Thermoplastic modifiers

Thermoplastics added to thermoset matrices can also improve the degree of toughness and it is believed that PES and PEEK are used for this application in many of the new generation epoxy systems. 

Thermoplastics with a high Tg have been blended with epoxy resins to confer good thermal stability and decrease the tendency to absorb water. PES has been used [86 91] as well as PEI [91-93]. 

Stenzenberger, in his papers [1,29] for toughening bismaleimide resins, cites the use of thermoplastics such as polyhydantoins [55] (Bayer s Resistofol N, Resistherm PH-10), 

System Tensile strength unidirectional (0°) GPa Tensile modulus unidirectional (0°) GPa Compressive strength Orientation (0°, 45°, 90°) GPa Compressive modulus Orientation (0°, 45°, 90°) GPa 

Source Reprinted with permission from Cam CY, Walker JV, Toughened Composites, Johnston NJ ed., ASTM STP937, American Society for Testing and Materials, Philadelphia, 9-22, 1987. Copyright 1987, ASTM International. 

---

Diglycidyl ether of bisphenol-A (DGEBA), epoxy resin (YD 128, Kuk Do Chem., Mn = 378), and bisphenol-A dicyanate (BPACY, Arocy B-10, Ciba-Geigy) were used as the thermoset resin. 4,4 -diaminodiphenyl sulfone (DDS, Aldrich Chem. Co.) was used as a curing agent for epoxy. Polyetherimide (PEI, Ultem 1000, General Electric Co., M = 18,000) and 2-methyl imidazole (2MZ, Aldrich Chem. Co.) were used as the thermoplastic modifier and catalyst. 

Many factors contribute to the toughness of a polyphase BMI/thermoplastic system, such as solubility parameters, phase adhesion, phase morphology, particle size and particle size distribution. Another important factor is the molecular weight of the thermoplastic modifier. It has been demonstrated for a particular poly(arylene-ether) backbone that high molecular weights increase the toughness of the blend system more than the low molecular weight counterparts (92). 

Other transitions such as degradation and phase separation may be also observed during the formation of the polymer network. Degradation is usually present when high temperatures are needed to get the maximum possible conversion. Phase separation may take place when the monomers are blended with a rubber or a thermoplastic, to generate rubber-modified or thermoplastic-modified polymer networks. In these cases, formulations are initially homogeneous but phase-separate during the polymerization reaction. This process is discussed in Chapter 8. 

Figure 8.9 TEM photographs of thermoplastic-modified epoxy networks. The system is DGEBA - 4,4 -methylene bis [3-chloro-2,6 diethylaniline], MCDEA, with 10 wt% polyphenylene ether, PPE (a) is cured at 135°C (T < Tg,ppE) and (b) at 240°C (T > Tg,PPE). (a) Epoxy-rich substructures are visible inside PPE-rich particles (b) the particle sizes decrease with an increase in cure temperature and a binodal distribution is observed. (From LMM Library.)... 

The precursors of thermosetting polymers are usually one of the ingredients of complex formulations. They may be present in very small amounts, as in the manufacture of abrasive disks where the thermoset acts as an aggluti-nant in medium amounts, as in the case of filler-reinforced thermosets or as the only components, in formulations used for encapsulation purposes. Apart from fillers, fibers, pigments, etc., some formulations contain rubber or thermoplastic modifiers that phase-separate upon the polymerization reaction (cure). 

The hyperbranched architecture does not always afford an advantage in toughness or viscosity compared with low-molar-mass thermoplastic modifiers. 

The thermoplastic resins are usually blended with the epoxy resin in a solvent solution. Early researchers realized that to make this approach effective, it was necessary to increase the compatibility and interfacial adhesion of the thermoplastic modifier and the epoxy resins. The problem of poor miscibility of the thermoplastic resins and poor processabihty of the final product are the main reasons that these materials have not achieved commercial success. 

The properties of silicones can be modified by combination with organic polymers. Block copolymers of poly(organo-siloxanes) and poly(ethers) as well as thermoplastic-modified silicones are industrially important, in addition to silicone combination resins. 

Thermoplastic-modified silicones are produced by polymerization of monomers, such as styrene, in the presence of polysiloxanes... 

Thermoplastic-modified siloxanes are produced by polymerization of monomers such as styrene, methyl methacrylate or vinyl acetate in the presence of e.g. ot,(i)-dihydroxy-poly(dimethylsiloxane). The reaction can be so controlled that the thermoplastic particles are formed as rods. These, depending upon their type and quantity, determine the mechanical properties of I he vulcanized. silicone resin obtained via cro.sslinking of the silicone component. Such products are utilized in the porcelain, electric, electronic and metal industries. 

Ultrathene [Quantum], TM for a series of ethylenevinyl acetate copolymer resins for adhesives, conversion coatings, and thermoplastic modifiers. Wide range of melt indexes. Improves specific adhesion of hot-melt, solvent-based, and pressure-sensitive adhesives. 

Several basic morphologies are observed in thermoplastic-modified epoxies and, indeed, other thermosets. Homogeneous [Fig. 6(A), in which no phase separation is observed] and particulate [Fig. 6(B), in which the modifier phase separates to produce small domains] morphologies occur at low concentrations of modifier. In these cases, the thermoplastic modifier is encapsulated within a thermoset matrix, whereas in the phase-inverted morphology, [Fig. 6(C)] the minor thermoplastic component is the continuous phase surrounding large, discontinuous domains of the major... 

There has been much work on characterizing the phase separation and network development for thermoplastic-modified epoxy-resin systems. Initial work (Yamanaka and Inoue,... 

Rheological Monitoring of Phase Separation Induced by Chemical Reaction in Thermoplastic-Modified Epoxy... 

Chen et al. (12) showed that phase separation in elastomer-modified epoxies can also be detected with much more sensitivity using small-angle X-ray scattering (SAXS). Then, the choice of the cloud point as a criterion for detecting the beginning of phase separation can be discussed. We have not performed SAXS studies on our systems because phase separation is much faster in thermoplastic-modified epoxy and the scanning time is too long. 

Determination and Use of Phase Diagrams for Rubber- or Thermoplastic-Modified Poly(cyanurate) Networks... 

Table 12.2 Moisture content of carbon fibre composites from thermosetting and thermoplastic matrices also shown is the benefit of modifying epoxy resin with thermoset and thermoplastic modifiers to reduce Mco...

---