J Polym Environ (2010) 18:407–412 DOI 10.1007/s10924-010-0169-0
ORIGINAL PAPER
Recycling of Paper Mill Sludge as Filler/Reinforcement in Polypropylene Composites J. Girones • G. Pardini • F. Vilaseca M. A. Pelach • P. Mutje
•
Published online: 17 April 2010 Ó Springer Science+Business Media, LLC 2010
Abstract The reutilization of paper mill sludge as filler/ reinforcement of polypropylene composites has been evaluated. The results show that the increasing content of sludge in composites resulted in materials with higher Young’s modulus, although with lower tensile strength and deformation at break. Thus, in composites submitted to tensile stress paper mill sludge behaved mainly as filler. On the contrary, when materials were submitted to flexural stresses sludge worked partially as reinforcement. The addition of a coupling agent enhanced the fiber–matrix interface. As a result, composite materials with improved mechanical properties, either to tensile, flexural and impact tests were obtained. Considering the mechanical properties of the materials obtained, the substitution of traditional mineral fillers by paper mill sludge is a feasible alternative for composites used in low mechanical demand applications. Keywords Paper mill sludge Mechanical properties PP composites Wood plastic composites
J. Girones F. Vilaseca M. A. Pelach P. Mutje Grup Lepamap, Departament d’Enginyeria Quı´mica Agra`ria i Tecnologia Agroalimenta`ria, Universitat de Girona, Campus Montilivi s/n, 17071 Girona, Spain G. Pardini Unitat de Cie`ncia del So`l, Departament d’Enginyeria Quı´mica Agra`ria i Tecnologia Agroalimenta`ria, Universitat de Girona, Campus Montilivi s/n, 17071 Girona, Spain J. Girones (&) Instituto de Ciencia y Tecnologı´a de Polı´meros, Consejo Superior de Investigaciones Cientı´ficas (CSIC), C/Juan de la Cierva, 3, 28006 Madrid, Spain e-mail:
[email protected]
Introduction Cellulosic fibers constitute the raw material in the production of paper products. In fact, basic paper is obtained by mixing cellulosic fibers with mineral or synthetic fillers that provide the required economical and technical properties such as opacity, strength and smoothness, with filler content approaching 40% (w/w) for some grades of paper [1]. Depending on specific demands, industries use either virgin fibers from pulping factories and/or recycled fibers from selective recovery of paper and board. Whilst virgin fibers are used for higher standards products, recycled fibers are mainly used in the field of packaging. Paper mill sludge is mainly composed of fibrous fines and some inorganic salts and mineral fillers, such as kaolin clay and calcium carbonate [2]. Paper industries using recycled fibers as raw material generate solid wastes that may represent up to 45% of the raw material [1, 3]. In fact, total production of paper mill sludge has been estimated to be around 35–45 kg per ton of paper [4, 5]. In 1995, the US pulp and paper industry generated about 5.3 million metric tons of mill wastewater-treatment residuals, which is equivalent to about 15 million metric tons of dewatered (moist) residuals [6]. Driven by economics, industries are optimizing their inmill fibre recovery systems, in order to increase their raw material yields and reduce operating costs, and primary sludge production [7]. As a result of fibre recovery, industries working with the more expensive virgin fibres, produce paper mill sludge with higher mineral content. Despite these in-mill loss-control measures the production of dry sludge is approximately 4–6% of the final product [1, 8, 9]. For a long time, economy has been marked by a narrow focus on the production of goods, their supply and
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consumption, whilst waste management has been dealt secondarily and in isolation. Now, in order to achieve a sustainable waste policy, the principles of waste management had to be re-examined and upgraded. Nowadays, the main topics leading waste management are minimization, reutilization, recycling and compounding. For sludge, as obtained or after an appropriate pre-treatment, several alternatives to its waste disposal have been proposed during the last 20 years. For instance, due to its good geomechanical properties, chemical inertness and microbiological stability, paper mill sludge has been used as alternative hydraulic barrier layers for landfill construction and covering instead of the more expensive clay [10, 11]. Reliable results has been obtained when paper mill sludge has been used as soil conditioner in degraded and weakly structured soils, providing noticeable improvements in the physical and chemical soil properties [12, 13]. The use of pre-treated waste sludge has also been successfully proved as a heated and powdered raw material for manufacturing ceramic products [9] and, after composting, for toxicity attenuation in heavy metal polluted soils [14, 15]. On the other side, the industry of plastic processing has also commonly use mineral fillers and fibers to adapt the mechanical properties of composites to market demands. In composite production, fillers are mostly used for economical reasons, whilst fibers, mainly fiberglass, are used to obtain materials with upgraded properties. Concerning this last topic, in recent years some works have been done in order to apply natural fibers as substitutes of more expensive synthetic fiberglass [16, 17]. Ismail et al. [18, 19] have recently published a series of works on the reutilization of paper sludge in thermoplastic and rubber composites. The province of Girona, NE Spain has a powerful paper industry. However, most of the sludge from water-treatment plants is transported to disposal and few attempts to re-use the sludge have been carried out. Paper mill sludge produced in one of these factories was characterized in order to establish its possible reutilization. Physicochemical studies emphasized the presence of highly porous cellulose fibers and calcium carbonate (full characterization is reported in Table 1). Its hydrophilic capacity makes its use suitable for soil structure amelioration. However, the high C/N ratio suggests the necessity of a nitrogen source addition in order to favor bacterial and fungal activity. After proper nitrogen enrichment (C/N ratio between 15 and 20) this by-product may be used in agriculture as soil conditioner. Its reversibility in hydration-dehydration processes also makes it suitable as a seed sink. In dry conditions it may be used as a seed carrier because with lack of water seeds remain in a dormant state. When wetted, its hydrophilic capacity make the seed germinate and its presence may be useful in soil as
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J Polym Environ (2010) 18:407–412 Table 1 Main characteristics of paper mill sludge Parameter
Value
Particle size (mm)
1–5
Bulk density (g/cm3)
0.50 ± 0.03
Moisture (%)
55 ± 7
Water holding capacity (%)
102 ± 11
pH
7.4 ± 0.8
Electrical conductivity (dS/m)
2.36 ± 0.15
Organic matter (%)
(C6H10O5)n & 30%
Total nitrogen (%)
0.25 ± 0.06
C/N
44 ± 7
CaCO3 (%)
&70%
amendment. Nevertheless, the economical revenue for any of these applications is fairly low. Given the characteristics of paper mill sludge, we believe it can be recycled as filler in composite materials. In this paper we analyzed the possible utilization of paper mill sludge as filler/reinforcement in polypropylene composites.
Experimental Materials Paper mill sludge, with 30% fibers and 70% CaCO3 content, was provided by Aconda Paper SA (Flac¸a, Spain). Supplied with a high humidity, the paper sludge was dried to water content below 15% before being disintegrated and homogenized by means of a blade mill equipped with a nominal 5 mm mesh. Prior to use, disintegrated sludge was further dried for 24 h in an air-circulating oven at 80 8C. Polypropylene was provided by Repsol YPF under the trade name ISPLEN PP 090 G2M, nominally with a melt flow index of 30 g/10 min (at 230 8C, 2.16 kg) and density 0.905 g/cm3, according to manufacturer data sheet. A maleic anhydride-grafted polypropylene (MAPP) has been used as coupling agent. Supplied by Eastman Chemical, under the trade name Epolene G 3015, this material has an acid number of 15 mg KOH/g, a density of 0.913 g/cm3 and number average and weight average molecular weights of Mw = 47,000 and Mn = 24,800. Physicochemical Characterization Calcium carbonate content was evaluated with a Bernhard calcimeter and corroborated by thermogravimetry (TGA) from the residue at 900 8C (considered as calcium oxide). Difference between calcium carbonate content and dry residue at 105 8C was considered as organic matter (fiber). Average particle size was determined by sieving oven dried
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sludge with juxtaposed sieves ranging from 0 to 5 mm. Bulk density was determined by the core method using 5 9 5 cm steel cylinders inserted in the original bulk material, cut in the edge and weighed after drying for 12 h in an oven at 105 8C. To determine the water holding capacity, 150 g of dry sample were saturated by capillary action. Then, the residual water content was measured after 3 and 24 h of free leaching. Composite Preparation Composite materials based on polypropylene reinforced with paper mill sludge have been obtained by means of a laboratory extruder (BrabenderÒ plastograph) working at 180 8C for 10 min. Mixtures have been prepared with sludge content of 25, 37.5 and 50 wt% based on dry weight. Coupling agent (MAPP) was added directly in the extruder at 4 wt% over the dry sludge weight content. Composite blends were homogenized and granulated in a blade mill before being injected, using a Meteor 40 (Mateu, Sole) injection machine, to obtain the test specimens. Injectionmolding was carried out in a steel mould according to ASTM D3641 standards. Before testing, specimens were conditioned (3 days at 23 8C, 50% humidity) according to ASTM D618 standards. Tensile, flexural and impact tests were conducted following standard protocols (ASTM D638, D790, D256, respectively). For each mechanical property at least five tested specimens were averaged. Results and Discussion PP-based composites had been filled with different proportions of paper mill sludge (25–50 wt%) following the procedure showed in Fig. 1. Composites had subsequently been evaluated under tensile and flexural strain and impact conditions. The mechanical responses of the different composites submitted to tensile strain are summarized in Table 2. In absence of coupling agent, the tensile strength of the composites diminished progressively as reinforcement content was increased from 25 to 50 wt%. Compared with the tensile strength of pure polypropylene matrix, the decreases recorded represented 16.5, 17.5 and 21% for composites containing respectively 25, 37.5 and 50 wt% sludge content. Commonly, incorporation of fillers to thermoplastic matrices generates similar responses. In the case of natural fibers, their incorporation usually generates only very slight increases. In fact, in a previously reported work [20], addition of 40% CaCO3 to PP090 caused a 16% fall in the tensile strength of PP, similar to that obtained with sludge. In another work [21] we detailed an increase of about 7% in tensile strength when hemp strands
Fig. 1 Flow chart of sample preparation
(20 wt%) were added to the PP matrix. Therefore, we can conclude that the response of composites filled with sludge agrees with the typical behavior of this kind of materials. Cellulosic fibers do not provide effective reinforcement to PP matrices, something that has been profusely reported and commonly ascribed to the poor compatibility between the hydrophilic fibers and the hydrophobic PP. The lack of interaction between the two main composite components results in deficient fiber–matrix interface that does not allow an effective stress transfer from the matrix towards the reinforcement. In natural fiber-reinforce materials a coupling agent is commonly added to solve this deficiency. Among several coupling agents reported, MAPP has been providing very good results. It has been accepted that MAPP binds covalently with natural fibers through its anhydride groups, whilst its main PP structure ensures a good interaction with the matrix [22, 23].
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Table 2 Tensile mechanical properties of sludge-reinforced composites Sludge content (wt%)
MAPP content (wt/wt%)
Tensile strength (MPa)
Young’s modulus (GPa)
Elongation at break (%)
Tenacity (KJ/m2)
0
0
27.6 (0.5)
1.5 (0.1)
9.3 (0.2)
475.5 (9.4)
25
0
23.7 (0.6)
2.7 (0.1)
3.2 (0.3)
61.2 (5.6)
25
4
25.8 (0.7)
2.6 (0.1)
3.8 (0.3)
70.9 (9.3)
37.5
0
23.4 (0.5)
3.3 (0.1)
2.3 (0.3)
40.0 (5.5)
37.5
4
25.9 (0.7)
3.1 (0.1)
2.8 (0.1)
48.7 (3.2)
50
0
22.7 (0.6)
3.9 (0.1)
1.8 (0.1)
28.6 (1.6)
50
4
27.1 (1.5)
3.9 (0.1)
2.3 (0.1)
39.2 (4.0)
The addition of MAPP coupling agent to sludge-reinforced composites prevented the fall in tensile strength observed in the uncoupled formulations. In spite of the low fiber content in the sludge, the quality of the interface provided by MAPP was enough to prevent the tensile strength loss caused by the carbonate filler. Composite materials with 50 wt% sludge content presented only a minor decrease in tensile strength (2% below that of the neat matrix). At this point, we must take into consideration the high carbonate content (roughly 70%) in the paper mill sludge used in this work. Nevertheless, one must consider that composition of paper plant sludge depends on the kind and quality of the paper fabricated. Sludge residue from newspaper plants contains almost no mineral fillers and in consequence, when applied as reinforcement, it may provide materials with better ultimate tensile strength. In fact, it has been documented that natural fiber-reinforced composites can provide almost 80% of tensile strength of materials containing fiberglass [24]. The evolution of Young’s modulus as function of sludge content showed a continuous and significant increase. Since stiffness of composites is independent of the interface quality, this can be considered as a typical trend for this kind of materials [25]. The Young’s modulus of the composites increased progressively from 1.55 GPa of neat PP up to 3.9 GPa as more paper mill sludge was added to the composite formulation. These increments are typical of
matrices with mineral fillers, and considerably lower than those containing effective reinforcing fibers. Addition of MAPP coupling agent caused a small but significant increase in deformation capabilities of the composites. The enhanced fiber–matrix interface provided by MAPP enabled a more effective stress transfer towards the reinforcement. As a result, composites presented higher elasticity and tenacity. Table 3 reports the flexural and impact strength characteristics for the PP matrix and the composites filled with different proportions of paper mill sludge with and without coupling agent. Opposite to the tensile stress response, composite materials presented enhanced flexural strength with addition of paper mill sludge, even in the absence of coupling agent. Thus, in this case one can assume that sludge acts not only as filler but also as a reinforcing element. Compared with plain PP090, paper mill sludge addition caused slight increases in flexural strength, ranging between 10 and 20%. For the same sludge content, the presence of MAPP coupling agent resulted in even bigger flexural strength increases. Changes observed on flexural modulus and on elongation at break followed similar patterns. Increases on flexural modulus followed a lineal correlation with sludge content. Again, the relatively small increases in the flexural modulus followed typical tendency for mineral-filled rather than for fiber-reinforced materials [26].
Table 3 Flexural and impact characteristics of sludge-reinforced composites Sludge content (wt%)
MAPP content (wt/wt%)
Flexural strength (MPa)
Flexural modulus (GPa)
Elongation at break (%)
Impact resistance (J/m)
0
0
40.2 (1.0)
1.1 (0.2)
9.6 (0.2)
Not break
25
0
45.1 (1.1)
1.7 (0.1)
6.4 (0.3)
8.9 (4.3)
25
4
47.9 (1.1)
1.6 (0.1)
8.5 (0.6)
13.7 (4.3)
37.5
0
47.8 (0.7)
2.1 (0.1)
5.4 (0.2)
8.6 (5.5)
37.5
4
48.6 (0.9)
1.9 (0.1)
5.7 (0.3)
11.1 (1.6)
50
0
44.0 (1.2)
2.4 (0.1)
3.8 (0.3)
8.0 (0.4)
50
4
49.6 (1.5)
2.2 (0.1)
4.5 (0.3)
10.8 (1.3)
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Composites presented a slight decrease in their impact strength as sludge content augmented. This behavior can be related with the lower ability of the materials to absorb impact energy with filler proportion. According to the elastic deformation, addition of MAPP coupling agent increased the impact strength of the composites. Enhanced stress transfer allowed reaching values at least 30% higher than those of the same formulation without coupling agent. During the last years, wood plastic composite materials have been introduced in the market as an environmentalfriend alternative, being profusely used in exterior nonstructural composite building products such as decking, fencing, roof tiles, etc. [27]. In order to evaluate the possible utilization of sludge-reinforced composites for these purposes, we determined the actual mechanical properties of exterior fences currently being used in Spain. For this purpose, a commercial fence made of PP copolymer commonly used in public construction works in Spain (Ecovalla II by Multiplastic) was granulated and mechanically characterized. The obtained results are summarized in Fig. 2. From the results, one can conclude that the mechanical properties of composites filled/reinforced with paper mill sludge are competitive and in consequence suitable for replacing currently commercialized materials. Characteristics of mill sludge are affected by the raw material and the in-mill fibre recovery systems implanted on each paper industry. Consequently, mechanical properties of composites prepared in this work can not be extrapolated to materials obtained from other paper mill sludges and should only be taken as a trend. Nevertheless, results presented demonstrate the technical viability of using paper mill sludge in polypropylene composites for low mechanical demanding applications. Considering the rising cost of raw materials and the cost of sludge disposal, utilization of sludge as reinforcing agent has big economical implications. In order to be commercially Tensile strength (MPa) Young Modulus (GPa)
Flexural strength (MPa) Flexural Modulus (GPa)
4,5
60
4 50
3 2,5
30 2 20
1,5
Modulus (GPa)
Strain (MPa)
3,5 40
1
Table 4 Costs required for paper mill sludge conditioning (considering treatment of 7200 tonne wet sludge per year) €/tonne Investment required (drying/homogenization)
200.000 €
Amortization (3 y-including interests)
?20
Sludge manipulation and transport to plant
?1.5
Energetic consumption
?3.5
Total cost
?25.0
Transport and disposal in authorized landfill
&33
and environmentally viable, a landfill must be adjusted to many specific requirements. For this reason, there are increasing difficulties to expand or find new locations for landfill disposal. As a consequence, waste disposal is becoming steadily more expensive over the years. In the case of the analyzed industry, last year the cost of paper mill sludge transport/disposal in a properly authorized landfill represented roughly 33 €/tonne. This cost is referred to raw sludge comprising 50% humidity. In order to avoid water interference during extrusion/injection, composites were prepared with sludge humidity levels below 1%. Sludge draining should allow reducing water content to 12–15%. Therefore, a new drying unit and the adaptation of the industry steam pipes would be required in order to further dry the sludge. The current annual production of sludge in this industry is around 7200 tonne/year. Table 4 shows the costs of installing and transforming the raw sludge into a material suitable for composite reinforcement. Energetic consume was measured considering the steam necessary to reduce the sludge humidity from 15 to 1% at current steam production costs. The term denoted as manipulation costs refers to sludge disposal outside the plant for its draining and includes the additional human resources required. According to these assumptions, the analyzed paper industry would obtain 3600 tonne/year of dry sludge at roughly 25 €/tonne. Thus, transforming sludge into a reinforcing material should be cheaper than its landfill disposal, although there is no market for such a product yet. Regardless, addition of 50% sludge in a composite formulation would halve the utilization of neat PP (&1 €/Kg), without decreasing neither tensile nor flexural strength of the material. In consequence, this paper demonstrates that the paper mill sludge could be used as low cost reinforcing material for composites intended to low demanding applications.
10 0,5 0
0 PP090
Commercial fence PP090 + 50wt% sludge
PP090 + 50wt% sludge + MAPP
Fig. 2 Mechanical properties comparison between a commercial PP fence and PP090 filled-reinforced with paper mill sludge
Conclusions The calcium carbonate-rich paper mill sludge used in this work, behaved mainly as filler when incorporated to a
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polypropylene matrix. As a result, composites with higher Young’s modulus were obtained by increasing sludge content. Nevertheless, the higher stiffness of these formulations results in lower tensile strength and elongation at break. However, sludge showed a feeble reinforcing effect when material was submitted to flexural stresses. Addition of MAPP coupling agent improved the reinforcing behavior of paper mill sludge. Composite formulations including 50 wt% of sludge and MAPP showed the same tensile strength as plain PP whilst offering higher flexural resistance. Considering the evaluated mechanical properties, we believe this composite can be used as substitute for mineral-filled composites in low mechanical demand applications like decking or fencing.
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