Improvement in the heating of plastic materials by infrared radiation

The present invention relates to heating plastics, especially thermoplastics.

Heating of thermoplastics is used in many fields. The most used in the industry is part molding. However, there are multiple molding techniques, and each technique requires different types of heating.

In a particular molding technique, a thermoplastic is heated above the melting point of the plastic, thereby making it fluid and molding from a fluid state. Examples may include extrusion, injection molding or other rotomolding.

In another technique, the thermoplastic is heated above the glass transition temperature but below the melting point, thereby imparting ductility to the material and making it moldable. Examples include thermoforming or blow molding, in particular injection blow molding or biaxially oriented injection blow molding. These techniques heat the material more slowly than the other techniques described above, because precise control of heating is essential. Such heating is generally performed by radiation in the furnace and does not bring the object to be heated into contact with the heating element.

Needless to say, in the industrial situation, the heating time is an important parameter because the production rate imposed on the production is generally extremely high. Therefore, it is considered desirable to shorten the heating time as much as possible without compromising other parameters, especially heating uniformity, penetration of radiant heat into the material, and energy efficiency.

Of the entire spectrum, only infrared radiation is known to be truly useful for heating.

The industry, especially the container manufacturing industry from thermoplastic plastic blanks (preforms or intermediate containers), uses ovens with halogen lamps, which have the advantage of being powerful, but the entire spectrum. It has the disadvantage that it radiates over and thus radiates part of the power consumed as a net loss, reducing the overall efficiency of the oven.

In order to increase the efficiency of the oven while shortening the heating time, Applicants in International Application WO 2006/056673 emitted a coherent beam, in particular a monochromatic electromagnetic beam, more specifically from a laser diode. It has recently been proposed to use a laser that may be used, thereby heating the preform.

This technique is promising and worth improving, as we have found that such heating does not produce certain results depending on the material selected.

<p> One object of the invention is thus a solution that can improve the versatility of heating, thereby allowing a compromise between heating time, heating uniformity, and energy efficiency for a variety of different materials. To provide. </p>

<p> To that end, the present invention provides a process of heating a plastic by at least one electromagnetic radiation source, which emits the radiation in the infrared region at a wavelength within one of the following ranges or in a wavelength spectrum. :: 1110 ~ 1160 nm, 1390 ~ 1450 nm, 1610 ~ 1650 nm, 1675 ~ 1700 nm, 1880 ~ 2100 nm, 2170 ~ 2230 nm, And preferably one of the following ranges: 1110 ~ 1150 nm, 1400 ~ 1430 nm, 1627-1647 nm, 1680 ~ 1695 nm, 1890 ~ 1906 nm, 1920 ~ 1950 nm, 2074-2094 nm, 2188 ~ 2216 nm.</p><p> According to a preferred embodiment, the infrared emission wavelength value may be approximately equal to one of the following values: 1130 nm, 1414 nm, 1637 nm, 1688 nm, 1898 nm, 1935 nm, 2084 nm, 2205 nm.</p><p> Further, in one particular embodiment, a plurality of radiations may be emitted at different wavelengths or different spectra, each wavelength being included in one of the above ranges or having one of the values listed above. .. </p><p> Electromagnetic radiation is preferably monochromatic or quasi-monochromatic. The electromagnetic radiation source is, for example, a laser.</p><p> This process, especially: Heating the blank to make a container from the blank, or Heating a foil or sheet to produce an article by thermoforming, May be applied to.</p>

Other objects and effects of the present invention will become apparent in the light of the following embodiments.

The purpose of the proposed process is to heat the intermediate molded product made of thermoplastic plastic with a view to the next deformation to obtain the finished product. The intermediate object to be molded is blown to obtain a thin sheet or narrow-mouthed container (bottle, etc.) intended to be thermoformed, in particular to obtain an object to be molded, such as a wide-mouthed container (for example, a box). It may be a preform intended for molding or stretch blow molding.

Among the assumed materials, polyethylene terephthalate (PET), polypropylene (PP) and polylactic acid (PLA) are particularly mentioned, and their main thermal properties are shown in Table 1 below. It should be noted that these materials are widely used in container manufacturing.<img file="JP2010520084A_D0001.tif" />

We started with the observation that, at a constant emission spectrum and constant power, heating generally does not give the same results, depending on the material selected, and the heating rate varies from material to material. In particular, significant differences were observed between one PET and the other PP.

Experiments were performed on multiple materials, holding the exact same source of electromagnetic radiation for the entire spectrum to select wavelengths that may be suitable for industrial heating of all of these materials. It was.

The experimental conditions are as follows.

The test sample is a thin sheet with a thickness of 3 mm made from the following materials: PET EASTMAN 9921 PET DAK Laser + PLA NATUREWORKS 7000D PP NOVOLEN 3348 PET / nylon 2% mixture.

Infrared rays are incident on these samples unilaterally (ie, with respect to one side, known as the incident surface), but the power of the infrared rays is 2 W / cm.²The wavelength selection range was set to 800 nm to 2500 nm.

For each sample, at each selected wavelength, the following was measured: --Radiation absorption rate ρ, that is, the ratio of incident power to the power absorbed by the sample, --Material heating rate V1 on the incident surface (in other words, depth of about 100 μm), and --Material heating rate V2 on the opposite side (in other words, about 100 μm in depth).

The measurement results were collated for each sample in Tables 2.1 to 2.5 below.

(Table 1.1) PET Eastman 9921<img file="JP2010520084A_D0002.tif" />

(Table 1.2) PET DAK Laser +<img file="JP2010520084A_D0003.tif" />

(Table 1.3) PLA 7000D<img file="JP2010520084A_D0004.tif" />

(Table 1.4) PP NOVOLEN 3348<img file="JP2010520084A_D0005.tif" />

(Table 1.5) PET Nylon 2%<img file="JP2010520084A_D0006.tif" />

It was observed that a good compromise was made between efficiency (high enough) and V1 / V2 thermal gradient (low enough) for a certain wavelength, which is relatively uniform and versatile. It proves that it is a certain heating.

On the other hand, for the other wavelengths marked with an asterisk in the table above, it was observed that this compromise could not be achieved for at least one selection material.

For example, at 880 nm, the efficiency was too low for the PET EASTMAN 9921.

At 1662 nm, the thermal gradient was too high for PET DAK Laser + and PET nylon.

At 1736 nm, the thermal gradient was too high for PP NOVOLEN3348.

At 2136 nm, the thermal gradient was too high for all PET, including PET nylon.

As a result of this analysis, it was derived to select the following wavelengths (unit: nm) for electromagnetic radiation from the radiation source used to heat thermoplastics: 1130, 1414, 1637, 1688, 1898, 1935, 2084. And 2205.

It can be assumed that these wavelengths are combined by juxtaposing a plurality of radiation sources having different values selected from the wavelengths listed above.

In practice, the electromagnetic radiation emitted by a commercially available infrared radiation source is not concentrated in a single wavelength but diffused into a roughly wide spectrum (several nm to several tens of nm).

As a result, through various assemblies (such as interposing an interference filter in front of the halogen lamp), with a single wavelength (monochromatic) or with tolerances of several nanometers (quasi-monochromatic), It is possible to limit the spectrum of the emitted radiation to radiate, but more rationally, select an inexpensive source (such as a powerful laser diode) and include the selected wavelength in the spectrum emitted from that source, 1 Avoid spreading to one (or more) unwanted wavelengths.

Shown below, on the opposite side of the selected wavelength, is an acceptable difference, i.e., a wavelength range that contains the wavelength or spectrum emitted by the selected source.

Table 3.1 shows the first set of relatively tight tolerances. Table 3.2 shows the second set of wider tolerances. Outside of these tolerances, the emitted radiation would not be able to obtain the objects described above.

(Table 2.1)<img file="JP2010520084A_D0007.tif" />

(Table 2.2)<img file="JP2010520084A_D0008.tif" />

As mentioned above, emitting multiple wavelengths or multiple spectra, for example, juxtaposed multiple sources, in one of their respective selected ranges (monochromatic, quasi-monochromatic, or over one spectrum). Depending on the mixture, it may be advantageous.

As proposed above, in the infrared region, a thermoplastic consisting of irradiating a material with electromagnetic radiation emitted in a spectrum contained in one of the selected wavelengths or one of the selected ranges is heated. The process may be successfully applied to the heating of blanks (which may be preforms or intermediate containers) for making containers.

This process may also be applied to heating a sheet or foil to produce an object to be molded by thermoforming.