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Formulation and characterization of leather and rubber wastes composites.


Footwear industry in Europe, and particularly in Portugal, concentrates in the production of added value shoes and uses extensively chromium tanned leather for the shoes upper part. Leather waste scraps resulting from upper materials cutting operations, represent 50-70% of the solid waste produced by footwear companies, resulting in about 150,000 metric tones of this type of waste being produced in Europe annually [1],

Attachment of the shoe upper and lower parts requires bottom upper materials carding and/or roughing. Certain sole materials are also rouged in the bonding margin to better prepare them for adhesion and/or in the outer face, laterally, to enhance their appearance. These operations generate leather and sole materials fibers, dusts, or small scraps, which represent 5-15% (w/w) of the solid wastes generated by shoe-making companies.

Disposal of leather upper wastes is expensive due to the increasing taxes associated to the land filling option as well as the presence of chromium that may become them as hazardous waste, according the Portuguese and European laws. Therefore, extensive research has been directed towards not land filling the generated wastes but giving value to them. Leather wastes intrinsic fibrous nature opens a potential window to use them as reinforcement additive for rubber composites as recently reported by Przepiorkowska et aL and Chronska et al. [2, 3].

Sole materials wastes include leather (vegetable tanned), thermoplastics [polystyrene-polybutadiene-poly-styrene (SBS), polystyrenepoly(ethylene-cobutylene)polystyrene (SEBS), polyethylvinyl acetate (EVA), polyvinylchloride (PVC), polyurethane (PU)] and thermosets [vulcanized rubber (VR), EVA, PU].

VR is a non biodegradable thermoset also extensively used in tires, pipes, belts, and a variety of technical pieces. VR waste management has received considerable research attention during the last decades, resulting in a number of recycling options, namely [4-8]: (i) renew (tires renew, reuse in soles); (ii) scrap size reduction to obtain powered rubber (PR) with sizes depending on the preparation technology and procedure, that can be used as such in rubber compounds or composites, reclaimed, or surface modified (for example with nitric acid, hydrogen peroxide, or sulfuric and chlorosulfonic acids to be used as ion exchanger); (iii) physico-mechanical, chemical, and/or biotechnological treatments to obtain regenerated/reclaim rubber; (iv) introduction in asphalt; (v) use as fuel due to its high calorific power; and (vi) making use of thermal decomposition products (charcoal, coal gas, oil).

Recycling of VR as PR appears today a sustainable option; therefore, in Germany and UK, the production of regenerated rubber has almost stopped and converted to the production of fine PR (75-300 /um) [4]. The production of activated PR in Sweden has also tended to substitute regenerated rubber [4J. In Portugal, the main option for VR is also PR production.

PR recycling in rubber compounds has been studied extensively over the past decade. Marvin assessed both tire and non-tire PR scrap rubber treated and with no treatment, in compounds, concluding that both could be used to provide better level of properties [9, 10]. The results are improved when the scrap composition is more like the raw rubber or compound into which is added. Treatment provides better results only at lower incorporation levels (below 40%). A mix of finely divided footwear waste materials (450-550 [micro]m)including vulcanized rubber, has also been used in the formation of new footwear articles [11]. Ravichandran and Natchimuthu studied scrap rubber recycling combined with leather waste particles in natural rubber compounds. Mechanical properties were found to increase on increasing scrap rubber loading in the absence of leather. The addition of leather was found to increase the loading levels of scrap rubber with marginal effect on properties over the compounds with no leather [12, 13].

Recent studies on leather fibers, as well as on PR, indicate that obtaining performing and environmental friendly composite materials as possible. Therefore, the main goal of the present work is to study the use of footwear industrial waste in high concentrations as reinforcing additive and filler for shoe components, through processes that can be feasible at the industrial scale. With this objective, a comparative study on the effect of incorporating: (i) finished leather fibers (grinded [less than or equal to]1 mm) in different quantities (10-25 parts per hundred parts of rubber), and (ii) upper (leather based) and sole (rubber based), roughing/carding wastes, collected at an industrial site, in the range of 20-100 phr, on the physical, chemical, morphological, rheometric, and swelling properties of the elastomers more frequently used in shoe components (SBR and NBR) has been carried out. Hazardousness of the waste materials used as well as that of the residues originated by the developed composites was evaluated, too, to compare with both Portuguese [14] and European Union [15] legislations.


Materials and Composites Preparation

Forty finished tanned leathers, which may be considered representative of the materials used in footwear industry were collected at five industrial companies and characterized regarding animal source. The materials were shredded to ~ [less than or equal to]1 mm (using a Pegasil [R]--ZIPOR [R] mill with rotating knives and a 1 mm sieve), to obtain short fibers and granules of leather, and referenced as MIX0.

Following, 20 kg of upper roughing/carding leather waste and 20 kg of roughing soles waste were collected at one industrial site, thoroughly homogenized, and referenced, respectively, as UD and SD. Very often these two types of wastes are mixed at the companies where they are generated. Therefore, two types of mixtures were prepared, respectively, 50% UD + 50% SD and 75% UD + 25% SD, both in w/w%. These mixtures were thoroughly homogenized and referenced as MJX1 and MIX2, respectively.

Then, all the materials were conditioned in standard laboratory atmosphere (296 K [+ or -] 2 K and 50% [+ or -] 5% relative humidity) for chemical characterization and composites preparation.

SBR and NBR materials were from Kumho Petrochemical [R], medium viscosity (45-50 Mooney), grades SBR 1502 and NBR 35 LM, respectively. All other chemicals used are listed in Table 1 and were of rubber grade.
TABLE 1. Rubber compounds and composites formulations.

Formulation components                                 phr

Synthetic polymers (Styrene-butadienc                         100
 rubber--SBR, Acrvlonitrile butadiene

Filler (Silica VN3)                                            30

Activator (Polyethylene glycol. Stearic acid,    6.0 [+ or -] 1.0
 Zinc carbonate)

Antioxidant (Lovinox[R] 22 M46, Vulcanox[R]      3.5 [+ or -] 0.5
 MB2, Antiozonant AC052)

Accelerator (Diben/othiazyl disulfide--MBTS,     1.5 [+ or -] 0.5
 Tetra methyl thiuram disulfide--TMTD)

Vulcanization agent (Sulfur SU95)                             1.5

Additive in study                                        Addition

 MIXO                                          10, 12.5. 15,20,25

 UD, SD                                               20, 50, 100

 MIX1                                             20, 50, 75. 100

 MIX2                                                 50, 75. 100

phr, parts per hundred of rubber; g of component per 100 g of rubber

The SBR and NBR rubber compounds were prepared according to the recipe listed in Table 1 using an industrial Banbury[R] mixer. Following, the SBR and NBR compounds were charged with the footwear wastes on a laboratory-scale open two-roll rubber mixing mill from Roca and Guix [R], allowing dispersion observation and preventing industrial production equipment contamination. This equipment with stainless steel cylinders of 30 cm long and 15 cm diameter rotates at 14 rpm and has distance between cylinders adjustable. The vulcanization agent was added to the compound prior to the waste addition incorporation. The formulations of the composites are presented in Table 1, too. In the first experimental series, the aim was assessing the effect that a mix of 40 leather materials normally used in the shoe industry, after grinded to [less than or equal to]1 mm, have in the physical and chemical properties of rubber composites. Thus, composites incorporating 10, 12.5, 15, 20, and 25 phr of M1X0 leather to the SBR and NBR rubber compound were prepared. In the second series of blends, the aim was assessing the feasibility of using fibers and dusts residues generated in the footwear process, with any pretreatment, to prepare rubber composites. Therefore, using the same base procedure, samples with (i) 20, 50, and 100 phr of UD; (ii) 20, 50, and 100 phr of SD; (iii) 20, 50, 75, and 100 phr of MIX1; and (iv) 50, 75, and 100 phr of MIX2 to the NBR rubber compound were prepared. A control mix with no residue was also prepared for comparison.

The time of vulcanization of the compounds developed was determined at 423 K using an oscillating disc Gibitre K Rheometer. Vulcanization was carried out in a lab hydraulic heated press at 423 K for the optimum cure times obtained by rheometry, using a 37 cm X 26 cm with 0.4 and 0.6 cm thickness mold. All the composites plaques were produced in triplicate and stored at 296 K [+ or -] 2 K. Specimens for characterization were mechanically cut out from the vulcanized plaques.


The 40 finished leathers collected, as well as the MIXO, UD, and SD samples were chemically characterized following the standards listed in Table 2 [16-35]. The size distributions of UD and SD residues were also determined.
TABLE 2. Chemical and physical methods for characterization of the
materials, waste additives, and developed composites [16-35].

Parameter                                          Method

Chemical characterization of leather. additives: MIX 0, UD, SD, and

pH                                                  ISO 4045:2008

Chromium (VI)                                      ISO 17075:2007

Amines (azo colorants)                          ISOATS 17234:2003

Pentachlorophenol                                  ISO 17070:2006

Formaldehyde                                     ISO 17226-2:2008

Organostanic compounds                             ISO 17353:2004

Cadmium and lead                            Acid digestion and US

Sulfated ash                                        ISO 4047:2006

Total ash                                         ASTM D2617:2006

Total organic carbon (TOC)                          EN 13137:2001

Total Cr                                Acid digestion and US EPA

Ehiiite from leaching test of additives; MIX 0, UD. SD. and

PH                                                  ISO 4045:2008


 Chromium (VI)                                     ISO 17075:2007

 Total Cr                                        US EPA7190B:1986

Physical characterization of developed

Densitv                                             ISO 2781:2008

Hardness                                    ISO 868:2003--Shore A

Abrasion resistance                                 EN 12770:1999

Tear strength                                       EN 12771:1999

Tensile strength                                    EN 12803:2000

Elongation at break                                 EN 12803:2000

Water absorption                                EN ISO 20344:2004

Water desorption                                EN ISO 20344:2004

Fatigue (Ross Ilex resistance)                   BS 5131-2.1:1991

The composites were physically and chemically characterized according to the standards listed in Table 2, too.

The morphology of MIXO, UD, and SD residues and selected composites was registered in images. Morphological studies of selected composites were carried out in micrometer (Anglia Scientific [R]) cut surfaces using a Nikon[R] optical microscope (Labophot-2) and in the cryofractured surfaces, after gold coated, using an Environmental Scanning Electron Microscope (Schottky) incorporating X-Ray microanalysis and electrons diffraction patterns analysis (FEG-ESEM/EDS/EBSD) FEI Quanta 400FEG/EDAX Genesis X4M [R].

Vulcanization characteristics determined following the ASTM D2084:200l standard [36], at ~423 K, with a Gibitre[R] Rheometcr, gave data for calculating the minimum torque [M.sub.L] (dN m); the increase in torque moment [DELTA]M (dN m); the optimal vulcanization time [t.sub.90]: and the activity of leather [a.sub.f] based on the equation:

([DELTA]M)/([DELTA][M.sub.0) -1 = [[alpha].sub.f]([m.sub.f])/([m.sub.p]) (1)

where [DELTA]M is the increase in torque moment of the compound with leather; [[DELTA]M.sub.0] is the increase in torque moment of the formulation with no leather; [m.sub.f]is the mass in the parts by weight of leather and [m.sub.p] the mass in parts by weight of polymer [2].

The number of crosslinks, n (number of moles per [cm.sup.3]), was determined on the basis of solvent-swelling measurements following ISO 1817:2005 [37J (toluene at 296 K [+ or -] 2 K during 24 h) by application of the Flory-Rhener equation [38]:


where [v.sub.r] is the volume fraction of polymer in the swollen mass; X is the Flory-Huggins polymer-solvent dimension-less interaction term (0.413 and 0.479, for SBR and NBR, respectively); and [V.sub.0] is the molar volume of the solvent [(V.sub.o] = Molar mass of solvent/Density of the solvent that for toluene is 106.3 [cm.sup.3]/mol).

The volume fraction of the polymer (volumetric content of elastomer in the sample under test in toluene) may be calculated by the expression [39]:


where [W.sub.r] and [W.sub.s] are the weights of the rubber (rubber sample initial weight) and solvent (rubber swollen weight minus rubber initial weight), respectively; and [p.sub.r] and [p.sub.s] are the rubber and solvent densities (0.8623 [g/em.sup.3] for toluene); or using the following expression [2]:


where Q is the equilibrium swelling coefficient, defined by [40]:


with [] the weight of the swollen sample.

The swelling ratios (percentage swelling) ot the vulcanizates were calculated using the following equation [12]:

[S.sub.ratio] = 100([] - [W.sub.r])/([W.sub.r]) (6)


Additive Residues Characteristics

Table 3 reflects some characteristics of the 40 leather materials collected whose origin distribution is 5%, 22.5%, and 72.5% for ovine, suine, and bovine, respectively. One characteristic is its acidic leather nature as seen through the eluates of the ISO 4045:2008 test giving pH values between 3 and 4. However, an abnormal high value of pH 6.4 was detected. Hexavalcnt chromium according to ISO 17075:2007 is below the actual threshold value of 3 mg/kg in 31 of the samples tested.
TABLE 3. pH, chromium (VI) of the 40 leather samples collected.

             Chromium (VI)               Chromium (VI)
Sample   pH       (mg/kg)   Sample   pH       (mg/kg)

     1  3.3            0.5      21  3.6            1.0
     2  3.1            0.1      22  3.4            1.2
     3  3.7            1.1      23  3.3           10.8
     4  3.6            3.5      24  3.1          155.0
     5  3.8            0.1      25  3.4            0.2
     6  3.6            1.3      26  3.3            0.2
     7  3.7            1.8      27  3.3            0.7
     8  3.7            0.2      28  3.4            0.7
     9  3.1            0.1      29  3.3            4.3
    10  3.7            0.1      30  3.4            1.3
    11  3.4            0.4      31  3.3            2.1
    12  3.0            3.2      32  3.4            1.8
    13  3.2            0.4      33  3.7            3.0
    14  6.4           10.5      34  3.3            0.9
    15  3.4            0.7      35  3.6            0.2
    16  3.9            0.5      36  3.4            0.2
    17  4.2            1.9      37  3.7            1.4
    18  3.4           14.4      38  3.4            2.1
    19  3.3            0.6      39  3.4            3.6
    20  3.4            0.1      40  3.5            2.3

Animal species: 2 ovine, 9 swine, and 29 bovine.

Table 4 presents the characteristics of the additive residues used in the compounding tests. MIX 0 has hexavalent chromium below the threshold value of 3 mg/kg. On the contrary, UD waste presents hexavalent chromium above that value. The three samples may be considered to present azo colorants, pentachlorophenol, formaldehyde, organostanics, and metals (cadmium and lead) below respective threshold values. Ashes and total organic carbon of these samples are the usual in the footwear materials.
TABLE 4. Characteristics of the additives used.

      Parameter            MIXO         UD           SD

Chemical characterization of MIX0, UD, SD

pH                             4.1      6.6

Chromium (VI)                   <3      7.2              -

Amines (mg/kg)                 <30      <30

Phenols (tetra e                <5       <5              -
penta) (mg/kg)

Formaldehyde (ma/kg)          85.0     68.9        ND (<1)

Organostanics                    -        -            <10

Cadmium (mg/kg)                <10      <10            <50

Lead (mg/kg)                   <50      <50              -

Sulfated ash (%)               7.2     10.5           16.2

Total ash (%)                    -

TOC (%)                       49.2     58.9           44.3

Total Cr (g/kg)       21.9 (~2.2%)  13.9 (~   4.7 (~ 0.5%)

Ehtate from leaching test according to EN 12457-2:2002 (ralion 1:10)

pH                               4.0      6.9            6.9

Analytics (mg/L)

 Cr (VI)                       <0.01    <0.01          <0.01

 Total Cr                        7.0      0.2            0.6

Environmental law specification regarding total Cr and Cr(VI)

Analytic                  PT law limit            EU law limit

Total Cr (g/kg)                3 inert                      NA

Total Cr (%)           5 non-hazardous                      NA

Chromium (VI) in     0.1 non-hazardous                      NA
eluate (mg/L)

Total Cr in eluate     2 non-hazardous      10 m Dii-hazardous

ND. non detected; NA, non applicable; PT, Portugal; EU, European union.

MIXO, UD, and SD samples have, respectively, about 21.9, 13.9, and 4.7 g/kg (2.2, 1.4, and 0.5%) of chromium. These values for total chromium being above 3 g/ kg and below 5% make these wastes as non-inert and non-hazardous according to the Portuguese law [141 regarding this parameter. Chromium variability, defined as the ratio between the range and average values on Table 4, in percentage, is 4.6, 7.1, and 1.3%, respectively, for the MIX 0, UD, and SD samples.

The more acid MIXO nature reflects in the eluate of the EN 12457-2:2002 leaching test with lower pH than for the UD and SD eluates. All these eluates have Less than0.01 mg/L of hexavalent chromium. The MIXO eluate has a total Cr higher than samples UD and SD, and above the threshold value of 2 mg/kg, thus this material is considered a hazardous waste according to Portuguese law, contrarily to the European Union Law [15] that establishes a threshold value of 10 mg/kg for this parameter.

Composites Physical Properties

Selected physical properties of the first and second series of experiments are shown in Figs. 1 and 2, respectively.

According to the ISO 2781:2008 Method B for density determination, addition of MIXO has a negligible effect in the density of both NBR and SBR base formulations in the first series of trials (data not shown). In the second set of tests, the density of all NBR composites is in the range of 1.11 [+ or -] 0.05 Mg/m3.

Hardness increases gradually with increasing addition of MIXO from about 60 to--80 Shore(A). Hardness also increases with the addition of UD, SD, MIX I, and MIX2 residues from about 60 up to 85, 80, 85, and 90 Shore(A), respectively. These results can be attributed to the more rigid material nature of the additives, which added higher hardness to the softer elastomer phase compared with the vulcanizates containing no or lesser amount of additive, as has been reported for other systems [12].

With MIX 0 additions; tear resistance increased for both SBR and NBR composites, in general, till 25 phr (Fig. lc and d, respectively). A higher reinforcement is obtained in NBR base composites whose tear resistance increased about 72, 65, 84, 91, and 99% relatively to the base formulation, respectively, for 10, 12.5, 15, 20, and 25 phr additions. An increment in this property is observed in the second set of experiments (Fig. 2b), too: (i) UD (mainly leather fibers granules and dusts) incorporation in 20, 50, and 100 phr increases average tear resistance in composites of 100, 122, and 44% relatively to the base formulation; (ii) the same incorporation of SD (mainly rubber sole granules and dusts) promotes average tear resistance increase of 13, 51, and 42% relatively to the base formulation; (iii) MIX1 (50% UD + 50% SD) composites with 20, 50, 75, and 100 phr incorporation increases average tear resistance of 128, 83, 80, and 41% relatively to the base formulation; and (iv) MIX2 (75% UD + 25% SD) incorporation in 50, 75, and 100 phr produces composites with tear resistance on average increased by 95, 71, and 21%, relatively to the base formulation. All the composites are adequate for applications in soles for normal and demanding footwear, whose minimum values of tear resistance are, respectively, 7 N/mm and 10 N/mm. The considerable increase in tear strength up to specified limits when leather containing residue additive is added is due to the fibrous nature of leather that effectively prevents the growth of the test specimen crack. The above referred increases on tear resistance appear to be essentially a physical phenomenon. However, the higher results in the NBR composites may be related with higher matrix--leather fibers affinity due to the active functional groups present in both materials [2].



Abrasion resistance decreases with MTXO addition. None of the SBR base composites reaches the more demanding footwear soles specification of Less than 150 mm3 (Fig. la). The NBR composites till 20 phr additive incorporation fulfill this specification (Fig. lb). Till 25 phr of incorporation the normal use specification, whose maximum is 300 mm3, is accomplished by both rubber bases composites. In the second set of tests (Fig. 2a), abrasion decreases with increasing additive incorporation; nevertheless, all the composites prepared have abrasion resistance adequate for application in soles of day-to-day use footwear (<300 [mm.sup.3]).

Tensile strength and elongation at break decrease relevantly with MIXO addition in both base polymers. Till 12.5 phr addition for SBR (Fig. le and g) and 15 phr for NBR (Fig. If and h), the composites fulfill the minimum tensile strength specification of 8 MPa and elongation at break above 300%. The UD, SD, MIX1, and MIX 2 additions gave results in good agreement. An increase in tensile strength (Fig. 2c) relative to the base NBR formulation is observed for the composites: (i) 20 phr UD; (ii) 20 phr SD; and (hi) 20 phr MIX1. An increase in elongation at break (Fig. 2d) is also observed for the following composites: (i) 20 phr UD; (ii) all SD additions; (iii) 20 and 50 phr MIX1; and (iv) 50 phr MIX2. Therefore, tilf 20 phr of UD, SD, and MIX1 incorporation, the composites present tensile strength and elongation properties satisfactory for soles application.

According to BS 5131/2.1:1991, both SBR and NBR composites till 25 phr of MIX 0 incorporation gave around 0.01 mm/kcycle, thus a satisfactory behavior with respect to flexing resistance. The same behavior was obtained in the second series till 50 phr of residue additives incorporation.

The decrease of abrasion resistance, tensile strength, elongation at break, and flexing resistance with increasing waste additives incorporation may partly be explained by the size of the additive materials and consequent voids in the matrix. Upon abrasion, tension, and stretching the voids grow in size and interact between them, leading to material de-bonding and failure. As indicated by Ravichandran and Natchimuthu [12, 13], a further negative contribution comes from the acidic and hydrophilic leather nature, thus leading to poor adhesion between the leather and the matrix. The affinity between leather and elastomer matrix could be increased by pretreating the leather residue as proposed by these authors. However, it would make the recycling process more complex and expensive.

Water absorption increases with leather fibers addition due to their hydrophilic characteristics (results not shown). Till 25 phr of MIXO incorporation composites show moderate water absorption of about 2% at the maximum. All of the composites obtained in the second series of experiments gave water absorption below 3-4% increasing with the leather fibers content in the composites, too. These results indicate that even though leather fibers have high affinity with water, when they are consolidated in the composite, the water uptake is reduced due to the more limited interaction of leather fibers with water and the hydrophobic nature of the rubber matrix.

Chemical Properties and Hazardousness of the Composites

The base SBR and NBR vulcanizates as well SBR MIXO 12.5 phr, NBR MIXO 15 phr, NBR UD 20 phrT NBR UD 100 phr, NBR SD 20 phr, and NBR SD 100 phr composites, here re-called as SBR1, NBR1, NBR2, NBR3, NBR4, and NBR 5, respectively, were chemically characterized following the procedures listed in Table 2 giving the results presented in Table 5. All the samples gave hexavalent chromium, total chromium, cadmium, lead, azo colorants, chlorophenols, formaldehyde, and oreanostanics below the threshold values.
TABLE 5. Chemical characteristics of the composites.*

Parameter            SBR      NBR     SBR 1    NBR 1

Analysis of the

 Chromium (VI)         <3       <3       <3       <3

 Amines (mg/kg)       <30      <30      <30      <30

 Phenols (mg/kg)       <5       <5       <5       <5

 Formaldehyde         <10      <10      <10      <10

 Organostanics    ND (<1)  ND (<1)  ND (<1)  ND (<1)

 Cadmium (mg/kg)      <10      <10      <10      <10

 Lead (mg/kg)         <50      <50      <50      <50

 Total Cr (g/kg)     <0.1     <0.l      4.1      4.0

Analysis of the

 Cr (VI) (mg/L)     <0.01    <0.0l    <0.01    <0.01

 Total Cr (mg/L)     <0.3     <0.3     <0.3     <0.3

Parameter          NBR 2    NBR 3    NBR 4    NBR 5

Analysis of the

 Chromium (VI)         <3       <3       <3       <3

 Amines (mg/kg)       <30      <30      <30      <30

 Phenols (mg/kg)       <5       <5       <5       <5

 Formaldehyde         <10      <10      <10      <10

 Organostanics     ND(<1)   ND(<1)   ND(<1)   ND(<1)

 Cadmium (mg/kg)      <10      <10      <10      <10

 Lead (mg/kg)         <50      <50      <50      <50

 Total Cr (g/kg)      4.0      4.7      0.2      0.8

Analysis of the

 Cr (VI) (mg/L)      <0.l     <0.1     <0.1     <0.1

 Total Cr (mg/L)     <0.3      0.4     <0.3     <0.3

* SBR 1, SBR M1X0 12.5 phr: NBR 1, NBR MIX0 15 phr; NBR 2,
NBR UD 20 phr; NBR 3, NBR UD 100 phr: NBR 4, NBR SD 20 phr;
NBR 5, NBR SD 100 phr; ND.
non detected.

The SBR, NBR, SBR1, and NBR1 samples have, respectively, <0.1, <0.1, 4.1, and 4.0 g/kg of total chromium. According to these values, since the SBR1 and NBR1 composites have total chromium between 3 g/kg and 5%, according to the Portuguese law, they are considered neither inert nor hazardous, but non-hazardous wastes. The leachine test of these materials following EN 12457-2:2002 protocol gave eluates whose hexavalenl and total chromium are below the PT and EU threshold values, thus confirming them as non-hazardous wastes at the end of life. Thus, despite the MIXO leather waste additive was considered hazardous according Portuguese law, the composites obtained with their incorporation in both elastomers matrices will be non-hazardous wastes at their end of life.

The NBR2, NBR3, NBR4, and NBR 5 samples have, respectively, 4.0, 4.7, 0.2, and 0.8 g/kg of total chromium, therefore, NBR2 and NBR3 are non-hazardous wastes while NBR4 and NBR5 are inert wastes according to the Portuguese law. The eluates obtained following EN 12457-2:2002 leaching test gave hexavalcnt chromium and total chromium concentrations that confirmed those classifications.


Optical and Morphological Studies of the Composites

Eye and ESEM views of selected additives and composites are presented in Fig. 3. The top area presents the general views of the waste additives with no pretreatment. UD (Fig. 3a) is a heterogeneous material composed mainly (>95%) by leather granules/agglomerates, fibers, and dusts that observed through optical microscope shows leather libers usually >0.1 mm and dusts <0.1 mm, with predominance for fibers and dusts agglomerates with sizes in the range of >0.2-0.3 mm to 10 mm, and relatively small percentage of threads with size in the range of 2-3 mm up to 3-6 cm. SD (Fig, 3b) is a finer and move homogeneous rubber waste material (SBR and NBR based according to the source of this material). Its size distribution determined by sieving is presented in Fig. 4, indicates a median of 843,40 [micro]m, with 13.5, 12.1, 12.1, 11.9, 12.0, 10.6, 9.0, 7.3, 5.1, 2.9, and 0.9% of the particles having average size distribution of 1550, 1290, 1090, 925, 675, 400, 275, 215, 165, 128, 90.5, 64, and 26.5 [micro]m, respectively.

The natural and optical microscope views (not presented) of the composites indicate a reasonable dispersion of the leather waste in the matrices. The ESEM micrographs of selected leather-rubber composites (Fig. 3c and d) reveal that fibers in certain areas are well embedded in the polymer matrix and show some degree of orientation; in other areas, fibers are visible in bundles/agglomerates not stretched, and presenting no orientation, SEM studies show that fracture of a short fiber reinforced composite can be caused by fiber breakage or debonding of the fibers [41] In the present study, both fiber breakage and debonding of the fibers are observed, too. In general, better apparent liaison between matrix-fiber is observed with lower leather fibers loads, thus contributing to explain the better mechanical properties in this range of incorporation.


Rheometric and Swelling Studies on the Composites

The rheometric properties of the composites obtained in the two series of experiments carried out are in general agreement as seen in Table 6.
TABLE 6. Rheometric properties of SBR and NBR [[[composites.

   Reference         [M.sub.L]  [DELTA] M  [t.sub.90]  [a.sub.f]
                       (dN m)    (dN m)     (min)

First series of experiments
 SBR                      24.9       68.5        1:31          -
 SBR MIXO l0 phr          26.6       72.7        1:21       0.61
 SB R MIXO 12.5 phr       28.2       74.6        1:26       0.90
 NBR MIXO 15 phr          30.5       77.8        1:23       0.81
 NBR MIXO 20 phr          32.4       79.6        1:23       1.26
 NBR MIXO 25 phr          34.4       83.3        1:28       0.86
 NBR                      21.0       63.7        1.08          -
 NBR MIXO 10 phr          25.8       70.4        1:09       1.05
 NBR MIXO 12.5 phr        27.3       74.4        1:09       1.36
 NBR MIXO 15 phr          27.0       75.6        1:08       1.24
 NBR MIXO 20 phr          29.3       78.3        1:09       1.14
 NBR MIXO 25 phr          30.1       79.9        1:09       1.02

Seond series of experiments
 NBR                      23.9       65.6        0:59          -
 NBR SD 20 phr            29.9       72.3        0:58       0.51
 NBR UD 50 phr            34.4       72.8        1:00       0.22
 NBR UD 100 phr           35.2       51.5        0:57         <0
 NBR UD 20 phr            24.1       57.2        0:55         <0
 NBR SD 50 phr            27.7       55.1        0:53         <0
 NBR SD 100 phr           28.0       51.2        0:59         <0
 NBR MIX1 20 phr          27.9       66.5        1:03       0.07
 NBR MIX1 50 phr          32.6       66.9        3:03       0.04
 BR MIX1 75 phr           39.2       72.1        1:02       0.13
 NBR MIX1 100 phr         39.1       68.9        1:02       0.05
 NBR MIX2 50 phr          34.1       70.5        1:01       0.15
 NBR MIX2 75 phr          32.6       60.1        1:05         <0
 NBR MIX2 100 phr         35.0       59.0        1:03         <0

From the rheometric parameters obtained, it can be concluded that the addition of MIXO to both SBR and NBR till 25 phr: (i) increases the viscosity of the mix [(M. usb.L]) as its percentage increases; (ii) slightly increases the torque [DELTA]M, suggesting a small increase in the crosslinking degree of vulcanizates with fibers addition and a small reinforcing action connected with the additive [2, 3]; (iii) slightly decrease vulcanization time, kxh particularly for SBR, suggesting that as more fibers are incorporated shorter the vulcanization reaction of the elastomers; and (iii) develop an almost negligible additive activity in the case of NBR [(a.sub.f]).

Adding footwear upper (UD) and sole (SD) roughing/carding wastes and their mixes (MIX1 and MTX2) to NBR till 100 phr: (i) increases the viscosity of the mixes [(M.sub.L]), especially for the composites with UD leather granules, fibers, and dusts; (ii) slightly increases the torque of the mixes, [DELTA]M, except in the case of NBR UD 100 phr, NBR SD, NBR MIX2 75 phr, and NBR MIX2 100 phr composites. The reduction in the maximum torque in the composites with higher loading of waste additive has been reported for other systems 112] and may be due to a low amount of chemicals available to the vulcanization; (iii) has a small change in the vulcanization time, [t.sub.90]; and (iii) develop no additive activity [(a.sub.f]).


The increase in the minimum torque upon addition of the leather based residues (MIXO and LID) can partially be attributed to the fibrous structure of leather and its more rigid material nature. In the case of SD residue, it can be related with the rigid nature of the material particles (vulcanized rubber).

The reduction in torque [DELTA]M of SD containing composites, specially at higher loadings, could be due to the introduction of more rubber into which the chemicals can migrate, which is expected to result in further crosslinking of the discontinuous SD phase and reduction in the state of cure of the continuous matrix phase as suggested by other works [12, 13]

The crosslink density values (n) of the vulcanizates and composites of the first and second experimental series measured in toluene are presented in Fig. 5a and b. The crosslink density increase (n increase) and swelling ratio, expressed as the variation in weight %, for both series are presented in Fig. 5c and d, respectively. The swelling of vulcanizates in an organic solvent such as toluene could indicate the extension of crosslinking and possible interactions between the matrix and the additive. On the basis of equilibrium swelling in toluene, it was observed that the incorporation of leather containing additives brought an increase in the crosslink density of the vulcanizates, which could indicate that leather takes part in the crosslinking of vulcanizates as observed with other systems [2, 3], This increase is considerably higher for the NBR MIXO composites than for the SBR MIXO ones which may be related to its active functional groups and contribute to explain the better physical properties of NBR based composites. From the second series of experiments it is evident that higher incorporations of UD leather based residue, either alone or in its mixture with SD, have a pronounced effect in the crosslink density increase.

When leather based additives were added, the swelling ratio values in toluene were considerably reduced. This can be due to the physical constraining role of leather at the elastomers matrix in the organic solvent. Thus, swelling tests suggest, too, that there is some reinforcing action connected with leather based additives.


The use of finished chromium tanned leather residues grinded to [less than or equal to]1 mm, as additives in SBR and NBR composites is possible with good results in production of shoe rubber soles. When these residues are added in the range of 10-15 phr originate composites that fulfill the day-today soles standard criteria defined by CTCP for materials with this application.

Furthermore, the residues from footwear roughing and carding operations, with any pretreatment could be recycled as day-to-day footwear rubber soles additive up to 20 phr incorporation.

Despite these wastes are generally considered as non-hazardous, chromium in leachates is sometimes above threshold value, so, they may be hazardous waste under the criteria for admission of wastes in landfills. However, when added to the elastomers, it works as a stabilization/ solidification binder and the composites obtained may be considered non-hazardous wastes at their end-of-life,


The authors acknowledge Portuguese Programs POE Mcdida 2.2 B and PRIME Medida 3.1 A, Projectos Mobilizadores, Projecto SHOEMAT n. 03/81, the rubber production company Procalcado--Produtora de Componentes para Calcado, S.A., particularly Dr. Jose Pinto for facilitating the use of lab, scale-up, and industrial materials and equipments that made the work possible and Eng. Rui Russo for interesting technical discussions about this subject, as well as CTCP and FEUP colleagues Dr. Isabel Santos and Eng. Silvia Pinho, for chemical tests and TOC and size distribution tests, respectively.


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Maria J. Ferreira, (1) Manuel F. Almeida (2), Fernanda Freitas (1)

(1) Centro Tecnologico do Calgado de Portugal (Portuguese Footwear Technological Centre), CTCP, Rua de Fundoes, Devesa Velha, Sao Joao da Madeira 3700-121, Portugal

(2) Faculty of Engineering, LEPAE, University of Porto, Rua Dr. Roberto Frias, Porto 4200-465, Portugal

Correspondence to: Maria J. Ferreira; e-mail:

DOI 10.l002/pen.21643

Published online Wiley Online Library (

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Author:Ferreira, Maria J.; Almeida, Manuel F.; Freitas, Fernanda
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EUPR
Date:Jul 1, 2011
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