Comparing Glacial Acrylic Acid and Glacial Methacrylic Acid

Glacial acrylic acid (GAA) and glacial methacrylic acid (GMAA) are polymerizable, unsaturated, monocarboxylic acid monomers.  Virtually all acrylic copolymers produced commercially contain either of these two monomers at some level, depending on the specific attributes desired in the copolymers.

Low levels (≤ 5%) of incorporation in copolymers enhance latex stability, improve adhesive properties and wettability, or allow post-crosslinking reactions.  Copolymers with higher levels of these two acidic monomers are used in applications like dispersants, sizing agents and paper wet strength improvement, cleaners, and water treatment chemicals.  At high acidic monomer levels, copolymers can be water-soluble or converted to water-soluble polymers as their alkali or ammonium salts. Applications include thickeners, viscosity modifiers, and fluid loss control agents in cements. Homopolymers are water-soluble or can be converted to high water-absorbing hydrogels through crosslinking. GAA is the monomer of choice when high acid levels are employed.

Formulators use both GAA and GMAA  in copolymers to improve performance in adhesives, caulks, sealants, paint & coatings, varnishes, printing inks, and textile finishes.  At higher acid monomer levels, formulators use GAA and GMAA to produce copolymers for applications like thickeners, dispersants, wetting agents, and flocculants.

The table below displays a comparison of the general physical properties of GAA and GMAA.

GAA, which is lower in cost than GMAA,  is the dominant unsaturated acid used in the acrylic polymers market.  The global acrylic acid market size was about 7,300,000 metric tons in 2018, with acrylate esters production accounting for almost 55 percent of this volume.  The GAA market is forecasted to grow at a CAGR of ~ 5.0 percent. In polymer applications, consumption in super absorbent polymers (SAPs), in water-based paints & coatings, and in oil field water-soluble polymer applications are major growth drivers.  The GMAA market volume is less than 10 percent of the GAA volume, reflecting the high use of GAA in SAPs and higher production volumes of acrylate monomers vs. methacrylate monomers. The CAGR growth rate for GMAA is about 2 percent.

The discussion that follows address considerations in selecting GAA or GMAA in the design and formulation of copolymers.

Copolymerization Reactivity Ratios for GAA and GMAA

Monomer reactivity ratios (r₁ and r₂) in free-radical polymerizations are important parameters in designing the compositional architecture of copolymers.  The relative reactivity of monomer pairs controls their copolymer compositional sequences, from random to alternating sequences and to blocky structures.  Monomer feed ratios also affect the compositional distributions in a copolymer.

The reactivity ratio for a pair of monomers is the reaction rate constant for addition of the first monomer to itself versus addition to the second monomer.  Accordingly, r₁ = k₁₁/k₁₂ and r₂ = k₂₂/k₂₁ wherein k₁₁ is the rate constant for propagation with the same monomer and k₁₂ is the rate constant for propagation of the first monomer with the second monomer.

The following monomer reactivity ratios for GAA and GMAA with styrene and MMA have been determined for copolymerizations at low conversion levels (< 3%) in benzene at 65°C.

Acrylic Acid (M₁)

M₂  Styrene:           r₁ = 0.075 and r₂= 0.29

M₂  MMA                 r₁ = 0.29 and r₂= 0.1  

M₂  AAM                  r₁ = 0.23 and r₂= 1.33

Methacrylic Acid (M₁)

M₂  Styrene:           r₁ = 0.37 and r₂= 0.15

M₂  MMA                 r₁ = 0.96 and r₂= 0.35  

The reactivity ratios show that both GAA and GMAA have a high propensity for propagation through addition of the second monomer.  That is, the rate constant k₁₁ is lower than the rate constant k₁₂ for the addition of the comonomer.

As might be expected, the reactivity ratios for GAA and GMAA are affected by pH in water-borne systems.  Their reactivity ratios decrease with increasing pH, owing to formation of the carboxylate anion. This decrease in reactivity for k₁₁ is mainly due to charge repulsion of the growing polymer chain with the acrylate ion. 

In another interesting study, r₁ of GAA showed a unique pH affect. GAA and N-vinylpyrrolidone (NVP) were copolymerized in an aqueous system at 30°C and the monomer reactivity ratios were determined as a function of pH at high conversions. The value of r₁ decreased from 5.2 at pH 4 to a minimum of 1.3 at pH 5 and then increased to 8.1, 6.6, and 7.2 at pH 7, 8, and 9, respectively. Addition of 1M NaCl at pH 6.5 restored the r₁ nearly to that at pH 4, the kinetics for the predominantly un-ionized acid. This analysis is complicated by the fact that at low pH values the NMP would be present in a protonated state which would cause a decrease in reactivity k₂₂ due to charge repulsion. Accordingly, the effect of pH on the reactivity ratios for GAA and GMAA pairs must be taken into consideration.  This is very relevant since copolymers of GAA with acrylamide and NMP have large uses in the oilfield and mining operations.  

Published summaries of the reactivity ratios for a large cross-section of monomer pairs are readily available from many literature sources. See for example:  https://onlinelibrary.wiley.com/doi/abs/10.1002/masy.19870100118

Crosslinking of Carboxylic Acid-Modified Acrylic Resins

Formulators can achieve an increase in durability, moisture resistance, and chemical resistance by crosslinking all or a portion of the available carboxylic acid sites in copolymers modified with GAA or GMAA at levels < 5 percent.  A delineation of crosslinking agents for the carboxylic acid-modified resins is provided below. Melamine-formaldehyde resins are particularly effective in crosslinking via the pendant carboxylic moieties. Many crosslinking agents are suitable for use in aqueous environments.  The final basis for the selection of the cross-linking agent would be cost considerations, processing parameters, and substrates.

• Melamine-formaldehyde crosslinking agents, e.g. Cymel® types

• Urea-formaldehyde crosslinking agents

• Phenol-formaldehyde crosslinking resins

• Glycidyl type epoxy resins and TGIC

• Multi-valent metal complexes like zinc, aluminum or zirconium

• Polyamines and aziridines

• Polycarbodiimides

• Multifunctional isocyanates

• Organofunctional silanes coupling agents 

Organofunctional silanes that can be used to modify GAA and GMAA copolymers include the aminopropyl silanes like A-1100 and glycidyl type silanes like A-187.  These organofunctional coupling agents serve as a strong bridge between the carboxylic modified acrylic resin and various substrates.

Major Points of Differentiation between GAA and GMAA in Copolymer Systems

The tables above showed that the many similarities between GAA and GMAA.  The major differentiators between the two acidic monomers are associated with the lower price point for GAA, the copolymerization kinetics and the lower contribution of GAA to the copolymer Tg vs. GMAA.  The table below summarizes some of the differences to consider when selecting an unsaturated carboxylic acid monomer for copolymerizations.

We can estimate the ultimate Tg of a copolymer from the homopolymer Tg of each of the component monomers.  Thus, the lower Tg of GAA would favor flexible and soft or tacky copolymers; a higher Tg would be useful in hard polymer compositions.

The pKa value is a measure of the strength and dissociation of an acid.  The lower the pKa value, the higher the acidity.

An insightful analysis of the kinetics of GAA and GMAA polymerization has been carried out in the following reference: https://d-nb.info/1046446487/34 . The analysts reached the following conclusions:

• Backbiting reactions (1,5-position intramolecular radical transfer to the polymer chain) were observed with AA, but not with MAA.  Backbiting creates a more stable radical, reduces the propagation rate, lowers the polymer chain length, and creates short-chain branches.

• The Norrish-Trommsdorff effect is weaker for AA polymerization than for MAA polymerizations.  The Trommsdorff “gel effect” causes an auto-acceleration in the propagation rate at high monomer conversions.

• As the pH increased, they saw a reduction in the termination rate  for both monomers. As the pH is raised with copolymers containing GAA or GMMA, electrostatic repulsive forces operate between the charged carboxyl moieties, which increases hydration of the copolymers.  Swelling and related effects are greater with the GAA copolymers because the hydrophobic methyl group affects hydration of the GMAA copolymers.
  
  

Handling of GAA and GMAA 

GAA and GMMA will readily self-polymerize if not properly inhibited, stored, and handled.  Polymerization can be rapid and violent, generating large amounts of heat and pressure. Note that it is important to store these stabilized monomers under an air atmosphere and to replenish oxygen over time, in order for inhibitors like MEHQ to effectively function.

GAA and GMAA also require special attention to avoid freezing, because the crystallized monomers exclude the inhibitor and the solid will contain a deficiency of inhibitor and oxygen.  During thawing, processors must mix the contents to redistribute the inhibitor and resupply dissolved oxygen They must limit temperatures to 35-45 °C.

An additional complication with GAA is that it will slowly dimerize in storage to form diacrylic acid.  While this dimer formation has a slow rate and is not hazardous, diacrylic acid at high concentrations will affect the polymerization of GAA by interfering with the free-radical polymerization process.  For this reason, the recommended upper storage temperature for GAA is 25 °C.  GMAA, to the contrary, does not form this thermal dimer.

An intercompany committee prepared excellent reference manuals on the safe handling and storage of inhibited GA and GMAA.

Sales Specifications for GAA and GMAA

Gantrade sells GAA and GMAA in 250 lb./204 Kg. drums and GAA in 44,000 lb./20 MT road tank trucks.  The following table charts our sales specifications.

Contact Gantrade to Discuss Your Acrylic Monomer

To understand which unsaturated carboxylic acid monomer best suits your needs, partner with Gantrade.  Our wealth of technical knowledge and expertise, combined with our broad line of acrylate monomers, can guide you to tailoring the best solutions for your applications.  Gantrade’s global procurement and logistics teams can help to create value in your sourcing requirements. Contact Gantrade today.