1. Introduction
Polymer composite materials (PCM) are one of the most popular and rapidly developing types of materials today. Carbon fiber is used as a reinforcing fiber in the production of structural elements. The widespread use of PCM is growing every day in both strategic and civil industries. Polymer composites are used in the aerospace and nuclear industries, in the automotive industry, power engineering, construction, shipbuilding, bridge building, pipeline transport, and consumer goods. The level of technologies for serial production of carbon composite materials and the degree of their application in industry determines the scientific and technical potential of any country.
One of the main disadvantages of PCM is their high flammability due to the organic nature of the matrix [
1,
2,
3]. This reason limits the wider application of PMC in the transport interior industry. Thus, the search for new benzoxazine resin mixtures with improved properties is an urgent task.
Benzoxazines have been developed as a new class of resins for PCM that appeared on the market at the beginning of the 21st century [
4]. This is an addition-type phenol-formaldehyde resin rather than a condensation type. Benzoxazines are beneficial compared to the latter as they do not release volatiles during curing and have the following advantages: self-curing at elevated temperatures, high modulus, surpassing most of the known types of resins, near zero shrinkage during curing, low water absorption, excellent dielectric properties, flexibility of molecular design, enhanced flame resistance [
5,
6]. So, most polybenzoxazines fit V-1 category according to the UL-94 standard, but some of them relate to the highest V-0 flammability category [
1,
7].
Nowadays, benzoxazine monomers are a promising basis for creating resins that fit the requirements of different PCM technologies such as prepreg, RTM (Resin Transfer Molding), autoclave and so forth. This makes it possible to replace traditional phenol-formaldehyde and epoxy resin [
8]. For example, many corporations have branded benzoxazine resins and prepregs. Flexible molecular design allows researchers to create benzoxazines with a wide variety of properties. For example, phenolphthalein benzoxazine resin has a glass transition temperature of 195–200 °C and belongs to the V-0 non-combustible category, while the bisphenol A based benzoxazine has a glass transition temperature of 173 °C and belongs to the category V-2.
One of the technological disadvantages of neat benzoxazine monomers is the glassy state of resin at room temperature and a narrow processing window. The lack of tack and drape at room temperature makes them difficult to use as a resin for prepreg technology. The viscosity at a processing temperature (80–130 °C) above 1 Pa·s limits their application, for example, in vacuum infusion technology, where the necessary condition for impregnation is a viscosity of the resin that is less than 0.5 Pa·s.
Thus, along with developing modifiers that improve mechanical, thermal and flame retardant properties, it is of great importance to search for viscosity regulator components that allow us to control the technological properties of benzoxazine resins without deteriorating the performance characteristics of polymer [
9,
10,
11].
Such components can be active diluents in the form of monobenzoxazines or low-viscosity di- and monofunctional epoxy resins. Good compatibility of benzoxazine with epoxy resins and the ability to copolymerize with them due to hydroxyl groups formation during the self-cure process allows us to obtain a dense polymer network. Thanks to such favorable aspects, blends of benzoxazines and epoxy or novolac resins are widely reported in the literature [
12,
13,
14,
15].
A higher crosslinking density of benzoxazines and epoxy copolymerrization leads to a great change in the mechanical properties of the cured composition. Thus, in [
11], with an addition of epoxy resin of 30 wt%, the glass transition temperature, flexural strength and deformation increased significantly in comparison with benzoxazine polymers. By varying the ratio of copolymers, this type of resin could be applied to specific performance requirements.
Another important advantage of composites based on a benzoxazine/epoxy mixture is the decrease of processing temperature at which an appropriate viscosity is reached. The high processing temperature could yield preliminary curing. So, mixtures with a concentration of cycloaliphatic epoxy resin of more than 25 wt% provided a proper viscosity for infusion processing [
15].
The knowledge of the rheological properties of curing is important from the point of view of prepreg, infusion, RTM and other technologies for producing composite materials. Rheological parameters directly depend on the extent of the chemical reaction, if chemical processes with an increase in molecular weight occur, in particular leading to the formation of a three-dimensional network. Viscosity values could be determined by imposing constant (rotational) or periodic (oscillatory) shear strains on the material. Oscillatory mode is applied when a wide range of curing processes, including gelation and vitrification, are to be studied. In this case, small-amplitude oscillations are necessary conditions for measurement to proceed in a linear viscoelasticity region [
16].
Some works combine different monitoring methods to conduct a comprehensive review of the process of curing. For example, in the study [
17], the authors investigated the kinetics of curing with the two methods being compared: rheology and traditional differential scanning calorimetry. Various aspects of the rheology of curing processes including T–T–T diagrams (time–temperature–transformation) [
18,
19], homogeneous and heterogeneous curing, and the possibility of relaxation transitions in curing were studied in the works of Malkin A. and Kulichikhin S. [
20,
21].
In the case of prepreg materials, tack is one of the major properties that governs the ability of prepreg to be laid up. However, there is no standard method for measuring the tackiness of material. The tack of a material can be estimated by the sensation one experiences when removing one’s finger from any sticky material (finger test) but it is only a qualitative assessment [
22]. One of the quantitative tack measurements is the probe tack test [
23,
24]. It consists of a probe that comes into contact with the material to characterize it, then the probe is removed and the force and/or energy of separation is measured. In this case, contact formation and separation phases are clearly dissociated, so the contact parameters are easy to control, and each step of debonding could be clearly observed.
This work was devoted to the study of the dilution effect of 1,4-butanediol diglycidyl ether (BD) and furfuryl glycidyl ether (FUR) in blends with benzoxazine monomer (BA-mt) based on bisphenol A and m-toluidine. The practical purpose was to reach a favorable tack, expand the processing window and enhance or maintain some thermal and mechanical properties. The fundamental issue was to compare the effect of mono- and difunctional epoxy resins on the properties of mixtures with benzoxazine.
4. Conclusions
This work disclosed that 1,4-butanediol diglycidyl ether (BD) and furfuryl glycidyl ether (FUR) in compositions with benzoxazine monomer BA-mt yielded improved tackiness of blends, reduced viscosity and higher temperature of curing. Some of the compositions demonstrated enhanced thermal and mechanical properties.
The benzoxazine/epoxy blends possess good tack at RT compared to neat benzoxazine, which is nearly in a glassy state at RT. Epoxy diluents efficiently decreased the glass transition point of uncured resin: a decrease in Tg was more than 20 °C for compositions with 10 phr of BD or FUR. However, epoxy additives affected the polymerization process. The peak polymerization temperature shifted to a high temperature region in the case of both BD and FUR addition.
The use of epoxy compounds in an amount of 10 phr provided a significant decrease in viscosity. The favorable tackiness was achieved with 10 phr of epoxy components in mixtures. The shift of the DSC polymerization peak resulted in an increased gel time and lifetime of the compositions. The viscosity increase rate constants of BB-10 and BF-10 were 5.5 and 1.5 times less than for neat BA-mt. Structure formation was characterized by the Malkin–Kulichikhin equation, which appeared to be applicable to the benzoxazine curing process. The characteristic time corresponding with the beginning of the three-dimensional cross-linking process was in a good agreement with the gelation time.
Moreover, the addition of 10 phr FUR decreased Tg at 13 °C and tensile strength at 4%. On the other hand, the addition of 10 phr BD decreased Tg only at 1 °C and led to an increase in tensile strength at 17%. The difference in the properties of the resulting polymers was associated with the difference in the functionality of the used epoxy resins BD and FUR.