Pultrusion Exotherm Study Gauge Robert D. Spaans, Werner Co. And James S. Tedesco, AOC Sensitivity And Application To Process September 25-27, 2002 Engineering Problems ABSTRACT between styrene and the sites of unsaturation in The Pultrusion Die Exotherm Test is broadly used the polyester alkyd. The styrene crosslinks tie the and discussed in the pultrusion industry. In this polyester alkyd together to create a 3-dimensional test, a thermocouple is inserted in the wetted, network. The result is a thermoset matrix that is moving reinforcement laminate and is pulled hard and stiff, making it very suitable for pultrud- through a pultrusion die. The curve of tempera- ed structural profiles. Typically multiple (in this ture vs. die position is used to interpret reaction case, 3) peroxide initiators are employed, as each kinetics in the die, and often applied to process decomposes at progressively higher temperatures engineering and quality control problems. In this with very short half-lives under typical process study, the authors have executed the Die Ex- conditions. Many factors are known to affect the otherm Test on numerous dies, the results are cure reaction, which in turn influences both the examined to establish gauge sensitivity of this cosmetic and structural quality of pultruded pro- test method, and comments on accuracy are of- files. These include material parameters (such as fered. Comparisons are made between die ex- resin chemistry, reinforcements, the chemistry otherm data and DSC as well as SPI gel data. and concentration of peroxide initiators and other These analysis techniques are then applied in a additives), and process parameters (including lo- designed experiment to optimize initiator levels cation and set-point of heaters, die mass and for a common channel profile relative to cost, heat flux, and puller speed). Since materials se- productivity, and product performance. lection and productivity both have significant im- pact on cost, it is important to have a means of BACKGROUND optimizing all of the above relative to the required Pultrusion is an efficient process for the continu- degree of cure for the part. Since there is no ous manufacture of composite articles of con- known continuous monitor for degree of resin stant cross-section. In this process, reinforce- cure, pultruders commonly employ the “Die Ex- ments and surface veils are pulled through a res- otherm Test” to obtain a fingerprint of the thermal in-impregnation bath or chamber and then into a profile in the moving laminate within the die. This heated die. The cured composite is continuously fingerprint then relates to the cure reaction under- pulled from the die through the use of traction way in the matrix resin. In this procedure, a sacri- (caterpillar) or reciprocating clamp pullers, and ficial thermocouple is inserted in the moving rein- then cut to length by a flying cut-off saw timed to forcement laminate upstream of the die, and re- the speed of the process. A wide range of rein- cording is initiated as the sensing junction enters forcements and resins can be pultruded, but most the die. As this study will highlight, the interpreta- commonly, E-glass reinforcements are combined tion of the resulting exotherm curves can be com- with saturated polyester surface veils, and un- plex. saturated polyester resins employing styrene as both diluent and crosslinking agent. Peroxide ini- The Die Exotherm Test has been discussed in pri- tiators are incorporated in the resin mixture to or literature. Examples are: Sumerak et. al., who initiate cure/crosslinking. They decompose when demonstrated the utility of the measurement exposed to heat to initiate free-radical reactions technique in solving process engineering prob- Pultrusion Exotherm Study, continued lems related to line speed [1], part quality [2], and lyzed relative to full-section deflection and die design [3]. Vaughan et. al [4] have used differ- exotherm data. An attempt is also made to relate ential scanning calorimetry to characterize resin exotherm measurements to other thermal charac- formulations, and predict the cure profile and die terization techniques, differential scanning calo- exotherm curve with the use of a numerical mod- rimetry, and SPI Gel data. el. However, the authors are not aware of any lit- erature that addresses the reliability of the meas- METHOD urement technique itself. This study employed a 3.15” X 1.17” X 0.120” channel section commonly used as ladder rail. Example exotherm curves are shown in Figure 1. The composite laminate was comprised of 2 lay- Here, 9 runs (3 replicates performed by each of ers of polyester veil, and 3 layers of 1 oz/yd2 E- three operators) are overlaid on the same graph. glass continuous filament mat, sandwiching two What is immediately obvious is the significant de- layers of unidirectional E-glass rovings. The resin gree of variation between these 9 replicates, was an unsaturated polyester resin containing though all 9 were performed on the same pultru- 30% by weight styrene. Other additives included 3 sion run within a span of 30 minutes. This begs peroxide initiators, inorganic fillers, orange pig- the question: How consistent and thus reliable is ment dispersion, and a commercially available the method? A fundamental prerequisite to the internal lubricant. This laminate was pultruded at use of any test method for engineering analysis or 84”/min through a steel die, 56” in length first 8” process control is to establish an acceptable comprises an un-heated injection chamber. The Gauge Repeatability and Reproducibility, or 48” cure chamber is heated in three zones by sur- GR&R. [The formulas for GR&R are shown in Fig- face-mounted electrical resistance heaters set to ure 2.] In other words, it is necessary that the 250, 350 and 330 degrees Fahrenheit in zones 1 variations between measurements performed by through 3 respectively. Exotherm measurements the same operator (repeatability) and the varia- were made by inserting a 30 gauge Teflon coated tion operator-to-operator (reproducibility) measur- Type K thermocouple into the center mat layer, ing the same object is much less than the part-to- such that the sensing junction would be centered part variation. Otherwise, variation in the gauge on the height and the thickness of the channel may be interpreted as a real change in the object flange. Temperature was continuously recorded or process being studied, leading to incorrect con- with an Omron OM3000 data logger at an interval clusions. For purposes of this paper, we will focus of 200 milliseconds. Recording was initiated at on “% Study Variation”, which is that variation as- the time the sensing junction entered the die. sociated with distinguishing differences between DSC measurements were made with a Mettler TC process conditions or resin formulations. In this 10A (heating rate=10oC/min, sample size=9.5- case, gauge variation should be no more than 15.5 mg). The SPI gel time measurements were 30% of the total variation. It should also be noted made according to Draft ASTM Protocol dated 4- that even tighter requirements (no more than 19-2002. In both the DSC and SPI test protocols, 10% gauge variation) should be applied to duplicate runs were averaged. Full section deflec- “tolerance variation”. Tolerance variation is em- tion measurements were made by supporting a ployed when the gauge is intended to establish pair of channels with flange tips facing each other the acceptability of a product for release to the on a 108” span, centered under a 1” dia. loading customer, based on customer acceptance crite- bar. The channels were fixtured with 0.5” thick ria. plastic plates spaced 12” apart along the entire length. The loading fixture was equipped with PURPOSE Satec control and data collection software, and In this paper, we establish the repeatability and rails were loaded to 250 lbs. Displacement of the reproducibility of the Die Exotherm Test through a loading bar was continuously logged, and the statistical “Gauge R&R” study, and then apply the peak deflection reported. Statistical analysis was technique to the solution of a process engineering performed using Minitab statistical analysis soft- problem: the optimization of peroxide concentra- ware, with alpha=0.1. tions in a polyester resin system. A designed ex- periment with three initiators at two levels is ana- Pultrusion Exotherm Study, continued GAUGE R&R valuable, as the gauge error for peak position was Initially, 7 pultrusion runs were performed at two reduced to 30%. This established the procedure as locations (AOC and Werner) with a consistent res- an acceptable gauge for peak position. However, the in formula. On each run, 9 replicate exotherm error for peak height was not improved. This necessi- measurements were performed, three operators tates the use of a “workaround gauge”, wherein the each made three measurements. The twisted % gauge variation can be reduced by the inverse of thermocouple junction was inserted into the mov- the square root of the number of replicate results ing center mat, approximately 5/8” from the edge averaged. Therefore, to cut the variation in half, it is of the mat (determined by eye by each operator). necessary to average the results of at least 4 meas- A black mark was made on the moving reinforce- urements. Nine replicates were averaged for the ment pack, which would be visible at the die face. pultrusion trials in the DOE initiator study, though 4 When the black mark reached the die face, re- would have been sufficient. cording was initiated until after the black mark exited the die. INITIATOR STUDY The data from both locations was analyzed sepa- A designed experiment was performed to optimize rately for Gauge R&R using Minitab statistical the levels of three peroxide initiator concentrations software. Data from both locations failed to pass relative to peak exotherm (location and height), the GR&R.