If the washed white sheets appear macula, the use of oxalic acid treatment and failed to remove, can rule out the possibility of rust or iron ions. The use of bleaching water treatment also failed to remove, can be ruled out is the possible cause of macular pigment.
Then here, analyze one of the main factors that cause yellow spots after washing of white fabrics. Some fabrics such as white bed sheets, white shirts, white underwear, white tablecloths, etc. all contain certain cotton components (not included, but few). Some fabrics (except tablecloths) contain a large proportion of cotton, and these fabrics (except tablecloths) are more or less directly contacted with human skin during use, this will inevitably contain some skin secretions, skin secretions are mainly sweat gland secretions-sweat, and sweat 99% for water, evaporation residue is 1%, which contains a certain proportion of protein. The content of protein in sweat will be different due to the human body, food structure, season, etc. When these proteins remain on white clothes or sheets, dry or wet state does not show any color, which can be considered as a "hidden state". But when it encounters a certain amount of heat, it is easy to solidify and firmly fixed on the substrate, and make it yellow.
The use of bleach, oxalic acid and the like in the treatment is extremely unsatisfactory. This yellowing phenomenon occurs mostly on white or light-colored fabrics containing cotton, and there is no way to save the yellowing to restore the original color of the fabric. Enzymes have a decomposing effect on proteins, but the treatment of proteins that have solidified and are "closely" bound to the fabric is not as effective. The best way to reduce yellowing of white fabrics is to take effective measures in the washing process. For example, when washing white sheets, the main washing is generally 60-70 ℃, and when heating, do not quickly heat to the preset temperature. It can be carried out separately. First, add washing powder when the water temperature is 40 ℃, and then gradually heat to the preset temperature after a period of time. If it is a white shirt or underwear can be treated with enzyme-added washing powder, or it can be treated with 5% salt water before washing, and then washed with detergent. And in the tablecloth for food dirt, will also be rich in protein, in the treatment can also refer to the bed sheet processing method.