Impact Testing of Polycarbonate Laboratory Safety Glasses and Facial Shields

Abstract

Polycarbonate laboratory safety glasses and facial shields were tested for impact resistance. Impacts from 22-caliber bullets fired from a firearm were compared with impacts of pellets fired from an air rifle. A low-weight pellet fired from an air rifle is a suitable and safer alternative to the use of a firearm. The results show that if there is a need for protection from flying projectiles, one should use multiple layers of protection. Furthermore, already-damaged protective equipment, even if the damage appears to be minor, may not provide any protection and should not be used. The resulting impacted polycarbonate lenses and sheets were used in a classroom discussion with the undergraduate chemistry students about polymer properties and adequate safety protection.

1. Introduction

The need for safe laboratories is now generally recognized [1,2] and is the primary concern in teaching laboratories [3]. Some useful guidelines for laboratory safety are provided by NIOSH and OSHA [4,5]. Early chemists were infamous for their lax attitude towards safety. It was only after the Second World War that laboratory safety became an important concern, and the importance of various shields as protective equipment was recognized relatively early [6].

Polycarbonate is a rigid polymer with excellent optical properties [7]. It is used in the manufacture of protective glasses, goggles, shields and bullet-proof windows. When it shatters, the pieces do not contain sharp edges. Therefore, it is safer than glass. Injection-molded bowl-shaped polycarbonate lenses, such as those shown in Figure 1a, are used to make safety glasses or goggles for motorcycle riders or skiers and glasses in general. Such protective glasses and goggles are designed to provide at least some protection against small flying objects. In the course of manufacture, each bowl-shaped piece is cut to provide a pair of lenses. Alternatively, a pair of laboratory safety glasses is made by injection molding polycarbonate into a suitable mold (Figure 1b).

Polycarbonate protective equipment has been tested against explosive blast impact [6], including detonation, to then generate glass or porcelain fragments [8]. Testing of the impact resistance of various polymers by firing a 22-caliber bullet at them has been reported [7,9,10,11]. To reproduce these results, several different types of 22-caliber long rifle (22 LR) bullets were fired at a polycarbonate lens [12].

As with the previous studies [7,8,9,10,11], this was only a qualitative study with a goal to determine the scope and limitation of personal protective equipment (PPE). The objectives were to examine safety glasses and facial shields used in undergraduate chemistry laboratories and illustrate to the undergraduate students the need for protective equipment while at the same time showing them the scope and limitations of it.

In an attempt to find a more convenient and safer alternative to the use of a firearm, several pellet guns of different designs were tested. A break-barrel air-powered rifle yielded good results. It was a suitable and safer alternative for the testing resistance of polycarbonate lenses to a high-velocity impact.

2. Materials and Methods

2.1. Use of a 22 Rifle

A polycarbonate lens was taped to a letter-size paper. The paper with the lens was attached to a target holder, and different types of bullets were fired at it from a semi-automatic 22-caliber rifle from a distance of 18 m (20 yards). Besides serving as a holder for the lens, the paper showed a bullet hole when the projectile passed through the lens. A single bullet was fired at each lens. Several such lenses were selected, and each was impacted by a second bullet of the same type.

2.2. Use of an Air Rifle

Various pellet guns were tested by firing projectiles at a polycarbonate plate, a pair of protective glasses, or a facial shield from distances ranging from 5 to 30 cm. A ruler was taped to the pellet gun barrel to serve as a spacer. Due to the presence of the air suppressor (Figure 2), the minimal distance was 5 cm. The distance of 30 cm caused damage to the polycarbonate sheet while the entire setup was still safely contained in a box. After several tests, it was determined that the distance of 20 cm was optimal.

Targets were placed in a cardboard box (36 × 36 × 44 cm). The sides and the top of the cardboard box were of sufficient thickness so that ricochet projectiles were not able to pass through them. A laminated wood panel covered by ~2 cm of old newsprint was placed at the bottom of the box to absorb the impact of any projectiles that could pass through the target. The target was placed on the top of the newsprint (in a suitable holder if needed). The pellet gun barrel was inserted through a hole on the top of the box and fired into the target.

2.3. Materials and Supplies

Polycarbonate lenses (thickness 2.7 mm) were donated by a manufacturer [12]. Polycarbonate sheets (8” × 10”, thickness 0.93 in or 2.36 mm) were purchased at a hardware store, and the protective wrapping was removed before use. A total of six pairs of polycarbonate protective laboratory glasses were used in this exercise. Four pairs were tested for impact from a 22-caliber rifle and two pairs were tested for impact from an air rifle. They were new and were not used before. The polycarbonate facial shield was several years old. During that time, it was used very seldom. The shield did not exhibit any appreciable wear and showed no damage. It was used because a new set of facial shields was purchased, and that one was a surplus.

2.4. Hazards

A pellet gun is not a toy. Pellets fired by it may cause serious injury. Protective equipment (safety glasses) should be worn.

Lead is toxic. Gloves should be worn when lead pellets are handled. Handling them with bare hands may be hazardous.

3. Results and Discussion

3.1. Impact of a 22 Long Rifle Bullet

There is a difference in the construction of ski goggles and various glasses and laboratory safety glasses. The former has frames into which lenses are inserted, while the latter are made of a single piece of polycarbonate. Upon impact, they behave differently. In the case of motorcycle riders and ski goggles, the impact usually results in a lens popping out of the frame. In the case of the laboratory safety glasses, the impact is absorbed by the entire unit.

Large bowl-shaped polycarbonate lenses were suitable for this study as they have a symmetrical shape and represent a relatively large target (d = 10 cm) for projectiles fired from a rifle at a distance of 18 m (20 yards). The results are summarized in

Table 1. Bullets used in each experiment and the results of the impact.

Entry	Bullet Type	Description	Weight 1 (g)	Velocity 2 (m s−1) 3	Result of the Impact
1	22 LR	Wax-coated lead bullet	2.59	326.1	Crater with a central peak
2	22 LR subsonic	Wax-coated lead bullet	2.59	216.4	Crater with a central peak
3	22 LR copper jacket	Copper-jacketed lead bullet	2.59	376.4	Lens shattered
4	22- LR copper jacket subsonic	Copper-jacketed lead bullet	2.14	225.6	Lens shattered
5	22 LR copper hollow point	Copper-jacketed hollow-point lead bullet	2.33	384.0	Passed through and left a large round hole.

¹ Bullet weight is provided by the manufacturer in grains (1 grain = 0.0648 g). ² Muzzle velocity is listed on the box. The actual velocity depended on the length of the barrel, which in turn determined how much of the charge burned out before the bullet left the barrel and how much drag the barrel exerted on the bullet. It also depended on the type of firearm. A bolt action rifle utilizes the entire charge to force the bullet out of the barrel, while a semiautomatic firearm utilizes only a part of it. A part is used to recycle the bolt and insert a new round. ³ Muzzle velocity is provided by the manufacturer in ft s⁻¹.

. The impact of an ordinary 22 LR and a subsonic 22 LR lead bullets left a crater-like impression on the lens (Figure 3 and Table 1, entries 1 and 2). Two lenses, one impacted by a standard and another by a subsonic 22 LR bullet, were each impacted by a second bullet of the same type. Each lens shattered into several large pieces.

While it may be tempting to interpret the results by treating polymers as viscous liquids, so viscous that they cannot flow, polycarbonate is not a liquid. It is a glassy (T_g = 147 °C), amorphous polymer [13]. A penetrating impact provided enough kinetic energy to its molecules to temporarily liquefy it. As the bullet passed through, the liquid flowed back to close the hole and solidified. This is why the resulting impact site had a crater-like appearance.

It is interesting to note that the impact of the second bullet shattered the lens. Apparently, as a result of the impact of the first bullet, there was a strain-induced crystallization. Thus, chain mobility was reduced, and the original elastic (ability of polycarbonate to undergo reversible deformation) and plastic (ability to undergo irreversible deformation without breaking) properties of the polycarbonate lens were lost. This effect was not present when polycarbonate was impacted by lower-energy projectiles from an air rifle (vide infra). Polycarbonate lenses are manufactured by injection molding. The resulting lens is a rigid amorphous polymer. It means that the individual polymer chains are randomly folded and oriented. Under the influence of various factors, such as pressure and shear, chains may undergo ordered folding and orientation to form microcrystalline regions [14]. Zhang and coworkers have shown that both pressure and flow are needed to induce crystallization [15]. In this case the flow resulted from brief melting of the polycarbonate due to the projectile impact, and the shockwave provided the necessary pressure.

Copper-jacketed lead bullets, regardless of their velocity, shattered the lens on the first impact (Table 1, entries 3 and 4). Thus, when impacted by a bullet made of a harder outside material (copper), polycarbonate did not exhibit a plastic response. A copper-jacketed hollow-point 22 LR bullet produced a large round impact hole (Figure 4 and Table 1, entry 5).

The results of impact on laboratory safety glasses were similar. The impact of a single 22 lead bullet resulted in the bullet passing through without leaving a hole (Figure 5a). Thus, it melted through, leaving crater-like impact. The impact of the second bullet resulted in considerably more damage. Three pairs of laboratory glasses were tested. Two shattered upon the impact of the second bullet, while the third only cracked and showed a visible bullet hole (Figure 5b).

3.2. Impact of an Air Gun Pellet

3.2.1. Pellets

Pellets of various sizes, shapes and weights were examined as an alternative to the use of a firearm. A standard 0.177” air gun pellet has a weight of 7.9 gr (grains) or 0.51 g. Lighter and heavier specialty pellets are available. The reduced-weight pellets needed for this exercise were hand-made by using an Exacto knife to cut off a part of a standard pellet (Figure 6a). The pellet was cut at the “neck” where the “skirt” met the pellet “head.” The resulting “skirt” was used. They were weighed, and initially only those weighing 0.30–0.31 g were used. However, after all the pellets prepared in this manner were tested, there was no difference in their penetrating power, and all of them were used. They weighed in the range of 0.24–0.35 g. Alternatively, a commercially available Crosman lead-free 5.4 gr (0.35 g) pellets were used (Figure 6b) [16].

3.2.2. Pellet Guns

Several pellet guns, along with different types of pellets, were tested in an attempt to reproduce the above effect without having to resort to the use of a firearm. All the pellet guns investigated were purchased specifically for this project and thus were new. None of the tested CO₂-powered or pump-action guns were suitable as they were not powerful enough to damage the protective glasses. However, a single-shot break-barrel air rifle provided good results. The air rifle used in this exercise was a break barrel with a caliber of 4.5 mm (0.177”). The pellet speed is listed as 1200 ft/s (366 m/s). The rifle has an air suppressor which cannot be removed and extends 5.0 cm past the end of the barrel (Figure 2). This break-barrel air rifle was also safer compared to other similar models. Upon breakage of the barrel, the safety engages, and the rifle cannot be fired until the safety lever is moved forward.

Air gun pellets were fired at a polycarbonate target from various distances ranging from 5 to 30 cm. Due to the presence of the air suppressor, the barrel could not be placed any closer than 5.0 cm from the target. Projectiles from the break barrel air rifle initially had unexpectedly high power and penetrated the polycarbonate from a distance of 5 cm, leaving a clean hole. From distances of 10–30 cm, they melted through without leaving a hole (crater-effect) (Figure 7). However, soon the power diminished, and the pellets dented but did not penetrate the polycarbonate plate even when fired from a distance of 5.0 cm.

Through use of lower-weight pellets, either made by cutting off a part of a regular lead pellet or using commercially available low-weight pellets, the crater effect was reproduced. As the energy of a projectile is linearly proportional to the mass of the projectile and exponentially to its speed (E = mv²/2), it may be tempting to attribute increased penetrating power to an increased speed and hence energy of the pellet. However, the rapid decrease and then leveling off the penetrating power of pellets, along with the presence of burn marks, points to dieseling as the cause [17]. Use of low-weight pellets offered less resistance to the compressed air which resulted in the piston moving faster. This, in turn, caused the air rifle to diesel again, which increased the speed and hence the energy of the fired pellets.

3.2.3. Dieseling

Dieseling is a phenomenon where lubricating the oils present in the cylinder of a break-barrel air rifle ignites when rapidly compressed [17]. The result is that pellets are propelled at a considerably higher speed. Dieseling also explains why the initial pellets fired from a new air rifle had so much energy. Due to the presence of a large amount of lubricating oil, the pellets were propelled at a rather high speed. This also explains the presence of burn marks on the sites of impact of the initial pellets. Furthermore, when fired in a closed space (a small box in this case), a very strong odor of burning oil, similar to that of a diesel engine, was noticed. As the excess oil burned out, dieseling either stopped or was minimal. The use of a lower-weight pellet caused dieseling to recur. Finally, this explains why, despite numerous efforts, it was not possible to damage the lenses with either pump-action or CO₂ pellet guns. Due to their design, none of them are capable of dieseling.

Dieseling can be caused on purpose. It may be tempting to a chemist, with the resources of a chemistry laboratory available, to do so. It has been well established that this practice is unsafe and should not be attempted. The only safe way to produce dieseling in an air rifle is to use a low-weight pellet. A low-weight pellet allows the piston to move forward faster and, in the process, generates more heat, resulting in the ignition of the oil droplets. A heavier pellet produces more resistance (“back pressure”), which slows down the piston and prevents, or minimizes, dieseling.

3.3. Initial Tests on Polycarbonate Sheets

Thus, after the initial excessive dieseling stopped and the power of the air rifle leveled off, ordinary commercially available lead pellets no longer produced crater effect. After some experimenting, crater effects were reproduced by using low-weight pellets, commercially available Crosman lead-free low-weight pellets (Figure 8a–c) or hand-made by cutting off a part of the regular lead pellet (Figure 8d,e). Unlike 22-caliber LR lead bullets fired from a firearm, pellets fired from an air rifle apparently did not have enough power to cause significant crystallization of the polycarbonate polymer. Thus, the second and subsequent impacts did not result in the breaking or shattering of the polycarbonate sheet.

3.4. Testing of Safety Glasses and Facial Shield

Various protective equipment, such as protective googles, safety glasses and facial shields, is made of polycarbonate. Polycarbonate has a reputation for its high strength and toughness and is a component of laminated bullet-proof glasses. An earlier study showed that polycarbonate protective glasses do not offer protection against even the lowest-power bullet (22 short) [9]. Search of the web for “bullet-proof glasses” resulted in a number of websites claiming that particular protective glasses resist impact of low-power bullet such as 22 LR. In view of the previously mentioned study [9], the results presented here and the fact that even the level 1 bullet-proof polycarbonate requires a thickness of 19 mm or more [18], such claims are doubtful.

According to the ANSI/ISEA Z87.1 standard, protective goggles should withstand a high-velocity impact of 6.35 mm steel ball traveling at 76.2 m/s or 6.00 mm steel ball traveling at 84.7 m s⁻¹ [19]. The fact that several of the air guns were not suitable for testing as they were not capable of causing damage to the protective glasses shows that the protective glasses exceeded the standard. For example, a 4.50 mm steel ball fired from Umarex CO₂-powered gun at 114 m s⁻¹ and a 5.56 mm (22 caliber) steel ball fired from Crosman 1322 pump-action air pistol at 140 m s⁻¹ did not cause visible damage to the protective glasses when fired from the distance of 5 cm.

The impact-protective ability of safety glasses and facial shields was tested by firing a projectile from a break-barrel air rifle in the same manner as that described in the previous section. A ruler taped to the rifle barrel served as a spacer, and pellets were fired from the distance of 20 cm. Polycarbonate glasses provided excellent protection and, while in some instances the surface was dented, various pellets did not penetrate the googles. Reduced-weight hand-made lead pellets easily penetrated polycarbonate sheet when fired from a distance of 5 cm (Figure 8). However, they did not penetrate safety glasses when fired from a distance of 20 cm (Figure 9). The only projectile to penetrate safety glasses from the distance of 20 cm was a Crosman lead-free 5.4 gr pellet, which is made of a harder material [16].

This example illustrates the need to occasionally wear both protective glasses and a facial shield (Figure 10). Thus, the 5.4 gr Crosman lead-free pellet penetrated the facial shield but was stopped by the safety glasses, which were dented.

3.5. Recommendations

Our tests have shown that the first impact of a 22 LR bullet resulted in crater-like damage to a polycarbonate lens and that the second one completely shattered it. Therefore, one should not use damaged polycarbonate protective glasses. One may be tempted to think that if nothing else is available, damaged glasses are better than nothing. However, as a result of an earlier impact, the polycarbonate may have undergone crystallization, and such glasses will easily shatter upon impact. Even though the pieces are not sharp, they can still cause injury. Thus, not only do such glasses not provide protection, they are hazardous to use. Protective glasses should be regularly inspected for damage and damaged ones discarded. One should have protective glasses, and other necessary protective equipment should be available in excess so that there is no temptation to use damaged items. For the same reason, a fume hood sash must be regularly inspected for damage, not just during the regular annual inspection but ideally before any work where there is even a remote possibility that a deflagration or an explosion may occur is carried out. In such an instance, a damaged sash may not provide any protection at all.

Impacts of air gun pellets on safety glasses and shields show a need for multiple layers of protection. While laminated shields are a possible alternative, they are bulky, thick and heavy. Multiple layers of personal protection are a better alternative in most cases as they are lighter and, since they are worn by the workers, offer protection as long as they are worn. Shields do not offer 360° protection and may not offer protection from ricochet projectiles.

Occasionally one is given an advice that if the work is carried out in a fume hood and the sash is down well below the face of the worker, protective glasses are not necessary as the fume hood sash provides sufficient protection. A fume hood sash may be made of tempered glass, laminated glass or polycarbonate. We have shown the advantages of multiple layers of protection and, therefore, even with the fume hood sash down, one should wear protective glasses and/or a facial shield. Furthermore, if one needs to adjust the equipment or perform an operation that requires lifting the fume hood sash, that person may not remember to put protective glasses on.

3.6. Testing of Protective Glasses and Facial Shields for Damage

Protective glasses and facial shields can be qualitatively examined for damage by the use of an improvised polariscope (Figure 11). One needs a light source and two polarizing sheets. The first sheet is the polarizer and is placed on the light source. The second polarizing sheet (analyzer) is placed above it, and they are crossed so that light does not pass through. Lens to be examined are placed between the sheets. The theoretical details are beyond the scope of this paper. In short, areas of high strain exhibit sharp color changes due to birefringence which is a result of the presence of microcrystalline regions. Completely strain-free, amorphous regions do not exhibit any color change. Regions with a low strain show smooth color change. It is normal for protective glasses to exhibit smooth color change as during the injection molding some low-level strain is generated. In fact, by examination of such lens by means of an improvised polariscope, one can identify the injection point. Lenses that exhibit sharp color changes are damaged and should not be used.

3.7. Classroom Discussion

Authors of earlier studies have established that boredom presents a significant obstacle when teaching laboratory safety to students [20] and that interesting safety demonstrations keep students engaged, help them better understand safety concepts and provide understanding as to why particular safety measures are being implemented [21]. Thus, one of the goals was to have undergraduate students become engaged and make the topic interesting to them. The results presented here were discussed with students enrolled in several sections of undergraduate organic chemistry classes. A lens, shown in Figure 3, was given to the students. They were told that the impact was caused by a 22 LR bullet and were asked to analyze the outcome. There was agreement among the students that the bullet dented the lens surface and then bounced off without penetrating it. Students were then told that the lens was attached to a sheet of paper, which was then shown to them. Students were surprised and at a loss to explain the presence of a bullet hole.

Next, the students were given sheets of polycarbonate that sustained impacts of air gun pellets (Figure 8). Initially, they had difficulties distinguishing between a penetrating impact and an impact that only dented the polycarbonate plate without penetrating it. Turning the sheet over and examining it from the other side helped students both identify impacts that passed through the sheet and explain the process (Figure 12). The impact that dented but did not penetrate the plate left a semispherical dome which was smooth to the touch (Figure 12a), while impacts of pellets that penetrate the plate left jagged back surfaces that were sharp to the touch (Figure 12b,c). The appearance of a frozen liquid was obvious. This may be an opportunity to discuss properties of polymers with the simplest explanation being that the projectile “melted through” the polycarbonate sheet.

When shown the impact of a pellet in the Styrofoam head equipped with protective glasses and facial shield (Figure 10), students concluded that there was a need for multiple layers of protection.

This also may be an opportunity to discuss the chemistry and physics behind dieseling in air rifles, even though it is not directly related to the properties of safety glasses and to the chemistry of polymers.

4. Conclusions

A break-barrel air rifle was a suitable alternative to the use of a firearm when protective glasses and facial shields were tested for resistance to a high-velocity impact. The results show that one should preferably use multiple layers of protection such as wearing both protective glasses and facial shield. Furthermore, impact-damaged protective equipment may not provide any protection, even if it appears that the damage is minor, and such protection should not be used.

For undergraduate students, analysis of protective equipment that suffered an impact allowed them to better understand the importance of laboratory safety and to be engaged in selecting and testing protective equipment and the consequences of not using it.