Volume 7, June 2012

03| 2012

Magazine for the Polymer Industry

preview

Plastics and mobility

Silicone elastomers

Carbon nanotubes

Mold surface heating

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02.-05.07.2012

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156 RFP 3/2012 – Volume 7

Silicone elastomers

1. Overview on elastomers

In comparison to the global rubber mar-ket, silicone elastomers continues to play a niche role and represent only approximately 1.5 % of the global elastomers market [1, 2] (fi g. 1). Due to their -Si-O- backbone, sili-cones are considered as inorganic elastomers as opposed to all other synthetic elastom-ers and natural rubber which are based on a –C-C- backbone. This difference is root cause reason for the various single advantages and disadvantages of silicone elastomers against their functional competitors (fi g. 2).

2. Positioning

The ASTM D 2000 was created to make elastomeric materials easier to specify and to standardize for a variety of properties and specifications. Plotting typical per-formance of standard elastomers in the defi ned classes on heat-stability and oil-resistance, the position of silicones vs. or-ganic elastomers is visualized. The oil-swell test differentiates the elastomers more or less by polarity (and their capability to han-dle unipolar media such as oils, grease or fuel). The heat-resistant position is often a consequence of double-bonds (to the low end) or strong bonding energies (on the high end) (fi g. 3).

A universal elastomer does not exist. Ideal properties like

• the dynamic fl exibility and strength of NR,• the barrier properties of BR or FKM,• the heat-stability and acid resistance of FKM,• the low temperature performance and sta-

ble physical properties of silicone (VMQ)

can not be realized in one rubber prod-uct. However, certain design changes and optimization on critical properties of an ap-plication often open a window for material alternatives. Silicone elastomers are not the

optimum material selection for all individual needs. When many different properties are important at the same time and in the same product, there is a good chance that a sili-cone elastomer is a suitable choice for the application.

3. Markets for silicone applications

The main market areas for silicone elas-tomers are:

• Transportation (automotive, aviation, aero space, railway, and marine)

• Energy (high voltage insulation, wire and cable)

• Consumer goods (E&E, baby care, kitchen-ware and other)

• Healthcare.

Within those market segments a variety of specialized formulations have been de-veloped which often add value vs. standard elastomers. Examples include:

• Self-bonding formulations with primer-less adhesion to many plastic and metal substrates

• Self-lubricating LSR which exude a bleed-ing oil to ease assembly

• Antimicrobial silicone elastomers for healthcare applications

• Conductive compounds for energy appli-cations

• High transparent silicone rubber for opti-cal applications

Winning against functional competition

The success story of siliconesO. Franssen, H. Bayerl*

In the global elastomers market, silicone elastomers continue to be a niche with approximately 1.5 % of the global demand in rubber. However silicones grow faster than many industries and economies. Due to megatrends including aging population, for healthcare applications, environmental awareness in automotive and energy or consumer perception and legislation for example in consumer good applications.While silicones have an almost universal set of physical properties for rubber ap-plications, based on material cost considerations they are often not immediately selected - instead of judging by system costs. Successful silicone elastomer appli-cations can be achieved, when engineers understand the silicone product features and creatively apply their benefits for new applications and designs. This paper will help to position silicone elastomers vs. functionally competing materials including rubber materials like FKM, ACM, EPDM or natural rubber and metal in a spring or thermoplastics and glass in optical applications. Examples show how specific properties of silicone elastomers lead to successful new applications win-ning against functional competition.

* Oliver Franssen,

oliver.franssen@momentive.com,

Global Marketing Director

Elastomers Transportation

Heiko Bayerl,

Marketing Manager Elastomers

Automotive Europe

Momentive Performance Materials GmbH,

Leverkusen, Germany

Paper, Silicone Elastomers 2012,

27 – 28 March 2012, Berlin, Germany

Smithers Rapra, Shawbury, Shrewsbury,

Shropshire, UK

Fig. 1: Global rubber market 2011

25.9 million t in 2011

Natural rubber11.2 million tSynthetic rubber

14.4 million t

Silicone elastomers0.3 million t

RFP 3/2012 – Volume 7 157

4. Winning against functional competition

The decision drivers for new rubber mate-rial can be grouped under these headlines:

4.1 Integration of functionality

Example: A new part design is based on two-component molding. It saves the manu-facturing and assembly of seals or dampen-ing elements as well as assembly mistakes. LSR is available in self-bonding formulations and is compatible in cycle time to molding of many engineering thermoplastics. In many cases LSR can outperform lower cost organic elastomer materials.

4.2 Specifi cation limits

Example: Globalization drives part and system specifications to wider applica-tion windows. In the transportation sec-tor, functionality has to be maintained at very low and relatively high temperatures (from starting the engine at –40 °C to full speed performance on German motorway). Here, the engineers need elastomers which are fl exible far below freezing point and

have a good heat resistance for under the hood applications, preferably with little change of mechanical properties. Silicone elastomers are the only available products for this range.

4.3 Lower system costs

Example: Classical rubber manufacturing is consuming a lot of machine and labor time. The amount of raw material is of-ten much smaller than the costs for con-version, de-fl ashing, fi nal inspection plus added cost for scrap (of fi nal parts plus the cured material of the runner systems). Cycle times in the minute range make in-vestment and payoff of decent automa-tion equipment a fi nancial challenge. The LSR injection molding process enables fully automated, fl ash-less, waste-less molding with short cycle times. For those pro cesses, material costs are the biggest share of a part calculation. For high volume part se-ries LSR can often outperform organic elas-tomers.

4.4 New technologies

Example: Lighting is one of the global drivers in innovation. Energy saving, CO2 reduction, and safety accelerate the trans-formation to LED light. The share of LED in lighting is forecasted to grow from 15 % in 2010 to 45 % in 2015, substituting tra-ditional lighting like the historical Edison lamp [5]. The light power of LEDs is devel-oping rapidly, in combination with emis-sion of strong blue light and increasing temperatures. High transparent silicones combine resistance against high tempera-tures and against UV light at high optical transparency.

4.5 Feature to benefi t

For a winning development with silicone elastomers, a pure material replacement cal-culation from an existing drawing/applica-tion is often not enough. In LSR, elastomer innovations allow redesigning of parts and systems benefi tting from values resulting from simple feature differences to individual organic elastomers. Those include:

• Low viscosity-enables very thin walls and long thin fl ow paths

• Low hardness without plasticizers• Easy to color, even glass-clear products are

available• Silicones mechanics are almost linear/simi-

lar to a Hooke spring• Hydrophobicity• Excellent insulation properties.

Those features enhance the number of functional competitors (and application potentials) signifi cantly and silicone elas-tomers therefore fi nd usage in:

• Insulators for high voltage, replacing por-celain

• Replacements of metal springs• Replacements of transparent plastics and

glass in optical applications as illustrated in following examples.

5. Ultra-transparent LSR vs. glass and plastics

Dispensable, high transparent silicones have replaced engineering thermoplastic res-ins in packaging of semiconductors in LEDs for some years. Higher temperatures and the hard blue light radiation of the highly effi -cient new generation LEDs requires UV light resistance and low yellowing for the optical packaging materials. These properties com-bined with improved crack resistance are provided by silicone-based packaging ma-terials which are basically used in LED pack-aging nowadays.

Innovation in lighting design and the success of highly effi cient LEDs are driving the research for new materials which can be processed by injection molding, enabling high complex design at high production vol-

R = -CH3

= -CH = CH2

= -Ph= -CH2-CH2-CF3

Silicone elastomers:MQ Methyl-siliconeVMQ Vinyl-methyl-siliconePVMQ Phenyl-vinyl-methyl-siliconeFVMQ Fluoro-vinyl-methyl-silicone

R – Si - O – Si - O – Si - R

R

R

R

R R

Rn

Volume swell in IRM 903 oil / %

70100125150175200225250275

Tem

pera

ture

/ °C

FPM

FFKM

FVMQVMQ= HCR / HV / HTV

EPDM

NBR, ECOCR

CPE / CSMIIR

NR

notrequired

140 120 100 80 60 40 30 20 10

SBR

FSL

= LSR= FSE / FQE / FFSL

AEM, ACM, HNBR

Fig. 3: Graphical position ac-cording to ASTM D 2000

Fig. 2: The structure of silicones

158 RFP 3/2012 – Volume 7

Silicone elastomers

umes. The new LSR 7000 family from Mo-mentive provides high optical transparency, excellent UV and blue light resistance. The new ultra-transparent LSR is a good candi-date to replace transparent plastics like PC and PMMA in lighting applications which re-quire high temperature resistance and fl ex-ibility in design (fi g. 5, 6).

An obvious next step is to use high trans-parent LSR for secondary optical lenses which can be directly assembled to the LED (fi g. 4). The light of a semiconductor is spread in a very simple way by the packaging material, e. g. a globe-top. In order to form and to guide the light, secondary lenses and light guides are needed. So far such optics are mainly made from PC or PMMA. In order to protect these materials against the high tem-perature of a high power LED, those optical elements can not be assembled directly onto the light source – a 1 – 2 mm gap is necessary to reduce the junction temperature. Result-ing refl ections from plastic to air and from air to plastic reduce the effi ciency of such optical components. Secondary lenses and light guides made out of high transparent, temperature resistant LSRs can be assembled directly to the LED and higher effi ciency can be achieved. Additionally, LSR allows a higher freedom of design than PC and PMMA.

6. LSR vs. NR in baby care

When LSR was brought to the market some 30 years ago, baby bottle nipples were the fi rst application, the industry im-mediately started to work on. LSR brought a transparent appearance, was easy to in-jection mold at high yield and neutral in odor and taste. At the time the market was

dominated by pacifi ers and bottle nipples made from natural rubber latices. Studies show that LSRs not only fulfi ll necessary food compliancy, their allergene potential is far less than the natural rubber product. LSR is UV resistant and will survive cook-ing water without signifi cant change of appearance or physical properties. The de-cision to choose LSR in baby care at the end was a decision from many mothers and even more babies, which preferred the sili-cone elastomer vs. natural rubber. The baby care market today is amongst the biggest LSR application volumes globally. Also, in this mature segment the elastomer inno-vation continues to fulfi ll expectations for the next generation of products e. g. with higher tear resistance. A new development is for example Momentive’s LSR 2645 with improved hot tear resistance according to EN 1400-2 (fi g. 7).

7. Silicone vs. porcelain in high voltage applications

Silicone elastomers in high voltage ap-plications came up in the late 1960s when experts tried to fi nd a lighter and mainte-nance-free solution to replace glass and porcelain in composite insulators like hol-low cores (fi g. 9).

LSR 7070 (thickness 1.97 mm)

010

2030

405060

7080

90100

200 300 400 500 600 700 800 900 1,000Wavelength / nm

%

LSR7070 transmission at 6° incidence angleLSR7070 reflection at 6° incidence angle

LSR 7070, 655 nm: 2,8 dB/m⇒ 30 cm: 82 % transparency⇒ 100 cm: 53 % transparency

0

20

40

60

80

100

120

140

LSR 2640 LSR 2645 Competitor

Perf

orm

ance

/ %

Cooked in water Aged – hot air 70 °C Tear resistance

0

50

100

150

200

250

132 kV 275 kV

Silicone compositeCoated ceramic discCeramic disc with washing

USA

/ yea

r

Fig. 4: Multifunctional lens based on LSR 7070 [4]

Fig. 6: Transparency over wave-length of LSR 7070 [7]

Fig. 7: LSR 2645 in comparison [6] Fig. 8: Life cycle cost for insulators [3]

Fig. 5: Plastics and LSR 7000 after heat aging 6480 h 150 °C [6]

PMMA

PMMITyp 1

PMMI Typ 2

PMMITyp 1

PC COP LSR 7060

LSR 7005

LSR 7070 LSR 7080

WRL 4984

RFP 3/2012 – Volume 7 159

Key expectations for this application are:

• Low weight – high mechanical strength • Easy handling • Pollution performance – low maintenance

costs• Flexibility/explosion safety (seismic behav-

ior).

A functional elastomer competitor to sili-cones is EPDM with as good weatherability and insulation properties. Silicone insulators benefi t from standard silicone features, the most important are:

• Flexibility• Low density • Hydrophobicity• Low viscosity.

Hydrophobicity (which prevents contami-nations and fl ashovers on the insulator sur-face) and low viscosity (enables casting of big shapes) are key properties differentiat-ing silicones vs. EPDM. Silicone also has the ability to transfer its water repellent proper-ties into contamination layers that might be built up in heavily polluted environments. A good example here is Momentive’s Silopren Electro 242-0, that additionally features ex-cellent tracking and erosion resistance (1A 4.5 kV according to IEC 60587).

Looking at manufacturing costs, silicone leads to the most expensive part. However, in system costs which include assembly, long-term maintenance and disassembly, silicone insulators are the most cost effi cient solution [3] (fi g. 8). One of the main reasons is the need for regular cleaning of conventional insulators including those made with EPDM which is un-necessary with the hydrophobic silicones.

8. Heat-resistant LSR against ACM and FKM in charge-air-cooler gaskets

The demand for more effi cient vehicles continues to drive the share of turbo-charged engines. Since turbochargers are running in a loss-oil lubrication with engine oil, their compressed hot charge air contains oil. The compressed air passes through a cooler to increase the density. The charge-air-cooler- gasket is normally a big square seal. In manu-facturing, the necessary fl ow length of the thin shape can cause problems in mold and process design with organic elastomers be-ing high in viscosity. Materials have to com-

bine necessary oil resistance with high tem-perature performance and acceptable static low temperature performance. Most speci-fi ed materials have been FKM (expensive and problematic in low temperatures) and ACM (better in low temperatures and cheaper, but limited in heat resistance). HCR silicone com-pounds came up (modifi ed with mineral fi ll-ers to achieve lower oil swell), but also these compounds had the same potential scorch problems in traditional rubber molding. In the early 2000s, a major OEM qualifi ed LSR 2670 black from Momentive. The LSR fea-tures fi t perfectly to the diffi cult to manu-facture geo metry, however LSR has higher swell in oil than any of the mentioned func-tional competitors.

• The very low viscosity of the shear-thin-ning LSR allows fast fi lling.

• The inhibition of LSR prevents scorch of the material.

• The platinum catalyzed addition cure makes full automation affordable.

• No material losses with an automated waste-less and trim-free molding system.

• Good high temperature resistance and compression set.

LSR 2670 contributed to signifi cant cost savings in this application and is in success-ful mass production.

9. LSR against a metal spring – Silicone HCR against NR in engine mounts

Movement is creating vibration and noise. Electrical motors as well as combustion engines need to be fi xed and in the same time decou-pled. The ideal material provides constant re-bound resilience combined with good mechan-ical properties along the working temperature range of the application (fi g 10). Silicone elas-tomers are relatively constant in modulus and spring properties from very low temperatures –40 °C and up to high temperatures above 150 °C. Thanks to such outstanding properties, LSR substituted TPE as the decoupling element in the fi xing device of the electrical cooling van under the hood. A special self-bonding LSR allows two-component injection molding of a thermoplastic fi xing device with chemi-cally bonded dampening elements made out

Silicone housing

Core

Flange/fitting

Mechanical strenth:Core: glass fiber enforced tubeepoxy or polyesterFittings: aluminumHousing:silicone (RTV, HTV, LSR)

10,000

1,000

100

10

1

100

10

1

-100 -50 0 50 100 150 200

0.1

0.01

Temperature / °C

Stor

age

mod

ulus

/ M

Pa

Loss

mod

ulus

/ M

Pa

-71.62 °C

0.15

0.05

0.10

tan

δ

DMA

Crystallization temperature: –72 °CLow temperature standard < –80 °C

Remarks:According to DBL 5555, issue 06-2004,the low temperature standard is definedby the main maximum of the loss modulus.

Fig. 9: Silicone hollow core insulator

Fig. 10: DMA LIM 8040 [6]

160 RFP 3/2012 – Volume 7

Silicone elastomers

of silicone rubber to keep the device in place to dampen the noise and vibration over a wide range of temperature (fi g 11).

Another example is the substitution of NR by a special, low dampening silicone rubber used in the engine mount of the combustion engine of a power station. NR is destroyed at temperatures above 120 °C. High effi cient combustion engines have to be encapsulated for noise reduction. As a consequence the temperatures around the engine are increas-ing and new materials are required to replace NR as dampening material. Engine mounts made out of silicone elastomer work consist-ently at temperatures of 150 °C (fi g. 12).

10. Outlook

The next successful application of silicone replacing a functional competitor requires bright engineers and innovative raw material suppliers teaming up with each other. Unique silicone formulations can add value in use above and beyond just higher processing ef-fi ciency. More important are creative innova-

tors who can think outside of the box and uti-lize necessary freedom of design to function.

11. References

[1] Dr. Stephen Evans, IRSG, Presentation on

China Rubber Conference, March 2011

[2] Momentive Market Estimates 2011

[3] EPRI, Transmission Line Reference Book,

115 – 138 kV Compact Line Design, RP 260, pp: 29.

[4] Foto by KIMW Lüdenscheid, Germany

[5] Philips Global Lighting Market Study

[6] Momentive Lab Data

[7] Prof. Neyer, Techn. University Dortmund

Fig. 11: Dampening cage for cooling fan motor

Fig. 12: Silicone engine mounts

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