POLYURETHANE DISPERSIONS PROVIDE SUSTAINABLE TECHNOLOGY FOR COMBINING PERFORMANCE WITH THE EASE OF APPLICATION.

Transcription

1 Page 1 (11) POLYURETHAE DISPERSIOS PROVIDE SUSTAIABLE TECHOLOGY FOR COMBIIG PERFORMACE WITH THE EASE OF APPLICATIO. Presentation by Robert C. Jansson, Perstorp R&T&D at the Abrafati conference Sao Paolo, Brazil Sep 35, 2003 ITRODUCTIO Since their introduction some fifty years ago, aqueous polyurethanes have proved to become versatile products for a great variety of applications both in coatings and adhesives, but also in a variety of special treatments in the textile and leather industry. Polyurethane dispersions represent clearly growing markets, with an annual growth rate varying between 8 and 12%, depending on the application. Primarily because of their versatility and toughness, they can be formulated to fit a wide variety of applications. They form a respected and appreciated class of high performance, high value added coating materials. Besides the typical polyurethane performance, polyurethane dispersions provide additional benefits by combining the typical resistance and elastic properties with low VOC content and superior ease of application. This paper focuses on the design, applications and properties of polyurethane dispersions (PUD). A discussion is provided on the impact of formulation parameters on the properties of polyurethane dispersions. Critical preparation steps will be described and the impact of polymer composition on the end properties will be discussed in detail. It will be shown that polyurethane dispersions can be formulated for a range of different properties, suitable for a variety of different applications and for the whole range of elasticity requirements. The products can be formulated as very soft and flexible, suitable for e.g. textile and leather, as moderately elastic for products like wood and plastics or to the other extreme, as hard coatings for metals and mineral surfaces. Aqueous polyurethane dispersions share common composition features, although they can be prepared by several, slightly different methods. This paper focuses on the prepolymer dispersion method, which is a straightforward process, accomplished easily with relatively simple manufacturing equipment. Dispersions can be produced at mild reaction conditions, with temperatures no higher than 80 C needed. Water dispersion of the intermediate prepolymer takes place spontaneously, with only moderate shear forces required. Thus, with careful attention to chemical safety issues, preparation of polyurethane dispersions might provide a straightforward, advantageous mean towards a versatile range of high performance, lowvoc finishes and adhesives. Dispersed colloidal systems require a stabilisation mechanism for providing stability on storage. Stabilisation of the dispersion can be anionic, cationic or nonionic. onionic dispersions suffer often from poor dispersion stabilisation and from permanent water sensitivity. Cationic PUD is possible, but not widely utilised commercially. In this paper we have selected to present examples of anionic PUD formulated with 2,2bis (methylol) propionic acid, as ionic moieties based on carboxylic anions have been shown to provide best options on least water sensitivity left in the dry polymer. Many of the basic features, however, like toughness and elasticity apply to PUD prepared by other types of stabilisation mechanism, as well. Perstorp Specialty Chemicals AB Perstorp Polyols Inc. SE Perstorp 600 Matzinger Road Sweden Toledo Ohio 43612, USA Tel Tel Fax Fax

2 Page 2 (11) FUDAMETALS OF POLYURETHAE DISPERSIOS Aqueous polyurethane dispersions are binary colloidal systems, in which the urethane polymer is dispersed in water as continuous phase. Dispersed polymeric systems show viscosity, which depends on the particle size and volume proportion of the dispersed polymer, but which is insensitive to the variation of molecular weight of the dispersed polymer. In the formulation of one component coatings advantage can thus be taken from the resistance properties resulting from high molecular weight, without suffering from extensive solvent content or extremely high viscosity. As a consequence, dispersion technology enables preparation of high performance, one component dispersions with low viscosity and comprising simultaneously both low VOC content and high molecular weight. Dispersed colloidal systems require a stabilisation mechanism for providing stability on storage. Coupling the ionic groups in the polymer backbone provides means of preparing stable dispersions without the use of any external tensides. The internal stabilisation used in PUD formulations provides migration free products, which, in many cases, show such a low tendency of foaming, that normally no defoamer is needed in the formulations. PREPARATIO Preparation of anionic polyurethane dispersions can be accomplished by means of two slightly different methods, the prepolymer mixing and the acetone process. Few other processes exist, and these are of far minor commercial interest. All the above mentioned processes utilise basically the same components for building up the urethane interlinked and waterdispersed macromolecular product. In the later lines we shall focus in more detail on the prepolymer mixing process, which is a versatile method with several advantages: o expensive special equipment required Mild reaction conditions Little sensitivity against formulation changes Enables preparation of cross linked dispersions PUD comprises a dispersion of a urethane polymer, which is prepared from a mixture of an oligomeric diol and a product containing an ionisable group, in our case 2,2bis (methylol)propionic acid (BisMPA). The lateral carboxylic groups of BisMPA are neutralised for water dispersibility and the polymer is chainextended for the appropriate increase of molecular weight. Preparation of PUD by the prepolymer mixing method is conveniently accomplished in the four successive steps, illustrated in fig 2: 1. Preparation of (a solution of) an isocyanatefunctional urethane prepolymer 2. eutralisation of the carboxyls 3. Dispersion of the product in water 4. Chain extension of the dispersed polymer The process is based on the reaction of isocyanates with hydroxyl groups, resulting in formation of a urethane linkage between the reacted groups. Primary hydroxyls react readily with isocyanates, whereas the hindered carboxylic groups of BisMPA remain unreacted. Dispersion of the isocyanateterminated prepolymer in water involves a danger of undesired sidereaction of the isocyanate with water. Fortunately, however, the reaction with water is rather slow at the temperatures applied for the dispersion process. The reaction of isocyanate with amine proceeds much faster. Consequently, sudden addition of amine provides practically instant chainextension within the dispersed polymer particles. The most essential reactions involved in PUD preparation are shown in fig 3. The prepolymer mixing process described above leaves a lot of opportunities for tailoring the properties of the end product according to specific desires. Products with different molecular geometry, molecular weights and chainlength can be selected, but also the ratio of the components can be modified within broad limits. Basic variation options in PUD composition are presented schematically in table 5.

3 Page 3 (11) IOIC MOIETY FOR AIOIC DISPERSIOS 2,2bis(methylol) propionic acid (later abbreviated BisMPA) is a versatile resin building block with a molecular structure containing two primary hydroxyls and a tertiary carboxylic acid group. Incorporation of BisMPA in the polymer backbone forms the basis of anionic polyurethane dispersions. Incorporation is accomplished by the reaction of the two hydroxyls with diisocyanate. The primary hydroxyls of BisMPA react readily with isocyanates and will be linked by urethane bond to the backbone of the isocyanate. The carboxyl group is sterically hindered and will not react with isocyanate under similar, mild reaction conditions. Consequently the carboxylic acid group remains unrelated and can either be neutralised with base forming ionic moieties for anionic stabilisation of the dispersion or, alternatively, may be utilised for further reactions e.g. with crosslinking agents. The molecular structure of BisMPA molecule is shown in fig. 4. Theoretical and practical properties of the substance are shown in table 1. Besides the PUD application discussed in this paper, BisMPA is used in a variety of other industrial applications, such as: Waterborne alkyds and polyesters Powdercoatings Epoxy esters Electrodeposition primers Rheology additives for aqueous coatings Polymer modifiers Fibre, textile and paper sizing A EXAMPLE OF PUD FORMULATIO In the following procedure a practical example of PUD formulation is shown. The dispersion was successfully prepared by the prepolymer mixing process. Resulting dispersion showed storage stability for more than two years. Film cast from the dispersion generates rapidly substantial tensile strength and high elasticity in the dry film. Composition and properties of the dispersion are shown in table 2. Preparation method utilised is described in the lines below: Procedure for PUD preparation (prepolymer mixing process, table 2): I Preparation of the prepolymer: The prepolymer is formed by reacting an excess of isocyanate with the mixture of polyols and dimethylolpropionic acid. The diisocyanate (1) is charged first to the preheated reactor (55 C), purged with dry nitrogen. Dimethylolpropionic acid (3) is predissolved in Methylpyrrolidone (MP) (4) and added under constant stirring to the reaction mixture as a premix with the polymeric diol (2). Care is be taken during the addition in order to control that the liberated reaction enthalpy does not result in the temperature increasing above 80 C. Maintaining the reaction temperature at 80 C, the reaction is allowed to proceed until a constant COvalue is reached (determined by dibutylamine back titration). (ote: Some diisocyanates may require higher reaction temperatures for successful prepolymerisation) II eutralisation of the prepolymer: The carboxylic groups of the prepolymer are neutralised by the base (5). The prepolymer is conveniently neutralised at the last stage of prepolymer preparation. The degree of carboxylic neutralisation is controlled to 80% of the theoretical. (ote: eutralisation degree may be selected differently according to the application. Very small levels do not provide adequate dispersion stabilisation.)

4 Page 4 (11) III Dispersion and chain extension A part of required amount of water (7) is charged in a container, equipped with a powerful mixer. Stirring is started at ambient temperature. The neutralised prepolymer is added while maintaining constant mixing. The rest of the water is added once the viscosity starts to increase, indicating a formation of colloidal dispersion. The rest of the prepolymer water is then added. Chain extension is performed immediately after a homogenous mixture is formed. Chain extension is accomplished by adding the diamine (6) as 1050% water solution to the constantly stirred dispersion. The process of chain extension is fast. A mixing period of ca ½ hour was used to ensure complete reaction and as a consequence, formation of a stabile dispersion with milky appearance. The product dries at ambient conditions and forms a homogenous, transparent, firm film. SAFETY ISSUES The hazardous nature of some of the components involved must be appropriately recognised and, as a consequence, careful attention needs to be paid to the safety measures during PUD production. Safe handling of monomeric isocyanates belongs without any doubt to the key challenges of qualified PUD production. Monomeric isocyanates are known for their toxicity and sensitisation potential. Yet isocyanate based products can be produced safely by paying careful attention to the storing, dosing and manufacturing equipment in order to efficiently prevent humans from becoming in contact with the isocyanates or vapours emerging from the production. In case of planning new production, handling of hazardous rawmaterials involved monomeric isocyanates, amines, solvent etc. might give rise to authorisation required by the local authorities. Once the final polyurethane dispersion is formed, the product is completely reacted and does not contain free isocyanate. PUD product as such is not harmful. Optional need for classification and risk labelling of the PUD product is dominated by the presence of optional solvent and free amine components. APPLICATIOS OF POLYURETHAE DISPERSIOS Polyurethane dispersions provide for many applications a versatile means of combining recognised polyurethane property with low VOC and the ease of application. A list of typical properties obtained with PUD is shown in table 4. Leather and textile applications of PUD are based on the high elasticity obtained with PUD. Both applications take additional advantage of the durability and good weathering properties of PUD, too. Elasticity is important for coating on wood and plastics, as well. Resistance towards weathering is also an important aspect for these applications. Good wear and abrasion resistance of PUD is important for parquet and other floor coating applications. Weathering resistance of PUD is successfully utilised in coating for windowframes. Adhesive applications of PUD take advantage of the polar nature of the urethane polymer. Adhesion combined with elasticity provides PUD with good options in formulating adhesives for flexible foils and films. Barrier properties of PUD can be utilised for constructing multilayer foils with barrier properties for flexible packaging. Besides the above discussed cases, PUD finds commercial use in several other applications. PUD is also used as sizing agent for textile, fibre and paper treatments. Various elastomeric coatings, adhesives and printing inks are currently formulated from PUD. Further application options are constantly invented by incorporating specific groups into the PUD backbone to give specific properties as durability, reactivity or protection characteristics.

5 Page 5 (11) STRUCTURE PROPERTY RELATIOSHIP The final film properties are influenced by the selection of the formulation variables, as shown in table 5. In the following, a short discussion is provided on the impact of component selection on the dispersion performance. Typical PUD formulation technology comprises the basic components: Hydroxyl functional soft segment, ionic moiety, isocyanate, chain extender and water as dispersion medium. Optionally also other components, such as low molecular weight polyols or solvent can be employed. A lot of opportunities exist to tailor the properties of the end product. Products with different molecular geometry, molecular weights and chainlength may be selected, but also the ratio of the components can be modified within broad limits. A summary of typical PUD composition, along with short descriptions on the impact of formulation variables is shown in table 5. Great differences in the properties of resulting film can be obtained by careful selection of the constitutional components. Few examples will be discussed further below: Tailoring the soft segment The chain length (molecular weight) of the soft segment contributes to the elasticity of the resulting polymer. Consequently, an increase of the soft segment molecular weight provides soft, elastomeric products. In case high tensile strength is favoured for the elasticity, low molecular weight polyols provide coatings with less elasticity but considerable hardness. The type of the soft segment influences the properties, too. E.g. polyethylene glycol soft segment enables films with high water vapour permeation rate, whereas polyesters with hydrophobic constituents may be used for applications calling for moisture barrier properties. Further differences in the impact of polyether and polyester building blocks are shown in table 4. Effect of neutralisation degree eutralisation converts the carboxyls to corresponding carboxylate anions. Appropriate neutralisation is required for the dispersion stability. Complete neutralisation provides dispersions with smallest possible particle size. In cases where the carboxyl groups are not completely neutralised, the unreacted carboxyls can be used to improve adhesion and as reaction sites, for crosslinking or additional modification. Incorporation of ingredients for special features The process of PUD preparation is based on the reaction of isocyanates with organic compounds containing hydroxyl or amino groups. Thus the process provides straight forward options on incorporating other functional elements to the polymer backbone, in case they contain appropriate amine or hydroxyl functionality in their molecular structure. As an example, incorporating hydroxyacrylates can be used as a means to provide unsaturation for the dispersion. COCLUSIOS Polyurethane dispersions (PUD) present a sustainable technology for a variety of coatings, adhesives leather, textile and fibre treatment applications. It has been demonstrated that the prepolymer mixing process is a straightforward process for the preparation of polyurethane dispersions. The 2,2bis(methylol) propionic acid provides a favourable practical means of linking an anionic moiety into the polymer backbone. Preparation of PUD is a versatile process permitting great variations in the formulations and consequently providing means to prepare products with a variety of elasticity and resistance properties. The onecomponent PUD products formed are easy to apply and they combine the features of low VOC content with the great performance of the resulting polymer films.

6 Page 6 (11) REFERECES Polyurethane Handbook, 2 nd Ed., Oertel, G., (Ed.), Carl Hanser Verlag, Waterborne Solvent Based Surface Coating Resins and their Applications, Volume III, Polyurethanes, Thomas, P., (Ed.), John Wiley &Sons and SITA technology Ltd., London, Progress in Organic Coatings, 9 (1981), , Dietrich, D., Aqueous Emulsions, Dispersions and Solutions of Polyurethanes; Synthesis and Properties. COO H CO Water Polyurethane (with neutralised carboxyls) Anionic polyurethane dispersion Figure 1. Dispersing urethane polymer in water is accomplished by aid of the stabilising effect, provided by the polymer linked carboxyl anions. HO OH + HO OH + OC CO 1. OC H COO CO COOH Macrodiol 2,2bis(methylol) Diisocyanate Urethane propionicacid prepolymer 2. RX Base COO (aq.) Dispersion HCO H H 2 H 2 Diamine 4. COO (aq.) Dispersion CO Water 3. OC COO HRX+ CO Anioinic poly(urethaneurea) dispersion eutralised prepolymer Figure 2. Stepwise reaction scheme for the preparation of aqueous polyurethane dispersions. (1) Preparation of urethane prepolymer. (2) eutralisation of the prepolymer. (3) Dispersion of the prepolymer in water. (4) Chain extension of the polymer dispersion in the water.

7 Page 7 (11) A. CO+ HO H O C O isocyanate alcohol urethane O B. CO + H 2 H C isocyanate amine urea C. CO + isocyanate HOH water H O C OH [ ] H 2 + CO2 amine carbon dioxide O D. CO + H H C isocyanate (branching) Figure 3. Important reactions involved in the preparation of polyurethane dispersions: A). Formation of urethane bond. B) Formation of urea bond. C) Sidereaction with water. D) Reaction of isocyanate with urethane polymer. HO OH COOH Figure 4. 2,2bis(methylol) propionic acid (BisMPA). BisMPA is utilised in PUD preparation as a difunctional compound, capable of reacting with isocyanate by the two primary hydroxyls. BisMPA introduces the ionic centres to the urethane polymer. When neutralised by a base, the carboxyls of BisMPA form hydrophilic sites along the polymer backbone, providing anionic stabilisation for PUD.

8 Page 8 (11) Table 1. Typical properties of 2,2bis(methylol)propionic acid Parameter Value Unit Theoretical values Physical form Colourless crystalline solid Molecular weight g/mol Density 1.37 kg/dm 3 Hydroxyl equivalent weight 67 g/eq. Hydroxyl contents 25.4 wt% Acid value 418 mgkoh/g OHvalue 836 mgkoh/g Melting point 190 C (final) Commercial product *) Appearance White, crystalline powder Purity 99 % Hydroxyl value mgkoh/g Acid equivalent weight g/eq. Acid value mgkoh/g Melting point 180 C (final) Solubility in Water 11 g/100g solvent MP 41 Acetone 2.3 Toluene insoluble Registry status On TSCA, DSL, EIECS, AICS, ECL, PICCS. *) Perstorp AB, Sweden

9 Page 9 (11) Table 2. PUD starting point formulation o. Component Role g Mwt f mol ote 1. Desmodur W a) 0.52 Prepolymer Diisocyanate CO:OH = 2:1 2. Polymeric diol 0.13 Experimental PGadipate, OHV 86 Polyester adipate BisMPA b) 2OH 0.13 Ionic centre DPMA:polyester=1:1 1acid wt% for the polymer MP Solvent 87.5 solids % of the carboxyl Triethylamine Base equivalents 6. mol%% of the pendant Ethylenediamine Chain extender isocyanate groups 7. wt% of the final Water Dispersion medium dispersion Total 1000 g Prepolymer 5.5 % Supplier: a) Bayer Properties Viscosity (70 C) 2.99 Pas b) Perstorp Dispersion Solids 35 % Properties Cosolvent 8.8 wt% PH 9.2 Film properties Film hardness (Koenig pendulum) 86 Sec Elongation at break 201 % Table 3. Typical properties obtained by aqueous polyurethane Property High elasticity over a wide hardness range Good weathering resistance Good resistance to oil, grease and many solvents Excellent wear resistance Good adhesion to a variety of substrates Low (or no) content of VOC o external emulsifiers required ormally no defoamer necessary Table 4. Impact of polyether and polyester soft segment on PUD properties Property Polyester Polyether Wear resistance + Mechanical properties + Low temperature flexibility + Hydrolytic resistance + Heat ageing + Weather resistance, UV stability + Microbe and fungus resistance + Swelling in oil, grease, solvents + + favourable unfavourable

10 Page 10 (11) Table 5. PUD components, formulation variables, their functions and effects on the end properties. Component Function Variable Example Advantages Polymeric diol Mn ,2,bis(methylol) propionic acid Diisocyanate Base Soft segment Polyester Polyether Other polymeric diol or polyol Polyester adipate Polycaprolactone Polycarbonate PEG, PPG, PTMG Poly(hydroxyacrylate), polybutadiene glycol, α,ωbishydroxypolydimetylsiloxane Low Mwt of the diol contributes to higher rigidity, tensile strength and low elongation. High Mwt provides softer films with low modulus and high elongation. Incorporation of diols or of low Mwt polyols increase strength and hardness. A portion of trifunctional polyol may be added if inter particle crosslinking is desired. Ionic centre Abrasion resistance, weathering resistance, solvent resistance. Hydrolytic resistance, high rate of water diffusion. Hybrid dispersions, special formulations, extreme hydrolytic resistance. The amount of BisMPA can be varied in the formulation within the limits of 1:2 2:1 mol/mol, in relation to the polymeric diol. Typically 1:1 molar amount is used. Formulations with up to 2:1 may be used for special purposes in order to increase adhesion, but such an amount is prone to reduce film appearance and induce water sensitivity to the dry film. Lower amount of BisMPA may provide advantages in improved water resistance. Formulations with very low amount of BisMPA do not provide adequate dispersion stabilisation, and ratios below 0.5 to 1 should thus be evaluated very carefully for dispersion stabilisation. Elasticity, weathering Aliphatic isocyanate HDI resistance IPDI Cycloaliphatic isocyanate Weathering resistance Reactive component H12MDI Araliphatic isocyanate TMXDI Elasticity Aromatic isocyanate MDI, TDI, Strength, lower price Typically added from 1.4 to 2.0 times the equivalents of hydroxyl components of the polyols and BisMPA. Cycloaliphatic diisocyanates are preferred choice for combining good reactivity and weathering resistance. Aromatic diisocyanates react rapidly already at the water dispersion and may prove to be difficult to handle. They promote good rigidity in the film. Aliphatic diisocyanates are sluggish to react and require either catalysis or elevated temperatures for completing the prepolymer reaction. eutralising agent Tertiary amine Inorganic base TEA, DMAE, DMAMP aoh, KOH

11 Page 11 (11) eutralisation degree can be varied with broad limits, ranging from 40% to over 100%. Decreasing BisMPA content calls for the higher level of neutralisation. Increased neutralisation decreases the particle size of resulting dispersion but increases water sensitivity. oncomplete neutralisation leaves carboxylic groups for adhesion promotion, further reactions or crosslinking. Water Solvent Chain extender (optional) Reaction partner for linking the isocyanate groups Diamine Polyamine (Glycol) EDA, hydrazine Amine addition is controlled by the CO content of the prepolymer. Addition is related to react 85100% of the CO groups with amine. Diamines react rapidly with the isocyanate thus increasing the polymer size by forming urea linkages. Glycols react much slower with the isocyanate and are thus applicable mainly in the acetone process, where the polymer is formed under nonaqueous conditions. Polyamines of higher functionality can be used if inter particle crosslinking is desired. Prepolymer viscosity reduction MP DMF Acetone Good solvency power Can be distilled off The solvent reducer the viscosity of the prepolymer and makes the handling much easier. Typically the prepolymer process is performed with 80% polymer solids in MP. The VOC content of the formulation is increased by addition of solvent, yet far less than the by using conventional, solventborne polyurethanes. Acetone or other easily evaporating solvents can be stripped from the formulation by treatment in vacuum, if totally solventfree formulations are desired. Dispersion medium A safe dispersion medium, insensitive to viscosity effects due to high molecular weight of the dispersed polymer. Solid content of finalised waterdispersion ranges typically between 30 and 40%wt.