Preparation,water absorbent and mechanical properties of water swellable rubber

Abstract

Water swellable rubber was prepared by blending styrene butadiene rubber as a matrix, cross-linked poly sodium polyacrylate (PAANa) as a high absorbent water resin, copolymer styrene maleic anhydride as a compatibiliser and polyethylene glycol as an absorbent accelerant. The preparation process was described. The effects of various preparation conditions, such as amount of sulphur, ratio of silicon dioxide to nanocalcium carbonate, amount of PAANa and amount of compatibiliser, on its water absorbency and mechanical properties were investigated by orthogonal tests. The optimal reaction conditions were 2 portion (phr) sulphur, 28∶12 silicon dioxide/nanocalcium carbonate, 60 phr PAANa and 5 phr compatibiliser. Furthermore, the vulcanisation property of the rubber was measured.

Keywords

Water swellable rubber Water absorbency Water absorbing rate Mechanical property Vulcanisation property

Introduction

Water swellable rubber (WSR) was a kind of elastomeric material, which possesses not only properties of general rubber (such as high resilience and good tensile strength), but also the water swellability. Water swellable rubber can be used as a water proof sealing material, so it can be widely used in civil engineering and building construction. In the case of its water proof applications, it demands not only water absorbent properties, but also mechanical properties.1, 2

Much research work on the preparation and properties of WSR has been reported in recent years; however, it is mainly confined to patent literature, and data on the relation among the components, structure and properties of the rubber are scarce. Water swellable rubber is normally prepared through multicomponent mechanical blending of a rubber matrix, a supper water absorbent resin and other hydrophilic ingredients. But owing to the hydrophilic polar nature of the water absorbent resin and the hydrophobic non-polar nature of the rubber matrix, the aggregation and poor dispersion inside the rubber often lead to the instability and degradation of water swelling ability, mechanical properties, long term water retention, and so on. And the practical application has been strongly limited. Therefore, how to improve the miscibility of the water absorbent resin and rubber becomes a problem that must be solved in its application.3^–6

In general, utilising a copolymer as a compatibiliser to improve a blend's miscibility is an effective method that is usually used. Directed against the hydrophilicity of an absorbent resin and the hydrophobicity of rubber, if an amphiphilic copolymer is used in the blend as a compatibiliser, the miscibility of hydrophilic dispersion and hydrophobic continuous phase is expected to be improve greatly, and the loss of super water absorbent resin from the rubber matrix to be reduced. In such a way, the water swellable ability and mechanical properties may be enhanced.7^–9

In this study, styrene butadiene rubber (SBR) matrix with cross-linked poly sodium polyacrylate (PAANa), with excellent elasticity was selected as a matrix. Possessing a high degree of absorption and water absorbing rate as well as stability, PAANa was selected as an absorbent resin and polyethylene glycol (PEG) as an assistant absorbent accelerant. At the same time, styrene maleic anhydride (SMA) was selected as a compatibiliser to improve the miscibility of SBR and PAANa. By the multicomponent blending method, a kind of water swellable rubber with excellent water absorptivity was prepared. The effects of various preparation conditions, such as amount of sulphur, ratio of silicon dioxide to nanocalcium carbonate, amount of PAANa and amount of compatibiliser, on its water absorbency, water absorbing rate and mechanical properties, were investigated by orthogonal tests. The optimal reaction conditions were 2 portion sulphur, 28∶12 silicon dioxide/nanocalcium carbonate, 60 phr PAANa and 5 phr compatibiliser. Furthermore, the vulcanisation property of rubber was measured.

Experimental

Materials

Styrene butadiene rubber, tetramethylthiuram disulphide and N-cyclohexyl-N′-phenyl-p-phenylene diamine (Antioxidant 4010) was obtained from Shi Hua Rubber Fartory (Lanzhou, China). Polyethylene glycol (PEG-1000, chemical pure) was obtained from SHANGHAI SENHAO FINE CHEMICALS CO.,LTD., Shanghai, China. Cross-linked PAANa was synthesised by the author and colleagues (particle diameter <50 μm, the water absorptivity, 500 g/g), so was copolymer SMA (maleic anhydride concentration, 12%). Other chemicals were of analytical grade and no further purification was required.

Preparation of samples

The samples were first prepared by blending at room temperature, and then vulcanised. The details of the preparing process are as follows: SBR (100 phr) was firstly masticated on a two roll mill for 1 min. After that, zinc oxide (5 phr), stearic acid (2 phr), vulcanisation accelerator tetramethylthiuram disulphide (1·5 phr), antioxidant 4010 (1 phr) and absorbent accelerant PEG-1000 (10 phr) were added one by one for 3 min. In succession, copolymer SMA as a compatibiliser, cross-linked PAANa as a high absorbent water resin, silicon dioxide and nanocalcium carbonate as padding were added for 10 min. After mixing for the given time, the rubber sheet was removed to cool down before returning to the mill and further mixed with sulphur for 3 min. At last, the blended materials were put in a vulcanising machine at 140±1°C. Thus, the water swellable rubber was obtained.

Properties test

The vulcanised strips of the water swellable rubber was cut into sheets with dimensions of 20×10×2 mm, and each was weighted and immersed into distilled water at room temperature. The samples were removed at specified intervals and gently blotted with tissue paper to remove the excess water on the surface; the weight of each swollen sample was recorded. This process was repeated at several time intervals until they reached their equilibrium states. Then the samples were dried at 90°C in vacuum to a constant weight. The swelling ratio (SR) was measured according to the following equation (1) where M₁ and M₂ are the weights of the sample before immersing into water and after swelling to a equilibrium state with water respectively.

Swelling rate index ν was calculated by the following equation (2) where SR₂₄ and SR_e are the swelling ratio after swelling for 24 h and swelling ratio after swelling to equilibrium state respectively.

Mass percentage loss L_m was calculated by the following equation (3) where M₃ is the weight of the dried swollen sample, which has reached its equilibrium state.

The tensile test was carried out at a crossshead speed of 500 mm min⁻¹, at room temperature, using an Instron tensile strength tester (T2000E, Beijing, China). The load cell was 5000 N, and the initial gauge length was 25 mm. Tensile strength T_sb, elongation at break E_b and modulus at 300% M_300% were then calculated using normal equations.

Shore A hardness was obtained using a hardness testing device (LX-A, Shanghai, China). The vulcanisation property of the sample was measured at 140°C using a moving die rheometer (MZ-4010, jiangdu mingzhu testing machine factory, Jiangsu, China).

Results and discussion

Optimisation of reaction condition

The best condition was selected from the orthogonal tests. Four independent variables: amount of sulphur (phr), ratio of silicon dioxide (phr) to nanocalcium carbonate (phr), amount of PAANa (phr) and amount of SMA (phr) were chosen, each at three levels. The investigated variables and their test levels are listed in Table 1. Reference to the experimental design theory, the orthogonal array L₉ (3⁴) was selected to arrange the test program. The water absorbency and mechanical properties of the samples were criterions of each test. The test results were listed in Table 2.

Table 1

Investigated variables and their levels

Levels of each variables	A: amount of sulphur/phr	B: ratio of silicon dioxide to nanocalcium carbonate/(phr/phr)	C: amount of poly sodium polyacrylate/phr	D: amount of styrene maleic anhydride/phr
1	1	12∶28	40	0
2	2	20∶20	60	5
3	3	28∶12	80	10

Table 2

Orthogonal experiment arrangement and test result

Experiment number	A	B	C	D	Test results
Experiment number	A	B	C	D	SR/%	ν	L_m/%	T_sb/MPa	E_b/%	M_300%/MPa	Hardness (Shore A)
1	1	1	1	1	122·4	0·317	6·38	3·87	569·6	1·48	68
2	1	2	2	2	227·2	0·464	9·38	4·37	567·0	1·77	75
3	1	3	3	3	327·7	0·947	20·7	3·73	600·0	1·62	70
4	2	1	2	3	291·4	0·266	7·62	4·23	500·0	2·05	78
5	2	2	3	1	351·4	0·486	20·2	3·07	398·4	2·08	72
6	2	3	1	2	332·5	0·367	13·7	4·87	451·2	2·81	80
7	3	1	3	2	335·5	0·317	15·7	2·78	350·4	2·34	80
8	3	2	1	3	120·6	0·273	1·33	3·43	280·0	3·10	78
9	3	3	2	1	261·4	0·433	14·0	3·76	332·0	3·38	80
SR		K-A₁	225·8		K-B₁	249·8	K-C₁	191·8	K-D₁	245·0
		K-A₂	325·1		K-B₂	233·1	K-C₂	260·0	K-D₂	298·4
		K-A₃	224·2		K-B₃	307·2	K-C₃	338·2	K-D₃	246·6
		R_a	100·9		R_b	74·1	R_c	146·4	R_d	53·4
ν		K-A₁	0·576		K-B₁	0·300	K-C₁	0·319	K-D₁	0·412
		K-A₂	0·373		K-B₂	0·408	K-C₂	0·388	K-D₂	0·383
		K-A₃	0·341		K-B₃	0·582	K-C₃	0·583	K-D₃	0·495
		R_a	0·235		R_b	0·282	R_c	0·264	R_d	0·112
L _m		K-A₁	12·12		K-B₁	9·87	K-C₁	7·10	K-D₁	13·50
		K-A₂	13·83		K-B₂	10·29	K-C₂	10·35	K-D₂	12·93
		K-A₃	10·36		K-B₃	16·16	K-C₃	18·86	K-D₃	9·88
		R_a	3·47		R_b	6·29	R_c	11·76	R_d	3·62
T _sb		K-A₁	3·99		K-B₁	3·63	K-C₁	4·06	K-D₁	3·57
		K-A₂	4·06		K-B₂	3·62	K-C₂	4·12	K-D₂	4·01
		K-A₃	3·32		K-B₃	4·12	K-C₃	3·19	K-D₃	3·79
		R_a	0·74		R_b	0·50	R_c	0·93	R_d	0·44
E _b		K-A₁	579		K-B₁	473	K-C₁	433	K-D₁	433
		K-A₂	450		K-B₂	415	K-C₂	466	K-D₂	456
		K-A₃	321		K-B₃	461	K-C₃	449	K-D₃	460
		R_a	258		R_b	58	R_c	33	R_d	27
M _300%		K-A₁	1·62		K-B₁	1·96	K-C₁	2·46	K-D₁	2·31
		K-A₂	2·31		K-B₂	2·32	K-C₂	2·40	K-D₂	2·30
		K-A₃	2·94		K-B₃	2·60	K-C₃	2·01	K-D₃	2·56
		R_a	1·32		R_b	0·64	R_c	0·45	R_d	0·26

The order of influence of each variable on the properties of the samples was shown in Table 3. Table 3 indicates that variable C (PAANa) has the greatest influence and variable D (SMA) has the smallest influence. Therefore, the optimum reaction conditions were as follows: amount of sulphur, 2 phr; ratio of silicon dioxide to nanocalcium carbonate, 28∶12; amount of PAANa, 60 phr; amount of SMA, 5 phr.

Table 3

Influence extent of each variable

Properties	order of influence
SR/%	C>A>B>D
Swelling rate index ν	B>A>C>D
Mass percentage loss L_m/%	C>B>D>A
Tensile strength T_sb/MPa	C>A>B>D
Elongation at break E_b/%	A>B>C>D
Modulus at 300% M_300%/MPa	A>B>C>D

Vulcanisation property

The vulcanisation property of the sample under the optimum reaction conditions was measured at 140°C using a vulkameter (MZ-4010, Jiangsu, China). The result is shown in Fig. 1. As shown in Fig. 1, the sample is fully vulcanised in 10 min and there is no phenomenon of excess vulcanisation.

Figure 1

Vulcanisation curve at 140°C

Effect of sulphur on properties of WSR

Effect of sulphur on water absorbency

The SR of the WSR with different contents of sulphur is shown in Fig. 2. When the amount of sulphur was 2 phr, the SR of the WSR was maximal. The reason is that lower sulphur dosage means a lower degree of cross-linking, and a lower degree of cross-linking could arise not only a higher SR, but also a larger loss of water absorbent component. As a result, when the amount of sulphur was 1 phr, the SR was lower. When the amount of sulphur was 3 phr, the SR was also lower. This is because of the exorbitant cross-linking density.

Figure 2

Effect of sulphur on SR

The influence of sulphur on swelling rate index ν is shown in Fig. 3. With an increase in the content of sulphur, the ν decreases markedly. This is because water molecule cannot enter the interior of the network easily when the cross-linking density is great.

Figure 3

Effect of sulphur on swelling rate index

Effect of sulphur on mechanical properties

Figures 4–6 reveal the effects of sulphur on T_sb, E_b and M_300% of the WSR respectively. And the results would be explained by the different cross-linking densities. From a practical point of view, if high tensile strain is a primary important parameter in specification of a rubber product, the blends should be prepared using a relatively low amount of sulphur. On the other hand, if a high tensile modulus is a primary important parameter in terms of the specification, the rubber should be compounded with a relatively high amount of sulphur.

Figure 4

Effect of sulphur on tensile strength

Figure 5

Effect of sulphur on elongation at break

Figure 6

Effect of sulphur on modulus at 300%

Effect of PAANa on properties of WSR

Effect of PAANa on water absorbency

The water absorbent resin PAANa plays an important role in the degree of swelling of the WSR. In the studied range, the more the PAANa, the greater the SR, so is the swelling rate index (Figs. 7 and 8). It is easy to explain this phenomenon by the relationship between the water absorbency and the PAANa content.

Figure 7

Effect of PAANa on SR

Figure 8

Effect of PAANa on swelling rate index

Effect of PAANa on mechanical properties

In Figs. 9 and 10, mechanical properties of the WSR are presented. T_sb decreased with increasing PAANa loading. Like T_sb, M_300% also followed the same trend, showing a decrease with an increase in the content of PAANa, which was the result of poor compatibility. The more the amount of PAANa, the more poor compatibility, and the lower of the strength the rubber has. As far as E_b is concerned, the amount of PAANa has little effect on it.

Figure 9

Effect of PAANa on tensile strength

Figure 10

Effect of PAANa on elongation at break

Effect of padding on properties of WSR

The component of padding has a great influence on the water absorbency of WSR. Figures 11 and 12 indicate that as the content of silicon dioxide in padding increases, the SR and swelling rate index increase. This is because the hydrophilicity of silicon dioxide is stronger than that of nanocalcium carbonate.

Figure 11

Effect of PAANa on modulus at 300%

Figure 12

Effect of padding on SR

As shown in Figs. 13–16, the effect of padding on the mechanical properties of the WSR is litter.

Figure 13

Effect of padding on swelling rate index

Figure 14

Effect of padding on tensile strength

Figure 15

Effect of padding on elongation at break

Figure 16

Effect of padding on modulus at 300%

Effect of compatibility on mass percentage loss of WSR

In order to increase the compatibility of the rubber phase and the water absorbent phase, decrease mass loss of the WSR after swelling and increase the stability of the WSR, we used the copolymer SMA as a compatibiliser. As shown in Fig. 17, as the amount of SMA increases, the mass loss of the WSR decreases, which indicates that SMA could operate as a compatibiliser.

Figure 17

Effect of compatibiliser on mass percentage loss

Conclusions

In this study, water swellable rubber was prepared by blending SBR as a matrix, cross-linked PAANa as a high absorbent water resin, copolymer SMA as a compatibiliser and PEG as an absorbent accelerant. The reaction conditions were optimised by orthogonal tests and the synthesised product had better water absorbency and mechanical properties.

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