EP1891253B1

EP1891253B1 - Natural or synthetic yarns with heat transmission barrier property obtained by aerogel deposition

Info

Publication number: EP1891253B1
Application number: EP06745363A
Authority: EP
Inventors: Alessandro Sannino; Alfonso Maffezzoli; Antonio Licciulli; Mauro Pollini
Original assignee: Megatex SpA
Current assignee: Megatex SpA
Priority date: 2005-05-24
Filing date: 2006-05-22
Publication date: 2010-08-11
Anticipated expiration: 2026-05-22
Also published as: ATE477358T1; ITLE20050008A1; DE602006016115D1; EP1891253A1; WO2006126232A1

Abstract

Production of natural or synthetic yarns with heat transmission barrier property, obtained by deposition of non-hypercritical synthesized aerogel. Characterized by high porosity, the aerogel confers to the yarn thermal insulation properties. This enables the production of fabrics which can be utilized, as an example, under hot and cold extreme conditions. The invention is characterized by a new technique of aerogel non-hypercritical synthesis and by a process of aerogel deposition by impregnation and then yarn exposition to the UV-rays.

Description

Technical field

The present invention relates to natural or synthetic yarns obtained by aerogel deposition.
In the field of advanced ceramic material, aerogel is among the most studied materials by researchers. Aerogel is usually called "Frozen smoke", due to its appearance, which is, at a glance, very similar to frozen smoke. A siliceous aerogel is normally composed of at least 90% of air and the residual is Silicon Dioxide (SiO₂). In some cases its composition reaches 99.9% of air and only 0.1% of SiO₂. Among the known solid materials, aerogel has the lowest density. Its remarkable features are the very low thermal conductivity, a good acoustic and electric insulation and the fire-retardant property. It is very interesting the fact that its preparation, by means of modern techniques, does not generate toxic wastes; aerogel disposal is also simple, by compression until a powder like sand is obtained.
There are many application of the aerogel as thermal insulator. Due to its high costs, however aerogel has been utilized only for scientific purposes. For example, in 1997, Mars Rover W.E.B. during the Pathfinder mission to Mars used aerogel as Rover thermal insulator, allowing the electronic components inside the Warm Electronics Box to resist against the low temperature, which is normally -100°C during the Martian nights. Aerogel is also used as catalyst or absorbent to remove or recover pollutants from water or air. Examples are: catalyst for production of catalytic mufflers, emission control in production plants, filters and anti-gas masks production for civil and military applications. Further applications of aerogel are thermal insulation of windows glasses; in fact, an aerogel panel has the same insulating capacity of 32 glass sheets; its installation reduces energy consumption (considering that 30% of home energy loss is due to a deficient insulation) and atmospheric pollution as well. In most cases aerogel production is made by drying a hydrogel under hypercritical conditions. The first step of the production process is made of numerous washings of the gel with acetone; the first washings take about fifteen minutes, the remaining one or more hours. The gel is solvent exchanged to replace the water and alcohol retained in the gel pores with suitable organic solvent like Acetone, to avoid the presence of water during the next step when the gel is dried by supercritical drying using carbon-dioxide (CO₂). Once the autoclave is switched on, the cooling system and the compressor are activated. When the temperature reaches about 10°C, liquid CO₂ is flushed through the vessel at a pressure of about 50 bar. After about 5 minutes, the vessel is emptied and then replenished with fresh CO₂. Many (up to 10) washing cycles are performed: the first three take few minutes, the second three 20 minutes, the last cycles 60 minutes. After the washing process, the vessel is filled once more with CO₂ and heated at a temperature that reaches 40°C, with 100 bar pressure. Under these conditions, CO₂ is a supercritical fluid. The subsequent phase is the vessel depressurisation, by a fluid slow release. It takes about 5 hours; after which the hypercritical aerogel sample is ready and is extracted from the autoclave. The gel drying under hypercritical conditions has many drawbacks due to high temperature and pressure, long process duration, high costs, and the use of very complex tools and processes. This leads to an alternative production technique, which must be cheaper, safer and faster.
US 6,378,229 B1 discloses a method for the subcritical drying of lyogels to produce aerogel particles.
At the present, special fabrics based on hypercritical aerogel are available on the market, by ASPEN AEROGELS INC. They are utilized to produce Antarctic, space, marine, military or fire-protection clothing. US 2004/0142168 B1 discloses fabrics which are impregnated with silica aerogel particles to impart thermal and fire protection. The aerogel particles fill voids in the fibers and/or between the fibers. On the other side, the non-hypercritical synthesized aerogel has not yet been introduced in the textile market; therefore, under extreme climatic conditions the most used materials are the modem synthetic and non synthetic fibers, which can assure thermal insulation without compromising comfort. The drawback is that the known materials cannot be used for a long period of time.
Therefore, the technical problem is to produce yarns, fabrics and end products (socks or stockings, as an example) having thermal barrier properties and characterized by high comfort under all extreme climatic conditions, both very cold (e.g. mountain climbing) and very hot (e.g. fire-protection application), and having a long lifetime.

Disclosure of the invention

The deposition of aerogel on yarns provides it with thermal insulation properties due to the nature and morphology of the aerogel itself. In fact, this material is characterized by high porosity and a very low thermal coefficient. The aerogel is synthesized by a technique, based on the use of non hypercritical fluids.
By using organic-inorganic hybrid systems, a gel (immersed) in an alcohol solution has been obtained by photo-polymerization. Such gel has been dried under atmospheric pressure and then synthesized; as a result, a micro-porous structure has been obtained, having a porosity equal to 90%.
These and other advantages will be pointed out in the detailed description of the invention that will refer to the figures of the tables 1/8 - 818. Both are examplifying and not restrictive.

Wary of carrying out the invention

With reference to the above mentioned tables:

Figure 1 is the TEOS chemical formula;
Figure 2 is the flowchart to obtain the gel, as aerogel precursor;
Figure 3 (Table 1) shows the molar ratios and the quantities (grams) needed to produce 100g of SiO₂ sol;
Figure 4 is the TMSPMA chemical formula;
Figure 5 (Table 2) shows the aerogel density values, before and after stabilization;
Figure 6 (Table 3) shows the thermal conductivity values of three aerogel samples;
Figure 7 is the regression to evaluate the aerogel thermal conductivity;
Figure 8 shows the thermal conductivity of the cotton and the aerogel additivated cotton.

The aerogel is synthesized by a technique, based on the use of non-hypercritical fluids. By using organic-inorganic hybrid systems, a gel immersed in alcoholic solution has been obtained by photo-polymerisation. Such gel have been dried under atmospheric pressure and then synthesized forming a micro-porous structure with a porosity of 90%.
According to the protocol, silica aerogel is synthesized. It contains large sterical organic molecules, the 3-(trimethoxysilyl)propylmethacrylate (TMSPMA), which, during drying, are the load bearing structure, that prevents the gel to shrink and break. As a precursor, the tetra ethyl ortho-silicate (TEOS) 98% by ALDRICH is the chosen alcohol-oxide. In Fig. 1, the TEOS chemical formula is shown. The used alcohol-oxide and organic concentration has been calculated to obtain a SiO₂ final yield equal to 4%, by assuming, for both TEOS and organic, all precursor moles to be converted in SiO₂. The solution contains the same TEOS and TMSPMA moles number. As a catalyst, 37% chloride acid has been used; while as a solvent, ethanol has been added in the solution. Fig. 2 shows the flowchart to obtain the gel, as aerogel precursor. Fig. 3 (Table 1) shows the molar ratios and the quantities (grams) needed to produce 100g of SiO₂ sol.
Finally, Fig. 4 shows the TMSPMA chemical formula.
The solutions have been prepared at ambient temperature and homogenized by mechanical mixing: in the first one (soul 1), 1/3 ethanol quantity with TEOS has been added; in the second one (sol 2), 1/3 ethanol quantity, HCl and H₂O have been added; in the third one (sol 3), the TMSPMA and the remaining 1/3 ethanol have been added. All beakers have been shaken by a magnetic shaker for 20 minutes. Then, sol 1 and sol 2 have been mixed (obtaining sol 4); in these phases, hydrolysis occurs together with heat release and consequent beakers warming-up. Sol 4 is shaken for 1.5-2 hours. Soon after, sol 3 has been added to sol 4 and the obtained solution has been shaken at least 2 hours. After hydrolysis, pH is equal to 2.5; by adding a water-ammonia solution (water to ammonia ratio: 8 to 2), the sol turns to a more basic pH (pH=6÷8). Further, a 1-Hydroxi-cycle-esil-fenil-cheton photo-initiator equal to 4% in weight of the available TMSPMA is added. The 1-Hydroxi-cycle-esil-fenil-cheton is available as powder or grains; therefore, before being added to the solution, it has been finely powdered, to improve its solubility in alcohol. Once in the solution, it has been mixed for 10 minutes by ultrasounds. Then, a smoked silica quantity, equal to 2 times the silica weight efficiency in the starting solution, is added; the solution is also exposed to ultrasounds until a full homogenization is obtained. Later, the final product is exposed to the UV-rays for about 2 minutes, after that it appears white coloured and without any shrinkage. The product has been dried into a furnace at 60°C. After drying, the sample is stabilized in the furnace under atmospheric pressure and temperature of 550°C, to eliminate all organic compounds. In Fig. 5 (Table 2), the aerogel density values, before and after stabilization, are shown. After the treatment at 550°C, only a slight density increase can be observed.
In summary, such process allows to obtain a silica aerogel characterized by high porosity and low thermal conductivity, therefore very suitable as a thermal barrier, which can be integrated in the fabrics, as further described below.
The fiber impregnation process with silica aerogel, obtained in a sub-critical way comprised 4 steps:

1. Silica sol preparation
2. Fiber impregnation in the sol
3. Fiber exposition to the UV-rays
4. Fiber drying

Sintering treatment is not performed, to avoid at 550°C the fiber pyrolysis. The fiber exposition to the UV-rays allows aerogel photo-polymerization directly on the yarn. Washing tests on the yarn or on the end product confirm the good adherence between the silica and the yam.
An alternative way of aerogel deposition is to start with a bulk aerogel and later, after powdering it, to connect the aerogel with the fiber surface, by means of a polymeric binding. The process to produce the yarn is the following:

1. Silica sol preparation
2. Sol exposition to UV-Rays
3. drying until a bulk is obtained
4. Sintering treatment
5. Bulk powdering
6. Powder deposition on the yarn or end product, by binding utilization

In this way a fabric having heat transfer barrier feature is obtained, using a yarn with a powder surface coating, which is bonded by means of a polymeric binding.
A further aerogel deposition technique is based on the short fiber impregnation in the sol, then its exposition to the UV-rays and finally the spinning.
One of the most important aerogel property is their low thermal conductivity, which becomes even lower in vacuum. Inside an insulating material the heat transmission takes place by three different mechanisms:

solid state conduction
gaseous state conduction (including convection)
radiant transmission (infrared radiance)

The addition of these three components gives the material thermal conductivity, which is quite high in dense silicates. Instead, the aerogel contains a very low fraction of solid silica (about 5%, in average): in this case, the solid is connected like a net having some death branches, which further decrease the thermal conductivity. The hollow space inside the material are generally filled with air or other gases, which allow the heat propagation, while the open aerogel pores allow the gas flowing through the material, even with difficulties. The other heat transmission component is the infrared radiance. Aerogel is transparent to the visible electromagnetic radiation and to infrared one, mostly to the wave lengths between 3 and 5 micron. At low temperature, the radiant component of the thermal transportation is low and therefore negligible, while at high temperatures the radiance contribution is not negligible. To minimize the heat transmission, such components have been controlled and an excellent material has been obtained, which can be integrated in the fabrics, to assure very high comfort in case of very high or very low temperatures. More precisely, a thermal conductivity calculation for three aerogel samples with different thicknesses has been carried out. Then, the thermal conductivity law has been interpolated. Table 3 shows the results of the measurements. The thermal conductivity behaviour is shown in Fig. 7. After the interpolation, the thermal conductivity value is equal to 0.0298 [W/mK] ± σ = 0.00368 [W/mK] (standard deviation). Also the thermal conductivities of the cotton and the aerogel additivated cotton have been evaluated and the results are shown in Fig. 8. the aerogel reduces the thermal conductivity of about 1/3 compared with that of the cotton without aerogel.
Both process and product have many advantages. Concerning the process, a non hypercritical technique to obtain integral and light silica aerogel samples has been developed, avoiding the gel drying in autoclave and, consequently, reducing both costs and preparation duration. Further, the risk of working with high pressure and temperature, and with CO₂ and ethanol (completely absent) is reduced. The simple and economic aerogel preparation allows the use of such advanced material in the textile sector; as an example, some activities practised under extreme climate conditions, need specific yarns. Simple natural yarns (cotton, wool) cannot assure a prolonged comfort. By aerogel deposition on yarns, fabrics (starting from both natural or polymeric yarns) or end products (like socks or stockings), new advanced technical products have been realized, such products being characterized by a low thermal conductivity, which are perfect under very cold or very hot conditions.

Claims

Fiber impregnation process with silica aerogel characterized in that it is obtained in a subcritical way by fiber impregnation in the sol and comprises the following steps:
• Silica sol preparation

• Fiber impregnation in the sol

• Fiber exposition to the UV-rays

• Fiber drying
Fiber deposition process with silica aerogel characterized in that it is obtained in a subcritical way by sintering treatment and comprises the following steps:
• Silica sol preparation

• Sol exposition to UV -Rays

• Drying until a bulk is obtained

• Sintering treatment

• Bulk powdering

• Powder deposition on the end product, by binding utilization
Process according to claim 1 or 2, wherein said silica aerogel comprises large sterical organic molecules as the 3-(thrimetoxysilyl) propylmethacryle.
Process according to one of the previous claims wherein said UV-rays have the following ranges: wave length 285- 400nm, power 20÷ 10000 W/m² , exposure time 5 seconds-30 minutes.
Process according to one of the previous claims wherein said UV-rays have the following ranges: wave length 360- 370nm, power 500 W/m², exposure time 1-2 minutes.
Fabric, yarn, sock or stocking obtained by the process as in one of the previous claims.