Main Technical Indexes and Requirements of Plastic Support Net

Tensile strength is an important index to evaluate the mechanical properties of plastic support nets and it is generally measured by non-metallic tensile testing machines. The tensile strength of the handmade packing belt, the plastic braid net and the steel plastic composite mesh is the single strength of the mesh belt. For bidirectional stretching, a certain number of test pieces are taken at random in the direction of longitudinal tensile bars and transverse tensile bars respectively. Test their tensile strength according to the tensile direction and take the arithmetic average. And then, multiply the average number of steel bars per meter in each direction, which is called tensile strength of per meter. Its symbol is kN/m. At present, the single tensile strength of the handmade packing belt and plastic braid net can generally reach 200 MPa and the single tensile strength of steel plastic composite net maximum reaches 240 MPa. The longitudinal and transverse tensile strength of the bidirectional stretch plastic mesh is over 30 kN/m and the tensile strength of the single tensile plastic net is more than 240 MPa.

Workers are installed the plastic support net in the coal mine.

IRP-01: The performance of flame retardant is very important to the application of plastic support net in coal mine.

This is the detail display of the plastic support net.

IRP-02: Tensile strength is an important index to evaluate the mechanical properties of plastic support nets.

Fire-retardant property

Most of the plastics have combustibility. In the fire, the main results of plastic combustion are smoke, poisonous gas, flammable gas and combustion droplet. The combustion process of plastics is a very complex thermal oxidation reaction and the basic elements leading to combustion are heat, oxygen, and combustible gases. It is generally believed that the combustion of plastics has undergone the following three stages:

The first stage is the thermal initiation process. Heat from an external heat source or source of fire results in phase changes (that is from solid to liquid) and chemical changes of the plastic.
The second stage is thermal degradation process, which is an endothermic reaction. When the plastic absorbs enough heat to overcome some of the weak core energy between the atoms of the molecule, the plastic begins to degrade. The essence of this reaction is a free radical chain reaction in the presence of oxygen in the air. The result of the reaction is the formation of gaseous phase combustible object, various flammable hydrocarbon monomer and so forth.
The third stage is the ignition process. After the concentration of fuel which generates by thermal degradation reaction in the second stage reaches the ignition limit, it meets and burns with the oxygen in the atmosphere. For example, the commonly used plastic raw materials such as PE, PVC and PP in coal mine will produce poisonous gas and smoke when burning. Its toxic components are mainly carbon monoxide and other gases, which are more toxic if combined with carbon monoxide. Therefore, the performance of flame retardant is very important to the application of plastic support net in coal mine.

At present, the indexes of fire-retardant property of non metallic polymer products used in underground coal mines in China are oxygen index value and the United States UL94 standard. The oxygen index value refers to the minimum concentration of oxygen which can just keep plastic to produce a flaming combustion. Indicated by the volume percentage concentration of oxygen, the higher the oxygen index, the harder the plastic is to burn. in addition, plastics with an oxygen index value of less than 21 are flammable materials and with an oxygen index value of more than 24 are flame retardant material. Such as the oxygen index value of nylon material used underground should be greater than or equal to 26. The United States UL94 standard determines the SMC/BMC material (plastic sheet / resin material) flame retardant performance standards, generally divided into V-0, V-1, V-2 and HB grade, defined as follows:

The flame retardant grades of plastics decrease progressively from V-0, V-1 and V-2 to HB.

V-0: After twice 10 seconds of combustion test, the flame is extinguished in 30 seconds. No burning substance falls.
V-1: After twice 10 seconds of combustion test, the flame is extinguished in 60 seconds. No burning substance falls.
V-2: After twice 10 seconds of combustion test, the flame is extinguished in 60 seconds. Burning substance is allowed to fall.
HB: It is the lowest flame retardant grade in UL94. It requires that samples of 3 to 13 mm thick, should have burning speed less than 40 mm per minute; samples of less than 3 mm thick, should have burning speed less than 70 mm per minute; or extinguished before 100 mm mark.

At present, the fire-retardant property of plastic support net which is used in underground coal mine in China has MT141-2005 "Plastic net top belt for underground coal mine" as basic standard. Its regulations for flame retardancy are the same with MT113-85. It is mainly on the basis of the establishment of UL94 standard, but higher than it, and defines the alcohol burner combustion test (intense combustion) and alcohol lamp burning test (slow combustion) project. Specific indicators are shown in table 1.

Table 1 - Flame retardant index in MT141-2005 standard
Serial number	Item		Technical index (s)
1	The combustion test of alcohol blast burner	Average time of flaming combustion	≤3
2		Maximum burning time of single flaming combustion	≤10
3		Average time of flameless combustion	≤10
4		Maximum burning time of single flameless combustion	≤30
5	The combustion test of alcohol lamp	Average time of flaming combustion	≤6
6		Maximum burning time of single flaming combustion	≤12
7		Average time of flameless combustion	≤10
8		Maximum burning time of single flameless combustion	≤30

Anti-static property

Plastics are excellent electrical insulators whose dielectric constant and loss factor are very low. PP (polypropylene) has high frequency insulation performance. Because of its almost no water absorption, its insulation performance is not affected by humidity. It also has high dielectric coefficient, breakdown voltage and static electricity. The static electricity produced by plastics is mainly caused by air friction and self friction accumulation. Its energy is enough to ignite combustible gas and make it burns or explodes. Being used in the coal mine, it will be very dangerous if the anti-static effect is not good.

The provision on anti-static property in MT141-2005 "Plastic net top belt for underground coal mine" standard is that arithmetic average of electric resistance of both upper and lower surfaces of product samples must less than or equal to 1.0 × 109 Ω.