partial dentures 7 :denture base polymers Flashcards
What is a denture base? 🤔
The denture base is the part of the denture that supports and attaches the prosthetic teeth 🦷.
It replaces both hard and soft tissues (like the gingiva and alveolar bone) 🧱.
It also has an aesthetic 🌸 and functional 💪 role.
In all-acrylic dentures, it may act as the major connector too 🔗.
What are the essential requirements for a denture base material? ✅
🦷 Bonds to denture teeth
🧪 Biocompatible
✨ Highly polishable for aesthetics
🌈 Colour stable & translucent
🚫 No porosity (no bubbles in America 🇺🇸 during polymerisation!)
🧬 Low residual monomer (otherwise toxic)
💧 Low sorption/solubility
💥 Flexural strength > 65 MPa
📐 Flexural modulus > 2 GPa
📜 ISO 1567 defines these properties!
What other properties are desirable but not in ISO standard? 🧐
🔁 Resilience – energy absorbed until deformation
💥 Toughness – energy absorbed until fracture
🧊 High impact strength (dentures get dropped!)
📏 Dimensional stability
🛠 Repairable if broken
🔥 Good thermal conductivity (helps patients sense hot foods!)
🩻 Radio-opaque (can be seen on radiographs if swallowed)
💰 Cheap
What is PMMA and when was it developed? 🧪
🧫 PMMA = Polymethyl methacrylate
🔬 Developed in 1932, introduced for denture bases in 1935
By 1940, it replaced vulcanite! - car tyres
Used in other industries (e.g., windows/lenses – transmits 90% of light, more than glass!) 🌞🔍
Describe the 4 stages of polymerisation of PMMA 🔄
Activation - heat ,light,chemcial
Initiation 🔥 – free radical from initiator (benzoyl peroxide) starts reaction
Propagation ➕ – monomer units keep joining
Termination ⛔ – no more monomer = end of chain
What is the most common activation method for PMMA denture base? 🔥
Heat activation
Benzoyl peroxide (initiator) + heat > 60°C creates free radicals ⚡
Heat = activator, usually via water bath immersion 🛁
What happens during polymerisation in terms of volume? 📉
As monomers join to form polymers, space between them reduces = shrinkage
🧪 Pure MMA shrinkage = 21%!
To reduce this: pre-polymerized PMMA beads are added to limit volumetric shrinkage to 6% and 0.5% linear
What’s in the powder & liquid of heat-cured acrylic resin? 🧴🧂
Powder:
PMMA beads
Benzoyl peroxide (initiator)
Pigments, dyes 🎨
Optical pacifiers ✨
Plasticisers
Synthetic & coloured fibres 🧵
Liquid:
Methyl methacrylate (MMA)
Inhibitor 🛑
Cross-linking agent 🔗
What are the stages during mixing PMMA + MMA? 🧑🔬
Ratio: Powder:Liquid = 2:1
Sandy – beads just start melting
Stringy – swelling and thickening
Dough – gelation, best stage for moulding
Rubbery – monomer penetrates bead cores
What is the denture processing sequence? 🧰
Wax pattern
Flasking
Boiling out wax ♨
Dough moulding
Boiling (polymerisation)
Deflasking
⏳ Typically done overnight, ~8 hours
DENTURE MADE DURIGN LOS WAX TEHCNIQUE
WAX BOILED OUT - leaving space
What are the 3 types of porosity problems in processing? 🫧
Gaseous porosity- due to monomer boiling before polymerises
occurs deep within thickest parts
MMA boils at 100°C
Exothermic polymerisation → temp can hit 170°C 😬
Causes bubbles inside
Prevention: slow heating
Contraction porosity
Shrinkage due to polymerisation
Prevention: pressure & excess dough
Granular porosity
Occurs if dough is left too long and dries out
Leads to incomplete bonding ❌
What are the advantages of PMMA? 💎
😄 Great aesthetics
🔧 Easy to repair
✨ High polishability
💸 Cheap
🏗 Easy to manufacture
🦷 Bonds to teeth
🌬 Low water sorption
🧱 Good physical properties
What is Tg and why is it important? 🌡
Glass transition temperature (Tg) = temp at which an amorphous polymer goes from glass-like to rubbery
💧 Needs to be >70°C to withstand hot food/drink in mouth
🔝 Only PMMA has a Tg high enough!
What affects Tg? 🔬
Molecular structure 🧬
Large pendant groups = more spacing = lower Tg
Plasticisers also lower Tg
What are the disadvantages of PMMA? 👎
💥 Poor impact strength
❄ Poor thermal conductivity
💔 Denture fractures are common (costs NHS £9 million/year!)
📉 68% fracture within 3 years
upper dentures
🔺 Common fracture in midline (stress from palatal tori, notches)
How can properties of PMMA be improved? 🛠
Metal bars/mesh – 🛑 May worsen stress - areas
Co-polymerisation (e.g., with butadiene-styrene for high-impact acrylic) 💪 increases toughness
Reinforcement
Injection moulding – high tech, expensive 💰
What are the features of plastic acrylic teeth? 🦷
✅ Bonds chemically to denture base
🔧 Adjustable
🧩 Matches colour well
not wear of opposing teeth
⚠ Minor wear under high force occlusion
⚠ May stain over time ☕
What are other types of denture teeth materials? 🦷
🔹 Porcelain (high fusing ceramic) – aesthetic, hard, but brittle ❗
🔹 Fibre-reinforced – experimental
🔹 Polycarbonate/Nylon teeth – less common
Nylon used in flexible partials (Valplast, Flexiplast)
Aesthetic & flexible
BUT may ↑ bone resorption due to pressure 👎
What are the curing methods for denture base polymers? 💡🔥🧪
Chemically cured
Benzoyl peroxide + tertiary amine
Hydroquinone as inhibitor
Heat cured
Heat + pressure = good conversion, no porosity
Light cured
Blue light + photo-initiators
Used for custom trays, repair, record bases
What is the most common activation method for PMMA denture base? 🔥
Heat activation using a water bath 🛁
Benzoyl peroxide = initiator
Heat (>60°C) = activator
⚡ Heat + initiator = free radicals → start polymerisation
What causes shrinkage during polymerisation? 📉
Polymerisation pulls monomer chains closer together 🤏
Pure MMA shrinkage = 21% 😱
To reduce shrinkage:
Add pre-polymerised PMMA beads = 6% volumetric + 0.5% linear shrinkage 📉
