Advances in addition-cure phenolic resins

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Abstract

Recent developments in the area of addition curable phenolic resins are reviewed. The article highlights the chemistry of addition-cure phenolic resins and discusses the different strategies involved in their molecular design. Structural modification through incorporation of thermally stable, addition curable groups on the novolac backbone is one strategy. The transformation of phenolic hydroxyl groups to addition curable functions forms an alternate approach. Cross-linking of novolac or its derivatives with a suitable curative also leads to addition-curable phenolic resin systems. This article examines the synthesis, characterization and curing of noted addition curable phenolic systems. Their thermal, physical and mechanical properties are discussed and the structure–property correlations examined. In selected cases, the adhesive properties of the systems have been examined. The review includes discussions on the properties of the composites in relevant cases. The systems discussed here include mainly allyl- and maleimide-functional phenolics, epoxy–phenolic, polybenzoxazine, bisoxazoline–phenolic, acetylene-functional and propargyl ether phenolics and phenolic-triazine. The relative advantages and demerits of these systems are discussed and their application potentials are considered.

Introduction

Despite the emergence of several new classes of thermosets, high performance polymers and several other new generation materials that are superior in some respects, phenolic resins retain industrial and commercial interest, a century after its introduction. Phenolic resins are preferred in a wide range of applications, from commodity and construction materials to high technology aerospace industry. This recognition emerges from the fact that these resins have several desirable characteristics, such as superior mechanical strength, heat resistance and dimensional stability, as well as, high resistance against various solvents, acids and water. They are inherently flame resistant, and evolve low smoke upon incineration. Although phenolics cannot be substitutes for epoxies and polyimides in many engineering areas, their composites still find a major market in thermo-structural application in the aerospace industry due to good heat and flame resistance, excellent ablative properties and low cost. These key properties add to their market growth, and as a result of innovative research, new products and applications continue to emerge, demonstrating the versatility and the potential of phenol resins to cope with the ever-changing requirements and challenges of advanced technology [1], [2], [3], [4], [5].

Undisputedly, classical phenolic resins based on resole and novolac dominate the resin market. However, their acceptance as a universal material in many engineering areas is hampered by some of the inherent qualities derived from their special chemical structures. These resins cure at moderately high temperature by a condensation mechanism with the evolution of volatiles, which necessitates application of pressure during molding to form void-free components. The need for the use of catalyst for curing and the limited shelf life of resin at ambient conditions are also major shortcomings of these systems. When compared to many known thermally stable polymers, their thermo-oxidative stability is low. The rigid aromatic units tightly held by the short methylene linkages make the matrix brittle. In view of this, a new chemistry is needed to modify the cure of phenolic resins, in particular, a new method is needed to chain extend and/or to cross-link phenolic resins without production of volatiles and allow for extended shelf stability at ambient conditions for the formulated thermosets. In doing so, it is imperative that the modifications do not impair the thermo-mechanical characteristics of the resultant system. The concept of addition cure phenolics gains significance in this context.

Several approaches have been reported for modification of phenolic resins and their cure chemistry. Structural modification to confer addition-cure character has been one thrust area of research [6], [7]. Addition-curable phenolic resins with improved thermal and pyrolysis characteristics will be the desirable resins in composites for thermo-structural applications [8]. Higher char-yield leads also to a better heat shielding. Such high char phenolics could be potential candidates as matrices in carbon/carbon composites too with obvious advantages [9]. The major strategies in designing addition-cure phenolics are:

  • (i)

    Incorporation of thermally stable addition-curable groups on to novolac backbone

  • (ii)

    Structural modification (transformation) involving phenolic hydroxyl groups

  • (iii)

    Curing of novolac by suitable curatives through addition reactions of OH groups

  • (iv)

    Reactive blending of structurally modified phenolic resin with a functional reactant

This article gives an account of recent research efforts in these directions for realizing addition curable phenolic resins.

Section snippets

Allyl-functional phenolics

Allyl phenol–formaldehyde novolac, synthesized by the allylation of novolac can cure thermally at 180 °C without the evolution of volatiles. On heating, the O-allyl derivative rearranges to the C-allyl polymer prior to cross-linking. The thermal curing of this resin takes place by polyaddition at allyl double bonds. The curing rate and cross-link density depend on the content of the reaction centers in the polymer molecule [10]. The allyl derivatives of phenols have been used for the

Bisoxazoline–phenolics

The unusual addition co-reaction of novolac phenolic resins with phenylene bisoxazoline has been explored to derive a new class of non-conventional phenolic thermosetting resin by Culbertson et al. [49]. The polymerization involves a tertiary phosphine-catalyzed reaction of bisoxazoline with a phenol-free novolac resin leading to an ether–amide copolymer as shown in Scheme 8.

The systems are suited for high performance composite applications [50]. The key features, which foretell the great

Polybenzoxazines (PBZ)

Another interesting addition-cure phenolic system is based on oxazine-modified phenolic resin that undergoes a ring-opening polymerization to give polybenzoxazine, which is effectively a poly(amino-phenol). The precursors are formed from phenol and formaldehyde in the presence of amines. The choice for phenol and amine permits design flexibility and polymer property tailoring. The as-synthesized mixture consists of monomer, and oligomers that contain phenolic groups. For practical applications,

Phenol–epoxy systems

Curing of epoxy with novolac type phenolic resin, making use of the OH–epoxy reaction, appears to be the simplest way to design addition-cure phenolic system. Although less preferred, polyphenols are used as curative for epoxies, since the addition-curing results in void-free products which are comparatively tougher due to the formation of flexible ether network [125], [126], [127], [128]. Phenol–epoxy thermosets are preferred in void-free composite structures. The interest in these systems has

Phenolic resins with phenyl maleimide functions

Novel phenolic novolac resins, bearing maleimide groups (PMF resin) and capable of undergoing cure principally through addition polymerization of these groups were synthesized by polymerizing a mixture of phenol and N-(4-hydroxy phenyl) maleimide (HPM) with formaldehyde in the presence of an acid catalyst [172]. The synthesis is shown in Scheme 28. The maleimide-content was varied by regulation of the stoichiometry in the feed. The resins were characterized by chemical, spectral and thermal

Pendant phenol-functional thermoplastics

Linear vinyl polymers with pendant phenolic groups were realized by free radical copolymerization of N-(4-hydroxy phenyl) maleimide (HPM) with butyl acrylate (BuA) and acrylonitrile (AN) and were characterized [178]. These thermoplastics (BNM) could form good films, with mechanical and adhesive properties dependent on the maleimido phenol-content in the chain, as given in Table 32. The structure of the terpolymers can be found in Scheme 34. The polymer films could be directly served as

Propargyl ether functional phenolics

Although less commercially exploited, propargyl ether-functional phenolic resins were developed as a potential hydrophobic substitute for epoxies in advanced composites, electronics, adhesives and coatings. The majority of thermosets, such as epoxy, BMI, etc. absorb moisture up to 5%, resulting in low hot/wet physico-chemical properties. For advanced applications, the required hot/wet performances for many composites are to exceed temperatures of about 230 °C. Hydrophilicity can lead to easy

Phenolic resins with terminal acetylene groups

Addition curable phenolic resins, bearing terminal ethynyl groups, anchored to a benzene ring through a phenyl azo linkage (ethynyl phenyl azo novolac, EPAN), were realized by a novel and simple synthesis strategy involving the coupling reaction between novolac and 3-ethynyl phenyl diazonium sulfate [205]. The synthesis is shown in Scheme 40. The diazo coupling was limited to the para position of the novolac and occurred to a maximum of 50 mol%. The molar mass, determined from GPC showed a

Phenolic resins with phenyl ethynyl groups

Replacement of terminal acetylene groups by phenyl ethynyl function gives scope for improving further the thermal stability of phenolic networks due to the higher aromatic-content of the cross links. Phenyl ethynyl groups have, of late, received a great deal of attention as a means of thermally chain extending and cross-linking polymers [211], [212]. On thermal curing, they provide a three-dimensional network exhibiting an excellent combination of properties including high glass transition

Comparative thermal property of PMF, PN, EPAN and PEPFN resins

On comparing the thermal stability of the addition-cure phenolic resins belonging to PMF, PN, EPAN and PEPFN class as a function of composition, it was observed that in many cases, thermal stability and char-yield increase with increased cross linking via enhanced functionalization. Exceptions were noted in the case of the blend of PMF with allyl novolac and for propargylated novolac (PN resins). In these two cases, the thermal stability decreased with cross-linking due to the increase in the

Phenolic–triazine resin (P–T resins)

Phenolic triazine (P–T) precursor resin is a reaction product of novolac resin and cyanogen halide. P–T network is formed by the thermal cyclotrimerization of the cyanate ester of novolac as shown in Scheme 44 [219], [220]. It is an ideal matrix system for composites, because it combines the processibility convenience of epoxies and the thermal capabilities of polyimides and fire resistance of phenolics. The absence of volatile by-products during cure renders them attractive matrices for

Outlook

The foregoing discussion has presented a consolidated view of the recent developments in non-conventional, addition curable phenolic resins. Phenolic resin still commands considerable research and industrial interest. Innovative research is focused on means to address the shortcoming of these systems in terms of processibility and oxidative resistance. The introduction of addition-cure phenolics is a partial answer to these problems. The allylphenol–BMI system is suited for void-free composite

Acknowledgements

At many phases of the work described here, the author has associated with his colleagues, R.L. Bindu, C. Gouri and Dona Mathew. The encouragement and support received from V.C. Joseph, R. Ramaswamy, K.N Ninan and K.S. Sastri are thankfully acknowledged. Permissions granted by John Wiley and Sons, Elsevier Sciences, Springer Verlag, M/s. Lonza Ltd, SAMPE, SAGE Publications, Kluwer Academics, Brill Academic Publishers, Rapra Technology and American Chemical Society for reproduction of data from

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