What are Polyurethane Adhesives?

Polyurethane adhesive refers to an adhesive containing a urethane group (-NHCOO-) or an isocyanate group (-NCO) in the molecular chain. Polyurethane adhesives are divided into two categories, polyisocyanates and polyurethanes. Polyisocyanate contains isocyanate (-NCO) and urethane groups (-NH-COO-) in the molecular chain, so polyurethane adhesives show a high degree of activity and polarity. It has excellent chemical adhesion to substrates containing active hydrogen, such as foams, plastics, wood, leather, fabrics, paper, ceramics and other porous materials, as well as metal, glass, rubber, plastic and other smooth surface materials.

Polyurethane adhesive

Polyurethane adhesive refers to an adhesive containing a urethane group (-NHCOO-) or an isocyanate group (-NCO) in the molecular chain. Polyurethane adhesives are divided into two categories, polyisocyanates and polyurethanes. Polyisocyanate contains isocyanate (-NCO) and urethane groups (-NH-COO-) in the molecular chain, so polyurethane adhesives show a high degree of activity and polarity. It has excellent chemical adhesion to substrates containing active hydrogen, such as foams, plastics, wood, leather, fabrics, paper, ceramics and other porous materials, as well as metal, glass, rubber, plastic and other smooth surface materials.

Chinese name

Polyurethane adhesive

Foreign name

Polyurethane Adhesive

Alias

Ulidang

mainly divided

Polyisocyanates and polyurethanes

Polyurethane adhesives are an important component of the rapidly developing polyurethane resins. They have excellent properties and have been widely used in many aspects. They are one of the eight major varieties of synthetic adhesives.

Polyurethane adhesives have excellent shear strength and impact resistance. They are suitable for various structural bonding fields and have excellent flexibility characteristics.

Polyurethane adhesive has excellent rubber characteristics, can adapt to the adhesion of substrates with different thermal expansion coefficients. It forms a soft-hard transition layer between the substrates, which not only has strong adhesion, but also has excellent cushioning and reducing properties. Shock function. The low-temperature and ultra-low-temperature properties of polyurethane adhesives surpass all other types of adhesives.

Water-based polyurethane adhesive with low

In 1940, German researchers discovered that triphenylmethane-4,4'-triisocyanate could bond metal to sodium butadiene rubber and used it on tank tracks in World War II. After the 1950s, Bayer developed the esmorhurs series (diisocyanates and polyisocyanates) and the esmophens series (low molecular weight hydroxyl-terminated polyester polyols). According to a certain amount of esmbd and esmoplen can be formulated into: Polystal series products (two-component solvent-based polyurethane adhesive).

Japan introduced German and American technology in 1954, produced polyurethane materials in 1960, began producing polyurethane adhesives in 1966, successfully developed vinyl polyurethane water-based adhesives, and began industrial production in 1981. The research and production of polyurethane adhesives in Japan is very active, and together with the United States and Western Europe, it has become a major polyurethane producer and exporter.

China developed and produced triphenylmethane triisocyanate (Lexner gum) in 1956, and soon produced toluene diisocyanate (TDI), a two-component solvent-based polyurethane adhesive. In 1966, the polyurethane adhesive was put into production. It is still the largest production of polyurethane adhesives in China. Since the 1980s, many advanced production lines and products have been introduced from foreign countries. Among them, a large number of imported polyurethane adhesives are needed to support them. Therefore, it has promoted the domestic research units to accelerate the development of polyurethane adhesives, especially after 1986. China's polyurethane industry has entered a period of rapid development. In 1994, the State formally approved the establishment of the "China Polyurethane Industry Association", which has a "Polyurethane Adhesive Committee", which has become the center of national polyurethane adhesive technology and information exchange.

(1) Classification by reaction composition According to the reaction composition, it can be divided into polyisocyanate adhesives, polyurethane adhesives containing isocyanate groups, hydroxyl-containing polyurethane adhesives, and polyurethane resin adhesives.

(2) Classification by use and characteristics According to use and characteristics, they can be divided into general-purpose adhesives, food packaging adhesives, shoe adhesives, paper-plastic composite adhesives, construction adhesives, and structures. Adhesives, adhesives for ultra-low temperature, foaming adhesives, anaerobic adhesives, conductive adhesives, hot-melt adhesives, pressure-sensitive adhesives, closed adhesives , Water-based adhesives and sealing adhesives. ^[1]

No matter what kind of polyurethane adhesive, the isocyanate group in the system reacts with the active hydrogen-containing substances in the system or outside the system to form polyurethane groups or polyureas, thereby greatly improving the strength of the system and achieving the purpose of bonding.

High-performance water-based polyurethane adhesives have the following characteristics:

(1) Good water and medium resistance.

(2) High bonding strength and large initial adhesion.

(3) Good storage stability.

(4) Resistant to freeze-thaw and higher temperature.

(5) The drying speed is fast, and the film forming property is good at low ambient temperature.

(6) Good construction technology.

To achieve the above points, in addition to synthesizing high-performance water-based polyurethane, the selection of film-forming additives is also particularly important.

The diversity of polyurethane adhesives has prepared solutions for many bonding problems, and it is particularly suitable for places where other types of adhesives cannot or are difficult to bond.

In addition, polyurethane adhesives have the characteristics of adjustable toughness, simple bonding process, excellent low temperature resistance and excellent stability. It is precisely because of its excellent adhesive properties and adaptability to a variety of substrates that polyurethane adhesives have continuously expanded their application fields, and have become the fastest growing adhesives in recent years at home and abroad.

1. Polyurethane adhesive for automobiles

The introduction of a large number of lightweight metals, composite materials and plastics into new automotive structures has resulted in the continued growth of automotive adhesives and sealants. The most widely used polyurethane adhesives in automobiles are mainly one-component moisture curing for windshield assembly.

Surface treatment

One of the conditions for good adhesion is the necessary treatment of the substrate surface.
Weak interface layers such as grease, dust, etc. often exist on the surface of the adherend. Due to the influence, it is difficult to improve the bonding strength of the adhesion established on the weak interface layer. Substrate surfaces that do not match the surface tension of the adhesive must also be chemically treated. Surface treatment is one of the first steps to improve the bonding strength.

Cleaning and degreasing

The surface of some metal and plastic substrates is often easily contaminated by sweat, oil, dust, etc. In addition, the surface of the plastic also has a release agent, so such plastics and adhesive layers only form a weak bonding interface.

For polyurethane adhesives, the grease on the metal or plastic surface is poorly compatible with polyurethane, and the existing moisture will react with an NCO group in the adhesive to generate air bubbles, which will reduce the contact surface area of the glue and the substrate and make the cohesive force of the adhesive layer Lower, so surface cleaning and drying must be performed before bonding. Generally, it is washed with alkaline water containing a surfactant and an organic solvent, and then washed with water and dried, or directly washed with an organic solvent (such as acetone, carbon tetrachloride, ethanol, etc.). For rusty metals, the surface rust is usually removed with a sandpaper or wire brush.

roughening treatment

The smooth surface is generally roughened to increase the contact area between the glue and the substrate. The adhesive penetrates into the recesses or pores on the surface of the substrate, and after curing, it acts like a nail, hook, stick, and stick, which can firmly stick the substrate together.

Commonly used methods are sand blasting wood files, roughening sanding paper, etc. However, too rough will affect the wetting of the adhesive on the surface, the recesses are easy to remain or generate bubbles, but will reduce the bonding strength. If sanding and other methods are easy to damage the substrate, it is advisable to use primer coating, etching, corona treatment and other methods to change its surface properties and make it easy to be bonded by polyurethane adhesive.

Metal surface chemical treatment

At the same time, the metal surface can be derusted, degreased, and slightly corroded. Many treatment agents are available. It is usually an acidic treatment solution. For aluminum-aluminum alloy, use a dichromate / concentrated sulfuric acid / water (mass ratio of about 10/100/300) mixed solution, soak at 70-12 ° C for 5-10min, wash with water, neutralize, wash with water, and dry. For iron, mix concentrated sulfuric acid (hydrochloric acid) with water 1: 1, soak at room temperature for 5-10min, wash with water, and dry. Or use potassium dichromate / concentrated sulfuric acid / water mixture.

Surface chemical treatment of plastic and rubber

Most polar plastics and rubbers need only be roughened and solvent degreased. However, the surface energy of polyolefin is very low, and chemical methods can be used to increase its surface polarity, such as solution oxidation method, corona method, and oxidation flame method.
a. The chemical treatment solution can be potassium dichromate / concentrated sulfuric acid / water (mass ratio 75/1500/12, or 5g / 55ml / 8ml, etc.), PP or PE soak at 70 for 1-10min or soak at room temperature for 5h , Washed with water and washed with water and dried.
b. Corona treatment uses high-frequency and high-voltage discharge to partially oxidize the plastic surface by oxygen in the air, generating polar groups such as carbonyl groups. Often a combination of several surface treatment methods, such as sanding corrosion cleaning drying.

top primer

In order to improve the bonding performance, a thin layer of primer (primer) can be applied to the surface of the treated substrate. The primer can also protect the surface of the adherend just after being treated from corrosion and contamination, and prolong the storage time. .
Primers commonly used in polyurethane adhesives and sealants are: polyurethane varnishes (such as dilute solutions of polyurethane adhesives or coatings); polyisocyanate adhesives (such as dilute solutions of PAPI); dilute solutions of silicone coupling agents; dilute solutions of epoxy resins, etc. .

Adhesive preparation

One-component polyurethane adhesives generally do not need to be formulated, and can be used directly according to operating requirements. This is also the convenience of using one-component adhesives.
For two-component or multi-component polyurethane adhesives, it should be formulated according to the instructions. If the hydroxyl content and isocyanate group content of the components are known, the distribution ratio of each group can be determined through chemical calculations. The isocyanate index R = NCO / OH is generally at 0.5-1.4 range.

In general, when two-component solvent-based polyurethane adhesives are compounded, the tolerance ratio of the two groups is larger than that of non-solvent types. However, if the NCO groups in the compound are too much, the curing is not complete, and the cured adhesive The adhesive layer is hard, even brittle; if there are too many hydroxyl components, the adhesive layer is soft, with low cohesion and poor adhesive strength. The latitude of the solvent-free two-component adhesive ratio is smaller than that of the solvent type. This is because the initial molecular weight of each component is small. If one of the components is excessive, it will cause slow curing and difficult to complete, and the surface of the adhesive layer will be sticky. Low strength.
The prepared glue should be used up every day, because the glue has a limited pot life. The pot life is the time that the formulated adhesive can maintain its operable construction. Viscosity increases with standing time, so it is difficult to operate until the glue loses fluidity, gelation occurs and it fails. Different types of brand polyurethane adhesives have different application periods, ranging from minutes to days. When it is used in large quantities in industrial production, a pot life test should be performed in advance.
If the adhesive component contains a catalyst, or a catalyst is added during compounding in order to speed up the curing speed, the pot life is short. In addition, the ambient temperature has a greater impact on the applicable period, with a shorter applicable period in summer and a longer winter. The two-component polyurethane adhesive diluted with urethane-grade organic solvents can extend the pot life. General solvent-based two-component adhesives, such as two-component polyurethane adhesives for soft plastic composite films, should be used for more than 8 hours (that is, one working day).
If the prepared glue can not be used up that day, it can be diluted properly, sealed with a lid, and stored in a cool place. Check for turbidity or gelation at work the next day. If there is no obvious change in the appearance of the glue, the fluidity is good. It can still be used, generally it can be used in small batches in the newly prepared glue. If it has deteriorated, it should be discarded.
In order to reduce the viscosity, facilitate the operation, make the glue solution evenly spread, and help control the sizing thickness, organic solvents can be added for dilution. Available diluents for polyurethane adhesives include acetone, methyl ethyl ketone, ethyl toluene, ethyl acetate and the like.
Adding a catalyst can speed up the curing speed of the glue. The curing catalyst is generally an organotin-based compound.

Adhesive construction

a. Glue coating (gluing) methods include spray coating, brush coating, dip coating, roller coating, etc. It is generally determined according to the type of glue, viscosity, and production requirements. The key is to ensure that the glue layer is uniform, free of bubbles, and lack of glue. .
The amount of glue (actually related to the thickness of the glue layer) is also an important factor affecting the shear strength, which is usually higher in a certain range. If the adhesive layer is too thin, the adhesive cannot fill the uneven gap on the surface of the substrate, leaving voids, and the adhesive strength is low. As the thickness of the adhesive layer increases, the bonding strength decreases. It is generally believed that when a lap shear sample is loaded, the adherend and the adhesive layer deform themselves, and the adhesive layer is broken into a peeling state. The effect of the peeling force reduces the apparent shear strength value.
b. Air-drying For solvent-based polyurethane adhesives, it takes a few minutes to tens of minutes to dry after coating, so that most of the solvent in the adhesive is volatilized, which is conducive to improving the initial adhesion. If necessary, it should be appropriately heated and air-dried (such as a composite film lamination process). Otherwise, since a large amount of solvent remains in the glue, it is easy to form bubbles in the glue layer during the curing process, which affects the bonding quality. For solvent-free polyurethane adhesives, the adherend can be adhered after application.
c. Adhesion This step is to glue the adhered surface of the adhered object. You can also use a clamp to fix the adhesive to ensure that the adhesive surface is fully fitted and positioned. If necessary, apply a certain pressure to Adhesives better produce plastic flow to wet the surface of the adherend and make the adhesive reach the maximum contact with the surface of the substrate.

Adhesive curing

Most polyurethane adhesives do not immediately have a high bond strength when they are bonded, and they need to be cured. The so-called curing refers to the process in which the liquid adhesive becomes a solid. The curing process also includes post-cure, that is, the reactable groups in the adhesive after preliminary curing further react or generate crystals to obtain the final curing strength. For polyurethane adhesives, the curing process is a process in which the NCO groups in the adhesive are completely reacted, or the solvent is completely volatilized, and the polyurethane molecular chain is crystallized, so that the adhesive and the substrate have a sufficiently high adhesion.

Polyurethane adhesives can be cured at room temperature. For reactive polyurethane adhesives, if curing at room temperature takes a long time, a catalyst can be added to promote curing. In order to shorten the curing time, heating can be used. Heating not only facilitates the curing of the adhesive itself, but also accelerates the reaction between the NCO groups in the adhesive and the active hydrogen groups on the surface of the substrate. Heating can also soften the adhesive layer to increase the infiltration of the surface of the substrate and facilitate molecular movement. Find a "partner" at the bonding interface that generates molecular forces.

Heating is good for improving adhesion. The curing heating methods include oven or drying tunnel heating, fixture heating, etc. For metal substrates with fast heat transfer, fixture heating can be used, and the adhesive layer is heated faster than the oven.
The heating process should be gradually increased. Pay attention to the volatilization rate of solvents in solvent-based polyurethane adhesives. During the drying process, most of the solvent has evaporated, and the remaining solvent slowly diffuses outward through the adhesive layer. If heated too quickly, the solvent vaporizes and bubbling in the softened adhesive layer, forming air bubbles in the joint. Seriously, most of the uncured concrete can be extruded from the joint, forming voids will affect the bonding strength. For two-component solvent-free adhesives and one-component moisture-curing adhesives, the heating should not be too fast, otherwise the NCO groups will accelerate the reaction with the moisture in the glue or the surface of the substrate and the air, and the generated CO2 gas will have time to diffuse and the glue layer The viscosity increases quickly and air bubbles remain in the glue layer.
Single-component moisture-curing polyurethane adhesives are mainly cured by moisture in the air, so a certain amount of air humidity should be maintained, and slow curing at room temperature should be appropriate. If the air is dry, a small amount of water can be applied to the rubberized surface to promote curing. If the glue is sandwiched between dry and hard adherends, and the glue layer is thick, the interface and external moisture will not easily penetrate into the glue, and the curing will be incomplete. Moisture.