Fiber reinforced products and method for producing same

A fiber reinforced composite composed of a scrim/mat. The scrim/mat includes a layer of fiber scrim to which is bonded a layer of fiber mat. This product can be formed into a tubular member such as a fuse tube. The tube is made by pultruding the scrim/mat.

BACKGROUND OF THE INVENTION 
This invention relates to fiber reinforced composite materials, and in 
particular to pultruded articles such as fuse tubes. 
There are various applications for hollow elongated members having high 
burst strengths, such as the tubular casings for electric current limiting 
fuses. Current limiting fuses generally include a fusible element which is 
severed when the electrical current in an electrical power line being 
monitored exceeds a predetermined limit, and a filler for quenching the 
arc created when current severs the fusible element. Since the foregoing 
arc releases substantial heat to in turn generate high gas pressures and 
thermal shock, a casing or tube must be provided around the fuse to 
contain the explosive forces released and to prevent arcing to ground when 
the fuse is blown. Such fuse casings should have high burst strength and a 
high resistance to heat shock. Furthermore, such casings should be capable 
of manufacture at high production rates and low cost. Compactness is a 
desired feature, with the casing having thin walls and light weight, but 
the casing must still perform its intended functions. The fuse casing must 
be an electrical insulator and resistant to thermal shock. Fuse casings 
should have dimensional stability in longitudinal, radial and peripheral 
(i.e. circular for tubular shapes) directions. 
Virtually all fuse tubes in current use are composed of vulcanized fiber, 
composite materials and ceramics. The composite materials are reinforced 
with multiple layers of fiber glass--some are composed of only woven fiber 
glass fabric, and others are combinations of various types of fiber glass 
reinforcement. Fiber glass conducts electricity poorly. Fiber glass is 
particularly suitable for fabricating elongated pieces of uniform cross 
section by the manufacturing process of pultrusion ("uniform cross 
section" means that the cross section is constant along the length of the 
piece; many pultruded products have non-uniform regions at a given cross 
section). Glass fibers can be arranged to provide high strength. Fiber 
glass products are often flexible, facilitating the assembly of end caps 
used in fuse tubes. Fuse tubes composed of glass fibers or rovings wound 
about the inner components of the fuse are disclosed in U.S. Pat. No. 
2,929,900 (White 1960) and U.S. Pat. No. 3,846,727 (Harmon 1974). U.S. 
Pat. No. 2,727,961 (Smith 1955) discloses a fuse tube having an inner 
liner for generating arc extinguishing gases, and an outer tubular member 
wound about the liner and composed of a woven fiber glass cloth or fabric 
which has been impregnated with resin to bond the glass fibers together. 
U.S. Pat. No. 3,911,385 (Blewitt et al, 1975) discloses a fuse casing 
composed of a glass fiber cloth which is impregnated with a melamine resin 
and has an epoxy resin coating. U.S. Pat. No. 3,979,709 (Healy 1975) 
discloses a pultruded composite fuse tube construction having an inner 
layer of woven glass fabric, an intermediate layer of glass fiber mat 
having non-woven, randomly oriented fibers, and an outer layer of woven 
glass fiber fabric. The latter patent further discloses the incorporation 
of multiple layers of mat and a layer of fiber glass rovings between the 
mats. In U.S. Pat. No. 3,984,800 (Salzer et al., 1976), another pultruded 
fuse casing is described which includes an inner layer of glass fiber 
rovings, an intermediate layer of non-woven glass fiber mat and an outer 
layer of woven glass fiber fabric. In such composite fuse tube 
constructions, fiber glass fabric is employed because the axial and 
circumferential fibers combine to provide high burst strength, the fiber 
glass mat is employed to provide additional strength at a lower cost than 
the fabric, and rovings are incorporated to facilitate the pulling of the 
product through pultrusion dies during the manufacturing process. 
U.S. Pat. No. 3,986,157 (Salzer et al., 1976) discloses a prismatic fuse 
casing (the term "tube" as used herein means any hollow, elongated member 
including those of prismatic configuration; however, U.S. Pat. No. 
3,986,157 specifically discloses a prismatic member) having an outer layer 
of woven glass fiber fabric, an intermediate layer of non-woven glass 
fiber mat, an inner layer of fabric, and four bundles of glass fibers at 
the respective corners of the member. U.S. Pat. No. 4,124,836 (Wilks, 
1978) discloses a composite fuse tube having layers of nonwoven fiber 
glass mat sandwiched between layers of woven fiber glass cloth, with a 
layer of rovings interposed between layers of mat according to one 
embodiment; an inner liner of high purified asbestos known as Quintex II 
is further included. U.S. Pat. No. 4,161,714 (Jacobs, Jr., 1979) describes 
another composite resin-impregnated fuse tube, this one having an outer 
layer of glass cloth, one or more intermediate layers of mat and an inner 
layer of cloth. 
Despite these proposals, the fuse tubes known in the art do not adequately 
meet the various criteria discussed above. There remains a need for fuse 
tubes and other tubular products which have an improved burst strength 
while being compact, which can be produced accurately, efficiently and 
inexpensively with reduced amounts of downtime caused by product breakage 
and the like, which have improved dimensional stability, and which perform 
their intended purposes reliably and for long periods of time. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a fiber reinforced composite 
product of improved strength. 
It is a further object of the present invention to provide an improved 
tubular electrical insulating member having high burst strength. 
Another object is to provide an improved fuse tube capable of being 
manufactured at a high production rate. 
A further object of the invention is to provide an improved fuse tube which 
can be produced in a highly efficient manner. 
It is also an object to provide an improved fuse tube product having an 
improved tensile strength. 
The provision of a pultruded fuse tube having high dimensional stability is 
yet another object. 
Another object is to provide a fuse tube which can be made with tight 
dimensional control. 
It is an additional object of the invention to provide an improved 
pultruded fuse tube having decreased production downtime resulting from 
product failure during the production process. 
A still further object of the invention is to provide a pultruded glass 
fiber fuse tube producible at a lower cost than known fuse tubes. 
Yet another object is to provide an improved fiber glass fuse tube which is 
practical, practicable and inexpensive to manufacture, and effective and 
efficient to use. Other objects will be apparent from the discussion to 
follow and from the appended claims. 
The foregoing objects are achieved according to the preferred embodiments 
of the invention by the provision of a fiber reinforced composite product 
made of layers of mat and scrim which are bonded together. (As used 
herein, the term "scrim/mat" shall mean woven scrim which is bonded to 
non-woven mat.) Scrim/mat can be formed into a fuse tube by pultrusion, 
with the incorporation of rovings being an optional addition to aid in the 
pultrusion process and to enhance the strength of the tube. The layers of 
mat and scrim can be in various orders and a fabric liner can be provided 
as well.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As discussed above, the employment of layers of various resin-impregnated 
fiber glass mats in the formation of fuse tubes has proven to be the best 
construction available. However, the various known fuse tubes made as 
glass fiber composites all use relatively expensive woven glass fiber 
fabric for the burst strength it provides. Resort has not previously been 
made to glass fiber scrim in fuse tubes and the like, either as an 
addition to glass fiber fabric or instead of the fabric. Glass fiber scrim 
is similar to woven fabric in that scrim is composed of woven glass 
fibers. However, scrim is of far less weight than fabric, scrim is thinner 
than fabric and scrim has far fewer thread counts than fabric. Glass fiber 
scrim often has thicknesses and thread counts of 1/4 to 1/3 that of glass 
fiber fabric, and a cost which often is less than 10% of the cost of the 
fabric. Scrim is commercially available, yet despite its cost advantage 
over glass fiber fabric, it has not been used in fuse tubes and the like 
because of its low burst strength. 
In the course of the development of the present invention, various problems 
were encountered which would apparently suggest that fuse tubes made of 
glass fiber scrim and glass fiber mat, without woven glass fiber fabric, 
would not adequately perform their intended purpose. The burst strength 
was found to lie in the 800 psi to 3200 psi range, which especially at the 
low end was deemed insufficient. An effort to make the outermost layer or 
ply of glass fiber mat was found to impede the pultrusion process by 
hindering the pulling of the material through the pultrusion die. Mat 
consistency was not satisfactory. Indeed, prior fuse tubes which 
incorporate glass fiber mat employ that mat as an interior layer because 
of pultrusion difficulties and a tendency of the glass fibers to unravel 
from the mat. 
It has been found that an improved fuse tube can be constructed by a 
composite arrangement of glass fiber scrim and glass fiber mat, with the 
mat and scrim being mechanically and chemically bound by a binder on the 
engaged surfaces of the two layers. The bonding of scrim to the mat to 
produce scrim/mat increased the wet tensile strength of the mat from 12 
lbs. to 100 lbs. for the scrim/mat. As a result of the bonding of the 
scrim to the mat and the increased wet tensile strength and formability, 
pultrusion of the scrim/mat was accomplished without difficulty at a lower 
cost than other fuse tube constructions using fiber glass fabric. 
Furthermore, the burst strengths were found to be improved over those of 
other composite tubes having unbound layers of fabric, mat and roving. 
Referring to FIG. 1, a laminated structure 1 of glass fiber scrim 3 and 
glass fiber mat 5 (i.e. scrim/mat) is shown in a flat state as it would 
appear prior to being changed into a tubular construction as by the 
process of pultrusion. The scrim is a woven grid-like construction 
composed of two groups of parallel lengths of glass fibers, one group 
extending in the longitudinal or warp direction and the other extending in 
the transverse or fill direction. The mat is a random array of loosely 
bound, non-woven glass fibers. The two layers are bound together through 
the intermediary of binder material on one or both of the engaging 
surfaces of the scrim and mat. 
The schematic diagram of FIG. 3 shows the manner in which the product of 
FIGS. 1 and 2 is made. A chain link endless belt 11 is driven by a set of 
drive rollers 13 in the clockwise direction. Woven scrim which has been 
pre-coated with a binder is fed onto belt 11 from a scrim supply roll 15, 
and chopped, loose glass fibers (or random continuous fibers) 17 are 
discharged on top of the scrim from a glass fiber reservoir 19 as the 
scrim is conveyed beneath the discharge port of reservoir 19. The fiber 
laden scrim is carried by belt 11 and moved through a resin binder 
applicator 21 wherein the fiber glass layers are impregnated with the 
resin. 
The resin-impregnated material is next conveyed through a compaction device 
23 which wrings or otherwise removes the excess solution carrying the 
resin binder out of the laminated material. Finally, the material is 
carried into an oven 27 where any remaining resin-carrying fluid is 
removed and the binder resin is cured. This enhances the bond between the 
glass fibers to strengthen the mat and render the mat coherent, and 
further makes the bond between the mat and the scrim stronger and 
permanent. The scrim/mat product is next packaged in some convenient form 
such as in rolls for subsequent use. 
In order to form the fuse tubes, the scrim/mat is first slit to the desired 
width. The tubes are formed by pultrusion, with the composite being pulled 
by a set of pullers through a forming guide and an appropriately 
configured and dimensioned heated mold. Pultrusion is a continuous 
process, and the fuse tubes are accordingly made by pulling the thermoset 
tubular workpiece from the heated die and severing it with an automatic 
saw to the desired length. 
As noted above, fiber glass rovings can be incorporated in the fuse tube to 
facilitate the pultrusion process, and to strengthen the finished product. 
The rovings are pulled from supply spools through the forming guide and 
the heated pultrusion die along with the scrim/mat. 
FIG. 4 shows a pultrusion system in schematic form. Scrim/mat as described 
above is withdrawn from a supply roll 31 and directed into a resin 
impregnator 33 shown here as a resin bath over a guide 35. Rovings, when 
used, are drawn from a set of supply rolls 37 and also directed over guide 
31 into impregnator 33. The work in process is pulled through the system 
by a set of pullers 39. The two products are impregnated with resin in by 
impregnator 33. Pullers 39 pull the resin impregnated scrim/mat and roving 
combination through a forming die 41 where the scrim/mat formed into a 
tubular shape with the rovings in a generally equally spaced relationship 
extending longitudinally in the tube against the inner or outer surfaces 
of the tubular product. The formed product is next pulled through a heated 
mold 43 where the resin is cured. 
The resin in impregnator 33 is provided to bind all of the materials in the 
pultruded product, as well as to add strength and rigidity. Such resins 
can include phenolics, melamines, unsaturated polyesters, epoxies, 
silicones and the like. To these resins, various fillers, pigments and 
other property modifiers can be added. 
Fillers include trihydrate of alumina, clay, calcium carbonate, gypsum and 
the like. Examples of pigments include black iron oxide, carbon black, 
titanium dioxide and the like. Other property modifiers include processing 
aids, such as fumed silica rheology control agent, flame retardants such 
as halogenated parafins or antimony compounds and the like. 
The mat, scrim and rovings can be arranged in different ways by controlling 
the orientation of these components as they are fed to the pultrusion 
apparatus. The cross sections of various fuse tubes according to the 
invention are shown in FIGS. 5-8. 
FIG. 5 shows a fuse tube 51 composed of scrim/mat having an outer layer 53 
of scrim and an inner layer 55 of mat. FIG. 6 depicts a fuse tube 61 
which, considering the layers in order from the outermost to the innermost 
layers, is composed of two juxtaposed layers of scrim/mat having layers, 
respectively, of mat 63 and scrim 65, and mat 67 and scrim 69. FIG. 7 
shows a fuse tube 71 also having two juxtaposed layers of scrim/mat 
composed respectively of an inner layer 73 of scrim and intermediate 
layers 75 of mat, and an intermediate layer 76 of mat and an outer layer 
77 of scrim. Layers 75 and 76 sandwich between them a layer of rovings 79. 
In FIG. 8, a fuse tube 81 is illustrated also having juxtaposed layers of 
scrim/mat, having respectively an inner layer 83 of mat, an intermediate 
layer 85 of scrim, and an intermediate layer 87 of scrim and an outer 
layer 89 of mat. Individual layers of scrim and mat, additional layers of 
rovings might in some instances be appropriate. Also, a liner of fabric 
might for some applications be appropriate. 
The components of the foregoing products are available commercially. A 
satisfactory scrim is style No. 1659 of the Clark-Schwebel Fiber Glass 
Corp. This scrim has a warp yarn of 150-1/0, a filling yarn of 75-1/0, a 
count of 20.times.10 Leno weave, a weight of 1.60 oz./sq.yd., a thickness 
of 0.0042 inches and a breaking strength of 65.times.70 pounds/inch. 
Fuse tubes made according to the invention have been found to have 
outstanding properties. Burst strengths in the range of 1700-3400 psi were 
achieved, and the product was 50% lower in raw material cost than 
comparable products made with woven fabric. The wall thickness is less 
than that of other fuse tubes even though the burst strength is higher. 
Less water absorption and retention, and greater dimensional control and 
dimensional stability have been experienced. 
The description above has been generally confined to glass fiber mat and 
glass fiber scrim. However, other fiber reinforcements can be used in 
addition to or in place of these glass fibers. These other fibers include 
inorganic fibers such as graphite, boron and the like, and organic fibers 
such as aramids, polypropylene, polyethylene and the like. 
The invention has been described in detail with particular reference to the 
preferred embodiments, but it should be understood that variations and 
modifications within the spirit and scope of the invention may occur to 
those skilled in the art to which the invention pertains.