Processing, thermosets liquid resins

Figure 6.6 Hand lay-up process for producing highly reinforced parts with fiber matting and polyester, epoxy, or other thermosetting liquid resin systems.

The cure of thermoset resins involves the transformation of a liquid resin, first with an increase in viscosity to a gel state (rubber consistency), and finally to a hard solid. In chemical terms, the liquid is a mixture of molecules that reacts and successively forms a solid network polymer. In practice the resin is catalyzed and mixed before it is injected into the mold thus, the curing process will be initialized at this point. The resin cure must therefore proceed in such a way that the curing reaction is slow or inhibited in a time period that is dictated by the mold fill time plus a safety factor otherwise, the increase in viscosity will reduce the resin flow rate and prevent a successful mold fill. On completion of the mold filling the rate of cure should ideally accelerate and reach a complete cure in a short time period. There are limitations, however, on how fast the curing can proceed set by the resin itself, and by heat transfer rates to and from the composite part. 

Phenolic Resins. Phenolic resins [9003-354] (qv) are thermosets prepared by the reaction of phenol with formaldehyde, through either the base-catalyzed one-stage or the acid-catalyzed two-stage process. The liquid intermediate may be used as an adhesive and bonding resin for plywood, particle board, fiberboard, insulation, and cores for laminates. The physical properties for typical phenolic laminates made with wood are listed in Table 1. 

Thermoset - Thermosets are resin and plastic compounds which, after final processing, are substantially infusible and insoluble. Thermosets are often liquids at some stage in their manufacture or processing and are cured by heat, oxidation, radiation, or other means often in the presence of curing agents and catalysts. Curing proceeds via polymerization and/or crosslinking. Cured... 

For prototyping or for limited production runs, liquid resin casting offers simplicity of process, relatively low investment in equipment, and fast results. Thermoset casting resins may be epoxies, polyesters, phenolics, etc. The resins harden by a polymerization or cross-linking reaction. Such resins are often poured into open molds or cavities. Because pouring is done at atmospheric pressure, molds are simple, often made of soft metals (Fig. 6.2). 

Figure 6.22 Schematic illustration of pultrusion process, using liquid polyester or other reactive thermosetting resin systems.

Thermosetting resins can be combined with reinforcing fiber directly in a molding press by pouring a liquid resin onto a fiber mat or fabric or by combining the resins and reinforcement in a treater. The molding press process is nsed to produce continuous low-pressure laminates of finite thickness. The treater process is used to produce high-pressure laminates in sheet form. 

Transfer, or plunger molding, is a process in which high-flow B-stage resins, such as epoxies, are liquefied under heat and pressure in a transfer pot, after which the resultant liquid resin is transferred under pressure into mold cavities. The transferred resin is then heated to form cured, final parts that are then removed from the mold. Although compression and transfer molding are used principally with thermosetting compounds, the processes are occasionally used with thermoplastic material, often thermoplastic composites. 

Chapters 9 through 11 all belong to a class of processes called liquid composite molding (LCM). In this class of processes, one usually transfers the resin from a reservoir into a network of fibers. The fiber network is formed from any type of reinforcement from random mat to woven, stitched, noncrimp and bidirectional fabrics. This class of processes is very attractive as one can make net shape parts from composites using continuous fibers and have good control over the orientation of the fibers so one can tailor the properties as required by the design. Usually thermoset matrices are used... 

Cure kinetics of thermosets are usually determined by DSC (71,72). However, for phenolic resins, the information is limited to the early stages of the cure because of the volatiles associated with the process. For pressurized DSC cells, the upper limit on temperature is 170°C. Differential scanning calorimetry is also used to measure the kinetics and reaction enthalpies of liquid resins in coatings, adhesives, laminations, and foam. Software packages that interpret DSC scans in terms of the cure kinetics are supplied by instrument manufacturers. 

Polymer matrix composites (PMC) are used to increase the rather low stiffness or strength of polymers by adding stiffer or stronger fibres. If thermoplastics are used as matrix material, short fibres can be added to the granulate material that is subsequently softened by heating and processed, for example in injection moulding. If thermoset resins (duromers) are used, the fibres can be placed into the liquid resin before curing. 

In the early days of rheological simulation, only the standard injection molding process of thermoplastics had been considered. Today, there are solutions for elastomers, thermosets, liquid silicone resin polymers, and also polymers for the reaction injection molding (RIM) process. Simulation programs have been developed for various materials as well as for various processing techniques on the shell model and are available in all these varieties. 

Harada M, Ando J, Hattori S, Sakurai S, Sakamoto N, Yamasaki T, Masunaga H, Ochi M (2013) In-situ analysis of the structural formation process of liquid-crystalline epoxy thermosets by simultaneous SAXSAVAXS measurements using synchrotron radiation. Polym J 45 43-49 Hirano Y, Akiba M, Yokota A, Nakamura H, Naitoh S (1999) Epoxy resin, resin composition, and resin-encapsulated semiconductor device. U.S. Patent 5,939,473 Hsu SH, Chen RS, Chang YL, Chen MH, Cheng KC, Su WF (2012a) Biphenyl liquid crystalline epoxy resin as a low-shrinkage resin-based dental restorative nanocomposite. Acta Biomater 8 4151 161... 

This cross-linking process is also utilized on an industrial scale— and not just for glues, caulks, and sealing materials. In the aerospace industry, thermoset materials are often used in the fabrication of composite structures, where fibers made of pure carbon—either in long strands or woven into fabric—are saturated with a liquid resin which hardens (cures) into a solid matrix, trapping and bonding with the fibers... 

Stereolithography (SLA) is an additive method and uses a liquid-based systan. The liquid polymer is photocured layer by layer. The liquid polymers are (UV) ultraviolet-curable photopolymer (thermoset) resins such as epoxy, acrylates, and vinyl ethers. The system is illustrated in Figure 15.7. A tub (vat) is filled with the liquid resin. A platform supports the model as it is built. The platform is first raised to just below the liquid surface at a depth of the first layer to be cured. Then a computer-controlled laser is focused so that it solidifies the first layer to the required depth as it is moved in the x, y directions within the 2D area of the first slice. Subsequently, the platform is lowered a distance equal to the second layer thickness to be cured and the process is continued. Layer thicknesses as small as 0.025 mm (0.001 in.) up to 0.5 mm (0.020 in.) can be cured in this manner. 3D Systems is the company that supplies its patented SLA equipment to a worldwide market. 

The thermoset polyurethane resin and the mold were both supplied by the sponsor. The resin came in two separate sealed five gallon buckets. One bucket contained isocyanate (A-component), and the other bucket contained liquid polyurethane resin (B-component). To optimize the reaction, the A and B components were mixed in a 30 70 ratio respectively. The mixed components had a working time of approximately 20 to 30 seconds, and fully cured parts were formed in approximately 3 to 4 minutes after mixing. To create parts, the materials were mixed and heated to 38.0° C to promote the curing process. The specific grade or manufaeturer of polyurethane resin was not specified by the sponsor. The two halves of mold were composed of solid thermoset polyurethane that had been cast and cleaved into a two-part mold that designed to create parts that resembled human wrist bones. 

---