The Tribologic and Thermomechanic Properties of Polypropylene Filled with CaCO 3 and Anhydrous Borax EROL FEYZULLAHOG ˘ LU* AND TU ¨ LIN SAHIN Department of Mechanical Engineering, Kocaeli University, Kocaeli, Turkey ABSTRACT: There are few studies showing the industrial application of boron compounds. In this study, boron compounds are used as fillers in plastic materials in order to increase the use of these raw materials. The effect of anhydrous borax mineral in the thermomechanical and tribologic properties of both calcium carbonate (CaCO 3 ) unfilled and CaCO 3 filled polypropylene (PP) mate- rial is investigated. Wear and thermomechanical properties of specialized PP material containing additives are examined by using pin-on disc wear tester, DSC, TMA and Shore D hardness testers. This study is the first to use boron (anhydrous borax) compounds as an additive in industrial plastics. As a consequence of the increase in the amount of boron in the samples, the friction coefficient also increases. An increase in the amount of CaCO 3 used as an additive in the samples containing the same percentage of boron reduces the friction coefficient. Using anhydrous borax and CaCO 3 as additives in a natural PP material did not create a considerable change in the crystalline melting temperature of the material. Weight loss increases for anhydrous borax, while it decreases for CaCO 3 compared with natural PPB. If the amount of anhydrous borax is increased, the weight loss increases too. The minimum weight loss is seen for 10% CaCO 3 . This increases with an increas- ing amount of CaCO 3 and is close to natural PPB. KEY WORDS: tribology, thermomechanical properties, polypropylene, anhydrous borax, CaCO 3 . INTRODUCTION P OLYPROPYLENE (PP) IS an industrial plastic, which has a wide usage in industry due to its easy workability, high strength, and high wear resistance [1,2]. The wear resistance of PP is lower than that of other engineering plastics, such as high density polyethylene and nylon [3]. The tribologic and wear behaviors of plastic materials are all important within machine elements, such as gears, journal bearings, and sledges. Tribologic behaviors of PP, which is a thermoplastic material, differ from that of metals. Thermoplastics have lower static friction coefficient than dynamic friction coefficient. This gives the feature of ‘sliding/adhesion’ or discontinuous sliding behavior to the thermoplastics working together with other metal or plastics [4]. The increase of friction and wear depends on the surface energy. When the load arises a limit value, this increase occurs. Increase of temperature on the contact surfaces causes the molecule chains of polymer to relax [5]. *Author to whom correspondence should be addressed. E-mail: [email protected]Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 29, No. 16/2010 2498 0731-6844/10/16 249815 $10.00/0 DOI: 10.1177/0731684409355200 ß The Author(s), 2010. Reprints and permissions: http://www.sagepub.co.uk/journalsPermissions.nav
Transcript
The Tribologic and ThermomechanicProperties of Polypropylene Filled with
CaCO3 and Anhydrous Borax
EROL FEYZULLAHOGLU* AND TULIN SAHIN
Department of Mechanical Engineering, Kocaeli University, Kocaeli, Turkey
ABSTRACT: There are few studies showing the industrial application of boron compounds. In thisstudy, boron compounds are used as fillers in plastic materials in order to increase the use of theseraw materials. The effect of anhydrous borax mineral in the thermomechanical and tribologicproperties of both calcium carbonate (CaCO3) unfilled and CaCO3 filled polypropylene (PP) mate-rial is investigated. Wear and thermomechanical properties of specialized PP material containingadditives are examined by using pin-on disc wear tester, DSC, TMA and Shore D hardness testers.This study is the first to use boron (anhydrous borax) compounds as an additive in industrialplastics. As a consequence of the increase in the amount of boron in the samples, the frictioncoefficient also increases. An increase in the amount of CaCO3 used as an additive in the samplescontaining the same percentage of boron reduces the friction coefficient. Using anhydrous borax andCaCO3 as additives in a natural PP material did not create a considerable change in the crystallinemelting temperature of the material. Weight loss increases for anhydrous borax, while it decreasesfor CaCO3 compared with natural PPB. If the amount of anhydrous borax is increased, the weightloss increases too. The minimum weight loss is seen for 10% CaCO3. This increases with an increas-ing amount of CaCO3 and is close to natural PPB.
POLYPROPYLENE (PP) IS an industrial plastic, which has a wide usage in industry dueto its easy workability, high strength, and high wear resistance [1,2]. The wear resistance
of PP is lower than that of other engineering plastics, such as high density polyethylene andnylon [3]. The tribologic and wear behaviors of plastic materials are all important withinmachine elements, such as gears, journal bearings, and sledges. Tribologic behaviors of PP,which is a thermoplastic material, differ from that of metals. Thermoplastics havelower static friction coefficient than dynamic friction coefficient. This gives the featureof ‘sliding/adhesion’ or discontinuous sliding behavior to the thermoplastics workingtogether with other metal or plastics [4]. The increase of friction and wear depends on thesurface energy. When the load arises a limit value, this increase occurs. Increase oftemperature on the contact surfaces causes the molecule chains of polymer to relax [5].
*Author to whom correspondence should be addressed. E-mail: [email protected]
Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 29, No. 16/2010 2498
0731-6844/10/16 2498�15 $10.00/0 DOI: 10.1177/0731684409355200� The Author(s), 2010. Reprints and permissions:http://www.sagepub.co.uk/journalsPermissions.nav
The wear of UHMWPE is dependent on the surface temperature. When the temperatureexceeds a critical value, wear proceeds in a series of discrete steps caused by the sudden lossof a molten or softened layer of polymer [6]. When friction coefficient is lower, less frictionheat will be produced; therefore the surface temperature will be lower, which indicates betteranti-wear properties [7]. The end product costs and product quality are improved by addingmineral additives, such as calcium carbonate (CaCO3), talc, and barium sulfate (BaSO4) toPP materials. It is known that PP material affects all physical and mechanical properties aswell as the amount of the filler other than its type [8]. Sole and Ball investigated the effects ofmineral fillers on the wear resistance of PP. They used talc, CaCO3, and BaSO4 as mineralfillers. The addition of mineral fillers to the PP decreases the wear resistance under severeabrasive conditions [9]. The mechanical and thermal properties of polymers can be greatlyimproved by the addition of inorganic additives. The PPs with different additives are widelyused to make packing and architectural material, because of their high strength/densityratio. The strength of PP can be enhanced by filling with CaCO3 particles [10]. Numerousstudies have been performed on the wear properties of PP composites by considering thetraditional fillers, such as non-ferrous metallic powders, kaolin, and some oxides.The kaolin fillers improve tensile strength of polymer materials [11]. It is widely knownthat the tribologic behaviors of polymers can sometimes be greatly improved by filling themwith inorganic particulate compounds. Kaolin filled at certain content is effective to reducefriction and wear of polymer materials in sliding against steel [12].
Addition of kaolin particulates to PTFE causes an increase in wear resistance [13].Nedorezova and Tsvetkova investigated the properties of PP filled boron nitride. Intheir study, the strength of materials is increased and friction properties are improved [14].
Nowadays, industrial application of studies as-built with boron compounds is consid-erably low. In this study, boron compounds are used as filler in plastic materials in orderto increase the use of these raw materials. In literature, there are very few studies regardingthe use of boron compounds as an additive in plastic materials.
The effect of anhydrous borax mineral on the thermomechanical and tribologic proper-ties of both CaCO3unfilled and CaCO3 filled PP material was investigated. Wear andthermomechanical properties of specialized PP material containing additives are examinedby using pin on disc wear tester, DSC, TMA, and Shore D hardness testers. This study isconsidered as a new one, which uses boron (anhydrous borax) compounds as additive inindustrial plastics.
EXPERIMENTAL STUDY
Material
In this study, polypropylene block copolymer (PPB) produced in BEC5015 trade nameof Borealis and in its natural color and granular state is used as a basic polymer material(http://www.borealisgroup.com). The important physical and mechanical properties ofsamples which are produced with the injection molding method are shown in Table 1.CaCO3 mineral filler, which is produced as granulated in Imercab-2XG trade nameby Mikromineral, is preferred when obtaining mineral filled PPB raw materials(http://www.mikromineral.com.tr). The basic characteristics of CaCO3 mineral filler arepresented in Table 2. In this study, green anhydrous borax mineral is used by pulverizing itto a mean particle diameter of 5 mm.
Properties of Filled Polypropylene 2499
Preparation of Samples
Double driven screw extrusion, which turns parallel to each other, was used when produ-cing the samples. Samples were prepared by adding CaCO3 in 10%, 15%, and 20% byvolume (25%, 34.62%, and 42.86% by weight) and anhydrous borax mineral filler in 0.5%,1%, and 2% by weight to PP materials when extruding as shown in Table 3. ArburgAllrounder 370 CMD model injection machine was used to prepare the samples.
Table 3. Anhydrous borax__CaCO3 mixing list.
By volume CaCO3 By weight anhydrous borax
0 % 0%0.5%1%2%
10 % 0%0.5%1%2%
15 % 0%0.5%1%2%
20 % 0%0.5%1%2%
Table 1. The physical and mechanical properties of PPB.
Properties Test method Test conditions Unit system BEC5015 natural
Tensile yield stress ISO 527-2 50 mm/min MPa 30Tensile yield strain ISO 527-2 50 mm/min % 10Tensile modulus of elasticity ISO 527 1 mm/min MPa 1500Charpy impact (notched) ISO 179/1eU +23�C kJ/m2 70Charpy impact (notched) ISO 179/1eU �20�C kJ/m2 7Hardness ISO 2039-2 Rockwell-R scale 72Melt flow index ISO 1133 230�C / 2.16 kg g/10 min 0.3Density ISO1183 g/cm3 0.9
Table 2. Basic characteristics for CaCO3 mineral filler.
Properties Test method Test conditions Unit system Mineral material
CaCO3 ratio � � % 99.28Mean particle diameter (D 50%) � � mm 2.15Maximum particle diameter (D 97%) � � mm 11.6Density ISO787/10 � g/cm3 2.7Brightness DIN 53163 Ry, C/2� % 97�98
2500 E. FEYZULLAHOGLU AND T. SAHIN
Wear Experiments
The samples with a 12� 12� 3mm3 dimension were prepared for wear tests. TE 53 Slimmodel pin-on disc wear testing machine was used for analysis of friction and wearbehaviors of test samples (Figure 1). The wear tests were performed with 5 N load and1.57m/s sliding speed in accordance with ISO 4649. The disc of wear testing device had a
diameter of 60mm and was produced of 95 HRB hardness SAE 1040 steel. Wear valueswere obtained as weight loss (mg) at the end of 4 h. Wear rates and friction coefficientswere calculated. In wear tests, 22.6 km sliding distance was achieved at the end of 4 h.In wear tests, weights of samples were measured with Precisa 125 A precision balance.The balance has 0.0001g precision. Tests were carried out in 22�C and in a 50% RH.Wear rate was estimated by the formula [7]:
Ws ¼�W
�� L
where Ws is the wear rate (cm3/m), �W is the weight loss (g), q is the density (g/cm3)q=0.9 g/cm3 (density of PP), L is the sliding distance (m).
DSC Experiments
Weight samples of 10mg were prepared from each test group. Differential scanningcalorimeter (DSC) test was started at 25�C and terminated at 220�C with a heating rateof 10�C/min and nitrogen was used as the inert gas according to ISO 3146 standard.
Sample
Load
Sample clip
Sample
Steel disc
Figure 1. Pin on disc wear test.
Properties of Filled Polypropylene 2501
Crystalline melting temperatures of samples and crystallizing degrees were determined inpercentages (% c) by using the test equipment trademark of Metller Toledo DSC1.
TMA Experiments
Samples of dimensions 6� 6� 3mm3 were analyzed by heating from 25�C to 170�C at arate of 5�C/min and using 0.5 gram weight in thermomechanical analyzer (TMA) test.The coefficient of thermal expansion (a) and the glass transition temperatures (Tg) of thesamples were acquired using a thermomechanical analyzer trademark of TMA-50Shimadzu in ASTM D 1545 and ASTM E 1363 standards.
Shore D Experiments
Shore D hardness tests of the samples were carried out using Zwick 3100 Type Dhardness tester according to ISO 868 standard.
SEM Examinations
Wear surfaces of the samples resulting from wear test were examined with a SEM testertrademark of Jeol 6060.
Wear Results
The relationship between sliding distance and weight loss in natural PP materials mixedwith anhydrous borax and CaCO3 is shown in Figure 2. As the sliding distance increases,
Figure 9. The sliding distance�wear rate in the PP material containing anhydrous borax mixed with20% CaCO3.
2506 E. FEYZULLAHOGLU AND T. SAHIN
The values of the samples containing high percentages of CaCO3 are close to the valuesof natural materials. As the amount of anhydrous borax in the samples containing anhy-drous borax increases, the wear rate also increases. As a consequence of the increase in theamount of anhydrous borax in the samples, the friction coefficient also increases. In thesamples containing anhydrous borax in the same percentages, the increase in CaCO3 thatwas used as an additive reduces the friction coefficient (Figure 10).
DSC Results
Figure 11 shows that using CaCO3 as an additive in the natural material reduces thecrystallizing degree of the material. Using additives such as anhydrous borax and CaCO3
in the PP material as shown in Figure 12 did not create a remarkable variation in crys-talline melting temperature of the samples. Since using additives does not affect the crys-talline melting temperature, the resultant temperature increase also does not affect thephysical properties of the material. During wear, the temperature of the contact areaincreases, the bond structure of the material impairs, and as a result of these the dissocia-tions occur in the material [15]. Reaching high melting temperatures enhances the thermalstability. In this study, acquired DSC results demonstrate the validity of this stability [1].
TMA Results
Figure 13 shows that no considerable changes have occurred to the Tg of the samplescontaining anhydrous borax mixed with CaCO3 depending on the percentage of boron.
0.70
0.75
0.80
0.65
0.60
0.55
0.50
0.45
Fric
tion
coef
ficie
nt
0.40
0.35
0.30
0.25
0.20
0 0.5 1
Anhydrous borax (%)
2
0% CaCO3
10% CaCO3
20% CaCO3
Figure 10. The relationship between the amount of anhydrous borax and the friction coefficient in thesamples.
Properties of Filled Polypropylene 2507
100
110
120
90
80
70
60
50
Cry
stal
lizin
g de
gree
(J/
G)
(%)
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00 0.5 1
Anhydrous borax (%)
2
0% CaCO3(Natural)
10% CaCO3
15% CaCO3
20% CaCO3
Figure 11. Anhydrous borax�crystallizing degree relationship in the PP material containing anhydrous boraxand CaCO3.
106
108
110
104
102
100
98
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Cry
stal
line
mel
ting
tem
pera
ture
(°C
) (%
)
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9010
0
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94
92
9010
0
0 0.5 1
Anhydrous borax (%)
2
0% CaCO3 (Natural)
10% CaCO3
15% CaCO3
20% CaCO3
Figure 12. Anhydrous borax�crystalline melting temperature relationship in the PP material containinganhydrous borax and CaCO3.
2508 E. FEYZULLAHOGLU AND T. SAHIN
In Figure 14, as the amount of anhydrous borax increases, a considerable decrease occursin the coefficient of thermal expansion of the sample unfilled with CaCO3. The thermalexpansion coefficient has been increased depending on the percentage of the boron in thesample mixed with 15% CaCO3 (Figure 14).
Hardness Results
The change in shore D hardness values in the PP samples containing CaCO3 dependingon the anhydrous borax amount used as an additive over natural samples are given inpercentages in Figure 15. In the examination, it was seen that the shore D hardness reducesin case of adding anhydrous borax in natural PP, the shore D hardness increases in PPmixed with CaCO3 depending on the CaCO3 amount (Figure 15).
SEM Results
The scanning electron micrographs under conditions for same load and same slidingdistance of both unmixed and mixed samples are shown in Figures 16 and 17. On the wearsurfaces of natural samples, more evident breakings are seen than in the mixed samples.Re-breakings on the wear surfaces with more than 10% CaCO3 filler are also shown inFigure 16(c).
However, these breakings are not seen on the wear surface of the sample containing 2%anhydrous borax as an additive due to good adhesion. No evident dissociations are seen
175
170
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155
Gla
ss tr
ansi
tion
tem
pera
ture
(T
g)
150
145
140
0
0 0.5 1
Anhydrous borax (%)
2
0% CaCO3
10% CaCO3
15% CaCO3
20% CaCO3
Figure 13. Glass transition temperature change depending on the amount of anhydrous borax mixed withCaCO3.
Properties of Filled Polypropylene 2509
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The
rmal
exp
ansi
on c
oeffi
cien
t (10
–6/°
C )
0 0.5 1
Anhydrous borax (%)
2
PPB+15% CaCO3+Anhydrous borax
PPB
Figure 14. The thermal expansion coefficient depending on the boron amount of the natural PP materialcontaining anhydrous borax filled with CaCO3.
50
80
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60
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0
Sho
re D
har
dnes
s
0 0.5 1
Anhydrous borax (%)
2
0% CaCO3
10% CaCO3
15% CaCO3
20% CaCO3
Figure 15. The shore D hardness�anhydrous borax amount relationship in the PP material mixed with CaCO3.
2510 E. FEYZULLAHOGLU AND T. SAHIN
according to the natural materials as shown in Figure 17(b) and (c) due to the effect ofadding both anhydrous borax and CaCO3 filler.
CONCLUSIONS
(1) The effects of anhydrous borax and CaCO3 on the tribological properties of PPB aredifferent. The weight loss is increased for anhydrous borax while it is decreased forCaCO3 compared with natural PPB. If the amount of anhydrous borax is increased,the weight loss is increased too. The minimum weight loss is seen for 10% CaCO3. It isincreased with increasing the amount of CaCO3 and closed to natural PPB.
(2) As the amount of boron in the samples containing only anhydrous borax increases, thewear rate also increases. The increase in the amount of CaCO3 also increases the wearrate values generally.
(3) As a consequence of increase in boron amount in the samples, the friction coefficientalso increases. Increase in the amount of CaCO3 used as an additive in the samplescontaining the same percentage of boron reduces the friction coefficient.
(4) Using anhydrous borax and CaCO3 as an additive in a natural PP material does notcreate considerable change in the crystalline melting temperature of the material.
(5) There has been no considerable change in the Tg of the samples containing anhydrousborax mixed with CaCO3.
(6) As the amount of anhydrous borax increases, considerable change in the thermalexpansion coefficient of the sample unfilled with CaCO3 is seen. An increase is seenin the thermal expansion coefficient depending on the boron percentage in case of thesample mixed with 15% CaCO3.
Figure 17. The scanning electron micrographs of the wear surfaces of the samples containing anhydrousborax and CaCO3: (a) 2% anhydrous borax, (b) 10% CaCO3 and 2% anhydrous borax, and (c) 15% CaCO3 and2% anhydrous borax.
Figure 16. The scanning electron micrographs of the wear surfaces of the natural and mixed with CaCO3
(7) In case of adding anhydrous borax in the PP material unmixed with CaCO3, it isdetermined that the shore D hardness decreases; in case of PP material mixed withCaCO3 depending on the CaCO3 amount, the shore D hardness increases.
(8) When the scanning electron micrographs of the wear surfaces of natural samples wereinvestigated, more breakings were observed than in the mixed materials. However,these breakings on the wear surface of the sample containing 2% anhydrous borax asan additive are seen due to being a good adhesion on the surface of the sample.
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