Emphasis on Surface Treatment Pre-Treating Plasticsin the Medical Industry
Catheter pad printing machine incorporating corona discharge pre-treatment method.
Catheters must be marked so that medical practitioners can determine insertion depth. Syringes and pipettes must be labeled so that doctors can see how much they are administering. For such applications, the pre-treatment of plastics is crucial. This article examines the three major methods in use today.Julian Joffe is the founder and president of Pad Print Machinery of Vermont, 201 Tennis Way, East Dorset, VT 05253. The company is a leader in the pad printing industry with such innovations as the ability to print on medical devices as small as 0.04 in. Joffe can be reached at firstname.lastname@example.org.
By Julian Joffe
The medical industry is perhaps the most innovative of all manufacturing sectors, and medical device makers exhibit an insatiable quest for new materials for their products. These products are often created with one major criterioninertness. The importance of inertness becomes obvious when considering a pregnancy tester container, which simply cannot leach plastic components into its test kit. Another example involves the human immune system itself. If a stent is inserted into the venous system, an adverse reaction could prove fatal.
While chemical engineers have managed to create a long list of materials that are extremely stable and won't react with other compounds, many manufacturers face the dilemma of identifying the end products with some sort of label. Catheters for neo-natal applications, as well as other applications, are often color-coded and marked at intervals so that medical practitioners are not guessing at the depth of the insertion. Syringes and pipettes are always labeled in order for doctors to know what and how much they are administering. For such applications, the pre-treatment of plastics is crucial.
To Treat or Not to TreatPre-treated substrates, including pre-treated polyethylene and polypropylene, are logical choices for medical device manufacturers. It's better to use inert materials that won't react to solvents nor cause havoc with the immune system than to employ materials just because they can be printed on easily.
When examining pre-treated polyethylene and polypropylene on a molecular level, you'll find no free radicals (cations or anions) on their outer surfaces. In order for most glues and inks to stick to a surfaceespecially a smooth onethey need to bond to free ions and thus produce an acceptable level of adhesion. After all, chemical reactions occur in order to find either the lowest level of activity or the best level of stability. When ink hits the surface of a compound, it remains unbonded unless the ink and the surface have free ions to create bonding.
Bonding & AdhesionVery often, in situations where extremely rough surfaces are being printed on, mechanical bonding alone offers sufficient adhesion. Of course, the best results are found when combining mechanical and chemical or "ionic" bonding. It is sometimes well worth considering a sandblasted mold surface that creates a matt finish on the final part because that can aid substantially in ink's adhesion. The rough surface dramatically increases the available surface area and produces a far greater potential for ionic bondingprovided some sort of pre-treatment has occurred.
The pre-treatment method you choose for your application depends on a number of factors including the characteristics of the substrate. Your options include flame, corona discharge, and chemical.
Flame treating, the oldest technique, uses the ozone created by combustion to add oxygen to the surface. It's not advisable on thin-walled or delicate surfaces due to the heat created by the combustion. Its major advantage is its range, which makes it well suited for erratic surfaces with deeper hills and valleys. The byproduct of heat aids adhesion even further by exciting molecules to higher levels of activity. Flame treating is the most reliable method of pre-treatments currently available, but many engineers, concerned with open flame on the manufacturing floor, opt to stay away from natural gas or propane flamers.
Corona discharge uses ozone, too, but the ozone is created from a plasma arc produced by an electrical discharge. It works well, is safe, and creates reasonably high levels of treatment. Corona discharge's shortcoming is its limited range, rendering this method best suited for flat or nearly flat surfaces.
Chemical pre-treatment, which employs a liquid that oxidizes the surface through chemistry, is cumbersome. Chemical pre-treats have to be applied wet and take several seconds to dry before printing and ink adhesion can occur. This method is best for sampling or short production runs.
The Final QuestionThe most common question regarding pre-treatment methods involves the length of time the pre-treatment lasts before the surface returns to its old stable state. The answer is not simple and depends on the environment. In all likelihood, pre-treated parts placed in a vacuum will hold the effect permanently. At the other end of the environmental spectrum, parts in a dusty and humid environment will lose their pre-treat within hours. The most efficient technique is to incorporate pre-treating and printing on the same piece of equipment whenever possible. This gets the surface printed while the treatment is seconds fresh. It also eliminates additional parts handling.
ONLINEFor additional information on the technologies discussed in this article, see Medical Design Technology online at www.mdtmag.com or Pad Print Machinery of Vermont at www.padprintmachinery.com.
Catheter pad printing machine incorporating corona discharge pre-treatment method