KANG, Byoung, Sam (14601 Waters Shore Drive, Midlothian, Virginia, 23112, US)
LEVIT, Mikhail, R. (5120 Dorin Hill Court, Glen Allen, Virginia, 23059, US)
KANG, Byoung, Sam (14601 Waters Shore Drive, Midlothian, Virginia, 23112, US)
| CLAIMS What is claimed is: 1. A muitiiayer structure comprising (a) a first iayer containing aramid and cellulose wherein (i) th aramid is present as an meta-aramid in an amount of 0 to 50 weight percent floe and 50 to 100 weight percent fibridi, Of) the ceiluiose is present in the form of ceiiuiosic puip fiber and; (in) the aramid is present in an amount of 18 to 75 weight percent and the ceiluiose is present in an amount of 25 to 84 weight percent, said percentages on the basis of the aramid and cellulose and: (b) a second iayer containing oeSSulossc puip fiber with the proviso that the second iayer does not contain aramid, 2. The muitiiayer structure of claim 1 , wherein the aramid is poiy(metaphenyiene isophtha!amide). 3. The multilayer siructure of claim , wherein the first layer contains no polymeric binder. 4. The muitiiayer structure of claim 1 , wherein the first layer contains 25 to 50 weight percent aramid and 50 to 75 weight percent cellulose, 5. The multilayer structure of claim 1 , wherein the first layer contains floe and florid present In a weight ratio of 0 5 to 4.0. 6. The muitiiayer structure of claim 5, wherein the weight ratio is 0.8 to 2.0, 7. The multilayer structure of claim 1 with at least one additional layer with the additional layer comprising (a) a layer containing aramid and cellulose wherein (i) the aramid is present as an eta-aramid in an amount of 0 to 50 weight percent floe and 50 to 100 weight percent fibrid, (it) the cellulose is present in the form of ceilulosic pul fiber and (iii) the aramid Is present in an amount of 16 to 75 weight percent and the cellulose is present in an amount of 25 to 84 weight percent, said percentages on the basis of the aramid and cellulose or (b) an additional layer containing ceilulosic pulp fiber with the proviso that the second layer does not contain aramid, 8 The multilayer structure of claim 7 with both additional layers present. 9. The multilayer structure of claim 7 with an additional Iayer being identical to the first layer or the second layer, 10. A device comprising an electrical conductor and an electrically insulating multilayer structure material comprising (a) a first laye containing aramid and cellulose wherein (i) the aramid is preseni as an meta-aramid in an amount of 0 to 50 weight percent floe and 50 to 100 weight percent fibrid, (si) the cellulose is present in the form of ceilulosic pulp fiber and (iii) the aramid is present in an amount of 8 to 75 weight percent and the cellulose is present in an amount of 25 to 84 weight percent, said percentages on the basis of the aramid and cellulose and (b) a second layer containing ceilulosic pulp fiber with the proviso that the second Iayer does not contain aramid, 11 , The device of claim 10 which is a transformer, 12. The transformer of ciaim 1 1 which is oil filled. 13. The transfofmer of ciaim 10 which has a capacity of at least 200 kVA. 14, The transformer of ciaim 13 whic has a capacity of at least 400 kVA. |
MULTILAYER STRUCTURE USEFUL FOR ELECTRICAL INSULATION BACKGROUND OF THE INVENTION
F ieid of the S nyentiort
The present invention relates to a multilayer structure useful for electrical insulation.
Description of the Related Art
Kraft paper, made from: cellulose pulp, is widely used as a solid electrical insulation in oil filled transformers because of its good insulating properties and economy. However, the cellulose polymer is susceptible to hydrolysis from long term exposure to high temperatures. Thermal stability as well as mechanical strength can be improved by blending high temperature synthetic fibers with the cellulose pulp. A polymer binder Is added to facilitate bonding between the synthetic fibers and the cellulose pulp. However, the polymer binder can cause non-uniformity in the paper and sticking problems during paper processing and its usage in contact with transformer coils during high temperature operations.
It is therefore desirable to prepare a paper for electrical insulation with improved thermal stability and mechanical strength while reducing its sticking in processing and the final end-use.
SUMMARY OF THE INVENTION
The present invention is directed to a multilayer structure comprising
(a) a first layer containing aramid and cellulose wherein
(i) the aramid is present as a meta-aramid in an amount of 0 to 50 weight
percent floe and 50 to 100 weight percent fibrid s
(ii) the cellulose is present in the form of cellulosic pulp fiber and (iii) the aramid Is present in an amount of 16 to 75 weight percent and
the cellulose is present in an amount of 25 to 84 weight percent,
said percentages on the basis of the aramid and cellulose and
(b) a second layer containing ceiluiosic pulp fiber wit the proviso that the second layer does not contai aramid.
Further embodiments of the present invention include one or more additional layers on a side of the second laye which does not face the first layer, i.e. the additional layer or iayers are not intermediate the first and second Iayers. It is considered that the first layer represents an outer layer in a construction in the event at ieast three or four Iayers are present.
Any ■ additional layer or Iayers may have the construction of the first and second Iayers. Examples of a four layer construction include the composition of the first and second layers (in any order).
The multilayer construction set forth above is useful an electrical insulation in a device with an electrical conductor. A preferred use is in a transformer.
DETAILED DESCRIPTION Of THE INVENTION
In the present disclosure the term "layer" is employed. However in some Instances the layer can be described as a "paper 8 . Generally a paper will have a thickness not greate than 1.0 millimeter. Accordingly i a preferred use each of the disclosed Iayers will have a thickness not greater than 1.0 millimeter,
A first required layer in the multilayer construction is a layer which comprises
aramid and cellulose wherein
(i) the aramid is present as an meta-aramid in an amount of 0 to 50 weight percent floe and 50 to 100 weight percent fibrid,
(ii) the cellulose is present in the form of ceiluiosic pulp fiber.
The term "aramid", as used herein, means aromatic poiyamide, wherein at least 85% of the amide (-CONH-) linkages are attached directly to two aromatic rings. Optionally, additives can be used with the aramid and may be dispersed throughout the polymer structure. It has been found that u to as much as about 10 percent by weight of other polymeric material can be blended with the aramid. It has aiso been found that copolymers can be used having as much as 10 percent of other diamines substituted for the diamine of the aramid or as much as 10 percent of other diacid chlorides substituted for the diacid chloride of the aramid. Meta-aramid are those aramids where the amide linkages are in the me a position relative to each other, A preferred meta-aramid is
poly(metapheny!ene Isophthalamide}.
The term "floe", as used herein, means fibers that are cut to a short length and whic are customarily used in the preparation of wet-laid sheets. Typically, floe has a length of from 3 to 20 millimeters, A preferred length is from 3 to 7 millimeters. Floe is normally produced by cutting continuous fibers into the required lengths using well-known methods in the art.
The term "fibrids", as used herein, means nongranular, fibrous or film-like particles with at least one of their three dimensions being of minor magnitude relative to the largest dimension. These particles can be prepared by precipitation of a solution of polymeric material using a non- solvent under high shear. The f!brids have a largest dimension length in a range from 0,2 mm to 1 mm with a length-to-width aspect ratio of 5:1 to 10:1. The thickness dimension is on the order of a fraction of a micron, for example, 0,1 microtis to about 1.0 micron. The fibrids, before being dried, can be used wet and can be deposited as a paper forming component.
In the event a combination of floe and fibrid is employed for the meta-aramid, a preferred weight ratio of floe to fibrid is in a range from 0.5 to 4,0 and more preferably 0,8 to 2.0. The term "ceiiulosic pulp", means a fibrous ceiiu sic material prepared by chemical or mechanical separation of fibers from wood, fiber crops, or waste paper, Ge!Suiosic pulp fiber is a required constituent of the multilayer structure of this invention. Preferred ceiiulosic pulp is unbleached softwood pulp.
A further requirement is present in the first layer of the muitilayer structure namely the aramid is present in an amount of 18 to 75 weight percent and the cellulose is present in an amount of 25 to 84 weight percent, said percentages on the basis of the aramid and cellulose. An example of a more preferred range is 25 to 50 weight percent aramid and 50 to 75 weight percent cellulose.
The required second layer comprises ceiiulosic pulp fiber with the proviso that the second layer does not contain aramid. The muitilayer structure of the present invention is typically formed on conventional paper making machinery. Conventional additives may be used in the formation of the second !ayer although an additional additive is not necessary.
Examples of suitable additives include a polymeric binder such as polyvinyl alcohol, polyvinyl acetate, polyamide resin, epox resin, phenolic resin, poiyurea, poiyurethane, melamine formaldehyde, and poiyester.
Improved performance can be obtained if one or more additional layers are employed in combination wit the required first and second layers, it is desirable in the additional construction that the first layer be an exterior iayer. One other additional layer may have the composition of the first iayer (either with the same or different aramid ceilulose ratios) or the second ceitulosic layer without aramid can be identical or different from the second layer,
The multilayer structure of the present invention can be used as an electrically insulating paper and the structure is particularly suitable in manufacture of a transformer, namely a device that transfers electrical energy from one circuit to another through inductively coupled conductors (the transformers coils). Suitable transformers include large scale including transformers which have the capacity to handle at least 200 kVA and more generally at least 400 kVA. Such large scale transformers typically will contain an oil which is well known. Preferably, when the electrically insulating paper is wrapped around an electrical conducting coii of a transformer, the first aramsd containing layer of the insulating paper is located adjacent to the coil .- An absence of polymeric binder in the first aramid containing layer is desirable to prevent adhesion and to reduce thermal degradation of the multilayer structure during usage.
TEST METHODS
The following test methods were used in the Examples provided below.
Basis Weight was measured according to ASTM D 646 and ASTM D 645-M-96 and reported in g/m 2
Thickness was measured according to ASTM D 846-96 and reported in mm.
Tensile Strength was measured according to ASTM D 828-93 with
2.54 cm wide test specimens and a gage length of 18 cm and reported in Pa.
Aging Studies were conducted in accordance with ASTM D 2413. A singte temperature ceil aging system was placed in the conventional lab oven with nitrogen filled head space at 80°G for 340 to 2,720 hours. The oil impregnation was conducted by thoroughly drying test samples at temperature of 115±5 C C and an absolute pressure of 75 Pa (0.5 Torr) or less for at least 18 hours, followed by exposure for 8 hours or more at atmospheric pressure in oii for the paper to become completely
impregnated in the oil.
Canadian Standard Freeness was measured in accordance with ISO 5267/2 and TAPPI T227 and reported in ml.
Sehopper-Riegjler Freeness was measured in accordance with ISO 5267/1 and reported in ml.
Multilayer paper structures were prepared: using conventional paper making techniques. Comparative Exampie A
AA ttwwoo llaayyeerr ppaappeerr ssttrruuccttuurree wwaass mmaaddee ffrroomm ttwwoo ilaayyeerrss ooff 110000 wweeiigghhtt ppeerrcceenntt cceelliiuulloossiicc ppuull f fiibbeerr.. E Eaacchh llaayyeerr wwaass pprreeppaarreedd ffrroomm aan aaqquueeoouuss ddiissppeerrssiioonn ooff 33 gg ooff eceeilliuulloossiicc wwoooodd ppuullpp ((ssooftftwwoooodd)) ffrroomm C Ceeiiccoo 55 CCoommppaannyy ((CChhiillee)).. TThhee ppuu!ipp wwaass rreeffiinneedd ttoo 225500 mmll ooff CCaannaaddiiaann SSttaannddaarrdd FFrreeeenneessss,, TThhee aaqquueeoouuss ddiissppeerrssiioonn wwaass ppoouurreedd wwiitthh 88 lliitteerrss ooff wwaatteerr iinnttoo aa 2211 xx 2211 ccmm hhaannddsshheeeett m mooildd aanndd aa w weett--llaaiidd s shheeeett wwaass ffoorrmmeedd.. TTwwoo wweett-- llaaiidd sshheeeettss ooff cceelliiuulloossiicc ppuullpp ffiibbeerr wweerree ppllaacceedd ttooggeetthheerr bbeettwweeeenn ttwwoo ppiieecceess ooff bblloottttiinngg ppaappeerr,, hhaanndd ccoouucchheedd wwiitthh aa rrooiilliinngg ppiinn aanndd ddrriieedd iinn aa 1100 hhaannddsshheeeett ddrryyeerr aatt 115500°°CC f foorr 00 mmiinnuutteess.. TThhee ddrriieedd ttwwoo llaayyeerr sshheeeett wwaass ccaalleennddeerreed aatt 22880000 NN//ccrrnn lliinneeaarr pprreessssuurree bbeettwweeeen aa mmeettaall rroollll aanndd aa ssooftft rrooiill wwiitthh mmeettaall rroollll bbeeiinngg hheeaatteedd ttoo 8800°°CC.. PPrrooppeerrttiiees ooff tthh r reessuullttiinngg ttwwoo llaayyeerr ppaappeerr ssttrruuccttuurree aarree lliisstteedd iinn tthhee TTaabbllee..
Two layer paper structures in accordance with the presen invention were made from one aramid containing layer and one aramid free layer. The aramid: containing layer was prepared from an aqueous dispersion of a never dried slurry of poly(metaphenylene isophthalamsde) fibrids. The
20 meta-aramid fibrids were made in a manner generally described in U.S.
Pat. 3,758,908 and the fibrids had freeness of 330 mi Sehopper-Riegier, The aramid free layer was prepared with celiulosic wood pulp as described in Comparative Exampie A.
The aramid to celiulosic pulp fiber ratio was adjusted so that the
25 Iayers would have 20, 30, 40 and 50 weight percent aramid content. The aqueous dispersions were mixed together for 3 minutes in the British Puip Evaluation Apparatus and then poured with 8 liters of water into a 21 x 21 cm handsheet mold, and a wet-laid sheet was formed. The aramid free layer was prepared from an aqueous dispersio of the same celiulosic
30 wood pulp using the same equipment. The aramid containing wet-laid sheet and the aramid free wet-laid sheet were placed together between two pieces of blotting paper, hand couched with a roiling pin and dried in a handsheet dryer at 150°C. The dried two layered sheet was calendered
8 as described in Comparative Example A. Properties of ihe resulting two layer paper structures are listed in the Table,
Comparative . Exampie B
A two layer paper structure was made from two layers of 100 weight percent aramid content. The layers were made using ihe aramid containing layer method described in Examples 1-4 except 100% aramid content was used with no ce!iulosic pulp fiber. Properties of the resulting two layer paper structure are listed in the Table,
Comparative Example C
A single layer paper structure was made illustrating a typical prior art paper structure. The layers were mad using the aramid containing layer method described in Examples 1-4 except 70 weight percent ce!!uSosic wood pulp, 20 weight percent E!vanol© 70-82 polyvinyl alcohol) (available from the DuPont Company) as a polymeric binder, and 10 weight percent aramid floe was used as the aqueous dispersion. The meta-aramid floe was poly(metaphenylene isop thalamide) floe of linear density 0,22 tex and length of 0.64 cm (sold by the DuPont Company under the trade name NQ!V!EX©). Properties of the resulting two layer paper structure are listed in the Table,
Examples 4 and 5 below illustrate the preparation of multilayer structure of this invention in a continuous process using papermaking machinery.
EXAMPLE 5
A paper with 3~layer structure was formed on multicyfinder papermaking machine utilizing 4 forming cylinders,
The slurry, which fed first and fourth cylinders, consisted of water with aramids fibnds and cellulose pulp described in exampies 1-4, with 25 weight percent aramid fibrids and 75 weight percent of cellulose pulp in their total content. The slurry, which fed second and third cylinder, consisted of water and cellulose pulp only.
The solids from the slurries were deposited on the screens of the forming cylinders and from them were transferred on the moving felt, The wet multilayer paper sheet passed wet press and drying sections, and finally, a machine calender with metal roils heated to about 80 °C.
The produced pape had 3-iaye structure with the first layer being ara id-ceSluSose, the second layer 100% cellulose, and the third layer aramid - cellulose. Basis weight of each layer is 39, 78, and 39 g/m 2 accordingly. Total basis weight of the paper was 158 g/cm 3 , thickness 0.130 mm., and apparent density 0.85 g/cm 3 ,
EXAMPLE 6
A 2~layer structure was formed on the Fourdrinier papermaking machine equipped with primary and secondary headboxes.
The slurry, whic fed the primary headbox, consisted of water with cellulose puip described in examples 1-4.
The slurry, which fed the secondary headbox, consisted of water with cellulose pulp, meta-aramid fibrids and meta-aramid floe. The solid composition of this slurry was:
- Cellulose pulp: 50 weight percent;
- Meta-aramid fibrids; 30 weight percent;
- Meta-aramid floe: 20 weight percent.
Cellulose pulp, meta-aramid fibrids, and meta-aramid floe were the same as in examples 1 -4 with the exception that the floe length was 3 mm After the wire, a 2-!ayer sheet passed wet press and dryer section. The dried sheet was additionally calendered between two metal rolls heated to 180 °C.
The final 2-layer paper structure had basis weight of 130 g/m 2 . thickness 0.125 mm, and apparent densit 0.85 g/cm 3 . Its initial tensile strength in machine direction was 82 MPa and in cross direction - 43 MPa. TABLE
MULTILAYER PAPER STRUCTURES AND PROPERTIES
Example Total ' Cellulose Aramid Containing Basis Thickness Tensile Strength
Aramid Onl Layer Weight (mm) ( Pa) (% Retention)
Content Layer Cellulose Aramid (g/rrf) Unaged Aged Aged Aged Aged
(wt. %) Present (wt. %) (wt. %} 340 h 680 h 1360 h 2720 h
Q 0 072
A yes na * na * 64 S3 " 25 (477 ' 22 (42 T9 ' T36)
1 10 yes 60 20 66 0.075 43 27 (63) 26 (60) - 24 (56)
2 15 yes 70. 30 67 0.072 38 - 29 (78) 26 (68) 27 (71)
3 20 yes 60 40 68 0.081 49 32 (65) 30 (61) 29 (59) 27 (55)
4 26 yes 50 50 71 0.082 48 35 (73) - 32 (67) 28 (58)
100 no 0 100 91 0.083 46 45 (98) 43 (93) 46 (104) 36 (78)
10 no 70% cell tulose 65 0.131 39 25 (64) 21 (54) 18 (48) 15 (38)
20% polyvinyl
alcohol)
10% aramid
* na - not applicable
* * B is a two layer paper structure with two aramid containing layers and no cellulose only Iayer,
* ** C is a single layer paper structure with one aramid containing Iayer and no cetluio.se only layer.
The tensile strength data from the Table show that the multilayer paper structure comprising an aramid containing layer of Examples 1-4 of the invention has a higher percent tensile strength retention under accelerated thermal aging as compared to Comparative Exampie A containing no aramid content and Comparative Exampie C representing the prior art paper structure containing aramid and a polymeric binder in a single layer construction.
it is considered when the aramid containing layer of the multila er paper structure of the invention is placed in contact with a transformer coil the paper structure resists thermal: degradation and resists sticking problems due to the absence of polymeric binder material in the aramid containing layer.
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