EP2024137B1

EP2024137B1 - Cryoblasting method for surface treatment

Info

Publication number: EP2024137B1
Application number: EP07736820A
Authority: EP
Inventors: Andrea Seghezzi
Original assignee: SAPIO PRODUZIONE IDROGENO OSSIGENO Srl
Current assignee: SAPIO PRODUZIONE IDROGENO OSSIGENO Srl
Priority date: 2006-05-02
Filing date: 2007-04-30
Publication date: 2012-02-22
Anticipated expiration: 2027-04-30
Also published as: EP2024137A1; ATE546257T1; WO2007125565A1; ITVR20060080A1

Abstract

A delivery device f or cryosandblasting machines designed f or the treatment of surfaces, in particular in the sector of historical buildings and artistic and architectural property, comprises a spray gun (10) f ormin g p art o f equ ipme nt c on s i s t ing o f a cryosandblasting machine (11), the spray gun comprising a handgrip (17) positioned below a body (18) housing a nozzle (19) and equipped with a pipe (20) positioned at the exit of the noz zle (19) and facing towards the exterior; the holes (23) in the flange (22) of the nozzle ( 9) are inclined with respect to the output direction axis of the nozzle.

Description

TECHNICAL FIELD

This invention concerns a method for surface treatment, in particular in the sector of the historical buildings and artistic and architectural property, using a cryosandblasting machine.
More specifically, this invention, which is part of the cryosandblasting research project for artistic restoration, proposes a method using a cryosandblasting machine provided with a delivery device, able to optimise the spray of CO₂ from the cryosandblasting machine in the form of pellets.
The innovation to the cryosandblasting system, which is described below, was suggested by the results of tests carried out during the previous stages of the project. The aim is, on one hand, to contain the abrasive effect which the cryosandblasting system has been shown to have on some types of stone, and on the other to improve the efficacy of removing the surface deposits from difficult materials for which the use of traditional equipment has not proved to be particularly efficient, with particular reference to the treatments for the restoration of surfaces of historical-artistic interest.
The invention in question can be applied in the industrial sector which uses CO₂ technology for the treatment or cryosandblasting of surfaces, in particular in the cryosandblasting sector as an innovative method in the treatment of surfaces of historical-artistic interest.

BACKGROUND ART

It is known that sandblasting is one of the techniques used for the abrasive treatment of surfaces, and it is also known that, in some cases, traditional sandblasting is replaced by so-called cryosandblasting, which instead of sand used solid state CO₂ in the form of pellets.
Cryosandblasting equipment, or cryosandblasting machines, which use dry ice require a compressed air generator and a power supply.
Appropriately treated compressed air is conveyed into the cryosandblasting machine in which the pellets are fed. The pellets and the carrier air are pushed into a first conduit, while high output compressed air is conveyed into a second conduit. The conduits are connected, at their other end with respect to the machine, to a spray gun, designed to receive the two conduits and to convey the jet towards the surface to be treated.
Inside the gun is a nozzle which joins the two air and CO₂ flows and directs them into the pipe positioned at the exit of the nozzle towards the exterior.
The nozzle in turn consists of a cylindrical tubular element fitted with a flange with a plurality of axial holes, forming a single body.
The tubular body and the holes in the flange are arranged, according to known concepts, in a coaxial direction.
The flow of compressed air passes through the flange, while the air with CO₂ passes through the cylindrical body.
In traditional sandblasting gun nozzles, the pressurised gas is emitted in the same direction as the CO₂ pellets, so that they are accelerated with a rectilinear motion in the pipe at the delivery device exit (photo 1), reaching the surface of the material to be treated with a limited collision cross-section (around 2-3 cm with the gun held at a distance of 20-30 cm).
A number of functional limitations have been encountered in the use of these solutions and will be described below.
While this technology is suitable for a sector generally concerning the restoration and cleaning of surfaces with a high resistance, it has been found that for particular uses in which the use of cryosandblasting would be advisable, mainly in the sector of historical buildings and artistic and architectural property, the results have instead been extremely unsatisfactory.
In fact, cryosandblasting used in the conventional way did not give the desired results, since during the tests carried out the treated surfaces were damaged due to the excessive violence of the jets emitted.
An attempt was made to rectify the situation by adjusting the gas delivery pressure, but this too did not produce satisfactory results as the reduction in pressure did not provide adequate surface treatment.
Document US-A-5725154 discloses a spray gun for cleaning using a dense fluid such as CO₂, providing snow pellets of controllable size and kinetic energy. A heated or thermal ionized gas is passed at a substantial pressure and accelerates the snow particles with an enveloping stream of gas.
Document WO-03/22525 discloses a method according to the preamble of claim 1.

DESCRIPTION OF THE INVENTION

This invention proposes to provide a method for surface treatment using a cryosandblasting machine that can eliminate or at least limit the problems described above.
This is achieved by means of a method for surface treatment, in particular for use in the sector of historical buildings and artistic and architectural property, with the features described in the main claim.
The dependent claims describe advantageous embodiments of the invention.
The main advantages of this solution, in addition to those deriving from the construction simplicity, concern first of all the fact that it makes it possible to use cryosandblasting for the restoration of artistic and architectural property, that is to say historical buildings in general.
The modifications according to the invention to the cryosandblasting system were suggested by the results of the tests carried out during the various stages of the project and are aimed on one hand at limiting the abrasive effect demonstrated by the cryosandblasting system on some types of stone, and on the other at improving the efficacy in the removal of surface deposits from difficult materials for which the use of the original pellets had not proved particularly effective.
As stated above, in the traditional sandblasting gun nozzles, the pressurised gas is emitted in the same direction as the CO₂ pellets, so that they are accelerated with a rectilinear motion in the pipe at the nozzle exit, reaching the surface of the material to be treated with a limited collision cross-section.
According to the invention, on the other hand, the gas output holes in the new nozzles are inclined so as to form an angle with respect to the output direction axis.
Output hole inclinations with a broad range of angles from 15° to 30° with respect to the output direction axis, corresponding to the axis of the pipe positioned at the exit of the nozzle, have been tested with particularly advantageous results.
According to the invention, the dry ice pellets are thus subjected to a lateral acceleration causing the flow to perform a helical motion inside the pipe.
The collision cross-section of the pellets with the treated material is therefore greater, when used at the same distance, with respect to a traditional nozzle, the angle of incidence also being modified.

DESCRIPTION OF THE DRAWINGS

other features and advantages of the invention will become evident on reading the description given below of one embodiment of the invention, given as a non-binding example, with the help of the accompanying drawings in which:

figure 1 represents a schematic view of equipment for cryosandblasting treatment;
figure 2 shows a schematic view of a cryosandblasting gun;
figure 3 represents a schematic view of a first nozzle according to the invention in which the channels on the flange are inclined at 30° with respect to the output axis;
figure 4 represents a schematic view of a second nozzle according to the invention in which the channels on the flange are inclined at 15° with respect to the output axis;
figure 5 shows a schematic view of a nozzle with holes in line with the axis of the pipe;
figure 6 shows a schematic view of a delivery device according to the invention;
figure 7 is a schematic view of one of the holes in the flange and its possible inclined axis shapes;
figure 8 shows a schematic view of a gun constructed according to the concepts on which the invention is based.

DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION

With reference to the accompanying figures, the delivery device 10 used in the method according to the invention is used together with a cryosandblasting machine according to a scheme indicated in figure 1.
With reference to figure 1 the cryosandblasting equipment used in the method according to the invention therefore consists of a cryosandblasting machine 11, generally on wheels, connected to a power supply and a source of compressed air.
The cryosandblasting machine 11 is designed to be supplied with CO₂ in pellet form which, together with the compressed air, passes through delivery conduits 12 and 13 conveying the high output air and the air with the CO₂.
The two conduits 12 and 13 are connected to the couplings 14 and 15 on the rear part of the delivery device 10, which is also attached to a connecting cable 16.
The delivery device 10 presents the classic shape of a spray gun and consists of a handgrip 17 positioned below a body 18 in which a nozzle 19 is housed.
The body 18 is equipped with a pipe 20, or barrel, positioned at the exit of the nozzle 19 facing towards the exterior.
As shown in figures 3 to 7, the nozzle 19 presents a cylindrical tubular body 21, to which a flange 22 provided with a plurality of holes 23 is fitted, forming a single body.
The high output compressed air flows through the holes in the flange 22, while the air carrying the CO₂ passes through the tubular body 21.
The holes 23 in the flange 22, which according to known solutions are arranged coaxially, are according to the invention arranged with an inclined axis.
In the new nozzles the exit holes for the pressurised gas are inclined respectively at angles of 15° and 30°.
The inclination of the exit holes 23 in the nozzle allows the dry ice pellets to be subjected to a lateral acceleration causing the flow to perform a helical motion inside the pipe 20.
The collision cross-section of the pellets with the treated material is therefore greater, when used at the same distance, with respect to a traditional nozzle, the angle of incidence also being modified.
The exit holes 23 of the nozzle present an average diameter of 7 mm.
More generally, the diameter of the holes 23 is within a range of from 2 to 12 mm., with a preference for the intermediate sizes.
As regards the dry ice pellets, it possible to decrease the diameter to 1.5 mm and the average length to 3 mm.
In the following description of the tests carried out, the new smaller pellets will be identified with the letter "P", to distinguish them from the larger traditional ones which will instead be identified with the letter "G".
The tests were carried out to assess whether the introduction of the above modifications to the cryosandblasting system according to the invention can limit the surface abrasion demonstrated by the original system on some materials such as vicenza stone, granite, Carrara marble and bricks.
In addition to the abrasion tests, cleaning tests were also carried out on samples of Carrara marble, botticino limestone and Istria stone.
The test methods used differ according to the type of stone considered and were also designed on the basis of the results obtained in previous abrasion tests.
To evaluate surface abrasion, the surface morphology of the various samples was assessed before and after the cryosandblasting tests, using a laser profilometer. This makes it possible to obtain the three-dimensional mapping of the treated part and to calculate the average roughness (Ra) as a characteristic parameter. The efficacy of the system in removing surface dirt was, instead, evaluated by macro- and microscopic observation of the samples before and after the treatment.
The tests carried out on samples of granite in the preliminary stage of the project had shown that this type of stone had little resistance to cryosandblasting since, due to its high rigidity and morphological nature, it tended to lose surface clasts with the formation of particularly visible cavities.

In order to ascertain whether the modifications made to the system are able to limit these deterioration phenomena, abrasion tests were carried out on a total of 6 samples, three pink granite and three white granite, maintaining the output of dry ice and the pressure of the compressed air at constant values of, respectively, 70 kg/h and 7 bar and modifying only, for each test piece, the type of pellets and nozzle as shown in table 1.
The treatment time and the distance between the gun and the surface were also maintained at constant values, respectively 30 seconds and 30 centimetres. The list of the granite test pieces and the treatment conditions is shown below.

MATERIAL	TEST PIECE	OUTPUT/ PRESSURE	PELLET TYPE	NOZZLE TYPE
Pink granite	A5	70 kg/h 7 bar	G	Traditional
	A6		P	Traditional
	A7		P	Holes inclined 15°
White granite	G1	70 kg/h 7 bar	G	Traditional
	G2		P	Traditional
	G3		P	Holes inclined 30°

visual examination of the samples at the end of the treatment already showed that the test pieces treated with the larger pellets and the traditional nozzle (type A) are subjected to considerable surface damage; when type P pellets are used the surface damage is much more limited and when type B or C nozzles are used, no visible signs of deterioration can be observed.
The surface profilometric measurements confirm what was observed macroscopically, as can be seen from the following table which shows the average roughness of the various samples before and after the treatment: a net reduction can be observed in average roughness after treatment when changing to smaller pellets, all the other treatment conditions remaining the same. With the use of the modified gas/pellet nozzles (the type with holes inclined at 15° and 30°) compared to the traditional device, the variation in roughness with respect to before the treatment is even more limited, confirming the lower degree of damage to the sample.

TEST PIECES

Before treatment After treatment

A5 2.9 19.7

A6 1.1 9.1

A7 0.8 1.7

G1 1.4 39.3

G2 0.9 18.7

G3 0.6 0.9
The tests carried out show that, even using particularly high CO₂ outputs and pressure levels, the modifications made to the system, and in particular to the gun, made it possible to almost completely eliminate the undesired surface abrasion of the material.
The test was also carried out on samples of Carrara marble. The previous tests performed using the traditional system had shown that, macroscopically, this type of stone can resist the cryosandblasting treatments sufficiently well; the profilometric analyses carried out on the material before and after the treatment had in any case shown a progressive reduction in surface roughness when the CO₂ output and compressed air pressure were decreased, confirming that, even if not visibly apparent, the material is slightly damaged.
The tests on the new gun and pellets were carried out on 4 test pieces, maintaining the pressure of the gas and the dry ice output constant and varying the type of pellet and nozzle for each test piece. The test conditions and the average roughness calculated for each test piece before and after the treatment are shown in the following table:

TEST PIECE OUTPUT / PRESSURE TREATMENT TIME PELLET TYPE NOZZLE TYPE

before After treatment

C3 85 Kg/h
7 bar 30 sec. G A 0.2 1.7

C4 P A 0.2 1.5

C5 P B 0.2 0.3

C6 P C 0.2 0.3
Visual observation after the treatment did not reveal any evident signs of surface abrasion on any sample; only on samples C3 and C4 was there a slight loss of polish, which the profilometric analysis showed to be the result of an increase in average roughness which, expressed in µm (micron), rose from 0.2µm to 1.7µm and 1.5µm respectively. Samples C5 and C6, treated on the other hand with the smaller pellets and the modified nozzles (type B and type C - where with B the holes are inclined at 15° and with C at 30°), the variation in roughness from before to after the treatment was almost negligible (from 0.2 µm to 0.3 µm).
The results obtained show clearly that by using the two types of modified gas/pellets nozzles (types B and C - with holes at 15° and 30°) it is possible to considerably reduce the abrasive effect of cryosandblasting with respect to the use of the traditional solution.
The tests also showed that just the use of the small pellets with the traditional nozzle (type A) did not significantly decrease the abrasive effect compared to the use of the large pellets, at the same pressure and output. Contrary to what would be expected, due to the reduced mass of the type P pellets compared to type G pellets, this result can be attributed to the fact that when the smaller pellets are accelerated by the gas they strike the sample at a considerably higher speed and the quantities of motion of the two types of pellets, at the time of impact against the sample, are substantially comparable.
The aim of the tests carried out was to evaluate the modifications to the size of the pellets and to the cryosandblasting spraying system, assessing both the abrasive effect and the cleaning efficacy.
The overall evaluation of the abrasion tests showed that the modifications, particularly to the gas/pellet nozzle, make it possible to considerably reduce the surface alterations that the traditional system has on some types of stone.
The results obtained on compact materials, such as Carrara marble and granite, were particularly interesting, showing that even when the treatment was carried out at high pressure and output levels the morphological features of the surfaces remained substantially unaltered.
The cleaning tests performed on unpolished samples of Carrara marble and botticino limestone characterised by the presence of particle deposits proved to be effective, although for the first material it was necessary to use limited nitrogen pressure levels.
The Istria stone sample, characterised by the presence of stratifications of recrystalized calcite and a compact black surface crust, showed on the other hand a high degree of deterioration due to the breakage of the recrystalization layers, considerably more fragile than the more internal stone matrix.
In view of the good results obtained in terms of compatibility in the compact materials, it is deemed advisable, when treating objects of particular historical-artistic value, to use only the modified pellets and nozzles which, the other conditions being equal, have been shown to be less invasive than the traditional system which could, however, be used for other building structures that are not works of art and made from compact materials.
The invention is described above with reference to a preferred embodiment. The scope of the invention is defined by the appended claims.

Claims

A method for the treatment of stone surfaces, in particular in the sector of historical buildings and artistic and architectural property, making use of a cryosandblasting machine (11) comprising a spray gun (10) with a body (18) housing a nozzle (19), the nozzle (19) presenting a cylindrical tubular body (21) through which air and dry ice is delivered, the nozzle (19) furthermore presenting a flange (22) through which compressed air is delivered so that solid state CO₂ in the form of a flow of pellets is pushed through a pipe (20) at the exit of the nozzle (19), characterized in that said pellets have a diameter of about 1.5 mm and a length of about 3 mm, and in that said pellets are subjected to a lateral acceleration inside the cryosandblasting machine causing the flow of pellets to perform a helical motion inside said pipe (20),
wherein the flow of compressed air inside the pipe (20) is inclined in respect of the longitudinal axis of said pipe (20), and
the inclination of the flow of compressed air is obtained by means of a series of exit holes (23) of the flange (22) inclined in respect of the longitudinal axis of said pipe (20).
A method according to claim 1, wherein said holes (23) are inclined by an angle comprised between 1° and 45°, preferably between 15° and 30°.
A method according to any one of claims 1 and 2, wherein said holes (23) have a diameter ranging from 2 and 12 mm.
A method according to any one of the preceding claims, wherein the output of solid state CO₂ pellets is kept at a constant value of about 70Kg/h to 85 Kg/h and the pressure of compressed air at a constant value of about 7 bar.