Process for devolatilizing molten oxymethylene polymer

A process is provided for reducing black speck formation which occurs during melt processing of oxymethylene polymer compositions containing polyamide stabilizers having a melting or softening point below the melting point of the oxymethylene polymer and for simultaneously removing volatile materials from a molten oxymethylene polymer, according to which from about 0.05 to about 5.0 weight percent based on the weight of the oxymethylene polymer of said polyamide stabilizer and from about 0.05 to about 10.0 weight percent, based on the weight of the oxymethylene polymer, of hindered phenol antioxidant is added while the molten polymer is passed through a rotating disk polymer processor having at least three stages; a first devolatilization stage, then a stabilization stage and then a second devolatilization stage. The molten polymer in the first devolatilization stage is maintained at a temperature above its melting point in the temperature range of from about 160.degree. C. to about 220.degree. and at a vapor space pressure of from about 0.1 to about 500 Torr. The polymer in the stabilization stage is maintained in the molten state at a temperature of from 160.degree. C. to about 220.degree. C. The polymer in the last devolatilization stage is maintained in the molten state at a temperature of from about 160.degree. C. to 220.degree. C. and at a vapor space pressure of from about 0.1 to about 200 Torr.

FIELD OF THE INVENTION 
This invention relates to an improved method of devolatilizing and 
stabilizing molten oxymethylene polymers containing a polyamide stabilizer 
and a hindered phenol antioxidant. More particularly, this invention 
relates to a method of removing formaldehyde and other volatile materials, 
such as triethylamine and water from molten, extrudable and moldable 
oxymethylene polymers while at the same time improving or reducing the 
black speck formation which frequently occurs during the processing and 
compounding of oxymethylene polymers containing a polyamide stabilizer. 
BACKGROUND OF THE INVENTION 
Oxymethylene polymers, as that term is used herein and further defined 
below, means those polymers having recurring --OCH.sub.2 -- units directly 
attached to each other. Such polymers have been known for many years. They 
may be prepared by the polymerization of anhydrous formaldehyde or by the 
polymerization of trioxane, which is a cyclic trimer of formaldehyde. 
Oxymethylene copolymers have at least one chain containing recurring 
oxymethylene units interspersed with --OR-- groups in the main polymer 
chain, where R is a divalent radical containing at least two carbon atoms 
directly linked to each other and positioned in the polymer chain between 
the two valences, with any substituents on the R radical being inert, that 
is those which are free of interfering functional groups and will not 
induce undesirable reactions. Particularly preferred are copolymers which 
contain from 60 to 99.9 mol percent of recurring oxymethylene groups. The 
R may be, for example, an alkylene or substituted alkylene group 
containing at least two carbon atoms. 
Among the copolymers which may be utilized are those having a structure 
comprising recurring units having the formula: 
##STR1## 
wherein n is an integer from zero to 5 and wherein n is zero in from 60 to 
99.9 percent of the recurring units. R.sub.1 and R.sub.2 are inert 
substituents, that is, substituents which are free of interfering 
functional groups and will not induce undesirable reactions. 
Particularly preferred oxymethylene copolymers are those having 
incorporated therein oxyalkylene units having adjacent carbon atoms which 
are derived from cyclic ethers having adjacent carbon atoms. These 
copolymers may be prepared by copolymerizing trioxane with a cyclic ether 
having the structure: 
##STR2## 
wherein n is an integer from zero to 2. 
Examples of preferred polymers include copolymers of trioxane and cyclic 
ethers containing at least two adjacent carbon atoms such as the 
copolymers disclosed in U.S. Pat. No. 3,027,352, incorporated herein by 
reference. Among the specific cyclic ethers which may be used are ethylene 
oxide; 1,3-dioxolane; 1,3,5-trioxepane; 1,3-dioxane; trimethylene oxide; 
pentamethylene oxide; 1,2-propylene oxide; 1,2-butylene oxide; neopentyl 
glycol formal; pentaerythritol diformal; paraldehyde; tetrahydrofuran and 
butadiene monoxide. As used in the specification and claims of this 
application, the term "copolymer" means polymers having two or more 
monomeric groups, including terpolymers and higher polymers. 
After polymerization, oxymethylene polymers, such as those comprising 
trioxane-ethylene oxide copolymer chains, contain unstable 
polyformaldehyde ends, which must be removed in order to improve the 
thermal stability and other properties of the acetal copolymer. For this 
purpose, a hydrolysis process, such as that disclosed in U.S. Pat. No. 
3,219,623, incorporated herein by reference, may be used. More 
specifically, a melt hydrolysis process, such as that disclosed in U.S. 
Pat. Nos. 3,318,848 and 3,418,280, incorporated herein by reference, is 
preferably utilized. 
In the past, it was very difficult to remove enough volatile material (e.g. 
trioxane, formaldehyde, formic acid, water, hydrolysis agent, etc.) to 
provide a product suitable for direct use. Subsequent devolatilization and 
compounding were required to remove volatiles in separate steps, to avoid 
excessive color formation. 
Copending U.S. patent application Ser. No. 664,796 of Auerbach et al, filed 
Oct. 25, 1984, incorporated herein by reference, describes and claims a 
moldable oxymethylene polymer containing a minor amount of a polyamide 
stabilizer which must be in a dispersion. The composition may also contain 
a hindered phenol antioxidant and an amidine stabilizer. 
The incidence of black speck contamination in conventional extrusion or 
compounding equipment is accelerated when processing molten oxymethylene 
polymers containing polyamide stabilizers. Under such circumstances it is 
common to require cleaning the equipment as frequently as every two or 
three days, to produce oxymethylene molding compositions having an 
acceptably low level of black speck contamination. 
Accordingly, there exists a need in the art for an improved method of 
devolatilizing oxymethylene polymers containing polyamide stabilizers, to 
remove formaldehyde and other volatile materials in a manner such that 
black speck generation is minimized and the need for frequent cleaning of 
extrusion or compounding equipment is substantially reduced. The process 
of the present invention provides an answer to this need and also provides 
a product which has a low level of free formaldehyde, low tendency to form 
mold deposits, low molding odor, good heat aged color, in addition to a 
low level of black specks, an does not require the polyamide stabilizer to 
be added as a dispersion. 
SUMMARY OF THE INVENTION 
A process is provided for minimizing or reducing black speck formation and 
for removing formaldehyde and other volatile materials from a molten 
oxymethylene polymer containing a minor amount of both a hindered phenol 
antioxidant and a polyamide stabilizer. Such a molten oxymethylene polymer 
is passed through a rotating disk polymer processor having at least three 
stages--a first devolatilization stage, then a first stabilization stage 
and then a last devolatilization stage. When an oxymethylene copolymer is 
employed in the present process, it is preferably hydrolyzed before it is 
subjected to devolatilization. 
The polyamide stabilizer used in the instant invention can be any polyamide 
having a melting or softening point below the melting point of the 
oxymethylene polymer into which it is incorporated or with which it is to 
be admixed. The polyamide may be crystalline, partially crystalline or 
amorphous. In the case of an amorphous polyamide, its softening point or 
temperature should be below the melting point or temperature of the 
oxymethylene polymer. In short, so long as an amorphous polyamide is 
processable in the compounding and molding equipment at a temperature 
below the melting temperature of the oxymethylene polymer it can be used. 
In all such instances the softening point of the polyamide will be below 
the melting point of the oxymethylene polymer. Generally, the melting 
temperature or softening temperature of the polyamide should be from about 
3.degree. C. to about 10.degree. C. or more below the melting temperature 
of the oxymethylene polymer. The melting temperature is measured by 
differential scanning calorimetry. 
The polyamide must also be one which is stable and does not decompose 
during the oxymethylene polymer compounding and molding operations. 
The polyamides, within the above limitations, can vary widely in 
composition and molecular weight. They are selected from the many 
macromolecular polyamides known in the art in which carboxamide linkages 
##STR3## 
form an integral part of the polymer chain and which upon hydrolysis 
yields monomeric compounds selected from (1) mixtures of dicarboxylic 
acids and diamines and (2) omega-aminomonocarboxylic acids. These 
polyamides preferably have carboxamide linkages in which R is hydrogen, 
alkyl or alkoxy. The molecular weight of the polyamides can vary widely, 
with degrees of polymerization ranging from about 50 to 500. 
The preferred polyamides have a melting temperature or softening 
temperature of from about 105.degree. C. to about 160.degree. C. A 
particularly preferred polyamide is a terpolymer of nylon 6,6/6,10/6, 
which has a melting point of about 150.degree. C. to about 157.degree. C. 
The amount of polyamide used will vary depending upon the particular 
oxymethylene polymer used and the degree of stability desired. Generally, 
the amount of polyamide used is from about 0.05 to about 5.0 weight 
percent, based on the weight of the oxymethylene polymer, preferably from 
about 0.1 to about 5.0 weight percent, and most preferably from about 0.1 
to about 3.0 weight percent. 
While the addition of the lower melting polyamides to oxymethylene polymers 
has provided excellent melt stability, black specks are commonly formed 
during both the compounding and the molding operations. As a result, the 
final molded article often contains black specks, thereby adversely 
affecting the appearance of the molded article. It is believed that the 
black specks are formed during processing and compounding due to the 
reaction of the free formaldehyde present in the oxymethylene polymer with 
the lower melting polyamide. The formaldehyde and polyamide are believed 
to react to form a gel-like substance which adheres to the extrusion 
screws used in the compounding and the molding equipment. The gel becomes 
black with the passage of time at the compounding and molding 
temperatures; portions eventually break apart from the screws as black 
specks and end up in the compounded molding composition and in the final 
molded object or article. 
The polyamide stabilizer is used in conjunction with a hindered phenol 
antioxidant. Suitable phenol antioxidants are disclosed, for example, in 
U.S. Pat. Nos. 3,103,499 and 3,240,753. A particularly effective group of 
phenol antioxidants are those containing two phenol groups each with up to 
two alkyl substituents on the benzene ring, each alkyl substituent 
containing 1 to 4 carbon atoms; these include bis-phenolic diesters such 
as the diester of hexanediol-1,6 and 
3-(3',5'-di-tert-butyl-4-hydroxy)phenylpropionic acid and alkylene 
bisphenols such as 2,2'-methylene bis-(4-methyl-6-tertiary butyl phenol), 
2,2'-ethylene bis-(4-methyl-6-tertiary butyl phenol), 4,4'-ethylidene 
bis-(6-tertiary butyl-3-methyl phenol) and 4,4'-butylidene bis-(6-tertiary 
butyl-3-methyl phenol). Suitable phenol stabilizers other than alkylene 
bis-phenols include 2,6-ditertiary butyl-4-methyl phenol, octyl-phenol and 
p-phenyl phenol. The hindered phenol antioxidant is used in an amount from 
about 0.05 to about 10.0 wt. %, preferably from about 0.1 to about 5.0 wt. 
% and most preferably from about 0.1 to about 1.0 wt. % of the 
composition. The polyamide stabilizer and the hindered phenol antioxidant 
are added to the oxymethylene polymer by any conventional means, such as 
by addition to the molten oxymethylene polymer in an extruder. Preferably, 
a portion of the hindered phenol antioxidant (preferably from 1/3 to 2/3 
of the hindered phenol antioxidant added) is added prior to the present 
devolatilization process. 
In the process of the present invention, the oxymethylene polymer is 
devolatilized such that the oxymethylene polymer in the first 
devolatilization stage is maintained at a temperature above its melting 
point, in the range of from about 160.degree. C. to about 220.degree. C. 
and at a vapor spaced pressure of from about 0.1 to about 500 Torr. The 
polymer in the stabilization stage is maintained in the molten state at a 
temperature of from about 160.degree. C. to about 220.degree. C. The vapor 
space pressure in the stabilization stage is not critical, since it is a 
holding stage during which stabilizers and other additives may be added to 
the molten polymer. Whatever vapor space pressure would facilitate, or at 
least no inhibit, the flow of stabilizer may be utilized, usually not too 
far from amospheric pressure, i.e. about 500 to about 2000 Torr. The 
polymer in the last devolatilization stage is maintained in the molten 
state at a temperature of from about 160.degree. C. to about 220.degree. 
C. and at a vapor space pressure of from about 0.1 to about 200 Torr. 
The maximum total residence time in the polymer processor is desirably from 
about 10 to about 120 seconds, preferably from about 10 to about 90 
seconds and most preferably from about 20 to about 45 seconds. It will, 
however, be recognized that the number of stages in the claimed process 
increases, the residence time will normally increase. The process is 
capable of producing a molten oxymethylene polymer having an extractable 
formaldehyde level of less than about 200 parts per million and in an 
optimized process, less than about 125 parts per million, based on the 
weight of the molten oxymethylene polymer, and a natural color, as 
measured by Hunter "b" value, of less than about 4, preferably less than 
about 2.5. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The process of the present invention removes formaldehyde and other 
volatile materials from molten, extrudable and moldable oxymethylene 
polymers while minimizing the formation of black specks and providing a 
molding composition wherein the occurrence of black specks is minimized in 
the resulting molded objects. The process comprises adding polyamide 
stabilizer and the hindered phenol antioxidant to a molten oxymethylene 
polymer while passing such molten oxymethylene polymer through a rotary 
disk polymer processor having at least three stages. Polyamide stabilizer 
and hindered phenol antioxidant is normally added in a stabilization stage 
of the present process. 
The vapor space pressure in the first devolatilization stage may range from 
about 0.1 to about 500 Torr, preferably from about 1 to about 300 Torr and 
most preferably from about 5 to about 200 Torr. In the second stage (first 
stabilization stage), in order to exclude oxygen, the vapor space pressure 
is maintained at about atmospheric or slightly above atmospheric pressure, 
preferably from about 500 to about 2000 Torr, most preferably from about 
600 to about 1000 Torr. The last devolatilization stage is maintained at a 
vapor space pressure of from about 0.1 to about 200 Torr, preferably from 
about 0.1 to about 100 Torr and for optimum devolatilization efficiency, 
from 0.1 to about 50 Torr. 
Throughout the process the polymer is maintained in the molten state at a 
temperature within the range of about 160.degree. C. to about 220.degree. 
C. In the last devolatilization stage the molten polymer is preferably 
maintained within the range of from about 160.degree. C. to about 
190.degree. C. to further minimize thermal degradation. 
A preferred process utilizes a rotary disk polymer processor having at 
least six stages. These include: (1) a feed stage for smoothing out flow 
variations to provide smoother processor operation, (2) a first 
devolatilization stage, (3) a stabilization stage, (4) a second 
devolatilization stage, (5) a holding stage to minimize surging and (6) a 
pumping stage for pressurizing polymer through the extrusion die. A third 
devolatilization stage and a second stabilization stage may also be added 
to provide an eight stage process. 
Throughout the process the molten oxymethylene polymer is maintained at a 
temperature of from about 160.degree. C. to about 220.degree. C. In a six 
stage process the vapor space pressure in the second devolatilization 
stage is maintained in the range from about 0.1 to about 200 Torr, 
preferably from about 0.1 to about 100 Torr, most preferably from about 
0.1 to about 50 Torr. The vapor space pressure in the second stabilization 
stage is not critical and the stabilization stages are normally maintained 
at about the same vapor space pressure, i.e. at or slightly above 
atmospheric pressure, preferably from about 500 to about 2000 Torr, most 
preferably from about 600 to about 1000 Torr. The vapor space pressures in 
the feed stage, holding stage and pumping stage are not critical. The feed 
stage and holding stage polymer vapor space pressures are normally 
maintained in the range of from about 500 to about 2000 Torr, preferably 
at about atmospheric pressure. The pumping stage is maintained at a 
positive vapor space pressure to facilitate pumping the molten polymer 
through the die. 
Rotary disk polymer processors particularly suitable for use in practicing 
the process of the present invention are described in U.S. Pat. Nos. 
4,141,805, 4,194,841 and 4,529,320, each of which is incorporated by this 
reference. However, other equivalent polymer processors known to those 
skilled in the art or subsequently developed may be used in the process of 
the present invention and are included within the scope of the present 
invention. 
The number of revolutions per minute (RPM) of the disks of the rotating 
disk polymer processor utilized in the present process is not critical. 
However, if the RPM is too low, the flow of polymer will be so slow that 
the machine will flood. If the RPM is too high, it will be extremely 
difficult to maintain proper control of the polymer temperature. 
The process of the present invention provides molding compositions which 
may be processed in the thermoplastic state, for example, by injection 
molding or extrusion molding, into shaped articles, for example, bars, 
rods, plates, sheets, films, ribbons, or tubes and the like. The process 
provides oxymethylene polymer molding compositions wherein the occurrence 
of black specks is minimized in the molding composition itself as well as 
in the resulting molded objects. Black specks previously had been found 
objectionable in objects molded from oxymethylene polymer molding 
compositions. 
It is within the ambit of the present invention that the oxymethylene 
polymer molding composition also include, if desired, plasticizers, other 
formaldehyde scavengers, mold lubricants, antioxidants, nucleating agents, 
fillers, colorants, reinforcing agents, light stabilizers, pigments, other 
stabilizers, and the like, so long as such additives do not materially 
affect the desired properties of the resulting molding composition and the 
articles molded therefrom. The additional additives can be admixed at any 
convenient stage in the molding composition preparation. 
Other suitable formaldehyde scavengers which may be used include 
cyanoguanidine, melamines, amine-substituted triazines or other amidines, 
ureas, hydroxyl salts of calcium, magnesium, and the like, salts of 
carboxylic acid, and metal oxides and hydroxides. Suitable mold lubricants 
include alkylene bisstearamide, long-chain amides, waxes, oils, and 
polyether glycides. Among the nucleating agents which can be utilized are 
oxymethylene terpolymers, such as butanediol diglycidyl ether/ethylene 
oxide/trioxane terpolymers containing from about 99.89 to 89.0 weight 
percent trioxane, 0.1 to 10.0 weight percent of the ethylene oxide and 
0.01 to 1 weight percent of the diglycidyl ether. 
The oxymethylene polymer may be blended with the various additives 
discussed previously using conventional blending or mixing equipment and 
procedures. Thus, the oxymethylene polymer and additives may be blended in 
any convenient sequence using, for example, conventional mills such as 
rubber mills, mixers and blenders as Henschel mixers and tumble benders 
and extruders. In some cases, the components are first dry-mixed, e.g. in 
a tumble or Henschel blender, followed by melt blending, e.g. in an 
extruder. Some of the additives may be added to finely divided 
oxymethylene polymer in the form of a solution in an appropriate solvent 
before the final step of intimate mixing.

The following Examples are given as specific illustrations of the claimed 
invention. It should be understood, however, that the specific details set 
forth in the examples are merely illustrative and not limitative. All 
parts and percentages in the Examples are by weight of the total 
composition, unless otherwise specified. 
POLYMER PREATION 
Oxymethylene copolymer containing about 98 weight percent of recurring 
--OCH.sub.2 -- groups derived from trioxane and about 2 weight percent of 
comonomer units having the formula --OCH.sub.2 --CH-- and derived from 
ethylene oxide was prepared as described in the aforementioned U.S. Pat. 
No. 3,027,352, and melt hydrolyzed according to the procedure described in 
the aforementioned U.S. Pat. Nos. 3,318,848 and 3,418,280 to stabilize the 
ends of the polymer chains. The oxymethylene copolymer possessed an 
inherent viscosity (I.V.) of approximately 1.0 (measured at 60.degree. C. 
in a 0.1 weight percent solution in p-chlorophenol containing 2 weight 
percent of alpha-pinene) and a softening temperature of about 160.degree. 
C. A portion of the oxymethylene monomer was polymerized to obtain a 
copolymer having a weight average molecular weight (m.sub.w) of 
approximately 68,000 and a Melt Index of about 9 g./10 min. when tested in 
accordance with ASTM method D1238-82. A second portion of the oxymethylene 
prepared monomer was polymerized to obtain a copolymer having a weight 
average molecular weight of approximately 50,000 and a Melt Index ranging 
from 27.4 to 31.7. 
In the following Examples, the parameters and values shown are average 
values for a one-hour run, unless otherwise indicated. 
The discoloration characteristics of the oxymethylene polymer is measured 
using the Hunterlab color technique before and after it has been exposed 
to a heat history in a Melt Indexer. The change in Hunterlab "b" value is 
a measure of the tendency to discolor during melt processing. 
Apparatus: 
1. Melt Indexer--equipped with an adjustable thermoregulator (obtainable 
from Precision Thermometer and Instrument Co., South Hampton, Pa., range 
0.degree. C. to 400.degree. C.), 2160 g and 4900 g loads, and a load 
support. 
2. Screw jack with stainless steel head beveled to match the opening in the 
Melt Indexer orifice retainer plate. 
3. Hydraulic press with electrically heated platens. 
4. Two and one quarter inch diameter plug mold, with hardened steel plugs 
and chrome plated, polished finish. 
5. Water cooled knock-out press. 
6. Aluminum dishes. 
7. Hunterlab color difference meter. 
8. 0.22 caliber gun cleaning patches. 
9. Timer 
10. Balance, accurate to 0.1 g. 
11. Phospher Bronze--tipped spatula for cutting the extruded sample. 
Procedure: 
1. Remove orifice plate from Melt Indexer and thoroughly clean the barrel 
with butyrolactone and gun cleaning patches. Set the screwjack in position 
to seal the bottom of the barrel. Load the barrel with 10-12 g of 
oxymethylene polymer, using both the 2160 and 4900 g loads, and extrude 
after 15 minutes. Discard the extrudate and clean the apparatus. 
2. Start the timer, and add 12.5 (.+-.0.2) g of sample to the Melt Indexer. 
Sample addition should take 1-2 minutes. Place the 2160 and 4900 g loads 
atop the sample. 
3. After 30 minutes (.+-.15 seconds) total elapsed time, lower the 
screwjack and extrude the sample into a clean, labelled, aluminum dish, 
and let it cool. The sample will weigh about 11 (.+-.1) g. 
4. Reclean the barrel, piston, and screwjack, and set up the Melt Indexer 
for the next run. 
5. Place the extruder sample into a 21/4" plug mold preheated to 191 
(.+-.3).degree. C. [375 (.+-.5).degree. F.]. Transfer the sample and mold 
to the hydraulic press, which is also preheated to 191.degree. C. Just 
close the platens, applying no pressure, for one minute. Then, apply about 
10000 psi. The pressure will slowly drop to 4000 psi in 11/2-2 minutes. 
Manually maintain this pressure (4000 psi) until the total time under 
pressure is 4 minutes. 
6. Transfer the mold to the sink and cool with water for 3-4 minutes. Cool 
both the top and bottom of the mold equally (if one side cools faster than 
the other, the disk will be concave). Remove the sample from the mold 
using the knock-out press. 
7. Determine the Hunterlab color of the disk, MX "b". 
Mold odor is a measurement of evolved formaldehyde. 
The formaldehyde level evolved from oxymethylene polymers is measured by 
capturing the gases around the extrudate that issue from an extruder and 
using on-line sampling for GC analysis. Typical conditions were those 
required to extrude an oxymethylene copolymer through a 3/4" Barbender 
extruder at barrel temperatures from 380.degree. F. to 440.degree. F. and 
a throughput of 5 lbs/hr. The measurement is intended to give a relative 
measure of the propensity to evolve formaldehyde upon processing in the 
melt state. 
A method is provided for determining the formaldehyde concentration in 
oxymethylene polymers, which can be extracted by refluxing with distilled 
water at 100.degree. C. The formaldehyde in the neutralized water extract 
is determined by the standard sulfite method. 
Apparatus: 
1. Extraction apparatus, Soxhlet, Lab Line 5000. CSM Cat. No. 119-362. 
2. Balance, Top loading, accurate to 0.1 g 
3. Flask, Iodine, 500 ml. CSM Cat. No. 103-127. 
4. Cylinder, graduated, 100 ml. 
5. Interval Timer, Spring mechanism. CSM Cat. No. 063-891. 
Reagents: 
1. H.sub.2 SO.sub.4, 0.10N, standardized. 
2. NaOH, 0.10N, standardized. 
3. Sodium sulfite, 1.0M 
4. Thymolphthalein indicator. 
Procedure: 
1. Place a clean 500 ml. iodine flask on the top loader balance and tare. 
Weigh 100.+-.0.1 g of the sample into the flask. 
2. Using a 100 ml. graduate cylinder, add 100.+-.5.0 ml. distilled water to 
the flask. 
3. Place the flask and contents on the hot plate and connect the reflux 
condenser. Turn on the cooling water. 
4. When reflux begins, set the timer for 60.+-.5.0 minutes. 
5. After refluxing the sample for the required time, turn off the heat, 
slip a plate under the flask and cool to room temperature. 
6. Wash the condenser down with 15 ml. of distilled water and remove the 
flask. NOTE: Do not titrate the sample while still warm. The sample must 
be at room temperature for accurate results. 
7. When the sample has been properly cooled, add 6 drops of thymolphthalein 
indicator and titrate to a faint blue endpoint with 0.10N NaOH. Record as 
titration A. 
8. Add 50 ml. sodium sulfite, which has been previously neutralized to a 
faint blue endpoint, to the sample. Swirl the sample and let stand for 15 
minutes. 
9. Titrate the sample to a faint blue endpoint with 0.1N H.sub.2 SO4. 
Record as titration B. 
Calculation: 
Aldehyde as Formaldehyde 
##EQU1## 
OR 
EQU % Formaldehyde=Titration B.times.0.0030 
TEST SUMMARY 
Test: MX color. 
Measures: The degree of yellowing of oxymethylene polymer under harsh 
stress conditions. 
Conditions: Hold sample for 30 minutes at 230.degree. C. in a melt indexer, 
extrude, compression mold and read the "b" value of the chip. 
EXAMPLES 1-8 
Molten oxymethylene copolymer prepared as described above was fed to the 
first stage of a six stage rotating disk polymer processor. The process 
conditions and properties of the polymers for Examples 1 to 8, which 
exemplify the process of the present invention, are set forth in Table I 
below. In examples 1-8, 0.2% of the hindered phenol antioxidant was added 
prior to the devolatilization process. During the devolatilization 
process, in the first stabilization stage (Stage 3), additional stabilizer 
was added (0.3% of the hindered phenol antioxidant, 0.25% of the polyamide 
stabilizer, 0.2% of a conventional lubricant and 0.5% of a nucleating 
agent). PG,24 
TABLE I 
__________________________________________________________________________ 
Example No. 1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
MI Initial 8.9 
9.2 
8.0 
8.9 
7.6 
9.5 
9.3 
9.3 
MI Final 8.5 
9.2 
8.2 
8.3 
8.2 
9.3 
9.9 
10.0 
xHCHO Initial 
730 
790 
750 
730 
690 
660 
810 
810 
xHCHO Final 80 90 110 
100 
90 90 80 70 
Initial b Color 
2.2 
1.8 
2.1 
2.2 
1.4 
1.4 
1.5 
1.5 
Final b Color 
1.7 
2.5 
2.2 
1.6 
2.4 
2.2 
2.3 
2.2 
Flow Rate 82 90 115 
55 66 83 91 104 
Disk RPM 30 47 50 30 30 30 50 50 
Temperatures: 
Water Bath 45 45 45 45 45 45 45 45 
Barrel Oil 182 
143 
127 
166 
182 
166 
127 
143 
Rotor Oil 165 
144 
128 
149 
165 
149 
128 
144 
Stage 1 211 
211 
211 
211 
218 
211 
209 
208 
Stage 2 184 
191 
183 
187 
196 
193 
179 
185 
Stage 3 186 
174 
174 
173 
184 
179 
170 
177 
Stage 4 187 
181 
181 
176 
183 
179 
175 
180 
Stage 5 190 
183 
191 
177 
189 
189 
182 
192 
Stage 6 181 
162 
169 
166 
179 
171 
148 
163 
Die 197 
198 
212 
186 
199 
197 
205 
214 
Vapor Space Pressures: 
Stage 1 760 
760 
760 
760 
760 
760 
760 
760 
Stage 2 35 29 21 38 37 35 32 31 
Stage 3 1000 
1000 
1000 
1000 
1000 
1000 
1000 
1000 
Stage 4 19 15 19 19 19 16 18 16 
Stage 5 760 
760 
760 
760 
760 
760 
760 
760 
Stage 6 760 
760 
760 
760 
760 
760 
760 
760 
Die Pressure 954 
1030 
1010 
843 
701 
801 
966 
1030 
Residence Time 
40 35 25 45 45 40 35 25 
__________________________________________________________________________ 
Units: 
xHCHO ppm extractable formaldehyde 
Feed Rate kg/hr 
Temperature 
degrees Centrigade (.degree.C.) 
Vapor Space Pressure 
mm Hg absolute (Torr) 
Die Pressure 
psig 
Residence Time 
seconds 
Table II sets forth the process conditions and polymer properties for 
comparative Examples 9 to 16 which correspond to Examples 1 to 8 of 
copending U.S. application Ser. No. 797,054, which discloses and claims a 
process for devolatilizing oxymethylene polymers. In these examples, 0.2% 
of the hindered phenol antioxidant was added prior to the devolatilization 
process. During devolatilization, in the first stabilization stage, 
additional stabilizer was added (0.3% of the hindered phenol, 0.1% of a 
conventional thermal stabilizer, 0.2% of a conventional lubricant and 0.5% 
of a nucleating agent. 
TABLE II 
__________________________________________________________________________ 
Comparative 
Example No. 9 10 11 12 13 14 15 16 
__________________________________________________________________________ 
MI Initial 9.3 
8.7 
7.6 
7.6 
8.0 
9.3 
8.9 
9.3 
MI Final 9.4 
8.8 
9.1 
9.1 
7.2 
9.2 
8.7 
9.9 
xHCHO Initial 
730 
750 
690 
690 
750 
690 
730 
790 
xHCHO Final 220 
160 
160 
140 
200 
140 
130 
210 
Initial b Color 
1.5 
1.6 
1.4 
1.4 
2.1 
1.9 
2.2 
1.8 
Final b Color 
1.5 
1.7 
2.1 
1.8 
2.4 
2.0 
1.7 
3.6 
Flow Rate 82 91 118 
59 59 82 91 127 
Disk RPM 30 50 50 30 30 30 50 50 
Temperatures: 
Water Bath 45 45 45 45 45 45 45 45 
Barrel Oil 182 
143 
127 
166 
182 
166 
127 
143 
Rotor Oil 165 
144 
128 
149 
165 
149 
128 
144 
Stage 1 209 
213 
209 
214 
215 
208 
212 
211 
Stage 2 192 
191 
188 
186 
195 
188 
188 
193 
Stage 3 187 
174 
172 
173 
184 
176 
168 
179 
Stage 4 187 
178 
177 
175 
184 
179 
173 
186 
Stage 5 198 
184 
188 
178 
191 
181 
182 
192 
Stage 6 183 
165 
166 
166 
182 
168 
159 
171 
Die 205 
205 
207 
191 
203 
188 
205 
206 
Vapor Space Pressures: 
Stage 1 760 
760 
760 
760 
760 
760 
760 
760 
Stage 2 36 32 24 33 36 34 27 26 
Stage 3 1000 
1000 
1000 
1000 
1000 
1000 
1000 
1000 
Stage 4 19 12 19 19 19 19 18 19 
Stage 5 760 
760 
760 
760 
760 
760 
760 
760 
Stage 6 760 
760 
760 
760 
760 
760 
760 
760 
Die Pressure 714 
847 
951 
742 
771 
1040 
921 
1200 
Residence Time 
40 35 25 45 45 40 35 25 
__________________________________________________________________________ 
Units: 
xHCHO ppm extractable formaldehyde 
Feed Rate kg/hr 
Temperature 
degrees Centigrade (.degree.C.) 
Vapor Space Pressure 
mm HG absolute (Torr) 
Die Pressure 
psig 
Residence Time 
seconds 
Table III below shows the unexpected improvement in removing formaldehyde 
when, according to the process of the present invention, the combination 
of a polyamide stabilizer and a hindered phenol antioxidant are added to 
the oxymethylene polymer prior to devolatilization in a rotary disk 
polymer processor. 
TABLE III 
______________________________________ 
Comparison of Tables I and II 
Present 
Application 
Invention 
Ser. No. 797,054 
______________________________________ 
Average xHCHO, Initial 
746.25 727.5 
Average xHCHO, Final 
88.75 170.0 
Average xHCHO, Reduction 
657.5 557.5 
______________________________________ 
Table IV below sets forth a comparison between oxymethylene copolymers 
having a Melt Index of approximately 9, prepared as set forth previously 
at page 14 wherein Sample A is oxymethylene copolymer produced by the 
process of the present invention. Sample B was produced by adding about 
0.25% of a polyamide stabilizer and about 0.5% of a hindered phenol 
antioxidant, according to the process of the present invention, but 
utilized a vented twin screw extruder to devolatilize the polymer, rather 
than the process of the present invention; Sample C was produced by adding 
0.5% of a hindered phenol antioxidant, 0.1% of a conventional thermal 
stabilizer, 0.2% of a conventional lubricant and 0.5% of a nucleating 
agent to the oxymethylene copolymer, which was then devolatilized by 
passing the molten polymer through a vented twin screw extruder; Sample D 
was produced by adding 0.5% of a hindered phenol antioxidant, 0.1% of a 
conventional thermal stabilizer, 0.2% of a conventional lubricant and 0.5% 
of a nucleating agent to the oxymethylene polymer which was then 
devolatilized utilizing a rotary disk polymer processor, according to the 
process disclosed in copending U.S. application Ser. No. 06/797,054. 
TABLE IV 
______________________________________ 
Sample Designation 
A (1) B (2) C (3) D (4) 
______________________________________ 
xHCHO, Final (5) 
110 210 280 150 
Mx Hunter b color 
4.3 12.1 25.1 19.0 
Mold Odor (HCHO) (6) 
0.19 0.89 0.98 -- 
______________________________________ 
(1) average of 8 week run on 200 mm, rotary disk polymer processor 
(2) average of 1 year commercial production 
(3) average of 1 year commercial production 
(4) average of 26 runs on 200 mm rotary disk polymer processor 
(5) xHCHO extractable formaldehyde expressed in PPM. 
(6) GC peak height 
EXAMPLE 9 
Molten oxymethylene copolymer prepared as described above, having a Melt 
Index of about 27.4, an initial Hunter b color of 1.8 and an extractable 
formaldehyde level of about 1190 PPM, was fed to the first stage of a 
rotating disk polymer processor. The molten polymer was fed to the polymer 
processor at a temperature of about 191.degree. C. The disk processor 
utilized had an internal diameter of 200 mm. and contained six stages, as 
described above. The molten polymer entered the feed stage at a feed rate 
of about 112 kilograms per hour and the disks of the rotary disk processor 
were rotating at about 55 revolutions per minute. In the first 
stabilization stage, additional stabilizer was added (0.3% of a 
conventional hindered phenol type antioxidant, 0.25% of a polyamide, 0.2% 
of a conventional lubricant and 0.5% of a nucleating agent). The total 
polymer residence time through the disk processor was about 25 seconds. 
The temperature of the oil circulated within the outer shell of the 
polymer processor (barrel oil temperature) was about 171.degree. C. The 
oil temperature in the rotating disks (rotor oil temperature) was about 
149.degree. C. 
The temperature and pressure conditions for each of the six stages are 
shown below in Table 9: 
TABLE 9 
______________________________________ 
Stage 1 2 3 4 5 6 
______________________________________ 
Temp. .degree.C. 
191 189 190 189 191 173 
Vapor Space 
760 4 1000 20 760 760 
Press. (Torr) 
______________________________________ 
The devolatilized molten oxymethylene polymer was pumped from stage 6 
through a die maintained at a temperature of about 191.degree. C. and 
having a die pressure of about 656 psig. The strands were passed through a 
water bath maintained at a temperature of about 45.degree. C. to solidify 
the molten polymer. The solidified strands were then passed through an air 
knife to a pelletizer operating at about 59 RPM to produce the final 
product in pellet form. The extractable formaldehyde level of the final 
product was about 90 parts per million (PPM), the final Hunter b color was 
2.1 and the final Melt Index was about 26.6. 
EXAMPLE 10 
Molten oxymethylene copolymer, having a Melt Index of about 31.7, an 
initial Hunter b color of 1.5 and an extractable formaldehyde level of 
about 860 PPM, prepared as described above, was fed to the feed stage of a 
six stage disk polymer processor. The molten polymer was fed to the 
polymer processor at a temperature of about 193.degree. C. The molten 
polymer entered the first stage at a feed rate of about 83 kilograms per 
hour and the disks of the rotary disk processor were rotating at about 55 
revolutions per minute. In the first stabilization stage, stabilizer was 
added as in Example 9. The total polymer residence time through the disk 
processor was about 30 seconds. The barrel oil temperature was about 
188.degree. C. and the rotor oil temperature was about 165.degree. C. 
The temperature and pressure conditions for each of the six stages are 
shown below in Table 10: 
TABLE 10 
______________________________________ 
Stage 1 2 3 4 5 6 
______________________________________ 
Temp. .degree.C. 
193 184 184 185 197 177 
Vapor Space 
760 16 1000 19 760 760 
Press. (Torr) 
______________________________________ 
The devolatilized molten polymer was pumped from stage 6 through a die 
maintained at about 203.degree. C. and having a die pressure of about 657 
psig. The strands were passed through a water bath maintained at a 
temperature of about 45.degree. C. to solidify the molten polymer. The 
solidified strands were then passed through an air knife and a pelletizer 
operating at about 59 RPM to produce the final product in pellet form. The 
extractable formaldehyde level of the final product was about 120 PPM, the 
final Hunter b color was 2.5 and the Melt Index was about 27.8. 
EXAMPLE 11 
Example 11 represents an eight week run. The operating parameters and 
results are the average values for the eight week run. Molten oxymethylene 
copolymer prepared as described above, having a Melt Index of about 8.5, 
an initial Hunter b color of 1.9 and an extractable formaldehyde level of 
about 520 PPM, was fed to the feed stage at a six stage disk polymer 
processor. The molten polymer was fed to the polymer processor at a 
temperature of about 202.degree. C. The molten polymer entered the first 
stage at a feed rate of about 81 kilograms per hour and the disks of the 
rotary disk processor were rotating to about 30 revolutions per minute. In 
the first stabilization stage, stabilizer was added as in Example 9. The 
total polymer residence time through the disk processor was about 40 
seconds. The barrel oil temperature was about 188.degree. C. and the rotor 
oil temperature was about 130.degree. C. 
The temperature and pressure conditions for each of the six stages are 
shown below in Table 11: 
TABLE 11 
______________________________________ 
Stage 1 2 3 4 5 6 
______________________________________ 
Temp. .degree.C. 
202 192 190 183 177 176 
Vapor Space 
760 14 1000 17 760 760 
Press. (Torr) 
______________________________________ 
The devolatilized molten polymer was pumped from stage 6 through a die 
maintained at about 177.degree. C. and having a die pressure of about 739 
psig. The strands were passed through a water bath maintained at a 
temperature of about 45.degree. C. to solidify the molten polymer. The 
solidified strands were then passed through an air knife and a pelletizer 
operating at about 59 RPM to produce the final product in pellet form. The 
extractable formaldehyde level of the final product was about 110 PPM, the 
final Hunter b color was 1.8 and the Melt Index was about 8.5.