Solvent-borne

Water-borne primers may or may not contain chromate-based corrosion inhibitors. The limited solubility of chromate salts in water makes them less than ideal for use in water-based primers, and much work has gone into developing alternatives [36], but the performance of recently developed water based primers using strontium chromate as the corrosion inhibitor is excellent, however, and appears equivalent to the solvent-borne analogues [37]. 

Adhesion of paints and adhesives to TPOs is especially problematical due to the aliphatic nature of the substrate material. In Europe, plasma and corona treatment is employed to render these surfaces wettable and obtain strong adhesion by adhesives and paints in automotive manufacture. In the United States, however, primers based on solvent-borne chlorinated polyolefin oligomers (CPOs) have become the treatment of choice for these substrate materials. The VOC emissions from these primers are considerable (as in all solvent-borne adhesives), but the less... 

Although waterborne systems were developed in the 1960s, the form of this chemistry that dominates the industry utilizes end-functional, high molecular weight base polymers dissolved in organic solvents. Work on solventless condensation systems continues, but has not yet become commonplace [45,47]. Solvent-borne condensation cure systems are convenient for their ease of pro-... 

The viscosity of elastomeric adhesives determines their method of application. Typically, solvent-borne rubber compounds require low viscosities for application. Thus, viscosities lower than 0.6 Pas for spray, curtain or dip applications are required, whereas for brush application viscosities lower than 5 Pa s are generally used. Roll- coating technology requires viscosities between 5 and 15 Pa s. 

In recent years, the use of solvent-borne adhesives has been seriously restricted. Solvents are, in general, volatile, flammable and toxic. Further, solvent may react with other airborne contaminants contributing to smog formation and workplace exposure. These arguments have limited the use of solvent-bome adhesives by different national and European regulations. Although solvent recovery systems and afterburners can be effectively attached to ventilation equipment, many factories are switching to the use of water-borne rubber adhesives, hot melts or 100% solids reactive systems, often at the expense of product performance or labour efficiency. 

Typieal composition of a solvent-borne rubber adhesive Elastomer 100 phr... 

Several types of polychloroprene elastomers have been commercialized. The most important polymers used in solvent-borne adhesives are the following. 

Neoprene AF ( 963). It is a polychloroprene modified with methacrylic acid. Although it is a slow-crystallizing elastomer, the cohesive strength develops very rapidly and it has improved creep resistance at high temperature compared with Neoprene AC or AD. The improved properties of Neoprene AF are derived from the interaction between the carboxyl functionality with the metal oxides added in the solvent-borne polychloroprene adhesives. 

Neoprene AH (1975). It is a methylacrylate-modified elastomer which is non-crystallizing and is chemically peptizable in aliphatic solvents. However, it is generally prepared as a dispersion in hexane, and has balanced properties between conventional solvent-borne adhesives and aqueous systems. 

The polymer type influences several properties of solvent-borne CR adhesives, mainly the molecular weight and rate of crystallization. 

Butyl phenolic resin is a typical tackifier for solvent-borne polychloroprene adhesives. For these adhesives, rosin esters and coumarone-indene resins can also be used. For nitrile rubber adhesives, hydrogenated rosins and coumarone-indene resins can be used. For particular applications of both polychloroprene and nitrile rubber adhesives, chlorinated rubber can be added. Styrene-butadiene rubber adhesives use rosins, coumarone-indene, pinene-based resins and other aromatic resins. 

Compatibility. Clear definition of compatibility is rather difficult. Compatibility has been defined as the ability of two or more materials to exist in close and permanent association for an indefinite period without phase separation and without adverse effect of one on the other [28]. On the other hand, compatibility is easily recognized in solvent-borne adhesives as a homogeneous blend of materials without phase separation. Normally, compatibility is understood as a clear transparent mixture of a resin with a given polymer. But, compatibility is a more complex thermodynamic phenomenon which can be evaluated from specific... 

For viscosity or sag control. When the rubber base adhesive is applied on a vertical surface, addition of a filler prevents the adhesive from running down the wall. In solvent-borne formulations, fumed silica can be used as anti-sag filler. In water-borne systems, clays impart yield stress and excellent sag control. 

Resistance to weathering. Zinc oxide and magnesium oxide stabilize poly-chloroprene against dehydrochlorination. Further, zinc oxide helps vulcanize the rubber, and magnesium oxide reacts with /-butyl phenolic resin to produce a resinate which improves heat resistance of solvent-borne polychloroprene adhesives. 

Metal oxides. Magnesium oxide is used to cure polychloroprene by converting its few active allylic chloride from 1,2 addition into ether cross-links. There is a synergistic effect when magnesium oxide is used in combination with t-butyl phenolic resins in solvent-borne polychloroprene adhesives. When solvent is removed, the phenolic group in the resin reacts with the magnesium oxide to cross-link [49]. 

Very good water and moisture resistance. Solvent-borne adhesives have an advantage over latex, which contains surfactants. 

Most mineral fillers can be easily incorporated into solvent-borne and water-borne (adding adequate surfactants and wetting agents) adhesives. 

Hydrocarbon resins, rosin, rosin ester, coumarone indene resins, and terpene resins can be directly added to solvent-borne adhesives. For latex adhesives, resin emulsification must be produced before addition. 

Plasticizers and oils, curatives and accelerators may also be added. In solvent-borne adhesives, curing agents are generally packed separately. 

Solvent-borne adhesives. Although the NR polymer is inherently tacky, tack-ifying resins are generally added to improve bonding to polar surfaces. Because the solids content in these adhesives is lower than 35 wt%, they are not suitable for gap filling. The quick-grab (cements) adhesives are particular because they contain about 65 wt% rubber, and set within a few seconds under finger pressure. 

The solvent-borne NR adhesives show an important mechanical component in the bonding process, and therefore bulk mechanical and rheological properties (addition of fillers is quite effective) are important. In fact, these adhesives are mainly suitable when at least one of the surfaces to be joined is water-porous (paper, concrete, leather, textiles). 

Solvent-borne NR and quick-grab adhesives are commonly used in the manufacturing of leather footwear for temporary bonding and in rubber footwear... 

Adhesion promoter. Epoxy silane can be added to increase adhesion to glass (mainly for BR sealants). In certain textile applications, isocyanates can be added to solvent-borne BR cements. 

Specific formulations of BR and PIB adhesives can be found in [5]. These adhesives are supplied in forms quite similar to those of natural rubber solvent-borne and water-borne dispersions, and pressure-sensitive pre-coated films. 

A broad range of solvents can be used in solvent-borne BR and PIB adhesives. Hydrocarbon (hexane, heptane, naphtha) and chlorinated solvents (perchloroethy-lene) can be used they provide higher viscosities. The presence of small amounts of stabilizer may cause cloudy solutions and settling is rarely produced. There is a logarithmic relationship between viscosity and solids content. In fact, a small... 

Although BR adhesives are used as solvent-borne dispersions, pre-formed tapes and hot-melt PSAs are more common forms. 

SBR adhesives can be used as latices or as solvent-borne, pressure-sensitive and tape adhesives. 

Solvent-borne CR adhesives and polychloroprene latices will be considered separately. 

Formulation of a solvent-borne CR. A typical formulation of a solvent-borne CR adhesive may include the following components (fillers are not commonly added and curing agents are added to improve heat resistance) (1) polychloroprene elastomer (2) metal oxides (3) resins (4) antioxidants (5) solvents (6) fillers (7) curing agents (8) other modifiers. 

Metal oxides. Metal oxides provide several functions in solvent-borne polychloroprene adhesives. 

Acid acceptor. This is the main function of metal oxides in CR adhesive formulations. Upon age, small amounts of hydrochloric acid are released which may cause discolouration and substrate degradation. Magnesium oxide (4 phr) and zinc oxide (5 phr) act synergistically in the stabilization of solvent-borne polychloroprene adhesives against dehydrochlorination. 

The high heat resistance produced by adding phenolic resins to solvent-borne CR adhesives is due to the formation of the infusible resinate, which reduces the thermoplasticity of the adhesive and provides good bond strength up to 80°C (Table 11). The resinate also increases the adhesive bond strength development by accelerating solvent release. 4 phr of magnesium oxide for 40 phr of phenolic resin are sufficient to produce a room temperature reaction. A small amount of water (1-2 phr) is necessary as a catalyst for the reaction. Furthermore, the solvent... 

Terpene phenolic resins can also be added to solvent-borne CR adhesives to increase open tack time and to provide a softer glue line than /-butyl phenolic resins. To provide adequate hot bond strength, these resins are used in combination with a polyisocyanate curing agent. 

Chlorinated rubber is also used to promote the adhesion of solvent-borne CR adhesives to metals and plasticized PVC. Addition of a low molecular weight chlorinated rubber (containing about 65 wt% chlorine) improves the shear strength and creep resistance of polychloroprene adhesives [75] but a reduction in open time is also produced. A heat reactivation (process in which the surface of the adhesive film is raised to 90-100°C to destroy the crystallinity of the film and allowing diffusion to produce polymer chain interlocking more rapidly) restores tack to the polychloroprene adhesives. 

Isocyanates can be added to solvent-borne CR adhesive solutions as a two-part adhesive system. This two-part adhesive system is less effective with rubber substrates containing high styrene resin and for butadiene-styrene block (thermoplastic rubber) copolymers. To improve the specific adhesion to those materials, addition of a poly-alpha-methylstyrene resin to solvent-borne CR adhesives is quite effective [76]. An alternative technique is to graft a methacrylate monomer into the polychloroprene [2]. 

Antioxidants. A good antioxidant should be added to CR adhesives to avoid oxidative degradation and acid tendering of substrates. Derivatives of diphenyl amine (octylated diphenyl amine, styrenated diphenyl amine) provide good performance but staining is produced. To avoid staining, hindered phenols or bisphenols can be added. 2 phr antioxidant is sufficient in solvent-borne CR adhesives formulations. 

Solvents. Solvents affect adhesive viscosity, bond strength development, open time, cost, and ultimate strength. Blends of three solvents (aromatic, aliphatic, oxygenates, e.g. ketones, esters) are generally added, and in their selection environmental and safety regulations must be considered. A graphical method has been proposed to predict the most adequate solvent blends for solvent-borne CR... 

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