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Recycling: commingled plastics/newspaper composites.

Composites with promising properties were prepared by a high-intensity mixing process from commingled polystyrene, polypropylene, and polyethylene with defibrillated newspaper.

Two strategies for recycling plastic wastes have been developed and begun to be implemented. One is the reprocessing or remanufacturing of industrial scrap and post-consumer plastics after separation, cleaning, and pelletization. The other is the recovery of post-consumer plastics by making hybrid products with minimal sorting and cleaning. A large number of studies on blending post-consumer plastics have been conducted. Unfortunately, the products were shown to have limited utility because the incompatibility between the constituents gave rise to poor adhesion at the interfaces and poor mechanical properties.

Wood products such as paper, paperboard, and building products constitute the largest portion of solid waste in landfills. Thus, the recovery of waste wood products is also very important, and the use of waste newspaper to reinforce mixed plastics would be particularly advantageous. The newspaper fiber itself is usually very brittle, and its strength and stiffness cannot be fully realized. The presence of newspaper in the plastics matrix protects the fiber and transfers the load to it. This gives a composite that combines the good properties of the fiber and the matrix, improving the strength, stiffness, and creep resistance of the mixed plastics alone. Recently, polystyrene/newspaper and polypropylene/newspaper composites with promising properties were prepared in our laboratory. By intensive mixing, a well-dispersed mixture was produced that resulted in a stronger interface even though the compatibility TABULAR DATA OMITTED was poor.

For this article, composites with different mixing ratios of polystyrene, polypropylene, polyethylene, and newspaper were prepared, and the interactions between the various plastics and newspaper were evaluated. The specific energy requirement for processing and the mechanical and thermal dynamic properties of the composites were determined, and the fracture surface morphologies were examined by scanning electron microscopy.

Experimental

Newspaper was defibrillated in a mechanical blender by a process described by D.N.-S. Hon and S.T. Sean in J. Thermoplastic Composites, 4,300 (1991). Commercial modified high-impact polystyrene (MPS), polypropylene (PP), and high-density polyethylene (HDPE) in a designated weight ratio were introduced into the intensive-mixture-measuring heads of a Brabender PL2000 Plasti-Corder and mixed at a 175|degrees~C temperature setting. After 3 min at 60 rpm, the mixed plastics were basically melted. A designated amount of newspaper fiber was then added, and mixing continued for another 37 min. The specific energy requirement for processing was monitored.

The uniformly dispersed mixture was then transferred to a compression mold. Dumbbell-type test specimens (ASTM D 638, Type V) were prepared using a hot press at 235|degrees~C. The mixture was first heated without pressure for 10 min; 500 psi was then applied for 5 min, followed by 1000 psi for 5 min. The mold apparatus was immediately cooled to below 100|degrees~C, and the molded samples were removed.

The Young's modulus and tensile strength at break were measured on an Instron 4204 Testing Machine according to ASTM D 638. Reported values are averages from six to eight specimens. The viscoelastic properties were determined from -100|degrees~C to 300|degrees~C on a Polymer Laboratories PL-DMTA dynamic mechanical thermal analyzer. Dual clamping was used with strain set at X4, a 30-Hz single-scan frequency, and a 4|degrees~C/min heating rate. Fracture surfaces of the tensile samples were coated with gold on a Hummer X sputter coater and examined on a JEOL SEM.
Table 2. Summary of Stepwise Procedure.
Specific energy Tensile strength Young's modulus
1. Fiber/.885 HDPE/.685 Fiber/.856
2. MPS/.070 MPS/.162 MPS/.080
3. PP/.018 PP/.063 PP/.034
4. HDPE/.011 Fiber/.024 HDPE/.021
5. MPS*PP*fiber/.004 MPS*PP*fiber/.023 MPS*PP*fiber/.002
6. MPS*PP/.001 MPS*HDPE/.007 MPS/HDPE/.000
7. MPS*HDPE/.001 MPS*PP/.005 --
8. HDPE*fiber/.001 PP*fiber/.004 --
9. -- PP*HDPE/.000 --


Experimental Design

To maximize the mechanical properties of the composites, specific energy, tensile strength, and Young's modulus were analyzed as a function of the proportional composition of the four components. The proportions of the three plastics ranged from 0 to 1 and the newspaper fiber from 0.1 to 0.6.

The effects of each component and the interactions between them were statistically analyzed using the General Linear Models procedure of base SAS. A special cubic polynomial equation with four variables was fitted by regression analysis. The model coefficients and their significant levels were determined. The equation is:

Y = a|X.sub.1~ + b|X.sub.2~ + c|X.sub.3~ + d|X.sub.4~ + e|X.sub.1~|X.sub.2~ + f|X.sub.1~|X.sub.3~ + g|X.sub.1~|X.sub.4~ + h|X.sub.2~|X.sub.3~ + i|X.sub.2~|X.sub.4~ + j|X.sub.3~|X.sub.4~ + k|X.sub.1~|X.sub.2~|X.sub.3~ + l|X.sub.1~|X.sub.2~|X.sub.4~ + m|X.sub.1~|X.sub.3~|X.sub.4~ + n|X.sub.2~|X.sub.3~|X.sub.4~ + o|X.sub.1~|X.sub.2~|X.sub.3~|X.sub.4~

where Y denotes specific energy, tensile strength, or Young's modulus, and |X.sub.1~, |X.sub.2~, |X.sub.3~, and |X.sub.4~ denote, respectively, weight fractions of MPS, PP, HDPE, and newspaper fiber.

Energy Requirements

The specific energy requirement for processing was strongly dependent upon the polymeric structures and physical properties of the materials, as well as their amounts and their interactions. The more rigid the material, the more energy was required. The specific energies required for processing MPS, PP, and HDPE on the Plasti-Corder were 1.5, 1.03, and 0.47 kNm/g, respectively. Because fiber cannot be processed alone, its specific energy was not measured. The specific energy requirements for mixing the four components at the mixing ratios studied ranged from 0.65 to 2.05 kNm/g.

The statistical analysis for the specific energy indicates that at the 99% confidence level the amounts of MPS, PP, HDPE, and the MPS*PP interaction were significant. The amount of fiber and the interaction PE*fiber were also significant at the 95% level. Other significant variables at the 95% level in the model were further analyzed with a stepwise procedure. The results (Table 2) show that the most significant effect on specific energy required is clearly the amount of fiber, followed by the amounts of MPS, PP, and HDPE. Because the fiber is highly crystalline, it is more rigid than the thermoplastics, and it required more energy to be processed. Interactions between the components played only a minor role. Figure 1 shows pictorial views at 0.48 fiber portion that were generated to better understand the relationship of the variables. Coefficients derived from SAS's PROC GLM analysis for the model equation for specific energy are given in Table 1.

Mechanical Properties

Mechanical properties of the composites are major criteria for determining their applications. The tensile strengths for MPS, PP, and HDPE, are 27.17, 31.47, and 25.15 MPa, respectively. The tensile strengths of the commingled plastics/newspaper fiber composites ranged from 5 to 26 MPa. These results imply that the more species of plastic present in the composite, the lower the tensile strength, and also suggest a lack of compatibility between MPS, PP, HDPE, and fiber. Modification of the interface to enhance compatibility should be considered.

The tensile strength was significantly influenced by the amounts of MPS, PP, HDPE, and fiber, and the MPS*PP interaction at the 99% confidence level (Table 1). The MPS*HDPE and MPS*PP*fiber interactions significantly jeopardized tensile strength at the 95% confidence level. The stepwise analysis (Table 2) indicated that the amount of HDPE was the dominant influence on tensile strength, followed by the amounts of MPS, PP, and fiber.

The Young's moduli of MPS, PP, and HDPE are 1700, 1670, and 1260 MPa, respectively; the moduli of the commingled composites ranged from 1380 to 2850 MPa. The statistical data (Table 1) show that the amount of each plastic played an important role at the 99% confidence level, but interactions between components are unimportant. At the 86% confidence level, the amount of fiber affected the modulus. In the right combination, the modulus of the commingled composite with fiber was higher than that of any single plastic component. Figures 2 and 3 show pictorial views at 0.48 fiber proportion. Coefficients derived for SAS's PROC GLM analyses for the mathematical models for tensile strength and modulus are given in Table 1.

Morphologies

Scanning electron microscopy (SEM) of the fracture surfaces of the tensile specimens was used to study the interactions between components. The fracture surfaces of the MPS/PP composites show an irregular rod-shaped MPS uniformly embedded in the PP. The fracture surfaces of MPS were quite smooth, and those of PP were like torn ribbons. It may be possible that voids were formed between MPS and PP during molding. Some cavities with smooth surfaces indicated that MPS was pulled out easily from the PP, suggesting that no strong adhesion existed between MPS and PP.

The interaction between MPS and HDPE was quite similar to that of MPS and PP. Fracture surfaces of composites showed a webbed network for HDPE, while the MPS had a bulky shape. As with the MPS/PP composites, the MPS/HDPE micrographs showed a large number of voids in the matrix, which probably contributed to the low tensile strength.

A unique fracture surface was observed for the PP/HDPE composite. As a result of good dispersing and packing between PP and HDPE, few voids were seen. Consequently, this composite had the greatest tensile strength of any combination.
Table 3. Experimental Glass Transition Temperatures,
|degrees~C.
Material |T.sub.g~ |T.sub.g1~ |T.sub.g2~ |T.sub.g3~
MPS 125
PP 185
HDPE 145
MPS/PP 124.5 183
MPS/HDPE 124.5 149
PP/HDPE 180 138
MPS/PP/
fiber 125 175
MPS/HDPE/
fiber 124 140
PP/HDPE/
fiber 140 178
MPS/HDPE/
PP/fiber 122 143 180


Micrographs of the four-component system showed newspaper fiber coated with plastic protruding from the fracture surface of the matrix. The plastics appeared to be uniformly mixed. Nonetheless, voids were observed, indicative of weak interaction between plastics and fiber, which is quite probably due to the hydrophilic nature of newspaper fiber and the hydrophobic nature of plastics.

Dynamic Mechanical Properties

The dynamic mechanical analysis substantiated that the incompatibility of the four components was a factor that prevented the composites from attaining higher moduli. The storage modulus (E') and damping factor spectra are shown in Figs. 7 and 8, respectively, for MPS, PP, MPS/PP, MPS/PP/fiber, and MPS/PP/HDPE/fiber. The E' of the components, either singly or in mixed composition, was reduced sharply at the glass-transition temperatures. MPS and PP exhibited |T.sub.g~s at 128|degrees~C and 185|degrees~C, respectively. The temperature zone for loss of E' for PP/MPS was between those of the MPS and PP. The addition of fiber, however, raised the inflection temperature because of the higher modulus of the fiber. Although the |T.sub.g~ for MPS/PP was higher than that of MPS and lower than that of PP, this did not indicate interaction between MPS and PP. The individual components still exhibited their own damping peaks even after mixing. Figure 8 also shows independent damping peaks for MPS and PP in the composites.

The statistical data do show some interaction between MPS and PP. It may be that the structural similarity between polystyrene and polypropylene allowed them to mix and disperse well. Unfortunately, this does not mean that they are compatible. Similar phenomena were observed for the other components. The DMTA results are summarized in Table 3.

Conclusions

Composites of modified polystyrene, polypropylene, high-density polyethylene, and newspaper were made by an intensive mixing process at 175|degrees~C for 40 min. Experimental data from compression-molded samples were used to fit the specific energy requirement for processing, the tensile strength, and Young's modulus to three special cubic polynomial equations with four variables (components). Statistical analysis revealed that the effects of MPS, PP, and HDPE levels and the MPS*PP interaction on both the specific energy and tensile strength were highly significant at the 99% confidence level. At the 95% confidence level, the amount of newspaper fiber and the HDPE*fiber interaction were significant for specific energy, and the MPS*PE and MPS*PP*fiber interactions significantly jeopardized tensile strength. Increasing the number of plastic components in the composite decreased tensile strength from about 22 MPa to 13 MPa. The amount of each component highly influenced Young's modulus at the 99% confidence level, but interactions were insignificant. SEM micrographs showed that the plastics were uniformly dispersed but that there was poor fiber/matrix adhesion. The dynamic mechanical spectra further proved that chemical interaction did not take place between the individual components.
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Author:Hon, David N.-S.; Chao, Wayne Y.; Buhion, Caroline J.
Publication:Plastics Engineering
Date:Oct 1, 1992
Words:2170
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