A Study On The Preparation And Commerical Test Of Conductive PET Fabrics Used For Electromagnetic Shielding

As the use of electronic products and communication instruments increases, electromagnetic interference (EMI) has been a problem for the lifetime and efficiency of the instruments. In addition, the regulation of EMI shielding such as by the Federal Communications Commission (FCC) has become more rigorous. Because of good conductivity and special shape, conductive fabrics coated with copper and nickel can be used for shielding electromagnetic radiation and interference. The electroless copper plating using formaldehyde as reducing agent is the main method for prepare copper-coated fabrics. However, formaldehyde is volatile and toxic, which is forbidden to use by many countries. So, this paper aims to study the electroless copper plating process using hypophosphite as reducing agent and improve the structure and properties of copper deposits, and apply it for preparing conductive PET fabrics. The process of pretreatment for PET fabric before electroless copper plating was also investigated. The process for prepare conductive PET fabrics is etching, sensitization, activation, electroless nickel plating, electroless copper plating and electroless nickel plating. The properties of conductive PET fabrics such as surface resistance, shielding effectiveness and adhesion of metal deposit to PET fabrics were investigated deeply.The best etching condition of PET fabrics is NaOH 200g/L, 70℃and 10min。At this condition, the weight loss of PET fabrics after etching is about 9.8% and the adhesion of metal deposit to fabric is most strong.The most proper process before electroless copper plating is sensitization(SnCl₂ 8¹0 g/L) , activation(PdCl₂ 0.05 ⁰.06 g/L)and electroless nickel plating with alkaline bath.The electroless copper plating process using hypophosphite as reducing agent was investigated by orthogonal test. According to influencing sequence, the factors affecting the surface resistance and deposition rate are as followed: the concentration of nickel sulphate>pH value> the concentration of citrate > the concentration of hypophosphite >temperature. The best basic process of electroless copper plating is 8g/L CuSO₄, 0.9g/L NiSO₄, 40g/L NaH₂PO₂, 20g/L C₆H₈O₇, 30g/L H₃BO₃, pH 9.5 and 65℃.The deposition rate of electroless copper plating at different concentration of Cu²⁺, Ni²⁺, NaH₂PO₂, C₆H₈O₇, pH and temperature respectively, were measured when maintaining other factors as constant. The kinetic equation of electroless copper plating is obtained by regression analysis:The trace of H₂ emission was removed by adding polyethylene glycol. However, the deposition rate increased obviously and the copper deposits became loose and porous, which resulted in higher surface resistance of copper-coated fabrics.The deposition rate on the 210T PET fabric decreased from 2.56 to 1.8 g/(min·m²) when adding 20 mg/L 2,2'-dipyridyl to the electroless copper plating bath. The copper deposit became compact and the color became copper-bright at the presence of 20mg/L 2,2'-dipyridyl in the bath. The copper deposits has preferred orientation of (220) crystal plane after adding 20mg/L 2,2'-dipyridyl and the mole ratio of NaH2PO2/CuSO4 consumed during operation decreased from 7.05 to 5.5. The surface resistance of 210T PET fabrics with 30g/m² copper deposit decreased from 79 to 28 mΩ/sq after adding 20mg/L 2,2'-dipyridyl .The deposition rate on the 210T PET fabric decreased from 2.56 to 1.95 g/(min·m²) when adding 2⁴ mg/L K4Fe(CN)6 to the electroless copper plating bath. The copper deposit became compact and the color became copper-bright at the presence of 2⁴ mg/L K4Fe(CN)6 in the bath. The copper deposits have preferred orientation of (220) crystal plane after adding 2⁴ mg/L K4Fe(CN)6 and the mole ratio of NaH2PO2/CuSO4 consumed during operation decreased from 7.05 to 3.5. The surface resistance of 210T PET fabrics with 30g/m² copper deposit decreased from 79 to 19 mΩ/sq after adding 2⁴ mg/L K4Fe(CN)6.The surface resistance of copper-coated PET fabric (210T,260T and 290T) decreased with the increase of the deposit weight on the fabric. When the deposit weight on the fabric reached to 30g/m², the surface resistance of 3 types of PET fabrics are less than 22 mΩ/sq. When the weight of the copper deposits was same, the surface resistance of copper-coated fabrics increased slightly with the increase of woven tightness.The shielding effectiveness of conductive fabric increased obviously with the increase of copper deposit weight or fabric conductivity. When the deposit weight on the 210T fabric reached to 30g/m², the shielding effectiveness are more than 82 dB in the range from 10MHz to 20GHz. The shielding effectiveness of conductive fabric decreased with the increase of electromagnetic frequency and increased with the increase of woven tightness of fabrics.The commercial test with the capacity of 120 Km²/y conductive fabric was carried out. The results show that conductive fabrics with low surface resistance and high shielding effectiveness can be produced by the followed process: scouring, etching, sensitization, activation, electroless nickel plating, electroless copper plating and electroless nickel plating. The technology is friendly to environment, easy to operate and control, of lower production cost, and has strong resistance to risk, which can be applied commercially.The innovations in these investigations are as followed:(1) The process of electroless copper plating using hypophosphite as reducing agent was investigated systemly. K4Fe(CN)6 and 2,2'-dipyridyl were firstly used to improve the structure and the conductance of copper deposits.(2) The technology of electroless copper plating using hypophosphite as reducing agent, which has such advantages as high stability, low cost, and relative safety, was firstly used to prepare conductive PET fabrics.(3) Conductive fabrics with low surface resistance and high shielding effectiveness were produced by next process: scouring etching, sensitization, activation, electroless nickel plating, electroless copper plating and electroless nickel plating.(4) A commercial test line was designed and manufactured to produce conductive PET fabrics continuously. The commercial test line has such advantages as cramped construction, low cost, being easy to operate and control.