| 
- Productos
Digestión por microondas
- Determinación de mercurio
- Química limpia
- Calcinación por microondas
- Extracción por microondas para análisis
- Extracción por microondas para procesamiento
- Síntesis por microondas
- Industrias
- Recursos
- Compañia
- Contacto
- Negocio
- Conviértete en miembro
-
Written by Gianpaolo Rota, Product Manager | 20 June 2024
Volume fraction measurement of fibre-reinforced polymer via selective microwave sample preparation
INTRODUCTION
Fibre volume fraction and void content of fibre-reinforced polymer composites are important properties that influence material performance and are therefore widely used in material analysis. The acid digestion method, standardised in ISO 14127:20083 and ASTM D3171-154 procedures, is one of the most widely used techniques by industry to measure volume fraction properties of carbon fibre reinforced polymers (CFRP). The method involves removing the polymer matrix phase of a composite sample through digestion in acid to leave the reinforcing fibres only. Fibre volume fraction and void content are then calculated considering the constituent polymer and fibre densities, the initial mass and density of the composite sample and the mass of the fibres following matrix removal.
For common epoxy matrices containing composites, the method can be carried out safely as polymer digestion can be performed with concentrated nitric acid. However, other polymer types (e.g. phenolic and cyanate esters) typically require more hazardous reagents to achieve complete matrix digestion, including sulphuric acid and hydrogen peroxide mixtures.
A milder chemical approach using nitric acid alone is desirable, allowing analysts to reduce the number of chemicals used and the associated risks. Although this approach reduces health and safety risks, the omission of such reagents can affect the digestion process, requiring optimisation of the digestion conditions for a particular type of material. This is particularly important as the method assumes that the polymer matrix is completely digested by the acid without attacking and removing material from the reinforcing fibre.
Microwave sample preparation has been successfully and widely applied for both elemental and molecular analysis thanks to its efficiency and selectivity achievable with volumetric heating.
In this application report, a selective microwave assisted sample preparation protocol has been developed by National Physical Laboratory (NPL) for the volume fraction determination of fibre-reinforced polymer composites. The innovative microwave protocol ensures a selective digestion of polymer matrix without attacking the reinforcing fibre or left regions of undigested polymer.
Microwave sample preparation has been successfully and widely applied for both elemental and molecular analysis thanks to its efficiency and selectivity achievable with volumetric heating.
In this application report, a selective microwave assisted sample preparation protocol has been developed by National Physical Laboratory (NPL) for the volume fraction determination of fibre-reinforced polymer composites. The innovative microwave protocol ensures a selective digestion of polymer matrix without attacking the reinforcing fibre or left regions of undigested polymer.
EXPERIMENTAL
INSTRUMENT AND REAGENTS
The protocol was performed with ETHOS UP with the aim to develop a fast and easy-to-apply protocol for routine high throughput analysis, based on nitric acid only.
EQUIPMENT
For the filtration process, a filter paper was pre-weighted. After the filtration, the filter was washed several times with deionized water and acetone before being dried at constant mass at 105°C in an electric oven.
SAMPLES
To evaluate the polymer composites degradation when submitted to acid digestion, hard to digest CFRP were used such as phenolic, cyanate ester and polyfurfuryl alcohol polymer.
Samples for digestion, approximately 1 g, were machined from supplied material using a CompCut 500 composite plate saw and dried to constant mass at 105°C before testing. For cyanate ester approximately 0.5g of sample was used due to the sample type.
SAMPLE PREPARATION
In this study, 0.5-1 g of fibre-reinforced polymer composites samples have been treated with a mixture of 5 mL of 70% HNO3 and 5 mL of deionised water over a range of time and temperatures. Diluted acid (35%) obtained by diluting 70% HNO3 in deionised water at a 1:1 ratio allows to better control the digestion process of the polymeric part. Before digestion, the initial mass and density of each sample was determined in accordance with the immersion method described in ISO 1183-1:2019 (method A)5. The sample and acid mixture were placed directly into the TFM digestion vessel and then the system was properly sealed with a PTFE cap and a spring to ensure hermetically closure during the run. In order to evaluate the efficiency of the polymer matrices digestion, the residual fibres were determined by SEM.
MICROWAVE PROCESS AND QUANTIFICATION
The MW program used for this work was the following:
After the microwave program, the digestates were cooled to ~65°C and the reinforcing fibres separated from the acid solution using a vacuum pump to pull the acid and digestion residue through a pre-weighed filter. The residual fibres were washed several times through the filter with deionised water and acetone before being dried to constant mass at 105°C in an oven.
The protocol was performed with ETHOS UP with the aim to develop a fast and easy-to-apply protocol for routine high throughput analysis, based on nitric acid only.
EQUIPMENT
- Milestone’s ETHOS UP
- SK-15 (TFM 100 mL vessels)
- CompCut 500 composite plate saw
- Drying oven
- Analytical balance
- Scanning Electron Microscopes (SEM)
For the filtration process, a filter paper was pre-weighted. After the filtration, the filter was washed several times with deionized water and acetone before being dried at constant mass at 105°C in an electric oven.
SAMPLES
To evaluate the polymer composites degradation when submitted to acid digestion, hard to digest CFRP were used such as phenolic, cyanate ester and polyfurfuryl alcohol polymer.
Samples for digestion, approximately 1 g, were machined from supplied material using a CompCut 500 composite plate saw and dried to constant mass at 105°C before testing. For cyanate ester approximately 0.5g of sample was used due to the sample type.
SAMPLE PREPARATION
In this study, 0.5-1 g of fibre-reinforced polymer composites samples have been treated with a mixture of 5 mL of 70% HNO3 and 5 mL of deionised water over a range of time and temperatures. Diluted acid (35%) obtained by diluting 70% HNO3 in deionised water at a 1:1 ratio allows to better control the digestion process of the polymeric part. Before digestion, the initial mass and density of each sample was determined in accordance with the immersion method described in ISO 1183-1:2019 (method A)5. The sample and acid mixture were placed directly into the TFM digestion vessel and then the system was properly sealed with a PTFE cap and a spring to ensure hermetically closure during the run. In order to evaluate the efficiency of the polymer matrices digestion, the residual fibres were determined by SEM.
MICROWAVE PROCESS AND QUANTIFICATION
The MW program used for this work was the following:
After the microwave program, the digestates were cooled to ~65°C and the reinforcing fibres separated from the acid solution using a vacuum pump to pull the acid and digestion residue through a pre-weighed filter. The residual fibres were washed several times through the filter with deionised water and acetone before being dried to constant mass at 105°C in an oven.
ANALYTICAL PROCEDURE
The fibre volume fraction (𝑉𝑓) was subsequently calculated according to the following equation:
Vf=(Mfx ρc)/(M"i " x" ρf" ) x100
Where:
-Mf is the mass of the fibres after digestion -Mi is the initial mass of the sample -𝜌𝑓 is the constituent fiber density -𝜌𝑐 is the composite density
Vf=(Mfx ρc)/(M"i " x" ρf" ) x100
Where:
-Mf is the mass of the fibres after digestion -Mi is the initial mass of the sample -𝜌𝑓 is the constituent fiber density -𝜌𝑐 is the composite density
RESULTS AND DISCUSSION
The microwave sample preparation protocol on polymer composites developed in this study has been designed to determine the fibre volume fraction via scanning electron microscopy technique. The applicability of such protocol strongly depends on the capability of the digestion method to selectively digest the polymer matrix without degrading the reinforcing fibres.
Initial digestion runs evaluated typical conditions when digesting with HNO3 as recommended in ISO 14127:2008 and ASTM D3171-15. Specimens were digested in 20 ml of 70% concentrated HNO3 for 30 mins at temperatures between 120 and 160°C. These conditions were not suitable for the purpose since as reported in the SEM micrographs (Figure 2) fibres were attacked, and undigested polymer was detected.
Initial digestion runs evaluated typical conditions when digesting with HNO3 as recommended in ISO 14127:2008 and ASTM D3171-15. Specimens were digested in 20 ml of 70% concentrated HNO3 for 30 mins at temperatures between 120 and 160°C. These conditions were not suitable for the purpose since as reported in the SEM micrographs (Figure 2) fibres were attacked, and undigested polymer was detected.
Following an in deep method optimization, the most selective condition in terms of temperature, time and acid concentration were 1:1 HNO3: deionized H2O digestion mixture working at 160°C for 60 min.
As reported in Figure 3, SEM micrographs shown integrity of the fibre morphology and no evidence of residual polymer.
Tables 3, 4 and 5 report the fibre volume fraction of different polymer matrices working under optimized conditions. The high level of repeatability observed from the low coefficients of variation (CV) further indicate optimal digestion. Large variability would be expected if either under- or over-digestion was present. The higher CV for cyanate ester composites is likely a result of these samples being of smaller volume (0.5 g) and therefore individual samples potentially encompassing different sections of the unit cell of the composite.
These results have been achieved by creating a more controlled digestion of the material. By reducing the HNO3 concentration, the polarity of the acid mixture is lowered enabling slower microwave absorption during heating. This, combined with the weaker acid reducing pressure inside the reaction vessel, would result in a more controlled, less aggressive digestion process, allowing the polymer to be fully digested without fibre attack.
Despite the described protocol is outside the ISO 14127:2008 and ASTM D3171-15 recommendations, the data reported in this study candidate the microwave method to a good complementary and alternative approach to the traditional protocol, using a much less harsh chemicals.
Despite the described protocol is outside the ISO 14127:2008 and ASTM D3171-15 recommendations, the data reported in this study candidate the microwave method to a good complementary and alternative approach to the traditional protocol, using a much less harsh chemicals.
Milestone™ Srl
Via Fatebenefratelli 1/5 - 24010 Sorisole (BG) - Italy
Teléfono: +39 035.573857 | Fax: +39 035.575498
Via Fatebenefratelli 1/5 - 24010 Sorisole (BG) - Italy
Teléfono: +39 035.573857 | Fax: +39 035.575498
Milestone™ Srl
Via Fatebenefratelli 1/5 - 24010 Sorisole (BG) - Italy
Teléfono: +39 035.573857 | Fax: +39 035.575498
Via Fatebenefratelli 1/5 - 24010 Sorisole (BG) - Italy
Teléfono: +39 035.573857 | Fax: +39 035.575498