Bonding mechanism for you to explain polyurethane-Iota

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Service > Bonding mechanism for you to explain polyurethane

Bonding mechanism for you to explain polyurethane

Adhesive A, metal, glass, ceramics, etc.
High surface tension of metal materials, glass, etc., are high-energy surface, comprising the cohesive energy higher urethane bond and urea bond in the cured PU adhesive, under certain conditions, be able to gather in the bonding surface, to form a high surface tension adhesive layer. In general, the isocyanate adhesive or a derivative thereof percentage content, the greater the surface tension of the adhesive layer, the more tenacious gum can be a good match with the base metals, the adhesive strength generally more high.
An NCO group containing adhesive for bonding metal mechanism is as follows:
Usually there is a metal surface absorbed water (even after polished metal surface there traces of adsorbed water or metal oxide hydrate), between a NCO water reacts with the urea bond with the metal oxide due to the hydrogen bonding chelation hydantoin formation - complex metal oxide, an NCO group and the metal hydrate can form a covalent bond and the like.
In the absence of an NCO case, van der Waals forces generated between the metal surface and hydrogen hydrate and metal atom with a urethane bond and urea bond, and with TDI, MDI-based polyurethane adhesive containing a benzene ring, with redundant electronic systems, with the metal forming coordinate bond metal surface composition is more complex, a variety of chemical bond or secondary bonds formed between the PU glue (such as hydrogen) types are also very complicated.
Glass, stone, ceramics and other inorganic materials are generally made Ah09, S02, CaO and Na20 or other component, also contain surface absorbed water, hydroxyl, roughly the same as the bonding mechanism and metal o
Adhesive B, plastics, rubber
Bonding rubber generally used in polyisocyanate adhesives or rubber-based adhesive modified polyisocyanate adhesives, organic solvent-free adhesives contained make rubber surface swell polyisocyanate adhesives smaller molecular weight, can penetrate the rubber surface inside presence and rubber active hydrogen reaction to form a covalent bond. polyisocyanate also with moisture reacts urea groups or biuret, and heat curing isocyanate occur from polyethylene, forming a cross-linked structure with rubber molecules crosslinked network to form a polymer crosslinked interpenetrating network (IPI), thus the adhesive layer with good physical properties. with conventional polyurethane adhesive bonding rubber, since the chemical and physical interactions between the various materials groups, but also to produce good adhesion.
PVC, PET, FRP plastic surface polar groups with urethane adhesive bond, ester, ether and other groups form hydrogen bonds formed with certain bonding strength of the joint. Some people think of glass fiber reinforced plastic ( FRP) containing an OH group, in which a chemical adhesion OH PU adhesive NCO reaction with the formation of a surface.
Non-polar plastics such as PE, PP, its surface is very low, with a polyurethane adhesive bonding polarity may encounter difficulties, which can be used a variety of methods for surface treatment of polyolefin plastics to be addressed. One way is corona on its surface oxidation, increasing polarity: Another approach is to be glued on a plastic surface using polyisocyanate adhesives for the anchor coating agent (primer, primer) as melt extrusion film in recess. When extrusion lamination on PET and other plastic film, due to the presence of the low molecular weight Xing weak surface boundary layer, the adhesive strength is not satisfactory, the use of primer, a polyisocyanate on the polyethylene surface of the heat diffusion, so weak interface layer to strengthen composite film is very good peel strength.
Adhesive C, fabric, wood, etc.
Fabric, wood and other substrates composed of fibers, and the fiber has a certain moisture absorption, and often contain an ether bond, an ester bond, an amide bond and other polar bond, and a carboxyl group, a hydroxyl group, etc. and the water and hydroxyl groups readily PU adhesives an NCO groups form a strong urethane bonds and urea bonds and other bond, and the formation of hydrogen bonds between the fibers in the polar group and the gum polar groups, and adhesive molecules also easy to penetrate between the fibers for .PU Such materials are generally capable of forming a strong bond.
Bonding process polyurethane

1. Surface Treatment
One of the conditions to form a good bond to the substrate surface is necessary processing.
Sticky surface there is often grease, dust and other weak interface layer, its impact, based on the weak boundary layer of the adhesive bond strength is not easy to raise income. For those with the adhesive surface of the substrate surface tension does not match, you must chemical treatment. Surface treatment is one of the first steps to improve the adhesion strength.
2. Clean degreasing
Some metal surface of the plastic substrate is often susceptible to sweat, oil, dust and other pollution, in addition, there is a release agent plastic surface, so this plastic and the adhesive layer is formed only weak bonding interface. Polyurethane adhesives terms , metal or plastic surfaces grease polyurethane poor compatibility, moisture will exist with the adhesive in an NCO groups reactive bubbles that reduce the surface area of adhesive contact with the substrate, so that the adhesive layer and reduce cohesion, thus bonding the front surface must be cleaned, a drying process generally is carried out using alkaline surfactant-containing cleaning agent and an organic solvent, and then washed with water and dried, or directly washing with an organic solvent (e.g. acetone, carbon tetrachloride, ethanol, etc.). to have rust metal generally first with sandpaper, wire brush to remove surface rust.
3. roughening treatment
Generally required for a smooth surface roughened to increase the contact area between the adhesive and the substrate. Adhesive penetrate the substrate surface concave gaps or voids, the cured newcomer "nails, hooks, stick" like Nails effect, can be firmly The substrate stick together. Common methods are sandblasting, rasps roughened sandpaper etc. But too rough surface makes adhesives infiltration affected pockets or bubbles are liable to remain, but will reduce the bonding strength If a sand base and other methods and easy to damage, so should adopt the primer coating, etching, corona treatment and other methods to change its surface properties, making it susceptible to the polyurethane adhesive bonding.
The chemically-treated metal surface
The metal surface can be simultaneously rust, degreasing, etching slightly, many available treatment agent generally is the acidic treating liquid, such as aluminum, aluminum alloys, can be used potassium dichromate / concentrated sulfuric acid / water (mass ratio of about 10 / 100/300) mixture at 70-12 ℃ dip 5-10min, washing, and then washed with water and dried.
Iron can be concentrated sulfuric acid (hydrochloric acid) and water 1: 1 mixture at room temperature dip 5-10min, washing, drying or potassium dichromate / concentrated sulfuric acid / water mixture treatment.
5. The chemical surface treatment of plastics and rubber
Most polar plastics and rubber only surface roughening and degreasing solvents. However, the low surface energy polyolefin, chemical methods may be employed to increase the surface polarity, it has a solution oxidation, corona discharge, flame oxidation method and so on.
(1) Chemical treatment solution available potassium dichromate / concentrated sulfuric acid / water (mass ratio 75/1500/12, or 5g / 55ml / 8ml ratio, etc.), PP or PE or at room temperature for 70 ℃ immersion dip 1-10min 1.5 After h, washed, and, washed and dried.
(2) with a high frequency high voltage corona discharge treatment, the plastic surface by air oxygen partial oxidation to produce a carbonyl group and other polar groups. Often a combination of several surface treatment methods, such as sanding → etching → washing → drying.
6. On the primer
In order to improve the adhesive properties, has been handled well in the substrate surface coated with a thin primer (primer), primer can also protect the surface of the adhesive material just processed from corrosion and contamination, prolong storage time .
Polyurethane adhesives and sealants used in the primer are: polyurethane varnish (such as polyurethane adhesive or coating a dilute solution); polyisocyanate adhesives (eg PAPI dilute solution); silicone coupling agent dilute solution; dilute solution, such as an epoxy resin .
7. The adhesive formulation
One-component polyurethane adhesive generally do not need preparation, according to operational requirements directly, which is used at the convenience of one-component adhesive.
For the two-component or multi-component polyurethane adhesive, specification requirements should be formulated, if they know the content of a hydroxyl group and an isocyanate group content of components, each component ratio can be determined by stoichiometric isocyanate index R = NCO / OH in general 0.5-1.4 range. Generally, when a two-component solvent-free polyurethane adhesive with plastic, two distribution ratio tolerance larger than the non-solvent-based, but if with glue in excess NCO groups too much, not completely cured, and curing the adhesive layer is hard, and even brittle; if a large excess of hydroxyl component, the layer of soft sticky, low cohesion, tolerance of poor bonding strength, solvent free two-component adhesive solvent-stoichiometric ratio. The smaller, because smaller initial molecular weight of each component, if one component is excessive, resulting in slow and difficult to fully cured, tacky surface layer, low intensity.
Has deployed a good glue should run out of days is appropriate, because of the limited dubbed the glue pot. Pot life that is formulated to maintain its operational after construction time. Viscosity with storage time increases, and thus difficult to operate, until Glue loss of mobility, gelation and fail. Different varieties of polyurethane adhesive suitable for different grades, ranging from a few minutes to a few days. When widely used in industrial production, should be applicable in advance of the test.
If the adhesive component containing a catalyst, or in order to accelerate the curing speed at which the catalyst is added with glue, then apply a shorter period. In addition, environmental temperature on the pot is large, short pot life summer and winter long. After urethane grade diluted organic solvent two-component polyurethane adhesive, applicable period may be extended. Usually solvent-based two-component adhesive such as soft-plastic composite film with two-component polyurethane adhesive, pot should be greater than 8h (ie a working day).
If you use up the day with a good glue, can be properly diluted, and the cover closed, cool place for storage, check the phenomenon become turbid or gel work the next day, if no significant changes in the appearance of glue, good liquidity, it can still be used against the person generally small batches with the new glue. Ruoyi deterioration should discarded.
In order to reduce the viscosity, is easy to operate, so that glue is applied evenly, and help control the thickness sizing, an organic solvent may be added to dilute polyurethane adhesive available diluents are acetone, methyl ethyl ketone, toluene, ethyl acetate and the like.
Adding a catalyst to speed up the adhesive curing speed curing catalyst is usually organic tin compounds.
8. bonding construction
1. Coating
Coating (glue) methods are spraying, brushing, dipping, roll coating, generally based on the type of glue, viscosity and production requirements and decisions, it is critical to ensure that layer uniform, no bubbles, no lack of plastic.
The amount of glue (practically relevant layer thickness) is also an important factor shear strength, usually within a range of high shear strength. If the layer is too thin, the adhesive does not fill the uneven surface of the substrate gap, leaving a vacancy, the adhesive strength is low. When the layer thickness increases, the bonding strength decreases generally believed that when lap shear specimens carry the load, are sticky and glue themselves deformed layer is broken into - kind of release state, the release force of the lower apparent shear strength values.
2. airing
For solvent-based polyurethane adhesive, the post-coated gum need a good airing a few minutes to tens of minutes, so that the majority of adhesives solvent evaporation, which will help improve tack. Also suitable heating necessary, blow dried (composite film lamination process). Otherwise, since a large number of solvent remains in the adhesive, the curing process is easy to form bubbles in the layer, the effect of the adhesive quality for solvent-free polyurethane adhesive, it can be the glue stick after was fit.
3. adhesive
This step is already coated plastic adherends bonded together with an adhesive surface, can also be fixed using a jig Bonds, ensure fully fit bonding surface positioning, apply some pressure when necessary, the adhesive better plastic flow to infiltration is stuck the surface, the adhesive surface of the substrate to achieve maximum contact.
9. The adhesive curing
Most polyurethane adhesive when the adhesive does not immediately have a high adhesive strength, the need for curing. Curing refers to the so-called liquid adhesive into solid process, the curing process also includes post-curing, i.e. the initial curing the adhesive after the reactive group can be further reacted or crystallization, to obtain a final cured strength. For polyurethane adhesive, the curing process is to make the gum NCO groups reacted completely or completely evaporate the solvent, crystallization of the polyurethane molecular chain, so that the adhesive with the substrate to produce Process sufficiently high adhesive strength. The room temperature curable polyurethane adhesive, the reactive polyurethane adhesive, it takes a long time when cured at room temperature, the catalyst may be added to promote curing. In order to shorten the curing time, heating method may be heating only conducive adhesive curing itself, but also help accelerate gum NCO groups to the substrate surface with active hydrogen groups reactive heating can also make glue softened surface of the substrate to increase infiltration and facilitate molecular movement, found on the bonding interface produce intermolecular forces "partner." heating to improve the adhesion favorable. A two-component polyurethane adhesive bonding steel, at different curing temperature, time of bonding strength.
There curing oven heating or drying tunnel, drying room heating, heating fixtures, etc. For fast heat of the metal substrate can be heated clamps, glue heat faster than the oven heating.
Heating process should be gradual warming is appropriate. Solvent-free polyurethane adhesive to note solvent evaporation rate in the airing process, most of the solvent has evaporated, the remaining solvent slowly spread out through the adhesive layer, if heated fast vaporization of the solvent in the softened layer bubbled bubble formation in the joint, can be most severe uncured state was fluid viscosity adhesive extrusion joints, forming vacancies will affect bond strength. For two component solvent-free adhesives and moisture-curing one-component adhesives, heat can not be too fast, otherwise the NCO groups in the gum or the substrate surface, moisture in the air to accelerate the reaction, CO2 gas generated too late diffusion, viscosity increases and layer Soon, the bubbles remain in the gel layer.
One-component moisture curing polyurethane adhesive mainly by moisture in the air curing, it should maintain a certain humidity, should be slow to solidify at room temperature is appropriate. If the air is dry, flat A small amount of water in the adhesive surface to facilitate curing. If dried gel was sandwiched between rigid adherends, and when the layer is thick, the interface and the outside water can not penetrate the gel, it is easy to not completely cured, in which case you can inject a small amount of water in the gel .

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