Method for mixing raw materials for producing oxymethylene copolymers

In a method for mixing a trioxane, a polymerization catalyst and a comonomer for producing of oxymethylene copolymers by continuously copolymerizing trioxane with a cyclic ether or a cyclic acetal as a comonomer, the improved method comprises following steps: PA1 spouting a comonomer and a polymerization catalyst from nozzle openings which are set adjacent to each other, PA1 while washing both openings at the tip end of the nozzles with a flow of trioxane, PA1 whereby mixing said comonomer, said polymerization catalyst and said trioxane, PA1 and then feeding the mixture to a polymerization reactor to effect copolymerization. The raw materials are mixed together before they are fed to the polymerization reactor, and no clogging trouble at the tip end of the nozzle for feeding a polymerization catalyst occurs. Further the resulting copolymer has a much superior heat stability. In addition, since each of the reaction raw materials is quantititively fed, the molecular weight and conversion of the copolymer extended from the polymerization reactor is so constant that a stabilized operation of the polymerization reactor can be secured.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for mixing polymerization raw 
materials by continuously mixing trioxane, a comonomer which is 
copolymerizable therewith and a polymerization catalyst and feeding them 
into a polymerization reactor in the production of polyacetals, and an 
apparatus therefore. 
Oxymethylene copolymers containing about 60 to 99.6% of oxymethylene units 
and a balance of repetition units of (--c--c-- ) have been known as a very 
useful plastic for industrial uses. 
Processes for producing the above-mentioned oxymethylene polymers have 
heretofore been studied, and above all, processes of employing trioxane as 
a starting raw material have been particularly developed due to easy 
purification of trioxane. 
Processes for producing oxymethylene copolymers employing trioxane include 
a batch system and a continuous system. 
In the batch production process, liquid trioxane, a comonomer and a 
polymerization catalyst, each in a fixed amount, are fed into a reactor 
and mixed together with stirring to obtain polymer. This process, however, 
has drawbacks that the reaction product forms a mass with the progress of 
a rapid polymerization reaction, adequate temperature control of 
polymerization substances is difficult, milling of final polymer is not 
easy and discharge of product is difficult. 
On the other hand, in the continuous process, trioxane and a comonomer ring 
are subjected to bulk polymerization while the reaction temperature is 
adjusted, and oxymethylene copolymer is obtained as a milled product. In 
an example, a reaction mixture is fed into a continuous polymerization 
apparatus from its inlet, and polymerized in a reaction zone. The 
resulting polymer is mechanically milled, carried and taken out of its 
exit. According to such a continuous bulk polymerization, the amounts of 
solvent and catalyst employed are small and a copolymer can be obtained at 
a high conversion, and hence the process is a very advantageous one as a 
commercial process for polyacetal resins. 
However, for completing this technique, certain problems should be solved. 
One of them is to develop a mixing method for enabling to continuously mix 
together trioxanes, a comonomer and a polymerization catalyst and 
quantitatively feed them into a continuous polymerization reactor. When 
trioxane containing substantially no solvent is mixed with a 
polymerization catalyst, polymerization reaction immediately starts and 
the reaction mixture solidifies. Thus, polymer adheres to the tip end of 
the catalyst-feeding nozzle, and feed often becomes impossible, resulting 
in an obstacle to a smooth continuous polymerization. In order to prevent 
this, processes such as incessant washing of the tip end of the 
catalyst-feeding nozzle with a large amount of a solvent, or adding large 
amount of a solvent to trioxane and comonomer to be fed, have been 
proposed, but these processes are deviated from the object of the bulk 
polymerization and commercially unadvantageous. Treatments such that the 
tip end of the catalyst-feeding nozzle is often taken out and cleaned may 
be carried out, but, in these cases, such problems are raised that 
mixing-in of moisture in air or variability of the amount of catalyst fed 
occurs, resulting in constant molecular weight of polymer extruded from a 
continuous polymerization reactor as well as inconstant conversion. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a method for mixing to 
obtain oxymethylene copolymers according to a commercially advantageous 
continuous bulk polymerization, and the invention resides in the following 
process: 
In the continuous polymerization reaction wherein trioxane is copolymerized 
with a cyclic ether or a cyclic acetal to obtain an oxymethylene 
copolymer. 
A method which comprises spouting a comonomer and a polymerization catalyst 
from nozzle openings which are set adjacent to each other, while washing 
both openings at the tip end of the nozzles with a flow of trioxane, 
whereby mixing said comonomer, said polymerization catalyst and said 
trioxane, and then feeding the mixture to a polymerization reactor to 
effect copolymerization.

DETAILED DESCRIPTION OF THE INVENTION 
As described above, the present invention comprises, as basic steps, 
spouting a comonomer and a polymerization catalyst from nozzle openings 
which are set adjacent to each other, while washing both openings at the 
tip end of the nozzles with a flow of trioxane, whereby mixing said 
comonomer, said polymerization catalyst and said trioxane, and then 
feeding the mixture to a polymerization reactor to effect 
copolymerization. 
The reaction mixture referred to herein means a mixture of trioxane, a 
comonomer and a polymerization catalyst as main components and besides, 
small amounts of a solvent and other additives. 
In the present invention, it is indespensable that the opening at the tip 
end of a nozzle for feeding a comonomer is made adjacent to the opening at 
the tip end of a nozzle for feeding a polymerization catalyst. If both the 
openings are not made adjacent, polymer is immediately formed at the 
opening at the tip end of a nozzle for feeding a polymerization catalyst 
and clogs the opening, resulting in incapability of feeding the 
polymerization catalyst. Further, even in the case of a process of pouring 
a polymerization catalyst into a mixture of trioxane with a comonomer, the 
clogging of the opening at the tip end of a nozzle for feeding a 
polymerization catalyst cannot be also avoided. 
Comonomer is herein employed for stabilizing the polymerization into 
trioxane polymer. Besides this effectiveness, according to the mixing 
process of the present invention, the comonomer exhibits the following 
effectiveness: 
If the tip end of the nozzle for feeding a comonomer is separate from that 
for feeding a polymerization catalyst, a considerably hard polymer is 
formed since just after the flow of trioxane has been mixed with a 
polymerization catalyst, and its polymer clogs the tip of the nozzle for 
feeding a catalyst. It goes without saying that in case where the 
respective tip ends of the nozzle for feeding a comonomer and that for 
feeding a polymerization catalyst are not washed, polymer is also formed 
there and clogs them as well, resulting in incapability of feeding 
catalyst. Further, for the flow of trioxane which washes the tip ends of 
both the nozzles as well as the flow of the reaction mixture downstream 
from the above-mentioned flow, a fixed flow rate or higher is required. 
Namely, at the tip ends of both the nozzles and downstream from them, it 
is desirable that the rate be 20 cm/sec. or higher, preferably 50 cm/sec. 
or higher. Further the retention time of the reaction mixture in the 
apparatus of the present invention is preferably 10 second or shorter, 
more preferably 5 seconds or shorter. 
The comonomer employed in the present invention is a cyclic ether or a 
cyclic acetal and a compound expressed by the general formula (I) 
mentioned below. The amount of the comonomer employed is 0.4-40% by mol, 
preferably 0.4-10% by mol. 
##STR1## 
wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are the same or different 
and each represent hydrogen atom, an alkyl group of a halogen-substituted 
alkyl group; R.sub.5 represents methylene group or oxymethylene group or 
an alkyl group-substituted or a halogenated alkyl group-substituted 
methylene group or oxymethylene group, (in this case, n being an integer 
of 0--3), or a compound of --(CH.sub.2).sub.m --O--CH.sub.2 -- or 
(--O--CH.sub.2 --CH.sub.2 --).sub.m O--CH.sub.2 -- (in this case, n is 
equal to 1 and m represents an integer of 1-4); and said alkyl group has 
1-5 carbon atoms and can be replaced by 0-3 halogen atoms, particularly 
chlorine atom(s). 
For the cyclic acetal or cyclic ether, ethylene oxide, glycol formal, 
diglycol formal, and among these, diglycol formal are particularly 
suitable. Further, for example, propylene oxide and epichlorohydrin can be 
also employed. Still further, a cyclic formal of long chain .alpha., 
.omega.-diol, e.g. butanediol formal (1, 3-dioxcepane) or hexanediol 
formal thereof is also suitable. 
As for the polymerization catalyst, known cationic polymerization catalysts 
are employed, and particularly one kind or more of boron fluoride, boron 
fluoride hydrate and coordinate compounds of boron fluoride with an oxygen 
atom--or sulfur atom--containing compound can be employed in the form of 
vapor or a solution thereof in a suitable organic solvent. Coordinate 
compounds of boron fluoride, particularly boron fluoride etherate and 
boron fluoride butyrate are preferably polymerization catalysts. 
Besides these, molecular weight modifiers such as methylal, 
polyoxymethylene dimethoxide, alcohols, e.g. methanol, ethanol, etc., 
phenolic compounds, etc. and other additives may be mixed with trioxane in 
advance. Further, it is also possible to mix them with polymerization raw 
materials by feeding them into the apparatus for feeding polymerization 
raw materials, of the present invention, by means of another nozzle. 
In the present invention, it is preferable that trioxane contain no solvent 
or 20% by weight or less of a solvent. Comonomer is fed as it is or 
diluted with a solvent and fed. Polymerization catalyst is fed as it is or 
diluted with a solvent and fed in the form of vapor or liquid. 
Next, the apparatus for carrying out the process for mixing polymerization 
raw materials, of the present invention will be illustrated referring to 
FIGS. 1-4. Numeral 1 shows an apparatus for mixing polymerization raw 
materials; numeral 2, a flow-in path of trioxane; numeral 3, a nozzle for 
feeding a comonomer; numeral 4 its tip end; numeral 5, a nozzle for 
feeding a catalyst; and numeral 6, its tip end. Downstream from both the 
tip ends is located a mixing chamber 8, and preferably an orifice 7 is 
provided above the mixing chamber 8. Numeral 9 shows a port for feeding a 
reaction mixture and numeral 10 shows a polymerization reactor. Two 
nozzles 3 and 5 as shown in FIG. 1 are contacted at the upper parts of the 
tip ends 4 and 6, and opened in an opposed manner. Accordingly, the 
comonomer and the polymerization catalyst are contacted with each other at 
the same time when they are extruded from the nozzles, and further mixed 
with trioxane. While they are passed through the orifice 7, they are 
uniformly mixed together, and their flow rate is somewhat reduced at the 
lower part of the mixing chamber 8 and then quietly fed into a 
polymerization reactor 10. FIGS. 2-4 show a double pipe structure, the 
outer pipe being a nozzle for feeding a comonomer 3 and the inner pipe 
being a nozzle for feeding a catalyst 5, the tip end 6 of which is 
protruded somewhat outwards from the tip end 4 of the nozzle for feeding a 
comonomer 3 and opened. Thus, in either cases, the tip ends 4 and 6 are 
incessantly washed by a strong flow of trioxane. In FIG. 3, a projection 
11 is provided on the inner wall of a mixing chamber 8 in a ring form, and 
supports a cylinder 12 thereon. This cylinder 12 is so arranged that a 
reaction mixture formed by contact of the extrusion from the tip ends 4 
and 6 of the nozzles with trioxane immediately flows into the inside 13 of 
the cylinder. By properly selecting cylinders 12 having the same diameters 
but different thicknesses, it is possible to obtain a desired linear 
velocity inside the cylinder. A part of trioxane fed passes through a 
space 14 between the outer diameter of the cylinder 12 and the inner 
diameter of the mixing apparatus 1, passes through between the projection 
11, and is combined with the reaction mixture having passed through the 
inside 13 of the cylinder. Further they are mixed together and then fed to 
the polymerization reactor through a port for feeding the reaction mixture 
9. In FIG. 4, a nozzle for feeding a molecular weight modifier 15 is 
further provided, and in this case, it is not always necessary to provide 
the tip end of this nozzle 15 adjacently to the tip ends 4 and 6 of the 
double pipe. 
As for the continuous polymerization reactor, there is the one disclosed in 
Japanese Patent Publication No. 5234/1969 and sold under a tradename of 
Kokneader. Further, U.S. Pat. No. 3,442,866 discloses a reactor containing 
a pair of engaged parallel screws in an elongated case. On the other hand, 
according to the polymerization process developed by the inventors of the 
present invention, raw material monomers and a polymerization catalyst are 
fed into a prior stage polymerization reactor having a self-cleaning 
property to carry out polymerization, and after a conversion in the range 
of 40-70% has been attained, the resulting reaction mixture is taken out 
of the exit of the prior stage polymerization reactor in the form of 
particle. The reaction mixture is then fed into a posterior stage 
polymerization reactor having no self-cleaning property but an agitating 
function to carry out a posterior stage polymerization reaction, and after 
a conversion of 95-100% has been attained, the resulting polymerization 
product is taken out of the exit of the posterior stage polymerization 
reactor, in the form of particle. FIGS. 5 and 6 show a state where the 
apparatus of the present invention is fixed to the above-mentioned prior 
stage polymerization reactor. Numeral 16 shows a jacket which controls the 
polymerization temperature through a heating medium therein. The reaction 
mixture fed through a port for feeding the reaction mixture 9 is led into 
the inside of a shell. Inside the shell, at least two horizontal agitating 
shafts 17 having a plurality of paddles 18 in the form of a 
pseudo-triangle plate fixed thereonto are provided. These shafts are 
constructed as follows: When the horizontal agitating shafts 17 are 
rotated at the same time in the same direction, the top edge 19 of the 
pseudo-triangle plate of the paddle 18 or one of the horizontal agitating 
shafts 17 is contacted with the inner surface of the reaction shell or the 
surface 20 of the corresponding paddle 18 fixed onto the other horizontal 
agitating shaft, while maintaining a slight clearance. Thus the 
polymerization reaction product adhered onto the inner surface of the 
reaction shell and the outer peripheral surface of the paddle is 
incessantly peeled off and gradually carried away from the inlet of the 
reactor to the exit. The polymerization reaction product extruded from the 
prior polymerization reactor is carried into the posterior polymerization 
reactor having a jacket outside it, an agitating mechanism inside it and 
no self-cleaning mechanism, where the polymerization reaction is completed 
with a conversion of 95-100%. 
According to the present invention, the tip end of the nozzle for feeding a 
comonomer is made adjacent to the tip end of the nozzle for feeding a 
polymerization catalyst, and the reaction mixture is fed to the 
polymerization reactor while the tip ends of both the nozzles are washed 
by liquid trioxane, and hence the polymerization raw materials are 
sufficiently mixed together before they are fed to the polymerization 
reactor, and also the clogging trouble at the tip end of the nozzle for 
feeding a polymerization catalyst does not occur. Further the heat 
stability of the resulting copolymer is also much superior. Still further 
since the respective raw materials are quantitatively fed, the molecular 
weight of the copolymer and the conversion are constant and also the 
stabilized operation of the polymerization reactor is secured. 
Furthermore, according to the present invention wherein a premixing 
process is carried out, the conversion is superior to that in case where a 
mixture of trioxane with a comonomer and a polymerization catalyst are fed 
to the polymerization reactor through separate nozzles. Thus, by employing 
the mixing apparatus of the present invention in a continuous bulk 
polymerization reactor, a commercial production of oxymethylene copolymers 
has become very advantageous. 
The present invention will be concretely described by way of Examples and 
Comparative examples. 
EXAMPLE 1 
Employing an apparatus for mixing polymerization raw materials shown in 
FIG. 3, trioxane was fed at a rate of 20 kg/hr. and 1,3-dioxcepane was fed 
at a rate of 700 g/hr. through the outer pipe of the double pipe i.e. a 
nozzle for feeding the comonomer 3. Further, 0.20 m.mol of boron 
trifluoride etherate per mol of trioxane was fed through the inner pipe 
i.e. a nozzle for feeding a polymerization catalyst 5. The boron 
trifluoride etherate was employed in the form of a solution in bezene as 
solvent, containing 0.6 m.mol of boron trifluoride etherate per ml of the 
solution. The reaction mixture flowed down through the inside of the 
cylindrical pipe 12 at a linear velocity of 80 cm/sec., and one second 
after mixing, fed into the polymerization reactor. Further, employing a 
prior stage polymerization reactor shown in FIG. 5 and having an inner 
diameter of the reaction shell of 102 mm, a prior stage polymerization 
reaction was carried out and successively a posterior stage polymerization 
reaction was carried out. The posterior stage polymerization reactor had a 
reaction shell equipped with a jacket outside it, and a pair of shafts 
having a number of mixing blades fixed thereonto, the shafts being a mixer 
of a non-cleaning property, by which the contents were mixed while the 
shafts were rotated in different directions to each other. The inner 
diameter of the reaction shell was 200 mm. 
The polymerization temperature was adjusted to 90.degree. C. at the prior 
stage and to 60.degree. C. at the posterior stage. The polymerization 
reaction was continued over 100 hours during which it was 
maintenance-free, and even after 100 hours, no abnormality was observed. 
Further, during the reaction, a polymer having an intrinsic viscosity of 
1.43-1.48 as measured in p-chlorophenol containing 2% .alpha.-pinene at 
60.degree. C. (this measuring method was employed also in the Example and 
Comparative examples mentioned below) and a polymer content of 99.5-99.9% 
was stably extruded from the posterior stage polymerization reactor. 
EXAMPLE 2 
Employing an appratus for mixing polymerization raw materials, shown in 
FIG. 1, trioxane was fed at a rate of 2 kg/hr. and liquefied ethylene 
oxide was fed at a rate of 50 g/hr. through the nozzle for feeding a 
comonomer 3. Further 0.18 m.mol of boron trifluoride etherate per mol of 
trioxane was fed through the nozzle for feeding a polymerization catalyst 
5. The boron trifluoride etherate was employed in the form of a solution 
in benzene as solvent, containing 0.1 m.mol of boron trifluoride etherate 
per ml of the solution. Further, the linear velocity of the reaction 
mixture in the orifice 7 was about 30 cm/sec. The period of time since 
mixing together of trioxane, ethylene oxide and the polymerization 
catalyst till feeding into the polymerization reactor, i.e. the retention 
time inside the mixing chamber 8, was one second. For the continuous 
polymerization reactor, the one consisting of a prior stage polymerization 
reactor and a pin mixer connected thereto was employed. This pin mixer had 
a shaft with many pins in a elongated case and was employed as a posterior 
stage polymerization reactor. The inner diameter of the interior 
polymerization reactor was 50 mm, and two horizontal agitating shafts 17 
were so constructed that onto the respective shafts were fixed paddles 18 
consisting of a number of ellipsoidal plates, engaged with the couterpart 
plates, and being capable of cleaning the inner surface of the reaction 
shell and the surface of the counterpart ellipsoidal plates with the major 
axis parts of the ellipsoidal plates. The polymerization temperature was 
adjusted to 80.degree. C. A crude polymer having a polymer content of 
68.0% by weight was extruded from the posterior stage polymerization 
reactor, and fed to a pin mixer where it was subjected to a further 
mixing. From this pin mixer was extruded a polymer having a polymer 
content of 99.5% by weight and an intrinsic viscosity of 1.62. A 
continuous polymerization was continued over 300 hours during which no 
trouble such as clogging due to polymer formed, etc. in the mixing 
apparatus occurred. The polymerization reaction was also stationary and 
the intrinsic viscosity and polymer content of the extruded polymer were 
also nearly constant. Polymerization reaction was stopped after 300 hours, 
and no abnormality was observed in this apparatus. 
To the polymer thus obtained was added triphenylphosphine in an amount of 
twice the mols of the polymerization catalyst employed to deactivate the 
catalyst, and then 0.5 part of Irganox 295 (tradename), 0.2 part of 
polyvinyl pyrrolidone and 0.1 part of calcium hydroxide were added, and 
further, they were kneaded, as they were, at 200.degree. C. for 20 minutes 
by means of a kneader to stabilize the polymer. Thus stabilized polymer 
had an intrinsic viscosity of 1.60 and its reduction rate 
K.sub.222.sup.air in the air at 222.degree. C. due to thermal 
decomposition was 0.01% by weight. Further the yield of the stabilized 
polymer from the crude polymer at the time of the stabilization was 94%. 
In addition, for comparison, the tip ends 4 and 6 of the nozzles were 
provided so as to be 3 cm apart away from each other, in the apparatus for 
mixing polymerization raw materials, shown in FIG. 1, and except for this, 
trioxane, ethylene oxide and boron trifluoride etherate were fed in the 
same manner as mentioned above. After 10 minutes, polymer began to be 
formed at the tip end 6 of the nozzle 5, and before long the feed of the 
catalyst solution became impossible. 
COMATIVE EXAMPLE 1 
Employing the apparatus shown in FIG. 7, a mixture of trioxane with 2.5% by 
weight based on trioxane of ethylene oxide was fed through a flow-in path 
for trioxane at a rate of 2 kg/hr. Boron trifluoride etherate in an amount 
of 0.18 m. mol per mol of trioxane was fed through a nozzle for feeding a 
catalyst 5. The boron trifluoride etherate was employed in the form of a 
solution in benzene as solvent, containing 0.1 m.mol of boron trifluoride 
ether per ml of the solution. The reaction mixture had a linear velocity 
of 50 cm/sec at the orifice part 7, and one second after mixing, was sent 
into the polymerization reactor. As a result, after 30 minutes, polymer 
adhered onto the tip end 6 of the nozzle for feeding a catalyst 5, and 
clogged it. The orifice part 7 was also clogged by this polymer to make 
the feeding of the liquid impossible. 
COMATIVE EXAMPLE 2 
As shown in FIG. 8, a mixture of trioxane with 2.5% by weight based on 
trioxane of ethylene oxide was fed directly to the polymerization reactor 
through a flow-in path for trioxane 2 at a rate of 20 kg/hr. On the other 
hand, boron trifluoride etherate was fed directly to the polymerization 
reactor through a nozzle for feeding a catalyst 5. The polymerization 
reactor employed was the same as that employed in Example 1. The boron 
trifluoride etherate was employed in the form of a solution in benzene as 
solvent containing 0.6 m.mol of the etherate per ml of the solution, and 
0.2 m.mol of boron trifluoride per mol of trioxane was fed. Thirty minutes 
after start of the polymerization, polymer adhered onto the tip end 6 of 
the nozzle for feeding a catalyst 5, and feed of liquid became impossible. 
This compelled operations of taking off the nozzle for feeding a catalyst 
5 each 15 minutes-30 minutes and removing polymer at the tip end 6. 
Accordingly the polymerization reaction became unstationary, and the 
polymer content of the polymer extruded from the prior stage 
polymerization reactor varied greatly between 50% and 60%, and also the 
intrinsic viscosity varied between 1.3 and 1.5. 
COMATIVE EXAMPLE 3 
As shown in FIG. 9, a mixture of trioxane with 2.5% by weight based on 
trioxane of ethylene oxide, through the outer pipe of the double pipe, and 
boron trifluoride etherate, through the inner pipe thereof i.e. a nozzle 
for feeding a catalyst 5, were fed directly to the polymerization reactor, 
which was the same as that employed in Example 1. The tip end of the 
double pipe was so constructed that the top edge 19 of the paddle 18 of a 
pseudo-triangle form, fixed onto the horizontal agitating shaft of the 
polymerization reactor was contacted with the tip end while maintaining a 
slight clearance when the shaft is rotated. The mixture of trioxane with 
ethylene oxide was fed at a rate of 20 kg/hr., and the boron trifluoride 
etherate was fed in the form of a benzene solution containing 0.6 m.mol of 
boron trifluoride etherate per ml of the solution, at a rate of 0.20 m.mol 
of boron trifluoride per mol of trioxane. One hour after initiation of 
polymerization, polymer began to adhere onto the inside of the tip end of 
the nozzle for feeding a catalyst 5, and feed of liquid became difficult.