Anti-microbial Polycarbonate

A number of our products are used in touch screen displays. In these applications customers often require Indium Tin Oxide coatings to conduct electricity and anti-reflective coatings to improve viewing characteristics.

Increasingly we are being asked about two other properties, anti-fingerprint and anti-microbial. We will save the discussion about anti-fingerprint properties for another day. Today I will give an overview of anti-microbial properties for Polycarbonate.

Touch screen displays are an ideal product for anti-microbial Polycarbonate. Touch screen displays are often touched by a large number of people and they therefore provide ideal transfer conditions for microbes. With touch screen displays becoming more common as payment points in fast food restaurants and monitors in hospitals, the market for anti-microbial Polycarbonate is small but growing. In addition to touch screen displays there are many other applications where anti-microbial properties are desirable.

It should be noted that when we talk about anti-microbial properties, we are talking about anti-microbial properties built into the Polycarbonate sheet to solely protect the sheet against micro-organisms. The anti-microbial properties are not designed to extend beyond the surface of the sheet itself. No public health claims that extend beyond the Polycarbonate sheet itself are being claimed implicitly or explicitly.

There are three broad groups of anti-microbial agents that can be used in Polycarbonate applications; these anti-microbial agents include silane, silver and triclosan based additives.

Silane based anti-microbials are nano-engineered structures that physically attract the microbes and then mechanically puncture the cell wall, killing the organism. Because the mechanism relies on mechanical damage to the cell, it does not allow the cell to mutate and become resistant. Also the anti-microbial does not need to detach from the surface of the sheet to enter the microbe and therefore does not leach into the environment.

Silver based anti-microbials release ionic free radicals that react with the cell DNA disrupting critical life processes in the cell. Silver based anti-microbials often rely on moisture to function and so have reduced effectiveness in dry environments. Over time certain microbes can also build up resistance to silver based anti-microbials as the organisms adapt. Silver based anti-microbials are perhaps the most common form of anti-microbial available.

Triclosan based anti-microbials release toxic bis Chlorinated Phenols that are consumed or absorbed by the cells, causing lethal mutations in the cells. In order to work the anti-microbial additives must leach from the Polymer into the environment. As with silver based anti-microbials, there is strong evidence that some organisms adapt and become resistant to this type of anti-microbial.

At HighLine Polycarbonate we typically favor using Silane based anti-microbial products, however, we have worked with customers that prefer to use silver based anti-microbials. Once the anti-microbial additive is chosen, there are two main ways to add the additive to the sheet. For small quantities we typically use proprietary technology to formulate a coating to add to the surface of the sheet. This coating technology can be combined with many of our other coating technologies such as hard-coats and anti-reflective coatings. This solution works well as it is only necessary to have the anti-microbial additives at the surface of the sheet and in many applications a coating needs to be applied anyway.

For larger volumes of products that do not require a coating it is possible to add the anti-microbial additive to the Polycarbonate resin and make either the entire sheet or a cap layer anti-microbial. For a limited number of applications this method can be more cost effective. It can also be a better choice where the sheet is cut into small parts requiring the cut edges to contain anti-microbial additives.

At HighLine Polycarbonate we are happy to assist customers specifying anti-microbial products.

Temperatures for thermoforming Polycarbonate

When Polycarbonate is cooled below 150 C / 302 F, it transitions from a flexible structure to a rigid structure that locks into what ever shape it is in; this temperature is known as the glass transition temperature. Conversely, when Polycarbonate it heated above its glass transition temperature it becomes flexible and can be bent into various shapes. This property is used in the process of thermoforming.

Thermoforming can be carried out at any temperature above the glass transition temperature and below the melt temperature of 267 C / 512 F, although in practice the Polycarbonate becomes more flexible the higher the temperature and it is not necessary to approach the melt temperature. The Polycarbonate actually becomes difficult to use much above a temperature of 215 C / 450 F.

There are three broad categories of forming – Cold forming, Low temperature thermoforming and high temperature thermoforming.

Cold forming.

Cold forming uses a frame to hold the Polycarbonate sheet in the desired shape. The sheet is then heated to between 302 F and 340 F for several hours until the entire sheet (interior and not just the surface) rises above the glass transition temperature. The sheet is then cooled below the glass transition temperature to set the shape. Cold forming is a simple process, but can only be used for relatively simple shapes (often two dimensional) without tight radius bends.

Low temperature thermoforming.

Low temperature thermoforming is carried out between 350 F and 370 F. This process is often used for simple shapes where the Polycarbonate sheet drapes over a mold or into a mold. While it is possible to achieve relatively simple 3D shapes with low temperature thermoforming, complex shapes with lots of detail are not possible. One advantage of low temperature thermoforming is that pre-drying of the sheets is not necessary.

High temperature thermoforming.

High temperature thermoforming is carried out between 370 F and 420 F. Complex shapes, sharp details and deep draws are all possible with high temperature thermo-forming. Many thermoforming processes use vacuum to achieve some of the complex shapes. One of the disadvantages of high temperature thermoforming is that all moisture must be removed from the sheet by drying the sheet prior to thermoforming. If this drying is not done, the higher temperatures will cause moisture evaporation bubbles to appear in the sheet during thermoforming.

Drying needs to be carried out above the boiling point of water and it is recommended that the sheet is heated to 120 C / 250 F to dry the material. The drying time is dependent upon the sheet thickness. For 0.118” thick sheet about 10 hours of drying is recommended, for 0.236” sheet, this can increase to closer to 30 hours. After drying the sheet should be used within a reasonably short time frame to prevent the sheet re-absorbing moisture from the air.

Hard coatings.

One thing to remember with thermoforming Polycarbonate sheet is that raising the temperature above the glass transition temperature will make the sheet flexible; any hard coating on the sheet will probably not be flexible and will crack during the thermoforming process. When purchasing Polycarbonate sheet for thermoforming it is important to use only hard coatings designed for thermoforming. These coatings are slightly more expensive than standard hard coats, but are considerably cheaper than the alternative of post coating any thermoformed parts.