Method of Preparation of Carbon Transfer Inks (1965)
U.S. Patent No. 3,214,282, granted on October 26, 1965, to Hansel L. McGee and Paul M. Schwartz (assigned to International Business Machines Corporation), addresses a critical supply and consistency challenge in office technology: the formulation of hot-melt transfer inks for typewriter ribbons and carbon paper.
Prior to this invention, carbon transfer inks relied heavily on imported natural waxes—primarily carnauba, ouricury, and montan waxes. While natural carnauba wax produced a hard, non-smearing, high-gloss coating, it suffered from severe commercial drawbacks: inconsistent purity, market adulteration with low-grade paraffin, and volatile pricing. McGee and Schwartz developed a class of synthetic amido-ester waxes derived from standard raw materials that matched or exceeded the performance of natural waxes.
The Innovation: Tailored Amido-Esters
Natural carnauba wax derives its hardness and high melting point from a “bi-dipolar” ester structure. The inventors realized that introducing an amide linkage into a synthetic fatty ester would create strong intermolecular hydrogen bonding.
This molecular arrangement delivers two critical properties:
- Increased Hardness and Melting Point: Amido-esters display melting points between 80°C and 130°C, significantly higher than ordinary acid esters, preventing premature softening or smudging under friction.
- Orderly Molecular Packing: Hydrogen bonding aligns the hydrocarbon chains into a dense, compact crystal matrix, yielding a tough, non-tacky film that retains oils while resisting smear.
Key Formulation Components
In a representative carbon transfer ink formulation, the ingredients serve distinct, complementary roles:
| Component | Concentration | Function |
| Amido-Ester Synthetic Wax (e.g., N-methyl docosyl succinamate) | 30% to 50% by weight | Provides the crystalline wax matrix; ensures high melting point, film hardness, oil retention, and non-smearing characteristics. |
| Paraffin Ink Oil | 10% to 50% by weight | Polycyclic, high-boiling petroleum fraction that acts as the vehicle and carrier for the pigment. |
| Channel Carbon Black (e.g., Peerless, Kohinoor, Mogul) | 2% to 20% by weight | Long-flow pigment providing deep black coloration and opacity. |
| Nigrosine Oleate Toner | Up to 10% (optional) | Organic dye precipitated onto carbon black to enhance color tone, intensity, and dispersion. |
Performance: Printability and Transfer
The synthetic ink was subjected to mechanical printability tests on standard carbon-paper coating equipment:
- Five-Layer Copy Clarity: Formulations using amido-esters (such as N-methyl docosyl succinamate or docosyl glutaramate) demonstrated sharp legibility through the fifth carbon copy.
- Smear and Offsetting Resistance: The coatings duplicate carnauba’s clean release without excessive tack, strike-through, or flake-off.
- Versatility Beyond Inks: Because of their high gloss and water-repellent qualities, the synthesized waxes also proved directly usable in high-luster automobile paste polishes and synthetic high-temperature lubricants.
The Manufacturing Process
The inventors outlined a precise hot-melt preparation process:
- Charge the synthetic amido-ester wax and paraffin ink oil into a stainless steel mill.
- Heat the mixture until the wax fully liquefies (typically 83°C to 84°C for succinamate derivatives).
- Add channel carbon black and nigrosine toner, continuing heat input until the slurry reaches 100°C.
- Introduce heated steel balls and agitate the mill vigorously for approximately 10 minutes to thoroughly disperse the pigments as the temperature drops to roughly 75°C.
- Transfer the homogeneous hot-melt ink directly to a standard carbon-paper coating machine for application onto a paper backing.
Summary of Claims
The patent explicitly claims:
- A method of preparing hot-melt carbon transfer inks utilizing synthetic amido-ester wax compounds defined by the general formula R1-O-C(=O)-R2-C(=O)-NR3R4 (where R1 is a C16–C22 alkyl radical, R2 is a C2–C8 alkylene radical, and R3/R4 are hydrogen or C1–C22 alkyl radicals).
- Ink formulations combining 30% to 50% amido-ester wax, 10% to 50% paraffin oil, and 2% to 20% carbon black.
- The specific hot-melt milling process using heated steel balls to ensure thorough dispersion and homogeneity before web coating.
