Types of Polymers and Resins

Vat photopolymerization processes primarily use radiation-curable resins as their primary materials.

Some visible light systems are used in photopolymerization processes.

Photopolymers were developed in the late 1960s and soon became widely applied in several commercial areas.

Stereolithography was the first vat photopolymerization process.

Longer polymer molecules yield higher molecular weights, indicating a more complete reaction

Fig. 4.4 shows the P–I term indicating a photoinitiator, $I$ symbol as a free radical, and $M$ as a monomer

Polymerization terminates from recombination, disproportionation, or occlusion

Occlusion occurs when free radicals become 'trapped' within a solidified polymer, affecting aging and mechanical properties

Cationic photopolymerization involves a photoinitiator generating a cation, reacting with a monomer, and a termination process

Basic raw materials like polyols, epoxides, and (meth) acrylic acids are used for radiation curing

Resin formulations include multifunctional monomers, polyol polyacrylates, and various oligomers

Photosensitizers are often used with photoinitiators, and various additives are used to achieve optimal performance

The photoinitiator's role is to convert light energy into reactive intermediates, either radicals or cationic species

In VP, radical photoinitiator systems include compounds that undergo unimolecular bond cleavage upon irradiation

The Irgacure family of radical photoinitiators from Ciba Specialty Chemicals is commonly used in VP

Photoinduced cationic polymerizations have received much less attention but have seen rapid development since the 1970s

Thermoplastic polymers are typically injection molded with a linear or branched molecular structure, while VP photopolymers are cross-linked and do not melt, exhibiting less creep and stress relaxation.

The first US patents for SL resins were published in 1989 and 1990, prepared from acrylates, which produced weak parts due to shrinkage and curling.

Acrylate-based resins typically could only be cured to 46% completion when exposed to the laser, leading to accuracy issues and curling during or after part fabrication.

Patents for epoxide composition for SL resins appeared in 1988 (Japanese), producing more accurate, harder, and stronger parts with lower shrinkage and reduced susceptibility to oxygen inhibition.

Epoxy resins have disadvantages of slow photospeed and brittleness, requiring the addition of acrylate to rapidly build part strength and reduce brittleness.

Most commercially available SL resins today are epoxides with some acrylate content to combine the advantages of both curing types and improve accuracy.

VP photopolymers consist of photoinitiators, reactive diluents, flexibilizers, stabilizers, and liquid monomers, reacting with UV radiation to start polymer chains and cross-link.

Two main types of photopolymer chemistry are evident: free-radical photopolymerization (acrylate) and cationic photopolymerization (epoxy and vinylether).

Acrylate photopolymers exhibit high photospeed but significant shrinkage and a tendency to warp and curl, thus rarely used without epoxy or other elements.

Epoxy SL resins typically have much smaller shrinkages and less tendency to warp and curl due to ring-opening polymerization, resulting in minimal volume change.

Almost all commercially available SL resins have significant amounts of epoxies, with polymerization being an exothermic reaction requiring a catalyst.

The free radical-initiated polymerization process involves the production of radicals from photons, which can lead to the polymerization of over 1,000 monomers.

Photopolymerization was developed in the late 1960s and found wide applications in commercial areas such as coating and printing industries.

Stereolithography (SL) technology was introduced in the mid-1980s.

Vat photopolymerization encompasses SL and related processes, using various types of radiation including gamma rays, X-rays, electron beams, UV, and visible light to cure commercial photopolymers.

Photopolymers are predominantly used in dentistry for sealing teeth surfaces and preventing cavities.

Vector scan and mask projection approaches in vat photopolymerization require scanning laser beams.

UV-curable photopolymers are used in a variety of applications including photoresists in the microelectronics industry.

In SL systems, UV radiation is used exclusively, although other types could be used.

Vat photopolymerization processes include vector scan, mask projection, and two-photon approaches.

3D Systems was created to market SL machines as 'rapid prototyping' machines to the product development industry.

In mask projection DMD-based systems, UV and visible light radiation are used.

The two-photon approach in vat photopolymerization fabricates parts below the resin surface without the need for recoating.

Vat photopolymerization processes use only visible light for curing commercial photopolymers.