Using Barcol Hardness Measurements to Correlate Hardness and Degree of Cure - C103

 

Using Barcol Hardness Measurements to Correlate Hardness and Degree of Cure
Case study
Document Type	Case study
Document Identifier	103
Themes	Materials deposition and consolidation management
Tags	Cure and crystallization Thermoset cure kinetics Thermoset polymers Demoulding Discontinuous fibre reinforcement Inspection
Prerequisites	Cure of thermosetting polymers Degree of cure Thermoset polymers

Summary

A manufacturer of bathtubs and other glass fibre products in the interior of British Columbia, Canada, approached the CKN with a desire to optimize their manufacturing process by establishing a method to perform in-situ degree of cure checks with a Barcol hardness impressor. This was in order to reduce the amount of delamination and splintering that can happen at the trimming step of their process, which occurs when the part is trimmed too early after material deposition, before the resin has reached a suitable degree of cure.

Their current manufacturing process is a spray up process with a polyester resin and chopped glass fibre. The deposition is rolled after spraying to consolidate the fibre an produce an even surface before being left at room temperature to cure. After the part is deemed to be suitably cured, it is taken off the mould and the edges are trimmed.

CKN performed DSC tests on neat resin and composite panels. They also made tests panels for the client to draw conclusions on which model of impressor is best suited and how it could be implemented in production. The impressors are not able to give a consistent reading, but are able to indicate when the cure has progressed enough for the part to be moved on to the next manufacturing step.

Challenge

The manufacturer needs an in-situ method to determine when the part is ready to be demoulded and trimmed without the part delaminating and splintering. There is no rapid and cost effective way to determine degree of cure in a production part, but there are some methods that test a property directly linked to the degree of cure. One of them being the hardness of the material.

Prerequisites

• Thermoset polymers
• Curing of thermosetting polymers
• Degree of cure

Also see "Resin Hardness Measurement (Barcol Impressor)" in How to measure curing time and degree of cure.

Approach

Differential Scanning Calorimetry

DSC tests were performed with the aim of determining the degree of cure that can be reached by holding the material at various temperatures. This proved to be difficult to do in practice as the initial curing reaction rate was very quick, such that the cure reaction had progressed significantly by the time the resin was in the DSC and cooled to below the Tg. To combat this, the resin and the catalyst were submerged in liquid nitrogen, then mixed as they were heating up once the viscosity was conducive to mixing. It was quickly distributed into DSC pans and either put into the freezer or into the DSC to start the run.

Another method employed was to dispense resin into the DSC pans, then submerge into liquid nitrogen. Then before the resin heats up, dispense catalyst into the pan and seal without mixing the two components. The DSC results from this method are questionable as there is no guarantee that the reaction occurred completely.

The DSC tests provided insight into the cure and how the curing reaction progressed at different temperatures. This became useful when using the impressors on the material and when also monitoring temperature during cure.

see A192

Barcol Hardness Impressor Models

Two models of Barcol Hardness Impressor (GYZJ-934-1 and -935) were acquired. Model 934-1 is designed for hard plastics and soft metals such as aluminum and GFRP. The range is approximately 25 to 150 Brinell (10mm ball 500 kg load). The 935 model is designed for softer plastics and very soft metals. Since a correlation between very soft materials and different hardness measuring systems is challenging to uniformly establish, the documentation for the 935 model does not provide a conversion curve to other hardness systems. However, it is estimated that the 935 model measures 40 Brinell and lower. The impressors work by placing the needle and sleeve part of the impressor on a flat surface, ensuring the support is level with the needle. A reading is prompted by pressing firmly on the handle and reading off the value on the dial.

Barcol Hardness Impressors. A: Model 934-1. B: Model 935

Thick and Thin Panel Fixtures

The bathtubs are produced by using a spray up onto an acrylic backing/gel coat, so the test panels were made by depositing the material onto 10x10 cm squares of the acrylic provided to CRN by the manufacturer. Their product line also follows two thicknesses of deposited material, so two fixtures were 3D printed so the acrylic plates could be placed inside. This allows the material to be deposited on top and when it sits flush with the frame, it is at the correct thickness. The two different panels are simply referred to as "thick" and "thin" in this case study.

3D printed fixtures to manufacture test plates to the correct thickness, with the acrylic backing placed inside.

Hardness Testing with Fibres

The first sets of tests were done with resin and 5% volume fraction of fibres. The time of mixing the resin and the catalyst was recorded and the mix of resin and fibres were spread over the acrylic plates and evened out to the correct thickness. The resin was left to gel at room temperature then hardness measurements were taken at regular intervals for a total of five hours. Even though the gel time of the resin is around 10-20 minutes at room temperature, the hardness impressors did not start giving readings until 70 minutes for the 935 model and 145 minutes for the 934-1 model. This highlights that although the resin might feel hard and fully cured to the touch, it may still be far from suitably cured.

From these tests we learned that the 935 model is much more appropriate for this application as the impressor started giving readings earlier in the curing process and the spread of readings is narrower. However, the spread of readings is also relatively high for the 935 model, suggesting that the uneven surface of the panel is affecting the readings. It is also evident that the thick panel cures faster than the thin panel, most likely due to the extra heat that is generated during curing.

Barcol hardness test plates with fibres. Visible marks from the impressor can be seen covering the surface.

Barcol hardness vs time from mixing of resin with fibres present. Comparing the 935 and 934-1 model.

Hardness Testing Without Fibres and Monitoring Temperature

The next set of tests were conducted with thermocouples embedded into the resin and no fibre. This test provided valuable insight into the exothermic reaction and how that correlated to the hardness and progression of cure. The test also showed that there is still a considerable spread in the hardness readings in the initial stages of recording hardness. However, when the readings begin to plateau, the spread is less than when fibre is included in the deposition. The exothermic peak seen by the thermocouple also matches the shape of the exotherm readings in the isothermal DSC tests. This test also reconfirmed that the thick panels cure faster than the thin panels.

Barcol hardness and temperature vs time after mixing resin without fibres. Results shown is for the thicker panel.

Outcomes

The cure of the polyester resin is deceptively slow when comparing to the gelation time of 15-20 minutes. The exothermic peak occurs at around 60 minutes, but the hardness readings do not begin until around 100 minutes, and the full cure is at around 240 minutes. The resin feels cured to the touch after 30 minutes, so the need for a qualitative method to determine cure progression on the shop floor is clearly needed. The issue with the Barcol hardness measurement method is that the readings are very varied depending on surface evenness, this is influenced by the fibres and the shape of the part. However, there is a certain hardness and degree of cure where the impressor will start to give readings and the average of the readings follows an upwards trend.

The Barcol hardness model 935 has been implemented into the process by using it as a check for minimum cure, where the criteria is: if the impressor gives a reading, the cure has progressed far enough to move to the next step.

Future Steps

To further optimize this process on the shop floor there are two paths that can be taken. Firstly, a study to determine the optimal degree of cure that a part can be trimmed at. This can then be related back to the Barcol hardness readings to provide a range of readings where the part is acceptable for demould. The second path will be to use temperature monitoring by thermal camera or thermocouple. This will monitor the relative temperature and exothermic reaction observed, to then determine how far along the cure has progressed, further determining when to demould the part.

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Related pages

• Practice for developing a thermal transformation process step
• Curing of thermosetting polymers
• Degree of cure

About	Help

Degree of cure (DOC) is an indication of how far the chemical curing reaction (crosslinking process) has advanced in a thermoset resin.

DOC is defined with a number between 0 and 1 (or 0% and 100%) where 100% is a fully cured resin. It does not have to fully reach 100% for the resin to become solid or the part to be used. In some aerospace applications, resins are only cured to about 90%. Higher the degree of cure, higher the mechanical properties.

The failure mode characterized by a partial or full separation of adjacent laminae. Delamination is unique to laminated structures, and can be induced by impact load, fatigue load, and quasistatic load. (Also known as interlaminar separation)

The act of combining reinforcement fibre, resin to shape on a mould (tool).

There are three main methods to combine resin and fibre to deposit onto a tool (i.e. material deposition):

• Wet Processes: Mixing liquid resin and dry fibre directly in the mould, e.g. Wet Compression Moulding process for parts in BMW 7 series
• Liquid Composite Moulding (LCM) processes: Infusing dry fabric with resin using a pressure gradient, e.g. High Pressure Resin Transfer Moulding process (HP RTM) for parts in BMW i3
• Prepreg Processes: Laying up using prepreg where resin and fibre are already combined, e.g. Automated Tape Laying (ATL) process for some of the parts in Boeing 787.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Differential Scanning Calorimetry (DSC).

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).