid for two or three washings. Such a behavior may be attributed to the fact that during the recycling process itself the fiber bundles are able to open thoroughly and thus be dispersed more uniformly into the matrix, affecting positively modulus o f elasticity and tensile strength . The better performance of the composites containing recycled fibers washed once in comparison with those containing unwashed fibers may be explained as follows: Microscopy observations revealed that for the former case the ionomer content remained on the fibers reduces, but this ionomer phase is much better dispersed through the whole mass of the fibers resulting in a better cooperation between them and the new ionomer matrix. On the contrary, the composites containing recycled fibers of two or three washings lose such a potential for better cooperation as the ionomer deposited on the fibers gradually becomes less. The conclusions derived from the modulus results meet very good verification when the tensile strength at yield and the tensile strength at break are studied (Figs. 4 and 5). 400- /
Fig. 6. LDPE based composites:Modulus of elasticity vs. different fiber grades. ! = unused glass fibers;2 = recycled glass fibers, no washings; 3 =recycled glass fibers, one washing; 4= recycled glass fibers, two washings; 5 = recycled glass fibers, three washings.
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Fig. 7. LDPE based composites: Tensile strength at yield vs. different fiber grades. 1 = unused glass fibers; 2=recycled glass fibers, no washings; 3 = recycled glass fibers, one washing; 4=recycled glass fibers, two washings; 5 = recycled glass fibers, three washings.
3.3. L D P E based composites
Recycled glass fibers containing different amounts of polyethylene (originated from the composites recycling procedure) were employed for the preparation of the polyethylene based composites. In contrast with the ionomer case, the unwashed fibers appear to contain much more polymer (Fig. 2), a fact probably associated with the higher viscosity encountered in the PE solutions. Nevertheless, washings are very effective in removing the polymer traces, as even one washing with hot toluene results in a sharp reduction of the quantity deposited. Fig. 6 shows the modulus data from the composites made, based on different 'grades' of glass fibers. Like the ionomer based composites one can readily observe a clear improvement of the modulus. Such a behavior may be correlated again with the better dispersion of the fibers into the matrix polymer melt due to the recycling procedure itself. Contrary with the ionomer ease, two washings ensure here the best performance. This should be associated 2O
1 2 3 4 5 Fig. 8. LDPE based composites: Tensile strength at break vs. different fiber grades. 1 =unused glass fibers; 2 = recycled glass fibers, no washings; 3 = recycled glass fibers, one washing; 4 = recycled glass fibers, two washings; 5 = recycled glass fibers, three washings.
C.D. Papaspyrides et aL / Resources, Conservation and Recyclbag 14 (1995) 91-101
with the fact that the polymer traces during washing are also redistributed on the fiber bundles, resulting in a more uniform deposition. Since the polyethylene solution is more viscous compared with the ionomer solution more solvent is required for this rearrangement. Figs. 7 and 8 present the corresponding data of tensile strength at yield and at break. The behavior is a little different as the composites containing recycled fibers after three washings, i.e., practically free from polymer, exhibit the best results. In agreement with previous studies [47,48] it seems that the favorable, in terms of matrix-fiber cooperation, polymer traces turn to be in this case stress concentration points with negative effect on tensile strength performance. It is worthwhile to mention here that a decrease in the length of the glass fibers was observed due to the recycling procedure. However, as revealed from the aforementioned discussion, the mechanical properties of the so modified composites are enhanced. Therefore, this decrease seems not to be in the critical range to affect negatively the tensile behavior encountered.
4. Conclusion The goal of this study is to investigate recycling aspects in glass fiber based thermoplastic composites. The technique followed involves the use of solvents in order to recycle both the polymer matrix and the reinforcing agent. The recycled fibers were used for the preparation of new composites, which generally exhibited better tensile performance than the ones containing unused fibers. The dispersion of the glass fibers into the matrix polymer melt and the distribution of the polymer deposited on the fibers seem to comprise the most important factors affecting the tensile properties of the derived composites. In other words, the recycling technique employed makes it feasible not only to recycle the polymer composites, but at the same time to improve the performance of the second generation composites based on the recycled fibers.
We wish to thank Olefini Co., Greece, for kindly supplying the ionomer Surlyn 8528, and Argo S.A., Greece for the low density polyethylene Alcudia 2202F.
[ 1 ] Ehrig, R.J, 1992. PlasticsRecycling:Products and Processes,Carl HanserVerlag, New York.  Matthews, V., 1990. Kunststoffe Europe, 1( 10): 139.
 Sassaroli,G., CarloNotoLa Diega,R., 1991.The Salvageof PolyethylenefromSolidUrbanWasteSystems in AutomaticSelectionPlants, Recycle'91, 3-5 April 1991,Davos,Switzerland,pp. 17.1-I7.12. [4l Saffert, R., 1988. Kunststoffe, 78( i 1): 1089.  Guillet,J.E,, 1979.J. Appl. PolymerSci., AppliedPolymerSymposium,35: 599.  Cinquin, J., Chabert,B., Chauchard,J., Morel, E. and Trofignon,J.P., 1990.Composites,21 (2): 141. [7l Webling, B., 1990. Kunststoffe,80(4): 463.
C.D. Papaspyrides et al. / Resources, Conservation and Recycling 14 (1995) 91-101
 Richter, H., 1985. Kunststoffe, 75(5): 266.  Bartkowski, K.-D. and Wesslein, M., 1990. Kunststoffe, 80(4): 485. [ 10] New, R., 1986. Plastics Waste Recycling in the United Kingdom, Department of Trade and Industry, Warren Spring Laboratory, pp. 1-32.  Yen, T.F., 1974. Recycling and Disposal of Solid Wastes, Industrial Agricultural Domestic, Ann Arbor Science Publishers Inc. [ 12] Leaversush, R.D., 1991. Modern Plastics International, pp. 26-29. [ 13] Menges, G., Eysmondt, B. v., Feldhaus, A. and Offergeld, H., 1988. Kunststoffe, 78(7): 573. [ 14] Warren, L.M. and Burns, R., 1988. Plastics Technology, pp. 41-63. [ 15] Papaspyrides, C.D., Gouli, S. and Poulakis, J.G., 1993. Recovery of poly(methyl methacrylate) by the dissolution/reprecipitation process: a model study. Polymer Recovery, in press. [ 16] Gouli, S., Poulakis, J.G. and Papaspyrides, C.D., 1993. Solvent recycling of poly(methyl methacrylate) decorative sheets. Polymer Recovery, in press.  Papaspyrides, C.D., Poulakis, J.G. and Varelidis, P.C., A model recycling process for low density polyethylene by means of solvent systems. Resources, Conservation and Recycling, 12: 177-184. [ 18] Kampouris, E.M., Papaspyrides, C.D. and Lekakou, C.N., 1988. Polymer Eng. Sci., 28(8): 534. [ 19] Papaspyrides, C.D. and Diakoulaki, D.C., 1986. J. Vinyl Technol., 8(2): 83.  Kampouris, E.M., Papaspyrides, C.D. and Lekakou, C.N., 1987. Conservation and Recycling, 10(4): 315.  Kampouris, E.M., Diakoulaki, D.C. and Papaspyrides, C.D., 1986. J. Vinyl Technol., 8(2): 79.  Drain, K.F., Murphy, W.R. and Otterburn, M.S., 1981. Conservation and Recycling, 4(4): 201.  Drain, K.F., Murphy, W.R. and Otterburn, M.S., 1983. Conservation and Recycling, 6(3): 123.  Murphy, W.R., Otterburn, M.S. and Ward, J.A., 1979. Polymer, 20: 333.  Berthold, H., 1991. Car Shredder, ABS Waste, Carpet PP/PA Scraps and PC/PMMA Materials Recovery by Use of Solvents, Recycle '91, 3-5 April 1991, Davos, Switzerland, pp. E5. I-E5.15. [ 26] Murphy, C.V., 1986. HP Ionomers: A Reinforced, Low Cost Composite with Outstanding Low Temperature Properties, Antec '86 Plastics Proc. of the 44th Annual Technical Conf., April 28-May 1 1986, Boston, pp. 73-78  Blumentritt, B.F., Vu, B.T. and Cooper, S.L., 1975. Polymer Eng. Sci., 6( 15): 428.  Avalos, F., Arroyo, M. and Vigo, J.P., 1990. J. Polymer Eng., 9: 3.  Eagles, D.B., Blumentritt, B.F. and Cooper, S.L., 1976. J. Appl. Poly. Sci., 20: 435.  Takayanagi, M., Kajiyama, T. and Katayose, T., 1982. J. Appl. Poly. Sci., 27: 3903.  Malzahn, J.C. and Friedrich, K., 1984. J. Mat. Sci. Lett., 3: 861.  Wake, W.C., 1978. Polymer, 19: 291.  Davis, J.H., 1970. Plastics Poly.: 137.  Ehrburger, P. and Donnet, J.B., 1980. Phil.Trans. R. Soc. Lond. A, 294: 495.  Hull, D., 1981. An Introduction to Composite Materials, Cambridge University Press, Cambridge. [ 36] Oleesky, S.S. and Mohr, J.G., 1964. Handbook of Reinforced Plastics, Reinhold Publishing Co., New York. [37 ] Weatherhead, R.G., 1980. FRP Technology, Applied Science Publishers Ltd., London.  Carosseli, R.F., 1967. Glass Textile Fibres, In: Mark, H.F., Arias, S.M. and Cernia, E. (Eds.), Man-Made Fibers: Science and Technology, Vol. 3, Interscience Publishers, pp. 430--436.  Papaspyrides, C.D. and P0ulakis, J., 1992. Polymer Int., 27: 139.  Papaspyrides, C.D. and Poulakis, J., 1992. Polymers for Advanced Technologies, 3: 367. [41 ] Skourlis, T., Duvis, T. and Papaspyrides, C.D., 1993. Composites Sci. Technol., 48:119.  Skourlis, T., Duvis, T. and Papaspyrides, C.D., 1993. Composites Sci. Technol., 48: 127.  Submmanian, R.V. and Jakubowski, J.J., 1978. Polymer Eng. Sci., 18: 590.  Cranmer, J.H., Tesoro, G.C. and Uhlmann, D.R., 1982. Ind. Eng. Chem. Prod. Res. Dev., 21: 185.  Kampouris, E.M. and Papaspyrides, C.D., 1987. J. Appl. Poly. Sci., 33: 205.  Papaspyrides, C.D. and Duvis, T., 1990. J. Appl. Poly. Sei., 41: 361.  Papanicolaou, C.G. and Papespyrides, C.D., 1987. Mat. Chem. Phys., 17: 453.  Papaspyrides, C.D. and Papanicolaou, G.C., 1988. J. Appl. Poly. Sci., 36: 309.