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LSR sponge having open cell structure.

Silicone rubber sponge has been widely used in many fields, such as electrical and electronic parts, business machines and automobiles. It has two excellent properties: One is as a silicone rubber and the other is as a rubber sponge. Therefore, silicone sponge has been used in many sealing, heat-insulating sheet and roll applications for printers, photocopiers and fax machines. In recent years, its use in roll applications has grown rapidly. As figure 1 shows, the fuser roll fixes the toner image on the paper by heating. Recent trends require heater rolls to have higher thermal conductivity for high-speed printing. At the same time, pressure rolls are trending toward lower thermal conductivity. A low thermal conductivity roll can keep the heat inside the system. This leads to a low energy type system and enables the production of a compact personal printer.

[FIGURE 1 OMITTED]

First of all, we will explain the difference between current gas blown type sponge and our new sponge. Current silicone rubber sponges are prepared by adding pyrolytic blowing agents to high consistency silicone rubber (HCR type). As the HCR is crosslinked at elevated temperatures, the foaming reaction is initiated by decomposition of the blowing agents.

On the other hand, the new types of sponge described here are prepared by blending hollow fillers with liquid silicone rubber (LSR), and require only a crosslinking reaction. Therefore, this new type sponge has several advantages compared to the conventional gas blown type.

In figure 2, an electron microscope picture shows the clear difference:

* New type sponge has much smaller and more uniform cell size;

* new type sponge can be cured in a mold since it does not require any gas blowing process;

* a flat-skin surface can be made (no necessity of polishing process);

* very thin (less than 1 mm) sponge parts can be produced;

* very low hardness (lower than 10 durometer) sponge can be made; and

* open-cell structure sponge can also be made by adding special agents.

[FIGURE 2 OMITTED]

Hollow fillers are of two types. One type is an inorganic filler such as glass filler and ceramic filler; the other is organic filler made of organic resins. In the case of inorganic fillers, it is very difficult to make soft (low durometer) sponge because of the filler's hard shell. Also, it is not possible to obtain the low thermal conductivity required, owing to the high thermal conductivity of the filler itself.

Figure 3 shows the hollow filler loading effect on the thermal conductivity. It demonstrates the difficulty of obtaining a low thermally conductive type sponge using inorganic hollow filler. If very thin-shelled inorganic hollow filler is used to decrease thermal conductivity, the product is very difficult to blend and mold its fragility.

[FIGURE 3 OMITTED]

There are two types of organic hollow filler: One is a pre-expanded type that contains gas inside, mad the other is a heat expansion type that contains low boiling point liquid inside. The latter type filler requires a filler expansion process, which causes the same problems as gas blowing types already mentioned.

In this article, we introduce the characteristics of a new liquid silicone sponge that is obtained by organic hollow filler (microballoon) blending. In addition, a low compression set, open-cell sponge made by blending this new sponge material with special agents will be shown.

Experimental

Materials

The properties of the new liquid rubber sponge (X-34-2273A/ B) are shown in table 1. X-93-3027, a clear liquid to make an open-cell structure, was also used.

Thermal conductivity

Thermal conductivity measurements were carried out on 6 mm thick rubber sheets by a Kemtherm QTM-D3.

Methanol immersion test

Samples for methanol immersion testing were prepared by cutting 6 mm x 30 mm x 30 mm sheets from molded slabs. These samples were immersed in 1L methanol for two, four and 16 hours with stainless mesh above the sample to prevent the sample from floating.

Results and discussion

Heat aging results of X-34-2273A/B are shown in figures 4 and 5. The materials were press cured at 120[degrees]C for 10 minutes. They show that at the initial stage of aging, both hardness and specific gravity are changed rapidly. After the drastic change at initial stages, both properties show stable lines. It can be assumed that this change is caused by the nature of microballoons blended in the material.

[FIGURES 4-5 OMITTED]

Microballoons are made of thermoplastic resins that cannot be easily broken, but are deformed by a mechanical pressure. These properties are a result of their elasticity. In the liquid silicone material, microballoons shrink since they receive even pressure on each surface. Therefore, if the liquid material containing microballoons is poured into the mold cavity and vulcanized with heat, it cures in the compressed state. As in the case of ordinary rubber, if the volume of the material exceeds that of the mold cavity, overflow or flash results.

Figures 6 and 7 show the effect of mold pressure on the material. Various weights of material are poured and cured in the same cavity at 120[degrees]C for 10 minutes (figure 6). Figure 6 indicates that the specific gravity rises with increasing pressure on the material.

[FIGURES 6-7 OMITTED]

Figure 7 shows the specific gravity of press cured (nonpost cure) and post cured materials at 200[degrees]C for four hours. Not only the as molded specific gravity but also the change in specific gravity during post cure, becomes higher as the mold pressure is increasing. Figure 8 shows the reasons why specific gravity changes with the mold pressure. During molding, the microballoon is compressed, causing the material to shrink. The compressed microballoon is then vulcanized into place. After the material is demolded from the cavity, it expands because the shrunken microballoon needs to restore itself to its original size by releasing pressure.

[FIGURE 8 OMITTED]

When the microballoon expands, the rubber undergoes strain, which induces the force to shrink the rubber. So the size after molding is determined by the balance between microballoon expanding force and rubber shrinking force. Then, during post cure above 180[degrees]C, the cured rubber shrinks into the original mold size as a result of microballoon decomposition.

Next, we will explain compression set behavior of this new sponge. For example, the compression set resistance of X-34-2273A/B is around 50% (at 150[degrees]C for 22 hours, 25% compression), too high for the purpose of general sealing materials, let alone for toner fixing rolls. However, this deformed rubber almost recovers its original shape (compression set [congruent to] 0) when it is left in the oven at 150[degrees]C for a few hours.

The specific gravity of the deformed rubber also exhibits higher value than that of the initial state, then goes back to almost the initial value as the deformed material changes into its initial shape in the oven. It is considered that, during compression in the oven, cell volume of the rubber shrinks. This is proven by the specific gravity increase.

However, it is very difficult for the shrunken cell to recover its initial volume owing to low gas permeability at the room temperature after mold pressure is released, even if the rubber force for recovering the initial shape remains sufficient. Therefore, it is easily estimated that a low compression set sponge can be obtained if each cell is linked together, so that air can permeate the cells even at room temperature.

In figure 9, the electron microscope photo and low compression set of the open-cell structure sponge are shown prepared with mixing X-34-2273A/B and X-93-3027 (open-cell agent) and compared with the original closed-cell type. In the open-cell micrograph, there are many cells constructed by microballoon, in which a small hole connecting with each cell can be seen. In figure 10, X-34-2273 mixed with the X-93-3027 shows higher methanol absorption compared with the close-cell type (without X-93-3027) and also shows that by changing the X-93-3027 mixing ratio, the open-cell rate of the sponge can be easily controlled.

[FIGURES 9-10 OMITTED]

Lastly, one possible mechanism of the X-93-3027 effect to change closed-cell into open-cell sponge is shown in figure 11. In the case of X-34-2273A/B without X-93-3027, the microballoons are dispersed uniformly in the liquid rubber, resulting in closed cell sponge. When X-93-3027 is mixed in this composition, microballoons partly gather with the aid of the reagent, since it does not dissolve in silicone but has good affinity for microballoon surfaces. Then, as the rubber crosslinks at high molding temperatures, some of the balloon shells decompose. This is because X-93-3027 decreases the softening point of thermoplastics, forming microballoon shells. During post-cure, all the microballoons decompose and all of the reagents evaporate, leading to an open-cell type rubber sponge.

[FIGURE 11 OMITTED]

Conclusion

Liquid silicone rubber sponge containing elastic hollow fillers has many advantages, as mentioned in this article. This new type of sponge was compared with the current gas blown type sponge. As for the heat resistance, the excellent properties of silicone rubber can be kept by decomposing the organic hollow filler through post curing. Furthermore, by adding a specific reagent into the material, this liquid silicone rubber sponge can be made into open-cell structure sponge, which shows very low compression set (less than 10% at 180[degrees]C for 22 hours). The specific reagent helped to allow the air to pass through the silicone rubber. In table 1, several examples of new liquid sponge are shown.
Table 1--list of new liquid silicone rubbers

 Standard Low hardness
 X-34-2273A/B X-34-2365A/B
2273A/2273B/X-93-3027 100/100/8 100/100/10
Appearance A Reddish brown Reddish brown
Appearance B Reddish brown Reddish brown
Viscosity (A/B) (Pa.s) 48/32 300/200
Specific gravity 0.59 0.55
Hardness (Asker C) 49 35
Hardness (durometer A) 26 10
Tensile strength (MPa) 0.4 0.3
Elongation (%) 50 80
Compression set (%) 5 14
Thermal conductivity (W/mk) 0.1 0.1
Heat aging (at 437[degrees]F for 70 hours)
 Hardness change
 Tensile strength change (%)
 Elongation change (%)
Oil resistance (at 302[degrees]F for 70 hours in ASTM#3)
 Hardness change
 Tensile strength change (%)
 Elongation change (%)

 Oil-resistant
 X-34-1605A/B
2273A/2273B/X-93-3027 100/100/0
Appearance A White
Appearance B Reddish brown
Viscosity (A/B) (Pa.s) 350/340
Specific gravity 0.54
Hardness (Asker C) 62
Hardness (durometer A) 35
Tensile strength (MPa) 2.1
Elongation (%) 140
Compression set (%) 45
Thermal conductivity (W/mk) 0.1
Heat aging (at 437[degrees]F for 70 hours)
 Hardness change -4
 Tensile strength change (%) -22
 Elongation change (%) -16
Oil resistance (at 302[degrees]F for 70 hours in ASTM#3)
 Hardness change -3
 Tensile strength change (%) -11
 Elongation change (%) -11
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Article Details
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Author:Meguriya, Noriyuki
Publication:Rubber World
Date:Jun 1, 2003
Words:1778
Previous Article:Effective process for precuring tire components. (Process Machinery).
Next Article:An overview of silicone rubber.


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