Unipace extends LSR bonding to more thermoplastics

Düsseldorf, Germany — Multicomponent molding of liquid silicone rubber to nylon and polybutylene terephthalate with good bond strength is well established.

The Unipace application development department at Kassel University in Germany has been working on extending that to other thermoplastics.

For nylon and PBT, processors use ultraviolet-C to boost the bond strength with LSR. That has especially been true with the development of adhesion-optimized LSR grades containing additional organofunctional silanes that act as adhesion promoters between inorganic silicon monoxide in LSR and functional groups of the thermoplastic.

Unipace has been working on extending the use of UV-C to boost bond strength with other thermoplastics. In 2017, for example, Unipace gave a live molding demonstration with LSR and polycarbonate at Arburg Technology Days in Germany.

Researchers also published trial results showing strong bond strength with LSR and polypropylene in 2022.

At the recent Silicone Elastomers World Summit in Düsseldorf, Michael Hartung of Unipace presented research using UV-C to mold polyketone with LSR.

Hartung described how Unipace obtained bond strength improvements between the Wacker Chemie adhesion-optimized Elastosil 3071/40 grade of LSR and the M330F easy-flow polyketone from South Korean plastics producer Hyosung. This was done by two methods. One was 285-nanometer-wavelength UV-C irradiation with an ozone-generating mercury arc lamp with 650 joules-per-mole photon energy to generate reactive species through absorbance of O2 and H2O.

The other method was Pyrosil flame pyrolytic deposition of a 20- to 40-nanometer amorphous rough silicone dioxide layer, a process developed by Jena, Germany-based Sura Instruments GmbH in which a gas flame doped with a silicone type of precursor is burnt. This results in formation of a thin reactive Si-OH group layer.

With an untreated sample achieving 1.22 Newtons per square millimeter peel force, one second UV-C exposure resulted in a much higher bond strength of 4.21 Newtons per square millimeter.

Pyrosil treatment gave 2.65 N/mm2, also a large improvement over the untreated sample, but lower than with UV-C irradiation.

Oven storage at 120°C for seven days created a tempering effect, which increased untreated sample bond strength to slightly under 4 N/mm2, while a Pyrosil-treated sample achieved around 4.6 N/mm2.

Tempering brought no improvement over high initial adhesive strength on UVC-irradiated samples, but it did bring a significant improvement with Pyrosil-treated samples.

One area where both types of treatment excelled was after seven days in storage in water at 70°C, with untreated samples recording only around 0.4 N/mm2, UVC treatment around 2.75 N/mm2 and Pyrosil treatment just under 4 N/mm2.

In looking at polyketone mold temperature, there was higher adhesion with higher crystallization with untreated samples, but lower adhesion with higher crystallization with UV-C samples.

The degree of LSR vulcanization also plays an important role in adhesion, Hartung observed. A major conclusion of the trials was that the surface treatment enables polyketone to substitute PBT in two-component thermoplastic/LSR potable water applications.

Although not within the scope of his presentation, Hartung has also recently worked on LSR/ABS two-component molding. In summarizing this work, Unipace head Ralf-Urs Giesen said standard thermoplastic ABS cannot be bonded to silicone rubbers by injection molding because of low softening temperature.

But Unipace has sought to address this deficiency within the MeKoMed project supported by the BMBF Federal German Ministry of Education & Research. The project aims at development of a multicomponent injection molding process for production of medical parts from standard thermoplastics and silicone rubbers. The project is coordinated by Hartung of Unipace and Dieter Göppert, managing director of Lahr, Germany-based mold maker Polar-Form Werkzeugbau GmbH.

Other project partners are medical component producers B. Braun Melsungen AG and Kaiserslautern, Germany-based Freudenberg Medical Europe GmbH, and Frankenberg, Germany-based hot runner produced Ewikon Heisskanalsysteme GmbH.

Unipace was founded by Kassel University with B. Braun Melsungen AG in 2013, as a cooperative development center conducting research "close to industry needs."

Unipace has investigated UV-C irradiation in the MeKoMed project along with the Openair plasma treatment process from Steinhagen, Germany-based Plasmatreat GmbH. It chose Elastosil LR3271/45 from Wacker Chemie as the LSR component, as this fast cure time limits heat exposure on the two types of ABS investigated: Ineos Styrolution Terluran GP 22 and Elix Polymers ABS M203FC.

Although ABS is a polar plastic, it requires surface activation for good LSR/ABS bond strength. With differences between the two ABS materials, UV-C produced similar or slightly higher peel strengths than plasma treatment. Peel strength fell significantly in all cases with dry and humid 70° C (158° F) aging tests, but less so with UV-C-irradiated samples.

Values were also lower after ethylene oxide or gamma sterilization, with plasma doing slightly better here with Terluran ABS than with Elix ABS.

The differences led Giesen to conclude: "No general assumptions or predictions can be made about adhesion of the joining partners in respect of material pairing, activation process and component conditioning."

But the material pairing factor had a particularly strong influence on adhesion, justifying investigation of many further material combinations.

"For components that are subject to only moderate mechanical requirements or offer a sufficiently large adhesion surface, pretreatment by means of UV or plasma, which can be easily integrated into clean room production, can already be used today," Giesen added.