Phthalide compounds, which are a type of colorless dye (color former), develop color via a ring-opening reaction when interacted with acids (developer), as shown in the following scheme. Because this reaction is reversible, decoloration occurs in a reversible manner under basic conditions.

Pressure and heat sensitive recording systems utilize this color developing mechanism in which color formers, developers and sensitizers are intimately incorporated. In pressure sensitive recording, the coloration occurs as follows: Capsules containing color former are destroyed by pressure and the color former makes contact with the developer coated support material. In heat sensitive recording, both color former and developer, which are dispersed on the support material, are thermally melted, thus contacting each other to initiate the coloring reaction. Furthermore, a low melting point sensitizer is also added to achieve a high sensitivity. Recently, the technique has been applied to re-writable recording paper, which can be used for repeated writing and erasing. This technique has been applied to print on commuter passes or prepaid cards. By combining acid-generating methods with the coloring mechanism, new applications can be expected for these materials.

Color Formers

Developers

Sensitizers

Color Formers

B0207	B2628	C1946
C0741	D3204	D3203
D3202	D3205	D3201

B0207	Benzoyl Leuco Methylene Blue
B2628	3',6'-Bis(diethylamino)-2-(4-nitrophenyl)spiro[isoindole-1,9'-xanthene]-3-one
C1946	2'-(2-Chloroanilino)-6'-(dibutylamino)fluoran
C0741	Crystal Violet Lactone
D3204	2'-(Dibenzylamino)-6'-(diethylamino)fluoran
D3203	6'-(Diethylamino)-1',2'-benzofluoran
D3202	6'-(Diethylamino)-1',3'-dimethylfluoran
D3205	3',6'-Dimethoxyfluoran
D3201	3-(1,2-Dimethyl-3-indolyl)-3-[4-(diethylamino)-2-methylphenyl]phthalide

Developers

T1428	B1563	H0209
B0464	B0813	B2748
B2752	B2749	B2750
B2751	B2753	B1567
B1568	B1715	B0494
B2664	D1356	B0495
B0735	D2121	B0819
D3267	E0664	E0432
H1166	M1684	M1410
H0216	N0026	N0027
P0201	R0008	T0032
T0144	T2259	T1254

T1428	alpha,alpha,alpha'-Tris(4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene
B1563	alpha,alpha'-Bis(4-hydroxyphenyl)-1,4-diisopropylbenzene
H0209	Benzyl 4-Hydroxybenzoate
B0464	4,4'-Biphenol
B0813	2,2'-Biphenol
B2748	2,2-Bis(3-sec-butyl-4-hydroxyphenyl)propane
B2752	1,1-Bis(3-cyclohexyl-4-hydroxyphenyl)cyclohexane
B2749	2,2-Bis(3-cyclohexyl-4-hydroxyphenyl)propane
B2750	2,2-Bis(2-hydroxy-5-biphenylyl)propane
B2751	2,2-Bis(4-hydroxy-3-isopropylphenyl)propane
B2753	1,1-Bis(4-hydroxy-3-methylphenyl)cyclohexane
B1567	2,2-Bis(4-hydroxy-3-methylphenyl)propane
B1568	1,1-Bis(4-hydroxyphenyl)cyclohexane
B1715	9,9-Bis(4-hydroxyphenyl)fluorene
B0494	2,2-Bis(4-hydroxyphenyl)propane
B2664	1,3-Bis[2-(4-hydroxyphenyl)-2-propyl]benzene
D1356	Bis(4-hydroxyphenyl) Sulfide
B0495	Bis(4-hydroxyphenyl) Sulfone
B0735	4-tert-Butylphenol Zone Refined (number of passes:19)
D2121	4,4'-Dihydroxydiphenyl Ether
B0819	4,4'-Dihydroxydiphenylmethane
D3267	4,4'-(1,3-Dimethylbutylidene)diphenol
E0664	4,4'-(2-Ethylhexylidene)diphenol
E0432	4,4'-Ethylidenebisphenol
H1166	4,4'-(2-Hydroxybenzylidene)bis(2,3,6-trimethylphenol)
M1684	4,4'-Methylenebis(2-methylphenol)
M1410	1,1'-Methylenedi-2-naphthol
H0216	Methyl 4-Hydroxybenzoate
N0026	1-Naphthol
N0027	2-Naphthol
P0201	4-Phenylphenol
R0008	Resorcinol
T0032	Tetrabromobisphenol A
T0144	4-(1,1,3,3-Tetramethylbutyl)phenol
T2259	2,3,4-Trihydroxydiphenylmethane
T1254	1,1,1-Tris(4-hydroxyphenyl)ethane

Sensitizers

B1248	B1519	B1619
D0159	D0001	D1598
T0015	D2017	C0043
D0910	P0231	D0866
F0016	F0017	H0067
C0024	L0011	H0283
M0476	O0238	O0107
P0002	H0797	S0075
S0314	T0018

B1248	Behenic Acid
B1519	4-Benzylbiphenyl
B1619	Benzyl 2-Naphthyl Ether
D0159	1,4-Dibenzyloxybenzene
D0001	Dibenzyl Sulfoxide
D1598	2,6-Diisopropylnaphthalene
T0015	Dimethyl Terephthalate
D2017	1,4-Diphenoxybenzene
C0043	Diphenyl Carbonate
D0910	1,3-Diphenyl-1,3-propanedione
P0231	Diphenyl Sulfone
D0866	Ethylene Glycol Diphenyl Ether
F0016	Fluoranthene
F0017	Fluorene
H0067	Hexadecanamide
C0024	Hexanamide
L0011	Lauric Acid
H0283	Methyl 3-Hydroxy-2-naphthoate
M0476	Myristic Acid
O0238	n-Octanamide
O0107	Oleamide
P0002	Palmitic Acid
H0797	Phenyl 1-Hydroxy-2-naphthoate
S0075	Stearamide
S0314	Stearanilide
T0018	m-Terphenyl