Tris(hydroxydietylene)-2-hydroxypropane-1,2,3-tricarboxylate for rigid PUR-PIR foams

2.2.1 Preparation of new compound (THT)

THT was synthesized by esterification reaction by the solvent method. The alcohol component was DEG and the acid component was 2-hydroxy-1,2,3-propanetricarboxyl acid.

Esterification was conducted in three-neck glass flask (volume of 500 ml) equipped with a reflux condenser, stirrer and Dean-Stark’s head. The reaction was performed in xylene medium. The released water was collected in the head. The mixture consisting of 2-hydroxy-1,2,3-propanetricarboxyl acid (96 g) and glycol DEG (159 g) was poured into the flask. The flask was heated in an electric bath until acid dissolution. Citric acid was dissolved at a temperature of about 85°C during 20 min. The time of reaction was counted from the moment when the mixture in the flask began to boil (about 125°C). Temperature of reaction increased within the range from 128°C to 156°C. Parameters of esterification are presented in Table 1.

THT was synthesized from 1 mole (96 g) of 2-hydroxy-1,2,3-propanetricarboxyl acid (citric) and 3 moles (135 g) of DEG [Eq. (I)]:

Every 30 min, the quantity of liberated water was measured. Every 30 min reactions were controlled by determination of acid number and pH of the product obtained. The product delaminated into two layers: the upper was xylene and the lower was polyol. Xylene was distilled off to Dean-Stark’s head. Residual xylene was evaporated in a vacuum drier. Properties of the obtained compound were determined.

In order to examine the usefulness of the newly obtained compound for synthesis of PUR-PIR foams, their properties were determined according to the obligatory standards.

Very important in the case of using the polyurethane polyol is the knowledge of their content of water. Water is added as a blowing agent in the foam. Excess water can cause cracking of the foam cells or even a fall. The water content was tested by Karl Fisher (PN-81/C-04959), in which the solvent used was a mixture of methanol and carbon tetrachloride in a 1:3 ratio. The titration reagent was Combo. Marking is to dissolve the appropriate test portion of the product in Titraqual (titrant for titration) and potentiometric titration of the solution to the equivalence point. The scope of jurisdiction of polyol was also included to determine its technological features, such as hydroxyl number (WT/06/07/PURINOVA), acid number (WT/06/07/PURINOVA) and the viscosity at a temperature of 25°C (EN ISO 12058-1).

The hydroxyl number was determined according to the standard of PURINOVA, Bydgoszcz. Into the conical flask was weighed 250 ml about 1 g of the product to an accuracy of 0.0002 g, loaded with a magnetic stirrer. Using pipette, 25 ml of the catalyst solution was added and it was stirred with a magnetic stirrer for at least 5 min to dissolve the sample. All was mixed by hand to rinse the walls of the flask and add 10 ml of a mixture of acylation. The contents were stirred with a magnetic stirrer for 15 min and 10 ml of distilled water was added. The flask contents were shuffled by hand and then for 5 min with a magnetic stirrer. After this time, 50 ml of acetone was added and the sides of the flask rinsed. Five drops of indicator solution were added and titrated with a solution of potassium hydroxide to the first change of color to blue. The hydroxyl number expressed in mg KOH/g are calculated according to the equation:

LOH=(V1-V2)⋅56.11m+Lk

where: V₁ is volume of the solution of potassium hydroxide C_KOH=1000 mol/l used for titration of the blank; V₂ is volume of the solution of potassium hydroxide C_KOH=1000 mol/l used for titration of the sample; 56.11 is the number of mg of potassium hydroxide equivalent to 1 ml of a solution of potassium hydroxide having a concentration of C_KOH=1000 ml/l; m is weighted in quantity of the product in grams (g); and L_k is the acid number of the sample. The result of the arithmetic mean was adopted for two studies and differed from each other by no more than 3 mg KOH/g.

Similarly, the acid number was determined. In an Erlenmeyer flask, 100 cm³ 1.0 g of sample was weighed to the nearest 0.001 g and 20 cm³ solvent was added from a burette. The flask contents were stirred with a magnetic stirrer to dissolve the sample. Phenolphthalein (3–5 drops) was then added and titrated with 0.5 mol alcoholic solution of KOH to the first pink color of the solution. In parallel, a blank test was performed. The acid number was calculated according to the equation:

Lk=(V1-V2)⋅M⋅56.1m, mgKOH/g

where: V₁ is the volume of alcoholic KOH solution used for titration of the sample (cm³); V₂ is the volume of alcoholic KOH solution used for titration of the blank test (cm³); m is the mass of the sample (g); and M is KOH solution molarity.

The viscosity was determined using a Hoeppler viscometer. The dynamic viscosity was calculated according to equation: η=t·(p1-p2)·K·F in which η is dynamic viscosity, (m·Pa·s); t is time of descent from top to bottom annular mark (s); p1 is density of ball, g/cm³; p2 is density of fluid, g/cm³; K is ball constant, mPa·cm³/g; F is multipilcator working angle. Used ball number 5 with the following data: diameter=14.002 mm, weight=11.071 g, density=7.7019, has balls: the upper limit was 6.790 and the lower limit was 6.789.

The density of polyols in the pycnometer was measured at 25°C (298 K) according to PN-92/C-04504. This depends on the hydroxyl number of the polyol used in the foam and the polyisocyanate necessary to produce a foam. Density and viscosity are very important during the processing process. The pH of polyols was also measured using the microcomputer CP-551 pH-meter. Organoleptically rated color of THT was milk, bright yellow.

To assess the chemical structure of the polyols, a spectrophotometer Vector of Brücker was used. Foam analysis by IR spectroscopy using the KBr technique was performed in the range of 400 cm^-1–4000 cm^-1.

Presence of citric acid in the obtained product was determined by dissolving 0.5 g of its sample in 5 ml of water and then, the solution was neutralized with 1 mole NaOH and 10 ml of CaCl₂ was added. The mixture was heated to boiling point. As a result of the reaction, a white precipitate was formed, which testified the presence of citric acid.

Qualitative analysis of THT was performed by gas chromatography with flame ionization detection (GC-FID). Analyses were performed by the use of chromatograph (USA TRACE 2200) equipped with a capillary column with a length of 30 m and diameter of 0.32 mm×0.25 μm, 007-5-30W DB5 and detector: GC-FID (gas chromatography flame ionization detector).

2.2.2 Preparation of PUR-PIR foams

The foams were prepared in laboratory scale by a one-stage method from a two-component system at equivalent ratio of NCO to OH groups equal to 3:1. Component A was obtained by thorough mixing (stirrer rotation speed of 1800 rpm, mixing time 10 s) of the appropriate amount of Rokopol RF551, the new polyol, catalysts (catalyst 12; 6.4 g, 2.1% ww and Dabco 2.76 g, 0.9% ww.), flame retardant (Antiblaze TMCP; 46.09 g, 15% ww), surfactant (Silicone L6900; 4.6 g, 1.5% ww) and blowing agent (water). The amount of water was reduced by content of water in the new compound THT; component B was Ongromat 30-20 (added in amount 250.67 g, 3,7R).

Both components were mixed (1800 rpm, 10 s mixing time) in appropriate mass ratio and poured into a metal open rectangular mold with dimensions of 195 mm× 195 mm×240 mm. A series, P2.1–P2.5, of foam were obtained with participation of THT. The newly prepared compound was used to foam composition in amount from 0.1 to 0.5 of chemical equivalent with respect to the amount of Rokopol-RF 551, whose content was reduced from 0.9 to 0.5 of chemical equivalent (Table 2). During the synthesis of PUR-PIR foams, the process of foaming reaction was monitored and measured using a stopwatch at appropriate technological times: start time, rise time, gel time. The start time was the time measured by a stopwatch from mixing all the components until the so-called “state of cream”. This is shown by starting to increase the volume of the foam. The rise time was the time measured by a stopwatch from mixing all components of the foam until the maximum volume of the foam. The gel time was the time measured by a stopwatch from mixing all foam components, until the free surface of the foam stops sticking to a clean glass rod.

The foams obtained, after removing them from the mold, were thermostated for 4 h at a temperature of 120°C. Then, they were seasoned for 48 h at a temperature of 20±4°C, cut into pieces and basic properties were determined according to valid standards.

It was noted that there is a close relationship between the properties of polyurethanes and their physical and chemical properties. Research directions of rigid polyurethane foams due to their properties: physical (density, water absorption, flammability), mechanical (compressive strength, brittleness) and electrical equipment (thermal conductivity).

The apparent density of the foams tested was determined using a sample in the form of a cube of side 50 mm according to ISO 845-1988, and the weight ratio of the foam to its geometric volume. Before cutting samples for determination of apparent density, foams were aged for 24 h at room temperature. The samples were then cut to the nearest 0.1 mm and weighed to the nearest 0.1 g.

Determination of water absorbing capacity was performed according to the standard: DIN 53433. The determination method consists of measuring the strength of buoyancy of a sample measuring 150 mm×150 mm×25 mm. They were immersed in distilled water for 24 h. This method applies to any rigid, porous materials, which doesn’t react with water. Water absorption is the ratio of the volume of absorbed water to the original volume of the sample expressed as a percentage.

The structure of foams was determined using an optical microscope, Axiotech Carl Zeiss, Hal 100, which simultaneously transmitted and reflected light (magnification 5×). Thermal resistance was measured by a derivatograph operating in the Paulik-Paulik Erdey system, MOM – Budapest). Thermal resistance of the foams was determined under dynamic conditions in air with a heating rate of 5°C/min, at a temperature from 20°C to 800°C.

The content of closed cells was determined according to the standard ISO 4590:1994 method II, using the foam test sample dimensions of 100 mm×30 mm×30 mm. This method involves the determination of the relative pressure drop, previously calibrated for volume models, and the difference in readings on the pressure gauge, one arm of which is open to the atmosphere. This measurement method is designed for the determination of the percentage of closed cells in rigid porous plastics.

Flammability was performed using a thermal imaging camera Vigo V-20E2-25 equipped with a thermoelectrically cooled HgCdTe detector.

The second method was a simplified chimney flammability test (test vertical – Butler) according to ASTM D3014-73. The apparatus used for the flammability test of the vertical test consists of a vertical column with dimensions 300 mm×57 mm×54 mm of which three walls are made of sheet metal, and the fourth is a movable window. The assay was performed on six samples with dimensions of 150 mm×19 mm×19 mm. Before combustion, the sample was weighed to an accuracy of 0.0001 mm, and then placed inside the chimney. Established glass and the sample was applied to the flame of the burner fueled by propane-butane at the time of 10 s. Then, the torch was moved away and a stopwatch measured the time of free samples of smoking and retention (residue after burning) in the vertical test. The retention was calculated according to the equation:

Re=mm0⋅100%

where R_e is retention; m₀ is mass of the sample before burning (g); and m is mass of the sample after ignition (g).

The chemical structure of the foams was determined by IR spectroscopy (Vector spectrophotometer, Brücker).

Compressive strength was performed with the Tira Test 2200 tensile testing machine, standard ISO 844:1993: DIN 53420. In the foams, skins and sides of the sample were cut in the shape of cubes with an edge length of a side of 50±1 mm. Then the samples were subjected to 10% compressive strain in the direction of foam rise.

Brittleness was determined in accordance with ASTM C-421-61. On the basis of quoted standards, friability is calculated as the percentage loss of foam blocks 12, (cubes having sides of 25 mm) when tested in a standardized unit, in relation to the initial weight. The instrument used to test the fragility of polyurethane foam is a cubic box made of oak, with dimensions 190 mm× 197 mm×197 mm, rotating around an axis at a speed of 60 rpm. Filling the box are 24 blocks with dimensions of 20 mm×20 mm×20 mm oak.

Weight loss in % foam, a measure of its fragility, is determined by the equation:

K=m1-m2m1⋅100%

where m₁ is the mass of the samples before the test (g); and m₂ is the mass of the samples after the test (g).

A softening point was determined using a sample in the form of a cube of sides 20 mm, in the direction of foam rise, according to DIN 53424. The foam samples were subjected to a compressive load of 24.52 kPa at a temperature of 50°C/h. For the softening point, the temperature at which the compression of the foam sample of 2 mm.

The thermal conductivity of foams was determined by examining the coefficient of thermal conductivity. The study sample was subjected to foam dimensions: 200 mm×200 mm×25 mm. To carry out these studies, the camera FOX 200 from Lasercomp was used. It allows the determination of the values in the range of 20–100 mW/(mK). The method of measurement is to determine the amount of heat flowing through the material per unit of time during a given heat flow at a constant temperature difference on opposite sides of the test sample material.