Rubber-resin compatibility

A good PSA can be made if we understand the rubber-resin compatibility. The compatibility of natural rubber with various resins is explained in papers published by Class and Chu.GO-13) The compatibility of resin and rubber can be determined by measuring viscoelastic properties of the blend. Compatibility is identified (in the G vs. temperature plot) by a pronounced shift of the tan 8 peak maximum temperature (Tg), associated with a decrease in the storage modulus in the plateau. An incompatible system is confirmed by a minimal shift of the tan 8 peak maximum temperature along with an increase in the storage modulus in the plateau (Figure 21). A second peak in tan 8 may be apparent in the incompatible system. Compatibility of rubber-resin systems depends on the structure, molecular weight, and concentration of the resin in the blends. The compatible systems exhibit pressure-sensitive adhesive performance at some ratio of rubber to resin. The incompatible systems, on the other... 

Figure 24 shows the viscoelastic properties of Kraton block copolymer blends with low-molecular-weight modifying resins. The rubber-resin compatibility data can be obtained from these curves. As mentioned in the previous section, a block copolymer can have two glass transition temperatures. A value at -90°C corresponds to polybutadiene domains (rubbery... 

At this point, we explained the principle of the rubber-resin compatibility using the dynamic mechanical properties of the blend. We also introduced the conventional pressure-... 

Nitrile rubber is compatible with phenol-formaldehyde resins, resorcinol-formaldehyde resins, vinyl chloride resins, alkyd resins, coumarone-indene resins, chlorinated rubber, epoxies and other resins, forming compositions which can be cured providing excellent adhesives of high strength, high oil resistance and high resilience. On the other hand, NBR adhesives are compatible with polar adherends such as fibres, textiles, paper and wood. Specific formulations of NBR adhesives can be found in [12]. 

Action of the plasticizer depends also on the stracture of the rabber. For example, a low amount of chlorine in chlorinated rubber decreases compatibility of chlorinated rubber with the plasticizer and the other resins. ... 

They reach the general conclusion that the compatibility of the various resins with the various rubbers impacts on the viscoelasticity of the blends and therefore on their pressure-sensitive performance. To the extent that resin structure and molecular weight determine compatibility, they are the variables of importance. They also reach the conclusion that the tan delta peak temperature for compatible rubber-resin blends can be estimated by the inverse temperature law of Fox. 

An examination of rubber-resin blends shows that compatibility is very sensitive to structure, even with resins that have apparent molecular weights below 1,000. Polystyrene resin 900, 1.6) was found to be... 

As the concentration of a normally compatible resin is increased in a rubber-resin blend, a level is reached where two phases appear. Natural rubber blended with a poly (vinyl cyclohexane) resin and styrene-butadiene rubber blended with a polystyrene resin, systems which were compatible at the 50% resin level, appear to have two phases at the 75% resin level. 

Tan 6 peak temperature vs concentration for blends of natural rubber with compatible resins. 

The plots demonstrate the applicability of the Fox equation to estimate the tan 6 peak temperature of rubber-resin systems. Using the Fox equation, the tan 6 peak temperature can be predicted for any concentration of a compatible rubber-resin system by determining the value for a single composition, as well as for the unmodified elastomer. 

The tan 6 peak temperature for compatible rubber-resin blends can be estimated using the reciprocal relationship proposed by T.G. Fox. 

The plateau modulus of compatible rubber-resin blends can be calculated from the volume fraction of the polymer. 

Chlorinated rubber is compatible with alkyds of similar linearity and low polarity. Most of the highway marking points in the United States are based on a combination of chlorinated rubber and a compatible alkyd resin based on soybean oil. 

An extremely tough epoxy based vinyl ester having 20% elongation This is a non-rubber resin, developed for SPI Liner applications to minimize cracking or crazing due to thermal or mechanical shock and physical abuse. Retains very good chemical resistance. Is 100% compatible with the other Co-Rezyn Vinyl Esters to add additional toughness. Other uses ... 

Other resins which find use as tackifiers for Neoprene include polyterpene resins, hydrogenated wood rosins, rosin esters, and couma-rone-indene resins. Chlorinated rubber is used to promote metal adhesion and as an ingredient of two-component adhesives. Poly-alpha-methyl styrene is used to obtain better specific adhesion to thermoplastic rubber. The compatibility of Neoprene with resins and other polymeric materials is detailed in Ref. 11. 

An adhesive formulator has limitations on the amount of resin which can be incorporated into an elastomer in an attempt to obtain the desired combination of rheological properties. All resin-elastomer blends show a variation in adhesive properties vs. the concentration of resin. Fig. 5 shows probe tack as a function of resin concentration for a natural rubber-resin ester blend. This type of resin response curve is typical of all adhesive systems. In Fig. 5, little enhancement of tack is seen up to 40% resin concentration. Between 40 and 65% resin, there is a rapid increase in tack, followed by an equally rapid drop off in tack above 65% resin. Above, 65%, the system becomes overloaded in resin, incompatibility develops, and tack drops. The maximum in a resin response curve will be determined by the general compatibility... 

Envelope sealing adhesives are usually remoistenable, and typically blends of dextrin and compatible polyvinyl aeetate homopolymers as well as all dextrin or all resin types. Some water-borne and hot melt pressure sensitive adhesives based on acryhc or rubber resin liquids and rubber resin hot melts are also used. Natural rubber latex is the basis for self sealing envelopes. 

In appearance and on handling the material is somewhat intermediate between a wax and a rubber. It is also semi-tacky. Like isotactic polypropylene it is attacked by oxygen but unlike the isotactic material it swells extensively in aliphatic and aromatic hydrocarbons at room temperature. It is also compatible with mineral fillers, bitumens and many resins. 

Being either brittle or soft, these resins do not have the properties for moulding or extrusion compounds. These are, however, a number of properties which lead to these resins being used in large quantities. The resins are chemically inert and have good electrical insulation properties. They are compatible with a wide range of other plastics, rubbers, waxes, drying oils and bitumens and are soluble in hydrocarbons, ketones and esters. 

Because of their wide compatibility and solubility, coumarone resins are used considerably in the paint and varnish industry. The resins also find application as softeners for plastics and rubbers such as PVC, bitumens and natural rubber. 

Class and Chu demonstrated that if a tackifier is chosen that is largely incompatible with the elastomer, a modulus increase due to the filler effect is observed and little change in Ta results, and once again a PSA would not be obtained. This was observed for mixtures of low molecular weight polystyrene resin and natural rubber. The same polystyrene resin did tackify SBR, a more polar elastomer that is compatible with the resin. Hydrogenating the polystyrene to the cycloaliphatic polyvinylcyclohexane changed the resin to one now compatible with the less polar natural rubber and no longer compatible with SBR. These authors also provide... 

Other natural product-based resins also became widely used, such as the light colored Lewis acid oligomerized products of terpenes such as a-pinene, p-pinene, and limonene. These natural product resins are relatively expensive, however, and formulators now often use the newer, less expensive synthetic resins in present day natural rubber PSAs. These are termed the aliphatic or C-5 resins and are Lewis acid oligomerized streams of predominately C-5 unsaturated monomers like cis- and /rawi-piperylene and 2-methyl-2-butenc [37]. These resins are generally low color products with compatibility and softening points similar to the natural product resins. Representative products in the marketplace would be Escorez 1304 and Wingtack 95. In most natural rubber PSA formulations, rubber constitutes about 100 parts and the tackifier about 75-150 parts. 

In the earlier art, there was some consideration that partial incompatibility of the tackifier resin with the rubber was responsible for the appearance of tack, but this no longer is seriously held in light of continuing studies by many investigators. Aubrey [38] has addressed this in his review of the mechanism of tackification and the viscoelastic nature of pressure sensitive adhesives. Chu [39] uses the extent of modulus depression with added tackifier as a measure of compatibility. Thus in a plot of modulus vs. tackifier concentration, the resin that produces the deepest minimum is the most compatible. On this basis, Chu rates the following resins in order of compatibility for natural rubber rosin ester > C-5 resin > a-pinene resin > p-pinene resin > aromatic resin. 

In general, resins are compatible with a large number of materials (oils, plasticizers, polyethylene waxes, rubbers). Compatibility depends on resin type, molecular weight and its distribution, resin structure and configuration, and finally on application requirements. 

A more quantitative estimation of compatibility can be obtained with the solvent cloud point test. The solvent cloud point is based on the idea that resins will be compatible with elastomers of similar chemical nature. Thus aliphatic resins will be effective tackifiers for aliphatic elastomers, such as natural rubber, while aromatic solvents are needed for aromatic elastomers, such as SBR. Solvent cloud point tests are carried out in three solvent systems which represent aliphatic, aromatic, or polar systems [16j ... 

Rosin esters show low cloud points and would have wide compatibility with most elastomers. Aliphatic hydrocarbon resins, however, will only be compatible with aliphatic elastomers (e.g. natural rubber). 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

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