New technology to produce silicone sponge without chemical blowing agents or VOCs.

Silicone sponge materials are used in applications requiring the benefit of siloxane polymers such as low temperature flexibility, thermal resistance, UV and ozone resistance, and electrical insulating properties. In addition, sponge rubber provides light weight, damping effects and thermal insulation. With today's more competitive and demanding markets, silicone sponge provides substantial cost reduction versus a compact elastomer because less material is used as a consequence of the lower specific gravity of sponge. Adding chemical blowing agent(s) to a rubber compound produces conventional silicone sponge. During vulcanization, heating causes the blowing agents to decompose, creating bubbles of gas entrapped in the rubber, which form the cells of the sponge. These conventional blowing agents may produce toxic by-products.

New closed cell silicone sponge technology has been developed that replaces the traditional reactive chemical blowing agents and use of volatile organic materials (VOC) as the expansion source. This new sponge technology utilizes water as the blowing agent to create a uniform cellular sponge structure that can be used in food contact applications. Performance characteristics include:

* Resilience;

* low compression set;

* eliminates environmental controls related to the sponge process;

* reduces the need for a venting system;

* no unpleasant odor;

* easily molded and extruded;

* smooth and uniform extrusion surface; and

* acceptable for food contact applications.

These sponge products can be enhanced with additives and modifiers.

This article will present the technology advancements, properties and performance characteristics, and the potential benefits of the Silastic sponge for rubber fabricators and end users.

Sponge rubber can be open or closed cell. The cell structure is defined by the linkage of the open areas (open cell) or by the discrete individual nature of the sponge structure openings (closed cell). Silicone rubber can be made into sponge in either an open or closed cell structure by the use of blowing agents. These blowing agents provide gas bubbles when heated in order to develop the cell structure while, at the same time, the vulcanization reactions occur to crosslink the rubber.

The physical and cost benefits for using expanded sponge have been summarized by Hamilton (ref. 1). In addition to lower sealing pressures, shape conformance and lower density and weight, silicone sponge rubber also provides enhanced temperature range suitability (from -60[degrees]C to 250C for long-term performance and up to 400[degrees]C for short-term application). This provides a wider operating temperature range compared to organic rubber sponge. Traditionally, silicone sponge rubber is produced using peroxide cure systems with blowing agents that could release toxic by-products. In some cases, food grade silicone sponge rubber was possible, but these materials may suffer from process constraints and poorer properties, including compression set and density.

The new technology silicone sponge rubber uses platinum cure (addition cure) and water as the blowing medium. This new technology provides a consistent cell structure, odor free sponge fabrication, uniform part surface and stable process conditions.

Applications

Silicone sponge rubber is used in a wide range of applications. Some of the more common ones include automotive gaskets for sealing compartments, and office equipment rollers used in copy machines, printers and facsimile machines. Consumer uses requiring food contact safe materials include appliance seals for dishwashers and ovens, and lunch boxes for transporting food. Silicone sponge rubber is used preferentially where solid rubber creates too high sealing or mating force, reduced density can lower overall material cost, thermal insulation is required and irregular surfaces are in contact with the sponge rubber seal, which include insulation, sponge gaskets, sheets, packing, roller and extruded profiles.

Sponge technology

Traditionally, silicone sponge rubber has been produced by adding chemical blowing agents to the rubber formulation. A chemical blowing agent will decompose under the heated conditions used for curing the rubber. The resulting liberation of gas (nitrogen is the most common) provides the cells or bubbles that result while crosslinking of the rubber occurs in a parallel process. 2,2 azobisisobutyronitrile (AIBN) is often used as a nitrogen-generating chemical blowing agent within peroxide cured high consistency silicone sponge rubber. While the resultant sponge structure from this technology is satisfactory in terms of consistency and cell size, a major disadvantage is the generation of a toxic and environmentally unfriendly by-product, tetramethylsuccinonitryl. This has been recognized as a major limiting factor for the use of silicone sponge in the rubber fabrication industry, and non-toxic and environmentally friendly products are desired.

Other less hazardous chemical blowing agents are available. Dinitroso pentamethylene tetramine (DNPT) and azodicarbonamide (ADCA) are two of the less toxic blowing agents, but poorer cell structure and less consistent physical properties are likely to result from their use.

Recently, another manufacturer has reported the development of silicone sponge that employs hollow fillers based on ceramic or organic materials (ref. 2). A second manufacturer reported the use of flexible microspheres as the method to obtain sponge structure (ref. 3). However, Dow Corning's technology for sponge rubber is silicone based. The new technology uses water as the blowing agent, instead of chemical agents that decompose. In addition, the system employs the use of proprietary silicone technology that supports the water-to-steam bubble creation that results in regular and consistent cell structure. Upon heating, the water dispersed in the silicone rubber matrix develops bubbles to give a uniform cell structure.

The new sponge technology allows the fabrication process temperature conditions to be varied to achieve desired volume expansion characteristics. This is shown in figure 1, where the curing/sponging temperature is increased from 200[degrees]C to 250[degrees]C, resulting in a reduction of specific gravity and Asker hardness.

[FIGURE 1 OMITTED]

The net result of this new sponge technology is a consistent cell structure that can be adjusted to vary the volume expansion. This allows the fabricator to use curing/sponging temperature to control the specific gravity. Figure 2 compares the new silicone sponge technology to traditional sponge using a chemical blowing agent. A uniform and finely divided cell structure is provided by this technology.

[FIGURE 2 OMITTED]

The new silicone sponge technology eliminates the use of chemical blowing agents and the potentially hazardous byproducts. As a result, this material has no distinctive odor and can be processed with less ventilation than conventional sponge materials.

Test data

The new technology sponge consists of three components. These components are HB-ES 5000 sponge base, AMS-5000 sponge curing agent and AMS-5001 sponge surface modifier. This system provides flexibility to the fabricator by providing them a choice to mix their own finished compound using the three materials or to use a finished compound directly from Dow Coming. These three materials can be used alone, or other silicone bases can also be formulated with these system components to make sponge with differing densities and compression deflections. This offers the fabricator simple formulation control and process flexibility to produce a variety of sponge products to meet differing end use requirements.

There are various industry specifications that control and categorize flexible cellular rubber products. In this article, we have tested and compared this new sponge material to several of the widely used industry specifications in order to show the ability of this new system to flexibly meet the various requirements commonly needed in the sponge rubber fabrication industry. Table 1 summarizes the industry specifications that can be used to evaluate performance of this new silicone sponge technology.

Table 2 details the test results of this new sponge technology compared to elements of the previously identified industry specifications. There are three sponge formulations listed in this table.

* A is formulated with the three system components;

* B is formulated with the three system components and 12.5 phr of dimethyl silicone rubber base (70 durometer A hardness); and

* C is formulated with the three system components and 25 phr of dimethyl silicone rubber base (70 durometer A hardness).

Sponge samples were extruded, hot air vulcanized for 10 minutes at 240[degrees]C and post cured four hours at 200[degrees]C. The results indicate that a range of compression-deflections can be achieved by simple formulation adjustment to meet specific targets. In addition, the previous discussion regarding sponge specific gravity variation with curing/sponging temperature suggests that the compression-deflection can also be varied with process temperature adjustment. This is possible since the addition cure system used in this new technology sponge is capable of curing the rubber over a range of temperatures. As a result, by using simple formulation adjustment and process temperature control, the fabricator using the Silastic sponge system can meet a variety of sponge requirements. The cure system and the water blowing agent are odorless, the sponge is uniform with a small cell structure and the surface is smooth and not sticky. When properly post cured, the new sponge technology is acceptable for use in food grade and low compression set (at 25% and 50% deflection) applications.

Sponge fabrication

Extrusion

The same extrusion machinery and curing equipment used for standard silicone extrusion can also be used for this new silicone sponge technology. Stock temperature at the screw tip should be 25[degrees] to 50[degrees]C, so slight cooling is recommended. Extrusion die shape should reflect the linear expansion rate of silicone rubber sponge, at a factor of 1.45 to 1.65 compared to solid materials (volume increase by factor 3.2 to 4.4, depending on cure conditions).

Die geometry needs to be adjusted to compensate for:

* The die swell of the solid material (figure 3); and

* corner areas due to fast heat transfer which expands less than straight areas of the profile (additional convex shape of areas intended to be straight on the profile cross section, as seen in figure 4).

[FIGURE 4 OMITTED]

The extrusion speed that can be attained depends on the geometry of the profile and on the equipment's capability (length and efficiency of heat transfer of the curing tunnel).

Curing

The cure temperatures depend strongly on the heating equipment that is used. Cure temperatures for the new sponge technology should be set higher than temperatures for conventional, chemically blown sponge. The higher the cure temperature level, the lower the density. The temperature at the entry of the curing tunnel should exceed 210[degrees]C to obtain good sponge. The optimum conditions will be based on a combination of size, shape, thickness of the profile and number and length of the curing ovens.

Sponge molding

A pre-form of adequate shape should be inserted into the mold, leaving enough space to allow for expansion to fill the mold and obtain the desired shape. The pre-form weight is normally determined by tests to determine the best quantity to provide complete mold fill. The density of the resulting molded sponge part is influenced by the geometry of the mold. Mold temperatures of 200-250[degrees]C are recommended, with typical cure times of 60 seconds per millimeter part thickness.

The processing advantages of the new technology sponge include the ability to use it in molding and extrusion fabrication processes, the addition cure provides a more environmentally friendly process, and the use of non-toxic blowing agent reduces environmental and personnel exposure hazards.

Conclusion

The new technology silicone sponge eliminates the use of chemical blowing agents and their potentially hazardous decomposition by-products. Water is used as the blowing agent and addition cure is used to vulcanize the rubber. The following benefits result from this new sponge technology:

* Acceptable for food contact applications;

* no unpleasant odors evolve during processing;

* reduced requirements for process venting;

* uniform and finely divided sponge cell structure;

* smooth, non-sticky sponge surface; and

* control sponge characteristics with simple formulation or process temperature changes.

Data were presented showing resilience, compression deflection, density and compression set that can be obtained to allow the fabricator to meet the requirements of the primary industry specifications for silicone sponge. The Silastic sponge is a three-component system that provides flexibility to the fabricator by allowing them a choice to mix their own compound or to use a finished compound directly from Dow Corning.

Table 1--industry specifications

Type                               Description

ASTM D [1056-00.sup.4] Standard    Covers flexible cellular
Specification for Flexible         rubber products known as sponge rub-
Cellular Materials--Sponge or      ber and expanded rubber, but
Expanded Rubber                    does not apply to latex foam rub-
                                   ber or ebonite cellular rubber.
ASTM D [6576-00.sup.5] Standard    This specification establishes
Specification for Flexible         requirements for chemically
Cellular Rubber Chemically         blown cellular rubber.
Blown
SAE AMS [3195E.sup.6] Silicone     This specification covers a sili-
Rubber Sponge Closed Cell,         cone rubber sponge in the form
Medium                             of sheets, strips, extrusions and
                                   molded shapes.
SAE AMS [3196F.sup.7] Sponge,      This specification covers a sili-
Silicone Rubber, Closed Cell,      cone rubber sponge in the form
Firm                               of sheets, strips, extrusions and
                                   molded shapes.

Table 2--new technology sponge results compared to industry
specifications

                                             Sponge formulation

                                            A        B        C

Specific gravity                          0.37      0.34      0.37
Density, kg./[m.sup.3]                     370       340       370
Resilience, %                               52        51        44
Compression-deflection, Kpa
  25%                                     27.6      37.9      55.2
  50%                                     89.6     106.9     127.6
Recovery (min. %)                         97.3      --        --
  after 50% deflection
% compression set, 22 hrs. @ 150[degrees]C
  25% deflection, 1/2 hr. rest            19.24    11.14     11.05
  25% deflection, 24 hr. rest             13.1      4.25      8.16
% compression set, 22 hrs. @ 100[degrees]C
  50% deflection, 24 hr. rest              5.56     4.65      4.27
Low temperature brittleness,               No       No        No
  ASTM D746-98, -55[degrees]C            cracks   cracks    cracks
Water absorption, %                        1.5      --        --
  change in weight

                                          ASTM 1056-00
                                              Type        ASTM 6576-00

                                               2          requirement

Specific gravity
Density, kg./[m.sup.3]
Resilience, %
Compression-deflection, Kpa
  25%                                      35-65 Kpa       35-63 Kpa
  50%
Recovery (min. %)                                            70% min.
  after 50% deflection
% compression set, 22 hrs. @ 150[degrees]C
  25% deflection, 1/2 hr. rest
  25% deflection, 24 hr. rest
% compression set, 22 hrs. @ 100[degrees]C
  50% deflection, 24 hr. rest              60% max.
Low temperature brittleness,                  No              No
  ASTM D746-98, -55[degrees]C               cracks          cracks
Water absorption, %                         5% max.         5% max.
  change in weight

                                           SAE AMS       SAE AMS

                                            3195E        3196F

Specific gravity
Density, kg./[m.sup.3]                    692 max.      830 max.
Resilience, %
Compression-deflection, Kpa
  25%                                     41-96 Kpa    83-138 Kpa
  50%
Recovery (min. %)
  after 50% deflection
% compression set, 22 hrs. @ 150[degrees]C
  25% deflection, 1/2 hr. rest
  25% deflection, 24 hr. rest
% compression set, 22 hrs. @ 150[degrees]C
  50% deflection, 24 hr. rest             60% max.      60% max.
Low temperature brittleness,                 No            No
  ASTM D746-98, -55[degrees]C              cracks        cracks
Water absorption, %
  change in weight

References

(1.) J.R. Hamilton II, Rubber World 228, No. 3, 35 (June 2003).

(2.) S. Sekiguchi, S. Hirabayashi and N. Meguriya, "New silicone rubber sponge," 3rd International Silicone Conference, Cleveland, OH (2003).

(3.) D. Klingberg and B. Griffin, "A novel approach for the fabrication of silicone rubber sponge," 3rd International Silicone Conference, Cleveland, OH (2003).

(4.) ASTM Standard D1056-00, "Standard Specification for Flexible Cellular Materials--Sponge or Expanded Rubber," Annu. Book ASTM Stand. 08.01, 1 (2000).

(5.) ASTM Standard D6576-00, "Standard Specification for Flexible Cellular Rubber Chemically Blown," Annu. Book ASTM Stand. 08.03, 1 (2000).

(6.) AMS 3195E, "Silicone Rubber Sponge Closed Cell, Medium, "SAE, 1 (1996).

(7.) AMS 3196F, "Sponge, Silicone Rubber, Closed Cell, Firm," SAE, 1 (2001).