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Phenolic resins in rubber compounds: applications and new developments.

By the middle of the 19th century, driven by curiosity and market needs, several chemists made empirical discoveries without any pertinent theoretical knowledge or background. One of the most remarkable discoveries was the vulcanization of rubber by Charles Goodyear and Nelson Goodyear, the inventor of Ebonite; the difference being the addition of small and respectively larger amounts of sulfur which upon heating generated the crosslinking reactions capable of producing useful end products. The reaction of phenol with aldehydes leading to noncrystallizable compounds was reported way back in 1872 by Bayer. However, it was 30 years later when Baekeland (refs. 1-3) set out to develop a synthetic shellac, to be used in varnishes, and discovered several methods for the hardening of phenol-formaldehyde condensate resins. Later on (1917), he was granted another patent (ref. 4) claiming an improved method for producing printing matrices via impregnating fabrics with phenolic condensation products. By 1924, the phenolic resin production technology was already well established and had grown to become a large profitable business capable of supplying critical materials to other new industries of those days. Table 1 shows the phenoplast production increase compared to other thermosetting resins covering the 1975-1985 period.

Early developments

As early as 1905, Baekeland recognized the importance of functionality during his experiments. Through this visionary anticipation, he came to the correct conclusion that a trifunctional phenol would produce rigid network polymers. Thus, later on he was able to produce soluble linear paint resins (novolacs) by using ortho or para substituted phenols via alkaline or acid condensation of phenolic derivatives such as para-cresol.

The first attempts to modify rubber compounds with phenolic resins were made by Aylsworth in 1914. By incorporating C stage powder resins in proportions of up to 90%, the results were no different than those from any other inert filler. In 1916 even Baekeland became interested in exploring these blends, presumably looking for a way to make some B stage molding powders more flexible. However, the results were disappointing due to unacceptable porosities, possibly caused by the elimination of residual water at the high molding temperatures.

In spite of these initial failures, the potential of upgrading and/or developing new rubber products has continued to challenge the creativity of many researchers in the following decades as attested by the numerous patents in the early 1920s and 1930s. During this period, their efforts were more focused and boosted by the nascent science of polymer chemistry based on the pioneering work of Staudinger who advanced the idea that the rubber molecule was indeed a giant molecule or macromolecule with the following characteristics: Long chain bonded covalent C atoms; polymer chains entangled via intermolecular forces; and these forces were stronger for fibers, intermediate for plastics and weakest for elastomers.

Thus, one of the first issued patents (ref. 5) describes the development of rubber-phenolic resin compounds via mill addition of chlorohydrine or cyclohexanol which were claimed to act as co-solvents. The addition of sulfur was also claimed as beneficial. Another patent (ref. 6) claims the discovery of a flexible rubber product with good resistance to oils, fatty acids and lubricants, as well as acids in a liquid or gaseous state. The pertinent compound had the following composition: 40% rubber; 25% phenolic resin; and 20% graphite. The balance of additives was made up by inert fillers and ZnO.

Waterman, Van Vlodroop and Weldman (ref. 7) describe a procedure where a finely dispersible powder of a rubber-resin blend can be obtained by adding, under slow agitation, an alkaline solution of a phenolic resin to latex. This blend is subsequently coagulated with a diluted acid and converted into a molding powder, that after addition of fillers and curatives, leads to gaskets with desirable insulating properties. During an interesting follow up study (ref. 8), it was noticed that the condensation reaction of formaldehyde with polyphenols, such as pyrogallol (1,3,5 trihydroxybenzene) or resorcinol (1,3 hydroxybenzene) was much faster than with the classical phenol. Furthermore, the resorcinol novolacs showed a better solubility under alkaline conditions, as well as an improved conversion yield into resin solutions with good latex miscibility. Thus, by preblending the latex with curing agents (ZnO, diethyldithiocarbamate, etc.) and after heating the dried resinlatex compound for one hour at 100[degrees]C, it was possible to obtain a leather-like material with excellent physico-mechanical properties. Wildschut (ref. 9) investigated thoroughly the modification of phenolic resins with natural rubber. His main objective was to assess if, and under what conditions, a chemical reaction can occur in the above system. He theorized that preferably the hardening of the resin should not proceed too fast to prevent a condensation to a hard C stage resin, and the employed rubber should not contain any curatives. In one of his most relevant experiments, blends containing 100 parts rubber and 40 parts of various phenolic resol resins were heated between the platens of a curing press for two hours at 155[degrees]C. The resins with adequate vulcanizing properties produced easily removable flexible samples, whereas non-reactive resols acted more as plasticizers, leading to a mass of partially sticky decomposed rubber and chunks of hard resin. After solvent immersion, the swelling of the flexible specimens was similar to those cured with sulfur under standard conditions. The author summarized his findings as follows: Resols made from substituted phenols and compounded with synthetic nitrile rubber appeared to exhibit the most promising overall results.

At this point, it should be mentioned that the production and commercialization of synthetic rubbers provided a new incentive and major interest in the development of rubber-resin compounds with superior oil and heat resistant properties, especially for military applications. It is thus not surprising that R.C. Bascom (ref. 10) pursued a more scientific approach based on structure-property correlations. He was the first to clarify the general marginal finished product properties obtained heretofore with phenolic resin-natural rubber blends by stressing the mutual incompatibility of the above polymers. As proof, he stated simply that natural rubber is soluble in hydrocarbon distillates, but not in alcohols, known as good resol solvents. Follow-up work by other researchers showed that certain nitrile rubber grades (Hycar OR-25) blended with phenolic resins in a 100/50 phr ratio and in the presence of the usual curing resins exhibit an acceptable balance of properties with the additional benefit of higher filler loadings. Furthermore, compared to mechanical parts made of straight phenolic molding powders, those containing nitrile rubber exhibit better impact resistance, a broad range of service temperature (-40[degrees] to 120[degrees]C) and lower specific gravity, as well as higher extrusion rates combined with an upgraded finished product appearance. Finally, also mentioned for the first time is the suitability of nitrile rubber-phenolic resins as reciprocal property modifiers and multipurpose adhesives. The subsequent availability of nitrile elastomers in crumb form facilitated greatly the processing technology for the mentioned applications.

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A screening study carried out in the early 1950s involving natural rubber, SBR, NBR and two different resol resins confirmed, by and large, previously reported data. In terms of heat and solvent swelling resistance, SBR performed better than natural rubber. Best overall results were obtained with high nitrile content NBR grades, a phenolic resin (20 phr) and additional crosslinking agents (sulfur 3; ZnO 10; HMTM 1). The vulcanizations were carried out at 143[degrees]C for 30 and 60 minutes. As anticipated, longer curing time was found beneficial.

The rubber-resin system

Since these systems are by their very nature essentially incompatible, they can be broadly classified in the following four types according to the main component in the dispersed phase:

* Resin as the dispersed phase;

* rubber as the dispersed phase;

* grafted rubber latex particles as the dispersed phase; and

* filled graft as the dispersed phase.

Resin as the dispersed phase

Several resins, phenolic, coumarone, etc., have been used to reinforce natural rubber (ref. 11). Pressure sensitive adhesives represent another important system belonging to this class. They contain generally a rather low molecular weight resin (800-1,200) acting as a tackifier. In 1957, Wetzel (ref. 12) and Hock and Abbot (ref. 13) found that these adhesives were actually two-phase systems (figure 1). They also reported a phase inversion at high resin concentration (>60 phr) (figure 2), beyond which the tack value of the adhesive dropped to zero.

Based on previous findings, diffusion is ruled out unless both resin and rubber have identical solubility parameters. Thus, the major interfacial interaction is physical adsorption which in turn determines adhesion.

Rubber as dispersed phase

A rubber is milled and masticated with a resin or dissolved in a resin solution. At the conclusion of blending, the rubber is dispersed in a resin as particles of spherical or irregular shape. According to the major intermolecular forces governing adhesion, this system can be further subdivided into three classes: By dispersion forces, blend of two incompatible polymers; by dipole interactions, PVC and NBR; and by covalent bond epoxy resin with a carboxy functional elastomer.

Rubber particles in all these classes are nonporous and compact. The interfacial adhesion between the two incompatible polymers can be increased by addition of a third component, such as an adhesive or compatibilizer.

Since details regarding the two other systems involving grafting (ref. 14) are beyond the scope of this review, we shall mention only the following relevant conclusions:

* Incompatibility is an advantage for reinforcement provided that the adhesion at the interface is strong enough to withstand the applied stresses.

* The graft rubber was found to function as a compatibilizer and as an adhesive or coupling agent for the rubber-resin interface.

* Besides grafting, good interfacial bonding can be achieved by forming hydrogen bridges, by causing dipole interaction or by creating covalent linkages.

* Crosslinking and thermal/oxidative degradation of the rubber phase are the most adverse factors.

Major resin types

Raw materials

Phenolic resins are formed by the condensation of a phenol with a baldheaded-primarily formaldehyde using either base or acid catalysis. Although there are many phenols which can be used, only a few have gained commercial importance. They can be divided into two major classes, unsubstituted and substituted.

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To this day, phenol itself, and more recently resorcinol, which reacts readily at room temperature, are the most important representatives of the first class. Of the substituted phenols, the most important are alkylated phenols like m-cresol which reacts with formaldehyde more readily than phenol, and 3,5 xylenol which reacts even faster than does m-cresol. It has been recommended for low temperature resins. Furthermore, p-cresol is less active than phenol and thus is considered less suitable for curing resins. Other representative raw materials are: p-tertiary butyl, amyl and octyl phenol. Small amounts of nonyl, and dodecyl phenols are also used.

By contrast, the aryl substituted, o- and p-phenyl phenols have been gradually phased out due to their scarcity and high cost. A similar fate, for controversial pollution and health reasons, could be in store for bisphenol A, one of the most useful raw materials in the production of phenolic/epoxy condensation resins. The chemical structures of these derivatives are shown in illustration 1.

Since formaldehyde in its pure state is a highly reactive gas, it is commercially used as a 40% water solution known as formalin.

Finally, hexamethylenetetramine [([CH.sub.2]).sub.6][N.sub.4] abbreviated as HMTA or "hexa," a reaction product of 6 mol of formaldehyde and 4 mol of ammonia, is used as an alkaline curing catalyst for C stage novolac molding powders, as well as in rubber formulations compounded with resol type resins.

The condensation reaction

The behavior of phenol when condensed with formaldehyde can be understood when considering the chemical reactions which might occur. The active hydrogens responsible for the phenolic resin formation are situated at the ortho and para positions to the hydroxyl group of phenol. In an alkaline solution, formaldehyde adds preferentially to these positions; and when used in excess at moderate temperatures forms the pertinent methylol derivatives, as shown in illustration 2. At higher temperatures, elimination of water and or [CH.sub.2]O from pairs of methyl groups yields structures, as shown in illustration 3. If desirable, these A stage resol resins may be advanced almost to incipient gelation (B-stage) by additional heating, and even cured to the C-stage via heating at higher temperature or with the addition of acids.

Linear novolac resins containing -[CH.sub.2]- linkages may be produced under acid conditions (usual1y [H.sub.2][SO.sub.4]) by condensing formaldehyde with an excess of phenol. These A-stage resins are advanced to the B-stage with HMTA, and after being mixed with other essential ingredients pertinent to the envisioned end use (wood flour, inert fillers, pigments, lubricants, recycled rubber, etc.) and further polymerized by passing through steam heated differential rolls, an extruder or heavy duty ribbon blender. The term phenolic molding compound is applied to the granulated product containing the Bstage resin which is subsequently cured to the C-stage in a mold. A structural formula of this hard high molecular weight polymer is shown in illustration 4.

Typical properties

Although phenoplasts, the generic designation of these thermosetting polymers, are considered as highly mature plastic materials, they have earned their longevity due to a combination of desirable properties such as good heat resistance, dimensional stability, good resistance to most solvents and excellent dielectric properties. By varying the nature of phenol, the ratio of phenol to formaldehyde and the conditions of preparation, a broad variety of resins can be made. Within the scope of this review we shall focus on the type of resins of special interest for elastomeric adhesives and related applications.

These end uses range from adhesives for tire cord, plywood and brake linings, to molded automotive and electrical parts such as printed circuits, composite materials and even missile nose cones.

As adhesive components they are used as tackifiers, heat reactive curing resins, reinforcing agents, etc. The simplest definition of tack is the ability of two materials to resist separation. However, practically one has to deal with two types of tack. One is pressure sensitive tack or the "quick stick" characteristic for pressure sensitive adhesives, capable of displaying tack upon application of light finger pressure, and thus facilitating the lifting of the backing on which the adhesive is coated. The second is defined as building or "green tack" based on the autoadhesion of adherents and directly related to the specific adhesion of the rubber compound to itself. The magnitude of the bond depends on the cohesive or internal strength of the pertinent rubber compound or adhesive, and is also referred to as "green strength." Tack and especially "building tack" are very important in the tire manufacturing process and related products (hose, belts, etc.) where multiple layers must be plied together and remain in place until curing is achieved. During this final step, good interlayer adhesion is another critical factor to prevent formation of voids and/or bubbles leading to defects in the finished product. While rosin derivatives and coumarone indene resins were originally used with natural rubber, the picture has changed drastically with the advent of synthetic rubbers, their blends, etc., which has lead to the need for more effective tackifiers. Judging from consumption data, phenolic resins have filled this need, representing the largest volume used in the production of tires. They are, in essence, alkylated (t-butyl) phenol-formaldehyde condensation resins referred to broadly as novolacs. Besides tires, they are also used in compounds for the production of hose, airspring, conveyor and power transmission V belts. In addition to several grades of the above resins, there are also commercially available terpene-phenolic tackifymg resins designed for use in adhesives based on NBR, PC or SBR, either alone or in conjunction with other resins.

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Since these resins are commercially available in a number of grades which differ in terms of a variety of factors, such as chemical composition, molecular weight, solubility, softening point or Tg, specific gravity, color, aged tack retention, polymer/cure system and cost, proper selection certainly represents a daunting task. The following lab screening test was found helpful: SBR is broken down on a tight mill for 15 minutes and tackifier resins like Amberol ST 137X and SP1068 are added at the 10 phr level and mixing is continued for a total of 30 minutes. Next, the stock was sheeted off at 3/16" thickness and allowed to stand for two hours. At the end of this time, 11/2" wide strips were cut from the stock, superimposed and rolled firmly together with a 5 lb. hand roller. Three test specimens of each resin were prepared next, set aside and tested after 10 minutes, two hours and 15 hours. The test consists of pulling the strips apart manually and judging the force required to separate them. Two percent of zinc naphthenate was found to enhance the adhesive properties.

Generally, resins with higher mw and high softening points tend to exhibit higher tack after high temperature/humidity aging conditions. In addition to the type and mw of the selected tackifier, the loadimg is also important. For the majority of tire compounds, common loadings are up to 5 phr. At low loadings, some tackifiers are not too effective and must be added in sufficient quantity to achieve desired results (ref. 15).

Heat reactive curing resins

This combined classification could sound confusing since any curing resin is obviously thermo-reactive. However, from the previously described chemistry of phenolic resins it should be kept in mind that only a resol resin characterized by the presence of dibenzylether groups and of a terminal benzene ring containing a methylol group can react by itself in the presence of heat to become a thermoset.

In contrast, a novolac by itself is thermoplastic and the alkyl phenols in their structure are joined by a -[CH.sub.2]- methylene bridge. Some of the previously described alkyl-phenol tackitiers, which are in essence novolacs, can be further reacted with additional formaldehyde and/or a methylene donor such as "hexa" (HMTA) and become a thermoset. On the other hand, to accelerate the action of a resol curing resin, a halogen donor like polychloroprene or hydrated stannous chloride can be added to the rubber formulation. In terms of end uses, the heat reactive novolac resins are primarily used as modifiers for polychloroprene cements to control some critical properties such as open time and heat resistance. In contrast, resole type resins like SP1045 at the 10 phr level are preferred for curing butyl bladder compounds. More details on the latter application including reaction mechanism, as well as pertinent controlling factors, etc., can be found in chapter 18 of reference 16. A significant contribution involving the chemistry and curing mechanism of phenolic resins in the vulcanization of EPDM was made by Duin and Souphanthong (ref. 17). By using sophisticated analytical procedures (GPC, IR, NMR), they provided the first experimental evidence to the existence of methylene bridges in the structure of the SP1045 cured rubber which was only suggested by van Der Meer in the early 1940s. Under certain conditions chromane bridge structures were also detected. Pending results from a follow up investigation, the authors may propose a curing mechanism involving both structures.

As a result of dynamic mechanical analysis (DMA) carried out on resols at 110-125[degrees]C with a rapid heating schedule to simulate the pressing conditions used in the manufacture of oriented strand board and plywood, it was found that this analytical method is useful for measuring the cure time, vitrification time as well as other relevant parameters of phenolic resins as affected by the temperature and resin composition (ref. 18).

Reinforcing resins

For some time it has been found that different kinds of resins provide some reinforcement to natural and synthetic rubber. However, in contrast to carbon black and other traditional fillers, for best results special methods of incorporation must be employed. Thus, one process consists of preliminary dispersion of the resin in the rubber followed by subsequent hardening, as the dispersion in the form of a masterbatch is subjected to additional milling, via refiner, prior to vulcanization. In the patent granted to Simmons (ref. 19), it is claimed that compounds of high modulus, high resilience and reasonably high tensile strength can be obtained in this way. Reinforced carbon black compounds of equivalent hardness to resin compounds have considerable lower resilience, but better tear and abrasion resistance. The resin may be one that hardens just by generation of heat or may require, just as in the case of phenol formaldehyde resins, the addition of a methylene donor such as HMTA. Van Alphen (ref. 20) found that many resins can be formed in a latex which has been stabilized against acid coagulation by means of a cationic or nonionic surfactant. The rubber coagulates along with the resin and the whole mass can be crumbled, leached with ammonia and subsequently dried. The product is then mixed on a mill with or without fillers and vulcanized in the usual way. The vulcanized compounds with up to 30% resin are rubber-like materials; with 30-50% resin they become leatherlike, and at still higher loadings become brittle. According to Van Alphen, the resin formed in the latex generates chemical bonds between the resin particles, which are much stronger than those of a reinforcing black and the surrounding rubber molecules. Further work related to the use of phenol-formaldehyde novolac reinforcing resins to increase the hardness of EPDM based rubber extruded profiles which are used in the automotive industry for weatherproofing windows, doors and car hoods, was reported in 1996. More detailed data on the above and on the effects of various modifiers such as hydroxy polybutadiene, cashew nut oil, cresol and tall oil on alkyl novolac reinforcing resins utilized in conjunction with NBR, EPDM as well as SBR/NR blends in tire related compounds were reported in a follow-up lab study carried out subsequently (ref. 21). The resin level was 8 and 15 phr, the rheometer cure time (tc90%) 5-40 minutes at a temperature range of 150-182[degrees]C and in the presence of HMTA and HMMM methylene donors.

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The overall results for some critical parameters (hardness, tear resistance, scorch safety, abrasion, etc.) were encouraging awaiting follow-up confirmation. It should be pointed out that in contrast to the novolac tackifiers, which have a linear structure, the reinforcing novolacs are more branched. The compatibility of reinforcing novolacs with pertinent elastomers is schematically shown in table 2.

As seen from the above, the highly polar NBR offers the highest possible loadings, and via incremental additions, has the potential to also achieve higher loadings of resin for the less compatible elastomers. Some of the cashew modified novolac reinforcing resins are already commercially available and claim to be highly compatible with NBR at all practical loading levels. They are claimed to prevent scorching and precuring for resins blended with HMTA. Also available are tall oil modified novolacs, recommended for SBR compounds at the 10-12 phr levels. They are claimed to facilitate scorch free processing, acting like plasticizers. HMTA can be subsequently mill added to ensure a desirable reinforcing action. More details on these resins can be found via data sheets and/ or supplier (Akrochem) websites.

Most significant applications

Elastomeric adhesives

After the recorded commercialization of phenolic resins in 1909, it did not take long for Baekeland to also discover their usefulness as adhesives. Thus, in 1912 he was granted a patent (ref. 22) describing "a method for cementing or joining articles by applying to the surfaces an adhesive containing partially condensed phenol-formaldehyde resins followed by moderate heating to an infusible state without risking premature hardening." During the 1930-35 period, researchers focused their efforts toward a better understanding of various factors affecting the phenol-formaldehyde condensation reaction. Practically, it was not until 1938 when the use of a resorcinol-formaldehyde latex was announced for bonding fabrics to rubber in the production of tires. Prior to WWII it became obvious that the brittleness of phenolic adhesives is a negative factor which had to be overcome to expand their use in terms of cleavage resistance. A first step in this direction was made by De Bruyne who described the use of polyvinyl-acetal resins as modiiers for the phenolic adhesives. As defined by Pocius in his manual (ref. 23), these were the first so-called "structural adhesives" formulated to resist cleavage forces. Thus, bonds to aluminum made with this type of adhesive were capable of sustaining at RT loads of up to 20.7 MPa (3,000 psi), whereas unmodified phenolic adhesives exhibited lap-shear strengths of only 13.8 MPa (2,000 psi). The "redux" systems, although very stable and still in use, have certain technological drawbacks such as cumbersome application techniques and the need for high pressures to ensure proper bonding. These led to the development of structural film adhesives based on NBR-phenolic resin blends capable of developing bond strengths for aerospace aluminum alloys in the range of 27 MPa (4,000 psi) and t-peel strength of 5.3 Kn (30 piw) at relatively low pressures and similar cure temperatures. These data obviously depend on the resin/rubber ratio employed in the compound which is roughly 2:1 in an illustrative formula. The employed phenolic resins are a 1:1 blend of Bakelite 18773 and Durez 7031A and the rubber component Hycar 1001.

These types of nitrile-phenolic adhesives are used for a wide range of applications ranging from automotive brake linings to clutch surfaces and aerospace fuselages based on a combination of desirable features such as heat/lubricant resistance, together with fatigue resistance and vibration damping. The Tg of the adhesive system is claimed to be below RT, suggesting that the cured adhesive exhibits electrometric properties over a wide range of usage temperatures.

Meanwhile, the production of several NBR grades in readily soluble crumb form like Hycar 1432 facilitated the development of many useful oil resistant cements, adhesives and binders for cork gaskets and brake linings. Whenever desirable, due to the increased polarity of the high nitrile content grades, it became possible to also upgrade their heat resistance via incorporation of resol phenolic resins. In addition, nitrile rubber can also be used as a compounding modifier in phenolic structural adhesives imparting flexibility and oil resistance. Hycar 1072 provides good compatibility with phenolic curing resins, low temperature resistance to brittleness and good hot tear in addition to excellent abrasion resistance.

Polychloroprene (Neoprene) is undoubtedly a prominent elastomer for adhesives because of its polarity and crystallinity. The polarity provides a broader formulating range in bonding a variety of materials, and the crystallinity gives improved cohesive strength. For certain less demanding applications, good bonds can be achieved just by the evaporation of the solvent from the adhesives using a higher crystallization grade.

In order to enhance specific adhesion, improve cohesive strength and hot bond strength, alkylphenolic heat reactive resins like SP 126 or even terpene-phenolic resins may provide a desirable balance of properties. Their compounding level is usually within the 15-35 phr range. It has also been reported that tert-butyl phenolic resins also used as tackifiers in CR adhesives have the ability to react with MgO upon solvent evaporation. However, if the phenolic resin is increased beyond a certain level, the "tack open time" decreases, which is undesirable for practical application of the adherent due to reduced tackiness.

The "one part" solvent contact bond adhesives are the so-called workhorses of the industry. They usually consist of a medium crystallyzation grade like Neoprene AC which is dissolved in a combination of organic solvents such as toluene, hexane and MEK. A typical formula may consist of Neoprene AC 100, [M.sub.g]O 7.5 (phr), antioxidant 3.0, Cabosil M5 5.0, SP 134 17.0, SP 154 17.0 and solvent blend 337.0 (phr).

To comply with environmental rulings, latex contact bond adhesives are gradually gaining acceptance in spite of their longer drying times and relative inferior tack.

Pressure sensitive adhesives

Pioneered by 3M in the mid 1930s, these adhesives used initially for sandpaper and masking tapes based on natural rubber have gradually seen a tremendous development into a high volume industrial sector. Table 3 shows data providing an illustration of the challenging technological trends faced and overcome by the pressure sensitive tape manufacturers in the past decades. However, the use of phenolic and terpene-phenolic heat reactive tackifiers has been rather limited to specialty applications such as creped paper automotive masking tapes to prevent any residue after removal from painted autobody parts, electrical insulating tapes based on vinyl, paper and polyester film backings, thermal bar code printing, electronic assembly, etc., as well as for other specialty applications requiring resistance to extreme operational conditions (space exploration, etc.).

Hot melt adhesives

Defined as adhesives applied from the melt and gaining strength upon RT solidification, hot melt adhesives have recently gained broad acceptance due to environmental regulations, since they are solvent-free. The operational range of a hot melt adhesive is directly dependent on the relationship between the melt temperature, Tin, and the glass transition temperature, Tg, of the respective compound formulation. This correlation can be simply expressed via the equation: [DELTA]T = [T.sub.m]-[T.sub.g].

The compounder must overcome the challenge of raw material selection to maximize the AT and make sure, at the same time, that the level of the plateau modulus is high enough so that it can support the loads for which the adhesive must perform according to its end use (book binding, sealing of corrugated boxes, etc.). This is quite a difficult task since any overheating of the melt can create hazardous working conditions as well as undesirable darkening and adhesive property changes. Some of the polymers used are primarily polyvinylacetate/polyethylene (EVA), SBS and SIS block copolymers of the Kraton type, phenoxy resins like high mw DGEBPA (diglicidyl ether of bisphenol A), phenol or epoxy terminated, and also phenolic and terpene phenolic resins like SP553 as tackifiers. Their proportion in relation to the elastomer can be as high as 1:1 depending upon the adhesive end use which can also include pressure sensitive duct, paper and strapping tapes. It is understood that high mw and high melt point tackifiers are preferred since they must withstand the high melt temperatures with no or only minimal color changes.

Specialty adhesives

Specialty adhesives find increased use in the electronic industry. In one of the more interesting patents granted to Santorelli (ref. 24) it is claimed that a thermosetting adhesive based on heat reactive resol or novolac resins capable of being modified with 5-40% epoxies, nitrile rubber, cashew nut shell oil, tall oil or even polyvinyl-butyric in conjunction with an ethylene acrylic elastomer like Du Pont's Vamac provides a thermosetting adhesive with improved bond strength and flexibility, capable of withstanding various manufacturing conditions necessary to produce flexible circuitry (soldering, etching, etc.). It is also claimed that this kind of adhesive can be applied as a solution to a Kapton polyimide film, copper or nickel foil and also cast as a free standing film itself which in turn can be utilized for laminates. As a possible replacement, even a nitrile rubber like Hycar 1032 carboxy terminated nitrile rubber or carboxy terminated butadiene rubber, carboxy functionality, can also be employed. The catalyst for the preparation of the formaldehyde/phenol resin (mole ratio 0.64) was oxalic acid and the preferred curing agent an imidazole derivative.

A composition for sealing semiconductors with good mold ability, solder, heat and impact resistance is described in a Japanese patent (ref. 25). Thus, a composition containing epoxy resins, a cresol novolac modified epoxy resin, a brominated phenol novolac resin, modified siloxanes, silica and other additives was blended, crushed into a powder, transfer injection molded and cured for eight hours at 175[degrees]C to give a product having good spiral flow and crack resistance after 500 cycles at +150[degrees]C and -196[degrees]C and also good solder resistance (failure after 72 hours in steam and the next 10 seconds in solder at 260[degrees]C). It is claimed that the above composition is superior versus those based on methyl siloxane derivatives.

Recent developments

New tackifiers

The development of a new class of "super tackifiers" was recently reported (ref. 26). The experimental work was based on a model formula consisting of a natural rubber/high cis-polybutadiene (75/25) blend in conjunction with carbon black (60 phr) and a vinyl modified novolac resin at the 4 phr level. It is claimed that the cure and tack properties of the above novel resin are dramatically better than those of the standard p-tert octyl phenol tackifier, equivalent to p-tert butyl phenol resins and under severe hot/humid storage conditions even net superior to the Koresin (tea-butyl acetylene condensate) type super tackifier.

Presently, these new reins are commercially available under the trade designations Elaztobond T-6000 and T-8000. More details on these and other newer enviro-friendly resins can be obtained from the supplier web site. Another recently developed solvent free tackifier dispersion, based on an alkyl phenol-ethoxilate resin, is being promoted for use in pressure sensitive adhesives containing emulsions of acrylic, NR and SBR polymers. Marketed under the designation Snowtack (Hexion S.C.), major applications include, besides tapes, also paper and film backed labels, as well as automotive, building and construction adhesives.

Resorcinol-formaldehyde condensation resins are now available in solid and liquid form under the designation Penacolite (R-50, R-220, 2170). They are formaldehyde-free and recommended to increase rubber adhesion to steel, polyester, rayon, aramid and other industrial fabrics as single step dipping systems. Similar versions of the above are also available for two-step dipping systems. Another aqueous dispersion resin, 1-868D, is a diisocyanate modified resoreinol derivative which could be used in conjunction with standard RFL adhesive systems as an adhesion promoter and also for the same purpose in the above described single- and two-step dipping systems. Other Penacolite resins have also been developed for dry bonding systems to promote adhesion to organic fibers as well as to brass plated wire in standard NR/SBR tire compounds. Detailed data sheets can be obtained from the supplier (Indspec Chemical).

Composites'

Composites are generally combinations of plastic materials with reinforcing fibers and sheet materials. This designation has been extended lately to a wide range of materials as diverse as concrete, fiber reinforced cements and metals, as well as wood and even bone powder. Plywoods are important commercial composites; they are laminates bonded with solvent and lately also waterborne amino and amino formaldehyde resins. The latter resins are also used in making particle boards from wood shavings. An important class of composite materials used for tires, belts, hoses, etc., is based on elastomers and fibers. Earlier, satisfactory bonding at the cotton/rubber belt interface was achieved via dipping systems based on natural or vinyl pyridine latexes in conjunction with resorcinol-formaldehyde emulsions. However, with the development of NR, SBR, EPDM, etc., blends in the tire manufacturing process, the switch from the bias ply to the radial ply construction with the subsequent evolution toward new synthetic fibers and even the use of steel cord, the need for higher strength bonding systems becomes obvious. Nevertheless, the authors of a recent review study (ref. 27) involving fiber adhesion to rubber compounds came to the conclusion that the RFL treatment, to which rayon and all synthetic fibers are subjected, is still widely used for almost all fiber/rubber composites, and that none of the alternatives that have been developed since the early 1930s have reached the same or higher levels of adhesion. It is also stated that the RFL dipping process is still more an art than a science and that the mechanism by which the adhesion is obtained for every fiber/rubber combination has to be adjusted accordingly. They are resorcinol/formaldehyde ratio; resin/latex ratio; latex type; pH of the dip; dip pick-up; cure time and temperature of the dip, etc.

Notwithstanding the above conclusions, the authors did not address the bonding of steel cord which has posed a challenging task for the development of more specific bonding systems.

In an extensive follow-up review involving the above issue, Van Ooij, Harakuni and Buytaert (ref. 28) have indicated that compared to the standard use of Co to upgrade the adhesion between rubber and brass coated cords, silane and others offer more formulating latitude to achieve low heat build-up without compromising the rubber-cord adhesion. The same authors claim that comparatively a one-component system based on melamine resins provides similar favorable results vs. a twocomponent system containing both the Co salt and an HMMM/resorcinol/formaldehyde bonding system. Other researchers confirmed the above findings by stating as an additional benefit the removal ofresorcinol, while at the same time lowering the sulfur and Co content in the rubber. Hotaka, Ishikawa and Mory have studied the effect of compound ingredients on the rubber steel cord adhesion. Their findings based on a formula comprising the B 18-S Penacolite resorcinol resin (2 phr) and HMMM (4 phr) among other ingredients (NR, HAFLS, etc.) indicate that the above dual system showed better interlayer adhesion, especially at the moisture aged stage (ref. 29). More recently, a comprehensive review of the above topic mentions the polymerization of a resorcinol resin with HMMM and that the resulting compound incorporated at the 3 phr level and after diffusing to the cord surface provided a protective barrier, thus enhancing the bond stability under high humidity aging conditions (ref. 30).

Heat reactive phenolic resins like SP 1045 used in conjuntion with stannous chloride, as well as CR as a halogen donor, have gained broad acceptance for curing butyl bladder compounds since they provide excellent crosslinking density combined with good flex and heat aging properties. However, if incorporated as such in powder form they cause processing problems due to intermix stickiness leading to batch-to-batch inconsistencies and time wasting mixer clean-up problems. According to a recent paper by Monthey, this can be prevented via the use of a polymeric binder premixed cure system package. In addition, it is claimed that the amount of curing resin, usually 10 phr, can be reduced by 20% leading to an overall improved cost-effectiveness (ref. 31).

Thermoplastic elastomers have gained a rather wide acceptance due to the fact that they have many rubber-like properties, but are processed as thermoplastics. Thus, they offer a substantial economic advantage since they do not have to be vulcanized into end parts. However, their usefulness for certain critical applications is limited due to their marginal thermostability and rather poor resistance to attack by hot oils. As described by Coran (ref. 32), these and other deficiencies have been successfully eliminated via dynamic vulcanization. This process can be described as follows: After sufficient melt mixing of rubber and plastic in an internal mixer or compounding extruder, vulcanizing agents are added. Vulcanization then occurs while mixing continues. After being discharged from the mixer or extruder, the blend can be briefly homogenized on a mill, cut into strips, pelletized, injection molded, etc. More specifically, an EPDM-PP blend was dynamically vulcanized in the presence of SP 1056, a dimethyl-p-octylphenol curing resin catalyzed by stannous chloride (ref. 33). It is claimed that this remarkable invention ensures complete vulcanization of the rubber component which is crucial in achieving a very strong elastomer with improved critical properties. Subsequent follow-up work in the field of these new EPDM-modified TPVs showed that it is possible to estimate crosslinking densities in the rubber phase by using swollen state NMR spectroscopy.

Challenges and opportunities

The collapse of the building industry in our recessionary economy, combined with the health hazards posed by the formaldehyde emissions from the strand board utilized in low cost prefab homes, has dealt a serious blow to the growing phenolic resin market in the last decade. Fortunately, this "call to order" has triggered a burst in R&D activities focused on the development of alternate eco-friendly materials and technologies.

Suppliers of phenolic resins for the rubber industry had little choice but to adjust quickly to the market realities via development of modified resins requiring reduced amounts of formaldehyde or its total replacement via increased use of methylene donors combined with stannous chloride or even CR to facilitate the transition to a desirable level of thermosetting properties. Another step involved the replacement of phenol with the more reactive resorcinol in gradually reduced amounts to minimize any degassing health hazards.

Waterborne tackifier resin systems are now commercially available for standard or carboxy SBR or NBR latexes for a variety of adhesive applications (carpet backings, pressure sensitive tapes, etc.). Standard terpene-phenolic resins utilized in hot melt adhesives can be replaced now with lower softening point versions which exhibit broader polymer compatibility without slowing down the melt rate, while keeping the hot melt viscosity in line with lower application temperature requirements. Although the above represent only a fraction of the recent accomplishments, they surely did not come the easy way, and attest to the diligence and dedication with which polymer scientists and practitioners have responded to the challenges posed by the task of creating enviro-friendly products and working conditions.

In terms of opportunities for future advancements, here are a few suggestions for possible consideration:

* Develop phenolic type resins from renewable resources, including even via biosynthesis.

* Explore new applications via hybridization and compatibility enhancement of phenolic resins with other polymers for electric car auto parts or to promote adhesion to nonpolar hydrophobic substrates.

* Design a simple and reliable top performance bonding system for the steel cord/rubber interface.

* Investigate structural modifications of present resins with styrene and/or related derivatives to upgrade reinforcing properties.

* Modify phenoxy resins via compatibilization with carboxy functional nitrile or butadiene elastomers.

* Investigate the use of phenolic resins in conjunction with recycled rubber or other ground polymers to create new composites.

Summary

Although the discovery of phenolic resins dates back to the past millenium, they are not only still around, but the production fundamentals related to the molar ratios of reactants are about the same. The key to their longevity can be explained only if one considers the potential of applications offered via their functionality also leading to the development of alkyl tackifiers. Rubber compounders have recognized their utility, especially after WWII when synthetic elastomers like NBR became commercially available. This, in turn, facilitated major applications, especially in the field of adhesives which are described in more detail. The development of super tackifiers offers the compounder a potential avenue for also achieving reinforcement via a possible controlled curing system due to the presence of vinyl structures comprising allylic hydrogen. It should be stressed that resin manufacturers have responded in a timely manner to regulatory constraints and continue their efforts to come up with more enviro-friendly products. Special consideration was also given to the developments in the field of rubber/steel cord bonding systems. Due to the conservative attitude of the tire industry and their reluctance to make changes in their present bonding systems, as long as they have worked and have proven themselves, the described efforts, although encouraging, should still be considered as work in progress. Far from being all comprehensive, it is hoped that this review will inspire to day's researchers to emulate Baekeland's visionary accomplishments.

References

(1.) L.H, Baekeland, U.S. Patent 942,699 (1907).

(2.) L.H Baekeland, U.S. Patent 942,809 (1907).

(3.) L.H Baekeland and A.H. Gotthelf U.S. Patent 1,217,115 (1917).

(4.) L.H. Baekeland U.S. Patent 1,233,298 (1917).

(5.) Chemisches Zentralblatt N, 607 (1923).

(6.) Chemisches Zentralblatt 11, 1,295 (1930).

(7.) H.I Waterman, C. Van Vlodroop and R. Weldman, Chem. Weekbl 32, 622 (1935).

(8.) Chem. Abstracts, 35, 1,385 (1941).

(9.) H.L. Wildschut, Rev. Gen. Caoutchouc, 20, 251 (1943)

(10.) R.C. Bascom, Modern Plastics, 27, 84 (1949),"

(11.) H. Dalesch-Paetsch, Kautschuk und Gummi, 9, 312 (1956).

(12.) F.H. Wetzel, Rubber Age, 82, 291 (1957).

(13.) C. W Hock and A. N Abbot, Rubber Age, 82, 471 (1957).

(14.) A.L. Alexander, "Interaction of Liquids at Solid Substrates, ACS, Advances in Chem. Series, 8i." 85 (1968).

(15.) C.K Rhee and J C. Andries, Rubber Chem. & Technology 54, 107 (1981).

(16.) B. Stuck in "Rubber Technology, "J.S. Dick, ed., p. 443, Hanser-Gardner, Cincinnaiy (2001).

(17.) M. van Duin and A. Souphanthong, Rubber Chem. & Technology, 68, 71 7 (1995).

(18.) K.G. Moon, L.S. Worm and R.M. Meacham, Ind.Engn. Chem. Res. 30, 798 (1991).

(19.) D.N. Simmons, Brit. Patent 686757 (1953).

(20.) J. Van Alphen "Rubber Reinforcement By Resins formed in Latex" Proceedings 3rd Rubber Tech. Conference 670, London (1954).

(21.) B. Stuck, M.C. Morel-Fourier and L.S. Souchet, presented at ACS Rubber Div. Meeting, Montreal (1996).

(22.) L.H. Baekeland U.S. Patent 1,019,407 (1912).

(23.) A.K. Pocius, "Adhesion and Adhesives Technology, " 2nd Ed. pp.259-264, Hanser-Gardner, Cincinnati (2002).

(24.) M. Santorelli U.S.. Patent 4,578,315 (1986).

(25.) C.A. Select Adhesives, Jap. Patent 03, 79, 623 (1991).

(26.) T.E. Banach, L.S. Howard and M. Bellil, Rubber & Plastic News, 16-19 (2006).

(27.) K.B. Wennekes, R.N Dotta and J. W Noordermeer, Rubber Chem. & Technology 81, 523 (2008).

(28.) W.J. Van Ooij, P.B. Harakuni and G. Buyaert "Adhesion of Steel cord to Rubber," J, Rubber Chem. and Technology 82, 315 (2009).

(29.) T Hotaka, Y. Ishikawa and K. Mory, Rubber Chem. and Technology, 78, 175 (2005).

(30.) P. Patil, W. Van Ooij and A. Tisler, Rubber & Plastics News, 15-21 (2004).

(31.) S. Monthey, "Using resin cure packages to improve bladder life" Rubber & Plastics News, 15 (2010).

(32.) A. Y Coran, "Vulcanization, Conventional and Dynamic, " Rubber Chem. and& Technology 68, 351 (1995).

(33.) S. A. Saber and M.A. Fath, U.S.. Patent 4,311,628 (1982).

by Alfred W. Blum, Chemintertech Associates
Table 1--U.S. production of thermosetting
resins (millions of pounds)

Resin type/year 1975 1980 1985

Phenolics 1,100 1,500 2,600
Polyesters 800 950 1,200
Ureas 690 1,200 1,200
Epoxies 200 320 390
Melamines 120 170 290

* R.B. Seymour & C.F. Carraher, Jr., "Polymer Chemistry,"
Marcel Dekker (1988)

Table 3--pressure sensitive adhesive trends
(% of total consumption)

Technology 1974 1980 1985 1990

Solvent 95 70 25 25
Water-borne 3 18 45 15
Hot melt 2 12 30 60
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