The formulation design of polyurethane adhesives.

Polyurethane adhesives are designed for end-use performance, so the design of polyurethane adhesives must take into account the construction (operability), curing conditions and other operating conditions of the polyurethane adhesives made, as well as bonding strength, heat resistance, chemical resistance, durability and other performance requirements. Below, we will explain how to design the formulation of polyurethane adhesives from the following aspects:
1, from the structural and performance point of view of the design of polyurethane molecules
For example, in terms of the appearance of its ontogenal material (i.e., solvent-free), it is available from soft to hard elastomers and foam

. Polyurethane is basically elastomer in terms of its ontogenous properties (or curings), and some of its physical and chemical properties, such as bonding strength, mechanical properties, durability, low temperature resistance and drug resistance, depend mainly on the chemical structure of the resulting polyurethane curing. Therefore, in order to design the polyurethane adhesive, we must first carry out molecular design of the raw material polyurethane, that is, start with the effect of chemical structure and composition on performance.
II, from the raw material point of view of the preparation of polyurethane adhesive design
polyurethane adhesive formulation is generally used in three types of raw materials: one is - NCO raw materials (i.e. isocyanate and its modifiers and Polycyanate), a class of - OH raw materials (i.e., hydroxyl-containing lysate polyols and chain-expanding agents, etc., broadly speaking, is a compound containing active hydrogen, so also includes polyamines, water, etc.), and another type of solvents and catalysts and other additives. From the point of view of raw materials for the formulation of polyurethane adhesives, there are two specific methods:
1, by the above three types of raw materials directly formulated
This method is not commonly used in the formulation design of polyurethane adhesives because of the low molecular weight of most low-polymer polyols (usually the molecular weight of polyether polyols Mr 6000, the molecular weight of polyester polyols MR 3000), so the formulation of polyurethane adhesive composition is small viscosity, low initial viscosity. However, in some cases, even with the addition of catalysts, the curing speed of the adhesive is still relatively slow, and the resulting curing material is low in strength and of little practical value. Moreover, due to the high unaltered TDI vapor pressure, large odor, volatile toxicity, and MDI at room temperature for solid state, inconvenient to use, so only a few commercialized polycyanate can be used as a raw material for polyurethane adhesives. However, the above methods can be used to configure polyurethane adhesives:
(1) organic solution composed of high molecular weight polyester polyol (molecular weight Mr between 5000 and 50000) and polyocyanate solution composed of two-component polyurethane adhesive performance is good, can be used for composite laminated film and other purposes. This is because its main component, high molecular weight polyester polyol itself has a high initial adhesion, so that the composition of polyurethane adhesive has a strong cohesion force;
(2) by polyether (or polyester) polyol, water, polycyanate and catalyst components configured composition, can be used as foam-type polyurethane adhesives, adhesives, for insulation materials, such as bonding, manufacturing, etc., has a certain practical value.
2, the -NCO and -OH raw materials pre-emulsified modified
As mentioned above, due to the low molecular weight of most POLYMER polyols, and TDI volatile toxicity, MDI at room temperature is solid, so the direct configuration of adhesives generally poor performance. Therefore, in order to improve the initial viscosity of polyurethane adhesives and reduce the time required to produce a certain bonding strength, polyether polyols or polyester polyols are usually reacted with TDI or MDI monomers to produce pre-polyurethane pre-polypicles containing isocyanate or end hydroxyl, which are used as -NCO or -OH components.
3. Designing the preparation of polyurethane adhesives from the requirements of the use form
Type:
1, single-group polyurethane adhesive
The advantage of single-group polyurethane adhesive is that it can be used directly, compared with the two-group adhesive is more convenient is that there is no need to adjust before use. There are two main types of single-group polyurethane adhesives:
wet-cured polyurethane adhesives based on isocyanate-based polyurethanes
This type of polyurethane adhesive is generally solvent-free polyurethane adhesive, in order to facilitate the application of glue, the viscosity of the adhesive can not be too large. Single-component wet-cured polyurethane adhesives are mostly polyether type, i.e. its main hydroxyl-containing raw material is polyether polyol. The degree of content of isocyanate base free in this kind of adhesive should be determined according to the viscosity of the adhesive (affecting the operability of polyurethane adhesive in construction), the way of applying the glue, the thickness of the coating and the type of adhesive, in addition, the storage stability of the polyurethane adhesive should be considered.
A single-component solvent-based polyurethane adhesive based on thermoplastic polyurethane elastomers
The main component of such polyurethane adhesives is high molecular weight end hydroxylline polyurethanes, which have a small hydroxyl base, and the viscosity of the adhesive increases rapidly when the solvent starts to evaporate, producing initial viscosity. When the solvent is basically volatile completely, a large enough bond is produced, and then placed at room temperature for a period of time, most of the medium chain segment of this type of polyurethane elastomer will crystallize, further improving the bonding strength of the adhesive. This type of single-component polyurethane adhesive is generally crystalline polyester polyols as the main raw material for polyurethane.
addition to this, single-group polyurethane adhesives include polyurethane hot melts and single-group water-based polyurethane adhesives.
2, two-component polyurethane adhesive
two-component polyurethane adhesive consists of the main agent containing end hydroxyl and the curing agent of the end-containing isocyanate group, with a single-component polyurethane adhesive Compared with the adhesive, the performance of the two-group polyurethane adhesive is good, the bonding strength is high, and the distribution ratio of the same two-group polyurethane adhesive allows for appropriate adjustment within a certain range, which can be used to adjust the performance of the curing. The main agent is generally polyether polyols or high molecular weight polyester polyols. The ratio of the two groups to curing agent slightly excessive, that is, there is a trace isocyanate group surplus is appropriate, so as to make up for the possible water and isocyanate group reaction caused by the loss of isocyanate group, so as to ensure that the adhesive can produce sufficient cross-linking reaction.
, according to the performance requirements of the preparation of polyurethane adhesive design
when there are special performance requirements for polyurethane adhesive, should be based on the structure and performance of polyurethane relationship to the formulation design. Different substrates, different applications and application environment, often have some special requirements for the performance of polyurethane adhesives, for example, in industrial production lines using polyurethane adhesive requirements for rapid curing, and composite soft packaging film polyurethane adhesives require acid-resistant and hydrolytic properties, among them, cooking-resistant soft packaging with polyurethane adhesives also require a certain degree of high temperature bonding and so on. So how can these special requirements be met?
1, high temperature
polyurethane adhesive generally has the disadvantage of low resistance to high temperature performance. Polyurethane adhesives can be designed in advance when they need to be used in special heat-resistant situations. The heat resistance of polyurethane adhesives can be improved to a certain extent through the following ways:
uses polyethers or polyester polyols containing benzene rings and isocyanate as raw materials;
increases the content of isocyanates and chaining agents (which form hard segments) in polyurethane adhesives;
increase the amount of curing agents in polyurethane adhesives;
uses high-temperature pyrolytic polycyanate (e.g. polycyanate containing isocyanate rings), or isocyanate is produced during curing;
is mixed with polyurethane with resins such as high temperature-resistant epoxy resins or polyamides, and PN technology is an effective way to improve polymer compatible.
2, hydrolysolytic
polyurethane adhesive is less hydrolytic, can be improved by adding a moderate amount of hydrolytic stabilizers (e.g. carbonized diamin, epoxy compounds, etc.). In order to improve the water resistance of polyester polyols themselves, long-chain binary acids and binary alcohols can be used as raw materials for the preparation of polyester polyols (e.g. diacids, 1,6-hexanols, etc.), in addition, the use of binary alcohols containing branch chains (e.g. new glycol) as raw materials can also improve the water resistance of polyester polyols. Polyether polyols are hydrolytically resistant compared to polyester polyols, so polyether polyols can sometimes be used to prepare polyurethane adhesives with better hydrolytic resistance by combining polyether polyols with polyester polyols. In addition, adding a small amount of silicone coupled agent to the formulation of polyurethane adhesives can also improve the water resistance of the adhesive layer to a certain extent.
3, improve the curing speed of polyurethane adhesive
A major way to improve the curing speed of polyurethane adhesive is to make polyurethane adhesive has a certain initial adhesive force, that is, after bonding is no longer easy to disengage. Therefore, improving the molecular weight of the main agent and using raw materials that can produce crystalline polyurethane adhesives are effective methods to improve the initial adhesion and curing speed. Sometimes the addition of catalytic crosslinkers such as triethanolamine can also help improve initial stickiness. In addition, the addition of catalysts is one of the main ways to speed up curing.
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