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Hot-melt polyurethane adhesive

October 29, 2019

1 Overview

Hot-melt adhesives are solid at room temperature and melt to a viscous liquid when heated to a certain temperature. After cooling to room temperature, they become solid and have a strong bonding effect. Therefore, people call it hot melt. (Type) adhesive (referred to as hot melt adhesive). Because the hot melt adhesive has the advantages of fast bonding speed, non-toxicity, simple bonding process, good adhesive strength and flexibility, etc., it is widely used in books, such as wireless binding, packaging sealing, footwear, and textiles. Application, and in recent years there have been new developments in variety and performance. Most hot-melt adhesives are made of hot-melt resins such as ethylene-vinyl acetate (EVA), Polyester, and polyamide. Because EVA and polyester hot-melt adhesives have poor strength and elasticity, they cannot withstand large external forces. Because of the higher melting point and hardness of the polyamide hot melt adhesive, it is limited in use. The principle of the preparation of hot-melt Polyurethane adhesive is similar to that of Thermoplastic Polyurethane Elastomer (or rubber), and it is mainly used to physically cross-link the hydrogen bonds in the composition, so that the polyurethane hot-melt adhesive has excellent elasticity and strength. Hot-melt type Polyurethane Adhesive loses its hydrogen bonding effect when heated, and turns into a molten viscous liquid, which returns to its original properties after cooling. Therefore, the reversible characteristic of polyurethane resin can be used to make hot melt adhesives with high adhesive strength, solvent resistance and wear resistance, which can be applied to the bonding of various materials, and is particularly suitable for bonding fabrics.

Polyamide hot melt adhesives can be divided into two categories: one is thermoplastic Polyurethane Elastomer hot melt adhesive, and the other is reactive polyurethane hot melt adhesive. The latter is divided into closed type and moisture curing hot melt type. Closed type has already been described, because the sealant dissociation temperature up to 100 °C above, will cause the glue layer to cause air bubbles, etc., only for maintenance and treatment. The moisture-curing hot-melt polyurethane rubber is a one-component, solvent-free type that meets the environmental protection regulations. It is easy to use and the performance is comparable to the solvent-reaction type. Therefore, the development prospect is very good.

According to the shape of hot melt adhesive, there are three types of adhesive film, tape and powder. The adhesive tape is backed with a reinforcing material or carrier, and the adhesive film is supported by the adhesive itself.

The reasons for the use of hot-melt polyurethanes are: high reliability of use, good chemical and physical homogeneity, simple bonding process (lower production cost), low waste (unused glue can be used after storage), no mixing Problems can make the adhesive achieve the best physical properties. In addition, because the hot melt adhesive does not use organic solvents, the production site will not be contaminated and will be welcomed by users.

2. Thermoplastic hot melt adhesive

Thermoplastic polyurethane hot melt adhesives use more polyester polyols, and their physical properties are superior to those of Polyether polyols. Polyisocyanates use MDI and HDI (no discoloration).
(polybutylene adipate-MDII hot melt adhesive)
1. Film Preparation 100 g (1/20 mol) of polybutylene adipate (molecular weight 2000) was placed in a 500-ml three-necked flask and heated to 120° C. by heating. Dewatering under reduced pressure (degree of vacuum 0.67 kPa) for 30 min under stirring. Into a three-neck flask, MDl melted in advance at 60 DEG C was added into a three-necked flask, a reflux condenser was set, and 200 ml of toluene was added to the bottle in 4 hours with stirring, and the reaction temperature was controlled at 110. -115°C. The reaction was continued for 30 minutes plus 100 ml of ethyl acetate, and the reaction was stopped after stirring. The glue solid content is 30%. The viscosity measured by the falling ball method is 4 Pa·s.

The glue liquid was scraped on the glass plate to volatilize the solution naturally and it could be removed after 24 hours.

2. Film properties (1) The film melt temperature is 90-100 °C. (2) Peel strength: Polyester-polyester fabric is greater than 3.92KN/m, and cotton-cotton fabric is greater than 4.9KN/m. (3) Hydrolysis resistance: Long-term immersion in water of 40-50°C (more than one month) has no effect on the strength of the rubber. The peel strength began to decrease after flexing 800 times in water at 40-50°C. Soaking in boiling water has no effect. (4) The retention period is more than 1 year.

3. application

The adhesive film is suitable for the bonding of textiles and glass, for example, for attaching a liquid crystal device and not for bonding a liquid crystal container on an instrument (computer).

The MDI hot melt adhesive (1) has a tensile strength of (32±2) MPa. (2) The elongation is 600%. (3) Peeling strength: cotton cloth-cotton 7±1kN/m, polyester-polyester 5kN/m. (4) Solvent resistance: insoluble in common organic solvents, slightly soluble in cyclohexanone, and soluble in DMF. (5) The characteristics of the cement are 0.65dL/g. (6) Heat-resistant water performance: In water of 100°C for 2 hours, there is no change in performance of (1)-(4); in water of 40°C for 30 days, there is no change in performance of (1)-(4). There was no effect on flexing in water at 40°C for 800 times. (7) Atmospheric aging can maintain its original performance for 3 months (open air, Beijing, August-November). (8) Hygrothermal aging (55°C, relative humidity 98%, 240h), tensile strength (27±1) MPa, elongation 80%. (9) Ultraviolet accelerated aging (1000W UV, distance 1m, light 100h), film yellowing, tensile strength decreased with different thickness and varying degrees, and is no longer soluble in DMF, indicating that UV accelerated aging, occurred Cross-linked. (10) The storage period is more than 1 year.

The HDI hot-melt adhesive polyurethane size is 0.3-0.5 mm and is dispersed through a sieve onto an undyed cotton fabric with crossed diagonal stripes. When the cotton fabric has a weight of 200 g per m2, the polyurethane gel used is 28 g/m2. After the cotton fabric is covered with rubber particles, it is heated with a 1500W infrared radiator for 20 seconds. If it is, the rubber particles can be uniformly applied to the fabric (the heating area of the infrared radiator is 300cm2, and the distance from the fabric is 10cm). After cooling it can be used for bonding. A piece of cotton knitted fabric is put on the hot-melt fabric after the above treatment. On an iron press, the ironing process is performed at a pressure of 64 kPa and 160° C. for 120 seconds, and the two fabrics are firmly bonded. Together. The tear strength of the fabrics bonded by this method is 0.57-0.69 kN/m, the tear strength after washing in a washing machine at 60°C is 0.57-0.67 kN/m, and the tear strength after washing with boiling water is 0.55-0.65 kN/m. , Such as dry cleaning with perchloroethylene tear strength is still 0.55 ~ 0.67kN / m. It was demonstrated that polyurethane hot melt adhesive particles were used to bond fabrics.

3. Reactive Hot Melt Adhesives In 1984, reactive hot melt polyurethane adhesives appeared on the American market, marking the new development of hot melt polyurethane adhesive technology. In Europe in 1986, the number of reactive hot melt adhesives for automobiles was 50 tons, which increased to 300 tons in 1989, an average annual increase of 80%, and reached 500 tons in 1994, an average annual increase of 11%. The reactive hot melt adhesive is heated and melted under the condition of inhibiting the chemical reaction, so as to facilitate the coating; after the two adherends are cooled together, the adhesive layer agglutinates and acts as an adhesive; after that, the presence of air or the adherend is used. Moisture adhered to the surface reacts with and expands the chain to form a high-molecular polymer with high cohesive force, which significantly improves adhesion, heat resistance, and the like. This type of adhesive is based on NCO end-group prepolymers, with thermoplastic resins and tackifying resins that are not reactive with isocyanate groups, and additives such as antioxidants, catalysts, and fillers to ensure long product pot life and storage. period.

Due to the excellent performance of the moisture curing reactive hot melt adhesive, in the case where the hot melt adhesive or the reactive adhesive is originally used for bonding, it is possible to convert to a moisture curing reactive hot melt adhesive to complete the bonding work. It is because of the strict isolation of moisture during its manufacture, storage, and sizing, and its promotion has been subject to certain restrictions. In recent years, due to breakthroughs in technology and equipment, these problems have gradually been overcome and have partially replaced hot melt adhesives. In the future, with its further improvement and the development of special plastics, its application will be further expanded.

1. Polytetrahydrofuran (molecular weight 1000) 100 g, MDl 45 g was charged into a reaction vessel, stirred and heated to 80° C. and reacted for several hours to obtain a prepolymer, and the viscosity at 25° C. was 5000 mPa·s. After mixing 65 g of this prepolymer with a polystyrene oligomer, a moisture-curing reactive polyurethane hot melt adhesive was prepared. The viscosity at 100°C is 50-600mPa·s, which gives good adhesion to polyester film and nylon fabric.

2. Polyoxypropylene polyol (molecular weight 400) and MDI were used to prepare 850 g of prepolymer with liquid NCO end groups at room temperature, and 94 g of solid prepolymer made of polyoxypropylene polyol (molecular weight 4000) and MDI was added. The mixture was uniformly mixed at about 120° C. to obtain a moisture-curing reactive polyurethane hot melt adhesive. The viscosity at 130°C is 1200 mPa·s. The initial shear strength of the hot melt adhesive to wood is 1.8 MPa, and the shear strength after aging at room temperature is 7.8 MPa.

3. Using adipic acid, terephthalic acid, 1,4-butanediol to synthesize a polyester polyol (polyadipate-terephthalic acid-1,4-butanediol) having a molecular weight of 2500-4500. Isocyanate using MDI, r value is 1.4-1.6, NCO content is controlled at 1.2%-1.8%, in the prepolymer made, add molecular weight regulators, stabilizers, catalysts, to obtain a moisture curing reactive polyurethane hot melt adhesive . The hot melt adhesive is applied in the shoe industry as follows.

1 The outer bottom and inner sole of the flowing water bottom assembly line are pre-applied and hardened at room temperature; the chain is extended at a temperature of 19-25° C. and a relative humidity of 40%-65% (RH) for 5 days, and is thermally activated at 60-80° C. under pressure. On-press pressure fits 8-10s. It is used for bonding six points with female horses' boots. The width of the peeling blade is 20mm and the peeling strength is 100N/cm. For 2 1/2 type men's leather shoes, with a blade width of 10mm and peel strength of 99N/cm.

2 Sandals Strip Bonding At present, the processing method for sandal strips is to temporarily glue the glutinous rice paste after the PU leather is cut into strips. In order to increase the strength, a cloth tape is added as a reinforcing strip in the middle and then sewn with a sewing machine. This method is complicated and unsightly. After being hot-melted at 100° C., the hot-melt adhesive was applied to a synthetic leather fabric, cooled and hardened, and then was chain-extended at room temperature for 5 days. The strips were cut and the strips of different widths were continuously pressed and formed on a special gluing machine. Bonded sandals are novel in terms of style, flexibility and comfort, and good water resistance; they do not need to be reinforced in the middle of the cloth, and can be directly used in various colors and sizes of molding strips to make the sandals production components excellent.

4. Powder Hot Melt Powder Powdered polyurethane hot melt adhesives are formulated by mixing thermoplastic urethane resins with additives such as colorants, plasticizers, stabilizers, and curing agents to provide an economical and energy-efficient manufacturing process. The powder mixing process is moderated, and the integrity of the resin structure and the stability of the performance are maintained, thereby ensuring its performance.

Another advantage of the powder is its fluidity, which facilitates transportation, dispersion, and use, and therefore does not require the help of other media (such as organic solvents or water), so the powder can be delivered and dispersed without melting.

There are mainly three methods for preparing polyurethane powders.
1. The polyurethane elastomer is ground under conditions of no oxygen (oxygen can cause dust explosion). If an impact pulverizer is used, the softer the product to be ground, the lower the required operating temperature, and the use of frozen brine, reduced temperature in ice or liquid nitrogen.

2. In the solvent, the addition polymerization method is used for the preparation. The selected solvent requires that only one reaction component be dissolved and the final polyurethane product (such as the hydrocarbon that only dissolves the isocyanate) be dissolved. Polyurethane resin produced by this solution polymerization method consumes a large amount of energy regardless of the manner in which the solvent is recovered.

3. In the presence of a very small amount of emulsifiers or in the case of extremely low levels of hydrophilic groups, an aqueous polyurethane dispersion is prepared by emulsion polymerization, and the powder is obtained after demulsification and drying. The manufacturing technology of polyurethane emulsions is complex, and there is a big gap between its product performance and overall polymerization.

In view of the above three methods for preparing polyurethane powder, the first method is mainly adopted at present. The particulate polyurethane resin is mixed with the additives and then extruded from a twin-screw extruder at a temperature of 80-130[deg.] C. (extrusion time 0.5-2 min). After the extruded material is cooled and cut, it is crushed in a dedicated pulverizer into granules having an average diameter of 40 to 60 um, and coarse particles are removed by screening. The polyurethane powder prepared by this method has good fluidity and stability (its particles do not stick together when stored at 50°C).

The Ultramoll PU produced by Bayer, Germany, is a highly flexible polyurethane powder containing a polymeric plasticizer. The powder is colorless and odorless and has a particle size of less than 1000 μm. It contains (5 ± 3)% PVC and is specially designed for mixing with PVC. Shore hardness is 72 ± 3. . Injection moulded shoe soles can be greatly improved when Ultramoll PU is used to bond the upper material

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