A 2D/3D VISION SYSTEM TO QUANTITATIVELY ASSESS THE EVOLUTION OF A PROTECTIVE LAYER ON FLAME RETARDED POLYMERS

*R. Dos Santos1,2, A. Castrovinci1, M. Banfi1, M. Floris1, G. Malucelli2, M. Marchizza3, G. Tognola3 1Department of Innovative Technologies, University of Applied Sciences and Arts of Southern Switzerland, 6928 Manno, Switzerland. E-mail: rosane.dossantos@supsi.ch

2 Department of Applied Sciences And Technology, Politecnico di Torino, Viale T. Michel 5, 15121 Alessandria, Italy3 SIP Industrial Promotion SA , La Monda 191A , 6534 S. Vittore , Switzerland

2D SENSOR SPECIFICATIONSManufacturer and model IDS uEye UI-1220SE-M-GLSensor Technology CMOS MonoResolution (h x v) 752 x 480Color depth (sensor) 10 bitColor depth (camera) 8 bitPixel Class WVGASensor Size 1/3"Shutter Global shuttermax. fps in Freerun Mode 87.2

SURFACE PROTECTIVE LAYER EXTRACTED CHARACTERISTICS

CONCLUSIONS

PROTECTIVE LAYER GROWTH RATE

Is the video over?

no

Process next frame

Is the image too bright? (excess of flames) yes

no

Dynamic Threshold: (Fig. 3a)Highlights the areas of the image that are darker than their surrounds

Select Shape: (Fig. 3b)Keeps only the zones that match specific criteria (area, countour length, shape) associated

with the presence of a crack

START

END

yes

(a) (b)

(a) (b) (c)

Fig. 5: Image from the acquired video(a)During UL94 test (b)Superposition of image and processing result

Fig. 3: Steps of cracks identification

Fig. 2: Images of cracks in the material surface before and after processing

The videos obtained during the UL 94 tests were processed in order to quantify the char formation rate in the material surface. The developed algorithm used to process the images is described below:

Fig. 4 Steps to obtain the char formation rate

(a) (b)

A TPU flame retarded with phosphorous system has been selected as key-study. The following graphs show the surface protective layer growth rate as a function of time for three formulations, a V0, V1 and a N.C. (UL94 standard). The plots are the average of at least 10 runs. Column A reports the growth rate of the surface protective layer for the whole UL94 test. Columns B and C report the growth rate, in its linear interval, for the first and second flame application, respectively:

This vision apparatus is an effective tool that provided a deep insight of the dynamic behavior of a phosphorous-based flame retarded material.The measurement of the surface protective layer growth rate brings to a better understanding of the behavior of the flame retardant systems, correlating the speed of the chemical reaction with the performances of the material.Dynamic monitoring of cracks formation can help to adjust the flame retardant formulations in order to minimize/avoid this behavior.The 3D scan can measure the volume expansion and inspect features that are not visible to the 2D camera (Work In Progress)

The apparatus identifies the cracks formation in the material surface using the following algorithm:

FORMATION OF CRACKS

Fig. 1: New experimental apparatus. (a) System representation; (b) Picture of the experimental set-up.

(a) (b)

EXPERIMENTAL METHODSMore than hundred experiments have been carried out to test and validate the apparatus presented in this work. Composites were blended by a twin-screw extruder type ZSE 18 HP – 40 D from Leistritz, and samples (125x 13x 0.3mm3) for testing were moulded by an injection machine Allrounder 270/320C from Arburg. Short movies were recorded during the UL 94 standard tests.

Table 1: Specification details of the 2D (a) and 3D (b) sensors

3D SENSOR SPECIFICATIONSManufacturer and model microEpsilon scanCONTROL 2810-25Measurement range, z axis 25 mmMeasurement range, x axis 25 mmResolution z axis 10 μm Resolution x axis 1024 points/profileLight source Laser diode 658 nm, 15 mWPermissible extraneous light 10.000 lxOperating temperature 0 ... +50 °C

(a) (b)

Measure the 5mm circular molding mark and scale its radius from pixels to real world coordinates (Fig. 4a)

Is the video over?

no

Process next frame

Is the image too bright? (excess of flames) yes

no

High pass filter: (Fig. 4b) Grays out the flat surface areas leaving only the small structures visible.

Filters out the influence of smoke and flames

Binary threshold: (Fig. 4c)Highlights the areas with the features of interest, i.e. those that are brighter

and darker with respect to the mean gray value

Count the highlighted pixels and convert to mm2

START

END

yes

3D SCAN OF THE MATERIAL SURFACE

A B CA

[mm

2 ]

T [s]

UL 94- V1

A [m

m2 ]

T [s]

A [m

m2 ]

T [s]

3,28 mm2/s

First flame application

A [m

m2 ]

T [s]

UL 94- V0

A [m

m2 ]

T [s]

Second flame application

A [m

m2 ]

T [s]

13,94 mm2/s

T [s]

UL 94- n.c (no classified)

A [m

m2 ]

ACKNOWLEDGMENTSWe thank the CTI / KTI for the financial support of this work. KTI / CTI Project No. 14714.1 PFIW-IW. We also thank MVTec Software GmbH for providing a trial of the Halcon software and microEpsilon Swiss AG for providing the 3D scanner.

ABSTRACT

This work describes an imaging acquisition system developed to monitor the surface of a burning polymer. The apparatus records a video of the burning surface, tracking the evolution of its features, i.e. 2D/3D surface texture and char volume expansion. The system minimizes the disturbances of the flames on the image sensors. It consists of 2D/3D cameras with customized lighting, acquiring images of the burning specimen. Surface swelling, the formation of cracks, and the carbonization front growth rate are continuously monitored and subsequently analysed by a dedicated software. The UL94 test procedure has been used as test bed for the system. The processed images allow identifying and quantitatively measuring key features of the protective layer that cannot be monitored by the operator’s naked eye.

1) Protective layer growth rate2) Formation of surface cracks3) 3D scan of the material surface during UL-94 test

The apparatus is designed to continuously monitor the material surface providing a video of the swelling sample in a non-invasive way. The experimental set-up of the 2D/3D imaging system consists of a CMOS industrial camera from IDS and a laser scanner from microEpsilon. The specification of the imaging sensors are shown in Tables 1a and 1b. A PC acquires and processes the surface images. The graphical unit interface (GUI) is written in C# using MS Visual Studio. The acquired videos are then post-processed with the MVTec Halcon software to extract the key characteristics.

SYSTEM DESCRIPTION