butadiene-acrylonitrile rubber · nbr · natural fiber

PRÜFEN UND MESSEN – PMA 2017 TESTING AND MEASURING – PMA 2017

26 KGK · 05 2018 www.kgk-rubberpoint.de

Butadiene-acrylonitrile rubber · NBR · cellulose fillers · modification of cellulose · physical properties

The effect of powdered cellulose and two samples of its modification on the physical properties of rubber composi-tions were investigated. Pulps were modified by acetylation and plasma treatment in gas mixture air, argon and vapor of HMDSO. The properties of cellulosic fillers were studied by FTIR analysis, wetting and swelling kinetics. The results indicate that the major changes in the monitored parameters had pulp modified by acetylation. The main objective was to determine an optimal amount of filler from point of view of curing characteristics and physical-mechanical properties of rubber blends. The filler was applied in concentration scale from 10 to 50 phr and the properties of filled rubber blends were compared to those of reference, unfiled sample. The achieved results revealed the possibility to apply the selected fillers in real rubber blends.

Natürliche Faserfüllstoffe und ihr Einfluss auf die Eigenschaf-ten von Kautschuk Butadienacrylnitrilkautschuk · NBR · Zellulose-Füllstoffe · Modifizierung von Zellulose · Physikalische Eigenschaften

Es ist der Effekt von pulverförmiger Zellulose und zwei Typen einer Modi- fikation auf die physikalischen Eigen-schaften von Kautschukmischungen untersucht worden. Pulpe wurde durch Acetylierung und durch Plasmabehand-lung in Gasgemischen aus Luft, Argon und HMDSO-Dampf modifiziert. Die Eigenschaften der Zellulose-Füllstoffe wurden mittels FT-IR-Analyse, Benet-zungsverhalten und Quellkinetik stu-diert. Die durch Acetylierung modifi-zierten Pulpe zeigt die größten Ände-rungen der betrachteten Parameter auf. Hauptziel war es, den optimalen Füll-stoffanteil hinsichtlich der Vernetzungs-eigenschaften und der physikalisch- mechanischen Eigenschaften in Kaut-schukverschnitten zu ermitteln.

Figures and Tables:By a kind approval of the authors.

IntroductionIn the recent years is important topic, the search and application of new natural resources in the chemical industry. These resources are often the only by-product with advantageous price. The example of natural resources are lignin, starch, cel-lulose etc.

Cellulose is the most abundant renew-able biomass material in nature and is al-so major component of the plant cell walls. Cellulose is an almost inexhaustible polymeric raw material with fascinating structure and properties. The cellulose macromolecules are made up of repeat-ing D-glucose units is a hydrophilic linear biomacromolecule with promising biode-gradable and reinforcing properties(1, 2). The high degree of polymerization and number of potential chemical modifica-tions of cellulose also make it an attractive candidate for chemical industry and rub-ber products. To achieve the latter, howev-er, its microfibrillar (micronscale) or whisk-er (nanoscale) versions are preferred (3).

Cellulose fibers represent a potential for application in rubber blends as a filler primarily for their excellent environmen-tal impact and also for giving them a cer-tain more fibrous reinforcements skills in rubber composites (4,5,6,7). Applying pulp to the rubber composites requires the pur-poseful modification. Pulp fibers useful in rubber compositions should be hydropho-bic and therefore it should have a higher hydro stability and affinity to the rubber matrix. These properties can be relatively easy achieved to surface acetylation of cellulose fibers. Acetylation is one of the most interesting reactions which can im-prove lignocellulosic fibers to the polymer matrices. Acetylation of pulp fibers can be carried out in room temperature in solvent free system with acetic anhydride in presence of sulfuric acid as a catalyst. The effect of esterification on pulp struc-ture can be investigated by FTIR spectros-copy. Structural changes in fibers were monitored by SEM and hydrophobic/hy-drophilic character was determined by contact angle measurement (8).

The esterification of pulp fibres re-duces their hydrophilic character by re-placing hydroxyl groups by acetylic ones.

Acetylation can take place in a homoge-neous or heterogeneous phase with or without a solvent. The solvent-free acet-ylation is preferred because the solvent dissolves the reactants while reducing the reaction rate. Also, the use of solvent complicates the potential technology and exaggerates the process (6).

As is mentioned above the acetylation reaction can be easily investigating using FTIR spectroscopy. The structural chang-es of acetylated pulp can be investigat-ing by intense adsorption around 3300, 1740, 1366 a 1215-1230 cm-1 (5).

The wettability or surface energy changes can be measured by measuring the contact wetting angle. Various liq-uids may be used to determine the sur-face energy of substrates made from acetylated pulp (5). However, this meas-urement does not refer to the dimen-sional stability of acetylated fibres.

The aim of this work is to examine surface dependant properties three types of powdered cellulose and it influ-ence to the final properties of filled rub-ber blends. The main objective is to de-termine an optimal amount of filler from point of view of curing characteristics and physical-mechanical properties of rubber blends. The filler was applied in concentration scale from 10 to 50 phr and the properties of filled rubber blends were compared to those of reference, unfiled sample. The achieved results re-vealed the possibility to apply the se-lected fillers in real rubber blends.

Natural Fiber Fillers and their Influence on the Properties of Rubber

AuthorsŠtefan Šutý, Ivan Hudec, Petra Holičková, Bratislava, Czech Republic

Corresponding Author:Štefan ŠutýSlovak University of Technology in Bratislava, Faculty of Chemical and Food Technology, Institute of Natural and Synthetic PolymersRadlinského 9812 37 Bratislava, Czech RepublicE-Mail: [email protected]


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Experimental

MaterialsLignocellulosic fibers (NC) used in this study were powdered pulp GW400 F sup-plied by Greencell s.r.o, in Slovakia. The powdered pulp is producing dry process grounding from Kraft pulp fibers pro-duced Bukocel a.s.

Physical and chemical properties of pulp: cellulose content: 99,5 %, humidity less than 7 %, bulk: 70 g/L–90 g/L, ash (850 °C, 4 h): max. 0,5 %, colour: white, pH of water extract: 6 ± 1, mesh analysis (STN EN ISO 4610) >100 µm max. 5 %, < 32 µm min. 25 % (7).

The butadiene-acrylonitrile rubber NBR, under the trade name SKN 3375 (content of acrylonitrile 31-35 %), was processed in order to prepare a model rubber blends. Besides the processing additives and components of curing system, rub-ber compounds contained different amount of cellulose in three forms as filler. The content of applied filler varied from 10 to 50 phr. The rubber com-pounds were prepared in laboratory mix-er Brabender in two mixing steps. In the first step, the rubber, the processing ad-ditives and the fillers were mixed to-gether. In the second step, the curing system was introduced.

The tensile properties of cured rubber compounds were measured by using ZWICK ROELL/Z 2.5 appliance at cross-head speed of 500 mm/min at laborato-ry temperature in accordance with the valid technical standards.

Acetylation of pulp The acetylation of fibers with acetic an-hydride was carried out in heterogene-ous conditions, without solvent and us-ing sulfuric acid as a catalyst. A sample of a total weight of more than 500 g for incorporation into the rubber blends was prepared in two batches. The first batch was prepared using 200 g of pow-dered cellulose with 600 ccm of acetic anhydride with the addition of 2 ccm of sulfuric acid. The second batch was pre-pared using 300 g of powdered cellulose with 900 ccm of acetic anhydride with the addition of 2 ccm of sulfuric acid. The reaction time of the two batches was 15 minutes. The volume of added liquid (anhydride and sulfuric acid) was not sufficient to obtain viscous material. The fibers were only impregnated by the liquid phase. Acetylation was performed at room temperature (25°C), with occa-

sionally handmade mixing, for different reaction times. The reaction was termi-nated by the addition of 3 liters of water and the product was mixed in a mixer for about 2 minutes. The sample (ACC) was stripped of water by pressing and dried at 70°C in a dryer to the 5% of humidity.

Plasma treatment of pulpThe sample (ARC) was processed in pow-der plasma equipment at the Depart-ment of Applied Physics and Technology at the Faculty of Education at South Bo-hemia University in České Budějovice. For the plasma treatment were used microwave low pressure plasma dis-charge 500 W, pressure of gases were 100 Pa, mix of gases O2 / Ar / Air with flow 100 ccm/s with HMDSO vapour during 90 min.

Infrared SpectroscopyFourier transform infrared (FTIR) spec-troscopy was used to evaluate the effect of the reaction conditions for the chemi-cal structure of pulp fibers. To measure the FTIR spectra we used the Excalibur FTS 3000MX FTIR spectrophotometer.

Fig. 1: FTIR - spectra of pulp samples

1

Fig. 2: Kinetics of relative swelling of pulps

2

The spectral range of the instrument is 7800-400 cm-1. The instrument was cali-brated to air. The entire measurement was performed by ATR’s fully attenuated reflection technique.

Contact angle measurement For contact angle measurement were use SeeSystem equipped with the SeeSoftware 6.0 program to measure the water droplet contact angle. The sample was prepared in advance by placing product on the double-sided adhesive tape, pressing it through the hourglass and checking under the optical micro-scope of covering the entire surface of the tape. On this sample was drip the droplet of the water that was applied as a testing liquid. Measurement on one sample was repeated 4 times. Since the adhesive tape can affect the measured wetting effect, we have made further measurements on the tablets. The tab-lets were prepared in a press so that they were pressurized at constant pressure 20 MPa. The tablet shape diameter was about 13 mm. In this way, tablets were also prepared to determine swelling abil-ity in water.


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1 Measured characteristics of different type of pulpsSample NC ACC ARCContact angle ° 41 73 49Relative swelling (t = 1000 s), % 115 86 130Maximum absorbancy at 3300 cm-1 wavenumber 0,0318 0,0241 0,0322Maximum absorbancy at 1730 cm-1 wavenumber 0,0047 0,0420 0,0047Maximum absorbancy at 1220 cm-1 wavenumber 0,0158 0,0589 0,0158

Swelling kineticsThe degree of swelling was the next pa-rameter which was monitored. Used equipment consisted of a computer and program puci.exe, EDK93, 4 pcs of induc-tion dimensional sensors and a measur-ing vessel placed in a stabilization vessel. The device is usually used for the ability to swell the wood. Water was used as the swelling liquid.

The compressed tablets were placed under inductive sensors, their thickness was recorded and the device was adjust-ed to record the changes every second from the start of the measurement when the samples were poured with water. The program allowed us to monitor the rela-tive (%) and absolute change in thickness (mm) over time. To compare the swelling we took the swelling values at 1000 s.

Results and discussionResults of measured characteristics of differend type of pulps are summarized in Table1. From results it is conclude that the natural powdered cellulose has a very similar characteristics as a ARC cel-lulose treated with argon plasma. In con-trast, acetylated cellulose has different properties in all parameters compared to native cellulose.

FTIR spectra (Fig. 1) were evaluated by maximal signal bands characteristic for acetylation. Absorbent peaks between 3000 - 3500 cm-1 are OH-linked cellulose bonds. It is this area that serves to com-pare whether there has been a decrease in OH groups and their replacement by acetyl groups. The range of 3000 to 2800 cm-1 is saturated with a saturated hydrocarbon, namely CH and CH2 groups. The peak in the region of about 1650 cm-1 is attributed to the absorbed water. A peak in the range of about 1430 cm-1 belongs to the CH2 groups of cellulose. OH groups in addition to the range of 3000 - 3500 cm-1 also include a peak with a wavelength of 1335 cm-1. C-O linkages are from 1170 to 1100 cm-1, and the last characteristic area is C = C bonds and circular vibrations in cellulose aromatic nuclei (898 cm-1). The structural change of acetylated pulp is confirmed by the appearance of three peaks characteristic of the acetyl group. The spectra show an intense adsorption changes after acetylation near 1730 (C=O), 1360 and 1220 cm-1 (C-H bond bending in –O(C=O)-CH and C-O stretching of acetyl) wave number.

The swelling kinetics of pulps were measured on tablets immersed in water. For a simple comparison, we took into account swelling values at 1000s. From Fig. 2 we can see that the biggest swell-ing ability has a plasma treated pow-dered cellulose. A little bit lower values had a natural cellulose and lowest swell-ing ability had a acetylated pulp.

From Fig. 3 it is evident that the ap-plication of all natural fillers decreases optimum curing time tc90 of the pre-pared compounds in comparison with reference sample without filler. The more evident decline was recorded in case of cellulose (ACC) and (ARC). The change of optimum curing time of NBR rubber compounds in consequence of increasing amount of ARC was recorded too. In com-pounds containing native cellulose (NC), the optimum curing time reduces with the increasing filler content. By incorpo-ration of 10 phr of acetylated cellulose

Fig. 3: The influence of filler content on the optimum curing time of NBR rubber compounds

3NBR + NC NBR + ACC NBR + ARC

content of filler [phr]

0 10 20 30 40 50

tc(9

0) [m

in]

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35

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Fig. 4: The influence of filler content on the tensile strength at break of NBR vulcanizates

4NBR + C NBR + ACC NBR + ARC


0 10 20 30 40 50

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ile st

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th a

t bre

ak [M

Pa]

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(ACC), the optimum curing time of NBR vulcanizates decreased, but with next increasing of filler content, the values of tc90 fluctuated in low range, almost in-dependently on the amount of acetylat-ed cellulose (ACC) incorporated.

The obtained values of physical-me-chanical properties of prepared compos-ites are illustrated in Fig. 4 and 5. The tensile strength at break of NBR vulcani-zates showed the decreasing tendency with increasing amount of ACC and ARC types of cellulose in comparison with reference sample as shown in Fig. 2. In the case of vulcanizates based on NBR, higher values of tensile strength at break were achieved by applying of native cel-lulose (NC) up to 20 phr filler compared to reference sample. With application of higher content of cellulose in NBR blends was recorded decreasing of tensile strength at break with minimum at the content of 30 phr filler.

As seen in Fig. 5 the all type of applied cellulose filler was caused almost linear decrease of elongation at break with in-creased amount of cellulose, only with content of 10 phr filler was recorded higher value in comparison with refer-ence sample.

Deterioration of physical-mechanical properties is probably due poor homoge-nization of natural fillers in rubber mat-rix based on NBR.

ConclusionFrom the results we can conclude that the modification of powdered cellulose by acetylation significantly changes the surface properties of the fibers toward hydrophobicity. This change by acetyla-tion significantly affects swelling ability in comparison with unmodified cellu-lose. Plasma powder treatment under argon and a pair of HMDSO do not lead to significant changes in surface energy and swelling.

The results achieved by study revealed that the increasing content of natural fillers in chosen rubber matrix leads to the deterioration of physical-mechanical properties. The reason might be attrib-uted to the structures of natural fillers and the presence of hydroxyl groups on the surface of cellulose (C), which tend to form intra- and intermolecular hydrogen bonds. This leads to the forming of ag-gregates and agglomerates of filler parti-cles in the rubber matrix, what to a large extent contribute to the deterioration of observed properties. Weak mutual inter-actions and adhesion between the rub-

ber matrix and the particles of fillers are next reason, why natural fillers do not act as reinforcing fillers in tested rubber systems. The results revealed that better properties of composites were achieved at lower contents of cellulose (NC), up to 20 phr. The efficient methods of modifi-cation should be developed in order to improve the compatibility and adhesion between the rubber matrix and the par-ticles of natural fillers, for broader utili-zation of such materials.

AcknowledgementThis work was supported by the Slovak Research and Development Agency un-der the contract No. APVV-¬0694¬-12, APVV-14-0393, APVV-14-0301, APVV-15-0052, by the VEGA Grant No. 1/0848/17, and by Ministry of Education of Slovak Republic project No. 26210120008 by the Research & Devel-opment Operational Program funded by the ERDF.

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Fig. 5: The influence of filler content on the elongation at break of NBR vulcanizates

5NBR + C NBR + ACC NBR + ARC


0 10 20 30 40 50El

onga

tion

at b

reak

[%]

500450400350300250200150100

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butadiene-acrylonitrile rubber · nbr · natural fiber

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