Frequently Asked Questions
This page presents information from Minerva's project files covering frequently asked questions and other items which we think may be of interest.
Are the mouldings you use made from 'pure plastics'?
There is no such thing as a moulded product made from a 'pure plastic'! Most of us know that a PVC part is not just PVC – but the same applies to a polypropylene product or a polystyrene moulding or any other generic plastic material. They ALL contain various additives or processing aids either to give the material certain characteristics, or to make it processable through the moulding or extrusion operation. These additives would be plasticisers, lubricants, stabilisers etc. which are, by nature, very reactive chemicals. In a medical or diagnostic application where these plastics come into contact with bio-materials or reagents, complex cross-reactions can be created between the chemicals and bio-materials which are often not understood because it is assumed that a plastic such as polypropylene is relatively inert – which it is, but the additives are not.
Our products are made by an outside moulder – how do you overcome quality problems?
A trade moulder, just like a computer, can be only as good as the information given to them. Sometimes problems are not due to a poor quality supplier but due to insufficient product specifications or end use information provided by the customer. Many bio-technologists or other specialists without a background in plastics processing believe that injection moulding of plastics is a relatively 'low tech' operation. It most certainly is not. Especially when the application is using the material in a very critical way or the design is at the cutting edge of technology. The balance of tooling design, processing conditions, material specification and product test parameters are critical and will often need to be evolved by close co-operation and understanding between moulder and customer. The top end trade moulders have a wealth of experience which can be applied to a new application, but if your application is a very specialist one, it is up to you, the customer, to help the moulder understand about your own field.
We have often been asked to resolve problems between customer and processor which are due to a lack of understanding and communication on both sides about the way the materials are processed and how this impinges on the end application. However, it is also the case that sometimes an 'interpreter' who understands both technologies is needed, and that is where Minerva can help.
What's wrong with PVC?
PVC has received possibly the worst publicity of any plastic, especially for medical and food applications. Much of this has been the result of misunderstood research into migration of additives used in PVC. After publication of one of the papers showing the appearance of plasticiser in blood stored in plastic bags, it was found that the plasticiser came from the PVC tubing used in the analytical equipment! PVC does contain many additives and these additives are much more mobile than those in other plastics – they can move around within the plastic and often move up to the surface where they can be leached off by materials in contact with the plastic, especially fatty substances. However, the exact mechanism of this leaching process and the methods of analysis needed to measure it have been shown to be very complex. For example, some additives will migrate not by themselves but 'riding on the back of' another additive. To cure this problem, it may be necessary to change the carrier additive rather than the identified migrating material. The main concerns have centred around the plasticiser DEHP (di(2-ethylhexyl) phthalate). This is used because it is very effective, low cost and has relatively low toxicity but it does migrate rapidly. Attempts to replace it with non-migrating plasticizers have proved of limited success. The approach of the regulatory bodies has been to put limits on the amount of acceptable migration for different applications. However, this approach is questionable because of the difficulty of deciding what the limits should be and also the high profile publicity given to the whole issue has made the public aware that the use of PVC does cause 'contamination' of contact substances such as food or blood and how harmful this contamination is will always be hotly debated. So, alternatives to PVC are available now but all have their own disadvantages and their selection depends very much on the application.
How do you bond filters to plastic housings?
Traditionally, the rules of bonding materials by use of glues or welding have been that only similar materials can be welded and only high surface energy materials can be glued (low energy plastics like polypropylene have been very difficult). It is often necessary to bond a filter made from one material to a housing made from another (e.g. a nitrocellulose filter to a polypropylene housing). To do this, you have to break the rules but with the help of modern technology and sometimes a bit of lateral thinking, it can be done. In medical applications, though, we must be wary about the use of adhesives which can have harmful effects on the application either by being toxic, as some adhesives are, or by causing an interfering chemical reaction with, say, the biochemistry used in a clinical diagnostic test. At Minerva, we enjoy breaking a few rules and have been able to defy the academics by successfully bonding 'the impossible'. One way of doing this is to use ultrasonic welding to melt the plastic housing at the bond interface and then to force the molten plastic into the pores of the filter to create a mechanical lock. Another technique is to use an inert intermediate material which bonds to each of the main materials to be bonded. Where appropriate, we have developed new techniques for bonding filters into housings and are happy to do this for any new applications.