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Copolymers of NVP and esters
Esters are the most studied class of compounds for PVP modification. Generally speaking, ester compounds copolymerized with NVP refer to esters containing unsaturated double-bond vinyl groups. After copolymerization of NVP and esters, on the one hand, due to the good molecular flexibility of the ester group, the chain segment activity is increased, thereby improving the flexibility of the copolymer film. On the other hand, the presence of hydrophobic alkyl groups in the ester units will reduce the water absorption capacity of the copolymer, thereby improving the copolymer's ability to maintain its shape under high humidity. Usually, esters include dimethylaminoethyl propylene formate, propylene acetate, methacrylate, propylene diacetate, methacrylate, etc.
N-vinyl pyrrolidone (NVP)/vinyl acetate (VAc) copolymer (VAP resin) is the earliest improved product of PVP and ester compounds. It is a colorless and transparent solid, slightly soluble in water, soluble in organic solvents such as isopropanol, ethanol, butyl acetate, etc., non-toxic and not easy to burn. VAP resin retains the function of PVP as a polymer, while overcoming the disadvantage of PVP's high price. In addition, VAP resin has good film-forming properties. Since it introduces hydrophobic vinyl acetate units into its molecular structure, it reduces the film's sensitivity to humidity, so it will not become sticky in a high humidity environment and prolongs the ability to maintain the hairstyle. In 1985, due to the ban on Freon, propane/butane became a new propellant, and the solubility of PVP/VA copolymer in propane/butane was only 10% (3% solid ethanol solution), so BASF in Germany launched PVP/VA/vinyl propionate copolymer. The increase in the proportion of vinyl propionate in the copolymer makes the hydrocarbon solubility of the copolymer reach about 40%. In general, VP:VA:vinyl propionate = 30:40:30 in the copolymer.The content of VP in the composition is reduced, which makes it have a higher curl retention.
Copolymers of NVP and acids
Methacrylic acid is currently the most commonly used acid for copolymerization with NVP. Methacrylic acid needs to be neutralized when used, and the neutralization degree is preferably 80-100%. Since NVP is easily hydrolyzed under acidic conditions, and the acidity and alkalinity also have a great influence on the stability of PVP, the pH value must be strictly controlled during copolymerization. A certain amount of KOH can be added before polymerization to ensure alkaline conditions for polymerization. The polymer obtained by copolymerization of NVP and acidic substances is anionic, has good gloss, elasticity and durability, is easy to wash off, has good moisture resistance and shaping ability, and is not easy to absorb moisture. For example, the most popular film-forming polymer PVP/tert-butyl acrylate/methacrylic acid copolymer, whose product is a 50% ethanol solution, has excellent performance and its advantages are:
(1) The compatibility with propane/butane is as high as 70% or more;
(2) The elasticity of the film is very good, and no plasticizer needs to be added;
(3) The degree of neutralization does not affect the hair-fixing performance and is easy to wash off;
(4) Good gloss and not easy to peel off;
(5) Strong curl retention. In humid climates, the hair has good elasticity and shape retention and will not be sticky.
Copolymers formed by NVP and quaternary ammonium salts
Olenic substances containing tertiary amine groups in the molecules (such as dimethylaminoethyl methacrylate methyl dimethylamine ethyl ester DMAEMA) can be quaternized and then copolymerized with NVP to form polyquaternary ammonium salts containing cationic units in the molecular chain. At normal pH values, the surface of hair fibers has anionic characteristics. The use of cations helps to deposit and adsorb, improves the directness and affinity to hair, and has a certain conditioning effect. Polyquaternary ammonium compounds provide the typical long-lasting binding, workability and strength that are unique to mousse hair foams. Such copolymers are widely used in hair conditioning rinses, shampoos and lotions, as well as general styling mousse products. They make wet hair easy to comb without tangling, and when the hair dries, it is naturally fluffy, shiny, and curly, just like the hairstyle when wet hair.
The use of PVP-based polyquaternary ammonium compounds was the key factor in the sudden success of mousse hair foams in the early 1980s. According to the report of the Cosmetic, Toiletries and Fragrance Association (CTFA), polyquaternium-11, polyquaternium-16, polyquaternium-28, polyquaternium-55, etc., which are used in hair styling agents, are all copolymers of NVP and quaternary ammonium salts. For example, polyquaternium-16 represents four polymers: PVP/DMAEMA ammonium chloride salt, PVP/quaternized vinyl imidazole, PVP/DMAEMA ethyl sulfate salt, etc., which have very good combing and styling capabilities.
Copolymers formed by NVP and amides
NVP itself belongs to the lactam class of substances, and its properties are similar to those of amides, and it has good compatibility. Choosing amides with more methyl groups as comonomers can reduce the water absorption of PVP while maintaining its original performance. Adjusting the ratio of NVP to amides in the copolymer, or choosing different ones, can change the solubility, viscosity and other performance properties of the copolymer resin. The solubility, viscosity and other performance properties of the copolymer resin can also be adjusted by adjusting the composition ratio of NVP to comonomers in the resin, such as vinyl caprolactam/VP/dimethylaminoethyl methacrylate copolymer, in which the monomer vinyl caprolactam has two more methyl groups than PVP, and has excellent compatibility with hydrocarbon propellants. Its use can also reduce the hygroscopicity of the polymer. Another monomer, dimethylaminoethyl methacrylate, can increase the curl retention ability of the polymer on the hair, which can reduce the amount of polymer used. The components VP and dimethylaminoethyl methacrylate in the copolymer are water-soluble and have good affinity, while vinyl caprolactam has excellent film-forming and hair-fixing properties, so that the amount of film-forming polymer used can be reduced, while its hair curl retention is still very good. The compatibility of this copolymer with propylene/butane is as high as 70%. Once the hair is treated with products formulated with it, it will show "wet" and shiny curls once it is completely dry. The solubility range of this terpolymer is extremely wide and can be used in all types of hair styling agents. It combines styling and conditioning in one, and is a new type of film-forming polymer with great development prospects.
Summary
Since PVP replaced natural shellac as a styling resin, the development trend and current status of PVP styling resin products are roughly as follows:
(1) From a single PVP homopolymer to a high-performance PVP modified copolymer formed by copolymerization of multiple monomers
(2) From a single styling function requirement to a styling polymer with multi-functional requirements such as conditioning and UV resistance.
(3) Due to environmental protection needs, the content of volatile organic compounds (VOC) in sizing resin propellants is limited. Therefore, when designing and synthesizing PVP series sizing resins, how to improve their compatibility (hydrocarbon solubility and water solubility) and compatibility has become an indispensable consideration and a research hotspot.