Patent GB-1233569-A (GB)
IPC: G01N19/10
Filed: 1968-09-18
Abstract: 1,233,569. Measuring humidity. JOHNSON SERVICE CO. Sept.18, 1968 [Sept.27, 1967], No.44421/68. Heading G1S. [Also in Division B2| A synthetic humidity sensing element comprises a first layer of the reaction product of a compound containing glucoside chains and a stabilizing monomer or partial polymer capable of crosslinking with the hydroxyl group of the glucoside chains, and a second layer extending over a substantial portion of the surface of the first layer and bonded thereto, the second layer being a non-cross-linked compound containing glucoside chains, or a partially cross-linked reaction product of a compound containing glucoside chains and a stabilizing monomer or partial polymer capable of cross-linking with the hydroxyl groups of the glucoside chains. The cross-linked first layer prevents creep of the element, and the second layer provides toughness for the element. The first layer may be the reaction product of cellulose, cellulose esters or ethers with monomers or partial polymers or urea-formaldehyde, phenolformaldehyde, melamine-formaldehyde, triazineformaldehyde, hexamethoxymethyl melamine, glyoxal, 2-hydroxy-adipaldehyde. A catalyst and a catalyst stabilizer may be added to the reaction mixture. The second layer may be formed of non-cross-linked cellulose, cellulose ester or ether or a partially cross-linked product utilizing the stabilizing monomers similar to those described in the formation of the first layer. The second layer can be applied by coating the first layer with a solvent solution of the reactants and subsequently evaporating the solvents and heating the laminated structure to achieve the crosslinking reaction; alternatively the second layer can be bonded to the fully cross-linked first layer by use of an adhesive. The sensitivity of the element can be increased by hydrolyzing the outer surface of the cellulosic second layer to regenerated cellulose by an alkaline or an acid medium. In an Example an un-cross-linkable solution comprising 10% cellulose acetate butyrate and 90% ethyl acetate was cast on to a clean glass plate with an adjustable strike-off bar. After evaporation of the solvent, a cross-linkable solution comprising 6À2% cellulose acetate butyrate, 6À18% urea-formaldehyde, 1À6% triethyl amine, 0À15% para-toluene sulphonic acid, 2À21% n-butyl alcohol, 2À06% ethyl alcohol, 40À6% diacetone alcohol, 41% ethyl acetate was then cast on the dried layer by means of the strike-off bar. Following the evaporation of solvent an outer layer of the first solution was cast on the dried layers and solvent evaporated to provide a 3-layer laminate with each outer layer having a thickness of 0À3 mils and the core having a thickness of about 0À4 mils. The composite structure while on the glass plate was heated to 350 F. for 12 minutes and the resulting laminate cut into strips which were employed as humidity sensing elements showing high sensitivity to moisture, excellent resistance to creep when subjected to stress and resistance to chemical attack. Various constructions of the element are described. In one embodiment as shown in Fig.2 the ends of a looped element 4 are disposed around rollers or supports 5 and mounting brackets 6 are attached to the rollers and serve to connect the rollers to a supporting structure. The looped element is subjected to tensile stress by providing a spring-load connection between one of the brackets and the supporting structure. In a second embodiment the element is in the form of a strip, the ends of the elements being secured to clamps which are connected to a supporting structure. A mechanical type of humidity sensing device, as shown in Fig. 4, uses the variation in the length of the element 7 to rotate a gauge needle 10 and an electrical type utilizing the element is also described Fig. 5 (not shown).