High-temperature resistant stacking support

A high-temperature resistant stacking support is composed of a, preferably self-supporting, core which at least partially is surrounded by a high-temperature resistant fiber composite made of polyimide fibers of the general formula ##STR1## wherein n is an integer larger than 1 and A represents a four-valent aromatic group. The stacking support can be produced by shrinking a polyimide fiber nonwoven consisting of polyimide fibers of the general formula (I) onto the core under heat exposure. These stacking supports are particularly useful for stacking heated aluminum sections.

The invention relates to a high-temperature resistant stacking support as 
it is used as a spacer in the stacking of hot sections in the metal 
processing industry. 
Cardboard or wood strips, which are commonly used as stacking supports, 
offer an extremely limited field of application due to their low 
thermostability. For instance, they cannot be used for the stacking of 
continuously cast metal sections that are subjected to heat treatments up 
to 200.degree. C. in a furnace for several hours. Even at lower 
temperatures, cardboard strips have the disadvantage that low-molecular 
weight substances are emitted from the cardboard and deposit on the 
sections and, thus, impairing the quality of the product. 
Stacking supports of fiberglass reinforced epoxy resins do not have these 
disadvantages. Yet, the surfaces of the metal sections get scratched on 
the faces of the support by the hard resin matrix and by glass fibers 
standing out of the matrix, so that high-temperature resistant textile 
sheet-like structures, such as fabrics or felts or para- or meta-aramide 
fibers, must be adhered for protection. However, adhered textiles have 
relatively low strengths limiting their mechanical wear resistance 
considerably. Add to this that the matrix strength gradually decreases at 
temperatures above 200.degree. C. 
It is the object of the invention to eliminate the disadvantages pointed 
out above and to provide a high-temperature resistant stacking support 
that may be used, in particular, at temperatures of above 200.degree. C. 
and does not affect the surfaces of the stacked goods.

In accordance with the invention, this high-temperature resistant stacking 
support is composed of a, preferably self-supporting, core which at least 
partially is surrounded by a high-temperature resistant fiber composite 
made of polyimide fibers of the general formula 
##STR2## 
wherein n is an integer larger than 1 and A represents a four-valent 
aromatic group selected from 
##STR3## 
in which X is selected from the group consisting of CO, CH.sub.2, O, S and 
CF.sub.2, and R represents at least one of the following divalent aromatic 
groups selected from 
##STR4## 
Polyimide fibers of this type may be produced and processed by the process 
described in PCT application WO89/08161. They can be processed to 
manipulatable nonwovens that densify to a fiber composite under heat 
exposure and, if desired, under pressure at temperatures in the glass 
transition range between 280.degree. and 350.degree. C. 
A substantial advantage of nonwovens made of polyimide fibers over 
nonwovens made of para- and meta-aramide fibers consists in that, due to 
the exposure to heat, the thickness, the density and, thus, the strength 
of the fiber composite may be adjusted within wide ranges. 
Preferably, the fiber composite exhibits the following properties in 
combination: 
a thickness of 0.50 to 8.0 mm, preferably of 1.0 to 3.0 mm; 
a density of 0.3 to 1.1 g/cm.sup.3 ; 
a tensile strength of at least 15 N/mm.sup.2. 
Stacking supports of this type are very resistant to mechanical stresses 
and are reusable several times. 
An advantageous embodiment of the stacking support according to the 
invention comprises a core of a fiber glass composite, in particular of a 
glass-reinforced synthetic resin, or of a metallic material, the modulus 
of elasticity of the core preferably ranging between 10,000 N/mm.sup.2 and 
40,000 N/mm.sup.2. Stacking supports having moduli of elasticity larger 
than 10,000 N/mm.sup.2 are very well suited for use in automated stacking 
operations. 
The stacking support according to the invention is particularly suited for 
stacking heated aluminum sections, in particular during a tempering 
treatment of aluminum sections. Temperatures up to 280.degree. C. are 
typically employed. 
The stacking support according to the invention is capable of being 
produced in that the thermally densified polyimide fiber composite is 
pressed or adhered to the, preferably self-supporting, core. A further 
manufacturing process, which is particularly simple, is characterized in 
that a polyimide fiber nonwoven consisting of polyimide fibers of the 
general formula 
##STR5## 
wherein n, A and R have the meanings indicated above, is shrunk onto the 
core under heat exposure. 
The invention will be explained in more detail by the following examples: 
EXAMPLE 1 
A polyimide fiber composite thermally densified at 315.degree. C., having a 
thickness of 0.50 mm and a density of 0.30 g/cm.sup.3 as well as a 
strength of 15 N/mm.sup.2 was adhered by means of a polyimide adhesive to 
a core produced by pultrusion of glass-reinforced polyester and having a 
width of 50 mm and a thickness of 4 mm. 
The structure of the stacking support obtained is schematically illustrated 
in the FIGURE, the polyimide fiber composite being denoted by 1, the 
adhesive layer being denoted by 2 and the polyester core being denoted by 
3. 
EXAMPLE 2 
A resin-impregnated glass cloth was combined with a polyimide fiber 
nonowoven on both sides by pultrusion and thermally bonded at a 
temperature of 100.degree. C. The densified polyimide fiber composite had 
a thickness of 1 mm, a density of 0.30 g/cm.sup.3 and a strength of 15 
N/mm.sup.2. 
Glass mats or glass rovings might as well be used instead of a glass cloth 
as reinforcement in the matrix. 
It has proved that thus produced stacking supports according to the 
invention having textile surface character are capable of being produced 
continuously. Depending on the starting materials chosen, stacking 
supports having moduli of elasticity of between 10,000 N/mm.sup.2 and 
40,000 N/mm.sup.2 could be produced. 
EXAMPLE 3 
Glass-reinforced prepregs of phenolic, epoxy or polyester resins were 
pressed on both sides with a polyimide fiber composite thermally compacted 
at 315.degree. C. and having a thickness of 1.0 mm and a density of 0.30 
g/cm.sup.3. In doing so, the composite structure was intended to reach a 
modulus of elasticity of 10.times.10.sup.3 at 40.times.10.sup.3 N/mm.sup.2 
with a multidirectional or unidirectional arrangement of the glass fibers. 
EXAMPLE 4 
A round-needled polyimide fiber nonwoven having a weight per unit area of 
350 g/m.sup.2 and a thickness of 3.0 mm was drawn on a parallelepipedic 
self-supporting core of aluminum and subsequently exposed to a temperature 
of 350.degree. C. for 30 minutes. The polyimide fiber composite shrunk 
onto the core adopting its shape. 
Instead of the round-needled nonwoven, a sewn nonwoven could be processed. 
Furthermore, it was possible to produce a stacking support by using steel 
or glass as core materials.