Article from composite material controlled by temperature and humidity and method of its production

FIELD: process engineering.

SUBSTANCE: invention relates to method of controlling humidity absorption in article mounted in aircraft. Proposed article comprises multiple layers of material from resin matrix reinforced fibrous material to be hardened by pressure and heat. Heating electric resistor and temperature metre connected with control means are arranged between said layers.

EFFECT: control over article operation.

13 cl, 10 dwg, 2 tbl, 1 ex

This invention relates to a product made of composite material according to the preamble of paragraph 1 of the claims.

As it is known that polymeric materials are macromolecular structure, i.e. composed of long polymer chains that have a relative movement, which varies depending on the structure, but in all cases allows access of the molecules with a lower molecular weight, which penetrate between macromolecules, thus forming a real solution.

Of course, the amount of these substances depends on the molecular nature of the polymer and substances with a low molecular weight. The interaction may be chemical and/or physical.

Chemical interaction for some of the more chemically aggressive substances such as acids and/or some organic solvents, may cause the modification of the polymer and sometimes also the dissolution of the polymer. Physical interaction instead largely linked to the reversible mixing. It creates between the polymer molecules and substances with a low molecular weight solution with physical characteristics different from the characteristics of the pure polymer. As for substances with a low molecular weight, they usually increase the relative mobility between macromolecules, causing, as a rule, the reduction in t is mperature glass transition (T _article); from the mechanical point of view is usually reduced within the yield strength σ_y(yield strength tensile) and τ_y(yield strength shear) and, as a rule, the modulus of elasticity E (modulus of tensile elasticity, or young's modulus) and the modulus of elasticity shear (G). All of these effects, taken as a whole, is generally defined as "the effects of plasticization".

Plasticization depends on the nature of the polymer, and the nature and quantity of substances with a low molecular weight.

Among the substances that cause plasticization, including organic solvents such as MEK, methyl alcohol, ethyl alcohol, hexane, acetone)and water. When the polymer is immersed in a plasticizing liquid, it tends to absorb such liquid, and absorbs some speed-dependent diffusion coefficient of the plasticizer in the polymer. When it reaches a state of equilibrium, so that there is no further absorption of the plasticizer in the polymer (in fact, at the molecular level, the number of incoming molecules equivalent to the number of molecules leaving), States that achieved the content of saturated plasticizer, which depends on the chemical nature of the polymer, and plasticizer, and may vary with temperature.

When the polymer is immersed in the environment in which partially present plasticizer, the amount of saturation depends on the percentage of plasticizer present in the environment; more specifically, in terms of thermodynamics, we are talking about the activity of the plasticizer. In the case of gas mixtures, the activity depends on partial pressure: if x is the volume fraction of the plasticizer, the partial pressure is equal to x*π, where π is the total pressure of the mixture. When the water is dispersed in the air in a gaseous state and established a balance between gaseous water and liquid water, gaseous activity of water is equivalent to the activity of liquid water. In this case, the environment is defined as water-saturated, the relative humidity is 100%, and the partial pressure of water in the gaseous state is equivalent to the vapor pressure of water at the same temperature.

In the case of polymers exposed to the environment containing some water, approximate linear law relates the relative humidity and the percentage of absorbed water at equilibrium.

On the contrary, with respect to the change amount of absorbed water as a function of temperature, typically the temperature dependence is not very strong; for epoxy resins used as the matrix of the composite for aircraft structural applications, the contents of absorbi vannoy resin water at saturation in liquid water (or, equivalent, in air with 100% relative humidity) changes in the composition of the resin is from 1 to 3% and is almost constant for the same resin in the range from 25°C to 80°C.

Instead, the time required to achieve saturation in different environments, is controlled by the diffusion of water in the polymer and, therefore, depends on the diffusion coefficient, which depends exponentially on temperature. The integration of the law of diffusion can find quadratic correlation between the time of saturation and thickness of the details.

On the basis of the previous considerations we can conclude that polymer materials, including, for example, matrix, polymer matrix composites, over time, undergo absorption of water from the surrounding atmosphere, the nature of which depends on external conditions. Due to the extreme variation of external conditions for the design needs of the precautions to be considered the most unfavorable external conditions, which for use in aviation were adopted on 28^°C and relative humidity of 85% for the entire service life of the aircraft (usually 30 years). Therefore, for most composite structures for certification should be considered saturation at 85%.

From the point of view of the temperatures usually minimum temperature (maximum to aserca height) is 55^°C, maximum temperature (on earth, the maximum exposure to sunlight) is 80^°C.

On the basis of what is referred to plasticization, high temperature effect acts in the same direction as the absorption of water; therefore, certification of material and structure is carried out by evaluation of the material at high temperature and after absorption of water ("hot wet" conditions) and at low temperature, usually without absorption of water ("cold dry" conditions).

The requirement to consider these conditions in programs aviation certification, which is already very demanding in terms of mechanical tests at ambient temperature (in any case they refer to the experimental samples, parts, components, sub-components and full-scale components), is very expensive in terms of additional testing activity (also due to the exposure to the effects of the test samples) and time. In fact, because of the foregoing, the absorption is very slow, and modeling of absorption within a period of 30 years at room temperature requires several months, and also when the use conditions of accelerated aging (at high temperature).

Therefore, the aim of the present invention is to offer a product consisting of a composite material which, having the ability to overcome the above difficulties, caused by the influence of humidity on polymeric materials.

Thus, the subject invention is a product comprising a composite material having the characteristics described in claim 1 of the claims.

Due to the fact that the product of composite material integrated heating and temperature measurement, it is possible to manage permanent and definitive way the working conditions of the product. The resulting benefits depend on the choice of dimensions of the structures, without taking into account the deterioration of material properties caused by high concentration of absorbed water and low temperature. This, in particular, means:

permission to use a wider resolved design standards that do not take into account the deterioration caused by humidity and ultimately low temperature, therefore, making a lighter structure;

permission to certify patterns without conducting wet tests at the level of prototypes, components, subcomponents and components.

The preferred way of carrying out the invention are defined in dependent claims.

The following objects of the invention are methods of use of the product according to the invention, with p snake, described in the claims with claim 9 in item 11, respectively, and a computer program product, loadable in the memory of a computer and comprising part of the system software for implementing the method, when the program product runs on a computer, and a system to control working conditions in the product according to the invention, having the characteristics defined in the claims from 13 to 15, respectively.

Next, some preferred, but not restrictive embodiment of the invention described with reference to the accompanying drawings, where:

figure 1 is a schematic representation of a top view on the panel of composite material according to the invention, showing some characteristics of the panel.

figure 2 is a schematic representation of the cross section of the panel in figure 1 in accordance with II-II;

figure 3 is an additional schematic representation of a top view on the panel of figure 1, showing other features of the panel; and

figure 4 is a schematic representation of a cross section of the panel 3 in accordance with IV-IV;

figure 5 is a schematic representation of a top view of the installation tool for use in the processes of layering panels of figure 1;

6 is an enlarged view of a portion of the tool in figure 5, indicated by the arrow VI of figure 5;/p>

7 is a schematic sectional view of part 6 according to VII-VII; and

Fig-10 are graphs representing the curves of absorption of moisture in the panels of composite material according to the invention.

The drawings show the product 1, consisting of a composite material according to the invention, in particular, the panel proposed to be installed on the aircraft. The panel 1 includes a known per se manner, multiple layers of material 10, cured by pressure and heat, in which each layer of material 10 is formed of a resin matrix reinforced with fibrous material. The polymer matrix may be a thermoplastic or thermosetting and reinforced fibers, in particular long fibers, for example, carbon, glass, or Kevlar. Between the layers of material 10 is defined areas of the boundary surfaces 11, 12 and 13.

According to the invention, the panel 1 includes a heating means 20 and the temperature measuring 30 embedded in a composite material, which are located, respectively, in at least one area of a surface of the partition 11, 13 between the layers 10 and is suitable in order to make it possible to control the temperature inside the panel 1 in the operation.

The heating means 20 is suitable to be the United States the mi when working with the tool 40 control to activate the heating means, to raise the temperature of the part above the level of the surrounding environment, thereby creating the effect of moisture loss or managing a minimum operating temperature of the material. Preferably the heating means 20 are local resistance, made of copper wires embedded in a composite material placed on the surface of the partition 11 in the middle of the composite product 1. In this case, the means 40 controls include the generator current or voltage.

An example of the placement of the resistance of a length of 20 m in the composite panel 1 m × 1 m is shown in the top view in figure 1. In this example, the resistance goes meanttobe way area of a surface of the partition 11 between the Central layer 10.

The power consumed by the electric resistance can be calculated on the basis of the following considerations.

Managed heating the product at a temperature above the ambient temperature may make it possible to effect drying. In fact, in conditions of equilibrium between the flow of plasticizer (water)leaving the product and included in the product at a temperature above the ambient temperature, the vapor pressure of water at the temperature of the polymer, T_p(i.e. the temperature of the matrix of the composite) is equivalent to the partial pressure of water p_wthat is the product of the vapor pressure of the odes at ambient temperature and relative humidity environment R.H.

Summary table 1 shows that slight heating causes a significant decrease of water content in the polymer at equilibrium conditions.

The coefficient of relative saturation, defined as the water content of/attributed to saturation/relative humidity environment), equal to:

the vapor pressure of water at ambient temperature)/pressure of water vapor at the temperature of the polymer).

For example, if the air temperature is 20°C coefficient of relative saturation, when the polymer is heated at 20°C (polymer at 40°C. environment at 20°C), 0.32, and for heating the polymer at 30°C. the polymer at 50°C environment at 20°C) is 0,19.

This means that if, for example, ambient relative humidity R.H. 85%, at equilibrium with the polymer heated at 20°C, the percentage of saturation of the polymer x_p0.32*0,85*100=27% of saturation, while the balance, when heating of the polymer is 30°C, the percentage saturation of x_pis 0,19*0,85*100=16% of saturation. For example, if the weight of the dry composite as a result of absorption of water in the environment with R.H. 100%, e.g. 2%, the equilibrium weight gain at 85% R.H. usually should be the Aven of 1.7%, but would have to be equal to 0.32%, if heating of the composite was 30°C.

Always under steady-state conditions, thermal power required to maintain the temperature difference between the polymer and the environment depends on the heat exchange with the environment by convection. So, if h is the convection exchange and S is the surface of exchange between the observed item and the environment, thermal power required to obtain the temperature difference ΔΤ is W=h×S×ΔΤ.

For the vertical bar in the air with heat transfer only by natural convection simplified dimensionless equation found in the literature (Perry - Chemical Engineers' Handbook - McGraw-Hill), which allows to calculate the coefficient of h for various values of the dimensionless numbers Grashof (Gr) and Prandtl (Pr):

h=b(ΔΤ)^m*L^3m-1

where the values of b and h are given for various conditions in the following table 2.

and the dimensions are:

h=(BTU)/HR*sq ft*°F

L - ft

ΔΤ - °F.

Based on these data when considering flat composite panels 1×1 m (thus, the 2 square meters open area) in a vertical position in the air that exchanges heat by natural convection, the necessary power to maintain the temperature difference ΔΤ between the panel and the external environment, which varies with different values of the dimensionless numbers for ΔΤ=20°C is 115-135 W, for ΔΤ=30°C is 192-231 W, for ΔΤ=40°C is 275-340 watts.

If the heat get electric resistance, it is governed by Ohm's law, and should be considered the following equations:

then:

and from equation (4) shows that the power of W for a given voltage ΔV linearly depends on the sectional area of the resistance S and is backward LINEST is th function of its length L.

When using copper resistor (electrical resistivity of copper ρ=0,0000000168 Ohm×m) calculation is shown for two cases (the minimum and maximum required power, 115 and 340 watts)above.

When considering copper resistor with a cross section S=0.025 mm²and length L=10 m generated power for voltage ΔV=8,8 IN is 115 watts, voltage 15,1 At 340 watts.

As reported above, the measurement means 30 is placed in the composite to measure the temperature by placing at different depths in thickness (preferably in the areas of the surfaces of the partition 13, close to the two outer surfaces of the panel 1 and the area of the surface section 11 of the Central zone, in the positions properly spaced from the surface. Preferably also insert measuring 50 to determine the moisture content, having them like temperature sensors 30. thermal sensors 30 are preferably thermocouples, while the moisture sensor 50 is based on the property change of the material of the sensor when humidity changes (usually material is a hygroscopic polymeric material that changes its index of diffraction).

Figure 3 and 4 shows an example layout of sensors, mainly used for the humidity sensor 50, and thermal sensors 30.

When the panel 1 is installed on Board lettering the apparatus (not shown), the sensors 30, 50 is functionally connected to a control unit 60, which receives the resulting measurement data and processes them by following a specific algorithm. The control unit 60 in turn is functionally connected to a generator voltage of 40 to control the heating panel 1 on the basis of measured data provided by the sensors 30 and 50.

The process of manufacturing a product according to the invention is essentially a process typical of the parts formed by the prepreg-based stacking prepregs with the geometry and orientation prescribed in the scheme of manufacture. For illustrative purposes, the following describes an example of manufacturing the panel, made from layers of prepreg with a thermosetting resin reinforced by long fibers. The first layer is placed directly on the snap, properly treated antiadhesion to prevent bonding of the composite to the snap. Then stack these layers, using their stickiness. After laying close bag (also using suitable auxiliary materials) and process in a loop with a defined temperature and pressure.

The only difference between the panel object according to the invention and the panel obtained in the usual way, is that in the template placement of resistances and/or sensors placed between the nth and (n+1)-m with the panorama layers, carried out after installation of the n-th layer and before the (n+1)-th layer. For accurate setting of the resistance and/or sensors in a specified position can be used with the installation tool 100, shown in figure 5-7, in which is formed a cavity 120 for resistance and sensors, responsible, respectively, the provisions that it is desirable to make the resistance and/or sensors in composite parts. The specified tool 100 set, turning it upside down and placing it on the layer on which you want to set the resistance and/or sensors, and then removed, leaving at the same time the layer to the desired resistance and/or sensors. For example, for resistance, shown in figure 1, the installation tool has a groove 120 with meanttobe geometry equivalent to the geometry of the resistance, and with a slightly wider cross-section than the cross section of the resistance. To prevent any problems demolding can be included several excavation needles 121, sliding in guides a crossing the groove 120.

Some embodiments of the method of use of products made of composite material according to the invention are described below.

In the first embodiment, the method provides that the product 1 made of composite material mounted on Board lette inogo apparatus therefore to have the heating means 20 and at least the temperature measuring 30 attached to the means 40, 50 control, placed on Board the aircraft. Alternatively, you can conceive that controls have been set on earth and to the heating means and temperature measurement joined the controls while parked aircraft.

Simple design and manufacture of the composite with the ability of self-heating is already enough to allow good control of humidity, when provided periodic cycles of heating by activating the heating means 20. In this case, the calculation of these cycles on the basis of knowledge of the law of diffusion of water and working environment, you can enjoy the benefits of the invention even without the use of humidity sensors. Thermal sensors in any case necessary in order to properly control the temperature rise in order to get the desired effect of reducing the humidity.

As an example, calculation was performed with the computer program, developed by the applicant using the algorithm based on the diffusion law Fika (one-dimensional case), the following:

Typically, the diffusion coefficient D varies with temperature according to the law of Arrhenius equation:

D=D₀*exp(-Ea/RT)

where T is the temperature in K, Ea is the activation energy, and R is the universal gas constant.

The following examples show the calculation of the percentage of water content as a function of time for the panel, subject to periodic cycles of absorption in the process, alternating with periods of heat, which causes desorption, with the result that the absorbed humidity is maintained below a predetermined threshold, even in the worst conditions.

Examples

Considered composite panel thickness th, consisting of a composite material, whose coefficient of diffusion of water is such that at 28°C, the material reaches the moisture content equal to 90% of the moisture content at saturation after 10 years, or the equivalent, it reaches the same content of moisture at 80°C after one month, starting in both cases with dry condition.

On the basis of such data can be calculated diffusion coefficient of water in the material for a specific case, but for a certain class of compounds, the level of the diffusion coefficient as a function of temperature can be obtained from the test absorbance at different the temperatures.

For the considered material and thickness of the analyzed working conditions are a constant ambient temperature T=28°C and relative humidity of 85% (this case is generally regarded as the worst for certification purposes), while ensuring periodic periods of heating panels with ΔΤ=30°C, i.e. at 58°C. the Heat causes the effect desorption: in fact, on the basis of the values shown in table 1, from the point of view of relative environmental humidity, relative humidity 85% appears in the heated panel relative humidity 85%*0,19=16%.

On Fig-10 shows the curves of moisture absorption, which were calculated for the panel in the above-mentioned conditions.

On Fig shows a typical increase of moisture in the panel with alternating periods of absorption/desorption during prolonged periods of desorption.

Fig.9 shows a graph relating to the periods of work in 1 month, alternating with periods of desorption at 12 o'clock

Figure 10 shows a graph relating to the periods of work in 1 month, alternating with periods of desorption 36 PM

One can observe that the duration of desorption also affects the asymptotic value of the maximum absorbed moisture, which is kept below a preset value only on the basis of theoretical and numerical forecast and when using the appropriate period of the s desorption without the use of moisture sensors.

As should be clear advantages of the invention consist in the possibility to set the dimensions of the structures, without taking into account the deterioration in material properties due to low temperature and high moisture absorption, which includes:

the ability to use a broader permitted design standards, which are not affected by the deterioration caused by absorbed moisture getting through this lighter structure;

the ability to certify patterns without testing in wet conditions on the level of a prototype, nor at the level of components and subcomponents.

The use of temperature sensors provides additional advantages according to the second variant of implementation of the present method, where the heat is used to raise the minimum operating temperature (typically for applications in aviation -55°C) by activating the heating at a low temperature. For this purpose a stage of heating by the activation of the heating means 20, when the working temperature of the product 1 measured by the temperature measuring 30 reaches a temperature below a preset minimum level. In the result, it is possible to design the item for a temperature interval with a higher minimum temperature, preventing the deterioration of certain properties, call the data low temperatures, using a wider resolved design standards and making it more light patterns. But in this case you may need a more powerful generation of heat, as possible the conditions of heat exchange with high forced convection (high cruising speed), and this generation should be required in the work. Instead, heating to effect the drying may be included in the service regulations on the earth during Parking of the aircraft.

The third variant of the method involves the use of humidity sensors 50 to enable the removal of moisture, when the water content exceeds the specified level.

1. The method of controlling the absorption of moisture in the product (1), installed on an aircraft, with the mentioned product is made from a composite material comprising multiple layers of material (10), cured by the application of pressure and heat, in which each layer of material consists of a resin matrix reinforced with fibrous material, characterized in that the method comprises the following steps:
- provision of the heating means (20) and means (30) temperature measurement, embedded in a composite material, which are placed respectively in at least one zone (11, 13) the interface between such layers of material;
- obespechenie means (40, 60) control, which are connected with said heating means and means for measuring temperature; and
programming mentioned controls to activate mentioned heating means to heat the said product according to periodic cycles of heating, these cycles of heating is calculated on the basis of the law of diffusion of moisture within the composite material and on the basis of the data about the temperature and relative humidity environment, and said data is defined with respect to working conditions of the aircraft.

2. The method according to claim 1, in which the product is made in the form of a panel and the heating means placed in the Central zone (11) the boundary surface of such panel.

3. The method according to claim 1, in which the product is made in the form of a panel and means for measuring temperature is placed in the Central zone (11) the boundary surface of such panel and zone (13) the boundary surface close to the outer surfaces of such panel.

4. The method according to claim 1, wherein the heating means comprise at least one electrical resistance.

5. The method according to claim 4, in which the electrical resistance includes a metal wire passing meanttobe, by zone (11) the boundary surface between the layers of material.

6. How to control absorb what their moisture in the product (1), installed on an aircraft, with the mentioned product is made from a composite material comprising multiple layers of material (10), cured by the application of pressure and heat, in which each layer of material consists of a resin matrix reinforced with fibrous material, characterized in that the method comprises the following steps:
- provision of the heating means (20) and means (30) temperature measurement, embedded in a composite material, which are placed respectively in at least one zone (11, 13) the interface between such layers of material;
- securing means (50) humidity measurement, built in such a composite material, which is placed in at least one area of the boundary surface between the layers of material;
- providing means (40, 60) control installed on the aircraft and connected with said heating means, means for measuring the moisture content and means of temperature measurement; and
programming mentioned controls to activate mentioned heating means to heat the said product, when the relative humidity in the product is determined by measuring the moisture content reaches values above the maximum threshold.

7. The method according to claim 6,in which the product is made in the form of a panel and the heating means placed in the Central zone (11) the boundary surface of such panel.

8. The method according to claim 6, in which the product is made in the form of a panel and means for measuring temperature is placed in the Central zone (11) the boundary surface of such panel and zone (13) the boundary surface close to the outer surfaces of such panel.

9. The method according to claim 6, in which the product is made in the form of a panel and means for measuring moisture content is placed in the Central zone (11) the boundary surface of such panel and zone (13) the boundary surface close to the outer surfaces of such panel.

10. The method according to claim 6, in which the heating means comprise at least one electrical resistance.

11. The method according to claim 10, in which the electrical resistance includes a metal wire passing meanttobe, by zone (11) the boundary surface between the layers of material.

12. The method according to claim 6, in which the temperature measurement consists of at least one sensor type thermocouple.

13. The method according to claim 6 in which the means of measuring humidity consist of at least one sensor based on the hygroscopic polymer.