Cyclotriphosphazene Synthesis Essay

The two synthesis essay questions below are examples of the question type that has been one of the three free-response questions on the AP English Language and Composition Exam as of the May 2007 exam. The synthesis question asks students to synthesize information from a variety of sources to inform their own discussion of a topic. Students are given a 15-minute reading period to accommodate the additional reading required for the question.

Sample 1

Below is a sample synthesis essay question, sample scoring guidelines, comments from the Chief Reader about the sample student essays, seven sample student responses, and scoring commentary for each sample.

Approximately 300 AP English Language and Composition students from eight schools in New York, Maine, Texas, Tennessee, Washington, Florida, and New Mexico wrote responses to this synthesis topic. Students from these schools were given a 15-minute reading period followed by a 40-minute writing period in which to complete the sample synthesis assignment.

Sample 2

An additional sample synthesis essay question is provided here.

1. Introduction

Textiles play an important role in everyday life: one of their main drawbacks refers to their structure, as they are mainly made of organic polymers, which conversely, if not inherently flame-retarded (such as polyaramides and polyphosphonate fibers), are flammable and potentially dangerous species. Specifically referring to fibers and fabrics, the annual UK fire statistics have clearly demonstrated that most of fire incidents occur in houses, involving upholstering furniture, bedding and nightwear [1].

In this context, the ease of flammability of textiles has been faced by designing and synthesizing suitable flame retardants (FRs), i.e., additives that are able to suppress or delay the appearance of a flame and/or reducing the flame-spread rate (flame retardants) or delaying ignition or reducing the rate of combustion when needed (fire retardants) [2].

From an overall point of view, combustion in the presence of flames is a gas-phase process exploiting the oxygen taken from the surroundings. As a consequence, before the occurrence of the combustion process, the textile undergoes degradation: some of the so-obtained degradation products turn into combustible volatile species that, in combination with oxygen, fuel the flame. If the heat generated in the combustion is sufficient, it can be easily transferred to the textile substrate, hence giving rise to further degradation phenomena and supporting a self-sustaining combustion cycle (Figure 1).

On the basis of the chemical composition and their thermal and fire characteristics, if they are not inherently flame retarded, fibers and fabrics have to be treated with additives that may contain halogen, nitrogen, phosphorus, sulphur, boron, metals, etc., hence becoming flame retarded. The aforementioned additives can be added during spinning processes performed on synthetic fibers, or deposited on the synthetic or natural fiber/fabric surface, hence creating a protective layer/coating. Both finishing and coating methods can be exploited: concerning the former, the fiber/fabric is impregnated with a solution or a stable suspension that contains the FR additive. Conversely, the coating method involves the application of a continuous or discontinuous layer/film on both surfaces of the textile.

From an overall point of view, the self-sustaining combustion cycle of Figure 1 can be broken in the presence of FR additives, thus achieving the extinction of the flame or reducing the burn rate. In particular, they can:
  • Lower the developed heat to below that necessary to carry on the combustion process.

  • Modify the pyrolysis process to lower the quantity of flammable volatiles developed, favoring, at the same time, the creation the char, i.e., a carbonaceous residue that also limits the heat and mass transfer between the textile material and the flame.

  • Isolate the flame from the oxygen/air supply.

  • Release flame inhibitors (like chlorinated and brominated compounds) when the textile approaches the ignition temperature.

  • Lower the heat flow back to the fabric, hence limiting or preventing further pyrolysis.

  • Favor the formation of a char or an intumescent protective layer when the textile interacts with a flame or a heat source.

A review published in 2011 succeeded in summarizing the state of the art for the different commercially available flame retardants for textile materials, which were classified according to the following periods [3]:
  • 1950–1980: The “golden period” of flame retardant research, involving the appearance of the first patents on FRs based on organophosphorus compounds for cellulosic textiles (i.e., cotton). During this period, inherently FR synthetic fibers bearing aromatic structures were also developed.

  • 1980–late 1990s: Very limited advances of the research in FRs were achieved during this period.

  • 2000 onward: Several efforts were carried out in the design of char-former flame retardant additives, possibly containing phosphorus-based products. Another goal was the investigation into the possibility of replacing bromine derivatives with other less toxic and efficient products. Furthermore, during this period, nanotechnology was demonstrated to show outstanding potential for conferring flame retardant features to fibers and fabrics, through the use of nanoparticles having different aspect ratios. In particular, the exploitation of both top-down (using preformed nanoparticle suspensions) and bottom-up (exploiting the generation of single nanoparticles or nanoparticle assemblies—even hybrid organic-inorganic structures) was successfully considered [4,5].

Specifically referring to the textile field, FRs can also be classified according to their “laundry durability”: indeed, a non-durable FR is washed off immediately when soaked in water, but may resist dry cleaning. Conversely, semi-durable FRs are able to resist water-soaking and possibly a few washes, while durable FRs endure some 50 or 100 washing cycles.

However, according to the very stringent directives recently promoted by the EU community and USA, some of the halogenated compounds (such as brominated diphenyl derivatives) have been banned, as they have clearly shown a high toxicity for both animals and humans [6].

The aforementioned disadvantages stimulated the scientific community toward the design and development of phosphorus-based compounds, which seem less toxic and may represent a suitable alternative to their halogen-based counterparts. Though it is not a general case that all phosphorus compounds are non-toxic, the development of new flame retardants based on phosphorus compounds has shown that they have lower toxicity profiles as compared to halogen-based counterparts [7,8]. In general, the development of any new flame retardant should involve a complete assessment of its performance in material as well as its toxicity.

In this context, some new products have been designed and nowadays are commercially available. In particular, Trevira CS®, which is based on the use of a phosphorus-containing comonomer (in the form of propionylmethylphosphinate), has been exploited for conferring flame retardant properties to polyester fibers and fabrics [9,10]. Regarding cotton and cellulosic-rich substrates, the present focus is either on the synthesis of effective non-halogenated additives for coatings and back-coated fabrics or on the utilization of hydroxymethylphosphonium salts (Proban®) or N-methylol phosphonopropionamide derivatives (Pyrovatex®).

The chemistry of the Proban® process exploits a tetrakis(hydroxymethyl) phosphonium–urea condensate, which, after padding, is crosslinked by ammonia gas in a dedicated plant and then subjected to peroxide oxidation for stabilizing the resulting polymeric matrix [11]. The washing fastness of this treatment is due to the deposition of the chemical within the fibers by a construction of a polymer network during the heating process: as a consequence, Proban® is not linked to the fibers but is mechanically retained within the fiber interstices. One of its major disadvantages refers to the possible release of formaldehyde during the fabric use [12].

Conversely, the chemistry behind Pyrovatex® is based on a conventional pad-dry-cure process in the presence of a methylolated crosslinking agent, which is responsible for the formation of covalent bonds with the hydroxyl groups of the cellulosic substrate. Nonetheless, about 50% of Pyrovatex® FR treatment has been reported to be lost during the first laundry occurrence, because of the extraction of unreacted products, though it remains stably linked thereafter.

Despite the efficiency of such commercially available treatments, both academic and industrial researchers are still seeking worthy alternatives, also taking into account the requirements that have to be fulfilled by the new products. In particular, with respect to the already existing FRs:
  • Any new flame retardant should show equivalent or superior ease of application.

  • Any new flame retardant should not emit formaldehyde during application or service.

  • Any new flame retardant should provide comparable textile service-life features, specifically referring to durability, comfort, tensile properties, outward appearance and aesthetics.

  • Any new flame retardant should possess an overall comparable cost-effectiveness to the already existing chemicals and preferably be less costly.

  • Any new flame retardant should possess equivalent or even lower toxicity and environmental impact.

  • Air permeability of the treated textiles should be maintained, irrespective of the possible high amounts of chemicals needed to provide flame retardant features.

  • Any new flame retardant should not cause any alteration in the hue of the dye and/or dyeability of the fibers/fabrics.

Nowadays, the approach adopted by the scientific community is being slightly changed: indeed, the durability of any new flame retardant is still needed, but the novel processes and methods developed in last five years seem to be more addressed to the design of low impact and eco-friendly systems.

In this context, the present paper aims at providing an overview of the recent advances in the design of phosphorus-based FRs, also in combination with nitrogen- or silicon-containing structures, for different fibers and fabrics: in particular, the evolution from chemical to low environmental impact products will be thoroughly described, highlighting the current achievements and limitations, as well as the open challenges and perspectives.

2. Assessment of the Fire Behavior of Textiles

This section summarizes the current methods that allow assessing the reaction of a textile toward the exposure to a flame or a heat flux. All the developed methods take into account that, for textiles, the high fiber surface to mass ratio favors their easy ignition; in fact, these materials burn faster than other bulk polymers.

Generally speaking, ignition occurs when a small flame is applied to flammable fabrics for no more than 12 s and the textile continues to burn after the removal of the flame. Therefore, most of the work on flammable fabrics focuses on the evaluation of the facility of ignition, the rate and extent of flame spread, the duration of flame propagation, the heat release and heat of combustion. All these parameters are merged with a quantitative portrayal of burning wreckages, such as melt dripping. It is very difficult to find a single test method able to measure all the aforementioned parameters. Concerning the fabrics that show self-extinction, such as flame-retarded fabrics, tests comprise the evaluation of time of afterflame and afterglow and extent of fire damage (specifically referring to char length, dimension of holes, or damaged sample length).

The measurement of textile flammability involves either scientific (i.e., research) test or the standard test methods. The former provide information suitable for assessing the burning behavior and are exploited for the design of new FRs or fire-retardant treatments.

Limiting oxygen index (LOI), also called oxygen index (OI), is one of the most popular scientific methods, used in many standards, such as ISO 4589 and ASTM D2863. LOI denotes the minimum concentration (vol %) of O2 in a mixture of O2 and N2 that will just sustain flaming combustion of a material in a candle-like manner. Textile materials burn rapidly when they exhibit LOI values up to 21 vol %, while they burn slowly when LOI is in between 21 vol % and 25 vol %. LOI values beyond 26 vol % indicate some flame retardant features [13].

The obtained LOI values may be affected by several fabric structural parameters, when measured for the same fiber type: this makes LOI values relative and not absolute data. In addition, the textile material is ignited at the top and thus it burns vertically downward (i.e., in candle-like manner) which is opposite to the burning of any material freely suspended.

Simple ignition tests (used in many standards, such as BS 5438 and EN ISO 6941) represent another usual approach for assessing the flammability of a textile material: more specifically, a standard gas flame is applied to the face or lower edge of a vertically oriented fabric sample; ignition is examined by visual observations and the time needed to ignite the specimen is recorded. The textile does not pass the test when, after the removal of ignition source, the flame achieves any end of the sample. If the flame reaches extinction, the char length, dimension of holes, afterglow, and type of any wreckage (molten drops, etc.) are thoroughly evaluated.

Flame spread (UL-94, which contains EN 60695 11-10, ASTM D 635-03 and D 3801-00) is a bench-scale test which measures the rate of flame spread usually calculated as the ratio of the distance to the time taken of the advancing flame front to reach defined distances marked on the fabric specimen. The upward fire spread is far faster than downward and horizontal flame spread and, hence, adopted as a better means of assessing the fire hazard of a fabric.

Although not specifically designed for fabrics, cone calorimetry tests (according to ISO 5660) have become a standard bench scale model of early flaming [14,15]. In particular, the cone calorimeter mimics the penetrative burning seen as fire burning into a specimen. It evaluates the heat release rate and the effective heat of combustion from a burning material exposed to a controlled radiant heat source (ISO 5660 part 1). Usually, such parameters as Time To Ignition (TTI), Total Heat Release (THR), Heat Release Rate and corresponding peak (pkHRR), Effective Heat of Combustion (EHC), Mass Loss (ML) and Mass Loss rate (MLR) can be evaluated. The cone calorimeter can also be utilized to evaluate smoke generation (ISO 5660 part 2): in this case, the measured parameters include the determination of CO and CO2 concentrations, as well as the assessment of smoke density (Specific Extinction Area–SEA, Total Smoke Production–TSP, etc.).

The micro-combustion calorimeter (MCC) has recently been standardized (ASTM D7309-13) and exploited for evaluating the flammability of polymers [16,17]. In this process, a small specimen (about 2–10 mg) undergoes pyrolysis through a fast heating up in inert atmosphere (with a heating rate below 1 °C/s). The obtained pyrolysed products are then mixed with O2/N2 mixture to expedite combustion. The oxygen concentration and flow rates of the combustion gases are evaluated, and the amount of generated heat is calculated on the basis of oxygen consumption calorimetry.

3. Phosphorus Chemistry in FRs: An Overview

Phosphorus-based flame retardants are quite versatile in their flame retardant action. Phosphorus compounds often exhibit both condensed and gas phase activity [18]. A simplified scheme of various flame retardant actions of phosphorus is presented in Figure 2.

An additive is considered to be active in condensed phase if it alters the thermal decomposition characteristics of the polymer by a chemical reaction. Hydrolysis, dehydration, chain scission or de-polymerization are some of the main chemical reactions occurring in condensed phase activity. This activity is usually characterized by a reduction in the decomposition temperature of the polymer and increased formation of char residue at elevated temperatures [19].

In some cases, the de-polymerization of thermoplastic polymer chains in the presence of a heat source reduces the viscosity of the system and enables it to retreat from the fire without producing any residue.

The efficiency of phosphorus compounds to change the decomposition and combustion characteristics of polymers makes their fire suppressant use imperative. Depending on the substrate and their chemistry, there could be chemical interactions in the condensed phase at elevated temperatures, which lead to changes in the decomposition pathway of the polymer and possible formation of carbonaceous char residues on the surface of decomposing polymer, hence preventing its further oxidation. In other instances, the phosphorus compounds and some of their decomposed products preferably volatilize from the polymer substrate when heated. These phosphorus species further decompose to release reactive phosphorus species, which then interact with the combustion intermediates in the gas phase as inhibitors. In most cases, such interactions lead to recombination of the H and OH radicals and prevent their oxidation.

The condensed or gas phase activity of phosphorus compounds significantly depends on their structure, as well as on the polymer substrate. For example, in case of natural polymers like cellulose and wool, the phosphorus compounds primarily exhibit condensed phase activity where dehydration of the polymer, leading to the formation of a thermally stable char, is the predominant mechanism. Referring to synthetic polymers containing oxygen and nitrogen atoms in their structure, catalytic hydrolysis of the ester or amide groups by phosphorus acids promotes an enhanced melt dripping and fast shrinkage from flame. As far as olefin-based polymers are considered, the phosphorus compounds mainly act in the gas phase by recombining the key fuel species such as H and OH radicals and preventing their oxidation. Some minor physical effects due to volatilization of phosphorus compounds and dilution of the fuel can also occur.

4. Chemical Phosphorus-Based FRs for Textiles

The following paragraphs will describe the recent advances concerning the use of phosphorus-based chemical products, suitable for conferring flame retardant properties to different fibers and fabrics. In general, Table 1 summarizes the recent findings of P-based flame retardants and their performance on textile fabrics.

4.1. Dioxaphosphorinane Derivatives for PET Fibers

These new flame retardants were designed to exhibit similar performances in their activity to Antiblaze 19®, i.e., a trimethylolpropane methylphosphonate oligomer obtained from the reaction of trimethylolpropane phosphite with dimethyl methylphosphonate [54], employed for poly(ethyleneterephtalate) (PET) fibers. In particular, Negrell-Guirao and co-workers [20] synthesized 2,5-ethyl-5-(allyloxymethyl)-2-oxo-1,3,2-dioxaphosphorinane (1), 2-butyl-5-ethyl-5-(allyloxymethyl)-2-oxo-1,3,2-dioxaphosphorinane (2) and 2-benzyl-5-ethyl-5(allyloxymethyl)-2-oxo-1,3,2-dioxaphosphorinane (3) (Figure 3).

Due to the two pseudo-asymmetric centers for dioxaphosphorinane monomers, compounds (1), (2) and (3) exist as diasterioisomers. However, the radical polymerization of dioxaphosphorinane monomers shows the influence of the presence of a chain transfer agent (CTA) on the efficiency of the radical polymerization reaction. Moreover, dimethyl phosphite can play a role by enhancing the fire retardant efficacy of dioxaphosphorinane derivatives. As the CTA concentration increases, the monomer conversion increases as well and the degree of polymerization decreases (Figure 4).

The products of the polymerization reaction were isolated as mono- and di-adduct, rather than high MW polymers. Furthermore, the flame retardancy of these dioxaphosphorinane derivatives was not assessed: therefore, it was not possible to make a real comparison with Antiblaze 19®.

4.2. UV-Curable Flame Retardants Coatings

Xing and co-workers designed and applied UV-curable flame retardants coatings able to protect cotton fabrics from heat penetration and flame spread [21]. In particular, as shown in Figure 5, tri (acryloyloxyethyl) phosphate (4) and triglycidyl isocyanurate acrylate (5) were synthesized and employed for impregnating cotton fabrics, using 1:1 weight ratio of the two monomers in acetone solution with 0.05 g/mL, 0.10 g/mL and 0.20 g/mL of reactive compounds. The monomers were then cured using 4 wt % of photoinitiator under a UV-lamp.

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