CA1152819A - Coating process and product - Google Patents

Abstract

ABSTRACT

This invention relates to a process for coating a substratum sheet material, such as woven fabrics, by first removing from the substratum material any moisture that does normally exist therein, and then coating the substratum material with a coating composition with no moisture, air or other impurities being trapped between the substratum and the coating. Novel products produced by this process comprise new compositions of sheet materials having improved weathering characteristics including stability against ultraviolet degradation for improved service in highly transparent or translucent coverings for roofs and solar devices.

Description

COATING P~OCESS A~D PRODIJCT
.

The present invention relates to a process for eoating a substratum such as a cloth or fabrle wlth a water resistant composition to produce a weather-proof fabrle reslstant to exposure to the weather and sunlight.
Water-proof coatings are well ]cnown and have been used on fabrie and eloth for a number of years. Sueh eoatings however, are generally used with elo-thes, eanvas tarps and the like. There is presently a requirement for a waterproof eoating to be used on highly transparent or translueent 10 flexible material sueh as fabrie ancl clo-th, in either permanent or removable struetures. Such an application of a water-proof material is in a building system such as that diselosed in my eo-pending patent appllcation relating to a canopy system for a bullding, Canadian Patent Applieation 15 Serial No. 352286.
In the past, water-proof eoatings have been used on yarns, fabries and the like, ~ithout any speclfic preparation to the~ yarns and fabries before the eoating step. The coatings have previously been applied to the fabric by 20 spraying, dipping, brushing, flow coating, transfer coating and sometimes by laminating. It is common practice to employ two or more layers of coatings to improve the product quality.
In rnost cases the coating does not pene-trate the yarns, but, rather adheres to the surfaee of the eloth. Thus, especially 25 with multi-filament yarns, there is often air, moisture and perhaps other eontaminants trapped within the yar~s thus coated over~
It was observed that in dip eoating processes where -fabrics woven of multi-filamented yarns are drawn through a ., ~.7~ B~9 bath of low viscosity coa-ting resin the final produc-t shows many flaws in the coating due to air bubbles. This air is brought in-to the bath with the Eabric since i-t is entrained within the yarns. As the fabric leaves the hath some of the 5 bubbles are picked up, and upon dryiny and curing, leave voids and pinholes in the coating. The faster the rate of fabric travel through the ~ath the more serious this problem becomes. ~lowever slower rates of coating will increase the cost per unit. Therefore a trade-off may be necessary 10 between the cos-t and the quality of the coated product. -Furthermore, it was noted tha-t the serviceabili-ty of the coated product, when exposed to weathering and sunlight, is impaired due, in part, to the presence of air and moisture , trapped in the material by the coating. Chemical reactions 15 that cause weakening and embrittlement of the yarn filaments take place in the presence of oxygen, moisture and other impurities. Exposure to sunlight, especially the ultra-violet rays thereof, initiates these degradation reactions in the presence of such impurities.
The high tensile strength yarns are composed of very fine rnulti filaments. Such types of fabric ma-terials are therefore especially susceptible to degradation reactions because a very large surface area is exposed to the contaminates. Thus although glass is known to be very stable 25 under weathering, fiberglass fabrics are embrittled, weakened -and yellowed rather rapidly if not protected by a coating material. If the coating develops fine fissures known as "crazing" due -to fatigue, then water and moisture can reach the fiberglass and wick along the yarns. This is the usual 30 mode of failure of composites with fiberglass reinforcements embeded in thermo-se-tting polyester resin.

The mode of failure oE hydrocarbon synthe-tic fibers is similar to that of glass fibers as described a~ove. There ~ is however, in addition -to the adsorbtion of water into the ; yarn, the absorption of moisture by the polymer itself.
5 These fibers have an affinity for mois:ture, and tend to have a low to-medium moisture content. The presence o~ the moisture at the sites of chemical degradation reactions is especially important for decomposition and deterioration on exposure to ultraviolet light. Thus Eor these fabrics their 10 s-tability under weathering is improved by preventing -entrap-ment of air and moisture in the yarns as previously discussed, and also by preventing the normal moisture regain of the fibers.
Similarly as for synthetics, it is important for 15 fabrics composed of natural fibers to prevent the entrapment of air and~moisture in the yarns and to prevent the normal moisture regain of the fibers in order -to achieve stability and long service life under exposure to weathering and sunlight.
; 20 The failure of such compositions of material is - caused primarily by the deterioration of the protective coating which after a certain time in service allows the sub-stratum to be exposed to oxygen and mois-ture from the atmosphere. However, it is apparent that the material is 25 often exposed to the said impurities by their entrapment within the material by the coating. Further, it is known that the degradation reactions produce reactive agents, so that these reactions are self sustaining and propaga-ting.
Therefore the material rnay be well on the way to its 30 deterioration from wi-thin before the coating that protects from outside agents loses its effec-tiveness.

In general, wa-ter-proof coatings have been placed on yarns, fabrics and sheet material such as films and sheet cast or extruded material, and the like, without any specific prepara-t:ion of these subs-tra-tum materials before the coating 5 step. Thus existing moisture is entrapped within these substratums to the extent of their affinity for adsorption and absorption of moisture. Furthermore, air and other contaminants can be trapped in the substratum by the coating.
The resul-t i-s often tha-t when ultraviolet light hits this 10 coated fabric~ decompositlon and deterioration of the fabric - occurs at least partially d~e to the impurities, mois-ture and air trapped beneath the waterproof coating.
Review of the past art indicates tha-t no specific effort has been made to avoid the presence of moisture in the 15 fibers of a substratum or to avoid the presence of moisture, air and contaminants adjacent to the filiments of the yarns as a means of producing fabric or cloth substratums wi-th improved stability and durability when exposed to sunlight and to the weather.
The presence of the said impurities in the fabric is not considered ln the coating of high viscosity resins onto fabric since they cannot penetrate the yarns but, rather, over-coat the fabrics. Therefore these coatings are usually applied by flow coating. A good quality product is obtained 25 by spreading the coating with a knife blade supplied with air pressure. Since the coatings are usually pigmented and otherwise treated for improved weatherability the ultra-violet radiation does not act upon the fabric substratum.
The presence of -the said impurities in the fabric has 30 been considered in the coating of low viscosity resins on-to fabric to the extent that it has been recognized that -the air within the fabric will cause flaws in the final produc-t as a result of the air forming bubbles i.n. the low viscosity resin.
- To avoid this problem of bubbles produced as the fabric moves through low viscosity resins it was found tha-t the coating 5 res Ln will not produce bubbles if the resin is transported at the same rate of travel as that of -the fabric. This method is typical of the "transfer type" coa-ting process. The low viscosity resin, transported on a moving belt or wheel, :~ contacts the fabric and wicks into it, while moving at the lO same speed as the fabric. Wetting the fabric by contact or spraying and allowing time for penetration and then dip coating in a resin bath will reduce :Eoaming in the bath. This method is described in Canadian Patent No. 998,300 and US
3,843,386. Dip coating may be avoided altogether if 15 successive transfer coa-ting runs can build up a sufficiently ; ~ thick coating over the fabric.
Previously, the above mentioned art has been employed to produce opaque products and thus the role of the said : impurities entrapped in the material as agents for the 20 degradation of the fabric has not been important/ slnce the ~ ultraviolet light which inltiates the degradation reactions does not reach the fabric substratum. There is presently a need for architectural fabrics of high transparency or translucency for capture of solar energy by the buildings and 25 for natural illumination within -the building. It is there-fore the object of the present invention to produce novel . products wherein a clear, hi.ghly transparent and mois-ture resistant composition penetrates and surrounds yarn filaments avoiding entrapment of air or other contaminants adjacent to 30 the fibers, and avoiding the natural moisture regain and the presence of other volatile contaminants as can exist in the in the fibers. The moisture resistance of the composition used on the fabric is such as to prevent the fabric's natural affinity for moisture from attractlng the moisture from the environment through the coating and into the fibers. Thus the 5 permeability of the coating to water vapour is very low.
The present invention provides a coating process for a fabric or cloth wherein moisture, air and impurities are first removed. The fabric then makes a pass through a coating bath which can employ resins of any specific viscosi-10 ty, which are clear when curred and do not absorb moisturenor deteriorate when exposed to weathering and sunlight. A
preferred resin for use in this process is available from ow Corning Corporation under the trade mark "R-4~3117 conformal coating" or any other resin type having equivalent . :
15 characteristics can also be used.
~; Another object of the process is to attain high rates of fabric travel while producing a product of excellent ~i ~ quality. Thls is achieved by preventing air entrainment into ;~ the bath and by the specific method employed for the resin 20 bath.
A further object of the process is to obtain control over volatiles released during the process. These volatiles can be recovered and liquified for reuse thus preventing pollution which is a common problem in prior coating proces- -25 ses.
In one embodiment a process is defined as shown in Figure 1, wherein the process consists of multiple stages of fabric travel, first, through a liquid bath and then immediately into a vacuum chamber. In one embodiment of the 30 invention two such stages are employed. The fabric(9), in the off loom condition and with no pretreatments is taken from a pay-out reel(7) and run through the entire machinery to be attached to the take-up reel(8) where -the coated fabric(10) , is taken up as a finished product in a roll. In the first stage the fabric enters the first bath(l) -through a slit(6), 5 the lips of the slit being fabricated of low fric-tion and abrasion resistant material. The fabric passes through the first bath chamber(l) and exits through another slit(6) into the first vacuum'chamber(2). Similarly the fabric(9), leaves the first vacuum chamber(2),through another sli.t(6) proceeding 10 directly into the second bath chamber(3? and -thence through ' another slit(6) into the second vacuum chamber(4) and'exiting by a final-exit slit(6).
The purpose of -the first bath(l) is primarily to serve as a seal-against entry of air into the first vacuum 15,chamber(2) in which a high vacuum is sustalned. Wa-ter or some other liquid or solution is constantly fed to the chamber which provides that the overflow is returned for resupply.
It is preferable that this liquid is quite ho-t so as to heat the fibers to as high a temperature as is consistant with .
20 safeguarding their physical properties. Thus the travel distance and the temperature of the bath(l) is to be deter-mined according to the type of ma-terial. Then, when the fabric(9) leaves the bath(l) and enters the first vacuum chamber(2) the liquid will flash oef as will t,he absorbèd 25 mo'isture due to the high temperature of -the fibers and high vacuum maintained in this chamber(2). It is provided that the first vacuum chamber can be portioned off into regions of progressively higher vacuum by means of pai.rs of soft rubber rollers or some other device.
Any air entering the first vacuum chamber(2) in the yarn is instantly released from the yarn and removed by the vacuum pump. Also since the liquid feed -to bath(l) can be pressurized the liquid will tend to wick out the incoming slit counter directionwise to the fabric travel so that air in the yarn will be largely displaced by the liquid before the 5 yarn(9) travels into the bath(l). Cross flow of the bath(l) liquid through the fabric can further assist to remove air and physical impurities from the fabric(9). The bath(l) supply and recycle circuits should ~herefore be fitted appropriately with filters and de-airing holding tanks.
The first bath(l) also may serve some pretreatment of the yarns such as heat setting to provide shrink resistance, or provide some chemical modification of the fibers or any combination of such uses including dying and the like prior to coating. The liquids employed in the first bath(l) must 1~ howe~er be highly volatile so as not to remain in the yarns on entering the first vacuum chamber. Thus there is no inter-ference from the ~irst bath that reduces the effectiveness of the release of absorbed moisture from the Eibers.
Therefore, according to the method of the present ~- 20 invention there will not exist any moisture in the fibers of the fabric(9) nor any mois-ture or air in the yarns, except in very small traces. And any such small traces of vapours will be easily displaced by liquid coating bath(3), which, supplied under pressure will penetrate completely and fill 25 the voids in the yarn, surrounding every filament. In the embodiment of the invention as we are now describing, the second bath chamber(3) is the coating chamber(3). The coating liquid supply should be at such a flow rate -that a cross flow exists with an over-flow from the bath(3) which is 30 resupplied. In this circuit a de-airing holding tank is used.
The coating liquid is supplied under pressure to cause a certain amount of counter direction flow and such that the coating liquld is extruded from the coa-tlng charnber(3) in-to the second vacuum chamber at a rate correspondlny to the fabric ra-te of travel. The exit slit(6) can have hard edged 5 lips or hard rollers to nip back the coa-ting resin and allow a specific thickness of the coating film over the surface of the fabric. A predetermined nip pressure controls the quantity of resin remaining on the fabric.
The second vacuum chanber(4) is provided for the lO drying and curring of the resin. A preferred embodiment employs a low vacuum with an inert atmosphere such-as nitrogen. The low vacuum assist the evapouration of the volatiles from the coating resin so that curring can begin.
Direct or infrared radlant heating can be employed for lS promoting the fast curring of the thermo-setting resin.
: The penetration of the resin into the fabric provides improved adhesion. Pressure, and elevated temperature assists high viscosity resins to properly penetrate the fabric. Since the fabric can travel quite quickly through 20 the coating chamber(3) a very short in-terval exposure to high temperatures often will not harm the fabric subs-tratum.
This is especially tr~le for glass fabrics which can be exposed briefly to temperatures exceeding 1000 F and also for polyesters and some other syn-thetic fabrics which can be 25 exposed briefly to temperatures exceeding ~00F. Thus the method of coating application of the present invention provides that thermoplastic materials can be supplied to the coating chamber(3) as a hot melt. This method may also employ volatile solvents to assist in loweriny the 30 temperature of the mel-t or improving the viscosity. ~rhe volatiles can then be recovered and liqulfied by exhausting -them from the second vacuum chamber(4). A low vacuum will assist the evaporation of the solvents.
- In another embodiment the coatiny is a liquid resin, supplied at room temperature to the coating chamber(3), which .5 cures by means of thermo-setting mechanism. There~ore upon exiting from the coating chamber(3) the fabric may enter a two : or three stage curing chamber(4) as is common practice with coating resins. The use of a catalyst and elevated . temperature can allow the coated fabric -to exit and be taken-10 up on a reel within 30 minutes of travel time within thecuring chamber.
The method herein of utilizing the baths as seals against entry of air into the process improves the quality of the resulting product but also at the same time preven-ts 15 exposure. of the process volatiles to the environment, and contains them for collection and reuse. This has both environmental and economic benefits. Many of the solvents. or treatments that could be employed evolve very toxic volatiles which pollute the environmen-t and are not easily eliminated 20 or removed from the exhaust of coating process employed in -- the prior art.
In a further embodiment a 3 or 4 stage process is visualized including pretreatment bath for substratum preparation, coating, secondary coating and post coati.ng 25 treatments.
By way of example, metalic ions may be deposited on opposite si.des of the coated fabric by reaction in seccessive bath and vacuum chambers following after the coating stage.
The final exit from the mechanism is through a slit 30 or through a pair of resilient surface rollers, or through a final bath with the dr~ving following in the open air. This --ll--last bath could be used for a final wa-ter cooling, fixing or washing bath or other purpose. The material is then taken up on a reel The present invention prodcues improved products by a 5 novel coating process for various types of compositions that are already known and used. The improved product is a fabrie with no moisture, air bubbles or impurities trapped between the fabric and the coating resin. Mositure which normally exist in the fibers is removed before the coating,and the 10 cured resin coating has a laek of affinlty for water, thus moisture eannot reaeh the fibers and normal moisture regain of the fibers lS prevented. Known eompositions that ean benefit by improved properties and process technology of this invention include glass fabric coated with silicone resin or poly-15 tetrafluoroethylene polymers. The invention relates to novelcompositions that are provided stability against ultraviolet ; degradation on exposure to weathering, which stability is provided by means of the process. The novel compositions include synthetic and natural fibers having low to medium 20 normal moisture regain, for example polyester fabrics, coated over with a polysiloxane resin such as is available from Dow Corning under the trade name R-4-3117 conformed coating silieone resin.
The present-invention offers improved process teeh-25 nology for produets other than~the transparent or translucent architectural Eabrics as discussed above. Indeed any eoating on a substratum ean advantageously employ the process deseribed. Thus films, slit and woven films, scrims, non-wovens, knits, tube-knit and tube-woven goods of any 30 deseription can advantageously be coated by means of a process embodying this invention, the scope of the process being limited only by the claims.

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for coating a substratum sheet material by first removing from the substratum material any moisture that does normally exist therein, and then coating the substratum material with a coating composition with no moisture, air or other impurities being trapped within the substratum nor between the substratum and the coating; the said process comprising the steps of:
(a) substratum travel first into a bath chamber of hot volatile liquid followed by travel through a vacuum drying chamber having zones of increasing vacuum for removal of substantially all air and volatiles from the substratum or its proximity; then (b) into a second bath chamber for application of a liquid coating to the substratum, which together extrude through a slit having a predetermined nip pressure into an inert atmosphere drying chamber provided with zones for recovery of volatiles evolved during drying and curring the coating composition.

2. The process of claim 1 wherein the first bath chamber liquid heats the substratum material to sufficiently high temperature to cause any absorbed moisture existing in the material due to the material's natural affinity for moisture, to immediately flash off and leave the material upon entering the vacuum chamber.

3. The process of claim 1 wherein the bath chambers act as seals against the entry of any substantial quantity of air into the drying chamber following, except that air which is entrained in the substratum material entering the first bath chamber.

4. The process of claim 1 wherein the coating bath is a liquified thermoplastic delivered to the bath as a hot melt and employing volatile solvents if required to lower the melt temperature or viscosity.

5. The process of claim 1 wherein the coating bath is a thermo-setting liquid resin, modified by solvents and employing catalyst and elevated temperature cure as may be required.

6. The process of claim 1 wherein the coating bath is a liquid with the coating compound as a particulate suspension therein and is following by a vacuum chamber for drying and sintering.

7. The process of claim 1 wherein the second drying/

curring chamber is provided with a means for removal and recovery by liquification of the volatiles evolved from coating curring.

8. The process of claim 7 wherein removal and recovery of volatiles is provided by means of recycling of a low pressure inert atmosphere such as nitrogen.

9. The process of claim 1 wherein additional pre-coating or postcoating treatments are achieved by means of any number of additional steps comprised of:
(a) a bath chamber; followed by (b) a vacuum drying chamber, or an inert atmosphere drying and/or curring chamber;
provided that a vacuum chamber preceeds a coating chamber, especially in the case of porous substratums, to prevent entrapment of gas or formation of bubbles in the coating.

10. The process of claim 1 wherein substratums include extruded and cast films and sheet material, slit and woven films, reinforced films or sheets and fibrous substratum material composed of glass, metal, synthetic hydrocarbons, and natural plant or animal fibers comprising continuous sheet material such as woven, non-woven, knit, tube-knit, scrim or tube woven goods.

11. The process of claims 43 5 or 6 wherein coatings are compositions including any of the following:

ABS, acetal,polytetrafluoroethylene, polychlorotrifluoro-ethylene, nylon, phenoxy, polycarbonate, polyimide, poly-phenlyene oxide, polyethylene, polyvinyl chloride, poly-vinylidene chloride, polyvinylidene fluoride, polyester, cellulose acetate, acrylic, epoxy, silicone elastomers, siloxane resins, hexa-fluoropropylene, polyvinyl fluoride and copolymers of any of the foregoing.

12. The products produced by the process of claim 1 comprising new compositions of sheet materials having improved weathering and stability against ultraviolet degradation for improved service in highly transparent or translucent coverings for roofs and solar devices.

13. The products produced by the process of claim 1 comprising any of the substratums coated by the coating composition where the normal moisture regain of the substratum is prevented by means of the coating's resistance to and non-affinity for moisture.

14. The products of claim 12 and 13, including archi-tectural coated fabrics wherein the substratums are composed of woven high tenacity fabrics including glass, nylon, and polyesters.