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.
The Mil-P-83310 Polycarbonate specification for sheet was written by the United States Air Force for the material that is used to construct the canopies on modern fighter jets.
Polycarbonate is used for these canopies due to its strength, clarity and resistance to damage from bird strikes.
The US Navy tends to use Stretched Acrylic for its canopies rather than polycarbonate. The main reason for the difference is assumed to be due to the different requirements for ejection from the aircraft. Ejections from Air Force jets tend to be at higher altitude and the canopy is discharged before the ejection seat is activated. For Navy aircraft there is a requirement to eject quicker due to the low altitude during carrier take off and landing operations. For this reason there is no time to eject the canopy first, so explosive charges detonate the canopy. Acrylic is able to shatter upon detonation, whereas the unbreakable nature of polycarbonate makes it unsuitable for this process.[Note: Stretched Acrylic is significantly different to standard extruded or cast Acrylic PMMA sheet and there is a specification that covers this material for aerospace applications].
Moving on to the Mil-P-83310 Polycarbonate specification, we will now detail why the material is different from standard polycarbonate sheet and why it is so difficult to produce to the specification.
The first and most obvious difference from standard polycarbonate sheet is the physical appearance of sheet. Most polycarbonate sheets have large amounts of blue dye to overcome the natural yellowness of the polycarbonate and any additional yellowness caused by adding regrind. The dyes do not remove the yellowness but rather mask it with a blue color. This makes the product appear brighter and cleaner. Even though the product appears brighter, the blue dyes actually lower the light transmission slightly, particularly in the blue part of the spectrum. This lowering of light transmission is not acceptable to the Air Force for canopies.
At HighLine we therefore use a special resin produced by one of our partners specifically for aerospace applications. These types of resins are only produced by a limited number of resin producers and are strictly controlled. We also use some proprietary technology to maximize the light transmission to meet the demands of the specification.
The next requirement of the specification is to control the internal strain or shrinkage of the material to less than 1%. Shrinkage has been discussed at length in another blog post by HighLine that can be found on this website. Most sheet manufacturers do not control shrinkage and those that do usually have a specification of 10%. Achieving the 1% target is a difficult proposition in sheet extrusion and requires advanced production techniques. Even though this target can be achieved by a well-run sheet extrusion operation using the latest sheet extrusion line, it can also be achieved post-production using advanced annealing techniques. These techniques will be discussed in a future blog post.
One of the most difficult parts of the specification to achieve is the control of minor defects such as bubbles, distortion and indents. The specification is significantly tighter than other optical grade polycarbonate applications such as transparent armor and ballistics glass for automobiles. Indeed some of the defects are difficult to detect with the naked eye and can only be observed with advanced viewing techniques (some of which are confidential to aerospace canopy manufacturers and their suppliers).
Other requirements of the specification that make it difficult to supply this grade of material include testing requirement, sampling, quality reporting and custom packaging. All of these technical and procedural requirements mean that it is very difficult to consistently produce and supply material against this specification. This difficulty is probably why only one or two suppliers other than HighLine Polycarbonate are able to produce material that meets the specification for this reasonably small, niche market.
If you are involved in the construction of military aircraft canopies and would like to discuss HighLine’s polycarbonate material meeting Mil-P=83310, please contact us by either email or phone.
As electronic displays are becoming more commonplace in new automobiles, and as these displays are becoming ever larger, the problem of glare on the displays is an increasingly important issue.
Sunlight reflecting off the display can make it difficult or even impossible to read the information. The solution is to make the clear plastic part on the front of the display (usually polycarbonate or acrylic) have anti-glare properties. Up until recently a clear plastic part was coated with an coating containing small particles giving anti-glare properties. Indeed, this was the solution offered by HighLine Polycarbonate to a number of our customers.
Even though the anti-glare coating did achieve the aim of reducing glare from the surface of the display and had a number of other advantages, it is important to note that it was not a solution without problems. The aim of this post is to identify some of the advantages and disadvantages of anti-glare coatings for this application and discuss an alternative approach.
The main advantage of an anti-glare coating is that by varying the amount of anti-glare additive in the coating, you can vary the level of anti-glare properties or “gloss” of the finished sheet. We offered anti-glare coated sheet with gloss levels of between 20% and 90%. By taking this approach you are able to produce a custom gloss level, which is optimized for the customer. Perversely, most manufacturers of anti-glare sheet choose to standardize gloss levels and only offer two or three gloss levels in their range – this standardization has the result of almost eliminating the main advantage of anti-glare coatings.
The next advantage is that the coating provides a barrier on the surface of the sheet that offers some protection against scratches and solvents damaging the sheet. Even though it is undeniable that there is some increased protection, it is minimal when the claim is examined in detail. The scratch protection offered by the coating offers little extra protection to real world damage when compared to uncoated product. This can be easily seen when car keys are thrown lightly at the coated surface (a not uncommon occurrence when car keys are no longer inserted into an ignition but rather stored near the center display). The coating, like the base polymer is easily scratched.
Also when the claim of protection against solvents is examined, the protection is not as great as implied. The coating looses protection in the area of any scratches and only the face of the sheet is protected. The edges of the sheet are not coated and so any solvents getting to the edge of the sheet would do damage.
The main disadvantage of coatings is that it is very difficult to control the level of anti-glare or “gloss” properties from batch to batch, within a batch and over the surface of an individual sheet. To explain the above statement, we need to examine how the coating is applied.
The coating is usually applied in a flow coating process (other processes such as spray and dip coating can cause even greater variation). Flow coating involves hanging the sheet vertically and then flowing a coating solution down the sheet over the surface. Excess coating flows off the bottom edge of the sheet and is recycled for use on the next sheet. The sheet is then cured in an oven.
The problem is that the coating contains heavy anti-glare additives and these additives tend to flow towards the bottom of the sheet. This process makes the bottom of the sheet have much less gloss than the top of the sheet. This variation in gloss can be minimized by coating a smaller sheet; giving less distance between the top and bottom of the sheet. When we produced by this method we were able to reduce the difference in gloss between the top and bottom of the sheet to less than 7% by reducing the sheet size to 24” wide. Most coaters prefer to coat sheets of 48” and 72” wide to improve the economics; this size of sheet has even more significant variation in gloss. When automobile manufacturers are increasingly trying to reduce variations in gloss to 1-2%, coating becomes very problematic.
Also as the coating process moves from sheet to sheet, the solvent level in the coating can vary. This makes variation between sheets and between batches notoriously difficult to control.
Another problem with anti-glare coatings is the cost. Coating is naturally an expensive process; when coupled with a low yield for trying to get a tight tolerance on the gloss levels, it can become very expensive. Coupled with other problems associated with coating such as delamination and dust particles in the coating (which become critical in this application), it is unsurprising that automobile manufactures are moving away from coatings for anti-glare displays.
HighLine Polycarbonate has moved away from offering anti-glare coated product for automobile displays. We now offer an anti-glare texture on the surface of the sheet that is applied during the extrusion of the sheet. This texture has been specifically developed for display applications. It maintains clarity of the information displayed while significantly reducing glare.
The main advantage is that it eliminates variation in glare between the top and bottom of the sheets, between individual sheets in a batch and from batch to batch. The gloss level specification can be held to within 1%.
The greater control of the gloss level makes for a much more consistent product at a much lower cost than coating can achieve.
The sheet does loose some of the resistance to scratches and solvents, but as discussed earlier, these claims are really little more than marketing ploys and offer little real world protection.
The other issue is that at this time we are only able to offer one level of anti-glare that has been selected for this application. Coatings can offer a wide range of anti-glare levels in theory, but in practice, manufactures have avoided offering their customers this option.
HighLine Polycarbonate is able to offer samples of their anti-glare polycarbonate upon request.