1. Field of the Invention
This invention is concerned with the design, production technology, tools and machinery for safety helmets by using three-dimensional cellular textile composite structures. The invented safety helmets possess a high specific energy absorption capacity, a long stroke of impact and constant reaction force during impact as well as good air permeability.
2. Description of Prior Art
Safety helmets for sports like cycling consist of several main components and each of them has different functions. The hard outer shell is used to withstand penetration, abrasion and initial impact. The common materials used for making the outer hard shell nowadays including polyvinyl chloride (PVC), etc. The most important component in the helmet is the shock-absorbing liner that is used for absorbing and distributing the impact energy. Desired characteristics of suitable shock-absorbing liner include:
long stroke with a constant reactive force;
high specific energy absorption capability;
permanently deformation with irreversible energy absorption mechanisms, such as plastic deformation involving fibre and matrix fracture, fibre/matrix debonding and delamination, etc.
stable mode of deformation under various impact conditions, e.g., not adversely affected by dirt, corrosion or other environmental factors; and
low costs and ease for manufacturing and maintaining.
Polyurethane and polystyrene foams are widely used as the shock absorbers in different aspects due to their shock absorption capacity, strength and their good adhesion to metal, plastic and wood. Such as the impact cushioning in dashboards and doors, and even the sports helmets. The foams compress and/or crack during impact and slow down the transmission of the impact force, and hence made them suitable material for using as the shock-absorbing liner in the helmets. However, the safety helmets utilizing such foams do not have good air ventilation and are relatively heavy.
Two kinds of chemical processes have been commonly applied into PU foam manufacturing.
The first one is a one-shot process, exothermic process. Polyisocyanate, polyol, additives and a blowing agent are combined together at one time during the process. The foam is either poured in place or rolled out as a continuous slab.
The second type is a prepolymer process, which is more expensive as compared with the previous one. Polyisocyanate or polyol is used in a preliminary chemical reaction to form long chemical chains. Then the long-chain polymer is mixed with the desired additives or blowing agents.
After foaming, molding occurs by either a pour-in place or continuous slab process. For the pour-in place process, the chemical components are mixed in batches and then the mixture is poured into an opening where it is going to be used. The mixture then expands and fills in the area. The mixture can usually expand to about 30-40 times its original volume. The continuous slab method usually requires a large working area and is more suitable for high volume production. During the process, the chemical components are poured and pumped to a mixer head at a fixed rate. The mixer head is then moved across a conveyor. Through this kind of process, the foams can have a more uniform density compared with the foam made by the pour-in place process.
Four main US standards have been used by the industry for such helmets. CPSC Bicycle Helmet Standard (1998), xe2x80x9cThe Final Rule, Published in the Federal Resisterxe2x80x9d, which becomes law in US after March 1998, is comparable to ASTM (American Society for Testing and Materials) Standard. American Society for Testing and Materials (ASTM), (1995), xe2x80x9cStandard Test Methods for Equipment and Produces used in Evaluating the Performancexe2x80x9d has been widely used since 1995. The B-95 standard is established by the Snell Memorial Foundation. Most helmets with a Snell sticker meet only the earlier B-90 standard, which is comparable to ASTM. The old American National Standards Institute (ANSI), Inc., (1984), xe2x80x9cAmerican National Standard for Protective Headgear for Bicyclistsxe2x80x9d has become eliminated some years ago.
According to the CPSC standard, tests should include peripheral vision, personal stability, retention system and impact attenuation. The basic set up that is used to test the energy absorption capacity of the helmet follows the CPSC standard. There are several kinds of anvil used for testing, including a flat anvil, hemisphere anvil and a curbstone. The impact velocity used for the flat anvil test is about 6.2 m/s and the impact velocity used for the hemisphere and curbstone is about 4.8 m/s. The peak acceleration should not be greater than 300 g""s. The headform used in the testing should conform to the A, E, J, M or O geometries specified in ISO DIS 6220-1983. The helmet is strapped on a headform and turned up side down. The helmet is then dropped in a guided free-fall on to the anvil.
Compared to foams, cellular textile composite energy absorbers are relatively new. Energy absorbing textile composites can be designed into different structural forms. such as tubes, plates, shells, as well as cellular structures. The textile composites compose of textiles as reinforcing material and a matrix system. The reinforcing textiles can be in many shapes and forms, such as continuous filaments, chopped strands, mats, various fabric structures, which in many cases, are made from glass, carbon, ceramics, aromatic, ultra high molecular weight polymeric or metallic fibres. The matrix material is typically a thermosetting or thermoplastic polymer such as epoxy, polyester, polypropylene, polyurethane, polyamides, polyvinylester, etc.
The processing idea of manufacturing a flat panel of three-dimensional cells by using textile materials is disclosed in U.S. Pat. No. 5,364,686 by D. Disselbeck. It describes a process for manufacturing a dimensionally stable, three-dimensionally shaped, sheet-like textile material using one or more layers of a deep-drawable textile material, preferably a knitted material. This textile structure is constructed from reinforcing fibres and a thermoplastic matrix material in fibre form. Several steps are taken to produce such a composite material. The material is first heated to a lower temperature than the melting temperature reinforcing fibres and formed into the shape desired for the core material by an area-enlarging shaping process, for example by deep drawing. The temperature is then reduced to below the melting point of the thermoplastic matrix material and keeping the shaped material in the mold until the thermoplastic matrix material has been sufficiently hardened. Demolding is the last step to give the resulting shaped textile material.
Similar patent is also found in U.S. Pat. No. 5,731,062 by Kim et al. Three-dimensional fibre networks were made in a semi-rigid and dimensionally stable form from textile fabrics that have regular conical projections and optional depressions which are compressible and return to their original shape after being compressed. The fibre networks are made by the thermo-mechanical deformation of textile fabrics that are in turn made from thermoplastic fibres. The fibre networks have flexibility to be used as cushioning and impact absorbing materials. In making these structural fibre network to textile composites, the two-dimensional textile fabric that is utilized in making the three-dimensional composite structures is selected from some simple classes of fabrics, such as knit, woven or non-woven textile fabrics.
The idea of three-dimensional textile composite structure was used in producing a cushioning inlay of shoes in U.S. Pat. No. 5,896,680 by Kim et at. A shoe midsole is made from the formed fibre network with projections of varying size to contour to the shape of the bottom of the foot. This fibre network is made from a textile fabric that has an array of projections made from the same fabric rising from the plane of the fabric.
Similar inventions of such structural projections from a textile material were also found in EP 0559969A1, entitled xe2x80x9cEmbossed Fabric, Process for Preparing the Same and Devices Therefor; EP 0469558A1, entitled xe2x80x9cFormable Textile Material and the Shape of the Mould Obtainedxe2x80x9d; and EP 0386687B1, entitled xe2x80x9cWeb-Like Boundary Layer Connection and Method to Make Samexe2x80x9d.
In U.S. Pat. No. 4,890,877 by A. Zarandi et al., a shaped energy-absorbing panel is used on a vehicle door. It is a stretchable lightweight resin coated fabric having a plurality of spaced apart circular conical projections rising from the planar sheet. The stretching effect is achieved by using weft knit plain fabric and warp knit fabric. Several molded panels were cut in a size and shape of the desired energy absorbing panel structure and assembled with adhesive coated interface panels to give the desired thickness of the energy absorbing panel structure. The energy absorbing structure is then mounted on a vehicle door above the arm rest and below the window opening between the door trim panel and the door inner panel to absorb energy in the event that the occupant contacts the door inner panel.
Another invention U.S. Pat. No. 5,435,619 by Nakae et al. was also found demonstrating a different design modification to the energy absorber in an automobile door. The purposes of the invention was to improve the shock absorbing characteristics and to reduce the weight of an automobile door. The shock absorber consists of a plurality of tiered foam main members, and polypropylene resin foam auxiliary members disposed between and connecting the main members to form chambers open transverse to the direction along which the main members are tiered. The connecting members were modified having semicircular ridges, wave-like form and sectional squared ridges.
The previously mentioned cellular composites are flat panels provided as solid structures. Holes for voids are revoided in such structures with the matrix material providing a solid panel in casing and interconnecting all the strands. There is no porosity retained from the original textile material. Such composites have disadvantages when considered for use in items such as safety helmets.
One disadvantage of such composite materials is that they do not provide particularly large in-plane plastic deformation. The reactive pore sets to be higher in order to absorb the required impact energy.
A further disadvantage is that the lack of porosity leads to a compropably heavier composite structure and the structure is unable to breathe or provide any form of air ventilation.
A further disadvantage with such solid composite structures is that, when used to make a three-dimensional shaped item such as a safety helmet or liner for such a helmet, the energy absorption behaviour under impact may be relatively poor and even worse than that of a flat panel made from the same material. With such safety helmets or liners being generally hemispherical and providing curved surfaces, relatively a few composite cells are involved in the dynamic response to any impact.
It is an object of the present invention to provide a safety helmet and methods of manufacture thereof that overcome some of the disadvantages of prior art helmets and manufacturing methods or at least provide the public with a useful choice.
Accordingly, in a first aspect, the invention may broadly be said to consist in a safety helmet comprising:
an outer shell;
an energy-absorbing liner within said outer shell; and
wherein said energy-absorbing liner includes a cellular textile composite material in which at least a portion is a porous textile material supported in a matrix material wherein a plurality of pores are retained in said portion of the composite.
Accordingly, in a second aspect, the invention may broadly be said to consist in a method of manufacturing a liner for a safety helmet comprising the steps of:
providing a sheet of textile fabric;
forming said textile fabric at controlled temperatures to provide a plurality of projecting cells;
forming said fabric with said cells into a generally hemispherical shape for use as a helmet liner;
forming linkages of adjacent cells;
applying resin as a matrix material at one or more stages throughout the method to form a textile fabric composite structure retaining a plurality of pores of the fabric in at least a portion of the resulting composite; and
curing said composite for use as a helmet liner.