CA1169230A

CA1169230A - Phosphated asbestos fibers

Info

Publication number: CA1169230A
Application number: CA000375388A
Authority: CA
Inventors: Jean-Marc Lalancette; Jacques Dunnigan
Original assignee: Societe Nationale de lAmiante
Current assignee: Societe Nationale de lAmiante
Priority date: 1981-04-29
Filing date: 1981-04-29
Publication date: 1984-06-19
Also published as: IT1226474B; IT8183392A0; CS253560B2

Abstract

A B S T R A C T

There is disclosed chemically modified chrysotile asbestos fibers, more particularly phosphated asbestos fibers containing from 0.5 to 5% by weight of phosphate groups and heat treated phosphated asbestos fibers, both of which have an infrared spectrum which exhibits substan-tially no absorption within the range of 954-1080 cm-1.
The novel phosphated asbestos fibers are characterized by reduced haemolytlc and cytotoxic activities.

Description

i~ 69~3~

FIELD OF THE INVENTION
_ _ _ _ The present invention relates to a method of treating chrysotile asbes~os fibers to reduce some of the undesirable effects associated with asbestos fibers and to industrial products made therefrom.
BACKGROUND OF THE lNVENTION
. .. ___ . . _ The term "asbestos" refers to a group of natu-rally occurring silicate mlnerals of commercial im-portance because of their fibrous nature. Chrysotile asbestos, by far the most importan~ variety, is a hydrated magnesium silicate having the general formula Mg3Si205(OH)4 which is characteri2ed by the presence of magnesium hydroxide groups on the extexior surface of the fibers.
There are a number of publications dealing with the surface modification of asbestos fiber with a view of modifying certain physico-chemical properties of asbestos fibers such as improved filtration characteristics of the fibers, enhanced tensile strength, improvement of flame ~; 2a resistance, water-proofing of the fibers for the manu-facture of water repellent fabrics, dispersion of the ~; ~ fibers, and reduction of the emission of the fibers during handling and use of finished asbestos-containing products.
Of particular interest to both producers and users of asbestos ~ibers has been the potential health problem allegedly associated with asbestos exposure. The National Safety Council has reported that persons in-haling large amounts of asbestos dust can develop dis-abling or fatal pulmonary and pleural fibrosis also known as asbestosis and various types of malignancy of the respiratory tract ("Asbestos", National Safety Council Newsletter, R & D Section, June 1974). There is also a belief that asbestos may cause various forms of carcino-genesis, particularly carcinoma of the lung.
Because of the apparent pathogenicity of ~ ~92~

asbestos fibers, there has been a general reaction of the public and certain health authorities regarding the use of products containing asbestos fibers. This has led to a certain amount of research to modify asbestos fibers in such a way as to reduce as much as possible the undesira-ble biological effects of asbestos ~ibers.
Various materials have been examined which interact with the surface of asbestos fibers and reduce its haemolytic activity. Such material includes disodium ethylenediamine tetraacetic acid (EDTA), simple phos-phates, disodium versenate, polyvinylpyridine N-oxide and aluminum (G. Macnab and J.S. Harrington, Nature 214, 522-3 (1967), and certain acidic polymers (R.~. Schnitzer and F~Lo Pundsack, En~ironmental Research 3, 1-14 (1970).
Some of these known materials, such as EDTA~
are solubiliæed in body fluids and do not reduce the long term haemolytic activity of the asbestos. There is therefore a need to determine materials which will adhere to the asbe~tos and reduce its haemolytic activity. Such 2Q passivating materials should not adversely affect the useful commercial properties o~ asbestos.
More recently, it has also been found that asbestos fibers with at least one metal molybdate (USP.
4,171,405) or metal tUngState (4,168,347) deposited thereon have reduced haemolytic activity in comparison with untreated asSestos fibers.
It should also be appreciated that in con-ceiving a treatment of asbestos fibers to reduc0 their cytotoxic and haemoly~ic activity, some attention must be 3Q given to the cost involved,since in any treatment so developed,the cost of treatment must be such that the price of the treated asbestos fiber is not such that i~
is priced out of the market. One of the disadvantages of prior art treatments of asbestos fiber to reduce its haemolytic activity 5uch as those prescribed in USP.
4,171,405 and 4,168,346 is that such treatments generally lnvolve carrying~-out the process in an aqueous medium with the disadvantage that, after the treatment, the water must be removed, the treated asbestos fiber washed and then dried thus increasing the cost of treatment to a point where no such treated asbestos fibers have yet been commercialized because of the high costs involved in their produc~ion. Another feature which adds to the cost of such fibers is the relatively high cost of molybdenum and tungsten salts which have to be used and which will contribute to double the cost of the thus treated asbestos fibers.
Accordingly, it would appear highly desirable to provide a modified asbestos fiber which would possess reduced haemolytic and cytotoxic activities along with a process allowing for the commercial production o such new fibers at a reasonable cost.
SUM~ARY OF THE INVENl'ION
The present invention thus provides a novel chemically modified asbestos fiber containing from 0.5 to 5~ by weight of phosphate groups affixed thereto.
The present invention also provides a method for trea~ing asbestos fibers by depositing phosphate groups on at least a por~ion of the asbestos fibers. The thus treated asbestos fibers have been found to possess reduced haemolytic activity as evidenced by using red blood cells,and~reduced cy~otoxicity using the rat pulmo-nary macrophage test. Furthermore, it has also been found that the phosphated asbestos fibers of the present invention possess an unexpectedly high degree oP freeness which correspondingly increases the drainage rate in aqueous slurries of products containing asbestos fibers such as in the manu~acture of asbestos-cemen~ products.
A further improved phosphated asbestos fiber can be obtained by subjecting the thus obtained phos-phated asbestos iber to a heat treatment between 300 and 500C. It is surprising that this heat treatment

2 3 ~

decreases the cytotoxicity of the phosphated asbestos fiber of the present invention which is contrary to what has been reported when ordinary asbestos fibers are heated to a temperature of from about 500C, where the toxicity of the fibers is substantially increased (Hayashi, H. Envir. Health Persp. 9: 267-270, 1974).
The novel phosphated asbestos fibers of the present invention are characterized by the absence of the characterizing peak at 1021 cm 1 which is one of the three characterizing peaks of un~reated asbestos fibers or of phosphated asbestos fibers prepared by the reaction of a phospha~e salt with asbestos fibers in an aqueous medium as disclosed in USP. 3,535,150, both of which possess characterizing peaks at 1080 cm 1, 1021 cm l and 954 cm 1 when subjected to infrared ana~ysis. In other words the novel phosphated fibers of the present in-ven~ion have an infrared spec~rum which exhibits substan-tially no absorption within the range of 954-1080 cm 1.
It is also a feature of the present invention to provide ~or ~he incorporation of the phosphate-containing asbestos fibers in industrial products~
thereby reducing the health hazards usually associated with the handling of asbestos fibers prior to their incorporation in industrial products, and thus confering to such industrial products the inherent safety factors of the novel phosphated fibers of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
.
In accordance with the present invention, asbestos fibers are treated to deposit from 0.5 to 5%
by weight of phosphate groups.
More specifically, the process of the present invention comprises contacting asbestos fibers under agitation with circulating dry vapors o a phosphorous compound selected from~the group consisting of phos-phorous oxychloride and phosphorous pentachloride in an inert atmosphere which is unreactive to the phosphorous compound vapors, whereby a portion of the terminal hydroxyl groups attached to the magnesium atoms are con-verted to phosphate groups, the amount of said phosphate groups being from 0.5 to 5% by weight o~ the treated fibers. The process of the presen~ invention can be carried out at normal temperature and presswre.
The presen~ invention will be more readily understood by referring to the drawings wherein:
Figure 1 is a schematic diagram of inter-connected parts for use in the practice of the process of the present invention;
Figure 2, where A i5 the infrared spectrum of asbestos fibers treated with phosphorous oxychloride in accordance with the present invention, and B is the infrared spectrum of asbestos fibers treated with phos-phorous oxychloride in accordance with the present invention and then subjec~ed to a hea~ treatment, and Figure 3, where C is the inXrared spectrum of natural and untreated asbestos fibers and D is the infra-red spectrum of asbestos fibers treated with NaH2PO4 in an aqueous medium.
The term "asbestos" as used herein is intended to be applied to chrysotile asbestos which is the most important of the naturally occurring fibrous silicates and represents about 95% of the ~world's asbestos pro-duction, and the term "asbestos `fibers" is intended to apply to commercial fibers of grades 2 to 7 (Quebec Standard). Essentially the asbestos fibers are those obtained directly from ~he separation from asbestos rocks.
The phosphorous compound selected is one which readily vaporizes at;room temparature and of all the phosphorou~ compounds, only phosphorous oxychloride and phosphorous pentachloride are suitable. In practice, the phosphorous compound vapors are obtained by passing a non-reactive dry gas through the phosphorous compound whereby dry vapors of the phosphorous compound are entrained for reaction with the a bestos fibers.
The amount o~ phosphorous oxychloride or penta-chloride gas used varies with the amount of asbestos fibers to be treated and the amount of phosphate it is ~ ~6923~

desired to a~fix to the asbestos fibers. For example, for 50 grams of asbestos fibers placed in the reaction cylinder, a stream of dry nitrogen of about 2 liters per minute is passed through a bottle of phosphorous oxy-chloride for a period of 20 minutes whereby from 4 to 8 ml of phosphorous.oxychloride are contacted with the asbestos fibers, thus yielding a modified asbestos fiber having fromØ5 ~o 1.5% by weight of phosphate groups : affixed thereto. The amount of phosphorous compound in vapor form contacted with the asbestos fiber can be increased or decreased by increasing or decreasing the temperature of the phosphorous compound through which the inert carrying gas passes.
In practice, the reaction i5 carried out in a rotating container 10 having an inlet 12 and an outlet 14 :~ and mixing blades 36 attached to the wall. The rotating means are not shown. The inlet 12 and the outlet 14 are connected with a perforated tube 16 which is closed at its center ~y wall 18. A nitrogen container 20 provided : 20 with a pressure gauge 22 adjusted to allow a flow of nitrogen under the desired pressure through conduit 24 to a closed container 26 of sulfuric acid 26a whereby the nitrogen gas is dehydrated,after which it is directed through co~dui.t 27 to a c.losed container 28 of a solution of phosphorous oxych:loride 30. As the p~ressure builds up ~ in the rotating container 28,vapors of phosphorous oxy-.~ . chloride are directed to the central inlet 12 of the rotating container 10 through conduit 32. The vapors of phosphorous oxychloride permeate through the openings 16a of the central conduit 12, thu~ coming into con~act with the rotating asbestos fiber~ 34 and any unreac~ed phos-phorous oxychloride will flow out of th.e reaction con-tainer 10 through the opening of the~second portion of the central tube 16. If desired the unreacted phos-phorous oxychloride can be recycled. A further improved phosphated asbestos fiber can be obtained by heating the ~ ~ ~923~

phosphated fibers at a temperature of from 300 to 500C.
It is understood that the apparatus illustrated can be readily modified without departing from the concept of carrying out the reaction of the present invention.
A main ad~antage of the novel process,of the present invention is that the reaction i9 carried out under dry conditions contrary to prior art procedures where the reaction was carried out in an aqueous medium thus requiring the expense of energy required to elimi-nate the water after the reaction has been completed.
The phosphate containing asbestos fibers obtained by the process of the present invention not only possess increased freeness,but have al~so been found to possess reduced haemolytic and cytotoxic effects. The fibers trea~ed in accordance with the present invention along with untreated fibers and phosphated asbestos fibers prepared in an aqueous medium in accordance with the teachings of U5P. 3,535/lS0 were subjected to com-2Q parative tests to determine their haemolysis and cyto-toxic activities.
Another aspect of the present invention is that the novel phosphated asbestos,fib~ers reduce the hazards normally associated with the handling of asbestos fibers prior ~o their incorporation into industrial products and consequently this adv~ntage will, to a high degree, be reflected in the industrial products made therefrom.
~mongst the specific products in which the modifled as~estos fibers prepared in accordance with the present invention can be incorporaked and substituted for un-treated asbestos fibers, ther,e may be mentioned, wall boards, asbestos cement products, such as pipes, plates roofing tiles etc., friction materials such as brakes, brake pads, clutch facings, paper backings, yarn and woven material, gasket materials,,felt for roofing or floor lining and insulating materials and the like.

23~

BIOLOGICAL TESTS
The chemically modified phosphated asbestos fibers of the present invention, having retained their fibrous structure, were tested ~or possible alterations of their haemolytic and cytotoxic effects. All compari-sons were made with the untreated chrysotile asbestos fibers samples, phosphated asbestos fibers prepared in accordance with USP. 3,535,150 and the tests were con-ducted in the following manner:
1 a H~EMOLYSIS
.
For each experiment, whole blood was obtained from the inferior vena cava of two ether-anaes~hetized adult male Long Island rats (250-300 g/body weight). The whole blood was then immediately suspended in 400 ml of Veronal~ buffer solution (290 + 5 mOsm) of pH 7.28.
- Erythrocytes were washed 3 times, and a 4% by volume suspension of the ra~t~red blood cells (RBC) was prepared in the Veronal buffer.
Weighed amounts of asbestos samples were sus-2Q pended in 12.5 ml of Veronal~ buffer using a~Dounce~tube. The concentrations of fibers studied varied from 100 to 1000 ~g/ml. Suspensions of dispersed fibers were placed in~30 ml Falcon~ flasks with 12.5 ml of the RBC
suspension (final concentration of R~C: 2%). Flas~s were incubated at 37C in a Dubnoff~ metabolic shaking incu-bator~ From each test tube and control, 3 ml samples were taken after 15, 30 and 60 minutes of incubation.
Samples were centxi~uged for 5 minutes to precipitate ghosts and intact RBC. One ml of supernatant was diluted with 3 ml of Veronal~ buffer and the absorbance was de-term.ined at 541 nm. Complete haemolysis was obtained by the addition of Triton X-100 to a 2% suspension of RBC
in distilled water, and determined as described before.
C OTOXICIT
For the procurement of rat alveolar macro-phages, the cells were harvested by bronchoalveolar 2 3 ~

lavages. Male Long-Evans black hooded rats (250-300 g/body weight) were killed by an i.p. overdose of sodium pentobarbital. After tracheotomy, serial lung lavages were carried-out in s~tu by ins~illating calcium- and magnesium- free Hanks'~ balanced salt solution (pH 7.4 at 37C) supplemented with glucose (1 g/l).
Free lung cells (2107 cells/rat) were isolated by low speed centrifugation and resuspended in a solution of isotonic NH4Cl for 10 min. on ice. This step was introduced to rule out any contamination by RBC. After serial washings, the cells (>95~ macrophages) were counted on a haemocy~ometer and the viability (93-97%) was estimated by the trypan blue test (0.4% solution).
Unless otherwise mentioned, all operations were performed at 4C and with steriliæed material.
The cells (106 cells/2.5 ml of medium) wexe then incubated at 37C for 24 hrs in covered siliconized glass vials. The incubation was performed in filtered normal air and the humidity in the incubation chamber was maintained around 80%. The alveolar macrophages were incubated in sterile MEM medium (Hanks' salts) supple-mented with 10 mM CaC12 and 10 mM MgC12, 2 mM L-Gluta-mine, 4~ (VfV) heat-inactivated foetal calf serum and antibiotics (initial pH: 6.8 at 37C).
Each fibrous material was autoclaved for 45 min. at 121C before its use and gently resuspended in sterile MEM medium with a Dounce~glass homogenizer.
Aliquots up to 250 ~/106 cells (10, 40 and 100 ~g/ml o~
incubation medium) were selected for the~assay.
After a 24 hrs incubation period, the cell-free incubation mediums were collected and assayed for:
- Cell viability (membrane integrity) - Lactic Dehydrogenase, or LDH (cytoplasmic leakage) - B-N-~cetylglucosaminidase, or B-NAG (lysosomal damage).
All spectrophotometric analyses were done on a 2 3 ~

double-beam spectrophotometer. ~
It is seen from TABLE I that contact of red blood cells (RsC) with untreated chrysotile, or with NaH2P04-treated chry~otile leads to a 82%, 72~ and 57 degree of haemolysis respectively wh~n compared to control, whereas treatment with POC13-treated fibers results in a 16% degree of haemolysis, after 15 min. of contact.
Concurrently, TABLE II shows the effects on parameters of pulmona~y macrophage response, which are widely acceptad as indices o~ cytotoxicity: viability, and enzyme leakage after exposure to the mineraI fibers.
It can be seen that at all levels of trea~ments, all the parame~ers show a definite decrease in t~le cytotoxic a~ects produced by POC13-treated fibers when compared to those produced by untreated or NaH2P04-treated fibers.
These data must be viewed in th~ light of the observation that there is a good correlation between the haemolytic potential, the effect on macrophage and the ~; 20 ~ibrogenic activity o~ mineral dusts, including asbestos (Allison, A.C. et al, 1977, Ann. Rheum. Dis. 36 (Suppl.) 3). Furthermore, it has been shown (Chamberlain, M. et al, 1978, Br. J. Exp. Path. 59: 183-189) that there is a correlation between the cytotoxic activities of mineral dusts and their ability to induce mesothelial tumors.

:~

2 3 ~

'~ er In CO O CO
o : E~ : .
'~ r~ ~ ,~
. ~ ~ ~1 ~ ~

~ ~ : O Z ~1 a ~ ' ~ ~ : U~

. ~ ~ '~: ~ .~ :
~: ~ ~n ~. P~ O In : : D
,. ~. U~ ~' ~
~ ~ po ~ ~ ~

. ~ ~ :
: . ~ O ~ *
. 1~ ~ O O ' . ~ : a ~ ~ ~, . :
~: ~ ~ ~ o ~: O ~i _ ~rl Z Z
: ~ ~ : E~ ~ E~ ~ ~ ~ ~ ~ '~
~ E~ ~ ~ ~ . . O
1~4 2 s ~ ~ t) ~ ~ o ~ u lC1 C~ a.
~:; c~
E~ , I o .r~ ~1 h a) _ ~ O O I h , O C~

Z ~ U ~ 1~, ' .

~ .
1 ~9230 ~ 00 ~

o~ ~O O O ~ ~ O ' . ~ ~ r al t~ o -o ~ ~ o~ 3 o ~
R tq ~.1::
. ~ ~
:, ~:1 ~ . ~ ~ "~
~ ' ~ CO CC~ U~O ~ 1 o r-l ~ ~ 0'1-l o : 2 ~ r~ o~ z ~
. U~ . _ ~ ~ s ~ o U~. ~ . ~ s~
~ . ~ _ ~ ~ ~r O
~ U~ dP ~; 3 . ~ . O
1~ t~ u~ S~l a) ~ o u~ 1 .~ ~ ~, tJ~
~ ~r ~ ~ a co ~co t~) r~ u7 1~ o ~ ~ D t : E~ ~ 2 8: ~ ~ Ln ~ ~ o ~ H ~ O O
. Z ~ t~l r~ l O U~
. ~ ~ m o ~ Z; ~P . ~0 ~
~; ~n ~ ~ o . ~ ~ s~ ~ o . ~
E~ o d~~ O ~ Q. O O
w : w ~ .o ~ ~ u E~ ;~ ~1 P~ ~ ~ ~ ~ W _ .,, 1~: ~ ~ , r~ ~ ~ o Ln ~r ~ ~ ca ~1 ~ X ol u E~ o p~ O U~ 0~: S~ ~01 ~ ~ z ~ ~ ~o ~ ~ o ~ ~o ~
: E~ ~ : : O
Z ~1 ~ o ~ h . W ~ ~ ~1 0 0 ~;:1 ,~: ~ O 1 G~ . ~ ~ ~ Ln O ~ In 1~ ~ u~ co u~ 1 0 O ~ ~ ) ~
H P:i ~ ~ ~ u~ S ~ o L: ,C
~ :z ~ ,, ~ ~ 8 .. ~
~ o :, ~ ~ ~ X * o o o o . O ~ ~ ~ H
C~ I ~ _l td -rl ~ ~~ ~
Q .~ ~ 3 !¢ al u ~ ~c E~ ~ ~ 000 000000 ~ *
~~ ~\ ~1 ~ 1-1 ~ 0 ~1 ~ 0 _I ~r o U ~
li~ ~J ~0 -I ~ .~

O ~ Z
U~ ~ E-/
O H s:: ~ W
~ U~-r1 ~ ~ O'' O H ~ U Z
R ~ ~ m ~ ~ ~
h ~ I H
~1 U~ ~ ~ CQH~rl O

~ ~6923(~

It is noteworthy that -the passivation of bio-logical effects, observed after treatment with POC13-treated fibers, relates with the disappearance of the charac~erizing peak at 1021 cm 1 of the infrared spectrum.
This phenomenon has been observed by Langer Selikoff et al, (1978) J. Toxicol. Env. Health 4: 173-188, when chry~o~ile was suhmitted to ball-milling to produce short fibers for experlmental purposes. The resulting fibers were far less haemolytic, and this coincided with an 10 alteration of i.r. spectral data, precisely at 1021 cm 1 and were in fact described as no longer asbestos.
Consequently, since two basically different methods of modifying fiber, i.e.: a mechanical treatment and a chemical treatment both gave much decreased biolog-~ ical activity associated with the alteration of the peak `~ at 1021 cm 1 (+ 2 cm 1), it is obvious that biological activity-is related to a structural feature in chrysotile, giving absorption at that~ frequency in the infrared.
Therefore any treatment of the chrysotile fiber, either physical or chemical, inducing a decrease or a disap-pearance of the absorption characteristic around 1021 cm 1 in the infrared, should result iII a substantial passi-vation of cytotoxic effects of such treated fiber.
Contrary to mechanical treatment, ~hich at the same time induces severe destruc~ion of the fi~rous structure o~ chrysotile and decreases th~ physiological activity. of the resulting material, treatment with POC13, while deactivating the fiber, leaves its fibrous struc-ture es~qentially intact.
The following examples are given to illustrate the preparation of the modified asb~stos fiber.

50 grams of the asbestos fiber (Grade 4T30 Quebec Standard) prepared sample were placed in a tumbler made of a plexiglass cylinder (30 cm x 12 cm) sealed at both ends. In~ide the cylinder are a number of fixed blades as shown in Fig. 1. A stainles~ steel tubing is -2 3 (~

inserted through both ends of the cylinder. Through a first set of holes in the tubing, the vapors of the gas are released inside the ~umbler, and exit through another set of holes at the othe~ end of the tubing. The tumbler is rotated at a rate of 33 RPM, by any convenient mechanical device.
controlled (2 l/min) stream of nitrogen dried by bubbliny through a bottle o~ H2SO4 is passed through a bottle of phosphorous oxychloride (POC13) at room temper-ature. The POC13 vapors are carried along with the nitrogen stream into the rotating tumbler, thus affording a very discrete contact of the POC13 vapors with the fibers. The treatment is pursued until approximately 4 to 8 ml of the POC13 have been used. Then the fibers ~` are purged with nitrogen only for a few hours, and are finally removed from the tumbler, and placed in a humidity chamber overni~ht to hydrolyze excess chlorides.
Analysis of PO4 content ~he treated fibers (Chemical Analysis Vol. 8 DF Boltz ed. method D p. 38, : 20 1958) shows that between 1.5~ by weight of PO4 have be,en fixed onto the fibers.

_ .
The phosphated fibers of Example 1 were divided in two lots. One lot was heated in an oven at 325C for a period of one hour after which all the fibers in the ` oven were at 325C.
Separate infrared analyses were performed with untreated asbestos fibers, asbestos~fibers phosphated in accordance with the procedure of U5P. 3,535,150, and asbestos fibers prepared according to Examples 1 and 2 herein.
The presence or absence of relevant peaks is_ reported in Table 3.

~ ~923~

~ _ .~ ~_ _ I.R. character.izing peaks in cm~l i.r.
Fiber 10801021 954 spectrum _~ . . _ _ __ ~
; flbers yesyes yes Fig. 2A
2. phosphated asbestos yes yes yes Fig. 2B
fibers (wet process : B.P. 1,143,842) ; 3. fibexs.of Example 1 yes no- yes Fig. 2C
4. treated fibers ofyes no yes Fig. 2D
Example 2 ~

It will be observed that the phosphated asbestos fibers o~ the present in-~ention:are character-ized by the absence of the characterizing peak at 1021 cm which is observed in untreated asbes~os fibers and asbestos ibers which have:been phosphated in an ~ aqueous medium.
:` : :

.
-

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for preparing a modified chryso-tile asbestos fiber containing from 0.5 to 5% by weight of phosphate groups, which comprises contacting chryso-tile asbestos fibers under dry conditions and agitation with vapors of a phosphorous compound selected from the group consisting of phosphorous oxychloride and phos-phorous pentachloride in an atmosphere inert to the phosphorous compound whereby a portion of the hydroxyl groups of the asbestos fiber are converted to phosphate groups, the amount of said phosphate groups being from 0.5 to 5% by weight of the fiber.

2. A process as in Claim 1, where the source of phosphorous compound is a phosphorous oxychloride.

3. The process of Claim 1 or 2, wherein the phosphated asbestos fibers obtained are heated to a temperature of from 300 to 500°C.

4. Asbestos fibers having phosphate groups bonded on the fibers in an amount of about 0.2 to 5% by weight based on the dry fibers, said phosphated asbestos fibers being characterized by the absence of the charac-terizing peak located at 1021 cm-1 obtained by the infra-red analysis of untreated asbestos fibers and of phos-phated asbestos fibers obtained by phosphating asbestos fibers in an aqueous medium.

5. Heat treated asbestos fibers having a portion of the hydroxyl groups substituted by from about 0.2 to about 5.0% by weight of phosphate groups based on the weight of the dry asbestos fibers, said heat treated phosphated asbestos fibers being characterized by the absence of the characterizing peak located at 1021 cm-1 in the infrared analysis of untreated asbestos fibers and of phosphated asbestos fibers obtained by phos-phating asbestos fibers in an aqueous environment.

6. A phosphated asbestos fiber containing from 0.5 to 5% by weight of phosphate groups.

7. A phosphated asbestos fiber containing from 0.5 to 5% by weight of phosphate groups, said phos-phated fiber having been heat treated at a temperature of from 300 to 500°C.