Hollow fibers are prepared by spray drying a solution of a film-former in a volatile solvent. The viscosity of the solution must be 1.5 to 10 times the viscosity that usually results in spheres or microspheres. The preferred film-forming substance is a combination of sodium silicate and a "polysalt."

BACKGROUND OF THE INVENTION 
This invention relates to hollow fibers which can be employed as 
reinforcing and density altering additives for polymer systems. In 
particular, the invention involves hollow fibers formed by spray drying 
solutions of film-forming materials. 
The use of various fibers, especially glass fibers, for reinforcing various 
polymer systems is very well known. Examples of an apparatus and method 
for preparing such fibers are found in U.S. Pat. No. 3,265,483. While such 
solid fibers are suitable for reinforcing various polymer systems, they 
are not effective in altering the density of such materials because of 
their own relatively high density. Numerous other materials are used as 
fillers for polymers and to control the density. Among these materials are 
hollow microspheres such as those described in U.S. Pat. No. 2,797,201, 
among many others. These materials provide no reinforcing effect. 
The problem of reinforcing and altering the density of polymer systems has 
been recognized by others. U.S. Pat. No. 4,039,718 describes hollow glass 
filaments sealed at both ends. These materials are difficult and expensive 
to prepare, involving a separate sealing step after forming the open 
hollow fiber. U.S. Pat. No. 3,692,507 describes silicate fibers that are 
formed from attenuating flows of molten silicate glass, hydrating the 
fibers so produced and foaming the fiber by applying heat. These methods 
involve costly and relatively complex processing steps, and the materials 
have not been adapted for fabricating reinforced plastics. 
It is an object of this invention to provide a hollow reinforcing fiber by 
a simplified production method. 
SUMMARY OF THE INVENTION 
I have found that hollow fibers useful in reinforcing polymers can be 
prepared by spray drying solutions of numerous film-forming substances. 
Many of these film-forming substances have also been used to produce 
hollow or solid spheres by spray-drying, but surprisingly small process 
changes produce hollow fibers instead of the hollow spheres. Increased 
viscosity of the feed to the spray-dryer favors the formation of fibers, 
as does an increased feed rate. The spray-dryer temperatures do not appear 
to affect product particle shape. I prefer inorganic silicate based 
materials as the film-forming substance. I most prefer a combination of 
sodium silicate and a "polysalt" such as a polyborate or polyphosphate. 
THE INVENTION 
Nearly any film-forming substance that can be dissolved in a volatile 
solvent can be used as a feed for the process of my invention to provide 
hollow fibers. Some examples of synthetic film-forming systems that may be 
used include polyvinyl alcohol, phenol-formaldehyde resin, 
urea-formaldehyde resin, melamine-formaldehyde resin, alkyd resin, 
polysiloxanes, cellulose esters, polyvinyl chloride, 
polyvinylchloride-polyvinylalcohol copolymers, polyvinyl butyrol, 
polystyrene, polyvinylidine chloride, polymethyl methacrylate, and 
polyamide resins. So-called "natural" film-forming systems are also useful 
and include soybean protein, zein protein alginates, cellulose xanthate 
and cuprammonium cellulose. Inorganic film-formers are useful as well, and 
include silicates, borates, and polyphosphates. 
Some of these film-forming substances require the inclusion of a so-called 
"blowing agent" to form and expand the hollow fibers while they are still 
plastic and to prevent breakage under atmospheric pressure when the walls 
have set. Examples of useful blowing agents include inorganic or organic 
salts of carbonates, nitrates, carbamates, oxalates, formates, benzoates, 
sulfites, and bicarbonates. Strictly organic substances are also of value, 
such as p-hydroxy phenylazide, di-N-nitropiperazines, polymethylene 
nitrosamine, urea and many others. Selection of a particular blowing agent 
is based upon compatibility with the film-forming system and the intended 
use of the product. 
Film-forming systems that are of particular value in carrying out the 
process of this invention and which do not require the addition of a 
so-called blowing agent are disclosed in U.S. Pat. No. 3,796,777 which is 
hereby incorporated by reference. The film-forming system comprises a 
sodium silicate and a "polysalt" such as polyborate or polyphosphates. 
Other descriptive information of my preferred film-forming system is 
disclosed in U.S. Pat. Nos. 3,794,503 and 3,888,957. These patents are 
hereby incorporated by reference. U.S. Pat. 3,794,503 describes the system 
most fully and, in Column 3, specifies the requirements of the "polysalt" 
needed to prepare hollow bodies, in this case hollow fibers. For this 
system "polysalts" are considered to be those salts with anion to cation 
ratios that are reduced when the salt is dissolved and becomes hydrolyzed. 
Ammonium pentaborate, sodium pentaborate and sodium hexametaphosphate are 
preferred polysalts. The composition of the preferred film-forming system 
can be 0.03 to 2.0 parts by weight (pbw) of "polysalt" solids per each pbw 
of silicate solids. 
Any conventional spray drying equipment can be used to implement the 
process of this invention. The feed material can be atomized into the 
spray tower with either an atomizer wheel or a spray nozzle. Since a wide 
range of film-forming materials and solvents can be used in my process, a 
wide range of temperature is employed to provide removal of solvent, and 
formation and expansion of the fibers in the spray tower. Inlet 
temperatures of 50.degree. to 500.degree. C. and outlet temperatures of 
40.degree. to 350.degree. C. may be used successfully depending on the 
film-former and solvent employed. More particularly, inlet temperatures of 
175.degree. to 500.degree. C. and outlet temperature of 100.degree. to 
300.degree. C. are suitable when using the preferred film-forming system 
of sodium silicate and a "polysalt." 
The viscosity of the solution containing the film-forming substance which 
is the feed to the spray dryer appears to be the most important variable 
influencing the preparation of hollow fibers. The feed solution must have 
a sufficiently high viscosity to maintain relatively continuous structures 
when the solution is sprayed or subjected to centrifugal force as is done 
in the usual atomization step of spray drying. The viscosity is considered 
sufficient if these fibers have a length that is at least 5 times the 
diameter. If the viscosity is low, drops are formed during atomization and 
spheres or hollow spheres are produced. It appears that good yields of 
hollow fibers are produced with solutions that have viscosities of 1.2 to 
10 times the upper limit of the viscosity range that results in hollow 
spheres. For example, when using my preferred sodium silicate polysalt 
system, hollow microspheres are produced from solutions of up to about 150 
cp, while feed solutions of 300 cp or more provide good yields of hollow 
fibers with aspect ratios of 8 or more. 
The feed rate of the film-forming solution to the dryer also has some 
influence on the formation of hollow fibers rather than spheres. This 
relationship is difficult to quantify, but it appears that higher feed 
rates lead to a higher yield of fibers with a somewhat larger aspect 
ratio. 
The product removed from the spray dryer comprises irregular convoluted 
fibers with large hollow regions separated by solid walls of varying 
thickness. The length of said fibers can vary widely, with aspect ratios 
of 8 or more. I prefer the fibers to be more than 100 microns in length 
and to have aspect ratios more than about 12 and up to about 65. These 
fibers have 8 to 20% loss on ignition (LOI) and a true particle density of 
0.7 to 1.1 g/ml. These fibers can be best treated to further reduce the 
LOI and slightly expand the fibers so that the density is somewhat 
reduced. The heat treatment must be carried out carefully so that the 
walls of the fibers do not rupture. One successful heat treatment 
comprises heating the fibers to 100.degree. C. and holding for an hour, 
then raising the temperature to 200.degree. C. and holding for an hour, 
then raising the temperature to 300.degree. C. and holding for 3 hours, 
and then cooling slowly. Any equivalent treatment would be satisfactory. 
The fibers treated in this manner have a LOI of 2 to 5% and a true 
particle density of 0.6 to 0.85 g/ml. 
These fibers are useful as lightweight reinforcing agents for polymers, 
lightweight insulation and components in synthetic foams or cores. 
EXAMPLES 
The following examples illustrate certain embodiments of my invention. They 
are not intended to establish the scope of the invention, said scope being 
set forth in the disclosure and the claims. All proportions are in parts 
by weight (pbw) and percent by weight (%) unless otherwise indicated.

EXAMPLES 1-4 
A series of feed solutions for the spray dryer were prepared by combining 
sodium silicate and ammonium pentaborate (APB). N.TM. sodium silicate was 
used as the silicate raw material and contains 8.9% Na.sub.2 O and 28.9% 
SiO.sub.2. N is a registered trademark of the PQ Corporation. 
The following table summarizes the composition of the feed solutions 
prepared. 
TABLE I 
______________________________________ 
Run # 1 2 3 4 
______________________________________ 
N.RTM. Silicate (pbw) 
81.3 82.9 84.6 84.0 
APB (pbw) 2.3 2.3 2.4 2.4 
H.sub.2 O (pbw) 
16.4 14.8 13.0 13.5 
Viscosity* (cP) 
125 176 460 552 
______________________________________ 
*Brookfield RVT Viscometer, #2 spindle @ 20 RPM at 25.degree. C. 
These feed solutions were prepared by dissolving the ammonium pentaborate 
in the water and then dispersing the resulting solutions into the silicate 
solution. 
The solutions were spray dried in a Bowan 7-foot diameter spray dryer with 
a centrifugal atomizer. The inlet temperature was controlled at about 
200.degree. to 220.degree. C., while the outlet temperature was 
145.degree. to 155.degree. C. The feed rate remained constant throughout 
the four runs. 
The product of feed solution run #1 was hollow microspheres with a LOI of 
13.2%. The product of run #2 was a mixture of hollow microspheres and 
hollow fibers with a LOI of 13.5%. The products of both runs #3 and #4 
were predominantly hollow fibers; only an occasional hollow microsphere 
was produced. The product of run #3 had a LOI of 17.5%, and the true 
density of the fibers was 0.92 g/ml. The LOI for run #4 was 11.9% with a 
true density of 0.87 g/ml. 
EXAMPLE 5 
The hollow fibers produced as a result of run #4 were further heat treated. 
The heat treatment was carried out by heating the material to 100.degree. 
for an hour, then raising the temperature to 200.degree. C. for an hour 
and then raising the temperature to 300.degree. C. and holding for 3 
hours. These hollow fibers had a LOI of 2.7% and a true density of 0.80 
g/ml and were between about 100 microns to 1.25 cm in length, with an 
aspect rate range of 8 to 26.