Understanding Rheology of Thermosets revised by A.J. Franck, TA Instruments 2 Rheology of Thermosets Rheological Testing of Thermosetting Polymers General Considerations the controlled influence of heat and pressure over time. Thus, Thermoset Polymer Uses thermosets build their final struc- Thermoset polymers form the ture during processing, forming a matrix in filled plastics and fiber- three-dimensional internal struc- reinforced composites used in a tural network of highly crosslinked diversity of products. These range polymer chains. And the final from consumer items and auto product is insoluble and not ther- body panels to advanced compos- mally reformable. ites for printed circuit boards Elastomers share characteris- (PCBs), aerspace structural tics of thermoplastics and compo-nents such as the Space thermosets. Elastomers begin Shuttle payload bay door and jet as thermoplastic polymers with engine cowls and ducts, and ex- discrete chains that later develop pensive, high-performance sports a network of covalent crosslinks. equipment. Also, thermosets are However, elastomers are distin- used extensively as adhe-sives, guished from thermosets by the molding compounds, and surface fact that the crosslink network is coatings, including protective sol- formed in a separate post-polyme- der masks for PCBs. rization step called vulcanization. Another factor distinguish-ing the Thermosets versus Ther- three classes of polymers is the moplastics and Elastomers glass transition temperature Tg – the temperature at which poly- Thermoset polymers are distin- mers reversibly transmute be- guished from elastomers and tween rubbery and glassy states. thermoplastic polymers in several For elastomers, the glass transi- ways. tion tempe-ature is below ambi- Thermoplastics are processed in ent temperatures; for thermoplas- the molten state, their final shape tics and thermosets, it is substan- and internal structure established tially above ambiant. by cooling, and they can be sof- tened and reshaped by reapplica- Studying the Crosslinking tion of heat and pressure. Also, Reaction their polymer chains, whether lin- ear or branched, remain di-screte The formation of a thermoset after molding. Thermoset poly- crosslinked network is shown mers usually go through three schematically in figure 1. stages. In the A-stage, some- times called a resole, the resin is Understanding of this pro-cess still soluble and fusible. In the B- has been advanced substantially stage, thermosets are nearly in- by use of rheological analysis – soluble, but are still thermoplas- measurements of resin viscosity, tic. They can, however, spend only shear modulus, and damping. a rela-tively short time in the mol- More sensitive than even Fourier ten state because the tempera- transform infrared spectroscopy tures that promote flow also cause for measuring extent of cure, the material to crosslink. The rheological testing has become crosslinking reaction is accom- a vital supplement to DSC, chro- plished in the final stages of po- matography, and wet chemical lymerization – the C-stage – dur- analysis in thermoset polymer ing molding of the product under re-search and develop-ment. The reasons for this reside in the na- AAN015 3 Rheology of Thermosets ture of the processes involved larly epoxies, must be protected and the rheological changes in- from moisture and refrigerated to herent in these. forestall ”advancement”, that is, conti-nuation of the reaction. Thermoset Processing To fabricate a product, a multilayer Multistage Processing sandwich of prepreg is prepared, placed in a press or autoclave, Quite commonly, thermosetting and heated to ”cure”. This resins are processed in two or processing step must accom- more stages, as, for example, are plish several things: wet the those used in the production of fibers thoroughly with resin, con- fiber reinforced laminates. solidate the laminate to the de- First, a laminating varnish is pro- sired thickness, remove excess duced from resin monomer, cur- resin, exhaust trapped air, mois- ing agent, other functional addi- ture, and solvent to avoid poro- tives, and a suitable solvent. sity, and advance the cross-link- Then this varnish is applied to a ing reaction to the desir-ed level. reinforcing fabric made of glass As the temperature is raised, the fiber or other high modulus prepreg melts and its viscosity fibers, the solvent is removed, drops precipitously, then levels off and the resin is partially reacted and again climbs. This viscosity to form a ”prepreg”. At this point, behavior is the consequence of the B-stage, the prepreg is a par- the intervention of a competing tially soluble, low-melting ther- circumstance: The rising tem- moplastic solid composed of perature triggers further reaction unreacted monomer, adduct (the among monomer, curing agent, re-action product of a one-to-one and oligomers increasing the pre- ratio of resin and curing agent), preg’s viscosity. Because the and higher molecular weight reactants are multi-functional, oligomers. Some resins, particu- crosslinks form among the poly- mer chains quickly in all direc- tions. Suddenly the system gels and flow ceases. At this point, the matrix is solid and the thickness and shape of the composite are fixed. Additional heating is need- ed, however, for the matrix to form the balance of ist potential crosslinks and to develop, for the most part, ist ultimate level of stiff- ness. To accomplish this, an ad- ditional out-of-mold post-cure step is generally used. Single-stage Processing Thermosets are also pro-cessed in a single-stage method called reaction injection molding (RIM). In use on a major scale since late in the seventies, RIM has become popular for a variety of automotive Figure 1: Schematic representation of structural development and recreational applications. during thermoset curing: (A) unreacted monomers; (B) formation of small branched molecules; (C) the gel point: a path of covalent In some respects, RIM resem- bonds exists across the sample; (D) the cured, crosslinked polymer bles thermoplastic injection with some unreacted groups and reactants. molding (TIM). The key difference, however, is that RIM uses polym- A.Franck 10/04 V1 4 Rheology of Thermosets erization in the mold rather than ing. In structural laminates, resin cooling to form a solid polymer. flow influences porosity, cured part dimensional uniformity, and Other reaction molding processes process economics. In multilayer also use polymerization to solidify printed wiring boards, resin flow the molded piece; however, in properties also influence unifor- thermoset injection molding, for mity of etched circuit encap- example, reactants are heated to sulation, copper-to-epoxy adhe- around 200 °C to activate the reac- sion, and final press thickness. tion. In RIM, the reactants are Therefore, at each stage of the combined at a relatively lower tem- pro-cess, the rheological state perature (about 40°C) because the should be known to employ these reaction is activated by impinge- materials effectively and economi- ment mixing, not external heat. cally. In the RIM process, complex The rheology of thermosetting res- plastic parts are quickly produced ins can be studied using both directly from low viscosity react- steady shear and dynamic oscil- ants. Two or more liquids are latory tests (sometimes referred impin-gement mixed at low tem- to as dynamic mechanical analy- peratures and pres-sures as they sis (DMA) or dynamic mechani- enter the mold. Solid polymer cal rheological testing ( DMRT). forms by phase separation of seg- How steady shear, dynamic, and mented block copolymers (but transient tests are run is de- structural buildup by crosslinking scribed in a later section. occurs with some urethane sys- tems) and parts can be removed Steady shear measurements can from the mold in under a minute. characterize only the initial por- tion of a thermoset’s viscosity Process Rheology range. Near the gel point the Thermoset structural development steady shear viscosity in- is accompanied by enormous creases rapidly and becomes rheological changes: The resin unmeasurable. Eventually the changes from a low-melting ther- stiffening sample fractures or moplastic solid to a low viscosity tears. liquid, to a gel, and then to a stiff In contrast, dynamic oscillatory solid. measurements of the resin’s vis- Thus, the properties are vitally cosity can be made as the reac- important to successful process- tion proceeds through the gel point – and beyond – until the resin becomes a stiff solid. This is pos- sible because the measurements can be made at a strain ampli- liquid solid oo tude low enough to prevent dis- G ruption of the gel structure as it is *, h* h being formed. Steady shear vis- , o cosity data are compared to dy- h G namic viscosity data for a curing Log epoxy resin in figure 2. h Being able to measure the viscos- ity throughout the thermoset cur- ing reaction is an important ca- pability. Still, it provides only a partial picture of the phenomenon tc Reaction time taking place. Because the mate- rial is viscoelastic, elasticity data are needed to complete the char- Figure 2: Measurement of the viscosity of a curing epoxy resin in acterization. These are obtained in a dynamic test concurrent with AAN015 5 Rheology of Thermosets measurement of the viscosity. neously, measuring tan d in the Figure 3 shows the storage (G’) time scale of the developing gel. and loss (G”) moduli and com- Figure 4 shows dynamic oscilla- plex viscosity h* measured dur- tory measurements of tan d made ing an epoxy molding compound simultane-ously at three frequen- cure. Besides providing essen- cies on a crosslinking polydi- tial mini-mum viscosity data, the methylsiloxane (PDMS) resin. cross-over point of the two modu- Because tan d is independent of lus curves gives an estimate of frequency at the gel point, the the time at which the resin be- three curves pass through a sin- gins to gel. gle point and unambiguously de- But this is only an ap-proximate fine the instant of gelation.