Sistema de Evaluación de Impacto Ambiental

III. EN MATERIA DE INSTRUMENTOS DE GESTION AMBIENTAL

2. Sistema de Evaluación de Impacto Ambiental

5.1 Introduction

Combatting the hazards associated with fires in the home and workplace is an

ongoing effort requiring constant reevaluation and improvement of fire safety

procedures and technologies. Damage to homes in the United States due to fires dropped

from $10.2 billion/year in 1977 to $7 billion/year in 2011, due partially to improved fire

safety standards and fire-mitigation technologies

. Despite the decreasing trend, the

National Fire Protection Association reports that 56% of fire deaths are of individuals

above the age of 50, suggesting that a disproportionate number of individuals in older

demographics fall victim to house fires

. These data make it apparent that there is need

for passive flame-retardants.

One important realm of flame-retardant research is in polymeric materials. Since

the middle of the 20

century, the use of polymers in the home and industry has

skyrocketed. However, in the event of a fire, the energy-dense hydrocarbon composition

of polymers provides a source of fuel, making their widespread application an item of

concern.

This is an Accepted Manuscript of an article published by Taylor & Francis in Plastics, Rubber, and

Composites on July 28, 2016, available online:

58 In response, an immense amount of research has focused on reducing the

flammability of polymeric materials. One early research focus was into halogenated

flame-retardants, which degrade into radical scavenging products that quench free-

radical reactions in the vapor phase

. These materials are often paired with a synergistic

additive such as antimony oxide or a phosphorous-based flame-retardant to enhance char

formation

_{. Some of the most common halogenated flame-retardants include}

tetrabromobisphenol A derivates, polybrominated diphenyl ethers, and polybrominated

biphenyls, among others

.

Another focus of research has been on phosphorous-based flame-retardants.

Various phosphorous-based materials have been developed and incorporated into

polymers, resulting in dramatic improvements to flammability

_{. Generally the efficacy}

of phosphorous-based approaches is due to the formation of char from degraded polymer

and in some cases, the generation of vapor, which can dilute or quench gas-phase

reactions

. Furthermore, vapor generation in phosphorous-based flame-retardants can

help increase the volume of produced char, producing thick, expanded, intumescent

barriers

_{. Examples of phosphorous-based flame-retardants include various types of}

ammonium polyphosphate and triphenyl phosphate, among many others

. Halogenated

phosphorous-based materials also exist, including tris(2-chloro-ethyl) phosphate and

tris(chloropropyl)phosphate, which take advantage of both halogenated and

phosphorous-based approaches

.

Phosphorous-based materials are only one type of intumescent flame-retardant.

Generally, intumescent systems involve the incorporation of a charring agent, a source

59 of carbon for the charring agent, and a blowing agent to generate vapor, which increases

the volume of the char to produce a thick insulating physical barrier. Some state-of-the-

art flame-retardants incorporate all three of these materials into a single intumescent

additive, such as in studies with melamine salts of pentaerythritol phosphate or the

familiar deoxyribonucleic acid (DNA)

. In addition, significant research focuses on

combining intumescent flame-retardant systems with nanofillers in an effort to reinforce

char and create a high quality, continuous barrier

.

However, some of the most commonly used flame-retardants have received

criticism due to their toxicity or environmentally persistent nature. Many halogenated

flame-retardants have been phased-out by manufacturers, including pentabrominated

diphenyl ethers (pentaBDEs), which accumulate in ecosystems where they are difficult

to remove

. Some phosphorous-based materials have been shown to pose environmental

concerns, being found in significant concentrations in the environment

. Furthermore,

many halogenated phosphorous-based flame-retardants are potentially toxic, such as tris

(1,3-dichloroisopropyl) phosphate (TDCPP) which been shown to be a neurotoxin in

mice cells and a mutagen for animals

10-12

_.

One technology that has the potential to replace potentially toxic and

environmentally persistent flame-retardants is the polymeric nanocomposite. These

materials, which are traditionally composed of a polymeric matrix embedded with

inorganic nanoparticles, possess high thermal stability and in some cases, the ability to

produce a physical barrier at the burning surface of the material

13-17

_{. The barrier that}

forms consists mainly of accumulated, thermally-stable inorganic nanoparticles and

60 carbonaceous char which act as shield, reducing mass and heat transport to the incident

flame, while protecting unburned material behind the barrier

18, 19

.

As with intumescent flame-retardants, a trend in previous research is to focus on

how conventional flame-retardants interact with nanofillers to synergistically enhance

the formation of a physical barrier, measured in part by an increase in high temperature

residuals

37-43

_{. Many different conventional flame-retardants have been utilized in}

conjunction with nanofiller, including phosphorous containing compounds such as

polyphosphoric acid, Friedel-Craft reagents such as phosphotungstic acid, or other

chemically active additives

37-41

.

Polymer cross-linking can be used to increase the char yield of polymers as well

as dramatically reduce internal mass transfer compared to linear polymers, but little has

been done to directly quantify the interactions between common polymer cross-linking

and nanocomposite degradation

. Furthermore, modern chemistry allows for the surface

treatment and customization of silica nanoparticles, transforming them into cross-linking

agents. Specifically, nanosilica can be surface treated with KH570, introducing many

vinylidene groups (RC=CH

) to the surface of the nanosilica. These surface vinylidene

groups can participate in free-radical polymerization, incorporating into growing

polymer chains. The results are polymeric nanocomposites with silica-based cross-

linkages, synthesized without the use of a separate cross-linking agent. Since methyl

methacrylate (MMA), the monomer of poly(methyl methacrylate) (PMMA) closely

resembles the active KH570 surface treatment, polymerization is more likely than with

other dissimilar monomers, making PMMA a good polymer candidate for this study.

61 5.2 Experimental Methods

5.2.1 Materials

Methyl methacrylate monomer (MMA) and 1,1'-Azobis(cyclohexanecarbonitrile)

(ABCN) initiator were supplied by Polysciences. Nanosilica (average diameter of 20-

30nm) was supplied by US Nano with two different surface treatments. One treatment,

KH550, does not participate in polymerization reactions and was used for linear PMMA

nanocomposites, while the second treatment, KH570, does participate in polymerization

reactions, and was used to produce silica cross-linked nanocomposites. Both nanosilicas

contain 3 wt-% to 4 wt-% surface treatment, as confirmed by per-batch elemental

analysis by the vendor.

5.2.2 Nanocomposite Synthesis

MMA was placed into a glass vial with silicone septum and PTFE liner, which

acts as a polymerization vessel. An appropriate amount of silica surface-modified with

KH570 was added to the monomer to make one of four loadings as follows: neat

PMMA, 1 wt-%, 2 wt-%, or 4 wt-% silica in MMA. As previously discussed, the KH570

silica surface functionalization can be polymerized with multiple PMMA chains to form

a cross-linked material. The solution was stirred with a magnetic stirrer for 30 minutes,

after which the solution was sonicated at room temperature for 30 minutes to degas and

complete mixing. An amount of ABCN is added to the mixture corresponding to 0.2 wt-

62 % ABCN solution in MMA. Under gentle stirring, nitrogen is bubbled through the

solution for 10 minutes to inert the polymerization vial and reduce the dissolved oxygen

concentration. The vial is submerged into a mineral oil bath which is maintained at 70°C

± 1°C. A hot plate provides heat and gentle magnetic stirring to both the oil bath and a

small magnetic stirrer in the polymerization vial.

5.2.3 Casting Nanocomposites

Just prior to the solution gelling, the solution is cast into multiple 2 mL sealable

polypropylene vials. Each vial, fitted with a piece of buoyant foam, is allowed to float in

an oil bath at 70°C ± 1°C for an additional 24 hours to finalize curing, after which

samples are removed from the polypropylene vials. Sections of cross-linked samples

were placed in acetone for several hours to assure solvent-swelling behavior, indicating

the presence of polymer cross-linkages.

In the case of Cone Calorimetry samples, instead of casting nanocomposites into

a small polypropylene vial, polymer solutions are transferred to a specialized

polymerization mold, developed from previously used methods

. The mold was

designed to produce panels of nanocomposite material which were cut into 10cm x 10cm

x 0.5cm samples for Cone Calorimetry testing. The mold consists of a length of silicone

tubing, sandwiched between two panes of glass, forming a cavity between the panes.

Small metal clamps are applied across the surface of each glass plates to keep the

silicone tubing in place and to assure the mold does not leak. The polymerization

63 solution is poured in the top of the cavity formed between the glass panels, and an

additional piece of silicone tubing is used to seal the top of the mold. The entire

apparatus is then submerged into a large, well-mixed oil bath at 70°C ± 1°C for an

additional 24 hours. The final thickness of the sample is defined by the thickness of the

silicone tubing and the force applied to the glass panels by the metal clamps. After

curing is complete, the mold is removed from the oils bath and thoroughly cleaned. The

metal clamps are then removed, allowing the glass plates to separate, releasing the cured

sample within. This molding process produces well-formed polymeric sheets and

removes the need for expensive specialized ovens to prepare Cone Calorimetry samples.

Using the large Cone Calorimetry mold, neat PMMA and 1 wt-% silica cross-

linked samples were tested. In addition, traditional 1 wt-% silica nanocomposites were

produced to compare the flammability of material in this study to more traditional

silica/PMMA nancomposites. Traditional linear nanocomposites are made using the

synthesis methods already discussed, except instead of surface treating silica with

KH570, silica is treated with KH550, which does not engage in free-radical

polymerization.

5.2.4 Material Characterization and Analysis

Scanning Electron Microscopy (SEM) was used to characterize the surface-

treated nanoparticles before being embedded by first dispersing the nanosilica in

isopropyl alcohol and then applying it to a substrate, followed by the evaporation of

64 isopropyl. Thermogravimetric Analysis (TGA) and Derivative Thermogravimetric

Analysis (DTG) were done using a Texas Instrument Q50 Thermogravimetric Analyzer.

Nanocomposite samples prepared for TGA consisted of neat PMMA, or a

nanocomposite containing 1 wt-%, 2 wt-%, or 4 wt-% nanosilica cross-linked to PMMA.

Each TGA sample had a mass of approximately 3-5mg and was tested at a temperature

ramp rate of 10°C/min or 20°C/min in an aluminum pan. TGA results were also used

with three kinetic methods to determine the activation energy and kinetic form of

degradation. X-Ray Diffraction (XRD) studies were done using a Bruker D8 powder

diffractometer and Cu k-α as a radiation source. Nanocomposite samples were ground

into a coarse powder for these XRD tests. Lastly, flammability and combustion

experiments were conducted using a Fire Testing Technology Cone Calorimeter, with an

external heat flux of 50 kW/m

. Neat PMMA, traditional 1 wt-% silica in linear PMMA

nanocomposites, and 1 wt-% silica cross-linked to PMMA nanocomposites were tested

using these Cone Calorimetry methods.

5.3 Results and Discussion

5.3.1 Nanocomposite Morphology

Before being used in nanocomposites, the surface treated nanosilica was

characterized using SEM, as shown in Figure 29 for a KH570 surface treated nanosilica.

The spherical nanosilica appears to be reasonably uniform, with a diameter of 20-30nm.

The SEM images show that the silica appears in small agglomerated clusters alongside a

65 number of individual particles. The morphology of silica surface treated with KH550

appeared similar to silica treated with KH570, being essentially indistinguishable.

Figure 29: SEM image of KH570 modified nanosilica.

X-Ray Diffraction measurements were used to better understand the morphology

of the cross-linked silica nanocomposites. XRD is particularly useful for observing

significant mass fraction of agglomerated particles. As seen in Figure 30, the XRD plots

for each nanocomposite sample look similar when compared to pure PMMA, except the

66 peaks are broader and shifted slightly towards higher 2Θ values, indicating a slightly

denser material. These results are consistent with other XRD measurements for

PMMA

_{. The first broad peak near 2Θ = 14°C corresponds to chain-to-chain lengths in}

PMMA while the second broad peak near 2Θ = 30°C corresponds to intermolecular

lengths within each PMMA repeat unit. The data in Figure 30 show each sample is

essentially amorphous, suggesting that agglomeration is not a major issue.

Figure 30: XRD results. Results for pure PMMA, 1 wt-%, 2 wt-%, and 4 wt-% silica

cross-linked to PMMA.

67 5.3.2 Thermogravimetric Analysis

The results for TGA and DTG at a ramp rate of 10°C/min are shown in Figure 31

and Figure 32, respectively. The TGA and DTG results for a ramp rate of 20°C/min have

similar shapes when compared with Figure 31 and Figure 32, but with shifted

degradation regions, as expected. In general, the degradation of PMMA in air occurs in a

single broad mass loss step, shown as a mass loss peak in Figure 32. This is in good

agreement with previous TGA measurements of PMMA degradation in air

. In addition,

the DTG data in Figure 32 show that increasing nanoparticle concentration reduces the

peak mass loss rate (PMLR), resulting in a maximum reduction of 30% for a 4 wt-%

cross-linked nanocomposite, comparable to the results of a 13 wt-% traditional silica

nanocomposite from literature

. These results indicate that silica cross-linkages enhance

the thermal stability of PMMA significantly, allowing for a reduced mass of silica to be

used.

68 Figure 31: TGA results. Results for cross-linked silica loadings between 0wt% to 4wt%.

Measured at a ramp rate of 10°C/min.

69 Figure 32: DTG results. Results for cross-linked silica loadings between 0wt% to 4wt%.

Measured at a ramp rate of 10°C/min.

5.3.3 Degradation Kinetics

Thermal degradation kinetics were first used to determine a suitable pseudo-step

kinetic form. Since PMMA is often cited as following first-order pseudo-step

degradation, a method for n-th order reactions was first used. In Section 4: α –Zirconium

Phosphate/PMMA Nanocomposites, it was shown that combining the mass balance

70 kinetic form, shown in Equation 3 of Section 1.4: Relevant Polymer Degradation

Kinetics, resulted in Equation 18.

According to Equation 18, if the inverse of T is plotted against the left side of

Equation 18 and a straight line is achieved, the degradation can be described by a single

first-order pseudo-step reaction. The results from this analysis, shown in Figure 33,

produce a set of straight lines for the majority of the degradation, suggesting PMMA and

all silica cross-linked PMMA samples are largely explained by a first-order reaction.

However, at high degree of conversion, degradation becomes more complex and is not

well described by a single kinetic form.

71 Figure 33: Linearized TGA data for PMMA and different loadings of KH570 silica. A

linear plot indicates the reaction is well described by first order kinetics.

Since single-sample methods are sometimes unreliable for determining kinetic

parameters, two model-free kinetic methods were chosen to calculate the activation

energy of thermal degradation. The first method, developed by Friedman, is shown in

Equation 15 in Section 1.4: Relevant Polymer Degradation Kinetics and assumes

Arrhenius Rate Laws. The second method, developed by Kissinger, Akahira, and

Sunose, then modified by Starink, assumes Arrhenius Rate Laws and uses an integral

approximation. The resulting linear equation is shown in Equation 17 in Section 1.4:

72 Relevant Polymer Degradation Kinetics. Both the Kissinger-Akahira-Sunose, and the

modified Starink methods linearize data across multiple temperature ramp rates, making

parameter determination more reliable than single-sample methods.

Using these two kinetic methods, the activation energy was determined with

respect to degree of conversion. Both methods provide good agreement with each other

and show that activation energy appears to increase with respect to particle loading,

summarized in Figure 34. The differences between pure PMMA, 1 wt-%, 2 wt-%, and 4

wt-% nanosilica in Figure 34 are dramatic, showing an increase in the activation energy

by a factor of roughly two. Compared to conventional silica in PMMA previously

studied, the activation energy is markedly improved with the nanocomposites presented

here. Similar kinetic methods with traditional silica-PMMA previously showed an

enhancement of roughly 20-50 kJ mol

-1

for similar silica loadings, while the

nanocomposites in this study show that covalently bonded nanosilica enhanced

activation energy by roughly 100 kJ mol

-1

, dramatically stabilizing degradation reactions

and producing more thermally stable materials

. In general, traditional nanofillers have

a stabilizing effect on polymers due to a reduction in polymer mobility and free-radical

capturing by silica, measured as an increase in the activation energy of degradation

.

The enhanced thermal stability is attributed to reduced polymer mobility and free-radical

trapping by silica, similar to traditional PMMA-silica nanocomposites

. However, the

effect in this case is more pronounced, producing higher activation energies. This is

attributed to increased interaction between nanofiller and polymer by covalent bonds.

73 Figure 34: Activation energy for thermal degradation. Shown with respect to particle

loading. Error bars indicate the standard deviation of the samples.

The final, high-temperature residuals from TGA and DTG tests were also

examined. An increase in the mass of high temperature residuals is beneficial for

forming a physical barrier as the material burns. In addition, increased residuals are often

due to incomplete combustion, meaning less volatile fuel leaves the condensed phase to

burn in a fire, remaining as thermally inert char. As shown in Figure 35, the residual

mass after testing was greater than the sum of the pure PMMA char and the inorganic

74 filler combined for all nanocomposites, indicating an increase in char yield due to silica.

Furthermore, the cross-linked silica nanocomposites studied in this work appear to

produce more char than traditional silica nanocomposites in other studies, indicating

there are most likely contributions from both cross-linkages and nanofiller

. This effect

is attributed to the decreased mass transport caused by polymer cross-linkages, shown

previously in polymers without nanofiller

_{. By reducing the internal mass transport}

within the material, reaction intermediates spend more time on the interior on the

material, closer to the nanoparticle-polymer interface at which char forming reactions

occur

.

Figure 35: Percentage of total mass remaining after TGA and DTG tests. Error bars

indicate sample standard deviation.

75 5.3.4 Cone Calorimetry Studies

The heat release rate (HRR) for neat PMMA, traditional 1 wt-% silica in PMMA

nanocomposites, and 1 wt-% silica cross-linked to PMMA nanocomposites are shown in

Figure 36. The results show that even for the low concentration of 1 wt-% silica cross-

linked to PMMA, there is a reduction of 18% in the peak heat release rate (pHRR)

compared to neat PMMA, and an improvement of 13% compared to a 1 wt-% silica in

linear PMMA nanocomposite. There was a small reduction in the total heat released

(THR) (7%) compared to neat PMMA, but this was similar for both cross-linked and

linear nanocomposites. The decrease in pHRR is attributed to both enhanced char

formation and enhanced thermal stability. While greater reductions in pHRR have been

achieved, they generally require large amounts of additives

. In this case, the presence of

covalently bonded nanofillers suggests benefits over traditional surface treated

76 Figure 36: HRR results. Results for neat PMMA, 1 wt-% silica in PMMA

nanocomposites, and 1 wt-% silica cross-linked to PMMA nanocomposites.

5.4 Conclusions

Nanosilica surface treated with KH570 was used to covalently cross-link

poly(methyl methacrylate) (PMMA), producing nanocomposites with silica-based cross-

linkages. The materials were synthesized using an in-situ method with nanosilica

loadings of 0 wt-%, 1 wt-%, 2 wt-%, and 4 wt-% in PMMA. Morphological studies

using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) showed

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