Optimize the Curing of Composites with Kinetics Neo

BLACKS Composites Srl and NETZSCH Analyzing & Testing made the next milestone in its already proven Partnership: Improving the curing of a bike rim with a simultaneous drastic reduction of the cycle time during cross-linkage. The bike rim was processed through hand lay-up technology and a subsequent autoclave curing. Saving cycle time with a better curing is not possible? It is! Based on the data of a differential scanning calorimeter and the simulation software Kinetics Neo, the cycle time was reduced by 46 % compared to the cure cycle suggested by the material data sheet. Additionally, a higher degree of cure could be achieved.

Collecting Data with a Differential Scanning Calorimeter

Differential Scanning Calorimetry (DSC) is the most known and widespread thermal analysis technique for the characterization oof polymers. During DSC analysis, a sample and a reference are subjected to a controlled heating profile, thus enabling to determine changes endothermal and exothermal effects of the sample. As a result reversible and non-reversible phase transitions of a polymer sample can be detected. These are:

• the glass-transition temperature,
• the melting and crystallization of semi-crystalline thermoplastics and
• the curing of cross-linking polymers.

To get more information about the working principle of a DSC, please visit our NETZSCH website: Principle of a heat-flux DSC. In the case of the work together with Blacks Srl. the target was to meet the requirements in curing of a thermosetting carbon fiber prepreg for the production of a bike rim with 95 %. And the suggested cycle time of the material data sheet with a total autoclave time of 480 minutes had to be shortened. To qualify the achieved degree of cure level, DSC measurements have been performed using the DSC214 Polyma on the uncured resin and the manufactured parts to evaluate the total heat enthalpy of the material \(H_{tot}\) and the residual cure \(H_{res}\). Using these values, the degree of cure \(\alpha\) can be calculated as follows: \(\alpha = \frac{H_{tot} – H_{res}}{H_{tot}}\)                    (1) The DSC measurements have been performed under dynamic conditions with a temperature ramp of 10 K/min. The following picture shows the entire curing compared to to the residual cure after producing a bike rim based on the conventional 480 minutes cycle suggested by the material data sheet. Using equation (1), the achieved degree of cure is 95.9 %. To be able to perform kinetic studies, thermal analysis measurement data for at least three different heating ramps or at three different isothermal temperatures are inevitable. In this case, dynamic ramps at the rates 1, 2, 5 and 10 K/min have been performed.

Performing Kinetic Studies and Predicting the Material Behavior

In order to be able to predict the material behavior for different process scenarios, the measured DSC data at the four different heating ramps have been uploaded to the NETZSCH Kinetics Neo software. The conversion fit below shows a model-free approach to the measurement data using a very easy-to-use klick solution. The mathematics behind the model is a newly developed model-free approach, called “numerical optimization”, specially designed for users without any experience in kinetic’s simulation. In order to avoid overheating and therefore material damage, it has been ensured that the maximum reaction rate of the conventional cycle won’t be exceeded in the new shorter cycle and was limited in the software settings. If you want to get more information about how to use Kinetics Neo to model the curing process please visit the following website: Epoxy Resin – Curing, Partial Diffusion Control

Dramatic Reduction in Cycle Time

The case shown here, combining the widely used Differential Scanning Calorimetry and the powerful Kinetics Neo software, demonstrates impressively how production cycles can be optimizes. Compared to trial and error manufacturing, material characterization and simulation is much more efficient and can bring the composite industry big cost savings in terms of material and time. The autoclave manufacturing cycle of the CFRP bike rim of Blacks Srl could be reduced by 46 % compared to the conventional curing cycle of the material data sheet meeting the requirements of the degree of cure and avoiding overheating at the same time.

Cycle	Thermal Cycle	Durata totale ciclo termico (min)	ΔH (J/g)	Degree of cure (%)
Prepreg	total cure	–	-223.9	100
conventional	RT-70°C HR=2.3°C/min 70-130°C HR=1.3°C/min Isothermal at 130°C x 270min Cooling	480	-9.25	95.9
new	RT-110°C HR=3°C/min Isothermal at 110 x 10min 110-130°C HR=3°C/min Isothermal at 130°C x 60min Cooling	260	-8.63	96.1

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