Rubber-based additives

The increased polarity of the acrylic polymers puts more stringent requirements on the properties of the tackifiers or plasticizers that can be used. The very low polarity additives commonly found in rubber based PSAs are not useful in most acrylic PSA formulations. For example, materials like paraffin waxes, mineral oils, and synthetic hydrocarbon tackifiers have little or no value in most acrylic PSAs. 

Some rubber base adhesives need vulcanization to produce adequate ultimate strength. The adhesion is mainly due to chemical interactions at the interface. Other rubber base adhesives (contact adhesives) do not necessarily need vulcanization but rather adequate formulation to produce adhesive joints, mainly with porous substrates. In this case, the mechanism of diffusion dominates their adhesion properties. Consequently, the properties of the elastomeric adhesives depend on both the variety of intrinsic properties in natural and synthetic elastomers, and the modifying additives which may be incorporated into the adhesive formulation (tackifiers, reinforcing resins, fillers, plasticizers, curing agents, etc.). 

Variety of form. Rubber base adhesives can be supplied for assembly operations as solvent or water-borne dispersions, hot melts, precast films, extruded tapes or reinforced films. In addition solvent and water-borne dispersions can be supplied as single or two-components systems. 

Rubber base adhesives develop strength faster than most other polymeric types. Fig. 1 [3J shows the differences in the development of peel strength for several rubber polymers (without additional additives, except an antioxidant). Natural... 

Tg values of resins (30-90°C) are higher than those of rubbers (—70 to —30°C), so the addition of resin can be used to raise the average Tg of the rubber base formulations. 

In rubber base adhesives, fillers may affect properties such as cohesion, cold flow, rheology and peel adhesion. Most fillers increase cohesion and reduce cold flow. In some formulations, even a small addition of filler dramatically reduces peel strength either because of interactions with the tackifier or because filler particles at the surface reduce the area of contact between the adhesive and the substrate. 

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. 

Density. Most fillers added in rubber base formulation have a density between 2 and 2.7 g/cm-, except barium sulphate (4-4.9 g/cm- ) and zinc oxide (5.6 g/cm ). Addition of filler increases the free volume of the polymer and, in general, there is a critical concentration of filler at which the density of the formulation increases. The method of incorporation of filler in the adhesive formulation is important because air voids may appear when a poor dispersion is produced. 

Some inorganic fillers are used as flame retardants in rubber base formulations. Flame retardants act in two ways (1) limiting or reducing access of oxygen to the combustion zone (2) reacting with free radicals (especially HO ), thus acting as terminator for combustion-propagation reaction. The additives most widely used as flame retardants for polymers are antimony oxides and alumina trihydrate. 

For instance, rubbers based on polydimethylsiloxane obtained from me-thyltrichlorosilane with 55% chlorine content, have a tensile strength of 6 MPa, whereas rubbers based on the same elastomer, but obtained from me-thyltrichlorosilane with 55.7% chlorine content (which shows the presence of methyltrichlorosilane impurities), have a tensile strength of 2 MPa. Thus, if technical dimethyldichlorosilane contains a considerable amount of methyltrichlorosilane, it is subjected to additional rectification. Since the boiling point of methyltrichlorosilane differs from the boiling point of dimethyldichlorosilane only by 4 °C, the mixture should be rectified in towers with a large number of theoretical plates. 

Copolymers with butadiene, ie, those containing at least 60 wt % butadiene, are an important family of rubbers. In addition to synthetic rubber, these compositions have extensive uses as paper coatings, water-based paints, and carpet backing. Because of unfavorable reaction kinetics in a mass system, these copolymers are made in an emulsion polymerization system, which favors chain propagation but not termination (199). The result is economically acceptable rates with desirable chain lengths. Usually such processes are mn batchwise in order to achieve satisfactory particle size distribution. 

Economy For many applications, rubber-based adhesives and sealants are inexpensive relative to the base cost of many other polymer systems. In addition, the costs for dispensing and application tooling are often low. The speed of assembly operations with quick-stick adhesives often provides a labor and total cost advantage, and may even lower tooling costs. 

Virtually all rubber materials, and plastic materials, can be made into an adhesive or sealant compound. This is because many elastomers begin as monomers dispersed in water or solvent and are polymerized in situ. Latex products can remain so, while solid elastomers that are extracted from either water or solvent systems can be solvated with an appropriate organic solvent system. In addition, most solid elastomers exhibit thermal flow characteristics which can make them suitable for hot-melt formulations. And since there are many different rubber polymer families, it stands to reason that there will be many different rubber-based adhesives to identify and describe. Some, however, have... 

Tables 9.30-9.35 display a series of model formulations on which further compound optimization can be based. Additional formulations are available in industry publications such as those from the Malaysian Rubber Producer s Research Association (Malaysian Rubber Producers Research Association, 1984 Waddell et al., 1990).

In 1893, the French chemist Moreau described two routes for the synthesis of acrylonitrile that were based on the dehydration of either acrylamide or ethylene cyanohydrin [10]. There was very little interest in acrylonitrile until 1937 when synthetic rubber based on acrylonitrile-butadiene copolymers was first developed in Germany. A process based on the addition of hydrogen cyanide to acetylene was developed at that time and in the 1950s, the acrylic fiber industry provided the stimulus for further process developments. Today acrylonitrile is made commercially by one of three possible methods (a) from propylene, (b) from acetylene and hydrogen cyanide, and (c) from acetaldehyde and hydrogen cyanide. 

Polymers, fillers, softeners and some additives are normally contained in the rubber base, whilst crosslinkers and tin catalysts, as well as extenders and dyes or pigments (if added for visual mixing control), are included in the curing agent. 

Bromobutyl rabber is relatively impermeable to air and moisture and it is often a major portion of the tire innerliner composition. The addition of the starch/plasticizer dispersion composite further reduces the already low air permeability of the butyl rubber-based com-positioa Starches with high softening points are difficult to incorporate it into rabber composition without plasticizer. ... 

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