FIELD OF THE INVENTION
[0001] The invention relates to a hair styling device using dielectric heating.
BACKGROUND OF THE INVENTION
[0002] US 2017/0360174 discloses an apparatus that manipulates the shape of hair using dielectric heating.
Typically, the apparatus includes opposing first and second electrodes respectively
provided on first and second arms that are movable towards and away from one another.
Drive circuitry supplies electrical energy to the first and second electrodes, to
cause an alternating electric field to be produced in the vicinity of the electrodes
in use, and thereby cause dielectric heating of hair placed between the electrodes
in use. Sensing circuitry senses a change in coupling of energy from the alternating
electric field to the hair during heating of the hair. Control circuitry controls
the drive circuitry to vary the electrical energy supplied to the first and second
electrodes in dependence upon the sensed change in coupling.
[0003] The range of frequencies is preferably from around 1 MHz to around 100 MHz. More
preferably the range of frequencies is from around 10 MHz to around 100 MHz. Even
more preferably the range of frequencies is from around 20 MHz to around 40 MHz, these
frequencies being well suited for consumer products since they have limited wave propagation
(unlike microwaves) and hence do not present a risk to health or undesirable EMC (electromagnetic
compatibility) effects. A voltage of around 50 V AC is used.
SUMMARY OF THE INVENTION
[0004] It is, inter alia, an object of the invention to provide an improved hair styling
device using dielectric heating. The invention is defined by the independent claim.
Advantageous embodiments are defined in the dependent claims.
[0005] Embodiments of the invention are based on the recognition that in the frequency range
of below 40 MHz, as preferred by the prior art
US 2017/0360174, the loss tangent and heat generation significantly depend on the moisture content.
So, with frequencies below 40 MHz, as preferred by the prior art, the power levels
need to be adapted depending on the water content of the hair. Moreover, since the
water content changes significantly during the styling process, heat control requires
real-time accurate monitoring of water content to provide optimal heating effects,
to prevent undesirable over-heating effects and poor efficacy due to under-treatment
from occurring. In that sense, it is desirable to have a method that is less dependent
on the water content and which requires less critical sensing and control of treatment
parameters. In accordance with an aspect of the invention, this can be achieved by
using frequencies in the range from 50 MHz to 90 MHz which will make the treatment
less dependent on water content and at the same provides about the same efficacy as
around 1 MHz for dry hairs. Preferably, a voltage of the radio-frequent signal does
not exceed 30 V, while a voltage not exceeding 10 V would work well in a configuration
that applies another heat source for heating hair up till a first temperature that
is no more than 150 °C (and thus lower than a critical temperature at which hair cuticle
damage will occur), and radio-frequent electrodes for - in combination with heat from
the other heat source - selectively heating a hair cortex to a second temperature
exceeding the first temperature and sufficiently high for hair styling.
[0006] These and other aspects of the invention will be apparent from and elucidated with
reference to the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007]
Fig. 1 shows a graph of loss tangent vs. frequency; and
Fig. 2 shows an embodiment of a hair styling device in accordance with the present
invention.
DESCRIPTION OF EMBODIMENTS
[0008] Fig. 1 shows a graph of loss tangent vs. frequency, for wet hair W and dry hair D,
respectively. The graph shows a relatively large difference in loss tangent between
wet hair W and dry hair W for frequencies below 40 MHz, while the difference is much
smaller for higher frequencies above 50 MHz, so that at such higher frequencies, it
is not necessary to accurately and constantly measure the moisture content of the
hair while styling it. While the graph shows that the same advantage also occurs at
frequencies higher than 90 MHz, an efficacy of the RF heating is less at frequencies
exceeding 90 MHz.
[0009] Fig. 2 shows an embodiment of a hair styling device 100 in accordance with the present
invention, similar to our previous application
WO 2017/080957 (attorneys' docket 2015PF01581), with the difference that in the embodiment of Fig.
2, radio-frequency (RF) heating is used instead of light. The hair styling device
100 comprises two jaws 101 which are hinged to pinch strands of hairs. Each jaw 101
comprises an array of RF electrodes or large area electrodes 102, which will selectively
heat the hair cortex to a temperature of about 170 °C required for styling. This temperature
can be controlled to prevent other non-desirable heating effects such as melting and
vaporization expected at 250 °C and 1200 °C respectively.
[0010] Each jaw 101 comprises optionally another heat source 103 which may directly or indirectly
pre-heat the hairs to a first temperature of no more than 150°, preferably no more
than 140 °C, e.g. 120 °C, to enhance the dielectric properties of the hair. The combined
heating effect from the other heat source 103 and the RF electrodes 102 will lead
to the temperature of about 170 °C required for styling.
[0011] Additional hair conductivity sensing controls 104 can be used to measure hair dielectric
properties and optimize the treatment by adjusting the power. Additional heat control
systems 105 can be used to regulate the temperature settings and treatment time. To
straighten/style the hairs, the strands of hairs are inserted between the plates and
are moved until the hair is styled.
[0012] Contact resistive RF heating is preferred over non-contact capacitive RF heating
for a consumer proposition because of the high voltage and power or longer cutting
time required for the latter one. A voltage below 10 V is suitable for use with high
frequencies in the range between 50 MHz and 90 MHz in the embodiment of Fig. 2.
[0013] It should be noted that the above-mentioned embodiments illustrate rather than limit
the invention, and that those skilled in the art will be able to design many alternative
embodiments without departing from the scope of the appended claims. For example,
while the embodiment shown in Fig. 2 is a hair straightener, the invention can alternatively
be used for hair curling. In the claims, any reference signs placed between parentheses
shall not be construed as limiting the claim. The word "comprising" does not exclude
the presence of elements or steps other than those listed in a claim. The word "a"
or "an" preceding an element does not exclude the presence of a plurality of such
elements. The invention may be implemented by means of hardware comprising several
distinct elements, and/or by means of a suitably programmed processor. In the device
claim enumerating several means, several of these means may be embodied by one and
the same item of hardware. Measures recited in mutually different dependent claims
may advantageously be used in combination.
1. A hair styling device (100) comprising
electrodes (102) for applying a radio-frequent signal to hair, characterized in that a frequency of the radio-frequent signal is between 50 MHz and 90 MHz.
2. A hair styling device (100) as claimed in claim 1, wherein a voltage of the radio-frequent
signal does not exceed 30 V.
3. A hair styling device (100) as claimed in claim 2, wherein the hair styling device
(100) further comprises another heat source (103) for heating hair up till a first
temperature that is no more than 150 °C, and wherein a voltage of the radio-frequent
signal does not exceed 10 V.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only.
It does not form part of the European patent document. Even though great care has
been taken in compiling the references, errors or omissions cannot be excluded and
the EPO disclaims all liability in this regard.
Patent documents cited in the description