Chapter 30
Recycling of Mixed Plastics Using Cellulosic Reinforcement 1
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Plastics, Rubber, and Paper Recycling Downloaded from pubs.acs.org by YORK UNIV on 12/23/18. For personal use only.
P. Gatenholm , P. Hedenberg , and C. Klason
1Department of Polymer Technology and2Departmentof Polymeric Materials, Chalmers University of Technology, S-412 96 Göteborg, Sweden
The aim of this research was to create novel structural composites based on waste paper and plastics that show good mechanical properties and long-term exposure properties. In this report, we summarize the results of a model study on the effect of combining chemithermomechanical pulp (CTMP) fibers and MAH-SEBS compatibilizer (maleic acid anhydride-grafted styrene-ethylene/butylene-styrene block copolymer) with a simulated waste plastic fraction composed of LDPE and HIPS (70:30). Experimental work demonstrated that an essential improvement of the mechanical properties of waste plastics can be obtained by the presence of cellulosefibers.Achieving proper strength in a material of this kind, composed of several phases, requires the addition of a compatibilizer.
Growing environmental awareness has brought about great activity in thefieldof plastic recycling. Plastic waste is now being collected in many countries and, after being sorted, is added to feed stock of virgin materials. Other concepts for plastic recycling include plastic product re-use and the conversion of material into monomers (chemical recycling). However, a substantial part of the municipal solid waste stream is composed of paper-contaminated plastics that are difficult to separate (1-2). Such a waste stream is a potentially inexpensive source of materials that can be converted into valuable composites. Cellulose fibers offer several advantages when combined with the plastic. Among them are low density, high modulus and high strength (3-5). The most important requirement for a discontinuous fiber to be able to act as reinforcement in a composite material is that the fiber have a sufficient length to diameter ratio (fiber aspect ratio) (4-7). Cellulosefibersin a typical paper product used together with synthetic polymer in a typical packaging material originate often from wood and exhibit a typical length of 0.2-2 mm (8). During processing, fibers are often 0097-6156/95/0609-0367$12.00/0 © 1995 American Chemical Society
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broken into smallerfragments,which commonly makes them too short to be useful as reinforcement (9). As the critical fiber length is dependent on the efficiency of the stress transfer from the matrix to the fibers, one way to overcome the problem of fibers that are too short is to improve interfacial adhesion between the cellulose fibers and polymeric matrix. There are several possible strategies for improving adhesion between cellulose and thermoplastic matrices. Surface treatments of cellulose with coupling agents, plasma, corona and ozone are among the most extensively used methods for improving the properties of cellulose-polymer composites (10-13). Another technical problem associated with plastic waste, such as the household plasticfractionof waste, is a heterogeneous composition. Instead of one well defined polymer, a blend of incompatible polymers is present. Owing to the lack of compatibility of different polymers, the properties of blends are inferior when no addition of a compatibilizing agent has been made (14). In this paper, we summarize the results of a model study on the effect of combining chemithermomechanical pulp (CTMP) fibers with a simulated waste plastic fraction composed of LDPE and HIPS (70:30). Maleic acid anydride-grafted styrene-ethylene/butylene-styrene block copolymer (MAH-SEBS) is used as a compatibilizer. Experimental Methodology
Materials. Low density polyethylene (LDPE), NCPE 1800, was supplied by Borealis AB, Stenungsund, Sweden. High impact polystyrene (HIPS), Polystyrol 456M, was supplied by BASF. Maleic acid anhydride-grafted styrene-ethylene/butylene-styrene block co-polymer (MAH-SEBS), Kraton FG 1901X, was supplied by Shell Chemical Co. Chemithermomechanical pulp (CTMP) composed of 95% spruce and 5% pine was from S.C.A., Sundsvall, Sweden. Regenerated cellulose fibers (rayon) were supplied by Svenska Rayon AB. Preparation of composites and mechanical testing.
LDPE and HIPS were
blended in a 70:30 weight proportion together with the CTMP fibers in a BussKneader PR 46. After homogenization, the samples were injection-molded in an Arburg 221E/170R. The mechanical properties of the test bars (DIN 53455) were evaluated by tensile testing (Instron 1193) with the deformation rate 4.5xlO"V . The fractured surfaces of the test bars were examined in a scanning electron microscope (Jeol JSM-5300). Impact strength tests were performed using a Frank KMO 79, pendulum type tester. The dynamic mechanical properties were evaluated in the bending mode using a Rheometrics RSA 11 dynamic mechanical analyzer. The frequency of the forced oscillation was kept at 1 Hz. l
Single fiber fragmentation test. Specimens for the singlefiberfragmentation test were prepared by placing rayonfibersbetween thin LDPE films, which were melted together in a press at 140°C for 2 minutes at a maximum pressure of 8 MPa. An optical microscope equipped with crossed polarizers was used to study the fragmentation process in the specimens when elongated in a Minimat Miniature Materials tester (Polymer Labs Thermal Sciences). Modification of regenerated fibers
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took place in 100°C toluene, where MAH-SEBS was dissolved. The fibers were reacted for 10 minutes with a concentration of MAH-SEBS in solution calculated as 5% by weight, relative to the rayon fibers. All fibers were sohxlet-extracted in toluene for 12h prior to specimen fabrication. Results and Discussion Mechanical properties of composites. The plastic fraction of municipal solid waste (PFMW) was simulated in this study by mixing 70% LDPE with 30% HIPS. Figure 1 shows the effect on the tensile modulus of the presence of various amounts of cellulose fibers (CTMP) and compatibilizer (MAH-SEBS) in the simulated plastic waste fraction. The stiffness of the plastic composite is almost linearly increased with an increased amount of fiber. Samples with 30% fibers and 5% MAH-SEBS had an elasticity modulus 180% higher than that of samples without cellulose and MAH-SEBS. The presence of 30% cellulose in an LDPE/HIPS 70:30-blend offers a load bearing capability to rather low-value materials, making them comparable with a lower range of engineering plastics. The stiffness of samples with no compatibilizer added was even higher (13). This can be explained by the mechanical characteristics of the compatibilizer (MAH-SEBS) itself, which decrease the tensile modulus. The elastic modulus of the MAH-SEBS modified material was determined to be 0.15 GPa, and the elongation at break was over 300%. However, the higher stiffness of the unmodified samples will be at the expense of a loss of impact strength. The impact strength for unnotched samples containing 30% cellulose fibers was measured as 14,4 kJ/m and 19,9 kJ/m for unmodified and MAH-SEBSmodified composites, respectively. Figure 2 shows the effect of cellulose with and without the addition of compatibilizer on the tensile strength of composites. Without the addition of compatibilizer, cellulosefibersact only as a filler, and do not reinforce the plastic blend. The tensile strength increased by 77% when both 30% fibers and compatibilizer were present. The improvement in tensile strength is the result of the reinforcing effect of the cellulosefibersthat is achieved when the cellulose fibers are surface-modified with MAH-SEBS. The surface modification will most probably increase compatibility and adhesion between the plastic phase and cellulose fibers, which is necessary to achieve such improvement of tensile strength. Dispersion of fibers is another important point to consider in respect to the mechanical performance of the cellulosefiber/wasteplastic composite materials. In samples without MAH-SEBS, the highfibercontent yields agglomeration and causes fractural impressions, whereas improved compatibility between the fibers and the matrix in MAH-SEBS-modified samples will be expected to improve the dispersion of fibers. Agglomeration may explain the decreased tensile strength registered for unmodified composites, containing 30% CTMP fibers. The earlier reported effect of SEBS-based compatibilizers of improving dispersion in pure polyethylene/polystyrene blends is also likely to take place in the MAH-SEBS modified composites. This consequently would have a number of positive effect on the mechanical properties (15,16). Hence, as shown in Figure 2, the tensile strength for the LDPE/HIPS material without fibers is increased by the addition of MAH-SEBS. 2
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Figure 1. The effect of the incorporation of cellulose and compatibilizer on the E-modulus of simulated plastic waste.
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Figure 2. The effect of the incorporation of cellulose and compatibilizer on the tensile strength of simulated plastic waste.
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Role of adhesion in short-fiber composites. Short fibers have the ability to reinforce a thermoplastic matrix material by carrying load that is applied to the composite. However, load can only be carried when the applied stress is transferred from the matrix to the fiber by shear along thefiber/matrixinterface. Consider an isolated short fiber as in Figure 3a. If the matrix is extended by tension along the fiber axis, then, according to the shear law, the tensile stress builds upfromzero at the fiber ends to a maximum in the center portion of the fiber. To understand the relationship between the fiber length and the load bearing capability, consider now a fiber longer than some specific length, l . The length of this fiber is sufficient to allow the transferred stress that builds up along the fiber axis to reach the ultimate stress in the fiber before break. A fiber that is shorter than l will be too short to have the ability to build up enough tensile stress to reach thefracturepoint. In other words, the fiber is does not fully utilize its strength. The limiting length for which the stress that builds up can reach the maximum strength of the fiber is called the critical fiber length and is denoted l . Thus, in a composite with a fiber length distribution surrounding the criticalfiberlength, improved fiber/matrix adhesion will lead to a more efficient stress transfer, which consequently will be seen as a decreased critical fiber length as compared with the earlier case. This is illustrated in Figure 3b. The criticalfiberlength for native cellulosicfibersin a common thermoplastic matrix is difficult to establish owing to the natural inhomogeneity of the fibers. Therefore, in order to perform such studies with cellulosics, it is common to use regenerated cellulose fibers (rayon), which offer a route to achieve reproducible measurements. The critical fiber lengths that have been determined for rayon fibers and cottonfibersin thermoplastics matrices (LDPE, PP) have typically been around 0.5-1 mm (11,17). As mentioned earlier, the length of woodfibersin thermoplastic composites have a broad range, typically between 0.2 and 2 mm (18). This means that some proportion (large or small) of the fibers in the LDPE/HIPS/CTMP composites will have lengths that are below the critical fiber length, as a woodfiber also normally has a higher tensile strength as compared with a rayon fiber. Increasing thefiber/matrixadhesion in such a system would thus inevitably increase the composite strength, as a larger fraction of fibers can utilize their maximum strength. To establish that the surface modification has indeed improved the cellulose fiber/waste plastics adhesion and that the critical fiber length is lower in the MAHSEBS-modified composites, we utilized the single fiber fragmentation (SFF) test (11). The test involves a dogbone-shaped specimen of the matrix material in which a fiber is embedded, aligned in the direction of elongation. When the specimen is elongated, stress that builds up in the matrix is transferred to the fiber via shear at thefiber/matrixinterface, as described earlier in Figure 3. When the stress in the fiber reaches the ultimate strength of the fiber, the fiber breaks. Further elongation of the specimen will give rise to new stress build-up in the fiber and cause additionalfractures.Finally, the fiber consists offragmentsall shorter or equal to the critical fiber length, and no further breakage occurs. At this stage, the fragment length distribution is used to determine the critical fiber length. In our study, we used regenerated cellulose fibers (rayon) as a model system instead of CTMP in order to achieve reproducible measurements. The important c
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Figure 3. Stress distribution along the fiber axis.
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issue in this type of comparison of the effect of the surface modification is that the surface chemistry is similar, which indeed is the case with these two types of cellulosics. Also, the model system involved pure LDPE as matrix material, as it is the component in excess in the composite and therefore will be the major component in contact with the fibers. This is seen on the SEM micrograph shown below. Unmodified and MAH-SEBS-modified fibers embedded in an LDPE matrix were subjected to the SFF test and the result was followed in an optical microscope equipped with crossed polarizers, which made it possible to study the stress patterns that developed in the matrix. Figures 4a and b are representative for several tests made. The figures show the fiber fragments in the matrix and the light intensity curves obtained from the birefringence patterns that developfromstress concentrations in the LDPE. Thefracturingprocess is completed for both samples, and the matrix is at equal elongation. In Figure 4a, the unmodified rayon fiber, the stress in the matrix is concentrated to the center portion of a relatively longfiberfragment. There is also stress surrounding the fractures, indicated by arrows, where no fiber prevents the flow of the matrix material. Between these two light intensity maxima, there is a rather extensive distance of low light intensity, indicating a low stress level. The low light intensity may be interpreted in terms of insufficient stress transfer to the fiber owing to a lack of adhesion. It is even possible that there is delamination between the fiber and the matrix at this point. When comparing the unmodified rayonfiberwith the MAH-SEBS-modified fiber shown in Figure 4b, the effect of the compatibilizer becomes obvious. In the modified, sample there is an intense fluctuation between nodes and maxima in the light intensity curve. Again, stress concentrations are located at the center portion of the fiber fragments and at the fractures, but the part of the fiber that is surrounded by low light intensity is dramatically shorter than the unmodified sample in Figure 4a. Thus the stress transfer between cellulosefibersand LDPE seems to be more efficient, and this will naturally be a consequence of an improved adhesion. Determinations can also be made of the length of thefragmentsin order to achieve a measurement of the interfacial adhesion. Obviously, the length of the modifiedfragmentsin Figure 4b are only about 30% of the unmodified fragments represented in Figure 4a. However, it is not of major importance to present numerical values of the criticalfiberlength, as the system chosen is merely a model of the actual, more complicated system containing wood fibers in an LDPE/HIPS mixture. Still, it is obvious that the adhesion between cellulosicfibersand LDPE can be improved considerably with the use of MAH-SEBS. Fractured surfaces. The fracture surfaces of composites were investigated using Scanning Electron Microscopy. Figure 5 shows a typical fracture surface of a fiber containing composite sample without the addition of compatibilizer. This figure supports the earlier discussed results of the SFF test. It shows the ability of MAHSEBS to yield a more intimate contact between the cellulose fiber and the LDPE/HIPS matrix. The plastic matrix covers the cellulosic fiber surface, when ,normally, the wetting of cellulosic fibers with thermoplastic melts is inferior, resulting in a poor coating of the material on thefibers(19).
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Figure 4a. Unmodified rayon fiber in LDPE during SFF testing. Light intensity curves from birefringence patterns are overlaid on the picture.
Figure 4b. MAH-SEBS modified rayonfiberin LDPE during SFF testing. Light intensity curves from birefringence patterns are overlaid on the picture.
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Figure 5. SEM micrograph of a fractured LDPEIHIPSICTMP composite with MAH-SEBS.
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The effect of MAH-SEBS on composite properties can probably be explained on an elementary chemical level on the basis of SFF and SEM results. The following mechanism has been proposed to be responsible for the enhancement of adhesion: Maleic acid anhydride grafted on SEBS chains is highly reactive towards hydroxyl groups at the cellulose surface and, when compounding at elevated temperatures, both covalent and hydrogen bonds develop between the runctionalized compatibilizer and the cellulose surface (14). Furthermore, MAH-SEBS covers the cellulose surface and, thus, wetting by the polyolefinic LDPE and HIPS phases is simplified, and better contact between cellulose and plastics is achieved. Dy nam ic Mechanical Properties. One of the limitations of thermoplastic materials is their heat sensitivity. Samples of composites with various fiber contents were evaluated with regard to their thermal response using DMTA. Investigation of the dynamic elasticity modulus, shown in Figure 6, confirms the overall stiffening effect of cellulose fibers. In fact, measurements at 60°C show that the stiffness of a composite containing 30% fibers is comparable with that of a composite without fibers at 10°C. This also proves that cellulose preserves the stiffening effect under the influence of heat. Conclusions Experimental work demonstrated that an essential improvement in the mechanical properties of waste plastics can be obtained by the presence of cellulose fibers.
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Figure 6. Dynamic Elasticity Modulus for samples with various fiber contents. Achieving proper strength in a material such as this, composed of several phases, requires the addition of a compatibilizer. Not only does a reactive compatibilizer improve compatibility between the phases, but also improves adhesion between cellulose fibers and the waste matrix. Improved adhesion is maintained by the formation of covalent bonds between the maleic acid anhydride group of the compatibilizer and the hydroxyl groups at the cellulose surface. Hence, cellulosic fibers can act as reinforcement and provide both strength and stiffness to mixed waste plastic matrices. Furthermore, the cellulosicfibersalso preserve the stiffness of the material over a wider temperature range than is seen for the pure waste plastic blend. Acknowledgm en ts
The authors are grateful to Dr. A. Mathiasson for his assistance in the experimental work. Financial support from the Swedish Waste Research Council is gratefully acknowledged. Literature Cited
1. Klason C.; Kubat J.; Gatenholm P. In Viscosity of Biomaterials; Glasser W.; Hatakeyama, H., Ed.; ACS Symposium Series; American Chemical Society: Washington D.C., 1992. 2. Mathiasson A., PhD. Thesis; Chalmers University of Technology: Göteborg, Sweden, 1992. 3. Zadorecki P.; Mitchell A.J. Polym. Compos. 1989, 10, 2. 4. Maldas D.; Kokta B.; Daneault C. Intern. J. Polymeric Mater. 1989, 12, 297. 5. Dalvag H.; KlasonC.;Strömvall H.-E. Intern. J. Polymeric Mater. 1985, 11, 9.
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6. Cox H.L. Br. J. Appl. Phys. 1952, 3, 72. 7. Felix J.M.; Gatenholm P. J. Appl.Pol.Sci. 1993, 50, 699. 8. Mitchell A.J.; Vaughan J.E.; Willis D. J. Appl. Polym.Sci.1978, 22, 2047. 9. Gatenholm P.; Mathiasson A. J. Appl. Pol. Sci. 1994, 51, 1231. 10. Bataille P.; Ricard L.; Sapieha S. Polym. Comp. 1989, 10, 103, (1989) 11. Felix J.; Carlsson G.M.; Gatenholm P. J. Adh. Sci. Technol. 1994, 8, 1. 12. Felix J.M.; Gatenholm P.; Schreiber H.P. J. Appl. Pol. Sci., 1994, 51, 285. 13. Gatenholm P.; Felix J.M. In New Advances in Polyolefins; Chung T.C., Ed.; ACS Symposium Series; Aerican Chemical Society: Washington D.C., 1993. 14. Hedenberg P.; Gatenholm P. J. Appl. Pol. Sci., in press. 15. Welander M.; Rigdahl M . Polymer, 1989, 30, 207. 16. Brahami B., Ait-Kadi A.; Ajji A.; Fayt R. J. Polym. Sci., Part B: Polym. Phys., 1991, 29, 945. 17. Felix J.M.; Gatenholm P. J. Mater. Sci., 1994, 29, 3043. 18. Gatenholm P., Bertilsson H.; Mathiasson A. J. Appl. Polym. Sci., 1993, 49, 197. 19. Felix J.M.; Gatenholm P. J. Appl. Polym. Sci., 1991, 42, 609. RECEIVED June 20, 1995
