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HIGH MOLECULAR WEIGHT THERMOSETTING PHENOL-
FORMALDEHYDE OLIGOMER: STUDY OF CURING CONDITIONS
Tokhirov M.I.
Alimukhamedov M.G.
Tashkent Institute of Chemical Technology, Uzbekistan
https://doi.org/10.5281/zenodo.14915374
Abstract.
This study investigates the synthesis of high molecular weight
thermosetting phenol-formaldehyde oligomers and the conditions influencing
their gelation and gel fraction yield. The results confirm the suitability of these
oligomers for processing at 180–200°C and their potential applications in
producing high-quality polymer products.
Keywords:
high molecular weight oligomer, phenol-formaldehyde resins,
thermosetting properties, gelation time, gel fraction, processing temperature,
copolycondensation, co-oligomer.
Introduction.
Traditional phenol-formaldehyde resins are characterized by
various properties that are thermoplastic or thermosetting. They can initially be
liquid or solid. Based on them, mixing with different polymers opens wide
opportunities for the production of a material that combines the advantages of
several polymers [1, 2, 3].
Research on the creation of this type of thermosetting oligomers usually
includes the following most necessary steps: synthesis of monomers, synthesis
of oligomers based on them, study of their physico-chemical properties and
structure, study of the effect of synthesis conditions on their technological
properties [4,5].
Phenol-formaldehyde oligomers provide durable and high-quality coatings
used in various industries [6, 7]. Phenol-formaldehyde oligomers provide long-
term protection against corrosion, scratches and cracks when used as a coating.
Heat treated phenol-formaldehyde resin coatings have maximum chemical
resistance even in highly acidic environments [8, 9]. Such materials are used as
coatings for chimneys, pumps, pipes and tanks [10]. Therefore, one of the main
technological properties of such composite materials is viscosity strength and
thermosetting stability, which significantly depends on the degree of
solidification (gel fraction).
Results and analysis.
To do this, we synthesized our monomers
phenolalcohol based on phenol and formaldehyde, diethyleneglycoladipinatol
based on adipic acid and diethyleneglycol. The copolycondensation process of
our two synthesized monomers, diethyleneglycoladipinitol and phenolalcohol,
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in a ratio of 1:40 mol/mol, was carried out in the presence of a catalyst (30%
solution of maleic anhydride) under conditions (pH=3). As a result, a high
molecular weight thermosetting phenol-formaldehyde oligomer was
synthesized.
We studied the technological properties of the synthesized high molecular
weight thermosetting phenol-formaldehyde oligomer. The main focus is on
studying the conditions for the transition (solidification) of oligomers to the
fixed state. As a result of studying these conditions, the ability to obtain press
compositions will improve and labor productivity will increase.
The gelation times of the synthesized (PhA:DEGA 40:1 mol/mol) high
molecular weight thermosetting phenol-formaldehyde co-oligomer were
performed according to GOST R 57779-2017 (ISO 8987:2005).
The gelation times of the synthesized co-oligomer at temperatures of
130⁰C, 150⁰C, 180⁰C, and 200⁰C were studied. The temperature dependence
diagram of the gelation time of a high molecular weight thermosetting phenol-
formaldehyde co-oligomer is presented in Figure 1.
The gelation time decreases with increasing synthesis time. At 130°C, the
gelation time is 1496 seconds 20 minutes after the start of synthesis, while after
90 minutes this indicator is reduced to 420 seconds. When the temperature was
raised to 200°C, the gelation times were reduced to 156, 108, and 68 seconds.
0
200
400
600
800
1000
1200
1400
1600
130ºC
150ºC
180ºC
200ºC
1496
558
210
156
889
375
125
108
420
178
83
68
G
ell
in
g tim
e,
se
c
Temperature, ºC
1-Figure. Susceptibility to gellanization times of high molecular mass
thermoresponsive phenol-formaldehyde co-oligomer at FS:DEGA
ratio of 40:1 mol/mol
1-synthesis duration 20 minutes; 2-synthesis duration 60 minutes;
3-synthesis duration 90 minutes;
1
1
1
1
2
2
2
2
3
3
3
3
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Changing the synthesis conditions improved the thermoresponsive properties of
the oligomer.
The diagram above shows that we observed that the gelation time
increased with increasing synthesis time for our high molecular weight
thermosetting phenol-formaldehyde co-oligomers. At the same time, we can see
that the high molecular weight thermosetting phenol-formaldehyde co-
oligomers we synthesized have a longer lifetime than traditional phenol-
formaldehyde oligomers. This is because simple thermosetting phenol-
formaldehyde oligomers in solid form (not in solution) partially gel after 20-30
days and become unusable [1].
While the gelatinization time of the thermosetting oligomers discussed
above determines the time they remain in a liquid state at a given temperature,
the degree of cross-linking is determined by the yield of the gel fraction during
extraction. This technological property of thermosetting oligomers determines
the processing time of a particular oligomer or a composition based on it under
certain temperature conditions.
We also studied the gel fraction yield of a high molecular weight
thermosetting phenol-formaldehyde co-oligomer synthesized at a 1:40 mol/mol
ratio of DEGA:PhA. We conducted this study in accordance with GOST R 59112-
2020 (ISO 10147:2011). The gel fraction was obtained by annealing the studied
oligomer at temperatures of 150⁰C, 180⁰C, and 200⁰C for 10 min., 30 min., 1 h., 5
h., and 10 h. The solidified oligomers were placed in filter paper bags and
extracted in a Soxhlet apparatus. A mixture of ethyl alcohol-dimethylformamide
in a 50:50 volume ratio was chosen as the extractant. The extracted products
0
20
40
60
80
100
0
2
4
6
8
10
G
el
f
rac
ti
on,
%
Freezing time, hours
2-Figure. Graph showing the dependence of gel fraction
yield on processing temperature and time for a high-
molecular-weight thermoreactive phenol-formaldehyde
co-oligomer
1- 150⁰C
2- 180⁰C
3- 200⁰C
2
1
3
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were dried and the degree of solidification (gel fraction) was calculated based on
the calculations. The result is presented in Figure 2 as a graph of the gel fraction
yield versus processing temperature and time.
Figure 2 describes the gel fraction yield of a high molecular weight
thermosetting phenol-formaldehyde co-oligomer synthesized at a DEGA:PhA
ratio of 1:40 mol/mol. As can be seen from the data in Figure 2, in curve 1, when
the sol-oligomer was treated at 150⁰C for 10 minutes, the gel fraction yield was
3%, and when the curing time was increased to 30 minutes, the gel fraction yield
increased sharply to 63%. Then, when the curing time was increased to 1, 5, and
10 hours, the gel fraction yield slowed to 78-86-90%, respectively.
Increasing the processing temperature to 180⁰C (curve 2) and 200⁰C (curve
3) resulted in a slight increase in the gel fraction yield from 4.5% to 6.4% at 10
minutes. Increasing the processing time to 30 minutes resulted in a sharp
increase in the yield of the gel fraction to 77-83%. Thus, increasing the
processing temperature and time leads to a gradual increase in the yield of the
gel fraction. When the processing time was increased to 10 hours, the gel
fraction yield was 93% at 180⁰C (curve 2) and 95.5% at 200⁰C (curve 3).
Usually, in the production of various types of plastics based on
thermosetting oligomers, depending on the type of binders and the
thermosetting oligomer, it is enough to achieve 70-80% productivity of gel
fractions. The high molecular weight thermosetting phenol-formaldehyde co-
oligomer we have achieved as a result of our scientific research allows for a gel
fraction yield of >98% at processing temperatures, which ensures high
production technological tests (thermal-physical, physical-mechanical, dielectric
properties) of the resulting plastic products.
Therefore, all synthesized high molecular weight thermosetting phenol-
formaldehyde co-oligomer products can be processed at temperatures of 180-
200⁰C under technologically acceptable conditions without reducing the
productivity of processing equipment.
The conclusion.
In conclusion, this study investigated the technological
properties of high molecular weight thermosetting phenol-formaldehyde co-
oligomers. It was found that the synthesized co-oligomers are more stable and
have longer-term use compared to traditional phenol-formaldehyde resins. The
effect of temperature and time on gelation time and gel fraction yield was
studied. The results show that optimal processing conditions are provided at a
temperature of 180–200°C, at which the gel fraction yield exceeds 98%.
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