Overmolding for soft-touch designs

New self-bonding LSR technology created to optimize Eastman Tritan copolyesters

The medical industry has a great and growing demand for innovative soft-hard designs in devices, housings, and other equipment. A recent advance in liquid silicone rubber (LSR) technology makes it easier to satisfy this demand with medical grades of Eastman Tritan copolyester. 

The advantages of Tritan are well-known to readers of this blog. Medical grades of Tritan offer a unique combination of properties including:
      •  Outstanding resistance to medical disinfectants and solvents
      •  Excellent impact strength and durability
      •  Made without BPA and halogens
      •  Excellent clarity and color retention after sterilization by ethylene oxide (EtO),
         e-beam, and gamma irradiation

Transparent and opaque formulations of medical grade Tritan also feature a lower Tg and require a lower processing temperature than some engineering polymers—a potential challenge to achieving strong in-mold adhesion with elastomers. 

Specially designed to optimize Tritan
Momentive Performance Materials, a leader in LSR solutions, recently introduced a self-bonding LSR technology specifically for one-step overmolding of silicone to Tritan medical grades. This is the first commercially available process of its kind and is tested against USP Class VI and/or ISO 10993 biocompatibility standards.
The Momentive Silopren* LSR 47x9 series provides strong in-mold adhesion with Tritan without the need for primers. It also cures rapidly at relatively low temperatures—hitting the sweet spot for achieving functional performance and efficient processing with Tritan medical grades.  
The combination is ideal for adding soft-touch designs for applications such as:
     •  Respiratory devices
     •  Sealing elements
     •  Gaskets for joints in housings and hardware
     •  Buttons and switches on electronic housings and hardware
     •  Vibration reduction
     •  Membranes and lenses for electronic device housings
The combination also demonstrates excellent adhesion strength, as summarized in this table. 

Adhesion performance of medical grades of Tritan with Silopren* LSR 4739 liquid silicone rubber
Engineering thermoplastic Peeling force (N/mm) 24h/RT Peeling force (N/mm) 4h/100°C
Tritan MX711 copolyester 6.7 6.8
Tritan MX731 copolyester 6.4 7.1
Tritan MX811 copolyester 6.6 7.2
Tritan MXF121 copolyester 6.0 6.5

For more information about how to optimize overmolding with Tritan medical grades, contact an Eastman Technical Service Representative.