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issue: December 2005 APPLIANCE Magazine

Plastics
An Overview of a New Ultra High Temperature Plastic (UHT)


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by David Ward, Ph.D., new product development director, Fulgor Appliances and Ing. Enzo De Candido, independent plastics consultant

A new silicone compound plastic is able to resist extreme thermal conditions greater than 900 degrees fahrenheit (482 degrees celcius), while maintaining the typical properties of plastics, such as formability, low density, a high degree of electrical insulation, and low thermal expansion.

Figure 1. Chemical structure of UHT Plastic

The Ultra High Temperature Plastic (UHT) is taking a foothold in areas where other options, such as metals or ceramics, are either more complex, provide design limitations or offer low reliability and inferior performance. The new plastic is not only dopable, so that it can include natural lubricants or pigments for coloring, but can also be structured with the inclusion of fibers to customize its mechanical properties. Applications range from self-lubricating sleeves for fireplace window guides to lamp holders, but more challenging applications are foreseen.

Today, the world plastics market is roughly 80 percent thermoplastic and 20 percent thermoset plastics. However, there are certain niche markets, like the UHT plastic presented here, that represent a small percentage, have low quantities, and, therefore, have the handicap of being expensive.

Accordingly, from an environmental point of view, there is mounting evidence that, if sensibly employed, plastics can actually pollute less than other, supposedly cleaner materials such as metals, ceramics, etc.

This is acknowledged in the ISO 140000 certification and in the concept of Life Cycle Analysis (LCA). In fact, in using LCA experts are realizing that the process of producing plastic materials and subsequent elimination or recycling can be more environmentally friendly. The difference is not only in the material but its fabrication, use and reuse from birth to recycling.

The new material discussed here is a stable silicone compound thermoset plastic that is more durable than other materials (including non-plastics) and therefore has a longer service life. It does not react with water or air.

Cost is always a driver in domestic appliances and many innovations have fallen short of launch simply because of issues with cost (capital investment, material costs or otherwise). To this end, plastics are sometimes considered a cheap alternative to metal. More recently, due to an escalation of petroleum costs, they have also seen cost as an innovation brake. The raw materials cost of the UHT plastic discussed here is standing at about 20 euro/kg (approx. U.S. $24/kg) but this would change for the better if consumption increased.

But perhaps plastics biggest handicap lies in the mediocre properties of some of the materials. A drawback of many plastic materials has always been temperature compatibility, especially working temperature. Even some specialty plastic materials cannot sustain temperatures in excess of 220 degrees Celcius to 250 degrees Celcius in engineering applications.

The UHT plastic has a working temperature of up to around 500 degrees Celcius, but can exceed 800 degrees Celcius, depending on working conditions. The UHT plastic is a silicone resin with the chemical scheme shown in Figure 1.

Thanks to the Si-Si bonds the resulting breakdown temperature is much higher than thermoplastics and standard thermoset plastics. In fact, many thermoset materials have a breakdown temperature of approximately 350 degrees celcius or lower because of their C-C bonds.

A typical silicone resin composition would be:

25 percent to 35 percent silicone resin
60 percent to 75 percent filler (glass fiber, carbon fiber etc.)
2 percent to 3 percent lubricants or mold release agents
1 percent to 2 percent catalyzer-accelerator
1 percent to 3 percent colorant (thermally stable)

UHT Plastics: Typical Properties

Based on knowledge gained so far, UHT plastics have essentially five main technical/technological characteristics:

1. They are thermally more stable that organic polymers and very resistant to oxidization.
2. They conserve their physical characteristics better and over a long time frame.
3. They are synthetic in nature and therefore, structurally speaking, completely different to their organic counterparts. They do not stick or dissolve in them either.
4. The resulting surface (even in its simplest and purest form) is hydro-repellent.
5. The resulting molded component is precise and has almost no shrinkage.

Successful Applications

Two aspects need to be considered from an application standpoint to correctly apply UHT plastic: the economics of the solution and the technical application characteristics.

Use of UHT thus far have been driven by a need to address applications in which other materials failed to meet working conditions, to address environmental concerns or to address application limitations, such as space restrictions versus working temperature. Current UHT applications are mostly driven by considerations in weight, spatial restrictions and mechanical and thermal conditions. Typically the challenge has been to prove better performance and qualities, such as:

reduced defects (less chipping)
easier use (better dimensional tolerance and control)
improved reliability (better mechanical properties)
easier to apply in practice (since it solves or simplifies tribological issues like self-lubrication)

 

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