Section 1 - Material Composition
Thermoset Resins
This material is the "glue" which is needed to hold glass fibers together in a composite helmet shell. Thermoset resins are a family of plastics that do not melt but chemically degrade at high temperatures. Thermoset resins are created by mixing two base materials just like epoxy glues. (Epoxy glues are thermoset resins). One of the ingredients is a catalyst when combined with the other agents and heat during molding, will solidify the mixture locking itself and the glass fibers into a rigid state. In compression molding applications, very little catalyst is used so that the liquid resin remains stable at room temperature; the heat and pressure of the molding operation initiates the chemical reaction to solidify the resin.
High Performance Thermoplastics
As a general rule, thermoplastic materials become softer and tougher as they get warmer, and harder and more brittle as the temperature goes down. Until relatively recently, it was possible to obtain either great heat resistance or great impact resistance, but not both in the same material.
In recent years, new thermoplastic materials have been developed which successfully combine both heat resistance and impact resistance. Polycarbonate began this trend in the 1960s; more recent materials such as GE's Ultem (a special high impact grade developed by GE in cooperation with Bullard specifically for fire helmet applications) and Amoco's Radel now provide comparably high levels of impact resistance with heat resistance far exceeding that of polycarbonate. While the cost of these resins is very high, it is justified in certain demanding applications by their exceptional performance. As technology moves forward, these materials will continue to improve and expand in applications.
Glass Fiber
Thermoset resins by themselves have relatively little strength: the strength of a thermoset composite material comes primarily from the fibers of glass or other materials that are bonded together by the resin. There are three types of reinforcing fiber in common use today: Plastic materials (Kevlar, PBI) have very high strength and toughness but very low stiffness, and perform most efficiently when they are allowed to flex under load; in addition, they tend to be more affected by heat than other reinforcing fiber materials. Carbon fibers provide both strength and very high rigidity but are electrically conductive and therefore unsuitable for applications requiring high levels of electrical insulation. The third material family, glass fiber, provides the best combination of high strength, high stiffness, electrical insulation and cost of any reinforcing material in common use for fire helmet applications.
The challenge in designing an effective composite material is getting the right mix of a good thermoset resin and high content of glass. The performance of the composite is a function of the structural strength and adhesive properties of the resin, the length of the glass fibers and the amount of glass reinforcement in the composite. By increasing the strength of the resin and/or the length of the glass fibers, it may be possible to reduce the content of glass without sacrificing performance. This may result in a product which is easier to mold and has a better surface appearance. The glass fiber is heavier than the resin so getting the right mix also creates the best potential for a lighter helmet shell. Most fire helmets today have a glass content of approximately 50%: preform molded shells tend to use a longer fiber length and a slightly lower glass content, while SMC shells use a higher content of shorter fibers.
KEVLAR
The following information is for clarification of questions concerning the use of Kevlar in fire helmet shells. Our justification is solely based upon NFPA standard performance criteria.
Kevlar helmets are a member of the composite family. This means helmet shells constructed of fiberglass, kevlar and or other materials. All of these materials are held together by a resin. Without the resin as a common bond none of these materials will perform as needed in a helmet.
The resin is the common denominator in all composite helmet shells. It also becomes the weakest link in performance qualities. This simply means that when you have reached the limits of the resins you have for all intents reached the limits of the helmet shell. Therefore, adding exotic materials such as kevlar to the matrix of a helmet shell only increases cost of the product and does nothing to enhance protection qualities of the helmet. Since protection is the true reason for wearing a fire helmet, Bullard is more concerned with providing the most useful, economical helmet possible which exceeds NFPA requirements.
The NFPA Standard for Structural Firefighter Helmets is the only performance criteria by which all helmets are measured in the United States. To be compliant with this standard, a helmet must meet or exceed all performance tests stipulated in this document. This includes a certification by an independent third party. SGS US Testing, Inc has certified Bullard fire helmets meet and or exceed all the latest NFPA requirements.
The purpose of the NFPA standard for fire helmets is to ensure that a minimum level of protection is met. The minimums set in this standard are so severe that they often translate into protection levels in excess of human survival. This is where you must ask the question, if the minimums are in excess of survival, why would you need to enhance their performance? To have a better looking corpse? Any helmet that meets the latest revision of the NFPA standard will outlive any firefighter wearing it.
Molding Processes
The most common processes for molding helmet shells are compression and injection. Compression molding is used for composite materials, and injection is used fir thermoplastics. Tooling costs and production rates are similar between these two processes, the composites being slightly slower and in some cases slightly less costly to tool.
A number of other molding processes exist for composites, although most are not suited to either the high production volumes or the high service temperatures associated with fire helmets. These include 'Hand Layup' and Resin Transfer Molding (RTM). In both of these cases, the resin is cured at room temperature and without the application of pressure. Tooling costs are extremely low for these proceses: in many cases the tools themselves can be made of plastic materials. A number of practical problems result, however: the resin must be fully catalyzed, making it unstable once it has been mixed; even so, without heat and pressure, it can take up to ten times as long to solidify. More significantly, however, the low-pressure process produces a lower density finished part which may have pockets of trapped air, surface pinholes and lower heat resistance than a part produced by compression molding.
Injection Molding
Injection molding is used for thermoplastics. The molds comparable in cost and complexity to the most sophisticated composite/compression molds and require more sophisticated equipment for processing. However, the quality, consistency, and speed for manufacturing are greatly improved.
Plastics used in injection molding processes come in small pellets, about the size of grains of rice. The material can be and often is pigmented with the final color of the end product. In this form, the plastic pellets are funneled into the molding machine screw. This screw looks like a giant drill bit that feeds the pellets in a tumbling action toward the steel mold. Heating elements are wrapped around the outside walls of the screw housing to heat and liquefy the plastic. As the now liquid plastic reaches the steel mold it pools at the front of the screw in the exact amount needed to fill the mold. The end of the screw becomes a ram, which injects this proportioned liquid through a small inlet hole called the sprue, and fills the mold cavity. The ram end of the screw creates a plug to hold the plastic in the mold until it solidifies. The mold separates and the finished part is ejected. The entire molding process of a single helmet shell as described takes about 1 minute.
The injection-molding machine used to process the hybrid plastics in today’s fire helmet shells requires capabilities of developing temperatures in excess of 700ºF and pressures up to 400 tons.
The specialized thermoplastics used in these processes require very tight tolerances in moisture content, heat consistency and material composition to insure repeatable molding process. Any variance can create a product that looks good but may actually be useless.
Compression Molding
In appearance, a composite compression mold resembles a plastic injection mold. However a compression mold receives raw material by separating the two halves of the mold and hand placing the material in the mold and closing the mold on the material. Through compression, the material fills the mold cavity.
Composite material used in compression molding comes in many forms. Common forms of Compression Molding include BMC, SMC, and Preform molding. The time required to produce a part by Compression Molding is typically about two minutes.
BMC Molding
Bulk Molding Compound (BMC) is a material made by mixing a thermoset resin and fiberglass in a manner much like an industrial grade bread dough mixer. This process and materials are commonly used for making products such as auto headlight housings.
Preform Molding
Preform composites are created by building a "nest" of glass fiber approximating the finished shape of the product and laying it in the mold cavity. A proportioned amount of thermoset resin is poured over this fiberglass nest and the mold closes to force the resin around all of the glass fibers filling the mold. The resin can be pigmented to match the finished part color. Many fire helmet shells are molded with this process.
SMC Molding
Sheet Molding Compound (SMC) is made by a process of laying a giant sheet of liquid resin and chopping glass fiber over this sheet. This process runs the mixture through a series of rollers squeezing the two materials together in preparation for use in a mold. These sheets are than cut into specific weighted amounts and laid into a mold. The mold closes and compresses this material filling the mold cavity. Several fire helmet shells are molded with this SMC process. The resin used in this process can also be pigmented with the finished part color.
RTM Molding
Resin Transfer Molding (RTM) is a process that uses a preform of glass fiber as above, but injects catalyzed resin into the mold after it has closed. The process takes place at room temperature, and there is no pressure involved beyond that created by the pump which injects the resin. Due to the low temperature and pressure, it can take up to twenty minutes to produce a part by resin Transfer Molding. The pressure is also often insufficient to force all of the air out of the mold, resulting in porous regions at the surface or in the interior of the part.