Synthesis and degradation profile of cast films of PPG-DMPA-IPDI aqueous polyurethane dispersions based on selective catalysts

Abstract

Waterborne polyurethane (WBU) dispersions synthesized from poly(propylene glycol) (PPG), dimethylolpropionic acid (DMPA), and isophorone diisocyanate (IPDI) with catalysts of different selectivity were prepared via by the conventional prepolymer isocyanate process. Two types of chain extenders were used, ethylene glycol (EG) and propylene glycol (PG), producing polyurethanes. The dispersions were neutralized by the addition of triethylamine. The thermal stability of the materials, obtained as cast films prepared from aqueous dispersions was evaluated by thermogravimetry (TG). It was observed that initial degradation temperatures were above 140 °C, with two-step degradation profiles. The use of a more selective catalyst in the formulations led to materials with higher thermal stability. DTG curves exhibited stages not perceptible in the curves of weight loss, which were mainly influenced by the differences in the formulations. Thermal decomposition of the obtained polyurethanes was followed by TG coupled with FTIR spectroscopy.

Introduction

The continuous reduction in costs and the control of volatile organic compound emissions are increasing the use of aqueous-based resins, motivating the development of polyurethanes dispersed in water. These products present many of the features related to conventional solvent-borne coatings with the advantage of low viscosity at high molecular weight and good applicability [1]. The use of aqueous polyurethane dispersions is steadily expanding in textile coatings [2], sizing, and in adhesives for a number of polymeric materials, glass, etc. Aqueous polyurethane dispersions can be tailor-made and show many of the features of conventional solvent-born coatings and many technological advantages, such as low viscosity and good applicability as coatings for different types of substrates, wood, concrete, leather, metal and some polymers. Polyurethanes obtained from aqueous dispersions have superior properties when compared with similar materials obtained from organic media [1].

The development of aqueous polyurethane applications has been motivated primarily by environmental considerations to reduce solvent emissions. The superior quality of waterborne polyurethanes over solvent-borne polyurethanes, e.g., enhanced mechanical properties and excellent adhesion on many surfaces such as glass and polymeric fibres due to the existence of Columbic forces at the ionic centres, is another important reason for this solvent substitution.

The polyols used in polyurethane dispersions synthesis are of polyethers, polyesters, polycaprolactone, and polycarbonate origin. Polyesters have some advantages over polyethers with respect to strength, oil resistance, etc. Polyols used are either linear or slightly branched. Short chains result in greater number of urethane links and thus different physical properties.

Nearly any type of di- and/or higher functionality isocyanate can be used in polyurethane dispersions. The important point is that the isocyanate must exhibit sufficient stability toward water during processing so that urethane bonds form instead of urea bonds. However, only a few diisocyanates are used commercially, such as toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, etc. Aromatic isocyanates, which were neglected in earlier stages of PU dispersion chemistry are nowadays used in some newly developed techniques. Processing is preferably performed below 10 °C to minimize formation of urea groups. The blocking of isocyanate (NCO) groups at the end of prepolymer molecules with a blocking agent is a useful technique to prevent isocyanate–water reactions during dispersing. The relative reactivity of the NCO group with other groups can be placed in the following order: aliphatic NH₂ > aromatic NH₂ > primary OH > water > secondary OH > tertiary OH > phenolic OH > COOH and, RONHOCOONHOR > ROCOONHOR > RONHOCOOOR [3], [4].

Aliphatic isocyanates improve resistance to discoloration, and to thermal and hydrolytic attack. Aliphatic diisocyanates are less reactive than aromatic ones and they must be used with certain catalysts. Isophorone diisocyanate (IPDI) is the most commonly used aliphatic diisocyanate, due to its unsymmetrical structure. In the dispersion process, water can diffuse into a hard segment domain more easily when the domain has a less ordered structure [4].

Aqueous polyurethanes can be prepared by emulsification of hydrophobic polyurethanes in water with the aid of emulsification agents and protective colloids. However, the emulsification with built-in hydrophilic groups result in better dispersion. This process does not require strong shear forces, yields finer particle size, gives better dispersing stability, and the product has reduced water sensitivity after evaporation of water. Building hydrophilic groups into the backbone means replacing other reactive ingredients (i.e., polyols, isocyanates) by special materials that contain ionic groups, ionic group precursors, or other water-soluble segments in their molecular structure. These units are called hydrophilic monomers or internal emulsifiers. The sulfonate diamines and diols, or dihydroxy carboxylic acids, are the most important compounds. Among the dihydroxy carboxylic acids, DMPA has been frequently used as ionic building block. The advantage of using this compound is related to the steric hindrance of the COOH group so that its interaction with isocyanates is minimized [3], [4].

Depending on the type of neutralization, polyurethane dispersions can be cationic or anionic. Quaternisation at the tertiary nitrogen atom yields cationic urethanes, whereas the neutralization of carboxylic acid groups gives anionic urethanes. The pendent carboxylic acid groups are neutralized with base to form internal-salt containing prepolymers that can be easily dispersed in water. The microphase separation between the incompatible soft- and hard-segment sequences contributes to the unique properties of polyurethane ionomers.

The polyurethane chains with NCO terminating groups can be extended with glycol forming urethane groups. Chain extenders are low molecular weight hydroxyl and amine terminated compounds that play an important role in the morphology of polyurethane fibres, elastomers, adhesives, and certain integral skin and microcellular foams. The elastomeric properties of these materials are derived from the phase separation of the hard and soft copolymer segments of the polymer, such that the urethane hard segment domains serve as cross-links between the amorphous polyether (or polyester) soft segment domains. This phase separation occurs because the mainly non-polar, low melting soft segments are incompatible with the polar, high melting hard segments. The soft segments, which are formed from high molecular weight polyols, are mobile and are normally present in coiled formation, while the hard segments, which are formed from the isocyanate and chain extenders, are stiff and immobile. Because the hard segments are covalently coupled to the soft segments, they inhibit plastic flow of the polymer chains, thus creating elastomeric resiliency. Upon mechanical deformation, a portion of the soft segments is stressed by uncoiling, and the hard segments become aligned in the stress direction. This reorientation of the hard segments and consequent powerful hydrogen bonding contributes to high tensile strength, elongation, and tear resistance values. The choice of chain extender also determines flexural, heat, and chemical resistance properties. The most important chain extenders are ethylene glycol, propylene glycol, 1,4-butanediol (1,4-BDO or BDO), 1,6-hexanediol, cyclohexane dimethanol and hydroquinone bis(2-hydroxyethyl) ether (HQEE). All of these glycols form polyurethanes that phase separate well and form well defined hard segment domains, and are melt processable. This gives advantageous properties to polyurethanes.

One of the inherent drawbacks of this new technology is the formation of carbon dioxide due to the side reaction of isocyanate with water [5]. When an isocyanate reacts with water, the products are a urea linkage (via an amine intermediate) and carbon dioxide. Carbon dioxide formation is problematic in that it causes imperfections in the coating during cure, such as blistering and pin-hole formation. An excess of isocyanate relative to polymer hydroxyl functionality is recommended due to this side reaction. It is quite common to prepare such water-based formulations with a 2.0/1.0 isocyanate to hydroxy-polymer mole ratio.

Side reactions are ubiquitous in urethane chemistry. Depending on whether a catalyst is present, these reactions can be favoured over urethane formation. The incorporation of the side reaction linkages, for example allophanate and isocyanurate, adds branch points, both increases the viscosity of the prepolymer and changes the onset of gelation during cure. These linkages also have different thermal stabilities than the urethane linkage [6].

One of the objectives is to reduce the amount of side reaction of isocyanate with water, which generates carbon dioxide, and can cause imperfections as the formulation dries to a finished coating. This reaction is reduced to a minimum by use of the non-tin catalysts [7]. One novel approach to control the water side reaction is the use of catalysts which selectively catalyze the isocyanate–polyol reaction and not the isocyanate–water reaction.

The relative selectivity (S), can be obtained from Eq. (1), was measured as urethane IR peak area (Purethan)/ urea IR peak area (Purea) ratio, by method Werner Blank [8]:

$$\text{S}={\text{P}}_{\text{uretan}}/{\text{P}}_{\text{urea}}$$

After the integration of characteristic absorption max of urethane (1700 cm⁻¹, 1540 cm⁻¹) and urea (1640 cm⁻¹, 1570 cm⁻¹) was done, the relative selectivity was calculated.

A manganese catalyst, a complex of Mn(III)–diacetylacetonatomaleate with various ligands based on the acetylacetonate and the maleic acid, used in some of the experiments [9], [10], shows a high selectivity for the isocyanate–hydroxyl reaction in comparison to the commercially available zirconium catalyst [8].

Zirconium catalyst is a proprietary zirconium tetra-dionato complex in the reactive solvent with the metal content of 0.4% [11].

Thermogravimetry (TG) is a suitable method to evaluate the thermal properties of several types of polyurethane elastomers. The thermal stability of polyurethanes has been studied extensively because of the great importance of this group of materials [12]. These thermoplastic elastomers generally are not very thermally stable, especially above their softening temperatures [13], and their mechanism of thermal degradation is very complex due to the variety of products formed. It was proposed that the thermal degradation of polyurethane is primarily a depolycondensation, which starts at about 250 °C. Commonly, it presents a bimodal profile where the first mode is related to the hard segments of polyurethanes. Usually, at a low heating rate, the degradation results in differential weight loss (DTG) curves with several peaks, which is an indication of the complexity of the degradation [1].

In this study, several water-based polyurethane dispersions with various formulations were synthesized. The effects of the presence of catalyst, the length of the hard segment and the presence of urethane linkages on the thermal stability of the materials are discussed.