The current solvent-based polyurethane adhesive to, but the organic solvents flammable, explosive, volatile, smell great, use of air pollution, has some toxicity. With the increasingly strict environmental regulations, the development trend of the world's synthetic adhesive outstanding performance for environmental protection and high performance. Water-soluble polyurethane adhesive for its excellent overall performance, in all kinds of water-soluble adhesive is unique, in recent years widespread glue stick attention at home and abroad, in particular the development of modified water-soluble polyurethane adhesive research has become a hot topic .
A water-soluble polyurethane adhesive performance characteristics
Compared with solvent-based polyurethane adhesive, water-soluble polyurethane adhesive has no solvents, no smell, no pollution, good film, strong adhesive and easy to emulsion polymer blending is conducive to modification advantages to the 90's have gradually bonding in automotive interiors, kitchen, furniture adhesive PVC decorative films and laminated films and soles, the upper adhesive applied. However, there are some shortcomings which limited its application.
(1) small viscosity. The viscosity of water-soluble polyurethane and polyurethane general the molecular weight and solid content has nothing to do, can be water-soluble thickening agent and water to adjust, plus the polymer thickener in addition to, the impact of water-soluble polyurethane adhesive viscosity of the important factors There are ion charge, structure and size of such particles. Polymer molecular ion and counter ion on the more, the greater the viscosity, the same solid content, the viscosity of water-soluble adhesive viscosity than solvent-based adhesives small.
(2) slow drying. As the water volatile organic solvents than the poor, it is slow drying water-soluble polyurethane adhesive, and because the surface tension of water is about 7.3 × 10-4N/cm, as the organic solvent is usually 3 times, based on surface hydrophobicity material wetting ability, when most of the water is not from the adhesive layer, evaporate into the air or be absorbed on the porous substrate sudden drying layer is difficult to get continuity.
(Three) water resistance, heat resistance is poor. Has been commercialized in the most water-soluble polyurethane is a linear thermoplastic polyurethane, and the molecular structure contains a hydrophilic group or side access of the ion groups, and sometimes also contain water-soluble thickening agent, dry film can not form to a certain extent cross-linking, so its water proof, heat resistance, solvent resistance, chemical resistance and film strength properties are poor.
2 modified soluble factors Polyurethane Adhesive
Modified water-soluble polyurethane adhesive to demand a water-soluble polyurethane has good mechanical properties, but also requires the use of its excellent processing performance, both requirements can be well applied to its various processing techniques, but also higher production efficiency.
Anionic water-soluble polyurethane to the present, since the emulsion-based [1]. Anionic water-based polyurethane is divided Sulfonate and carboxylate-type categories. Application is now more carboxylic acid salt. Carboxyl containing hydrophilic chain extenders are generally semi-ester and 2,2-dihydroxy - Double-hydroxymethyl propionic acid (DMPA). DMPA relative molecular mass is small, less able to provide sufficient amount of hydroxyl equivalent, but because of DMPA is solid, and high melting point, in the reaction process will produce uniform phenomenon, but more than half of acid good compatibility with polyols However, a large amount of water-soluble and are made of polyurethane film adhesive [2], DMPA as the hydrophilic chain extender is very popular abroad, but the water-soluble polyurethane modified or both over a sulfonate sulfonate and hydroxyl salt type, has launched the so-called "fourth-generation shoe adhesive" that is water-soluble sulfonate polyurethane adhesive, which can meet the water vapor permeability, viscosity, initial adhesive strength, peel strength, water resistance, heat resistance and cost requirements.
2.1 Water-soluble polyurethane dispersions of the crystallization rate
Both solvent or water-soluble polyurethane adhesive, high crystallization rate, while fast drying adhesive, forming at low ambient temperature with good, high bond strength, initial adhesive power and impact dispersion crystallization rate The main factors polyol component of the structure, composition and molecular weight and the hard segment structure and composition of the polyol molecular structure of the more structured, higher molecular weight, the crystallization of the stronger. As the ester polar than the ether of molecular force big, so the general polyester-based polyether crystallinity than good, especially for the PPG polyether molecular structure of its methyl side chain will hinder section of ordered structure, less crystalline, crystalline polyester generally refers to the synthesis of binary acid and polyester diols. Polyurethane adhesive, the crystalline polyester content higher, stronger crystallinity, the faster the crystallization rate. But with the increasing amount of crystalline polyester, will reduce the water vapor transmission, is therefore important to grasp its balance. The crystalline polyester and polyester containing sulfonate or carboxylic acid containing polyester base and used as ionic strength of the Coulomb force and intermolecular force and a high degree of crystallinity, will reduce the intertwining between the molecular chains, favorable micro-phase separation of polymer to increase the number of latex particle size decreased accordingly, film of the initial modulus increases almost linearly with the tensile strength also increased [3]. Therefore, obtained by water-soluble polyurethane adhesive has a faster crystallization rate, a higher initial adhesive strength and adhesive strength, and gives very good heat resistance, solvent resistance and good adhesion.
2.2 Thermal activation and initial adhesive strength
In the thermal activation process, the polyurethane dispersion applied to the substrate, evaporation of water completely, the adhesive layer activated by heat into a bonding state, the adhesive should be modified to show a low initial activation temperature and high viscosity strength. In general, high crystallization rate of the adhesive should have low thermal activation temperature and high initial adhesive strength.
Water-soluble polyurethane dispersion biggest drawback is that the activation required activation temperature adhesive layer is too high, so frequently to be sticky substrate damage. Such as the use of existing water-soluble polyurethane dispersion adhesive thermoplastic rubber molded soles can not be satisfactory, because the temperature activated adhesive layer will often sole deformation.
To reduce the activation temperature, often using a solvent, plasticizer or other resin approach, but often these methods will further loss of heat resistance of polyurethane adhesive cost.
Stability of low pH value of 2.3
Usually the new cationic and non-ionic polyurethane dispersion has a lower pH (below 7) the stability of the anionic polyurethane dispersion only when the pH value was higher (7), is stable. Many patent coverage, with the sulfonated polyurethane dispersions with low pH value (5 ~ 7) stability. Introduced in the molecular structure of polyurethane sulfonate has the following several ways.
(1) by containing sulfonate groups of the chain extender. Commonly used are: 2-amino-alkyl sulfonates, unsaturated dicarboxylic acid and sodium bisulfite adduct, 2,4 - amino acid and so on.
(2) Preparation of sulfonated sulfonated polyether diol end-isocyanate prepolymers. Many patents reported the preparation of sulfonated polyether diols and polyether diols obtained by the sulfonation of terminal isocyanate prepolymer dispersed, chain extender, while the system was stable polyurethane dispersions.
(3) Preparation of Sulfonated polyester diol sulfonated isocyanate prepolymers. Sulfonated polyester polyol can be binary acid, binary alcohol and sulfonated diacids, diols obtained by the transesterification. Reagents are commonly used sulfonated isophthalic acid monosodium salt, 2-methyl-sodium sulfonated isophthalic acid, such as a single. Reagents with aromatic sulfonated polyurethane dispersions can also be given to the crystallization and heat resistance, but it should be proper control of acid ions in the polyurethane dispersion of the concentration. Too high will make polyurethane dispersion improve the thermal activation temperature, is too low will reduce the stability of polyurethane dispersion, or even difficult to disperse a stable dispersion was obtained.
Polyester diols obtained by the sulfonation of the sulfonated aqueous polyurethane dispersion has a low pH value (5 ~ 7), stability, high crystallization rate and low thermal activation temperature, while with the other water-based polymer and crosslinker has good compatibility.
(4) with a sulfonated polyester diol obtained by sulfonation of the isocyanate prepolymer with chain extender containing carboxyl obtained both sulfonate and carboxylic acid base polyurethane dispersion, such dispersion finer granularity, more stable. And its structure has a strong hydrogen bond and Coulomb force, these polymers have strong intermolecular forces and a high degree of crystallinity, has good heat resistance, solvent resistance and excellent adhesion .
2.4 viscosity and solid content
The viscosity of the prepolymer with the ionic groups of the increase of the viscosity of sulfonated polyurethane prepolymers are generally higher than the polyurethane prepolymer containing carboxyl of viscosity, for ease of dispersion, which require the reaction system with 10% 30% of high boiling point solvents such as N-methyl pyrrolidone. As the sulfonated polyurethane soft segment and better dispersion in water, the prepared polyurethane dispersion of the solid content of up to 60%, while the viscosity is usually in the tens to hundreds mPa · s. Improve the solid content can improve the dryness. High solid content, low viscosity of water-soluble polyurethane dispersions have good drying coating film and processing properties.
2.5 with rubber additives
System was modified to water-soluble polyurethane adhesive, in addition to synthesis of high-performance water-soluble polyurethanes, rubber additives with the selection and application are especially important.
(1) If required thin coating occasions, in order to reduce surface tension, surface active agent to achieve this objective, surfactant to fluorine-containing surfactant is better.
(2) thick layer of glue to adapt to prevent the loss of water-soluble thickener can be added to increase the viscosity of water-soluble polyurethane dispersions.
(3) the coating containing the surfactant foaming, should join the defoamer.
With many excellent plastic additives are produced abroad. China is still lacking in this regard. This is also the development of modified water-soluble polyurethane adhesive to overcome the bottleneck.