CMC vs CMS: Carboxymethyl Cellulose or Carboxymethyl Starch — Cost, Performance and Where Each Wins

CMC and CMS are made by the same carboxymethylation chemistry on different polysaccharides: CMC on cellulose and CMS on starch. CMS is usually the lower-cost option per kilogram and performs well where moderate viscosity, film formation and water retention are enough, such as paper surface sizing, textile sizing and fluid-loss control in moderate-temperature muds. CMC wins where you need high viscosity at a low dose, high DS or purity, clarity, salt tolerance or long-term solution stability.

Chemistry: same reaction, different backbone

Both products start from a glucose polymer that is alkalised with sodium hydroxide and etherified with monochloroacetic acid, and both carry sodium chloride and sodium glycolate as by-products unless these are washed out. The difference lies in the backbone. Cellulose is a linear chain of β-1,4-linked glucose units with a high degree of polymerisation, so its derivatives build high viscosity. Starch, from maize, cassava, potato or wheat, consists of mostly linear amylose and highly branched amylopectin with α-1,4 and α-1,6 links; branched molecules occupy less hydrodynamic volume, so CMS thickens less per gram.

Sentai's CMS grades have a DS of 0.30–0.85 and purity of ≥ 80 %, with a solution pH of 8–12 (7–11 for the paper grade) and 60–80 mesh fineness. Its CMC grades span DS 0.4 to ≥ 1.2 and purity from 55 % for technical grades to ≥ 99.5 % for refined grades.

Performance compared

Stability and temperature

Starch's α-glycosidic links are the natural target of amylase enzymes, which many microbes produce, so CMS solutions, sizing liquors and muds lose viscosity faster if they are stored or recirculated without a preservative. In drilling fluids, starch-based fluid-loss additives are typically limited to around 120 °C (250 °F), whereas PAC and CMC are typically used up to around 120–150 °C. Starch derivatives handle salt well, however, and have long been used in salt-saturated muds.

Clarity, purity and residues

Refined CMC produces clear, smooth solutions with very little salt. CMS is typically supplied at lower purity and gives hazier solutions, which does not matter in paper, muds or sizing but rules it out for clear beverages, battery slurries and other high-purity applications.

Cost logic: price per kilogram versus cost in use

Starch is a cheaper raw material than refined cellulose, so CMS is usually cheaper per kilogram. What matters, though, is the cost of reaching the performance target. A useful formula is: cost in use = price per kilogram × dose required to reach the target. As an illustration only, if a CMS costs 60 % as much per kilogram as a CMC but needs 1.4 times the dose for the same water retention, its cost in use is 84 % of the CMC's, which is still a saving but a much smaller one than the price list suggests.

Hidden costs belong in the calculation too: preservatives, longer dissolution, larger volumes to store and dose, and the risk of an out-of-specification batch. Customs classification also differs, with CMC under HS 3912.31 and CMS under HS 3505.10, so check the duty rate that applies in your market.

Where each wins

Blending and partial substitution

The two chemistries are compatible, and the most economical answer is often a blend. Replace CMC with CMS in steps, for example a quarter and then a half, and at each step measure the property that matters: water retention and runnability in coating colours, API filtrate in muds, or weaving efficiency in sizing. Stop when the property or the process window starts to move.

How Sentai can help

Sentai makes both chemistries on one site in Xiangtan: 10,000 t/yr of CMC and PAC, and a dedicated 5,000 t/yr CMS line producing CMS-H, CMS-M, CMS-L and the paper grade CMS-P. For paper mills, the paper-grade CMC PC-Series covers 7–600 mPa·s (2 % solution) alongside CMS-P, so the applications team can propose a CMC, a CMS or a blend and compare them in the same trial. Free 100–300 g samples are available.