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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