Silicone Coated Fabric

Silicone rubber is a kind of elastomer with excellent heat resistance, low temperature resistance, ozone resistance, weather resistance, electrical properties and high permeability. It is widely used in the buttons of mobile phones, telephones, notebook computers, fax machines, copiers, home appliances remote control and other products. The buttons is high frequency, easy to wear, so need wearproof and good decorative coating. But the silicone rubber surface energy is low, adhesion of the coating is poor, and easy to fall off. At present, the cost of UV curing and organic silicone and other coating is high, it is difficult to meet the rapidly expanding market demand. The blocking polyurethane coating reacts with the functional groups such as the hydroxyl groups on the surface of the treated silicone rubber to obtain a high adhesion, and the polyurethane coating of the layer has excellent abrasion resistance and surface decorative performance. In this research, polycarbonate diol (PCDL) with excellent mechanical properties, hydrolysis resistance, heat resistance, oxidation resistance and abrasion resistance was selected and reacted with excess isophorone diisocyanate (IPDI). The effects of blocking agent type, amount of blocking agent, temperature and time of blocking reaction and synthesis technology on the sealing rate were studied.

Experiment
Main raw material
IPDI, PCDL, dibutyltin dilaurate, toluene, di-n-butylamine, trimethylolpropane (TMP), diethyl malonate, chemically pure. Acetylacetone; ethyl acetoacetate; ethyl acetate; isopropanol , Phenol; methyl ethyl ketone oxime, purity 98%, all of the above is AR.

The synthesis of blocking polyurethane
A certain amount of PCDL, dibutyltin dilaurate and ethyl acetate were added to a four-necked flask equipped with a collector-type magnetic stirrer, a reflux condenser, a thermometer and a nitrogen inlet and outlet, and IPDI was added dropwise at 80℃, reacts 2 hours. After cooling, the NCO group was added and reacted at 60℃ for a period of time. The residual NCO group content was measured. Then, the crosslinking agent TMP was added for 2 hours to obtain a blocking polyurethane solution with a solid mass fraction of 46%.

Analysis and testing
The determination of NCO content: Test by dibutylamine re-titration method.
Measurement of the blocking rate: The blocking rate of the NCO group was calculated according to (W1-W2) x 100% / W1, and W1 and W2 were calculated as the NCO group contents of the prepolymer before and after the blocking.
Resistance to yellowing test: the obtained blocking products evenly coated on the slide, until the solvent volatile, put into 160℃ oven for 1h, observe the appearance of the product.
Alcohol resistance test: impregnated with cotton group of anhydrous alcohol, wrapped in a dedicated test head (after wrapping with cotton, the test head area of about 1cm2), applying 500g load to 40 to 60 cycles per minute, 20mm around, wipe back and forth on the surface of the bottom, 3 samples, the coating thickness of 0.06 ~ 0.09mm. The number of falling off cycles is the test result.

Resistant to sweat test: clean cloth soaked in the ammonia containing 1. 07%, sodium chloride 0.48% of the mixed water solution, which stick on the surface of the product, and sealed with a plastic bag, placed in the room temperature after 24 hours. After the test is completed, the test solution on the surface of the product will be wiped clean and the coating adhesion will be tested after 2h or more at room temperature. 3 samples, the coating thickness is 0.06 ~ 0.09 mm.

Using CMT4340 electronic testing machine to test the tensile strength : the tensile speed is 50mm/min, the sample made of the thickness of 1 ~ 2mm dumbbell shape, after standing at room temperature for 24 h, according to GB / T 528-92 standard determination.

Using the Spectrum Fourier transform infrared spectrometer (FT-IR) of Sprectrum from United States, after pure KBr tabletting, the solution was dropped onto the KBr sheet with a capillary and tested after evaporation of the solvent.

The decompression temperature of the blocked product was measured by a differential thermal analyzer (DSC821e) manufactured by METTLER. The flowing rate of N2 was controlled at 80 mL/min, from 20℃ to 160℃, and the heating rate was 10℃/min.

Results and discussion
Effect of blocking selection on blocking rate

The requirement of silicone rubber surface coating decorative effect is high high, the selected blocking agent required resistance to yellowing and has a low decapsulation temperature. In this study, phenols, oximes and dicarbonyl compounds were used as blocking agents. The molar ratio of blocking agent to NCO group (-H: -NCO) was 1: 1, and reacted at 60℃ for 4 h to synthesize blocked polyurethane solution , the experimental results in Table 1.
Table 1 Effect of blocking selection on blocking rate

In Table 1, the sealing rate of phenol and methyl ethyl ketoxime is very high, but the high temperature curing release of the blocking agent is toxic, and the coating is yellowing. The resistance to yellowing of the products of three β-dicarbonyl compounds, such as diethyl malonate, acetylacetone and ethyl acetoacetate, was relatively good. Although acetylacetone and ethyl acetoacetate are able to achieve higher blocking rate than diethyl malonate, the first two are significantly higher in the decapsulation temperature and are accompanied by slight yellowing and are difficult to meet the high decorative requirements of the coating. Therefore, in this study, the prepolymer was blocked with diethyl malonate without yellowing.

Effect of the amount of blocking agent on the blocking rate
The reaction temperature was 60℃ and the reaction time was 3 h. The effect of the molar ratio of blocking agent to NCO group on the blocking rate was investigated, and the experimental results are shown in Pic 1

The molar ratio of —H :—NCO Blocking rate/%
Pic 1 Effect of the amount of blocking agent on the blocking rate

In Pic 1, when the molar ratio of -H: -NCO is 1: 1 in the blocking reaction, the blocking rate of the blocking reaction is only 35%, but with the increase the molar ratio of -H: -NCO , the blocking rate is improved. After the molar ratio is more than 1.9, the rise of the blocking rate tends to ease. This largely depends on the reactivity of active hydrogen (-H) in diethyl malonate, because the active hydrogen on the methylene group in the diethyl malonate is affected by the strong absorption and symmetry structure of the dicarbonyl, the activity is far less than active in phenolic, amine-based sealants. Secondly, when the coating of small molecules contain a high proportion of the blocking, it will seriously affect the coating solvent resistance and other properties. Thus the fixed molar ratio of -H: -NCO is 1. 9.

Effect of reaction temperature on blocking rate
The reaction time is 4h, the molar ratio of -H: -NCO molar ratio is 1. 9, and the effect of the blocking reaction temperature on the blocking rate is investigated and results are shown in Pic 2.

Pic 2 Effect of closed reaction temperature on the sealing rate

In Pic 2, when the temperature rose from 40℃ to 80℃, the blocking rate showed a trend of rise and fall, the suspension of NCO is the exothermic reaction, the reaction of diethyl malonate is very low under the low temperature conditions, and affecting the blocking efficiency. However, the high temperature will inhibit the exothermic reaction in the direction of positive reaction, and the high activity of IPDI will produce many side effects, which will affect the blocking effect of the product. The suitable reaction temperature is 60℃.

Effect of blocking reaction time on blocking rate
The molar ratio of -H: -NCO was 1.9, and the blocking reaction temperature was 60℃. The blocking rate of the reaction time was from 1h to 6h. The reaction results are shown in Pic 3.

Tim/h Blocking rate/%
Pic 3 Effect of blocking reaction time on the blocking rate

In Pic.3, when the blocking reaction is 1 h, the blocking rate has reached 45%, and in the flowing few hours, the closure rate increased by only about 15%, within 1 h after feeding, the initial reaction concentration of the blocking agent and NCO group reached the maximum, so that sufficient contact between the reactants, completed most of the reaction. When the blocking reaction is carried out for 4 hours, a dynamic equilibrium state is reached, and the effect of increasing the reaction time on the reaction degree is very small. Generally, the appropriate blocking reaction time is 4h.

Effect of crosslinking agent on polyurethane coating
In order to overcome the shortcomings of linear polyurethane water resistance, solvent resistance and heat resistance, the blocked residual isocyanate groups and cross-linking agent react together, the polyurethane molecules to form a cross-linked network structure, but the introduction of cross-linking agent will increase the viscosity of the early polymerization system too fast, it’s easy to gel. In this study, TMP was used as an internal crosslinking agent to synthesize a suitable cross-linked polyurethane solution. Because the blocked reaction has a low blocking rate under the premise that the coating is not yellowing, the free NCO groups still exist in the system, which affects the stability of the system. Moderately crosslinking can not only improve the solvent resistance of the coating, but also can improve the blocking rate of closed reaction. The crosslinked and uncrossed polyurethane prepolymers were characterized by FT-IR after the end of the blocking reaction. The results are shown in Pic. 4.

In Pic. 4 that there is still a characteristic absorption peak (2264 cm-1) of the NCO group in the polyurethane prepolymer without the crosslinking agent after the end of the blocking reaction, mainly due to the blocked reaction efficiency. After the cross-linking, the characteristic absorption peak of the NCO group disappears, and the NCO group has completely reacted by the crosslinking agent.

Wave number/cm – 1
1 – non crosslinked product, 2 – crosslinking products
Pic 4 The reaction product FT – IR of pre polymers before and after blocking and crosslinking

Performance test results before and after crosslinking
—H: —NCO is 1. 9 ：1, the reaction temperature was 60℃ and the reaction time was 4h. To investigate the effect of the amount of crosslinking agent on the properties of the coating, curing on the surface of the silicone rubber treated by plasma, place it for 24 h to test its performance, the results shown in Table 2.

Note: The crosslinker content is the mass fraction of PCDL in the prepolymer.
In Table 2 that the tensile strength and the alcohol resistance of the coating gradually increase with the increase of crosslinking, meanwhile, the content of the NCO group decreases with the increase of the crosslinking. And the silicone rubber surface groups to reduce the probability of reaction, resulting in resistance to hand sweat after coating adhesion decreased.

Conclusion
The molar ratio of -H: -NCO was 1.9, the reaction temperature was 60℃, the reaction time was 4h, and the cross-linking was achieved with TMP to obtain the closed polyurethane solution. The decarburization temperature of the prepolymer was tested to be about 116℃. The tensile strength and the alcohol resistance of the coating gradually increased with the increase of the crosslinking, and the adhesion decreased after the sweat resistance test.

If you want to know more information of this product, pls feel free to contact our engineer (Email: system@corefrp.com), we will reply you within 24 hours.

Silicone rubber surface coating compounded with blocking polyurethane.pdf

Nickel-chromium heating sheet is commonly used in a heating element of Aviation equipment, instrument, it is also named heating zone, the larger product called electric heating blanket. Its role is to rely on power through the heating sheet itself after the heat generated by the temperature control system to control the instrument’s operating temperature, because its small size, light weight, long using life, good process, safe and reliable, easy to use, Which is widely used in aviation and aerospace industrial equipment, instrumentation, temperature control system. Due to using various types of nickel-chromium heating sheet, to ensure that equipment, instruments to work in requested temperature for long time.

In general, there are two processing methods to manufacture the nickel-chromium heating sheet. namely molybdenum wire cutting method, and lithography etching method. The former process is simple, easy to process, suitable for stereotypes of large-scale production of products, the latter which cartographic flexibility, it can be changed while using, and it is suitable for small batch development. Our processing of nickel-chromium heating sheet, which is produced by the two methods.

The performance and features of nickel-chromium heating sheet
It can be designed according to user requirements, also can be designed into a variety of shapes and sizes, and can be flat or bent wrapped in the instrument.
Using silicone coated glass cloth as the cover, temperature performance is good, can be in -50 ~ 200℃ temperature range of long-term normal work.
Insulation performance is good. It does not occur flashover breakdown phenomenon under the voltage of 500 volts, the power of 0.5 KVA, anti-electricity 1 minute.
Its lead wire connected firmly, at least to withstand 0.2kg of tension. The insulation resistance between the lead line and the end ≥ 200MΩ (checked by 500V megger).
Resistance values can range from a few ohms to several hundred ohms. Through the DC or AC power can work long-term.
Anti-wet performance is good. The bonding strength between Silicone glass fiber cloth and nickel-chromium alloy sheet is high, will not fall off.
Long service life, which can work continuously for 1000 hours. With the GPS-4 or GD402 adhesive bonding, it bonded firmly with aluminum, copper, steel and other metal, and it is not corrosive to metal.

The structure and materials of nickel chrome heating sheet
It is composed of a resistive material – nickel chrome alloy sheet, lead wire, substrate and cap layer and other components. Making the nickel chrome heating sheet, according to different working temperature, conditions of use and requirements, the appropriate choice of various types of raw materials.
Resistance material. Ni-Cr alloy sheet is an important part of the heating film, according to the use of requirements, should be used high resistivity, corrosion resistance of the precision resistance alloy sheet.
Leads. From the end of nickel-chromium alloy sheet leads to the two fine insulated wire, using high temperature resistance of PTFE insulated installation line, model AF-250 (110 volts), the nominal cross-section of 0.013 mm2.
Substrate and cap layer. The substrate is used to maintain the geometry and relative position of the nichrome sheet, the cap layer protects both the shape and the relative position of the nichrome sheet and the nichrome sheet.

The substrate and cap layer are different with respect to the function of the heating sheet. In fact, the material used for substrate and cap layer is the same. In order to meet the production requirement of the curved heating sheet, besides selecting thinner nickel alloy sheet, the substrate and cover materials should also be also selected. Thus, it will not only meet the needs of the heat resistance and electrical insulation, but also have a certain deflection performance. Therefore, taking the silicone coated glass fiber cloth as the substrate and cap layer materials, the thickness of 0.18 ~ 0.20mm. Since the material is not vulcanized, so that taking the sticky performance of silicone rubber which coated on the glass fiber cloth, fix the shape and the position of a nickel-chromium alloy sheet, there will be strong bonding force between the silicone coated glass fiber cloth and nickel-chromium alloy sheet after heating vulcanization .

Production of Ni – Cr heating sheet by lithography etching method
Lithography etching method, through photochemical, corrosion and other means to print the desired shape of the line on the Ni – Cr heating sheet. The steps of the surface processing technology are as follows:
Graphic design→surface treatment of alloy→copy graphics→solid film processing→paste silicone glass fiber cloth→chemical corrosion→cleaning→measurement of welding wire and resistance→cover glue.
Graphic design. The thickness, the shape and the voltage of the required nickel chrome heating sheet in the instrument are used as the basis to calculate the required resistance value to determine the density, the direction and the length and width of the line on the heating sheet and draw 2: 1 ~ 5: 1 enlarged drawings of line, keep the negative film after photographing.
Pre-treatment of plate and nichrome sheet. Pre-treat the nickel-chromium alloy plates, firstly, the nickel-chromium film according to film size, the appropriate amplification on each side, cut with a cutter. And then using the magnesium oxide powder to remove oil on the surface, and placed in ferric chloride solution for weak corrosion treatment, so that the surface roughness, to improve its surface adhesion.

Copy graphics
The preparation and coating of photoresist, we use polyvinyl alcohol (PVA) as a photoresist material, the formula as below:
Polyvinyl alcohol 100 g
Water 1000ML
Ammonium dichromate 12 g
Preparation Methods: Firstly, PVA mixed with water, and then placed in a water bath to about 90℃, so that the PVA dissolved fully, remove and cool to room temperature, after the aqueous ammonium dichromate solution was poured, the mixture was sufficiently stirred. In order to remove the photoresist in the impurities and air bubbles, should also through net light filtering. Generally with three or four gauze filter for twice, then with the centrifuge to spin coating on the sensitive adhesive. And placed near to the oven until the glue is not sticky.

Notes for coating:
(A) The thickness of the photoresist has a great influence on the development and coloring of the subsequent exposure. Too thin to take off easily, and no protective effect, too thick will be insufficient, and lines blurred easily.
(B) The dryness of the photoresist also has a certain impact on the quality. If it’s not dry completely, the film will tend to stick to the film in the exposure, so the film damages, on the other side, it will make the Ni-chrome graphics incomplete, and for long time baking, it will cause later in the coloring and cleaning Process, it will difficult to show a complete graphics on the nickel-chromium alloy sheet.
(C) Coat sensitive glue on the Nickel-chromium alloy sheet, after baking, it should be placed in the dark place to avoid light, so that the development of lines blurred., should be placed in the dark to avoid light, so that the development of lines blurred.

Exposure. Put the sensitive glue coated nickel-chromium alloy sheet on the baseplate, which close to emulsion side of negative film (Attention to prevent light scattering, so as not to cause blurred lines), then put it under the cold light. The lamp power of 250 ~ 300 watts, the light source from the film is about 20 cm, time is 3 minutes, in addition, the sun light be used in hot days, time is 3 to 5 minutes, thus forming the required graphics of the “latent image.”

Solid film. The exposed nickel-chromium alloy sheet was washed with warm water and colored in 0.5% eosin water solution immediately. Rinse with warm water, remove the photosensitive part of the photosensitive glue from the metal sheet, when the line is clear, immerse it in the fixing solution immediately.
The fixative formulation is as follows:
Chromic anhydride 5 g
Water 100ML

Take the the nickel-chromium alloy sheet out after 20 seconds. After rinsed with water thoroughly, dry the surface with a hair dryer, and then put it into the electric oven, bake it under the temperature of 140℃. The baked nickel-chromium sheet, the color of line color was brown, and the film is very strong. Remove the extra part of the line on the nickel-chromium sheet, the incomplete part is filled with white nitrocellulose.

Paste silicone glass fiber cloth. We take the nickel-Chromium heating sheet as the base plate, which the glass cloth is coated with vinyl silicone rubber compound without vulcanization of semi-finished products.

The processing of silicone coated glass fiber fabric
(A) Dewaxing: Silicone coated glass fiber cloth thickness can be selected according to the request of product. We use the glass cloth of 0.06mm thick twill E-glass cloth, treated under 250 ℃ temperature for about 10 to 15 minutes, until smoke up. Thus, the wax on the glass cloth is volatiled completely.
(B) Chemical treatment: the dewaxing glass cloth immersed in the treatment solution for 3 to 5 minutes, dry it after removing.
Treatment solution formula:
Vinyltriethoxysilane 4 g
Industrial alcohol 50 g
Water 50ML
(C) Coating: After dewaxing and chemical treatment, glass cloth can be well integrated with silicone rubber, after coating the two sides by the coating machine, the total thickness is 0.19 ~ 0.20mm.
Formula:
Vinyl silicone rubber 100 units
Diphenylsilanediol 3 units
Fumed white carbon black 30 units
2,4-dichlorobenzoyl peroxide (DCBP) 4 units

The surface treatment of nickel-chromium sheet. Place the sheet for 5 minutes after cleaning the bottom of the nickel-chrome sheet with acetone, then coat 2 layers chemical tackifier evenly. The formula of tackifier:
Vinyl triethoxy silane (referred to as A-151) 10 g
Boric acid 0.05 g
Dichlorobenzoyl peroxide (DCBP) 0.5 g

A-151 PH value of which is adjusted to 2 ~ 3 with hydrochloric acid, and the general PH value of A-151 is 5 ~ 6, effect of tackifying is poor. The nickel-chromium plate coated with the tackifier was allowed to stand at room temperature for 30 minutes and then placed in an electric oven at 60℃for 1 hour. Take treated nickel-chromium sheet out, the sheet and the silicone coated glass fiber cloth fit together tightly, And caught between two layers of Teflon film, and then taking the bovine bone scraper to remove bubbles in the sandwich.

The bonding of nickel-chromium sheet and silicone: carry the one side of nicker-chromium sheet with silicone coated glass cloth into the 135℃electric oven, blast insulation for 10 minutes, so that nickel-chromium and silicone coated glass fiber cloth firmly bonded together.

Chemical corrosion. The nickel-chromium heating plate after washing with water, FeCl3 solution used for a weak corrosion. Then FeCl3 Mixed with water to adjust to the Baume’s reading on the table for the 36 ~ 38, temperature control in the 60 ~ 70 ℃, through the compressed air for chemical corrosion. Time is about 10 minutes, remove any unnecessary parts. Be noted that, besides the fluidity of the solution, the cleanliness of the surface of the nickel-chromium sheet have a great impact on the corrosion rate and quality .

Cleaning. Wipe off one layer yellow film of the nickel-chromium line with banana oil, or concentration of 60% sulfuric acid water solutions, and immersed for 30 seconds at temperature of 70℃ to remove the surface yellow film and then rinsed with water.

The resistance measurement of lead wire. In the beginning of nickel-chromium alloy sheet welding lead wire (design the length of wire as requested). The resistance value of heater sheet is determined by Wheatstone bridge. If the resistance value is greater than the index value, it will be removed, and the qualified one will be glued.

Cover glue. For the qualified nickel-chromium heating sheet, covered with a layer of the same size silicone coated glass fiber cloth on the side of wire, and then . After pressing, and then taking the bovine bone scraper to remove bubbles from the sandwich, and then into the electric oven temperature of 135℃, the blast insulation for 10 minutes, and then take it out.

Take the GPS-4 adhesive to pate the nickel-chromium heating sheet on the metal shell. Before the adhesion, cleaning the adherend (shell) surface with solvent, then coat thin layer surface treatment of NO1〜NO8. The mixture was placed at room temperature for 1 hour and then baked at 80℃ for 10 minutes. After being dried, coat the glue on the surface of the treated metal and non-metal surface (shell), the thickness of coating is about 0.2 ~ 0.3 mm, paste after a few minutes. And it can be parked at room temperature for 3 days to get enough adhesive strength. If it is allowed to stand at room temperature for one day, the vulcanization is carried out at a temperature of 80℃to 90℃ for 4 hours, so that the adhesion strength can be improved.

The experimental situation: We produced 22 heating plate totally, the maximum is 66.90Ω and the minimum is 49.1Ω, the difference is more than ten ohms, of course, the majority of nickel-chromium alloy sheet can adjust the resistance value, but the production efficiency is low.

Process of manufacturing nickel – chromium heater by molybdenum wire cutting method
The design of the heating sheet, the selection of material as the above 4.1, the image should be 1 : 1.

The production of the heating sheet.
(1) The processing of nickel-chrome alloy sheet. According to line shape and size, using wire cutting technology to cut out the required nickel-chromium sheet. This batch of 31 pieces of a cutting, the resistance is about 36.9Ω (design resistance of 36±1.5Ω).
(2) Welding wire. In the beginning and end of nickel-chromium alloy sheet, welded PTFE insulated installation line.
(3) Cleaning. Clean the lead with the nickel-chromium alloy tablets with ethanol, gasoline, acetone and other solvent, in order to improve the dry bonding strength. Special attention should be paid to the solder at the solder joint, otherwise the acidic substance of the residual flux will corrode the wire.
(4) Coat surface treatment agent. To increase bonding effect between the nickel-chromium alloy sheet and silicone coated glass fiber cloth. The treatment agent coating on the nickel-chromium alloy sheet placed 40 to 60 minutes, with vulcanization.
(5) Put the nickel-chromium alloy sheet on the substrate, cover layer, remove the bubbles between substrate and cap layer.
(6) The pasted heating sheet on the electric oven at 135℃within 10 minutes of heat preservation blast, remove it, and to be requested silicone coated glass cloth heating sheet. Because decomposition of DBP is 2.4 dichlorobenzoic acid and 2.4 dichlorobenzene, relatively less volatile, so the pressure is not vulcanized to avoid bubbles.

With the further development of space technology, the application range of nickel chrome heating sheet is expanding. Besides of the heating element used in aviation, the large nickel-chromium heating sheet is also used in technology equipment. Practice has proved that the use of wire cutting technology to produce nickel-chromium-clad heating sheet is a high-efficiency, good quality method. While the use of lithography etching method manufacturing heating sheet, although there are some shortcomings, such as low pass rate of quality instability and so on. But taking a series of remedial measures, the quality of products can be improved a lot. In general, in the processing of nickel-chromium heating sheet, taking the method of using lithography etching for small batch production as possible. Once the product is finalized, should use molybdenum wire cutting method to the mass production.

In addition, because the welding wire is still used brazing method commonly , so the thickness of the solder joint is still large. The flat fluorine plastic wire should be used in the future, and to replace the current circular wire, thereby reducing the thickness of the solder joints, and improving the bonding strength. In the future, in order to use thinner heating sheet and the use of a wider range of heating temperature range, the polyimide film as the substrate and cap layer can be considered, through the flexible accelerometer silicone coated glass fiber cloth wrapped nickel chrome heating sheet, and open up various types of temperature control heating sheet manufacturing methods.

If you want to know more information of this product, pls feel free to contact our engineer (Email: system@corefrp.com), we will reply you within 24 hours.

The production of silicone coated glass fiber fabric covered nickel-chromium heating sheet.pdf