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The heterogeneous formalization of poly(vinyl alcohol) accompanying crosslinking reaction Ц I.

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Die Angewandte Makromolekulnre Chemie 24 (1972) 89-93 ( N r . 331)
From the Department of Textile Industrial Chemistry, Faculty of Textile Science
and Technology, Shinshu University, Ueda, Nagano, Japan
The Heterogeneous Formalization
of Poly(viny1 alcohol) accompanying
Crosslinking Reaction - I
By KENJIOGASAWARA,
NORIO
NAKAMURA,
and SHUJIMATUZAWA
(Eingegangen am 6. Oktober 1971)
SUMMARY:
The effect of the solvents of poly(viny1alcohol) (PVA) species on the crosslinking
reaction accompanying in the heterogeneous formalization of PVA was studied. The
crosslinking reaction was prevented when PVA film was allowed to react with formaldehyde in the solution containing methanol, ethanol or propanol, whereas it occurred in the solution containing acetone. The crosslinkingreaction occurred more easily
in syndiotacticity-rich PVA than in atactic PVA (commercial PVA).
ZUSAMMENFASSUNG :
Der EinfluD des Losungsmittels und der Art des Polyvinylalkohols (PVA) auf die
durch heterogene Formalisierung entstehende Vernetzung des PVA wurde untersucht. Die Vernetzungsreaktion wird zuruckgehalten, wenn man PVA-Film in
methanol-, athanol- oder propanolhaltigen Losungen formalisiert, wahrend es in
acetonhaltiger Losung zur Vernetzung kommt. Die Vernetzungsreaktion erfolgt
leichter in starker syndiotaktischem PVA als in ataktischem PVA (handelsublichem
PVA).
1. Introduction
The heterogeneous formalization of poly(viny1 alcohol) (PVA) has been carried out by steeping it in the aqueous solution containing formaldehyde, hydrochloric acid as a catalyst and some salts (sodium chloride or sodium sulfate) as
coagulating reagents. I n this reaction the formalization reaction has been considered for a long time t o occur mainly between the two intramolecular neighbouring hydroxyl groupsl.
The authors and KAWASE
e t a1.2 have reported that the reaction of PVA with
formaldehyde occurs not only in the intramolecular hydroxyl groups b u t also in
the intermolecular ones and the probability of the occurrence of the latter reaction is mainly affected by the formaldehyde concentration.
K. OGASAWARA,
N. NAKAMURA,
and S. MATUZAWA
The authors have found t h a t PVA reacts with formaldehyde in the aqueous
solutions containing methanol, ethanol, propanol or acetone, formaldehyde and
an acid catalyst and the crosslinking reaction is prevented when PVA is allowed
t o react in the aqueous solution containing alcohol, whereas not in the aqueous
solution containing acetone. Moreover we have found t h a t the crosslinking reaction occurs more easily in syndiotacticity-rich PVA film than in atactic PVA
(commercial PVA) film.
I n this paper, we wish t o report about these experimental results.
2. Experimental
2.1 Samples
2.1.1 A t a c t i c P V A ( A - P V A )
Commercial PVA (Kurare Poval 1 2 0 P = 2000) and its fractions were used
after the perfect saponification. The fractionation was carried out by the nonsolvent addition method using water-n-propanol system a t 30 "C, starting from the
solution of 1yopolymer concentration. The PVA was divided into 12 parts. The degree of polymerization was determined by viscometry using the equation3 [ ? I ] =
7.5 . 10-3 . ~ 0 . 6 4 .
2.1.2 S y n d i o t a c t i c i t y - r i c h P V A ( S - P V A )
Vinyl trifluoroacetate (VTFA) was prepared by the addition of trifluoroacetic
acid to acetylene4. The VTFA which had a boiling point of 38.4 to 39.4OC was polymerized in bulk a t 60°C with 0.7 yo benzoyl peroxide. The catalyzed monomer was
sealed off under nitrogen and allowed to polymerize for 24 hrs. to almost complete
conversion. The polymer was purified by several reprecipitations from acetone into
petroleum benzine and vacuum dried a t 50 "C for 24 hrs.
Poly(viny1 trifluoroacetato)(PVTFA) was converted to PVA by dissolving in diethylenetriamine and acetylated again in apyridine-acetic anhydride mixture and the
degree of polymerization was determined by measuring the solution viscosity of
the acetylated sample using benzene as a solvent a t 30
0.1 "C and the equation5
= 5.63 . 10-4 . ~ 0 . 6 2 .
The fractionation of acetylated S-PVA was carried out with water-acetone system
a t 30 "C, starting from the solution wiLh a polymer concentration of 1yo.The acetylated S-PVA was saponified after the fractionation.
+
2.1.3 P r e p a r a t i o n of f i l m s
PVA films of A-PVA of about 0.1 mm in thickness were prepared by casting,
which were exposed to thermal treatment at 50°C for 24 hrs. (A-PVA) and 120°C
for 20 min (S-PVA).
2.2 Degree of crystallization
The density of the PVA films was determined with use of a floating method (benzene-carbontetrachloride) at 30 "C and the degree of crystallization was calculated
from the following equation :
Heterogeneous Formalization of Poly(viny1 alcohol)
1
1
- _-d
dcr
+-(1-X)
dam
where d is the density of film, d,, and dam,the densities of crystalline region and
amorphous region, respectively.
2.3 Tacticity
The ratio of optical density a t 916 cm-l to that a t 850cm-l, Dgl6/D850, was estimated from the infrared spectrum of PVA film of 0.01 to 0.03 mm in thickness. The
infrared spectrum was taken using a Nippon Bunko DS-301 IR-spectrometer. The
content of syndiotactic diads (S-(diad)%)was calculated using the following equation6:
S-(diad)% = (72.4
1.09) . (D916/D850)0.43* 0.0°6.
2.5 Formalization
The PVA films were formalized in a mixture of formaldehyde, hydrochloric acid
as a catalyst, water and a poor solvent for PVA (methanol, ethanol, propanol and
acetone) or in a mixture of water, formaldehyde, sodium chloride and hydrochloric
acid a t 50°C.
Formaldehyde used was a 37% solution of guaranteed reagent chemical. Hydrochloric acid used was a 35% solution of reagent grade chemical.
After the given time of reaction, the formalized film was drawn off the reaction
vessel, washed with distilled water repeatedly. The formaldehyde content was determined by the colorimetric method using the chromotropic acid7. The intensity of
the color of the solution, expressed as the optical density, was measured a t 570 mp
by means of Hitachi Perkin-Elmer 139 UV-VIS spectrophotometer.
2.6 Estimation of the crosslink content
I n the study about the effect of the species of PVA on crosslinking reaction, the
degree of swelling of formalized PVA was measured in the ethylenediamine a t 30°C,
because syndiotacticity-rich PVA was crosslinked during acetylation in the pyridineacetic anhydride mixture.
On the other hand, in the study about the effect of the various solvents on the
crosslinking reaction, formalized PVA was acetylated a t 120 "C for 4 hrs. in a pyridine-acetic anhydryde mixture8 and the degree of swelling of the acetylated sample
was measured in chloroform a t 30°C.
The degree of swelling was calculated using the following equation :
DS = (W-W,)/W,
where W is the weight of the swollen film and W,, the weight of the swollen and
dried film.
3. Results and Discussion
3.1. The eflect of the solvent on the crosslinking reaction
Table 1 shows the degrees of swelling of samples formalized in several kinds
o f aqueous solutions and acetylated in a pyridine-acetic anhydride mixture.
91
K. OGASAWARA,
N. NAKAMURA,
and S. MATUZAWA
Table 1. Results of the formalization in several solutions.
HC1: 5% reaction temperature: 50°C.
Formalization conditions
CHz0
(70)
1
5
10
20
Reaction
Medium
time
(hr)
k/g)
CH30H/Hz0
CHaCOCH3/H20
NaCljHzO
CH30H/Hz0
CzH50H/Hz0
C3H70H/Hz0
CH&OCH3/HzO
NaCl/HZO
83.6/10.4
10.0/84.5
7 1.9/18.1
10.0/80.0
CH30H/H20
59.2/25.8
CH~COCH~/HZO
10.0/75.0
NaCl/H20
CH30H/HzO
33.0/42.0
CH~COCH~/HZO
10.0/65.0
NaCl/HZO
48
0.5
0.17
48
24
5
0.17
0.17
0.5
24
0.5
1
5
4.2
0.5
Degree of
formalizatjon
(mole-yo)
31.2
43.5
32.7
56.0
61.2
73.9
63.0
48.5
66.7
75.0
70.6
72.7
76.2
70.7
71.8
Degree of
swelling
*
43.0
37.5
*
*
*
83.9
9.6
8.6
*
8.21
5.4
28.8
10.1
7.9
* Soluble in the acetylating solution.
These results suggest that the crosslinking reaction is prevented in the solution
containing alcohol, and not in the aqueous solution containing acetone or sodium chloride.
One of the reasons for the occuring of crosslinking reactions is supposed to be
the distance between neigbouring molecules. The crosslinking reaction accompanying the formalization in alcoholic solutions is prevented if the degree of
swelling of PVA film in the alcoholic solution a t the time of formalization is larger than those of the solutions containing acetone or sodium chloride under
the same formaldehyde and acid contents. As is evident fiom table 2, however,
the degree of swelling of PVA films in the alcoholic solutions is smaller than that
of the solution containing acetone or sodium chloride. The effect of the distance
between the two neighbouring intermolecular hydroxyl groups on the crosslinking reaction cannot be recognized.
The other factors preventing the crosslinkingreaction are supposed to be chemical factors. The chemical structure and physical properties of formalized cellulose have been studied for a long time. R O F F ~has pointed out that intermolecular crosslinking was present in formaldehyde treated cellulose. G U T H R I E
has
~~
assumed that short crosslinks, possibly monomeric, were produced by means of
dilute formaldehyde solutions and longer crosslinks were introduced by more
92
Heterogeneous Formalization of Poly(viny1 alcoh,ol)
Table 2. The extent of swelling of PVA films in several solutions containing hydrochloric acid of 5% at 50°C for 1 hr.
I
Svstem
Coagulating reagent(%)
-
NaCl
CH3COCH3
CH30H
CzH50H
C3H70H
~
1
Water(%)
Extent of swelling
(ml/g PVA)*
I
10
85
4.4
75
20
20
2.1
1.8
20
1.8
20
1.9
75
75
75
~~~~~~
* Volume of absorbed solution.
concentrated solutions by measuring the wet crease recovery angle of the formaldehyde treated cotton cloth.
METTA et al.11 have suggested that the dimeric crosslink might be formed
from cellulose hemiformal according to the following mechanism :
Cell-OCHZOH
+
Cell-OCH2
H+
Cell-OCH20H2
+ -HzO
+
Cell-06H2
+ HO-CH2-O-Cell - Cell-OCH~-O-CH2-O-Cell
+ H+ .
PATEL
et a1.12 have also suggested the presence of the dimeric crosslinks from
the study ofthe chemical structure ofthe methylated celluloseformals.GuTHRIEI3
has estimated directly the length of the oxymethylane bridges introduced into
cotton cellulose by formaldehyde, measuring the lost of weight before and after
the hydrolysis of the formalized polymer and concluded that the crosslinks are
mainly polymeric and the length of the crosslinks is effected by the reaction
conditions.
Thus the crosslinks in the formalized PVA seem to be polymeric. Methanol is
considered to prevent the formation of polymeric formaldehyde, which is considered as trimeric, or the formation of dimeric crosslinks because of the formation of methylal. Theincrease in the content of crosslinks with increasing formaldehyde concentration can be attributed to increasing concentration of polymeric formaldehyde.
KAWASEet al. 2, on the contrary, have recently reported that the content of
crosslinks depends only on the rate of formalization regardless of polymeric formaldehyde, i. e. the content of crosslinks increases with increasing rate offormalization, because the probability of intermolecular reaction of formaldehyde increases with it. No crosslinks, however, could be found for the PVA formalized in
the alcohol solution containing 10 yo of formaldehyde, 5 yo of hydrochloric acid
and 59.4 yoof water, whereas it could be for that formalized in aqueous solution
93
K . OGASAWARA,
N. NAKAMURA,
and S. MATUZAWA
containing 1 yo of formaldehyde, 5 yo of hydrochloric acid and 10 yo of sodium
chloride, though the rate of formalization for the two systems was the same. The
dependence of the degree of swelling of the formalized PVA in ethylenediamine
on the reaction time is shown in Fig. 1. The dependences of the degree of swelling during formalization on the reaction time or the degree of formalization are
shown in Fig. 2 and 3.
I
-0
20 40 60 80
100 120
Reaction time ( min 1
Fig. 1. Relation between the degree of swelling of formalized PVA film in ethylenediamine a t 30 "C and reaction time for formalization. The numbers in
brackets represent the degree of formalization (mole-yo).The formalization condit,ions are: CHzO lye, HC15°/o, NaCl lOyoand H2O 84% a t 50°C.
0
20 40 60
80
100 120
Reaction time (mln)
Fig. 2 .
94
Relation between the degree of swelling of the formalized PVA film in the
solution for formalization and reaction time. The degree of swelling is represented in cc. of absorbed solution/g.PVA. The formalization conditions
of these samples are: 0 ;
CHzO 176, HC1 5y0, NaCl loyo,and HzO 84%
a t 50°C.
0 ; CHzO lo%, HC1 524, CH30H 59.2% and H2O
25.8% a t 50°C.
Heterogeneous Formalization of Poly(viny1 alcohol)
0
20 40 60 80 100
Degree of formalizationhole%)
Fig. 3.
Relation between the degree of formalization and the degree of swelling.
The samples shown in Fig. 2 were used.
The states of PVA during formalization are not different from eachother. This
suggests that the polymeric formaldehyde decreases in alcoholic solution to prevent the crosslinks. The crosslinks with polymeric formaldehyde seem to be important. Now it is not determined experimentally whether the crosslinks are polymeric or not, so a detailed study will be necessary.
We wish to discuss t a mechanism of the formalization reaction of PVA with
formaldehyde in the presence of the low aliphatic alcohols:
1. Activated formaldehyde reacts with hydroxyl groups of PVA directly.
2 . Hemiacetal which is produced by the reaction of formaldehyde with alcohol
is decomposed and the liberated formaldehyde is activated and reacts with
the hydroxyl groups of PVA. Therefore the rate of formalization in alcoholic
solutionis smaller than that in the aqueoussolution containing sodium chloride.
3. The hemiacetal produced reacts with hydroxyl groups of PVA directly and
produces alcoxyl groups. The alcoxyl groups may prevent th-, crosslinking
reaction due to steric hindrance. The mzbhoxyl groups, however, could not
be detected.
3.2 Eeect of P V A species on the crosslinking reaction
It has recently become possible to synthesize PVA of various degrees of steet al.14 have reported that the rate of formalization
reoregularity. SHIBATANI
reaction of PVA with formaldehyde is affected by the tacticity of PVA. The
crosslinking reaction is also supposed t o be affected by the tacticity of PVA.
95
K . OGASAWARA,
N. NAKAMURA,
and S. MATUZAWA
I n this study, fractionated A-PVA (syndiotactic diad content was 47 yo)and
fractionated S-PVA (syndiotactic diad content was 56Y0) were formalized in
solution containing sodium chloride. The degree of crystallization of the two
kinds of PVA was about 30 yo.
0
20
40
60
80
100
Degree of formalization (mole %)
Fig. 4. Relation between the degree of swelling and the degree of formalization.
Reaction conditions for formalization are: CH2O 5y0,HCI lyo,NaCl 10%
and H2O 84% at 40°C.
Fig. 4 shows the relation between the degree of swelling in the ethylenediamine
a t 30°C and the degree of formalization. The crosslink content of formalized
S-PVA is smaller than that of A-PVA within the degree of formalization of
about 30 mole-%, but this relation is inverse beyond the degree of formalization
of about 30 mole-%.
Table 3 shows the degree of swelling of PVA films in water. The degree of
swelling of S-PVA in water is smaller than that of A-PVA. This suggests that
the solid state structure of S-PVA is more compact than that of A-PVA because
of the strong hydrogen bonding between the hydroxyl groups.
The reason why the crosslinkingreaction easily occurs beyond the degree of formalization of about 30 mole- yofor A-PYA is that the formalized PVA becomes
hydrophobic and the degree of swelling of formalized PVA decreases with increasing degree of formalizationl5.
From these experimental points of view, the principal factor controlling the
crosslinking reaction seems to be the degree of swelling a t the time of formalization.
We wish to discuss about the inversion of the degree of swelling in Fig. 4. As is
shown in Fig. 5, the degree of swelling of the sample formalized under the same
96
Heterogeneous Formalization of Poly(viny1 alcohol)
'Table 3. Degree of swelling of atactic and syndiotacticity-richPVA films in water.
I
I
PVA
Atactic
Syndiotacticityrich
DP
2030
2010
I
Degree of' swelling
I
1
I
40°C
5.2
1.2
60°C
Solubility (yo)
20
1.7
30°C
24
0
I
60°C
63
0
reaction conditions decreased with increasing degree of polymerization of PVA.
The degree of swelling of formalized S-PVA was larger than that of A-PVA, because the degree of polymerization of S-PVA (P = 720) used in this experiment
is smaller than that of A-PVA (P = 880). If the degrees of polymerization are
equal, the degree of swelling of S-PVA may be smaller than that of A-PVA.
'I!able 4 shows the results of S-PVA formalized under the various formaldehyde
concentrations a t 60°C and 100°C. The crosslink content increased with increaet al.2. They have
P .Z 15'
u)
Y-
0
\
o P=660
o P = I425
to -
Q)
2
m 5 -
0"
0
0
20
40
60
80
100
Degree o f formalization (mole %)
Fig. 5. Effect of formalization of atactic PVA on the degree of swelling. The conditions for formalization are: CH2O 5y0, HC1 lye, NaCl 10% and H20 84%
at 60°C.
described that the rate of formalization was important for the crosslinking
reaction. I n this case, however, the degree of swelling of PVA film is more important. As is evident from Fig. 6, the reaction rate of heterogeneous formalization of
A-PVA a t the initial stage is faster than that of S-PVA similar to the results of
the homogeneous formalization of PVA14.
97
K. OGASAWARA,
N. NAKAMURA,
and S. MATUZAWA
0
2
0
4
A-PVA
S-PVA
6
12
Reaction time (hr 1
Fig. 6.
Effect of tacticity of PVA on the reaction with formaldehyde at CH20 5y0,
HC1 lye, and NaCl 10% a t 60°C.
Table 4. Effect of formaldehyde concentration on the crosslinking reaction of syndiotacticity-rich PVA.
T
~
rature
60
Reaction
conditions*
~
~
~
-
1
4.5
5
5
10
100
0.5
0.5
0.5
5
1
Degree of
formalization
(mole-yo)
Degree of swelling
0.3
50.1
59.3
0.17
0.67
0.08
48.1
65.2
48.8
10.3
5.6
6.4
0.5
2
13
0.17
50.4
61.2
62.6
66.1
16.0
11.2
11.7
3.8
*NaCI: 10%.
1
2
98
I. SAKURADA
et al., Kobunshi Tenbo 6 (1952) 58.
S. MATUZAWA,T. IMOTO,
and M. OKAZAKI,
Kobunshi KtLgaku 25 (1968) 25.
S. MATUZAWA,
T. IMOTO,
and K. OGASAWARA,
ibid. 25 (1968) 173.
H. KAWASE
et al., J. Chem. SOC.Japan Ind. Chem. Sect. 74 (1971) 1014, 1218,
1223, 1228.
Heterogeneous Formalization of Poly(viny1 alcohol)
3
4
5
6
7
A. NAKAJIMA
and K. FURUDATE,
Kobunshi Kagaku 6 (1949) 460.
H. C. HAAS,E. S. EMERSON,
and N. W. SCHULER,
J. Polym. Sci. 22 (1956) 291.
T. ITO, K. NOMA,and I. SAKURADA,
Kobunshi Kagaku 16 (1959) 115.
Y.Go, S. MATUZAWA,
and K. NAKAMURA,
ibid. 25 (1968) 55.
A. NAKAJIMA,
Kobunshi Kagaku 11 (1954) 42.
S. MURAHASHI,
S. NOZAKURA,
M. SUMI,H. PUKI, and K. HATADA,
Kobunshi
Kagaku 23 (1966) 605.
K. OGASAWARA,
N. NAKAMURA,
and S. MATUZAWA,Makromol. Chem. 149 (1971)
291.
S. MATUZAWA,
T. IMOTO,
and M. OKAZAKI,
Kobunshi Kagaku 25 (1968) 25.
W. J. ROFF,J. Textile Inst. 54 (1963) 281.
J. D. GUTHRIE,Textile Res. J. 33 (1963) 955.
1 1 H. U. MEHTA,
N. T. BADDI,R. S. PARMAR,
K. A. PATEL,and R. S. PATEL,Textile Res. J. 39 (1969) 387.
l'z S. PATEL,
J. RIVLIN,T. SAMUELSON,
0. A. STAMM,
and H. ZOLLINGER, Textile
Res. J. 38 (1968) 226.
l:) J. D. GUTHRIE, Textile Res. J. 37 (1967) 40.
l 4 K. SHIBATANI,
K. FUJII,Y. ~ Y A N A G
J.IUKIDA,
,
and M. MATSUMOTO,
J. Polym.
Sci. C 23 (1968) 647.
B:l
N. TAKAHASHI
and K. ONOZATO,
J. Chem. SOC.Japan, Ind. Chem. Sect. 63 (1960)
1540.
8
99
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