Dorota GŁOWACZ-CZERWONKA ? Faculty of Chemistry, Rzeszów University of Technology, Rzeszów

Abstract:
This paper presents methods for obtaining reactive solvents for melamine. The mechanisms for solubilising melamine in these solvents and the opportunities for applying such solutions to produce solid, filled and foamed (expanded) melamine-formaldehyde-ketone plastics were discussed as well.

Please cite as: CHEMIK 2013, 67, 4, 289-300

Obtaining reactive solvents for melamine
Melamine is a non-toxic and a non-flammable raw material applied in the chemical industry. It is a compound with six functional groups, composed of thermally resistant aromatic s-triazine ring, whose presence makes plastics produced from melamine more thermally resistant. Since there is high nitrogen content in its structure, melamine is more and more often used as an excellent flame retardant [1]. There is not much information published on melamine plastics, except for melamine-formaldehyde resins. It is caused by low solubility of melamine in organic solvents in which the reaction with melamine could take place. Effective solvents for melamine have been searched for many years. If such solvents were found, they would considerably broaden a range of melamine applications and would modify the technology for processing polymers obtained from melamine [2].

The most important solvents for melamine are: water, DMSO and polyhydroxyl alcohols in which it has the highest solubility of 2-5.7 g in diols and of 10.0 g in glycerol in 100 g of solvent, at 140°C [1]. Melamine also has good solubility in formalin at a temperature exceeding 80°C and its solubility is a chemical reaction producing melamine ? formaldehyde resins. For that reason, melamine was considered to have good solubility in so called hydroxymethyl derivatives of ketone, too [2]. Wirpsza conducted the first studies on this issue in 1994 [2÷4]. He found that melamine had good solubility in a reaction product of 1 mole of acetone with 3 moles of formaldehyde, and the produced solution contained 40?50% by weight of solubilised melamine. Melamine is not only physically soluble, but in the solution it also reacts with the obtained product forming a reactive resinous system in which polycondensation occurs at an increased temperature and in the presence of acidic or alkaline catalysts. This reaction results in the formation of a resin and then leads to its gelation and setting [2].

Further studies showed that melamine had good solubility in hydroxymethyl derivatives of aliphatic compounds containing electronegative groups activating C-H bond in ? orientation a [5÷7]. This has led to the discovery of a group of reactive solvents for melamine which are obtained in the alkaline environment as a result of formaldehyde binding to C-H aliphatic bond activated by a strong electronegative group (Y), e.g. -C=O or ?NO2, [2].

The course of reaction and properties of reactive solvents depend on, among other things, a type of the source compound (containing active C-H bonds), molar ratio of that compound to formaldehyde reacting with it, duration and type of reaction and a quantity of catalyst. The reactions for formation of reactive solvents were conducted at temperatures of 40°C and 80°C. The duration of the reaction was considerably reduced at a higher temperature, and the obtained products, in comparison to synthesised products, solubilised melamine more effectively at 40°C [5]. The higher pH of the environment is, the higher the reaction rate of formaldehyde binding to ketone is. Catalysts in the synthesis of reactive solvents for melamine can include, among other things: sodium hydroxide, calcium hydroxide, potassium carbonate and tertiary amines [2, 5]. Sodium hydroxide is the cheapest catalyst and provides the fastest rate of a reaction as it ensures that a high pH level of the environment (ca. 11-12) is kept. It can also catalyse the polycondensation process of hydroxymethyl groups and it catalyses the Cannizzaro reaction which neutralises a base and reduces pH until the medium is completely neutralised:

In practice, calcium hydroxide, potassium carbonate and tertiary amines do not catalyse the Canizzaro reaction and the polycondensation process, but they catalyse binding of formaldehyde and the formation of hydroxymethyl groups occurs slowly in their presence [2]. It is convenient to use calcium hydroxide which can remove calcium ion by precipitation, and then filtrate insoluble sulphate salt or carbonate salt precipitates. On the other hand, such hydroxide has poor solubility in a reactive mixture, and even in water [6]. Potassium chloride, produced by neutralising a catalyst with hydrochloric acid, has to be removed from the post-reaction mixture, which is a drawback of using potassium carbonate. Moreover, the products obtained in the presence of such catalysts as calcium hydroxide or potassium carbonate require a high temperature, even up to 120°C, to solubilise melamine [2].

The drawbacks of using the above mentioned catalysts can be easily avoided by using triethylamine as a catalyst. When the reaction is completed, this catalyst can be easily removed from a post-reaction mixture by distilling it along with water. Triethylamine slightly changes the course of a reaction by facilitating the formation of hemiacetal groups in the products at the expense of the direct reaction between acetone and formaldehyde [5]. The formation of hemiacetals in the presence of triethylamine can be explained by the possibility for deprotonation of a hydroxyl group and the formation of alkoxide anion which is capable of reacting with another formaldehyde molecule:

The presence of hemiacetals is advantageous because, as it was demonstrated in the papers [5; 7÷11], they facilitate melamine solubilisation. Melamine solubility is increased by adding water to a reactive solvent. During solubilisation, added water evaporates from the solution.
During melamine solubilisation in the reactive solvents, the following processes occur [2, 5, 7 ÷ 11]:
? physical solubility of melamine in a reactive solvent (RS)
? formaldehyde dissociation from O-hydroxymethyl group of RS

? addition of amino groups of melamine to formaldehyde and the formation of N-hydroxymethyl groups

? condensation of N-hydroxymethyl groups of melamine and C-hydroxymethyl groups of a reactive solvent

? condensation of N-hydroxymethyl derivatives of melamine with each other

While melamine is solubilised, also its hydroxymethyl derivatives are formed. They are characterised by a higher degree of substitution of hydrogen atoms from ?NH2 groups with hydroxymethyl groups

Melamine solubility in reactive solvents was found to increase as the solubilisation temperature was increasing. At a temperature above 100°C, condensation water or alternatively, water added to a reactive solvent and other volatile compounds evaporated. At a temperature above 120°C, liquid and practically anhydrous solutions of low viscosity were obtained. Solubilised melamine reacted with a reactive solvent and formed a liquid melamine resin which could be subjected to thermosetting or catalytic curing [2, 5].

Further studies were focused on using ketones, other than acetone, to obtain the reactive solvents. The following substances were used as ketones: cyclohexanone, cyclopentanone, methyl ethyl ketone, acetophenone, benzoyloacetone, acethylacetone and biacetyl [12].

The reactive solvents are usually thick and clear liquids, but some of them (particularly higher hydroxymethyl derivatives) after 7-14 days of seasoning become turbid, their density increases and they turn into greasy semi-solid or solid substances which return into their clear state after being heated up to a temperature of 80-90°C [11 ÷ 13]. For the obtained products regarded as reactive solvents, a degree of melamine solubilisation in these products is regarded as their relevance criterion. Melamine solubilisation depends on a type of ketone and hydroxymethyl derivative of ketone as well as on a method of its dosing and water content in a solvent [12]. An amount of solubilised melamine was lower if it was dosed gradually as it was solubilised. Such a phenomenon was caused by the reactions of cyclisation and linear condensation occurring between molecules of the same solvent [12]:

A number of hydroxymethyl groups became smaller when such structures were formed, but melamine solubility was not completely reduced because 1,3-dioxane rings were not very stable at an increased temperature (reaction 11) and they could be decomposed, and then could react with melamine. Cyclic groups additionally enhanced steric hindrance in a molecule of a reactive solvent and made it more difficult for hydroxymethyl derivative of melamine to react with this solvent. After adding water to the system, an increase in melamine solubility was observed [12].

Water added to two samples had a double function ? it solubilised a part of melamine and facilitated the decomposition of hemiacetal groups and the reaction between released formaldehyde and melamine. During heating, a part of added water evaporated from the solution.

It was proposed to use the reactive solvents for melamine to form water-resistant polymer coatings, filled plastics and expanded polyurethane plastics [14].

Water-resistant polymer coatings
Melamine solutions in reactive solvents (RS) obtained from cyclohexanone (CH) and methyl ethyl ketone (MEK) demonstrating the highest capacity for melamine solubilisation, were used to form melamine-formaldehyde-ketone (Mel-F-Ket) coatings [11÷13, 15]. The coatings were obtained as a result of curing melamine solutions in the reactive solvents. Water-resistance of the cured coatings was measured by the loss of plastic mass during its exposure to boiling water adopting as the test criteria the loss of plastic mass exposed to boiling water smaller than 1% [16] and the amount of formaldehyde released from the plastic to boiling water [17, 18]. By using a solution of 80% formic acid acting as a hardening catalyst, coatings from Mel-F-CH solutions were characterised by the best water-resistance (Fig. 1) [17].

Among the coatings obtained from Mel-F-MEK solutions, the ones cured in the presence of acetic acid had the best waterresistance (Fig. 2) [18]. Water left after its adding to a reactive solvent during melamine solubilisation, had a considerable impact on the appearance and properties of plastics. Generally, the larger the amount of water (> 30% by weight) added to the RS during melamine solubilisation, the worse the water resistance becomes. As melamine content was increasing in the solution, water-resistance of plastics was clearly enhanced. Adding more than 60 g of melamine per 100 g of RS did not change the mass of plastics after their exposure to water (Dm = 0%) and did not cause formaldehyde release in boiling water [18].

Moreover, new melamine-formaldehyde-ketone (Mel-F-Ket) plastics are clear and hard in comparison to the typical solid melamineformaldehyde (Mel-F) plastics [17, 18].
Computer methods, that is, a multi-dimensional cluster analysis implemented into SKANKEE system, were also used to predict the properties of polymer coatings. This analysis showed the relations between generally assumed causes, that is, the composition and parameters of the process of obtaining coatings and generally defined results, that is, the properties of obtained samples The computer simulations confirmed that this method could be used to classify coatings [19].

Moulding compounds and melamine-formaldehyde-ketonefilled plastics

Fillers in thermoset molding compounds primary strengthen the obtained plastics and enhance their performance characteristics which depend on a type of the used filler and its uniform distribution. By changing a filler type, the price of moulding compounds, their mechanical properties, the appearance of articles and the processing technology can be affected to a large extent.

Mel-F-Ket moulding compounds belong to composites composed of melamine resin (obtained by melamine solubilisation in RS containing water), cellulose or wood flour as a filler. Melamine solutions in RS obtained by reacting 1 mole of cyclohexanone or methyl ethyl ketone with 7 moles of formaldehyde (7-HMCH or 7-HMMEK) containing 20% by weigth of water, are the main components of moulding compounds. They demonstrate satisfactory strengthening properties [20, 21]. The obtained moulding compounds are characterised by moderate or high plasticity. For Mel-F-Ket moulding compounds with a cellulose filler, the plasticity is 130-156 mm, and for Mel-FCH moulding compounds, this parameter reaches 125-165 mm [22]. Plasticity of Mel-F-Ket moulding compounds with a wood flour filler is 110-150 mm. Their higher thermal resistance in comparison to that of a standard moulding compound determines their application as a potential raw material in the process of obtaining thermosetting plastics with increased thermal resistance [20, 21].

Melamine-formaldehyde-ketone moulding compounds undergo compression moulding which results in producing new filled Mel- F-Ket plastics. Regarding Mel-F-Ket plastics, moulded pieces filled with sulphite cellulose are characterised by greater hardness. The comparison of plastics with respect to a type of ketone used to obtain Mel-F-Ket resin demonstrated that moulded pieces from Mel-F-CH resins had greater hardness (Fig. 3) [20].

Mechanical properties of obtained Mel-F-Ket plastics were compared with a commercial cured melamine-formaldehyde moulding compound known under the trade name Polomel MEC-3, manufactured in ZTS ERG [Plastics Manufacturing Plant ERG] in Pustków, Poland. It is a thermoset moulding compound obtained by saturating cellulose with melamine-formaldehyde resin [20 21].

In comparison to the commercial cured moulding compound Polomel MEC-3, the hardness of this compound was considerably improved. The plastic material contains 30% of cellulose as a filler and its hardness strength is 82.0 MPa, while the hardness of moulded pieces of Mel-F-CH moulding compounds (containing a comparable amount of filler) is 109.4 MPa [16]. Impact strength (3.76 kJ/m2) (Fig .4) and bending strength (73.1 MPa) slightly greater than these of the mentioned moulding compound (3.68 kJ/m2 and 71.6 MPa respectively) were improved in a similar way [20, 21].

Melamine foam plastics expanded with blowing agents

Melamine solutions in RS can be applied to produce melamine foam plastics (MelFP) composed of only 2-3% of polymer, and just 97-98% of expanding gas. MelFP plastics were obtained by expanding anhydrous melamine solutions containing 100% of polymeric substances, in RS. Tris(hydroxymethyl)acetone, that is, a reaction product of 1 mole of acetone and 3 moles of formaldehyde, was exclusively used as a reactive solvent [4]. Such solutions can be moulded both with physical and chemical blowing agents. However, regarding technological and economical issue, the application of chemical blowing agents, e.g. azodicarbonamide (ADN) and sodium hydrocarbonate (NaHCO3) turned out to be more advantageous [3]. A structure of melamine foams expanded with NaHCO3 is less uniform than a structure of expanded ADN, hower their thermal resistance is greater. Foams expanded with ADN have a completely uniform cellular structure and smaller dimensions of cells (0.1-0.3 mm) [23].

A method for manufacturing foams is based on melamine solubilisation in RS at 140°C, cooling the solution to a temperature of ca. 100°C and adding a blowing agent ? ADN, a surfactant (sodium alkylbenzene sulphonate) and a catalyst (weak organic acid) to the solution while stirring it continuously. Then, the reaction mixture was reheated until its expansion started and it was put into a thermostat heating chamber at a temperature of 150-200°C, in which the foam expanded [4]. The obtained foam plastics contained 40-50% of melamine and were characterised by density of 20-140 kg/m3, compressive strength of 0.1-1.0 MPa, regular cellular structure (majority of closed cells) and self-extinguishing properties. During combustion, they released a small amount of fume which differentiated them from the commonly applied foam plastics [23]. Holding MelFP at a temperature of 100-200°C did not reduce its compressive strength but even increased it under specific conditions. Its exposure to a temperature of 150°C resulted in a 3.5-fold increase in compressive strength in comparison to the initial strength [4]. Moreover, MelFP foams demonstrated better water-resistance than urine foams and the majority of their cells were closed. Thus, such foams can provide satisfactory thermal insulation [4].

Melamine foam plastics obtained from two-component systems by using a single-stage method

Melamine foam plastics were also obtained from two-component systems by using a single-stage method. Component A contained polyol constituents (melamine solutions in RS) and adequate additives including: a surfactant (Silicon 5340), a catalyst (triethylamine) and a foaming agent (water). Isocyanate, e.g. MDI ? 4,4?-diphenylmethane diisocyanate was added as the component B to the component A. The mixture was vigorously stirred, and then poured into a mould in which foaming process occurred [24, 25]. For testing purposes, anhydrous reactive solvents obtained by reacting 1 mole of cycloheanone or methyl ethyl ketone with 5 or 7 moles of formaledhyde (5-HMCH, 7-HMCH, 5-HMMEK, 7-HMMEK respectively), were used. Optimal quantities of used raw materials depending on a type of RS are presented in Table 1 [24, 25].

The foams obtained from melamine solutions in 5-HMCH and 7-HMCH were characterised by greater stiffness than the ones obtained from 5-HMMEK and 7-HMMEK. The reason is the structure of original ketones ? a ring in cyclohexanone stiffened the foam structure, while an aliphatic chain in methyl ethyl ketone increased a number of flexible segments [24, 25]. Melamine foam plastics are characterised by apparent density with the maximum value of 54 kg/m3, dimensional stability of 0.0÷0.3% and water absorption within a range of: 0.7-6.4% by volume after 24 hours (Tab.2) [24, 25].

The obtained foam compositions are self-extinguishing. The tests carried out at the temperatures of 150, 175 and 200°C showed that these foams had increased thermal resistance. The foams obtained from melamine solutions in hydroxymethyl derivatives of cyclohexanone had lower mass loss than the foams obtained from hydroxymethyl derivatives of MEK [24, 25]. Depending on the temperature, they demonstrated mass loss of 10-25% by weight for 5-HMCH and 11.6-19% by weight for 7-HMCH (Tab.3.) [24].

The mass loss of the foam was reduced as melamine content in this foam increased. The lowest mass losses. and thus the best thermal resistance, were achieved by solubilising the maximum amount of melamine, at which gelation of the solution hadn?t yet occurred, in RS (Fig. 5).

After their annealing, the tested foams were characterised by greater stiffness and considerably greater compressive strength when exposed to temperatures (Tab. 4).

The foams obtained from melamine solutions in RS prepared from cyclohexanone demonstrated greater compressive strength and thermal resistance. Melamine foam plastics obtained from melamine solutions in RS can be classified as rigid or semi-rigid foams. Compressive strength of 0.05-0.35 MPa and regular pore structure are characteristic for such foams [26]. Compressive strength of foams is different and depends on annealing temperature and a type of used RS. However, the process of annealing foams at the temperatures of 150 and 175°C seems to be the most advantageous as their strength increases in comparison to the unexposed composition. The largest increase in strength was observed for foam compositions obtained from melamine solutions in 5-HMCH (Fig. 6) [24, 25].

Oligoetherols from melamine solutions in reactive solvents

So far, oligoetherols with s-triazine ring in their structure were obtained from the reaction of melamine with the excess of oxiranes [27] or alkylene carbonates [28].

Such oligoetherols were then used to obtain polyurethane foams with increased thermal resistance [29]. Oligoetherols can be obtained from melamine solutions in reactive solvents synthesised from cyclohexanone [30, 31] and methyl ethyl ketone [32]. They were obtained by adding to an autoclave a melamine solution in RS, water (0-20% by weight), propylene oxide and triethylamine as a catalyst. The reaction took place at a temperature of 50-70°C. As a result, clear yellow-orange resins were obtained [31, 32]. Foams obtained from these products were rigid or semirigid. Compressive strength of foam compositions from RS-based oligoetherols is greater than that of foam compositions obtained from melamine solutions in RS and this strength increases as the exposure temperature increases. This parameter is the highest after annealing the compositions from anhydrous oligoetherols at 175°C [30, 31].

Conclusions
The reactions of some ketones with an excess of formaldehyde produce the compounds that can be used as reactive solvents for melamine.
When melamine was added to the reactive solvent, a series of reactions occurred. These reactions led to the solubilisation of melamine and the formation of 100% solution of polymeric substances similar to melamine-formaldehyde-ketone resins.

It is expected that melamine solutions in reactive solvents can be applied in polymer chemistry to produce:
? polymer coatings of high water-resistance
? new moulding compounds and filled melamine-formaldehydeketone plastics
? polyurethane foams (expanded with chemical blowing agents or water in the presence of isocyanates)
? oligoetherols suitable for the production of polyurethane foams with increased thermal resistance

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Dorota GŁOWACZ-CZERWONKA ? Ph.D.,(Eng), graduated from the Faculty of Chemistry at the Rzeszów University of Technology in 1995. She has been working at this university since completing the studies. She defended her doctoral thesis at the Faculty of Chemistry at the Rzeszów University of Technology in 2004. Research interests: synthesis of oligoetherols with s-triazine ring and expanded plastics with increased thermal resistance, modification in structures of some azacyclic compounds to enhance their solubility in organic solvents.
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