formaldehyde-scavenging nanoparticles for high performance ... · urea-formaldehyde resin is the...
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Formaldehyde-Scavenging Nanoparticles for High Performance Resins
Inês Ponce Dentinho Chemical Engineering Department, Instituto Superior Técnico, Lisbon, Portugal
ARTICLE INFO ABSTRACT
Date: November 2017
Mesoporous silica nanoparticles, MSNs, have been developed in order to carry a strong acid, used as a
catalyst, into the curing step of urea formaldehyde (UF) resin synthesis. This synthesis has the huge challenge of
reduction of formaldehyde emissions, a carcinogenic agent, during resin curing. Since it was proved that using a
strong acid as a catalyst reduces formaldehyde emissions, if encapsulated until the hot pressing/curing of the resin
in order to avoid pre-curing, mesoporous silica nanoparticles with thermo-responsive behaviour seem the most
promising solution.
Mesoporous silica nanoparticles, MSN, were used as reservoirs to different acids and, subsequently
encapsulated with polymer. MSNs filled with p-toluenesulfonic acid in their pores, functionalized with
trimethoxypropylsilane and coated with PLURONIC polymer obtained the best acid release results when heated up.
Their size and morphology were analysed by Transmission electron microscopy (TEM), their stability in water was
observed through Dynamic Light Scattering (DLS) and the amount of trimethoxypropylsilane grafted on the surface
was calculated by 1H NMR. The particles were then tested in UF resin synthesis. Different amounts of particles with
different acid loadings were tested. Non-washed-MSNs at 7,6% on UF resin cured the resin with success.
Keywords: Urea formaldehyde resins, Hybrid mesoporous silica nanoparticles Formaldehyde emissions.
1. Introduction
Wood particleboards production involves mixing up
wood particles with an adhesive system, mat forming,
pressing between two plates and curing with heat. [1] Then,
the resulting product is cut into boards that are used to
fabricate the final products. Urea-formaldehyde resin is the
most used resin in this kind of materials, [2] due to its high
reactivity, low cost, capacity of curing at low temperatures,
transparency and excellent adhesion to wood [3, 4]. On the
other hand, they present poor UV durability [5,6] and low
stability as far as moisture and high temperatures are
concerned [2]. Still, formaldehyde emission represents its
major disadvantage. Formaldehyde emissions from
particleboard come from two sources: unreacted free
formaldehyde and polymer hydrolysis (breakdown of the
urea-formaldehyde linkages). [3] In order to reduce the
emissions, urea-formaldehyde producers have been working
in different approaches to change the way the polymer
polymerizes, tightening production controls, using additives
such as formaldehyde scavengers, resorting to ammonium
sulphate and other catalysts or reducing the
formaldehyde/urea molar ratio (F/U) (1.3 to less than 1.0). [3]
The reduction of F/U molar ratio isn’t favourable for industry
since the minimum limit has already been attained so, further
lowering of it might impair resin curing due to the excessively
low free formaldehyde content. [7]. The addition of
formaldehyde scavengers is another approach recently
studied. [8] The most common formaldehyde scavengers are
compounds such as urea, ammonia, melamine, and
dicyandiamide. Other additives such as casein, tannin,
resorcinol, peroxides, and ammonia had been proved to
effectively suppress the formaldehyde emission from wood-
based composite panel. Unfortunately, these additives are
considered expensive [9] as well as they consume free
formaldehyde available for the cure reaction [8].
One of the possible alternatives to decrease
formaldehyde emissions is changing the catalyst, from a
latent catalyst, such ammonium sulphate, to an ordinary
catalyst, like a strong acid. When applying ammonium
sulphate, hexamine is formed as by-product. Hexamine
hydrolysis may contribute to formaldehyde release during the
lifetime of wood-based panels produced with UF resins.
Orthophosphoric acid, on the other hand, catalyses resin
curing without forming by-products.
Orthophosphoric acid can provide a sufficiently
acidic environment to induce cure of UF resins. When this
acid is used, formaldehyde content is significantly higher
when compared with ammonium sulfate, after panel
2 production. However, after free formaldehyde has been
released during storage, the board cured with
orthophosphoric acid presented the lowest formaldehyde
content. Resin combined with orthophosphoric acid presents
a shorter pot-life than ammonium sulfate. The short pot-life of
orthophosphoric acid is a limitation common to all acid
catalysts, but it could be, in principle, overcome by resorting
to encapsulation of the acid. [8]
E. Roumeli et al. [7] studied the impact of silica
nanoparticles on UF resins’ mechanical properties. This
interaction leads to an increasing effect on the calculated
activation energy of the curing reactions that gets higher and
higher as the concentration of nanoparticles increases.
Furthermore, nanoparticles behave as a physical obstacle
disrupting the continuity in the matrix, making it more difficult
for the reactive groups of urea and formaldehyde to come
close and interact. [10, 7] Although the consequential small
retarding of the condensation reactions and the slightly
inhibition of the curing process of the UF resin [7], silica
nanoparticles also present great benefits as far as
mechanical properties are concerned. By producing
particleboards with different amounts of silica, Roumeli [7]
observed great improvements as far as internal bond,
rupture module and thickness swelling are concerned [7].
Regarding all improvements brought up by the
introduction of silica nanoparticles on UF resins mechanical
properties and all the advantages that the change of catalyst
from ammonium sulfate to a strong acid can bring, the
project here developed involves the transport of a strong acid
inside silica mesoporous nanoparticles (MSNs) into the
curing step of UF resins. Mesoporous silica nanoparticles are
synthesized by reacting tetraethyl orthosilicate (TEOS) or
another source of silica with a template made of micellar
rods. The result is a collection of rods that are filled with a
regular arrangement of pores [11].
The surfactant first forms rod-like micelles that
subsequently align into hexagonal or circle arrays. This
structure is as a template. After adding silica species, these
will cover the rods. [12]
MCM-41 is one of the most common types of
mesoporous nanoparticles. These consist on a regular
arrangement of hexagonally or circle shaped mesopores that
form a one-dimensional pore system. [12,13]. The most used
surfactant in MCM-41 production is CTAB
(cetyltrimethylammonium bromide). After producing the silica
structure, the template can then be removed by washing with
a solvent adjusted to the proper pH (chemical extraction) or
through calcination[12].
Regarding the goal of this project here developed,
after producing MSNs with a diameter inferior to 100 nm [14],
these will be filled up with acid and involved with a
thermoresponsive compound. Thus, when subjected to high
temperatures, MSNs will let the acid out, allowing UF resins
to cure.
2. Experimental
2.1 Materials Tetraethyl orthosilicate (TEOS) (≥99,0%, GC),
Hexadecyltrimethylammonium bromide (CTAB) (BioXtra,
≥99,0%), Ethanol puriss. p.a., absolute (≥99,8%, GC),
Trimethoxypropylsilane (97%), Trioxane (>99%) and p-
Toluenesulfonic acid monohydrate (ACS reagent, ≥98,5%)
were purchased from Sigma-Aldrich. Sodium hydroxide EKA
pellets puro were obtained from eka. Hydrochloric acid (37%)
and PLURONIC® PE 10400 Muster were supplied by VWR
and BASF Aktiengesellschaft, respectively. Deuterium oxide
(D2O) (D, 99,9%) and Dimethylsulfoxide-d6 (DMSO-d6) (D,
99,9%) were purchased from Cambridge Isotope
Laboratories, Inc.. Commercial toluene was distilled over
calcium hydride before use. All samples prepared with Milli-
Q water resorted to water from a Millipore system Milli-Q≥18
MΩ cm. Ammonium Sulphate (30%), UF resin 3C28 (63%)
and UF resin 3C32 were produced in Sonae.
2.2 Instruments TEM was carried on a Hitachi, model H-8100, of
high voltage (200 KV) and LaB6 filament. This equipment
has a bottom mounted CCD keenview camera (1376 x 1032
pixels) and EDS thermonoran light elements detector. With a
resolution of 2,7 Å point-to-point, the device is also
characterized by its -45o/ +45o double tilt holder. The
samples were prepared in absolute ethanol. NMR data was
collected on a Bruker Avance III 400 spectrometer (Bruker
BioSpin GmbH, Rheintetten, Germany) operating at 400
MHz. Trioxane was used as internal standard, NaOH
solution destroyed the sample and DMSO was used to
solubilize. Each measurement took 10 mg of the sample to
be analysed, 100 µL of a solution of trioxane in deuterated
water (5 g/ L), 300 µL of NaOH solution in deuterated water
(0,6 M) and 200 µL of DMSO. The pH was measured with a
VWR pHenomenal pH1000L pH meter equipped with a VWR
pHenomenal MIC 220 glass microelectrode and a VWR
pHenomenal PT1000 1M temperature
sensor. Centrifugations at 80 000 g were carried on a
3 Beckman Coulter, model Avanti J-30I at 20ºC.
Centrifugations at 25 200 g were carried on a B.Braun,
model Sigma 2K15 at 20ºC. Centrifugations at 25 20000 g
were carried on a VWR, model CT15E/CT15RE at 20ºC
2.3 Methods
MSN synthesis and characterization In order to produce mesoporous silica
nanoparticles, after pre-heating the oil bath to 80 oC, 240 g of
water Millli-Q were added to CTAB BioXtra (0,5 g) in a plastic
flask at room temperature. The solution was then immersed
in the oil bath with agitation. When it reached 40 oC, 1,75 mL
of Ammonium hydroxide solution (1 M) was added.
Afterwards, 2,5 mL of TEOS was added drop by drop. Two
hours later, the suspension was removed from the bath. After
cooling down, particles were separated from the mixture (80
000g, 30 minutes), washed twice with 1:1 ethanol and water
solution and once with ethanol (80 000g, 20 minutes). TEM
analysed both morphology and size. CTAB of MSN1 was
removed by mixing up a proportion of 100 mg of particles in
5 mL of a 0,5 M-HCl-solution in ethanol for 4 hours at a
40ºC-oil bath. MSN1 were separated from the suspension
(25 200 g, 15 minutes), washed three times with ethanol (25
200 g, 15 minutes) and dried.
MSN for acid release synthesis On T1, 0.04 g of MSN1 particles were mixed up with
hydrochloric acid (37%) [32] for an hour in order to fill the
mesopores, then the acid release was performed.
On the third test, T2, the particles (0.04 g) were also
blended in hydrochloric acid (37%) then, a mixture of water
and PLURONIC (2,5 mL to 500mg) was added and it
remained under stirring during 12h after being subjected to 2
minutes of ultrasounds. The suspension was subjected to
ultrasounds so the micelles of PLURONIC would disintegrate
and reintegrate around particles.
Synthesis of functionalized MSNs for acid release MSN2 was first functionalized through graft method.
MSN2 particles were added with anhydrous toluene in an
inert atmosphere in a proper size round-bottom flask. After
15 minutes under ultrasounds, TMPS was added and the
mixture was led into reflux for 24 hours. For this
functionalization step, it was considered that a gram of
particles would need 36 mL of anhydrous toluene and also
that 4 molecules of TMPS would occupy 1 nm2 of the particle
surface. This procedure resorted to anhydrous toluene in
order to guarantee the bond between the hydroxy group of
the silica nanoparticle and the silicon atom of TMPS. The
resulting particles were separated from the mixture through
centrifugation (25 200 g, 15 minutes), washed three times
with ethanol (25 200 g, 10 minutes) and dried.
CTAB was then removed following the same
procedure previously described. CTAB was kept inside in
order to prevent TMPS intrusion, which wasn’t necessary in
the first attempt since PLURONIC is too big to get in. After
separating the particles from the mixture (25 200 g, 15
minutes), wash them three times (25 200g, 10 minutes) and
dried them under vacuum over night, the functionalization
results were analysed through 1H NMR spectroscopy. When
reliable functionalization results were obtained, the above
described procedures of acid filling and PLURONIC coating
were followed.
Synthesis of Optimized MSNs for acid release
p-Toluenesulfonic acid was diluted in ethanol (in a
proportion of 1g for 1,14mL). Similar procedures to fill the
pores were done in all samples, except for the time of stirring
in acid, which was raised from 1 hour to an hour and a half.
T4, the same amount of particles used in all tests was
functionalized with TMPS, then it was separated from the
mixture through centrifugation (25 200 g, 15 minutes),
washed three times with ethanol (25 200 g, 10 minutes) and
dried. After that, CTAB was removed and the particles were
separated from the mixture (25 200 g, 15 minutes) and
washed three times with ethanol (25 200 g, 10 minutes). To
finalize, the particles were filled with p-toluenesulfonic acid
and dried for 24 hours. This test was performed in order to
assess if TMPS was playing a hampering role. T5
preparation was similar. All steps of T4 were performed. The
solution of PLURONIC and water was added and it stayed
under stirring for 12 hours.
Acid Release After preparing all the different samples, T1 and T2
were centrifuged (25 20000 g, 10 minutes) several times until
the pH value of the supernatants stabilized. On T3, T4 and
T5, the pH of the sample was analysed at room temperature
till the supernatant pH matched the milli-Q water pH (pH≅6-
7). After the acid release test was performed in the particles
containing PLURONIC at room temperature, the same ones
were lead into water and the suspension was put under
stirring for 1 hour in an 120ºC-oil-bath. After cooling down,
the suspensions were centrifuge (2520000 g, 10 minutes)
4 and the pH supernatants were measured.
Preparation of MSNs for UF resins Neutral MSNs for UF resins (MSN3-MSN14) were
produced following the same production steps as T5. They
differed from T5 on the amount of particles to be
functionalized, which was the whole batch in each
functionalization (from MSN3 to MSN14). After cleaning the
CTAB of each batch, all batches were inserted on a 250 mL-
glass-flask and acid was added (the same portion added in
T5 production). Instead of 24 hours drying, the whole batch
dried for 4 days under vacuum at 60ºC. After drying,
PLURONIC was added on the same portions added in T5.
The excess of acid was washed (25 200 g, 20 minutes) till
the supernatant pH reached the milli-Q pH value (pH≈6).
Optimized MSNs for UF resin (MSN15-MSN18)
were produced following similar steps of MSN3-MSN14.
Since the drying time after adding acid increased
considerable when the amount to be dried increased, the
four batches were divided in seven batches (B1-B7), in which
the acid was added. The seven batches dried for 24 hours.
PLURONIC was added separately. B1 was separated (25
200 g, 10 minutes) from the excess of PLURONIC but wasn’t
washed. B2, B3, B4 and B5 were washed until the
pHsupernatant≈3, B6 until pHsupernatant≈4 and B7 until until
pHsupernatant≈5 (16 128 g, 10 minutes). On table 5 are present
production amounts of the applied compounds. TMPS
amount represented on table 5 is related to MSN15-MSN18.
Catalysis Curve 20 g of resin were prepared by diluting UF resin
(from 63% to 50%) and by adding the intended amount of
catalyst. Different percentages of catalyst were tested on UF
resins for both catalysts (ammonium sulphate and MSNs).
The amount of catalyst applied was calculated regarding the
amount of resin instead of the based on the total solution
prepared.
Different test tubes were filled with 250µL of each
preparation; lead into boiling water and stirred resorting to a
glass rod. The time it took to gelify the resin was clocked.
The first test performed with B1 followed the same
procedure performed in MSN3-MSN14 for 1% of catalyst. On
the rest of the tests performed with each catalyst the whole
preparation was lead into a 100ºC-water-bath on a trap tube
while mixing with a glass rod. The time of gelification was
clocked. B2 MSNs were tested at 7,6%.
3. Results and Discussion
3.1 MSN synthesis and characterization Two batches of mesoporous silica nanoparticles
(MSNs) were produced and their diameter and morphology
were analysed by TEM (Figure 4, (A) and (B)). About a
hundred random particles of each sample were measured by
Fiji software [15]. The average diameter was (62±11) and
(50±11) nm for MSN1 and MSN2, respectively.
Figure 1 -‐ TEM image of MSN1 (A) and MSN2 (B).
As the morphology was approximately spherical
and the diameter was near 50 nm the tests proceeded.
Some of the functionalization tests were done with CTAB
inside the particles, so this was only removed from MSN1.
3.2 MSN for acid release By coating the surface of MSNs filled with acid with
a stimulus-responsive-polymer, the produced nanoparticles
will be able to release acid only when stimulated. For the
system in study, temperature might be the best option as a
stimulus since a fast release of protons is needed when the
temperature rises and UF resin and wood particles are
subjected to press. Therefore, it’s needed a commercially
available thermoresponsive molecule to coat the surface.
PLURONICs are widely used polymers composed
by poly(ethylene oxide)−poly- (propylene
oxide)−poly(ethylene oxide)[16,17]. This amphiphilic
copolymer is able to self-assemble into micelles in an
aqueous solution [18]. Consequently, micelles can host a
variety of guest molecules.[19] As far as the acid choice,
hydrochloric acid seemed to be appropriate since it is strong,
it’s commercially available and frequently used in industry.
In order to verify if MSNs filled with hydrochloric
acid and coated with PLURONIC would accomplish the
intended task, two samples were prepared. The first, T1,
consisted on MSN1 filled with hydrochloric acid. This test
was performed in order to confirm if the acid could be easily
released. On T2, also produced with MSN1, PLURONIC
was solubilized in water, before mixing it up with dried MSN1
5 filled with hydrochloric acid. T2 approach is represent on
Figure 2.
Figure 2 -‐ Representation of the preparation steps of MSN1s filled with acid and coated with PLURONIC. The large green sphere represents the mesoporous particle, blue circles represent pores filled with CTAB, the white ones are empty pores and the red ones are pores filled with acid. The last step consists in the particles functionalization with PLURONIC, an amphiphile molecule [40]. The orange lines represent the hydrophilic part, while the blue ones represent the hydrophobic part.
3.2.1 Acid release The acid release test permits realise if the acid
inside MSN pores is released properly. Figure 3 and Figure 4
present the test to MSN1s filled with hydrochloric acid, T1,
and the test to MSN1s filled with hydrochloric acid and
coated with PLURONIC, T2.
Figure 3 -‐ pH evolution of MSN1 filled with acid, T1. Blue dots represent pH values of washing supernatants at room temperature.
Figure 4 -‐ pH evolution of MSN1s coated with PLURONIC. PLURONIC was added as a solution in water, T2. Blue dots represent pH values of washing supernatants at room temperature and the red one after MSNs being heated at 120ºC.
From figure 3 (T1), it’s possible to conclude that
there are no constraints as far as the release of acid in
mesoporous particles filled with acid is concerned, which is
positive since a fast release of acid is needed when
pressing. However, it can also be concluded that the attempt
to keep acid inside the particles at room temperature through
T2 (Figure 4) wasn’t well succeeded by comparing the last
pH value of the last washing step (the last blue dot of the
chart) with the pH value of the supernatant after being
heated (red dot of the chart). This might mean that
PLURONIC isn’t coating the nanoparticles and probably this
prefers to be in contact with the dispersant instead of being
coated with PLURONIC micelles. Thus, PLURONIC is likely
forming empty micelles. Thereby, T2 behave like T1, losing
all acid at room temperature probably because these aren’t
coated with PLURONIC. Therefore, when heated, it T2
MSNs don’t release any acid (Figure 4). Thus, the last
washing pH value (blue dot) is similar to the pH value after
being heated up (red dot).
3.3 Functionalized MSNs for acid release The previous result showed that other procedures were
necessary to achieve the intended goal. After concluding that
PLURONIC wasn’t probably coating the surface and that the
mesoporous structures were preferring being dispersed in
water instead of being inside PLURONIC micelles, it was
clear that it was required a hydrophobic molecule coating the
silica nanoparticle surface. Regarding the fact that this
molecule easily bonds with the hydroxyl group of the silica
surface (Figure 5), trimethoxypropylsilane (TMPS) was
chosen for the effect.
Figure 5 -‐ Silica functionalization reaction with Trimethoxypropylsilane.
This way, T3 was prepared with similar synthesis
steps (Figure 6). MSN2s were functionalized with TMPS,
before the CTAB extraction in order to avoid the inside pores
functionalization and coated with PLURONIC after acid
adsorption.
Extrac'on*of*CTAB*
Addi'on*of*acid*
PLURONIC*coa'ng*
0 2 4 6 8
10
0 5 10
pH
Number of washes
0
5
10
0 5 10 15 20
pH
Number of washes
OH# O"
MSN
"
MSN
"Si#
OCH3#
OCH3#
H3CO#CH3# Si#
OCH3#
OCH3#
CH3#
Trimethoxypropylsilane0
6
Figure 6 -‐ Representation of MSN2s preparation steps involving a functionalization step with TMPS. The large green sphere represents the mesoporous particle, blue circles represent porous filled with CTAB, the white ones are empty porous, the red ones are porous filled with acid and the purple spheres represent TMPS. The orange lines represent the hydrophilic part, while the blue ones represent the hydrophobic part of PLURONIC.
3.3.1 Acid release T3 test (Figure 7) tested the acid release and simulate
the real acid release in the UF resins. On the other hand,
despite an identical preparation to T2, T3 was washed until a
supernatant pH around 6 instead of washing MSNs until the
pH value stabilizes.
Figure 7 -‐ pH evolution of MSN2 functionalized with TMPS and coated with PLURONIC, T3. pH evolution until a pHsupernatant≈6. Blue dots represent pH values of washing supernatants at room temperature and the red one after MSNs being heated at 120ºC.
The graphic represented in figure 7 showed that
functionalize the surface with TMPS before coating it with
PLURONIC doesn’t change the results. Hydrochloric acid is
still capable of coming out through PLURONIC at room
temperature. The evolution of pH remained, even after
heating the sample instead of decreasing (Figure 7). This
result has proved that this approach hasn’t accomplished the
intended results and acid keeps coming out when it’s not
supposed to.
3.4 Optimization of MSNs for acid release The result obtained for the previous approach revealed
that protons were getting out of the pores even when
PLURONIC was absorbed on the MSN surface. Since
PLURONIC is a polymer and it probably wasn’t that the
problem in the system that was causing the acid to get out,
the last attempt was the change of acid. Hydrochloric acid is
a very soluble acid in water and it is a really small molecule,
which might cause an easier diffusion in water and through
PLURONIC. Supported by these facts, hydrochloric acid was
replaced on these new tests by p-toluenesulfonic acid [20].
Beside its notorious larger size, this acid is also less soluble
in water than hydrochloric acid.
For this last attempt, two samples were prepared.
The first test, T4 consisted on MSN2s filled with p-
toluenesulfonic acid and functionalized with TMPS. This test
was performed to confirm a good dispersion of acid even
with TMPS in the surface of the particle. As for T5, this test
followed the scheme present on Figure 6.
3.4.1 Acid release Figure 8 reports the results obtained for T4, which is, as
previously stated, MSN2s filled with p-toluenesulfonic acid
and functionalized with TMPS. T5 (figure 9) represent
MSN2s filled with p-toluenesulfonic acid, functionalized with
TMPS and coated with PLURONIC.
Figure 8 -‐ pH evolution of MSN2s filled with p-‐toluenesulfonic acid and functionalized with TMPS, T4. Blue dots represent pH values of washing
supernatants at room temperature.
Figure 9 -‐ pH evolution of MSN2s filled with p-‐toluenesulfonic acid, functionalized with TMPS and coated with PLURONIC, T5. Blue dots
represent pH values of washing supernatants at room temperature and the red one after MSNs being heated at 120ºC.
In figure 8, T4, it’s notorious a slowdown on the pH
evolution which might reflect the influence that the size and
lower solubility of the acid might have in the system in study.
T4 presented positive results, showing that TMPS didn’t act
as a barrier for the acid release.
After all attempts, T5 (Figure 9) obtained the
desired result and acid was released when the temperature
rose. After heating, the pH value decreases from 6 to 3.
Addi$on'of'TMPS'
Extrac$on'of'CTAB'
PLURONIC'coa$ng'
Addi$on'of'acid'
0
5
10
0 2 4 6 8
pH
Number of washes 0 2 4 6 8
0 5 10 15 20
pH
Number of washes
0 2 4 6 8
0 2 4 6 8
pH
Number of washes
7 3.5 UF resin production with MSNs
The impact of the produced MSNs on UF resins is
studied here. Since a larger quantity of particles is needed
and since there’s still no scale up study, twelve batches of
optimized MSN for acid release (MSN3-MSN14) were
produced (Table 1).
Table 1 -‐ Main Characteristics of MSN3-‐MSN14: quantity produced, average diameter, D, TMPS functionalized per mole and TMPS molecules
functionalized per area of surface.
MSN (MSN3-
MSN14) (g)
D
(nm)
TMPS/particles
(mole/g)
Total produced
8,817 - -
Average - 90±14 1,2 x 10-3
Before applying the produced particles on UF
resins, all batches were cleaned together several times until
the pH of the supernatant matched the water pH. The pH
curve obtained for the particles is represented on Figure 10.
Figure 10 -‐ pH evolution (left) and cumulative release of H+(right) of particles filled with p-‐toluenesulfonic acid, functionalized with TMPS and coated with PLURONIC. Blue dots represent pH values of washing supernatants at room temperature and the red one after MSNs being heated at 120ºC.
3.5.1 Catalysis Curve After achieving a good release of acid in water (test T5),
the acid release on UF resins was tested with different
amounts of particles in order to obtain a catalysis curve. A
catalysis curve relates the percentage of catalyst applied
with the time it takes to gelify the resin when heated. When
applied in UF resins, the produced particles didn’t gelify UF
resin. Thus no curve was obtained since, even when MSNs
were applied in different amounts (1, 5%, 10, 20, 30, 40 and
50%), the resin didn’t gelify. As the resin didn’t gelify, one of
the possible reasons could be an excessive cleaning of the
acid inside the particles. In order to check this hypothesis,
the same acid release test performed before in water was
preceded, that is to say, a dispersion of particles in water
(10%) was heated for 1 hour. After cooling down, the pH of
the dispersion was measured, in which the pH of distilled
water decreased from 7.20 to 4.88 (Figure 10), which means
there was still acid inside the pores.
As previously stated, the presence of silica
nanoparticles inhibits the curing process of the UF resins
[21], which might mean that the amount of acid released by
the MSNs might not be enough to overcome the barrier
created by silica nanoparticles on the curing process of the
resin.
3.6 Optimization of MSNs for UF resins production Through the previous experiments on UF resin with the
prepared batches (MSN3-MSN14), it was notorious that the
amount of acid brought up by MSNs wasn’t enough and, for
this reason, new more acid batches were prepared.
Therefore, four batches were produced. Table 2 present their
main characteristics.
Table 2 -‐ Main Characteristics of the produced MSN15-‐MSN18: quantity
produced, average diameter, D, TMPS functionalized per mass.
MSN (MSN3-
MSN14) (g)
D
(nm)
TMPS/particles
(mole/g)
Total
produced 3,722 - -
Average - 70±26 1,6 x10-6
The four batches were divided in 7 portions (B1-B7) and
washed at different degrees in order to study how many
times MSNs needed to be washed before applied. This way,
B1 wasn’t washed, B2 was washed to reach a supernatant
pH near three, B6 was washed until its supernatant pH
reached four and B7 was washed till it reached a pH value
around five. Since the amount produced was low, the other
batches were washed till a supernatant pH of about three so
different percentages particles were applied in UF resins with
this supernatant pH value.
3.6.1 Catalysis Curve The first MSNs tested were B1. Therefore, 250 µL of the
preparation for the catalysis curve were lead into 100ºC-
water-bath. As result, the resin didn’t gelify, which means
either the quantity of the particles wasn’t enough or the B1
weren’t well mixed up and when 250 µL were pipetted out no
particles were there. Thereby, a different catalysis curve was
tried out. Instead of testing 250 µL of the whole preparation
0
2
4
6
8
0 10 20 30 40
pH
Number of washes
8 in a catalysis curve test, all the UF resin prepared (20g)
with different percentages of each catalyst were tested.
Table 3 contains the results for B1 MSNs at 1% and
7,6% while, Table 4 presents the results for ammonium
sulphate at the same amount as B1 MSNs as well as the
current amount applied on industry. MSNs B1 weren’t
applied at 3% since B1 sample was scarce.
The results obtained when resorting to MSNs (table 3)
weren’t well succeeded when these were applied at 1% but
they can be considered quite positive when MSNs were
applied in UF resins at 7,6 %, which means that results tend
to improve when the amount of MSNs applied increases.
Table 3 -‐ Curing time of 20g of UF resin with 1% and 7,6% of B1.
MSNs (%) Time (minutes)
1 >90
7,6 5,2
Table 4 -‐ UF resin pH and curing time of 20g of UF resin with 1%, 3% and
7,6% of ammonium sulphate.
NH4SO4 (%) Time (minutes)
1 3,2
3 2
7,6 2,1
The last test with particles was done with B2 ones. This
test was done at just 7,6% in order to compare with the result
obtained with B1 (Table 3). The resin with B2 MSNs didn’t
gelify the resin as well.
4. Conclusion The ultimate goal of this project was to encapsulate
acid inside MSNs pores and coat them with a polymer.
These hybrid structure once in contact with UF resins should
ensure that resin only cures upon temperature increase,
desadsorbing polymer from the MSN surface and releasing
the acid. This new strategy will decrease formaldehyde
emissions. MSNs were prepared with an average diameter of
80 nm and used in three different approaches for acid
release. On the first approach, MSNs were filled with
hydrochloric acid and coated with PLURONIC, though, after
testing the acid release, it was concluded that a different
approach should be tried out because all the acid was being
released before heating the particles. In the second
approach, the external surface of MSNs was functionalized
with trimethoxypropylsilane, a hydrophobic molecule, and 1H
NMR confirmed the functionalization. The particles were
filled with hydrochloric acid and coated with PLURONIC.
After washing, MSNs were heated in water and the pH was
measured. The acid was unsuccessfully encapsulated inside
these new MSNs and, for that reason, after being heated, the
pH of water remained the same (pH≈6). Regarding that a
good particle coating with PLURONIC was guaranteed when
MSNs were functionalized with trimethoxypropylsilane and
observing the adverse results obtained in the last approach,
it was established that hydrochloric acid was getting out of
pores because of its high solubility in water and small size.
For this effect, p-toluenesulfonic acid replaced hydrochloric
acid on the tests, maintaining all preparation steps of the
second approach. The acid was released when MSNs were
heated up in water and pH decreased to pH=3. When used
in UF resins, MSNs filled with p-toluenesulfonic acid didn’t
gelify the resin at any percentage applied on. Despite the
success in decreasing water pH, mesoporous nanoparticles
didn’t accomplish the same results in UF resin. In a new set
of MSN, the number of washing steps was reduced, to avoid
leading the acid from the pores. MSNs were applied on UF
resins at different percentages and resin curing was obtained
with non-washed-MSNs at 7,6%. Despite of the positive
results obtained, further studies must be performed to
improve the results achieved in this thesis.
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