Abstract:
In a method for prime coating fiberboard products the improvement wherein a foaming agent is added to a conventional aqueous latex based prime coating and the coating aerated to give a frothed prime coating having a consistency of 500 to 750 grams per liter, the frothed coating applied to the board product at a rate of 220 to 320 grams per square meter, and heated to collapse the froth and dry the coating.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improved method for finish coating fiberboard products such as mineral wool and wood acoustical ceiling tile. 
     2. Description of the Prior Art 
     Mineral wool and wood fiber ceiling tile products are ordinarily formed from slurries of the board forming ingredients using conventional Fourdrinier and Oliver board forming equipment. After drying and cutting to size the board products are sanded and then painted by first prime coating the board and then finish coating the board. 
     SUMMARY OF THE INVENTION 
     Because of the porous nature of the board, conventional prime coats bleed into the board product requiring both large amounts of prime coat to give the required hiding power and high energy usage for drying. I have found that I can froth a slightly modified standard prime coat formulation to form a prime coat having a consistency similar to shaving cream. When this prime coat is utilized in finishing board products, I can get equivalent or better hiding power at considerably lower usage rates and can conserve a considerable amount of energy in the finishing operation. 
     DETAILED DESCRIPTION 
     Conventional prime coats used in finishing fiberboard products are water base paints conventionally containing clay, thickeners, wetting agents, aqueous latex binders and plasticizers, along with defoamers. By simply deleting the defoamers from these conventional formulations and adding a foaming agent, aerating the paint to form a froth coating having a consistency of 500 to 750 grams per liter, fiberboard products can now be primed at a much lower usage rate of coating for equivalent hiding power and also require less energy to properly dry the prime coated product. The following example illustrates a preferred embodiment of my invention. 
    
    
     EXAMPLE 
     
         ______________________________________           Standard    Froth           Prime Coat  Prime CoatIngredients     Parts by Weight                       Parts by Weight______________________________________Water           12.78       12.49Klondyke Clay Slurry           59.64       58.29(70% Solids)Sodium alginate thickener           0.088       0.086(Keltex)Aryl alkyl sulfonate           0.052       0.051surfactant(Polywet ND-2, 25% Solids)Nuclay Slurry   20.31       19.86(70% Solids)Plasticizer (35% Dibutyl           0.208       0.204Phthalate, 65% diisobutyrateester of neopentyl glycol)Eastman 135Polyvinyl acetate latex.sup.(1)           6.64        6.49(58% Solids-Wallpol 40-311)Formaldehyde (37%)           0.08        0.078Ammonia         0.104       0.102Sodium Lauryl Sulfonate           --          0.745Surfactant(Tannawet LS)Foam Aid. No. 1 --          1.49Sodium SuccinateDefoamer        0.048(NDW-Napco Chemical Co.)______________________________________ .sup.(1) Viscosity (CPS) 1500  Tg 33 
    
     The modified prime coat above containing the foaming agent and foaming aid was frothed using a conventional Oaks foamer to produce a frothed prime coating having a consistency of 625 grams per liter. Actually this coating had a consistency similar to shaving cream. 
     Generally, the preferred latex paints of this invention comprise, on a solids basis, 49 to 61 parts by weight clay; 3 to 5 parts by weight latex, preferably polyvinyl acetate latex commercially available such as Wallpole 40-311, Polyco 2113, MCar 130 and Duroset SB321; and from 0.5 to 1.5 parts by weight surfactant. Other preferred surfactants include N-Tallow Sulfo Succinimate and Ether Sulfo Succinimate. 
     Wood fiberboard products prepared from water laid webs, which were then dried and sanded to give a smooth outer surface, were primed and then finish coated using conventional finishing procedures and the froth prime coated products were then compared to equivalent wood fiberboard products which has been primed and finished with the standard paints. The composition of the boards was as follows: 
     
         ______________________________________Ingredient         Parts by Weight______________________________________Wax                0.2Starch Binder      6.0Newsprint (pulped) 8.0Broke (Ground Board Scrap)              36.0Woodpulp           49.8______________________________________ 
    
     A standard board finishing line containing a roll coater was utilized for applying the prime coat, an oven for drying the prime coat, another roll coater for applying the finishing coat and a further oven for drying the finishing coat. Sanded board products of the above composition were prime coated with both the standard formulation and with the frothed formulation set forth above. The standard prime coat was applied at a rate of 400 grams per square meter and the frothed prime coat was applied at a rate of 260 grams per square meter. The primed boards were dried at temperatures of respectively 525° F. for the standard and 300° F. for the froth. The lower temperature required for collapsing and drying the froth prime coat resulted in a savings of approximately 40% of the energy required for drying the conventional prime coat, i.e. 40% of the burners were turned off. 
     A finish coat was applied at a rate of 300 grams per square meter using the standard prime coated board whereas, with the board which has been primed with the froth coating, a 32 gram reduction was achieved (268 grams per square meter). 
     
         ______________________________________Finish Coat FormulationIngredient         Parts by Weight______________________________________Water              48.525Sodium Alginate Thickener              0.194(Keltex)Tetrasodium Pyrophosphate              0.209TiO.sub.2 (pigment grade)              4.109Talc               8.217Calcium Carbonate  27.639Polyvinyl Acetate Latex              10.652Wallpole 40-31(58% solids)Plasticizer (35% Dibutyl              0.336Phthalate - 65% diisobutyrateester of neopentyl glycol)Defoamer           0.0598(NDW-Napco Chemical Co.)______________________________________ 
    
     Tile produced in accordance with the above were subjected to accelerating aging exposures in hot, wet and hot, dry rooms. They were then tested for color stability, adhesion, scrub resistance, surface durability, blocking and warp. These properties were reexamined after two and four week exposures in the hot, wet room and hot, dry room. Results indicated comparable performance between those tile prepared using the standard prime coat and those tile prepared using the frothed prime coat. It was noted that there was no detectable migration of the foaming agent. The only noticeable difference in these coatings resulted in the fact that the foamed or frothed prime coat did not strike into the board surface as far as the conventional prime coat. Thus the very upper fraction of the board is not reinforced by the coating. However, evaluations of surface properties of the coating indicated comparable toughness.