Cresol novolac–epoxy networks: properties and processability
Introduction
Phenolic resins are ideal for structural adhesives and composites since they are flame retardant [1] and cost-effective [2], [3]. The major drawback thus far for typical phenolic networks, including both phenolic novolacs cured with hexamethylenetetramine (HMTA) and thermally cured resoles, has been their brittle nature originating from high crosslink densities and high void contents. These typical crosslinking reactions allow little control over network structures and generally lead to highly crosslinked networks. Volatiles such as water and ammonia released during the cure reactions cause the networks to have high void contents.
Curing phenolic novolac resins with epoxies instead of HMTA yields tough, void-free materials. The cure reaction proceeds via nucleophilic addition of the phenolic hydroxyl onto the epoxy group without releasing volatiles [4], [5], [6]. These types of materials, in which high compositions of an epoxy component have been cured with phenolic novolacs, have been important in microelectronic packaging applications. However, such materials do not retain the excellent flame properties characteristic of typical phenolic networks since they are mostly comprised of flammable epoxy resins. Previous work from our laboratories has demonstrated that flame retardance, combined with toughness, can be achieved by crosslinking high compositions of relatively high molecular weight novolacs with minor concentrations of diepoxides [7], [8]. High phenol to epoxy reagent stoichiometries were utilized to only crosslink the minimum amount of phenols required to achieve low sol fractions, and good toughness and moduli. Hence, relatively high molecular weights between crosslinks (on the phenolic component) in the networks lead to both good mechanical properties and good flame properties.
Commercial phenolic novolacs lack molecular weight control. The phenol monomer is trifunctional, and therefore, branching and even gelation are inevitable as higher molecular weights develop. For example, branching has been shown to occur significantly once the molecular weights reach 900–1000 g/mol [9]. This paper describes properties of related networks with high phenol to epoxy stoichiometries, prepared from linear novolac resins with well-controlled molecular weights [10]. Difunctional ortho- or para-cresol monomers allow for preparing higher molecular weight linear oligomers without branching. In addition, the presence of a methyl group adjacent to the hydroxy group on ortho-cresol was expected to affect the chemistry and the properties of resulting networks. In this study, a 2000 g/mol ortho-cresol novolac oligomer was crosslinked with two epoxies, and the network properties and processability were investigated.
Section snippets
Materials
Epon 828, a bisphenol-A based diepoxide with an epoxy equivalent weight (EEW) of 187 g/mol was obtained from Shell Chemicals. D.E.N. 438, an epoxidized novolac with a functionality of ∼3.5, was supplied by Dow Chemical. A phenolic novolac resin with a molecular weight of ∼700 g/mol was donated by Georgia Pacific (Product #GP-2073). ortho-Cresol (99+%), 2,6-dimethylphenol (99%), paraformaldehyde (powder, 95%), formaldehyde (37 wt% solution in water), oxalic acid dihydrate (99%) and
Results and discussion
Tough, flame retardant, melt processable, phenolic novolac–epoxy networks were previously studied in our laboratories [7], [8]. The phenolic novolac resins were prepared from trifunctional phenol and formaldehyde and were thus branched structures. The goal of the present research was to improve the processability and reduce the network moisture absorption of novolac–epoxy networks while maintaining thermal and mechanical properties as well as flame retardance.
A 2000 g/mol ortho-cresol novolac
Conclusions
ortho-Cresol novolac–epoxy networks with high compositions of the novolac component had good toughness, high glass transition temperatures, and low moisture sorption, combined with reasonably good flame properties. The 70:30 and 60:40 wt/wt cresol novolac–epoxy compositions exhibited KIC toughness values >1 MPa m1/2 and glass transition temperatures >140 °C. These properties were superior to the epoxy control, the phenolic control and the phenolic novolac–epoxy networks (65:35 wt/wt ratio).
ortho
Acknowledgements
The authors wish to thank the National Science Foundation under NSF Goali program (DMR 993672) and to the Dow Chemical Co. for funding and close technical cooperation. They are also grateful to Dow Chemical Co., Georgia-Pacific Corp., and Shell Chemicals for materials donations.
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Present address: Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD, 20899-8543, USA.