Lyn I can't vouch for the accuracy of WIKI but it states that it is used for sizing see some extract and link below:
Construction materials
Methyl cellulose finds a major application as a performance additive in construction materials. It is added to mortar dry mixes to improve the mortar's properties such as workability, open and adjustment time, water retention, viscosity, adhesion to surfaces etc. Construction grade methyl cellulose is to not to be identified with food and pharmaceutical grade methyl cellulose and hydroxypropyl methyl cellulose, since it may be cross-linked with glyoxal for easy dispersion in water.
The construction materials can be cement based or gypsum based. Notable examples of dry mixture mortars which utilize methyl cellulose include: tile adhesives, EIFS, insulating plasters, hand-trowed and machine sprayed plaster, stucco, self-leveling flooring, extruded cement panels, skim coats, joint & crack fillers, and tile grouts. Typical usage is about 0.2% ~ 0.5% of total dry powder weight for dry mixture
Derivatives of methyl cellulose, which improve upon the performance characteristics, include hydroxypropyl methyl cellulose (HPMC) and hydroxyethyl methyl cellulose (HEMC). These derivatives typically improve the characteristics such as water retention, vertical surface slip-resistance, open time, etc.
[edit]Glue and binder
Methyl cellulose can be employed as a mild glue which can be washed away with water. This may be used in the fixation of delicate pieces of art as well as in book conservation to loosen and clean off old glue from spines and bookboards.
Methyl cellulose is the main ingredient in many wallpaper pastes. It is also used as a binder in pastel crayons and also as a binder in medications.
[edit]Paper and textile sizing
Methyl cellulose is used as sizing in the production of papers and textiles as it protects the fibers from absorbing water or oil.
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Methyl cellulose - Wikipedia, the free encyclopedia