Putting the emphasis on fast cure

By: David Vink
LSR World

23 February 2018

Düsseldorf, Germany — Medical molders need liquid silicone rubber materials that cure quickly and at lower temperatures. Dow Performance Silicones featured the novel, innovative elastomers at two recent presentations.
Stéphane Cornelis, responsible for medical application engineering and technical service at Dow Performance Silicones in Seneffe, Belgium, spoke Nov. 14 about development and benchmark evaluation of the new QP1-33XX series medical quality LSR.
The material, launched at the Compamed trade fair in Düsseldorf, cures well below 140° C, which enables molders to use thermally sensitive additives and to do two-component molding of LSR with thermoplastics with lower heat resistance than glass-fiber-reinforced nylon or polybutylene terephthlate, which are the plastics most frequently used for LSR/thermoplastic molding.
Starting in 2016, Dow submitted QP1-3350 (Shore A50) to benchmarking tests on its Engel all-electric e-Max 100 liquid injection molding press. The machine, in Dow's Seneffe technical center, has 100 metric tons of clamping force.
Dow compared QP1-33XX with two standard LSR grades available at that time from two competing LSR producers, varying injection speed, holding pressure and mold temperature.
It also included its standard QP1-50 in the tests and another new Dow medical grade, QP1-250, a slow storage modulus and low “structure rebuild” Shore A50 LSR with its low dynamic viscosity rheology enabling easier flow for fast molding at lower injection pressure.
Cornelis observed “slow structure rebuild capability was demonstrated by fast constant rate metering without any delay after holding time.”
Dow showed the molded part of a respiratory mask at its Compamed booth, molded in QP1-250 by Sankt Augustin, Germany-based Intersurgical GmbH.
There was little difference between the materials in cure time at 180° C, but there was 15 seconds faster cure with QP1-3350 than the fastest curing of the competitors’ LSR at 120° C, which widened to a 133-second advantage at 100° C.
Although QP1-3350 tear strength at 30 kN/m (without post-cure) was lower than 44 and 46 kN/m for QP1-50 and QP1-250, it was higher than the 21 and 25 kN/m values for the competitors’ LSRs. There was little difference in tensile strength values between the different LSRs.
QP1-3350 post-molding compression set at 52 percent was slightly lower than competitors’ 60 to 62 percent values, but it was higher than the 39.1 percent for QP1-50. Post-cure tests were made at 200° C/4h and 200° C/8h, the latter showing significant differences, with QP1-3350 almost twice as high at 19 percent than competitors’ grades at 10 percent and much higher than the 6 percent value for QP1-250, while QP1-50 achieved 17 percent.
Based on 180 mm x 130 mm x 2 mm test plaque molding, Cornelis pointed out that one of the competitor’s grades had been found to need 100 tonnes clamping force due to 608 bar injection pressure, but that QP1-3350 with 325 bar and QP1-250 with 365 bar (injection speed 50 cm3/s) could be molded on a 50 tonnes clamping force machine.
Cornelis supported this by showing a chart in which the new QP grades had lower injection pressure at a given injection speed. Overall, 44-second cycle time at 120 °C (560 bar) was faster for QP1-3350 than the 78 seconds needed for the above-mentioned competitor’s material (540 bar).
So aside from widening additive and thermoplastic co-molding options, there are productivity and cost benefits from QP1-3350 with a smaller molding machine and faster cycle time, Cornelis said. He said it is also easier to mold thick wall parts, such as optical lenses, faster with this new material.
QP1-250, on the other hand, also offers particular advantages when producing LSR parts in molds with large numbers of mold cavities.

Biocompatible LSR
In a separate Dow Performance Silicones presentation at the Nov. 15 Compamed suppliers forum, Pennadam Sivanand, EMEA manager in Seneffe medical technology development, application engineering and technical service, talked about another new low-temperature and fast-cure Dow material, the QP1-5040 self-adhesive (SA) Shore A40 hardness medical-grade LSR.
Sivanand illustrated the material’s low-temperature cure by showing the same 100° C cure profile chart for QP1-3350 as Cornelis had done the previous day — the one with up to 133 seconds faster cure than competitors’ LSR grades.
Sivanand said medical industry OEMs seek biocompatible elastomer solutions that withstand disinfection requirements and can be molded with fast cycle times. He said attractiveness of the new SA LSR is due to ability to eliminate mechanical interlocking and primers, with the higher efficiency and faster molding cycle time resulting in cost reduction.
Dow chose to investigate QP1-5040 bond strength in two-component molding with Tritan copolyester from Capelle aan den Ijssel, Netherlands-based Eastman Chemical BV, a thermoplastic resin similar in cost to medical-grade polycarbonate, yet free of bisphenol A and with higher stress cracking resistance to aggressive chemicals. Sivanand also talked about Tritan’s high clarity and UV resistance.
The SA technology behind QP1-5040 is based on use of “well-known system D adhesion promoters” acting as coupling agents to establish bonded interfaces through “diffuse interphase interpenetrating networks,” Sivanand said. LSR/copolyester adhesive pull-off tests on compression molded plaques with co-polyesters with 66 psi (0.46 MPa) heat distortion temperatures (HDT) of 95° C, 100° C and 110° C showed pull-off force rising from 154 N/25 mm for one of two 95° C HDT grades to 183 N/25 mm for the 110° C HDT copolyester.
Cohesive failure, meaning extent of damage to the substrate with which the LSR was bonded, was understandably 0 percent for stainless steel, with large differences between PC grades (below 1 percent and 65 percent), yet highest at 95 percent with copolyester.
Target applications for the new SA-LSR/Tritan material combinations include respiratory care masks (as an alternative to more expensive transparent nylon and similar-cost PC substrates). Potential is also seen in medical components, devices, enclosures, wearable monitors and various molded-on gaskets and seals.
Aside from the presentations by Cornelis and Sivanand in the Compamed suppliers forum, Dow Medical global strategic marketing director Gary Lord held a short material science choices presentation in the Medica medical industry fair. Dow exhibited at both of the parallel shows.
Lord talked about how Dow contributes with its silicone materials to the “growing phenomenon of the internet of medical things.”
Dow Performance Silicones is a global business unit of DowDuPont Inc.