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Preparation and properties of fly ash/ reclaimed rubber powder composites.

With the development of the car industry, more and more rubbers are needed, but the supply of the raw rubber fails to meet the demands in China. Moreover, it is mainly imported (ref. 1). In addition, the rubber price has been increasing in recent years. The price of natural rubber (NR) ranges from several thousands to thirty thousand Yuan for one ton (ref. 2). Because the content of the exploited crude oil is less and less, the production of synthetic rubber descends, and its price ascends. Therefore, the attention is focused on the utilization of used rubber at present (refs. 3-7).

Various damaged tires can be found in the world, and people have already turned their attention to environmental contamination (ref. 8). However, there are two approaches to utilize used rubber: The former is to reclaim rubber, the latter is to reclaim rubber powder (RP) (ref. 9). Because the used rubber is the thermoset polymer, it is difficult to degrade (ref. 10), the cost of the reclamation and utilization is very high, and it is technically difficult (ref. 11). Developed countries have discarded reclaimed rubber since the 1970s, and they are mainly engaged in producing RP. More and more RP is produced in China now.

Similar to the utilization of RP, more countries have realized that exploiting industry-used residue is important (refs. 12-14), and it may effectively relieve the shortage of natural and energy resources, and prevent environmental pollution (refs. 15 and 16). China is one of the largest coal-producing countries in world, and coal will be the major energy in the future (ref. 17). Consequently, making use of fly ash (FA) is of significant interest at present, because the focus is on recycling materials and decreasing environmental contamination. This research is based on using two industrial used materials, that is, RP was blended with FA and silane coupling agent to make new composites, where FA was used as reinforcement and filler in the composites (refs. 18 and 19), but the silane coupling agent 3-aminopropyltriethoxysilane (KH-550) was used as a crosslinking agent. The FA/RP composite material is featured in the excellent performance.




As industry products, RP and FA were produced by Chongqing Maolin Industry Trade Ltd. and Qiqihar power plant respectively. RP is fine rubber powder with particle diameter less than 0.05 mm from used tread rubber (cis-polybutadiene rubber). FA is fine fly ash with particle diameter less than 0.05 mm (60% Si[O.sub.2], 25% [Al.sub.2][O.sub.3], 7% [Fe.sub.2][O.sub.3], 5% CaO). Silane coupling agent of grade KH-550 (3-aminopropyltriethoxysilane, [H.sub.2]NC[H.sub.2]C[H.sub.2]C[H.sub.2]Si-[(O[C.sub.2][H.sub.5]).sub.3]) was supplied by Nanjing Shuguang Chemical Factory. Aluminate coupling agent of grade DL-441 was supplied by Polymer Experimental Factory of Fujian Normal University. Dicumyl peroxide (DCP), a pure chemical, was supplied by Shanghai chemical reagents company of Chinese medicine group. The other ingredients, such as sulfur, stearic acid (SA) and so on, were all common commercially available materials on the market.

Preparation of samples

RP and FA were dried previously at 100[degrees]C for 30 minutes (the moisture content is no more than 0.5%), then placed, and mixed with the ingredients in a high speed mixer at 45-50[degrees]C for 20 minutes. After that, the mixed rubber was prepared by adding sulfur, DCP and coupling agent on a XK-160 two-roll mill for 15 minutes at 55-60[degrees]C, while the nip gap was about 4.0 ram. The mixtures of different compositions were molded into the vulcanized rubber in an electrically heated hydraulic press (XLB-D350x350) at 150[degrees]C for 25 minutes under 8 MPa pressure. Then, the vulcanized rubbers were cut into the appropriate samples. The sequence of reactions involved in the KH-550 coupling agent modified FA/RP is given in scheme 1.



Tensile tests were performed on dumbbell-shaped specimens according to ISO 37-1994 at 100 mm/minute. Durometer A hardness was measured on the thickness of 6 mm according to ISO 48-1994. Wear attrition was determined according to BS903A9 by using an Akron Abrader machine (MN-74). The specimens are made by using a wheel cutter. The vulcanized rubbers were fractured in liquid nitrogen, then the fractured surface was sprayed with gold, and the fracture morphologies of the blending samples were observed by a JSM-5600LV (JEOL Co.) scanning electron microscope (SEM). Heat aging of the samples was performed by a 401-B air aging oven at 200[degrees]C for 24 hours (Jiangsu Test Mechanical Ltd.) according to ISO 188-1998. The IR spectrum of the blends were recorded at a resolution of 4,000 [cm.sup.-1] on an IR-7685 Fourier transforms infrared (FTIR) spectrometer (Shanghai analyzer plant). For each of the measurements, an average of at least five readings was taken. Errors in the measurement of mechanical and thermal properties were 10% and 11%, respectively.

Results and discussion

Effect of fly ash content on the composites properties

In Table 1, with increasing amount of FA, tensile strength, durometer A hardness and wear of the composites gradually increase, and FA performs the function of filler and reinforcement in the composites, but elongation at break is in downturn.

Since FA and RP blended on a two-roll mill are too difficult to prepare the sample, it does not show results when FA is more than 35 phr. In addition, the change of hardness before and after hot aging (200[degrees]C x 24 hours) becomes less, more change is seen in tensile strength, elongation at break and wear, and it seems the best turning point is when FA is 25 phr. The more the property change before and after hot aging is, the worse thermal stability is. Thinking over thermal properties, 25 phr fly ash is the more suitable. The SEM photos of tensile fracture of the FA/RP composites are shown in figure 1.

When fly ash content is too little to completely fill into the rubber, the rubber is not adsorbed as well on the surface of fly ash. More holes in figure 1 (a) illuminate the worse filling effect. When used at 25 phr, fly ash is shown in figure l(b), where the interfaces between FA and RP are not clear, have fewer holes and show better compatibility. It suggests that the fly ash fully fills into the rubber and leads to better adsorption, and it improves mechanical properties of the FA/RP composites. When 30 phr FA is used, as in figure l(c), FA and RP present respective phases and worse compatibility, so the mechanical properties are bad. This further validates the results in table 1. When FA content is 35 phr, there is too much FA in the composites, which makes the fluidity and viscosity of the composites rapidly descend. Moreover, there is difficulty blending and bubbles in the composites appear from the outside. There, the processing is quite difficult.

Effect of coupling agent on composites properties

FA is inorganic material, and RP is organic material, so they are difficult to blend. As shown in table 1, the tensile strength and elongation at break of the samples are low. The coupling agent was introduced in order to improve their crosslinking. There are two categories of coupling agent in the rubber industry, aluminate and silane coupling agent, and their effects on the properties of composites are shown in table 2.

The molding process of the composites is slower with aluminate coupling agent, but the samples with silane coupling KH-550 mold fast and the properties of the vulcanized rubbers are better than the former. It can be seen that the trimethoxy groups [(O[C.sub.2][H.sub.5]).sub.3] in the silane coupling KH-550 easily bond with Si[O.sub.2] in fly ash to form a stable chemical construction, so that FA/RP composites use silane coupling KH-550 as the coupling agent. As seen in table 2, the mechanical properties of the samples with the coupling agent are better than those without, and also better than pure rubber powder (as seen in table 1). It suggests that the coupling agent with two functional groups can crosslink FA and RP to form a chemical bond and physical entanglement. In addition, the property changes before and after aging (200[degrees]C x 24 hr.) are less than the change of those without silane coupling agent. It suggests that the thermal properties are improved.

As more silane coupling agent KH-550 is added in the recipe, hardness of the samples gradually increases, but the tensile strength, elongation at break, wear and thermal properties show the best result when KH-550 is 2 phr. It can be seen that the redundant ethoxies of the silane coupling agent KH-550 would decompose at high temperature, which makes them bond with RP, and results in the macromolecule chain of the rubber breaking down (ref. 20). So, when the coupling agent level is too high, some does not take effect, and it makes hardness of the samples gradually increase, and other properties firstly increase, and then decrease.

Effect of the ingredients addition order on composites' properties

The order of adding ingredients affects the FA/RP composites' properties, as seen in table 3. The mechanical and thermal properties were the worst when all ingredients were added together, as in sample A, sample C was next, and sample B was the best. It can be seen that the decomposed ethoxy groups of the coupling agent would be bonded with curing accelerator in sample A, the curing accelerator is not as effective, and the mechanical and thermal properties decrease. Moreover, the free radical sulfur is self-cured when the sulfur and accelerator M were added together. When the curing agent and accelerator M were respectively mixed with the rubber, such as sample B, the curing action would be notable, and curing agent decomposes to crosslink with rubber. In summary, it is the best adding order that the accelerator M and SA are firstly added, then sulfur and DCP, and KH-550 finally.

Influence of cure condition on composite properties

The vulcanization is an absolutely necessary and important process for the rubber processing. The cure conditions are primarily temperature, pressure and time (that is to say, three parameters).

As shown in table 4, when the cure temperature is 130[degrees]C, the mechanical and thermal properties are lower. As the cure temperature increases, hardness, tensile strength, elongation at break, wear resistance and thermal properties increase. The mechanical and thermal properties are the best at 145[degrees]C. Since the lower cure temperature of 130[degrees]C results in a lack of vulcanization, RP and FA don't blend completely. The higher cure temperature of 160[degrees]C leads to over-cure, crosslinked networks are broken down, thus the properties decrease.

The composites' properties show a turning point at the curing pressure of 9 MPa, and the worst at 7 MPa, because the pressure is too small to press rubber into a mold and make samples delaminate. But, too much pressure accelerates the sample breaking down at 10 MPa, so it affects mechanical and thermal properties.

In addition, when the cure time increases, hardness of the vulcanized rubber increases, but tensile strength, elongation at break, wear and thermal properties show a turning point for a cure time of 40 minutes. However, the properties were worse when the cure time was 15 minutes or 50 minutes. This is because the PR can't crosslink with FA for the lacking time of 15 minutes, but the crosslinked bonds are broken down and an over-cure phenomenon appears at over 40 minutes. Therefore, the best cure conditions are at 145[degrees]C for 40 minutes under 9 MPa pressure.


IR analysis of FA/RP

Infrared (IR) spectra of reclaimed rubber powder with (b) and without fly ash (a) are shown in figure 2. Both spectra presented are very different below 2,000 [cm.sup.-1]. The spectrum of the FA/RP composite (b) shows not only Si-O new stretching peaks at 1,079.41 [cm.sup.-1] and 1,030.41 [cm.sup.-1], but also three stretching peaks of C-H at 900-600 [cm.sup.-1]. This suggests that the RP and the organic group of the silane coupling agent form the crosslinking bonds C-H. The two curves mainly differ because of crosslinking bonds with fly ash. In addition, both present the stretching peaks of hydroxyl O-H at 3,400 [cm.sup.-1]. The peak without fly ash was larger than that with, because hydrolyzing the silane coupling takes some of the water, and it shows the characteristic peak at 3,400 [cm.sup.-1] for the O-H.


The results obtained in this work show that RP can be reinforced with FA; but an overdose of FA should not be added, because it would make blending difficult and it cannot form samples. Moreover, the silane KH-550 was used as a coupling agent. That the coupling agent crosslinked with FA and RP was proved by IR spectrogram, and the formation of Si-O and C-H bonds improves the compatibility and processing characteristic of the FA/RP composites. The order of adding ingredients affects composite properties; the best order is that the accelerator M and SA firstly are added, then sulfur and DCP, and KH-550 finally. The best mass ratio and curing condition of the FA/RP composites was 25/100, and 145[degrees]C for 40 minutes under 9 MPa pressure. The mechanical and thermal properties of FA/RP composites were improved obviously with KH-550 coupling agent. The results encourage the use of FA and RP as new filler and matrix for the composites. This will reduce the cost of composites, prevent environmental contamination and cut down landfill area required for the disposal of FA and RP.


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by Weili Wu, Qiqihar University, China (
Table 1--effect of fly ash contents on composites properties


Fly ash/phr 0 10 20 25 30 35
Durometer A hardness 80 89 92 92 93 -
Tensile strength MPa 3.6 3.3 3.6 4.2 4.0 -
Elongation at break % 329 237 210 201 140 -
Wear ([cm.sup.3]/1.6 km) 1.3 1.8 2.1 2.1 2.40 -
Change before and after aging (200[degrees]C 24 hr.)
Durometer A hardness 1 -1 0 1 -3 -
Tensile strength, % -57 -48 -28 -24 -53 -
Elongation at break, % -56 -50 -48 -42 -45 -
Wear, % 1.3 2.0 2.2 2.0 2.4 -

RP 100, DCP 2, SA 4, accelerant M 1.5,
antioxidant D 2, sulfur 1.5

Table 2--effect of coupling agent on composites properties

 Aluminate coupling agent/phr

Sample 1.0 2.0 3.0
Durometer A hardness 88 89 90
Tensile strength, MPa 8.1 8.6 8.4
Elongation at break, % 266 301 289
Wear ([cm.sup.3]/1.6 km) 1.4 1.4 1.3
Change before and after aging (200[degrees]C x 24 hr.)
Durometer A hardness 2 2 1
Tensile strength, % -36 -28 -35
Elongation at break, % -47 -33 -40
Wear, % 2.2 1.9 1.8

 Silane coupling agent KH-550/phr

Sample 1.0 1.5 2.0 2.5 3.0
Durometer A hardness 90 92 92 94 95
Tensile strength, MPa 8.6 8.9 9.5 9.1 9.0
Elongation at break, % 340 360 389 337 320
Wear ([cm.sup.3]/1.6 km) 1.1 1.0 0.8 0.8 1.0
Change before and after aging (200[degrees]C x 24 hr.)
Durometer A hardness 1 -1 0 0 -2
Tensile strength, % -32 -28 -26 -23 -35
Elongation at break, % -34 -32 -28 -30 -32
Wear, % 1.8 1.6 1.3 1.3 1.5

RP 100, FA 25, DCP 2, SA 4, accelerant M 1.5, antioxidant D 2,
sulfur 1.5, (coupling agent as shown)

Table 3--effect of the ingredients addition order on composites

Sample A B C

Durometer A hardness 88 94 91
Tensile strength MPa 8.5 9.4 9.1
Elongation at break % 199 289 225
Wear ([cm.sup.3]/1.6 km) 1.0 0.8 0.9
Change before and after aging (200[degrees]C x 24 hr.)
Hardness 1 0 0
Tensile strength, % -35 -26 -34
Elongation at break, % -29 -28 -30
Wear, % 2.1 1.3 1.5

RP 100, FA 25, KH-550 2, DCP 2, SA 4, accelerant M
1.5, antioxidant D 2, sulfur 1.5

A: All ingredients mixed directly in the previous mixing.

B: Accelerant M and SA firstly are added in the previous
mixing, mixed sulfur and DCP, and KH-550 finally.

C: Accelerant M and SA firstly are added in the previous
mixing, and mixed sulfur, DCP and KH-550.

Table 4--effect of cure condition on composites properties

 Cure temperature (b), [degrees]C

Sample (a) 130 140 145 150 160
Durometer A hardness 84 90 94 94 92
Tensile strength, MPa 8.3 8.8 9.5 9.4 9.0
Elongation at break, % 158 204 294 289 199
Wear ([cm.sup.3]/1.6 km) 1.3 1.0 0.8 0.8 0.9
Change before and after aging (200[degrees]C x 24 hr.)
Hardness 1 -1 0 -2 -2
Tensile strength, % -38 -28 -26 -27 -29
Elongation at break, % -40 -37 -28 -32 -35
Wear, % 1.9 1.7 1.3 1.5 1.8

 Cure pressure (c), Mpa

Sample (a) 7 8 9 10
Durometer A hardness 85 90 94 93
Tensile strength, MPa 9.4 8.9 9.5 9.1
Elongation at break, % 163 244 294 232
Wear ([cm.sup.3]/1.6 km) 1.1 0.8 0.7 0.8
Change before and after aging (200[degrees]C x 24 hr.)
Hardness 1 2 1 -3
Tensile strength, % -31 -22 -19 -20
Elongation at break, % -56 -35 -22 -21
Wear, % 1.1 1.8 1.3 1.7

 Cure time (d), minutes

Sample (a) 15 20 30 40 45 50
Durometer A hardness 80 89 92 94 93 94
Tensile strength, MPa 8.1 8.9 9.1 9.5 9.4 8.7
Elongation at break, % 192 239 279 301 289 199
Wear ([cm.sup.3]/1.6 km) 2.3 1.5 1.0 0.7 0.9 0.9
Change before and after aging (200[degrees]C x 24 hr.)
Hardness 2 -1 -1 0 1 1
Tensile strength, % -21 -16 -17 -11 -18 -20
Elongation at break, % -47 -36 -34 -25 -25 -33
Wear, % 2.1 3.0 1.7 1.4 1.5 1.8

(a) RP 100, FA 25, KH-550 2, DCP 2, SA 4, accelerant M 1.5,
antioxidant D 2, sulfur 1.5

(b) Cure pressure 8.0 MPa, cure time 25 min.

(c) Cure temperature 145[degrees]C, cure time 25 min.

(d) Cure temperature 145[degrees]C, cure presssure 9.0 MPa.
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Author:Wu, Weili
Publication:Rubber World
Date:Feb 1, 2009
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