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Preparation and properties of gas barrier resin/rubber nanolaminated composites.

INTRODUCTION

Tire air pressure is an important factor of safe driving. Proper air pressure increases the tire's life, fuel economy, and vehicle handling and stability, while also reducing rolling resistance. An excellent tire gas barrier property is the key to keeping the tire pressure constant. At present, most schemes to improve rubber gas barrier property involve using either special rubber or high barrier surface coatings; however, these approaches are not cost effective. Another way is to add fillers of excellent barrier property to the rubber matrix to form a nanoscale laminated structure, which can effectively increase the degree of path twisting as the gas molecules go through the material. This multilayer structure can significantly enhance the gas barrier of the composite [1-3].

Because nanoparticles can greatly improve the compatibility of the polymer interfaces and the mechanical properties of the composite, polymer nanocomposites attract considerable attention in materials science and engineering [4]. Polymer nanolamination technology refers to the technology for the formation of dozens or even thousands of complex alternating layers of two or more polymers. The dispersive nanoparticles can give the rubber substrate many excellent properties, and the thickness of the composite can reach the nanometer range [5]. The limited micro-layer space produced by the nanolamination technology can effectively promote the orientation of the polymer chains and crystallization, thereby enhancing the barrier property of the material [6, 7]. The nanocomposite has excellent gas barrier property because the nanoparticles form a barrier region with a high aspect ratio in the vertical direction. The barrier region turns into a network of circuitous channels which impede molecular diffusion [8, 9]. The nanoparticles in the filled layers of the rubber can play a role in the reinforcing performance; the lamellar structure can enhance the gas barrier property of the rubber matrix, so the mechanical properties of the rubber composite can be enhanced. But the toughness decreased associated to the crystallization process [10]. Especially when the nanoparticles are dispersed in the rubber matrix with the two-dimensional continuous lamellar structure, the degree of twists and turns will be effectively increased as the gas molecules go through the rubber composite [11].

The nanolamination technology has been used in the preparation of the inner liner of a tire. It combines the advantages of the plastic and the rubber in the composite. Rubber is prone to orientation during stretching and has excellent flexibility; however, the gas barrier property of pure rubber is poor [12]. Filling rubber with a resin having excellent gas barrier property improves the gas barrier property of the rubber. As a result, the same gas barrier property of a tire inner liner can be obtained at reduced thickness if the liner is made of the resin/rubber composite instead of the pure rubber. At present, the preparation techniques for resin/rubber composites are at the experimental stage because it is very difficult to evenly disperse the resin in the rubber matrix at a nanometer thickness and the production costs are too high. Using special lamination equipment and existing production devices, we dispersed Polyvinyl Alcohol (PVA) resin with extremely high gas barrier property into a highly oriented rubber matrix to form a structure with alternate layers of resin and rubber. The resulting composite had excellent gas barrier property. The tire inner liners were prepared from this nanolaminated composite with multilayer structures. The nanolaminated composite can improve the gas barrier property of the tire inner liner and reduce the costs of the tire production. In this study, we focused on the structure, gas barrier principle and performance, and the applications of the lire inner liners made of the nanolaminated composite.

EXPERIMENTAL

PVA powder (BP-26 for industrial use) was supplied from Changchun Chemical (Jiangsu). PVA has good chemical nature, such as poisonless, tasteless and stable in its performance. And the alcoholysis degree of PVA powder (BP-26 for industrial use) was 88%, this powder was good solubility in water. The main facilities of equipment were thermostatic water bath and electric muddler.

The powder was immersed into water forming an emulsion. The mass concentration of the resin was 10%. Then the emulsion and styrene-butadiene rubber (SBR), which was taken as rubber matrix, are added into two extruders with mechanical lamination mixing device. The lamination part and the cascading process of the device are showed in Fig. 1. Then the sheet was prepared by the calender. During calendering, in order to obtain the specified thickness of the sheet, the sheet was rolled at least three times. Finally, composite sheets with a 16-layer structure were set by plate vulcanization machine (Fig. 2). But the tires were prepared by 4-roll Calender Group and capsule vulcanizer. The sheets were used as the tire inner liners. The tire inner liners required that there was no bubble on the surface of the sheets, and the thickness of the sheet was 1.75 [+ or -] 0.05 mm. The thickness and surface quality of the prepared inner liners were able to meet the requirements of the tires (Fig. 3), and experimental sample tires of size 185/60R14 were trial produced (Fig. 4).

A series of tests were conducted on the experimental sample tire and comparison samples produced by existing technology. The tests included permeability (tested by VAC-V2 Gas Permeability Instrument produced by Labthink Instruments), tensile strength (tested by electronic tensile testing machine CMT4104-20, produced by MTS), tire pressure (tested by precision digital pressure gauge, produced by Beijing ConST) and endurance property (tested by tire endurance and high speed performance test machine TJR-2PC(Y), produced by Tianjin Jiu Rong).

RESULTS AND DISCUSSION

Barrier Property

Morphology. The gas permeability of the nanolaminated composite depends on three factors: the structure of the composite, the properties of the gas, and the environment [13, 14]. In this study, we looked at the effect of the structure of the nanocomposite only, and the effects of the gas properties and the environment were not considered.

Figure 5 shows a SEM photograph of the nanocomposite, and Fig. 6 shows a schematic diagram of the structure of the nanocomposite. The resin barrier layers are evenly dispersed in the rubber matrix (Fig. 5). The distribution as well as the orientation of each barrier layer is roughly the same, and the layers were almost parallel to each other. Only a small part of the layers fractured, and the orientation of each fractured layer is almost the same. This phase structure was different from the island structure and the dual-continuous phase structure in traditional blending composites. The interfaces of the composite are clear, and there was no stripping between the layers. The barrier layers are aligned in the rubber matrix because the composite is under shearing during processing. By controlling the two dimensional nanoparticle orientations on the flow plane, the lamellae are deformed. The rubber is stretched longer under the shear flow, and the barrier layers are oriented in the shear flow direction.

Gas Barrier Mechanism. A comparison of the gas barrier performance of our composite with that of other rubber materials is shown in Fig. 7. The gas barrier property of the composite is significantly better than that of NR, SBR and BIIR. Figures 8 and 9 compare the pressure test results for the inner liners of the sample tire and the commercial BUR tire. It can be seen from Fig. 8 that at the same initial tire inflation pressure, the difference in pressure between the sample tire and the commercial tire increases with time, the inner liner made of the composite has a better gas barrier property than the inner liner of the commercial BIIR tire. During the pressure measurement, the total pressure reduction for the tire with the BIIR inner liner was 15%, while that for the tire with the composite inner liner was 12%.

As the rubber matrix is filled by the barrier layers in the nanometer range, the gaps originally present in the rubber matrix are now closed in the rubber composite. As a result, the density of the composite is higher and the permeable free volume of the composite is lower than those of the rubber matrix. The barrier layers aligned parallel in the rubber matrix form a barrier unit to limit the freedom of movement of the polymer molecules. As shown in Fig. 10, the gas molecules diffuse through the composite in two ways. A part of gas cannot directly pass through the surface of the barrier layers, and they need to circumvent these barrier layers and diffuse through the gaps of the fractured barrier layers. The diffusion path of the gas molecules indicated by the dashed arrows in Fig. 10 is more tortuous and longer than a direct pass-through, thus reducing the gas diffusion rate and increasing the time taken by the gas to pass through the composite, so the total amounts of the gas diffusion reduce. This layered structure can effectively improve the gas barrier property of the composite by reducing the total amount of gas diffusion. This effect is called the "nanobarrier wall" or "multipath effect" [15]. The dashed line shown in Fig. 11 is probably the motion path of the gas molecules in the laminated composite. In addition, the continuity of the barrier layers in the composite is very good, and there are very few gaps to allow the gas molecules to bypass the barrier layers. Therefore, the gas molecules can only pass through the barrier layers by permeation. But the barrier layers themselves have excellent gas barrier properties; thus, it is difficult for the gas molecules to penetrate these layers. Only the small gas molecules can penetrate the barrier layers. In addition, because the gas molecules can directly penetrate the layers or bypass the layers through the gaps in the layers, the gas barrier property is good.

Calculation of Relative Permeability of Gas. For a composite filled by flake-like fillers, the relative permeability of the composite can be quantitatively analyzed by using the modified Nielsen gas barrier model [16, 17]. The model considers the radius to thickness ratio [alpha] = w/t of the flake-structured filler as an important factor. Besides, the number of fillers and the side gap width of the lamella have large effects on the barrier property. There exist two phenomena: the gas molecules going around and permeating the lamellar barriers. Assuming that the layered barrier materials are highly oriented in the rubber matrix, the dispersed lamellar barriers are aligned parallel to each other. But the impact factor of the "occluded rubber" is not considered in the model. The layers were assumed into standard rectangular in shape. The edges of rectangular were smooth, and the adjacent sides of rectangular were perpendicular to each other. So the shape of all barriers is assumed into neat rectangular [18-20]. The relative permeability of the composite can be calculated by the following equation:

[R.sub.p] = P/[P.sub.0] = 1 - [phi]/1 + w/2 [(t/[phi]).sup.1/2] [(t+h).sup.-1/3] (1)

where [R.sub.p] is the relative permeability of the composite, P is the permeability coefficient of the composite, [P.sub.0] is the permeability coefficient of the pure rubber, [phi] is the volume fraction of the resin filler, t is the thickness of the dispersible lamella, w is the width of the dispersible lamella, and h is the distance between the two dispersible lamellae.

The volume fraction [phi] of the filler can be calculated by the following equation:

[phi] = ([w.sub.1]/[w.sub.2]) x % (2)

where [w.sub.1] is the volume of the resin layers and [w.sub.2] is the total volume of the composite.

According to the SEM micrograph of the composite shown in Fig. 11, the thickness of a rubber layer is 800-900 nm and the thickness of a barrier layer is 5-15 nm. Because the thickness of a barrier layer is very small compared with the thickness of a rubber layer, the distance (h) between two barrier layers can be ~850 nm, and the volume fraction of the filler is 2%. The relative permeability of the composite was calculated for different widths and thicknesses of the composite, and the calculated results are shown in Table 1.

Table 1 shows that the relative permeability of the composite is low. Especially when the barrier layers are dispersed continuously, the relative permeability of the composite is ideal low, mainly because the radius to thickness ratio [alpha] = wit, a key factor of the barrier material to the permeability, has a great impact on the relative permeability of the composite. According to the model, the higher the radius to thickness ratio [alpha], the lower the relative permeability of the composite. And the better the continuity of the barrier layers, the better the gas barrier property of the composite. Because the lamellar barrier layers in the composite have good continuity and uniform distribution, even the continuity of some lamellar barriers is ideal; there is little breakage of the barrier layers. So the relative permeability of the composite is low. The present rubber nanocomposite which was prepared by mechanical mixing was silicate nanocomposite. The radius to thickness ratio a of the silicate nanocomposite prepared in the present study was 10-50 [21], while the value of a of the resin/ rubber composite was several times higher. At the same volume fraction of filler in the composite, the relative permeability of the resin/rubber composite is lower than that of the silicate nanocomposite because the barrier layers of the resin/rubber composite have a high degree of orientation and a good continuity.

Mechanical Properties

Figure 12 shows that the longitudinal tensile strength of the inner liner made of the resin/rubber composite is more than three times higher than that of the BIIR inner liner.

The results of the endurance and high-speed tests of the sample tire are presented in Tables 2 and 3. The sample tire showed good endurance and high-speed performances. Table 3 shows that when the speed was increased to 230 km [h.sup.-1], the sample tire was damaged after 4 min. On the other hand, the normal tire, with a BIIR inner liner, was damaged after 10 min, but at a lower speed of 210 km [h.sup.-1]. So the high-speed performance of the sample tire is better than normal tire.

The results show that using the new inner liner made of the composite will not weaken the existing tire structure. Because the parallel barrier layers in the nanometer range were dispersed into the rubber matrix, they effectively filled the gaps in the rubber matrix. On the other hand, the resin barrier layers had a very good reinforcement effect on the rubber sheets and could prevent cracking from propagating. Thus, the mechanical properties of the composite are higher than those of the rubber matrix, and the endurance and high-speed performances of the tire increased. In summary, a small amount of resin nanolayers can significantly improve the tensile strength of the rubber composite.

CONCLUSION

With the use of multilayer technology and micro-layer coextrusion technology, a resin with extremely high gas barrier property can be distributed with high orientation and in alternate neat-layers into a rubber matrix to form a laminated composite. The resin/rubber nanolaminated composite has excellent gas barrier property.

As we know, SBR is much cheaper than BIIR, and PVA is cheaper than BIIR. So the resin/rubber nanolaminated composite has a good cost performance, and its excellent gas barrier property can make it a good replacement for expensive butyl gas barrier materials, especially in tire inner liners. The excellent gas barrier property can enable the tire to maintain constant pressure, thus eliminating the safety issues and additional energy consumption caused by insufficient tire pressure. In addition, the raw materials that are used to prepare the resin/rubber nanolaminated composite are readily available, and the process is simple. This nanolamination technique can solve the shortage problem of butyl rubber material applied to tire inner liner. This technology can greatly reduce the manufacturing costs of tires by improving the production efficiency and simplifying the tire manufacturing process, and it is in line with the green tire production requirements and has broad application prospects.

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Qin Liu, (1,2) Yang Wei-Min, (1,2) Ding Yu-Mei, (1,2) Jiao Zhi-Wei (1,2)

(1) College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

(2) National Engineering Laboratory for Tire Design and Manufacture, Weihai 264200, China

Correspondence to: Jiao Zhi-wei; e-mail: jiaozw@mail.buct.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 50973009, 21174015; contract grant sponsor: Doctoral Fund of the Ministry of Education of China; contract grant number: 20090012110005.

DOI 10.1002/pen.23884

TABLE 1. The gas relative permeability of the resin film layer
filler of different dimensions.

                   Thickness,               Thickness,
                   t = 5 (nm)               t = 10 (nm)
Width,
w (nm)       [alpha] = w/t     Rp     [alpha] = w/t     Rp

300               60         0.8951        30         0.8649
500               100        0.8463        50         0.8021
1000              200        0.7447        100        0.6789
[infinity]    [infinity]       0       [infinity]       0

                   Tthickness,
                   t = 15 (nm)
Width,
w (nm)       [alpha] = w/t     Rp

300               20         0.8438
500               40         0.7723
1000              70         0.6372
[infinity]    [infinity]       0

TABLE 2. The endurance test results of the sample tire.

Total testing time (h)      Total distance (km)

124.2                            14,902.00

TABLE 3. The high speed test of the sample tire.

Test stage                     1      2     3     4     5     6

Test load (kg)                380    380   380   380   380   380
Test speed (km [h.sup.-1])   0-150   150   160   170   180   190
Test time (min)
  Sample tire                 10     10    10    10    10    10
  Normal tire                 10     10    10    10    10    10

Test stage                    7     8     9    10    11

Test load (kg)               380   380   380   380   380
Test speed (km [h.sup.-1])   193   200   210   220   230
Test time (min)
  Sample tire                10    10    10    10     4
  Normal tire                10    10    10

FIG. 12. A comparison of the tensile strengths of BUR and composite.

BIIR          7.64
Composite    22.57

Note: Table made from bar graph.
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Author:Liu, Qin; Wei-Min, Yang; Yu-Mei, Ding; Zhi-Wei, Jiao
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jan 1, 2015
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