Alternatives to PPSU and PSU sheets for medical applications, selecting the right material for Sterilization
There are a number of different thermoplastic resins that are used to make sheet for medical applications. These resins include Polyphenylsulfone (PPSU), Polysulfone (PSU), Polyetheretheketone (PEEK), and Polyaryletherketone (PAEK), as well as many others. The cost of these resins is often ten or twenty times higher than Polycarbonate resin. One of the reasons that these resins are selected is their ability to withstand some of the sterilization techniques used in the medical market to destroy living organisms such as bacteria.
The main sterilization techniques include electron beam or gamma radiation, Steam Autoclaving, Ethylene Oxide (ETO) and Vaporized Hydrogen Peroxide (VHP).
Some of the resins such as PPSU and PSU are able to withstand most if not all of the sterilization techniques; however, due to the very high resin cost, this high performance often translates to high prices. Even though Highline can offer both PPSU and PSU sheets, the aim of this blog post is to look at whether there are more economic alternatives if the finished part only needs to withstand a known sterilization technique. To do this analysis, we will look at each sterilization technique in turn.
Electron beam or Gamma Radiation
Standard Polycarbonate does not perform well under repeated radiation sterilization. It tends to loose mechanical properties and yellow. However, there are some grades of polycarbonate that are designed to resist radiation and reduce the negative effects. These grades do not have the same level of resistance as PPSU and PSU, but could be an alternative where the product only requires a limited number of sterilization cycles during its life.
Another alternative is Transparent Polyamide. This material, even though it is more expensive than polycarbonate, is significantly cheaper than PPSU. Transparent Polyamide has excellent resistance to radiation an would be an ideal choice of material. HighLine does not currently produce or stock polyamide sheet, however we have previously produced the material and have the production capabilities to make it. If a customer had a large enough application for this material, we could consider custom production in partnership with the customer.
Standard Polycarbonate is not suited to steam autoclaving. However, there are high temperature grades of polycarbonate available for superheated steam sterilization. These grades have typically been produced for the injection molded part market rather than the extruded sheet market. If a customer had a significant need for this type of extruded sheet, HighLine could investigate whether it is possible to produce sheet from high temperature polycarbonate.
Polycarbonate has a fair degree of resistance to ETO, however it is certainly not at the level of PPSU and PSU. If the final product only needs to undergo a limited number of sterilization cycles during its life, polycarbonate may be an option, but full testing would be needed to verify. For repeated ETO sterilization cycles, transparent Polyamide sheet is an excellent lower cost alternative. Again HighLine has proved that it has the capability to produce polyamide sheet, but it would need to partner with a customer with a significant demand to make it economically viable.
Vaporized Hydrogen Peroxide
Polycarbonate has excellent resistance to vaporized hydrogen peroxide. If this sterilization technique is the only sterilization method used on the finished product, there would be no need to consider higher priced materials.
To summarize, if only one or two sterilization techniques are to be used on the finished product, significant cost savings can often be achieved by using lower cost materials than some of the high end thermoplastics such as PPSU and PSU. Not only does this make the finished items more economically viable, but it also allows them to benefit from some of the other properties of polycarbonate or polyamide such as transparency, clarity and strength. If a customer has a significant need for material to withstand sterilization, we would recommend that they contact us so that we can have a full discussion on the options.
As a final thought, it could also be possible to add anti-microbial additives to the polycarbonate or polyamide sheet to further improve their use in the medical market.
HighLine is pleased to be the first company to offer Carbon Neutral Polycarbonate and PET-G sheet. We have committed to quantifying, reducing and offsetting the global greenhouse gas (GHG) emissions associated with our operations and production of the polycarbonate and PET-G sheets.
We have responded to many of our customers that are looking for ways to reduce their environmental impact beyond token measures such as including regrind in the product. By quantifying all direct and indirect GHG emissions associated with the product, from oil extraction and refining, to polycarbonate and PET-G resin production, to sheet manufacture and transportation to the customer, we are able to identify areas where we can change the process to reduce emissions. Over time we can improve the accuracy of this emissions estimate to a point where we will be able to certify products to Green House Gas Protocol standard.
We have already found areas where we have been able to reduce the GHG emissions, such as:
- Encouraging Polycarbonate and PET-G resin suppliers to reduce emissions and preferentially purchasing from suppliers that demonstrate a commitment to reducing GHG emissions.
- Utilizing green electricity for sheet production.
- Using container ships with a high Environmental Ship Index (ESI) to reduce GHG emissions.
- Shipping material by road using carriers with low GHG emissions and plans for further reducing emissions.
- Eliminating air-freight shipments.
Each pound of Polycarbonate sheet generates over 5 pounds of GHG emissions by the time it is delivered to the customer, Our aim is to continue to reduce this amount and we are setting aggressive goals for reduction. However, we recognize that as the product is derived from oil, it will not be possible to eliminate these GHG emissions completely. HighLine is therefore working with Carbonfund.org to offset any GHG emissions that cannot be eliminated. This offset is carried out by investing in North America based renewable energy projects that have been certified by a third party. These projects reduce GHG emissions and this reduction is used to offset the remaining GHG emissions associated with the polycarbonate and PET-G sheets.
Our aim is not just to reduce the emissions associated with our extruded sheet, but also to encourage resin producers and other extruded sheet manufacturers to also reduce GHG emissions. We are therefore reaching our to other sheet manufacturers in the hope of establishing a working group. We have formed a LinkedIn Group called “Carbon Neutral Plastic Sheet” as the starting point for information exchange.
When talking to people about the advantages of polycarbonate over acrylic for ice hockey spectator shielding, one topic that often comes up is the higher cost of polycarbonate compared to acrylic and whether the benefits justify the cost.
There multiple sources of evidence that flexible boards lead to a reduction in injures. The following link to the British Medical Journal shows an article where injury rates at the IHF World Championships were studied over a seven year period. It shows that injury rates (including those to the shoulder and head regions) were significantly lower when flexible boards were used compared to glass boards. Increasing this flexibility by using polycarbonate rather than acrylic is likely to lead to further reductions in injury rates.
When looking at the cost of polycarbonate versus acrylic sheet, there is no doubt that a sheet of 0.545″ or 0.472″ sheet of 48″ x 96″ polycarbonate is more expensive than an equivalent sized sheet of acrylic. This increase in cost may not be significant for arenas at the higher levels of hockey but could be an important factor for community level rinks.
However, it is important to remember that with polycarbonate sheet, it is not necessary to use the same thickness sheet as for acrylic sheet because the polycarbonate sheet will not break. In fact it is better to use a thinner sheet as it further increases the flexibility. It is perfectly possible to replace 0.545″ or 0.472″ acrylic sheet with 0.39″ polycarbonate sheet; this reduction leads to 30% weight saving in the case of the 0.545″ sheet and 20% weight saving in the case of the 0.472″ sheet. This reduction in the weight means that the price of the polycarbonate sheet drops and becomes very competitive with the thicker acrylic sheet. Even though switching from polycarbonate to acrylic is unlikely to give an material cost saving, it is also unlikely to contribute to a cost increase.
Although the capital cost is likely to be similar, the labor cost of installing the thinner sheet is likely to be lower as it can be installed quicker and with less personnel. Also shielding breakage will be eliminated by using polycarbonate, reducing replacement cost. There is also the very real cost saving associated with reducing player injuries, which we will not attempt to calculate here.
In conclusion, because thinner sheet can be used when switching from polycarbonate to acrylic, it is likely that installing polycarbonate sheet will not be any more expensive than installing acrylic sheet. Cost is therefore eliminated as a reason for not switching to polycarbonate to improve hockey player safety.
As we discussed in the first two posts of this series, polycarbonate has a number of advantages over acrylic for hockey spectator shielding:
- Polycarbonate is more flexible than acrylic. This increased flexibilty can contribute to reducing the risk of concussion and improving player safety. Tests have shown it can be over 60% more flexible.
- Polycarbonate sheet will not break unlike acrylic. This strength leads to less risks to spectators. Also the sheets can be made thinner, leading to less potential for injuries to arena personnel during installation.
Given these significant benefits, why then did arenas choose acrylic when moving away from tempered glass? The answer lies in the optical properties of the sheet. Even though acrylic is not as flexible or as strong as polycarbonate, it was a significant improvement over tempered glass.
The acrylic sheet used in hockey arena shielding is made using a cast process. This process is a batch process where liquid monomer is poured between two sheets of glass separated by an edge gasket. The whole structure is then heated to polymerize the liquid monomer into the solid acrylic sheet. Once this process is complete, the two pieces of glass are then removed. Although the process is slow and has some limitations (such as the difficulty in controlling thickness), it produces a very clear sheet with very little optical distortion.
Polycarbonate sheet cannot be made in the same way due to the chemistry involved. To make polycarbonate sheet, it is necessary to polymerize polycarbonate in a reactor and then form plastic pellets of polycarbonate resin. These pellets are then melted in an extruder and the molten polymer is then passed between some chrome rolls to make a smooth sheet. The process of extruding sheet is a continuous process that produces a very consistent product. Unfortunately, until recently, the optical properties of polycarbonate sheet were no-where near as good as acrylic sheet and had significant distortion. This distortion would make viewing hockey through the sheet a poor experience. This distortion issue became more severe the thicker the sheet. Most of the polycarbonate sheet on the market still has a distortion issue, particularly on the thicknesses required for hockey spectator shielding.
Believing that polycarbonate offers many advantages over acrylic for the hockey arena market, HighLine has devoted considerable resources to improving the optical distortion of 0.39″ to 0.585″ polycarbonate sheet. We are now able to offer a sheet that has optical properties as good as cast acrylic sheet. We test the optical properties by projecting a line pattern through a sheet that has been inclined. The test equipment set up is shown in the diagram below. This test method is based on a German test standard used to measure distortion on front windshields of automobiles.
With a sheet with optical distortion, this pattern becomes distorted when it passes through the sheet. The projected pattern from a competitor’s polycarbonate sheet is shown below.
Viewing a game through a sheet with this level of distortion would be difficult at best.
In comparison below is the projected pattern from a HighLine Polycarbonate sheet that we used for testing for the hockey arena shielding:
It can be seen that virtually no distortion is detectable.
Sheet from HighLine Polycarbonate can now be used to improve the safety of players, spectators and arena staff without compromising the viewing experience at the hockey game.
In the first part of this blog series we showed that 0.47″ Polycarbonate is 20-25% more flexible than the same thickness Acrylic under simulated hockey player impact conditions. This increased flexibility could potentially lead to a reduced number of player concussions and reduce concussion severity.
In this article we will discuss how the thickness of the shielding affects the flexibility. Acrylic is traditionally used at a thickness of 0.47″. As Acrylic can break, it is not advisable to go thinner than this for spectator shielding. Indeed, some hockey arenas even increase the thickness to 0.545″ to prevent breakage, unfortunately increasing the rigidity in the process.
Polycarbonate does not suffer from the same breakage problems as Acrylic, therefore it is possible to reduce the thickness without having the risk of material breaking. To see how reducing the thickness of the Polycarbonate affected the flexibility, we tested 0.47″, 0.39″ and 0.31″ Polycarbonate under simulated hockey impact conditions (a 180lb weight hitting the shielding at a speed of 14mph). We found that 0.39″ Polycarbonate was 40% more flexible than 0.47″ Polycarbonate. We also found that 0.31″ Polycarbonate was 45% more flexible than 0.47″ Polycarbonate. We concluded that the largest benefit in flexibility was in reducing the Polycarbonate from 0.47″ to 0.39″ and that further reductions to 0.31″ only had marginal benefits.
Overall changing from 0.47″ Acrylic to 0.39″ Polycarbonate increased the flexibility by over 60%; a significant change from a player safety perspective. Even with this increased flexibility and reduced thickness, the Polycarbonate would virtually eliminate the current breakage issue.
Another factor to consider for arena safety is that changing from 0.545″ x 50″ x 80″ Acrylic to 0.39″ x 50″ x 80″ Polycarbonate would reduce the weight of each sheet from over 90 lbs per sheet to 65 lbs per sheet. This decrease is very significant when considering the safety of arena personnel lifting the panels into place and when considering the consequences of a panel falling into the crowd following an impact by the players.
One consequence of the increased flexibility of the panel is there is slightly increased movement of the panel in the frame. A 0.47″ Acrylic sheet moved 0.34″ in the frame whereas the 0.39″ Polycarbonate moved 0.71″ in the frame. This increased movement needs to be considered when designing the frame and HighLine can provide assistance if required.