Elsevier

Composites Science and Technology

Volume 66, Issue 16, 18 December 2006, Pages 3273-3280
Composites Science and Technology

Real time monitoring of cure and gelification of a thermoset matrix

https://doi.org/10.1016/j.compscitech.2005.07.009Get rights and content

Abstract

The consolidation of a commercial thermoset resin has been completely monitored by using a unique fiber optic sensor integrating a refractometer and a fiber Bragg grating. In particular, the advancement and the evolution of the resin polymerization reaction have been followed by measuring the optical properties changes induced by the thermo-chemical transformations. On the other hand, the fiber Bragg grating has been used to detect on-line the resin gelification onset. The experimental results demonstrated the capability and the efficiency of the developed sensor system to provide complete information on the main phenomena occurring during the cure cycle of a thermoset based system.

Introduction

In recent years, the strong demand for high performance structural and mechanical systems for aerospace, defences and advanced manufacturing industries has promoted the development and the evolution of thermoset based advanced composite materials for several applications. In fact, polymer composites could provide many advantages over traditional materials like metals including above all the light weight coupled with high mechanical properties and the processability characteristics.

It is though well established that the quality and the performance of a composite part are determined by the manufacturing cycle that is accompanied mainly by the exothermal polymerization reaction of the thermoset resin. In fact, the activation and the advancement of the cure reaction implies the consolidation of the polymer resin which changes its physical state passing from a low molecular weight liquid to a rubbery and, then, transforming into a glassy at the end of the processing cycle by chemical reactions of the active groups present in the system which develop progressively a denser polymeric network until reaching the so-called gel point and forming an insoluble material. Therefore, due to the continuous variation of the resin mechanical and rheological properties, it is extremely important to ensure a uniform evolution of the cure reaction that, coupled with the heat transport conditions, can lead to exothermal peaks, thermal and chemical conversion gradients determining residual stresses and gradients of the matrix mechanical properties. In addition, it should be considered that the polymer resin together with fiber reinforcement is placed into a mold that is usually made of a different material and, thus, reacts in a different way to the environmental conditions encountered during the processing. In particular, different thermal expansion coefficients and mechanical properties can characterize the system constituents (mold and fibers) and the thermoset resin which, on the other hand, can contract significantly during the crosslinking being subjected to chemically induced volumetric shrinkage. At the end of the processing cycle, these thermal and chemical deformations coupled to the mold constraints are balanced within the part and induce residual stresses that can determine low performance, shape distortions, warpage, matrix cracks and delaminations.

Thus, to meet the quality control requirements and have the desired properties and dimensions of the composite part, one of the most critical issues is the capability to acquire real-time information about the manufacturing cycle by using sensors able to monitor the cure advancement, the gel formation and the process-induced residual strains.

In the past, many non-destructive techniques, based on different operating principles, have been developed and employed for the cure reaction monitoring including dielectric analysis, ultrasonic scanning, acoustic emission, X radiography, thermochromic analysis, near infrared spectroscopy and refractive index measurements [1], [2], [3], [4], [5], [6], [7]. In general, most of these techniques are used with success in the laboratory environment, but few of them have been implemented in production being limited for the on-line process monitoring [1].

For example, the most mature technology, the dielectrometry, provides an indirect measurement of the cure giving information on the viscosity of the thermoset resin [8]. In this approach, a small sensor element, behaving as a radio frequency (RF) antenna, is placed in contact with the curing resin. An RF voltage is applied to the element by an electronic impedance analyzer that is connected to the sensor and measure its impedance. The sensor response is affected by the resin polar side groups, so, as the polymerization reaction advances and the mobility of the polar groups decreases, the resin dielectric constant and the impedance of the resin-sensor combination change. Thus, the resin viscosity may be related with the impedance of the sensor. The main disadvantages of this method are: the difficulty to relate the sensor measurement to the resin mechanical properties, the requirement of a good contact between the sensor and the composite, i.e., a poor connection and impurities on the composite surface determine irreproducible data [8].

Further, while ultrasonic techniques have been demonstrated useful to detect the changes of the composite viscoleastic properties, they suffer some disadvantages associated mainly with the use of an acoustical impedance matching coupling medium between the piezoelectric transducer and the test material. The presence of the coupling medium can cause large transit time errors, change of the waveform and, hence, effect on the velocity measurement accuracy [8].

At this stage, the optical techniques seem to be the most adequate for the real time monitoring of the composite processing. In fact, the sensor systems can be implemented by optical fibers that have several advantages over common sensing elements, being unperturbed by electromagnetic interference, low cost, very light and small enough to be embedded into the composite with minimum detrimental effects on the host structure [9]. Fiber optic refractive index measurements and near infrared fiber optic spectroscopy have been tested with success for the measurement of the cure advancement and the viscosity changes demonstrating great capability to perform on line and in situ cure kinetic monitoring [3], [4], [5], [6], [7].

On the other hand, fiber optic sensors are considered one of the most recent and promising methods also for the monitoring of the process-induced residual strains. It is important to outline that, in this area, most of the works have been concentrated on global measurements, such as curvature, warpage and spring-in of unsymmetrical laminates [10], [11], [12], [13] at the end of the process cycle, while only more recently sensing techniques, based on Fabry–Perot and Bragg fiber optic have been implemented to get local information about the strain fields [14], [15], [16], [17], [18], [19], [20] during the whole cure stage. In fact, the major characteristics of these tiny sensing elements (few hundreds of μm in diameter) enable them to be embedded in the body of the product under examination, without having almost any effect on the stress/strain field transfer between the reinforcement and the polymeric matrix. The use of extrinsic Fabry–Perot fiber optic sensors (EFPI) [14], [15], [16] has been proposed due to the strong dependence of interferometer response to the local strain field and the great insensitivity to the low thermo-optic effect associated to air cavities. Further, fiber Bragg sensors have been adopted by Partridge et al. [17] which proposed an internal and external temperature compensated fiber optic sensing system involving three fiber Bragg grating sensors combined with dielectric sensors. The overall system was able to monitor the development of residual strains in all phases of the curing stage. Recently, Asundi et al. [18] used a single fiber Bragg grating sensor for cure monitoring in composites. In this case, only the residual strains at the end of the cure were clearly identified.

In this work, the non-isothermal consolidation of an epoxy resin placed within an aluminum mold has been monitored by using a unique optical fiber integrating a refractometer and a single Bragg grating sensor able to measure simultaneously the resin refractive index changes and the strain induced on the optical strain gauge (fiber Bragg grating). In particular, due to the constitutive equation, the Lorenz–Lorentz law, relating the refractive index and the density of the resin, the measurements of the resin optical properties by the refractomer give information about the evolution and the level of the polymerization reaction. In addition, the build-up of the process-induced strain during the whole non-isothermal cure cycle was measured by the single fiber Bragg grating embedded within the epoxy resin. The experimental results demonstrated the capability and the efficiency of the fiber optic sensor for the on-line monitoring of the main phenomena occurring during the processing cycle. In particular, adaptive calibration allowed to directly evaluate the polymer conversion from the fiber optic refractometer and the gel onset from the Bragg sensor.

Section snippets

Fiber optic refractometer

The developed cure sensor system exploits the principle that the refractive index of a thermoset resin may be related to its properties by the Lorenz–Lorentz law (Eq. (1)) [21], that defines the relationship between the refractive index n, the density ρ and the polarizability β:n2-1n2+2=N3Mηρβ,where N is the Avogadro number, M is the molecular weight of polymer repeat unit and η is the free space permittivity. In other terms, the refractive index reflects the variation of the polymer density

Materials

The studied resin system was a mixture of a DGEBA (diglycidyl ether of bisphenol A) epoxy resin (EPON 828 supplied by Miller-Stephenson Chemical Company, USA) characterized by an epoxy equivalent weight of 188 g/mol and an aliphatic amine curing agent (triethylenetetramine, supplied by Vantico, Spain). The amine to epoxy ratio was 10.6 Phr (part of amine for hundred parts of resin by weight).

Set-up

Aim of the experiment was to study and highlight the dual functionality of an optic fiber including both a

Results and discussion

The fiber optic sensor has been used to analyze the non-isothermal polymerization of a commercial epoxy resin (EPON 828) that has been preliminary characterized by calorimetric tests. For details about the kinetic model of the analyzed epoxy–amine resin, see [24]. As described above, the cure cycle consisted of a first heating to 43 °C, followed by a second heating to 133 °C (see Fig. 1). During the whole cycle the resin optical properties have been recorded by the fiber optic refractometer that

Conclusion

The dual functionality of a fiber optic sensor including both a refractometer and a single Bragg grating has been explored and tested to monitor in real time the main phenomena occurring during the non-isothermal polymerization of a commercial thermoset resin: the cure reaction and the gelification onset. In particular, the refractometer sensor has been used to measure during the whole cycle the changes of the resin optical properties that are linearly related to the temperature variations and

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