Liquid Reaction Rotational Moulding of a Biostable Silicone-Urethane Co-polymer
Ajay D Padsalgikar1, Elenora Stepanova1 , Paul Nugent2
1AorTech Biomaterials, 677 Springvale Road, Mulgrave, Vic 3150, Australia
2International Rotational Moulding Consultancy, Reading, PA 19606, USA
Medical devices in a variety of applications such as cardiology, gastro-enterology and plastic surgery require thin, flexible and biologically stable polymer shells or balloons. Liquid reaction rotational moulding (LRRM) is a promising technique to fabricate such devices. Elast-Eon™, a co-polymer of silicone and urethane has been established in several studies as having a very high degree of biological stability and thus suitable for long term in-vivo applications. The combination of LRRM and Elast-Eon™, to fabricate thin, biostable, polymeric shells, is the subject of this study. Parameters such as the chemo-rheological behaviour of Elast-Eon™ systems and its effect on the process of LRRM are investigated.
The isocyanate used for Elast-Eon™ is Methylene diphenylene ioscyanate (MDI), Butane diol (BDO) and bis hydroxy tetra methyle disiloxane (BHTD) are the main chain extenders. The isocyanate and the chain extender together form the hard block of the polymer. The soft block comprised a mixture of hydroxy allyloxy terminated poly(dimethylene siloxane) (PDMS) and poly(hexamethylene oxide) (PHMO). The typical hard to soft block ratio used was 40/60. Elast-Eon™ was produced in a two step process where the MDI was reacted with the polyol mixture to produce a pre-polymer. The pre-polymer was mixed with the chain extender and injected into the mould of the rotational moulding machine. The rotational moulding trials were mainly carried out on Kent Ridge Instruments (KRi) laboratory roto moulder. The trials were carried out on a Teflon coated, hemispherical mould, 90 mm in diameter. The rheological profiles were measured using a parallel plate set up on the Rheometrics RS1 rheometer.
Different research groups have analysed the process of rotational moulding in general and liquid rotational moulding in particular 1, 2 . However, not too much work has been done in the area of LRRM. The little work done so far has been concentrated in the area of thicker articles. Rotational moulding of a liquid can be mathematically approximated by the flat plat withdrawal theory first developed by Groenveld 3 . This approximation can be reduced to equation 1 for the case of LRRM of polymeric liquids.
v = 2.3 r g d 2 / h (1)
or,
d µ ( h v) 1/2 (2)
Where v is the velocity of withdrawal, h is the fluid viscosity and d is the thickness of the deposited layer.
Equations 1 and 2 show that the thickness (d) is directly proportional to the square root of the fluid viscosity and the velocity of the rotational moulder.
Figure 1 shows the typical rheological behaviour of the Elast-Eon™ system as it undergoes phase separation during the chain extension period. This phase separation leads to vitrification and subsequent increase in the viscosity of the systems. In rotational moulding trials a few aspects of the rheology and the process stood out as the most critical ones and these matched quite well with earlier theoretical analysis as illustrated by equations 1 and 2.
The two main rheological parameters that influenced the thickness of the part produced were the initial viscosity and ‘gel’ time. The gel time can be described as the point where there is a dramatic increase in the viscosity of the mixture. It was observed that lower the initial viscosity, the greater the possibility of achieving a thinner article. The gel time influenced the time available for the liquid to build up a layer on the walls of the mould. Increasing the temperature of the mixture reduced the viscosity of the mixture but at the same time, reduced the gel time of the mixture.
The speed of rotational moulding machine along both axes and the ratio of the two speeds clearly influenced the uniformity of the deposition of the liquid layer on the mould. A specific ratio of 1:4 in terms of the ratio of the major axis versus minor axis speeds, gave the best result. As the starting viscosity of the mixture was fairly high, the major axis speed had to be reduced to very low levels in order to get the best uniformity of thickness distribution.
It can, therefore, be concluded that the initial viscosity, gel time, the ratio of major to minor axes speeds and the value of the major axis speed play a critical role in the liquid reaction rotational moulding process. It was possible to mould a thin shell using the LRRM process with a biologically stable polymer, Elast-Eon™, thickness of up to 300 microns.
References
1. R.C. Progelhof and J.L. Throne, Polym. Eng. Sci., 16, 680 (1976)
2. S.H. Teoh, et al, Rotation, III-3, 10 (1994).
3. P. Groenveld, Chem. Eng. Sci., 25, 33 (1970)
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