Process for producing mixed poly(vinyl acetal)s

A process for producing mixed poly(vinyl acetals) containing butyral acetal groups and long chain acetal groups derived from one or more aldehydes containing at least six C atoms, by conducting an acetalization reaction forming long chain acetal groups in the substantial of absence butyraldehyde until gel break occurs, adding butyraldehyde after gel break and continuing acetalization until the desired reaction endpoint is reached. Reaction temperature is from about 40.degree. C. up to the boiling point of water at the pressure of the acetalization reaction and preferably between about 75.degree. C. and about 98.degree..

BACKGROUND OF THE INVENTION 
This invention relates to a process for producing mixed poly(vinyl acetal)s 
and more particularly to a process for producing partial polyvinyl butyral 
containing unreacted vinyl alcohol groups which is modified with long 
chain acetal groups. 
Polyvinyl butyral is used as sheet in transparent laminated glazings, 
adhesive compositions, protective coatings, photoimaging applications, 
binders for ceramics and the like. Such resin contains 10 to 30 weight % 
unreacted vinyl alcohol groups and is hereinafter called "partial 
polyvinyl butyral" or "partial PVB". Partial polyvinyl butyral can be 
modified by including long chain acetal groups in the polymer for internal 
plasticizing, such partial polyvinyl butyral containing long chain acetal 
groups hereinafter called "mixed poly(vinyl acetal)s". In this regard note 
U.S. Pat. No. 5,137,954, issued Aug. 11, 1992. Such long chain acetal 
groups can broaden the utility of a mixed poly(vinyl acetal). For example, 
U.S. Pat. No. 5,137,954 discloses use of mixed poly(vinyl acetal)s with 
reduced amounts of external plasticizer and U.S. Pat. No. 5,019,624 
discloses mixed poly(vinyl acetal)s without external plasticizer. 
The size of resin particles formed during synthesis of partial PVB and 
mixed poly(vinyl acetal)s is important, particularly for laminated glazing 
applications. Particles too large can vary in hydroxyl content within the 
particle and can appear as visual defects in the transparent sheet of the 
glazing; particles too small present handling problems during formulation 
with other materials and can be an explosion hazard. 
Partial PVB and mixed poly(vinyl acetal)s have been synthesized by 
acetalizing polyvinyl alcohol with butyraldehyde or mixtures of 
butyraldehyde and another aldehyde at low temperature on the order of 
5.degree.-20.degree. C., followed over time as the reaction continues by a 
slow increase to about 50.degree.-90.degree. C. In this regard, note 
Example 1 of U.S. Pat. No. 5,137,954. When two aldehydes were used in such 
prior art, simultaneous formation of both acetal linkages occurred during 
the acetalization reaction. As far as is known, this acetalization 
temperature profile in the prior art was apparently intended to provide 
the desired resin particle size and minimize problems just referred to. 
SUMMARY OF THE INVENTION 
Now improvements have been made to optimize the process for producing mixed 
poly(vinyl acetal)s. 
Accordingly, a principal object of this invention is to improve the process 
for producing mixed poly(vinyl acetal)s. 
Another object is to provide an industrially valuable, high temperature, 
reduced cycle time process for preparing mixed poly(vinyl acetal)s. 
A further object is to provide a process technique reducing the size and 
improving the size distribution of particles of mixed poly(vinyl acetal)s 
during their synthesis. 
Another object is to make mixed poly(vinyl acetal)s without using 
low-temperature in the synthesis process. 
Other objects will in part be obvious and will in part appear from the 
following description and claims. 
These and other objects are accomplished by a process for producing mixed 
poly(vinyl acetal)s containing butyral acetal groups and long chain acetal 
groups derived from-one or more aldehydes containing at least six C atoms 
which comprises a) conducting an acetalization reaction forming long chain 
acetal groups in the substantial absence of butyraldehyde until gel break 
(hereinafter defined) occurs, b) adding butyraldehyde after gel break, and 
c) continuing acetalization until the desired reaction end point is 
reached. 
In another aspect, there is provided, in a process for producing partial 
polyvinyl butyral modified with long chain acetal groups which comprises 
a) acetalizing polyvinyl alcohol with butyraldehyde in the presence of an 
acid catalyst to form partial polyvinyl butyral having about 10 to 30 
weight percent unreacted vinyl alcohol groups and b) neutralizing the 
partial polyvinyl butyral with a base, the step in combination therewith 
of: c) condensing the polyvinyl alcohol with one or more aldehydes 
containing at least six C atoms before conducting the butyraldehyde 
acetalizing step. 
In a further aspect, a process is provided for producing mixed poly(vinyl 
acetal)s which comprises a) acetalizing polyvinyl alcohol with one or more 
long chain aldehydes, each containing at least six C atoms, at a 
temperature between about 40.degree. C. to about 98.degree. C. in an 
aqueous acidic solution until polymeric particles containing long chain 
acetal groups derived from said one or more long chain aldehydes 
precipitate out of solution, b) adding butyraldehyde to the reaction 
medium of a) while such medium is within the temperature range recited in 
a), and then c) continuing acetalization of the polyvinyl alcohol with 
butyraldehyde at a temperature greater than about 80.degree. C. until the 
desired reaction end point is reached. 
In a specific aspect, a process is provided for improving the efficiency of 
reaction during synthesis of mixed poly(vinyl acetal)s containing butyral 
acetal groups and long chain acetal groups derived from aldehydes having 
at least six C atoms which comprises a) conducting the acetalization 
reaction forming the long chain acetal groups at a temperature above about 
50.degree. C. in the substantial absence of butyraldehyde until gel break 
occurs, b) adding butyraldehyde to the reaction medium after a time 
interval greater than one minute subsequent to gel break without 
decreasing the temperature below about 50.degree. C., and c) continuing 
acetalization until the desired reaction end point is reached. 
In another aspect, a process is provided for producing mixed poly(vinyl 
acetal)s which comprises a) sequentially forming butyral acetal groups 
after first forming long chain acetal groups derived from one or more 
aldehydes having at least six C atoms during an acetalization reaction at 
a temperature which is always below boiling and greater than about 
40.degree. C.

DETAILED DESCRIPTION 
The process of the invention produces mixed poly(vinyl acetal)s containing, 
based on weight of the poly(vinyl acetal), 10 to 30% by weight vinyl 
alcohol groups, about 0 to 3 % by weight vinyl acetate groups, 20 to 85 
weight % butyral groups and an effective amount (weight %) sufficient to 
functionally modify the properties of the poly(vinyl acetal) of long chain 
acetal groups derived from one or more aldehydes containing at least 6 C 
atoms (LCA). Such effective amount of long chain acetal groups in a 
poly(vinyl acetal) ranges from more than incidental impurities up to 65 
weight %. For laminated glazing applications, a preferred range is 5 to 55 
weight % long chain acetal groups, most preferably 20 to 40 weight %. 
In the process of the invention, acetalization of an aqueous, homogenous, 
polyvinyl alcohol (PVOH) solution to form long chain acetal groups is 
first conducted in the substantial absence of butyraldehyde. In this 
regard, trace amounts of butyraldehyde up to about 5 weight percent of the 
LCA in the first phase of the process do not adversely affect performance. 
After a brief period of operating this way, the length of which depends on 
the temperature of the reaction and is usually on the order of about 5 to 
40 minutes, white polymeric particles precipitate out of the PVOH solution 
as an emulsion of condensed polyvinyl acetal containing such long chain 
acetal groups. This usually sudden, crisp change from clear homogeneous 
solution to a heterogeneous phase is referred to herein as "gel break". 
After gel break, the condensation reaction of PVOH with LCA is preferably 
continued, still without the presence of substantial butyraldehyde, for a 
finite variable time greater than one minute and which is usually about 15 
to 90 minutes, to gradually further reduce the hydroxyl level of the 
reacting PVOH. Then, after a certain variable level of hydroxyl groups of 
the PVOH have reacted with the LCA, butyraldehyde is added and 
acetalization of the PVOH under the influence of the prior-formed long 
chain acetal groups is continued in situ in the single reaction zone to 
form butyral acetal groups until the desired end point of the condensation 
reaction is reached. Such end point is determined by the unreacted 
hydroxyl level (calculated as vinyl alcohol groups) desired in the final 
mixed poly(vinyl acetal) product. 
After addition of butyraldehyde (or during addition if added over time) to 
the emulsion of PVOH partially reacted with LCA, significant growth of the 
mixed poly(vinyl acetal) particles begins. This can be visually detected 
in the transparent glass reactor embodiments of the Examples following and 
results in an average resin particle diameter of about 300-400 microns 
developed after gel break. When the reaction is conducted in this manner, 
i.e. sequentially forming butyral acetal groups after first forming long 
chain acetal groups, the tendency of the polymer particles to undesirably 
agglomerate or stick to each other is unexpectedly significantly reduced. 
In other words, acetalization with the LCA occurs to an extent effective 
to counteract and stabilize against later excess agglomeration as the 
particles increase in size in the presence of the reacting butyraldehyde 
and become richer in butyral content. This occurs to such an extent as to 
permit conducting the entire acetalization reaction at elevated 
temperature (to be further described) and avoids the low temperature phase 
so prominently used in the prior art. 
In the process of the invention, phases of the acetalization reaction can 
be conducted at different temperatures. For example, though unnecessary 
pursuant this invention, PVOH and LCA can be reacted at temperatures as 
low as 20.degree. C. It is, however, preferred for industrial practicality 
that acetalization be conducted at elevated temperature, which broadly is 
between (on the low end) about 40.degree. C. (e.g. 50.degree. C.) to (on 
the high end) just below (i.e. 2.degree.-5.degree. C.) the boiling point 
of water which varies with the pressure naturally developed in the 
water-containing reaction phase. Below about 40.degree. C. the rate of LCA 
reaction with PVOH is usually too slow to be industrially practical. At or 
above the boiling point of the reaction medium containing water in which 
the PVOH is dissolved results in undesirable frothing of the polymerizing 
reactants, though this can be reduced by lowering agitation intensity 
and/or gradually adding butyraldehyde over a finite time period (i.e. 
greater than one minute) as opposed to adding it at once in a single 
charge. At reaction pressures above atmospheric, liquid butyraldehyde may 
be pumped under pressure into the reaction zone. Alternatively, at 
atmospheric reaction pressure the butyraldehyde may be added at a reaction 
temperature of about 73.degree.-75.degree. C. which is just below (i.e. 
within about 5.degree. C.) the atmospheric boiling point of butyraldehyde. 
Under some circumstances, e.g. when using a very reactive LCA, the initial 
phase of acetalization prior to adding butyraldehyde can be conducted 
within the range of about 50.degree. C. to about 150.degree. C., but below 
the particular temperature within such range of butyraldehyde addition. 
For example, the initial LCA-PVOH reaction temperature up to and beyond 
gel break could be at least about 60.degree. C. and then the temperature 
raised to about 73.degree. C. before butyraldehyde addition. For 
convenience of operation and optimal results, the entire reaction is 
preferably conducted between about 75.degree. to about 98.degree. C., most 
preferably at least about 80.degree. C., e.g. 80.degree. to 95.degree. C. 
at atmospheric pressure. 
The high temperature process of the invention for synthesizing mixed 
poly(vinyl acetal)s wherein butyral acetal groups are sequentially formed 
after first forming long chain acetal groups during an acetalization 
reaction at a temperature which preferably is always greater than about 
40.degree. C., provides a facile way to control the size and size 
distribution(preferably narrow) of the polymer particles formed. 
Relatively small particles of narrow size distribution and therefore of 
improved size uniformity are more thoroughly neutralized after the 
reaction is complete and more uniformly plasticized downstream, which in 
turn provides improved control of sheet properties for safety glazing 
applications. Moreover, with the process of the invention reaction 
conversion efficiency (weight of reacted aldehydes divided by weight of 
aldehydes charged to the reaction, times 100) is improved (on the order of 
about 5%) over processes employing a low temperature phase in the reaction 
cycle. This environmentally compatible advantage is commercially important 
in reducing the amount of unconverted aldehyde for disposal or recycle, 
particularly considering the relatively expensive LCA components. 
Any long chain aldehyde is usable which has at least six carbon atoms and 
is capable of reacting with PVOH to form at gel break an emulsion of 
polymer particles containing chemically combined long chain acetal groups. 
Usable LCA's include those which are acyclic, alicyclic or aromatic 
(containing one or more optionally substituted phenyl rings). Such LCA's 
may be substituted (singly or plurally) anywhere along the aldehyde chain 
or unsubstituted, straight or branched chain, saturated or unsaturated. 
Representative usable LCA's include n-hexyladehyde, 2-ethylbutyraldehyde, 
n-heptaldehyde, n-octylaldehyde, n-nonyladehyde, n-decylaldehyde, 
undecylaldehyde, benzaldehyde, cinnamaldehyde, 3-phenyl-1-butyraldehyde, 
undecylenic aldehyde (i.e. H.sub.2 C.dbd.CH(CH.sub.2).sub.8 CHO), phenexal 
##STR1## 
hexyl cynnamic aldehyde 
##STR2## 
LCA's of any of the foregoing having 6 to 20 C. atoms are preferred; most 
preferred are aliphatic aldehydes containing 6 to 20 C. atoms such as 
2-ethylhexaldehyde. The LCA's may be used either singly or in combination 
with one another. 
Any known partially or completely hydrolyzed polyvinyl alcohol (including 
blends of different grades) is suitable as starting PVOH in the 
acetalization. It is preferred to use highly hydrolyzed polyvinyl alcohols 
having 0.5-3% by weight of vinyl acetate units and a viscosity as measured 
in accordance with DIN 53,015 at 20.degree. C. on a 4% strength aqueous 
solution of between 5 and 45 mPa.s, preferably between 15 and 35 mPa.s. 
The acetalization reaction occurs in the presence of catalytically acting 
acids used in customary concentration, for example between 0.1 and 10% by 
weight, preferably between 0.2 and 2% by weight, based on the aqueous PVOH 
phase. Since the acid can coarsen the polymer particles, it is advisable 
to work at sufficiently low concentrations as will enable the reaction to 
be carried out at industrially acceptable rates. Suitable acids and 
mixtures thereof are in particular strong mineral acids such as HCl, 
H.sub.2 SO.sub.4, HNO.sub.3, HClO.sub.4, H.sub.3 PO.sub.4 and the like, as 
well as aromatic and aliphatic sulfonic acids. 
The acetalization reaction is started in customary manner by adding the LCA 
and acid in single successive charges or after first mixing together or by 
adding a little at a time of one or the other or both to the aqueous 
solution of PVOH. Butyraldehyde is later added as a single charge or, 
preferably, continuously during a time interval up to about 90 minutes, 
for example, 5 to 30 minutes. The reaction mixture must be continuously 
stirred, preferably vigorously, throughout the entire acetalization 
process. 
The initial concentration (solids content) of PVOH in aqueous solution is 
usually about 4 to 10 weight % although other concentrations are usable. 
As solids content increases, the emulsion at gel break becomes denser and 
the final particle size distribution improves insofar as becoming narrower 
with the maximum size shifting toward a smaller size. 
At completion of acetalization, the mixed poly(vinyl acetal) is in the form 
of a strongly acidic aqueous suspension with a solids content of between 5 
and 25% by weight. The solids are isolated and converted into a flowable 
product by customary methods, for example, by centrifuging, washing and 
drying including neutralizing or alkalizing before, during or after these 
steps. The thoroughness of washing required depends on the acid content in 
the completed reaction suspension. The mixed poly(vinyl acetal) particles 
vary in size and have an average mean particle diameter of between about 
1000 to 75 microns in which particles passing through a 30 M screen and 
retained on a 100 M screen (i.e. having a particle diameter of 600 to 150 
microns) account for at least 50 weight % of all particles. They can be 
spherical, oval or the like in shape and usually have a smooth surface. 
Conventional property-enhancing additives at customary concentrations (e.g. 
0.05 to 1% by weight based on the precipitating acetalized polymer) may 
optionally be introduced to the reaction medium before or during 
acetalization. These additives include crosslinkers, emulsifiers, 
stabilizers, antioxidants, ultraviolet absorbers, glass adhesion control 
agents and the like. 
The mixed poly(vinyl acetal) products are suitable for any use known for 
this type of polymer. As disclosed in U.S. Pat. No. 5,137,954, they are 
particularly suitable as sheeting in the production of laminated glass. 
For such applications, the mixed poly(vinyl acetal) is melt mixed and 
shaped, usually in a screw extruder in the optional presence of a 
plasticizing amount of external plasticizer, in a manner known to those 
skilled in the art, into a sheet for use with one or more rigid panels 
such as glass in a safety glazing. 
The invention is further described in the following Examples which are not 
intended to limit or restrict the invention. Unless otherwise indicated, 
quantities and percentages are expressed in weight. PVOH means polyvinyl 
alcohol. 
EXAMPLE 1 
Preparation of poly(vinyl 2-ethyl hexyl acetal)-poly(vinyl butyral) by 
reacting PVOH with 2- ethyl hexaldehyde and n-butyraldehyde. 
PVOH having a residual polyvinyl acetal content of less than 2% and 
molecular weight of 77,000 is dissolved in deionized water at 
85.degree.-90.degree. C. in an agitated fluted glass reactor to obtain 
7994 g of 8.7% solution. The temperatures noted in this and the other 
Examples are achieved by varying the temperature of a water bath in which 
the reactor is immersed. When temperature is given, both the temperature 
of the bath and the reaction mixture are measured with a thermometer. 
269.3 g of 35% nitric acid catalyst and 227.7 g of 2-ethyl hexanal is 
added to the PVOH solution at 85.degree.-95.degree. C. After reacting for 
5-7 minutes the initially clear solution becomes a creamy white emulsion. 
This is gel break. The reaction is continued for one hour after gel break. 
While the reaction mixture is at 85.degree.-90.degree. C., 280.5 gm of 
n-butyraldehyde is added over about one hour and the reaction is continued 
for about four more hours at 85.degree.-90.degree. C. Total reaction cycle 
time (after PVOH solution reaches 85.degree.-90.degree. C.) is about 6.5 
hours. The reactor contents are washed to pH 4.0, neutralized with 45% 
potassium hydroxide to pH 10.0, held at this pH for one hour at 75.degree. 
C. and then washed with water to pH 7.0. The product is then filtered and 
dried to less than 2% moisture. The composition (weight fraction) has the 
following analysis: 
______________________________________ 
vinyl acetate 0.020 
vinyl alcohol 0.181 
vinyl butyral 0.499* 
vinyl 2-ethylhexyl acetal 
0.300* 
______________________________________ 
*estimated, by mass balance using the efficiency noted below. 
Particle size analysis (% retained on noted screen size) is as follows: 
______________________________________ 
Mesh size 
% 
______________________________________ 
10 6.47 
16 10.72 
20 6.69 
30 6.66 
40 8.29 
50 45.55 
100 13.74 
140 0.15 
200 0.32 
______________________________________ 
Screen analysis uses U.S. Standard Testing Sieve ASTME 11 specification 
wherein 30M has 600 microns openings and 100M has 150 microns openings. 
Combined aldehyde efficiency is 95%. 
EXAMPLES 2, 3 
The procedure of Example 1 through completion of the acetalization reaction 
is repeated except in one instance, (Ex. 2) 2-ethyl hexanal is replaced 
with phenexal, a branched aldehyde of 12 C atoms containing a substituted 
phenyl ring and in a second instance (Ex. 3) it is replaced with decanal, 
a saturated aldehyde of 10 C atoms. Reaction specifics are as follows: 
Using phenexal 
3302 g of 8.0% PVOH solution 
68 g HNO.sub.3 
95 g phenexal 
110 g butyraldehyde 
gel break--at two minutes; held for 30 minutes; add butyraldehyde over 40 
minutes; continue reaction for 2.5 hours after all butyraldehyde added. 
Using decanal 
3302 gm 8.0% PVOH solution 
68 gm HNO.sub.3 
90 gm decanal 
107.5 gm butyraldehyde 
gel break--at five minutes; hold for 30 minutes; add butyraldehyde over 43 
minutes; continue reaction for 2.5 hours after all butyraldehyde added. 
The dried product particles are visually similar to those of Example 1 with 
Example 2 particles appearing somewhat larger than Example 3. Particle 
size distribution and aldehyde efficiency, if measured, should be similar 
to the results of Example 1. 
Other LCA's can be interchangeably substituted for those used in the 
procedure of Examples 1-3 to form mixed poly(vinyl acetal) polymers having 
relatively narrow particle size distribution and high aldehyde to polymer 
reaction efficiency at commercially reasonable cycle times. 
EXAMPLE 4 
This explores a different reaction temperature schedule according to the 
invention. 
The procedure of Example 1 is repeated except 2-ethyl hexanal is added to 
the PVOH solution at 40.degree. C. Time elapsed between addition and gel 
break is 35 minutes. After gel break, the reaction temperature is 
increased to 85.degree. C. and the remaining procedure of Example 1 
followed. Reaction specifics are as follows: 
3302 g of 8% PVOH 
102 g of 35% HNO.sub.3 
86.2 g of 2-ethyl hexanal 
After gel break the reaction is continued for 35 minutes without 
butyraldehyde at 85.degree. C. Add butyraldehyde over 30 minutes and 
continue reaction after butyraldehyde addition is complete for 21/2 hours. 
The product particles are visually observed to be comparable to those of 
Example 1. Though not measured, aldehyde efficiency and particle size 
distribution are predicted to be comparable to Example 1. 
EXAMPLE C1 
This control example conducts the acetalization reaction not according to 
the invention--i.e. with a significant low temperature period during the 
reaction cycle during which butyraldehyde and LCA simultaneously react 
with PVOH. 
The procedure of Example 1 is repeated except as follows. The temperature 
of the initial PVOH solution is decreased from 85.degree.-90.degree. C. to 
16.degree. C. Then the following is added as a single charge: 
81.6 g HNO.sub.3 
251.6 g n-butyraldehyde 
219.7 g 2-ethyl hexanal 
The mixture is allowed to react at 16.degree. C. with gel break occurring 
in 25 minutes. 97.9 g of 35% HNO.sub.3 are added, the temperature 
increased to 85.degree. C. and reaction continued for 4 hours. Total 
reaction time (after PVOH solution reaches 85.degree.-90.degree. C.) is 
7.5 hours which includes 1.5 hours to lower the temperature to 16.degree. 
C. The polymer composition (weight fraction) has the following analysis: 
______________________________________ 
vinyl acetate 0.020 
vinyl alcohol 0.191 
vinyl butyral 0.489* 
vinyl 2-ethyl hexyl acetal 
0.300* 
______________________________________ 
*estimated, by mass balance using the efficiency noted below. 
Particle size analysis is as follows: 
______________________________________ 
Mesh Size 
% 
______________________________________ 
10 14.25 
16 32.03 
20 26.69 
30 22.44 
40 4.23 
50 0.02 
100 0.00 
140 0.00 
200 0.00 
______________________________________ 
The combined aldehyde efficiency is 90% 
The above product data of this Example C1 is to be contrasted with Example 
1. Reaction efficiency is lower by 5%, particle size is shifted to larger 
size and particle size distribution is broader. Total reaction cycle time 
is 15% longer. 
Contrary to the traditional understandings and expectations of the prior 
art, in forming mixed poly(vinyl acetal)s the process of the invention can 
eliminate the low temperature phase of the acetalization process thereby 
conserving energy otherwise consumed in cooling the PVOH solution from an 
initial dissolution temperature of about 80.degree.-90.degree. C. to about 
10.degree.-20.degree. C. followed, after polymer precipitation, by heating 
again to about 60.degree.-80.degree. C. to complete the reaction. 
Eliminating such a low temperature phase reduces polymerization time 
leading to increased capacity from existing and new facilities. Such 
energy savings are considerable and reduced cycle times important in 
commercial size manufacturing systems. 
The preceding description is for illustration and should not to be taken as 
limiting. Various modifications and alterations will be readily suggested 
to persons skilled in the art. It is intended, therefore, that the 
foregoing be considered as exemplary only and that the scope of the 
invention be ascertained from the following claims.