WO2022241557A1

WO2022241557A1 - Helmet sheer layer

Info

Publication number: WO2022241557A1
Application number: PCT/CA2022/050788
Authority: WO
Inventors: Farid Golnaraghi; Adrian Wikarna
Original assignee: Shield-X Technology Inc.
Priority date: 2021-05-18
Filing date: 2022-05-18
Publication date: 2022-11-24
Also published as: CA3120169A1; CN115643793A; EP4340669A1

Abstract

An apparatus and method for reducing tangential acceleration of a head with a helmet having a sheer release layer between the helmet and the head of the user. The release layer includes a low-friction surface against another surface such that the helmet shell acceleration does not translate to equal acceleration of the layer immediately adjacent to the head of the user. Coatings and materials are disclosed as well as padding and attachment materials and methods.

Description

HELMET SHEER LAYER

This present application is an International Application of U.S Application Serial No. 17/323,397 filed May 18, 2021 and Canada Application Serial No. 3,120,169 filed May 28, 2021, which claims priority to U.S Provisional Application Serial No. 63/026,207 filed May 18, 2020; the contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

[0001] The present disclosure details improvements to protective headgear and, more specifically, to a sheer release layer for integration or securement to headgear such as to the interior of a helmet. The sheer release reduces rotational acceleration felt by the brain during an impact to an outside surface of protective headgear.

BACKGROUND OF THE INVENTION

[0002] Studies have shown that rotational forces on the head are a major source of concussions in an impact scenario. As such, designers have proposed headgear that, in addition to absorbing normal forces, reduces the tangential component of an impact to a head. United States Patent 10,143,255 (Golnaraghi, et. al.) describes a danger of injuries occurring to a head of a person wearing protective headgear due to rotational acceleration. The reason for this is that, upon impact, an outer shell of the protective headgear stops instantly, not allowing the head to keep moving. Stopping the human skull too quickly tends to cause concussions. A solution proposed by Golnaraghi et. al. is to provide an impact diverting mechanism that includes a top layer attached to the shell of the helmet and a bottom layer attached to the liner of the helmet. The top layer is disposed adjacent to and mechanically connected to the bottom layer. Upon impact, the top layer shifts and stretches relative to the bottom layer. The sliding and stretching dissipates kinetic energy.

[0003] Golnaraghi et. al. further describes embodiments that have an intermediate layer positioned between the top layer and the bottom layer. The intermediate layer may include a liquid-gel lubricant to facilitate relative slipping movement of the top layer and the bottom layer. As research in this area continues, there have been various structures proposed that include an intermediate layer to facilitate relative slipping movement of the top layer and the bottom layer. U.S. Patent publication 20130040524 (Halldin et al) titled “Intermediate Layer of Friction Decreasing Material” proposes the use of fibers as friction decreasing material.

SUMMARY OF THE INVENTION

[0004] In a first embodiment of the invention a protective liner includes a liner body having a padding layer and a low-friction layer. The padding layer faces the body of the user, such as the head, elbow, knee, or shoulder. The padding layer may also provide impact absorption, especially from normal forces. The padding layer provides comfort to the user interface, such as padding and breathability. It is preferably covered with a thin fabric.

[0005] The low-friction layer is preferably positioned outwardly from the padding layer relative to the user’s body. This layer is either positioned under or outside a layer of fabric (or other material). For example, a top fabric layer is positioned over the low-friction (i.e., slippery) layer in one preferred embodiment. In such an embodiment, the low-friction layer is an intermediate layer that overlies and is secured to the padding layer positioned on the bottom of the intermediate layer, such as with a helmet. The top fabric layer overlies the intermediate layer. The top fabric layer has a perimeter and a central portion. The top fabric layer is preferably secured on the perimeter leaving the central portion of the top fabric layer free to accommodate relative sliding movement of the intermediate layer.

[0006] The top fabric layer embodiment provides some distinct advantages. The slippery intermediate layer slides freely on fabric. Even when the fabric is secured at the perimeter, the central portion of the fabric still accommodates sliding movement of the slippery intermediate layer. In one preferred embodiment, a limited movement of 5mm to 15mm is sufficient to dissipate a significant amount of the kinetic energy and reduce rotational acceleration of the head to much safer levels in many instances.

[0007] If a greater range of sheer movement is desired to dissipate more kinetic energy in an intended application, there are a number of ways that this can be accommodated. For example, one could use a stretchable top fabric layer, such that the top fabric layer facilitates increased sliding movement over the slippery intermediate layer.

[0008] There are immediate benefits in the use of the helmet liner, as summarized above. The helmet liners that use the liquid-gel lubricant are more difficult and more expensive to manufacture. More things can go wrong with helmet liners that use the liquid-gel lubricant. If the seal confining the liquid-gel lubricant is compromised, the liquid-gel lubricant will leak out or the liquid-gel lubricant will dry out when exposed to air. In either event, movement of the top layer in relation to the bottom layer is adversely affected. [0009] In one preferred embodiment, the protective liner is a helmet liner attached to an interior surface of a helmet (such as a bicycle or motorcycle helmet) through the use of hoop-and-loop fasteners, commonly known by the trademark brand name VELCRO®. This attachment can be simplified when the top fabric layer can serve as a loop portion of a hook- and-loop fastener and mates with a hook portion of the hook-and-loop fastener secured to the top fabric layer. In some embodiments, the use of hook-and-loop fasteners improves the sheer release performance of the helmet liner when the top fabric layer serves as the loop portion of the hook-and-loop fastener. The hook portion anchors the top fabric layer, which tends to localize the impact force (thus the sliding motion) and also ensures the sliding motion happens optimally in specific locations on the liner.

[0010] In another preferred embodiment the low-friction material is held on a body material with the low-friction layer facing the user. The low-friction layer in such embodiment preferably includes a low-friction coating or surface treatment. In one embodiment, the low- friction layer is positioned beneath a liner material that interfaces with the body of the user. The liner material, such as a liner having an open-cell foam covered in a thin fabric, bears against the user’s body on one side and bears directly or indirectly against the low-friction material on the other side. The low friction material is preferably positioned between such liner and an impact shell, such as a helmet shell or other impact shell such as an elbow, knee, or shoulder guard. A further impact layer, such as a closed-cell foam layer (e.g., expanded polystyrene (EPS)) can be positioned between the low-friction layer and the shell. Furthermore, a body with further padding or a support layer can be positioned between the low-friction layer and the shell and/or further impact layer. [0011] Thus, the low-friction layer reduces rotational acceleration during an impact to an outside surface of protective gear, such as headgear. The body having the low-friction layer is positioned between a head of a person and an inside surface of protective headgear. The body has a first face and a second face, with the first face providing a slippery exterior surface. The method involves positioning the second face of the body against one of the head or the inside surface of the protective headgear, with the slippery exterior surface on the first face engaging the other of the head or the inside top surface of the protective headgear. Upon impact, a sliding movement and material stretching along the slippery exterior surface takes place between the head of the user and the inside top surface of the protective headgear engaging along the slippery exterior surface. Thus, the rotational acceleration of the head of the user is reduced.

[0012] Likewise, if the arrangement of the slippery surface between a shell and user is employed on other protective gear, damage to impact points on the body is reduced. Thus, for example, an elbow or knee may be less likely to abraded or broken.

[0013] Those familiar with the prior art might appreciate that the method described above is a marked departure from the teachings of the prior art. The sheer-release body does not have a top layer and a bottom layer that slide relative to each other. There is no intermediate sliding layer to assist relative movement of the top layer and the bottom layer. Instead, the body has a slippery exterior surface and facilitates movement to dissipate kinetic energy using this slippery exterior surface.

[0014] There will hereinafter be described a series of alternative embodiments that demonstrate alternative ways of implementing the apparatus and method using a slippery exterior surface. In a first embodiment, the low-friction layer slides under a layer of fabric secured about the low-friction layer. In another embodiment, the body is secured to the inside surface of the protective headgear and the head of a person slides along the slippery exterior surface. In a further embodiment, the inside surface of the protective headgear slides along the slippery exterior surface. In still another embodiment, a helmet liner (with the head of a person positioned within it) slides along the slippery exterior surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein:

[0016] Figure 1 is a schematic diagram illustrating the forces involved in an impact on a protective headgear common to all disclosed embodiments;

[0017] Figure 2 is a top plan view of a helmet liner;

[0018] Figure 3 is a partial side-elevational view, in section, of a first embodiment illustrating sheer between the protective gear and the user;

[0019] Figure 4 is a partial side-elevational view, in section, of another embodiment showing the basic common components;

[0020] Figure 5 is a partial side-elevational view showing a sheer-release body secured to protective gear;

[0021] Figure 6 is a partial side-elevational view, in section, of another embodiment using the body of Figure 4; and [0022] Figure 7 is a partial side-elevational view, in section, of another embodiment using the body of Figure 4.

DETAILED DESCRIPTION

[0023] Figure 1 illustrates the force vectors acting upon protective gear when an impact occurs. The primary focus of the present invention is the tangential component of the force. Protective gear that has long been available having compressible layers, such as crushable EPS foam, addresses the normal forces to some extent. The present invention principally addresses increased protection for the tangential component of impact forces while also adding some additional protection from normal forces in some embodiments. The tangential forces are addressed with various arrangements of a low-friction layer sandwiched with various other layers between the user and the shell of a piece of protective gear.

[0024] A helmet liner identified by reference numeral 10, will now be described with reference to Figures 2 and 3. As one can appreciate from Figure 2, the liner may be formed to fit any particular piece of protective gear, such as a helmet. Liner fingers (or other shapes) can be arranged to follow the particulars gear, such as along the inner ridges of a vented bicycle helmet, for example.

[0025] As shown in Figure 3, helmet liner 10 has a liner body 12 with multiple layers. When viewed in section, it can be seen that liner body 12 has an impact absorbing bottom layer 15, a low-friction (“slippery”) intermediate layer 17, and atop moveable layer 19. Moveable layer 19 is preferably fabric but could alternatively be another layer, such as a thin elastomer layer with properties that allow it slide freely on intermediate layer 17. Fabric of various materials has been found to provide the right properties to create a low-friction interface between low-friction interface with intermediate layer 17. In this preferred embodiment, slippery intermediate layer 17 overlies and is secured to bottom layer 15. Top fabric layer 19 overlies slippery intermediate layer 17.

[0026] Referring to Figure 3, top fabric layer 19 has a perimeter 21 and a central portion 23. If secured at all, top fabric layer 19 is secured to either bottom layer 15, slippery intermediate layer 17, or both preferably only at locations along perimeter 21. The securement can be all along the perimeter or just at intermittent locations along the perimeter. This leaves central portion 23 of top fabric layer 19 free to accommodate relative sliding movement of slippery intermediate layer 17. In order to suit most applications, a sliding movement of at least 5mm should be achieved. Some fabrics are inherently more “slippery” and more readily accommodate sliding movement than other fabrics. The selection of the fabric material for top fabric layer 19 can, therefore, impact performance. Where increased sliding movement is desired, a stretchable fabric material may be selected for top fabric layer 19. This enables top fabric layer 19 to facilitate increased sliding movement of slippery intermediate layer 17 as top fabric layer 19 stretches.

[0027] Referring to Figure 3, it is preferred that top fabric layer 19 be capable of serving as a loop portion of a hook-and-loop fastener. A hook portion 26 of the hook-and-loop fastener mates to the fabric for the purpose of connecting to an interior surface 32of a helmet 30.

[0028] An ideal material for top layer 19, such as a fabric, is slippery on the intermediate layer, provides the desired amount of movement, and is capable of serving as a loop portion of a hook-and-loop fastener. Nylon and spandex blended fabrics as well as polyester and spandex blended fabrics provide beneficial results. Various materials are suitable for impact absorbing bottom layer 15, such as a polymer foam and, more particularly, an open- celled polymer foam such as an EVA foam. Certain elastic materials and auxetic materials would also be suitable. Various materials are suitable for slippery intermediate layer 15. The preferred material is a polymer plastic and, more particularly, a thermoplastic.

[0029] Referring to Figure 3, to increase comfort, a second fabric layer 25 is adhered to bottom layer 15 for direct contact with a human head. Second fabric layer 25 is for the comfort of the wearer and increases the durability of the liner by protecting the open-cell foam of bottom layer 15. Otherwise, second fabric layer 25 does little to contribute to performance of helmet liner 10 in the event of an impact.

[0030] Referring to Figure 3, helmet 30 represents atypical piece of protective gear. Helmet 30 is shown as a single layer in the schematic view of Figure 3. However, it will be a layered construction such as in known in the art and may include a hard outer shell with an inner layer of crushable foam such as a closed-cell polystyrene foam. The helmet 30 can have additional layer or layers such as fabric or other impact absorbing materials. For our purposes, all such layers are represented by helmet 30 of Figure 3.

[0031] In the event of an impact having a tangential component, the exterior surface or shell will immediately accelerate opposite the direction of impact due to the frictional sheer forces applied to the shell. Depending on the construction of helmet 30, including the sheer properties of the materials of construction, the interior surface 32 of helmet 30 will accelerate along with the shell in a direction indicated by arrow 27. Thus, if the helmet is initially moving and impacts a non-moving surface or object (e.g., the ground, pavement, a rock, etc.) it can come to an abrupt stop upon impact with the non-moving surface.

[0032] An acceleration force will likewise be applied to a head 40 of a wearer in the direction indicated by arrow 27. However, helmet liner 10 helps to protect the head 40 of wearer by dissipating kinetic energy such that head 40 does not experience the same degree of acceleration (i.e., impact). Top fabric layer 19 is anchored to interior surface 32 of helmet 30 by hook portion 26 of a hook-and-loop fastener, which engages top fabric layer 19. As previously described, top fabric layer 19 serves as the loop portion of the hook-and-loop fastener. Impact absorbing bottom layer 15 with attached slippery intermediate layer 17 abut directly or indirectly against head 40 of the wearer. However, there is relative sliding movement between slippery intermediate layer 17 and top fabric layer 19. This results in slippery intermediate layer 17 moving relative to head 40 such that the acceleration of head 40 is less than that of intermediate layer 17. Thus, head 40 is less likely to be concussed or incur other damage due to the impact forces being spread over a longer time interval (i.e., lower acceleration).

[0033] Helmet impact tests were performed using a guided free-fall oblique impact test rig dropping a helmet against an anvil at an impact angle of 45 degrees. The impact speed generated was 6.5 meters per second. A test was first conducted with a helmet having a standard helmet liner. This established a base line for comparing the performance of the helmet line described above. Tests were conducted in five impact locations and orientations: 1. front, 2. right side toward back, 3. left side toward back, 4. left side toward front, and 5. right side toward front. The rotational acceleration percentage reduction achieved by helmet liner 10 was as follows: 1. front an improvement of 22%, 2. right side toward back an improvement of 35%,

3. left side toward back an improvement of 6%, 4. left side toward front an improvement of 31% and 5. right side toward front an improvement of 45%. Note that in orientation #3 improvement gained was only 6%. The reason for this is believed to be due to the shape of the helmet tested, which created a geometric lock reducing the amount of motion possible.

[0034] Turning to another preferred embodiment, Figure 4 illustrates a body 12. If only the tangential force were being addressed, body 12 could consist only of polymer plastic portion 14 with low-friction coating or treatment 22 and would not require impact absorbing properties. However, to provide body 12 with additional impact-absorbing properties above that already provided by helmet 30 to address the normal component of the impact force, it is preferred that body 12 include an open-celled polymer foam portion 16. The open-cell structure provides breathability and improved fit that can enhance comfort. Body 12 has a first face 18 and a second face 20. First face 18 provides body 12 with a slippery exterior surface. This can be accomplished by using a self-lubricating polymer plastic or, as illustrated here, by applying a coating 22 to a hard polymer plastic layer 14. Other surface treatments may alternatively be used. As will hereinafter be further described, body 12 is secured by mating hook-and-loop tape fasteners, having a hook tape fastener portion 26 and a loop tape fastener portion 28. A loop tape fastener portion 28 is secured to second face 20.

[0035] Figure 5 illustrates body 12 positioned between the head 40 of a person and an inside surface 32 of the protective gear, in this instance headgear 30. Second face 20 of body

12 is secured to inside top surface 32 of protective headgear 30 by mating hook tape fastener portion 26 secured to inside top surface 32 with loop tape fastener portion 28 secured to second face 20.

[0036] When assembled as shown and described the slippery exterior surface on first face 18 engages head 40. Upon impact, a sliding movement takes place relative to head 40 along the slippery exterior surface on first face 18.

[0037] At isolated locations where the hook-and-loop fasteners are placed, there is little or no motion relative to helmet 30. However, in areas where there are no hook-and-loop fasteners and the slippery exterior surface is in direct contact with the head or helmet surface, there is nothing stopping motion from occurring relative to the helmet shell. In some preferred embodiments, the slippery exterior surface will allow up to 15mm of motion, which has proven to be enough to reduce the rotational acceleration of the head by up to 30%.

[0038] Although open-cell polymer foam portion 16 is preferred, other materials that are known for their impact absorbing properties could be substituted such as closed-cell foams, memory-foams, or other types of shock-absorbing foams. Although hard polymer plastic layer 14 is preferred for slippery exterior surface, other materials could be substituted and coated to provide the desired slippery surface, such as conventional thermoplastic, thermoset elastomers, natural or synthetic rubber, plasticized foams, low-density polyethylene, or high-density polyethylene. Preferred coating materials include a matte acrylic coating and a Teflon® (PTFE) coating. Although hook-and-loop tape fasteners are preferred, other types of mechanical fasteners could be used such as buttons, snap fasteners, stitching, adhesives, etc.

[0039] A further embodiment is illustrated in Figure 6 Body 12 is again positioned between the head 40 of a person and an inside top surface 32 of protective headgear 30. In this embodiment, body 12 serves as a helmet liner and second face 20 of body 12 is positioned against, but not necessarily secured to, head 40.

[0040] When assembled as shown and described, the slippery exterior surface on first face 18 engages inside top surface 32 of protective headgear 30. Upon impact, the tangential force creates a sliding movement of inside top surface 32 sliding relative to the slippery exterior surface on first face 18. It is to be noted that body 12 is secured to inside top surface 32 of protective headgear 30 by mating hook tape fastener portion 26 secured to inside top surface 32 with loop tape fastener portion 28 secured to first face 18. The idea is to secure body 12 to a portion of inside top surface 32, such as an edge of a rib, which is not directly facing head 40 and allowing sliding to occur on that portion of inside top surface 32 that is directly facing head 40.

[0041] In another embodiment shown in Figure 7, body 12 is again positioned between the head 40 of a person and an inside top surface 32 of protective headgear 30. In this embodiment, second face 20 of body 12 is secured to inside top surface 32 of protective headgear 30 by mating hook tape fastener portion 26 secured to inside top surface 32 with loop tape fastener portion 28 secured to second face 20. As with the embodiment described immediately above, body 12 is secured to inside top surface 32 of protective headgear 30 by mating hook tape fastener portion 26 secured to inside top surface 32 with loop tape fastener portion 28. The idea is to secure body 12 to a portion of inside top surface 32, such as an edge of a rib, which is not directly facing head 40 and allowing sliding to occur on that portion of inside top surface 32 that is directly facing head 40. [0042] When assembled as shown and described, the slippery exterior surface on first face 18 faces head 40 and engages therewith indirectly, by engaging with helmet liner 50 that is secured to head 40. Upon impact, a sliding movement takes between head 40 (along with helmet liner 50) and first face 18 by sliding along the slippery exterior surface on first face 18.

[0043] The embodiments described above share the feature of a low-friction surface sliding in a layered configuration relative to a portion of a human body to be protected from excessive sheer forces causing tangential acceleration. The acceleration of the helmet upon impact is greater than that of the body part (e.g., head) being protected. Thus, the likelihood of injury is reduced.

[0044] The scope of the claims should not be limited by the illustrated embodiments set forth as examples but should be given the broadest interpretation consistent with a purposive construction of the claims in view of the description as a whole.

Claims

We claim:

1. Protective gear for reducing tangential acceleration of a body during an impact to a surface, the protective gear comprising: an outer layer, an inner layer, an intermediate layer between the outer layer and the inner layer, the intermediate layer including a low-friction surface abutting the inner layer or the outer layer.

2. The protective gear of claim 1, further comprising an impact layer having impact absorbing properties to absorb energy from normal forces.

3. The protective gear of claim 2, wherein the impact layer comprises an open-celled polymer foam.

4. The protective gear of claim 1, wherein the low-friction surface comprises a polymer plastic layer.

5. The protective gear of claim 1, wherein the low-friction surface includes a low-friction coating on the intermediate layer.

6. The protective gear of claim 1, wherein a body includes the intermediate layer and an impact layer, the body being held in position within the outer layer by mechanical fasteners.

7. The protective gear of claim 6, wherein the mechanical fasteners are hook-and-loop tape fasteners.

8. The protective gear of claim 6, wherein the protective gear is a helmet and the body includes a first face being the low-friction surface and a second face positioned against an inside top surface of the helmet and a wearer’s head slides along the slippery exterior surface on the first face.

9. The protective gear of claim 6, wherein the protective gear is a helmet and the body includes a first face being the low-friction surface and a second face of the body is positioned against the head and an inside top surface of the protective headgear slides along the slippery exterior surface on the first face.

10. The protective gear of claim 9, wherein the body is secured to a portion of the inside top surface that does not directly face the head, with sliding occurring on that portion of the inside top surface that is directly facing the head.

11. The protective gear of claim 9, wherein the body is a helmet liner.

12. The protective gear of claim 6, wherein the protective gear is a helmet and wherein a second face of the body is positioned against an inside top surface of the helmet and a helmet liner is position on the head and the head, covered by the helmet liner, slides along the low-friction surface of the intermediate layer.

13. The protective gear of claim 12, wherein the body is secured to a portion of the inside top surface that does not directly face the head, with sliding occurring on that portion of the inside top surface that is directly facing the head.

14. A method for reducing rotational acceleration during an impact to an outside surface of protective headgear, comprising: positioning a body between a head of a person and an inside top surface of a protective headgear, the body having a first face and a second face, the first face providing a slippery exterior surface; and positioning the second face of the body against one of the head or the inside top surface of the protective headgear, with the slippery exterior surface on the first face engaging the other of the head or the inside top surface of the protective headgear, such that, upon impact, a sliding movement takes place along the slippery exterior surface of the first face.

15. The method of claim 14, wherein the body has impact absorbing properties.

16. The method of claim 15, wherein the body is an open-celled polymer foam.

17. The method of claim 14, wherein the slippery exterior surface is a polymer plastic layer on the first face.

18. The method of claim 14, wherein the body is held in position within the protective headgear by mechanical fasteners.

19. The method of claim 18, wherein the mechanical fasteners are hook-and-loop tape fasteners.

20. The method of claim 14, wherein the second face of the body is positioned against the inside top surface of the protective headgear and the head slides along the slippery exterior surface on the first face.

21. The method of claim 14, wherein the second face of the body is positioned against the head and the inside top surface of the protective headgear slides along the slippery exterior surface on the first face.

22. The method of claim 21, wherein the body is secured to a portion of the inside top surface that does not directly face the head, with sliding occurring on that portion of the inside top surface that is directly facing the head.

23. The method of claim 21, wherein the body is a helmet liner.

24. The method of claim 14, wherein the second face of the body is positioned against the inside top surface of the protective headgear and a helmet liner is position on the head and the head, covered by the helmet liner, slides along the slippery exterior surface on the first face.

25. The method of claim 24, wherein the body is secured to a portion of the inside top surface that does not directly face the head, with sliding to occurring on that portion of the inside top surface that is directly facing the head.

26. A helmet liner for a helmet having a shell, the helmet liner comprising: a liner body securable within the shell, the liner body comprising a low-friction layer and a fabric layer, the fabric layer abutting the low-friction layer, the fabric layer having a perimeter and a central portion, the central portion of the top fabric layer free to accommodate sliding movement relative to the low-friction layer.

27. The helmet liner of claim 26, further comprising an impact absorbing bottom layer secured to the low-friction layer.

28. The helmet liner of claim 27, wherein the fabric layer is secured on at least a portion of its perimeter to the low-friction layer.

29. The helmet liner of claim 26, wherein the fabric layer is stretchable, such that the fabric layer accommodates increased sliding movement of the low-friction layer as the fabric layer stretches.

30. The helmet liner of claim 26, wherein the fabric layer is capable of serving as a loop portion of a hook-and-loop fastener and mating with a hook portion of the hook-and-loop fastener for the purpose of being anchored to an interior surface of a helmet.

31. The helmet liner of claim 27, wherein the bottom layer is a polymer foam.

32. The helmet liner of claim 26, wherein the low-friction layer is a polymer plastic.