Application of x-ray spectroscopic scattering topography to rubber based composites.

X-ray projection imaging contains absorption radiography and diffraction topography. The former has been widely used in various fields such as medical diagnosis and non-destructive testing of industrial materials. The latter reveals structural irregularities and/or x-ray wave fields inside a single crystal with fairly high perfection (low dislocation density) which limits fields of application (ref. 1). To alleviate the limitation of the specimen observed, a topography using scattered x-rays diffracted ones, had been proposed (refs. 2-5). This is known as x-ray scattering topography and can be applied to any kind of material, such as single crystals with imperfections, polycrystals, composites, biological crystals and amorphous materials (ref. 6). The imaging technique can be regarded as a generalized x-ray diffraction topography system.

Figure 1 shows the relation between the absorption radiography, diffraction topography and scattering topography. Essentially the first proposal of x-ray spectroscopic scattering topography was made when scattering topography was applied to observations of the orientation distribution in a single crystal. Further improvements in the detection system made this method more applicable in the field of material science. This article describes the concept of the topography, the outline of the apparatus and several examples of the observations made on rubber based composites.

[Figure 1 ILLUSTRATION OMITTED]

Apparatus and method

A Rigaku 200B rotating molybdenum anode was used under the operating condition of 45Kv/40mA. A pencil beam of about 50 [Micro] diameter was used. Figure 2a is a schematic diagram of scanning type scattering topography set-up equipped with a multichannel detector (PSPC or SSD). Figures 2b and 2c show the PSPC and SSD respectively used in the system shown in 2a. At a favorable angular position of the specimen, it is X-Y two dimensionally scanned step by step, and scattered x-rays are recorded by the multichannel detection system. Several extracted quantities in the x-ray spectrum measured at individual step positions are transferred to the microcomputer within a short time during the one-step translation. Because the data for a number of x-ray spectra are too many to be saved in the computer memory, the channel number giving the maximum x-ray intensity and the integrated intensity at channels corresponding to an analyzed diffraction peak are normally selected for the data input. From this data assembly in the computer, topographs can be constructed directly or in a kind of reconstructed mode by computing if necessary.

[Figure 2 ILLUSTRATION OMITTED]

Results and discussion

Tire

A tire is representative of a composite materials product. It is composed of rubber compounds, steel cords and organic fibers. The specimen studied was cut from a tire to be about 10 mm thick, suitable for a transmission case. The geometry of the observations is illustrated in each figure. Although steel cords in the tire can be easily noticed in an ordinary x-ray absorption radiograph, the organic fibers have hardly been observed. This is because the atomic density of the organic fiber is almost the same as that of the matrix rubber. It has been also quite difficult to distinguish between the different kinds of rubber. Scattered x-rays from the tire have an x-ray energy spectrum as shown in figure 3 when MoK[Alpha] is used. X-rays corresponding to the energies of MoK[Alpha] ZnK[Alpha] and BrK[Alpha] are seen in the spectrum and AA and BrK[Alpha]Bra are fluorescent x-rays from components in the tire. Figure 4 represents scattering topographs of tread side position taken with the channel numbers for MoK[Alpha], and BrK[Alpha] radiation, respectively. The color scale in the sidebar indicates relative intensity of the detected x-rays (top - strong; bottom - weak). Figures 4a and b clearly reveal the existence of the organic fiber cords and different rubber compound layers. Figure 4c for BrK[Alpha] shows a bromobutyl rubber layer (innerliner). Figure 5 shows the topographs of carcass sidewall. The x-ray beam is irradiated perpendicular to the surface. The existence of organic fiber cords and the difference in thickness of bromobutyl innerliner can be recognized. Figure 6 shows the topographs of the carcass sidewall position where a part of bead filler is located. Rubber compound layers and the existence of organic fiber cords can be seen clearly.

[Figures 3 to 6 ILLUSTRATION OMITTED]

Unvulcanized calender sheet

Two different unvulcanized calender sheets were examined. One of them is composed of rubber compound and nylon fiber cords and the other is composed of rubber compound and polyethyleneterephthalate (PET) fiber cords. The rubber compound contains zinc oxide. The scattering topographs were taken with the channel number for ZnK[Alpha] radiation. The rubber compound only area gives stronger ZnK[Alpha] radiation than the area where fiber cords are embedded. From figures 7 and 8, it can be seen that the locations of the fiber cords embedded in rubber compound are clearly detected.

[Figures 7 & 8 ILLUSTRATION OMITTED]

Detecting the position of rubber compound only area between fiber cords, it is possible to cut the unvulcanized calender sheet parallel to the fiber cord direction without any damage to fiber cords.

Vulcanized rubber sheet

Vulcanized rubber sheet (rubber 100, carbon black 50, zinc oxide 5, zinc stearate 3, antioxidant 1, accelerator 1, sulfur 1.8) 2 mm in thickness was examined. The x-ray beam was irradiated perpendicular to the sample surface. Anomalous structure was observed in the topographs shown in figure 9. It is known that zinc stearate forms nodule (ref. 7). Judging from the characteristics (element, size) observed in the topographs, the anomalous structure is considered to be closely related to the nodule.

[Figure 9 ILLUSTRATION OMITTED]

Some application cases of the x-ray spectroscopic scattering topography have been presented here. The application of this method is not limited to the presently demonstrated materials. It can be applied to various kinds of materials using sophisticated techniques. With the improvement of resolution and the use of a strong x-ray source (synchrotron radiation), it will become possible for this method to contribute to clarifying the unsolved problems.

Conclusions

Application of x-ray spectroscopic scattering topography to rubber based composites has been made successfully. It has become possible to distinguish:

* the different rubber compounds and organic fiber cords in tires;

* organic fiber cords embedded in unvulcanized rubber sheet; and

* the anomalous structure in rubber vulcanizate.

With this topography system, the internal structure of materials can be revealed, which had not hitherto been found by ordinary x-ray absorption radiography.

References

(1.) A.R. Lang, 1970 modern diffraction and imaging techniques in material science, Ed S. Amelincks et al (Amsterdam: North-Holland).

(2.) Y. Chikaura, Y. Yoneda and G. Hildebrant, J. Appl. Crystallogr. 15, 48 (1982).

(3.) Y. Yoneda and Y. Chikaura, Z. Naturforsch. 37a 412 (1982).

(4.) Y. Chikaura, Y. Shiraishi, T. Ogawa and H. Uematsu, Applied Physics. 55, 983 (1986) (in Japanese).

(5.) Y. Suzuki and Y. Chikaura, J. Appl. Phys. 70, 1290 (1991).

(6.) Y. Chikaura, Y. Suzuki and Y. Udagawa, J. Phys. D: Appl. Phys, 26, 2212 (1993).

(7.) R.W. Smith and A.L. Black, Rubber Chem. Technol. 37, 338 (1964).