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On the surface of polymers

19th June 2015


Untreated PTFE surface Untreated PTFE surface
Plasma treated PTFE surface Plasma treated PTFE surface

Engineering polymers, such as PEEK, POM, polyamides and PTFE, have witnessed a remarkable growth in their use in recent years in medical product manufacturing, such as catheters, micro-catheters, nasogastric feeding tubes and endotracheal tubes to name but a few. They are typically chosen for unique properties which include resistance to chemicals, high strength to weight ratio and of course relatively low cost.

However, there are fundamental differences between polymers and other engineering materials which create unique technical challenges in a production environment.

One important property is the characteristic low surface energy of polymers (see Table 1) and the resulting intrinsically poor adhesion characteristics. This is an important obstacle in achieving reliable glue joints and  PAD printing steps, where various types of markings must be permanent. Various methods of improving adhesion are available but often don’t lend themselves to production settings and frequently involve the use of harsh and environmentally unfriendly chemicals to physically attack and etch the surface of the material. Plasma surface modification offers a reliable and environmentally friendly alternative surface preparation for most engineering polymers.

Plasmas can be a vacuum types (batch) or atmospheric types (in-line) and contain reactive gas species which, by careful choice of gas type and process parameters, can be used to increase the surface energy of a wide range of engineering polymers, and in doing so significantly improve wetting characteristics and therefore adhesion characteristics.

In-line atmospheric plasma surface treatment has been successfully demonstrated to increase the surface energy of PEEK from 35mN/m to >72mN/m, ensuring permanent PAD print adhesion. The treatment is active on PEEK for several weeks and so parts can be stored until needed.

For PTFE catheters, air is ineffective due to the strength of the C-F bond. Batch processes are preferred which allow different plasma gases to be used and which are more effective in fluorine extraction from the surface. This process also increases the effective surface area which in turn improves ink adhesion as shown in the SEM images below for untreated and plasma-treated PTFE. The surface energy of PTFE is raised from 18mN/m to >72mN/m in this case also.

Conclusions

Both batch and in-line plasma treatments offer a reliable and repeatable surface preparation method for improving adhesion to a wide range of engineering polymers used in medical device manufacture. Applications include gluing and PAD printing of catheter tubes for example.

Table 1. Typical surface energies of different materials

Surface Energy (mN/m)

Surface Energy (mN/m)                   18.5

Polytetrafluoroethylene (PTFE)        30-40

PEEK                                               24

Silicone                                            25

Poly(vinylidene fluoride)                   31

Polyethylene (PE)                            31

Polypropylene (PP)                          33

Polystyrene                                      39

Poly(vinyl chloride) (PVC)                43

Nylon-66

Aluminium                                       ~500

Glass                                              ~1000        

Henniker Plasma is a UK-based supplier of plasma surface treatment equipment and processes, document the advantages of plasma treatment to improve adhesion on a wide range of engineering polymers.







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