SEMICOSIL^® 811

Mixing & Dispensing Application

ELASTOSIL® CAT Pt or Pt-F contain the Pt-catalyst, ELASTOSIL® CAT UV contains the UV active catalyst, while SEMICOSIL® 811 contains the crosslinker. Even traces of the catalysts may cause gelling of the component containing the crosslinker. Therefore tools (spatula, stirrers, etc.) used for handling of the catalyst containing component must not come into contact with this component.

The two components should be thoroughly mixed at a 10 : 1 ratio by weight or volume as a target process value. Acceptable mixing tolerances range from 9,7:1  to 10,3:1 (w/w).  Necessary dosing accuracy is provided by state-of-the-art equipments (piston dispenser, endless piston or gear pumps) for shot weights in the gram scale.

Machine capability tests should be conducted to safeguard processes especially if significantly lower shot weights are considered.

 For homogenous mixing we recommend the use of a 2-part dispensing machine equipped with preferentially a dynamic mixing system or a static mixer. Sufficient mixing performance can be achieved by using a static mixing tube with maximum <6 mm diameter and at least >32 helical turnings. Manual mixing/dispensing is not recommended.

Degassing

SEMICOSIL® 811 is filled into hobbock packaging avoiding inclusions of macroscopic air bubbles. Like all silicones significant volumes of air are dissolved in the material at ambient pressure during storage.

To eliminate any air introduced during dispensing or trapped under components or devices a vacuum deairing process in the material supply feeding system is recommended. In case of permanent material preparation in vacuo, it is recommended to not exceed room temperature or go below 50 mbar.

In all steps of the mixing process, transfer and bonding of the substrates (manually or by robots) a potential contamination of the substrate surface should be avoided. Substrates pretreated should be used in the intended time period.

Because of the pronounced shear thinning effect and the strongly reduced viscosity under shear SEMICOSIL ® 811 / ELASTOSIL ® Cat. can be dispensed with high dispensing performance and enables superior wetting of substrated in the bonding process.

UV Cure

Discharge Lamps

SEMICOSIL ® 811 / ELASTOSIL ® Cat UV is activated by direct UV irradiation using UV B and UV A light . UV-irradiation should use emissions in the wavelength range between 250 and 350 nm. Typically, D-bulbs (Fe-doped Hg-light sources) using ozone-free quartz should be used and are commercially available. H-bulbs with emissions below 250 nm are not recommended.

UV-intensity levels here referred to are corresponding to the integral intensity of a Fe-doped ozone free discharge lamp as measured with a UV-intensity sensor array that is sensitive in the UV A region (Hönle AG  area sensor UVA (330 nm - 400 nm).

Curing time of SEMICOSIL ® 811 / ELASTOSIL ® Cat UV is highly dependent both on the intensity  and dose of the UV-light and the spectral intensity distribution. Curing is also dependent on the layer thickness, the optical properties of the substrate and temperature. Increase of the temperature  will fasten curing reaction. 

Approx. 50 mW/cm² can be regarded as the minimum intensity that is required to activate thin layers of 1-2 mm thickness. With increasing UV intensity the light propagates into deeper layer sections. For 120 mW/cm² light intensity the following dependency of the curing time on layer thickness can be given for an open layer of stable thickness which is not bonded to another substrate. Maximum layer thickness at this intensity is approx. 8 mm

UV-LED

SEMICOSIL ® 811 / ELASTOSIL ® Cat UV  alternatively can be cured using high power 365 nm UV-LEDs. Currently, the use of  UV-LEDs can be recommended in case distance of light source to silicone can be set to a mimimum distance (< 0,5 cm), in case of matching of size of silicone area to be irradiated and LED array.  Moreover, wet film thickness should be less than 3 mm as depth of UV light penetration ir reduced compared to discharge lamp at comparable overall UV intensity.

General

SEMICOSIL ® 811 / ELASTOSIL ® Cat. cures at room-temperature or after UV irradiation (with ELASTOSIL CAT UV) and generates unprimed adhesion. Depending on the specific substrate and the intended time period for adhesion built-up a minimum of energy input via short heat tests or IR irradiation may be utilized to further accelerate cure and adhesion built-up.

We recommend to run preliminary tests to optimize conditions for the particular application.

For the processing  of  SEMICOSIL ® 811 / ELASTOSIL ® Cat Pt (Pt-F) potlife (purging of static mixing tube with a frequency of < 2 min) has to be considered.

Morover for processing of  SEMICOSIL ® 811 / ELASTOSIL ® Cat Pt (Pt-F) and SEMICOSIL ® 811 / ELASTOSIL ® Cat UV open times hav to be considered.

The open time defines the maximum recommended time between start of the reaction (e.g. mixing in case of combination with ELASTOSIL® CAT Pt) or between UV irradiation and the subsequent bonding step in order to allow optimum adhesion performance.

Open times are influenced by temperature of the silicone prior to bonding as well as temperature, UV intensity and dose (in the case of ELASTOSIL ® Cat UV catalyst). Above figure shows an open time of approx. 60 sec, which allows automated processing, for a 5 to 10 sec UV irradiation at 120 mW/cm² whithout major effect on adhesion performance on the model substrate.

General Consideration / Discharge Lamps

Curing time of SEMICOSIL ® 811 / ELASTOSIL ® Cat UV is highly dependent both on the intensity  and dose of the UV-light and the spectral intensity distribution. Curing is also dependent on the layer thickness, the optical properties of the substrate and temperature. Increase of the temperature  will fasten curing reaction. 

Approx. 50 mW/cm² can be regarded as the minimum intensity that is required to activate thin layers of 1-2 mm thickness. With increasing UV intensity the light propagates into deeper layer sections. For 120 mW/cm² light intensity the following dependency of the curing time on layer thickness can be given for an open layer of stable thickness which is not bonded to another substrate. Maximum layer thickness at this intensity is approx. 8 mm

UV-LED

SEMICOSIL ® 811 / ELASTOSIL ® Cat UV  alternatively can be cured using high power 365 nm UV-LEDs. Currently, the use of  UV-LEDs can be recommended in case distance of light source to silicone can be set to a mimimum distance (< 0,5 cm), in case of matching of size of silicone area to be irradiated and LED array.  Moreover, wet film thickness should be less than 3 mm as depth of UV light penetration ir reduced compared to discharge lamp at comparable overall UV intensity.

SEMICOSIL ® 811 / ELASTOSIL ® Cat UV is provided as an UV active silicone. UV irradiation activates the catalyst starting an addition polymerization (hydrosilylation) of directly irradiated areas. Even after stop of the UV irradiation the reaction is not interrupted, but proceeds in degree dependent on time and temperature.

This allows the use of non UV transparent substrates. 

Curing speed and open times are adjustable by UV intensity and dose.

The UV active sealing adhesive SEMICOSIL ® 811 / ELASTOSIL ® Cat UV allows therefore the application (e.g. dispensing) of the silicone to one substrate surface, followed by UV irradiation and then in a separate process step the subsequent bonding to the counter substrate with the given open time.

SEMICOSIL ® 811 / ELASTOSIL ® Cat (Pt; Pt-F and UV) shows good primerless adhesion to many substrates. We recommend to run preliminary tests to optimize conditions for the particular application.

Safeguarding reproducable and suitable surface qualities is recommended. State of the art methods include in-line surface pretreatments like plasma processes or laser pretreatment.

SEMICOSIL ® 811 / ELASTOSIL ® Cat (Pt; Pt-F and UV) offers adhesion built up at moderate temperatures. First cohesive adhesion at 25°C may be accomplished on suitable substrates may be observed after approximately 60 min.

Time to first adhesion (allowing tightness tests of bonded parts) can be significantly shortened by moderate energy input. A short exposure to 80°C (2 min) can reduced time to first adhesion to less than 5 min.

Moderate energy input not only will boost speed of adhesion built-up, but also will support to increase concentration of established chemical links to the anchor groups of the substrate contribution to enhanced adhesion performance.

In the interest to safeguard robust processes and fitness of the product for an application dedicated tests after different exposure scenarios of application are recommended to customers for specific substrate surface qualities. 

Moreover, in order to enable optimum results it is recommended to consider potlife (combination with ELASTOSIL ® Cat (Pt; Pt-F) and open times (SEMICOSIL ® 811 / ELASTOSIL ® Cat (Pt; Pt-F and UV)).

Typically, at 25°C dispening speed and bonding process design should be designed that bonding is performed within 2 min after mixing of the material. Purging frequency has to be aligned, respectively.

In combination with ELASTOSIL ® UV potlife does not need to be considered (during 1-3 days). However, after UV-irradiation the bonding process of the involved substrates has to be affected within open time. Open times are depending on and can be tuned by UV intensity and dose. For a 120 mW/cm² / 10 sec irradiation with an ozone free discharge lamp open time is approx. 60 sec, which allows integration in automated processing.