US3650800A

US3650800A - Textile products and production thereof

Info

Publication number: US3650800A
Application number: US809932A
Authority: US
Inventors: Martin Reiner
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1968-04-25
Filing date: 1969-03-24
Publication date: 1972-03-21
Anticipated expiration: 1989-03-21
Also published as: AT307360B; BE728538A; NL6906271A; DE1760260A1; FR2006872A1; SE342767B; ES366439A1; BR6906701D0; GB1220561A

Abstract

Coated textiles are produced by a process in which 4-rail knitted fabric which is napped on one face is coated on the napped face with a polyurethane and subsequently grained mechanically.

Description

llnited States Patent Reiner 5] Mar. 21, 1972 [54] TEXTILE PRODUCTS AND PRODUCTION THEREOF References Cited [72] Inventor: Martin Reiner, Plochingen, Germany UNITED STATES PATENTS [73] Assignee: Farbenfalbriken Bayer Aktiengesellschaft, 1,406,710 2/ 1922 Wilson ..117/11 Leverku'sen, Germany 2,914,836 12/1959 Montgomery 1 ..26/29 X 3,387,989 6/1968 West et a1. ...117/161 X [22] mm Mar-24,1969 3,483,015 12/1969 Fukushima 61111 ..1 17/161 x [21] App1.No.: 809,932

. Primary Examiner-Ralph S. Kendall Attorney-Robert A. Gerlach and George W. Rauchfuss, Jr. [30] Foreign Applicatron Priority Data Apr. 25, 1968 Germany ..P 17 60 260.7 [57] ABSTRACT Coated textiles are produced by a process in which 4-rail [52] U.S. Cl ..l17/37, 26/29, 28/74, knitted fabric which i napped on one face is coated on the 7/11, 7/411 7 7/109! 7/161 KP napped face with a polyurethane and subsequently grained [51 Int. Cl ..B44d 1/44 mechanically [58] Field ofSearch ..l6l/67; 28/74; 26/29; 117/1 1,

117/161 KP, 37,41,109,119.6,119.8,76T

2 Claims, No Drawings TEXTILE PRODUCTS AND PRODUCTION THEREOF This invention relates to coated textiles and to a process for producing the same. More particularly, this invention relates to textiles coated with polyurethane and to a method of producing the same.

lt has been heretofore known that various types of textile materials can be coated with higher molecular weight com positions of the polyaddition, polymerization or polycondensation type. It has also been heretofore known that the textile materials can be coated by either the direct coating process or by the reverse coating process both in their originally smooth or smoothed form and on a napped surface. The coating of napped woven fabrics by special coating processes using polyurethane coating compounds leads to production of materials which are permeable to water vapor and air. The napped fabric used for this purpose is preferably a cotton fabric. The use of cotton fabric has the disadvantage, however, in that it is very difficult to obtain the uniformly rough surface required for certain purposes because this depends very greatly on the cotton fiber used, the yarn twist and the construction of the weave. To obtain the suitable qualities requires passage of the fabric through the napping machine up to times and even then fabric has to be shorn short in between the passages. It is thus apparent that both the handle and the surface characteristics of these coated napped woven fabrics are often unsatisfactory because the density of the underlying web cannot be controlled with sufficient accuracy. The problem, therefore, was to find material in which the napped surface could be made substantially absolutely uniform.

The coating of knitted materials has itself also been heretofore known for a long time. Most of the knitted goods which are coated, however, are not completely satisfactory in their dimensional stability and their response to stretching (longitudinal, diagonal and transverse directions). Difficulties therefore, often arise in the direct coating process. In many cases, the knitted fabrics can only be coated by the reverse process. It is, therefore, commercially desirable to provide a knitted material which is of such a construction that it can be reliably be coated both by the direct coating process and by the reverse coating process.

It is, therefore, an object of this invention to provide coated textiles and a process for producing the same which is devoid of the foregoing problems and disadvantages. Another object of this invention is to provide coated textile material that is of such a construction that it can be acceptably coated by both the direct coating process and the reverse coating process. A further object of this invention is to provide a coated textile and a process for producing the same wherein a coated knitted fabric having a grain which is irregular and which can be as varied as desired and yet which does not repeat itself periodically is obtained. A still further object of this invention is to provide such coated textile fabric grained by mechanical means. Yet another object of this invention is to provide coated textiles and a process for producing the same wherein said coated textiles have a substantially constant weight per unit area over large production lengths.

The foregoing objects and others which will become apparent from the following description are accomplished in accordance with this invention, generally speaking, by a process for producing coated textiles, utilizing as the starting material a knitted fabric which is stable under tension and which is made of a pattern that can be napped and which comprises tearing open the threads of the knitted fabric introduced on the upper guide bar (rough laying) during the napping process and then coating the fabric with a polyurethane coating composition. A suitable starting material for this purpose is 4-rail warped knitted material in which the fiber pile produced by the napping process is of such uniformity that no subsequent shearing of the napped product is required. in the 4-rail construction the knitted material produced in this manner is so stable in the uncoated state that it can be conveniently washed and dyed on ajig.

It is frequently desired to provide coated textiles with a grain or an embossed pattern. Hitherto, a grain could only be imparted to the fabric by a manual operation which is obviously not practical when production is on a commercial scale. Furthermore, a grain could be obtained by impregnating the coated textile material on impregnating calenders under the action of heat and pressure. When the reverse process is employed, one can also obtain a pattern if grained transfer materials, especially transfer papers are used. Now, it has suprisingly been found that a grain which is irregular and which can be varied as desired can be obtained on the products of the process of this invention if the knitted fabric which has been coated with polyurethanes is treated for a certain time in an axial spin drier with alternating direction of rotation at temperatures of, for example, from about 20 C. to about C. This effect is surprising since the frequent previous attempts to produce such a grain on a coated textile material, such 25, for example, by means of washing machines, cylinder mills or similar apparatuses, have always resulted in unsightly surfaces, that is, having heavy creases or squeeze folds.

Thus, a particularly preferred form of this invention relates to a process for the production of textiles which are coated with polyurethanes by a process in which a 4-rail knitted fabric which has been napped on one side is coated on the napped face with a polyurethane coating composition and is subsequently grained by mechanical means. The invention also produces in this preferred form a coated textile of 4-rail warp knitted material which is napped on one side and coated on the napped face with a polyurethane coating composition and subsequently grained mechanically.

The knitted fabrics to be used in the process according to the invention can be produced from one or more fibers of known types. Both non-twisted and twisted or folded yarns as well as endless filaments can be used. Mixed fiber constructions can be produced, for example, by supplying the four rails with fibers of different origin. Suitable fibers are, for example, those obtained from polyamides such as polyamide-6 from caprolactam, polyamide-66 from adipic acid and hexamethylene diamine, or polyamide-l2 and the like, terephthalic acid polyesters such as, for example, polyethylene terephthalate, polyacrylonitrile, cellulose-2 /2- acetate, cellulose triacetate, cotton, wool, rayon, regenerated cellulose, polyolefins such as polyethylene, polypropylene and the like, vinyl derivatives such as polyvinyl chloride, polyvinylidene chloride and the like, and polyurethanes.

The fibers may be pure white, matted or spun dyed. They can be colored by the usual dyeing and printing processes employed in the textile industry. The knitting data, that is, for example, the warp thread count, pattern, stitch count, rod count or guide rails may, of course, be varied and be of any suitable parameter. These knitted materials can be coated in the napped state both on the napped surface and on the smooth surface by the usual processes generally known to those skilled in the art.

The process according to the invention is generally carried out according to the following procedure:

1. Knitting of a stable knitted fabric, for example, on a 4-rail warp knitting loom;

2. Washing of the fabric in the usual washing apparatus;

3. If necessary, heat fixing on a tentering frame;

4. Dyeing on a jig, HT beam dyeing apparatus, winch vat or HT vat with simultaneous softening;

5. Drying on a tentering frame;

6. Napping the goods, such as, for example, on a hosiery napping machine with 24 rollers, on a napping machine for woven fabric with 32 rollers, or on any other suitable roller scraper napping machines which have both stroke and counterstroke rollers;

7. Felt calendering on a steel/felt calender or any other suitable smoothing apparatuses which have a similar effect;

. Coating on the usual coating apparatuses employed in the textile industry, which are equipped with rollers or with squeegees and which are covered with rubber sheets. In addition, the coating apparatuses should be suitable both for use in the direct coating process and in the reverse coating process.

9. Application of a finish by means of doctor blades or by spraying or any other suitable coating technique;

10. Mechanical graining of the coated fabric in an axial tumbler drier;

l1. Laying out the goods and stretching them on suitable drying frames, and at the same time dampening the goods, or winding the goods on rollers.

Steps 4 and 5 may be omitted in the case of spun dyed or predyed yarn or filaments. The individual steps are described in more detail below.

i. Knitting A stretch resistant or dimensionally stable warp knitted fabric, for example, made of a 4-rail construction, is suitable for use in production of a stable basic fabric which is substantially equivalent to a woven fabric as regards resistance to stretching in width wise and length wise.

Production Guide har L Pattern: Satin 1-0/3-4 Draft: full, even patterned Material: 40 den or 44 dtex polyamide 6 Guide bar L,,: Pattern; Fringe O-l/l-O Draft: full, even patterned Material: 40 den or 44 dtex polyamide-6 Guide bar L Pattern: Fringe l-O/O-l Draft: full. even patterned Material: 40 den or 44 dtex polyamide6 Guide bar L,: Pattern: Satin l-0/3-4 Draft: full, even patterned Material: 80 den or 88 dtex cuprammonium rayon If a material which has a final width of about 150 cm. or a coating width of 140 cm. to 145 cm. is to be produced, the following data, for example, may generally be used as a basis:

:1. Width of untreated material: about I80 cm.

b. Fineness of machine: 28 English inches 0. Thread count or needle count: I920 d. Stitch count of untreated material: l8/cm. e. Draft ratio: L,=l 2| cm. per A-rack L,=66'/z cm. per /g-rack L ,=66le cm. per fa-rack L,=l2l cm. per -raek ft Stitch count of finished goods: -20.5/cm. g. Rod count of finished goods: l2.5l2.8/cm.

The 4-rail warp knitted goods knitted according to these data have a final weight per square meter (prior to coating) of about 230 to about 240 go, it has been found in practice that such a weight per square meter is very suitable for textile purposes. The weight per square meter may, however, be varied within wide limits. For example, 40, 60, 80, 100 or 120 and the like den cuprammonium rayon filaments may be used in the upper guide bar (rough patterning) or alternations may be effected by altering, for example, the feed ratio, the stitch count and rod count and the pattern. Changes in the stability of the fabric can be produced, for example, by supplying all the guide bars with nylon-6 or nylon-66 or other fibers in contrast to that used in the above-mentioned examples. The guide bars may also be supplied with the same or different fibers or filamentary materials as previously disclosed. Warp knitted goods which are strong or dimensionally stable, made, for example, from a 4--rail construction, have certain advantages Over woven goods. Spinning and variations in the material are eliminated since one can always work with the same den or dtex numbers on capillary numbers and the same stitch count, rod count, pattern, feed ratio and the like. Furthermore, it is much easier to produce a certain weight per square meter within narrow tolerances.

2. Washing The goods must be prewashed to remove spinning preparations, melts, etc. This can be done in winch vats, roller vats, full width washing machines, chemical cleaning parts, etc.

3. Heat fixing When using synthetic fibers, especially of the polyamide type, a heat fixing step may have to be employed which may be carried out in the usual manner on suitable tentering frames. Accurate control of the temperature, keeping it constant, is especially important.

4. Dyeing Dyeing may also be carried out on the rear face ofthe white raw material. Suitable dyeing apparatuses are, for example, jigs, and HT beam dyeing apparatuses and the like.

5. Drying The dyed goods are dried on a tentering frame in the usual manner known to those skilled in the art.

6. Napping Napping of the knitted goods to be used according to the invention can be carried out without any significant shrinkage in width, for example, on a 24 roller hosiery napping machine, or, for example, on a 32 roller napping machine for woven goods. Four to six passages through such machines are generally necessary to achieve sufficient napping. This is an important advantage over the process of napping woven fabric to achieve a more or less comparable effect since, for example, duvetyn, moleskin or velvetone require about 30 to passages through napping machines to obtain a suitable quality fabric. Owing to the fact that the fiber floats are always the same in the guide bar 4 (rough laying) napping is easier to achieve and no subsequent or intermediate shearing is necessary. These shearing processes are unavoidable when achieving a uniform pile in woven goods. Another advantage is that, in contrast to comparable woven napped fabric, napped knitted fabric need not be dampened or brushed.

Napping of the fabric is generally carried out in the direction of the pile but may also be carried out against the pile, according to the napping system or types of napping machines employed. The process is started at a low napping 0 energy and this is gradually increased during the subsequent passages through the machine. It has been found to be desirable that, for example, the 4-rail warp knitted goods does not undergo any increase in density on napping owing to mechanical shrinkage in width and that the floats placed in the guide bar 4 can be napped completely and uniformly in the napping machine with practically no shrinkage in width. This effect is especially advantageous in allowing one to obtain constant weights per square meter over large production lengths.

7. Felt calendering Felt calendering should be used to obtain a satisfactory handie in the case ofa uniform compacted surface. Calendering is carried out in the direction of the pile. The temperature may, for example, be from about C. to about C., and the pile face is placed on steel and the rear face on cotton. Other equipment, such as, for example, cylinder presses or smoothing calenders, are also suitable.

It is also important to calender the material to obtain a desirable grain in the finished fabric. MAterials which have not been calendered have a relatively coarse grain with full fabric handle. A finer grain is obtained with each passage through the felt calender (2 to 4 passages). It is, therefore, surprising that the appearance of the finished goods can even be influenced at this stage of the production.

8. Coating Various processes can be employed for coating the fabric. in the direct coating process, for example, one can use a transparent undercoat followed by a pigmented filling or top coat. Alternatively, for example, the fabric may first be provided with a transparent undercoat and a pigmented top coat is then applied by the reverse coating method. Furthermore, the fabric can be directly provided with a pigmented top coat by the process ofreverse coating without the use ofan undercoat. According to another variation, the transparent undercoat is applied by the reverse process and after the reaction is complete, the surface is coated by the direct process with pigmented solution of high molecular weight compounds. To obtain a better bond, it is preferable, in the case of a fabric which has not been provided with a preliminary coat in a direct coating process, to arrange for the pile surface to be in the counter stroke direction for the reverse coating. After the coating, it is always advisable to modify the surface character of the coating byapplying a so-called finish to vary the handle of the fabric in various ways as required. Silicones, are frequently applied as finishers, either as solutions or in the form of dispersions. Other agents which affect the surface may, for example, be prepared from polyacrylates or cellulose acetobutyrate. The finish should not be applied to the fabric until it has been sufficiently cross-linked. This means, for example, that in the case of reverse coatings the fabric must remain for about 6 to about 24 hours on the roller. It is obvious, of course, that in the case ofa material which is first coated by reverse coating which is then coated by direct coating, sufficient time must be allowed to elapse between these two stages to allow the coat which has been applied by the reverse process sufficient time for cross-linking.

According to the invention, the above-mentioned transparent coats consist of polyurethanes which may be prepared, for example, from higher molecular weight polyhydroxyl compounds such a hydroxyl polyesters, polyethers, polyester amides or polyacetals and the like, preferably polyhydroxyl polyesterurethanes, such as, for example, those disclosed in German Pat. Nos. 1,012,456; 957,294; 897,625; and 847,502, and polyisocyanates. Suitable polyisocyanates are those disclosed, for example in German Pat. Nos. 957,294; 1,012,456; 870,400; 953,012 and 1,090,196. The polyurethanes which are to be used according to the invention adhere very firmly on the textile substrates.

The transparent undercoating can be produced from a combination of the following components;

a. 1,000 parts of a 30 percent solution of a polyesterurethane prepolymer (polyester of adipic acid, hexanediol and neopentylglycol pre-lengthened with toluylene diisocyanate) in ethyl acetate;

b. 50 parts of a 75 percent solution of the reaction product of 1 mol of l,l,l-tri-methylolpropane and 3 mols of toluylene diisocyanate in ethyl acetate;

. 50 parts of a solution of a basic accelerator which consists of the following individual components; 7 parts of phenyl diurethane 3 parts of tetraoctadecyl titanate 45 parts of ethylene chloride 45 parts of ethyl acetate 2 parts of acetic acid 025 parts of acetic acid anhydride.

The additive c) is not necessary if a transparent undercoat is applied to the direct process and this is followed by reverse coating.

In another variation, one may use an undercoat having a composition in which, for example, as component a) 1,000 parts of a 30 percent solution of a polyesterurethane prepolymer (polyester of adipic acid and hexane diol prelengthened with toluylene diisocyanate) in acetic acid ester is reacted with 50 parts of a component analogous to b) and 50 parts ofa component analogous to c) are used.

An undercoat can also be prepared, for example, by reacting 1,000 parts of a 30 percent solution of polyesterurethane prepolymer (polyester of adipic acid and diethylene glycol pre-lengthened with toluylene diisocyanate) with 50 parts of a component analogous to b) and adding 50 parts of a component analogous to c).

The pigmented coating may, for example be a paste of 600 parts of 30 percent polyesterurethane prepolymer solution in ethyl acetate, 35 parts of a solution analogous to b) and 35 parts of a solution analogous to c) in which 30 parts of a silicic sol which has a very large surface area, 80 parts ofa titanium dioxide pigment of the rutile type, 3 parts of a green chromium oxide pigment and 1 part of a brown iron oxide pigment have been ground in. An ivory color is thereby obtained.

A deep red coating is obtained, for example, by adding 35 parts of a cadmium red pigment to the above-mentioned pigment containing composition.

If 30 parts of carbon black are used instead of the cadmium red pigment, a black coating is obtained.

The single colored coating can easily be made more decorative by applying certain color effects. This can be done, for example, by any suitable known roller printing or film techniques. Ornamental effects can also be obtained by spraying the fabric with differently colored solutions of high molecular weight compounds or dyes. It is also known that pre-printed ornaments on special supports may be applied to the coating in the manner of transfers. Furthermore, when using grained transfer paper, one may employ an undercoat which is of a different color to the top coat.

9. Finish A finish is generally applied in order to obtain an elegant smooth flowing handle. Silicone products are particularly suitable for this purpose in the case of textile materials which are to be used for clothing. A finish can be applied by means of doctor blades, spraying or by immersion in a bath of the finish. Finishes comprising, for example, cellulose acetobutyrate or mixed esters of ethyl and methyl silicic acid are preferably used in the production of upholstery articles. The finish should not be applied until the coated fabric has been cross-linked to such an extent that it can easily withstand being rolled out.

10. Graining It has hitherto not been possible to obtain a grain which is irregular and yet which does not repeat itself periodically by mechanical coating methods. In all the known processes the fabric is first embossed and therefore the resulting grain is repeated. The use of grained paper which is subsequently separated also results in the production of repeating surface effects.

It has hitherto only been possible to obtain a grain on a fabric which is uniform and yet not repetitive which has been coated with polyurethanes by intensive handling.

It is now possible in the process according to the invention to achieve this effect mechanically. The fabric is treated for this purpose in an axial spin drier the direction of rotation of which can be altered as desired. Machines of this type are known as tumblers. The fineness of the grain can also be influenced by this method. The treatment temperature is generally between about 20 C. and about C., preferably about 50 C. to about 70 C. The periods of residence in the tumbler are generally between about 3 and about 10 minutes. The higher the treatment temperature, the shorter will be the required period of residence and the finer will be the grain. The felt calendering carried out in the previous stage and the application of the coating in the case of the reverse process coating also have an important effect on the final nature of the grain. The axial spin drier should have a drum diameter of at least about em but preferably more. The fabric should first be provided with a suitable finish to prevent the fabrics sticking when applying the grain. The coated fabric should be allowed to stand for about 24 to about 28 hours before graining to avoid formation of surface cracks. Different but reproducible grain effects can be achieved in this way. The fabric may subsequently be stretched with slight dampening and at the same time rolled up to remove creasing in the grained fabric.

Depending on the construction of the original fabric and the choice of coating materials, coloring or patterning, the products obtained according to the invention may be used, for example, for clothing, millinery material, upholstery materials, protective coverings, household textiles, bags and luggage, decorative material and material for wood facings.

The materials may be worked up by the methods normally employed in the art, for example, they may be backed with foam plastics and provided with decorative quilted seams.

The invention is further illustrated but is not intended to be limited by the following example in which all parts and percentages are by weight unless otherwise specified.

EXAMPLE A 4-rail warp knitted fabric is produced as follows:

cellulose a. Fineness b. of raw material cv Feed ratio:

28 English inches the material is washed on a winch vat to which a nonionic washing agent has been added for about 20 minutes at moderate tetnperature. It is then heat fixed on a tentering frame at the temperature usually employed for the particular type of fiber, in the present case at about 188 C. to about 192 C. for poly-e-caprolaetam. The material is dyed a black shade on a jig with acid dyes. After rinsing and drying the fabric is napped. This is carried out in six passages through a 24 roller hosiery napping machine. Napping is carried out in the direction of the nap, starting with a low napping energy which is gradually increased during the subsequent passages. The napped fabric is passed three times at about 130 C. over a felt calender which has a diameter of about 2 m. Coating is carried out by first applying a clear undcrcoat having the following composition in a quantity of about parts of solids (setting ofdoctor wiper: doctor blade: 1 mm.).

1000 parts of a percent solution of a polyester-polyurethane prepolymer (polyester of adipic acid and diethylene glycol pre-lengthened with toluylene diisocyanate) are reacted with 50 parts of a 75 percent solution of the reaction product of 1 mol of lzlzl-trimethylolpropane and 3 mols of toluylene diisocyanate in ethyl acetate.

Coating of this material is carried out using a doctor applicator cover with a rubber sheet without the use of a supporting roller. The reverse coating has the following composition:

a. 1,000 parts ofa 30 percent solution of polyester urethane prepolymer (polyester of adipic acid, hexanediol and neopentyl glycol pre-lengthened with tolylene diisocyanate in ethyl acetate; 50 parts of a 75 percent solution of the reaction product ofl mol of l,l,l-trimethyl propane and 3 mols oftolylene diisocyanate in ethyl acetate; c. 50 parts ofa solution ofa basic accelerator which consists ofthe following individual components: 7 parts of phenyldiurethane, 3 parts of tetraoctacyltitanate, 45 parts of ethylene chloride, 45 parts of ethyl acetate 2 parts ofacetic acid, and 0.25 parts of acetic acid anhydride. The coating material is colored black and applied to a smooth silicone separating paper and united directly under slight pressure with the fabric which has been provided with an undercoat. I

The fabric is rolled up together with the separable material and unrolled after about 48 hours. A silicone finish is then applied by a doctor blade, After drying, the material is placed in a tumbler where it is rolled around for about 3 minutes at 70 C., the direction of rotation of the drum changing after every 10 revolutions. The diameter of the drum is about 150 cm. The finished fabric has a fine grain and attractive handle. It is taken out of the tumbler, spread out, stretched, while slightly damp and is at the same time rolled up.

It is to be understood that any of the components and conditions mentioned as suitable herein can be substituted for its counterpart in the foregoing examples and that although the invention has been described in considerable detail in the foregoing, such detail is solely for the purpose of illustration. Variations can be made in the invention by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A process for producing coated textiles which comprises napping a tension stable 4-rail knitted fabric on one face and coating the napped face with a polyurethane coating composition and mechanically graining by tumbling the polyurethane coated napped fabric in an axial spin drier by rotating said dryer in alternate directions of rotation at a temperature of from about 20 C. to C.

2. The process of claim 1 which comprises a: knitting a stable knitted fabric on a 4-rail wrap knitting loom;

b. washing the knitted fabric;

c. heat-fixing the knitted fabric on a tentering frame;

d. dyeing the knitted fabric;

e. drying the knitted fabric;

f. napping the knitted fabric on one face;

g. felt calendering the napped fabric;

h. coating the napped fabric with a polyurethane coating composition;

i. applying a silicone finish to said polyurethane fabric; and

j. mechanically graining said coated fabric in axial tembler drier at a temperature of from about 20 Cv to about 100 C. for a period of from about 3 to about 10 minutes by rotating said drier in alternate direction.

Claims

2. The process of claim 1 which comprises a. knitting a stable knitted fabric on a 4-rail wrap knitting loom; b. washing tHe knitted fabric; c. heat-fixing the knitted fabric on a tentering frame; d. dyeing the knitted fabric; e. drying the knitted fabric; f. napping the knitted fabric on one face; g. felt calendering the napped fabric; h. coating the napped fabric with a polyurethane coating composition; i. applying a silicone finish to said polyurethane fabric; and j. mechanically graining said coated fabric in axial tembler drier at a temperature of from about 20* C. to about 100* C. for a period of from about 3 to about 10 minutes by rotating said drier in alternate direction.