Newspaper reinforced plastic composite laminates: mechanical and water uptake characteristics.

INTRODUCTION

The opportunity of adding value to low-cost forest or agro based cellulosic material by formulating them in valuable composite materials has of late been exploited (1). Large sources of the fiber or similar feed-stock can come from recycling (2) fiber based products, such as paper, waste wood, and point source agricultural residues. In light of dwindling forests and the search for a sustainable future fiber supply, future strategies for the recycling of products made from natural fibers have been contemplated. Important industries such as paper and composites need to adopt a combined approach for recycling waste papers directly into new paper, while some waste papers, including all inks, adhesives, and inorganics, can be used directly to make composites, as the integrated paper recycling program will allow recycled fibers to flow into highest possible market with a minimum of reprocessing. Waste newspapers could be an ideal source and should be converted into high value composite materials using a dry process. The currently adopted practice of recycling paper products into paper requires wet processing and removal of inks, inorganics, and adhesives, and as such may become rather expensive. Newspaper material consists of lignocellulosic material and other inorganic fillers. Waste newspaper invariably contains printing inks and other process aid materials. Newspaper-reinforced plastic composites similar to wood-based composites (3) may find applications as structural materials for the housing industry, such as load bearing roof systems, sub-flooring, and framing components, and nonstructural products such as doors, windows, furniture, automotive, and interior parts.

The Macromolecular Research Centre, R. D. University, Jabalpur, India, has initiated a program to develop a composite material using waste newspaper and a suitable resin matrix. The problem of compatibility, i.e., newspapers containing cellulosic materials in a resin matrix, is primarily addressed in scientific study. Matrix compositions of epoxy with phenolic resin were reported (4) wherein the novolac type of phenolic resin was used to cure the epoxy resin matrix. Advancements in manufacturing techniques have led to the development of prepreg techniques; the basic resin system should preferably be in a homogeneous liquid phase. From these considerations, combinations of phenolic resin with medium viscosity epoxy resin systems may become very attractive. Resol phenolic resin has not been used as a matrix resin because of its short shelf life, high reactive nature, and the evolution of volatile matter during its curing reaction. Of late, suitable modifications in the production of resol type phenolic resins have been made that have resulted in the development of resins with long shelf lives and moderate viscosity, and which need no solvent.

A matrix resin consisting of a resol type phenolic and diglycidyl ether of bisphenol-A type epoxy resin has been chosen utilizing the basic chemical reactions studied previously (4). A fixed ratio of phenolics and epoxy was used, which emerged from studies conducted in our laboratories for the development of carbon fiber composites. In this paper, compositions of the composite are reported wherein the weight fraction of the paper in the waste newspaper-reinforced phenolic-epoxy matrix composite is varied. The mechanical properties and water uptake characteristics of these composites are evaluated, and possible applications are suggested, keeping in view the properties achieved in this study.

EXPERIMENTAL

Material

Phenolic resin: The phenol formaldehyde resin used in this study was PR-100, commercially available from ABR Organics, Hyderabad, India, which is solventless resol type phenolic resin. The total solid content in resin is 65-75% (w/w polymer) and 25-35% (w/w oligomer). The physicochemical characteristics of PR-100 resin are: dark brown liquid, viscosity (30 [degrees] C) = 0.6 to 0.9 Pa.S, pH = 7.5 to 7.8 and gel time at 150 [degrees] C = 6 min.

Epoxy resin: Diglycidyl ether or Bisphenol - A type epoxy resin (LAPOX B-11) was obtained from Cibatul Ltd., Mumbai, India, which is basically unmodified resin. The LAPOX B-11 has epoxy value 5.2-5.5 Equiv./kg. and its viscosity at 25 [degrees] C was 0.9 to 1.2 Pa.S.

Newspaper. The general characteristics of waste newspapers were (i) basic weight = 40 to 60 g/[m.sup.2] (ii) moisture content = 8 to 12% (iii) residue content approximately 20%. The mechanical and other characteristics were neither reported nor determined in this laboratory. Papers of uniform thickness, preferably from a single source, were used for preparing the samples.

Newspaper Composite Preparation

Prepregs of newspaper and phenol-formaldehyde and epoxy resin matrix were prepared in which the ratio of phenolic to epoxy resin was kept constant (50 parts of epoxy resin per hundred parts of phenolic resin) and the combined matrix has epoxy equivalent of 1.70 to 1.85 Equiv./Kg. A calculated quantity of uniformly mixed resin was coated on either side of the paper and the same was heated for 3 to 10 min at 140 to 170 [degrees] C in a heavy duty oven. The resultant prepregs of newspaper and PR-EP were stacked in sufficient number to build up the composite thickness to 1.5 to 2 mm. The stacked prepregs were kept in a three-piece leaky mold, to which pressure was applied. The time-temperature profile used for the curing was: 150 [degrees] C for 1 h and further 200 [degrees] C for 3 h. The mold was brought to room temperature for removal of the sample. Five newspaper composites were prepared; they were designated PC-I, PC-II, PC-III, PC-IV and PC-V for the newspaper weight fractions of 0.30, 0.40, 0.50, 0.60, and 0.65, respectively.

Mechanical Characterization

The determination of tensile strength, modulus, and elongation was performed on an Instron Universal Testing Machine according to ASTM test method D-638. The crosshead speed (initial strain rate) was 5 mm/min and the grip length was 80 mm. The flexural tests were performed on an Instron Universal Testing Machine according to ASTM test method D-790. The crosshead speed and gauge length were 1.3 mm/min and 25 mm, respectively. Heat deflection temperature (HDT) was determined as per ASTM test method D-648 wherein the fiber stress was 1.81 MPa and the rate of heating was 2 [degrees] C per min. The span length was 100 mm. The average of the five results was reported.

Water Absorption, Absorption Isotherm and Diffusion Coefficient

Water absorption was determined as per ASTM test method D-570. The specimens of material were dried in an oven for 1 h at 110 [degrees] C and then the specimens were placed in a container of distilled water and the equilibrium value was determined at the end of 24 h. The water diffusion behavior of the composites was determined by the method primarily developed by Shen and Springer (5) for moisture absorption of composite materials. A 30 x 32 mm specimen was cut from the 2 mm thick composite. All the faces of the specimen were then polished to remove any surface roughness and the specimen was then dried to a constant weight in an oven at 100 [degrees] C prior to testing. The samples were then kept in water for different intervals of time and the water absorbed at different intervals of time and at equilibrium conditions were determined. The moisture absorption isotherms for composites were obtained as a plot of water absorption against function of [-square root of time/thickness]. From the gradient (m) of the linear portion of this isotherm curve, the diffusion coefficients for the composites were calculated using the relationship given by Shen and Springer (5).

RESULTS AND DISCUSSION

A cellulosic filler/fiber based composite with a thermoplastic or thermoset matrix may exhibit varied physicomechanical properties depending upon the lignocellulosic content and the interaction between the filler and the matrix. A matrix consisting of resol phenolic and epoxy resin for newspaper-based composites fulfills these requirements to a great extent. Being in liquid form, the combined phenolic-epoxy matrix can penetrate deep into the pores of the newspaper and can form a chemical bond with the lignocellulosic moieties present within, as well as on the surface of the newspaper. This interaction of the resin matrix and polar lignocellulosic fiber will be critical in determining the properties (especially mechanical, water uptake, and heat deflection temperature) of the composite material. It is expected that the phenolic-epoxy combined matrix may manifest hydrogen bonding with hydroxyl groups of cellulose during the uncured stage, and the strong covalent bonding may appear during the prepreg preparation or cure reaction of the matrix on the newspaper. The possible formation of covalent linkage between the hydroxyl functionality of the lignocellulosic material of the newspaper with this resin matrix is being studied by FT-IR and other analytical instruments in our laboratory and will be reported in a future paper.

As mentioned earlier, the desired mechanical, physical (i.e. low water uptake and low density), and thermal properties are essential for structural and nonstructural applications of these composites. Tensile and flexural strength and impact resistance are three major parameters that are generally evaluated for these composites to assess their suitability for desired applications. In this study, the tensile strength and modulus and flexural strength and modulus have been determined for five paper-based composites wherein the resin matrix composition has been kept same for all. However, the weight fraction of the newspaper filler has been varied from 0.30 to 0.65. It may also be recalled that the waste newspaper composites with phenolic-epoxy matrix were prepared with utmost care so that the resin content is uniform throughout, and as well, the volatiles are reduced to a minimum, for void-free composite samples. Variations in tensile strength and modulus with increases in the weight fraction of newspaper are shown in Fig. 1. The tensile strength varies from 39.5 to 62.5 MPa for all five samples. The highest tensile strength of 62.5 MPa is exhibited by the composite having high resin content, i.e., having the lowest newspaper weight fraction of 0.30, whereas the lower tensile strengths of 39.5 MPa, and 42.35 MPa are exhibited by the composites having 0.60 and 0.65 weight fractions respectively. The tensile moduli of these composites show a declining trend with an increase of the weight fraction of the newspaper, as can be seen from the right-hand side of Fig. 1. Tensile moduli of 2.3 GPa and 2.9 GPa were shown for newspaper weight fractions of 0.40 and 0.30, respectively. More or less similar values of tensile moduli (i.e., 1.5 GPa) were realized at newspaper weight fractions of 0.50 and 0.60, and the tensile modulus was further reduced to 1.2 GPa at 0.65 weight fraction. The decrease in both tensile strength and modulus with an increase in the newspaper weight fraction is probably due to either the filler effect or insufficient chemical bonds between the resin matrix and the fiber component of newspaper. The latter, however, could not be authenticated, for want of experimental data. The results of the "elongation at break" parameter with an increase in the newspaper content show that elongation can be enhanced by reducing the resin matrix level in the composite. Figure 2 shows that elongation at break increases from 1.8% at a newspaper weight fraction of 0.30 to the highest of 4.7% at a weight fraction of 0.6. These results suggest that the newspaper component contributes more to the elongation value, unlike tensile strength and modulus, where the resin matrix plays the important role in improving these properties.

The effect on both the flexural strength and flexural modulus with variations in newspaper weight fraction is shown in Fig. 3. The increase in the newspaper weight fraction from 0.3 to 0.5 did not affect the flexural strength, and the values are within 10% from 160.5 MPa for the initial newspaper weight fraction (i.e. 0.3). Nevertheless, a steep reduction was observed at higher (0.6-0.65) newspaper weight fractions. It is evident from Fig. 3 that the flexural modulus was found to be maximum at 0.3 and 0.4 newspaper weight fractions, and a further increase in the newspaper component resulted in the reduction in flexural modulus values. The decrease in both flexural strength and flexural modulus of the newspaper-based composite can be attributed to two different causes. The insufficient wetting of reinforcing material by the matrix resin could be one cause for the decrease in the flexural properties. However, the possibility of such a situation can be ruled out, as the matrix resin was spread uniformly over the newspaper material. There was not much variation among the five samples prepared for each testing. As mentioned previously, the phenolic epoxy matrix may form covalent bonds with the hydroxyl groups of the cellulose moiety present in newspaper. This possible formation of covalent linkages between the hydroxyl functionality of the lignocellulose material of the newspaper with the functional groups of the matrix resin may cause a stoichiometric imbalance of the reactive components (i.e. phenolic and epoxy resins) of the resin matrix. It is, therefore, visualized that the resultant composite material with newspaper weight fractions of 0.3 to 0.5 might have the desired stoichiometric compositions, thereby resulting in enhanced flexural properties. The resol type phenolic resin prepared by the reaction of phenol and formaldehyde in alkaline conditions comprised complicated mixtures of mono- or polynuclear hydroxymethyl phenols (6, 7) and para-quinone methides (8). Since the resol type phenol-formaldehyde resin used in this study was procured from a commercial source, the actual phenol/formaldehyde ratio is not known. Owing to the multiplicity of the reaction products, it is very difficult to calculate the stoichiometric ratio of the phenolic and epoxy groups. This complexity is fixer increased because of the reaction of the matrix resin component with the reactive site of the lignocellulosic material. The overall results on the variation of the mechanical parameters with five different newspaper weight fractions reflect the efficiency of prepreg method used in composite making as very uniform, and consistent results were accomplished.

In recent years, there has been a shift toward the use of thermoplastic matrices (9) in place of thermoset resins for making composites using cellulosic reinforcing fillers such as newspaper, wood fiber, and flour. The use of thermoplastics may result in easier processing and also significant property advantages. However, the upper limit of service temperature for the use of the composites may be restricted to the lower temperature because of the low glass-transition temperature of thermoplastics (polypropylene). Owing to these constraints associated with thermoplastics, the use of thermoset resins such as phenolics or epoxy offer advantages in terms of service temperature. Heat deflection temperature (HDT) is a property that would provide a basis for the selection of the material to be used at higher temperatures. The HDT values for newspaper-based composites were determined; the results are shown in Fig. 4. Composites with the low newspaper weight fractions (i.e. 0.30 and 0.40) have HDTs of 135 [degrees] C and 127 [degrees] C, respectively, whereas there is a drastic reduction in HDT values at newspaper weight fraction values of 0.50 upward. An HDT of 90 [degrees] C to 99 [degrees] C was observed for the composites with newspaper weight fractions between 0.50 and 0.60. The reduction in HDT with increase in paper content could be attributed to poor or insufficient encapsulation of the paper with the matrix resin, and there exists the possibility of improvement by adopting better prepreg preparation conditions. These results on the HDT of paper-based composites demonstrate that the samples with lower newspaper weight fractions or higher matrix contents would be useful for higher temperature applications. Similarly, composites containing high newspaper weight fractions may be as good or even better than polypropylene reinforced with cellulosic filler/fiber.

Water absorption and specific gravity of lignocellulosic fiber composites are two important physical properties that determine their end-use applications. Water absorption by composite materials could lead to a decrease in some of the properties, such as mechanical and thermal, and this should be considered when selecting their applications. It is difficult to entirely eliminate the absorption of moisture in the composite without using expensive surface barriers on the composite surface. Water absorption in paper-based composites can lead to a buildup of moisture in the lignocellulosic fiber cell wall and also in the paper-matrix interphase region. The moisture buildup in the cell wall could result in fiber swelling and affect the dimensional stability. These paper-based composites may have exposure to very high relative moisture, and at times they may be in constant contact with water, as on rainy days. Thus the water absorption for these composites, as determined by 24 h water soak test (ASTM-D570), was determined. The water absorption coefficients were also determined for evaluating the relative water absorption capability of the newspaper composites using phenolic-epoxy matrices. The water diffusion isotherms for newspaper composites are shown in Fig. 5 and from the gradient (m) of the linear portion of the curve. Water diffusion coefficients of these materials were calculated using the relationship (as given in Eq 1) analytically worked out by Shen and Springer (5).

D = [Pi] [[mh/4[M.sub.[infinity]].sup.2] [[1 + (h/L) + (h/n)].sup.2] (1)

where D = water diffusion coefficient corrected for edge effect

m = gradient of linear portion of the water content against [-square root of time/thickness] curve

[M.sub.[infinity]] = equilibrium moisture content, which is the value of the water absorbed such that there is no further change in the water absorption with time

h = thickness

L = length

n = width

The consolidated data on percentage absorbed water determined as per ASTM D-570 and the values [TABULAR DATA FOR TABLE 1 OMITTED] [TABULAR DATA FOR TABLE 2 OMITTED] of the water diffusion coefficient for newspaper composites are shown in Table 1. The water absorption by the composite samples was found to be dependent on their newspaper weight fractions. The low water absorption was observed at newspaper weight fractions of 0.30 and 0.40, and significant amounts of water get absorbed at higher, i.e. 0.60 and 0.65, newspaper weight fractions. The maximum water absorption of 8.6% at higher (i.e. 0.65) paper weight fractions can itself be considered very low in comparison with newspaper, which absorbs over 200% water within 2 or 3 h. The water diffusion coefficient for the composite with the newspaper weight fraction of 0.30 was calculated to be 4.9 x [10.sup.-4] [mm.sup.2] [s.sup.-1], and an almost one order of magnitude decrease in this value was observed for the rest of the samples, wherein the newspaper weight fraction was varied from 0.40 to 0.65. It can be inferred that samples PC-I and PC-II will be useful from the water absorption point of view, and the end applications of these newspaper reinforced phenolic-epoxy matrix need to be selected on the basis of the water absorption characteristics. As mentioned earlier that the thermoplastic like polypropylene has been chosen as a matrix for lignocellulosic fibers (9) and recently a composite consisting recycled newspaper fiber reinforced polypropylene has been reported (10). Table 2 presents a comparison of the mechanical and physical properties of the composites developed at Macromolecular Research Centre, Jabalpur, India, with those of recycled newspaper fiber-reinforced polypropylene (10). It can be seen that the composite PC-II prepared from newspaper and a phenolic-epoxy resin matrix has mechanical properties comparable to the one prepared at an equivalent newspaper weight fraction from recycled newspaper fiber-reinforced polypropylene. A marginally higher specific gravity and high water uptake shown by the newspaper composites could be compensated by the low processing cost. The studies, comprising detailed physicochemical, mechanical, and thermal characterizations and further optimization of the physicochemical parameters of the production of newspaper-based composites, are under way.

CONCLUSION

Newspaper-reinforced plastic composites are prepared wherein a matrix comprising resol type phenolics and epoxy resin is used as prepreg of reinforcing materials. The mechanical properties in respect of tensile strength and modulus, elongation at break, flexural strength and modulus and heat deflection temperature (HDT) are determined for these composites containing varied newspaper weight fractions from 0.30 to 0.65. The relatively poor mechanical properties at higher newspaper weight fractions may be attributed to the possible covalent bonds formation between hydroxyl group of cellulose moiety in newspaper material and reactive functionalities in combined phenolic epoxy resin matrix. The results of water absorption and diffusion coefficients of composites are reported and these along with mechanical properties could be used as determinant for the end applications of these composites. Both mechanical and water uptake characteristics of newspaper reinforced phenolic-epoxy matrix composites are found quite comparable to those exhibited by recycled newspaper-reinforced polypropylene.

ACKNOWLEDGMENTS

The authors express their gratitude to Dr. S. Sethy, TIFAC, Dept. of Science & Technology, Govt. of India, for fruitful suggestions. The enormous help extended by Dr. S. S. Bisen and Miss Anamika Shukla both of the Tropical Forest Research Institute, Jabalpur, India, is gratefully acknowledged. Thanks are due to Mr. Rajendra Dubey for his secretarial assistance.

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