I. Chemical Classification and Mechanism of Dry Strength Agents

  1. Cationic Starch

    • Molecular Structure: Cationic starch is modified through etherification by introducing quaternary ammonium groups (such as 2,3-epoxypropyltrimethylammonium chloride), with a typical degree of substitution (DS) of 0.02-0.05. 
      Starch-O-CH2-CH(OH)-CH2-N+(CH3)3 Cl-
    • Mechanism of Action:
      • Charge Neutralization: Binds electrostatically with negatively charged fiber groups (carboxyl, sulfate ester groups), increasing the Zeta potential to +5mV~+15mV.
      • Hydrogen Bond Enhancement: Hydroxyl groups form a three-dimensional network with fibers, increasing inter-fiber hydrogen bond density by approximately 2×10^18 bonds/g.
    • Experimental Data:
      • The addition of 1% cationic starch can increase the internal bond strength of paper (Scott Bond) by 30%-50% (TAPPI T569).
      • Optimal molecular weight range: 200,000-500,000 Da (Carbohydrate Polymers, 2021).

  2. Polyacrylamide (PAM) Dry Strength Agents

    • Anionic PAM:
      • Contains carboxyl (-COO⁻) groups, which bond with fibers via Ca²⁺ bridging (most effective at pH >6).
    • Cationic PAM:
      • Contains quaternary ammonium groups, directly binding to negatively charged fiber sites (optimal at charge density >2 meq/g).
    • Comparison of Strength Enhancement Efficiency:
      Type Tensile Strength Increase Burst Strength Increase Applicable pH Range
      Cationic Starch 30%-50% 20%-35% 4-9
      Cationic PAM 40%-60% 30%-45% 3-8
      Anionic PAM 20%-40% 15%-30% 6-10
      Data Source: Paper Chemistry and Technology, 2019

  3. Modified Natural Polymers (e.g., Chitosan Derivatives)

    • Carboxymethyl Chitosan (CMCS):
      • At a substitution degree (DS) >0.6, water solubility significantly improves, forming ionic-dipole interactions with fibers (Langmuir, 2020).
    • Environmental Benefits:
      • Biodegradation rate >90% (ISO 14855), with 65% lower carbon emissions than synthetic polymers (Carbon Trust certification).

II. Chemical Kinetics Optimization in the Application Process

  1. Chemical Compatibility in Wet-End Addition

    • Synergistic Effect of Cationic Starch and Aluminum Salts:
      • The presence of aluminum sulfate (Al₂(SO₄)₃) increases cationic starch adsorption on fiber surfaces by 2-3 times (Colloids and Surfaces A, 2018).
      • Optimal Al³⁺ concentration: 50-100 ppm (excessive amounts cause over-flocculation).
    • Enzyme Degradation Prevention:
      • Starch enzyme activity inhibition: Adding 0.05% methylene bis(thiocyanate) (MBT) maintains starch molecular weight retention above 95% (Bioresource Technology, 2022).

  2. Rheological Control in Size Press Applications

    • Oxidized Starch Viscosity Regulation:
      • The relationship between oxidation degree (carboxyl content) and viscosity: 
        η (mPa·s) = 250 - 15×[COOH] (mmol/g)
      • Industrially common oxidation degree: 0.08-0.12 mmol/g (viscosity range 80-120 mPa·s, Brookfield DV2T, 20°C).
    • Anti-Retrogradation Technology:
      • Adding 0.1%-0.3% hydroxypropylation agents (propylene oxide) prevents amylose recrystallization (XRD shows a 70% reduction in crystallinity).

  3. In-Situ Gelatinization in Formation Section Spraying

    • Thermodynamic Parameters:
      • Native starch gelatinization activation energy: 120-150 kJ/mol (DSC analysis).
      • Critical gelatinization conditions: Temperature >70°C for 30 seconds (Starch/Stärke, 2020).

III. Chemical Engineering Practices in Production Management

  1. Chemical Inhibition of Starch Retrogradation

    • Dynamic Storage Stability:
      Storage Time (h) Viscosity Retention (%) Bonding Strength Retention (%)
      0 100 100
      4 85 80
      8 60 50
      Solution: Add 0.05% sodium citrate (chelates Ca²⁺ to inhibit cross-linking)

  2. Microbial Control in the System

    • Biocide Selection:
      Type Minimum Inhibitory Concentration (ppm) Enzyme Inhibition Rate
      Isothiazolinone 50 92%
      DBNPA 30 88%
      Glutaraldehyde 100 75%

IV. Case Study: Chemical Process Design for High-Strength Corrugated Paper

Objective: Production of 200 g/m² AA-grade corrugated board with a required ring crush strength of ≥8.5 kN/m (ISO 3035).

Solution:

  1. Wet-End Chemistry:
    • Cationic starch (DS=0.03): 1.2% dosage, combined with 0.8% cationic PAM.
    • Aluminum sulfate: 0.6% dosage (based on dry pulp weight).
  2. Surface Enhancement:
    • Size press: 12% oxidized starch (carboxyl content 0.1 mmol/g), viscosity 90 mPa·s.
    • Spraying system: Native corn starch (particle size 15-25 μm), spray dosage 4 g/m².

Results:

  • Ring crush strength: 9.2 kN/m (38% improvement).
  • Starch total retention rate: 92% (traditional process: 75%-80%).

V. Future Technology Outlook

  1. Nanocellulose Reinforcement Systems:
    • Blending 2-5% nanocellulose (diameter 20 nm, aspect ratio >50) with starch further improves tensile strength by 15% (ACS Sustainable Chemistry & Engineering, 2023).
  2. Enzyme-Modified Starch:
    • Partial hydrolysis with α-amylase (DE value 5-8) enhances molecular chain flexibility, reducing gelatinization temperature by 10-15°C (Enzyme and Microbial Technology, 2022).
  3. Smart-Responsive Dry Strength Agents:
    • pH-sensitive polyelectrolytes (e.g., poly(dimethylaminoethyl methacrylate)) undergo self-crosslinking during drying (Angewandte Chemie, 2021).

VI. References

  1. Heinze, T. (2018). Starch Chemistry and Technology. Springer.
  2. TAPPI T569 om-15: Internal bond strength of paperboard.
  3. Zhang, Y. et al. (2021). “Cationic starch-fiber interactions: A combined QCM-D and AFM study”. Carbohydrate Polymers, 256, 117582.
  4. ISO 14855: Determination of the ultimate aerobic biodegradability under controlled composting conditions.