UDK 678.7	ISSN 1580-2949
Professional article/Strokovni članek	MTAEC9, 49(5)821(2015)
FLAME RESISTANCE AND MECHANICAL PROPERTIES OF COMPOSITES BASED ON NEW ADVANCED RESIN SYSTEM
FR4/12
NEGORLJIVOST IN MEHANSKE LASTNOSTI KOMPOZITOV NA OSNOVI NOVIH NAPREDNIH SISTEMOV SMOL FR4/12
Vladimir Rusnak1, Sona Rusnakova2^Ladislav Fojtl2, Milan Zaludek2,
Alexander Capka2
1Faculty of Metallurgy and Materials Engineering, VSB-Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava-Poruba, Czech
Republic
2Faculty of Technology, TBU in Zli'n, Nad Stranemi 4511, 760 05 Zli'n, Czech Republic vladimir.rusnak@form-composite.com
Prejem rokopisa - received: 2014-09-10; sprejem za objavo - accepted for publication: 2014-11-24
doi:10.17222/mit.2014.223
Composite materials used in the transport industry and also in other sectors must have a certain degree of flame resistance. For this purpose, commonly used flame retardants are based on halogen compounds in the liquid state or aluminum hydroxide in the solid state. Solid flame retardants have a negative effect on the processing and mechanical properties. Low viscosity and rapid wettability of fibers are very important, especially in an resin transfer molding (RTM) process.
Therefore, a new advanced matrix system based on phosphorus flame retardants was developed. The flame resistance and mechanical properties of the composite materials produced from the new resin system were tested. Furthermore, the processing parameters and tests are described in the article.
Keywords: flame-retardant composites, phosphorus, RTM, vacuum infusion
Kompozitni materiali, ki se uporabljajo v industriji transporta in tudi v drugih sektorjih, morajo ustrezati določeni stopnji negorljivosti. Za ta namen so splošno uporabljani zaviralci gorenja, ki temeljijo na osnovi spojin halogenov v tekočem stanju ali na aluminijevem hidroksidu v trdnem stanju. Zaviralci gorenja v trdnem stanju imajo negativen vpliv na izdelavo in mehanske lastnosti. Posebno majhna viskoznost in hitra omočljivost vlaken sta pomembni, posebno še pri RTM-procesu. Zato je bila razvita nova napredna matrična zasnova na podlagi fosfornega zaviralca gorenja. Preizkušene so bile odpornost proti vžigu in mehanske lastnosti kompozitnega materiala, izdelanega iz novega sistema smol. V članku so opisani parametri izdelave in preizkusi.
Ključne besede: negorljivi kompoziti, fosfor, RTM, vakuumska infuzija
1 INTRODUCTION	Flame retardants are the materials that prevent or
retard the burning of a material and are indispensable for Composite materials based on a polymer matrix and plastic products, electrical applications, construction glass reinforcement are often used in transport vehicles. materials, textiles, etc. Generally, they can be divided
The main reasons for their utilization are their low mass, i^to two main groups: the reactive organic and non-reac-
excellent formability and relatively low production costs tive inorganic compounds (additives). The reactive orga-
for low and medium manufacturing series.	^ic compounds include molecules of halogens (bromine
When composite and sandwich structures are used in and chlorine), melamine compounds, phosphate com-
train applications they haveto fulfill different require-	pou^^s and many others.2 Their main advantage is that ments. The first (or the main) requirement is the fire
1	they do not change the physical properties of the resin. safety.1 In addition to the fire safety of these materials,
relatively high mechanical properties, dimensional and Generally' thhe amount ofthe flame retardant in a poly-
thermal stabilities and health safety are required. For mer matrix depends on the retarding effect. However, it example the French standard NF F01-28: Fiber rein- is necessary to take into account the possibility of a forced plastic in railway rolling stock requires a mini- negative effect influencing the mechanical and pro-
mum bending strength of reinforced plastics of up to 150	cessing properties of the polymer. MPa, while maintaining the self-extinguishing properties The reactive inorganic compounds include aluminum
according to another French standard, NF F16-101.	hydroxide (ATH) and magnesium hydroxide, red phos-
Polyester matrices reinforced with fiber reinforce-	phorus, ammonium compounds, antimony compounds of
ments in various forms are primarily used for these	antimony or boron, graphite, etc.3,4 These compounds are
applications. In these composite systems self-extinguish-	insoluble in a polymer matrix and need to be added in
ing properties are primarily achieved with additions of	higher concentrations (30-150 %) to achieve the right
flame retardants to the synthetic matrices.	fire-retardancy level. They have a very negative influence
on the viscosity of the resulting mixture, thus limiting their use in the preparation of a composite with the vacuum infusion or RTM technology. And, last but not least, they negatively influence the mechanical properties - the composites become less stiff and brittle.
They slow down the combustion process in different ways, physically or chemically. The physical processes include cooling the material, creating a protective layer (that continues to protect the remaining material, blocking the combustion by limiting the access of air) or diluting the gas flame with retardant components.
With respect to the chemical processes, flame retardancy is a reaction in the gaseous phase, leading to a release of significant amounts of water or carbon dioxide. Thus, a reaction at this stage limits and slows down a burning process. This group includes the flame retardants based on halogens. These compounds create carbon ash on the polymer surface or a swelling under the protective layer of the ash to provide a better thermal insulation.5
Some flame retardants based on halogens, such as polychlorinated biphenyls (PCB), or various brominated flame retardants (polybrominated diphenyl ethers, poly-brominated biphenyls) may pose a health risk. The fire retardancy of a halogenated flame retardant is based on the formation of a volatile-combustion hydrobromic or hydrochloric acid in the presence of hydrocarbons (e.g., toxic dioxins) at a temperature of around 350 °C.
Flame retardants based on phosphorus are the compounds that are more environmentally friendly and their consumption is slowly growing. Phosphor flame retard-ants are already applied in matrices with phosphorus amounts of about 5-8 %. They work very well for the polymers containing oxygen in the chain.
2 EXPERIMENTAL WORK 2.1 Materials
2.1.1 Matrix
An unsaturated polyester resin diluted with styrene, containing a phosphorus flame retardant incorporated into the chain molecules was developed under the working title FR4/12 (Table 1). The goal was to prepare a resin, which enables, due to its viscosity, a preparation of the composite parts for rail vehicles using the RTM method or the vacuum infusion method. Pure resin would also meet the requirement for a flame resistance without the additives - the limited oxygen index (LOI) must be higher than 30. The requirement for the maximum viscosity was 600 mPa s.6 The amount of phosphorus in the polyester chain is naturally limited since it greatly affects the mechanical properties. The resulting formulation is designed in such a way that the mechanical properties of pure resin match the mechanical properties of the halogenated resins, normally used for the production of the composite parts for rail vehicles.
Table 1: Mechanical and processing properties of FR4/12 matrix Tabela 1: Mehanski in procesni parametri osnove FR4/12
Material description	Matrix FR4/12
Viscosity (Brookfield LV 2/12, 20 °C) (mPa s) 6	475
Tg/°C	105
HDT/°C	68
Acid number, the KOH content (mg g-1)	2.5
Hydroxyl number, the KOH content (mg g-1)	50
Tensile strength (MPa)	63
Tensile modulus (MPa)	3400
Elongation (%)	2.2
Flexural strength (MPa)	3600
Flexural modulus (MPa)	130
Oxygen index	32-34
2.1.2 Reinforcements, coatings
A multiaxial nonwoven fabric (bidiagonal and tri-axial fabric) was selected as the reinforcement. A nonwoven fabric guarantees a high proportion of glass in a composite (hence, better mechanical properties). It also has a suitable drapability. In line with the requirements of the process technology, mats with the trade name UNIFILLO ® were used for the RTM technology.
To determine the effect of the coating on the combustibility, two samples of the BÜFA gelcoat layer were provided.
2.2 Production methods and testing
The testing of the resin system was carried out at two levels. One level was the production of the test samples. These samples were produced on a simple plate or in a simple RTM mold. The second level of the matrix testing consisted of a production of real products (prototypes). Production methods were chosen with respect to the environmental, health and economic aspects (productivity), including two closed-production technologies -VIP (vacuum infusion process) and RTM (resin transfer molding).
In addition to the newly produced matrix, a total of five composite materials (Table 2) of various compositions as well as other polymer systems were produced (Sample D was made from a phenolic prepreg, Sample E was based on a commercial matrix). Samples B and E were provided with a gelcoat. Firstly, the flexural test was realized according to ISO 178.7 With respect to the fire retardancy the oxygen index was recorded, and the tests of the flame spread and the (partial) heat release were performed using the cone calorimeter method.8 9
3 RESULTS AND DISCUSSION
The measured values of the mechanical properties and fire behavior are shown in Figure 1. As can be seen, the highest values for both flexural properties of the samples prepared with the wet technology were obtained
Table 2: Material characterization of composite materials Tabela 2: Karakterizacija materialov v kompozitu
Material description	Sample A	Sample B	Sample C	Sample D	Sample E
	1 X BIDI glass fabric*	1 X BIDI glass fabric*	1 X BIDl glass fabric*	10 X phenolic/glass*	1 X BIDI glass fabric*
Reinforcement lay-up	1 X CGM **	1 X CGM**	1 X CGM**	prepreg	1 X CGM**
	1 X BIDI glass fabric*	1 X BIDI glass fabric*	1 X BIDI glass fabric*		1 X BIDI glass fabric*
Matrix	matrix FR4/12	matrix FR4/12	matrix FR4/12	phenolic matrix	Norester 880
Additive	-	-	-	-	ATH (100 % to matrix)
Coating	—	gelcoat BÜFA	-	-	gelcoat BÜFA
Production technology	RTM	RTM	VIP	prepreg	RTM
(*bidiagonal glass fabric, **continuous-filament glass mat)
for Sample C. Sample D prepared from prepregs showed excellent flexural performance and fire properties compared to the samples prepared with RTM and VIP; however, these materials are very expensive.
Moreover, comparing FR4/12 to the commercial matrix, it is obvious that the obtained mechanical properties are almost twice as high as for the samples prepared from FR4/12.
The tests of the fire properties (Figure 2) showed that the gelcoat layer also influences the flame spread and the heat release. Further, the oxygen index of Samples B (matrix FR4/12) and E (the commercial matrix) was the same for both samples. Sample C exhibited the lowest oxygen index of all the produced composites. Regarding the requirement for the oxygen index (higher than 30), all the samples passed the test. The heat-release parameter (Figure 3) was measured only for Samples A, B and C, where individual values were 211.6, 154.7 and 37.7. The curves in Figure 3 show the heat release during the combustion of the samples. The flame spread (Figure 4) for Samples A and C is the same (due to the same material composition).
4 CONCLUSION
The main objective of the mechanical testing was to determine whether the strengths of the material combinations reached 150 MPa (this value is the requirement
Figure 2: Fire properties of prepared samples Slika 2: Požarne lastnosti pripravljenih vzorcev
Figure 1: Flexural properties of prepared samples Slika 1: Upogibne lastnosti pripravljenih vzorcev
Figure 3: Heat release during the flame test of composite materials: a) sample A, b) sample B
Slika 3: Sproščanje toplote pri preizkusu kompozitnega materiala v plamenu: a) vzorec A, b) vzorec B
Table 3: Table guide to selecting the test classification1 Tabela 3: Tabelarni napotek za izbiro klasifikacije preizkusa1
ASTM 2863: LIMITED OXYGEN INDEX (LOI)
UK: BS 476 PART 6,7
GERMANY: 4102 5510
FRANCE: NF P 92-501
USA: ASTM E84 UL94
EUROCLASS: SINGLE BURNING ITEM (SBI)
41- 50
Class 0
Class A2
S4
M1
25
5V
34.5-41
Class 1
Class B1
S4
M2
50
B/C
28.5- 34.5
Class 2
Class B2
S3
M3
100
C/D
22-28.5
Class 3
Class B3
S2
M4
>100
1
3
D/E
Figure 4: Composite samples after the flame-spread test Slika 4: Kompozitni vzorci po preizkusu širienia ognja
of French standard NF F01-281). All the material combinations met this requirement with a certain safety margin, except for Sample E. The result was expressed with a high amount of the filler in the resin (the ratio of 1 : 1). It is obvious that after adding the additives the value of the bending strength decreases.
The objective of testing the fire resistance of the materials was to determine the level of the flame retardancy of the composites made from the new matrix system. Using a simple test - determination of the oxygen index - the materials could be classified into the mid-level category of flame-retardant resins. It was assumed that with the addition of non-reactive additives the oxygen index, together with the overall resistance to fire, would increase. According to all the flame-retard-ancy tests, the flame-retardant gelcoat plays an important role in improving the fire resistance.
The oxygen index (Table 3) for various composites based on the new matrix is very similar to the oxygen index of pure resin. Increasing the glass amount has a minimum impact on the oxygen index. However, it is assumed that with the additions of non-reactive additives to a composite system, the oxygen index will increase. The values of the heat release for the phenolic systems are relatively large in comparison with the values for the composite systems based on the new matrix.
The developed matrix exhibits excellent processing properties, good and fast fiber wetting, adjustable reactivity and curing times. Another advantage is that the used promoter is based on iron compounds.
From the overview of the properties of all the systems, it is clear that the polyester systems do not reach the same fire-resistance values as the phenolic systems. On the other hand, due to other advantages (surface quality, price and processing methods) the development of the composite systems based on polyester systems will dominate.
Acknowledgement
This work and the project were realized with the financial support of the Ministry of Industry and Trade of the Czech Republic, project number FR-T13/433. Further, this study was also supported by an internal grant of TBU in Zlm, No. IGA/FT/2015/001, funded from the resources for specific university research.
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