Silicone's made great strides in short time

CUYAHOGA FALLS, Ohio—The silicone industry has gone a long way, even all the way to Mars, according to Oliver Franssen, global marketing director, elastomers/transportation for Momentive Performance Materials Inc.

Though the history of silicone is still relatively recent, it's made some great strides, he said in his presentation "The World Records of Silicones" at the recent International Silicone Conference held in Cuyahoga Falls.

After GE Chemist Eugene Rochow discovered the direct reaction process for making methylchlorosilanes in the early 1940s, it led to the development of applying silicones in our daily lives, he said. Silicone innovation has continued over a 75-year timeline, but time wasn't the measurement Franssen wanted to use.

"I want to invite you to look at this time axis on a whole different scale," he said. "We will talk about silicone applications in the distance to the human body."

Starting at a distance of -1 inch, Franssen began with silicones with the highest purity and intrinsic chemical stability compared to organic elastomers, with a baby pacifier.

"That's a really unique selling point of silicone elastomers," he said. "The inertness of the polymer and the fact that it is inorganic, it has a low absorption for UV and is therefore is more or less UV transparent. It is not attacked like natural rubber would be for baby pacifiers, so it does not need to be protected."

Silicone can make very soft materials without the risk of nitrous amines, which normally would not be healthy for the baby, Franssen said.

"That is the foundation of the big success of silicone elastomers in the medical industry," he said. "It is odorless and can comply with the USP standards, FDA and ISO standards."

Moving to a distance of 0 inches, silicone provides the highest permeability, making gas permeable contacts a possibility, he said.

"If I talk to somebody making tires about permeability, it is negative," he said. "By structure, silicone elastomers are highly, highly permeable to media. That can be fluid, that can be gaseous."

Silicone elastomers have high freedom of polymer chain motion and high solubility and diffusivity of penetrants. For an application like gas permeable contact lenses, the application calls for a product that has a high oxygen permeability, and "that is the reason for success of the silicone compounds for contact lenses," Franssen said.

Clarity and viscosity

With the highest transparency of all the elastomers, silicone elastomers do well at a distance of about an inch from the human body for items, such as underwater goggles, he said.

"There's nothing which can beat the silicone elastomers," he said. "Especially since 2008, when a new generation was brought to the market in terms of LSR with optical quality."

Polydimethylsiloxane polymers are highly transparent to visible light, making them a strong fit for applications requiring ultraclear elastomers and opening new opportunities for consumer goods and LED lights, Franssen said.

"Instead of tensile strengths and tear resistance and compression set, we now deal with a yellowing index, with Abbe number with chromatic aberration," he said.

Silicone elastomers boast the lowest hardness without plasticizer, so they work well at about 1.5 feet within arm's reach for items like the transparent sensor coupling pad for an automotive driver assistance camera, Franssen said.

"For plasticizer-free EPDMs, it's impossible to go 40 Shore A, but if you work with silicone rubber you can have no limit to the low end," he said.

The range of LSR hardness runs from about 5-90 Shore A, with substances of about 3-5 Shore A having "the consistency of pudding," he said.

Franssen borrowed an example from a fellow presenter at the event, Gabriel Geyne, application engineer for Sigmasoft Virtual Molding, to show off lowest viscosity for silicone, at about 2.5 feet. The part was a designer webbed wine bottle net, with thin material that still needed to keep enough strength to hold a bottle without dropping and breaking it.

Silicone has a very low viscosity with limited temperature dependency and excellent pumpability, he said.

"Viscosity of LSR is a factor two magnitudes lower than organic elastomers," Franssen said. "This is allowing you to make shear thinning, so it is getting even more liquid under stress, and has a very high flow length."

Strong resistance

At a distance of about 3 feet, kitchenware can be a useful application for silicone elastomers, thanks to its high hydrophobicity, Franssen said.

"That's a very unique property of silicone elastomers, which also differentiates the product from the rest of the organic elastomers," he said. "Here you see a nice replacement of steel as a functional competitor of silicone elastomer. Silicone has a food compliancy, has a flexibility."

A cook easily can demold a treat like a cake without sticking, making the process that much straightforward, he said.

"Of course, you can also create hydrophilic materials, which you would need for contact lenses, so here you benefit from two things," he said.

For silicone elastomer's low compression set, Franssen reached a distance of about 6 feet for connector seals in automotive applications.

"It's very important, if you look at car breakdowns in the field, 30 percent of those can be related to failures in the steering unit," he said. "You're totally depending meanwhile on these 2.5 miles of cables, these 220 connectors, plus managing the end devices."

Because of the introduction of autonomous vehicles, optimizing compression set for LSRs looks different than it has in the past, running the test to failure rather than testing to specifications, he said.

Moving out to about 60 feet, silicone elastomers hold up as some of the best substances to use for weather resistance applications, Franssen said.

He showed data from test samples under accelerated weathering conditions over a 28-year test in Florida weather, between silicones, polyurethanes and an acrylic terpolymer.

"The only material in this table which is showing excellent rating in toughness and stability, elastomeric recovery and so forth from this 28-year test is silicone elastomer," he said.

The distances begin to widen at about 140 feet, for the highest heat application with heat shield sleeves for the two nozzles of the Ariane 5 boosters, under the European Space Agency. The sleeves are made with gasket production using LSR injection molding.

"People ask me, 'What is the temperature resistance of silicone?' and to the best of my knowledge, the highest temperature a silicone application sees right now is the heat shield of the booster rocket of the Ariana," he said. "It is seeing more than 3,000°C, for less than two minutes, though.

"We've never received any complaints or had parts shipped back," he added.

Silicone's lowest temperature flexibility shows up at about 240,000 miles away, as part of the construction of moon boots used by astronauts. Studies have confirmed that the silicone chain is "flexible at -196°C, which is the temperature of liquid nitrogen, and this is for sure a world record of silicones," Franssen said.

But the Moon isn't even the farthest distance silicone elastomers have traveled from the human body, Franssen said. That clocks in at about 140 million miles away: Mars.

"I didn't receive any permission to talk about this silicone application of Momentive in this one because it is confidential, but we can safely say that's probably the longest traveled elastomer, as well," he said.