Polyurethane adhesives hardening

Polyurethane adhesives are formed by the reaction of various types of isoeyanates with polyols. The polar urethane group enables adhesion to various surfaees. Depending on the raw materials, glue lines with rubber-like elastic to brittle-hard behavior ean be aehieved. The presence of reactive terminal groups provides a ehemieally hardened adhesive. When polymerized to a high enough molecular weight, the adhesive ean be physically rather than chemically hardened, i.e. a hot melt. 

Toluene diisocyanate is commonly produced as a mixture of the 2,4- and 2,6-iso-niers, that is used as a monomer in the preparation of polyurethane foams, elastomers and coatings, as a cross-linking agent for nylon-6, and as a hardener in polyurethane adhesives and finishes. Polyurethane elastomers made from toluene diisocyanates are used in coated fabrics and clay-pipe seals. Polyurethane coatings made from toluene diisocyanates are used in floor finishes, wood finishes and sealers, and in coatings for aircraft, tank trucks, truck trailers and truck fleets (United States National Library of Medicine, 1997). 

Many interior fittings (e.g., instrument panel, door and sidewall panels, package trays, seat linings, boot carpetings, rear window shelves, etc.) are often manufactured by vacuum drawing and laminating the cover sheet material (leatherette, textile, leather, etc.) onto the trim panel. The use of hot-melt (also cross-linking) adhesives and water-based polyurethane adhesives (with hardener) is state of the art for this type of component production. 

Polyurethanes Thermosetting polyurethane adhesives can be used for structural applications. They are normally two-component systems based on an isocyanate resin, the second component being one of a number of hardeners. Some thermoplastic solvent-based polyurethanes are available. 

The chemistry of 2C-Polyurethane adhesives is outlined in Fig. 61. Cross-linking occurs after mixing a polyol-based, filled resin with an isocyanate-based hardener which can either be a monomeric diisocyanate or an isocyanate terminated prepolymer synthesised from a polyol with the isocyanate-containing compound. The active hydrogen groups, such as OH, NH, SH etc. react in an electrophilic manner with the isocyanate groups. 

One-component polyurethane adhesive Adhesive containing isocyanates ernes on exposure to moisture or to heat, when latent hardener is included Primer Special coating designed to improve adhesion between adhesive and substrate - applied before adhesive application Reactive adhesives Adhesives that cure or set when exposed to heat, moisture, radiation, etc. 

Two-part polyurethane adhesive Addition reaction of two components, main component and hardener... 

The generic term polyurethane adhesives covers both adhesives that already contain polyurethane compounds or compounds with a urethane bond and also reactive adhesives in which urethane groups are formed only during the hardening process. 

Polyurethane Adhesives. The search for formaldehyde-ffee binders resulted in a new generation of polyurethane-based glues for chipboard production. The usage of polyurethane based adhesives for assembly of wooden materials has also increased. These liquid polyurethane adhesives are single-component adhesives that harden by a chemical cross-linking reaction which is initiated by moisture in the material. The water... 

Figure 1.1 A caravan sandwich panel being sprayed with a moisture-hardening rubbery polyurethane adhesive, in a solvent, prior to bonding on the inner skin. The absence of mechanical fasteners greatly increases the production rate and results in a more pleasing appearance.

For the steel sector, two-component cross-linking polymerization adhesives of polyester, vinyl ester, or acrylate base are important. They are relatively tolerant when it comes to the mixing ratio of resin and hardener and can also be used at low temperatures. They are the most important representatives of polyaddition adhesives including 2-part epoxy and 2-part polyurethane adhesives. 

Some recent literature has been published dealing with moisture-reactive hot-melt polyurethane adhesives (Frisch Jr 2002). Most moisture-curing hot-melt adhesives are obtained by reacting a crystallizable polyol such as poly(hexamethylene adipate) and monomeric MDl (NCO/OH ratios = 1.5-2.2). A catalyst such as dimorpholinediethyl ether is also necessary. The polyester polyol plays a key role because open time, viscosity, and glass transition temperature can be adequately tailored. A mixture of hydroxyl-terminated polyesters having different characteristics allows to control the adhesion and hardening time of the adhesive. 

Polymers. AH nitro alcohols are sources of formaldehyde for cross-linking in polymers of urea, melamine, phenols, resorcinol, etc (see Amino RESINS AND PLASTICS). Nitrodiols and 2-hydroxymethyl-2-nitro-l,3-propanediol can be used as polyols to form polyester or polyurethane products (see Polyesters Urethane polymers). 2-Methyl-2-nitro-l-propanol is used in tires to promote the adhesion of mbber to tire cord (qv). Nitro alcohols are used as hardening agents in photographic processes, and 2-hydroxymethyl-2-nitro-l,3-propanediol is a cross-linking agent for starch adhesives, polyamides, urea resins, or wool, and in tanning operations (17—25). Wrinkle-resistant fabric with reduced free formaldehyde content is obtained by treatment with... 

In many adhesives formulations, the resin portion is the same or similar and what determines its properties is the hardener or catalyst that is used to cure the resin. Thus, depending on the hardener, epoxy adhesives may be referred to as amine-cured, anhydride-cured, polyamide-cured, or novolac-cured. Polyurethanes may be polyol-cured or hydroxypolyester-cured. 

Thus, the hardening of the toluene diisocyanate-based polyurethane system in the presence of microquantities of the surface-active agent KEP-2 occurs at all stages of the process in conditions different from the ordinary hardening mixture of TMP-TDI-oligoglycol. These differences become apparent in kinetic and structural studies of the hardening process. The final product is a polymer with very weak adhesion compared with the initial specimen (hardening temperature 353 K). 

The reduced equilibrium moisture content of heat-treated wood can influence the hardening of adhesives that need water for the curing reaction. This is significant for 1-component polyurethane (PUR) adhesive, which cures slower when bonding heat-treated wood with a smaller content of hydroxyl groups than non-treated wood... 

PVAc-based commercial wood adhesives are evaluated using standard tests for non-structural applications, as reported in EN 205 [8], and they are classified in agreement with the standard EN-204 [9]. This standard allows to classify wood adhesives in 4 categories from D1 to D4. D1 adhesives show a good resistance only in dry conditions D2 adhesives should withstand a rather low water presence, such as in occasional exposure in kitchens and bathrooms D3 adhesives are suitable to come in contact with cold water, such as for outside windows and doors, kitchen and bathrooms furniture D4 adhesives are suitable to be used in extreme conditions (resistance to hot water). Vinyl acetate homopolymer can be used to formulate D1 or D2 adhesives. Vinyl acetate based adhesives cross-Unked with hardeners and urea-formaldehyde (UF) adhesives belong to class D3. Only the phenol-formaldehyde (PE), resorcinol-formaldehyde (RF) and melamine-formaldehyde (MF) adhesives, some special 2-component polyurethanes (PUs), and cross-linking vinyl adhesives belong to class D4. 

Bisphenol-A-based epoxy with a poly(amido amine) hardener system cured Mesuaferrea L. seed oil-based hyperbranched polyurethane (HBPU)/ clay nanocomposites obtained by an ex situ solution technique, was also reported. The partially exfoliated nanocomposites showed a two-fold improvement in adhesive strength and scratch hardness, 10 MPa increments in tensile strength and thermostability at 112°C with little effect on impact resistance, bending and elongation at break compared to a pristine epoxy-modified HBPU system. However, similar epoxy-cured Mesua ferrea L. seed oil-based HBPU/clay nanocomposites exhibited a two-fold increase in tensile strength, a 6°C increase in melting point and thermostability at 111°C after nanocomposite formation using an in situ technique. An excellent shape recovery of about 96-99% was observed for the nanocomposites. The above observations confirm that the performance characteristics of nanocomposites are influenced by their preparation technique. 

As structural adhesives, epoxies are the most widely accepted and used. They typically contain several components, the most important being the resin. To the base resin is added a variety of materials, for example hardeners, flexibilisers, tougheners and fillers. These all contribute to the properties of the resulting adhesive. Formulations may be further varied to allow for curing at either ambient or elevated temperatures. The epoxies and polyesters, together with acrylics, polyurethanes and synthetic polymer lattices will be... 

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